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Plate Tectonics

PLATE TECTONICS

CONCEPT

The earth beneath our feet is not dead; it is constantly moving, driven by forces deep in its core. Nor is the planet's crust all of one piece; it is composed of numerous plates, which are moving steadily in relation to one another. This movement is responsible for all manner of phenomena, including earthquakes, volcanoes, and the formation of mountains. All these ideas, and many more, are encompassed in the concept of plate tectonics, which is the name for a branch of geologic and geophysical study and for a powerful theory that unites a vast array of ideas. Plate tectonics works hand in hand with several other striking concepts and discoveries, including continental drift and the many changes in Earth's magnetic field that have taken place over its history. No wonder, then, that this idea, developed in the 1960s but based on years of research that preceded that era, is described as "the unifying theory of geology."

HOW IT WORKS

Tectonics and Tectonism

The lithosphere is the upper layer of Earth's interior, including the crust and the brittle portion at the top of the mantle. Tectonism is the deformation of the lithosphere, and the term tectonics refers to the study of this deformation, including its causes and effects, most notably mountain building. This deformation is the result of the release and redistribution of energy from Earth's core.

The interior of Earth itself is divided into three major sections: the crust, mantle, and core. The first is the uppermost division of the solid earth, representing less than 1% of its volume and varying in depth from 3 mi. to 37 mi. (5-60 km). Below the crust is the mantle, a thick, dense layer of rock approximately 1,429 mi. (2,300 km) thick. The core itself is even more dense, as illustrated by the fact that it constitutes about 16% of the planet's volume and 32% of its mass. Composed primarily of iron and another, lighter element (possibly sulfur), it is divided between a solid inner core with a radius of about 760 mi. (1,220 km) and a liquid outer core about 1,750 mi. (2,820 km) thick.

Tectonism results from the release and redistribution of energy from Earth's interior. There are two components of this energy: gravity, a function of the enormous mass at the core, and heat from radioactive decay. (For more about gravity, see Gravity and Geodesy. The heat from Earth's core, the source of geothermal energy, is discussed in Energy and Earth.) Differences in mass and heat within the planet's interior, known as pressure gradients, result in the deformation of rocks.

DEFORMATION OF ROCKS.

Any attempt to deform an object is referred to as stress, and stress takes many forms, including tension, compression, and shear. Tension acts to stretch a material, whereas compressiona type of stress produced by the action of equal and opposite forces, whose effect is to reduce the length of a materialhas the opposite result. (Compression is a form of pressure.) As for shear, this is a kind of stress resulting from equal and opposite forces that do not act along the same line. If a thick, hardbound book is lying flat and one pushes the front cover from the side so that the covers and pages are no longer perfectly aligned, this is an example of shear.

Under the effects of these stresses, rocks may bend, warp, slide, or break. They may even flow, as though they were liquids, or melt and thus truly become liquid. As a result, Earth's interior may manifest faults, or fractures in rocks, as well as folds, or bends in the rock structure. The effects of this activity can be seen on the surface in the form of subsidence, which is a depression in the crust, or uplift, which is the raising of crustal materials. Earthquakes and volcanic eruptions also may result.

There are two basic types of tectonism: orogenesis and epeirogenesis. Orogenesis is taken from the Greek words oros ("mountain") and genesis ("origin") and involves the formation of mountain ranges by means of folding, faulting, and volcanic activity. The Greek word epeiros means "mainland," and epeirogenesis takes the form of either uplift or subsidence. Of principal concern in the theory of plate tectonics, as we shall see, is orogenesis, which involves more lateral, as opposed to vertical, movement.

Continental Drift

If one studies a world map for a period of time, one may notice something interesting about the shape of Africa's west coast and that of South America's east coast: they seem to fit together like pieces of a jigsaw puzzle. Early in the twentieth century, two American geologists, Frank Bursley Taylor (1860-1938) and Howard Baker, were among the first scientists to point out this fact. According to Taylor and Baker, Europe, the Americas, and Africa all had been joined at one time. This was an early version of continental drift, a theory concerning the movement of Earth's continents.

Continental drift is based on the idea that the configuration of continents was once different than it is today, that some of the individual landmasses of today once were joined in other continental forms, and that the landmasses later moved to their present locations. Though Taylor and Baker were early proponents, the theory is associated most closely with the German geophysicist and meteorologist Alfred Wegener (1880-1930), who made the case for continental drift in The Origin of Continents and Oceans (1915).

PANGAEA, LAURASIA, AND GONDWANALAND.

According to Wegener, the continents of today once formed a single supercontinent called Pangaea, from the Greek words pan ("all") and gaea ("Earth"). Eventually, Pangaea split into two halves, with the northern continent of Laurasia and the southern continent of Gondwanaland, sometimes called Gondwana, separated by the Tethys Sea. In time, Laurasia split to form North America, the Eurasian land-mass with the exception of the Indian subcontinent, and Greenland. Gondwanaland also split, forming the major southern landmasses of the world: Africa, South America, Antarctica, Australia, and India.

The Austrian geologist Eduard Suess (1831-1914) and the South African geologist Alexander du Toit (1878-1948), each of whom contributed significantly to continental drift theory, were responsible for the naming of Gondwanaland and Laurasia, respectively. Suess preceded Wegener by many years with his theory of Gondwanaland, named after the Gondwana region of southern India. There he found examples of a fern that, in fossilized form, had been found in all the modern-day constituents of the proposed former continent. Du Toit, Wegener's contemporary, was influenced by continental drift theory and improved on it greatly.

FORMATION OF THE CONTINENTS.

Today continental drift theory is accepted widely, in large part owing to the development of plate tectonics, "the unifying theory in geology." We examine the evidence for continental drift, the arguments against it, and the eventual triumph of plate tectonics in the course of this essay. Before going on, however, let us consider briefly the now-accepted timeline of events described by Wegener and others.

About 1,100 million years ago (earth scientists typically abbreviate this by using the notation 1,100 Ma), there was a supercontinent named Rodinia, which predated Pangaea. It split into Laurasia and Gondwanaland, which moved to the northern and southern extremes of the planet, respectively. Starting at about 514 Ma, Laurasia drifted southward until it crashed into Gondwanaland about 425 Ma. Pangaea, surrounded by a vast ocean called Panthalassa ("All Ocean"), formed approximately 356 Ma.

In the course of Pangaea's formation, what is now North America smashed into northwestern Africa, forming a vast mountain range. Traces of these mountains still can be found on a belt stretching from the southern United States to northern Europe, including the Appalachians. As Pangaea drifted northward and smashed into the ocean floor of Panthalassa, it formed a series of mountain ranges from Alaska to southern South America, including the Rockies and Andes. By about 200 Ma, Pangaea began to break apart, forming a valley that became the Atlantic Ocean. But the separation of the continents was not a "neat" process: today a piece of Gondwanaland lies sunken beneath the eastern United States, far from the other landmasses to which it once was joined.

By about 152 Ma, in the late Jurassic period, the continents as we know them today began to take shape. By about 65 Ma, all the present continents and oceans had been formed for the most part, and India was drifting north, eventually smashing into southern Asia to shape the world's tallest mountains, the Himalayas, the Karakoram Range, and the Hindu Kush. This process is not finished, however, and geologists believe that some 250-300 million years from now, Pangaea will re-form.

EVIDENCE AND ARGUMENTS.

As proof of his theory, Wegener cited a wide variety of examples, including the apparent fit between the coastlines of South America and western Africa as well as that of North America and northwestern Africa. He also noted the existence of rocks apparently gouged by glaciers in southern Africa, South America, and India, far from modern-day glacial activity. Fossils in South America matched those in Africa and Australia, as Suess had observed. There were also signs that mountain ranges continued between continentsnot only those apparently linking North America and Europe but also ranges that seemed to extend from Argentina to South Africa and Australia.

By measurements conducted over a period of years, Wegener even showed that Greenland was drifting slowly away from Europe, yet his theory met with scorn from the geoscience community of his day. If continents could plow through oceanic rock, some geologists maintained, then they would force up mountains so high that Earth would become imbalanced. As for his claim that matching fossils in widely separated regions confirmed his theory of continental drift, geologists claimed that this could be explained by the existence of land bridges, now sunken, that once had linked those areas. The apparent fit between present-day landmasses could be explained away as coincidence or perhaps as evidence that Earth simply was expanding, with the continents moving away from one another as the planet grew.

Introduction to Plate Tectonics

Though Wegener was right, as it turned out, his theory had one major shortcoming: it provided no explanation of exactly how continental drift had occurred. Even if geologists had accepted his claim that the continents are moving, it raised more questions than answers. A continent is a very large thing simply to float away; even an aircraft carrier, which is many millions of times lighter, has to weigh less than the water it displaces, or it would sink like a stone. In any case, Wegener never claimed that continents floated. How, then, did they move?

The answer is plate tectonics, the name both of a theory and of a specialization of tectonics. As an area of study, plate tectonics deals with the large features of the lithosphere and the forces that fashion them. As a theory, it explains the processes that have shaped Earth in terms of plates (large movable segments of the lithosphere) and their movement. Plate tectonics theory brings together aspects of continental drift, seafloor spreading (discussed later), seismic and volcanic activity, and the structures of Earth's crust to provide a unifying model of Earth's evolution.

It is hard to overemphasize the importance of plate tectonics in the modern earth sciences; hence, its characterization as the "unifying theory." Its significance is demonstrated by its inclusion in the book The Five Biggest Ideas of Science, cited in the bibliography for this essay. Alongside plate tectonics theory in that volume are four towering concepts of extraordinary intellectual power: the atomic model, or the concept that matter is made up of atoms; the periodic law, which explains the chemical elements; big bang theory, astronomers' explanation of the origins of the universe (see Planetary Science); and the theory of evolution in the biological sciences.

THE PIECES COME TOGETHER.

In 1962 the United States geologist Harry Hammond Hess (1906-1969) introduced a new concept that would prove pivotal to the theory of plate tectonics: seafloor spreading, the idea that seafloors crack open along the crest of mid-ocean ridges and that new seafloor forms in those areas. (Another American geologist, Robert S. Dietz [1914-1995], had published his own theory of seafloor spreading a year before Hess's, but Hess apparently developed his ideas first.) According to Hess, a new floor forms when molten rock called magma rises up from the asthenosphere, a region of extremely high pressure underlying the lithosphere, where rocks are deformed by enormous stresses. The magma wells up through a crack in a ridge, runs down the sides, and solidifies to form a new floor.

Three years later, the Canadian geologist John Tuzo Wilson (1908-1993) coined the term plates to describe the pieces that make up Earth's rigid surface. Separated either by the mid-ocean rifts identified earlier by Heezen or by mountain chains, the plates move with respect to one another. Wilson presented a model for their behavior and established a global pattern of faults, a sort of map depicting the movable plates. The pieces of a new theory were forming (an apt metaphor in this instance!), but as yet it had no name.

That name appeared in 1967, when D. P. Mackenzie of England and R. L. Parker of the United States introduced the term plate tectonics. They maintained that the surface of Earth is divided into six major as well as seven minor movable plates and compared the continents to enormous icebergsmuch as Wegener had described them half a century earlier. Subsequent geologic research has indicated that there may be as many as nine major plates and as many as 12 minor ones.

To test these emerging ideas, the U.S. National Science Foundation authorized a research voyage by the vessel Glomar Challenger in 1968. On their first cruise, through the Gulf of Mexico and the Atlantic, the Challenger 's scientific team collected sediment, fossil, and crust samples that confirmed the basics of seafloor spreading theory. These results led to new questions regarding the reactions between rocks and the heated water surrounding them, spawning new research and necessitating additional voyages. In the years that followed, the Challenger made more and more cruises, its scientific teams collecting a wealth of evidence for the emerging theory of plate tectonics.

REAL-LIFE APPLICATIONS

Early Evidence of Plate Tectonics

No single person has been as central to plate tectonics as Wegener was to continental drift or as the English naturalist Charles Darwin (1809-1882) was to evolution. The roots of plate tectonics lie partly in the observations of Wegener and other proponents of continental drift as well as in several discoveries and observations that began to gather force in the third quarter of the twentieth century.

During World War II, submarine warfare necessitated the development of new navigational technology known as sonar (SO und N avigation A nd R anging). Sonar functions much like radar (see Remote Sensing), but instead of using electromagnetic waves, it utilizes ultrasonic, or high-frequency, sound waves projected through water. Sonar made it feasible for geologists to study deep ocean basins after the war, making it possible for the first time in history to map and take samples from large areas beneath the seas. These findings raised many questions, particularly concerning the vast elevation differences beneath the seas.

EWING AND THE MOUNTAINS UNDER THE OCEAN.

One of the first earth scientists to notice the curious aspects of underwater geology was the American geologist William Maurice Ewing (1906-1974), who began his work long before the war. He had gained his first experience in a very practical way during the 1920s, as a doctoral student putting himself through school. Working summers with oil exploration teams in the Gulf of Mexico off the coast of Texas had given him a basic understanding of the subject, and in the following decade he went to work exploring the structure of the Atlantic continental shelf and ocean basins.

His work there revealed extremely thick sediments covering what appeared to be high mountainous regions. These findings sharply contradicted earlier ideas about the ocean floor, which depicted it as a flat, featureless plain rather like the sandy-bottomed beaches found in resort areas. Instead, the topography at the bottom of the ocean turned out to be at least as diverse as that of the land above sea level.

HEEZEN AND THE RIFT VALLEY.

During the 1950s, a team led by another American geologist, Bruce Charles Heezen (1924-1977), worked on developing an overall picture of the ocean basin's topography. Earlier work had identified a mountain range running the length of the Atlantic, but Heezen's team discovered a deep valley down the middle of the chain, running parallel to it. They described it as a rift valley, a long trough bounded by two or more faults, and compared it to a similar valley in eastern Africa.

Around the same time, a group of transatlantic telephone companies asked Heezen to locate areas of possible seismic or earthquake activity in the Atlantic. Phone company officials reasoned that if they could find the areas most likely to experience seismic activity, they could avoid placing their cables in those areas. As it turned out, earthquakes tended to occur in exactly the same region that Heezen and his team had identified as the rift valley.

The Plates and Their Interactions

The most significant plates that make up Earth's surface are as follows:

Selected Major Plates

  • North American (almost all of North America and Mexico, along with Greenland and the northwestern quadrant of the Atlantic)
  • South American (all of South America and the southwestern quadrant of the Atlantic)
  • African (Africa, the southeastern Atlantic, and part of the Indian Ocean)
  • Eurasian (Europe and Asia, excluding the Indian subcontinent, along with surrounding ocean areas)
  • Indo-Australian (India, much of the Indian Ocean, Australia, and parts of the Indonesian archipelago and New Zealand)
  • Antarctic (Antarctica and the Antarctic Ocean)

In addition to these plates, there are several plates that while they are designated as "major" are much smaller: the Philippine, Arabian, Caribbean, Nazca (off the west coast of South America), Cocos (off the west coast of Mexico), and Juan de Fuca (extreme western North America). Japan, one of the most earthquake-prone nations in the world, lies at the nexus of the Philippine, Eurasian, and Pacific plates.

MOVEMENT OF THE PLATES.

One of the key principles of geology, discussed elsewhere in this book, is uniformitarianism: the idea that processes occurring now also occurred in the past. The reverse usually is also true; thus, as we have noted, the plates are still moving, just as they have done for millions of years. Thanks to satellite remote sensing, geologists are able to measure this rate of movement. (See Remote Sensing for more on this subject.) Not surprisingly, its pace befits the timescale of geologic, as opposed to human, processes: the fastest-moving plates are careening forward at a breathtaking speed of 4 in. (10 cm) per year. The ground beneath Americans' feet (assuming they live in the continental United States, east of the Juan de Fuca) is drifting at the rate of 1.2 in. (3 cm) every year, which means that in a hundred years it will have shifted 10 ft. (3 m).

WHEN PLATES INTERACT.

Plates interact by moving toward each other (convergence), away from each other (divergence), or past each other (transform motion). Convergence usually is associated with subduction, meaning that one plate is forced down into the mantle and eventually undergoes partial melting. This typically occurs in the ocean, creating a depression known as an oceanic trench. Divergence results in the separation of plates and most often is associated either with seafloor spreading or the formation of rift valleys.

There are three types of plate margins, or boundaries between plates, depending on the two types of crusts that are interacting: oceanic with oceanic, continental with continental, or continental with oceanic. The rift valleys of the Atlantic are an example of an oceanic margin where divergence has occurred, while oceanic convergence is illustrated by a striking example in the Pacific. There, subduction of the Philippine Plate by the Pacific Plate has created the Mariana Trench, which at 36,198 ft. (10,911 m) is the deepest depression on Earth.

When continental plates converge, neither plate subducts; rather, they struggle against each other like two warriors in a fight to the death, buckling, folding, and faulting to create huge mountain ranges. The convergence of the Indo-Australian and Eurasian plates has created the highest spots on Earth, in the Himalayas, where Mount Everest (on the Nepal-Indian border) rises to 29,028 ft. (8,848 m). Continental plates also may experience divergence, resulting in the formation of seas. An example is the Red Sea, formed by the divergence of the African and Arabian plates.

Given these facts about the interactions of oceanic and continental plates with each other, what occurs when continental plates meet oceanic ones is no surprise. In this situation, the oceanic plate meeting the continental plate is like a high-school football player squaring off against a National Football League pro tackle. It is no match: the oceanic plate easily subducts. This leads to the formation of a chain of volcanoes along the continental crust, examples being the Cascade Range in the U.S. Pacific Northwest (Juan de Fuca and Pacific plates) or the Andes (South American and Nazca plates).

Transform margins may occur with any combination of oceanic or continental plates and result in the formation of faults and earthquake zones. Where the North American Plate slides against the Pacific Plate along the California coast, it has formed the San Andreas Fault, the source of numerous earthquakes, such as the dramatic San Francisco quakes of 1906 and 1989 and the Los Angeles quake of 1994.

Paleomagnetism

As noted earlier, plate tectonics brings together numerous areas of study in the geologic sciences that developed independently but which came to be seen as having similar roots and explanations. Among these disciplines is paleomagnetism, an area of historical geology devoted to studying the direction and intensity of magnetic fields in the past, as discerned from the residual magnetization of rocks.

Earth has a complex magnetic field whose principal source appears to be the molten iron of the outer core. In fact, the entire planet is like a giant bar magnet, with a north pole and a south pole. It is for this reason that the magnetized material in a compass points north; however, Earth's magnetic north pole is not the same as its geographic north pole. It so happens that magnetic north lies in more or less the same direction as geographic north, but as geologists in the mid-nineteenth century discovered, this has not always been the case. (For more about magnetic north and other specifics of Earth's magnetic field, see Geomagnetism.)

In 1849 the French physicist Achilles Delesse (1817-1881) observed that magnetic minerals tend to line up with the planet's magnetic field, pointing north as though they were compass needles. Nearly 60 years later, however, another French physicist, Bernard Brunhes (1867-1910), noted that in some rocks magnetic materials point south. This suggested one of two possibilities: either the planet's magnetic field had reversed itself over time, or the ground containing the magnetized rocks had moved. Both explanations must have seemed far-fetched at the time, but as it turned out, both are correct.

Earth's magnetic field has shifted, meaning that the magnetic north and south poles have changed places many times over the eons. In addition, the magnetic poles have wandered around the southern and northern portions of the globe: for instance, whereas magnetic north today lies in the frozen islands to the north of Canada, at about 300 Ma it was located in eastern Siberia. The movement of magnetic rocks on Earth's surface, however, has turned out to be too great to be explained either by magnetic shifts or by regional wandering of the poles. This is where plate tectonics and paleomagnetism come together.

CONFIRMATION OF PLATE TECTONIC THEORY.

Rocks in Alaska have magnetic materials aligned in such a way that they once must have been at or near the equator. In addition, the orientation of magnetic materials on South America's east coast shows an affinity with that of similar materials on the west coast of Africa. In both cases, continental drift, with its driving mechanism of plate tectonics, seems the only reasonable explanation.

Thus, paleomagnetic studies have served to confirm the ideas of continental drift and plate tectonics, while research conducted at sea bolsters seafloor spreading theory. Using devices called magnetometers, geologists have found that the orientation of magnetic minerals on one side of a rift mirrors that of materials on the other side. This suggests that the new rock on either side of the rift was formed simultaneously, as seafloor spreading theory indicates.

Earthquakes and Volcanoes

Several findings relating to earthquakes and volcanic activity also can be explained by plate tectonics. If one follows news stories of earthquakes, one may begin to wonder why such places as California or Japan have so many quakes, whereas the northeastern United States or western Europe have so few. The fact is that earthquakes occur along belts, and the vast majority of these belts coincide with the boundaries between Earth's major tectonic plates.

The same is true of volcanoes, and it is no mistake that places famous for earthquakesthe Philippines, say, or Italyoften also are known for their volcanoes. Although they are located near the center of the Pacific Plate, the islands of Hawaii are subject to plate movement, which has helped generate the volcanoes that gave those islands their origin. At the southern end of the island chain, many volcanoes are still active, while those at the northern end tend to be dormant. The reason is that the Pacific Plate as a whole is moving northward over a stationary lava source in the mantle below Hawaii. The southern islands remain poised above that source, while the northern islands have moved away from it.

The Oceanic and Continental Crusts

Given what we have seen about continental drift and seafloor spreading, it should come as no surprise to learn that, generally speaking, the deeper one goes in the ocean, the newer the crust. Specifically, the crust is youngest near the center of ocean basins and particularly along mid-ocean ridges, or submarine mountain ridges where new seafloor is created by seafloor spreading.

It also should not be surprising to learn that oceanic and continental crusts differ both in thickness and in composition. Basalt, an igneous rock (rock formed from the cooling of magma), makes up the preponderance of ocean crust, whereas much of the continental crust is made up of granite, another variety of igneous rock. Whereas the ocean crust is thin, generally 3-6 mi. (5-10 km) in depth, the continental crust ranges in thickness from 12.5-55 mi. (20-90 km). This results in a difference in thickness for the lithosphere, which is only about 60 mi. (100 km) thick beneath the oceans but about 2.5 times as thick150 mi. (250 km)under the continents.

WHERE TO LEARN MORE

Erickson, Jon. Plate Tectonics: Unraveling the Mysteries of the Earth. New York: Facts on File, 1992.

Gallant, Roy A. Dance of the Continents. New York: Benchmark Books, 2000.

Geology: Plate Tectonics (Web site). <http://www.ucmp.berkeley.edu/geology/tectonics.html>.

Kious, W. Jacquelyne, and Robert I. Tilling. This Dynamic Earth: The Story of Plate Tectonics. U.S. Geological Survey (Web site). <http://pubs.usgs.gov/publications/text/dynamic.html>.

Miller, Russell. Continents in Collision. Alexandria, VA: Time-Life Books, 1987.

Plate Tectonics (Web site). <http://www.platetectonics.com/>.

Plate Tectonics (Web site). <http://observe.ivv.nasa.gov/nasa/earth/tectonics/Tectonics1.html>.

Plate Tectonics, the Cause of Earthquakes (Web site). <http://www.seismo.unr.edu/ftp/pub/louie/class/100/plate-tectonics.html>.

Silverstein, Alvin, Virginia B. Silverstein, and Laura Silverstein Nunn. Plate Tectonics. Brookfield, CT: Twenty-First Century Books, 1998.

Wynn, Charles M., Arthur W. Wiggins, and Sidney Harris. The Five Biggest Ideas in Science. New York: John Wiley and Sons, 1997.

KEY TERMS

ASTHENOSPHERE:

A region of extremely high pressure underlying the lithosphere, where rocks are deformed by enormous stresses. The asthenosphere liesat a depth of about 60-215 mi. (about100-350 km).

COMPRESSION:

A form of stress produced by the action of equal and opposite forces, the effect of which is to reduce the length of a material. Compression is a form of pressure.

CONTINENTAL DRIFT:

The theory that the configuration of Earth's continents was once different than it is today; that some of the individual landmasses of today once were joined in other continental forms; and that these landmasses later separated and moved to their present locations.

CONVERGENCE:

A tectonic process whereby plates move toward each other. Usually associated with subduction, convergence typically occurs in the ocean, creating an oceanic trench. It is one of the three ways, along with divergence and transform motion, that plates interact.

CORE:

The center of Earth, an area constituting about 16% of the planet's volume and 32% of its mass. Made primarily of iron and another, lighter element (possibly sulfur), it is divided between a solid inner core with a radius of about 760 mi.(1,220 km) and a liquid outer core about1,750 mi. (2,820 km) thick.

CRUST:

The uppermost division of the solid earth, representing less than 1% of its volume and varying in depth from 3-37 mi.(5-60 km). Below the crust is the mantle.

DIVERGENCE:

A tectonic process whereby plates move away from each other. Divergence results in the separation of plates and is associated most often either with seafloor spreading or with the formation of rift valleys. It is one of the three ways, along with convergence and transform motion, that plates interact.

EPEIROGENESIS:

One of two principal forms of tectonism, the other beingorogenesis. Derived from the Greek words epeiros ("mainland") and genesis ("origins"), epeirogenesis takes the form of either uplift or subsidence.

FAULT:

An area of fracturing between rocks resulting from stress.

FOLD:

An area of rock that has been bent by stress.

GEOPHYSICS:

A branch of the earth sciences that combines aspects of geology and physics. Geophysics addresses the planet's physical processes as well as its gravitational, magnetic, and electric properties, and the means by which energy is transmitted through its interior.

HISTORICAL GEOLOGY:

The study of Earth's physical history. Historical geology is one of two principal branches of geology, the other being physical geology.

LITHOSPHERE:

The upper layer of Earth's interior, including the crust and the brittle portion at the top of the mantle.

MA:

An abbreviation used by earth scientists, meaning "million years." When an event is designated as, for instance, 160 Ma, it means that it happened 160 million years ago.

MANTLE:

The thick, dense layer of rock, approximately 1,429 mi. (2,300 km) thick, between Earth's crust and its core.

MID-OCEAN RIDGES:

Sub marine mountain ridges where new seafloor is created by seafloor spreading.

OCEANIC TRENCH:

A deep depression in the ocean floor caused by the convergence of plates and the resulting subduction of one plate.

OROGENESIS:

One of two principal forms of tectonism, the other being epeirogenesis. Derived from the Greek words oros ("mountain") and genesis ("origin"), orogenesis involves the formation of mountain ranges by means of folding, faulting, and volcanic activity. The processes of oro-genesis play a major role in plate tectonics.

PALEOMAGNETISM:

An area of historical geology devoted to studying the direction and intensity of magnetic fields in the past, as discerned from the residual magnetization of rocks.

PLATE MARGINS:

Boundaries between plates.

PLATE TECTONICS:

The name both of a theory and of a specialization of tectonics. As an area of study, plate tectonics deals with the large features of the lithosphere and the forces that fashion them. As a theory, it explains the processes that have shaped Earth in terms of plates and their movement. Plate tectonics theory brings together aspects of continental drift, seafloor spreading, seismic and volcanic activity, and the structures of Earth's crust to provide a unifying model of Earth's evolution. It is one of the dominant concepts in the modern earth sciences.

PLATES:

Large movable segments of the lithosphere.

RADIOACTIVITY:

A term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles (positively charged helium nuclei), beta particles (either electrons or subatomic particles called positrons ), or gamma rays, which occupy the highest energy level in the electromagnetic spectrum.

REMOTE SENSING:

The gathering of data without actual contact with the materials or objects being studied.

RIFT:

A split between two bodies (for example, two plates) that once were joined.

RIFT VALLEY:

A long trough bounded by two or more faults.

SEAFLOOR SPREADING:

The theory that seafloors crack open along thecrests of mid-ocean ridges and that new seafloor forms in those areas.

SHEAR:

A form of stress resulting from equal and opposite forces that do not act along the same line. If a thick, hard-bound book is lying flat and one pushes the front cover from the side so that the covers and pages are no longer perfectly aligned, this is an example of shear.

STRESS:

In general terms, any attempt to deform a solid. Types of stress includetension, compression, and shear. More specifically, stress is the ratio of force to unit area F/A, where F is force and A area.

SUBDUCTION:

A tectonic process thatresults when plates converge and one plate forces the other down into Earth's mantle. As a result, the subducted plate eventually undergoes partial melting.

SUBSIDENCE:

A depression in Earth's crust.

TECTONICS:

The study of tectonism, including its causes and effects, most notably mountain building.

TECTONISM:

The deformation of the lithosphere.

TENSION:

A form of stress produced by a force that acts to stretch a material.

THEORY:

A general statement derived from a hypothesis that has withstood sufficient testing.

TRANSFORM MOTION:

A tectonic process whereby plates slide past eachother. It is one of the three ways, along with convergence and divergence, that plates interact.

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Plate Tectonics

Plate tectonics

Plate tectonics is the theory explaining geologic changes that result from the movement of lithospheric plates over the asthenosphere (the molten, ductile, upper portion of the earth's mantle). The visible continents, a part of the lithospheric plates upon which they ride, shift slowly over time as a result of the forces driving plate tectonics. Moreover, plate tectonic theory is so robust in its ability to explain and predict geological processes that it is equivalent in many regards to the fundamental and unifying principles of evolution in biology, and nucleosynthesis in physics and chemistry .

Based upon centuries of cartographic depictions that allowed a good fit between the western coast of Africa and the eastern coast of South America , in 1858, French geographer Antonio Snider-Pellegrini, published a work asserting that the two continents had once been part of larger single continent ruptured by the creation and intervention of the Atlantic Ocean. In the 1920s, German geophysicist Alfred Wegener's writings advanced the hypothesis of continental drift depicting the movement of continents through an underlying oceanic crust . Wegner's hypothesis met with wide skepticism but found support and development in the work and writings of South African geologist Alexander Du Toit, who discovered a similarity in the fossils found on the coasts of Africa and South Americas that derived from a common source.

The technological advances necessitated by the Second World War made possible the accumulation of significant evidence now underlying modern plate tectonic theory.

Plate tectonic theory asserts that Earth is divided into core, mantle, and crust. The crust is subdivided into oceanic and continental crust. The oceanic crust is thin (34.3 mi [57 km]), basaltic (<50% SiO2), dense, and young (<250 million years old). In contrast, the continental crust is thick (18.640 mi [3065 km]), granitic (<60% SiO2), light, and old (2503,700 million years old). The outer crust is further subdivided by the subdivision of the lithospheric plates, of which it is a part, into 13 major plates. These lithospheric plates, composed of crust and the outer layer of the mantle, contain a varying combination of oceanic and continental crust. The lithospheric plates move on top of mantle's asthenosphere.

Boundaries are adjacent areas where plates meet. Divergent boundaries are areas under tension where plates are pushed apart by magma upwelling from the mantle. Collision boundaries are sites of compression either resulting in subduction (where lithospheric plates are driven down and destroyed in the molten mantle) or in crustal uplifting that results in orogeny (mountain building). At transform boundaries, exemplified by the San Andreas fault, the continents create a shearing force as they move laterally past one another.

New oceanic crust is created at divergent boundaries that are sites of sea-floor spreading . Because Earth remains roughly the same size, there must be a concurrent destruction or uplifting of crust so that the net area of crust remains the same. Accordingly, as crust is created at divergent boundaries, oceanic crust must be destroyed in areas of subduction under-neath the lighter continental crust. The net area is also preserved by continental crust uplift that occurs when less dense continental crusts collide. Because both continental crusts resist subduction, the momentum of collision causes an uplift of crust, forming mountain chains . A vivid example of this type of collision is found in the ongoing collision of India with Asia that has resulted in the Himalayan Mountains that continue to increase in height each year. This dynamic theory of plate tectonics also explained the formation of island arcs formed by rising material at sites where oceanic crust subducts under oceanic crust, the formation of mountain chains where oceanic crust subducts under continental crust (e.g., Andes mountains), and volcanic arcs in the Pacific. The evidence for deep, hot, convective currents combined with plate movement (and concurrent continental drift) also explained the mid-plate "hot spot" formation of volcanic island chains (e.g., Hawaiian Islands) and the formation of rift valleys (e.g., Rift Valley of Africa). Mid-plate earthquakes , such as the powerful New Madrid earthquake in the United States in 1811, are explained by interplate pressures that bend plates much like a piece of sheet metal pressed from opposite sides.

As with continental drift theory two of the proofs of plate tectonics are based upon the geometric fit of the displaced continents and the similarity of rock ages and Paleozoic fossils in corresponding bands or zones in adjacent or corresponding geographic areas (e.g., between West Africa and the eastern coast of South America).

Modern understanding of the structure of Earth is derived in large part from the interpretation of seismic studies that measure the reflection of seismic waves off features in Earth's interior. Different materials transmit and reflect seismic shock waves in different ways, and of particular importance to the theory of plate tectonics is the fact that liquid does not transmit a particular form of seismic wave known as an S-wave. Because the mantle transmits S-waves, it was long thought to be a cooling solid mass. Geologists later discovered that radioactive decay provided a heat source within Earth's interior that made the asthenosphere plasticine (semi-solid). Although solid-like with regard to transmission of seismic S-waves, the asthenosphere contains very low velocity (inches per year) currents of mafic (magma-like) molten materials.

Another line of evidence in support of plate tectonics came from the long-known existence of ophiolte suites (slivers of oceanic floor with fossils) found in upper levels of mountain chains. The existence of ophiolte suites are consistent with the uplift of crust in collision zones predicted by plate tectonic theory.

As methods of dating improved, one of the most conclusive lines of evidence in support of plate tectonics derived from the dating of rock samples. Highly supportive of the theory of sea floor spreading (the creation of oceanic crust at a divergent plate boundary (e.g., Mid-Atlantic Ridge) was evidence that rock ages are similar in equidistant bands symmetrically centered on the divergent boundary. More importantly, dating studies show that the age of the rocks increases as their distance from the divergent boundary increases. Accordingly, rocks of similar ages are found at similar distances from divergent boundaries, and the rocks near the divergent boundary where crust is being created are younger than the rocks more distant from the boundary. Eventually, radioisotope studies offering improved accuracy and precision in rock dating also showed that rock specimens taken from geographically corresponding areas of South America and Africa showed a very high degree of correspondence, providing strong evidence that at one time these rock formations had once coexisted in an area subsequently separated by movement of lithospheric plates.

Similar to the age of rocks, studies of fossils found in once adjacent geological formations showed a high degree of correspondence. Identical fossils are found in bands and zones equidistant from divergent boundaries. Accordingly, the fossil record provides evidence that a particular band of crust shared a similar history as its corresponding band of crust located on the other side of the divergent boundary.

The line of evidence, however, that firmly convinced modern geologists to accept the arguments in support of plate tectonics derived from studies of the magnetic signatures or magnetic orientations of rocks found on either side of divergent boundaries. Just as similar age and fossil bands exist on either side of a divergent boundary, studies of the magnetic orientations of rocks reveal bands of similar magnetic orientation that were equidistant and on both sides of divergent boundaries. Tremendously persuasive evidence of plate tectonics is also derived from correlation of studies of the magnetic orientation

of the rocks to known changes in Earth's magnetic field as predicted by electromagnetic theory. Paleomagnetic studies and discovery of polar wandering, a magnetic orientation of rocks to the historical location and polarity of the magnetic poles as opposed to the present location and polarity, provided a coherent map of continental movement that fit well with the present distribution of the continents.

Paleomagnetic studies are based upon the fact that some hot igneous rocks (formed from volcanic magma) contain varying amounts of ferromagnetic minerals (e.g., Fe3O4) that magnetically orient to the prevailing magnetic field of Earth at the time they cool. Geophysical and electromagnetic theory provides clear and convincing evidence of multiple polar reversals or polar flips throughout the course of Earth's history. Where rock formations are uniformi.e., not grossly disrupted by other geological processesthe magnetic orientation of magnetite-bearing rocks can also be used to determine the approximate latitude the rocks were at when they cooled and took on their particular magnetic orientation. Rocks with a different orientation to the current orientation of Earth's magnetic field also produce disturbances or unexpected readings (anomalies) when scientists attempt to measure the magnetic field over a particular area.

This overwhelming support for plate tectonics came in the 1960s in the wake of the demonstration of the existence of symmetrical, equidistant magnetic anomalies centered on the Mid-Atlantic Ridge. Geologists were comfortable in accepting these magnetic anomalies located on the sea floor as evidence of sea floor spreading because they were able to correlate these anomalies with equidistant radially distributed magnetic anomalies associated with outflows of lava from land-based volcanoes.

Additional evidence continued to support a growing acceptance of tectonic theory. In addition to increased energy demands requiring enhanced exploration, during the 1950s there was an extensive effort, partly for military reasons related to what was to become an increasing reliance on submarines as a nuclear deterrent force, to map the ocean floor. These studies revealed the prominent undersea ridges with undersea rift valleys that ultimately were understood to be divergent plate boundaries. An ever-growing network of seismic reporting stations, also spurred by the Cold War need to monitor atomic testing, provided substantial data that these areas of divergence were tectonically active sites highly prone to earthquakes. Maps of the global distribution of earthquakes readily identified stressed plate boundaries. Improved mapping also made it possible to view the retrofit of continents in terms of the fit between the true extent of the continental crust instead of the current coastlines that are much variable to influences of weather and ocean levels.

In his important 1960 publication, History of Ocean Basins, geologist and U.S. Navy Admiral Harry Hess (19061969) provided the missing explanatory mechanism for plate tectonic theory by suggesting that the thermal convection currents in the asthenosphere provided the driving force behind plate movements. Subsequent to Hess's book, geologists Drummond Matthews (19311997) and Fred Vine (19391988) at Cambridge University used magnetometer readings previously collected to correlate the paired bands of varying magnetism and anomalies located on either side of divergent boundaries. Vine and Matthews realized that magnetic data revealing strips of polar reversals symmetrically displaced about a divergent boundary confirmed Hess's assertions regarding seafloor spreading.

See also Dating methods; Earth, interior structure; Fossil record; Fossils and fossilization; Geologic time; Hawaiian Island formation; Lithospheric plates; Mantle plumes; Mapping techniques; Mid-ocean ridges and rifts; Mohorovicic discontinuity (Moho); Ocean trenches; Rifting and rift valleys; Subduction zone

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Plate Tectonics

Plate Tectonics

Plate tectonics is the unifying theory of geology that describes and explains that all earthquakes, volcanic activity, and mountain-building processes are caused by the gradual movement of rigid slabs of rock, called plates, that make up the Earth's surface layer. Given the expanse of geologic time, even modest movementsmeasured in centimeters or inches per yearresult in substantial changes in the distribution of lands and oceans over millions of years.

Earth Structure

The Earth's internal structure can be viewed in two ways: either in terms of compositional layers, or in terms of layers of varying strength. There are three main compositional layers: the crust, mantle, and core. The crust, the outermost layer, is relatively buoyant and very thin compared to the mantle and core.

Beneath the oceans, the oceanic crust varies very little in thickness, generally extending only about 5 kilometers (3.1 miles), and is composed of basalt . The crust beneath the continents, however, is much more variable in thickness, averaging about 30 kilometers (18.6 miles); under large mountain ranges it can extend to depths of up to 100 kilometers (62.1 miles). Continental crust is mostly formed of granite, which is less dense than basalt. This density difference is important in driving the motion of plates, as described below.

Below the crust is the mantle, a dense, hot layer approximately 2,900 kilometers (1,802 miles) thick. At the center of the Earth lies the core, which is composed of an ironnickel alloy. It is divided into two regionsa liquid outer core and solid inner core. As the Earth rotates, the liquid inner core spins, creating the Earth's magnetic field.

Within the crust and mantle, there also are two important mechanical layersthe lithosphere and asthenosphere. The lithosphere is the outermost of these layers, and comprises the crust and uppermost mantle. The lithosphere is relatively cool, making the rock strong and resistant to deformation. The lithosphere is broken into the moving tectonic (or lithospheric) plates.

Below the lithosphere is a relatively narrow, mobile zone of the mantle called the asthenosphere. The asthenosphere is a weak zone, formed of mostly solid rock (with perhaps a little magma mixed in), and flows very slowly, in a manner similar to the ice at a bottom of a glacier. The rigid lithosphere is believed to "float" or move about on the slowly flowing asthenosphere.

Plate Tectonic Theory is Developed

The plate tectonic theory known today evolved in the 1950s, owing to four major scientific developments:

  1. Demonstration of the young age of the ocean floor;
  2. Confirmation of repeated reversals of the Earth's magnetic field in the geologic past;
  3. Emergence of the seafloor-spreading hypothesis and associated recycling of the oceanic crust; and
  4. Precise documentation that the Earth's earthquake and volcanic activity was concentrated along subduction zones and mid-ocean ridges.

Youthful Seafloor.

Before the nineteenth century, the depth of the open ocean was a matter of speculation, although most scientists believed it to be flat and featureless. Only in 1855 did the first bathymetric maps reveal the first evidence of underwater mountains in the central Atlantic. In 1947, seismologists found that the sediment layer on the floor of the Atlantic was much thinner than previously thought. Scientists believed that the oceans were over 4 billion years old, and were perplexed by the distinct lack of sediment cover. The answer to this question would prove vital to advancing the theory of plate tectonics.

Magnetic Field Reversals.

In the 1950s, scientists began recognizing magnetic variations in the rocks of the ocean floor. This was not entirely unexpected, since it was known that basalt contained the mineral magnetite, and this mineral was known to locally distort compass readings. In the early part of the twentieth century, geologists recognized that oceanic rocks had normal or reverse polarity (i.e., in normal polarity, the rocks have the same orientation of today's magnetic field). This can be explained by the ability of the magnetite grains to align themselves in the molten basalt with the Earth's magnetic field. When the rock cools, these grains are "locked" in, recording the magnetic orientation or polarity (normal or reversed) at the time of cooling. As more of the ocean floor was mapped, patterns of alternating stripes of normal and reverse polarity were noted; this became known as magnetic striping.

Seafloor Spreading.

With the discovery of magnetic striping at mid-ocean ridges, scientists began to theorize that mid-ocean ridges mark structurally weak zones where magma from deep within the Earth rises and erupts at the surface. This theory, called seafloor spreading, quickly gained acceptance, but raised an additional question: If new crust is continually being formed at mid-ocean ridges, and the Earth is not increasing in size, what is happening to the old crust? Harry Hess and Robert Dietz postulated that the old crust must be destroyed in the deep canyon-like oceanic trenches, while new crust if formed at the mid-ocean ridges. This theory explained why the Earth is not expanding, there is little sediment on the ocean floor, and oceanic crust is much younger than continental rocks.

Subduction Zones.

The final scientific discovery that cemented the theory of plate tectonics occurred with improvements in seismic detection in the 1950s. Seismologists identified regions of earthquake activity that coincided with Hess's predicted areas of ocean crust generation (mid-ocean ridges) and oceanic lithosphere destruction (subduction zones). Today scientists know that tectonic plates move, because they can measure their motion directly using the global positioning system (GPS).

Plate Tectonic Boundaries

Plate tectonic boundaries are regions where lithospheric plates meet. There are three types of plate tectonic boundaries: divergent, convergent, and transform.

Divergent.

Divergent boundaries occur along spreading ridges where plates are moving apart and new crust is being created by ascending magma from the mantle. An example of a divergent plate boundary is the Mid-Atlantic Ridge. This submerged mountain chain extends from the Arctic to the southern tip of Africa, and is one part of the global ridge system that extends around the Earth.* The Mid-Atlantic Ridge spreads at a rate of approximately 2.5 centimeters (1 inch) per year.

Convergent.

Convergent boundaries are regions where lithospheric plates collide. The type of convergence depends on the types of plates involved: namely, (1) oceanicoceanic convergence; (2) oceaniccontinental convergence; (3) continentalcontinental convergence (see figure).

  • OceanicOceanic. When two oceanic plates collide, one plate is subducted beneath the other. This occurs as one lithospheric plate becomes older, colder, and denser than the underlying hot, weak asthenosphere. As the lithosphere sinks slowly through the asthenosphere, the uppermost sediments are melted, and the resulting magma reaches the surface to form volcanoes. As a result, subduction zones are marked by an arc of volcanoes parallel to and about 150 kilometers (93 miles) from the plate margin. An example of oceanic-oceanic collision is the Marianas Trench and the Aleutian Islands in the Pacific Ocean.
  • OceanicContinental. When oceanic and continental plates collide, the oceanic plate is the one that is subducted beneath the continental plate, because the continental crust is lighter and less dense. An example of oceanic-continental collision is seen at the Cascadia Subduction Zone, where the Pacific Plate is being subducted beneath the North American Plate.
  • ContinentalContinental. Continentalcontinental convergence results in spectacular mountain ranges such as the Himalayas, the Alps, and the Appalachians. Because continental crust is buoyant, neither plate will subduct, and a collision zone is the result.

Transform.

Transform boundaries mark regions where plates slide past one another. Transform boundaries are great vertical fractures that extend down through the lithosphere. An example of a transform boundary is the San Andreas Fault in Southern California (see the photograph of the fault on page 202).

Plate Movements

The lithospheric plates do not randomly meander about the Earth's surface, but are driven by internal forces. The mantle is believed to move in circular motions rather like soup boiling on a stovetop, wherein the heated soup rises to the surface, cools, and sinks back to the bottom of the pot, where it is heated and rises again. This cycle is called convective flow, and it is the same process that occurs in the mantle today. However, the heat source within the Earth is radioactive decay of minerals and residual heat from the formation of the Earth.

Until the early 1990s, scientists believed that mantle convection, seafloor spreading, and magma intrusion at mid-ocean ridges (called "ridge push") were the predominant mechanisms that drove plate motion. However, in recent years, the significance of subduction mechanisms over mid-ocean ridge processes has taken precedence. The gravity-controlled sinking of a cold, dense, oceanic slab into a subduction zone (called "slab pull") now is considered the driving mechanism behind plate tectonics.

Although scientists know that forces deep within the Earth drive plate motion, they may never know the exact details, because no mechanism can be directly tested. The fact that lithospheric plates have moved in the past and are still in motion today is beyond dispute, but the exact mechanisms of how and why they move will continue to challenge scientists in the future.

see also Geothermal Energy; Hot Springs on the Ocean Floor; Mid-Ocean Ridges; Ocean Basins; Ocean-Floor Bathymetry; Volcanoes, Submarine.

Alison Cridland Schutt

Bibliography

Skinner, Brian J., and Stephen C. Porter. This Dynamic Earth. New York: John Wiley & Sons, 1992.

Internet Resources

This Dynamic Earth. U.S. Geological Survey. <http://www.pubs.usgs.gov/publications/text/>.

* See "Mid-Ocean Ridges" for an image illustrating the mid-ocean ridge system.

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Plate Tectonics

Plate tectonics

Plate tectonics is the geologic theory that Earth's crust is made up of rigid plates that "float" on the surface of the planet. Tectonics comes from the Greek word meaning "builder." The movement of the plates toward or away from each other either directly or indirectly creates the major geologic features at Earth's surface.

Plate tectonics revolutionized the way geologists view Earth. Like the theory of evolution in biology, plate tectonics is the unifying concept of geology. It explains nearly all of Earth's major surface features and activities. These include faults and earthquakes, volcanoes and volcanism, mountains and mountain building, and even the origin of the continents and ocean basins.

Continental drift

Plate tectonics is a comparatively new idea. The theory of plate tectonics gained widespread acceptance only in the 1960s. About 50 years earlier, German geophysicist Alfred Wegener (18801930) developed a related theory known as continental drift. Wegener contended that the positions of Earth's continents are not fixed. He believed instead that they are mobile and over time drift about on Earth's surfacehence the name continental drift.

Wegener's most obvious evidence for his theory was the fact that several of the world's continents fit together like pieces in a jig-saw puzzle. Based on this, he proposed that the continents of the world were previously joined together in one large continental mass, a supercontinent he called Pangaea (pronounced pan-JEE-ah). Wegener believed that this supercontinent had subsequently broken up into the six present-day continents. However, Wegener could not provide a convincing explanation as to what moved the continents around the surface of the planet. That answer came with the theory of plate tectonics.

Plate structure

Earth's tectonic plates are rigid slabs of rock. Geologists divide the interior of Earth into layers, based on their composition (from solid to liquid).

Words to Know

Asthenosphere: Portion of the mantle beneath the lithosphere composed of partially melted material.

Convection current: Circular movement of a fluid in response to alternating heating and cooling.

Convergence: The movement of two plates toward one another.

Crust: Thin, solid outer portion of Earth.

Divergence: Separation of two plates as they move in opposing directions.

Lithosphere: Rigid uppermost section of the mantle combined with the crust.

Mantle: Thick, dense layer of rock that underlies Earth's crust.

Ocean trench: Deep depression in the seafloor, created by an oceanic plate being forced downward into the subsurface by another, overriding plate.

Plate margin: The boundaries where plates meet.

Plates: Large regions of Earth's surface, composed of the crust and uppermost mantle, which move about, forming many of Earth's major geologic surface features.

Seafloor spreading: Process in which new seafloor forms as molten rock from Earth's interior rises toward the surface, pushing the existing seafloor out of its way.

Subduction: Tectonic process that involves one plate being forced down into the mantle at an oceanic trench, where it eventually undergoes partial melting.

Transform motion: Horizontal plate movement in which one plate slides past another.

The thin outer portion of the planet is the crust. Beneath that is the mantle, which is solid near the top and "soft" or partially melted beginning at a depth of about 40 miles (65 kilometers) beneath the surface. The crust and the rigid portion of the mantle compose the lithosphere. The soft portion of the mantle is called the asthenosphere.

It is the lithosphere that is broken up into plates, which move about while floating upon the underlying asthenosphere. There are about eight major plates and several smaller ones that are in constant contact with each other. When one plate moves, it causes other plates to move. These plates have many different shapes and sizes. Some, such as the Juan de Fuca plate off the west coast of Washington State, have surface areas of a few thousand square miles. The largest, the Pacific plate, underlies most of the Pacific Ocean and covers an area of hundreds of thousands of square miles.

Plate movement

Most modern geologists believe convection currents in the asthenosphere are the driving force for plate motion. The heat energy at the center of the planet is carried to the surface by currents. As they reach the surface, the currents cool and begin to sink back toward the center. Below the crust, pressure exerted on the bottom of the plates by the convection currents helps to push the plates along. Plates move at rates of

about 1 inch (2.5 centimeters) per year. The fastest plates move more than 4 inches (10 centimeters) per year.

Plate interactions

Tectonic plates can interact in one of three ways. They can move toward one another, or converge. They can move away from one another, or diverge. Or they can slide past one another, or transform. The boundaries where plates meet are known as plate margins. The types of geologic activity that occur when two plates interact is dependent on the nature of the plate interaction and of the margins. Plate margins come in three varieties: oceanic-oceanic, continental-continental, and continental-oceanic.

Oceanic-oceanic plates. When two oceanic plates converge, one of the plates subducts or sinks underneath the other, forming a deep depression called an ocean trench. The subducted plate sinks downward into the mantle where it begins to melt. Molten rock from the melting plate rises toward the surface and forms a chain of volcanic islands, or a volcanic island arc, behind the ocean trench. When oceanic plates diverge, a ridge (mountain chain) develops and seafloor spreading occurs. Molten rock pushes up at the divergent margin, creating mountains and an expanding seafloor. Today, Europe and North America move about 3 inches (7.5 centimeters) farther apart every year as the Atlantic Ocean grows wider.

Continental-continental plates. Continental-continental convergent plates act quite differently than oceanic-oceanic plates. Continental crust is too light to be carried downward into a trench. At continental-continental convergent margins neither plate subducts. The two continental plates converge, buckle, and compress to form complex mountains ranges of great height. Convergence of this sort produced the Himalayas when the Indian-Australian plate collided with the Eurasian plate.

Continental-continental divergence causes a continent to separate into two or more smaller continents when it is ripped apart along a series of fractures. The forces of divergence literally tear a continent apart as the two or more blocks of continental crust begin slowly moving apart and magma pushes into the rift formed between them. Eventually, if the process of continental rifting continues, a new sea is born between the two continents. Rifting between the Arabian and African plates formed the Red Sea in this way.

Continental-oceanic plates. When continental and oceanic plates converge, the oceanic plate (which is denser) subducts below the edge of the continental plate. Volcanoes form as result, but in this setting, the chain of volcanoes forms on the continental crust. This volcanic mountain chain, known as a volcanic arc, is usually several hundred miles inland

from the plate margin. The Andes Mountains of South America and the Cascade Mountains of North America are examples of volcanic arcs. No continental-oceanic divergent margins exist today. They are unlikely to form and would quickly become oceanic-oceanic divergent margins as seafloor spreading occurred.

Transform motion. In addition to convergence and divergence, transform motion may occur along plate margins. Transform margins are less spectacular than convergent and divergent ones, and the type of plates involved is really of no significance. As two rock plates slide past one another at a margin, a crack or fault develops. The energy generated by the movement is often released in the form of an earthquake. The best known example of a transform plate margin is the San Andreas Fault in California, where the Pacific and North American plates are in contact.

[See also Earthquake; Earth's interior; Fault; Geologic map; Ocean; Volcano ]

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plate tectonics

plate tectonics, theory that unifies many of the features and characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized geologists' understanding of continents, ocean basins, mountains, and earth history.

Development of Plate Tectonics Theory

The beginnings of the theory of plate tectonics date to around 1920, when Alfred Wegener, the German meteorologist and geophysicist, presented the first detailed accounts of how today's continents were once a large supercontinent that slowly drifted to their present positions. Others brought forth evidence, but plate tectonics processes and continental drift did not attract wide interest until the late 1950s, when scientists found the alignment of magnetic particles in rock responded to the earth's magnetic field of that time. Plotting paleomagnetic polar changes (see paleomagnetism) showed that all continents had moved across the earth over time.

Synthesized from these findings and others in geology, oceanography, and geophysics, plate tectonics theory holds that the lithosphere, the hard outer layer of the earth, is divided into about 7 major plates and perhaps as many as 12 smaller plates, c.60 mi (100 km) thick, resting upon a lower soft layer called the asthenosphere. Because the sides of a plate are either being created or destroyed, its size and shape are continually changing. Such active plate tectonics make studying global tectonic history, especially for the ocean plates, difficult for times greater than 200 million years ago. The continents, which are c.25 mi (40 km) thick, are embedded in some of the plates, and hence move as the plates move about on the earth's surface.

The mechanism moving the plates is at present unknown, but is probably related to the transfer of heat energy or convection within the earth's mantle. If true, and the convection continues, the earth will continue to cool. This will eventually halt the mantle's motion allowing the crust to stabilize, much like what has happened on other planets and satellites in the solar system, such as Mars and the moon.

Plate Boundary Conditions

There are numerous major plate boundary conditions. When a large continental mass breaks into smaller pieces under tensional stresses, it does so along a series of cracks or faults, which may develop into a major system of normal faults. The crust often subsides, forming a rift valley similar to what is happening today in the Great Rift Valley through the Red Sea. If rifting continues, a new plate boundary will form by the process of seafloor spreading. Mid-ocean ridges, undersea mountain chains, are the locus of seafloor spreading and are the sites where new oceanic lithosphere is created by the upwelling of mantle asthenosphere.

Individual volcanoes are found along spreading centers of the mid-ocean ridge and at isolated "hot spots," or rising magma regions, not always associated with plate boundaries. The source of hot-spot magmas is believed to be well below the lithosphere, probably at the core-mantle boundary. Hot-spot volcanoes often form long chains that result from the relative motion of the lithosphere plate over the hot-spot source.

Subduction zones along the leading edges of the shifting plates form a second type of boundary where the edges of lithospheric plates dive steeply into the earth and are reabsorbed at depths of over 400 mi (640 km). Earthquake foci form steeply inclined planes along the subduction zones, extending to depths of about 440 mi (710 km); the world's most destructive earthquakes occur along subduction zones.

A third type of boundary occurs where two plates slide past one another in a grinding, shearing manner along great faults called strike-slip faults or fracture zones along which the oceanic ridges are offset. Continental mountain ranges are formed when two plates containing continental crust collide. For example, the Himalayas are still rising as the plates carrying India and Eurasia come together. Mountains are also formed when ocean crust is subducted along a continental margin, resulting in melting of rock, volcanic activity, and compressional deformation of the continent margin. This is currently happening with the Andes Mts. and is believed to have occurred with the uplift of the Rockies and the Appalachians in the past.

Movement of the Continents

According to plate tectonics, the ocean basins are viewed as transient features that have periodically opened and closed, first rending and then suturing the continental masses, which are permanent features on the earth's surface. Geologists now believe that the continents were sutured together 200 million years ago at the beginning of the Mesozoic era to form a supercontinent named Pangaea. Initial rifting along the Tethys Sea formed a northern continental mass, Laurasia, and a southern continental mass, Gondwanaland. Then plate movements caused North American and Eurasian separation coincidentally with the separation of South America, Africa, and India. Australia and Antarctica were the last to separate. The major plates are named after the dominant geographic feature on them such as the North American and South American plates.

Plate motions are believed to have transported large crustal blocks several thousand miles, suturing very different terrains together after collision with a larger mass. These "exotic" terrains may include segments of island arcs quite unrelated to the history of the continent onto which they are sutured. Some geologists believe that continents grow in size primarily by the addition of exotic terrains.

Bibliography

See E. M. Moores and R. J. Twiss, Tectonics (1995); B. F. Windley, The Evolving Continents (3d ed. 1995); K. C. Condie, Plate Tectonics and Crustal Evolution (4th ed. 1997); L. P. Zonenshain et al., Paleogeodynamics: The Plate Tectonic Evolution of the Earth (1997).

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plate tectonics

plate tectonics The unifying concept that has drawn continental drift, sea-floor spreading, seismic activity, crustal structures, and volcanic activity (see VOLCANICITY) into a coherent model of how the outer part of the Earth evolves.The theory proposes a model of the Earth's upper layers in which the colder, brittle, surface rocks form a shell (the lithosphere) overlying a much less rigid asthenosphere. The shell comprises several discrete, rigid units (tectonic plates) each of which has a separate motion relative to the other plates. The plate margins are most readily defined by present-day seismicity, which is a consequence of the differential motions of the individual plates. The model is a combination of continental drift and sea-floor spreading. New lithospheric plates are constantly forming and separating, and so being enlarged, at constructive margins (ridges), while the global circumference is conserved by the subduction and recycling of material into the mantle at destructive margins (trenches). This recycling results in andesitic volcanism and the creation of new continental crust, which has a lower density than the oceanic crust and is more difficult to subduct. Many features of the Earth's history are explicable within this model which has served as a unifying hypothesis for most of the Earth sciences. Previous mountain systems are now recognized as the sites of earlier subduction, often ending with continental crustal collision: the movement of plates has been used with varying success in interpreting orogenic belts as far back as the early Proterozoic. Plate motions are driven by mantle convection and are likely to have occurred throughout Earth history, although the resultant surface features are likely to have changed with time. See RIDGE-PUSH; and SLAB-PULL.

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plate tectonics

plate tectonics The theory that the surface of the earth is made of lithospheric plates, which have moved throughout geological time resulting in the present-day positions of the continents. The theory explains the locations of mountain building as well as earthquakes and volcanoes. The rigid lithospheric plates consist of continental and oceanic crust together with the upper mantle, which lie above the weaker plastic asthenosphere. These plates move relative to each other across the earth. Six major plates (Eurasian, American, African, Pacific, Indian, and Antarctic) are recognized, together with a number of smaller ones. The plate margins coincide with zones of seismic and volcanic activity.

A constructive (or divergent) plate margin occurs when two plates move away from each other. It is marked by a mid-oceanic ridge where basaltic material wells up from the mantle to form new oceanic crust, in a process known as sea-floor spreading. The production of new crust at constructive plate margins is compensated for by the destruction of material along a destructive (or convergent) plate margin. Along these margins, which are also known as subduction zones and marked by an oceanic trench, one plate (usually oceanic) is forced to plunge down beneath the other (which may be continental or oceanic). The crust becomes partially melted and rises to form a chain of volcanoes in the upper plate parallel to the trench. When two continental plates collide the compression results in the formation of mountain chains. A third type of plate margin – the transform plate margin – occurs where two plates are slipping past each other.

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plate tectonics

plate tectonics Theory or model to explain the distribution, evolution, and causes of the Earth's crustal features. It proposes that the Earth's crust and part of the upper mantle (the lithosphere) consists of several separate, rigid slabs, termed plates, which move independently forming part of a cycle in the creation and destruction of crust. The plates collide or move apart at the margins, and these produce zones of earthquake and volcanic activity. Three types of plate boundary can be identified. At a constructive or divergent margin, new basaltic magma originating in the mantle injects into the plate. The crusts are forced to separate and an oceanic ridge is formed. At a destructive or convergent margin, plates collide and one plate moves under the other. This occurs along oceanic trenches. The recycling of crust by subduction results in the melting of some crustal material, and volcanic island arcs (such as the islands of Japan) are produced. Material that cannot be subducted is scraped up and fused onto the edge of plates. This can form a new continent or add to existing continents. Mountain chains are explained as the sites of former subduction or continental collision. At a conservative margin, plates move past each other along a transform fault. Plate movement is thought to be driven by convection currents in the mantle. See also seafloor spreading

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plate tectonics

plate tec·ton·ics • pl. n. [treated as sing.] a theory explaining the structure of the earth's crust and many associated phenomena as resulting from the interaction of rigid lithospheric plates that move slowly over the underlying mantle. DERIVATIVES: plate-tec·ton·ic adj.

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Plate Tectonics

Plate tectonics


Anyone who looks carefully at a map of the Atlantic Ocean is likely to be struck by an interesting point. The eastern coastline of South America bears a striking similarity to the western coastline of Africa. Indeed, it looks almost as if the two continents could somehow fit together--provided, of course, one could find a way to slide the land masses across the ocean floor.

The match of coastlines was noticed by scholars almost as soon as good maps were first available in the late fifteenth century. In 1620, for example, the English philosopher Sir Francis Bacon commented on this fact and argued that the match was "no mere accidental occurrence."

One obvious explanation for the South America-Africa fit was that the two continents had once been connected and had somehow become separated in the past. A few early scientists tried to use the Biblical story of The Flood to show how that might have happened. But as Biblical explanations for natural phenomena began to lose credibility, this approach was discarded.

That made the concept of moving continents even more difficult to believe. The earth's crust was generally thought to be solid and immoveable. How could one or two whole continents somehow slide through such a material?

Yet, over time, more and more evidence began to accumulate, supporting the notion that South America and Africa might once have been joined to each other. Much of that early evidence came from the research of the German geographer, Alexander von Humboldt. Von Humboldt spent a number of years traveling through South America, Africa, and other parts of the world. In his journeys, he collected plant and animal specimens and studied geological and geographic patterns. He was struck by the many similarities he observed between South America and Africa, similarities that went far beyond an obvious geographic fit of continental coastlines.

For example, he observed that mountain ranges in Brazil that end at the sea appear to match other mountain ranges in Africa that began at the coastline. He noted similar patterns among mountain ranges in Europe and North America.

During the nineteenth century, similarities among fauna and flora on either side of the Atlantic were observed. Although species in eastern South America do differ to some extent from those in western Africa, their similarities are often striking. Before long, similarities across other oceanic gaps began to be noted. Plant and animal fossils found in India, for example, were often remarkably similar to those found in Australia .

Attempts to explain the many similarities in various continental properties were consistently stymied by the beliefs that the earth's crust was solid and immoveable. One way around this problem was the suggestion that mammoth land bridges existed between continents. These large bridges would have allowed the movement of plants and animals from one continent to another. But no evidence for such bridges could be found, and this idea eventually fell into disrepute.

By the mid-1850s, an important breakthrough in geological thought began to occur. A few geologists started to accept the hypothesis that the earth's crust is not as solid and immovable as it appears. In fact, they said, it may be that the earth's outer layer is actually floating (and, thus, is moveable) on the layer below it, the mantle.

Still, it was not until the early years of the twentieth century that a new theory of "floating continents" was seriously proposed. Then, in a period of less than four years, two distinct theories of this kind were suggested. The first was offered in a December 29, 1908, paper by the American geologist Frank B. Taylor. Taylor outlined a theory that described how the continents had slowly shifted over time with a "mighty creeping movement."

Taylor's paper met largely with indifference. Such was not the fate, however, of the ideas of a German astronomer and meteorologist, Alfred Wegener. While browsing through the University of Marburg library in the fall of 1911, Wegener was introduced to the problem of continental similarities. Almost immediately, he decided to devote his attention to this question and began a study that was to dominate the rest of his professional life and to revolutionize the field of geology.

By January 1912, Wegener had developed a theory to explain continental similarities. Such similarities cannot be explained by sunken land bridges, he said, but are the result of continents having moved slowly across the face of the planet. Three more years of research were needed before Wegener's theory was completed. In 1915, he published The Origins of Continents and Oceans, summarizing his ideas about continental similarities.

According to Wegener's theory, the continents were once part of one large land mass, which he called Pangaea. Eventually this land mass broke into two parts, two super-continents, which he called Gondwanaland and Laurasia. Over millions of years, Gondwanaland broke apart into South America, Africa, India, Australia, and Antarctica , he suggested, while Laurasia separated into North America and Eurasia.

The basic problem Wegener faced was to explain how huge land masses like continents can flow. His answer was that the materials of which the earth's crust is made are of two very different types. One, then call "sial," is relatively light, but strong. The other, then called "sima," can be compared to very thick tar. Continents are made of sial, he said, and sea floors of sima. The differences in these materials allows continents to "ride" very slowly across sea floors.

Wegener's theory was met with both rejection and hostility. Fellow scientists not only disagreed with his ideas, but also attacked him personally even for suggesting the ideas. The theory of continental drift did not totally disappear as a result of these attitudes, but it fell into disfavor for more than three decades.

Research dating to the mid-1930s revealed new features of the sea floor which made Wegener's theory more plausible. Scientists found sections of the ocean bottoms through which flows of hot lava were escaping from the mantle, somewhat like underwater volcanoes. These discoveries provided a crucial clue in the development of plate tectonics, the modern theory of continental drift.

According to the theory of plate tectonics, the upper layer of the earth is made of a number of plates, large sections of crust, and the upper mantle. About ten major plates have been identified. The largest plate is the Pacific Plate, underlying the Pacific Ocean. The North and South American, Eurasian, African, Indian, Australian, Nazca, Arabian, Caribbean, and Antarctica are the other major plates.

Scientists believe that plates rest on an especially plastic portion of the mantle known as the "asthenosphere." Hot magma from the asthenosphere seeps upward and escapes through the ocean floor by way of openings known as rifts. As the magma flows out of the rift, it pushes apart the plates adjoining the rift. The edge of the plate opposite the rift is ultimately forced downward, back into the asthenosphere. The region in which plate material moves down into the mantle is a trench.

Plates move at different speeds in different directions at different times. On an average, they travel 0.42 in (1-5 cm) per year. To the extent that this theory is correct, a map of the earth's surface ten million years from now will look quite different from the way it does now.

The theory of plate tectonics explains a number of natural phenomena that had puzzled scientists for centuries. Earthquakes, for example, can often be explained as the sudden, rather than gradual, movement of two adjacent plates. One of the world's most famous earthquake zones, the San Andreas Fault, lies at the boundary of the Pacific and the North American plates. Volcanoes often accompany the movement of plates and earthquakes. The boundaries of the Pacific Plate, for example, define a region where volcanoes are very common, a region sometimes called the Ring of Fire.

Plate tectonics is now accepted as one of the fundamental theories of geology. Its success depends not only on the discovery of an adequate explanation for continental movement (sea-floor spreading, rifts, and trenches), but also on the discovery of more and more similarities between continents.

For example, modern geologists have re-examined Humboldt's ideas about the correlation of mountain ranges in South America and Africa. They have found that rock strata, or layers, in Brazil lie almost exactly where they should be expected if strata in Ghana are projected to the west. If the Atlantic Ocean could somehow be removed, the two strata could be made to coincide almost perfectly.

Some interesting data have come from studies of paleomagnetism also. Paleomagnetism refers to the orientation of iron crystals in very old rock. Since the earth's magnetic poles have shifted over time, so has the orientation of iron crystals in rock. Scientists have found that the orientation of iron crystals in one continent correspond very closely to those found in rocks of another continent thousands of miles away.

Fossils continue to be a crucial way of confirming continental drift. For example, scientists have learned that the fossil population of is much less like the fossil population of Africa (300 miles [483 km] to the west) than it is like the fossil population of India (3,000 miles/4,830 km to the northeast). This finding would make almost no sense at all unless one recognized that the theory of continental drift has Madagascar breaking off from India millions of years ago, not off Africa.

The flow of magma out of the asthenosphere often results in the formation of ores, regions that are rich in some mineral. For example, the high temperatures characteristic of a rift or a trench may be sufficient to cause the release of metals from their compounds. The flow of magma across rock may also result in a phenomenon known as contact metamorphism, a mechanism that also results in the formation of metals such as lead and silver.

See also Biogeography; Endemic species; Evolution; Geosphere; Topography

[David E. Newton ]


RESOURCES

BOOKS

McGraw-Hill Encyclopedia of Science & Technology. 7th ed. New York: McGraw-Hill, 1992.

Miller, R., and the Editors of Time-Life Books. Continents in Collision. Alexandria, VA: Time-Life Books, 1983.

Moran, J. M., M. D. Morgan, and J. H. Wiersma. Environmental Science. Dubuque, IA: W. C. Brown, 1993.

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Plate Tectonics

Plate Tectonics

Continental drift versus plate tectonics

An overview of plate tectonic theory

Evidence supporting plate tectonic theory

Rates of plate movement

Scale and number of plates

Plate interactions

Oceanic-oceanic plates

Continental-continental plates

Continental-oceanic plates

Transform margins

Continent formation

Driving mechanism

Importance of plate tectonics

Resources

Plate tectonics is the theory explaining geologic changes that result from the movement of Earths rigid lithospheric plates over the ductile asthenosphere. Plates move and shift their positions relative to one another, and the movement of and contact between plates accounts for most of the major geologic features on Earths surface.

The visible continents, which a part of the lithospheric plates upon which they ride, move slowly over time. Plate tectonic theory is so robust in its ability to explain and predict geological processes that it is equivalent in many regards to the fundamental and unifying principles of evolution in biology, and nucleosynthesis in physics and chemistry.

Continental drift versus plate tectonics

Based upon centuries of cartographic depictions showing a resemblance between the Western coast of Africa and the Eastern coast of South America, in

1858, French geographer Antonio Snider-Pellegrini, published a work asserting that the two continents had once been part of larger single continent ruptured by the creation and intervention of the Atlantic Ocean. In the 1920s, German geophysicist Alfred Wegeners writings advanced the hypothesis of continental drift depicting the movement of continents through an underlying oceanic crust. Wegners hypothesis met with wide skepticism from most geologists but found support and development in the work and writings of South African geologist Alexander Du Toit, who discovered a similarity in the fossils found on the coasts of Africa and South Americas that derived from a common source.

Wegeners continental drift theory was dismissed, and even ridiculed, by leading geologists of his day because it could not explain how the continents moved across the solid surface of Earth. He perished on a meteorological expedition to Greenland in 1930.

Technological advances during World War II made possible the accumulation of significant evidence that gave rise to modern plate tectonic theory, which explained how continents could move. The theory of plate tectonics gained widespread acceptance only in the late 1960s through the early 1970s.

An overview of plate tectonic theory

Earth is divided into a core, mantle, and crust, and the crust is further divided into oceanic and continental crust. The oceanic crust is thin (34.3 mi [57 km]), basaltic (<50% SiO2), composed primarily of dense basalt and gabbro, and young (<250 million years old). In contrast, the continental crust is thick (18.640 mi [3065 km]), composed primarily of comparatively light granitic rocks, light, and old (250 3,700 million years old). The crust and the uppermost portion of the mantle constitute the lithosphere, which is divided into 13 major and several minor tectonic plates. Beneath the lithosphere lies the asthenosphere. The lithospheric plates, which are rigid, move on top of the more ductile asthenosphere.

There are three major types of boundaries between lithospheric plates. Divergent boundaries are those along which plates move apart from each other, allowing magma to move upward from the mantle and form new crust. Convergent boundaries are those across which plates move towards each other, resulting in subduction (in which one plate is over-ridden or subducted by the other) or in uplift that results in orogeny (mountain building). At transform boundaries, for example the San Andreas fault along the boundary between the North American and Pacific plates, the continents move laterally past one another.

New oceanic crust is created at divergent boundaries. Because Earth remains roughly the same size, there must be a concurrent destruction or uplifting of crust so that the net area of crust remains the same. Accordingly, as crust is created at divergent boundaries, oceanic crust must be destroyed in areas of subduction underneath the lighter continental crust. The net area is also preserved by continental crust uplift that occurs when less dense continental crust collides with continental crust. Because both continental crusts resist subduction, the momentum of collision causes an uplift of crust, forming mountain chains.

The best modern example of this is the ongoing collision of India with Asia, which has created the Himalayan Mountains. The dynamic theory of plate tectonics also explains the origin of island arcs (such as the Aleutian Islands in Alaska and the Philippine Islands), formed by rising material at sites where oceanic crust subducts under oceanic crust and the formation of mountain chains where oceanic crust subducts under continental crust (such as the Andes Mountains or the Cascade Range of western North America). The evidence for deep, hot, convective currents combined with plate movement also explains mid-plate hot spot formation of volcanic island chains (e.g., Hawaiian islands and the Yellowstone region of Montana) and the formation of rift valleys (e.g., Rift Valley of Africa and the Rio Grande Rift of western North America). Mid-plate earthquakes, such as the powerful New Madrid earthquake in the United States in 1811, are explained by interplate pressures that bend plates much like a piece of sheet metal buckled from opposite sides.

Evidence supporting plate tectonic theory

As with continental drift theory, two lines of evidence supporting plate tectonics are based upon the geometric fit of the displaced continents and the similarity of rock ages and Paleozoic fossils in corresponding bands or zones in adjacent or corresponding geographic areas (e.g., between West Africa and the eastern coast of South America).

Ocean topography also provided evidence in support of plate tectonic theory. Nineteenth century surveys of the oceans indicated that rather than being flat featureless plains, as was previously thought, some parts of ocean floors are mountainous while others contain deep depressions. Surveys in the 1950s and 1960s provided an even more detailed picture of the ocean floor. Long and continuous mountain chains as well as deep troughs were discovered. Geoscientists later identified the mountainous features as the midoceanic ridges (MORs) where new plates form, and the deep ocean trenches as subduction zones where plates descend into the subsurface.

Modern understanding of the structure of Earth is derived in large part from the interpretation of seismic studies that measure the reflection of seismic waves off features in Earths interior. Different materials transmit and reflect seismic waves in different ways, and of particular importance to theory of plate tectonics is the fact that liquid does not transmit a particular form of seismic wave known as an S-wave. Because the mantle transmits S-waves, it was long thought to be solid. Geologists later discovered that radioactive decay provided a heat source with Earths interior that made part of the mantle, known as the astheno-sphere, a semi-solid plastic material. Although solid with regard to transmission of seismic S-waves, material within the asthenosphere flows or creeps very slowly (in a manner similar to glacial ice) in response to the temperature difference between the surface and interior of Earth. The mantle rock moves in nearly circular patterns known as convection cells that serve to redistribute the heat generated deep within Earth.

Another line of evidence in support of plate tectonics came from the long-known existence of rocks known as ophiolte suites (slivers of oceanic floor containing a characteristic combination of igneous rock, sedimentary rock, and fossils) found in some mountain ranges. The existence of ophiolte suites are consistent with the uplift of crust in collision zones predicted by plate tectonic theory.

As methods of radiometric dating improved, one of the most conclusive lines of evidence in support of plate tectonics derived from the ages of rock samples. Highly supportive of the theory of sea floor spreading (the creation of oceanic crust at a divergent plate boundary such as Mid-Atlantic Ridge) was evidence that rock ages are similar in equidistant bands symmetrically centered on the divergent boundary. More importantly, dating studies showed that the age of the rocks increases as their distance from the divergent boundary increases. Accordingly, rocks of similar ages are found at similar distances from divergent boundaries, and the rocks near the divergent boundary where crust is being created are younger than the rocks more distant from the boundary. Eventually, radioisotope studies offering improved accuracy and

precision in rock dating also showed that rock specimen taken from geographically corresponding areas of South America and Africa showed a very high degree of correspondence, providing strong evidence that at one time these rock formations had once coexisted in an one area and were subsequently separated by movement of lithospheric plates.

Similar to the age of rocks, studies of fossils found in once adjacent geological formations showed a high degree of correspondence. Identical fossils are found in bands and zones equidistant from divergent boundaries. Accordingly, the fossil record provides evidence that a particular band of crust shared a similar history as its corresponding band of crust located on the other side of the divergent boundary.

The line of evidence, however, that firmly convinced modern geologists to accept the arguments in support of plate tectonics derived from studies of the magnetic signatures or magnetic orientations of rocks found on either side of divergent boundaries. Just as similar age and fossil bands exist on either side of a divergent boundary, studies of the magnetic orientations of rocks reveal bands of similar magnetic orientation that were equidistant and on both sides of divergent boundaries. Persuasive evidence of plate tectonics is also derived from correlation of studies of the magnetic orientation of the rocks to known changes in Earths magnetic field as predicted by electromagnetic theory. Paleomagnetic studies and discovery of polar wandering, a magnetic orientation of rocks to the historical location and polarity of the magnetic poles as opposed to the present location and polarity, provided a coherent map of continental movement that fit well with the present distribution of the continents.

Paleomagnetic studies are based upon the fact that some hot igneous rocks (formed from volcanic magma) contain varying amounts of ferromagnetic minerals (e.g., Fe3O4) that magnetically orient to the prevailing magnetic field of Earth at the time they cool. Geophysical and electromagnetic theory evidence of multiple polar reversals or polar flips throughout the course of Earths history. Where rock formations are uniformi.e., not grossly disrupted by other geological processesthe magnetic orientation of magnetite-bearing rocks can also be used to determine the approximate latitude the rocks were at when they cooled and took on their particular magnetic orientation. Rocks with a different orientation to the current orientation of Earths magnetic field also produce disturbances or unexpected readings (anomalies) when scientists attempt to measure the magnetic field over a particular area.

This overwhelming support for plate tectonics came in the 1960s, in the wake of the demonstration of the existence of symmetrical magnetic anomalies centered on the Mid-Atlantic Ridge. During magnetic surveys of the deep ocean basins, geologists found areas where numerous magnetic reversals occur in the ocean crust. These look like stripes, oriented roughly parallel to one another and to the MORs. When surveys were run on the other side of the MORs, they showed that the magnetic reversal patterns were remarkably similar on both sides of the MORs. After much debate, scientists concluded that new ocean crust must form at the MORs, recording the current magnetic orientation. This new ocean crust pushes older crust out of the way, away from the MOR. When a magnetic reversal occurs, new ocean crust faithfully records it as a reversed magnetic stripe on both sides of the MOR. Older magnetic reversals were likewise recorded; these stripes are now located farther from the MOR.

Geologists were comfortable in accepting magnetic anomalies located on the sea floor as evidence of sea floor spreading because they were able to correlate these anomalies with radially distributed magnetic anomalies associated with outflows of lava from land-based volcanoes.

Additional evidence continued to support a growing acceptance of plate tectonic theory. In addition to increased energy demands requiring enhanced exploration, during the 1950s there was an extensive effort, partly for military reasons related to what was to become an increasing reliance on submarines as a nuclear deterrent, to map the ocean floor. These studies revealed the prominent undersea ridges with undersea rift valleys that ultimately were understood to be divergent plate boundaries. An ever-growing network of seismic reporting stations, also spurred by the Cold War (late 1940searly 1990s) need to monitor atomic testing, provided substantial data that these areas of divergence were tectonically active sites highly prone to earthquakes. Maps of the global distribution of earthquakes readily identified stressed plate boundaries. Earthquake experts recognized an interesting pattern of earthquake distribution. Most major earthquakes occur in belts rather than being randomly distributed around Earth. Most volcanoes exhibit a similar pattern. This pattern later served as evidence for the location of plate margins, that is, the zones of contact between different crustal plates. Earthquakes result from friction caused by one plate moving against another.

Improved mapping also made it possible to view the retrofit of continents in terms of the fit between the true extent of the continental crust instead of the current coastlines that are much variable to influences of weather and ocean levels.

In his important 1960 publication, History of Ocean Basins, geologist and U.S. Navy Admiral Harry Hess (19061969) provided the missing explanatory mechanism for plate tectonic theory by suggesting that the thermal convection currents in the athenosphere provided the driving force behind plate movements. Subsequent to Hesss book, geologists Drummond Matthews (19311997) and Fred Vine (19391988) at Cambridge University used magnetometer readings to correlate the paired bands of varying magnetism and anomalies located on either side of divergent boundaries. Vine and Matthews realized that magnetic data reveling strips of polar reversals symmetrically displaced about a divergent boundary confirmed Hesss assertions regarding seafloor spreading.

In the 1960s ocean research ships began drilling into the sediments and the solid rock below the sediment in the deeper parts of the ocean. Perhaps the most striking discovery was the great age difference between the oldest continental bedrock and the oldest oceanic bedrock. Continental bedrock is over a billion years old in many areas of the continents, with a maximum age of 3.6 billion years. Nowhere is the ocean crust older than 180 million years.

Marine geologists discovered another relationship. The age of the oceanic bedrock and the sediments directly above it increase from the deep ocean basins to the continental margins. That is, the ocean floor is oldest next to the continents and youngest near the center of ocean basins. In addition, ocean crust on opposing sides of MORs show the same pattern of increasing age away from the MORs.

The great age of continental rocks results from their inability to be subducted. Once formed, continental crust becomes a permanent part of Earths surface. We also know that the increase in age of ocean crust away from ocean basins results from creation of new sea floor at the MORs, with destruction of older sea floor at ocean trenches, which are often located near continental margins.

Plate movement an today be measured by sophisticated GPS and laser-based measuring systems. A much slower but certainly more spectacular proof of plate movement is exemplified by the still-ongoing formation of the Hawaiian Islands. The Pacific plate is moving north over a stationary lava source in the mantle, known as a hot spot. Lava rises upwards from this hot spot to the surface and forms a volcano. After a few million years, that volcano becomes extinct as it moves north, away from the hot spot, and a new volcano begins to form to the south. As of the early 2000s a new volcano is forming on the ocean floor south of the island of Hawaii.

Rates of plate movement

Plates move at rates of about an inch (a few centimeters) per year. Scientists first estimated the rate of plate movement based on radiometric dating of ocean crust. By determining the age of a crustal sample, and knowing its distance from the MOR at which it formed, they estimate the rate of new ocean floor production and plate movement. As of 2006, satellites capable of measurement of plate motion provide a more direct method. Results from these two methods agree fairly closely. The fastest plates move more than 4 in (10 cm) per year. The rate of motion of the North American plate averages 1.2 in (3 cm) per year.

Scale and number of plates

Estimates of the number of plates differ, but most geologists recognize at least fifteen and some as many as twenty. These plates have many different shapes and sizes. Some, such as the Juan de Fuca plate off the west coast of Washington State, have surface areas of a few thousand square miles. The largest, the Pacific plate, underlies most of the Pacific Ocean and covers an area of hundreds of thousands of square miles. In the distant geologic past, Earths lithosphere perhaps consisted of many more of these smaller plates, rather than the comparatively few, larger plates now present.

Plate interactions

Tectonic plates can interact in one of three ways. They can move toward one another, or converge; move away from one another, or diverge; or slide past one another, a movement known as transform motion. All plate margins have one thing in commonearthquakes. Most earthquakes occur along plate margins. The other types of activity that occur when two plates interact depend on the nature of the plate interaction and of the margins. Plate margins (or boundaries) come in three varieties: oceanic-oceanic, continental-continental, and continental-oceanic.

Oceanic-oceanic plates

Recall that plates in continental areas are thicker and less dense than in oceanic areas. When two oceanic plates converge (an oceanic-oceanic convergent margin) one of the plates subducts into a trench. The subducted plate sinks downward into the mantle where it begins to melt. Molten rock from the melting plate rises toward the surface and forms a chain of volcanic islands, or a volcanic island arc, behind the ocean trench. Subduction of the Pacific plate below the North American plate along the coast of Alaska formed the Aleutian Trench and the Aleutian Islands, a volcanic island arc. At oceanic-oceanic divergent margins, sea floor spreading occurs and the ocean slowly grows wider. Today, Europe and North America move about 3 in (7.6 cm) farther apart every year as the Atlantic Ocean grows wider.

Continental-continental plates

Because of their lower density and greater thickness, continental-continental convergent plate margins behave differently than oceanic-oceanic margins. Continental crust is too light to be carried downward into a trench, so neither plate is subducted in a continent-continent collision. The two plates converge, buckle, fold, and fault to form complex mountains ranges of great height. Continental-continental convergence produced the Himalayas when the Indian-Australian plate collided with the Eurasian plate.

Continental-continental divergence causes a continent to separate into two or more smaller continents when it is pulled apart along a series of faults to create a continental rift. Eventually, if the process of continental rifting continues (it may fail, leaving the continent fractured but whole), a new sea is born between the two continents. In this way rifting between the Arabian and African plates formed the Red Sea. The Rio Grande rift of New Mexico and Colorado, through which flows the Rio Grande, is a failed continental rift that is essentially inactive today.

Continental-oceanic plates

When continental and oceanic plates converge, the scenario is a predictable one. Because of its greater density, the oceanic plate easily subducts below the edge of the continental plate. Again subduction of the oceanic plate leads to volcano formation, but in this setting, the chain of volcanoes forms on the continental crust. This volcanic mountain chain, known as a volcanic arc, is usually several hundred miles inland from the plate margin. The Andes Mountains of South America and the Cascade Mountains of North America are examples of volcanic arcs formed by subduction along a continental-oceanic convergent margin. Continental-oceanic convergence may form a prominent trench, but no continental-oceanic divergent margins exist today. They are unlikely to form and would quickly become oceanic-oceanic divergent margins as sea floor spreading occurred.

Transform margins

In addition to convergence and divergence, transform motion may occur along plate margins. Along transform margins, plate movement produces periodic earthquakes as the two plates slide past one another. The best known example of a transform plate margin is the San Andreas fault in California, which separates the Pacific and North American plates.

Continent formation

Knowledge of the processes involved in the formation of continents is limited, but geologists infer that formation of the early continents resulted from subduction at oceanic-oceanic convergent margins. When a plate is subducted, a process known as partial melting occurs. Partial melting of mafic rock results in the production of magma that is more felsic in composition; that is, it has a composition intermediate between basalt and granite. In addition, weathering of mafic rock at Earths surface also produces sediments with a more felsic composition. When these sediments subduct, they yield magma of felsic composition via partial melting.

Repeated episodes of subduction and partial melting, followed by volcanic eruption, produced lava of increasingly felsic composition. This cycle eventually formed volcanic island arcs that were too buoyant to be subducted and became a permanent part of Earths surface. When sea floor spreading pushes one of these buoyant volcanic island arcs toward a subduction zone, it is added or accreted onto the side of the volcanic island arc forming on the other side of the trench. Over time, microcontinents grew by accretion to form larger continental masses.

Continents have either passive or active margins. Passive margins are found where the continents edge is on the same plate as the adjacent ocean, and it is along passive margins that accretionary wedges form. Active margins are found where the continent and the bordering oceanic crust are on separate plates. In these situations, a subduction zone is usually present. In general, the continents bordering the Atlantic Ocean have passive margins, while those surrounding the Pacific Ocean, which has a very active MOR, have active margins.

Driving mechanism

Geologists have inferred that convection cells in Earths interior are the driving force for plate motion. Convection cells in the mantle bring molten rock to the surface along MORs where it forms new ocean crust.

KEY TERMS

Accretion The addition of sediment or rock to a plates margin at a subduction zone. Material is scraped off the subducting plate and adheres to the edge of the overriding plate.

Basalt A dense, dark colored igneous rock, with a composition rich in iron and magnesium (a mafic composition).

Convection cells The circular movement of a fluid in response to alternating heating and cooling. Convection cells in Earths interior involve molten rock that rises upwards below midoceanic ridges. ConvergenceThe movement of two plate margins toward one another; usually associated with plate subduction or the collision of two continents.

Crust The outermost layer of Earth, situated over the mantle and divided into continental and oceanic crust.

Divergence The separation of two plate margins as they move in opposing directions; usually associated with either sea floor spreading or continental rifting.

Granite A light-colored igneous rock that is less dense than basalt due to an abundance of lighter elements, such as silicon and oxygen (a felsic composition).

Hot spots Areas in the mantle, associated with rising plumes of molten rock, which produce frequent, localized volcanic eruptions at Earths surface.

Magnetic reversals Periods during which Earths magnetic poles flip-flop; that is, the orientation of Earths magnetic field reverses. During these periods of reversed magnetism, compass needles point toward the south pole.

Mantle The thick, dense layer of rock that underlies Earths crust.

Microcontinents Volcanic islands of intermediate to felsic composition that were too buoyant to subduct, and therefore formed the first continental crust.

Mid-oceanic ridges Continuous submarine mountain ranges, composed of basalt, where new sea floor is created.

Ocean trench A deep depression in the sea floor, created by an oceanic plate being forced downward into the subsurface by another, overriding plate.

Plates Large regions of Earths surface, composed of the crust and uppermost mantle, which move about, forming many of Earths major geologic surface features.

Sea-floor spreading The part of plate tectonics that describes the movement of the edges of two of the plates forming Earths crust away from each other under the ocean. Sea-floor spreading results in the formation of new submarine surfaces.

Subduction In plate tectonics, the movement of one plate down into the mantle where the rock melts and becomes magma source material for new rock.

Transform motion Horizontal plate movement in which one plate margin slides past another.

Below the crust, pressure is exerted on the bottom of the plates by the convection cell, helping to push the plates along, and causing divergence. At the trenches, the cells may also exert a downward force on the descending plates, helping to pull them down into the mantle.

Importance of plate tectonics

Plate tectonics revolutionized the way geologists view Earth. This new paradigm brings together nearly all the divisions of geologic study. Like the theory of evolution in biology, plate tectonics is the unifying concept of geology. The initial appeal and rapid acceptance of plate tectonics resulted from its ability to provide answers to many unresolved questions about a variety of seemingly unrelated phenomena.

Plate tectonics also revitalized the field of geology by providing a new perspective from which to interpret many old ideas. Finally, plate tectonics explains nearly all of Earths major surface features and activities. These include faults and earthquakes, volcanoes and volcanism, mountains and mountain building, and even the origin of the continents and ocean basins.

See also Earth science.

Resources

BOOKS

Blatt, H., R. Tracy, and B. Owens. Petrology: Igneous, Sedimentary, and Metamorphic. New York: Freeman, 2005.

Fowler, C.M.R. The Solid Earth: An Introduction to Global Geophysics. Cambridge, United Kingdon: Cambridge University Press, 2004.

Rogers, J.J.W., and M. Santosh. Continents and Supercontinents. Oxford, United Kingdom: Oxford University Press, 2004.

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, New Jersey: Prentice Hall, 2004.

K. Lee Lerner

Clay Harris

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Plate Tectonics

Plate tectonics

Plate tectonics, is the theory explaining geologic changes that result from the movement of lithospheric plates over the asthenosphere (the molten, ductile, upper portion of Earth's mantle). Plates move and shift their positions relative to one another. Movement of and contact between plates either directly or indirectly accounts for most of the major geologic features at Earth's surface.

The visible continents, a part of the lithospheric plates upon which they ride, shift slowly over time as a result of the forces driving plate tectonics. Moreover, plate tectonic theory is so robust in its ability to explain and predict geological processes that it is equivalent in many regards to the fundamental and unifying principles of evolution in biology , and nucleosynthesis in physics and chemistry .


Continental drift versus plate tectonics

Based upon centuries of cartographic depictions that allowed a good fit between the Western coast of Africa and the Eastern coast of South America , in 1858, French geographer Antonio Snider-Pellegrini, published a work asserting that the two continents had once been part of larger single continent ruptured by the creation and intervention of the Atlantic Ocean. In the 1920s, German geophysicist Alfred Wegener's writings advanced the hypothesis of continental drift depicting the movement of continents through an underlying oceanic crust. Wegner's hypothesis met with wide skepticism but found support and development in the work and writings of South African geologist Alexander Du Toit who discovered a similarity in the fossils found on the coasts of Africa and South Americas that derived from a common source.

What Wegener's continental drift theory lacked was a propelling mechanism. Other scientists wanted to know what was moving these continents around. Unfortunately, Wegener could not provide a convincing answer. Therefore, other scientists heavily disputed his theory and it fell into disrepute.

The technological advances necessitated by the Second World War made possible the accumulation of significant evidence now underlying modern plate tectonic theory.

The theory of plate tectonics gained widespread acceptance only in the late 1960s to early 1970s.


An overview of tectonic theory

Plate tectonic theory asserts that Earth is divided into core, mantle, and crust. The crust is subdivided into oceanic and continental crust. The oceanic crust is thin (3–4.3 mi [5–7 km]), basaltic (<50% SiO2), dense, and young (<250 million years old). In contrast, the continental crust is thick (18.6–40 mi [30–65 km]), granitic (>60% SiO2), light, and old (250–3,700 million years old). The outer crust is further subdivided by the subdivision of the lithosperic plates, of which it is a part, into 13 major plates. These lithospheric plates, composed of crust and the outer layer of the mantle, contain a varying combination of oceanic and continental crust. The lithospheric plates move on top of mantle's athenosphere.

Boundaries are adjacent areas where plates meet. Divergent boundaries are areas under tension where plates are pushed apart by magma upwelling from the mantle. Collision boundaries are sites of compression either resulting in subduction (where lithospheric plates are driven down and destroyed in the molten mantle) or in crustal uplifting that results in orogeny (mountain building). At transform boundaries, exemplified by the San Andreas fault , the continents create a shearing force as they move laterally past one another.

New oceanic crust is created at divergent boundaries that are sites of sea-floor spreading. Because Earth remains roughly the same size, there must be a concurrent destruction or uplifting of crust so that the net area of crust remains the same. Accordingly, as crust is created at divergent boundaries, oceanic crust must be destroyed in areas of subduction underneath the lighter continental crust. The net area is also preserved by continental crust uplift that occurs when less dense continental crust collides with continental crust. Because both continental crusts resist subduction, the momentum of collision causes an uplift of crust, forming mountain chains. A vivid example of this type of collision is found in the ongoing collision of India with Asia that has resulted in the Himalayan mountains that continue to increase in height each year. This dynamic theory of plate tectonics also explained the formation of island arcs formed by rising material at sites where oceanic crust subducts under oceanic crust, the formation of mountain chains where oceanic crust subducts under continental crust (e.g., Andes mountains), and volcanic arcs in the Pacific. The evidence for deep, hot, convective currents combined with plate movement (and concurrent continental drift) also explained the mid-plate "hot spot" formation of volcanic island chains (e.g., Hawaiian islands) and the formation of rift valleys (e.g., Rift Valley of Africa). Mid-plate earthquakes, such as the powerful New Madrid earthquake in the United States in 1811, are explained by interplate pressures that bend plates much like a piece of sheet metal pressed from opposite sides.

Proofs of tectonic theory

As with continental drift theory two of the proofs of plate tectonics are based upon the geometric fit of the displaced continents and the similarity of rock ages and Paleozoic fossils in corresponding bands or zones in adjacent or corresponding geographic areas (e.g., between West Africa and the eastern coast of South America).

Ocean topography also provided evidence of plate tectonic theory. Nineteenth century surveys of the oceans indicated that rather than being flat featureless plains, as was previously thought, some ocean areas are mountainous while others plummet to great depths. Contemporary geologic thinking could not easily explain these topographic variations, or "oceanscapes." Surveys in the 1950s and 1960s provided an even more detailed picture of the ocean bottom. Long, continuous mountain chains appeared, as well as numerous ocean deeps shaped like troughs. Geoscientists later identified the mountainous features as the mid-oceanic ridges (MORs) where new plates form, and the deep ocean trenches as subduction zones where plates descend into the subsurface.

Modern understanding of the structure of Earth is derived in large part from the interpretation of seismic studies

that measure the reflection of seismic waves off features in Earth's interior . Different materials transmit and reflect seismic shock waves in different ways, and of particular importance to theory of plate tectonics is the fact that liquid does not transmit a particular form of seismic wave known as an S wave. Because the mantle transmits S-waves, it was long thought to be a cooling solid mass . Geologists later discovered that radioactive decay provided a heat source with Earth's interior that made the athenosphere plasticine (semi-solid). Although solid-like with regard to transmission of seismic S-waves, the athenosphere contains very low velocity (inches per year) currents of mafic (magma-like) molten materials.

Another line of evidence in support of plate tectonics came from the long-known existence of ophiolte suites (slivers of oceanic floor with fossils) found in upper levels of mountain chains. The existence of ophiolte suites are consistent with the uplift of crust in collision zones predicted by plate tectonic theory.

As methods of dating improved, one of the most conclusive lines of evidence in support of plate tectonics derived from the dating of rock samples. Highly supportive of the theory of sea floor spreading (the creation of oceanic crust at a divergent plate boundary (e.g., Mid-Atlantic Ridge) was evidence that rock ages are similar in equidistant bands symmetrically centered on the divergent boundary. More importantly, dating studies show that the age of the rocks increases as their distance from the divergent boundary increases. Accordingly, rocks of similar ages are found at similar distances from divergent boundaries, and the rocks near the divergent boundary where crust is being created are younger than the rocks more distant from the boundary. Eventually, radioisotope studies offering improved accuracy and precision in rock dating also showed that rock specimen taken from geographically corresponding areas of South America and Africa showed a very high degree of correspondence, providing strong evidence that at one time these rock formations had once coexisted in an area subsequently separated by movement of lithospheric plates.

Similar to the age of rocks, studies of fossils found in once adjacent geological formations showed a high degree of correspondence. Identical fossils are found in bands and zones equidistant from divergent boundaries. Accordingly, the fossil record provides evidence that a particular band of crust shared a similar history as its corresponding band of crust located on the other side of the divergent boundary.

The line of evidence, however, that firmly convinced modern geologists to accept the arguments in support of plate tectonics derived from studies of the magnetic signatures or magnetic orientations of rocks found on either side of divergent boundaries. Just as similar age and fossil bands exist on either side of a divergent boundary, studies of the magnetic orientations of rocks reveal bands of similar magnetic orientation that were equidistant and on both sides of divergent boundaries. Tremendously persuasive evidence of plate tectonics is also derived from correlation of studies of the magnetic orientation of the rocks to known changes in Earth's magnetic field as predicted by electromagnetic theory. Paleomagnetic studies and discovery of polar wandering, a magnetic orientation of rocks to the historical location and polarity of the magnetic poles as opposed to the present location and polarity, provided a coherent map of continental movement that fit well with the present distribution of the continents.

Paleomagnetic studies are based upon the fact that some hot igneous rocks (formed from volcanic magma) contain varying amounts of ferromagnetic minerals (e.g., Fe3O4) that magnetically orient to the prevailing magnetic field of Earth at the time they cool. Geophysical and electromagnetic theory provides clear and convincing evidence of multiple polar reversals or polar flips throughout the course of Earth's history. Where rock formations are uniform—i.e., not grossly disrupted by other geological processes—the magnetic orientation of magnetite-bearing rocks can also be used to determine the approximate latitude the rocks were at when they cooled and took on their particular magnetic orientation. Rocks with a different orientation to the current orientation of the Earth's magnetic field also produce disturbances or unexpected readings (anomalies) when scientists attempt to measure the magnetic field over a particular area.

This overwhelming support for plate tectonics came in the 1960s in the wake of the demonstration of the existence of symmetrical, equidistant magnetic anomalies centered on the Mid-Atlantic Ridge. During magnetic surveys of the deep ocean basins, geologists found areas where numerous magnetic reversals occur in the ocean crust. These look like stripes, oriented roughly parallel to one another and to the MORs. When surveys were run on the other side of the MORs, they showed that the magnetic reversal patterns were remarkably similar on both sides of the MORs. After much debate, scientists concluded that new ocean crust must form at the MORs, recording the current magnetic orientation. This new ocean crust pushes older crust out of the way, away from the MOR. When a magnetic reversal occurs, new ocean crust faithfully records it as a reversed magnetic "stripe" on both sides of the MOR. Older magnetic reversals were likewise recorded; these stripes are now located farther from the MOR.

Geologists were comfortable in accepting these magnetic anomalies located on the sea floor as evidence of sea floor spreading because they were able to correlate these anomalies with equidistant radially distributed magnetic anomalies associated with outflows of lava from land-based volcanoes.

Additional evidence continued to support a growing acceptance of tectonic theory. In addition to increased energy demands requiring enhanced exploration, during the 1950s there was an extensive effort, partly for military reasons related to what was to become an increasing reliance on submarines as a nuclear deterrent force, to map the ocean floor. These studies revealed the prominent undersea ridges with undersea rift valleys that ultimately were understood to be divergent plate boundaries. An ever-growing network of seismic reporting stations, also spurred by the Cold War need to monitor atomic testing, provided substantial data that these areas of divergence were tectonically active sites highly prone to earthquakes. Maps of the global distribution of earthquakes readily identified stressed plate boundaries. Earthquake experts recognized an interesting pattern of earthquake distribution. Most major earthquakes occur in belts rather than being randomly distributed around Earth. Most volcanoes exhibit a similar pattern. This pattern later served as evidence for the location of plate margins, that is, the zones of contact between different crustal plates. Earthquakes result from friction caused by one plate moving against another.

Improved mapping also made it possible to view the retrofit of continents in terms of the fit between the true extent of the continental crust instead of the current coastlines that are much variable to influences of weather and ocean levels.

In his important 1960 publication, "History of Ocean Basins," geologist and U.S. Navy Admiral Harry Hess (1906–1969) provided the missing explanatory mechanism for plate tectonic theory by suggesting that the thermal convection currents in the athenosphere provided the driving force behind plate movements. Subsequent to Hess's book, geologists Drummond Matthews (1931–1997) and Fred Vine (1939–1988) at Cambridge University used magnetometer readings previously collected to correlate the paired bands of varying magnetism and anomalies located on either side of divergent boundaries. Vine and Matthews realized that magnetic data reveling strips of polar reversals symmetrically displaced about a divergent boundary confirmed Hess's assertions regarding seafloor spreading.

In the 1960s ocean research ships began drilling into the sediments and the solid rock below the sediment, called bedrock , in the deeper parts of the ocean. Perhaps the most striking discovery was the great age difference between the oldest continental bedrock and the oldest oceanic bedrock. Continental bedrock is over a billion years old in many areas of the continents, with a maximum age of 3.6 billion years. Nowhere is the ocean crust older than 180 million years.

Marine geologists discovered another curious relationship as well. The age of the oceanic bedrock and the sediments directly above it increase as you move from the deep ocean basins to the continental margins. That is, the ocean floor is oldest next to the continents and youngest near the center of ocean basins. In addition, ocean crust on opposing sides of MORs show the same pattern of increasing age away from the MORs.

The great age of continental rocks results from their inability to be subducted. Once formed, continental crust becomes a permanent part of Earth's surface. We also know that the increase in age of ocean crust away from ocean basins results from creation of new sea floor at the MORs, with destruction of older sea floor at ocean trenches, which are often located near continental margins.

Plate movement an today be measured by sophisticated GPS and laser-based measuring systems. A much slower but certainly more spectacular proof of plate movement is exemplified by the still-ongoing formation of the Hawaiian Islands. The Pacific plate is moving north over a stationary lava source in the mantle, known as a hot spot . Lava rises upwards from this hot spot to the surface and forms a volcano . After a few million years, that volcano becomes extinct as it moves north, away from the hot spot, and a new volcano begins to form to the south. A new volcano is forming today on the ocean floor south of the island of Hawaii.


Rates of plate movement

Plates move at rates of about an inch (a few centimeters) per year. Scientists first estimated the rate of plate movement based on radiometric dating of ocean crust. By determining the age of a crustal sample, and knowing its distance from the MOR at which it formed, they estimate the rate of new ocean floor production and plate movement. Today, satellites capable of measurement of plate motion provide a more direct method. Results from these two methods agree fairly closely. The fastest plates move more than 4 in (10 cm) per year. The rate of motion of the North American plate averages 1.2 in (3 cm) per year.


Scale and number of plates

Estimates of the number of plates differ, but most geologists recognize at least fifteen and some as many as twenty. These plates have many different shapes and sizes. Some, such as the Juan de Fuca plate off the west coast of Washington State, have surface areas of a few thousand square miles. The largest, the Pacific plate, underlies most of the Pacific Ocean and covers an area of hundreds of thousands of square miles. In the distant geologic past, Earth's lithosphere perhaps consisted of many more of these smaller plates, rather than the comparatively few, larger plates now present.

Plate interactions

Tectonic plates can interact in one of three ways. They can move toward one another, or converge; move away from one another, or diverge; or slide past one another, a movement known as transform motion. All plate margins along which plate movement is occurring have one thing in common—earthquakes. In fact, most earthquakes happen along plate margins. The other types of activity that occur when two plates interact are dependent on the nature of the plate interaction and of the margins. Plate margins (or boundaries) come in three varieties: oceanic-oceanic, continental-continental, and continental-oceanic.


Oceanic-oceanic plates

Recall that plates in continental areas are thicker and less dense than in oceanic areas. When two oceanic plates converge (an oceanic-oceanic convergent margin) one of the plates subducts into a trench. The subducted plate sinks downward into the mantle where it begins to melt. Molten rock from the melting plate rises toward the surface and forms a chain of volcanic islands, or a volcanic island arc, behind the ocean trench. Subduction of the Pacific plate below the North American plate along the coast of Alaska formed the Aleutian Trench and the Aleutian Islands, a volcanic island arc. At oceanic-oceanic divergent margins, sea floor spreading occurs and the ocean slowly grows wider. Today, Europe and North America move about 3 in (7.6 cm) farther apart every year as the Atlantic Ocean grows wider.


Continental-continental plates

Due to their lower density and greater thickness, continental-continental convergent plate margins act quite differently than oceanic-oceanic margins. Continental crust is too light to be carried downward into a trench. At continental-continental convergent margins neither plate subducts. The two plates converge, buckle, fold , and fault to form complex mountains ranges of great height. Continental-continental convergence produced the Himalayas when the Indian-Australian plate collided with the Eurasian plate.

Continental-continental divergence causes a continent to separate into two or more smaller continents when it is ripped apart along a series of fractures. The forces of divergence literally tear a continent apart as the two or more blocks of continental crust begin slowly moving apart and magma pushes into the rift formed between them. Eventually, if the process of continental rifting continues (it may fail, leaving the continent fractured but whole), a new sea is born between the two continents. In this way rifting between the Arabian and African plates formed the Red Sea.


Continental-oceanic plates

When continental and oceanic plates converge, the scenario is a predictable one. Due to its greater density, the oceanic plate easily subducts below the edge of the continental plate. Again subduction of the oceanic plate leads to volcano formation, but in this setting, the chain of volcanoes forms on the continental crust. This volcanic mountain chain, known as a volcanic arc, is usually several hundred miles inland from the plate margin. The Andes Mountains of South America and the Cascade Mountains of North America are examples of volcanic arcs formed by subduction along a continental-oceanic convergent margin. Continental-oceanic convergence may form a prominent trench, but not always. No continental-oceanic divergent margins exist today. As you can imagine, they are unlikely to form and would quickly become oceanic-oceanic divergent margins as sea floor spreading occurred.


Transform margins

In addition to convergence and divergence, transform motion may occur along plate margins. Transform margins, in many ways, are less spectacular than convergent and divergent ones, and the type of plates involved is really of no significance. Along transform margins, about all that occurs are faults and earthquakes. Plate movement produces the earthquakes, as the two rock slabs slide past one another. The best known example of a transform plate margin is the San Andreas fault in California, where the Pacific and North American plates are in contact.


Continent formation

If sea floor spreading only produces basaltic (oceanic) rock, where did the continents come from? Knowledge of the processes involved is somewhat limited, but formation of the early continents resulted from subduction at oceanic-oceanic convergent margins. When plates subduct, a process known as partial melting occurs. Partial melting of mafic rock results in the production of magma that is more felsic in composition; that is, it has a composition intermediate between basalt and granite. In addition, weathering of mafic rock at the earth's surface also produces sediments with a more felsic composition. When these sediments subduct, they yield magma of felsic composition via partial melting.

Repeated episodes of subduction and partial melting, followed by volcanic eruption, produced lavas of increasingly felsic composition. Finally, this cycle formed volcanic island arcs that were too buoyant to be subducted and became a permanent part of Earth's surface. When sea floor spreading pushes one of these buoyant volcanic island arcs toward a subduction zone, rather than subducting, it welds, or accretes, onto the side of the volcanic island arc forming on the other side of the trench. Over time, these microcontinents, through accretion, formed larger continental masses.

Continents "float" on the plastic material making up the mantle like a block of wood floats on water . As erosion occurs, sediments are carried from mountains and higher elevations out to sea, where they accumulate on the continental shelf , forming wedges of sediment. Such accretionary wedges can extend far out to sea, depending on the size and shape of the continental shelf. As erosion moves sediments from the interior of the continent to the edges, the continent gets thinner but its surface area becomes larger. If conditions remain stable, accretionary wedges can go on accumulating for a very long time, reaching hundreds of miles out into the ocean. Sometimes, the wedge becomes so thick it rises above sea level to become dry land.

Continents have either passive or active margins. Passive margins are found where the continent's edge is on the same plate as the adjacent ocean, and it is along passive margins that accretionary wedges form. Active margins are found where the continent and the bordering oceanic crust are on separate plates. In these situations, a subduction zone is usually present. In general, the continents bordering the Atlantic Ocean have passive margins, while those surrounding the Pacific Ocean, which has a very active MOR, have active margins.


Driving mechanism

Most geologists believe convective cells in the earth's interior are the driving force for plate motion. If you have ever seen a rapidly boiling pot of water, then you know about convection cells. In the center of the pot, bubbles rise to the surface and push water to the sides. Along the sides, the water cools and descends back down to the bottom of the pot to be heated again.

In a similar way, convection cells in the mantle bring molten rock to the surface along MORs where it forms new ocean crust. Below the crust, pressure is exerted on the bottom of the plates by the convection cell , helping to push the plates along, and causing divergence. At the trenches, the cells may also exert a downward force on the descending plates, helping to pull them down into the mantle.


Importance of plate tectonics

Plate tectonics revolutionized the way geologists view Earth. This new paradigm brings together nearly all the divisions of geologic study. Like the theory of evolution in biology, plate tectonics is the unifying concept of geology . Plate tectonics' initial appeal and rapid acceptance resulted from its ability to provide answers to many nagging questions about a variety of seemingly unrelated phenomena. Plate tectonics also revitalized the field of geology by providing a new perspective from which to interpret many old ideas. Finally, plate tectonics explains nearly all of Earth's major surface features and activities. These include faults and earthquakes, volcanoes and volcanism, mountains and mountain building, and even the origin of the continents and ocean basins.

See also Earth science.


Resources

books

Hancock, P.L., and B.J. Skinner, eds. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.

Tarbuck, Edward. D., Frederick K. Lutgens, and Tasa Dennis. Earth: An Introduction to Physical Geology. 7th ed. Upper Saddle River, NJ: Prentice Hall, 2002.

Winchester, Simon. The Map That Changed the World: WilliamSmith and the Birth of Modern Geology. New York: Harper Collins, 2001.

periodicals

Buffett, Bruce A. "Earth's Core and the Geodynamo." Science (June 16, 2000): 2007–2012.

Hellfrich, George, and Bernard Wood. "The Earth's Mantle." Nature (August 2, 2001): 501–507.


other

United States Department of the Interior, U.S. Geological Survey. "This Dynamic Earth: The Story of Plate Tectonics." February 21, 2002 [cited March 11, 2003]. <http://pubs.usgs.gov/publications/text/dynamic.html>.


K. Lee Lerner

Clay Harris

KEY TERMS


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Accretion

—The addition of sediment or rock to a plate's margin at a subduction zone. Material is scraped off the subducting plate and adheres to the edge of the overriding plate.

Basalt

—A dense, dark colored igneous rock, with a composition rich in iron and magnesium (a mafic composition).

Convection cells

—The circular movement of a fluid in response to alternating heating and cooling. Convection cells in the earth's interior involve molten rock that rises upwards below midoceanic ridges.

Convergence

—The movement of two plate margins toward one another; usually associated with plate subduction or the collision of two continents.

Crust

—The outermost layer of the earth, situated over the mantle and divided into continental and oceanic crust.

Divergence

—The separation of two plate margins as they move in opposing directions; usually associated with either sea floor spreading or continental rifting.

Granite

—A light-colored igneous rock that is less dense than basalt due to an abundance of lighter elements, such as silicon and oxygen (a felsic composition).

Hot spots

—Areas in the mantle, associated with rising plumes of molten rock, which produce frequent, localized volcanic eruptions at Earth's surface.

Magnetic reversals

—Periods during which the earth's magnetic poles flip-flop; that is, the orientation of Earth's magnetic field reverses. During these periods of reversed magnetism, compass needles point toward the south pole.

Mantle

—The thick, dense layer of rock that underlies Earth's crust.

Microcontinents

—Volcanic islands of intermediate to felsic composition that were too buoyant to subduct, and therefore formed the first continental crust.

Mid-oceanic ridges

—Continuous submarine mountain ranges, composed of basalt, where new sea floor is created.

Ocean trench

—A deep depression in the sea floor, created by an oceanic plate being forced downward into the subsurface by another, overriding plate.

Plates

—Large regions of the earth's surface, composed of the crust and uppermost mantle, which move about, forming many of Earth's major geologic surface features.

Sea-floor spreading

—The part of plate tectonics that describes the movement of the edges of two of the plates forming Earth's crust away from each other under the ocean. Sea-floor spreading results in the formation of new submarine surfaces.

Subduction

—In plate tectonics, the movement of one plate down into the mantle where the rock melts and becomes magma source material for new rock.

Transform motion

—Horizontal plate movement in which one plate margin slides past another.

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