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T W O
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
KELLMC02_0132251507.QXD
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Internal Structure of Earth
and Plate Tectonics
Written with the assistance of Tanya Atwater
Learning Objectives
The surface of Earth would be much different—
relatively smooth, with monotonous topography
—if not for the active tectonic processes within
Earth that produce earthquakes, volcanoes,
mountain chains, continents, and ocean
basins.1 In this chapter we focus directly on
the interior of Earth, with the following learning
objectives:
쐍 Understand the basic internal structure and
processes of Earth
쐍 Know the basic ideas behind and evidence
for the theory of plate tectonics
쐍 Understand the mechanisms of plate tectonics
쐍 Understand the relationship of plate tectonics
to environmental geology
The San Andreas fault in southern California is the major
boundary between the Pacific and North American plates. Here in
the Indio Hills, the fault is delineated by lines of native palm trees.
(Edward A. Keller)
37
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Internal Structure of Earth and Plate Tectonics
CASE HISTORY
Two Cities on a Plate Boundary
California straddles the boundary between two tectonic plates,
which are discussed in detail in this chapter. That boundary
between the North American and Pacific plates is the notorious San Andreas Fault (Figure 2.1). A fault is a fracture along
which one side has moved relative to the other, and the San
Andreas Fault is a huge fracture zone, hundreds of kilometers
long. Two major cities, Los Angeles to the south and San
Francisco to the north, are located on opposite sides of this
fault. San Francisco was nearly destroyed by a major earthquake in 1906, which led to the identification of the fault. Many
of the moderate to large earthquakes in the Los Angeles area
are on faults related to the San Andreas fault system. Most of
the beautiful mountain topography in coastal California
near both Los Angeles and San Francisco is a direct result of
processes related to movement on the San Andreas Fault.
However, this beautiful topography comes at a high cost to
society. Since 1906, earthquakes on the San Andreas fault system or on nearby faults, undoubtedly influenced by the plate
boundary, have cost hundreds of lives and many billions of dollars in property damage. Construction of buildings, bridges,
and other structures in California is more expensive than elsewhere because they must be designed to withstand ground
shaking caused by earthquakes. Older structures have to be
retrofitted, or have changes made to their structure, to withstand
the shaking, and many people purchase earthquake insurance
in an attempt to protect themselves from the “big one.”
Los Angeles is on the Pacific plate and is slowly moving
toward San Francisco, which is on the North American plate.
Figure 2.1
San Andreas Fault
Map showing the San Andreas Fault
and topography in California. Arrows
show relative motion on either side of
the fault. (R. E. Wallace/National Earth-
quake Information Center. U.S.G.S.)
S
REA
ND
N A LT
SA FAU
SAN ANDREAS
FAULT
SAN ANDREAS
FAULT
SAN ANDREAS
FAULT
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Internal Structure of Earth
In about 20 million years the cities will be side by side. If people are present, they might be arguing over which is a suburb
of the other. Of course, there will still be a plate boundary
between the Pacific and North American plates 20 million
years from now, because large plates have long geologic lives,
on the order of 100 million years. However, the boundary
39
may not be the San Andreas Fault. The plate boundary will
probably have moved eastward, and the topography of what
is now California may be somewhat different. In fact, some
recent earthquake activity in California, such as the large 1992
Landers earthquake, east of the San Andreas fault, may be the
beginning of a shift in the plate boundary.
2.1 Internal Structure of Earth
You may be familiar with the situation comedy Third Rock from the Sun, a phrase
that refers to our planet Earth. Far from being a barren rock, Earth is a complex
dynamic planet that in some ways resembles a chocolate-covered cherry. That is,
Earth has a rigid outer shell, a solid center, and a thick layer of liquid that moves
around as a result of dynamic internal processes. The internal processes are incredibly important in affecting the surface of Earth. They are responsible for the
largest landforms on the surface: continents and ocean basins. The configuration
of the continents and ocean basins in part controls the oceans’ currents and the
distribution of heat carried by seawater in a global system that affects climate,
weather, and the distribution of plant and animal life on Earth. Finally, Earth’s
internal processes are also responsible for regional landforms including mountain
chains, chains of active volcanoes, and large areas of elevated topography, such as
the Tibetan Plateau and the Rocky Mountains. The high topography that includes
mountains and plateaus significantly affects both global circulation patterns of air
in the lower atmosphere and climate, thereby directly influencing all life on Earth.
Thus, our understanding of the internal processes of Earth is of much more than
simply academic interest. These processes are at the heart of producing the multitude of environments shared by all living things on Earth.
The Earth Is Layered and Dynamic. Earth (Figure 2.2a) has a radius of about
6,300 km (4,000 mi) (Figure 2.2b). Information regarding the internal layers of the
Earth is shown in Figure 2.2b. We can consider the internal structure of Earth in
two fundamental ways:
쐍 by composition and density (heavy or light).
쐍 by physical properties (for example, solid or liquid, weak or strong).
Our discussion will explore the two ways of looking at the interior of our planet.
Some of the components of the basic structure of Earth1 are
쐍 A solid inner core with a thickness of more than 1,300 km (808 mi) that is
roughly the size of the moon but with a temperature about as high as the
temperature of the surface of the Sun.2 The inner core is believed to be primarily metallic, composed mostly of iron (about 90 percent by weight), with
minor amounts of elements such as sulfur, oxygen, and nickel.
쐍 A liquid outer core with a thickness of just over 2,000 km (1,243 mi) with a
composition similar to that of the inner core. The outer core is very fluid,
more similar to water than to honey. The average density of the inner and
outer core is approximately 10.7 grams per cubic centimeter (0.39 pounds
per cubic inch). The maximum near the center of Earth is about 13 g/cm3
(0.47 lb/in3). By comparison, the density of water is 1 g/cm3 (0.04 lb/in3)
and the average density of Earth is approximately 5.5 g/cm3 (0.2 lb/in3).
쐍 The mantle, nearly 3,000 km (1,864 mi) thick, surrounds the outer core
and is mostly solid, with an average density of approximately 4.5 g/cm3
(0.16 lb/in3). Rocks in the mantle are primarily iron- and magnesium-rich
silicates. Interestingly, the density difference between the outer core and the
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Internal Structure of Earth and Plate Tectonics
Sea level
Marine sediment
Mohorovicic discontinuity
Oceanic crust
Continental
crust
AVERAGE DENSITY, g/cm3
0
60
Rigid
km
40
e
Mantl
20
80
phere
Lithos
Continental crust
2.8
Oceanic crust
2.9
Mantle
4.5
Core
10.7
Entire Earth
Crust
5.5
Asthenosphere
0 km
1000
Mantle
2000
(a)
3000
Outer core
4000
Figure 2.2 Earth and its interior (a) Earth from space. (National Geophysical Data
Center, National Oceanic and Atmospheric Administration) (b) Idealized diagram showing
the internal structure of Earth and its layers extending from the center to the surface.
Notice that the lithosphere includes the crust and part of the mantle, and the
asthenosphere is located entirely within the mantle. Properties of the various layers
have been estimated on the basis of (1) interpretation of geophysical data (primarily
seismic waves from earthquakes); (2) examination of rocks thought to have risen from
below by tectonic processes; and (3) meteorites, thought to be pieces of an old Earthlike planet. (From Levin, H. L. 1986. Contemporary physical geology, 2nd ed. Philadelphia:
5000
Inner
core
(b)
6000
Saunders)
overlying mantle is greater than that between the rocks at the surface of
Earth and the overlying atmosphere! In the case of the outer core and mantle, the more fluid phase of the outer core is beneath the solid phase of the
mantle. This is just the opposite of the case of the rock-atmosphere relationship, where the fluid atmosphere overlies the solid lithosphere. Because it is
liquid, the outer core is dynamic and greatly influences the overlying mantle
and, thus, the surface of Earth.
쐍 The crust, with variable thickness, is the outer rock layer of the Earth. The
boundary between the mantle and crust is known as the Mohorovičić
discontinuity (also called the Moho). It separates the lighter rocks of the
crust with an average density of approximately 2.8 g/cm3 (0.10 lb/in3) from
the denser rocks of the mantle below.
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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41
How We Know about the Internal Structure of Earth
Pan filled
with water
Cool wa
water
ol
Hot water
Co
Convection cell
ter
Continents and Ocean Basins Have Significantly Different Properties and
History. Within the uppermost portion of the mantle, near the surface of Earth
our terminology becomes more complicated. For example, the cool, strong outermost layer of Earth is also called the lithosphere (lithos means “rock”). It is much
stronger and more rigid than the material underlying it, the asthenosphere
(asthenos means “without strength”), which is a hot and slowly flowing layer of
relatively weak rock. The lithosphere averages about 100 km (62 mi) in thickness,
ranging from a few kilometers (1 to 2 mi) thick beneath the crests of mid-ocean
ridges to about 120 km (75 mi) beneath ocean basins and 20 to 400 km (13 to
250 mi) beneath the continents. The crust is embedded in the top of the lithosphere. Crustal rocks are less dense than the mantle rocks below, and oceanic crust
is slightly denser than continental crust. Oceanic crust is also thinner: The ocean
floor has a uniform crustal thickness of about 6 to 7 km (3.7 to 4.4 mi), whereas
the crustal thickness of continents averages about 35 km (22 mi) and may be up to
70 km (44 mi) thick beneath mountainous regions. Thus, the average crustal thickness is less than 1 percent of the total radius of Earth and can be compared to the
thin skin of a tangerine. Yet it is this layer that is of particular interest to us because
we live at the surface of the continental crust.
In addition to differences in density and thickness, continental and oceanic
crust have very different geologic histories. Oceanic crust of the present ocean
basins is less than approximately 200 million years old, whereas continental crust
may be several billion years old. Three thousand kilometers (1,865 mi) below us,
at the core-mantle boundary, processes may be occurring that significantly affect
our planet at the surface. It has been speculated that gigantic cycles of convection
occur within Earth’s mantle, rising from as deep as the core-mantle boundary up
to the surface and then falling back again. The concept of convection is illustrated
by heating a pan of hot water on a stove (Figure 2.3). Heating the water at the
bottom of the pan causes the water to become less dense and more unstable, so it
rises to the top. The rising water displaces denser, cooler water, which moves
laterally and sinks to the bottom of the pan. It is suggested that Earth layers
contain convection cells and operate in a similar fashion.
A complete cycle in the mantle may take as long as 500 million years.1 Mantle
convection is fueled at the core-mantle boundary both by heat supplied from
the molten outer core of Earth and by radioactive decay of elements (such as
uranium) scattered throughout the mantle. Let us now examine some of the
observations and evidence that reveal the internal structure of Earth.
Gas stove
Figure 2.3
Convection
Idealized diagram showing the concept of convection. As the pan of water
is heated, the less dense hot water
rises from the bottom to displace the
denser cooler water at the top, which
then sinks down to the bottom. This
process of mass transport is called
convection, and each circle of rising
and falling water is a convection cell.
2.2 How We Know about the Internal
Structure of Earth
What We Have Learned about Earth from Earthquakes. Our knowledge
concerning the structure of Earth’s interior arises primarily from our study of
seismology. Seismology is the study of earthquakes and the passage of seismic
waves through Earth.3 When a large earthquake occurs, seismic energy is released
and seismic waves move both through Earth and along its surface. The properties
of these waves are discussed in detail in Chapter 6 with earthquake hazards.
Some waves move through solid and liquid materials while others move
through solid, but not liquid materials. The rates at which seismic waves propagate are on the order of a few kilometers per second (1 or 2 miles per second).
Their actual velocity varies with the properties of the materials through which the
waves are propagating (moving). When the seismic waves encounter a boundary,
such as the mantle-core boundary, some of them are reflected back. Others cross the
boundary and are refracted (change the direction of propagation). Still others fail to
propagate through the liquid outer core. Thousands of seismographs (instruments
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Internal Structure of Earth and Plate Tectonics
that record seismic waves) are stationed around the world. When an earthquake
occurs, the reflected and refracted waves are recorded when they emerge at the
surface. Study of these waves has been a powerful tool for deducing the layering
of the interior of Earth and the properties of the materials found there.
In summary, the boundaries that delineate the internal structure of Earth are
determined by studying seismic waves generated by earthquakes and recorded
on seismographs around Earth. As seismology has become more sophisticated, we
have learned more and more about the internal structure of Earth and are finding
that the structure can be quite variable and complex. For example, we have been
able to recognize
쐍 where magma, which is molten rock material beneath Earth’s surface, is generated in the asthenosphere
쐍 the existence of slabs of lithosphere that have apparently sunk deep into the
mantle
쐍 the extreme variability of lithospheric thickness, reflecting its age and history
2.3 Plate Tectonics
The term tectonics refers to the large-scale geologic processes that deform Earth’s
lithosphere, producing landforms such as ocean basins, continents, and mountains. Tectonic processes are driven by forces within the Earth. These processes are
part of the tectonic system, an important subsystem of the Earth system.
Movement of the Lithospheric Plates
What Is Plate Tectonics? The lithosphere is broken into large pieces called
lithospheric plates that move relative to one another (Figure 2.4a).4 Processes associated with the creation, movement, and destruction of these plates are collectively
known as plate tectonics.
Locations of Earthquakes and Volcanoes Define Plate Boundaries. A lithospheric plate may include both a continent and part of an ocean basin or an ocean
region alone. Some plates are very large and some are relatively small, though
they are significant on a regional scale. For example, the Juan de Fuca plate off
the Pacific Northwest coast of the United States, which is relatively small, is
responsible for many of the earthquakes in northern California. The boundaries
between lithospheric plates are geologically active areas. Most earthquakes
and many volcanoes are associated with these boundaries. In fact, plate boundaries are defined by the areas in which concentrated seismic activity occurs
(Figure 2.4b). Over geologic time, plates are formed and destroyed, cycling materials from the interior of Earth to the surface and back again at these boundaries
(Figure 2.5). The continuous recycling of tectonic processes is collectively called
the tectonic cycle.
Seafloor Spreading Is the Mechanism for Plate Tectonics. As the lithospheric
plates move over the asthenosphere, they carry the continents embedded within
them.5 The idea that continents move is not new; it was first suggested by German
scientist Alfred Wegener in 1915. The evidence he presented for continental drift
was based on the congruity of the shape of continents, particularly those across
the Atlantic Ocean, and on the similarity in fossils found in South America and
Africa. Wegener’s hypothesis was not taken seriously because there was no
known mechanism that could explain the movement of continents around Earth.
The explanation came in the late 1960s, when seafloor spreading was discovered.
In seafloor regions called mid-oceanic ridges, or spreading centers, new crust is
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Plate Tectonics
80°
NORTH AMERICAN PLATE
JUAN DE FUCA
PLATE
40°
63
PHILIPPINE
PLATE
CARIBBEAN
PLATE
92
ATLANTIC
OCEAN
COCOS
PLATE
PACIFIC PLATE
40°
23
San Andreas
Fault
20°
0°
35
EURASIAN PLATE
19
AFRICAN PLATE
32
CAROLINE
PLATE
0°
FIJI
PLATE
Equator
PACIFIC
OCEAN
INDIAN
OCEAN
141
NAZCA
PLATE
40°
SOUTH AMERICAN
PLATE
PACIFIC
PACIFIC
PLATE
OCEAN
49
72
33
62
INDO-AUSTRALIAN PLATE
91
0°
Transform fault
Convergent
25
(a)
160°
0
Uncertain
plate boundary
Divergent
(spreading ridge
offset by transform
faults)
80°
40°
120°
80°
ANTARCTIC PLATE
0
120°
1,500
1,500
60°
3,000 Miles
3,000 Kilometers
Direction of
plate motion
(relative motion
rates in mm/yr)
40°
0°
40°
80°
120°
160°
80°
40°
40°
ATLANTIC
OCEAN
PACIFIC
20°
OCEAN
20°
PACIFIC
OCEAN
0°
Equator
0°
INDIAN
OCEAN
40°
Volcanoes
60°
0
0
(b)
1,500
1,500
3,000 Miles
60°
Earthquakes
3,000 Kilometers
Figure 2.4
Earth’s plates (a) Map showing the major tectonic plates, plate boundaries, and direction of plate movement. (Modified from Christopherson, R. W. 1994. Geosystems, 2nd ed. Englewood Cliffs, NJ:
Macmillan) (b) Volcanoes and earthquakes: Map showing location of volcanoes and earthquakes. Notice the
correspondence between this map and the plate boundaries. (Modified after Hamblin, W. K. 1992. Earth’s
dynamic systems, 6th ed. New York: Macmillan)
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
KELLMC02_0132251507.QXD
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Internal Structure of Earth and Plate Tectonics
Figure 2.5
Model of plate
tectonics Diagram of the model of
plate tectonics. New oceanic lithosphere is being produced at the spreading ridge (divergent plate boundary).
Elsewhere, oceanic lithosphere returns
to the interior of Earth at a convergent
plate boundary (subduction zone).
(Modified from Lutgens, F., and Tarbuck, E.
1992. Essentials of geology. New York:
Macmillan)
Divergent boundary
Convergent boundary
Transform fault
Transform fault
Transform fault
Lithosphere
Hot rock
rising
Oceanic
spreading
ridge
Cool rock
sinking
Subduction
zone
Asthenosphere
continuously added to the edges of lithospheric plates (Figure 2.5, left). As oceanic
lithosphere is added along some plate edges (spreading centers), it is destroyed
along other plate edges, for example, at subduction zones (areas where one plate
sinks beneath another and is destroyed) (Figure 2.5, right). Thus continents do not
move through oceanic crust; rather they are carried along with it by the movement of
the plates. Also, because the rate of production of new lithosphere at spreading
centers is balanced by consumption at subduction zones, the size of Earth remains
constant, neither growing nor shrinking.
Sinking Plates Generate Earthquakes. The concept of a lithospheric plate sinking into the upper mantle is shown in diagrammatic form in Figure 2.5. When the
wet, cold oceanic crust comes into contact with the hot asthenosphere, magma
is generated. The magma rises back to the surface, producing volcanoes, such as
those that ring the Pacific Ocean basin, over subduction zones. The path of the
descending plate (or slab, as it sometimes is called) into the upper mantle is clearly
marked by earthquakes. As the oceanic plate subducts, earthquakes are produced
both between it and the overriding plate and within the interior of the subducting
plate. The earthquakes occur because the sinking lithospheric plate is relatively
cooler and stronger than the surrounding asthenosphere; this difference causes
rocks to break and seismic energy to be released.6
The paths of descending plates at subduction zones may vary from a shallow
dip to nearly vertical, as traced by the earthquakes in the slabs. These dipping
planes of earthquakes are called Wadati-Benioff zones (Figure 2.6). The very
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
KELLMC02_0132251507.QXD
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Page 45
Plate Tectonics
45
Volcanism
re
he
p
s
e
ho er
Lit sph
o
en
ts h
SL
Trench
Depth (km)
100
250
Upper mantle
Figure 2.6
Subduction zone
Idealized diagram of a subduction
zone showing the Wadati-Benioff
zone, which is an array of earthquake
foci from shallow to deep that delineate the subduction zone and the
descending lithospheric plate.
A
Magma
Wadati-Benioff zone
400
Earthquake focal depth
600
Shallow (<75 km)
Intermediate (75–325 km)
Deep (>325 km)
existence of Wadati-Benioff zones is strong evidence that subduction of rigid
“breakable” lithosphere is occurring.6
Plate Tectonics Is a Unifying Theory. The theory of plate tectonics is to geology
what Darwin’s origin of species is to biology: a unifying concept that explains an
enormous variety of phenomena. Biologists now have an understanding of evolutionary change. In geology, we are still seeking the exact mechanism that drives
plate tectonics, but we think it is most likely convection within Earth’s mantle. As
rocks are heated deep in Earth, they become less dense and rise. Hot materials,
including magma, leak out, and are added to the surfaces of plates at spreading
centers. As the rocks move laterally, they cool, eventually becoming dense enough
to sink back into the mantle at subduction zones. This circulation is known as
convection, which was introduced in Section 2.1. Figure 2.7 illustrates the cycles
of convection that may drive plate tectonics.
Oceanic
ridge
So
ut
h
A
m
er
ic
a
Ocean
floor
fr
A
Asthe
nosp
her
e
a
ic
Lithosphere
Trench
Mantle
Core
Figure 2.7
Plate movement Model of plate movement and mantle.The outer layer (or lithosphere) is
approximately 100 km (approximately 62 mi) thick and is stronger and more rigid than the deeper asthenosphere, which is a hot and slowly flowing layer of relatively low-strength rock.The oceanic ridge is a spreading
center where plates pull apart, drawing hot, buoyant material into the gap. After these plates cool and become
dense, they descend at oceanic trenches (subduction zones), completing the convection system.This process
of spreading produces ocean basins, and mountain ranges often form where plates converge at subduction
zones. A schematic diagram of Earth’s layers is shown in Figure 2.2b. (Grand, S. P. 1994. Mantle shear structure
beneath the Americas and surrounding oceans. Journal of Geophysical Research 99:11591–621. Modified after
Hamblin, W. K. 1992. Earth’s dynamic systems, 6th ed. New York: Macmillan)
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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TABLE 2.1
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Internal Structure of Earth and Plate Tectonics
Types of Plate Boundaries: Dynamics, Results, and Examples
Plate Boundary
Plates Involved
Dynamics
Results
Example
Divergent
Usually oceanic
Spreading. The two plates move
away from one another and molten
rock rises up to fill the gap.
Mid-ocean ridge forms and new
material is added to each plate.
African and North American
plate boundary (Figure 2.4a)
Mid-Atlantic Ridge
Convergent
Convergent
Ocean-continent
Ocean-ocean
Oceanic plate sinks beneath
continental plate.
Mountain ranges and a
subduction zone are formed with
a deep trench. Earthquakes and
volcanic activity are found here.
Nazca and South American
plate boundary (Figure 2.4a)
Older, denser, oceanic plate sinks
beneath the younger, less dense
oceanic plate.
A subduction zone is formed with
a deep trench. Earthquakes and
volcanic activity are found here.
Fiji plate (Figure 2.4a)
Andes Mountains
Peru-Chile Trench
Fiji Islands
Convergent
Continent-continent
Neither plate is dense enough to
sink into the asthenosphere;
compression results.
A large, high mountain chain
is formed, and earthquakes
are common.
Indo-Australian and Eurasian
plate boundary (on land)
(Figure 2.4a)
Transform
Ocean-ocean or
continent-continent
The plates slide past one another.
Earthquakes common. May result
in some topography.
North American and Pacific
plate boundary (Figure 2.10)
Himalaya Mountains
San Andreas fault
Types of Plate Boundaries
There are three basic types of plate boundaries: divergent, convergent, and transform, shown in Figures 2.4 and 2.5 and Table 2.1. These boundaries are not narrow
cracks as shown on maps and diagrams but are zones that range from a few to
hundreds of kilometers across. Plate boundary zones are narrower in ocean crust
and broader in continental crust.
Divergent boundaries occur where new lithosphere is being produced and
neighboring parts of plates are moving away from each other. Typically this
process occurs at mid-ocean ridges, and the process is called seafloor spreading
(Figure 2.5). Mid-ocean ridges form when hot material from the mantle rises up
to form a broad ridge typically with a central rift valley. It is called a rift valley,
or rift, because the plates moving apart are pulling the crust apart and splitting, or
rifting, it. Molten volcanic rock that is erupted along this rift valley cools and
forms new plate material. The system of mid-oceanic ridges along divergent plate
boundaries forms linear submarine mountain chains that are found in virtually
every ocean basin on Earth.
Convergent boundaries occur where plates collide. If one of the converging
plates is oceanic and the other is continental, an oceanic-continental plate collision
results. The higher-density oceanic plate descends, or subducts, into the mantle
beneath the leading edge of the continental plate, producing a subduction zone
(Figure 2.5). The convergence or collision of a continent with an ocean plate can
result in compression. Compression is a type of stress, or force per unit area. When
an oceanic-continental plate collision occurs, compression is exerted on the lithosphere, resulting in shortening of the surface of Earth, like pushing a table cloth
to produce folds. Shortening can cause folding, as in the table cloth example,
and faulting, or displacement of rocks along fractures to thicken the lithosphere
(Figure 2.8a). This process of deformation produces major mountain chains and
volcanoes such as the Andes in South America and the Cascade Mountains in
the Pacific Northwest of the United States (see A Closer Look: The Wonder of
Mountains). If two oceanic lithospheric plates collide (oceanic-to-oceanic plate
collision), one plate subducts beneath the other, and a subduction zone and arcshaped chain of volcanoes known as an island arc are formed (Figure 2.8b) as, for
example, the Aleutian Islands of the North Pacific. A submarine trench, relatively
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
KELLMC02_0132251507.QXD
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47
Plate Tectonics
Continental
plate
Volcanoes
Mountains
Mountains
Trench
Island
arc
Volcanic island
chain
Trench
Oceanic plate
Subduction
zone
Oceanic
plate
Oceanic plate
(a)
Continental
plate
Suture zone
Subduction
zone
Shortening, thickening
uplift
Magma
Continental
plate
Magma
(c)
(b)
Shortening, thickening
uplift
Figure 2.8 Convergent plate boundaries Idealized diagram illustrating characteristics of convergent plate boundaries: (a) continental-oceanic plate collision, (b) oceanic-oceanic plate collision, and
(c) continental-continental plate collision.
narrow, usually several thousand km long and several km deep depression on the
ocean, is often formed as the result of the convergence of two colliding plates
with subduction of one. A trench is often located seaward of a subduction zone
associated with an oceanic-continental plate or oceanic-oceanic plate collision.
Submarine trenches are sites of some of the deepest oceanic waters on Earth. For
example, the Marianas trench at the center edge of the Philippine plate is 11 km
(7 mi) deep. Other major trenches include the Aleutian trench south of Alaska and
the Peru-Chile trench west of South America. If the leading edges of both plates
contain relatively light, buoyant continental crust, subduction into the mantle of
one of the plates is difficult. In this case a continent-to-continent plate collision
occurs, in which the edges of the plates collide, causing shortening and lithospheric thickening due to folding and faulting. (Figure 2.8c). Where the two plates
join is known as a suture zone. Continent-to-continent collision has produced some
of the highest mountain systems on Earth, such as the Alpine and Himalayan
mountain belts (Figure 2.9). Many older mountain belts were formed in a similar
way; for example, the Appalachians formed during an ancient continent-tocontinent plate collision 250 to 350 million years ago.
Figure 2.9
Mountains in Italy
Mountain peaks (the Dolomites) in
southern Italy are part of the Alpine
mountain system formed from the
collision between Africa and Europe.
(Edward A. Keller)
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Internal Structure of Earth and Plate Tectonics
A CLOSER LOOK
The Wonder of Mountains
Mountains have long fascinated people with their awesome
presence. We are now discovering a fascinating story concerning their origin. The story removes some of the mystery as to
how mountains form, but it has not removed the wonder. The
new realization that mountains are systems (see Chapter 1)
resulting from the interaction between tectonic activity (that
leads to crustal thickening), the climate of the mountain, and
Earth surface processes (particularly erosion) has greatly
expanded our knowledge of how mountains develop.7,8
Specifically, we have learned the following:
쐍 Tectonic processes at convergent plate boundaries lead to
crustal thickening and initial development of mountains.
The mean (or average) elevation that a mountain range
attains is a function of the uplift rate, which varies from
less than 1 mm to about 10 mm per year (0.04 to 0.4 in. per
year). The greater the rate of uplift, the higher the point to
which the mean elevation of a mountain range is likely to
rise during its evolution.
쐍 As a mountain range develops and gains in elevation, it
begins to modify the local and regional climate by blocking
storm paths and producing a “rain shadow” in which the
mountain slopes on the rain-shadow side receive much less
rainfall than does the other side of the mountain. As a result,
rates of runoff and erosion on the side of the rain shadow are
less than for the other side. Nevertheless, the rate of erosion
increases as the elevation of the mountain range increases,
and eventually the rate of erosion matches the rate of uplift.
When the two match, the mountain reaches its maximum
mean elevation, which is a dynamic balance between the
uplift and erosion. At this point, no amount of additional
uplift will increase the mean elevation of the mountains
above the dynamic maximum. However, if the uplift rate increases, then a higher equilibrium mean elevation of the
range may be reached. Furthermore, when the uplift ceases
or there is a reduction in the rate of uplift, the mean elevation of the mountain range will decrease.7 Strangely, the
elevations of individual peaks may still increase!
쐍 Despite erosion, the elevation of a mountain peak in a range
may actually increase. This statement seems counterintuitive until we examine in detail some of the physical
processes resulting from erosion. The uplift that results from
the erosion is known as isostatic uplift. Isostasy is the
principle whereby thicker, more buoyant crust stands topographically higher than crust that is thinner and denser. The
principle governing how erosion can result in uplift is illustrated in Figure 2.A. The fictitious Admiral Frost has been
marooned on an iceberg and is uncomfortable being far
above the surface of the water. He attempts to remove the ice
that is above the water line. Were it not for isostatic (buoyant)
uplift, he would have reached his goal to be close to the water
line. Unfortunately for Admiral Frost, this is not the way the
world works; continuous isostatic uplift of the block as ice is
removed always keeps one-tenth of the iceberg above the
water. So, after removing the ice above the water line, he still
stands almost as much above the water line as before.7
쐍 Mountains, of course, are not icebergs, but the rocks of
which they are composed are less dense than the rocks of
the mantle beneath. Thus, they tend to “float” on top of the
denser mantle. Also, in mountains, erosion is not uniform
but is generally confined to valley walls and bottoms.
Thus, as erosion continues and the mass of the mountain
range is reduced, isostatic compensation occurs and the
entire mountain range rises in response. As a result of the
erosion, the maximum elevation of mountain peaks actually may increase, while the mean elevation of the entire
mountain block decreases. As a general rule, as the equivalent of 1 km (0.6 mi) of erosion across the entire mountain
block occurs, the mean elevation of mountains will rise
approximately five-sixths of a kilometer (one-half mile).
In summary, research concerning the origin of mountains
suggests that they result in part from tectonic processes that
cause the uplift, but they also are intimately related to climatic
and erosional processes that contribute to the mountain building process. Erosion occurs during and after tectonic uplift,
and isostatic compensation to that erosion occurs for millions
of years. This is one reason it is difficult to remove mountain
systems from the landscape. For example, mountain systems
such as the Appalachian Mountains in the southeastern
United States were originally produced by tectonic uplift
several hundred million years ago when Europe collided
with North America. There has been sufficient erosion of the
original Appalachian Mountains to have removed them as
topographic features many times over were it not for continued isostatic uplift in response to the erosion.
Transform boundaries, or transform faults, occur where the edges of two plates
slide past one another, as shown in Figure 2.5. If you examine Figures 2.4a and 2.5,
you will see that a spreading zone is not a single, continuous rift but a series of rifts
that are offset from one another along connecting transform faults. Although the
most common locations for transform plate boundaries are within oceanic crust,
some occur within continents. A well-known continental transform boundary is
the San Andreas Fault in California, where the rim of the Pacific plate is sliding
horizontally past the rim of the North American plate (see Figures 2.4a and 2.10).
Locations where three plates border one another are known as triple junctions.
Figure 2.10 shows several such junctions: Two examples are the meeting point of the
Juan de Fuca, North American, and Pacific plates on the West Coast of North America (this is known as the Mendocino triple junction) and the junction of the spreading
ridges associated with
and Nazca plates west of South America.
P R Ethe
L I M Pacific,
I N A R Y P RCocos,
OOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Plate Tectonics
49
10,000
kg
90,000
kg
(a)
(b)
9000
kg
81,000
kg
90,000
kg??
(c)
(d)
Figure 2.A Isostasy Idealized diagram or cartoon showing the principle of isostatic uplift. Admiral Frost is left adrift on an iceberg and is uncomfortable being so far above the surface of the water (a). He decides to remove the 10,000 kg (22,046 lb) of ice that is above the water line on the
iceberg on which he is standing (b). Were it not for isostatic (buoyant) uplift, Admiral Frost would reach his goal (c). However, in a world with isostasy,
uplift results from removal of the ice, and there is always one-tenth of the iceberg above the water (d). What would have happened if Admiral Frost
had elected to remove 10,000 kg of ice from only one-half of the area of ice exposed above the sea? Answer: The maximum elevation of the iceberg
above the water would have actually increased. Similarly, as mountains erode, isostatic adjustments also occur, and the maximum elevation of mountain peaks may actually increase as a result of the erosion alone! (From Keller, E. A., and Pinter, N. 1996. Active tectonics. Upper Saddle River, NJ: Prentice Hall)
Rates of Plate Motion
Plate Motion Is a Fast Geologic Process. The directions in which plates move are
shown on Figure 2.4a. In general, plates move a few centimeters per year, about as
fast as some people’s fingernails or hair grows. The Pacific plate moves past the
North American plate along the San Andreas Fault about 3.5 cm per year (1.4 in.
per year), so that features such as rock units or streams are gradually displaced
over time where they cross the fault (Figure 2.11). During the past 5 million years,
there has been about 175 km (about 110 miles) of displacement, a distance equivalent to driving two hours at 55 mph on a highway along the San Andreas Fault.
Although the central portions of the plates move along at a steady slow rate, plates
interact at their boundaries, where collision or subduction or both occur, and
movement may not be smooth or steady. The plates often get stuck together. Movement is analogous to sliding one rough wood Pboard
Movement
R E L I M I Nover
A R Y P another.
ROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Internal Structure of Earth and Plate Tectonics
AFRICAN
PLATE
ATLANTIC
OCEAN
63
NORTH
AMERICAN
PLATE
53
PACIFIC
OCEAN
San Andreas
Fault
JUAN DE FUCA
PLATE
PACIFIC PLATE
Subduction zone
Spreading center
Transform fault
63
35
CARIBBEAN
PLATE
Triple
Junction
Plate motion
(rate in mm/yr)
Direction of relative displacement
on transform fault
(rate in mm/yr)
35
SOUTH
AMERICAN
PLATE
COCOS
PLATE
127
58
Triple
Junction
91
NAZCA
PLATE
Figure 2.10 North American plate boundary Detail of boundary between the North American
and Pacific plates. (Courtesy of Tanya Atwater)
Figure 2.11 The San Andreas
Fault The fault is visible from the
lower left to upper right diagonally
across the photograph, as if a gigantic
plow had been dragged across the
landscape. (James Balog/Getty
Images Inc.)
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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A Detailed Look at Seafloor Spreading
51
occurs when the splinters of the boards break off and the boards move quickly by
one another. When rough edges along the plate move quickly, an earthquake is
produced. Along the San Andreas Fault, which is a transform plate boundary, the
displacement is horizontal and can amount to several meters during a great earthquake. During an earthquake in 1857 on the San Andreas Fault a horse corral across
the fault was reportedly changed from a circle to an “S” shape. Fortunately, such an
event generally occurs at any given location only once every 100 years or so. Over
long time periods, rapid displacement from periodic earthquakes and more continuous slow “creeping” displacements add together to produce the rate of several
centimeters of movement per year along the San Andreas Fault.
2.4 A Detailed Look at Seafloor Spreading
When Alfred Wegener proposed the idea of continental drift in 1915, he had
no solid evidence of a mechanism that could move continents. The global extent
of mid-oceanic ridges was discovered in the 1950s, and in 1962 geologist Harry
H. Hess published a paper suggesting that continental drift was the result of the
process of seafloor spreading along those ridges. The fundamentals of seafloor
spreading are shown in Figure 2.5. New oceanic lithosphere is produced at the
spreading ridge (divergent plate boundary). The lithospheric plate then moves
laterally, carrying along the embedded continents in the tops of moving plates.
These ideas produced a new major paradigm that greatly changed our ideas about
how Earth works.3,6,9
The validity of seafloor spreading was established from three sources: (1) identification and mapping of oceanic ridges, (2) dating of volcanic rocks on the floor
of the ocean, and (3) understanding and mapping of the paleomagnetic history
of ocean basins.
Paleomagnetism
We introduce and discuss Earth’s magnetic field and paleomagnetic history in
some detail in order to understand how seafloor spreading and plate tectonics
were discovered. Earth has had a magnetic field for at least the past 3 billion
years2 (Figure 2.12a). The field can be represented by a dipole magnetic field with
lines of magnetic force extending from the South Pole to the North Pole. A dipole
magnetic field is one that has equal and opposite charges at either end. Convection occurs in the iron-rich, fluid, hot outer core of Earth because of compositional
Magnetic
axis
N
Magnetic
axis
S
Magnetic
equator
S
N
Axis of rotation
(a) Normal polarity
Axis of rotation
(b) Reversed polarity
Figure 2.12 Magnetic reversal Idealized diagram showing the magnetic field of Earth under
(a) normal polarity and (b) reversed polarity. (From Kennett, J. 1982. Marine geology. Englewood Cliffs,
NJ: Prentice Hall)
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Internal Structure of Earth and Plate Tectonics
changes and heat at the inner-outer core boundary. As more buoyant material in
the outer core rises, it starts the convection (Figures 2.3 and 2.7). The convection in
the outer core, along with the rotation of Earth that causes rotation of the outer
core, initiates a flow of electric current in the core. This flow of current within the
core produces and sustains Earth’s magnetic field.2,3
Earth’s magnetic field is sufficient to permanently magnetize some surface
rocks. For example, volcanic rock that erupts and cools at mid-oceanic ridges
becomes magnetized at the time it passes through a critical temperature. At that
critical temperature, known as the Curie point, iron-bearing minerals (such as
magnetite) in the volcanic rock orient themselves parallel to the magnetic field. This
is a permanent magnetization known as thermoremnant magnetization.3 The term
paleomagnetism refers to the study of the magnetism of rocks at the time their
magnetic signature formed. It is used to determine the magnetic history of Earth.
The magnetic field, based on the size and conductivity of Earth’s core, must be
continuously generated or it would decay away in about 20,000 years. It would
decay because the temperature of the core is too high to sustain permanent
magnetization.2
Earth’s Magnetic Field Periodically Reverses. Before the discovery of plate tectonics, geologists working on land had already discovered that some volcanic
rocks were magnetized in a direction opposite to the present-day field, suggesting
that the polarity of Earth’s magnetic field was reversed at the time the volcanoes
erupted and the rocks cooled (Figure 2.12b). The rocks were examined for whether
their magnetic field was normal, as it is today, or reversed relative to that of today,
for certain time intervals of the Earth’s history. A chronology for the last few million years was constructed on the basis of the dating of the “reversed” rocks. You
can verify the current magnetic field of the Earth by using a compass; at this point
in Earth’s history the needle points to the north magnetic pole. During a period of
reversed polarity, the needle would point south! The cause of magnetic reversals
is not well known, but it is related to changes in the convective movement of
the liquid material in the outer core and processes occurring in the inner core.
Reversals in Earth’s magnetic field are random, occurring on average every few
hundred thousand years. The change in polarity of Earth’s magnetic field takes a
few thousand years to occur, which in geologic terms is a very short time.
What Produces Magnetic Stripes? To further explore the Earth’s magnetic field,
geologists towed magnetometers, instruments that measure magnetic properties
of rocks, from ships and completed magnetic surveys. The paleomagnetic record
of the ocean floor is easy to read because of the fortuitous occurrence of the volcanic rock basalt (see Chapter 3) that is produced at spreading centers and forms
the floors of the ocean basins of Earth. The rock is fine-grained and contains sufficient iron-bearing minerals to produce a good magnetic record. The marine geologists’ discoveries were not expected. The rocks on the floor of the ocean were
found to have irregularities in the magnetic field. These irregular magnetic patterns were called anomalies or perturbations of Earth’s magnetic field caused by
local fields of magnetized rocks on the seafloor. The anomalies can be represented
as stripes on maps. When mapped, the stripes form quasi-linear patterns parallel
to oceanic ridges. The marine geologists found that their sequences of stripe width
patterns matched the sequences established by land geologists for polarity reversals in land volcanic rocks. Magnetic survey data for an area southwest of Iceland
are shown on Figure 2.13. The black stripes represent normally magnetized rocks
and the intervening white stripes represent reversed magnetized rocks.10 Notice
that the stripes are not evenly spaced but have patterns that are symmetrical on
opposite sides of the Mid-Atlantic Ridge (Figure 2.13).
Why Is the Seafloor No Older than 200 Million Years? The discovery of patterns
of magnetic stripes at various locations in ocean basins allowed geologists to infer
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
KELLMC02_0132251507.QXD
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A Detailed Look at Seafloor Spreading
60° W
30° W
10° W
0°
GREENLAND
Arctic Circle
ICELAND
60° N
Mid ocean ridg
e
60° N
EUROPE
40° W
20° W
Figure 2.13 Magnetic
anomalies on the seafloor Map
showing a magnetic survey southwest
of Iceland along the Mid-Atlantic
Ridge. Positive magnetic anomalies are
black (normal) and negative magnetic
anomalies are white (reversed). Note
that the pattern is symmetrical on the
two sides of the mid-oceanic ridge.
(From Heirtzler, J. R., Le Pichon, X., and
Baron, J. G. 1966. Magnetic anomalies
over the Reykjanes Ridge. Deep-Sea
Research 13:427–43)
10° W
Rid
g
ax e
is
50° W
53
numerical dates for the volcanic rocks. Merging the magnetic anomalies with
the numerical ages of the rocks produced the record of seafloor spreading. The
spreading of the ocean floor, beginning at a mid-oceanic ridge, could explain the
magnetic stripe patterns.11 Figure 2.14 is an idealized diagram showing how
seafloor spreading may produce the patterns of magnetic anomalies (stripes).
The pattern shown is for the past several million years, which includes several
periods of normal and reversed magnetization of the volcanic rocks. Black stripes
represent normally magnetized rocks, and brown stripes are rocks with a reversed
magnetic signature. Notice that the most recent magnetic reversal occurred approximately 0.7 million years ago. The basic idea illustrated by Figure 2.14 is that
rising magma at the oceanic ridge is extruded, or pushed out onto the surface,
through volcanic activity, and the cooling rocks become normally magnetized.
When the field is reversed, the cooling rocks preserve a reverse magnetic signature, and a brown stripe (Figure 2.14) is preserved. Notice that the patterns of
magnetic anomalies in rocks on both sides of the ridge are mirror images of one
another. The only way such a pattern might result is through the process of
seafloor spreading. Thus, the pattern of magnetic reversals found on rocks of
the ocean floor is strong evidence that the process of spreading is happening.
Mapping of magnetic anomalies, when combined with age-dating of the magnetic reversals in land rocks creates a database that suggests exciting inferences;
Figure 2.15 shows the age of the ocean floor as determined from this database. The
pattern, showing that the youngest volcanic rocks are found along active midoceanic ridges, is consistent with the theory of seafloor spreading. As distance
from these ridges increases, the age of the ocean floor also increases, to a maximum of about 200 million years, during the early Jurassic period (see Table 1.1).
Thus, it appears that the present ocean floors of the world are no older than
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Internal Structure of Earth and Plate Tectonics
Magnetic field anomaly
observed at sea surface
Age
(millions
of years)
Seafloor
0
Magnetized
rocks
2
4
6
8
Black = normal polarity
Brown = reversed polarity
200
0
200
Distance from spreading center (kilometers)
(b) Seafloor spreading
(a) Polarity reversal
time scale
Figure 2.14 Magnetic reversals and seafloor spreading Idealized diagram showing an oceanic
ridge and the rising of magma, in response to seafloor spreading. As the volcanic rocks cool, they become
magnetized. The black stripes represent normal magnetization; the brown stripes are reversed magnetization. The record shown here was formed over a period of several million years. Magnetic anomalies
(stripes) are a mirror image of each other on opposite sides of the mid-oceanic ridge. Thus, the symmetrical bands of the normally and reversely magnetized rocks are produced by the combined effects of the
reversals and seafloor spreading. (Courtesy of Tanya Atwater)
200 million years. In contrast, rocks on continents are often much older than
Jurassic, going back about 4 billion years, almost 20 times older than the ocean
floors! We conclude that the thick continental crust, by virtue of its buoyancy, is
more stable at Earth’s surface than are rocks of the crust of the ocean basins.
Continents form by the processes of accretion of sediments, addition of volcanic
materials, and collisions of tectonic plates carrying continental landmasses. We
will continue this discussion when we consider the movement of continents
during the past 200 million years. However, it is important to recognize that it is
the pattern of magnetic stripes that allows us to reconstruct how the plates and the
continents embedded in them have moved throughout history.
Hot Spots
What Are Hot Spots? There are a number of places on Earth called hot spots,
characterized by volcanic centers resulting from hot materials produced deep in
the mantle, perhaps near the core-mantle boundary. The partly molten materials
are hot and buoyant enough to move up through mantle and overlying moving
tectonic plates.3,6 An example of a continental hot spot is the volcanic region of
Yellowstone National Park. Hot spots are also found in both the Atlantic and
Pacific Oceans. If the hot spot is anchored in the slow-moving deep mantle, then,
as the plate moves over a hot spot, a chain of volcanoes is produced. Perhaps
the best example of this type of hot spot is the line of volcanoes forming the
Hawaiian-Emperor Chain in the Pacific Ocean (Figure 2.16a). Along this chain,
volcanic eruptions range in age from present-day activity on the big island of
Hawaii (in the southeast) to more than 78 million years ago near the northern
end of the Emperor Chain. With the exception of the Hawaiian Islands and some
coral atolls (ringlike coral islands such as Midway Island), the chain consists of
submarine volcanoes known as seamounts. Seamounts are islands that were
eroded by waves and submarine landslides and subsequently sank beneath the
ocean surface. As seamounts
move
off the hot spot, the volcanic rocks
PRELIMINARY
P R O O F farther
S
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Pangaea and Present Continents
55
Age of Ocean Floor (millions of years)
0–2
58–66
2–5
66–84
5–24
84–117
24–37
117–144
37–58
144–208
Figure 2.15 Age of the ocean floor Age of the seafloor is determined from magnetic anomalies
and other methods. The youngest ocean floor (red) is located along oceanic ridge systems, and older rocks
are generally farther away from the ridges. The oldest ocean floor rocks are approximately 180 million
years old. (From Scotese, C. R., Gahagan, L. M., and Larson, R. L. 1988. Plate tectonic reconstruction of the Cretaceous
and Cenozoic ocean basins. Tectonophysics 155:27–48)
the islands are composed of cool and the oceanic crust they are on becomes
denser, and sinks.
Seamounts constitute impressive submarine volcanic mountains. In the
Hawaiian Chain the youngest volcano is Mount Loihi, which is still a submarine
volcano, presumably directly over a hot spot, as idealized on Figure 2.16b. The ages
of the Hawaiian Islands increase to the northwest, with the oldest being Kauai,
about 6 million years old. Notice in Figure 2.16a that the line of seamounts makes a
sharp bend at the junction of the Hawaiian and Emperor Chains. The age of the
volcanic rocks at the bend is about 43 million years, and the bend is interpreted to
represent a time when plate motions changed.12 If we assume that the hot spots are
fixed deep in the mantle, then the chains of volcanic islands and submarine volcanoes along the floor of the Pacific Ocean that get older farther away from the hot
spot provide additional evidence to support the movement of the Pacific plate. In
other words, the ages of the volcanic islands and submarine volcanoes could
systematically change as they do only if the plate is moving over the hot spot.
2.5 Pangaea and Present Continents
Plate Tectonics Shapes Continents and Dictates the Location of Mountain
Ranges. Movement of the lithospheric plates is responsible for the present
shapes and locations of the continents. There is good evidence that the most
recent global episode of continental drift, driven by seafloor spreading, started
about 180 million years ago, with the breakup of a supercontinent called Pangaea
(this name, meaning “all lands,” was first proposed by Wegener). Pangaea
(pronounced pan-jee-ah) was enormous, extending from pole to pole and over
halfway around Earth near the equator (Figure
2.17).
PRE
L I M I N APangaea
R Y P R O O F Shad two parts
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Internal Structure of Earth and Plate Tectonics
RUSSIA
Alaska
Aleutian
Kuril
Trench
50˚N
40˚N
Islands
Aleutian Trench
78 MY
Emperor Seamounts
56
2/2/07
Volcanoes of Hawaiian–
Emperor Chain
Seamounts
Subduction zone
Hawaiian–Emperor
Bend
Present plate motion
43 MY
30˚N
Haw
PACIFIC
OCEAN
aiian
Ridg
e
Hawaii
20˚N
6 MY
0
0
160˚E
325
325
Present
volcanic
activity
750 Miles
750 Kilometers
170˚E
180˚
170˚W
160˚W
(a)
Kauai
3.8–5.6 MY
Oahu
2.2–3.3 MY
Hawaiian Islands
Molokai
1.3–1.8 MY
Maui
All less than 1.0 MY
Hawaii
0.8 to present MY
Loihi (submarine)
present
Oceanic lithosphere
Hot spot
(deep in mantle)
Dates in millions of years
MY = Million years old
(b)
Figure 2.16
Hawaiian hot spot (a) Map showing the Hawaiian-Emperor Chain of volcanic islands
and seamounts. Actually, the only islands are Midway Island and the Hawaiian Islands at the end of the
chain, where present volcanic activity is occurring. (Modified after Claque, D. A., Dalrymple, G. B., and Moberly, R.
1975. Petrography and K-Ar ages of dredged volcanic rocks from the western Hawaiian Ridge and southern Emperor
Seamount chain. Geological Society of America Bulletin 86:991–98) (b) Sketch map showing the Hawaiian
Islands, which range in age from present volcanic activity to about 6 million years old on the island of
RELIMINARY PROOFS
Kauai. (From Thurman, Oceanography, 5thUnpublished
ed. PColumbus,
OH:
plate 2)
Work © 2008
by Merrill,
Pearson Education,
Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Pangaea and Present Continents
57
70˚
A
A U
S I
R A
L
P
E
NORTH
AMERICA
EU
160˚
RO
A S I A
120˚
80˚
SOUTH
AMERICA
120˚
160˚
120˚
160˚
TETHYS SEA
AFRICA
G ON
DW
A
N INDIA
A
AUSTRALIA
ANTARCTICA
(a) 180 million years ago
Direction of plate
motion
Subduction zone
70˚
U
R A S I A
P
E
L
NORTH
AMERICA
A
EU
160˚
RO
A S I A
120˚
80˚
AFRICA
SOUTH
AMERICA
G
O
N
A
AN
DW
INDIA
AUSTRALIA
ANTARCTICA
(b) 135 million years ago
Figure 2.17 Two hundred million years of plate tectonics (a) The proposed positions of the
continents at 180 million years ago; (b) 135 million years ago; (c) 65 million years ago; and (d) at present.
Arrows show directions of plate motion. See text for further explanation of the closing of the Tethys Sea,
the collision of India with China, and the formation of mountain ranges. (From Dietz, R. S., and Holden, J. C.
1970. Reconstruction of Pangaea: breakup and dispersion of continents, Permian to present. Journal of Geophysical
Research 75(26):4939–56. Copyright by the American Geophysical Union. Modifications and block diagrams from
Christopherson, R. W. 1994. Geosystems, 2nd ed. Englewood Cliffs, NJ: Macmillan)
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Internal Structure of Earth and Plate Tectonics
160˚
E
NORTH
AMERICA
EU
RO
P
A S I A
120˚
80˚
120˚
160˚
120˚
160˚
AFRICA
SOUTH
AMERICA
INDIA
AUSTRALIA
ANTARCTICA
(c) 65 million years ago
ce
O
ic
an
Convergent plate boundary—
plates converge, producing a subduction zone,
mountains, volcanoes, and earthquakes
h
nc
tre
Plate
Plate
E
Asthenosphere
NORTH
AMERICA
EU
RO
P
A S I A
AFRICA
160˚
20˚
60˚
SOUTH
AMERICA
Divergent plate boundary—
plates diverge at mid-ocean
ridges
AUSTRALIA
40˚
Plate
ge
ANTARCTICA
rid
M
id
-
oc
ea
n
60˚
(d) Present
Plate
Transform fault—
plates move laterally past each other
between seafloor spreading centers
Fracture zone
Transform
fault
Asthenosphere
Figure 2.17
Two hundred million years of plate tectonics (Continued)
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Pangaea and Present Continents
59
(Laurasia to the North and Gondwana to the South) and was constructed during
earlier continental collisions. Figure 2.17a shows Pangaea as it was nearly 200 million years ago. Seafloor spreading over the past 200 million years separated
Eurasia and North America from the southern land mass; Eurasia from North
America; and the southern continents (South America, Africa, India, Antarctica,
and Australia) from one another (Figure 2.17b–d). The Tethys Sea, between Africa
and Europe-Asia (Figure 2.17a–c), closed, as part of the activity that produced the
Alps in Europe. A small part of this once much larger sea remains today as the
Mediterranean Sea (Figure 2.17d). About 50 million years ago India crashed into
China. That collision, which has caused India to forcefully intrude into China a
distance comparable from New York to Miami, is still happening today, producing the Himalayan Mountains (the highest mountains in the world) and the
Tibetan Plateau.
Understanding Plate Tectonics Solves Long-Standing Geologic Problems.
Reconstruction of what the supercontinent Pangaea looked like before the most
recent episode of continental drift has cleared up two interesting geologic
problems:
쐍 Occurrence of the same fossil plants and animals on different continents that
would be difficult to explain if they had not been joined in the past (see
Figure 2.18).
Fossil remains of Cynognathus, a Triassic
land reptile approximately 3 m long, have
been found in Argentina and southern Africa.
Remains of the freshwater reptile
Mesosaurus have been found in both
Brazil and Africa.
Fossils of the fern Glossoptens, found in
all of the southern continents, are proof
that they were once joined.
Africa
India
South America
Evidence of the Triassic land reptile
Lystrosaurus have been found in Africa,
Antarctica, and India.
Australia
Antarctica
Figure 2.18 Paleontological evidence for plate tectonics This map shows some of the paleontological (fossil) evidence that supports continental drift. It is believed that these animals and plants could not
have been found on all of these continents were they not once much closer together than they are today.
Major ocean basins would have been physical barriers to their distribution. (From Hamblin, W. K. 1992.
Earth’s dynamic systems, 6th ed. New York: Macmillan)
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Internal Structure of Earth and Plate Tectonics
쐍 Evidence of ancient glaciation on several continents, with inferred directions of
ice flow, that makes sense only if the continents are placed back within Gondwanaland (southern Pangaea) as it was before splitting apart (see Figure 2.19).
Late Paleozoic
glacial boundary
Late Paleozoic
glacial deposits
Direction of
glacier motion
(a)
Late Paleozoic
glacial boundary
Late Paleozoic
glacial deposits
Direction of
glacier motion
GO
ND
N
WA
ALA
ND
(b)
Figure 2.19
Glacial evidence for plate tectonics (a) Map showing the distribution of evidence
for late Paleozoic glaciations. The arrows indicate the direction of ice movement. Notice that the arrows
are all pointing away from ocean sources. Also these areas are close to the tropics today, where glaciation
would have been very unlikely in the past. These Paleozoic glacial deposits were formed when Pangaea
was a supercontinent, before fragmentation by continental drift. (b) The continents are restored (it is
thought that continents drifted north away from the South Pole). Notice that the arrows now point outward as if moving away from a central area where glacial ice was accumulating. Thus, restoring the position of the continents produces a pattern of glacial deposits that makes much more sense. (Modified after
Hamblin, W. K. 1992. Earth’s dynamic systems, 6th ed. New York: Macmillan)
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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How Plate Tectonics Works: Putting It Together
61
2.6 How Plate Tectonics Works:
Putting It Together
Driving Mechanisms That Move Plates. Now that we have presented the concept that new oceanic lithosphere is produced at mid-oceanic ridges because of
seafloor spreading and that old, cooler plates sink into the mantle at subduction
zones, let us evaluate the forces that cause the lithospheric plates to actually move
and subduct. Figure 2.20 is an idealized diagram illustrating the two most likely
driving forces, ridge push and slab pull.
The mid-oceanic ridges or spreading centers stand at elevations of 1 to 3 km
(3,000 to 9,000 ft) above the ocean floor as linear, gently arched uplifts (submarine
mountain ranges; see Figure 2.21) with widths greater than the distance from
Florida to Canada. The total length of mid-oceanic ridges on Earth is about twice
the circumference of Earth. Ridge push is a gravitational push, like a gigantic
landslide, away from the ridge crest toward the subduction zone (the lithosphere
slides on the asthenosphere). Slab pull results because as the lithospheric plate
moves farther from the ridge, it cools, gradually becoming denser than the
asthenosphere beneath it. At a subduction zone, the plate sinks through lighter,
hotter mantle below the lithosphere, and the weight of this descending plate pulls
on the entire plate, resulting in slab pull. Which of the two processes, ridge push
or slab pull, is the more influential of the driving forces? Calculations of the
expected gravitational effects suggest that ridge push is of relatively low importance compared with slab pull. In addition, it is observed that plates with large
subducting slabs attached and pulling on them tend to move much more rapidly
than those driven primarily by ridge push alone (for example, the subduction
zones surrounding the Pacific Basin). Thus, slab pull may be more influential in
moving plates than ridge push.
Volcanoes
Spreading center (mid-oceanic ridge)
offset by transform fault
Transform
fault
Lit
ho
sp
he
re
As
th
en
os
ph
er
e
Trench
ll
ab
Sl
pu
n
tio
c
du
z
e
on
Ridge
push
Lithosphere
b
Su
Figure 2.20 Push and pull in moving plates Idealized diagram showing concepts of ridge push
and slab pull that facilitate the movement of lithospheric plates from spreading ridges to subduction
zones. Both are gravity driven. The heavy lithosphere falls down the mid-oceanic ridge slope and subducts
down through the lighter, hotter mantle. (Modified after Cox, A., and Hart, R. B. 1986. Plate tectonics. Boston:
Blackwell Scientific Publications)
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Internal Structure of Earth and Plate Tectonics
Transform fault
England
Canada
U.S.A.
Mid-Atlantic Ridge
Subduction
zone
Pacific
Ocean
Subduction
zone and
trench
Atlantic Ocean
0
1,500
3,000 km
Figure 2.21
Mid Atlantic Ridge Image of the Atlantic Ocean basin showing details of the seafloor.
Notice that the width of the Mid-Atlantic Ridge is about one-half the width of the ocean basin. (Heinrich C.
Berann/NGS Image Collection)
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Plate Tectonics and Environmental Geology
63
2.7 Plate Tectonics and Environmental Geology
Hudso
n River
Plate Tectonics Affects Us All. The importance of the tectonic cycle to environmental geology cannot be overstated. Everything living on Earth is affected by
plate tectonics. As the plates slowly move a few centimeters each year, so do the
continents and ocean basins, producing zones of resources (oil, gas, and minerals), as well as earthquakes and volcanoes (Figure 2.4b). The tectonic processes
occurring at plate boundaries largely determine the types and properties of the
rocks upon which we depend for our land, our mineral and rock resources, and
the soils on which our food is grown. For example, large urban areas, including
New York and Los Angeles, are developed on very different landscapes, but both
have favorable conditions for urban development. New York (Figure 2.22a) is
sited on the “trailing edge” of the North American plate, and the properties of the
coastline are directly related to the lack of collisions between plates in the area.
The divergent plate boundary at the Mid-Atlantic Ridge between North America
and Africa is several thousand kilometers (over 1,500 miles) to the east. The collision boundaries between the North American and Caribbean plates and between
the North American and Pacific plates are several thousands of kilometers (over
1,500 miles) to the south and west, respectively (see Figure 2.4a). The passive
processes of sedimentation from rivers, glaciers, and coastal processes, depositing
sediments on rifted and thinned continental crust, instead of the more active
crustal deformation that produces mountains, have shaped the coastline of the
eastern United States north of Florida. The breakup of Pangaea about 200 million
years ago (Figure 2.17) produced the Atlantic Ocean, which, with a variety of
geologic processes, including erosion, deposition, and glaciation over millions
San Gabriel Mts.
Santa
Monica
Mts.
Long Island
Jersey
City
Los
Angeles
Long
Beach
Coney
Island
N
0
10 km
0
10 km
N
(b)
(a)
Figure 2.22 Los Angeles and New York Satellite images of (a) New York City and (b) the city of
Los Angeles. Both are coastal cities; however, Los Angeles is surrounded by mountains, whereas New York
is sited in a relatively low-relief area characteristic of much of the Atlantic coastal environment. For these
images, healthy vegetation is red, urban development is blue, beaches are off-white, and water is black.
(Science Source/Photo Researchers, Inc.)
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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Internal Structure of Earth and Plate Tectonics
of years, eventually led to the development of the beautiful but subdued topography of the coastal New York area. In contrast, the Los Angeles metropolitan area
is near the “leading edge” of the boundary between the North American and
Pacific plates (Figure 2.10), characterized by active, vigorous crustal deformation
(uplift; subsidence, or sinking of the ground’s surface; and faulting) associated
with the San Andreas fault, a transform boundary. The deformation has produced
the Los Angeles Basin, rimmed by rugged mountains and uplifted coastline
(Figure 2.22b).
Plate motion over millions of years can change or modify flow patterns in the
oceans and the atmosphere, influencing or changing global climate as well as regional variation in precipitation. These changes affect the productivity of the land
and its desirability as a place to live. Plate tectonics also determines, in part, what
types of minerals and rocks are found in a particular region. We will explore how
rocks and minerals are influenced by plate tectonics in Chapter 3.
SUMMARY
Our knowledge concerning the structure of Earth’s interior is
based on the study of seismology. Thus we are able to define
the major layers of Earth, including the inner core, outer core,
mantle, and crust. The uppermost layer of Earth is known as
the lithosphere, which is relatively strong and rigid compared
with the soft asthenosphere found below it. The lithosphere
is broken into large pieces called plates that move relative
to one another. As these plates move, they carry along the
continents embedded within them. This process of plate tectonics produces large landforms, including continents, ocean
basins, mountain ranges, and large plateaus. Oceanic basins
are formed by the process of seafloor spreading and are destroyed by the process of subduction, both of which result
from convection within the mantle.
The three types of plate boundaries are divergent (midoceanic ridges, spreading centers), convergent (subduction
zones and continental collisions), and transform faults. At
some locations, three plates meet in areas known as triple
junctions. Rates of plate movement are generally a few centimeters per year.
Evidence supporting seafloor spreading includes paleomagnetic data, the configurations of hot spots and chains
of volcanoes, and reconstructions of past continental
positions.
The driving forces in plate tectonics are ridge push and
slab pull. At present we believe the process of slab pull is
more significant than ridge push for moving tectonic plates
from spreading centers to subduction zones.
Plate tectonics is very important in environmental geology
because everything living on Earth is affected by it.
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
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KELLMC02_0132251507.QXD
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Critical Thinking Question
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Key Terms
asthenosphere (p. 41)
isostasy (p. 48)
seafloor spreading (p. 42)
continental drift (p. 42)
lithosphere (p. 41)
seismology (p. 41)
convection (p. 41)
magnetic reversal (p. 52)
spreading center (p. 42)
convergent boundary (p. 46)
mantle (p. 39)
subduction zone (p. 44)
core (p. 39)
mid-oceanic ridge (p. 42)
submarine trench (p. 46)
crust (p. 40)
Moho (p. 40)
transform boundary (p. 48)
divergent boundary (p. 46)
paleomagnetism (p. 52)
triple junction (p. 48)
hot spot (p. 54)
plate tectonics (p. 42)
Wadati-Benioff zone (p. 44)
Review Questions
1. What are the major differences
between the inner and outer cores
of Earth?
4. What is the major process that is
thought to produce Earth’s magnetic field?
2. How are the major properties of
the lithosphere different from
those of the asthenosphere?
5. Why has the study of paleomagnetism and magnetic reversals
been important in understanding
plate tectonics?
3. What are the three major types of
plate boundaries?
6. What are hot spots?
7. What is the difference between
ridge push and slab pull in the
explanation of plate motion?
Critical Thinking Question
1. Assume that the supercontinent Pangaea (Figure 2.17)
never broke up. Now deduce how Earth processes, landforms, and environments might be different than they
are today with the continents spread all over the globe.
Hint: Think about what the breakup of the continents did
in terms of building mountain ranges and producing
ocean basins that affect climate and so forth.
PRELIMINARY PROOFS
Unpublished Work © 2008 by Pearson Education, Inc.
From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights
reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic,
mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.