PHYSICAL
GEOLOGY EXPLORING THE EARTH
James S. Monroe
ReedWicander
The Rock Cycle
(Figure 1-15)
Ridge axis '
The
Transform
Subduction zone
Zones
of extension within continents
Earth's Plates (Figure 1-13)
gp"
Upwelling
Asthenospnere Upwelling Lithosphere
"^ Three
Principle
Types of Plate Boundaries (Figure
1
-
14)
Uncertain plate boundary
PHYSICAL
GEOLOGY EXPLORING THE EARTH James
S.
Monroe
Reed Wicander Central Michigan University
WEST PUBLISHING COMPANY St.
Paul
New York
Los Angeles
San Francisco
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Printed in the United States of America
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99 98 97 96 95 94 93 92
8
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1
Edwin Church was one of America's premier landscape painters of the mid-nineteenth century. His paintings were magnificent in scope and sought to integrate realism with the majesty of nature. Cotopaxi, which shows the Ecuadoran volcano erupting, is an excellent example of Church's work. This painting was chosen for the cover because of its realism and to show how geology plays an integral part in the human Frederic
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1992 By WEST PUBLISHING 610 Opperman Drive P.O. Box 64526
LIBRARY OF CONGRESS CATALOGING-INPUBLICATION DATA Monroe,
J. S.
(James'S.)
Physical geology
:
exploring the Earth
/
James
S.
Monroe,
Reed Wicander. cm. p.
ISBN 0-314-00559 1.
Physical geology.
QE28.2.M655
550-dc20
-5
I.
Wicander, Reed, 1946-
.
II.
Title.
1992 91-29160
CIP (go)
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BRIEF CONTENTS
Preface
xvii
Developing Critical Thinking and Study 1
Understanding the Earth: Physical Geology 2
2
A
An
Skills
xxiii
Introduction to
17 Groundwater
80
6 Weathering, Erosion, and
158
Metamorphism and Metamorphic Rocks
9 Geologic Time
450 484
214
546
Shorelines and Shoreline Processes
190
Answers to Multiple-Choice and Review Questions 599 Glossary
250
11 The Interior of the Earth
514
572
134
Soil
7 Sediment and Sedimentary Rocks
10 Earthquakes
376
19 The Work of Wind and Deserts
20
340
and the
Building,
414
18 Glaciers and Glaciation
5 Igneous Rocks and Intrusive Igneous Activity 110
8
Unifying Theory
14 Deformation, Mountain
16 Running Water
54
4 Volcanism
A
Plate Tectonics:
Evolution of Continents
26
and Planets
314
13
15 Mass Wasting
History of the Universe, Solar System,
3 Minerals
12 The Sea Floor
Index
286
ys
$?*
Credits
601
617 633
Fill-in-the-Blank
1
ryyy%3
CONTENTS Preface
Chapter Summary 22 Important Terms 23 Review Questions 23 Additional Readings 24
xvii
Developing Critical Thinking and Study xxiii
Skills
CHAPTER
1
CHAPTER
UNDERSTANDING THE EARTH: An
Introduction to Physical Geology
Prologue 3 Introduction 5 What Is Geology?
5
Geology and the Human Experience 7 How Geology Affects Our Everyday Lives
Perspective 1—1: Public
Need
The Earth
as a
to
How Much
Know?
Dynamic
Science
8
Does the
10 Planet
1
Perspective 1-2: The Gaia Hypothesis
12
Geology and the Formulation of Theories 12 The Formulation of Plate Tectonic Theory 14
Guest
Essay: Science:
Plate Tectonic
Theory
Our Need
to
Know
16
The Rock Cycle 16 Geologic Time and Uniformitarianism
15
2
A HISTORY OF THE UNIVERSE, SOLAR SYSTEM, AND PLANETS Prologue 27 Introduction 28 The Origin of the Universe 29 The Origin and Early Development of the Solar System 30 General Characteristics of the Solar System 30 Current Theory of the Origin and Early History of 31 the Solar System Meteorites 33 The Planets 35
The Terrestrial Planets Mercury 35
35
Perspective 2—1: The Tunguska Event 21
Venus
36
38 Contents
vii
Perspective 2—2: The Evolution of Climate on the Terrestrial Planets 40
Mars 43 The Jovian Planets Jupiter 44 Saturn 44
Ferromagnesian Silicates 66 Nonferromagnesian Silicates 67 Carbonate Minerals 67
Perspective 3-1: Quartz— A
44
Useful Mineral
Other Mineral Groups
Uranus 45 Neptune and Pluto 45 The Origin and Differentiation of the Early Earth The Origin of the Earth-Moon System 49 Chapter Summary 51 Important Terms 52 Review Questions 52 Additional Readings 53
68
Physical Properties of Minerals
46
Common
68
Color and Luster 69 Crystal Form 70 Cleavage and Fracture Hardness 72 Specific Gravity 72
69
71
Perspective 3-2: Diamonds and
Pencil Leads
73
Other Properties 74 Important Rock-Forming Minerals 74 Mineral Resources and Reserves 76 Chapter Summary 78 Important Terms 78 Review Questions 78 Additional Readings 79
CHAPTER MINERALS Prologue 55 Introduction 56 Matter and Its Composition Elements and Atoms 56
56
Bonding and Compounds 58 Ionic Bonding 58 Covalent Bonding 59 Metallic and van der Waals Bonds Minerals 60
VOLCANISM Prologue
60
Naturally Occurring, Inorganic Substances
Guest
Essay: Mineralogy:
Pursuits
A
61
The Nature of
Crystals 62 Chemical Composition 62 Physical Properties 64 Mineral Diversity 64 Mineral Groups 64 Silicate Minerals 65
Contents
81
84 and Lava 85 Composition 85 Temperature 86 Viscosity 86 Volcanism 87 Volcanic Gases 87 Lava Flows and Pyroclastic Materials Introduction
Magma
60
Career with Diverse
Perspective 4—1: Volcanism System
87
in the Solar
88
Perspective 4-2: Volcanic Gases and Climate
90
Volcanoes 92 Shield Volcanoes
Perspective 5-1: Ultramafic Lava Flows 93
Andesite-Diorite
Perspective 4—3: Monitoring Volcanoes and Forecasting Eruptions 94 Cinder Cones 97 Composite Volcanoes Lava Domes 98 Fissure Eruptions 99
Guest
Other Igneous Rocks
122
Intrusive Igneous Bodies: Plutons
Dikes and
98
Laccoliths
Essay: Monitoring Volcanic Activity
Pyroclastic Sheet Deposits
Rhyolite-Granite
100
101
102 102 Plate Tectonics and Volcanism Volcanism at Spreading Ridges 103 Volcanism at Subduction Zones 105 106 Intraplate Volcanism Chapter Summary 107 Important Terms 107 Review Questions 108 Additional Readings 109 Distribution of Volcanoes
Sills
120
121 121 123
123
125
Volcanic Pipes and Necks 125 Batholiths and Stocks 125
Mechanics of Batholith Emplacement 126 Pegmatites 128 Plate Tectonics and Igneous Activity 129
Perspective 5-2: Complex Pegmatites
130
Chapter Summary 132 Important Terms 132 Review Questions 133 Additional Readings 133
CHAPTER
CHAPTER
5
WEATHERING, EROSION,
AND IGNEOUS ROCKS AND INTRUSIVE IGNEOUS ACTIVITY Prologue 111 Introduction 112 Igneous Rocks 113 Textures 113
Composition 115 Bowen's Reaction
Series
116 Assimilation 117 Magma Mixing 118 Classification 118 Ultramafic Rocks 119 Basalt-Gabbro 119 Crystal Settling
115
SOIL
Prologue 135 Introduction 136 Mechanical Weathering 137 Frost Action 138 Pressure Release 139 Thermal Expansion and Contraction
139
Perspective 6 — 1: Bursting Rocks and 140 Sheet Joints Activities of Organisms 141 Chemical Weathering 141 Solution 141 Oxidation 142 Hydrolysis 143
Perspective 6-2: Acid Rain
144
Contents
Chemical Sedimentary Rocks Limestone-Dolostone 168
Factors Controlling the Rate of Chemical
Weathering Particle Size
144 145
Climate 146 Parent Material Soil
The
Perspective 7—1: The Mediterranean Desert
146
Chert 171 Coal 172 Sedimentary Facies
148
Factors Controlling Soil Formation
Climate
149
149
Parent Material
Organic Activity
151 151
-"-Guest Essay: Environmental Geology: Sustaining
152
the Earth
and Slope 153 Time 153 153 Soil Erosion Weathering and Mineral Resources Chapter Summary 155 Important Terms 156 Review Questions 156 157 Additional Readings Relief
154
CHAPTER
Perspective 7-2: Persian Gulf Petroleum
CHAPTER METAMORPHISM AND METAMORPHIC ROCKS 162
Guest
Gas
Essay: Exploring for Oil and Natural
164 Sedimentary Rocks 165 166 Detrital Sedimentary Rocks Conglomerate and Sedimentary Breccia Sandstone 166
167
184
7
Prologue 159 Introduction 160 Sediment Transport and Deposition 160 Lithification: Sediment to Sedimentary Rock
Contents
173 Marine Transgressions and Regressions 174 Environmental Analysis 175 Sedimentary Structures 175 Fossils 177 Environment of Deposition 179 Sediments, Sedimentary Rocks, and Natural Resources 180 Petroleum and Natural Gas 181 Uranium 183 Banded Iron Formation 183 Chapter Summary 187 Important Terms 188 Review Questions 188 189 Additional Readings
SEDIMENT AND SEDIMENTARY ROCKS
Mudrocks
170
170
Evaporites
147 Soil Profile
168
Prologue 191 Introduction 193 The Agents of Metamorphism Heat 193 Pressure
194
Fluid Activity
166
193
Perspective
195
8 — 1:
Asbestos
196
Types of Metamorphism 197 Contact Metamorphism 197
1
Dynamic Metamorphism 200 Regional Metamorphism 200 Classification of Metamorphic Rocks 201 Foliated Metamorphic Rocks 201 Nonfoliated Metamorphic Rocks 205 Metamorphic Zones and Facies 206 Metamorphism and Plate Tectonics 208 Metamorphism and Natural Resources 208 Perspective 8—2: Graphite
210
Chapter Summary 211 Important Terms 211 Review Questions 211 Additional Readings 212
Radiocarbon Dating Methods
Perspective 9-2: Radon: The
239 Silent Killer
"•-Guest Essay: Paleontology: Tracing Life through
Time
244
Chapter Summary 247 Important Terms 248 Review Questions 248 Additional Readings 249
CHAPTER
CHAPTER
Prologue
251
Introduction
Prologue 215 Introduction 216 Early Concepts of Geologic Time and the Age of the Earth 216 James Hutton and the Recognition of Geologic
218
Methods 219 Fundamental Principles of Relative Dating 219 Unconformities 222 Applying the Principles of Relative Dating to the
Relative Dating
Reconstruction of the Geologic History of
223 227 Absolute Dating Methods an Area
Correlation
23 Atoms, Elements, and Isotopes
Perspective 9-1: Subsurface Correlation and the Search for Oil and Natural Gas 232
234
Long-Lived Radioactive Isotope Pairs
253
Rebound Theory 254 Seismology 255 The Frequency and Distribution of Earthquakes Elastic
Guest
Essay: Geology Meets Public Policy
Seismic Waves
258
260
261
Body Waves 261 Surface Waves 263 Locating an Earthquake 263 Measuring Earthquake Intensity and Magnitude Intensity 264 Magnitude 266 The Destructive Effects of Earthquakes 269 Ground Shaking 269
264
Perspective 10-1: Designing Earthquake-Resistant Structures 270
231
Radioactive Decay and Half-Lives Sources of Uncertainty 235
10
EARTHQUAKES
9
GEOLOGIC TIME
Time
240
Tree-Ring and Fission Track Dating Methods 242 The Development of the Geologic Time Scale 243
239
273 Tsunami 274 Fire
Ground Failure 275 Earthquake Prediction 276 Earthquake Precursors 276 Contents
xi
Dilatancy
Model
278
Earthquake Prediction Programs 279 Earthquake Control 280 -^Perspective 10-2: A Predicted Earthquake That Didn't Occur
Chapter Summary 312 Important Terms 312 Review Questions 312 Additional Readings 313
281
Chapter Summary 283 Important Terms 284 Review Questions 284 Additional Readings 285
THE SEA FLOOR Prologue
THE INTERIOR OF THE EARTH Prologue 287 Introduction 288
The Discovery of the Earth's Core 290 Density and Composition of the Core -•-Guest Essay: Geology:
Rewarding Career
An Unexpected But
293
297 Internal Heat
295
Earth's Crust Earth's
297
the Mantle
298
^Perspective 11-2: Seismic Tomography
302
303 Earth's Magnetic Field 306 Inclination and Declination of the Magnetic Field 307 Magnetic Anomalies 309 Magnetic Reversals 310
The The
Principle of Isostasy
Contents
320
322
323
329
Seamounts, Guyots, and Aseismic Ridges 329 -^Perspective 12-2: Maurice Ewing and His Investigation of the Atlantic
300
Measuring Gravity
Submarine Fans 322 Types of Continental Margins The Deep-Ocean Basin 325 Abyssal Plains 325 Oceanic Trenches 326 Oceanic Ridges 326 Fractures in the Sea Floor
-^Perspective 11-1: Kimberlite Pipes -Windows to
Heat Flow
Rise
Turbidity Currents, Submarine Canyons, and
291
Structure and Composition of the Mantle
The The
-
The Continental Slope and
294
The Mantle
316 '
Oceanographic Research 317 Continental Margins 318 The Continental Shelf 319 ^Perspective 12-1: Lost Continents
289
Seismic Waves
315
Introduction
301
Deep-Sea Sedimentation
Ocean
330
330
332 Composition of the Oceanic Crust Resources from the Sea 334 Chapter Summary 337 Important Terms 338 Review Questions 338 Additional Readings 339
Reefs
334
and the Distribution of
Plate Tectonics
Natural Resources 371 Chapter Summary 373 Important Terms 373 Review Questions 374 Additional Readings 375
CHAPTER
13
PLATE TECTONICS:
A Unifying Prologue
Theory
341
CHAPTER
342
Introduction
Alfred Wegener and the Continental Drift
Hypothesis
The Evidence
DEFORMATION, MOUNTAIN AND THE EVOLUTION OF CONTINENTS
344
BUILDING,
345
for Continental Drift
Continental Fit
345
Rock Sequences and Mountain Ranges 346 Glacial Evidence 347 Fossil Evidence 349 Paleomagnetism and Polar Wandering 349 Similarity of
Sea-Floor Spreading 351 "^ Perspective 13 — 1: Paleogeographic Maps
Prologue 377 Introduction 378
Deformation 379 Strike and Dip 379 Folds
352 """
384
Domes and Joints Faults
Basins
385
386 389
^"Perspective 14—1: Folding, Joints, and
Convergent Boundaries 361 Oceanic-Oceanic Boundaries 362 Oceanic-Continental Boundaries 363 Continental-Continental Boundaries 364
"^ Guest Essay: Geoscience Careers— The Diversity Unparalleled 365 Plate
368
Plate Tectonics
381
Guest Essay: Studying the Earth: Reflections of an Enthusiast
^Perspective 13-2: Tectonics of the Terrestrial Planets 358
The Driving Mechanism of
380
Monoclines, Anticlines, and Synclines Plunging Folds 383
Deep-Sea Drilling and the Confirmation of Sea-Floor Spreading 355 Plate Tectonic Theory 357 Plate Boundaries 357 Divergent Boundaries 357
Transform Boundaries 366 Movement and Motion 366 Hot Spots and Absolute Motion
14
343
Early Ideas about Continental Drift
369
Arches
390
Dip-Slip Faults Strike-Slip Faults
is
391 393
Oblique-Slip Faults 394 Mountains 395 Types of Mountains 396 Mountain Building: Orogenesis 397 Plate Boundaries and Orogenesis 397
Orogenesis at Oceanic-Oceanic Plate Boundaries 397
Contents
xiii
Orogenesis at Oceanic-Continental Plate Boundaries 399 Orogenesis at Continental-Continental Plate Boundaries 399 ^"Perspective 14—2:
The Origin of Rocky Mountains 400
the
The Origin and Evolution of Continents Shields, Cratons, and the Evolution of Continents 405
Flows
433
Complex Movements
437
Recognizing and Minimizing the Effects of
Mass Movements ""'Perspective
439
15-2: The Vaiont
Dam
Disaster
440
Chapter Summary 448 Important Terms 448 Review Questions 449 Additional Readings 449
405
^Perspective 14—3: Plate Tectonic History of the Appalachians 406 Microplate Tectonics and Mountain Building Chapter Summary 410 Important Terms 411 Review Questions 411 Additional Readings 412
408
CHAPTER
16
RUNNING WATER
CHAPTER
Prologue 451 Introduction 452 The Hydrologic Cycle
15
MASS WASTING
452 Running Water 454 Sheet Flow versus Channel Flow Stream Gradient 456 Velocity and Discharge 457
455
"^ Guest Essay: Managing Our Water Resources Prologue 415 Introduction 417
Mass Wasting 418 419 Weathering and Climate 420 Water Content 420 Vegetation 420 Overloading 421 Geology and Slope Stability 421 Triggering Mechanisms 421 "^ Perspective 15—1: The Tragedy at Aberfan, Wales 422
Factors Influencing
Slope Gradient
Types of Mass Wasting Falls
Slides
424
425 426
"•'Guest Essay: Cleansing the Earth— Waste
Management xiv
Contents
427
Stream Erosion 459 Transport of Sediment Load 460 Stream Deposition 461 Braided Streams and Their Deposits 462 Meandering Streams and Their Deposits 463 Floodplain Deposits
464
"^ Perspective 16—1: Predicting and Controlling Floods 465 Deltas
466
Alluvial Fans
469
Drainage Basins and Drainage Patterns Base Level 472 The Graded Stream 474 Development of Stream Valleys 475 Superposed Streams 476 Stream Terraces 477 Incised
Meanders
478
470
458
"^ Perspective 16—2: Natural Bridges
479
Chapter Summary 480 Important Terms 480 Review Questions 481 Additional Readings 482
CHAPTER GLACIERS
CHAPTER
17
Prologue 485 Introduction 486
524 U-Shaped Glacial Troughs 524 Hanging Valleys 526 Cirques, Aretes, and Horns 526 Erosional Landforms of Continental Glaciers 528 Glacial Deposits 528 Landforms Composed of Till 528 End Moraines 528 Lateral and Medial Moraines 530 Drumlins 530 Landforms Composed of Stratified Drift 531 Outwash Plains and Valley Trains 531 Karnes and Eskers 531 532 Glacial Lake Deposits Pleistocene Glaciation 533
Groundwater and the Hydrologic Cycle 486 Porosity and Permeability 487 The Water Table 488 Groundwater Movement 489 Springs, Water Wells, and Artesian Systems 489 Springs 490 Water Wells 491 "^ Perspective 17—1: Mammoth Cave National
492
493 Groundwater Erosion and Deposition 495 Sinkholes and Karst Topography 495 Caves and Cave Deposits 496 Modifications of the Groundwater System and Their Effects 498 Lowering of the Water Table 500 Saltwater Incursion 500 Subsidence 502 Groundwater Contamination 504 "^ Perspective 17—2: Radioactive Waste Disposal Artesian Systems
Hot
Springs and Geysers
506 Geothermal Energy 509 Chapter Summary 511 Important Terms 512 Review Questions 512 Additional Readings 513
AND GLACIATION
Prologue 515 Introduction 516 Glaciers and the Hydrologic Cycle 516 The Origin of Glacial Ice 517 Types of Glaciers 518 The Glacial Budget 519 Rates of Glacial Movement 520 Glacial Erosion and Transport 522 Erosional Landforms of Valley Glaciers
GROUNDWATER
Park, Kentucky
18
^Perspective 18 — 1: Glacial Lake Missoula and the Channeled Scablands 534
536 and Proglacial Lakes
Pleistocene Climates Pluvial
506
"^ Perspective 18—2: Great Lakes 538
A
537
Brief History of the
539 540 Causes of Glaciation 540 The Milankovitch Theory 541 Short-Term Climatic Events 541 Chapter Summary 542 Changes
in
Sea Level
Glaciers and Isostasy
Contents
xv
Important Terms 543 Review Questions 543 Additional Readings 544
CHAPTER
20
SHORELINES AND SHORELINE PROCESSES
CHAPTER
19
Prologue 573 Introduction 574
THE WORK OF WIND
Wave Dynamics 575 Wave Generation 576
AND DESERTS Prologue 547 Introduction 549 Sediment Transport by
^Guest
Wind
549
on Mars
Wind
Wave
Wind
Activity
552
^Perspective 19—2: Death Valley National
562
Weathering and
Soils 564 Mass Wasting, Streams, and Groundwater Wind 566 Desert Landforms 566 Chapter Summary 569 Important Terms 570 Review Questions 570
Additional Readings xvi
Contents
Refraction and Longshore Currents Rip Currents 580 Shoreline Deposition 581 Beaches 582 Seasonal Changes in Beaches 583
and Bay mouth Bars 584 585 The Nearshore Sediment Budget Shoreline Erosion 587
580
Spits
552 The Formation and Migration of Dunes 553 Dune Types 554 Loess 556 Air Pressure Belts and Global Wind Patterns 558 The Distribution of Deserts 559 Characteristics of Deserts 561 Temperature, Precipitation, and Vegetation 561 Deposits
Monument
577
^Perspective 20—1: Waves and Coastal Flooding 579
Bed Load 549 Suspended Load 550 Wind Erosion 550 Abrasion 550 Deflation 551 ^Perspective 19 — 1: Evidence of
576
Essay: Geophysics and the Search for Oil
Shallow- Water Waves and Breakers Nearshore Currents 578
571
565
Barrier Islands
587
^ Perspective 20—2: Rising Sea Level and Coastal
Management
588
Wave-cut Platforms and Associated Landforms Types of Coasts 592 Submergent and Emergent Coasts 592 Tides 594 Chapter Summary 596 Important Terms 597 Review Questions 597 Additional Readings 598
591
Answers to Multiple-Choice and Fill-in-the-Blank Review Questions 599 Glossary 601 Index 617 Credits 633
T^^^^^^mj^^r» ^^m. ^^^^^^^^^^K^^m.^^ ^^^^^^^^ ^
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PREFACE
The Earth
dynamic planet that has changed contin4.6 billion years of existence. The size, shape, and geographic distribution of the continents and ocean basins have changed through time, as have the atmosphere and biota. Over the past 20 years, bold new theories and discoveries concerning the Earth's origin and how it works have sparked a renewed interest in geology. We have become increasingly aware of how fragile our planet is and, more importantly, how inter-
students can see, through relevant and interesting exam-
dependent all of its various systems are. We have learned that we cannot continually pollute our environment and that our natural resources are limited and, in most cases, nonrenewable. Furthermore, we are coming to realize how central geology is to our everyday lives. For these and other reasons, geology is one of the most important college or university courses a student can take. Physical Geology: Exploring the Earth was designed for a one-semester introductory course in geology that serves both majors and nonmajors in geology and the Earth sciences. It was written with the student in mind. One of the problems with any introductory science course is that the students are overwhelmed by the amount of material that must be learned. Furthermore, most of the material does not seem to be linked by any unifying theme and does not always appear to be rele-
logic
is
a
uously during
vant to their
One
its
lives.
of the goals of this book
ples,
how
geology impacts our
lives.
^ TEXT ORGANIZATION is the unifying theme of geology book. This theory has revolutionized geology because it provides a global perspective of the Earth and allows geologists to treat many seemingly unrelated geo-
Plate tectonic theory
and
this
phenomena
as part of a total planetary system.
Because plate tectonic theory
duced
in
Chapter
1,
and
is
is
so important,
it is
intro-
discussed in most subsequent
chapters in terms of the subject matter of that chapter.
We have organized Physical Geology: Exploring the Earth into several informal categories. Chapter 1 is an introduction to geology,
its
relevance to the
human
perience, plate tectonic theory, the rock cycle, logic time
ex-
and geo-
and uniformitarianism. Chapter 2 discusses and planets,
the origin of the universe, the solar system
and the Earth's place in the evolution of this larger system. Chapters 3-8 examine the Earth's materials (minerals and igneous, sedimentary, and metamorphic rocks) and the geologic processes associated with them including the role of plate tectonics in their origin and distribution. Chapter 9 discusses geologic time, introduces several dating methods, and explains how geologists
10—14
is to provide students with a basic understanding of geology and its processes
correlate rocks. Chapters
and, more importantly, with an understanding of how geology relates to the human experience; that is, how geology affects not only individuals, but society in gen-
deformation and mountain building, and plate tectonics. Chapters 15-20 cover the Earth's surface processes.
eral.
With
this goal in
mind,
we
introduce the major
themes of the book in the first chapter to provide students with an overview of the subject and enable them to see how the various systems of the Earth are interrelated. We also discuss the economic and environmental aspects of geology throughout the book rather than treating these topics in separate chapters. In this
way
deal with the related
topics of the Earth's interior, the sea floor, earthquakes,
We have found, as have many of the reviewers of this book, that presenting the material in this order works well for most students. We know, however, that many instructors prefer an entirely different order of topics depending on the emphasis in their course. We have therefore written this
book
so that instructors can present
the chapters in any order that suits the needs of their course.
Text Organization
xvii
^ CHAPTER ORGANIZATION All chapters have the
Prologues
same organizational format. Each
chapter opens with a photograph relating to the chapter material, a detailed outline,
and a prologue, which
is
designed to stimulate interest in the chapter material by discussing
The
some aspect of
text
is
introductory prologues focus on the
human
aspects of geology such as the eruption of Krakatau
(Chapter
1),
the
Loma
Prieta earthquake (Chapter 10),
or the story of Floyd Collins (Chapter 17).
the chapter in detail.
written in a clear informal style,
comprehend
easy for students to
Many of the
making
it
Numer-
the material.
Economic and Environmental Geology
ous diagrams and photographs complement the text, providing a visual representation of the concepts and
The
information presented. Each chapter contains at least two Perspectives that present a brief discussion of an
in separate chapters at the
interesting aspect of geology or geological research.
nomic and environmental geology with the chapter material helps students see the importance and relevance of
The end-of-chapter
materials begin with a concise
topics of environmental
and economic geology are
discussed throughout the text rather than being treated
many
end of the book as
is
done
in
other physical geology books. Integrating eco-
many
review of important concepts and ideas in the Chapter
geology to their
Summary. The Important Terms, which are printed in boldface type in the chapter text, are listed at the end of each chapter for easy review, and a full glossary of important terms appears at the end of the text. The Review
with a section on resources, further emphasizing the im-
book; they include multiple-choice questions with answers as well as short answer and essay questions. Each chapter Questions are another important feature of
concludes with a
list
which are written
lives. In
addition,
portance of geology in today's world.
Perspectives
this
of Additional Readings,
many
of
at a level appropriate for beginning
students interested in pursuing a particular topic.
The chapter
perspectives often focus
asbestos and graphite (Chapter 8), radioactive waste dis-
posal (Chapter 17), and wind activity on 19).
The
it
The
many
fascinating
perspectives can be assigned as
part of the chapter reading, used as the basis for lecture
number of special
or discussion topics, or even used as the starting point features that set
apart from other physical geology textbooks.
them
Mars (Chapter
topics for the Perspectives were chosen to pro-
aspects of geology.
» SPECIAL FEATURES
on aspects of en-
vironmental, economic, or planetary geology such as
vide students with-tan overview of the
This book contains a
chapters close
Among
and study skills section, the chapter prologues, guest essays by people who chose
for student papers.
are a critical thinking
geology or geologically related the integration of
fields for their careers,
economic and environmental geologic
throughout the book, and a set of multiple-choice questions with answers for each chapter.
Guest Essays
A number of guest essays
are interspersed throughout the
book. These essays focus on three themes— how and
issues
the individuals
became
career, their current areas of research,
and the possible
ciopolitical ramifications of their specific field.
Study
why
interested in geology as a potential so-
The essayists
Randolph H. Bromery (University of MassachuAmherst and former president of the Geological Society of America), Susan M. Landon (a consulting geologist), Michael L. McKinney (a paleontologist at the University of Tennessee), Malcolm Ross (United States Geological Survey), and Steve Stow (head of nuclear waste include
Skills
setts at
Immediately following the Preface is a section devoted to developing critical thinking and study skills. This section contains hints to help students improve their study habits, prepare for
exams, and generally get the most tips can be
out of every course they take. While these helpful in any course,
relevant to geology.
many
Whether you
are just beginning col-
about to graduate, take a few minutes to read over this section as these suggestions can help you in your studies and later in life. lege or
xviii
Preface
disposal at
Oak
Ridge National Laboratories).
of them are particularly
Planetary Geology Planetary geology at the
is
discussed in Chapter 2 rather than
end of the book as
it is
in
many
other physical
geology textbooks. This early coverage of comparative planetary geology allows meaningful examples to be in-
try
troduced later in the book. Furthermore,
priate topical films.
student to understand
it
enables the
how the origin and early evolution
organized by region, all images from the textbook, animated sequences, quiz frames, and clips from appro-
Two
slide sets will
be provided. The
first set will
and
include 150 of the most important and attractive figures
The book has been planned,
however, so that Chapter 2 can be covered at any time
and photographs of rocks and minerals, as well as photographs from the book, and the second set will contain
in the course or omitted altogether
at least
of the Earth
fit
into the larger context of the origin
history of the solar system.
wishes.
The planetary examples
if
the instructor
later in the
book are not
dependent on the student having read Chapter
300 slides illustrating important geologic feaThe majority of these photographs will be from
North America, but examples from around the world and the solar system will also be provided.
2.
Transparency masters of the important charts, graphs, and figures will be available as well as a set of full-color
Review Questions Most
tures.
physical geology books have a set of review ques-
transparency acetates to provide clear and effective
illus-
An important
end of each chapter. This book, however, includes not only the usual essay and thought-provoking
trations of important
questions, but also a set of multiple-choice questions,
same
something not found in other physical geology textbooks. The answers to the multiple-choice questions are at the end of the book so that students can check their answers and increase their confidence before taking an
example, volcano and earthquake distributions and plate
examination.
disclosures. This will ensure that
tions at the
artwork from the
feature of the transparencies size,
is
that the
text.
maps will
all
be the
so they can be used as overlays to show, for
boundaries.
A
Newsletter will be provided to adopters each year book with recent and relevant research
to update the
most current information
your students have the
available.
Lastly, in addition to publishing a separate student
Unique
Illustrations
study guide,
we
have incorporated
much
of the material
usually found in such guides into the
depicting geologic processes or events are block dia-
book itself. This saves students time and money and also makes the book a more valuable learning tool. For those students who want fur-
grams rather than cross sections so that students can
ther study aid, a study guide
The
figures include
many
pieces of original artwork de-
signed especially for this book.
Many
of the illustrations
more
easily visualize the salient features of these pro-
cesses
and
human
on the
events. In an effort to focus attention
aspects of geology,
paintings, drawings,
and
we have
also included
many
also available.
^ ACKNOWLEDGMENTS As the authors, we
historical photographs.
is
are, of course, responsible for the
organization, style, and accuracy of the text, and any mistakes, omissions, or errors are our responsibility.
» INSTRUCTOR ANCILLARY
finished product
MATERIALS To
assist
you
in
teaching this course and supplying your
students with the best in teaching aids, West Publishing
Company
has prepared a complete supplemental pack-
age available to
all
Instructor's
Manual
will include
teaching ideas, lecture outlines (including notes on ures
and photographs available
videodisc for use in lecture has been developed to
accompany
the text.
work during which we received numerous comments and advice from many geologists who reviewed parts of the text.
We
wish to express our sincere appreciation to whose contributions were in-
the following reviewers
The videodisc
includes,
Gary C. Allen
fig-
as slides), teaching tips,
Consider This lecture questions, Enrichment Topics, global examples, slides, transparency masters and acetates as well as a computerized test bank.
A
The
the culmination of several years of
valuable:
adopters.
The Comprehensive
is
among
other things, a wealth of images from around the coun-
University of
New
Orleans
R. Scott Babcock
Western Washington University
Kennard Bork Denison University
Thomas W. Broadhead University of Tennessee at Knoxville
Acknowledgments
xix
Anna
James F. Petersen Southwest Texas State University
Buising
Hayward
California State University at F. Howard Campbell HI James Madison University
Katherine H. Price
Larry E. Davis
Washington State University
William D. Romey St. Lawrence University
Noel Eberz
Gary Rosenberg
California State University at San Jose
Indiana University, Purdue University at Indianapolis
Allan A. Ekdale
David B. Slavsky Loyola University of Chicago
DePauw
University of Utah
Stewart
S.
Edward
Farrar
University
F.
Stoddard
Eastern Kentucky University
North Carolina
Richard H. Fluegeman,
Charles
Jr.
J.
State University
Thornton
Pennsylvania State University
Ball State University
William
P.
Samuel
Fritz
B.
Upchurch
Georgia State University
University of South Florida
Kazuya Fujita Michigan State University
John R. Wagner Clemson University
Norman Gray
We
University of Connecticut
Jack Green
also wish to
thank Professor Emeritus Richard
V.
Dietrich of Central Michigan University for reading var-
California State University at
Long Beach
David R. Hickey Lansing Community College
ious drafts of the book, providing us with several pho-
tographs, and discussing various aspects of the text with
on numerous occasions.
us
In addition,
we
are grateful
University of Texas at Austin
Geology Department of Central Michigan University for reading various drafts and providing us with photographs. They are David J. Matty, Jane M. Matty, Wayne E. Moore, and Stephen D. Stahl. We also thank Mrs. Martha Brian of the Geology Department, whose word processing skills and general efficiency were invaluable during the preparation of the manuscript, and Bruce M. C. Pape of the Geography Department for providing photographs. David Hickey de-
Richard H. Lefevre
serves special thanks for his assistance with the devel-
Grand
opment of many of
R.
to the other membtJrs of the
W. Hodder
University of Western Ontario
Cornells Klein University of
New
Mexico
W
Lawrence Knight William Rainey Harper College Martin
I. P.
B.
Lagoe
Valley State University
Martini
University of Guelph, Ontario
Michael McKinney University of Tennessee
at Knoxville
California State University at Fresno
Carleton Moore Arizona State University P.
Morris
Harold Pelton
Preface
are also grateful for the generosity of the various
Community
College
many
countries
who
pro-
vided photographs.
must go to Jerry Westby, college ediWest Publishing Company, who made many valuable suggestions and patiently guided us Special thanks
torial
University of Texas at San Antonio
Seattle Central
We
agencies and individuals from
Robert Merrill
Alan
the excellent ancillaries for the text,
and for proofing all of the illustrations in the text. Additionally, we wish to acknowledge the fine efforts of Kathleen Chiras in coordinating the Guest Essay feature.
manager
for
through the entire project. His continued encouragement provided constant inspiration and helped us pro-
duce the best possible book. We are equally indebted to our production manager, Barbara Fuller, whose atten-
tion to detail
and consistency
is
greatly appreciated as
are her unflagging efforts and diligence in securing
many
sponsible for
We would
of the photographs and paintings used in the book. Bar-
them.
bara was especially helpful in responding to our
tion manager,
last-
minute concerns as she guided the book through final We would also like to thank Patricia Lewis
production.
for her excellent copyediting
and indexing
skills.
We
appreciate her help in improving our manuscript. Be-
cause geology
is
such a visual science,
thanks to Carlyn Iverson
and
to the artists
who
we extend
of the rest of the art program. They
we enjoyed working with
also like to
Ann
acknowledge our promo-
Hillstrom, for her help in the devel-
opment of
the promotional poster that is available with book, and Maureen Rosener, marketing manager, who developed the excellent videodisc that accompanies this book. this
Our
special
rendered the reflective art at Precision Graphics who were re-
much
did an excellent job, and
families
were patient and encouraging when most
of our spare time and energy were devoted to this book.
We
thank them for their support and understanding.
Acknowledgments
xxi
DEVELOPING CRITICAL THINKING AND STUDY SKILLS * INTRODUCTION
beneficial, waiting until the last
demanding and important time, a time when your values will be challenged, and you will try out new ideas and philosophies. You will make personal and career decisions that will affect your entire life. With this new freedom you will enjoy, one of the most important things you must learn is how to balance your time among work, study, and recreation. If you develop good time management and study skills early in your college career, you will find that your college years will be successful and rewarding. This section offers some suggestions to help you maximize your study time and develop critical thinking and College
study
is
a
skills
that will benefit you, not only in college, but
throughout your course
and
is
life.
While mastering the content of a
obviously important, learning
to think critically
portant. Like
is,
most things
in
many ways,
how far
to study
more im-
in life, learning to think crit-
and study efficiently will initially require addiand effort, but once mastered, these skills save you time in the long run.
ically
tional time will
You may already be gestions
and may
familiar with
find that others
to you. Nevertheless,
if
many
do not
specific goals
basis,
It is easy to fall into the habit of eating nothing but junk food and never exercising. To be mentally alert, you must be physically fit. Try to develop a program of fit.
regular exercise. ergy, feel better,
to read this
avoiding pro-
While procrastination provides temporary you have avoided doing something you did not want to do, in the long run procrastination leads to stress. While a small amount of stress can be crastination.
satisfaction because
You
will find that
and study more
you have more en-
efficiently.
^ GENERAL STUDY SKILLS Most courses, and geology vious material, so
it is
in particular, build
upon
pre-
extremely important to keep up
with the coursework and
set aside regular time for study each of your courses. Try to follow these hints, and you will find you do better in school and have more time
in
for yourself:
tively. is
greatly reduce the temptation to procras-
better to
of the sug-
and apply the appropriate suggestions to your we are confident that you will become a better and more efficient student, find your classes more rewarding, have more time for yourself, and get better grades. We have found that the better students are usually also the busiest. Because these students are busy with work or extracurricular activities, they have had to learn to study efficiently and manage their time effecof the keys to success in college
is
which is usually what happens when you procrastinate. Another key to success in college is staying physically
•*»
situation,
One
clear,
and working toward them on a regular
work efficiently for short periods of time than to put in long, unproductive hours on a task,
section
own
you can
tinate. It
directly apply
you take the time
minute usually leads to
mistakes and a subpar performance. By setting
»
»
Develop the habit of studying on a daily basis. Set aside a specific time each day to study. Some people are day people, and others are night people. Determine when you are most alert and use that time for study. Have an area dedicated for study. It should include a well-lighted space with a desk and the study materials you need, such as a dictionary, thesaurus, paper, pens and pencils, and a computer if you have one. Study for short periods and take frequent breaks, usually after an hour of study. Get up and move around and do something completely different. This will help you stay alert, and you'll return to your studies with renewed vigor.
General Study
Skills
xxiii
Try to review each subject every day or at least the day of the class. Develop the habit of reviewing lecture material from a class the same
example, pt (plate tectonics), iggy (igneous), meta (metamorphic), sed (sedimentary), rx
day.
years),
"v Become familiar with the vocabulary of the course. Look up any unfamiliar words in the glossary of your textbook or in a dictionary.
(rock or rocks), ss (sandstone),
and
my
(million
gts (geologic time scale).
Rewrite your notes soon after the lecture. Rewriting your notes helps reinforce what you heard and gives you an opportunity to
Learning the language of the discipline will help
determine whether you understand the material.
you learn the
^ GETTING THE MOST FROM
By learning the vocabulary of the discipline before the lecture, you can cut down on the amount you have to write— you won't have to write down a definition if you already know
YOUR NOTES
the word.
material.
you are to get the most out of a course and do well on exams, you must learn to take good notes. This does not mean you should try to take down every word your If
good note taker is knowing what is important and what you can safely leave out. Early in the semester, try to determine whether the
professor says. Part of being a
lecture will follow the textbook or be
predominantly
much
covered in the
new
material.
If
when
the material
is
new. In any case, the
is
make you
following suggestions should
taker and enable you to derive the
a better note
maximum amount of
information from a lecture: -^-
would appear on a
(They were usually
to class regularly,
what
if
the screen, If
somewhat
familiar with the
everything. Later a few key
words or phrases
your memory as to what was said. Before each lecture, briefly review your notes from the previous lecture. Doing this will refresh your memory and provide a context for will jog
material.
own style of note taking. Do not down every word. These are notes
It is
sit
near the front of
easier to hear
and there are fewer
the professor allows
it,
distractions.
tape record the
but don't use the recording as a
lecture,
is
down
and
and see on the board or projected onto
possible.
written
is
chapter the lecture will cover before class. This
substitute for notes. Listen carefully to the
and write down the important points; in any gaps when you replay the
lecture
then
fill
tape.
and they are available, These are usually taken by a graduate student who is familiar with the
If
your school allows
buy
it,
class lecture notes.
Develop your
material; typically they are quite
try to write
comprehensive. Again use these notes to supplement your own. Ask questions. If you don't understand
you're taking, not a transcript. Learn to abbreviate and develop your
own set of common words
abbreviations and symbols for
example, w/o (without), w (equals), (above or increases),
and phrases: (with),
=
for
A
(below or decreases),
a
Pay particular attention to the professor's examples. These usually elucidate and clarify an important point and are easier to remember
way you
new
test.
Check any unclear points in your notes with classmate or look them up in your textbook.
Go
as the textbook or supplements
being said rather than trying to write
xxiv
when I stated something twice during a lecture, they knew it was important and probably
the class
the
down and highlight it told me (RW) that
it
some way. Students have
same material
concepts and can listen critically to what
-*«•
he or
than an abstract concept.
the reading assignment, read or scan the
-w-
in
If
important or repeats a
point, be sure to write
Regardless of whether the lecture discusses the
will be
is
right!)
of the material
textbook, your notes do not have to be as extensive or detailed as
Learn the mannerisms of the professor. she says something
V
<
(less than),
>
(greater
something, ask the professor. are reluctant to
do
lecture hall, but
if
Many
students
this, especially in a large
you don't understand
a
point, other people are probably confused as
you can't ask questions during
than), &c (and), u (you).
well. If
Geology lends itself to many abbreviations that can increase your note-taking capability: for
lecture, talk to the professor after the lecture or
Developing Critical Thinking and Study
Skills
during office hours.
a
^
GETTING THE MOST OUT OF
Whenever you encounter new facts, ideas, or concepts, be sure you understand and can
WHAT YOU READ
define all of the terms used in the discussion.
"you get out of something what you put into it" is very true when it comes to reading textbooks. By carefully reading your text and following these suggestions, you can greatly increase your under-
Determine
how
derived.
the facts were derived from
standing of the subject:
repeated?
The old adage
that
fusion
is
an excellent example.
Two
scientists
claim to have produced cold fusion reactions using simple experimental laboratory
chapter before you start to read in depth.
apparatus, yet other scientists have as yet been unable to achieve the same reaction by repeating the experiments. •-
logical or
bold face or
on previous
material,
it is
Look
What
critically
is
particularly important in learning
it to what you already know. Although you can't know everything, you can learn to question effectively and arrive at conclusions consistent with the facts. Thus, these suggestions for critical thinking can help you in all your courses:
material and relating
how
dam
how
across a river that
will be the
consequences to the beaches
One of the most important lessons you can learn from your geology course is how interrelated the various systems of the Earth river?
When you alter one numerous other features are.
Thinking
determine
that will be deprived of sediment from the
if you were taking a test. Only when you see your answer in writing will you know if you really understood the material.
and white, and it is important to be able to examine an issue from all sides and come to a logical conclusion. One of the most important things you will learn in college is to think critically and not accept everything you read and hear at face value.
at the big picture to
flows to the sea affect the stream's profile?
imperative that you
are black
the underlying
were known were not accepted until of overwhelming evidence.
will constructing a
out your answers as
life
all,
various elements are related. For example,
over the end-of-chapter questions. Write
things in
flawed?
ideas. After
the 1970s in spite
Because geology builds
^ DEVELOPING CRITICAL THINKING SKILLS
somehow
early in this century, yet
understand the terminology.
Go
is it
principles of plate tectonic theory
of the key terms, especially those italic type.
the source?
Be open to new
make
you don't highlight everything. Make notes in the margins. If you don't understand a term or concept, look it up in the glossary. »• Read the chapter summary carefully. Be sure you in
is
Consider whether the conclusions follow from the facts. If the facts do not appear to support the conclusions, ask questions and try to determine why they don't. Is the argument
sure
all
not accept any statement at face value. is the source of the information? How
reliable
unconformities.
understand
Do
What
As you read your textbook, highlight or underline key concepts or sentences, but
new
Can they be The current controversy over cold
executed and free of bias?
is
•^ Pay particular attention to the tables, charts, and figures. They contain a wealth of information in abbreviated form and illustrate important concepts and ideas. Geology, in particular, is a visual science, and the figures and photographs will help you visualize what is being discussed in the text and provide actual examples of features such as faults or
Few
was
about and how it flows from topic to topic. If you have time, skim through the material
^
the facts or information
experiments, were the experiments well
"» Look over the chapter outline to see what the
^
If
feature,
IMPROVING YOUR is
affect
MEMORY
Why do you remember some things reason
you
as well.
and not others? The
that the brain stores information in different
ways and forms, making it easy to remember some things and difficult to remember others. Because college requires that you learn a vast amount of information, any suggestions that can help you retain more material will help you in your studies: "» Pay attention to what you read or hear. Focus on the task at hand, and avoid daydreaming. Repetition of any sort will help you remember
Improving Your
Memory
xxv
Review the previous
material.
lecture before
"•"
important.
questions as you read.
Try to
Use mnemonic devices to help you learn unfamiliar material. For example, the order of the Paleozoic periods (Cambrian, Ordovician,
facts to
Devonian, Mississippian,
Pennsylvanian, and Permian) of the geologic time scale can be remembered by the phrase,
Campbell's Onion Soup Does Make Peter Pale, or the order of the Cenozoic epochs (Paleocene, Eocene, Oligocene, Miocene, Pliocene, and Pleistocene) can be remembered by the phrase,
example, pyroclastic comes from pyro meaning fire and clastic meaning broken pieces. Hence a pyroclastic rock is one formed by volcanism
and composed of pieces of other rocks.
remember
much
body of
easier than learning
discrete facts.
Looking
^ The most important advice
particularly helpful in geology because so
t -*
things are interrelated. For example, plate tectonics explains
how mountain
volcanism, and earthquakes are
building,
all
related
(Chapter 13). The rock cycle relates the three major groups of rocks to each other and to subsurface and surface processes (Chapter to tie concepts
1).
•^ Use deductive reasoning
Remember
together.
what you learned as
that geology builds
your foundation and see
material relates to
the
new
If
it.
you can draw
parts,
its
material.
type of
how
a picture and you probably understand the Geology lends itself very well to this
•w Draw a picture. label
on
previously. Use that material
device because so much is example, instead of memorizing a of glacial terms, draw a picture of a
memory
is
to study regularly
cram everything into one massive study session. Get plenty of rest the night before an exam, and stay physically fit to avoid becoming susceptible to minor illnesses that sap your strength and lessen your ability to concentrate on the subject at hand. Set up a schedule so that you cover small parts of the material on a regular basis. Learning some concrete examples will help you understand and remember the material. Review the chapter summaries. Construct an outline to make sure you understand how everything fits together. Drawing diagrams will help you remember key points. Make up flash cards to help you remember terms and concepts.
•*r
many
part of a course.
rather than try to
related material
is
tests are the critical
well
examination:
unconnected and
for relationships
and use the
in the details.
on an exam, you must be prepared. These suggestions will help you focus on preparing for the
To do
Outline the material you are studying. This will help you see how the various components are
is
fill
^ PREPARING FOR EXAMS
their definitions.
interrelated. Learning a
Form
a study group, but
make
sure your group
on the task at hand, not on socializing. Quiz each other and compare notes to be sure you have covered all the material. We have found that students dramatically improved their focuses
grades after forming or joining a study group. -v Write out answers to all of the end-of-chapter questions. Review the key terms. Go over all of the key points the professor emphasized in class. If
you have any questions,
visit
the professor or
review sessions are offered, be sure to attend. If you are having problems with the material, ask for help as teaching assistant.
If
soon as you have difficulty. Don't wait end of the semester. If
what
long
are asked. Find out whether the
list
and label its parts and the type of topography it forms.
Developing Critical Thinking and Study
Skills
all
until the
old exams are available, look at them to see is emphasized and what type of questions
visual. For
glacier
can't
on the
visualize the big picture,
For most students,
We
have provided the roots of many important terms throughout this text to help you
You
so focus
important points of the lecture or the chapter.
Put Eggs On My Plate Please. Using rhymes can also be helpful. »' Look up the roots of important terms. If you understand where a word comes from, its meaning will be easier to remember. For
xxvi
is
remember everything,
class,
Silurian,
^
Focus on what
or look over the last chapter before beginning the next. Ask yourself
going to
objective or
all
exam
will be
essay or a combination.
you have trouble with
a particular type of
If
question (such as multiple choice or essay), practice answering questions of that
study group or a classmate
may
Furthermore, the multiple-choice questions
type— your
contain
question as your opening sentence to the answer. Get right to the point. Jot down a quick outline for longer essay questions to
now
time to take the exam. The most important thing to remember is not to panic. This, of course, is easier said than done. Almost everyone suffers from test anxiety to
exam
some
degree. Usually,
begins, but in
some
cases,
it
passes as soon as the
it is
If
you are one of those people, get help as soon as possible. Most colleges and universities have a program to help students overcome test anxiety or at least keep it in check. Don't be afraid to seek help if you suffer test anxiety. Your success in college depends to a large extent on how well you perform on exams, so by not seeking help, you are only hurting yourself. In addition, the fol-
"w
may
First of all, relax. briefly to see its
Then look over
sure you cover everything. you don't understand a question, ask the examiner. Don't assume anything. After all, it your grade that will suffer if you misinterpret If
If
you have time, review your exam to make you covered all the important points and
sure
»
answered all the questions. you have followed our suggestions, by the time you finish the exam, you should feel confident that you did well and will have cause If
for celebration.
the
exam
format and determine which If it
helps,
^ CONCLUDING COMMENTS
quickly jot
We
afraid
benefit to
down any information you are you might forget or particularly want to remember for a question. *• Answer the questions that you know the best first. Make sure, however, that you don't spend too much time on any one question or on one that is worth only a few points. exam
is a combination of multiple choice answer the multiple-choice questions first. If you are not sure of an answer, go on to the next one. Sometimes the answer to one question can be found in another question.
If
the
and
essay,
is
the question.
be helpful:
questions are worth the most points.
-*"
make
"»-
so debilitating that
the individuals do not perform as well as they should.
lowing suggestions
may
of the facts needed to answer
some of the essay questions. Read the question carefully and answer only what it asks. Save time by not repeating the
be able to help.
^ TAKING EXAMS It is
many
hope that the suggestions we have offered will be of you not only in this course, but throughout your college career. While it is difficult to break old habits and change a familiar routine, we are confident that following these suggestions will make you a better student. Furthermore,
you work more
many
efficiently,
of the suggestions will help
not only in college, but also
throughout your career. Learning is a lifelong process that does not end when you graduate. The critical thinking skills that you learn now will be invaluable throughout your life, both in your career and as an informed citizen.
Concluding Comments
xxvii
PHYSICAL
GEOLOGY EXPLORING THE EARTH
CHAPTER
1
UNDERSTANDING THE EARTH: to
An Introduction Physical Geology ^OUTLINE PROLOGUE INTRODUCTION WHAT IS GEOLOGY? GEOLOGY AND THE HUMAN EXPERIENCE
HOW GEOLOGY AFFECTS OUR EVERYDAY LIVES w Perspective 1-1: How Much
~
'
the Public
THE EARTH
T
Need AS A
Perspective 1-2:
to
Science
Does
Know?
DYNAMIC PLANET The Gaia Hypothesis
GEOLOGY AND THE FORMULATION OFTHEORIES The Formulation of
Plate Tectonic
Theory
IT Guest Essay: Science: Our Need PLATE TECTONIC THEORY
to
Know
THE ROCK CYCLE GEOLOGIC TIME AND UNIFORMITARIANISM CHAPTER SUMMARY
Volcanic peaks of the island of Moorea, part of the French Polynesian Islands chain. These islands formed as a result of volcanic eruptions caused by plate movement.
PROLOGUE On
August 26, 1883, Krakatau, a
small, uninhabited volcanic island in
the
Sunda
between Java and Sumatra, exploded than one day, 18 cubic kilometers of rock were erupted in an ash cloud 80 Straits
(Fig. 1-1). In less
(km 3
)
The explosion was heard as far and Rodriguez Island, 4,653 km to the west in the Indian Ocean. Where the 450 meter (m) high peak of Danan once stood, the water was now 275 m deep, and only one-third of the km island remained above sea level (Fig. 1-2). The explosions and the collapse of the chamber that held kilometers (km) high.
away
as Australia
5x9
the magma (molten rock) beneath the volcano produced giant sea waves, some as high as 40 m. On nearby islands, at least 36,000 people were killed and 165 coastal villages destroyed by the sea waves that hurled ashore coral blocks weighing more than 540
metric tons.
So much ash was blown into the stratosphere that Sunda Straits were completely dark from 10 a.m., August 27, until dawn the next day. Ash was reported the
falling on ships as far away as 6,076 km. The sun appeared to be blue and green as volcanic dust, ash, and aerosols circled the equator in 13 days. As these airborne products spread to higher latitudes, vivid red
sunsets were
common around
three years (Fig. 1-3).
the world for the next
The volcanic dust
in the
stratosphere not only created spectacular sunsets,
it
"^ FIGURE
1-1 Krakatau's climactic explosion in August 1883 was preceded by several smaller eruptions. This photograph was taken on May 27, 1883, one week after Krakatau's initial eruption. It shows ash and steam erupting from a vent at Perbawatan on the south side of the island.
incoming solar radiation back into space; the average global temperature dropped as also reflected
much
as 1/2°C during the following year
and did not
eruption, a few shoots of grass appeared, and three
Why have we chosen the eruption of Krakatau as an introduction to physical geology? The eruption was dramatic and interesting in its own right, but it also illustrates several of the aspects of geology that we will be examining, including the way the Earth's interior, surface, and atmosphere are all interrelated. Sumatra, Java, Krakatau, and the Lesser Sunda
years later 26 species of plants had colonized the
Islands are part of a 3,000
island, thus providing a suitable habitat for animals.
islands that
return to normal until 1888.
Of
animal life was destroyed on Krakatau. The remaining portion of the original island was blanketed by tens of meters of volcanic ash and pumice; two months later, the ash and pumice were still so hot that walking was difficult! A year after the course,
all
The
first creatures to reach Krakatau probably flew or were lofted in by the wind; later, others either swam or were rafted to the island on driftwood or other
flotsam.
Upon
multiplied,
arrival, the various
and today most of the
are widely distributed.
animals rapidly species
on Krakatau
location
is
make up
km
long chain of volcanic
the nation of Indonesia. Their
a result of a collision between
two
pieces
of the Earth's outer layer, generally called the crust.
The theory plates that
that the Earth's crust
move
is
over a plastic zone
divided into rigid is
known
as plate
tectonics (see Chapter 13). This unifying theory
explains and
ties
together such apparently unrelated Prologue
Lampong Bay Krakatau'^
'"•'
FIGURE
Indonesia,
Sumatra,
is
(b)
1-2
(a)
Krakatau, part of the island nation of
located in the Sunda Straits between Java and Krakatau before and after the 1883 eruption.
Krakatau Island-After
After the eruption, only one-third of the island remained
above sea
(b)
level.
"^" FIGURE 1-3 Airborne volcanic ash and dust particles from the eruption of Krakatau soon encircled the globe, producing exceptionally long, beautiful sunsets. This sunset was sketched by William Ascroft in London, England, at 4:40 p.m. on November 26, 1883, three months after Krakatau erupted.
geologic
phenomena
as volcanic eruptions,
earthquakes, and the origin of mountain ranges. In tropical areas such as Indonesia, physical
chemical processes rapidly break lava flows, converting for agriculture (see
them
Chapter
down
ash
and and
falls
into rich, productive soils 6).
These
soils
can
support large populations, and, in spite of the dangers of living in a region of active volcanism, a strong correlation exists between volcanic activity
and
population density. Indonesia has experienced 972 eruptions during historic time, 83 of which have
caused
fatalities.
Yet these same eruptions are also
ultimately responsible for the high food production that can support large
numbers of people.
Volcanic eruptions also affect weather patterns; recall that the eruption of Krakatau caused a global cooling of 1/2°C. More recently, the 1982 eruption of El
Chichon
in
Mexico
resulted in lower global
temperatures and abnormal weather patterns (see
Chapter 4
Chapter
1
An
Introduction to Physical Geology
4).
As you read
book, keep in mind that the you are studying are parts of dynamic
interrelated systems, not isolated pieces of
and surface. These eruptions not only have an immediate effect on the surrounding area, but also contribute to climatic changes that affect the
information. Volcanic eruptions such as Krakatau are
entire planet.
this
different topics
the result of
complex interactions involving the
^ INTRODUCTION One major
benefit of the space age
is
the ability to look
back from space and view our planet in its entirety. Every astronaut has remarked in one way or another on how the Earth stands out as an inviting oasis in the otherwise black void of space
The Earth system
in that
is it
(Fig. 1-4).
unique among the planets of our solar supports life and has oceans of water, a
hospitable atmosphere, and a variety of climates. ideally suited for life as
we know
bination of factors, including
sphere, oceans, and, to
by
life
some
it
crust, oceans, in
processes.
In
and
at-
the Earth's atmocrust have been
turn,
these physical
changes have affected the evolution of life. The Earth is not a simple, unchanging planet. Rather,
complex dynamic body
which innumerable many components. The continual evolution of the Earth and its life makes geology an exciting and ever-changing science in which new discoveries are continually being made. it
is
a
interactions are occurring
among
structural geology, the study of the deformation of the
Earth's crust; geophysics, the application of physical laws and principles to the study of the Earth, particularly its interior; paleontology, the study of fossils; and paleogeography, the study of the Earth's past geographical features.
its
extent,
mineralogy, the study of minerals; petrology, the study of rocks; stratigraphy, the study of the sequence of geologic events as recorded in successive layers of rock;
It is
because of a com-
distance from the Sun
its
and the evolution of its interior, mosphere. Over time, changes influenced
Earth's interior
in
its
Nearly every aspect of geology has some economic or environmental relevance, so it is not surprising that
many
geologists are involved in exploration for mineral
and energy resources. Geologists use
their specialized
"^ FIGURE 1-4 The Earth as seen from Apollo 17. Almost the entire coastline of Africa is visible in this view, which extends from the Mediterranean Sea area to the Antarctic south polar ice cap. The Asian mainland is on the horizon toward the northeast, where the Arabian Peninsula can be seen, and Madagascar is visible off the eastern coast of Africa. In addition, numerous storm systems can be seen over the Atlantic and Indian oceans.
^ WHAT IS GEOLOGY? what is geology and what is it that geologists do? Geology, from the Greek geo and logos, is defined as
Just
"the study of the Earth."
It is
generally divided into
two
broad areas — historical geology and physical geology. Historical geology examines the origin and evolution of the Earth,
its
and
continents, oceans, atmosphere,
However, before one can interpret the Earth's
life.
an understanding of physical geology is needed. This involves the study of Earth materials, such as minerals and past,
rocks, as well as the processes operating within the
Earth and upon
The
its
surface.
discipline of geology
many shows many of
vided into
is
so broad that
it is
subdi-
different fields or specialties. Figure 1-5
the diverse fields of geology
and their chem-
relationship to the sciences of astronomy, physics, istry,
and biology. Some of the
specialties of
geology are
What
is
Geology?
,
Geomorp ho|fogy
**
(landscape " an aP6,t>rn fc>r»—
-T.
»"o!
^ ^"A#
0?V
FIGURE
knowledge
1-5
Some
of geology's
many
subdivisions and their relationship to the other sciences.
to locate the natural resources
industrialized society
is
on which our
based. Such mineral resources as
ways in the search and energy resources (Fig. 1-6). Although locating mineral and energy resources is ex-
geology
in increasingly sophisticated
for mineral
and gravel are nonrenewand once known deposits of them are depleted, new deposits or suitable substitutes must be found. As the world demand for these nonrenewable resources in-
problems.
creases, geologists are applying the basic principles of
water for the ever-burgeoning needs of communities and
coal, petroleum, metals, sand, able,
Chapter
1
An
Introduction to Physical Geology
tremely important, geologists are also being asked to use their expertise to help solve
Some
many
of our environmental
geologists are involved in finding ground-
industries or in monitoring surface ter pollution ical
and suggesting ways
engineering
is
and underground wa-
to clean
it
up. Geolog-
being used to find safe locations for
dams, waste disposal
sites,
and power
plants, as well as to
help design earthquake-resistant buildings.
long-range predictions about earthquakes and volcanic In addition, they are
to help
working with
civil
may
result.
defense planners
draw up contingency plans should such natural
disasters occur.
As
emwide variety of pursuits. As the world's population increases and greater demands are made on the Earth's limited resources, the need for geologists and ployed
this
brief survey illustrates, geologists are
in a
their expertise will
become even
lives
discussion of these topics).
Geologists are also involved in making short- and
eruptions and the potential destruction that
which we depend on geology in our everyday and also at the numerous references to geology in the arts, music, and literature (see the articles by R. V. Dietrich listed at the end of this chapter for an extensive tent to
Rocks and landscapes are realistically represented in sketches and paintings. Examples by famous artists include Leonardo da Vinci's Virgin of the Rocks and Virgin and Child with Saint Anne, Giovanni Bellini's Saint Francis in Ecstasy and Saint Jerome, and Asher Brown Durand's Kindred Spirits (Fig. 1-7). In the field of music, Ferde Grofe's Grand Canyon Suite was, no doubt, inspired by the grandeur and timelessness of Arizona's Grand Canyon and its vast rock exposures. The rocks on the Island of Staffa in the Inner
many
Hebrides
greater.
provided
the
inspiration
for
Felix
Men-
delssohn's famous Hebrides Overture (Fig. 1-8). In literature, references to geology
^ GEOLOGY AND THE HUMAN EXPERIENCE Most people
are aware of the importance of geology in
the search for energy resources
and
abound in The Ger-
man Legends of the Brothers Grimm. Jules Verne's jour-
in the prediction
and
minimization of damage caused by various natural disasters. Many people, however, are surprised at the ex-
ney to the Center of the Earth describes an expedition into the Earth's interior (see Chapter 10 Prologue). On one level, the poem "Ozymandias" by Percy B. Shelley deals with the fact that nothing lasts forever
and even under the ravages of time and weathering. References to geology can even be solid rock eventually disintegrates
^ FIGURE
1-6
(a)
Geologists
measuring the amount of erosion on a glacier in Alaska, (b) Geologists
increasingly use computers in their
search for petroleum and other natural resources.
Geology and the
Human
Experience
found in comics, two of the best known being B.C. by Johnny Hart and The Far Side by Gary Larson (Fig. 1-9). Geology has also played an important role in history. Wars have been fought for the control of such natural resources as oil, gas, gold, silver, diamonds, and other valuable minerals. Empires throughout history have risen and fallen on the distribution and exploitation of natural resources. The configuration of the Earth's surface, or its topography, which is shaped by geologic agents, plays a critical role in military tactics. Natural barriers such as
mountain ranges and
rivers
have
fre-
quently served as political boundaries.
^ HOW GEOLOGY AFFECTS OUR EVERYDAY LIVES Destructive
volcanic
eruptions,
devastating
earth-
quakes, disastrous landslides, large sea waves, floods,
and droughts are headline-making events that affect people (Fig. 1-10). Although we are unable to prevent most of these natural disasters, the more we know about them, the better we are able to predict, and
many
possibly control, the severity of their impact.
FIGURE
Kindred
1-7
Spirits
by Asher Brown Durand
(1849) realistically depicts the layered rocks occurring along gorges in the Catskill Mountains of New York State. Asher Brown Durand was one of numerous artists of the nineteenth-century Hudson River School, who were known for their realistic landscapes.
"^ FIGURE
1-8
Mendelssohn was on the Island of Staffa
Felix
inspired by the rocks
in
when he wrote the famous known as Fingal's Cave)
the Inner Hebrides,
Hebrides (also
Overture. Mendelssohn wrote the opening bars of this overture while visiting Staffa.
8
Chapter
1
An
Introduction to Physical Geology
The
envi-
ronmental movement has forced everyone to take a closer look at our planet and the delicate balance between its various systems. Most readers of this book will not go on to become professional geologists. However, everyone should have a basic understanding of the geological processes that ultimately affect all of us. Such an understanding of geology is important so that one can avoid, for example,
building in an area prone to landslides or flooding. Just
ask anyone
who
purchased a
home
in the
Portuguese
jtted
hits
Caucasus region, 40 de
Bend area of southern California during the 1950s (Fig. 15-31) or who built along a lakeshore and later saw the lake level rise and the beach and sometimes even their house disappear.
As
society
becomes increasingly complex and technowe, as citizens, need an understand-
<;^:
logically oriented,
ing of science so that
we can make informed
'§5
choices
#*•<&
about those things that affect our lives (see Perspective 1-1). We are already aware of some of the negative aspects of an industrialized society, such as problems relating to solid waste disposal, contaminated groundwater, and acid rain. We are also learning the impact that humans, in increasing numbers, have on the environ-
ment and
we can no
that
longer ignore the role that
we
play in the dynamics of the global ecosystem.
-^ FIGURE 1-10 As these headlines from various newspapers indicate, geology affects our everyday lives.
Most people
"^ FIGURE found
in
1-9 References to geology are frequently comics as illustrated by this Gary Larson Far Side
are
unaware of the extent
ology affects their everyday
lives.
For
to
many
which
ge-
people, the
connection between geology and such well-publicized
cartoon.
THE FAR SIDE
By
GARY LARSON
problems as nonrenewable energy and mineral resources, let alone waste disposal and pollution, is simply too far removed or too complex to be fully appreciated. But consider for a moment just how dependent we are on geology in our daily routines. Much of the electricity for our appliances comes from the burning of coal, oil, or natural gas or from uranium
consumed
who
in
nuclear-generating plants.
It is
geologists
and uranium. The copper or other metal wires through which electricity travels are manufactured from materials found as the result of mineral exploration. The buildings that we live and locate the coal, petroleum,
work
in
owe
their very existence to geological resources.
A
few examples are the concrete foundation (concrete is a mixture of clay, sand, or gravel, and limestone), the drywall (made largely from the mineral gypsum), the
windows
(the mineral quartz
is
the principal ingredient
manufacture of glass), and the metal or plastic plumbing fixtures inside the building (the metals are from ore deposits and the plastics are most likely manufactured from petroleum distillates of crude oil). Furthermore, when we go to work, the car or public transportation we use is powered and lubricated by some type of petroleum by-product and is constructed of metal alloys and plastics. And the roads or rails we ride over come from geologic materials, such as gravel, in the
'You know,
Seems
I
like
used to like this hobby ... But shoot! everybody's got a rock collection."
asphalt, concrete, or steel. All of these items are the result of processing geologic resources.
How
Geology Affects our Everyday Lives
Perspective 1-1
HOW MUCH SCIENCE DOES
THE PUBLIC NEED TO KNOW? We
live in an age of increasingly greater complexity in which scientific and technological innovations are emerging at an astonishingly rapid rate. New discoveries in medicine, chemistry, and electronics are announced almost daily. Advances in computer technology have revolutionized the way we live and work. For example, we use computers to type letters and documents, to get cash from automated teller machines, and to read the prices of our purchases through bar code scanners at the supermarket. Computers even control the engines of our cars and operate robots in many of our mines and factories. As jobs become more technologically oriented, it is imperative that everyone know more science and how it impacts on our lives, particularly in terms of its application to technology. According to a 1985 National Science Board report, however, the last time most high school students ever take a math or science
course
the tenth grade. Furthermore, students in the
is
United States spend only one-half to one-third as
much
time learning science as do students in Germany, the Soviet Union, China, and Japan. If our nation is to compete in the global marketplace, we must have a scientifically literate work force. It is becoming increasingly clear that the American public knows and understands very little science. In 1988, one American in five knew what DNA was, yet we are debating whether and under what conditions the genetic code of organisms should be purposely altered. About 50 percent of American adults said
victim to charlatans.
How
we make informed
can
decisions about nuclear power, toxic water,
of other critical issues that affect us
all if
and a host
we cannot
separate fact from fiction and logically follow debates
about issues involving science and technology? In a 1985 Gallup poll, 55% of American teenagers believed that astrology works, and a poll in Great Britain revealed that
idea that astrology
look
much beyond
is
75%
of the population accepts the
scientific.
One does not
have to
the supermarket checkout counter to
see the public's interest in
and fascination with such
pseudosciences as astrology, parapsychology, UFOs, and
New Age and
"science," particularly the belief in the healing
spiritual
An
powers of
crystals.
unquestioning obedience to the dictates of some
pseudosciences, or to a discredited scientific theory, can
have devastating
results.
One
of the most tragic
examples of adherence to a disproved
scientific
theory
who
involved Trofim Denisovich Lysenko (1898-1976)
became president of the So\aet Academy of Agricultural Sciences in 1938. Lysenko endorsed the theory of inheritance of acquired characteristics according to
which plants and animals could be changed in desirable ways simply by exposing them to a new environment. For example, according to Lysenko, seeds exposed to dry conditions would acquire a resistance to drought,
and this trait would be inherited by future generations. Lysenko accepted inheritance of acquired characteristics because of
its
apparent compatibility with
Marxist-Leninist philosophy.
As
president of the
they did not understand the concept of radiation. Yet
academy, Lysenko did not allow any other research to
are being asked to decide whether we should have our homes checked for radon concentrations (see
be conducted concerning inheritance mechanisms.
we
Perspective 9-2) close
down
and to vote on measures to build or
nuclear
power
plants.
Based on a study conducted
public
was
estimated that only
scientifically literate.
scientific literacy
and appreciating the only the
5%
common
social
at
way
S.
scientific
vocabulary of science,
impact of
scientific
essential point of Dr. Miller's survey
1 in
Northern
of the U.
According to Dr. Miller,
means understanding the
method, knowing the
The
scientific
1985, John Miller,
Opinion Laboratory
director of the Public Illinois University,
in
advances.
was
that
every 20 Americans understands science and
science works.
we, as consumers and
One
implication of this
citizens,
Unfortunately for the Soviet people, inheritance of acquired characteristics had been discredited as a
is
that
run the risk of falling
results of
theory more than 50 years before.
The
Lysenko's belief in the political correctness
of this theory were widespread crop failures,
and misery for millions of people. drawn from this example is that scientific research must be based on scientific realities, not philosophical beliefs. Science proceeds on the basis of the scientific method, and all persons need at least a rudimentary knowledge of this method and the way science works if they are to make intelligent and starvation,
The
lesson to be
informed decisions.
"^ FIGURE
1-11
A
cross section of the Earth
illustrating the core, mantle,
and
crust.
The enlarged
portion shows the relationship between the lithosphere, composed of the continental crust, oceanic crust, and upper mantle, and the underlying asthenosphere and
lower mantle.
It is
quite apparent that as individuals and societies,
the standard of living
we
enjoy
is
directly
dependent on
the consumption of geologic materials. Therefore,
we
need to be aware of geology and of how our use and misuse of geologic resources may affect the delicate balance of nature and irrevocably alter our culture as well as
ers as a function of variations in composition, temper-
ature,
The
and pressure. core, the innermost part of the Earth, has a cal-
culated density of 10 to 13 grams per cubic centimeter 3
(g/cm ) and occupies about 16% of the Earth's total volume. Seismic (earthquake) data indicate that the core
and a larger, apparThe core is inferred to consist largely of iron and a small amount of nickel. The mantle surrounds the core and comprises about
our environment.
consists of a small, solid inner core
ently liquid, outer core.
^ THE EARTH AS A DYNAMIC PLANET The Earth
is
a
dynamic planet
83%
that has continuously
changed during its 4.6-billion-year existence. The size, shape, and geographic distribution of continents and ocean basins have changed through time, the composition of the atmosphere has evolved, and life-forms existing today differ from those that lived during the past. We can easily visualize how mountains and hills are worn down by erosion and how landscapes are changed by the forces of wind, water, and ice. Volcanic eruptions and earthquakes reveal an active interior, and folded and broken rocks indicate the tremendous power of the Earth's internal forces.
The Earth
consists of three concentric layers: the
and the crust (Fig. 1-11). This orderly from density differences between the lay-
ot t he Earth's volume. It is less dense than the core g/cm J and is thought to be composed largely^ of peridotite, a dark, dense rock containing abundant iron and magnesium. The mantle is divided into three distinct zones. The lower mantle is solid and forms most of the volume of the Earth's interior. The asthenosphere, which surrounds the lower mantle, is also solid, although it behaves plastically and slowly flows. Partial (475
)
melting within the asthenosphere generates
some of which less
rises to the Earth's surface
dense than the rock from which
it
was
magma,
because
it is
derived.
The
upper mantle surrounds the asthenosphere. This solid upper mantle and the overlying crust constitute the lithosphere, which is broken into numerous individual
move over
the asthenosphere.
core, the mantle,
pieces called plates that
division results
Interactions of these plates are responsible for such phe-
The Earth
as a
Dynamic
Planet
11
Lovelock proposed a mathematical model that he called Daisyworld, in which an imaginary planet is populated only by white and black
Daisyworld
and
rises,
daisies. If the
temperature on
the black daisies absorb too
die, thus leaving
mostly white daisies that
much
heat
the planet
temperature controls. Unquestionably, biologic processes are important, but those
who
accept the Gaia hypothesis also claim that the
proportions of various gases in the atmosphere are kept in
balance by purposeful feedback mechanisms. They
point out that the present-day atmosphere
is
it is
body
in
which there
is
is
according to
If,
was
itself,
able to
why have
there
been periods of biological instability? As one would expect, there are strong objections to the Gaia hypothesis. Many biologists dismiss it because
it is
teleological; that
is, it
appeals to design
Some
or purpose in nature and thus cannot be tested.
geologists point out that plate tectonics alone can
While the Gaia hypothesis controversial,
eventually
it
become an acceptable
endeavor,
scientific
is,
to say the least,
remains to be seen whether
new and
it
will
As in any ideas must
theory.
radical
field
of
theoretical postulates, or
scientists investigate it
may
its
be rejected or
modified depending on future discoveries. In any case,
Gaia has forced
scientists to critically evaluate the
relationship between
a theory are derived predictive statements that can
can be assessed. The law of universal gravitation
is
an example of a theory describing the attraction between masses (an apple and the Earth in the popularized account
Newton and
regulate the environment to suit
Gaia will be supported as
a giant self-
an intimate connection
be tested by observation and/or experiment so that their validity
by the various mass
extinctions seen in the fossil record.
hypothesis, evidence testing, and prediction. Perhaps
between the evolution of the living and nonliving components of the planet. As some critics point out, however, the
From
of these changes were
as indicated
demonstrate their worth in the competitive
mechanisms, according to proponents of the Gaia regulating
life,
of carbon dioxide (see Perspective 2-2).
on Venus. Such feedback
hypothesis, indicate that the Earth
And many
detrimental to
control the Earth's temperature through the recycling
dominated
by nitrogen and oxygen, both reactive gases that should have long ago combined with other elements to form nitrates. Furthermore, they claim that without life, carbon dioxide should have become the dominant atmospheric gas, as
geologic time.
the Gaia hypothesis, the biosphere
reflect
down. When Daisyworld cools sufficiently, black daisies thrive again and absorb more heat. In short, there is a feedback mechanism for
more heat and cool
composition of the atmosphere has changed through
The
life
and the global environment.
fact that a scientific theory
can be tested and
is
subject to such testing separates science from other
forms of
human
inquiry. Because scientific theories
can
be tested, they have the potential of being supported or even proved wrong. Accordingly, science must proceed
as the scientific
without any appeal to beliefs or supernatural explanations, not because such beliefs or explanations are necessarily untrue, but because we currently have no way to
ical
investigate them.
of
his discovery).
Theories are formulated through the process
known
method. This method is an orderly, logapproach that involves gathering and analyzing the facts or data about the problem under consideration. Tentative explanations or hypotheses are then formulated to explain the observed phenomena. Next, the hypotheses are tested to see
if
what they predicted
actually
For
this
reason, science
makes no
claim about the existence or nonexistence of a supernatural or spiritual realm.
Each
scientific discipline
has certain theories that are
of particular importance for that discipline. In geology,
occurs in a given situation (see Perspective 1-2). Finally,
the formulation of plate tectonic theory has changed the
one of the hypotheses is found, after repeated tests, to explain the phenomena, then that hypothesis is proposed as a theory. One should remember, however, that
way
if
in science,
even a theory
is still
subject to further testing
and refinement as new data become
available.
geologists view the Earth. Geologists
now
view
Earth history in terms of interrelated events that are part of a global pattern of change. Before plate tectonic theory was generally accepted by geologists,
however, numerous interrelated hypotheses
Geology and the Formulation of Theories
13
''
— FIGURE 1-12 T;*t
\:~.
:
"'
-r."
~.:.\
.
r.
~zs. '''-
"i
11:
--
i:;r
;
'
:ttr
:';-.rr
f -
•
.-.
ir:
\
-:tr
Tr.t
-.:
3;e
\
:.-.t
..-r'iit
:
•
•-;
•
warm -
:: :.-.;
:m.;. i- : :-t-. .r-'.r. .: -;;-. :r;;t-i; ri:v ;-:•
:
The Earth's ~ t —:-•;
convection oeQs in which
Ii~-
-------
~-
- _ -
= .-:
trr.tr.:
of these coarectioa cells is believed to be the mechanism :;
;
:.-; .:.; :::
:.-.;
~:
--.—.it.:
::
the Earth's plates, as shown in this diagrammatic cross section.
were proposed and ory
tested.
illustrates the scientific
Thus, the evolution of this the-
and plants are found on
method
rocks indicating glacial conditions are
at
work. Because plate
and unifies so many aspects of geology, we examine the formulation of this theory.
tectonics affects will briefly
and why now found on
different continents,
continents located in the tropics.
Wegener's hypothesis and its predictability could be by asking what type of rocks or fossils would one
tested
The Formulation of
Plate Tectonic
expect to find at a given location on a continent
Theory
continent
idea that continents moved during the past goes back to the time when people first noticed that the margins of eastern South America and western Africa looked as if they fit together. Geologists also noticed that similar or identical fossils occur on widely separated continents, that the same types of rocks from the same time period are found on different continents, and that ancient rocks and features indicating former glacial conditions occur in today's tropical areas. As more and more facts were gathered, hypotheses were proposed to explain them. In 1912, Alfred egener, a German meteorologist, amassed a tremendous amount of geologTcal, paleontologicai, and cGmatological ^lata thatlhdicated continents movortirough time; he proposed the Hypothesis of continental drift to explain and synthesize this myriad of tacts. Wegener stated that at one time all of the continents were united into one single supercontinent that he named Pangaea. Pangaea later broke apart, and the in-
The
W
dividual continents drifted to their current locations.
The
continental drift hypothesis explained
shorelines of different continents ferent
fit
together,
why how
mountain ranges were once part of a larger conwhy the same fossil animals
tinuous mountain range,
14
the dif-
Chapter
1
An
Introduction to Physical Geology
test the
was
if
that
180 million years ago. To
in the tropics
hypothesis of continental
drift, all
researchers
had to do was to go into the field and examine the rocks and fossils for a particular time period on any continent to see
if
they indicated
what the hypothesis
predicted for
the proposed location of that continent. In almost cases, the data
fit
one problem with Wegener's hypothesis: plain
how
all
the hypothesis. However, there was
continents
moved over
it
did not ex-
oceanic crust and
the mechanism of continental movement was. During the late 1950s and early 1960s, new data abouTthe sea floor emefgBihhat enabled geologists to propose the hypothesisof sea-floor spreading. This hypothesis suggested that the continents and segments of oceanic crust move together as single units, and that
what
some type of thermal convection
cell
system operating
within the Earth was the mechanism responsible for plate movements Fig. 1-12 .
Sea-floor spreading
combined plates are
into a single
and continental drift were then hypothesis in which moving rigid
composed of continental and/or oceanic crust underlying upper mande. These plates are
and the bounded by mid-oceanic ridges, oceanic trenches, faults, and mountain belts. In this hypothesis, plates move away from mid-oceanic ridges and toward oceanic
JAMES
Guest Essay
WATKINS
D.
rrmrnTTTTTTTTTmnTTT
>
i
*?TtT TTT TT T TTTTTfT TT T T »?> ?TTT TTTT
KNOW
SCIENCE: OUR NEED TO The following essay
is
based on a speech ghen by
genuine problems from
newspaper
toy Watkhis at the annual competition of the
American Neuspaper Publishers .\ssoaation
One a
of the goals of our society
is
comprehensive energy strategy that
we need
in
May
1°^1.
the development oi will assure us
of
economic growth into the next century. But such a strategy cannot be limited to energy production alone— it must also take steps to curb our growing demand for energy and must include the energy
to sustain
measures that will protect the environment.
A
major impediment to achieving
energy strategy
is
to understand the
of the debate on global climate change
half the adults in
when
one survey did not know that the Earth
false alarms.
Washington
was "Uranium in the Asparagus" grown locally. Wiat the article did not say— although it was reported two days later in a tiny item rucked behind the sports section — is that you would have to eat 5,000 metric tons of the asparagus per year for 50 yean one ski trip to Aspen. At and editor of that story did not know
a radiation dose equivalent to best, the writer
that radioactivity occurs naturally in life.
Nor
How
can
we
radiation level meant. Basic scientific literacy, including
knowledge of the most elementary facts about radiation, would have enabled the newspaper and its readers to focus their attention on real problems.
our duties as
expect to compete on a world scale in
the twenty-first century
when most of our population
cannot speak the language of the disciplines of mathematics and the various sciences? For example, the units
we use to compute energy consumption are quads. Does a quad have any meaning for you? It should — it is that
the energy equivalent to burning half a million barrels of oil
each day for one year, or a quadrillion BTU. The
each year— nearly
25%
of world consumption. That
we can
citizens.
How
tell if
do
is
essential
we
are to
scientists
the carbon dioxide
fulfill
measure carbon dioxide so
things are getting better or worse?
come from power
plants?
Does
all
How
is produced naturally — by rice fields and cows — and how much is generated by human activities? These are the kinds of issues that we and our elected representatives must increasingly grapple with. How
much
we
are in our polio- choices will be determined
by the amount of informed knowledge is
if
Consider the issue of global
we have
at
our
The fact is that informed public policy decision making can no longer be made without a fundamental disposal.
something that should concern
You may
warming.
prudent
United States uses about 85 quads of primary energy
some forms of what that
did they have any idea of
Finally, scientific literacy
rakes one year to revolve around the Sun?
For example, a
State reported, with big
headlines, that there
plant
comprehensive
the lack of scientific literacy in America.
How can we expect our population intricacies
this
in
all
of us.
is not a problem you personally; as long as our society produces enough scientists to fill its laboratories, there is no crisis. But you would be wrong. Scientific literacy has become mandatory for even citizen. Today even- American household contains products and equipment that defy
think that scientific literacy
for
understanding of science and technology. A
James D. Watkins graduated from the United Stares Naval Academy and later earned a master's degree in mechanical
-
explanation by the average rocket the average adult, ^ihat
does use
it
is
scientist,
much
a microwave oven,
less
and how
work? What happens to the water table when we and pesticides in our gardens? Is the
fertilizers
asbestos insulation in the attic dangerous? that's the health risk of radon?
we have
it
in
How
can
we
determine whether
our homes? What's the link between the
engines in our cars and the atmosphere? Very few
American adults can answer these questions, yet these issues— and others like them— are crucial to our survival, both as individuals and as a nation. Even- day the American people must make decisions about the behaviors that influence their health and well-being. They must also be able to distinguish
engineering from the Naval Postgraduate School California.
He
then joined the navy's
new
in
Monterey.
nuclear submarine
program where he became an administrative assistant to Admiral H>tnan G. Rickover. Eventually. Watkins commanded the nuclear submarine IBS Snook and the navy's first nudear-powered cruiser, the USS Long Beach. In 19S2, he was
named
chief of naval operations
and became a member of the Joint Chiefs of Staff. Watkins retired from the navy in 1986, announcing that henceforth he would devote pan of his fame to promoting excellence — in education, health, and motivation— among the nar youth. In 1989, President George Bush named Watkins to be secretary of energy.
trenches. Furthermore, new crust is added along the mid-oceanic ridges and consumed or destroyed along
oceanic trenches, and mountain chains are formed adjacent to the oceanic trenches.
According to this later hypothesis, Europe and North America should be steadily moving away from each other at a rate of up to several centimeters per year. Precise measurements of continental positions by satellites
have verified
plate
movement
Furthermore,
this,
thus confirming the validity of the
hypothesis. plates are
if
moving away from mid-
oceanic ridges as predicted by the plate tectonic hypothesis,
then rocks of the oceanic crust should become progres-
sively older
with increasing distance from the mid-oceanic
ridges (Fig. 13-12).
ment and oceanic scientific
To
deep-sea sedi-
test this prediction,
crust
were
drilled as part of a
Drilling Project. Analysis of the oceanic crust
of sediment immediately above
it
showed
massive
Deep Sea
study of the ocean basins called the
and the
layer
that the age of
the oceanic crust does indeed increase with distance
from
the mid-oceanic ridges, and that the oldest oceanic crust
is
adjacent to the continental margins.
With
and other predictions
the confirmation of these
of the plate tectonic hypothesis, most geologists accept that the hypothesis
is
plate tectonic theory.
correct Its
and therefore
call
it
the
acceptance has been so wide-
spread not only because of the overwhelming evidence
supporting
it
but also because
relationships between logic features
it
appears to explain the
many seemingly
unrelated geo-
and events.
The magma solidifies to form rock, which attaches moving plates, thus increasing their size. The margins of divergent plate boundaries are marked by midoceanic ridges in oceanic crust and are recognized by linear rift valleys where newly forming divergent boundaries occur beneath continental crust. The separation of South America from Africa and the formation of the South Atlantic Ocean occurred along a divergent plate boundary, the Mid- Atlantic Ridge (Fig. 1-13). Pl ates moving to ward one another collide at conver1-14).
to the
gent plate b oundaries
(Fig. 1-14).
Ingof upper mantle and
When
a plate consist-
oceanic crust collides with one
composed of upper mantle and continental
crusfT_for
example, the denser oceanic plate sinks beneath the continental plate along a subduction zone. As Iheoceanic plate descends into the Earth's interior,
creasingly hot until
erating a
it
it
becomes
in-
melts, or partially melts, thus gen-
magma. This magma may erupt
at the Earth's
mountain range. The Andes Mountains on the west coast of South America are a goocT example of a volcanic mountain range formed as a result of subduction of the Nazca plate beneath the South American plate along a convergent plate boundary (Fig. 1-13). Transform plate b oundaries are s^tes where plates slide sideways_past each otKeFTFigTT-14). The SarTAndreas fault in California, an example of a transform plate boundary, separates the Pacific plate from the North American plate (Fig. 1-13). The earthquake activity along the San Andreas fault results from the Pacific plate moving northward relative to the North American surface, thus forming a volcanic
^ PLATE TECTONIC THEORY
plate.
The acceptance of
1960s, plate tectonic theory has had significant and far-
a
major milestone
plate tectonic theory
is
A
recognized as
in the geological sciences. It
is
com-
revolutionary theory
reaching consequences in
when
it
all fields
parable to the revolution caused by Darwin's theory of
provides the basis for relating
evolution in biology. Plate tectonics has provided a
geologic phenomena.
framework for interpreting the composition, structure, and internal processes of the Earth on a global scale. It has led to the realization that the continents and ocean
Mountains
was proposed
in the
of geology because
many seemingly
it
unrelated
For example, the Appalachian
North America and the mountain ranges of Greenland, Scotland, Norway, and Sweden are in eastern
not the result of unrelated mountain-building episodes,
basins are part of a lithosphere-atmosphere-hydrosphere
but rather are part of a larger mountain-building event
(water portion of the planet) system that evolved together
that involved the closing of an ancient "Atlantic
with the Earth's
(known
interior.
According to plate tectonic theory, the lithosphere divided into seven major plates as well as a smaller ones,
all
of which move,
more or
number of
less,
indepen-
Zones of occur along bound-
dently over the asthenosphere (Fig.
earthquake and volcanic activity aries between plates.
is
1-13).
At divergent plate boundaries, plates move apart as rises tolhe surface from the asthenosphere (tig.
magma
16
Chapter
1
An
Ocean"
and the formation of the supercontinent Pangaea about 245 million years ago. as Iapetus)
Introduction to Physical Geology-
^ THE ROCK CYCLE Geologists
recognize
three
major groups of rocks —
metamorphic—each of which is characterized by its mode of formation. Each group contains a variety of individual rock types that differ from one igneous, sedimentary, and
Ridge axis ***'.
that
FIGURE
Transform
1-13
move over
The
Subductxxi zone
Earth"s lithosphere
is
Zones
of extension within continents
Uncertain plate boundary
divided into rigid plates of various sizes
the asthenosphere.
"^ FIGURE
1-14 An idealized cross section illustrating the relationship between the and the underlying asthenosphere and the three principal types of plate boundaries: divergent, convergent, and transform. lithosphere
-.-.'
:::;;
:
"^ FIGURE
1-15 The rock cycle showing the interrelationships between the Earth's and external processes and how each of the three major rock groups is related
internal
to the others.
another on the basis of composition or texture, that
is,
The rock
cycle
a
is
way
It
relates
sive igneous rocks
and external prothe three rock groups to each
Rocks exposed at the Earth's surface are broken into particles and dissolved by various weathering processes. The particles a nd dissolved material may be transported by wind, water, or ice and eventually deposited as sed iment. This sediment may then be compacted or c emented into sedimentary rock.
other; to surficial processes such as weathering, trans-
and deposition; and to internal processes generation and metamorphism. Plate moveme nt i< th p mechanism responsible for recyding
portation,
such as
magma
ro^kmatejials-and therefore drives trie rocTTcvcle. Igneous rocks result from the crystallization
of
magma lize,
(Fig. 1 -16). As a magma cools, minerals crystaland the resulting rock is characterized by interlock-
ing
mineral grains.
18
Chapter
1
An
that cools at the Earth's surface produces ex tru-
of viewing the interrelation-
ships between the Earth's internal cesses (Fig. 1-15).
Earth's surface produces intrusive igneous rocks, whil e
magma
the size, shape, and arrangement of mineral grains.
,Magma
that cools
beneath the
Introduction to Physical Geology
.
Sedimentary rocks originate by consolidation of rock fragme nts, precipitation of mineral matter from soluti on, or compaction ot plant or animal rema ins (big. 1-17). Because sedimentary rocks
Earth's surface, geologists can
form
make
at or near the
inferences about
^
FIGURE 1-16 Hand specimens of two common igneous rocks, {a) Basalt, a common igneous rock, forms by the rapid cooling of
magma
at the Earth's
surface, (b) Granite,
another
common
igneous
rock, forms by the slow
cooling of
magma below
the Earth's surface. (Photos
courtesy of Sue Monroe.)
which they were deposited, the type and perhaps even something about the source from which the sediments were derived the environment in
(see
of transporting agent,
useful for interpreti ng Earth history
Chapter
7).
Accordingly, sedimentary rocks are very
Metamorphic rocks
result
.
from the transformation of
"•' FIGURE 1-17 Hand specimens of various sedimentary rocks, (a) Sandstone
forms by the consolidation of sand-sized mineral grains, (b)
Marine
limestone forms by the extraction of mineral matter from seawater
by organisms or by inorganic precipitation of the mineral calcite
from seawater. (e) Coal forms by the accumulation and compaction of plant material. (Photos
courtesy of Sue
Monroe.)
The Rock Cycle
19
•^ FIGURE 1-18 Hand specimens of two common metamorphic rocks, (a) Schist foliated
rock
in
is
a
metamorphic which the mineral
grains have a preferred
orientation as a result of
pressure applied to the
parent rock. (£>) Marble, a nonfoliated
metamorphic rock, is formed by metamorphism of the sedimentary rock limestone. (Photos courtesy of Sue Monroe.)
preexisting rocks under the influence of elp varpH tem-
peratures or pressure, or as a consequence p f composi-
brought about by fluid activity (F ig. These changes generally occur beneath the Earth's surface For example, marble, a rock preferred by many sculptors and builders, is a metamorphic roc k produced when the agents of meramnrprikm arp applipH to the sedimentary rock limestone or dolostone tional changes 1-18).
.
.
^ FIGURE
1-19
As Figure 1-15 and
rock groups are between plates determine, to a certain extent, which one of the three kinds of rock will form (Fig. 1-19). For example, weathering produces sediment that is transported by various means from the continents to the oceans, where it is deposited. This sediment, along with the oceanic crust, is part of a moving plate. When plates converge, heat and pressure interrelated,
illustrates, the three
interactions
Plate tectonics
and the rock cycle. The cross section shows how the three major rock groups, igneous, metamorphic, and sedimentary, are recycled through both the continental and
Sediment
oceanic regions.
Metamorphism Asthenosphere
Upper
Magma and igneous
mantle
activity
Melting
20
Chapter
1
An
Introduction to Physical Geology
generated along the plate boundary
may
lead to igneous
and metamorphism within the descending oceanic plate. Some of the sediment and sedimentary rock is subducted and melts, while other sediments and sedimentary rocks along the boundary of the nonsubducted plate are metamorphosed by the heat and pressure genactivity
Earth formed 4.6 billion years ago corresponds to 12:00 midnight, January 1. On this calendar, we see that the oldest fossils, simple, microscopic bacteria, which first appeared about 3.6 billion years ago, are in mid-March; di-
nosaurs, which existed between 242 million and 66 million years ago, are
erated along the converging plate boundary. Later, the
26; and
mountain range or chain of volcanic islands formed along the convergent plate boundary will once again be worn down by weathering and erosion, and the new sediments will be transported to the ocean to begin yet
last
another rock cycle.
a geologist, recent geologic events
are those that occurred within the last million years or so.
One popular analogy
geologists use to convey the imis
to
compare the
1-1
it
strikes midnight!
scale resulted
nineteenth-century geologists
from the work of
who pieced
covery of radioactivity in 1895, and the development of various radiometric dating techniques, geologists have since been able to assign absolute age dates in years to
the subdivisions of the geologic time scale (Fig. 1-20).
jQne of the cornerstones of geology
is
the principle of
based on the premise tha t present-day processes have operated throughout geouniformitarianism. logic time.
It
Therefore,
pret the rock record,
day processes and
is
in
order to understand and inter-
we must
first
understand present-
their results.
Uniformitarianism
is
a
powerful principle that allows
us to use present-day processes as the basis for inter-
preting the past and for predicting potential future
history of the
evenis^_We should keep in mind that uniformitarianism
when
does not exclude such sudden or catastrophic events as
Earth to a calendar year (Table 1-1). The time
— TABLE
tick of the clock before
the Earth's biota through time. However, with the disis
fundamental to an understanding of geology. Indeed, time is one of the main aspects that sets geology apart from the other sciences. Most people have difficulty comprehending geologic time because they tend to think in terms of the human perspective— seconds, hours, days, and years. Ancient history is what occurred hundreds or even thousands of years ago. When geologists talk of ancient geologic history, however, they are referring to events that happened hundreds of millions or even bil-
mensity of geologic time
history occurs during the
togeth er information from numerous rock exposures and constructed a sequential chronology based on changes in
appreciation of the immensity of geologic time
To
human
few seconds of December 31. Furthermore, all of the scientific and technological discoveries that have brought us to our present level of knowledge take place in the final
many
UNIFORMITARIANISM
lions of years ago.
between December 12 and December
of recorded
The geologic time
^ GEOLOGIC TIME AND An
all
the
We know
constant through time. Era
Epoch
Period
was more
years ago than
Recent Quaternary
0.01
2 5
Miocene
that volcanic activity
North America 5 to 10 million today, and that glaciation has been
intense in it is
more prevalent during the last 3 million years than in the previous 300 million years. What uniformitarianism means is that even though the rates
and
have var-
intensities of geological processes
and chemical laws of nature have remained the same and cannot be violated. Although the Earth is in a dynamic state of change and ied during the past, the physical
24 Oligocene
37
Eocene
has been ever since 58
Paleocene
have shaped
it
it
are the
was formed, the processes that same ones in operation today.
66 Cretaceous
144 Jurassic
208
^ CHAPTER SUMMARY
Triassic
245 1.
286
Carboniferous
Pennsylvanian
Geology is the study of the Earth. two broad areas: physical geology
It is is
divided into
the study of the
composition of Earth materials as well as the processes that operate within the Earth and
upon its and
Missis-
surface; historical geology examines the origin
sippian
evolution of the Earth,
atmosphere, and Devonian
2.
its
continents, oceans,
life.
Geology is part of the human experience. We can examples of it in the arts, music, and literature.
find
Silurian
A
438
basic understanding of geology
for dealing with the
Ordovician
505
and
Cambrian
3.
570
is
also important
many environmental problems
issues facing society.
Geologists engage in a variety of occupations, the
main one being exploration for mineral and energy resources. They are also becoming increasingly involved in environmental issues and making shortand long-range predictions of the potential dangers from such natural disasters as volcanic eruptions and earthquakes. 4.
right of the
1-20
The geologic time
columns are ages
scale.
Numbers
is
differentiated into layers.
The
outermost layer, or crust, is divided into co ntinent al an d oceanic p ortions. Below the crust is the upp er mantle. T he crust and upper mantle comprise the ^lithospherej which is broken into a series of plates.
3800
"^ FIGURE
The Earth
to the
in millons of years before the
present.
The
lithosphere
moves over the asthenosphere,
a
zone that behaves plastically. Below the as thenosphere is the solid lower mantle The Earth's core, which is beneath the lower mantle, is divide d into an outer liquid portion and an inner soli d .
volcanic eruptions, earthquakes, landslides, or flooding that frequently occur.
modern world, and,
These are processes that shape our in fact, some geologists view the
history of the Earth as a series of such short-term or
punctuated events. Such a view is certainly in keeping with the modern principle of uniformitarianism. Furthermore, uniformitarianism does not require that the rates and intensities of geological processes be
22
Chapter
1
An
Introduction to Physical Geology
portion. 5.
--
approach and analyzing facts abou t a pa rticular phenomenon, formulat ing h ypotheses to explain the phenomenon, testipgjh e_hypothgse,s, and
Theftcientific method/is an orderly, lo gical
that involves gathering
finally
proposing a theory. A( theory!? an
explanation for some natural
phenomenon
that has
a large
body of supporting evidence and can be
2.
tested.
many geological features and events Plates can move away from each other, toward each other, or slide past each other. The nteraction between plat es for
.
i
3.
responsible for volcanic eruptions, earthquake sT
is
and the forma tion of mountain ranges and ocean basins. 7.
and metamorphic rocks are major groups of rocks. Jgneous rocks r esult from the crystallization of magma. ^pHimpntary rocks are formed by the consolidation of rock fragments, precipitation of mineral matter from solution, or compaction of plant or animal remains Metamorphic rocks are produced when preexisting frocks are changed in response t o ele vated Igneous,
s edimentar y,
the three
.
temperature, pressure or fl'iiH heneafh the F arth'c cnrfarp
activity,
,
gpnpnll y
8.
The rock
9.
between the internal and external processes of the Earth and among the three major rock groups. Time sets geology apart from the other sciences, is
the
Which
of the following is not a subdivision of geology? a paleontology; b. J<_ astronomy; c mineralogy; d petrology;
a 5.
1;
The
2;
the Earth
is
c.
is
X_ 3;
d
4; e
5.
inferred to be:
composed of rock with a high completely molten; d. i composed mostly of iron and nickel; e. completely solid. 6. The asthenosphere: hollow;
silica
content;
a.
7.
d. 8.
b.
c.
beneath the lithosphere;
lies
b.
is
composed primarily of peridotite; c. behaves plastically and flows slowly; d. is the zone over which plates move; e. a all of these. The layer between the core and the crust is the: a.
X
mantle; b
sima;
lithosphere; c
innersphere.
sial; e.
What fundamental
process
believed to be
is
that the laws of nature have been constant through
responsible for plate motion?
t ime and that the same processes operating tod ay have operated in the past, albeit at different rates.
c.
hot spot activity; b. ^ subduction; spreading ridges;(dy y&C convection cells;
e.
density differences.
a.
9.
IMPORTANT TERMS
Which
not true? an explanation for some natural phenomenon; b. it has a large body of supporting evidence; c. )r it is a conjecture or
metamorphic rock
asthenosphere continental drift
plate tectonic theory
convergent plate
plate
boundary crust
uniformitarianism rock cycle
divergent plate
scientific
boundary geology hypothesis igneous rock
it is
a.
Hutton;
d.
Lyell;
©X
c.
J(
Indonesia; d.
12.
13.
Japan;
e.
A a.
Australia.
^\
Wegener;
Hess;
c.
Lovelock.
examples of what type of
divergent; b.
2.
convergent;
subduction;
d.
e.
answers
and (d). The San Andreas fault separating the Pacific plate from the North American plate is an example of what type of boundary? convergent;
divergent; b. J>C transform; d.
(b)
the United States;
these.
(b)
c.
is in:
b.
e.
transform;
c.
a.
Italy; b.
none of
e.
the hypothesis of
boundary?
^ REVIEW QUESTIONS a
testable;
11. Mid-oceanic ridges are
mantle
Krakatau
it is
The man who proposed continental drift was:
sedimentary rock subduction zone theory transform plate
boundary
lithosphere
10.
method
sea-floor spreading
geologic time scale
is
guess; d.
principle of
core
of the following statements about a scientific
theory a.
1.
b
Earth's core
a.
basic to the
interpretation of Earth histor y. This principle hold s
concentric layers
divided?
The
is
how many
Into
calendar geologists use to date past events. principle of uniformitarianism
stratigraphy.
e.
4.
cycle illustrates the interrelationships
except astronomy. The geologic time scale 10.
of Krakatau: thousands of people; b. created giant sea waves; c. produced spectacular sunsets around the world: d caused a global cooling of about 1/2°C; e. _a_ all of these. killed
a.
Plate tectonic theory provides a unifying explanation
6.
The eruption
and
plate
subduction;
e.
answers
(d). is
composed of
the:
core and lower mantle;
and asthenosphere;
c.
b. lower mantle asthenosphere and upper
Review Questions
23
\
upper mantle and crust; mantle; d. continental and oceanic crust.
24. Briefly describe the Gaia hypothesis. 25. Briefly describe the plate tectonic theory, and explain
e.
14.
Which a.
^V volcanic;
b.
sedimentary;
d.
15.
not a major rock group? igneous; c. metamorphic;
of the following
is
none of
e.
Which rock group forms from magma? "& *>C igneous; b. sedimentary; c.
27.
these.
the cooling of a
all
29.
of these;
e.
none
be
What
is the principle of uniformitarianism? Does allow for catastrophic events? Explain.
it
30. Briefly discuss the importance of having a
of these. 16.
28.
why it is a unifying theory of geology. What are the three types of plate boundaries? What are the three major groups of rocks? Describe the rock cycle, and explain how it may related to plate tectonics.
metamorphic; d.
26.
The premise
that present-day processes have
operated throughout geologic time
is
known
scientifically literate
populace.
as the
principle of: a.
plate tectonics; b.
c.
continental drift; d.
e.*
17.
^X
sea-floor spreading;
Gaia;
uniformitarianism.
The rock
cycle implies that:
metamorphic rocks are derived from magma; rock type can be derived from any other rock type; c. igneous rocks only form beneath
~^ any
the Earth's surface; d.
sedimentary rocks only
form from the weathering of igneous rocks; e
18.
19.
all
Why
of these.
21. 22. 23.
theory.
24
Chapter
1
An
Calif.:
Introduction to Physical Geology
C,
Jr.
1980. The abyss of time. San Francisco,
Freeman, Cooper &c Co.
Dietrich, R. V. 1989.
Rock music. Earth Science 42,
no. 2:
24-25.
&
1990. Rocks depicted in painting and sculpture. Rocks Minerals 65, no. 3: 224-36. 1991. Rocks
Dietrich, R. V.,
and
in literature.
B.
J.
Rocks
Skinner. 1990.
& Minerals Qems,
66.
granites,
and
New
York: Cambridge University Press. Ernst, W. G. 1990. The dynamic planet. Irvington, N.Y.: gravels.
important for people to have a basic understanding of geology? /-_ ^ivJor. Describe some of the ways in which geology affects c <^*our everyday lives. Explain both the difference between physical and historical geology and how they are related. Describe two industries that employ geologists, and briefly discuss what geologists do in each industry. Name the major layers of the Earth, and describe their general composition. Describe the scientific method, and explain how it may lead to a scientific theory. Define scientific is it
'
20.
ADDITIONAL READINGS
Albritton, C.
a.
b.
^
Columbia University Press. P., and S. Self. 1983. The eruption of Krakatau. Scientific American 249, no. 5: 172-87. Hively, W. 1988. How much science does the public understand? American Scientist 76, no. 5: 439-44. Lovelock, J. E. 1988. The ages of Gaia: A biography of our living Earth. New York: W. W. Norton & Co. Mirsky, A. 1989. Geology in our everyday lives. Journal of Geological Education 37, no. 1: 9-12. Rhodes, F. H. T, and R. O. Stone. 1981. Language of the Earth. Elmsford, N.Y.: Pergamon Press. Siever, R. 1983. The dynamic Earth. Scientific American 249, no. 3: 46-55. Francis,
CHAPTER
2
HISTORY OF THE UNIVERSE, SOLAR SYSTEM, AND PLANETS
A
»
OUTLINE
PROLOGUE INTRODUCTION THE ORIGIN OF THE UNIVERSE THE ORIGIN AND EARLY DEVELOPMENT OF THE SOLAR SYSTEM
j.
General Characteristics of the Solar System
Current Theory of the Origin and Early History of the Solar System Meteorites
THE PLANETS The Terrestrial Mercury
Planets
""* Perspective 2-1:
The Tunguska Event
Venus "** Perspective 2-2:
on the
The Evolution of Climate
Terrestrial Planets
Mars The Jovian
Planets
Jupiter
Saturn
Uranus
Neptune and Pluto
THE ORIGIN AND DIFFERENTIATION OF THE EARLY EARTH THE ORIGIN OF THE EARTH-MOON SYSTEM CHAPTER SUMMARY
This impressive impact crater Aurelia, named after Julius Caesar's mother, measures 31.9 km in diameter. It was discovered by the Magellan space probe that began orbiting and radar mapping Venus during the summer of 1990.
"
PROLOGUE BS^sij^ll
On
August 20 and September 5, 1977, Voyagers 1 and 2 were launched on
an ambitious mission to explore the outer planets. They both flew by Jupiter and Saturn, but Voyager 1 took a course out of the solar system while Voyager 2 went on
Uranus and Neptune. Twelve years and 7.13 billion after it was launched, Voyager 2 radioed back spectacular images of the blue planet Neptune (Fig. 2-1) and its pink and blue mottled moon Triton. Its primary mission completed, and with all but a few of its instruments turned off to conserve power, Voyager 2's last act will be to measure the exotic fields and subatomic particles it passes through on its voyage to infinity. By 2018, its generator will be too weak to power communication with Earth, and the most successful space probe ever launched will fall silent. The discoveries made by these two space probes were truly fantastic and in many cases totally to
km
""" FIGURE 2-1 A shimmering blue planet set against the black backdrop of space, Neptune reveals itself to Voyager 2's instruments during its August 1989 flyby. Shown here is Neptune's turbulent atmosphere with its Great Dark Spot and various wispy clouds.
"^ FIGURE
2-2 Europa, the second moon out from covered by a thick surface layer of ice that is crisscrossed by numerous fractures. These fractures appear to be rifts where water has risen to the surface and frozen. This system of fractures and the lack of craters are evidence
Jupiter,
that
is
Europa
is
a geologically active
unexpected by three
addition to discovering
scientists. In
new moons
moon.
of Jupiter, the Voyagers found dusty
rings encircling the planet, thus demonstrating that
rings are a
common
They showed persistent
feature in the outer solar system.
that the Great
eddy
in Jupiter's
Red Spot
is
an enormous,
atmosphere, and they
detected lightning discharges that are 10,000 times
more powerful than those on
Earth.
The Voyagers
sent back images of one of Jupiter's
moons, Io, spewing forth hot sulfurous gases 320 km into space (see Perspective 4-1, Fig. 1). Another Jovian moon, Europa, a liquid
is encrusted with a thick shell of ice covering ocean several kilometers below its surface
(Fig. 2-2).
This
ice
is
crisscrossed with
what appear
to
be cracks that occasionally open to erupt water and then refreeze.
As the Voyagers flew past Saturn, they revealed the its 70,000-km-wide ring system and sent back images of spiral bands of debris only 35 m thick. The Voyagers also discovered that spectacular complexity of
moon Titan has an atmosphere rich in hydrocarbons and nitrogen. As Voyager 2 passed by Uranus on January 24, 1986, it found nine dark, compact rings encircling the the Saturnian
Prologue
27
planet, discovered 10
new moons, and
corkscrew-shaped magnetic
field that
revealed a
extends for
from the planet. Voyager 2 reached its final target, Neptune, in August 1989 and sent back spectacular images and data that were, for the most part, completely millions of kilometers
unanticipated. Instead of a quiet, placid planet,
Neptune turned out to be a dynamic world cloaked in a thin atmosphere composed predominantly of hydrogen and helium mixed with some methane. Winds up to 2,000 km/hour blow over the planet creating tremendous storms, the largest of which, the Great Dark Spot, is in the southern hemisphere. It is nearly as big as the Earth and is similar to the Great Red Spot on Jupiter. Indeed, one of the mysteries raised by Voyager 2's discovery is where Neptune gets the energy to drive such a storm system.
Equally intriguing were the discoveries of six new Neptunian moons and three rings encircling the planet. However, the most astonishing discoveries were found on Neptune's largest moon Triton, which has a diameter of 2,720 km, 700 km less than our
own moon
(Fig. 2-3). Triton,
with a mottled surface
of delicate pinks, reds, and blues,
one of the most colorful objects
is
turning out to be
in the solar system.
surface consists primarily of water ice, with minor amounts of nitrogen and a methane frost. There is good evidence that geysers are erupting frozen nitrogen crystals and organic compounds. If this activity is confirmed, Triton would be only the second Its
Sifc'LrffSS
"^ FIGURE
2-3 Neptune's moon Triton is described by "a world unlike any other." In this composite of numerous high-resolution images taken by Voyager 2 during its August 1989 flyby, various features can be seen. The large south polar ice cap at the bottom consists mostly of frozen nitrogen that was deposited during the previous Tritonian winter and is slowly evaporating. The dark plumes in the lower right may be the result of volcanic activity. Smooth plains and fissures in the upper half are evidence of geologic activity in which the surface has been cracked and flooded by slushy ice that refroze. scientists as
gravitational field soon after the formation of the solar
system. However,
much
still
place other than Earth undergoing active volcanism
Triton and Neptune's other
(see Perspective 4-1).
hypothesis can be accepted.
Some
areas of Triton are
smooth while others have numerous
the end of the
episodes of deformation. Heavily cratered areas bear
began
bombardment by
meteorites or the collapse
most intriguing aspect of Triton is that it may have once been a planet— much like Pluto, which it resembles in size and possibly composition — that was captured by Neptune's of
its
^ Of
surface. Perhaps the
INTRODUCTION all
the
Earth, has
known life
on
planets it.
and
moons only
one,
This unique planet, revolving
around the Sun every 365.25 days, is a dynamic and complex body. When viewed from the blackness of
28
Chapter 2
A
first
era of planetary exploration that
1962 with a flyby of Venus. However, an ambitious program of unmanned space exploration in
planned for the 1990s, including placing spacecraft orbit around Venus (see Perspective 13-2), Mars, Jupiter, and Saturn and a space probe designed to
is
in
rendezvous with the comet Kopff.
space, the Earth their
before this
Voyager's dazzling encounter with Neptune marked
a very irregular appearance indicating
witness to
needs to be learned about
moons
wrapped
is
in a veil
a brilliant, shimmering, bluish planet,
of swirling white clouds (Fig. 1-4).
Beneath these clouds
is
a surface covered by oceans
and
seven continents and numerous islands.
The Earth has not always looked
History of the Universe, Solar System, and Planets
the
way
it
does
today. Based
on various
lines of evidence,
many
scien-
homogeneous mass of rotating dust and gases that contracted, heated, and differentiated during its early history to form a mediumsized planet with a metallic core, a mantle composed of iron- and magnesium-rich rocks, and a thin crust. Overlying this crust is an atmosphere currently composed of 78% nitrogen and 21% oxygen. As the third planet from the Sun, Earth seems to have formed at just the right distance from the Sun (149,600,000 km) so that it is neither too hot nor too tists
think that the Earth began as a
cold to support is
just right to
life
as
we know
it.
Furthermore,
hold an atmosphere.
gravity
would be so weak
smaller,
its
little, if
any, atmosphere.
If
its
-— TABLE
it
1.
2.
it.
Two
fundamental phenomena indicate that the Big first is the expansion of the universe. When astronomers look beyond our own solar system, they observe that everywhere in the universe galaxies are apparently moving away from each other at tremendous speeds. By measuring this expansion rate, they can calculate how long ago the galaxies were all together at a single point. Secondly, a background radiation of 2.7° above absolute zero (absolute zero equals — 273°C) permeates the entire universe. This background radiation is thought to be the faint afterglow of the Big Bang. At the time of the Big Bang, matter as we know it did not exist, and the universe consisted of pure energy. Within the first second after the Big Bang, the four basic forces— gravity, electromagnetic force, strong nuclear force, and weak nuclear force (Table 2-1)— had all separated, and the universe experienced enormous expansion. Matter and antimatter collided and annihilated each other. Fortunately, there was a slight excess of mat-
Bang occurred. The
over that would become the universe.
was
three minutes old, temperatures
When
body toward
The electromagnetic force combines electricity and magnetism into the same force and binds atoms
gamma
rays (shortest) to radio waves (longest)
through massless particles called photons. 3.
The strong nuclear
force binds protons
in the
and
nucleus of an atom.
The weak nuclear force is responsible for the breakdown of an atom's nucleus, producing radioactive decay.
scientists think that the universe originated be-
universe
the attraction of one
neutrons together
tween 13 and 20 billion years ago in what is popularly called the "Big Bang." In a region infinitely smaller than an atom, both time and space were set at zero. As explained by Einstein's theory of relativity, space and time are unalterably linked to form a space-time continuum. In other words, without space there can be no time. Therefore, there is no "before the Big Bang," only what
ter left
is
into molecules. It also transmits radiation across the various spectra at wavelengths ranging from
^ THE ORIGIN OF THE UNIVERSE
occurred after
Gravity another.
4.
Most
all
interactions of matter:
size
could retain
Basic Forces
Four forces appear to be responsible for
the Earth were
that
The Four
2-1
of the Universe
the
were cool
enough for protons and neutrons to fuse together to form the nuclei of hydrogen and helium atoms. Approx-
SOURCE: Adapted
by permission from Table 7-2, page 175 of Historical Geology: Evolution of the Earth and Life through Time by Reed Wicander and James S. Monroe. Copyright © 1989 by West Publishing Company. All rights reserved.
imately 100,000 years
later,
previously formed nuclei to
electrons joined with the
make complete atoms
of
hydrogen and helium. At the same time, photons (the energetic particles of light) separated from matter, and light burst forth for the first time.
As the universe continued expanding and cooling, and galaxies formed, and the chemical makeup of
stars
the universe changed. Early in
its
history, the universe
was 100% hydrogen and helium, whereas
it is
now 98%
hydrogen and helium by weight. Over the course of their history, stars undergo many nuclear reactions whereby lighter elements are converted into heavier elements by nuclear fusion in which atomic nuclei combine to form more massive nuclei. Such reactions, which convert hydrogen to helium, occur in the cores of all stars. The subsequent conversion of helium to heavier elements, such as carbon, depends on the mass of the star. When a star dies, often explosively, the heavier elements that were formed in its core are returned to interstellar space and are available for inclusion in
new
stars.
When new
stars
form, they will
have a small amount of these heavier elements, which may be converted to still heavier elements. In this way, the amount of heavier elements in the chemical composition of the galaxies, each of which consists of billions of stars,
is
gradually enhanced.
The Origin of
the Universe
29
<«r-
TABLE
2-2
Jupiter
Pluto
Terrestr a
Venus
""' FIGURE 2-4 Diagramatic representation of the solar system showing (a) the relative sizes of the planets and (b) their orbits around the Sun.
Neptune '.a's
EarIh
Mercury
Jovian planets
..a:
dicating that they are
elements. These are
composed of rock and
known
metallic
that cloud, should have a very rapid rate of rotation,
as the terrestrial planets be-
leisurely 25-day rotation. any theory of the origin of the solar system must accommodate the nature and distribution of the various interplanetary- objects such as the asteroid belt,
cause they are similar to terra, which
is
Latin for Earth.
The next four planets— Jupiter, Saturn, Uranus, and Neptune — are called the Jovian planets because they all resemble Jupiter. The Jovian planets are large and have low mean densities, indicating that they are composed mostly of lightweight gases such as hydrogen and helium, as well as frozen compounds such as ammonia and methane. The outermost planet, Pluto, is small and has 3 a low mean density of slightly more than 2.0 g/cm The slow rotation of the Sun is another feature that must be accounted for in any comprehensive theory of the origin of the solar system. If the solar system formed from the collapse of a rotating cloud of gas and dust as is currently accepted, the Sun, which was at the center of .
instead of
its
Finally,
comets, and interplanetary gases and dust.
Current Theory of the Origin and Early History of the Solar System Various scientific theories of the origin of the solar system
have been proposed, modified, and discarded since the scientist and philosopher Rene Descartes first proposed in 1644 that the solar system formed from a gigantic whirlpool within a universal fluid. Most theories have involved an origin from a primordial rotating cloud of gas
French
The Origin and Early Development of
the Solar System
31
cording to the laws of physics should be rotating rapidly.
Ionized gases
This problem was finally solved with the discovery of solar
wind, which
is
an outflow of ionized gases from the Sun its magnetic field and slow down its
that interact with
rotation through a magnetic braking process (Fig. 2-5).
According to the currently accepted solar nebula theory Magnetic force lines
(Fig. 2-6), interstellar
Milky this
flattened
90%
called a solar nebula.
and
dust.
Through
the forces of gravity
and rotation,
this
cloud then shrank and collapsed into a rotating disk. Detached rings within the disk condensed into planets, and
Sun condensed in the center of the disk. The problem with most of these theories is
the
failed to explain the
that they
slow rotation of the Sun, which ac-
—
own
its
gravitational attraction, then
and
(b)
contracting, rotating,
(c)
flattening into a disk, with
(d) the
Sun forming
in the center
and eddies gathering up material to form planets. As the Sun contracts and begins to visibly shine, (e) intense solar radiation blows away unaccreted gas and dust until finally, (f) the Sun begins burning hydrogen and the planets complete their formation.
32
Chapter 2
A
The
inner portions of this nebula
were hot and the outer regions were cold. The turbulence in this solar nebula formed localized eddies where gas and solid particles condensed. Every element and compound has a temperature and pressure combination at which it condenses from the gaseous phase, just as frost forms from water vapor on a cold night. Elements that condense easily at high tempera-
FIGURE 2-6 The solar nebula theory for the origin of our solar system involves (a) a huge nebula condensing under
As
and began rotating counterclockof its mass concentrated in the central part of the cloud. As the rotation and concentration of material continued, an embryonic sun formed, surrounded by a turbulent, rotating cloud of material it
wise, with about
2-5 The slow rotation of the Sun is the result of the interaction of its magnetic force lines with ionized gases of the solar nebula. Thus, the rotation is slowed by a magnetic braking process.
the
cloud gradually collapsed under the influence of
gravity,
~^ FIGURE
arm of
collapsing.
material in a spiral
Way Galaxy condensed and began
History of the Universe, Solar System, and Planets
tures,
are
such as iron, magnesium, silicon, and aluminum,
known
formed
as refractory elements,
and these elements
solid particles in the hot inner region of the solar
The volatile elements, such as hydrogen, heliu m, ammonia, and methane, conden se at very low temperatures; consequently^ they "remained gaseous in the ho t nebula.
iTlrTer
region of the
nebula, but formed ices in
s olar
i
ts
coIcToTIter-portion.
and
rotation of the planets and their moons, the differences in
composition of the
terrestrial
and Jovian
planets,
and
the slow rotation of the Sun, as well as the presence of the asteroid belt. Based
on the available data, the solar
nebula theory best explains the features of the solar system and provides a logical explanation for its evolutionary history.
As condensation took solid particles
solar system accounts for the similarities in orbits
place, gaseous, liquid,
and
began accreting into ever-larger masses
became true planeThe composition and evolutionary history
called planetesimals that eventually
Meteorites
of the planets are indicated, in part, by their distance
Meteorites are thought to be pieces of material that originated during the formation of the solar system 4.6 billion
from the Sun. For example, the
years ago, and as such they provide important informa-
tary bodies.
composed of rock and
terrestrial planets are
metallic elements that condensed
The Jovian planets, all of which have small central rocky cores compared to their overall size, are composed mostly of hydrogen, helium, ammonia, and methane, which condense at low temperatures. Thus, the farther away from the Sun that condensation occurred, the lower the temat the high temperatures of the inner nebula.
tion about
origin.
its
bardment occurred of the
many
A
pieces of material that
into planetary bodies or tivity
bomwas clearing itself had not yet accreted
period of heavy meteorite
as the solar system
moons. Since then, meteorite
has greatly diminished.
Most of the
ac-
meteorites that
currently reach the Earth's surface are probably frag-
ments resulting from
perature, and hence the higher the percentage of volatile
collisions
between asteroids.
Meteorites are classified into three broad groups
elements relative to refractory elements.
based on their proportions of metals and
While the planets were accreting, material that had been pulled into the center of the nebula also condensed, collapsed, and was heated to several million degrees by
als (Fig. 2-7).
are
posed of iron
(minerals
gravitational compression.
The
result
was
the birth of a
our Sun. During the early accretionary phase of the solar system's history, collisions between various bodies were star,
common,
as indicated by the craters
and moons.
An
nus could explain
why
it
rotates clockwise rather than
com-
known as stones (Fig. 2-7b). There many varieties of stones, and they provide geologists with much information about the origin and history of oxygen) and thus are are
the solar system. Irons, the second group, accounting for all
meteorites, are
composed primarily of
a
about 6% of combination
of iron and nickel alloys (Fig. 2-7c). Their large crystal
and chemical composition indicate that they must
counterclockwise, and a collision could also explain
size
why Uranus and
have cooled very slowly
Pluto do not rotate nearly perpendic-
miner-
containing the elements iron, magnesium, silicon, and
on many planets
unusually large collision involving Ve-
silicate
About 93% of all meteorites and magnesium silicate minerals
in large objects
such as asteroids
planetesimals in a localized eddy between
where the hot iron-nickel interior could be insulated from the cold of space. Collisions between such slowly cooling asteroids produced the iron meteorites that we
ally
find today.
ular to the plane of the ecliptic. It is
thought that the asteroids probably formed as
what eventubecame Mars and Jupiter in much the same way as other planetesimals formed the terrestrial planets. However, the tremendous gravitational field of Jupiter prevented this material from accreting into a planet. The comets, which are interplanetary bodies composed of loosely bound rocky and icy material, are thought to have condensed near the orbits of Uranus and Neptune. Each time the comets pass by Jupiter and
composed of nearly
Stony-irons, the third group, are
equal amounts of iron and nickel and they
make up
less
than
1%
of
all
silicate
minerals;
meteorites (Fig. 2-7d).
Stony-irons are generally believed to represent frag-
ments from the zone between the
silicate
and metallic
portions of a large differentiated asteroid.
Astronomers have
identified
at
least 40 asteroids whose orbits cross
Saturn, however, the gravitational slingshot effect of
larger than a kilometer in diameter
those planets increases their speed, forcing them further
the Earth's and estimate that there
out into the solar system.
1,000 such asteroids. A collision between a large asteroid and the Earth formed the famous Meteor Crater in
Thus, the solar nebula theory of the formation of the
The Origin and Early Development of
may
be as
many
the Solar System
as
33
"^ FIGURE 2-7 {a) Relative proportions of the three groups of meteorites, (b) Polished slab of a stony meteorite from the Pinto Mountains, (c) Polished slab of an iron meteorite from Bogou, Upper Volta, Brazil, (d) Polished slab of a stony-iron meteorite from Thiel Mountain, Antarctica. (Photo
(b)
courtesy of
New
Ken Nichols,
Institute of Meteorites,
Mexico. Photos (c) and Brian Mason, Smithsonian Institution University of
(d)
courtesy of
mmr^mm^imm^%mt^~2~TTr (d)
34
Chapter 2
A
History of the Universe, Solar System, and Planets
'
Arizona if
While asteroid-Earth collisions are do happen and could have devastating results
(Fig. 2-8).
rare, they
they occurred in a populated area (Fig. 2-9) (see Per-
spective 2-1). a meteorite
Many
scientists think that a collision
about 10
km
in
with
diameter led to the extinc-
and several other groups of animals 66 million years ago. Such a collision would have generated a tremendous amount of dust that would have tions of dinosaurs
blocked out the Sun, thereby lowering global temperaand preventing photosynthesis, which, in turn,
tures
would have
triggered a collapse of the ecosystem
massive extinctions.
We know
and
that the ash released into
the atmosphere from volcanic eruptions has affected
cli-
mates (see Chapter 4), and studies indicate that a collision with a large meteorite could produce enough dust to similarly affect global climate.
»
THE PLANETS
about each planet in the solar system has been derived from Earth-based observations and measurements as well as from the numerous space probes launched during the past 30 years. Such information as a planet's size, mass, density, composition, presence of a magnetic field, and atmospheric composition has allowed scientists to formulate hypotheses concerning the origin and history of the planets and their moons. As with any scientific endeavor, hypotheses have been modified or abandoned as is
2-8 Meteor Crater, Arizona, is the result of an Earth-asteroid collision that occurred between 25,000 and 50,000 years ago. It produced a crater 1.2 km in diameter and 180 m deep.
Mercury
A tremendous amount of information
available. This
"^ FIGURE
new
information has become
especially true in the area of planetary
geology. Images and data radioed back by the various
Mercury, the changed very its
closest planet to the Sun, apparently has little
since
it
was heavily cratered during Most of what we know diameter) planet comes from
early history (Fig. 2-10).
about
this small
(4,880
km
"^ FIGURE
2-9 Artistic rendition of what the moment of impact would look like if the nucleus of a comet, 48 km in diameter, hit northern New Jersey. Everything visible in this picture, including the buildings of lower Manhattan in the foreground, would be vaporized, and a plume of fine material would be ejected into the atmosphere and circulated
around the Earth.
space probes have forced scientists to reexamine and
modify
The It
Earth-based hypotheses about many of and the forces that formed and shaped them.
earlier
the planets
Terrestrial Planets
appears that
all
of the terrestrial planets had a similar
early history during
which volcanism and cratering from
meteorite impacts were
common.
After accretion, each
planet appears to have undergone differentiation as a result of heating
by radioactive decay. The mass, density,
and composition of the planets indicate that each formed a metallic core and a silicate mantle-crust during this phase. Images sent back by the various space probes also clearly show that volcanism and cratering by meteorites
continued during the differentiation phase. Vol-
canic eruptions produced lava flows, and an atmosphere
developed on each planet by a process called outgassing (see Perspective 2-2).
The
Planets
35
••
,'ft<^
Perspective 2-1
THE TUNGUSKA EVENT On
June 30, 1908, a bright object crossed the sky
moving from southeast to northwest over central Siberia, and a few seconds later a huge explosion occurred in the Tunguska River basin (Fig. 1). The noise from the explosion was heard up to 1,000 km away, a column of incandescent matter rose to a height of about 20 km, the shock wave from the
explosion traveled around the world twice, and
seismographs around the world registered an earthquake. Eyewitnesses reported that the concussion
wave threw people to the ground as much as 60 away from the blast site. What the object was that caused this massive
explosion remains uncertain. Part of the uncertainty
Arctic
p-
Ocean
^JORWAYj
SWEDEN,
&^
TINI-Ar*
>a fe
«f
¥
ESTONIA LATVIA LITHUANIA
• Moscow SOVIET UNION
Black
\Sea
Karaganda
TURKEY/
/"^Aral
0>Sea '
MONGOLIA
Caspian
Sea
IRAQ,
CHINA
IRAN
"^FIGURE
36
1
Chapter 2
The Tunguska explosion occurred
A
km
in central Siberia in the
History of the Universe, Solar System, and Planets
Soviet Union.
is
-»- FIGURE 2 Evidence of the Tunguska event is still apparent in this photograph taken 20 years later. The destruction was caused by some type of explosion in central Siberia in 1908.
in an extremely remote 1921 that an expedition was launched to investigate. Unfortunately, illness and exhaustion prevented this expedition from reaching the explosion site. Finally, in 1927, 19 years after the explosion, an expedition led by Leonid Kulik successfully reached the Tunguska basin. A vast peat bog called the Southern Swamp was identified as the site above which the explosion occurred; subsequent
because the event occurred
area,
and
it
was not
investigations
and
occurred about 8
until
studies indicate that the explosion
km
above the surface, and estimated to have been about 12.5 megatons (equivalent to 12.5 million tons of
1,000
km 2
it is
TNT). More than
of forest were leveled by the explosion,
from a meteorite impact. In for investigation
fact,
part of the incentive
may have been economic;
the Soviets
was present and could be mined for its iron content. However, when investigators finally reached the site, no evidence of meteor crater was ever identified. During the 1930s, two Americans proposed that the devastation in the Tunguska River basin was caused by a small, icy comet that exploded in the believed that a meteor
atmosphere. According to
perhaps 50
m
this hypothesis, a
in diameter, entered the
began heating up; as
this
a
comet,
atmosphere and
heating occurred, frozen
gases were instantaneously converted to the gaseous state, releasing a
tremendous amount of energy and
and, according to earlier accounts, tens of thousands
causing a large explosion. The comet hypothesis was
of animals perished
subsequently endorsed by E. L. Krinov of the Soviet
(Fig. 2). Fortunately, there were no human casualties. Even before the explosion site was reached, scientists had hypothesized that the explosion resulted
Academy
of Sciences, and currently
is
the
most widely
accepted explanation for the Tunguska event.
The
Planets
37
"""
FIGURE
2-10
(a)
Mercury has
surface that has changed very
little
a heavily cratered
since
its
early history.
Seven scarps (indicated by arrows) can clearly be seen this image. It is thought that these scarps formed when Mercury cooled and contracted early in its history. (c) Internal structure of Mercury, showing its large solid (b)
core relative to
(b)
its
in
overall size.
measurements and observations made during the flybys
bly escaped into space very quickly. Nevertheless, very
of Mariner 10 in 1974 and 1975 (Table 2-2).
high
small quantities of hydrogen and helium, thought to
has a large
have originated from the solar winds that stream by Mercury, were detected by Mariner 10.
overall density of 5.4
g/cm
3
indicates that
metallic core measuring 3,600
accounts for
80%
in
it
diameter; the core
of Mercury's mass (Fig. 2- 10). Fur-
thermore, Mercury has a
1%
km
Its
weak magnetic
field
(about
as strong as the Earth's), indicating that the core
is
Images sent back by Mariner 10 show a heavily cratered surface with the largest impact basins filled with
what appear to be lava flows similar to the lava plains on the Moon. However, the lava plains are not deformed, indicating that there has been little or no tectonic activity. Another feature of Mercury's surface is a large number of long
cliffs,
called scarps (Fig. 2- 10b).
gested that these scarps formed
and contracted. Because Mercury tion
is
38
it
Chapter 2
is
may A
all the planets, Venus is the most similar in size and mass to the Earth (Table 2-2, Figure 2-11). It differs, however, in most other respects. Venus is searingly hot with a surface temperature of 475°C and an oppressively thick atmosphere composed of 96% carbon dioxide and 3.5% nitrogen with traces of sulfur dioxide and
It is
sug-
when Mercury cooled
sulfuric
and hydrochloric
acid.
From information ob-
tained by the various space probes that have passed by,
orbited Venus, and descended to
its
surface,
we know
composed of droplets of planet. Furthermore, winds up
that three distinct cloud layers
so small,
its
gravitational attrac-
atmospheric gases; any athave held when it formed proba-
insufficient to retain
mosphere that
Venus
Of
probably partially molten.
sulfuric acid envelop the
to
360 km/ hour occur
the planet's surface
History of the Universe, Solar System, and Planets
is
at the top of the clouds,
calm.
whereas
'*' FIGURE 2-11 (a) Venus has a searingly hot surface and is surrounded by an oppressively thick atmosphere composed largely of carbon dioxide, (b) This relief map of Venus shows the three major highland areas: Ishtar Terra at the top, Beta Regio at left center,
and Aphrodite Terra
at right center, (c)
The
internal structure of Venus.
The
Planets
39
Perspective 2-2
THE EVOLUTION OF CLIMATE ON THE TERRESTRIAL PLANETS The
origins
and early evolution of the
history,
terrestrial
hold a somewhat different view
planets has acquired a dramatically different climate.
For example,
Why?
water vapor
All four planets were initially alike, with atmospheres high in carbon dioxide and water vapor derived by outgassing, a process whereby light gases from the interior rise to the surface during volcanic
eruptions. Mercury, because of
proximity to the Sun, lost evaporation early
its
its
small size and
in its history.
Venus, Earth, and
all
their early histories to
climate capable of supporting
The reason
is
related to the recycling of
carbon
(carbon-silicate geochemical cycle) as well as their
Carbon dioxide
recycling
is
an
important regulator of climates because carbon dioxide, other gases, and water vapor allow sunlight to pass
"through" them but trap the heat the planet's surface.
Heat
is
reflected
back from
thus retained, and the
temperature of the atmosphere and surface increases in
what is known as the greenhouse effect. Carbon dioxide combines with water in the atmosphere to form carbonic acid. When this slightly acidic rain falls, it decomposes rocks, releasing calcium and bicarbonate ions into streams and rivers and, ultimately, the oceans. In the oceans, marine organisms use some of these ions to construct calcium carbonate.
When
shells of
the organisms die, their
shells
become part of the
some
of which are eventually subducted at convergent
plate boundaries.
total
1—2).
is present in the atmosphere and there is The amount of carbon dioxide leaving the atmosphere thus decreases and less decomposition of rocks occurs. However, there is no overall long term change in the amount of carbon dioxidefeturned to it is
continually replenished
by plate subduction and volcanism. This leads to a temporary increase in carbon dioxide in the atmosphere, greater greenhouse warming, and, thus, higher surface temperatures.
would happen
if
the surface
temperature should increase. Oceanic evaporation
dioxide between the atmosphere and the crust distance from the Sun.
(see Perspective
the Earth's surface cools, less
less rain.
Just the opposite
life.
that these three planets evolved such
different climates
when
the atmosphere because
atmosphere by
were temperate enough during have had fluid water on their surfaces, yet only Earth still has surface water and a Mars, however,
carbonate sediments,
During subduction these carbonate
would then increase, leading to greater rainfall and more rapid decomposition of rock; as a result, carbon dioxide would be removed from the atmosphere. Greenhouse warming would then decrease and surface temperatures would fall. Venus today is almost completely waterless. However, many scientists think that during its early history, when the Sun was dimmer, Venus perhaps had vast oceans. During this time, water vapor as well as carbon dioxide was being released into the atmosphere by volcanism. The water vapor condensed and formed oceans, while carbon dioxide cycled (by plate tectonics) just as it does on Earth. As the Sun's energy output increased, however, these oceans
Once
eventually evaporated. there
was no water
the oceans disappeared,
to return carbon to the crust,
and
carbon dioxide began accumulating in the atmosphere, creating a greenhouse effect and raising temperatures. Mars, like Venus and Earth, probably once had a moderate climate and surface water, as indicated by
network of
on
sediments are heated under pressure and release
the crisscrossing
carbon dioxide gas that reenters the atmosphere primarily through volcanic eruptions (Fig. 1).
it had formed and hence cooled rapidly. Eventually, the interior of Mars became so cold that it no longer released carbon dioxide. As a
The
terrain.
Chapter 2
A
Because Mars
less internal
recyling of carbon dioxide has allowed the
Earth to maintain a moderate climate throughout
40
although proponents of the Gaia hypothesis
planets appear to have been similar, yet each of these
its
History of the Universe, Solar System, and Planets
heat
is
when
valleys
its
oldest
smaller than the Earth,
it
Weathering of continental
rocks
s
Calcium and
Carbon dioxide released back into atmosphere
.bicarbonate ions
by volcanism
carried to
ocean Trench
Marine organisms construct calcium
carbonate shells
Carbonate sediment I
Upper mantle Continental crust
t~^~
Carbon dioxide in
magma
1 The carbon-silicate geochemical cycle illustrates how carbon dioxide is Carbon dioxide is removed from the atmosphere by combining with water and forming slightly acidic rain that falls on the Earth's surface and decomposes rocks. This decomposition releases calcium and bicarbonate ions that ultimately reach the oceans. Marine organisms use these ions to construct shells of calcium carbonate. When they die, the shells become part of the carbonate sediments that are eventually subducted. As the sediments are subjected to heat and pressure, they release carbon dioxide gas back into
FIGURE
recycled.
the atmosphere primarily through volcanic eruptions.
result, the
amount
creased to
its
of atmospheric carbon dioxide de-
current low
level.
The greenhouse
effect
was thus weakened, and the Martian atmosphere became thin and cooled to its present low temperature. If Mars had been the size of Earth or Venus, it very likely would have had enough internal heat to
continue recycling carbon dioxide, thus offsetting the
low sunlight levels caused by its distance from the Sun. In other words, Mars would still have enough carbon dioxide in its atmosphere so that it effects of
could maintain a "temperate climate."
The
Planets
41
Radar images from orbiting spacecraft as well as from the Venusian surface indicate three general types of terrain (Fig. 2-1 lb). Rolling plains, characterized by numerous craters and circular basins, cover about 65% of the planet; lowlands cover another 27%; and highlands, similar to continents, occupy the remaining 8%.
42
Chapter 2
A
Even though no active volcanism has been observed on Venus, the presence of volcanoes, numerous lava flows, folded mountain ranges, and a network of fractures indicate internal and surface activity during the past (see Perspective 12-2). There is, however, no evidence for active plate tectonics such as on Earth.
History of the Universe, Solar System, and Planets
"^ FIGURE 2-12 (a) (left) Dawn rises over Mars as the Viking 2 orbiter passes by. One of the largest volcanoes on Mars, Ascreaus Mons, can be seen near the top of this photograph, while near the bottom is the Argyre basin, formed from the impact of a large meteorite early in the history of Mars. The largest canyon known in the solar system, Valles Marineris, can be seen on the right side of Mars. To gain some perspective on the size of Valles Marineris, consider that it would nearly stretch across the United States and its width and depth would dwarf the Grand Canyon
(see insert), (b)
known volcano
Olympus Mons,
in the solar system,
the largest
can be seen rising above
white clouds of frozen carbon dioxide, (c) To illustrate the size of the Martian volcanoes, a map of the western United States is shown superimposed over Olympus Mons and three companion volcanoes, (d) The internal structure of Mars.
Mars Mars, the red planet, has a diameter of 6,787 km and a mass one-tenth that of the Earth (Table 2-2; Fig. 2-12). It is
differentiated, as are all the terrestrial planets, into
and a silicate mantle and crust. The thin Martian atmosphere consists of 95% carbon dioxide, 2.7% nitrogen, 1.7% argon, and traces of other gases. Rotating once every 24.6 hours, a Martian day is only slightly longer than an Earth day. Mars also has distinct seasons during which its polar ice caps of frozen carbon dioxide expand and recede. Perhaps the most striking aspect of Mars is its surface, many features of which have not yet been satisfactorily explained. Like the surfaces of Mercury and the a metallic core
The
Planets
43
Moon,
the southern hemisphere
is
heavily cratered, at-
bombardment. Hellas, a crater with a diameter of 2,000 km, is the largest known impact structure in the solar system and is found in the Martian southern hemisphere. The northern hemisphere is much different, having large smooth plains, fewer craters, and evidence of extensive volcanism. The largest known volcano in the solar testing to a period of meteorite
system,
Olympus Mons
(Fig.
2-12b), has a basal diameter
27 km above the surrounding plains, and is topped by a huge circular crater 80 km in diameter. The northern hemisphere is also marked by huge canyons that are essentially parallel to the Martian equator. of 600
One
km,
rises
of these canyons, Valles Marineris,
km long, 250 km wide,
and 7
km
is
at least
deep and
is
4,000
the largest
were present on Earth, it would stretch from San Francisco to New York (Fig. 2-12a)! It is not yet known how these vast canyons
yet discovered in the solar system.
If it
formed, although geologists postulate that they may have started as large rift zones that were subsequently modified by running water and wind erosion. Such hypotheses are based on comparison to
rift
structures
found on Earth and topographic features formed by geologic agents of erosion such as water and wind (see Chapters 16 and 19). Tremendous wind storms have strongly influenced the surface of Mars and led to dramatic dune formations (see Perspective 19-1, Fig. 3). Even more stunning than the dunes, however, are the braided channels that appear to be the result of running water (Fig. 16-1). It is currently too cold for surface water to exist, yet the channels strongly indicate that there was running water on Mars during the past.
The fresh-looking strongly suggest that
its
Mars was a and may still
many volcanoes
tectonically active
There is, howno evidence that plate movement, such as occurs
planet during the past ever,
surfaces of
be.
on Earth, has ever occurred.
Jupiter Jupiter
the largest of the Jovian planets (Table 2-2;
is
With its moons, rings, and radiation belts, it most complex and varied planet in the solar sys-
Fig. 2-13). is
the
tem. Jupiter's density
but because (Table 2-2). 2.5 times
it
It is
from the time of
its
formation.
When Jupiter
formed,
heated up because of gravitational contraction
and
the planets) insulates
its
is
still
it
all
cooling. Jupiter's massive size
and hence
interior,
did
(as
it
has cooled very slowly.
Jupiter has a relatively small central core of solid
rocky material formed by differentiation. Above this core is a thick zone of liquid metallic hydrogen followed by a thicker layer of liquid hydrogen; above that is a thin layer of clouds (Fig. 2-13b). Surrounding Jupiter
are a strong magnetic field
and an intense radiation
belt.
Jupiter has a dense atmosphere of hydrogen, helium,
methane, and ammonia, which some believe are the same gases that composed the Earth's first atmosphere.
atmosphere is divided into a series of bands as well as a variety of spots (the Great Red Spot) and other features, all interacting in incredibly complex motions. Revolving around Jupiter are 16 moons varying greatly in tectonic and geologic activity (see Perspective 4-1). Also surrounding Jupiter is a thin, faint ring, a
Jupiter's cloudy
different colored
feature shared by
all
the Jovian planets.
Saturn Saturn
is
slightly smaller
than Jupiter, about one-third as
massive, and about one-half as dense, but has a similar
and atmosphere (Table 2-2; Fig. 2-14). more energy (2.2 times as gets from the Sun. Saturn's most conspic-
internal structure
Saturn, like Jupiter, gives off it
is its
ring system, consisting of thousands
of rippling, spiraling bands of countless particles.
Planets
planets are completely unlike any of the ter-
restrial planets in size
it has 318 times the mass an unusual planet in that it emits almost
more energy than it receives from the Sun. One is that most of the excess energy is left over
uous feature
The Jovian
only one-fourth that of Earth,
explanation
much) than
The Jovian
is
so large,
is
or chemical composition (Table
The composition of Saturn is similar to Jupiter's, but more hydrogen and less helium. Sat-
consists of slightly
urn's core
is
not as dense as Jupiter's, and as
in the case
and followed completely different evolutionary histories. While they all apparently contain a small core in
of Jupiter, a layer of liquid metallic hydrogen overlies
relation to their overall size, the bulk of a Jovian planet
helium, and,
composed of volatile elements and compounds that condense at low temperatures such as hydrogen, helium, methane, and ammonia.
cause liquid metallic hydrogen can exist only at very
2-2)
is
44
Chapter 2
A
the core, followed by a zone of liquid hydrogen and lastly,
a layer of clouds (Fig. 2-14b). Be-
high pressures, and since Saturn
is
smaller than Jupiter,
such high pressures are found at greater depths
History of the Universe, Solar System, and Planets
in Sat-
"^ FIGURE 2-14 Saturn and three of its moons, (a) This image of Saturn was taken by Voyager 2 from several million kilometers away and shows the ring system of the planet as well as its banded atmosphere. Saturn has an atmosphere similar to that of Jupiter, but has a thicker cloud cover and contains little ammonia, (b) The internal structure of Saturn,
(c)
Mimas (392 km in diameter) exhibits Some areas of Enceladus (500 km
a large impact crater, (d) in
diameter) have fewer craters, suggesting recent volcanic Hyperion (350 x 200 km) has an irregular
activity, (e) Little
shape and several impact craters; Saturn.
46
Chapter 2
A
History of the Universe, Solar System, and Planets
it
tumbles as
it
orbits
"^ FIGURE 2-15 (a) Images of Uranus taken by Voyager 2 under ordinary' light show a featureless planet, (b) When color is enhanced by computer processing techniques, Uranus is seen to have zonal flow patterns in its atmosphere. (c) The internal structure of Uranus.
With
a diameter of only 2,300
est planet
and,
strictly
Jovian planets (Table 2-2). but recent studies indicate a mixture of
km, Pluto
speaking, Little
it
is
it
is
is
the small-
not one of the
known about
Pluto,
has a rocky core overlain by
methane gas and
ice (Fig. 2-17). It also
has
a thin, two-layer atmosphere with a clear upper layer
overlying a
more opaque lower
Pluto differs from
all
highly eccentric orbit that
plane of the that
is
differ
to those of Jupiter.
The
internal structure of
Neptune
is
Uranus (Table 2-2); it has a rocky core approximately 17,000 km in diameter surrounded by a semifrozen slush of water and liquid methane (Fig. 2-16). Its atmosphere is composed of hydrogen and helium with some methane. Encircling Neptune are three similar to that of
faint rings
and eight moons.
ecliptic. It
nearly half
its
markedly from
layer.
the other planets in that is
tilted
has one
size
it
has a
with respect to the
known moon, Charon,
with a surface that appears to
Pluto's.
^ THE ORIGIN AND DIFFERENTIATION OF THE EARLY EARTH As matter was accreting in the various turbulent eddies that swirled around the early Sun, enough material eventually gathered together in one eddy to form the planet Earth. Recall from Chapter 1 that the Earth is a
The Origin and
Differentiation of the Early Earth
47
24,500
km
The differentiation into a layered planet is probably most significant event in the history of the Earth. Not only did it lead to the formation of a crust and eventually to continents (see Chapter 14), but it was the
probably responsible for the outgassing of light volatile elements from the interior that eventually led to the formation of the oceans and atmosphere.
» THE ORIGIN OF THE EARTH-MOON SYSTEM We
probably
know more about our Moon
than any
other celestial object except the Earth (Fig. 2-19). Nevertheless,
even though the
Moon
centuries through telescopes rectly,
many
has been studied for
and has been sampled
di-
questions remain unanswered.
The Moon
is
one-fourth the diameter of the Earth, has
low density (3.3 g/cm 3 relative to the terrestrial planets, and exhibits an unusual chemistry in that it is bone-dry, having been largely depleted of most volatile elements (Table 2-2). The Moon orbits the Earth and rotates on its own axis at the same rate, so we always see the same side. Furthermore, the Earth-Moon system is unique among the terrestrial planets. Neither Mercury nor Venus has a moon, and the two small moons of Mars— Phobos and Deimos — a
)
FIGURE 2-19 The side of the Moon as seen from Earth. The light-colored areas are the lunar highlands which were heavily cratered by meteorite impacts. The dark-colored areas are maria, which formed when lava flowed out onto the surface.
"'•'
are probably captured asteroids.
The major
surface of the
Moon
can be divided into two
parts: the low-lying dark-colored plains, called
maria, and the light-colored highlands
highlands are the oldest parts of the
""'
FIGURE
2-18
(a)
The
early Earth
(Fig. 2-19).
Moon
The
and are
heavily cratered, providing striking evidence of the massive meteorite
bombardment
was probably of uniform composition and
density throughout, (b) Heating of the early Earth reached the melting point of iron
and
which, being denser than silicate minerals, settled to the Earth's center. At the same time, the lighter silicates flowed upward to form the mantle and the crust. (c) In this way, a differentiated Earth formed, consisting of a dense iron-nickel core, an iron-rich silicate mantle, and a silicate crust with continents and ocean basins. nickel,
that occurred in the solar
system more than four billion years ago.
Study of the several hundred kilograms of rocks returned by the Apollo missions indicates that three kinds of materials dominate the lunar surface: igneous rocks,
and dust. Basalt, a common dark-colored igneous rock on Earth, is one of the several different types of igneous rocks on the Moon and makes up the greater breccias,
The presence of igneous rocks that are essentially the same as those on Earth shows that magmas similar to those on Earth were generated on the part of the maria.
Moon
long ago.
The lunar "soil") that
is
surface
is
covered with a regolith (or thick. This gray
m
estimated to be 3 to 4
composed of compacted aggregates of rock fragments called breccia, glass spherules, and covering, which
is
small particles of dust,
is
thought to be the result of
interior structure of the
Moon
from that of the Earth, indicating a ary history (Fig. 2-20).
The highland
diately following the
are
12%
is
quite different
different evolution-
crust
is
thick (65 to
Moon's volbillion years ago, immeMoon's accretion. The highlands
100 km) and comprises about ume. It was formed about 4.4
thin covering (1 to 2
of the
composed principally of the igneous rock anwhich is made up of light-colored feldspar
km
thick) of basaltic lava
17%
fills
of the lunar surface,
mostly on the side facing the Earth. These maria lavas came from partial melting of a thick underlying mantle of silicate composition. Moonquakes occur at a depth of
about 1,000 km, but below that depth seismic shear waves apparently are not transmitted. Because shear waves do not travel through liquid, their lack of transmission implies that the innermost mantle may be partially molten. There is increasing evidence that the Moon has a small (600 km to 1,000 km diameter) metallic core comprising 2 to 5% of its volume.
The
origin
and
earliest history of the
unclear, but the basic stages in
ment
are well understood.
ago
years
debris formed by meteorite impacts.
The
A
the maria; lava covers about
and shortly
It
its
Moon
are
still
subsequent develop-
formed some 4.6
thereafter
was
billion
partially
or
wholly melted, yielding a silicate melt that cooled and crystallized to form the mineral anorthite. Because of the
low density of the anorthite
crystals
and the lack
of water in the silicate melt, the thick anorthosite
highland crust formed. The remaining
melt
silicate
cooled and crystallized to produce the zoned mantle, while the heavier metallic elements formed the small
orthosite,
metallic core.
minerals that are responsible for their white appearance.
The formation of the lunar mantle was completed by about 4.4 to 4.3 billion years ago. The maria basalts, derived from partial melting of the upper mantle, were extruded during great lava floods between 3.8 and 3.2
^" FIGURE
2-20
The
internal structure of the
Moon
is
from that of the Earth. The upper mantle is the source for the maria lavas. Moonquakes occur at a depth of 1,000 km. Because seismic shear waves are not transmitted below this depth, it is believed that the innermost mantle is liquid. Below this layer is a small metallic core. different
Mare
basalt
billion years ago.
Numerous models have been proposed for the origin Moon, including capture from an independent
of the
formation with the Earth as part of an integrated two-planet system, breaking off from the Earth during
orbit,
and formation resulting from a collision between the Earth and a large planetesimal. These various models are not mutually exclusive, and elements of some occur in others. At this time, scientists cannot agree on a single model, as each has some inherent problems. However, the model that seems to account best for the Moon's particular composition and structure inaccretion,
volves an impact by a large planetesimal with a
Earth
young
(Fig. 2-21).
In this model, a giant planetesimal, the size of
Mars
or larger, crashed into the Earth about 4.6 to 4.4 billion years ago, causing the ejection of a large quantity of hot
Moon. The material that was was mostly in the liquid and vapor phase and came primarily from the mantle of the colliding planetesimal. As it cooled, the various lunar layers crystalmaterial that formed the ejected
lized
50
Chapter 2
A
History of the Universe, Solar System, and Planets
out
in the
order
we have
discussed.
"'' FIGURE 2-21 According to one hypothesis for the origin of the Moon, a large planetesimal the size of Mars crashed into the Earth 4.6 to 4.4 billion years ago, causing the ejection of a mass of hot material that formed the Moon. This computer simulation shows the formation of the Moon as a result of an Earth-planetesimal collision.
CHAPTER SUMMARY
2.
The
universe began with a Big Bang approximately 13 to 20 billion years ago. Astronomers have deduced this age from the fact that celestial objects are
moving away from each other
to be
an ever-expanding universe.
in
what appears
3.
The
universe has a background radiation of 2.7° above absolute zero, representing the cooling remnant of the Big Bang. About 4.6 billion years ago, the solar system formed from a rotating cloud of interstellar matter. As this cloud condensed, it eventually collapsed under the influence of gravity and flattened into a
Chapter Summary
51
The age
counterclockwise rotating disk. Within this rotating disk, the Sun, planets, and moons formed from the turbulent eddies of nebular gases and solids. 4. Meteorites provide vital information about the age and composition of the solar system. The three 5.
major groups are stones, irons, and stony-irons. Temperature as a function of distance from the Sun played a major role in the type of planets that evolved. The terrestrial planets are composed of rock and metallic elements that condense at high
forces ?
electromagnet^; c. strong photon. e. The composition of the universe has been changing since the Big Bang. Yet 98% of it by weight still
hydrogen and carbon; b. helium and hydrogen and helium; d. carbon c. hydrogen and nitrogen. and nitrogen; e.
seem to have had a similar which volcanism and cratering from meteorite impacts were common.
Which
Venus; e Mars. The age of the solar system
and crust, and all had an early atmosphere of carbon dioxide and water vapor. The Jovian planets differ from the terrestrial planets in size and chemical composition and followed
Earth;
c.
is
generally accepted by
scientists as:
4.6 billion years;
a.
10 billion years;
b.
20 billion years; 50 billion years. The major problem that plagued most early theories 15.5 billion years; d.
c.
completely different evolutionary histories. All of the Jovian planets have a small core compared to their overall size, but they are mainly composed of
e.
of the origin of the solar system involved the:
at
distribution of elements throughout the solar
a.
low temperatures, such as hydrogen, helium, methane, and ammonia. The Earth formed from one of the swirling eddies of nebular material 4.6 billion years ago and, by at least 3.8 billion years ago, was differentiated into its present-day structure. It accreted as a solid body and then underwent differentiation during a period of
rotation of the planets around their slow rotation of the Sun; revolution of the planets around the Sun;
system; axes; d.
The
b.
c.
source of meteorites and asteroids.
e.
surface of the
Moon
light-colored highlands
is
divided into
and low-lying, dark-colored
plains called:
internal heating.
Moon
probably formed as a result of a Mars-sized planetesimal crashing into Earth 4.6 to 4.4 billion years ago and ejecting a large quantity of hot material. As it cooled, the various lunar layers crystallized, forming a zoned body.
^ IMPORTANT
not a terrestrial planet?
is
Jupiter;
b.
d.
core, mantle,
The
of the following
Mercury;
a.
7. All the terrestrial planets are differentiated into a
10.
nuclear;
a.
terrestrial planets
and compounds that condense
weak
consists of the elements:
early history during
9.
gravity; b.
a.
nuclear; d.
carbon;
volatile elements
4.6 billion years;
million years; b.
8 to
temperatures.
The
generally accepted by
is
15 billion years; d. 13 to 20 billion greater than 50 billion years. years; e. Which of the following is not one of the four basic
The Jovian planets plus Pluto are composed mostly of hydrogen, helium, ammonia, and methane, all of which condense at lower
8.
570
a. c.
temperatures.
6.
of the universe
scientists as:
a
anorthosites; b
d.
nebulas;
regolith; c
cratons;
maria.
e.
The most widely accepted theory regarding origin of the
Moon
the
involves:
an capture from an independent orbit; b. breaking independent origin from the Earth; c. off from the Earth during the Earth's accretion; formation resulting from a collision between d. none of the Earth and a large planetesimal; e. a.
TERMS
these.
Big Bang greenhouse effect
refractory element
irons
stones
Jovian planets meteorites
stony-irons
outgassing
volatile
10.
solar nebula theory
Images radioed back by Voyagers
1
and 2 revealed
that:
terrestrial planets
1.
52
11
REVIEW QUESTIONS The most abundant meteorites a.
stones; b.
d.
acondrites;
Chapter 2
A
irons; e.
c.
peridotites.
Neptune
is
c. Uranus has and Neptune;
a placid planet;
d.
Pluto has an atmosphere similar to that of
Mars;
e.
The
all
of these.
planets can be separated into terrestrial and
Jovian primarily on the basis of which property? density; atmosphere; c. a. size; b.
are:
stony-irons;
all
b.
a large spot like those of Jupiter
element
planetesimal
^
of the Jovian planets have rings;
a.
d.
12.
It is
color;
was caused by
History of the Universe, Solar System, and Planets
none of
e.
these.
currently believed that the a(n):
Tunguska explosion
meteor;
a.
13.
Which of
asteroid;
b.
the following events did
terrestrial planets
14.
e.
all
21.
comet.
of the
experience early in their history?
a.
accretion; b.
c
volcanism;
e.
all
Which of
nuclear
c.
volcanic eruption;
explosion; d.
differentiation;
meteorite impacting;
d.
22.
How
24.
How
does the solar nebula theory account for the general characteristics of the solar system? 23. What are the three major groups of meteorites?
of these.
its
the following
surface;
flows;
not characteristic of
is
25.
heavy cratering
numerous
scarps; d.
c.
b.
similar to Earth's;
d.
thin, like that of
The
surface of
Mars
Mars;
none of
e.
huge
c.
large craters; d.
Which
these.
smooth
plains;
all
e.
Jupiter; b.
d.
answers
(a)
Saturn;
and
Uranus; answers (a) and
c.
(b); e.
Both Jupiter and Saturn have a core overlain by a zone of: helium;
c.
frozen
e.
carbon dioxide.
b.
relatively small
ammonia;
The only planet whose
hydrogen;
d.
axis of rotation nearly
parallels the plane of the ecliptic
Venus;
rocky
liquid metallic hydrogen;
a.
is:
Uranus; Neptune; e. Pluto. 20. What was the main source of heat for the Earth b.
Saturn;
c.
d.
early in
its
history?
meteor impact; b. radioactivity; c. gravitational compression; d. an initial molten condition; e. spontaneous combustion. a.
how
Earth-Moon system.
the Voyager space probes have changed by.
^
ADDITIONAL
READINGS
American 262, no. 6: 50-59. Grieve, R. A. F. 1990. Impact cratering on the Earth. Scientific American 262, no. 4: 66-73. Horgan, J. 1990. Universal truths. Scientific American 263, no. 4: 108-17. Ingersoll, A. P. 1987. Uranus. Scientific American 256, no. 1: 38-45. Kasting, J. F., O. B. Toon, and J. B. Pollack. 1988. How climate evolved on the terrestrial planets. Scientific American 258, no. 2: 90-97. Kinoshita, J. 1989. Neptune. Scientific American 261, no. 5: 82-91. Kuhn, K. F. 1991. In quest of the universe. St. Paul, Minn.: West Publishing Co. McSween, H. Y., Jr. 1989. Chondritic meteorites and the formation of planets. American Scientist 77, no. 2: 146-53. Saunders, R. S. 1990. The surface of Venus. Scientific American 263, no. 6: 60-65. Taylor, S. R. 1987. The origin of the Moon. American Scientist 75, no. 5: 468-77. Benzel, R. 1990. Pluto. Scientific
a.
a.
30. Discuss
more energy than they
(c).
19.
and history of the four Jovian planets?
our ideas about the planets they have flown
receive?
18.
are the similarities and differences in the
origin
massive volcanoes;
valleys; b.
planets give off
What
into three concentric layers.
of these. 17.
and history of the four terrestrial planets? why Venus, Earth, and Mars currently have
29. Discuss the origin of the
possesses:
a.
are the similarities and differences in the
origin
28. Discuss the origin and differentiation of the Earth
nonexistent;
c.
What
quite different atmospheres.
27.
hydrogen and helium. 15. The atmosphere of Venus is: a. thick and composed of carbon dioxide;
16.
the terrestrial planets differ from the Jovian
26. Discuss
lava
small amounts of atmospheric
e.
do
planets?
a strong magnetic field; b.
of
indicate that the
Big Bang occurred?
Mercury? a.
What two fundamental phenomena
Additional Readings
53
CHAPTER
3
MINERALS *=
OUTLINE
PROLOGUE INTRODUCTION MATTER AND ITS COMPOSITION Elements and Atoms
Bonding and Compounds
MINERALS Naturally Occurring, Inorganic Substances
"^ Guest
Essay: Mineralogy: Diverse Pursuits
The Nature of
A
Career with
Crystals
Chemical Composition Physical Properties
MINERAL DIVERSITY MINERAL GROUPS Silicate
Minerals
Carbonate Minerals r" Perspective 3-1: Quartz— A
'"
Common
Useful Mineral
Other Mineral Groups
PHYSICAL PROPERTIES OF MINERALS Color and Luster Crystal
Form
Cleavage and Fracture
Hardness Specific Gravity
y*
Perspective 3-2:
Diamonds and
Pencil
Leads
Other Properties
IMPORTANT ROCK-FORMING MINERALS MINERAL RESOURCES AND RESERVES CHAPTER SUMMARY "Steamboat"— red and green tourmaline and
From the Tourmaline King mine, near Pala, San Diego County, California. The specimen is about 28 cm high. National Museum of Natural History specimen #R51. (Photo by D. Penland, courtesy of Smithsonian Institution.) colorless quartz crystals.
^^^^^^^^^^^^ ^m>^^^^»jk^
*^«^6
;"
«r-*r
PROLOGUE
the Europeans' lust for gold
fact,
was responsible
for
the ruthless conquest of the natives in those areas. In the United States, gold
Among
the hundreds of minerals used
by humans none is so highly prized and eagerly sought as gold (Fig. 3-1). This deep yellow mineral has been the cause of feuds and wars and was one of the incentives for the exploration of the Americas. Gold has been mined for at least 6,000 years, and archaeological evidence indicates that
North Carolina
was
first
1801 and
profitably
flocked to California to find riches. Unfortunately,
only a few found what they sought. Nevertheless, during the five years from 1848 to 1853, which
people in Spain possessed small quantities of gold
constituted the gold rush proper, million in gold
many
Why in tools
benefits for those is
who
possessed
it is
too soft and pliable to
hold a cutting edge. Furthermore, it is too heavy to be practical for most utilitarian purposes (it weighs about
much
During most of historic time, gold has been used for jewelry, ornaments, and ritual objects and has served as a symbol of wealth and as a monetary standard. Gold is so desired for several twice as
reasons: (1)
which
it
scarcity
its
as lead).
pleasing appearance, (2) the ease with
can be worked, (it is
much
(3) its durability,
and
more than $200
was recovered.
Another gold rush occurred
in
1876 following the
report by Lieutenant Colonel George Armstrong
it.
gold so highly prized? Certainly not for use
or weapons, for
in
in
40,000 years ago. Probably no other substance has caused so much misery, but at the same time provided so
mined
Georgia in 1829, but the truly spectacular finds occurred in California in 1848. This latter discovery culminated in the great gold rush of 1849 when tens of thousands of people in
(4) its
rarer than silver).
Central and South American natives used gold
Custer that "gold in satisfactory quantities can be obtained in the Black Hills [South Dakota]." The flood of miners into the Black Hills, the
War
in the
during which Custer and some 260 of his
were annihilated
Montana
at the Battle of the Little
Indian
men
Bighorn
in
June 1876. Despite this stunning victory, the Sioux could not sustain a war against the U.S. Army, and in September 1876, they were forced to in
relinquish the Black Hills.
For 50 years following the California gold rush, the
United States led the world in gold production, and
it
produces a considerable amount, mostly from
extensively long before the arrival of Europeans. In
still
"^ FIGURE
"^ FIGURE
3-1 Specimen of gold from Grass Valley, California— National Museum of Natural History (NMNH) specimen #R121297. (Photo by D. Penland, courtesy of Smithsonian Institution.)
Holy
Wilderness of the Sioux Indians, resulted
3-2
Homestake Mine headworks is the
The headworks (upper at Lead,
South Dakota,
right) of the in
1900. The
cluster of buildings near the
opening to a
mine.
Prologue
55
mines
Nevada and South Dakota
in
(Fig. 3-2).
Currently, however, the leading producer
is
South
Union a distant second, followed by Canada and the United States. Much gold
Africa with the Soviet
still is
used for jewelry, but in contrast to
uses, gold
=*=
now
its
earlier
has some more practical applications
as well, including the chemical industry, gold plating, electrical circuitry,
and
glass
making. Consequently,
the quest for gold has not ceased or even abated. In
many
industrialized nations, including the United
domestic production cannot meet the demand, and much of the gold used must be imported. States,
INTRODUCTION
The term "mineral" commonly brings to mind dietary substances that are essential for good nutrition such as calcium, iron, potassium, and magnesium. These sub-
mineral quartz, and ore deposits are natural concentra-
stances are actually chemical elements, not minerals in
using mineral resources such as iron, copper, gold, and
is also sometimes used to any substance that is neither animal nor vegetable. Such usage implies that minerals are inorganic substances, which is correct, but not all inorganic substances are minerals. Water, for example, is not a mineral even though it is inorganic and is composed of the same chemical elements as ice, which is a mineral. Ice is, of course, a solid whereas water is a liquid; minerals are
many
the geologic sense. Mineral
tions of economically valuable minerals. Indeed, our in-
dustrialized society depends directly
upon
finding
and
others.
refer to
^ MATTER AND
ITS
COMPOSITION
lme~-means~it has a regular internal structure. Further-
Anything that has mass and occupies space is matter. The atmosphere, water, plants and animals, and minerals and rocks are all composed of matter. Matter occurs in one of three states or phases, all of which are important in geology: solids, liquids, and gases (Table 3-1). Atmospheric gases and liquids such as surface water and groundwater will be discussed later in this book, but here we are concerned chiefly with solids because all
more, a mineral has a narrowly deTmgd~ch eniIcal co m-
minerals are solids.
solids rather than liquids or gases. In fact, geologists
have a very specific definition of the teririmjneral: a naturally occurring,jnorganic crystalline solid. Crystal-
position and characteristic physicaj^ropejrtie^uchas
and hardness. Most rocks are solid agoTone orjnor e minerals, and thus mjneraJs^are
density, color,
"gregates
~~ObviouiIy7 minerals are important to geologists as the constituents of rocks, but they are important for
Many gemstones such as diamond and topaz are actually minerals, and rubies are simply red-colored varieties of the mineral corundum. The sand used in the manufacture of glass is composed of the
other reasons as well.
""»"
TABLE
Characteristics
Solid
Rigid substance that retains
Liquid
Flows a
is
the characteristics of an element. Ninety-one naturally
occurring elements have been discovered, some of which are listed in Table 3-2, and more than a dozen additional
elements have been
its
shape unless distorted by a force
and conforms to the shape of the containing vessel; has well-defined upper surface and greater density than a gas
Flows
Chapter 3
made
in laboratories.
Each naturally
Examples
easily
easily
and expands to
a well-defined upper surface;
56
is made up of chemical elements, each of composed of incredibly small particles called atoms. Atoms are the smallest units of matter that retain
which
Phases or States of Matter
3-1
Phase
Gas
Elements and Atoms All matter
the building blocks of rocks.
Minerals
fill
is
all parts of a containing vessel; lacks compressible
Minerals, rocks, iron,
wood
Water, lava, wine, blood, gasoline
Helium, nitrogen,
air,
water vapor
— TABLE
3-2
^ FIGURE
3-4
Schematic
representation of isotopes of carbon. A carbon atom has an
atomic number of 6 and an atomic mass number of 12, 13, or 14 depending on the number of neutrons in its nucleus.
2
atoms of the same element may have different atomic mass numbers. For example, different carbon (C) atoms have atomic mass numbers of 12, 13, and 14. All of these atoms possess 6 protons, otherwise they would not be carbon, but the number of neutrons varies. Forms of the same element with different atomic mass numbers are isotopes (Fig. 3-4).
isotope but many,
such as uranium and carbon, have several
(Fig. 3-4).
*C(Carbon-14)
but the outermost shell never contains more than eight (Table 3-2).
The
electrons in the outermost shell are
those that are usually involved in chemical bonding.
Two
types of chemical bonds are particularly impor-
and covalent, and many minerals Two other types of chembonds, metallic and van der Waals, are much less
tant in minerals, ionic
contain both types of bonds. ical
A number of elements have a single Some
^C(Carbon-13)
C(Carbon-12)
common, but
are extremely important in determining
the properties of
some very
useful minerals.
isotopes are unstable and spontaneously change to
a stable form. This-proress. c3]icdj^adio active dec ay.
occurs because the forces t hat bind the _nucleus_together
are_not_strong enough. Such decay occurs at
and is the mining age that
rates
known
basis for several techniques for deter-
Chapter 9. Neveran element behave the same chemically. For example, both carbon 12 and carbon 14 are present in carbon dioxide (C0 2 ). will be discussed in
theless, all isotopes of
eight electrons in
complete outer are
known
of
.
The noble
react reacfiTy^with other elements to
because
of
this
electron
gases do not form compounds
configuration.
two or more
elements arebonded, the resulting substance
atoms
is
different is
a
com-
pound. Thus, a chemical substance such as gaseous oxygen, which consists entirely of oxygen atoms, is an element, whereas ice, which consists of hydrogen and oxygen, is a compound. Most minerals are compounds although there are several important exceptions, such as gold and silver. To understand bonding, it is necessary to delve
eight electrons, unless the is
Interactions
tend to produce electron configurations
That
is,
act such that their outermost electron shell
are joined to other
When atoms
contain ing_£Jght electrons; they
similar to those of the noble gases.
Bonding and Compounds called bonding.
s hells
as the noble gases
among atoms
The process whereby atoms
ou tgr most electronjsheU. Some
t heir
elements, however, including neon and argon, have
first shell
(with
atoms
inter-
is filled
two
with
electrons)
also the outermost electron shell as in helium.
One way
in
which the noble gas configuration can be
by the transfer of one or more electrons from one atom to another. Common salt, for example, is attained
is
composed of
sodium (Na) and chlorine when combined chemically, they form the compound sodium chloride (CI),
the elements
each of which
is
poisonous, but
(NaCl), the mineral halite or
common
salt.
Notice
in
Figure 3-5a that sodium has 11 protons and 11 elec-
deeper into the structure of atoms. Recall that negatively
trons; thus, the positive electrical charges of the protons
charged electrons
are exactly balanced by the negative charges of the elec-
in electron shells orbit the nuclei
of
and the atom
atoms. With the exception of hydrogen, which has only
trons,
one proton and one electron, the innermost electron shell of an atom contains no more than two electrons.
chlorine with 17 protons
The other
rine has eight electrons in
58
shells
Chapter 3
contain various numbers of electrons,
Minerals
neutral (Fig. 3-5a).
is
electrically neutral. Likewise,
and 17 electrons is electrically However, neither sodium nor chloits
outermost electron
shell;
sodium has only one whereas chlorine has seven. In order to attain a stable configuration, sodium loses the electron in
its
outermost electron
with eight electrons as the outermost one
shell
sodium ions are bonded to chlorine on all sides, and chlorine ions are surrounded by sodium ions (Fig. 3-5b). neutrality. In halite,
next
ions
(Fig.
However, sodium now has one fewer electron
3-5a).
(negative charge) than
an
it is
shell, leaving its
dimensional framework that results in overall electrical
electrically
it
electron lost by
ermost electron
Such a particle is an + symbolized Na
Covalent bonds form between atom£ when th eir elecmnr-slTeTIs"overlap ancTelectrons are~shared. FoTexarn ple, atoms of the same element, such as oxygen in oxygen gas, cannot bomTBytransferring electrons from o ne atom to another. Carbon (C), which forms the minerals graphite and diamond, has four electrons in its outermost electrqrTshell (Fig. 3-6a). If these four electrons
particle.
ion and, in the case of sodium,
The
Covalent Bonding
has protons (positive charge) so
charged
sodium
is is
shell of chlorine,
.
transferred to the out-
which had seven
elec-
more
trons to begin with. Thus, the addition of one
electron gives chlorine an outermost electron shell of eight electrons, the configuration of a noble gas. Its total
number of electrons, however, is now 18, which exceeds by one the number of protons. Accordingly, chlorine also
An
becomes an
ion, but
bond forms
it is
negatively charged (Cl~
were transferred to another carbon atom, the atom ceiving the electrons
1
would have
ration of eight electrons in
).
between sodium and chlo rine be-
its
re-
the noble gas configu-
outermost electron
shell,
charged sodium ion and the negatively charged chlorine
but the atom contributing the electrons would not. In such situations, adjacent atoms share electrons by overlapping their electron shells. For example, a carbon
ion (Fig. 3-5ay
atom
ionic
cause of th e attrac tive force between the positively
fiT ionic
mineral
compounds, such
halite),
the
ions
as
are
sodium chloride arranged in
a
in
diamond shares
all
four of
its
outermost
elec-
trons with a neighbor to produce a stable noble gas
(the
configuration (Fig. 3-6a).
three-
-^ FIGURE
3-5
{a)
I
onic
bonding The electron in the outermost shell of sodium is transferred to the outermost .
Transfer of electron
<
electron shell of chlorine.
Once
the
/
transfer has occurred,
I*
chlorine are positively
sodium and and
negatively charged ions, respectively.
The
sodium The showi the relative sizes of sodium and chlorine ions, and the diagram on the right shows the locations of the ions in (b)
crystal structure of
chloride, the mineral halite.
diagram on the Chlorine
(CM)
Sodium (Na +1
)
left
the crystal structure.
Matter and
its
Composition
59
(b)
(a)
"^"
FIGURE
(c)
Covalent bon ds forme d by^adjacenLatompharing.eleetrons in diamond, (b) The three-dimensional framework of carbon atoms in diamond. (c) "Covalent bonding also occurs in graphite, but here the carbon atoms are bonded together to form sheets that are held to one another by van der Waals bonds. The sheets themselves are strong, but the bonds between sheets are weak. 3-6
(a)
^ MINERALS
Covalent bonds are not restricted to substances composed of atoms of a single kind. Among the most com-
mon ter),
compounds of two or more elements. is generally shown by a chemical formula, which is a shorthand way of indicating the numbers of atoms of different elements composing a
Most minerals
minerals, the silicates (discussed later in this chapthe element silicon forms partly covalent
ionic
and partly
bonds with oxygen.
Metallic
mineral.JQie mineral quartz, for example, consists of nnp sjiJjmiWSi) atom for every two oxygen (O) atoms,
and van der Waals Bonds
Metallic bondin&jes«4ts-fxQm_an extrejnejype of j^lecJxon_sharing. The electrons of the outermost electron shell
of such metals as gold,
readily lost
silver,
and thus has the formula Si0 2 indicates the
and copper are
eight
per, for its
example,
is
oxygen atoms so
Before
the smallest unit of a substance having the properties
A water molecule (H 2 0), two hydrogen atoms and one oxygen atom. of that substance.
60
Chapter 3
Minerals
we
our formal \
for example, possesses
KAlSi 3 O g Some .
Known
as
discuss minerals in
definition: a mineral
more is
detail, let us recall
a naturally occurring,
inorganic, crystalline solid, with a narrowly
denned chem-
ical composition and characteristic physical properties. (The next sections will examine each part of this definition.
Naturally Occurring, Inorganic Substance s, "Naturally occurring" excludes from minerals stances that are manufactured by synthetic
is
is
p latinum (Pt), and graphite_a nd diamo nd, both of which are composed of carbon (C).
high electrical conductivity.
*A molecule
formula
Silver (Ag),
used for electrical wiring because of
electrically neutral atoms and molecules* have no electrons available for ionic, covalent, or metallic bonding. They nevertheless have a weak attractive force between them when in proximity. This weak attrac tive force is a van der Waals or resid uadbond^Jhs^oar^on atoms in the mineral graphite are covalently bonded to form sheets, but the^shegts _are w eakly held together by van der Waals bonds (Fig. 3-6c).
is
native_ejements, they include such minerals as joldJAu),
useful; cop-
Some
its
number composed silicon, and
the subscript
;
of atoms. Orthoclase
minerals are composed of a single element.
have a metallic luster (their appearance in reflected light), provide good electrical and thermal conductivity, and can be easily reshaped. Only a few minerals possess
do are very
number
of one potassium, one aluminum, three
and move about from one atom to another.
This electron mobility accounts for the fact that metals
metallic bonds, but those that
are
Mineral composition
tificially
erals
diamonds and rubies and
all
sub-
humans. Accordingly, a
number of other
ar-
synthesized substances are not regarded as min-
by most
geologists. This criterion
portant to those
who buy and
sell
is
particularly im-
gemstones,
all
of which
1
Guest Essay
MALCOLM
ROSS
MINERALOGY: A CAREER WITH DIVERSE PURSUITS My path
becoming a professional geoscientist was As an undergraduate, I majored in forestry and then zoology, intending to become a marine biologist. After graduation, I obtained a temporary position with the U.S. Geological Survey where I learned to operate the electron microscope and then applied my newfound skills to the study of clay minerals. A whole new world opened up to me as I photographed mineral to
somewhat
circuitous.
particles magnified as
experience convinced
went on
My
much me to
as half a million times. This redirect
my
interests,
and
I
to graduate school to study mineralogy.
first
serious scientific studies concerned the
elucidation of the crystal symmetries of fine-grained
vanadium-bearing minerals. This work was followed by studies of the crystal structures of several
uranium-
bearing minerals as part of the U.S. Atomic Energy
program; next
I
asbestos minerals have been used in international
commerce, but by
most widely utilized is which accounts for 95% of world production. Until recently, few investigators had paid any attention to the relationship between the type of asbestos disease (lung cancer, mesothelioma, and asbestosis) and the type of asbestos to which the individual was exposed. By comparing medical studies of miners and millers who were exposed to only one form of asbestos, I found that the common form of asbestos, chrysotile, is not a hazard at low to moderate exposures and offers no danger to children attending school in buildings that contain this mineral. Since most buildings contain only chrysotile, removal of asbestos from such buildings is unnecessary and even counterproductive.
undertook an extended study of the
and physical chemistry of several silicate minerals that make up a major portion of the Earth's crust— the micas, pyroxenes, and amphiboles. In 1968 I submitted a proposal to the National Aeronautics and Space Administration to study the important silicate minerals composing the surface of the Moon. At that time, many (but not all) geoscientists thought that the lunar surface was composed of rocks and minerals similar to those found on Earth. The first lunar samples were returned to Earth by the Apollo 1 crew on July 24, 1969. Within a few weeks, samples of lunar rocks and soils were sent to earth scientists all over the world, and after four months of intensive study, over 500 of these scientists converged on Houston, Texas, to report on their investigations. Indeed, the rocks and minerals of the Moon and Earth turned out to be similar in many respects. Some striking differences were noted, however, between lunar and terrestrial rocks. The crystal structures
far the
chrysotile asbestos,
Recently,
rain"
I
completed a study of the
on limestone and marble building
of "acid
effects
materials.
I
found
that air pollutants generated within large cities are
primarily responsible for stone deterioration rather than sulfur dioxide originating
from midwestem power
This observation has particular importance as the
we
plants.
look for
most effective and economical ways of mitigating acid Although I am still involved with asbestos studies, I
rain.
am now the
Hot
also
engaged
titanium-
mineral resources in
and vanadium-bearing minerals are located
and understanding locate
in investigating
Springs area of Arkansas. Important deposits of
new
their
mode
here,
of formation will help us
deposits of these valuable metals.
Perhaps
this essay
interesting
and
laboratory
work with
can give the reader some idea of
diversified geoscience
can be;
it
how
combines
field studies in fascinating localities
and encompasses both basic research and research related to humans' benefit.
directly
lunar rocks are very old (3.7 to 4.3 billion years); the
absence of younger rocks implies that geochemical
JVlalcolm Ross earned
Moon's history in contrast to Earth processes that still go on today. New minerals were found on the Moon that had not been observed on the Earth, and a complete absence of water-bearing lunar minerals was also noted. In the late 1970s, I became involved in the "asbestos and health" issue due to the increasing concern over the
Ph.D. degree in geology from Harvard University in 1962.
processes stopped very early in the
effects
of exposure to asbestos dust. Four types of
He
his
has been employed by the
U.S. Geological Survey since that time, specializing in studies related to the occurrence,
chemistry, structure,
and
health effects of a wide variety
of minerals.
kAAAAAAAAAAAAAAAAAAAAAi**AAAA*A**Ati*4i*itAAAAAiAAAAAAAAAAAAAAiA<^UAAAAAAAAA*ti.AAAA Minerals
61
(b)
'*'
FIGURE
(d)
(c)
Mineral crystals occur in a variety of shapes, several of which are shown here, (a) Cubic crystals typically develop in the minerals halite, galena, and pyrite. (b) Dodecahedron crystals such as those of garnet have 12 sides, (c) Diamond has octahedral or 8-sided crystals, (d) A prism terminated by pyramids is found in 3-7
quartz.
are minerals, because
some humanmade substances
very difficult to distinguish from natural
Some
are
minerals.
geologists think the term "inorganic" in the
mineral definition
mind
gem
is
superfluous.
It
does, however, re-
us that animal matter and vegetable matter are not
some organisms produce compounds that are minerals. For example, corals, clams, and a number of other animals construct their shells of the compound calcium carbonate (CaC0 3 ), which is either aragonite or calcite, both of which are minerals. minerals. Nevertheless,
well-formed mineral crystals are rare,
all
minerals of a
given species have the same internal atomic structure.
As early as 1669, a well-known Danish scientist, Nicholas Steno, determined that the angles of intersection of equivalent crystal faces on different specimens of Si nce then th cconstancv of interbeen demonstrated formany other minerals,~regardless of their size, shape, or geographic occurrence (Fig. 3-8). Steno postulated that mineral crystals
quartz_ are identica l. facial angles has
are
composed of very
small, identical building blocks
and
that the arrangement of these blocks determines the ex-
By
the external
definition minerals are cryjtalline_solids.|Recall that
a solid
is
a rigid substance that retains
its
shape unless
deformed by an applied force (Table 3-1). A crystalline solid is a soli d in wh ich the constituent atoms are arranged in aj-egular, three-dimensional framework, as in the mineral halite (Fig 3-5b). Under ideal conditions, such as in a cavity, mineral crystals can grow and form sharp corners, and straight edges
minerals lacking obvious crystals. For example,
many
minerals possess a property called cleavage, meaning that they break or split along closeh/-SBac£d,.j;rnooth I he tact that these min erals can be sp litjdong ^ch smooth planar surfaces indicates that the mineral's
planes.
(crystal
internal structure controls such breakager The~Behavior
(Fig. 3-7). In
ortight and X^fay beamsTtransmitted through minerals
perfect crystals that possess planar surfaces faces),
form of the crystals (Fig. 3-8). Such regularity of form of minerals must surely mean that external crystal form is controlled by internal structure. The crystalline structure can be demonstrated even in ternal
Jhe Nature of Crysta ls
other words, the regular geometric shape of a well-
also provides compelling evidence for an orderly ar-
formed mineral crystal is the exterior manifestation of an ordered internal atomic arrangement. Not all rigid substances are crystalline solids, however; natural and manufactured glass, for example, lack the ordered arrangement of atoms and are said to be amorphous, meaning without form.
rangement of atoms within minerals.
many
numerous minerals grow in proximity, as in a cooling lava flow, and thus do not have an opportunity to develop well-formed crystals. Even though In
62
cases
Chapter 3
Minerals
Chemical The
Com position
definition of a mineral contains the phrase "a nar-
rowly defined chemical composition," because some minerals actually have a range of compositions. When the compositions of these minerals vary, they do so within a specific range. For many minerals the chemical compo-
^20^ (a)
"""
FIGURE
lb)
(c)
Side views and cross sections of three quartz crystals showing the constancy of interfacial angles: (a) a well-shaped crystal; (b) a larger crystal; and (c) a poorly shaped crystal. The angles formed between equivalent crystal faces on different specimens of the same mineral are the same regardless of the size or shape of the
3-8
specimens.
is constant: quartz is always composed of silicon and oxygen (Si0 2 ), and halite contains only sodium and chlorine (NaCl). Other minerals have a range of compositions because one element may substitute for another
sition
2+
3-
atoms of two or more elements are nearly the same and the same charge. Notice in Figure 3-9 that iron and magnesium atoms are about the same size, and therefore they can substitute for one another. The chemical if
the
size
— FIGURE
1
3-9
Electrical charges
and
relative sizes of ions
common
0.39 (
0.99
)
O
(uoj
O
in minerals.
The numbers within the ions are the radii
shown
in
Angstrom
units.
Sodium
Calcium
Aluminum
Silicon
0.15 (O-K'l
Potassium
lron
2+
Iron
0.72
(
3
" 1 "
O
Carbon
0.62
J
Magnesium
Chromium
Minerals
63
%
Magnesium (Mg) 50
Q)
— TABLE
3-4
(a)
(b)
(c)
(d)
commonly dark colored and more dense than nonferromagnesian silicates. Some o f the co mmon ferromagnesian silicate minerals" are oli vine, thejjy rQXfnes, rhe amphiboles, and biotite (Fig. 3-13). Olivine, an olive erals are
uncommon
in
common
in
some
ijmec>uj_xooks, but
most otKeTrocIc
types.
The pyroxenes and
green mineral,
amphiboles are
is
a'cjually^ mineral
^V
f ct til, P" Nonferrbmagnesian Silicates hc\dSpM£> The nonferromagnesian silicates, as their name
3-14).
less
The
dense than ferromagnesian
most common minerals
are nonferromagnesian silicates
major rock groups, especially in such rocks as granite, and sandstone. It is a framework silicate that can far V' usually be recognized ^rvusuairy recog by its glassy appearance and hard/,
implies,
known
(Fig^
asffeldspars^ -
is
a general
)
)
""''
FIGURE
3-13
Common
ferr omagnesian silicates
(a)
olivine ; (b) augite, a p yroxene gro up mineral ;
(c)
hornbIende,~an amjjhibole^group mineral; and biotite mica. (Photo courtesy of Sue Monroe.)
(tj)
:
closely
distinctive,
Jgneiss,
r^
name, however, and two distinct groups are recognized, each of which includes several species. The potassium feldspars, represented by microcline and orthoclase (KAlSi 3 O g ), are common in igneous, metamorphic, and some sedimentary rocks. Like all feldspars, microcline and orthoclase have two internal planes of weakness along which they break or cleave. The second group of feldspars, the plagioclase feldspars, range from calcium-rich (CaAl 2 Si 2 8 to sodiumrich (NaAlSi 3 8 varieties. They possess the characterFeldspar
not a good properly
common nonferromagnesian common in the three
Qua_r tz (Si0 2 ), another
silicates
is
Plagioclase cleavage sur-
silicate (see Perspective 3-1), is
Biotite
in the Earth'sfcru st
.
commonly show numerous,
faces
lack iron and magnesium, are generally light colored,
and are
gray. Color, however,
in identifying feldspars
spaced, parallel lines called striations.
silicate
•
feldspar cleavage and typically are white or cream
medium
jo use
groups, but the variet-
ies iygite and hornblen de are the most common. mica is a common, dark-colored ferromagnesian with a distinctive sheet structure. structure C
^t
istic
to
>ess(Fig.3-14a Another
,
fair!fairly
common
muscovite, which
is
muscovite
silicate,
but
whereas
biotite
is
nonferromagnesian
a mica. Like biotite is
typically
dark colored
(Fig.
it
nearly
silicate is
is
a sheet
colorless
3-14d).
si
Carbonate Minerals Carbonate minerals are those that contain the negatively 2 charged carbonate ion (C0 3 )~ An example is calcium .
carbonate
(CaC0 3
),
the mineral calcite (Table 3-4). I^akitex
main constituent of the sedimentary rock limestone. of other carbonate minerals ^rejcnown, but only one of these need concern us-.^dolomitsJ'lCaMg (C0 3 2 is formed by the chemical alteration of calcite by the addition of magnesium. Sedimentary rock composed is
the
A number )
]
of the mineral dolomite
is
dolostone (see Chapter
"^ FIGURE
3-14 Common nonferromagnesian silicates t he potassium fplrk par nrrhnrlacp; (c) pjagi oclas e_ieldspar; and (d) muscov te mi ca. (Photo courtesy of Sue Monroe.) (a)
quart z;
6).
:
(b)
i
Mineral Groups
67
Perspective 3-1
QUARTZ-A COMMON USEFUL MINERAL Agate
a very finely
During the Middle Ages, quartz crystals' were thought to be ice frozen so solidly that they would not melt (Fig. 1). In fact, the term "crystal" is derived from a Greek word meaning ice. Even
pink to deep rose)
today, crystal refers not only to transparent quartz,
semiprecious stone for jewelry. For example, the term
but also to
clear, colorless glass of
high quality,
such as crystal ware, crystal chandeliers, or the transparent glass or plastic cover of a watch or clock dial.
Quartz
Most
is
a
common
mineral in the Earth's crust.
of the sand on beaches, in sand dunes, and in
Sand deposits composed mostly of quartz are called silica sands and are used in the manufacture of glass. Quartz is also used in stream channels
is
quartz.
(Fig. 1).
crystalline variety of quartz
is
commonly used
as a
decorative stone (Fig. Id). Colorless quartz in particular has been used as a
"rhinestone" originally referred to transparent quartz
made in Germany. Herkimer "diamonds" are simply colorless quartz crystals from Herkimer County, New York. During the past, large, transparent quartz crystals were shaped into spheres crystals used for jewelry
for the fortune teller's crystal ball.
The property of piezoelectricity (which literally means "pressure" electricity) is what enables quartz be such an accurate time-keeper.
When
pressure
optical equipment, for abrasives such as sandpaper,
applied to a quartz crystal, an electric current
and in the manufacture of steel alloys. Quartz occurs in several color varieties. Milky white quartz is a common variety and frequently
generated.
occurs as well-formed crystals.
A
milky white
quartz crystal weighing 11.8 metric tons and
measuring 3.5
m
long and 1.7
m
in
diameter was
discovered in Siberia. Color varieties of quartz include
amethyst (purple), smoky (smoky brown to black), citrine (yellow to yellowish
"^ FIGURE
3-15
brown), and rose (pale
View of an
iron
mine near Palmer,
If
an
electric current
crystal, the crystal
is
to
is
is
applied to a quartz
expands and compresses extremely
rapidly and regularly (about 100,000 times per
second). In a quartz
movement watch,
a thin wafer of
a quartz crystal vibrates because of the electrical
current supplied by the watch's battery.
The
first
developed are
clock driven by a quartz crystal
in
was
1928. Today quartz clocks and watches
commonplace, and even inexpensive quartz
Other Mineral Groups
Michigan. In addition to silicates
and carbonates,
several other
mineral groups are recognized (Table 3-4). The oxides
combined with oxygen as in heHematite and another iron oxide called magnetite are both commonly present in small quantities in a variety of rocks. Rocks containing high concentrations of hematite and magnetite, such as those in the Lake Superior region, are important sources of iron ores for the manufacture of steel (Fig. 3-15). The sulfides have a positively charged ion combined consist of an element
matite (Fe 2
3
).
with sulfur (S~
2 ),
such as in the mineral galena (PbS),
which contains lead (Pb) and sulfur (Fig. 3- 16a). Sulfates contain an element combined with the complex sulfate ion 2 (S0 4 )~ gypsum (CaS0 4 -2H 2 0) is a good example (Fig. ;
68
Chapter 3
Minerals
"^ FIGURE (a)
Varieties of quartz.
1
Colorless crystals from the Jeffrey
Stone Quarry, Arkansas— National Museum of Natural History specimen #R12804. (Photo by Chip Clark, courtesy of Smithsonian Institution.) (b) Smoky quartz. (Photo courtesy of Sue Monroe.) (c) Amethyst— specimen #C6647. (Photo by V. E. Krantz, courtesy of Smithsonian Institution.) (d) Agate, a variety of very finely crystalline quartz. (Photo courtesy of B. J. Skinner.) {e) Rose quartz. (Photo courtesy of Sue Monroe.)
NMNH
from Brazil needed prompted the development of artificially synthesized quartz, and now most of the quartz used in watches and clocks
timepieces are extremely accurate. Precision-
difficulty obtaining quartz crystals
manufactured quartz clocks used in astronomical observatories do not gain or lose more than one second every 10 years.
for
An
interesting historical note regarding quartz
that during
3- 16b). rine (CI
The -1 )
erals halite
World War
II
the United States
is
fluorine (F
(NaCl) and
_1
examples include the min-
);
fluorite
(CaF2
)
radios. This shortage
synthetic.
had
halides contain halogen elements such as chlo-
and
is
making
(Fig. 3-16c).
Color and Luster For some minerals, especially those that have the ap-
pearance of metals, color
many
^ PHYSICAL PROPERTIES
others
purities.
The
it
is
rather consistent, but for
amounts of imsome minerals occur in a variety
varies because of minute
fact that
distressing to beginning students because the
OF MINERALS
of colors
All minerals possess characteristic physical properties
for identification.
that are determined
by their internal structure and
be made, however Ferromagnesian
chemical composition.
Many
black,
physical properties are re-
markably constant for a given mineral species, but some, especially color, may vary. Though a professional geologist
may
use sophisticated techniques in studying
and identifying minerals, most identified
is
most obvious mineral property
common
minerals can be
by using the following physical properties.
Some .
is
not particularly useful
generalizations about color can silicates are typicall y
brown, or dark green, although olivine is oliv e green Fig. 3-13). Nonferromagnesian silicates, on the (
other hand, can vary considerably in color, but are only rarely dark (Fig. 3-14).
busier fyiot to he confused with rnlnr) is fhp appearance of a mineral in reflected ligh t. Two major types of
Physical Properties of Minerals
69
"^"
FIGURE
3-17 Luster is the appearance of a mineral in Galena (left), the ore of lead, has the appearance of a metal and is said to have a metallic luster, whereas orthoclase has a nonmetallic luster. reflected light.
luster are recognized: ^ metallic
3-17).
They
and nonmetallic
(Fig.
are distinguished by observing the quality of
light reflected
from a mineral and determining
if it
has
the appearance of a metal or a nonmetal. Several types
of nonmetallic luster are also recognized. Quartz possesses a glassy or vitreous type of nonmetallic luster,
and
other minerals have lusters characterized as greasy,
waxy,
brilliant (as in
Crystal
diamond), and dull or earthy.
Form
As previously noted, mineral crystals are rare. Thus, many mineral specimens you encounter will not show the perfect crystal form typical of that mineral species. Keep in mind, however, that even though crystals may not be apparent, minerals nevertheless possess the atomic structure that would have yielded well-formed crystals if they
had developed within an unconfined space. Some minerals do typically occur as crystals. For example, 12-sided crystals of garnet are
and 12-sided
grow
common,
as are 6-
crystals of pyrite (Fig. 3-18). Minerals that
in cavities
or are precipitated from circulating hot
water (hydrothermal solutions) rocks also commonly occur as
in cracks
and crevices
in
crystals.
Crystal form can be a very useful characteristic for min-
but a number of minerals have the same example, pyrite (FeS 2 ), galena (PbS), and halite (NaCl) all occur as cubic crystals. However, such minerals can usually be easily identified by other propereral identification,
crystal form. For
""^ (a)
FIGURE
3-16
CaSGy2H 2 0);
70
Representative examples of minerals from
the sulfides (galena
and
Chapter 3
— PbS);
(c)
(b)
the sulfates (gypsum
—
the halides (halite -NaCl). ties
Minerals
such as color,
luster,
hardness, and density.
(a)
(b)
Cleavage in one direction
Cleavage
Micas— biotite and muscovite
in
two directions at right angles
(c)
Cleavage
in
three directions at right
(d)
Halite,
galena
angles
Cleavage
in
three directions, not at right
angles
(e)
Cleavage
in
four directions
(f)
(b)
Cleavage
in
Sphalerite
six directions
3-18 (a) Crystals of pyrite from Spain — specimen #R18657. (Photo by D. Penland, courtesy of Smithsonian Institution.) (b) Garnet crystals from Alaska. '**'
FIGURE
NMNH
"^" (a)
FIGURE 3-19 Several types of mineral cleavage: one direction; (b) two directions at right angles; (c) three
directions at right angles; (d) three directions, not at right angles;
Cleavage and Fracture
Qeay age Not
all
is
a property
oMndiyidual mineral
four directions; and
(f)
six directions.
crystals.
minerals possesscleavageTbut those that do tend
to break, or split, along a smooth plane or planes of weakness determined by the strength of the bonds within the mineral structure. Cleavage can be characterized in terms of quality (perfect, good, poor), direction, and angles of intersection of cleavage planes. Biatkca commo n ferromagnesian silicate, has p erfect cleavage in one direction (Fig. 3-19a). The fact that biotite preferentially cleaves along a allel
(c)
planes
is
related to
number of its
closely spaced, par-
structure;
it is
a sheet silicate
silica tetrahedra weakly bonded to one another by iron and magnesium ions (Fig. 3-12c). Feldspars possess two directions of cleavagejthat intersect at right angles, and the mineral halite has three directions of cleavage, all of which intersect at right an-
with the sheets of
gles (Fig. 3-19c). Calcite also possesses three directions
of cleavage, but none of the intersection angles angle, so cleavage fragments of calcite are
drons
(Fig.
is
a right
rhombohe-
3-19d). Minerals with four directions of
Physical Properties of Minerals
71
cleavage include fluorite and diamond. Ironically, diamond, the hardest mineral, can be easily cleaved (see Perspective 3-2).
A
few minerals such as sphalerite, an (Fig. 3-19f).
ore of zinc, have six directions of cleavage Cleavage is a very important diagnostic property of minerals, and ing between
its
recognition
some
is
essential in distinguish-
minerals. For example, the pyroxene
mineral augite and the amphibole mineral hornblende look much alike: both are generally dark green to black,
have the same hardness, and possess two directions of
However, the cleavage planes of augite intersect at about 90°, whereas the cleavage planes of hornblende intersect at angles of 56° and 124° (Fig. 3-20). cleavage.
In contrast to cleavage, fracture
along irregular surfaces.
enough force
is
applied.
is
mineral breakage
Any mineral can be The
fractured
commonly uneven
all
Hardness
is
the resistance of a mineral to abrasion.
An
Austrian geologist, Friedrich Mohs, devised a relative
hardness scale for 10 minerals.
He
arbitrarily assigned a
hardness value of 10 to diamond, the hardest mineral known, and lesser values to the other minerals. Relative hardness can be determined easily by the use of Mohs hardness scale (Table 3-5). For example, quartz will scratch fluorite but cannot be scratched by fluorite, gyp-
sum can be ness
is
scratched by a fingernail, and so on. Hard-
controlled mostly by internal structure. For ex-
ample, both graphite and diamond are composed of carbon, but the former has a hardness of 1 to 2 whereas the latter has a hardness of
10.
if
fracture surfaces will not be
smooth, however, which implies that the internal bonds are equally strong in
Hardness
directions. Fracture surfaces are
or conchoidal (smoothly curved).
Specific Gravity
The
specific gravity of a mineral
to the weight of an equal
is
the ratio of
volume of water.
weight
mineral
three times as heavy as
with a specific gravity of 3.0
is
water. Like
gravity
all ratios, specific
its
A
is
not expressed in
grams per cubic centimeter — it is a dimensionless number. Specific gravity varies in minerals depending upon their composition and structure (Fig. 3-21). Among the units such as
common
example, the ferromagnesian silifrom 2.7 to 4.3, whereas the nonferromagnesian silicates vary from 2.6 to 2.9. Obviously, the ranges of values overlap somewhat, silicates, for
cates have specific gravities ranging
but for the most part ferromagnesian specific gravities
eral, the metallic minerals,
"" TABLE
(b)
"^ FIGURE
in augite and hornblende. Augite crystal and cross section of crystal showing cleavage, {b) Hornblende crystal and cross section of crystal
3-20
(a)
showing cleavage.
Cleavage
silicates
than nonferromagnesian
3-5
have greater
silicates. In
gen-
such as galena (7.58) and he-
Perspective 3-2
DIAMONDS AND PENCIL LEADS You may be
surprised to learn that
diamonds and
pencil
"lead" (graphite) are composed of the same substance, carbon. Both diamonds and graphite are crystalline solids
and are therefore minerals; because they each
contain only a single element, they are also native
diamond common: diamond is the
elements. Other than composition, however,
and graphite have
little
in
hardest mineral, whereas graphite
is
so soft that
be scratched by a fingernail; diamond
may
red, yellow, blue, gray, or black, while graphite
invariably steel gray (Fig.
1).
it
can
be colorless, is
Obviously, the same
chemical substance occurs in vastly different forms, so
what could
3-6c). Graphite can be used for pencil leads because
has good cleavage in one direction. lead
structure— both are crystalline but the
atoms within
crystals of
diamond and graphite
differently.
are
Such minerals sharing the
same composition but differing in structure are called polymorphs (poly = many; morph = shape or form). Notice in Figure 3-6 that in a diamond crystal the carbon atoms are arranged such that all of them are bonded to one another. In graphite the carbon atoms are bonded together to form sheets, but the sheets are weakly held together by van der Waals bonds (Fig.
across a piece of paper, small pieces of
van der Waals bonds and adhere to the paper. Most of the diamonds mined are not of gem quality and are used in such industrial applications as diamond drill bits, diamond-tipped cutting blades, or abrasives. Most gem-quality diamonds are mined in South Africa, although in terms of total diamond production South Africa is in fifth place, with Australia being the largest producer.
How
graphite differ mostly because of
their internal
arranged quite
moved
it
a pencil
graphite flake off along the planes held together by
possibly control such differences?
Diamond and
is
When
does one "cut" a diamond, the hardest
known? Diamond
substance
cutting
by several processes, one of which
Diamond a
is
is
actually
done
cleaving.
possesses four directions of cleavage, and
diamond
is
cleaved such that
all
if
four cleavage planes
are perfectly developed, the resulting "stone" will be
shaped
like
Diamonds
two pyramids placed base
to base.
are cleaved by placing a knife parallel with
and then tapping the knife with a diamonds are commonly preshaped by
a cleavage plane mallet. Large
cleaving
them
into smaller pieces that are then further
shaped by sawing and grinding with diamond dust.
"^" FIGURE (a)
1 Two minerals composed of carbon. Graphite. (Photo courtesy of Sue Monroe.) (b) The
Oppenheimer diamond—
NMNH
specimen #117538. (Photo by D. Penland, courtesy of Smithsonian Institution.)
(b)
Physical Properties of Minerals
73
on
graphite writes
magnetic
paper, halite tastes salty, and magnetite
(Fig. 3-22). Calcite
possesses the property of double refraction, meaning that an object when viewed through a transparent piece of calcite will have a double image (Fig. 3-22c). Some minerals are plastic and, when is
new
bent into a
shape, will retain that shape, whereas
others are flexible and,
when
position
A simple
if
bent, will return to their original
the forces that bent
chemical
them are removed.
test to identify the
minerals calcite
and dolomite involves applying a drop of chloric acid to the mineral specimen. calcite,
it
will react vigorously
If
dilute hydro-
the mineral
is
with the acid and release
carbon dioxide, which causes the acid to bubble or effervesce. Dolomite, on the other hand, will not react with hydrochloric acid unless it is powdered.
Mg 2 SiO
^ IMPORTANT ROCK-FORMING
Specific gravity
FIGURE 3-21 The specific gravity of olivine group minerals increases as a function of increasing iron content.
"^r
MINERALS Rocks are generally defined of one or more minerals.
as solid aggregates of grains
Two
important exceptions to such as obsidian (see
this definition are~natural glass
matite (5.26), are heavier than nonmetals. Structure as a control of specific gravity
is
illustrated
by the native
ment carbon (C): the specific gravity of graphite from 2.09 to 2.33; that of diamond is 3.5.
ele-
varies
Chapters 4 and 5) and the sedimentary rock coal (see Chapter 7). Although it is true that many minerals occur in
various kinds of rocks, only a few varieties are com-
mon enough Most of
to be designated as rock-forming minerals.
the others occur in such small
amounts that
they can be disregarded in the identification and classi-
Other Properties
fication of rocks; these are generally called accessory
A number of other physical properties characterize some minerals. For example, talc has a distinctive soapy
"^ FIGURE
3-22
Chapter 3
minerals. Granite, an igneous rock consisting largely of potassium feldspar and quartz, commonly contains such
Various properties of minerals, (a) Graphite, the mineral from (b) Magnetite is magnetic, (c) Calcite
which pencil leads are made, writes on paper, shows double refraction.
74
feel,
Minerals
"•'
FIGURE
3-23
The
igneous rock granite composed largely of
is
potassium feldspar and quartz, lesser
amounts of and
plagioclase feldspar,
accessory minerals such as biotite mica, (a) Hand
specimen of granite. (b) Photomicrograph
showing the various minerals.
accessory minerals as sodium plagioclase, biotite, hornblende, muscovite, and, rarely, pyroxene (Fig. 3-23).
We
have already emphasized that the Earth's crust
compos ed
is
largely~o t silicate minerals. This being the
one would suspect that most rocks are also composed of silicate minerals, and this is correct. Only a few of the hundreds of known silicates are common in rocks, however, although many occur as accessories. The comcase,
mon
summarized in Table 3-6. of clay minerals, all of which are sheet
rock-forming
Several varieties
"*»"
TABLE
3-6
silicates are
Rock-Forming Minerals
common rock-forming minerals. These form mostly by the chemical alteration of other silicate minerals, such as feldspars, and are particularly common in some sedimentary and metamorphic rocks, as well as in soils (see Chapter 6). The most common nonsilicate rock-forming joi nerals are"ftTe~two~carbonates, EalcifeJtCaC0 3 ) and \dolom ite; [CaMg((J0 3 2 J, the primary constituents o f the sedime ntary roc ks^imest^ne^ncTHol oston eTrespectively. Among the sulfates and halides, gypsum (CaS0 4 -2H 2 0) and hasilicates,
are also
clay minerals
)
Nonmetallics
Clays 195 kg
•^ FIGURE 3-24 The approximate amounts of mineral resources used by every resident of North America during 1988.
Salt
170 kg
cn Iron
and
Phosphate 145 kg
czy
Mineral Resources
Major Producing Countries
Brazil,
Canada
Manganese
USSR, South
Tantalum
Brazil,
Bauxite*
Jamaica, Australia, Guinea
Chromium
South Africa,
Cobalt
Zaire,
Africa, Brazil
Canada
USSR
Zambia
Platinum group Tin
Malaysia,
USSR,
Brazil,
Thailand
USSR, Canada, New Caledonia,
Australia
USA, Canada, Australia Mercury
USSR, Spain. Algeria
Zinc
Canada,
Tungsten
China, USSR, South Korea
Australia,
Mexico
Gold
South Africa, USSR, Canada,
Titanium (ilmenite)
Australia,
Silver
Mexico, Peru, USSR, USA,
Antimony
China. USSR, South Africa
USSR,
Norway,
USA
USSR
Brazil, Australia,
Canada
China
South Africa, USSR, China
Copper
Chile,
USA, USSR, Canada, Zaire
Australia,
USSR, USA
*Ore of aluminum.
What constitutes a resource as opposed to a reserve depends on several factors. For example, iron-bearing minerals occur in many rocks, but in quantities or ways that make their recovery uneconomical. As a matter of fact, most minerals that are concentrated in economic quantities are mined in only a few areas; 75% of all the metals mined in the world come from about 150 locations. Geographic location is also an important consideration. A mineral resource in a remote region may not be mined because transportation costs are too high, and what may be considered a resource in the United States
-»-
FIGURE
3-25
The
percentages of some mineral resources imported by the United States. The lengths of the blue bars correspond to the amounts of resources imported.
or Canada may be mined in a third-world country where labor costs are low. The market price of a commodity is, of course, important in evaluating a potential resource. From 1935 to 1968, the United States government maintained the price of gold at $35 per troy ounce (= 31.1 g). When this restriction was removed and the price of gold became subject to supply and demand, the price rose (it reached an all-time high of $843 per troy ounce during January 1980). As a consequence, many marginal deposits became reserves and many abandoned mines were reopened.
Mineral Resources and Reserves
77
Technological developments can also change the staexample, the rich iron ores of the
depleted. In order to ensure continued supplies of essen-
tus of a resource. For
tial
Great Lakes region of the United States and Canada had been depleted by World War II. However, the develop-
cated geophysical and geochemical mineral exploration
ment of a method of separating the iron from previously unusable rocks and shaping it into pellets that are ideal for use in blast furnaces made it feasible to mine poorer
reau of Mines continually assess the status of resources
grade ores.
Most of
the largest
and
richest mineral deposits have
probably already been discovered and,
in
some
minerals, geologists are using increasingly sophisti-
techniques.
The
and the U.S. Bu-
U.S. Geological Survey
view of changing economic and political conditions and developments in science and technology. In the following chapters, we will discuss the origin and distribution of various mineral resources and reserves. in
cases,
t.^^^^^^^^^^^^^^^^^^^^^ «^C ^. ^ ^^ « «.-« ^'« '« TL « «^g IMPORTANT TERMS ^ CHAPTER SUMMARY ,
1.
is composed of chemical elements, each of which consists of atoms. Individual atoms consist of a nucleus, containing protons and neutrons, and
All matter
electrons that circle the nucleus in electron shells. 2.
3.
Atoms are characterized by their atomic number (the number of protons in the nucleus) and their atomic mass number (the number of protons plus the number of neutrons in the nucleus). whereby atoms are joined to atoms of different elements are bonded, they form a compound. Ionic and covalent bonds are most common in minerals, but metallic and van der Waals bonds also occur in a few. Most minerals are compounds, but a few, including gold and silver, are composed of a single element and are called native elements. All minerals are crystalline solids, meaning that they possess an orderly internal arrangement of atoms. Some minerals vary in chemical composition because atoms of different elements can substitute for one Bonding
is
the process
other atoms.
4.
5.
6.
If
another provided that the electrical charge is balanced and the atoms are of about the same 7.
Of
the
more than 3,500 known
silicates.
Ferromagnesian
size.
minerals, most are
silicates
contain iron
(Fe)
and magnesium (Mg), and nonferromagnesian silicates lack these elements.
8.
In addition to silicates, several other mineral groups are recognized, including carbonates, oxides, sulfides, sulfates,
9.
The
and
halides.
physical properties of minerals such as color,
hardness, cleavage, and crystal form are controlled
by composition and structure. few minerals are common enough constituents of rocks to be designated rock-forming minerals.
10.
A
11.
Many
resources are concentrations of minerals of
economic importance. 12. Reserves are that part of the resource base that can
be extracted economically.
atom
'
,
,
.
,
,
.
.
.
,
.
1
4
most abundant elements
in the Earth's crust
are:
carbon and potasand magnesium; b. sodium and nitrogen; d^JC^silicon and oxy gen; e sand and clay. The sharing of electrons by adjacent atoms is a type of bonding called: a. van der Waals; b. /^ covalent; c. silicate; iron
a.
sium; c
v^Br-
tetrahedral;
d.
J^.
7.
c. J^\ common rock-forming carbonate minerals; d. minerals used in the manufacture of pencil leads; e. important energy resources. 16. How does a crystalline solid differ from a liquid and
distinctive sheet structure;
f/CWAUAJr'(-
ionic.
e.
A
chemical element is a substance made up of atoms, all of which have the same: number of a. atomic mass number; b. neutrons; cyt number of protons; d. size; weight. e. Many minerals break along closely spaced planes
and are said to possess: a.
specific gravity; b.
c.
covalent bonds;
>8L cleavage; fracture;
d.
double
e.
refraction.
\%,
The chemical formula for olivine is (Mg,Fe) 2 Si0 4 which means that in addition to silica: a. 2is_ magnesium and iron can substitute for one another; b. magnesium is more common than iron; c. magnesium is heavier than iron; all olivine contains both magnesium and iron; d. e. more magnesium than iron occurs in the
<$.
The
what 18.
all silicate
minerals
is
What
21.
What
b
Why
the:
oxygen-silicon cube;
c.^fi^z silica tetrahedron; d.
e
^JOr
silica
of a
mineral calcite;
a.
23.
double chain;
common
^^T specific
ionic
and covalent bonding.
native element.
accounts for the fact that some minerals have
are the angles between the
same
is
a silicate mineral?
crystal faces
on
How
do
the
two
minerals differ from one
silicate
24. In sheet silicates, individual sheets
composed of
tetrahedra possess a negative electrical charge. biotite;
c.
halite.
e.
equal volume of water a.
is
another?
npnferro magnesian
b.j£_ quartz;
ratio of a mineral's
lost,
specimens of a mineral species always the same?
What
weight to the weight of an
is its:
gravity; b.
hardness;
luster; c
atomic mass number; e. cleavage. 12. Those chemical elements having eight e lectrons their outermost electron shell are the: a. tk' noble gases; b. native elements;
silica
How
charge satisfied? carbonate minerals have in common? 26. Describe the mineral property of cleavage, and explain what controls cleavage. 27. What are rock-forming minerals? is
The
outer electron shell are
is:
hematite;
d.
11.
its
magnesium ion? the atomic mass number of the magnesium
subgroups of
framework.
An example silicate
silicate
electrons in
the electrical charge of the
a range of chemical compositions?
all
silicon sheet;
is
atom shown above? 19. Compare and contrast 20. Define compound and
22.
basic building block of
a
two
the
If
,
Earth's crust.
this negative
25.
What do
^
ADDITIONAL
all
d.
carbonates; d.
c.
halides;
e.
in
isotopes.
Ji. Minerals are solids possessing an orderly internal arrangement of atoms, meaning that they are: f
amorphous substances; b. ^f crystalline; composed of at least three different elements; composed of a single element; e. d. ionic compounds. The silicon atom has a positive charge of 4, and a.
c
14.
oxygen has a negative charge of ion group (Si0 4 has a:
2.
Accordingly, the
2;
negative charge of 1; d.
c.
of 4;
e.
15. Calcite a.
b.
/{
negative charge of positive charge
negative charge of 4.
and dolomite
Berry, L. G., B.
are:
oxide minerals of great value; ferromagnesian silicates possessing a
Mason, and R.
READINGS V. Dietrich.
1983. Mineralogy.
San Francisco, Calif.: W. H. Freeman and Co. Blackburn, W. H., and W. H. Dennen. 1988. Principles of mineralogy. Dubuque, Iowa: William C. Brown Publishing Co. Dietrich, R. V, and B. J. Skinner. 1979. Rocks and rock minerals. New York: John Wiley Sons. 1990. Gems, granites, and gravels: Knowing and using
2d
ed.
&
New
rocks and minerals. Klein,
C, and
(after
)
positive charge of 2; b.
a
«
C^VyW-A CXTr^r, ojui, CuXtyyy^ I /W«c< 'bdiAUMvjj An atom of the element magnesium is shown^below. r a gas?
17.
York: Cambridge University Press.
Hurlbut Jr. 1985. Manual of mineralogy James D. Dana). 20th ed. New York: John Wiley C.
S.
&
Sons.
Pough,
F.
H. 1987.
A
field
guide to rocks and minerals. 4th ed.
Boston, Mass.: Houghton Mifflin.
Vanders,
I.,
and
P. F.
York: John Wiley
Kerr. 1967.
&
Mineral recognition.
New
Sons.
Additional Readings
79
<
.
V
H A ()
V
L
'
C
4
AXIS M
* O UT
L
'
1
JNTKODtrCFION
MAG&4A AM> LAVA
;\r
-.
.:
' .
\
x -.':':
.
~*r
"•"
.
-.
"-
--..:
:
;
.-
''.x-f.i
:
Pmpeeinre 4-1: Vokasasna am System :
.
-. -.-
-•_;--.-
--:
-.-.-
- 1
:
:.
tine
'..-=;.r.
Sdbr
;.-.:
-,;- -.e;
Shield Volcanoes *~ '': ;*:-'.
'•(_--.--;
-:
;.- :
':.-.'.--
-
-.
.":
_i.-
.r.
z.r.z- '-.
Cinder Cones
Composite Volcanoes
Lam Domes Fissure Eruption*
"" Guest Essay:
Monitoring Volcanic
Activity Pyroclastic Sheet Deposits
DISTRIBUTION OF VOLC\NOES PLATE TECTONICS AND VOLCANTSM Volcanism at Spreading Ridges
Volcanism at Subduction Zones Intraplate Volcanism
CHAPTER SUMMARY Mount Pinatubo in the Philippines is one of many volcanoes in a belt nearly encirding Ocean basin. It is shown here erupting on June 12, 1991. A huge, thick cloud of ash and steam rises above Clark Air Force Base, from which about 15,000 people had already been evacuated to Subic BayNaval Base. Following this eruption, the the Pacific
remaining 900 people evacuated.
at the base
were also
PROLOGUE "•— -
-
-
—
r-
:-:.;:
-'
-
-
:-3:-:'
viojn
-
."* Although no one could predict precisely when Mount St. Helens would erupt, the USGS report included maps showing areas in which damage from an eruption could be expected. Forewarned with such data, local officials were better prepared to formulate policies when the
eruptions during the last 4,500 years
.
.
.
eruption did occur.
On March
27, 1980,
Mount
St.
Helens began
erupting steam and ash and continued to do so during
March and most
of April. By late March, a had developed on its north face as molten rock was injected into the mountain, and the bulge continued to expand at about 1.5 m per day. On May 18, an earthquake shook the area, the unstable bulge collapsed, and the pent-up volcanic gases below expanded rapidly, creating a tremendous the rest of
visible bulge
northward-directed lateral blast that blew out the
"^ FIGURE
4-3
southwest
1978.
in
View of Mount
St.
Helens from the
north side of the mountain
(Figs. 4-4, 4-5).
from 350
to 1,080 km/hr,
blast accelerated
obliterating virtually everything in
km 2
its
path.
The
lateral
Some 600
of forest were completely destroyed; trees were
snapped off
at their bases
and strewn about the km from the bulge
countryside, and trees as far as 30 it
erupted violently, causing the worst volcanic disaster
in U.S. history.
The awakening of Mount
St.
Helens came as no
were seared by the intense heat. Tens of thousands of animals were killed; roads, bridges, and buildings were destroyed; and 63 people perished.
surprise to geologists of the U.S. Geological Survey
(USGS) who warned in 1978 that Mount St. Helens is an especially dangerous volcano because [of] its past behavior and [its] relatively high frequency of ".
.
.
-^ FIGURE
* D. R. Crandell and D. R. Mullineaux, "Potential Hazards from from Future Eruptions of Mt. St. Helens Volcano, Washington," United States Geological Survey Bulletin 13S3-C, (1978):C1.
The eruption of Mount St. Helens on May 18, 1980. (a) The lateral when the bulge on the north face of the mountain collapsed and reduced the pressure on the molten rock within the mountain, {b) Part of the lateral blast zone. Many of the trees in this view were more than 30 m tall. 4-4
blast that occurred
82
Chapter 4
Volcanism
"^ FIGURE
4-5 Mount St. Helens on September 10, 1980. The large crater formed as a result of the avalanche
and
lateral blast.
Shortly after the lateral blast, volcanic ash and
steam erupted and formed a cloud above the volcano 19 km high (Fig. 4-6). The ash cloud drifted east-northeast, and the resulting ash fall at Yakima, Washington, 130 km to the east, caused almost total darkness at midday. Detectable amounts of ash were deposited over a huge area. Flows of hot gases and
— FIGURE
4-6 Shortly after the lateral blast of May 18, 1980, Mount St. Helens erupted a steam and ash cloud that rose about 19 km high.
down the north flank of the mountain, causing steam explosions when they encountered bodies of water or moist ground. Steam volcanic ash raced
explosions continued for weeks, and at least one
occurred a year
later.
Snow and
glacial ice on the upper slopes of Mount Helens melted and mixed with ash and other surface debris to form thick, pasty volcanic mudflows.
"^ FIGURE surged
down
4-7
A
house surrounded by the mudflow that North Fork of the Toutle '
the valley of the
River.
St.
The
and most destructive mudflow surged North Fork of the Toutle River (Fig. 4-7). Ash and mudflows displaced water in lakes and streams and flooded downstream areas. Ash and other particles carried by the flood waters were deposited in stream channels; many kilometers from largest
down
the valley of the
Mount
St.
Helens, the navigation channel of the
Columbia River was reduced from 12 m to less than 4 m as a result of such deposition. Although the damage resulting from the eruption of Mount St. Helens was significant and the deaths were tragic, it was not a particularly large or deadly
Prologue
83
-~-
TABLE
4-1
=»=
MAGMA AND LAVA
Magma
is molten rock material below the Earth's surand lava is magma at the Earth's surface. Magma is less dense than the solid rock from which it was derived, thus it tends to move upward toward the surface. Some magma is erupted onto the surface as lava flows, and some is forcefully ejected into the atmosphere as particles called pyroclastic materials (from the Greek pyro, "fire", and klastos, "broken") (Fig. 4-8). Igneous rocks (from the Latin ignis meaning fire) form when magma cools and crystallizes, or when pyroclastic materials such as volcanic ash become consol-
face,
idated.
Magma extruded onto the Earth's surface as lava
and pyroclastic materials forms volcanic (or extrusive igneous) rocks, whereas magma that crystallizes within the Earth's
rocks
crust forms plutonic
{intrusive
igneous)
(Fig. 4-9).
Composition Recall from Chapter 3 that the
most abundant minerals silicates, composed of
comprising the Earth's crust are silica
and the other elements
when
listed in
Table 3-3. Accord-
and form magma, the magma is typically silica rich and also contains considerable aluminum, calcium, sodium, iron, magnesium and potassium as well as many other elements in lesser ingly,
crustal rocks melt
Not
all magmas originate by melting of however; some are derived from upper mantle rocks that are composed largely of ferromagnesian silicates. A magma from this source contains comparatively less silica and more iron and magnesium. Although silica is the primary constituent of nearly all
quantities.
crustal rocks,
magmas,
silica
"^ FIGURE 4-8 Lava fountains such as this one Hawaii are particularly impressive at night.
in
content varies and serves to distinguish
Jejsjc^uitennediate, and mafic
magmas
(Table 4-2).
felsicjTiagma, for example, contains morertian
65%
A
sil-
ica_ansLcansiderable sodium, potassium, and aluminum,
but little calcium, iron, and magnesium. Coolingofjelsic
magma yieldsTgneous rocks, such as rhyolite and granite, which are composed largely of the nonferromagnesian silicates potassium feldspar, sodium-rich plagioclase, and
'•'
TABLE 4-2 The Most Common Type Magmas and Their Characteristics
of
Crystallizes to
In contrast to felsic
Form
magmas, mafic magmas are more calcium,
poor, and contain proportionately
and magnesium.
Silica
Type of
quartz (Table 4-2).
When
mafic
magma
silica
iron,
cools and crystal-
Magma
Content (%)
Mafic
45-52%
Intermediate
53-65
igneous rocks suchasjjasalt and g ahhro, which contain high percentages of ferromagnesian silicaTeTarTcTcalcium plagioclase (Table 4-2). As^one^would expect, igneous rocks that crystallize from intermediate
Felsic
>
magmas have
65
Volcanic
Plutonic
Rock
Rock
lizes, it yields
ate
mineral compositions that are intermedibetween those of mafic and felsic rocks (Table 4-2).
Magma
and Lava
85
FIGURE
4-9
The rock
cycie,
with emphasis on extrusive igneous rocks.
masses of
Temperature
No
direct measurements of temperatures of magma below the Earth's surface have been made. Erupting lavas
generally have temperatures in the range of 1,000° to
1,200°C, although temperatures of 1,350°C have been
recorded above Hawaiian lava lakes where volcanic gases reacted with the atmosphere.
Most
felsic
magma, have been measured
at a dis-
tance by using an instrument called an optical pyrome-
measurements have been little or no explosive where geologists can safely approach the lava. direct temperature
The surfaces of these domes have temperatures up to 900°C, but the exterior of a dome is probably much cooler than its interior. When Mount St. Helens erupted in 1980, it ejected felsic magma as particulate matter in pyroclastic flows. Two weeks later, these flows still had temperatures between 300° and 420°C. ter.
taken at volcanoes characterized by activity
Therefore, lavas,
little is
known
of the temperatures of
tures of
86
when The tempera-
because eruptions of such lavas are rare, and
they do occur, they tend to be explosive.
some
lava domes,
Chapter 4
Volcanism
Viscosity
felsic
most of which are bulbous
Magma
is
also characterized by
tance to flow. water,
is
The
viscosity of
its
viscosity, or resis-
some
liquids,
such as
very low; thus, they are highly fluid and flow
readily.
The
viscosity of
however, that they flow
some other much more
liquids
is
so high,
Motor oil but become
slowly.
and syrup flow readily when they are hot, stiff and flow very slowly when they are cold. Thus, one might expect that temperature controls the viscosity of magma, and such an inference is partly correct. We can generalize and say that hot lava flows more readily than cooler lava. However, temperature is not the only con-
vapor. Lesser amounts of carbon dioxide, nitrogen, sul-
and hydrogen sulfide, and very small amounts of carbon monoxide, hydrogen, and chlorine are also commonly emitted. In areas of fur gases, especially sulfur dioxide
recent volcanism, such as Lassen Volcanic National
Park
in California, gases
continue to be emitted.
When magma
trol of v iscosity; other controls include the presence of mineraTcrystals and gas bubbles, the amount of dis-
reduced and the contained gases begin to expand.
solved water, and, most importantly, composition.
ever, in felsic
Magma
viscosity
tent. In a felsic lava,
is
strongly controlled by silica con-
numerous networks of
silica tetra-
hedra retard flow, because the strong bonds of the networks must be ruptured for flow to occur. Mafic lavas,
on the other hand, contain fewer silica tetrahedra networks and consequently flow more readily. Felsic lavas form thick, slow-moving flows, whereas mafic lavas tend to form thinner flows that move rather rapidly over great distances. One such flow in Iceland in 1783 flowed about 80 km, and some ancient flows in the state of Washington can be traced for more than 500 km.
its
m
toward the
surface, the pressure
is
How-
is
inhibited
magmas allow
In contrast, low-viscosity mafic
expand and escape
Accordingly, mafic
easily.
gases to
magmas
generally erupt rather quietly.
Although the amount of gases contained in magmas it is rarely more than a few percent by weight. Obviously, the gases can be sampled only when they are
varies,
expelled at the surface.
mine
how much is
It is
difficult,
of these gases
is
contaminant; that
a
however, to deter-
of magmatic origin and is,
gas that originated
from reactions between the magma and surrounding rocks or groundwater. Even though volcanic gases con-
VOLCANISM
Volcanis
rises
magmas, which are highly viscous, expanand gas pressure increases. Eventually, the pressure may become great enough to cause an explosion and produce pyroclastic materials such as ash. sion
how much **.
One
cannot help but notice the rotten-egg odor of hydrogen sulfide gas in such areas.
refers to the proces ses
whereby magma and
associated gases rise through the Earth's crust and are
extruded onto the surface or into the atmosphere
(Fig.
stitute a small
proportion of a
gerous, and, in
matic effects
some
cases,
magma,
they can be dan-
have had far-reaching
cli-
(see Perspective 4-2).
4 :FJ7Currentfy, more than 500 volcanoes are active— that
is,
they have erupted during historic time. Well-
known examples
of active volcanoes include
Mauna
Loa and Kilauea on the island of Hawaii, Mount Etna on Sicily, Fujiyama in Japan, and Mount St. Helens in Washington (Fig. 4-4). Only two other bodies in the
known
solar system are
to possess active volcanoes (see
In addition to active volcanoes,
numerous dormant
volcanoes exist that have not erupted recently but
may
Mount Vesuvius in Italy had human memory until a.d. 79 when it
again. For example,
not erupted in
erupted and destroyed the
Lava flows are frequently portrayed
in
movies and on
television as fiery streams of incandescent rock material
posing a great danger to humans. Actually, lava flows are the least dangerous manifestation of volcanism,
though they may destroy buildings and cover
Perspective 4-1).
do so
Lava Flows and Pyroclastic Materials
cities
of Herculaneum and
al-
agricul-
Most lava flows do not move particularly and because they are fluid, they follow existing low areas. Thus, once a flow erupts from a volcano, determining the path it will take is fairly easy, and anyone tural land. fast,
in areas likely to
be affected can be evacuated.
Some volcanoes have not erupted during recorded history and show no evidence of doing so again;
The geometry of lava flows pending on their viscosity and
thousands of these extinct or inactive volcanoes are
phy. Unless they are confined to a valley, comparatively
known.
fluid flows are thin and widespread, whereas more viscous flows tend to be lobate and to have distinct margins (Fig. 4-10). The surfaces of lava flows may be marked by
Pompeii.
Volcanic Gases Samples of gases taken from present-day volcanoes indicate that 50 to 80% of all volcanic gases are water
differs considerably, de-
the preexisting topogra-
such features as pressure ridges and spatter cones. Pressure ridges are buckled areas on the surface of a lava flow
(Fig.
4-1 la) that form because of pressure on the
Volcanism
87
Perspective 4-1
VOLCANISM IN THE SOLAR SYSTEM From data obtained during
the
first
phase of planetary
exploration that ended with Voyager 2's encounter
with Neptune, solar system
it
appears that only three bodies in the signs of present-day volcanism;
show any
the Earth, the Jovian
Neptunian
common
moon
moon
Io,
was
a
occurrence during the formation and early
history of the terrestrial planets
Recall that
known
and perhaps the
Triton. However, volcanism
and of many moons.
Olympus Mons on Mars
is
the largest
volcanic mountain in the solar system (Fig.
2-12b). Images from the Magellan spacecraft orbiting
Venus reveal numerous volcanic features, including dome-shaped volcanoes, but it is not certain whether any of these are active. Io, the
Jupiter,
is
innermost of the four large moons of
probably the most volcanically active body
yet observed in the solar system (Fig.
of Voyager
1).
Prior to the
1979, scientists expected that Io had a heavily cratered ancient surface similar to those of Mercury and the Moon. It turns out, however, that fly by
Io
is
1 in
not cratered. Instead,
it is
brilliantly
is
so volcanically active that
its
surface
is
constantly changing, and any impact craters that
may
form are very quickly obliterated by volcanic eruptions. To date, at least 10 active volcanoes have been discovered on
Io. It
years and erupt continually during this time, spewing
plumes of material 70 to 320
The source of heat
km
into space.
for the volcanic activity
on
Io
comes from the continual gravitational pull exerted by Jupiter and Io's sister moon, Europa. As Io revolves
88
Chapter 4
Volcanism
One
1
of Io's volcanoes
around
Jupiter,
it
gravitational field
is
shown erupting
in
moves in and out of Europa's and is therefore repeatedly squeezed
by gravitational tidal forces. This repeated squeezing generates an enormous amount of internal frictional heat that apparently keeps Io's interior molten and its volcanoes erupting. Heating and continual eruptions have depleted Io of any water or other volatile (easily vaporized) compounds that may have been present initially; thus,
only sulfurous
denser than any of the other
appears that individual
volcanoes remain active from a few months to a few forth
FIGURE
colored in
and yellows and has a variegated surface with many fractures and steep escarpments and numerous circular objects resembling volcanic craters. The colors result from the various sulfur compounds spewed forth by volcanoes and geysers. reds, oranges,
Io
~^~
the upper part of this image.
system because
it is
compounds are left. Io is moons of the outer solar
devoid of the lighter elements. also appears to be
The Neptunian moon Triton
volcanically active (Fig. 2-3). Evidence from images
returned by Voyager 2 indicate that Triton has geysers that are erupting frozen nitrogen crystals
compounds 35 km
into space.
and organic
"" FIGURE the
4-10
(a)
A
flow erupted during
fluid lava
1969-1971 Mauna Ulu
eruption of Kilauea volcano, Hawaii, (b) A viscous lava at Mount Shasta in California showing distinct margins.
partly solid crust of a
when
moving
flow. Spatt er cones
form
gases escaping from a flow hurl globs of molten
lava into the
air. 1
hese globs
fall
back to the surface and
adhere to one another, forming these small, steep-sided cones
(Fig.
Two
flow will not change to pahoehoe in a downflow direction,
however. Pahoehoe flows are
into blocks
and move forward
Columnar
4-1 lb).
less
flows; indeed, the latter are viscous
joints are
viscous than aa
enough to break up
as a wall of rubble.
common
in
many
lava flows,
named
especially mafic flows, but they also occur in other kinds
for Hawaiian flows, are generally recognized.. A paho ehoe (pronounced pah-hoy-hoy) flow has a ropy surface
of flows and in some intrusive igneous rocks (Fig. 4-13). A" lava How contracts as it cools ancT thus produces forces that cause fractures called 7b zwrs to"open up. On
almost
types of lava flows, both of which were
Tike tally (Fig. 4""-12a]7The surface of
ndTTncecTalT-ah) flow
is
an aa (pro-
characterized by roughTjagged
Some flows solidify as pahoehoe or aaUifoughout, but some pahoehoe flows change to aa in the downflow direction; an aa angular blocks and fragments
•^ FIGURE spatter cones
(Fig.
4-12b).
the surface of a How," these joints
commonlyTorfn po-
lygonal (often six-sided) cracksT These cracks also ex-
downward into the flow, thus forming parallel columns with their long axes perpendicular to the principal tend
{a) Pressure ridge on a 1982 lava flow in Hawaii, [b) A row of formed on February 25, 1983, on a flow at Kilauea volcano, Hawaii.
4-11
Volcanism
89
Perspective 4-2
VOLCANIC GASES AND CLIMATE Most
Volcanic ash erupted into the upper atmosphere
volcanic gases quickly dissipate in the
danger to humans, but on several occasions such gases have caused numerous fatalities. In 1783, toxic gases, probably sulfur
has some effect on climate, but
dioxide, erupted from Laki fissure in Iceland had
more important
atmosphere and pose
devastating effects.
little
About
75%
effect. Sulfur
all
particles except the
and produce no long-lasting
gases emitted during large eruptions have effects; small gas
molecules remain in
the upper atmosphere for years, absorbing incoming
of the nation's
and the hare resulting from the gas
livestock died,
smallest settle quickly
solar radiation
and
reflecting
it
back into space. In
caused lower temperatures and crop failures; about 24% of Iceland's population died as a result of the
1816, a persistent "dry fog" caused unusually cold spring and summer weather in Europe, the eastern
ensuing Blue Haze Famine.
United States, and eastern Canada. In North America,
1816 was
Obviously, large volcanic eruptions can devastate local areas, but they
can also affect climate over
much
regions— in some cases worldwide. The 1783 Laki
larger
produced what Benjamin Franklin called
fissure eruption
was responsible for dimming the The severe winter of Europe and eastern North America is
a "dry fog" that
intensity of sunlight in Europe.
1783-1784
in
called
"The Year without
occurred through the
summer
in
New
Philippines during the previous year
225
waters of Lake Nyos, which occupies a volcanic
agreement
exists
from the
crater.
on what caused the gas to suddenly once
it
did,
downhill along the surface because
it
was denser than
burst forth
lake, but
it
flowed
air. In fact, the density and velocity of the gas cloud were great enough to flatten vegetation, including trees, a few kilometers from the lake. Unfortunately, thousands of animals and many people, some as far as
23
km
from the
lake,
contributed to the cool spring
in the
may have and summer of 1816
as
Another large historic eruption that had widespread climatic effects was the eruption of Krakatau in 1883 (see the Prologue to Chapter 1). In comparison with Tambora and Krakatau, the 1980 Mount St. Helens eruption was small. Furthermore, it did not emit much sulfur gas, and its explosion was directed laterally so that most of the particulate matter did not enter the upper atmosphere. In fact, the much smaller 1982 eruption of El Chichon in Mexico had a greater effect on the climate, because it erupted so much sulfur gas and its gases and ash were ejected vertically so that much of them entered the upper atmosphere. well.
years.
More recently, in 1986, in the African nation of Cameroon 1,746 people died when a cloud of carbon dioxide engulfed them. The gas accumulated in the
No
England,
The eruption of Mayon volcano
historic time.
coldest winter in
or
and food shortages. The particularly cold spring and summer of 1816 are attributed to the 1815 eruption of Tambora in Indonesia, the largest and most deadly eruption during
atmosphere. In Iceland, the winter temperature was
its
Summer"
resulting in crop failures
attributed to the presence of this "dry fog" in the upper
4.8°C below the long-term average; the country suffered
a
"Eighteen Hundred and Froze to Death." Killing frosts
were asphyxiated.
can_bj^seen at Devil's Postpile National
whenjava is rapidly chilled beneath water, butitsJormation was notoBserved until 1971. Divers near Hawaii saw
Cal ifornia
pillows form
c ooling
s urface.
(Fig.
Excellent examples of columnar joints
4-13a), Devil's
Monument in Tower National Monu-
ment in Wyoming (see Chapter 5 Prologue), the Giant's Causeway in Irelaijdr-aail many other areas.
Much
of the igneous rock jn the upper part of the o ce-
anic crust
is
of a distinctive type;
it
consists of bulbous
masses of basalt resembling pillows, hence the
low
90
lava. It
was long recognized
Chapter 4
Volcanism
name
pil-
that pillow lava forms
when
a blob of lava broke through the crust
of an underwater lava flow and cooled almost instantly,
forming a glassy
exterior.
Remaining
fluid
inside then
broke through the crust of the pillow, resulting
in
an
ac-
cumulation of interconnected pillows (Fig. 4-14). "fh py r "HaS tic material is erupt rH as ath -a_A'<:ig-
M
nation for pyroclastic particles measurin g
|
less t han
2.0
(b)
"^ FIGURE
4-12
(a)
Pahoehoe flow
in the east rift
zone
of Kilauea volcano in 1972. (b) An aa flow in the east rift zone of Kilauea volcano, Hawaii in 1983. The flow front is
about 2.5
m
high.
"*"
fall
or an ash flow. During an ash
fall,
ash
is
ejected into
the atmosphere and settles to the surface over a wide area. In 1947, ash that erupted fell
3,800
from Mount Hekla
in Iceland
km away on Helsinki, Finland. About 10 million
years ago, in
what
is
now
northeastern Nebraska, numer-
ous rhinoceroses, horses, camels, and other
mammals were
buried by volcanic ash that was apparently erupted in
New
4-13
(a)
Columnar joints in a lava flow at Monument, California, (b) Surface
view of the same columnar joints showing their polygonal pattern. The straight lines and polish resulted from glacial ice
mm (Fig. 4- 15a). Ash may be erupted in two ways: an ash
FIGURE
Devil's Postpile National
moving over
this surface.
Pyroclastic materials larger than ash are also erupted
by explosive volcanoes. Particles measuring from 2 to 64 are known as lapilli, and any particle larger than 64 is called a bomb or block depending on its shape.
mm mm
Bombs have
twisted, streamlined shapes that indicate
they were erupted as globs of fluid that cooled and solidified
during their
flight
through the
air (Fig. 4-15b).
Blocks are angular pieces of rock ripped from a volcanic
Mexico, more than 1,000 km away. Ash is also erupted in ash flows, which are coherent clouds of ash and gas that
conduit or pieces of a solidified crust of a
commonly flow along or close to the land surface. Such flows can move at more than 100 km per hour, and some
cumulations are not nearly as widespread as ash deposits; instead, they are confined to the immediate area of
of them cover vast areas.
eruption.
cause of their large
size,
volcanic
magma.
bomb and
Be-
block ac-
Volcanism
91
-»'
FIGURE 4-14 magma
form when
These bulbous masses of pillow lava erupted under water.
is
Volcanoes Conical mountains formed around a vent where lava and pyroclastic materials are erupted are volcanoes. Volcanoes, which are named for Vulcan, the Roman deity of fire, come in many shapes and sizes, but geologists recognize several major categories, each of which has a distinctive eruptive style. One must realize, however, that
each volcano
is
unique
in
terms of
its
overall
and development. The frequency of example, varies considerably; the Hawai-
history of eruptions
eruptions, for
ian volcanoes have erupted repeatedly during historic
time, whereas others, such as
Mount
Helens, have
St.
erupted periodically with long periods of inactivity. of the duties of the U.S. Geological Survey active volcanoes
is
One
monitoring
and developing methods of forecasting
eruptions (see Perspective 4-3).
Most volcanoes have
a circular depression or crater
summit. Craters form as a result of the extrusion of gases and lava from a volcano and are connected via a conduit to a magma chamber below the surface. It is not unusual, however, for magma to erupt from vents at their
on the
flanks of large volcanoes
where
cones develop. For example, Shastina sitic
cone on the flank of
(Fig. 4-16),
and Mount Etna on on its flanks.
smaller vents
92
Mount
Chapter 4
Volcanism
smaller, parasitic is
a
major para-
Shasta in California Sicily
has some 200
(b)
-^- FIGURE 4-15 Pyroclastic materials, (a) Volcanic ash being erupted from Mount Ngaurauhoe, New Zealand during January 1974. (b) Volcanic bombs collected in Hawaii.
d
"^ FIGURE 4-16 Mount Shasta in northern California is one of the large volcanoes of the Cascade Range. The cone in the right foreground is Shastina, a parasitic cone that developed from flank eruptions on Mount Shasta. (Photo courtesy of Wayne E. Moore.) Some volcanoes
are characterized by a caldera rather
than a crater. Craters are generally
less
than
km
1
in
diameter, whereas calderas exceed this dimension and
have steep country
sides.
Crater Lake
is
One
of the best-known calderas in this
misnamed Crater Lake
the
is
about 6,600 years ago drained the
tially
in
actually a caldera (Fig. 4-17).
Oregon — It formed
voluminous eruptions parchamber. This drainage left the
after
magma
summit of the mountain, Mount Mazama, unsupported, and it collapsed into the magma chamber, forming a caldera more than 1,200 m deep and measuring 9.7 by 6.5 km. Many calderas have probably formed when a summit has collapsed during particularly large, explosive eruptions as in the case of Crater
have apparently formed
when
Lake, but a few
the top of the original
volcano was blasted away.
Shield Volcanoes Shield volcanoes resemble the outer surface of a shiel d
on the ground w ith the co nvex side uo_(Fig. They have low, rounded profiles with gentle slopes ranging from about 2 to 10 degrees. Their low slopes reflect the fact that they are composed mostly of
ly ing
4-18a).
m afic
flows-thatJiadjQW-jascosi ty, so the jlogs ^prea
out a nd fo rmed thin layers. Eruptions from shield volcanoes, sometimes called Ha waiian-type volcano es, are
compared to those of volcanoes such as Mount St. Helens; lavas most commonly rise to the surface with
quiet
little
to
explosive activity, so they usually pose
humans. Lava fountains, some up
contribute
some
to
little
400
danger
m
high,
"^ FIGURE
4-17
The sequence of
origin of Crater Lake, Oregon, (a-b)
flows partly drain the
Mazama.
(c)
The
events leading to the
Ash clouds and ash
magma chamber
collapse of the
beneath
Mount
summit and formation of
the caldera. (d) Post-caldera eruptions partly cover the caldera floor, and the small volcano known as Wizard Island
forms,
Wizard
[e)
View from
the rim of Crater
Lake showing
Island.
pyroclastic materials to shield vojca-
Volcanism
93
.
Perspective 4-3
MONITORING VOLCANOES AND FORECASTING ERUPTIONS Two
facilities in this
Of
country staffed by geologists of
critical
importance
in
volcano monitoring and
the U.S. Geological Survey (USGS) are devoted to
eruption forecasting are a sudden increase in
volcano monitoring; Hawaiian Volcano Observatory on the rim of the crater of Kilauea volcano and the
earthquake activity and the detection of harmonic tremor.
Harmonic tremor
David A. Johnston Cascades Volcano Observatory in Vancouver, Washington. The latter was established in 1981 and named in memory of the USGS geologist killed during the 1980 Mount St. Helens eruption. This facility is responsible for monitoring the various
motion
as
Cascade Range volcanoes (Fig. 4-2). Numerous volcanoes on the margins of the Earth's tectonic plates have erupted explosively during historic time and have the potential to do so again. As a matter of fact, volcanic eruptions are not as unusual as one might think; 376 separate outbursts occurred between 1975 and 1985. Fortunately, none of these compared to the 1815 eruption of Tambora; nevertheless, fatalities occurred in several instances,
1985 in Colombia where about 23,000 perished in mudflows generated by an eruption (see the Prologue to Chapter 13). Only a small minority of these potentially dangerous volcanoes are the worst being in
monitored, including some
in Italy,
Japan,
New
Zealand, the Soviet Union, and the Cascade Range.
Many
of the methods for monitoring active volcanoes
were developed at the Hawaiian Volcano Observatory. These methods involve recording and analyzing various changes
in
continuous ground
is
opposed to the sudden
earthquakes.
It
precedes
all
produced by
jolts
eruptions of Hawaiian
Mount magma is
volcanoes and also preceded the eruption of St.
Helens. Such activity indicates that
moving below the surface. The analysis of data gathered during monitoring not by
itself sufficient
history of a particular volcano
To determine
is
to forecast eruptions; the past
must
also be
known.
the eruptive history of a volcano, the
record of previous eruptions as preserved
in
rocks
must be studied and analyzed. Indeed, prior to 1980, Mount St. Helens was considered one of the most likely Cascade volcanoes to erupt because detailed studies indicated that it has had a record of explosive activity for the past 4,500 years. For the better monitored volcanoes, such as those in
Hawaii,
it is
now
possible to
make
accurate
short-term forecasts of eruptions. For example, in
1960 the warning signs of an eruption of Kilauea were recognized soon enough to evacuate the residents of a small village that was subsequently buried by lava flows. Unfortunately, forecasting for more than a few months cannot be done at present.
both the physical and chemical attributes of
volcanoes. Tiltmeters are used to detect changes in the slopes of a volcano into
it,
when
it
inflates as
horizontal distances, which also inflates (Fig. 1). Geologists also
and changes
in the local
is
injected
beam to measure change when a volcano
Chapter 4
monitor gas emissions
magnetic and
volcanoes.
94
magma
while a geodimeter uses a laser
electrical fields of
~^ FIGURE show
volcano: reaches
(a)
its
Volcano monitoring. These diagrams Hawaiian
The volcano begins
peak;
returning to
Volcanism
1 (right)
three stages in a typical eruption of a
its
(c)
to inflate; (b) inflation
the volcano erupts
normal shape.
and then
deflates,
Horizontal
and
vertical distances
increase from Stage
1
Distance measurement points Ti\vnete<
A
B
Magma Stressed rockszone of earthquakes
(b)
Stage 2
Eruption t
A'
B'
.
A
\
reservoir
Shield volcano
(a)
Pyroclastic
Central vent filled
layers
with
rock fragments
(b)
Cinder cone (c)
FIGURE
Composite volcano
Examples of the shapes and internal structures of the three basic types of volcanoes, {a) A shield volcano. Each layer shown consists of numerous, thin basalt lava flows. (£>) Cinder cones are composed of layers of angular pyroclastic materials, (c) Composite volcanoes are the typical, large volcanic mountains on continents. They are composed of lava flows, pyroclastic layers, and volcanic mudflows. -*»'
4-18
(Fig. 4-8), but-Otherwise these volcan oesare comp osed largely of fasalt J ava flows; flows comprise more than 99% of the Hawaiian volcanoes above sea level. Although eruptions of shield volcanoes tend to be rather quiet, some of the Hawaiian volcanoes have, on occasion, produced sizable explosions. Such explosions
noes
occur
when magma comes
causing
it
were
Chapter 4
stands
floor. Its
Volcanism
island of
is
nearly 100
km
in the
world.
across at the base and
more than 9.5 km above the surrounding sea 3 volume is estimated at about 50,000 km By
with groundwater,
by a cloud of hot volcanic gases. Shield volcanoes are most common in oceanic areas, T\ such as those of the Hawaiian Islands and Iceland, but 96
Mauna Loa
such explosion oc-
killed
The
waiian volcanoes are the largest volcanoes
One
Keoua was leading about 250 warriors across the summit of Kilauea volcano to engage a rival chief in battle. About 80 of Keoua's warriors
are also present
in east Africa.
in contact
to instantly vaporize.
curred in 1790 while Chief
.
on the continents — for example, Hawaii consists of five huge shield volcanoes, two of which, Kilauea and Mauna Loa, are active much of the time (Fig. 4-19). These Ha-
some
.
contrast, the largest volcano in the continental United States,
Mount
Shasta in northern California (Fig. 4-16),
has a volume of only about 205
km
.
summit crater or caldera and a number of smaller cones on their flanks through which lava is erupted (Fig. 4- 18a). For example, a vent opened Shield volcanoes have a
^ FIGURE much
4-19
The Hawaiian volcanoes
are active
of the time.
on the flank of Kilauea and grew to more than 250 high between June 1983 and September 1936.
m
Cinder Cones Volcanic peaks composed of pyroclastic materials that resemble cinders are
known as cinder cones
(Fig. 4- 18b).
They form when pyroclastic materials are ejected into the atmosphere and fall back to the surface to accumulate around the vent, thus forming small, steep-sided cones. The slope angle may be as much as 33 degrees, depending on the angle that can be maintained by the shaped pyroclastic materials. Cinder cones m high, and many have a large, bowl-shaped crater. Many cinder cones are very nearly symmetric in shape; that is, the pyroclastic materials accumulate uniformly around the vent, forming a symirregularly
are rarely
more than 400
metric cone.
The symmetry may be
less
'"•'
FIGURE
4-20
Paricutin,
Mexico, shown soon
after
it
1943. Mostly ash and cinders were erupted, but some lava flows broke through the flanks and the base of the volcano.
formed
in
than perfect,
however, when prevailing winds cause the pyroclastic materials to build up higher in the
Many
downwind
direction.
cinder cones form on the flanks or within the
calderas of larger volcanic mountains and appear to rep-
slopes built
up
to a height of
more than 300 m. Shortly
after the initial explosive stage of eruption, lava flows
broke through the base and flanks, spread outward, and two nearby towns (Fig. 4-20). Pari-
resent the final stages of activity, particularly in areas
eventually covered
formerly characterized by basalt lava flows. Wizard
1952 and then ceased. on the Icelandic island of Heimaey, the town of Vestmannaeyjar was threatened by a new cinder cone. The initial eruption began on January 23, and within two days a cinder cone, later named Eldfell, rose to about 100 m above the surrounding area (Fig. 4-21). Pyroclastic materials from the volcano buried parts of the town, and by February a massive aa lava flow was advancing toward the town. The flow's leading edge ranged from 10 to 20 m thick, and its central part was as much as 100 m thick. By spraying the leading edge of the flow with sea water, which caused it to cool and
land in Crater Lake, Oregon,
is
Is-
a small cinder cone that
formed after the summit of Mount Mazama collapsed to form the caldera (Fig. 4-17). Cinder cones are common in the southern Rocky Mountain states, particularly New Mexico and Arizona, and many others occur in northern California, Oregon, and Washington. One cinder cone of particular interest is Paricutin in
Mexico
(Fig. 4-20).
On
February 20, 1943, a farmer ob-
served fumes emanating from a crack in his cornfield, and
within a few minutes pyroclastic materials were erupted. Within a month a symmetrical cone with 30-degree
cutin's activity continued until
In 1973,
Volcanism
97
"^ FIGURE
The town
of Vestmannaeyjar in Iceland from Eldfell, a cinder cone, that formed in 1973. Within two days of the initial eruption on January 23, the new volcano had grown to about 100 m high. Another cinder cone called Helgafell is also visible.
4-21
was threatened by
lava flows
~^r FIGURE 4-22 Mayon volcano in the Philippines is one of the most nearly symmetrical composite volcanoes in the '
world.
flows).
Some
form when
lahars
rain falls
loose pyroclastic materials and creates a solidify, the residents of Vestmannaeyjar successfully di-
verted the flow before
it
did
much damage
to the town.
Composite Volcanoes
on
layers of
muddy
slurry
moves downslope. On November 13, 1985, mudflows resulting from a rather minor eruption of Nevado del Ruiz in Colombia killed about 23,000 people (see
that
the Prologue to Chapter 13).
Composite volcanoes, also called stratovolcanoes, are composed of both pyroclastic layers and lava flows (Fig. 4-18c). Typically, both materials have an intermediate composition, and the flows cool to form andesite (Table 4-2). Recall that lava of intermediate
composition
is
more viscous than mafic lava. In addition to lava flows and pyroclastic layers, a significant proportion of a composite volcanoe is made up of lahars (volcanic mud-
FIGURE 4-23 A cross section showing the internal dome. Lava domes form when a viscous mass of magma, generally of felsic composition, is forced up through a volcanic conduit. "*''
structure of a lava
Composite volcanoes are steep sided near their summuch as 30 degrees, but the slope decreases toward the base where it is generally less than 5 degrees. Mayon volcano in the Philippines is one of the most perfectly symmetrical composite volcano on Earth. Its concave slopes rise ever steeper to the summit with its central vent through which lava and pyroclastic materimits, perhaps as
als are periodically
erupted
(Fig. 4-22).
Composite volcanoes are the typical large volcanoes of the continents and island arcs. Familiar examples include Fujiyama in Japan and Mount Vesuvius in Italy as well as many of the volcanic peaks in the Cascade Range of the northwestern United States.
Lava Domes If
the
upward pressure
conduit
in a volcanic
enough, the most viscous
is
great
magmas move upward and
form bulbous, steep-sided lava domes
(Fig. 4-23).
Lava
domes are generally composed of felsic lavas although some are of intermediate composition. Because such magma is so viscous, it moves upward very slowly; the lava dome that formed in Santa Maria volcano in Guatemala in 1922 took two years to grow to 500 m high and 1,200 m across. Lava domes contribute significantly to
many composite
number
98
Chapter 4
Volcanism
of lava
volcanoes. Beginning
domes were emplaced
in
1980, a
in the crater
of
'*"'
FIGURE
erupted from
Mount
St.
4-24
St. Pierre,
Mount
Martinique
Pelee in 1902.
after
it
Only 2 of the
was destroyed by city's
a nuee ardente 28,000 inhabitants survived.
Helens; most of these were destroyed during
Mount St. Helens dome growth.
subsequent eruptions. Since 1983, been characterized by sporadic
has
Lava domes are often responsible for extremely exmagma accumulated beneath the summit of Mount Pelee on the island of
plosive eruptions. In 1902, viscous
Martinique. Eventually, the pressure within the tain increased to the point that
it
moun-
could no longer be
contained, and the side of the mountain blew out in a
tremendous explosion.
When
this occurred, a mobile,
dense cloud of pyroclastic materials and gases called a nuee ardente (French for "glowing cloud") was ejected and raced downhill at about 100 km/hr, engulfing the city of St. Pierre (Fig. 4-24). This nuee ardente had internal temperatures of in its
path
700°C and incinerated everything
(Fig. 4-24).
Of
the 28,000 residents of
St.
Pierre, only 2 survived, a prisoner in a cell below the ground surface and a man on the surface who was terribly burned by the nuee ardente. Following the disastrous 1902 eruption of Mount Pelee, a spine of
magma was much
as
20
almost completely solidified viscous
forced up through the conduit.
m per day,
but chunks continually
surface forming a pile of rubble.
tained a height of its
own
When
more than 300 m,
it
It
rose as
fell
off
its
the spine at-
collapsed under
weight.
Fissure Eruptions During the Miocene and Pliocene epochs (between about 17 million and 5 million yeari ago), some 2 164,000 km of eastern Washington and parts of Ore-
Volcanism
99
DAVID
Guest Essay
P.
HILL
MONITORING VOLCANIC ACTIVITY
My interest in geology stems
the interaction of science
fascination with the outdoors
of
from a long-standing and nature. This fascination was strongly influenced by my early childhood in Yellowstone National Park and later visits with
my
Service)
(who worked for the National Park to Yosemite and other western parks. I began father
vague idea of becoming a naturalist with the National Park Service. In response to a growing interest in the more analytical aspects of the physical sciences, however, I switched to college studying biology with the
geology at the end of to study geophysics I
began
my
my sophomore
and seismology
year and went
in
on
graduate school.
research with the U.S. Geological
Mammoth
Long
Valley caldera,
met with
society.
The
resort
town
the southwestern margin of
and the
initial
news
1982 that
in
was and anger by the residents of Lakes and Mono County. Responding to
disbelief
Mammoth
the continuing activity in the caldera over subsequent
years has proved to be an educational process for
both local residents and
monitoring the
activity.
scientists studying
The
and
local residents have
come
to better appreciate the geologic processes that have
sculpted the spectacular setting in which they the scientists have
involved
Earth's crust beneath the western United States.
is
and physical properties of
and
lies at
the activity might be related to volcanic activity
Survey using seismic waves and small variations in the Earth's gravity field to study the structure of the Scientifically, the thickness
Lakes
come
in effectively
live,
and
to appreciate the challenge
communicating the
their research to the public in a useful
results of
way. The
latter
a challenge that faces scientists in general as
taxpayers and politicians increasingly
demand
they are getting for their money;
to
the crust and upper mantle are key elements for
know what
understanding the geologic processes that form the outer layers of the Earth. They are also keys to
particularly acute challenge for those of us pursuing
understanding seismic wave propagation
landslides,
in the Earth.
Support for this work derived from a national interest in using seismology to discriminate underground nuclear explosions from earthquakes and, ultimately,
means for ban treaty.
to provide a technical
comprehensive
More
test
recently,
my
verifying a
research has turned toward the
study of earthquakes and the clues they provide on
deformation of the Earth's crust (seismotectonics) and volcanic processes. Since 1983, 1 have been in charge of the U.S. Geological Survey's efforts to monitor and
it is
a
on geologic hazards (earthquakes, volcanoes, and the like) because the results of our research and the manner in which we present them can have an immediate impact on the economy and research
social well-being of the
as
on public safety. The Earth sciences
communities involved as well
are central to our understanding
of the risks posed by geologic hazards and a growing
number of environmental issues that modern society faces. They will continue to offer a wide range of scientifically exciting
and
socially significant
opportunities in the future.
A
better understand the recurring episodes of earthquake
swarms and ground uplift in Long Valley caldera in eastern California. Long Valley caldera is a large oval depression (15 by 30 km) at the base of the eastern escarpment of the Sierra Nevada that was formed by a massive volcanic eruption 730,000 years ago. Volcanic activity
has continued in the area with the most recent
500 years ago. The current earthquake activity and ground deformation are symptomatic of the movement of magma in the upper 5—10 km of the crust and are typical of the sort of
eruptions occurring just
geological unrest that often precedes volcanic eruptions.
This activity raises a host of fascinating scientific questions, but
100
Chapter 4
it
also raises important issues regarding
Volcanism
David
P. Hill graduated from San Jose State University and earned an M.S. in geophysics from the Colorado School of Mines and a Ph.D. from the California Institute of Technology. He has been a
geophysicist with the U.S.
Geological Survey since 1961.
His work in the Mammoth Lakes area has been widely cited in newspapers and magazines,
News, as a good example of how scientific research can have an impact on including Science
the general public.
Fissures
gon and Idaho were covered by overlapping basalt lava flows. These Columbia River basalts, as they are called,
now well exposed in the walls of the canyons eroded by the Snake and Columbia rivers (Fig. 4-25). These lavas, which were erupted from long fissures, were so fluid that volcanic cones failed to develop. Such fissure eruptions yield flows that spread out over large areas
"^ FIGURE
4-26 A block diagram showing fissure eruptions and the origin of a basalt plateau.
are
and form basalt plateaus (Fig. 4-26). The Columbia River basalt flows have an aggregate thickness of about 1,000 m, and some individual flows cover huge areas — for example, the Roza flow, which is 30 m thick, advanced along a front about 100 km wide and covered 2 40,000 km Fissure eruptions and basalt plateaus are not common, although several large areas of such features are known (Table 4-3). The only area where such activity is .
currently occurring
is
in Iceland.
mountains are present island
Two
is
composed of
major
4-3
one
Basalt Plateaus
of volcanic
but the bulk of the
basalt flows erupted
fissure eruptions,
"^ TABLE
A number
in Iceland,
in a.d.
from fissures. 930 and the
other in 1783, account for about half of the
erupted
in Iceland
magma
during historic time. The 1783 erup-
tion occurred along the Laki fissure,
which
is
25
km
long; lava flowed several tens of kilometers from the fissure and 200 m.
in
one place
filled
a valley to a depth of about
Pyroclastic Sheet Deposits
More than 100
years ago, geologists were aware of vast
areas covered by felsic volcanic rocks a few meters to
hundreds of meters thick. It seemed improbable that these could have formed as vast lava flows, but ir also seemed equally unlikely that they were ash fall deposits. Based on observations of historic pyroclastic flows, such
and the Alaskan volcanoes
in the
Aleutian Island arc.
The belt continues on the western side of the Pacific Ocean basin where it extends through Japan, the Philippines, Indonesia, and New Zealand. The circumPacific belt also includes the
cano, at
Mount Erebus
southernmost active vol-
and a
in Antarctica,
large caldera
Deception Island that erupted during 1970 (Fig. 4-28). About 20% of all active volcanoes are in the Medi-
terranean belt
(Fig. 4-28).
Included
in this belt are the
famous Italian volcanoes such as Mount Etna, Stromboli, and Mount Vesuvius. Most of the large volcanoes in the circum-Pacific and Mediterranean
"^ FIGURE
4-27 The Yellowstone Tuff in the walls of Grand Canyon of the Yellowstone, Yellowstone National Park, Wyoming. Tuff is a volcanic rock composed of
the
consolidated ash.
belts
are composite volcanoes, but a
number of them have had lava domes emplaced in their craters or calderas. The fact that most of the volcanoes in these two belts are composite volcanoes is significant. Recall that such volcanoes are composed of lava flows and pyroclastic layers of intermediate and felsic composition whereas those within the ocean basins are composed primarily of mafic
as the nuee ardente erupted
by
Mount
Pelee in 1902,
now
seems probable that these ancient rocks originated as pyroclastic flows. They cover far greater areas than any observed during historic time, however, and apparently erupted from long fissures rather than from a central vent.
The
pyroclastic materials of
many
of these
flows were so hot that that they fused together to form welded tuff (tuff is a volcanic rock composed of consolidated ash). It
from
now
appears that major pyroclastic flows issue formed during the origin of calderas. For
fissures
example, the Yellowstone Tuff was erupted during the formation of a large caldera Yellowstone National Park
in the area of present-day in
Wyoming
(Fig.
4-27).
Bishop Tuff of eastern California appears to have been erupted shortly before the formation of the Long Valley caldera. Interestingly, earthquake activity in the Long Valley caldera and nearby areas beginning in Similarly, the
1978 may indicate that
magma
is
moving upward
lavas.
of the rest of the active volcanoes are at or near
Most
it
the mid-oceanic ridges (Fig. 4-28).
The
longest of these
which is near the middle of the Atlantic Ocean basin and curves around the southern tip of Africa where it continues as the Indian Ridge. Branches of the Indian Ridge extend into the Red Sea and East Africa. Mount Kilimanjaro in Africa is on this latter branch (Fig. 4-28). Most of the volcanism along the mid-oceanic ridges is submarine, and much of it goes undetected; but in a few places, such as Iceland, it occurs above sea level. Volcanism is occurring in a few other areas at present, most notably on and near the island of Hawaii (Fig. 4-28). Only two volcanoes are currently active on the island, Mauna Loa and Kilauea, although a submarine volcano named Loihi exists about 32 km to the south; Loihi rises more than 3,000 m above the sea floor, but its summit is still about 940 m below sea level. ridges
the Mid-Atlantic Ridge,
is
be-
neath part of the caldera. Thus, the possibility of future
^
eruptions in that area cannot be discounted.
AND VOLCANISM
^ DISTRIBUTION OF VOLCANOES
At
PLATE TECTONICS this point,
volcanoes: (1)
two questions might be
What
raised regarding
accounts for the alignment of volWhy do magmas erupted within
Rather than being distributed randomly around the
canoes in belts?
Earth, volcanoes occur in well-defined zones or belts.
ocean basins and magmas erupted at or near continental margins have different compositions? Recall from Chapter 1 that the outer part of the Earth is divided into large
More than 60%
of
all
active volcanoes
are in the
circum-Pacific belt that nearly encircles the margins of the Pacific
Ocean basin
(Fig. 4-28).
This belt includes the
volcanoes along the west coast of South America, those in Central America, Mexico, and the Cascade Range,
102
Chapter 4
Volcanism
plates,
(2)
which are sections of the lithosphere. Litho-
sphere can consist of upper mantle and oceanic crust or upper mantle and continental crust, called oceanic and
Convergent plate margins
Spreading ridges
"^ FIGURE
Most volcanoes
4-28
are at or near plate boundaries.
belts are recognized: the circum-Pacific belt contains
about 20% are in the Mediterranean mid-oceanic ridges.
belt,
about
and most of the
60%
Two
all
major volcano
active volcanoes,
along
rest are located
Most volcanism
continental lithosphere, respectively.
of
occurs at spreading ridges where plates diverge or along
subduction zones where plates converge.
spreading
because
at Spreading Ridges
Red
Sea, the Gulf of Aden, and east Africa and along the Indian Ridge and the East PacificTCIse.
Some
Volcanism
also currently occurring_at_diyergent -mar-
is
gins in the
of the volcanism at spreading ridges it
occurs above sea
level,
is
apparent
but, as previously
much of it is submarine and goes undetected. However, research submarines have descended into the rifts at the crests of spreading ridges where scientists have observed pillow lavas that formed during submanoted,
Spreading ridges are areas where new oceanic lithosphere is produced by volcanism as plates diverge and
move away from one another
Most spreadsome extend into
(Fig. 1-14).
ing ridges are in the ocean basins, but
'continents as in east Africa (Fig. 4-28). According to plate tectonic theory, the Atlantic
developing
when
rifting
Ocean basin began
of a large plate and subsequent
plate divergence resulted in the breakup of the super-
continent Pangaea about 250 million years ago.
Mid-Atlantic Ridge plate divergence
is
The
fact that
undisputed, but ridges
is
not
volcanism occ urs at spreading ridges
how magma
fully
understood.
is
originates beneath the
One
e xplanation
is
re-
"
whirh rhe Farrh's temperature increases with depth. We know from deep mines and
lat ed to the
jnanne r
which
deep~driilhojeirTHat~a temperature increase, called the
at present. Similar
geothertnal gradient, do es occur and that, on average,
the spreading ridge along
began and continues
The
rine eruptions (Fig. 4-14).
Plate Tectonics
and Volcanism
103
"^ FIGURE
4-29
the continents - Subcontinental geothermal gradient
'
/
- Suboceanic geothermal gradient
,
j
,' ji
/
/ /
/
Dry peridotite
// /
/
/
Calculated geothermal gradients under
and ocean basins (somewhat
That melt + peridotite
is,
rifting at
1
.000
Temperature
1
in
,500
°C
overcome
(Fig. 4-29).
spreading ridges probably causes a
decrease in pressure on the hot rocks at depth, thus initiating melting. Rifting
500
The
Rising hot rock beneath spreading ridges maintains a geothermal gradient well above average, and locally the temperature exceeds the melting temperature, at least in part, because pressure effects are
Dry basaltic
speculative).
melting of dry basalt has been experimentally investigated only in the pressure-temperature region indicated by the solid line. Melting of dry peridotite occurs between 100 and 125 km beneath the ocean basins. However, should the pressure be reduced, as occurs at spreading ridges, melting might occur at even shallower depths.
is
unlikely to be the sole cause
of melting, however, because melting also occurs in
some
areas
where there appears
to be
no
rifting,
such as
beneath the Hawaiian Islands.
Another explanation about 25°C/km. Accordingly, rocks at depth are hot, but remain solid because their melting temperature rises with increasing pressure. the gradient
"^ FIGURE
is
4-30
Some
for spreading ridge
volcanism
called mantle
plumes
spread outward in
rise
all
beneath spreading ridges and
directions (Fig. 4-30). Perhaps
of the "hot spots" in the Earth's crust that are thought to
overlie rising mantle plumes.
"^^
104
Chapter 4
Volcanism
is
that localized, cylindrical plumes of hot mantle material,
N
\.
Cocos /
Middle^P
1
^^
gence and spreading ridge volcanism occur at the East Pacific Rise. As a consequence of spreading, the Nazca plate moves east and collides with the South American plate (which
is
moving west
as a result of plate diver-
gence at the Mid- Atlantic Ridge). Thus, a collision occurs between oceanic and continental lithosphere, and because the Nazca plate is denser, it plunges beneath the South American plate (Fig. 4-3 lb). Th^belFoTIarge composite volcanoes near the western margin of South America formed from the magma created by partial melting of the subducted plate. As the
Nazca
plate de scends
toward the asthenosphe re,
it is
heated by the Earth's geothermal gradient. When the descendingjlate reaches a depth where the temperature is high enough, partial melting occurs and magma is generated~(Fig- 4-3 lb). Additionally, the _wel_oceanic crust descends to a depth at which dewatering occurs. AsThFwater rises Into the overlying mantle, it enhances
melting, ancTa
m agma
may
be generated
(Fig.
4-3 lb).
one phenomenon_accounting^or_the factlhajTmagm as generated ajjmbduction zones are intermediate and felsic in composition. Recall that partial melting of ultramafic rock of the upper mantle yields Partial melting
is
Kauai
$
maKc_rnagrna. Likewise, partial melting of oceanic crust,
which has a mafic composition, may in silica
yield
magma
margins are probably carried downward with the subducted plate and contribute their silica to the magma (Fig. 4-3 lb). In addition, mafic magma rising through the lower continental crust may be contaminated with felsic materials,
which change
Intermediate and
felsic
its
Direction of plate motion
felsic
magma
is
ea'
\
—
million
years
Hawaii 0.7 million years
are typically pro-
ducecPat"convergent plate margins where subduction occurs. The intermediate magma that is erupted is more viscous than mafic magma and tends to form composite
Much
Maui
^^0-1.3
composition.
magmas
years
^^-?(\
years
sediments and sedimentary rocks of continen-
tal
volcanoes.
1.3-1.8 million
2.3-3.3 million
km
Molokai
0ahuW5
richer
than the source rock. Additionally, some of the
silica-rich
100
3.8-5.6 million years
Recent volcanism
^
to
present
^_ ^^ ^f
Indicates
Undersea
active volcano
volcano
(b)
intruded into the con-
where it forms various intrusive igneous bodies (see Chapter 5), but some is erupted as pyroclastic materials or emplaced as lava domes, thus accounting
"^ FIGURE
4-32
origin of the
Hawaiian
for the explosive eruptions that characterize convergent
only present-day volcanism occurs on Hawaii and beneath the sea just to the south. (£>) Map showing the age of the
tinental crust
moves over
plate margins.
Intraplate
(a)
Generalized diagram showing the Islands.
As the
lithospheric plate
a hot spot, a succession of volcanoes forms.
islands in the
The
Hawaiian chain.
Volcanism
Mauna Loa and
Kilauea on the island of Hawaii and
Even though these Hawaiian volcanoes are unrelated
Loihi just to the south are within the interior of a rigid
to spreading ridges or subduction zones, the evolution
plat e far fro
m any spreading ridge or subduction zone 4-28)nFl?postulated that aTnantle plume creates a local "h ot spot" ben eath' Hawaii? Themagma is mafic and thus relatively fluid, so it buildsup shield volcanoes
of the Hawaiian Islands
(Fig.
Notice
(FIg74^32a7^ 106
Chapter 4
in
is
related to plate tectonics.
Figure 4-32b that the ages of the rocks com-
posing the islands in the Hawaiian chain increase tos
ward
the northwest; Kauai
formed 3.0 to 5.6 million
years ago, whereas Hawaii began forming less than one
Volcanism
and Loihi began forming even more Continuous motion of the Pacific plate over the "hot spot," now beneath Hawaii, has created the various islands in succession. Mantle plumes and "hot spots" have also been proposed to explain volcanism in a few other areas. A man-
plume may be beneath Yellowstone National Park
million years ago,
tle
recently.
Wyoming. Some source
^ CHAPTER SUMMARY 1.
for the present-day hot springs Faithful, but
heat
is
whereby magma and
the process
associated gases erupt at the surface.
erupts as lava flows, and
some
is
its
Only
of gases, most of which
water vapor. Sulfur gases emitted during large eruptions can have far-reaching is
climatic effects. viscosity of lava flows depends mostly on their temperature and composition. Sili ca-rich (felsic) lava is more viscous th an silica-poor (mafic) lava.
4.
The
5.
Many
lava flows are characterized by pressure ridges
and spatter cones. Columnar lava flows
when
joints
form
in
some
they cool. Pillow lavas are erupted
formed
deposits.
Most active volcanoes are distributed in linear belts. The circum-Pacific belt and Mediterranean belt c ontain more than 80% of all active volcanoes.
13.
Volcanism
in the circum-Pacific and Mediterranean convergent plate argins where subduction occurs. Partial melting of the subducted
belts
is
m
at
plate generates intermediate
consists
fissures
12.
Some magma
magma
Old
body of intruded magma that has not yet com-
from
ejected explosively
a few percent by weight of a
as
during the origin of calderas cover vast areas! Such eruptions of pyroclastic materials form sheetlike
as pyroclastic materials. 3.
and geysers such
11. Pyroclastic flo ws erupted
magmas. Volcanism
a
in
responsible
geologists think that the source of
mantle plume.
surface,
2.
is
many
is
pletely cooled rather than a
Magma
is molten rock material below the Earth's whereas lava is magma that reaches the surface. The silica content of magmas varies and serves to differentiate felsic, intermediate, and mafic
of heat at depth
Migma^deriyed by
and
felsic
magmas.
upper mantle beneath spreading ridges accounts for the ^\ mafic lavaTof ocean basins. Melting in these areas may T5e caused by reduction in pressure and/or hot mantle plumes. 15. The two active volcanoes on the island of Hawaii and one just to the south are thought to lie above a hot mantle plu me. The Hawaiian Islands developed as a series of volcanoes that formed on the Pacific plate as it moved over the mantle plume. 14.
partial melting of the
\g
under water and consist of interconnected bulbous masses. 6.
Volcanoes are conical mountains
built
up around
IMPORTANT TERMS
a
vent where lava flows and/or pyroclastic materials are 'erupted. 7.
Shield volcanoes have low, rounded profiles
and are
mafic
basalt plateau
pyroclastic materials that resemble cinders are
cinder cone
intermediate composition, layers of pyroclastic materials,
9.
lava flow
composed mostly of matirftows that have cooled and formed basaltc Cinder jxmes form where erupted and accumulate as small, steep-sided cones. (fCompositt volcanoes are composed of lava flows of
8.
aa
ash
and volcanic mudflows.
circum-Pacific belt
felsic
of volcanoes are characterized by a
calderas form by
10. Fluid mafic lava erupted
is
partly drained.
from long
spatter cone
lava
magma
viscosity
volcanism volcano
dome
^ REVIEW QUESTIONS
fissures (fissure
eruptions) spreads over large areas to form basalt plateaus.
shield volcano
larger caldera.
summit collapse when an
magma chamber
pyroclastic materials
magma
intermediate lava
underlying
pressure ridge
fissure eruption
lahar
Many
pillow lava
crater
nuee'ardenTes.
much
pahoehoe
columnar joint composite volcano
because they erupt explosively and frequently eject
circular or oval crater or a
mantle plume Mediterranean belt nuee ardent
caldera
(stratovolcano)
Viscous masses of lava, generally of felsic composition, are forced up through the conduits of some volcanoes and form bulbous, steep-sided lava domes. Volcanoes with lava domes are dangerous
The summits
magma magma
1.
Which of humans?
the following
is
most dangerous to
Review Questions
107
*
a.
A
pahoehoe;
3.
4.
b
vesicular;
lapilli;
obsidian;
c.
aa;
)f
""'
pyroclastic sheet deposit.
e.
Most calderas form by: a. JC summit collapse;
<
explosions;
c.
fissure eruptions; d.
e.
erosion of lava domes.
15.
forceful injection;
b.
widespread ash
16.
accumulation of
falls; c.
on composite volcanoes; e. Jf- eruptions of fluid lava from long fissures. One other Cascade Range volcano besides Mount
a.
17.
6.
18.
b.
Ar consolidation
magma
beneath the surface;
A
felsic
magnesium; as basalt; d.
is
c.
-^
contains
more than
65%
characterized as silica poor;
silica;
contains
e.
mostly sodium and potassium. 8.
9.
10
The
viscosity of
magma
is
a.
temperature; b.-^i
c.
pressure; d.
primarily controlled by: silica content,;
texture; e
elevation. __—
The most commonly emitted volcanic gas hydrogen
a
carbon dioxide;
b.
c.
nitrogen; d.
chlorine;
e. ^**
is:
sulfide;
water vapor.
Shield volcanoes have
composed a
low slopes because they are
Mount Mount
area where fissure eruptions are currently is:
Red
western South America;
e.
Japan.
Sea; b.
controls the viscosity of a lava flow?
23. Explain
how
pyroclastic materials
and volcanic gases
can affect climate. 24. How do spatter cones and columnar joints form? 25. What accounts for the fact that volcanic ash can cover vast areas, whereas pyroclastic materials such as cinders are not very widely distributed? 26. Explain how most calderas form. 27.
What
kinds of warning signs enable geologists to
forecast eruptions?
do shield volcanoes have such low slopes? do pahoehoe and aa lava flows differ? a cross section of a
Indicate
its
composite volcano.
constituent materials, and
show how and
where a flank eruption might occur. 31. Why do composite volcanoes occur in belts near convergent plate margins? Are such volcanoes present at 32. 33.
Why
all
are lava
Compare and
convergent plate margins? domes so dangerous? contrast basalt plateaus and
pyroclastic sheet deposits.
mostly pyroclastic layers; b. lahars and c. J^cT fluid mafic lava flows; felsic magma; e. pillow lavas.
Chapter 4
b.
Fujiyama, Japan; d. jjfel Mauna Loa, Hawaii.
rock?
pyroclastic blocks.
of:
viscous lava flows; d.
e.
e.
is:
the Pacific Northwest; d.^^-Iceland;
Draw
lapilli; c.
c.
world
Helens, Washington;
c.
30.
parasitic cones;
St.
the
How
pillow lava; b.
Mount
a.
Why
material; d.
yf^ rhe Hawaiian
d.-
Iceland.
e.
The only
29.
columnar joints; d. pahoehoe; volcanic bombs. Much of the upper part of the oceanic crust is composed of interconnected bulbous masses of
the mid-oceanic ridges;
b.
largest volcano in the
28.
a. if-
108
The
lava flows
igneous rock called:
12
East Africa;
the Cascade Range;
What
e.
11
a.
a c.
(b) only.
22.
Small, steep-sided cones that form on the surfaces of
where gases escape are: lava tubes; b. <&~ spatter cones;
and
21.
cools to form volcanic rocks such
b.
(a)
Why is silica the major component of magma? How can a mafic magma be derived from ultramafic
20.
contains a high percentage of iron and
a.
is
are unrelated to either a
c.
occurring
of pyroclastic materials;-
magma:
answers
e.
The volcanoes of
Vesuvius, Italy; 19.
heating of sedimentary rocks atmosphere; d. beneath lava flows; e. all of these. 7.
ridges
intermediate;
felsic; c.
of these;
Etna, Sicily;
reaction of volcanic gases with the
c.
mafic; b. all
a
cooling and crystallization of lava flows and the: crystallization of
Jf.
Islands;
c.
Volcanic or extrusive igneous rocks form by the a.
the oceanic ridge belt.
divergent or a convergent plate margin.
St.
Mount Garibaldi, British Mount Adams, Washington; Columbia; d. Mount Mazama, Oregon. e.
the Hawaiian
Iceland; d.-ii!_ ihe circum-Pacific belt;
c.
The magma generated beneath spreading
d.
Helens has erupted during this century. It is: a Mount Hood, Oregon; b.^fe_ Mount Lassen, California;
spatter cones.
e.
mostly:
the
thick layers of pyroclastic materials; d.
dome.
active volcanoes are in:
the Mediterranean belt; b.
e.
repeated eruptions of cinder cones;
origin of lahars
5.
Most
Islands;
a.
lava
e.
y
a.
Basalt plateaus form as a result of:
basalt plateau;
a:
shield
c.
The volcanic conduit of a lava dome is most commonly plugged by: a mafic magma; b columnar joints; viscous, felsic magma; d. c. volcanic mudflows;
b.
an excellent example of
is
cinder cone;
b.
volcano; d. 14.
is
termed: d.
Oregon
in
jf caldera;
a.
lava flow with a surface of jagged blocks
a
Lake
13. Crater
lava flows;
b.
pillow lava.
e.
2.
nuee ardente;
volcanic bombs; d.
c.
Volcanism
34. Give a brief
summary
of the origin and development
of the Hawaiian Islands.
^
ADDITIONAL
READINGS
T. G., and V. Aylesworth. 1983. The Mount St. Helens disaster: What we've learned. New York: Franklin
Aylesworth,
Wans.
M.
1984. Volcanoes of the Earth. 2d ed. Austin, Tex.: University of Texas Press. Decker, R. W., and Decker, B. B. 1991. Mountains of fire: The nature of volcanoes. New York: Cambridge University Press. Bullard,
F.
Erickson,
1988. Volcanoes
J.
Summit, Harris,
Tab Books. 1976. Fire and
& earthquakes.
P.
W,
eruptions of
St.
ice:
The Cascade volcanoes.
Seattle,
al.
1981. Volcanoes of the world:
A
regional
and chronology of volcanism during the
last 10,000 Hutchison Ross Publishing Co. 1987 Eruptions of Mount St. Helens: Past, present,
years. Stroudsburg, Pa.:
and
I.
future. U.S. Geological Survey. I.,
C. Heliker, and T. L. Wright. 1987. Eruptions of Past, present, and future. U. S.
Hawaiian volcanoes: eds. 1981.
The 1980
Helens, Washington. United States
Geological Survey Professional Paper 1250. McClelland, L., T. Simkin, M. Summers, E. Nielsen, and C. Stein, eds. 1989. Global volcanism 1975-1985. Englewood
T
Cliffs, N.J.: Prentice-Hall.
gazetteer,
Tilling, R.
and D. R. Mullineaux,
Mount
16:73-99.
Simkin, T. et
Tilling, R.
Wash.: The Mountaineers.
Lipman,
Rampino, M. R., S. Self, and R. B. Strothers. 1988. Volcanic winters. Annual Review of Earth and Planetary Sciences
Blue Ridge
Pa.:
S. L.
The eruptions of Mt. Pelee, 1929-1932. Washington, D.C.: Carnegie Institution of Washington, Publication No. 458.
Perret, F. A. 1937.
Geological Survey.
Volcanoes and the Earth's interior. 1982. Readings from Scientific American. San Francisco, Calif.: H. Freeman and Co.
W
Wenkam,
R. 1987. The edge of fire: Volcano and earthquake country in western North America and Hawaii. San Francisco, Calif.: Sierra Club Books.
Additional Readings
109
CHAPTER
5
IGNEOUS ROCKS AND INTRUSIVE IGNEOUS ACTIVITY =*=
OUTLINE
PROLOGUE INTRODUCTION IGNEOUS ROCKS Textures
Composition
Bowen's Reaction
Series
Crystal Settling
Assimilation
Magma Mixing Classification
Ultramafic Rocks
Basalt-Gabbro
"V
Perspective 5-1: Ultramafic Lava Flows
Andesite-Diorite
Rhyolite-Granite
Other Igneous Rocks
INTRUSIVE IGNEOUS BODIES: PLUTONS Dikes and
Sills
Laccoliths
Volcanic Pipes and Necks Batholiths
and Stocks
MECHANICS OF BATHOLITH EMPLACEMENT PEGMATITES PLATE TECTONICS ACTIVITY "•' Perspective 5-2:
AND IGNEOUS
Complex Pegmatites
CHAPTER SUMMARY
Intrusive igneous rock exposed in Yosemite
National Park, California.
PROLOGUE
song, the rock grew to the present size of Devil's Tower. In both legends, the bear's attempts to reach the Indians
About 45
50 million years ago, several small masses of magma were to
intruded into the Earth's crust in
what
is
m above its base and stands more above the level of the nearby Belle Fourche River (Fig. 5-1). The tower is visible from 48 km away and served as a landmark for early travelers nearly
than 390
left
deep scratch marks
in the tower's
(Fig. 5-2).
Geologists have a less dramatic explanation for the The near vertical striations (the bear's
tower's origin.
now
northeastern Wyoming. These cooled and solidified, forming intrusive igneous rock bodies; the best known of these, Devil's Tower, was established as our first national monument by President Theodore Roosevelt in 1906. Devil's Tower is a remarkable landform; it rises
rocks
260
m
scratch marks) are simply the lines formed by the
columnar joints. As explained in columnar joints form in response to cooling and contraction in some intrusive igneous bodies and in some lava flows (see Fig. 4-13). Many of the columns are six sided, but columns with four, five, and seven sides occur as well. The larger columns measure about 2.5 m across. The pile of rubble at the intersections of
Chapter
4,
in the area.
Devil's
Tower and other similar, nearby bodies such Dakota are important in the
as Bear Butte in South
legends of the Cheyenne and Lakota Sioux Indians.
"^" FIGURE 5-2 An artist's rendition of a Cheyenne legend about the origin of Devil's Tower.
These native Americans call Devil's Tower Mateo Tepee, which means "Grizzly Bear Lodge." It was also called the "Bad God's Tower," and reportedly, "Devil's
Tower"
is
a translation of this phrase.
According to one Indian legend, the tower formed when the Great Spirit caused it to rise up from the ground, carrying with it several children who were trying to escape from a gigantic grizzly bear. Another legend
tells
woman who
of six brothers and a
also being pursued by a grizzly bear.
brother carried a small rock, and
"^ FIGURE
5-1
Devil's
Tower
in
were
The youngest
when he sang
Wyoming
a
exhibits
well-developed columnar jointing.
Prologue
111
is an accumulation of columns that have from the tower.
tower's base fallen
Geologists agree that Devil's
Tower
originated as a
small intrusive body, and that subsequent erosion
exposed
it
in its
present form.
body and the extent of
=*=
In
its
4,
we
modification by erosion are
discussed volcanism and the origin of
and and the origin of volcanic or extrusive igneous rocks. In this chapter we continue our discussion of igcalderas,
5-3
The rock
cycle,
Chapter 5
Igneous Rocks and Intrusive Igneous Activity
conduit and that
magma it
that solidified
has been
little
modified by erosion.
neous processes and activity in general, but here we are concerned primarily with the textures, composition, and classification of igneous rocks and with plutonic or intrusive igneous activity (Fig. 5-3).
Although volcanism and intrusive igneous
activity are
discussed in separate chapters, they are related
with emphasis on intrusive igneous rocks. Weathering
112
simply the remnant of the
in a volcanic
different volcanic landforms, such as volcanoes
FIGURE
Devil's
it is
The type of igneous
INTRODUCTION
Chapter
Some geologists believe that Tower is the eroded remnant of a more extensive body of intrusive rock, whereas others think debatable, however.
phenom-
ena. Volcanic rocks are widespread, but they probably
Rapid cooling
Slow cooling
Fine-grained
Coarse-grained
(aphanitic) texture
(phaneritic) texture
(a)
(b)
represent only a small portion of the total rocks formed by the cooling and crystallization of
magma. Most magma
cools below the Earth's surface
and forms bodrcs-of-Feek calEd^gfarcOTsrThTiame types of magmas~areinvolved in both volcanism and plutonism, although mafic magmas, because of their greater mobility, more commonly reach the surface. Plutons typically underlie areas of extensive volcanism and were the sources of the overlying lavas and pyroclastic materials. Furthermore, like volcanism,
most
plutonism occurs at or near plate margins.
^
IGNEOUS ROCKS
As previously discussed,
geologists recognize
two major
categories ofTgnebiis^rocks: (1) volcahic^br extrusive
igneouTrocksTwhich fofm^when
magma
extruded onto
the Earth's surface cools^and crystallizes or
when
pyro-
become consolidated, and (2) plutonic or jntrusive igneous rocks, which crystallize from clastic materials
magma E arth's The
intruded into jX_forrned in pla££_adthin the c rust (Fig. 5-3). process of crystallizing minerals from
magma
"^ FIGURE
The effect of the cooling rate of a magma on nucleation and growth of crystals: {a) Rapid 5-4
involves the formation of crystal nuclei and subsequent
cooling results in
The atoms in a magma are in constant motion, but when cooling begins, some atoms bond to form small groups, or nuclei, whose arrangement of atoms corresponds to the arrangement in min-
texture, {b)
growth of these
nuclei.
As other atoms in the liquid chemically bond to these nuclei, they do so in an ordered geometric arrangement, a nd the nuclei grow into crystalline mi n-
eral crystals.
eratgrains, the individual parucle_sjhaic£nipns^^rock.
During rapid cooling, the rate of nuclei formation exceeds the rate of growth, and an aggregate of many small grains results (Fig. 5-4a). With slow cooling, the rate of growth exceeds the rate of nucleation, so relatively large grains form (Fig. 5-4b).
small grains and a fine-grained results in a coarse-grained texture.
Rocks with porphyritic textures have a somewhat more complex cooling history. Such rocks have a combination of mineral grains of markedly different
The
mineral grains within
The
ture.
it
it
ing history of a
magma
or lava. For example, rapid cool-
ing, as occurs in lava flows or
some near-surface
sions, results in a fine-grained texture
In
intru-
termed aphanitic.
an aphanitic texture, individual mineral grains are
too small to be observed without magnification
(Fig.
line
are extruded onto the Earth's
cools rapidly, forming an aphanitic tex-
resulting igneous rock
eral grains (phenocrysts)
Several textures of igneous rocks are related to the cool-
would have
suspended
groundmass, and the rock would be characterized
as a porphyry.
A
may
its constituent atoms become arranged in the ordered, three-dimensional frameworks typical of minerals. As a
lava
cool so rapidly that
do not have time
to
consequence of such rapid cooling, anatura^gj ass such Even though obsidian is not
as obsid ian forms (Fig. 5-6a).
visible
or phaneritic texture have mineral grains that are easily without magnification (Fig. 5-5 b). Such large
rock;
mineral grains indicate slow cooling and generally an
of rock as an aggregate of grains of one or
intrusive origin; a phaneritic texture can develop in the
erals.
some
thick lava flows as well.
large min-
in a finely crystal-
5-5a). In contrast, igneous rocks with a coarse-grained
interiors of
sizes.
and the smaller ones are referred to as groundmass (Fig. 5-5c). Suppose that a magma begins cooling slowly as an intrusive body, and that some mineral-crystal nuclei form and begin to grow. Suppose further that before the magma has completely crystallized, the remaining liquid phase and solid larger grains are phenocrysts,
surface where
Textures
many
Slow cooling
composed of minerals, it is
it is still
one of the exceptions
considered to be igneous to the general definition
more minSuch substances that lack a crystalline structure are said to be amorphous, meaning without form. Igneous Rocks
113
PWp?SS£rTv
'"'
;^^H
Olivine
7\»
Dtion
Pyroxene
A
%
Amphibole
Reaction
m Biotite
mica Potassium feldspar
Muscovite
1
mica
"^ FIGURE series.
5-7
that
Bowen's reaction it
consists of a
discontinuous branch and a continuous branch.
Some magmas contain large amounts of water vapor and other gases. T hese gases may_be trappgdjn cool ing lava where th ey form nu m erou s small holes or^ cavities called^vesicles; rocks possessing
termed vesicular' as cinder cones are
numero us
vesicles ar e
Many
in vesicular basalt (Fig. 5-6b).
composed of fragments containing so
many
vesicles that the rock,
more
cavities
A
Note
than solid rock
known
(see
Table 4-2). The parent
magma
plays a significant
role in determining the mineral composition of igneous
However,
rocks.
it is
same magma
possible for the
yield different igneous rocks because
its
can change as a consequence of contamination or the sequence in which minerals crystallize.
as scoria, contains
Bowen's Reaction
(Fig. 5-6c).
pyroclastic or fragmental texture characterizes ig-
Series
During the early part of
this century,
Bowen hymagmas magma. He knew N.
neous rocksTormed by explosive volcanic activity. For example, ash may be discharged high lntcTthe atmo-
pothesized that mafic, intermediate, and
sphere and eventually
that minerals
cumulates;
if it is
settle to the surface
turned into solid rock,
where
it is
it
ac-
considered
to be a pyroclastic igneous rock.
(53—65%
all
derive from a parent mafic
do not
silica),
or
(45—52%
felsic
(>
65%
silica),
silica)
all crystallize
L.
felsic
simultaneously from
magma. Based on his observations and laboratory experiments, Bowen proposed a mechanism, now called a
magma
are characterized as mafic
intermediate
could
Bowen's reaction intermediate and
Composition
Magmas
to
composition
series, to felsic
account for the derivation of
magmas from
a basaltic (mafic)
Bowen's reaction series consists of two branches: a discontinuous branch and a continuous branch (Fig. 5-7). Crystallization of minerals occurs (Fig. 5-7).
Igneous Rocks
115
along both branches simultaneously, but for convenience
we
will discuss
them
Calcium-rich plagioclase crystallizes the
separately.
magma
first.
As cooling of
proceeds, calcium-rich plagioclase reacts
In the discontinuous branch, which contains only ferromagnesian minerals, one mineral changes to another over specific temperature ranges (Fig. 5-7). As the temperature decreases, a temperature range is reached in which a given mineral begins to crystallize. However, a previously formed mineral reacts with the remaining liquid magma (the melt) such that it forms the next mineral in the sequence. For example, olivine [(Mg, Fe) 2 Si0 3 is the first ferromagnesian mineral to crystallize. As the magma continues to cool, it reaches the temperature range at which pyroxene is stable; a reaction occurs between the olivine and the remaining melt, and pyrox-
with the melt, and plagioclase containing proportion-
ene forms.
potassium, aluminum, and silicon. These elements com-
]
A
between pyroxene and the melt as further cooling occurs, and the pyroxene structure is rearranged to form amphibole. Further cooling causes a reaction between the amphibole and the melt, and its structure is rearranged such that the sheet structure typical of biotite mica forms. Although the reactions just described tend to convert one mineral to the next in the series, the reactions are not always complete. For example, olivine might have a rim of pyroxene, indicating an incomplete reaction. In any case, by the time biotite has crystallized, essentially all magnesium and iron present in the original magma have been used up. similar reaction occurs
Plagioclase feldspars are the only minerals in the con-
tinuous branch of Bowen's reaction series
(Fig. 5-7).
more sodium crystallizes until all of the calcium and sodium are used up. In many cases, however, cooling is too rapid for a complete transformation from ately
calcium-rich to sodium-rich plagioclase to occur. Plagioclase
that
forming under these conditions it
gressively richer in
FIGURE
Photomicrograph of zoned plagioclase crystals. The magma in which these crystals formed cooled too quickly for a complete transformation from calcium-rich 5-8
They contain cores rich calcium surrounded by zones progressively richer in sodium. (Photo courtesy of R. V. Dietrich.)
zoned, meaning
sodium
(Fig. 5-8).
Magnesium and iron on the one hand and calcium and sodium on the other are used up as crystallization occurs along the two branches in Bowen's reaction ries.
Accordingly, any
magma
left
over
bine to form potassium feldspar (KAlSi 3
water pressure will form.
(Si0 2 ). quartz
is
se-
enriched in
8 ),
and
if
the
high, the sheet silicate muscovite mica
is
Any remaining magma
icon and oxygen
is
predominantly
sil-
and forms the mineral quartz The crystallization of potassium feldspar and
is
(silica)
not a true reaction
series,
however, because
they form independently rather than from a reaction of the orthoclase with the remaining melt.
Crystal Settling Crystal settling involves the physical separation of min-
by crystallization and gravitational settling (Fig. example, olivine, the first ferromagnesian mineral to form in the discontinuous branch of Bowen's reaction series, has a specific gravity greater than that of erals
5-9). For
the remaining "^"
is
has a calcium-rich core surrounded by zones pro-
magma and
thus tends to sink
in the melt. Accordingly, the
downward
remaining melt becomes
to sodium-rich plagioclase to occur. in
FIGURE 5-9 Differentiation by crystal settling. Early-formed ferromagnesian minerals have a specific gravity
""*"
greater than that of the
accumulate
116
Chapter 5
Igneous Rocks and Intrusive Igneous Activity
in the
magma
so they settle and
lower part of the
magma
chamber.
relatively rich in silica,
much
of the iron and
sodium, and potassium, because magnesium were removed when
minerals containing these elements crystallized.
Although crystal settling does occur in magmas, it does not do so on the scale envisioned by Bowen. In some thick, tabular, intrusive igneous bodies called sills, the
formed minerals
first
concentrated.
more
in the reaction series are
The lower
indeed
parts of these bodies contain
and pyroxene than the upper parts, which However, even in these bodies, crystal has yielded very little felsic magma from an orig-
olivine
are less mafic. settling
inal mafic
Assimilated pieces of country rock
magma.
magma could be derived on a large scale from magma as Bowen believed, there should be far more mafic magma than felsic magma. In order to yield If felsic
mafic
a particular
volume of granite
(a felsic
igneous rock),
magma would
about 10 times as much mafic
have to be
"^ FIGURE 5-10 As magma moves upward, fragments of country rock are dislodged and settle into the magma. If they have a lower melting temperature than the magma, they may be incorporated into the magma by assimilation. Incompletely assimilated pieces of country rock are inclusions.
present initially for crystal settling to yield the volume of granite in question.
If this
were
so, then mafic intrusive
much more common than
igneous rocks should be
ones. However, just the opposite
is
felsic
the case. Thus,
it
appears that mechanisms other than crystal settling
must account
we noted crust
in
and
for the large
Chapter
volume of
4, partial
silica-rich
felsic
magma. As
melting of mafic oceanic
an
ice
melted
magma richer in silica than magma rising through
limited
source rock. Furthermore,
the the
some felsic materials by become more enriched in silica.
continental crust can absorb
Assimilation
The composition of a magma can be changed by assimilation, a process whereby a magma reacts with preexisting rock, called country rock, with which it comes in contact (Fig. 5-10). The walls of a volcanic conduit or magma chamber are, of course, heated by the adjacent magma, which may reach temperatures of 1,300°C. Some of these rocks can be partly or completely melted,
magma
itself, and this would have the effect of magma. This process is analogous to placing
cube
in a
hot drink: the
cools, but only a very limited
sediments of continental margins
during subduction yields
assimilation and thus
from the
cooling the
in a
ice melts
and the drink
amount of
ice
can be
drink of a given volume. Likewise, only a
amount of rock can be assimilated by a magma, and that amount is usually insufficient to bring about a major compositional change. Neither crystal settling nor assimilation can produce a significant amount of felsic magma from a mafic one. However, both processes, if operating concurrently, can change the compositon of a mafic magma much more
"^ FIGURE in California.
5-11 Dark-colored inclusions in granitic-rock (Photo courtesy of David J. Matty.)
provided their melting temperature is less than that of the magma. Since the assimilated rocks seldom have the
same composition
magma The
is
as the
magma,
the compositon of the
changed.
fact that
assimilation occurs can be
demon-
strated by inclusions, incompletely melted pieces of rock that are fairly
clusions
common
within igneous rocks.
Many
in-
were simply wedged loose from the country
rock as the
magma
forced
its
way
into preexisting frac-
5-10 and 5-11). There is no doubt that assimilation occurs, but its effect on the bulk composition of most magmas must be slight. The reason is that the heat for melting must come tures (Figs.
Igneous Rocks
117
would be
ing Felsic
magma
a modified version of the parent
For example, suppose that a rising mafic
with a 5-12).
felsic
The
magma
magmas.
magma
mixes
same volume (Fig. "new" magma would have a more
resulting
of about the
intermediate composition.
Classification rnagma
Most igneous rocks features
"^ FIGURE
5-12 mix and produce a
from
Magma magma
either of the parent
mixing.
Two
5-13 that
all
pairs; the
members of
on the
basis of textural
Notice
(Fig. 5-13).
in
a pair have the
same composition
and granite are compositional and rhyolite are aphanitic and most commonly extrusive, whereas gabbro, diorite, and granite have phaneritic texand
with a composition different
magmas.
diorite,
and
rhyolite
(mineralogical) equivalents, but basalt, andesite,
tures that generally indicate an intrusive origin.
Some geologists believe many intermediate magmas
than either process acting alone.
is one way that form where oceanic lithosphere
that this
Figure
of the rocks, except peridotite, constitute
but different textures. Thus, basalt and gabbro, andesite
magmas
rising
are classified
and composition
is
subducted beneath
How-
continuum. The extrusive and intrusive members of each pair can usually be differentiated by texture, but many shallow intrusive
ever, all of these pairs exist in a textural
rocks have textures that cannot be readily distinguished
continental lithosphere.
from those of extrusive igneous rocks.
Magma Mixing
The igneous rocks shown
in Figure
5-13 are also
dif-
composition indicates that magmas of differing composition must be present. Thus, it seems likely that some of these magmas would come into contact and mix with
Reading across the chart from rhyolite to andesite, to basalt, for example, the relative proportions of nonferromagnesian and ferromagnesian minerals differ. However, the differences in
one another. If this is the case, we would expect that the composition of the magma resulting from magma mix-
composition are gradual so that a compositional continuum exists. In other words, there are rocks whose
The
fact that a single
ir FIGURE
5-13
of igneous rocks. illustrates relative
the chief mineral
common
volcano can erupt lavas of different
Classification
Diagram
Texture Aphanitic:
Rhyolite
Andesite
Basalt
Phaneritic:
Granite
Diorite
Gabbro
proportions of
components of
ferentiated by composition.
igneous rocks.
Darkness and specific gravity increase i
118
Chapter 5
Igneous Rocks and Intrusive Igneous Activity
s
"•'
FIGURE
5-14
The
ultramafic rock peridotite. (Photo
(a)
courtesy of Sue Monroe.)
compositions are intermediate between rhyolite and andesite,
and so on.
Ultramafic Rocks Ultramafic rocks are composed largely of ferromagnesian silicate minerals (Fig. 5-14). For example, the ultra-
mafic rock peridotite contains mostly olivine, lesser
amounts of pyroxene, and generally a little plagioclase feldspar (Fig. 5-13). Another ultramafic rock (pyroxenite) is composed predominantly of pyroxene. Because these minerals are dark colored, the rocks are generally
black or dark green. Peridotite
is
thought to be the rock
type composing the upper mantle (see Chapter 11), but ultramafic rocks are rare at the Earth's surface. In fact, ultramafic lava flows are rare in rocks younger than 2.5 billion years (see Perspective 5-1). Ultramafic rocks are
generally believed to have originated by concentration
of the early-formed ferromagnesian minerals that separated from mafic
magmas.
Basalt-Gabbro (45-52% silica) are the fine-grained and coarse-grained rocks, respectively, that crystallize from mafic magmas (Fig. 5-15). Thus, both have the same composition — mostly calcium-rich plagioclase and pyroxene, with smaller amounts of olivine and amphibBasalt and gabbro
ole (Fig. 5-13). Because they contain a large proportion
of ferromagnesian minerals, basalt and gabbro are dark colored; those that are porphyritic typically contain cal-
cium plagioclase or Basalt
is
(c)
olivine phenocrysts.
generally considered to be the
most
common
extrusive igneous rock. Extensive basalt lava flows were
"*»"
(b)
FIGURE
5-15
Mafic igneous rocks:
basalt lava flows near
Twin
Falls,
(a) basalt;
Idaho; and
(c)
gabbro.
(Photos courtesy of Sue Monroe.)
erupted in vast areas in Washington, Oregon, Idaho, and
Igneous Rocks
119
Perspective 5-1
ULTRAMAFIC LAVA FLOWS Geologists refer to the interval of geologic time from 3.8 to 2.5 billion years ago as the Archean Eon. Some of the most interesting rocks that formed during the Archean Eon are ultramaflc lava flows because such
flows are rare in younger rocks and none are forming at present. Archean ultramafic lava flows are generally parts of large,
complex associations of rocks known
as greenstone belts.
An
idealized greenstone belt
major rock units: the lower and middle units are dominated by volcanic rocks, and the upper unit is composed mostly of sedimentary rocks (Fig. 1). The lower volcanic units of some Archean
consists of three
greenstone belts contain ultramafic lava flows. Why did ultramafic lava flows occur during early
Earth history, but only rarely later? The answer is related to the heat produced within the Earth. When
it
formed, the Earth possessed a considerable amount of residual heat inherited from the formative
first
processes (see Chapter 2). As
we noted
earlier,
rock
is I
poor conductor of heat, so this primordial heat was slowly lost. Another source of heat within the Earth is a
related to the
phenomenon
of radioactive decay.
Recall from Chapter 3 that as the isotopes of
"**
some
FIGURE 2 The ratio of heat produced by radioactive decay during the past and at the present. The shaded band encloses the ratios according to different models. 8
d
a.
E
<3
O..C ra
c
c in _ o O
Q.
°8
Compl'
"^ FIGURE
1
their structure
Two adjacent greenstone belts showing and sequence of rock types. The lower some greenstone belts contain ultramafic
volcanic units of lava flows.
northern California
(Fig.
4-25 and 5-15b). Oceanic
is-
lands such as Iceland, the Galapagos, the Azores, and
Hawaiian Islands are composed mostly of basalt. Furthermore, the upper part of the oceanic crust is composed almost entirely of basalt. the
Gabbro
much
is
less
common
than basalt, at least in
where it can be easily observed. bodies of gabbro do occur in the conti-
the continental crust or
Small intrusive
nental crust, but less mafic intrusive rocks such as diorite
and granite are much more common. The lower part of the oceanic crust is composed of gabbro, however. Andesite-Diorite
Magmas
intermediate in composition (53-65%
silica)
form andesite and diorite, which are compositionally equivalent fine- and coarse-grained igneous rocks (Fig. 5-16). Andesite and diorite are composed predominantly of plagioclase feldspar, with the typical ferromagnesian component being amphibole or biotite (Fig. 5-13). Andesite is generally medium to dark gray, but diorite has a salt and pepper appearance because of its white to light gray plagioclase and dark ferromagnecrystallize to
sian minerals (Fig. 5-16).
Andesite
is
a
common
from lavas erupted
extrusive igneous rock formed
in volcanic chains
at
convergent
The volcanoes of the Andes Mountains of South America and the Cascade Range in the northwestern United States are composed in part of andesite. Intrusive bodies composed of diorite are fairly common in plate margins.
However, diorite is not nearly as abundant as granitic rocks and is uncommon outside the areas where andesites occur. the continental crust.
(b)
"^ FIGURE and
5-16
(b) diorite.
Intermediate igneous rocks: (a) andesite (Photos courtesy of Sue Monroe.)
Rhyolite-Granite Rbyolite and granite (>
magmas and
sic
65%
silica) crystallize
from
are therefore silica-rich rocks
fel-
(Fig.
and granite consist largely of potassium feldspar, sodium-rich plagioclase, and quartz, with perhaps some biotite and rarely amphibole (Fig. 5-13). Because nonferromagnesian minerals predominate, rhyolite and granite are generally light colored. Rhyolite is fine grained, although most often it contains phenocrysts of potassium feldspar or quartz, and granite is coarse grained. Granite porphyry is also fairly common. 5-17). Rhyolite
Rhyolite lava flows are
much
less
common
than
andesite and basalt flows. Recall that the greatest control if
of viscosity in a
a felsic
magma
the pressure
on
it
magma
is
the silica content. Thus,
rises to the surface,
decreases,
it
and gases
begins to cool, are released ex-
plosively, usually yielding rhyolitic pyroclastic particles.
The
rhyolitic lava flows that
highly viscous and thus
Among
do occur are thick and
move only
geologists, granite has
short distances.
come
to
mean any
coarsely crystalline igneous rock with a composition
corresponding to that of the Strictly speaking,
not
all
field
shown
rocks in this
in
Figure 5-13.
field are granites.
For example, a rock with a composition close to the line separating granite and diorite is usually called granodiorite.
To avoid
the confusion that might result from
introducing more rock names,
we
will follow the prac-
of referring to rocks to the
left
of the granite-diorite
tice
line in Figure
5-13 as granitic.
Granitic rocks are by far the most
common
intrusive
igneous rocks, although they are restricted to the continents.
Most
granitic rocks
were intruded
at or near con-
vergent plate margins during episodes of mountain
Igneous Rocks
121
r
forms as particles erupted from explosive volcanoes. If pumice falls into water, it can be carried great distances because it is so porous and light that it floats.
» INTRUSIVE IGNEOUS PLUTONS
BODIES:
Unlike volcanism and the origin of extrusive or volcanic igneous rocks, which can be observed, intrusive igneous activity
can be studied only
indirectly. Intrusive
when magma
bodies called plutons form
igneous
cools and crys-
within the Earth's crust (Fig. 5-20). Although
tallizes
plutons can be directly observed after erosion has ex-
posed them
we cannot
at the surface,
duplicate the con-
ditions that exist deep in the crust, except in small-scale
laboratory experiments. Thus, geologists face a greater challenge in interpreting the mechanisms whereby plutons originate than in studying the origins of extrusive
igneous rocks. Several types of plutons are recognized,
all
of which
are defined by their geometry (three-dimensional shape)
and
their relationship to the country
Geometrically, plutons
may
rock
(Fig. 5-20).
be characterized as massive
or irregular, tabular, cylindrical, or
mushroom
shaped.
Plutons are also described as concordant or discordant.
A
concordant pluton, such as a
has boundaries that
sill,
are parallel to the layering in the country rock.
A
dis-
cordant pluton, such as a dike, has boundaries that cut across the layering of the country rock (Fig. 5-20).
Dikes and
Sills
Both dikes and
sills
are tabular or sheetlike plutons, but
dikes are discordant whereas 5-20). Dikes are
Many
common
sills
are concordant (Fig.
intrusive features (Fig. 5-21).
are small bodies measuring
1
or 2
they range from a few centimeters to thick.
m
across, but
more than 100
m
Dikes are emplaced within preexisting zones of
weakness where fractures already exist or where the fluid pressure is great enough for them to form their own fractures during emplacement. Erosion of the Hawaiian volcanoes exposes dikes in rift
zones, the large fractures that cut across these vol-
canoes.
The Columbia River
ter 4) issued
from long
basalts (discussed in
fissures,
and the
Chap-
magma
cones as on
Mount Etna,
marked by rows of
Italy.
Some
fissure eruptions are underlain
^ FIGURE
5-19
(a)
Obsidian and
(b)
pumice. (Photos
courtesy of Sue Monroe.)
that
cooled in the fissures formed dikes. In some cases, dikes that reach the surface are
(b)
spatter
of the large historic
by dikes; for example,
dikes underlie both the Laki fissure eruption of 1783 in
Iceland
where erup300 km long. are concordant plutons, many of which are a
and the Eldgja
tions occurred in a.d. Sills
meter or
less thick,
(Fig. 5-20).
fissure, also in Iceland,
950 from
a fissure
although some are
For example, the
Whin
Sill
much
thicker
of northern En-
Intrusive Igneous Bodies: Plutons
123
Cinder cone
Volcanic neck
Composite volcano
Volcanic pipe
Stock
Laccolith
"^ FIGURE
S-20
-^ FIGURE
5-21
Block diagram showing the various types of plutons. Notice that some of these plutons cut across the layering in the country rock and are thus discordant, whereas others parallel the layering and are concordant.
the rock layers
because
it
is
The dark layer cutting diagonally The other dark layer is a sill
across
a dike.
parallels the layering.
is up to 100 m thick, underlies an area of 2 5,000 km and has an estimated volume of 215 km 3 Probably the best-known sill in the United States is the Palisades sill that forms the Palisades along the west side of the Hudson River in New York and New Jersey (Fig. 5-22). It is exposed for 60 km along the river and is up to 300 m thick. Most sills have been intruded into sedimentary rocks.
gland, which at least .
Many
of these parallel the layering for
some
distance
and then cut through these strata in abrupt steps. Thus, sills many change laterally into dikes. Eroded volcanoes also reveal that
sills
are
volcanic rocks. In fact,
commonly injected into piles of some of the inflation of volca-
noes preceding eruptions of
sills
In contrast to dikes, sills
124
Chapter 5
Igneous Rocks and Intrusive Igneous Activity
may
be caused by the injection
(see Perspective 4-3).
are emplaced
when
which follow zones of weakness, the fluid pressure
is
so great that
the intruding
magma
actually
lifts
Because emplacement requires
the overlying rocks.
fluid pressure
exceeding
the force exerted by the weight of the overlying rocks, sills
are typically shallow intrusive bodies.
Laccoliths Laccoliths are similar to
sills
in that
they are concor-
dant, but instead of being tabular, they have a
roomlike geometry floor
(Fig. 5-20).
and are domed up
They tend
mush-
to have a flat
in their central part. Like
sills,
laccoliths are rather shallow intrusive bodies that actu-
up the overlying strata. In this case, however, the upward over the pluton (Fig. 5-20). Most laccoliths are rather small bodies. The best-known laccoliths in this country are in the Henry Mountains of ally
lift
strata are arched
^ FIGURE
5-22
The
Palisades
sill
of the
Hudson
River.
southeastern Utah.
Nevada
Volcanic Pipes and Necks The conduit connecting the crater of a volcano with an underlying magma chamber is a volcanic pipe (Fig. 5-20). In other words,
magma
it is
the structure through
When
rises to the surface.
eroded as
it is
acids.
The volcanic mountain
magma
it is
which
a volcano ceases to
attacked by water, gases, and
erupt,
batholith of California (Fig. 5-25)
was em-
placed over a period of millions of years during a
eventually erodes away,
mountain-building episode
known
as
the
Nevadan Nevada
orogeny. Later uplift and erosion of the Sierra
exposed this huge composite pluton at the Earth's surface. Other large batholiths in North America include the Idaho batholith and the Coast Range batholith in British Columbia, Canada.
is more resisand erosion and is often left as an erosional remnant, a volcanic neck (Fig. 5-20). A num-
but the
that solidified in the pipe
tant to weathering
ber of volcanic necks are present in the southwestern
United States, especially (Fig. 5-23),
in
Arizona and
FIGURE
5-23
A
volcanic neck in northern Arizona.
New Mexico
and others are recognized elsewhere.
Batholiths and Stocks Batholiths are the largest intrusive bodies. By definition 2
km ofTurface area, and most are much larger than this (Fig. 5-20). Stocks have the same general features as batholiths but are smaller, although some stocks are simply the exposed parts of they mustliave at least 100
much
larger intrusions, that once revealed by erosion
are batholiths (Fig. 5-24). Batholiths are generally dis-
cordant, and most consist of multiple intrusions. In is a large composite body produced by repeated, voluminous intrusions of magma in the same area. The coastal batholith of Peru, for example, was emplaced over 60 to 70 million years and consists of perhaps as many as 800 individual plutons. The igneous rocks composing batholiths are mostly
other words, a batholith
granitic,
although diorite
may
also occur.
Most batho-
emplaced near continental margins during episodes of mountain building. For example, the Sierra
liths are
Intrusive Igneous Bodies: Plutons
125
during the early history of the Earth, and the mountains
were once present have long since been eroded
that
away. Thus, the remaining rocks represent the eroded
"roots" of these ancient mountains.
^ MECHANICS OF BATHOLITH EMPLACEMENT Geologists realized long ago that the emplacement of '""'
FIGURE
5-24 Some stocks are small intrusive bodies, but others are simply exposed parts of larger plutons. In this example, erosion to the dashed line would expose a batholith.
A number
of mineral resources occur in rocks of
and in the country rocks adjacent example, silica-rich igneous rocks, such as granite, are the primary source of gold, which forms from mineral-rich solutions moving through cracks and batholiths and stocks to them. For
fractures of the igneous body.
Butte,
Montana,
The copper
deposits at
are in rocks near the margins of the
Near mined from the mineralized rocks adjacent to the Bingham stock, a composite pluton composed of granite and granite porphyry. As noted above, batholiths appear to be emplaced in the cores of mountain ranges that resulted from plate collisions. However, large exposures of granitic rocks also occur within the interiors of continents where mountains are absent. For example, a large area in Can-
granitic rocks of the Boulder batholith (Fig. 5-26). Salt
ada
Lake
is
City, Utah,
copper
is
underlain by extensive granitic rocks as well as by
other rock types. These granites were apparently em-
placed during mountain-building episodes that occurred
"^ FIGURE
5-25
View of part of
the Sierra
Chapter 5
is,
what hap-
pened to the rock that formerly occupied the space now occupied by a granite batholith? One solution to this space problem was to propose that no displacement had occurred, but rather that the granite had been formed in place by alteration of the country rock through a process called granitization. According to this view, granite did not originate as a magma, but rather from hot, ionrich solutions that simply altered the country rock and transformed it into granite. Granitization is a solid-state phenomenon so it is essentially an extreme type of metamorphism (see Chapter 8). Many granites show clear evidence of an intrusive origin. For example, the contact between these granites and the adjacent country rock is sharp rather than gradational, and elongated mineral crystals are commonly oriented parallel with the contact (Fig. 5-27). nitic
Some
Igneous Rocks and Intrusive Igneous Activity
gra-
rocks lack sharp contacts, however, and gradually
change in character until they resemble the adjacent country rocks. Some of these have likely been altered by granitization.
Most
geologists think that only small
formed by this process, and that cannot account for the huge granite batholiths of the world. These geologists believe an igneous origin for granite is clear, but then they must deal with the space quantities of granite are it
Nevada
batholith in Yosemite National Park, California.
126
batholiths posed a space problem; that
"*»"
FIGURE
5-26
A
copper mine
in Butte,
Montana.
"**
FIGURE
between
5-27
this granite
indicates that
it
A
sharp rather than gradational contact
and the dark-colored country rock
had an
intrusive origin.
The
granite also
contains an inclusion of the country rock.
One
problem.
solution, proposed by
some
geologists,
way
that these large igneous bodies melted their crust. In other
try
is
into the
words, they simply assimilated the coun-
rock as they
moved upward
(Fig. 5-10).
The presence
"^"
FIGURE
batholith.
5-28 Emplacement of a hypothetical As the magma rises, it shoulders aside and
deforms the country rock.
of inclusions, especially near the tops of such intrusive bodies, indicates that assimilation does occur. Nevertheless, as
we noted previously, assimilation is a limited magma is cooled as country rock is as-
process because
similated; calculations indicate that far too available in a
magma
little
heat
to assimilate the necessarily
is
huge
quantities of country rock.
Most
geologists
magma and
agree that batholiths were em-
magma, being less dense rock from which it was derived, moved upward
placed as
than the
now
that the
toward the surface. Recall, however, that granite is derived from viscous felsic magma and, thus, it rises slowly. It appears that the magma deforms and shoulders aside the country rock, and as it rises further, some
of the country rock
fills
the space beneath the
magma
A somewhat
analogous situation was discovered in which large masses of sedimentary rock called rock salt rise through the overlying rocks to form (Fig. 5-28).
salt
domes.
Salt
domes are recognized
in several areas of the
world
including the Gulf Coast of the United States. Layers of salt exist at some depth, but salt is less dense than most other types of rock materials. Thus, when under pressure, it rises toward the surface even though it remains solid, and as it moves upward, it pushes aside and deforms the country rock (Fig. 5-29). Natural examples
rock
"^ FIGURE 5-29 Three stages in the origin of a salt dome. Rock salt is a low-density sedimentary rock that {a) when deeply buried (b) tends to rise toward the surface, (c) pushing aside and deforming the country rock and forming a dome. Salt domes are thought to originate in much the same manner as batholiths are intruded into the Earth's crust.
Mechanics of Batholith Emplacement
127
H9
Lava Dikes
-
Dikes
•^ FIGURE
Gabbro
Magma Oceanic crust
Mantle
complex pegmatites
eral others are
Some complex pegmatites
(see Perspective 5-2).
few of which are important economically. In addition, several gem minerals such as emerald and aquamarine, both of which are varieties of the silicate mineral beryl, and tourmaline are found in some pegmatites. Many rare minerals of lesser value and well-
nuclei liquid
pegmatites are similar to those processes
in
crystallize
However, some do form, and because the appropriate ions in the can move easily and attach themselves to a grow-
nity to
grow
to very large sizes (see Perspective 5-2).
» PLATE TECTONICS AND IGNEOUS ACTIVITY
magma,
In Chapter 4
vapor phase from inhibits the formation of
we
discussed plate tectonics and the oc-
currence of volcanism at spreading ridges (see Fig. 4-28)
critical difference: the
which pegmatites
tetrahedra are inhibited from forming
ing crystal, individual mineral grains have the opportu-
formed crystals of common minerals, such as quartz, are also mined and sold to collectors and museums. The formation and growth of mineral-crystal nuclei but with one
silica
Intrusive
the ordered configuration of minerals.
species, a
in
The
nuclei.
contain 300 different mineral
5-32
igneous activity at a spreading ridge. The oceanic crust is composed largely of vertical dikes of basaltic composition and gabbro that appears to have crystallized in the upper part of a magma chamber.
and subduction zones
(see Fig. 4-31).
^ FIGURE
Volcano
magma and Magma
Trench
Lithosphere-
Continental crust
f-Lithosphere
v
Oceanic crust
Asthenosphere v
Upper mantle
5-33
the
Plutons are also
Generation of
emplacement of
plutons at a convergent plate margin.
1
kfili 11
li^i
"«*
FIGURE 3 Giant spodumene crystals in the Etta pegmatite in the Black Hills of South long. Dakota. The crystal above the miner's head measures more than 12
m
Jefferson, Lincoln,
Many
Rushmore were carved
for various resources.
Granite
Etta pegmatite, which contains crystals of spodumene,
(Fig. 2).
and Theodore Roosevelt on Mount into rocks of the Harney Peak These pegmatites formed about 1.7 during the Late Proterozoic Eon,
billion years ago,
when
was emplaced as a composite pluton of numerous sills and dikes. Subsequent
the granite
consisting
during the Late Cretaceous Period resulted in erosion of the overlying rocks, thus exposing the uplift
granite
and
Most
its
associated pegmatities.
of the Black Hills pegmatites are simple, with
compositions closely resembling that of the Harney Peak Granite; about 1% are complex pegmatites.
form of pillow lavas
Magmas
(see Fig. 4-14).
generated by partial melting of mafic oce-
of these complex pegmatites have been mined
One
of the best
a lithium-bearing silicate mineral, that
m
is
the
commonly
spodumene crystals are the size of large logs, and one was more than 14 m long (Fig. 3)! Micas and tin were originally mined from the Etta pegmatite, and for many years it was a measure
1 to
3
long; the larger
-
major producer of lithium. It closed in 1960, however, because more economical sources of lithium are available from arid region lake deposits.
(see Fig. 4-31).
Some
of this
magma
is
erupted at the
surface and forms the typical large composite volcanoes
anic crust and silica-rich continental margin sediments
that characterize such plate margins.
margins where subduction takes place are mostly intermediate and felsic in composition
ever,
at convergent plate
known
Much
of
it,
how-
simply cools at depth as large plutons, especially
batholiths (Fig. 5-33, page 129).
Plate Tectonics
and Igneous Activity
131
*=
CHAPTER SUMMARY magma and
9.
2.
small crystal nuclei form and grow. Volcanic rocks generally have aphanitic textures because of their rapid cooling, whereas slow cooling
lava
laccoliths
(tabular geometry, concordant);
(mushroom shaped, concordant); and
batholiths
and stocks
(irregular geometry,
discordant). 10.
and phaneritic textures characterize plutonic rocks.
By
definition batholiths
must have
at least
pyroclastic.
period of time.
is determined by the composition of the parent magma. possible, however, for an individual magma to
smaller.
Under
11.
series, consists
series
can
from a vapor-rich
phase left over after the crystallization of granite accounts for the very large mineral crystals in pegmatites. in
form first in and become concentrated near the base of a magma chamber or intrusive body. Such settling of iron- and magnesium-rich minerals causes a chemical change Bowen's reaction
overall composition similar
to that of granite. Crystallization
14.
the crystal structure.
The ferromagnesian minerals
magma moves upward and
most of which have an
involves changes only in
sodium replaces calcium
felsic
13. Pegmatites are very coarse-grained igneous rocks,
sequence. plagioclase feldspar as
geologists think that granite batholiths are
The upward movement of rock salt and the formation of salt domes provide a somewhat analogous situation.
ferromagnesian minerals, each of which reacts with the melt to form the next mineral in the
The continuous branch
Some
shoulders aside and deforms the country rock.
of a
discontinuous branch and a continuous branch. a. The discontinuous branch contains only
b.
batholiths appear to have formed in the cores
emplaced when
within specific temperature ranges. This sequence,
Bowen's reaction
plutons emplaced over a long
building. 12.
yields a sequence of different minerals that are stable
called
Most
batholiths are large composite bodies
many
of mountain ranges during episodes of mountain
It
magma
ideal cooling conditions, a mafic
Many
consisting of
The composition of igneous rocks is
remaining melt. can be changed compositionally when it assimilates country rock, but this process usually has only a limited effect. Magma mixing may also bring about compositional changes in magmas. Most igneous rocks are classified on the basis of their textures and composition. Two fundamental groups of igneous rocks are recognized: volcanic or
in areas where volcanism occurs, such as at spreading ridges and above subduction
zones.
that
settle
Most plutons form
15. Ancient batholiths within the interiors of continents
where no mountains are present probably represent the eroded "roots" of former mountain ranges.
in the
7.
A magma
aphanitic
natural glass
assimilation
pegmatite phaneritic
batholith
Bowen's reaction
most of which are aphanitic, and plutonic or intrusive rocks, most of which are
concordant country rock
phenocryst pluton plutonic rock
phaneritic.
crystal settling
porphyritic
dike
pyroclastic
discordant
sill
volcanic rock.
extrusive igneous rock
stock
Common
granitization
stoping
igneous rock
vesicle
Plutons are igneous bodies that formed in place or
inclusion
volcanic neck
were intruded into the Earth's
intrusive igneous rock
volcanic pipe
plutons are classified by their geometry and whether
laccolith
volcanic rock
they are concordant or discordant.
magma
Common andesite,
b.
volcanic rocks include rhyolite,
and
basalt. Tuff
is
another
common
plutonic rocks include granite, diorite,
and gabbro.
132
IMPORTANT TERMS
extrusive rocks,
a.
8.
km 2
of surface area; stocks are similar to batholiths but
yield igneous rocks of differing compositions.
6.
100
Igneous rocks with a porphyritic texture have mineral crystals of markedly different sizes. Other igneous rock textures include vesicular and
largely
5.
sills
volcanic necks (cylindrical geometry, discordant);
when
Minerals crystallize from
4.
plutons include dikes (tabular geometry,
discordant);
1.
3.
Common
Chapter 5
crust.
Various types of
Igneous Rocks and Intrusive Igneous Activity
mixing
series
^
REVIEW QUESTIONS
13.
An
igneous rock possessing mineral grains large to be seen without magnification is said to have a texture.
enough 1.
The
first
minerals to crystallize from a mafic
magma
are: a.
quartz and potassium feldspar;
b.
calcium-rich plagioclase and olivine;
c.
biotite
pyroxene; 2.
3.
a.
basalt; b.
d.
obsidian;
granite;
of a concordant pluton having a tabular
d.
lava flow;
volcanic neck;
c.
are essentially:
magma
a.
sodium-rich plagioclase;
c.
quartz; d.
olivine;
is
likely to
The process whereby
by crystal
11.
settling?
reacts with
crystal differentiation; b.
plutonism;
e.
assimilation.
magma
d.
and
granitization;
Why
Volcanism" in Chapter 21. What is a welded tuff? 22. How do dikes and sills emplaced?
pyroclastic; b.
c
intermediate; d.
ultramafic; felsic; e.
mafic.
the following pairs of igneous rocks have
same mineral composition? granite-tuff; b.
c.
pumice-diorite; d.
e.
peridotite-andesite.
the following stock;
b.
a.
sill;
d.
dike;
basalt-gabbro;
is
differ?
How
is
each
a volcanic neck.
25.
What
how
are pegmatites? Explain
batholiths form.
why some
pegmatites
contain very large mineral crystals.
or are these completely separate phenomena? 27. In
what
plate tectonic settings does intrusive igneous
activity occur?
^
ADDITIONAL READINGS
Baker, D.
c.
S.
1983. Igneous rocks. Englewood
Cliffs, N.J.:
M.
G. 1982. Igneous and metamorphic petrology. San W. H. Freeman and Co. Dietrich, R. V, and B. J. Skinner. 1979. Rocks and rock Sons. minerals. New York: John Wiley Dietrich, R. V. and R. Wicander. 1983. Minerals, rocks, and Francisco, Calif.:
&
fossils.
Ernst,
W
New
York: John Wiley &c Sons. G. 1969. Earth materials. Englewood
Cliffs, N.J.:
Hall, A. 1987. Igneous petrology. Essex, England: Scientific
Hess,
a concordant pluton?
volcanic neck;
batholith.
e.
12. Batholiths are
and
4.)
Prentice-Hall.
andesite-rhyolite;
a.
composed of gabbro and
24. Briefly explain where and
Best,
a.
the oceanic crust
is
Prentice-Hall.
mixing;
Igneous rocks composed largely of ferromagnesian minerals are characterized as:
Which of
and dissimilar?
is:
a.
the
how does it form? how are granite and diorite
potassium
e.
c.
Which of
form
and
a natural glass,
is
20.
muscovite;
b.
magma
a
incorporates preexisting rock
10.
crystals
contrast the continuous and discontinuous branches of Bowen's reaction series. 19. Describe how the composition of a magma can be changed by crystal settling; by assimilation. Cite
be
feldspar.
9.
whereby mineral
are volcanic rocks generally
26. Are extrusive and intrusive igneous activity related,
a tuff.
e.
of the following minerals
separated from a mafic
8.
Why
Compare and
formed by explosive volcanism;
c.
a porphyry;
d.
differ?
23. Describe the sequence of events in the formation of
cylindrical plutons.
Which
What
similar
igneous rock possessing a combination of
mineral grains with markedly different sizes is: a. a natural glass; b. the product of very
7.
do they
vesicular.
e.
two major kinds of igneous rocks?
basalt? (Refer to the section "Plate Tectonics
thick light-colored gabbro; b. very accumulations of pyroclastic materials; c. rhyolite porphyry; coarse-grained granite; d.
rapid cooling;
How
18.
dike.
e.
Most pegmatites
An
are the
evidence indicating that both of these processes occur. batholith:
sill; b.
e.
What
17. In terms of composition,
is a:
a.
a
6.
phaneritic;
aphanitic? 16.
specific gravity.
e.
An example
aphanitic;
fragmental; d.
and grow.
pumice;
c.
rhyolite.
e.
porphyritic; b.
c.
15. Describe the process
is:
Volcanic rocks can usually be distinguished from plutonic rocks by: composition; c. irona. color; b. the size of their mineral magnesium content; d.
geometry
5.
amphibole and and muscovite; d. andesite and basalt.
The most common aphanitic igneous rock
grains; 4.
e.
14.
a.
composed mostly of what type of
rock?
P.
Longman
and Technical.
C. 1989. Origins of igneous rocks. Cambridge, Mass.:
Harvard University
Press.
McBirney, A. R. 1984. Igneous petrology. San Francisco, Calif.: Freeman, Cooper and Co. MacKenzie, W. S., C. H. Donaldson, and C. Guilford. 1982. Atlas of igneous rocks and their textures. New York: Halsted Press.
a.
granitic; b.
gabbro;
d.
andesite;
peridotite.
e.
c.
basalt;
Middlemost, E. A. K. 1985. Magma and magmatic rocks. London: Longman Group.
Additional Readings
133
CHAPTER
6
WEATHERING, EROSION, AND SOIL W OUTLINE PROLOGUE INTRODUCTION MECHANICAL WEATHERING Frost Action Pressure Release
Thermal Expansion and Contraction
"^
Perspective 6-1: Bursting
Rocks and Sheet
Joints Activities of
Organisms
CHEMICAL WEATHERING Solution
Oxidation Hydrolysis **" Perspective 6-2:
Acid Rain
FACTORS CONTROLLING THE RATE OF
CHEMICAL WEATHERING Particle Size
Climate Parent Material
SOIL
THE SOIL PROFILE FACTORS CONTROLLING FORMATION
SOIL
Climate Parent Material
Organic Activity ~^~
Guest Essay: Environmental Geology: Sustaining the Earth
Relief
and Slope
Time
SOIL EROSION
WEATHERING AND MINERAL RESOURCES CHAPTER SUMMARY
Weathering and erosion of sedimentary rocks is responsible for the scenery in Bryce Canyon National Park, Utah.
PROLOGUE ^j^JlV^jj
The stock market crash of 1929 ushered in the Great Depression, a
when millions of people were unemployed and many had no means to provide food and shelter. time
Urban areas were
affected
most severely by the
depression, but rural areas suffered as well, especially
during the great drought of the 1930s. Prior to the 1930s, farmers had enjoyed a degree of success
World War I, the price of wheat soared, and after the war when Europe was recovering, the government subsidized wheat prices. High prices and mechanized farming practices resulted in more and more land being tilled. Even the weather cooperated, and land in the western United States that would otherwise have been marginally productive was plowed. Deep-rooted prairie grasses unparalleled in U.S. history. During
that held the soil in place
where it settled on New York City, Washington, D.C., and other eastern cities as well as on ships some 480 km out in the Atlantic Ocean. The Soil Conservation Service reported that dust storms of regional extent
occurred on 140 occasions during 1936 and 1937.
Dust was everywhere.
It
seeped into houses,
suffocated wild animals and livestock, and adversely
human
affected
healt
The dust was, of
,
course, the material derived
the tilled lands; in other words,
many
regions
was not
m jch
from
of the soil in
was simply blown away. Blowing dust
the only problem; sand piled up along fences,
drifted against houses
and farm machinery, and
covered what otherwise might have been productive
FIGURE 6-1 The Dust Bowl of the 1930s. Drought conditions extended far beyond the boundaries shown here, but this area was particularly hard hit by drought and wind erosion.
were replaced by
shallow-rooted wheat.
Beginning
in
about 1930, drought conditions
two and Vermont— were not drought-stricken. Drought conditions varied from moderate to severe, and the consequences of the drought were particularly severe in the southern Great Plains. Some rain fell, but in amounts insufficient to
prevailed throughout the country; only states
— Maine
maintain agricultural production. And since the land, even marginal land, had been tilled, the native vegetation was no longer available to keep the soil
from blowing away. And blow away it did — in huge quantities. Nothing stopped the wind from removing large quantities of top soil.
A was and
large region in the southern Great Plains that particularly hard hit by the drought, dust storms,
came to be known as the Dust Bowl. boundaries were not well defined, it included parts of Kansas, Colorado, and New Mexico as well as the panhandles of Oklahoma and Texas soil
erosion
Although
its
Bowl and its less affected more than 400,000 km Dust storms became common during the 1930s, and some reached phenomenal sizes (Fig. 6-2). One of the largest storms occurred in 1934 and covered more (Fig. 6-1);
together the Dust
fringe area covered
!
2
than 3.5 million km It lifted dust nearly 5 km into the air, obscured the sky over large parts of six states, and blew hundreds of millions of tons of soil eastward .
Explanation
Severe wind erosion in
1935-36
Severe wind erosion in
1938
Severe wind erosion in 1940
Most severe wind erosion in 1935-38
Prologue
135
^ FIGURE
6-2 The huge dust storm of April 14, 1935, also known as Black Sunday, photographed at Hugoton, Kansas.
soils.
Agricultural production
fell
precipitously in the
Dust Bowl, farmers could not meet their mortgage payments, and by 1935 tens of thousands were leaving. Many of these people went west to California and became the migrant farm workers immortalized in John Steinbeck's novel The Grapes of Wrath. The Dust Bowl was an economic disaster of great magnitude. Droughts had stricken the southern Great Plains before, and have done so since, but the drought of the 1930s was especially severe. Political and economic factors also contributed to the disaster. Due in part to the artificially inflated wheat prices, many
^
farmers were deeply in debt— mostly because they had purchased farm machinery in order to produce more
and
benefit
from the high
marginal land, and employed few, conservation measures.
economic
if
any, soil
If the Dust Bowl has a bright side, it is that the government, farmers, and the public in general no
longer take soil for granted or regard
it
as a substance
no nurturing. In addition, a number of conservation methods developed then have now become standard in agriculture. that needs
the weathered materials
INTRODUCTION
prices. Feeling
pressure because of their huge debts, they tilled
water,
wind
is
known
as erosion.
(see the Prologue), or glaciers
soil
Running
commonly
Weathering is the physical breakdown (disintegration) and chemical alteration (decomposition) of rocks and
transport the weathered materials elsewhere where they
minerals at or near the Earth's surface.
tary rock (Fig. 6-3).
It is
the process
whereby rocks and minerals are physically and chemically altered such that they are more nearly in equilibrium with a new set of environmental conditions. For example, many rocks form within the Earth's crust where little or no water or oxygen is present and where temperatures, pressures, or both are high. At or near the surface, however, the rocks are exposed to low temperatures and pressures and are attacked by atmospheric gases, water, acids, and organisms. Geologists
are
interested
weathering because cycle (Fig. 6-3).
weathered,
is
it
is
an
in
the
phenomenon
of
essential part of the rock
The parent
material, or rock being
broken down into smaller
pieces,
and
some of its constituent minerals are dissolved or altered and removed from the weathering site. The removal of 136
Chapter 6
Weathering, Erosion, and Soil
which may become sedimenWhether they are eroded or not,
are deposited as sediment,
weathered rock materials can also be further modified to form a soil. Thus, weathering provides the raw materials for both sedimentary rocks and soils. Weathering is also important in the origin of some mineral resources such as aluminum ores, and it is responsible for the enrichment of other deposits of economic importance. Weathering is such a pervasive phenomenon that many people take it for granted or completely overlook it. Nevertheless, it occurs continuously although its rate and impact vary from area to area or even within the same area. Rocks are not homogeneous throughout; because they vary in structure and composition, some
weather more rapidly than others. This weathering, as weathering at different rates
is
differential called, yields
uneven surfaces. In Bryce Canyon National Park
in Utah,
FIGURE
differential
6-3
The rock
cycle,
with emphasis on weathering.
weathering and erosion of sedimentary rocks
cut by intersecting fractures have produced oddly shaped
rock formations in
road
Rocks in natural exposures and mines, and tombstones disintegrate
(Fig. 6-4).
cuts, quarries,
"^ FIGURE
6-4 The scenery of Bryce Canyon National in Utah is the result of differential weathering and erosion of sedimentary rocks.
Park
and decompose, as do the rocklike materials of roadways, sidewalks, and foundations (Fig. 6-5). Two types of weathering are recognized, mechanical and chemical. Both types occur simultaneously at the weathering site, during erosion and transport, and even in the environments where weathered materials are deposited.
MECHANICAL WEATHERING /;Wy* Mechanical weathering occurs when physical forces break rock materials into smaller pieces that retain the chemical composition of the parent material. For examMechanical weathering
137
"^ FIGURE 6-6 Mechanically weathered granite. The sandy material consists of small pieces of granite (rock fragments) and minerals such as quartz and feldspars liberated from the parent material.
pie, granite
may
be mechanically weathered to yield
smaller pieces of granite, or disintegration
may
liberate
individual mineral grains from
The
physical
it
(Fig. 6-6).
processes responsible for mechanical weathering include """
FIGURE
6-5
Weathering of the rocklike material of a
frost action, pressure release, thermal
Frost Action /F***+ c^cd^^y
"^ FIGURE
6-7
Frost wedging occurs
when water
/
nticsny ne*"f*
seeps
and expands as it freezes. Repeated freezing and thawing pry loose angular pieces of rock. into cracks
expansion and
contraction, and the activities of organisms.
bridge. (Photo courtesy of R. V. Dietrich.)
Frost action involves the repeated freezing and thawing
When water expands by about 9%
of water in cracks and crevices in rocks. seeps into a crack and freezes,
it
and exerts great force on the walls of the crack, thus widening and extending it by frost wedging. As a consequence of repeated freezing and thawing, pieces of rock are eventually detached from the parent material (Fig. 6-7).
Frost wedging
is
particularly effective
if
the
wedgeshaped opening, much of the force of expansion is released upward toward the surface. Frost action is most effective in areas where temperatures commonly fluctuate above and below freezing. For example, in the high mountains of the western United States and Canada, frost action is very effective even during summer months. In the tropics and in areas where water is permanently frozen, frost action is of little or no importance. The debris produced by frost wedging in mountains crack
is
convoluted.
If
commonly accumulates
the crack
is
a simple
as large cones of talus lying at
the bases of slopes (Fig. 6-8).
The materials
that form
the talus are simply angular pieces of rock from a larger
138
Chapter 6
Weathering, Erosion, and Soil
a
"^ FIGURE
6-8
Talus in the Canadian
Rocky Mountains.
•—
"^ FIGURE Nevada of
6-9
Sheet join ts in granite in the Sierra
California.
body that has been mechanically weathered. Most rocks have a system of fractures called joints along which frost action is particularly effective. Water seeps along the
Thermal Expansion and Contraction
and eventually wedges pieces of rock loose; these then fall downslope to accumulate with
of solids, such as rocks, changes in response to heating
joint surfaces
other loosened rocks.
phenomenon known
mass of sediment or soil undergoes freezing, expansion, and actual lifting, followed by thawing, contraction, and lowIn the
as frost heaving, a
ering of the mass. Frost heaving
where water
freezes beneath
is
particularly evident
roadways and sidewalks.
During thermal expansion and contraction the volume
where the temperature may one day, rocks expand when heated and contract as they cool. Expansion jnd co nt raction do not occur uniformly throughout rocks, h owever. For one thing, a rock is a poor conductor of heat, so its outside heats up more than the inside. Consequently, the surface expands more than the interior, and cooling.
vary as
In a desert,
much
as
30°C
in
may cause fracturing. Furthermore, dark minerals absorb heat faster than light-colored minerals, so differential expansion occurs even between the causing stresses that
Pressure Release Pressure release is
is
a
mechanical weathering process that formed as deeply bur-
especially evident in rocks that
ied intrusive bodies such as batholiths, but
other types of rocks as well.
magma
When
it
a batholith forms, the
"•"
under tremendous pressure (the weight of the overlying rock) and is stable under these pressure conditions. When the batholith is uplifted and crystallizes
the overlying rock
sure is
is
is
stripped
mineral grains of some rocks.
occurs in
away by
FIGURE
6-10
Exfoliation
domes
in
Yosemite National
Park, California.
erosion, the pres-
reduced. However, the rock contains energy that
released by expansion and the formation of sheet
joints, large fractures that
more or less parallel the rock bounded by sheet joints
surface (Fig. 6-9). Slabs of rock
may
slip, slide,
or spall (break) off of the host
rock—
process called exfoliation— and accumulate as talus.
The
large
rounded domes of rock resulting from
this
process are exfoliation domes; examples are found in
Yosemite National Park in California and Stone Mountain in Georgia (Fig. 6-10). Sheet-jointing and exfoliation constitute an engineering problem in many areas (see Perspective 6-1).
Mechanical Weathering
139
Perspective 6-1
BURSTING ROCKS AND SHEET JOINTS The
can expand and produce well-known phenomenon. In deep mines, example, masses of rock suddenly detach from the fact that solid rock
fractures for
is
a
sides of the excavation, often with explosive violence.
Such rock bursts generally occur below depths of about 600 m; spectacular rock bursts have been recorded in deep gold mines in South Africa and Canada and in zinc mines in Idaho. Obviously, rock bursts and related phenomena, such as less violent popping, pose a danger to mine workers. In South Africa, about 20 miners are killed by rock bursts every year. In
some quarrying operations,* the removal of
surface materials to a depth of only 7 or 8
m
has led
to the formation of sheet joints in the underlying rock (Fig. 1).
At quarries
in
Vermont and Tennessee,
for
example, the excavation of marble exposed rocks that
were formerly buried under great pressure. When the overlying rock was removed, the marble expanded and sheet joints formed. Some slabs of rock that were bounded by sheet joints burst so violently that quarrying machines weighing more than a ton were thrown from their tracks, and some quarries had to be
"»*"
FIGURE
2
Sheet joints in granite of the Sierra
Nevada
in California.
abandoned because fracturing rendered the stone useless.
Sheet joints paralleling the walls of the Vaiont
*A quarry
is
a surface excavation, generally for the extraction of
building stone.
River valley in Italy contributed to the worst reservoir disaster in history.
FIGURE 1 Sheet joints formed by Mount Airy Granite in North Carolina.
r^i"
expansion in the (Photo courtesy of
240 million
3
On
October
9,
1963, more than
of rock slid into the Vaiont Reservoir.
Although several factors contributed
moved
W. D. Lowry.)
m
to this slide,
it
The slide displaced water in the reservoir, causing a large wave to overtop the dam and flood the downstream area where nearly 3,000 people drowned. (See Perspective 15-2 for a more complete discussion of the Vaiont partly along a system of sheet joints.
Reservoir disaster.)
The
Sierra
Nevada of
granitic rocks,
many
California are
composed of
of which contain numerous sets
of sheet joints parallel to the canyon walls. Large slabs of granite
bounded by sheet
joints
lie
on
steeply
above highways and railroad tracks where they pose a danger to the road or trackway below (Fig. 2). Occasionally, a mass of this unsupported rock slides or falls, blocking highways and railroad tracks. inclined surfaces
140
Chapter 6
Weathering, Erosion, and Soil
($1
* K'f^ka^^
"^ FIGURE
6-12 The contribution of organisms to mechanical weathering. Tree roots enlarge cracks in rocks.
bring material from depth to the surface where further
^ FIGURE
weathering This desert rock appears to have been
6-11 weathered by repeated heating and cooling.
worms
Repeated thermal expansion and contraction
is
a
but are the forces generated sufficient to overcome the internal strength of a rock? Experiments in which rocks are heated and cooled repeatedly to simulate years of such activity indicate that thermal expansion and contraction is not an important agent of mechanical weathering.* Despite these experimental results, many rocks in deserts do indeed appear
show
the effects of this process (Fig. 6-11).
cause very rapid expansion. During a forest fire, rocks may heat very rapidly, especially near the surface since
The heated
surface layer
interior, and thin become detached.
expands more rapidly than the paralleling the rock surface
Activities of
sheets
Organisms
Animals, plants, and bacteria all participate in the mechanical and chemical alteration of rocks. Burrowing
worms, reptiles, rodents, and many constantly mix soil and sediment particles and
animals, such as others,
Even materials ingested by and animal burrows
trees,
wedge themselves
widen them
(Fig. 6-12).
into cracks in rocks
and further
Tree roots that grow under or
through sidewalks and foundations able damage.
may do
consider-
^ CHEMICAL WEATHERING jdwrttd M™*-** is the process whereby rock madecomposed by chemical alteration of the parent material. A number of clay minerals, for example, form as the chemically altered products of other minerals. Some minerals are completely decomposed during chemical weathering, but others, which are more
Chemical weathering terials are
Daily temperature variation is the most common cause of alternate expansion and contraction, but these changes occur over periods of hours. In contrast, fire can
they conduct heat so poorly.
occur.
allow gases and water to have easier access to greater depths. The roots of plants, especially large bushes and
common phenomenon,
to
may
are further reduced in size,
resistant, are
simply liberated from the parent material. is accomplished by the action of atmo-
Such weathering
and water and acids Organisms also play an important role in chemical weathering. Rocks that have Hcii£ns (co mposite orga nisms consisting of fungi and algae growing on their surfaces undergo more extensive chemical alteration than lichen-free rocks (Fig. 6-13). Plants remove ions spheric, gases, especially oxygen,
.
)
from
soil
water and reduce the chemical
stability
of
soil
minerals, and their roots release organic acids.
Solution 'Thermal expansion and contraction may be a significant Moon where extreme
mechanical weathering process on the temperature changes occur quickly.
During solution the ions of a substance become dissociated from one another in a liquid, and the solid sub-
Chemical Weathering
141
A
There are several sources of carbon dioxide that
may
combine with water and react to form acid solutions. The atmosphere is mostly nitrogen and oxygen, but about 0.03%
carbon dioxide, causing rain to be
is
Human activities have added materials to
slightly acidic.
the atmosphere that contribute to the
problem of acid Carbon dioxide is also prodecay of organic matter and the
rain (see Perspective 6-2).
duced
in soil
by the
respiration of organisms, so groundwater
is
also gener-
Climate affects the acidity, however, with arid regions tending to have alkaline groundwater ally slightly acidic.
is, it has a low concentration of hydrogen ions). Whatever the source of carbon dioxide, once an
(that
acidic solution
"^ FIGURE
is
present, calcite rapidly dissolves ac-
cording to the following reaction:
6-13
Lichen-covered rocks are chemically weathered more rapidly than lichen-free rocks.
CaC0 3 + H 2 calcite
C0 2
+
water
+
?± Ca"
carbon
calcium
2HC0 3 bicarbonate
dioxide
stance dissolves. Water
is
a remarkable solvent because
its molecules have an asymmetric shape; they consist of one oxygen atom with two hydrogen atoms arranged such that the angle between the two hydrogens is about 104 degrees (Fig. 6-14). Because of this asymmetry, the oxygen end of the molecule retains a slight negative electrical charge, whereas the hydrogen end retains a slight
positive charge.
m ineral
halit e
When
a solu ble sjjbsiance
NfaQ) comes (
this reaction
CaC0 3 + calcite
may
H+
HCO3-
hydrogen
bicarbonate
ion
The
dissolution of the calcite in limestone
many
positively charged end of the water molecule (Fig. 6-14). Thus, ions are liberated from the crystal structure, and
small cavities to large caverns such as
the solid dissolves.
overcome theTorces between
minerals. For example, the mTneraTcalcite
pure water, but rapidly dissolves present.
the
An
easy
way
to
if
a small
make water
amount acidic
is
of
by
H2
+
C0 2 carbon
H 2 C0
?±
this
3
^
H
+
+
HC0
3
carbonic
hydrogen
bicarbonate
acid
ion
ion
dioxide
According to
chemical equation, water and carbon
dioxide combine to form carbonic acid, a small
amount
of which dissociates to yield hydrogen and bicarbonate ions.
in
Mammoth Cave in New Mexico (see
The concentration of hydrogen ions determines the more hydrogen ions present,
acidity of a solution; the
the stronger the acid.
Oxidation The term oxidation has ists,
meanings to chemmeaning is more to reactions with oxygen to
a variety of
but in chemical weathering
its
Oxidation refers form oxides or, if water is present, hydroxides. For example, iron rusts when it combines with oxygen to form
restricted.
the iron oxide hematite:
dissociating the ions of carbonic acid as follows:
water
Kentucky and Carlsbad Caverns Chapter 17).
and marble
places ranging from
particles in
(CaC0 3 ),
major constituent of the sedimentary rock limestone and the metamorphic rock marble, is practically insoluble in is
2HCCV bicarbonate
are not very soluble in pure water
because the attractiveTorces~oTwater molecules are not
acid
+
at-
and
has had dramatic effects in
suffic ient to
^±
calcium
the negatively charged chloride ions are attracted to the
Most mi nerals
carbonic acid,
ion
Ca ++
such as the
tracted to the negative end of the water molecule,
in
also be written as
contact with a water
in
molecule7~tne positively charged sodium ions are
'
Because of the dissociation of the ions
4Fe
Water molecule
©o© o> n o o©p ©o® D o ©o©
^ FIGURE
and
biotite. Iron in these
minerals combines
with oxygen to form the reddish iron oxide hematite (Fe 2 3 or the yellowish or brown hydroxide limonite. )
and red colors of many soils and sedimentary rocks are caused by the presence of small amounts of hematite or limonite. An oxidation reaction of particular concern in some
The
yellow, brown,
areas
is
the oxidation of iron sulfides such as the mineral
pyrite (FeS 2 ). Pyrite is commonly associated with coal, so in mine tailings* pyrite oxidizes to form sulfuric acid
place
p ositive ions
ble salts
As an
thoclase (KAlSi 3
is
the chemical reaction between the hydro-
(OH~)
ions of water
and a
mineral's ions. In hydrolysis hydrogen ions actually re-
"Tailings are the rock debris of mining; they are considered too
poor
for further processing
and are
left
as heaps
on the
surface.
replacement
chemical
8)
are
common
in
many rock
types, as
are the plagioclase feldspars (which vary in composition
from CaAl 2 Si 2 8 to NaAlSi 3 8 ). All feldspars are framework silicates, but when altered, they yield soluble salts and clay minerals, such as kaolinite, which are
The chemical weathering of potassium hydrolysis occurs as follows:
orthoclase
ions and hydroxyl
Such
illustration of hydrolysis, consider the
2KAlSi 3 O s
)
minerals.
in
alteration of feldspars. Potassium feldspars such as or-
Hydrolysis +
The
and iron that then may be oxidized.
sheet silicates.
)
gen (H
The
changes the composition oFmuierals by liberating solu-
(H 2 S0 4 and iron oxide. Acid soils and waters in coalmining areas are produced in this manner and present a serious environmental hazard (Fig. 6-15).
Hydrolysis
(a)
asymmetric arrangement of the hydrogen atoms causes the molecule to have a slight positive electrical charge at its hydrogen end and a slight negative charge at its oxygen end. (£>) The dissolution of sodium chloride (NaCl) in water.
(b)
phiboles,
6-14
structure of a water molecule.
+
feldspar by
Perspective 6-2
ACID RAIN one of the consequences of most industrialized nations, such as the United States, Canada, and the Soviet Union, have actually reduced their emissions into the atmosphere, but many developing nations continue to increase theirs. Some of the consequences
form nitric acid (HN0 3 ). Although carbon dioxide and nitrogen gases contribute to acid rain, the greatest culprit is sulfur dioxide (S0 2 ), which is primarily
of atmospheric pollution include smog, possible
acid rain.
Atmospheric pollution
is
industrialization. Several of the
disruption of the ozone layer, global
Chapter
18),
and acid
warming
released by burning coal that contains sulfur.
form
in
The
net effect of this reaction
slightly acidic.
Thus, acid rain
is
is
that
in
(H 2 S0 4 ),
acid rain
was
England by Robert Angus Smith
first
in
recognized
1872, about a
century after the beginning of the Industrial
As we noted previously, water and carbon dioxide atmosphere react to form carbonic acid that dissociates and yields hydrogen ions and bicarbonate is
sulfuric acid
The phenomenon of
(see
rain.
It was not until 1961, however, that acid become a public environmental concern. At that time, it was realized that acid rain is corrosive and irritating, kills vegetation, and has a detrimental effect on surface waters. Since then, the effects of acid rain
Revolution.
in the
ions.
Once
oxygen to the main component of
the atmosphere, sulfur dioxide reacts with
rain
all rainfall
the direct
consequence of the self-cleansing nature of the atmosphere; that is, many suspended particles of gases in the atmosphere are soluble in water and are removed from the atmosphere during precipitation
have been recognized in Europe, especially in Eastern Europe where so much coal is burned, the eastern United States, and southeastern Canada. During the last 10 years, the developed countries have made
events.
Several natural processes, including volcanism and
efforts to
reduce the impact of acid rain;
Act of 1990 outlined
in the
United
the activities of soil bacteria, introduce gases into the
States the Clean Air
atmosphere that cause acid rain. Human activities, however, produce added atmospheric stress. For
steps to reduce the emissions of pollutants that cause
example, the burning of fossil fuels (oil, natural gas, and coal) has added carbon dioxide to the
The areas most affected by acid rain invariably lie downwind from coal-burning power plants or other
atmosphere. Nitrogen oxide (NO) from internal combustion engines and nitrogen dioxide (N0 2 ),
industries that emit sulfur gases.
which
is
formed
in the
atmosphere from
NO,
acid rain.
smelters (plants
react to
hydrogen ions attack the ions in the some liberate d ions are inco rp orated in a dev elo ping clay mi neral. The potassium andbicar bonate ions go into solution and comSne-to
In this reaction
orthoclas e structure, a nd
orm
f
a soluble salt
excess
silica
that
.
On
the right side of the equation
would not
fit
an important source of cement in sedimentary rocks (see Chapter 7). Plagioclase feldspars are altered by hydrolysis in the silica is
same way
difference
as orthoclase.
The only
is
that sol-
uble calcium and sodium salts are formed rather than potassium salt. In fact, these dissolved salts are what make hard water hard. Calcium salts in water are a
144
Chapter 6
Weathering, Erosion, and Soil
discharge large
Chemical plants and where metal ores are refined) quantities of sulfur gases and other
problem because they inhibit the reaction of detergents with dirt and precipitate as scaly mineral matter in water pipes and water heaters.
is
into the crystal structure
of the clay mineral. Such dissolved
specific
^ FACTORS CONTROLLING THE RATE OF CHEMICAL WEATHERING Chemical weathering processes operate on the surfaces is, chemically weathered rocks or minerals are altered from the outside inward. Several factors including particle size, climate, and parent material conof particles; that
trol the rate
of chemical weathering.
substances such as heavy metals. rain in these areas
may
geology. For example,
limestone or alkaline
The
plants built before 1975 have
no emission controls and must be addressed if emissions are to be reduced to an acceptable level. The most effective way to reduce emissions from these older plants is with flue-gas desulfurization (FGD), a process that removes up to 90% of sulfur dioxide from exhaust gases. There are drawbacks to FGD, however. One is that
effect of acid
be modified by the existing
if
an area
is
underlain by
acid rain tends to be
soils, the
soil. Areas underlain by granite, on the other hand, are acidic to begin with and have little or no effect on the rain.
neutralized by the limestone or
The effects of acid rain vary. Small lakes become more acid as they lose the ability to neutralize the acid rainfall. As the lakes increase in acidity, various types of organisms disappear, and, in some cases, all
some
life-forms eventually die. Acid rain also causes
sulfur wastes, the lack of control
increased weathering of limestone and marble (recall
emissions, and reduced efficiency of the
that both are soluble in
weak
plants are simply too old to be profitably upgraded; the 85-year-old Phelps Dodge copper smelter in Douglas, Arizona, closed in 1987 for
reason. Other problems with
FGD
this
include disposal of
on nitrogen gas power plant, percent more coal.
which must burn several Other ways to control emissions include the
acids) and, to a lesser
degree, sandstone. Such effects are particularly visible
used, the
on buildings, monuments, and tombstones; a notable example is Gettysburg National Military Park in Pennsylvania, which lies in an area that receives some of the most acidic rain in the country. While the effects on vegetation in the immediate
conservation of electricity; the
areas of sulfur-gas-emitting industries are apparent,
attributed to other causes. In
problem that knows no currents may blow pollutants from the source in one country to another where the effects are felt. Developed nations have the economic resources to reduce emissions, but many underdeveloped nations cannot afford to do so.
the needles of
Furthermore,
alternate energy source
many
United States show signs of
Acid rain
firs,
yellow and falling
cannot be
Germany's Black Forest, spruce, and pines are turning
would require the
this
installation
furnaces in existing plants.
a global
many
Wind
nations have access to only
high-sulfur coal and cannot afford to install
off.
FGD
devices. Nevertheless, acid rain can be controlled only
Currently, about 20 million tons of sulfur dioxide
by the cooperation of problem.
are released yearly into the atmosphere in the United
mostly from coal-burning power plants. Power
States,
is
national boundaries.
forests in the eastern stress that
new
of expensive
some people have questioned whether acid rain has much effect on forests and crops distant from such sources. Nevertheless,
less that is
lower the emissions of pollutants. Natural gas contains practically no sulfur, but converting to
Particle Size
all
nations contributing to the
3 ume remains the same at 1 m We can make two important statements .
jq
Because chemical weathering affects particle surfaces, the greater the surface area, the
weathering.
It is
more
effective
is
the
important to realize that small particles
have larger surface area s compared t o t heir volume_than do large particles. Notice in Figure 6-16 that a block ~melisuring~TTfroh a side has a total surface area of 6
when
m
,
measuring 2 0.5 m on a side, the total surface area increases to 12 m And if these particles are all reduced to 0.25 m on a side, 2 the total surface area increases to 24 m Note that while the surface area in this example increases, the total volbut
the block
is
broken into
particles
.
.
block
in Figure 6-16. First, as
smaller blocks,
its
ical
weather ing
is
regarding the
split into a
number of
total surface area increases.
the smaller any single block
has compared to
it is
its
volume
is,
the
more
(Fig. 6-16).
Second,
surface area
it
Because chem -
a su rface proce ss, the fact that small
objects have proportionately
more
surface area
com-
pared to volume than do large objects has profound implications. We can conclude that mechanical weath-
which reduces the size of particles, contributes to chemical weathering by exposing more surface area. ering,
Factors Controlling the Rate of Chemical Weathering
145
more
effective in tropical regions than in arid
and
arctic
regions because temperatures and rainfall are high and
evaporation rates are low. In addition, vegetation and animal life are much more abundant in the tropics than in arid or cold regions. Consequently, the effects of weathering extend to depths of several tens of meters in the tropics, but commonly extend only centimeters to a few meters deep in arid and arctic regions. One should
however, that chemical weathering goes on everywhere, except perhaps where Earth materials are perrealize,
manently frozen.
Parent Material It
should be apparent that some rocks are chemically stable than others and thus are not altered as rap-
more idly
by chemical processes. For example, the metamoris an extremely
phic rock quartzite, composed of quartz,
stable substance that alters very slowly compared to most other rock types. In contrast, rocks such as granite, which contain large amounts of feldspar minerals, decompose rapidly because feldspars are chemically unstable.
Ferromagnesian minerals are also chemically unstawhen chemically weathered, yield clays, iron
ble and,
oxides,
"^ FIGURE
6-15 The oxidation of pyrite in mine tailings forms acid water as in this small stream. More than 11,000 km of U.S. streams, mostly in the Appalachian region, are contaminated by abandoned coal mines that leak sulfuric acid.
mon
and ions
minerals
is
in solution. In fact, the stability
just the
Bowen's reaction series (Fig. 6-17): the minerals that form last in this series are chemically statallization in
ble,
whereas those that form early are
easily altered
One
manifestation of chemical weathering
more
processes occur
rapidly at high
temperatures and in the presence of liquids. Accord-
not surprising that chemical weathering
*~ FIGURE 6-16 Particle size and chemical weathering. As a rock is reduced into smaller and smaller particles,
but
its
its
Surface area = 6
m-
is
ers to
Surface area = 12
m2
i
surface area increases
volume remains the same.
Thus,
m2 m2
in (a) the surface area is 6 2 in {b) it is 12 , and in (c) 24
m
,
,
but the volume remains the same at 3 1 Accordingly, small particles have more surface area in proportion to their volume than do large
m
.
particles. (a)
146
spheroi-
one that is rectangular to begin with, weathform a spheroidal shape because that is the most stable shape it can assume. The reason is that on a rectangular stone the corners are attacked by weathering stone, even
is
is
dal weathering (Fig. 6-18). In spheroidal weathering, a
Most chemical it
by
chemical processes because they are most out of equilibrium with their conditions of formation.
Climate
ingly,
of com-
opposite of their order of crys-
Chapter 6
Weathering, Erosion, and Soil
(b)
(c)
.
--
:
;
'
:
.--'
:
:
V
1
I
1
i i
A 1GL"RE 6-1"
«-f-
•
rr _ -
:-:
—
::::::
:::
i.-i
tatrz the
.-r.
its.
_ir
:
rser ;;
zzir
.r.
-
Fluids follow the joint :
-:-:
:
jr.izi
r.
:
;•:-
:
.
:
:
r.i
:
iztz.
::
:
i
.:.
zr
rr~ rr" :~r
Most
land-
soil for their ex-
and most of their ::-;.- :::;:: i
--j.zz~.z-Zi
-j.--.-n-i.
i.'.z.
::.: ;.i
:oocL fertile soil for garden-
:
-19.
r.=. ; is
humus
— _~
r.i
Truer.: :: sue.-
is
_
organic matter, b ;"-zr.z :
:
SOIL
-.::
most places the land surface is covered by a .-;::•:..::::: :::: ir.J r..:.::i. :::r:::: ; ji ..;_
.
:-"
-
;„__-.£
-
.
:
.z
r.
-
zz.'r.z
above.
nutrients
:
: ;:
:-_•
..-•:
i.
life
endent on :
.:
zr.iztr.i..
_:::::
.-
--t—
-
:"
:-i
-
~
torn*
-.' r- i- i
-
:
-zt.zjzi-
In
~:.iin
re-
Regouth may consist of volcanic ash, sediment deposited by wind, streams, or glaciers, or weathered ::_>: — iztzi. z:—.tz s. z.izz if i zzi.ZJz S ~ e regootfa.
an :
essential source
-r_rr ztztT.T.
onh of ;
:
r.
:
>:_
14"
"^ FIGURE
6-18
Spheroidal weathering,
(a)
are attacked by chemical weathering processes,
The rectangular blocks outlined by (£>)
joints
but the corners and edges are
weathered most rapidly, (c) When a block has been weathered so that it is spherical, entire surface is weathered evenly, and no further change in shape occurs.
and silt-sized mineral grains, especially quartz, but other weathered materials may be present as well. Such solid particles are important because they hold soil particles apart, allowing oxygen and water to circulate more freely. Clay minerals are also important constituents of soils and aid in the retention of water as well as supplying nutrients to plants. Soils with excess clay minerals, however, drain poorly and are sticky when wet and hard
when
dry.
is
(Fig.
6-20a). For example,
if
a
body of granite
weathers, and the weathering residue accumulates over
and
is
converted to
soil,
the soil thus formed
residual. In contrast, transported soils are developed
on weathered material eroded and transported from the weathering site to a new location (Fig. 6-20b). Many fertile transported soils of the Mississippi River valley and the
Pacific
windblown dust
=»
Residual soils are formed where parent material has
weathered
the granite
its
THE
Northwest developed on deposits of called loess (see Chapter 19).
SOIL PROFILE
Soil-forming processes begin at the surface and
downward, so
the upper layer of soil
from the parent material than the
"^ FIGURE
served in vertical cross section, a 6-19
Spheroidal weathering of granite in
Australia.
more
layers below.
soil consists
work
altered
Ob-
of distinct
from one another in and color (Fig. 6-21).
layers or soil horizons that differ
texture, structure, composition,
WWMV^Wg^:
is
Starting
from the top, the horizons
typical of soils are
designated O, A, B, and C, but the boundaries between
horizons are transitional rather than sharp.
The
O
horizon, which
is
generally only a few centi-
meters thick, consists of organic matter. The remains of plant materials are clearly recognizable in the upper part
O
lower part consists of humus. is called top soil (Fig. 6-21). This layer contains more organic matter than those below. It is also characterized by intense biological activity because plant roots, bacteria, fungi, and animals such as worms are abundant. Threadlike soil bacteria give freshly plowed soil its earthy aroma. In
of the
horizon, but
its
Horizon A, lying beneath horizon O,
soils
148
Chapter 6
Weathering, Erosion, and Soil
developed over a long period of time, the
A horizon
(b)
(a)
^p" FIGURE 6-20 (b)
Transported
Residual soil developed on bedrock near Denver, Colorado. developed on windblown dust deposit.
(a)
soil
consists mostly of clays
and chemically
such as quartz. Water percolating
zon
A
dissolves the soluble minerals that
present and carries them
stable minerals
down through
hori-
were originally
away or downward
to lower
by a process called leaching. Horizon B, or subsoil, contains fewer organisms and less organic matter than horizon A (Fig. 6-21). Horizon levels in the soil
B
is
also called the zone of accumulation, because sol-
uble minerals leached from horizon irregular masses.
If
horizon
sion leaving horizon well,
and
stickier
if
A
B exposed,
horizon B
is
when wet than
clayey,
other
Horizon C, the lowest
A
accumulate as away by ero-
stripped
is
do not grow as harder when dry and
plants
it is
soil
in
horizon B
of partially
(Fig.
6-22a).
Pedocals are soils characteristic of arid and semiarid regions and are found in States, especially the
name
rives
its
Such
soils
horizon
horizons.
soil layer, consists
symbols for aluminum (Al) and iron (Fe). Because these soils form where abundant moisture is present, most of the soluble minerals have been leached from horizon A. Although it may be gray, horizon A is generally dark colored because of abundant organic matter, and aluminum-rich clays and iron oxides tend to accumulate
in part
contain
much
southwest
from the
less
of the western United 6-22b). Pedocal de-
(Fig.
three letters of calcite.
first
organic matter than pedalfers, so
A is generally lighter colored
and contains more
unstable minerals because of less intense chemical weath-
altered to unaltered parent material (Fig. 6-21). In horizons
A and B, the composition and texture of the parent material have been so thoroughly altered that the parent material is no longer recognizable. In contrast, rock fragments and
"'''
mineral grains of the parent material retain their identity
or mature
horizon C. Horizo n
C
contains
litt le
in
FIGURE
6-21
soil.
The
soil
horizons in a fully developed
^O
«t
'
J;.. ,
organic matte r.
^ FACTORS CONTROLLING SOIL
Horizons
O
=
thin layer of
organic matter
FORMATION A = zone
of leaching
B = zone
of
Climate It
has long been acknowledged that climate is the single factor in soil origins. A very general
most important
classification recognizes three
major
soil
teristic
of different climatic settings. Soils that develop in
humid
regions such as the eastern United States and
much
of
Canada
are pedalfers, a
Greek word pedon, meaning
soil,
accumulation
types charac-
C =
partially altered to
unaltered parent material
name
derived from the and from the chemical
Factors Controlling Soil Formation
149
-^ FIGURE
Caliche on
6-23
Mormon Mesa
in
southern
Nevada.
soil
water evaporation
intense yields alkali soils that
is
are so alkaline that they cannot support plants. Laterite
a soil
is
weathering
is
formed in the tropics where chemical and leaching of soluble minerals is
intense
complete. Such
soils are red,
commonly extend
to depths
of several tens of meters, and are composed largely of
aluminum hydroxides,
iron oxides, and clay minerals;
even quartz, a chemically stable mineral, leached out
Although not very
is
generally
(Fig. 6-24a).
laterites
fertile.
The
support lush vegetation, they are is sustained by from the surface layer of or-
native vegetation
nutrients derived mostly
ganic matter, but
little
humus
present in the soil
is
because bacterial action destroys
it.
When
such
itself
soils are
cleared of their native vegetation, the existing surface
accumulation of organic matter is rapidly oxidized, and there is little to replace it. Consequently, when societies practicing slash-and-burn agriculture clear these soils,
they can raise crops for only a few years at best. the soil
is
Then
completely depleted of plant nutrients, the
clay-rich laterite bakes brick hard in the tropical sun,
and the farmers move on process
One
"^ FIGURE
6-22
(a)
Pedalfer
is
the type of soil that
develops in humid regions, whereas arid and semiarid regions.
{b)
pedocal
is
typical of
If
is
to another area
aspect of laterites
the parent material
is
is
of great economic importance.
rich in
150
Chapter 6
Weathering, Erosion, and Soil
aluminum, aluminum hy-
may
accumulate in horizon B as bauxite, the ore of aluminum (Fig. 6-24b). Because such intense chemical weathering currently does not occur in North America, we droxides
are almost totally dependent
As soil water evaporates, calcium carbonate leached from above commonly precipitates in horizon B where it forms irregular masses of caliche (Fig. 6-23). Precipitation of sodium salts in some desert areas where ering.
where the
repeated.
num
Some aluminum
on
foreign sources for alumi-
do exist in Arkansas, Alabama, and Georgia, which had a tropical climate about 50 million years ago, but currently it is cheaper to import aluminum ore than to mine these deposits. ores.
ores
(b)
(a)
"^ FIGURE forms
6-24
(a) Laterite,
shown
here in Madagascar,
is
a deep, red soil that
response to intense chemical weathering in the tropics, {b) Bauxite, the ore of in horizon B of laterites derived from aluminum-rich parent materials. (Photo courtesy of Sue Monroe.) in
aluminum, forms
Much humus
Parent Material The same rock type can yield different soils in different climatic regimes, and in the same climatic regime the same soils can develop on different rock types. Thus, it seems that climate is more important than parent material in determining the type of soil that develops. Nevertheless, rock type does exert some control. For example, the metamorphic rock quartzite will have a thin soil because
chemically stable, whereas an adja-
over
it
cent
body of granite
it is
will
have a
much deeper
in soils is
provided by grasses or leaf
decompose to obtain food. In so doing, they break down organic compounds within plants and release nutrients back into the soil. Additionally, organic acids produced by decaying soil organisms litter
that microorganisms
are important in further weathering of parent materials
and soil particles. Burrowing animals constantly churn and mix soils, and their burrows provide avenues for gases and water. Soil organisms, especially
some
types of bacteria, are
soil (Fig.
6-25).
^ SoiHhat develops on basalt will be rich in iron oxides because basalt contains abundant ferromagnesi an min -
buTfocksTacking such minerals will not yield an iron oxide-rich soil no matter how thoroughly they are
erals,
"^ FIGURE 6-25 The influence of parent material on soil development. Quartzite is resistant to chemical weathering, whereas granite is altered quickly.
weathered. Also, weathering of a pure quartz sandstone will yield
no
clay,
whereas weathering of clay
will yield
nqj and. Organic Soils
Activity-
not only depend on organisms for their
fertility,
but
from microscopic, single-celled bacteria to large burrowing animals such as ground squirrels and gophers. Earthworms — as many as one million per acre— ants, sowbugs, termites, centipedes, millipedes, and nematodes, along with various types of fungi, algae, and single-celled animals, make their also provide a suitable habitat for organisms ranging
homes
in the soil.
AUof these
contribute to the formation
and provide humus when they die and are decomposed by bacterial action. of
soils
Quartzite
Factors Controlling Soil Formation
151
STEPHEN
Guest Essay
H.
STOW
TTVTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT»T»TT»TTTTTTTrm
ENVIRONMENTAL GEOLOGY: SUSTAINING THE EARTH
We
can think of Earth as a spaceship, upon which all live. Our existence depends on our learning
The demand
resources are another crucial area.
for
humans
petroleum will continue, as will the need for geologists
about our home, the Earth, about its behavior, its limits, and about how we, as passengers on this spaceship, can most efficiently live with our environment.
in that industry.
Earth science touches almost every aspect of our It
lives.
make
importance
is
mineral, energy, are limited,
we
and
use owes
its
environment sciences.
We
is
human
race
all
fragile
in
cities.
though not yet fully understood, may of humans' release of materials into the
evolved over millions of years. Another problem
and costs of assessing and correcting these are immense, but must be undertaken.
All earth scientists, including geologists, are
much
in
market for geologists was driven by the petroleum industry, but today there is an scientists to
undertake
environmental studies. For instance, hydrology studies dealing with waste disposal issues are needed as are studies of
how
water resources respond to changes
in
global climates. Deciphering the rock record to identify
past fluctuations in climate
may
help us predict future
As populations grow, the proper use of become an increasingly important issue, and earth scientists are becoming fluctuations.
precious land and resources has
intimately involved in the decision process. Energy
many
of the environmental studies.
interest in the Earth goes
days; in high school,
My
field trips
interest
I
Chapter 6
Weathering, Erosion, and Soil
childhood
geology in
was aroused by mineral-hunting
My
who encouraged my
professional interests are no initial
enthusiasms, but that
to be expected because the profession has changed,
It is gratifying to be applying fundamental knowledge to the solution of issues that confront us daily— issues that absolutely must be solved if our
too.
future existience
is
to be ensured.
A
Otephen H. Stow earned a Ph.D. in geochemistry from Rice University. He has worked as a research scientist for Continental Oil
Company and
has served on
the faculty at the University of
Alabama. Currently, he heads the Geosciences Section of the
Environmental Sciences Division at the Oak Ridge National Laboratory in Tennessee.
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAA AAAAAAAAAAAAAAAAAAA AAAAAAAAAAAA 152
my in
with the geology club as well as by an
excellent chemistry teacher,
is
back to
decided to major
longer the same as those
should be
Historically, the job
unprecedented need for earth
My
sites,
field
interests in science.
aware of the fragility of our planet and the impact that we can have on it.
demand.
that guides
college.
is
groundwater contamination due to unrestricted disposal of waste products over the last several decades. The
scientists,
requires sophisticated understanding of
and computer modeling of data and laboratory studies. To function effectively in this area, earth scientists must not only have a sound base in their discipline, but must also be familiar with other sciences, mathematics, and legislation obtained from
become aware of the ozone hole warming with a resultant
Everyone, not just professional earth
it
aspects of disposal
atmosphere, altering the delicate heat balance that
situations
involves the study of the
contaminant transport, structural and stratigraphic
situations,
challenges
company,
Earth processes, such as groundwater flow and
on the Earth's
increase in sea level that could inundate coastal
be the result
My present position
United States;
the atmosphere and global
These
oil
deals almost entirely with the
massive effort being undertaken throughout the entire
another important aspect of the earth
have
work
existence to water,
throughout the world, so shortages often arise, sometimes leading to confrontations between nations. the
current
But most resources
Of equal
processes.
and they are not distributed evenly
The impact of
job after graduate school involved
sites where the Department of Energy (and its predecessor agencies) disposed of nuclear and chemical wastes from nuclear energy and weapons manufacturing. This cleanup is a
its
soil resources.
first
cleanup of historical waste disposal
our dependence on the Earth's resources,
virtually everything
my
studies.
us acutely aware of the dynamics of the
Earth and the need to understand
my
application of the earth sciences to environmental
encompasses natural disasters— volcanoes,
earthquakes, tropical storms, and floods. Such natural events
Although
exploration and geochemistry for a major
extremely important into a
form of
Relief Relief
soil
in
changing atmospheric nitrogen
nitrogen suitable for use by plants.
will
From
of geologic time.
the difference in elevation between high
low points
it
soil-forming process occurs at a rapid rate in the context
and Slope
is
develop faster on unconsolidated sediment than on solid bedrock.* Under optimum conditions of soil formation, the
soil will
and
Because climate changes with
in a region.
formation
soil
is
the
human
perspective, however,
a slow process; consequently, soil
is
regarded as a nonrenewable resource.
elevation, relief affects soil-forming processes largely
through elevation. For example, on the west slope of the Bighorn Mountains in Wyoming, soils change laterally from pedocal at low elevation to pedalfer at the crest of the mountains.
One
Slope affects soils in two ways.
simply slope
is
angle: the steeper the slope, the less opportunity for soil
development because weathered material is eroded faster than soil-forming processes can work. The other slope control
is
the direction the slope faces. In the
Northern Hemisphere, north-facing slopes receive sunlight than south-facing slopes. is
steep,
it
may
receive
If
no sunlight
less
a north-facing slope
at
all.
Consequently,
north-facing slopes have soils with cooler internal temperatures,
may
support different vegetation, and,
if
in a
cold climate, remain frozen longer.
SOIL EROSION
=»
Unquestionably, construction and farming can accelerate the rate of soil erosion,
some
and
soil losses to
erosion are
magnitude of the problem varies. For one thing, a problem in one area may be only a minor inconvenience someplace else; the critical in
areas. Nevertheless, the
two of a thin than the same loss on a deep, fertile loss of a centimeter or
soil soil.
more critical The Soil Con-
is
servation Service of the U.S. Department of Agriculture
has determined that
soil losses
exceeding
tons per
five
acre per year adversely affect the productivity of the
Most
than
less
13%
that
maximum. This same agency
this
of
all
soil.
United States are being eroded at rates
soils in the
estimates
agricultural land accounts for
71%
of
some parts of the world, however, soil much more serious problem. Madagascar,
the erosion. In
erosion
Time Recall our statement that soil-forming processes begin at the surface
and work downward. Thus, the degree of
alteration of parent material in horizon
is
complete
its
soil to
pulverized by plowing, the fine particles are easily
soil is
properties of a soil are determined by the
blown away. The Dust Bowl of the 1930s is a poignant reminder of just how effective wind erosion can be (see the Prologue). Falling rain disrupts soil particles, and
and organisms altering parent matethrough time; the longer these processes have operated, the more fully developed the soil will be. If a soil is weathered for extended periods of time, however, its fertility
decreases as plant nutrients are leached out, un-
new
materials are delivered. For example, agricul-
tural lands adjacent to
major streams such as the Nile
River in Egypt have their
soils
replenished during yearly
floods. In areas of active tectonism, uplift
and erosion
provide fresh materials that are transported to adjacent areas where they contribute to
How much
soils.
needed to develop a centimeter of soil a meter or so deep? No definitive answer can be given because weathering proceeds at vastly different rates depending on climate and time
is
or a fully developed
parent material, but an overall average might be about 2.5
percentage of
practices, overgrazing,
The
rial
soil
lost a large
and deforestation. Most soil erosion occurs by the action of wind and water. When the natural vegetation is removed and a poor farming
has been undergoing change for the longest
factors of climate
less
a
it
because time.
A
is
example, has
for
cm per century. However, a lava flow a few centuries may have a well-developed soil on it,
when is
it
runs off at the surface,
it
carries soil with
it.
This
on steep slopes from which vegetative cover has been removed by overgrazing or
particularly devastating
the
deforestation.
Two
types of erosion by water are recog-
nized: sheet erosion
Sheet erosion the surface
is
and
erosion.
rill
more or
less
and removes thin
evenly distributed over
layers of soil. Rill erosion,
on the other hand, occurs when running water scours small channels. If these rills become too deep to be eliminated by plowing (about 30 cm), they are gullies (Fig. 6-26). Where gullying becomes extensive, croplands can no longer by tilled and must be abandoned. If
the rate of soil erosion
is
less
than
five
tons per year
most parts of the United States — soil-forming processes can keep pace, and the
per acre— as
is
the case in
old in Hawaii
whereas
a flow the
siderably less
soil.
same age in Iceland will have conGiven the same climatic conditions,
"Bedrock
is
a general term for the rock underlying soil or
unconsolidated sediment.
Soil
Erosion
153
"^ FIGURE 6-27 One soil conservation practice is contour plowing, which involves plowing parallel to the contours of the land. The furrows and ridges are perpendicular to the direction that water would otherwise flow downhill and thus inhibit erosion.
ported elsewhere, perhaps onto neighboring cropland, onto roads, or into channels. Sediment accumulates in canals and irrigation ditches, and agricultural fertilizers
and
insecticides are carried into streams and lakes. Problems experienced during the past, particularly during the 1930s, have stimulated the development of methods to minimize soil erosion on agricultural lands.
Various practices including crop rotation, contour plowing,
and the construction of
terraces have
all
proved
helpful (Fig. 6-27). Other practices include no-till plant-
ing in which harvested crop residue to protect the surface
is left on the ground from the ravages of wind and
water.
^ WEATHERING AND MINERAL RESOURCES we
In a preceding section,
discussed intense chemical
and the origin of bauxite, the chief ore of aluminum. Such an accumulation of valuable minerals formed by the selective removal of soluble weathering
""^"
FIGURE
6-26
rainstorm. This gully
is
rill
(a) Rill
was
erosion in a
later
plowed
field
during a
over, (b) This small
too deep to be plowed.
remains productive. If the maximum is exceeded, however, the upper layers of soil— the most productive first,
is
a residual concentration.
It
represents an
insoluble residue of chemical weathering. In addition to
bauxite, a number of other residual concentrations are economically important; for example, ore deposits of
soil
layers— are removed
substances
in the tropics
thus exposing horizon B. Such
iron, manganese, monds, and gold.
Some
clays,
nickel,
phosphate,
tin,
dia-
limestones contain small amounts of iron car-
bonate minerals.
When
the limestone
is
dissolved during
losses are problems, of course, but there are additional
chemical weathering, a residual concentration of insol-
consequences. For one thing, the eroded
uble iron oxides accumulates.
154
Chapter 6
soil is trans-
Weathering, Erosion, and Soil
Some
of the sedimentary
-
iron deposits (see Chapter 7) of the Lake Superior region were enriched by chemical weathering when the soluble constituents that were originally present were carried away. Residual concentrations of insoluble manganese oxides form in a similar fashion from manganese-rich
Country rock
source rocks.
Most commercial clay deposits were formed by hydrothermal alteration of granitic rocks or by sedimentary processes. However, some have formed in place as residual concentrations. For example, a olinite
deposits
in
the
number of ka-
southern United States were
formed by the chemical weathering of feldspars
in peg-
matites and of clay-bearing limestones and dolostones. Kaolinite
is
a type of clay mineral used in the manufac-
ture of paper
Water table
""'
FIGURE
6-28
A
showing a gossan and and the supergene enrichment of
cross section
the origin of oxidized ores ores.
and ceramics.
Gossans, oxidized ores, and supergene enrichment of ores are interrelated, and all result from chemical weathering (Fig. 6-28).
composed
A gossan is
a yellow to reddish deposit
largely of hydrated iron oxides that
formed
by the oxidation and leaching of sulfide minerals such as pyrite (FeS 2 ). The dissolution of such sulfide minerals
forms sulfuric acid, which causes other metallic minerals to dissolve, and these tend to be carried downward toward the groundwater table (Fig. 6-28). Oxidized ores form just above the groundwater table as a result of chemical reactions with these descending solutions.
Some
of the minerals formed in this zone contain cop-
per, zinc,
and
other metals such as lead, zinc, nickel, and copper that
have a greater
affinity for sulfur. Indeed,
)
source of copper than the
latter.
Gossans have been used occasionally as sources of iron, but they are far more important as indicators of underlying ore deposits.
lead.
supergene chal-
(Cu 2 S), an important copper ore, forms as a replacement of primary pyrite (FeS 2 and chalcopyrite (CuFeS 2 ). Notice that both chalcocite and chalcopyrite are copper-bearing minerals, but the former is a richer
cocite
One
of the oldest
known
un-
Supergene enrichment of ores occurs where metalbearing solutions penetrate below the water table (Fig.
derground mines exploited such ores about 3,400 years ago in what is now southern Israel. Supergene enriched
6-28). Such deposits are characterized by the replace-
ore bodies are generally small but extremely rich sources
primary deposit with sulfide minerals introduced by the descending solutions. For example, the iron in iron sulfides may be replaced by
of various metals.
ment of
sulfide minerals of the
^ CHAPTER SUMMARY 1.
4.
that
it is
more nearly
in
soluble salts,
The
can be deposited as sediment, which may become sedimentary rock. Mechanical weathering includes such processes as frost action, pressure release, thermal expansion and contraction, and the activities of organisms. Particles liberated by mechanical weathering retain the chemical composition of the parent material. soil,
or
5.
Ch emical we ather ing p roceeds most
6.
wet environments, but it occurs in all areas, except perhaps where water is permanently frozen. Mechanical weathering aids chemical weathering_ by
in solution.
residue of weathering can be further modified to
form 3.
and ions
various ions in solution, and soluble salts are formed during chemical weathering.
equilibrium
with new physical and chemical conditions. The products of weathering include solid particles,
Solution, oxidation, and hydrolysis are chemical
weathering processes; they result in a chemical change of the weathered products. Clay minerals,
Mechanical and chemical weathering are processes whereby parent material is disintegrated and
decomposed so
2.
The largest copper mine in the world, Bingham, Utah, was originally mined for supergene ores, but currently only primary ores are being mined. at
rapidly inhot,
"breaking parent material intojj maller piec es, thereby
it
7.
exposing more surface a rea. Mechanic al and~ch emical weath eri ng produ ce r egolith , air,
8.
some ofwhich is soil if ft consists^ of solids, humus and supports plant growth.
water, and
Soils are characterized
by horizons that are
designated, in descending order, as O, A, B, and C;
Chapter Summary
155
horizons differ from one another in texture,
soil
structure, composition, 9.
The
and
factors controlling soil formation include
3.
and time.
as the eastern United States Arid and semiarid regions soils are pedocals, many of which contain irregular masses of caliche in
a.
4.
horizon B.
12.
from intense chemical Such soils are deep, red, and sources of aluminum ores if derived from aluminum-rich parent material. Soil erosion, caused mostly by sheet and rill erosion, is a problem in some areas. Human practices such as construction, agriculture, and deforestation can a soil resulting
is
5.
clay. is
activities of
debris produced mostly
e.^_ soil
organisms;
and supergene enrichment of from chemical weathering. 7.
IMPORTANT TERMS
produced
by intense weathering in the tropics. When the ions in a substance become dissociated, the substance has been: weathered mechanically; b. altered to a.
^
c.
oxidized;
dissolved; d.
converted to
soil.
The process whereby hydrogen and hydroxyl water replace ions in minerals is: supergene enrichment; b. a.
14. Gossans, oxidized ores, all result
residual manganese;
d.
an accumulation of: calcium carbonate in horizon B of pedocals; angular rock fragments at the base of a slope; valuable minerals formed by selective removal
Talus
e.
clay.
ores
present.
is
calcium sulfate;
clay:
6.
the mineral calcite
silicon dioxide;
b.
e.
by the
of which
contain valuable minerals such as iron, lead, copper,
if
of soluble substances; d.
responsible for the
many
composed of
is
c.
c.
accelerate losses of soil to erosion. origin of residual concentrations,
pressure
e.
nearly insoluble in pure water but
carbonic acid;
x
b.
in the tropics.
13. Intense chemical weathering
and
X
a.
weathering as
is
dissolves rapidly
humid regions such and much of Canada.
is
Limestone, which
(CaC0 3 ),
and
10. Soils called pedalfers develop in
11. Laterite
oxidation and reduction;
\
release.
climate, parent material, organic activity, relief slope,
d.
color.
c.
laterization; d.
e.
carbonization.
Which of most
X
ions of
oxidation;
hydrolysis;
the minerals in Bowen's reaction series
is
stable chemically?
calcium plagioclase; k s\ quartz; biotite; e olivine. pyroxene; d. Granite weathers more rapidly than quartzite because it contains abundant:
a.
chemical weathering differential weathering
pedocal
erosion
regolith
pressure release
exfoliation
rill
dome
exfoliation
c.
frost action
erosion
minerals;
spheroidal weathering
wedging
soil soil
hydrolysis
solution
laterite
talus
leaching
thermal expansion and
horizon 10.
12.
13.
is:
2.
laterite; b.
d.
bauxite;
e.
A
pedocal;
c.
gossan;
domes? heating and cooling;
exfoliation
contraction;
156
Chapter 6
c.
parent material; top
Y
soil
talus.
e.
known as the: humus layer; c.
also
is
soil; b.
zone of accumulation;
alkali
organic-
e.
The
chief ore of
a.
caliche; b.
d.
gossan;
aluminum
e.
is:
pedalfer;
X^
subsoil;
c.
bauxite.
The removal of thin layers of soil by water over a more or less continuous surface is: a.
gullying; b.
)(
c.
weathering;
d.
sheet erosion; leaching;
e.
exfoliation.
Oxidation and leaching of sulfide minerals yield a yellow to red deposit of hydrated iron oxides known
pedalfer.
Which mechanical weathering process forms a.
d.
Horizon B of a
rich layer.
11.
of soil typical of arid and semiarid regions
a.
a. ~yr
and unconsolidated rock material covering most places are: humus; regolith; b. laterite; c.
soil
a.
transport
weathering zone of accumulation
-V
The
zone; d.
contraction
» REVIEW QUESTIONS The type
9.
carbonate
d.
caliche.
e.
the Earth's surface in
humus
pedalfer
ferromagnesian minerals;
sheet joint
heaving
quartz;
—f- feldspars; b.
c.
frost
1.
a.
sheet erosion
frost
mechanical weathering oxidation parent material
8.
a.
residual deposit; b.
exfoliation
b.
expansion and
the activities of organisms;
Weathering, Erosion, and Soil
dome;
clay deposit. sheet joint; e. )^ gossan; d. 14. Bacterial decay of organic matter yields c.
which
is/are essential to soil fertility.
humus;
sand; b. /\
a
ways in which soil erosion can be minimized on agricultural lands.
30. Discuss several
pedalfer;
c
ferromagnesian minerals. 15. How does mechanical weathering differ from and contribute to chemical weathering? 16. What is differential weathering, and why does it subsoil;
d.
e.
31.
How
^
does frost wedging differ from frost heaving?
18. Explain
how
sheet joints
and exfoliation domes
most minerals not very soluble
are
22.
What
in
pure
role
do hydrogen ions play
why
panicle size
is
Press.
in the hydrolysis
American Planning Association, Planning Advisory Service Report No. 386. Courtney, F. M., and S. T. Trudgill. 1984. The soil: An introduction to soil study. 2d ed. London: Arnold. Gibbons, B. 1984. Do we treat our soil like din? National agricultural erosion.
an important factor
in
chemical weathering. 24. Describe spheroidal weathering. 25.
Draw each
26.
a soil profile
soil
What
is
and
the characteristics of
list
Geographic 166, no. 3:350-89. Loughnan, F. C. 1969. Chemical weathering of the
horizon. the significance of climate
minerals.
and parent
material in the development of soil?
Oilier, C.
How
Parfit,
do organisms contribute to soil formation? 28. Compare and contrast pedalfer, pedocal, and laterite. 29. Explain how plowing, overgrazing, and deforestation 27.
P.
Carroll, D. 1970. Rock weathering. New York: Plenum Press. Coughlin, R. C. 1984. State and local regulations for reducing
process? 23. Explain
1986. Earth: The stuff of life. 2d revised ed. Okla.: University of Oklahoma Press.
W. 1984. Soils and geomorphology. New York: Oxford University Press. Buol, S. W., F. D. rlole, and R. J. McCracken. 1980. Soil genesis and classification. Ames, Iowa: Iowa State University
is an acid solution, and why are acid solutions important in chemical weathering?
What
E.
Birkeland,
water? 21.
F.
Norman,
whereby soluble minerals such
as halite (NaCl) are dissolved.
Why
and how do they
ADDITIONAL READINGS
Bear,
originate.
19. Describe the process
20.
are residual concentrations,
form?
occur? 17.
What
New
silicate
York: Elsevier.
1969. Weathering. New York: Elsevier. dust bowl. Smithsonian 20, no. 3:44-54,
M. 1989. The
56-57.
contribute to soil erosion.
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Additional Readings
157
CHAPTER
7
SEDIMENT AND SEDIMENTARY ROCKS p OUTLINE PROLOGUE INTRODUCTION SEDIMENT TRANSPORT AND DEPOSITION LITHIFICATION: SEDIMENT
TO
SEDIMENTARY ROCK ""•'
Guest Essay: Exploring for Oil and Natural Gas
SEDIMENTARY ROCKS Detrital Sedimentary
Rocks
Conglomerate and Sedimentary Breccia Sandstone
Mudrocks Chemical Sedimentary Rocks Limestone-Dolostone ^-Perspective 7-1: The Mediterranean Desert Evaporites
Chert
Coal
SEDIMENTARY
FACIES
Marine Transgressions and Regressions
ENVIRONMENTAL ANALYSIS Sedimentary Structures Fossils
Environment of Deposition
SEDIMENTS, SEDIMENTARY ROCKS,
AND NATURAL RESOURCES Petroleum and Natural Gas
Uranium Banded Iron Formation **r Perspective 7-2: Persian
Gulf Petroleum
CHAPTER SUMMARY
Sedimentary rocks exposed
in the
Sheep
Rock area of John Day Fossil Beds National Monument, Oregon. This small hill is capped by the remnants of a lava flow.
PROLOGUE
The Green River Formation its
huge deposits of
oil
is
About 50
million years ago,
lakes existed in
what
are
two
now
large
parts of
substance
known
known
for
and an organic
consists of small clay particles
^^pl^|
also well
shale (Fig. 7-2). Oil shale
as kerogen.
When
the appropriate
extraction processes are used, liquid oil and
Wyoming, Utah, and Colorado. Sand, mud, and
combustible gases can be produced from the kerogen
where they accumulated as layers of sediment that were subsequently converted into sedimentary rock. These sedimentary rocks, called the Green River Formation,
of
contain the fossilized remains of millions of
the Green River Formation. During the
dissolved minerals were carried into these lakes
and
fish, plants,
and are a potential source of large quantities of oil, combustible gases, and other substances. Thousands of fossilized fish skeletons are found on single surfaces within the Green River Formation, indicating that mass mortality must have occurred insects
The cause of these events is not with certainty, but some geologists have
repeatedly (Fig. 7-1).
known
suggested that blooms of blue-green algae produced toxic substances that killed the fish. Others propose that rapidly changing water temperatures or excessive salinity at times of increased
evaporation was
Whatever the cause, the fish died by the thousands and settled to the lake bottom where their decomposition was inhibited because the water contained little or no oxygen. One area of the formation in Wyoming where fossil plants are particularly abundant has been designated as Fossil responsible.
Butte National
Monument.
-"^ FIGURE 7-1 Fossil fish from the Green River Formation of Wyoming. (Photo courtesy of Sue Monroe.)
oil shale.
To be designated
as a true oil shale,
however, the rock must yield a gallons of oil per ton of rock.
source of fuel
is
not new, nor
people in Europe used
oil
minimum
The use of is oil
of 10 oil
shale as a
shale restricted to
Middle Ages,
shale as solid fuel for
domestic purposes, and during the 1850s, small
oil
shale industries existed in the eastern United States;
were discontinued, however, when drilling and pumping of oil began in 1859. Oil shales occur on all continents, but the Green River Formation contains the most extensive deposits and has the potential to yield huge quantities of oil. Oil can be produced from oil shale by a process in C which the rock is heated to nearly 500 C in the absence of oxygen, and hydrocarbons are driven off as gases and recovered by condensation. During this process, 25 to 75% of the organic matter of oil shale can be converted to oil and combustible gases. The Green River Formation oil shales yield from 10 to 140 gallons of oil per ton of rock processed, and the total the latter
amount of
oil
recoverable with present processes
is
estimated at 80 billion barrels. Currently, however,
little oil is
produced from
oil
shale in the United
that
would be necessary would have considerable What would be done with
States except at experimental plants, because
environmental impact.
conventional drilling and pumping
billions of tons of processed rock?
Nevertheless, the Green River
is
less
expensive.
shale constitutes one
oil
of the largest untapped sources of oil in the world.
more
effective processes are developed,
more than
eventually yield even
it
If
could
realize,
and sedimentary rocks
(Fig. 7-3).
Any
type of rock
be completely dissolved or chem-
Chapter 6). Such weathered materials are commonly eroded and transported to another location and deposited as sediment. Thus, all sediment is derived from preexisting rocks and ically altered to
form clay minerals
can be characterized
in
is
in
an
already in short supply?
considered by scientists and industry. Perhaps at some future time, the
Green River Formation
some of our energy
any
can weather mechanically to yield small rock fragments and individual mineral grains, and some of a rock's min-
may
huge volumes of water
come from— especially
These and other questions are currently being
Mechanical and chemical weathering disintegrate and decompose rocks yielding the raw materials for both
eral constituents
will the
however, that at the current
INTRODUCTION
soils
Where
necessary for processing area where water
and expected consumption rates of oil in the United States, oil production from oil shale will not solve all of our energy needs. Furthermore, large-scale mining
**
mining be conducted with minimal
disruption of wildlife habitats and groundwater
systems?
the currently
estimated 80 billion barrels.
One should
large-scale
the
Can such
particle, regardless of
1/16 to 2.0
composition, that measures
mm. Gravel- and sand-sized particles are large
enough to be observed with the unaided eye or with lowpower magnification, but silt- and clay-sized particles are too small to be observed except with very high magnification.
Gravel generally consists of rock fragments,
whereas sand,
silt,
and
clay particles are mostly individ-
We
should note, however, that clay
ual mineral grains.
(see
has two meanings: in textural terms, clay refers to sed-
imentary grains
less
than 1/256
mm in size, and in com-
positional terms, clay refers to certain types of sheet icate minerals (see Fig. 3-12).
two ways:
will provide
needs.
sil-
However, most clay-sized
particles in sedimentary rocks are, in fact, clay minerals. 1.
Detrital sediment,
which
consists of rock
fragments and mineral grains. 2.
Chemical sediment, which consists of the minerals precipitated from solution by inorganic chemical processes or extracted from solution by organisms.
In
SEDIMENT TRANSPORT
AND DEPOSITION Detrital sediment can be transported by
any geologic
move
particles of a
agent possessing enough energy to
any case, sediment
is
deposited as an aggregate of
Much accumulated sediment such as mud in a lake, or from
loose solids (Fig. 7-4).
set-
from a fluid, the atmosphere as dust. The term sediment is derived from the Latin sedimentum, meaning settling. Most sedimentary rocks formed from sediment that was transformed into solid rock, but a few sedimentary tled
^
given
size.
Glaciers are very effective agents of transport
and can move any
sized particle.
Wind, on the other
hand, can transport only sand-sized and smaller sediment. Waves and marine currents also transport sediment, but by far the most effective way to erode sediment
rocks skipped the unconsolidated sediment stage. For
^* TABLE
example, coral reefs form as solids when the reef organ-
7-1
Classification of
Sedimentary Particles
isms extract dissolved mineral matter from seawater for their skeletons.
However,
if
a reef
is
broken apart during on
>2
the sea floor are sediment.
One important
mm
Name
Gravel
1/16-2 mm 1/256-1/16
criterion for classifying detrital sedi-
ments and the rocks formed from them is the size of the Gravel refers to any sedimentary particle measuring more than 2.0 mm, whereas sand is
Sediment
Size
a storm, the solid pieces of reef material deposited
Sand
mm
< 1/256 mm
particles (Table 7-1).
160
Chapter 7
Sediment and Sedimentary Rocks
*
Mixtures of
silt
and clay are generally referred to
as
mud.
FIGURE
7-3
The rock
from the weathering
site
cycle,
with emphasis on sediments and sedimentary rocks.
and transport
it
elsewhere
is
by
areas of sand accumulation^Glaciers and mudflows,
streams.
however, are unselective, because their energy allows
During transport, abrasion reduces the size of sedimentary particles. The sharp corners and edges are abraded the most as the particles, especially gravel and sand, collide with one another and become rounded (Fig. 7-5a). Another sediment property modified during
them
transport
is
sorting. Sorting refers to the size distribu-
tion in an aggregate of sediment;
if all
the particles are
approximately the same size, the sediment is characterized as well sorted, but if a wide range of grain sizes occur, the sediment is poorly sorted (Fig. 7-5b). Sorting
from processes that selectively transport and deposit particles by size. Wi ndblown dunes are composed of _well-sorted_ sand, because wind cannot transport gravel effectively and it blows silt and clay beyond_the results
to transport
many
different-sized particles,
and
their deposits tend to be poorly sorted.
Sediment may be transported a considerable distance from its source area, but eventually it is deposited. Some of the sand and mud being deposited at the mouth of the Mississippi River at the present time came from such distant places as Ohio, Minnesota, and Wyoming. Any geographic area in which sediment is deposited is a depositional environment.
Although no completely satisfactory
classification of
depositional environments exists, geologists generally
recognize three major depositional settings: continental, transitional,
depositional
and marine (Fig. 7-6). Major continental environments include stream systems, Sediment Transport and Deposition
161
v^
.
t
^CA^HQp
Desert dunes
Playa lake Alluviarfan
Gi aC a environment j
environment
i
Barrier island
Delta
Beach
Tida
|
f)at
Shallow marine
— environment
Shallow marine environment
Lagoon Continental
Organic reef
shelf
Organic reef
Submarine fan
FIGURE
7-6
Major depositional environments
are
shown
in this generalized
be compacted and/or cemented and thereby converted into
carbonate
sedimentary rock; the process by which sediment
ing a small
is
trans-
formed into sedimentary rock is lithification. When sediment is deposited, it consists of solid particles and pore spaces, which are the voids between particles. The amount of pore space varies depending on the depositional process, the size of the sediment grains, and sorting. When sediment is buried, compaction, resulting from the pressure exerted by the weight of overlying sediments, reduces the amount of pore space, and thus
volume of the deposit (Fig. 7-7b). When deposits of mud, which can have as much as 80% water-filled pore space, are buried and compacted, water is squeezed out, and the volume can be reduced by up to 40%. Sand may have up to 50% pore space, although it is generally somewhat less, and it, too, can be compacted so that the sand grains fit more tightly together. However, once the
sand grains are arranged in a best fit, sand resists further compaction because the rigid mineral-grain framework supports the weight of overlying sediments.
diagram.
(CaC0 3
readily dissolves in water contain-
)
amount of carbonic
acid,
weathering of feldspars and other silica
may
and that chemical
silicate
minerals yields
(Si0 2 ) in solution. These dissolved compounds
pore spaces ot sediments, cement that effectively binds the sediment together (Fig. 7-7c). Calcite cement is easily be pre cipitated
where They
'"•'
FIGURE
in the
act as a
7-7
Lithification of sand, {a)
When
initially
deposited, sand has considerable pore space between grains. (b) Compaction resulting from the weight of overlying sediments reduces the amount of pore space, (c) Sand is converted to sandstone as cement is precipitated in pore spaces from groundwater.
Pore space
Feldspar
Quartz
Compaction alone is generally sufficient for lithificamud, but for sand and gravel deposits cementa-
tion of tion
is
necessary to convert the sediment into sedimen-
tary rock (Fig. 7-7c). Recall
(b)
from Chapter 6 that calcium
Lithification:
and compaction
Burial
(c)
Cementation
Sediment to Sedimentary Rock
163
Guest Essay SUSAN M. LANDON TTTTfTTTTTTTTTTTTTTTTTTrnrTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
EXPLORING FOR OIL AND NATURAL GAS am
an independent petroleum geologist. I specialize applying geological principles to frontier areas— places where little or no exploration has occurred and few or no hydrocarbons have been I
in
discovered.
It is
very
much
like solving a mystery.
The
earth provides a variety of clues— rock type, organic content, stratigraphic relationships, structure, and the
like— that geologists must piece together to determine the potential for the presence of hydrocarbons.
An example of an exploration frontier is the Precambrian Midcontinent Rift located in the north central portion of the United States. Some rifts, like the Gulf of Suez and the North Sea, are characterized by significant
hydrocarbon
unexplored
rift
reserves,
and the presence of an
basin in the center of North America
is
Rocks deposited in this rift basin are exposed along the shores of Lake Superior where they serve as the host for copper ores. One of the mines in the Upper Peninsula of Michigan, the White Pine Mine, has intriguing.
historically
been plagued by
in the shale.
For
many
oil
bleeding out of fractures
years, this
had been documented
as academically interesting because the rocks are
much
older than those that typically have been associated with
hydrocarbon production. Oil and natural gas are generated from organic material preserved in sediment that is subjected to increased temperature through time.
provided the prospect.
We
final
data necessary to generate a specific
then had to convince management that
this prospect had high enough potential to contain hydrocarbon reserves to offset the significant risks and costs. An economic evaluation was conducted to determine the worth of the project given a probability
of success. In this case,
was
management agreed
offset
authorized.
Amoco
was dry (economically
well
drilling sites in the
My
Midcontinent
Rift.
began very early as a result of collecting rocks and growing up in an oil field in the Midwest. I completed my undergraduate work at a small liberal arts college and earned a master's interest in geology
degree from a larger state university.
well-rounded education provided
me
have contributed to
My career Amoco, and, the company
began after
to
my
petroleum industry with
15 years,
work
I
made them
the organic content.
evaluating a Cretaceous chalk in the
history of the basin
was modeled
oil.
drill
area.
I
is
the decision to leave
independently.
prospects in
adequate organic material to be the source of the
believe that a
with a sound
successful career.
in the
My goal
The thermal
I
geological background and communication skills that
and
that the
to
the well will be used to continue to define prospective
projects.
Mine contained
Iowa
unsuccessful), but the
organisms (algae, fungi, and bacteria) to contribute to
and laboratory work documented
well in
geologic information obtained as a result of drilling
variety of companies, assisting
Field
m
drilled a .5,441
prospect at a cost of nearly $5 million. The
test the
However, the sediments associated with the onebillion-year-old rift had a very limited source of
copper-bearing shale at the White Pine
that the
by the potential for a very large accumulation of hydrocarbons, and a well was
risk
I
to have the opportunity to develop
new
frontier areas.
me
I
am
currently
Rocky Mountain
also teach courses for industry.
provided
consult for a
in exploration
My
career has
with the opportunity to travel to a wide
variety of places.
a
to
determine the timing of hydrocarbon generation.
If
hydrocarbons had been generated prior to deposition of
an effective seal and formation of a trap, the hydrocarbons would have leaked naturally out into the
Jusan M. Landon began
atmosphere.
Further
work
identified sandstones with
enough
porosity to serve as reservoirs for hydrocarbons.
Analogy with other hydrocarbon productive rifts gave the exploration team models for trap types. Seismic data were acquired and interpreted to identify specific traps. Coordination with geophysicists and engineers
career in
her
1974 with Amoco
Company and, in opened her own consulting
Production 1989,
office in
Denver, Colorado. In
1990, she was elected president of the American Institute of Professional Geologists.
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 164
Chapter 7
Sediment and Sedimentary Rocks
.
^ FIGURE
7-8
sedimentary rocks
These in the Valley
of
the Gods, Utah are red because they
contain iron oxide cement. (Photo courtesy of Sue Monroe.)
detected because acid.
canyons of Utah and Arizona are colored by small amounts of iron oxide or hydroxide cement (Fig. 7-8).
effervesces with dilute hydrochloric
it
Rocks cemented by
silica are the
hardest,
most
durable sedimentary rocks.
Calcium carbonate and cements
droxides,
such
[FeO(OH)],
some
silica are
the
most common
=*=
sedimentary rocks, but iron oxides and hy-
in
as
hematite
(Fe 2
and
3)
form a chemical cement in of the iron oxide cement is derived
respectively, also
Much
rocks.
from the oxidation of iron
in
ferromagnesian minerals
present in the original deposit, although
some
is
SEDIMENTARY ROCKS
Even though about 95% of the Earth's crust is composed of igneous and metamorphic rocks, sedimentary rocks are the most common at or near the surface. About 75% of the surface exposures on continents consist of sediments or sedimentary rocks, and they cover
limonite
carried
by circulating groundwater. The yellow, brown, and red sedimentary rocks exposed in the walls of the vast
most of the sea
in
floor.
classified as detrital or
Sedimentary rocks are generally chemical (Tables 7-2 and 7-3). "N>
rc.
^ TABLE
7-2
Classification of Detrital Sedimentary
Rocks -cfcp-
Sediment
and
Size
Gravel (>2
Sand
Mud
7
Name mm)
('/i6-2
(
mm) mm)
*Mudrocks possessing
-*"p»
_5^
the property offissilim
Description
Rock Name
Roun ded grave l Angular_gravel
Conglo merate Sedime ntary Breccia
Mostly quartz Quartz wit h >25% feldspar
Arkose
Mostly silt Silt and clay Mostly clay meaning they break along
juartz sandstone
Mudrocks
~> Claystone*
.
closely spaced, paral lel planes, are corn monly called shale.
Sedimentary Rocks
165
"*"
/
\
TABLE
Classification of
7-3
Chemical and Biochemical Sedimentary Rocks
Texture
Composition
Clastic or crystalline
Calcite
Rock Name
(CaC0 3
Limestone (includes coquina
)
and Dolostone chalk,
Dolomite [CaMg (C0 3
^
)
)
2
Halite (NaCl)
Rock gypsum Rock salt
Microscopic Si0 2 shells Altered plant remains
Chert Coal
Gypsum (CaS0 4 -2H 2 0)
Crystalline
^?\
Usually crystalline
oolitic limestone)
Carbonates
I
Evaporites
Hi.ff^u
Detrital Sedimentary
Rocks
gravel gravel,
Detrital sedimentary
rocks consist of detritus, the solid
particles of preexisting rocks.
texture,
Such rocks have a
clastic
meaning that they are composed of fragments
or particles also
known
as clasts (Fig. 7-9a). Several
sedimentary rocks are recognized, characterized by the size of its constit-
varieties of detrital
each of which
is
uent particles (Table 7-2).
Conglomerate and Sedimentary Breccia
4vw<-t-'< J
gular gravel called rubble.
Conglomerate imentary breccia sized particles
a fairly
is is
common
rather rare.
rock type, but sed-
The reason
is
that gravel-
become rounded very quickly during
transport. Thus, if a sedimentary breccia is encountered, one can conclude that the rubble that composes it was not transported very far. High-energy transport agents such as rapidly flowing streams and waves are needed to
-
Both conglomerate and sedimentary breccia consist of gravel-sized particles (Table 7-2; Fig. 7- 10a and b). Usually,
particles; conglomerate consists of rounded whereas sedimentary breccia is composed of an-
transport gravel, so gravel tends to be deposited in high-
energy
environments such as stream channels and
beaches
(Fig. 7-11).
the particles measure a few millimeters to a few
centimeters, but boulders several meters in diameter are
Sandstone
glomerate and sedimentary breccia
sometimes present. The only difference between conis the shape of the
The term sand is simply a size designation, so sandstone may be composed of grains of any type of mineral or rock
""'
clastic texture
FIGURE
7-9
(a)
Photomicrograph of a sandstone showing a
consisting of fragments of minerals, mostly quartz in this case, (b) Photomicrograph of the crystalline texture of a limestone
166
Chapter 7
showing a mosaic of
Sediment and Sedimentary Rocks
calcite crystals.
(
S^T/i/67?C
(C)
-^-
jfc^Wa
FIGURE
breccia;
(c)
7-10
(d )
Detrital sedimentary rocks: (a) conglomerate; (b) sedimentary
sandstone; and
the
(d)
mudrock
shale. (Photos courtesy of
fragment. However, most sandstones consist primarily of
amounts of
the mineral quartz (Fig. 7- 10c) with small
number of other
a
minerals. Geologists recognize several
types of sandstones, each characterized by
its
composi-
is the most common and, as its composed mostly of quartz. Arkose, which contains more than 25% feldspars, is also a fairly
Quartz sandstone
tion.
name
suggests,
common It
is
variety of sandstone (Table 7-2).
may seem odd
that quartz
stones since feldspars are so
is
so
common
in
sand-
much more abundant
in
source rocks.* However, the chance that any specific type of mineral will end up in a sedimentary rock de-
pends on
its
*The Earth's
crust
12%
)
availability,
Sue Monroe.)
chemical
stability.
dant, but quartz cally stable,
Both quartz and feldspars are ahunhard, lacks cleavage, and is chemi-
is
whereas feldspars have two directions of
cleavage and are readily chemically weathered to clays, soluble salts, and ions in solution (see Chapter 6).
The only other particles of much consequence in sandstones are the micas (muscovite and biotite) and fragments of chert, a rock composed of microscopic crystals of quartz. The ferromagnesian silicates, other than biotite, are uncommon in sandstones because they are chemically unstable. One of the most common accessory minerals is the iron oxide magnetite.
mechanical durability, and
Mudrocks is
estimated to contain
potassium feldspar, and
12%
quartz.
39%
plagioclase feldspar,
The mudrocks include all detrital sedimentary rocks composed of silt- and clay-sized particles (Fig. 7-10d).
Sedimentary Rocks
167
Limestone-Dolostone Calcite (the
main component of limestone) and dolomite
comprising the rock dolostone) are both carbonate minerals; calcite is a calcium carbonate (CaC0 3 ), whereas dolomite [CaMg(C0 3 ) 2 ] is a calcium magnesium carbonate. Thus, limestone and dolostone are carbonate rocks. Recall from Chapter 6 that calcite readily (the mineral
dissolves in water containing a small
amount of acid, but
the chemical reaction leading to dissolution
is
reversible,
so solid calcite can be precipitated from solution. Accordingly,
much,
"^ FIGURE
7-11
Gravel
is
deposited in high-energy
depositional environments such as this mountain stream.
some limestone, although probably not very
results
from inorganic chemical reactions.
Most limestones have a large component of calcite that was originally extracted from seawater by organisms. Corals, clams, algae, snails, and a number of other marine organisms construct their skeletons of aragonite, which is an unstable form of calcium carbonate that
Among
mudrocks we can differentiate between siltstone, mudstone, and claystone. Siltstone, as the name implies, is composed of silt-sized particles; mudstone contains a mixture of silt- and clay-sized particles; and claystone is composed mostly of clay (Table 7-2). Some mudstones and claystones are designated as shale if they are fissile, which means they break along closely spaced the
alters to calcite.
tons
may
silt-,
rocks. Turbulence in water keeps
and must therefore be
at a
silt
minimum
and clay suspended if
they are to
settle.
Consequently, deposition occurs in low-energy deposi-
where currents are weak such as in the quiet offshore waters of lakes and in lagoons (Fig. 7-12).
such organisms
Most
Many
chemical sedimentary rocks are composed of
an interlocking mosaic of mineral crystals and are said to have a crystalline texture (Fig. 7-9b). Some, however, have a
clastic texture;
ken pieces of
168
many
limestones consist of bro-
shells.
Chapter 7
Sediment and Sedimentary Rocks
and
limestones are conveniently classified as bio-
chemical sedimentary rocks, because organisms play such a significant role in their origin (Table 7-3). For
example, the limestone tirely
known
as
coquina consists en-
of broken shells cemented by calcium carbonate,
and chalk is a soft variety of biochemical limestone composed largely of microscopic shells of organisms (Fig. 7-13a and b).
One
distinctive
type of limestone contains small
spherical grains called ooids.
"^ FIGURE Chemical sedimentary rocks originate from the ions and salts taken into solution in the weathering environment (Table 7-3). Such dissolved materials are transported to lakes and the oceans where they become concentrated. Inorganic chemical processes remove these substances from solution, and they accumulate as solid minerals. Biochemical sedimentary rocks, which constitute a subcategory of chemical sedimentary rocks, result from the chemical processes of organisms (Table 7-3).
lithified
(Fig. 7-13).
tional environments
Chemical Sedimentary Rocks
die, their skele-
and clay-sized sediment that becomes
forms limestone
parallel planes (Fig. 7-10d).
The mudrocks comprise about 40% of all detrital sedimentary rocks, making them the most common of these
When
be broken up and accumulate as gravel-, sand-,
7-12
Ooids have a small nu-
Exposure of mudstone that was
deposited in an ancient lake.
(a)
^-FIGURE is
7-13
composed of
Three types of limestones,
(a)
Coquina
the broken shells of organisms. (Photo
courtesy of Sue Monroe.) (b) Chalk cliffs in Denmark. Chalk is made up of microscopic shells. (Photo courtesy of R. V. Dietrich.) (c) Photomicrograph of ooids in an oolitic in diameter. limestone. These ooids measure about 1
mm
cleus, a sand grain or shell fragment perhaps — around which concentric layers of calcite precipitate; lithified deposits of ooids form oolitic limestones (Fig. 7-13c). The near-absence of recent dolostone and evidence from chemistry and studies of rocks indicate that most
was originally limestone that has been changed to dolostone. Many geologists think most dolostones originated through the replacement of some of the calcium in calcite by magnesium. Figure 7-14 shows one way this can occur. Note that in a restricted environment, such as a lagoon, where evaporation rates are dolostone
high,
much
of the calcium in solution
goes into calcite
is
extracted as
it
(CaC0 3 and gypsum (CaS0 4 -2H 2 0). )
i
High evaporation rate
Magnesium (Mg), on
the other hand, becomes concenwhich then becomes denser and permeates the preexisting limestone and converts it to dolostone by the addition of magnesium. trated in the water,
*
— FIGURE
7-14
One way
in
which limestone can be converted to dolostone. In this example, the
seawater in a lagoon becomes enriched in magnesium as evaporation occurs. This magnesium-rich water is denser than normal seawater so it permeates the older limestones and converts them to dolostone.
Limestone converted dolostone
to
Sedimentary Rocks
169
Perspective 7-1
THE MEDITERRANEAN DESERT not for the connection with the Atlantic Ocean at the
Vast, thick evaporite deposits are present in several
one of the most notable is beneath the Mediterranean Sea. At the present time, the Mediterranean Sea is in an arid region where the rate of evaporation of seawater exceeds the rate at which water is added to the sea by rainfall runoff. If it were areas, but
"^ FIGURE About
1
Strait of Gibraltar (Fig. 1), the
Mediterranean would
eventually dry up and form a vast desert basin far
below sea level. Some geologists think that the Mediterranean did dry up during the Cenozoic Era, resulting in the deposition of rock gypsum and rock
Panoramic view showing the submarine topography of the present-day Mediterranean Basin. was probably a vast desert lying 3,000 m below sea level.
6 million years ago, the Mediterranean Basin
tion limit, the point at
Evaporites
Rock
Evaporites include such rocks as rock salt and rock gypsum, which form by inorganic chemical precipitation of minerals from solution (Table 7-3; Fig. 7-15). Both are characterized by a crystalline texture. In Chapter 6
we
which precipitation must occur.
composed of the mineral halite (NaCl), is simply sodium chloride that was precipitated from seawater or, more rarely, lake water (Fig. 7-15a). Rock gypsum, the most common evaporite rock, is composed of the mineral gypsum (CaS0 4 H 2 0), which also presalt,
A
noted that some minerals are dissolved during chemical
cipitates
weathering, but a solution can hold only a certain vol-
number of other evaporite rocks and minerals are known, but most of these are rare. Some are important, however, as sources of various chemical compounds; for
ume
of dissolved mineral matter.
solution
is
If
the
volume of a
reduced by evaporation, the amount of
dis-
solved mineral matter increases in proportion to the vol-
ume
170
of the solution and eventually reaches the satura-
Chapter 7
Sediment and Sedimentary Rocks
from evaporating solutions
(Fig.
7- 15b).
example, sylvite, a potassium chloride (KC1), is used the manufacture of fertilizers, dyes, and soaps.
in
salt
now
present beneath the floor of the sea.
Atlantic
Studies of the Mediterranean evaporites indicate
deposition in shallow water, rather than in a deep-ocean basin,
which
is
According to
what
the Mediterranean
this hypothesis, the
is
now.
Mediterranean
lost its
connection with the Adantic, evaporated to near dryness in as little as
1,000 years, and became a vast desert
basin lying 3,000
m
below sea
level (Fig. 2).
Simply evaporating the Mediterranean to dryness would yield only 40 to 45 m of evaporites, however,
km thick. It appears that once the Mediterranean evaporated, it formed a low-lying basin in which vast, shallow saline lakes existed. Evaporites formed in these lakes and simply accumulated, forming thick deposits. Furthermore, apparently the not a layer 2
connection with the Atlantic was periodically reestablished so that the Mediterranean refilled.
During the times when an oceanic connection existed, sand and gravel were deposited near the margins of the sea, and deep-sea sediments, mostly clay, were deposited in the offshore areas. Subsequently, the
Mediterranean was again isolated from the Atlantic, and the evaporation sequence was repeated, perhaps several times.
Supporting evidence for history
this
view of Mediterranean
comes from southern Europe and north
Africa.
The present-day Mediterranean controls the level to which streams can erode downward: they can erode no lower than sea level. In Europe and Africa, however, there are canyons cut into solid bedrock that extend to depths far below present sea contention
is
level.
The
that during periods of lower sea level,
Ocean
Western Mediterranean
(a)
(c)
-^ FIGURE (c)
Core of rock salt from a well Chert. (Photos courtesy of Sue Monroe.) 7-15
cherts result
(a)
(Fig.
Michigan,
7-16). Unfortunately, the shells of
partly altered plant remains accumulate as layers of or-
biochemical origin of
of
silica-
many bedded
cherts
is
obscured.
Coal a biochemical sedimentary rock
land plants (Table 7-3; Fig. 7-17).
and bogs where the water
composed of the
is
It
forms in swamps oxygen or where
deficient in
organic matter accumulates faster than
it
decomposes.
decompose vegetation in swamps can exist without oxygen, but their wastes must be oxidized, and because no oxygen is present, the wastes accumu-
172
late
commonly smell of hydrogen sulfide odor of swamps). When buried, this organic muck becomes peat, which looks rather like ganic muck, which (the rotten-egg
bacteria that
Chapter 7
Sediment and Sedimentary Rocks
Where peat
is abundant, as in Ireburned as a fuel. Peat that is buried more deeply and compressed, especially if it is heated too, is altered to a type of dark brown coal called lignite, in which plant remains are still clearly visible. During the change from organic muck to coal, such volatile elements of the vegetation as oxygen, hydrogen, and nitrogen are partly vaporized and driven off, thus enriching the residue in carbon; lignite contains about
coarse pipe tobacco.
is
compressed, altered remains of organisms, especially
The
Rock gypsum.
and kill the bacteria. Thus, bacterial decay ceases and plant materials are not completely destroyed. These
shells
these organisms are easily altered, so the evidence for a
Coal
(b)
organisms such as radiolarians
from accumulations of
secreting, single-celled
and diatoms
in
land and Scotland,
it
is
LT
¥.-x-j*r-
\ .<Mm (a)
(b)
•^ FIGURE
7-16
single-celled (a)
Bedded chert is composed of microscopic radiolarians and (b) diatoms.
70%
carbon as opposed to about 50% in peat. Bituminous coal, which contains about 80% carbon, is a higher grade coal than lignite. It is dense and black and has been so thoroughly altered that plant remains can only rarely be seen. The highest grade coal is anthracite, which is a metamorphic type of coal (see Chapter 8). It contains up to 98% carbon and, when burned, yields
more heat per unit volume than other types of coal. most of the coal mined in the United
Historically,
States has been bituminous coal
from the coal
fields
of
shells of silica-secreting,
of physical, chemical, and biological attributes. Such distinctive bodies of sediment, or sedimentary rock, are
sedimentary
Any
facies.
aspect of sedimentary rocks that
makes them
recognizably different from adjacent rocks of the same age, or approximately the
same
age, can be used to es-
tablish a sedimentary facies. Figure 7-19 illustrates three
sedimentary
carbonate
facies: a
facies.
If
sand
facies, a
these sediments
mud
facies,
become
and a
lithified,
they are called sandstone, mudstone (or shale), and
the Appalachian coal basin (Fig. 7-18). These coal de-
limestone facies, respectively.
formed in coastal swamps during the Pennsylvanian Period between 286 and 320 million years ago. Huge lignite and subbituminous coal deposits also exist in the western United States, and these are becoming
^ FIGURE
7-17
composed of
the altered remains of land plants. (Photo
posits
Coal
is
a biochemical sedimentary rock
courtesy of Sue Monroe.)
increasingly important resources (Fig. 7-18).
^ SEDIMENTARY FACIES If
a layer of sediment or sedimentary rock
erally,
both.
it
It
is
traced
lat-
generally changes in composition, texture, or
changes by lateral gradation resulting from the
simultaneous operation of different depositional processes in adjacent depositional environments. For
exam-
sand may be deposited in a high-energy nearshore environment while mud and carbonate sediments accumulate simultaneously in the laterally adjacent lowenergy offshore environments (Fig. 7-19). Deposition in each of these environments produces a body of sediment, each of which is characterized by a distinctive set ple,
Sedimentary Facies
173
Explanation
Type
of
coal
|
I
I
4
[
environments. The strata
in
Percentage of
all
coal
| Anthracite
1
^\ Bituminous coal
48%
%
^| Subbituminous coal 34% ^\
Lignite
17%
L_J
600
km
"^ FIGURE
7-18
Distribution of coal deposits in the United States.
Marine Transgressions and Regressions Many
sedimentary rocks in the interiors of continents show clear evidence of having been deposited in marine
^ FIGURE
7-19
Deposition in
adjacent environments yields distinct bodies of sediment, each of
which
is designated as a sedimentary facies.
174
Chapter 7
Sediment and Sedimentary Rocks
Figure 7-20, for example,
consist of a sandstone facies that
was deposited
in a
nearshore marine environment overlain by shale and limestone facies that were deposited in offshore environ-
ments. Such a vertical sequence of facies can be explained by deposition occurring during a time level rose
with respect to the continents.
when
When
sea
sea level
with respect to a continent, the shorelin£_mxiY£S.
rises
^nland, giving rise to a marine tra nsgression ( Fig. 7-20).
As the shoreline advances ronments
inland, the depositional envi-
parallel to the shoreline
do
likewise.
Remem-
ber that each laterally adjacent environment in Figure
7-20
is
the depositional site of a different sedimentary
As a result of a marine transgression, the facies that formed in the offshore environments are superposed over the facies deposited in the nearshore envifacies.
ronment, thus accounting for the vertical succession of sedimentary facies (Fig. 7-20). Another important aspect of marine transgressions is that an individual facies can be deposited over a huge geographic area (Fig. 7-20). Even though the nearshore environment is long and narrow at any particular time, deposition occurs continuously as the environment migrates landward during a marine transgression. The sand deposited under these conditions may be tens to hundreds of meters thick, but have horizontal dimensions of length and width measured in hundreds of kilometers.
The opposite of
a marine transgression
is
a
marine
regression. If sea level falls with respect to a continent,
and environments that parallel the shoreseaward direction. The vertical sequence produced by a marine regression has facies of the nearshore environment superposed over facies of offshore environments. Marine regressions can also account for the shoreline line
move
in a
the deposition of a facies over a large geographic area.
^ ENVIRONMENTAL ANALYSIS When geologists
investigate sedimentary rocks in the field,
they are observing the products of processes that operated
during the past. The only record of these processes
is
pre-
served in the rocks, so geologists must evaluate those aspects of sedimentary rocks that allow inferences to be
and the environment of
made about
the original processes
deposition.
Sedimentary textures such as sorting and
rounding can give clues to the depositional process. Windblown dune sands, for example, tend to be well sorted and well rounded.
The geometry or three-dimensional shape of
rock bodies
another important criterion in environmen-
is
tal interpretation.
Marine transgressions and regressions
yield sediment bodies with a blanket or sheetlike geometry, in stream channels tend to be long and narrow and are therefore described as having a shoestring geometry (Fig. 7-21). Other aspects of sedimentary rocks
whereas deposits
that are important in environmental analysis include sed-
imentary structures and
fossils.
Mudrock
— Sandstone "^FIGURE 7-21 Two different geometries of sedimentary rock bodies. The limestone, sandstone, and mudrock all have blanket geometries. Within the mudrock, however, elongate sandstones have a shoestring geometry.
composition, grain tures (Fig. 7-22).
size, color,
Almost
some kind of bedding;
all
"^ FIGURE 7-22 Most sedimentary rocks show some kind of layering or bedding as these sandstones in Montana.
or a combination of fea-
sedimentary rocks show
a few, such as limestones that
they reach the relatively level sea or lake floor. There, they rapidly slow
down and
begin depositing trans-
formed as coral reefs, lack this feature, however. Graded bedding involves an upward decrease in grain size within a single bed (Fig. 7-23). Most graded bedding appears to have formed from turbidity current deposition, although some forms in stream channels during the waning stages of floods. Turbidity currents are un-
ported sediment, the coarsest
derwater flows of sediment-water mixtures that are denser than sediment-free water. Such flows move
and
sediments in stream channels and shallow marine environments. Invariably, cross-beds result from trans-
downslope along the bottom of the sea or
port by wind or water currents, and the cross-beds are
a lake until
"»" FIGURE 7-23 Graded bedding in an ancient stream channel in Montana. Notice that the sizes of the sedimentary particles decrease upward; the base of the deposit consists of conglomerate, whereas the top, just above the hammer handle, is sandstone.
176
Chapter 7
Sediment and Sedimentary Rocks
first
followed by progres-
sively smaller particles (Fig. 7-23).
Many
sedimentary rocks are characterized by cross-
bedding; cross-beds are arranged such that they are at an angle to the surface upon which they accumulated (Fig. 7-24).
Cross-bedding
is
common
in desert
dunes
in
"^ FIGURE
Cross-bedding forms when the beds are upon which they accumulate. Cross-beds indicate ancient current directions 7-24
inclined with respect to the surface
by
their dip, to the left in this case.
(b)
(a)
"^ FIGURE (b)
7-25
Wave-formed
inclined
(a)
Current ripple marks on the bed of a stream
ripples
on Heron
downward, or
dip, in the direction of flow. Be-
cause their orientation depends on the direction of flow, cross-beds are
good
in
Michigan.
Island, Australia.
indicators of ancient current direc-
intact.
The remains of organisms are called body fossils them from trace fossils such as tracks,
to distinguish trails,
and burrows
(Fig. 7-27),
which are indications of
tions or paleocurrents (Fig. 7-24).
ancient organic activity.
one can commonly observe smallmarks on bedding planes. Two common types of ripple marks are recognized. One type is asymmetrical in cross section and forms as a result of currents that move in one direction as in a stream channel. These are current ripple marks (Fig. 7-25a) and, like cross-bedding, are good paleocurrent indicators. In con-
For any potential fossil to be preserved, it must escape the ravages of such destructive processes as running water, waves, scavengers, exposure to the atmosphere, and bacterial decay. Obviously, the soft parts of organ-
In sand deposits
scale, ridgelike ripple
trast, ripples that
tend to be symmetrical in cross section
and are wave-formed ripple marks (Fig. 7-25b). cracks are found in clay-rich sediment that has
are produced by the to-and-fro motion of waves
known
Mud
isms are devoured or decomposed most rapidly, but even the hard skeletal elements will be destroyed unless they
and protected in mud, sand, or volcanic ash. Even if buried, skeletal elements may be dissolved by groundwater or destroyed by alteration of the host rock. are buried
as
dried out (Fig. 7-26).
When
such sediment
shrinks and forms intersecting fractures
(mud
dries,
it
cracks).
Such features in ancient sedimentary rocks indicate that the sediment was deposited where periodic drying was
"^ FIGURE when
7-26
Mud
cracks form in clay-rich sediments
they dry and shrink.
on a river floodplain, near a lake shore, or where muddy deposits are exposed on marine shorelines at low tide. possible as
Fossils
remains or traces of ancient organisms These remains are mostly the hard skeletal elements of organisms such as shells, bones, and teeth, but under exceptional conditions even the soft-part anatomy may be preserved. For example, several frozen woolly mammoths have been discovered in Alaska and Siberia with hair, flesh, and internal organs preserved Fossils are the (Fig. 7-27).
Environmental Analysis
177
"^ FIGURE
7-28
Specimen of
woody
original structure of the
petrified
tissues.
wood showing
the
(Photo courtesy of
Sue Monroe.)
Nevertheless, fossils are quite
common. The remains
of
microscopic plants and animals are the most common, but these require specialized methods of recovery, prep-
and study and are not sought out by casual of marine animals are also very common and easily collected in many areas, and even the bones and teeth of dinosaurs are much more common than most people realize. Some fossils retain their original composition and
aration,
fossil collectors. Shells
structure,
but
and thus are preserved as unaltered remains,
many have been
altered in
some way. For example,
dissolved mineral matter can be precipitated in the pores of bones, teeth, and shells or can fill the spaces within cells
of
ment of
wood. the
Wood may
woody
tissues
as petrified, a term that
be preserved by the replaceby silica; it then is referred to
means "to become stone"
(Fig.
7-28). Silicon dioxide (Si0 2 ) or iron sulfide (FeS 2 ) can
completely shells of
replace
calcium
the
marine animals
leaves, stems,
organism
(Fig.
show
7-29b). Shells in sediment
mold
is
pre-
the details of the
be dissolved leaving a cavity called a mold that like the shell. If a
)
and the
and roots of plants are commonly
served as thin carbon films that original
(CaC0 3
carbonate
(Fig. 7-29a). Insects
filled in, it
is
may
shaped
becomes a cast
(Fig. 7-29c).
"»" FIGURE 7-27 (a) Body fossils consist of the actual remains of organisms, trilobites in this case, (b) Trace fossils are an indication of ancient organic activity. These bird tracks are preserved in mudrock of the Green River Formation of Wyoming.
178
Chapter 7
Sediment and Sedimentary Rocks
If it were not for fossils, we would have no knowledge of such extinct animals as trilobites and dinosaurs. Thus, fossils constitute our only record of ancient life. They are not simply curiosities, however, but have sev-
eral practical uses. In
many
geologic studies,
it is
neces-
sary to correlate or determine age equivalence of sedi-
mentary rocks
in different areas.
Such correlations are
(d)
(c)
*** {b)
FIGURE
7-29 Various types of fossilization. (a) Replacement by iron sulfide (FeS 2 ). Carbonized leaf, (c) Mold, (d) Cast. (Photos courtesy of Sue Monroe.)
most commonly demonstrated with fossils; we will discuss correlation more fully in Chapter 9. Fossils are also useful in determining environments of deposition.
Environment of Deposition The sedimentary rocks
in the geologic
record acquired
their various properties, in part, as a result of the physical,
chemical, and biological processes that operated in
the original depositional environment.
One
of geolo-
major tasks is to determine the specific depositional environment of sedimentary rocks. Based on their knowledge of cross-bedding and present-day processes, such as sediment transport and deposition by streams, gists'
geologists can
make
inferences regarding the deposi-
tional environments of ancient sedimentary rocks.
While conducting field studies, geologists commonly make some preliminary interpretations. For example, some sedimentary particles such as ^ooid sjp limesto nes most commonly form in shallow marine environments
where cu rrents are vigorou s. Large-sca lg_c ross-beddin g is typicaTof, but not restricted to, des&rt dunes. Fossils of land plants and animals can be washed into transitional environments, but mos^of them are preserved in deposts of contin e ntal envir o nment s. Fossil sheuTof such i
marine-dwelling animals as corals obviously indicate marine depositional environments. Much environmental interpretation is done in the laboratory where the data and rock samples collected during field work can be more fully analyzed. Such analyses might include microscopic and chemical examination of rock samples, identification of fossils, and
Environmental Analysis
179
— West Central
Pennsylvania
Tuscarora
Sandstone
Grain size increases
Shale more
common Comparison with recent deposits
Sedimentary structures Cross-bedding
I
Mostly horizontal
Textural trends similar to
and
distribution of
sedimentary structures
those of Platte River, Colorado-Nebraska
Interpretation
•^ FIGURE
7-30
cross section
showing the
A
simplified
Generally west to northwest
Tuscarora
downstream
lateral
relationships for three rock units in
of braided
the eastern United States.
river
part
system
Shawangunk and Green Pondupper part of braided stream system
were derived from a source region
graphic representations showing the three-dimensional
indicates they
shapes of rock units and their relationships to other rock units. In addition, the features of sedimentary rocks are
area of the present-day Appalachian Mountains.
compared with those of sediments from present-day
mations are exposed, and many of these can be traced for great distances. Three of these, the Tapeats Sandstone, the Bright Angel Shale, and the Mauv Limestone,
depositional environments; the contention
is
that fea-
tures in ancient rocks, such as ripple marks,
formed
during the past in response to the same processes sponsible for
them now.
Finally,
when
all
re-
data have been
analyzed, an environmental interpretation
is
made.
The following examples illustrate how environmental interpretations are made. The Green Pond Conglomerate, Shawangunk Conglomerate, and Tuscarora Sandstone, three ancient formations* in the eastern United States, possess characteristic grain sizes,
rock types, and
sedimentary structures that indicate deposition in a con-
In the
occur
Grand Canyon of Arizona,
in vertical
a
number
in the
of for-
sequence and contain features, includ-
ing fossils, that clearly indicate that they were deposited
and marine environments (Fig. 7-31). In were forming simultaneously, but a marine transgression caused them to be superposed in the order in transitional fact, all three
now
observed (Fig. 7-20). Similar sequences of rocks of approximately the same age in Utah, Colorado, Wyoming, Montana, and South Dakota indicate that this marine transgression was widespread indeed.
tinental environment, particularly a system of streams
westward (the paleocurrent direcwas determined by the orientation of cross-beds) (Fig. 7-30). As supporting evidence for this interpretation, these ancient deposits possess textures and sedi-
that flowed generally tion
mentary structures very similar to those of the presentday deposits of the Platte River in Colorado and Nebraska. The composition of the sedimentary particles
*A formation
is
boundaries that
map. The term
a is
is
body of rock with distinctive upper and lower extensive enough to be depicted on a geologic generally applied to sedimentary rocks, but can be
used for some igneous and metamorphic rocks as well.
180
Chapter 7
Sediment and Sedimentary Rocks
^
SEDIMENTS, SEDIMENTARY ROCKS, AND NATURAL RESOURCES
The
uses of sediments
terials
and sedimentary rocks or the ma-
they contain vary considerably. Sand and gravel are
essential to the construction industry,
are used for ceramics,
and limestone
pure clay deposits
is
used in the manu-
where iron ore
facture of
cement and
in blast furnaces
refined to
make
Evaporites are the source of com-
mon
steel.
table salt as well as a
pounds, and rock gypsum board.
The
is
is
number of chemical comused to manufacture wall-
tiny island nation of
Nauru, with one of the
highest per capita incomes in the world, has an
economy
based almost entirely on mining and exporting phosphatebearing sedimentary rock that
is
used in
fertilizers.
an especially desirable resource because it burns hot with a smokeless flame. Unfortunately, it is the least common type of coal, so most coal used for heating buildings and for generating electrical energy is Anthracite coal
bituminous
make coke,
is
(Fig. 7-18).
Bituminous coal
ash of bituminous coal; coke
is
also used to
prepared by heating the
coal and driving off the volatile matter. fire
is
a hard, gray substance consisting of the fused
Coke
is
used to
blast furnaces during the production of steel (Fig.
and gas and a number of other prodmade from bituminous coal and lignite.
7-32). Synthetic oil ucts are also
Petroleum and Natural Gas Both petroleum and natural gas are hydrocarbons, meaning that they are composed of hydrogen and car-
"^ FIGURE 7-31 View of the Tapeats Sandstone, Bright Angel Shale (forming the slope in the middle distance), and Mauv Limestone in the Grand Canyon in Arizona. These formations were deposited during a widespread marine transgression.
"^ FIGURE
Diagrammatic representation of a which iron ore is refined. The raw made from bituminous coal, and limestone as a fluxing agent. The limestone combines with silica in the iron ore and forms a glassy slag that is drawn off near the bottom of the blast furnace. Much of the molten iron is further refined to produce steel.
Crushed ore limestone and coke
7-32
blast furnace in
materials needed are iron ore, coke
Exhaust gases
Preheated
air
or oxygen
Sediments, Sedimentary Rocks, and Natural Resources
181
bon. Hydrocarbons form from the remains of microscopic organisms that exist in the seas and in lakes.
When
some
large
these organisms die, their remains settle to
where little oxygen is available to decompose them. They are then buried under layers of sediment. As the depth at which they are buried increases, they are heated and transformed into petroleum and natural gas. The rock in which the hydrocarbons formed is generally called the source rock. For petroleum and natural gas to occur in economic quantities, they must migrate from the source rock into the sea or lake floor
^ FIGURE
7-33
some kind of rock in which they can be trapped. If there were no trapping mechanism, both would migrate upward and eventually seep out at the surface. Indeed, such seeps are known; one of the most famous is the La Brea Tar Pits in Los Angeles, California. The rock in which petroleum and natural gas accumulate is known as reservoir rock (Fig. 7-33). Effective reservoir rocks
contain a considerable
amount of pore space
so that
appreciable quantities of hydrocarbons can accumulate.
Furthermore, the reservoir rocks must possess high permeability, or the capacity to transmit fluids; otherwise
Oil and natural
The arrows
in both diagrams indicate the migration of hydrocarbons, (a) Two examples of
gas traps.
stratigraphic traps, (b)
Two
examples of structural traps, one formed by folding, the other by faulting.
Oil
seep
Source bed (b)
182
Chapter 7
Sediment and Sedimentary Rocks
hydrocarbons cannot be extracted in reasonable quantities. In addition, some kind of impermeable cap rock must be present over the reservoir rock to prevent upward migration of the hydrocarbons (Fig. 7-33). Many hydrocarbon reservoirs consist of nearshore marine sandstones in proximity with fine-grained, organic-rich source rocks. Such oil and gas traps are called stratigraphic traps because they
owe
their exist-
ence to variations in the strata. Ancient coral reefs are also
good
stratigraphic traps. Indeed,
the Persian Gulf region
is
trapped
spective 7-2). Structural traps result
formed by
some of
the oil in
in ancient reefs (Per-
when
rocks are de-
folding, fracturing, or both. In areas
where
sedimentary rocks have been deformed into a series of folds, hydrocarbons migrate to the high parts of such structures (Fig. 7-3 3 b). Displacement of rocks along faults (fractures
along which movement has occurred)
also yields situations conducive to trapping hydrocar-
bons
(Fig.
"^ FIGURE
7-34
dome
which
salt
in
An example oil
of structures adjacent to a
and natural gas may be trapped.
7-33b).
Coast region, hydrocarbons are commonly domes. A vast layer of rock salt was precipitated in this region during the Jurassic Period as the ancestral Gulf of Mexico formed
Most
when North America separated from North Africa. Rock salt is a low-density sedimentary rock, and when deeply
United States comes from the complex potassium-, uranium-, vanadium-bearing mineral carnotite found in
buried beneath more dense sediments such as sand and
some sedimentary
In the Gulf
found
mud,
in structures adjacent to salt
it
rises
toward the surface
domes. As the rock
salt rises,
it
in pillars
known
as salt
penetrates and deforms
the overlying rock layers, forming structures along
its
margins that may trap petroleum and gas (Fig. 7-34). Other sources of petroleum that will probably become increasingly important in the future include oil shales and tar sands. The United States has about twothirds of all known oil shales, although large deposits also occur in South America, and all continents have some oil shale. The richest deposits in the United States are in the Green River Formation of Colorado, Utah,
and Wyoming
(see the Prologue).
Tar sand is a type of sandstone in which viscous, asphaltlike hydrocarbons fill the pore spaces. This substance is the sticky residue of once-liquid petroleum
from which the volatile constituents have been lost. Liquid petroleum can be recovered from tar sand, but to do so, large quantities of rock must be mined and processed. Since the United States has few tar sand deposits, it
cannot look to
ergy resource.
this
of the uranium used in nuclear reactors in the
rocks. Some uranium is also derived from uraninite (UO z ), a uranium oxide that occurs in granitic rocks and hydrothermal veins. Uraninite is easily oxidized and dissolved in groundwater, transported elsewhere, and chemically reduced and precipitated in the presence of organic matter.
The
uranium ores in the United States are Colorado Plateau area of Colorado and adjoining parts of Wyoming, Utah, Arizona, and New Mexico. These ores, consisting of fairly pure masses and encrustations of carnotite, are associated with plant remains in sandstones that formed in ancient stream channels. Although most of these ores are associated with fragmentary plant remains, some petrified richest
widespread
in the
uranium. Large reserves of low-grade uranium ore also occur
trees also contain large quantities of
in the
Chattanooga Shale. The uranium
inated in this black, organic-rich
is
finely dissem-
mudrock
that underlies
large parts of several states including Illinois, Indiana,
Ohio, Kentucky, and Tennessee.
source as a significant future en-
The Athabaska
tar sands in Alberta,
Can-
ada, however, are one of the largest deposits of this type. These deposits are currently being mined, and it is esti-
mated
Uranium
that they contain several
of recoverable petroleum.
hundred
billion barrels
Banded Iron Formation Banded iron formation
is
a chemical sedimentary rock of
great economic importance. Such rocks consist of alter-
nating thin layers of chert and iron minerals, mostly the
Sediments, Sedimentary Rocks, and Natural Resources
183
Perspective 7-2
PERSIAN GULF PETROLEUM During the 1850s, the demand
for
petroleum was
of
increasing in the United States as people sought a cheap alternative to other sources to be used for lighting, as a
and as an ingredient in liniments. In 1859, Edwin L. Drake drilled an oil well 21 m deep at Titusville, Pennsylvania, and began lubricant for machinery,
pumping 10 to 35 barrels of oil per day (1 gallons). The United States quickly became leading producer, a position since that time,
it
= 42
the world's
(Fig. 1).
petroleum imports
in the
world come from the
Although large concentrations of petroleum occur in areas of the world, more than 50% of all proven reserves are in the Gulf region (Fig. 2)! Furthermore,
many
some of
the oil fields are gigantic; at least
expected to yield more than each,
and 7 had surpassed
20
are
five billion barrels
this figure
of
oil
by 1983.
Several factors account for the prolific quantities of
maintained until 1965;
has been in second place
it
barrel
all
Gulf countries.
oil in
the Gulf region.
By the beginning of the Mesozoic
of the continents had joined together to form the
90 years after the Drake well was drilled, the United States became a net petroleum importer. Currently, the United States imports more than half of all the petroleum it consumes, much of it from
Era,
the Persian Gulf region.
was
Even though petroleum was discovered as early as 1908 in Iran, the Gulf region did not become a
continental margin as opposed to an active margin
Nevertheless, barely
significant
petroleum-producing area until the
economic recovery after World War II. Following the war, Western Europe and Japan in particular became dependent on Gulf oil and still rely heavily on this region for most of their supply. The United States is also dependent on imports from the Gulf, but receives significant quantities of petroleum from other sources such as Mexico and Venezuela. Currently, fully 40%
t*- FIGURE 1
all
supercontinent Pangaea. However, they were arranged
such that present-day Africa and Eurasia were separated
by the Tethys Sea
(Fig. 3).
What
is
now
the Gulf region
a broad, stable, marine shelf extending eastward
from
Africa. Geologists refer to such a shelf as a passive
characterized by plate convergence, volcanism,
earthquake
activity,
Gulf Coast, which is
and strong deformation. The U.S.
is
also a passive continental margin,
another area of significant petroleum reserves.
During the Mesozoic Era, and particularly the when most of the petroleum formed, this continental margin lay near the equator where countless microorganisms lived in the surface Cretaceous Period
The top 10 oil-producing Numbers indicate barrels
countries for 1989.
of
oil
produced
daily.
Venezuela 1,732.000
184
Chapter 7
Sediment and Sedimentary Rocks
Kuwait 1,742,000
United Kingdom 1,743,000
'"'-
FIGURE
proven
2
The top 10 countries in 1989. Numbers indicate
oil reserves in
millions of barrels of
(Fig. 3). The remains of these organisms accumulated with the bottom sediments and were buried, beginning the complex process of oil generation and formation of source beds. Broad passive continental margins such as the one that existed in the Gulf region during the Mesozoic Era
waters
oil.
are particularly susceptible to transgressions
and
regressions. Several such events occurred during
some of
which
the reservoir rocks formed as extensive, thick
""•" FIGURE 3 The position of the continents during the Cretaceous Period. The Gulf region, which is part of the Arabian plate, was still connected to the African plate.
continued on next page
Sediments, Sedimentary Rocks, and Natural Resources
185
African plate
-•-FIGURE 4
The
oil
and
gas fields or discoveries in the Gulf region.
Arabian plate
is
The moving
north (arrow) and colliding with Eurasia along the
Zagros suture.
regressive sandstones.
of oolitic limestones
and
reefs
Other important reservoirs consist
(Fig. 7- 13c), algal reef
composed of
limestones,
the shells of clams. In any case,
because the shelf upon which they were deposited was
2,000 to 3,000 km wide and at least twice as long. Overlying the reservoir rocks are cap rocks that include units.
Equally important in the overall geologic history of the region
— and
the preservation of petroleum
—
is its
deformational history since the petroleum formed. In general, tectonism has not been extreme;
been,
much
if it
fields are
however
these reservoir rocks are geographically extensive
widespread shale and evaporite
Arabian plate against Eurasia causes continuing deformation in Iran and northern Iraq. Most of the
had
of the petroleum would have been
Many
oil
south of the area of strong deformation, (Fig. 4).
nations including the United States are
on imports of Gulf oil, a dependence that will increase in the future. Within a few decades, however, the world's petroleum resources
heavily dependent
will likely be nearly exhausted.
that
all
Most
geologists think
of the truly gigantic oil fields have already
been found, but concede that some significant discoveries are yet to be made.
One must view
these
potential discoveries in the proper perspective,
destroyed by metamorphism or lost by leakage
however. For example, the discovery of an
through extensive fractures. As a consequence of
comparable to that of the North Slope of Alaska (about 10 billion barrels) constitutes about a two-year
Red Sea and Gulf of Aden during the Cenozoic Era, the Arabian plate has separated from rifting in the
the African plate.
186
Chapter 7
Northward movement of
the
Sediment and Sedimentary Rocks
supply for the United States at the current
consumption
rate.
oil field
and magnetite (Fig. 7-35). Banded on all the continents and account for most of the iron ore mined in the world today. The origin of banded iron formations is not fully understood, and none are presently forming. Fully 92% of all banded iron formations were deposited in shallow seas between 2.5 and 2.0 billion years ago, during the Proterozoic Eon. Iron is a highly reactive element that in the presence of oxygen combines to form rustlike oxides that are not readily soluble in water. During early Earth history, however, little oxygen was present in the atmosphere, and thus little was dissolved in seawater. How+2 ever, soluble reduced iron (Fe and silica were present iron oxides hematite
iron formations are present
)
in seawater.
Geological evidence indicates that abundant photosynthesizing organisms were present about 2.5 billion
"^ FIGURE 7-35 Outcrop of banded iron formation in northern Michigan.
years ago. These organisms, such as bacteria, release
oxygen as a byproduct of respiration; thus, they released oxygen into seawater and caused large-scale precipitation of iron oxides and silica as banded iron formations.
^CHAPTER SUMMARY 1.
whereas chemical sediment consists of minerals extracted from solution by inorganic chemical processes and the activities of organisms. Sedimentary particles are designated in order of decreasing size as gravel, sand, silt, and clay. Sedimentary particles are rounded and sorted during transport although the degree of rounding and sorting depends on particle size, transport distance, and depositional process. Any area in which sediment is deposited is a
3.
4.
5.
is
7.
recognized.
Carbonate rocks contain minerals with the 2 carbonate ion (C0 3 )~ as in limestone and dolostone. Dolostone probably forms when
magnesium
partly replaces the calcium in
limestone. 8.
Evaporites include rock salt and rock gypsum, both
of which form by inorganic precipitation of minerals 9.
from evaporating water. Coal is a type of biochemical sedimentary rock
composed of
the altered remains of land plants.
depositional environment.
10. Sedimentary facies are bodies of sediment or
settings are continental,
sedimentary rock that are recognizably different from adjacent sediments or rocks. 11. Some sedimentary facies are geographically widespread because they were deposited during marine transgressions or marine regressions. 12. Sedimentary structures such as bedding,
each of which includes several specific depositional environments. Compaction and cementation are the processes of sediment lithification in which sediment is converted into sedimentary rock. Silica and calcium carbonate are the most common chemical cements, but iron oxide and iron hydroxide cements are important in
some 6.
Major depositional transitional, and marine,
A
subcategory called biochemical sedimentary rocks
Detrital sediment consists of mechanically weathered solid particles,
2.
or the biochemical activities of organisms.
classified as detrital
or chemical: a.
Detrital sedimentary rocks consist of solid
from preexisting rocks. Chemical sedimentary rocks are derived from ions in solution by inorganic chemical processes
particles derived b.
sediments
when
commonly form
in
or shortly after they are deposited.
Such features preserved
rocks.
Sedimentary rocks are generally
cross-bedding, and ripple marks in
sedimentary rocks aid
geologists in determining ancient current directions
and depositional environments. and sedimentary rocks are the host materials for most fossils. Fossils provide the only record of prehistoric life and are useful for correlation and environmental interpretations.
13. Sediments
Chapter Summary
187
14. Depositional
environments of ancient sedimentary
6.
rocks are determined by studying sedimentary
and structures, examining fossils, and making comparisons with present-day depositional
7.
arkoses. c./fc_ evaporites ^3^BL mudrocks; e. Most limestones havera large component of calcite
processes.
that
Many
a.
sediments and sedimentary rocks including
sand, gravel, evaporites, coal, and banded iron
formations are important natural resources. Most and natural gas are found in sedimentary rocks.
was
d.
8.
from seawater by:
originally extracted
inorganic chemical reactions;
£\y )*tr oil
sedimentary rocks are:
detrital
limestones; Jj*—-^ sandstones;
a. l
textures
15.
The most abundant
organisms; c. evaporation; weathering; e. lithification.
_mf chemical
Dolostone
formed by the addition of
is
to
limestone.
^
IMPORTANT TERMS .
bedding bedding plane biochemical sedimentary rock carbonate rock cementation chemical sedimentary rock
fossil
marine regression marine transgression
depositional environment detrital
mark
11.
rounding sediment sedimentary facies sedimentary rock sedimentary structure
crystalline texture
sedimentary rock
The most common evaporite rock
a.
superposition; b.
d.
invasion;
Which of
*
the following
broken sea
is
detrital
shells; b.
e.
3.
13.
4.
mud
cross-bedding;
e.
grain size.
together
Which of
the following is a trace fossil? dinosaur tooth; b. frozen mammoth;
d. e.
188
c.
sand deposits are typically well sorted; forms only by evaporation of water.
it
Chapter 7
Sediment and Sedimentary Rocks
clam
e.
and natural gas
resulting
from
traps.
rock; d.
Most of
stratigraphic;
structural;
the
known
e.
oil shales
a.
the Soviet Union; b.
c.
Venezuela;
e.
Australia.
d. If
is
cap
c.
dome.
are in;
China;
the United States; for nuclear
obtained from the mineral:
a.
aragonite; b.
d.
halite; e.
How
salt
most uranium
16. In the United States
17.
feldspars are chemically unstable;
bird bone;
d.
reservoir; b.
a
and binds
is:
burrow;
are
dissolved mineral matter
compaction; b. rounding; c. bedding; weathering; e. */ cementation. d. Sedimentary breccia is a rare rock type because: a. if gravel is rounded quickly during transport; clay is less abundant than other sedimentary b. particles;
if worm
reactors
a.
5.
turbidity currents;
d.
variations in the properties of sedimentary rocks
lithified.
precipitates in the pore spaces of sediment it
graded bedding;
cracks; b.
14. Traps for petroleum
15.
mm.
an aggregate of sediment consists of particles that are all about the same size, it is said to be: a. -jr well sorted; b. poorly rounded; c. completely abraded; d. sandstone;
The process whereby
facies;
c.
shell.
If
e.
regression;
transgression.
the following can be used to determine
a.
c.
sediment?
ions in solution;
quartz sand; d.
5
•$
is responsible for most: bedding planes; b. -f- graded bedding: c. wave-formed ripple marks; d. sedimentary facies; e. marine regressions.
e.
2.
is;
c.
a.
conglomerate; graded bedding. A clay-sized sedimentary particle measures: 2-10 cm; a. greater than 2 m; b. 1/4-1/2 mm; d.-^Z. less than 1/256 mm; c c.
e.
c. Sfc
a.
a.
chert;
b.
12. Turbidity current deposition
sorting
^ REVIEW QUESTIONS Which of
4h magnesium;
paleocurrent direction?
evaporite
1.
c.
migrates inland during a marine:
crack
ripple
cross-bedding
carbonate;
sodium.
e.
bituminous rock salt; e. siltstone. 10. The superposition of offshore facies over nearshore facies occurs when sea level rises and the shoreline
paleocurrent
compaction
iron;
coal; d. -y
lithification
clastic texture
calcium; b.
d.
rock gypsum;
graded bedding
mud
a.
«^
gypsum;
c.
kaolinite;
carnotite.
does the gravel
in
sedimentary breccia differ
from the gravel in conglomerate? 18. What are the two meanings of the term "clay"? 19. Explain why the sediment in windblown sand dunes is
20.
better sorted than that in glacial deposits.
What
are the
common
sedimentary rocks, and
chemical cements in how do they form?
21. Distinguish clastic and crystalline textures. Give an
example of a sedimentary rock with each
texture.
22.
Why
23.
What
is quartz the predominant mineral in most sandstones? What is a sandstone called that contains
at least
25%
and why are some mudrocks
what fundamental way do chemical sedimentary rocks differ from detrital sedimentary rocks? 25. Compare and contrast limestone and dolostone. 26. What are the common evaporites, and how do they 24. In
are banded iron formations, and why are they an important resource?
^ ADDITIONAL
READINGS
G. Middleton, and R. Murray. 1980. Origin of sedimentary rocks. New York: W. H. Freeman.
Blatt, H.,
Boggs,
1987. Principles of sedimentology and Columbus, Ohio: Merrill Publishing Co.
S., Jr.
stratigraphy.
J. D., and D. B. Thompson. 1982. Sedimentary structures. London: Allen Unwin.
originate?
Collinson,
&
27. Briefly describe the origin of coal.
Name
What
feldspar?
are mudrocks,
called shale?
28.
33.
three sedimentary structures and explain
how
they form. 29.
How
can
30.
What
are marine transgressions and regressions?
be used to interpret ancient depositional environments? fossils
Explain how a marine transgression can account for beach sand being deposited over a vast region. 31. What kinds of data do geologists use to determine depositional environment? 32. What is oil shale, and how can liquid oil be extracted from
it?
W.
N. Moore. 1988. Basics of physical stratigraphy and sedimentology. New York: John Wiley
Fritz,
J.,
and
J.
&
Sons.
LaPorte, L.
F.
1979. Ancient environments. 2d ed. Englewood
Cliffs, N.J.: Prentice-Hall.
Moody,
New York: Macmillan Publishing Co. 1978. Ancient sedimentary environments. Ithaca,
R. 1986. Fossils.
Selley, R. C.
N.Y.: Cornell University Press.
1982. An introduction to sedimentology. 2d ed. New York: Academic Press. Simpson, G. G. 1983. Fossils and the history of life. New York: Scientific American Books.
Additional Readings.
189
CHAPTER
8
METAMO RPHI M AND METAMORPHIC ROCKS S
^OUTLINE PROLOGUE INTRODUCTION THE AGENTS OF METAMORPHISM Heat Pressure Fluid Activity
"^Perspective 8-1: Asbestos
TYPES OF
METAMORPHISM
Contact Metamorphism
Dynamic Metamorphism Regional Metamorphism CLASSIFICATION OF METAMORPHIC
ROCKS Foliated
Metamorphic Rocks
Nonfoliated Metamorphic Rocks
METAMORPHIC ZONES AND FACIES METAMORPHISM AND PLATE TECTONICS
METAMORPHISM AND NATURAL RESOURCES -"» Perspective 8-2:
Graphite
CHAPTER SUMMARY
Marble quarry, northcentral Vermont. (Photo courtesy of R. V. Dietrich.)
-•ygg«."* ^^: *'«. ». «.'yr»^. *^-*'y^^'%Y ,
,
,
,
PROLOGUE
,
Although marbles
metamorphism
result
when
(heat, pressure,
the agents of
and
fluid activity) are
applied to carbonate rocks, the type of marble formed ^j^gJ|Vjjj|
Because of
its
Marble is a metamorphic rock that is formed from limestone or dolostone. homogeneity, softness, and textures,
marble has been a favorite rock of sculptors throughout history. As the value of authentic marble sculptures has increased through the years, the
number of forgeries has also increased. With the price of some marble sculptures in the millions of dollars, private collectors and museums need some means of assuring the authenticity of the work they are buying. Aside from the monetary considerations,
it is
important that such forgeries do not become part of the historical and artistic legacy of
human
Experts have traditionally relied on the
endeavor. artistic style
its weathering characteristics whether a marble sculpture is authentic or a forgery. Because marble is not very resistant to weathering, forgers have had to resort to a variety of methods to produce the weathered appearance of an authentic ancient work. Now, however, with new techniques of analyzing marble, geologists can differentiate a naturally weathered surface from one
of the object as well as to determine
that has been artificially altered.
s~—
'~'~ J
is
because each quarry yields marble with a
distinctive set of
carbon and oxygen isotope values
(Fig. 8-lb).
Recall from Chapter 3 that isotopes are forms of individual elements with different atomic mass
and oxygen isotope
the carbon
numbers.
If
sculpture
fall
ratios of a
outside the typical range of the locality
from which the marble supposedly comes, then
it is
probably a forgery. Using this technique, geologists showed that a marble head of Achilles owned by the J. Paul Getty Museum in Malibu, California, was a forgery.
When
from the Getty
the carbon
Museum
and oxygen isotope
those obtained from another marble head of authenticity, they did not
known
match, indicating that the two
sculptures were carved from marbles that
two
ratios
specimen were compared with
came from
different quarries.
Norman Herz
of the Geology Department of the
all of the major and many of the minor ancient marble quarries in the Aegean Sea region and assembled a large isotopic data
University of Georgia has sampled
base for these quarries. Using this data base for comparative purposes, Herz has been able to determine the source area of many marble pieces, as well as demonstrating that some marble sculptures
have been reassembled from marbles that came from
•^ FIGURE the is
53-cm
Carbon and oxygen isotopic analysis of Antonia Minor portrait showed that the head
8-2
tall
different localities
authentic, but unrelated to the other four pieces that
compose
and therefore were not part of the
original piece.
In one especially interesting case,
it.
determine that the
five
Herz was able
to
fragments composing the
in the Fogg Museum at Harvard University (Fig. 8-2) are not all the same marble. The portrait was purchased by the earl of Pembroke in 1678 and its restoration was completed
Antonia Minor portrait
depends, in part, on the original composition of the parent carbonate rock as well as the type and intensity of metamorphism. Therefore, one way to authenticate
in
1758. Since that time, art historians have debated
the portrait's authenticity
and method of restoration,
marble sculpture is to determine the origin of the marble itself. The major quarrying localities of the Preclassical, Greek, and Roman periods include the islands of Naxos, Thasos, and Paros in the Aegean Sea as well as the Greek mainland, Turkey, and Italy
with some claiming the portrait was assembled from
(Fig. 8-la).
shoulder and breast. Carbon and oxygen isotopic analysis of the five fragments revealed that three of
a
In order to determine the locality that the
various sculptures has
come from,
marble
completely different statues.
The
five
fragments composing the portrait are the
head, the end of the ponytail, the right shoulder and breast, the lower left shoulder,
in
geologists have
and the upper
left
and two were was concluded that the head
the pieces were of Parian marble
employed a wide variety of analytical techniques. These include hand specimen and thin-section analysis
Carrara marble.
of the marble, trace element analysis by X-ray
the right shoulder
fluorescence, stable isotopic ratio analysis for carbon
shoulder and breast being comparatively recent
and oxygen, and other more esoteric techniques. Currently, however, carbon and oxygen isotopic analysis has proven to be the most powerful and reliable method for source area determination. This
additions.
192
Chapter 8
Metamorphism and Metamorphic Rocks
It
is
authentic, but unrelated to the other pieces, with
and breast and the upper
left
which carbon and oxygen isotopic was the "Livia" head in the Ny Carlsberg Glyptotek in Copenhagen, Denmark. Its Another case
in
analysis proved useful
authenticity and identification
had also been debated by art historians. Isotopic analysis of the skullcap, head, and nose showed that the head is Parian marble, suggesting that
Roman
it is
authentic and
The skullcap
times.
is
was made
Many museums
in
Ephesian marble, which
a popular Roman source, and therefore could have come from any statue of that time. The nose is Carrara marble, where quarries have been operating
was
since
Roman
times,
new
rocks. These trans-
in the solid state,
morphism, and the amount of time the parent rock was subjected to the effects of metamorphism. large portion of the Earth's continental crust
composed of metamorphic and igneous
is
rocks. Together,
they form the crystalline basement rocks that underlie the sedimentary rocks of a continent's surface. This
basement rock
known
is
exposed widely
in regions of the
con-
as shields; these are areas that have been
very stable during the past 600 million years
Metamorphic
body of is
rocks such as marble and slate are used as building ma-
and the type of metamorphic rock formed depends on the original composition and texture of the parent rock, the agents of meta-
tinents
geological testing
analyzed.
^ INTRODUCTION
A
now making
data about the characteristics and origin of marble being amassed as more sculptures and quarries are
Metamorphic rocks (from the Greek meta meaning change and morpho meaning shape), the third major group of rocks, result from the transformation of other rocks, generally beneath the Earth's surface. As Figure 8-3 illustrates, metamorphic rocks can form from any other rock, including previously formed metamorphic rocks. Metamorphism usually takes place beneath the Earth's surface where rocks are subjected to sufficient heat, pressure, and fluid activity to change their mineral composiformations take place
are
is
marble sculptures an important part of
to authenticate
their curatorial functions. In addition, a large
and therefore could have been
tion and/or texture, thus forming
added at any time. When the skullcap from "Livia" removed, iconographically, the portrait is that of Agrippina and is now so labeled.
(Fig. 8-4).
rocks also constitute a sizable portion of
the crystalline core of large mountain ranges.
terials,
and certain metamorphic minerals are economFor example, garnets are used as gem-
ically valuable.
stones or abrasives; talc
is used in cosmetics, in the manufacture of paint, and as a lubricant; asbestos is used for insulation and fireproofing (see Perspective
and kyanite
8-1);
is
used
in the
production of refractory
materials such as sparkplugs.
» THE AGENTS OF METAMORPHISM As we have already mentioned, metamorphism involves the transformation of preexisting rock by the agents of heat, pressure, and fluid activity. During metamorphism, the originarpock undergoes change so as to come into equilibrium with its new environment. The changes may result in the
formation of
in the texture
new
minerals and/or a change
of the rock by the reorientation of the
original minerals. In
some
instances the change
and features of the parent rock can other cases the rock changes so
still
much
is
minor,
be recognized. In
that the identity of
the parent rock can be determined only with great difficulty, if at all.
Some of
known rocks, dated at 3.96 billion years from Canadian Shield, are metamorphic, indicating they formed from even older rocks. Why is it important to study metamorphic rocks? For one thing, they provide information about geological processes operating within the Earth and about the way the oldest the
these processes have varied through time.
From
the pres-
Heat Heat
is
an important agent of metamorphism because
increases the rate of chemical reactions that
may
duce new mineral assemblages different from those the original rock.
The heat may come from
it
proin
intrusive
ence of certain minerals in metamorphic rocks, geolo-
magmas
can determine the approximate temperatures and pressures that parent rocks were subjected to during
such as occurs during subduction along a convergent plate boundary.
metamorphism and thus gain insights and chemical changes that occur at
subjected to intense heat that affects the surrounding rock;
gists
into the physical different depths
within the Earth's crust. Furthermore, metamorphic
When the
or result from deep burial in the Earth's crust
rocks are intruded by bodies of
magma,
they are
most intense heating usually occurs adjacent to the The Agents of Metamorphism
193
""*" FIGURE 8-3 The rock cycle, showing sedimentary rocks are interrelated.
magma body and the intrusion.
forms
body
in the is
how metamorphic,
gradually decreases with distance from
The zone of metamorphosed rocks and easy
to recognize.
Recall from Chapter 4 that temperature increases
with depth and that the Earth's geothermal gradient averages about 25°C/km. surface
may
Rocks forming
at the Earth's
be transported to great depths by subduc-
tion along a convergent plate
boundary and thus subjected to increasing temperature and pressure. During subduction, some minerals may be transformed into other minerals that are more stable under the higher temperature and pressure conditions.
194
Chapter 8
Pressure
that
country rock adjacent to an intrusive igneous
usually rather distinct
igneous, and
Metamorphism and Metamorphic Rocks
When
rocks are buried, they are subjected to increas-
ingly greater lithostatic pressure; this pressure, results
from the weight of the overlying rocks,
equally in
directions (Fig. 8-5a).
A
which
applied
similar situation
immersed in water. For examthe deeper a styrofoam cup is submerged in the
occurs ple,
all
when an
is
object
ocean, the smaller
depth and
is
it
is
gets because pressure increases with
exerted on the cup equally in
thereby compressing the styrofoam
all
directions,
(Fig. 8-5b).
Just as in the styrofoam example, rocks are subjected to increasing lithostatic pressure with depth such that
E23 Precambnan
Sediments
shields
covering shields
Folded mountain belts
"^ FIGURE crystalline
8-4 Shields of the world. Shields are the exposed portion of the basement rocks that underlie each continent; these areas have been very
stable during the past
600 million
years.
may become more Under such conditions, the minerals may recrystallize; that is, they may form smaller and denser minerals either of the same chemical composition or of the mineral grains within a rock closely packed.
different mineral assemblages. In addition to the lithostatic pressure resulting burial, rocks
may
also experience differential pressures.
In this case, the pressures are not equal
the rock typically
is
from
on
all sides,
and
consequently distorted. Differential pressures
occur during deformation
associated
with
mountain building and can produce distinctive metamorphic textures and features (Fig. 8-6).
Fluid Activity In almost every region where metamorphism occurs, water and carbon dioxide (C0 2 are present in varying amounts along mineral grain boundaries or in the pore spaces of rocks. This water, which may contain ions in solution, enhances metamorphism by increasing the rate of )
The Agents of Metamorphism
195
Perspective 8-1
ASBESTOS Asbestos (from the Latin, meaning unquenchable)
a
is
general term applied to any silicate mineral that easily separates into flexible fibers (Fig.
1).
The combination
of such features as noncombustibility and flexibility
makes asbestos an important considerable value. In
known
3,000
uses.
fact,
industrial material of
asbestos has
more than
These include brake linings and
clutch facings, fireproof fabrics, heat insulators,
cements, shingles, acid and chemical equipment,
and binders for various plasters, porcelains, and electrical insulators to name only a few. Commercial users consider asbestos fibers to be insulation,
either spinning or nonspinning. Spinning fibers are
more valuable because they can be spun into thread and yarn that can be woven into a variety of fireproof textiles.
Nonspinning
fibers are
used mainly in various
types of fireproofing and insulation.
The unique
properties of asbestos were certainly
known in the ancient world. The Romans used it to make lamp wicks that never burned out and also wove it into cremation clothes for the nobility. The modern asbestos industry really began, however, in 1868 when Italy produced approximately 200 tons of raw material. A decade later, huge discoveries were made in Quebec, enabling Canada to become one of the world's leading producers.
Asbestos can be divided into two broad groups,
is
magnesium
a hydrous
Mg 3 Si 2
silicate
5 (OH)4, is
The
with the chemical
the fibrous form of
when even
small amounts of fluid
are introduced, reaction rates speed up, mainly because
ions can
move
readily through the fluid
and thus enhance
chemical reactions and the formation of
The following
how new
reaction provides a
new
seawater moving through hot basaltic rock transforms
metamorphic mineral serpentine:
2Mg 2 Si0 4 + 2H 2 olivine
water
-
Mg3 Si 2
5
(OH) 4 +
serpentine
carried in
solution
196
Chapter 8
vast majority of chrysotile asbestos occurs in
The chemically
active fluids that are part of the meta-
morphic process come primarily from three sources. The first is
water trapped
in the
pore spaces of sedimentary
rocks as they form; as these rocks are subjected to heat is
heated, thus accelerating the
various chemical reaction rates. volatile fluid within
magma;
A
second source
is
the
as these hot fluids disperse
through the surrounding rock, they frequently react alter the mineralogy of the country rock by adding or removing ions. The third source is the dehydration of water-bearing minerals such as gypsum
with and
MgO away
the
all
and pressure, the water
minerals.
good example of
minerals can be formed by fluid activity. Here,
olivine into the
it is
serpentine that has been altered from such ultramafic
chemical reactions. Under dry conditions, most minerals react very slowly, but
most valuable type and commercial asbestos. Chrysotile's strong, silky fibers are easily spun and can withstand temperatures up to 2,750°C. serpentine asbestos;
constitutes the bulk of
serpentine and amphibole asbestos. Cbrysotile, which
formula
•^
FIGURE 1 Hand specimen of chrysotile from Thetford, Quebec, Canada. Chrysotile is the fibrous form of serpentine asbestos.
Metamorphism and Metamorphic Rocks
(CaS0 4 -2H 2 0) and some clays; when these minerals, which contain water as part of their crystal chemistry,
igneous rocks as peridotite under low- and medium-grade metamorphic conditions. Serpentine is believed to form from the alteration of olivine by hot, chemically active, residual fluids emanating from the cooling
magma. The
chrysotile asbestos forms veinlets
of fiber within the serpentine and
20%
may comprise up when the
to
recently been raised, however, concerning the threat
posed by asbestos.* Central to the debate
whether
all
varieties of
whose
tend to be curly, does not become lodged in the lungs. Furthermore, the fibers are generally soluble fibers
and disappear
of the rock. Other chrysotile results
is
asbestos should be lumped together. Chrysotile,
in tissue. In contrast, crocidolite
has
metamorphism of magnesium limestone or dolostone
long, straight, thin fibers that penetrate the lungs
produces discontinuous serpentine bands that develop
stay there. Thus, crocidolite, not chrysotile,
within the limestone beds.
overwhelmingly responsible for asbestos-related lung cancer. Because about 95% of the asbestos in place in the United States is chrysotile, many people are
At least five varieties of amphibole asbestos are known, but crocidolite, a sodium-iron amphibole with
Na 2 (Fe +3
the chemical formula is
the most
common.
as blue asbestos, is
stronger but
more
little
(Fe
+2 )
22 (OH) 2 ,
3 Si 8
which
is
also
known
a long, coarse, spinning fiber that
is
brittle
than chrysotile and also
The other
less resistant to heat.
asbestos have
)2
Crocidolite,
varieties of
commercial value and are used
Crocidolite
and
is
Removing asbestos from some recent
in
buildings where
much
as
$150
it
has
billion,
such metamorphic rocks as
and
studies have indicated that the air in
amount of airborne
found
schists. It is
been somewhat exaggerated.
buildings containing asbestos has essentially the
chiefly for insulation.
slates
questioning whether the dangers from asbestos have
been installed might cost as
amphibole
and
is
same
asbestos fibers as the air outdoors.
In fact, unless the material containing the asbestos
thought that crocidolite forms by
the solid-state alteration of other minerals within the
improper removal of asbestos can lead to
high temperature and high pressure environment that
contamination. In most cases of improper removal,
results
rarely
from deep
burial.
Unlike chrysotile, crocidolite
found associated with igneous
In spite of
its
is
intrusions.
widespread use, the federal
Environmental Protection Agency recently enacted a all new asbestos products. The ban was imposed because asbestos can cause cancer and scarring
gradual ban on
of the lungs
if its
fibers are inhaled.
The
threat of lung
is
disturbed, asbestos does not shed fibers. Furthermore,
the concentration of airborne asbestos fibers
higher than
if
the asbestos
had been
left in
is
far
place.
The problem of asbestos contamination is a good example of how geology affects our lives and why a basic knowledge of science is important. Asbestos is certainly a health hazard, but not all varieties of
asbestos are equally dangerous.
cancer has resulted in legislation mandating the removal of asbestos already in place in all
many
buildings, including
public and private schools. Important questions have
are subjected to heat
and pressure, the water may be
driven off and enhance metamorphism.
*P. H. Abelson, "The Asbestos Removal 4946 (1990): 1017.
Fiasco," Science
247 no.
boundary between them is not always disand depends largely on which of the three metamorphic agents was dominant. arately, the
tinct
» TYPES OF METAMORPHISM Three major types of metamorphism are recognized: contact metamorphism in which magmatic heat and fluids act to produce change; dynamic metamorphism,
which
is
principally the result of high differential pres-
and regional and is caused primarily by mountain-building forces. Even though we will discuss each type of metamorphism sepsures associated with intense deformation;
metamorphism, which occurs within
a large area
Contact Metamorphism Contact metamorphism takes place when a body of magma alters the surrounding country rock. At shallow depths an intruding magma raises the temperature of the surrounding rock, causing thermal alteration. Furthermore, the release of hot fluids into the country rock by the cooling intrusion can also aid in the formation of
new
minerals.
Types of Metamorphism
197
Vertical pressure (kbar
Surface
—*
1 kilobar (kbar) = 1,000 bars Atmospheric pressure at sea
level
=
1
""^ FIGURE 8-6 Differential pressure is pressure that is unequally applied to an object. Rotated garnets are a good example of differential pressure applied to a rock during
bar
(a)
metamorphism. These rotated garnets come from
a
calcareous schist of the Waits River Formation, north of Springfield, Vermont. (Photo courtesy of John L. Rosenfeld, University of California, Los Angeles.)
magmas mal
(see
effect
Chapter
4)
on the rocks
size of the intrusion
is
and hence have a greater
ther-
surrounding them. The
directly
also important. In the case of
small intrusions, such as dikes and
sills,
usually only
those rocks in immediate contact with the intrusion are affected.
Because large intrusions, such as batholiths,
take a long time to cool, the increased temperature in
"^ FIGURE
the surrounding rock 8-5
applied equally in all directions in the Earth's crust due to the weight of the overlying rocks. Thus, pressure increases with depth, {b) A similar situation occurs when 200 ml styrofoam cups are {a)
Lithostatic pressure
is
lowered to ocean depths of approximately 750 m and 1,500 m. Increased pressure is exerted equally in all directions on the cups, and they consequently decrease in volume, while still maintaining their general shape. (Styrofoam cups courtesy of David J. Matty and Jane M. Matty. Photo courtesy of Sue Monroe.)
may
last
long enough for a larger
area to be affected. Fluids also play an important role in contact meta-
morphism.
Many magmas
chemically active fluids that
are
wet and contain
may emanate
rounding rock. These fluids can react with the rock and aid in the formation of new minerals. In addition, the country rock may contain pore fluids that, when heated by the magma, also increase reaction rates. Temperatures can reach nearly 900°C adjacent to an intrusion, but they gradually decrease with distance. effects
Important factors in contact metamorphism are the temperature and size of the intrusion as well as the fluid content of the magma and/or country rock. The initial temperature of an intrusion is controlled, in part, by its composition: mafic magmas are hotter than felsic initial
198
Chapter 8
Metamorphism and Metamorphic Rocks
hot,
into the sur-
usually occur in concentric zones 8-7).
The
of such heat and the resulting chemical reactions
The boundary between an
may be
known
as aureoles (Fig.
intrusion
and
its
aureole
either sharp or transitional (Fig. 8-8).
Metamorphic aureoles vary in width depending on and composition of the intrusion
the size, temperature,
Inner andalusite-cord hornfels
"^ FIGURE
zone
with
some
biotite
A
8-7
metamorphic
aureole typically surrounds
Intermediate zone
many
igneous intrusions. The metamorphic aureole associated with this idealized granite batholith contains three zones of mineral assemblages reflecting the decreases in temperature with distance from the intrusion.
An
andalusite-
cordierite hornfels forms the inner
zone adjacent to the batholith. This is followed by an intermediate zone of extensive recrystallization in
which some biotite develops, and farthest from the intrusion is the outer zone, which is characterized by spotted
baked when
slates.
Sometimes the baking of
as well as the mineralogy of the surrounding country
is
rock. For example, small intrusive bodies such as
country rock produces a metamorphic rock
sills
fired in a kiln.
most
known
as
common where
and dikes may produce an aureole only a few centimeters wide, whereas large intrusive bodies such as batholiths may give rise to an aureole several kilometers wide.
spotted slate
Typically, these large intrusive bodies have several meta-
the formation of large, scattered crystals during baking.
morphic zones, each characterized by assemblages indicating the decrease
distinctive mineral
in
distance from the intrusion (Fig. 8-7). to the intrusion,
peratures,
may
minerals (that
and hence subject
temperature with
The zone
8-9).
This
is
metamorphosed. from the growth of new minerals or
clay-rich rocks have been thermally
The "spots"
result
During the
magma
final stages
of cooling
when an
intruding
begins to crystallize, large amounts of hot, wa-
closest
to the highest tem-
contain high-temperature metamorphic
is,
(Fig.
minerals in equilibrium with the higher
temperature environment) such as sillimanite. The outer zones may be characterized by lower temperature meta-
morphic minerals such as chlorite, talc, and epidote. The formation of new minerals by contact metamorphism depends not only on proximity to the intrusion, but also on the mineralogy of the country rock. Shales, mudstones, impure limestones, and impure dolostones, for example, are particularly susceptible to the formation of
"^ FIGURE 8-8 A sharp and clearly defined boundary occurs between the intruding light-colored igneous rock on the left and the dark-colored metamorphosed country rock on the right. The intrusion is part of the Peninsular Ranges Batholith, east of San Diego, California. (Photo courtesy oi David
J.
Matty.)
new
minerals by contact metamorphism, whereas pure sandstones or pure limestones typically are not.
Two
types of contact metamorphic rocks are gener-
ally recognized:
those resulting from baking of country
rock and those altered by hot solutions. Many of the rocks resulting from contact metamorphism have the texture of porcelain; that grained. This
is
is,
they are hard and fine
particularly true for rocks with a high
clay content, such as shale. Such texture results because
the clay minerals in the rock are baked, just as a clay pot
Types of Metamorphism
199
*j>(* '.:•:' »*"--
»-
i
"•
£
«»•«
».
^ FIGURE
8-10 This light-colored, 15-cm thick mylonite unit is part of the Carthage-Colton Mylonite Zone exposed along Route 3, south of Harrisville, New York. (Photo courtesy of Eric Johnson.)
; !5fri*S' ~r'
"^"
FIGURE
Hand specimen
of a spotted slate. The "spots" result from the growth of new minerals during the baking of a clay-rich rock. (Photo courtesy of Con Gillen, The University of Edinburgh, Scotland.)
8-9
pressure applied to the rock. High shearing pressure
completely pulverizes the country rock and essentially
"smears" the These solutions may react with the country rock and produce new metamorphic minerals. This process, which usually occurs near the Earth's surface, is called hydrothermal alteration. One source of hydrothermal alteration occurs at or near mid-ocean ridges, and some of these ocean-floor alterations become mineral deposits such as the Kuroko sulfide deposit in Japan (see Chapter 13). Geologists think that many of the world's ore deposits result from the migration of metallic ions in hydrothermal solutions. Examples include copper, gold, iron ores, tin, and zinc in various localities including Australia, Canada, China, Cyprus, Finland, the Soviet Union, and the western United States. tery solutions are often released.
acteristic
fine particles together,
producing a char-
mylonite texture. Fault breccias, which are
composed of broken particles, are not, strictly speaking, metamorphic rocks. Examples of tectonic settings in which mylonites occur include the Moine Thrust Zone in northwest Scotland and portions of the San Andreas fault in California.
Regional Metamorphism Most metamorphic rocks
are the result of regional meta-
morphism, which occurs over a large area and is usually the result of tremendous temperatures, pressures, and deformation within the deeper portions of the Earth's crust. Regional metamorphism
is
most obvious along conver-
gent plate margins where rocks are intensely deformed
lonites are differentiated
and recrystallized during convergence and subduction. Within these metamorphic rocks, there is usually a gradation of metamorphic intensity from areas that were subjected to the most intense pressures and/or highest temperatures to areas of lower pressures and temperatures. Such a gradation in metamorphism can be recognized by the metamorphic minerals that are present. Regional metamorphism is not confined to convergent margins. It also occurs in areas where plates diverge, though usually at much shallower depths in the Earth's crust because of the high geothermal gradient
are broken
associated with these areas.
Dynamic Metamorphism Most dy namic metamorphism
is
associated with fault
zones where rocks are subjected to high differential
The metamorphic rocks that result from pure dynamic metamorphism are called mylonites. They are typically restricted to narrow zones adjacent to faults (fractures along which movement has occurred). Mylonites are hard, dense, fine-grained rocks, many of which pressures.
are characterized by thin laminations (Fig. 8-10).
200
up by
Chapter 8
fault
My-
from fault breccias (rocks that movement) by the intensity of the
Metamorphism and Metamorphic Rocks
-"•-
TABLE
8-1
-*"
TABLE
8-2
Classification of
Common Metamorphic
Rocks
I:
I;:
' !
.,!!!!!!!
'
,
jij
1
I'l
!!'!!
i:;:nii; il'lii'iiiU'iHi'li
Random arrangement
Elongated minerals arranged in a parallel fashion as a result of pressure applied to two
elongated minerals before pressure is applied to two sides of
sides (a)
(b)
^ FIGURE
8-12
(a)
When
rocks are subjected to
differential pressure, the mineral grains are typically in a parallel fashion, producing a foliated texture. Photomicrograph of a metamorphic rock with a foliated texture showing the parallel arrangement of mineral grains.
arranged (b)
Slate
monly sult of rarely,
is
a very fine-grained
metamorphic rock that com-
exhibits slaty cleavage (Fig. 8-13b). Slate
is
the re-
low-grade regional metamorphism of shale or, more volcanic ash. Because it can easily be split along
cleavage planes into
flat pieces, slate is
an excellent rock
and pool table tops, and blackboards. The different colors of most slates are caused by minute amounts of graphite (black), iron oxide (red and for roofing
and
floor
tiles, billiard
purple), and/or chlorite (green). Phyllite
is
similar in composition to slate, but
coarser grained. However, the minerals are
still
is
(b)
^
FIGURE 8-13 (a) Hand specimen of slate, (b) This panel of Arvonia Slate from Albemarne Slate Quarry, Virginia, shows bedding (upper right to lower left) at an angle to the slaty cleavage. (Photo (a) courtesy of Sue Monroe; photo (b) courtesy of R. V. Dietrich.)
too
small to be identified without magnification. Phyllite can
be distinguished from slate by its glossy or lustrous sheen. It represents an intermediate grain size between slate
and
schist.
Schist is most commonly produced by regional metamorphism. The type of schist formed depends on the intensity of metamorphism and the character of the par-
Classification of
Metamorphic Rocks
203
ent rock (Fig. 8-14).
can yield
schist,
Metamorphism of many rock
types
but most schist appears to have formed
from clay-rich sedimentary rocks. All schists contain
gated minerals,
all
more than
50%
platy
and elon-
of which are large enough to be
clearly visible. Their mineral
composition imparts a
schistosity or schistose foliation to the rock that
com-
wavy type of parting when
split.
monly produces
a
common
low- to high-grade metamorphic environments. Because a schist's mineral grains can be readily identified, each type is known by its most conspicuous mineral or minerals, for example, mica Schistosity
is
in
and talc schist. metamorphic rock that is streaked or has segregated bands of light and dark minerals. Gneisses are composed mostly of granular minerals such as schist, chlorite schist,
Gneiss
is
a
"^ FIGURE out at
-"^ (b)
8-15
Gneiss
is
characterized by segregated
and dark minerals. This folded gneiss crops Wawa, Ontario, Canada.
bands of
light
FIGURE 8-14 Schist, (a) Garnet-mica schist. Hornblende-mica-garnet schist. (Photos courtesy of Sue quartz and/or feldspar with lesser percentages of platy
Monroe.)
or elongated minerals such as micas or amphiboles (Fig. 8-15).
Quartz and feldspar are the principal
light-
colored minerals, while biotite and hornblende are the typically dark-colored minerals.
an irregular manner,
much
Most
gneiss breaks in
like coarsely crystalline
non-
foliated rocks.
Most gneiss probably
from
results
recrystallization of
clay-rich sedimentary rocks during regional
metamor-
phism. Gneiss also can form from crystalline igneous rocks such as granite or older metamorphic rocks.
Another
fairly
amphibolite.
It is
common dark
metamorphic rock is and composed mainly of
foliated
in color
hornblende and plagioclase. The alignment of the hornblende crystals produces a slightly foliated texture. Many amphibolites result from medium- to high-grade
metamorphism of such ferromagnesian mineral-rich
ig-
neous rocks as basalt. In
some areas of
regional
metamorphism, exposures
of "mixed rocks" having both igneous and high-grade
metamorphic
characteristics are present.
These rocks,
called migmatites, usually consist of streaks or lenses of
granite intermixed with high-grade ferromagnesian-rich
metamorphic rocks
Most migmatites
(Fig. 8-16).
are thought to be the product of
extremely high-grade metamorphism, and several models for their
lem
in
how
origin have been proposed. Part of the prob-
determining the origin of migmatites
the granitic
is
explaining
component formed. According
to
one
model, the granitic magma formed in place by the partial melting of rock during intense metamorphism. Such an origin is possible providing that the host rocks con-
204
Chapter 8
Metamorphism and Metamorphic Rocks
uniform texture, and its various colors have the favorite rock of builders and sculptors throughout history (see the Prologue). ble,
its
made
it
Quartzite is a hard, compact rock typically formed from quartz sandstone under medium-to-high-grade metamorphic conditions during contact or regional metamorphism (Fig. 8-19). Because recrystallization is so complete, metamorphic quartzite is of uniform strength and therefore usually breaks across the component quartz grains rather than around them when it is struck. Pure quartzite is white, but iron and other impurities commonly impart a reddish or other color to
it.
Quartzite
is
commonly used
as
foundation material for road and railway beds.
The name greenstone "**"
FIGURE
8-16 Migmatites consist of high-grade metamorphic rock intermixed with streaks or lenses of granite. This Precambrian(P) migmatite crops out at Thirty Thousand Islands of Georgian Bay, Lake Huron, Ontario, Canada. (Photo by Ed Bartram, courtesy of R. V. Dietrich.)
is
applied to any compact,
dark-green, altered, mafic igneous rock that formed un-
der low-to-high-grade
metamorphic conditions. The
green color results from the presence of chlorite, epidote,
and hornblende.
Hornfels
is
a fine-grained, nonfoliated
metamorphic it is com-
rock resulting from contact metamorphism; tained quartz and feldspars and that water
was present. components
Another possibility is that the granitic formed by the redistribution of minerals by recrystallization in the solid state, that is, pure metamorphism.
posed of various equidimensional mineral grains. The composition of hornfels is directly dependent upon the composition of the parent rock, and many compositional varieties are known. However, the majority of hornfels are apparently derived from contact metamorphism of clay-rich sedimentary rocks or impure dolostones.
ferred orientation of their mineral grains. Instead, they
hard coal that concarbon and a low percentage of volatile matter. It usually forms from the metamorphism of lower grade coals by heat and pres-
generally consist of a mosaic of roughly equidimen-
sure and
and are characterized as nonfoliated (Fig. 8-17). Most nonfoliated metamorphic rocks result from contact or regional metamorphism of rocks in which no platy or prismatic minerals are present. Fre-
metamorphic rock.
Anthracite
Nonfoliated Metamorphic Rocks
Some metamorphic
rocks do not
show
discernible pre-
sional minerals
quently, the only indication that a granular rock has
been metamorphosed
from
is
is
a black, lustrous,
tains a high percentage of fixed
is
thus considered by
many
geologists to be a
"^ FIGURE 8-17 Nonfoliated textures are characterized by a mosaic of roughly equidimensional minerals as in this photomicrograph of marble.
the large grain size resulting
metamorphic rocks composed largely of example, marble or quartzite; and
recrystallization. Nonfoliated
are generally of
two
only one mineral, for
types: those
those in which the different mineral grains are too small to be seen without magnification, such as greenstone
and hornfels. Marble is a relatively well-known metamorphic rock composed predominantly of calcite or dolomite; its grain size ranges from fine to coarsely granular (Figs. 8-2 and 8-18). Marble results from either contact or regional metamorphism of limestones or dolostones. Pure marble is snowy white or bluish, but varieties of all colors exist because of the presence of mineral impurities in
the parent sedimentary rock.
The
softness of mar-
Classification of
Metamorphic Rocks
205
~^ FIGURE
8-18
Marble
results
from the metamorphism of the sedimentary rock
limestone. (Photos courtesy of Sue Monroe.)
^ METAMORPHIC ZONES AND The
Note that these are the metamorphic minerals produced from clay-rich sediments. Other mineral assemblages and index minerals are produced from rocks with dif-
FACIES
first
systematic study of metamorphic zones
was
ferent original compositions (Table 8-1).
The
1800s by George Barrow and other British geologists working in the Dalradian schists of the southwestern Scottish Highlands. In this area of Scotland, clay-rich sedimentary rocks have been subjected to regional metamorphism, and the resulting metamorphic rocks can be divided into different zones based on the presence of distinctive silicate mineral assemblages. These mineral assemblages, each recognized by the presence of one or more index minerals, reflect different degrees of metamorphism. The index minerals
an isograd. The region between isograds
Barrow and his associates chose to represent increasing metamorphic intensity were, in order, chlorite, biotite, garnet, staurolite, kyanite, and sillimanite (Table 8-1).
metamorphic zone. The rocks within each zone represent a metamorphic grade. By noting the occurrence of metamorphic index minerals, geologists can construct a map
conducted during the
late
successive appearance of
tensity of
metamorphism. Going from lower toward
higher grade zones, the
Chapter 8
Metamorphism and Metamorphic Rocks
first
appearance of a particular
index mineral indicates the location of the
minimum
temperature and pressure conditions needed for the for-
mation of that mineral.
When
the locations of the
first
appearances of that index mineral are connected on a
map, the
result
**' FIGURE 8-19 Quartzite results from the metamorphism of quartz sandstone. (Photos courtesy of Sue Monroe.)
206
metamorphic index
minerals reflects gradually increasing or decreasing in-
is
a line of equal
metamorphic is
intensity or
known
as a
METAMORPHIC ZONES
I
Increasing
|
Younger, nonmetamorphosed rocks
|
Chlorite
metamorphic intensity
FIGURE
WISCONSIN 50 I
l_j
i
8-20
i
building and minor granitic intrusion during the Proterozoic
I
Eon, about 1.5
showing the metamorphic zones of an
entire area (Fig.
8-20).
Numerous
studies of different
metamorphic rocks
have demonstrated that while the texture and mineralogy of any rock may be altered by metamorphism, the overall chemical composition may be little changed.
Thus, the different mineral assemblages found in increasingly higher grade metamorphic rocks derived from the same parent rock result from changes in temperature and pressure (Table 8-1).
A
metamorphic
facies
is
a
Metamorphic
zones in the upper peninsula of Michigan. The zones in this region are based on the appearance of distinctive silicate mineral assemblages resulting from the metamorphism of sedimentary rocks during an interval of mountain
group of metamorphic
billion years ago.
rocks were pure quartz sandstones or pure limestones or dolostones. Such rocks would yield only quartzites and marbles, respectively.
"^ FIGURE 8-21 A pressure-temperature diagram showing where various metamorphic facies occur. A facies is characterized by a particular mineral assemblage that formed under the same broad temperature-pressure conditions. Each facies is named after its most characteristic rock or mineral.
rocks that are characterized by particular mineral as-
j
i
i
.
i
semblages formed under the same broad temperaturepressure conditions (Fig. 8-21). Each facies after
its
most
is
characteristic rock or mineral. For
named
the green
1
Blueschist
/^
40
\
*
\
35
\
relatively
Granulite
30
i
25 20 Pumpellyite
<£>
/
—7/ //
granulite facies develop.
Although usually applied to areas where the original rocks were clay rich, the concept of metamorphic facies can be used with modification in other situations. It cannot, however, be used in areas where the original
50
-45
Eclogite
v
examwhich
metamorphic mineral chlorite, low temperatures and pressures, forms under yields rocks said to belong to the greenschist facies. Under increasingly higher temperatures and pressures, other metamorphic facies, such as the amphibolite and
ple,
55 -
Zeolite
100
200
/
/<*
300
400
500
^ /
/
600
h 15
/
700
10 Sanidinite
800
5
900 1000
Temperature (°C)
Metamorphic Zones and
Facies
207
High-temperature, high-pressure zone
High-temperature, low-pressure zone
(amphibolite-granulite facies)
(contact metamorphism)
Low-temperature, high-pressure zone (blueschist facies^
^-, Sediment
V
Lithosphere
^fwfc
^ FIGURE
cr,,sl
Upper
Metamorphic from various
8-22
facies resulting
mantle
temperature-pressure conditions produced along an oceanic-
Asthenosphere
continental convergent plate
boundary.
^ METAMORPHISM AND
As subduction along the oceanic-continental plate boundary continues, both temperature and pressure increase with depth and can result in high-grade metamor-
PLATE TECTONICS Although metamorphism is associated with types of plate boundaries (Fig. 1-14), it is most
all
three
common
along convergent plate margins. Metamorphic rocks form at convergent plate boundaries because temperature
and pressure increase
Figure
8-22
as a result of plate collisions.
illustrates
the
various
temperature-
pressure regimes that are produced along an oceaniccontinental convergent plate boundary and the type of
metamorphic
facies
and rocks that can
result.
When
an
oceanic plate collides with a continental plate, tremen-
dous pressure is generated as the oceanic plate is subducted. Because rock is a poor heat conductor, the cold descending oceanic plate heats very slowly, and metamorphism is caused mostly by the rising pressure as depth increases. Metamorphism in such an environment produces rocks typical of the blueschist facies (low temperature, high pressure),
which
is
characterized by the blue-
colored amphibole mineral glaucophane (Fig. 8-21). Thus, geologists use the occurrence of blueschist facies rocks as evidence of ancient subduction zones. An excellent example of blueschist metamorphism can be
found
in the California
Franciscan
Coast Ranges. Here rocks of the
Group were metamorphosed under low-
phic rocks. Eventually, the descending plate begins to
melt and generates a rising
magma may
magma
alter the
that moves upward. This surrounding rock by contact
metamorphism, producing migmatites
in
the deeper
portions of the crust and hornfels at shallower depths.
Such an environment is characterized by high temperaand low to medium pressures. While metamorphism is most common along convergent plate margins, many divergent plate boundaries are characterized by contact metamorphism. Rising magma from mid-oceanic ridges heats the adjacent rocks, producing contact metamorphic minerals and textures. In addition to contact metamorphism, fluids emanating from the rising magma — and from the reaction of the magma and sea water— very commonly produce hydrotures
thermal solutions that
may
precipitate minerals of great
economic value.
^ METAMORPHISM AND NATURAL RESOURCES Many metamorphic
rocks and minerals are valuable
temperature, high-pressure conditions that clearly indi-
natural resources. While these resources include
cate the presence of a former subduction zone (Fig. 8-23).
types of ore deposits, the
208
Chapter 8
Metamorphism and Metamorphic Rocks
two most
familiar
many
and widely
Ophiolite
"»Great Valley
Group Franciscan
Group —
Low-temperature,
Oceanic crust Sediment
high-pressure zone where blueschist facies develops
FIGURE
8-23
Index
map
of
California showing the location of the Franciscan Group and a diagrammatic reconstruction of the environment in which it was regionally metamorphosed under
low-temperature, high-pressure subduction conditions during the Jurassic Period, approximately 150 million years ago.
used metamorphic rocks, as such, are marble and slate, which, as previously discussed, have been used for centuries in a variety of
Many phism
in
ways.
ore deposits result from contact metamor-
which hot, ion-rich
fluids
migrate from igneous
intrusions into the surrounding rock, thereby producing rich ore deposits.
The most common
sulfide ore minerals
with contact metamorphism are bornite, chalcopyrite, galena, pyrite, and sphalerite, while two common oxide ore minerals are hematite and magnetite. associated
Tin and tungsten are also important ores associated with contact metamorphism (Table 8-3). Other economically important metamorphic minerals include talc for talcum powder; graphite for pencils and dry lubricants (see Perspective 8-2); garnets and corundum, which are used as abrasives or gemstones, depending on their quality; and andalusite, kyanite, and sillimanite, all of which are used in the manufacture of high-temperature porcelains and refractives for products such as sparkplugs
and the
linings of furnaces.
Metamorphism and Natural Resources
209
Perspective 8-2
GRAPHITE Graphite
(
from the Greek grapbo meaning write)
soft mineral that
is
gray to black, has a greasy
is
feel,
a
and
composed of the element carbon. Graphite occurs in two varieties: crystalline, which consists of thin, flat, nearly pure black flakes, and amorphous, a noncrystalline, impure variety found in compact masses. Graphite has the same composition as diamond (see Perspective 3-2), but its carbon atoms are strongly bonded together in sheets, with the sheets weakly held together by van der Waals bonds (Fig. 3-6). Because is
(CaC0 3 by an inorganic process. Graphite is also found in igneous rocks, pegmatite dikes, and veins; it is thought to have formed in these environments from the primary constituents of the magma or from the hot fluids and vapors released by the cooling magma. The major producers of graphite are Mexico, the Soviet Union, Ceylon, Madagascar, Korea, and Canada. In the United States, graphite has been mined )
in
to
Graphite
the sheets are loosely held together, they easily slide
over one another, giving graphite
its
ability to
mark
paper and serve as a dry lubricant.
metamorphic rocks produced by contact and regional metamorphism. It is found in marble, quartzite, schist, gneiss, and even in anthracite. Contact metamorphism of impure limestones by igneous intrusions produces some of the graphite found in marbles. The graphite resulting from regional metamorphism of sedimentary rocks probably came from organic matter present in the sediments. However, some evidence indicates that the graphite in Graphite occurs mainly
in
Precambrian aged rocks (>570 million years) may be the result of the reduction of calcium carbonate
"•"
TABLE
8-3
27 states, but production Alabama and New York.
is
is
used for
in pencil leads,
is
many
where
it is
now
generally limited
purposes. finely
The
oldest use
ground, mixed
clay, and baked. The amount of clay and the baking time give pencil leads their desired hardness. Other important uses include batteries, brake linings,
with
carbon brushes, crucibles, foundry facings, lubricants, refractories, and steel making. Synthetic graphite can be produced from anthracite coal or petroleum coke and
graphite production.
99.5% purity
pure) is
makes
Its it
now
accounts for most
extreme purity (99% to
especially valuable
where high
required such as in the rods that slow
the reaction rates in nuclear reactors.
down
SUMMARY
CHAPTER 1.
Metamorphic rocks
result from the transformation of other rocks, usually beneath the Earth's surface,
consequence of one or a combination of three and fluid activity. Most of the heat for metamorphism comes from as a
agents: heat, pressure,
2.
intrusive
magmas. Pressure
is either lithostatic or trapped in sedimentary rocks or emanating from intruding magmas can enhance chemical changes and the formation of new
differential. Fluids
minerals. 3.
4.
The
three major types of metamorphism are contact, dynamic, and regional. Metamorphic rocks are classified primarily according to their texture. In a foliated texture, platy minerals
have a preferred orientation. A nonfoliated texture does not exhibit any discernible preferred orientation of the mineral grains. 5.
Foliated metamorphic rocks can be arranged order of grain size and/or perfection of their foliation. Slate
phyllite
and
is
in
very fine grained, followed by
schist; gneiss displays segregated
of minerals. Another fairly
metamorphic rock
is
common
bands
foliated
amphibolite.
6.
Common
7.
marble, quartzite, greenstone, and hornfels. Metamorphic rocks can be arranged into
nonfoliated metamorphic rocks are
metamorphic zones based on the conditions of metamorphism. Individual metamorphic facies are characterized by particular minerals that formed under specific metamorphic conditions. Such facies
named for a characteristic rock Metamorphism can occur along all are
8.
plate boundaries.
or mineral. three kinds of
Most, however, occurs
at
convergent plate margins. 9. Metamorphic rocks formed near the Earth's surface along an oceanic-continental plate boundary result from low-temperature, high-pressure conditions. As a subducted oceanic plate descends, it is subjected to increasingly higher temperatures and pressures that result in higher grade metamorphism. 10. Many metamorphic rocks and minerals are valuable natural resources, for example, marble, slate, graphite, talc,
^
and asbestos.
IMPORTANT
10.
What
the correct
is
metamorphic sequence of
19.
—* slate —» gneiss —* schist; —» phyllite —* schist —» gneiss; gneiss —» phyllite -» slate — » schist; schist —» gneiss — » phyllite —» slate; slate —* schist —* gneiss —* phyllite.
a.
b. c.
d. e.
11
An
explain
22.
16.
tiles,
and blackboards
a.
marble;
d
hornfels; e
gneiss;
b.
is:
Mixed rocks containing
c.
amphibolites; d.
e.
greenstones.
212
24.
What
explain
Along what type of plate boundary most common? a.
convergent;
c.
transform;
Which of
d.
the following
a.
graphite; b.
d.
garnet;
is
metamorphism
Which
asbestos;
of the following
static.
c.
talc;
a
chrysotile; b.
crocidolite; c
d.
actinolite; e.
anthophyllite.
Metamorphic rocks form shields; b.
c.
oceanic crust;
e.
answers
Chapter 8
(b)
mountain ranges; (a) and (b);
answers
d.
and
tremolite;
a significant proportion of:
the cores of
a.
may
texture,
and
be produced.
What
the difference between a metamorphic zone metamorphic facies? 29. What types of metamorphic rocks and facies are produced along a convergent plate margin? 30. Name some common metamorphic rocks or minerals that are economically valuable, and describe their
28.
and
is
a
^
ADDITIONAL
READINGS
M. G. 1982. Igneous and metamorphic petrology. San Francisco, Calif.: W. H. Freeman and Co. Bowes, D. R., ed. 1989. The encyclopedia of igneous and metamorphic petrology. New York: Van Nostrand Reinhold. Gillen, C. 1982. Metamorphic geology. London: George Allen Unwin. Hyndman, D. W. 1985. Petrology of igneous and metamorphic rocks. 2d ed. New York: McGraw-Hill Book Co. Margolis, S. V. 1989. Authenticating ancient marble sculpture. Scientific American 260, no. 6: 104-11.
&
the dangerous variety of
is
they
Best,
mantle plume; e. not a metamorphic
is
gypsum.
e.
occur?
eclogite.
e.
divergent;
b.
how
it
two types of metamorphic
uses.
blueschist;
d.
determine the effects of metamorphism? is regional metamorphism, and under what conditions does
greenschist;
granulite; b.
produce? can they be used to
describe their characteristics.
from each other reflect a metamorphic grade; by isograds; c. none of these. all of these; e. d. To which metamorphic fades do metamorphic rocks formed under low-temperature, low-pressure
amphibolite;
it
How
what metamorphic rocks would be produced by increasing heat and pressure? 27. Name the three common nonfoliated rocks, and
are separated
c.
are aureoles?
25. Describe the
are characterized by distinctive mineral
a.
contact metamorphism occur, and what
26. Starting with a shale,
hornfels;
asbestos?
18.
What
the characteristics of both
Metamorphic zones: b.
Where does
23. phyllite;
c.
slate.
mineral?
17.
metamorphism, and
how
type of changes does
and
conditions belong?
15.
the three agents of
each contributes to metamorphism. 21. What are the two types of pressure? What type of metamorphic textures does each produce?
assemblages;
14.
Name
20.
excellent rock for billiard table tops, floor
a.
metamorphic rocks, and how do they
slate
igneous and high-grade metamorphic rocks are: migmatites; mylonites; b. a.
13.
are
phyllite
roofing
12
What form?
increasingly coarser grain size?
(c).
Metamorphism and Metamorphic Rocks
F. J. 1981. Metamorphic petrology. 2d McGraw-Hill Book Co.
Turner,
ed.
New
York:
CHAPTER
9
GEOLOGIC TIME ^ OUTLINE PROLOGUE INTRODUCTION EARLY CONCEPTS OF GEOLOGIC TIME AND THE AGE OF THE EARTH JAMES HUTTON AND THE RECOGNITION OF GEOLOGIC TIME RELATIVE DATING
METHODS
Fundamental Principles of Relative Dating Unconformities
Applying the Principles of Relative Dating to the Reconstruction of the Geologic History of an Area
CORRELATION ABSOLUTE DATING METHODS Atoms, Elements, and Isotopes "** Perspective 9-1: Subsurface Correlation
and the Search
for Oil
and Natural Gas
Radioactive Decay and Half-Lives
Sources of Uncertainty
Long-Lived Radioactive Isotope Pairs
Radiocarbon Dating Methods
"^
Perspective 9-2:
Radon: The
Silent Killer
Tree-Ring and Fission Track Dating Methods
THE DEVELOPMENT OF THE GEOLOGIC TIME SCALE ~^~
Guest Essay: Paleontology: Tracing Life through Time
CHAPTER SUMMARY
Massive cross-bedded sandstones of the Jurassic-aged Navajo formation as viewed
from Emerald Pool Park, Utah.
Trail,
Zion National
PROLOGUE is time? We seem obsessed with and organize our lives around it with the help of clocks, calendars, and appointment books. Yet most of us feel we don't have enough of it— we are always running "behind" or "out of time." According to biologists and psychologists, children less than two years old and animals exist in a "timeless present," where there is no past or future. They have no conscious concept of time. Some scientists believe that our early ancestors may also have lived in a state of timelessness with little or no perception of a past or future. According to Buddhist, Taoist, and Mayan beliefs, time is circular, and like a circle, all things are destined to return to where they once were. Thus, in these belief systems, there is no
What
(jgS^ij^fl
it,
beginning or end, but rather a cyclicity to everything.
and moves can be measured and
For most people though, time flowing stream.
like a
We
subdivided.
which there
is
It
can place events
is
linear
in a
chronology
a history of past events
expectations for the future.
in
and
Most people
accept Sir
Newton's belief that time is absolute and has a of its own; that is, it "flows equably without
and then by Pope Gregory XIII in 1582 led to the Gregorian calendar, which is accurate to within one day per 3,323 years. Su Song, an eleventh-century Chinese scholar, is credited with building one of the first mechanical water clocks. It was not until the thirteenth century, however, that the first mechanical clock was built in Europe. The age of precise timekeeping really began two centuries later when the Dutch scientist Christian Huygens constructed the first pendulum clock. Today the quartz watch is the most popular timepiece. Powered by a battery, a quartz crystal vibrates approximately 100,000 times per second. An integrated circuit counts these vibrations and converts them into a digital or dial reading on your watch face. An inexpensive quartz watch today is more accurate than the best mechanical watch, and precisionmanufactured quartz clocks are accurate to within one second per 10 years. Precise timekeeping
is
important
in
our
technological world. Ships and aircraft plot their locations by satellite, relying
on
a time signal that has
an accuracy of a millionth of a second. Deep-space probes such as the Voyagers (see Chapter 2) require radio
commands timed
to billionths of a second, while
Isaac
physicists exploring the
life
an atom deal in trillionths of a second as easily as we talk about minutes. To achieve such accuracy, scientists use atomic clocks. First developed in the 1940s, these clocks rely
relation to anything external."
Albert Einstein, however, changed that view in 1905
with his special theory of time
is
a
dimension and
relativity. Einstein
is
not absolute. In
The
showed
that
fact, like
mass of an object, the greater its gravitational attraction, and thus the slower that time moves relative to an object of lesser mass. For example, if you had two identical clocks and placed one on Jupiter and one on Earth, the clock on Jupiter would run detectably slower than the clock on Earth because Jupiter has 318 times the mass of the Earth and thus exerts a greater gravitational attraction. Therefore time is unique to any particular location in space,
it is
bent by gravity.
greater the
some
respects, time
is
defined by the methods
used to measure it. The Babylonians defined a year as 360 days and divided it into 12 lunar months of 30 days each. Babylonian astronomers knew that there were 365V4 days in a year, but their priests believed that the
number 360 possessed magical
Improvements
in
properties.
time measurement by the
Romans
inside the nucleus of
oscillating electrons, a
rhythm so regular
that they are accurate to within a few thousandths of a second per day. Cesium atomic clocks were used to prove Einstein's prediction that a clock will slow down as its speed increases. While physicists deal with incredibly short intervals of time, astronomers and geologists deal with "deep time," millions or billions of years. When astronomers
look at a distant galaxy, they are seeing what it looked like billions of years ago. Geologists looking into the
the universe.
In
on an atom's
motion
Grand Canyon
are viewing nearly
two
billion
years of Earth history preserved in the rocks of the
canyon
walls. Geologists
can measure decay rates of
such radioactive elements as uranium, thorium, and rubidium to determine how long ago an igneous rock
formed. Furthermore, geologists
know that the Earth's down a few
rotational velocity has been slowing
thousandths of a second per century as a result of the
Prologue
215
ice.
*
is
day
is still
,
what
gies
geology apart from
sets
all
of the other
what occurred hundreds or even thousands of
when
geologists talk in terms of ancient
geologic history, they are referring to events that hap-
pened millions or even Geologists use
two
different frames of reference
when
their position in the rock record. Relative Hating
us
how
long agu a panicular event occurred,
only that one event preceded another. The various prin-
determine relative dating were discovered hundreds of years ago and used to construct the relative geologic time scale. They are still widely used today. Absolute dating results in speci fic dates for rock un its or events expressed in year s before the presen t. Radiociples used to
metnc dating
is
the
most common method of obtaining
absolute age dates. Such dates are calculated from the natural rates of decay of various radioactive elements
occurring in trace amounts in some rocks. until the discovery of Radi oactivity
It
was not
near the end of the
nineteenth century that absolute ages could be accurately applied to the relative geologic time scale.
geologic time scale
is
Today
the
really a dual scale: a relative scale
based on rock sequences with radiometric dates expressed as years before the present
added to
it
found
in Scripture.
more than about 6,000 years
old.
The idea of a
very
young
Earth provided the basis for most western chronologies of Earth history prior to the eighteenth century.
During the eighteenth and nineteenth centuries, sevwere made to determine the age of the Earth on the basis of scientific evidence rather than revelation. One scholar assumed that the Earth gradually cooled to its present condition from a molten beginning. eral attempts
To simulate
this history,
he melted iron balls of various
(Fig. 9-2).
temperature. By extrapolating their cooling rate to a ball the size of the Earth, he determined that the Earth
was
75,000 years old. While this age was much older than that derived from Scripture, it was still vastly younger than we now know the Earth to be. Other scholars were equally ingenious in their attempts to calculate the age of the Earth. For example, if deposition rates could be determined for various sediments, geologists reasoned that they could calculate how long it would take to deposit any rock layer. Furthermore, they could then extrapolate how old the Earth was from the total thickness of sedimentary rock in the Earth's crust. However, even for the same type of rock, rates of deposition vary. Furthermore, it is impossible to estimate how much rock has been removed by erosion, or how much a rock sequence has been reduced by compaction. As a result of these variables, estimates ranged from less than a million years to more than a billion years. Another attempt to determine the Earth's age involved ocean salinity. Scholars assumed that the Earth's ocean waters were originally fresh and that their present at least
salinity
was
the result of dissolved salt being carried into
Knowing the volume of John Joly, a nineteenthmeasured the amount of salt cur-
the ocean basins by streams.
^
EARLY CONCEPTS OF GEOLOGIC TIME AND THE AGE OF THE EARTH
ocean water and
The concept of geologic time and
would have taken
its
measurement have
changed through human history. For example, many early Christian scholars and clerics tried to establish the date of creation by analyzing historical records
216
Based on their analyses, they genand all of its features were no
erally believed that the Earth
diameters and allowed them to cool to the surrounding
billions of years ago!
speaking of geologic time. Relative dating involves pla cing gfo)o p;ic events in a s equential order as determined
tell
watch works, deep time, or geologic not easy for most people to comprehend.
a quartz
time,
^^ ^^^^^^^^^^m.^^»r^- «g7»r^^g^^-^^^ ^^^^^m.^ '^^i
.
years ago, but
from
we can
how
a
hending geologic time because they tend to view time from the perspective of their own existence. Ancient his-
will not
although
now
and an appreciation of the immensity of geologic time is fundamental to understanding both the physical and biological history of our planet (Fig. 9-1). Most people have difficulty compre-
is
And
grasp concepts such as milliseconds and understand
sciences except astronomy,
tory
a fascinating topic that has been the subject
at the current
INTRODUCTION
Time
is
of numerous essays and books.
Five hundred million years
ago a day was only 20 hours long, and rate of slowing, 200 million years from will be 25 hours long.
^^^'^m
Time
ocean currents, and varying
frictional effects of tides,
thicknesses of polar
Chapter 9
Geologic Time
and the genealo-
its
salinity,
century Irish geologist,
rently in the world's streams. at least
He
then calculated that
90 million years
to reach their present salinity level. This
it
for the oceans
was
still
much
younger than the now accepted age of 4.6 billion years for the Earth, mainly because Joly had no way of cal-
3.96
BYA
^ FIGURE Otc&St rucks
time of
its
9-1 Geologic time is depicted in this spiral history of the Earth from the formation 4.6 billion years ago to the present. (B.Y. = billion years;
M.Y. = million
years.)
Early Concepts of Geologic
Time and
the
Age of
the Earth
217
Millions
Epoch
of
Years
Major Geologic and Biologic Events
Ago Recent or Holocene
Quaternary
0.01 1.6
Pliocene 5.3
23.7
Oligocene 36.6
Eocene 57.8
66 Cretaceous 144
Permian
Carbon-
Pennsylvanian
iferous
Missis-
sippian
Devonian Silurian
Ordovician
Cambrian
Proterozoic Eon
— FIGURE
9-2
The geologic time scale.
Some
of the
major biological and geological events are
indicated along the right-hand margin.
Ice
Age ends
a quandary. They either had to accept and squeeze events into a shorter time frame or reject his calculations. However, Kelvin's quantitative measurements and arguments seemed unassailable. While Kelvin's reasoning and calculations were sound, his basic premises were false, thereby invalidating his conclusions. Kelvin was unaware that the Earth has an internal heat source, radioactivity, that has allowed it to maintain a fairly constant temperature through time.* His 40-year campaign for a young Earth ended with the discovery o f radioactivity n ear the end of the nineteenth century. His "unassailable calculations" were no longer valid, and his proof for a geologically young Earth collapsed. Moreover, while the discovery of
he concluded that the Earth must be very old and wrote
"we
that
find
no
ogists
and no prospect
vestige of a beginning,
of an end."
Observing the processes of wave action, erosion by running water, and sediment transport, Hutton concluded that given enough time these processes could ac-
count for the geologic features of his native Scotland. He believed that "the past history of our globe must be explained by what can be seen to be happening now." Thjs_as snmption t h at present-day processes have o perated throughout geologic time
was
the basis for the_prin-
cipleof_uniformita riariism (see Chapter
were
in
Kelvin's dates
1).
Unfortunately, Hutton's ideas were not widely disseminated or accepted. In 1830, however, Charles Lyell pub-
landmark book, Principles of Geology, in which he championed Hutton's concept of uniformitarianism.
radioactivity destroyed Kelvin's arguments,
Instead of relying on catastrophic events to explain var-
age and validate what geologists had been saying
ious features of the Earth, Lyell recognized that imper-
along, namely, that the Earth
ceptible changes brought about by present-day processes
than
could, over long periods of time, have tremendous cu-
heat, radiometric calculations
lished a
mulative
effects.
Through
for,
and instrumental
in,
1866
to have de-
stroyed the uniformitarian foundation of HuttonianLyellian geology. Starting with the generally accepted belief that the
sumed that,
Earth was originally molten, Kelvin as-
that the Earth has gradually been losing heat
by measuring
this
knew from deep mines
that the Earth's tem-
perature increases with depth, and he reasoned that the
Earth
is
therefore losing heat from
its
interior.
By know-
ing the melting temperatures of the Earth's rocks, the size of the Earth, and the rate of heat loss, Kelvin was
able to calculate the age at which the Earth
molten.
From
was
entirely
these calculations, he concluded that the
Earth could not be older than 100 million years or younger than 20 million years. This wide range in age reflected
uncertainties
creases with depth
-^•Six fundamental geologic prin ciples are used '
in relat ive
dating: s uperpositio n, origi nal horizontall y, lateral continuity, cro ss-cuttins relationsh ips, in clusion s,
anH fossil
s ucce ssion.
The Danish anatomist, Nicolas Steno (1638-1686), observed that during flooding, streams spread out across their floodplains and deposit layers of sediment that
bury organisms dwelling on the floodplain. Subsequent new layers of sediments that are
flooding events produce
deposited or superposed over previous deposits.
When
and the various melting points of the
After finally establishing that the Earth
how
Fundamental Principles of Relative Dating
over average temperature in'Actually, the Earth's temperature has decreased through time
Earth's constituent materials.
and showing
dating tech-
and
heat loss, he could determine the
age of the Earth. Kelvin
r adiometric
had no acceptable means oT absolute gjgTdat ing and t hus depended on relative dating method s. These methods only allow events to be placed in sequential order and do not tell us how long ago an event took place. While the principles of relative dating may now seem self-evident, their discovery was important because they provided geologists with a means to interpret Earth history and develop a relative geologic time scale.
a highly respected English physicist, in
the Earth's oldest rocks.
niques, geologists
have operated over vast periods of time, geologists were nevertheless nearly forced to accept a very young age for
when
some of
Before the development of
After finally establishing that present-day processes
Lord Kelvin (1824-1907), claimed
radium generated were providing ages, of
the
acceptance of Darwin's 1859 theory of evolution.
the Earth
all
old! Less
^ RELATIVE DATING METHODS
geolo gy. Furthermore, the recognitionof virtually limittime was also necessary
was indeed very
10~ years after the discovery that
billions 6T yeariFfor
his writings, Lyell firmly es-
tablished uniformitarianism as the guid ing jphilosophy of
less
provided
it
geologists with a clock that could measure the Earth's
was very
because the original amount of radioactive materials has been
old,
present-day processes operating over
long periods of time can explain geological features, geol-
decreasing and thus
temperature
is
is
not supplying as
much
heat.
However, the
decreasing at a rate considerably slower than would
be required to lend any credence to Kelvin's calculations.
Relative Dating
Methods
219
-»'
FIGURE
The Grand Canyon of Arizona
9-3
illustrates
three of the six fundamental principles of relative dating. The sedimentary rocks of the Grand Canyon were originally
deposited horizontally in a variety of marine and continental environments (principle of original horizontality). The oldest rocks are therefore at the bottom of the canyon, and the youngest rocks are at the top, forming the rim (principle of superposition).
some distance
lithified,
The exposed rock
layers extend laterally for
(principle of lateral continuity).
these layers of sediment
become sedimentary
rock. Thus, in an undisturbed succession of sedimentary
rock layers, t he oldest layer is at the bottom and the youngest layer is at the top T his principle of supe rposition is th e basis for relative age det erminations_ of .
a nd their contained fossil s (Fig. 9-3). Steno also observed that because sedimentary parti-
s trata
from water under the influence of
cles settle
gravity,
deposited in essentially horizontal laye rs, thus illustrating the principle of original horizontality (Fig. y73)TTherefore, a sequence of sedimentary rock
sediment
is
layers that
have been
is
from the horizontal must and lithificatio n. the pri nciple of lateral con ti-
steeply inclined
tilte d
after deposition
Steno's third principle,
sediment extends laterally in all dire cons until it thins and pinch es. ""* "r rprminarps against th e edge of the depositional ba sin (Fig. 9-3). James Hutton is credited with discove ring the principle of cross-cutting relationships. Based on his detailed studies
nuity, states that ti
and observations of rock exposures recognized that an
i
pTienii<;
in Scotland,
intrusion or fault
Hutton must be
younger than the rocks it intrudes or cut s (Fig. 9-4). While this principle illustrates that an intrusive igneous structure is younger than the rocks it intrudes, the association of sedimentary and igneous rocks may cause problems in relative dating. Buried lava flows and intrusive igneous bodies
sequence of strata
220
Chapter 9
such as
sills
(Fig. 9-5).
look very similar in a
However, a buried lava
Geologic Time
^ FIGURE
9-4 The principle of cross-cutting relationships. dark-colored dike has been intruded into older light-colored granite, north shore of Lake Superior, Ontario, Canada, (b) A small fault displacing horizontal beds in central {a)
A
Texas. (Photo
(b)
courtesy of David
J.
Marty.)
'
— FIGURE lava flows,
9-5
sills,
Relative ages of
and associated
may
sedimentary rocks
be difficult buried lava flow (4) baked the underlying bed, and bed 5 contains inclusions of the lava flow. The lava flow is younger than bed 3 and older than beds 5 and 6. (b) The rock units above and below the sill (3) have been baked, to determine, (a)
A
indicating that the
sill is
younger
than beds 2 and 4, but its age relative to bed 5 cannot be determined.
flow
is
older than the rocks above
it
w hile a
it
(
principle of super -
According
.
if
ati on
by heat_
(see
Chapter
8,
Contact Metamorphism).
than the igneous rock with which
it
is
in contact. In
produces a zone of baking immediately above and below it because it intruded into previously existing sedimentary rocks. A lava flow, on Figure 9-5, for example, a
sill
the other hand, bakes only those rocks
Another way to dete rmine
below
rela tive ages
is
it.
by
usi ng
it
is
sometimes
to this principle, fossil assemblages suc-
The validity and successful use of this prindepend on three points: (1) life has varied thro ugh
able order. ciple
alter-
sedimentary rock showing such effects must be older
floral succession as
ceed one another through time in a regular and predict-
the sedimentary rocks in contact
with the igneous rocks show signs of baking or
of faunal and
called (Fig. 9-7).
s ill is
ogists look to see
A
ple
younger than all the beds below and younger than the be d immediately above it To resolve such relative age problems as these, geol-
position ),
time, (2) fossil assemblages are recognizably differe nt "
from one another, and
(3)
the re l ative ages of the foss il
assemblages can be determined. Observations of in older versus
younger
strata clearly
life-forms have changed. Because this
fossils
demonstrate that true, fossil as-
is
semblages (point 2) are recognizably different. Furthermore, superposition can also be used to demonstrate the relative ages of the fossil assemblages.
the principle of inclus ions. This principle holds that in clusions7"o~f~lrag ments of l
one rock contained within a
ayer of another, are older than the rock lay er
example, the batholith shown
in
itself.
For
Figure 9-6a contains
sandstone inciulfi6hs7"and the sandstone unit shows the
we
conclude that the
effects of baking.
Accordingly,
s andstone is ol der^
than the batholi th. In Figure 9-6b,
however, the sandstone contains granite rock fragments, indicating that the batholith inclusions Fossils
and
is
was the source rock
known
mapping was not
fully
Chapand geologic
for centuries (see
ter 7), yet their utility in relative dating
therefore
is
older.
f
for the
therefore older than the sandstone.
have been
"^ FIGURE 9-6 (a) The batholith is younger than the sandstone because the sandstone has been baked at its contact with the granite and the granite contains sandstone inclusions, (b) Granite inclusions in the sandstone indicate that the batholith was the source of the sandstone and
ff
appreciated until the early nine-
teenth century. William Smith (1769-1839), an English civil
in
engineer involved in surveying and building canals
southern England, independently recognized the prin-
by reasoning that the fossils at the sequence of strata are older than those at
ciple of superposition
bottom of
a
the top of the sequence. This recognition served as the
^Ti^e
.
basis for the principle of fossil succession or the princi-
Relative Dating
Methods
221
Section
ir FIGURE
9-7
This generalized William Smith to identify strata of the
diagram shows used
fossils
same age
how
in different areas
(principle of fossil succession).
The
composite section on the right
shows the
relative ages of all strata
in this area.
have no record of the conversations that were occurring this period of time, we have no record of the
Unconformities
during
Our
discussion thus far has been concerned with conformable sequences of strata, sequences in which no depositional breaks of any consequence occur. A sharp bedding plane (Fig. 7-22) separating strata may represent a depositional break of minutes, hours, years, orjflM even tens of years, but it is inconsequential when con-
—
\p
sidered in the context of geologic time. S urfaces
of
discontinuity
a mounts of geologic time are in terva l
representing
is
arates
.
Three types of unconformitie s are recognized. A disrnnfnrnyfy k * cn r fcr p rrf erosiorfor non deposition between
younger and older beds that a re
another
(Fig. 9-9).
face separates the older
anrl
anv jy
ficult to
a surface of nondepositi on or erosion that__£cp-
tilted
or folded
st rata
hppp
rlppnglfprl
(hlg
An
from older rock s. As such,
it
rep-
/
many
disconformities are dif-
recognize and must be identified on the basis of
fossil
strata
one
from the younger parallel beds, an ordinary bed-
JWBing plane. Accordingly,
significa nt
un conformities,
of g eologic time not represented by strata in a is a hiatus (Fig. 9-8). "T hus, an unconfo r-
younger
parallel with
Unless a well-defined erosional sur-
I|
particular area
mity
events that occurred during a hiatus. ,
assemblages.
angular unconformity
J
is
an erosional su rface on
over which younger strata"fiave H-\i\)
Kr>fK~~y7inngPi-~
qnH
n\r\* r
b reak_Jr^ our j-ecordoi geologic rime. The strata may dip, but if their dip angles are different (genfamous 12-minutegaplrTthe Watergate tapesof Richard ^pL erally the older strata dip more steep ly), an angu lar un Nixon's presidency is somewhat analogous. Just as we, nir conformity is present. res ents a
A^ 222
Chapter 9
Geologic Time
•**
FIGURE
9-8
A
simplified
diagram showing the development of an unconformity and a hiatus. (a) Deposition began 12 million years ago (M.Y.A.) and continued more or less uninterrupted until 4 M.Y.A. (b) A 1-million-year episode of erosion
occurred, and during that time strata representing 2 million years of geologic
time were eroded, (c) A hiatus of 3 million years exists between the older strata
and the
formed
strata that
during a renewed episode of deposition that began 3
M.Y.A.
stratigraphic record. is
{d) The actual The unconformity
the surface separating the strata and
represents a major break in our record
of geologic time.
The angular unconformity is
probably the most famous
Siccar Point, Scotland, that
illustrated in Figure 9- 10b in the
world.
James Hutton
It
was here
at
w ere
worn away and covered by The erosional surface between
then
younger, flat-lying rocks.
and the younger flat-lying strata was a significant gap in the rock record. Although Hutton did not use the term unconformity, he was the first to understand and explain the the older tilted rocks
meant
is
very impor-
tant since they represent different sequences of events.
realized that
severe up heavals had tilted the lower rocks and formed
mountains that
nonconformity and an intrusive contact
that there
Applying the Principles of Relative Dating to the Reconstruction of the Geologic History of an Area
We
can decipher the geologic history of the area repre-
sented by the block diagram in Figure 9-12 by applying
The example are the same as
the various relative dating principles just discussed.
significance of such discontinuities in the rock record.
methods and
Th e third type of un conformity is a nonconformity. Here an erosion surface cut into metamorphic or igrie ous rocks isco vered by sedimentary rocks (Fig. 9-11).
those applied by nineteenth-century geologists in con-
This type of unconformity closely resembles an intrusive
nal horizontality, beds A, B, C, D, H,
igneous contact with sediment ary rock s. The principle
posited horizofiTattyTthen they were either
of inclusions
is
helpful in determining whether the rela-
tionship between the underlying igneous rocks and the is the result of an intrusi on o r ero sion. I n the case of an imTfusio^the igneous rock s are "ymffrger, but in the case of qfosiorT^the sedimentar y rock? areyounger. Being able to distinguish between a
overlying sedimentary rocks
logic used in this
structing the geologic time scale.
According to the principles of superposition and
origi-
jmdjg_wgrg
F,
tilted,
de-
faulted
and e roded or afte r de position, they were faultedTH), andtnen eroded (Fig. 9- 13 a, b, and c). Because the fauTTcuts beds A^G, it must be younger than the beds (H),
,
tilted,
according to the principle of cross-cutting relationships.
Beds
J,
K, and L were then deposited horizontally
over this erosional surface producing an angular uncon-
Relative Dating
Methods
223
c
Deposition
•^ FIGURE
9-9
(a)
Formation of a disconformity.
Mississippian and Jurassic strata in Montana. on Jurassic strata, and his right foot is resting
224
-•
(b)
(a)
Chapter 9
Geologic Time
(b)
Disconformiry between
The geologist at the upper upon Mississippian rocks.
left is sitting
""*'
FIGURE
unconformity
9-10
(a)
Formation of an angular unconformity, (b) Angular (Photo courtesy of Dorothy L. Stout.
at Siccar Point, Scotland.
Relative Dating
Methods
225
/~
.
^ J-
£L,^
or~L<>
K i-
U>
s.vl **)
.O/odM**-^
^CrocL(\ SvMcjuct
I
t*tf*
lM
m^'V^e-.lV.wv cUX
T^ 1
^
FIGURE 9-12 A block diagram of a hypothetical area in which the various relative dating principles can be applied to reconstruct the geologic history.
formity
(I)
(Fig.
9-13d). Following deposition of these
three beds, the entire sequence
was intruded by
a dik e
(Ml which, according to the principle of cross-cuttin g must be younger than all the rocks it in,
relationships,
trudes (FigT~9-13e).
was then uplifted and eroded: next were deposite d, producing a discon formity (N b etween bed s L and P and a nonconformity (O) and the sedimentary between the igneous intrusion bed P (Fig. 9-13f~and g). We know that the relationship and the overlying sed ibetween igneous intrusion m entary bed P is a n onconformity because of the presin P (principle of inclusions). ence of inclusions of
The
e ntire area
beds P and
Q
)
M
M
M
At thispoint, there are several
possibilities for recon-
structing the geologic history of this area. According to the principle of cross-cutting relationships, dike
we cannot
after
Q,
determine whether
right after S, or after
poses of this history,
we will
R was
T was
say that
deposition of bed JD (Fig. 9-13g and
formed
right
formed. For pur-
it
,
depositedjjn the Earth's surf a ce
Thus,
we have
established a relative chronology for
the rocks and events of this area by using the principles of_relative dating.
wayoFTcnowirig curred unless
Remember, however,
how many
we can
that
we have no
years ago these events oc-
obtain radiome tric_ date s for the
igneous rocks. With these dates
we can
establish the
range of absolute ages between which the different sed-
imentary units~wereTdeposited and also determine
much
time
is
how
represented by the unconformities.
R must
be younger than bed Q because it intrudes into it. It can have intruded anytime after bed Q was deposited; however,
Following the intrusion of dike R, lava S flowed over bed Q, followed by the deposition of bed T (Fig. 9-13i and j). Although the lava fl ow (S) is not a se djmejitary unit the principle of superposition still applies beca use it flowed "on the Earth's su rface, Justus sediments a re
intruded after the
h).
^^^
\U
^
CORRELATION
/->M'<JU.,;-K
\/\A
£,
bed f.
If geologists are to reconstruct Earth history; they must demonstrate the time equivalency of rock units in different areas. This process is known as c orrelat ion
J^CU^vp \OlOL;
Qi
\ ,o)-Cb
J« //ii/^L -A Lfc-OJ*
t
rsTX
i-CLA>n
j.
'JO
i?
C
T~
Correlation
227
/£&
with the lowermost equivalent rocks of another area,
of time during the geologic past. Fossils that are easily
the history of the entire region can be deciphered.
identified, are geographically
Although geologists can match up rocks on the basis of similar rock type and stratigraphic position, correlation of this type can only be done in a limited area where beds can be traced from one site to ano ther. In order to
a rather short geologic time are particularly useful. Such
correlate rock units over a large area or to correlate
guide
fossils are called
fossil
ratnus meet
9-14
Correlation of rock units,
of these criteria and are therefore
all
fossils. In contrast,
identified
the brachiopod Lingula
and widespread, but
Because most
fossils
it
have
of
its
good easily
geologic range of Or-
little
fairly
is
use in correlation.
long geologic ranges,
geologists constructl assemblage range zones to determine
the remains of organisms that lived for a certain length
FIGURE
(
dovician to Recent makes
succession must be used.
Fossils are us eful as time in dic ators because they are
'*'
guide fossils or index fossils Fig. 9-16).
For example, the trilobite Isotelus and the clam Inoce-
age-equivalent units ^>f differenPcornpbsition, fossils
and the principle of
widespread, and existed for
In areas of adequate exposures, rock
(a)
(£>) Correlation by similarities rock type and position in a sequence. The sandstone in section 1 is assumed to intertongue or grade laterally into the shale at section 2. (c) Correlation using a key bed, a distinctive black limestone.
units can be traced laterally even
occasional gaps exist.
if
in
I
I
|.!i
I
|
I
I
i|i|i|
I
I
.
|i
I
I
i|. !
!
I
I
.|
.
I
I
I
iiiiii
|
I
I
i!
.
'
1
'I
I
!i|i|i
i| |
.
I
I
m
'''
I
|i|i|i
i
!
' '
'
PC
ffig
Correlation
229
Precambrian Eon
Fm =
Formation
230
Ss = Sandstone
Chapter 9
Ls = Limestone
Geologic Time
•
Rocks
ol
Ordovician and Silurian age are not present
in
the
Grand Canyon
FIGURE
"**"
9-15
Correlation of rocks within the
(left)
Colorado Plateau. By correlating the rocks from various locations, the history of the entire region can be deciphered.
Atoms, Elements, and Isotopes As we discussed
in
Chapter
3, all
matter
th e age of_thc_sedimentary roclcs-contatBiBfr^he^fossils.
Assemblage range zones are established by plotting the overlapping geologic ranges of different species of
The
first
establish
and
last
fossils.
occurrences of two species are used to
an assemblage zone's boundaries
(Fig. 9-17).
Correlation of assemblage zones generally yields correlation lines that are considered time equivalent. In
is
made up
of
composed of extremely small particles called atoms. The nucleus of an atom is composed of protons and neutrons with electrons encircling it (Fig. 3-3). The number of protons defines an element's atomic number and helps determine its properties and characteristics. The combined number of protons and neutrons in an atom is its atomic mass number. However, not all atoms of the same element have the same number of neutrons in their nuclei. These variable forms of the same element are called isotopes. chemical elements, each of which
is
other words, the strata encompassed by the correlation
thought to be the same age. Geologists are aware, however, that such zones are not exactly the lines are
same age everywhere, because no fossil organism appeared and disappeared simultaneously over its entire geographic range. Even so, first and last appearances do not differ greatly from origins and extinctions in geologic time; thus, correlation of assemblage zones can still
^ FIGURE
The geologic ranges of three marine The brachiopod Lingula is of little use in correlation because of its long geologic range. The trilobite hotelus and the bivalve Inoceramus are good guide fossils 9-16
invertebrates.
because they are geographically widespread, are easily identified, and have short geologic ranges.
be very precise. For example, during the 1840s and
1850s, Albert Oppel was able to subdivide the Jurassic
based on the overlapping ranges of ammonites found in Europe. Most of these
strata into zones fossils called
zones are
less
than a million years in duration
Tertiary
(later
by correlation with radiometrically dated beds) and can be used to correlate Jurassic rocks accurately throughout the world.
verified
Cretaceous
Inoceramus
^ ABSOLUTE DATING METHODS Thus
far,
our discussion has largely concerned the con-
cept of geologic time and the formulation of principles
used to determine relative ages.
It is
somewhat
ironic
that radioactivity, the very process that invalidated Kelvin's calculations,
now
Permian
Lord
serves as the basis for deter-
Pennsylvaman
mining absolute dates. Mississippian
Although most of the isotopes of the 91 naturally occurring elements are stable, some are radioactive and
spontaneously decay to other more stable isotopes of elements, releasing energy in the process. The discovery, in
1903 by
Pierre
and Marie Curie, that radioactive de-
cay produces heat as a by-product meant that geologists finally had a mechanism for explaining the internal heat
Ordovician
of the Earth that did not rely on residual cooling from a
molten origin. Furthermore, geologists and paleontolohad a powerful tool to date geologic events accurately, and thus verify the long time periods postulated by Hutton, Lyell, and Darwin.
gists
Cambrian
Absolute Dating Methods
231
Perspective 9-1
SUBSURFACE CORRELATION AND THE SEARCH FOR OIL AND NATURAL GAS During the early years of the petroleum industry, geologists relied almost exclusively in their search for oil
and
gas.
techniques, they constructed
on surface
Among
studies
other
maps showing rocks and
geologic structures such as folds and faults. Interpretation of such
maps sometimes
interpretation of data regarding geologic features
revealed
subsurface structures, such as those in Figure 7-33,
which oil and natural gas might be trapped. Surface methods are still important in petroleum geology, particularly in unexplored regions, but most exploration is now done using subsurface methods. Subsurface geology is the acquisition and
in
beneath the Earth's surface. Drilling operations have
provided a wealth of data on subsurface geology.
When """
FIGURE
Core and (b) rock chips are the two types of samples recovered from drill holes. (Photos courtesy of Sue Monroe.) 1
(a)
drilling for oil or natural gas, cores or
rock
chips called well cuttings are usually recovered from 1). These samples are studied under and reveal such important information as rock type, porosity (the amount of pore space) and permeability (the ability to transmit fluids), and the
the drill hole (Fig. the microscope
presence of
oil stains.
In addition, the samples can
also be processed for microfossils that can aid in
determining the geologic age of the sediments
(Fig. 2).
Cores are very useful for correlating rock units from well to well and locating oil- or gas-producing zones. Geophysical instruments may be lowered down a drill hole to record such rock properties as electrical resistivity, density,
and
radioactivity, thus providing a
well log of the rocks penetrated (Fig. 3). (text
"*"""
FIGURE
2
continued on page 234)
Microscopic one-celled animals called
foraminifera can be used to determine the age of the rock they are found in and can be used to correlate rock units between wells. (Scanning electron micrograph by Dee Breger, Lamont-Doherty Geological Observatory.)
232
Chapter 9
Geologic Time
Magnetic recording
Down
hole
logging tool
(a)
"•^ FIGURE 3 {a) A schematic diagram showing how well logs are made. A logging tool is down the drill hole. As the tool is withdrawn, data are transmitted to the surface where they are recorded and printed out as a well log. (b) Electrical logs and correlations of rocks in two wells in Colorado. The curves labeled SP are plots of self-potential (electrical potential caused by different conductors in a solution that conducts electricity) with depth, and the curves labeled R are plots of electrical resistivity with depth. lowered
Absolute Dating Methods
233
Energy source
Satellite
navigation
system
Hydrophones
/^^
^ FIGURE
4 {a) A diagram showing the use of seismic reflections to detect buried rock units at sea. Sound waves are generated at the energy source. Some of the energy of these waves is reflected from various horizons back to the surface where it is detected by hydrophones. Buried rock units can also be detected on land, but here explosive charges are detonated as an energy source, (b) Seismic record and depositional sequences defined in the Beaufort Sea. Boundaries of seismic sequences are shown by solid black lines. The scale on the right shows seismic wave travel time. Notice the sloping lines indicating faults in the right part of the seismic record.
have made it possible to work out problems that could not otherwise have been solved. Such logs have saved oil companies tremendous amounts of money in coring expenses and, by enabling the companies to determine the subsurface fluid content, have helped them discover additional oil that might otherwise have been missed. Electrical logs have also been used for very accurate Electrical logs
structural
correlation, particularly over short distances (Fig. 3).
Subsurface rock units
may
also be detected
and
traced by the study of seismic profiles. Energy pulses,
such as those from explosions, travel through rocks at a velocity determined by rock density, and this
Most
energy
is
reflected
some
of
from various horizons (contacts
isotopes are stabl e, but
s
ome
are unstable
spontane ously cteca~y~to~a more~itirjIe~rbrm.
It
is
and the
between contrasting it is
recorded
continental shelves where
is
to
map
234
Chapter 9
Geologic Time
it is
very expensive to
drill
the structure to see
most well
if it
has the
and gas. Another important use is in predicting where an oil- or gas-producing horizon might occur outside the limits of a known oil field. The choice of subsurface correlation methods depends on the information geologists are seeking, the general geology of the area, and the cost and time
potential for trapping oil
available to run different logs.
atomic nucleus of a different element. radioact ive decay are recognized, the nucleus emits
is the process whereby an unstable atomic nucleuses spontaneously transformed into an
where
is
In petroleum exploration, the purpose of correlations
c hange
Radioactive decay
to the surface,
holes and other techniques have limited use.
j\ o
Radioactive Decay and Half-Lives
back
Seismic stratigraphy
particularly useful in tracing units in areas such as the
^dec ay
rate of u nstable jsotopes tKatgeologi sts meas ure determ ine the absoluteage~oFrocIci^
layers)
(Fig. 4).
all
of
Three
types, of
w hichj-esult in a
o f atomic structure (Fig. 9-18). Injdpjia_decay, two protons and two neutrons with the result that the atomic number decreases by two and the atomic mass number decreases by four. B eta decay is the emission of a fast-moving electron from a neutron in the nucleus; the neutron
is
changed to
consequently the atomic number
is
a proton,
and
increased by one,
"" FIGURE
9-17
Correlation of two
sections by using assemblage range
zones. These zones are established by the overlapping ranges of fossils
A
through E.
with no resultant atomic mass number change. Electron capture results
an electron
when
shell
and
a proton captures an electron is
as a result, the atomic
from
thereby converted to a neutron;
number decreases by one, but
the
atomic mass number does not change. Some elements undergo only one decay step in the conversion from an unstable form to a stable form. For example, rubidium 87 decays to strontium 87 by a sin-
and potassium 40 decays to argon 40 by a single electron capture. Other radioactive elements undergo several decay steps (see Perspective 9-2). Uranium 235 decays to lead 207 by seven alpha and six beta steps, while uranium 238 decays to lead 206 by eight gle beta emission,
alpha and six beta steps
When to
them
discussing decay .rates,
act ive element a"
is
the time
it
it is
convenient to refer
The half-life of
given radioactive element
from
less
is
By measuring the parent-daughter
ratio
and knowing
geologists can calculate the age of a sample containing
The parent-daughter
usually determined by a
mass spectrometer, an
constant
and can be
in the laboratory. Half-lives
active elements range
instruments.
the radioactive element.
t
toms of the original unstable parent element to deca y atoms of a new, more stable daughter elemen t. The
measured
,
he
takes for one-half of
gardless of external conditions
.
a rad io-
to
halt-lite of a
hav e 500,000 parent atom s and 500,000 daugh ter atoms after one half-life After two half-lives, it will have 250,000 parent atoms (one-half of the previous parent atoms "which is equivalent to one-fou rth ot the original parent a toms) and 750,000 daughter atoms. After three half-lives, it will have 125,000 parent atoms (one-half of the previous parent atoms or one-eighth of the original parent atoms) and 875,000 daughter atoms, and so on until the number of parent atoms remaining is so few that they cannot be accurately measured by present-day
the half-life of the parent (determined in the laboratory),
(Fig. 9-19).
in term^oLhalf-Jiyes)
For example, an element with 1.000,000 parent atoms will
ment
that
meas uresjhe proportions
ratio
is
instru-
of_eleme_nts_of dif-
ferent masses.
re-
precisely
of various radio-
than^a-bjllionth of a
second to 49 billion yea rsRadioac tive decay occurs at a geometric rate rath er t han a li negxiatejherefore, a graph of the decay rate produces a curve rather than a straight line (Fig. 9-20).
Sources of Uncertainty
The most accurate radiometric dates are obtained from i gneous rock s. As a magma cools and begins to crystallize, radioactive
parent atoms are separated from previ-
ously formed daughter atoms. Because they are the right size,
some radioactive parent atoms
are incorporated
Absolute Dating Methods
235
Changes in atomic number and atomic mass number
Alpha particle
Atomic number = -2 Atomic mass number = -4
Alpha decay
Beta particle
Atomic number = +1 Atomic mass number = Beta decay
-»-
FIGURE
9-18
radioactive decay,
Three types of Alpha decay,
(a)
Atomic number = -1 Atomic mass number =
which an unstable parent nucleus emits two protons and rwo neutrons, (b) Beta decay, in which an electron is emitted from the in
nucleus,
(c)
Electron capture
Electron capture, in
which a proton captures an electron and is thereby converted to a
Q
Protron
neutron.
into the crystal structure of certain minerals.
daughter atoms, however, are a different
The
size
stable
than the
radioactive parent atoms and consequently cannot into the crystal structure of the
parent atoms. Therefore crystallize, the
when
same mineral the
magma
fit
as the
begins to
mineral will contain radioactive parent
atoms but no stable daughter atoms (Fig. 9-21). Thus, the time that is being measured is the time of crystallization of the mineral containing the radioactive atoms,
not the time of formation of the radioactive atoms.
Exay3t_jnj musual circumstan ces, sedimentary rocks ca nnot be radiometrically dated, be cause one
would be
measuring the age of a particular mineral rather than the time that it was deposited as a sedimentary particle. One of the few instances in which radiometric dates can be obtained on sedimentary rocks is when the mineral glauconite
236
is
present. Glauconite
Chapter 9
is
a greenish mineral cbn-
Geologic Time
#
Neutron
Electron
taining radioactive potassium 40, which decays to argon
40 (Table
marine environments du ring the convers ion from sediments to sedimentary rock. Thus, it forms when the sedimentary rock forms, and a radiometric date indicates the time of the sedimentary rock's origin. However, because the daughter product argon is a gas, it can easily escape from a mineral. Therefore, any date obtained from glauconite, or any other mineral containing the potassium 40— argon 40
~a" s~a
pair,
9-1).
It
forms
in certain
result of chemical reactions with clay minerals
must be c onsidered
a
minimum
To obtain accurate radiometric
ag e.
dates, geologists
must
be sure that they are dealing with a closed system, mean-
atoms have been added or removed from the s ystem since crystallization and that the ratio between them results only from raing that neither parent nor daughter
dioactive decay. Otherwise, an inaccurate date will re-
Magma
^ FIGURE
9-21
(a)
A magma
contains both radioactive and stable atoms, (b) As the magma cools and begins to crystallize,
some
radioactive atoms are incorporated into certain minerals because they
are the right size
and can
fit
into the
crystal structure. Therefore, at the
time of crystallization, the mineral will contain 100% radioactive
parent atoms and 0% stable daughter atoms, (c) After one half-life, 50% of the radioactive parent atoms will have decayed to stable daughter atoms.
daughter ratio of two different radioactive elements
in
same mineral. For example, naturally occurring uranium consists of both uranium 235 and uranium 238 isotopes. Through various decay steps, uranium 235 decays to lead 207, whereas uranium 238 decays to lead 206 (Fig. 9-19). If the minerals containing both uranium the
isotopes have remained closed systems, the ages ob-
tained from each parent-daughter ratio should be in close agreement
and therefore should indicate the time magma. If the ages do not closely agree, other samples must be taken and ratios measured to see which, if either, date is correct. of crystallization of the
Long-Lived Radioactive Isotope Pairs Table 9-1 shows the
five
common,
long-lived parent-
daughter isotope pairs used in radiometric dating. Longlived pairs have half-lives of millions or billions of years. All of these still
were present when the Earth formed and are
present in measurable quantities. Other shorter-lived
radioactive isotope pairs have decayed to the point that
only small quantities near the limit of detection remain.
The most commonly used isotope pairs are the and thorflimjeji^jienes., > which^ are_used prmcTpairyto date ancient igneous intrusives, lunar sam ples, and some meteorites The r ubidium-strontium pa ir tranium-lead
.
is'also
used ~t or very old samples and has been effective
d ating _thiie_Qidest rocks on E artrTas well as meteorites he ggtassium- argor^method is typically used for dating
in 1
.
finegrained v olcanic roc ks from which individual crys-
cannot be separated; hence the whole rock is anaHowever, argon is a gas, so great care must be taken to assure that the sample has not been subjected to heat, which would allow argon to escape; such a sample tals
lyzed.
would
yield
an age that
is
too young. Other long-lived
radioactive isotope pairs exist, but they are rather rare
and
"^ FIGURE
9-22 The effect of metamorphism in driving out daughter atoms from a mineral that crystallized 700 million years ago (M.Y.A.). The mineral is shown immediately after crystallization (a), then at 400 million years (b), when some of the parent atoms had decayed to daughter atoms. Metamorphism at 350 M.Y.A. (c) drives the daughter atoms out of the mineral into the surrounding rock, (d) Assuming the rock has remained a closed chemical system throughout its history, dating the mineral today yields the time of metamorphism, while dating the rock provides the time of its crystallization, 700 M.Y.A.
are used only in special situations.
Radiocarbon Dating Methods
£"/
?^
is an important el ement in nature and is one o fthe ^ba sic elements found in all forms of l ife. It has three isotopes; two of these, carbo n 12 and 13, are stable, where as ?n 14 is radioactive. Carbon 14 has a halt-life of pears plus or minus 30 years. The carbon 14 dating^, ^techniq ue is based on the ratio of carbon 14 to carbon 12 and is generally used to date once-livin g material. The short half-life of carbon 14 makes this dating^ technique pj-gctical only for specimens you nger than abourJZQJDOO years. Consequently, the carbon 14 dating method is especially useful in archaeology and has
,V Carbon
greatly aide d in unraveling the events of the latter por-
p
tion of rh flfl^istocene EpocT
Carbon 14 sphere by the
is
constantly formed in the upper atmo-
bombardment
of cosmic rays, which are
high-energy particles (mostly protons). These high-energy particles strike the atoms of upper-atmospheric gases, splitting their nuclei into protons
When
and neutrons. atom
a neutron strikes the nucleus of a nitrogen
(atomic number 7, atomic mass number 14), it may be absorbed into the nucleus and a proton emitted. Thus, the atomic number of the atom decreases by one,
Absolute Dating Methods
239
Perspective 9-2
RADON: THE SILENT KILLER What
is
radon, what makes
how
so dangerous, and
it
worried should you be about it in your home, school, or business? According to the U.S. National Research Council, approximately 20,000 people die prematurely
home, however, radon can accumulate levels (>4 pCi/L). Continued exposure
to unhealthy
to these
elevated levels over several years can greatly increase the risk of lung cancer.
As one of the natural decay products of uranium
each year from cancers induced by exposure to indoor radon. In fact, radon is the second leading cause of
238, radon
lung cancer in the United States.
elements called radon daughters
Your chances of being adversely affected by radon depend on numerous interrelated factors such as your
time you breathe, these daughter elements become
geographic location, the geology of the area, the
releasing high-energy alpha
climate,
much
how
the building
time you spend
as yet,
no
constructed, and
is
in the building.
how
While there
(Fig.
are,
federal standards defining unacceptable
Environmental Protection Agency (EPA) recommends radon levels not exceed indoor radon
levels, the
four picocuries per
liter
(pCi/L) of air (a curie
is
standard measure of radiation, and a picocurie
the
is
one-trillionth of a curie).
Radon
is
part of the uranium
238—lead 206
series (Fig. 9-19). It
occurs
in
and
the atmosphere
where
it is
harmless levels (0.2 pCi/L
any rock or
level of radon). In
1
Some
of the
diluted is
soil that
Chapter 9
and
dissipates to
the average ambient
an enclosed area such as a
common
radon can enter a house.
240
(Fig. 9-19).
Every
your lungs and eventually break down,
9-18) that
and beta decay particles tissue and can cause lung
damage lung
cancer.
Concern about the health arose during the 1960s
when
risks
the
posed by radon
first
news media revealed
some homes in the West had been built with uranium mine tailings. Since then, geologists have found that high indoor radon levels can be caused by natural uranium in minerals of the rock and soil on
that
is
radioactive decay
contains uranium 238. Outdoors, radon escapes into
"^ FIGURE
in
decays into other radioactive
a colorless, odorless, naturally occurring
radioactive gas that has a three-day half-life
outdoor
trapped
itself
Geologic Time
entry points where
"^"
FIGURE
2
Two
of the most popular commercially
available radon-testing devices are (a) the charcoal canister
and (b) alpha track detectors. Both are left open and exposed to the air and then sent to a laboratory for analysis.
FIGURE 3 Areas in the United States where granite, phosphate-bearing rocks, carbonaceous shales, and uranium occur. These rocks are all potential sources of radon gas.
"'•'
left open and your house and then sent to a
which buildings are constructed. In response to the high cost of energy during the 1970s and 1980s, old buildings were insulated, and new buildings were constructed to be as energy efficient and airtight as
track detectors (Fig. 2). Both devices are
possible. Ironically, these energy-saving measures also
levels of
sealed in radon.
Radon
enters buildings through dirt floors, cracks
in the floor
or walls, joints between floors and walls,
sumps, and utility pipes as well as any cracks or pores in hollow-block walls (Fig. 1). Radon can also be released into a building whenever the water is turned on if the water comes from a private floor drains,
well.
Municipal water
is
generally safe because
it
inexpensive, simple
home
it
gets to
testing devices.
The two
most popular are the charcoal canister and alpha
air in
laboratory for analysis.
radon readings are above the recommended EPA 4 pCi/L, several remedial measures can be taken to reduce your risk. These include sealing up all cracks in the foundation, pouring a concrete slab over If
a dirt floor, increasing the circulation of air
basement and
throughout the house, especially
in the
crawl space, providing
drains and other
utility
filters for
openings, and limiting the time spent in areas
with higher concentrations of radon.
has
your home. To find out if your home has a radon problem, you must test for it with commercially available, relatively usually been aerated before
exposed to the
It is
important to remember that although the radon
hazard covers most of the country, some areas are
more
likely to
radon than others
have higher natural concentrations of (Fig. 3).
For example, such rocks as
uranium-bearing granites, metamorphic rocks of granitic (continued on next page)
Absolute Dating Methods
241
composition, and black shales (high carbon content) are quite likely to cause indoor radon problems. Other
rocks such as marine quartz sandstone, noncarbonaceous shales and siltstones, most volcanic rocks, and igneous and metamorphic rocks rich in iron and magnesium typically do not cause radon
problems. The permeability of the
soil
overlying the
rock can also affect the indoor levels of radon gas.
Some
soils are more permeable than others and allow more radon to escape into the overlying structures. The climate and type of construction affect not only how much radon gets into a structure, but how much
escapes. Concentrations of radon are highest during the
winter
northern climates because houses are sealed as
in
tightly as possible. likely to
Homes
with basements are more
have higher radon
levels
than those built on
homes in Gunderson of the U.S. Geological Survey found that homes with a basement had average radon levels two to three times higher than homes built on a concrete slab. Furthermore, homes that had cracks in their basement walls or that were constructed with hollow-block walls (such blocks are very gas permeable) had higher radon readings than those with solid, poured concrete walls. While research continues into the sources of indoor radon and ways of controlling it, the most important thing people can do is to test their home, school, or business for radon. In this way more data will be available for analysis, some preventive measures can be taken, and a solution to this major problem will be concrete slabs. In a recent study of 3,000 Atlanta, Georgia, Linda
found sooner.
while the atomic mass number stays the same. Because
Tree- ring datingjs^a usefujjriethod forjd atingjecent
number has changed, a new element, carbon 14 (atomic number 6, atomic mass number 14), is formed. The newly formed carbon 14 is rapidly assim-
even ts. The age of a tree can be determined by counting
the atomic
carbon cycle and, along with carbon 12 and 13, is absorbed in a nearly constant ratio by all living organisms (Fig. 9-23). When an organism dies, however, carbon 14 is not replenished, and the ratio of carbon 14 to carbon 12 decreases as carbon 14 decays back to nitrogen by a single beta decay step (Fig. 9-23). The ratio of carbon 14 to carbon 12 is remarkably constant in both the atmosphere and living organisms, and geologists assume that it has also been constant for the past 100,000 years. Comparing ages established by carbon 14 dating of wood samples with ages obtained by counting annual tree rings in the same samples yields slight differences (Fig. 9-24). It appears that the production of carbon 14 and hence the ratio of carbon 14 to carbon 12 has varied slightly over the past several thousand years, in part, because the amount of C0 2 has ilated into the
As a result, corrections in carbon 14 ages have been made to account for such variations in the past.
varied.
Tree-Ring and Fission Track Dating Methods In addition to radiometric dating, various other
ods can yield accurate absolute dates.
common
242
include tree-ring
Chapter 9
and
Two
meth-
of the most
fission track dating.
Geologic Time
the
growth rings
in the
lower part of the trunk. Each
and the pattern of wide and narrow rings can be compared among trees to establish the exact year in which the rings were formed. The procedure of matching ring patterns from numerous trees and wood fragments in a given area is referred to as cross-dating. By correlating distinctive tree-ring sequences from living to nearby dead trees, a time scale has been constructed extending back to about 14,000 years ago (Fig. 9-25). By matching ring patterns to the composite ring scale, wood samples whose ages are not ring represents one year's growth,
known can The ited
be accurately dated.
applicability of tree-ring dating
because
it
is
somewhat
lim-
can only be used where continuous tree
records are found.
It
is
therefore most useful in arid
regions, particularly the southwestern United States. Fissi on
track dating
is
a useful techn ique that can be
applied in dating samples ranging in age from only a tew
hundred to hundreds of millions of years. It is most usetul tor dating samples between about 40,000 and one million years ago, a period for which other dating techniques are not particularly effective.
When
a
uranium isotope
in a
mineral emits an alpha
decay particle, the heavy, rapidly moving alpha particle
damages the
crystal structure.
The damage appears
as
small linear tracks that are visible only under a high-
"^ FIGURE
9-23
The carbon
(right)
cycle
showing the
formation, dispersal, and decay of carbon 14.
powered microscope and only after etching the mineral with hydrofluoric acid. The age of the sample is determined by the number of fission tracks present and the
amount of uranium
number of
One of the problems in when the rocks have been tures. If this
The
the sample contains.
sample, the greater the
older the
Cosmic
tracks (Fig. 9-26).
radiation
fission track dating occurs
subjected to high tempera-
happens, the damaged crystal structures are
Neutron capture
Nitrogen 14
Carbon 14
\
"repaired" by annealing, and consequently, the tracks disappear. In such instances, the calculated age will be
younger than the actual age.
y
C 14 C
^ THE DEVELOPMENT OF scale
is
in
time units of varying duration
absorbed C 12 and
is
(Fig. 9-2).
a
into the tissue
organisms
fairly
constant
ratio.
a hierarchical scale in
the 4.6-billion-year history of the Earth
13
of living
THE GEOLOGIC TIME SCALE The geologic time
is
along with
which
divided into
The geologic
time scale was not developed by any one individual, but rather evolved, primarily during the nineteenth century,
through the efforts of
many
people. By applying relative
dating methods to rock outcrops, geologists in England
and western Europe defined the major geologic time units without the benefit of radiometric dating tech-
niques
and
(Fig. 9-27).
Using the principles of superposition
fossil succession,
When an organism dies, C 14 back to N 14 by beta decay.
they were able to correlate the
converts
various exposures and piece together a composite geoBeta decay
Nitrogen 14
Beta*v particle
"^ FIGURE 9-24 (below) Discrepancies exist between carbon 14 dates and those obtained by counting annual tree rings. Back to about 600 B.C., carbon 14 dates are too old, and those from about 600 b.c to about 5,000 b.c are too young. Consequently, corrections must be made to the carbon 14 dates for this time period.
2.000
1
,000
1
,000
2,000
•
3,000
Proton
4,000
Tree-ring dates
The Development of
the Geologic
Time
Scale
243
H Even as a fossils.
I
Guest Essay MICHAEL L. McKINNEY TTTfTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTI
PALEONTOLOGY
LIFE THROUGH TIME
child,
I
being interested in rocks and
recall
know now
same reasons
the
that
that
I
I
them for enjoy teaching and doing
was
still
attracted to
research in historical geology. For one thing, rocks
and
fossils are a
my
constant reminder that time did not
knowledge leads to a more relaxed view of what I— and the human species for that matter— am doing here. One's self-importance is continually diminished when you work with fossils begin with
existence. This
that are millions of years old.
A
bigger part of
my
motivation, however, comes
from the "detective" work involved
in historical geology.
Like a police detective, the historical geologist trys to reconstruct past events from fragmentary evidence.
Whether an
oil
as a sedimentologist trying to determine
when
basin formed, or a paleontologist trying to find
the ancestors of
modern mammals,
use whatever limited information
be frustrating, but as with
when
many
the challenge
is
is
available. This
puzzles, the
to
can
moment
"come together" is very satisfying. Furthermore, new evidence is always being found so new puzzles always arise and old answers often prove inadequate. Most satisfying of all is the knowledge that the work is more than idle amusementr you are contributing to our understanding of how the Earth and its life came to be what they are today. ideas
Besides being fun, the study of fossils
sedimentary rocks has
many
and
is
Our
built
materials formed. For example,
work
for oil companies,
cores brought
up by
many
on
for a Ph.D. After receiving
paleontologists
examining microfossils
in
rock
this
choice because
projects of
it
my own
number of graduate
allows
me
choosing. students
laboratory, doing research in
United States.
have never worked
offered jobs by
my
two
oil
in industry
submitting grant applications
Some
companies when
master's degree in geology. Instead,
I
I
I
was
completed
chose to go
iiAiiAHiiilititliliilti iilii t iiAil i
244
Chapter 9
Geologic Time
teach I
made
to carry out research
I am helped by a who work in my their own particular
aimed
of
my own
at finding
if
their research
favorite research
is
many
information on the
relevance today,
an alarming
when
over
99%
of
have ever existed have died out, the
amount
contains a vast
we have
species of animals (such as
more
generally
become
likely to
We
extinctions
much
all
at
species that
fossil
record
of useful data about
extinctions. For instance,
some
costly.
becoming extinct
species are
rate. Since
is
currently
seen in the fossil record. This research has
already learned that
mammals)
are
extinct than others
have also discovered that habitat
destruction has been the
main cause of extinction
throughout geologic time, just as it is today. The only difference is that today humans destroy the habitats,
whereas
in the past
changes
impacts, and other natural
in climate, meteorite
phenomena caused
A
destruction.
IVLichael
L.
McKinney
is
an
associate professor in the
Geology and Ecology Programs at the University of Tennessee,
He
has published
books and many technical articles on evolution, paleontology, and environmental three
although
I
one is making highly sophisticated measurements of fossil shapes by using a television camera connected to a computer. Much of this work is supported by grants from agencies such as the National Science Foundation. Funding from these agencies is very competitive, and the grants usually last only a couple of years. Therefore, scientists must often spend a significant amount of time writing and
Knoxville. I
joined the
I
where
areas. For instance,
specializing in sedimentology
fields in the
degree,
undergraduate and graduate courses. I'm glad
drilling rigs. Historical geologists
and stratigraphy are also employed in the search for oil and minerals; they examine the physical characteristics of the rock cores and correlate rock layers. Environmental firms are currently the major employers of geologists, and environmental careers are among the fastest growing
my
faculty at the University of Tennessee
(such as clams).
practical applications.
on ores and energy (such as fossil fuels) that come from the Earth. By studying the history of the Earth, we learn how and, more importantly, where these society
TRACING
:
topics.
the
C. This
beam came
from an old house
VV^^^i
11
ll
1
^ This date obtained by counting back from bark of
A
through B
Specimens taken from
ruins,
when matched and overlapped as indicated, progressively extend
the dating back into prehistoric times.
"^ FIGURE are
9-25
In the cross-dating
matched against each other
method, tree-ring patterns from different woods
to establish a ring-width chronology
logic section. This composite section
is,
in effect, a rel-
ative time scale because the rocks are arranged in their
correct sequential order.
Geologists also recognized that the different fossil as-
semblages, representing distinct time periods in the past, could be used to correlate rock units elsewhere even if the rock types were different.
The names of
these time
backward
in time.
•^ FIGURE
9-26
Each
fission track
length) in this apatite crystal
is
(about 16
p.
in
the result of the radioactive
decay of a uranium atom. In order to make the fission tracks visible, the apatite crystal has been etched with hydrofluoric acid. This apatite crystal comes from one of the dikes of Shiprock, New Mexico, and indicates a calculated age of 27 million years. (Photo courtesy of Charles W. Naeser, U.S. Geological Survey.)
periods were thus based on the areas in which the rock units were originally described. For example, the Camis taken from the Roman word for Wales (Cambria), whereas the Ordovician and Silurian periods are named after the Silures and Ordovices, tribes that
brian Period
Wales during the Roman conquest (Fig. 9-27). By the beginning of the twentieth century, geologists had developed a relative geologic time scale, but did not yet have any absolute dates for the various time unit lived in
boundaries. Following the discovery of radioactivity near the end of the last century, radiometric dates were relative geologic time scale (Fig. 9-2). Because sedimentary rocks, with rare exceptions, cannot be radiometrically dated, geologists have had to
added to the
The Development of
the Geologic
Time
Scale
245
Carboniferous (Coneybeare and Phillips, 1822) ,'
Cambrian (Sedgwick, 1835)
Ordovician (Lapworth, 1879) Silurian
(Murchison, 1835)
^ FIGURE
9-27 The names of the time periods of the geologic time scale were based on areas in England and Europe where the rock units were originally described.
Note
that the
Carboniferous, which is recognized in Europe, is represented by two systems in North America, the Mississippian and Pennsylvanian.
FIGURE 9-28 Absolute ages of sedimentary rocks can be determined by dating associated igneous rocks. In {a) and (b), sedimentary rocks are bracketed by rock bodies for which absolute ages have been determined. "*•*
Nonconformity
150 M.Y.
—
(a)
> 600 to <675
bQ^°rs^Q°v^R°:
~"
•Lava flow (600 M.Y.)
M.Y. -Sill
(675 M.Y.)
675-750 M.Y.
- - Sedimentary rocks regionally
metamorphosed (750 M.Y.)
246
Chapter 9
Geologic Time
FIGURE 9-29 (right) Ash falls and lava flows can be used to correlate sections and determine the absolute ages (in millions of years) of the intervening sedimentary rocks. "*»-
rely
on interbedded volcanic rocks and igneous
intru-
sions to apply^absolute dates to the bo undanesTof the
-
various subdivisions of the geologic time scale (Fig.
9-28). An ash fall or lava flow provides an excellent marker bed that is a time-equivalent surface, supplying a minimum age for the sedimentary rocks below and a
maximum
age for the rocks above. Ash
ularly useful because they
may
fall
falls
are partic-
over both marine and
nonmarine sedimentary environments and can provide a connection between these different environments. Multiple ash falls, lava flows, or a combination of b oth in a
Ash
n
fall
Lava flow
roc k sequence are particularly useful in determining a bs edimentary rocks and their c ontained 9l$). Thousands of absolute ages are now known for sedimentary rocks of known relative ages, and these absolute dates have been added to the relative time scale. In this way, geologists have been able to determine the absolute ages of the various geologic periods and to deter-
so lute ages of fossilsJFig.
mine
^m.
their durations (Fig. 9-2).
^^%-%^m.^^ ^x^^.^^.xyg^^^^i.^.TLi m.x » ^ ^ ->l ^m % i 1
^ CHAPTER SUMMARY 1.
5.
A bsolute
.
da tingr esults
g eologic history of the Eart h. 6.
evidence rather than
While some attempts were quite
ingenious, they yielded a variety of ages that are 3.
known
to be
much
7.
now
too young.
Uniformitarianism as articulated by Charles Lyell, soon becarnet he guiding principle of geology. It holds that~tRelaws of nature have been constant through time and that the same processes operating today have operated in the past, although not .
necessarily at the
same
rates.
is
by correlating all
observations were instrumental in establishing the basis for the principle of uniformitarianism. 4.
Correlation
the stratigraphic practice of
demonstrating equivalency of units in different areas. Time equivalence is most commonly demonstrated
James Hutton believed that present-day processes operating over long periods of time could explain the geologic features of his native Scotland. His
Surfaces of discontinuity that encompass significant
amounts of geologic time are common in the geologic record. Such surfaces are unconformities and result from times of nondeposition, erosion, or both.
During the eighteenth and nineteenth centuries, attempts were made to determine the age of the scientific
Inaddit ion to uniformitari anisnu_rhe prinriples-of
andTossil succe ssion_a_re basic fo r determining relative geologic ages and for interpreting the
in sp ecific
present.
revelation.
.
.
continuity, cros s-cutting relation ships, inclusions,
dates for events, expressed in years before the
Earth based on
.
.
superposition, original horizontality, lateral
Relative dat ing involves placing geologic events in a sequential order as determined from their position in
theT ock record
2.
.-«.
,
-
8.
Radioactivity
strata containing similar fossils.
was discovered during
the late
nineteenth century, and soon thereafter radiometric
dating technique s allowed geologists to determin e ah solute ages jor_g eologic events 9. Absolute age dates for rock samples are usually obtained by determining how many half-lives o f a radioactive parent elerrienTrrave~elapsed since t he sa mple originally crys tallised. A halt-life is the tim e .
it
takes for one-half of the radioactive jjargpt
element to decay to a stable daughter element.
Chapter Summary
247
10.
The most accurate radiometric
dates are obtained
date will be obtained. This date will be actual date.
from long-lived radioactive isotope pairs in igneous rocks. The most reliable dates are those obtained by using at least two different radioactive decay series in the
same rock.
wood
and shells and is effective back to about 70,000_years ago. Carbon 14 ages are determined by the ratio of radio active carbon 14 to stable carbon_12. 12. Through theefforts of many geologists applying the ,
4.
bones.,
scale
was
Most
obtained indirectly by dating associated metamorphic or igneous rocks. fossils are
6.
IMPORTANT TERMS
assemblage range zone beta decay
fission track dating
carbon 14 dating
guide
8.
9.
principle of superposition
radioactive decay relative dating
succession
tree-ring dating
unconformity
principle of inclusions
lateral continuity; b.
c.
original horizontality; d.
e.
cross-cutting relationships.
principle of lateral
which type of radioactive decay are two protons and two neutrons emitted from the nucleus? In
alpha;
beta;
b.
The author
of Principles of Geology and the
and
a
Hutton; b
d.
Smith;
The
era younger than the
Proterozoic; b
d.
Phanerozoic;
Which of
b.
e.
the following
angular unconformity; e. none of
when
d.
the dated mineral
a sedimentary rock;
e.
when
the
was formed.
a radioactive element has a half-life of 4 million
amount?
b Vie; Vs; d %; e Vi. 12. In carbon 14 dating, which ratio is being measured? a. the parent to daughter isotope; b C 14/N 14 c C 12/C 13 ; d C 12/N 14 ; C 12/C 14 <£
in the
rock record
is
13.
'/3 2
;
How many
half-lives are required to yield a mineral 238 206
with 625 atoms of
called: a.
absolute dating; b.
e.
historical dating.
relative dating; d.
248
is
;
Placing geologic events in sequential order as
uniformitarianism; correlation;
heated during metamorphism and the daughter atoms migrate out of a mineral that is subsequently radiometrically dated, an inaccurate If
If
the original
parallel
hiatus;
determined by their position
3.
the:
amount of parent material remaining after 12 million years of decay will be what fraction of
these. 2.
is
Archean; c. Paleozoic; Cenozoic. not a long-lived
years, the
a
nonconformity;
Mesozoic
uranium-lead; b. thorium-lead; potassium-argon; Sk carbon-nitrogen; e. none of these. 10. What is being measured in radiometric dating? a. the time when the radioactive isotope formed; the time of crystallization of a mineral $)>. containing an isotope; c. the amount of the
11.
to each other?
disconformity; d.
Lyell;
c.
Playfair.
4>
became part of
which type of unconformity are the beds
a.
interpreter of
Steno;
&
parent isotope only;
^ REVIEW QUESTIONS
^
electron capture;
c.
radiocarbon.
fission track; e.
stable daughter isotope
In
succession;
superposition;
radioactive isotope pair?
continuity
1.
fossil
c
uniformitarianism
relationships
ff.
a
principle of
principle of fossil
geologic principle states that the
is
fossil
horizontality
principle of cross-cutting
of these.
Which fundamental
uniformitarianism was:
principle of original
key bed nonconformity parent element
all
radiometric dating; position in a sequence;
fossils; d.
principal advocate
half-life
hiatus
guide
a.
daughter element disconformity electron capture
technique
lateral tracing; b.
c.
(0f 7.
correlation
alpha decay angular unconformity
a
on the bottom of a vertical succession of sedimentary rocks and the youngest is on top?
established.
absolute dating
of the following methods can be used to demonstrate age equivalency of rock units?
oldest layer
absolute ages of sedimentary rocks and their
contained
none
Which
® 5.
principles of relative dating, a relative geologic time
13.
the
as; d.
of these.
11. Carbonl4_daiirig_can be applied only-oruofganic
matter such as
younger than; b. older than; c. it cannot be determined; e.
*&)
same
the
a rock
is
Chapter 9
Geologic Time
a 14.
What
15.
What
£
U
5; c 4; is the difference
and 19,375 atoms of Pb ? 10. 6; d 8; e between relative and absolute
dating of geologic events? are the six fundamental principles used in
relative age dating?
Why
are they so important in
deciphering Earth history?
16. Describe the contributions to the
development of
geology made by each of the following men: James Hutton, Lord Kelvin, Charles Lyell, and Nicolas Steno. 17. Define the three types of unconformities.
Why
are
unconformities important in relative age dating? 18. Explain how a geologist would determine the relative ages of a granite batholith and an overlying
sandstone formation. 19.
Why
is
the principle of uniformitarianism important
to geologists?
20. Are volcanic eruptions, earthquakes, and storm deposits geologic events encompassed by
uniformitarianism? 21.
What
is
radon, and
why
is it
so dangerous to
Unconformity
humans? 22.
23
24
are assemblage range zones? How can such zones be used to demonstrate time equivalency of strata in widely separated areas? If you wanted to calculate the absolute age of an intrusive body, what information would you need?
What
Assume
product?
What
are
some of
the potential sources of error in
How
can geologists be sure that the absolute age
dates they obtain from igneous rocks are accurate?
Why
is it difficult to date sedimentary and metamorphic rocks radiometrically? 28. How does the carbon 14 dating technique from uranium-lead dating methods?
27.
29.
How
ADDITIONAL READINGS C, Jr. 1980. The abyss of time. San Francisco, Freeman, Cooper and Co. 1984. Geologic time. Journal of Geological Education
Albritton, C. Calif.:
32, no.
1:
W.
B.
Berry,
29-37. N. 1987. Growth of a prehistoric time
scale.
2d
ed.
Palo Alto, Calif.: Blackwell Scientific Publications. Boslough, J. 1990. The enigma of time. National Geographic 177, no. 3: 109-32.
Geyh, M. A., and H. Schleicher. 1990. Absolute age
radiometric dating? 26.
^
a hypothetical radioactive isotope with an
atomic number of 150 and an atomic mass number of 300 emits five alpha decay particles and three beta decay particles and undergoes two electron capture steps. What are the atomic number and atomic mass number of the resulting stable daughter 25.
"^ Fault
differ
did the geologic time scale evolve?
30. Using the principles of relative dating, give the
geologic history for the diagram at top right.
New York: Springer-Verlag. 1987. Time's arrow, time's cycle. Cambridge,
determination.
Gould,
S. J.
Mass.: Harvard University Press. Harland, W. B., R. L. Armstrong, A. V. Cox, L. E. Craig, A. G. Smith, and D. G. Smith. 1990. A geologic time scale
1989.
New
Ramsey, N.
F.
York: Cambridge University Press. 1988. Precise measurement of time. American
42-49. W. 1982. Dating very old 14-20.
Scientist 76, no. 1:
Wetherill, G.
91, no. 9:
objects.
Natural History
Additional Readings
249
CHAPTER
10 *
% m. "w"**
yr,yi-
"y
EARTHQUAKES * OUTLINE PROLOGUE INTRODUCTION ELASTIC REBOUND THEORY SEISMOLOGY
THE FREQUENCY AND DISTRIBUTION OF EARTHQUAKES """
Guest Essay: Geology Meets Public Policy
SEISMIC WAVES Body Waves Surface Waves
LOCATING AN EARTHQUAKE MEASURING EARTHQUAKE INTENSITY AND MAGNITUDE Intensity
Magnitude
THE DESTRUCTIVE EFFECTS OF EARTHQUAKES Ground Shaking
"w
Perspective 10-1: Designing
Earthquake-Resistant Structures Fire
Tsunami
Ground
Failure
EARTHQUAKE PREDICTION Earthquake Precursors Dilatancy Model
Earthquake Prediction Programs
EARTHQUAKE CONTROL "y Perspective 10-2: A Predicted
Earthquake
That Didn't Occur
CHAPTER SUMMARY
Serpentine Fence west of Yellowstone National Park along Highway 287. This fence was bent when seismic waves passed through the ground during the August 17, 1959 earthquake (magnitude 7.1) at Hebgen Lake, Montana.
PROLOGUE ^^Jf^^i ^™--
-^S
In the early evening of
Interstate 880 freeway in Oakland sent it crashing down, killing 42 unfortunate motorists (Fig. 10-lc). The shaking from this earthquake lasted less than 15
October 17,
1989, millions of baseball fans around
the country turned
on
their television sets expecting to
buildings were
transportation,
results of another, far
taking place 100
km
more important event
that
was
south of San Francisco's
Candlestick Park. At a few minutes past 5
p.m.,
near
peak in the Santa Cruz Mountains, a 40 km long segment of the San Andreas fault ruptured beneath the Earth's surface, triggering a major earthquake (Fig. 10-1). The energy released by the sudden movement between the North American and Prieta
Pacific plates
km 2
was
felt
by people within a
1
million
area that included most of California, western
Nevada, and southern Oregon. Within seconds of the break, southward-moving shock waves demolished the downtown area of Santa Cruz. The shock waves also damaged or destroyed
much
of the
town of Watsonville
(Fig.
10-le) as well
damaging several other nearby communities. The northward-racing shock waves shattered homes and businesses in Los Gatos. They shook San Jose, Palo Alto, and Menlo Park, although most of the as
As 50 million
structures in these cities survived intact.
stunned viewers watched on television, Candlestick Park and 62,000 fans shook and swayed seismic waves passed beneath
it
when
the
(Fig. 10-ld).
was built on solid bedrock, and thus the shaking was short and sharp, resulting in only minor damage. Those districts of the San Francisco-Oakland Bay Area that were built on artificial fill or reclaimed bay mud were not so fortunate, however. Even though the earthquake waves had by then traveled nearly 100 km and were losing energy, the soft fill amplified the shaking effects of the waves with devastating results. In the Marina district of San Francisco, numerous buildings were destroyed, and a fire, fed by broken gas lines, lit up the night sky (Fig. 10-lb). A 15 m section of the upper deck of the San FranciscoOakland Bay Bridge collapsed when bolts holding it in place snapped because of the swaying. The failure of the columns supporting a portion of the two-tiered Fortunately, the stadium
left
totaling almost
game
of the World Series between the and the San Francisco Giants. Instead of the baseball game, viewers witnessed the
Loma
12,000 people
Athletics
see the third
Oakland
seconds but resulted in 63 deaths, 3,800
$6
injuries, at least
homeless, and property billion.
damage
Approximately 28,000
damaged or destroyed, and utility, and communication networks in Santa Cruz and the San Francisco Bay Area suffered major disruptions. Despite the damage, most observers believe San Francisco fared quite well. If the shaking had lasted even a few seconds longer,
more
it is
very likely that
would have from property damage and human
buildings and freeways
losses
would have been much
many
failed,
and the
suffering
higher.
Although the Loma Prieta earthquake was a major one in terms of energy released and damage done, it was not the "Big One" that Californians have long been expecting. That is not to say, however, that it was totally unexpected. There are sections along the San Andreas fault that
have not experienced any significant
movement "locked."
for
many
When
years and can be thought of as
a portion of a fault
is
locked, instead of
and releasing energy by small earthquakes, the essentially sticks. Potential energy builds up in the
slipping fault
rocks adjacent to the fault until
it
finally snaps, releasing
the energy as a major earthquake.
Several segments of the San Andreas fault are
and have the potential of producing One. A 1988 study by the U.S. National Earthquake Prediction Evaluation Council estimated that there was a 50% probability that a major earthquake of magnitude 7.0 or greater would occur in the Bay Area within 30 years. Despite the 1989 Loma Prieta earthquake, the council, in January 1990, revised its estimate of a major earthquake occurring in the Bay Area within the next 30 years to a probability of 60 to 65%. In anticipation of such an earthquake, what lessons can be learned from the Loma Prieta earthquake? As
currently locked the Big
was
so dramatically demonstrated, the underlying
geology and type of building construction are probably the two most important factors determining the
amount of damage
that can occur. Furthermore,
the importance of careful planning and preparation in
earthquake-prone areas was strongly reinforced. For instance, none of the structures in San Francisco that
were constructed
in
compliance with current building
Prologue
251
252
Chapter 10
Earthquakes
preparation for just such an emergency. Certainly,
codes collapsed.
in
Within hours after the earthquake, shelters were open and emergency relief services were in place and
more can be done
One. demonstrated that California putting into practice what has been learned from a long history of dealing with earthquakes.
However, Loma
operating smoothly. This was due, in part, to the
numerous
rehearsals that various agencies conducted
^ INTRODUCTION Earthquakes are violent and usually unpredictable; typically, they produce a feeling of helplessness. As one of
most frightening and destructive phenomena, fear. Even when an earthquake begins, there is no way to tell how strong the nature's
they have always aroused a sense of
shaking will be or
how
long
it
will last.
more than 13
estimated that
It is
million people have
died as a result of earthquakes during the past 4,000 years,
and approximately
1
million of these deaths oc-
curred during the last century (Table 10-1). The two
most destructive earthquakes
in history in
terms of loss
China. The worst took place on January 23, 1556, near the city of Xian in Shanxi province, with a death toll estimated at 830,000. The second struck Tangshan (160 km east of Beijing) on July 28, of
life
occurred
1976. The
in
and
city
environs are one of the most
its
densely populated areas in China, and this density cer-
which was of242,000 by the Chinese govern-
tainly contributed to the high death toll, ficially
announced
ment;
however, others estimated
at
that
as
many
as
700,000 people may have died. If you have never experienced an earthquake, try to imagine that as you are reading this book, the ground suddenly and without any warning starts shaking and everything around you begins to sway. If the shock waves are severe enough, you might be knocked down and have trouble standing up. The first thought that would probably go through your mind is, "how long is the shaking going to last and is it going to get any stronger?" You want to do something, but you don't know exactly
may
"*•'
what
to do.
break, and
FIGURE
if
10-1
If
the shaking
you are
(left)
(a)
is
severe,
to prepare for the Big
Prieta
is
from the ceiling and walls, and there will be loud creaking and groaning noises as the building sways. In most cases the shaking will stop almost as suddenly as it began, and you will realize that you have survived one of nature's most terrifying natural disasters. What seemed like eternity was probably only tens of seconds or less. Depending on the circumstances, you also may experience a gentle rolling motion as the slowest of the four types of earthquake waves pass below you. You also may feel numerous aftershocks, which typically are not as strong as the main shock. Having described what it is like to experience an earthquake, we should ask, how do geologists define an earthquake?
An
earthquake
is
the vibration of the Earth
caused by the~sudden release of energy, usually as a result of displacement of rocks along fractures, or faulting,
beneath the Earth's surface.
humans and cultures had much more imaginaand colorful explanations of earthquakes than this
Early tive
scientific
explanation. For example,
lieved that the Earth rested
many
cultures be-
on some type of organism
whose movements caused the Earth to shake. In Japan, it was a giant catfish (Fig. 10-2); in Mongolia, a giant frog; in China, an ox; in India, a giant mole; in parts of
"'•*'
FIGURE
10-2
This painting from the Edo period
to subdue a giant catfish. According to Japanese legend, earthquakes are caused by the movement
shows people trying of a giant catfish.
windows
in a building, objects will fall
An
outline
map
of the area
1989 Loma Prieta earthquake, (b) Marina caused by broken gas lines, (c) Aerial view
affected by the district fire
looking west at part of the collapsed two-tiered Interstate 880 in Oakland. Only 1 of the 51 double-deck spans did not collapse, (d) Candlestick Park was filled with 62,000 fans awaiting the start of the third game of the World Series when the earthquake struck, (e) Damage to buildings along
Main and Second
Street in
downtown
Watsonville.
Introduction
253
^ TABLE
10-1
Rupture and release
of
energy
(a)
"**" FIGURE 10-3 (a) According to the elastic rebound theory, when rocks are deformed, they store energy and bend. When the inherent strength of the rocks is releasing the energy in the form of earthquake waves that they rupture, exceeded, radiate outward in all directions. Upon rupture, the rocks rebound to their former undeformed shape, (b) During the 1906 San Francisco earthquake, this fence in Marin County was displaced 2.5 m.
m
during the 50site sides of the fault had moved 3.2 year period prior to breakage in 1906, with the west side
moving northward
side of the fault
moved
that
undeformed shape, releasing the energy
had been
internally stored.
stored in rocks undergoing elastic defor-
The energy
(Fig. 10-3).
According to Reid, rocks on opposite sides of the San Andreas fault had been storin g energy and bendi ng slightly for at least 50 year s Before the 1906 earthquake. Any straight line such as a fence or road that crossed the San Andreas fault would gradually be bent, as rocks on
one
their original
relative to rocks
on the other was
mation
is
analogous to the ene rgy stored
-
spring that
wound, energy
the
is
tightly
is
more energy it
as the spring rapidly
exceeded, and rupture occurred. When this happened, the rocks on opposite sides of the fault rebounded or
=*=
undeformed shape, and the energy stored was released as earthquake waves radiating outward from the break (Fig. 10-3). Additional field and laboratory studies conducted by Reid and ot hers have confirmed tha^^srir-reboTrncT is) the mechanism by which earthquakes are generated. In laboratory studies, rocks subjected to forces equivalent to those occurring in the Earth's crust initially their shape.
However,
as
more
force
is
change
applied, they
deformation until their internal strength is exceeded. At that point, they break and snap back to resist further
If
in a
and
thus, the
the spring
unwinds and
watgTT
tighter the spring
is
breaks, then the stored energy
original shape.
to their former
is~stored,
available for release.
tightly that
side (Fig. 10-3). Eventually, the strength of the rocks
"snapped back"
wound. The
wound is
is
more so
released
partially regains
its
SEISMOLOGY
Seismology, the study of earthquakes, began emerging as
aTrugjgjence around
1
88U with
the development of in-
struments that effectively recorded earthquake waves. Much of what we know about the interior of the Earth comes from the study of natural and artificially generated (see Chapter 11). The data from seismology have led to many important discoveries about the Earth and are an integral part of plate tectonic theory. The earliest earthquake detector was invented by the Chinese scholar Chang Heng sometime around a.d. 132
earthquake waves
Seismology
255
vibrations produced by an earthquake (Fig. 10-5).
The made by a seismograph is a seismogram. Although modern seismographs are very sophisticated inrecord
struments that electronically record the motion onto a
seismogram or enter
it
directly into a computer, they
still
follow the basic principles of operation that were used
seismographs.
in the earliest
To construct a seismograph that measures horizontal movement, a heavy mass is suspended by a supporting cable and attached at one end by a tapering arm to a frame secured in the bedrock (Fig. 10-5b). At the other end of the mass is a marker resting on paper wrapped around a rotating drum that is also attached to the frame. During an earthquake, the heavy mass remains stationary because of
marker
its
inertia,
drum move with
rotating
is
the
attached to the heavy mass,
stationary and records the ground "^"
FIGURE
The world's first earthquake detector was invented by Chang Heng sometime around a.d. 132. 10-4
tating
drum.
If
the
movement
mass to move
of the ground
first
sure that earthquake waves from a large hollow jar with
eight dragon heads evenly spaced
around it; each dragon's mouth contained a metal ball. Underneath each dragon's head was a frog with its head tilted back and its mouth open. Earthquake waves passing beneath the instrument would shake fall
into the
it
mouths of the
causing some of the balls to frogs below. In this
way
the
general direction of the earthquake could be deterif the balls from the dragons on the and west sides of the jar were dislodged, then the earthquake waves must have come from either the east or the west. Although what was inside the jar is not known, it is believed that there must have been some type of pendulum that would swing when the earth moved, thus knocking balls from the dragons' mouths. A story is told that one day Chang Heng's instrument indicated that there had been an earthquake, but no one in the area had felt a tremor. Most people regarded his experiment as a failure. However, a few days later, a rider arrived with the news of an earthquake that had
mined. For example, east
occurred
in a distant
province
in the direction indicated
by Chang's instrument.
Over the succeeding centuries, other instruments were invented to study earthquakes, but it was not until the late nineteenth century that the
first
effective seis-
mograph was developed. A seismograph is an instrument that detects, records, and measures the various
256
Chapter 10
Earthquakes
is
the ro-
parallel to
as well. Therefore a second seismograph,
oriented perpendicular to the
The instrument was
also remains
it
movement on
the length of the frame, the rod will cause the heavy
Movement of the vase dislodged a ball from a dragon's mouth into the waiting mouth of a frog below.
(Fig. 10-4).
while the frame and ground. Because the
one, all
is
needed to en-
directions will be
To record vertical ground movement, the mass must be suspended from a spring hanging from the frame (Fig. 10-5c). When an earthquake occurs, energy in the form of seismic waves radiates outward in all dTre^tionsTrom the point of release. These seismic waves are analogous to the ripples that result when a stone is thrown into a quiet body of water; the ripples move outward in concentric circles from the point of the stone's impact. recorded.
Most earthquakes_rejujt_when^:ocks
in^
the Earth's
crust rupture along a fault because of the buildup of
excessive pressure, which
movement. Once
is
usually caused by plate
it moves along the km/sec for as long as conditions for failure exist. The length of the fault along which rupture occurs can range from a few meters to several hundred kilometers. The longer the rupture, the
a rupture begins,
fault at a velocity of several
more time
it
takes for
all
of the stored energy in the
rocks to be released, and therefore the longer the ground will shake. In a small earthquake, the rupturing
is
usu-
completed within a few seconds. For a large earthquake, however, it will take much longer. For example, during the 1906 San Francisco earthquake, the San Andreas fault took over a minute to rupture more than 400 km along its length. The location within the crust where rupture initiates, and thus where the energy is released, is referred to as ally
"^ FIGURE
10-5 (a) Modern seismographs record earthquake waves electronically. A geophysicist points to the trace of an earthquake recorded by a seismograph at the National Earthquake Information Service, Golden, Colorado. (b) A horizontal-motion seismograph. Because of its inertia, the heavy mass that contains the marker will remain stationary while the rest of the structure moves along with the ground during an earthquake. As long as the length of the arm is not
ground movement, the marker will record the earthquake waves on the rotating drum, (c) A vertical-motion seismograph. This seismograph operates on the parallel to the direction of
same
principle as a horizontal-motion instrument
vertical
and records
ground movement.
the iocus orjry pocenter
.
The point on
the Earth's sur-
which is news reports on earthquakes (Fig. 10-6). The depth from the epicenter to the focus can range from a few kilometers to several hundred kilometers; the depth usually relates to the plate tectonic setting in which the earthquake occurred face vertically
above the focus
the location that
is
is
the epicenter
usually given in
Support
(discussed later in this chapter).
recognize three categories of earth-
Seismologists
quakes based on the depth of their foci. Shallow-focus earthquakes have a focal depth of less than 70Tcm. Earthquakes with foci between 70 and 300 km are referred to as intermediate focus,
greater than
300
km
and those with .
are not evenly distributed
among
these three categories.
.Shallnw-fnriispartJTqi^gkeSJirp, with
in
most destructive.
it
arecaTIed de ep focus Earthquakes
Approximately 90% of all earthquake depth of less than 100 km. the
foci
Base anchored into bedrock and moves with
All of the
known
foci
occur at a
few evrppHnns,
large earthquakes
California have been shallow focus, and most have
originated within the upper 10
km
of the Earth's crust.
The 1964 Alaska earthquake, the strongest yet recorded in the United States, had a focal depth near 30 km. There is an interesting relationship between earthquake foci and plate margins. Earthquakes generated along divergent or transform plate boundaries are alfocus, while almost all intermediate- and
ways shallow
deep-focus earthquakes occur within the circum-Pacific belt
along convergent margins
a pattern emerges
when
(Fig. 10-7).
Furthermore,
the focal depths of earthquakes
near island arcs and their adjacent ocean trenches are plotted. Notice in Figure 10-8 that the focal depth in-
creases beneath the
Tonga Trench
in a
narrow, well-
defined zone that dips approximately 45°. Dipping
seis-
mic zones, called Benioff zones, are a feature common to island arcs and deep ocean trenches. Such zones indicate the angle of plate descent along a convergent
Seismology
257
'"'
FIGURE
10-6
The focus of an earthquake
location where rupture begins and energy
place
on the Earth's surface
vertically
is
the
is
released.
above the focus
The is
the
epicenter.
and that angle varies greatly depending on subduction conditions.
plate boundary, to 90°)
(
from 20°
^ THE FREQUENCY AND DISTRIBUTION OF EARTHQUAKES While earthquakes occur tribution
(almost
is
all
over the world, their dis-
certainly not haphazard.
95%) occur
Most earthquakes
in seismic belts that
correspond to
where stresses develop as plates converge, diverge, and slide past each other. Earthquake activity distant from plate margins is minimal, but can plate boundaries
~**"
FIGURE
The
10-7
relationship between the distribution of earthquake epicenters
80% of earthquakes occur within the within the Mediterranean-Asiatic belt, and the remaining within the interiors of plates or along oceanic spreading ridge systems. Each dot represents a single earthquake epicenter. and
plate boundaries.
circum-Pacific belt,
258
Approximately
15%
Chapter 10
Earthquakes
5%
be devastating plate margins
when
it occurs. The relationship between and the distribution of earthquakes is
readily apparent
when
the locations of earthquake epi-
centers are superimposed
on
a
map showing
the bound-
aries of the Earth's plates (Fig. 10-7).
The majority of all earthquakes (approximately 80%) occur in the c ircum-Pac ific belt, a zone of seism ic activity that encircles the Pacific Ocean basin. Most of these earthquakes are a resul t of conver gence along plate margins.
Some
of the world's most devastating
earTrrqaakes, resulting in billions of dollars of property
damage and more than 500,000
deaths, have occurred
within this belt (Table 10-1).
The second major Asiatic belt
seismic bel t
is
where approximately
the Mediterranean-
15%
of
all
earth-
quakes occur. This belt extends westerly from Indonesia through the Himalayas, across Iran and Turkey, and westerly through the Mediterranean region of Europe.
The devastating earthquake in
1988
killing
that struck Soviet
Armenia
25,000 people and the 1990 earthquake 40,000 are recent examples of the
in Iran that killed
destructive earthquakes that strike this region.
The remaining
5%
of earthquakes occur mostly in
January 23 and February 7, 1812. These three earthquakes killed approximately 20 people (the region had a very small population at the time) and nearly destroyed the town of New Madrid. So strong were these earthquakes that they were felt from the Rocky Mountains to the Atlantic Ocean and from the Canadian border to the Gulf of Mexico. In addition, the earthquake caused church bells to ring as far away as Boston, Massachusetts (1,600 km). Within the immediate area, numerous buildings were destroyed and forests were flattened; the land sank several meters in some areas, causing flooding; and the Mississippi River is said to have reversed its flow during the shaking and changed its course slightly.
Another major intraplate earthquake struck Charleson August 31, 1886, killing 60 people and causing $23 million in property damage (Fig. ton, South Carolina,
10-9).
Most
recently,
another large intraplate earth-
quake struck near Tennant Creek ern Territory in December 1988.
The cause of
in Australia's
intraplate earthquakes
is
not well un-
derstood, but geologists~beheve they arise trom localized stesses
caused by the compression that most plates ex-
perience along their margins.
The release of these stresses
resulting intraplate earthquakes are due to
the interiors of plates and along oceanic spreadingxidge
and hence the
system s. Tflost oFthese earthquakes are not very strong
local factors. Interestingly,
although there have been several major intraplate earth-
are associated with very ancient and
quakes that are worthy of mention, especially the 1811 and 1812 earthquakes near New Madrid, Missouri. The Missouri earthquake was actually three major
faults that are reactivated at various intervals.
shocks that occurred on December 16, 1811, and on
many
intraplate earthquakes
presumed
inactive
More than 150,000 earthquakes that are strong enough to be felt by someone are recorded every year by the worldwide network of seismograph stations. Some
^ FIGURE
Tonga volcanic
North-
Tonga Trench
10-8
Focal depth
increases in a well-defined zone that
dips approximately 45° beneath the
Tonga volcanic
arc in the South Dipping seismic zones are features of island arcs and deep ocean basins. Pacific.
common
0-
Magma
q
400-
Earthquake focus^
The Frequency and Distribution of Earthquakes
259
Guest Essay
DANIEL SAREWITZ
GEOLOGY MEETS PUBLIC POLICY I
had always assumed that
naturally
During
I
my
I
would be
started college intending to
freshman year,
it
a novelist, so
Others suggested that
major
Department of
occurred to
read and write fiction regardless of
my
me
in English.
that
I
could
profession, but
I would never understand the origins of mountains and oceans unless I spent some time learning about geology. At some point during my education, I realized that my vague aspirations to write fiction were overshadowed by the fact that I had become— without ever planning to do so— a geologist. As time passed, I began to consider my future options. One possibility was to become a professor, but I doubted that I was suited to academic life. The obvious alternative was to work for an oil or mineral company. This set of options— academia or industry— seemed unacceptably circumscribed, but when I looked for examples of geologists who had gone on to
that
nontraditional careers,
As an
alternative,
I
I
to Washington, D.C., as part
scientific issues if
have done.
I began working as a fellow the week before the October 1989 earthquake in Loma Prieta, California, and spent a good part of the next year attempting to transform the publicity generated by the earthquake into a renewed federal commitment to research on earthquake hazard reduction. Much of my work was educational: congressional staff and members of Congress alike needed to understand that bigger, more damaging earthquakes were inevitable in the future; that earthquakes occurred throughout the United
not just in California; and that federal funding earthquake research could save lives and money.
States, for
Congressional action often comes only on the heels crisis. With no major U.S. earthquakes in almost 20 years, funding for the federal earthquake program had declined significantly. In the wake of the Loma Prieta event, however, Congress voted to more than double funding over a period of four years. This victory was short-lived. One year after the earthquake, the president asked Congress to cut
in science,
would still have to be made between political and scientific considerations. For example, a member of Congress from a state whose economy depends on high-sulfur coal production may obliged to vote against regulations that prohibit
feel
using such coal to generate electricity, even though he
or she understands that burning this coal contributes to acid rain. All the same, to
relative
who
I
were well versed
legitimate trade-offs
year in congressional staff positions. Although most fellows
choose to stay on, as
the
astonishingly ill-informed. But even
is
elected officials
data.
some
why
was made, it illustrates that federal science policy is commonly based not on science, but on politics and fiscal concerns. At times, the quality of congressional debate over decision
of a fellowship program that places about 25 scientists a
return to academia after their year in Washington,
which administers the USGS,
led to the requested cuts. But, regardless of
make
members of Congress must be
couldn't find any.
came
political rifts within the
Interior,
importance of
wise decisions,
able to weigh the
political pressures
They cannot do so without
and
scientific
the advice of staff
are scientifically literate.
In the
coming
years, Congress will be increasingly
faced with complex decisions that are intimately
warming, energy water supply, nuclear and solid waste disposal, and federal funding of academic research facilities are related to the geosciences. Global
policy,
a few of the issues that will be
on
the national
agenda. Only two members of the House of Representatives have degrees in science or engineering.
Few congressional
staff
members have
scientific
backgrounds.
Thus, the geoscience community should recognize it can make an important contribution to the
that
formulation of public policy and that careers in public policy represent a legitimate— and
professional opportunity.
growing— area
of
A
of a
earthquake research funding at the U.S. Geological Survey (USGS) back to pre— Loma Prieta levels. Some said this request was simply part of the attempt to reduce federal spending and balance the budget.
Uaniel Sarewitz
is
a science
Committee on Science, Space, and Technology of the U.S. House of policy analyst for the
He earned his Ph.D. in geological sciences from
Representatives.
Cornell University in 1985 and served as a Geological Society of
America Congressional Science Fellow from September 1989 to August 1990.
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 260
Chapter 10
Earthquakes
"^ FIGURE
10-9
to Charleston,
Damage done
South Carolina, by
the earthquake of August 31,
1886
This earthquake is the largest reported in the eastern United States.
of these, such as the examples
we have
already given, are
major earthquakes that cause tremendous damage and loss of life. In addition, it has been estimated that about 900,000 earthquakes occur annually that are recorded by seismographs, but are too small to be individually cataloged. These small earthquakes result from the energy released as continual adjustments between the Earth's various plates occur.
^ SEISMIC WAVES The shaking and destruction resulting from earthquakes are caused by two different types of seismic waves: body waves, which travel through the Earth and are some-
what
like
sound waves; and surface waves, which
travel
only along the ground surface and are analogous to
ocean waves.
Bo dy Wave s An
earthquake generates two types of body wav es: P-wayesand S-waves. P-waves or prvmary_uMU.es anTthe fastest sejsmicjKaves. and can travel through sohdsjjiquids. and gases. P-waves are compressional, or pushpull, waves and are similar to sound waves in that they move material forward and backward along a line in the
waves themselves are moving material through which P-waves travel is expanded and compressed as the wave moves through it and returns to its original size and shape after the wave passes by. In fact, some P-waves emerging from within the Earth are transmitted into the atmosphere as sound waves that can be heard by humans and animals at certain frequencies. S -waves or secondary waves a re some whatd gwer than P-waves and c an only travel through Sahjjs^ S-waves are shear waves because they move the material
same
direction that the
Thus,
10-10b).
(Fig.
the
perpendicular to the direction of travel, thereby producing shear stresses in the material they (Fig.
move through
10-10c). Because liquids (as well as gases) are not
rigid, they
have no shear strength and S-waves cannot be
transmitted through them.
The
and S-waves are determined by through which they travel. For example, seismic waves travel more slowly through rocks of greater density, but more rapvelocities of P-
the density
idly
and
elasticity of the materials
through rocks with greater
elasticity. Elast icity is_a
property of solids T _such as rocks, jmd means~that once t hey hav e been deformed by an applied force, they ret urn
when the force is no longer P-wave velocity is greaterlhan S-wave materials, however, P-waves always arrive
to their ori gina l shape
present. Because velocity in
all
at seismic stations
first.
Seismic Waves
261
"*""
FIGURE 10-10 Seismic waves. Undisturbed material. (b) Primary waves (P-waves) compress and expand material in (a)
same direction as the wave movement, (c) Secondary waves the
(S-waves) move material perpendicular to the direction of wave movement, (d) Rayleigh waves
(R-waves) move material in an elliptical path within a vertical plane oriented parallel to the direction of wave movement. (e) Love waves (L-waves) move material back and forth in a horizontal plane perpendicular to the direction of
wave movement.
(a)
Undisturbed material
Surface Waves
As Figure 10-11
waves seismogram patterns. The first waves to arrive, and thus the fastest, are the P-waves, which travel at nearly twice the velocity of the S-waves that follow. Both the P- and S-waves travel directly from the focus to the seismograph through the interior of the Earth. The last waves to arrive are the L- and R-waves, which are the slowest and also travel the longest route along the Earth's
produce Surface waves travel along the surface of the ground, or
and are slower than body waves. Unlike and shaking that body waves cause, surface waves generally produce a rolling or swaying motion, much like the experience of being on a boat. Surface waves can be divided into several different types of waves. The two most important are Rayjeigh waves (R-waves) and Love waves (L-waves), named after the BrTtisTrs^ieTrmts^whtrdtscovered them, Lord Rayleigh and A. E. H. Love. Rayleigh waves are generally the slower of the two and behave like water waves in just
below
it,
the sharp jolting
move forward while the individual particles of material move in an elliptical path within a vertical plane oriented in the direction of wave movement (Fig. 10-10d). The motion of a Love wave is similar to that of an
surface (Fig. 10-11).
Because the Earth
move back and
forth in a horizontal plane perpendicu-
lar to the direction of
wave
travel (Fig. 10-10e). This
type of lateral motion can be particularly damaging to building foundations.
» LOCATING AN EARTHQUAKE The various
seismic waves travel at different speeds and
The exwave can be determined
thus arrive at a seismograph at different times. act arrival time of each seismic
by a time scale on the seismogram.
is
not homogeneous, the speeds of
the different seismic waves vary, depending rials
is
on
the mate-
through which they move. Thus, the farther a
mograph
that they
S-wave, but the individual particles of the material only
illustrates, the different seismic
distinctive
is
from an earthquake's
focus, the
that the velocity of the seismic waves
is
more
seis-
likely
it
not constant.
By accumulating a tremendous amount of data over the years, seismologists have determined the average travel times of P- and S-waves for any specific distance. These P- and S-wave travel times are published as timedistance graphs and illustrate that the difference between the arrival times of the P- and S-waves is a function of the distance of the seismograph from the focus; that is, the farther the waves travel, the greater the time between arrivals of P- and S-waves (Fig. 10-12). As Figure 10-13 demonstrates, the epicenter of any earthquake can be determined by using a time-distance graph and know ing the arrival times of the F^lind
S-waves at any three seismograph locations. Subtracting the arrival time of the first P-wave from the arrival time of the first S-wave gives the time interval between the
"^"
FIGURE 10-11 A schematic seismogram showing the arrival order and pattern produced by P-, S-, and L-waves. When an earthquake occurs, body and surface waves radiate outward from the focus at the same time. Because P-waves are the fastest, they arrive at a seismograph first, followed by S-waves, and then by surface waves, which are the slowest waves. The difference between the arrival times of the P- and the S-waves is the P-S time interval; it is a function of the distance of the seismograph station from the focus. Body waves
Arrival of
Arrival of
P-wave
S-wave
—>\
P-S time
interval
Surface waves
[«—
Locating an Earthquake
263
^ MEASURING EARTHQUAKE INTENSITY
AND MAGNITUDE
Geologists measure the^strength of an earthquake in two is annalirative as-
different ways. Thj^first, in tensity ,
sessment of the kinds ofdamage done by anelrthquake. The secondzjnagnitudejjs ajmantitative measurem ent
amoun t of en ergy_ released by an earthq uake. Each methocTprovidesgeologists with important data about earthquakes and their effects. This information
of the
can then be used to prepare for future earthquakes.
Intensity I ntensity is
measure of the kind of damage
a subjective
done by an earthquake
as well as people's reactiorTtcTit.
Since the mid-nineteenth century, geologists have used
rough approximation of the size and The most common intensity scale used in the United States is the Modified Mercalli Intensity Scal e, which has values ranging from I txTXlI (Table 10-2). This scale was originally developed by the Italian seismologist Giuseppe Mercalli in 1902 and was later modified for use in the United States by H. O. intensity as a
strength of an earthquake.
2,000
4,000
6,000
8,000
10,000
12.000
Distance from focus (km)
"^ FIGURE 10-12 A time-distance graph showing the average travel times for P- and S-waves. The farther away a seismograph station is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S-waves, and hence the greater the distance between the curves on the time-distance graph as indicated by the P-S time interval.
and F. Neumann of the California Institute of Technology Seismological Laboratory in 1931. After an assessment of the earthquake damage is
Wood
made, isoseismal lines (lines of equal intensity) are drawn on a map, dividing the affected region into various intensity zones.
zone
two waves for each seismograph location. Each time interval is then plotted on the time-distance graph, and a line is drawn straight down to the distance arrivals of the
how
away each station is from the focus of the earthquake. Then a circle whose radius equals the distance shown on the timedistance graph from each of the three seismograph locations is drawn on a map (Fig. 10-13). The intersection axis of the graph, thus indicating
of the three circles epicenter.
is
A minimum
far
the location of the earthquake's
of three locations
is
needed be-
cause two locations will provide two possible epicenters and one location will provide an infinite number of possible epicenters. It
now
should be noted that computers are
used to determine the epicenter of an earthquake,
and many seismic stations are used for redundancy and to determine the most accurate location.
264
Chapter 10
Earthquakes
is
the
The
maximum
intensity value given for each
intensity that the earthquake pro-
duced for that zone. Even though intensity maps are not precise because of the subjective nature of the measurements, they do provide geologists with a rough approximation of the location of the earthquake, the kind and extent of the damage done, and the effects of local geology and types of building construction (Fig. 10-14). In fact, because intensity is a measure of the kind of damage done by an earthquake, insurance companies classify
earthquakes on the basis of
still
intensity.
While it is generally true that a large earthquake will produce greater intensity values than a small earthquake, many other factors besides the amount of energy released by an earthquake affect its intensity. These include the distance from the epicenter, the focal depth of the earthquake, the population density and local geology of the area, the type of building construction employed, and the duration of shaking.
A
comparison of the
intensity
map
Francisco earthquake and a geologic
for the
map
1906 San
of the area
"^ FIGURE 10-13 Three seismograph stations are needed to locate the epicenter of an earthquake. The P-S time interval is plotted on a time-distance graph for each seismograph station to determine the distance that station is from the epicenter. A circle with that radius is drawn from each station, and the intersection of the three circles is the epicenter of the earthquake.
shows a strong correlation between the amount of damage done and the underlying rock and soil conditions (Fig. 10-15). Damage was greatest in those areas under-
terials,
by poorly consolidated material or artificial fill because the effects of shaking are amplified in these ma-
reinforced by the 1989
lain
whereas damage was rather low
in areas of solid
bedrock. The correlation between the geology and the
amount of damage done by an earthquake was
many
Loma
of the same areas that were extensively
Measuring Earthquake
further
when damaged
Prieta earthquake
Intensity
and Magnitude
265
^ TABLE
10-2
Modified Mercalli Intensity Scale
Not
felt except by a very few under especially favorable circumstances. only by a few people at rest, especially on upper floors of buildings. Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing automobiles may rock slightly. During the day felt indoors by many, outdoors by few. At night some awakened. Sensation like heavy truck striking building, standing automobiles rocked noticeably. Felt by nearly everyone, many awakened. Some dishes, windows, etc. broken, a few instances of cracked plaster. Disturbance of trees, poles, and other tall objects sometimes noticed. Felt by all, many frightened and run outdoors. Some heavy furniture moved, a few instances of fallen plaster or damaged chimneys. Damage slight. Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken. Noticed by people driving automobiles. Damage slight in specially designed structures; considerable in normally constructed buildings with possible partial collapse; great in poorly built structures. Fall of chimneys, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Damage considerable in specially designed structures. Buildings shifted off foundations. Ground noticeably cracked. Underground pipes broken. Some well-built wooden structures destroyed; most masonry and frame structures with foundations destroyed; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Water splashed over river banks. Few, if any (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Damage total. Waves seen on ground surfaces. Objects thrown upward into the air.
Felt
VI VII
VIII
IX
XI XII
SOURCE:
in the
aged
United States Geological Survey.
1906 earthquake were once again heavily dam-
Magnitude iLe arthquakes are to be compared q uantitatiyely. we must use a scale that measures the amount of energy released and is independent of intensity. Such a scale was developed in 1935 by Charles F. Richter, a seismologist at the California Institute of Technology. The Richter Magnitude Scale measures earthquake magnitude, which is the total amount of energy released by an earthquake at its source. It is an open-ended scale with values beginning at 1. The largest magnitude recorded has been 8.6, and though values greater than 9 are theoretically possible, they are highly
improbable because rocks are
not able to withstand the buildup of pressure required to release that
much
km
from the epicenter would be the standard diswould be measured. Amplitude heights for other distances are thus converted to what they would be if the seismograph were located 100 km from the epicenter (Fig. 10-16). 100
tance at which the amplitude of the seismic waves
(see the Prologue).
energy.
Since
Richter
was dealing only with shallow-focus
earthquakes, the distance from the epicenter and the distance from the focus were almost the same. Other
conversions must be
made
for intermediate-
and deep-
focus earthquakes.
Richter also realized that different types of seismo-
graphs can provide somewhat different amplitudes even
if
maximum wave He
they are at the same location.
therefore established the
Wood-Anderson seismograph wave
as the standard instrument to be used in measuring
amplitudes.
If
other types of seismographs are used,
their amplitudes
must be converted to what they would
be on a Wood-Anderson seismograph.
The mag nitude of an earthquakejs determined by measuring the amplitude of the largest seismic wave as recordethorfir seismogram (Fig. 10-16). However, be-
ventional base-10 logarithmic scale to convert the am-
cause the amplitude of seismic waves decreases with dis-
ical
tance from the epicenter, Richter decided arbitrarily that
266
Chapter 10
Earthquakes
Finally, to avoid large
numbers, Richter used a con-
wave to a numermagnitude value (Fig. 10-16). Therefore, each integer increase in magnitude represents a 10-fold inplitude of the largest recorded seismic
San 'Francisco Limits of felt
California I
Pacific
— IV
area
Bay mud by
(in
artificial
m
thick)
Alluvium (<30
m
thick)
H Very H
violent
Violent
Very strong |
]] Strong
"^ FIGURE 10-15 A comparison between {a) the general geology of the San Francisco peninsula and (b) a Modified Mercalli Intensity map of the same area for the 1906 San Francisco earthquake. Notice the close correlation between the geology and the intensity. Areas of bedrock correspond to the lowest intensity values, while areas of poorly consolidated material (alluvium) or bay mud have the highest intensity values.
268
Chapter 10
Earthquakes
as of 1906)
~\ Alluvium (>30
Bedrock
|
places covered
fill
Weak
Perspective 10-1
DESIGNING
EARTHQUAKE RESISTANT STRUCTURES One way
damage, injuries, and loss and build structures as earthquake-resistant as possible. While no society can spend unlimited monies on making all structures completely safe during an earthquake, many things can be done to improve the safety of current structures and of new buildings as well. California has a Uniform Building Code that sets
of
life is
to reduce property
to design
minimum
standards for building earthquake-resistant
structures
and
is
used as a model around the world. The
more stringent than federal earthquake building codes and requires that structures be able to withstand a 25-second main shock. California code
Unfortunately,
is
far
many earthquakes
duration. For example, the
are of far longer
main shock of the 1964
Alaskan earthquake lasted approximately three minutes
during moderate to major earthquakes than those built before
damaged
buildings in this earthquake
had been built according to the California code, they were not designed to withstand shaking of such long duration (Fig. 1). Nevertheless, in California and elsewhere in the world, structures built since the California code
went
^ FIGURE
Damage done
into effect have fared
much
better
Alaska, as a result of ground shaking during the 1964 earthquake. Close-up of Government Hill School.
minimizing the loss of
understanding of the area's geology
life,
is
also very
important because certain ground materials such as water-saturated sediments or landfill can lose their strength and cohesiveness during an earthquake. Such
materials should be avoided
engineers must be aware of
if
at all possible. Finally,
how
different structures
behave under different earthquake conditions.
With the
level of
technology currently available, a
well-designed, properly constructed building should be able to withstand small, short-duration earthquakes of
than 5.5 magnitude with little or no damage. In moderate earthquakes (5.5 to 7.0 magnitude), the damage suffered should not be serious and should be repairable. In a major earthquake of greater than 7.0 magnitude, the building should not collapse, although less
may
later
Many to structures in Anchorage,
is
and damage. To achieve this goal, engineers must understand the dynamics and mechanics of earthquakes including the type and duration of the ground motion that occurs and how rapidly the ground accelerates during an earthquake. An injuries,
it
1
implementation.
objective in designing earthquake-
resistant structures
and was followed by numerous aftershocks. While many of the extensively
its
The major
have to be demolished.
factors enter into the design of an
earthquake-resistant structure, but the most important is
that the building be tied together; that
is,
foundation, walls, floors, and roof should
the
all
be joined
together to create a structure that can withstand both
horizontal and vertical shaking caused by an
earthquake
(Fig. 2). Structural
continuity can be
assured by requiring that the walls of a building be securely anchored to the foundation
beam and
joist
and that the
supports of the walls, floors, and roof
be securely joined to each other. Almost structural failures that have resulted
all
of the
from earthquake
ground movement have occurred at weak connections, where the various parts of a structure were not securely tied together (Fig. 3).
The
size
and shape of a building can also affect box-shaped
resistance to earthquakes. Rectangular
buildings are inherently stronger than those of
270
Chapter 10
Earthquakes
its
"
' ' •
.
:" '••-:
Secure vent
^ FIGURE potential
2
damage
This diagram shows some of the things a homeowner can do to reduce the to a building because of ground shaking during an earthquake.
irregular size or shape because different parts of an irregular building
may sway
-^-
FIGURE
During the 1971 San Fernando, California stair tower broke away from the main building. The hospital was built to standards, but still suffered major federal earthquake damage. 3
earthquake, the Olive View Hospital's
at different rates,
and likelihood of structural with open or unsupported first stories are particularly susceptible to damage. Some reinforcement must be done or collapse is a increasing the stress
failure (Fig. 4b). Buildings
distinct possibility. Tall buildings, such as skyscrapers,
must be
designed so that a certain amount of swaying or flexing can occur, but not so much that they touch
neighboring buildings during swaying (Fig. 4d). If a building is brittle and does not give, it will crack and fail.
In addition to designed flexibility, engineers
make
must
sure that a building does not resonate at the
same frequency earthquake. the seismic
as the
When waves
ground does during an
that happens, the force applied by at
ground
level
is
multiplied several
times by the time they reach the top of the building (continued on next page)
The Destructive
Effects of Earthquakes
271
-+ FIGURE 4 The effects of ground shaking on various tall buildings of Damage
differing shapes, (a)
occur
if
will
two wings of a building
are joined at right angles
and
experience different motions. (b) Buildings of different heights sway differently leading to damage at the point of connection. (c) Shaking increases with height and is greatest at the top of a will
building, (d) Closely spaced
buildings
may
crash into each
other due to swaying, (e) building whose long axis
A is
parallel
to the direction of the seismic
sway less than a building whose axis is
waves
will
perpendicular,
Two
(f)
buildings of
different design will behave
when
differently even
the
subjected to
same shaking conditions. A sways as a unit and
Building
remains standing while building B first story is composed of only tall columns collapses because most of the swaying takes place in
Direction of
the "soft"
first story.
seismic
(Fig. 4c).
This condition
whose
wave
particularly troublesome in
is
areas of poorly consolidated sediment (Fig. 5). Fortunately, buildings can be designed so that they will
sway
at a different
What about every city and
frequency from the ground.
structures built
town has
many
years ago?
Almost
older single and multistory
structures, constructed of unreinforced brick
masonry,
poor-quality concrete, and rotting or decaying wood.
new
most important thing that and safety of older structures is to tie the different components of each building together. This can be done by adding a steel
Just as in
buildings, the
can be done to increase the
stability
frame to unreinforced parts of a building such as a garage, bolting the walls to the foundation, adding reinforced joist
beams
to the exterior,
and using beam and
connectors whenever possible. Although such
modifications are expensive, they are usually cheaper
than having to replace a building that was destroyed by
an earthquake.
272
Chapter 10
Earthquakes
""•*
FIGURE
5
This 15-story reinforced concrete building
collapsed due to the ground shaking that occurred during the 1985 Mexico City earthquake. The soft lake bed
sediments on which Mexico City is built enhanced the seismic waves as they passed through.
^ FIGURE
10-17
The
amplitude, duration, and period of seismic waves vary in different types of materials. The amplitude and duration of the waves generally increase as they pass from bedrock to poorly consolidated or water-saturated material. Thus,
Bedrock
Poorly
Bay mud
structures built
consolidated
consolidated
typically suffer greater
sediments
sediments
(water saturated)
S-wave amplitude than those on bedrock. In addition to fill and water-saturated sediments tend to liquefy, or behave as a fluid, a process known as
greater shaking,
liquefaction.
When
shaken, the individual grains lose
cohesion and the ground flows. This
been well documented
in
on weaker material damage than structures built on bedrock.
Well-
many major
phenomenon has earthquakes.
magnitude of an earthquake and the underlying geology, the material used and the type of construction also affect the amount of damage done (see Perspective 10-1). The tremendous loss of life in many In addition to the
similar
earthquakes results from the collapse of buildings that
were not designed to withstand earthquakes. Adobe and mud-walled structures are the weakest of all and almost always collapse during an earthquake. Unreinforced brick structures and poorly built concrete structures are also particularly susceptible to collapse. For example,
thousands of people were
killed
by collapsing structures
during the 1988 Soviet Armenian earthquake. The 1976
earthquake in Tangshan, China, completely leveled the city because almost none of the structures were built to resist seismic forces. In fact,
most of them had unreinflexibility, and conse-
forced brick walls, which have no
^
FIGURE 10-18 Many of the approximately 242,000 people who died in the 1976 earthquake in Tangshan, China, were killed by collapsing structures. Many of the buildings were constructed from unreinforced brick, which has no flexibility, and quickly fell down during the earthquake. A few tents and temporary shelters can be seen in this oblique aerial view of a part of Tangshan.
quently they collapsed during the shaking
(Fig. 10-18).
During the 1906 San Francisco earthquake, 10% of the buildings were destroyed as a direct result of ground shaking. Many of the buildings in San Francisco at the time were constructed of brick and were not designed to withstand the violent shaking unleashed by an earth-
quake
(Fig.
10-19).
Fire
many earthquakes, particularly in urban areas, fire is major hazard. Almost 90% of the damage done in the 1906 San Francisco earthquake was caused by fire. The shaking severed many of the electrical and gas lines, which touched off flames and started numerous fires all over the city. Because water mains were ruptured by the earthquake, there was no effective way to fight the fires. Hence, they raged out of control for three days, destroying much of the city. During the 1989 Loma Prieta earthquake, a fire broke out in the Marina district of San In a
Francisco (Fig. 10-lb) but
was contained within
The Destructive
Effects of Earthquakes
a small
273
along the banks of the Sumida River to escape the raging fires.
Suddenly, a firestorm swept over the area, killing
people. The fires from this earthquake were so devastating because most of the buildings were constructed of wood; many fires were started by chemicals and fanned by 20 km/hr winds.
more than 38,000
S eismic sea
waves or t sunami are destructive sea wave s
that are usually produced by^alrtEquakes but can also
be caused by submarine landslides or volcanic eruptions (see the
Prologue to Chapter
1).
Tsunami
are popularly
do from the sudden movement of the sea floor, which sets up waves within the water that travel outward, much like the ripples that form when a stone is thrown into a pond. Tsunami travel at speeds of several hundred km/hr and are commonly not felt in the open ocean because their wave height is usually less than 1 m and the distance between wave crests is typically several hundred kilometers. However, when tsunami approach shorelines, the waves slow down and water piles up to heights of up to 65 m (Fig. 10-20). The tsunami that resulted from the Chilean earthquake of May 22, 1960, caused extensive death and damage, not only in Chile, but also in Hawaii, the Philippines, Okinawa, and Japan. Twenty-two hours later and 17,000 km from the epicenter, the tsunami hit the coasts of Honshu and Hokkaido, Japan, killing more than 180 people and causing extensive property damage (Fig. 10-21). Following a 1946 tsunami that killed 159 people and caused $25 million in property damage in Hawaii, the U.S. Coast and Geodetic Survey established a Tsunami called tidal waves, although they have nothing to
with
^ FIGURE
10-19
Approximately
10%
of the total
destruction during the 1906 San Francisco earthquake
was
ground shaking. Very few buildings were designed to withstand the violent shaking that took place. Many buildings were constructed of brick or masonry and quickly collapsed. The City Hall dome remained standing because it was supported by a steel framework, but the walls and rest of the building collapsed. the direct result of
San Francisco in 1989 had a its water and gas pipeline system so that lines could be isolated from breaks. During the September 1, 1923, earthquake in Japan, fires destroyed 71% of the houses in Tokyo and practically all the houses in Yokohama. In all, a total of 576,262 houses were completely destroyed by fire, and 143,000 people died, many as a result of the fire. A horrible example occurred in Tokyo where thousands of people gathered area. In contrast to 1906,
system of valves throughout
"^ FIGURE
10-20
A
tsunami destroying the tsunami
pier at Hilo, Hawaii, in 1946. This
was generated by an earthquake in the Aleutian Islands. The man in the path of waves was never seen again.
274
Chapter 10
Earthquakes
the
tides.
Instead, tsunami result
Early Warning System in Honolulu, Hawaii, in an at-
tempt to minimize tsunami devastation. This system combines seismographs and instruments that can detect earthquake-generated waves. Whenever a strong earthquake occurs anywhere within the Pacific basin, its location is determined, and instruments are checked to see if a tsunami has been generated. If it has, a warning is sent out to evacuate people
may be
from low-lying areas that
affected (Fig. 10-22). Since
its
inception, the
Tsunami Early Warning System has saved many
Ground
lives.
Failure
Landslides and liquefaction are the two most
common
types of ground failure resulting from earthquakes.
Landslides are particularly dangerous in mountainous
"^ FIGURE
10-21
and have been responsible for tremendous amounts of damage and many deaths. For example, the 1959 earthquake in Madison Canyon, Montana, generated a major rock slide (Fig. 10-23), while the 1970 Peru
that occurred
on
regions
FIGURE
10-22
Tsumani
travel times within the Pacific
A tsunami generated by an earthquake May 22, 1960, off the coast of Chile km before striking the coast
traveled approximately 17,000
of Japan. The force of the tsunami tossed this fishing boat on top of a house.
Ocean basin
to Honolulu, Hawaii.
USSR
NEW ZEALAND • Reporting
tidal stations
The Destructive
Effects of
Earthquakes
275
"•"
FIGURE
On
10-23
August 17, 1959, an earthquake
""•"
FIGURE
10-24
The
effects of
ground shaking on
are dramatically illustrated by the
Madison River in Montana and created Earthquake Lake. The slide began on
water-saturated
one side of the valley, demolished a campsite at the valley bottom, killing approximately 26 people, completely filled the river forming an earthen dam, and continued up the opposite valley slope. This view shows the slide in the background and Earthquake Lake in the foreground.
earthquake. The buildings were designed to be earthquake-resistant and fell over on their sides intact.
started a landslide that blocked the
collapse of these buildings in Niigata, Japan, during a
15).
Most
quake
in
of loess
of the 100,000 deaths from the 1920 earth-
Gansu, China, resulted when (wind-deposited
20,000 people were
collapsed.
silt)
when
killed
cliffs
composed
More than
two-thirds of the
town
1964
One method of long-range earthquake forecasting is based on the distribution and intensity of previous earthquakes.
earthquake caused an avalanche that completely destroyed the town of Yungay (see the Prologue to Chapter
soil
From an analysis of historic records and known faults^eismic risk map s can
the distribution of
be constructed that indicate the likelihood and potential severity of future earthquakes based
on the
past earthquakes (Fig. 10-25). Although such predict
when
the next
major earthquake
intensity of
maps cannot
will occur, they
are useful in helping people plan for future earthquakes.
of Port Royal, Jamaica, slid into the sea following an
earthquake on June 7, 1692. Liquefaction can also be a problem in earthquakeactive areas. Dramatic examples in addition to San Francisco include Niigata, Japan, where large apartment buildings
were tipped to
their sides after the water-saturated soil
of the hillside collapsed
(Fig.
10-24),
and Turnagain
many homes were destroyed when Cove Clay lost all of its strength when
Earthquake Precursors Studies conducted over the past several decades indicate
most earthquakes are preceded by both short-term and long-term changes within the Earth. Such changes that
are called precurs ors.
One
Heights, Alaska, where the Bootlegger
shaken by the 1964 earthquake
be predicted?
A
successful prediction
must include a time frame for the occurrence of the earthquake, its location, and its strength. In spite of the tremendous amount—of information geologists have gathered about the cause of earthquakes, successful predictions are
still
dictions can be
quite rare. Nevertheless,
reliable pre-
made, they can greatly reduce the num-
ber of deaths and injuries.
276
if
Chapter 10
Earthquakes
major
earthquakes and their aftershocks to detect areas that have had major earthquakes in the past but are currently
(Fig. 15-21).
^ EARTHQUAKE PREDICTION Can earthquakes
long-range prediction technique used in seismi-
cally active areas involves plotting the location of
inactive.
Such regions are locked and not releasing enis continuing to accumulate
ergy. Nevertheless, pressure in these regions
due to plate motions, so_th£se-seismic
gaps are prime locations for future earthquakes. Several seismic gaps along the San Andreas fault have the potential for future major earthquakes (Fig. 10-26). A major
earthquake that damaged Mexico City
in
1985 oc-
curred along a seismic gap in the convergence zone along the west coast of Mexico (see the Prologue to
Chapter
13).
I
I
I
I
I
I
I
I
No damage Intensities
I
to IV
damage Intensities V and
Minor
VI
Moderate damage Intensity VII
Major
damage
Intensities VIII (a)
<%>
and greater
^ FIGURE
10-26
Three seismic
gaps are evident in this cross section along the San Andreas fault from north of San Francisco to south of Parkfield. The first is between San Francisco and Portola Valley, the
second near
Loma
Prieta
Mountain,
and the
third southeast of Parkfield.
The top
section
shows the
epicenters of earthquakes between
January 1969 and July 1989. The bottom section shows the southern Santa Cruz Mountains gap after it was filled by the October 17, 1989, Loma Prieta earthquake (open
and
circle)
its
aftershocks.
measure tilting of the ground surface that is believed to result from increasing pressure in the rocks. Data from measurements in central California indicate significant tilting occurred immediately preceding small earthFurthermore, extensive tiltmeter work perJapan prior to the 1964 Niigata earthquake
quakes.
formed
in
showed a relationship between increased tilting and the main shock. While more research is needed, such changes appear to be useful in making short-term
clearly
earthquake predictions. Other earthquake precursors include fluctuations in the water level of wells and changes in the Earth's magnetic field and the electrical resistance of the ground. These fluctuations are believed to result from changes in the
amount of pore space
pressure.
A change
in
in rocks due to increasing animal behavior prior to an earth-
also is frequently mentioned. It may be that animals are sensing small and subtle changes in the Earth prior to a quake that humans simply do not sense. The Chinese used all of the precursors just mentioned,
quake
except seismic gaps, to successfully predict a large earth-
quake
278
in
Haicheng on February
Chapter 10
Earthquakes
4, 1975.
The earthquake
had a magnitude of 7.3 and destroyed hundreds of buildings but claimed very few lives because most people had been evacuated from the buildings and were outdoors when it occurred. While this was not the first successful earthquake prediction, it was the first to predict a major earthquake and thus saved thousands of lives. Another possible earthquake precursor was discovered following the 1989 trical
Loma
Prieta earthquake. Elec-
engineers at Stanford University, California, no-
ticed that the amplitude of ultra-low frequency radio
waves increased about three hours before the earthquake. Furthermore, they noticed that the background all frequencies abruptly increased 12 days before the earthquake and then suddenly decreased one day before the tremor hit. At this time it is not known why such a change should occur, but it is hoped that it may prove useful in short-term prediction of fu-
radio noise for
ture earthquakes.
Dilatancy Model
Many
of the precursors just discussed can be related to which is based on changes occurring
the dilatancy model,
in rocks subjected to very high^pressures^Laboratory-experiments have shown that rocks undergo an increase in
volume,
known
As numerous small
as dilatancy, just before rupturing.
pressure builds in rocks along faults,
cracks are produced that alter the physical properties of the rocks.
Water enters the cracks and increases the fluid volume of the rocks
pressure; this further increases the
and decreases
their inherent strength until failure even-
producing an earthquake. The dilatancy model is consistent with many earthquake precursors (Fig. 10-27). Although additional research is needed, it appears that this model has the potential for pretually occurs,
dicting earthquakes under certain circumstances.
laboratory and
activity
along major active
Most earthquake
of the behavior of rocks before,
faults.
prediction
work
in
United
the
done by the United States Geological Survey (USGS) and involves a variety of research into all aspects of earthquake-related phenomena. One of the more ambitious programs undertaken by the USGS is the Parkfield earthquake prediction experiment. Over the past 130 years, moderate-sized earthquakes have occurred on an average of every 21 to 22 years along a 24 km segment of the San Andreas fault at Parkfield, California. Based on the regularity of these earthquakes and the States
is
fact they
Earthquake Prediction Programs
field studies
during, and after major earthquakes as well as monitoring
have
all
been very similar, the
that another moderate-sized earthquake this
region in 1988, plus or minus
USGS
forecast
would occur
five years.
in
At the time
Currently, only four nations— the United States, Japan, the
of this writing (1991), the predicted moderate-sized
and China— have government-sponsored earthquake prediction programs. These programs include
earthquake has not yet occurred. During the mid-1980s, the USGS set up a variety of instruments to monitor
Soviet Union,
"*•"
FIGURE
10-27
The
relationship between dilatancy
and
various other earthquake precursors.
Earthquake Prediction
279
TABLE Anyone who
10-4
What You Can Do
lives in
an area that
is
to Prepare for an Earthquake
subject to earthquakes or
certain precautions to reduce the risks
and
losses resulting
who
will be visiting or
moving
to such
an area can take
from an earthquake.
Before an earthquake: 1. Become familiar with the geologic hazards of the area where you live and work. 2. Make sure your house is securely attached to the foundation by anchor bolts and that the walls, floors, and roof are all firmly connected together. 3. Heavy furniture such as bookcases should be bolted to the walls; semiflexible natural gas lines should be used so that they can give without breaking; water heaters and furnaces should be strapped and the straps bolted to wall studs to prevent gas-line rupture and fire. Brick chimneys should have a bracket or brace that can be anchored to the roof. 4. Maintain a several-day supply of fresh water and canned foods, and keep a fresh supply of flashlight and radio batteries as well as a fire extinguisher. 5. 6. 7.
Maintain a basic first-aid kit, and have a working knowledge of first-aid procedures. Learn how to turn off the various utilities at your house. Above all, have a course of action planned for when an earthquake strikes.
During an earthquake: 1. Act calmly and avoid panic. 2. If you are indoors, get under a desk or table if possible, or stand in an interior doorway or room corner as these are the structurally strongest parts of a room; avoid windows and falling debris. 3. In a tall building, do not rush for the stairwells or elevators. 4. In an unreinforced or other hazardous building, it may be better to get out of the building rather than stay in it. Be on the alert for fallen power lines and the possibility of falling debris. 5. If you are outside, get to an open area away from buildings if possible. 6. If you are in an automobile, stay in the car, and avoid tall buildings, overpasses, and bridges if possible. After an earthquake: 1.
2. 3. 4. 5. 6. 7. 8.
you are uninjured, remain calm and assess the situation. Help anyone who is injured. Make sure there are no fires or fire hazards. Check for damage to utilities and turn off gas valves if you smell Use your telephone only for emergencies. Do not go sightseeing or move around the streets unnecessarily. Avoid landslide and beach areas. Be prepared for aftershocks. If
conditions along this segment of the San Andreas fault in
order to study earthquake precursors and to assess
the possibility of short-term predictions of moderatesized earthquakes.
The Chinese have perhaps one of the most ambitious earthquake prediction programs anywhere in the world, which
is
understandable considering their long history of
The Chinese program on earthquake prediction was initiated soon after two large earthquakes occurred at Xingtai (300 km southwest of Beijing) in 1966. The Chinese program includes extensive study and monitoring of all possible earthquake precursors. In addition, the Chinese also emphasize changes in phenomena that can be observed by seeing and hearing destructive earthquakes.
without the use of sophisticated instruments. The Chinese have had remarkable success in predicting earthquakes, particularly in the short term, such as the 1975 Haicheng earthquake. They failed, however, to predict 280
Chapter 10
Earthquakes
gas.
1976 Tangshan earthquake that killed at 242,000 people. Great strides are being made toward dependable, accurate earthquake predictions, and studies are underway to assess public reactions to long-, medium-, and short-term earthquake warnings. However, unless short-term warnings are actually followed by an earthquake, most people will probably ignore the warnings as they frequently do now for hurricanes, tornadoes, and tsunami (see Perspective 10-2). Perhaps the best we can hope for is that people will take measures to minimize their risk from the next major earthquake (Table 10-4). the devastating least
^ EARTHQUAKE CONTROL If
earthquake prediction
is still
in the future,
can any-
thing be done to control earthquakes? Because of the
tremendous forces involved, humans are certainly not
Perspective 10-2
PREDICTED EARTHQUAKE THAT DID N'T OCCUR
A
3, 1990, passed without incident when a major earthquake that had been predicted publicly for
December
a portion of the
Midwest
failed to materialize. For
St.
Louis
months, a five-state region overlying the New Madrid fault zone braced for a potentially devastating earthquake (Fig. 1). During the months leading up to December
MISSOURI
3,
insurance salespeople did a brisk business selling
KENTUCKY
earthquake insurance to homeowners and businesses; entrepreneurs cashed in on the sale of such kits, and on earthquake preparedness and survival drew large crowds; and public officials reviewed disaster plans and
earthquake-related items as T-shirts, survival gas-line shutoff safety devices; seminars
coordinated emergency
relief efforts,
New
Madrid
fault
zone
while schools
So great was the concern that an earthquake would occur as predicted, many practiced earthquake
drills.
5*
school districts canceled classes and numerous
Memphis
businesses closed for several days.
The reason for such massive preparation and media was a prediction made by Iben Browning, a 72-year-old New Mexico scientist. Browning, who has
attention
a Ph.D. in physiology, genetics,
was previously
best
known
for
and bacteriology and his work on climates,
T
FIGURE 1 A devastating earthquake was predicted to occur on December 3, 1990, somewhere within the five-state region that overlies the New earthquake did not happen.
Madrid
fault zone. Luckily, the
claims to have correctly predicted the dates of several
major earthquakes and volcanic eruptions. He also is said to have predicted, within a day of its occurrence, the devastating 1989 Loma Prieta earthquake (see the Prologue) as well as the 1971 San Fernando Valley, California, and the 1972 Nicaragua earthquakes. Based on the apparent accuracy of his previous predictions, Browning's New Madrid prediction was taken very seriously by many people and received
wide media coverage. Browning predicted that there was a 50% chance of a magnitude 6.5 to 7.5 earthquake occurring somewhere within the New Madrid fault zone on December 3, 1990 (plus or minus a day). He also predicted for the same time that there
was
a lesser chance of a similar earthquake
a straight line, they exert greater than
normal
gravitational forces (although the forces are
still
weak) that some believe could trigger fault movement. Such a hypothesis is not new. Whenever relatively
natural orbits around the Sun, doomsayers are always predicting
some type
of natural disaster will occur,
and it never does. Earthquakes are the result of complex interactions within the Earth and occur under varied geologic conditions. Consequently, no one factor can be used to predict when and where an earthquake
will occur.
Seismologists do admit that based on past
occurring along California's San Andreas or Hayward faults and an even greater chance of an 8.2 magnitude
earthquake activity in the New Madrid fault zone area, there is a high probability of another major
earthquake striking Tokyo. However, none of the predicted earthquakes occurred. All of Browning's predictions are based on tidal
earthquake
in the area
Yet exactly
when
forces.
When
the Earth,
Moon, and Sun
are aligned in
-
the various planets are aligned as a result of their
within the foreseeable future.
that will be,
no
willing to predict because far too
geoscientist
is
many complex
variables are involved.
Earthquake Control
281
going to be able to prevent earthquakes. However, there
that the earthquakes in
may
the injection of contaminated waste water into a dis-
be ways to dissipate the destructive energy of major
earthquakes by releasing
it
in small
amounts that
posal well 3,674
will
m
Denver were
directly related to
deep at the Rocky Mountain Arse-
The U.S. Army initially denied was any connection, but a USGS study concluded that the pumping of waste fluids into the disposal well was the cause of the earthquakes. Figure 10-28 shows the relationship between the av-
not cause extensive damage.
nal, northeast of Denver.
During the early to mid-1960s, Denver experienced numerous small earthquakes. This was surprising because Denver had not been prone to earthquakes in the past. In 1962, David M. Evans, a geologist, suggested
•** FIGURE 10-28 (a) A block diagram of the Rocky Mountain Arsenal well and the underlying geology, (b) A graph showing the relationship between the amount of waste injected into the well per month and the average number of Denver earthquakes per month.
that there
Rocky Mountain Arsenal well
P
7
ro
6
£ o
5
1
Average gallons
of
waste injected per month
in
arsenal disposal well
4
oi
3
W 2
1 I
Maximum
Maximur
injection
pressure 550
lbs.
injectior
No waste
1
Injected by gravity flow
injected i
i
i
i
i
i
pi
10501b';
i
5<5tt<$OZO™£s<2-3-?OZQ 1962 (b)
282
Chapter 10
Earthquakes
1963
1964
1965
erage
number of earthquakes in Denver per month and amount of contaminated waste fluids in-
the average
month. Obviously,
jected into the disposal well per
a
high degree of correlation between the two exists, and the correlation
particularly convincing considering
is
when no waste fluids were injected, earthquake activity decreased dramatically. The geology of the area consists of highly fractured gneiss overlain by that during the time
sedimentary rocks. fractures,
it
When
water was pumped into these
decreased the friction on opposite sides of
the fractures and, in essence, lubricated
movement
them so that
occurred, causing the earthquakes that Den-
ver experienced.
Experiments conducted in 1969 at an abandoned oil near Rangely, Colorado, confirmed the arsensal hypothesis. Water was pumped in and out of abandoned field
pumped out. What the geologists were doing was starting and stopping earthquakes at will, and the relationship between pore-water pressures and earthquakes was established. Based upon these results, some geologists have proposed that fluids be pumped into the locked segments of the fluids were
active faults to cause small- to moderate-sized earth-
quakes. They believe that this would relieve the pressure
on the
and prevent a major earthquake from ocWhile this plan is intriguing, it also has many potential problems. For instance, there is no guarantee that only a small earthquake might result. Instead a major earthquake might occur, causing tremendous property damage and loss of life. Who would be responsible? Certainly, a great deal more research is needed before such an experiment is performed, even in an area of low fault
curring.
oil wells,
the pore-water pressure in these wells was measured, and seismographs were installed in the area
population density.
measure any seismic activity. Monitoring showed that small earthquakes were occurring in the area when fluid was injected and that earthquake activity declined when
accurately predicted or controlled, the best
to
^ 1.
2.
on opposite
sides of a fault until the is
exceeded and
rupture occurs. When the rocks rupture, stored energy is released as they snap back to their original position.
4.
Seismology is the study of earthquakes. Earthquakes are recorded on seismographs, and the record of an earthquake is a seismogram. The focus of an earthquake is the point where energy is released. Vertically above the focus on the Earth's surface
5.
is
circum-Pacific
6.
The
epicenter of an earthquake can be located by
the use of a time-distance graph of the P- and
S-waves from any given distance. Three seismographs are needed to locate the epicenter of an earthquake. 9. Intensity is a measure of the kind of damage done by an earthquake and is expressed by values from I to XII in the Modified Mercalli Intensity Scale. 10. Magnitude measures the amount of energy released by an earthquake and is expressed in the Richter -
Magnitude Scale. Each increase in the magnitude number represents about a 30-fold increase in energy released.
occur
in the
Mediterranean- Asiatic belt, and the remaining 5% mostly in the interior of plates or along oceanic spreading ridge systems. The two types of body waves are P-waves and S-waves. Both travel through the Earth, although S-waves do not travel through liquids. P-waves are the fastest waves and are compressional, while S-waves are shear.
7.
means of
and preparation.
Ground shaking is the most destructive of all earthquake hazards. The amount of damage done by an earthquake depends upon the geology of the area, the type of building construction, the magnitude of the earthquake, and the duration of shaking. 12. Tsunami are seismic sea waves that are usually produced by earthquakes. They can do a tremendous amount of damage to coastlines, even thousands of kilometers away from the earthquake 11.
seismic belts.
80% of all earthquakes belt, 15% within the
Approximately
careful planning
are directed horizontally. 8.
the epicenter.
Most earthquakes occur within
is
and Love waves. Rayleigh waves behave like water waves, and Love waves are similar to S-waves, but
Earthquakes are vibrations of the Earth caused by the sudden release of energy, usually along a fault. The elastic rebound theory states that pressure inherent strength of the rocks
appears that until such time as earthquakes can be
defense
CHAPTER SUMMARY
builds in rocks
3.
It
Surface waves travel along or just below the Earth's surface. The two types of surface waves are Rayleigh
epicenter.
13. Seismic risk
maps
are helpful in
making long-term
predictions about the severity of earthquakes based
on past occurrences.
Chapter Summary
283
The
Earthquake precursors are any changes preceding an earthquake that can be used to predict when an earthquake will occur. Precursors include seismic gaps, changes in surface elevation, tilting, fluctuations in water well levels, and anomalous animal behavior. 15. A variety of earthquake research programs are underway in the United States, Japan, the Soviet Union, and China. However, studies indicate that most people would probably not heed a short-term earthquake warning. 16. Fluid injection into locked segments of an active fault holds great promise as a means of possible earthquake control. 14.
vast majority of
depth of
less
20; b 100.
a e
earthquake
all
than
foci
occur at a
kilometers.
60; d
40; c
With few exceptions, the most earthquakes are: a. shallow focus;
80;
destructive
intermediate focus;
b.
precursor
deep focus; d. answers (a) and (b); e. answers (b) and (c). The majority of all earthquakes occur in the: a. Mediterranean-Asiatic belt; b. interior of plates; c. circum-Atlantic belt; d. circumPacific belt; e. along spreading ridges. Body waves are: a. P-waves; b. S-waves; c. Rayleigh waves; d. answers (b) and (c); e answers (a) and (b). The fastest of the four seismic waves are: a. P; b S; c. Rayleigh; d Love;
P-wave
e.
Rayleigh wave Richter Magnitude Scale
An
elasticity
seismic gap
point on the Earth's surface vertically above the
epicenter
seismic risk
focus intensity
seismogram seismograph
liquefaction
seismology
Love wave magnitude Modified Mercalli
S-wave
IMPORTANT Benioff zone dilatancy
model
earthquake elastic
rebound theory
c.
TERMS
tsunami. epicenter
a.
map
focus;
is:
the location where rupture begins; b. the
c.
same
location where energy
the
as the hypocenter; d. is
released;
the
none of
e.
these. 10.
time-distance graph
tsunami
11,
Intensity Scale
What
is the minimum number of seismographs needed to determine an earthquake's epicenter? a 1; b 5. 2; c 3; d 4; e A qualitative assessment of the kinds of damage done by an earthquake is expressed by: a. seismicity; b. dilatancy: c. magnitude;
d.
^ REVIEW QUESTIONS
12.
e. none of these. more energy is released by
intensity;
How much
a
magnitude
5 earthquake than by one of magnitude 2? 1.
2.
3.
According to the elastic rebound theory: earthquakes originate deep within the Earth; a. b. earthquakes originate in the asthenosphere where rocks are plastic; c. earthquakes occur where the strength of the rock is exceeded; d. rocks are elastic and do not rebound to their former position; e. none of these. A seismogram is: a. an instrument that records earthquake waves; b. the record made by a seismograph; c. the slowest of the seismic waves; d. a unit of energy released by an earthquake; e. none of these. To ensure that earthquake waves from all directions will be recorded, one needs a minimum of seismographs that are not oriented parallel .
284
2; b.
Chapter 10
.3;
c.
4;d.
Earthquakes
5;e
6.
2.5 times: b.
3 times;
c.
30 times;
1,000 times; e 27,000 times. 13. Which of the following usually causes the greatest amount of damage and loss of life? a. fire; b. tsunami; c. ground shaking; d
d.
14.
liquefaction;
e.
landslides.
A
tsunami is a: measure of the energy released by an a. earthquake; b. seismic sea wave; precursor to an earthquake; d. c. locked portion of a fault;
e.
seismic gap.
Define an earthquake.
How does the elastic rebound theory explain energy is released during an earthquake? Describe how a seismograph works. What
to each other,
a
a.
is
the difference between
surface waves?
how
body waves and
19.
How
do P-waves
differ
from S-waves?
How
do
^
ADDITIONAL
READINGS
Rayleigh waves differ from Love waves?
20
What
is
the difference between the focus
and the
epicenter of an earthquake?
21
How
22.
What
is is
the epicenter of an earthquake determined?
the relationship between plate boundaries
and earthquakes? 23.
What and
is
focal depth?
24. Explain the difference between intensity and
25
magnitude and between the Modified Mercalli Intensity Scale and the Richter Magnitude Scale. Why is ground shaking so destructive during an
26
earthquake? Explain how tsunami are produced and
Why
28.
How
29 30
are seismic risk
M. 1966. Man-made earthquakes
they
Johnston, A.
Denver.
useful to planners?
can earthquake precursors be used to predict earthquakes? What is the dilatancy model? How does it help explain how earthquake precursors are related? Explain how fluid injection may be useful in
C, and
L. R. Kanter. 1990.
Earthquakes
continental crust. Scientific American 262, no. 3: Penick,
maps
in
11-18. Frohlich, C. 1989. Deep earthquakes. Scientific American 260, no. 1: 48-55. Gere, J. M., and H. C. Shah. 1984. Terra non firma. New York: W. H. Freeman and Co. Hanks, T. C. 1985. National earthquake hazard reduction program: Scientific status. U.S. Geological Survey Bulletin 9:
1659.
why
are so destructive.
27.
Evans, D.
Geotimes 10, no.
the relationship between plate boundaries
New
York: W. H. Freeman and Co. Canby, T. Y. 1990. California earthquake— prelude to the big one? National Geographic 177, no. 5: 76-105. Bolt, B. A. 1988. Earthquakes.
J. L., Jr.
1981. The
New Madrid
in stable
68-75.
earthquakes. 2d ed.
Columbia, Mo.: University of Missouri Press. and R. S. Yeats. 1989. Hidden earthquakes. Scientific American 260, no. 6: 48-59. Wesson, R. L., and R. E. Wallace. 1985. Predicting the next great earthquake in California. Scientific American 252, no. Stein, R. S.,
2:
35-43.
controlling earthquakes.
Additional Readings
285
CHAPTER
11
THE INTERIOR OF THE EARTH * OUTLINE PROLOGUE INTRODUCTION SEISMIC WAVES THE DISCOVERY OF THE EARTH'S CORE Density and Composition of the Core
^ Guest
Essay: Geology: But Rewarding Career
An Unexpected
THE MANTLE Structure and Composition of the Mantle
THE EARTH'S CRUST THE EARTH'S INTERNAL HEAT "^"Perspective 11-1: Kimberlite
Pipes— Windows
to the
Mantle
Heat Flow "^" Perspective 11-2: Seismic
Tomography
MEASURING GRAVITY THE PRINCIPLE OF ISOSTASY THE EARTH'S MAGNETIC FIELD Inclination
and Declination of
the
Magnetic
Field
Magnetic Anomalies Magnetic Reversals
CHAPTER SUMMARY
Probing the Earth's interior. The world's deepest hole, more than 12 km deep, is on the Kola Peninsula in the northwestern Soviet Union. The 30-story building in this image houses the
drill rig.
PROLOGUE (jg^aj^M
The
Earth's interior has always been
an inaccessible, mysterious realm.
During most of historic time, it was perceived as an underground world of vast caverns, heat, and sulfur gases, populated by demons (Fig. 11-1). By the 1800s, scientists had some sketchy ideas about the Earth's structure, but outside scientific circles, all kinds of
bizarre ideas were proposed. In 1869, for example,
Cyrus Reed Teed claimed that the Earth was hollow and that humans lived on the inside. As recently as 1913, Marshall B. Gardner held that the Earth is a large sphere with a 1,300-km-thick outer shell surrounding a central sun. Although making no claim to present a reliable picture of the Earth's interior, Jules Verne's
1864
A
Journey to the Center of the Earth described the adventures of Professor Hardwigg, his nephew, and an Icelandic guide as they descended into the Earth through the crater of Mount Sneffels in Iceland. During their travels, they followed a labyrinth of passageways until they finally arrived 140 km below the surface. Here, they encountered a vast cavern containing "the central sea" illuminated by some novel
electrical
phenomenon
related to the northern lights.
Along the margins of the sea, they saw forests of and palms and a herd of mastodons
prehistoric ferns
complete with a gigantic human shepherd. Dwelling in the central sea were Mesozoic-aged marine reptiles and gigantic turtles. Their adventure ended when they
were carried upward to the surface on a raft by a rising plume of water. Scientists in 1864 knew what the average density of the Earth was and that pressure and temperature increase with depth.
They
also
knew
that the fabled
passageways followed by Professor Hardwigg could not exist, but little else was known, even though humans had probed the Earth through mines and wells for centuries. Even the deepest mines (the gold mines in South Africa) penetrate only about 3 km
below the surface. The deepest drill hole is currently about 12 km deep, although when completed, it will reach a depth of about 15 km. A drill hole 12 km deep
is
impressive, but
it is
less
than
0.2% of
the
"»"
FIGURE
11-1
In 1678, Athanasius Kircher
(1602-1680) published Mundus Subterraneus, which contained this drawing showing what he believed was the "ideal system of subterranean fire cells from which volcanic mountains arise, as it were, like vents."
distance to the Earth's center. Indeed,
were the
size
of an apple, this
drill
if
the Earth
hole would be
roughly equivalent to a pinprick penetrating less than halfway through the skin of the apple! In short, mines and wells have barely penetrated the Earth's surface. Because the Earth's interior is hidden from direct observation, it is more inaccessible than the surfaces of the Moon and Mars. Nevertheless, scientists have a -
reasonably good idea of the Earth's internal structure
and composition.
No
vast openings or passageways exist
as in Jules Verne's story; the deepest
extend to depths of
less
known
caverns
than 1,500 m. Even at the
modest depths to which Professor Hardwigg and his companions are supposed to have descended, the pressure and temperature are so great that rock actually flows even though it remains solid. In deep mines the rock is under such tremendous pressure that rock bursts and popping are constant problems (see Perspective 6-1). In short, the behavior of solids at depth where the temperature and pressure are great is very different from their brittle behavior at the surface.
Prologue
287
^ The
Earth's interior
from 2.5
so inaccessible that most people
is
about it. One can appreciate the stunning beauty of the northern lights and yet be completely unaware that they exist because of the interaction between the magnetic field that is generated within the Earth and think
little
the solar wind,
continuous stream of electrically
a
charged particles emanating from the Sun. Much of the Earth's geologic activity including volcanism, earthquakes, the movements of plates, and the origin of mountains
is
caused by internal heat.
A
continual slow ex-
change of material occurs as magma rises from within the Earth, and solid Earth materials are subducted and returned to the interior. Scientists
have
known
the Earth's interior
is
for
more than 200 years
not homogeneous.
that
New-
Sir Isaac
ton (1642-1727) noted in a study of the planets that the Earth's average density density of
1
g/cm
3 ).
is
5.0 to 6.0 g/cm
In 1797,
3
(water has a
Henry Cavendish
lated a density value very close to the 5.5
accepted.
'""
288
The
FIGURE
Earth's average density
11-2
Chapter
1 1
The
is
calcu-
g/cm 3
Interior of the Earth
now
considerably
internal structure of the Earth.
The
most of which range 3 g/cm Thus, in order for the average 3 density to be 5.5 g/cm much of the interior must consist of materials with a density greater than the Earth's greater than that of surface rocks,
INTRODUCTION
to 3.0
.
,
average density.
The Earth
is
generally depicted as consisting of con-
and density that from adjacent layers by rather distinct
centric layers that differ in composition
are separated
boundaries or the crust,
(Fig. 11-2). Recall that the is
outermost
the very thin skin of the Earth.
layer,
Below the
and extending about halfway to the Earth's center which comprises more than 80% of the Earth's volume (Table 11-1). The central part of the Earth consists of a core, which is divided into a solid inner core and a liquid outer part (Fig. 11-2). Because no direct observations of the Earth's interior can be made, this model of the Earth's internal structure is based on indirect evidence, mostly from the study of seismic waves. Nevertheless, the model is widely accepted by scientists and is becoming increasingly refined as more sophisticated methods of probing what some crust is
the mantle,
call
"inner space" are developed.
^ TABLE
11-1
" th Data on the Eart!
Mass
Volume (thousands of
km 3
Percentage of the Total
7,512,800 169,490,000
15.68
Mantle
896,990,000
83.02
4,760,800 1,747,200
0.44 o.i6
Continental crust
Oceanic crust
Atmosphere, water,
Several aspects of seismic waves were discussed in Chap-
10 (see Fig. 10-10). They are caused by any disturbance such as a passing train or construction equipment, but only those generated by large earthquakes, explosive volcanism, asteroid impacts, and nuclear explosions can travel completely through the Earth. Seismic waves ter
outward as wave fronts from their source areas, it is most convenient to depict them as wave
although
19,000,000,000
31.79%
40,500,000,000
67.77
J~
—
1
r
ice
^ SEISMIC WAVES
travel
Percentage of the Total
(trillions
0.70% ]_
Outer core
Inner core
rays,
of
metric tons)
which are
lines
250,000,000
0.42
14,351,000
0.02
showing the direction of movement
of small parts of wave fronts
and
(Fig. 11-3).
The behavior
and S-waves within the Earth provide geologists with much information about its intravel times of P-
ternal structure.
As we noted in Chapter 10, the velocities of P- and S-waves are determined by the density and elasticity of the materials through which they travel. Both the density
and
elasticity of
rocks increase with depth, but elas-
ticity increases faster
than density, resulting
*^ FIGURE 11-3 r
outward
in a general
Seismic wave fronts move from their source, the
in all directions
focus of an earthquake in this example. Wave rays drawn perpendicular to wave fronts.
are lines
Seismic Waves
289
a depth of about 2,900 km (Fig. 11-6). Such marked changes in the velocity of seismic waves indicate a
~2*
boundary
called a discontinuity ^across
ca nt change in Earth
curs^uch
m at erials
which
g_g|gnifi-
or their pro pertigs^oc-
discontinuities are the basis for subdividing
the Earth's interior into concentric layers.
The contribution of seismology (a)
"^ FIGURE 11-4 {a) If the Earth had the same composition and density throughout, seismic wave rays would follow straight paths, (b) Because density increases with depth, wave rays are continually refracted so that their paths are curved.
increase in the velocity of seismic waves. P-waves travel
than S-waves through
faster like
all
materials.
However, un-
P-waves, S-waves cannot be transmitted through a
no shear strength
liquid because liquids have
(rigidity)
—
they simply flow in response to a shear stress.
Earth were a homogeneous body, P- and
the
If
S-waves would travel
in straight
paths as
ure ll-4a. However, as a seismic
wave
shown
travels
in Fig-
from one
material into another of different density and elasticity, its
velocitvand jjirection of travel chan ge. Thatis^-the
wayjL_is
bent,
(Fig. 1 l-4b).
a
phenomenon known—as—refraction
Since seismic waves pass through materials
of differing density and elasticity, they are continually refracted so that their paths are curved; the only exception
is
that
of travel
is
wave rays
are not refracted
if
perpendicular to a boundary
to the study of the
Earth's interior cannot be overstated. Beginning in the
(b)
early 1900s, scientists recognized the utility of seismic
wave studies and, between 1906 and 1936, largely worked out the internal structure of the Earth on the basis of these studies.
» THE DISCOVERY OF THE EARTH'S CORE In 1906, R. D.
Oldham
of the Geological Survey of
India discovered that seismic waves arrived later than
expected at seismic stations more than 130° from an earthquake focus. He postulated the existence of a core that transmits seismic waves at a slower rate than shal-
lower Earth materials. We now know that P-wave velocity decreases markedly at a depth of 2,900 km, thus indicating a major discontinuity
core-mantle boundary
now
recognized as the
(Fig. 11-6).
The sudden decrease in P-wave velocity at the coremantle boundary causes P-waves entering the core to be refracted in such a way that very little P-wave energy reaches the Earth's surface in the area between 103° and
their direction (Fig. 11-5). In
that case they travel in a straight line.
In
addition
reflected,
much
to
refraction,
as light
is
seismic 'rays
reflected
are
also
from a mirror.
Seis-
mic rays that encounter-a^oundary separating materials of different density or elasticity within the Earth are refracted as they pass through the boundary,
and some
back to the Earth's surface wave velocity and the time required for it to travel from its source to the boundary and back to the surface, we can calculate the depth of the reflecting boundary. Such information is useful in
of their energy (Fig. 11-5). If
is
~^ FIGURE refracted,
and some of
common
may
Reflected waves
the
contain petroleum. Seismic reflection
is
Focus
a
tool used in petroleum exploration (see Per-
spective 9-1).
Although changes
in seismic
wave
velocity occur con-
tinuously with depth, P-wave velocity increases sud-
denly at the base of the crust and decreases abruptly at
290
Chapter
1 1
The
Interior of the Earth
their energy
surface.
determining not only the depths of the various layers within the Earth, but also the depths of sedimentary rocks that
Refraction and reflection of P-waves.
seismic waves pass through a boundary separating Earth materials of different density or elasticity, they are
reflected
we know
11-5
When
Outer core
is
reflected
back to the
Lithosphere
Asthenosphere
1412 -
Solid
inner
core
^FIGURE
11-8
Inge Lehmann, the
(a)
Danish seismologist who in 1936 postulated that the Earth has a solid
inner core, (b)
Lehmann
P-wave
proposed that reflection from an inner core could
shadow zone
explain the arrival of
weak P-wave energy in the P-wave shadow zone.
(b)
may
represent the dif-
ferentiated interiors of large asteroids
and approximate
and nickel
alloys (see Fig. 2-7),
be dense enough to yield an average density of 5.5 g/cm 3 for the Earth.
Both the outer and inner core are thought largely of iron, but pure iron is too
and composition of the Earth's core. The 3 density of the outer core varies from 9.9 to 12.2 g/cm and that of the inner core ranges from 12.6 to 13.0 3 g/cm (Table 11-2). At the Earth's center, the pressure is equivalent to about 3.5 million times normal atmo-
orites indicate that
spheric pressure.
sists
the density
,
The core cannot be composed
common
at the Earth's surface,
tremendous pressures
at great
of the minerals most
because even under the
depth they would
still
not
to be
composed
dense to be the sole constituent of the outer core. Thus, it must be "diluted" with elements of lesser density. Laboratory experiments and comparisons with iron mete-
perhaps
12%
of the outer core con-
of sulfur, one of the few elements sufficiently
abundant to account for the estimated density. In addition, some silicon and small amounts of nickel and potassium are also probably present (Table 11-2). In contrast,
pure iron
is
not dense enough to account
for the estimated density of the inner core. """
FIGURE
11-9 The presence of an S-wave shadow zone indicates that S-waves are being blocked within the Earth.
Most
geol-
10 to 20% of the inner core also consists of nickel. These metals form an iron-nickel alloy that under the pressure at that depth is thought to
ogists think that perhaps
be sufficiently dense to account for the density of the inner core.
Any model also
of the core's composition and physical
must explain not only the variations
state
why
solid,
the outer core
is
in density,
but
liquid while the inner core
and how the magnetic
field is
is
generated within the
core (discussed later in this chapter).
When
the core
formed during early Earth history, it was probably completely molten and has since cooled to the point that its interior has crystallized.
mantle boundary
is
The temperature
at the core-
estimated at 3,500° to 5,000°C, yet
the high pressure within the inner core prevents melting. In contrast, the outer core
is
subject to less pressure.
Even more important than the differences fe shadow zC
292
Chapter
1 1
The
Interior of the Earth
in pressure,
however, are the compositional differences between the inner and outer core. The sulfur content of the outer
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TABLE
11-2
Composition and Density of the Earth Density 3 (g/cm )
Composition
20%
Inner core
Iron with 10 to
Outer core
Iron with perhaps
Mantle Oceanic crust Continental crust
core helps depress
its
melting temperature.
An
iron-
sulfur mixture melts at a lower temperature than does
pure iron, or an iron-nickel pressure, the outer core
is
alloy, so despite the
high
molten.
* THE MANTLE Another
significant discovery
was made
12.6-13.0 9.9-12.2
nickel
12% sulfur; also silicon and small amounts of nickel and potassium Peridotite (composed mostly of ferrogmagnesian silicates) Upper part basalt, lower part gabbro Average composition of granodiorite
about the Earth's interior
1909 when the Yugoslavian seismologist Andrija Mohorovicic detected a discontinuity at a depth of about 30 km. While studying arrival times of P-waves in
3.3-5.7
—3.0 ~2.7
from Balkan earthquakes, Mohorovicic noticed that P-waves arrived sooner at seismic stations more than 200 km from an earthquake's epicenter than at stations closer than 200 km (Fig. 11-10). From his observations Mohorovicic concluded that a sharp boundary separating rocks with different properties exists at a depth of about 30 km. He postulated that P-waves below this boundary travel at 8 km/sec, whereas those above the boundary travel at 6.75 km/sec. Thus, when an earthquake occurs, some waves travel directly from the focus to the seismic station, while oth-
^
FIGURE 11-10 Andrija Mohorovicic studied seismic waves and detected a seismic discontinuity at a depth of about 30 km. (a) At a 200 km from an earthquake's epicenter, the waves traveling through the crust arrive first, even though the deeper waves travel faster. (£>) At distances greater than 200 km, the deeper, faster seismic waves arrive at seismic stations first, even though seismic station less than
they travel farther.
294
Chapter
1 1
The
Interior of the Earth
Direct
wave
Epicenter
East Pacific Rise
Peru-Chile
South
Mid-Atlantic
Trench
America
Ridge
Oceanic
Oceanic
crust
crust
•^ FIGURE
11-11
The Moho
is
present everywhere except beneath
spreading ridges such as the East Pacific Rise and the Mid-Atlantic Ridge. However, the depth of the
Moho
varies considerably.
ers travel
through the deeper layer and some of their refracted back to the surface (Fig. 11-10). Waves traveling through the deeper layer travel farther to a seismic station but they do so more rapidly than those in the shallower layer. The boundary identified by
averages 35 km, but ranges from 20 to 90
energy
the sea floor
Mohorovicic sepa ratejjh e crust from the mantle and is now called the Mohorovici c discontinuity, or simpl y the Nloho. IFTsTpr esent everywhere except beneath spread-
Although seismic wave velocity
is
ing ridges, but
its
depth varies: beneath the continents
it
it is
5 to 10
km
deep
km; beneath
(Fig. 11-11).
Structure and Composition of the Mantle in the
mantle generally
increases with depth, several discontinuities also exist. Be-
tween depths of 100 and 250 km, both P- and S-wave velocities decrease markedly (Fig. 11-12). This layer be-
"^ FIGURE 11-12 Variations in P-wave velocity in the upper mantle and transition zone.
7^
The Mantle
295
rween 100 and 250
km
deep
is
the low-velocity zone;
it
corresponds closely to the asthenosphere, a layer in which the rocks are close to their melting point and thus are less elastic; this
decrease in elasticity accounts for the observed
The asthenosphere is an important zone because it may be where some magmas are generated. Furthermore, it lacks strength and flows plastically and is thought to be the layer over which the plates decrease in seismic
wave
velocity.
of the outer, rigid lithosphere move.
Even though the low-velocity zone and the asthenosphere closely correspond, they are still distinct. The asthenosphere appears to be present worldwide, but the
'•'
FIGURE
wave
11-13
(a)
Seismic
discontinuities in the mantle
are thought to be caused by structural changes in minerals with
depth,
(b) In olivine,
the
dominant
mineral in peridotite, a silicon atom is surrounded by four oxygen atoms, (c) At greater depth, the olivine structure is rearranged into the denser structure of spinel, which also has four oxygen atoms surrounding a silicon atom, {d) At a depth of about 700 km, another
change occurs, and the spinel structure is converted to that of perovskite,
which has a silicon atom six oxygen atoms.
surrounded by
-i
low-velocity zone
is
not. In fact, the low-velocity zone
appears to be poorly defined or even absent beneath the ancient shields of continents.
Other discontinuities have been detected at deeper levwithin the mantle. However, unlike those between the crust and mantle or between the mantle and core, these probably represent structural changes in minerals rather than compositional changes. In other words, geologists believe the mantle is composed of the same material els
throughout, but the structural states of minerals such as olivine change with depth (Fig. 11-13). At a depth of 400
km, seismic wave
velocity increases slightly as a conse-
Oceanic
Mid-oceanic
Continental
crust
ridge
crust
quence of such changes in mineral structure (Fig. 11-12). Another velocity increase occurs at 640 to 720 km where the minerals break
and
(iron oxide)
dioxide (Si0 2 )
down
MgO
into metal oxides, such as
(magnesium oxide), and
A
11-13).
(Fig.
FeO
silicon
third discontinuity exists
about 1,050 km where P-waves once again increase in velocity. These three discontinuities are within what is called a transition zone separating the upper mantle from the lower mantle (Fig. 11-12). Although the mantle's density, which varies from 3.3 3 to 5.7g/cm can be inferred rather accurately from seisat
,
mic waves,
its
composition
less certain.
is
The igneous
considered the most likely component.
most rocks have densities of 2.0 to 3.0 and the overall density is about 2.70 g/cm 3 (Table 11-2). P-wave velocity in the continental crust is about 6.75 km/sec; at the base of the crust, P-wave velocity abruptly increases to about 8 km/sec. The continental crust varies considerably in thickness. It averages about 35 km thick, but is much thinner in such areas as the Rift Valleys of East Africa and a large area called the Basin and Range Province in the iron ore deposits,
g/cm
3
,
western United States. The crust stretched
and thinned
in
in these areas is
what appear
being
to be the early
stages of rifting. In contrast, continental crust beneath
mountain ranges
much
spars (see Fig. 5-13). Peridotite
pyroxene) with about 10% feldis considered the most
and projects deep into Himalayas of Asia, the continental crust is as much as 90 km thick. Crustal thickening beneath mountain ranges is an im-
likely candidate for three reasons. First, laboratory ex-
portant point that will be discussed in "The Principle of
periments indicate that
Isostasy" later in the chapter.
rock peridotite Peridotite
(60%
is
mostly
contains
olivine
and
30%
would account
that
it
ferromagnesian
minerals
possesses physical properties
for the mantle's density
and ob-
wave transmissions. Second,
is
thicker
the mantle. For example, beneath the
Although variations also occur
in
oceanic crust, they
peri-
are not as distinct as those for the continental crust. For
dotite forms the lower parts of igneous rock sequences
example, oceanic crust varies from 5 to 10 km thick, being thinnest at spreading ridges. It is denser than con-
served rates of seismic
believed to be fragments of the oceanic crust and upper
mantle emplaced on land
(see
Chapter
12).
And
third,
peridotite occurs as inclusions in volcanic rock bodies
known
tinental crust, averaging
about 3.0 g/cm
3 ,
and
it
trans-
mits P-waves at about 7 km/sec. Just as beneath the
come
continental crust, however, P-wave velocity increases at
from great depths. These inclusions are thought to be
the Moho. The P-wave velocity of oceanic crust is what one would expect if it were composed of basalt. Direct observations of oceanic crust from submersibles and deep-sea drilling confirm that its upper part is indeed
such as kimberlite pipes that are
to have
pieces of the mantle (see Perspective 11-1).
^ THE EARTH'S CRUST The of
Earth's crust
its
is
the
most
concentric layers, but
and best studied also the most complex Whereas the core and
accessible
it is
both chemically and physically. mantle seem to vary mostly in a vertical dimension, the
shows considerable vertical and lateral variation. (More lateral variation exists in the mantle than was once believed, however.) The crust along with that part of the upper mantle above the low-velocity zone constitutes the crust
lithosphere of plate tectonic theory.
Two
types of crust are recognized
— continental crust
and oceanic crust— both of which are
less
dense than the
more comwide variety of igneous, sedimentary, and metamorphic rocks. It is generally described as "granitic," meaning that its overall composition is similar to that of granitic rocks. Specifically, its overall composition corresponds closely to that of granodiorite, an igneous rock having a chemical composition between granite and diorite (see Figure 5-13). Continental crust varies in density depending on rock underlying mantle. Continental crust
is
the
plex, consisting of a
type, but with the exception of metal-rich rocks, such as
composed of basalt. The lower part of the oceanic crust is composed of gabbro, the intrusive equivalent of basalt (see Chapter 12 for a more detailed description of the oceanic crust).
^ THE EARTH'S INTERNAL HEAT During the nineteenth century, scientists realized that the Earth's temperature in deep mines increases with depth. Indeed, very deep mines must be air conditioned so that the miners can survive. More recently, the same trend has been observed in deep drill holes, but even in these we can measure temperatures directly down to a depth of only a few kilometers. The temperature increase with depth, or geothermal gradient, near the surface is about 25°C/km, although it varies from area to area. For example, in areas of active or recently active volcanism, the geothermal gradient is greater than in adjacent nonvolcanic areas, and temperature rises faster beneath spreading ridges than elsewhere beneath the sea floor. Unfortunately, the geothermal gradient is not useful for estimating temperatures deep in the Earth. If we were sim-
The
Earth's Internal
Heat
297
Perspective 11-1
KIMBERLITE PIPES-WINDOWS
TO THE MANTLE Diamonds have been economically important throughout history, yet prior to 1870, they had been found only in river gravels, where they occur as the result of weathering, transport,
and deposition.
In
1870, however, the source of diamonds in South Africa was traced to cone-shaped igneous bodies
found near the town of
called kimberlite pipes
Kimberly
(Fig. 1).
Kimberlite pipes are the source
rocks for most diamonds.
The in
greatest concentrations of kimberlite pipes are
southern Africa and Siberia, but they occur in
many
other areas as well. In North America they have been
found
in the
Canadian
Arctic, Colorado,
Wyoming,
Missouri, Montana, Michigan, and Virginia, and one at
Murfreesboro, Arkansas, was
briefly
worked
for
diamonds. Diamonds discovered in glacial deposits in some midwestern states indicate that kimberlite pipes are present farther north. The precise source of these diamonds has not been determined, although some
kimberlite pipes have recently been identified in
A
o
sea
J
A o
°
northern Michigan. Kimberlite pipes are composed of dark gray or blue
igneous rock called kimberlite, which contains olivine, a
potassium- and magnesium-rich mica, serpentines, and calcite
and
silica.
Some
of these rocks contain inclusions
l^^JMMBM
of peridotite that are thought to represent pieces of the Tfr
mantle brought to the surface during the explosive
pipe.
volcanic eruptions that form kimberlite pipes. If
magma
1
Generalized cross section of a kimberlite
kimberlite pipes measure less than
500
m
in
in kimberlite
pipes originated at a depth of at least 30 km. Indeed, the presence of
diamonds and the
structural
form of
the silica in the kimberlite can be used to establish
minimum and maximum
depths for the origin of
ply to extrapolate from the surface
perature at 100 great pressure, for pockets of
km would all
known
magma,
it
downward,
the tem-
be so high that in spite of the
rocks would melt. Yet except
appears that the mantle
is
solid
it transmits S-waves. Accordgeothermal gradient must decrease markedly. Current estimates of the temperature at the base of the crust are 800° to 1,200°C. The latter figure seems to be an upper limit: if it were any higher, melting would
rather than liquid because ingly, the
298
Most
diameter at the surface.
peridotite inclusions are, in fact, pieces of the
mantle, they indicate that the
both
FIGURE
Chapter
1 1
The
Interior of the Earth
the
magma. Diamond and
graphite are different
forms of carbon (see Fig. 3-6), but diamond forms only under high-pressure, high-temperature conditions. The presence of diamond and the absence crystalline
be expected. Furthermore, fragments of mantle rock in kimberlite pipes (see Perspective 11-1), thought to have
come from depths of about 100
to
300 km, appear
to
have reached equilibrium at these depths and at a temperature of about 1,200°C. At the core-mantle boundary, the temperature is probably between 3,500° and
5,000°C; the wide spread of values indicates the uncertainties of such estimates. If these figures are reasonably accurate, however, the geothermal gradient in the man-
Temperature (°C)
600
800
1,000
1,200
1
,400
1
,600
^- FIGURE 2 The forms of carbon silica in kimberlite pipes provide information on the depth at which the magma formed. The presence of and
diamond and
coesite in kimberlite
indicates that the
magma
probably
formed between 100 and 300
shown by
km
as
the intersection of the
calculated continental geotherm with the graphite-diamond and coesite-stishovite inversion curves.
of graphite existed
The
in
kimberlite indicate that such conditions
magma
where the
originated.
calculated geothermal gradient
and the
shown
in
in kimberlite,
is
on
maximum
the other hand,
is
a
form that
depth of about 300 km. Quartz
the form of silica found under low-pressure,
low-temperature conditions. Under great pressure,
pressure increase with depth beneath the continents are
found
indicates a
however, the crystal structure of quartz changes to
Figure 2. Laboratory experiments have
its
established a diamond-graphite inversion curve
high-pressure equivalent called coesite, and at even
showing the pressure-temperature conditions at which graphite is favored over diamond (Fig. 2). According
pipes contain coesite but no stishovite, indicating that
greater pressure
to the data in Figure 2, the intersection of the
the kimberlite
diamond-graphite inversion curve with the geothermal
of
gradient indicates that kimberlite
magma came from
minimum depth of about 100 km. Diamond can establish only a minimum depth kimberlite because
it is
stable at
silica
only about l°C/km. Recently, considerable temper-
new
technique called seismic tomography (see
Perspective 11-2).
Considering that the core uncertainties exist regarding
general estimates of 11-14).
The dashed
its
line
is
its
so remote and so
many
composition, only very
temperature can be made (Fig. in Figure 11-14 is an admittedly
speculative melting point curve for Earth materials
have come from a depth
as indicated by the intersection of
the coesite-stishovite inversion curve with the (Fig. 2).
for
ature variation has been inferred within the mantle by a rather
km
geothermal gradient
any pressure greater
than that occuring at a depth of 100 km. The
tie is
a
changes to stishovite.* Kimberlite
magma must
than 300
less
it
com-
*
Coesite and stishovite are also
known from
environments such as meteorite impact
other high-pressure
sites.
posed mostly of iron. Notice that the melting point curve is above the temperature estimates until the outer core is reached. Recall from earlier discussions that the S-wave shadow zone indicates that the outer core is liquid, whereas P-wave velocities indicate that the inner core
is
solid. Therefore, the postulated
remains within the
field
melting curve
of temperature estimates until
the depth corresponding to the outer core— inner core
boundary
is
reached. According to these considerations,
The
Earrh's Internal
Heat
299
E
— FIGURE
3,000
Outer core
Mantle
11-14
Temperature
estimates for the Earth's interior. The range of estimates increases
with depth indicating greater uncertainties. The dashed line is a speculative melting curve for iron.
Depth (km)
11-15). Higher values are also recorded in areas of con-
maximum temperature at the center of the core is 6,500°C, very close to the estimated temperature for the
tinental volcanism, such as in Yellowstone National
surface of the Sun!
Park
the
in
Wyoming, Lassen National Park
Heat Flow Even though rocks are poor conductors of heat, detectable amounts of heat from the Earth's interior escape at the surface by heat flow. The amount of heat lost from within the Earth is small and can be detected only by Heavy, cylindrical probes are dropped into soft sea-floor sediments, and temperatures are measured at various depths along the cylinder. On sensitive
in California,
Washington. Any area possessing higher than average heat flow values is a potential area for the development of geothermal energy
and near Mount
instruments.
(see
Chapter
Most
St.
Helens
in
17).
of the Earth's internal heat
is
generated by ra-
dioactive decay. Recall from Chapter 3 that isotopes of
some elements spontaneously decay state and, in
doing
to a
so, generate heat.
result of heat flow studies
is
more
One
stable
surprising
that, discounting local vari-
ations, the average values for the continents
and sea
surprising because con-
made
at
in areas
of
oceanic crust. Thus, one would expect the continents to
active or recently active volcanism. For example, greater
have higher heat flow values. Geologists postulate that convection cells and mantle plumes of hot mantle rock beneath the oceanic crust account for the oceanic crust's
the continents, temperature measurements are drill holes and mines. As one would expect, heat flow is greater
heat flow occurs at spreading ridges, and lower than
average values are recorded at subduction zones
Chapter
1 1
The
Interior of the Earth
about the same. This
tinental crust contains
various depths in
300
floor are
(Fig.
is
more radioactive elements than
Perspective 11-2
TOMOGRAPHY
SEISMIC The model of
the Earth's interior consisting of an
iron-rich core
and a rocky mantle
but
is
is
Seismometer
probably accurate
also rather imprecise. Recently, however,
geophysicists have developed a
new technique
called
tomography that allows them to develop three-dimensional models of the Earth's interior. In seismic tomography numerous crossing seismic waves are analyzed in much the same way radiologists analyze CAT (computerized axial tomography) scans. In CAT scans, X-rays penetrate the body, and a two-dimensional image of the inside of a patient is formed. Repeated CAT scans, each from a slightly different angle, are computer analyzed and stacked to
seismic
produce a three-dimensional picture. In a similar fashion geophysicists use seismic to
probe the interior of the Earth. From
its
waves
time of
and distance traveled, the velocity of a seismic computed at a seismic station. Only average
arrival
ray
is
velocity
is
determined, however, rather than variations
tomography numerous wave rays are analyzed so that "slow" and "fast" areas of wave travel can be detected (Fig. 1). Recall that seismic wave velocity is controlled partly by elasticity; cold rocks have greater elasticity and therefore transmit seismic waves faster than hot rocks.
Earthquake
in velocity. In seismic
Using
this technique, geophysicists
"^ FIGURE
1
Numerous earthquake waves
are analyzed
to detect areas within the Earth that transmit seismic waves
than adjacent areas. Areas of fast wave correspond to "cold" regions (blue), whereas "hot"
faster or slower
travel
regions (red) transmit seismic waves
more
slowly.
have detected
areas within the mantle at a depth of about 150
km
where seismic velocities are slower than expected. These anomalously hot regions lie beneath volcanic areas and beneath the mid-oceanic ridges, where convection cells of rising hot mantle rock are thought
several kilometers into the mantle.
Of
course, the base
of the mantle possesses the same features in reverse; geophysicists have termed these features
to exist. In contrast, beneath the older interior parts
"anticontinents" and "antimountains."
of continents, where tectonic activity ceased hundreds
the surface of the core
of millions or billions of years ago, anomalously cold
sinking and rising masses of mantle material.
spots are recognized. In effect, tomographic
three-dimensional diagrams
show heat
maps and
variations
within the Earth. Seismic tomography has also yielded additional and
sometimes surprising information about the core. For example, the core-mantle boundary is not a smooth surface, but has broad depressions and rises extending
As a
is
result of seismic
It
appears tbat
continually deformed by
tomography,
picture of the Earth's interior
is
a
much
emerging.
It
clearer
has
already given us a better understanding of complex convection within the mantle, including upwelling
convection currents thought to be responsible for the
movement Chapter
of the Earth's lithospheric plates (see
13).
The
Earth's Internal
Heat
301
Oceanic ridge (spreading ridge)
3-
CD
X
p
— FIGURE
11-16
(a)
The
gravitational attraction of the Earth pulls
all
objects
mass. Objects
1
toward its center of and 2 are the same
distance from the Earth's center of
mass, but the gravitational
on one is greater because more massive. Objects 2 and 3 have the same mass, but the gravitational attraction on 3 is four times less than on 2 because it is attraction it is
twice as far from the Earth's center of mass, (b) The Earth's rotation generates a centrifugal force that partly counteracts the force of gravity. Centrifugal force
the poles
and maximum
is
zero at
at the
equator.
a
mass deficiency exists over the unconsolidated sediment
because the force of gravity
is
less
than the expected av-
erage (Fig. 11-18). Large negative gravity anomalies also exist over salt
domes
(Fig.
11-19) and at subduction
zones, indicating that the crust
—
"
FIGURE
from a spring
11-17
is
not
The mass suspended shown
in the gravimeter,
diagrammatically, is pulled downward more over the dense body of ore than is
in
in equilibrium.
"" FIGURE gravity
PRINCIPLE OF ISOSTASY
More than 150
years ago, British surveyors in India
m when they compared two measurements between points 600 km
detected a discrepancy of 177 the results of
11-18
anomaly over
structure. it
adjacent areas, indicating a positive
^ THE
A
negative
a buried
-»-
FIGURE
11-19
Rock
salt
is
dense than most other types of rocks. A gravity survey over a salt less
dome shows
a negative gravity
anomaly.
gravity anomaly.
The
Principle of Isostasy
303
^^ Expected \^ plumb
N.
of
"^ FIGURE
deflection
1
1-20
(a)
A plumb
line
is
normally
vertical,
pointing to the Earth's center of gravity. Near a mountain range, one would expect the plumb line to be deflected as shown if the mountains were simply thicker, low-density
line
Himalayas
on denser material, (b) The actual deflection plumb line during the survey in India was less than It was explained by postulating that the
material resting of the
expected.
Himalayas have a low-density
root.
suspended weight) of their surveying instruments from the vertical, thus accounting for the error. Calculations revealed, however, that if the Himalayas were simply thicker crust piled
on denser
material, the error should
have been greater than that observed
(Fig.
11-20).
George Airy proposed that in addition to projecting high above sea level, the Himalayas— and other mountains as well — also project far below the surface and thus have a low-density root (Fig. 11-20). In effect, he was saying that mountains float on denser rock at depth. Their excess mass above sea level is compensated for by a mass deficiency at depth, which would In 1865, Sir
account for the observed deflection of the plumb during the British survey
(Fig.
line
11-20).
Gravity studies have revealed that mountains do indeed have a low-density "root" projecting deep into the mantle. If it were not for this low-density root, a gravity survey across a mountainous area would reveal a huge
The fact that no such anomaly mass excess is not present, so some of the dense mantle at depth must be displaced by
positive gravity anomaly. exists indicates that a
apart.
Even though
this
discrepancy was small,
it
was an
unacceptably large error. The surveyors realized that the gravitational attraction of the nearby tains
probably deflected the plumb
Himalaya Moun-
line (a
cord with a
^
FIGURE 11-21 (a) Gravity measurements along the line shown would indicate a positive gravity anomaly over the excess mass of mountains
if
the
simply thicker crust resting on denser material below, (b) An actual gravity survey across a mountain region shows no departure from the expected and thus no gravity anomaly. Such data indicate that the mass of the mountains above the surface must be compensated for at depth by low-density material displacing denser material.
wave
shown
in
Figure 11-21. (Seismic
studies also confirm the existence of low-density
roots beneath mountains.)
Positive gravity
s~>^
the mountains were
lighter crustal rocks as
anomaly
— FIGURE
An
11-22
iceberg
sinks to an equilibrium position
with about 10% of its mass above water level. The larger iceberg sinks farther
below and
rises
higher above
the water surface than does the
some of
smaller one.
If
above water
level
icebergs will rise
the ice
should melt, the to maintain the
same proportion of ice above and below water level. The Earth's crust floating in more dense material below is analogous to this example.
Airy's proposal is now called the principle of isostasy. According to this principle, the Earth's crust is in floating equilibrium with the more dense mantle below. This phenomenon is easy to understand by an analogy to an iceberg (Fig. 11-22). Ice
and thus
is
slightly less
dense than water,
However, according to Archimedes'* principle of buoyancy, an iceberg will sink in the water until it displaces a volume of water that equals its total weight. When the iceberg has sunk to an equilibrium position, only about 10% of its volume is above water level. If some of the ice above water level should melt, the iceberg will rise in order to maintain the same proportion of ice above and below water (Fig. 11-22).
The in that
Where it
it
floats.
Earth's crust it
is
similar to the iceberg, or a ship,
sinks into the mantle to
the crust
sinks further
is
thickest, as
down
its
equilibrium
level.
ice.
higher above the equilibrium surface (Fig. 11-21). Con-
crust also responds isostatically to widespread (Fig.
11-24).
Unloading of the Earth's crust causes
it
to respond by
upward until equilibrium is again attained. This phenomenon, known as isostatic rebound, occurs in arrising
eas that are deeply eroded
and
in areas that
covered by a vast is still
ice sheet until
rebounding
for-
century
about 10,000 years ago,
isostatically at a rate of
up to
1
m per
ll-25a). Coastal cities in Scandinavia have
(Fig.
been uplifted sufficiently rapidly that docks constructed
now
several centuries ago are
rebound has also occurred land has risen as
much
as
far
from shore. Isostatic Canada where the during the last 6,000
in eastern
100
m
years (Fig. 11 -25 b). If
the principle of isostasy
is
correct,
it
implies that
the mantle behaves as a liquid. In preceding discussions,
however,
we
must be
said that the mantle
transmits S-waves, which will not
solid because
move through
and less dense than oceanic crust stands higher than the ocean basins. Should the crust be loaded, as where widespread glaciers accumulate, it responds by sinking further into the mantle to maintain equilibrium (Fig. 11-23). In Greenland and
When
Antarctica, for example, the surface of the crust has
riods of time,
been depressed below sea level by the weight of glacial
time scales can be considered a viscous liquid.
tinental crust being thicker
were
merly glaciated. Scandinavia, for example, which was
beneath mountain ranges,
into the mantle but also rises
The
erosion and sediment deposition
it
liquid.
How
can
this
considered in terms of the short time necessary
for S-waves to pass through solid.
a
apparent paradox be resolved?
it,
However, when subjected it
will yield
the mantle
is
indeed
to stress over long pe-
by flowage and thus at these
The
A familiar
Principle of Isostasy
305
Crust
Continental crust
(d)
"^ FIGURE 11-23 A diagrammatic representation of the response of the Earth's crust to the added weight of glacial ice. (a) The crust and mantle before glaciation. (b) The weight of glacial ice depresses the crust into the mantle. (c)
When
and the rebound is
the glacier melts, isostatic rebound begins,
crust rises to
its
former position,
(d) Isostatic
complete.
substance that has the properties of a solid or a liquid depending on how rapidly deforming forces are applied is silly
putty.
It
sufficient time,
will flow
under
but shatters as a
its
own
weight
brittle solid if
if
given
struck a
~^ FIGURE
11-24
diagrammatic representation
isostatic
shown in Figure 11-26 is dipolar, meantwo unlike magnetic poles referred to as the north and south poles. The Earth possesses a dipolar magnetic field that resembles, on a large scale, magnetic ing that
field
it
possesses
that of a bar
sharp blow.
A
response of the crust to erosion (unloading) and widespread deposition (loading).
showing the
What
is
magnet
(Fig. 11-27).
the source of this magnetic field?
A number
^ THE EARTH'S MAGNETIC FIELD
of naturally occurring minerals are magnetic, with magnetite being the most common and most magnetic. It is
A
very unlikely, however, that the Earth's magnetic field is generated by a body of buried magnetite because mag-
simple bar magnet has a magnetic field, an area in which magnetic substances are affected by lines of magnetic force radiating from the magnet (Fig. 11-26). The
306
Chapter
1 1
The
Interior of the Earth
netic substances lose their
magnetic properties when
Germany
Poland
(a)
lb)
""'
FIGURE
in centimeters last
11-25
(a) Isostatic
per century,
rebound in Scandinavia. The lines show rates of uplift rebound in eastern Canada in meters during the
(b) Isostatic
6,000 years.
heated above a temperature called the Curie point. The Curie point for magnetite its
is
580°C, which
is
far
below
melting temperature. At a depth of 80 to 100
within the Earth, the temperature
km
high enough that
is
magnetic substances lose their magnetism. The fact that the locations of the magnetic poles vary through time also indicates that buried magnetite
is
not the source of
and Declination
Notice in Figure 11-27 that the lines of magnetic force around the Earth parallel the Earth's surface only near the equator. As the lines of force approach the poles, they are oriented at increasingly large angles with respect to the surface, and the strength of the magnetic
the Earth's magnetic field. Instead, the magnetic field
Inclination
of the Magnetic Field
is
generated within the
Earth by electrical currents (an electrical current
is
a
flow of electrons that always generates a magnetic field). These currents are generated by the different rotation
at the equator and strongest compass needle mounted so can rotate both horizontally and vertically not
field increases;
it is
weakest
at the poles. Accordingly, a
that
it
only points north, but
is
also inclined with respect to the
speeds of the outer core and mantle.
Earth's surface, except at the magnetic equator.
conducting liquid outer core rotates
gree of inclination depends
The electrically more slowly than
and this differential rotation around the Earth's axis generates the electrical currents that create the magnetic field. the surrounding mantle,
on the
along a line of magnetic force
is
field
called magnetic inclination.
The
de-
(Fig. 11-28).
This deviation o f the magnetic zontal
The
needle's location
from the
hori-
To compensate
Earth's Magnetic Field
for
307
"* FIGURE lines of
this,
11-26 Iron filings align themselves along the magnetic force radiating from a magnet.
compasses used
small weight
in the
Northern Hemisphere have a
on the south end of
erty of the Earth's magnetic field
the needle. This propis
important
in deter-
mining the ancient geographic positions of tectonic plates (see Chapter 13). Another important aspect of the magnetic field is that the magnetic poles, where the lines of force leave and enter the Earth, do not coincide with the geographic
— FIGURE inclination.
11-28
The
Magnetic
strength of the
magnetic field changes uniformly from the magnetic equator to the magnetic poles. This change in strength causes a dip needle to parallel the Earth's surface only at
the magnetic equator, whereas
its
inclination with respect to the
surface increases to 90° at the
magnetic poles.
308
Chapter
1 1
The
Interior of the Earth
"^ FIGURE lines
11-27 The magnetic field of the Earth has of force just like those of a bar magnet.
(rotational) poles.
tween the two netic field
At present, an IIV2 angle
(Fig. 11-29). Studies
show
exists be-
of the Earth's mag-
that the locations of the magnetic poles
vary slightly over time, but they
still
correspond closely
on the average with the locations of the geographic poles. A compass points to the north magnetic pole in the Canadian Arctic islands, some 1,290 km away from
Magnetic
Geographic
north pole
north pole
the geographic pole (true north); only along the line
shown
in
Figure 11-29 will a compass needle point to
both the magnetic and geographic north poles. From any other location, an angle called magnemrdeclination exis t s be t we e n
tinesdrawn fromThe iuinpa ss pusi i iorr to
the magnetic pole~aTRLthe~geographic pole (Fig. 11-29).
Magnetic declination must be taken into account during surveying and navigation because, for most places on Earth, compass needles point east or west of true north.
Magnetic Anomalies Variations in the strength of the Earth's magnetic
field
occur on both regional and local scales. Such variations from the normal are called magnetic anomalies. Regional variations are probably related to the complexities
of convection within the outer core where the mag-
netic
field
is
generated.
accounted for by
Local
variations
can be rock
lateral or vertical variations in
types within the crust.
An
instrument called a magnetometer can detect
slight variations in the strength of the
magnetic
""•"
FIGURE
11-29
Magnetic declination.
A
compass
needle points to the magnetic north pole rather than the
geographic pole (true north). The angle formed by the lines from the compass position to the two poles is the magnetic declination.
field,
and deviations from the normal are characterized
as
positive or negative. For example, a positive magnetic
anomaly
exists in areas
iron-bearing
where the rocks contain more
minerals than elsewhere.
In
the
Great
underlain by basalt lava flows, such as the Columbia
River basalts of the northwestern United States
(Fig.
Lakes region of the United States and Canada, huge iron ore deposits containing hematite and magnetite add
4-25), possess positive magnetic anomalies, whereas an
magnetism to that of the Earth's magnetic field; the result is a positive magnetic anomaly (Fig. 11-30). Positive magnetic anomalies also exist where extensive ba-
negative magnetic anomaly (Fig. 11-30).
their
saltic
volcanism has occurred because basalt contains
appreciable quantities of iron-bearing minerals. Areas
Positive
magnetic anomaly
Negative
magnetic anomaly
adjacent area underlain by sedimentary rocks shows a Geologists have used magnetometers for magnetic sur-
veys for decades because iron-bearing rocks can be easily detected by a positive magnetic
anomaly even
if
they are
deeply buried. In addition, magnetometers can defect a
Positive
magnetic
anomaly t
"^ FIGURE
11-32 Magnetic reversals recorded in a shown diagrammatically by red arrows, whereas the record of normal polarity events is shown by black arrows. The lava flows containing a record of such magnetic-polarity events can be radiometrically dated so that a magnetic time scale as in Figure 11-33 can be constructed. succession of lava flows are
"""'
FIGURE
salt
dome.
A
11-31
negative magnetic anomaly over a
domes, which show negative magnetic anomalies (Fig. 11-31); these can be detected by gravity surveys as well. variety of buried geologic structures, such as salt
Magnetic Reversals
When
a
magma
cools through the Curie point,
its
iron-
located roughly at the north and south geographic poles.
However, as early
sals occur, the Earth's
themselves with the Earth's magnetic
that the north~arrow
its
direction
and
strength.
As long
subsequently heated above the Curie point, serve that magnetism. However,
if
recording
field,
as the rock it
the rock
is
not
will preis
heated
above the Curie point, the original magnetism is lost, and when the rock subsequently cools, the iron-bearing minerals will align with the current magnetic field.
The iron-bearing minerals of some sedimentary rocks formed on the deep sea floor) are
were discov-
When these magneti c revermagnetic polarity is reversed, so
geologic past (Fig. 11-32).
bearing minerals gain their magnetization and align
both
as 1906, rocks
showed reversed magnetism. Paleomagnetic studies initially conducted on continental lava flows have clearly shown that the Earth's magnetic field has completely reversed itself numerous times during the ered that
on
a
compass would poinFsouth
rather than north.
Rocks that have
a record of
magnetism the same as the
present magnetic field are describedas jiaving larity ,_whe reas
reversed polarity.
norm al po-
magnetism have The ages ofthlTnormal aricTreversed
rocks with
"th e_opposite
polarity events for the past several million years have been
determined by applying absolute dating techniques to con-
sediments are deposited. These rocks also preserve a
and have been used to construct a magThese same patterns of normal and reversed polarity were soon discovered in
record of the Earth's magnetic
the oceanic crust (see Chapter 13).
(especially those that
also oriented parallel to the Earth's magnetic field as the
the time of their
field at
formation. Such information preserved in lava flows and
some sedimentary rocks can be used
to determine the
directions to the Earth's magnetic poles
of the rock
when
it
Paleomagnetism
and the
latitude
was formed.
is
tinental lava flows
netic reversal time scale (Fig 11-33).
The cause of magnetic reversals is not completely known, although they appear to be related to changes in the intensity of the Earth's magnetic indicate that the magnetic field has
simply the remanent magnetism in
during the
last century. If this
field.
Calculations
weakened about
5%
trend continues, there will
when
ancient rocks that records the direction and strength of
be a period during the next few thousand years
the Earth's magnetic field at the time of their formation.
magnetic
Geologists refer to the Earth's present magnetic
After the reversal occurs, the magnetic field will rebuild
normal, that
310
is,
field as
with the north and south magnetic poles
Chapter 11
The
Interior of the Earth
itself
field will
the
be nonexistent and then will reverse.
with opposite polarity.
^ FIGURE
11-33
(a)
Normal
and reversed polarity events the last 66 million years. Rocks in northern Pakistan
(black) for (b)
correlated with the
magnetic-polarity time scale.
XXX =
Volcanic ash
I
xxxxxxxx
I
xxxxxxxx
xxxxxxxx«xxxxxxxx
1 (b)
60'
The
Earth's Magnetic Field
311
^ CHAPTER SUMMARY
12.
The by
1.
2.
The Earth
is
concentrically layered into an iron-rich
13.
of the information about the Earth's interior has been derived from studies of P- and S-waves that travel through the Earth. Laboratory experiments,
magnetic force
The
lines
of magnetic
phenomenon 14.
of magnetic inclination.
Although the magnetic poles are close
to the
comparisons with meteorites, and studies of inclusions in volcanic rocks provide additional
declination exists between lines
drawn from a compass location to the magnetic and geographic
The
Earth's interior
on the
is
subdivided into concentric
basis of changes in seismic
north poles.
wave
15.
Density and elasticity of Earth materials determine the velocity of seismic waves. Seismic waves are refracted when their direction of travel changes. reflection occurs at boundaries across
The behavior
A
magnetometer can detect departures from the normal magnetic field, which can be either positive or negative.
16.
Although the cause of magnetic reversal understood,
which
shadow zones allow
and composition of and to estimate the size and depth of the core and mantle. The Earth's inner core is thought to be composed of iron and nickel, whereas the outer core is probably composed mostly of iron with 10 to 20% sulfur and the Earth's interior
other substances in lesser quantities. Peridotite most likely component of the mantle.
is
the
and granitic in composition, respectively. The boundary between the crust and the mantle is the Mohorovicic
The oceanic and continental
is
not fully
clear that the polarity of the
magnetic field has completely reversed times during the past.
crusts are basaltic
^
many
itself
IMPORTANT TERMS
The geothermal gradient of 25°C/km cannot continue to great depths, otherwise most of the Earth would be molten. The geothermal gradient for the mantle and core is probably about l°C/km. The temperature at the Earth's center
is
estimated to be
6,500°C. 9. Detectable amounts of heat escape at the Earth's surface by heat flow. Most of the Earth's internal
magnetic field magnetic inclination magnetic reversal mantle Mohorovicic
asthenosphere continental crust
core crust
Curie point
normal polarity
geothermal gradient
anomaly and negative)
gravity
(positive
oceanic crust
paleomagnetism
heat flow isostatic
(Moho)
discontinuity
discontinuity
discontinuity.
peridotite
rebound
principle of isostasy
lithosphere
P-wave shadow zone
low-velocity zone
reflection
magnetic anomaly
refraction
(positive
and negative)
reversed polarity
S-wave shadow zone
magnetic declination
REVIEW QUESTIONS
generated by radioactive decay. 10. According to the principle of isostasy, the Earth's crust is floating in equilibrium with the denser
1.
mantle below. Continental crust stands higher than oceanic crust because it is thicker and less dense. 11. Positive and negative gravity anomalies can be
2.
heat
it is
of P- and S-waves within the Earth and
geologists to estimate the density
is
detected where excesses and deficiencies of mass
312
lines of
geographic poles, they do not coincide exactly. For most places on Earth, an angle called magnetic
the presence of P- and S-wave
8.
surrounded by
except at the equator, thus accounting for the
Much
the properties of rocks change.
7.
is
crust.
Wave
6.
The Earth
force are inclined with respect to the Earth's surface,
velocities at discontinuities.
5.
thought to be generated
similar to those of a bar magnet.
layers
4.
is
core with a solid inner core and a liquid outer part, a rocky mantle, and an oceanic crust and continental
information. 3.
Earth's magnetic field
electrical currents in the outer core.
The average
line
occur, respectively. Gravity surveys are useful in
c.
exploration for minerals and hydrocarbons.
gradient.
Chapter 11
The
Interior of the Earth
is
6.75; d.
3 - g/cm
.
1.0;
showing the direction of movement of a small wave front is a: P-wave reflection; seismic discontinuity; b. seismic particle beam; e. wave ray; d
part of a a
5.5; c
2.5.
e
A
density of the Earth
12.0; b
a
3.
When
seismic waves travel through materials having
14. Iron-bearing minerals in a
different properties, their direction of travel changes.
phenomenon
This
4.
is
a.
elasticity; b.
c.
refraction; d.
A major seismic km is the:
wave: energy dissipation; deflection;
6.
reflection.
e.
oceanic
b.
crust-continental crust boundary;
5.
field
discontinuity at a depth of 2,900
core-mantle boundary;
a.
reflected.
lithosphere-asthenosphere boundary.
18.
Why
is
sulfur; b.
d.
potassium;
Which
probably composed mostly iron.
e.
a.
inclusions in volcanic rocks; b.
c.
meteorites; d.
zone;
peridotite;
iron-nickel alloy;
spreading ridges;
the:
Moho;
determine that a discontinuity,
less dense than continental crust; primary source of magma.
Most
of the Earth's internal heat
a.
moving
c.
earthquakes;
e.
meteorite impacts.
plates; b.
is
According to the principle of isostasy: a. more heat escapes from oceanic crust than from continental crust; b. the Earth's crust is floating in equilibrium with the more dense mantle below; c. the Earth's crust behaves both as a liquid and a solid; d. much of the asthenosphere is molten; e. magnetic anomalies result when the crust is loaded by glacial ice. 12. The magnetic field is probably generated by: 11
a.
the
b.
the solar wind;
tilt
of the Earth's rotational axis; c.
electrical currents in the
deformation of the asthenosphere; e. a large deposit of magnetite at the North Pole. 13. Except at the magnetic equator, a compass needle in the Northern Hemisphere points to the magnetic north pole and downward from the horizontal. This outer core; d.
phenomenon
is:
magnetic declination; b. magnetic reflection; c. magnetic reversal; d. magnetic polarity; e. magnetic inclination. a.
it
geologists account for the fact that heat
is
the continental crust is deeply eroded in one area and loaded by widespread, thick sedimentary If
how
will
it
respond
isostatically
at each location?
25.
generated by:
volcanism; radioactive decay;
d.
do
deposits in another,
the
in
about the same through oceanic crust and it should be greater through the latter? 24.
e.
How flow
thinnest at
b.
Moho,
continental crust even though
granitic in composition;
c.
called the
decrease within the Earth? 23.
gabbro.
e.
now
between the crust and the mantle. 21. How do oceanic and continental crust differ composition and thickness? 22. What is the geothermal gradient? Why must
high-velocity
d.
10
is
transition zone.
Oceanic crust is: a 20 to 90 km thick;
probably
exists
Continental crust has an overall composition corresponding closely to that of: a. basalt; b. sandstone; c. granodiorite; d.
is
mantle. What accounts for these discontinuities? 20. Explain the reasoning used by Mohorovicic to
diamonds; S-wave
e.
at the base of the crust
magnetic anomaly; b. geothermal gradient; d. e.
the inner core thought to be
19. Several seismic discontinuities exist within the
of the following provides evidence for the
shadow zone. The seismic discontinuity
shadow zone? composed of
the significance of the S-wave
is is
iron and nickel whereas the outer core composed of iron and sulfur?
of:
nickel;
silica; c.
Curie
magnetic-polarity
magnetic declination. determines the velocity of P- and S-waves? 16. Explain how seismic waves are refracted and
e.
Earth's core
isostasy curve; d.
field; e.
What
What
a.
9.
c.
17.
a.
they cool through the:
point;
inner core-outer core boundary;
The
gain their
negative magnetic anomaly; b.
d.
c
8.
when
magma
align themselves with the magnetic
a.
15.
Moho;
c.
composition of the core?
7.
magnetism and
What
is meant by positive and negative gravity anomalies? Give examples of where each type of anomaly might occur.
What
is the magnetic field, and how is it thought to be generated? 27. Explain the phenomenon of magnetic inclination.
26.
28. Illustrate
how
a vertical succession of ancient lava
flows preserves a record of magnetic reversals.
^
ADDITIONAL READINGS
Anderson, D. L., and A. M. Dziewonski. 1984. Seismic tomography. Scientific American 251, no. 4: 60-68. Bolt, B. A. 1982. Inside the Earth: Evidence from earthquakes. San Francisco: W. H. Freeman and Co. Brown, G. C. 1981. The inaccessible Earth. London: George Allen Unwin. Fowler, C. M. R. 1990. The solid Earth. New York: Cambridge
&
University Press.
Heppenheimer, T. A. 1987. Journey to the center of the Earth. Discover 8, no. 10: 86-93. Jeanloz, R. 1983. The Earth's core. Scientific American 249, no. 3: p.
56-65.
McKenzie, D.
P.
1983. The Earth's mantle. Scientific American
249, no. 3: p. 66-78. Monastersky, R. 1988. Inner space. Science
News
136:
266-268.
Additional Readings
313
CHAPTER
12
THE SEA FLOOR ^ OUTLINE PROLOGUE INTRODUCTION OCEANOGRAPHIC RESEARCH CONTINENTAL MARGINS The Continental
Shelf
"^"Perspective 12-1: Lost Continents
The Continental Slope and Rise Turbidity Currents, Submarine Canyons, and
Submarine Fans
TYPES OF CONTINENTAL MARGINS THE DEEP-OCEAN BASIN Abyssal Plains
Oceanic Trenches Oceanic Ridges Fractures in the Sea Floor
Seamounts, Guyots, and Aseismic Ridges "*r Perspective 12-2:
Maurice Ewing and His
Investigation of the Atlantic
Ocean
DEEP-SEA SEDIMENTATION REEFS
COMPOSITION OF THE OCEANIC CRUST RESOURCES FROM THE SEA CHAPTER SUMMARY
Pillow lava on the floor of the Pacific Ocean near the Galapagos Islands.
PROLOGUE |^gJ)lV~||
j
n 1979^ researchers aboard the
submersible Alvin descended about
2,500
m
to the
Galapagos Rift
in the eastern Pacific
Ocean basin and observed hydrothermal vents on sea floor (Fig. 12-1).
the
Such vents occur near spreading
where seawater seeps down into the oceanic and fissures, is heated by the hot rocks, and then rises and is discharged onto the sea floor as hot springs. During the 1960s, hot metal-rich brines apparently derived from hydrothermal vents ridges
crust through cracks
were detected and sampled in the Red Sea. These dense brines were concentrated in pools along the axis of the sea; beneath them thick deposits of metal-rich sediments were found. During the early 1970s, researchers observed hydrothermal vents on the Mid-Atlantic Ridge about 2,900 km east of Miami, Florida, and in 1978 moundlike mineral deposits were sampled from the East Pacific Rise just south of the Gulf of California.
When the submersible Alvin descended to the Galapagos Rift in 1979, mounds of metal-rich sediments were observed. Near these mounds the researchers saw what they
called black
smokers (chimneylike vents)
discharging plumes of hot, black water (Fig. 12-1). Since
1979
similar vents have been observed at or near
spreading ridges in several other areas.
"^ FIGURE 12-1 The submersible Alvin sheds light on hydrothermal vents at the Galapagos Rift, a branch of the East Pacific Rise. Seawater seeps down through the oceanic crust, becomes heated, and then rises and builds chimneys on the sea floor. Communities of organisms, including tubeworms, giant clams, crabs, and several types of fish, live
Submarine hydrothermal vents are interesting for Near the vents live communities of
several reasons.
organisms, including bacteria, crabs, mussels,
starfish,
and tubeworms, many of which had never been seen before (Fig. 12-1). In most biological communities,
near the vents.
"**'
FIGURE
12-2
Formation of a black smoker. The is simply heated water saturated
plume of "black smoke"
with dissolved minerals. Precipitation of anhydrite (CaS0 4 ) and sulfides of iron, copper, and zinc forms the chimney.
months
When
photosynthesizing organisms form the base of the
1979 was
food chain and provide nutrients for the herbivores and carnivores. In vent communities, however, no
activity ceases, the vents eventually collapse
sunlight
is
available for photosynthesis,
inactive six
and the base
The economic is
chemosynthesis; they oxidize sulfur compounds from
Deep of
the
and the nutrients
for other
own
members of
tons of metals, including iron, copper, zinc, the
gold. These deposits are fully as large as the
mined on land.
sulfide deposits
then reacts with the crust and
throughout geologic time.
transformed into a
metal-bearing solution. As the hot solution discharges onto the sea floor, iron, copper,
and zinc
sulfides
it
rises
and
and other minerals that
more common than it is at present because the Earth possessed more heat, and this activity is believed to have been responsible for the formation of the atmosphere and surface water. As we noted in previous chapters, volcanoes emit a variety of gases, the most abundant of
water vapor. The atmosphere and surface wa-
thought to have derived within the Earth and been emitted at the surface by volcanoes in a process called outgassing* (Fig. 12-3). As the Earth cooled, waters are
vapor began condensing and fell as rain, which accumulated to form the surface waters. Geologic evidence clearly indicates that an extensive ocean was present more than 3.5 billion years ago. During most of historic time, people knew little of the oceans and, until fairly recently, believed that the sea floor was flat and featureless. Although the ancient Greeks had determined the size of the Earth rather acter
*The alternate hypothesis— that much of the Earth's surface water was derived from comets — is not yet widely accepted.
316
Chapter 12
The Sea Floor
and major
silver,
of these sulfide
Troodos Massif on have formed on the sea floor
Cyprus, are believed to by hydrothermal vent activity.
Hydrothermal vent
sulfide deposits
None
have formed
are currently being
mined, but the technology to exploit them determined that
exists. In fact,
and Sudanese governments have
it is
feasible to recover such deposits so.
in
Although the oceans are distinct enough to be designated by separate names such as Pacific, Atlantic, and Indian, a single interconnected body of salt water covers more than 70% of the Earth's surface. During its very earliest history, the Earth was probably hot, airless, and lacking in surface water. Volcanic activity, however, was
is
Many
II
million
land, such as the
from the Red Sea and are making plans to do
INTRODUCTION
which
now on
deposits
the Saudi Arabian
cools, precipitating
accumulate to form a chimneylike vent (Fig. 12-2). These vents are ephemeral, however; one observed
^
in the Atlantis
Red Sea contain an estimated 100
food chain. Another interesting aspect of these submarine hydrothermal vents is their economic potential. When seawater circulates downward through the oceanic crust, it is heated to as much as 400°C. The hot water is
and are
potential of hydrothermal vent
tremendous. The deposits
deposits
nutrients
their
incorporated into a moundlike mineral deposit.
of the food chain consists of bacteria that practice the hot vent waters, thus providing their
later.
curately,
Western Europeans were not aware of the vast-
ness of the oceans until the fifteenth and sixteenth cen-
when
turies
various explorers sought
to the Indies.
August
When
new
trade routes
Christopher Columbus set
sail
on
an attempt to find a route to the Indies, he greatly underestimated the width of the Atlantic
3,
1492,
in
Ocean. Contrary to popular
belief,
Columbus was
not attempting to demonstrate that the Earth sphere
is
a
— the Earth's spherical shape was well accepted by
The controversy was over the Earth's circumference and what was the shortest route to China. During these and subsequent voyages, Europeans sailed to the Americas, the Pacific Ocean, Australia, New Zealand, the Hawaiian Islands, and many other islands previously unthen.
known
to them.
Such voyages of discovery added considerably to our knowledge of the oceans, but truly scientific investigations did not begin until the late 1700s. Great Britain was the dominant maritime power, and in order to maintain that dominance, the British sought to increase their knowledge of the oceans. The earliest British scientific voyages were led by Captain James Cook in 1768, 1772, and 1777. In 1872, the converted British warship H.M.S. Challenger began a four-year voyage, during which seawater was sampled and analyzed, oceanic depths were determined at nearly 500 locations, rock and sediment samples were recovered from the sea floor, and more than 4,000 new marine species were classified.
Escapes
Hydrogen Water
h Nitrogen N,
To atmosphere
Carbon dioxide
Erosional debris
—
FIGURE 12-4 The Glomar Challenger 122-m long oceanographic research vessel.
a larger,
is
a 10,500-ton,
more advanced research vessel, the JOIDES* made its first voyage in 1985.
Resolution,
In addition to surface vessels, submersibles, both re-
"
r
motely controlled and manned by
FIGURE
Gases derived from within the Earth by outgassing formed the early atmosphere and surface waters. 12-3
Continuing exploration of the oceans revealed that the sea floor
is
not
flat
and
featureless as formerly be-
lieved. Indeed, scientists discovered that the sea floor
possesses varied topography including oceanic trenches,
submarine ridges, broad plateaus, hills, and vast plains. Some people have suggested that some of these features are remnants of the mythical lost continent of Atlantis (see Perspective 12-1).
Drilling Project,
scientists,
have been
to the research arsenal of oceanographers. In
1985, for example, the Argo, towed by a surface vessel and equipped with sonar and television systems, provided the first views of the British ocean liner R.M.S. Titanic since it sank in 1912. The U.S. Geological Survey is using a towed device to map the sea floor (Fig. 12-5). The system uses sonar to produce images resembling aerial photographs. Researchers aboard the submersible Alvin have observed submarine hydrothermal vents (see the Prologue) and have explored parts of the oceanic ridge system.
The
measurements of the oceanic depths were a weighted line to the sea floor and measuring the length of the line. Now, however, an instrument called an echo sounder is used. Sound waves from a ship are reflected from the sea floor and detected by instruments on the ship, thus yielding a continuous profile of the sea floor. Depth is determined by knowing the velocity of sound waves in water and the time it takes for the waves to reach the sea floor and return to first
made by lowering
^ OCEANOGRAPHIC RESEARCH The Deep Sea
added
an international program
sponsored by several oceanographic institutions and funded by the National Science Foundation, began in 1968. Its first research vessel, the Glomar Challenger, was capable of drilling in water more than 6,000 m deep (Fig. 12-4). It was equipped to drill into and recover long cores of sea-floor sediment and the oceanic crust. During the next 15 years, the Glomar Challenger drilled more than 1,000 holes in the sea floor. The Deep Sea Drilling Project came to an end in 1983 when the Glomar Challenger was retired. However, an international project, the Ocean Drilling Program, continued where the Deep Sea Drilling Project left off, and
the ship.
Seismic profiling
more
similar to echo sounding but even waves are generated at an energy
is
useful. Strong
source, the waves penetrate the layers beneath the sea floor,
and some of the energy
*JOIDES is an acronym Deep Earth Sampling.
for Joint
is
reflected
from various
Oceanographic Institutions for
Oceanographic Research
317
"^ FIGURE
12-6 Diagram showing how seismic profiling used to detect buried layers at sea. Some of the energy generated at the energy source is reflected from various horizons back to the surface where it is detected by hydrophones. is
"^ FIGURE 12-5 The sonar system used by the U.S. Geological Survey for sea-floor mapping.
acquired since World
War
II.
This statement
with respect to the sea
larly true
floor,
is
particu-
because only in
recent decades has instrumentation been available to
The data
geologic horizons back to the surface (Fig. 12-6). Recall
study this largely hidden domain.
from Chapter 11 that seismic waves are reflected from boundaries where the properties of Earth materials
not only important in their own right but also have provided much of the evidence that supports plate tec-
change. Seismic profiling has been particularly useful in mapping the structure of the oceanic crust beneath sea-
tonic theory (see Chapter 13).
^ CONTINENTAL MARGINS
floor sediments.
Oceanographers also use gravity surveys to detect domes beneath the continental margins are recognized by negative gravity anomalies, and oceanic trenches also exhibit negative gravity anomalies. Magnetic surveys have also provided
bounded by continental margins, zones separating the part of a continent above sea level
gravity anomalies. For example, salt
important information regarding the sea floor
All continents are
from the deep-sea
-^ FIGURE
12-7
A
generalized
showing
features of the continental margins.
The
vertical
The
continental margin consists
clined continental slope, and, in
(see
the continental margin
is
'»
_
in-
cases, a deeper,
Seaward of
the deep-ocean basin. Thus,
the continental margin extends to increasingly greater
depths until
it
merges with the deep-sea
floor.
Continental margin
Continental margin
*
some
gently sloping continental rise (Fig. 12-7).
\
Continental shelf
Continental shelf
dimensions of the
/
features in this profile are greatly
Sea
level
exaggerated because the vertical and horizontal scales
floor.
of a gently sloping continental shelf, a more steeply
Chapter 13). Although scientific investigations of the oceans have been yielding important information for more than two hundred years, much of our current knowledge has been
profile of the sea floor
collected are
Oceanic ridge
differ.
Oceanic trench Continental slope Continental slope i
i
i
i
i
I
500
I
I
i
i
i
1,000
i
i
I
i
I
1,500
i
I
I
i
i
i
2,000
i
I
I
i
2,500
i
I
I
3,000
Distance (km)
318
Chapter 12
The Sea Floor
f'ni^^r^^^^^rT 3,500
4,000
4,500
5,000
-^ FIGURE
12-8
The
transition
from continental to oceanic crust, and hence the geological margin of a continent, occurs beneath the
continental slope.
Most people
perceive continents as land areas out-
by sea level. However, the true geologic margin of a continent— that is, where continental crust changes to oceanic crust— is below sea level, generally somewhere lined
beneath the continental slope
(Fig. 12-8).
Accordingly,
marginal parts of continents are submerged.
The Continental Shelf Between the shoreline and continental slope of all continents lies the continental shelf, an area where the sea floor slopes very gently in a seaward direction. Its slope is much less than 1° (Fig. 12-7); it averages about 2 m/km, or 0.1°.
The outer edge of
erally taken to
the continental shelf
is
gen-
correspond to the point at which the
in-
clination of the sea floor increases rather abruptly to several degrees; this shelf-slope
depth of about 135
m
break occurs at an average
(Fig. 12-7).
Continental shelves
eral
hundred kilometers across
along the west coast
it is
in
some
extend well up onto the continental
but some of them shelf.
associated with streams
more As
on
They are discussed
land.
a
consequence of lower sea level during the Pleismuch of the sediment on continental
shelves accumulated in stream channels
much
as sev-
of these
fully in the following section.
meters to more than 1,000 km. For example, the shelf as
Some
canyons lie offshore from the mouths of large streams. At times during the Pleistocene Epoch (1,600,000 to 10,000 years ago), sea level was more than 100 m lower than at present, so much of the continental shelves were above sea level. Streams flowed across these exposed shelves and eroded deep canyons that were subsequently flooded when sea level rose. However, most submarine canyons extend to depths far greater than can be explained by stream erosion during periods of lower sea level. Furthermore, many submarine canyons are not
tocene Epoch,
is
whereas
Deep, steep-sided submarine canyons are most characteristic of the continental slope,
vary considerably in width, ranging from a few tens of
along the east coast of North America
places,
only a few kilometers wide.
(Fig. 12-9). In fact, in areas
and floodplains
such as northern Europe and
-^ FIGURE lower sea
At times of during the
12-9
level
Pleistocene Epoch, large parts of the
continental shelves were exposed. Accordingly, much of the sediment deposited during these times accumulated in various continental
environments such as stream channels and lakes.
Continental Margins
319
Perspective 12-1
LOST CONTINENTS Most people have heard of
the mythical lost continent
True Continent
of Atlantis, but few are aware of the source of the Atlantis legend or the evidence that
former existence of
this continent.
cited for the
is
Only two known
sources of the Atlantis legend exist, both written in
about 350
B.C.
by the Greek philosopher Plato. In two
of his philosophical dialogues, the Timaeus and the Critias, Plato tells of Atlantis, a large island continent
according to him, was located
that,
Ocean west of the call the Strait
in the Atlantic
of Gibraltar (Fig.
now
which we
Pillars of Hercules,
Plato also wrote
1).
that following the conquest of Atlantis by Athens, the
continent disappeared: .
.
day and night came when
disappeared beneath the sea.
now
the sea there has
which the
island
.
And
.
.
Atlantis
.
.
it
is
produced as
by the it
mud
one assumes that the destruction of Atlantis was one conjured up by Plato to a philosophical point, it
was supposed
Critias,
who
he nevertheless lived long
to have occurred.
turn told
in
it
to Plato.
two types of evidence
claim that Atlantis did indeed exist.
supposed cultural Atlantic
Ocean
similarities
to support their First,
on opposite
W. Ramage,
ed., Atlantis: Fact
or
Fiction? (Bloomington, Ind.: Indiana University Press, 1978), p. 13.
320
Chapter 12
The Sea Floor
the Azores,
Bermuda, the Bahamas, and the
Mid-Atlantic Ridge are alleged to be remnants of Atlantis. If a continent
Atlantic, however,
it
had actually sunk
in the
could be easily detected by a
gravity survey. Recall that continental crust has a
and a lower density than oceanic were actually present beneath the Atlantic Ocean, there would be a huge negative gravity anomaly, but no such anomaly has granitic composition
Thus,
if
a continent
been detected. Furthermore, the crust beneath the
Secondly, supporters of the legend assert that remnants
in E.
No "mud
Atlantic has been drilled in
and those of Central and South America. They contend that these similarities are due to cultural diffusion from the highly developed civilization of Atlantis. According to archaeologists, however, few similarities actually exist, and those that do can be explained as the independent development of analogous features by different cultures.
Quoted
call
shallows" exist in the Atlantic as Plato claimed, but
sides of the
basin, such as the similarity in shape of
the Timaeus.
we now
they point to
the pyramids of Egypt
*From
of the sunken continent can be found.
crust.
Present-day proponents of the Atlantis legend generally cite
According to Plato, Atlantis was a large
1
the Strait of Gibraltar.
sank.*
According to Plato, Solon, an Athenian who lived about 200 years before Plato, heard the story from Egyptian priests who claimed the event had occurred 9,000 years before their time. Solon told the story to his grandson, after
"^ FIGURE
continent west of the Pillars of Hercules, which
shallows
a real event, rather than
make
True Continent
.
for this reason even
become unnavigable and
unsearchable, blocked as
If
and floods and one
there were violent earthquakes
.
terrible
many
samples recovered indicate that
same
places,
its
and
all
composition
the
is
the
as that of oceanic crust elsewhere.
In short, there
is
some may be based on a Nevertheless,
no geological evidence
for Atlantis.
archaeologists think that the legend real event.
About 1390
B.C.,
a huge
volcanic eruption destroyed the island of Thera in the
Mediterranean Sea, which was an important center of
Greek civilization. The eruption was one of the most violent during historic time, and much of the island disappeared when it subsided to form a caldera
early
(Fig. 2).
Most
of the island's inhabitants escaped
(Fig. 3),
but the eruption probably contributed to the demise of
km p^j Pre-collapse island
y
—
.]
I
Collapsed material
Possible pre-collapse
shape
ol island
?* FIGURE
2 The island of Thera was destroyed by a huge eruption about 1390 b.c. Ash was carried more than 950 km to the southeast, and tsunami probably devastated nearby coastal areas. The inset shows the possible profile of the island before the eruption and its shape immediately after the caldera
formed.
culture on Crete. At least 10 cm of ash on parts of Crete, and the coastal areas of the island were probably devastated by tsunami. It is possible that Plato used an account of the destruction the
Minoan
fell
of Thera, but fictionalized
it
for his
own
purposes,
thereby giving rise to the Atlantis legend.
"*»" FIGURE 3 (right) An artist's rendition of the volcanic eruption on Thera in about 1390 b.c. that destroyed most of inhabitants escaped the island's island. Most of the the
devastation.
Continental Margins
321
Shelf-slope
break
Submarine fan
"^r_
FIGURE
12-11
Submarine fans formed by the down submarine canyons by
deposition of sediments carried
Much
turbidity currents.
of the continental rise
is
composed
of overlapping submarine fans.
monly descend
directly into
continental rise
is
The
absent
shelf-slope break
an oceanic trench, and a
(Fig. 12-7). is
a very important feature in
terms of sedimentation. Landward from the break, the
"^ FIGURE 12-10 {a) Turbidity currents flow downslope along the sea floor (or lake bottom) because of their density. (b) Graded bedding formed by deposition from a turbidity current.
parts of
North America,
glaciers
extended onto the ex-
posed shelves and deposited gravel, sand, and mud. Since the Pleistocene Epoch, sea level has risen submerging the shelf sediments, which are now being reworked by marine processes. That these sediments were, deposited on land
is
human mammoths and mastodons
indicated by evidence of
settlements and fossils of (extinct
in fact,
members of the elephant
family)
and other land-
by waves and tidal currents. Seaward of bottom sediments are completely unaffected by surface processes, and their transport onto the slope and rise is controlled by gravity. The continental slope and rise system is the area where most of the sediment derived from continents is eventually deposited. shelf
is
affected
the break, the
Much
of this sediment
rents through
Canyons, and Submarine Fans Turbidity currents are sediment-water mixtures denser
than normal seawater that flow downslope to the deep-
An
flows onto the relatively
deposited
The seaward margin of
the continental shelf
by the shelf-slope break
(at
marked an average depth of 135 m) is
relatively steep continental slope begins (Fig.
12-7). Continental slopes average about 4°, but range
from
1° to 25°. In
many
places, especially
around the
margins of the Atlantic, the continental slope merges with the more gently sloping continental rise. In other places, such as
322
around the
Chapter 12
Pacific
The Sea Floor
flat
individual turbidity current sea floor
where
it
slows and
begins depositing sediment; the coarsest particles are
The Continental Slope and Rise
where the
transported by turbidity cur-
Turbidity Currents, Submarine
sea floor (Fig. 12-10).
dwelling animals.
is
submarine canyons.
Ocean, slopes com-
cles,
first,
followed by progressively smaller parti-
thus forming graded bedding (Fig. 12-10). These
deposits accumulate as a series of overlapping submarine fans,
which constitute a large part of the continental At their seaward margins, these fans
rise (Fig. 12-11).
grade into the deposits of the deep-ocean basins.
No
one has ever observed a turbidity current
progress, so for
many
years there
was considerable
in
de-
bate about their existence. In 1971, however, abnor-
mally turbid water was sampled just above the sea floor in the
North
perhaps play some role
Atlantic, indicating that a turbidity current
in their origin.
bidity currents periodically
and are
had occurred recently. Furthermore, sea-floor samples from many areas show a succession of graded beds and the remains of shallow-water organisms that were ap-
now
Furthermore, tur-
move through
these canyons
thought to be the primary agent responsi-
ble for their erosion.
parently displaced into deeper water.
» TYPES OF CONTINENTAL MARGINS
Perhaps the most compelling evidence for the existence of turbidity currents
is
the pattern of trans-Atlantic
Newfoundland on it was asoccurred on that date
cable breaks that occurred south of
November sumed
18,
1929
(Fig.
Two
12-12). Initially,
that an earthquake that
had ruptured several trans-Atlantic telephone and telegraph cables. However, while the breaks on the continental shelf near the epicenter occurred
when
The broke was known, so
which each cable
in succession.
oceanic lithosphere
was
continental margin logically
It
apparently
moved
at
when
it
a simple
is
tion of land-derived sediments. tal
margins are on the
(Fig.
fully understood. It is known that move through submarine canyons and
12-13b).
narrow, and
activity of the conti-
These passive continen-
edge of a continental plate
They possess broad continental shelves and rise; vast, flat abyssal plains
a continental slope
are
commonly
present adjacent to the rises (Fig. 12-
13b). Furthermore, passive continental margins lack the
100
03:03
trailing
and
Southeast
Time intervals between quake and cable breaks
is
The continenwas stretched, thinned, and fractured as rifting proceeded. As plate separation occurred, the newly formed continental margins became the sites of deposi-
• Breaks due to • Breaks due to
-
andesitic volca-
crust
tal
Northwest
5,000
characterized by seismicity, a geo-
young mountain range, and
the rifting of the supercontinent Pangaea.
reached
Breaks due
a
considerably from their western margins. In the east,
fer
not
00:59"
is
the continental margins developed as a consequence of
However, many have no such association, and
strong currents
is
(Fig. 12-13a). The west good example. Here, the
subducted
The configuration and geologic
yons can be traced across the shelf to associated streams their origin
is
nental margins of eastern North and South America dif-
As mentioned previously, submarine canyons occur on the continental shelves, but they are best developed on continental slopes (Fig. 12-11). Some submarine canland.
margin
Chile Trench.
the continental rise.
on
active continental
the continental slope descends directly into the Peru-
about 80 km/hr on the continen-
but slowed to about 27 km/hr
An
nism. Additionally, the continental shelf
matter to calculate the velocity of the turbidity current. tal slope,
active.
coast of South America
the earth-
precise time at it
and
develops at the leading edge of a continental plate where
quake struck, cables farther seaward were broken later and in succession. The last cable to break was 720 km from the source of the earthquake, and it did not snap until 13 hours after the first break occurred (Fig. 12-12). In 1949, geologists realized that the earthquake had generated a turbidity current that moved downslope, breaking the cables
types of continental margins are generally recog-
nized, passive
to turbidity current
shock, slumps turbidity current
'
Continent
Continental shelf
Continental
slope
Oceanic trench
Upper mantle
(a)
Continent
Continental shelf
Abyssal plain
(b)
"•'
FIGURE
12-13
Diagrammatic views of
passive continental margin.
324
Chapter 12
The Sea Floor
(a)
an active continental margin and
(b) a
^ Oceanic ridge system
Rift
| Abyssal
Oceanic trench
"^ FIGURE
plain
12-14
The
valley
distribution of oceanic trenches, abyssal plains,
and the
oceanic ridge system.
(Fig.
the temperature is generally just above 0°C, and the pressure varies from 200 to more than 1,000 atmospheres depending on depth. Submersibles have carried scientists to the greatest oceanic depths, so some of
12-13). Active continental margins obviously lack a
the sea floor has been observed directly. Nevertheless,
continental rise because the slope descends directly into
much
intense seismic
and volcanic
activity characteristic of ac-
margins.
tive continental
Active and passive continental margins share features, but in other respects they differ
markedly
some
an oceanic trench. Just as on passive continental margins, sediment is transported down the slope by turbidity currents, but it simply fills the trench rather than
forming a
rise.
The proximity of
tinent also explains
why
the trench to the con-
the continental shelf
is
so nar-
life exists,
of the deep-ocean basin has been studied only by echo sounding, seismic profiling, and remote devices that have descended in excess of 11,000 m. Although oceanographers know considerably more about the deepocean basins than they did even a few years ago, many questions remain unanswered.
row. In contrast, the continental shelf of a passive continental
margin
is
much wider because
land-derived
sedimentary deposits build outward into the ocean.
^ THE DEEP-OCEAN BASIN Considering that the oceans are an average 3,865
Abyssal Plains Beyond the continental
rises of passive continental
gins are abyssal plains,
flat
of the sea floor. In
m deep,
most of the sea floor lies far below the depth of sunlight penetration, which is rarely more than 100 m. Accordingly, most of the sea floor is completely dark, no plant
some
flattest, flat
osition
areas they are interrupted by
km, but in general they are the most featureless areas on Earth (Fig. 12-14).
peaks rising more than
The
mar-
surfaces covering vast areas
topography
is
1
a consequence of sediment dep-
on the rugged topography of the oceanic
The Deep-Ocean Basin
crust.
325
60
Miles
"*** FIGURE 12-15 Seismic profile showing the burial of rugged sea-floor topography by sediments of the Northern Madeira Abyssal Plain.
Where sediment accumulates rugged sea floor
ment
in sufficient quantities, the
buried beneath thick layers of sedi-
is
Ocean basin
abyssal plains are covered with fine-grained sediment
derived mostly from the continents and deposited by
Some
turbidity currents.
of this sediment
meaning that
it
is
character-
was deposited
far
from
up to 25° sites
12-13). Oceanic trenches are also the
(Fig.
of the greatest oceanic depths; a depth of more than
11,000 m has been recorded in the Challenger Deep of Marianas Trench. Oceanic trenches show anomalously low heat flow
the
the land by the settling of fine particles suspended in
compared
seawater. Abyssal plains are invariably found adjacent
pears that the crust here
to the continental rises,
which are composed mostly of
overlapping submarine fans that
owe
their origin to dep-
Along active continental margins, sediments derived from the shelf and slope are trapped in an oceanic trench, and abyssal osition by turbidity currents (Fig. 12-11).
plains
fail
common Pacific
Pacific
of oce-
anic trenches, the continental slope descends at angles of
(Fig. 12-15).
Seismic profiles and sea-floor samples reveal that the
ized as pelagic,
common around the margins of the (Fig. 12-14). On the landward side
they are
to develop. Accordingly, abyssal plains are
in the Atlantic
Ocean basin
Ocean
basin, but rare in the
to the rest of the oceanic crust; thus, is
it
ap-
cooler and slightly denser
than elsewhere. Furthermore, gravity surveys reveal that trenches
show
a
huge negative gravity anomaly, indicatis held down and is not in isostatic
ing that the crust
equilibrium.
Seismic activity also occurs at or near
trenches. In fact, trenches are characterized by Benioff
zones in which earthquake foci become progressively deeper in a landward direction
(Fig. 10-8).
Most
of the
Earth's intermediate and deep earthquakes occur in such
(Fig. 12-14).
zones. Finally, oceanic trenches are associated with vol-
canoes, either as an arcuate chain of volcanic islands
Oceanic Trenches
(island arc) or as a chain of volcanoes
Although oceanic trenches constitute a small percentage
arc) adjacent to a trench
of the sea floor, they are very important, for
as in western South
it is
consumed by subduction Oceanic trenches are long, narrow
here
that lithospheric plates are
(see
Chapter
fea-
13).
tures* restricted to active continental margins; thus,
326
Chapter 12
The Sea Floor
km
long,
America
(Fig.
12-13).
Oceanic Ridges
A feature called "The Peru-Chile Trench west of South America is 5,900 but only 100 km wide. It is more than 8,000 m deep.
on land (volcanic
along the margin of a continent
the Atlantic
tury
when
the Telegraph Plateau
Ocean basin during
the
first
was discovered
in
the late nineteenth cen-
submarine cable was
laid
between
North America and Europe. Following the 1925-1927 voyage of the German research vessel Meteor, scientists proposed that this plateau was actually a continuous feature extending the length of the Atlantic Ocean basin (see Perspective 12-2). Subsequent investigations revealed that this proposal this feature the
was
correct,
Mid-Atlantic Ridge
and we now
(Fig.
call
rises
about 2.5
is more than 2,000 km wide km above the sea floor adjacent to
terminate where they are offset along major fractures oriented
more or
less at right angles to ridge
much
submarine 65,000 km long. The oceanic ridge system runs from the Arctic Ocean through the middle of the Atlantic, curves around South Africa, and passes into the Indian Ocean, continuing
mountainous topography
from there into the
larger system of
at least
Pacific
Ocean basin
(Fig.
12-14).
This oceanic ridge system's length surpasses that of the
mountain range on land. However, the latter composed of granitic and metamorphic rocks and sedimentary rocks that have been folded and fractured by compressional forces. The oceanic ridges, on the other hand, are composed of volcanic rocks (mostly basalt) and have features produced by tenlargest
ranges are typically
sional forces.
ologists are convinced that
some geologic
Where
these fractures offset oceanic ridges, they are
characterized by shallow seismic activity only in the area
between the displaced ridge segments
earthquakes, basaltic volcanism, and high heat flow. Direct observation of the ridges and their
rift
valleys
began in 1974. As a part of Project FAMOUS (FrenchAmerican Mid-Ocean Undersea Study), submersible craft descended into the rift of the Mid-Atlantic Ridge,
and more recent dives have investigated other rifts. Although no active volcanism was observed, the researchers did see pillow lavas (Fig. 4-14), lava tubes, and sheet lava flows, some of which appear to have formed very recently. In addition, hydrothermal vents such as black smokers have been observed (see the Prologue).
Profile across the
well-developed central
Continental Slope Rise
Fur-
adjacent to them, the offset segments yield vertical relief
on the sea floor. For example, nearly vertical escarpments 3 or 4 km high develop, as illustrated in Figure 12-17. We will have more to say about such fractures, called transform faults, in Chapter 13.
Seamounts, Guyots, and Aseismic Ridges
large
Chapter 13); ridges are characterized by shallow-focus
(Fig. 12-17).
thermore, because ridges are higher than the sea floor
Rise lack such a feature. These rifts are commonly one to two kilometers deep and several kilometers wide. Such rifts open as sea-floor spreading occurs (discussed in
12-16
on
sion of such fractures into continents.
plain, except for the abyssal plains,
its
ge-
the continents can best be accounted for by the exten-
As noted
FIGURE
Many
features
they are buried beneath sea-floor sediments.
forces (Fig. 12-16), although portions of the East Pacific
Ridge with
(Fig.
kilometers, although they are difficult to trace where
Running along the crests of some ridges is a rift that appears to have opened up in response to tensional
"**
axes
it.
part of a
It is, in fact,
Oceanic ridges are not continuous features winding without interruption around the globe. They abruptly
12-17). Such large-scale fractures run for hundreds of
12-14).
The Mid-Atlantic Ridge and
Fractures in the Sea Floor
previously, the sea floor
underlain by rugged topography
number of volcanic
is
not a
flat,
featureless
and even these are
(Fig.
12-15). In fact, a
seamounts, and guyots
hills,
above the sea floor. Such features are present in all ocean basins, but are particularly abundant in the Pacific. All are of volcanic origin and differ from one another mostly in size. Seamounts rise more than one kilometer rise
above the sea
floor;
if
they are
flat
guyots rather than seamounts
topped, they are called
(Fig. 12-18).
volcanoes that originally extended above sea
Guyots are level.
How-
upon which they were situated continued to grow, they were carried away from a spreading ridge, and the oceanic crust cooled and descended to greater oceanic depths. Thus, what was once an island slowly sank beneath the sea, where it was eroded by ever, as the plate
waves, giving
North Atlantic Ocean showing
it
the typical flat-topped appearance.
the Mid-Atlantic
rift.
Shelf
Bermuda
Mid-Atlantic Ridge
Is.
1
1
1,000
itmm+Mmm
UMte
1.500
The Deep-Ocean Basin
327
"^ FIGURE
12-17
Fractures in the sea floor of the Atlantic
line indicates the crest
of the Mid-Atlantic Ridge.
The
inset
is
basin. The dark diagrammatic view of a
Ocean a
fracture offsetting a ridge. Earthquakes occur only in the segments between offset ridge crests.
Other volcanic features are also known to exist on most of these are much smaller than seamounts, but probably originated in the same way. These so-called abyssal hills average only about 250 m high. the sea floor;
328
Chapter 12
The Sea Floor
They
are
common on
the sea floor
and underlie thick
sediments on the abyssal plains.
Other
common
linear ridges
features in the ocean basins are long, and broad plateaulike features rising as
— FIGURE
Submarine up above sea level to form seamounts. As the plate upon which these volcanoes rest moves away from a spreading volcanoes
12-18
may
build
ridge, the volcanoes sink
sea level
much
km
as 2 to 3
They are known seismic activity.
A
above the surrounding sea
floor.
as aseismic ridges because they lack
few of these ridges are thought to be
small fragments separated from continents during ing.
rift-
Such fragments, referred to as microcontinents, are
"^ FIGURE
12-19
Map
represented by such features as the Jan the
North Atlantic
Most
(Fig.
Mayen Ridge
in
12-19).
aseismic ridges form as a linear succession of
hot spot volcanoes. These
may
develop at or near an
oceanic ridge, but each volcano so formed
showing the locations of some of the aseismic
beneath
and become guyots.
is
carried
ridges.
^75
|
Aseismic ridge
Oceanic ridge system
Oceanic trench
The Deep-Ocean Basin
329
Perspective 12-2
MAURICE EWING AND HIS INVESTIGATION OF THE ATLANTIC OCEAN In 1935,
when Maurice Ewing began
his studies of the
continental shelf off Norfolk, Virginia,
known about
little
was
itself
the deep-sea floor. Ewing's analysis of
seismic evidence had indicated that the continental shelf
is
covered by a thin layer of sediments, but the floor
composed of sediment as much as 4,000 m had been deposited on ocean-floor bedrock.
was of
geologically recent origin.
led two more expeditions to the Mid- Atlantic Ridge, and in 1949 he founded the Columbia Lamont Geologic Observatory, whose main In 1948,
Ewing
studying the ocean
thick that
mission
Since these thick sediments probably contained
discovered that the oceanic crust
hydrocarbons, he tried to interest oil companies in supporting further studies of the continental shelf. was told that oil was so easily found on land that
was no reason
there
to look for
it
under the
is
sea.
Undiscouraged, he pursed his ocean-floor research and made many important discoveries. In 1947, the National Geographic Society commissioned Ewing to explore the little-known Mid-Atlantic Ridge and the adjacent sea floor. Using seismic and echo-sounding techniques as well as equipment for sampling seawater, he determined water temperature at various depths and sampled the sea floor itself. His initial samples and seismic investigations produced surprising results. The data
km
thick,
much
thinner than continental crust.
During the early 1950s, Ewing decided to transfer all of the available seismic profiles of the North Atlantic Ocean floor onto a topographic map. He assigned the job to Bruce Heezen, a graduate student who enlisted the help of Marie Tharp, a cartographer (mapmaker) at the observatory. As the profiles were converted into a map, both Heezen and Tharp were surprised to see a deep canyon (or rift valley) running
down
the center of the Mid-Atlantic Ridge. Initially,
they did not believe that such a large-scale so Heezen and
Ewing began
What emerged was
200
million years of
deposition. Furthermore, dredging across the slopes of
the Mid-Atlantic Ridge brought up pieces of pillow lava (see Fig. 4-14).
Not only was
the ocean floor
rift
existed,
plotting the locations of
mid-ocean earthquakes for which they had data. a band of earthquakes running
all
sediment that had accumulated for billions of years, the sediments were only several hundred meters thick to
Early on, he
composed of
he determined that the oceanic crust
indicated that rather than a thick layer of sea-floor
and represented 100
is
sunken continental material. Furthermore, is only 5 to 10
basalt, not
He
floor.
through not only the middle of the
rift
valley
mapped
by Tharp, but through all the world's oceans. In 1959 Ewing, Heezen, and Tharp published a spectacular three-dimensional map of the North Atlantic Ocean. The
map showed
vast plains
and conical
with the plate upon which it originated. The net such activity is a sequence of seamounts/guyots extending from an oceanic ridge (Fig. 12-18); the Walvis
coarse-grained sediment (sand and gravel) far from land.
Ridge in the South Atlantic is a good example (Fig. 1219). Aseismic ridges also form over hot spots unrelated
the ocean basins, but only trivial
laterally
result of
to ridges.
formed
in
The Hawaiian-Emperor chain such a manner (Fig. 12-19).
in the Pacific
Coarse sediment
in icebergs
Deep-sea sediments consist mostly of fine-grained deposits because few mechanisms exist that can transport
330
Chapter 12
The Sea Floor
its
amounts are
way
into
actually
transported by such processes.
Most of the fine-grained sediment in the deep sea is windblown dust and volcanic ash from the continents and oceanic islands and the
^ DEEP-SEA SEDIMENTATION
or trapped in floating veg-
etation, such as the roots of a tree, can find
isms that
live in the
shells of
microscopic organ-
near-surface waters of the oceans.
Other sources of sediment include cosmic dust and defrom chemical reactions in seawater. The manganese nodules that are fairly common in all the posits resulting
*" FIGURE 1 This map of the sea floor resulted from the work of Maurice Ewing, Bruce Heezen, and Marie Tharp.
seamounts, as well as the Mid-Atlantic Ridge with mysterious
still
rift
valley (Fig. 1).
As more of
its
the
world's ocean floors were explored, this original regional
map was expanded
km
to reveal a
long winding through
The recognition of
all
mountain chain 65,000
the world's oceans.
a curving ridge located
midway
ocean basins are a good example of the latter (Fig. 1220). These nodules are composed mostly of manganese and iron oxides, but also contain copper, nickel, and cobalt.
Such nodules may be an important source of
between and parallel to the coasts of South America and Africa forced geologists to reexamine their theories about the Earth. The realization that new crust was forming along the rift valley of the Mid-Atlantic Ridge hastened the acceptance of sea-floor spreading
and plate tectonic theory.
The bulk of the sediments on the deep-sea floor meaning that they settled from suspension
pelagic,
from land.
Two
ognized: pelagic clay and ooze
(Fig.
12-21). Pelagic clay
covers most of the deeper parts of the ocean basins.
interested in this potential resource.
sized particles derived
is
The contribution of cosmic dust negligible. Even though some
to deep-sea sediment
researchers estimate
360,000 metric tons of cosmic dust may fall to Earth each year, this is a trivial quantity compared to the volume of sediments derived from other sources.
that as
much
as
far
categories of pelagic sediment are rec-
some metals in the future; the United States, which imports most of its manganese and cobalt, is particularly
generally
are
brown or reddish and
is
composed of
It is
clay-
from the continents and oceanic Ooze, on the other hand, is composed mostly of shells of microscopic marine animals and plants. It is characterized as calcareous ooze if it contains mostly calcium carbonate (CaC0 3 skeletons of tiny marine organisms such as foraminifera (see Perspective 9-1, Fig. islands.
)
Deep-Sea Sedimentation
331
^ REEFS Reefs are moundlike, wave-resistant structures composed of the skeletons of organisms are called coral reefs, but
(Fig. 12-22).
many
Commonly they
other organisms in addi-
make up reefs. A reef consists of a solid framework of skeletons of corals, clams, and such encrusting organisms as algae and sponges. Reefs grow to a depth of about 45 or 50 m and are restricted to shallow tropical seas where the water is clear, and the temperature does not fall below about 20°C. Three types of reefs are recognized: fringing, barrier, and atoll (Fig. 12-23). Fringing reefs are solidly attached to the margins of an island or continent. They have a rough, tablelike surface, are as much as one kilometer wide, and, on their seaward side, slope steeply down to tion to corals
-»-
FIGURE
12-20
Manganese nodules on
the sea floor
south of Australia.
the sea floor. Barrier reefs are similar to fringing reefs,
except that they are separated from the mainland by a lagoon. Probably the best-known barrier reef in the 2). Siliceous
ooze
composed of the
is
silica
world
(Si0 2 ) skel-
is the Great Barrier Reef of Australia. It is more than 2,000 km long and is separated from the continent by a wide lagoon (Fig. 12-24).
etons of such single-celled organisms as radiolarians (animals) and diatoms (plants) (Fig. 7-16).
""»'
FIGURE
The
12-21
Calcareous ooze
distribution of sediments
Siliceous
|
332
Chapter 12
The Sea Floor
ooze
on the deep-sea
~~\
floor.
Pelagic clay
"•'
FIGURE
12-22
Reefs such as this one fringing an island in the Pacific are composed of the skeletons of organisms.
wave-resistant structures
The
last
type of reef
is
an
atoll,
which
is
shallow water. However, the island eventually subsides
a circular to
oval reef surrounding a lagoon (Fig. 12-23). Such reefs
below sea
form around volcanic islands that subside below sea level as the plate upon which they rest is carried progressively farther from an oceanic ridge (Fig. 12-18). As subsidence occurs, the reef organisms construct the reef
a more-or-less
upward so
"^"
FIGURE
a lagoon.
12-23
Three-stage development of an
As the island disappears beneath the
atoll. In
continuous reef
common in Many of
are particularly
basin (Fig. 12-25). reefs,
that the living part of the reef remains in
reef forms, but as the island sinks, a barrier reef
lagoon surrounded by 12-23). Such reefs the western Pacific Ocean
level, leaving a circular
first
these began as fringing
but as subsidence occurred, they evolved
barrier reefs
the
(Fig.
and
first
to
finally to atolls.
stage, a fringing
becomes separated from the
island by
sea, the barrier reef continues to
grow
upward, thus forming an atoll. An oceanic island carried into deeper water by plate movement can account for this sequence. Fringing reef
Barrier reef
Atoll
Reefs
333
FIGURE
Deep-sea
View of an
12-25
drill
atoll in the Pacific
Ocean.
holes have penetrated through the upper
oceanic crust into a sheeted dike complex, a zone consisting
26).
almost entirely of vertical basaltic dikes
What
lies
below
this sheeted dike
been sampled. Even though the oceanic crust
is
(Fig.
12-
complex has not
km thick and
5 to 10
can be penetrated only about 1 km by drill holes, geologists have a good idea of the composition of the entire
As mentioned previously, oceanic crust is continconsumed at subduction zones, but a tiny amount of this crust is not subducted. Rather it is emplaced in mountain ranges on continents, where it usually arrives by moving along large fractures called thrust faults (thrust faults and mountain building are discussed more fully in Chapter 14). Such slivers of oceanic crust and upper mantle now on continents are called ophiolites (Fig. 12-26). They are crust.
uously
"*"
FIGURE
of Australia.
12-24 It is
Aerial view of the Great Barrier Reef
more than 2,000
from the continent
km
long and separated
the background) by a wide lagoon.
(in
structurally complex, but detailed studies reveal that an ideal ophiolite consists of a layer of deep-sea sedimen-
tary rocks underlain by pillow basalts
This particular scenario for the evolution of reefs from
and a sheeted dike
fringing to barrier to atoll
complex, the same layers as in deep-sea cores. Further downward in an ophiolite is massive gabbro, and below
years ago by Charles
that
naturalist
on
the
has revealed that
was proposed more than 150 Darwin while he was serving as a ship H.M.S. Beagle. Drilling into atolls they do indeed rest upon a basement of
volcanic rocks, thus confirming Darwin's hypothesis.
^ COMPOSITION OF THE OCEANIC CRUST Sampling and direct observations of the oceanic ridges
pillow lavas
334
Much
(Fig. 4-14),
Chapter 12
of this basalt
may
comthe form of
is
in
represent
magma chamber
magma (Fig.
that
12-26).
Beneath the gabbro is peridotite— sometimes altered by metamorphism to assemblages containing serpentine— that probably represents the upper mantle. Thus, a complete ophiolite consists of deep-sea sedimentary rocks, (Fig. 12-26).
^ RESOURCES FROM THE SEA
is
but sheet flows are also present.
The Sea Floor
layered gabbro that
oceanic crust, and upper mantle
reveal that the upper part of the oceanic crust
posed of basalt.
is
cooled at the top of a
Seawater contains
many
which are extracted
elements in solution, some of
for various industrial
and domestic
Oceanic ridge
"•»•
FIGURE
12-26
New
oceanic
Layered
crust consisting of the layers
gabbro
here forms as
Pendotite
Upper mantle
magma
shown
beneath oceanic ridges. The composition of the oceanic crust is known from ophiolites, sequences of rock on land consisting of deep-sea sediments, oceanic crust, and upper rises
mantle.
uses. For
ble salt)
in many places sodium chloride (taproduced by the evaporation of seawater, and
example,
is
a large proportion of the world's
magnesium
is
^ FIGURE
12-27
120°E
extracted from seawater, but for
many, such as gold, the cost
pro-
duced from seawater. Numerous other elements and
to the United States
compounds can be
is
prohibitive.
on the becoming
In addition to substances in seawater, deposits
sea floor or within sea-floor sediments are
The Exclusive Economic Zone (EEZ) includes and its possessions.
a vast area adjacent
150°E
Resources from the Sea
335
"^ FIGURE
12-28
Exclusive Economic
Sedimentary basins within the
Zone
in
which known or potential
reserves of hydrocarbons occur.
336
Chapter 12
The Sea Floor
increasingly
sources
lie
important.
Many
of these potential re-
well beyond the margins of the continents, so
the ownership of such resources is a political and legal problem that has not yet been resolved. Most nations bordering the ocean claim those resources occurring
The United
within their adjacent continental margin.
example, by a presidential proclamation issued on March 10, 1983, claims sovereign rights over an area designated as the Exclusive Economic Zone (EEZ). States, for
The EEZ extends seaward 200 nautical miles (371 km) from the coast, giving the United States jurisdiction over an area about 1.7 times larger than its land area (Fig. 12-27).* Also included within the EEZ are the areas adjacent to U.S. territories, such as Guam, American
Samoa, Wake
and Puerto Rico (Fig. 12-27). In huge area of the sea floor and any resources on or beneath it. Numerous resources occur within the EEZ, some of which have been exploited for many years. For example, sand and gravel for construction are mined from the continental shelf in several areas. About 17% of U.S. oil and natural gas production comes from wells on the continental shelf. Some 30 sedimentary basins occur within the EEZ, several of which are known to contain hydrocarbons whereas others are areas of potential hydrocarbon production (Fig. 12-28). Ancient shelf deposits in the Persian Gulf region contain the world's largest Island,
short, the United States claims a
CALIFORNIA
Mendocino
fracture
zone
~^~
FIGURE 12-29 Massive sulfide deposits formed by submarine hydrothermal activity have been identified on the Gorda Ridge within the Exclusive Economic Zone.
reserves of oil (see Perspective 7-2).
Other resources of
interest include the massive sulfide
deposits that form by submarine hydrothermal activity
spreading ridges (see the Prologue). Such deposits containing iron, copper, zinc, and other metals have at
EEZ at the Gorda and Oregon; similar deposits the Juan de Fuca Ridge within the Canadian
Ridge off
been identified within the the coasts of California
occur at
EEZ
(Fig.
12-29).
Other potential resources nodules discussed previously
manganese 12-20), and metallif-
include the
(Fig.
erous oxide crusts found on seamounts. Manganese nodules contain manganese, cobalt, nickel, and copper; the United States first
also claim sovereign rights to resources
heavily dependent
on imports of
the
EEZ, however, manganese nodules occur near Johnston Island in the Pacific Ocean and on the Blake Plateau off the east coast of South Carolina and Georgia. In addition,
EEZ
seamounts and seamount chains within the
the Pacific are
*A number of other nations
is
three of these elements (see Fig. 3-25). Within the
known
in
to have metalliferous oxide crusts
several centimeters thick
from which cobalt and man-
ganese could be mined.
within 200 nautical miles of their coasts.
J3K>^^*:^--«^«£^g3^^
Ti
Continental margins separate the continents above sea level from the deep ocean basin. They consist of
^ CHAPTER SUMMARY 1.
Scientific investigations of the
oceans began during
equipped to investigate the sea floor by sounding, and seismic profiling.
drilling,
a continental shelf, continental slope,
cases a continental
the late 1700s. Present-day research vessels are
echo
and
in
some
rise.
Continental shelves slope gently in a seaward direction and vary in width from a few tens of
Chapter Summary
337
4.
meters to more than 1,000 km. The continental slope begins at an average depth of 135 m where the inclination of the sea floor increases rather abruptly
from
less
15.
The United
States has claimed rights to all resources occurring within 200 nautical miles (371 km) of its shorelines. Numerous resources including various
metals occur within this Exclusive Economic Zone.
than 1° to several
degrees. 5.
Submarine canyons are characteristic of the some of them extend well up onto the shelf and lie offshore from large streams. Stream erosion of the shelf during the Pleistocene Epoch may account for some submarine canyons, but many have no association with streams on land and were probably eroded by turbidity currents. Turbidity currents commonly move through submarine canyons and deposit an overlapping series of submarine fans that constitutes a large part of the
IMPORTANT TERMS
continental slope, but
6.
continental 7.
rise.
Active continental margins are characterized by a
narrow
and a slope that descends directly into an oceanic trench with no rise present. Such margins are also characterized by seismic activity and shelf
volcanism. 8.
Passive continental margins lack volcanism exhibit
little
seismic activity.
The
and
active continential
aseismic ridges are oriented more-or-less
continental margin
margin
continental rise
pelagic clay
continental shelf
reef
continental slope
seamount
echo sounder
seismic profiling
Exclusive Economic
Zone
guyot
submarine canyon submarine fan
oceanic ridge
turbidity current
oceanic trench
^ REVIEW QUESTIONS 1.
2.
Much
of the continental rise
a.
calcareous ooze; b.
c.
fringing reefs; d.
e.
ophiolite.
The
sheeted dikes;
greatest oceanic depths occur at:
shelf-slope break; d.
guyots;
Abyssal plains are most
common:
a.
around the margins of the Atlantic;
b.
adjacent to the East Pacific Rise;
in the rift
valley of the Mid-Atlantic Ridge;
on
4.
A
circular reef enclosing a lagoon
a.
barrier reef; b.
ridge; d. 5.
guyot;
e.
continental slopes; d. fractures in the sea floor.
Deep-sea drilling and the study of fragments of sea floor in mountain ranges on land reveal that the oceanic crust is composed in descending order of pillow lava, sheeted dikes, and gabbro.
Chapter 12
The Sea Floor
7.
c.
aseismic
Submarine canyons are most characteristic of
e.
composed of
a(n):
atoll.
c.
6.
is
seamount;
Deep-sea sediments consist mostly of fine-grained particles derived from continents and oceanic islands and the microscopic shells of organisms. The primary types of deep-sea sediments are pelagic clay
atoll.
e.
continental shelves.
chain of seamounts and/or guyots.
and
along
c.
the west coast of South America; d.
continental shelves; b.
Reefs are wave-resistant structures
the
c.
oceanic trenches;
passive continental margins.
e.
3.
of:
submarine fans;
aseismic ridges; b.
a
composed
is
a.
reefs are recognized: fringing, barrier,
338
passive continental
perpendicular to oceanic ridges and consist of a
animal skeletons, particularly corals. Three types of 14
ophiolite
aseismic ridge
and ooze. 13
margin
continental shelf
along such margins is broad, and the slope merges with a continental rise. Abyssal plains are commonly present seaward beyond the rise. 9. Oceanic trenches are long, narrow features where oceanic crust is subducted. They are characterized by low heat flow, negative gravity anomalies, and the greatest oceanic depths. 10. Oceanic ridges consisting of mountainous topography are composed of volcanic rocks, and many ridges possess a large rift caused by tensional forces. Basaltic volcanism and shallow-focus earthquakes occur at ridges. Oceanic ridges nearly encircle the globe, but they are interrupted and offset by large fractures in the sea floor. 11. Other important features on the sea floor include seamounts that rise more than a kilometer high and guyots, which are flat-topped seamounts. Many
12
ooze
abyssal plain
the:
abyssal plains; rift
valleys;
The
Earth's surface waters probably originated through the process of: a. dewatering; b. subduction; c.
outgassing; d.
e.
erosion.
crustal fracturing;
Continental shelves: a.
are
composed of
pelagic sediments; b.
lie
between continental slopes and rises; c. descend slope gently to an average depth of 1,500 m; d. from the shoreline to the shelf-slope break; e.
are widest along active continental margins.
8.
9.
The
flattest,
most
c.
continental slopes; d.
e
continental margins. settles
the:
b.
aseismic ridges;
from suspension pelagic;
a.
abyssal; b.
d.
generally coarse grained;
far
from land
volcanic;
c.
a
is
correct?
most of the continental margins around the oceanic ridges are
Atlantic are passive; b.
c.
Summarize the evidence indicating that turbidity currents transport sediment from the continental shelf onto the slope and rise. 21. Where do abyssal plains most commonly develop? Describe their compositon. 22.
the following statements
composed
others.
characterized
e.
by graded bedding.
Which of
largely of
deformed sedimentary rocks;
the deposits of turbidity currents consist of
What
the significance of oceanic trenches,
is
where are they found? 23. How do mid-oceanic ridges ranges on land?
how
24. Describe
differ
their relative importance.
intermediate and deep earthquakes occur at or near oceanic crust is thicker than oceanic ridges; e.
26. Describe the sequence of events leading to the origin
continental crust.
27. Illustrate and label an ideal sequence of rocks in an
of an
atoll.
Massive
28.
12.
as on passive continental margins; b. accumulations of microscopic shells on the sea floor; by precipitation of minerals near c. from sediments derived hydrothermal vents; d. in oceanic trenches. from continents; e. The most useful method of determining the structure
of the oceanic crust beneath continental shelf
Anderson, R. N. 1986. Marine geology.
sulfide deposits form:
ophiolite.
a.
sediments a.
d.
echo sounding;
observations from
b.
What
seismic profiling;
is
25°; b 40°.
e.
How
4°;
c.
rise.
d
0.1°;
is
a characteristic of: turbidity current
pelagic clay; d.
siliceous ooze;
manganese nodules. do sulfide mineral deposits form on the sea
floor?
17.
What
is
an echo sounder, and
how
is it
used to
study the sea floor? 18.
What
are the characteristics of a passive continental
margin?
How
Economic Zone? What types
^
it?
ADDITIONAL READINGS New
York: John Wiley
Bishop,
J.
M.
1984. Applied oceanography.
An
New
York: John
introduction to the
marine environment. Dubuque, Iowa: W. C. Brown. J. M., and K. Von Damm. 1983. Hot springs on the ocean floor. Scientific American 248, no. 4: 78-93. Gass, I. G. 1982. Ophiolites. Scientific American 247, no. 2:
Edmond,
122-31. Kennett,
J.
R
1982. Marine geology. Englewood
Cliffs, N.J.:
Prentice-Hall. reefs, seamounts, and guyots. Sea 143-49. Pinet, P. 1992. Oceanography: An introduction to the planet oceanus. St. Paul, Minn.: West Publishing Co. Rona, P. A. 1986. Mineral deposits from sea-floor hot springs. Scientific American 254, no. 1: 84-93. Ross, D. A. 1988. Introduction to oceanography. Englewood
Mark, K. 1976. Coral Frontiers 22, no. 3:
continental shelves; b.
deposits;
16.
1°; c
Graded bedding a.
continental
the average slope of the continental slope?
a
the Exclusive
Davis, R. A. 1987. Oceanography:
underwater
e.
volcanic arc; e
e
is
of metal deposits occur within
Wiley &c Sons.
dredging;
c.
photography. 13. Which of the following is not characteristic of an active continental margin? oceanic earthquakes; c. volcanism; b. a. trench; d.
What
8c Sons.
is:
submersible research vessels;
15.
from mountain
an aseismic ridge forms.
11.
14.
and
25. List four sources of deep-sea sediments, and explain
most of the Earth's
calcareous ooze; d.
rise
20.
is:
10.
and explain why a occurs at some continental margins and not at
19. Describe the continental rise,
abyssal plains;
oceanic ridges;
Sediment that
on Earth are
featureless areas
a.
Cliffs, N.J.: Prentice-Hall.
Thurman, H. V. 1988. Introductory oceanography. 5th ed. Columbus, Ohio: Merrill Publishing Co. Tolmazin, D. 1985. Elements of dynamic oceanography. Boston, Mass.: Allen & Unwin.
does such a continental margin
originate?
Additional Readings
339
CHAPTER
13
PLATE TECTONICS: A Unifying Theory OUTLINE PROLOGUE INTRODUCTION EARLY IDEAS ABOUT CONTINENTAL DRIFT
ALFRED WEGENER AND THE CONTINENTAL DRIFT HYPOTHESIS THE EVIDENCE FOR CONTINENTAL DRIFT Continental Fit Similarity of
Rock Sequences and Mountain
Ranges Glacial Evidence Fossil
Evidence
PALEOMAGNETISM AND POLAR
WANDERING SEA-FLOOR SPREADING "^
Perspective 13-1: Paleogeographic
Maps
Deep-Sea Drilling and the Confirmation of Sea-Floor Spreading
PLATE TECTONIC THEORY PLATE BOUNDARIES Divergent Boundaries
"*
Perspective 13-2: Tectonics of the Terrestrial Planets
Convergent Boundaries
"^ Guest
Essay: Geoscience Careers— The
Diversity
Is
Unparalleled
Transform Boundaries
PLATE
MOVEMENT AND MOTION
Hot Spots and Absolute Motion
THE DRIVING MECHANISM OF PLATE TECTONICS PLATE TECTONICS AND THE DISTRIBUTION OF NATURAL
RESOURCES CHAPTER SUMMARY Vertical
view of the Himalayas, the youngest
and highest mountain system in the world. The Himalayas began forming when India collided with Asia 40 to 50 million years ago.
PROLOGUE
Both of these events occurred along the eastern portion of the Ring of Fire, a chain of intense seismic
and volcanic
activity that encircles the Pacific
basin (Fig. 13-1).
Two
tragic events that occurred
Ocean
of the world's greatest
disasters occur along this ring because of volcanism
during 1985 serve to remind us of the dangers of living near a convergent plate margin. September 19, a magnitude 8.1 earthquake killed
Some
On
and earthquakes generated by plate convergence. For example, the 1989 volcanic eruptions in Alaska, the
1980 eruption of Mount
St.
Helens, and the 1970
more than 9,000 people in Mexico City. Two months later and 3,200 km to the south, a minor eruption of Colombia's Nevado del Ruiz volcano partially melted its summit glacial ice, causing a mudflow that engulfed Armero and several other villages and killed more than 23,000 people. These two tragedies resulted in more than 32,000 deaths, tens of thousands of injuries, and billions of dollars in
earthquake that killed 66,000 people in Peru all occurred as a consequence of plate convergence. Although earthquakes and volcanic eruptions are very different geologic phenomena, both are related to the activities occurring at convergent plate margins. The Mexico City earthquake resulted from subduction of the Cocos plate at the Middle America Trench (Fig. 13-1). Sudden movement of the Cocos plate beneath
property damage.
Central America generated seismic waves that traveled
*•'
FIGURE
13-1
The Ring of
convergence as illustrated
Fire
is
a zone of intense earthquake
Ocean basin. Most of by the two insets.
activity that encircles the Pacific
and volcanic from plate
this activity results
Mexico City
Volcanoes
Earthquakes
Prologue
341
the mountain; the meltwater rushed
down
mixed with the sediment, and turned
it
the valleys,
into a deadly
viscous mudflow.
The
city
of Armero, Colombia,
lies in
the valley of
the Lagunilla River, one of several river valleys inun-
dated by mudflows. Twenty thousand of the city's 23,000 inhabitants died, and most of the city was destroyed (Fig. 13-2). Another 3,000 people were killed in nearby valleys. A geologic hazard assessment study completed one month before the eruption showed that
Armero was in a high-hazard mudflow area! These two examples vividly illustrate some
of the
dangers of living in proximity to a convergent plate
boundary. Subduction of one plate beneath another "•'
FIGURE
The 1985 eruption of Nevado del Ruiz in Colombia melted some of its glacial ice. The meltwater mixed with sediments and formed a huge mudflow that destroyed the city of Armero and killed 20,000 of its 13-2
inhabitants.
outward
in all directions.
The
violent shaking
experienced in Mexico City, 350
km
away, and
elsewhere was caused by these seismic waves.
The
strata underlying
Mexico City
consist of
unconsolidated sediment deposited in a large ancient lake.
Such sediment amplifies the shaking during
earthquakes with the unfortunate consequence that buildings constructed there are heavily
damaged than those
commonly more on
built
solid
bedrock
(see Perspective 10-1, Fig. 5).
Less than
two months
Mexico City
after the
earthquake, Colombia experienced
recorded natural disaster.
Nevado
several active volcanoes resulting
magma
Nevado
^
from the
(Fig. 13-1).
A
is
is
one of
rise
of
subducted
minor eruption on
del Ruiz partially melted the glacial ice
felt far
from
their epicenters.
Since 1900, earthquakes have killed
more than
112,000 people in Central and South America alone. While volcanic eruptions in this region have not caused nearly as many casualties as earthquakes, they have, nevertheless, caused tremendous property damage and have the potential for triggering devastating events such as the 1985 Colombian mudflow. Because the Ring of Fire is home to millions of people, can anything be done to decrease the devastation that inevitably results from the earthquake and volcanic activity occurring in that region? Given our present state of knowledge, most of the disasters could not have been accurately predicted, but better planning and advance preparations by the nations bordering the Ring of Fire could have prevented much life. As long as people live near convergent plate margins, there will continue to be
disasters.
However, by studying and understanding
geologic activity along convergent as well as divergent
and transform plate margins, geologists can help minimize the destruction.
tion
that the Earth's geography has changed
and distribution of many important natural
sources,
now
continuously through time has led to a revolution in the
boundaries, and geologists are
tectonic theory into their prospecting efforts.
the way they view the Earth. Although many people have only a vague notion of what plate tectonic theory
continents, ocean basins,
profound effect on all of our lives. It is now realized that most earthquakes and volcanic eruptions occur near plate margins and are not plate tectonics has a
342
Chapter 13
Plate Tectonics:
A
Unifying Theory
re-
such as metallic ores, are related to plate
geological sciences, forcing geologists to greatly modify
is,
to
merely random occurrences. Furthermore, the forma-
INTRODUCTION
The recognition
which are frequently
tragic loss of
greatest
generated where the Nazca plate
beneath South America of
its
del Ruiz
repeatedly triggers large earthquakes, the effects of
The movement of in turn affects the
incorporating plate
plates determines the location of
and mountain systems, which
atmospheric and oceanic circulation
patterns that ultimately determine global climates. Plate
movements have
also profoundly influenced the geo-
graphic distribution, evolution, and extinction of plants
During the ologist
and animals. Since at least the early 1900s, abundant evidence has
late nineteenth century, the
Edward Suess noted
Late Paleozoic plant
fossils
Austrian ge-
the similarities between the
of India, Australia, Africa,
moving through-
Antarctica, and South America as well as evidence of
out geologic time. Nevertheless, most geologists rejected
glaciation in the rock sequences of these southern con-
was no suitable mechanism to explain such movement. By the early 1970s, however, studies of the Earth's magnetic field, its interior, and the ocean basins (see Chapters 11 and 12) convinced most
tinents. In
geologists that continents are parts of plates that are
where, along with evidence of extensive glaciation,
indicated that the continents have been
the idea because there
moving
in
response to some type of heat transfer system
Plate tectonic theory geologists,
and
is
many
as
we
will use here) for a supercontinent
composed of these southern landmasses. The name came from Gondwana, a province in east-central India abundant
fossils
of the Glossopteris flora occur (Fig.
its
and
now almost universally accepted application has led to a greater
understanding of how the Earth has evolved and continues to do so. This powerful, unifying theory accounts for apparently unrelated geologic events, allowing geol-
view such phenomena as part of a continuing
ogists to
1885 he proposed the name Gondwanaland
Gondwana
13-3). Suess believed the distribution of plant fossils
within the Earth.
among
(or
story rather than as a series of isolated incidents.
Before discussing plate tectonic theory, the various hypotheses that preceded
it
we will
review
"•" FIGURE 13-3 Representative members of the Glossopteris flora. Fossils of these plants are found on all five of the Gondwana continents. Glossopteris leaves from (a) the Upper Permian Dunedoo Formation and (b) the Upper Permian Illawarra Coal Measures, Australia. (Photos courtesy of Patricia G. Gensel, University of North
Carolina.)
and examine the
some people to accept the idea of conmovement and others to reject it. Because plate
evidence that led tinental
quiries
from numerous scientific inand observations, only the more important ones
will be
covered
tectonic theory has evolved
in this chapter.
^ EARLY IDEAS ABOUT CONTINENTAL DRIFT The
idea that the Earth's geography
the past
is
was
different during
not new. During the fifteenth century, Leon-
ardo da Vinci observed that "above the plains of Italy where flocks of birds are flying today fishes were once moving in large schools." In 1620, Sir Francis Bacon commented on the similarity of the shorelines of western Africa and eastern South America but did not make the connection that the Old and New Worlds might once have been sutured together. Alexander von Humboldt made the same observation in 1801, although he attributed these similarities to erosion rather than the splitting apart of a larger continent.
One
of the earliest specific references to continental
drift is in
and
Its
that
all
Antonio
Snider-Pellegrini's
1858 book Creation
Mysteries Revealed. Snider-Pellegrini suggested
of the continents were linked together during the
Pennsylvanian Period and later conclusions
on
split apart.
He
based his
the similarities between plant fossils in the
Pennsylvanian-aged coal beds of Europe and North America.
However, he thought that continental separation was
a consequence of the biblical deluge.
Early Ideas About Continental Drift
343
was a consequence of
glacial deposits
extensive land
bridges that once connected the continents
and
later
sank beneath the ocean.
One
of the
continental
first
Frank
B. Taylor
ing his
own
who
propose a mechanism for
in
the American geologist 1910 published a paper present-
theory of continental
the formation of eral
to actually
movement was
drift. In it
mountain ranges as
movement of
continents.
He
he explained
a result of the lat-
also envisioned the
Geological Association in Frankfurt, Germany, Wegener first
presented his ideas for moving continents. His evi-
dence for continental drift and his conclusions were published in 1915 in his monumental book, The Origin of Continents and Oceans. According to Wegener's comprehensive hypothesis, all of the landmasses were originally united into a single supercontinent that he
named Pangaea, from Wegener portrayed
Greek meaning "all land." grand concept of continental of maps showing the breakup of the
his
present-day continents as parts of larger polar conti-
movement
nents that had broken apart and migrated toward the
forces
Pangaea and the movement of the various continents to their present-day locations. Wegener had amassed a tremendous amount of geological, paleontological, and climatological evidence in support of continental drift, but
Moon
the initial reaction of scientists to his then-heretical ideas
equator because of a slowing of the Earth's rotation due to gigantic tidal forces. According to Taylor, these tidal
were generated when the Earth captured the about 100 million years ago. Although we now know that Taylor's mechanism is incorrect, one of his most significant contributions was his suggestion that the Mid-Atlantic Ridge, discoverd by
1872-1876 might mark the
H.M.S. Challenger expeditions, site along which an ancient continent broke apart to form the present-day Atlantic Ocean. the
British
^ ALFRED WEGENER AND THE CONTINENTAL DRIFT HYPOTHESIS Alfred Wegener, a
German
meteorologist
(Fig. 13-4), is
generally credited with developing the hypothesis of
continental
drift. In
a
1912
lecture before the
German
in a series
can best be described as mixed. Opposition to Wegener's ideas became particularly in North America after 1928 when the American Association of Petroleum Geologists held an international symposium to review the hypothesis of continental drift. After each side had presented its arguments, the opponents of continental drift were clearly in the majority, even though the evidence in support of continental drift, most of which came from the Southern Hemisphere, was impressive and difficult to refute. One problem with the hypothesis, however, was its lack of a mechanism to explain how continents, composed of gra-
widespread
nitic rocks,
could seemingly
move through
the denser
basaltic oceanic crust.
Nevertheless, the eminent South African geologist Alexander du Toit further developed Wegener's arguments
— FIGURE
13-4 Alfred Wegener, a German meteorologist, proposed the continental drift hypothesis in 1912 based on a tremendous amount of geological,
paleontological,
and climatological evidence. He
is
shown
here waiting out the Arctic winter in an expedition hut.
and gathered more geological and paleontological evidence in support of continental drift. In 1937, du Toit published Our Wandering Continents, in which he contrasted the glacial deposits of posits of the
same age found
Gondwana with in the
coal de-
continents of the
Northern, Hemisphere. In order to explain the origin and distribution of these rocks, both of which form under different climatic conditions, du Toit
Gondwana continents
to the South Pole
moved
the
and brought the
northern continents together such that the coal deposits at the equator. He named this northern
were located
Jandm ass Laurasia. It consisted -America. Greenland, Europe, and
of present-da y North Asia (except tor India).
In spite of what seemed to be overwhelming evidence, most geologists still refused to accept the idea that continents moved. It was not until the 1960s when ocean-
ographic research provided convincing evidence that the continents had once been joined together and subsequently separated that the hypothesis of continental drift finally
344
Chapter 13
Plate Tectonics:
A
Unifying Theory
became widely accepted.
THE EVIDENCE FOR CONTINENTAL DRIFT =»
The evidence used by Wegener, du support the hypothesis of continental
Continental Fit Wegener, Toit,
and others
drift includes the
to fit
same same age on
of the shorelines of continents; the appearance of the
rock sequences and mountain ranges of the
now widely separated; the matching of glacial and paleoclimatic zones; and the similarities of many extinct plant and animal groups whose fossil remains are found today on widely separated continents.
like
some before him, was impressed by
the
close resemblance
between the coastlines of continents on opposite sides of the Atlantic Ocean, particularly between South America and Africa. He cited these similarities as partial evidence that the continents were at one
continents
time joined together as a supercontinent that subse-
deposits
quently
split apart.
As
his critics pointed out,
however,
the configuration of coastlines results from erosional
depositional processes and therefore
— FIGURE
is
and
continually being
13-5
The
best
fit
between continents occurs along the continental slope at a depth of 2,000 m.
Areas of overlap
Gaps
The Evidence
for Continental Drift
345
modified. Thus, even
if
the continents
had separated
during the Mesozoic Era, as Wegener proposed, likely that the coastlines
A
more
realistic
would
approach
is
fit
exactly.
to
fit
it is
not
the continents to-
gether along the continental slope where erosion
would
be minimal. Recall from Chapter 12 that the true margin of a continent— that
is,
where continental crust
Similarity of
If
the continents were at one time joined together, then
Edward Bullard, an Enand two associates showed that the
slope (see Fig. 12-8). In 1965 Sir glish geophysicist,
best
fit
between the continents occurs along the conti-
nental slope at a depth of about 2,000
m
(Fig. 13-5).
Since then, other reconstructions using the latest ocean
basin data have confirmed the close nents
"•"
when
FIGURE
fit
between conti-
they are reassembled to form Pangaea.
13-6
and mountain ranges of the same age in adon the opposite continents should match. Such is the case for the Gondwana con(Fig. 13-6). Marine, nonmarine, and glacial rock
the rocks
joining locations closely tinents
changes to oceanic crust— is beneath the continental
Rock Sequences
and Mountain Ranges
sequences of Pennsylvanian to Jurassic age are almost identical for all five
is
that of the Glossopteris flora.
J*
continents, strongly in-
The
trends of several major mountain ranges also
These mounone continent only to apparently continue on another continent across the ocean. For example, in a reconstructed support the hypothesis of continental tain ranges seemingly
Marine, nonmarine, and glacial rock sequences of Pennsylvanian to same for all Gondwana continents. Such close similarity strongly suggests that they were at one time joined together. The range indicated by G
Jurassic age are nearly the
Gondwana
dicating that they were at one time joined together.
end
drift.
at the coastline of
(a)
•^ FIGURE
Various mountain ranges of the deformation are currently widely separated by oceans, (b) When the continents are brought together, however, a single continuous mountain range is formed. Such evidence indicates the continents were at one time joined together and were subsequently separated.
same age and
13-7
{a)
style of
Gondwana, the east-west trending mountain range at the Cape of Good Hope in South Africa abruptly terminates at the coast. However, a mountain range of the same age and
style of
gentina.
deformation occurs near Buenos Aires, ArSouth America and Africa are brought
When
two seemingly different mountain ranges continuous structure (Fig. 13-7). In North America, the folded Appalachian Mountains trend northeastward through the eastern United
together, these
form ,
a single
3,000 I
and Canada and terminate abruptly at the Newfoundland coastline. Mountain ranges of the same age
i
i
i
I
km
States
(b)
The Evidence
for Continental Drift
347
"^ FIGURE
13-8
(a) If
the continents did not
move
in the past, then Late Paleozoic
bedrock in Australia, India, and South America indicate that glacial movement for each continent was from the oceans onto land within a subtropical to tropical climate. Such an occurrence is highly unlikely, (b) (right) If the continents are brought together, such that South Africa is located at the South Pole, then the glacial movement indicated by the striations makes sense. In this situation, the glacier, located in a polar climate, moved radially outward from a thick central area toward its periphery. glacial striations preserved in
and deformational
style
occur in eastern Greenland,
Ire-
and Norway. Even though these mountain ranges are currently separated by the Atlantic Ocean, they form an essentially continuous mountain
land, Great Britain,
range
when
the continents are positioned next to each
All of the
Gondwana
tropical climates.
Mapping
of glacial striations in bed-
rock in Australia, India, and South America indicates that the glaciers moved from the areas of the present-
day oceans onto land
other (Fig. 13-7).
continents except Antarctica
are currently located near the equator in subtropical to
(Fig. 13-8a).
However,
this
would
be impossible because large continental glaciers (such as
occurred on the
Glacial Evidence
Gondwana
Massive glaciers covered large continental areas of the Southern Hemisphere during the Late Paleozoic Era. Ev-
accumulation toward the
idence for this glaciation includes layers of
would have
till
(sedi-
ments deposited by glaciers) and striations (scratch marks) in the bedrock beneath the till. Fossils and sedimentary rocks of the same age from the Northern Hemisphere, however, give no indication of glaciation. Fossil plants found in coals indicate that the Northern Hemisphere had a tropical climate during the time that the Southern Hemisphere was glaciated.
348
Chapter 13
continents during the Late
Paleozoic Era) flow outward from their central area of
Plate Tectonics:
A
Unifying Theory
If
move during
the past, one
how glaciers moved from the and how large-scale continental gla-
to explain
oceans onto land ciers
sea.
the continents did not
formed near the equator. But
if
the continents are
reassembled as a single landmass with South Africa located at the south pole, the direction of movement of Late Paleozoic continental glaciers makes sense. Fur-
thermore, this geographic arrangement places the northern continents nearer the tropics, which
is
consistent
Furthermore, even
if
the seeds
had
floated across the
ocean from one continent to another, they probably would not have remained viable for any length of time in salt water.
The present-day
climates of South America, Africa,
and Antarctica range from
India, Australia,
much
polar and are
compose
plants that
tropical to
too diverse to support the type of
Wegener
the Glossopteris flora.
rea-
soned therefore that these continents must once have been joined such that these widely separated localities
were
the
all in
The
same
latitudinal climatic belt (Fig. 13-9).
remains of animals also provide strong ev-
fossil
drift. One of the best examples is Mesosaurus, a freshwater reptile whose fossils are found in Permian-aged rocks in certain regions of Brazil and South Africa and nowhere else in the world (Fig. 13-9).
idence for continental
Because the physiology of freshwater and marine ani-
mals
is
completely different,
it is
freshwater reptile could have
Ocean and found to
tical
its
could have that
how
a
across the Atlantic
a freshwater environment nearly iden-
former habitat. Moreover,
swum
across the ocean,
should be widely dispersed.
sume
hard to imagine
swum
Mesosaurus
It
is
fossil
more
lived in lakes in
Mesosaurus
if
its
remains
logical to as-
what
now
are
adjacent areas of South America and Africa, but were
then united into a single continent.
Cynognathus
and
Lystrosaurus
both
are
land-
dwelling reptiles that lived during the Triassic Period; their fossils are I
I
Glaciated area tal
Arrows indicate the direction of glacial movement based on striations preserved in bedrock.
rus
found only on the present-day continen-
fragments of
Gondwana
(Fig. 13-9).
Since Lystrosau-
and Cynognathus are both land animals, they
tainly could not have
separating the
swum
Gondwana
cer-
across the oceans currently continents. Therefore, the
(b)
continents must once have been connected.
with the
fossil
and climatological evidence from Laur-
The evidence favoring continental drift seemed overwhelming to Wegener and his supporters yet the lack of a suitable mechanism to explain continental movement prevented
asia (Fig. 13-8b).
its
widespread acceptance. Not
until
new
ev-
idence from studies of the Earth's magnetic field and
oceanographic research showed that the ocean basins Fossil
Some
Evidence
were geologically young features did renewed
of the most compelling evidence for continental
comes from the fossil record. Fossils of the Glosfound in equivalent Pennsylvanianand Permian-aged coal deposits on all five Gondwana
drift
sopteris flora are
continents.
The
Glossopteris flora
is
characterized by
the seed fern Glossopteris (Fig. 13-3) as well as by
many
interest in
continental drift occur.
^ PALEOMAGNETISM AND POLAR WANDERING Some
of the most convincing evidence for continental
came from
other distinctive and easily identifiable plants. Pollen
drift
and spores of plants can be dispersed over great distances by wind, but Glossopteris-type plants produced seeds that are too large to have been carried by winds.
tively
new
some
geologists
the study of paleomagnetism, a rela-
During that time, were researching past changes of the
discipline during the 1950s.
Earth's magnetic field in order to better understand the
Paleomagnetism and Polar Wandering
349
Lystrosaurus Glossopteris
^^ FIGURE
Some
13-9
of the animals and plants whose fossils are found today on
the widely separated continents of South America, Africa, India, Australia, and Antarctica. These continents were joined together during the Late Paleozoic to form the southern landmass of Pangaea. Glossopteris and similar plants are Pennsylvanian- and Permian-aged deposits on all five continents. Mesosaurus a freshwater reptile whose fossils are found in Permian-aged rocks in Brazil and South Africa. Cynognathus and Lystrosaurus are land reptiles who lived during the Early Triassic Period. Fossils of Cynognathus are found in South America and Africa, while fossils of Lystrosaurus have been recovered from Africa, India, and Antarctica.
Gondwana, found
in
present-day magnetic
field.
As so often happens
in sci-
ence, these studies led to other discoveries. In this case,
they led to the discovery that the ocean basins are geologically
indeed
young
features,
moved during
and that the continents have Wegener and oth-
the past, just as
mine the location of the Earth's magnetic poles and the latitude of the rock
when
Recall from Chapter 11 that the Earth's magnetic
it
formed.
Research conducted during the 1950s by the English geophysicist
S.
K.
Runcorn and
his associates
that the location of the paleomagnetic pole, as
by the paleomagnetism
had proposed.
ers
is
in
ferent ages, varied widely.
showed
measured
European lava flows of They found that during
dif-
the
recording both the direction and the intensity of the
500 million years, the north magnetic pole has apparently wandered from the Pacific Ocean northward through eastern and then northern Asia to its presentday location near the geographic north pole (Fig. 1310). This paleomagnetic evidence from Europe could be
magnetic
interpreted in three ways: the continent remained fixed
poles correspond closely to the location of the geo-
graphic poles (see Fig. 11-27).
When
a
magma
cools, the
iron-bearing minerals align themselves with the Earth's
magnetic
350
field
field.
when
they reach the Curie point, thus
This information can be used to deter-
Chapter 13
Plate Tectonics:
A
Unifying Theory
past
and the north magnetic pole moved; the north magnetic still and the continent moved; or both the continent and the north magnetic pole moved. When paleomagnetic readings from numerous lava flows of different ages in North America were plotted on
pole stood
a
to different magnetic pole
map, however, they pointed
same ages
locations than did flows of the
in
Europe
13-10). Furthermore, analysis of lava flows from
had
tinents indicated that each continent
of magnetic poles! Does this
had a
mean
its
(Fig.
con-
all
own
series
that each continent
That would be
different north magnetic pole?
highly unlikely and difficult to reconcile with the laws of
physics and netic field
is
what we know about how
the Earth's
,,
mag-
/jl
Path of
v
European paleomagnetic
generated (see Chapter 11).
pole
Therefore, the best explanation for the apparent
wandering of the magnetic poles
is
that they have re-
mained at their present locations near the geographic poles and the continents have moved. When the continents are fitted together so that the paleomagnetic data
point to only one magnetic pole,
we
find, just as
We-
gener did, that the rock sequences, mountain ranges,
and
glacial deposits
matic evidence
leogeography
match, and that the
fossil
and
cli-
consistent with the reconstructed pa-
is
(see Perspective 13-1).
"•'' FIGURE 13-10 The apparent paths of polar wandering for North America and Europe. The apparent
location of the north magnetic pole is shown for different periods on each continent's polar wandering path.
» SEA-FLOOR SPREADING In addition to the paleomagnetic research in the 1950s,
movement. Hess proposed
oceanographic research led to extensive mapping of the world's ocean basins (see Perspective 12-2). Such mapping revealed that the Mid-
move
a
renewed
interest in
Atlantic Ridge
is
part of a worldwide oceanic ridge
system more than 65,000
km
long.
It
was
also discov-
ered that oceanic ridges are characterized by high heat flow, basaltic volcanism,
and
seismicity.
Furthermore,
magnetic reversals, as recorded in oceanic-crust rocks, and the age of deep-sea sediments immediately above the oceanic crust occur in distinct patterns with respect to ridges.
Harry H. Hess of Princeton University conducted
much
of his oceanographic research while serving in the
central Pacific during
World War
II.
His discovery of
guyots (submerged, flat-topped volcanic islands) prois movaway from the oceanic ridges (see Fig. 12-18). As a result of his discovery of guyots and other re-
vided geologists with evidence that the sea floor ing
search conducted during the 1950s, Hess published a
landmark paper
in
1962
in
which he proposed the hy-
pothesis of sea-floor spreading to account for continental
that the continents
do not
across or through oceanic crust, but rather that the
continents and oceanic crust
move
together and are both
parts of large plates. According to Hess, oceanic crust
new
formed by newly formed oceanic crust moves laterally away from the ridge, thus explaining how volcanic islands that formed
separates at oceanic ridges where
upwelling
magma. As
the
at or near ridge crests later
magma
crust
is
cools,
become guyots
the
(Fig. 12-18).
Hess revived the idea (proposed in the 1930s and 1940s by Arthur Holmes and others) of a heat transfer system — or thermal convection cells— within the mantle as a mechanism to move the plates. According to Hess, hot magma rises from the mantle, intrudes along rift zone fractures defining oceanic ridges, and thus forms new crust. Cold crust is subducted back into the mantle at deep-sea trenches where it is heated and recycled.
How crust
is
could Hess's hypothesis be confirmed? If new forming at oceanic ridges and the Earth's mag-
netic field
is
periodically reversing
itself,
then these mag-
netic reversals should be preserved as magnetic lies in
anoma-
the rocks of the oceanic crust (Fig. 13-11).
Sea-Floor Spreading
351
Perspective 13-1
PALEOGEOGRAPHIC MAPS The
to any reconstruction of world paleogeography is the correct positioning of the continents in terms of latitude and longitude and the
and animals provides a on the latitudes determined by paleomagnetism and can provide additional limits on
proper orientation of the paleocontinent relative to the paleonorth pole. The main criteria used for paleogeographic reconstructions are paleomagnetism,
longitudinal separation of continents.
The key
biogeographic patterns indicated by
continents. For the
Paleozoic Era, however, the paleomagnetic data are
Tectonic activity
the effects of
may
be acquired through
ophiolites.
is
fossil
""'' FIGURE 1 Three paleogeographic maps and one modern during the (a) Late Cambrian Period, {b) Early Triassic Period, and (d) Recent.
Uplands and
I
I
Lowlands
mountains
352
Chapter 13
Plate Tectonics:
A
evidence.
indicated by deformed
Such features allow geologists to recognize (text
PyiSil
known
ancient mountain chains and zones of subduction.
metamorphism or weathering.
(a)
well
sediments associated with andesitic volcanics and
often inconsistent and contradictory because
secondary magnetizations
It is
and animals is controlled by both climatic and geographic barriers. Such information can be used to position continents and ocean basins in a way that accounts for the that the distribution of plants
biogeography, tectonic patterns, and climatology. Paleomagnetism provides the only quantitative data
on the orientations of the
distribution of plants
useful check
Unifying Theory
continued on page 354)
map (c)
depicting the Earth Late Cretaceous Period,
I
I
Shallow sea
I
I
Deep sea
Sea-Floor Spreading
353
These mountain chains may subsequently have been separated by plate movement, so the identification of large, continuous mountain chains provides important information about continental positions in the geologic past. Climate-sensitive sedimentary rocks are used to interpret past climatic conditions. Desert dunes are
and cross-bedded on a large and associated with other deposits, they indicate an arid environment. Coals form in freshwater swamps where climatic conditions promote abundant
exceeds precipitation, such as in desert regions or Tillites result from glacial and indicate cold, wet environments. By combining all relevant geologic, paleontologic, and climatologic information, geologists can construct paleogeographic maps (Fig. 1). Such maps are simply interpretations of the geography of an area for a
along hot, dry, shorelines. activity
The majority
typically well sorted
particular time in the geologic past.
scale,
paleogeographic maps show the distribution of land
plant growth. Evaporites result
when evaporation
Around 1960, magnetic data gathered by scientists Institution of Oceanography in Cali-
and
sea,
probable climatic regimes, and such
geographic features as mountain ranges, swamps, and glaciers.
L.
W. Morley, a Canadian geologist, independently armodel that explained this pattern of magnetic
from the Scripps
rived at a
fornia indicated an unusual pattern of alternating posi-
anomalies.
and negative magnetic anomalies for the Pacific ocean floor off the west coast of North America. The
magma
tive
pattern consisted of a series of roughly north-south parallel stripes,
but they were broken and offset by essen-
It was not until 1963 that F. Vine and D. Matthews of Cambridge University and
tially
354
east-west fractures.
Chapter 13
Plate Tectonics:
A
Unifying Theory
of
These three geologists proposed that when basaltic intruded along the crests of oceanic ridges, it would record the magnetic polarity at the time it cooled. As the ocean floor moved away from these oceanic ridges, repeated intrusions would form a symmetrical series of magnetic stripes, recording periods of normal
Oceanic ridge
Normal magnetism
Reversed magnetism
Magnetic profile as recorded by a
Continental
sequence
magnetometer
of
Continental lava flows
magnetic reversals ""'
FIGURE
crust
The sequence of magnetic anomalies preserved within
13-11
on both
the oceanic
an oceanic ridge is identical to the sequence of magnetic reversals continental lava flows. Magnetic anomalies are formed when intrudes into oceanic ridges; when the magma cools below the Curie
sides of
already
known from
basaltic
magma
records the Earth's magnetic polarity at the time. Subsequent intrusions split formed crust in half, so that it moves laterally away from the oceanic ridge. Repeated intrusions produce a symmetrical series of magnetic anomalies that reflect periods of normal and reversed polarity. The magnetic anomalies are recorded by point,
it
the previously
a magnetometer,
which measures the strength of the magnetic
and reverse polarity
(Fig. 13-11).
Shortly thereafter, the
field.
million years old, whereas the oldest continental crust
is
was supported
3.96 billion years old; this difference in age provides
by evidence from magnetic readings across the Reyk-
confirmation that the ocean basins are geologically
janes Ridge, part of the Mid-Atlantic Ridge south of
young
Vine, Matthews, and Morley proposal
A
features
whose openings and
To many
oceanic ridges.
support of continental
Magnetic surveys for most of the ocean floor have been completed (Fig. 13-12). They demonstrate that the youngest oceanic crust is adjacent to the spreading ridges and that the age of the crust increases with distance from the ridge axis, as would be expected ac-
now
cording to the sea-floor spreading hypothesis. Further-
more, the age of the oldest oceanic crust
is
less
than 180
tially
closings are par-
responsible for continental movement.
group from the Lamont-Doherty Geological Observatory at Columbia University found that magnetic anomalies in this area did form stripes that were distributed parallel to and symmetrical about the oceanic ridge. By the end of the 1960s, comparable magnetic anomaly patterns were found surrounding most Iceland.
Deep-Sea Drilling and the Confirmation of Sea-Floor Spreading amassed in and sea-floor spreading was convincing. Results from the Deep-Sea Drilling Project (see Chapter 12) have confirmed the interpretations made by earlier paleomagnetic studies. Cores of deepsea sediments and seismic profiles obtained by the Glomar Challenger and other research vessels have provided
much
geologists, the paleomagnetic data drift
of the data that support the sea-floor spreading
hypothesis.
Sea-Floor Spreading
355
EaSr% | Pleistocene |
|
to
| Paleocene (58-66
Recent (0-2 M.Y.A.)
Pliocene (2-5 M.Y.A.)
^2 Miocene (5-24
|
M.Y.A.)
^| Oligocene (24-37 Eocene (37-58
M.Y.A.)
|
Late Cretaceous (66-88 M.Y.A.)
|
Middle Cretaceous (88-1 18 M.Y.A.;
Cretaceous (118-144 | B Late Jurassic (144-161 Early
M.Y.A.)
M.Y.A.)
M.Y.A.)
M.Y.A.)
"^ FIGURE 13-12 The age of the world's ocean basins established from magnetic anomalies demonstrates that the youngest oceanic crust is adjacent to the spreading ridges and that its age increases away from the ridge axis.
According to
this hypothesis,
oceanic crust
is
contin-
uously forming at mid-oceanic ridges, moving away
distribution.
Sediments
at a rate of less
sumed
basins were as
at
subduction zones.
If this is
the case, oceanic
and become progressively older with increasing distance away from them. Moreover, the age of the oceanic crust should be symmetrically distributed about the ridges. As we have crust should be youngest at the ridges
just
deep-sea sediments to be several kilometers thick.
How-
fossils from sediments overlying and radiometric dating of rocks found
islands both substantiate this predicted age
spreading. Accordingly, at or very close to spreading
noted, paleomagnetic data confirm these state-
the oceanic crust
356
than 0.3
from numerous drill holes indicate that deepsea sediments are at most only a few hundred meters thick and are thin or absent at oceanic ridges. Their near-absence at the ridges should come as no surprise, however, because these are the areas where new crust is continuously produced by volcanism and sea-floor
ments. Furthermore,
on oceanic
open ocean accumulate, on average, cm per 1,000 years. If the ocean old as the continents, we would expect
in the
from these ridges by sea-floor spreading, and being con-
Chapter 13
Plate Tectonics:
A
Unifying Theory
ever, data
.
Oceanic crust "•"
FIGURE
13-13
The
total
thickness of deep-sea sediments
away from oceanic ridges. because oceanic crust
increases
This Total thickness of
increases
sediment
away from
oceanic ridge
Magma
Upper mantle
ridges
Increasing age of crust
where the oceanic crust
have had
little
ness
increases
(Fig.
13-13).
is
young, sediments
time to accumulate, but their thick-
with distance away from the ridges
accumulate.
much as 250 km thick, whereas those of upper mantle and oceanic crust are up to 100 km thick. The lithosphere overlies the hotter and weaker semiare as
plastic asthenosphere. It
ing from
^ PLATE TECTONIC THEORY As
early as 1965,
J. T.
Wilson of the University of Tor-
He
on the nature of large fracand named them transform
also speculated
tures in the oceanic crust faults
(discussed later in this chapter).
Isacks,
J.
Oliver,
and
L. R.
In
1968, B.
Sykes of Columbia University
the concepts of continental drift, seajjioor spreading.
nnw-heerusharrenerl
Most
it
seemingly
is
it
is
overwhelming, and also
a unifying theory that can explain
unrelated
quently, geologists
now
many
phenomena. Conseview many geologic processes,
into the
geological
phenomena occurring
at their boundaries.
» PLATE BOUNDARIES move
relative to
one another such that
their
boundaries can be characterized as divergent, conver-
and transform. Interaction of plates
at
their
volcanic activity and, as will be apparent in the next chapter, the origin of
mountain systems.
Divergent Boundaries Divergent plate boundaries or spreading ridges occur
of the terrestrial planets have had a similar
where p lates are sepaf ating~and new oceanic lit hosphere is forming. Divergent boundaries are placeswKere the cfusi is "b eing extended, thinned, and fractured as magma, derived from the partial melting of the mantle, rises to the surface. The magma is almost entirely basaltic and intrudes into vertical fractures to form dikes and lava flows (Fig. 13-15). As successive injections of magma cool and solidify, they form new oceanic crust and record the intensity and orientation of the Earth's magnetic field (Fig. 13-11). Divergent boundaries most
cause
all
origin
and
early history, geologists are interested in de-
termining whether plate tectonics it
operates in the same
is
unique to Earth or
way on
the other terres-
planets (see Perspective 13-2).
based on a simple model of both oceanic and continental crust, as well as the underlying upper mantle, consists of numerous variable-sized pieces called plates (Fig. 13-14). The plates vary in thickness; those composed of upper mantle and continental crust Plate tectonic theory
the Earth.
such as at oceanic
and are subducted back
tectonics. Furthermore, be-
from the perspective of plate
trial
the asthenosphere, they separate, mostly
geologic
such as mountain building, seismicity, and volcanism,
whether
result-
boundaries accounts for most of the Earth's seismic and
geologists accept plate tectonic theory, in part
because the evidence for because
movement
transfer system within the
mantle. Individual plates are recognized by the types of
gent, t^4>late_iectonics
move over
trenches, they collide
Plates has:
believed that
at oceanic ridges, while in other areas
proposed the term new global tectonics to encompass
and^ansforrn jaults/Ihat rprm
is
some type of heat
asthenosphere causes the overlying plates to move. As plates
onto proposed that the Earth's crust is composed of several large rigid plates that move with respect to one another.
is
becomes older away from oceanic ridges, and thus there has been more time for sediment to
The
is
rigid outer lithosphere, consisting of
commonly occur along
the crests of oceanic ridges, for
Plate Boundaries
357
Perspective 13-2
TECTONICS OF THE TERRESTRIAL PLANETS Recall from Chapter 2 that the four terrestrial planets— Mercury, Venus, Earth, and Mars— all had a similar early history involving accretion,
and silicate mantle and formation of an early atmosphere by outgassing. Their early history was marked by widespread volcanism and meteorite impacts, both of which helped modify their surfaces. The volcanic and tectonic activity and resultant surface features (other differentiation into a metallic core
and
crust,
"^ FIGURE 2 {a) Western Ishtar Terra and mountain belts surrounding Lakshmi Planum. Surrounding Western Ishtar Terra are a transitional zone (blue) and lowlands plains (rust), (b) A radar image of Akna Montes, Freyja Montes, and a portion of Lakshmi Planum illustrating the folded and faulted nature of the Akna and Freyja montes.
than meteorite craters) of these planets are clearly related to the way in which they transport heat from their interiors to their surfaces.
The Earth appears is
broken up into a
to be
unique in that
series of plates.
The
its
surface
creation and
destruction of these plates at spreading ridges
and
subduction zones transfer the majority of the Earth's internally
produced heat. In addition, movement of
the plates, together with life-forms, the formation of
sedimentary rocks, and water,
is
responsible for the
cycling of carbon dioxide between the atmosphere
Sedna
and
Planitia
lithosphere and thus the maintenance of a habitable
climate
on Earth
340°
(see Perspective 2-2).
"^^
FIGURE 1 This radar image of Venus made by the Magellan spacecraft reveals circular and oval-shaped volcanic features. A complex network of cracks and fractures extends outward from the volcanic features. Geologists think these features were created by blobs of magma rising from the interior of Venus with dikes filling some of the cracks.
358
Chapter 13
Plate Tectonics:
A
Unifying Theory
(a)
350° 50°
,
50°
Heat
is
transferred between the interior
and surface of
both Mercury and Mars mainly by lithospheric conduction. This method
is
sufficient for these planets
because both are significandy smaller than Earth or Venus.
Because both Mercury and Mars have a
single, globally
continuous plate, they have exhibited fewer types of volcanic
and
The warming of Mercury and Mars produced
tectonic activity than has the Earth.
initial interior
expansional features such as normal faults (see Chapter 14)
and widespread volcanism, while their subsequent cooling produced folds and faults resulting from compressional forces, as well as a succession of volcanic activity.
Mercury's surface is heavily cratered and shows the way of primary volcanic structures.
little in
However,
it
does have a global system of lobate scarps These have been interpreted as
(see Fig. 2-10).
evidence that Mercury shrank a
little
soon
after its
crust hardened, resulting in crustal cracking.
Mars has numerous
features that indicate
early period of volcanism.
an extensive
These include Olympus Mons,
the solar system's largest volcano (see Fig. 2-12), lava flows,
uplifted regions believed to have resulted
from
convection. In addition to volcanic features,
Mars
and
mande
abundant evidence of tensional tectonics, numerous faults and large fault-produced valley structures. While Mars was tectonically active during the past, there is no evidence that plate tectonics comparable to that on Earth has ever occurred there. Venus underwent essentially the same early history as also displays
including
the other terrestrial planets, including a period of it is more Earth-like in its tectonics than Mercury or Mars. Initial radar mapping in 1990
volcanism, but either
by the Magellan spacecraft revealed a surface of extensive lava flows, volcanic domes, folded mountain ranges, and an extensive and intricate network of faults, all
of which attest to an internally active planet (Fig.
1).
broad plateau area named the Western Ishtar Terra, a series of mountain belts surrounds Lakshmi In a
Planum, a central smooth plain (Fig. 2). On the basis of detailed mapping from radar images and interpretation
FIGURE
movement. It is thought that the Freyja Montes region was the site of large-scale crustal convergence that is continuing as a result of the underthrusting of the North
Block diagram showing the geologic history region, (a) Crustal convergence and compression cause buckling and underthrusting of the crust and lithosphere. (b) Continued convergence, compression, and underthrusting produce crustal thickening, uplift, and the formation of new zones of underthrusting. (c) Continuing convergence, crustal thickening, and underthrusting cause numerous slabs of crust to overlap one another like shingles, producing the present-day
Polar Plains beneath Ishtar Terra (Fig. 3).
configuration of the region.
of the topography and geology of the
Akna and
"*r-
Freyja
montes, geologists believe that these structures represent
mountain
belts.
faults resulting
Features identified include folds and from compressive forces and horizontal
of the Freyja
3
Montes
Plate Boundaries
359
• Hot spot
—»- Direction
"^ FIGURE direction of
13-14
of
movement
A map
of the world showing the plates, their boundaries,
movement, and hot
spots.
'*"' FIGURE 13-15 Pillow lavas forming along the Mid-Atlantic Ridge. Their distinctive bulbous shape result of underwater eruption.
example, the Mid-Atlantic Ridge. Oceanic ridges are thus is
the
characterized by rugged topography with high relief resulting from displacement of rocks along large fractures,
shallow-focus earthquakes, high heat flow, and basaltic flows or pillow lavas.
Divergent b ound aries also occur under continents
during
trie early"
stages of continental breakup (Fig. 13-
When magma
16).
crust
is
wells
initially elevated,
up beneath a continent, the extended, and thinned (Fig.
13-16a). Such stretching eventually produces fractures
an d
rift
v alleys.
During IKIs
stage, magma~~typically in-
trudes into the faults and fractures forming
sills,
and
valley floor
(Fig.
lava flows; the latter often cover the
13-16b).
example of If
The East African rift valleys
this stage
rift
are an excellent
of continental breakup
spreading proceeds, some
rift
dikes,
(Fig. 13-17).
valleys will continue
and deepen until they form a narrow linear two continental blocks (Fig. 13- 16c). The Red Sea separating the Arabian Peninsula from Africa (Fig. 13-17) and the Gulf of California, which separates to lengthen
sea separating
360
Chapter 13
Plate Tectonics:
A
Unifying Theory
Crustal
upwarp
Narrow sea
"^
FIGURE 13-16 History of a divergent plate boundary, {a) Rising magma beneath a continent pushes the crust up, producing numerous cracks and fractures, (b) As the crust and thinned,
is
and lava flows onto the valley floors, (c) Continued spreading further separates the continent until a narrow seaway develops, (d) As spreading continues, an oceanic ridge system forms, and an ocean basin develops and grows. stretched
rift
valleys develop,
Baja California from mainland Mexico, are good exam-
advanced stage of rifting. As a newly created narrow sea continues enlarging, it may eventually become an expansive ocean basin such as the Atlantic, which separates North and South America from Europe and Africa by thousands of kilometers (13-16d). The Mid-Atlantic Ridge is the boundary between these diverging plates; the American plates are
ples of this
moving westward, and the Eurasian and African are moving eastward.
plates
Convergent Boundaries'^ Because new lithosphere
is
formed
at divergent plate
boundaries, older lithosphere must be destroyed and recycled in order for the entire surface area of the Earth to
Plate Boundaries
361
Most
SO°E
of these planes dip from oceanic trenches beneath
adjacent island arcs or continents, marking the surface of Levantine
Rift
slippage between the converging plates. ing plate
moves down
As the subduct-
into the asthenosphere,
and eventually incorporated subduction does not occur
it is
into the mantle.
when both
heated
However,
of the converging
plates are continental because continental crust
is
not
dense enough to be subducted into the mantle.
Convergent boundaries are characterized by deformamountain building, metamorphism, seis-
tion, volcanism,
micity,
and important mineral
convergent plate
boundaries
oceanic, oceanic-continental,
Oc eanic -Oceanic Carlsberg
Ridge
deposits.
Three types of
recognized:
are
oceanic-
and continental-continental.
Boundaries
When-twxLXiceanic plates^conterge, one of them is subducted beneath t he other along an oceanic-oceanic plate
boundary
13-18). The subducting plate bends an angle between 5° to 10° to form the
(Fig.
downward
at
outer wall of an oceanic trench.
The
inner wall of the
trench consists of a subduction complex
composed of
wedge-shaped slices of highly folded and faulted marine sediments and oceanic lithosphere scraped off from the descending plate. This subduction complex is elevated Rift
T
as a result of uplift along faults as subduction continues
I
'
Rift valley
I
I
Oceanic crust
I
I
(Fig. 13-18).
As the subducting plate descends into the asthenosit is heated and partially melted, generating a
Stretched continental
phere,
crust
magma, commonly
magma and
is
less
of
andesitic
This
composition.
dense than the surrounding mantle rocks
rises to the surface
overriding plate where
through the nonsubducting or forms a curved chain of vol-
it
canoes called a volcanic island arc (any plane intersect-
Madagascar
makes an arc). This arc is nearly parallel to and is separated from it by up to hundred kilometers — the distance depends on
ing a sphere
the oceanic trench several Kilometers
•^ FIGURE
13-17
The East African
the angle of dip of the subducting plate (Fig. 13-18).
L
J
being formed by the separation of eastern Africa from the rest of the continent along a divergent plate boundary. The Red Sea represents an advanced stage of rifting, in which two continental blocks are separated by a narrow sea. rift
valley
is
Located between the volcanic island arc and the subduction complex of the oceanic trench (Fig. 13-18). It typically
362
Chapter 13
Plate Tectonics:
A
Unifying Theory
a fore-arc basin
generally flat-lying detrital sediments up to 5 km thick. These sediments are derived from the weathering and erosion of the island arc volcanoes and reflect a progressive shallowing as the basin
remain constant. Otherwise, we would have an expanding Earth. Such plate destruction occurs at convergent plate boundaries where two plates collide. At a convergent boundary, the leading edge of one plate descends beneath the margin of the other_by_sjibdiigtion. A dipping plane of earthquake foci, referred to as a Benioff zone, defines subduction zones (Fig. 10-8).
is
contains a diverse assortment of
In those areas
where the
fills
up.
rate of subduction
is
faster
than the forward movement of the overriding plate, the lithosphere
arc
may
on the landward
and thinned,
resulting in the formation of a back-arc
basin. This back-arc basin
magma
side of the volcanic island
be subjected to tensional stress and stretched
may grow by
spreading
breaks through the thin crust and forms
if
new
Sea
level
—
FIGURE 13-18 Oceanic-oceanic plate boundary. An oceanic trench forms where one oceanic plate is subducted beneath another. As a result of subduction, a complex of highly folded and faulted marine sediment and scraped-off pieces of oceanic lithosphere forms along the inner Magma
Asthenosphere
wall of the trench.
On
the
nonsubducted plate, a volcanic island arc forms from the rising magma generated from the subducting plate.
The
and Antillean (Caribbean)
oceanic crust (Fig. 13-18). In any case, the back-arc ba-
pine Islands.
with a mixture of volcanic rocks and detrital sediments. A good example of a back-arc basin associated with an oceanic-oceanic plate boundary is the Sea
land arcs are present in the Atlantic Ocean basin.
of Japan between the Asian continent and the islands of
When
sin will
fill
Japan.
Most present-day active volcanic island arcs are in Ocean basin and include the Aleutian Islands,
the Pacific the
Kermadec-Tonga
arc,
and the Japanese and
Philip-
Scotia
Oc eanic-Continen ta
l
is-
Boundaries
an oceanic and a continental plate c onverge, the oceanic plate is subducted under the continental plate alo ng an oceanic-continental pla te_boundary (Fig. 1319).
The oceanic
plate
is
subducted because
it is
denser
than continental crust. Just as at oceanic-oceanic plate
— FIGURE
13-19
Oceanic-continental plate boundary.
Continental interior
When
Trench
Sea level
an oceanic plate is subducted beneath a continental plate, an andesitic volcanic mountain range is
formed on the continental plate result of rising
Magma
as a
magma.
Continental crust
Asthenosphere
Plate Boundaries
363
boundaries, the descending oceanic plate forms the
of subduction, and the Andes Mountains are the result-
outer wall of an oceanic trench; a subduction complex
ing volcanic
forms the inner wall of the trench and between continent
is
it
and the
mountain chain on the overriding plate
(see Fig. 4-31).
a fore-arc basin.
The oceanic trenches of oceanic-continental boundaries typically contain
sediments derived from the ero-
Continental-Continental Boundaries
rocks. These
converge ;dong a boundary, one platem av partially slide undg£the other, but neither plate wil l be subductej becausej^Lt heir low and equal de nsities and
well as
great thickness (Fig. 13-20). These continents are
The subduction complex consists of wedge-shaped slices of complexly folded and faulted sion of continents.
wedges contain continental sediments as some of the sediment and pieces of crust that are scraped off by the overriding continental plate. The subduction complex is elevated as new slices are added by the underthrusting of subduction. The fore-arc basin of the
continental
plates
rtinental plate
ini-
separatecTfrom ojiejmojhgr_ by oceanic crust that being subducted under one of the continents. The edge
tially is
of that continent will display the characteristics of an
oceanic-continental boundary contains detrital sediments
oceanic-continental boundary with the development of
derived from the erosion of the continent. These sediments
a deep-sea trench,
are typically flat-lying or only mildly deformed.
and volcanic arc (Fig. 13-19). Eventually, the oceanic crust is totally consumed and the two continents collide; the sediments and portions of sea floor caught between the two plates are deformed and uplifted. A new mountain range is thus formed, composed of deformed sedimentary rocks, scraped-off oceanic crust, and the vol-
As the
cold, wet,
and
slightly denser oceanic plate
descends into the hot asthenosphere, melting occurs and
magma
is
generated. This
riding continental plate
magma
rises
beneath the over-
and can extrude
at the surface,
producing a chain of andesitic volcanoes (also called a volcanic arc), or intrude into the continental margin as plutons, especially batholiths. filled
A
back-arc basin
may
be
with continental detrital sediments, pyroclastic
and lava flows, derived from and thickening toward the volcanic arc. An excellent example of an oceanic-continental plate boundary is the Pacific coast of South America where the oceanic Nazca plate is currently being subducted under South America. The Peru-Chile Trench is the site materials,
*»-
FIGURE
13-20
When two
canic arc of the overriding plate.
The Himalayas, the world's youngest and highest mountain system, resulted from the collision between India and Asia that began about 40 to 50 million years ago and is still continuing (Fig. 14-35). During this collision, the leading margin of the Indian plate was partially forced under the Asian plate, resulting in a thick accumulation of and the uplift of the Himalayas and the Tibetan Plateau. Other examples of mountain continental lithosphere
Deformed and metamorphosed subduction complex
Continental-continental plate
boundary.
subduction complex, fore-arc basin,
continental
is subducted because of their great thickness and low and equal densities. As the two
plates converge, neither
Oceanic crust fragments
continental plates collide, a
mountain range interior
formed in the of a new and larger is
continent.
Continental crust
Magma Asthenosphere -
364
Chapter 13
Plate Tectonics:
A
Unifying Theory
Oceanic crust
NICHOLAS
Guest Essay
B.
CLAUDY
GEOSCIENCE CAREERS-THE IS UNPARALLELED
DIVERSITY
The following essay originally appeared in the January 1991 issue of Geotimes, and has been adapted with permission from the author.
Department of Energy and the Environmental Numerous employment opportunities in energy-related programs will Protection Agency.
show moderateThe geosciences
offer unparalleled career opportunities
that reflect a unique blend of disciplines.
Whether you
many
scientific
are interested in scientific
sector for the next few years.
and development to problem solving, conserving and protecting natural resources, or disseminating geologic knowledge, the geosciences offer rewarding careers. research, applying research
consultants
into the 1990s.
retirements increase.
employment growth than for the labor force as a whole; and potential shortages of workers, due to depressed enrollments, too few new graduates, and the
The following
-
force
in
employed
sources,
on
More
areas.
qualified secondary
result,
preferred credentials for
its
However,
all
requisites.
A
its
list
of
employers seek a few basic
were
far
more
is
highly desirable.
B.A./B.S.
graduates than jobs available, but the situation
emphasis
was
quite the opposite for those with a master's
degree. Diversity of coursework
Domestically, there will be increased
valued, since
it
experience
(
is
Any work
full-time, part-time or
also a valuable asset. Skills in oral
and an energy
communication are
and viable option.
necessity for
Mining/minerals (9%): Worldwide metallicand nonmetallic-mineral exploration and
highly
allows the employee to be more
adaptable to employer needs.
on improved recovery technology
rather than exploration. Shortages of geoscientists
career remains a strong
own
new employees.
master's degree
In 1990, there
global expansion of energy
are likely in the next few years,
and high
placed on
is
markets and improved research and operations. concentration
summer) is and written
also frequently cited as a
new employees, a
the federal sector will probably not
B. Gaudy graduated from Brown University where he majored in Greek studies and earned a master's degree in Greek from the University of North Carolina at Chapel Hill. In 1979, he joined the American Geological Institute where he is responsible for
hiring significantly, although
preparing several publications. In
production will continue as current supplies decrease. Probable growth in nuclear power will increase interest in energy-related minerals,
such as uranium and plutonium. Federal/state (12%): Due to budget constraints,
efforts will require a larger
expand its some regulatory work force. State
agencies will continue to assume a greater role in regulatory activities. -
some
math literacy. Each category of employer has
alternative energy
and conservation. As a
will be placed
on
on
science skills and
in that area):
world's attention has been refocused oil, realistic
increased emphasis
sciences as increased emphasis
Oil/gas (50%): Since the invasion of Kuwait, the
dependence on
The
school teachers will be needed in the earth
force.
are the major geoscience employers
work
predicted for
environmental studies will perhaps allow growth
(the figure in parentheses indicates the percentage of
the geoscience
is
academia as enrollments begin to recover and
greater
work
And, for those
deal with environmental issues,
Academia (14%): Modest growth
demand for lower unemployment rate and far
aging of the current
who
faster-than-average growth should continue well
Several factors will contribute to the geoscientists: a far
to above-average growth.
Consulting (11%): This has been and will continue to be the fastest growing employment
1986, he became the
.
-
-i.
institute's
director of development. Claudy
notes that his general liberal arts
education
Research institutions/Department of Energy labs (4%): This employment category includes energy-related programs funded by the U.S.
AAAAAAAAAAAAAAAAA,AAAAAAAA«
JN icholas
is
an example of
how
geology-related positions are to people from diverse backgrounds.
open
AAA AAAAAAAAAAJ
Hiliit illi tiiti
j
ranges that formed by continent-continent collision are Sea
the Appalachians, Alps,
and Urals
(see
Chapter
14).
level
Transform Boundaries Thej hird ary
type of rjlaiejjoundary is a transform bounda long transform faults where plates
These occur
slide laterall y past
one another roughly parallel to the
directionof_plate
movemen t. Although
lithosphere
is
neither created nor destroyed along a transform boundary, the
Oceanic
movement between
intensely shattered rock
Upper
plates results in a zone of and numerous shallow-focus
earthquakes.
mantle
Transform
(a)
faults are particular types of faults that
'
transform" or change one type~of motion_betjveen plates lntoan otRer type of notion. The majority of transfoFm raultsconnect two oceanic ridge segments, but they '
Transform fault
Trench
Sea
level
/
can also connect ridges to trenches and trenches to trenches (Fig. 13-21). While the majority of transform faults
^,
occur
in
oceanic crust and are marked by distinct
fracture zones, they
One
may
also extend into continents.
of the best-known transform faults
is the San Andreas fault in California. It separates the Pacific plate from the North American plate and connects spreading ridges in the Gulf of California and the ridge separating the Juan de Fuca and Pacific plates off the coast of
northern California
(Fig.
13-22).
The many earthquakes movement along
that affect California are the result of this fault. (b)
Transform
Trench
Sea
fall
|
„ Oceanic
f
ridge
level
^ PLATE MOVEMENT AND MOTION How
and in what direction are the Earth's various moving, and do they all move at the same rate? Rates of movement can be calculated in several ways. The least accurate method is to determine the age of the sediments immediately above any portion of the oceanic crust and divide that age by the distance from the spreading ridge. Such calculations give an average rate fast
plates
of movement.
Magma
Oceanic
Ajnore
/
crust
the magnetic reversals in the crust of the sea floor. Recall
mantle (c)
'"•'
that magnetic reversals are distributed symmetrically
FIGURE
13-21 Horizontal movement between plates occurs along a transform fault, (a) The majority of transform faults connect two oceanic ridge segments. Note that relative motion between the plates only occurs between the two ridges, (b) A transform fault connecting two trenches, (c) A transform fault connecting a ridge and a trench.
366
accura te method of determining both the avmovement and relative motion is by dating
erage rate of
Upper
Chapter 13
Plate Tectonics:
A
Unifying Theory
about and parallel to the oceanic ridges (Fig. 13-12), and that the age of each reversal has been determined. Therefore, the distance from an oceanic ridge axis to any magnetic reversal indicates the width of new sea floor that formed during that time interval. Thus, for a given interval of time, the wider the strip of sea floor, the faster the plate has moved. In this way not only can the
British
Columbia
<s^Xeg'
&f
Juan cieFuca
.Seattle
Washington
plate
Montana
Oregon
*^ FIGURE 13-23 This map shows the average rate of year and relative motion of the Earth's plates. r
movement and
present average rate of
be determined
motion
13-23), but the average rate of
(Fig.
movement during
relative
movement
the past can also be calculated by
dividing the distance between reversals by the
amount
of
the information in Figure 13-23,
that the rate of
movement
per
movement and relative motion have also been calculated by measuring the difference between artion, rates of
rival times of radio signals
receiving stations
on
from the same quasar
different plates.
The
to
rate of plate
movement determined by
time elapsed between reversals.
From
in centimeters
varies
among
it is
obvious
plates.
The
lates closely
these two techniques correwith those determined from magnetic re-
versals (Fig. 13-23).
southeastern part of the Pacific plate and the Cocos plates are the
two
fastest
moving
plates, while the
Ara-
bian and southern African plates are the slowest.
The average
movement as motion between any two plates can by
rate of
satellite laser
station
on one
bounced
synchronous
orbit)
ferent plate.
As the plates move
there
beam
is
Plate motions derived
also be determined
and
ranging techniques. Laser beams from a
plate are
an increase
off a satellite (in geo-
and returned
to a station
on
a dif-
lasers give
respect to another.
must have
from magnetic
To determine absolute motion, we
a fixed reference
rection of plate
reversals, satellites,
only the relative motion of one plate with
from which the
rate
and
di-
movement can be determined. Hot spots,
of time that the laser
which may provide reference points, are locations where stationary columns of magma, originating deep within
relative to each other,
in the length
Hot Spots and Absolute Motion
well as the relative
takes to go from the sending station to the sta-
the mantle (mantle plumes), slowly rise to the Earth's
tionary satellite and back to the receiving station. This
surface and form volcanoes or flood basalts (Fig. 13-14).
used to calculate the rate of
One of the best examples of hot spot activity is that over which the Emperor Seamount— Hawaiian Island
difference in elapsed time
movement and
368
relative
Chapter 13
is
motion between
Plate Tectonics:
A
plates. In addi-
Unifying Theory
Aleutian
Kamchatka
Islands
Sea
level
/
Sea
/ Sea
level
level
Al
We also know that the ultimate energy source the plates
is
the Earth's internal heat,
heat gets to the surface by
within the mantle. heated,
it
When
some type of convection
a portion of the mantle
expands, becoming
less
rounding rock, and thus slowly face.
To
offset this
warm
driving
and much of that is
material must
vection
rising mass, cooler, denser
move downward.
In this
manner, a con-
which
warm
material rises to
generated
in
the surface, and cooler material descends back into the
Earth's interior.
Two
dense than the sur-
rises to the Earth's sur-
cell is
models involving thermal convection
been proposed to explain plate movement In
one model, thermal convection
have
cells
(Fig. 13-25).
cells are restricted to
model the
the asthenosphere, whereas in the second
en-
mantle is involved. In both models spreading ridges mark the ascending limbs of adjacent convection cells, while trenches occur where the convection cells descend back into the Earth's interior. Thus, the locations of spreading ridges and trenches are determined by the tire
"^ FIGURE
13-25 Two models involving thermal have been proposed to explain plate movement, (a) In one model, thermal convection cells are restricted to the asthenosphere. (b) In the other model, thermal convection cells involve the entire mantle. convection
cells
convection els,
Oceanic ridge
Oceanic trench
themselves. Furthermore, in both
cells
the lithosphere
is
mod-
considered to be the top of the
cell, and each plate therefore corresponds to a single convection cell. While most geologists agree that the Earth's internal heat plays an important role in plate movement, problems are inherent in both models. The major problem associated with the first model is the difficulty in explaining the source of heat for the convection cells and
thermal convection
Oceanic trench
why
they are restricted to the asthenosphere. In the sec-
ond model,
the source of heat
outer core, but
it is still
not
comes from the Earth's
known how
heat
is
trans-
from the outer core to the mantle. Nor is it clear convection can involve both the lower mantle and
ferred
Oceanic ridge
how
the asthenosphere.
Some Lithosphere
Oceanic ridge
geologists believe that in addition to being gen-
erated by thermal convection within the Earth, plate
movement
Oceanic trench
mechanism
also occurs, in part, because of a
involving "slab-pull" or "ridge-push" (Fig. 13-26). Both (a)
of these mechanisms are gravity driven, but Oceanic ridge
Oceanic trench
Oceanic trench
on thermal differences within the Earth.
depend
still
In "slab-pull,"
because the subducting cold slab of lithosphere
is
than the surrounding warmer asthenosphere,
pulls the
rest of the plate
along with
it
as
it
is
a
denser
descends into the
asthenosphere. As the lithosphere moves there
it
downward,
corresponding upward flow back into the
spreading ridge.
Operating
in
conjunction with "slab-pull"
"ridge-push" mechanism. As a result of rising
is
the
magma,
the oceanic ridges are higher than the surrounding oce-
anic crust. lithosphere
Oceanic ridge
It is
believed that gravity pushes the oceanic
away from
the higher spreading ridges
and
toward the trenches. Currently, geologists are fairly certain that
of convective system
Lithosphere
Oceanic ridge Oceanic trench (b)
370
Chapter 13
Plate Tectonics:
A
Unifying Theory
is
some type
involved in plate movement.
However, the extent to which other mechanisms such as "slab-pull" and "ridge-push" are involved is still unresolved. Consequently, no comprehensive theory of plate
SGa Trench |
Rising
eve
.
\
magma Convection
Asthenosphere
^^
cell
movement (a)
Tre nch ,
Sea
level
^ FIGURE
13-26 Plate is also thought to occur because of gravity-driven "slab-pull," or "ridge-push" mechanisms, (a) In "slab-pull," the edge of the subducting plate descends into the Earth's interior, and the rest of the plate is pulled
movement
downward, Rising
magma
^^
Convection cell
(b)
Asthenosphere
movement
"ridge-push,"
(b) In
magma
pushes the oceanic ridges higher than the rest of the oceanic crust. Gravity thus pushes the oceanic lithosphere rising
away from
the ridges
and toward
the trenches.
movement has been developed, and much to be learned
about the Earth's
still
remains
their search for
new mineral deposits and known deposits.
in
explaining
the occurrence of
interior.
Many
metallic mineral deposits such as copper, gold,
lead, silver, tin,
^
PLATE TECTONICS AND THE DISTRIBUTION OF
and zinc are related
sociated hydrothermal activity, so
to igneous it
is
and
as-
not surprising
that a close relationship exists between plate boundaries
and these valuable deposits. In the late 1960s, Frederick Sawkins of the University of Minnesota pointed out that
NATURAL RESOURCES and distribution of the Earth's natural resources. Con-
majority of metallic sulfides are located along present-day and ancient plate boundaries. The magma generated by partial melting of a sub-
sequently, geologists are using plate tectonic theory in
ducting plate rises toward the Earth's surface, and as
In addition to being responsible for the
the Earth's crust, plate
movement
major features of
affects the
formation
the
Plate Tectonics
and the Distribution of Natural Resources
it
371
• Porphyry copper deposits a. Subduction zone
— Divergent boundary "^ FIGURE
13-27
Important porphyry copper deposits North and South
are located along the west coasts of
America.
cools,
it
precipitates
and concentrates various metallic
Some of the major metallic ore deposits copper and molybdenum, for example) associ-
sulfide ores.
(such as
"^ FIGURE
Bingham Mine
13-28
copper deposits
in
the
when
world's gold
is
associated with sulfide deposits located
at ancient convergent plate
boundaries in such areas as South Africa, Canada, California, Alaska, Venezuela, Brazil, southern India, the Soviet Union, and western
less
than 60 million years ago
oceanic plates were subducted under the North
Divergent plate boundaries also yield valuable
The porphyry copper
deposits of western
North and
South America are an excellent example of the relationship between convergent plate boundaries and the distribution, concentration, and exploitation of valuable Chapter 13
huge
and South American plates. The rising magma and associated hydrothermal fluids carried minute amounts of copper, which was originally widely disseminated but eventually became concentrated in the cracks and fractures of the surrounding andesites. These low-grade porphyry copper deposits contain from 0.2 to 2% copper and are extracted from large open-pit mines (Fig. 13-28).
Australia.
372
a
The world's largest copper The majority of the Andes and the southwestern
the Andes, Rockies, the Coast Ranges of
region to Pakistan. In addition, the majority of the
is
metallic ores (Fig. 13-27).
United States were formed
South America, Japan, the Philippines, the Soviet Union, and a zone extending from the eastern Mediterranean
Utah
deposits are found along this belt.
ated with convergent plate boundaries include those in
North and
in
open-pit copper mine with reserves estimated at 1.7 billion tons. More than 400,000 tons of rock are removed each day. (Photo courtesy of R. V. Dietrich.)
Plate Tectonics:
A
Unifying Theory
sources.
As we discussed
vents are the sites of
Chapter 12, hydrothermal
much
metallic mineral precipita-
Cyprus in the Mediterranean is rich copper and has been supplying all or part of the
tion. in
The
re-
in
island of
world's needs for the last 3,000 years. tion of itation
The concentra-
copper on Cyprus formed as a result of precipadjacent to hydrothermal vents. This deposit was
stage in the growth of an ocean basin. Sediments sampled from three central basins within the Red Sea are rich in the
aforementioned elements; they are believed to
brought to the surface when the copper-rich sea floor collided with the European plate, warping the sea floor
result
from the interaction of the hot seawater and the
rising
magma.
and forming Cyprus.
becoming increasingly clear that if we are to keep up with the continuing demands of a global industrialIt is
Studies indicate that minerals of such metals as cop-
and zinc are currently formThe Red Sea is opening as divergence and represents the earliest
per, gold, iron, lead, silver,
ing as sulfides in the a result of plate
Red
Sea.
» CHAPTER SUMMARY 1.
origin
subsequently 2. Alfred
is
The average
each other. rate of movement and relative motion of plates can be calculated several ways. These slide past
different
plate
He
provided abundant geological and paleontological evidence to show that the continents were once united into one supercontinent he named Pangaea. Unfortunately, Wegener could not explain how the continents moved, and therefore most geologists ignored his ideas.
The hypothesis of continental drift was revived during the 1950s when paleomagnetic studies revealed that there apparently were multiple
magnetic north poles. This paradox was resolved by moving the continents into different positions. When this was done, the paleomagnetic data were
essential.
8.
generally credited with developing
the hypothesis of continental drift.
is
Three types of plate boundaries are recognized: divergent boundaries, where plates move away from each other; convergent boundaries, where two plates collide; and transform boundaries, where two plates
split apart.
Wegener
and distribution of mineral resources
7.
The concept of continental movement is not new. Early maps showing the similarity between the east coast of South America and the west coast of Africa provided people with the first evidence that the continents may once have been united and
3.
ized society, the application of plate tectonic theory to the
methods
all
movement and
yield similar average rates of
indicate that the plates
move
at
different average velocities. 9.
Absolute motion of plates can be determined by the movement of plates over mantle plumes. A mantle plume is an apparently stationary column of magma that rises to the Earth's surface
where
it
becomes a
hot spot and forms a volcano. 10. Although a comprehensive theory of plate
movement
has yet to be developed, geologists believe that some type of convective heat system is involved. 11. A close relationship exists between the formation of mineral deposits and plate boundaries. Furthermore, the formation and distribution of the Earth's natural
resources are related to plate movement.
consistent with a single magnetic north pole. 4.
Magnetic surveys of the oceanic crust reveal magnetic anomalies in the rocks indicating that the Earth's magnetic field has reversed itself in the past. Since the anomalies are parallel and form symmetric belts adjacent to the oceanic ridges,
crust
must have formed as the sea
new
floor
was
6.
back-arc basin continental-continental
spreading. 5.
IMPORTANT TERMS
oceanic
Sea-floor spreading has been confirmed by dating the
plate
boundary
oceanic-oceanic plate
boundary Pangaea
sediments overlying the oceanic crust and by radiometric dating of rocks on oceanic islands. Such dating reveals that the oceanic crust becomes older
continental drift
plate
convergent plate
plate tectonics
with distance from spreading ridges. Plate tectonic theory became widely accepted by the 1970s because of the overwhelming evidence supporting it and because it provides geologists with a powerful theory for explaining such phenomena as
divergent plate
Gondwana
volcanism, seismicity, mountain building, global climatic changes, past and present animal and plant
hot spot Laurasia
distribution,
and the distribution of mineral
boundary boundary fore-arc basin
Glossopteris flora
sea-floor spreading
spreading ridge
subduction thermal convection cell transform boundary transform fault volcanic island arc
oceanic-continental plate
boundary Important Terms
373
^
REVIEW QUESTIONS
11.
Which of
the following will allow you to determine
the absolute motion of plates? 1.
The man who a.
c.
d.
d.
satellite laser
credited with developing the
a.
directly
is:
Wilson; b. Hess; c. Vine; du Toit. Wegener; e. 2. The southern part of Pangaea, consisting of South America, Africa, India, Australia, and Antarctica, is a.
Gondwana;
d.
Laurentia;
Which
Laurasia;
b.
12.
Atlantis;
c.
Pacifica.
e.
of the following has been used as evidence continental
animals;
b.
fit;
13.
plants and
fossil
all of these. paleomagnetism; e. Magnetic surveys of the ocean basins indicate
the oceanic crust
a.
spreading ridges;
youngest the oceanic crust is
the oceanic crust c.
is
the youngest adjacent to spreading ridges; d. oceanic crust is the same age in all ocean basins; e. answers (a) and (b). Plates:
same thickness everywhere;
are the
a
vary in thickness; c. upper mantle; d. answers
include the crust and
b.
and
(a)
answers (b) and (c). Divergent boundaries are the areas where: new continental lithosphere is forming; a. b. V- new oceanic lithosphere is forming;
two
c.
7.
plates
come
two
together; d.
each other;
14.
e.
10.
transform boundaries;
oceanic-
e.
The driving mechanism of
plate
movement
is
magnetism;
isostasy; b.
thermal
c.
rotation of the Earth;
cells; d.
e. none of these. The formation and distribution of copper
deposits
boundaries.
convergent;
divergent; b.
answers (a) and (b); and (c). Name the type of plate boundary indicated in the illustration found at the top of page 375. c.
transform;
e.
answers
d.
(b)
15.
17. 18. 19.
What
evidence convinced Wegener that the
continents were once joined together and
subsequently broke apart? 20.
Why
can't the similarity between the coastlines of
continents alone be used to prove they were once
divergent; b.
c.
convergent;
d.
e.
answers
and
(b)
21.
transform;
answers
(a)
and
22.
(c).
The west coast of South America
is
divergent; b.
c.
oceanic-oceanic; d.
e.
transform.
oceanic-continental;
a.
divergent; b.
c.
transform;
e.
answers
(b)
and fault
plate
can magnetic anomalies be used to show that
What
answers
(a)
27.
convergent;
c.
transform; d.
e.
continental-continental.
oceanic-continental;
Plate Tectonics:
how
What
Unifying Theory
rates of
movement of
plates can be
are mantle
plumes and hot spots?
How
can
they be used to determine the direction and rate of 28.
movement of plates? are some of the
What
positive
and negative features
of the various models proposed to explain plate
movement?
A
features characterizing the
determined.
(b);
an example of a(n)
divergent; b.
Summarize the geologic
26. Explain
and
(c). is
25.
three different types of plate boundaries.
convergent;
d.
a
Chapter 13
How
theory?
Back-arc basins are associated with boundaries.
The San Andreas
the significance of polar wandering in
evidence besides magnetic anomalies convinced geologists of sea-floor spreading? 24. Why is plate tectonics such a powerful unifying 23.
continental-continental;
is
the sea floor has been spreading?
an example of
plate boundary.
a.
What
relation to continental drift?
(b);
boundary.
374
d.
hot
b.
divergent plate boundaries;
joined together?
a.
a(n)
9.
c.
a.
occur?
8.
spots;
are associated with
plates
answers (b) and (d). Along what type of plate boundary does subduction slide past
result of:
oceanic-oceanic plate boundaries;
16.
(c);
e.
6.
the island of Hawaii and the Loihi
a.
convection
that:
oldest adjacent to
is
b.
adjacent to the continents;
5.
The formation of Seamount are the
a.
d.
4.
of
believed to be:
similarity of rock sequences;
c.
all
e.
continental plate boundaries.
for continental drift? a.
ranging techniques;
these.
called:
3.
hot spots;
b. the age of the sediment above any portion of the ocean crust; magnetic reversals in the sea-floor crust;
is
continental drift hypothesis
Asthenosphere
Oceanic crust
29.
What
Moon is
features
would an astronaut look
or another planet to find out
currently active or
30. Briefly discuss
how
if it
was
if
for
on the
plate tectonics
active during the past?
a geologist could use plate
tectonic theory to help locate mineral deposits.
Condie, K. 1989. Plate tectonics and crustal evolution. 3d ed. New York: Pergamon Press. Cox, A., and R. B. Hart. 1986. Plate tectonics: How it works. Palo Alto, Calif.: Blackwell Scientific Publishers. Cromie, W. J. 1989. The roots of midplate volcanism. Mosaic 20, no. 4: 19-25. Kearey,
^ ADDITIONAL
P.,
Calif.:
READINGS
and
F. J.
Vine. 1990. Global tectonics. Palo Alto,
Blackwell Scientific Publishers.
Nance, R. D.,
T. R. Worsley,
and
J.
B.
Moody. 1988. The 1: 72-79.
supercontinent cycle. Scientific American 259, no.
The behavior of the Earth. Cambridge, Mass. Harvard University Press. Bonatti, E. 1987. The rifting of continents. Scientific American 256, no. 3: 96-103.
Allegre, C. 1988.
Saunders, R.
S.
1990. The surface of Venus. Scientific American
263, no. 6: 60-65. Vink, G. E., W. J. Morgan, and P. R. Vogt. 1985. The Earth's hot spots. Scientific American 252, no. 4: 50-57.
Additional Readings
375
CHAPTER
14
DEFORMATION, MOUNTAIN BUILDING, AND THE EVOLUTION OF CONTINENTS ^ OUTLINE PROLOGUE INTRODUCTION DEFORMATION Strike
and Dip
Folds
"^ Guest
Essay: Studying the Earth:
Reflections of an Enthusiast Joints Faults
T
Perspective 14-1: Folding, Joints,
and
Arches
MOUNTAINS Types of Mountains
MOUNTAIN
BUILDING: OROGENESIS
Plate Boundaries
and Orogenesis
'"' Perspective 14-2:
The Origin of Rocky Mountains
the
THE ORIGIN AND EVOLUTION OF CONTINENTS Shields, Cratons,
and the Evolution of
Continents
"^
Perspective 14-3: Plate Tectonic History of the Appalachians
MICROPLATE TECTONICS AND
MOUNTAIN BUILDING CHAPTER SUMMARY
Rocks deformed by folding and intruded by small granite dikes, Georgian Bay, Ontario, Canada. (Photo courtesy of R. V. Dietrich.)
K-^^^^ ** **'
:
"*-
"^ *« •* •*- *'
PROLOGUE ^HgJiV^JI
Of
the
many
scenic
in the continental
mountain ranges
United States, few
grandeur to the Teton Range of Wyoming (Fig. 14-1). The Native Americans of the region called these mountains Teewinot, meaning many pinnacles. This is an appropriate name indeed, for the Teton Range consists
compare
in
northwestern
numerous jagged peaks, the loftiest of which, the Grand Teton, rises to 4,190 m above sea level. There are higher and larger mountain ranges in the United States, but none rise so abruptly as the Tetons. They ascend nearly vertically more than 2,100 m above the of
""•"
FIGURE
View of
14-1
The Grand Teton
Jackson Hole, the valley to their east. This range and the surrounding region comprise Grand Teton National Park. Mountains began forming in this region about 90 million years ago. These early mountains were quite
is
the Teton
Range
in
Wyoming.
the highest peak visible.
floor of
different
from the present ones
in that the
were eroded, exposing the underlying metamorphic and plutonic rocks (Fig. 14-2). The fault is along the east side of the Teton block, so as uplift occurred, the block has been tilted ever more steeply toward the west (Fig. 14-2). Displacement of recent sedimentary deposits along the east flank of the Teton Range shows that uplift is continuing today. The spectacular, rugged topography of the Teton Range developed rather recently. Currently, the range supports about a dozen small glaciers, but periodically during the last 200,000 years it was more heavily
long axes
of these ranges were oriented northwest-southeast,
and they originated as the Earth's crust was contorted and folded. The present-day Teton Range, which runs north-south, began forming about 10 million years ago when part of the crust was uplifted along a large fracture called the Teton fault (Fig. 14-2). Most of the rocks exposed in the Teton Range are Precambrian-aged metamorphic and plutonic rocks formed at great depth beneath sedimentary rocks. Movement on the Teton fault resulted in uplift of the Teton block relative to the block to the east; the total displacement on this fault is about 6,100 m. As the
glaciated. Glaciers are particularly effective agents of
erosion; they scoured out valleys and intricately
sculpted the uplifted Teton block, producing excellent examples of glacial landforms. The Grand Teton, which is a horn peak, is one of the most prominent of these (see Chapter 18).
Teton block rose, the overlying sedimentary rocks
^ FIGURE I~~l
Cenozic rocks Mesozoic rocks Paleozoic rocks
EZ3
Precambrian granite gneiss,
and
A
14-2
of the Teton Range,
Teton Range
Grand Teton elevation 4,190
cross section
Wyoming.
,
m
\
schist
Prologue
377
>
^ —
sr-«g
^
^^
«^^
^
.
^
^
^^K-
^
^^^'^^^LL-^^c:^^^
^E.^ K.^ »l^m: ,
INTRODUCTION
Many
ancient rocks are fractured or highly contorted
—
an indication that forces within the Earth caused deformation during the past. Seismic activity is a manifestation of forces continuing to operate within the Earth, as is
Range
the Teton
uplift in
Wyoming
(Fig. 14-1).
Col-
and mountain building along convergent plate margins, and in so doing, they add material to the margins of continents by a process called accretion. Mountain systems within continents form when two continents collide and become sutured, thereby forming a larger landmass. Mountains also form when continents are stretched during rifting events. In short, deformation, mountain building, and the evolution of continents are interreliding plates generate forces causing deformation
lated,
although not
all
deformation results
in the origin
of mountains.
"^ FIGURE 14-3 Deformed layers of rock. The folded rock layers are considered to be ductile because they show considerable plastic deformation, whereas the fractured rocks are brittle.
The study of deformed rocks has several applications. For example, the geologic structures produced during deformation, such as folds and
faults,
provide a record
sources. For example, several geologic structures
and natural gas
of the kinds and intensities of forces that operated dur-
traps for petroleum
ing the past. Interpretations of such structures allow
thermore, geologic structures are considered
geologists to
make
inferences regarding Earth history.
Understanding the nature of the local geologic structures also helps geologists find
— FIGURE
14-4
Stress
and recover natural
re-
are selected for dams, large bridges, plants, especially
if
formation.
and
possible types of resulting
deformation, {a) Compression causes shortening of rock layers by folding or faulting, (b) Tension lengthens rock layers and causes faulting. (c) Shear stress causes deformation by displacement along closely spaced planes.
378
Chapter 14
Deformation, Mountain Building, and the Evolution of Continents
such
sites are in
form
(see Fig. 7-33). Fur-
when
sites
and nuclear power areas of active de-
^ DEFORMATION Fractured and contorted rock layers such as those in Figure 14-3 are said to be deformed; that is, their original shape or volume or both have been altered by stress, which is the result of force applied to a given area of
rock.
the intensity of the stress
If
is
greater than the
undergo strain, which is deformation caused by stress. Three types of stress are recognized: compressional, tensional, and shear. Compressional stress results when rocks are squeezed or compressed by external forces directed toward one another. Rock layers subjected to compression are commonly shortened in the direction of stress by folding or faulting (Fig. 14-4a). Tensional stress results from forces acting in opposite directions along the same line (Fig. 14-4b). Such stress tends to internal strength of the rock,
it
will
"^ FIGURE 14-5 This marble slab in the Rock Creek Cemetery, Washington, D.C., bent under its own weight about 80 years.
in
lengthen rocks or pull them apart. In shear stress, forces
one another but in opposite directions, by displacement of adjacent
act parallel to
resulting in deformation
layers along closely spaced planes (Fig. 14-4c).
Strain
is
characterized as elastic
return to their original shape laxed.
Squeezing a tennis
strain,
but once the stress
returns to
beyond
its
if
when
the
monly
deformed rocks
the stresses are re-
for
ball,
high temperature and high pressure, they are more com-
example, causes
is
released, the tennis ball
original shape.
Rocks that are strained
cannot recover their original shape, however, and retain the configuration produced by the stress. Such rocks either deform by plastic strain, as when they are folded, or behave as brittle solids and
and deform plastically rather than fracture The foci of most earthquakes are at depths of less than 30 km, indicating that deformation by fracturing becomes increasingly difficult with depth; no fracturing is known to occur at depths greater than 700 km. ductile
(Fig. 14-6).
their elastic limit
Strike
and Dip
As we observed tality
earlier, the principle
holds that
when sediments
of original horizon-
are deposited, they ac-
are fractured (Fig. 14-3).
The type of
strain that occurs
stress applied, the
the rock type,
amount of
depends on the kind of
pressure, the temperature,
and the length of time the rock
is
sub-
"^ FIGURE
14-6
Ductile versus brittle behavior in the
jected to the stress. For example, a small stress applied
The thickness of the brittle upper crust varies depending on the amount of heat, the presence of fluids, and
over a long period of time, as on the slab shown in
variations in pressure.
lithosphere.
Figure 14-5, will cause plastic deformation. By contrast, a large stress applied rapidly to the
hammer,
same
object, as
when
probably result in fracture. Rock type is important because not all rocks respond to stress in the same way. Rocks are considered to be either it is
struck by a
ductile or brittle
will
depending on the amount of plastic
strain they exhibit. Brittle rocks
show no
plastic strain
before fracture, whereas ductile rocks exhibit a great deal (Fig. 14-3).
Many
rocks
show
';2*
the effects of plastic deformation
must have occurred deep within the Earth's crust where the temperature and pressure' are high. Recall from Chapters 8 and 1 1 that rock materials behave very differently under these conditions compared to their behavior near the surface. At or near the surface, they behave as brittle solids, whereas under conditions of
~"
*>">"*
**
=*
J
>
in\
1 ,
•
that
«*
'„
*
'I*
%
ii
**
*
«•"* * •-"> -" * ' Ductile-brittle transition zone
t\
»,
;
Ductile lower crust
*
Ji'
%
*
//"*
/+ ~~ *"
=*
.
p
* „ xt
IK
J
1
>
and mantle
Deformation
379
such as a rock
layer.
For example,
in
Figure 14-8, the
surface of any of the tilted rock layers constitutes an inclined plane. The intersection of a horizontal plane with any of these inclined planes forms a line, the direction of which is the strike. The strike line's orientation is
determined by using a compass to measure its angle with respect to north. Dip is a measure of the maximum angular deviation of an inclined plane from horizontal, so it
must be measured perpendicular
to the strike direction
(Fig. 14-8).
Geologic maps indicate strike and dip by using a long line oriented in the strike direction
and a short
line per-
pendicular to the strike line and pointing in the dip direction (Fig. 14-9a). "''"
FIGURE
14-7 The principle of original horizontality holds that sediments are deposited in horizontal layers. These sedimentary rocks in Utah are inclined from horizontal, so we can infer that they were tilted after deposition and lithification. (Photo courtesy of David J. Matty.)
The number adjacent
to the strike
and dip symbol indicates the dip angle. A circled cross is used to indicate horizontal strata, and a strike symbol with a short crossbar indicates layers dipping vertically (Fig. 14-9b and c).
Folds cumulate in nearly horizontal layers (see Fig. 9-3). Thus, sedimentary rock layers that are steeply inclined must have been
tilted
following deposition and lithification
Some igneous
rocks, especially ash falls and form nearly horizontal layers. To describe the orientation of deformed rock layers, geol(Fig. 14-7).
many
lava flows, also
ogists use the concept of strike
Strike
is
and
dip.
the direction of a line formed by the inter-
section of a horizontal plane with an inclined plane,
^ FIGURE The
strike
is
14-8 Strike and formed by the
you place your hands on a tablecloth and move them toward one another, the tablecloth is deformed by compression into a series of up- and down-arched folds. SimIf
ilarly,
rock layers within the Earth's crust commonly
that
is,
to
the rocks have been strained plastically.
rocks at or near the surface are
dip.
(the water surface) with the surface of an inclined plane (the surface of the rock layer). Xhe_dip is th e maximum .angular deviation of the inclined plane from horizontal.
Chapter 14
Most
folding probably occurs deep within the crust because
intersection of a horizontal plane
380
re-
compression by folding. As opposed to the tablecloth, however, folding in rock layers is permanent;
spond
Deformation, Mountain Building, and the Evolution of Continents
brittle
and generally de-
-^ FIGURE
14-9
(a)
Strike
and
The long bar is oriented and the short bar points in the dip direction. The number indicates the dip angle. (£>) The symbol used to indicate horizontal rock layers, (c) The dip symbol.
in the strike direction,
symbol for
form by fracturing rather than by folding. The intensity of folding in
many rocks
is
quite impressive (Fig. 14-10).
^ FIGURE
14-10
vertical rock layers.
Intensely folded sedimentary rocks in
California. (Photo courtesy of
David
J.
Matty.)
Monoclines, Anticlines, and Synclines
A
monocline
is
a simple
bend or flexure
in
otherwise
horizontal or uniformily dipping rock layers (Fig. 1411a).
The large monocline in Figure 1 4- 1 1 b formed when Mountains of Wyoming were uplifted along
the Bighorn
a large fault. This fault did not penetrate to the surface, however, so as uplift occurred, the near-surface layers of rock were bent such that they appear to be draped over
the margin of the uplifted block (Fig. 14-1 lb).
An anticline is an up-arched fold, while a syncline is down-arched fold (Fig. 14-12). Both anticlines and synclines are characterized by an axial plane that divides them into halves; the part of a fold on opposite sides of the axial plane is a limb (Fig. 14-13). Because folds most a
commonly occur
as a series of anticlines alternating with
synclines, a limb
is
generally shared by an anticline and
an adjacent syncline.
important to remember that anticlines and synrock lasers arid not by the configuration of the Earth's surface. Thus, folds may or may not correspond to mountains and It is
clines are defined-hy. the oriejrtation of
valleys
surface
and may, is
rather
in fact, underlie areas flat (Fig.
where the Earth's com-
14-14). Indeed, folds are
Deformation
381
(b)
(a)
^ FIGURE
A
monocline. Notice the strike and dip symbols and the symbol for horizontal layers, (b) Uplift of the Bighorn Mountains in Wyoming formed
14-11
the monocline visible
(a)
on the
skyline.
monly exposed to view in areas that have been eroded. Even where the exposed view has been eroded, anticlines and synclines can easily be distinguished from each other by strike and dip and by the relative ages of the folded strata. As Figure 14-15 shows, in an eroded anticline, the strata of each limb dip outward or away from the center, where the oldest strata are located. In eroded synclines, on the other hand, the strata in each
-»t:
FIGURE
14-12
limb dip inward toward the center, and the youngest strata coincide
Thus folds in
far,
we
with the center of the fold. have described symmetrical, or upright,
which the
axial plane
limb dips at the same angle axial plane
is
inclined, the limbs dip at different angles,
Antidine_and
Calico Mountains of southeastern California.
Chapter 14
and each fold However, if the
vertical,
and the fold is characterized as asymmetrical (Fig. 4-16a). In an overturned fold, both limbs dip in the
s ynclinej n_the
382
is
(Fig. 14-13).
Deformation, Mountain Building, and the Evolution of Continents
^" FIGURE 14-14 These folded rocks in Kootenay National Park, British Columbia, Canada, illustrate that anticlines and synclines do not necessarily correspond to mountains and valleys Synclme
"^ FIGURE
14-13
axial plane, axis,
and
respectively.
Anticline
Syncline and anticline showing the fold limbs.
Plunging Folds Folds
may
be further characterized as nonplunging or
plunging. In the former, the fold axis, a line formed by
same
direction. In other
rotated
words, one fold limb has been
more than 90 degrees from
such that
it is
now
upside
down
its
(Fig.
original position
14-16b). Folds in
the intersection of the axial plane with the folded beds, is
horizontal (Fig. 14-13). However,
common
it
is
for the axis to be inclined so that
much more it
appears to
which theaxial_pjane is- horizontal-are, r eierre d_to_as recumbent (Fig. 14- 16c). Overturned and recumbent folds are particularly common in many mountain ranges
plunge beneath the surrounding strata; folds possessing
(discussed later in this chapter).
geologists use exactly the
an inclined axis are plunging folds (Fig. 14-17). To differentiate plunging anticlines from plunging synclines,
same
criteria
used for non-
•^ FIGURE 14-15 Identifying eroded anticlines and synclines.
Deformation
383
MARIE MORISAWA
Guest Essay
STUDYING THE EARTH: REFLECTIONS OF AN ENTHUSIAST on becoming a geologist; in fact, my major was mathematics. But in my junior year, friends convinced me to take an introductory geology course. That did it! I was fascinated by what I learned about the Earth and by how much we still did not know about it. It was too late to change my major, but my I
As
didn't plan
college
senior year
was
with as
filled
many
geology courses as
I
could take.
That
was held
interest
years, after
which
abeyance, however, for 10
in
decided to go back to graduate
I
school and study geology.
geology professor warned
Why? After all, my former me that I probably could not were not
get a position teaching geology because there
very
many geology departments
When
I
received
Wyoming, an not hire
me
in
company
then, did
I
women's
colleges.
geology at the University of
me he would would hire me as a
recruiter told
as a geologist— but
Why,
secretary.
my M.A.
oil
in
go on to obtain a Ph.D.
in
geology from Columbia University? In part because of students
my
and
my own
hold
interest
my
and encouragement of
the accepting attitude
professors. Then, too,
academically,
and enthusiasm
I
I
felt
that
could succeed.
for geology
fellow
if I
And
a geology teacher,
knowledge
felt
I
could do two things:
essential to their understanding of the
of
I could imbue some them with the same love and enthusiasm for
I have. So throughout my career I taught Brooklyn College, Bryn Mawr College, the University of Montana, Antioch College, and, finally,
geology that at
New York at Binghamton from which I recently retired. For a time, both as a student and as a professor, I also did research as a at the State University of
geologist for the U.S. Geological Survey.
As
I
worked
interested in
in geology,
how
I
became more and more humans and
the environment affects
how humans in turn affect the environment. Much of my research and teaching has been in that area. I found that human activity has upset the natural behavior of the Earth systems.
I
became
particularly interested in natural
(geologic) hazards such as
wave and river erosion, and volcanic eruptions
flooding, landslides, earthquakes,
how humans
and
finally
events.
I
came
have handled these catastrophic
to see that in order to cope with these
hazards in an environmentally compatible manner of
need,
What could be more interesting than the Earth on which we live? How was that rock formed? How do we know that a sheet of ice 915 m thick once covered the state of New York? Why did Mount St. Helens erupt? How did all the beautiful scenery that we see around us
processes at work. Only then can
to be? All these questions
answered.
many
And
and more need to be
good thing about geology
the
questions are
still
challenge— and even
I
unanswered. This
(or you) could
answer some of them. The delight these questions
is
is
is
that so
the
answer
the very complexity of the Earth's
and the continual change that
is
taking place in
itself is
the geologist's textbook
and
laboratory. Geomorphologists, such as myself, are the
who
study the landscape and the As an outdoor person, I combine work and recreation. Doing field work, hiking, canoeing, and camping are all part of a day's work. types of geologists
processes that form
it.
we
disasters. If
Chapter 14
we
do not understand the basic components of the Earth systems and how they work together, we increase the danger rather than mitigate the hazard. This is the me— to use our
present challenge of geology to
knowledge about the Earth to enhance the environment and to use it wisely. This makes geology worthwhile, a
JVlarie Morisawa graduated from Hunter College and earned an M.A. from the University of a Ph.D. from
Wyoming and
Her geomorphology and environmental geology. She has taught at several colleges and universities and recently retired from the State University of New York at Binghamton where she is University.
specialties are
professor emeritus.
lAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAJkAAAAAAAAAAAAAAA,
384
and
take suitable
measures to deal successfully with such
Columbia
the systems.
The Earth
all,
have a chance to
in trying to
physical systems, the interaction of one process with
another,
first
we
to understand the geologic setting
doubts about the future.
come
I
Earth environment. And, perhaps,
could
overcame any
I
could introduce a large number of students to the
Deformation, Mountain Building, and the Evolution of Continents
Axial plane
"•*
FIGURE
14-16
(a)
An
asymmetrical fold. The axial
and the fold limbs dip at different angles. (b) Overturned folds. Both fold limbs dip in the same direction, but one limb is inverted. Notice the special strike and dip symbol to indicate overturned beds, (c) Recumbent plane
is
not
vertical,
folds.
away from the fold whereas in plunging synclines all strata dip inward toward the axis. The oldest exposed strata are in the center of an eroded plunging anticline, whereas the youngest exposed strata are in the center of an eroded plunging syncline (Fig. 14-17b). In Chapter 7 we noted that anticlines form one type of structural trap for petroleum and natural gas (see Fig. 7-33). As a matter of fact, most of the world's petroleum plunging folds: that
is, all
strata dip
axis in plunging anticlines,
production comes from anticlinal traps, although several other types are important as well. Accordingly, geologists are particularly interested in correctly identifying the
geologic structures in areas of potential petroleum and natural gas production. Figure 14-18 shows hypothetical examples of how folds are identified from surface rock exposures and how buried folds are located.
Domes and
Basins
and synclines are elongate structures; that is, they tend to be long and narrow. Domes and basins, on Anticlines
the other hand, are the circular to oval equivalents of anticlines
and synclines
the oldest exposed rock the opposite
is
an eroded dome, whereas in a basin
(Fig. 14-19). In is
at the center,
true. All of the strata in a
dome
dip
away
from a central point (as opposed to dipping away from a fold axis, which is a line). By contrast, all the strata in a basin dip inward toward a central point (Fig. 14-19). Many domes and basins are of such large proportions that they can be visualized only on geologic maps or aerial photographs. The Black Hills of South Dakota, for example, are a large oval dome (Fig. 14-19b). One of the best-known large basins in the United States is the Michigan basin (Fig. 14-19d). Most of the Michigan
Deformation
385
Axial
plane
Angle of plunge
**-
FIGURE
14-17 Plunging folds schematic illustration of a plunging fold, (b) A block diagram (a)
A
showing surface and cross-sectional views of plunging folds. The long arrow at the center of each fold
shows the direction of plunge. (c) Surface view of the eroded, plunging Sheep Mountain anticline in
Wyoming.
basin
(c)
buried beneath younger strata so
is
it is
not
rectly observable at the surface. Nevertheless, strike
dip of exposed strata near the basin margin
sands of
drill
holes for oil and gas clearly
di-
and
and thou-
show
that the
deformed into a large structural basin. The Michigan basin was determined by using a combination of the methods shown in Figure 1418. It is a huge structure of overall basinal configuration, but much of its oil and gas production comes from small anticlines and domes.
Joints
which no movement has ocwhere movement has been perpendicular to
Joints are fractures along
curred, or
may
strata are
the fracture surface. In other words, the fracture
structure of the
open up, but no relative movement of the masses of rock on opposite sides of the fracture occurs parallel to the
386
Chapter 14
The term "joint" was originally used by coal miners long ago for cracks in rocks that appeared to be surfaces where adjacent blocks were "joined" together. fracture.
Deformation, Mountain Building, and the Evolution of Continents
•"-"
FIGURE
14-18
Identification of
geologic structures from surface
exposures,
[a)
Valley with rock exposures.
Data from these exposures are used to map and cross sections of the area. Strike and dip would be recorded at many places but only two (£>)
construct a geologic
are
Joints are the
commonest
structures in rocks; almost
near-surface rocks are jointed to
some degree
all
(Fig. 14-
The lack of any movement parallel to joint surfaces what distinguishes them from faults, which do show movement parallel with the fracture surface.
Joints can
shown
here.
form under a variety of conditions. For ex-
ample, anticlines are produced by compression, but the
20).
rock layers are arched such that tension occurs perpen-
is
dicular to fold crests,
and
joints
form
parallel to the long
axis of the fold in the upper part of a folded layer (Fig.
Deformation
387
I
14-19 (a) A block diagram of a dome. (b) A satellite view of an elongated dome, the Black Hills in western South Dakota, (c) A block diagram of a basin, [d) A map view of the Michigan basin.
14-21a). Joints also form in response to tension when rock layers are simply stretched (Fig. 14-21 b). Compressive stresses
can also produce joints as shown
in Figure
14-21c. Joints vary
388
from minute fractures to those of regional
Chapter 14
I
I
~~|
| Middle Devonian
Pennsylvanian
| Upper
"^ FIGURE
_H Upper Devonian
Jurassic
I
Mississippian
I
Lower Mississippian
|
H
Silurian
Ordovician
Mississippian and/or Devonian
(d)
extent (Fig. 14-20). Furthermore, they are often ar-
ranged
sets, and it is comtwo or perhaps three promiRegional mapping reveals that joints and joint
in parallel
or nearly parallel
mon
for a region to have
nent
sets.
sets are usually related to
Deformation, Mountain Building, and the Evolution of Continents
other geologic structures such
Weathering and erosion of jointed rocks Utah has produced the spectacular scenery of Arches
as large folds. in
National Park
One
(see Perspective 14-1).
type of joint pattern that
we have
already dis-
cussed consists of columnar joints that form in lava flows
and
in
some
some
intrusive igneous bodies. Recall
from Chapters 4 and 5 that as cooling lava contracts, it develops tensional stresses that form polygonal fracture patterns (see Figs. 4-13 and 5-1). Another type of jointing previously discussed is sheet jointing that forms in response to unloading (see Fig. 6-9).
Faults Faults are fractures along
which movement has occurred
parallel to the fracture surface.
A
tault plane
is
the frac -
"^ FIGURE
14-20
Jointed strata on the northeast flank of
the Salt Valley anticline, Arches National Park, Utah.
ture surface along which blocks of rock on opposite
"^ FIGURE anticline.
(£>)
14-21 Joints
{a) Folding and the formation of joints parallel to the crest of an produced by tension, (c) Joints formed in response to compression.
^-r^
(b)
(a)
(c)
Deformation
389
y
Perspective 14-1
FOLDING, JOINTS, AND ARCHES Arches National Park
in eastern Utah is noted for its which include such landforms as Delicate Arch, Double Arch, Landscape Arch, and many others (Fig. 1). Unfortunately, the term arch is
structures play a significant role in the origin of
panoramic
arches.
used for a variety of geologic features of different
vigorously along joints because these processes can
vistas,
we will restrict the term to mean an opening through a wall of rock that is formed by weathering and erosion. The arches of Arches National Park continue to origin, but here
form
as a result of
weathering and erosion of the
folded and jointed Entrada Sandstone, the rock
underlying
much
of the park. Accordingly, geologic
Where the Entrada Sandstone was folded into it was stretched so that parallel, vertical
anticlines, joints
formed. Weathering and erosion occur most
attack the exposed rock from both the top and the sides,
whereas only the top
adjacent joints. Figure 14-20.
^" FIGURE
sedimentary rocks, as shown
an arch.
2
Many
Some
sides have
such
fins
of rock between
fins are clearly visible in
parts of these fins are
Baby Arch shows the
early
more
development of
-'4
I-
TFault dip angle
attacked in unjointed
Erosion along joints causes them to enlarge, thereby forming long slender
"** FIGURE 1 Delicate Arch in Arches National Park, Utah formed by weathering and erosion of jointed in Figure 3.
is
strata (Fig. 14-20).
mov ed
relative to
one another. Notice
in Fig-
ure 14-22 that the blocks adjacent to the fault plane are labeled banging wall block and footwall block. The
11
hanging wall block is the block that overlies the fault, whereas the footwall block lies beneath the fault plane.
Hanging wall and footwall blocks can be defined with respect to any fault plane except those that are vertical. Understanding the concept of hanging wall and footwall
blocks
is
ment of
important because geologists use the move-
the hanging wall block relative to the footwall
Hanging Arrows
show
directions
of relative
390
movement
Chapter 14
w^tt-btock
-» FIGURE
14-22
Deformation, Mountain Building, and the Evolution of Continents
Fault terminology.
and erosion than others, and
susceptible to weathering
may
as the sides are attacked, a recess
form.
If it
does,
eventually pieces of the unsupported rock above the recess will fall away, forming an arch as the original recess
is
enlarged (Figs. 2 and
remnants of along
fins
joints.
Historical observations to
Thus, arches are
3).
formed by weathering and erosion
show
form today. For example,
enlarged
The park
when
a large block
in
that arches continue
1940, Skyline Arch was
fell
from
collapsed during prehistoric time.
* FIGURE
Arches National Park
recess, (b)
arches,
The arches continue
pinnacles, spires,
(c)
to enlarge until they finally
is
underside.
of arches that
When
collapse, they leave isolated pinnacles
(a) Weathering and erosion of a fin form a 3 These recesses expand and eventually develop into
its
many examples
also contains
arches
and
spires.
well worth visiting; the
and arches are impressive features
indeed.
collapse.
block to distinguish between two different types of
do wn
faults.
fault.
Like sedimentary beds, fault planes can be characterand dip (Fig. 14-22). Two basic types
ized by their strike
of faults are distinguished on the basis of whether the
blocks on opposite sides of the fault plane have
moved
relative to the block on the opposite side of th e Although it is not possible to tell how the blocks actually moved, it is usually easy to determine which block appears to have moved up or down in relation to the other. Thus, geologists refer to relative movement on faults. For example, in Figure 14-23a one cannot tell if
parallel to the direction of dip or along the direction of
the hanging wall block
strike.
block
moved down,
or
if
the footwall
both blocks moved. Nevertheless, the hanging wall block app ears to hav e moved dow nward "relative to the footwall bloc kT Such faults are
Dip-Slip Faults Dip-slipfaults are those on
moved
wh ich
all
movemen t
is
p ar-
called
up, or
normal
if
faults ,
whereas those where the hanging
alieTwithThe
wall block movedLug^relative to the footwall block are
words,
reverse faults (Fig. 14-23b).
all
dip of the fault p lane (Fig. 14-2j).~In other movement is such that one block moves up or
A
type of reverse fault in-
Deformation
391
Normal
Reverse
fault
fault
Rift
zone
Offset
stream
Strike-slip fault
Thrust
fault
0Wft£>, "'-w
^ FIGURE
Oblique-slip fault
14-23 Types of faults, (a), (b), and (c) are dip-slip faults, {a) Normal fault— hanging wall block down relative to footwall block. \b) and (c) Reverse and thrust faults— hanging wall block up. (d) Strike-slip fault— all movement parallel to strike of fault, (e) Oblique-slip fault— combination of dip-slip and strike-slip.
392
Chapter 14
Deformation, Mountain Building, and the Evolution of Continents
"•r FIGURE 14-24 east in
Owens
View of
uplifted along a large
normal
Nevada from the The mountains have been
the Sierra
Valley, California.
fault.
yojving a fault plane with a dip of
less
than 45°
is
a
thrust fauI t~(Fig~ 14-23c).
Normal
faults are
caused by tensional forces, s uch as
when the Earth's crust is stretched and by rifting. The mountain ranges of a large area
those that occur
thinned
called the Basin
and Range Province
in the
western
United States are bounded on one or both sides by major normal faults. A large normal fault is present along the east side of the Sierra
Nevada
in California; these
moun-
have been uplifted along this normal fault so that above the lowlands they now stand more than 3,000 tains
m
Continued normal faulting is also found along the eastern margin of the Teton Range
to the east (Fig. 14-24).
in
Wyoming
(Fig. 14-1).
Unlike normal jaults, reverse (and thrust) faults ar e
by compressio n (Fig. 14-25). Many large reverse and thrusfTauTti are present in mountain ranges that form at convergent plate margins (discussed later in the chapter). A well-known thrust fault is the Lewis overthrust of Montana. A large slab of Precambrian-aged rocks moved at least 75 km eastward on this fault and now rests upon much younger rocks of Cretaceous age c aused
(Fig.
14-26).
Strike-Slip Faults
Shearing forces are responsible for strike-slip faulting, a type~oTfau1tingTnvolving horizontal movement in which
(b)
opp osite sides of a^a^iltj^kne_sli de~siclewa vs past one~ano ther (Fig. 14-23d). In other words, all movement islrTthe direction of the fault plane's strike.
Mojave Desert, California, (b) Thrust fault in Sumter County, Alabama. The fault plane dips at 8°.
blocks o n
^ FIGURE
14-25
{a)
Reverse fault
in
welded
tuff,
Deformation
393
Precambrian rocks Chief Mountain
Cretaceous rocks (a)
(c)
(b)
"^ FIGURE
14-26
mountain,
Chief Mountain.
The Lewis overthrust fault in Glacier National Park, Montana. (a) Cross section showing the fault. As the slab of Precambrian rocks moved east along the fault, it deformed the rocks below. Chief Mountain is an erosional remnant of a more extensive slab of rock, (b) The trace of the fault is the light line on the side of the (c)
One of the best-known strike-slip faults is the San An dreas fau lt of California.* Recent movement on this fault caused the October zy, 1989 earthquake that damaged so much of Oakland, San Francisco, and several communities to the south and resulted in a 10-day delay
of the
World
Series (see the Prologue to
Chapter
10).
can be characterized as right-lateral depending on the apparent direction of
Strike-slip faults
or left-lateral, offset. In
Figure 14-23d, for example, an observer look-
ing at the block
mines whether to the
left.
on the opposite
it
side of the fault deter-
moved to the example, movement appears
appears to have
In this
'Recall from Chapter 13 that the San Andreas fault
transform fault in plate tectonics terminology.
394
Chapter 14
is
been to the
left,
so the fault
lateral strike-slip fault. strike-slip fault, the
is
Had
characterized as a
this
left-
been a right-lateral
block across the fault from the ob-
to have moved to the right. The San Andreas fault is a right-lateral strike-slip fault (see Figs. 10-3b and 14-27), whereas the Great Glen fault in Scot-
server
land
would appear
is
left-lateral (Fig. 14-28).
Oblique-Slip Faults It is
possible for
movement on
a fault to
show compo-
right or
nents of both dip-slip and strike-slip. For example,
to have
movement may be accompanied by a dip-slip component giving rise to a combined movement that includes left-lateral and reverse, or right-lateral and normal (Fig. 14-23e). Faults having components of both dip-slip and strike-slip movement are oblique-slip faults.
also called a
strike-slip
Deformation, Mountain Building, and the Evolution of Continents
-»-
FIGURE
14-27 Right-lateral by the San Andreas southern California, the offset about 21 m.
offset of a gully fault in
gully
is
^ MOUNTAINS any area of land that stands
The term mountain
refers to
significantly higher
than the surrounding country. but
Some
much more
mountains are
single, isolated peaks,
commonly they
are parts of a linear association of peaks
FIGURE 14-28 Map view of the left-lateral offset along the Great Glen fault of Scotland. The body of granite has been displaced by about 105 km.
and/or ridges called mountain ranges that are related in age and origin.
A
mountain system
is
a
tainous region consisting of several or ranges; the
Porky Mountains and
complex mounmany mountain
A ppalachians
are ex-
amples of mountain system s. Major mountain systems are indeed impressive features
and represent the
effects of
erating within the Earth.
The
dynamic processes op-
forces necessary to elevate
Himalayas of Asia to nearly 9 km above sea level are comprehend, yet when compared with the size of the Earth, even the loftiest mountains are very the
difficult to
small features. In fact, the greatest difference in elevation
on 2
on Earth
a globe 1
is
m
mm. From
about 20 km;
if
we
depicted this to scale
in diameter, its relief
the
human
would be
less
than
perspective, however, major
mountain systems are large-scale manifestations of tremendous forces that have produced folded, faulted, and thickened parts of the crust. Furthermore, in some mountain systems, such as the Andes of South America
Mountains
395
can develop over a hot spot, but more commonly a
series
of volcanoes develops as a plate moves over the hot spot,
Hawaiian Islands (see Fig. 13-24). also forms where the crust has been intruded by batholiths that are subsequently uplifted and eroded (Fig. 14-29). The Sweetgrass Hills as in the case of the
Mountainous topography
of northern
Montana
consist of resistant plutonic rocks
exposed following uplift and erosion of the softer overlying sedimentary rocks. Yet another way to form mountains — block-faulting— involves considerable deformation (Fig. faulting involves
or
more blocks
classic
example
movement on normal
14-30). Block-
faults so that
one
are elevated relative to adjacent areas. is
A
the large-scale block-faulting currently
occurring in the Basin and Range Province of the western
United States, a large area centered on Nevada but extend-
and northern Mexico. This numerous north-south trending mountain ranges, each of which is separated from the next range by a valley (Fig. 14-31). In the Basin and Range Proving into several adjacent states
region
^ FIGURE
14-29
(a)
Pluton overlain by sedimentary
is
characterized by
ince, the Earth's crust
is
being stretched in an east-west
rocks, (b) Erosion of the softer overlying rocks reveals the
pluton and forms small mountains.
direction; thus, tensional stresses
produce north-south
ented, range-bounding faults. Differ ential
and down-dropped blocks called grabens (Fig. 14-30). Horsts and grabens are bounded on both sides by parallel normal faults. Erosion of the horsts has yielded the mountainous topography now present, and the grabens have filled with sediments eroded from the horsts (Fig. 14-30). The processes discussed above can certainly yield mountains. However, the truly large mountain systems of the continents, such as the Alps of Europe and the Appalachians in North America, were produced by compression along convergent plate margins.
these faultsjias yielded uplifted blocks called horsts
and the Himalayas of Asia, the mountain-building processes remain active today.
Types of Mountains Mountainous topography can develop in a variety of ways, some of which involve little or no deformation of the Earth's crust. For example, a single volcanic mountain
'"•'"
FIGURE
14-30
Block-faulting and the origin of a horst and a graben.
^ Graben Horst
396
Chapter 14
ori-
movement n n
Deformation, Mountain Building, and the Evolution of Continents
"^ FIGURE
14-31
and Range Province bounded by normal in Nevada.
(a)
Cross section of part of the Basin
Nevada. The ranges and valleys are faults, (b) View of the Humboldt Range in
* MOUNTAIN BUILDING: OROGENESIS An orogeny
is an episode of mountain building du ring which ntense deformation occurs, generally accom pan ied bymetamo rp hism and the emplacement of pluton s, i
especially batholiths. esis', is still
Mountain
building, called orogen-
not completely understood, but
to be related to plate
movements. In
it is
fact, the
known
advent of
changed the way mountain systems. Any theory accounting for orogenesis must adequately explain the characteristics of mountain systems such as their long, narrow geometry and their location at or near plate tectonic theory has completely
geologists view the origin of
The intensity of deformation increases from the continental interior into mountain systems whereToverturned and recumbent folds and reverse and thfusTTaults indica ting compression are common Furthermore, both shallow and deep marine sedimentary rocks in mountain systems have been elevated far above sea level — in some cases as high as 9,000 m! plate margins.
.
Plate Boundaries
and Orogenesis
of the Earth's geologically recent and present-day orogenic activity is concentrated in two major zones or
Most belts:
the
Alpine-Himalayan orogenic belt and the
circum-Pacific orogenic belt (Fig. 14-32).
Most of
the
number of
these orogens, such as the
Himalayan oro-
gen, are active today. Older orogenic belt s include the
areas of the present-day Appalachia n frJ
Mountains of
orth America and the Ural Mountains in the So viet
Union.
Most orogenies occur
at
convergent plate boundaries
where one plate is subducted beneath another or where two continents collide. Subduction-related orogenies are t hose involving oceanic-oceanic and oceamc^ontinental plate boundaries.
Orogenesis at Oceanic-Oceanic Plate Boundaries Orogenies occurring where oceanic lithosphere is subducted beneath oceanic lithosphere are characterized by the formation of a volcanic island arc and by deformation
and igneous
activity.
Deformation occurs when sed-
iments derived from the volcanic island arc are compressed
along
a
convergent plate
boundary.
These
Earth's volcanic
sediments are deposited on the adjacent sea floor and in
two
and seismic activity also occurs in these Figs. 4-28 and 10-7). Both belts are com-
the back-arc basin.
posed of a number of smaller segments called orogens; each orogen is a zone of deformed rocks, many of which have been metamorphosed and intruded by plutons. A
sediments deposited in the oceanic trench, are deformed
belts (see
Those on the sea
floor,
including
and scraped off against the landward side of the trench (Fig. 14-33), thus forming a subduction complex, or
Mountain
Building: Orogenesis
397
"^ FIGURE activity
is
14-32
Most of
concentrated
and present-day orogenic and Alpine-Himalayan orogenic belts.
the Earth's geologically recent
in the circum-Pacific
-»-
FIGURE 14-33 Orogenesis and the origin of a volcanic island arc at an oceanic-oceanic plate
boundary.
Volcanic island arc
Asthenosphere
398
Chapter 14
Deformation, Mountain Building, and the Evolution of Continents
— accretionary wedge, of intricately folded rocks cut by
Orogenesis at Continental-Continental
numerous compression-induced thrust
Plate Boundaries
tion, orogenesis in
faults. In addi-
generated by plate convergence results
low-temperature, high-pressure metamorphism char-
acteristic of the blueschist facies (see Fig. 8-22).
Deformation of sedimentary rocks also occurs in the where it is caused largely by the emplacement of plutons, and many rocks show evidence of high-temperature, low-pressure metamorphism. The
In contrast to the Andes, the
when to
India
first
Himalayas of Asia formed 40
collided with Asia beginning about
50 million years ago. Prior
to that time, India
was
far
island arc system
overall effect of island arc orogenesis
is
the origin of
two
more-or-less parallel orogenic belts consisting of a land-
ward volcanic
island arc underlain by batholiths
seaward belt of deformed trench rocks
and a
(Fig. 14-33).
Orogenesis at Oceanic-Continental
"***
FIGURE
Generalized diagrams showing three Andes of South America. (a) Prior to 200 million years ago, the west coast of South America was a passive continental margin, (b) Orogenesis began when the west coast of South America became an active continental margin, (c) Continued deformation, volcanism, and plutonism.
Plate Boundaries
Passive continental margin
Sea
Many major mountain
systems including the Alps of
Europe and the Andes of South America formed
at
The— Ande s
of
oceanic-continental
western South
plate
Amer ica
boundaries.
are perhaps the best
such continuing orogeny of the
(Fig. 14-32).
example of
Among the ranges
Andes are the highest mountain peaks
Americas and
many
in the
active volcanoe s. Furthermore, the
west coast of South America
ment of the
cir cum-Pacific
is
an extremely active seg-
earth quake belt.
One
of the
Earth's great ocea nic trenchsysteTnp, the Peru-Chile
Trench,
lies just
14-34
stages in the development of the
orTlhe west coast ^Fig. 12-14).
200 million years ago, the western margin of South America was a passive continental margin, where sediments accumulated on the continental shelf, slope, and rise much as they currently do along the east coast of North America. However, when Pangaea split apart in response to rifting along what is now the MidAtlantic Ridge, the South American plate moved westward. As a consequence, the oceanic lithosphere west of South America began subducting beneath the continent (Fig. 14-34). As subduction proceeded, sedimentary rocks of the passive continental margin were folded and faulted and are now part of the accretionary wedge Prior to
along the west coast of South America. Accretionary wedges here and elsewhere commonly contain fragments of oceanic crust and upper mantle called ophiolites (see Fig. 12-26). Subduction also resulted in partial melting of the descending plate prod ucing a~v ofcanic arc, and numerous large plutons were emplaced beneath the arc (Fig. 14-34t: The Rocky Mountains of North America also formed as a consequence of pl ate convergence and subdu ction. However, they differ from other mountain systems in several important aspects (see Perspective 14-2).
level
v K
Perspective 14-2
THE ORIGIN OF THE
ROCKY MOUNTAINS
are part of a complex mountainous region known as the North American Cordillera, which extends from Alaska into central
The Rocky Mountains
-"-FIGURE
1
Map
of the
North American Cordillera United States.
Mexico. In the western United States, the Cordillera widens to about 1,200 km and is one of the most complex parts of the circum-Pacific orogenic belt
in the
Cenozoic basins Coast
of Pacific
Pliocene-
Pleistocene volcanics
Oceanic
Forearc
Arc volcanoes
trench
seismicity
\
Backarc Continental crust
seismicity
Base
of
lithosphere
(a)
Block
uplift
and rupture
"^*
FIGURE 2 Orogenies resulting (a) steep and [b) shallow subduction at oceanic-continental plate boundaries. In the shallowsubduction model, the subducted slab moves nearly horizontally beneath the continent, and volcanism ceases. from
Subhorizontal seismic zone (b)
(Fig. 1).
Although the Cordillera has a long history of
much
less steep
angle and moves nearly horizontally
deformation, the most recent episode of large-scale
beneath the continental lithosphere, deforming
deformation was the Laramide orogeny, which began 85 to 90 million years ago. Like many other
continental crust far inland from the continental
orogenies,
it
occurred along an oceanic-continental
However, deformation in the area of present-day Wyoming and Colorado occurred much farther inland from the continental margin than is typical (Fig. 1). Furthermore, mountain building was not accompanied by significant intrusions of granitic plate boundary.
batholiths.
To account for these observations, geologists have modified the classic model for orogenies along convergent plate margins. Geologists think that when is subducted beneath continental descends at a steep angle (30° or more),
oceanic lithosphere lithosphere,
it
from the trench, and on the continental the Laramide style of
a volcanic arc develops inland
the thick sediments deposited
margin are deformed. In orogeny, the subducted oceanic slab descends at a
margin
(Fig. 2).
occur only
Furthermore, magmatism seems to
when
the descending plate penetrates as
deep as the asthenosphere, so orogeny,
magmatism
is
in the
Laramide type of
suppressed.
Another consequence of shallow subduction seems produced large-scale fracturing of the crust and uplift of fault-bounded blocks; such deformation differs from the intense folding and to be deformation that
thrust faulting that characterizes a typical
oceanic-continental plate boundary orogeny. the ranges in the present-day as large blocks that
The Laramide
Many
of
Rocky Mountains began
were elevated along such faults. deformation ceased about 40
style of
million years ago, but since that time the Rocky Mountains have continued to evolve. For example, the mountain ranges that formed during the orogeny were (continued on next page)
Older sedimentary rocks
Thrust
Volcanic ash
fault
falls
Younger sedimentary rocks
Older sedimentary rocks Valleys
filled
to overflowing
Normal
"^ FIGURE
3
(a)
through
fault
(c)
Sediments eroded from the
blocks uplifted during the Laramide orogeny (d)
filled
the
were nearly covered. The sediment-filled valleys are eroded, and deep canyons
valleys
between ranges
until the ranges
are cut into the uplifted blocks by streams.
eroded, and the valleys between ranges
sediments
buried in their
402
rilled
with
Many of the ranges were nearly own erosional debris, and their
(Fig. 3).
Chapter 14
present-day elevations are the result of renewed uplift that continues to the present in
Prologue).
Deformation, Mountain Building, and the Evolution of Continents
some
areas (see the
south of Asia and separated from (Fig. 14-35a). As the Indian plate
it
by an ocean basin
moved northward,
"""
a
FIGURE
14-35
subduction zone formed along the southern margin of
{a)
was consumed (Fig. 1435a). Partial melting generated magma, which rose to form a volcanic arc, and large granite plutons were emplaced into what is now Tibet. At this stage, the activity along Asia's southern margin was similar to what is now Asia where oceanic lithosphere
showing the and the origin of the Himalayas.
Simplified cross sections
collision of India with Asia
The northern margin of
India before
its
collision
with
Asia. Subduction of oceanic lithosphere beneath southern
Tibet as India approached Asia, (b) About 40 to 50 million years ago, India collided with Asia, but since India was too light to be subducted, it was underthrust beneath Asia. (c) Continued convergence accompanied by thrusting of rocks of Asian origin onto the Indian Subcontinent. (d) Since about 10 million years ago, India has moved
occurring along the west coast of South America.
beneath Asia along the main boundary fault. Shallow marine sedimentary rocks that were deposited along India's northern margin now form the higher parts of the Himalayas. Sediment eroded from the Himalayas has been deposited on the Ganges Plain.
Crust
Volcano
Main Central Thrust
(c)
20-40
m.y.
Main Boundary Fault
Main Central Thrust -
(d)
20-0
m.y.
Main Boundary Fault
Mountain
Building: Orogenesis
403
The ocean separating
India from Asia continued to and India eventually collided with Asia (Fig. 1435b). As a result, two continental plates became welded, or sutured, together. Thus, the Himalayas are now loclose,
northward, and two major thrust faults carried rocks of Asian origin onto the Indian plate (Fig. 14-35c and d).
Rocks deposited ern margin
14-32 and 14-35b). The exact time of India's collision with Asia is uncertain, but between 40 and 50 million years ago, India's rate of northward drift decreased abruptly— from 15 to 20 cm per year to about 5
cm
(Figs.
per year. Because continental lithosphere
dense enough to be subducted,
this
is
not
decrease in rate
seems to mark the time of collision and India's resistance to subduction. Consequently, the leading margin of India
was
thrust beneath Asia, causing crustal thick-
ening, thrusting, and uplift. Sedimentary rocks that
been deposited
in
had
the sea south of Asia were thrust
Chapter 14
uplifted,
they were also
eroded, but at a rate insufficient to match the
Much
uplift.
of the debris shed from the rising mountains
was
transported to the south and deposited as a vast blanket
of sediment on the Ganges Plain and as huge submarine fans in the Arabian Sea
14-36). Since
its
and the Bay of Bengal
(Fig.
collision with Asia, India has been un-
derthrust about 2,000
km beneath Asia.
Currently, India
moving north at a rate of about 5 cm per year. A number of other mountain systems also formed as a result of collisions between two continental plates. The Urals in the Soviet Union and the Appalachians of is
"•" FIGURE 14-36 Sediment eroded from the Himalayas has been deposited as a vast blanket on the Ganges Plain and as large submarine fans in the Arabian Sea and the Bay of Bengal.
404
shallow seas along India's north-
the higher parts of the Himalayas.
As the Himalayas were
cated within a continent rather than along a continental
margin
in the
now form
Deformation, Mountain Building, and the Evolution of Continents
North America both formed by such
collisions (see Per-
platforms are collectively called cratons, so shields are
simply the exposed parts of cratons. Cratons are con-
spective 14-3).
sidered to be the stable interior parts of continents.
^ THE ORIGIN AND EVOLUTION
In
much
OF CONTINENTS Rocks 3.8
billion years old that are
continental crust are
known from
ing Minnesota, Greenland,
North America, the Canadian Shield includes of Canada; a large part of Greenland; parts of the
thought to represent
several areas, includ-
and South
ologists agree that even older crust
Africa.
Most
ge-
probably existed,
and, in fact, rocks dated at 3.96 billion years were re-
Canada. According to one model for the origin of continents,
cently discovered in
the earliest crust
was
thin
and unstable and was com-
posed of ultramafic igneous rock. This early ultramafic crust was disrupted by upwelling basaltic magmas at
and was consumed at subduction zones (Fig. 14would therefore have been destroyed because its density was great enough to make recycling by subduction very likely. Apparently, only crust of a more granitic composition, which has a lower density, is resistant to destruction by subduction. A second stage in crustal evolution began when partial melting of earlier formed basaltic crust resulted in the formation of andesitic island arcs, and partial melting of ridges
37a). Ultramafic crust
lower crustal andesites yielded granitic
were emplaced
in the crust that
magmas
had formed
that
earlier (Fig.
14-37b). By 3.96 to 3.8 billion years ago, plate motions accompanied by subduction and collisions of island arcs had formed several granitic continental nuclei.
Shields, Cratons,
and the
Evolution of Continents Each continent is characterized by one or more areas of exposed ancient rocks called a shield (see Fig. 8-4). Extending outward from these shields are broad platforms of ancient rocks that are buried beneath younger sediments and sedimentary rocks. The shields and buried
^ FIGURE
14-37
continental crust.
The
Model
for the origin of granitic
earliest crust
may have been
composed of ultramafic rock but was disrupted by rising magmas, {a) Basaltic crust is generated at spreading ridges its high density, subduction zones and is form at convergent plate margins. Granitic continental crust forms by collisions of
underlain by mantle plumes. Because of basaltic crust
is
consumed
at
recycled, (b) Andesitic island arcs
island arcs
and intrusions of
granitic
Subduction zone
magmas.
The Origin and Evolution of Continents
405
Perspective 14-3
PLATE TECTONIC HISTORY OF THE APPALACHIANS (Fig. 1) of eastern North America have a long and complex history that includes continental rifting, opening and closure of the same ocean basin, continental collision, and finally renewed continental rifting. The relationship between mountain building and the opening and closing of ocean basins is known as the Wilson cycle in honor of the Canadian geologist J. T. Wilson. Wilson was the first to suggest that an ancient ocean had closed to form the Appalachians and then reopened and widened to form the present-day Atlantic Ocean. During the Late Proterozoic Eon, a large rift
The Appalachian Mountains
developed
in a
supercontinent consisting of what are
now North America and
As rifting proceeded, an ocean basin formed and continued to widen along a divergent plate boundary (Fig. 2a and b). During this time, the east coast of North America and the west coast of Europe were passive continental margins,
much
Eurasia.
central Massachusetts,
and Vermont, was the
first
of
several orogenies to affect the Appalachian region.
Radiometric age dating of igneous rocks from Georgia Newfoundland indicates that the Taconic orogeny
to
occurred 480 to 440 million years ago. Continuing closure of the ocean basin resulted
in the
Acadian orogeny during the Silurian and Devonian periods (Fig. 2d). It affected the Appalachian region
from Newfoundland to Pennsylvania as continental margin sedimentary rocks were deformed and thrust northward and westward. Like the Taconic orogeny, the Acadian orogeny occurred along an oceanic-continental plate boundary, but collision occurred
it
culminated
when
continental
during the Devonian Period.
The Acadian orogeny was of
greater magnitude
than the Taconic orogeny, as indicated by more
widespread regional metamorphism and granitic intrusions. Radiometric dates from these rocks cluster
between 350 and 400 million years ago, indicating
as they are at the present. Plate
was the time of maximum deformation.
separation continued until the Early Paleozoic Era, at
that
which time the plate motions reversed, forming oceanic-continental plate boundaries on both sides of the ocean basin (Fig. 2c).
During the Late Paleozoic Era, the southern parts of the Appalachian region from New York to Alabama
The
resulting Taconic orogeny,
named
for the
present-day Taconic Mountains of eastern
New
York,
were further deformed. This event, the Alleghenian orogeny,
was
the last in a succession of orogenies
beginning during the Early Paleozoic, and
it
coincides
with the amalgamation of the supercontinent Pangaea.
^ FIGURE
1
The folded Appalachian Mountains
eastern United States.
in the
During the Late Triassic Period, the first stage in the breakup of Pangaea began, with North America separating from Eurasia and North Africa. Along the
North America, from Nova Scotia to North Carolina, block-faulting occurred and formed numerous ranges with intervening valleys much like those of the present-day Basin and Range Province of east coast of
the western United States (Fig. 3). Great quantities of
poorly sorted red-colored nonmarine detrital sediments were deposited in the valleys, some of which are well-known for dinosaur footprints. Rifting was accompanied by widespread volcanism, which resulted in extensive lava flows and numerous dikes and sills (see Fig. 5-22).
Erosion of the block-fault mountains during the and Cretaceous periods produced a broad,
Jurassic
low-lying erosion surface.
Renewed
uplift
and erosion
during the Cenozoic Era account for the present-day
topography of the Appalachian Mountains.
406
Chapter 14
Deformation, Mountain Building, and the Evolution of Continents
(a)
Continental crust
Caledonian
AcadianCaledonian
Continental-
Tacontc Highlands
continental plate
bOL'
*- FIGURE 2 Early history of the Appalachian region. [a\ Opening of the Iapetus Ocean basin during the Late Proterozoic Eon. \b) The ocean continues to widen during the Early Paleozoic Era. (c) The ocean begins closing, and subducnon occurs on both sides, id) Final closure
'Oceanic-cc^' nenta (c)
plate
Ocean during
boundary
the
of the Iapetus
Devonian Period.
"• r FIGURE 3 Rifting of Pangaea during the Tnassic Period resulted in block-faulting in eastern North America. (j) Location of basins formed by block-faulting. [b-c\ Thick sedimentary deposits and dikes and sills filled the basins,
which were themselves broken by faults
Albany .
during
a
complex of normal
rifting.
^Connecticut Valley -'area
The Origin and Evolution of Continents
407
is not directly observable except in the Canadian where one can easily see the remnants of ancient mountains and early small cratons. Many of the exposed rocks are plutonic and metamorphic, and many of them show the structural complexities associated with
cretion
Shield
orogenesis.
^ MICROPLATE TECTONICS AND MOUNTAIN BUILDING In the preceding sections,
we
discussed orogenies along
convergent plate boundaries resulting cretion.
Much
during such events crust,
in continental ac-
of the material accreted to continents is
simply eroded older continental
but a significant amount of
to continents as well
— igneous
new
material
is
added
rocks that formed as a
consequence of subduction and partial melting, for example. While subduction is the predominant influence I
I I
on the tectonic history
Canadian Shield
I
in
many
regions of orogenesis,
other processes are also involved in mountain building Other exposed Precambrian rocks
and continental accretion,
Covered Precambrian rocks
I
"^ FIGURE
The North American
14-38
craton.
The
exposed Precambrianaged rocks. Extending from the shield are platforms of buried Precambrian rocks. The shield and platforms collectively make up the craton.
Canadian Shield
is
especially the accretion of mi-
croplates.
a large area of
During the
late
1970s and 1980s, geologists discovmany mountain systems are com-
ered that portions of
posed of small accreted lithospheric blocks that are clearly of foreign origin. These microplates differ completely in their fossil content, stratigraphy, structural
and paleomagnetic properties from the rocks of mountain system and adjacent craton. In fact, these microplates are so different from adjacent rocks that most geologists think that they formed elsewhere and were carried great distances as parts of other trends,
the surrounding
Lake Superior region in Minnesota, Wisconsin, and Michigan; and parts of the Adirondack Mountains of
New is
York
(Fig. 14-38). In general, the
a vast area of subdued topography,
Canadian Shield numerous lakes,
plates until they collided with other microplates or con-
and exposed ancient metamorphic, volcanic, plutonic, and sedimentary rocks. By about 2.5 billion years ago, the Canadian Shield area formed by the amalgamation of smaller cratons
tinents.
that collided along belts of deformation called orogens,
croplates are
thereby forming a larger craton
(Fig.
14-39a). Several
additional episodes of orogenesis resulted in further ac-
and eastern margins of the 570 million years ago, North America had a size and shape approximating that in Figure 14-39c. Further orogeny and accretion during the last 570 million years occurred mostly along the eastern, southern, and western margins cretion along the southern
craton as
shown
in
Figure 14-39b, so that by
Geologic evidence indicates that more than
25%
of
the entire Pacific coast from Alaska to Baja California
The accreting micomposed of volcanic island arcs, oceanic ridges, seamounts, and small fragments of continents that were scraped off and accreted to the continent's consists
of accreted microplates.
margin as the oceanic plate with which they were carwas subducted under the continent. It is estimated that more than 100 different-sized microplates have been added to the western margin of North America
ried
during the
The
last
200 million years
(Fig.
14-40).
basic plate tectonic reconstruction of orogenies
of the craton, giving rise to the present configuration of
and continental accretion remains unchanged, but the
North America.
details of such reconstructions are decidedly different in
Much younger
408
of the North American craton
is
covered by
strata, so the evidence for early continental ac-
Chapter 14
view of microplate tectonics. For example, growth along active continental margins is faster than along passive
Deformation, Mountain Building, and the Evolution of Continents
billions of
years
"*"
FIGURE
EZS3 >2.5
14-39
Hi 1.9-1.8
I
I
1.8-1.7
Three stages
I
1
1.7-1.6
I
1
1.2-1.0
in the early evolution
of the North American craton. (a) By about 2.5 billion years ago, North America consisted of the elements shown here, {b) and (c) Continental accretion along the southern and eastern margins of North America. By the
end of the Proterozoic Eon, 570 million years ago, North America had the size and shape shown diagrammatically in (c).
Microplate Tectonics and Mountain Building
409
FIGURE
""•*"
Some
14-40
of the accreted lithospheric
blocks called microplates that form the western margin of the North American craton. The light brown blocks
probably originated as parts of continents other than North America. The reddish brown blocks are possibly displaced parts of North America.
continental margins because of the accretion of microplates.
new
Furthermore, these accreted microplates are often
additions to a continent, rather than reworked older
continental material.
So far, most microplates have been identified in mountains of the North American Pacific coast region, but a number of such plates are suspected to be present in other ficult to
mountain systems as well. They are more difrecognize in older mountain systems, such as
the Appalachians, however, because of greater deforma-
and erosion. Nevertheless, about a dozen mi-
tion
croplates have been identified in the Appalachians, but their
boundaries are hard to
tectonics provides a
new way
identify.
Thus, microplate
of viewing the Earth and
of gaining a better understanding of the geologic history of the continents.
SUMMARY
CHAPTER 1.
Contorted and fractured rocks have been deformed or strained by applied stresses.
2.
Stresses are characterized as compressional,
tensional, or shear. Elastic strain
is not permanent, removed, the rocks return to their original shape or volume. Plastic strain and fracture are both permanent types of
meaning that when the
stress
is
deformation. 3.
The
orientation of deformed layers of rock
is
described by strike and dip. 4.
Rock layers that have been buckled into up- and down-arched folds are anticlines and synclines, respectively. They can be identified by the strike and dip of the folded rocks and by the relative age of the rocks
5.
in the center
Domes and
of eroded folds.
basins are the circular to oval
equivalents of anticlines and synclines, but are
commonly much 6.
Two
larger structures.
recognized: joints are fractures along which the only
410
7.
types of structures resulting from fracturing are
Joints,
form 8.
On
which are the commonest geologic
in
structures,
response to compression, tension, and shear.
dip-slip faults, all
movement
Two
is
in the dip
movement, if any, is perpendicular to the fracture surface, and faults are fractures along which the blocks on opposite sides of the fracture move
to tension, while reverse faults are caused by
parallel to the fracture surface.
compression.
Chapter 14
direction of the fault plane. faults are recognized:
Deformation, Mountain Building, and the Evolution of Continents
normal
varieties of dip-slip
faults
form
in
response
Strike-slip faults are those
9.
in the direction
on which
movement
all
is
*F
characterized as right-lateral or left-lateral depending
on the apparent direction of
offset of
1.
one block
Some
faults
strike-slip;
11.
12.
13.
dip-slip
and
they are called oblique-slip faults. 2.
continental plates collide.
4.
3.
volcanic island arc, deformation, igneous activity,
oceanic lithosphere at an oceanic-continental plate
15.
boundary also results in orogeny. Some mountain systems, such as the Himalayas, are within continents far from a present-day plate boundary. Such mountains formed when two continental plates collided and became sutured. A craton is the stable core of a continent. Broad areas in which the cratons of continents are exposed are called shields; each continent has at least one
17.
characterized as
compression;
d.
plastic; e.
as a result of accretion, a process
b.
brittle; b.
sheared;
fractured;
a.
d.
Most
fracturing; b.
d.
convection;
An
syncline;
An
An
a central point
fault
down
d.
reverse;
Faults
on which both
normal
basin
oblique-slip fault
compressional stress
orogeny
craton
plastic strain
dip
plunging fold
dip-slip fault
reverse fault
dome
shear stress
elastic strain
shield
fault
strain
plane footwall block
stress
fracture
strike-slip fault
hanging wall block
syncline
joint
tensional stress
microplate
thrust fault
fault
normal;
strike-slip; c. joint.
e.
dip-slip
and
strike-slip
are referred to as:
recumbent; c. obliqueb. normal-slip. nonplunging; e. The range-bounding faults in the Basin and Range Province of the western United States plunging;
slip; d.
9.
fault
are
10.
faults.
a.
normal;
d.
strike-slip; e.
A a.
strike
to
is
fault.
a.
^ IMPORTANT
dome; recumbent
relative to the footwall block
movement has occurred
anticline
strata dipping a(n):
is
basin.
e.
thrust; b.
TERMS
is
on which the hanging wall block appears
a.
8.
all
plunging anticline; b. overturned syncline; d.
a. c.
a
microplates collide with
the axis
vertical; c.
oval to circular fold with
and igneous rocks to the margin of a craton during
when
is
the strata in one limb are horizontal;
outward from
continents.
basin;
c.
anticline.
the strata are faulted as well as folded.
e.
realize that continental accretion
the strata dip in
monocline;
e.
inclined; d.
A
all
a(n):
the axial plain
b.
7.
is
rifting;
overturned fold is one in which: both limbs dip in the same direction;
a.
6.
compaction; c. compression.
e.
elongate fold in which
d. 5.
ductile;
c.
folding results from:
a.
orogenesis. also occurs
plastic strain are
of these.
all
e.
have moved
now
tensional;
elastic; c.
shear.
involving the addition of eroded continental material
Geologists
deformed rocks
if
they are no longer subjected
Rocks that show a large amount of
syncline;
formed
when
a.
toward the center a. dome; b.
shield area. 16. Cratons
is
said to be:
and metamorphism characterize orogenies occurring at oceanic-oceanic plate boundaries. Subduction of
14.
Strain
to stress.
show components of both
Mountains can form in a variety of ways, some of which involve little or no folding or faulting. Mountain systems consisting of several mountain ranges result from deformation related to plate movements. Most orogenies occur where plates converge and one plate is subducted beneath another or where two
A
QUESTIONS
regain their shape
relative to the other.
10.
REVIEW
of strike of the fault plane. They are
graben
reverse;
b.
c.
thrust;
oblique-slip.
is a:
fold with a horizontal axial plane; b.
of reverse fault with a very low dip;
c.
type fracture
along which no movement has occurred; down-dropped block bounded by normal d. faults; e.
type of structure resulting from
compression. 11. In
which of the following
is
an orogeny currently
taking place? a.
east coast of
North America;
coast of South America;
d
central Africa;
e.
c.
b.
west
the Appalachians;
western Europe.
monocline
Review Questions
411
have have mainly
mainly vertical displacement;
c.
horizontal movement; d
are faults
movement has by
yet occurred;
Which of
What
c.
normal
are recumbent and overturned folds?
How
do
30.
Draw
subjected to
overturned.
from
joints differ
faults?
a simple cross section
showing the
displacement on a normal fault. 31. What type of stress is responsible for reverse 32. Explain
strike-slip fault;
basin;
fault; d.
recumbent
e.
33.
Draw on
fold.
which no movement has occurred monoclines;
joints; b.
axial planes;
transform
c.
fold limbs.
e.
intersection of an inclined plane with a
horizontal plane
is
the definition of:
a.
horizontal strata; b.
c
folded strata; d
movement;
dip-slip strike; e
mountain systems that form
joint.
at continental
is
meant by an oblique-slip fault. map showing the displacement
a left-lateral strike-slip fault.
two ways
in
which mountains can form with
or no folding and faulting.
little
faults; d.
what
a simple sketch
34. Discuss
are:
two examples of mountain systems in which mountain-building processes remain active. 36. Explain why two roughly parallel orogenic belts develop where oceanic lithosphere is subducted beneath continental lithosphere. 37. How do geologists account for mountain systems within continents, such as the Urals in the Soviet 35. Cite
Union?
margins: the Earth's crust
a.
between
faulting?
anticline; b.
17. In
criteria for distinguishing
What
folded; c
a
The
two
29.
the following might result from tensional
15. Fractures along
16.
are the
same patterns on two important ways.
28.
stresses?
a
the
have been:
elastically strained; e.
tension; d.
show
basins
deformed by movement along
sheared; b
Assume
them?
closely spaced slippage planes are said to
a
Domes and
geologic maps, but differ in
uplift of the footwall block.
13. Solids that have been
14.
27.
on which no
are characterized
e.
syncline.
that these folds plunge to the east.
are low-angle reverse faults; b.
a.
and an adjacent plunging
anticline
12. Strike-slip faults:
is
thicker than average;
model
38. Briefly outline the
most deformation is caused by tensional little or no volcanic activity occurs; stresses; c. stretching and thinning of the continental d. crust occur; e. most deformation results from
that
b.
was presented
39. Explain
40.
What
is
how
for the origin of continents
in this chapter.
continents
"grow" by
accretion.
the difference between a reverse fault and a
thrust fault?
rifting.
18
The
circular equivalent of a syncline
is
a(n):
joint; c. basin; monocline; b. overturned fault. asymmetric anticline; e. d. 19 Sediments deposited in an oceanic trench and then deformed and scraped off against the landward side of the trench during an orogeny form a(n): divergent margin complex; b. accretionary a. island arc wedge; c. back-arc basin facies; d. orogenic continental margin complex. system; e. 20. An excellent example of a mountain system forming a.
as a result of a continent-continent collision
is
the:
^ ADDITIONAL
READINGS
Davis, G. H. 1984. Structural geology of rocks
and
regions.
&
New
York: John Wiley Sons. J. G. 1987. Structural geology:
Dennis,
Dubuque, Iowa: Hatcher, R. D.,
Jr.
Wm.
An
introduction.
C. Brown.
1990. Structural geology: Principles, concepts,
and problems. Columbus, Ohio: Merrill Publishing Co. Howell, D. G. 1985. Terranes. Scientific American v. 253, no. 5:
116-125. 1989. Tectonics of suspect terranes: Mountain building and continental growth. London: Chapman and Hall. Jones, D. L., A. Cox, P. Coney, and M. Beck. 1982. The growth of western North America. Scientific American v. 247, no. 5: .
21
Rocky Mountains;
c.
Andes; b. Himalayas;
What
types of evidence indicate that stress remains
a.
d.
Alps;
e.
Appalachians.
70-84.
active within the Earth?
22
How
do compression, tension, and shear
differ
from
How
is it
possible for rocks to behave both
and plastically? meant by the elastic
elastically
24.
What
is
25. Explain
how
limit of rocks?
the factors of rock type, time,
temperature, and pressure influence the type of strain in rocks.
26.
412
Draw
R.
a simple geologic
Chapter 14
map showing
a plunging
J.
1988. Geological structures and maps:
A
practical
New
York: Pergamon Press. Miyashiro, A., K. Aki, and A. M. C. Segnor. 1982. Orogeny. guide.
one another? 23.
Lisle,
&
New York: John Wiley Sons. Molnar, P. 1986. The geologic history and structure of the Himalaya. American Scientist 74, no. 2: 144-154. 1986. The structure of mountain ranges. Scientific American v. 255, no. 1: 70-79. Spencer, E. W. 1988. Introduction to the structure of the Earth. New York: McGraw-Hill Book Company.
Deformation, Mountain Building, and the Evolution of Continents
CHAPTER
15
MASS WA STING ^OUTLINE PROLOGUE INTRODUCTION FACTORS INFLUENCING MASS WASTING Slope Gradient
Weathering and Climate
Water Content Vegetation
Overloading
Geology and Slope
Stability
Triggering Mechanisms
^"Perspective 15-1: The Tragedy at Aberfan, Wales
TYPES OF MASS WASTING Falls
Slides -^-
Guest Essay: Cleansing the Earth— Waste
Management Flows
Complex Movements
RECOGNIZING AND MINIMIZING THE EFFECTS OF MASS MOVEMENTS ""T Perspective 15-2: The Vaiont Dam Disaster
CHAPTER SUMMARY
Hong Kong's most
destructive landslide
occurred on Po Shan road on June 18, 1972. Sixty-seven people were killed when a 68-m wide portion of this steep hillside failed, destroying a four-story building and a 13-story apartment block.
^'» * TK^ric-'«r^3E^K^aEC .-^^•^-^^•^^.^TK.^.-Kr* -
:
>
PROLOGUE
.
more than 50,000,000 m3 mud, rock, and water, flowed over ridges 140 m
the avalanche, consisting of
of
high obliterating everything in
|||||IlV|j
On May
31, 1970, a devastating
earthquake occurred about 25 km in the Peruvian Andes, about 65 km to the east, the violent shaking from the earthquake tore loose a huge block of snow, ice, and west of Chimbote, Peru. High
rock from the north peak of
Nevado Huascaran
(6,654 m), setting in motion one of this century's
worst landslides. Free-falling for about 1,000 m, this block of material smashed to the ground, displacing
thousands of tons of rock and generating a gigantic debris flow (Fig. 15-1). Hurtling down the mountain's steep glacial valley at speeds
up to 320
km
per hour,
its
path.
About 3 km east of the town of Yungay, where the valley makes a sharp bend, part of the debris flow overrode the valley walls and within seconds buried Yungay, instantly killing more than 20,000 of its residents (Fig. 15-1).
down
The main mass of
the flow
overwhelming the town of Ranrahirca and several other villages and burying about 5,000 more people. By the time the flow reached the bottom of the valley, its momentum carried it across the Rio Santa and some 60 m up the continued
the valley,
opposite bank. In a span of roughly four minutes
from the time of the
initial
ground shaking,
"»»" FIGURE 15-1 An earthquake 65 km away triggered a landslide on Nevado Huascaran, Peru, that destroyed the towns of Yungay and Ranrahirca and killed more than 25,000 people.
Pacific
Ocean
Prologue
415
^ FIGURE part of
15-2
Yungay
Cemetery Hill was the only 1970 landslide that of the town. Only 92 people
to escape the
destroyed the rest survived the destruction by running to the top of the hill.
approximately 25,000 people died, and most of the area's transportation, power, and communication
network was destroyed. Ironically, the
only part of Yungay that was not
buried was Cemetery Hill, where 92 people survived
by running to geophysicist
its
top
who was
Yungay provided
(Fig. 15-2).
A
Peruvian
giving a French couple a tour of
a vivid eyewitness account of the
disaster:
breaker coming in from the ocean.
one-half to three-quarters of a minute
when
the
earthquake shaking began to subside. At that time I heard a great roar coming from Huascaran. Looking
saw what appeared to be a cloud of dust and it looked as though a large mass of rock and ice was breaking loose from the north peak. My immediate reaction was to run for the high ground of Cemetery Hill, situated about 150 to 200 m away. I began running and noticed that there were many others in Yungay who were also running toward Cemetery Hill. About half to three-quarters of the way up the hill, the wife of my friend stumbled and fell and I turned up,
down
hill
who was
carrying
two small
children
toward the hilltop. The debris flow caught him and he threw the two children toward the hilltop, out of the path of the flow, to
swept him
down
safety,
although the debris flow
the valley, never to be seen again.
I
remember two women who were no more than a few meters behind me and I never did see them again. Looking around, I counted 92 persons who had also
also
saved themselves by running to the top of the
was and
the most horrible thing I
I
hill. It
have ever experienced
will never forget it.*
I
to help her
The
416
estimated the
to be at least
meters
As we drove past the cemetery the car began to shake. It was not until I had stopped the car that I realized that we were experiencing an earthquake. We immediately got out of the car and observed the effects of the earthquake around us. I saw several homes as well as a small bridge crossing a creek near Cemetery Hill collapse. It was, I suppose, after about
I
80 m high. I observed hundreds of people in Yungay running in all directions and many of them toward Cemetery Hill. All the while, there was a continuous loud roar and rumble. I reached the upper level of the cemetery near the top just as the debris flow struck the base of the hill and I was probably only 10 seconds ahead of it. At about the same time, I saw a man just a few
wave
back to her
crest of the
Chapter 15
feet.
wave had
As was,
and devastating as was not the first time a
tragic it
had swept down
Mass Wasting
huge
avalanche
the Rio Shacsha valley. In January
1962, another large chunk of snow,
ice,
and rock
broke off from the main glacier and generated a large debris avalanche that buried several villages and killed
about 4,000 people. *B. A. Bolt et
a curl, like a
this debris
destructive landslide
al.,
Geological Hazards
1977), pp. 37-39.
(New York:
Springer-Verlag,
Mass wasting
^ INTRODUCTION Geologists use the term landslide in a general sense to
cover a wide variety of mass movements that loss of life,
(also called mass movement) is defined downslope movement of material under the direct influence of gravity. Most types of mass wasting are aided by weathering and usually involve surficial material. The material moves at rates ranging from almost
as the
may
cause
property damage, or a general disruption of
human
imperceptible, as in the case of creep, to extremely fast
the
as in a rockfall or slide.
activities. For example, in 218 B.C., avalanches in European Alps buried 18,000 people; an earthquake-generated landslide in Hsian, China, killed an estimated 1,000,000 people in 1556; another 200,000 people died when the side of a hill collapsed due to an earthquake in Kansu, China, in 1920; and 7,000 people died when mudflows and avalanches destroyed Huaraz, Peru, in 1941. What makes these mass movements so terrifying, and yet so fascinating, is that they almost always occur with little or no warning and are over in a very short time, leaving behind a legacy of death and
Mass wasting is an important geologic process that can occur at any time and almost any place. While most people associate mass wasting with steep and unstable
destruction (Table 15-1).
ceptible types, such as creep, usually
Every year about 25 people are killed by landslides
in
the United States alone, while the total annual cost of
damages from them exceeds $1 billion. Almost all of the major landslides have natural causes, yet many of the smaller ones are the result of human activity and could have been prevented or their damage minimized.
"^ TABLE
15-1
Selected Landslides, Their Cause,
While water can play an imporis the major force
tant role, the relentless pull of gravity
behind mass wasting.
slopes,
it
can also occur on near-level land, given the
right geologic conditions. Furthermore, while the rapid
types of mass wasting, such as avalanches flows, typically get the
most
and mud-
publicity, the slow, imper-
do the greatest
amount of property damage.
A
basic
knowledge of mass wasting
some
is
important to
have been knowledge can help one avoid selecting an unsafe building site for a house or business or can be useful in making decisions about land use. avoid a recurrence of mistakes,
made during
the past. Such
and the Number of People Killed
tragic, that
GRAVITATIONAL FORCE
-•'
FIGURE
on
material's strength
the
amount of
A
15-3
strength depends
slope's shear
the slope
and cohesiveness,
internal friction
between grains, and any external support of the slope. These factors
promote slope
stability.
The
force
of gravity operates perpendicular to the horizontal but has a component acting parallel to the slope. force,
which promotes
When
this
instability,
Component
exceeds a slope's shear strength, slope
* FACTORS INFLUENCING MASS WASTING When its
the gravitational force acting
on
ternal support of the slope (Fig.
resisting forces helping to
Opposing
a slope exceeds
maintain slope
ity.
a slope's shear strength
causing instability gle, the greater the
between grains, and any ex-
is
the force of grav-
but has a component acting parallel to the slope, thereby
include the slope material's strength and cohesion, the internal friction
These factors
Gravity operates perpendicular to the horizontal
stability
amount of
15-3).
collectively define a slope's shear strength.
resisting force, slope failure (mass wasting) occurs.
The
of gravitational
force acting parallel to slope
failure occurs.
the slope,
The
and the
(Fig. 15-3). The greater a slope's ancomponent of force acting parallel to greater the chance for mass wasting.
steepest angle that a slope can maintain without
collapsing
is its
angle of repose. At this angle, the shear
strength of the slope's material exactly counterbalances the force of gravity. For unconsolidated material, the angle
of repose normally ranges from 25° to 40°. Slopes steeper
than 40° usually consist of unweathered solid rock.
"^ FIGURE
15-4 Undercutting by stream erosion removes a slope's base, which increases the slope angle and (b) can lead to slope failure, (c) Undercutting by stream erosion caused slumping along this stream near Weidman, (a)
Michigan.
418
Chapter 15
Mass Wasting
All slopes are in a state of
means
dynamic equilibrium, which
that they constantly adjust in response to
new
Slope Gradient
con-
While we tend to view mass wasting as a disrupand usually destructive event, it is one of the ways that
ditions.
Slope gradient
tive
ing.
a slope adjusts to
new
conditions.
Whenever
a building or
is
probably the major cause of mass wast-
Generally speaking, the steeper the slope, the
stable
it
is.
Therefore, steep slopes are
more
on a hillside, the equilibrium of that The slope must then adjust, perhaps by mass wasting, to this new set of conditions. Many factors can cause mass wasting: slope gradient,
experience mass wasting than gentle ones.
weakening of material by weathering, increased water content, changes in the vegetation cover, and overloading. Although most of these are interrelated, we will examine them separately for ease of discussion, but will also show how they individually and collectively affect a
the slope angle,
slope's equilibrium.
are another
road
slope
is
is
constructed affected.
less
likely to
A number of processes can oversteepen a slope. One of the
most
common
is
undercutting by stream or wave ac-
tion (Fig. 15-4). This removes the slope's base, increases
and thereby increases the gravitational
force acting parallel to the slope.
Wave
action, especially
during storms, often results in mass movements along the shores of oceans or large lakes.
Excavations for road cuts and hillside building
major cause of slope
failure (Fig.
sites
15-5).
""' FIGURE 15-5 {a) Highway excavations disturb the equilibrium of a slope by [b) removing a portion of its support as well as oversteepening it at the point of excavation, (c) Such action can result in frequent landslides. (d) Cutting into the hillside to construct this portion of the
Pan American Highway in Mexico resulted in a rockfall that completely blocked the road. (Photo courtesy of R. V. Dietrich.)
Factors Influencing
Mass Wasting
419
30
—
"•"
FIGURE
15-7
A
California
Highway Patrol officer stands on top of a 2-m high wall of mud that rolled over a patrol car near the
Golden
State
Freeway on October
23, 1987. Flooding and mudslides also trapped other vehicles and closed the freeway.
up (Fig. 15-7). The soils of many hillZealand are sliding because deep-rooted native bushes have been replaced by shallow-rooted dollars to clean sides in
New
grasses used for sheep grazing.
When
heavy rains satucannot hold the
rate the soil, the shallow-rooted grasses
and parts of
slope in place,
it
rection as the slope, water can percolate along the var-
friction
particularly true
when
there are interbedded clay layers
when
because clay becomes very slippery
Even
slide downhill.
and decrease the cohesiveness and between adjacent rock units (Fig. 15-8a). This is
ious bedding planes
if
wet.
the rocks are horizontal or dip in a direction
may dip in the same Water migrating through them weathers the rock and expands these openings until the opposite to that of the slope, joints direction as the slope.
Overloading is almost always the result of human acand typically results from dumping, filling, or piling up of material. Under natural conditions, a material's load is carried by its grain-to-grain contacts, and a slope is thus maintained by the friction between the grains. The additional weight created by overloading, however, increases the water pressure within the material, which in turn decreases its shear strength, thereby weakening the slope material. If enough material is added, the slope will eventually fail, sometimes with
Overloading
weight of the overlying rock causes
it
to
fall (Fig.
15-8b).
tivity
tragic consequences.
Geology and Slope The
relationship between topography
of an area (Fig.
Stability
is
important
in
and the geology
determining slope stability
15-8). If the rocks underlying a slope dip in the
same direction to occur
than
as the slope, if
mass wasting
is
more
likely
the rocks are horizontal or dip in the
opposite direction.
When
the rocks dip in the
same
di-
Triggering Mechanisms While the factors previously discussed all contribute to slope instability, most— though not all — rapid mass movements are triggered by a force that temporarily disturbs slope equilibrium. The most common triggering mechanisms are strong vibrations from earthquakes and excessive amounts of water from a winter snow melt or a heavy rainstorm. Earthquakes are the most common type of strong vibrations and thus trigger many mass movements (see the Prologue and the Chapter 13 Prologue). In many cases, the resulting landslide causes far more damage and poses a greater threat to people than the earthquake
itself.
Volcanic eruptions, explosions, and even loud claps of thunder slope
is
may
be enough to trigger a landslide
sufficiently unstable.
Many
Factors Influencing
if
the
avalanches, which
Mass Wasting
421
Perspective 15-1
THE TRAGEDY AT ABERFAN, WALES debris brought out of underground coal mines in southern Wales typically consists of a wet mixture of
The
various sedimentary rock fragments. This material usually builds
dumped along
up
is
the nearest valley slope where
into large waste piles called tips.
A
it
tip is
long as the material composing it is and its sides are not oversteepened. Between 1918 and 1966, seven large tips composed of mine debris had been built at various elevations on the valley slopes above the small coal-mining village of Aberfan (Fig. 1). Shortly after 9:00 a.m. on October 21, 1966, the 250 m high, rain-soaked Tip No. 7 collapsed, and a black sludge flowed down the fairly stable as
relatively dry
it came 800 m from its starting place, the flow had destroyed two farm cottages, crossed a canal, and
valley with a loud train roar (Fig. 2). Before to a halt
buried Pantglas Junior School, suffocating virtually
A
all
144 people died in the flow, among them 116 children who had gathered for morning assembly in the school. the children of Aberfan.
total of
After the disaster, everyone asked,
tragedy occur and could
it
"Why
did this
have been prevented?" The
subsequent investigation revealed that no stability
•^ FIGURE 1 Aberfan, Wales, and a cross section showing the various tips built along the valley walls above Aberfan.
422
Chapter 15
Mass Wasting
could result from a combination of various geologic features including springs In 1939, 8
km
and seeps from the
tip.
to the south, a tip constructed
under
conditions almost identical to those of Tip No. 7
no one was injured, but was soon forgotten and the Aberfan tips continued to grow. In 1944 Tip No. 4 failed, and again no one was injured. By the time Tip No. 5 was closed in 1956, it had a large, ominous bulge growing on its lower side, but fortunately it collapsed. Luckily
unfortunately the failure
never
slid.
1958 Tip No. 7 was sited solely on the basis of available space, with no regard to the area's geology. The springs and seeps, though they were visible and well known, were completely ignored. In spite of previous tip failures and warnings of slope failure by tip workers and others, mine debris was being piled onto Tip No. 7 until the day of the disaster. What exactly caused Tip No. 7 and the others to In
fail?
The
official investigation
revealed that the
had become saturated with water from the springs over which they were built. In the case of the collapsed tips, pore pressure from the water exceeded the friction between grains, and the entire mass liquefied like a "quicksand." Behaving as a liquid, the mass quickly moved downhill spreading out laterally. As it flowed, water escaped from the mass, and the sedimentary particles regained their foundation of the
tips
cohesion.
Following the inquiry,
"^ FIGURE
2 Aerial view of the Aberfan which 144 people died.
tip disaster in
had ever been made on the
tips
and that
repeated warnings about potential failure of the as well as previous slides,
that a
new
tip sites.
and advise on the
Unfortunately, six years
Aberfan disaster, a similar incident occurred West Virginia, where a water-saturated, coal-mining refuse dam collapsed. The resulting mudflow swept down the valley killing 118 people. after the
tips,
had all been ignored. As warned that tip failures
early as 1927, a publication
was recommended
assess the dangers of existing tips
construction of studies
it
National Tip Safety Committee be established to
in
Factors Influencing
Mass Wasting
423
Water percolates through soil and sandstone, wetting the clay layer,
which swells and
becomes
"•"
FIGURE
dipping hill's
slippery
(a) Rocks same direction as a
15-8
in the
slope are particularly
susceptible to
mass wasting.
Undercutting of the base of the slope by a stream removes support
and steepens the slope at the base. Water percolating through the soil and into the underlying rock increases
its
weight and,
if
clay
layers are present, wets the clay
making them
slippery, (b) Fractures dipping in the same direction as a slope are enlarged by chemical weathering, which can remove enough material to cause mass
wasting.
are rapid
movements of snow and ice down steep mounby the sound of a loud gunshot
tain slopes, are triggered or, in rare cases,
even a person's shout.
^ TYPES OF MASS WASTING Geologists recognize a variety of mass ble 15-2).
Some
a combination of different types.
424
movements
(Ta-
are of one distinct type, while others are
Chapter 15
Mass Wasting
It is
not
uncommon
for
one type of mass movement to change into another along its course. For example, a landslide may start out as a slump at its head and, with the addition of water, become an earthflow at its base. Even though many slope failures are combinations of different materials and movements, it is still convenient to classify them according to their dominant behavior. Mass movements are generally classified on the basis of three major criteria (Table 15-2): (1) rate of move-
"^ TABLE
15-2
Classification of
Mass Movements and Their
Characteristics
"^ FIGURE
15-10 Numerous rockfalls have resulted from wedging of these bedded and fractured rocks at Alberta Falls, Rocky Mountain National Park, Colorado. Accumulations of talus can be seen at the base of these frost
outcrops.
Rockfalls range in size from small rocks falling from a cliff to massive falls involving millions of cubic meters
of debris that destroy buildings, block highways (Fig. 15-11),
and even bury towns. When
large blocks of rock
into restricted bodies of water, they
fall
may
generate
huge waves capable of tremendous damage. One of the largest of these occurred on July 9, 1958, in Lituya Bay, Alaska. An earthquake dislodged an estimated 30.5 mil-
m3
lion
of rock that
fell
on
level
its
opposite side (see Perspective 20-1, Fig.
Rockfalls are a eas
m into the bay, m above the bay's
more than 900
causing a surge of water that rose 524
common
where roads have been
built
2).
mountainous arby blasting and grading
hazard
in
through steep hillsides of bedrock. Anyone who has ever driven through the Appalachian Mountains, the Rocky Mountains, or the Sierra Nevada is familiar with the
"Watch
for Falling
Rocks"
warn
signs posted to
drivers
of the danger. Slopes particularly prone to rockfalls are
sometimes covered with wire mesh in an effort to prevent dislodged rocks from falling to the road below. Another tactic is to put up wire mesh fences along the base of the slope to catch or slow down bouncing or rolling rocks.
Slides
A
slide involves
more soil,
rock, or a combination of the two, and
apart during
426
movement of material along one or The type of material may be
surfaces of failure.
movement or remain
Chapter 15
Mass Wasting
intact.
it
A
may
break
slide's rate
•**- FIGURE 15-11 Rockfall in Jefferson County, Colorado. All eastbound traffic and part of the westbound lane of Interstate 70 were blocked by the rockfall. Heavy rainfall and failure along joints and foliation planes in Precambrian gneiss caused this rockfall.
of
movement can vary from extremely slow
to very
rapid (Table 15-2).
Two
types of slides are generally recognized:
(1)
slumps or rotational slides, in which movement occurs along a curved surface; and (2) rock or block glides, which move along a more-or-less planar surface. A slump involves the downward movement of material along a curved surface of rupture and is characterized by the backward rotation of the slump block (Fig. 15-12). Slumps occur most commonly in unconsolidated or weakly consolidated material and range in size from small individual sets, such as occur along stream banks, to massive, multiple sets that affect large areas
and cause considerable damage. Slumps can be caused by a variety of factors, but the most common is erosion along the base of a slope, which removes support for the overlying material. This local steepening may be caused naturally by stream erosion along its banks (Fig. 15-12) or by wave action at the base of a coastal cliff. Slope oversteepening can also be caused by human activity, such as the construction of highways and housing developments. Slumps are particularly prevalent along highway cuts and fills where they are generally the most frequent type of slope failure observed. While many slumps are merely a nuisance, large-scale slumps involving populated areas and highways can cause extensive damage. Such is the case in coastal southern California where slumping and sliding have been a constant problem. Many areas along the coast are underlain by poorly to weakly consolidated silts,
BONNIE ROBINSON
Guest Essay
rTTTTTTTTTTTTTTTTTTTTTTTTTTfTTTTTTTTyTT'TTTTT T TTTY T TTTTTTT
CLEANSING THE EARTHWASTE MANAGEMENT*
I
remember the moment when
in geology.
My
I
theory of continental
drift;
became interested was discussing the
first
fifth-grade teacher
using a
map
of the world,
showed us how North and South America could against Europe and Africa to form a single giant she
continent! This intriguing concept
made
so
fit
much
sense— it was like putting together the pieces of a giant jigsaw puzzle— and that is how I still view geology. From that time, I knew that the sciences were my
was an unusual pursuit
I
for an Africanan urban environment. always enjoyed being outdoors and examining maps.
I
was
I
went.
re. It
erican girl
I
a
growing up
in
rockhound, collecting rocks and
majored
broadened
in
my
wherever
fossils
geology at Oberlin College and
understanding of the
field
during
summer
Geology was fascinating because it linked all of the natural and physical sciences together with engineering and applied them to the study of the Earth. I learned that geology internships at the Smithsonian Institution.
human health and the environment, and and administrative controls on the generation, handling and disposal of the wastes. A national E&P waste management program would have far-reaching implications due to the complexity of the oil and gas industry, the wide range of environmental settings affected, and the variety of state regulatory programs. Oil and gas production is scattered throughout more than 30 states, where over 26,000 companies are involved in the exploration and production of oil and gas. Each year thousands of new wells are drilled and thousands of well sites are abandoned. The major wastes generated at these locations consist of water extracted with the oil and gas, drilling fluids, and a variety of lesser wastes. These wastes often contain varying amounts of potentially hazardous constituents. impacts on
legal
One
of the key issues facing the
determine the most
E&P
impacts on
land-use planning requires knowledge of geology, social
domestic production
and other
After college
I
but
skills.
worked
in
environmental geology at
oil
I
how
to
improving
and gas production. Continued is
vital to the nation's interest,
must be balanced with adequate environmental
protection.
Knowledge of
the U.S. Geological Survey, followed by graduate studies at the University of California, Santa Cruz.
it
is
waste management without significant adverse
influenced other fields of endeavor. For example, proper
sciences,
EPA
efficient alternatives for
literacy,
is
science
and technology, or science making
essential for intelligent decision
spent the next 13 years as a petroleum geologist,
regarding critical national issues. Opportunities exist
working on oil and gas exploration and development projects throughout the western United States. My
for full participation by minorities
interest in
environmental issues affecting the
petroleum industry led to field
In
my
desire to
work
technology. in
the
and women, who and that we encourage, develop, and
are severely underrepresented in science
utilize this
It is vital
pool of
talent.
A
of waste management.
my
position at the Environmental Protection
Agency (EPA), I am involved in the development of the program for improved management of wastes generated by crude oil and natural gas exploration and production (E&P) activities. The EPA's Office of Solid Waste is conducting studies of the characteristics of the wastes, waste handling methods and their "Opinions expressed in this paper are solely those of the author and do not necessarily represent those of the U.S. Environmental Protection Agency.
Oonnie Robinson earned an A.B. degree
in
geology from
Oberlin College in 1974, followed by graduate studies at the University of California at
Santa Cruz. She worked as a petroleum geologist in Denver,
Colorado, for 13 years and recently joined the U.S.
Environmental Protection Agency in Washington, D.C.
AAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAA AAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Types of Mass Wasting
427
-~-
FIGURE
material
15-12
In a slump,
moves downward along
the
curved surface of a rupture, causing the slump block to rotate backward. Most slumps involve unconsolidated or weakly consolidated material and are typically caused by erosion along the slope's base.
Surface
of rupture
some of
sands, and gravels interbedded with clay layers,
which are weathered ash California
is
falls.
In addition, southern
tectonically active so that
many
of these
deposits are cut by faults and joints, which allow the
infrequent rains to percolate
and lubricating the clay
downward
rapidly, wetting
Southern California dry most of the year.
tween November fall in
lies in
When
a semiarid climate
a short time.
,
canyon walls
Pacific Palisades '
Santa Monica
Los Angeles
"»»
FIGURE
wave action
15-13
Undercutting of steep sea
cliffs
by
resulted in massive slumping in the Pacific
on March 31 and April 3, 1958. Highway 1 was completely blocked. Note the heavy earth-moving equipment for scale. Palisades area of southern California
428
Chapter 15
Mass Wasting
Pacific
is
Thus, the ground quickly becomes
saturated, leading to landslides along steep as well as along coastal cliffs (Fig. 15-13).
layers.
and
does rain, typically beand March, large amounts of rain can it
Ocean
Most
of the
•^ FIGURE
15-14
Rock
glides occur
when
material
moves downslope along
a
generally planar surface.
slope failures along the southern California coast are the
about 21
These slumps have destroyed many expensive homes and forced numerous roads to be closed and relocated. A rock or block glide occurs when rocks move downslope along a more-or-less planar surface. Most rock glides occur because the local slopes and rock layers dip in the same direction, although they can also
tons.
result of slumping.
occur along fractures parallel to a slope
(Fig. 15-14).
known rock glide in the world is the preSaidmarreh landslide in southwestern Iran (Fig. 15-15). A slab of limestone 305 m thick, 14 km long, and 5 km wide became detached from the Kabir Kuh ridge and slid down and across the adjacent 8 km wide The
largest
historic
Saidmarreh Valley with enough momentum to climb over a ridge 460 m high before stopping nearly 18 km
from
its
source!
The volume of
the slipped material
was
it
km 3
When
,
and
it
weighed approximately 50
billion
the debris from the rock glide finally settled,
covered an area of 166
The causes of factors: (1)
km 2
.
rock glide probably involved three the massive limestone dipped in the same this
was un-
direction as the local slope; (2) the limestone derlain by a
weak
claystone;
and
(3) its
base was un-
dercut by the Karkheh River. In addition, the area seismically active,
and
it is
is
believed an earthquake prob-
ably triggered the slide. In addition to slumping, rock glides are also
common
occurrences along the southern California coast. At Point Fermin, seaward-dipping rocks with interbedded slippery clay layers are undercut by
merous
waves causing nu-
glides (Fig. 15-16a).
Farther south in the residents
watched as
a
town of Laguna Beach,
startled
rock glide destroyed or damaged
Types of Mass Wasting
429
"^ FIGURE 15-15 The world's largest known rock glide occurred in the Saidmarreh Valley, some 96 km northwest of Dizful, Iran. An earthquake is believed to have triggered this massive prehistoric slide that covered an area of 166 km*.
5
Rubble following rock glide
Karkheh River
^-
430
Chapter 15
Mass Wasting
/
/
/
km
(b)
(a)
ir FIGURE
A
combination of interbedded clay beds that become slippery in the same direction as the slope of the sea cliffs, and undercutting of the sea cliffs by wave action has caused numerous rock glides and slumps at Point Fermin, California, (b) The same combination of factors apparently activated a rock glide farther south at Laguna Beach that destroyed numerous homes and cars on October 2, 1978. (Photo (a) courtesy of Eleanora I. Robbins, U. S.
15-16 (a) when wet, rocks dipping
Geological Survey.)
50 homes on October
2,
1978
(Fig.
15-16b). Just as at
previous winter's heavy rains wet a subsurface clayey
Point Fermin, the rocks at Laguna Beach dip about 25° in the same direction as the slope of the canyon walls
siltstone, thus
and contain clay beds that "lubricate" the overlying rock layers, causing the rocks and the houses built on them to glide. In addition, percolating water from the
about five acres, it was part of a larger ancient slide complex. Not all rock glides are the result of rocks dipping in
reducing
activate the glide.
its
shear strength and helping to
Although the 1978
glide covered only
Types of Mass Wasting
431
^V
v
the
same direction
The rock
as a hill's slope.
glide at
Frank, Alberta, Canada, on April 29, 1903, illustrates
how
nature and
human
activity
can combine to create a
situation with tragic results (Fig. 15-17). It would appear at town of Frank, lying was in no danger from
many
first
at least
50%
silt-
and
clay-sized particles, (b)
Mudflow
in
Estes Park, Colorado.
glance that the coal-mining
at the base of Turtle
Mountain,
a landslide (Fig. 15-17). After
of the rocks dipped
"»" FIGURE 15-18 [a) Mudflows are the most fluid of flows and consist of large amounts of water combined with
away from
all,
the mining valley,
unlike the situations at Saidmarreh, Point Fermin, and
Laguna Beach. The
joints in the massive limestone
com-
posing Turtle Mountain, however, dip steeply toward
and are essentially parallel with the slope of mountain itself. Furthermore, Turtle Mountain is supported by weak limestones, shales, and coal layers that underwent slow plastic deformation from the weight of the overlying massive limestone. Coal mining the valley the
along the base of the valley also contributed to the stress
on the rocks by removing some of the underlying support. All of these factors, as well as frost action and chemical weathering that widened the joints, finally re3 sulted in a massive rock glide. Almost 40 million m of rock slid down Turtle Mountain along joint planes, killing 70 people and partially burying the town of Frank.
(a)
Flows Mass movements
which material flows as a viscous movement are termed flows. Their rate of movement ranges from extremely slow to extremely rapid (Table 15-2). In many cases, mass movements may begin as falls, slumps, or slides and fluid
in
or displays plastic
change into flows further downslope.
Mudflows are the most fluid of the major mass movement types (Fig. 15-18). They consist of at least 50% silt- and clay-sized material combined with a significant amount of water (up to 30%). Mudflows are common in arid and semiarid environments where they are triggered by heavy rainstorms that quickly saturate the regolith, turning it into a raging flow of mud that engulfs everything in
its
path.
Mudflows can
also occur in
mountain regions and in areas covered by volcanic ash where they can be particularly destructive (see Chapter 4). Because mudflows are so fluid, they generally follow preexisting channels until the slope decreases or the
channel widens, at which point they fan out. Mudflows are very dangerous types of mass move-
ments because they typically form quickly, usually move very rapidly (at speeds up to 80
capable of transporting
all
km
per hour), and are
different sizes of objects.
As
urban areas in arid and semiarid climates continue to expand, mudflows and the damage they create are beTypes of Mass Wasting
433
•^ FIGURE
15-19
Debris flows
contain larger-sized particles than mudflows and are not as fluid. Debris flows can be very destructive in
mountainous regions because of and
the steep slopes, loose material,
water available from melting snow.
coming problems. For example, mudflows are very common in the steep hillsides around Los Angeles where they have damaged or destroyed many homes. In addition to the damage they cause on hillsides, mudflows are also a hazard to structures built along the bases of steep mountain fronts. This danger arises because mudflows forming in the mountains follow valleys down the mountainside until they reach the base where they fan out onto the
flat
highway, or railroad tracks will be quickly
flow tive
moved or
valley floor. in the
Any
building,
path of the mudflow
buried. For example, a
mud-
Cajon Pass near Los Angeles carried a locomoa distance of more than 600 m before burying it. in
Debris flows are composed of larger-sized particles
much water. Conmore viscous than mudflows,
than mudflows and do not contain as sequently, they are usually typically
do not move
as rapidly,
and
rarely are confined
to preexisting channels. Debris flows can, just as
however, be large ob-
damaging because they can transport
jects (Fig. 15-19).
In semiarid regions, debris flows, like mudflows, are
quite destructive,
and depending on the amount of water
commonly
wet regolith
mudflows and debris any size, and are frequently destructive. They occur, however, most commonly in (Fig.
15-20). Like
flows, earthflows can be of
humid heavy
climates
on grassy soil-covered slopes following
rains.
Some clays spontaneously liquefy and flow like water when they are disturbed. Such quick clays have caused serious damage and loss of lives in Sweden, Norway, eastern Canada, are
composed of
and Alaska (Table 15-1). Quick clays fine silt and clay particles made by the
grinding action of glaciers. Geologists believe these fine
sediments were originally deposited
in a
marine envi-
ronment where their pore space was filled with salt water. The ions in the salt water helped establish strong bonds between the clay particles, thus stabilizing and strengthening the clay. However, when the clays were subsequently uplifted above sea level, the salt water was flushed out by fresh groundwater, reducing the effectiveness of the ionic bonds between the clay particles and thereby reducing the overall strength and cohesiveness of the clay. Consequently,
when
a sudden shock or shaking,
it
the clay
is
disturbed by
essentially turns to a liquid
part of a hillside, leaving a scarp, and flows slowly
and flows. An example of the damage that can be done by quick clays occurred in the Turnagain Heights area of Anchorage, Alaska, in 1964 (Fig. 15-21). Underlying most of the Anchorage area is the Bootlegger Cove Clay, a massive clay unit of poor permeability. Because the Bootlegger Cove Clay forms a barrier preventing groundwater from
downslope
flowing through the adjacent glacial deposits to the sea,
present, they
intergrade. Debris flows are also
mountainous regions because of the combination of steep slopes, great amounts of loose debris, and large volumes of water from melting snow. particularly destructive in
Earthflows
move more slowly than
or debris flows.
434
An
either
mudflows
earthflow slumps from the upper
as a thick, viscous, tongue-shaped
Chapter 15
Mass Wasting
mass of
considerable hydraulic pressure builds up behind the clay.
Some of this water has
the clay
and
flushed out the salt water in
also has saturated the lenses of sand
associated with the clay beds.
Good
When
Friday earthquake struck on
and
silt
the 8.5-magnitude
March
27, 1964, the
shaking turned parts of the Bootlegger Cove Clay into a quick clay and precipitated a series of massive slides in the coastal bluffs that destroyed
most of the homes
in the
Turnagain Heights subdivision. Solifluction is the slow downslope movement of water-saturated surface sediment. Solifluction can occur in
any climate where the ground becomes saturated with
water, but
is
most common
in cold climates
where the
upper surface periodically thaws and freezes. Permafrost is ground that remains permanently frozen. It
covers nearly
20%
of the world's land surface
(Fig.
15-
During the warmer season when the upper portion of the permafrost thaws, water and surface sediment form a soggy mass that flows by solifluction and produces a 22a).
topography (Fig. 15-22b). As might be expected, many problems are associated
characteristic lobate
A good what happens when an uninsulated building is constructed directly on permafrost. In this instance, heat escapes through the floor, thaws the ground below,
Construction of the Alaska pipeline from the oil fields Prudhoe Bay to the ice-free port of Valdez raised numerous concerns over the effect it might have on the permafrost and the potential for solifluction. Some in
thought that
oil
warm enough
flowing through the pipeline would be
to melt the permafrost, causing the pipe-
ground and possibly rupture. were conducted, scientists concompleted in 1977, could safely
line to sink further into the
numerous
After
studies
cluded that the pipeline,
be buried for more than half of its 1,280 km length; where melting of the permafrost might cause structural problems to the pipe, it was insulated and installed above ground. Creep is the slowest type of flow. It is also the most widespread and significant mass wasting process in terms of the total amount of material moved downslope and the monetary damage that it does annually. Creep involves extremely slow downhill rock. Although
mate,
it
is
most
it
movement of
soil
can occur anywhere and in any
effective
and
or cli-
significant as a geologic
agent in humid rather than arid or semiarid climates. In the most
common form
of mass wasting in the
with construction in a permafrost environment.
fact,
example
southeastern United States and the southern Appala-
is
and turns
ground into the
is
it
into a soggy, unstable
no longer
mush. Because the
solid, the building settles
unevenly
ground, and numerous structural problems
15-20
chian Mountains.
Because the rate of movement
is
essentially impercep-
we are frequently unaware of creep's existence unwe notice its effects: tilted trees and power poles,
tible, til
broken
streets
foundations
sult (Fig. 15-23).
"^ FIGURE
re-
it is
and sidewalks, cracked retaining walls or 15-24). Creep usually involves the
(Fig.
Earthflows form tongue-shaped masses of wet regolith that in humid climates on grassy An earthflow near L'Anse, Michigan.
{a)
move slowly downslope. They occur most commonly soil-covered slopes, {b)
Types of Mass Wasting
435
"^ FIGURE (a)
15-21
Groundshaking by the 1964
Alaska earthquake turned parts of Cove Clay into a quick clay, causing numerous slides the Bootlegger (b)
that destroyed
many homes
in
the Turnagain Heights subdivision
of Anchorage.
436
Chapter 15
Mass Wasting
•
ii&
<^*W«M'
O^ontinuo'us zone / Discontmuou zone
/
*}&*
(a)
(b)
"^ FIGURE
15-22
is
Distribution of permafrost areas in the Northern Hemisphere.
flows near Suslositna Creek, Alaska,
(b) Solifluction
that
{a)
show
the typical lobate topography
characteristic of solifluction conditions.
whole hillside and probably occurs, to some extent, on any weathered or soil-covered, sloping surface.
Not only to control.
is
creep difficult to recognize,
stabilize creep,
many
is
difficult
times the only course of action
to simply avoid the area
of creep
it is
Although engineers can sometimes slow or if
at all possible or,
if
"^ FIGURE
15-23
This house, located south of
Fairbanks, Alaska, has settled unevenly because the
underlying permafrost thawed.
in fine-grained silts
and sands has
is
the zone
relatively thin, design structures that
can be
anchored into the solid bedrock.
Complex Movements many mass movements are combinations of movement types. When one type is dominant, the movement can be classified as one of the movements described thus far. If several types are involved, howRecall that different
it is called a complex movement. The most common type of complex movement is the slide-flow in which there is sliding at the head and then some type of flowage farther along its course. Most slide-
ever,
flow landslides involve well-defined slumping at the
Types of Mass Wasting
437
^ FIGURE
15-24
(a)
Some
evidence of creep: (A) curved tree trunks; (B) displaced monuments; (C) tilted power poles; (D) displaced
and
tilted fences; (E)
roadways
moved out of alignment; (F)
hummocky
surface, (b)
Creep
has bent these sandstone and shale beds of the Haymond Formation near Marathon, Texas, (c) Trees bent by creep, Wyoming, (d) Stone wall tilted due to creep, Champion, Michigan. (Photo courtesy of David J.
Matty.)
*»-
Slump block
15-25 A complex which slumping occurs the head followed by an
FIGURE
movement
/
Surface of
at
rupture
earthflow.
head, followed by a debris flow or earthflow (Fig. 15-25).
Identifying areas with a high potential for slope
ure
out as rockfalls when large quantities of rock, ice, and snow are dislodged from a mountainside, frequently as a result of an earthquake. The material
previous mass wasting
then slides or flows
down
up The
the mountainside, picking
additional surface material and increasing in speed.
motion the debris avalanche that destroyed the town of Yungay (see the Prologue). 1970 Peru earthquake
set in
fail-
any hazard assessment study; such studies include identifying former landslides as well as sites of potential mass movement. Because of the effects
However, any combination of different mass movement types can be classified as a complex movement. A debris avalanche is a complex movement that often occurs in very steep mountain ranges. Debris avalanches typically start
in
important
is
in
of weathering, erosion, and vegetation, the evidence for
may
be obscured. However,
hummocky surface, and sudden changes in vegetation are some of the features indicating former landslides or an
scarps,
open
fissures, displaced or tilted objects, a
area susceptible to slope failure.
and bedrock samples are also studied, both in the and laboratory, to assess such characteristics as
Soil field
composition, susceptibility to weathering, cohesiveness,
and gists
» RECOGNIZING AND
of conditions.
The information derived from
MINIMIZING THE EFFECTS OF MASS MOVEMENTS way,
former landslides and areas susceptible to mass movements can be identified and perhaps avoided (see Perspective 15-2).
By assessing
the risks of possible
a
hazard assessment
study can be used to produce slope stability maps of the area. These maps allow planners and developers to
The most important factor in eliminating or minimizing the damaging effects of mass wasting is a thorough geologic investigation of the region in question. In this
Such studies help geoloand engineers predict slope stability under a variety
ability to transmit fluids.
mass
wasting before construction begins, steps can be taken to eliminate or minimize the effects of such events.
make
decisions about
and housing or
where
industrial
to site roads, utility lines,
developments based on the
relative stability or instability of a particular location. In
addition, the landslide that
may
maps
problem
also indicate is
how
extensive an area's
and the type of mass movement
occur. This information
is
important for de-
signing slopes or building structures to prevent or min-
imize slope failure damage.
Recognizing and Minimizing the Effects of Mass Movements
439
Perspective 15-2
THE VAIONT DAM DISASTER On
October
1963, a glacial valley in the Italian of the worst dam disaster in history.
9,
Alps was the
site
More than 240
million
m3
of rock and
soil slid into
the Vaiont Reservoir, triggering a destructive flood that killed nearly 3,000 people (Fig.
1).
To
fully
appreciate the enormity of this catastrophe, consider
30 seconds, mass of debris 2 above the reservoir
the following: Within a period of 15 to the slide
km
filled
the reservoir with a
long and as high as 175
water that
"^ FIGURE Vaiont
Dam
m
wave of overflowed the dam by 100 m and was
The impact of
level.
1
more than 70
m
the
dam
itself
still
wind
that
survived the disaster (Fig. 2)!
The dam was
the debris created a
km
downstream. The slide shook houses, broke windows, and even lifted the roof off one house in the town of Casso, which is 260 m above the reservoir on the opposite side of the slide; it also set off shock waves recorded by seismographs throughout Europe. Considering the forces generated by the slide, it is a tribute to the designer and construction engineer that high 1.6
also set off a blast of
built in a glacial valley that
is
underlain by a thick sequence of folded and faulted
Location of the
disaster
and features
associated with the landslide.
Reservoir water I
I
Reservoir area
by 1963
filled
slide
Path of wave in reservoir caused by slide
••••
440
Chapter 15
Mass Wasting
1963 landslide
limit
1960 landslide
limit
limestones and interbedded clay layers and marls that are further
weakened by
H
jointing (Fig. 3). Signs of
'
v
„
tfiMi*
previous slides in the area were obvious, and the few
boreholes in the valley slopes revealed clay seams and small-scale slide planes. In spite of the geological
evidence of previous mass wasting in the area and objections to the site by
some of
the early
investigators, construction of the 265.5
Dam began. A combination
m
high Vaiont
of both adverse geological features
and conditions resulting from the dam construction
Among
contributed to the massive landslide.
the
which the same
geological causes were the rocks themselves,
were weak to begin with and dipped
in
direction as the valley walls of the reservoir. Fractured
limestones
make up
the bulk of the rocks
and are
interbedded with numerous clay beds that are particularly prone to slippage. Active solution of the
limestones by slightly acid groundwater further weakened them by developing and expanding an extensive network of cracks, joints, fissures, and other
openings.
During the two weeks before the
slide occurred,
heavy rains saturated the ground, adding extra weight
and reducing the shear strength of the rocks. In addition to water from the rains, water from the reservoir infiltrated the rocks of the lower valley walls,
further reducing their strength.
It is
believed that the
actual slope failure
was caused by an
water pressure that
facilitated slippage
^ FIGURE
increase of
lubricated clay layers.
Soon
after the
dam was
small slide of one million
completed, a relatively
m
3
of material occurred on
the south side of the reservoir. Following this slide,
it
was decided to limit the amount of water in the reservoir and to install monitoring devices throughout the potential slide area. Between 1960 and 1963, the eventual slide area
moved an average
of about
1
cm
Aerial view of the Vaiont
Dam.
Around October 1, animals grazing on the slopes of Mount Toe seemingly sensed danger and moved off the hillside. The mayor of Casso ordered the evacuation of the town in anticipation of a landslide and water wave from the reservoir. By October almost 39
was not
on September 18, 1963, however, numerous monitoring stations reported movement had increased to about 1 cm per day. It was assumed that these were individual blocks moving, but in reality it was the entire slide area! Heavy rains fell between September 28 and October 9, increasing the amount of subsurface water. per week. Beginning
2
along the wet,
cm
8, the
creep rate had increased to
per day, and engineers realized that
it
individual blocks that were moving, but the
and quickly began lowering the October 9, the rate of movement in the slide area had increased still further, in some locations up to 80 cm per day, and there were reports entire slide area,
reservoir level.
On
that the reservoir level
be expected
if
was actually rising. This was bank was moving into the
to
the south
(continued on next page)
Recognizing and Minimizing the Effects of Mass Movements
441
'
Cretaceous Limestone Dashed where marl is present
«-"* — »— —
— *
Malm Formation
=
Contains clay interbeds
Principal strike plane
' Fault
Dogger Formation
I
-^FIGURE
A
3
Lias Formation
generalized geologic cross section through the slide area of the
Vaiont Reservoir area. The
reservoir
line of the section
and displacing water.
Finally, at
is
shown
in
Figure
10:41 p.m.
1.
study should examine the geology of the area, identify
conducted before major construction begins. Such a
mass movements, assess their potential for and evaluate the effects that the project will have on the rocks, including how it will alter their shear strength over time. Without these precautions, dams will continue to fail and lives will needlessly be lost.
The importance of slope
later
that night, during yet another rainstorm, the south
bank of the Vaiont valley slid into the reservoir. The lesson to be learned from this disaster is that a complete and systematic appraisal of an area must be
unstable areas
is
stability
well illustrated by
maps in delineating what happened in
San Clemente, California (Fig. 15-26). After the area had already been developed, a relative slope stability
map
town was made that classified on a scale ranging from relatively stable to unstable. The house indicated by the arrow in Figure 15-26b was built on material identified as unstable and was of a portion of the
areas
442
Chapter 15
Mass Wasting
past
recurrence,
destroyed by a landslide
stability
(Fig.
15-26c).
If a
slope
study had been conducted before development
began, construction might not have been allowed in unstable areas.
Although most large mass movements usually cannot be prevented, geologists and engineers can employ various
methods to minimize the danger and damage refrom them. Because water plays such an impor-
sulting
FIGURE
15-26
(a)
Relative
slope stability map of part of San Clemente, California, showing areas
delineated according to relative stability, (b)
the
The house indicated by built on unstable
arrow was
material,
(c)
A
landslide later
destroyed the home.
,....
}\$wv£mmpm*i*!!*im*
(b)
Recognizing and Minimizing the Effects of Mass Movements
443
^J
-^ FIGURE
15-27
(a)
Driving
drainpipes that are perforated on
one side into a
hillside
with the
perforated side up can remove some subsurface water and thus help stabilize a hillside, (b)
A
drainpipe
driven into the hillside at Point
Fermin, California, helps remove subsurface water and stabilize the slope.
many landslides, one of the most effective and inexpensive ways to reduce the potential for slope tant role in
failure or to increase existing slope stability
is
through
and subsurface drainage of a hillside. Drainage serves two purposes. It reduces the weight of the material likely to slide and increases the shear strength of the slope material by lowering pore pressure. surface
444
Chapter 15
Mass Wasting
Surface
waters
can
be
drained
and diverted by
ditches, gutters, or culverts designed to direct water
away from
slopes. Drainpipes perforated along one surand driven into a hillside can help remove subsurface water (Fig. 15-27). Finally, planting vegetation on hillsides helps stabilize slopes by holding the soil together and reducing the amount of water in the soil.
face
Another way to help slope.
stabilize a hillside
is
to reduce
its
Recall that overloading or oversteepening by
grading are
common
causes of slope failure. By reducing
the gradient of a hillside, the potential for slope failure is
decreased.
Two common
methods are generally em-
ployed to reduce a slope's gradient. In the cut-and-fill
method, material
"^ FIGURE
is
removed from the upper part of
the
slope and used as
at the base, thus
fill
providing a
flat
and reducing the slope (Fig. 15-28). The second method, which is called benching,
surface for construction
involves cutting a series of benches or steps into a
hill-
This process reduces the average slope, and the benches serve as collecting sites for small landslides or
side.
rockfalls.
Benching
is
most commonly used on steep
hill-
Two common methods used to help stabilize a hillside and reduce method, material from the steeper upper part of the removed, thereby reducing the slope angle, and is used to fill in the base. This provides some additional support at the base of the slope, (b) Benching involves making its
15-28
slope, {a) In the cut-and-fill
hillside
is
several cuts along a hillside to reduce the overall slope.
This material has
been removed
Before
Former slope
Recognizing and Minimizing the Effects of Mass Movements
445
Vegetation planted on slope
Stable bedrock
Drain pipe
Unstable rock layers
(a)
(a)
(b)
"^ FIGURE 15-29 (a) Retaining walls anchored into bedrock, backfilled with gravel, and provided with drainpipes can support a slope's base and reduce landslides (b) Steel retaining wall built to stabilize the slope and keep falling and sliding rocks off of the highway.
sides in conjunction with a system of surface drains to
divert runoff. In
some
situations, retaining walls
can be constructed
to provide support for the base of the slope (Fig. 15-29).
These are usually anchored well into bedrock, backfilled with crushed rock, and provided with drain holes to prevent the buildup of water pressure in the hillside.
Rock
bolts, similar to those
employed
in tunneling
and mining, have been used to fasten potentially unstable rock masses into the underlying stable bedrock (Fig. 15-30). This technique has been used successfully on the hillsides of Rio de Janeiro, Brazil, and to help secure the slopes at the Glen Canyon Dam on the Colorado River. Recognition, prevention, and control of landslideprone areas is expensive, but not nearly as expensive as the damage can be when the warning signs of mass 446
Chapter 15
Mass Wasting
"^ FIGURE
15-30
(a)
Rock
bolts secured in
bedrock can
help stabilize a slope and reduce landslides, (b) Rock bolts are used to help secure rock above the outlet of the west diversion tunnel of the Glen Canyon Dam. As can be seen, however, some portions of rock still broke away.
wasting are ignored or not recognized. We end this chapter with a discussion of the Portuguese Bend land-
one of the most damaging landslides in California and one in which all of the warning signs of impending disaster were ignored. The Portuguese Bend area of southern California is
slide,
part of a large ancient landslide complex. Signs of
former mass wasting such as scarps, erally
hummocky ground
fissures,
and a gen-
surface are obvious on aerial
photographs and geologic maps of the area (Fig. 15-31). In spite of such evidence, nothing was done to prevent the construction of roads and houses in the area during the 1950s.
Movement began
sliding.
movement
in
2
began in 1956 when a 1 km area During the period from 1956 to 1978, Portuguese Bend was essentially continufirst
*» FIGURE
s* Santa Monica
slide area in
Los Angeles
movements, (b) Aerial view of the Portuguese Bend slide area on November 29, 1966, in which some of the features in (a)
•mrrnTt
15-31 (a) Diagram of the Portuguese Bend 1957. The arrows indicate the direction of slide
can be seen.
"Pull-away" scarp or trench Fracture, fissure, or slip surface
~^=-~^=.
Swarm Toe
of
of small fractures
major slide mass
(a)
Recognizing and Minimizing the Effects of Mass Movements
447
ous, averaging between 0.3 and 1.3
and
the late 1970s
more than
2.5
cm
per day. During
early 1980s, the rate accelerated to
cm per day, probably
years of above-average rainfall. In
as a result of several
more than 150
all,
vating the landslide at Portuguese Bend.
homes were
destroyed, and the highway at the base of
is
had
to be relocated several times because of the
landslide. In addition,
the slide
and rock gliding that occurred. In addition, property damage caused by the landslide was estimated at more than $10 million. The cause of this landslide became the subject of a lawsuit brought by the homeowners' association against the County of Los Angeles. After years of litigation, the court ruled in favor of the homeowners, and the county compensated them for damage to their property. A variety of factors were apparently responsible for reacticreep, slumping,
^
thought that the extra weight helped to
Mass wasting
is
the
downslope movement of
which contributed to the lubrication of the subsurface clay layers and the subsequent movement. As a result of the Portuguese Bend event, Los Angeles County now requires detailed geological engineering studies before any hillside home construction can begin. Since the plan was adopted, the percentage of homes damaged or destroyed by landslides has been greatly
reduced.
are aided by weathering and involve
Several types of mass wasting are geologic hazards that are frequently responsible for loss of
Millions of dollars
in
damage
life.
are caused annually by
mass wasting. 3.
Mass wasting occurs when
the gravitational force
acting parallel to a slope exceeds the slope's strength. 4.
Mass wasting can be caused by many
common factors
material, the material's water content, overloading
of the slope, and removal of vegetation. Usually, several of these factors in combination contribute to
slope failure.
Mass movements are generally classified on the basis of their rate of movement, type of movement, and type of material.
6.
Rockfalls are a
rocks 7.
Two
common mass movement
free-fall.
types of slides are recognized. Slumps are
rotational slides involving surface; they are
most
movement along
common
in
9.
448
Rock
glides
movement, type of material, and
amount of water. Mudflows consist of mostly clay- and particles and contain more than 30%
Chapter 15
Mass Wasting
of permafrost. is
the imperceptible
downslope movement of soil or rock. Creep is the most widespread of all types of mass wasting. 15. Complex movements are combinations of different types of mass movements in which one type is not dominant. Most complex movements involve sliding and flowing. 16. The most important factor in reducing or eliminating the damaging effects of mass wasting is a thorough geologic investigation of the area including mapping, soil and rock analysis, and the construction of slope stability
a curved
occur when movement takes place along a more or less planar surface; they usually involve solid pieces of rock. Several types of flows are recognized on the basis of their rate of
in areas
susceptible to
maps
to outline areas
mass movements.
poorly
consolidated or unconsolidated material.
8.
which
in
larger-sized particles
less
14. Creep, the slowest type of flow,
including a slope's gradient, weathering of the slope
5.
composed of
water than mudflows. They are more viscous and do not flow as rapidly as mudflows. 11. Earthflows move more slowly than either debris flows or mudflows; they move downslope as thick, viscous, tongue-shaped masses of wet regolith. 12. Quick clays are clays that spontaneously liquefy and flow like water when they are disturbed. 13. Solifluction is the slow downslope movement of water-saturated surface sediment and is most
and contain
Most mass
surficial material.
2.
septic
most common in semiarid and arid environments and generally follow preexisting channels.
material under the influence of gravity.
movements
initiate the
most of the houses had
systems, and the residents watered their lawns, both of
10. Debris flows are 1.
of the
are
SUMMARY
CHAPTER
One
main causes was construction of roads in the area. Large quantities of fill from the road construction were dumped at the top of what became the slide area, and it
silt-sized
water.
They
^ IMPORTANT
TERMS
complex movement
mudflow
creep debris avalanche
permafrost quick clay
debris flow
rapid mass
earthflow
rockfall
mass wasting
rock glide
movement
shear strength
slump
slide
solifluction
12.
was
movement
slow mass
An
ancient landslide at Portuguese Bend, California, reactivated by:
a
road construction;
c
above-average
13.
none of these. Define mass wasting.
14.
What
septic systems;
b.
rainfall; d.
all
of these;
e.
REVIEW QUESTIONS Shear strength includes: a.
the strength
b.
the
grains;
gravity; d.
c.
Which of
and cohesion of material; internal friction between
15.
amount of
answers
e.
and
(a)
not a factor influencing
is
17.
weathering;
gravity; b.
water content;
gradient; d.
Which of
none of
e.
these.
the following factors can actually enhance
water content;
c.
overloading; d.
direction as the slope;
Mass wasting can
18.
rocks dipping
none of
e.
in the
same
c.
in flat-lying areas; d.
on steep slopes; all
in
solifluction;
slide; b.
flow; d.
fall; c.
none of these. Downslope movement along an
slip;
a.
slump;
landslide;
essentially planar
rockfall;
b.
earthflow;
c.
rock glide.
e.
which area can good examples of slumps, and flows be found? In
a.
Wyoming;
c.
Alberta, Canada; d.
e.
all
b.
slides,
Anchorage, Alaska;
of these.
of the following are the most fluid of mass earthflows; b.
debris flows;
slumps. mudflows; d. solifluction; e. The most widespread and costly type of mass wasting in terms of total material moved and monetary damage is: a. creep; b. solifluction; c. mudflow; c.
10.
d.
11.
debris flow;
Which of
the relationship
e.
slumping.
the following features indicate former
landslides or areas susceptible to slope failure? a.
displaced objects;
c.
hummocky
e.
all
of these.
are rockfalls?
Where
most
are they
common
What
is
glide?
Why
the difference between a
slump and a rock
are slumps particularly
common
along
fills?
two
different
ways
Why
25.
What
26.
Why
are quick clays so dangerous?
precautions must be taken when building permafrost areas? is
creep so prevalent, and
why does
it
in
do so
27.
How
can creep be controlled once
it
has started?
complex movements, and how do they differ from other types of mass movements? 29. What are some of the indications of previous mass wasting? How can you recognize areas that are susceptible to mass movement? 28.
What
^
ADDITIONAL
are
southern California;
movements? a.
dangerous?
is it
much damage?
a:
d.
What
24.
e.
Which
why how
earthflow.
rockfalls;
is a:
is
overloading and
23. Differentiate between a mudflow, debris flow, and
is:
c.
slides; e.
surface
Give
that rock glides might occur.
mountainous
mudflows. Movement of material along a surface or surfaces of a.
is
22. Discuss and give examples of
common
regions in which talus accumulates
failure
What
road cuts and
of these;
these.
type of mass wasting
d.
stability?
and why? 21.
slopes; b.
creep; b.
20.
these.
occur:
on gentle
a.
does vegetation affect slope
affects slope stability.
vegetation;
b.
a.
none of
mass
between topography and the underlying geology
a.
e.
How
19. Give several examples of
slope stability?
A
in
several examples.
slope
c.
do climate and weathering play
what ways does the ground's water content affect slope stability? Give an example of how excessive water content has resulted in slope failure.
(b).
the following
roles
16. In
of these;
all
What
wasting?
mass wasting? a.
are the forces that help to maintain slope
stability?
b.
surfaces; d.
scarps;
open
fissures;
READINGS
and B. L. Harrod, eds. 1989. Landslides: Extent and economic significance. Brookfield, Va.: A. A. Balkema. Crozier, M. J. 1989. Landslides: Causes, consequences, and environment. Dover, New Hampshire: Croom Helm. Fleming, R. W., and F. A. Taylor. 1980. Estimating the cost of Brabb, E.
E.,
landslide
damage
in the
United States. U.S. Geological Survey
Circular 832. Kiersch, G. A. 1964. Vaiont reservoir disaster. Civil Engineering 34: 32-39. McPhee, J. 1989. The control of nature.
New
York: Farrar,
Straus &C Giroux. J., and M. J. Clark. 1982. Slopes and weathering. New York: Cambridge University Press. Zaruba, Q., and V. Mencl. 1982. Landslides and their control. 2d ed. Amsterdam, The Netherlands: Elsevier Publishing Co.
Small, R.
Additional Readings
449
CHAPTER
16
RUNNING WAT E R ^ OUTLINE PROLOGUE INTRODUCTION THE HYDROLOGIC CYCLE RUNNING WATER Sheet Flow versus Channel Flow-
Stream Gradient Velocity
and Discharge
T" Guest
Essay:
Managing Our Water
Resources
STREAM EROSION TRANSPORT OF SEDIMENT LOAD STREAM DEPOSITION Braided Streams and Their Deposits
Meandering Streams and Their Deposits Floodplain Deposits
"^
Perspective 16-1: Predicting
and
Controlling Floods Deltas Alluvial Fans
DRAINAGE BASINS AND DRAINAGE PATTERNS BASE LEVEL
THE GRADED STREAM DEVELOPMENT OF STREAM VALLEYS SUPERPOSED STREAMS STREAM TERRACES INCISED MEANDERS T^
Perspective 16-2: Natural Bridges
CHAPTER SUMMARY
Grand Sable
Falls in Alger County, Michigan. (Photo courtesy of R. V Dietrich.)
PROLOGUE In 1877, the Italian astronomer
Giovanni Virginio Schiaparelli viewed and was convinced that he saw straight lines. In his report he called these lines
Mars through
his telescope
canali, the Italian
word
for channel, but
it
was
icecaps to the equatorial regions of a planet that
was
becoming progressively drier. When Mariner 4 flew past Mars on July 15, 1965, however, it sent back images indicating that no canals were present; apparently, they were simply an optical illusion. Although no Martians or their works were discovered, Mariner 4 and the subsequent Viking missions did reveal evidence for running water. Currently, liquid water cannot exist on the Martian
During the 1890s, Percival Lowell, who founded the Lowell Observatory near Flagstaff, Arizona, published numerous maps showing interconnecting canals on Mars. Many
surface because the atmospheric pressure is too low. If any water were present, it would rapidly vaporize. Mars does, however, show clear evidence of volcanism
astronomers could not see Lowell's canals, but others thought they could. By the early 1900s, the public had
volcanoes are
translated into English as "canal."
accepted the idea that intelligent beings had constructed the canals to divert water from the polar
(see Fig. 2-12),
although
still
it is
doubtful that any of the
active. Nevertheless, just as
dioxide and water vapor by outgassing (see Fig. 12-3). Furthermore, the gravitational attraction of Mars is sufficient to retain these gases, so
that originated during ,*r
FIGURE
16-1
of chaotic terrain directions of flow.
Outflow channels extending from areas (CT) on Mars. Arrows show inferred
on
Earth, Martian volcanoes no doubt emitted carbon
its
most of the water
early history
may
still
some form. The polar icecaps of Mars composed of frozen carbon dioxide and frozen there in
and large quantities of water in the
ice are
be are
water,
probably present
pore spaces of the surface deposits.
"^ FIGURE 16-2 Runoff channels on Mars. Although runoff channels resemble dry channels called arroyos in the southwestern United States, they may not have been formed by running water.
\
-
Prologue
451
where the water no doubt ponded and
Studies of Mariner and Viking images reveal areas called chaotic terrane that appear to consist of loosely
piled rubble.
Winding
valleys,
these areas of chaotic
terrane (Fig. 16-1). These channels are 10 to 100
wide, some are more than 2,000
them are
number
a
km
long,
km
through a liquid phase),
and within
of features indicating fluid flow.
Apparently, the channels formed
when huge
The overlying rock then subsided, forming the chaotic terrane, and the water was released at the surface as flash floods. Judging from the size of these outflow channels, the flash floods
volcanic activity.
What became
known on
Earth.
of these flash flood waters?
its
surface
likely percolated
percolated
down
fell
and where it
as snow, melted,
into the surface deposits
refroze.
quantities
of subsurface ice suddenly melted, perhaps because of
probably exceeded any
The water beneath very
downward and froze once again. The surface ice probably sublimated (vaporized without going
termed outflow
some of
channels, extend from
froze.
The
Much smaller networks of runoff channels are remarkably similar to the dry channels called arroyos found in the southwestern United States (Fig. 16-2). Nevertheless, the Martian channels do not possess features unequivocally associated with running water. Thus, even though the runoff channels resemble channels on Earth, running water may not have been responsible for their origin.
outflow channels terminate at closed depressions
»
INTRODUCTION
Among
the terrestrial planets, the Earth
is
unique
in
having abundant liquid water. Both Mercury and the Earth's
moon
are too small to retain any water,
and
runaway greenhouse effect, is too hot to retain surface water. Mars has some frozen water and trace amounts of water vapor in its atmosphere (see the Prologue). In marked contrast, 71% of the Earth's surface is covered by water, and a small but important
Venus, because of
its
quantity of water vapor
is
present in
The volume of water on Earth
km 3
is
its
atmosphere.
estimated at 1.36
most of which (97.2%) is in the oceans. About 2% is frozen in glaciers, and the remaining 0.8% constitutes all the water in streams, lakes, swamps, groundwater, and the atmosphere (Fig. 16-3). Thus, only a tiny portion of the total water on Earth is in streams, but running water is nevertheless the most imbillion
,
portant erosional agent modifying the Earth's surface. Despite the importance of running water as an agent of erosion, sediment transport, and deposition,
some
its
role
is
by glacial ice, such as Greenland and Antarctica, running water is currently not important. Some parts of deserts are also little affected by running water. Even in most desert regions, however, the effects of running water are manifest, although the channels are dry most of the time (Fig. 16-4). limited in
areas. In areas covered
In addition to
its
significance as a geologic agent,
important for many other reasons as well. It is a source of fresh (nonsaline) water for industry, domestic use, and agriculture. About 8% of the elec-
running water
452
is
Chapter 16
Running Water
tricity used in North America is generated by falling water at hydroelectric stations (Fig. 16-5). Streams have been, and continue to be, important avenues of com-
merce.
Much
of the interior of North America
was
explored by following such large streams as the
Lawrence, Mississippi, and Missouri
first
St.
rivers.
^ THE HYDROLOGIC CYCLE Although the quantity of water in streams is small at any one time, during the course of a year very large volumes of water move through stream channels. In fact, water is continually recycled from the oceans, through the atmosphere, to the continents, and back to the oceans. This continual recycling of water is called the hydrologic cycle (Fig. 16-6). (The hydrologic cycle will also be relevant to our discussions of groundwater in Chapter 17 and glaciers in Chapter 18.) The hydrologic cycle, which is powered by solar radiation, is possible because water changes phases easily under Earth surface conditions. Huge quantities of water evaporate from the oceans each year as the surface waters are heated by solar energy. The amount of ocean water evaporated yearly corresponds to a layer about 1 m thick from all the oceans. Approximately 85% of all water that enters the atmosphere is derived from the oceans; the remaining 15% comes from evaporation of water on land. When water evaporates, the vapor rises into the atmosphere where the complex processes of condensation and cloud formation occur. About 80% of the precipi-
/
1
sess ae SBXBUF. ~tr-T
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zam
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--;-—
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Groundwater to rivers and oceans
"*"'
flow, on the other hand, occurs in almost all streams. The primary control on the type of flow is velocity; roughness of the surface over which flow occurs also plays a role. Laminar flow occurs when water flows very slowly as when groundwater moves through the tiny pores in sediments and soil. In streams, however, the flow is usually fast enough and the channel walls and bed rough enough so that flow is fully turbulent. Laminar flow is so slow, and generally so shallow, that it causes little or no erosion. Turbulent flow is much more energetic and thus is capable of considerable erosion and sediment transport.
Flow versus Channel Flow
The amount of runoff in any area during a rainstorm depends on its infiltration capacity, the maximum rate at which soil or other surface materials can absorb water. Infiltration capacity depends on several factors, including the intensity and duration of rainfall. Loosely packed,
dry
soils
absorb water faster than
tightly
16-6
"^ FIGURE parallel to
The hydrologic
16-7 (a) In laminar flow, streamlines are one another, and little or no mixing occurs
cycle.
all
between adjacent layers in the fluid, (b) In turbulent flow, streamlines are complexly intertwined, indicating mixing between adjacent layers in the fluid. Most flow in streams turbulent.
Water surface
(a)
Sheet
FIGURE
is
'"•'"
"^ FIGURE
gradient of this stream is 2 m/kra. Gradient can be calculated for any segment of a stream as shown in this example. Notice that the gradient is
16-8
The average
steepest in the headwaters area
downstream
and decreases
in a
FIGURE
16-9 In a stream, flow velocity varies as a with the banks and bed. The maximum flow velocity is near the center and top of a stream in a straight channel. The lengths of the arrows in this illustration are proportional to the velocity. result of friction
direction.
Streams receive water from several sources, including and rain falling directly into stream channels.
sheet flow
packed, wet
soils. Hard, dry surfaces, such as those that
develop during droughts, also have low infiltration capacities. Therefore, still
If
when
they do receive rain, there
is
considerable runoff. rain
occurs.
is
absorbed as
fast as
However, should the
it
falls,
no surface runoff
infiltration capacity
be ex-
become saturated, on the surface and, if a slope exists, moves downhill. Even on steep slopes such flow is initially slow, and hence little or no erosion occurs. As the water moves downslope, however, it accelerates and
ceeded, or should surface materials excess water collects
may move by
sheet flow, a more-or-less continuous film is not con-
of water flowing over the surface. Such flow
and it accounts for sheet erosion, a particular problem on some agricultural lands (see Chapter 6). fined to depressions,
In channel flow, surface runoff is confined to long, troughlike depressions. Channels vary in size from rills
containing a trickling stream of water to the
Amazon
is 6,450 km long and up wide and 90 m deep. Channelized flow is described by various terms including rill, brook, creek, stream, and river, most of which are distinguished by size and volume. The term stream carries no connotation of size and is used here to refer to all runoff con-
River of South America, which to 2.4
km
fined to channels regardless of size.
456
Chapter 16
Running Water
more important, however, is the water supplied by moisture and groundwater, both of which flow downslope and discharge into streams (Fig. 16-6). In humid areas where groundwater is plentiful, streams Far
soil
may maintain
a fairly stable flow year round, even dur-
ing dry seasons, because they are continually supplied
by groundwater. In contrast, the amount of water in streams of arid and semiarid regions fluctuates widely because such streams depend more on infrequent rainstorms and surface runoff for their water supply.
Stream Gradient Streams flow downhill from a source area to a lower where they empty into another stream, a lake,
elevation
or the sea.*
The
slope over which a stream flows
gradient. For example,
stream
500
is
km
1,000
m
if
is its
the source (headwaters) of a
above sea level and the stream flows it drops 1,000 m vertically over a
to the sea,
km
horizontal distance of
500
'The flow
streams diminishes
in certain desert
(Fig. 16-8). Its
in a
direction by evaporation
and
Some streams
with numerous caverns
in regions
below the ground.
infiltration until the
gradient
is
downstream
streams disappear.
may
disappear
Broad, shallow channel
— FIGURE
16-10
All three of
these channels have the
same
cross-sectional area, but each has a different shape.
Cross-sectional
10
m2
in
contact
12
m
with water
channel.
calculated by dividing the vertical drop by the horizontal
semicircular
channel has the least perimeter in contact with the water and thus causes the least frictional resistance to flow. If other variables, such as channel roughness, are the same in all of these channels, flow velocity will be greatest in the semicircular
area
Perimeter
The
distance; in this example,
it is
1,000 m/500
km =
2
semicircular channels flows
counters
maximum
m/km. Gradients vary considerably, even along the course of
more
rapidly because
less frictional resistance. In
it
en-
many streams
the
flow velocity occurs near the surface at the
center of the channel;
it
occurs slightly below the surface
a single stream. Generally, streams are steeper in their
because of frictional resistance from the air above.
upper reaches where their gradients may be tens of meters per kilometer, but in their lower reaches the gradient may be as little as a few centimeters per kilometer.
However,
Some streams
center only along straight reaches (Fig. 16-11).
States
mountains of the western United have particularly steep gradients of several hunin the
maximum
in
sinuous (meandering) channels, the line of
flow velocity switches from one side of the
channel to the other and corresponds to the channel
dred meters per kilometer.
Velocity
and Discharge
Stream velocity and discharge are closely related variables. Velocity is simply a measure of the downstream distance traveled per unit of time. Velocity
expressed in feet per second (m/sec)
(
ft/sec)
is
"^ FIGURE 16-11 In a sinuous (meandering) channel, flow velocity varies from one side of the channel to the other. As the water flows around curves, it flows fastest near the outer bank. The dashed line in this illustration follows the path of maximum flow velocity.
usually
or meters per second
and varies considerably among streams and even
within the same stream. Variations in flow velocity occur not only with distance along a stream channel but also across a channel's
width. For example, flow velocity
is
slower and more
turbulent near a stream bed or stream banks than
it is
from these boundaries. The bed and banks cause frictional resistance to flow, whereas the water some distance away is unaffected by friction and thus has a farther
higher velocity (Fig. 16-9).
Other controls on velocity include channel shape and roughness. Broad, shallow channels and narrow, deep
channels have proportionally more water in contact with their perimeters than do channels with semicircular cross sections (Fig. 16-10). Consequently the water in
Running Water
457
Guest Essay
BURTON KEMP
E.
III
MANAGING OUR WAT E R RESOURCES
My
when
geology began
interest in
I
was
a senior in
high school. At that time, the petroleum industry was
and development were beginning to expand dramatically. at a high point. In particular, overseas exploration
The
possibility of extensive travel to foreign countries,
groundwater levels and supply potential, and hazardous toxic waste contamination. These studies have led to such projects as the evaluation and design of an emergency dewatering system to prevent excessive uplift during dewatering of a critical contn
New
coupled with the potential for a comfortable
structure in eastern
was attractive to me. At the same time, I had always had an interest in the natural and physical
evaluation and development programs in east and
livelihood,
seemed to be a perfect was a natural physical science, and
sciences. Therefore, geology
choice for me:
me
offered
it
the opportunity to travel, seek
and satisfy my phenomena.
My
was
original intent
to pursue a career in the
graduation from undergraduate school, the
is
was
it
fortune,
interest in naturally occurring
petroleum industry; however, at the time of industry
my
one of the
in
much
prone. After
cyclic
thought,
I
my
and gas downturns to which it oil
decided that a change in
would be prudent. I shifted my emphasis toward more engineering-related areas of study and direction
southwest Texas; design of a groundwater monitoring program for a hazardous holding pond in north-central Louisiana; detailed analysis of the
subsurface for hazardous toxic waste analysis at several industrial facilities in Louisiana, Arkansas,
Nuclear Waste Repository in central Mississippi. These studies included studying, examining, and monitoring the effects that changes in groundwater levels and chemistry have on our environment. Finally, through participation in detailed studies the Louisiana Coastal
predictions
graduate studies with emphasis on engineering geology
will increase
its
many
related fields such as geomorpholgy,
Zone marshes and wetlands, rates, make
have been able to help define land loss
losses
on future areas where land loss problems or diminish, and isolate areas where are minimal and offer the best potential to
new marshes. These make it possible to reduce
hydrogeology, environmental geology, and the
reduce future losses or create
application of geology to engineering structures.
coastal zone studies will
During
my
career,
participate in a projects,
and
I
have been privileged to
number
of investigations, studies,
activities that
have been extremely
losses within
our valuable wetlands and help
These studies and investigations, as well as participation
viewpoint. Specifically,
panels, study groups,
investigations
and
final
participated in the initial site
design for a
number
projects such as dams, control structures,
navigation locks. Cooper
Caddo
Dam
in
Dam
in
of large
and
northeast Texas;
northwest Louisiana; Old River
Control Auxiliary Structure in central Louisiana; Lock and Dam Numbers 1 through 5 on the Red River
Waterway
in
presi
these natural resources.
rewarding, from both a professional and a personal I
and
South Carolina; and evaluation of the potential for groundwater contamination at a proposed High Level
began a career as an engineering geologist with the U.S. Army Corps of Engineers. I also elected to pursue
and
Orleans; groundwater
on numerous committees, boards, revi and seminars, have afforded me
the opportunity to
work
closely with
disciplines in conserving, protecting,
natural environment.
I
many
other
and improving our
strongly that geology
feel
very dynamic and rewarding
field
and
is
a
will continue to
be equally exciting and challenging in the future, a
north-central Louisiana; and the Lake
Pontchartrain &C Vicinity Hurricane Protection Project in
some examples of these These structures are used to control floods
southeast Louisiana are
projects.
and hurricane tidal surges, thus reducing property damage and the potential for loss of life. They also help maintain our nation's waterways so that waterborne commerce can move efficiently. I have also been engaged in several ongoing groundwater studies involving foundation effects,
XL. Burton Kemp III earned an M.S. in geology from Tulane University Graduate School.
has worked
in the
He
area of
engineering geology for more
than 30 years and
is
currently a
with the U.S. Army Corps of Engineers for the New Orleans District. district geologist
LAAAAAAAAAAAAAA.AAAAAAAAAAAAAAAAAAAAAAA
*w TABLE
16-1
^ FIGURE particles
B.
M.
The
16-12
up to the
solid load of a stream consists of
size of boulders.
(Photo courtesy of
C. Pape.)
The
solid sediment carried in streams ranges
clay-sized particles to large boulders (Fig. 16-12).
of this sediment finds ing (Fig. 16-13), but
from
Much
way into streams by mass wastmuch is derived directly from the
its
stream bed and banks. For example, the power of running water, called hydraulic action, is sufficient to set particles in motion.
Everyone has seen the
loose
if
soil, a
from a garden hose is directed onto soon gouged out by hydraulic action.
the flow
hole
is
sand and gravel, the impact of these particles abrades exposed rock surfaces. One obvious manifestation of abrasion
is
the occurrence of potholes in the beds of
These circular to oval holes occur where eddying currents containing sand and gravel swirl around and erode depressions into solid rock. streams
(Fig. 16-14).
results of
hydraulic action, although perhaps not in streams. For
example,
"^ FIGURE 16-13 Streams such as the Snake River in Idaho receive some of their sediment load by mass wasting processes. (Photo courtesy of R. V. Dietrich.)
Another process of erosion in streams is abrasion, in which exposed rock is worn and scraped by the impact of solid particles. If running water contains no sediment, little or no erosion will result, but if it is transporting
-»- FIGURE 16-14 Potholes in the bed of the Chippewa River in Ontario, Canada.
^ TRANSPORT OF SEDIMENT LOAD Streams transport a solid load of sedimentary particles
and
a dissolved load consisting of ions taken into solu-
tion
by chemical weathering. Sedimentary
particles are
transported either as suspended load or as bed load.
Suspended load consists of the smallest particles, such as silt and clay, which are kept suspended by fluid turbulence (Fig. 16-15). Such particles are deposited only where turbulence is minimal as in lakes or the quiet offshore waters of the sea.
Bed load consists of the coarser particles such as sand and gravel (Fig. 16-15). Fluid turbulence is insufficient to keep such large particles suspended, so they
move
along the stream bed. Part of the bed load can be suspended temporarily, however. For example, an eddying current
may
swirl across a stream bed
grains into the water. These particles
and
lift
sand
move forward
at
approximately the flow velocity, but at the same time they settle toward the stream bed where they come to rest, to
be
moved again
later
by the same process. This
process of intermittent bouncing and skipping along the
stream bed
is
called saltation (Fig. 16-15).
Particles too large to
be suspended even temporarily
are transported by rolling or sliding (Fig. 16-15). Obvi-
460
Chapter 16
Running Water
Surface of stream
and
sliding
"^ FIGURE
16-15
Rolling
Stream bed
Methods of sediment transport by
running water. The velocity profile at the right indicates that and slowest along
the water flows fastest near the surface the stream bed.
ously, greater flow velocity
of these sizes.
is
required to
The maximum-sized
move
particles
particles
that a
stream can carry define its competence, a factor related to flow velocity. Figure 16-16 shows the velocities required to erode, transport, and deposit particles of various sizes.
As expected, high
velocity
is
necessary to erode
and transport gravel-sized particles, whereas sand is eroded and transported at lower velocities. Notice, however, that high velocity is needed to erode clay because clay deposits are very cohesive: the tiny clay particles
adhere to one another and can be disrupted only by energetic flow conditions. Once eroded, however, very little
needed to keep the clay particles in motion. is a measure of the total load a stream can carry. It varies as a function of discharge; with greater discharge, more sediment can be carried. Capacity and
energy
is
Capacity
competence may seem quite
similar,
ally related to different aspects
but they are actu-
of stream transport. For
example, a small, swiftly flowing stream
competence to move gravel-sized transport a large capacity.
A
may have
the
particles but not to
volume of sediment. Thus, it has a low slow-flowing stream, on the other
large,
may have a very large suspended load, and hence a large capacity. hand, has a low competence, but
^ STREAM DEPOSITION Streams can transport sediment a considerable distance from the source area. For example, some of the sediments
Mexico by the Mississippi River came from such distant sources as Pennsylvania, Minnesota, and Wyoming. Along the way, deposition may
deposited in the Gulf of
occur in a variety of environments, such as stream channels, the floodplains adjacent to channels, and the points
where streams flow into lakes or the seas or flow from mountain valleys onto adjacent lowlands.
"*•»
FIGURE
A
16-18
meandering stream, the Flathead
"^ FIGURE
16-19
The cut bank of
a
meandering stream.
River near Kalispell, Montana.
when
a stream
is
which over time within
its
is
supplied with excessive sediment,
meandering channel when
deposited as sand and gravel bars
resistant materials. Streams fed
channel. During high-water stages, these bars
are submerged, but during low-water stages, they are
exposed and divide a single channel into multiple channels (Fig. 16-17). Braided streams have broad, shallow channels. They are generally characterized as bed load transport streams, and their deposits are composed mostly of sheets of sand and gravel (Fig. 16-17). Braided streams are common in arid and semiarid regions where there
is little
vegetation and erosion rates are
high. Streams with easily eroded
become braided. its
banks are
** FIGURE
In fact, a
easily
eroded
banks are also
stream that
may have
is
likely to
braided where
a single, sinuous or
also
Two
small point bars in a meandering
flows into an area of
more
by melting glaciers are
commonly braided because the melting much sediment (see Chapter 18).
glacial ice
yields so
Meandering Streams and Their Deposits Meandering streams possess
a single, sinuous channel
with broadly looping curves called meanders
(Fig. 16-
Such stream channels are semicircular in cross section along straight reaches, but at meanders they are markedly asymmetric, being deepest near the outer bank, which commonly descends vertically into the channel. The outer bank is called the cut bank because flow velocity and turbulence are greatest on that side of the channel where it is eroded (Fig. 16-19). In contrast, 18).
flow velocity 16-20
stream.
it
is
at a
minimum
near the inner bank,
which slopes gently into the channel. As a consequence of the unequal distribution of flow velocity across meanders, the cut bank is eroded and deposition occurs along the opposite side of the channel.
The
meander migrates laterally, and more or less constant width because erosion on the cut bank is offset by an equal amount of deposition on the opposite side of the channel. The deposit formed in this manner is a point bar; it consists of cross-bedded sand or, in some cases, gravel net effect
is
that a
the channel maintains a
(Fig.
16-20). Point bars are the characteristic deposits
meandering stream channels. meanders to become so sinuous that the thin neck of land separating adjacent methat accumulate within It is
anders
not
is
floors of
462
Chapter 16
Running Water
uncommon
for
eventually cut off during a flood.
The
valley
meandering streams are commonly marked by
***
FIGURE
16-21
the origin of an
and
(b),
the
narrower,
Four stages
oxbow
in
lake. In (a)
meander neck becomes The meander neck is
(c)
cut off, and part of the channel
abandoned,
(d)
When
is
it is
completely isolated from the main channel, the abandoned meander is an oxbow lake.
crescent-shaped
meanders
oxbow
(Fig. 16-21).
some
lakes, which are actually cutoff These oxbow lakes may persist
streams
commonly have
a floodplain, but this feature
is
usually proportional to the size of the stream; thus,
time, but are eventually filled with
small streams have narrow floodplains, whereas the
organic matter and fine-grained sediment carried by
lower Mississippi and other large streams have floodmany kilometers wide. Streams restricted to deep, narrow valleys usually have little or no floodplain. Some floodplains are composed mostly of sand and gravel that were deposited as point bars. When a meandering stream erodes its cut bank and deposits on the opposite bank, it migrates laterally across its floodplain. As lateral migration occurs, a succession of point bars
as lakes for
floods.
Once
filled,
oxbow
lakes are called
meander
scars.
'
One immediate effect of meander cutoff is an in
increase
flow velocity; following the cutoff, the stream aban-
dons part of
its
old course and flows a shorter distance,
Numerous
cutoffs would, of meandering stream, but such streams usually establish new meanders elsewhere
thus increasing
its
gradient.
course, significantly shorten a
when
plains
old ones are cut off.
develops by lateral accretion isodes of sedimentation
Many floodplains
Floodplain Deposits Most streams
periodically receive
(Fig.
16-22b). That
is,
the
deposits build laterally as a consequence of repeated ep-
on the inner banks of meanders.
are dominated by vertical accretion
When a stream overflows its banks and floods, the velocity of the water spilling onto
of fine-grained sediments.
more water than
their
channel can carry, so they spread across low-lying,
rel-
the floodplain diminishes rapidly because of greater fric-
atively flat areas called floodplains adjacent to their
tional resistance to flow as the water spreads out as a
channels (Fig. 16-22a) (see Perspective 16-1). Even small
broad, shallow sheet. In response to the diminished ve-
Stream Deposition
463
Perspective 16-1
PREDICTING AND CONTROLLING FLOODS Occasionally, a stream receives
channel can handle, and all
of
its
floodplain.
it
more water than
floods,
its
occupying part or
To monitor stream behavior, the more than 11,000
U.S. Geological Survey maintains
stream gauging stations, and various state agencies also
monitor streams. Data collected at gauging hydrograph
stations can be used to construct a
showing
how
(Fig. 1).
Hydrographs are
a stream's discharge varies over time
and water supply better idea of
projects,
what
useful in planning irrigation
and they give planners a
to expect during flood events.
Stream gauge data are also used to construct (Fig. 2). To construct such a curve, the peak discharges are first arranged in order
flood-frequency curves
i
i
Feb. 13lFeb.
i
I
i
i
i
I
i
i
i
ulFeb. 15lFeb.
I
i
i
i
i
i
i
i
i
i
i
i
161 Feb. 17lFeb. 18lFeb. 19
of volume; the flood with the greatest discharge has a
magnitude rank of 1, the second largest is 2, and so on (Table 1). The recurrence interval— that is, the time period during which a flood of a given magnitude or larger can be expected over an average of many
**f FIGURE 2 Flood-frequency curve for the Rio Grande near Lobatos, Colorado. The curve was constructed from the data in Table
1.
-^"
FIGURE
Ashland
1
Hydrograph
Sycamore Creek near February 1989 flood. (From
for
City, Tennessee, for the
U.S. Geological Survey Water-Resources Investigations
Report 89-4207.)
350-
40
30 Recurrence
464
Chapter
1
6
Running Water
interval (years)
I
50
60
801100 90
I
70
"» TABLE
1
Some of
the
Data and Recurrence
Intervals for the
Rio Grande near Lobatos, Colorado
Year
Discharge (m 3 /sec)
Rank
1900 1901
133 103 16
23 35 69
3.35 2.20 1.12
362
2
38.50
1902 1903 1904
Recurrence Interval
— FIGURE
16-22 (a) The broad, area adjacent to the channel of stream a is its floodplain. (b) Floodplain deposits forming by lateral accretion of point bars. flat
locity, ridges
(b)
of sandy alluvium called natural levees are
deposited along the margins of the stream channel (Fig. 16-23). Natural levees are built up by repeated deposi-
numerous
These natural levees separate most of the floodplain from the stream
tion of sediment during
floods.
Deltas
The fundamental process of delta formation is rather simple: when a stream flows into another body of water, its
flow velocity decreases rapidly and deposition occurs.
commonly poorly drained and swampy. In fact, tributary streams may parallel the main stream for many kilometers until they find a way
As
a result of such deposition, a delta forms, causing the
through the natural levee system
larger
channel, so floodplains are
The
(Fig. 16-23).
flood waters spilling from a
main channel carry beyond the floodplain. During the
large quantities of fine-grained sediment
natural levees
waning
and onto the
stages of a flood, the flood waters
may
flow
local shoreline to build out, or
Deltas in lakes are
The
and
far
prograde
common, but marine
(Fig.
16-24).
deltas are
much
more complex.
simplest prograding deltas exhibit a characteris-
tic vertical
sequence
in
which bottomset beds are and topset beds
cesssively overlain by foreset beds
16-24a). Such sequences develop
when
suc(Fig.
a stream enters
and the suspended silt and clay eventually settle as layers of mud. Thus, floodplain mud deposits build upward by deposition during successive
another body of water, and the
flood events.
the river mouth, foreset beds are formed as sand and
very slowly or not at
466
Chapter 16
all,
Running Water
ried
finest
sediments are car-
some distance beyond the river mouth, where they from suspension and form bottomset beds. Nearer
settle
silt
Y
"»" FIGURE 16-23 Three stages in the formation of vertical accretion deposits on a floodplain. [a] Stream at low-water stage. (b) Flooding stream and deposition. Many such episodes of flooding form natural levees. [e]
are deposited in gently inclined layers.
The
topset beds
consist of coarse-grained sediments deposited in a net-
work of
distributary channels traversing the top of the
delta. In effect,
streams lengthen their channels as they
extend across prograding deltas
(Fig. 16-24).
Many
After flooding.
small deltas in lakes have the three-part divi-
sion described above, but large marine deltas are usually-
much more complex. Depending on
the relative impor-
tance of stream, wave, and tidal processes, three major types of marine deltas are recognized
(Fig.
Stream Deposition
16-25).
467
^ FIGURE
16-24
(a)
Internal
structure of the simplest type of
prograding in
delta, (b)
which bottomset,
A
small delta
foreset,
and
topset beds are visible.
Stream-dominated
deltas,
such as the Mississippi River
delta, consist of long fingerlike
sand bodies, each depos-
ited in a distributary channel that progrades far seaward. Such deltas are commonly called bird's-foot deltas because the projections resemble the toes of a bird. In contrast, the Nile delta of Egypt is wave-dominated, although it also possesses distributary channels; the seaward margin of the delta consists of a series of barrier
islands
formed by reworking of sediments by waves, and
the entire margin of the delta progrades seaward. Tide-
dominated
deltas,
such as the Ganges-Brahmaputra of
Bangladesh, are continually modified into tidal sand bodies
that parallel the direction of tidal flow (Fig. 16-25).
468
Chapter 16
Running Water
Coal can form in several depositional environments, such as the fresh water marshes between distributary channels of deltas inated by
(Fig.
nonwoody
domwhose remains accumulate
16-24a). Such marshes are
plants
form peat, the first stage in the origin of coal (see Chapter 7). If peat is buried, the volatile components of the plants are driven off leaving mostly carbon that eventually forms coal. Delta progradation is one way in which potential reservoirs for oil and gas form. Because of their porosity and permeability and association with organic-rich mato
rine sediments, distributary tain oil
and
gas.
Much
sand bodies commonly conoil and gas production of
of the
"^ FIGURE
16-25 (a) The Mississippi River delta of the U.S. Gulf Coast is stream-dominated, and (b) the Nile Delta of Egypt is wave-dominated, (c) The Ganges-Brahmaputra delta of Bangladesh is tide-dominated.
Coast of Texas comes from buried delta deposSome of the older deposits of the Niger River delta of
Alluvial Fans
the Gulf its.
Africa and the Mississippi River delta are also
contain vast reserves of
oil
and
gas.
known
to
on land (Fig. 16-26). They form best on lowlands adjacent to highlands in arid and semiarid regions where little or no vegetation Alluvial fans are lobate deposits
Stream Deposition
469
canyon onto the lswland area, it quickly spreads its velocity diminishes, and deposition ensues. The alluvial fans that develop by the process just described are mostly accumulations of sand and gravel, a large proportion of which is deposited by streams. In some cases, however, the water flowing through a mountain canyon picks up so much sediment that it becomes a viscous mudflow (see Chapter 15). Consequently, mudflow deposits make up a large part of many
the
out,
alluvial fans.
* DRAINAGE BASINS AND DRAINAGE PATTERNS Thousands of streams, most of which are parts of
larger
drainage systems, flow either directly or indirectly into the oceans. A stream such as the Mississippi River consists
of a main stream and
all
streams that supply water to
of the smaller tributary
it.
The
Mississippi
and
all
any other drainage system for that matter, carry surface runoff from an area known as the
of
its
tributaries, or
drainage basin (Table 16-1). Individual drainage basins are separated from adjacent ones by topographically higher areas called divides (Figs.
Some
divides are rather
modest
16-27 and 16-28).
rises,
such as that sep-
arating the Great Lakes' drainage basin from that of the Mississippi River, whereas others, such as the Continental
Divide along the crest of the Rocky Mountains, are
more
impressive.
Various drainage patterns are recognized based on the regional arrangement of channels in a drainage system.
The most common
"^ FIGURE
is
dendritic drainage,
which
16-27 Small drainage basins separated from one another by divides.
(b)
"^ FIGURE
16-26 {a) Alluvial fans form where a stream discharges from a mountain canyon onto an adjacent lowland, {b) Alluvial fans adjacent to the Black Mountains in
Death
Valley, California.
exists to stabilize surface materials.
When
periodic rain-
storms occur, surface materials are quickly saturated and runoff begins. During a particularly heavy rain, all of the surface flow in a drainage area is funneled into a
mountain canyon leading stream
is
cannot spread
470
to
an adjacent lowland. The
confined in the mountain canyon so that laterally.
Chapter 16
However,
Running Water
as
it
it
discharges from
^ FIGURE basin of the
16-28
The drainage
Amazon
River covers
more than 6,000,000
of a network of channels resembling tree branching (Fig. 16-29a). Dendritic drainage develops on
consists
km 2
.
are strongly controlled by geologic structures, particularly regional joint
systems that intersect at right angles.
where the materials respond more or less homogeneously to erosion. Areas of flatlying sedimentary rocks and some terrains of igneous or metamorphic rocks usually display a dendritic drainage
Rectangular drainage develops because streams more easily erode and establish channels along the traces of
pattern.
Virginia and Pennsylvania, erosion of folded sedimen-
gently sloping surfaces
In
marked contrast
to dendritic drainage in
which
joints.
In
some
tary rocks develops a landscape of alternating parallel
tributaries join larger streams at various angles, rectan-
ridges
characterized by channels with right
strata,
gular drainage angle bends
is
and
tributaries that join larger streams at
right angles (Fig. 16-29b).
The
positions of the channels
parts of the eastern United States, such as
and
valleys.
The
ridges consist of
more
resistant
such as sandstone, whereas the valleys overlie less resistant strata such as shale. Main streams follow the trends of the valleys. Short tributaries flowing from the
Drainage Basins and Drainage Patterns
471
FIGURE
16-29
Examples of
drainage patterns, (a) Dendritic drainage, (b) Rectangular drainage. (c) Trellis drainage, (d) Radial drainage,
(e)
Deranged drainage.
adjacent ridges join the main stream at nearly right angles,
hence the name
trellis
drainage
(Fig. 16-29c).
outward in all direcfrom a central high area (Fig. 16-29d). Radial drainage develops on large, isolated volcanic mountains, such as Mount Shasta in California (see Fig. 4-16), and where the Earth's crust has been arched up by the inIn radial drainage, streams flow
tions
it developed recently and has not yet formed an organized drainage system. In areas in Minnesota, Wisconsin, and
Michigan that were glaciated
until
about 10,000 years
ago, the previously established drainage systems were obliterated by glacial ice. Following the final retreat of the glaciers,
drainage systems became established, but have
not yet become
fully organized.
trusion of plutons such as laccoliths. In
some
areas streams flow in
and out of swamps and
lakes with irregular flow directions. Drainage patterns
characterized by such irregularity are called deranged (Fig. 16-29e).
472
The presence of deranged drainage
Chapter 16
Running Water
indicates that
^ BASE LEVEL Streams require a slope on which to flow, so they can downward only to the level of the body of water
erode
Rock to
resistant
erosion
Ultimate base // level
^ FIGURE and
(£>),
16-30
sea level
is
In both (a)
ultimate base
rock layer while in a local base level.
level. In (a) a resistant
forms a local base (a)
into
which they
(b)
flow.
A
example, cannot erode it
could,
it
would have
stream flowing into the sea, for valley lower than sea level
its
—
a lake
(b)
if
to flow uphill to reach the sea.*
is
present on the Northern Hemisphere continents, sea level
was more than 100
m
lower than at present. Ac-
cordingly, streams deepened their valleys by adjusting to
did the Nile River of Egypt
Thus, there exists a lower limit to which streams can
a
erode called base level
(see Perspective 7-1).
In addition,
tended
onto
stream could erode so sea level level.
is
its
(Fig.
16-30). Theoretically, a
entire valley to very near sea level,
commonly
referred to as ultimate base
Streams never reach ultimate base
because they must have some gradient
in
level,
level,
new, lower base their
level, as
valleys
shelves. Rising sea level at
many
streams ex-
exposed continental the end of the Pleistocene the
however,
order to main-
tain flow.
In addition to ultimate base level, streams
have local
or temporary base levels. For example, a lake or another
stream can serve as a local base level for the upstream
•^ FIGURE 16-31 Niagara Falls on the New York/Ontario, Canada border. The resistant rock forming the escarpment over which the falls plunge forms a local base level.
segment of a stream (Fig. 16-30b). Likewise, where a stream flows across particularly resistant rock, a waterfall may develop, forming a local base level. The escarp-
ment over which Niagara Falls plunges is a good example of a local or temporary base level (Fig. 16-31).
When
sea level rises or falls with respect to the land,
or the land over which a stream flows
is
uplifted or
subsides, changes in base level occur. For example, dur-
ing the Pleistocene
Epoch when extensive
'Streams flowing into depressions below sea
level,
glaciers
were
such as Death
Valley in California, have a base level corresponding to the lowest
point of the depression and are not limited by sea level (see Chapter 19).
Base Level
473
dam
neers are well aware th/t the process of building a
and impounding a reservoir creates a 16-32a).
Where
a stream enters a reservoir,
Ultimate
(Fig.
base
velocity diminishes rapidly
level
local base level its
and deposition occurs;
flow thus,
reservoirs are eventually filled with sediment unless they
are dredged.
Another consequence of building a dam
that the water discharged at the free,
but
dam
is
is
largely sediment
possesses energy to transport sediment.
it still
Commonly, such streams simply acquire a new sediment load by vigorously eroding downstream from the dam. Draining a lake along a stream's course may seem change that is well worth the time and expense to expose dry land for agriculture or commercial like a small
-— -T
development.
stream after drained
Profile of
lake
is
sult.
^ FIGURE
16-32
stream deposits a lake
is
{a)
The process of constructing a dam
a reservoir creates a local base level.
much
A
into a reservoir, (b)
when
Remember
draining
and impounding
A
its sediment load where it flows stream adjusts to a lower base level
it
valleys.
human
intervention, but not always
in anticipated or desirable
ways. Geologists and engi-
""»'
FIGURE
16-33 its
{a)
An ungraded
that a lake
is
a temporary base level, so
respond by rapid downcutting
(Fig.
likely
16-32b).
^ THE GRADED STREAM longitudinal profile shows the elevations of a
and
graded stream.
Stream
its
length as viewed in cross section (Fig.
The longitudinal profiles of many streams show number of irregularities such as lakes and waterfalls,
16-33). a
which are
local base levels (Fig. 16-33a).
irregularities tend to
Over time such
be eliminated by stream processes;
where the gradient is steep, erosion decreases it, and where the gradient is too low to maintain sufficient flow velocity
stream has
longitudinal profile, (b) Erosion
deposition along the course of a stream eliminate irrregularities and cause it to develop the smooth, concave profile typical of a
the
anticipates
lowers the base level for that part of the
channel along
irregularities in
one
stream above the lake, and the stream will very
A stream's level to rise,
Streams adjust to
unless
of
drained.
and the streams responded by depositing sediments and backfilling previously formed caused base
However,
stream's probable response, dire consequences can re-
for
sediment
transport,
deposition
occurs,
steepening the gradient. In short, streams tend to de-
velop a smooth, concave longitudinal profile of equiliball parts of the system are dynamione another (Fig. 16-33b). Streams possessing an equilibrium profile are said to be graded streams; that is, a delicate balance exists between gradient, discharge, flow velocity, channel characteristics, and sediment load such that neither significant erosion nor deposition occurs within the channel. Such a delicate balance is rarely attained; thus, the concept of a graded stream is an ideal. Nevertheless, many streams do indeed approximate the graded condition,
rium, meaning that cally adjusting to
profile
Erosion
although usually only temporarily. Even though the concept of a graded stream ideal,
we can
is
an
generally anticipate the responses of a
graded stream to changes altering its equilibrium. For example, a change in base level would cause a stream to adjust as previously discussed. Increased rainfall in a
stream's drainage basin
474
would
result in greater dis-
charge and flow velocity. In short, the stream would
(b)
Chapter 16
Running Water
now
possess greater energy
— energy
that
must be
dissi-
pated within the stream system by, for example, a
change
in
channel shape.
to a broad,
A
change from a semicircular
shallow channel would dissipate more enOn the other hand, the stream may
A
valley
may
begin where runoff has sufficient energy
and excavate a small rill. surface runoff and be-
to dislodge surface materials
Once formed,
a
collects
rill
comes deeper and wider
more
until a full-fledged valley develops
ergy by friction.
(Fig. 16-36). Several
respond by active downcutting in which it erodes a deeper valley and effectively reduces its gradient until it
and evolution of valleys, including downcutting, lateral erosion, mass wasting, sheet wash, and headward erosion. Downcutting occurs when a stream possesses more
is
once again graded. Vegetation inhibits erosion by having a stabilizing
ef-
and other loose surface materials. Thus, a decrease in vegetation in a drainage basin might lead to higher erosion rates, causing more sediment to be washed fect
on
soil
into a stream than
the stream
it
which increases the stream's gradient ciently steep to transport the greater
until
its it
some of
requires to transport
excess energy cuts
its
sediment load, so
valley deeper (Fig.
its
suffi-
eral erosion, creates unstable conditions so that part of
is
16-36a).
If
ing, valleys
a bank or valley wall may move downslope by any one or a combination of mass wasting processes (Fig. 16-
sediment load.
Furthermore, sheet wash and erosion of
13).
gully tributaries carry materials into the
STREAM VALLEYS common
it
channel,
^ DEVELOPMENT OF Valleys are
its
downcutting were the only process operatwould be narrow and steep sided as in Figure 16-35. In most cases, however, the valley walls are undercut by the stream. Such undermining, termed lat-
can effectively carry. Accordingly,
may respond by deposition within
energy than
processes are involved in the origin
rill and from the valley walls
main stream. becoming deeper and wider, stream
In addition to
landforms, and with few exceptions
they form and evolve as a consequence of stream erosion,
although other processes, especially mass wasting, also
The shapes and sizes of valleys vary considsome are small, steep-sided gullies, whereas othbroad and have gently sloping valley walls. Some
valleys are
commonly lengthened
as well. Valleys are
lengthened in an upstream direction by headward erosion as drainage divides are eroded by entering runoff
some
headward erosion
contribute.
water
erably;
eventually breaches the drainage divide and diverts part
ers are
steep-walled, deep valleys of vast proportions are called
canyons. The Grand
Canyon of Arizona,
for example,
is
an interconnected system of canyons eroded by the Col-
orado River and
its
narrow and deep
^ FIGURE
16-34
tributaries (Fig. 16-34). Particularly
valleys are gorges (Fig. 16-35).
The Grand Canyon
in Arizona is a Colorado River and E. Andrews.)
vast system of canyons eroded by the its
tributaries.
(Photo courtesy of L.
(Fig.
16-37a). In
cases
of the drainage of another stream by a process called
stream piracy
(Fig.
16-37b).
Once stream
piracy has oc-
must adjust; one now has more water, greater discharge, and greater potential to erode and transport sediment, whereas the other is dicurred, both drainage systems
minished
-^-
in all of these aspects.
FIGURE
16-35
River in Colorado
is
The Black Canyon of a gorge because
it is
the
Gunnison
steep-walled and
deep.
Development of Stream Valleys
475
—
FIGURE 16-36 Valley development, {a) If valleys formed mostly by downcutting, they would be narrow and steep sided. (b) Valleys are deepened by downcutting, but most of them are also widened by lateral erosion, mass wasting, and sheet wash.
^ SUPERPOSED STREAMS Streams flow downhill
in
response to gravity, so their
courses are determined by preexisting topography. Yet a
number of streams seem,
at first glance, to have defied fundamental control. For example, the Delaware, Potomac, and Susquehanna rivers in the eastern United States have valleys that cut directly through ridges lying in their paths. The Madison River in Montana meanders northward through a broad valley, then enters a narrow canyon cut into bedrock that leads to the next valley where the river resumes meandering. All of the streams cited above are superposed. In order to understand how superposition occurs, it is necthis
essary to
know
the case of the
the geologic histories of these streams. In
Madison
River, the valleys
it
now
occu-
were once filled with sedimentary rocks so that the river flowed on a surface at a higher level (Fig. 16-38). As the river eroded downward, it was superposed dipies
upon a preexisting knob of more resistant rock, and instead of changing its course, it cut a narrow, steep-walled canyon called a water gap.
rectly
Superposition also accounts for the fact that the Del-
aware, Potomac, and Susquehanna rivers flow through
water gaps. During the Mesozoic Era, the Appalachian Mountain region was eroded to a sediment-covered plain across which numerous streams flowed generally east-
•^ FIGURE
16-37
Two
stages in stream piracy, (a) In the
lower elevation extends its channel by headward erosion. In (b) it has captured some of flowing at the higher elevation. of the stream the drainage first
476
Chapter 16
Running Water
stage, the stream at the
yw FIGURE down
16-38
The
origin of a superposed stream, (a)
underlying structure.
A
A
stream begins cutting
removed by erosion, exposing the The stream flows across resistant beds that form the ridges.
into horizontal strata.
(£>)
horizontal layer
is
ward. During the Cenozoic Era, however, regional uplift commenced, and as a consequence of the uplift, the streams began eroding
downward and were superposed
on
forming water gaps
resistant strata, thus
(Fig. 16-38).
day floodplain
some
(Fig. 16-39). In
several steplike surfaces
above
its
cases, a stream has
present-day floodplain,
indicating that stream terraces formed several times.
Although
all
stream terraces result from erosion, they
are preceded by an episode of floodplain formation
^ STREAM TERRACES Adjacent to
many
floodplains formed
stream to cut
They
downward
Once
until
is
it
the stream again
once again graded
becomes graded,
streams are erosional remnants of
(Fig. 16-40).
when
begins eroding laterally and establishes a
the streams were flowing at a
higher level. These erosional remnants are stream terraces.
consist of a fairly flat upper surface
and
deposition of sediment. Subsequent erosion causes the
and
a
steep slope descending to the level of the lower, present-
at a
lower
level.
it
floodplain
Several such episodes account for the
multiple terrace levels seen adjacent to (Figs.
new
some streams
16-39 and 16-40).
-•-
FIGURE
16-39
Stream
terraces adjacent to the
River
in
Madison
Montana.
Stream Terraces
477
many stream
Floodplain.
terrace*, greater runoff in a stream's drain-
age basin can also result in the formation of terraces. Recall that one of the variables controlling velocity discharge. Thus, a stream can erode
change
in
is
downward with no
base level and form terraces.
^ INCISED MEANDERS Some streams
are restricted to deep, meandering can-
yons cut into solid bedrock, where they form features called incised meanders. For example, the San Juan River in Utah occupies a meandering canyon more than 390 meters deep (Fig. 16-41). Such streams, being reby solid rock walls, are generally ineffective
stricted
in
eroding laterally; thus, they lack a floodplain and oc-
cupy the entire width of the canyon floor. Some incised meandering streams do erode laterally, thereby cutting off meanders and producing natural bridges (see Perspective 16-2). It is
not
difficult to
downward
understand
how
a stream can cut
into solid rock, but forming a
pattern in bedrock
is
meandering
another matter. Because lateral
one must meandering course was established when the stream flowed across an area covered by alluvium. For example, suppose that a stream near base level has established a meandering pattern. If the land over which the stream flows is uplifted, erosion is initiated, and the
erosion
is
inhibited once downcutting begins,
infer that the
meanders become incised into the underlying bedrock.
-^ FIGURE
Uplift does not account for
Origin of stream terraces, {a) A stream has a broad floodplain adjacent to its channel, (b) The stream erodes downward and establishes a new floodplain at a lower level. Remnants of its old floodplain are stream terraces, (c) Another level of terraces originates as the
16-40
downward
stream erodes
again.
Where they
some
are cut into solid bedrock.
are cut into bedrock, the terrace surface
generally covered by a thin veneer of sediment. In
is
many
stream valleys, terraces are paired, meaning that they
occur at the same elevation on opposite sides of the channel
(Fig.
16-40b and
Renewed erosion and
c).
the formation of stream ter-
races are usually attributed to a change in base level.
which a stream flows or gradient and increased flow velocity, thus initiating an episode of downcutting. When the stream reaches a level at which it is once again graded, downcutting ceases. Although changes in base level no doubt account for Either uplift of the land over
lowering of sea
478
level yields a steeper
Chapter 16
Running Water
pattern provided that face.
tern
As is
level it
all
incised meanders.
A
can establish a meandering
flows over a gently sloping sur-
in the last case,
however, the meandering pat-
already established before erosion into bedrock
occurs.
Stream terraces are commonly cut into previously deposited sediment, but
stream far above base
^ FIGURE
16-41
Goose Necks of
the San Juan River.
Perspective 16-2
Af*.
-
NATURAL BRIDGES
The term natural bridge has been used
to describe a
variety of features including spans of rock resulting
from wave erosion, the partial collapse of cavern roofs, and weathering and erosion along closely spaced, parallel joints as in Arches National Park in Utah (see Perspective 14-1). Here, however, we are concerned only with natural bridges that span a valley eroded by running water.
The is
in
best place to observe this type of natural bridge
Natural Bridges National
Monument
in
southwestern Utah. Three natural bridges are present within the
way. it
Of
monument, and
all
originated in the
these three, Sipapu Bridge
stands 67
m
is
same
the largest (Fig. 1);
above White Canyon and has
a
span of
The process by which these natural bridges were formed is well understood, and, as a matter of fact, is still going on. In the first stage, a meandering stream was incised into solid bedrock (Fig. 2). In Natural Bridges National Monument,
^ FIGURE
1
Sipapu Bridge
Monument, Utah. (Photo
81.5 m.
this
it
rock unit
which consists of sandstone formed from windblown sand deposited during the Permian Period. When local meandering streams
the Cutler Formation,
became incised, lateral erosion created a thin wall of rock between adjacent meanders that was eventually breached (Fig. 2). As the breach was subsequently enlarged, the stream abandoned its old meander and
the stream flow previously,
process is
was
oxbow
Natural Bridges National
As we discussed formed by a similar
diverted.
lakes are
(Fig. 16-21).
in
courtesy of Sue Monroe.)
The only
significant difference
is
form natural bridges are incised. Natural bridges are temporary features. Once formed, they are destroyed by other processes. For example, rocks fall from the undersides of bridges, their surfaces are weathered and eroded, and that the streams that
eventually they collapse.
The monument contains
several examples of such collapsed bridges, but
ones are
in the process of forming.
-*r FIGURE 2 Origin of a natural bridge, (a) A meandering stream flows across a gently sloping surface, (b) Incised meanders develop as the stream erodes down into solid rock. (c) A thin wall of rock between meanders is eventually breached, forming a natural bridge.
new
CHAPTER SUMMARY
large
marine deltas are more complex. Marine
deltas are characterized as stream-, wave-, or
Water
is
rises as
continually evaporated from the oceans,
water vapor, condenses, and
20%
About
precipitation.
of
falls
as
tide-dominated.
land and eventually returns to
precipitation falls
surface runoff.
consist mostly of sand arid regions 15. Sea level
Running water moves by
either laminar or turbulent
which streams can erode. However, streams
one another, complexly intertwined.
streams, or the points where they flow across
in
streams
is
turbulent.
particularly resistant rocks.
Gradient generally varies from steep to gentle along
channels so that they develop a smooth, concave profile of equilibrium. Such streams are graded. In a graded stream, a balance exists between gradient,
the course of a stream, being steep in upper reaches
discharge, flow velocity, channel characteristics, and
and gentle in lower reaches. Flow velocity and discharge are related. A change in one of these parameters causes the other to change
within the channel.
sediment load so that
or no deposition occurs
processes including downcutting, lateral erosion,
stream and its tributaries carry runoff from its drainage basin. Drainage basins are separated from one another by divides.
Many
19.
meaning that they once flowed on a higher surface and eroded downward into resistant rocks. Renewed downcutting by a stream possessing a
ridges directly in their paths are superposed,
dissolution of soluble rocks.
The coarser part of
a stream's
sediment load
is
transported as bed load, and the finer part as
suspended load. Streams also transport a dissolved load of ions in solution.
measure of the maximum-sized and is related to velocity. Capacity is a function of discharge and is a measure of the total load transported by a stream. is
a
particles that a stream can carry
mass wasting, sheet wash, and headward erosion. streams flowing through valleys cut into
18.
Streams erode by hydraulic action, abrasion, and
Competence
little
17. Stream valleys develop by a combination of
A
commonly results in the formation of stream terraces, which are remnants of an older floodplain at a higher level. floodplain
20. Incised meanders are generally attributed to renewed
downcutting by a meandering stream such that occupies a deep, meandering valley.
now
Braided streams are characterized by a complex of dividing and rejoining channels. Braiding occurs
when sediment
transported by the stream
IMPORTANT
TERMS
is
deposited within channels as sand and gravel bars.
Meandering streams have a single, sinuous channel with broad looping curves. Meanders migrate laterally as the cut bank is eroded and point bars form on the inner bank. Oxbow lakes are cutoff meanders in which fine-grained sediments and
abrasion
hydraulic action
alluvial fan
hydrologic cycle
alluvium
incised
base level bed load braided stream
infiltration capacity
organic matter accumulate.
delta
oxbow
Floodplains are rather flat areas paralleling stream channels. They may be composed mostly of point
discharge
point bar
dissolved load
runoff
divide
floodplain
stream stream terrace superposed stream suspended load
graded stream
velocity
bar deposits formed by lateral accretion or
accumulated by
vertical accretion
mud
during numerous
floods.
drainage basin
drainage pattern
Deltas are alluvial deposits at a stream's mouth.
Many
small deltas in lakes conform to the three-part
division of bottomset, foreset,
480
local base levels such as lakes, other
16. Streams tend to eliminate irregularities in their
as well.
13.
rates are high.
ultimate base level, the lowest level to
commonly have
in the latter they are
streams.
12.
where erosion
gravel.
flow. In the former, streamlines parallel
Runoff can be characterized as either sheet flow or channel flow. Channels of all sizes are called
11.
is
and
whereas
Most flow
10.
on land that They form best in
14. Alluvial fans are lobate alluvial deposits
on the oceans, mostly by
all
Chapter 16
Running Water
and topset beds, but
gradient
meander
meandering stream natural levee lake
it
QUESTIONS
REVIEW
c.
Trellis
drainage develops on:
a.
natural levees; b.
granite;
fractured
c.
14
sedimentary rock layers; horizontal layers of volcanic rocks. e. Mounds of sediment deposited on the margin of a 15
stream are: a.
natural levees; b.
c.
bottomset beds;
e.
alluvial fans.
The
direct
saltation;
b.
cutoff;
atmosphere;
The
vertical
distance
the
d.
base
drainage pattern.
level; e.
velocity;
c.
rectangular; b.
d.
deranged;
dendritic;
trellis; c.
18.
radial.
e.
saltation; b.
dissolved load;
c.
capacity; d.
suspended load;
e.
alluvium. capacity of a stream
volume of water; d.
a
is
10.
measure of
discharge; e
its:
total
c.
a single, sinuous channel;
alluvial fans; b.
floodplains; d
Which of
the following
a.
lake; b.
d
point bar;
c.
channel;
a broad,
21.
22.
and
(a)
and
channel
c.
(b); e.
all
of
A
is
sediment carried by saltation and rolling bed is the:
sliding along a stream
natural levee;
divide; b.
d
valley;
drainage pattern
alluvial
c.
point bar.
e.
in
which streams flow
longitudinal;
a
radial; b.
d.
rectangular;
is
deranged;
graded.
e.
would you expect
to find
deposits?
point bar;
delta; b.
Why
c.
and out
in
is:
incised
c.
floodplain.
alluvial fan; e.
d.
the Earth the only planet that has
abundant
How
do solar radiation, the changing phases of and runoff cause the recycling of water from the oceans to the atmosphere and back to the
What
the difference between laminar and why is flow in streams usually
is
turbulent flow, and turbulent?
floodplain
alluvial fan.
from
a(an):
oceans?
a local base level? c.
is
water,
natural
Erosional remnants of floodplains that are higher than the current level of a stream are: stream cut banks; c. oxbow lakes; b a natural incised meanders; e. terraces; d. All of the
gradient;
b.
answers
d.
liquid water?
24. Explain
what
important 25.
A
infiltration capacity
km
is
and why
it is
in considering runoff.
stream 2,000
1,500
m
above sea
to the sea.
What
level at its source flows
is its
gradient?
Do you
think the gradient that you calculated would be accurate for all segments of this stream?
bridges. 13.
1,000;
feature separating one drainage basin
mudflow
point bars; deltas; e
ocean; e.
375; d
125; c
of the following controls flow velocity in
meanders; in its
a deep, narrow valley; shallow channel; d. e. long, straight reaches and waterfalls. In which of the following do foreset beds occur?
c
m" and
/sec.
20. In which of the following
ability to
23.
12.
The
a.
a.
stream can
variation in flow
of lakes with irregular flow directions
levees.
11
Which
fan;
19.
velocity;
b.
meandering stream is one having: numerous sand and gravel bars
b.
3
500; b 200.
a.
erode.
a.
vertical distance a level; e.
stream with a cross-sectional area of 250
anether
a.
load of sediment;
the:
these.
Sediment transport by intermittent bouncing and skipping along a stream bed is:
A
A
roughness;
tree.
a.
is
hydraulic action;
rate at
streams? channel shape; a.
drainage pattern resembles the
a
Infiltration capacity
m 17
gradient;
The
downcutting.
a flow velocity of 1.5 m/sec has a discharge of
is its:
discharge; b.
branching of a
vertical accretion; d.
e.
given horizontal
in a
a.
The
c.
a
drop of a stream
channel by:
its
headward erosion;
runoff; b.
streams;
lakes; d.
c.
16.
glaciers.
e.
channel
velocity across a stream channel.
level.
is in:
the groundwater system; b.
a.
stream can lengthen
a.
absorb water; d. erode below sea
hydraulic
base
e.
A
a.
is:
c.
on Earth
of the fresh water
capacity; e.
distance which a stream erodes; b. a stream flows from its source to the ocean; c. maximum rate at which surface materials can
lakes;
incised meanders;
d.
meander
action; d.
Most
oxbow
impact of running water
bed load;
a.
bed load;
pattern.
tilted
basalt; d.
_ drainage
suspended load; b. stream profile; d. _
a.
26.
How
do channel shape and roughness control flow
velocity?
Review Questions
481
27.
Is
the statement "the steeper the gradient, the greater
what can about the underlying rocks of the region? 29. How do streams erode and acquire a sediment load? 30. Explain the concepts of stream competence and 28.
If
How
braided streams look
like,
and what do
is it
maintain a more or
less
constant
do oxbow lakes and meander scars form? how floodplains can develop by both lateral
vertical accretion.
How
does a stream-dominated delta differ from a wave-dominated delta? 36. What are alluvial fans and where are they best developed?
is
ultimate base level for most streams.
sea level drops with respect to the land,
If
how would
a stream respond?
do streams tend
to eliminate irregularities in
their channels?
40.
What
is
a graded stream,
and why are streams
How
do headward erosion and stream piracy
Illustrate
a
482
York: John
J.,
ed. 1971. Introduction to fluvial processes.
Edward Arnold. Leopold, L.
B.,
M. G. Wolman, and
J. P.
Miller. 1964. Fluvial
how
a stream can be superposed
water gap.
Chapter 16
&Co.
Running Water
Straus,
J.
&
1989. The control of nature. Giroux.
and form
New
York. Farrar,
Morisawa, M. 1968. Streams: Their dynamics and morphology. New York: McGraw-Hill. Petts, G., and I. Foster. 1985. Rivers and landscape. London:
Edward Arnold.
New
York: John Wiley
&
Sons.
Schumm,
lengthen a stream channel?
42
New
London: Methuen. Crickmay, C. H. 1974. The work of the river. London: Macmillan. Frater, A., ed. 1984. Great rivers of the world. Boston: Little, Brown. Knighton, D. 1984. Fluvial forms and processes. London:
Rachocki, A. 1981. Alluvial fans. rarely
graded except temporarily? 41
The channels of Mars. Austin, Texas:
Carling, eds. 1989. Floods.
Wiley &c Sons.
McPhee,
alluvial fans?
38. Sea level
P.
processes in geomorphology. San Francisco: W. H. Freeman
37. What two depositional processes predominate on
Why
ADDITIONAL READINGS
Chorley, R.
Explain
39.
terraces form?
possible for a stream near base level to
University of Texas Press.
possible for a meandering stream to erode
laterally yet
How
^
Beven, K., and
channel width?
and
is it
Baker, V. R. 1982.
What do
they transport and deposit?
35
do paired "Stream
infer
capacity.
32
How How
erode a deep meandering valley?
a stream possesses rectangular drainage,
you
31
43. 44.
the flow velocity" correct? Explain.
&
S.
Sons.
A. 1977. The fluvial system.
New
York: John Wiley
CHAPTER
17
GROUND WAT E R ^ OUTLINE PROLOGUE INTRODUCTION
GROUNDWATER AND THE HYDROLOGIC CYCLE POROSITY AND PERMEABILITY THE WATER TABLE GROUNDWATER MOVEMENT SPRINGS, WATER WELLS, AND ARTESIAN SYSTEMS Springs
Water Wells
"^
Perspective 17-1:
Mammoth
Cave
National Park, Kentucky Artesian Systems
GROUNDWATER EROSION AND DEPOSITION Sinkholes and Karst Topography
Caves and Cave Deposits
MODIFICATIONS OF THE GROUNDWATER SYSTEM AND THEIR EFFECTS Lowering of the Water Table Saltwater Incursion
Subsidence
Groundwater Contamination "**r
Perspective 17-2: Radioactive Waste
Disposal
HOT
SPRINGS
AND GEYSERS
Geothermal Energy
CHAPTER SUMMARY
The Leaning Tower of is
Pisa, Italy.
partly the result of subsidence
removal of groundwater.
The
tilting
due to the
gT
K^^«CTE3KJg«r»^nr»rTK3*3Ka^^
»m
PROLOGUE For more than two weeks in February 1925, Floyd Collins, an unknown farmer and cave explorer, became a household word (Fig. 17-1). News about the attempts to rescue him
from a narrow subsurface
fissure
near
.
^ K^C^'yrrv information booths redirected unsuspecting tourists away from Mammoth Cave. It was in this
environment that Floyd Collins grew up. Seven years before his tragic death, Collins had discovered Crystal Cave on the family farm and opened it up for visitors. But like most of the caves
in
Mammoth
Cave, Kentucky, captured the attention of the nation.
The saga of Floyd
Collins is rooted in what is Cave War of Kentucky. The western region of Kentucky is riddled with caves formed by groundwater weathering and erosion. Many of them were developed as tourist attractions to help supplement meager farm earnings. The largest and best known is Mammoth Cave (see Perspective 17-1). So spectacular is Mammoth, with its numerous caverns, underground rivers, and dramatic cave deposits, that it soon became the standard by which all other caves were measured. As Mammoth Cave drew more and more tourists, rival cave owners became increasingly bold in attempting to lure visitors to their caves and curio shops. Signs pointing the way to Mammoth Cave
known
as the Great
frequently disappeared, while "official" cave
"^ FIGURE
17-1 {a) Location of the cave in which Floyd was trapped, (b) Collins looking out of a fissure near cave where he ultimately died, (c) Cross section showing fissure where Collins was trapped, the rescue shaft that
Collins the the
was sunk, and the
lateral tunnel that finally
reached him.
(O
Prologue
485
the area, Crystal visited
Cave attracted few tourists — they Cave instead. Perhaps it was the
attempts led by Floyd's brother
Mammoth
thought of discovering a cave rivaling Mammoth or even connecting to it that drove Collins to his fateful exploration of Sand Cave on January 30, 1925. As Collins inched his way back up through the narrow fissure he had crawled down, he dislodged a small oblong piece of limestone from the ceiling that immediately pinned his left ankle. Try as he might, he
Homer
continued.
Three days after he had become trapped, a harness was put around Collins's chest and rescuers tried to
numerous attempts to yank him abandon that plan because Collins was unable to bear the pain. Meanwhile at the surface, a carnival-like atmosphere had developed as hordes of up to 20,000 people converged on the scene, and the National Guard had to be called out to
pull
him
free.
out, workers
After
had
to
was trapped in total darkness 17 m below ground. As he lay half on his left side, Collins's left arm was partially wedged under him, while his right arm was
maintain order.
held fast by an overhanging ledge. During his
rescuers collapsed, sealing Collins's fate.
and further immobilizing him
struggles to free himself, he dislodged
small rocks to bury his legs,
and adding
enough
silt
several neighbors reached Collins
and
were able to talk to him, feed him, encourage him, and try to make him more comfortable, but they could not get him out.
Word
of his plight quickly
spread and the area soon swarmed with reporters. Eventually, volunteers were able to excavate an area
around Collins's upper body, but could not free his legs. While an anxious country waited, rescue
days after the attempt to pull Collins out of
hope now was
The only
to dig a vertical relief shaft
a lateral tunnel could be
dug
from which
to reach Collins. For 12
dug the on February 16, rescuers reached the chamber where Collins lay entombed. There was no sign of life. With the news of his death, Floyd Collins's place in American folklore was secured. His body was finally brought out and buried near Crystal Cave, where it is appropriately marked by a beautiful stalagmite and pink granite headstone.
more
to his anguish.
The next day
Two
the fissure failed, part of the passageway used by
days, volunteers using picks and shovels
shaft. Finally
pinned
^
INTRODUCTION
stored in the open spaces within underground rocks and unconsolidated material— is a
Groundwater— the water
valuable natural resource that
is
essential to the lives of all
importance to humans is not new. Groundwahave always been important in the western United States, and many legal battles have been fought over them. Groundwater also played a crucial role in the
people. ter
Its
rights
development of the U.S. railway system during the nineteenth century when railroads had to have a reliable source of water for their steam locomotives. Much of the water used by the locomotives came from groundwater tapped
by wells.
Today, the study of groundwater and its movement has become increasingly important as the demand for fresh water by agricultural, industrial, and domestic us-
an all-time high. More than 65% of the groundwater used in the United States each year goes for irrigation, with industrial use second, followed by do-
ers has reached
mestic needs. Such
demands have
severely depleted the
groundwater supply in many areas and led to such problems as ground subsidence and saltwater contamination. In other areas, pollution from landfills, toxic waste, and agriculture has rendered the groundwater supply unsafe. 486
Chapter 17
Groundwater
As the world's population and industrial development expand, the demand for water, particularly groundwater, will increase. Not only is it important to locate new groundwater sources, but, once found, these sources must be protected from pollution and managed properly to ensure that users do not withdraw more water than can be replenished. Consequently, geologists trained in groundwater exploration and management are in great demand. If we wish to maintain adequate supplies of clean groundwater in the future, we must ensure that the
groundwater supply is intelligently managed. To do this, a knowledge of where groundwater occurs, how it moves, and how it becomes polluted is essential.
^ GROUNDWATER AND THE HYDROLOGIC CYCLE Groundwater represents approximately
km 3
22%
(8.4 mil-
of the world's supply of fresh water (see Fig. 16-3). This amount is about 36 times greater than the total for all of the streams and lakes of the world (see lion
)
Chapter 16) and equals about one-third the amount in the world's ice caps (see Chapter 18). If the
locked up world's
groundwater were spread evenly over the it would be about 10 m deep.
Earth's surface,
Pore space
rocks, other types of porosity can include cracks, fractures, faults,
and
vesicles in volcanic rocks (Fig. 17-2).
Porosity varies
pendent on the
among
size,
different rock types
and
de-
is
shape, and arrangement of the
ma-
composing the rock (Table 17-1). Most igneous and metamorphic rocks as well as many limestones and dolostones have very low porosity because they are composed of tightly interlocking crystals. However, their poterial
rosity can be increased if they have been fractured or weathered by groundwater. This is particularly true for massive limestone and dolostone whose fractures can be
enlarged by acidic groundwater.
By
contrast, detrital sedimentary rocks
composed of
well-sorted and well-rounded grains can have very high
two grains touch only at a single open spaces between the grains (Fig. 17-2a). Poorly sorted sedimentary rocks, on the other hand, typically have low porosity because finer porosity because any
point, leaving relatively large
grains "^^
FIGURE
17-2
A
is
dependent on the
shape, and arrangement of the material composing the rock, {a) A well-sorted sedimentary rock has high porosity size,
while (b) a poorly sorted one has low porosity,
(c) In soluble rocks such as carbonates, porosity can be increased by
solution, while (d) crystalline rocks can be rendered
porous
by fracturing.
Groundwater is one reservoir of the hydrologic cycle. The major source of groundwater is precipitation that infiltrates the ground and moves through the soil and pore spaces of rocks (see Fig. 16-6). Other sources include water infiltrating from lakes and streams, recharge ponds, and wastewater treatment systems. As the groundwater moves through soil, sediment, and rocks, many of its impurities, like disease-causing
out.
Not
some
all soils
microorganisms, are
and rocks are good
filters,
serious pollutants are not removed.
eventually returns to the surface reservoir
filtered
however, and Groundwater
when
it
enters
lakes, streams, or the ocean.
* POROSITY AND PERMEABILITY Porosity and permeability are important physical properties
of rocks, sediment, and soil and are largely respon-
sible
for the
amount,
availability,
and movement of
groundwater. Water soaks into the ground because the soil, sediment, or rock has open spaces or pores. Porosity
volume that is pore While porosity most often consists of the spaces between particles in soil, sediments, and sedimentary is
the percentage of a material's total
space.
fill
in the
space between the larger grains, reduc-
ing the porosity (Fig. 17-2b). In addition, the rock's porosity
amount of
cement between grains can also decrease porosity. Although porosity determines the amount of groundwater a rock can hold, it does not guarantee that the water can be extracted. The capacity of a material for transmitting fluids
is its
permeability. Permeability
is
de-
pendent not only on porosity, but also on the size of the pores or fractures and their interconnections. For example, deposits of silt or clay are typically more porous than sand or gravel. Nevertheless, shale has low permeability because the pores between its clay particles are very small,
and the molecular attraction between the clay and
the water
"•-
is
great, thereby preventing
TABLE
17-1
Porosity
movement of
the
water. In contrast, the pore spaces between grains in sand-
stone and conglomerate are attraction
on the water
is
much larger, and the molecular
therefore low. Chemical
and
bio-
chemical sedimentary rocks, such as limestone and dolostone,
and many igneous and metamorphic rocks
that are
highly fractured can also be very permeable provided that the fractures are interconnected. In fact, as northern Georgia, for their
many
depend on fractured
areas, such
crystalline rocks
groundwater supply.
A permeable layer transporting groundwater is called an aquifer, from the Latin aqua meaning water. The
material that
ward its
it is
mof ing through and
progress. This region
water
is
called
is
halts
suspended water
(Fig. 17-3).
spaces in this zone contain both water and ing irregularly
upward
its
down-
the zone of aeration, and
The pore
air.
Extend-
a few centimeters to several
meters from the base of the zone of aeration
is
the cap-
Water moves upward in this region from the zone of saturation below because of surface tension. Such movement is analogous to the upward movement
illary fringe.
of water through a paper towel.
When
precipitation occurs over land,
Beneath the zone of aeration lies the zone of saturation which all of the pore spaces are filled with groundwater (Fig. 17-3). The base of the zone of saturation varies from place to place, but usually extends to a depth where an impermeable layer is encountered or to a depth where confining pressure closes all open space. The surface separating the zone of aeration from the underlying zone of saturation is the water table (Fig. 17-3). In general, the configuration of the water table is a subdued replica of the overlying land surface; that is, it has its highest elevations beneath hills and its lowest elevations in valleys. In most arid and semiarid regions, however, the water table is quite flat and is below the
rates,
some of
level of river valleys.
most
effective aquifers are deposits of well-sorted
and
well-rounded sand and gravel. Limestones in which fractures and bedding planes have been enlarged by solution are also good aquifers. Shales and many igneous and
metamorphic rocks, however, are typically impermeable. Rocks such as these and any other materials that prevent the movement of groundwater are called aquicludes.
^ THE WATER TABLE some of it evapoaway by runoff in streams, and the remainder seeps into the ground. As this water moves down from the surface, some of it adheres to the
^ FIGURE
it is
17-3
carried
The zone of
aeration contains both air and water within its open space, while all of the open space in the zone of is filled with groundwater. The water table is the surface separating the zones of aeration and saturation. Within the capillary fringe, water rises upward by surface tension from the zone of saturation into the zone of aeration.
saturation
488
Chapter 17
Groundwater
in
Several factors contribute to the surface configuration of a region's water table.
These include regional
** FIGURE 17-4 Groundwater moves downward due to the force of gravity. It moves through the zone of aeration to the zone of saturation where
some of
it
moves
along the slope of the water table and the rest of it moves through the zone of saturation from areas of high pressure toward areas of low pressure.
amount of rainfall, permeability, and groundwater movement. During periods of high rainfall, groundwater tends to rise beneath hills because it cannot flow fast enough into the adjacent valleys to maintain a level surface. During droughts, however, the water table falls and tends to flatten out
has been demonstrated that groundwater ve-
differences in the
methods,
the rate of
locity varies greatly
because
it is
not being replenished.
^ GROUNDWATER MOVEMENT Groundwater moves very slowly through the pore spaces It moves fastest in the central area of the pore space and decreases in velocity to zero along the edges because of friction and the molecular attraction between the water molecules and the material through which it moves. Gravity provides the energy for the downward movement of groundwater. Water entering the ground moves of Earth materials.
it
and depends on many factors. Vem per day in some extremely permeable material to less than a few centimeters per year in nearly impermeable material have been measured. In most ordinary aquifers, however, the average velocity of groundwater is a few centimeters per day. locities
ranging from 250
^ SPRINGS, WATER WELLS, AND ARTESIAN SYSTEMS Adding water to the zone of saturation is called recharge, and it causes the water table to rise. Water may be added by natural means, such as rainfall or melting snow, or artificially at recharge basins or wastewater treatment plants (Fig. 17-5). If groundwater is discharged without sufficient replenishment, the water table drops.
Groundwater discharges naturally whenever
through the zone of aeration to the zone of saturation (Fig. 17-4). When water reaches the water table, it con-
move through the zone of saturation from arwhere the water table is high toward areas where it
tinues to
"•»
eas
New
is
lower, such as streams, lakes, or
swamps
FIGURE
17-5
A
recharge basin in Nassau County"
York.
(Fig. 17-4).
Only some of the water follows the direct route along the slope of the water table. Most of it takes longer curving paths downward and then enters a stream, lake, or swamp from below. This occurs because groundwater moves from areas of high pressure toward areas of lower pressure within the saturated zone. Below the wagroundwater is under greater pressure beneath than at the same elevation beneath a valley. The rate at which groundwater flows can be deter-
ter table,
a
hill
mined in several ways. The most common method is to add dye to the groundwater in a well and measure how long the dye takes to appear in the groundwater at another well a known distance away. Using this and other Springs,
Water Wells, and Artesian Systems
489
the water table intersects the
ground surface as at a swamp. Groundwater
way
by pumping water
ally,
spring or along a stream, lake, or
can also be discharged
from
artificially
(Fig. 17-6).
Where percolating groundwater reaches
the water table or an impermeable layer,
and
if
this
it
flows later-
flow intersects the Earth's surface, the
water discharges onto the surface as a spring (Fig. 17-7). in Kentucky, for example, is underlain by fractured limestones that have been en-
wells.
The Mammoth Cave area Springs
A
larged into caves by solution activity (see Perspective
where groundwater flows or seeps out of the ground. Springs have always fascinated people because the water flows out of the ground for no apparent reason and from no readily identifiable source. spring
It is
is
a place
not surprising that springs have long been regarded
with superstition and revered for their supposed medic-
and healing powers. Nevertheless, there is nothing mystical or mysterious about springs. Although springs can occur under a wide variety of geologic conditions, they all form in basically the same inal value
where and caves intersect the ground surface allowing groundwater to exit onto the surface. Springs most commonly occur along valley walls where streams have cut valleys below the regional water table. 17-1). In this geologic environment, springs occur
the fractures
Springs can also develop wherever a perched water table intersects the Earth's surface (Fig. 17-8).
water table
may
Most commonly,
sandstone.
As water migrates through
Springs
they
form when percolating water reaches an impermeable layer and migrates laterally until
it seeps out Springs also can occur in areas underlain by fractured soluble rocks such as limestones where groundwater
at the surface.
moves
freely
cavities until
and flows
(£>)
through underground it
reaches the surface
out.
Water table
490
Chapter 17
Groundwater
perched
within a larger aquifer, such as a lens of shale within a
"»- FIGURE 17-6 Springs form wherever laterally moving groundwater intersects the Earth's surface, (a)
A
occur wherever a local aquiclude occurs
Springs
the zone of aera-
tion,
stopped by the local aquiclude, and a localized
it is
zone of saturation "perched" above the main water table is created. Water moving laterally along the perched water table
may
produce a spring.
intersect the Earth's surface to
Water Wells
A water well
is
made by digging or
drilling into the
zone
most water wells today are dug, particularly in areas where the
of saturation. Although drilled,
some
water table saturation
are
is
is
still
very close to the surface.
Once
the zone of
reached, water percolates into the well and
water table. Most wells must be groundwater to the surface. When a well is pumped, the water table in the area around the well is lowered, because water is removed from the aquifer faster than it can be replenished. A cone of depression thus forms around the well, varying in size according to the rate and amount of water being withdrawn (Fig. 17-9). If water is pumped out of a well faster than it can be replaced, the cone of depression grows until the well goes dry. This lowering of the water table normally does not pose a problem for the average fills it
to the level of the
pumped
to bring the
domestic well provided that the well ciently
is
drilled suffi-
deep into the zone of saturation. The tremendous
amounts of water used by industry and
irrigation,
how-
"^ FIGURE
ever,
may
17-7
Periodic Spring, near Afton,
Wyoming.
create a large cone of depression that lowers
the water table sufficiently to cause shallow wells in the
immediate area to go dry (Fig. 17-9). Such a situation is uncommon and frequently results in lawsuits by the owners of the shallow dry wells. Furthermore, lowering of the regional water table is becoming a serious problem in many areas, particularly in the southwestern United States where rapid growth has placed tremennot
"•-
FIGURE
17-8
If
a localized
aquiclude, such as a shale layer,
occurs within an aquifer, a perched water table may result with springs
Localized aquiclude
occurring where the perched water table intersects the Earth's surface.
Springs
Zone
of saturation
Springs,
Water Wells, and Artesian Systems
491
Perspective 17-1
MAMMOTH PARK,
CAVE NATIONAL
KENTUCKY
Within the limestone region of western Kentucky largest cave system in the world. In 1941,
lies
the
approximately
set aside and designated as Mammoth Cave National Park. In 1981 it became a World
51,000 acres were Heritage
Site.
Recently, the National Park Service has
been considering closing health hazard created by
groundwater
Mammoth
Cave because of the raw sewage and contaminated
in the area.
From ground
level,
the topography of the area
is
unimposing with numerous sinkholes, lakes, valleys, and disappearing streams. Beneath the surface, however, are
more than 230 km of interconnecting passageways whose spectacular geologic features have been enjoyed by numerous cave explorers and tourists alike. Based on carbon 14 dates from some of the many artifacts found in the cave (such as woven cord and wooden bowls), Mammoth Cave had been explored and used by Native Americans for more than 3,000 years prior to its rediscovery in 1799 by a bear hunter named Robert Houchins. During the War of 1812, approximately 180 metric tons of saltpeter (a potassium nitrate mineral), used in the manufacture of gunpowder, were mined from Mammoth Cave. At the end of the war, the saltpeter market collapsed, and
Mammoth
Cave was developed as a
overshadowing the other caves in the area. Over 150 years, the discovery of new passageways and caverns helped establish Mammoth Cave as the world's premier cave and the standard against which all others were measured (see the Prologue). Mammoth Cave formed in much the same way as all other caves (Fig. 17-18). Groundwater flowing through the St. Genevieve Limestone eroded a complex network of openings, passageways, and caverns. Flowing through the various caverns is the Echo River, a system of subsurface streams that eventually joins the Green River at the surface. The colorful cave deposits are the primary reason millions of tourists have visited Mammoth Cave. Here can be seen numerous stalactites, stalagmites, and easily
the next
columns, as well as spectacular travertine flowstone deposits (Fig. 1). Other attractions include the Giant's
m
and giant about 58 m high (Fig. 2). The cave is also home to more than 200 species of insects and other animals, including about 45 blind species; some of these can be seen on the Echo River Tour, which conveys visitors 5 km along the underground stream. Coffin, a 15
rooms such
as
collapse block of limestone,
Mammoth Dome,
which
is
tourist attraction,
FIGURE 1 Frozen Niagara is a spectacular example massive travertine flowstone deposits.
FIGURE 2 Looking up Mammoth Dome, in Mammoth Cave, Kentucky.
"••"
^r*
:>f
room
the largest
—
FIGURE 17-9 A cone of depression forms whenever water withdrawn from a well. If water withdrawn faster than it can be
is
is
replenished, the cone of depression will
grow
in
depth and
circumference, lowering the water
and causing nearby shallow wells to go dry. table in the area
Cone of depression
dous demands on the groundwater system. Unrestricted withdrawal of groundwater cannot continue indefinitely, and the rising costs and decreasing supply of groundwater should soon limit the growth of this region
well was drilled in a.d. 1126 and is still flowing today. The term artesian, however, can be applied to any sys-
of the United States.
able to rise above the level of the aquifer
People in rural areas and those without access to a
municipal water system are well aware of the problems of locating an adequate
groundwater supply. The
distri-
bution and type of rocks present, their porosity and permeability, fracture patterns, that determine (Fig.
whether
a
and so on are
all
factors
water well will be successful
17-10).
Artesian Systems
The word
artesian
province of Artois times) near Calais,
comes from the French town and (called Artesium during Roman where the first European artesian
tem
in
which groundwater
high hydrostatic
drilled
is
confined and builds up
(fluid) pressure.
through the confining
Water
layer,
in
such a well if
a well
is is
thereby reducing the
upward (Fig. 17-11). For an artesian system to develop, three geologic conditions pressure and forcing the water
must be present
(Fig.
17-12): (1) the aquifer must be
confined above and below by aquicludes to prevent wa-
from escaping; (2) the rock sequence is usually tilted and exposed at the surface, enabling the aquifer to be recharged; and (3) there is sufficient precipitation in the recharge area to keep the aquifer filled. ter
The elevation of the water table in the recharge area and the distance of the well from the recharge area determine the height to which artesian water rises in a well. The surface defined by the water table in the re-
•*r
FIGURE
17-10
Many
factors
determine whether a water well will be successful. Wells A and E were drilled to the same depth. Well A was successful because it tapped a perched water table, whereas well E did not. To be successful, it will have to be drilled below the water table like well C. Well B tapped a fracture below the water table and
Perched water
was
successful,
whereas well
D
missed the fractures and was dry.
ei^ Fractured crystalline
basement rock
Springs,
Water Wells, and Artesian Systems
493
artesian-pressure surfece. Friction, however, slightly re-
duces the pressure of the aquifer water and consequently the level to which artesian water rises. This is why the pressure surface slopes.
An only
artesian well will flow freely at the
if
the wellhead
is
at
pressure surface. In this
ground surface
an elevation below the artesiansituation, the water flows out of
it rises toward the artesian-pressure which is at a higher elevation than the wellhead. In a nonflowing artesian well, the wellhead is above the artesian-pressure surface, and thus the water will rise in
the well because surface,
the well only as high as the artesian-pressure surface. In addition to artesian wells,
many
also exist. Such springs can occur
if
artesian springs
a fault or fracture
intersects the confined aquifer allowing
water to
rise
commonly
arte-
Because the geologic conditions necessary for
arte-
above the
aquifer.
Oases
in deserts are
sian springs.
sian water can occur in a variety of ways, artesian sys-
^
FIGURE 17-11 Artesian well at Deep Well Ranch, South Fork of the Madison River, Gallatin County,
tems are quite
Montana.
in
many areas of the world unOne of the best-known
artesian systems in the United States underlies South
charge area, called the artesian-pressure surface, cated by the sloping dashed line in Figure 17-12.
were no
common
derlain by sedimentary rocks.
friction in the aquifer, well
tesian aquifer
would
is
indi-
If
there
water from an
rise exactly to the elevation
ar-
of the
Dakota and extends southward to central Texas. The majority of the artesian water from this system is used for irrigation. The aquifer of this artesian system, the Dakota Sandstone, is recharged where it is exposed along the margins of the Black Hills of South Dakota. in this system was originally
The hydrostatic pressure
—
FIGURE 17-12 An artesian system must have an aquifer confined above and below by aquicludes, the aquifer must be exposed at the surface, and there must be sufficient precipitation in the recharge area to keep the aquifer
filled.
The
elevation of the
water table
in the
which is dashed line
(the artesian-pressure
recharge area, indicated by a sloping
surface), defines the highest level to
which well water can
rise. If
elevation of a wellhead
is
the
below the
elevation of the artesian-pressure surface, the well will be free-flowing
because the water will
rise
toward which
the artesian-pressure surface, is
at a higher elevation than the
wellhead.
wellhead
If is
the elevation of a at or
above that of the
artesian-pressure surface, the well will be nonflowing.
494
Chapter 17
Groundwater
Artesian-pressure surface
~^~
FIGURE
17-13
The
distribution of the major limestone
produce free-flowing wells and to opThe extensive use of water for irrigation over the years, however, has reduced the pressure in many of the wells so that they are no longer freegreat
enough
to
erate waterwheels.
flowing and the water must be pumped.
These carbonates are exposed at the surface
in the
northwestern and central parts of the state where they are recharged, and they dip toward both the Atlantic and Gulf coasts
where they are covered by younger sediments. The
carbonates are interbedded with shales forming a series of confined aquifers and aquicludes. This artesian system
is
tapped in the southern part of the state where it is an important source of fresh water and one that is being rapidly depleted.
^ GROUNDWATER EROSION AND DEPOSITION When
soluble rock, groundwater sion and thus
is
is
the principal agent of ero-
responsible for the formation of
many
major features of the landscape.
common
sedimentary rock composed
primarily of the mineral calcite
(CaC0 3 ),
underlies large
areas of the Earth's surface (Fig. 17-13). Although lime-
stone
is
practically insoluble in pure water,
amount of weak acid
it
readily
Carbonic that forms when carbon acid (H 2 C0 3 is a + C0 2 -» H 2 C0 3 dioxide combines with water (H 2 (see Chapter 6). Because the atmosphere contains a small amount of carbon dioxide (0.03%), and carbon dioxide is also produced in soil by the decay of organic matter, most groundwater is slightly acidic. When groundwater percolates through the various openings in limestone, the slightly acidic water readily reacts with the calcite to dissolve the rock by forming soluble calcium bicarbonate, which is carried away in solution (see Chapter 6). dissolves
if
a small )
acid
is
present.
)
Sinkholes and Karst Topography
rainwater begins seeping into the ground,
mediately starts to react with the minerals
weathering them chemically. In an area underlain by
Limestone, a
Another example of an important artesian system is the Floridan aquifer system. Here Tertiary-aged carbonate rocks are riddled with fractures, caves, and other openings that have been enlarged and interconnected by solution activity.
and karst areas of the world.
it
it
im-
contacts,
In regions underlain
may
by soluble rock, the ground surface
be pitted with numerous depressions that vary in
Groundwater Erosion and Deposition
495
in this
way
are a serious hazard, particularly in
lated areas. In regions
popuprone to sinkhole formation, the
depth and extent of underlying cave systems must be mapped before any development to ensure that the underlying rocks are thick enough to support planned structures.
A
karst topography
is
by groundwater erosion
one that has developed
The name
(Fig. 17-15).
largely
karst
is
derived from the plateau region of the border area be-
tween Yugoslavia and northeastern of topography
is
Italy
where
this type
well developed. In the United States,
regions of karst topography include large areas of south-
western
Illinois,
southern Indiana, Kentucky, Tennessee,
northern Missouri, Alabama, and central and northern Florida (Fig. 17-13).
Karst topography
is
numerous caves, and disappearing
characterized by
springs, sinkholes, solution valleys,
streams
(Fig.
17-15).
When
adjacent sinkholes merge,
they form a network of larger, irregular, closed depressions called solution valleys. Disappearing streams are
another feature of areas of karst topography. They are so
named because
they typically flow only a short distance
and then disappear into a sinkhole. The water continues flowing underground through various at the surface
fractures or caves until
it
surfaces again at a spring or
other stream.
Karst topography can range from the spectacular high relief
landscapes of China to the subdued and pock-
marked landforms of Kentucky
common
(Fig.
17-16).
to all karst topography, however,
is
What
is
that thick-
(b)
*w FIGURE and
9,
1981,
17-14 (a) This sinkhole formed on May 8 Winter Park, Florida, due to a drop in the
in
water table after prior dissolution of the underlying limestone. The sinkhole destroyed a house, numerous cars, and the municipal swimming pool. It has a diameter of 100 m and a depth of 35 m. {b) This sinkhole in a rural area near Montevallo, central Alabama, formed on December 2, 1972. Its diameter is 130 m, and its depth is 45 m.
and shape. These depressions, called sinkholes or merely sinks, mark areas where the underlying rock is
bedded, readily soluble rock
is
present at the surface or
and enough water is present for solution activity to occur. Karst topography is, therefore, typically restricted to humid and temperate climates. At the present, however, some of the best karst topography can be found in arid and semiarid regions such as Bexar County, Texas, and the Carlsbad Caverns region in New Mexico. The examples of karst topography in these regions are relicts that originally formed when the climate was more humid. just
below the
soil,
size
soluble (Fig. 17-14). Sinkholes usually form in one of
two ways. The
first is
when
the soluble rock
below the
by seeping water. Natural openings in and filled in by the overlying soil. As the groundwater continues to dissolve the rock, the soil is eventually removed, leaving depressions that are typically shallow with gently sloping sides. soil is dissolved
the rock are enlarged
Sinkholes also form
when
a cave's roof collapses,
usually producing a steep-sided crater. Sinkholes formed
496
Chapter 17
Groundwater
Caves and Cave Deposits Caves are some of the most spectacular examples of the combined effects of weathering and erosion by groundwater. As groundwater percolates through carbonate rocks (limestone and dolostone), larges original fractures
and enform a complex caves, caverns, and
it
and openings
interconnecting system of crevices,
dissolves
to
underground streams. A cave is usually defined as a naturally formed subsurface opening that is generally con-
Solution valleys
Springs
Karst valley
Disappearing streams
Deeply intrenched permanent stream
•^ FIGURE
nected to the surface and enter.
A
cavern
is
is
large
enough
a very large cave or a
for a person to
system of
inter-
connected caves.
More than 17,000
17-15
Some
of the
features of karst topography.
Cave
caves are
known
in the
United
Most of them are small, but some are quite large and spectacular. Some of the more famous caves in the United States are Mammoth Cave, Kentucky (see Perspective 17-1); Carlsbad Caverns, New Mexico; Lewis States.
"^ FIGURE
17-16 (a) The Stone Forest, 126 km southeast of Kunming, People's Republic of China, is a high relief karst landscape formed by the dissolution of carbonate rocks, (b) Solution valleys, sinkholes, and sinkhole lakes dominate the subdued karst topography east of Bowling Green, Kentucky.
'"
M
Groundwater Erosion and Deposition
497
•^ FIGURE
17-17
Some
of the
spectacular cave deposits of
Meramec
Caverns, Missouri.
and Clark Caverns, Montana; Wind Cave and Jewel Cave, South Dakota; Lehman Cave, Nevada; and Meramec Caverns, Missouri, which Jesse James and his outlaw band often used as a hideout (Fig. 17-17). Caves and caverns form as a result of the dissolution of carbonate rocks (limestone, dolostone, and occasionally marble) by weakly acidic groundwater (Fig. 17-18). Groundwater percolating through the zone of aeration slowly dissolves the carbonate rock and enlarges its fractures and bedding planes. Upon reaching the water table, the groundwater migrates toward the region's surface streams (Fig. 17-4). As the groundwater moves through the zone of saturation,
it
same manner and are collectively known as dripstone. As water seeps through a cave, some of the dissolved carbon dioxide in the water escapes, and a small amount of calcite
is
precipitated. In this manner, the various
dripstone deposits are formed. Stalactites are icicle-shaped structures
dripping water
(Fig. 17-19).
thin layer of calcite
The water
continues to dissolve
a
from a cave's
ceiling also pre-
amount of calcite when it hits the floor. calcite is deposited, an upward growing
passageways through which the dissolved rock is carried to the streams. As the surface streams erode deeper valleys, the water table drops in response to the lower elevation of the streams. The water that flowed through the system of horizontal passageways now percolates down to the lower water table where a new system of passageways begins to form. The abandoned channelways now form an interconnecting system of caves and caverns that may continue to enlarge as groundwater percolates through them and dissolves the surrounding rock. As the caves increase in size, they may become unstable and collapse, littering the floor with fallen debris. When most people think of caves, they think of the seemingly endless variety of colorful and bizarre-shaped deposits found in them. Although a great many different types of cave deposits exist, most form in essentially the
As additional
Groundwater
With each drop of water,
deposited over the previous layer,
that drips
cipitates a small
Chapter 17
is
forming a cone-shaped projection that grows downward from the ceiling. While many stalactites are solid, some are hollow and are appropriately called soda straws.
the rock and gradually forms a system of horizontal
498
hanging from
cave ceilings that form as a result of precipitation from
projection called a stalagmite forms (Fig. 17-19).
If
a
and stalagmite meet, they form a column. Groundwater seeping from a crack in a cave's ceiling may form a vertical sheet of rock called a drip curtain, while water flowing across a cave's floor may produce stalactite
travertine terraces (Fig. 17-18).
»
MODIFICATIONS OF THE
GROUNDWATER SYSTEM AND THEIR EFFECTS Groundwater
is
a valuable natural resource that
idly being exploited
with
little
is
rap-
regard to the effects of
overuse and misuse. Currently, about
20%
of
all
water
^ FIGURE
The formation of caves, (a) As groundwater percolates through and flows through the zone of saturation, it dissolves the carbonate rocks and gradually forms a system of passageways, (b) Groundwater moves along the surface of the water table, forming a system of horizontal passageways through which dissolved rock is carried to the surface streams and thus enlarging the passageways. (c) As the surface streams erode deeper valleys, the water table drops, and the abandoned channelways form an interconnecting system of caves and caverns. 17-18
the zone of aeration
Modifications of the Groundwater System and Their Effects
499
1
•""
FIGURE
17-19
Stalactites are
the icicle-shaped structures seen
hanging from the ceiling, while the upward-pointing structures on the cave floor are stalagmites. Several columns are present where the stalactites and stalagmites have met in this chamber of Luray Caves, Virginia.
used in the United States age
is
groundwater. This percent-
is
and unless this resource is sufficient amounts of clean ground-
increasing, however,
used more wisely, water will not be available in the future. Modifications of the groundwater system may have many conse-
quences including
(1)
lowering of the water table, which
causes wells to dry up;
(2) loss
of hydrostatic pressure,
which causes once free-flowing wells to require pumping; (3) saltwater encroachment; (4) subsidence; and (5) contamination of the groundwater supply.
from
irrigated lands can be triple
viding the quantities of water that
some
parts of the
water
is
being
High
pumped
Consequently, water faster
than
it
is
Plains,
will
it
has
in the past. In
from 2 to 100 times more
annually than
is
is
being recharged.
being removed from the aquifer
being replenished, causing the water
table to drop significantly in
What
happen
many
areas (Fig. 17-20).
to this region's
economy
if
long-
term withdrawal of water from the High Plains aquifer
Lowering of the Water Table Withdrawing groundwater
what they would be
without irrigation. While the High Plains aquifer has contributed to the high productivity of the region, it cannot continue pro-
greatly exceeds
at a significantly greater rate
its
recharge rate such that
it
can no
longer supply the quantities of water necessary for
irri-
recharge
gation? Solutions range from going back to farming
effects. For example, the High Plains one of the most important aquifers in the United States. Underlying most of Nebraska, large parts of Colorado and Kansas, portions of South Dakota, Wyoming, and New Mexico, as well as the panhandle regions of Oklahoma and Texas, it accounts for approximately 30% of the groundwater used for irrigation in the United States (Fig. 17-20). Irrigation from the High
without irrigation to diverting water from other regions such as the Great Lakes. Farming without irrigation
than
it is
replaced by either natural or
artificial
can have serious aquifer
is
Plains aquifer
is
largely responsible for the high agricul-
tural productivity of this region.
A
significant percent-
age of the nation's corn, cotton, and wheat
is
grown
and half of our beef cattle are raised in this region. Large areas of land (more than 14 million acres) are
here,
currently irrigated with water Plains aquifer. Irrigation
500
Chapter 17
is
pumped from
the
High
so popular because yields
Groundwater
would result in greatly decreased yields and higher costs and prices for agricultural products, while the diversion of water from elsewhere would cost billions of dollars and the price of agricultural products would still rise.
Saltwater Incursion
The
excessive
can result
Long lines
pumping of groundwater
in saltwater
in coastal areas
incursion such as occurred on
Island, New York, during the 1960s. Along coastwhere permeable rocks or sediments are in contact
with the ocean, the fresh groundwater, being
less
dense
than seawater, forms a lens-shaped body above the un-
^
FIGURE 17-20 Areal extent of the High Plains aquifer and " changes in the water table, predevelopment to 1980.
When
become con-
derlying salt water (Fig. 17-21a). The weight of the fresh water exerts pressure on the underlying salt water. As long as rates of recharge equal rates of withdrawal, the contact between the fresh groundwater and the seawater
tained fresh water.
remain the same. If excessive pumping occurs, howdeep cone of depression forms in the fresh groundwater (Fig. 17-21b). Because some of the pressure from the overlying fresh water has been removed, salt water
is a major problem in many rapgrowing coastal communities. As the population in these areas grows, greater demand for groundwater creates an even greater imbalance between recharge and withdrawal. Natural recharge of the groundwater sys-
will
ever, a
migrates
upward
to
fill
the pore space that formerly con-
this occurs, wells
water and remain contaminated until recharge by fresh water restores the former level of the fresh groundwater water table. taminated with
salt
Saltwater incursion
idly
Modifications of the Groundwater System and Their Effects
501
Ocean
filtrate
the groundwater supply
may
also be constructed
Both of these methods are successfully used on Long Island, which has had a saltwater incursion problem for several decades. (Fig. 17-5).
Subsidence
Fresh groundwater
Salty
As excessive amounts of groundwater are withdrawn from poorly consolidated sediments and sedimentary rocks, the water pressure between grains is reduced, and
groundwater
(a)
the weight of the overlying materials causes the grains to pack closer together, resulting in subsidence of the ground. Subsidence is becoming a major hazard in many areas and can cause damage to buildings, water lines, utility lines, and roads. As more and more groundwater is pumped to meet the increasing needs of agriculture and population growth, subsidence is becoming more prevalent. The San Joaquin Valley of California is a major agricultural region that relies largely on groundwater for irrigation. Between 1925 and 1975, groundwater withdrawals in parts of the
Ocean
Fresh groundwater
Salty
groundwater
(b)
m
valley caused subsidence of almost 9
Other examples of subsidence
Ocean
clude
New
in the
(Fig.
17-22).
United States
in-
Orleans, Louisiana, and Houston, Texas,
both of which have subsided more than 2 m, and Las Vegas, Nevada, which has subsided 8.5
Elsewhere
"^ FIGURE
17-21 Saltwater incursion, (a) Because fresh not as dense as salt water, it forms a lens-shaped body above the underlying salt water, (b) If excessive pumping occurs, a cone of depression develops in the fresh groundwater, and a cone of ascension forms in the underlying salty groundwater that may result in saltwater contamination of the well, (c) Pumping water back into the groundwater system through recharge wells can help lower is
the interface between the fresh groundwater and the salty groundwater and reduce saltwater incursion.
is further decreased as large areas of the ground are covered by roads and buildings, which prevent water
tem
from
infiltrating the soil.
To counteract
the effects of saltwater incursion, re-
charge wells are often drilled to
pump
water back into
the groundwater system (Fig. 17-21c). Recharge
ponds
that allow large quantities of fresh surface water to in-
502
Chapter 17
Groundwater
world, the
tilt
m
(Table 17-2).
of the Leaning
Tower
groundwater withdrawal. The tower started tilting soon after construction began in 1173 because of differential compaction of the foundation. During the 1960s, the city of Pisa withdrew everlarger amounts of groundwater, causing the ground to subside further; as a result, the tilt of the tower increased until it was considered in danger of falling over. However, strict control of groundwater withdrawal and
of Pisa
water
in the
is
partly due to
stabilization of the foundation have reduced the
of tilting to about
1
mm
amount
per year, ensuring that the
tower should stand for several more centuries. A spectacular example of subsidence occurred
in
which is built on a former lake bed. As groundwater is removed for the ever-increasing needs of
Mexico
City,
the
the fine-grained lake sediments are compacting,
city,
and Mexico City is slowly and unevenly subsiding. Its opera house has settled more than 3 m, and half of the first floor is now below ground level. Other parts of the city have subsided more than 6 m, creating similar problems for other structures (Fig. 17-23). Withdrawal of groundwater is not the only cause of surface subsidence. The extraction of oil can also cause subsidence. a result of
Long Beach,
34 years of
California, has subsided 9
oil
production.
More
m as
than $100
1955
"^"
FIGURE
The dates on this power pole amount of subsidence the San Joaquin Valley has undergone since 1925. Due to withdrawal of groundwater for agricultural needs and the ensuing compaction of sediment, the ground subsided almost 9 m between 1925 and 1975. 17-22
dramatically illustrate the
damage was done to the pumping, transporand harbor facilities in this area because of subsidence and encroachment of the sea (Fig. 17-24). Once secondary recovery wells began pumping water back into the oil reservoir and stabilizing it, subsidence virmillion of tation,
tually stopped.
~^~
TABLE
17-2
Subsidence of Cities and Regions
1963
Groundwater Cofttamination
A
major problem facing our society is the safe disposal numerous pollutant by-products of an industrialized economy. We are becoming increasingly aware that our streams, lakes, and oceans are not unlimited reservoirs for waste, and that we must find new safe ways to of the
dispose of pollutants.
The most common sources of contamination
are sew-
age, landfills, toxic waste disposal sites (see Perspective
17-2), and agriculture. Once pollutants get into the groundwater system, they will spread wherever groundwater travels, which can make containment of the contamination difficult. Furthermore, because groundwater
moves very
slowly,
it
takes a very long time to cleanse a
groundwater reservoir once
many
In
way
it
of disposing of sewage.
leases
has become contaminated.
areas, septic tanks are the
A
sewage into the ground where
oxidation and microorganisms and
ment most
as
it
most common
septic tank slowly reit is
decomposed by
filtered
by the sedi-
percolates through the zone of aeration. In
situations,
by the time the water from the sewage it has been cleansed of
reaches the zone of saturation,
any impurities and is safe to use (Fig. 17-25a). If, howwater table is very close to the surface or if the rocks are very permeable, water entering the zone of saturation may still be contaminated and unfit to use. Landfills are also potential sources of groundwater contamination (Fig. 17-25b). Not only does liquid waste
ever, the
^ FIGURE
17-23
Lady of Guadalupe)
The right Mexico
in
side of this church
(Our
City has settled slightly
than a meter. (Photo courtesy of R. V. Dietrich.)
more
seep into the ground, but rainwater also carries dis-
^
FIGURE 17-24 The withdrawal of petroleum from the oil field in Long Beach, California,
m
of ground up to 9 subsidence because of sediment compaction. It was not until secondary recovery wells began resulted in
pumping water back
into the
reservoir to replace the petroleum
that
ground subsidence essentially 29 feet = 0.6 to 8.8
ceased. (2 to
meters)
504
Chapter 17
Groundwater
Drain pipes
Septic tank
•*r
Zone
of aeration
Average water table
Zone
of saturation
17-25
(a)
A
septic
supply.
(b)
solved chemicals and other pollutants
downward
into
groundwater reservoir. Unless the landfill is carefully designed and lined below by an impermeable layer such as clay, many toxic and cancer-causing compounds will find their way into the groundwater system. For example, paints, solvents, cleansers, pesticides, and battery acid are just a few of the toxic household items that end up in landfills and can pollute the groundwater supply. Toxic waste sites in which dangerous chemicals are either buried or pumped underground are an increasing the
source of groundwater contamination.
The United
States
alone must dispose of several thousand metric tons of
hazardous chemical waste per year. Unfortunately, much of this waste has been, and still is being, improperly
dumped and
FIGURE
system slowly releases sewage into the zone of aeration. Oxidation, bacterial degradation, and filtering by the sediments usually remove all of the natural impurities before they reach the water table. If, however, the rocks are very permeable or the water table is too close to the septic system, contamination of the groundwater can result, (b) Unless there is an impermeable barrier between a landfill and the water table, pollutants can be carried into the zone of saturation and contaminate the groundwater
is
contaminating the surface water,
soil,
and
groundwater.
Examples of indiscriminate dumping of dangerous and toxic chemicals can be found in every state. Perhaps the most famous is the Love Canal, near Niagara Falls, New York. During the 1940s, the Hooker Chemical
Company dumped approximately 19,000
tons of chem-
waste into the Love Canal. In 1953 it covered one of the dump sites with dirt and sold it for one dollar to the Niagara Falls Board of Education, which built an elementary school and playground on the site. Heavy rains and snow during the winter of 1976-1977 raised- the water table and turned the area into a muddy swamp in the spring of 1977. Mixed with the mud were thousands of different toxic, noxious chemicals that formed puddles in the playground, oozed into people's basements, ical
and covered gardens and lawns. Trees, lawns, and gardens began to die, and many of the residents of the area suffered from serious illnesses. The cost of cleaning up the Love Canal site and relocating its residents will eventually exceed $100 million, and the site and neighborhood are now vacant. Toxic wastes are also disposed of by injecting them into deep wells. These wells extend below all fresh water aquifers and are completely isolated from them to ensure that existing or potential water supplies are not
Modifications of the Groundwater System and Their Effects
505
Perspective 17-2
RADIOACTIVE WASTE DISPOSAL One
of the problems of the nuclear age
is
finding safe
until
around the year 2030, at which time and backfilled.
entrance
its
storage sites for the radioactive waste from nuclear
shafts will be sealed
power
The canisters holding the waste are designed to remain leakproof for at least 300 years, so there is
plants, the manufacture of nuclear weapons, and the radioactive by-products of nuclear medicine. Radioactive waste can be grouped into two categories: low-level and high-level waste. Low-level wastes are low enough in radioactivity that, when properly handled, they do not pose a significant environmental threat.
Most
fuel assemblies
used
in
Currently,
dump
first
it
high-level
Such a facility must be able to isolate high-level waste from the environment for at least 10,000 years, which is the minimum time such waste will remain dangerous. The Yucca Mountain site will have a capacity of 70,000 metric tons of waste and will not be completely filled radioactive waste
Under
dump
repository will be buried in a volcanic tuff at a depth
more than 15,000 metric tons of spent
southern Nevada as the nation's
isotopes from entering the groundwater system.
of about 300 m.
uranium fuel are awaiting disposal, and the Department of Energy (DOE) estimates that by the year 2000 the nation will have produced almost 50,000 metric tons of highly radioactive waste that must be disposed of safely. Near the end of 1987, Congress authorized the DOE to study the feasibility of using Yucca Mountain in
however, that
site must be located so that the groundwater from the site to the outside environment is at least 1,000 years. The radioactive waste at the Yucca Mountain
extremely
dangerous because of high amounts of radioactivity; therefore presents a major environmental problem.
believes,
travel time for
nuclear reactors and is
DOE
the geology of the area will prevent radioactive
a radioactive
High-level radioactive waste, such as the spent the material used in nuclear weapons,
possibility that leakage could occur over the
next 10,000 years. The
an Environmental Protection Agency (EPA) regulation,
low-level wastes can be safely buried in
controlled dump sites where the geology and groundwater system are well known and careful monitoring is provided.
uranium
some
(Fig. 1).
The water
table in the area will be
an additional 200 to 420 m below the dump site. Thus, the canisters will be stored in the zone of
which was one of the reasons Yucca selected. Only about 15 cm of rain fall in this area per year, and only a small amount of this percolates into the ground. Most of the water that does seep into the ground evaporates before it migrates very far. Thus, the rock at the depth the canisters are buried will be very dry, helping prolong aeration,
Mountain was
the lives of the canisters.
Geologists believe that the radioactive waste at
Yucca Mountain environment if it
is is
most
likely to
in liquid
contaminate the
form;
if
liquid,
it
could
seep into the zone of saturation and enter the
groundwater supply. But because of the low moisture in the zone of aeration, there is little water to carry the waste downward, and it will take well over 1,000
way
them
contaminated. Monitoring wells are usually drilled into
must
the aquifers to ensure that the waste
the contamination of our groundwater supply.
is
not migrating
find a
to dispose of
safely
and prevent
upward. One of the problems associated with deep well disposal, however, tential to initiate
is
that such injections have the po-
earthquakes (see Chapter 10).
Other sources of groundwater pollution include toxic chemicals from fertilizers, pesticides, and herbicides that are sprayed on fields and eventually percolate downward into the groundwater supply. As more chemicals come into industrial, agricultural, and domestic use, we
506
Chapter 17
Groundwater
^HOT SPRINGS AND The subsurface rocks
in regions
GEYSERS of recent volcanic ac-
hot for thousands of years. Groundwater percolating through these rocks is heated and, if returned to the surface, forms hot springs or geysers. Yellowstone National Park in the United States, Rotivity usually stay
Interior
view of Yucca Mountain
Volcanic rock
Storage tunnels
300
m
deep Emplacement ramp
truck
Exhaust
Excavation
equipment
ramp
Storage pile of rock removed during excavation I
High
(not to
/
level
m
300
.
/
/
Metal alloy
tunnels
sca e i
lining
radioactive Stainless-
waste
steel
container
Volcanic rock
Water
table
-
"^FIGURE
1 The location of Nevada's Yucca Mountain and a schematic diagram of the proposed high-level radioactive waste dump.
years to reach the zone of saturation. In fact, the
DOE
estimates that the waste will take longer than 10,000
years to
One
move from
the repository to the water table.
of the concerns of
some
geologists
is
that the
climate will change during the next 10,000 years. the region should will percolate
become more humid, more water
through the zone of aeration. This
increase the corrosion rate of the canisters
cause the water table to travel
If
rise,
will
and could
thereby decreasing the
saturation. This area of the country
humid between 2
was much more
million and 10,000 years ago (see
Chapter 18). While it appears that Yucca Mountain meets
all
of
the requirements for a safe high-level radioactive
waste dump, the
site is still controversial, and further must be conducted to ensure that the groundwater supply in this area is not rendered
studies
unusable by nuclear waste.
time between the repository and the zone of
New Zealand, and Iceland are all famous for hot springs and geysers. They are all sites of recent volcanism, and consequently their subsurface rocks and
more than 1,000
torua,
springs in the United States,
their
Far West, while the rest are in the Black Hills of South
groundwater are very hot.
Dakota, the Ouachita region of Arkansas, Georgia, and the Appalachian region (Fig. 17-27).
A
hot spring (also called a thermal spring or warm is a spring in which the water temperature is
spring)
warmer than the temperature of
human body (37°C) however, are much hotthe
17-26). Some hot springs, with temperatures ranging up to the boiling point in many instances. Of the approximately 1,100 known hot (Fig.
ter,
Hot world.
common
in
other parts of the
of the most famous
is
at Bath,
springs are also
One
are in the
England,
where shortly after the Roman conquest of Britain in a.d. 43, numerous bathhouses and a temple were built around the hot springs (Fig. 17-28). The heat for most hot springs comes from magma or
Hot Springs and Geysers
507
some hot springs, h6*wever, is circulated deep into the Earth, where it is warmed by the normal increase in temperature, the geothermal gradient. For example, the
Warm Springs, Georgia, is heated in this manner. This hot spring was a health and bathing resort long before the Civil War; later with the establishment of the Georgia Warm Springs Foundation, it was used to spring water of
help treat polio victims.
Geysers are hot springs that intermittently eject hot water and steam with tremendous force. The word comes from the Icelandic geysir which means to gush or
One of the most famous geysers in the world Old Faithful in Yellowstone National Park in Wyoming (Fig. 17-29). With a thunderous roar, it erupts a column of hot water and steam every 30 to 90 minutes.
rush forth. is
"" FIGURE 17-26 Hot springs are springs with a water temperature greater than 37°C. This hot spring is in West Thumb Geyser Basin, Yellowstone National Park, Wyoming.
cooling igneous rocks. activity in the
large
number
-»-FI
The
geologically recent igneous
western United States accounts for the
of hot springs in that region.
The water
in
Other well known geyser areas are found
New
in Iceland
and
Zealand.
Geysers are the surface expression of an extensive underground system of interconnected fractures within hot igneous rocks (Fig. 17-30). Groundwater percolating down into the network of fractures is heated as it comes into contact with the hot rocks. Since the water
dissolve
Due
more
rapidly in
warm
water than
in cold water.
to this high mineral content, the waters of
springs are believed by
some
many hot
to have medicinal proper-
Numerous spas and bathhouses have been built throughout the world at hot springs to take advantage of these supposed healing properties. ties.
When
the highly mineralized water of hot springs or
geysers cools at the surface,
solution
is
some of
the material in
precipitated, forming various types of depos-
its. The amount and type of precipitated mineral depend on the solubility and composition of the material through which the groundwater flows. If the groundwa-
ter
contains dissolved calcium carbonate
(CaC0 3 ),
then
travertine or calcareous tufa (both of
which are varieties of limestone) are precipitated. Spectacular examples of hot spring travertine deposits are found at Mammoth Hot Springs in Yellowstone National Park and at Pamukhale in Turkey (Fig. 17-31). Groundwater containing dissolved silica will, upon reaching the surface, precipitate a soft, white,
ter or geyserite,
opening
hydrated mineral called siliceous
sin-
which can accumulate around a geyser's
(Fig. 17-32).
Geothermal Energy Energy that is harnessed from steam and hot water trapped within the Earth's crust is called geothermal It is a desirable and relatively nonpolluting alform of energy. Approximately 1 to 2% of the world's current energy needs could be met by geothermal energy. In those areas where it is plentiful, however,
energy. ternate
^
FIGURE 17-28 One of the many bathhouses in Bath, England, that were built around hot springs shortly after the Roman conquest in a.d. 43.
near the bottom of the fracture system pressure than that near the top,
higher temperature before
it
it
is
under greater
must be heated
will boil.
geothermal energy can supply most,
if
not
all,
of the
•^ FIGURE 17-29 Old Faithful Geyser in Yellowstone National Park, Wyoming, is one of the world's most famous geysers, erupting approximately every 30 to 90 minutes. _
to a
Thus, when the
deeper water
is heated to very near the boiling point, a temperature or a drop in pressure, such as from escaping gas, will cause it to instantly change to
slight rise in
The expanding steam quickly pushes the water above it out of the ground and into the air, thereby producing a geyser eruption. After the eruption, relatively
steam.
cool groundwater starts to seep back into the fracture it is heated to near its boiling temperature and the eruption cycle begins again. Such a process explains how geysers can erupt with some regularity. Hot spring and geyser water typically contains large quantities of dissolved minerals because most minerals
system where
Hot Springs and Geysers
509
FIGURE 17-30 The formation of a geyser. Groundwater percolates downward into a network of interconnected openings and is heated by the hot igneous '**'
(a)
The water near the bottom of the fracture system is under greater pressure than that near the top and consequently must be heated to a higher temperature before it will boil, {b) Any rise in temperature of the water above its boiling point or a drop in pressure will cause the water to change to steam, which quickly pushes the water above it upward and out of the ground, producing a geyser eruption. rocks.
heated from geothermal wells. Direct heating
manner
heating and
The for
its
fields.
city
much
this
cleaner.
New
of Rotorua in
Zealand
is
world famous
volcanoes, hot springs, geysers, and geothermal Since the
more than 800
first
well
own
was sunk by hand
in the
1930s,
wells have been drilled to tap the hot
water and steam below. their
in
significantly cheaper than fuel oil or electrical
is
Many homes
in
Rotorua have
well for heating, hot water, and even steam
barbecuing. Geothermal energy in Rotorua variety of ways:
is
used
in a
home, commercial, and greenhouse
heating; powering refrigeration plants for air conditioning;
water
ture;
and
Research
distillation; the
commercial geothermal 1960 at The Geyabout 120 km north of San Francisco, California 17-33). Here, wells were drilled into the numerous
electrical generating plant
(Fig.
first
was
built in
near-vertical fractures underlying the region.
"***"
FIGURE
17-31
Minerva Terrace
Springs in Yellowstone National Park,
gneous
when calcium
other types of energy.
Some
of the countries currently
using geothermal energy in one form or another include Iceland, the United States,
Mexico,
Italy,
New
Zealand,
Japan, the Philippines, and Indonesia.
Geothermal energy has been successfully used
in Ice-
land since 1928. In Reykjavik, Iceland's capital, steam
and hot water from wells
pumped
drilled in
geothermal areas are
into buildings for heating
and hot water. Fruits
and vegetables are grown year-round
510
Chapter 17
Groundwater
in
hot houses
As
pres-
at Mammoth Hot Wyoming, formed
carbonate-rich hot spring water cooled,
precipitating travertine deposits.
energy needs, sometimes at a fraction of the cost of
furni-
Institute.
In the United States, the
sers,
manufacture of cane
for various research activities at the Forest
^ FIGURE
17-32
is
"^ FIGURE
Cap in Wyoming,
Liberty
Yellowstone National Park,
California.
mound produced by
a geyserite
17-33 The Geysers, Sonoma County, Plumes of steam can be seen rising from several
steam-generating plants.
repeated geyser eruptions. Each
eruption of hot silica-rich water
amount of
precipitated a small
geyserite, eventually building
large
mound.
sure
on the
up
this
groundwater decreases, the water
rising
changes to steam that
is
piped directly to
electricity-
The present electrical generating caThe Geysers is about 2,000 megawatts, which
ment have begun. While geothermally generated generally clean source of power,
ity is a
it
electric-
can also be
generating turbines.
expensive because most geothermal waters are acidic and
pacity at
very corrosive. Consequently, the turbines must either be
is
enough
to supply
about two-thirds of the
electrical
needs of the San Francisco Bay area.
As
built of expensive corrosion-resistant alloy metals or fre-
quently replaced. Furthermore, geothermal power
becoming
not
is
west-
The steam and hot water removed for geothermal power cannot be easily replaced, and eventually
ern United States, such as the Salton Sea area of southern
pressure in the wells drops to the point at which the geo-
oil
reserves decline, geothermal energy
an attractive alternative, particularly California,
is
in parts of the
where geothermal exploration and develop-
The water stored
in the
pore spaces of subsurface
rocks and unconsolidated material
is
called
groundwater. 2.
Groundwater
is
part of the hydrologic cycle and
represents approximately
22%
of the world's supply
Porosity soil
is
the percentage of a rock, sediment, or
consisting of pore space. Permeability
ability of a rock,
field
must be abandoned.
material that transmits groundwater is an aquifer and one that prevents the movement of groundwater is an aquiclude. The water table is the surface that separates the zone of aeration (in which pore spaces are filled with both air and water) from the zone of saturation (in which all
pore spaces are
filled
with water).
Groundwater moves very slowly through the pore
of fresh water. 3.
thermal
A
^CHAPTER SUMMARY 1.
inexhaustible.
sediment, or
soil to
is
the
transmit
fluids.
spaces of rocks, sediment, or soil (zone of aeration)
and moves through the zone of saturation to
outlets
such as streams, lakes, and swamps.
Chapter Summary
511
6.
A
spring occurs wherever the water table intersects
the Earth's surface.
Some
springs are the result of a
perched water table, that is, a localized aquiclude within an aquifer and above the regional water
water well zone of aeration zone of saturation
spring stalactite
stalagmite
water table
table. 7.
8.
Water wells are made by digging or drilling into the zone of saturation. When water is pumped out of a well, a cone of depression forms. If water is pumped out faster than it can be recharged, the cone of depression deepens and enlarges and may locally drop to the base of the well, resulting in a dry well. Artesian systems are those in which confined groundwater builds up high hydrostatic pressure. Three conditions must generally be met before an artesian system can form: the aquifer must be confined above and below by aquicludes; the aquifer is usually tilted and exposed at the Earth's surface so it can be recharged; and precipitation must be
keep the aquifer filled. 9. Karst topography results from groundwater, weathering, and erosion and is characterized by sinkholes, solution valleys, and disappearing streams. 10. Caves form when groundwater in the zone of saturation weathers and erodes soluble rock such as
^ 1.
2.
3.
4.
the correct order, from highest to lowest, of in the
United States?
b.
industrial, domestic, agricultural;
c.
domestic, agricultural, industrial;
d.
agricultural, domestic, industrial;
e.
industrial, agricultural, domestic.
What
percentage of the world's supply of fresh
water
is
a
5; b
The
represented by groundwater? 22; d
18; c
43;
porosity; b.
c.
solubility; d.
e.
saturation. table
is
is:
permeability;
a.
The water
50.
e
capacity of a material to transmit fluids aeration quotient;
a surface separating the:
zone of porosity from the underlying zone of
a.
permeability; b.
capillary fringe
underlying zone of aeration;
11. Modifications of the
from the capillary fringe
c.
from the underlying zone of saturation;
zone
d.
of aeration from the underlying zone of saturation;
zone of saturation from the underlying zone
e.
of aeration. 5.
Groundwater:
moves slowly through the pore spaces of b. moves fastest through the
a.
Earth materials;
central area of a material's pore space;
move upward
areas of low pressure;
eject
6.
A
7.
An
can
c.
against the force of gravity;
moves from areas of high pressure toward
d.
rocks. Geysers are hot springs that intermittently
IMPORTANT TERMS
is
agricultural, industrial, domestic;
a.
limestone. Cave deposits, called dripstone, result
hot water and steam. 14. Geothermal energy comes from the steam and hot water trapped within the Earth's crust. It is a relatively nonpolluting form of energy that is used as a source of heat and to generate electricity.
What
groundwater usage
sufficient to
from the precipitation of calcite. groundwater system can cause serious problems. Excessive withdrawal of groundwater can result in dry wells, loss of hydrostatic pressure, saltwater encroachment, and ground subsidence. 12. Groundwater contamination is becoming a serious problem and can result from sewage, landfills, toxic waste, and agriculture. 13. Hot springs and geysers may occur where groundwater is heated by hot subsurface volcanic
REVIEW QUESTIONS
all
e.
of these.
perched water table: a. occurs wherever there is a localized aquiclude within an aquifer; b. is frequently the site of springs; c. lacks a zone of aeration; d. answers (a) and (b); e. answers (b) and artesian system
water
a.
is
is
one
in
which:
water can
confined; b.
when
rise
aquiclude
groundwater
the level of the aquifer
aquifer
hot spring
artesian system
karst topography
capillary fringe
perched water table
water must be pumped; d. answers answers (a) and (b). Which of the following is not an example of
cave
permeability
column
porosity
a.
karst topography; b.
cone of depression
recharge
c.
sinkholes; d.
dripstone
saltwater
geothermal energy
512
incursion
sinkhole
geyser
Chapter 17
Groundwater
a well
is
c.
and
8.
above
drilled; (a)
(c); e.
groundwater erosion?
9.
caves;
stalactites; e.
caverns.
What
percentage of the water used in the United
States
is
a
50; b
provided by groundwater? 40; c 30; d
20; e
10.
10.
Rapid withdrawal of groundwater can result a cone of depression; b. ground
23.
in:
subsidence;
saltwater incursion; d.
c.
hydrostatic pressure; 11. In
which area are you
loss of
of these.
all
e.
least likely to find
hot springs
or geysers?
24.
eastern Canada; b.
c.
Iceland; d.
New
western United States; Zealand; e. none of
The water
in
hot springs and geysers:
is
believed to have curative properties;
b.
is
noncorrosive;
contains large
c.
(b); e.
answers
(a)
and
(a)
groundwater removal may have on
14.
The Geysers, California; Wyoming; d. Omaha, Nebraska. e. Which of the following is not a cave deposit?
the following
stalagmite; b. stalactite; e.
Discuss the role
make good
types of materials
^ADDITIONAL READINGS
subdued
What
does groundwater surface water?
20.
Where
21.
How What
ed.
American
Columbus,
Ohio: Merrill Publishing Co. J. 1990. Dreams of riches led Floyd Collins to a nightmarish end. Smithsonian 21, no. 2: 137-49. Freeze, R. A., and J. A. Cherry. 1979. Groundwater. Englewood Fincher,
Cliffs, N.J.: Prentice-Hall.
aquifers
and
replica of the
causes the water table
J.
N. 1983. Karst landforms. American
Scientist 71,
578-86.
no. 6:
1985. Karst geomorphology. 2d ed. Oxford, England:
Monastersky, R. 1988. The 10,000-year
so
much slower than
does a perched water table differ from a
is a cone of depression and important?
Science
News
133:
M. 1985. Introducing groundwater. London: Allen &c
Unwin. Rinehart,
are springs likely to occur?
test.
139-41. Price,
move
J. S.
1980. Geysers and geothermal energy.
York: Springer-Verlag. Sloan, B., ed. 1977. Caverns, caves, and caving.
New
New
Brunswick, N.J.: Rutgers University Press.
regional water table? 22.
cities.
38-47. W. 1988. Applied hydrogeology. 2d
Jennings,
level to fluctuate?
Why
and
Basil Blackwell.
the water table a
surface topography?
a thermal spring
what ways has geothermal energy been used?
.
is
groundwater system
Scientist 74, no. 1:
aquicludes?
19.
Give
a geyser?
Fetter, C.
How can a rock be porous and yet not be permeable? Why
a
Dolan, R., and H. G. Goodell. 1986. Sinking
cycle.
18.
a region.
is
room; c. dripstone; none of these. of groundwater in the hydrologic
a.
d.
ways that
may become contaminated. What is the difference between
30. In
(c).
not a geothermal site? Rotarua, New Zealand; b. Reykjavik, a. Yellowstone National Park; Iceland; c.
What
pumped?
does groundwater weather and erode?
How do caves and their various features form? 27. Discuss the various effects that excessive
29.
answers
Which of
17.
How
28. Discuss the various
a.
13.
16.
artesian wells free-flowing while
some examples.
quantities of dissolved minerals; d.
15.
some
26.
a.
and
are
25. List the surface features of karst topography and explain how they form.
these.
12.
Why
others must be
a.
why
is it
so
Additional Readings
513
CHAPTER
18
GLACIERS AND G
L
AC
I
AT O N I
^ OUTLINE PROLOGUE INTRODUCTION GLACIERS AND THE HYDROLOGIC CYCLE THE ORIGIN OF GLACIAL ICE TYPES OF GLACIERS THE GLACIAL BUDGET RATES OF GLACIAL MOVEMENT GLACIAL EROSION AND TRANSPORT Erosional Landforms of Valley Glaciers
U-Sbaped Glacial Troughs
Hanging
Valleys
Cirques, Aretes,
and Horns
Erosional Landforms of Continental Glaciers
GLACIAL DEPOSITS Landforms Composed of
Till
End Moraines Lateral
and Medial Moraines
Drumlins
Landforms Composed of
Outwash Plains and Karnes and Eskers Glacial
Stratified Drift
Valley Trains
Lake Deposits
PLEISTOCENE GLACIATION "^
Perspective 18-1: Glacial Lake Missoula
and the Channeled Scablands Pleistocene Climates Pluvial
"^
and Proglacial Lakes
Perspective 18-2:
A
Brief History of the
Great Lakes
Changes
in
Sea Level
GLACIERS AND ISOSTASY CAUSES OF GLACIATION The Milankovitch Theory Short-Term Climatic Events
CHAPTER SUMMARY Climbers ascending Ingraham Glacier on Mount Rainier, Washington.
^^ ^>ra^^3aagg^^
PROLOGUE Following the Great Ice Age, which ended about 10,000 years ago, a
warming trend occurred
general
that
was
periodically
interrupted by short relatively cool periods. cool period, from about a.d.
1500
One
such
to the mid- to
was characterized by the expansion of small glaciers in mountain valleys and the persistence of sea ice at high latitudes for longer periods than had late- 1800s,
occurred previously. This interval of nearly four centuries
The
is
known
most of the problems. Particularly hard hit were Iceland and the Scandinavian countries, but at times much of northern Europe was affected (Fig. 18-1). Growing seasons were shorter during many years, resulting in food shortages and a number of famines.
as the Little Ice Age.
climatic changes leading to the Little Ice
Age
began by about a.d. 1300. During the preceding centuries, Europe had experienced rather mild temperatures, and the North Atlantic Ocean was warmer and more storm-free than it is at the present. During this time, the Vikings discovered and settled Iceland, and by a.d. 1200, about 80,000 people resided there. They also discovered Greenland and North America and established two colonies on the former and one on the latter. As the climate deteriorated, however, the North Atlantic became stormier, and sea ice occurred further south and persisted longer each year. As a consequence of poor sea conditions and political problems in Norway, all shipping across the North Atlantic ceased, and the colonies in Greenland and North America eventually actually
"^ FIGURE 18-1 (a) During the Little Ice Age, many of the glaciers in Europe, such as this one in Switzerland, much farther down their valleys than they do at The Unterer Grindelwald painted in 1826 by Samuel Birmann (1793-1847). (b) This mid-1600s painting by Jan-Abrahamsz Beerstraten titled The Village of Nieukoop in Winter shows the canals of Holland frozen. These canals rarely freeze today. extended present.
disappeared.
During the Little Ice Age, many of the small Europe and Iceland expanded and moved
glaciers in far
down
their valleys, reaching their greatest historic
A small ice cap formed in where none had existed previously, and glaciers in Alaska and the mountains of the western United States and Canada also expanded to their greatest limits during historic time. Although glaciers caused some problems in Europe where they advanced across roadways and pastures, destroying some villages in Scandinavia and threatening villages elsewhere, their overall impact on humans was minimal. Far more important from the human perspective was that during much of the Little Ice Age the summers in northern latitudes were cooler and wetter. Although worldwide temperatures were a little lower during this time, the change in summer extent by the early 1800s. Iceland
conditions rather than cold winters or glaciers caused
Prologue
515
from its high of 80,000 40,000 by 1700. Between 1610 and
Age ended is debatable. end at 1880, whereas others ended as early as 1850. In any case, during 1800s, the sea ice was retreating northward, were retreating back up their valleys, and
when
Exactly
Iceland's population declined
the*Little Ice
Some
authorities put the
1870, sea
ice was observed near Iceland for as much months a year, and each time the sea ice persisted for long periods, poor growing seasons and
think
it
as three
the late
food shortages followed.
summer weather became more
in
1200
to about
m.^^ i^^.^ -g
m. -
-
g.^ -ic^g^^^ m ^L T
^ INTRODUCTION Most people have some idea of what a glacier is, but many confuse glaciers with other masses of snow and ice. A glacier is a mass of ice composed of compacted and recrystallized snow that flows under its own weight on
land. Accordingly, sea ice as in, for example, the
north polar region
is
not glacial
ice,
icebergs glaciers even though they
from
glaciers that flowed into the
high mountains
may
nor are drifting
may have derived sea. Snow fields in
persist in protected areas for years,
but these are not glaciers either because they are not
moving. At the present time, glaciers cover nearly 15 million km 2 or about one-tenth of the Earth's land surface (Table 18-1). Numerous glaciers exist in the mountains of actively
,
the western United States, especially Alaska, western
Canada, the Andes in South America, the Alps of Europe, the Himalayas of Asia, and other high mountains.
^ TABLE
18-1
glaciers
Present-Day Ice-Covered Areas
-
.
^
fc
.
^-
stable.
^ ^ ^'SK-^^-^^ ^ ^^g^i
•
'
-
-
"^ FIGURE
18-2
Glacier in Glacier
=»
Iceberg calving from the Margerie Bay National Park, Alaska.
THE ORIGIN OF GLACIAL
Ice is
crystalline structure cal
ICE
a mineral in every sense of the word;
and possesses
it
has a
characteristic physi-
and chemical properties. Accordingly, geologists
consider glacial ice to be rock, although
rock that
is
easily
forward manner
deformed. (Fig.
It
forms
When
18-3).
it is
a type of
in a fairly straight-
an area receives
more winter snow than can melt during the spring and
summer seasons, a fallen snow consists but
it
compacts
as
net accumulation occurs. Freshly
of about it
80%
air
refreezes; in the process, the original
verted to a granular type of ice called firn is cial
further
ice,
and
20%
solids,
accumulates, partly thaws, and
compacted and
consisting of about
is
snow
finally
90%
layer
is
con-
Deeply buried converted to gla-
firn.
solids
(Fig.
18-3).
When
accumulated snow and
ice
reach a
critical thick-
40 m, the pressure on the ice at depth is sufficient to cause deformation and flow, even though it remains solid. Once the critical thickness is reached and
ness of about
^ FIGURE
The conversion snow to firn and
18-3
of freshly fallen glacial ice.
The Origin of
Glacial Ice
517
'**'
FIGURE
18-5
Movement
of a glacier by a
combination of plastic flow and basal
slip. If
solidly frozen to the underlying surface,
it
a glacier
is
moves only by
plastic flow.
» TYPES OF GLACIERS Geologists generally recognize two basic types of gla-
and continental.
ciers:
valley
name
implies,
is
A
valley glacier, as
its
confined to a mountain valley or per-
haps to an interconnected system of mountain valleys (Fig. 18-6). Large valley glaciers commonly have several
(b) "•'"
FIGURE
The Margerie Glacier in Alaska can At lower latitudes glaciers exist only at high elevations as this one on Mount Cook, New Zealand. 18-4
(a)
exist at sea level, (b)
(Photo courtesy of R.
V. Dietrich.)
flow begins, the moving mass of polar regions where
little
ice
becomes
summer melting
a glacier. In
of
snow
oc-
curs, glaciers can exist at or very near sea level, but at
lower latitudes they are found only at higher elevations (Fig. 18-4).
which causes permanent deformation, is the primary way move. They may also move by basal slip,
Plastic flow,
occurs in response to pressure and that glaciers
which occurs when a glacier surface (Fig. 18-5). Basal slip
slides is
over the underlying
facilitated
by the pres-
ence of meltwater that reduces frictional resistance be-
tween the underlying surface and the
518
Chapter 18
Glaciers
glacier.
and Glaciation
much
smaller tributary glaciers,
as large streams have
from higher to lower elevations and are invariably small in comparison to continental glaciers, even though some may be more than 100 km long, several kilometers wide, and several hundred meters thick. tributaries. Valley glaciers flow
Continental glaciers, also called areas (at least 50,000
km 2
)
ice sheets,
cover vast
and are unconfined by
to-
pography (Fig. 18-7). In contrast to valley glaciers, which flow downhill within the confines of a valley, continental glaciers flow outward in all directions from a central area of accumulation. Valley glaciers flow in
the direction of an existing slope, whereas the direction a continental glacier flows ice thickness. Currently,
is
determined by variations
in
only two continental glaciers
one in Greenland and the other in Antarctica. Both are more than 3,000 m thick in their central areas, become thinner toward their margins, and cover all but exist,
"•*
FIGURE
18- T
The Antarctic
ice sheet,
one of two
continental glaciers existing at present.
»
THE GLACIAL BUDGET
Just as a savings account
grows and shrinks
as funds are
deposited and withdrawn, glaciers expand and contract in response to accumulation and wastage. Their behavior can be described in terms of a glacial budget, which is essentially a balance sheet of accumulation and wastage.
The upper pan of lation
surface
lower losses "**
FIGURE
A
18-6
is
perennially covered by snow. In contrast, the
pan of the same glacier is
a zone of wastage, where from melting, sublimation, and calving of icebergs
At the end of winter, a
(Fig.
18-8).
During the
Pleis-
with
covered
tocene Epoch, such glaciers covered large pans of the
snow recedes during
Northern Hemisphere continents. Many of the erosional and depositional landforms in much of Canada and the northern tier of the United States formed as a consequence of Pleistocene glaciation. Although valley and continental glaciers are easily differentiated by their size and location, an intermediate va-
limit (Fig. 18-9).
riety called ilar to,
an
ice
cap
is
also recognized. Ice caps are sim-
but smaller than, continental glaciers and cover
than 50,000
less
km 2 Some ice caps form when valley glaciers .
grow and overtop the divides and passes between adjacent valleys and coalesce to form a continuous ice cap. They also form on fairly flat terrain including some of the islands of the Canadian Arctic and Iceland.
(Fig. 18-9).
glacier's surface
is
usually
accumulated seasonal snowfall. During spring and summer, however, the snow begins to melt, first at lower elevations and then progressively higher up the glacier. The elevation to which completely
mountains
zone of accumuand the glacier's
a
is
losses,
exceed the rate of accumulation large valley glacier in Alaska. Notice
the tributaries to the large glacier.
the highest
a valley glacier
where additions exceed
the
a wastage season
One can
is
called the'firn
zones of accumulation and wastage by noting the position of the easily identify the
firn limit.
Observations of a single glacier reveal that the posifrom year to year.
tion of the firn limit usually changes If it
does not change or shows only minor fluctuations, is said to have a balanced budget; that is,
the glacier
additions in the zone of accumulation are exactly bal-
anced by losses in the zone of wastage, and the end or terminus of the glacier remains stationary. the firn limit
moves down
positive budget;
its
terminus advances
the glacier, the glacier has a
additions exceed (Fig.
distal
When
18-10b).
The
If
its
losses,
the budget
Glacial Budget
and is
its
nega-
519
i
70°
L H7S«_^grE'!sworth «5° 60° '
Mts.
#po(e
2000
Mirny
^ FIGURE
18-8
The two
existing continental glaciers. {a)
almost completely averaging thick and reaching thickness of about
Antarctica
covered by an about 2,160
is
Ungiaciated surface
ice sheet
m
a
maximum
4,000 m.
{b)
sheet has a
The Greenland
maximum
Land ice Ice shelf
ice
thickness
of approximately 3,350 m.
(a)
the glacier recedes— its terminus retreats
tive,
glacial valley (Fig. 18-10c).
But even though a
up the glacier's
may be receding, the glacial ice continues to move toward the terminus by plastic flow and basal slip. terminus
If
a negative budget persists long enough, however, a
glacier recedes
and
which
thins to the point at
it
no
longer flows, thus becoming a stagnant glacier.
Although we used a valley glacier as our example, the the flow of conti-
same budget considerations control
nental glaciers as well. For example, the entire Antarctic ice sheet
»
in the
is
the ocean
zone of accumulation, but
it
flows into
where wastage occurs.
RATES OF GLACIAL
In general, valley glaciers
MOVEMENT
move more
rapidly than con-
tinental glaciers, but the rates for both vary, ranging
from centimeters to tens of meters per day. Valley ciers
moving down
glaciers of
that
all
steep slopes flow
comparable
size
on
more
gla-
rapidly than
gentle slopes, assuming
other variables are the same.
The main glacier in volume of ice
a valley glacier system contains a greater
and thus has a greater discharge and flow 520
Chapter 18
Glaciers
and Glaciation
velocity than
"^ FIGURE 18-9 The glacial budget is the annual balance between additions in the zone of accumulation and losses in the zone of wastage. Ice and rock debris are progressively buried by newly formed ice in the zone of accumulation, but eventually reach the surface in the zone of wastage as the
Zone
of
accumulation Annual snow
line
\
(firn limit)
overlying ice melts.
Zone
of
wastage its
tributaries (Fig. 18-6).
Temperature exerts a seasonal
control on valley glaciers because although plastic flow
remains rather constant year-round, basal
important during warmer months
more abundant. Flow rates also vary within the
slip is
more
when meltwater
ice itself.
is
For example,
flow velocity generally increases in the zone of accumulation until the firn limit
is
reached; from that point, the
Zone of wastage
accumulation
-^ FIGURE 18-10 Response of a hypothetical glacier to changes in its budget, {a) If the losses in the zone of wastage, shown by stippling, equal additions in the zone of
accumulation,
shown by
crosshatching, the terminus of the
Gains exceed losses, and the glacier's terminus advances, (c) Losses exceed gains, and the glacier's terminus retreats, although the glacier continues to flow. glacier remains stationary, (b)
Rates of Glacial
Movement
521
•^ FIGURE
18-12
Crevasses and an
ice fall in a glacier in
Alaska.
FIGURE 18-11 Flow velocity in a valley glacier varies both horizontally and vertically. Velocity is greatest at the top-center of the glacier. Friction with the walls and floor of the glacial trough causes the flow to be slower adjacent to these boundaries. The length of the arrows in the figure is "•"
proportional to the velocity.
velocity
becomes progressively slower toward the
gla-
through a glacier at a velocity several times faster than the normal flow. Although surges are best documented in valley glaciers, they occur in ice caps and continental glaciers as well. During a surge, a glacier's terminus may
advance several kilometers during a year. The causes of surges are not fully understood, but some of them have occurred following a period of unusually heavy precipitation in the zone of accumulation. Others developed when excessive amounts of snow and ice were dislodged from mountain peaks and fell onto the upper parts of glaciers.
Continental glaciers ordinarily flow at a rate of cen-
cier's terminus. Valley glaciers are similar to streams, in
that the valley walls
and
floor cause frictional resistance
to flow. Thus, the ice in contact with the walls
moves more slowly than
the ice
some
and
floor
away
distance
Notice
in
Figure 18-11
upward
until the
that the flow velocity in-
top few tens of meters of
ice are
or no additional increase occurs after that point. This upper ice constitutes the rigid part of the glacier that is moving as a consequence of basal slip and reached, but
little
plastic flow below.
The
fact that this
of ice behaves as a brittle solid
is
m
upper 40 or so demonstrated
clearly
by large fractures called crevasses that develop when a valley glacier flows over a step in its valley floor where the slope increases or where it flows around a corner (Fig.
18-12). In either case, the glacial ice
is
rate of a meter or so per
Chapter 18
Glaciers
and Glaciation
move comparatively
day has a great cumu-
One
slowly
reason continenis
that they exist
and are frozen to the underlying surface most of the time, which limits the amount of basal
at higher latitudes
slip.
Some
basal slip does occur even beneath the Ant-
most of its movement is by plastic some parts of continental glaciers achieve extremely high flow rates. For exam-
arctic ice sheet, but
flow. Nevertheless,
manage ple, is
to
near the margins of the Greenland
forced between mountains in
glaciers. In
ing
100
m
some of
what
ice sheet, the ice
are called outlet
these outlets, flow velocities exceed-
per day have been recorded.
stretched
and large crevasses develop, but they extend downward only to the zone of plastic flow. In some cases, a valley glacier descends over such a steep precipice that crevasses break up the ice into a jumble of blocks and spires, and an ice fall develops (Fig. 18-12). The flow rates of valley glaciers are also complicated by glacial surges, which are bulges of ice that move (subjected to tension),
522
modest
lative effect after several decades. tal glaciers
(Fig. 18-11).
creases
timeters to meters per day. Nevertheless, even a rather
^ GLACIAL EROSION AND TRANSPORT Glaciers are currently limited in areal extent, but during the Pleistocene Epoch, they covered
much
larger areas
and were thus more important than their present distribution would indicate. Glaciers are moving solids that
»" FIGURE
18-14 Origin of a roche moutonnee. As the moves over a hill, it smooths the "upstream" side by abrasion and shapes the "downstream" side by plucking. ice
^
FIGURE 18-13 A glacial erratic near York. (Photo courtesy of R. V. Dietrich.)
Hammond, New
can erode and transport huge quantities of materials, especially unconsolidated sediment
areas of
Canada and
and
In
soil.
many
the northern United States, glaciers
transported boulders,
some of huge proportions,
for
form called a roche moutonnee, which is French for "rock sheep." As shown in Figure 18-14, a glacier smooths the "upstream" side of an obstacle, such as a small hill, and plucks pieces of rock from the "downstream" side by repeatedly freezing and pulling away from the obstacle. Sediment-laden glacial ice can effectively erode by abrasion. For example, bedrock over which sediment-
long distances before depositing them. Such boulders
laden glacial
are called glacial erratics (Fig. 18-13).
polish, a
Important erosional processes associated with glaciers include bulldozing, plucking,
and abrasion.
dozing, although not a formal geologic term,
is
Bullfairly
(Fig.
ice
has
moved commonly develops
smooth surface that
18-15a). Abrasion also yields glacial striations,
consisting of rather straight scratches (Fig.
a glacial
glistens in reflected light
on rock surfaces more than a
18-15b). Glacial striations are rarely
glacial ice freezes in the cracks
few millimeters deep, whereas glacial grooves are simibut much larger and deeper (Fig. 18-16). Abrasion also thoroughly pulverizes rocks so that they yield an
and crevices of a bedrock projection and eventually
aggregate of clay- and silt-sized particles having the con-
self-explanatory: a glacier simply shoves or pushes un-
consolidated materials in quarrying, occurs
pulls
it
loose.
W FIGURE (b)
when
One
18-15
its
path. Plucking, also called
manifestation of plucking
(a)
Glacial polish
on
is
a land-
lar
sistency of flour, hence the
name rock
flour.
Rock
flour
quartzite near Marquette, Michigan. Monument, California.
Glacial striations in basalt at Devil's Postpile National
Glacial Erosion and Transport
523
-~- FIGURE 18-16 Glacial grooves on Kelly's Island in Lake Erie.
is
so
common
in
streams discharging from glaciers that
Continental glaciers can derive sediment from
moun-
through them, and windblown dust seton their surfaces. Otherwise, most of their sediment
tains projecting tles
» FIGURE
18-17
derived from the surface over which they
move and
is
trast, valley glaciers
but
it is
(Fig.
carry sediment in
all
parts of the ice,
concentrated at the base and along the margins
18-17).
Some
of the marginal sediment
is
derived
by abrasion and plucking, but much of it is supplied by mass wasting processes. The sediments carried along the margins and center become lateral and medial moraine Sediment
is
transported in
all
parts of
The sediment carried along the margins is moraine; where two lateral moraines coalesce, they
a valley glacier. lateral
is
transported in the lower part of the ice sheet. In con-
the water generally has a milky appearance.
deposits, respectively, as discussed later in this chapter (Fig. 18-17).
form a medial moraine.
Erosional Landforms of Valley Glaciers
Some
of the world's most inspiring scenery
by valley
begin with, but
is
produced
Many mountain
ranges are scenic to
when modified by
valley glaciers, they
glaciers.
take on a unique aspect of jagged, angular peaks and ridges in the midst of
broad valleys
(Fig. 18-18).
Many
landforms resulting from valley glaciation are easily ognized. Such features enable us to appreciate the
mendous
erosive
power of moving
rectre-
ice.
U-Shaped Glacial Troughs
A U-shaped
glacial
trough
is
one of the most
features of valley glaciation (Fig 18-18c).
distinctive
Mountain
val-
eroded by running water are typically V-shaped in cross section; that is, they have valley walls that descend leys
steeply to a
narrow
trast, valleys
valley
bottom
(Fig.
18-18a). In con-
scoured by glaciers are deepened, widened,
and straightened such that they possess very steep or
524
Chapter 18
Glaciers
and Glaciation
U-shaped glacial trough
•^ FIGURE
18-18
Erosional landforms produced by valley glaciers,
area before glaciation. (b)
The same
area during the
maximum
(a)
A
mountain
extent of the valley
glaciers, (c) After glaciation.
vertical walls,
but have broad, rather
thus, they exhibit a
Many
glacial
U-shaped
contain
troughs
flat
valley floors;
profile (Fig. 18-19).
—
FIGURE 18-19 A U-shaped glacial trough northwestern Montana.
in
triangular-shaped
truncated spurs, which are cutoff or truncated ridges that extend
Another
into the preglacial valley
common
feature
basins in the valley floor
of varying resistance;
is
where the
many
(Fig.
18-18c).
a series of steps or rock glacier eroded rocks
of the basins
now
contain
small lakes.
During the Pleistocene, when glaciers were extensive, was about 130 m lower than at present, so glaciers flowing into the sea eroded their valleys to much greater depths than they do now. When the glaciers melted at the end of the Pleistocene, sea level rose, and the ocean filled the lower ends of the glacial troughs so sea level
that
now
they are long, steep-walled embayments called
fiords (Fig. 18-20).
Glacial Erosion and Transport
525
-^ FIGURE
18-20 Milford Sound, a fiord in New Zealand. (Photo courtesy of George and Linda Lohse.
Fiords are restricted to high latitudes where glaciers can be maintained even at low elevations, such as Alaska, western Canada, Scandinavia, Greenland, southern New Zealand, and southern Chile. Lower sea level during the Pleistocene was not entirely responsible for the formation of all fiords. Unlike running water,
can erode a considerable distance below sea 500 m thick can stay in contact with the sea floor and effectively erode it to a depth of about 450 m before the buoyant effects of water cause glaciers
level. In fact, a glacier
the glacial ice to float! pressive;
some
m
deep.
1,300
Hanging
in
The depth of some
Norway and
fiords
is
im-
southern Chile are about
which
is
a tributary valley
valleys meet, the
perched far above the
whose
floor
is
at a
mouth of the hanging main valley's floor (Fig.
valley
is
18-18c).
Accordingly, streams flowing through hanging valleys
plunge over vertical or very steep precipices. Although not all hanging valleys form by glacial erosion, many do. As Figure 18-18 shows, the large glacier in the
main valley vigorously erodes, whereas
the smaller
glaciers in tributary valleys are less capable of large-scale
erosion.
When
tary valleys
526
Yosemite
Falls in
Yosemite National
courtesy of Sue Monroe.)
higher level than that of the main valley. Thus, where the
two
18-21
Valleys
Although waterfalls can form in several ways, some of the world's highest and most spectacular are found in recently glaciated areas. For example, Yosemite Falls in Yosemite National Park, California, plunge 435 m vertically, cascade down a steep slope for another 205 m, and then fall vertically 97 m, for a total descent of 737 m (Fig. 18-21). The falls plunge from a hanging valley,
"^ FIGURE
Park, California plunge from a hanging valley. (Photo
the glaciers disappear, the smaller tribu-
remain as hanging
Chapter 18
valleys.
Glaciers and Glaciation
Cirques, Aretes,
and Horns
Perhaps the most spectacular erosional landforms in areas of valley glaciation occur at the upper ends of glacial troughs and along the divides separating adjacent glacial troughs. Valley glaciers form and move out from steepwalled, bowl-shaped depressions called cirques at the upper end of their troughs (Fig. 18-1 8c). Cirques are
on three sides, but one side is open and leads into the glacial trough. Some cirques typically steep-walled
slope continuously into the glacial trough, but many have a lip or threshold at their lower end (Fig. 18-22).
Although the details of cirque origin are not fully understood, they apparently form by erosion of a preexisting depression
accumulate
on
a
mountain
As snow and ice wedging and plucking
side.
in the depression, frost
takes on the typical cirque shape. In or threshold, the glacial ice apparently not only moves outward but rotates as well, scouring out
enlarge
it
until
cirques with a
it
lip
rimmed by rock. Such depressions commonly contain a small lake known as a tarn (Fig. 18-22). Cirques become wider and are cut deeper into mountain sides by headward erosion as a consequence of abrasion, plucking, and several mass wasting processes. a depression
For example, part of a steep cirque wall
may
collapse,
while frost wedging continues to pry loose other rocks
tumble downslope. Thus, a combination of promountain side depression into a large cirque; the largest one known is the Walcott Cirque in Antarctica, which is 16 km wide and 3 km deep. that
cesses can erode a small
The fact that cirques expand laterally and by headward erosion accounts for the origin of two other distinctive erosional features, aretes and horns. Aretes— narrow, serrated ridges — can form in two ways. In many cases, cirques form on opposite sides of a ridge, and headward erosion reduces the ridge until only a thin partition of rock remains (Fig. 18-18c). The same effect occurs when erosion in two parallel glacial troughs reduces the
^" FIGURE
18-22
Many
called tarns such as these
cirques contain small lakes
on Mount Whitney
in California.
intervening ridge to a thin spine of rock (Fig. 18-23).
The most majestic of these
steep-walled,
all mountain peaks are horns; pyramidal peaks are formed by
headward erosion of cirques. In order for a horn to form, a mountain peak must have at least three cirques on its flanks, all of which erode headward (Fig. 18-18c).
Excellent examples of horns include
Mount Assiniboine
Canadian Rockies, the Grand Teton in Wyoming 14-1), and the most famous of all, the Matterhorn
in the (Fig.
in
Switzerland
(Fig.
18-24).
—- FIGURE 18-23
The
knifelike
ridges adjacent to these glaciers in
the
North Cascades of Washington
are aretes.
Glacial Erosion and Transport
527
In a large part of Canada, particularly the vast Canadian Shield region, continental glaciation has stripped off the soil and unconsolidated surface sediment, revealing extensive exposures of striated and polished bedrock (Fig. 18-25). Similar though smaller bedrock exposures
are also widespread in the northern United States from
Maine through Minnesota. Farther south, however, one sees the deposits of these same glaciers. Another consequence of erosion in these areas is the complete disruption of drainage that has not yet become reestablished. Thus, much of the area is characterized by deranged drainage (Fig. 16-29e), numerous lakes and
swamps, low relief, extensive bedrock exposures, and little or no soil. Such areas are generally referred to as ice-scoured plains (Fig. 18-25).
^ GLACIAL DEPOSITS consequence of
All sediment deposited as a tivity is called glacial drift.
aged glacial -~-
FIGURE
18-24
The Matterhorn
in
Switzerland
is
a
well-known horn.
States
A
drift exists in the
glacial ac-
vast sheet of Pleistocene-
northern
and adjacent parts of Canada
tier
of the United
(Fig. 18-26).
Smaller
accumulations of similar material are found where valley
remain active. Glacial deposits in sevupper midwestern states are important sources of groundwater and rich soils, and in several states they are exploited for their sand and gravel.
glaciers existed or
Erosional Landforms of Continental Glaciers Areas eroded by continental glaciers tend to be smooth and rounded because such glaciers bevel and abrade high areas that projected into the ice. Rather than yielding the sharp, angular landforms typical of valley glaci-
produce a landscape of rather nous topography interrupted by rounded ation, they
flat,
monoto-
hills.
eral
Geologists generally recognize two distinct types of glacial drift,
till
and
stratified drift. Till consists of sed-
iment deposited directly by glacial stratified; that
or density, and
it
ice. It is
not sorted or
by
size
does not exhibit any layering.
Till
is, its
particles are not separated
deposited by valley glaciers looks
much
like the
till
of
continental glaciers except that the latter's deposits are
^ FIGURE
18-25
Territories of
Canada.
An
ice-scoured plain in the Northwest
much more extensive and have much farther.
generally been trans-
ported
Stratified drift
name
implies,
is
is
sorted by size and density and, as
layered. In fact,
its
most of the sediments
recognized as stratified drift are braided stream deposits;
which they were deposited received water and sediment load directly from melting gla-
the streams in
their
cial ice.
Landforms Composed of
Till
Landforms composed of till include several types of moraines and elongated hills called drumlins.
End Moraines The terminus of either may become stabilized
528
Chapter 18
Glaciers and Glaciation
a valley or a continental glacier in
one position for some period
"^ FIGURE drift
18-26
Exposure of Pleistocene-aged
glacial
of time, perhaps a few years or even decades. Such stabilization of the ice front does
has ceased flowing, only that
When
an
not mean that the glacier it
ice front is stationary,
dumped
is
terminus
An end moraine
18-27
which continue
in the
middle distance
to
grow as long as the ice front is staEnd moraines of valley glaciers are
bilized (Fig. 18-28).
commonly
flow within the glacier
valley occupied by the glacier.
upon
as a pile of rubble at the glacier's
(Fig. 18-27).
FIGURE
has a balanced budget.
continues, and the sediment transported within or the ice
"•*'
spans the valley of the Casement Glacier in Alaska.
near Plymouth, Massachusetts.
Such deposits are end moraines,
crescent-shaped ridges of
ciers similarly parallel the ice
till
spanning the
Those of continental glafront, but are much more
extensive.
Following a period of stabilization, a glacier
may
ad-
^ FIGURE as terminal
18-28 (a) The origin of an end moraine, (b) End moraines are described moraines or recessional moraines depending on their relative positions with produced them.
respect to the glacier that
Valley train
(a)
During glaciation
(b)
After glaciation
Glacial Deposits
529
vance or it
retreat,
depending on changes
in its
budget.
advances, the ice front overrides and modifies
If its
former moraine. Should a negative budget occur, howtoward the zone of accumu-
ever, the ice front retreats
As the ice front recedes, till is deposited as it is from the melting ice and forms a layer of ground moraine (Fig. 18-28b). Ground moraine has an lation.
liberated
irregular, rolling sists
topography, whereas end moraine con-
raines.
of long ridgelike accumulations of sediment.
After a glacier has retreated for
nus
Illinois. Their outermost end momarking the greatest extent of the glaciers, go by the special name terminal moraine (valley glaciers also deposit terminal moraines). As the glaciers retreated from the positions at which their terminal moraines were deposited, they temporarily ceased retreating numerous times and deposited dozens of recessional mo-
Ohio, Indiana, and
raines,
may once
again stabilize, and
it
some
time,
its
will deposit
termi-
another
end moraine. Because the ice front has receded, such moraines are called recessional moraines (Fig. 18-28b). During the Pleistocene Epoch, continental glaciers in the mid-continent region extended as far south as southern
"^ FIGURE
18-29
Lateral
and medial moraines on
a
and Medial Moraines
Lateral
As we previously
discussed, valley glaciers transport
considerable sediment along their margins.
Much
of
this
abraded and plucked from the valley walls, but a significant amount falls or slides onto the glacier's surface by mass wasting processes. In any case, when a glacier melts, this sediment is deposited as long ridges of till called lateral moraines along the margin of the glasediment
is
cier (Fig. 18-29).
glacier in Alaska.
Where two
lateral
moraines merge, as when a tribumoraine
tary glacier flows into a larger glacier, a medial
forms (Fig. 18-29). In fact, a large glacier often has sevdark stripes of sediment on its surface, each of which is a medial moraine. Thus, although medial mo-
eral
raines are identified by their position
on a
valley glacier,
they are, in fact, formed from the coalescence of two
moraines.
lateral
many tributaries
One can
generally determine
a valley glacier has by the
how
number of its
medial moraines.
Drumlins In
many
till,
the
areas where continental glaciers have deposited
till
has been reshaped into elongated
hills called
Some drumlins measure as much as 50 m high km long, but most are much smaller. From the
drumlins.
and
1
drumlin looks like an inverted spoon with the end on the side from which the glacial ice advanced, and the gently sloping end pointing in the diside, a
steep
rection of ice
movement
(Fig.
18-30). Thus, drumlins
ice movement. Drumlins are most often found in areas of ground moraine that were overridden by an advancing ice sheet. Although no one has fully explained the origin of drumlins, it appears that they form in the zone of plastic flow
can be used to determine the direction of
as glacial ice modifies preexisting
till
into streamlined
Drumlins rarely occur as single, isolated hills; instead they occur in drumlin fields in which hundreds or thousands of drumlins are present. Drumlin fields are found in several states and Ontario, Canada, but perhaps the finest example is near Palmyra, New York.
hills.
530
Chapter 18
Glaciers and Glaciation
"*" FIGURE 18-30 These elongated hills in Antrim County, Michigan are drumlins. (Photo courtesy of B.
(a)
M.
C.
Pape.)
Landforms Composed of As already noted,
Stratified Drift
stratified drift
posit that exhibits sorting
and
is
a type of glacial de-
layering, an indication
was deposited by running water. Stratified drift is and continental glaciers, but one would expect, it is more extensive in areas of
that
it
associated with both valley as
continental glaciation.
Outwash
Plains
and
Valley Trains
Glaciers discharge meltwater laden with sediment
most
of the time, except perhaps during the coldest months.
Such meltwater forms a
series of
braided streams that
from the front of continental glaciers over a wide region. So much sediment is supplied to these radiate out
much
streams that as
so
of
it is
deposited within the channels
sand and gravel bars. The vast blankets of sediments
formed are called outwash plains (Fig. 18-3 la). amounts of meltwater
Valley glaciers discharge huge
and, like continental glaciers, have braided streams ex-
tending from them. However, these streams are generally
confined to the lower parts of glacial troughs, and
their long,
narrow deposits of
stratified drift are
known
as valley trains (Fig. 18-31b).
Outwash numerous
plains
and
valley trains
commonly contain many of which
circular to oval depressions,
contain small lakes. These depressions are kettles; they
form when a retreating block of ice that (Fig.
18-32).
is
When
a depression;
if
ice sheet
or valley glacier leaves a
subsequently partly or wholly buried the ice block eventually melts,
it
leaves
the depression extends below the water
Sediment-filled
depressions
End moraine
(b)
"^ FIGURE
18-33
(a)
An
area of ground moraine and an
esker. (b) This small, conical hill
of B.
M.
a
is
kame. (Photo courtesy
C. Pape.;
"^ FIGURE 18-32 Two stages in the origin of kettles, kames, and eskers. (a) During glaciation. (£>) After glaciation.
they form in tunnels beneath stagnant ice and in meltwater channels on the surface of glaciers (Fig. 18-32).
Long sinuous ridges of stratified drift, many of which meander and have tributaries, are called eskers (Figs. 18-32 and 18-33a). Most eskers have sharp crests and about 30°. Some are quite high, as 100 m, and can be traced for more than 100 km. Eskers occur most commonly in areas once covered by continental glaciers, but they are also associated with large valley glaciers. The sorting and stratification of the sediments within eskers clearly indicate deposition by sides that slope at
much
as
Glacial
Lake Deposits
Numerous
consequence of glaciers scouring out depressions; others occur where a stream's drainage was as a
blocked
(see Perspective 18-1);
and others are the
Regardless of
how
they formed, glacial lakes, like
lakes, are areas of deposition.
into
them and deposited
Sediment
and observations of present-day
glacial lakes are
Chapter 18
Glaciers and Glaciation
may
all
be carried
as small deltas, but of special
interest are the fine-grained deposits.
532
result
of water accumulating behind moraines or in kettles.
running water. The physical properties of ancient eskers glaciers indicate that
Some have
lakes exist in areas of glaciation.
formed
commonly
Mud
deposits in
finely laminated, consisting
"•" FIGURE 18-34 with a dropstone.
of alternating light and dark layers. Each light-dark cou-
Each varve represents light layers form during the spring and summer and consist of silt and clay; the dark layers form during the winter when the smallest particles of clay and organic matter settle from suspen-
plet
is
called a varve (Fig. 18-34).
an annual episode of deposition; the
sion as the lake freezes over. dicates
how many
Another
The number of varves
in-
years a glacial lake has existed.
distinctive feature of glacial lakes containing
varved deposits
is
the presence of dropstones (Fig. 18-
some of boulder size, in otherwise very fine-grained deposits. The presence of varves indicates that currents and turbulence in such lakes was minimal, otherwise clay and organic matter would not have settled from suspension. How then can 34).
These are pieces of
we account ment? Most
gravel,
for dropstones in a low-energy environ-
of them were probably carried into the
lakes by icebergs that eventually melted
sediment contained
in the ice.
and released
Glacial varves
^ PLEISTOCENE GLACIATION In hindsight,
it is
hard to believe that so many compewere skeptical that
tent naturalists of the last century
widespread glaciers existed on the northern continents during the not-too-distant past. Many naturalists invoked the biblical flood to account for the large boulders throughout Europe that occur far from their sources. Others believed that the boulders were rafted to their present positions ters. It
was not
until
by icebergs floating
1837
in
floodwa-
that the Swiss naturalist Louis
Agassiz argued convincingly that the displaced boulders,
many
coarse-grained sedimentary deposits, polished and
and many of the valleys of Europe from huge ice masses moving over the land. We know today that the Pleistocene Ice Age began about 1.6 million years ago and consisted of several intervals of glacial expansion separated by warmer interglacial periods. At least four major episodes of Pleisstriated bedrock,
resulted
Pleistocene Glaciation
533
Perspective 18-1
GLACIAL LAKE MISSOULA AND THE CHANNELED SCABLANDS The term scabland
is
used in the Pacific Northwest to
interpretation based
on normal stream erosion over
describe areas from which the surface deposits have
long period of time. In contrast, Bretz held that the
been scoured, thus exposing the underlying rock. Such
scablands were formed rapidly during a flood of
an area exists in a large part of eastern Washington where numerous deep and generally dry channels are
glacial
present.
Some
flows, are
more than 70
m
deep, and their floors are
high and 70 to 100 of high
hills in
m
much
apart. Additionally, a
as
10
meltwater that lasted only a few days.
The problem with
Bretz's hypothesis
was
that he
could not identify an adequate source for his
of these channels, cut into basalt lava
covered by gigantic "ripple marks" as
a
m
number
the area are arranged such that they
appear to have been islands in a large braided stream. In 1923, J Harlan Bretz proposed that the
floodwater.
He knew
that the glaciers
had advanced
as
Spokane, Washington, but he could not explain how so much ice melted so rapidly. The answer to Bretz's dilemma came from western Montana where an enormous ice-dammed lake (Lake far south as
Missoula) had formed. Lake Missoula formed
when
channeled scablands of eastern Washington were
an advancing glacier plugged the Clark Fork Valley at
formed during a single, gigantic flood. Bretz's unorthodox explanation was rejected by most
western
geologists
~^»"
who
FIGURE
1
preferred a
more
Ice
Cork, Idaho, causing the water to
Montana
fill
the valleys of
At its highest level, Lake 2 Missoula covered about 7,800 km and contained an
traditional
(Fig. 1).
Location of glacial Lake Missoula and the channeled scablands
of eastern Washington.
Canada
Glacial Lake Clark
Montana Flathead
Lobe Alpine glaciers
534
Chapter 18
Glaciers and Glaciation
*^~
FIGURE
at Missoula,
2 The horizontal lines on Sentinel Mountain Montana are wave-cut shorelines of glacial
Lake Missoula.
estimated 2,090
km 3
of water (about
42%
into Washington.
The maximum
of the
rate of flow
estimated to have been nearly 11 million
m
3
is
/sec,
about 55 times greater than the average discharge of
Amazon
River.
When
these raging floodwaters
These gravel ridges are the so-called giant glacial Lake Missoula
this area
near
Camas Hot
Springs,
Montana.
Bretz originally believed that one massive flood formed the channeled scablands, but geologists now know that Lake Missoula formed, flooded, and re-formed at least four times and perhaps as many as seven times. The largest lake formed 18,000 to 20,000 years ago, and its draining produced the last great flood. How long did the flood last and did humans witness it? It has been estimated that approximately one month passed from the time the ice dam first broke and water
rushed out onto the scablands to the time the scabland streams returned to normal flow.
anyone witnessed the
reached eastern Washington, they stripped away the
if
and most of the surface sediment, carving out huge valleys in solid bedrock. The currents were so powerful and turbulent they plucked out and moved pieces of basalt measuring 10 m across. Within the channels, sand and gravel was shaped into huge ridges, the so-called giant ripple marks (Fig. 3).
evidence of
soil
3
marks that formed when
drained across
volume of present-day Lake Michigan). The shorelines of Lake Missoula are still clearly visible on the mountainsides around Missoula, Montana (Fig. 2). When the ice dam impounding Lake Missoula failed, the water rushed out at tremendous velocity and drained south and southwest across Idaho and
the
"^ FIGURE ripple
flood.
No
The
one knows for sure
oldest
known
from the Marmes Man site in southeastern Washington dated at 10,130 years ago, nearly 2,000 years after the last flood from
humans
in the region
is
Lake Missoula. However, it is now generally accepted that Native Americans were present in North America least
at
15,000 years ago.
Pleistocene Glaciation
535
•^ FIGURE 18-35 (a) Standard terminology for Pleistocene glacial and
interglacial stages in
America,
[b)
A
North
reconstruction
showing an idealized succession of deposits and soils developed during the glacial and interglacial stages.
tocene glaciation have been recognized in North America (Fig. 18-35),
and
and
six or seven
major
glacial
advances
now
appears,
retreats are recognized in Europe. It
Pleistocene Climates
As one would expect, Pleistocene
however, that at least 20 warm-cold cycles can be de-
popular
tected in deep-sea cores. In view of these data, the tra-
is
subdivision
four-part
ditional
of the
Pleistocene
of
the climatic effects responsible for
glaciation
belief,
were worldwide. Contrary to
however, the world was not as
commonly portrayed
in
vicinity of the glaciers experienced short
know
climates.
initely,
the present interglacial period will persist indef-
or whether
we
will enter
another glacial interval.
The onset of glacial conditions really began about 40 million years ago when surface ocean waters at high southern latitudes suddenly cooled. By about 38 million years ago, glaciers had formed in Antarctica, but a con-
tinuous ice sheet did not develop there until 15 million years ago. Following a brief
warming trend during
the
Late Tertiary Period, ice sheets began forming in the
Northern Hemisphere about 2 to 3 million years ago, and the Pleistocene Ice Age was under way. At their greatest extent, Pleistocene glaciers covered about three times as much of the Earth's surface as they do now and were up to 3 km thick (Fig. 18-36). Large areas of North America were covered by glacial ice as were Greenland, Scandinavia, Great Britain, Ireland, and a large area in the northern Soviet Union. Mountainous areas also experienced an expansion of valley glaciers and the devel-
opment of
536
ice caps.
Chapter 18
Glaciers and Glaciation
it
times of glacier growth, those areas in the immediate
North America must be modified. Based on the best available evidence, it appears that the Pleistocene ended about 10,000 years ago. However, geologists do not if
frigid as
cartoons and movies. During
long,
summers and
wet winters.
Areas outside the glaciated regions experienced varied During times of glacial growth, lower ocean temperatures reduced evaporation so that most of the world was drier than it is today. However, some areas that are arid today were
much
wetter. For example, since
the cold belts at high latitudes expanded, the temperate,
and tropical zones were compressed toward and the rain that now falls on the Mediterranean shifted so that it fell on the Sahara of North Africa enabling lush forests to grow in what is now desert. California and the arid southwestern United States were also wetter because a high-pressure zone over the northern ice subtropical,
the equator,
sheet deflected Pacific winter storms southward.
Following the Pleistocene, mild temperatures pre-
and 6,000 years ago. After this became cooler and moister favoring the growth of valley glaciers on the Northern Hemisphere continents. Careful studies of the deposits at the margins of present-day glaciers reveal that during the last 6,000 years (a time called the Neo-
vailed between 8,000
warm
period, conditions gradually
(b)
(a)
Centers of ice accumulation and maximum extent of Pleistocene glaciation in North America, (b) Centers of ice accumulation and directions of ice movement in Europe during the maximum extent of Pleistocene glaciation. "^"
FIGURE
18-36
(a)
glaciation), glaciers
expanded
The
several times.
last ex-
pansion, which occurred between 1500 and the mid- to late- 1800s,
Pluvial
was
Age
the Little Ice
(see the Prologue).
test, driest
North America. During the Pleisenough rainfall to lake 145 km long and 178 m deep. When the place in
tocene, however, that area received
maintain a
and Proglacial Lakes
During the Pleistocene, many of the basins in the western United States contained large lakes that formed as a result of greater precipitation and overall cooler temperatures (especially during the summer), which lowered
The largest of these was Lake Bonneville,
the evaporation rate (Fig. 18-37). pluvial lakes, as they are called,
which attained a
maximum
depth of at least 335
m
size of
(Fig.
50,000
18-37).
The
km
posits of the Bonneville Salt Flats west of Salt
Utah formed Great Salt Lake
in
and a
vast salt de-
Lake City
as parts of this ancient lake dried up: is
simply the remnant of this once great
lake.
Another large pluvial lake existed
in
California (see Perspective 19-2), which
is
Death
now
Valley,
the hot-
Arizona
"•"
FIGURE
18-37
Pleistocene pluvial lakes in the western
United States.
Pleistocene Glaciation
537
Perspective 18-2
BRIEF HISTORY OF THE GREAT LAKES A
Before the Pleistocene, no large lakes existed in the
of the
Great Lakes region, which was then an area of generally flat lowlands with broad stream valleys
level.
draining to the north (Fig.
1).
As
the glaciers
advanced southward, they eroded the stream valleys more deeply, forming what were to become the basins of the Great Lakes. During these glacial advances, the ice front moved forward as a series of lobes, some of which flowed into the preexisting lowlands where the ice
became thicker and moved more
rapidly.
As
a
consequence, the lowlands were deeply eroded— four
-^ FIGURE 1 Theoretical preglacial drainage in the Great Lakes region. The divide separating the preglacial Mississippi and St. Lawrence drainage basins was probably near its present location. The future sites of the Great Lakes are outlined by dotted lines.
At
Great Lakes basins were eroded below sea
five
their greatest extent, the glaciers
covered the
entire
Great Lakes region and extended
south
(Fig.
far to the
18-36a). As the ice sheet retreated
northward during the periodically stabilized,
late Pleistocene, the ice front
and numerous recessional
moraines were deposited. By about 14,000 years ago, parts of the Lake Michigan and Lake Erie basins were ice-free, and glacial meltwater began forming
As the retreat of the ice sheet continued— although periodically interrupted by minor readvances of the ice front— the Great Lakes basins were uncovered, and the lakes expanded until they eventually reached their present size and configuration proglacial lakes (Fig. 2).
(Fig. 2). Currently, the Great Lakes contain nearly 3 23,000 km of water, about 18% of the water in all fresh water lakes. Although the history of the Great Lakes just
presented
is
generally correct,
it is
oversimplified. For
and depths of the evolving Great Lakes fluctuated widely in response to minor instance, the areas
readvances of the filled,
ice front.
Furthermore, as the lakes
they spilled over the lowest parts of their
margins, thus cutting outlets that partly drained them.
And
finally, as
the glaciers retreated northward,
rebound raised the southern patts of the Great Lakes region, greatly altering their drainage systems. We shall have more to say about isostatic rebound in this region in a later section. The present-day Great Lakes and their St. Lawrence River drainage constitute one of the great commercial waterways of the world. Oceangoing vessels can sail into the interior of North America as far west as Duluth, Minnesota. To do so, however, isostatic
lake evaporated, the dissolved salts were precipitated
the other shorelines consist of moraines.
on the
named
valley floor;
some of
these evaporite deposits,
especially borax, are important mineral resources.
which form far from glaproglacial lakes are formed by the meltwater ac-
In contrast to pluvial lakes, ciers,
cumulating along the margins of glaciers. In fact, in many proglacial lakes, one shoreline is the ice front itself, while 538
Chapter 18
Glaciers and Glaciation
in
honor of the French
Lake Agassiz,
naturalist Louis Agassiz,
was a large proglacial lake covering about 250,000 km' of North Dakota and Manitoba, Saskatchewan, and Ontario,
Canada.
It
persisted until the glacial ice along
its
northern margin melted, at which time the lake was able to drain
northward into Hudson Bay.
^•>>^ Laurentide Ice Sheet
covered with vegetation. Indeed, a land bridge existed across the Bering Straits from Alaska to Siberia. Native
Americans crossed the Bering land bridge, and various animals migrated between the continents; the American bison, for example, migrated from Asia. The British Isles were connected to Europe during the glacial intervals because the shallow floor of the North Sea was above
When
sea level.
the glaciers disappeared, these areas
were again flooded, drowning the plants and forcing the animals to migrate farther inland. San Francisco
Lowering of sea
level
during the Pleistocene also
af-
most major streams. When sea level dropped, streams downcut as they sought to adjust to a new lower base level (see Chapter 16). Stream channels in coastal areas were extended and deepened along the emergent continental shelves. When sea level rose at the end of the Pleistocene, the lower ends of river valleys along the east coast of North America were flooded and are now important harbors (see Chapter 20). A tremendous quantity of water is still stored on land fected the base level of
in
present-day glaciers
(Fig.
should completely melt, sea flooding
many
16-3).
level
these
If
would
rise
Los Angeles
-*r
FIGURE
Large parts of North America— and
18-38
other continents— would be flooded by the (70 m) that
would
result
if all
all
rise in sea level
the Earth's glacial ice melted.
glaciers
about 70 m,
of the coastal areas of the world where
all
of the world's large population centers are located
the greatest crustal depression, occurred farther north in
(Fig. 18-38).
Canada
^ GLACIERS AND ISOSTASY
rebound has not been evenly distributed over the entire glaciated area: it increases in magnitude from south to north (see Fig. 11 -25b). As a result of this uneven isos-
In
Chapter
1 1
we
discussed the concept of isostasy and
noted that loading or unloading of the Earth's crust causes
it
to respond isostatically to
an increased or de-
creased load by subsiding and rising, respectively. There is
no question that
isostatic
rebound has occurred
as a
features in such areas can be explained only
consequence of
isostatic
adjustments of the Earth's
crust.
When
the Pleistocene ice sheets
in size, the
weight of the
ice
zones of accumulation. For these reasons,
rebound, coastal features
formed and increased
caused the crust to respond
above
their
former
levels in the
far we have examined the effects of glaciation, but have not addressed the central questions of what causes
large-scale glaciation
and why so few episodes of wide-
spread glaciation have occurred. For more than a cenprehensive theory explaining
at a rate of
about
1
m
per century (see Fig. ll-25a). In Perspective 18-2
we noted
that the Great Lakes
evolved as the glaciers retreated to the north. As one
would expect,
isostatic
retreated north.
rebound began as the
Rebound began
first
part of the region because that area
in the
was
ice front
southern
free of ice first.
Furthermore, the greatest loading by glaciers, and hence
540
Chapter 18
Glaciers and Glaciation
north and thus slope to
Thus
tury, scientists
rebounding
re-
elevated higher
^ CAUSES OF GLACIATION
was depressed as much as 300 m below preglacial elevations. As the ice sheets disappeared, the downwarped areas gradually rebounded to their former positions. As noted in Chapter 11, parts of still
Great Lakes
now
the south.
by slowly subsiding deeper into the mantle. In some places, the Earth's surface
Scandinavia are
in the
gion, such as old shorelines, are
in the
areas formerly covered by continental glaciers. In fact, a
number of
tatic
in the
have been attempting to develop a comall
aspects of ice ages, but
have not yet been completely successful. their lack of success sible
is
for glaciation,
One
reason for
that the climatic changes respon-
the cyclic occurrence of glacial-
and short-term events such as the Little Ice Age operate on vastly different time scales. Only a few periods of glaciation are recognized in the geologic record, each separated from the others by long intervals of mild climate. Such long-term climatic changes probably result from slow geographic changes interglacial episodes,
related to plate tectonic activity.
carry continents to high latitudes
Moving
where
plates can
glaciers
can ex-
— FIGURE
18-39
{a)
The
Earth's orbit varies from
nearly a circle (dashed line) to an ellipse (solid line)
and
back again in about 100,000 years, [b) The Earth moves around its orbit while spinning about its axis, which is tilted to the plane of the ecliptic at 23.5° and points toward the North Star. The Earth's axis of rotation slowly moves and traces out the path of a cone in space, (c) At present, the Earth is closest to the Sun in January when the Northern Hemisphere experiences winter, (d) In about 11,000 years, as a result of precession, the Earth will be closer to the Sun in July, when summer occurs in the Northern Hemisphere.
ist,
provided that they receive enough precipitation as
snow. Plate collisions, the subsequent uplift of vast areas
(a)
and the changing atmospheric and oceanic circulation patterns caused by the changing shapes and positions of plates also contribute to longfar
above sea
level,
Axis
in 1 1
approximately ,000 years
term climatic change. Intermediate-term climatic events, such as the glacial-
occur on time hundreds of thousands of years. The cyclic nature of this most recent episode of glaciation has long been a problem in formulating a compreheninterglacial episodes of the Pleistocene,
scales of tens to
sive theory of climatic change.
The Milankovitch Theory A
particularly interesting hypothesis for intermediate-
term climatic events was put forth by the Yugoslavian astronomer Milutin Milankovitch during the 1920s. He
proposed that minor irregularities in the Earth's rotation and orbit are sufficient to alter the amount of solar radiation that the Earth receives at any given latitude
and hence can
affect climatic changes.
Milankovitch theory,
it
was
initially
Now
(b)
called the
Conditions
received renewed interest during the last
20
years. January
Milankovitch attributed the onset of the Pleistocene Ice Age to variations in three parameters of the Earth's orbit (Fig. 18-39).
which
is
now
ignored, but has
The first of these is orbital eccentricity,
the degree to
(c)
which the orbit departs from a
perfect circle. Calculations indicate a roughly 100,000-
year cycle between times of
maximum
eccentricity.
Conditions
in
about
1
1.000 years
This
corresponds closely to 20 warm-cold climatic cycles that occurred during the Pleistocene. The second parameter is the angle between the Earth's axis and a line perpendic-
)
January
July
ular to the plane of the ecliptic (Fig. 18-39). This angle
i
(d)
about 1.5° from its current value of 23.5° during a 41,000-year cycle. The third parameter is the precession shifts
of the equinoxes, which causes the position of the equinoxes and solstices to shift slowly around the Earth's elliptical orbit in a
23,000-year cycle (Fig. 18-39). in these three parameters cause the
Continuous changes
amount of slightly
solar heat received at
however, remains
and
any
latitude to vary
over time. The total heat received by the planet, little
changed. Milankovitch proposed,
now many scientists agree, that the interaction of these Causes of Glaciation
541
three parameters provides the triggering
mechanism
for
space. Records kept over the past dicate that during this time the
the glacial-interglacial episodes of the Pleistocene.
has varied only energy
Short-Term Climatic Events
may
slightly.
75 years, however,
amount of
in-
solar radiation
Thus, although variations
in solar
influence short-term climatic events, such a
correlation has not been demonstrated.
Climatic events having durations of several centuries,
During large volcanic eruptions, tremendous amounts
Age, are too short to be accounted for by plate tectonics or Milankovitch cycles. Several hypotheses have been proposed, including variations in
of ash and gases are spewed into the atmosphere where
such as the
Little Ice
they reflect incoming solar radiation and thus reduce
at-
Variations in solar energy could result from changes
mospheric temperatures. Recall from Perspective 4-2 that small droplets of sulfur gases remain in the atmosphere for years and can have a significant effect on the
or from anything that would reduce
climate. Several such large-scale volcanic events have
The
been recorded, such as the 1815 eruption of Tambora, and are known to have had climatic effects. However, no
solar energy
and volcanism.
within the Sun the
itself
amount of energy
latter
the Earth receives from the Sun.
could result from the solar system passing through
clouds of interstellar dust and gas or from substances in
relationship between periods of volcanic activity
the Earth's atmosphere reflecting solar radiation back into
riods of glaciation has yet been established.
^ CHAPTER SUMMARY
and pe-
hanging valleys are also products of valley glaciation.
1.
Glaciers are masses of ice plastic flow
and basal
on land
slip.
that
move by
Glaciers currently cover
about 10% of the land surface and contain all water on Earth. 2.
2%
of
Valley glaciers are confined to mountain valleys and
flow from higher to lower elevations, whereas continental glaciers cover vast areas and flow
outward
from a zone of
in all directions
abrade and bevel high areas, producing a smooth, rounded landscape. 10. Depositional landforms include moraines, which are ridgelike accumulations of till. Several types of moraines are recognized, including terminal, recessional, lateral, and medial moraines. 11. Drumlins are composed of till that was apparently reshaped into streamlined hills by continental 9. Continental glaciers
accumulation. 3.
A
glaciers.
forms when winter snowfall in an area exceeds summer melt and therefore accumulates year after year. Snow is compacted and converted to glacial ice, and when the ice is about 40 m thick, glacier
pressure causes 4.
The behavior which
is
it
composed of on
its
budget,
13.
respectively.
move
depending on the and season. Valley glaciers tend to
at varying rates
slope, discharge,
Glaciers are powerful agents of erosion are particularly effective at eroding soil
7.
542
arid regions,
They
lower part of the ice, whereas valley glaciers may carry sediment in all parts of the ice. Erosion of mountains by valley glaciers yields several sharp, angular landforms including cirques, aretes, and horns. U-shaped glacial troughs, fiords, and Chapter 18
Glaciers
and Glaciation
and
sea level
was
as
are
now
130
m
what
much
as
lower than at present. 15.
Loading of the Earth's crust by Pleistocene
glaciers
caused isostatic subsidence. When the glaciers disappeared, isostatic rebound began and continues
unconsolidated sediment, and they can transport any size sediment supplied to them. Continental glaciers transport most of their sediment in the
8.
equator, large pluvial lakes existed in
and
about
widespread glaciation, separated by interglacial North America. The other Northern Hemisphere continents were also affected by widespread Pleistocene glaciation. 14. Areas far beyond the ice were affected by Pleistocene glaciation; climate belts were compressed toward the
and
transport because they are solids in motion.
glaciers covered
of the land surface. Several intervals of
periods, occurred in
flow more rapidly than continental glaciers. 6.
stratified drift.
During the Pleistocene Epoch,
30%
the relationship between accumulation and
If a glacier possesses a balanced budget, its terminus remains stationary; a positive or negative budget results in advance or retreat of the terminus,
Glaciers
by meltwater streams issuing from glaciers; it is found in outwash plains and valley trains. Ridges called eskers and conical hills called kames are also
to flow.
of a glacier depends
wastage.
5.
12. Stratified drift consists of sediments deposited in or
16.
in some areas. Major glacial intervals separated by
tens or
hundreds of millions of years probably occur as a consequence of the changing positions of tectonic plates, which in turn cause changes in oceanic and atmospheric circulation patterns.
17.
Currently, the Milankovitch theory is widely accepted as the explanation for glacial-interglacial
6.
intervals.
18.
Rocks abraded by is
The reasons
for short-term climatic changes, such as
Two
the Little Ice Age, are not understood.
proposed causes for such events are changes in the amount of solar energy received by the Earth and
may
glaciers
develop a smooth
surface that shines in reflected light. Such a surface
7.
volcanism.
called glacial:
a.
grooves;
d.
striations; e.
A
small lake
polish;
b.
cirque
in a
flour;
c.
till.
a.
pluvial lake; b.
c.
tarn; d.
is a:
proglacial lake;
salt lake; e.
trough
glacial
lake. 8.
IMPORTANT
TERMS
The most
recent ice age occurred during the:
c.
Archean Eon; b. Mesozoic Era; d.
e.
Tertiary Period.
a
abrasion
glacier
arete
drumlin
ground moraine hanging valley horn lateral moraine medial moraine
end moraine
Milankovitch theory
move
esker
outwash plain
a.
rock creep;
fiord
plastic flow
d.
surging;
firn
recessional moraine
firn limit
stratified drift
is a:
terminal moraine
a.
basal slip
cirque
continental glacier
glacial
budget
glacial drift
till
glacial erratic
U-shaped
glacial
groove
9.
Firn
the zone of wastage;
glacial trough
on
10. Pressure
e.
depth
causes
in a glacier
it
to
by: fracture;
b.
glacial erosion
medial moraine;
fiord; b.
basal slip;
c.
plastic flow.
e.
pyramid-shaped peak formed by
horn;
c.
hanging valley. 12. Glacial drift is a general term for: a the erosional landforms of continental cirque;
glacial ice
valley train
glaciers; b.
glacial polish
zone of accumulation zone of wastage
c.
glacial striation
ice at
a granular type of another name for a type of glacial groove.
b.
a valley train; d.
d.
valley glacier
snow;
freshly fallen
ice; c.
A
Cambrian Period;
is:
a.
11.
Pleistocene Epoch;
e.
all
the deposits of glaciers; the
icebergs floating at sea; d.
of glaciers by plastic flow and basal
movement the
slip; e.
annual wastage rate of a glacier. 13. The number of medial moraines on a glacier
^ REVIEW QUESTIONS 1.
Crevasses in glaciers extend
down
generally indicates the
to:
the base of the glacier; about 300 m; b. variable the zone of plastic flow; d. c. the depths depending on how thick the ice is; e.
2.
If
increases;
The bowl-shaped depression glacial trough a.
Which
is
cirque;
of the following
is
e.
16.
till.
U-shaped moutonnee.
d.
5.
lateral
glacial trough; e
is
a(an):
horn; moraine.
a.
fiord; b.
e.
lateral
Which
of the following
erosion of a group of cirques on the
flanks of a
mountain may produce
tarn; b.
d.
kettle; e
a glacial erratic?
a.
deposit of unsorted, unstratified
b.
glacially transported c.
e.
varve;
horn.
c.
18.
U-shaped
its
glacial
deposits consisting of light and dark
How
does glacial
ice
form, and
why
is it
how do
considered
valley glaciers differ
What
is
from
the relative importance of plastic flow
and low
19. Explain in terms of the glacial budget active glacier
a(an):
drumlin;
till;
boulder far from sand and gravel deposited in a
basal slip for glaciers at high
Headward a.
is
cirque;
arete; d.
c.
continental glaciers?
moraine; roche
valley
e.
to be a rock?
not an erosional
arete; c
plains;
knifelike ridge separating glaciers in adjacent
17. Other than size,
horn; b
outwash
layers.
lateral
c.
its
terminal moraines;
depression on a glacier; d.
upper end of a
landform? a
A
source;
no longer form.
at the
eskers; d.
trough;
drumlin;
d.
15.
a(an):
inselberg; b.
moraine; 4.
crevasses will
e.
c.
valleys
a glacier has a negative budget:
its the terminus will retreat; b. accumulation rate is greater than its wastage rate; the glacier's length all flow ceases; d c.
3.
14.
layer.
a.
tributary glaciers; b.
trains.
a
outwash
number of
a.
20.
What
is
becomes
a glacial surge
and
latitudes?
how
a once
a stagnant glacier.
and what are the probable
causes of surges?
Review Questions
543
21. Explain
how
glaciers erode
by abrasion and
ADDITIONAL
READINGS
plucking.
22.
Why
are glaciers
more
effective agents of erosion
and transport than running water? 23. Describe the processes responsible for the origin of a cirque, U-shaped glacial trough, and hanging valley. is an arete and how does one form? do the erosional landforms of continental glaciers differ from those of valley glaciers? 26. Discuss the processes whereby terminal, recessional, and lateral moraines form. 27. How does a medial moraine form, and how can one
24.
What
25.
How
determine the number of tributaries a valley glacier has by its medial moraines? 28. Describe drumlins, and explain how they form.
What
outwash plains and valley trains? 30. In a roadside outcrop, you observe a deposit of alternating light and dark laminated mud containing a few large boulders. Explain the sequence of events
29.
are
responsible for 31.
How
32
We
544
its
deposition.
do pluvial lakes differ from proglacial lakes? Give an example of each of these types of lakes. can be sure that the ancient shorelines of the Great Lakes were horizontal when they were formed, yet now they are not only elevated above their former level but they also tilt toward the south. How can you account for these observations?
Chapter 18
Glaciers and Glaciation
and G. H. Denton. 1990. What drives glacial cycles? Scientific American 262, no. 1: 49-56. Carozzi, A. V. 1984. Glaciology and the ice age. Journal of Geological Education 32: 158-70. Covey, C. 1984. The Earth's orbit and the ice ages. Scientific American 250, no. 2: 58-66. Drewry, D. J. 1986. Glacial geologic processes. London: Edward Arnold. Grove, J. M. 1988. The Little Ice Age. London: Methuen. Imbrie, J., and K. P. Imbrie. 1979. Ice ages: Solving the mystery. New Jersey: Enslow Press. John, B. S. 1977. The ice age: Past and present. London:
W.
Broecker,
S.,
Collins. .
1979. The winters of the world. London: David
&
Charles.
Kurten, B. 1988. Before the Indians.
New
York: Columbia
University Press.
— McClean, D. M. 1978. A lessons from the past. Science 201: 401-406. Schneider, S. H. 1990. Global warming: Are we entering the greenhouse century? San Francisco, Calif.: Sierra Club Books. Sharp, R. P. 1988. Living ice: Understanding glaciers and glaciation. New York: Cambridge University Press. terminal Mesozoic "greenhouse"
S., Jr. 1983. Glaciers: Clues to future climate? United States Geological Survey. Wright, A. E., and F. Moseley, eds. 1975. Ice ages: Ancient and modern. Liverpool, Great Britain: Seel House Press.
Williams, R.
CHAPTER
19
THE WORK OF WIND AND DESERTS * OUTLINE PROLOGUE INTRODUCTION SEDIMENT TRANSPORT BY WIND Bed Load Suspended Load
WIND EROSION Abrasion Deflation
^f
Perspective 19-1: Evidence of Activity
Wind
on Mars
WIND DEPOSITS The Formation and Migration of Dunes
Dune Types Loess
AND GLOBAL WIND PATTERNS THE DISTRIBUTION OF DESERTS AIR PRESSURE BELTS
CHARACTERISTICS OF DESERTS Temperature, Precipitation, and Vegetation "^Perspective 19-2: Death Valley National
Monument Weathering and
Soils
Mass Wasting, Streams, and Groundwater Wind
DESERT LANDFORMS
CHAPTER SUMMARY
Racetrack Playa, Death Valley, California,
famous
for
its
is
"sliding rocks." Geologists
winds push the rocks across a lake's exposed wet, slippery bed after a rainstorm. This limestone block was believe that strong
moved 24
m
by the wind.
PROLOGUE
fringe areas include large regions in several parts of
world (Fig. 19-1). While natural processes such as climatic change result in gradual expansion and contraction of desert the
During the last few decades, deserts have been advancing across millions of
regions,
much
recent desertification has been greatly
human
acres of productive land, destroying rangelands,
accelerated by
croplands, and even villages. Such expansion,
natural vegetation has been cleared as crop cultivation
estimated at 70,000
km
human
2
per year, has exacted a
activities. In
many
areas, the
has expanded into increasingly drier fringes to support
Because of the relentless advance of deserts, hundreds of thousands
the growing population. Because these areas are
of people have died of starvation or been forced to
common
migrate as "environmental refugees" from their
susceptible to increased
terrible toll in
homelands
to
suffering.
camps where
the majority are severely
especially
prone to droughts, crop
failures are
occurrences, leaving the land bare and
wind and water erosion. Because grasses constitute the dominant natural
malnourished. This expansion of deserts into formerly
vegetation in most fringe areas, raising livestock
productive lands
common economic
Most
is
called desertification.
regions undergoing desertification
the margins of existing deserts. delicately
lie
along
These margins have a
balanced ecosystem that serves as a buffer
between the desert on one side and a more humid environment on the other. Their potential to adjust to increasing environmental pressures from natural causes or
"^"
human
FIGURE
19-1
activity
is
limited. Currently, such
is
a
activity. Usually, these areas
achieve a natural balance between vegetation and livestock as
nomadic herders graze
the available grasses. In
many
their
animals on
fringe areas, however,
numbers have been greatly increasing in recent and they now far exceed the land's capacity to support them. As a result, the vegetation cover that livestock
years,
protects the soil has diminished, causing the soil to
Desert areas of the world and areas threatened by desertification.
Prologue
547
-*"
FIGURE
19-2
A
sharp line
marks the boundary between pasture and an encroaching dune in Niger, Africa. As the goats eat the remaining bushes, the dune will continue to advance, and more land will be lost to desertification.
crumble. This leads to further drying of the accelerated soil erosion by
wind and water
soil
desertification because important nutrients in the
and
are not returned to the
(Fig. 19-2).
Desertification captured the world's attention
Drilling water wells also contributes to desertification because
around a well
human and
site strips
away
during the Sahelian drought of
livestock activity
the vegetation.
With
its
The Sahel averages between 10 and 60 cm
starvation.
merge with the surrounding desert. In addition, the water used for irrigation from these wells sometimes contributes to desertification by increasing the salt content of the soil. As the water
of rainfall per year,
resultant bare areas
amount of
salt
is
deposited in the
1968-1973 when
nearly 250,000 people and 3.5 million cattle died of
vegetation gone, the topsoil blows away, and the
evaporates, a small
dung
soil.
falls.
90%
Because drought
is
of which evaporates
common
when
it
in the Sahel, the
region can support only a limited population of livestock
and humans. Traditionally, herders and
livestock existed in a natural balance with the
it would be in an area more rain. Over time, the salt concentration becomes so high that plants can no
vegetation, following the rains north during the rainy
that receives
season and returning south to greener rangeland
longer grow. Desertification resulting from soil
planted and
soil
and
is
not flushed out as
salinization
Middle
is
during the dry seasons.
a major problem in North Africa, the
East, southwest Asia,
and the western United
Collecting firewood for heating and cooking
is
another major cause of desertification, particularly
many
less-developed countries where
wood
is
major
fuel source. In the Sahel of Africa (a belt
1,100
km
wide that
lies
in
the
300
to
south of the Sahara), the
expanding population has completely removed all trees and shrubs in the areas surrounding many towns and cities. Journeys of several days on foot to collect firewood are common there. The use of dried animal dung to supplement firewood has exacerbated
548
Chapter 19
The Work of Wind and Deserts
Some
areas were alternately
fallow to help regenerate the
soil.
During fallow periods, livestock fed off the stubble of the previous year's planting, and their dung helped fertilize
States.
left
the
soil.
With the emergence of new nations and increased foreign aid to the Sahel during the 1950s and 1960s, nomads and their herds were restricted, and large areas of grazing land were converted to cash crops such as peanuts and cotton that have a short growing season. Expanding human and animal populations and more intensive agriculture put increasing demands on the land until the worst drought of the century brought untold misery to the people of the Sahel.
Without
rains, the crops failed
and the
livestock
denuded the land of what little vegetation remained. As a result, the adjacent Sahara expanded southward as much as 150 km. The tragedy of the Sahel and prolonged droughts in other desert fringe areas serve to remind us of the
delicate equilibrium of ecosystems in such regions.
Once
the fragile soil cover has been
erosion,
it
Chapter
6).
will take centuries for
3t3t3Eg3K^Tg^^rym^Cg^^
» INTRODUCTION Most people
Wind it
is
associate the
deserts.
an effective geologic agent in desert regions, but an important role wherever loose sediment
can be eroded, transported, and deposited, such as along shorelines or the plains (see the Prologue to Chapter 6).
we
will first consider the
work of wind
in
general and then will turn to the distribution, charac-
and landforms of
teristics,
deserts.
^ SEDIMENT TRANSPORT BY WIND ment wind
is
in
and therefore transports sedimuch the same way as running water. Although a turbulent fluid
typically flows at a greater velocity than water,
silt-size particles
as
suspended load. Sand and larger the ground as bed load.
moved along
Bed Load Sediments too large or heavy to be carried in suspension by water or wind are moved as bed load either by saltation or
by rolling and
ter 16, saltation is the
sliding.
As we discussed
in
Chap-
process by which a portion of the
bed load moves by intermittent bouncing along a stream
"^ FIGURE
.
Tfc.
**.
«.«».
VI
lifts
descending sand grains grains causing 19-3).
Wind
them
to
hit the surface, they strike other
bounce along by saltation
tunnel experiments have
shown
(Fig.
that once
sand grains begin moving, they will continue to move, if the wind drops below the speed necessary to start them moving! This happens because once saltation be-
even
it
sets off a
chain reaction of collisions between
grains that keeps the sand grains in constant motion. Saltating sand usually
even
when winds
moves near the
surface,
and
are strong, grains are rarely lifted
If the winds are very strong, wind-whipped grains can cause extensive abrasion (Fig. 19-4). A car's paint can be removed by sandblasting in a short time, and its windshield will become completely frosted and translucent from pitting.
higher than about a meter. these
it
has a lower density and, thus, can carry only clay- and particles are
TE
The wind starts sand and carries some grains short distances before they fall back to the surface. As the grains rolling and
gins,
Wind
TE.
(see
bed. Saltation also occurs on land.
work of wind with
also plays
Therefore,
removed by soil to form
new
Particles larger than sand can also be moved along the ground by the process of surface creep. This type of movement occurs when saltating sand grains strike the larger particles and push them forward along the ground.
"• r
FIGURE
The effects of wind abrasion can be Dunes National Recreation Area, Florence, Oregon. The glass is frosted as a result of pitting by windblown sand. 19-4
seen on this bottle at
is moved near the ground Sand grains are picked up by the wind falling back to the before and carried a short distance ground where they usually hit other grains, causing them to bounce and move in the direction of the wind.
19-3
Most sand
surface by saltation.
Sediment Transport by
Wind
549
— FIGURE Death
19-5
A
dust storm in
Valley, California.
Suspended Load
originated in the Sahara of Africa has been collected
on
Silt-
and clay-sized particles constitute most of a wind's suspended load. Even though these particles are much smaller and lighter than sand-sized particles, wind usually starts the latter moving first. The reason for this
the Caribbean island of Barbados.
phenomenon
that a very thin layer of motionless air
Recall that streams and glaciers are effective agents of
silt and clay remain undisturbed. The larger sand grains, however, stick up into the turbulent air zone where they can be moved. Unless the stationary air layer is disrupted, the silt and clay particles remain on the ground providing a smooth surface. This phenomenon can be
erosion, much more so than wind. Even in deserts, where wind is most effective, running water is still responsible for most erosional landforms, although stream channels are typically dry (Fig. 16-4). Nevertheless, wind action can still produce many distinctive erosional features and
lies
is
next to the ground where the small
particles
observed on a
road on a windy day. Unless a vehicle travels over the road, little dust is raised even though it is windy. When a vehicle moves over the road, it breaks
^ WIND EROSION
is
an extremely
Abrasion
the calm
Wind
layer of dust,
tion.
boundary layer of air and disturbs the smooth which is picked up by the wind and forms a dust cloud in the vehicle's wake. In a similar manner,
turbed,
silt-
and carried
when
and clay-sized in
a sediment layer
is
dis-
particles are easily picked
up
suspension by the wind, creating clouds
of dust or even dust storms (Fig. 19-5).
Once
these fine
particles are lifted into the atmosphere, they
may
be
from their source. For example, large quantities of fine dust from the southwestern United States were blown eastward and fell on New England during the Dust Bowl of the 1930s (see carried thousands of kilometers
the Prologue to Chapter 6). In addition, fine dust that
550
efficient sorting agent.
dirt
Chapter 19
The Work of Wind and Deserts
erodes material in two ways: abrasion and deflaAbrasion involves the impact of saltating sand grains on an object and is analogous to sandblasting (Fig. 19-4). The effects of abrasion, however, are usually
minor because sand, the most sion,
is
rarely carried
common
more than
1
m
agent of abra-
above the surface.
Rather than creating major erosional features, wind abrasion merely modifies existing features by etching, pitting, smoothing, or polishing. Thus, wind abrasion is most effective on soft sedimentary rocks. Ventifacts are a these are stones ted,
common
whose
product of wind abrasion;
surfaces have been polished, pit-
grooved, or faceted by the wind
(Fig. 19-6). If the
-^ <££> -^
^^
(a)
"^ FIGURE
19-6 (a) A ventifact forms when wind-borne abrade the surface of a rock (2) forming a flat surface. If the rock is moved, (3) additional flat surfaces are formed, (b) A granite ventifact in the dune corridor along the Michigan shore, Lake Michigan. (Photo courtesy of particles (1)
Marion A. Whitney.)
wind blows from different directions, or if the stone is moved, the ventifact will have multiple facets. Ventifacts are most common in deserts, yet they can also form wherever stones are exposed to saltating sand grains, as on beaches in humid regions and some outwash plains in
New
England.
Yardangs are larger features than ventifacts and also result from wind erosion (Fig. 19-7). They are elongated and streamlined ridges that look like an overturned ship's hull. They are typically found grouped in clusters aligned parallel to the prevailing winds. They probably
^ FIGURE
19-7 Profile view of a streamlined yardang in the Roman playa deposits of the Kharga Depression, Egypt. (Photo courtesy of Marion A. Whitney.)
(b)
form by allel to
differential erosion in
which depressions, par-
the direction of wind, are carved out of a rock
body, leaving sharp, elongated ridges. These ridges
may
then be further modified by wind abrasion into their
Although yardangs are fairly comthem was renewed when images radioed back from Mars showed that they are also widespread features on the Martian surface (see
characteristic shape.
mon
desert features, interest in
Perspective 19-1).
Deflation Another important mechanism of wind erosion is deflation, which is the removal of loose surface sediment by the wind.
Among
the characteristic features of deflation in
many
and semiarid regions are deflation hollows (also called blowouts). These shallow depressions of variable dimensions result from differential erosion of surface maarid
Wind
Erosion
551
~*ir
FIGURE
3
Large dune
fields
surrounding the north
polar ice cap are testimony to the incessant wind action
occurring on Mars.
'
.'.J**'
particles
have been discovered surrounding the north (Fig. 3). The origin of these dunes is still
polar ice cap 2 A planetary dust storm obscured Mariner view of the Martian surface for the first few weeks after went into orbit around Mars in 1971.
-^FIGURE 9's it
most of the debris on the northern plains and the dunes themselves consist of material eroded from the polar deposits. When the deposits of dust-sized particles were removed by the wind, the sand-sized particles were left behind and were transported by saltation to form controversial. Geologists think that
dunes.
clay that are deposited over large areas
commonly
far
from
downwind and
their source.
The Formation and Migration of Dunes The most
characteristic features associated with sand-
covered regions are dunes, which are
mounds
or ridges
Dunes form when the wind must flow over and around an obstruction. This results of wind-deposited sand.
*» FIGURE
19-8
A
deflation hollow in
Death
Valley,
California.
Wind
Deposits
553
Desert pavement ends)
(deflation
res
i1^"5^e»
*o
"o
<=>„'"5=£»
^r^
5
2k* <•• x7 "% <2k*<»*^
^ FIGURE
19-9 (a) Desert pavement forms when removes fine-grained material from the ground surface leaving behind larger-sized particles, (b) As deflation continues and more material is removed, the larger particles are concentrated and form a desert pavement, which protects the underlying material from additional deflation. (c) Desert pavement in the Mojave Desert, California. Several ventifacts can also be seen in the lower left of the photograph. (Photo courtesy of David J. Matty.) deflation
and force it to deposit any sand it carries. Most dunes have an asymmetrical profile, with a gentle windward slope and a steeper downwind or leeward
ers that reduce the wind's velocity
slope that
is
inclined in the direction of the prevailing
wind (Fig. 19-1 la). Sand grains move up the gentle windward slope by saltation and accumulate on the leeward side forming an angle between 30° and 34° from the horizontal, which is the angle of repose of dry sand.
When
this angle is
exceeded by accumulating sand, the down the leeward
slope collapses, and the sand slides
coming to rest at its base. Over time, as sand moves from a dune's windward side and periodically slides down its leeward slope, the dune slowly migrates in the prevailing wind direction (Fig. 19-1 lb). When slope,
preserved in the rock record, dunes help geologists de-
termine the prevailing direction of ancient winds 19-12).
dunes,
The expansion of is
a
deserts, in part
major problem because
it is
(Fig.
by migrating
destroying mil-
lions of acres of agricultural land (see the Prologue).
two zones of quiet air, called wind shadows, that form immediately in front of and behind the obstruction (Fig. 19-10). As saltating sand grains settle in these wind shadows, they begin to accumulate and build up a deposit of sand. As they grow, these sand deposits become self-generating in that they form ever-larger wind barriin
554
Chapter 19
The Work of Wind and Deserts
Dune Types Four major dune types are generally recognized {barchan, longitudinal, transverse, and parabolic), although intermediate forms
between the major types also
exist.
The
size,
Wind
Aerial view
(a)
Profile
Direction of
view
dune migration (a)
(b)
"^ FIGURE
19-11 (a) Profile view of a sand dune. Dunes migrate when sand moves up the windward side and slides down the leeward slope. Such movement of the sand grains produces a series of inclined beds that slope in the direction of wind movement. (b)
Aerial view
Profile
Longitudinal dunes (also called seif dunes) are long, sand aligned generally parallel to the
parallel ridges of
view
(b)
mr FIGURE
19-10
When wind
(a)
direction of the prevailing winds; they
form where the
"^ FIGURE
sandstone
flows around an
two wind shadows form, one in front of the and the other behind it. Sand accumulates in both wind shadows, (b) The accumulating sand forms a
obstacle,
obstacle
of these
may
mound
that
shape,
and arrangement of dunes
develop into a dune.
19-12
Cross-bedding
in this
in
Zion National Park, Utah, helps geologists determine the prevailing direction of wind that formed these ancient sand dunes.
result
from the
interac-
many factors, including sand supply, the direction and velocity of the prevailing wind, and the amount of vegetation. While dunes are usually found in deserts, they can also occur wherever there is an abundance of sand tion of
such as along the upper parts of
many
beaches.
Barchan dunes are crescent-shaped dunes whose tips point downwind (Fig. 19-13). They form in areas where there
is
a generally
flat,
dry surface with
little
vegetation,
a limited supply of sand,
and a nearly constant wind
Most barchans
are small, with the largest
direction.
m
Barchans are the most mobile of the major dune types, moving at rates that can exceed 10 per year. reaching about 30
in height.
m
Wind
Deposits
555
(b) ""*'
(b)
FIGURE
Barchan dunes form where there
FIGURE
and a generally flat, dry surface with little vegetation. The barchan dunes point downwind, (b) Several barchan dunes west of the Salton Sea, California.
19-14 (a) Longitudinal dunes form long, sand aligned roughly parallel to the prevailing wind direction. They typically form where sand supplies are limited, (b) Aerial view of the great seif dune field near Glamis, southern California.
sand supply is somewhat limited (Fig. 19-14). Longitudinal dunes result when winds converge from slightly different directions to produce the prevailing wind. They range in size from about 3 m to more than 100 m
dune field where there is less sand. Such intermediateform dunes are known as barchanoid dunes (Fig. 19-16). Parabolic dunes are most common in coastal areas with abundant sand, strong onshore winds, and a partial
and some stretch for more than 100 km. These dunes are especially well developed in central Australia,
(Fig. 19-17). Although parabolic dunes have a crescent shape like barchan dunes, their tips point upwind. Parabolic dunes form when the vegetation cover is broken and deflation produces a blowout. As the wind transports the sand out of the depression, it builds up on the convex downwind dune crest. The central part
a limited
19-13
{a)
amount of
sand, a nearly constant
wind
is
direction,
tips of
high,
where they cover nearly one-fourth of the continent. They also cover extensive areas in Saudi Arabia, Egypt, and Iran. Transverse dunes form long ridges perpendicular to the prevailing wind direction in areas where abundant sand is available and little or no vegetation exists (Fig. 19-15). When viewed from the air, transverse dunes have a wavelike appearance with crests and troughs and are therefore sometimes called sand seas. The crests of transverse dunes can be as high as 200 m, and the dunes
may
be as
much
as 3
km
wide.
Some
transverse dunes
develop a clearly distinguishable barchan form and
may
separate into individual barchan dunes along the edges of
556
Chapter 19
The Work of Wind and Deserts
""•*"
parallel ridges of
the
cover of vegetation
of the dune
is
excavated by the wind, while vegetation
holds the ends and sides fairly well in place.
Loess
Windblown
silt
and clay deposits composed of angular
quartz grains, feldspar, micas, and calcite are loess.
The
distribution of loess
from three main sources:
shows that
known
it is
as
derived
deserts, Pleistocene glacial out-
central Asia are the source for this loess.
Other important on the North European Plain from Belgium eastward to the Ukraine, Central Asia, and the Pampas of Argentina. In the United States, they occur in the Great Plains, the Midwest, the Mississippi River Valley, and eastern Washington (see Perspective 18-1). loess deposits are
"•" FIGURE 19-16 Barchanoid dunes at White Sands National Monument, New Mexico.
(b)
-^ FIGURE
19-15
Transverse dunes form long ridges
(a)
of sand that are perpendicular to the prevailing
wind
sand, (b)
or no vegetation and abundant Aerial view of transverse dunes, Great Sand Dunes
National
Monument, Colorado.
direction in areas of
wash
and the floodplains of rivers in semiarid must be stabilized by moisture and vegetation order to accumulate. Consequently, loess is not found deserts, even though they provide much of its matedeposits,
regions. in in
little
rial.
It
Because of
its
unconsolidated nature, loess
is
easily
eroded, and as a result, eroded loess areas are characterized
by steep
stream erosion
At present,
cliffs
and rapid
19-19).
The most
and
30
30%
10% of
of the United States
extensive and thickest loess de-
posits occur in northeast
greater than
and headward
loess deposits cover approximately
the Earth's land surface (Fig.
lateral
(Fig. 19-18).
China where accumulations
m are common. The extensive deserts in Wind
Deposits
557
—
FIGURE 19-17 dunes typically form where there
is
(a)
Parabolic
in coastal areas
a partial cover of
vegetation, a strong onshore wind,
and abundant sand, (b) Parabolic dune developed along the Lake Michigan shoreline, west of St. Ignace, Michigan.
Loess-derived soils are tile. It is
some of
the world's
most
fer-
therefore not surprising that the world's major
Earth's atmospheric circulation patterns. Air pressure the density of air exerted
grain-producing regions correspond to the distribution
weight).
of large loess deposits such as the North European Plain,
ing
the Ukraine,
and the Great Plains of the United
States.
GLOBAL WIND PATTERNS To understand deserts,
the
we need
work of wind and
the distribution of
to consider the global pattern of air
pressure belts and winds, which are responsible for the
558
Chapter 19
The Work of Wind and Deserts
When
mass
air
is
its
surroundings (that
is
is, its
it expands and rises, reducvolume and causing a decrease in
heated,
for a given
air pressure.
and
^ AIR PRESSURE BELTS AND
its
on
Conversely,
when
air
is
cooled,
it
contracts
air pressure increases. Therefore, those areas of the
Earth's surface that receive the
most
solar radiation,
such as the equatorial regions, have low air pressure, while the colder areas, such as the polar regions, have high air pressure. Air flows from high-pressure zones to low-pressure zones.
If
the Earth did not rotate,
winds would move
in
^ FIGURE River,
Yukon
19-18
These steep banks along the Yukon Canada are formed of loess.
Territory,
a straight line
from one zone to another. Because the
Earth rotates, however, winds are deflected to the right of their direction of motion (clockwise) in the Northern
Hemisphere and
to the left of their direction of
motion
(counterclockwise) in the Southern Hemisphere. Such a deflection of air
between latitudinal zones resulting from
the Earth's rotation
is
known
as the Coriolis effect.
Therefore, the combination of latitudinal pressure differences
and the Coriolis
effect
pattern of east- west-oriented
The energy, the air
produces a worldwide
wind
belts (Fig. 19-20).
most solar which heats the surface air, causing it to rise. As rises, it cools and releases moisture that falls as Earth's equatorial zone receives the
rain in the equatorial region (Fig. 19-20).
The
19-19
The
(Fig. 19-21).
rising air
is now much drier as it moves northward and southward toward each pole. By the time it reaches 20° to 30° north and south latitude, the air has become cooler and denser and begins to descend. Compression of the atmosphere warms the descending air mass and produces a warm, dry, high-pressure area, providing the perfect
FIGURE
conditions for the formation of the low-latitude deserts
of the Northern and Southern hemispheres
distribution of the Earth's
^ THE DISTRIBUTION OF DESERTS Dry climates occur
in the
low and middle
latitudes. In
these climates, the potential loss of water by evaporation exceeds the yearly precipitation (Fig. 19-21).
climates cover
major loess-covered
30%
Dry
of the Earth's land surface and are
areas.
3000 Km/
The
New Zealand
Distribution of Deserts
559
Subsiding
'J
air
c
Polar easterlies
Westerlies
Southeast trade winds
Westerlies
Polar easterlies
FIGURE
The general
19-20
circulation of the Earth's atmosphere.
re-
bian Desert in the Middle East, along with the majority
precipitation than arid regions, yet
of Pakistan and western India form the largest essen-
subdivided into semiarid and arid regions. Semiarid gions receive
more
are moderately dry. Their soils are usually well devel-
oped and
fertile
and support a natural grass cover. Arid
regions, generally described as deserts, are very dry; they receive less than
cm
of rain per year, typically have
soils,
and are mostly or completely
25
poorly developed
unbroken desert environment
Hemisphere.
More than 40%
most of the
rest
is
semiarid.
in
the Northern
of Australia It is
is
desert,
no wonder that
and it is
called the "desert continent" (Fig. 19-22).
The remaining dry
climates of the world are found in
the middle and high latitudes, mostly within continental
devoid of vegetation.
The majority of
tially
the world's deserts are found in the
interiors in the
Northern Hemisphere
(Fig.
19-21).
Many
(Fig. 19-
of these areas are dry due to their remoteness from moist
North America, most of the southwestern United and northern Mexico are characterized by this hot, dry climate, while in South America this climate is primarily restricted to the Atacama Desert of coastal Chile and Peru. The Sahara in Northern Africa, the Ara-
maritime air and the presence of mountain ranges that produce a rainshadow desert (Fig. 19-23). When moist marine air moves inland and meets a mountain range, it is forced upward. As it rises, it cools, forming clouds and
dry climates of the low and middle latitudes 21). In States
560
Chapter 19
The Work of Wind and Deserts
producing precipitation that
falls
on the windward
side
—' FIGURE 19-21
The
distribution of the Earth's arid
and
semiarid regions.
The air that descends on the leeward mountain range is much warmer and drier, producing a rainshadow desert. Three widely separated areas are included within the of the mountains.
Temperature, Precipitation, and Vegetation
side of the
The
mid-latitude dry climate zone (Fig. 19-21). of these
is
largest
the central part of Eurasia extending from just
north of the Black Sea eastward to north-central China.
The Gobi Desert in China is the largest desert in this region. The Great Basin area of North America is the second largest mid-latitude dry climate zone and results from the rainshadow produced by the Sierra Nevada (see Perspective 19-2). This region adjoins the southwestern deserts of the United States that
formed as
of the mid-latitude dry climate areas
is
from the rainshadow
effect of the
The remainder of the world's
The
small-
the Patagonian
region of southern and western Argentina. results
deserts are well
known. Many
of the deserts of the low latitudes have average summer temperatures that range between 32° and 38°C for sev-
months. It is not uncommon for some low-elevation inland deserts to record daytime highs of 46° to 50°C for
eral
weeks at a time. The highest temperature ever recorded was 58°C in El Azizia, Libya, on September 13, 1922. During the winter months when the angle of the Sun is lower and there are fewer daylight hours, daytime temperatures average between 10° and 18°C. Winter
a result of the
low-latitude subtropical high-pressure zone. est
The heat and dryness of
Its
dryness
found
19-22
The Nullarbor
Plain
is
one of the
larger desert regions of Australia, a continent that
than
Andes.
deserts are
^ FIGURE 40%
is
more
desert.
in the
cold, but dry high latitudes, such as Antarctica.
=»
CHARACTERISTICS OF DESERTS
To people who live in humid regions, deserts may seem stark and inhospitable. Instead of a landscape of rolling hills and gentle slopes with an almost continuous cover of vegetation, deserts are dry, have little vegetation, and consist of nearly continuous rock exposures or sand
dunes. deserts cesses
And yet despite the great contrast between and more humid areas, the same geologic proare at work, only operating under different cli-
matic conditions.
Characteristics of Deserts
561
Perspective 19-2
DEATH VALLEY NATIONAL MONUMENT Death Valley National Monument was established in 2 1933 and encompasses 7,700 km of southeastern California and part of western Nevada (Fig. 1). The hottest, driest, and lowest of the U.S. National Monuments and Parks, it receives less than 5 cm of rain per year and features normal daytime summer temperatures above 42°C. The highest temperature
was 57°C in the shade! The topographic Death Valley is impressive. Telescope Peak
ever recorded relief in
near the southwestern border
is
the lowest point in the Western
below sea level— is
less
than 32
3,368
m
high, while
Hemisphere— 86
km
m
to the east at
Badwater.
T FIGURE
1
Death Valley
National Monument, California, encompasses 7,700 km 2 of
Chapter 19
The Work of Wind and Deserts
its
bordering mountains
wide variety of and economically valuable evaporite deposits. In addition, numerous folds, faults, landslides, and considerable evidence of volcanic desert landforms
activity
can be seen.
The geologic history of Death Valley is complex and still being worked out, but rocks from every geologic era can be found in the valley or the
surrounding mountains. Although the geologic history of the region reaches back to the Precambrian, Death itself formed less than 4 million years ago. Death Valley formed during the Pliocene Epoch,
Valley
Anvil Spring
Owlshead
southeastern California and part of western Nevada.
562
Within Death Valley and
are found excellent examples of a
Wingate Wash
Canyon
Warmspring Canyon
?•* FIGURE 2 Ubehebe Crater, an explosion crater, erupted approximately 2,000 years ago.
last
^T FIGURE ridges
when
the Earth's crust
was
and pinnacles making
Borax Works was home
stretched and rifted,
it
very
difficult to traverse.
famous 20-mule teams wagons of borax (Fig. 4).
to the
that hauled out countless
forming horsts and grabens. Great blocks of rock were rotated and tilted along normal faults, and
The borax, used for ceramic glazes, fertilizers, glass, solder, and pharmaceuticals, was leached from volcanic ash by hot groundwater and then
Death Valley was further widened along various strike-slip faults. As faulting continued, streams carried tremendous amounts of sediments into the
accumulated
in layers of lake sediment.
Besides the
valley.
During the Pleistocene Epoch, when the climate of this region was more humid than it is today, numerous pluvial lakes spread over the valley (see Chapter 18). Lake Manly, the largest of these pluvial lakes (145 km long and 178 m deep), dried up about 10,000 years ago, when the climate became arid. Volcanic activity has been occurring during the
Course consists of a layer of formed a network of polygonal
Devil's Golf
3
solid rock salt that has
made Death
it is
features that have
also
home
to
more
than 600 species of plants as well as numerous animals.
last "rmr
thousand years. The most famous volcanic feature in Death Valley is Ubehebe Crater, an explosion crater that formed approximately 2,000 years ago (Fig. 2). Death Valley continues to subside along normal faults; it is sinking most rapidly along its western side. The movement has been so great along the normal faults that more than 3,000 m of sediments resulting from erosion are beneath the several
numerous geologic
Valley famous,
FIGURE
Death
4
Twenty-mule teams carried borax out of
Valley.
present valley floor. Furthermore, geologists estimate that at least 6,700
m
of total vertical
movement has
occurred on the faults that formed Death Valley. In addition to the usual desert features, Death Valley also includes
some unusual ones such
as the
Devil's Golf Course, a bed of solid rock salt
displaying polygonal ridges and pinnacles that are almost impossible to traverse (Fig. 3). The Harmony
*&fg&S8to
&&£, *&*#'**&*
Characteristics of Deserts
563
"^ FIGURE
19-23 Many deserts middle and high latitudes are rainshadow deserts, so named because they form behind mountain ranges. When moist marine air moving inland meets a mountain in the
range, it is forced upward where it cools and forms clouds that produce rain. This rain falls on the
windward side of the mountain. The air descending on the leeward side is much warmer and drier, producing a rainshadow
desert.
nighttime lows can be quite cold, however, with frost
plants have a widespread shallow root system to absorb
and freezing temperatures common in the more poleward deserts. Winter daily temperature fluctuations in
the
low-latitude deserts are
among
the greatest in the world,
dew
deserts
that forms each
and
may
there
to help
be. In addition,
for finding water far
known
many
from below 0°C
to
more than 38°C
and spring
in a single day!
The dryness of the rily
plants
low-latitude deserts results prima-
tiful
to
in all
some
in
but the driest
what
little soil
plants have deep roots
below the surface. In extreme cases, dormant during particularly dry years life after the first rain shower with a beau-
ranging between 18° and 35°C. Temperatures have been to fluctuate
morning
anchor the plant
lie
profusion of flowers.
from the year-round dominance of the subtropical
high-pressure belt, while the dryness of the mid-latitude is due to their isolation from moist marine winds and the rainshadow effect created by mountain ranges. The dryness of both is further accentuated by their high
Weathering and
Soils
deserts
Mechanical weathering
is
dominant
in
temperatures.
primary forms of mechanical weathering
Although deserts are defined as regions receiving, on average, less than 25 cm of rain per year, the amount of rain that falls each year is very unpredictable and unreliable. It is not uncommon for an area to receive more than an entire year's average rainfall in one cloudburst and then to receive very little rain for several years. Thus, yearly rainfall averages can be quite misleading. Deserts display a wide variety of vegetation (Fig. 1924). While the most arid deserts, or those with large
The breakdown of rocks by
areas of shifting sand, are almost devoid of vegetation, deserts support at least a sparse plant cover. Compared to humid areas, desert vegetation may appear monotonous. A closer examination, however, reveals an amazing diversity of plants that have evolved the ability
most
to live in the near-absence of water.
Desert plants are widely spaced, typically small, and
have low growth ally
tion
564
Their stems and leaves are usuto minimize water loss by evapora-
rates.
hard and waxy
and protect the plant from sand erosion. Most
Chapter 19
The Work of Wind and Deserts
desert regions.
Daily temperature fluctuations and frost wedging are the
roots
(see
and from
Chapter
6).
salt crystal
growth are of minor importance. Some chemical weathering does occur, but its rate is greatly reduced by aridity and the scarcity of organic acids produced by the sparse vegetation.
Most chemical weathering occurs during the winter there is more precipitation, particularly in
months when
the mid-latitude deserts.
An
interesting feature seen in
many
deserts
is
a thin,
brown, or black shiny coating on the surface of many rocks. This coating, called rock varnish, is composed of iron and manganese oxides (Fig. 19-25). Because many of the varnished rocks contain little or no iron and manganese oxides, the varnish is thought to result from either windblown iron and manganese dust that settles on the ground or from the precipitated waste red,
of microorganisms.
Desert soils, if developed, are usually thin and patchy because the limited rainfall and the resultant scarcity of vegetation reduce the efficiency of chemical weathering
"^ FIGURE 19-24 Tucson, Arizona. Desert vegetation is typically sparse, widely spaced, and characterized by slow growth rates. Cacti are an excellent example of the type of vegetation that has adapted to the harsh desert environment. (Photo courtesy of B. M. C. Pape.) and hence
soil
formation. Furthermore, the sparseness
While water
wind and water erosion
day, recall that
of the vegetative cover enhances of
what
little soil
Pleistocene
actually forms.
the major erosive agent in deserts to-
was even more important during the Epoch when these regions were more humid it
(see Chapter 18). During that time, many of the major topographic features of deserts were forming. Today
Mass Wasting, Streams, and Groundwater When
is
that topography
most people are impressed by the work of wind in the form of moving sand, sand dunes, and sand and dust storms. They may also notice the dry washes and dry stream beds. Because of the lack of running water, most people would conclude that wind is the most important erosional geologic agent in deserts. They would be wrong. Running water, even though it occurs infrequently, causes most of the erosion in deserts. The dry conditions and sparse vegetation characteristic of deserts enhance water erosion. If you look closely, you will see the evidence of erosion and traveling through a desert,
is
being modified by wind and infre-
quently flowing streams.
Most
desert streams are poorly integrated
only intermittently.
Many
because the water table
"^ FIGURE exposed
19-25
is
usually far deeper than the
The shiny black coating on
at Castle Valley,
and flow
of them never reach the sea
Utah,
is
composed of iron and manganese
rock varnish.
this
rock
It is
oxides.
transportation by running water nearly everywhere. Recall that or less
comes
annual
25 cm
most of
a desert's
in brief,
heavy, localized cloudbursts. Dur-
rainfall of
ing these times, considerable erosion occurs because the
ground cannot absorb vegetation to hinder
all
its
of the rainwater. With so
flow, runoff
is
little
rapid, especially
on moderately to steeply sloping surfaces, resulting in flash floods and sheetflows. Dry stream channels quickly fill with raging torrents of muddy water and mudflows, which carve out steep-sided gullies and overflow their banks. During these times, a tremendous amount of sediment is rapidly transported and deposited far downstream.
Characteristics of Deserts
565
orado River are leading to increased salt concentrations in its lower reaches and causing political problems between the United States and Mexico. The water table in most desert regions is below the stream channels and is only recharged for a short time after a rainfall. In deserts
with through-flowing streams,
the water table slopes
away from
The
the streams.
through-flowing streams help to recharge the ground-
water supply and can support vegetation along their banks. Trees, which have high moisture requirements, are rare in deserts, but may occasionally occur along the banks of both ephemeral and permanent streams, where their roots
can reach the higher water table.
Wind Although running water does most of the erosional work in deserts, wind can be an effective geologic agent and is capable of producing a variety of distinctive erosional and depositional features. Wind is an important geologic agent in deserts and is very effective in transporting and depositing unconsolidated sand, silt, and dust-sized particles. Contrary to popular belief, however, most deserts are not sand-covered wastelands, but rather consist of vast areas of rock exposures. Sand-
covered regions, or sandy deserts, constitute
25%
of the world's deserts.
The sand
less
than
in these areas
has
accumulated primarily by the action of wind.
^ DESERT LANDFORMS
(b)
"^ FIGURE
19-26 {a) Playa lake formed after a rainstorm filled Croneis Dry Lake, Mojave Desert, California. (b) Racetrack Playa, Death Valley, California. Inyo Mountains can be seen in the background.
Because of differences in temperature, precipitation, and wind, as well as the underlying rocks and recent tectonic events, the landforms in arid regions vary considerably.
Although wind
many channels of most streams, so they cannot draw upon
is
distinctive
an important geologic agent in deserts, landforms are produced and modified
by running water.
and
After an infrequent and particularly intense rain-
absorption into the ground. This type of drainage in
is not absorbed by the ground low areas and form playa lakes (Fig. 19-26a). Such lakes are very temporary, lasting from a few hours to several months. Most of them are very shallow and have rapidly shifting boundaries as water flows in or leaves by evaporation and seepage into the ground.
groundwater
which
to replace water lost to evaporation
a stream's load
is
called internal drainage
deposited within the desert
and
is
common
in
most
is
arid
regions.
While the majority of deserts have internal drainage, deserts have permanent through-flowing streams such as the Nile and Niger rivers in Africa, the Rio Grande and Colorado rivers in the southwestern United States, and the Indus River in Asia. These streams are able to flow through the desert region because their headwaters are well outside the desert and water is plentiful enough to offset losses resulting from evaporation and infiltration. However, demands for greater amounts of water for agriculture and domestic use from the Col-
some
566
Chapter 19
The Work of Wind and Deserts
storm, excess water that
may accumulate
in
Furthermore, the water
When
in
playa lakes
is
often very saline.
a playa lake evaporates, the dry lake bed
called a playa or salt
pan and
is
characterized by
is
mud-
cracks and precipitated salt crystals (Fig. 19-26b). Salts in
some playas
cially.
are thick
enough
mined commerin Death hundred years (see
to be
For example, borates have been mined
Valley, California, for
Perspective 19-2).
more than
a
— FIGURE an
19-27 Aerial view of Death Valley,
alluvial fan,
California.
Other common features of deserts, particularly in the Basin and Range region of the United States (Fig. 1431), are alluvial fans after a cloudburst,
and bajadas. Alluvial fans form
when sediment-laden streams
flow-
from the generally straight, steep mountain fronts deposit their load on the relatively flat desert floor. Because there are no valley walls to contain it, the ing out
sediment spreads out laterally, forming a gently sloping and poorly sorted fan-shaped sedimentary deposit (Fig.
19-27). Alluvial fans are similar in origin and shape to
Chapter 16) but are formed entirely on land. may coalesce to form a bajada. This broad alluvial apron typically has an undulating surface resulting from the overlap of adjacent fans (Fig. 19-28). Large alluvial fans and bajadas are frequently important sources of groundwater for domestic and agricul-
deltas (see
Alluvial fans
tural use. Their outer portions are typically
composed of
fine-grained sediments suitable for cultivation, and their
^ FIGURE
19-28 Coalescing forming a bajada at the base of the Black Mountains, Death
alluvial fans
Valley, California.
Desert Landforms
567
Barchan dunev
|
^ FIGURE
19-29 (a) Pediments bedrock surfaces formed by erosion along a mountain front, (b) Pediment north of Mesquite, Nevada. are erosional
good drainage of water. Many alluand bajadas are also the sites of large towns and cities, such as San Bernardino, California, Salt Lake City, Utah, and Teheran, Iran. Most mountains in desert regions, including those of the Basin and Range region, rise abruptly from gently
gentle slopes allow vial fans
sloping surfaces called pediments. Pediments are erosional bedrock surfaces of
low
away from mountain bases ments are covered by
relief that
(Fig.
19-29).
slope gently
Most
a thin layer of debris or
pedi-
by alluvial
fans or bajadas.
The
origin of pediments has been the subject of
controversy.
Most
sional features developed
568
Chapter 19
much
geologists agree that they are ero-
on bedrock
in association
The Work of Wind and Deserts
with
the erosion
and
retreat of a
mountain front
(Fig. 19-29a).
The disagreement concerns how the erosion has occurred. While not all geologists would agree, it appears that pediments are produced by the combined erosional activities of lateral
erosion by streams, sheet flooding,
and various weathering processes along the retreating mountain front. Thus, pediments grow at the expense of the mountain, and they will continue to expand as the mountain is eroded away or partially buried. Rising conspicuously above the flat plains of many deserts are isolated steep-sided erosional remnants called inselbergs, a German word meaning "island mountain" (Fig. 19-30). Inselbergs
have survived for a longer period
of time than other mountains because of their greater
4.
which effectively protects the underlying surface from additional deflation. The two major deposits of wind are dunes and loess.
Dunes
are
whereas 5.
mounds or
loess
is
The four major dune
silt
and
and parabolic. The amount the prevailing wind direction, the
and the amount of vegetation present determine which type will form. Loess is derived from deserts, Pleistocene glacial outwash deposits, and river floodplains in semiarid
wind
6.
Earth's land surface
productive
and weathers
10%
of the
to a rich
and
(less
than 25
soils,
cm
rain/year),
and are mostly or
completely devoid of vegetation. The winds of the major east-west—oriented air pressure belts resulting from rising and cooling
activities
of lateral erosion by streams, sheet
flooding,
and various weathering processes.
Inselbergs are isolated steep-sided erosional remnants that rise above the surrounding desert plains. Buttes
and mesas
^ IMPORTANT
11.
low and middle water by evaporation exceeds the yearly precipitation. Dry climates cover 30% of the Earth's surface and are subdivided into semiarid and arid regions. The majority of the world's deserts are in the low-latitude dry climate zone between 20° and 30° north and south latitudes. Their dry climate results from a high-pressure belt of descending dry air. The remaining deserts are in the middle latitudes where their distribution is related to the rainshadow effect and in the dry polar regions.
abrasion
inselberg
alluvial fan
internal drainage
bajada barchan dune barchanoid dune
loess
butte
parabolic dune
where the potential
Coriolis effect
pediment
deflation
playa
deflation
hollow
playa lake
rainshadow desert
desert
loss of
desert
pavement
rock varnish
desertification
transverse dune
dry climate
ventifact
dune
yardang
REVIEW 1.
can be found
a.
2.
12.
Pleistocene
Wind
Epoch when
pluvial climates resulted in
conditions. is
of minor importance as an erosional agent
in deserts,
but
is
very effective in transporting and
Between what
answers (b)
and
3.
suspension; b.
d.
precipitation;
filled
with
5.
Which
suspended load?
poorly sorted, fan-shaped sedimentary deposits that
a.
may
(a)
Chapter 19
The Work of Wind and Deserts
abrasion; e.
answers
particle size constitutes
water, they form playa lakes. Alluvial fans are
coalesce to form bajadas.
and
(c);
both hemispheres; d. only to the right for both hemispheres; e. not at all. 4. The primary process by which bed load is transported is: a.
when temporarily
(a)
(c).
do the world occur? 30° 10° and 20°; b 20° and 30°; c a 40° and 60°; e 60° and 80°. and 40°; d The Coriolis effect causes wind to be deflected: a. to the right in the Northern Hemisphere and the left in the Southern Hemisphere; b. to the left in the Northern Hemisphere and the right in the Southern Hemisphere; c. only to the left for
which
are dry lake beds;
of rain per
latitudes in both hemispheres
depositing unconsolidated fine-grained sediments. 15. Important desert landforms include playas,
cm
driest deserts in the
scarce.
Mechanical weathering is the dominant form of weathering in deserts. The sparse precipitation and slow rates of chemical weathering result in poorly developed soils. 13. Running water is the dominant agent of erosion in deserts and was even more important during the
answers
e.
it
middle, and high
are mostly or completely devoid of
vegetation; d.
c.
in the low,
receive less than 10
latitudes; b.
rates.
when
QUESTIONS
Deserts:
and high evaporation
Furthermore, rainfall is does occur, tends to be very intense and of short duration. As a consequence of such aridity, desert vegetation and animals are
570
mesa
year;
unpredictable and,
14.
longitudinal dune
Deserts are characterized by lack of precipitation
humid
and
TERMS
climates are located in the
latitudes
10.
are, respectively, pinnacle-like
flat-topped erosional remnants with steep sides.
control the world's climate.
Dry
controversial, although
air
are deflected by the Coriolis effect. These belts help
9.
The most that they form by the combined is
bases.
soil.
Deserts are very dry
have poorly developed 8.
17.
velocity,
regions. Loess covers approximately
7.
geologists believe
types are barchan,
away from mountain
sloping
origin of pediments
clay.
longitudinal, transverse,
of sand available,
relief gently
ridges of wind-deposited sand,
wind-deposited
low
16. Pediments are ert>sional bedrock surfaces of
sand; b.
and
(b); e.
(a)
most of clay; d.
silt; c.
answers
(b)
and
saltation;
c.
(c).
and a
(c).
wind's
answers
Which of the following wind erosion? a.
playa; b.
d.
yardang;
What
is
produced by
a feature
is
17. Describe
dune;
loess; c.
18.
none of these. the approximate angle of repose e.
for dry
19.
45°;
20.
sand? a
15°; b
e
55°.
tips point
a.
barchan;
d.
transverse;
of the following
Which of
10.
25°; c
Which whose
35°; d
a crescent-shaped
is
dune
21.
downwind? longitudinal;
b.
parabolic;
c.
barchanoid.
e.
22. 23.
the following dunes form long ridges of
24.
prevailing wind?
25.
What What
parabolic;
26.
Why
loess
27.
a.
barchan;
transverse;
Where
longitudinal;
b.
c.
low
barchanoid.
e.
and most extensive
are the thickest
deposits in the world? a.
United States;
c.
Belgium;
What
Pampas of Argentina;
b.
Ukraine;
d.
28.
northeast
e.
low-latitude deserts? isolation from moist dominance of the subtropical
effect; b.
Coriolis effect;
all
e.
desert vegetation
,
soils
mechanical,
b.
chemical,
e.
The major agent of erosion ,
in deserts
today
is
while during the Pleistocene Epoch
it
was
wind, wind; wind, running water; b. running water, running water, wind; d running water; e. wind, glaciers. 14. The dry lake beds in many deserts are: a.
d.
15.
An
bajadas;
playas; b.
inselbergs;
is:
a
alluvial fans; b
c
bajadas; d.
e.
16.
c.
mesas. pediments; e. important source of groundwater for domestic
and agricultural use
What
answers are
some
(a)
Why
dominant form of weathering and why is it so effective? the
in desert
are deserts characterized by internal drainage? role does
groundwater play
in this
type of
and
and describe
how
they form.
ADDITIONAL
READINGS
Agnew, C, and A. Warren. 1990. Sand trap. The Sciences March/April: 14-19. Brookfield, M. E., and T. S. Ahlbrandt. 1983. Eolian sediments and processes. New York: Elsevier Publishers.
W. S. 1987. Africa's Sahel: The stricken land. National Geographic 172, no. 2: 140-79. and J. Iversen. 1985. Wind as a geologic process. Cambridge, Mass.: Cambridge University Press. Hunt, C. B. 1975. Death Valley: Geology, ecology, archaeology. Greeley, R.,
c.
a.
is
Ellis,
diverse, thin. 13.
What
30. Explain the difference between a butte and a mesa,
^
is
and
mechanical, limited, thin;
c.
chemical, diverse, thick;
d.
in deserts
is
mechanical, limited, thick;
a.
diverse, thin;
and vegetation
of
these.
The dominant form of weathering ,
are temperature, precipitation,
drainage?
high-pressure belt; d.
12.
latitudes?
How
What
the primary cause of the dryness of
rainshadow a. marine winds; c.
loess
is
regions,
29.
is
and why is it important? meant by arid and semiarid climate? are most of the world's deserts located in the is
interrelated in desert environments?
China. 11.
the global distribution of air pressure
conditions necessary for their formation.
sand aligned roughly parallel to the direction of the
d.
how
and winds operates. What are the two ways that sediments are transported by wind? Describe the two ways that wind erodes. How effective an erosional agent is wind? What is the difference between a ventifact and yardang? How do both form? Why is desert pavement important in a desert environment? How do sand dunes migrate? Describe the four major dune types and the belts
playa lakes; answers (a) and (b);
Berkeley, Calif.: University of California Press.
Sheridan, D. 1981. Desertification of the United States. Washington, D.C.: Council on Environmental Quality.
Thomas, D.
S.
G., ed. 1989. Arid zone geomorphology.
New
York: Halsted Press. S. 1982. Deserts of China. American Scientist 70, 366-76. Wells, S. G., and D. R. Haragan. 1983. Origin and evolution of deserts. Albuquerque, N. Mex.: University of New Mexico
Walker, A. no. 4:
Press.
Whitney, M. A. 1985. Yardangs. Journal of Geological Education 33, no. 2: 93-96.
(c).
of the problems associated with
desertification?
Additional Readings
571
CHAPTER
20
SHORELINES AND SHORELINE PROCESSES ^ OUTLINE PROLOGUE INTRODUCTION WAVE DYNAMICS Wave Generation Guest Essay: Geophysics and the Search for Oil
Shallow-Water Waves and Breakers
NEARSHORE CURRENTS ^Perspective 20-1: Waves and Coastal Flooding
Wave
Refraction and Longshore Currents
Rip Currents
SHORELINE DEPOSITION Beaches Seasonal Changes in Beaches Spits
and Baymouth Bars
Barrier Islands
The Nearshore Sediment Budget
SHORELINE EROSION T' Perspective Coastal
20-2: Rising Sea Level and
Management
Wave-Cut Platforms and Associated Landforms
TYPES OF COASTS Submergent and Emergent Coasts
TIDES
CHAPTER SUMMARY
View of
the Pacific shoreline at Ecola State Park, Oregon.
^^'T^'^^^m m^ ^^^^'s is^jiJ*^^^ r'
.
PROLOGUE ^|^i\^jj|
i
n 1900, Galveston, Texas, was a busy
port city of 38,000 located on
Galveston Island, a long, narrow barrier island a short distance from the mainland.
On
September
8, a
hurricane swept in from the Caribbean, destroying
much
of the city and killing between 6,000 and 8,000
of Galveston's residents in the greatest natural disaster in U.S. history. When the hurricane struck, storm waves surged inland, eventually covering the entire island. Buildings and other structures near the shoreline were battered to pieces, and "great beams and railway ties were lifted by the [waves] and driven like battering rams into dwellings and business
"^" FIGURE 20-1 Construction of this seawall to protect Galveston, Texas from storm waves began in 1902.
houses"* farther inland. Finally, after the first four shoreline blocks were destroyed, the debris piled up high enough to form a protective barrier for the rest of the
city.
At about 10:00 p.m., the wind suddenly died down and soon thereafter the water began to subside. The next morning was calm and clear, but the city was in utter ruins; property
damage was estimated
at
more
than $20 million, and at least 15% of the city's killed. Hurricanes had swept
population had been
through Galveston before, some of them causing damage and deaths, and the residents were aware of how vulnerable the city was. The highest part of the island was only 2.7 m above mean low tide; thus
storm waves could sweep across the entire island. In order to protect the city from future hurricanes, a colossal two-part project was begun in 1902. First, a seawall 5.6 km long was constructed along the side of the city facing the shore (the south); with assistance, the seawall
km
(Fig. 20-1).
m
successful in doing so. However, the seawall alone would not prevent the city from flooding during storms. To protect against this hazard, the second part of the project had to be completed. This entailed filling
the area behind the seawall with sand
and
raising parts of the city to the level of the top of the wall. Filling such
simple task had
it
an area would have been a rather not been for the streetcar lines,
^
FIGURE 20-2 Some of the nearly 3,000 buildings in Galveston, Texas that were raised and supported on stilts until sand fill was pumped beneath them.
government to 16 its base and
was eventually extended
The wall
is
m
4.8
wide
at
top and has a concave face so that waves are deflected upward. Its top is just over 5 above the highest above mean low tide, about 0.4 1.5
wide
at
its
m
m
water level recorded during the 1900 hurricane. A wide apron of granite riprap (a layer of large stones to prevent erosion) protects the wall on its seaward side.
The
seawall
from the
direct
W.
Jr.,
*L.
Bates,
was constructed
to protect the city
impact of waves, and
"Galveston— A City
Built
it
has been
upon Sand,"
Scientific
American 95(1906): 64.
Prologue
573
power lines, roadways, sidewalks, and nearly 3,000 buildings that lay in the area to be filled. Before filling could begin, jacks were placed beneath the buildings so that they could be raised to sewers,
and supported on stilts until fill was pumped beneath them (Fig. 20-2). To raise a church estimated to weigh more than 2,700 metric tons required 700 jacks. In short, most of the city was raised anywhere from a few centimeters to as much as 3.6 m above its former level! 3 The last of the more than 8.5 million m of fill was the appropriate height
=»
INTRODUCTION
Shorelines are the areas between est level
we
on land
low
tide
and the high-
affected by storm waves. In this chapter,
are concerned mostly with ocean shorelines
where
processes such as waves, nearshore currents, and tides continually modify existing shoreline features. ever,
waves and nearshore currents are also
How-
effective
geologic agents in large lakes, the shorelines of which
many of the same features present along seaThe most notable differences are that waves and nearshore currents are more energetic on seashores, and exhibit shores.
even the largest lakes lack appreciable
tides.
In contrast to other geologic agents such as running
water, wind, and glaciers that operate over vast areas,
narrow zone at any particular time (Fig. 20-3). However, shorelines migrate landward or seaward depending on changes in sea shoreline processes are restricted to a
level
and
uplift or subsidence of the coastal region. Al-
in place
on August?, 1910. Seven years and more
than $3.5 million had been invested, and subsequent events indicate that the time and expense were
During 1961, hurricane Carla hit the city, and although some flooding occurred and some buildings were damaged by wind, the flooding was not serious and no deaths occurred. At the west end of the seawall, where the island is unprotected, the shoreline has been eroded back about 45 m. Had the seawall not been constructed, the shoreline along Galveston would no doubt have been eroded as well. justified.
though shorelines constitute an environment in which change occurs continuously, their appeal is so strong that about two-thirds of the world's population is concentrated in narrow bands adjacent to them. Many of the world's large cities such as New York, Los Angeles, New Orleans, Tokyo, London, Rio de Janeiro, and Shanghai are coastal
and Canada, to those with broad sandy beaches as in eastern North America from New Jersey southward. Whatever their type, on all shorelines there is a continual interplay between the energy levels of shoreline processes and the shoreline materials. In areas where energy levels are particularly high, erosion predominates and the shoreline
may
retreat landward.
supply from the land
is
Where sediment
great, deposition dominates.
shorelines with broad sandy beaches, beach sand tinually shifted
^ FIGURE
cities.
The continents possess more than 400,000 km of shorelines. They vary from rocky, steep shorelines, such as those in Maine and much of the western United States
is
On
con-
from one area to another by waves and
nearshore currents.
20-3 Building damaged at Nags Head, North Carolina during the storm of March 1989.
Living near a shoreline
many
is
appealing, but
it is
not with-
most of and much of Canada, sea level is rising, and buildings that were built some distance from the ocean are now being undermined and destroyed (Fig. 20-3). Slumps and slides are common along rocky, steep shorelines; narrow offshore barrier islands migrate landward by erosion on their seaward sides and deposition on their landward sides; and hurricanes expend much of their fury on shorelines and coastal regions in
out
risks. In
parts of the world, including
the United States
general. Scientists from several disciplines have contributed to our understanding of shorelines as dynamic systems.
communimust become familiar with shoreline processes so
Elected officials and city planners of coastal ties
they can develop policies that serve the public as well as
574
Chapter 20
Shorelines and Shoreline Processes
Direction of
wave
Crest
travel
Trough
qXDOOOQQO T Trough
-^ FIGURE
20-4
Wave
111
II
terminology.
protect the fragile shoreline environment. In short, the
study of shorelines
is
not only interesting, but has
Wave base
many
= 1/2 wave length
practical applications.
•^ FIGURE orbits.
^ WAVE DYNAMICS Waves are
oscillations of a
water surface. They occur on
height at the surface, but they decrease in magnitude with depth. Wave base is the depth at which the diameters of these orbits are essentially zero.
bodies of water, but are most significant in large lakes
all
and the oceans where they serve as agents of erosion, transport, and deposition. Many of the erosional and depositional features of the world's shorelines form and are modified by the energy of incoming waves. Figure 20-4 shows a typical series of waves in deep water and the terminology applied to them. The highest part of a wave is its crest, whereas the low point between crests is the trough. Wave length is the distance between successive wave crests (or troughs), and wave height is the vertical distance from trough to crest. The speed at which a wave advances, generally called celerity (C), can be calculated if one knows the wave length (L) and the wave period (T), which is the time required for two successive
wave
crests (or troughs) to pass a given point:
C = LIT The speed of wave advance (C) is actually a measure wave form rather than a measure of
of the velocity of the
are
affected by surface waves. will be
explored more
The
significance of
wave base
fully in later sections.
Wave Generation Waves can be generated by
several processes including
displacement of water by landslides, displacement of the
and volcanic explosions. However, most of the geologic work done on shorelines occurs from wind-generated waves. When wind blows over water, some of its energy is transferred to the water, causing the water surface to oscillate. The mechanism whereby energy is transferred from wind to water is related to the frictional drag resulting from one fluid (air) moving over sea floor by faulting,
another (water). In
an area of wave generation, as beneath a storm
waves
center at sea, sharp-crested, irregular waves called seas
waves moving across a moves forward and back a wave passes but has no net forward movement.
develop. Seas are an aggregate of waves of various
the speed of the molecules of water. In fact, water
somewhat
grass-covered as
20-5 The water in waves moves in circular The diameters of these orbits are equal to wave
similar to the
field;
the grass
Likewise, as waves
move
water "particles" rotate
across a water surface, the
in circular orbits,
with
little
or
no net movement in the direction of wave travel (Fig. 20-5). They do, however, transfer energy in the direction of wave advance. The diameters of the orbits followed by water particles in waves diminish rapidly with depth, and at a depth of about one-half wave length (L/2), called wave base, they are essentially zero (Fig. 20-5). Thus, at depths exceeding
wave
base, the water
and sea
floor,
or lake floor, are un-
heights and lengths, and one
wave cannot be clearly move out from the
distinguished from another. As seas
area of wave generation, however, they are sorted into
broad swells that have rounded, long crests and are all about the same size (Fig. 20-6). As one would expect, the harder and longer the wind blows, the larger are the waves generated. Wind velocity and duration, however, are not the only factors controlling the size of waves. For example, high-velocity wind blowing over a small pond will never generate large waves regardless of how long it blows. In fact, waves occur on ponds and most lakes only while the wind is
Wave Dynamics
575
Guest Essay
RICHARD
CHAMBERS
L.
GEOPHYSICS AND THE SEARCH FOR OIL My
involvement in geology came about by sheer
chance. During the
summer
of 1965, the year
their organizations,
my best friend and I Grand Tetons of Wyoming
graduated from high school, spent a
month
in the
making themselves
restructurings flooded the job
Many were
of geoscientists.
where we took a rock climbing course. One of our
fields,
campsite neighbors was not only a rock climber, but
sciences, while others
also a high school geology teacher.
Grand Tetons.
did
Little
I
chance encounter would eventually
realize that this
my career goals. When I entered college,
alter
I
intended to prepare for a
and gave little thought to my summer experience. Then, during my sophomore year, I enrolled in physical geology to learn more about the rocks and landforms I saw while hiking and climbing. From that time on, I was hooked on geology because it offered me the opportunity to combine my career in medicine
my
doctoral studies on depositional
processes in Lake Michigan,
I
obtained a
summer
eventually led to a full-time position with the Great
Lakes Environmental Research Laboratory. Here
my
the chemical and sedimentological
processes controlling the health of the Great Lakes.
By
late
1980,
I felt it
was time
for a change
and
began looking for opportunities outside the federal
worked as a Petroleum Company.
government. For the next eight years, research geologist for Phillips In mid-1985,
1
had the opportunity
I
high debt forced Phillips to restructure. After
thought,
Our
first
km
join a regional basin studies team.
I
unemployment was only temporary because I was offered a position with Amoco Production Company's Geophysical Research Division. Fortunately,
within two weeks Since joining
Amoco
have been involved in
many
fields.
My
latest
discovered basins. Educational requirements for careers in the petroleum
more stringent. More must be used in our quest for oil resources because major discoveries are more difficult to find. Today's geoscientist must be skilled in several areas. Geology is no longer just a descriptive discipline, industry are becoming
sophisticated technologies
but rather a
field
where computers and complex
mathematical models are required to analyze data.
Those of you willing to make a commitment
and computer programming are also recommended) can have an exciting career. A statistics,
project
of
and the integration of several thousand well and other data. The results provided explorationists
Ivichard his
L.
Chambers earned
Ph.D. from Michigan State
with a better understanding of the geological history of
University in 1975.
Mexico and helped them develop exploration plays and lease acquisition strategies. By the mid-1980s, oil companies were feeling the impact of rapidly declining oil prices. During this struggling economy, many companies restructured
works
the Gulf of
Shorelines and Shoreline Processes
to an
already rigorous curriculum (courses in mathematics,
seismic data
Chapter 20
I
types of research with the ultimate objective of
logs
576
much
decided to leave the petroleum industry.
to leave research
involved the interpretation of nearly 37,073
and
a mid-life career change.
January 1988 was a critical time in my career. Continued depressed oil prices and the stress of a very
research involves risk analysis in frontier and newly
Lake Survey Center in Detroit. My assignment was to analyze bottom samples from Lake Michigan, determine their physical characteristics, and write a report on their depositional environment. This job
on
make
forced to
position as a sedimentologist with the U.S.
research focused
such as hydrology and the environmental
increasing production from existing
avocation with a fascinating profession.
During
market with thousands
able to switch into allied
found academic positions. However, there simply were not enough jobs available, and many highly talented, experienced people were
Over the next
weeks, he occasionally gave us fascinating lectures on the geological history of the
smaller, in an
effort to increase profits. Unfortunately, these
I
for
Company
Amoco
He
currently
Production
in the area of lithology
prediction from seismic data to
provide reservoir descriptions for use in exploration and
development.
blowing; once the wind stops, the water quickly smooths out. In contrast, the surface of the ocean is continually in motion, and during storms, waves with heights of 20 or 30 m have been recorded; the highest ever reliably measured had a height of 34 m. The reason for the disparity between the wave sizes on ponds and lakes and on the oceans is the fetch, which is the distance the wind blows over a continuous water surface.
The
waves. Fetch
greater the fetch, the greater the size of the is
limited by the available water surface, so
on ponds and lakes it corresponds to their length or width, depending on wind direction. A wind blowing the length of Lake Superior, for example, can generate large waves, and even larger ones develop in the oceans. To produce waves of greater length and height, more energy must be transferred from wind to water; hence large waves form beneath large storms at sea.
Shallow- Water Waves and Breakers Waves moving out from the area of generation form swells and lose only a small amount of energy as they travel across the ocean. In
surface oscillates paths, with
little
the direction of
deep-water swells, the water
and water
particles
move
in orbital
net displacement of water occurring in
wave advance
(Fig. 20-7).
When
these
— FIGURE
20-6
Small swells
in the Atlantic
Ocean near
Massachusetts.
length decrease, and wave height increases. In effect, as waves enter shallower water, they become oversteep-
ened; the wave crest advances faster than the wave form, until eventually the crest plunges
forward as a breaker
commonly several times higher than deep-water waves, and when they plunge forward, (Fig. 20-8).
Breakers are
is expended on the shoreline. Excepwaves generated during storms or by faultvolcanic explosions, and rockfalls can cause serious
their kinetic energy
waves enter progressively shallower water, however, the water is displaced in the direction of wave advance (Fig.
tionally large
20-7).
flooding in coastal regions (see Perspective 20-1).
When deep-water waves enter shallow water,
ing,
they are
transformed from broad, undulating swells into sharpcrested waves. This transformation begins at a water depth of wave base; that is, it begins where wave base
^ NEARSHORE CURRENTS
waves "feel" the bottom, and the orbital motions of water particles within waves are disrupted (Fig. 20-7). As they move further shoreward, the speed of wave advance and wave
extending seaward from the shoreline to just beyond the
intersects the sea floor.
"^ FIGURE 20-7 water within them
At
this point, the
It is
convenient to identify the nearshore zone as the area
It includes a breaker zone and where breaking waves rush forward onto the shore followed by seaward movement of
area where waves break. a surf zone,
which
is
As deep-water waves move toward shore, the orbital motion of disrupted when they reach the point at which wave base intersects the sea floor. Wave length decreases while wave height increases, causing the waves to oversteepen and eventually break. is
Wave
length decreases
Nearshore Currents
577
Perspective 20-1
WAVES AND COASTAL FLOODING Wind-generated storm waves, especially those formed by hurricanes, are responsible for most geologic work on shorelines. Hurricanes, called typhoons in some parts of the world, are vast storms with winds that
may exceed 300
km/hr.
When
wind can cause considerable damage, but most of the damage and about 90% of all hurricane fatalities are caused by coastal flooding. Flooding during hurricanes is caused by large storm-generated waves being driven onshore and by intense rainfall, more than 60 cm in 24 hours in some cases. In addition, as the storm moves over the ocean, low atmospheric pressure beneath the eye of the storm
coastal areas, the intense
0.5 m.
When
upward
as
much
as
the eye of the storm reaches the
shoreline, the bulge coupled with wind-driven
m
high flooded the low-lying coastal areas of
the coastal areas of Bangladesh have been flooded several
more
times, the
most recent and most tragic when more than 100,000
being on April 30, 1991,
they sweep across
causes the ocean surface to bulge
to 10
Bangladesh, drowning 300,000 people. Since 1970,
waves
people were drowned.
Another type of wave that can cause extensive is a tsunami. As we explained in Chapter 10, tsunami are generated by fault displacement of the sea floor, submarine slides and
coastal flooding
slumps, and volcanic explosions (see the Prologue to
Chapter rarely
1).
In the
more than
open
0.5
lengths greater than
m
sea,
tsunami are low waves,
high, but they have
wave
km and can travel at speeds When such waves enter
200
in excess of 700 km/hr. shallow water, their wave height increases to as
much
65 m!
piles
up in a storm surge that can rise several meters above normal high tide and inundate areas several
as
kilometers inland. Several coastal areas in the United
rapid withdrawal of the sea from coastal regions,
States have been devastated
Galveston, Texas,
in
1900
by storm surges, including
(see the
1).
One
of
the greatest natural disasters of the twentieth century
occurred in 1970
when
a storm surge estimated at 8
•^ FIGURE
Charleston, South 1 Carolina was flooded by a storm surge produced by Hurricane Hugo on September 22, 1989.
578
Chapter 20
Shorelines and Shoreline Processes
first
indication of an approaching tsunami
is
a
followed a few minutes later by destructive waves. In
many
Prologue) and
Charleston, South Carolina, in 1989 (Fig.
The
tsunami come in as a rapidly rising tide, backwash, which undermines structures and carries loose objects out to sea, causes most of the damage and fatalities. Depending on shoreline
and
cases,
their
FIGURE 2 On July 9, 1958, a giant wave created by a rockfall from the cliff (r) (d) and to a distance of destroyed the forest over the light areas up to an elevation of 536 "•"-
m
1,097
m
from the high-tide shoreline
at Fish
Lake
(/).
Dam
configuration, offshore topography, and the direction
The Vaiont
wave approach, tsunami will have different effects even on the same shoreline. They sweep much farther inland on gently sloping shorelines and rise higher in narrow inlets than elsewhere. A tsunami that hit Hawaii in 1957 rose more than 10 m on one part of the island, but rose less than 1 m elsewhere on the same island. The largest of all waves occur in restricted bodies
Perspective 15-2,
of
of water, such as bays or lakes,
when water
is
disaster in Italy, discussed in
was one such
event.
The largest of on July 9,
these so-called landslide surges occurred
1958,
in
million
Lituya Bay, Alaska.
m3
An
estimated 30.5
of rock plunged into the bay from a height
of more than 900 m. The sudden displacement of water caused a surge on the opposite side of the bay that rose
536
moved out of it
m
above sea level (Fig. 2). The wave and into the open sea where
the harbor
quickly dissipated.
suddenly displaced by large landslides or rockfalls.
Nearshore Currents
579
""" FIGURE 20-8 Breakers pounding the shoreline Oregon. (Photo courtesy of Jane Duffield.)
"•'
in
FIGURE
are refracted
Wave
20-9
refraction.
and more nearly
These oblique waves
parallel the shoreline as they
enter progressively shallower water.
backwash
the water as
(Fig. 20-8).
The width of
the
nearshore zone varies depending on the wave length of the approaching waves, because long waves break at a greater depth,
Two
waves.
and thus farther offshore, than do short
types of currents are important in the near-
out to sea.
in is
which water moves seaward from which are narrow
in rip currents,
surface currents that flow out to sea through the breaker
zone
shore zone, longshore currents and rip currents.
One way
the nearshore zone
(Fig.
20-10). Surfers
rip currents for
commonly
take advantage of
an easy ride out beyond the breaker
zone, but such currents pose a danger to inexperienced
Wave Refraction and Longshore Currents
swimmers. Some
Deep-water waves are characterized by long, continuous
per hour, so
but rarely are their crests parallel with the shore-
crests,
line (Fig. 20-9). In other
words, they seldom approach a
shoreline head on. Thus, one part of a
low water where
it
wave
enters shal-
encounters wave base and begins
breaking before other parts of the same wave. As a wave begins breaking, its velocity diminishes, but the part of
rip currents flow at several kilometers
swimmer
is caught in one, it is useless back to shore. Instead, because rip currents are narrow and usually nearly perpendicular to the shore, one can swim parallel to the shoreline for a short distance and then turn shoreward with no
to try to
if
swim
a
directly
difficulty.
wave still in deep water races ahead until it too encounters wave base. The net effect of this oblique apthe
'*""
proach
is
that the waves
bend so that they more nearly Such a phenomenon is
parallel the shoreline (Fig. 20-9).
called
wave
refraction.
Even though waves are
refracted, they
still
usually
some angle, causing the water breaker zone and the beach to flow parallel
strike the shoreline at
between the
These longshore currents, as they are and narrow and flow in the same general the approaching waves (Fig. 20-9). These
to the shoreline. called, are long
direction as
currents are particularly important agents of transport
and deposition
in the
nearshore zone.
Rip Currents Waves carry water into the nearshore zone, so there must be a mechanism for mass transfer of water back 580
Chapter 20
Shorelines and Shoreline Processes
FIGURE 20-10
Suspended sediment, indicated by
discolored water, being carried seaward by rip currents.
Rip currents can be characterized as circulating cells When waves approach a shoreline obliquely, the amount of water moving paralfed by longshore currents.
lel
to the shoreline builds
up
until the excess
moves out
through the breaker zone. These rip currents are oriented at an angle to the shoreline, and they migrate in to sea
rents commonly develop where wave heights are lower than in adjacent areas. Such differences in wave height
are
commonly
For example, height of the
than
in
controlled by variations in water depth. if
waves move over a depression, the
wave over
the depression tends to be less
adjacent areas.
the direction of the longshore current (Fig. 20-1 la).
Where waves approach rip currents are fed in velocity
the shoreline head on, adjacent by nearshore currents that increase
from midway between each
rip current (Fig.
20-1 lb).
The configuration of
» SHORELINE DEPOSITION Depositional features of shorelines include beaches, spits,
the sea floor plays an important
role in determining the location of rip currents.
Rip cur-
baymouth
teristics
bars,
and barrier
islands.
The charac-
of beaches are determined by wave energy, and
they are continually modified by waves and longshore
"^ FIGURE (a)
20-11
Rip currents.
Where waves approach
the
shoreline obliquely, rip currents are
oriented at an angle to the
These rip currents migrate along the shoreline in the direction of the longshore current, (b) When waves approach the shoreline head shoreline.
on, rip currents are oriented
perpendicular to the shoreline and are fed on both sides by nearshore currents.
(a)
Rip current
Shoreline Deposition
581
currents. Spits
and baymouth bars both
result
from de-
eluding a backshore that
is
usually dry, being covered by
trans-
water only during storm waves or exceptionally high rides. The backshore consists of one or more berms, platforms composed of sediment deposited by waves;
port fine-grained sediment seaward through the breaker
the berms are nearly horizontal or slope gendy in a land-
position by longshore currents, but the origin of barrier islands is controversial. Rip currents play only a minor role in the configuration of shorelines, but they
do
direction. The sloping area below the berm that is exposed to wave swash is called the beach face Fig. 20-13 The beach face is part of the foreshore, an area covered by water during high ride but exposed duirng low ride Fig. 20-13). Some of the sediment on beaches is derived from weathering and wave erosion of the shoreline, but most of it is transported to the coast by streams and redistributed along the shoreline by longshore currents. Longshore drift is the phenomenon by which sand is transported along a shoreline by longshore currents As previously noted, waves usually strike Fig. 20-14 beaches at some angle, causing the sand grains to move up the beach face at a similar angle; as the sand grains are carried seaward in the backwash, however, they move perpendicular to the long axis of the beach. Thus, individual sand grains move in a zigzag partem in the
ward
.
Beaches Beaches are the most familiar of all coastal landforms, attracting millions of visitors each year and providing the economic base for many communities. They consist of a long, narrow strip of unconsolidated sediment, commonly sand, and are constandy changing. Depend-
on shoreline configuration and wave
ing
intensity,
may
be discontinuous, existing only in protected areas such as embayments. or they may be continuous for long distances. South Carolina, for example, beaches
proudly advertises its Grand Strand, 100 km of nearly continuous beach Tig. 20-12\ By definition a beach is a deposit of unconsolidated sediment extending landward from low tide to a change in topography such as a line of sand dunes, a sea clift. or the point where permanent vegetation begins Fig. 2013'
.
Typically, a beach has several
component
parts in-
.
direction of longshore currents. This
movement
is
not
however; it extends seaward to the outer edge of the breaker zone Fig. 20-14). In an attempt to widen a beach or prevent erosion, restricted to the beach,
shoreline residents often build groins, structures that
"^ FIGURE shown here
20-12
at
The Grand Strand of South Carolina,
Myrde Beach,
is
100
km
of nearly continuous
project seaward at right angles from the shoreline
20-15^.
They
Fig.
interrupt the flow of longshore currents
causing sand to be deposited on their upcurrent side,
beach.
thus widening the beach at that location. However, erosion inevitably occurs on the downcurrent side of a groin
Fig.
20-15
.
Most beaches are sandy, but in areas of parricularly vigorous wave activity, they are gravel covered. Most beach sand is composed of quartz, but a number of other minerals and rock fragments
"»"
FIGURE 20-13
showing
its
parts.
= e=:-
Chapter 20
Shorelines and Shoreline Processes
present as
Cross section of a typical beach
component
Backshore
-;-es-:-e
582
may be
a:e
Be™;
\
\
Breaker zone Direction of
longshore current
^ FIGURE
20-14
Longshore
currents transport sediment along the shoreline between the breaker
zone and the upper limit of wave action. Such sediment transport is longshore drift.
well.
One
of the most
beach sands gravity,
is
magnetite
minerals and
is
common
accessory minerals in
magnetite; because of is
commonly
its
high specific
separated from the other
visible as thin, black layers.
Although quartz is the most common mineral in most beach sands, there are some notable exceptions. For example, the black sand beaches of Hawaii are composed of sand-sized basalt rock fragments, and some Florida beaches are composed of the fragmented calcium carbonate shells of marine organisms. In short, beaches are composed of whatever material is available; quartz is most abundant simply because it is available in most areas and is the most durable and stable of the common
waves. In
many areas, beach profiles change with the we recognize summer beaches and winter
seasons; thus,
beaches, each of which
adjusted to the conditions
Summer
beaches
and are characterized by a wide berm, a gently sloping beach face, and a smooth offshore profile. Winter beaches, on the other hand, tend to be coarser grained and steeper; they have a small are generally covered with sand
^ FIGURE
20-15
These groins
at
Cape May,
interrupt the flow of longshore currents so sand
on
their upcurrent side.
groins, however, sand
rock-forming minerals.
is
prevailing at these times (Fig. 20-16).
longshore
is
On
the
downcurrent
New Jersey is
trapped
side of the
eroded because of continuing
drift.
Seasonal Changes in Beaches
A
beach
is
an area where wave energy is dissipated, so composing the beach are constantly af-
the loose grains
wave motion. However, the overall configuraunchanged as long as equilibrium conditions persist. The beach profile consisting of a berm or berms and a beach face shown in Figure 20-13 fected by
tion of a beach remains
can be thought of as a profile of equilibrium; that is, all parts of the beach are adjusted to the prevailing conditions of
wave
intensity
and nearshore
currents.
Tides and longshore currents affect the configuration of beaches to some degree, but by far the most important agent modifying their equilibrium profile is storm
Shoreline Deposition
583
"^ FIGURE 20-16 Seasonal changes in beach profiles. A winter beach showing offshore sand bars, {b) A summer beach with beach
its
wider berm and more gently sloping
face. (b)
"^ FIGURE berm or none
at
all,
and
their offshore profiles reveal
sand bars paralleling the shoreline (Fig. 20-16). Seasonal changes in beach profiles are related to changing wave intensity. During the winter, energetic storm waves erode the sand from the beach and transport
offshore where
it
it
is
The same sand
(a) Spits
form where longshore
extends across the mouth of a bay. of the Klamath River in California.
(b)
A
spit at the
mouth
stored in sand bars (Fig.
was eroded from the beach during the winter returns the next summer when it is driven onshore by the more gentle swells that occur during that season. Thus, the volume of sand in the system remains more or less constant; it simply moves farther 20-16).
20-17
currents deposit sand in deeper water as at the entrance to a bay. A baymouth bar is simply a spit that has grown until it
that
offshore or onshore depending on the energy of waves.
feature.
A
spit
is
simply a continuation of a beach form-
ing a point, or "free end," that projects into a
body of
commonly a bay. A baymouth bar is a spit that has grown until it completely closes off a bay from the
water,
open sea (Fig. 20-17). Both spits and baymouth bars form and grow
Where
as a
The terms winter and summer beach, although widely used, are somewhat misleading. A "winter beach" pro-
result of longshore drift (Fig. 20-17).
can develop at any time of the year if a large storm occurs, and likewise a "summer beach" profile can develop during a prolonged calm period in the winter.
longshore current velocity diminishes, and sediment is deposited, forming a sand bar. The free ends of many
file
weak, as
spits are
in the
currents are
deeper water at the opening to a bay,
curved by wave refraction or waves approach-
ing from a different direction. Such spits are called
Spits
and Baymouth Bars
Other than the beach itself, some of the most common depositional landforms on shorelines are spits and baymouth bars, both of which are variations of the same
584
Chapter 20
Shorelines and Shoreline Processes
hooks or recurved
A
spits (Fig. 20-17).
rarer type of spit, called a tombolo, extends out
into the sea
and connects an island
to the mainland.
Tombolos develop on the shoreward sides of islands shown in Figure 20-18. Wave refraction around an
as is-
Tombolo
"^"
FIGURE 20-19
Soon
after this
breakwater was
constructed offshore at Santa Monica, California a bulge appeared in the beach. Wave refraction around the
breakwater resulted tombolo.
in the origin of a feature similar to a
Barrier Islands Long, narrow islands composed of sand and separated from the mainland by a lagoon are called barrier islands (Fig. 20-22). On their seaward margins, barrier islands are smoothed by waves, but their lagoon sides are irregular. During large storms, waves completely overtop (b)
these islands
20-18 (a) Origin of a tombolo. Wave refraction around an island causes longshore currents to converge and deposit a sand bar that joins the island with the mainland, (b) Goat Rock is connected to the California shoreline by a tombolo.
Once
— FIGURE
in the
lagoon.
because they are protected from further wave action. are common on barrier islands and are generally the highest part of these islands.
Windblown sand dunes
"^ FIGURE land causes converging currents that turn seaward and
and deposit lobes of sand
deposited, these lobes are modified only slightly
20-20
A
small tombolo in Lake Superior at
Marquette, Michigan.
deposit a sand bar connecting the shore with the island.
A similar ter is
feature
may form when an
artificial
breakwa-
constructed offshore (Fig. 20-19).
Although spits, baymouth bars, and tombolos are most commonly found on irregular seacoasts, many examples of the same features occur in large lakes (Fig. 20-20).
Whether along seacoasts or lakeshores, these sand deposits present a continuing problem where bays must be kept open for pleasure boating or commercial shipping. The entrances to such bays must either be regularly dredged or protected. The most common way to protect entrances to bays is to build jetties, which are structures extending seaward (or lakeward) that protect the bay from deposition by longshore currents
(Fig.
20-21).
Shoreline Deposition
585
***"
FIGURE
20-21
The seaward-projecting
lines represent jetties that
protect the jetties
Ocean City
were constructed
Inlet at
have protected the
inlet,
net southerly longshore drift.
Ocean
City,
heavy, black
1930s to Maryland. The
in the
but they also disrupted the a consequence, Assateague and migrated about
As
Island has been starved of sediment
500
m
landward and
is
now
offset
from Fenwick Island to
the north. (b)
^
The origin of barrier islands has been long debated and is still not completely resolved. It is known that they form on gently sloping continental shelves with abundant sand in areas where both tidal fluctuations and wave energy levels are low. Although barrier islands occur in many areas, most of them are along the east coast of the United States from New York to Florida and along the U.S. Gulf Coast. According to one model, barrier islands formed as spits that became detached from the land, while another model proposes that they
586
Chapter 20
Shorelines and Shoreline Processes
FIGURE 20-22 {a) A barrier island with sandy beaches and a smooth profile on its seaward (right) side. (b) This chain of barrier islands comprises the Outer Banks of North Carolina. Cape Hatteras juts the furthest out into the Atlantic.
formed
as beach ridges
on coasts that subsequently sub-
sided (Fig. 20-23).
Because sea
level
is
currently rising,
most
barrier
is-
lands are migrating in a landward direction. Such migration
is
a natural
these islands, but
it is
consequence of the evolution of a problem for the island residents
,
and communities.
Barrier
generally
islands
rather slowly, but the rates for
many
are rapid
migrate
enough
Barrier island
to
Tidal inlet
cause shoreline problems (see Perspective 20-2).
The Nearshore Sediment Budget We
can think of the gains and losses of sediment in the in terms of a budget. If a nearshore sys-
nearshore zone
tem has a balanced budget, sediment is supplied to it as fast as it is removed, and the volume of sediment remains more or less constant, although sand may shift offshore and onshore with the changing seasons (Fig. 20-16). A positive budget means gains exceed losses, whereas a negative budget results when losses exceed gains. If a negative budget prevails long enough, a nearshore system is depleted and beaches may completely disappear.
Erosion of sea
cliffs
provides some sediment to
beaches, but in most areas probably no
10%
of the total sediment supply
is
more than 5
to
derived from this
source. There are exceptions, however. For example, al-
most all the sediment on the beaches of Maine is derived from the erosion of shoreline rocks. Most of the sediment on typical beaches is transported to the shoreline by streams and then redistributed along the shoreline by longshore
drift.
Thus, longshore
drift also plays a role in
the nearshore sediment budget because
it
continually
moves sediment into and away from beach systems. The primary ways in which a nearshore system loses sediment include offshore transport, wind, and deposition in submarine canyons. Offshore transport mostly
involves fine-grained sediment that
is
carried seaward
Wind is an removes sand from beaches and blows it inland where it commonly piles up as sand dunes. However, storm waves may erode dunes and
where
it
eventually settles in deeper water.
important process because
carry If
some of
their
it
sand back into the nearshore zone.
the heads of submarine canyons are nearshore,
huge quantities of sand are funneled into them and deposited in deeper water. La Jolla and Scripps submarine canyons off the coast of southern California funnel off 3 an estimated 2 million m of sand each year. In most areas, however, submarine canyons are too far offshore to interrupt the flow of sand in the nearshore zone. It should be apparent from the preceding discussion that if a nearshore system is in equilibrium, its incoming supply of sediment exactly offsets its losses. Such a delicate
balance tends to continue unless the system
somehow balance streams
disrupted.
One common way
in
which
is
this
affected is the construction of dams across the supplying sand. The sediment contribution
is
Sea
level rises
Perspective 20-2
RISING SEA LEVEL AND
COASTAL MANAGEMENT
Shorelines in the United States are eroding as sea level
According to one study, 54% of U.S. shorelines are eroding at rates ranging from millimeters per year to more than 10 m in a few areas (Fig. 1). Many other areas of the world are experiencing shoreline problems as well. During the last century, sea level rose about 12 cm rises.
worldwide, and to rise.
all
indications are that
The absolute
*" FIGURE left
1
will continue
on two
factors.
Shoreline erosion in the United States.
No
uncolored.
Annual shoreline change
| |
|
588
Severely eroding
Moderately eroding Relatively stable
Chapter 20
first is the volume of water in the ocean basins, which is increasing as a result of the melting of glacial ice and the thermal expansion of near-surface
seawater.
Many
resulting
scientists think that sea level will
consequence of global warming from concentrations of greenhouse gases in
continue to
rise as a
the atmosphere.
The second
factor controlling sea level
uplift or subsidence of a coastal area. In
rate of sea level rise in a
particular shoreline region depends
areas
it
The
Shorelines and Shoreline Processes
uplift
is
occurring so fast that sea level
data are available for shoreline
is
is
the rate of
some
areas,
actually
^ FIGURE after the U.S.
The beach
3
Miami Beach,
at
Florida, before
Army Corps
and
of
Engineers' beach nourishment project.
Other problems associated with sea level rise include increased coastal flooding during storms and saltwater incursions that may threaten groundwater supplies Chapter 17). Since nothing can be done to prevent sea level from rising, engineers and scientists must examine what can be done to prevent or minimize the effects of shoreline (see
erosion.
One
is
At present, only a few viable options exist. on coastal development.
to put strict controls
maintain. Furthermore, they retard erosion only in the area directly behind the seawall; recall that Galveston Island west of the seawall has been eroded back about 45 m. Another option, adopted by both Atlantic City, New Jersey, and Miami Beach, Florida, is to pump sand onto the beaches to replace that lost to erosion (Fig. 3). This, too, is expensive as the sand must be
replenished periodically because erosion
many
is
a
areas, groins are
North Carolina, for example, permits large structures to be sited no closer to the shoreline than 60 times the annual erosion rate. Although a growing awareness of
continuing process. In
shoreline processes has resulted in similar legislation
currents invariably erode sand from the downcurrent
elsewhere,
some
states
have virtually no restrictions on
commendable, but it has no impact on existing structures and coastal communities. A general retreat from the shoreline may Regulating coastal development
is
be possible, but expensive, for individual dwellings and small communities, but it is impractical for large population centers. Such communities as Atlantic City, New Jersey, Miami Beach, Florida, and Galveston, Texas, have adopted one of two strategies to
One
is
combat
to build protective barriers
such as seawalls. Seawalls, such as the one at Galveston, Texas (see the Prologue), can be effective, but they are tremendously expensive to construct and
590
Chapter 20
sand
is
artificially
supplied to the beaches, longshore
sides of the groins.
Rising sea level has already had a significant
coastal development.
coastal erosion.
constructed to preserve beaches, but unless additional
Shorelines and Shoreline Processes
economic impact, and
phenomenon
all
options for dealing with this
are expensive. Fortifying the shoreline
with seawalls, groins, and other structures is initially expensive, requires constant maintenance, and in the long run will be ineffective rise.
A
if
sea level continues to
general retreat from the shoreline
is
simply
impractical for most coastal communities. Perhaps the best option
is
to replace
sand
lost to erosion
by
pumping it from elsewhere, usually farther offshore. In some areas, however, little can be done to offset the effects of rising sea level.
consequence of corrosion, hydraulic action, and same processes that account for erosion by running water (see Chapter 16). Corrosion is an erosional process involving the wearing away of rock by as a
abrasion, the
chemical processes, especially the solvent action of seawater.
The
action,
is
force of the water a
particularly
itself,
effective
called hydraulic
erosional
process.
Waves exert tremendous pressure on shorelines by direct impact, but are most effective on sea cliffs composed of unconsolidated sediment or rocks that are highly fractured.
Abrasion
is
an erosional process involving the
grinding action of rocks and sand carried by waves.
Wave-Cut Platforms and Associated Landforms
"•*
The rate at which sea cliffs are eroded and retreat in a landward direction depends on wave intensity and the resistance of the coastal rocks or sediments. cliff
retreat occurs during storms and, as
expect, occurs
most rapidly
Most
composed of cliff comon Cape Cod,
in sea cliffs
glacial
Massachusetts, retreats as
much
and some parts of the White
drift
as
Cliffs
30
m
per century,
of Dover in Great
more than 100 m per By comparison, sea cliffs consisting of dense igneous or metamorphic rocks may retreat at negligible Britain are retreating at a rate of
century.
rates.
Sea
cliffs
retreat mostly as a
consequence of hydraulic
action and abrasion at their bases.
As
a sea cliff
dercut by such erosion, the upper part
is
left
is
un-
unsup-
ported and susceptible to mass wasting processes. Thus, sea cliffs retreat little by little, and as they do, they leave a beveled surface called a wave-cut platform that slopes gently in a seaward direction (Fig. 20-25). Broad wavecut platforms exist in
water over them action of waves
is
is
many
only effective to a depth of about 10 m.
Wave-cut platforms are surfaces of sediment transport. The sediment eroded from sea cliffs is transported seaward until it reaches deeper water at the edge of the wave-cut platform. There it is deposited and forms a seaward extension of the wave-cut platform called a Sea
cliffs
do not
(Fig.
where erosion rather
and b). Continued erosion generally causes the span of an arch to collapse, yielding isolated sea stacks on wave-cut platforms (Fig. 20-27c). In the long run, shoreline processes tend to straighten an initially irregular shoreline. They these join, they form a sea arch (Fig. 20-27a
do so because wave refraction causes more wave energy to be expended on headlands and less on embayments. Thus, headlands become eroded, and some of the sediment yielded by erosion is deposited in the embayments. The net effect of these processes is to straighten the shoreline (Fig. 20-26b).
Wave-cut platforms and their associated features are most common along seashores, but they are also present along the shores of large lakes.
A number
of such fea-
tures are present in the Great Lakes region,
many
of
areas, but invariably the
shallow because the abrasive planing
wave-built platform
shorelines
sea
one would
unconsolidated sediment. For example, a sea
posed of unconsolidated
On
FIGURE 20-24
than deposition predominates, a sea cliff develops. Wave erosion of sea cliffs causes them to migrate landward and leave a beveled surface.
Wave
20-25
erosion of a sea
cliff
produces a
forms a wave-built platform.
Wave
cut
platform
Wave
20-25).
retreat uniformly, however, because
some of the materials of which they are composed more resistant to erosion than others. Headlands
-^ FIGURE
gently sloping surface called a wave-cut platform. Deposition at the seaward margin of the wave-cut platform
built
platform
Original land
Sea
cliff
surface
,^'
Notch eroded by waves
are are
seaward-projecting parts of the shoreline that are eroded on both sides due to wave refraction (Fig. 20-26a). Sea caves
may form on
opposite sides of a headland, and
if
Shoreline Erosion
591
~-
Wave energy dispersed
in
bays
Wave
crest
(b)
(a)
-^ FIGURE
20-26 (a) Wave refraction acts to straighten shorelines by concentrating wave energy on headlands, (b) The same shoreline after extensive erosion of the headlands and deposition in the bays.
which have been raised above lake quence of isostatic rebound.
level as a conse-
are described as emergent (uplifted), these
may
monly possess
^ TYPES OF COASTS Coasts can be classified
them
in different
features allowing
them
to be classified in
several ways.
ways, but none of
Submergent and Emergent Coasts
are completely satisfactory because of variations in
and variaand configuration of coasts.
sea level rises with respect to the land or the land
the factors controlling coastal development
If
tions in the composition
subsides, coastal regions are flooded
Rather than attempt to categorize
all
coasts,
we
shall
simply note that two types of coasts have already been discussed, those dominated by deposition and those
dominated by erosion, and shall look further at the changing relationships between coasts and sea level. Depositional coasts, such as the U.S. Gulf Coast, are characterized by an abundance of detrital sediment and the presence of such depositional landforms as deltas and barrier islands. Erosional coasts are generally steep and irregular and typically lack well-developed beaches except in protected areas. They are further characterized by erosional features such as sea cliffs, wave-cut platforms, and sea stacks. Many of the beaches along the west coast of North America fall into this category. The following section examines coasts in terms of their changing relationships to sea level. But note that while some coasts, such as those in southern California,
592
same coasts
be erosional as well. In other words, coasts com-
Chapter 20
Shorelines and Shoreline Processes
and
said to be sub-
mergent or drowned (Fig. 20-28). Much of the east coast of North America from Maine southward through South Carolina was flooded during the post-Pleistocene rise in sea level, so that it is now an extremely irregular coast. Recall that during the
expansion of glaciers dur-
ing the Pleistocene, sea level
was
as
much
as
130
m
lower than at present, and that streams eroded their valleys more deeply as they adjusted to a lower base level.
When
sea level rose, the lower ends of these valleys
were drowned, forming estuaries such as Delaware and Chesapeake bays (Fig. 20-28). Estuaries are the seaward ends of river valleys where seawater and freshwater mix. The divides between adjacent drainage systems on submergent coasts project seaward as broad headlands or a line
of islands.
Submerged coasts also occur at higher latitudes where Pleistocene glaciers flowed into the sea. When sea
"^ FIGURE
20-27 (a) Erosion of and the origin of sea sea arches, and sea stacks.
a headland caves, (b)
This sea stack
in Australia
has
an arch developed in it. (c) Sea stacks south of La Push, Washington.
Types of Coasts
593
"**
FIGURE 20-29
An emergent coast in California. Such cliff, and they tend to be submergent coasts. (Photo courtesy of Jerry
coasts are characterized by a sea straighter than
Westby.)
races in
some
areas. In southern California, for
several terrace levels are present, each of
example,
which proba-
bly represents a period of tectonic stability followed by
The
uplift.
above sea
"^ FIGURE
20-28
Chesapeake Bay
highest of these terraces
is
now
about 425
m
level.
^ TIDES is
a large estuary.
It
formed when the east coast of the United States was flooded as sea level rose following the Pleistocene Epoch.
On seacoasts the surface of the ocean rises and
falls
once
or twice daily in response to the gravitational attraction
Moon
and Sun. Such regular fluctuations in the Two high tides and two low tides occur daily in most areas as sea level rises and falls anywhere from a few centimeters to more than 15 m (Fig. 20-31). During rising or flood tide, more and
of the
sea's surface are called tides.
lower ends of the glacial troughs were drowned, forming fiords (Fig. 18-20). Emergent coasts are found where the land has risen with respect to sea level (Fig. 20-29). Emergence can occur when water is withdrawn from the oceans as occurred during the Pleistocene expansion of glaciers. At present, however, coasts are emerging as a consequence of isostasy or tectonism. In northeastern Canada and the Scandinavian countries, for example, the coasts are irregular because isostatic rebound is elevating formerly glaciated terrain from beneath the sea. Coasts rising in response to tectonism, on the other level rose, the
hand, tend to be straight because the sea-floor topography being exposed as uplift proceeds is smooth. The
more of
the nearshore area
reached.
Ebb
tide occurs
is
tional attraction to exert tide-generating forces strong
enough have a
to
deform the
much
solid
body of the Earth, but they on the oceans. The Sun is
greater influence
27 million times more massive than the Moon, but it is 390 times as far from the Earth, and its tide-generating force
is
only
46%
as strong as that of the
cordingly, the tides are
Sun does play an important well.
Shorelines and Shoreline Processes
Moon. Ac-
dominated by the Moon, but the
consequence of plate tectonics. Distinctive features of such coasts are marine terraces (Fig. 20-30), which are old wave-cut platforms now elevated above sea level. Uplift in such areas appears to be episodic rather than continuous, as indicated by the multiple levels of ter-
Chapter 20
is
currents flow seaward
during a decrease in the height of the tide. Both the Moon and the Sun have sufficient gravita-
west coasts of North and South America are rising as a
594
flooded until high tide
when
role in generating tides as
If we consider only the Moon acting on a spherical, water-covered Earth, the tide-generating forces produce
two bulges on the ocean surface (Fig. 20-32a). One bulge extends toward the Moon because it is on the side
Moon's gravitational attraction is The other bulge occurs on the opposite side of the Earth, where the Moon's gravitational attraction is least. These two bulges always point toward and away of the Earth where the
greatest.
from the Moon, (Fig. 20-32a), so as the Earth rotates and the Moon's position changes, an observer at a particular shoreline location experiences the rhythmic rise and fall of tides twice daily. The heights of two successive high tides may vary depending on the Moon's inclination with respect to the equator.
The so
its
Moon
revolves around the Earth every 28 days,
position with respect to any latitude changes
each day. That is, as the Moon moves in its orbit and the Earth rotates on its axis, it takes the Moon 50 minutes longer each day to return to the same position it was in the previous day. Thus, an observer would experience a high tide at 1:00 p.m. on one day, for example, and at 1:50 p.m. on the following day. Tides are also complicated by the combined effects of the Moon and the Sun. Even though the Sun's tidegenerating force is weaker than the Moon's, when the Moon and Sun are aligned every two weeks, their forces are added together and generate spring tides, which are slightly
20% higher than average When the Moon and Sun are at about
another, also at
tides
(Fig.
20-32b).
right angles to
one
two-week intervals, the Sun's tidesome of that of the Moon, and
generating force cancels
~^ FIGURE
20-30
This gently sloping surface in Ireland
a marine terrace.
is
neap
tides
about
20%
lower than average occur
(Fig.
20-32c).
Tidal ranges are also affected by shoreline configuration.
Broad, gently sloping continental shelves as in the
Gulf of Mexico have low of
fall
tides.
Fundy
in
much
steep,
greater rise
and
some narrow, For example, in the Bay
Tidal ranges are greatest in
funnel-shaped bays and of
whereas
tidal ranges,
irregular shorelines experience a
Nova
inlets.
Scotia a tidal range of 16.5
— FIGURE and
(b)
high
20-31
(a)
Low
m
has
tide
tide.
Tides
595
Full
New
moon
moon
(b)
First-quarter
Spring tides
moon
Inertial
bulging and bulging
moon's
due
to
gravitational pull
Tidal bulge
due
to
inertia
Tidal bulge
due to
the moon's
'-
^- FIGURE
20-32
(a)
The
Earth
j
tides are
caused by the gravitational pull of the Moon and, to a lesser degree, the Sun. The Earth-Moon-Sun alignments at the times of the (b) spring and (c) neap tides are
f
pull
Third-quarter
moon Earth
shown.
(c)
been recorded, and ranges greater than 10
m
Neap
(a)
tides
occur in
narrow passages where tidal curenough to erode and transport sediment. Indeed, if it were not for strong tidal currents, some passageways would be blocked by sediments de-
shorelines, except in
rent velocity
several other areas.
Tides have an important impact on shorelines because the area of wave attack constantly shifts onshore and offshore as the tides rise and fall. Tidal currents themselves, however, have
little
modifying
effect
Tidal forces
is
great
posited by longshore currents.
on
:^^^^^^^ ^-F«r«^^^ ^^^^xm.-3t>^^X^ m. % ^^ ^ U « '»3 ,
l
.
» CHAPTER SUMMARY 1.
2.
3.
4.
The waves become oversteepened and plunge forward onto the shoreline, thus expending
Shorelines are continually modified by the energy of waves and longshore currents and, to a lesser degree,
Waves approaching
by
a longshore current.
Such currents are capable of
Rip currents are narrow surface currents that carry water from the nearshore zone seaward through the
length.
breaker zone. Beaches are the most
Little
or no net forward motion of water occurs in waves in the open sea. When waves enter shallow
They processes, and
water, they are transformed into waves in which
seasonal changes.
water does move in the direction of wave advance. Wind-generated waves, especially storm waves, are
bars, and tombolos all form and consequence of longshore current transport and deposition. Barrier islands are nearshore sediment deposits of uncertain origin. They parallel the mainland but are separated from it by a lagoon. The volume of sediment in a nearshore system
work on
features.
shorelines,
but waves can also be generated by faulting, volcanic
10.
explosions, and rockfalls.
596
a shoreline at an angle generate
considerable erosion, transport, and deposition.
on water surfaces that transmit energy in the direction of wave movement. Surface waves affect the water and sea floor only to wave base, which is equal to one-half the wave oscillations
responsible for most geologic
5.
their
kinetic energy.
tidal currents.
Waves are
.
Breakers form where waves enter shallow water and the orbital motion of water particles is disrupted.
Chapter 20
Shorelines and Shoreline Processes
11
common
shoreline depositional
are continually modified by nearshore their profiles generally exhibit
Spits,
baymouth
grow
as a
remains rather constant unless the system
somehow 12.
when dams
Erosion of a sea
is
are built across
cliff
produces a gently sloping
surface called a(an):
the streams supplying sand to the system.
a.
submergent coast; b
Many
c.
beach;
shorelines are characterized by erosion rather
than deposition. Such shorelines have sea cliffs and wave-cut platforms. Other features commonly
coast.
present include sea caves, sea arches, and sea stacks.
mainland by
Submergent and emergent coasts are defined on the
a
basis of their relationships to changes in sea level.
bars; d.
13.
14.
disrupted as
The
gravitational attraction of the
Moon
causes the ocean surface to rise and
twice daily in most shoreline areas. currents have
on
effect
little
fall
and Sun
Most
tidal
shorelines.
lagoon
a
sea stacks;
force of
waves impacting on shorelines
c.
hydraulic action; d.
e.
translation.
distance the
terracing;
berm;
fetch; b.
a.
a water surface
is
marine terrace rip current
paths but with
baymouth bar
shoreline
of
beach beach face
spit
a.
breakers; b.
submergent coast
c.
swells; d.
berm
tide
e.
rip currents.
breaker
tombolo
crest (wave)
trough (wave) wave base wave-cut platform
10.
11.
deep-water waves, the water moves
In
little
wave wave wave
e.
12.
drift
waves;
more nearly
that they
is:
wave oscillation; wave refraction;
translation; b. deflection; d. reflection.
The excess water the
are:
longshore
The bending of waves so
c.
in orbital
in the direction
refracted waves;
parallel the shoreline
wave height wave length wave period wave refraction
movement
net
wave advance. Such waves
a.
wave
spit; d.
c.
wave trough.
e.
barrier island
headland longshore current longshore drift
is:
oscillation;
wind blows over
backshore
foreshore
wave
corrosion; b
period;
fetch
sea arches.
the:
IMPORTANT TERMS
emergent coast
baymouth
atolls; c.
e.
a
The
the
are:
barrier islands; b.
The
wave-cut platform; emergent
e.
composed of sand and separated from
Islands
as tides
backshore;
d.
in the
nearshore zone returns to
b.
longshore currents;
open sea by:
a.
tombolos;
c.
wave
emergence;
refraction; d.
rip
e.
currents.
^ REVIEW QUESTIONS
13.
A
sand deposit extending into the mouth of a bay
is a:
1.
Which
of the following
is
not a depositional
2.
d.
a.
spit; b.
d.
beach;
The speed
at
water surface
3.
tombolo;
c.
baymouth
bar;
which a wave form advances over a
celerity; b.
d.
wave base;
wave
length;
sea stack.
c.
of sea
erosion of
cliffs; b.
streams; d.
breakers;
coastal submergence.
e.
15.
Erosional remnants of a shoreline
now
rising
above
a wave-cut platform are:
is:
a.
the distance offshore that waves break;
b.
the width of a longshore current;
c.
the
Waves approaching
a shoreline obliquely generate:
a
flood tides; b.
c.
tidal currents;
d
longshore currents; marine berms; e
16.
a.
barrier islands; b.
c.
beaches; d.
Which
Most beach sand
is
composed of what mineral?
a
basalt;
b
calcite; c
d
quartz; e
feldspar.
gravel;
drowned
c.
range;
How
sea stacks;
marine terraces;
of the following
emergent coasts? a. marine terraces;
What
terraces. 5.
wave erosion
a.
refraction;
c.
fetch.
e.
spit;
c.
Although there are exceptions, most beaches receive most of their sediment from: offshore reefs;
is:
depth at which the orbital motion in surface waves dies out; d. the distance wind blows over a the height of storm waves. water surface; e 4.
14.
sea stack.
e.
a.
Wave base
headland; b. beach; wave-cut platform; e.
a.
landform?
is
b.
e.
spits.
a distinctive feature of
estuaries;
very high tidal
river valleys; d.
fiords.
e.
do deep- and shallow-water waves differ? is wave base and how does it affect waves
as
they enter shallow water?
Explain
how
What
longshore drift?
is
a longshore current
is
generated.
Review Questions
597
21.
What and
is
the relationship between longshore currents
22. Sketch a north-south shoreline along which several groins have been constructed.
Assume
approach from the northwest. 23. Explain why quartz is the most composing beach sands.
25.
How
why
they
common
mineral a winter
differ.
does a tombolo form?
26. Explain the concept of a nearshore sediment budget. 27.
How
does a wave-cut platform develop? how an initially irregular shoreline
28. Explain 29.
Why
30.
What
A
is
may
be helpful. does an observer at a shoreline experience two
straightened.
sketch
Fox,
W.
Prentice-Hall. J. 1988. America in peril from the sea. New Scientist 118:54-59. Komar, P. D. 1976. Beach processes and sedimentation. Englewood Cliffs, N.J.: Prentice-Hall. 1983. CRC handbook of coastal processes and erosion. Boca Raton, Fla.: CRC Press. Pethick, J. 1984. An introduction to coastal geomorphology. London: Edward Arnold. Schneider, S. H. 1990. Global warming: Are we entering the greenhouse century? San Francisco, Calif.: Sierra Club Books.
Hecht,
summer beach and
24. Sketch the profiles of a
beach, and explain
that waves
F., and M. L. Schwartz. 1985. The world's coastline. York: Van Nostrand Reinhold Co. T. 1983. At the sea's edge. Englewood Cliffs, N.J.:
Bird, E. C.
New
rip currents?
high and two low tides each day? are the characteristics of a submergent coast?
.
Snead, R. 1982. Coastal landforms and surface features. Stroudsburg, Pa.: Hutchinson Ross Publishing Co.
Walden, D. 1990. Raising Galveston. American Heritage of Invention Technology 5:8-18.
&
Williams,
crisis. U.S.
^
ADDITIONAL
Abrahamson, D.
E., ed.
Washington, D.C.: Island Bird, E. C.
F.
1984. Coasts:
geomorphology.
598
New
Chapter 20
READINGS
1989. The challenge of global warming. Press.
An
introduction to coastal
York: Blackwell.
Shorelines and Shoreline Processes
K. Dodd, and K. K. Gohn. 1990. Coasts in Geological Survey Circular 1075.
S. J.,
•^ **- *«• "^
•*-
-^T^gr
ANSWERS TO MULTIPLE-CHOICE AND FILL-IN-THE-BLANK
REVIEW QUESTIONS CHAPTER 1. c; 2. e; 3.
CHAPTER
1
b; 4. c; 5. d; 6. e; 7. a; 8. d; 9. c; 10. b; 11. a;
11
b; 2. c; 3. c; 4. a; 5. e; 6. c; 7. b; 8. c; 9. b; 10. d; 11. b;
12. c; 13. d; 14. a; 15. a; 16. e; 17. b.
12. c; 13. e; 14. b.
CHAPTER
CHAPTER
1. a; 2.
2
d; 3. e; 4. c; 5. b; 6. a; 7. c; 8. e; 9. d; 10. a; 11. c;
12. e; 13. e; 14. a; 15. a; 16. e; 17. d; 18. b; 19. c;
20. b.
y CHAPTER 1.
b; 2. d; 3. a; 4. e; 5. c; 6. c; 7. d; 8. b; 9. b; 10. a; 11.
c;
12. d; 13. e; 14. c; 15. b.
CHAPTER 3
1.
b; 2. e; 3. c; 4. d; 5. b; 6. c; 7. b; 8. a; 9. c; 10. b; 11.
12
1.
13
d; 2. a; 3. e; 4. c; 5. e; 6. b; 7. c; 8. d; 9. b; 10. c; 11. a; 12.
b; 13. c; 14. b; 15. divergent; 16. oceanic-oceanic convergent;
a; 12. a; 13. b; 14. e; 15. c.
17. transform; 18. oceanic-continental convergent.
CHAPTER
CHAPTER
1. a; 2. c; 3.
4
a; 4. e; 5. b; 6. b; 7. c; 8. b; 9. e; 10. b; 11.
1.
14
b; 2. c; 3. e; 4. d; 5. a; 6. b; 7. c; 8. c; 9. a; 10. d; 11. b;
a; 12. c; 13. a; 14. c; 15. d; 16. a; 17. d; 18. e; 19. d.
12. c; 13. a; 14. c; 15. a; 16. d; 17. a; 18. c; 19. b; 20. c.
CHAPTER
CHAPTER
1.
^
1.
5
b; 2. a; 3. d; 4. a; 5. c; 6. d; 7. d; 8. e; 9. b; 10. d; 11.
1. e; 2. e; 3.
a; 12. a; 13. d.
12. d.
CHAPTER
CHAPTER
6
15
b; 4. d; 5. c; 6. a; 7. e; 8. e; 9. c; 10. a; 11. e;
16
1.
b; 2. e; 3. a; 4. b; 5. c; 6. d; 7. b; 8. a; 9. a; 10. d; 11.
1.
d; 2. a; 3. c; 4. e; 5. b; 6. c; 7. a; 8. c; 9. b; 10. d; 11. a; 12.
e;
12. b; 13. c; 14. b.
c;
13. d; 14. b; 15. c; 16. c; 17. e; 18. a; 19. c; 20. d.
CHAPTER 1. c; 2.
CHAPTER
7
d; 3. a; 4. e; 5. a; 6. d; 7. b; 8. c; 9. a; 10. e; 11. c;
12. b; 13. c; 14.
CHAPTER
"
b";
15. d; 16.
17
b; 4. d; 5. e; 6. d; 7. e; 8. b; 9. d; 10. e; 11.
a; 12. e; 13. e; 14. b.
e.
CHAPTER
8
1. c; 2. e; 3. a; 4. c; 5. a; 6. c; 7.
1. a; 2. c; 3.
d; 8. c; 9. d; 10. b; 11. e;
18
1. c; 2. a; 3. b; 4. c; 5. e; 6. b; 7. c; 8. b; 9. b;
12. b; 13. d; 14. b; 15. a; 16. e; 17. b; 18. d.
12. b; 13. a; 14. c; 15. b.
CHAPTER
CHAPTER
1. c; 2. c; 3.
9
a; 4. e; 5. d; 6. a; 7. c; 8. e; 9. d; 10. b; 11. c;
12. e; 13. b.
CHAPTER 1. c; 2.
10
b; 3. a; 4. e; 5. a; 6. d; 7. e; 8. a; 9. b; 10. c; 11. d;
12. e; 13. c; 14. b.
10. e; 11. c;
19
1.
d; 2. b; 3. a; 4. c; 5. e; 6. d; 7. c; 8. a; 9. b; 10. e; 11.
c;
12. b; 13. d; 14. a; 15.
CHAPTER
e.
20
1. e; 2. a; 3. c; 4.
b; 5. d; 6. b; 7. a; 8. c; 9. a; 10. c; 11. d;
12. e; 13. c; 14. c; 15. b; 16. a.
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•»"y%^3C3E
GLOSSARY
mainly of hornblende and
aa
A
lava flow with a surface of
plagioclase.
rough, jagged angular blocks and fragments.
angular unconformity An unconformity below which older
abrasion The process by which exposed rock is worn and scraped by the impact of solid particles.
strata dip at a different angle
absolute dating The process of assigning actual ages to geologic events. Various dating techniques
based on radioactive decay are used to determine absolute ages.
The
abyssal plain
flat
rises
of
margin
A
continental margin that develops at the leading edge of a continental plate
where oceanic lithosphere
is
subducted. alluvial fan A lobate deposit of sand and gravel deposited by a stream on lowlands adjacent to
highlands, usually in an arid or
A
general term for
detrital material deposited
by a
black, lustrous, hard
up-arched fold characterized by an axial plane that in half.
it
aphanitic A fine-grained texture in igneous rocks in which the individual mineral grains are too small to be seen without magnification. An aphanitic texture results from rapid
cooling of
magma.
aquiclude prevents the
Any material that movement of
groundwater.
A
allows the
permeable layer that
movement of
groundwater.
A
particle consisting of
two protons and two neutrons from the nucleus
artesian system
of an atom; emission of an alpha
groundwater is up high hydrostatic
foliated
A
dark-colored
metamorphic rock composed
assemblage range zone A type of biozone established by plotting the overlapping ranges of fossils that have different geologic ranges; the first
and
last
occurrences of
fossils
are used to establish assemblage
range zone boundaries.
A
assimilation
process in which a
reacts with preexisting rock it
comes
in contact.
asthenosphere The part of the mantle that lies below the lithosphere; behaves plastically and flows.
atom
The
smallest unit of matter
that retains the characteristics of an element.
atomic mass number The total of protons and neutrons in the nucleus of an atom,
number
atomic number protons
in the
The number of nucleus of an atom,
aureole A zone surrounding an igneous intrusion in which contact metamorphism has taken place.
narrow, serrated ridge
arete
two
glacial valleys or
B
adjacent cirques.
back-arc basin
A
system in which confined and builds (fluid) pressure.
aseismic ridge A long, linear ridge or broad plateaulike feature rising as
much
mm
erupted by a volcano.
with which
An
anticline
separating
amphibolite
is
lower-grade coals.
alpha decay A type of radioactive decay involving the emission of a
atomic number by two and the atomic mass number by four.
that
magma
stream.
particle decreases the
Uncemented pyroclastic material measuring less than 2
of volatile matter. Anthracite usually forms from the metamorphism of
aquifer
semiarid region.
alluvium
A
anthracite
coal that contains a high percentage of fixed carbon and a low percentage
divides
passive continental margins. active continental
strata.
surface of
the sea floor, covering vast areas
beyond the continental
(usually steeper) than the overlying
younger
ash
km
above the surrounding sea floor and lacking as 2 to 3
seismic activity.
A
basin formed on
the continent side of a volcanic island arc; thought to
form by
back-arc spreading; the site of a marginal sea, e.g., the Sea of Japan.
backshore is
The area of
a beach that
usually dry, being covered by
water only by storm waves or exceptionally high tides.
Glossary
601
bajada A broad alluvial apron formed at the base of a mountain range by coalescing alluvial fans.
barchan dune A crescent-shaped dune whose tips point downwind; found in areas with generally flat dry surfaces with
little
vegetation, limited
supply of sand, and nearly constant
wind
direction.
barchanoid dune A dune intermediate between transverse and barchan dunes; typically forms along the edges of a
dune
A
field.
narrow island composed of sand and separated from the mainland by
barrier island
long,
a
lagoon.
A
basal slip
type of glacial
that occurs when a glacier over the underlying surface.
movement slides
A
basalt plateau
large plateau built
fissure eruptions.
circular equivalent of a
dip toward a central point.
The
largest of intrusive
bodies, having at least 100 surface area.
Most
km 2
of
batholiths are
discordant and are composed chiefly of granitic rocks.
baymouth bar grown
until
it
A
A
spit that
has
completely cuts off a
bay from the open
beach
sea.
deposit of unconsolidated
sediment extending landward from low tide to a change in topography or where permanent vegetation begins.
beach face The sloping area below the berm that is exposed to wave swash.
The coarser part of
a
or slope gently in a landward
formed
direction.
resulting
beta decay A type of radioactive decay during which a fast-moving electron is emitted from a neutron and thus is converted to a proton; results in an increase of one atomic
number, but does not change atomic mass number.
A
Big Bang
model
for the evolution
state
is
followed by expansion,
and a
less
dense
state.
chemical processes of organisms; a subcategory of chemical sedimentary
sedimentary rocks.
The bounding
surface that separates one layer of strata
602
from another.
Glossary
large, steep-sided,
by summit collapse from the underlying magma chamber being partly drained, or by a large explosion in which the summit is blown away. either
The area extending upward a few centimeters
capillary fringe irregularly
to several meters
from the base of
the zone of aeration.
carbon 14 dating technique An absolute dating method that relies upon determining the ratio of C 14 C 12 in a sample; useful back to about 70,000 years ago; can be
to
applied only to organic substances.
A
carbonate mineral
mineral that
rocks.
bonding The process whereby atoms are joined to other atoms.
carbonate rock A rock containing predominately carbonate minerals,
Bowen's reaction series A mechanism that accounts for
cave A naturally formed subsurface opening that is generally connected
.
the
and
derivation of intermediate and felsic
to the surface
magmas from
for a person to enter.
a mafic
magma.
It
is
The
large
enough
consists of a discontinuous branch of
cementation
ferromagnesian minerals that change from one mineral to another over specific temperature ranges and a continuous branch of plagioclase feldspars whose composition changes as the temperature decreases.
binding material between and
precipitation of
around the grains of sediment, thus converting
it
to sedimentary rock.
chemical sedimentary rock Originates by precipitation of minerals derived from the ions and
braided stream A stream possessing an intricate network of
salts
dividing and rejoining channels.
chemical weathering The process whereby rock materials are decomposed by chemical alteration
when sediment
transported by the stream
is
and gravel
bedding plane
A
circular or oval volcanic depression
contains the negatively charged -2 carbonate ion (C0 3 )
sand and gravel.
in
underlying rocks.
caldera
deposited within channels as sand
Another name for layering
rapid erosion of the less resistant
The backshore area of a beach consisting of a platform composed of sediment deposited by waves; berms are nearly horizontal
stream's sediment load; consists of
bedding
found in arid and semiarid regions; formed by the breaching of a resistant cap rock, which allows
berm
Braiding occurs
bed load
feature of
biochemical sedimentary rock A sedimentary rock resulting from the
syncline. All of the strata in a basin
batholith
dipping seismic
island arcs and deep ocean trenches; such zones indicate the angle of plate descent along a convergent plate boundary.
cooling,
base level The lowest limit to which a stream can erode.
The
A
common
of the universe in which a dense, hot
up by numerous lava flows from
basin
Benioff zone zone that is a
breaker as
it
bars.
A
wave
enters shallow water until the
An
of the parent material.
cinder cone that oversteepens
crest plunges forward.
butte
taken into solution in the weathering environment.
isolated, steep-sided,
pinnacle-like erosional structure
A
small steep-sided
volcano that forms from the accumulation of pyroclastic material
around a
vent.
circum-Pacific belt
A
zone of
seismic and volcanic activity that
nearly encircles the margins of the Pacific
Ocean
basin; the majority of
the world's earthquakes
and volcanic
eruptions occur within this
cirque
A
belt.
steep-walled, bowl-shaped
concordant Refers to plutons whose boundaries are parallel to the layering in the country rock.
cone of depression The lowering of the water table around a well in
depression formed by erosion by a
the shape of a cone; results
valley glacier.
water
is
faster
than
clastic texture
A
texture of
when
removed from an aquifer it
can be replenished.
metamorphism Metamorphism in which
convergent plate boundary The boundary between two plates that are moving toward one another; three types of convergent plate
boundaries are recognized.
core
The
interior part of the Earth
which begins
at a depth of about 2,900 km; probably composed mostly of iron and nickel; divided into an outer liquid core and an
sedimentary rocks consisting of the broken particles of preexisting rocks or organic structures such as shells.
contact
cleavage The ability to break or split along a smooth plane of weakness. Cleavage is determined by the strength of the bonds within
rock.
Coriolis effect
continental-continental plate
winds to the right of their direction of motion (clockwise) in the Northern Hemisphere and to the left of their direction of motion (counterclockwise) in the Southern Hemisphere due to the
body
alters the
boundary plate
A
a
magma
surrounding country
type of convergent
boundary along which two
minerals.
continental lithospheric plates collide
column A cave deposit formed when stalagmites and stalactites
Asia).
the collision of India with
A
columnar jointing jointing that forms
The
igneous rocks.
rocks overlying the
type of
columns joints
in
commonly
form a polygonal (usually hexagonal) Columnar joints are most
pattern.
in
compaction lithification
mafic lava flows.
A method
correlation
The demonstration of
time equivalency of rock units in different areas.
igneous, sedimentary, and
country rock The rock that is invaded by and surrounds an igneous
metamorphic rocks. It has an overall composition corresponding closely to granodiorite and an overall density 3 of about 2.70 g/cm
intrusion.
covalent
bond
A bond
formed by
.
whereby the pressure
amount of pore space and thus volume of a deposit.
that
a single landmass that broke apart
the
A
combination of different types of mass movements in which one type is not dominant; most complex sliding
The theory
continental drift
the sharing of electrons between
atoms.
the continents were once joined into
overlying sediment reduces the
movements involve
deflection of
consisting of a wide variety of
of
exerted by the weight of the
complex movement
The
Earth's rotation.
The continental upper mantle and
continental crust
join.
common
(e.g.,
inner solid core.
and
flowing.
with the various fragments (continents) moving with respect to one another; proposed by Alfred Wegener in 1912. continental glacier covering a vast area
km 2
A
large glacier
(at least
50,000
and unconfined by topography. Also called an ice sheet.
crater
A
circular depression at the
summit of
a volcano resulting
the extrusion of gases
and
connected by a conduit to a
chamber below the Earth's craton
The name applied
from
lava;
magma
surface. to the
relatively stable part of a continent;
consists of a shield
and
a platform,
)
a buried extension of a shield;
the ancient nucleus of a
The area
composite volcano A volcano composed of both pyroclastic layers and lava flows typically of intermediate composition. Composite
continental margin
volcanoes, also called
continental rise
stratovolcanoes, are steep-sided near
the base of the continental slope
crest
summits (up to 30°), but decrease in slope toward their base where they are generally less than 5°.
with a gentle slope.
cross-bedding Beds that are deposited at an angle to the surface upon which they are accumulating.
their
A
above sea
level
from the deep-sea
floor.
slowest type of flow.
The area between and continental slope
continental shelf the shoreline
where the sea floor slopes very gently in a seaward direction.
different elements.
continental slope
substance resulting
compressional stress resulting
when
Stress
rocks are squeezed by
The imperceptible downslope movement of soil or rock; it is the
creep
The area beyond
from the bonding of two or more
compound
continent.
separating the part of a continent
The
relatively
steep area between the shelf-slope
break
(at
an average depth of 135 m)
crust
The
highest part of a wave.
The outermost
layer of the
Earth; the upper part of the lithosphere,
which
the mantle by the into continental
is
separated from
Moho;
divided
and oceanic
The
external forces directed toward one
and the more gently sloping
crystal settling
another.
continental rise or oceanic trench.
separation of minerals by
crust.
physical
Glossary
603
The expansion of
desertification
and gravitational
crystallization
A topographicaly high
divide •' i-
deserts into formerly productive
settling.
.'tZi'i'r:'.
'
-
J:'
'-
"
".''.
a
T.k'iS:
lands.
A solid in which atoms are arranged
crystalline solid
dome
A
an a dome dip
circular equivalent of
a regular, three-dimensional
detntal sedimentary rock Sedimentary rock consisting of
anticline. All strata in
framework.
detritus, the solid panic.
away from a
preexisting rocks. Such rocks have a
drainage basin The area occupied by a drainage system that contributes water to a given stream.
the constituent
A
crystalline texture
in
texture of
clastic texture.
rocks consisting of an interlocking
mosaic of mineral
Pressure that not applied equally to all sides of a rock body; results in distortion of the body. differential pressure
crystals.
is
Curie point The temperature at which iron-bearing minerals in a
magma
cooling
attain their
weathering
differential
magnetism.
of rock at different rates,
Weathering producing
an uneven surface.
A
dike
daughter element An element formed by the radioactive decay of another element, e.g., argon 40 is the daughter element of potassium 40.
A
debris avalanche
movement steep
mountain ranges;
starts
out as a rockfall.
A
movement
to
dip
in
high pressures.
A measure
of the
maximum
movement
is
A
':;
;:•:.-
Various cave deposits
from the deposition of
fault
drumlin An elongated hill of till measuring as much as 50 m high and 1 km long; formed by the movement
on which
A
dry climate
climate that occurs
low and middle
where the potential
perpendicular to the strike direction. dip-slip fault
resulting
in the
plane from horizontal; measured
water than a
:.-a..-.5i-e
dripstone
of a continental glacier.
rocks subjected
angular deviation of an inclined
typically
type of mass
less
changes occurring rj
that contains larger-sized
and
particles
A model u.sed on
predict earthquakes based
complex
that often occurs in very-
debris flow
discordant pluton.
model
drainage partem The regional arrangement of channels in a
calche.
tabular or sheetlike
dilatancy
central point.
all
parallel with the dip of
latitudes
loss of
water by
evaporation exceeds the yearly precipitation; covers 30% of the Earth's land surface and is divided
and arid
into semiarid
dune
A mound
regions.
or ridge of
the fault plane.
wind-deposited sand.
discharge The total volume of water in a stream moving past a particular point in a given period of
dynamic metamorphism Metamorphism associated with
zones where rocks are subjected to
depression of variable dimensions
time.
high differential pressures.
that results from the differential
disconformity An unconformity above and below which the strata
mudflow.
The removal of
deflation
loose
surface sediment by the wind,
deflation
hollow
A
shallow
erosion of surface materials by wind, delta the
An
mouth
alluvial deposit
formed
at
depositional environment An area in which sediment is deposited; a depositional
site differs in
aspects, chemistry,
physical
and biology from
adjacent environments, desert
Any
than 25
cm
are parallel.
discontinuity
of a stream.
area that receives
less
of rain per year.
A marked
change
in
scarp,
Earth materials or their properties,
as a thick, viscous, tongue-shaped
discordant Refers to plutons whose boundaries cut across the layering of country rock.
mass of wet
dissolved load
That part of a
taken into solution by chemical weathering.
divergent plate boundary
sand-sized and smaller panicles by
The boundary between two plates that are moving apart; new oceanic lithosphere forms at the boundary; characterized by volcanism and
wind.
seismicity.
A
surface mosaic
boulders found
in
many
dry regions
and formed by the removal of
604
Glossary'
flow that moves from
indicating a significant change in
developed soil and is mostly or completely devoid of vegetation,
pavement
A
the upper part of a hillside, leaving a
stream's load that consists of ions
of close-fitting pebbles, cobbles, and
earthflow
the velocity of seismic waves
Typically, a desert has poorly
desert
fault
and flows slowly downslope regolith.
The vibration of the Earth caused by the sudden release of energy, usually as a result of the earthquake
displacement of rocks along
faults.
echo sounder
An
sound signal to and return.
travel to the sea floor
instrument that determines the depth of the sea floor by measuring the time it takes for a
elastic
A theory earthquakes occur. rocks are deformed, they store rebound theory
that explains
When
how
dasr>.
•
dasnc
Ho.\«1n.
A
electron
Ilui. -
i
-
.
dectron capture
which an
.
n
rnmic numhci. Sm no atomic mass number
dectror. she.
shells.
Each
orbit
shell
can onl\ .
-rain
fault plane
numb:
dectro:
rilled.
dectrons move to the next
from the
farther
dement
A
n.
substance
same aroms.
shell
.
compos
cannot he changed into another clement b\ ordinary chemical means. all
the
fault
A
emergent coast
it
where the
coast
land has risen with respi
fctsk
magma
more
t!
A
....
considerable sodiu
aluminum, bin
lii.
.
.
.
.
um.
fcrromagnesian
\
silicate
-
1
mineral containing iron
.
level.
end moraine
A
pile of
ruhhle
deposited at the termini^
-
Such nun. are commonhj dark colored and densei than nonfcrromagncsian cm both.
esium
..I.
..I.
fossil
glacier.
.1.-
epicenter
The point on
the Earth's
The distance
fetch
surface vertically above the focus ot
an earthquake.
The removal
erosion
of weathered
material.
esker
A
long sinuous ridge of formed by deposition by running water in tunnels beneath stagnant ice or in melrwater channels
on the surface of evaporite
a glacier.
A
sedimentary rock that forms by inorganic chemical
precipitation of minerals solution.
A
from
lo;
>-
wmJ
Fr«< la..
.
:
tiosi
A
formed
gramilat type ol sm>v\ b> the melting
snow
and
.14
lion
llu
and refn
of snow. firn limit
..I
I
below firn
stratified drift
nnnuiuis
.
fiord
ch<
Irosi hi
The
elev.ition 10
whuh
recedes durinf
.
season. fission track dating
II. ,1
lllllMl.
follow?) I
he
dating samples by counting the number of small lineal tracks tissum mmii.il tracks) that result when .1
..,.1
pra ll.l si
".
.Iliiiu
I,.
I
wideni Iiii.iii;.
..|>.
mill,
.ii.l
•!..!
.nul
lli.it.
1
< <«"
Glossopteris flora A Late Paleozoic found only on the Southern
geologic time scale A vertical geologic chart arranged such that the designation for the earliest part of geologic time appears at the bottom,
flora
and progressively younger
gneiss
designations appear in their proper
that
chronologic sequence.
bands of
The
geology
and Earth
processes,
after
history.
is
A
conditions.
streaked or has segregated
One
the Triassic Period.
sedimentary bedding
temperature increase with depth. about 25°C/km near the Earth's
area.
A
steam.
The expansion and
budget
contraction of a glacier in response to
accumulation and wastage.
The sediment
glacial drift
type of in
activity.
A
boulder
transported by a glacier from
its
original source.
groove A deep straight scratch on a rock surface formed by glacial
Any
fossil
easily identifiable
has a wide geographic distribution and a short geologic range; used to determine the geologic fossil that
ages of strata and to correlate strata
which an
same
A
top.
volcanic origin rising
A stream possessing an equilibrium profile in which a delicate balance exists between gradient, discharge, flow velocity, channel characteristics, and sediment load such that neither significant erosion nor deposition occurs within
km
the channel.
decay to a stable daughter product, e.g., the half-life of potassium 40 is
characterized by a
graded stream
The
slope over which a
stream flows. Gradient generally from steep to gentle along the course of a stream, being steep in the
varies
glacial erratic
guide
guyot
gradient
deposited as a consequence of glacial
groundwater The water stored in the open spaces within underground rocks and unconsolidated material.
of the
is
It is
hot spring that intermittently ejects hot water and
glacial
A
ice as a
decrease in grain size from bottom to
individual bed
A
from area to
the
present-day continents of South
trapped within the Earth's crust.
surface, but varies
composed of
The sediment
from melting
terminus retreats.
glacier's
of the six major
Paleozoic continents;
ground moraine liberated
and dark minerals.
light
graded bedding
geyser
low-to-high-grade metamorphic
genus,
metamorphic rock
foliated
geothermal energy Energy that comes from the steam and hot water
geothermal gradient
known
best
its
Glossopteris.
America, Africa, Antarctica, Australia, and India; began fragmenting during
and internal
rocks), surface
named
Gondwana
science concerned
with the study of the Earth; includes studies of Earth materials (minerals
and
Hemisphere continents and India;
greenstone The name applied to any compact, dark green, altered, mafic igneous rock formed under
upper reaches and gentle lower reaches. granitization
A
in the
age.
flat-topped
above the sea
floor.
H The time required for number of
half-life
one-half of the original
atoms of a radioactive element to
1.3 billion years.
hanging valley A tributary glacial valley whose floor is at a higher level than that of the main glacial valley. hanging wall block
process whereby
seamount of more than 1
The block
different types of rocks are converted
headland
The seaward-projecting
to granite or granodiorite while in
part of the shoreline that
glaciers over bedrock. Glacial
the solid state.
on both
grooves are deeper than glacial
gravity
striations.
the expected force of gravity at a
movement of sediment-laden
the
Ice that has
glacial ice
from
formed
A
movement
glistening
of a sediment-laden
glacier over
A
of sediment-laden glaciers.
Glacial striations are rarely
more
than a few millimeters deep.
A
center of the Earth. gravity
is
a
mass
a
mass
A
on land that flow and basal slip.
mass of
plastic
Glossary
ice
effect
gases,
eroded
refraction.
agent of metamorphism;
the crust,
magma, and
applied
pressure.
The flow of heat from
the Earth's interior to
its
surface.
is
hiatus
deficiency.
some other
An
heat flow
negative
anomaly occurs when there
greenhouse straight scratch
a rock surface caused by the
movement
606
positive gravity
it.
glacial striation
moves by
A
heat
is
due to wave
sides
heat comes from increasing depth in
excess between the surface and the
smooth
bedrock surface formed by the
glacier
particular location.
departure from
anomaly occurs when there
firn.
glacial polish
on
A
anomaly
that
overlies a fault plane.
Carbon dioxide, and water vapor
allow sunlight to penetrate the atmosphere but trap the heat
The
interval of geologic time
not represented by strata
in a
sequence
of strata containing an unconformity.
horn A steep-walled, pyramidal peak formed by the headward
reflected
erosion of cirques.
surface, thus causing the
hornfels
increase in global temperatures.
metamorphic rock resulting from contact metamorphism.
back from a planet's atmosphere to heat up. The result is an overall
A
fine-grained, nonfoliated
hot spot A localized zone of melting below the lithosphere; detected by volcanism at the
intensity
earthquake as well as people's
densities, indicating that they are
surface.
reaction to
hot spring A spring in which the water temperature is warmer than the temperature of the human body
intermediate magma A magma having a silica content between 53 and 65% and an overall composition intermediate between felsic and mafic
composed mostly of lightweight gases such as hydrogen and helium, as well as frozen compounds such as ammonia and methane.
the kind of
(37°C).
humus gives
many
soils their
hydraulic action
it.
magmas.
The material derived by
bacterial decay of organic matter;
The subjective measure of damage done by an
it
dark color.
The power of
A
internal drainage
drainage found
in
K type of
semiarid and arid
regions in which a stream drains into
low area without
moving water.
a central
hydrologic cycle The continual recycling of water from the oceans, through the atmosphere, to the continents, and back to the oceans.
intrusive igneous rock
The chemical reaction + between the hydrogen (H ions and hydroxyl (OH~) ions of water and a
ion
hydrolysis
)
numerous that
from magma intruded or formed in place within the
into
An
A bond
bond
identification of the
group of meteorites composed primarily of iron and nickel alloys and accounting for about 6% of all meteorites.
intensity.
as ash.
isostatic
meandering canyon cut into bedrock by a stream. inclusion
An
solid
incompletely melted
piece of rock enclosed within an
igneous rock.
index mineral A mineral that forms only within specific temperature and pressure ranges. Index minerals allow geologists to recognize low-, intermediate-, and high-grade metamorphic zones. infiltration capacity rate at
which a
soil,
The maximum under a given
condition, can absorb rain.
inselberg
An
isolated steep-sided
concordant pluton with
mushroomlike geometry.
lahar
A
lateral
moraine
volcanic mudflow.
The sediment
deposited as a long ridge of
consolidation of volcanic ejecta such
deep,
A
laccolith a
A
igneous rock Any rock formed by cooling and crystallization of magma, or by the accumulation and
A
unit
that results
isograd A line on a map connecting the first appearances of a particular index mineral and thus indicating equal metamorphic
meander
same rock
in different areas.
from the attraction of positively and negatively charged ions. irons
rock unit that is allow
sufficiently distinctive to
charged atom
produced by adding or removing electrons from the outermost electron shell.
A
key bed
electrically
ionic
incised
caves, springs, sinkholes,
solution valleys, and disappearing
streams.
Earth's crust.
hypothesis A tentative explanation formulated to explain observed phenomena and used as a basis for further experimentation or
I
exiting.
Rock
A topography developed largely by groundwater erosion and characterized by karst topography
crystallizes
mineral's ions.
investigation.
Neptune) that resemble Jupiter. They are all large and have low mean
till
along
the margin of a glacier.
A soil formed in the tropics where chemical weathering is intense and leaching of soluble minerals is laterite
complete.
rebound The phenomenon in which unloading of
Laurasia A Late Paleozoic northern hemisphere continent composed of
the Earth's crust causes
the present-day continents of
upward
it
until equilibrium
to rise is
North
America, Greenland, Europe, and
again
Asia.
attained.
lava
Magma at the Earth's surface. dome A bulbous, steep-sided
isotope Two or more forms of an element having the same atomic
lava
number and
structure
similar chemical
formed by very viscous
properties, but a different atomic
magma moving upward
mass number.
volcanic conduit.
lava flow
A
stream of
through a
magma
flowing over the Earth's surface.
J
leaching
A
which no movement has occurred, or where joint
movement has been perpendicular the fracture surface.
erosional remnant that rises above
Jovian planets
the surrounding desert plains.
(Jupiter, Saturn,
The four
to
planets
dissolution or
The process by which and water-saturated sediments
liquefaction fill
Uranus, and
The
removal of soluble minerals from a soil or rock by percolating water.
fracture along
liquefy, or
behave as a
fluid,
when
shaken.
Glossary
607
composition are mixed together producing a modified version of the
matter
parent magmas.
meandering stream
possessing a single, sinuous channel
the Earth consisting of the upper
magnetic anomaly Any change, such as a change in average strength,
mantle, oceanic crust, and
of the Earth's magnetic
mechanical weathering
The process by which
lithification
sediment is transformed into sedimentary rock.
The
lithosphere
outer, rigid part of
continental crust;
lies
above the
asthenosphere. lithostatic pressure
resulting
Pressure
from the weight of the
overlying rock;
it is
applied equally
in all directions.
Windblown
loess
deposits; derived
sources
— deserts,
silt and clay from three main
Pleistocene glacial
field.
The
field
area in which
magnetic substances are affected by lines of magnetic force emanating from the Earth.
The
magnetic inclination
deviation
ridge of sand aligned generally
magnetic reversal The phenomenon in which the north and south magnetic
parallel to the direction of the
poles are completely reversed.
sand supply
is
where the
somewhat
limited.
magnitude
refraction.
longshore drift The movement of sediment along a shoreline by longshore currents.
its
surface
wave
in
which the individual particles of the material only move back and forth in a horizontal
plane perpendicular
to the direction of
wave
travel.
low-velocity zone The zone within the mantle between the depths of
km
100 and 250
where the
velocity
of both P- and S-waves decreases
markedly;
it
total
amount
corresponds closely to
the asthenosphere.
largely of peridotite.
A
stationary
column
that originates deep
within the mantle and slowly rises to the Earth's surface to
form volcanoes
mafic
silica-poor
magma
52%
and proportionately more calcium, iron, and magnesium than a silica
felsic
magma.
magma
generated within the Earth,
magma
mixing
The process
whereby magmas of
608
Glossary
different
A
broad, flat-topped
bounded on
all
by steep slopes; forms when the resistant cap rock is breached,
sides
allowing rapid erosion of the
less
resistant underlying sedimentary rock.
metallic
An
bond
electron sharing in
extreme type of which the electrons
of the outermost electron shells of
metals are readily lost and
move about
from one atom to another. facies A group of metamorphic rocks characterized by
particular mineral assemblages
formed under the same broad
calcite or dolomite.
temperature-pressure conditions.
marine regression The withdrawal of the sea from a continent or coastal
metamorphic grade
area resulting in the emergence of land
within a metamorphic zone,
as sea level falls or the land rises with
which are the same grade, medium, or high grade.
now
An
old wave-cut
elevated above sea
metamorphic rock
The rocks
Any rock
altered by high temperature
of
all
i.e.,
low,
type
and
pressure and the chemical activities
level.
marine transgression of coastal areas or
much
The
invasion
of a
continent by the sea resulting from a rise in sea level
Molten rock material
mesa
metamorphic
platform
magma A
through the Himalayas, across Iran and Turkey, and through the Mediterranean region of Europe; about 20% of all active volcanoes and 15% of all earthquakes occur in
marble A nonfoliated metamorphic rock composed predominantly of
marine terrace
containing between 45 and
Mediterranean belt A zone of seismic and volcanic activity that extends westerly from Indonesia
or flood basalts.
respect to sea level.
M
medial moraine A moraine formed where two lateral moraines merge.
erosional remnant
mantle plume
magma
that retain the chemical composition
this belt.
source.
composed
breaking of rock materials by
of
mantle The mantle surrounds the core and comprises about 83% of the Earth's volume; it is less dense than the core and is thought to be
of
A
Love wave
The
energy released by an earthquake at
longshore current A current between the breaker zone and the beach that flows parallel to the shoreline and is produced by wave
The
of the parent material.
magnetic
horizontal.
prevailing wind; forms
with broadly looping curves.
geographic pole.
of the magnetic field from the
long, parallel
stream
physical forces into smaller pieces
rivers in semiarid regions.
A
A
magnetic declination The angle between lines drawn from a compass position to the magnetic pole and the
outwash deposits, and floodplains of longitudinal dune
Anything that has mass and
occupies space.
or subsidence of the
of fluids
is
said to have been
metamorphosed,
e.g., slate, gneiss,
marble.
metamorphic zone
The region
land.
between isograds.
mass wasting The downslope movement of material under the
meteorite
influence of gravity.
to the Earth's surface.
A
mass of matter of
extraterrestrial origin that has fallen
A
microplate block that
Lava that cools so constituent atoms do
natural glass
small lithospheric
clearly of different
is
rapidly that
its
not have time to become arranged in the ordered, three-dimensional
origin than the rocks of the
surrounding mountain system and adjacent craton.
framework
migmatite A rock having both igneous and high-grade metamorphic
natural levee
characteristics; usually consists of
of a stream channel during floods.
streaks or lenses of granite
neutron
intermixed with high-grade
particle
ferromagnesian-rich metamorphic
atom.
rocks.
Milankovitch theory
A
theory that
explains cyclic variations in climate as a
consequence of
A
and
A
ridge of sandy
alluvium deposited along the margins
An found
electrically neutral in the
nucleus of an
nonconformity An unconformity in which stratified rocks above the erosion surface overlie igneous or
irregularities in
the Earth's rotation
mineral
typical of minerals.
metamorphic rocks.
orbit.
nonferromagnesian
naturally occurring,
silicate
inorganic, crystalline solid, with a narrowly defined chemical composition and characteristic
iron or magnesium. Nonferromagnesian
physical properties.
dense than ferromagnesian
Modified Mercalli Intensity Scale A scale having values ranging from to XII that is used to measure earthquake intensity based on damage.
mantle. Also called the
Moho.
in
silt-sized particles
A
oceanic crust The crust underlying the ocean basins. It ranges in thickness from 5 to 10 km and has a composition of basalt and an average 3 density of 3 g/cm .
oceanic-oceanic plate boundary A type of convergent plate boundary along which two oceanic lithospheric plates collide.
oceanic ridge A submarine mountain system found in all of the it is composed of volcanic rock (mostly basalt) and displays features produced by tensional
oceans;
forces.
oceanic trench
A
long,
narrow
feature restricted to active
ooze Deep-sea pelagic sediment composed mostly of shells of marine animals and plants. ophiolite
A
sequence of rock
now
on land consisting of deep-sea sediments, oceanic crust, and upper
moved downward
mantle.
relative to the
Rocks having a record of magnetism the same as the
The process of forming mountains, especially by folding and thrust faulting; an episode of
present magnetic
mountain building.
orogeny
nuee ardente
field.
A
mobile dense cloud
of highly heated pyroclastic material and gas ejected more or less
outgassing
The process whereby
gases derived from the Earth's interior are released into the
atmosphere by volcanic
ourwash plain
activity.
-
The sediment
deposited by the meltwater discharging from the terminus of a continental glacier.
hard, dense,
metamorphism; typically restricted narrow zones adjacent to faults.
N native element
A
mineral
a single element.
lake A cutoff meander with water. Oxbow lakes
oxbow
fine-grained metamorphic rock resulting from pure dynamic
composed of
dip-slip fault
path.
environments.
mylonite
A
than air, it rushes down the slope of a volcano engulfing everything in its
water; most
semiarid and arid
and by volcanism and
seismicity.
from tensional forces in which the hanging wall block has resulting
horizontally from a volcanic vent. Because a nuee ardente is denser
flow consisting of
30%
fault
plate
subduction occurs.
normal polarity
fractures.
common
normal
oceanic plate beneath a continental
continental margins and along which
A
nonfoliated texture
footwall block.
crack A sedimentary structure found in clay-rich sediment that has dried out. When such sediment dries, it shrinks and forms intersecting
and more than
silicate
orientation of mineral grains.
mud
mostly clay- and
less
metamorphic texture in which there is no discernible preferred
A
A
minerals
and
minerals. I
simple bend or flexure in otherwise horizontal or uniformly dipping rock layers.
mudflow
silicate
are generally light colored
Mohorovicic discontinuity The boundary between the crust and the monocline
A
silicate
mineral that does not contain
characterized by subduction of an
filled
o
form when meanders become so
to
oblique-slip fault
A
fault
having
sinuous that the thin neck of land is cut
both dip-slip and strike-slip
separating adjacent meanders
movement.
off during a flood, leaving a cutoff
oceanic-continental plate boundary A type of convergent plate boundary along which oceanic lithosphere and continental lithosphere collide;
meander.
oxidation The reaction of oxygen with other atoms to form oxides or, if water is present, hydroxides.
Glossary
609
A
pahoehoe
type of lava flow that
has a ropy surface.
The
paleocurrent
direction of an
ancient current; determined by
measuring the orientations of various sedimentary structures such as cross-bedding.
paleomagnetism The remanent magnetism in ancient rocks that records the direction and strength of the Earth's magnetic field at the time
of their formation.
Pangaea The name proposed by Alfred Wegener for a supercontinent that existed at the end of the Paleozoic Era and included all the Earth's landmasses.
A
parabolic dune
crescent-shaped
dune in which the tips point upwind; forms where the vegetation cover is broken and deflation produces a blowout.
parent material
The
The
passive continental margin trailing
soil.
edge of a continental plate
consisting of a broad continental shelf rise.
and a continental slope and
A
vast, flat abyssal plain
commonly rise.
is
present adjacent to the
Passive continental margins lack
intense seismic
pedalfer
humid
A
and volcanic
soil that
activity.
develops
in
regions and has an
A horizon and aluminum-rich clays and iron oxides in horizon B. organic-rich
pediment surface of
away from
An
erosional bedrock
low relief gently sloping a mountain base; most
result of stress in
cannot recover
its
plate
the continents and oceanic islands.
lithosphere that
perched water table A water table that may form where a local
asthenosphere. plate tectonic theory
aquiclude occurs within a larger
that large segments of the outer part
aquifer; water migrating through the zone of aeration is stopped by the local aquiclude, and a localized zone of saturation "perched" above the main water table is created.
of the Earth (lithospheric plates)
brown or composed
Generally
An
peridotite
igneous rock
containing about 90% ferromagnesian minerals (olivine and
original shape
A
permeability
material's capacity
for transmitting fluids.
A
coarse-grained texture
igneous rocks in which the
mineral grains are easily visible
without magnification.
A
phaneritic
texture results from the slow cooling
of a
magma. larger grains in a
porphyritic texture.
A
move
A general term that encompasses the concepts of continental
spreading,
drift, sea-floor
and transform
faults.
A dry lake bed found and characterized by mudcracks and chemically
in
precipitated rocks such as rock
gypsum; formed by the evaporation of water in a playa lake.
playa lake A temporary lake formed in a desert after a rainstorm.
plunging fold
A
fold with an
inclined axis.
pluton An intrusive igneous body that forms when magma cools and crystallizes within the Earth's crust.
The
phenocryst
phyllite
The theory
plate tectonics
deserts
of the world's land surface.
moves over the
like a very viscous fluid.
playa
permafrost Ground that remains permanently frozen; covers nearly
individual piece of
one another; lithospheric plates are rigid and move over the asthenosphere, which behaves much
component of
the mantle.
An
stress
relative to
pyroxene) and about 10% feldspar; thought to be the principal
in
weathered to yield sediment and
a material
of clay-sized particles derived from
material that
being chemically and mechanically
which
reddish deep-sea sediment
pelagic clay
phaneritic
decay.
The
plastic strain
and retains the configuration produced by the such as by folding of rocks.
20%
parent element An unstable element that is changed into a stable daughter element by radioactive
is
pegmatite A very coarse-grained igneous rock commonly associated with plutons.
fine-grained
metamorphic rock composition to
similar in
slate,
but slightly
plutonic rock Another name for an intrusive igneous rock, i.e., one that crystallizes from magma intruded into or
formed
in place
within the
Earth's crust.
coarser grained.
point bar
Bulbous masses of basalt resembling pillows. It forms when lava is rapidly chilled beneath water and is characteristic of much of the igneous rock in the upper part
the gently sloping side of a
of the oceanic crust.
porphyritic An igneous texture with mineral grains of markedly
pillow lava
planetesimal
A
large
mass of
Sediment deposited on
meander
loop.
porosity
The percentage of a volume that is pore
material's total space.
gaseous, liquid, and solid particles
different sizes that results
pediments are covered by a thin layer of debris or by alluvial fans or
that began accreting during the early
two-stage cooling history. The larger
history of the solar system and
grains are phenocrysts, and the
bajadas.
eventually
pedocal A soil characteristic of arid and semiarid regions with a thin A horizon and a calcium carbonate-rich B horizon.
body.
610
Glossary
became
plastic flow in
a true planetary
The flow
that occurs
response to pressure and causes
permanent deformation.
from a
smaller ones are referred to as groundmass.
precursor A short-term or long-term change within the Earth that precedes an earthquake.
pressure release A mechanical weathering process in which rocks
formed deep within the Earth, due to a release of pressure, expand upon being exposed at the surface. that
A
proton particle
positively charged
found
in the
occurs quite suddenly and the
nucleus of an
atom.
material
moves very quickly
downslope.
A
P-wave
push-pull wave; the fastest seismic
Rayleigh wave A surface wave which the individual particles of
the surface of a lava flow that forms
wave and one that can travel through solids, liquids, and gases; also
within a vertical plane oriented
because of pressure on the partly
known
the direction of
A
pressure ridge
buckled area on
moving
solid crust of a
flow.
in
An
important principle
determining the relative ages of
events; holds that an igneous intrusion
or fault must be younger than the
rocks that
it
on the work of
William Smith that holds that fossils, and especially assemblages of fossils, succeed one another through time in a regular and determinable order.
A
A
pyroclastic
A
principle of fossil succession
principle of inclusions
earthquake focus where little P-wave energy is recorded by seismographs. The P-wave shadow zone results from the fact that the Earth has a solid inner core.
intrudes or cuts.
principle based
primary wave.
P-wave shadow zone The area between 103° and 143° from an
principle of cross-cutting
relationships
as a
compressional, or
fragmental texture
material
move
and
ejected
from a volcano.
till
was deposited.
reef
A
moundlike, wave-resistant
composed of the skeletons
reflection
refraction
fragments in a sandstone are older than the sandstone rock unit. principle of isostasy
The
theoretical concept of the Earth's
underlying layer. principle of lateral continuity
hard, compact
nonfoliated metamorphic rock
formed from quartz sandstone under low-to-high-grade metamorphic conditions during typically
contact or regional metamorphism.
on a dense
crust "floating"
A
quartzite
A
quick clay A clay that spontaneously liquefies and flows like water when disturbed.
that holds that sediment layers in all directions until
they terminate.
R The spontaneous decay of an atom by emission of a
A
particle
radioactive decay
from
its
nucleus (alpha and
Steno that holds that sediment layers
beta decay) or by electron capture;
are deposited horizontally or very
the
nearly so.
different element.
principle of superposition
A
principle developed by Nicolas Steno that holds that
younger layers of
strata are deposited
on top of older
strata.
principle of uniformitarianism principle developed by
A
James Hutton
we
can interpret past events by understanding present-day processes; based on the assumption that holds that
that natural laws have not
through time.
changed
elasticity.
refractory element
Any
element,
such as iron, magnesium, silicon, or aluminum, that condenses easily at high temperature.
metamorphism Metamorphism that occurs over
a
and is usually the result of tremendous temperatures, pressures, and deformation within the deeper large area
principle of original horizontality principle developed by Nicolas
The change in direction and velocity of a seismic wave when it travels from one material into another of different density and
regional
principle developed by Nicolas Steno
extend outward
of a
within the Earth.
elasticity
fragments, in a rock unit are older granite
some when it
return of
encounters a boundary separating materials of different density or
principle
itself, e.g.,
The
seismic wave's energy
that holds that inclusions, or
than the rock unit
in
wave movement.
recharge The addition of water to the zone of saturation.
of organisms.
Fragmental
path
terminus of a glacier has stabilized
structure
material that has been explosively
elliptical
moraine A moraine formed by a retreating glacier; it marks the location where the
formed by explosive volcanic pyroclastic material
an
recessional
characteristic of igneous rocks activity.
in
in
atom
is
changed to an atom of a
rainshadow desert A desert found on the lee side of a mountain range; forms because moist marine air moving inland forms clouds and produces precipitation on the windward side of the mountain range such that the air descending on the leeward side is much warmer and
The layer of unconsolidated rock and mineral fragments that covers almost all the regolith
Earth's surface.
The process of determining the age of an event relative to other events; involves placing geologic events in their relative dating
correct chronologic order, but
involves
no consideration of when
the events occurred in terms of
number of years
drier.
rapid mass
portions of the Earth's crust.
movement
A
mass movement involving a
movement of
type of visible
material; usually
reserve
ago.
That part of the resource
base that can be extracted economically.
Glossary
611
A
resource
A
rock varnish
concentration of
naturally occurring solid, liquid, or
gaseous material in or on the Earth's crust in such form and amount that
thin, red,
brown,
or black shiny coating on the surface of many desert rocks; composed of iron
and manganese oxides.
cross-bedding,
mud
and
cracks,
animal burrows.
A
seismic gap
region that
locked
is
and not releasing energy; a prime
economic extraction of a commodity from the concentration is currently
rounding
sharp corners and edges of
seismic profiling
or potentially feasible,
sedimentary particles are abraded during transport and become
to echo sounding. Strong waves are
rounded.
penetrate the layers beneath the sea
A
reverse fault
dip-slip fault
from compressional forces which the hanging wall block has
resulting in
moved upward
runoff
The process by which
The
the
surface flow of streams.
relative to the
Rocks having
a
saltwater incursion
the present magnetic
displacement of fresh water by water as a result of excessive
Richter Magnitude Scale An open-ended scale that measures earthquake magnitude; values begin
scientific
narrow surface
generated at an energy source and floor.
Some
of the energy
is
reflected
in coastal
metamorphic rock most commonly produced by regional metamorphism. logical
can be mapped. seismic risk
map
A map
and
the distribution
foliated
method
An
based on
intensity of
previous earthquakes. Such
A
erosion Erosion by running water that scours small channels in the ground.
A
salt
areas.
schist
rip current
The
pumping of groundwater
at 1. rill
maps
cannot predict when the next major earthquake will occur, but do indicate the potential severity of
future earthquakes and are useful in orderly,
approach that involves
helping people plan for such
earthquakes.
current that flows out to sea through
gathering and analyzing the facts or
the breaker zone.
data about the problem under
seismogram
consideration.
earthquake waves made by a seismograph.
mark
Wavelike (undulating) structure produced in granular sediment such as sand; formed by wind, unidirectional water currents, ripple
or wave currents.
rock
A
consolidated aggregate of
The theory moves away from spreading centers and is eventually subducted and consumed at that the sea floor
although they are exceptions to
origin rising
and
aggregates of shells are also
more than
1
km
above
the sea floor.
is
rock cycle A sequence of processes through which Earth materials may pass as they are transformed from one rock type to another.
A common
structure of volcanic
Weathered material that derived from preexisting rock.
sediment
considered rocks.
rockfall
A
type of
sedimentary facies
aspect of a
sedimentary rock unit that makes it recognizably different from adjacent sedimentary rocks of the same, or approximately the same, age.
extremely rapid mass movement in which rocks of any size fall through
sedimentary rock
the
may
air.
Any
An
seismograph
Any rock
composed of sediment. The sediment be particles of various sizes such
The study
seismology
shear strength The resisting forces helping to maintain slope stability; includes the slope material's strength
and cohesion, the amount of internal friction between grains, and any external support of the slope.
The
shear stress
opposite directions; results in deformation by displacement of adjacent layers along closely spaced planes.
mineral that comprises a significant
animals or plants as in coal and some limestones, or chemicals in solution that are extracted by
sheet erosion
organic or inorganic processes.
soil.
sedimentary structure Any structure in sedimentary rock such as
more or
rock glide
movement
A in
type of rapid mass which rocks move
downslope along a more or planar surface.
612
Glossary
less
result of forces
acting parallel to one another but in
as gravel or sand, the remains of
portion of a rock.
of
earthquakes.
A common
rock-forming mineral
instrument that
and measures the various vibrations produced by an detects, records,
earthquake.
convergent plate margins.
seamount
definition, coal, natural glass,
The record of
sea-floor spreading
minerals or particles of other rocks; this
similar
crust beneath the sea-floor sediments
record of magnetism the opposite of field.
A method
from the various geologic horizons back to the surface, and in this manner, the structure of the oceanic
footwall block.
reversed polarity
location for future earthquakes.
or
less
Erosion that
is
more
evenly distributed over the
surface and removes thin layers of
sheet joint
Large fractures that are the rock
less parallel to
and
surface
from pressure
result
released by expansion of the rock.
shield An area of exposed ancient rock found on every continent.
volcano
shield
volcano;
and
has a
it
The largest type of low rounded profile
composed mostly of
is
basalt
The
line of intersection
The slow downslope movement of water-saturated surface solifluction
A compound
silica
of silicon and
oxygen atoms. tetrahedron
building block of minerals.
It
The
basic
silicon
A
silicate sill
A
mineral containing
silica.
tabular or sheetlike
A
sinkhole
depression
in the
ground that forms in karst regions by the solution of the underlying carbonate rocks or the collapse of a cave roof.
A
slate
very fine-grained foliated
metamorphic rock
from
resulting
low-grade regional metamorphism of
more
shale or,
rarely, volcanic ash.
93%
stony-irons
solid substance
of
all
meteorites.
A
group of meteorites composed of nearly equal amounts of iron and nickel and silicate minerals and comprising about 1% all
meteorites.
magma
A
process in which rising
detaches and engulfs pieces
of the surrounding country rock.
Deformation caused by
strain stress.
A
term referring to the
degree to which
all
particles in
A
stratified drift
both sorting and
sediment and sedimentary rock are about the same size.
stream
Drift displaying stratification.
Runoff that
is
channels regardless of small, steep-sided
cone that forms when gases escaping from a lava flow hurl globs of molten lava into the air that fall back to the surface and adhere to one another. spheroidal weathering A manifestation of chemical weathering in
about
become
spatter cone
concordant pluton.
silicate
stoping
dissolves.
atom and four oxygen atoms,
group of meteorites iron and magnesium minerals and comprising
reaction in which the
and the
sorting
one
of
dissociated from one another in a
all silicate
consists of
in areas
A
stones
composed of
of
A
ions of a substance liquid,
silica
common
permafrost.
solution
between the sea and the land,
theory for
and evolution of the solar system from an initial rotating cloud of gas that formed in a spiral arm of the Milky Way Galaxy,
sediment; most
flows.
shoreline
A
solar nebula theory the origin
which rock, even
if
rectangular to
begin with, weathers to form a spheroidal shape.
confined to
size,
stream terrace An erosional remnant of a floodplain that formed when the stream was flowing at a higher
level.
The force per unit area applied to a material such as rock within the Earth's crust. stress
strike
The
direction of a line
formed by the intersection of a horizontal plane with an inclined
A continuation of a beach forming a point that projects into a
plane, such as a rock layer.
movement of material along one or more surfaces of
body of water, commonly
horizontal
failure.
spreading ridge
A
slide
type of mass
movement
involving
slow mass movement
movement
Mass is
oceanic lithosphere
usually only
detectable by the effects of
its
A
spring
a bay.
location where
is
The downslope movement
icicle-shaped
carbonate structure hanging from a
of material along a curved surface of
cave ceiling; forms as a result of
rupture; characterized by the
precipitation
backward rotation of the slump
saturated dripping water.
block. soil
that rises
weathered material, water, air, and organic matter that can support
from carbonate-
A
carbonate projection floor; forms from carbonate-saturated water dripping from a cave ceiling. stalagmite
Regolith consisting of
from a cave
plants.
stock
horizon A distinct soil layer that differs from other soil layers in
surface area less than 100
soil
texture, structure, composition, color.
and
Many
A
discordant pluton with a
km 2
stocks are simply the
A
fault involving
movement
in
which
blocks on opposite sides of a fault another.
forming.
place where groundwater
An
stalactite
strike-slip fault
plane slide sideways past one
and new
flows or seeps out of the ground.
movement.
slump
A
plates are separating
that advances at an
imperceptible rate and
spit
subduction
The process whereby
the leading edge of
one plate
descends beneath the margin of another plate.
subduction zone An elongated, narrow zone at a convergent plate boundary where an oceanic plate descends relative to another plate,
Nazca beneath the South American
the subduction of the
e.g.,
plate plate.
submarine canyon A steep-sided canyon cut into the continental shelf and slope.
.
submarine fan
A
sedimentary
exposed parts of much larger
deposit located seaward of a
plutons.
submarine canyon.
Glossary
613
A
tectonics, a type of circulation of
earthquake but can also be caused by submarine landslides or volcanic
land or the land subsides.
material in the asthenosphere during
eruptions.
superposed stream A stream that once flowed on a higher surface and eroded downward into resistant
which hot material rises, moves laterally, cools and sinks, and is reheated and reenters the cycle.
submergent coast
coast in which
sea level rises with respect to the
rocks, while
maintaining
still
its
course.
The
suspended load
smallest
by a stream, such which are kept
particles carried
and clay, suspended by silt
A
S-wave
as
fluid turbulence.
shear
wave
that
moves
thermal convection
cell
In plate
thermal expansion and contraction A type of mechanical weathering in which the volume of rocks changes in response to heating and cooling. thrust fault
which the
in
A
type of reverse fault
fault plane dips less
than 45°. regular fluctuation in the
tide
direction of travel, thereby producing
sea's surface in response to the
shear stresses in the material
gravitational attraction of the
it
floor.
u
Moon
U-shaped
the
stream
S-wave shadow zone Those areas more than 103° from an earthquake focus where no S-waves are recorded. The S-wave shadow zone indicates that the outer core must be
stratified.
cannot travel
liquid since S-waves
through
liquid.
syncline
A
down-arched fold
characterized by an axial plane that divides
it
in half.
The weathered material that accumulates at the base of slopes,
talus
Forces acting in
tensional stress
opposite directions along the same
till
time-distance graph A graph showing the average travel times for
and S-waves for any specific distance from an earthquake's focus. P-
A
tombolo
type of spit that
terminal moraine The outermost end moraine, marking the greatest
The four
innermost planets (Mercury, Venus, Earth, and Mars) of the solar system. They are all small and have high
mean densities, indicating that they are composed of rock and metallic elements.
theory natural
An
explanation for some
phenomenon
velocity
or no vegetation exists. tree-ring dating
that has a large
trough
scientific, a
wave
be testable
(e.g.,
614
plate tectonic
tsunami that
Glossary
The lowest point between
crests.
is
A
destructive sea
wave
usually produced by an
long,
narrow deposit
of stratified drift confined within a
van der Waals bond attraction exerted by
A
The weak all
molecules in
measure of the
downstream distance water
travels
per unit of time. Velocity varies
among streams and even within the same stream. considerably
A
ventifact
stone whose surface
has been polished, pitted, grooved, or faceted by the wind; a common product of wind abrasion. vesicle
A
small hole or cavity
formed by gas trapped
in a
cooling
lava.
viscosity
The process of
determining the age of a tree or wood in structures by counting the number of annual growth rings.
considered theory).
long ridge of
sand perpendicular to the prevailing wind direction; forms in areas where abundant sand is available and little
body of supporting evidence;
to be theory must
A
transform plate boundary A plate boundary along which plates slide past one another, and crust is neither produced nor destroyed.
A
glacier confined to
valleys.
proximity.
dune
a glacier through a
mountain valley or perhaps to an interconnected system of mountain
motion.
running water, wind, or glaciers.
valley
a
glacial valley.
transverse
A
valley glacier
transform fault A type of fault that changes one type of motion between plates into another type of
The mechanism by which weathered material is moved from one place to another, commonly by
A
valley.
valley train
extent of a glacier. terrestrial planets
movement of
extends out into the sea and connects an island to the mainland.
transport
line.
glacial trough
with very steep or vertical walls and a broad, rather flat floor. Formed by
Sediment deposited directly by glacial ice. It is not sorted or
as a
erosional surface
that separates younger strata from
secondary wave, an S-wave only travels through solids.
also
An
unconformity
and Sun.
moves through;
downslope to the deep-sea
that flows
older rocks.
The
material perpendicular to the
known
A sediment-water mixture denser than normal seawater
turbidity current
A
fluid's resistance to
flow.
volatile elements
Elements such as hydrogen, helium, ammonia, and methane that condense at very low temperatures.
volcanic island arc A curved chain of volcanic islands parallel to a deep-sea trench where oceanic
lithosphere is subducted causing volcanism and the origin of volcanic
configuration of the water table
islands.
generally a subdued replica of the
volcanic neck An erosional remnant of a volcanic pipe after a volcano has eroded away.
overlying land surface.
volcanic pipe
The conduit
connecting the crater of a volcano
with an underlying chamber.
magma
materials
when
pyroclastic
become consolidated.
volcanism The process whereby magma and its associated gases rise through the Earth's crust and are extruded onto the surface or into the atmosphere.
volcano A conical mountain formed around a vent as a result of the eruption of lava and pyroclastic materials.
w water table The surface separating the zone of aeration from the
A
water well
made by
well
is
yardang
An
elongated and
streamlined ridge that looks like an
digging
overturned ship's hull; formed by wind erosion and typically found
or drilling into the zone of
grouped
saturation.
to the prevailing
wave base
A
aligned parallel
in clusters
wind
direction.
depth of about
one-half wave length, where the orbital
Another name for extrusive igneous rock that forms when magma is extruded onto the Earth's surface and cools and volcanic rock
crystallizes or
underlying zone of saturation; the
motion of water
particles
is
zone of accumulation
essentially zero.
A
wave-cut platform
terminology, another
beveled
surface that slopes gently in a
seaward direction and the retreat of a sea
wave height
The
from wave trough
wave
The
length
is
formed by
cliff.
vertical distance
to
wave
crest.
between successive wave
is
perennially
covered by snow.
crests or
zone of aeration
troughs.
The zone above
the water table that contains both
wave period two
for
horizon B where the soluble minerals leached from horizon A accumulate as irregular masses. In glacial terminology, the part of a glacier where additions exceed losses and the glacier's surface
distance
In soil
name
successive
The time required wave crests (or
for
water and air within the pore spaces of the rock or soil.
troughs) to pass a given point.
zone of saturation
wave
the zone of aeration in which
The bending of a more nearly parallels
refraction
wave so
that
The zone below all
the
with
the shoreline.
pore spaces are groundwater.
weathering
The physical breakdown and chemical alteration of rocks and minerals at or near the
zone of wastage The part of a glacier where losses from melting, sublimation, and calving of icebergs
Earth's surface.
exceed the rate of accumulations.
it
filled
Glossary
615
:^^-^^x^^^^t^«^s^^
INDEX
Aa
lava flow, 89, 91
Aberfan, Wales,
tip failure,
422-23
Abrasion, 161, 460 glacial,
523-24
waves, 591
wind, 549, 550-51 Absolute dating, 216, 231, fission track dating,
radioactive decay
234-43 242-43
and
half-lives,
234-39 radiocarbon dating methods, 239, 242 tree-ring dating,
242
329-30 plain, 323, 325-26
Abyssal
Amphibole, 67, 116, 119, 121 in sequence of index minerals, 201 silica tetrahedra of, 66 Amphibole asbestos, 196, 197 Amphibolite, 202, 204
Asbestos, 61, 193,
facies,
207
intermediate
Accessory minerals, Accretion, 378,
74—75
408
gravity,
463, 466
lateral vs. vertical,
and South America, 343, 345,
538-39 538
558-59
George, 304
Bootlegger Cove Clay
slides,
276,
488
493, 494, 495 Plains, 500, 501 perched water table and, 490-91
High
434-35, 436 earthquake of 1964, 257, 267, 270,
Aragonite, 168
276, 435
Arch,
435 Alkali soils, 150 pipeline,
390-91, 479
144-45
formation, 316, 317
Jovian planets, 44, 45, 47 mass, 289 regulation of (Gaia hypothesis), terrestrial planets, 35,
water vapor Atoll,
in,
12-13
40-41
452, 453, 454
333-34
Atom(s), 56-58, 231 basic structure, 57 bonding of, 58-60 Atomic mass number, 57-58, 231, 234-35, 239, 242 Atomic number, 57, 234-35, 239, 242 Augite, 67, 72 Aurelia (Venusian crater), 26
Aureole,
198-99
Avalanche, 421, 424 Axial plane (fold), 381
Axis (fold), 383
Archimedes, 305
469-70
Arete,
in deserts, 567, 568 Alluvium, 461
Alpha decay, 234, 236 Alpine-Himalayan orogenic 398
390-91
Arches National Park, Utah, 389,
Alleghenian orogeny, 406 Alluvial fans,
acid rain,
in artesian systems,
320-21
Atmosphere, 29, 35, 40-41, 90
Aphanitic texture, 113, 114
Aquifer(s),
Alaska, 341
322, 326
vs. Pacific,
Atlantis, 317,
354-55
Appalachian Mountains, 16, 395, 396, 410, 426, 435 and European mountains, 347-48 formation, 404, 406-7 Aquiclude, 488, 490-91, 493
347
Airy, Sir
Asymmetrical fold, 382 Athabaska tar sands, Canada, 183 Atlantic Ocean, 320, 361
381-82, 387
Anticline,
117—18
Asthenosphere, 11, 16, 17, 296, 357 convection cells in, 370
plunging, 383, 385
324, 325, 326, 408
Air pressure belts,
and, 85, 98, 121
247
Asteroids, 33, 35
Anthracite, 173, 181, 202, 205
Active continental margin, 184, 323,
Agassiz, Louis, 533,
use in dating,
Assemblage range zone, 229, 231, 235
Anorthite, 50
144-45
Acidity of solutions, 142
Agassiz, Lake,
Ash, volcanic, 4, 85, 90-91, 92, 122
302-3
magnetic, 309-10,
Accretionary wedge, 399 rain, 61,
magma
493-95 196-97 329-30
Artesian systems,
Aseismic ridges,
Assimilation,
209 Andes Mountains, 16, 121, 364, 395, 399, 561 Andesite, 405 composition and texture, 118, 121 Andalusite, 201, 202,
Angle of repose (slope), 418 Angular unconformity, 222-23, 225 Anomalies
Acadian orogeny, 406
Africa,
Armenia (Soviet), earthquake, 259, 273 Armero, Colombia, mudflow, 341, 342
hill,
Abyssal
Acid
Aluminum, 150 Ammonites, 231 Amorphous substance, 62, 113
527
Argon, 236, 238, 239 Arid region, 560 belt,
397,
Aristotle,
254
Arkose, 165, 167
B Back-arc basin, 362-63, 397 Backshore, 582
Bacon, Francis, 343
Index
617
Bajada,
567-68
187 Bangladesh, coastal flooding in, 578 Barchan dune, 555, 556 Barchanoid dune, 556, 557
Banded
iron formation, 183,
Barrier island, 581, 582,
585-87
583
and, 86, 98, 105, 119
on the Moon, 50 in
oceanic crust, 297, 330, 334, 355
487 472-74, 540 385-86, 388 Basin and Range Province, 297, 406, 567, 568 block-faulting, 396 normal faults, 393 Batholith(s), 125-28, 221 contact metamorphism and, 198 emplacement, mechanics of, 126—28 orogenesis and, 396, 397, 399 porosity of,
level (stream),
pressure release and, 139 Bauxite, 150, 154
Bomb
bar,
gravel, deposition of,
263-64
261-62
(pyroclastic material), 91, (of atoms),
92
58-60
Bonneville, Lake, Utah,
537
Bootlegger Cove Clay, Turnagain Heights, Alaska,
276, 434-35, 436
slides,
Bornite,
209
Boulder batholith, 126
Bowen, N. L., 115, 117 Bowen's reaction series, 115-16 and stability of minerals, 146, 147 Braided stream, 461-62, 528, 531 Breaker (wave), 577 zone, 577 fault,
lunar,
200 50
sedimentary, 165, 166
166
seasonal changes in, 583-84 Beach face, 582 Bear Butte, South Dakota, 111 Bedded chert, 171-72
volcanic, 122 Bretz, J Harlan, 534-35 Brittle rocks, 379
Browning, Iben, 281 Bryce Canyon National Park, Utah, 134, 136-37, 228, 230 Bullard, Edward, 346 Bureau of Mines, 76, 78
175-77
Bedding plane, 175-76, 222 Bed load
460 wind, 549 stream,
Butte,
Giovanni, 7
Benching,
Calcareous ooze, 331 Calcareous tufa, 509
234-35, 236
Bicarbonate, in carbon dioxide recycling,
Big Bang, 29
Bighorn Mountains, Wyoming, 381, 382 Bingham stock, 126 Biochemical sedimentary rocks, 165, 168
238
cleavage, 71 in
sequence of index minerals, 201,
206 Bird's-foot delta,
468
Bishop Tuff, California, 102
618
Index
Calcite, 67, 75,
205
cement, 163, 165 cleavage, 71
40,41
Biotite mica, 66, 67, 116, 167,
mass wasting, 420-21, 426, 428-29, 431 subsidence, 502, 503 Calving (icebergs), 516 Canada, isostatic rebound in, 305, 307 Canadian Shield, 193, 405, 408, 528 Cap rock, 183 Capacity (stream), 461 Capillary fringe, 488 Carbon carbon 14, use in dating, 239, 242 covalent bonds and, 59-60 isotopes of, 58, 191, 192, 239 Carbon dioxide in atmosphere, 12-13, 40-41 greenhouse
effect and,
40-41
Carbonate minerals, 67, 168 rock forming, 75 Carbonate rocks, 166, 168-69, 496-498 Carbonic acid, 163, 495 in carbon dioxide recycling, 40, 41, 144 formation, 142 Carlsbad Caverns, New Mexico, 142 Carnotite, 183 Cascade Range, volcanoes, 81, 84, 94, 98, 121 Cast, 178 Cavendish, Henry, 288 Caves/caverns,
Mammoth
485-86, 496-98, 499
Cave, 490, 492
Cementation, 163, 165 silicate, 66 Chalcocite, 155
Chain
Chalcopyrite, 155, 209
Chalk, 168, 169
534-35
257-58, 326, 362
Berm, 582 Beta decay,
Prieta
Channeled scablands, Washington,
445-46
Benioff zone,
Loma
Chang Heng, 255-56 Channel flow, 456 Channel roughness, 459
569
Bedrock, 153 Bellini,
earthquakes. See
sources, 142
Borax, 563
Breccia
.
581, 582, 584-85 Beach(es), 162, 581, 582-84
Bedding,
locating epicenters with,
Bonding
Basin,
Baymouth
Body fossil, 177 Body seismic waves types,
plateaus, 101
Base
208, 399
Blueschist facies,
121
magma
406
Blowout, 551
composition and texture, 118, 119, mafic
Blanket geometry (of sedimentary
Block-faulting, 396,
California
earthquake; San Andreas fault geothermal energy, 510-11
494 dome, 385, 388 Black smoker, 315 bodies), 175, 176 Block (pyroclastic material), 91
586-87, 589 334 Barrow, George, 206 Basal slip (glacier), 518 migration,
Barrier reef, 332, 333,
Basalt, 19, 151, 309, 405,
Bituminous coal, 173, 181 Black Hills, South Dakota, 55, 130-31,
dissolution,
142
dripstone deposits and, 498 identifying,
74
in limestone, 168, 495 Calcium carbonate, 67, 583 in carbon dioxide recycling, 40, 41 in cementation, 163 Caldera, 93 Caliche, 150
Chaotic terrain (Mars), 452 Charleston, South Carolina, 578
earthquake of 1886, 259
Charon (moon), 47 Chemical sediment, 160 Chemical sedimentary rocks, 166,
168-73 Chemical weathering, 141, 564
144-45
acid rain,
defined, 141
hydrolysis,
oxidation,
143-44 142-43
rate of, factors controlling,
solution,
141-42
144-47
Chert, 166, 167,
171-72, 183
Chlorite, 199, 201, 202, 204, 205, 206,
207 Chrysotile (asbestos), 61,
196-97
Cinder cone, 96, 97-98, 115 Circum-Pacific belt, 102, 103 earthquakes and, 257, 258, 259, 399 orogenic belt, 397, 398
526-27
Cirque, Clast,
Contact metamorphism, 197-200, 208, 221 ore deposits and, 209, 210 Continent(s), 378
early. lost,
Gondwana;
See
166
true
Clay
Laurasia; Pangaea
See Continental drift
of.
and evolution, 405, 408
maps margins, 319
paleographic
Clastic texture,
360-61
320-21
legends of,
movements
of,
and deposition,
461, 550, 552, 556 mass wasting and, 420, 421, 428, 431, 441, 448 minerals, 66, 75, 141, 143, 155, 160 porosity and permeability,
quick clays,
487
434-35
sedimentary grains, 160 soil,
Cleavage, 62, 67,
71-72
Climate
536-37, 540-42 karst topography and, 496 mass wasting and, 420, 435 soil formation and, 149-40, 153 volcanic gases and, 4, 90 Coal, 19, 205, 468 acid rain and, 144-45 characteristics and types, 172-73, 181 glaciation and,
distribution in the United States, 173,
174 also Shoreline
Coast Ranges, California, blueschist
Creep, 417, 425, 435, 437, 438 Crest (wave), 575
composition, volume, and density, 289,
Crevass, 522
294, 297 isostatic rebound, 305-6, 307
Crocidolite (blue asbestos), 197
heat flow, 300
Cross-cutting relationships, principle of,
14
drift,
Crust (Earth),
composition, 165, 167, 193, 294, 297
for,
Wegener and, 344 Continental glacier, 518-19
102-3
323, 324, 325
defined,
318
passive,
323-25
settling,
Curie, Marie/Pierre, 231
Curie point, 307
318, 322
Continental slope, 318, 322
Convection
cell.
162, 318, 319, 322,
at,
586
longshore, 580, 582
See Thermal convection
Columbus, Christopher, 316 Columnar joints, 89-90, 111-12, 389 Comets, 33, 37 Compaction, 163 Competence (stream), 461 Complex mass movements, 425, 437, 439 Composite volcano, 96, 98, 102, 106
Compound, 58-60
Convergent plate boundaries, 16, 17 characteristics,
mountain building at, 396, 397-405 volcanism and, 105, 106, 341 Cook, James, 316
discovery of,
temperature,
62
gradient),
445
Cynognathus, 349, 350 Cyprus, 372-73
D effect
on streams, 474
flood control, 465 hydroelectric power,
(Earth), 11
and composition
580-81
Current ripple marks, 177 Custer, George Armstrong, 55 Cut-and-fill method (of reducing slope
Dams
Copper, 155, 372-73 Coquina, 168, 169 Coral reef, 332 density
nearshore,
361-64, 366
earthquakes and, 257-58, 259, 341 metamorphism and, 193, 194, 200, 208 mineral resources and, 372
Core
Compressional stress, 379 Concordant pluton, 123 Cone of depression, 491, 501 Conglomerate, 165, 166, 167
Current(s)
345, 346
cell
Color (of minerals), 69 Colorado Plateau, 229, 230 Columbia River basalts, 101, 123, 309
of interfacial angles,
Crystalline texture, 166
Oceanic-continental convergence
fit
116-17
Crystalline solid, 56, 62, 63
Continental-oceanic convergence. See
continental
235-36
Bowen's reaction series and, 115-16 cleavage and fracture, 71-72 form, 70 from magma, 113-17 nature of, 62, 63 in pegmatites, 130
movements, 522
Cocite, 155
485-86
Crystal(s)/crystallization,
Continental margin active,
64
in,
movements, 305-6 volume and mass, 288, 289
erosional landforms, 528
Continental lithosphere,
elements
vertical
349-51
rise,
Coke, 181
16
evidence
shelf,
299
3, 11, 12, 14,
common
343-44 345-49
Continental
Collins, Floyd,
219, 220-21, 223, 227
early ideas about,
Continental
Constancy
Cross-bedding, 176-77, 214
Cross-dating, 242, 245
metamorphism in, 208 Coccos plate, 341-42, 368 Coesite,
59-60
92-93
volcanic,
paleomagnetism and polar wandering,
chemical weathering and, 146
592-94. See
mineral assemblages for different types,
319
Continental crust, 12, 14, 16, 117, 121,
Continental
203
Cosmic action, 330, 331 Country rock, 117
Crater Lake, Oregon, 93, 97 Craton, 405, 408
161-62
Claystone, 165, 168
Coast(s),
366
403-5
Continental depositional environment,
148
slaty,
399,
at,
and gas exploration, 232-34
Corrosion, 591
Crater(s), 33, 35, 44, 49. See also Meteorites
characteristics of, 364,
orogenesis
232
559
227-31
Correlation,
201 Covalent bonding,
352-54
Continental-continental convergence, 105
erosion, transport,
(well cutting),
Coriolis effect,
in oil
divergent boundaries under,
origin
166
364
collision of,
Core
,
291-92, 294
290-91 299-300
volume and mass, 288, 289, 291
452
Darwin, Charles, 16, 219, 231, 334 Daughter element, 235 Death Valley, California, 537-38, 546,
562-63, 566
Index
619
magma
Debris avalanche, 415-16, 439
intermediate
Debris flow, 425, 434 Deep-focus earthquake, 257, 266
pegmatite, 129
Dip angle,
in
Dip-slip fault,
seamounts, guyots, and aseismic ridges,
Dipolar,
Drilling Project, 16, 317,
355
551-52 551-2, 553
hollows,
Mountain building earthquakes and, 255
467
regional
Depositional environments, 161-62, 163
179-80
analysis of, 175,
high energy, 166, 173
173-75
sedimentary facies and,
360-61
at,
559-69 characteristics, 561-66 distribution, 559-61
Desert(s)/desertification,
547-49, 554
566-69 source of, 556-57
frequency and distribution,
372—73
Desert pavement, 552,
165-68
California, 90, 91
Tower National Monument, Wyoming, 90, 111-12 Diamond, 59, 60, 74, 210 Devil's
cleavage, 71, 72, 73
kimberlite pipes and,
298-99
structure and characteristics, 73 Diatoms, 172, 173, 332 Differential pressure, 195, 198
498
caves and, 496,
Double
74
Downcutting, 475 Drainage basin/pattern, 470-72 Drake, Edwin L., 184
field,
136-37 Earth, 47-49
592
559-61
379 Dune(s), 161, 552-56, 585 cross-bedding in, 176, 556 formation and migration, 553-54 types, 554-56, 557 Durand, Asher Brown, 7, 8 Dust Bowl of the 1930s, 135-36, 153, 550 Dynamic metamorphism, 197, 200 Ductile rocks,
metamorphism and, 198
278-79
Dinosaurs, extinction of, 35
297
composition and texture, 118, 121
Index
270-72
sea floor, 323, 326,
327
261-63 254-58
seismic waves,
Ebb tide, 594 Echo sounder, 317 Ecliptic,
plane
of,
30, 541
215 Chichon volcano, Mexico, 4, 90 Elastic rebound theory, 254-55 Elastic strain, 379 Elasticity, 261 and velocity of seismic waves, 289-90 Einstein, Albert, 29,
Eldfell volcano, Iceland,
97-98
Eldgja fissure, Iceland, 123
Electromagnetic force, 29 Electron(s), 29, in
bonding,
capture, 235, in
57
58-60 236
radioactive decay,
Electron shell, Element(s),
234-35
57
56-58 49
in early Earth, 48, in Earth's crust,
64
daughter, 235 native,
parent,
Differential weathering,
Dike, 123, 124, 357
276 — 80,
El
530
climates,
resistant structures,
seismology,
refraction,
coast,
for,
281
74
dynamic metamorphism and, 197, 200 foliated texture and, 201, 203
620
264-68 predicting and preparing
Dolostone, 20, 67, 166, 168, 169
Dry
Monument,
251-53
254
measuring intensity and magnitude,
Drowned
166
Devil's Postpile National
Prieta earthquake,
Dolomite, 67, 75, 205 cleavage, 71 in dolostone, 166, 168
Drumlin, 528, 530
Detrital sedimentary rocks,
Diorite,
Loma
258-61
263-64
major,
Dropstone, 533
554
160
Differentiation of the
locating,
metamorphism and, 200
Dripstone, 498
landforms,
273-76
rebound theory, 254-55
elastic
Dome, 385, 388
Descartes, Rene, 31
282-83
control, 280,
porosity and permeability, 487, 488
Deranged drainage, 472
contact
Earthquake(s), 16, 94, 250-81, 379, 421
Divide (drainage basin boundary), 470
identifying,
low energy, 168, 169, 173
Dilatancy model,
Earthflow, 425, 434, 435
destructive effects, 269,
mineral resources
Denver, Colorado, earthquakes, 282
Detritus,
water on, 316, 317, 452, 453
459-60
earthquakes and, 257
470-71
11-12,29,288
structure,
characteristics, 357,
466-69
Detrital sediment,
290
Discordant pluton, 123 Dissolved load (stream),
103-5
Dendritic drainage,
loess,
and the Moon, 49-51, 594-96 orbit of, and climate, 541-42 origin and differentiation, 47-49
Divergent plate boundaries, 46, 17,
strike and dip, 379-80 Deimos (moon), 49
of,
mantle. See Mantle
Disappearing stream, 496 Discharge (stream), 459, 461 Disconformity, 222, 224
Distributary channel,
389-94 folds, 380-86 joints, 386-89 faults,
expansion
391-93
306
Discontinuity,
Deformation, 376, 378, 379-94. See also
Delta, 162,
286-311 297-302, 370-71 magnetic field, 306-11
interior,
internal heat,
oceanic trenches, 326
Deflation,
Gaia hypothesis, 12-13
429
angular unconformities,
222
327-30
Crust
289 dynamic nature, 11-13 data,
slope stability and, 421,
325-26 ridges, 326—27
abyssal plains,
Deep Sea
crust. See
Dip, 380, 381, 390, 391
Deep-ocean basin, 318 oceanic
and, 85, 121
Earth, 5, age, air
28-29,
30, 358
216-19
pressure belts and wind patterns,
558-59 atmosphere, 12-13, 29, 40 composition and density, 294 core. See
Core
60 235
radioactive, refractory,
234-35
33
volatile, 33 Emergent coast, 594 Emperor Seamount-Hawaiian Island
chain, 330,
368-69
End moraine, 528-30
Energy resources, 76. See also Geothermal energy; Mineral resources; Natural gas; Petroleum
Flue-gas desulfurization, 145
acid rain, 61,
sedimentary,
144-45
and water, 143 197 base level of streams and human
acid soils
Andreas
asbestos,
block-faulting,
activities,
474
elastic
153-54
erosion,
floods, controlling
and
predicting,
464-65
396
rebound along, 254-55
joints, distinguished
from, 387
plunging, 383, 385
plane,
symmetrical, 382
394
Foliated
389
and,
Foraminifera, 232, 331
439-48
506-7
240-42
Environmental Protection Agency (EPA),
257 locating, 263-64, 265 Epidote, 199, 205 Erebus, Mount, Antarctica, 102 Erosion, 153—54. See also Mass wasting defined, 153, 456 in deserts, 565
143-44
in sandstone,
167
glaciers and, 516,
522-28
groundwater and, 495-98 headward, 475
rill,
153
shoreline, 582, 587,
591-92
459-60 wind, 550-52
domes and, 98
volcanic gases
Ferromagnesian minerals, 118, 119, 121 Ferromagnesian silicates, 66-67, 146 characteristics, 66-67, 69, 72 colors,
branch of Bowen's
reaction series, 116
66
359
Freyja Montes, Venus,
Frost action,
138-39,425
Frost heaving, 139 Frost wedging, 138
Fujiyama, Japan, 87, 98 Fumarole, 81
577 525-26, 594
earthquake hazard, 273-74 thermal expansion and, 141
Estuary,
87, 92, 102, 123
Etta pegmatite, 131
517 limit, 519
Firn,
Europa (moon), 27, 88 Evans, David M., 282 Evaporites, 166, 170-71, 181
Fission track dating,
Ewing, Maurice, 330-31
Flood(s),
Exclusive Economic
silicate,
Franklin, Benjamin, 90
Fire
Esker,
Sicily,
Framework
Fringing reef, 332, 333
69
in discontinuous
Fetch,
219, 221,
222, 229, 231 Fracture, 72
87
in,
National Monument,
Wyoming, 159 Fossil succession, principle of,
subduction zones and, 106
Fiord,
stream,
Zone (EEZ),
335-37 Exfoliation, 139
Extrusive (volcanic) igneous rocks, 18, 85, 113
115 — 16
rock forming, 75
456
231 349 at Green River Formation, 159 guide (index), 229 microfossils, 232, 244 sea-floor spreading and, 356 trace, 177 continental drift and, 14,
Fossil Butte series and,
mafic magma and, 85 oxidation, 142-43
475
532 592 Etna, Mount,
85, 115, 127, 198
117-18
response to, 305, 306
sheet, 153,
magma,
Bowen's reaction lava
177-79, 244
body, 177 correlation, use in, 229,
204
assimilation and crystal settling,
240, 427, 506
lateral,
Fossil(s),
hydrolysis of,
Felsic
362
582
Formation (rock), 180
200-201
floral succession, principle of,
Feldspars, 67, 75, 146, 202,
Epicenter (earthquake),
isostatic
393
Faunal and
cleavage, 71
588—90 dynamics, 574-75
rising sea level,
shoreline
Foreshore,
221
radioactive waste,
radon,
thrust,
Fore-arc basin,
393-94
Fault breccia, effects of,
metamorphic rocks, 201-5
Footwall block, 390
391-93
strike-slip,
382-83
recumbent, 383
391-93
oblique-slip,
rupture along, 256 activities
381-83 overturned,
reverse,
420, 421, 422-23, 426 mass wasting, minimizing
385-86
basins,
282-83
500-506 mass wasting, human
380-86
fluid injections into,
normal,
150
Fold(s),
monoclines, anticlines, and synclines,
contamination, 486, 493, 498, laterite soils, depletion,
72
Focus (earthquake), 257
domes and
391-93
locked, 251
Gaia hypothesis, 12-13 geology and, 6—7, 9, 11 groundwater, depletion and
cleavage, 71,
asymmetrical, 382
fault
dip-slip,
69
Fluorite,
173-75 Fall (mass movement), 425-26 Fault(s), 200, 389-94. See also San
Entrada Sandstone, 390 Environmental issues, 152, 384
557
Flow (mass movement), 425, 433-37 metamorphic, 207
66
Enstatite,
Floodplain, 219, 319, 463, 466, 477, Facies
242-43, 245
Fissure eruption, 99, 101 Flint,
171
463-66
channeled scablands and, 534-35 coastal, 573-74, 577, 578-79, 590 controlling
and
Flood
tide,
594
464-65 464-65
predicting,
Flood-frequency curve,
Gabbro, 85, 119, 128 composition and texture, 118, 119, 121 in
oceanic crust, 297, 334
Gaia hypothesis, 12-13, 40 Galapagos Rift, hydrothermal vents at, 315 Galena, 68, 70, 72, 209 cleavage, 71 Galveston, Texas, hurricane and flooding,
573-74, 578 Ganges-Brahmaputra
delta,
468, 469
Index
621
Glossopteris flora, 343, 346, 349, 350
Gansu, China, earthquake, 276 Gardner, Marshall B., 287 Garnet, 70, 71, 193, 209 in sequence of index minerals, 201,
206 Gas, 56 Gemstones, minerals as, 56, 60, 61, 68, 73 Geodimeter, 94 Geologic time, 216-47 absolute dating methods, 231, 234-43
227-31, 232-34
correlation,
216-18
early concepts,
Huttonand, 218-19 relative dating
219-27 243-47
methods,
216, 218, formation in, 153
scale, 21, 22, soil
Geology in art,
7-8
music, and literature,
career opportunities,
economic and environmental 5-7, 152 in everyday life, 8-9, 11
uses,
Gradient geothermal, 103-4,
in
126-27
migmatites and,
90
Glacial
lithification,
Gravity,
drift hypothesis,
crust depressed by, 305,
306
516
deposits,
528-33
erosion and transport, 160, 161,
522-28
519-20 517-18
glacial budget, glacial ice,
movement,
520-22
533-40 518-19
Pleistocene, types,
volume of Earth's water 516 Glauconite, 236 Glaucophane, 208
Index
water
487
302-3 418
Gravitational compression in early Earth,
48 Great Barrier Reef, Australia, 332, 334 Great Glen fault, Scotland, 394, 395 Great Lakes, 538-39 region and isostatic rebound, 540 Green River Formation, Wyoming, 159-60, 178, 183 Greenhouse effect, 40-41
Gregory XIII (pope), 215 Grofe, Ferde, 7 Groins, 582, 583, 590
in,
452, 453,
Ground
in,
452, 453
488-89,500
493
Guide (index) fossil, 229 Gully, 153, 475 Gunderson, Linda, 242 Gutenberg, Beno, 291 Guyot, 327, 329, 330, 351 chemical sedimentary rocks, 166,
H Haicheng, China, earthquake, 278, 280 Half-life,
235
rock forming,
29
measuring,
table,
wells, 491,
Halides, 65, 69
Greenstone, 202, 205
540
rates of,
and deposition, 461
Greenschist facies, 207
hydrologic cycle and, 516 isostasy and,
298-99
and time, 215
348-49 defined,
210
73, 74,
groundwater movement and, 489
540-42
and continental
uses,
163
slope failure and,
502—3
170
anomalies, 302-3, 318, 320, 326
514-42
braided streams and, 462 causes of,
subsidence and,
volume of Earth's water
in
and
500-502
490-91
springs,
297 207
sedimentary rocks from, 165, 166 Gravimeter, 302
523 groove, 523 lake, 532-33, 534-35 polish, 523 surge, 522
Glacier(s)/glaciation, 377,
622
characteristics
porosity,
Till
Glacial erratic,
Glacial
saltwater incursion,
Gypsum, 68-69, 70, 75-76, 169
weathering, 146, 147, 148
erosion, transport,
Glacial drift, 528. See also Stratified drift,
Glacial
204—5
487
porosity of,
hydrologic cycle,
movement, 489
stream, 456-57, 459 Grand Canyon, Arizona, 180, 181, 475 correlation and, 228, 230 relative dating and, 220 Grand Teton, Wyoming, 377, 527 Granite, 19, 140, 151,297,405 batholiths,
506-9 486-87
hot springs and geysers,
297-98
diamond and, 59, 60, Gravel, 160, 582 Ireland,
495-98
geothermal energy, 509-11
Graphite
Geyserite,
by,
Graded bedding, 176, 322 Graded stream, 474-75
Granodiorite, 121,
508-9
493-95 504-6
486 566 erosion and deposition
Graben, 396
Granulite facies,
Glacial
systems,
in deserts,
Geothermal energy, 509-11 Geothermal gradient, 103-4, 297-98
509 Giant's Causeway,
143
in artesian
defined,
Gossan, 155
Geophysics, 5
Geyser,
acid, 142,
Granitization, 126
232 12-14, 16
theories in,
Groundmass, 113 Groundwater, 456, 486-511
contamination,
composition and texture, 118, 121-22 felsic magma and, 85, 121
465
defined, 5
subsurface,
204 Gold, 55-56, 77, 372 Gondwana, 343, 346, 348, 349 Gorda Ridge, 337 Gorge, 475 Gneiss, 202,
failure (earthquake hazard),
275-76 Ground moraine, 530 Ground shaking, 269, 273
75-76 75-76
Halite, 69, 70, 74,
cleavage, 71
Hanging valley, 526 Hanging wall block, 390 Hardness (mineral), 72 Harmonic tremor, 94 Harney Peak Granite, 130, 131 Hart, Johnny, 8
Hawaiian
Islands, volcanism in, 84, 87,
93, 94, 96, 102,
106-7, 123, 369, 396
Headland, 591
Headward
erosion,
475
Heat. See also Temperature as by-product of radioactive decay, 231
Earth's internal, flow, 300,
297-302, 370
302
Kelvin's calculations of Earth's age
and, 219 metamorphism and, 193-94, 198
Heezen, Bruce, 330-31 Hekla, Mount, Iceland, 91 Hellas (Martian crater), 44 Hematite, 68 ,72, 74, 143, 165, 209, 309 Henry Mountains, Utah, 125 Herz, Norman, 192 Hess, Harry H., 351
223
Hiatus, 222,
radiometric dating, 235-36, 238
Highlands (lunar), 49 Himalayas, 340, 395, 396 continental crust beneath, 297,
formation, 364, 399,
304
403-4
Incised
landslide,
414
Horn, 527 Hornblende, 67, 202, 204, 205, 238 cleavage, 72 Hornfels, 202, 205, 208 Horst, 396
Hot Hot
spots,
368-69
Inselberg,
Hydrolysis,
516
486-87 143-44
Hydrosphere, 16 Hydrothermal alteration, 200 Hydrothermal vents, 315-16, 337, 373 Hypocenter, 257 Hypothesis, 13
264-66
magma,
Bowen's reaction
85, 115
series and,
Internal drainage (deserts),
metamorphism and, 193-94, 197-200 relative dating and, 220-21 Intrusive igneous rocks, 18, 85, 113. See
also Plutons Io (moon), 27, 88
Ion,
59
141-42 58-59
in solution,
fall,
sheets, Ice
Kilauea volcano, Hawaii, 87, 94, 96, 97,
banded iron formations, 183, 187 deposits, 68, 154-55, 309 oxidation, 142-43, 165 Irons (meteorites), 33, 34, 292 Isacks, B., 357 Ishtar Terra, Venus, 359
Kimberlite,
298-99
Kimberlite pipe, 297,
298-99
Kinetic energy (stream),
459
Kopff (comet), 28 Krakatau volcano, Indonesia, 3-4, Krinov, E. L., 37
Kuroko
200 209
sulfide deposit, Japan,
Kyanite, 193, 201, 202, 206,
264
90
5,
37
Kulik, Leonid,
206
540
rebound, 305-6, 307 emergent coasts and, 594 of Great Lakes region, 538
517-18
231 carbon and oxygen
La Brea Tar
Age, 516, 533, 536. 541. See also
sculptures, 191,
ratios of marble
192
long-lived radioactive pairs, 238,
515-16, 537, 540, 542
radioactive
Iceland
and absolute dating,
Lahar, 98 Lake(s), 474, 579, 585,
J
extrusive, 18, 85
Jan Mayen Ridge, 329 Japan
123-26
on the Moon, 50 pegmatites, 128-29, 130-31
129-31
591-92
532-33, 534-35
Great Lakes,
126-28 classification, 118-23
plate tectonics and,
239
glacial,
528
intrusive, 18, 85,
182
deltas,
Igneous rocks, 18, 19, 110-23, 487 batholiths,
California,
Lagoons, as depositional environment, 168, 169
466, 467 as depositional environment, 168
234-35
geothermal energy in, 507, 510 volcanism in, 84, 91, 97-98, 101, 102
Pits,
Laccolith, 125
Isotope(s), 58,
518
Ice-scoured plain,
102, 106 Kilimanjaro, Mount, Tanzania, 102
Isostatic
Pleistocene Little,
Kansu, China, landslides, 417 Kaolinite, 143, 155 Karst topography, 496 Kelvin, Lord, 219, 231 Keoua, Hawaiian chief, 96 Kerogen, 159 Kettle, 531
Key bed, 228
Iron
Isostasy, principle of, 303-6,
caps,
glacial,
88
Kame, 531, 532
Intrusive igneous bodies
Isoseismal lines,
519 522
44-47
Jurassic strata, 231
K
566
106-7
Intraplate volcanism,
volcanoes and, 326, 362
516
Jovian planets, 30, 31, 33,
Jupiter, 27, 30, 31, 33, 44, 45,
115 — 16
composite volcanoes and, 98 subduction zones and, 106
Isograds,
Ice,
389
Joly.John, 216
Intermediate-focus earthquake, 257, 266
earthquakes and, 257
Iapetus, 16
139
89-90
sheet, 139, 140,
Island arcs
I
in lava flows,
spheroidal weathering and, 147, 148
Intensity (earthquake),
Ionic bond,
in,
386-89
frost action and,
568-69
Intermediate
Huaraz, Peru, mudflow/avalanche, 417 Humboldt, Alexander von, 343 Humus, 147 Hutton, James, 12, 218-19, 220, 223, 231 Huygens, Christian, 215 Hydraulic action, 460, 591 Hydrocarbons, 181 Hydrograph, 464 Hydrologic cycle, 452, 454, 455
158
columnar, 89-90, 111-12, 389 faults, distinguished from, 387
455-56
Infiltration capacity,
506-8, 509 Houchins, Robert, 492 Hsian, China, landslide, 417
groundwater
at,
Joint(s),
relative dating and, 219, 221, 223, 227 Index (guide) fossil, 229 Index minerals, 201, 206-7
springs,
glaciers and,
Monument, Oregon, sedimentary rock
igneous rock, 117, 127
Holmes, Arthur, 351
Harold, 291 Fossil Beds National
John Day
Inclusions
Historical geology, 5
Hong Kong, 1972
Jeffreys,
220-21 113-15 meander, 478, 479
relative dating,
textures,
566 pluvial and
538-39
playa,
James, Jesse, 498
as back-arc basin,
earthquakes Jasper, 171
in,
363
274, 276
proglacial,
537-39
waves on, 575, 577 Laki
fissure, Iceland, 90,
101, 123
Lakshmi Planum, Venus, 359 Laminar flow, 454-55 Landslide(s), 415-16, 417, 446-48
Index
623
J
earthquakes and, Lapilli,
275-76
579
surges,
91
Laramide orogeny, 401 Larson, Gary,
9
8,
Laser ranging techniques, 368 Lassen,
Mount,
California, 81
Lateral continuity, principle of, 219,
Lateral moraine, 524, Laterites, 150,
Laurasia,
220
530
Longshore current, 580, 582 Longshore drift, 582, 587 Love, A. E. H., 263 Love Canal, New York, 505 Love waves (L-waves), 263 Lovelock, James, 12-13 Lowell, Percival, 451 Low-velocity zone (mantle), 296 Luster (mineral),
69-70
219, 231 Lysenko, Trofim Denisovich, 10
151
Lyell, Charles,
344
Magma;
Lava, 85. See also Lava flow;
Lystrosaurus, 349, 350
Volcanism; Volcano pillow, 90, 92, 103, 131, 314, 327,
334
M
113-15
textures of,
Lava dome, 98-99, 102 Lava flow(s), 85, 121, 357 columnar joints in, 89-90 dating, use in,
Madison Canyon, Montana, earthquake, 275, 276 Mafic magma, 85, 113, 115
220-21, 247
Bowen's reaction
geometry, 87, 89
on other planets, 38, 42, 50 paleomagnetism in, 350-51 types, 89 ultramafic, 119, 120 viscosity, 87 Leaching, 149 Lehmann, Inge, 291 Lewis overthrust, Montana, 393, 394 Lichens (role in weathering), 141 Lignite,
Limb
172-73
(fold),
and types, 166, 168-
69 dissolution, 142, 145,
495
porosity and permeability,
487
Limonite, 143, 165 Liquefaction, 423,
434
earthquakes and, 273, 275, 276 Lisbon, Portugal, earthquake,
254
162-63, 165
357 oceanic vs. continental, 102—3 at plate boundaries, 361-62, 366 Lithostatic pressure, 194-95, 198 Little Ice Age, 515-16, 537, 540, 542 Lithosphere, 11, 16, 17, 297,
Lituya Bay, Alaska, earthquake/rockfall,
426, 579 Liquid, 56 inability to transmit S-waves,
261, 290,
291
552-53, 556-58
369 Prieta, California, earthquake,
251-53, 260, 265, 273, 278 Long Valley caldera, 102 Longitudinal dune,
555-56
Longitudinal profile (stream),
624
Index
factors influencing,
minimizing
418-24 439-48
effects of,
424-38
103-4, 130-31.
56-58
elements and atoms,
Volcanism assimilation
Mauna Mayon
characteristics,
Mazama, Mount, Oregon, 93 Meanders (stream), 462-63
and mixing, 117-18 Bowen's reaction series and, 115 — 16
85-87
197-98 113-16 crystal settling, 116-17 Magnesium (in dolostone), 169 Magnetic anomalies, 309-10, 318 in oceanic crust, 354—55, 366, 368 Magnetic field, 306-11 inclination and declination, 306-9 polar wandering and, 349—51 source, 306-7 Magnetic Magnetic
polarity,
reversals,
310 310-11, 366, 368
Loa, Hawaii, 87, 96, 102, 106 volcano, Philippines, 90, 98
incised, 478 Mechanical weathering, 137-41, 564
chemical weathering, contribution
organisms,
138-39
activities, of,
thermal expansion and contraction, 139, 141 Medial moraine, 524, 530 Mediterranean Basin, evaporite deposits in,
170-71
Magnetometer, 309 Magnitude (earthquake), 266-67, 269
Mediterranean-Asiatic
Mammoth
Mendelssohn,
boundary with core, 290-91, 298 composition and structure, 294, 295-97, 305 cells in, 14, in,
370 298-99, 301
kimberlite pipes and, 297,
298-99
volume and mass, 288, 289 Mantle plume, 104-5, 106-7, 300, 368, 369
Maps 381
352-54
141
pressure release, 139, 140
Mediterranean
Cave, Kentucky, 142, 485, 486, 490, 492, 497 Manganese nodules, 330-31, 332, 337 Mantle, 11, 294-97
to,
145 frost action,
Magnetite, 68, 74, 167, 209, 306, 583
paleogeographic,
8
Mass wasting, 414-48, 460 defined, 417
Matthews, D., 354
geologic, 380,
474
Maria (lunar), 49 Marianas Trench, 326 Marine depositional environment, 162 Marine regression, 175 Marine terrace, 594 Marine transgression, 175 Mars, 28, 30, 33, 40-41, 43-44, 49, plate tectonics on, 359 water, on, 45 1 -42 wind activity on, 551, 552-53 Mass deficiency, 303 Mass excess, 302 Mass movement. See Mass wasting Mass spectrometer, 235, 238
See also Igneous rocks; Lava;
geothermal gradient
Loihi volcano, Hawaii, 102, 106, 107,
498
bonding and compounds, 58-60
87
11, 16, 18, 19,
convection
Loess, 148,
Loma
Magma,
in,
dissolution, 142, 145,
Matter, 56
spreading ridges and, 105 volcanic gases
191-93
authenticating sculptures,
types,
cooling,
Limestone, 19, 20, 67, 429, 433, 441 caves and, 490, 492, 496, 498
Lithification,
115-16
contact metamorphism and,
381
characteristics
series and,
shield volcanoes and, 93
Marble, 20, 190, 202, 205, 206, 209
102, 103
belt,
belt,
earthquakes
and, 258, 259 Felix, 7, 8
Mercalli, Giuseppe,
264
Mercury, 30, 35, 38, 40, 88, 452
359 Mesa, 569 Mesosaurus, 349, 350 Metallic bonding, 60 Metallic luster, 70 plate tectonics on,
Metallic resources, 76. See also Mineral resources
Metals, 73
bonding
in,
Metamorphic Metamorphic
60 facies,
207
rocks, 18,
19-20, 193,
487. See also Metamorphic facies;
Metamorphic zones; Metamorphism
201-5 201-5
exploration
classification,
foliated,
radiometric dating,
237-38
Metamorphic zones, 199, 201, 206-7 Metamorphism, 20, 21
193-97 197-200 dynamic, 200 agents,
contact,
natural resources and, plate tectonics and,
208-10
208
200-201 197-201
subsidence,
9,
232-34, 302,
metamorphism and, 208-10 oil shales and, 159-60 petroleum and natural gas, 181-83, 184-86, 232-34
371-73 334-37
plate tectonics and, sea,
sediment and sedimentary rocks and,
180-87
Meteor Crater, Arizona, i3, 35 Meteorites, 33-35, 48, 49, 291 dating, 239 Mexico City earthquake, 276,
5-6,
hydrothermal alteration and, 200 at hydrothermal vents, 315-16 imported by the United States, 76, 77
from the
regional, types,
for,
341-42
delta, 468, 469 Missoula, Lake, Montana,
534-35
351,360
Mohorovicic discontinuity (Moho), 295 Mohs, Friedrich, 72 Mohs hardness scale, 72 Mold, 178 Molecule, 60 Monocline, 381, 382
Ewing's research on, 330-31
Moon, 49-51,
327
volcanism and, 102, 103 Mid-oceanic ridges. See Oceanic ridges Migmatite, 202, 204-5, 208 Milankovitch, Milutin, 541 Milankovitch theory (of climatic changes),
Milky
Way
74
62—64 70-71
crystalline structure, 62,
index, 201,
and geysers, 509
206-7
metamorphic
physical properties, 64, silicate,
structural changes in mantle,
296-97
Mineral grain, 113 Mineral reserves, 75 Mineral resources, 76-78. See also specific resources, e.g., Iron
banded iron formations, 183, 187 in batholiths and stocks, 126 coal deposits in the United States, 173,
174 complex pegmatites, 128, 130-31 defined, 75 deltas and,
468-69
516
346-48
Neptune, 27, 28, 30, 31, 33, 45, 47, 48
Neumann,
(orogenesis),
121-22,
397-405
408-10
metamorphism and, 197
Rocky Mountains,
264
in radioactive decay, 234-35 Nevado del Ruiz volcano, Colombia, 98, 341-42 Nevado Huascaran, Peru, avalanche, 415
New
Madrid, Missouri, earthquakes,
259,281 Newton, Isaac,
13, 215, 288,
302
Niigata, Japan, earthquake, 276, 277,
278
gas,
58
Nonferromagnesian minerals, 118, 121 Nonferromagnesian silicates, 75, 85 characteristics, 67, 69, 72 Nonfoliated metamorphic rocks, 202, 205 Nonmetallic luster, 70 Nonmetallic resources, 76. See also
origin of,
Nonplunging
Normal Normal
fault,
fold,
383
391-93
polarity,
310
North America evolution of, 405, 408,
400-402
409
Pleistocene glaciation, 519,
522-23,
536-40
lithification,
163
sedimentary rocks from, 165, 167-68 crack,
F.,
Neutron, 57, 231, 239
Mineral resources
126, 364, 378,
Mud
364
595
320, 326 Negative magnetic anomaly, 309 Neoglaciation, 536-37
Mud, 160
65-67
in
567, 568
ranges and systems distinguished, 395 similarity of, on opposite continents,
regional
plate, 16, 17, 106, tide,
Nodules, 171 manganese, 330—31, 332, 337 Nonconformity, 223, 226
395-96
microplate tectonics and,
206-7 69-74
intensity and,
Nazca
Noble
Moons, planetary, 44, 45, 46, 47 Moorea, volcanic peaks on, 2
types,
64-69
332-34
Nile River delta, 468, 469, 473
594-96
396 Mountain building
60
at hot springs
tides and,
glaciers in,
chemical composition,
groups,
141
in desert regions,
carbonate, 67
defined, 56,
thermal expansion and contraction on,
Mountain(s),
60-78, 113
Mineral(s), 56,
88,452
30
Moraine, 528-30 Morley, L. W., 354
541-42 Galaxy, 30, 32
Miller, John, 10
accessory,
61,
characteristics,
for,
Natural glass, 133 Natural levee, 466 Native element, 60, 65 Navajo Formation, Zion National Park, Utah, 214
nearshore sediment budget, 587
Modified Mercalli Intensity Scale, 264, 266 Mohorovicic, Andrija, 294-95
Mid-Atlantic Ridge, 16, 17, 106, 344,
468-69
subsurface correlation and exploration
Negative gravity anomaly, 302-3, 318,
470
Michigan basin, 385-86, 388 Microcline, 67 Microcontinents, 329 Microplate tectonics, 408, 410
Utah, 479
Natural gas, 181-83, 337, 385,
Nearshore zone, 577, 580
Mississippi River, 459, 463,
502
Natural Bridges National Monument,
Neap
uranium, 183 weathering and, 154—55 Mineralogy, 5, 61
Micas, 66, 67 in sandstone, 167
profile,
N
309-10
nonfoliated, 205
177
Mudflow, 161, 417, 420, 425, 433-34, 470 at Armero, Colombia, 341, 342 lahars (volcanic), 98
Mudrocks, 165, 167-68 mineral assemblages produced for, 201 Mudstone, 165, 168 Muscovite mica, 66, 67, 116, 167, 238 cleavage, 71
Mylonite, 200
North American Cordillera, 400-401 North American plate, 16, 17 mountains in, 347-48 Nucleus (atom), 57 Nuee ardente, 99, 102 Nyos, Lake, Cameroon, 90
o Oblique-slip fault, 395
Obsidian, 113, 114, 122, 123
Index
625
Ocean(s)
changes in the level of. See Sea level convergence with continents. See
in
316
hydrologic cycle, 452, 454
volume of Earth's water in, 452, 453, 588 waves. See Waves Ocean basin. See Deep-ocean basin Ocean Drilling Program, 317 Oceanic-continental convergence, 105
363—64
characteristics,
at,
Oceanic crust, 12, 14, 16, 17, 117, 319 age and distance from oceanic ridges,
16,355-57 composition, volume, and density, 121,
289, 294, 297, 330, 334 formation, 351
pillow lava
90,
in,
in,
in
ooze,
351,
354-55
in
top
130-31, 334
119,296 chemical composition, 64 metamorphosed into serpentine, 196 silica
tetrahedra of,
65-66
specific gravity, 74 Olympus Mons, Mars, 43, 44, Ooids, 168-69
Oolitic limestone,
169
Ooze, 331-32 Ophiolites, 334,
399
Oppel, Albert, 231 Optical pyrometer, 86
Organisms
626
Index
359
in,
Petrified
143-44
Outlet glacier,
576
fold,
lake,
and subsidence, 502—3
extraction of,
522
plain, 531,
wood, 178
Petroleum, 181-83, 337, 385, 468-69,
oil
557
shales and,
159-60
184-86, 337 subsurface correlation and exploration Persian Gulf,
382-83
for,
463
232-34
Petrology, 5
142-43
Phaneritic texture, 113, 114
Oxides, 65, 68 Oxidized ores, 155 Oxygen, and carbon isotope analysis, 191, 192. See also Oxidation; Oxides
Phenocryst, 113
Phobos (moon), 49 Photon, 29 Phyllite, 202, 203 Physical geology, 5 Piezoelectricity,
68
Pillow lava, 90, 92, 103, 131, 314, 327,
334 Pacific
Ocean,
vs. Atlantic,
322, 326
Pinatubo, Mount, Philippines, 80 Pisa, Italy, subsidence at,
368
Pitted
ourwash
plain,
202, 204
Paleogeographic maps,
352—54
in
Bowen's reaction 71
Paleomagnetism, 310, 352
characteristics,
continental drift and, 349-51 Paleontology, 5, 244 Palisades, Hudson River, New York, 124,
cleavage, 71
125 Pangaea, 14, 16, 103, 185, 323, 344, 346, 399, 406 Parabolic dune, 556, 557 Parent-daughter ratio, 235
rate of chemical weathering and,
116
hydrolysis of, 143, 144 Planets, 30. See also specific planets, e.g.,
Venus
44-47
30-31, 33, 35, 38-44 518 Plastic strain, 379 Plate(s), 11-12, 14, 16, 17. See also terrestrial,
Plastic flow (glacier),
146-47
formation and, 151 Pan'cutin volcano, Mexico, 97 soil
106
Passive continental margin, 323, 324,
325, 408, 410 petroleum and, 184-85
series,
Planetesimals, 33, 30, 51
Jovian, 31, 33,
Parent material, 136
Partial melting, 105,
484, 502
531
Plagioclase feldspars, 67, 118, 121, 128,
Paleogeography, 5
Parent element, 235 88,
437
184-86,337
Pahoehoe lava flow, 89, 91 Paleocurrent, 177
Olivine, 69, 116, 117,
mantle, 11, 119, 297
Orthoclase, 65, 67, 70
Pacific plate, 16, 17,
Oldham, R. D., 290 Oliver, J., 357
in the
Permafrost, 435,
Peru-Chile Trench, 323, 326, 364, 399
Oxidation,
Oceanographic research, 317-18 Oil. See Petroleum Oil shale, 159-60, 183 Old Faithful geyser, Yellowstone National Park, Wyoming, 107
334
composition and texture, 118, 119
Orogens, 397, 408 Orogeny, 397
orogenesis
and,
102
490-91 487-88
Perched water table,
Persian Gulf region, petroleum
building
Oxbow
earthquakes and, 257, 258 at plate boundaries, 362, 364
331
Pelee volcano, Martinique, 84, 99,
Peridotite, 298,
weathering, 141
Overturned
370-71
Pelagic sediment, 326,
Permeability, 182,
Overloading, mass wasting and, 421
Oceanic ridges, 14, 326-37. See also Mid-Atlantic Ridge; Spreading ridge divergent boundaries at, 357, 360 hydrothermal alteration and, 200, 208 oceanic crust and, 16, 351, 355—57 sediment at, 356-57 volcanism and, 102 Oceanic trenches, 326. See also specific trenches, e.g., Marianas Trench
soils,
Pelagic clay, 331
148
soil,
Ourwash
362-63 397-99
cells
331-32
radiocarbon dating, 239, 242 reefs and, 332-34 soil formation, role in, 151, 153
characteristics,
convection
149-50 128-29 complex, 130-31
Pegmatite,
Outflow channels (Mars), 452 Outgassing, 35, 40, 316, 451
Oceanic lithosphere, 102-3 Oceanic-oceanic convergence, 105 at,
Pediment, 568 Pedocal
hydrolysis, of,
heat flow, 300, 302
magnetic anomalies
177-79
in
at
220, 223, 379-80 Orogenesis, 397-405. See also Mountain
400-402
399,
fossils,
468
Pedalfer soils, 149
Original horizontaliry, principle of, 219,
metamorphism and, 208 orogenesis
Peat, 172,
hydrocarbons, 182 hydrothermal vents, 315 — 16 limestone and, 168
Shoreline early,
bedded cherts and, 171-72 172
in coal,
Plate tectonics; specific plates
boundaries. See Convergent plate
boundaries; Divergent plate boundaries; Transform plate
boundaries intraplate earthquakes,
259
movement and motion, 366-69
357 342-43,
Plate tectonic theory, 13, 16, 17, Plate tectonics,
3-4,
12, 14,
276—79
Precursors (earthquake),
radiocarbon, 239, 242
Pressure release, 139
uncertainty, sources of,
357-73
Pressure ridge (lava flow), 87, 89
carbon dioxide recycling and, 40, 41 driving mechanism, 369-71 emergent coasts and, 594
Primary waves. See P-waves Principles. See specific principle, e.g.,
Uniformitarianism, principle of
540-41 activity and, 129-31
538-39
235-39
Radon, 240-42 Rainshadow desert, 560-61 Rapid mass movements, 425 Rayleigh, Lord, 263 Rayleigh waves (R-waves), 263
glaciation and,
Proglacial lake,
igneous
Prograde/prograding, 466, 468
Recessional moraine,
Proton, 57, 231, 239
Recharge (groundwater), 489 wells and ponds, 502 Recrystallization, 195 Rectangular drainage, 471
metamorphism and, 208 mountain building and, 397-405
in radioactive decay,
371-73
natural resources and,
Pumice, 122-23
plate boundaries, types of, 357,
P-wave(s)
360-66 plate movement and motion, 366-69 rock cycle and, 20-21 on terrestrial planets, 358-59 volcanism and, 102-7 Plato,
Playa,
234-35
in the crust,
297
289-90 262, 263—64
Earth's interior, study of,
earthquakes and, 261,
294-97
mantle,
in the
Pleistocene
566 Epoch
Reef,
90-91, 92 bombs and blocks,
lower seal
level during,
319, 322,
lapilli,
axis of rotation, 30,
Regional metamorphism, 197, 200-201,
101-2
203 Regolith, 147
Pyroclastic texture, 115
Pyroxene, 67, 116, 117, 119
lunar,
33
laccoliths,
Quarrying
(glacier),
123—25
146, 202,
125 126
in
299
varieties
and
uses,
68-69
Quartzite, 146, 151, 202, 205,
537-38
Quick
clays,
425,
206
434-35
Point bar, 462, 463
350-51
Polymorphs, 73 Popping, 140, 287 Pore spaces, 163
Radial drainage, 472
Radioactive decay, 48, 58, 120, 231,
487
234-39
Porphyritic texture, 113, 114
internal heat of Earth and, 300,
Porphyry, 113 Port Royal, Jamaica, earthquake,
Portuguese Bend, California, landslide,
9,
446-48 303 magnetic anomaly, 309
Positive gravity anomaly, 302, Positive
Potassium-argon dating, 236, 238, 239 Potassium feldspars, 116, 121, 128, 238 characteristics,
67
cleavage, 71
hydrolysis of,
143-44
239 radiocarbon dating, 239, 242 radiometric dating, 235-39 Radioactive waste, 506 Radioactivity,
on
soil
(fault),
Residual concentration, 154 soil, 148 Resource (vs. reserve), 76, 77 Reverse fault, 391-93 Reversed polarity, 310 Rhyolite, 85 composition and texture, 118, 121, 122 tuff, 122
Residual
Richter, Charles E,
266
Richter Magnitude Scale,
266
Ridge-push mechanism, 370-71 Rift(s)/rifting, 103, 104, 323, 406 divergent plate boundaries and,
360-61 360
Radiocarbon dating, 239, 242 Radiolarians, 172, 173, 332
Rift Valley, East Africa, 297,
Radiometric dating, 216, 219, 235-39,
Ring of Fire, 341-42 Rio Grande, data on, 464, 465 Rip current, 580-81 Ripple marks, 177
242-43 242-43
fission track,
Potholes (stream beds),
long-lived isotope pairs,
224-26
391
formation, 153
oceanic ridges and, 327
219
Potential energy (stream),
459 460
302
long-lived radioactive isotope pairs,
276
movement
Reserve (vs. resource), 76, 77 Reservoir rock, 182 Residual (van der Waals) bond, 60
sandstone, 165, 167
113
Relative
Relief, effect
beach sand, 582-83 63
crystals, 62,
Plutonic (intrusive igneous) rocks, 18, 85,
Porosity,
219-21
223, 227 unconformities and, 22-23,
523
Quartz, 54, 66, 67, 70, 116, 121, 128,
stocks, 125,
Polar wandering,
of,
reconstructing geologic history from,
volcanic pipes and necks, 125
Pluvial lake,
216
fundamental principles
discordant, 123 sills,
254-55
Relative dating,
concordant, 123 dikes and
50
Reid, H. E,
tetrahedra of, 66
125-28
batholiths,
290
water waves, 580, 584
113, 123-26, 397, 399
Pluton(s),
290
Refractory element, 33
91
Pyroclastic sheet deposit,
silica
362
332-34
seismic waves, 91, 92
cinder cones and, 97
373-74, 539-40, 592, 594 Plucking (glacier), 523 Plunging fold, 383, 385, 386 Pluto, 28, 30, 31,47, 48
315, 316
373
Refraction
ash,
552-53, 525, 530, 533, 536-40, 540-42, 565
in,
in,
Reflection (seismic waves), 234,
Pyroclastic material, 85
glaciation during, 519,
hydrothermal vents
plate divergence and, 360,
oxidation, 143, 146, 155
Playa lake,
383
fold,
mineral resources
shadow zone, 290-91 Pyrite, 70, 71, 209
320 566
Recumbent Red Sea
530
239
Rill erosion,
153
Index
627
303, 318
River, 456, 469. See also Stream
negative gravity anomaly
Roche moutonee, 523
negative magnetic anomaly
Rock(s), 16. See also Igneous rocks;
petroleum and natural gas and, 183
Metamorphic rocks; Sedimentary rocks defined, 74 and
elasticity,
289-90
517
glacial ice as,
oldest
241-42
formation and, 151
136-47
weathering,
Rock Rock Rock
bolt,
446
cycle, 16,
deep-sea,
studies of, in elastic
defined,
of,
Rotorua,
New
energy, 507,
intensity
510
lithification of,
565-66
on Mars, 451-52
velocity
455-56
See Stream
and discharge, 457, 459
Runoff channels on Mars, 452 defined,
455-56
infiltration capacity and,
548-49 429,
Helens, Mount, Washington, 81-84, 86, 87, 90, 92, 93, 94, 99, 300, 341
attempt to calculate Earth's age
from, 216, 218
566 dome, 127-28
202,
203-4
pelagic,
326
relative dating and,
219-20
shelf-slope break and,
322
459-63, 466-70, 523-24, 549-50 water-saturated and earthquakes, 269,
Sedimentary breccia, 165, 166, 167 Sedimentary facies, 173-75 Sedimentary rocks, 18-19, 158-60,
165-87 168-73 165-68
chemical, 166, detrital,
environmental analysis facies,
of,
175—80
173-75
lithification of sediment,
162—63, 165
mineral resources and, 158-60,
180-87 porosity and permeability,
487-88
radiometric dating, difficulty of, 236,
245
219-20
Scientific
Scoria, 114,
Seismic gap, 276 Seismic profiling, 234, Seismic risk map, 276,
317-18, 330 277
Sea (wave), 575
Seismic tomography, 299, 301
Sea arch, 591
Seismic waves, 256 Earth's interior and,
587, 588, 591
cliff,
Sea floor,
315-37 318-19, 322-25
oceanographic research and, 317—18 reefs,
332-34 335-37
resources on,
and, 581
rip currents
sedimentation,
289-90 263-64
locating epicenters with,
deep-ocean basin, 325-30 hydrothermal vents on, 315-16 oceanic crust, composition of, 334
Salt
Index
162-63, 165
Sedimentary structures, 175-77 Self dune, 555—56
Salinization, 548,
628
lithification,
relative dating and,
10
continental margins, glide,
430
Salinity,
porosity and permeability, 487, 488 Santa Maria volcano, Guatemala, 98 Saturn, 27, 30, 31, 44-45, 46 Sawkins, F., 371 Scandinavia, isostatic rebound in, 305, 307, 540 Scarps (Mercury), 38, 359 Schiaparelli, Giovanni Virginio, 451
Sea
desertification,
Saidmarreh Valley, Iran, rock
166-67 166-67
method, 13 115 Scotland, 219, 220 Secondary waves. See S-waves
Sahara, 548, 549, 550, 560
and
163
Sandstone, 19, 151, 165, formation, 163
Scientific literacy,
Rupture, 256
erosion and, 154. See also Erosion
273 and deposition,
sedimentary rocks from, 165,
Schist, 20,
454
Sahel, Africa, drought
554—56
461, 549
Zealand, geothermal
160 160
transport and deposition, 160-61,
erosion, transport,
Running water, 454-79
in streams.
map, 264-65, 268
on beaches, 582-83, 590 in deserts, 566 dunes,
160
330-32, 335-37, 356-57
nearshore budget, 587
Sand, 160
400-402
sheet flow vs. channel flow,
367
Valley, California,
San Francisco, California, 1906 earthquake, 254, 256, 273
Rounding, 161 Rubble, 166 Rubidium-strontium dating, 238, 239 Runcorn, S. K., 350 in deserts,
detrital,
earthquake, 267
Roosevelt, Theodore, 111
St.
classification of particles,
San Fernando
Rock flour, 523-24 Rock glide, 425, 429-33 Rock gypsum, 166, 170, 181 Rock salt, 166, 170 Rock varnish, 564 Rockfall, 420, 425-26, 579 Rocky Mountains, 395, 399, 426 formation
278 395 rebound theory,
as strike-slip fault, 394,
as transform fault, 366,
20-21
334-35
in,
seismic gaps along, 276,
burst, 140,
plate tectonics and,
573-74, 590
Seawater, resources
Sediment, 18, 20 chemical, 160
254-55
287 18-21
322, 373-74, 525, 539-40, 592 574, 586, 588-90, 592 Sea stack, 591, 592 Seamount, 327, 330 Seawalls,
251, 256, 277,
fault, 16,
'279 mylonites and, 200
346-48
continents, soil
354-57
rising,
492
San Andreas
sequence, similarity of on opposite
as evidence for continental drift, 351,
periods of lower, 170-71, 175, 319,
Saltation
500-502
74-76
Sea-floor spreading, 14
Sea level
Saltwater, incursion into groundwater,
known, 48, 193, 405
rock-forming minerals,
310
566
Salt pan,
Saltpeter,
radon, likely to contain,
at,
streams, 460, 461 wind, 549
deformed. See Deformation density
at,
330—32
magnitude and, 266 sea waves (tsunami), 274-75 types,
261-63
Seismogram, 256, 266 Seismograph, 256, 257
Wood-Anderson, 266 Seismology
289-90 255-58
Earth's interior and,
earthquakes and,
Seimiarid region,
560
Slide-flow, 437,
439
gradient,
419-20, 456-57
595 498 Stalagmites, 498
contact
reducing,
445-46
Stars, evolution of,
oil,
shear strength, 418, 444 soils, effect on, 153
334
Serpentine, 196, 197,
168
Shale, 165,
metamorphism and, 199, 200 159-60, 183 porosity and permeability, 487 spotted, 199, 200 Shallow-focus earthquake, 257, 266, 366 Shasta, Mount, California, 92, 93, 96, 472 Shear strength, 290, 418 Shear stress, 379 Shear waves, 261 Sheet erosion, 153
140,389
Sheet joint, 139, Sheet
66
silicate,
428-29
Snider-Pellegrini, Antonio,
149-51, 153
7 195
148-49
horizons,
Shield(s), 193,
infiltration capacity,
and continental evolution, 405, 408 Shield volcano, 93,
96-97
176
coasts, types of,
572-96
592-94
574
deposition,
581-87 591-92
nearshore currents, 577,
580-81
594-96
wave dynamics, 575-77 Sialic,
12
Siccar Point, Scotland, angular
unconformity, 223, 225
Nevada, California, 426, 561 batholith, 125, 126 normal faulting, 393 sheet joints, 140 Silica, 65 in cementation, 163, 165 Silica
tetrahedron,
Silicates,
65—66
65-67, 85
ferromagnesian,
66-67
65-66
332 sinter, 509
Siliceous ooze, Siliceous
Siltstone, 165,
168
209
198, 199
56 seismic waves and, 261 Solifluction, 425, 435, 437 Solon, 320
141-42
Sinkhole,
495-96
Slab-pull
mechanism, 370-71
Slate,
201-2, 203, 209
Slide,
425, 426,
428-33
456
in deserts,
565-66
discharge,
459
drainage basins and patterns,
474-75 456-57
gradient, piracy,
475
submarine canyons and, 319
476-77 477-78
superposed, terraces,
transport and deposition by, 160-62,
166,
460-63, 466-70
development of, 475-76 457, 459 volume of Earth's water in, 452, 453 Stream-dominated delta, 468, 469 Streamline, 454, 455 Stress, 379 Striations, 67, 348 glacial, 523 Strike, 380, 381, 390, 391 valleys,
velocity,
496
Sorting, 161
Source rock, 182 South America
74 71, 72, 209
Strike-slip fault,
393-94
Specific gravity, 72,
Stromboli volcano,
Sphalerite,
Strong nuclear force, 29
146-47
Italy,
Structural trap, 183
Spotted shale, 199, 200 Spreading ridge(s), 357, 360-61. See also
metamorphism and, 194
Oceanic ridge age of oceanic crust and, 16, 355-57 cells and, 370-71
earthquakes at, 259 volcanism at, 103-5, 130 hot,
490-91 506-8
102
Structural geology, 5
581, 582, 584-85 Spodumene, 131 Spit,
Spring(s),
470-72
459-60
and floodplains, 463-66
floods
convection
Sima, 12
455
level,
graded,
Spheroidal weathering,
Sillimanite, 199, 201, 202, 206, Sill(s), 117, 123-25 contact metamorphism and, relative dating, 220-21 Silt, 160, 487, 550, 552, 556
30-31 47-49 Earth-Moon system, 49-51 meteorites, 33-34, 36-37 origin and early history, 31-33 planets, 30-31, 3S, 38-47 in,
Solution valley,
531-32
Stream(s), 171, 418, 419,
Earth
Solution,
528,
kames and eskers, 531-32 ourwash plains and valley trains, 531
characteristics,
South American plate, 16, 17, 106 Spatter cone (lava flow), 89, 123
nonferromagnesian, 67
Stratified drift,
erosion,
and Africa, 343, 345, 347 orogenesis in, 399
bonding, 60
structure,
32—33
Solid(s),
Sierra
Stratigraphic trap, 183
defined,
Solar system
erosion, 587,
tides,
548
volcanic activity and, 4 Solar nebula theory,
Storm surge, 578 Strain, 379
472-74 bedding in, 176-77
transported, 148
Shoreline(s)/shoreline processes,
Stones (meteorites), 33, 34 Stony-irons (meteorites), 33, 34 Stoping, 128
base
148
salinization of,
219-20
299
Stishovite,
Stratovolcano, 96, 98
455-56
148-49
profile,
residual,
Shoestring geometry (of sedimentary
defined,
343
chloride, 58-59 147-48 desert, 564-65 erosion, 153-54
Sodium
322
Steno, Nicholas, 62, Stock, 125, 126
Smith, Robert Angus, 144 Smith, William, 221
Shelley, Percy B.,
bodies), 175,
Steinbeck, John, 136
stability maps, 439, 442, 443 Slow mass movements, 425
Slumps, 425, 426,
29 206
Staurolite, 201,
formation, factors controlling,
Shelf-slope break, 319,
tide,
Stalactites,
Soil(s),
456
Sheet flow,
Spring
Slope
Subduction zone, 16, 334, 362, 397 negative gravity anomalies
orogenesis
at,
volcanism
at,
at,
303
397-405, 408 105-6
Su Song, 215 Sublimation, 516 Submarine canyon, 319, 323, 587 Submarine fan, 322, 404 Submergent coast, 592, 594
Index
629
Subsidence, 484,
and pressure 207
502-3 588-89
in coastal areas,
thermal expansion and contraction, 139, 141
Edward, 343-44
Sulfates, 65,
68-69 75-76 68, 371-72 32, 33, 594-95
Tensional stress, 379 Terminal moraine, 530
rock-forming, Sulfides, 65,
Sun, 30, 31,
577
Surface seismic waves, 261, 263 Surface water, formation of, 316,
317
40-41 358-59 Teton Range, Wyoming, 377, 393 Tharp, Marie, 330-31 Theory, 12-14, 16 Thera, volcanic eruption
Suspended load
460 wind, 550 streams,
Thermal convection 351,
Suspended water, 488 S-waves
of,
cells,
320-21
12, 14, 300,
370-71
Thermal expansion and contraction, 139,
Earth's interior, study of and,
289-90,
305
263-64
earthquakes and, 261, 262, shadow zone, 291, 292
141 Thorium-lead dating, 238, 239 Thrust fault, 334, 393 Tide(s),
594-96
Tide-dominated delta, 468, 469 Till, 348, 528-30
Swell (wave), 575
357
drumlins, 530
170
moraines,
Symmetrical fold, 382 Syncline,
30-31, 35, 38-44
plate tectonics on,
Surface creep, 549
Sylvite,
477-78
composition, 31, 33 evolution of climate on,
221, 223, 227
Sykes, L. R.,
stream,
381-82
plunging, 383, 385
263-64
209
138-39, 425 Tambora volcano, Indonesia, 542
Tombolo, 584-85 Tonga Trench, 257
Titan (moon), 27, 45 Toit, Alexander du, 344
Top
soil,
148-49
Tourmaline, 54
Toxic waste, 505-6 Trace fossil, 177
326-27
earthquake precursors), 278 Ultramafic rocks, 119,405 lava flows and, 120 Unconformities, 222-23,
224-26
222-23, 225 disconformity, 222, 224 nonconformity, 223, 226 Undercutting, 418, 419 angular,
Uniformitarianism, principle
21-22,
of,
219 earthquake prediction
by,
279-80
sea-floor
464-65
mapping, 317, 318
volcanoes, monitoring and forecasting eruptions,
94-95
Universe, origin of, 29 Ural Mountains, Soviet Union,
404
238 Uranium, 183, 235 Uraninite, 183,
fission track dating,
242-43, 245
Uranium-lead radioactive decay
series,
238, 239, 240 Uranus, 27-28, 30, 31, 33, 45, 47
U-shaped
glacial trough,
Vaiont Reservoir,
Transitional depositional environment,
Valley(s)
524-26
297-300
454
Transported soil, 148 Transverse dune, 556, 557 Travertine,
Kelvin's calculation of the Earth's age
Tree-ring dating, 242, 245
mass wasting, 420
metamorphism and, 194
Valles Marineris,
140,
Mars, 43, 44
Trellis drainage,
solution,
496
stream, development of, train,
472
Trench, oceanic. See Oceanic trench Trilobite, 178, 229, 231
475-76
531
Valley glacier, 518,
509
geothermal gradient, 103-4, 297 and, 219
Italy, disaster,
440-42, 579
hanging, 526
Transpiration,
564
of Earth's interior, 292,
Index
Ultra-low frequency radio waves (as
fault, 327, 357, 366 Transform plate boundaries, 16, 17 characteristics, 366 earthquakes and, 257 Transition zone (mantle), 297
162
Curie point, 307 deformation and, 379
630
Valley, California,
Transform,
Temperature chemical weathering and, 146 contact metamorphism and, 198
of deserts, 561, 562,
Death
Trans-Atlantic cable, 323
Tar sands, 183 Taylor, Frank B., 344 Teed, Cyrus Reed, 287 Telegraph Plateau,
Crater,
mineral resources and, 76, 78
Taconic orogeny, 406 Tailings (mine), 143 Talc, 193, 199, 202, 204, 209
280
Ubehebe 563
flood prediction,
epicenters with,
Tarn, 527
u
Time, 215
Tin,
Tangshan, China, earthquake, 253, 273,
glide,
432, 433
U.S. Geological Survey (USGS)
528-30 277-78
magnetic reversal time scale, 310-11 Time-distance graph, determining
84, 90, 94,
322-23
Tiltmeter, 94, 95,
geologic. See Geologic time
Talus,
Trough (wave), 575 Truncated spur, 525 Tsunami, 274-75, 321, 578-79 Tuff, 102, 122 Tungsten, 209 Tunguska event, 36-37 graded bedding and, 176 Turbulent flow, 454-55 Turtle Mountain, Canada, rock
marine, 594 Terrestrial planets,
Triton (moon), 27, 28, 88
Turbidity currents,
Terrace
Supergene enrichment of ores, 155 Superposed stream, 476-77 Superposition, principle of, 219, 220, Surf zone,
facies,
12-13
Subsurface geology, 232 Suess,
metamorphic
regulation of Earth's (Gaia hypothesis),
149
Subsoil,
in
519
erosional landforms,
524-27
movement, 520-22 van der Waals bonds, 60 Varve, 533
Vegetation
notable eruptions, 84
and desertification and, 547-48, 564 erosion and, 475 mass wasting and, 420-21, 444 Velocity (stream), 457, 459, 461
shield, 93,
deserts
550-51
Ventifact,
Venus, 26, 28, 30, 33, 39, 42, 88
greenhouse effect on, 40 Verne, Jules, 7,
287
Vesuvius,
Mount,
Italy,
102
87, 98,
Vinci, Leonard da, 7, 343 Vine F, 354 Viscosity, 86 Volatile element, 33 Volcanic ash. See Ash, volcanic
Volcanic breccia, 122
Volcanic gases, 87, 90, 144 Volcanic island arcs, 326, 362-63, 397,
399 Volcanic neck, 125
Volcanism, 3-5, 80-107, 112-13, 122, 125, 144, 507, 542. See also Volcano active continental margins and, 323 carbon dioxide recycling and, 40, 41 at convergent boundaries, 362, 364
87 101
oceanic ridges and trenches, 326, 327 planets, 28, 35, 42, 44, 50,
88,358-59,451
102—7 deposits, 101-2
plate tectonics and, 16,
pyroclastic sheet
surface water and atmosphere, role in
forming, 316, 317 Volcano(es), 3-5,
active,
92-99. See
also
volcanoes
87
cinder cone, 96,
97-98
composite, 96 distribution,
102
565-66
Europe, 123-24
cycle,
Wilson,
J. T.,
406 357, 406
deposits (dunes and loess), in deserts,
erosion,
552-58
566
550-52
558-59, 560 on Mars, 552-53 global patterns,
transport of sediment by, 161,
549-50, 587 wave generation by, 575, 577 Wind shadow, 554 Wizard Island, Oregon, 93, 97 Wood, H. O., 264
490-91
well(s),
491, 493
desertification and,
548
Wave(s), 160
Xian, China, earthquake, 253
578-79 575—77
coastal flooding,
generation,
extinct (inactive),
guyots and seamounts, 327, 329
domes, 98-99 monitoring and forecasting eruptions,
583-84
Yardangs, 551 Yellowstone National Park,
Wyoming,
shallow-water and breakers, 577 shoreline erosion and, 587, 591-92
506, 508, 509 Yellowstone Tuff, Wyoming, 102
terminology, 575
Yosemite Falls, California, 526 Yuca Mountain, Nevada, 506—7 Yungay, Peru, earthquake/landslide,
tsunami, 274-75, 321,
578-79
base, 575, 577 Wave-built platform, 591
Wave
275-76, 415-16, 439
Wave-cut platform, 591-92 Wave-dominated delta, 468, 469 Wave-formed ripple marks, 177
Wave
rays,
Weak
nuclear force, 29
289
Weather/weather patterns. See Climate Weathering, 134-55 defined,
87
584
seasonal effect on beaches,
chemical,
dormant, 87
lava
Sill,
Wilson
Wind, 549-59
and mass wasting, 420, 442, 444 metamorphism, role in, 195-97 in oxidation, 142 running. See Running water as a solvent, 142 suspended, 488 volume of, on Earth, 452, 453 Water gap, 476-77 Water table, 488-89
refraction, 580,
and pyroclastic materials,
89-91
specific
tuff, 102, 122 Well log, 228, 232-34
nearshore sediment budget and, 587
heat flow and, 300
on other
Welded
Whin
486-87 on Mars, 451-52
perched,
Weight, 302
mass wasting and, 419, 426
fissure eruptions, 99,
87,
146-47
351
Walvis Ridge, 330 Water. See also Groundwater; Lake;
Water
154—55
rock cycle, 18,20, 136, 137
Wegener, Alfred, 14, 344, 345, 346, 349,
w
lowering, 500
Volcanic (extrusive igneous) rocks, 85, 113
lava flows
in
spheroidal,
desert,
Volcanic pipe, 125
defined,
mineral resources and,
hydrologic cycle, 452, 454, 455,
Vesicular texture, 114, 115
137-41
mechanical,
96-97 92-99
Ocean; Stream; Wave hard, 144
359
plate tectonics on,
types,
141-47
136
in deserts,
564
differential,
136-37
mass wasting and, 420
Zion National Park, Utah, 214, 228, 230 Zircon, 238 Zone of accumulation, 149 glaciers, 519 Zone of aeration, 488 Zone of saturation, 488 Zone of wastage (glacier), 519 Zoned plagioclase, 116
94-95
Index
631
^•^^^ *»» SB.'*. TKT^
^^3«^m.^ ^^^3gmsKr^ ».^^^ m.
^^ m. *.m. ^ -
'
Er «p^i
CREDITS
ENDSHEETS Front
Precision
Graphics.
CONTENTS
BRIEF Left: U.S.
Geological Survey. Middle: George and Linda
Lohse.
U.S.
2—1, Figure 2: TASS from Sovfoto. 2- 10b: National Space Science Data Center, Dr. Bruce C. Murray, principal investigator. 2-10c, 2-llc, 2-12d, 2-13b, 2-14b, 2-15c, 2—16, 2—17, and 2-18: Victor Royer. Perspective 2—2, Figure 1: Carlyn Iverson. 2— 12a inset and 2— 12b: Astronomical Society of the Pacific. 2—19: Lick Observatory. 2—21: Benz and W. Slattery, Los Alamos National Laboratories.
See chapter opener photo credits.
CHAPTER
1
— 1: Krakatau
1883, by Tom Simpkin and Richard S. Smithsonian Institution, 1983. 1—2, 1—5, and 1-13: Rolin Graphics. 1-3: The Geological Society, London. 1-4: NASA. l-6a: Patricia K. Armstrong/Visuals Unlimited. l-6b: American Association of Petroleum Geologists/IBM. 1-7: Collection of the New York Public Library Astor, Lenox, and Tilden Foundations. 1-8: British Fiske,
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1—11: Victor Royer. 1-12, 1-14, and 1-19: Carlyn Iverson. 1-15, 1-16, 1-17, and 1-18: Precision Graphics. Table 1—1: Modified from R.V. Dietrich and R. Wicander, Minerals, Rocks, and Fossils (New York: Sons, Inc., 1983): 160, Table IV- 2. 1-20: John Wiley Precision Graphics. From A.R. Palmer, "The Decade of North American Geology, 1983 Geologic Time Scale." Gerights reserved.
&
ology (Boulder, Colo.:
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1983): 504. Reprinted by permission of the Geological Society of
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CONTENTS
1
©1980, p. 11. Reprinted by permission of PrenticeEnglewood Cliffs, NJ 07632. 2-8: D.J. Roddy, Geological Survey. 2—9: Paul Dimare. Perspective
Earth,
Front right top: Rolin Graphics. Front right bottom: Carlyn Iverson. Back left: Darwen and Vally Hennings. Modified from Geologic Time. 1981. U.S. Geological Survey. left:
2
Opener and 2-lla: Finley Holiday Film. 2-1, 2-llb, and 2-15a,b: JPL/NASA. 2-2, 2-3, 2-10a, 2-12a, 2-12c, 2-13a, and 2-14a,c,d,e: NASA. 2-4a: John and Judy Waller. 2-4b and 2-7a: Precision Graphics. 2-5; Perspective 2-1, Figure 1; and 2-20: Rolin Graphics. 2-6: Precision Graphics.
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3-2: Wyoming State Museum. 3—3 and 3-4: Layout by Georg Klatt; final inking by Elizabeth Morales-Denney; 3-5, 3-6, 3-7, 3-8, 3-10, 3-11, 3-12, 3-19, 3-20, 3-21, and Review Question 17: Precision Graphics. 3—9: Precision Graphics. From R.V. Dietrich and Brian J. Skinner, Gems, Granites, and Gravels: Knowing and Using Rocks and Minerals (New York: Cambridge University
3- 18b: Ward's Natural Science Establishment, Inc. 3-24: Precision Graphics. From R.V. Dietrich and Brian J. Skinner, Gems, Granites, and Gravels: Knowing and Using Rocks and Minerals (New Press, 1990): 39, Figure 3.4.
York: Cambridge University Press, 1990): 97, Figure 6.1. Precision Graphics. From Brian J. Skinner, "Mineral Resources of North America." In Geology of North America, vol. A. (Boulder, Colo.: Geological Society of America, 1989): 577, Figure 2. Reprinted by permission of the Geological Society of America and Brian J. Skinner.
3—25:
CHAPTER
4
Opener: Reuters/Bettmann Archive. 4-2: Rolin Graphics. From R.I. Tilling, U.S. Geological Survey. 4-3: D.R. Cran-
America.
CHAPTER
CHAPTER
From
Eicher/McAlester, History of the
dell, U.S. Geological Survey. 4-4a: Keith Ronnholm. 4_4b, 4-6, 4-8, 4-10a, 4-llb, and 4-19: U.S. Geological Survey. 4-5: Harry Glicken, U.S. Geological Survey. 4-7: P.W Lipman, U.S. Geological Survey. 4-9, 4-18, 4-23, 4-26, and 4-32: Precision Graphics. Perspective 4-1, Figure 1: NASA. 4-10b: John S. Shelton. 4-lla: T.J. Takahashi, U.S. Geological Survey. 4- 12a: D.W. Peterson,
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igin (Berkeley, Calif.: University of California Press): Illus-
from page 84. Copyright ©1941 Regents of the ©renewed 1969 Howel Williams. Perspective 4-3, Figure 1: Precision Graphics. From R.I. trations
University of California,
4—20: K. Segerstrom, U.S. Geological Survey. 4-21: Solarfilma/GeoScience Features. 4—22: Lawrence R. Solkoski, consulting geologist, VancouTilling, U.S. Geological Survey.
4-24: I.C. Russell, U.S. Geological Sur4—25: Ward's Natural Science Establishment, Inc. 4-28: Rolin Graphics. Modified from R.I. Tilling, C. Heliker, and T.L. Wright, Eruptions of Hawaiian Volcanoes: Past, Present, and Future. 1987. U.S. Geological Survey. 4-29: Precision Graphics. WG. Ernst, Earth Materials, ©1969, p. 107. Reprinted by permission of Prentice-Hall, Inc., Englewood Cliffs, NJ 07632. 4-30: Rolin Graphics. From "Hot Spots on the Earth's Surface," copyright ©1976, by Scientific American, Inc., George V. Kelvin, all ver,
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CHAPTER
From
WR.
Dickinson and W.C. Luths, "A Model for Plate Tectonic Evolution of Mantle Layers." Science 174 (22 October 1971): 402, Figure 1. Copyright 1971 by the AAAS. Reprinted by permission of the AAAS and W.R. Dickinson. 5—21: Martin G. Miller/Visuals Unlimited. 5-22: Palisades Interstate Park Commission. 5-32 and 5-33: Carlyn Iverson. Perspective 5-2, Figure 1: Photo ©1985 by Wendell E. Wilson. Perspective 5-2, Figure 3: WT. Schaller, U.S. Geological Survey.
CHAPTER
6
Opener: Paul Johnson. 6-1: Rolin Graphics. Modified from Donald Worster, Dust Bowl (New York: Oxford University Press, 1979): 30. 6—2: Kansas State Historical So-
6-3, 6-14, 6-16, 6-17, 6-18, 6-21, 6-25, and 6—28: Precision Graphics. 6-4: Dietrich Stock Photos, Inc. 6—7: Precision Graphics. From A. Cox and R.R. Doell, "Review of Paleomagnetism." GSA Bulletin 71 (1960): 758, Figure 33. 6-8: University of Colorado. Perspective 6-1, Figure 2: N.K. Huber, U.S. Geological Survey. 6-11: B.C. Hunt, U.S. Geological Survey. 6-15: Bill Beatty/Viciety.
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CHAPTER
7
7-2: R.L. Elderkin, U.S. Geological Survey. 7-3, 7-7, 7-14, 7-19, 7-20, 7-21, 7-33, and 7-34: Precision Graphics. 7—6; 7—30; and Perspective 7—2, Figure 4: Rolin Graphics. Perspective 7—1, Figure 1: Michael Thomas Associates. Perspective 7-1, Figure 2: Precision Graphics. From M.B. Cita, "Mediterranean Evaporite: Paleontological Arguments for a Deep-Basin Desiccation Model." In C.W. Drooger, ed., Messinian Events in the Mediterranean, Geodynamics Scientific Report no. 7 (1973): 212, Figure 3. Reprinted by permission of M.B. Cita. 7—18: Rolin Graphics. From the U.S. Geological Survey. 7—31: Alan L. Mayo, GeoPhoto Publishing Company. 7—32: J&R Art Services. Perspective 7-2, Figures 1 and 2: Precision Graphics. Data from Saudi Aramco. Perspective 7—2, Figure 3: Michael Thomas Associates. From Robert S. Dietz and John C. Holden, "Reconstruction of Pangaea: Breakup and Dispersion of Continents, Permian to Present." Journal of Geophysical Research 75, no. 6 (10 September 1970): 4949, Figure 5. Copyright by the American Geophysical Union.
CHAPTER
5
5-2: Photo of painting by Herbert Collins, courtesy Devil's Tower National Monument. 5-3, 5-4, 5-7, 5-9, 5-10, 5-12, 5-20, 5-24, 5-28, 5-29, and 5-30: Precision Graphics. 5-13: Precision Graphics. Modified from R.V. Dietrich, Geology and Michigan: Fortynine Questions and Answers. 1979. Perspective 5-1, Figure 1: Rolin Graphics. Perspective 5 — 1, Figure 2: Rolin Graphics.
6— 20a and 6-23: John S. Shelton. 6-20b and 6— 22a, b: John D. Cunningham/Visuals Unlimited. 6-24a: Walt Anderson/Visuals Unlimited. 6-27: Science suals Unlimited.
8
8-la, 8-3, 8-7, and 8-12a: Precision Graphics. 8 — lb: Precision Graphics. From G. Rapp, Jr., and J. A. Gifford, eds., Archaeological Geology (New Haven, Conn.: Yale University Press, 1985): 338, Figure 13.3. Reprinted by
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8—2: Arthur M. Sackler Museum, Harvard University, Cambridge, Massachusetts, John Randolph Coleman Memorial Fund. 8-4: Rolin Graphics. Perspective 8 — 1, Figure 1: Smithsonian Institution. 8 -5a and 8-11: Precision Graphics. From C. Gillen, Metamorphic Geology (London: Chapman & Hall, 1982): 24 and 73, Figures 2.3 and 4.4. Reprinted by permission of Chapman & Hall and C. Gillen. 8—9 and Table 8 — 1: From C. Gillen, Metamorphic Geology (London: Chapman &C Hall, 1982): 49, Figure 3.2; and Table 4—1, p. 70. Reprinted by permission of Chapman &C Hall and C. Gillen. 8-20: Rolin Graphics. From H.L. James, GSA Bulletin 66 (1955): 1454, Plate 1. Reprinted by permission of the Geological Society of America. 8-21: Precision Graphics. Reprinted with permission from AGI Data Sheet 35.4, AGI Data Sheets, 3d ed., 1989, American Geological Institute. 8-22: Carlyn Iverson.
8—23 Figure
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8-23 from
8—23 bottom: Carlyn
Iverson.
"Effects of Late Jurassic-Early Tertiary
Subduction in California." Late Mesozoic and Cenozoic Sedimentation and Tectonics in California, San Joaquin Geological Society Short Course (1977): 66, Figure 5—9. Reprinted by permission of the San Joaquin Geological Society.
CHAPTER
9
Martin
9—1: Darwen and Vally Hennings. Modified from Geologic Time. 1981. U.S. Geological Survey. 9—2: Precision Graphics. From A.R. Palmer, "The Decade of North American Geology, 1983 Geologic Time Scale." Geology (Boulder, Colo.:
Geological Society of America, 1983): 504. Re-
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9-5; 9-6; 9-9a; 9-10a; 9-lla; 9-12; 9-13; 9-16; Perspective 9-1, Figure 4a; 9-18; 9-20b; 9-21; 9-22; Perspective 9—2, Figures 1 and 2; 9—23; and Review Question 30: Precision Graphics. 9—7: Precision Graphics. From The Story of the Great Geologists by Carroll Lane Fenton and Mildred Adams Fenton. Copyright ©1945 by Carroll Lane Fenton and Mildred Adams Fenton. Used by permission of Doubleday, a division of Bantam Doubleday Dell Publishing Group, Inc. 9-8, 9-14, 9-17, 9-28, and 9—29: Rolin Graphics. 9-15: Precision Graphics. From Geologic Time. 1981. U.S. Geological Survey. Photos by
Reed Wicander. Perspective 9-1, Figure 3a: Rolin Graphics. From M.H. Rider, The Geological Interpretation of Well Logs (Glasgow: Blackie and Son Limited, 1986): 2,
M.H. Rider. PerRolin Graphics. From B. Rascoe,
Figure 1.2. Reprinted by permission of spective
9—1, Figure 3b:
"Regional Stratigraphic Analysis of Pennsylvanian and Permian Rocks in Western Mid-Continent, Colorado, Kan-
Jr.,
Oklahoma, Texas." American Association of Petroleum Geologists Bulletin 46, no. 8 (1962): 1356, Figure 7. Perspective 9—1, Figure 4b: From O.R. Berg and D.G. Roberts, "Depositional Sequence Mapping as a Technique to Establish Tectonic and Stratigraphic Framework and Evaluate Hydrocarbon Potential on a Passive Continental Margin." In R.J. Hubbard, J. Pape, and R.G. Wolverton, eds., Seismic Stratigraphy II — An Integrated Approach, Memoir 39 (1985): 84, Figure 4. Reprinted by permission of the American Association of Petroleum Geologists. 9-19: Precision Graphics. Data from S.M. Richardson and H.Y. McSween, Jr., Geochemistry— Pathways and Processes (Englewood Cliffs, NJ: Prentice-Hall, 1989). 9-20a: Precision Graphics. From Don L. Eicher, Geologic Time 2d ed., ©1976, p. 120. Reprinted by permission of Prentice-Hall, Inc. Englewood Cliffs, NJ 07632. Perspective 9-2, Figure 3: Rolin Graphics. Data from Environmental Protection Agency. 9-24: Precision Graphics. From E.K. Ralph, H.N. Michael, and M.C. Han, "Radiocarbon Dates and Reality." MASG4 Newsletter 9 (1973): 5, Figure 8. 9-25: Rolin Graphics. From Stokes and Smiley, An Introduction to Tree-Ring Dating (Chicago: The University of Chicago Press, 1968): 6, Illustration #2. 9-27: Michael Thomas Associates. From L.W. Mintz, Historical Geology: The Science of a Dynamic Earth, 3d ed. (Westerville, Ohio: Charles E. Merrill Publishing Comsas,
pany, 1981): 27, Figure 2.18.
CHAPTER
10
Opener: J. P. Stacy, U.S. Geological Survey. 10-la; 10-13; and Perspective 10-2, Figure 1: Rolin Graphics. 10-lb:
E.
Klimek, Marin Independent Journal. 10- lc:
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physical Data Center,
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10- le:
Geo-
10- Id:
Boulder, Colorado. C.
Stover,
Data Center, NOAA, Boulder, Colorado. 10-2 and 10-21: Katsuhiko Ishida. 10-3a; 10-5b,c; 10-6; 10-11; Perspective 10-1, Figure 2; and 10-17: Precision Graphics. 10-3b: U.S. Geological Survey. 10-4: Reproduced by permission of the Trustees of the Science Museum, London. 10— 5a: Earthquake Information Bulletin 181, U.S. Geological Survey. 10-7: Rolin Graphics. Data from National Oceanic and Atmospheric Administration. 10-8: Carlyn Iverson. 10-9: J.K. Hillers, U.S. Geological Survey. 10-10: Precision Graphics. From Nuclear Explosions and Earthquakes: The Parted Veil by B.A. Bolt. Copyright ©1976 by W.H. Freeman and Co. Reprinted by permission. 10—12: Precision Graphics. Data from C.F. Richter, Elementary Seismology. 1958. W.H. Freeman and Co. 10-14 and 1025a: Rolin Graphics. From M.L. Blair and WW. Spangle, U.S. Geological Survey, National Geophysical
U.S. Geological Survey Professional Paper 941-B. 1979.
10-15: Rolin Graphics. From R.D. Borcherdt,
ed., U.S.
Geological Survey Professional Paper 941-A. 1975.
10-16
and Perspective 10-1, Figure 4: Precision Graphics. From Earthquakes by Bruce A. Bolt. Copyright ©1978, 1988 by W.H. Freeman and Co. Reprinted by permission. Perspective 10-1, Figure 3: R. Kachadoorian, U.S. Geological Survey. Perspective 10-1, Figure 5: M. Celebi, U.S. Geological Survey. 10—18: Hebei Provincial Seismological Bureau, U.S. Geological Survey. 10-19: San Francisco Public Library. 10-20; 10-24; and Perspective 10-1, Figure 1: National Geophysical Data Center, NOAA, Boulder, Colorado. 10-22: Rolin Graphics. Data from NOAA. 10-25b: Alice Thiede. Modified from S.T Algermissen and D.M. Perkins,
"A
Probabilistic Estimate of
Maximum
Accelera-
tion in the Contiguous United States." U.S. Geological Sur-
vey Open-File Report 76-416. July 1976. 10-26: From The Loma Prieta Earthquake of October 17, 1989. 1989. U.S. Geological Survey. 10-27: Precision Graphics. Reprinted with permission from Predicting Earthquakes, 1976. Published by National Academy Press, Washington, D.C. 10-28: Precision Graphics. Reprinted with permission from Geotimes 10 (1966): 17.
CHAPTER
11
Opener: Wilfred A. Elders, Professor of Geology, Univerof California, Riverside. 11-2: Victor Royer. 11-3; 11-4; 11-5; 11-7; ll-8b; 11-9; 11-10; 11-11; Perspective 11-1, Figure 1; 11-15; 11-16; 11-17; 11-18; 11-19; 11-20; 11-21; 11-22; 11-23; 11-24; 11-27; 11-28; 11-29; 11-30; 11-31; 11-32; and 11-33: Precision Graphics. 11-6 and 11-12: Precision Graphics. From G.C. Brown and A.E. Musset, The Inaccessible Earth Hall, 1981): 17 and 124, Figures (London: Chapman 12.7a and 7.11. Reprinted by permission of Chapman 5c sity
&
Credits
635
Hall.
11— 8a: Kort-og
Matrikelstyrelsen (National Survey
and Cadastre — Denmark). 11-13: Precision Graphics. From D.P. McKenzie, "The Earth's Mantle." Original illustration by Ian Worpole. Copyright ©September 1983 by Scientific American, Inc. All rights reserved. Perspective 11 — 1, Figure 2: Precision Graphics. From Keith G. Cox, "Kimberlite Pipes." Original illustration by Adolph E. Brotman. Copyright ©April 1978 by Scientific American, Inc. All rights reserved.
11 — 14: Precision Graphics.
Inside the Earth by Bruce A. Bolt. Copyright
From ©1982 by
W.H. Freeman and Co. Reprinted by permission. tive 11—2, Figure 1: Precision Graphics. Andrew Copyright
©1987
PerspecChristie/
Discover Publications. 11 -25a: Rolin
From Beno Gutenberg,
Physics of the Earth's Interior (Orlando, Florida: Academic Press, 1959): 194,
Graphics.
Figure 9.1. Reprinted by permission of Academic Press. 11 -25b: Rolin Graphics. From R.F. Flint, Glacial and
Quaternary Geology (New York: John Wiley &c Sons, Inc., 1971): 363, Figure 13-13. 11-26: Fundamental Photographs.
CHAPTER
Woods Hole Oceanographic Institution. 12—1; 12-3; Perspective 12-1, Figure 2; 12-17; 12-19; and 12-29: Rolin Graphics. 12-2: From John M. Edmond, "The Geochemistry of Ridge Crest Hot Springs." Oceanus 27, no. 3: 16. Copyright ©1984, Woods Hole Oceanographic Institution. 12-4: Scripps Institution of Oceanography, University of California, San Diego. 12—5: Precision Graphics. From U.S. Geological Survey. 12—6, 12—7, 12-8, 12-9, 12-10, 12-11, 12-13, and 12-23: Precision Graphics. Perspective 12-1, Figure 1: Rolin Graphics. From Phyllis Young Forsyth, Atlantis: The Making of a Myth (Montreal: McGill-Queen's
University Press):
13,
Figure 2. Perspective 12—1, Figure 3: Painting by Lloyd K.
©
National Geographic Society. 12— from Bruce C. Heezen and Charles D. Hollister, The Face of the Deep (New York: Oxford University Press, 1971): 297, Figure 8.15. 12-14: Rolin Graphics. From Alyn and Alison Duxbury, An Introduction to the World's Oceans. Copyright ©1984 Addison-Wesley Publishing Company, Inc., Reading, Massachusetts. Reprinted by permission of Wm. C. Brown Publishers, Dubuque, Iowa. All rights reserved. 12—15: From Bruce C. Heezen and Charles D. Hollister, The Face of the Deep (New York: Oxford University Press, 1971): 329, Figure 8.48. 12-16: Precision Graphics. From B.C. Heezen, M. Tharp, and M. Ewing, "The Floors of the Oceans, Part 1, The North Atlantic." Geological Society of America Special Paper 65. 1959. 12-18 and 12-26: Car-
Townsend copyright
12: Precision Graphics. Modified
lyn Iverson. Perspective
map by
12—2, Figure
1:
World Ocean
Bruce C. Heezen and Marie Tharp, 1977. Copyright ©Marie Tharp 1977. Reproduced by permission of Marie Tharp, 1 Washington Ave., South Nyack, NY 10960. 12-20: Dr. Bruce Heezen, Lamont-Doherty Geo-
636
Credits
,
Institution of
CHAPTER
13
NASA. 13-1
left; 13-7; 13-8; Perspective 13-1, Figure Id; 13-14; 13-22; 13-24 right; and 1327: Rolin Graphics. 13-1 right, 13-16, 13-18, 13-19, 13-20, 13-21, 13-24 left, and 13-26: Carlyn Iverson. 13-2: U.S. Geological Survey. 13-4: Bildarchiv Preussischer Kulturbesitz. 13—5: Rolin Graphics. From E. Bullard, J.E. Everett, and A.G. Smith, "The Fit of the Continents
Opener:
Around 12
Opener:
Floor
1
Observatory Columbia University, courtesy Scripps Oceanography, University of California, San Diego. 12-21: Rolin Graphics. From T.A. Davies and D.S. Gorsline, "Oceanic Sediments and Sedimentary Processes." In J. P. Riley and R. Chester, eds., Chemical Oceanography 5, 2d ed. (Orlando, Florida: Academic Press, 1976): 26, Figure 24.7. Reprinted with permission from Academic Press and T.A. Davies. 12-22: Bruce Berg/Visuals Unlimited. 12-24: Frink/Waterhouse, H. Armstrong Roberts. 12-25: John D. Cunningham/Visuals Unlimited. 12—27 and 12—28: Rolin Graphics. From U.S. Geological Survey. logic
the Atlantic." Philosophical Transactions of the Royal Society of London 258 (1965). Reproduced with permission of the Royal Society, J.E. Everett, and A.G. Smith. 13-6: Rolin Graphics. Reprinted with permission of Macmillan Publishing Company from R.J. Foster, General Geology, 4/e, Fig. 20-2, p. 351. Copyright ©1983 by Merrill Publishing Company. 13-9: Rolin Graphics. Modified from E.H. Colbert, Wandering Lands and Animals (1973): 72, Figure 31. 13-10: Michael Thomas Associates. From A. Cox and R.R. Doell, "Review of Paleomag-
netism."
GSA
Bulletin 71
(1968): 758, Figure 33. Re-
printed by permission of the Geological Society of America. Perspective 13 — 1, Figure la: Rolin Graphics.
From R.K.
Bambach, C.R. Scotese, and A.M. Ziegler, "Before Pangea: The Geographies of the Paleozoic World." American SciReprinted by permission of American Scientist, journal of Sigma Xi, The Scientific Research Society. Perspective 13-1, Figure lb,c: Rolin Graphics. From R.S. Dietz and J.C. Holden, "Reconstruction of Pangaea: Breakup and Dispersion of Continents, Permian to Present." Journal of Geophysical Research 75, no. 26 (1970): 4939-56. Copyright by the American Geophysical Union. Reprinted with permission from The American Geophysical Union, R.S. Dietz, and J.C. Holden. 13-11: Precision Graphics. From A. Cox, "Geomagnetic Reversals." Science 163 (17 January 1969): 240, Figure 4. Copyright 1969 by the AAAS. Reprinted by entist 68, no. 1 (Jan.-Feb. 1980): 29, Figure 5.
AAAS. 13-12: Rolin Graphics. From The Bedrock Geology of the World. Copyright ©1984 by R.L. Larson and WC. Pitman III. Reprinted with permission by W.H. Freeman and Company. 13-13, 13—25, and Review Questions 15 — 18: Precision Graphics. Perspective 13-2, Figure 1: AP/Wide World Photos. Perspective 13-2, Figure 2a: Rolin Graphics. From Geology 18 (February 1990): 99, Figure 1. Reprinted by permission of the Geopermission of the
America and James W. Head. Perspective 13-2, Figure 2b: From L.S. Crumpler, James W. Head, and Donald B. Campbell, "Orogenic Belts on Venus." Geology 14 (December 1986): 1031, Figure 1. Courtesy Department of Geological Sciences, Brown University. Perspective 13-2, Figure 3: Precision Graphics. From Geology 18 (February 1990): 102, Figure 4. Reprinted by permission of the Geological Society of America and James W. Head. 13-15: Woods Hole Oceanographic Institution. 13-17: Alice Thiede. 13-23: Rolin Graphics. Data from J.B. Minster and T.H. Jordan, "Present-day Plate Motions." Journal of Geophysical Research 83 (1978): 5331-51. logical Society of
CHAPTER
14
Copyright ©1990 by Merrill Publishing Company. 14Martin F. Schmidt, Jr. 14-19d; 14-22; 14-23; 14-
14:
28; 14-30; Perspective 14-3, Figure 3; and 14-38: Rolin Graphics. 14-20 and Perspective 14—1, Figure 2: S.W.
Lohman,
U.S. Geological Survey.
for Flysch Dispersal in the Appalachian-Quachita System
86 (1975). Perspective 14-3, Figure 1: Reproduced by permission of Earth Observation Satellite Company, Lanham,
Maryland, USA.
14-39: Michael Thomas Associates. Goes Back 2 Billion Years." Science 230 (20 Decmeber 1985): 1366. Copyright 1985 by the AAAS. Reprinted by permission of the AAAS. 14-40: Rolin Graphics. From Zvi Ben-Avraham, "The Movement of Continents." American Scientist 69: 291-
From R.A.
Kerr, "Plate Tectonics
299, Figure 9, p. 298. Reprinted by permission of American Scientist, journal of Sigma Xi, The Scientific Research Society.
14-2: Precision Graphics. From Structural Geology of North America by A.J. Eardley. Copyright ©1951 by Harper &c Row, Publishers, Inc. Copyright ©1951 by A.J. Eardley. Reprinted by permission of HarperCollins Publishers. 14-3 and 14- 19b: U.S. Geological Survey. 14-4; 14-8; 14-9; 14-lla; 14-13; 14-15; 14-16; 1417a,b; 14-18; 14-19a,c; 14-21; Perspective 14-1, Figure 3; 14-26a; 14-29; 14-31a; 14-33; 14-34; 14-37; and Perspective 14-3, Figure 2: Precision Graphics. 14-5, 14-17c, and 14-27: John S. Shelton. 14-6: Precision Graphics. Reprinted with permission of Merrill, an imprint of Macmillan Publishing Company, from Structural Geology: Principles, Concepts, Problems by Robert D. Hatcher, Jr.
14-36: Michael Thomas Associates. Modified from Graham, Dickinson, and Ingersoll, Himalayan-Bengal Model
14— 25a:
B. Bradley, Uni-
Colorado Geology Department. 14— 25b: W.H. 14—32: Michael Thomas Associates. Reproduced by permission of the Geological Society and A.M. Spencer from A.M. Spencer, ed., Mesozoic-Cenozoic Orogenic Belts (Bath: Geological Society Publishing House, 1974). Perspective 14—2, Figure 1: Michael Thomas Associates. From C.W. Stearn, R.L. Carroll, and T.H. Clark, Geological Evolution of North America, 3d ed. (New York: John Wiley Sons, Inc., 1979): 376, Figure 16-13. Perspective 14-2, Figure 2: Rolin Graphics. From WR. Dickinson and WS. Snyder, "Plate Tectonics of the Laramide Orogeny." In V. Matthews, ed., Laramide Folding Associated with Basement Block Fault-
CHAPTER Opener:
15
Hong Kong Government,
15-1
Geotechnical Control
15-13 right; 15-16 top; 15-17 top; 15-26a, and 15-31 top: Rolin Graphics. 15-1 right:
Office.
left;
15—2: U.S. Geo15-3; 15-4a,b; 15-5a-c; 15-6; 15-8; 15-9; 15-12; 15-14; 15-17a; 15-18a; 15-19; 1520a; 15-21 top, a; 15-24a; 15-25; 15-27a; 15-28; 1529a; and 15— 30a: Precision Graphics. 15-7: Boris Yaro, Los Angeles Times. Perspective 15 — 1, Figure 1 top: Rolin Graphics. Perspective 15—1, Figure 1 bottom: Precision Graphics. Figure 15-1 from A.C. Waltham, Catastrophe: The Violent Earth (New York: Macmillan, 1978): 71. Perspective 15-1, Figure 2: T. Spencer/Colorific! 15-11: W.R. Hansen, U.S. Geological Survey. 15-13 left: John S. George
Plafker, U.S. Geological Survey.
logical Survey.
Shelton.
15-15 bottom: PreFrom A.C. Waltham, Catastrophe: The Earth (New York: Macmillan, 1978): 51. 15-16b: 15-15
top: Rolin Graphics.
cision Graphics.
versity of
Violent
Monroe,
Steven R. Lower, GeoPhoto Publishing
U.S. Geological Survey.
&
GSA Memoir
151 (1978): 359, Figure 2. Reprinted with permission from W.R. Dickinson. Perspective 14-2, Fig. 3: Rolin Graphics. From S.H. Knight in D.L. Blackstone, Jr., "Traveler's Guide to the Geology of Wyoming." Geological Survey of Wyoming Bulletin 67 (1988): 43-44. 14-35: Rolin Graphics. From Peter Molnar, "The Geologic History and Structure of the Himalaya." American Scientist 74: 144—154, Fig. 4, pp. 148 — 149. Reprinted by permission of American Scientist, journal of Sigma Xi, The Scientific Research Society. ing in the Western United States.
Company. 15 -17b, 15-22b, and 15-24b: B. Bradley and the University of Colorado's Geology Department to National Geophysical Data Center, NOAA, Boulder, Colorado. 15-21b: Department of the Army, U.S. Army Engineer District, Alaska Corps of Engineers. 15— 22a: Rolin Graphics. From O.J. Ferrians, Jr., R. Kachadoorian, and G.W Greene, U.S. Geological Survey Professional Paper 678. 1969. 15—23: OJ. Ferrians, Jr., U.S. Geological Survey. Perspective 15—2, Figure
1:
Rolin
ASCE. From G.A.
Graphics.
Reprinted with
permission
Kiersch, "Vaiont Reservoir Disaster."
Civil Engineering 34 (1964). Perspective 15-2, Figure 2: UPI/Bettmann. Perspective 15-2, Figure 3: Precision Graphics. Reprinted with permission ASCE. From G.A. Kiersch, "Vaiont Reservoir Disaster." Civil Engineering 34 (1964). 15— 26b,c: Reprinted with permission of Merrill Publishing Company, an imprint of Macmillan Publishing Company, from Keller, Environmental Geology 6e, copyright ©1992. 15-29b: John D. Cunningham/Visuals Un-
15-30b: Dell R. Foutz/Visuals Unlimited. 15From R.H. Jahns, Bulletin 170, Geology of Southern California, California Division of
limited.
31a:
Rolin Graphics.
Credits
637
Mines. 15 -3 lb: Los Angeles County Department of Public
Works.
CHAPTER
Figure 1: Alice Thiede. 18-12: National Park Service photograph by Ruth and Louis Kirk. 18-22, 18— 31b, and
16
16-1 and 16-2: JPL. 16-3; 16-7; 16-8; 16-9; 16-10; 16-11; 16-15; 16-16; 16-21; Perspective 16-1, Figures 1 and 2; 16-22b; 16-23; 16-24a; 16-26a; 16-29; 1630; 16-32; 16-33; 16-36; 16-37; 16-38; and 16-40: 16-4: Martin G. Miller/Visuals UnRolin Graphics, 16-17, 1618, 16-22a, and 16-41: John S. Shelton. 16-25c: Rolin Graphics. From W.L. Fisher et al., Delta Systems in the Exploration for Oil and Gas— A Research Colloquium (1969). 16-26b: Alan L. Mayo, GeoPhoto Publishing Company. 16—28: Alice Thiede. 16-31: Alan Smith/Tony Stone Worldwide. 16-35: Petley Studios. 16-39: J.R. Stacy, U.S. Geological Survey. Perspective 16-2, Figure 2: Precision Graphics.
limited.
16-6 and 16-25a,b:
Precision Graphics.
From Natural
Bridges. National Park
Service.
CHAPTER
17
Opener: Sarah Stone/Tony Stone Worldwide. 17-la,c: Rolin Graphics. From Trapped by Robert K. Murray and Roger W. Brucker. Copyright ©1979 by Murray and Brucker, copyright renewed. Used by permission. 17- lb:
Brown
Brothers. 17-2, 17-3, 17-4, 17-6, 17-8, 17-9, 17-10, 17-12, 17-15, 17-18, 17-21, 17-25, and 1730: Precision Graphics. 17-5: G.E. Seaburn, U.S. Geological Survey. 17-7: Linda D. Mayo, GeoPhoto Publishing Company. Perspective 17-1, Figure 1: Ed Cooper. Perspective 17-1, Figure 2: W.L. McCoy. 17-11: J.R. Stacy, U.S. Geological Survey. 17—13: Alice Thiede. 17— 14a: Frank Kujawa, University of Central Florida, GeoPhoto Publishing Company. 17— 14b and 17-22 right: U.S. Geological Survey. 17-16b: John S. Shelton. 17-17: R.F. Ashley/Visuals Unlimited. 17-19: Daniel Gotshall/Visuals Unlimited. 17-20: Rolin Graphics. From J.B. Week:, et al.,
W
1400-A. 1988. Rolin Graphics. 17-24: City of Long Beach
U.S. Geological Survey Professional Paper
17-22
left:
Department of Oil Properties. Perspective 17-2, Figure 1: Precision Graphics. Modified from U.S. News World Report (18 March 1991): 72-73. 17-27: Rolin Graphics.
&
From G.A. Waring,
U.S. Geological Survey Professional Paper 492. 1965. 17-28: British Tourist Authority.
CHAPTER
Credits
John
Shelton.
S.
18-23:
Photographers.
Inc.
18 -la:
Bob and Ira 18-24: Swiss
National Tourist Office. 18-25: Alan Kesselheim/Mary Pat Ziter, ©JLM Visuals. 18-27: National Park Service photograph by Bruce Paige. 18-34: Canadian Geological Survey. Perspective
18-1, Figures 2 and
Geological Survey. 18—37: Michael
3:
P.
Thomas
Weis, U.S. Associates.
From H.E. Wright and D.G.
Frey, eds., Quaternary GeolCopyright ©1965 by Princeton University Press. Reprinted by permission of Princeton Uni-
ogy of the United
States.
18-2, Figure 1: Rolin Graphics. and Farrand, "The Glacial Lakes Around Michigan." Geological Survey Bulletin 4 (1967). Michigan Department of Natural Resources. Perspective 18-2, Figure 2: Rolin Graphics. From V.K. Prest, Geology and Economic Minerals of Canada: Department of Energy, Mines, and Resources Economic Geology Report 1, 5/e (1970): 90-91, Figure 7—6. Reproduced with the permission of the Minister of Supply and Services Canada, 1991. 18-38: Michael Thomas Associates. From J.T. Andrews, "Earth Science Symposium on Hudson Bay, Ottawa, 1968." Caversity Press. Perspective
From
Kelley
nadian Geological Survey Paper 68-53 (1969): 53. Reproduced with the permission of the Minister of Supply and Services Canada, 1991.
CHAPTER
19
Opener; 19— 13b; 19-26a,b; 19-31a; and Perspective 19-2, Figure 2: John S. Shelton. 19-1 and 19-19: Alice Thiede. 19-2: Steve McCurry/Magnum. 19-3; 19-6a 19-9a,b; 19-10; 19-11; 19-13a; 19-14a; 19-15a 19-17a; 19-23; and 19-29a: Precision Graphics. 19-4 Walt Anderson/Visuals Unlimited. 19-5, 19-8, and 1927: Martin G. Miller/Visuals Unlimited. Perspective 19—1, Figure 1: Mary A. Dale-Bannister, Washington University, St. Louis. Perspective 19-1, Figures 2 and 3: NASA. 1914b, 19-15b, and 19-28: Alan L. and Linda D. Mayo,
GeoPhoto Publishing Company. 19-16: Willard Clay/ Tony Stone Worldwide. 19-18: Steve McCutcheon/Visuals Unlimited. 19-20: Alex Teshin Associates. Based on F.K. Lutgens and E.J. Tarbuck, The Atmosphere: An Introduction to Meteorology (Englewood 1979):
150,
Figure
Graphics. Perspective 19—2, Figure
Kunstsammlung, Kupferstichkabinett Basel. 18- lb: Reproduced courtesy of the Board of Directors of the Budapest Museum of Fine Arts. 18-2: Tom Bean. 18-3, 18-10, 18-11, 18-14, 18-17, 18-18, 18-32, 18-35, and 18-39: Precision Graphics. 18-4a: National Park Service photograph by Bruce Paige. 18—5; 18—9; 1828; 18-36; and Perspective 18-2, Figure 1: Rolin Graphics. 18-6 and 18-29: Engineering Mechanics, Virginia
638
18-33a:
Spring/Kirkendall-Spring
Prentice-Hall,
18
Opener: David Hiser, Photographers/Aspen, Offentliche
Polytechnic Institute and State University. 18—7: Frank Awbrey/Visuals Unlimited. 18 — 8 and Perspective 18—1,
From
C.B.
1:
New
Jersey:
19-21:
Rolin
Cliffs,
7.3.
Precision Graphics.
Hunt and D.R. Mabey, U.S. Geological Survey 494 A (1966): A5, Figure 2. Perspective
Professional Paper
19—2, Figure 3: John D. Cunningham/Visuals Unlimited. 19-2, Figure 4: United States Borax 8e Chem-
Perspective ical
Corporation.
CHAPTER
20
Opener: Jim Pickerell/Tony Stone Worldwide. 20—1 and 20—2: Rosenberg Library, Galveston, Texas. 20—3: Susan
Trossbach, University of Virginia. 20-4; 20-5; Perspective 20-2, Figure 2; and 20-32: Rolin Graphics. 20-7,
20-11, 20-13, 20-14, 20-16, 20-17a, 20-18a, 2023, 20—25, 20-26, and 20-27a: Precision Graphics. Perspective 20-1, Figure 1: Steve Starr/SABA. Perspective 20-1, Figure 2: D.J. Miller, U.S. Geological Survey. 20-9, 20-10, 20-15, 20-19, 20-24, and 20-27c: John S. Shel-
ton.
20-12: Michael
Slear.
20-21 and
Perspective
20-2,
Rolin Graphics. From U.S. Geological Survey Circular 1075. 20-22a: P. Godfrey. 20-22b: NASA. Perspective 20-2, Figure 3: U.S. Army Corps of Engineers, 20-27b: Nick Harvey. 20-28. GEOPIC®, Earth Satellite Corporation. 20-31: Karl Kuhn. Figure
1:
Credits
639
""^ Geologic
Time Depicted
in a Spiral
History of the Earth (Figure 9-1)
"*» r
English-Metric Conversion Chart
_
.
textbook coop the asun. 6E0 PHYS oii99A081£ W1 i £5.50 Book Number Book Price
*
ISBN 0-314-00559-5
90000 PHVSICAL GE0L
9 "780314"005595
3773911 03951 073
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