Continents in Motion: The Search for a Unifying Theory · 2009-11-05 · 362 CHAPTER 13 † EARTH STRUCTURE, EARTH MATERIALS, AND PLATE TECTONICS Continents in Motion: The Search

C H A P T E R 1 3 • E A R T H S T R U C T U R E , E A R T H M AT E R I A L S , A N D P L AT E T E C TO N I C S362

Continents in Motion: The Search for a Unifying Theory

Scientists in all disciplines constantly search for broad explana-tions that shed light on the detailed facts, recurring patterns, and interrelated processes that they observe and analyze. Is there one broad theory that can help explain how and why Earth’s litho-spheric processes work? Can it explain such diverse phenomena as the growth of continents, the movement of solid rock be-neath Los Angeles, the location of great mountain ranges, dif-fering patterns of temperature in the rocks of the seafloor, and the violent volcanic eruptions on the island of Montserrat in the West Indies? The answer is yes, and the concept is that of the continual movement of landmasses on Earth’s surface over mil-lions of years of time.

Sometimes it requires years to develop, test, and refine a sci-entific theory to the point where it is more fully understood and broadly acceptable. As data and information are gathered and ana-lyzed, new methods and technologies contribute to the process of testing hypotheses via the scientific method, and bit by bit an acceptable explanatory framework emerges. Over the past cen-tury, the theoretical framework of continental drift has been refined into a well-established theory called plate tectonics, which has been tested by collecting a great deal of evidence from the lithosphere. The theory of plate tectonics has revolutionized the Earth sci-ences and our understanding of Earth’s history.

Long ago, some scientists believed that Earth’s landscapes were created by great cataclysms. They might have believed, for example, that the Grand Canyon split open one violent day and has remained that way ever since, or that the Rocky Mountains appeared over-night. This theory, called catastrophism, has been rejected. For almost two centuries, physical geographers, geologists, and other Earth scientists have accept instead the theory of uniformitarian-ism, which is the idea that internal and external Earth processes operate today in the same manner as they have for millions of years.

Uniformitarianism, however, does not mean that processes have always operated at the same rate or with equal strength everywhere on Earth. In fact, our planet’s surface features are the result of variations in the intensity of internal and external pro-cesses, influenced by their geographical location. These processes have varied in intensity and location throughout Earth’s history. Furthermore, regular or episodic changes in the Earth system that may seem relatively small to us can dramatically alter a landscape after progressing, even on an irregular basis, for millions of years.

Continental DriftMost of us have probably noted on a world map that the Atlantic coasts of South America and Africa look as if they could fit to-gether. In fact, if a globe were made into a spherical jigsaw puzzle, several widely separated landmasses could fit alongside each other without large gaps or overlaps ( ● Fig. 13.21). Is there a scientific explanation for this phenomenon?

In the early 1900s, Alfred Wegener, a German climatologist, proposed the theory of continental drift, the idea that con-

tinents and other landmasses have shifted their positions during Earth history. Wegener’s evidence for continental drift included the close fit of continental coastlines on opposite sides of oceans and the trends of mountain ranges on land areas that also match across oceans. He cited comparable geographical patterns of fossils and rock types found on different continents that he felt could not result from chance and did not reflect current climatic con-ditions. To explain the spatial distributions of these features, he reasoned that the continents must have been previously joined. Wegener also noted evidence of great climate change, such as an-cient evidence of glaciation where the Sahara Desert is today and tropical fossils found in Antarctica, that could be explained best by large landmasses moving from one climate zone to another.

Wegener hypothesized that all the continents had once been part of a single supercontinent, which he called Pangaea, that later divided into two large landmasses, one in the South-ern Hemisphere (Gondwana), and one in the Northern Hemi-sphere (Laurasia). Later, these two supercontinents also broke apart into sections (the present continents) and drifted to their current positions. Laurasia in the Northern Hemisphere con-sisted of North America, Europe, and Asia. Gondwana in the Southern Hemisphere was made up of South America, Africa, Australia, Antarctica, and India ( ● Fig. 13.22). Continued con-tinental movement created the geographical configuration of the landmasses that exist on Earth today.

● FIGURE 13.21The geographic basis for Wegener’s continental drift hypothesis. Note the close correlation of the edges of the continents that face one another across the width of the Atlantic Ocean. The ac-tual fit is even closer if the continental slopes are matched.

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The reaction of most of the scientific community to Wegener’s proposal ranged from skepticism to ridicule. A major objection to his hypothesis was that neither he nor anyone else could provide an acceptable explanation for the energy needed to break up huge land-masses and slide them over the rigid crust and across vast oceans.

Supporting Evidence for Continental DriftAbout a half century later, in the late 1950s and 1960s, Earth scientists began giving serious consideration to Wegener’s notion of moving continents. New information appeared from research in oceanography, geophysics, and other Earth sciences, aided by sonar, radioactive dating of rocks, and im-provements in equipment for measuring Earth’s magnetism. These scientific efforts discovered much new evidence that indicated the movement of portions of the lithosphere, in-cluding the continents.

As one example, scientists were originally unable to ex-plain the varied orientations of magnetic fields found in basaltic rocks that had cooled millions of years ago. They knew that iron-bearing minerals in rocks display the magnetic field of Earth as it existed when the rocks solidified, which is a phe-nomenon known as paleomagnetism. Scientists at that time also knew that the exact position of the magnetic poles wan-dered through time, but they could not account for the con-fusing range of magnetic field orientations indicated by the basaltic rocks they studied. Magnetic field orientations of rocks of the same age did not point toward a single spot on Earth, and the indicated positions for the magnetic north pole ranged widely, including some that pointed toward the present south magnetic pole. The observed variations were more than could be accounted for by the known magnetic polar wandering.

Scientists eventually used the paleomagnetic data to model where the sampled rocks would have to have been relative to

a common magnetic north pole. Successful alignment was only possible if the continents had been in different positions than they are today. Using rocks of different age, they recon-structed locations of the continents during past periods in geologic history ( ● Fig. 13.23). Paleomagnetic data revealed that the continents were grouped together about 200 million years ago, just as Wegener’s hypothesized two supercontinents began to split apart to form the beginnings of the modern Atlantic Ocean. Paleomagnetic data also revealed that the po-larity of Earth’s magnetic field had reversed many times in the past. A record of these polarity reversals was imprinted within the iron-rich basaltic rocks of the seafloor.

Supporting evidence for crustal movement came from a variety of other sources in the mid-20th century. The widely separated patterns of similar fossil reptiles and plants found in Australia, India, South Africa, South America, and Antarctica, previously noted by Wegener, were mapped in detail. The fos-sils represented organisms that in each instance were so similar and specialized that they could not have developed without their now-distant locations being either connected or at least much closer together than they are today. When the positions

of the continents were reassembled on a paleomap derived from paleomagnetic data and representing the time when the organisms were living, the fossil locations came together spatially. Other types of ancient environmental evidence, such as left by glaciations, could also be fit together in logical geographical patterns on reconstructed paleomaps of the continents or the world ( ● Fig. 13.24).

How well Earth’s landmasses match up when they are brought together on a paleomap was found to be even better

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● FIGURE 13.22The supercontinent of Pangaea included all of today’s major landmasses joined together. Pangaea later split to make Laurasia in the Northern Hemisphere and Gondwana in the Southern Hemisphere. Further plate motion has produced the continents as they are today.How has continental movement affected the climates of landmasses?

C O N T I N E N T S I N M OT I O N : T H E S E A R C H F O R A U N I F Y I N G T H E O R Y

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● FIGURE 13.23Paleomagnetic properties of rocks that formed when the Northern Hemisphere continents were joined point to the location of magnetic north at that time. It requires rejoining the continents to their original positions, as shown on this map, in order for the magnetic orientations to point to a common magnetic pole.

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arth’s magnetic field encircles the globe with field lines that converge at two opposite magnetic poles. The

geographic North and South Poles do not coincide with their magnetic counterparts, but outside of the polar regions the mag-netic poles are useful for navigation by compass. It is necessary to account for the magnetic declination (see again Figs. 2.26 and 2.27) for directional accuracy.

Earth’s magnetic field has changed over geologic time by increasing and decreasing intensity, and the polarity of the magnetic poles has reversed many times. Before the last reversal, about 700,000 years ago (to what we call nor-mal polarity today), a compass would have pointed to the south. Paleomagne-tism deals with changes in Earth’s mag-netic field through time. Paleomagnetic studies have yielded much evidence to help us understand plate tectonics and assist in reconstructing the shifting geo-graphical positions of landmasses during Earth history.

By studying orientations of magnetic fields in mineral crystals within rocks of varying ages, we know that magnetic pole reversals have occurred. Knowing the age of the rocks by radiometric dating, we can determine their location when they cooled as well as the nature of the magnetic field at that time. Ancient basaltic rocks, which are iron rich, are most commonly used for this research. When basalt solidifies, iron oxide crystals in the rock become magnetized in a way that records several magnetic properties, which are related to Earth’s magnetic field at the time of cooling.

Three important characteristics that these rocks record are polarity (normal, like that of today, or reversed), declina-tion, and inclination, which is measured with a vertically mounted compass needle. Each property provides different evidence about changes in the magnetic field and about how Earth’s paleogeography varied as plate tectonics moved the landmasses.

Numerous measurements of these three paleomagnetic qualities worldwide have given scientists a good picture of Earth’s continually changing paleogeography throughout the last several hundred million years.

• Polarity Seafloor spreading was con-firmed by polarity changes discovered in stripelike patterns of basalts that matched on opposite sides of the spreading center where they formed. Going farther away from the Mid-Atlantic Ridge, the rocks were pro-gressively older, and each stripe had a counterpart of the same age and same magnetic polarity on opposite sides of the ridge. The basaltic seafloor had recorded the polarity history of the magnetic field and the widening of the Atlantic Ocean.

• Declination Declination shows the direction to the magnetic pole. By studying basalts of the same age but on several continents, it is possible to triangulate directions to the magnetic north pole at the time they formed (see Fig. 13.23). The information provided by these paleodeclinations is the orien-tation of ancient landmasses, in other words, whether or not they rotated rela-tive to north as they drifted.

• Inclination The magnetic field sur-rounding Earth causes not only a mag-netic compass needle to point north but also to dip downward in a straight-line direction to north. This is called mag-netic dip, and a needle’s angle off of horizontal approximates its latitudinal location. Paleoinclinations recorded from ancient basalts provide the latitude of their location at the time of cooling.

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G E O G R A P H Y ’ S S P A T I A L S C I E N C E P E R S P E C T I V E

Paleomagnetism: Evidence of Earth’s Ancient Geography

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Earth’s magnetic field, circling the planet, makes a magnetized dip needle point downward at an angle that equals the latitude of the needle’s location. At the equator, the magnetic dip would be 0° (horizontal), and at the magnetic pole the needle would point straight down (90°).


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when using the true continental edges—the continental slopes—which lie a few hundred meters below sea level. In this case, as also had been noted by Wegener, mountain ranges on opposite sides of oceans line up and rock ages and types match where the continents join. Knowledge of the geographical distribution of Earth’s environments relative to latitude and climate zones provided additional insight. Evidence that ancient glaciation occurred simultaneously in India and South Africa while tropi-cal forest climates (represented by coal deposits) existed in the northeastern United States and in Great Britain could only be

P L AT E T E C TO N I C S

explained by the latitudinal movement of landmasses, and their locations came together well on paleogeographic reconstructions.

Plate TectonicsPlate tectonics, the modern theory to explain the movement of continents, suggests that the rigid and brittle outer shell of Earth, that is, the lithosphere (crust and uppermost mantle), is broken into several separate segments called lithospheric plates that rest on, and are carried along with, the flowing plastic asthenosphere( ● Fig. 13.25). Tectonics involves large-scale forces originating within Earth that cause parts of the lithosphere to move around. In plate tectonics, the lithospheric plates move as distinct and discrete units. In some places they pull away from each other (diverge), in other places they push together (converge), and elsewhere they slide alongside each other (move laterally). Seven major plates have proportions as large as or larger than continents or ocean basins. Five other plates are of minor size, although they have maintained their own identity and direction of movement for some time. Sev-eral additional plates are even smaller and exist in active zones at the boundaries between major plates. All major plates consist of both continental and oceanic crust although the largest, the Pacific plate, is primarily oceanic. To understand how plate tectonics op-erates and why plates move, we must consider the scientific evi-dence that was gathered to test this theory. We should also evaluate how well this theory holds up under rigorous examination. The supporting evidence, however, is overwhelming.

Seafloor Spreading and Convection CurrentsIn the 1960s, several keys to plate tectonics theory were found while studying and mapping the ocean floors. First, detailed under-sea mapping was conducted on a system of midoceanic ridges (also called oceanic ridges or rises) that revealed configurations remark-ably similar to the continental coastlines. Second, it was discovered in the Atlantic and Pacific Oceans that basaltic seafloor displayed parallel bands of matching patterns of magnetic properties in rocks of the same age but on opposite sides of midoceanic ridges. Third, scientists made the surprising discovery that although some conti-nental rocks are 3.6 billion years old, rocks on the ocean floor are all geologically young, having been in existence less than 250 million years. Fourth, the oldest rocks of the seafloor lie beneath the deep-est ocean waters or close to the continents, and rocks become pro-gressively younger toward the midoceanic ridges where the young-est basaltic rocks are found ( ● Fig. 13.26). Finally, temperatures of rocks on the ocean floor vary significantly, being hottest near the ridges and becoming progressively cooler farther away.

Only one logical explanation emerged to fit all of this evidence. It became apparent that new oceanic crust is being formed at the midoceanic ridges while older oceanic crust is being destroyed along other margins of ocean basins. The emergence of new oceanic crust is associated with the movement of great sections or plates of the lithosphere away from the midoceanic ridges. This phenomenon, which represents a major advance in our understanding of how

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● FIGURE 13.24A wide variety of paleogeographical evidence indicates the previous loca-tions and distributions of Earth’s landmasses in the geologic past: (a) rocks of ancient mountain ranges; (b) evidence of ancient glaciation.

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● FIGURE 13.25Earth’s solid exterior (the lithosphere) is broken into giant segments called plates. This map shows Earth’s major tectonic plates and their general directions of movement. Most tectonic and volcanic activity occurs along plate boundaries where the large segments separate, collide, or slide past each other. Barbs indicate boundaries where one plate is overriding another, with the barbs on the side of the overriding plate.Does every lithospheric plate include a continent?

● FIGURE 13.26The global oceanic ridge system and the age of the seafloor. Red represents the youngest seafloor, and blue the oldest. Detailed mapping and study of the ocean floors yielded much evidence to support the theory of plate tectonics by identifying the process of seafloor spreading.

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continents move, is called seafloor spreading ( ● Fig. 13.27). The rigid lithospheric plates diverge along the oceanic ridges and separate at an average rate of 2 to 5 centimeters (1–2 in.) per year as they are carried along with the flowing plastic asthenosphere in the mantle. The young age of oceanic crust results from the creation of new ba-saltic rock at undersea ridges and the movement of the seafloor with lithospheric plates toward ocean basin margins where the older rock is remelted and destroyed. As molten basalt cooled and crystallized in the seafloor, the iron minerals that they contain became magnetized in a manner that replicated the orientation of Earth’s magnetic field at that time. The iron-rich basalts of the seafloor have preserved a his-torical record of Earth’s magnetic field, including polarity reversals (times when the north pole became south, and vice versa).

Plate tectonics includes a plausible explanation of the mecha-nism for continental movement, which had eluded Wegener. The mechanism is convection. Hot mantle material travels upward to-ward Earth’s surface and cooler material moves downward as part of huge subcrustal convection cells ( ● Fig. 13.28). Mantle mate-rial rises to the asthenosphere where it spreads laterally and flows in opposite directions, dragging the lithospheric plates with it. Pulling apart the brittle lithosphere breaks open a midoceanic ridge. Mol-ten basalt wells up into the fractures, cooling and sealing them to form new seafloor. In this process, the ocean becomes wider by the width of the now-sealed fracture. The convective motion continues as solidified crustal material moves away from the ridges. In a time frame of up to 250 million years, older oceanic crust is consumed in the deep trenches near plate boundaries where sections of the litho-sphere meet and are recycled into Earth’s interior.

Tectonic Plate MovementThe shifting of tectonic plates relative to one another provides an explanation for many of Earth’s surface features. Plate tectonics theory enables physical geographers to better understand not only our planet’s ancient geography but also the modern global distribu-tions and spatial relationships among such diverse, but often related, phenomena as earthquakes, volcanic activity, zones of crustal move-ment, and major landform features ( ● Fig. 13.29). Let’s briefly ex-amine the three ways in which lithospheric plates relate to one an-other along their boundaries as a result of tectonic movement: by pulling apart, pushing together, or sliding alongside each other.

P L AT E T E C TO N I C S

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● FIGURE 13.27Seafloor spreading at an oceanic ridge produces new seafloor.

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● FIGURE 13.28Convection is the mechanism for plate tectonics. Heat causes convection currents of material in the mantle to rise toward the base of the solid lithosphere where the flow becomes more horizontal. As the astheno-sphere undergoes its slow, lateral flow, the overlying lithospheric plates are carried along because of friction at the boundary between the asthe-nosphere and lithosphere.Why is plate tectonics a better name than continental drift for the lateral movement of Earth’s solid outer shell?

Plate Divergence The pulling apart of plates, tectonic plate divergence, is directly re-lated to seafloor spreading (see again Fig. 13.27). Tectonic forces that act to pull objects apart cause the crust to thin and weaken. Shallow earthquakes are often associated with this crustal stretching, and asthenospheric magma wells up between crustal fractures. This creates new crustal ridges and new ocean floor as the plates move away from each other. The formation of new crust in these spread-ing centers gives the label constructive plate margins to these zones. Occasional “oceanic” volcanoes,



like those of Iceland, the Azores, and Tristan da Cunha, mark such boundaries ( ● Fig. 13.30).

Most plate divergence occurs along oceanic ridges, but this process can also break apart continental crust, eventually reducing the size of the landmasses involved ( ● Fig. 13.31a). The Atlantic Ocean floor formed as the continent that included South America and Africa broke up and moved apart 2 to 4 centime-ters (1–2 in.) per year over millions of years. The Atlantic Ocean continues to grow today at about the same rate. The best mod-ern example of divergence on a continent is the rift valley system of East Africa, stretching from the Red Sea south to Lake Malawi. Crustal blocks that have moved downward with respect to the land on either side, with lakes occupying many of the depressions, characterize the entire system, including the Sinai Peninsula and the Dead Sea. Measurable widening of the Red Sea suggests that it may be the beginning of a future ocean that is forming between Africa and the

Arabian Peninsula, similar to the young Atlantic between Africa and South America about 200 million years ago (Fig. 13.31b).

Plate Convergence A vide variety of crustal activity occurs at areas of tectonic plate convergence. Despite the relatively slow rates of plate movement (in terms of human perception), the in-credible energy involved in convergence causes the crust to crumple as one plate overrides another. The denser plate is forced deep be-low the surface in a process called subduction. Subduction is most common where dense oceanic crust collides with and descends be-neath less dense continental crust ( ● Fig. 13.32). This is the situation along South America’s Pacific coast, where the Nazca plate subducts beneath the South American plate, and in Japan, where the Pacific plate dips under the Eurasian plate. As oceanic crust, and the litho-spheric plate of which it forms a part, is subducted, it descends into the asthenosphere to be melted and recycled into Earth’s interior.

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● FIGURE 13.29Plate tectonic movement. Unlike the other major Earth systems, the plate tectonics system does not obtain its energy from the sun. Instead, movements of the lithosphere result from heat energy derived from Earth’s inte-rior. As lithospheric plates move due to heat-driven convection cells in the mantle, they interact with adjoining plates, forming different boundary types, each displaying distinct landform features. This diagram shows three major plate boundary types: spreading centers, subduction zones, and continental collision zones.

Spreading centers (far left and right of middle on diagram) are divergent plate boundaries. These are con-structional boundaries at which new crustal material emerges along active rift zones. Over time, newer material pushes older rock progressively away from the active rift zone in both directions. Earth’s oceanic divergent plate boundaries form the midoceanic ridge system, which extends through all of the major oceans.

Subduction zones (right side of diagram) occur where two plates converge, with the margin of at least one of them consisting of oceanic crust. This is a destructive type of boundary where crustal material returns to Earth’s interior. The denser oceanic plate is forced by gravity and plate movement to subduct beneath the less dense plate, whether that consists of continental crust or oceanic crust. Surface features common to subduction zones are deep ocean trenches and volcanic mountain ranges or island arcs. The best examples of subduction are found around the Pacific Ring of Fire, such as those along Japan, Chile, New Zealand, and the northwest coast of the United States.

Continental collision zones (middle of diagram) are found where two continental plates collide. Mas-sive mountain building occurs as the crust thickens because of compression. Volcanoes tend to be absent in these regions. The world’s highest mountains, the Himalayas, were formed when the Indian plate collided with Eurasia. The Alps were formed in a similar manner in a collision between the African and Eurasian plates.


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● FIGURE 13.30Iceland represents part of the Mid-Atlantic Ridge where it stands above sea level to form a volcanic island. The “striped” pattern of polarity re-versals documented in the basaltic rocks along the Mid-Atlantic Ridge helped scientists understand the process of seafloor spreading.

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● FIGURE 13.31(a) A continental divergent plate boundary breaks continents into smaller landmasses. (b) The roughly triangular-shaped Sinai Peninsula, flanked by the Red Sea (lower left) to the south, Gulf of Suez (photo center) on the west, and Gulf of Aqaba (lower right) toward the east, illustrates the breakup of a continental landmass. The Red Sea rift and the narrow Gulf of Aqaba are both zones of spreading. The irrigated valley of the Nile River (upper left) in Egypt can be seen heading northward across the desert into the Mediterranean Sea.

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Deep ocean trenches, such as the Peru–Chile trench and the Japanese trench, occur where the crust is dragged downward into the mantle. Frequently, hundreds of meters of sediments that are deposited on the seafloor or along continental margins are carried down into these trenches. At such convergent bound-aries, rocks can be squeezed and contorted between colliding plates, becoming uplifted and greatly deformed or metamor-phosed. These processes have produced many great mountain

ranges, such as the Andes, at convergent plate margins. A sub-ducting plate is heated as it plunges downward into the mantle. Its rocks are melted, and the resulting magma migrates upward into the overriding plate. Where molten rock reaches the sur-face, it forms a series of volcanic peaks, as in the Cascade Range of the northwestern United States. Where two oceanic plates meet, the older, denser one will subduct below the younger, less dense oceanic plate, and volcanoes may develop, creating major island arcs on the overriding plate between the continents and the ocean trenches. The Aleutians, the Kuriles, and the Marianas are all examples of island arcs near oceanic trenches that border the Pacific plate.



As the subducting plate grinds downward, enormous friction is produced, which explains the occurrence of major earthquakes in these regions. Subduction zones are sometimes referred to as Be-nioff zones, after the seismologist Hugo Benioff, who first plotted the position of earthquakes extending downward at a steep angle on the leading edge of a subducting plate (see again Fig. 13.29).

Continental collision causes two continents or major land-masses to fuse or join together, creating a new larger landmass ( ● Fig. 13.33). This process, which closes an ocean basin that once separated the colliding landmasses, has been called continental su-turing. Where two continental masses collide, the result is massive folding and crustal block movement rather than volcanic activity. This crustal thickening generally produces major mountain ranges at sites of continental collision. The Himalayas, the Tibetan Plateau, and other high Eurasian ranges formed in this way as the plate containing the Indian subcontinent collided with Eurasia some 40 million years ago. India is still pushing into Asia today to produce the highest mountains in the world. In a similar fashion, the Alps were formed as the African plate was thrust against the Eurasian plate.

Zones where plates are converging mark locations of major, and some of the tectonically more active, landforms on our planet: huge mountain ranges, chains of volcanoes, and deep ocean trenches. The distinctive spatial arrangement of these features worldwide can best be understood within the framework of plate tectonics.

Transform Movement Lateral sliding along plate boundar-ies, called transform movement, occurs where plates neither pull

Oceanic-continental convergence

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● FIGURE 13.32An oceanic–continental convergent plate boundary where continent and seafloor collide. The west coast of South America is an excellent example of this kind of plate margin. Collision has contributed to the develop-ment of the Andes and a deep ocean trench offshore.

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● FIGURE 13.33Continental collision along a convergent plate boundary fuses two land-masses together. The Himalayas, the world’s highest mountains, were formed when India drifted northward to collide with Asia.

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● FIGURE 13.34Along this lateral plate boundary, marked by the San Andreas Fault in western North America, the Pacific plate moves north-westward relative to the North American plate. Note that north of San Francisco the boundary type changes.What boundary type is found north of San Francisco and what types of surface features indicate this change?

apart nor converge but instead slide past each other as they move in opposite directions. Such a boundary exists along the San Andreas Fault zone in California ( ● Fig. 13.34). Mexico’s Baja peninsula and Southern California are west of the fault on the Pacific plate. San Francisco and other parts of California east of the fault zone are on the North American plate. In the fault zone, the Pacific plate is moving laterally northwestward in relation to the North Ameri-can plate at a rate of about 8 centimeters (3 in.) a year (80 km or about 50 mi per million years). If movement continues at this rate, Los Angeles will lie alongside San Francisco (450 mi northwest) in about 10 million years and eventually pass that city on its way to finally colliding with the Aleutian Islands at a subduction zone.

Another type of lateral plate movement occurs on ocean floors in areas of plate divergence. As plates pull apart, they usually do so along a series of fracture zones that tend to form at right


371G R O W T H O F C O N T I N E N T S

angles to the major zone of plate contact. These crosshatched plate boundaries along which lateral movement takes place are called transform faults. Transform faults, or fracture zones, are common along midoceanic ridges, but examples can also be seen elsewhere, as on the seafloor offshore from the Pacific Northwest coast be-tween the Pacific and Juan de Fuca plates (see again Fig. 13.34). Transform faults are caused as adjacent plates travel at variable rates, causing lateral movement of one plate relative to the other. The most rapid plate motion is on the East Pacific rise where the rate of movement is more than 17 centimeters (5 in.) per year.

Hot Spots in the MantleThe Hawaiian Islands, like many major landform features, owe their existence to processes associated with plate tectonics. As the Pacific plate moves toward the northwest near these islands, it passes over a mass of molten rock in the mantle that does not move with the lithospheric plate. Called hot spots, these almost station-ary molten masses occur in a few other places in both continental and oceanic locations. Melting of the upper mantle and oceanic crust causes undersea eruptions and the outpouring of basaltic lava on the seafloor, eventually constructing a volcanic island. This pro-cess is responsible for building the Hawaiian Islands, as well as the chain of islands and undersea volcanoes that extend for thousands of miles northwest of Hawaii. Today the hot spot causes active vol-canic eruptions on the island of Hawaii. The other islands in the Hawaiian chain came from a similar origin, having formed over the hot spot as well, but these volcanoes have now drifted along

with the Pacific plate away from their magmatic source. Evidence of the plate motion is indicated by the fact that the youngest is-lands of the Hawaiian chain, Hawaii and Maui, are to the south-east, and the older islands, such as Kauai and Oahu, are located to the northwest ( ● Fig. 13.35). A newly forming undersea volcano, named Loihi, is now developing southeast of the island of Hawaii and will someday be the next member of the Hawaiian chain.

Growth of ContinentsThe origin of continents is still being debated. It is clear that the continents tend to have a core area of very old igneous and metamorphic rocks that may represent the deeply eroded roots of ancient mountains. These core regions have been worn down by hundreds of millions of years of erosion to create areas of relatively low relief that are located far from active plate boundaries. As a result, they have a history of tectonic stability over an immense period of time. These ancient crystalline rock areas are called con-tinental shields ( ● Fig. 13.36). The Canadian, Scandinavian, and Siberian shields are outstanding examples. Around the peripheries of the exposed shields, flat-lying, younger sedimentary rocks at the surface indicate the presence of a stable and rigid rock mass below, as in the American Midwest, western Siberia, and much of Africa.

Most Earth scientists consider continents to grow by accre-tion, that is, by adding numerous chunks of crust to the main continent by collision. Western North America grew in this man-ner over the past 200 million years by adding segments of crust,

Kamchatka

Kauai3.8–5.6

Oahu2.3–3.3

Molokai1.3–1.8 Maui

0.8–1.3

Hawaii 0.7 topresent

EmperorSeamounts

Sea level

Hawaii

Oceanic crustHawaiian Islands

Upper mantle

Directionof plate movement

AleutianIslands

Sea level

Alaskancoast

Sea level

Asthenosphere Hotspot

● FIGURE 13.35The Hawaiian Islands were created by a mantle hot spot. A stationary zone of molten material in the mantle has caused volcanoes to form at the same location in the Pacific Ocean for millions of years. As the Pacific plate has drifted to the northwest, each of the Hawaiian Islands has moved with it, away from the active volcanic zone. The islands are progressively older toward the northwest (ages are in millions of years). The hot spot is currently located at the island of Hawaii. It is about 300 kilometers from the island of Hawaii to Honolulu on the island of Oahu.How long did it take the Pacific plate to move Oahu to its current position?



Quaternary

Tertiary

Cretaceous

Jurassic, Triassic

Permian,Carboniferous

Devonian, Silurian

Ordovician, Cambrian

Upper Precambrian(Includes Paleozoic metamorphic rock)

Lower Precambrian(Includes metamorphic and igneous rock)

Formation of earth

SEDIMENTARY ROCKS

2

63

138

240

360

435

570

4600

3800

2500

PR

EC

AM

BR

IAN

PALE

OZO

ICM

ES

OZO

ICC

EN

OZO

IC

Cenozoic, Mesozoic

MILLION YEARS AGO

ROCK AGES

Cenozoic, Mesozoic, Paleozoic

INTRUSIVE IGNEOUS ROCK

Continental shelf

Ice sheet

EXTRUSIVE IGNEOUS ROCK

CanadianShield

● FIGURE 13.36Map of North America showing the continental shield and the general ages of rocks.Going outward from the shield toward the coast, what generally happens to the ages of rocks? What does this suggest about the size of the continent during the time span represented by the rocks along the continental margins?


373

late tectonics explains that the continents are parts of lithospheric plates that act like rafts, moving

along with the slowly flowing astheno-sphere. The solid upper mantle, oceanic crust, and continental crust constitute the lithosphere, which lies on top of the flow-ing asthenosphere. The mantle material in the asthenosphere flows at about 2–5 centimeters (1–2 in.) per year, like a very thick fluid. Gravity does not cause the lithosphere to sink because its material is less dense than that of the asthenosphere.

The principle of buoyancy tells us that an object will sink if its density (mass di-vided by volume) is greater than that of the fluid. The volume of fluid displaced by a floating object will weigh the same as the object. If the object floats, the propor-tion floating above the surface equals the percentage of density difference between it and the fluid. An ice cube having 90% of the density of water floats with 10% of the cube extending above the water surface. As long as the weight of a cargo ship and its load is less than the weight of the water they displace, a balance (equilibrium) will be maintained and the ship will float. If the ship and its cargo become heavier than the volume of water they displace, the ship will sink. Ships float higher when empty and lower when full of cargo.

Isostasy is the term for the equalization of hydrostatic pressure (fluid balance) that affects Earth’s lithosphere and in turn its to-pography. One concept of isostasy suggests that material of the lithosphere exists in a density in equilibrium with the material of the asthenosphere. A column of lithosphere (and the overlying hydrosphere) anywhere on Earth weighs about the same as a col-umn of equal diameter from anywhere else regardless of vertical thickness. The litho-sphere is thicker (taller and deeper) where it contains a higher percentage of low-density materials. The lithosphere is thinner where it contains a higher percentage of high-density materials. Continental crust has a lower density than oceanic crust, which is why it is the thinner, denser oceanic crust that is subducted along ocean trenches.

If an additional load is placed in an area by a massive accumulation of sediments, lake water, or glacial ice, the lithosphere there will subside to a new equilibrium level. If these materials are later removed, the region will tend to rise in a process called isostatic rebound. Neither uplift nor subsidence of the lithosphere will be instantaneous because flow in the asthenosphere is only a few centimeters per year. Imagine a waterbed filled with molasses. If you lie on it, you will sink slowly, because molasses is thicker than water, until you reach a floating equilibrium.

When you get out of the bed, the depression that you made will slowly rise back up as the molasses fills in the space from below.

Isostasy suggests that mountains are made of relatively light crustal materials but exist in areas of very thick crust, while re-gions of low elevation have thin crust. Here the analogy is like that of an iceberg: A tall iceberg requires a massive amount of ice below the surface in order to expose ice so high above sea level, and as ice above the surface melts, ice from below will rise above sea level to replace it until the iceberg has completely melted.

Isostatic balance helps to explain many aspects of Earth’s surface, including the following:

• Why most of the continental crust lies above sea level

• Why wide areas of the seafloor are at a uniform depth

• Why many mountain ranges continue to rise even though erosion removes mate-rial from them

• Why some regions where rivers are de-positing great amounts of sediments are subsiding

• Why the crust subsided in areas that were covered by 2000 to 3000 meters of ice during the last glacial age and now continues to rebound after deglaciation

P

G E O G R A P H Y ’ S P H Y S I C A L S C I E N C E P E R S P E C T I V E

Isostasy: Balancing Earth’s Lithosphere

Oceanic crust MountainContinental crust

Mantle Mountain root

Because continental crust is considerably less dense than the material in the asthenosphere, where continental crust reaches high elevations it also extends far below the surface. Oceanic crust is also less dense than mantle material, but because it is denser than continental crust, it is thinner than continental crust.

G R O W T H O F C O N T I N E N T S

The density of ice is 90% water, thus icebergs (and ice cubes) float with nine tenths of their volume below the surface and 10% above.



known as microplate terranes (a term that should not be con-fused with the term terrain), as it moved westward over the Pacific and former oceanic plates. Paleomagnetic data show that parts of western North America from Alaska to California originated south of the equator and moved to join the continent. Terranes, which have their own distinct geology from that of the continent to which they are now joined, may have originally been offshore island arcs, undersea volcanoes, or islands made of continental fragments, such as New Zealand or Madagascar are today.

PaleogeographyThe study of past geographical environments is known as paleoge-ography. The goal of paleogeography is to try to reconstruct the past environment of a geographical region based on geologic and

climatic evidence. For students of physical geography, it generally seems that the present is complex enough without trying to know what the geography of ancient times was like. However, peering into the past helps us forecast and prepare for changes in the future.

The immensity of geologic time over which major events or processes (such as plate tectonics, ice ages, or the formation and erosion of mountain ranges) have taken place is difficult to picture in a human time frame of days, months, and years. The geologic timescale is a calendar of Earth history (Table 13.2). It is divided into eras, which are typically long units of time, such as the Mesozoic Era (which means “middle life”), and eras are divided into periods, such as the Cretaceous Period. Epochs, as for example the Pleistocene Epoch (recent ice ages), are shorter time units and are used to subdivide the periods of the Cenozoic Era (“recent life”), for which geologic evidence is more abundant. Today we are in the Holocene Epoch (last 10,000 years), of the

Pre

cam

bria

n

Pal

eozo

icM

esoz

oic

Cen

ozoi

c

Ter

tiary

Pal

eoge

neN

eoge

ne

Pha

nero

zoic

Period Epoch

Holocene (or Recent)

PleistoceneQuaternary

Pliocene

Miocene

Oligocene

Eocene

Cretaceous

Jurassic

Triassic

Permian

Devonian

Silurian

Ordovician

Cambrian

Proterozoic Eon

Archean Eon

Hadeon Eon

Pennnsylvanian

MIssissippian

Paleocene

EraEon

Ice Age ends

Ice Age beginsEarliest humans

Formation of HimalayasFormation of Alps

Extinction of dinosaursFormation of Rocky MountainsFirst birdsFormation of Sierra NevadaFirst mammalsBreakup of PangaeaFirst dinosaursFormation of PangaeaFormation of Appalachian Mountains

0.01

Millions of years ago

Major Geologic and Biologic Events

Abundant coal-forming swamps

First reptiles

First amphibians

First land plants

First fish

Earliest shelled animals

Earliest fossil record of life

1.6

5

24

34

56

65

144

206

248

290

323

354

417

443

490

543

2,500

3,800

~4,650

TABLE 13.2Geologic Timescale


375PA L E O G E O G R A P H Y

Quaternary Period (last 1.6 million years), of the Cenozoic Era (last 65 million years). In a sense, these divisions are used like we would use days, months, and years to record time.

If we took a 24-hour day to represent the approximately 4.6 billion-year history of Earth, the Precambrian, an era of which we know very little, would consume the first 21 hours. The cur-rent period, the Quaternary, which has lasted about 1.6 million years, would take less than 30 seconds, and human beginnings, over about the last 4 million years, about 1 minute.

Each era, period, and epoch in Earth’s geologic history had a unique paleogeography with its own distribution of land and sea, climate regions, plants, and animal life. If we look at evidence for the paleogeography of the Mesozoic Era (245 million to 65 million years ago), for instance, we would find a much dif-ferent physical geography than exists now. This was a time when the supercontinents, Gondwana and Laurasia, each gradually split apart as new ocean floors widened, creating the continents that are familiar to us today. Global and local Mesozoic climates were very different from those of today but were changing as North America drifted to the northwest. During the Cretaceous Period,

much of the present United States experienced warmer climates than now. Ferns and conifer forests were common. The Mesozoic was the “age of the dinosaurs,” a class of large animals that ruled the land and the sea. Other life also thrived, including marine plants and invertebrates, insects, mammals, and the earliest birds.

The Mesozoic Era ended with an episode of great extinctions, including the end of the dinosaurs. Geologists, paleontologists,and paleogeographers are not in agreement as to what caused these great extinctions. Some of the strongest evidence points to a large meteorite striking Earth 65 million years ago, disrupting global climate and causing global environmental change. Other evidence points to plate tectonic changes in the distribution of oceans and continents or increased volcanic activity, either of which could cause rapid climate changes that might possibly trigger mass extinctions.

Our maps of Earth in early geologic times show only approximate and generalized patterns of mountains, plains, coasts, and oceans, with the addition of some environmental character-istics. These maps portray a general picture of how global geogra-phy has changed through geologic time ( ● Fig. 13.37). Much of

30

30

Pangaea

LateTriassic~180 Ma

LateJurassic~135 Ma

Mid-ocean ridge

Ma = mega-annum, indicating millions of years ago

Island arc trench

Triassic~210 Ma

Late Cretaceous~65 Ma

Present

60

60

Stage 1

Stage 2

Stage 3

Stage 4

Stage 5

● FIGURE 13.37Paleomaps showing Earth’s tectonic history over the last 250 million years of geologic time. A preponderance of evidence from paleomagnetism, ages and distributions of rocks and fossils, patterns of earthquakes and volca-noes, configurations of landmasses and mountain ranges, and studies of the ocean floor supports plate tecton-ics. These lines of evidence make it possible to produce a generalized historic sequence of how Earth’s global geography has changed over that time frame.How has the environment at the location where you live changed through geologic time?



Chapter 13 ActivitiesDefine & Recall

seismic wavesseismographcoreinner coreouter coremantleplastic solidelastic solidlithosphere (as an element of planetary structure)asthenospheretectonic forcesMohovicic discontinuity (Moho)crustoceanic crustcontinental crustmineralrocksilicatebedrockregolithoutcrop

igneous rockmagmalavaextrusive igneous rockpyroclasticsintrusive igneous rockplutonic rockjointcolumnar jointsedimentary rocksclastsclastic sedimentary rockorganic sedimentary rockchemical sedimentary rockstratificationstratabedding planesunconformitycross beddingmetamorphic rockfoliationsrock cycle

catastrophismuniformitarianismcontinental driftPangaeapaleomagnetismplate tectonicslithospheric plateseafloor spreadingpolarity reversalconvectionplate divergenceplate convergencesubductionisland arccontinental collisiontransform movementhot spotcontinental shieldaccretionmicroplate terranepaleogeography

1. Identify the major zones of Earth’s interior from the center to the surface. How do these zones differ from one another?

2. Define and distinguish continental crust and oceanic crust. Define and distinguish the lithosphere from the astheno-sphere.

3. List the eight most common elements in Earth’s crust. What is a mineral? What is a rock?

4. Descr ibe the three major categor ies of rock and the principal means by which each is formed. Give an example of each.

5. What is the rock cycle?

6. What evidence did Wegener rely on in the formulation of his theory of continental drift? What evidence did he lack? What evidence has since been found to support the theory that landmasses at Earth’s surface move around?

7. What type of lithospheric plate boundary is found parallel-ing the Andes, at the San Andreas Fault, in Iceland, and near the Himalayas?

8. How does the formation of the Hawaiian Islands support plate tectonic theory?

9. Define paleogeography. Why are geographers interested in this topic?

Discuss & Review

the evidence and the rocks that bear this information have been lost through metamorphism or erosion, buried under younger sediments or lava flows, or recycled into Earth’s interior. The further back in time, the sketchier is the paleoenvironmental in-formation presented on the map. Paleomaps, like other maps, are simplified models of the regions and times they represent.

As time passes and additional evidence is collected, paleo-geographers may be able to fill in more of the empty spaces on those maps of the past that are so unfamiliar to us. These paleo-

geographic studies aim not only at understanding the past but also at understanding today’s environments and physical landscapes, how they have developed, and how processes act to change them. By applying the concept of uniformitarianism and the theory of plate tectonics to our knowledge of how the Earth system and its subsystems function, we can gain a better understanding of our planet’s geologic past, as well as its present, and this will facilitate better forecasts of its potential future.


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Continents in Motion: The Search for a Unifying Theory · 2009-11-05 · 362 CHAPTER 13 † EARTH STRUCTURE, EARTH MATERIALS, AND PLATE TECTONICS Continents in Motion: The Search