plate tectonics

plate tectonics
a theory of global tectonics in which the lithosphere is divided into a number of crustal plates, each of which moves on the plastic asthenosphere more or less independently to collide with, slide under, or move past adjacent plates.

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Theory that the Earth's lithosphere (the crust and upper portion of the mantle) is divided into about 12 large plates and several small ones that float on and travel independently over the asthenosphere.

The theory revolutionized the geological sciences in the 1960s by combining the earlier idea of continental drift and the new concept of seafloor spreading into a coherent whole. Each plate consists of rigid rock created by upwelling magma at oceanic ridges, where plates diverge. Where two plates converge, a subduction zone forms, in which one plate is forced under another and into the Earth's mantle. The majority of the earthquakes and volcanoes on the Earth's surface occur along the margins of tectonic plates. The interior of a plate moves as a rigid body, with only minor flexing, few earthquakes, and relatively little volcanic activity.

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 theory dealing with the dynamics of Earth's outer shell, the lithosphere (Earth), that revolutionized Earth sciences by providing a uniform context for understanding mountain-building processes (orogeny), volcanoes (volcano), and earthquakes (earthquake), as well as understanding the evolution of Earth's surface and reconstructing its past continental and oceanic configurations.

      The concept of plate tectonics was formulated in the 1960s. According to the theory, Earth has a rigid outer layer, known as the lithosphere, which is about 100 km (60 miles) thick and overlies a plastic layer called the asthenosphere. The lithosphere is broken up into about a dozen large plates and several small ones. These plates move relative to each other, typically at rates of 5 to 10 cm (2 to 4 inches) per year, and interact at their boundaries, where they either converge, diverge, or slip past one another. Such interactions are thought to be responsible for most of Earth's seismic and volcanic activity, although earthquakes and volcanoes are not wholly absent in plate interiors. Plate motions cause mountains to rise where they push together or continents to fracture and oceans to form where they pull apart. The continents are embedded in the plates and drift passively with them, which over millions of years results in significant changes in Earth's geography.

      The theory of plate tectonics is based on a broad synthesis of geologic and geophysical data. It is now almost universally accepted, and its adoption represents a true scientific revolution, analogous in its consequences to the Rutherford (Rutherford atomic model) and Bohr (Bohr atomic model) atomic models in physics or the discovery of the genetic code in biology. Incorporating the much older idea of continental drift, the theory of plate tectonics has provided an overarching framework in which to describe the past geography of continents and oceans, processes controlling the creation and destruction of landforms, and the evolution of Earth's crust, atmosphere, biosphere, ancient oceans, and climates.

      For details on the specific effects of plate tectonics, see the articles earthquake and volcano. A detailed treatment of the various land and submarine relief features associated with plate motion is provided in the articles tectonic landform and ocean.

Principles of plate tectonics
 In essence, plate tectonic theory is elegantly simple. Earth's surface layer, from 50 to 100 km (30 to 60 miles) thick, is rigid and is composed of a set of large and small plates. Together these plates constitute the lithosphere, from the Greek lithos, meaning “rock.” The lithosphere rests on and slides over an underlying, weaker layer of plastic and partially molten rock known as the asthenosphere, from the Greek asthenos, meaning “weak.” Plate movement is possible because the lithosphere-asthenosphere boundary is a zone of detachment. As the lithospheric plates move across Earth's surface, driven by forces as yet not fully understood, they interact along their boundaries, diverging, converging, or slipping past each other. While the interiors of the plates are presumed to remain essentially undeformed, plate boundaries are the sites of many of the principal processes that shape the terrestrial surface, including earthquakes (earthquake), volcanism (volcano), and orogeny (that is, formation of mountain ranges).

Continental and oceanic crust
 The lithosphere is subdivided on the basis of the presence of rock types with different chemical compositions. The outermost layer of the lithosphere is called the crust, of which there are two types, continental and oceanic, which differ in their composition and thickness. The distribution of these crustal types closely coincides with the division into continents and ocean basins. The continents have a crust that is broadly granitic (granite) in composition and, with a density of about 2.7 grams per cubic cm (0.098 pound per cubic inch), is somewhat lighter than oceanic crust, which is basaltic (basalt) in composition and has a density of about 2.9 to 3 grams per cubic cm (0.1 to 0.11 pound per cubic inch). Continental crust is typically 40 km (25 miles) thick, while oceanic crust (ocean) measures only 6 to 7 km (3.7 to 4.4 miles) in thickness.

      These crustal rocks both sit on top of the mantle, which is ultramafic in composition (that is, rich in iron- and magnesium-bearing minerals). The boundary between the continental or oceanic crust and the underlying mantle, named the Mohorovičić discontinuity (also called Moho) for Andrija Mohorovičić (Mohorovičić, Andrija), Croatian seismologist and its discoverer, has been clearly defined by seismic studies. The mantle is Earth's most voluminous layer, extending to a depth of 2,900 km (1,800 miles). However, increasing temperatures with depth cause mantle rocks to lose their rigidity at about 100 km (60 miles) below the surface. This change in rheology (a science examining the flow and deformation of materials) defines the base of the lithosphere and the top of the asthenosphere. This upper portion of the mantle, known as the lithospheric mantle, has an average density of about 3.3 grams per cubic cm (0.12 pound per cubic inch).

      The effect of the different densities of lithospheric rock can be seen in the different average elevations of continental and oceanic crust. The less-dense continental crust has greater buoyancy, causing it to float much higher in the mantle. Its average elevation above sea level is 840 metres (2,750 feet), while the average depth of oceanic crust is 3,790 metres (12,400 feet). This density difference creates two principal levels of Earth's surface.

Plate boundaries
      Lithospheric plates are much thicker than oceanic or continental crust. Their boundaries do not usually coincide with those between oceans and continents, and their behaviour is only partly influenced by whether they carry oceans, continents, or both. The Pacific Plate, for example, is purely oceanic, whereas the North American Plate is capped by continental crust in the west (the North American continent) and by oceanic crust in the east—it extends under the Atlantic Ocean as far as the Mid-Atlantic Ridge.

 In a simplified example of plate motion shown in the figure, movement of plate A to the left relative to plates B and C results in several types of simultaneous interactions along the plate boundaries. At the rear, plates A and B move apart, or diverge, resulting in extension. At the front, plates A and B overlap, or converge, resulting in compression. Along the sides, the plates slide past one another, a process called shear.

Divergent margins
      As plates move apart at a divergent plate boundary, the release of pressure produces partial melting of the underlying mantle. This molten material, known as magma, is basaltic (basalt) in composition and is buoyant. As a result, it wells up from below and cools close to the surface to generate new crust. Because new crust is formed, divergent margins are also called constructive margins.

Continental rifting
      Upwelling of magma causes the overlying lithosphere to uplift and stretch. If the diverging plates are capped by continental crust, fractures develop that are invaded by the ascending magma, prying the continents farther apart. Settling of the continental blocks creates a rift valley, such as the present-day East African Rift Valley (East African Rift System). As the rift continues to widen, the continental crust becomes progressively thinner until separation of the plates is achieved and a new ocean is created. The ascending partial melt cools and crystallizes to form new crust. Because the partial melt is basaltic in composition, the new crust is oceanic. Consequently, diverging plate boundaries, even if they originate within continents, eventually come to lie in ocean basins of their own making.

      As upwelling of magma continues, the plates continue to diverge, a process known as seafloor spreading. Samples collected from the ocean floor show that the age of oceanic crust increases with distance from the spreading centre—important evidence in favour of this process. These age data also allow the rate of seafloor spreading to be determined, and they show that rates vary from about 0.1 cm (0.04 inch) per year to 17 cm (6.7 inches) per year. Seafloor spreading rates in the Pacific Ocean are much more rapid than in the Atlantic or Indian oceans. At spreading rates of about 15 cm (6 inches) per year, the entire crust beneath the Pacific Ocean (about 15,000 km, or 9,300 miles, wide) could be produced in 100 million years.

      Divergence and creation of oceanic crust are accompanied by much volcanic (volcano) activity and by many shallow earthquakes (earthquake) as the crust repeatedly rifts, heals, and rifts again. These regions are swollen with heat and so are elevated by 2 to 3 km (1.2 to 1.9 miles) above the surrounding seafloor. Their summits are typically 1 to 5 km (0.6 to 3.1 miles) below the ocean surface. On a global scale, these features form an interconnected system of undersea mountains about 65,000 km (40,000 miles) in length and are called oceanic ridges.

      Oceanic crust, once formed, cools as it ages and eventually becomes denser than the underlying asthenosphere, and so it has a tendency to subduct. The life span of this crust is prolonged by its rigidity, but eventually this resistance is overcome. The geologic record suggests that subduction commences when oceanic crust is about 200 million years old.

Convergent margins
 Given that Earth is constant in volume, the continuous formation of new crust produces an excess that must be balanced by destruction of crust elsewhere. This is accomplished at convergent plate boundaries, also known as destructive plate boundaries, where one plate descends at an angle—that is, is subducted—beneath the other. Where two oceanic plates meet, the older, denser plate is preferentially subducted beneath the younger, warmer one. Where one of the plate margins is oceanic and the other is continental, the greater buoyancy of continental crust prevents it from sinking, and the oceanic plate is preferentially subducted. Continents are preferentially preserved in this manner relative to oceanic crust, which is continuously recycled into the mantle; this explains why ocean floor rocks are generally less than 200 million years old whereas the oldest continental rocks are almost 4 billion years old. Where two plates carrying continental crust collide, neither is subducted. Instead, towering mountain ranges, such as the Himalayas, are created. See the section on Mountain building (plate tectonics).

      Because the plates form an integrated system, it is not necessary that new crust formed at any given divergent boundary be completely compensated at the nearest subduction zone, as long as the total amount of crust generated equals that destroyed.

Subduction zone trenches
      The subduction process involves the descent into the mantle of a slab of cold, hydrated oceanic lithosphere about 100 km (60 miles) thick that carries a relatively thin cap of oceanic sediments. The path of descent is defined by numerous earthquakes along a plane that is typically inclined between 30° and 60° into the mantle and is called the Benioff zone, for American seismologist Hugo Benioff, who pioneered its study. Most, but not all, earthquakes in this planar dipping zone result from compression, and the seismic activity extends 300 to 700 km (185 to 435 miles) below the surface. At greater depths, the subducted plate melts and is recycled into the mantle.

      The site of subduction is marked by a deep trench between 5 and 11 km (3 and 7 miles) deep that is produced by frictional drag between the plates as the descending plate bends before it subducts. The overriding plate scrapes sediments and elevated portions of ocean floor off the upper crust of the lower plate, creating a zone of highly deformed rocks within the trench that becomes attached, or accreted, to the overriding plate. This chaotic mixture is known as an accretionary wedge.

      The rocks in the subduction zone experience high pressures but relatively low temperatures, an effect of the descent of the cold oceanic slab. Under these conditions the rocks recrystallize, or metamorphose, to form a suite of rocks known as blueschists, named for the diagnostic blue mineral glaucophane, which is stable only at the high pressures and low temperatures found in subduction zones.

Island arcs (island arc)
      When the downgoing slab reaches a depth of about 100 km (60 miles), it gets sufficiently warm to drive off its most volatile components, thereby stimulating partial melting of mantle in the plate above the subduction zone (known as the mantle wedge). Melting in the mantle wedge produces magma, which is predominantly basaltic in composition. This magma rises to the surface and gives birth to a line of volcanoes (volcanism) in the overriding plate, known as a volcanic arc, typically a few hundred kilometres behind the oceanic trench. The distance between the trench and the arc, known as the arc-trench gap, depends on the angle of subduction. Steeper subduction zones have relatively narrow arc-trench gaps. A basin may form within this region, known as a forearc basin, and may be filled with sediments derived from the volcanic arc or with remains of oceanic crust.

      If both plates are oceanic, such as in the modern western Pacific Ocean, the volcanoes form a curved line of islands, known as an island arc, that is parallel to the trench. If one plate is continental, the volcanoes form inland, as they do in the Andes of western South America. Though the process of magma generation is similar, the ascending magma may change its composition as it rises through the thick lid of continental crust, or it may provide sufficient heat to melt the crust. In either case, the composition of the volcanic mountains formed tends to be more silicon-rich and iron- and magnesium-poor relative to the volcanic rocks produced by ocean-ocean convergence.

Back-arc basins
      Where both converging plates are oceanic, the margin of the older oceanic crust will be subducted because older oceanic crust is colder and therefore more dense. As the dense slab collapses into the asthenosphere, however, it also may “roll back” oceanward and cause extension in the overlying plate. This results in a process known as back-arc spreading, in which a basin opens up behind the island arc. This style of subduction predominates in the western Pacific Ocean, in which a back-arc basin separates several island arcs from the Asian continent.

Continental collision
      If the rate of subduction in an ocean basin exceeds the rate at which the crust is formed at oceanic ridges, closure of the basin is inevitable, leading ultimately to terminal collision between the approaching continents. For example, the subduction of the Tethys Sea, a wedge-shaped body of water that was located between Gondwana and Laurasia, led to the accretion of terranes (crustal blocks or formations of related rocks) along the margins of Laurasia. These events were followed by continental collisions beginning about 30 million years ago between Africa and Europe and between India and Asia. These collisions culminated in the formation of the Alps and the Himalayas. Because continental lithosphere is too buoyant to be subducted, one continent overrides the other, producing crustal thickening and intense deformation that forces the crust skyward to form huge mountains.

Accretion and obduction
      As subduction leads to contraction of an ocean, elevated regions within the ocean basin such as linear island chains, oceanic ridges, and small crustal fragments (such as Madagascar or Japan), known as terranes, are transported toward the subduction zone, where they are scraped off the descending plate and added—accreted—to the continental margin. Since the Late Devonian (Devonian Period) and Early Carboniferous (Carboniferous Period) periods, some 360 million years ago, subduction beneath the western margin of North America has resulted in several collisions with terranes, each producing a mountain-building event. The piecemeal addition of these accreted terranes has added an average of 600 km (375 miles) in width along the western margin of the North American continent.

      During these accretionary events, small sections of the oceanic crust may break away from the subducting slab as it descends. Instead of being subducted, these slices are thrust over the overriding plate and are said to be obducted. Where this occurs, rare slices of ocean crust, known as ophiolites (ocean), are preserved on land. They provide a valuable natural laboratory for studying the composition and character of the oceanic crust and the mechanisms of their emplacement and preservation on land. A classic example is the Coast Range (Coast Ranges) ophiolite of California, which is one of the most extensive ophiolite terranes in North America. This oceanic crust likely formed during the middle Jurassic Period, roughly 170 million years ago, in an extensional regime within either a back-arc or a forearc basin. It was later accreted to the continental margin of Laurasia.

Mountain building (orogeny)
      Where the rate of seafloor spreading is outpaced by subduction, the oceanic crust eventually becomes completely destroyed, and collision between continental landmasses is inevitable. Because collision occurs between two buoyant plate margins, neither can be subducted. A complex sequence of events occurs, forcing one continent to override the other so that the thickness of the continental crust is effectively doubled. These collisions produce lofty landlocked mountain ranges such as the modern Himalayas. The buoyancy of continental crust eventually causes subduction to cease. Much later, after these ranges have been largely leveled by erosion, it is possible that the original contact, or suture, may be exposed.

      The balance between creation and destruction is demonstrated by the expansion of the Atlantic Ocean by seafloor spreading over the past 200 million years, compensated by the contraction of the Pacific Ocean, and the consumption of an entire ocean between India and Asia (the Tethys Sea). The northward migration of India led to collision with Asia some 40 million years ago. Since that time, India has advanced a further 2,000 km (1,250 miles) beneath Asia, pushing up the Himalayas and forming the Tibetan plateau. Pinned against stable Siberia, China and Indochina were pushed sideways, resulting in strong seismic activity thousands of kilometres from the site of the continental collision.

Transform faults
 Along the third type of plate boundary, two plates move laterally and pass each other along giant fractures in Earth's crust, called transform faults, so called because they are linked to other types of plate boundaries. The majority of transform faults link the offset segments of oceanic ridges. However, transform faults also occur between plate margins with continental crust—for example, the San Andreas Fault in California and the North Anatolian fault system in Turkey. These boundaries are conservative because plate interaction occurs without creating or destroying crust. Because the only motion along these faults is the sliding of plates past each other, the horizontal direction along the fault surface must parallel the direction of plate motion. The fault surfaces are rarely smooth, and pressure may build up when the plates on either side temporarily lock. This build-up of stress may be suddenly released in the form of an earthquake.

Hot spots
 Although most of Earth's seismic and volcanic activity is concentrated along or adjacent to plate boundaries, there are some important exceptions in which this activity occurs within plates. Linear chains of islands, thousands of kilometres in length, that occur far from plate boundaries are the most notable examples. These island chains record a typical sequence of decreasing elevation along the chain from volcanic island to fringing reef, to atoll, and finally to a submerged seamount. An active volcano usually exists at one end of an island chain, with progressively older extinct volcanoes occurring along the rest of the chain. Canadian geophysicist J. Tuzo Wilson (Wilson, J. Tuzo) and American geophysicist W. Jason Morgan have explained such topographic features as the result of hot spots.

 The number of these hot spots is uncertain (estimates range from 20 to 120), but most occur within a plate rather than at a plate boundary. Hot spots are thought to be the surface expression of giant plumes of heat, termed mantle plumes, that ascend from deep within the mantle, possibly from the core-mantle boundary, some 2,900 km (1,800 miles) below the surface. These plumes are thought to be stationary relative to the lithospheric plates that move over them. A volcano builds upon the surface of a plate directly above the plume. As the plate moves on, however, the volcano is separated from its underlying magma source and becomes extinct. Extinct volcanoes are eroded as they cool and subside to form fringing reefs and atolls, and eventually they sink below the surface of the sea to form a seamount. At the same time, a new active volcano forms directly above the mantle plume.

      The best example of this process is preserved in the Hawaiian-Emperor seamount chain. The plume is presently situated beneath Hawaii, and a linear chain of islands, atolls, and seamounts extends 3,500 km (2,175 miles) northwest to Midway and a further 2,500 km (1,550 miles) north-northwest to the Aleutian trench. The age at which volcanism became extinct along this chain gets progressively older with increasing distance from Hawaii—critical evidence that supports this theory. Hot spot volcanism is not restricted to the ocean basins; it also occurs within continents, as in the case of Yellowstone National Park in western North America.

Plate motion
Euler's contributions
 In the 18th century, Swiss mathematician Leonhard Euler (Euler, Leonhard) showed that the movement of a rigid body across the surface of a sphere can be described as a rotation around an axis that goes through the centre of the sphere, known as the axis of rotation. The location of this axis bears no relationship to Earth's spin axis. The point of emergence of the axis through the surface of the sphere is known as the pole of rotation. This theorem of spherical geometry provides an elegant way for defining the motion of the lithospheric plates across Earth's surface. Therefore, the relative motion of two rigid plates may be described as rotations around a common axis, known as the axis of spreading. Application of the theorem requires that the plates not be internally deformed—a requirement not absolutely adhered to but one that appears to be a reasonable approximation of what actually happens. Application of this theorem permits the mathematical reconstruction of past plate configurations.

      Because all plates form a closed system, all movements can be defined by dealing with them two at a time. The joint pole of rotation of two plates can be determined from their transform boundaries, which are by definition parallel to the direction of motion. Thus, the plates move along transform faults, whose trace defines circles of latitude perpendicular to the axis of spreading, and so form small circles around the pole of rotation. A geometric necessity of this theorem—that lines perpendicular to the transform faults converge on the pole of rotation—is confirmed by measurements. According to this theorem, the rate of plate motion should be slowest near the pole of rotation and increase progressively to a maximum rate along fractures with a 90° angle to it. This relationship is also confirmed by accurate measurements of seafloor-spreading rates.

Past plate movements
      It is possible that the entire lithosphere might slide around over the asthenosphere like a loose skin, altering the positions of all plates with respect to Earth's spin axis and the Equator. To determine the true geographic positions of the plates in the past, investigators have to define their motions, not only relative to each other but also relative to this independent frame of reference. The plumes provide an example of such a reference frame, since they probably originate within the deep mantle and since many appear to have relatively fixed positions over time. As a result, the motion of the lithosphere above these plumes can be deduced. The hot spot island chains serve this purpose, their trends providing the direction of motion of a plate; the speed of the plate can be inferred from the increase in age of the volcanoes along the chain relative to the distance between the islands.

      Earth scientists are able to accurately reconstruct the positions and movements of plates for the past 150 to 200 million years, because they have the oceanic crust record to provide them with plate speeds and direction of movement. However, since older oceanic crust is continuously consumed to make room for new crust, this kind of evidence is not available during earlier geologic time intervals, making it necessary for investigators to turn to other, less-precise techniques (see section on Paleomagnetism, polar wandering, and continental drift (plate tectonics)).

Development of tectonic theory

      The outlines of the continents flanking the Atlantic Ocean are so similar that many probably noticed the correspondence as soon as accurate maps became available. The earliest references to this similarity were made in 1620 by the English philosopher Francis Bacon (Bacon, Francis, Viscount Saint Alban (or Albans), Baron of Verulam), in his book Novum Organum, and by French naturalist Georges-Louis Leclerc, count de Buffon (Buffon, Georges-Louis Leclerc, count de), a century later. Toward the end of the 18th century, Alexander von Humboldt (Humboldt, Alexander von), a German naturalist, suggested that the lands bordering the Atlantic Ocean had once been joined.

      In 1858, French geographer Antonio Snider-Pellegrini proposed that identical fossil plants in North American and European coal deposits could be explained if the two continents had formerly been connected. He suggested that the biblical flood was due to the fragmentation of this continent, which was torn apart to restore the balance of a lopsided Earth. In the late 19th century, the Austrian geologist Eduard Suess (Suess, Eduard) proposed that large ancient continents had been composed of several of the present-day smaller ones. According to this hypothesis, portions of a single enormous southern continent—designated Gondwanaland, or Gondwana—foundered to become the Atlantic and Indian oceans. Such sunken lands, along with vanished land bridges, were frequently invoked in the late 1800s to explain sediment sources apparently present in the ocean and to account for floral and faunal connections between continents. These explanations remained popular until the 1950s and stimulated believers in the ancient submerged continent of Atlantis.

      In 1908, American geologist Frank B. Taylor postulated that the arcuate mountain belts of Asia and Europe resulted from the equatorward creep of the continents. His analysis of tectonic features foreshadowed in many ways modern thought regarding plate collisions.

Alfred Wegener (Wegener, Alfred Lothar) and the concept of continental drift
      In 1912, German meteorologist Alfred Wegener (Wegener, Alfred Lothar), impressed by the similarity of the geography of the Atlantic coastlines, explicitly presented the concept of continental drift. Though plate tectonics is by no means synonymous with continental drift, the term encompasses this idea and derives much of its impact from it.

      Wegener came to consider the existence of a single supercontinent from about 350 to 245 million years ago, during the late Paleozoic Era, and named it Pangea, meaning “all lands.” He searched the geologic and paleontological literature for evidence supporting the continuity of geologic features across the Indian and Atlantic oceans during that time period, which he assumed had formed during the Mesozoic Era (about 248 to 65 million years ago). He presented the idea of continental drift and some of the supporting evidence in a lecture in 1912, followed in 1915 by his major published work, Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans).

      Wegener pointed out that the concept of isostasy rendered large sunken continental blocks, as envisaged by Suess, geophysically impossible. He concluded that, if the continents had been once joined together, the consequence would have been drift of their fragments and not their foundering. The assumption of a former single continent could be tested geologically, and Wegener displayed a large array of data that supported his hypothesis, ranging from the continuity of fold belts across oceans, the presence of identical rocks and fossils on continents now separated by oceans, and the paleobiogeographic and paleoclimatological (paleoclimatology) record that indicated otherwise unaccountable shifts in Earth's major climate belts. He further argued that, if continents could move up and down in the mantle as a result of buoyancy changes produced by erosion or deposition, they should be able to move horizontally as well.

Driving forces
      The main stumbling block to the acceptance of Wegener's hypothesis was the driving forces he proposed. Wegener described the drift of continents as a flight from the poles due to Earth's equatorial bulge. Although these forces do exist, Wegener's nemesis, British geophysicist Sir Harold Jeffreys (Jeffreys, Sir Harold), demonstrated that these forces are much too weak for the task. Another mechanism proposed by Wegener, tidal forces on Earth's crust produced by gravitational pull of the Moon, were also shown to be entirely inadequate.

      Wegener's proposition was attentively received by many European geologists, and in England, Arthur Holmes pointed out that the lack of a driving force was insufficient grounds for rejecting the entire concept. In 1929, Holmes proposed an alternative mechanism—convection of the mantle—which remains today a serious candidate for the force driving the plates. Wegener's ideas also were well received by geologists in the Southern Hemisphere. One of them, the South African Alexander Du Toit, remained an ardent believer. After Wegener's death, Du Toit continued to amass further evidence in support of continental drift.

Evidence supporting the hypothesis
      The strikingly similar Paleozoic sedimentary sequences on all southern continents and also in India are an example of evidence that supports continental drift. This diagnostic sequence consists of glacial deposits called tillites (tillite), followed by sandstones (sandstone) and finally Coal Measures, typical of warm, moist climates. An attempt to explain this sequence in a world of fixed continents presents insurmountable problems. Placed on a reconstruction of Gondwana, however, the tillites mark two ice ages that occurred during the drift of this continent across the South Pole from its initial position north of Libya about 500 million years ago and its final departure from southern Australia 250 million years later. About this time, Gondwana collided with Laurentia (the precursor to the North American continent), which was one of the major collisional events that produced Pangea.

      Both ice ages resulted in glacial deposits—in the southern Sahara during the Silurian Period (more than 400 million years ago) and in southern South America, South Africa, India, and Australia from 380 to 250 million years ago, spanning the latter part of the Devonian, the Carboniferous, and almost all of the Permian (Permian Period). At each location, the tillites were subsequently covered by desert sands of the subtropics, and these in turn by coal measures, indicating that the region had arrived near the paleoequator.

      During the 1950s and '60s, isotopic dating (dating) of rocks showed that the crystalline massifs of Precambrian (Precambrian time) age (from about 4 billion to 542 million years ago) found on opposite sides of the South Atlantic did indeed closely correspond in age and composition, as Wegener had surmised. It is now evident that they originated as a single assemblage of Precambrian continental nuclei later torn apart by the fragmentation of Pangea.

      By the 1960s, although evidence supporting continental drift had strengthened substantially, many scientists were claiming that the shape of the coastlines should be more sensitive to coastal erosion and changes in sea level and are unlikely to maintain their shape over hundreds of millions of years. Therefore, they argued, the supposed fit of the continents flanking the Atlantic Ocean is fortuitous. In 1964, however, these arguments were laid to rest. A computer analysis by Sir Edward Bullard (Bullard, Sir Edward) showed an impressive fit of these continents at the 1,000-metre (3,300-foot) depth contour. A match at this depth is highly significant and is a better approximation of the edge of the continents than the present shoreline. With this reconstruction of the continents, the structures and stratigraphic sequences of Paleozoic mountain ranges in eastern North America and northwestern Europe can be matched in detail in the manner envisaged by Wegener.

Disbelief and opposition
      Sir Harold Jeffreys (Jeffreys, Sir Harold) was one of the strongest opponents of Wegener's hypothesis. He believed that continental drift is impossible because the strength of the underlying mantle should be far greater than any conceivable driving force. In North America, opposition to Wegener's ideas was most vigorous and very nearly unanimous. Wegener was attacked from virtually every possible vantage point—his paleontological evidence attributed to land bridges, the similarity of strata on both sides of the Atlantic called into question, and the fit of Atlantic shores declared inaccurate. This criticism is illustrated by reports from a symposium on continental drift organized in 1928 by the American Association of Petroleum Geologists. The mood that prevailed at the gathering was expressed by an unnamed attendant quoted with sympathy by the great American geologist Thomas C. Chamberlin (Chamberlin, Thomas Chrowder): “If we are to believe Wegener's hypothesis, we must forget everything which has been learned in the last 70 years and start all over again.” The same reluctance to start anew was again displayed some 40 years later by the same organization when its publications provided the principal forum for the opposition to plate tectonics.

Renewed interest in continental drift
      Ironically, the vindication of Wegener's hypothesis came from the field of geophysics, the subject used by Jeffreys to discredit the original concept. The ancient Greeks realized that some rocks are strongly magnetized, and the Chinese invented the magnetic compass in the 13th century. In the 19th century, geologists recognized that many rocks preserve the imprint of Earth's magnetic field as it was at the time of their formation. The study and measurement of Earth's ancient magnetic field is called paleomagnetism. Iron-rich volcanic rocks such as basalt contain minerals that are good recorders of paleomagnetism, and some sediments also align their magnetic particles with Earth's field at the time of deposition. These minerals behave like fossil compasses that indicate, like any magnet suspended in Earth's field, the direction to the magnetic pole and the latitude of their origin at the time the minerals were crystallized or deposited.

      During the 1950s, paleomagnetic studies, notably those of Stanley K. Runcorn (Runcorn, Stanley Keith) and his coworkers in England, showed that in the late Paleozoic the north magnetic pole—as reconstructed from European data—seems to have wandered from a Precambrian position near Hawaii to its present location by way of Japan. This could be explained either by the migration of the magnetic pole itself (that is, polar wandering) or by the migration of Europe relative to a fixed pole (that is, continental drift). The distinction between these two hypotheses came from paleomagnetic data from other continents. Each continent yielded different results, called apparent polar wandering paths. The possibility that they might reflect true wandering of the poles was discarded, because it would imply separate wanderings of many magnetic poles over the same period. However, these different paths could be reconciled by joining the continents in the manner and at the time suggested by Wegener.

      Impressed by this result, Runcorn became the first of a new generation of geologists and geophysicists to accept continental drift as a proposition worthy of careful testing. Since then, more sophisticated paleomagnetic techniques have provided both strong supporting evidence for continental drift and a major tool for reconstructing the geography and geology of the past.

Gestation and birth of plate tectonic theory
      After World War II, rapid advances were made in the study of the relief, geology, and geophysics of the ocean basins. Owing in large part to the efforts of Bruce C. Heezen and Henry W. Menard of the United States, these features, which constitute more than two-thirds of Earth's surface, became well enough known to permit serious geologic analysis.

Discovery of ocean basin features
 One of the most important discoveries was that of the oceanic ridge system. Oceanic ridges form an interconnected network about 65,000 km (40,000 miles) in length that nearly girdles the globe, has elevations that rise 2 to 3 km (1.2 to 1.9 miles) above the surrounding seafloor, and has widths that range from a few hundred to more than 1,000 km (600 miles). Their crests tend to be rugged and are often endowed with a rift valley at their summit where fresh lava, high heat flow, and shallow earthquakes (earthquake) of the extensional type are found.

      Long, narrow depressions—oceanic trenches (deep-sea trench)—were also discovered that contain the greatest depths of the ocean basins. Trenches virtually ring the Pacific Ocean; a few also occur in the northeastern part of the Indian Ocean, and some small ones are found in the central Atlantic Ocean. Elsewhere they are absent. Trenches have low heat flow, are often filled with thick sediments, and lie at the upper edge of the Benioff zone of compressive earthquakes. Trenches may border continents, as in the case of western Central and South America, or may occur in mid-ocean, as, for example, in the southwestern Pacific.

      Offsets of up to several hundred kilometres along oceanic ridges and, more rarely, trenches were also recognized, and these fracture zones (submarine fracture zone)—later termed transform faults—were described as transverse features consisting of linear ridges and troughs. In oceanic domains, these faults were found to occur approximately perpendicular to the ridge crest, continue as fracture zones extending over long distances, and terminate abruptly against continental margins. They are not sites of volcanism, and their seismic activity is restricted to the area between offset ridge crests, where earthquakes indicating horizontal slip are common.

Hess's seafloor-spreading (seafloor spreading hypothesis) model
      The existence of these three types of large, striking seafloor features demanded a global rather than local tectonic explanation. The first comprehensive attempt at such an explanation was made by Harry H. Hess of the United States in a widely circulated manuscript written in 1960 but not formally published for several years. In this paper, Hess, drawing on Holmes's model of convective flow in the mantle, suggested that the oceanic ridges were the surface expressions of rising and diverging convective mantle flow, while trenches and Benioff zones, with their associated island arcs, marked descending limbs. At the ridge crests new oceanic crust would be generated and then carried away laterally to cool, subside, and finally be destroyed in the nearest trenches. Consequently, the age of the oceanic crust should increase with distance away from the ridge crests, and, because recycling was its ultimate fate, very old oceanic crust would not be preserved anywhere. This explained why rocks older than 200 million years had never been encountered in the oceans, whereas the continents preserve rocks up to 3.8 billion years old.

      Hess's model was later dubbed seafloor spreading by the American oceanographer Robert S. Dietz (Dietz, Robert S.). Confirmation of the production of oceanic crust at ridge crests and its subsequent lateral transfer came from an ingenious analysis of transform faults by Canadian geophysicist J. Tuzo Wilson (Wilson, J. Tuzo). Wilson argued that the offset between two ridge crest segments is present at the outset of seafloor spreading. As each ridge segment generates new crust that moves laterally away from the ridge, the crustal slabs move in opposite directions along that part of the fracture zone that lies between the crests. In the fracture zones beyond the crests, adjacent portions of crust move in parallel (and are therefore aseismic—that is, do not have earthquakes) and are eventually absorbed in a trench. Wilson called this a transform fault and noted that on such a fault the seismicity should be confined to the part between ridge crests, a prediction that was subsequently confirmed by an American seismologist, Lynn R. Sykes.

The Vine-Matthews hypothesis

Magnetic anomalies
      In 1961, a magnetic survey of the eastern Pacific Ocean floor off the coast of Oregon and California was published by two geophysicists, Arthur D. Raff and Ronald G. Mason. Unlike on the continents, where regional magnetic anomaly patterns tend to be confused and seemingly random, the seafloor possesses a remarkably regular set of magnetic bands of alternately higher and lower values than the average values of Earth's magnetic field. These positive and negative anomalies are strikingly linear and parallel with the oceanic ridge axis, show distinct offsets along fracture zones, and generally resemble the pattern of a zebra skin. The axial anomaly tends to be higher and wider than the adjacent ones, and in most cases the sequence on one side is the approximate mirror image of that on the other.

Reversal of Earth's magnetic field
      A key piece to resolving this pattern came with the discovery of magnetized samples from a sequence of basalt lavas. These lavas were extruded in rapid succession in a single locality on land and showed that the north and south poles had apparently repeatedly interchanged. This could be interpreted in one of two ways—either the rocks must have somehow reversed their magnetism, or the polarity of Earth's magnetic field must periodically reverse itself. Allan Cox of Stanford University and Brent Dalrymple of the United States Geological Survey collected magnetized samples and showed that these samples from around the world displayed the same reversal at the same time, implying that the polarity of Earth's magnetic field periodically reversed. These studies established a sequence of reversals dated by isotopic methods (dating).

      Assuming that the oceanic crust is indeed made of basalt intruded in an episodically reversing geomagnetic field, Drummond H. Matthews of the University of Cambridge and a research student, Frederick J. Vine, postulated in 1963 that the new crust would have a magnetization aligned with the field at the time of its formation. If the magnetic field was normal, as it is today, the magnetization of the crust would be added to that of Earth and produce a positive anomaly. If intrusion had taken place during a period of reverse magnetic polarity, it would subtract from the present field and appear as a negative anomaly. Subsequent to intrusion, each new block created at spreading centre would split and the halves, in moving aside, would generate the observed bilateral magnetic symmetry.

      Given a constant rate of crustal generation, the widths of individual anomalies should correspond to the intervals between magnetic reversals. In 1966, correlation of magnetic traverses from different oceanic ridges demonstrated an excellent correspondence with the magnetic polarity-reversal time scale established by Cox and Dalrymple on land. This reversal time scale went back some three million years, but since then, further extrapolation based on marine magnetic anomalies (confirmed by deep-sea drilling) has extended the magnetic anomaly time scale far into the Cretaceous Period, which spans the interval from about 145 to 65 million years ago.

      About the same time, Canadian geologist Laurence W. Morley, working independently of Vine and Matthews, came to the same explanation for the marine magnetic anomalies. Publication of his paper was delayed by unsympathetic referees and technical problems and occurred long after Vine's and Matthews's work had already firmly taken root.

Toward a unifying theory
Determination of plate thickness
      After marine magnetic anomalies were explained, the cumulative evidence caused the concept of seafloor spreading to be widely accepted. However, the process responsible for continental drift remained enigmatic. Two important concerns remained. The spreading seafloor was generally seen as a thin-skin process, most likely having its base at the Mohorovičić discontinuity—that is, the boundary between the crust and mantle. If only oceanic crust was involved in seafloor spreading, as seemed to be the case in the Pacific Ocean, the thinness of the slab was not disturbing, even though the ever-increasing number of known fracture zones with their close spacing implied oddly narrow but very long convection cells. More troubling was the fact that the Atlantic Ocean had a well-developed oceanic ridge but lacked trenches adequate to dispose of the excess oceanic crust. This implied that the adjacent continents needed to travel with the spreading seafloor, a process that, given the thin but clearly undeformed slab, strained credulity.

      Working independently but along very similar lines, Dan P. McKenzie and Robert L. Parker of Britain and W. Jason Morgan of the United States resolved these issues. McKenzie and Parker showed with a geometric analysis that, if the moving slabs of crust were thick enough to be regarded as rigid and thus to remain undeformed, their motions on a sphere would lead precisely to those divergent, convergent, and transform boundaries that are indeed observed. Morgan demonstrated that the directions and rates of movement had been faithfully recorded by magnetic anomaly patterns and transform faults. He also proposed that the plates extended approximately 100 km (60 miles) to the base of a rigid lithosphere, which coincided with the top of the weaker asthenosphere. Seismologists had previously identified this boundary, which is marked by strong attenuation of earthquake waves, as a fundamental division in Earth's upper layers. Therefore, according to Morgan, this was the boundary above which the plates moved.

      In 1968, a computer analysis by the French geophysicist Xavier Le Pichon proved that the plates did indeed form an integrated system where the sum of all crust generated at oceanic ridges is balanced by the cumulative amount destroyed in all subduction zones. That same year, the American geophysicists Bryan Isacks, Jack Oliver, and Lynn R. Sykes showed that the theory, which they enthusiastically labeled the “new global tectonics,” was capable of accounting for the larger part of Earth's seismic activity. Almost immediately, others began to consider seriously the ability of the theory to explain mountain building and sea-level changes.

Plate-driving mechanisms and the role of the mantle
      By the late 1960s, details of the processes of plate movement and of boundary interactions, along with much of the plate history of the Cenozoic Era (the past 65 million years), had been worked out. Yet the driving forces that bedeviled Wegener continue to remain enigmatic because there is little information about what happens beneath the plates.

Mantle convection
      Most agree that plate movement is the result of the convective circulation of Earth's heated interior, much as envisaged by Arthur Holmes in 1929. The heat source for convection is thought to be the decay of radioactive elements in the mantle. How this convection propels the plates is poorly understood. In the western Pacific Ocean, the subduction of old, dense oceanic crust may be self-propelled. The weight of the subducted slab may pull the rest of the plate toward the trench, a process known as slab pull. In the Atlantic Ocean, however, westward drift of North America and eastward drift of Europe and Africa may be due to push at the spreading ridge, known as ridge push. Hot mantle spreading out laterally beneath the ridges or hot spots may speed up or slow down the plates, a force known as mantle drag. However, the mantle flow pattern at depth does not appear to be reflected in the surface movements of the plates.

      The relationship between the circulation within Earth's mantle and the movement of the lithospheric plates remains a first-order problem in the understanding of plate-driving mechanisms. Circulation in the mantle occurs by thermal convection, whereby warm, buoyant material rises, and cool, dense material sinks. Convection is possible even though the mantle is solid; it occurs by solid-state creep, similar to the slow downhill movement of valley glaciers. Materials can flow in this fashion if they are close to their melting temperatures. Several different models of mantle convection have been proposed. The simplest, called whole mantle convection, describes the presence of several large cells that rise from the core mantle boundary beneath oceanic ridges and begin their descent to that boundary at subduction zones. Some geophysicists argue for layered mantle convection, suggesting that more vigorous convection in the upper mantle is decoupled from that in the lower mantle. This model would be supported if it turned out that the boundary between the upper and lower mantle is coincident with a change in composition. A third model, known as the mantle plume model, suggests that upwelling is focused in plumes that ascend from the core-mantle boundary, whereas diffuse return flow is accomplished by subduction zones, which, according to this model, extend to the core-mantle boundary.

Seismic tomography
      A powerful technique, seismic tomography, is providing insights into this problem. This technique is similar in principle to that of the CT (computed tomography) scan and creates three-dimensional images of Earth's interior by combining information from many earthquakes (earthquake). Seismic waves generated at the site, or focus, of an earthquake spread out in all directions, similar to light rays from a light source. As earthquakes occur in many parts of Earth's crust, information from many sources can be synthesized, mimicking the rotating X-ray beam of a CT scan. Because their speed depends on the density, temperature, pressure, rigidity, and phase of the material through which they pass, the velocity of seismic waves provides clues to the composition of Earth's interior. Seismic energy is absorbed by warm material, so that the waves are slowed down. As a result, anomalously warm areas in the mantle are seismically slow, clearly distinguishing them from colder, more rigid, anomalously fast regions.

      Tomographic imaging shows a close correspondence between surface features such as ocean ridges and subduction zones to a depth of about 100 km (60 miles). Hot regions in the mantle occur beneath oceanic ridges, and cold regions occur beneath subduction zones. However, at greater depths, the pattern is more complex, suggesting that the simple whole mantle-convection model is not appropriate. On the other hand, subduction zones beneath Central America and Japan have been tracked close to the core-mantle boundary, suggesting that transition between the upper and lower mantle is not an impenetrable barrier to mantle flow. If so, convection is not decoupled across that boundary, again casting doubt upon the layered mantle model. Imaging the mantle directly beneath hot spots has identified anomalously warm mantle down to the core-mantle boundary, providing strong evidence for the existence of plumes and the possibility that the mantle plume hypothesis may indicate an important mechanism involved in mantle convection.

Dissenting opinions and unanswered questions
      After decades of controversy, the concept of continental drift was finally accepted by the majority of Western scientists as a consequence of plate tectonics. Sir Harold Jeffreys (Jeffreys, Sir Harold) continued his lifelong rejection of continental drift on grounds that his estimates of the properties of the mantle indicated the impossibility of plate movements. He did not, in general, consider the mounting geophysical and geologic arguments that supported the concept of Earth's having a mobile outer shell.

      Russian scientists, most notably Vladimir Vladimirovich Belousov (Belousov, Vladimir Vladimirovich), continued to advocate a model of Earth with stationary continents dominated by vertical motions. The model, however, only vaguely defined the forces supposedly responsible for the motions. In later years, Russian geologists came to regard plate tectonics as an attractive theory and a viable alternative to the concepts of Belousov and his followers.

      In 1958, the Australian geologist S. Warren Carey proposed a rival model, known as the expanding Earth model. Carey accepted the existence and early Mesozoic breakup of Pangea and the subsequent dispersal of its fragments and formation of new ocean basins, but he attributed it all to the expansion of Earth, the planet presumably having had a much smaller diameter in the late Paleozoic. In his view, the continents represented the preexpansion crust, and the enlarged surface was to be entirely accommodated within the oceans. This model accounted for a spreading ocean floor and for the young age of the oceanic crust; however, it failed to deal adequately with the evidence for subduction and compression. Carey's model also did not explain why the process should not have started until some four billion years after Earth was formed, and it lacked a reasonable mechanism for so large an expansion. Finally, it disregarded the evidence for continental drift before the existence of Pangea.

Unanswered questions
      In his famous book, The Structure of Scientific Revolutions (1962), the philosopher Thomas S. Kuhn (Kuhn, Thomas S.) pointed out that science does not always advance in the gradual and stately fashion commonly attributed to it. Most natural sciences begin with observations collected at random, without much regard to their significance or relationship between one another. As the numbers of observations increase, someone eventually synthesizes them into a comprehensive model, known as a paradigm. A paradigm is the framework, or context that is assumed to be correct, and so guides interpretations and other models. When a paradigm is accepted, advances are made by application of the paradigm. A crisis arises when the weight of observations points to the inadequacy of the old paradigm, and there is no comprehensive model that can explain these contradictions. Major breakthroughs often come from an intuitive leap that may be contrary to conventional wisdom and widely accepted evidence, while strict requirements for verification and proof are temporarily relaxed. If a new paradigm is to be created, it must explain most of the observations of the old paradigm and most of the contradictions.

      This paradigm shift constitutes a scientific revolution; therefore, it often becomes widely accepted before the verdict from rigorous analysis of evidence is completely in. Such was certainly the case with the geologic revolution of plate tectonics, which also confirms Kuhn's view that a new paradigm is unlikely to supersede an existing one until there is little choice but to acknowledge that the conventional theory has failed. Thus, while Wegener did not manage to persuade the scientific world of continental drift, the successor theory, plate tectonics, was readily embraced 40 years later, even though it remained open to much of the same criticism that had caused the downfall of continental drift.

      The greatest successes of plate tectonics have been achieved in the ocean basins, where additional decades of effort have confirmed its postulates and enabled investigators to construct a credible history of past plate movements. Inevitably in less-rigorous form, the reconstruction of early Mesozoic and Paleozoic continental configurations has provided a powerful tool with which to resolve many important questions.

      There is further evidence, as held by the American geophysicist Thomas H. Jordan, that the base of the plates extends far deeper into the asthenosphere below the continents than below the oceans. How much of an impediment this might be for the free movement of plates and how it might affect their boundary interactions remain open questions. Others have postulated that the lower layer of the lithosphere peels off and sinks late in any collision sequence, producing high heat flow, volcanism, and an upper lithospheric zone vulnerable to contraction by thrusting.

      It is understandable that any simple global tectonic model would work better in the oceans, which, being young, retain a record of only a brief and relatively uneventful history. On the continents, almost four billion years of growth and deformation, erosion, sedimentation, and igneous intrusion have produced a complex imprint that, with its intricate zones of varying strength, must directly affect the application of modern plate forces. Seismic reflection studies of the deep structure of the continents have demonstrated just how complex the events that form the continents and their margins may have been, and their findings sometimes are difficult to reconcile with the accretionary structures one would expect to see as a result of subduction and collision.

      Notwithstanding these cautions and the continuing lack of an agreed-upon driving mechanism for the plates, one cannot help but conclude that the plate tectonics revolution has been fruitful and has immensely advanced the scientific understanding of Earth. Like all paradigms in science, it will most likely one day be replaced by a better one; yet there can be little doubt that, whatever the new theory may state, continental drift will be part of it.

Plate tectonics and the geologic past
      The extent to which plate tectonics has influenced Earth's evolution through geologic time depends on when the process started. This is a matter of ongoing debate among geologists. The principal problem is that almost all oceanic crust older than about 200 million years has been obliterated by subduction. Although thick sequences of marine sedimentary rocks up to 3.5 billion years old imply that oceanic environments did exist early in Earth's history, virtually none of the oceanic crust that underlay these sediments has been preserved. Despite these disadvantages, there is enough fragmentary evidence to suggest that plate tectonic processes similar to those of today extend back in time at least as far as the Early Proterozoic (Proterozoic Eon) Era, some 2.5 to 1.6 billion years ago.

Wilson cycle
      The first step toward this conclusion was once again provided by J. Tuzo Wilson (Wilson, J. Tuzo) in 1966, when he proposed that the Appalachian-Caledonide mountain belt (Appalachian orogenic belt) of western Europe and eastern North America was formed by the destruction of an ocean that predated the Atlantic Ocean. Wilson was impressed with the similarity of thick sequences of Cambrian (Cambrian Period)– Ordovician (Ordovician Period) marine sediments to those of modern continental shelves. In a Pangea reconstruction, Wilson showed that these shelflike sediments extend along the entire length of the mountain chain from Scandinavia to the southeastern United States. However, these shelf sequences contain two distinct fossil assemblages on opposite sides of the mountain chain. The assemblage on the western side of the mountain chain he named the Pacific Realm, whereas the eastern side of the chain he named the Atlantic Realm. Both the Pacific and Atlantic realms could be traced for thousands of kilometres along the length of the mountain belt, but neither could be traced across it. Wilson concluded that these sediments with distinctive faunal realms represented two opposing flanks of an ancient ocean that was consumed by subduction to form the Appalachian-Caledonide mountain belt. According to this model, subduction in this ocean in Ordovician times led to the foundering of the continental shelves and the formation of volcanic arcs. The basaltic complexes of western Newfoundland were interpreted as ophiolitic complexes representing slivers of oceanic crust that escaped subduction as they were emplaced onto the continental margin. Continued subduction resulted in the closure of this ocean in the Devonian. Rocks representing each of these environments were found, lending strong support to this model. This ocean was subsequently named Iapetus, for the father of Atlantis in Greek mythology.

      The concept that oceans may close and then reopen became known as the Wilson cycle, and with its acceptance came the application of plate tectonic principles to ancient orogenic belts. But how far back these principles may be extended is still an open question.

      In the absence of the seafloor record, evidence of ancient oceans may be obtained from thick sedimentary sequences similar to those of modern continental shelves or from preserved features formed in or around subduction zones, such as accretionary wedges, glaucophane-bearing blueschists, volcanic rocks with compositions similar to modern island arcs, and remnants of oceanic crust preserved as ophiolites obducted onto continents. These features can be confidently identified in the Paleozoic Era (approximately 540 to 250 million years ago), and possibly the Neoproterozoic Era (the later part of the Proterozoic Eon, occurring approximately 1 billion to 540 million years ago), but their recognition is more problematic with increasing age. It is not known with any certainty whether this is because the dynamic nature of Earth's surface obliterates such evidence or because processes different from the modern form of plate tectonics existed at the time. Since the 1990s, however, most geoscientists have begun to accept that some form of plate tectonics occurred throughout the Proterozoic Eon, which commenced 2.5 billion years ago, and some extend these models back into the Archean (Archean Eon), more than 2.5 billion years ago.

      Some of the critical evidence supporting this assertion comes from a suite of rocks in the Canadian Shield known as the Trans-Hudson belt. This belt separates stable regions of continental crust, known as cratons (craton). Marc St-Onge and colleagues from the Geological Survey of Canada provided strong evidence that the formation of the Trans-Hudson belt represents the oldest documented example of a Wilson cycle in which the cratonic areas, once separated by oceans, were brought together by subduction and continental collision. They found thick sedimentary sequences typical of modern continental rifts that are about 2 billion years old, and they found ophiolites of about the same age, which indicated that rifting resulted in continental drift and formation of an ocean. About 1.85 billion years ago, volcanic rocks typical of modern island arcs were deposited on top of this sequence, indicating that the continental margins had foundered and become subduction zones. Finally, they dated the time of continental collision at about 1.8 billion years. This collisional event is particularly important because welding the cratons together provided the core of the continent that was ultimately to become North America.

Supercontinent cycle
      Although the Wilson cycle provided the means for recognizing the formation and destruction of ancient oceans, it did not provide a mechanism to explain why this occurred. In the early 1980s, a controversial concept known as the supercontinent cycle was developed to address this problem. When viewed in a global context, it is apparent that episodes of continental rifting and mountain building are not equally distributed throughout geologic time but instead are concentrated in relatively short time intervals approximately 350 to 500 million years apart. Mountain building associated with the formation of Pangea peaked at about 300 million years ago. This episode was preceded by other mountain-building events peaking at 650 million and at 1.1, 1.6, 2.1, and 2.6 billion years ago. Like Pangea, could these episodes represent times of supercontinent amalgamation? Similarly, the breakup of Pangea is documented by continental-rifting events that began about 200 million years ago. However, regionally extensive and thick sequences of similar deposits occur 550 million years ago and 1, 1.5, and 2 billion years ago. Could these represent times of supercontinent dispersal?

      If indeed a supercontinent cycle exists, then there must be mechanisms responsible for breakup and amalgamation. The first step is to examine why a supercontinent like Pangea would break up. There are several theories, the most popular of which, proposed by American geophysicist Don Anderson, attributes breakup to the insulating properties of the supercontinent, which blocks the escape of mantle heat. As a result, the mantle beneath the supercontinent becomes anomalously hot, and vast volumes of basaltic magma pond beneath it, forcing it to arch up and crack. Magma invades the cracks, and the process of continental rifting, ultimately leading to seafloor spreading, begins. This model implies that supercontinents have in-built obsolescence and can exist only for so long before the buildup of heat beneath them results in their fragmentation. The dating of emplacement of vast suites of basaltic magma, known as basaltic dike swarms, is consistent with the ages of continental rifting, suggesting that mantle upwelling was an important contributor to the rifting process.

Continental reconstructions
      Magnetic anomalies, transform faults, hot spots, and apparent polar wandering paths permit rigorous geometric reconstructions of past plate positions, shapes, and movements. These reconstructions, known as paleogeographic reconstructions, show the changing geography of Earth's past and can be determined with excellent precision for the past 150 million years. Before that time, however, the absence of the ocean-floor record makes the process significantly more challenging. A variety of geologic data are used to help determine the proper fit of continents through time. Some of the methods used to test these reconstructions are based on matching patterns from one continental block to another and are similar to the approach of Wegener. However, modern geoscientists have more precise data that help constrain these reconstructions. Of the many advances, perhaps the most significant are the improved analytical techniques for radiometric dating, allowing the age of geologic events to be determined with much greater precision.

      Since the 1990s, the database has improved so that reasonably constrained reconstructions can now be made as far back as 1 billion years. For example, the abundance of continental-collisional events about 1.1 billion years ago is one of the principal lines of evidence suggesting the presence of a supercontinent that is given the name of Rodinia. By about 760 million years ago, a number of continental-rift sequences had developed, suggesting that Rodinia had begun to break up. Between about 650 and 550 million years ago, however, a number of mountain belts formed by continental collision, which resulted in the amalgamation of Gondwana. The continental fragment that rifted away from Laurentia (the name given to ancestral North America) did not return to collide with North America as predicted by a simple Wilson cycle. Instead, it rotated counterclockwise away from Laurentia until it collided with eastern Africa.

Interactions of tectonics with other systems

      As plate tectonics changes the shape of ocean basins, it fundamentally affects long-term variations in global sea level. For example, the geologic record in which thick sequences of continental shelf sediments were deposited demonstrates that the breakup of Pangea resulted in the flooding of continental margins, indicating a rise in sea level. There are several contributing factors. First, the presence of new ocean ridges displaces seawater upward and outward across the continental margins. Second, the dispersing continental fragments subside as they cool. Third, the volcanism associated with breakup introduces greenhouse gases in the atmosphere, which results in global warming, causing continental glaciers to melt.

      As the ocean widens, its crust becomes older and denser. It therefore subsides, eventually forming ocean trenches. As a result, ocean basins can hold more water, and sea level drops. This changes once again when subduction commences. Subduction preferentially consumes the oldest oceanic crust, so that the average age of oceanic crust becomes younger, and the material therefore becomes more buoyant. This increased buoyancy causes sea level to rise once more.

Composition of ocean water
      Water's strong properties as a solvent mean that it is rarely pure. Ocean water contains about 96.5 percent by weight of pure water, with the remaining 3.5 percent predominantly consisting of ions such as chloride (1.9 percent), sodium, (1.1 percent), sulfate (0.3 percent), and magnesium (0.1 percent). The drainage of water from continents via the hydrologic cycle plays an important role in transporting chemicals from the land to the sea. The effect of this drainage is profoundly influenced by the presence of mountain belts. For example, the erosional power of the Ganges River, which drains from the Himalayas, carries 1.45 billion metric tons of sediment to the sea annually. This load is nine times that of the Mississippi River. The processes of weathering and erosion easily strip soluble elements such as sodium from their host minerals, and the relatively high concentration of sodium in ocean water is attributed to the effects of continental drainage.

      Until the advent of plate tectonics, uncovering the source of chlorine was problematic because chlorine is present in only very minor amounts in the continental crust. Scientists hypothesized that the source of this element may lie in underwater volcanic activity. In the late 1970s, three scientists investigating the oceanic ridge off the coast of Peru from a submersible craft documented the occurrence of superheated jets of water, up to 350 °C (660 °F), continuously erupting from chimneys that stood 13 metres (43 feet) above the ocean floor. These hot springs were found to be rich in chlorine and metals, confirming that the source of chlorine in the oceans lay in the tectonic processes occurring at oceanic ridges.

      The continuous rearrangement over time of the size and shape of ocean basins and continents, accompanied by changes in ocean circulation and climate, has had a major impact on the development of life on Earth. One of the first studies of the potential effects of plate tectonics on life was published in 1970 by American geologists James W. Valentine and Eldridge M. Moores, who proposed that the diversity of life increased as continents fragmented and dispersed and diminished when they were joined together.

Deep sea vents
      Plate tectonics has influenced the evolution and propagation of life in a variety of ways. The study of oceanic ridges revealed the presence of bizarre life adjacent to the chimneys of superheated water that together make up about 1 percent of the world's ecosystems. The existence of these life-forms in the deep ocean cannot be based on photosynthesis. Instead, they are nourished by minerals and heat. The energy released when hydrogen sulfide in the vent reacts with seawater is utilized by bacteria to convert inorganic carbon dioxide dissolved in seawater into organic compounds, a process known as chemosynthesis. Some scientists speculate that the cumulative effects of this process over time have had a significant effect on evolution. Others suggest that similar processes may ultimately be responsible for the origin of life on Earth.

      When Laurentia began to rift apart and the Atlantic Ocean started to open during the middle Mesozoic, the differences between the faunas of opposite shores gradually increased in an almost linear fashion—the greater the distance, the smaller the number of families in common. The difference increased more rapidly in the South Atlantic than in the North Atlantic, where a land connection between Europe and North America persisted until about 60 million years ago.

      After the breakup of Pangea, no land animal could become dominant because the continents were disconnected. As a result, separate landmasses evolved highly specialized fauna. South America, for example, was rich in marsupial mammals, which had few predators. North America, on the other hand, was rich in placental mammals. However, about three million years ago, volcanic activity associated with subduction of the eastern Pacific Ocean formed a land bridge across the isthmus of Panama, reconnecting the separate landmasses.

      The emergence of the isthmus made it possible for land animals to cross, forcing previously separated fauna to compete. Numerous placental mammals and herbivores migrated from north to south. They adapted well to the new environment and were more successful than the local fauna in competing for food. The invasion of highly adaptable carnivores from the north contributed to the extinction of at least four orders of South American land mammals. A few species, notably the armadillo and the opossum, managed to migrate in the opposite direction. Ironically, many of the invading northerners, such as the llama and tapir, subsequently became extinct in their country of origin and found their last refuge to the south.

      Perhaps the most dramatic example of the potential impact of plate tectonics on life occurred toward the end of the Permian Period (about 300 to 250 million years ago). During this time, several extinction events caused the permanent disappearance of half of Earth's known biological families. The marine realm was most affected, losing more than 90 percent of its species. This drop may be attributed in part to biogeographic changes associated with the formation of Pangea. Other factors, such as a sharp decrease in the area of shallow-water habitats, or a change in ocean fertility due to a lack of upwelling (marine ecosystem) of nutrient-rich deep currents, have also been invoked.

      The extinction had a complex history. High latitudes were affected first as a result of the waning of the Permian ice age when the southern edge of Pangea moved off the South Pole. The equatorial and subtropical zones appear to have been affected somewhat later by a global cooling. On the other hand, the extinctions were not felt as strongly on the continent itself. Instead, the vast semiarid and arid lands that emerged on so large a continent, the shortening of its moist coasts, and the many mountain ranges formed from the collisions that led to the formation of the supercontinent provided strong incentives for evolutionary adaptation to dry or high-altitude environments.

      Climate changes associated with the supercontinent of Pangea and with its eventual breakup and dispersal provide an example of the effect of plate tectonics on paleoclimate (paleoclimatology). Pangea was completely surrounded by a world ocean (Panthalassa) extending from pole to pole and spanning 80 percent of the circumference of Earth at the paleoequator. The equatorial current system, driven by the trade winds, resided in warm latitudes much longer than today, and its waters were therefore warmer. The gyres (ocean) that occupy most of the Southern and Northern hemispheres were also warmer, and consequently the temperature gradient from the paleoequator to the poles was less pronounced than it is at present.

      Early in the Mesozoic (Mesozoic Era), Gondwana split from its northern counterpart, Laurasia, to form the Tethys (Tethys Sea) seaway, and the equatorial current became circumglobal. Equatorial surface waters were then able to circumnavigate the world and became even warmer. How this flow influenced circulation at higher latitudes is unclear. From about 100 to 70 million years ago, isotopic (isotopic fractionation) records show that Arctic and Antarctic surface water temperatures were at or above 10 °C (50 °F), and the polar regions were warm enough to support forests.

      As the dispersal of continents following the breakup of Pangea continued, however, the surface circulation of the oceans began to approach the more complex circulation patterns of today. About 100 million years ago, the northward drift of Australia and South America created a new circumglobal seaway around Antarctica, which remained centred on the South Pole. A vigorous circum-Antarctic current developed, isolating the southern continent from the warmer waters to the north. At the same time, the equatorial current system became blocked, first in the Indo-Pacific region and next in the Middle East and eastern Mediterranean and, about 6 million years ago, by the emergence of the Isthmus of Panama. As a result, the equatorial waters were heated less and the midlatitude ocean gyres were not as effective in keeping the high-latitude waters warm. Because of this, an ice cap began to form on Antarctica some 20 million years ago and grew to roughly its present size about 5 million years later. This ice cap cooled the waters of the adjacent ocean to such a low temperature that the waters sank and initiated the north-directed abyssal flow that marks the present deep circulation.

      Also, at about six million years ago, the collision between Africa and Europe temporarily closed the Strait of Gibraltar, isolating the Mediterranean Sea and restricting its circulation. Evaporation, which produced thick salt deposits, virtually dried up this sea and lowered the salt content of the world's oceans, allowing seawater to freeze at higher temperatures. As a result, polar ice sheets grew, and sea level fell. About 500,000 years later, the barrier between the Mediterranean and the Atlantic Ocean was breached, and open circulation resumed.

      The Quaternary Ice Age arrived in full when the first ice caps appeared in the Northern Hemisphere about two million years ago. It is highly unlikely that the changing configuration of continents and oceans can be held solely responsible for the onset of the Quaternary Ice Age, even if such factors as the drift of continents across the latitudes (with the associated changes in vegetation) and reflectivity for solar heat are included. There can be little doubt, however, that it was a contributing factor and that recognition of its role has profoundly altered concepts of paleoclimatology.

Tjeerd H. van Andel J. Brendan Murphy

Additional Reading

Plate tectonics: scientific revolution
J. Tuzo Wilson (ed.), Continents Adrift and Continents Aground (1976), contains an excellent and readable set of articles on the plate tectonics revolution drawn from Scientific American, many written by its protagonists, with fine introductions by the editor. Similarly, Allan Cox (ed.), Plate Tectonics and Geomagnetic Reversals (1973), offers a well-chosen selection of the original classical papers that produced this revolution in the earth sciences, from Holmes's work in the early 1900s to contributions in the late 1960s.The history of ideas pertaining to continental drift and plate tectonics has been thoughtfully analyzed in Anthony Hallam, A Revolution in the Earth Sciences: From Continental Drift to Plate Tectonics (1973). The same subject is reviewed from a somewhat different vantage point—that of a science writer—in Walter Sullivan, Continents in Motion: The New Earth Debate (1974). A summary of the revolution, mostly in a critical vein by many of its principal opponents, is presented in Charles F. Kahle (ed.), Plate Tectonics: Assessments and Reassessments (1974), which is a bit dated but still a good substantive statement on the subject. More technical, though not forbiddingly so, are two books rich in detail and substance: Peter J. Wyllie, The Way the Earth Works: An Introduction to the New Global Geology and Its Revolutionary Development (1976); and Robert H. Dott, Jr., and Roger L. Batten, Evolution of the Earth, 3rd ed. (1981).

General works and applications
Introductory treatments of plate tectonics are provided in Stephen Marshak, Earth: Portrait of a Planet (2001); and Kent C. Condie and Robert F. Sloan, Origin and Evolution of Earth: Principles of Historical Geology (1998). Brendan Murphy and Damian Nance, Earth Science Today (1999), outlines the development of plate tectonics, the modern understanding of plate processes, and the influence of plate tectonics on Earth's evolution and environment. Eldridge Moores and Robert J. Twiss, Tectonics (1995), gives a modern, advanced treatment of plate tectonics. Eldridge M. Moores (ed.), Shaping the Earth: Tectonics of Continents and Oceans (1990), presents a set of modern articles drawn from Scientific American on the dynamic nature of Earth and its relationship to plate tectonics and mountain building. The concepts discussed include the nature of oceanic crust, the growth of continents, and the supercontinent cycle. Allen Cox and Robert Hart, Plate Tectonics: How It Works (1986), provides a treatment of the impact of the plate theory on research in Earth history.A more scholarly treatment, though still written for a lay audience, is offered in Tjeerd H. Van Andel, New Views on an Old Planet: Continental Drift and the History of Earth (1985), an application of plate theory to the climatic, oceanographic, and geographic history of Earth and the relation of the theory to the history of life. For those familiar with a fundamental understanding of plate tectonics, Stephen Stanley, Earth System History (1999), presents a more advanced and quantitative treatment of the topic.Tjeerd H. van Andel J. Brendan Murphy

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Universalium. 2010.

Look at other dictionaries:

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  • plate tectonics — N UNCOUNT Plate tectonics is the way that large pieces of the earth s surface move slowly around. [TECHNICAL] …   English dictionary

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