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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:

Understand the basic internal structure and

processes of Earth

Know the basic ideas behind and evidence

for the theory of plate tectonics

Understand the mechanisms of plate tectonics

Understand the relationship of plate tectonics

to environmental geology

37

Internal Structure of Earthand Plate Tectonics

Written with the assistance of Tanya Atwater

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)



38 Chapter 2 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 notori-ous San Andreas Fault (Figure 2.1). A fault is a fracture along

which one side has moved relative to the other, and the SanAndreas Fault is a huge fracture zone, hundreds of kilometerslong. Two major cities, Los Angeles to the south and SanFrancisco to the north, are located on opposite sides of thisfault. San Francisco was nearly destroyed by a major earth-quake in 1906, whichledto theidentification of thefault. Manyof the moderate to large earthquakes in the Los Angeles areaare on faults related to the San Andreas fault system. Most of the beautiful mountain topography in coastal Californianear 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 tosociety. Since 1906, earthquakes on the San Andreas fault sys-tem or on nearby faults, undoubtedly influenced by the plate

boundary, havecosthundreds of lives and manybillionsof dol-lars in property damage. Construction of buildings, bridges,and other structures in California is more expensive than else-where because they must be designed to withstand groundshaking caused by earthquakes. Older structures have to beretrofitted, or have changesmade totheir structure, towithstandthe shaking, and many people purchase earthquake insurancein an attempt to protect themselves from the “big one.”

Los Angeles is on the Pacific plate and is slowly movingtoward San Francisco, which is on the North American plate.

S N NDRE S

F ULT

SAN ANDREASFAULT

SAN ANDREASFAULT

SAN ANDREASFAULT

SAN ANDREASFAULT

SAN ANDREASFAULT

S A N A N D R E A

S

S

N

N

D

R

E

S

F A U L T

F

U

L

T

S A N A N D R E A

S

F A U L T

Figure 2.1 San Andreas Fault

Map showing the San Andreas Faultand topography in California. Arrowsshow relative motion on either side of the fault. (R. E. Wallace/National Earth-

quake Information Center. U.S.G.S.)



In about 20 million years the cities will be side by side. If peo-ple are present, they might be arguing over which is a suburbof the other. Of course, there will still be a plate boundary between the Pacific and North American plates 20 millionyears from now, because large plates have long geologic lives,on the order of 100 million years. However, the boundary

Internal Structure of Earth 39

may not be the San Andreas Fault. The plate boundary willprobably have moved eastward, and the topography of whatis now California may be somewhat different. In fact, somerecent earthquake activity in California, such as the large 1992Landers 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 phrasethat refers to our planet Earth. Far from being a barren rock, Earth is a complexdynamic 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 movesaround as a result of dynamic internal processes. The internal processes are in-credibly important in affecting the surface of Earth. They are responsible for thelargest landforms on the surface: continents and ocean basins. The configurationof 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’sinternal processes are also responsible for regional landforms including mountainchains, chains of active volcanoes, and large areas of elevated topography, such asthe Tibetan Plateau and the Rocky Mountains. The high topography that includesmountains and plateaus significantly affects both global circulation patterns of airin 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 thansimply academic interest. These processes are at the heart of producing the multi-tude 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 theEarth is shown in Figure 2.2b. We can consider the internal structure of Earth intwo fundamental ways:

by composition and density (heavy or light).

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

A solid inner core with a thickness of more than 1,300 km (808 mi) that isroughly the size of the moon but with a temperature about as high as thetemperature of the surface of the Sun.2 The inner core is believed to be pri-marily metallic, composed mostly of iron (about 90 percent by weight), with

minor amounts of elements such as sulfur, oxygen, and nickel. 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 andouter core is approximately 10.7 grams per cubic centimeter (0.39 poundsper 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).

The mantle, nearly 3,000 km (1,864 mi) thick, surrounds the outer coreand 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-richsilicates. Interestingly, the density difference between the outer core and the



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 man-tle, the more fluid phase of the outer core is beneath the solid phase of themantle. This is just the opposite of the case of the rock-atmosphere relation-ship, where the fluid atmosphere overlies the solid lithosphere. Because it isliquid, the outer core is dynamic and greatly influences the overlying mantleand, thus, the surface of Earth.

The crust, with variable thickness, is the outer rock layer of the Earth. The boundary between the mantle and crust is known as the Mohorovicicdiscontinuity (also called the Moho). It separates the lighter rocks of thecrust with an average density of approximately 2.8 g/cm3 (0.10 lb/in3) fromthe denser rocks of the mantle below.


(b)

(a)

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0

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AVERAGE DENSITY, g/cm3

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2.8

2.9

4.510.7

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L i t h o s p h e r e

R i g i d

M a n t l e

Figure 2.2 Earth and its interior (a) Earth from space. (National Geophysical Data

Center, National Oceanic and Atmospheric Administration) (b) Idealized diagram showingthe 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 theasthenosphere is located entirely within the mantle. Properties of the various layershave been estimated on the basis of (1) interpretation of geophysical data (primarily seismic waves from earthquakes); (2) examination of rocks thought to have risen frombelow by tectonic processes; and (3) meteorites, thought to be pieces of an old Earth-like planet. (From Levin, H. L. 1986. Contemporary physical geology, 2nd ed. Philadelphia:

Saunders)



How We Know about the Internal Structure of Earth 41

Continents and Ocean Basins Have Significantly Different Properties andHistory. Within the uppermost portion of the mantle, near the surface of Earthour terminology becomes more complicated. For example, the cool, strong outer-most layer of Earth is also called the lithosphere (lithos means “rock”). It is muchstronger 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-oceanridges to about 120 km (75 mi) beneath ocean basins and 20 to 400 km (13 to250 mi) beneath the continents. The crust is embedded in the top of the litho-sphere. Crustal rocks are less dense than the mantle rocks below, and oceanic crustis slightly denser than continental crust. Oceanic crust is also thinner: The oceanfloor has a uniform crustal thickness of about 6 to 7 km (3.7 to 4.4 mi), whereasthe crustal thickness of continents averages about 35 km (22 mi) and may be up to70 km (44 mi) thick beneath mountainous regions. Thus, the average crustal thick-ness is less than 1 percent of the total radius of Earth and can be compared to thethin skin of a tangerine. Yet it is this layer that is of particular interest to us becausewe 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 crustmay be several billion years old. Three thousand kilometers (1,865 mi) below us,at the core-mantle boundary, processes may be occurring that significantly affectour planet at the surface. It has been speculated that gigantic cycles of convectionoccur within Earth’s mantle, rising from as deep as the core-mantle boundary upto 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 itrises to the top. The rising water displaces denser, cooler water, which moveslaterally and sinks to the bottom of the pan. It is suggested that Earth layerscontain convection cells and operate in a similar fashion.

A complete cycle in the mantle may take as long as 500 million years.

1

Mantleconvection is fueled at the core-mantle boundary both by heat supplied fromthe molten outer core of Earth and by radioactive decay of elements (such asuranium) scattered throughout the mantle. Let us now examine some of theobservations and evidence that reveal the internal structure of Earth.

2.2 How We Know about the Internal

Structure of Earth

What We Have Learned about Earth from Earthquakes. Our knowledgeconcerning the structure of Earth’s interior arises primarily from our study of seismology. Seismology is the study of earthquakes and the passage of seismicwaves through Earth.3 When a large earthquake occurs, seismic energy is releasedand seismic waves move both through Earth and along its surface. The propertiesof these waves are discussed in detail in Chapter 6 with earthquake hazards.

Some waves move through solid and liquid materials while others movethrough solid, but not liquid materials. The rates at which seismic waves propa-gate 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 thewaves 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 topropagate through the liquid outer core. Thousands of seismographs (instruments

C o o l wat e r Coo l w

a t e r

H o t

w a t e r

Pan filledwith water

Gas stove

Convection cell

Figure 2.3 Convection

Idealized diagram showing the con-cept of convection.As the pan of wateris heated, the less dense hot waterrises from the bottom to displace thedenser cooler water at the top, whichthen sinks down to the bottom.This

process of mass transport is calledconvection, and each circle of rising and falling water is a convection cell.




that record seismic waves) are stationed around the world. When an earthquakeoccurs, the reflected and refracted waves are recorded when they emerge at thesurface. Study of these waves has been a powerful tool for deducing the layeringof the interior of Earth and the properties of the materials found there.

In summary, the boundaries that delineate the internal structure of Earth aredetermined by studying seismic waves generated by earthquakes and recordedon seismographs around Earth. As seismology has become more sophisticated, we

have learned more and more about the internal structure of Earth and are findingthat the structure can be quite variable and complex. For example, we have beenable to recognize

where magma, which is molten rock material beneath Earth’s surface, is gen-erated in the asthenosphere

the existence of slabs of lithosphere that have apparently sunk deep into themantle

the extreme variability of lithospheric thickness, reflecting its age and history

2.3 Plate TectonicsThe term tectonics refers to the large-scale geologic processes that deform Earth’slithosphere, producing landforms such as ocean basins, continents, and moun-tains. Tectonic processes are driven by forces within the Earth. These processes arepart of the tectonic system, an important subsystem of the Earth system.

Movement of the Lithospheric PlatesWhat Is Plate Tectonics? The lithosphere is broken into large pieces calledlithospheric plates that move relative to one another (Figure 2.4a).4 Processes associ-ated with the creation, movement, and destruction of these plates are collectivelyknown as plate tectonics.

Locations of Earthquakes and Volcanoes Define Plate Boundaries. A lithos-pheric plate may include both a continent and part of an ocean basin or an oceanregion alone. Some plates are very large and some are relatively small, thoughthey 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, isresponsible for many of the earthquakes in northern California. The boundaries between lithospheric plates are geologically active areas. Most earthquakesand many volcanoes are associated with these boundaries. In fact, plate bound-aries are defined by the areas in which concentrated seismic activity occurs(Figure 2.4b). Over geologic time, plates are formed and destroyed, cycling mate-rials 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 calledthe tectonic cycle.

Seafloor Spreading Is the Mechanism for Plate Tectonics. As the lithosphericplates move over the asthenosphere, they carry the continents embedded withinthem.5 The idea that continents move is not new; it was first suggested by Germanscientist Alfred Wegener in 1915. The evidence he presented for continental driftwas based on the congruity of the shape of continents, particularly those acrossthe Atlantic Ocean, and on the similarity in fossils found in South America andAfrica. Wegener’s hypothesis was not taken seriously because there was noknown 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



Plate Tectonics 43

JUAN DE FUCA PLATE

San Andreas

Fault

COCOSPLATE

NAZCA PLATE

SOUTH AMERICANPLATE

AFRICAN PLATE

ANTARCTIC PLATE

EURASIAN PLATE

PHILIPPINEPLATE

PACIFICPLATE

CAROLINEPLATE

FIJIPLATE

INDO-AUSTRALIAN PLATE

PACIFIC PLATE

CARIBBEANPLATE

NORTH AMERICAN PLATE

PACIFIC OCEAN

ATLANTICOCEAN

PACIFIC OCEAN

INDIAN

OCEAN

PACIFIC

OCEAN

OCEAN

PACIFIC

OCEAN

INDIAN

OCEAN

AT LANT IC

Equator

Equator

80°

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Transform fault

Direction of plate motion(relative motionrates in mm/yr)

Uncertainplate boundary

25

Volcanoes

Earthquakes

(a)

(b)

Figure 2.4 Earth’s plates (a) Map showing the major tectonic plates, plate boundaries, and direc-tion 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 thecorrespondence between this map and the plate boundaries. (Modified after Hamblin, W.K. 1992. Earth’s

dynamic systems, 6th ed. New York: Macmillan)




Transform fault

Transform fault

Lithosphere

Asthenosphere

Hot rockrising

Oceanicspreadingridge Cool rock

sinking

Subductionzone

Divergent boundary Convergent boundary

Transform fault

Figure 2.5 Model of plate

tectonics Diagram of the model of plate tectonics. New oceanic litho-sphere is being produced at the spread-ing ridge (divergent plate boundary).Elsewhere, oceanic lithosphere returnsto the interior of Earth at a convergentplate boundary (subduction zone).

(Modified from Lutgens, F., and Tarbuck, E.1992. Essentials of geology. New York:

Macmillan)

continuously added to the edges of lithospheric plates (Figure 2.5, left). As oceaniclithosphere is added along some plate edges (spreading centers), it is destroyedalong other plate edges, for example, at subduction zones (areas where one platesinks beneath another and is destroyed) (Figure 2.5, right). Thus continents do notmove 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 spreadingcenters is balanced by consumption at subduction zones, the size of Earth remainsconstant, neither growing nor shrinking.

Sinking Plates Generate Earthquakes. The concept of a lithospheric plate sink-

ing into the upper mantle is shown in diagrammatic form in Figure 2.5. When thewet, cold oceanic crust comes into contact with the hot asthenosphere, magmais generated. The magma rises back to the surface, producing volcanoes, such asthose that ring the Pacific Ocean basin, over subduction zones. The path of thedescending plate (or slab, as it sometimes is called) into the upper mantle is clearlymarked by earthquakes. As the oceanic plate subducts, earthquakes are produced both between it and the overriding plate and within the interior of the subductingplate. The earthquakes occur because the sinking lithospheric plate is relativelycooler and stronger than the surrounding asthenosphere; this difference causesrocks to break and seismic energy to be released.6

The paths of descending plates at subduction zones may vary from a shallowdip 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




Types of Plate BoundariesThere are three basic types of plate boundaries: divergent, convergent, and trans-form, shown in Figures 2.4 and 2.5 and Table 2.1. These boundaries are not narrowcracks as shown on maps and diagrams but are zones that range from a few tohundreds of kilometers across. Plate boundary zones are narrower in ocean crustand 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 thisprocess 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 upto 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, orrifting, it. Molten volcanic rock that is erupted along this rift valley cools andforms new plate material. The system of mid-oceanic ridges along divergent plate boundaries forms linear submarine mountain chains that are found in virtuallyevery ocean basin on Earth.

Convergent boundaries occur where plates collide. If one of the convergingplates is oceanic and the other is continental, an oceanic-continental plate collisionresults. 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 canresult in compression. Compression is a type of stress, or force per unit area. Whenan oceanic-continental plate collision occurs, compression is exerted on the litho-sphere, resulting in shortening of the surface of Earth, like pushing a table clothto 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 andvolcanoes such as the Andes in South America and the Cascade Mountains inthe Pacific Northwest of the United States (see A Closer Look: The Wonder of Mountains). If two oceanic lithospheric plates collide (oceanic-to-oceanic platecollision), one plate subducts beneath the other, and a subduction zone and arc-shaped 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

TABLE 2.1 Types of Plate Boundaries: Dynamics, Results, and Examples

Plate Boundary Plates Involved Dynamics Results Example

Divergent Usually oceanic Spreading. The two plates move Mid-ocean ridge forms and new African and Nor th American

away from one another and molten material is added to each plate. plate boundary (Figure 2.4a)

rock rises up to fill the gap.

Mid-Atlantic Ridge

Convergent Ocean-continent Oceanic plate sinks beneath Mountain ranges and a Nazca and South Americancontinental plate. subduction zone are formed with plate boundary (Figure 2.4a)

a deep trench. Earthquakes and

volcanic activity are found here. Andes Mountains

Peru-Chile Trench

Convergent Ocean-ocean Older, denser, oceanic plate sinks A subduction zone is formed with Fiji plate (Figure 2.4a)

beneath the younger, less dense a deep trench. Earthquakes and

oceanic plate. volcanic activity are found here. Fiji Islands

Convergent Continent-continent Neither plate is dense enough to A large,high mountain chain Indo-Australian and Eurasian

sink into the asthenosphere; is formed, and earthquakes plate boundary (on land)

compression results. are common. (Figure 2.4a)

Himalaya Mountains

Transform Ocean-ocean or The plates slide past one another. Earthquakes common. May result North American and Pacific

continent-continent in some topography. plate boundary (Figure 2.10)

San Andreas fault



Plate Tectonics 47

Subductionzone

Trench

Volcanoes

Mountains

Oceanic plate

Continental plate

Magma Shortening, thickeninguplift

(b)

Subductionzone

Trench

Oceanic plate

Magma(a)

Oceanicplate

Volcanic islandchain

Islandarc

Suture zone

Shortening, thickeninguplift

(c)

Continentalplate

Continentalplate

Mountains

Figure 2.8 Convergent plate boundaries Idealized diagram illustrating characteristics of conver-gent 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 theocean, is often formed as the result of the convergence of two colliding plateswith subduction of one. A trench is often located seaward of a subduction zoneassociated with an oceanic-continental plate or oceanic-oceanic plate collision.Submarine trenches are sites of some of the deepest oceanic waters on Earth. Forexample, 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 andthe Peru-Chile trench west of South America. If the leading edges of both platescontain relatively light, buoyant continental crust, subduction into the mantle of one of the plates is difficult. In this case a continent-to-continent plate collisionoccurs, in which the edges of the plates collide, causing shortening and litho-spheric 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 Himalayanmountain belts (Figure 2.9). Many older mountain belts were formed in a similarway; for example, the Appalachians formed during an ancient continent-to-continent plate collision 250 to 350 million years ago.

Figure 2.9 Mountains in Italy

Mountain peaks (the Dolomites) insouthern Italy are part of the Alpinemountain system formed from thecollision between Africa and Europe.

(Edward A. Keller)




Transform boundaries, or transform faults, occur where the edges of two platesslide 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 riftsthat are offset from one another along connecting transform faults. Although themost common locations for transform plate boundaries are within oceanic crust,some occur within continents. A well-known continental transform boundary isthe San Andreas Fault in California, where the rim of the Pacific plate is slidinghorizontally 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 themeeting pointof the Juan de Fuca, North American, and Pacific plates on the West Coast of North Amer-

ica(this is known as theMendocino triplejunction) andthejunctionof thespreadingridges associated with the Pacific, Cocos, and Nazca plates west of South America.

A CLOSER LOOK The Wonder of Mountains

Mountains have long fascinated people with their awesomepresence. We are now discovering a fascinating story concern-ing their origin. The story removes some of the mystery as tohow mountains form, but it has not removed the wonder. Thenew realization that mountains are systems (see Chapter 1)resulting from the interaction between tectonic activity (thatleads to crustal thickening), the climate of the mountain, andEarth surface processes (particularly erosion) has greatlyexpanded our knowledge of how mountains develop.7,8

Specifically, we have learned the following:

Tectonic processes at convergent plate boundaries lead tocrustal thickening and initial development of mountains.The mean (or average) elevation that a mountain rangeattains is a function of the uplift rate, which varies fromless than 1 mm to about 10 mm per year (0.04 to 0.4 in. peryear). The greater the rate of uplift, the higher the point towhich the mean elevation of a mountain range is likely torise during its evolution.

As a mountain range develops and gains in elevation, it begins to modify the local and regional climate by blockingstorm paths and producing a “rain shadow” in which themountain slopes on the rain-shadow side receive much lessrainfall than does the otherside of the mountain.As a result,ratesof runoffand erosion onthe side of the rain shadow areless than for the other side. Nevertheless, the rate of erosionincreases 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 maximummean elevation, which is a dynamic balance between theuplift and erosion. At this point, no amount of additionaluplift will increase the mean elevation of the mountains

above the dynamicmaximum. However, if theuplift rate in-creases, then a higher equilibrium mean elevation of therange may be reached. Furthermore, when the uplift ceasesor there is a reduction in the rate of uplift, the mean eleva-tion of the mountain range will decrease.7 Strangely, theelevations of individual peaks maystill increase!

Despiteerosion, theelevation of a mountain peak in a rangemay actually increase. This statement seems counterin-tuitive until we examine in detail some of the physicalprocesses resulting from erosion. Theuplift thatresults fromthe erosion is known as isostatic uplift. Isostasy is the

principle whereby thicker, more buoyant crust stands topo-graphicallyhigherthan crustthat is thinner anddenser. Theprinciple governing how erosion can result in uplift is illus-trated in Figure 2.A. The fictitious Admiral Frost has beenmarooned on an iceberg and is uncomfortable being farabove thesurface of the water. Heattempts toremovethe icethat is abovethewaterline. Wereit notfor isostatic (buoyant)uplift,he would have reached hisgoal tobe closeto thewaterline. Unfortunately for Admiral Frost, this is not the way theworldworks; continuous isostatic uplift of the block as ice isremoved always keeps one-tenth of the iceberg above thewater. So,after removing the iceabove the water line, he stillstandsalmost as muchabove the water line as before.7

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 thedenser 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 mountainrange is reduced, isostatic compensation occurs and theentire mountain range rises in response. As a result of theerosion, the maximum elevation of mountain peaks actu-ally may increase, while the mean elevation of the entiremountain block decreases. As a general rule, as the equiva-lent of 1 km (0.6 mi) of erosion across the entire mountain block occurs, the mean elevation of mountains will riseapproximately five-sixths of a kilometer (one-half mile).

In summary, research concerning the origin of mountainssuggests that they result in part from tectonic processes thatcause the uplift, but they also are intimately related to climaticand erosional processes that contribute to the mountain build-ing process. Erosion occurs during and after tectonic uplift,and isostatic compensation to that erosion occurs for millionsof years. This is one reason it is difficult to remove mountainsystems from the landscape. For example, mountain systemssuch as the Appalachian Mountains in the southeasternUnited States were originally produced by tectonic upliftseveral hundred million years ago when Europe collidedwith North America. There has been sufficient erosion of theoriginal Appalachian Mountains to have removed them astopographic features many times over were it not for contin-ued isostatic uplift in response to the erosion.



Figure 2.A Isostasy Idealized diagram or cartoon showing the principle of isostatic uplift. Admiral Frost is left adrift on an iceberg and is un-comfortable 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 theiceberg 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 Frosthad 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 moun-tain 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)

Plate Tectonics 49

Rates of Plate MotionPlate Motion Is a Fast Geologic Process. The directions in which plates move areshown on Figure 2.4a. In general, plates move a few centimeters per year, about asfast as some people’s fingernails or hair grows. The Pacific plate moves past theNorth 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 displacedover 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 equiva-lent 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, platesinteract at their boundaries, where collision or subduction or both occur, and

movement may not be smooth or steady. The plates often get stuck together. Move-ment is analogous to sliding one rough wood board over another. Movement

(a) (b)

(c)

(d)

10,000kg

90,000

kg

90,000

kg??

9000

kg

81,000

kg




Figure 2.11 The San Andreas

Fault The fault is visible from thelower left to upper right diagonally across the photograph, as if a giganticplow had been dragged across thelandscape. (James Balog/Getty

Images Inc.)

NORTH AMERICAN

PLATE

COCOSPLATE

CARIBBEANPLATE

SOUTH AMERICAN

PLATE

AFRICANPLATE

PACIFIC PLATE

JUAN DE FUCA PLATE

San AndreasFault

Triple Junction

Triple Junction

63

53

35

127

91

58

NAZCA PLATE

PACIFIC OCEAN

ATLANTIC OCEAN

Subduction zone

Spreading center

Transform fault

Plate motion(rate in mm/yr)

Direction of relative displacementon transform fault(rate in mm/yr)

63

35

Figure 2.10 North American plate boundary Detail of boundary between the North Americanand Pacific plates. (Courtesy of Tanya Atwater)



A Detailed Look at Seafloor Spreading 51

occurs when the splinters of the boards break off and the boards move quickly byone another. When rough edges along the plate move quickly, an earthquake isproduced. Along the San Andreas Fault, which is a transform plate boundary, thedisplacement is horizontal and can amount to several meters during a great earth-quake. During an earthquake in 1857 on the San Andreas Fault a horse corral acrossthe fault was reportedly changed from a circle to an “S” shape. Fortunately, such anevent generally occurs at any given location only once every 100 years or so. Over

long time periods, rapid displacement from periodic earthquakes and more contin-uous slow “creeping” displacements add together to produce the rate of severalcentimeters 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 hadno solid evidence of a mechanism that could move continents. The global extentof mid-oceanic ridges was discovered in the 1950s, and in 1962 geologist HarryH. Hess published a paper suggesting that continental drift was the result of theprocess of seafloor spreading along those ridges. The fundamentals of seafloorspreading are shown in Figure 2.5. New oceanic lithosphere is produced at thespreading ridge (divergent plate boundary). The lithospheric plate then moveslaterally, carrying along the embedded continents in the tops of moving plates.These ideas produced a new major paradigm that greatly changed our ideas abouthow Earth works.3,6,9

The validity of seafloor spreading was established from three sources: (1) iden-tification and mapping of oceanic ridges, (2) dating of volcanic rocks on the floorof the ocean, and (3) understanding and mapping of the paleomagnetic historyof ocean basins.

Paleomagnetism

We introduce and discuss Earth’s magnetic field and paleomagnetic history insome detail in order to understand how seafloor spreading and plate tectonicswere discovered. Earth has had a magnetic field for at least the past 3 billionyears2 (Figure 2.12a). The field can be represented by a dipole magnetic field withlines of magnetic force extending from the South Pole to the North Pole. A dipolemagnetic field is one that has equal and opposite charges at either end. Convec-tion occurs in the iron-rich, fluid, hot outer core of Earth because of compositional

Magneticequator

Magneticaxis

Magneticaxis

(a) Normal polarity

Axis of rotation

(b) Reversed polarity

Axis of rotation

S

N

N

S

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)



A Detailed Look at Seafloor Spreading 53

numerical dates for the volcanic rocks. Merging the magnetic anomalies withthe numerical ages of the rocks produced the record of seafloor spreading. Thespreading of the ocean floor, beginning at a mid-oceanic ridge, could explain themagnetic stripe patterns.11 Figure 2.14 is an idealized diagram showing howseafloor spreading may produce the patterns of magnetic anomalies (stripes).The pattern shown is for the past several million years, which includes severalperiods of normal and reversed magnetization of the volcanic rocks. Black stripesrepresent normally magnetized rocks, and brown stripes are rocks with a reversedmagnetic signature. Notice that the most recent magnetic reversal occurred ap-proximately 0.7 million years ago. The basic idea illustrated by Figure 2.14 is thatrising 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 signa-

ture, 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 oneanother. 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 magnet-ic 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. Thepattern, showing that the youngest volcanic rocks are found along active mid-oceanic ridges, is consistent with the theory of seafloor spreading. As distancefrom these ridges increases, the age of the ocean floor also increases, to a maxi-mum 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

20° W 10° W40° W50° W

60° N 60° N

30° W60° W 10° W

Arctic Circle

0°

EUROPE

ICELAND

GREENLAND

M i d

o c e a n

r i d g e

R i d g

e

a x i s

Figure 2.13 Magnetic

anomalies on the seafloor Mapshowing a magnetic survey southwestof Iceland along the Mid-AtlanticRidge. Positive magnetic anomalies areblack (normal) and negative magneticanomalies are white (reversed). Notethat 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)




Magnetic field anomalyobserved at sea surface

Distance from spreading center (kilometers)

Seafloor

Magnetizedrocks

(a) Polarity reversal

time scale

(b) Seafloor spreading

Age(millionsof years)

Black = normal polarityBrown = reversed polarity

200200 08

6

4

2

0

Figure 2.14 Magnetic reversals and seafloor spreading Idealized diagram showing an oceanicridge and the rising of magma, in response to seafloor spreading. As the volcanic rocks cool, they becomemagnetized. The black stripes represent normal magnetization; the brown stripes are reversed magnetiza-tion. 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 symmetri-cal bands of the normally and reversely magnetized rocks are produced by the combined effects of thereversals 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 oceanfloors! 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 volcanicmaterials, and collisions of tectonic plates carrying continental landmasses. Wewill continue this discussion when we consider the movement of continentsduring the past 200 million years. However, it is important to recognize that it isthe pattern of magnetic stripes that allows us to reconstruct how the plates and thecontinents embedded in them have moved throughout history.

Hot SpotsWhat Are Hot Spots? There are a number of places on Earth called hot spots,characterized by volcanic centers resulting from hot materials produced deep inthe mantle, perhaps near the core-mantle boundary. The partly molten materials

are hot and buoyant enough to move up through mantle and overlying movingtectonic 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 andPacific 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. Perhapsthe best example of this type of hot spot is the line of volcanoes forming theHawaiian-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 northernend of the Emperor Chain. With the exception of the Hawaiian Islands and somecoral 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 theocean surface. As seamounts move farther off the hot spot, the volcanic rocks



Pangaea and Present Continents 55

the islands are composed of cool and the oceanic crust they are on becomesdenser, and sinks.Seamounts constitute impressive submarine volcanic mountains. In the

Hawaiian Chain the youngest volcano is Mount Loihi, which is still a submarinevolcano, presumably directly over a hot spot, as idealized on Figure 2.16b. The agesof 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 asharp bend at the junction of the Hawaiian and Emperor Chains. The age of thevolcanic rocks at the bend is about 43 million years, and the bend is interpreted torepresent a time when plate motions changed.12 If we assume that the hot spots arefixed deep in the mantle, then the chains of volcanic islands and submarine volca-noes along the floor of the Pacific Ocean that get older farther away from the hotspot provide additional evidence to support the movement of the Pacific plate. In

other words, the ages of the volcanic islands and submarine volcanoes couldsystematically 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 MountainRanges. Movement of the lithospheric plates is responsible for the presentshapes and locations of the continents. There is good evidence that the mostrecent global episode of continental drift, driven by seafloor spreading, startedabout 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 overhalfway around Earth near the equator (Figure 2.17). Pangaea had two parts

0–2

2–55–24

24–37

37–58

58–66

66–8484–117

117–144

144–208

Age of Ocean Floor (millions of years)

Figure 2.15 Age of the ocean floor Age of the seafloor is determined from magnetic anomaliesand other methods.The youngest ocean floor (red) is located along oceanic ridge systems, and older rocksare 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)




E m p e r o r

S e a m o u n

t s

H a w a i i a n R i d g e

Aleutian Trench

Hawaiian–Emperor Bend

43 MY

78 MY

6 MY

Present volcanicactivity

KurilTrench

Alaska

Hawaii

RUSSIA

A l e u t i a n

Islands

PACIFIC OCEAN

180˚ 160˚W170˚W170˚E160˚E

20˚N

30˚N

40˚N

50˚N

0

325 750 Kilometers0

325 750 Miles

Volcanoes of Hawaiian–Emperor Chain

Seamounts

Subduction zone

Present plate motion

Oceanic lithosphere

Hot spot(deep in mantle)

Hawaii0.8 to present MY

MauiAll less than 1.0 MY

Molokai1.3–1.8 MY

Oahu2.2–3.3 MY

Kauai3.8–5.6 MY

Hawaiian Islands

Loihi (submarine)present

Dates in millions of yearsMY = Million years old

(a)

(b)

Figure 2.16 Hawaiian hot spot (a) Map showing the Hawaiian-Emperor Chain of volcanic islandsand seamounts. Actually, the only islands are Midway Island and the Hawaiian Islands at the end of thechain, 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 HawaiianIslands, which range in age from present volcanic activity to about 6 million years old on the island of Kauai. (From Thurman, Oceanography, 5th ed. Columbus,OH: Merrill, plate 2)




120˚160˚ 160˚120˚80˚

120˚160˚ 160˚120˚80˚

70˚

70˚

L A

U R A S I A

L A

U R A S I A

G O N D W

A N A

G O N D W A N

A

NORTH AMERICA

NORTH AMERICA

A S I A

A S I A

AFRICASOUTH

AMERICA

AFRICA

SOUTH AMERICA

INDIA

INDIA

AUSTRALIA

AUSTRALIA

ANTARCTICA

ANTARCTICA

E U R O

P E

E U R O

P E

TETHYS SEA

(a) 180 million years ago

(b) 135 million years ago

Direction of platemotionSubduction zone

Figure 2.17 Two hundred million years of plate tectonics (a) The proposed positions of thecontinents 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)




80˚ 120˚ 160˚160˚ 120˚

120˚60˚

40˚

60˚

20˚ 160˚160˚

(c) 65 million years ago

(d) Present

NORTH AMERICA

A S I A

AFRICASOUTH

AMERICA INDIA

AUSTRALIA

ANTARCTICA

E U R O

P E

ANTARCTICA

NORTH AMERICA A S I A

AFRICA

SOUTH AMERICA

AUSTRALIA

E U R O

P E

Fracture zoneTransform fault

Convergent plate boundary— plates converge, producing a subduction zone, mountains, volcanoes, and earthquakes

Divergent plate boundary—plates diverge at mid-oceanridges

Transform fault—plates move laterally past each otherbetween seafloor spreading centers

Asthenosphere

Plate Plate

Asthenosphere

P la te Plate

M

i d - o c e a n

r i d

g e

O c e a n i c t r e n c h

Figure 2.17 Two hundred million years of plate tectonics (Continued )




(Laurasia to the North and Gondwana to the South) and was constructed duringearlier continental collisions. Figure 2.17a shows Pangaea as it was nearly 200 mil-lion years ago. Seafloor spreading over the past 200 million years separatedEurasia and North America from the southern land mass; Eurasia from NorthAmerica; and the southern continents (South America, Africa, India, Antarctica,and Australia) from one another (Figure 2.17b–d). The Tethys Sea, between Africaand 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 theMediterranean Sea (Figure 2.17d). About 50 million years ago India crashed intoChina. That collision, which has caused India to forcefully intrude into China adistance comparable from New York to Miami, is still happening today, produc-ing the Himalayan Mountains (the highest mountains in the world) and theTibetan Plateau.

Understanding Plate Tectonics Solves Long-Standing Geologic Problems.Reconstruction of what the supercontinent Pangaea looked like before the mostrecent episode of continental drift has cleared up two interesting geologicproblems:

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 (seeFigure 2.18).

Africa

India

Australia

Antarctica

Fossil remains of Cynognathus, a Triassicland reptile approximately 3 m long, havebeen found in Argentina and southern Africa.

Evidence of the Triassic land reptileLystrosaurus have been found in Africa,Antarctica, and India.

Fossils of the fern Glossoptens, found inall of the southern continents, are proof that they were once joined.

Remains of the freshwater reptileMesosaurus have been found in bothBrazil and Africa.

South America

Figure 2.18 Paleontological evidence for plate tectonics This map shows some of the paleonto-logical (fossil) evidence that supports continental drift. It is believed that these animals and plants could nothave 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)




Evidence ofancient glaciation on several continents, withinferreddirections of ice flow, that makes sense only if the continents are placed back within Gond-wanaland (southern Pangaea) as it was before splitting apart (see Figure 2.19).

Late Paleozoicglacial boundary

Late Paleozoicglacial deposits

Direction of glacier motion

(a)

G O N D

W A N A L A N D

Late Paleozoicglacial boundary

Late Paleozoicglacial deposits

Direction of glacier motion

(b)

Figure 2.19 Glacial evidence for plate tectonics (a) Map showing the distribution of evidencefor late Paleozoic glaciations.The arrows indicate the direction of ice movement. Notice that the arrowsare 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 isthought that continents drifted north away from the South Pole). Notice that the arrows now point out- ward as if moving away from a central area where glacial ice was accumulating.Thus, restoring the posi-tion 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)




Canada

U.S.A.

England

Atlantic Ocean

Pacific Ocean

0 1,500 3,000 km

Subductionzone

Subductionzone andtrench

Transform fault

Mid-Atlantic Ridge

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)



Plate Tectonics and Environmental Geology 63

2.7 Plate Tectonics and Environmental Geology

Plate Tectonics Affects Us All. The importance of the tectonic cycle to environ-mental geology cannot be overstated. Everything living on Earth is affected byplate tectonics. As the plates slowly move a few centimeters each year, so do thecontinents and ocean basins, producing zones of resources (oil, gas, and miner-

als), as well as earthquakes and volcanoes (Figure 2.4b). The tectonic processesoccurring at plate boundaries largely determine the types and properties of therocks upon which we depend for our land, our mineral and rock resources, andthe soils on which our food is grown. For example, large urban areas, includingNew York and Los Angeles, are developed on very different landscapes, but bothhave favorable conditions for urban development. New York (Figure 2.22a) issited on the “trailing edge” of the North American plate, and the properties of thecoastline are directly related to the lack of collisions between plates in the area.The divergent plate boundary at the Mid-Atlantic Ridge between North Americaand Africa is several thousand kilometers (over 1,500 miles) to the east. The colli-sion boundaries between the North American and Caribbean plates and betweenthe North American and Pacific plates are several thousands of kilometers (over1,500 miles) to the south and west, respectively (see Figure 2.4a). The passiveprocesses of sedimentation from rivers, glaciers, and coastal processes, depositingsediments on rifted and thinned continental crust, instead of the more activecrustal deformation that produces mountains, have shaped the coastline of theeastern United States north of Florida. The breakup of Pangaea about 200 millionyears ago (Figure 2.17) produced the Atlantic Ocean, which, with a variety of geologic processes, including erosion, deposition, and glaciation over millions

(a)

(b)

H u

d s o n

R i v

e r

Long Island

ConeyIsland

JerseyCity

N

N

0 10 km

0 10 km

San Gabriel Mts.

SantaMonica

Mts.

LosAngeles

LongBeach

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 Yorkis sited in a relatively low-relief area characteristic of much of the Atlantic coastal environment. For theseimages, healthy vegetation is red, urban development is blue, beaches are off-white, and water is black.(Science Source/Photo Researchers, Inc.)



Critical Thinking Question 65

Key Terms

asthenosphere (p. 41)

continental drift (p. 42)

convection (p. 41)

convergent boundary (p. 46)

core (p. 39)crust (p. 40)

divergent boundary (p. 46)

hot spot (p. 54)

isostasy (p. 48)

lithosphere (p. 41)

magnetic reversal (p. 52)

mantle (p. 39)

mid-oceanic ridge (p. 42)Moho (p. 40)

paleomagnetism (p. 52)

plate tectonics (p. 42)

seafloor spreading (p. 42)

seismology (p. 41)

spreading center (p. 42)

subduction zone (p. 44)

submarine trench (p. 46)transform boundary (p. 48)

triple junction (p. 48)

Wadati-Benioff zone (p. 44)

Review Questions

1. What are the major differences between the inner and outer coresof Earth?

2. How are the major properties of the lithosphere different fromthose of the asthenosphere?

3. What are the three major types of plate boundaries?

4. What is the major process that isthought to produce Earth’s mag-netic field?

5. Why has the study of paleomag-netism and magnetic reversals been important in understandingplate tectonics?

6. What are hot spots?

7. What is the difference between

ridge push and slab pull in theexplanation of plate motion?

Critical Thinking Question

1. Assume that the supercontinent Pangaea (Figure 2.17)never broke up. Now deduce how Earth processes, land-

forms, and environments might be different than theyare today with the continents spread all over the globe.

Hint: Think about what the breakup of the continents didin terms of building mountain ranges and producing

ocean basins that affect climate and so forth.

ch02 internal structure of earth and plate tectonics written with

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