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The theory of plate tectonics, formulated during the late 1960s, rests on a broad synthesis of geologic and geophysical data. It is now almost universally accepted and has had a major impact on the development of the Earth sciences. Its adoption represents a true scientific revolution, analogous in its consequences to the Rutherford and Bohr 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 made the study of the Earth more difficult by doing away with the notion of fixed continents, but it has at the same time provided the means of reconstructing the past geography of continents and oceans. While its impact has, to a considerable degree, run its course in marine geology and shows signs of reaching the limits of usefulness in the study of mountain-building processes, its influence on the scientific understanding of the Earth's history, of ancient oceans and climates, and of the evolution of life is only beginning to be felt.
The plate tectonics theory has a long and tortuous history. Yet, the theory itself is elegantly simple. The surface layer of the Earth, from 50 to 100 kilometers (31 to 62 miles) thick, is assumed to be composed of a set of large and small plates, which together constitute the rigid lithosphere. The lithosphere rests on and slides over an underlying, weaker layer of partially molten rock known as the asthenosphere. The constituent lithospheric plates move across the Earth's surface, driven by forces as yet not fully agreed upon, and interact along their boundaries, diverging, converging, or slipping past each other. While the interiors of the plates are presumed to remain essentially undeformed, their boundaries are the sites of many of the principal processes that shape the terrestrial surface, including earthquakes, volcanism, and orogeny (i.e., the deformation that builds mountain ranges).
The most conspicuous feature of the Earth's surface is its division into continents and ocean basins, a division that owes its existence to differences in thickness and composition between the continental and the oceanic crust. The continents have a crust of granitic composition and hence are somewhat lighter than the basaltic ocean floor. Also, they are 30 to 40 kilometers thick as compared to the oceanic crust, which measures only 6 to 7 kilometers in thickness. Their greater buoyancy causes them to float much higher in the mantle than does the oceanic crust, thus accounting for the difference between the two principal levels of the Earth's surface. The boundary between the continental or oceanic crust and the underlying mantle, the Mohorovicic discontinuity, has been clearly defined by seismic studies.
As conceived by the theory of plate tectonics, the lithospheric plates are much thicker than the oceanic or the continental crust; their boundaries do not usually coincide with those between oceans and continents; and their behavior is only partly influenced by whether they carry oceans, continents, or both. The Pacific Plate, for example, is purely oceanic, but most of the others contain continents.
At a divergent plate boundary, magma wells up from below as the release of pressure produces partial melting of the underlying mantle and generates 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. In fact, most divergent plate boundaries seem to have formed within continents rather than in oceans, probably because a hot, weak layer, sandwiched at a depth of about 15 kilometers between two stronger ones, renders the continental crust more vulnerable to fragmentation than its oceanic counterpart. The creation of the new crust is accompanied by much volcanic activity and by many shallow tension earthquakes as the crust repeatedly rifts, heals, and rifts again.
The continuous formation of new crust produces an excess that must be disposed of elsewhere. This is accomplished at convergent plate boundaries where one plate descends--i.e., is subducted--beneath the other. At depths between 300 and 700 kilometers, the subducted plate melts and is recycled into the mantle. Because the plates form an integrated system that completely covers the surface of the Earth, 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.
It is in subduction zones that the difference between plates carrying oceanic and continental crust can be most clearly seen. If both plates have oceanic edges, either one may dive beneath the other; but, if one carries a continent, the greater buoyancy prevents this edge from sinking. Thus, it is invariably the oceanic plate that is subducted.
Continents are permanently preserved in this manner, while the ocean floor continuously renews itself. If both plates possess a continental edge, neither can be subducted and a complex sequence of events from crumpling to under- and overthrusting raises lofty mountain ranges. Much later, after these ranges have been largely leveled by erosion, their remains continue as a reminder that this is the "suture" where continents were once fused.
The subduction process, which involves the descent into the mantle of a slab of cold rock about 100 kilometers thick, is marked by numerous earthquakes along a plane inclined 30-60 into the mantle--the Benioff zone. Most earthquakes in this planar dipping zone result from compression, and the seismic activity extends 300-700 kilometers below the surface. At a depth of 100 kilometers or more the subducted oceanic sediments, together with part of the upper basaltic crust, melt to an andesitic magma, which rises to the surface and gives birth to a line of volcanoes a few hundred kilometers behind the subducting boundary. This boundary is usually marked by an oceanic deep, or trench, where the overriding plate scrapes off the upper crust of the lower plate to create a zone of highly deformed, largely sedimentary rock. If both plates are oceanic, the deformed sediments and volcanoes form two island arcs parallel to the trench. If one plate is continental, the sediments are usually accreted against the continental margin and the volcanoes form inland, as they do in Mexico or western South America.
Along the third type of plate boundary, two plates move laterally and pass each other without creating or destroying crust. Large earthquakes are common along such strike-slip, or transform, boundaries. Also known as fracture zones, these plate boundaries are perhaps best exemplified by the San Andreas fault in California and the North Anatolian fault system in Turkey.
Most of the seismic and volcanic activity on Earth is therefore concentrated along plate boundaries where mid-ocean ridges, trenches with island arcs, and mountain ranges are generated. Some seismic and volcanic activity also occurs within plates. Interesting examples of this interplate activity are linear volcanic chains in ocean basins, such as the Hawaiian Islands and their westward continuation as a string of reefs and submerged seamounts. An active volcano usually exists at one end of an island chain of this type, with progressively older extinct volcanoes occurring along the rest of the chain. Such topographic features have been explained by J. Tuzo Wilson of Canada and W. Jason Morgan of the United States as the product of "hot spots," magma-generating centers of controversial origin located deep in the mantle far below the lithosphere. A volcano builds at the surface of a plate positioned above a hot spot. As the plate moves on, the volcano dies, is eroded, and eventually sinks below the surface of the sea, while a new one forms above the hot spot. Hot spot volcanism is not restricted to the ocean basins; other manifestations occur within continents, as in the case of Yellowstone National Park in western North America.
The movement of a plate across the surface of the Earth can be described as a rotation around a pole, and it may be rigorously described with the theorem of spherical geometry formulated by the Swiss mathematician Leonhard Euler during the 18th century. Similarly, the motions of two plates with respect to each other may be described as rotations around a common pole, provided that the plates retain their shape. The requirement that plates are not internally deformed has become one of the postulates of plate tectonics. It is not totally supported by evidence, but it appears to be a reasonable approximation of what actually happens in most cases. It is needed to permit the mathematical reconstruction of past plate configurations.
It is, of course, conceivable that the entire lithosphere might slide around over the asthenosphere like a loose skin, altering the positions of all plates with respect to the spin axis of the Earth and the equator. To determine the true geographic positions of the plates in the past investigators have to define their motions, relative not to each other but rather to this independent frame of reference. 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. It is assumed, of course, that the hot spots themselves remain fixed with respect to the Earth, an assumption that appears to be reasonably accurate for at least some hot spots.
The theory of plate techtonics is originally credited to Alfred Wegener. Wegener was by training and profession a meteorologist (he was highly respected for his work in climatology and paleoclimatology), but he is best remembered for the foray into geology that led to his formulation of the concept of continental drift. In 1910, because of the geography of the Atlantic coastlines, Wegener came to consider the existence of a single supercontinent during the late Paleozoic era (about 350 to 225 million years ago) and named it Pangaea. He searched the geologic and paleontological literature for evidence attesting to the continuity of geologic features across the Indian and Atlantic oceans, which he assumed had formed during the Mesozoic era (about 245 to 66.4 million years ago). His efforts proved rewarding, and he presented the idea of continental drift and some of the supporting evidence in a lecture in 1912. This was followed in 1915 by his major work, Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans).
Much was thus to be said for the idea that the continents were joined together in the Paleozoic, and supporting evidence has continued to accumulate to this day. The opposing Atlantic shores match well, especially at the 1,000-metre (3,300-foot) depth contour, which is a better approximation of the edge of the continental block than the present shoreline, as Sir Edward Bullard cogently demonstrated in 1964 with the aid of computer analysis. Similarly, the structures and stratigraphic sequences of Paleozoic mountain ranges in eastern North America and northwestern Europe can be matched in detail. This fact was already known to Wegener and has been strengthened substantially in subsequent years.
The existing theory of plate tectonics leaves many unanswered questions. Unforeseen natural occurrences occur everyday which often contradict once believed theories. Luckily for us though, scientists such as Alfred Wegener have paved the way in reaching a better understanding of the natural environment and the way we can grow symbiotically with it.
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