Plate tectonics is an all-embracing concept that has revolutionized the Earth sciences in the 20th century. It posits that the lithosphere, or outermost shell of the Earth, is divided into a number of rigid plates floating on a viscous underlayer in the mantle. The generation of new crust between diverging plates accounts for the young oceans of the world, and the collision and destruction of converging plates explain the formation of the world's mountain belts.
Six major plates and a host of smaller ones have been distinguished. The Alpine fold belt extending from Gibraltar to the Middle East is made up of many mini-plates. Plates are either solely oceanic (for example, Cocos and Nazca), solely continental (for example, Iran), or both (for example, North America). In the case of North America, new oceanic crust has been welded onto older continental crust. Thus the margins of some continents, such as the western side of South America, are occupied by an active compressional plate boundary, whereas other continental margins, such as the western side of Africa, lie within a plate and are tectonically passive.
The plates consist of lithosphere, which includes both oceanic and continental crust plus the underlying upper part of the mantle. The plates are about 70 to 80 km (40 to 50 mi) thick under the oceans and about 100 to 150 km (60 to 90 mi) thick under the continents. The oceanic crust is about 7 km (4 mi) thick, whereas the continental crust ranges in thickness from an average of about 40 km (25 mi) to about 70 to 80 km (40 to 50 mi) under the highest mountain ranges.
The plates have three types of boundary: the mid-oceanic ridge, along which new oceanic lithosphere is created; convergent zones, such as oceanic trenches (where one plate bends down in a subduction zone and is consumed beneath another) or sutures (where two continental plates have collided); and transform faults, where two plates slide passively past each other with no creation or destruction of lithosphere. The earthquakes of the world most often occur along plate boundaries. Subduction zones are marked by shallow (less than 70 km/40 mi), intermediate (70 to 300 km/43 to 186 mi) and deep (300 to 700 km/190 to 430 mi) earthquakes, whereas oceanic ridges and transform faults experience only the shallow type. The depth of earthquakes that occur below trenches allows the dip direction of subduction zones to be defined beneath island arcs and Cordilleran mountain belts.
At the inception of the plate-tectonic scheme of events, the continental crust splits into a series of rifts and graben. First, a domal uplift forms (probably because of mineral changes in the Earth's mantle) that may be about 100 x 250 km (60 x 160 mi) across and 1 km (0.6 mi) high, and, ideally, three rift valleys develop that symmetrically meet at the center of the dome. Such domes and rift valleys are clearly seen today in Africa, especially in the northeast where the Ethiopian Rift meets the Red Sea and the Gulf of Aden (both of these have opened into narrow seas) at the triple junction at Afar. The three rifts may all widen and become narrow oceans that will eventually grow into an ocean as wide as the Atlantic. Alternatively, it is common for only two rifts to develop into oceans, the third being left as a failed rift arm, or aulacogen, extending into the continent. The Benue Trough can be regarded as an aulacogen, the other two rifts having opened up to give rise to the South Atlantic Ocean. Other examples of aulacogens that failed to become oceans are the east african rift system, the Midland Valley of Scotland, and Scandinavia's Oslo Graben. If the ocean reaches a mature stage, it may begin to contract by subduction, and eventually the subduction zone bordered by a continent may collide with the original continental margin, leaving the aulacogen as a sediment-filled rift striking at a high angle into a mountain belt.
These rifts and aulacogens are sites of marked heat flow and magma injection, phenomena that commonly give rise to volcano) and basic dikes. After a rift has evolved into an ocean, some of the early lavas and dikes may be preserved on the facing continental coasts. For example, Triassic rifts filled with continental sediments and volcanics occur along the eastern coast of the United States, and early-Jurassic, basic- dike swarms occur parallel to the coast in southwestern Greenland and Liberia. All of these examples date from the incipient stage of the development of the Atlantic Ocean.
OCEAN OPENING AND CLOSURE
The way in which two plates move apart and a new ocean is created at a mid-oceanic ridge can be demonstrated by the structure and mode of evolution of the oceanic crust on either side of the ridge. Seafloor Spreading takes place when magma upwells from the upper mantle and forms a new ocean-floor layer at a mid-oceanic ridge. As the magma cools, magnetic minerals in the new material take on the same sense of polarity as that of the magnetic field of the Earth at the time of crystallization. When the Earth's magnetic field periodically reverses its polarity, the new basaltic lava forming at the ridge acquires the reversed polarity magnetization. The seafloor spreading process takes place symmetrically, with the result that older lavas with a recognizable magnetic polarity are carried away equal distances on either side of the ridge, and a symmetrical pattern of magnetic stripes is created about the ridge axis. Thus the ocean crust contains a magnetic record of its own formation. The magnetic stripes are numbered symmetrically from 1 at the ridge to about 180-200 near the continental margins, which formed when the ocean was young. These magnetic patterns demonstrate that the ocean floors of the world have all been created since the early Jurassic and that the continents must have moved apart to accommodate the formation of new oceanic crust.
The world's oceans are in different stages of opening and closure. The Red Sea is in a narrow, embryonic stage. The Atlantic Ocean is still spreading on both sides. The Indian Ocean is opening on the west, but subducting on the east side. The Pacific Ocean is being subducted on both sides and will probably disappear as Asia collides with the Americas. In the Alpine fold system, caused by the collision of Africa and Europe, and in the Himalayas, caused by the collision of India and Asia, the only remnants of the former oceanic crust are seen in lenses and pods of ophiolites (see OPHIOLITE).
The rate of plate movement away from the ridge, which is half of the rate at which an ocean expands, varies from 1 cm/ yr (0.4 in/yr) in the North Atlantic and the Red Sea to 4.4 cm/ yr (1.7 in/yr) in the East Pacific.
If the oceans continued to grow without any compensating structures, the Earth would have to expand. No cogent, widely accepted evidence has been presented, however, that indicates that the Earth has expanded appreciably in the last 200 million years. Rather, the growing oceanic plates are subducted and consumed beneath other plates at approximately the same rate as the seafloor spreads.
In addition to the seafloor spreading data, much geologic and geophysical evidence from continental rocks corroborates the continental drift theory--that the present-day continents, originally part of one or two earlier supercontinents, separated and drifted aboard lithospheric plates to their present positions. To obtain a predrift assembly of the continents, Cambridge University geologists in the early 1960s fitted the continental edges together by computer using the 500 -fathom (3,000-ft) submarine contour, in order to take account of the continental shelves. The resultant reassembly has remarkably few overlaps or gaps. Confirmation that this reassembly is reasonably correct has been obtained by plotting the distribution of predrift geologic structures that cross from one continent to another, such as Precambrian and Paleozoic age provinces, cratons, and fold belts. Also, a large part of the present Southern Hemisphere suffered a major glaciation about 280 million years ago in Permo- Carboniferous time, when the Glossopteris land plant flourished; the glaciated and floral-distribution areas have a coherent grouping on a reconstruction of the southern continents.
Paleomagnetism provides a major quantitative confirmation of continental drift. The paleomagnetic method depends on the fact that when many igneous and sedimentary rocks form, their constituent magnetic particles are aligned according to the prevailing magnetic field of the Earth.
By carefully measuring the declination and inclination of the magnetic field built into oriented rock samples when they formed, geologists can calculate the paleolatitude of the rocks and the location of the paleopole at a given time. The paleolatitudes can be used in the construction of paleographic maps, and the paleopoles in the construction of apparent polar- wandering paths. Unfortunately, the paleolongitude cannot be determined by the paleomagnetic method, but this deficiency can be partly compensated for by taking into consideration the oceanic magnetic-anomaly patterns for the last 200 million years, and by geometric matching of continental margins.
If the paleopole positions are determined for a sequence of North American rocks of different ages ranging from the early Mesozoic to the present, these positions can be plotted on a map to define a polar-wandering curve for that continent. When the paleopoles of rocks from Europe of a similar age range are also calculated and plotted, the two separate curves clearly converge on the present pole. This convergence strongly suggests that the so-called polar-wandering paths do not represent a wandering of the poles, but rather the drift of the two continents. These two curves become congruous with a rotation of 40 degrees , only 2 degrees different from the rotation needed to close the Atlantic Ocean by matching the 500 -fathom submarine contours of the coasts of North America and Europe. After Pangea began to break up in the early Mesozoic Era, the individual continents moved off at different times in different directions before taking up their present positions, and their movement paths can be followed via their apparent polar-wandering curves.
Paleogeographical maps have now been constructed for all the continents for all the periods of the Phanerozoic Eon by using a combination of paleomagnetic, geometric-fit, and oceanic- magnetic data. These maps demonstrate how the continents were widely dispersed during the Cambrian Period, gradually moved together during the Paleozoic Era to form mountain belts, and eventually formed the Pangea supercontinent during the Permian and Triassic periods, after which they went their own way again during the Mesozoic and Cenozoic eras (paleogeography).
Most limestones, evaporites, red sandstones, and coal beds have formed within 30 degrees of the equator. A confirmation of continental drift comes from the fact that if similar fossil sediments are plotted on a present-day map of the continents they have a random distribution, but when plotted on the relevant paleomagnetically determined paleographic maps they tend to have symmetrical distributions across their respective paleoequators in a way that mirrors their modern equivalents.
A final test of the continental-drift theory is to consider the past diversity of fossil fauna. During the periods of extensive seafloor spreading (early to mid- Paleozoic, mid- to late Mesozoic, and Cenozoic), the growing mid-oceanic ridges had such a considerable volume that oceanic waters were appreciably displaced, causing a marine transgression of continental margins, which in turn increases the diversity of marine fauna. Conversely, during a period when a supercontinent is formed (Permian-Triassic), seafloor spreading stops and previously active ridges subside, causing a marine regression. The rapid fall in sea level eliminates most shallow-water faunal niches, and thus faunal diversity decreases.
The convergence and collision of two plates is the fundamental cause of mountain building. The three basic types of plate interaction are oceanic-oceanic, oceanic-continental, and continental-continental. In essence, these three types can be regarded as a progressive developmental sequence.
The Island Arc represents the incipient stage (oceanic-oceanic plate interaction) in mountain building. The inner wall of the oceanic trench contains a complicated zone where wedges of oceanic melange are thrust under older units of melange so that the youngest material continually occupies the base of the tectonic pile. This under-plating tends to uplift the evolving arc margin. High-pressure-low-temperature metamorphism in this zone causes the formation of glaucophane (a sodic amphibole) and high-pressure eclogite. Otherwise, the arc-trench gap is the site of deposition of flat-lying, shallow-water, shoreline sediments.
The arc is characterized by three features. First, intense volcanic activity is caused by magmas derived from the melting of the subducted lithospheric slab. Young arcs, such as the Tonga and the South Sandwich, have tholeiitic basalt; more mature arcs such as Japan, the Aleutians, and Indonesia have more highly evolved calc-alkaline andesite. Second, volcanic tuff, ash flows, and turbidites are deposited in intra-arc basins. Third, low-pressure-high-temperature metamorphism occurs; it is related to the high heat flow and the rise of magma deep in the arc, where granite may be intruded. The high- pressure-low-temperature and low-pressure-high-temperature belts of metamorphism make up paired belts.
Behind the arc, a small basin--floored by oceanic crust that has formed by a process of seafloor spreading roughly analogous to that which created the main oceanic crust--often develops. Extension in the back-arc basin separates the growing arc from a formerly adjoining arc or continent. Examples are the Sea of Japan, the Aleutian basin, and the Tasman Sea.
Cordilleran Mountain Belts
The next stage in mountain building is represented by Cordilleran mountain belts, which are caused by subduction of an oceanic plate beneath a continental plate. Much of what can be said about island arcs also applies to the Cordilleran mountain belts, because both are developed at subducting plate margins. For example, they both ideally have a trench, an arc- trench gap, an arc and back-arc basin structure, paired metamorphic belts, underthrusting in the trench, and andesitic volcanism in the arc.
The development of a Cordilleran mountain belt on a subducting continental margin, such as the Pacific margin extending from Alaska to Chile, can be superimposed on an Atlantic-type trailing continental margin. A classic modern example of the latter extends along the eastern United States, where two main tectonic environments are present. The Bahama Banks represents a shallow-water carbonate succession developed on the continental shelf; to its east, a deep-water sequence, consisting of shales and of greywackes deposited by turbidity currents, has developed on the continental rise. Such an Atlantic-type continental margin can become a Cordilleran-type margin if the oceanic crust-mantle breaks or decouples from the continental rocks and begins to descend beneath the continental rise.
A trench develops at the mouth of the subduction zone as oceanic material is underthrusted to form melanges. After the downgoing slab reaches depths of 100 to 150 km (60 to 90 mi), magmas released by partial melting uprise and are either extruded as lavas in the arc (for example, the andesites of Oregon) or intruded as granitic batholiths (as in the Sierra Nevada, Southern California, and Peruvian batholiths). The heated arc core expands and rises with appropriate low-pressure metamorphism and at a high level becomes the site of erosion and an axis of sedimentary transport. Flysch sediments are transported on one side over the arc-trench gap toward the trench and on the other toward the continent into the back-arc trough that develops from the downsagged continental shelf. Gradually the arc rocks are thrust toward the continent over the back-arc trough, which is eventually filled with post- orogenic sandstones and conglomerates.
Collisional Mountain Belts
The final stage in the plate-tectonic scenario occurs when an oceanic plate is totally consumed by subduction. Two continental plates collide to give rise to a new collisional mountain belt that will probably be superimposed on a Cordilleran-type belt. The prime example is the Himalayas, caused by the northward drift and collision of India against Asia. The formation of the Alpine fold system extending from Gibraltar to the Middle East is in general attributed to the collision of Africa with Europe and the destruction of the intervening Tethyan oceanic plate. The fold belt consists of a large number of small plates that have shuffled about with respect to each other, many colliding and giving rise to narrow collisional mountain belts, such as the French, Swiss, Austrian, and Dinaric alps; the Apennines; the Taurus Mountains; and the Zagros Mountains.
The zone of collision (suture) may be marked by lenses of chert, ophiolite, high-pressure glaucophane- bearing metamorphic rocks, and flysch sediments--all remnants of the former oceanic plate deposited in an early trench. The Indus suture is prominent in the Himalayas, and the Mediterranean belt contains innumerable suture rocks, especially in the Ivrea Zone in Italy and in the former Yugoslavia, Greece, and Turkey. A mountain belt may contain a zone of complex thrusts and nappes, the formation of which tends to thicken the evolving continental crust as splinters of one plate are stacked above those of another. Such stacking is clearly seen in the Swiss Alps.
Along the Alpine fold system, rock suites resulting from every stage in the plate-tectonic scheme of events are preserved. Basaltic lavas dating from the inception of continental rifting in the Triassic occur in the Othris Mountains of Greece. Evaporites in Italy and western Greece probably formed in Red Sea-type primordial oceans. Remnants of the original Tethyan plate are represented by the oceanic crust underlying the Caspian and Black seas. Geologists have observed a carbonate platform in Sicily, arc volcanics in western Italy, a back-arc marginal basin ophiolite in the Troodos complex of Cyprus, Cretaceous flysch in the Swiss Alps, and Oligocene-Miocene molasse in the Central Alps of Switzerland.
The northward movement of India into Asia is an example of indentation tectonics, analogous to driving a wedge into a sheet of plastic. The effects of stress release, which can be modeled, are seen not only in the thrust sheets of the collisional mountain belt, but also in transcurrent faults that occur widely throughout China and southern Siberia more than 3,000 km (1,900 mi) north of the Himalayas.
BASINS OF SEDIMENTATION
Plate tectonics also controls the formation of sedimentary basins, which tend to be localized along or near plate boundaries. The three main areas of sediment deposition are ocean basins, mountain belts, and continental interiors.
Following the early rifting of a continent, narrow basins or primordial oceans such as the Red Sea are formed by incipient seafloor spreading. Having formed from a dome or arch, the lips of the basin may still be uplifted, thus preventing fluvial runoff and deposition of terrigenous clastic debris. Given the right climate, carbonate reefs and evaporites will form.
As the ocean widens, arkose, sandstone, and shale may accumulate in clastic wedges along the continental margin, and marine limestone may be deposited. The successions on the coasts of Brazil and West Africa record symmetrically the change from nonmarine clastic sandstones and shales, via evaporites (about 120 million to 110 million years old), to marine sandstones and limestones. This sequence records the incoming of seawater into a continental rift valley, widened via a Red Sea-type narrow ocean into a mature ocean.
The mid-oceanic ridge, in the youngest part of a growing ocean basin, is usually free of sediment. Passing off the ridge, calcareous sedimentation, caused by microorganisms, is found. Clays and chert (silica rock) occur in deeper parts.
On the continental margins of mature oceans, the basal clastic wedge may be succeeded, if the climate is appropriate, by a carbonate platform, such as the Bahama Banks off the eastern United States and the Great Barrier Reef off northeastern Australia. Turbidity currents flow down the continental slope and deposit terrigenous sediments in the deeper water of the continental rise. River deltas locally add great thicknesses of clastic sediments to the trailing continental margin.
Sedimentation takes place in all the main tectonic segments-- the trench, trench-arc gap, arc, and back-arc marginal basin-- of island arcs and belts situated at subducting plate margins.
The trench is a bathymetric depression (up to 11 km/6.8 mi deep and 50 to 100 km/30 to 60 mi wide) at the flexure part of the downgoing plate, that is, at the throat of the subduction zone. The unconsolidated sedimentary zeolite clays and radiolarian cherts cannot be easily subducted because they are not sealed to the oceanic plate. They are thus easily offscraped and deposited in the trench in a tectonic melange, in which turbidites, derived from the uplifted margin and the adjacent plate, are intermixed.
Subsidence of the trench-arc gap creates a fore-arc basin possibly caused by descent of the ocean plate below the gap. The basin receives erosional debris from the arc, either high- level volcanic material or plutonic material exposed by uplift (geosyncline).
Fault-bounded intra-arc basins developed by extension along the axis of the main arc are probably a result of the arching associated with the thermal and mechanical uplift of the arc zone. Arc-domed volcaniclastic debris and turbidites are typically deposited in these basins.
The marginal basins that develop in the back-arc region by extension and seafloor spreading contain mostly clay, ooze, and volcaniclastic debris.
Only local sedimentary basins occur in the high area of Himalayan-type mountain belts. Forming late in the collision between the two plates and assisted by faults formed in connection with uparching and thrusting, these basins act as catchment zones for clastic debris derived from the rapidly eroding high mountains.
Some sedimentary basins form in the stable interiors of continents. Local faulting may control the formation of small basins. Extensive, thick basins may be underlain by partly attenuated continental basement and thus occupy wide zones of crustal subsidence. Clastic sediments predominate here.
Deep failed rift arms extend into continental interiors from an ocean or a collisional mountain belt. They are typically infilled with vast thicknesses of terrigenous sediments, particularly flysch and molasse.
During the evolution of new oceanic plates and mountain belts by plate tectonics, a large number of mineral deposits with vast accumulations of minerals form, particularly in association with plate boundaries. Many of these deposits may be economic for mining purposes (ore deposits). Specific types are diagnostic of different plate regimes.
Domes and Rifts
Before rifting begins, linear belts of alkaline granites with concentrations of tin develop in association with early topographic domes. Examples occur in Nigeria, Chad, and Damaraland in Namibia.
Lead-zinc-silver deposits occur in carbonate sediments in the Oslo Graben, and niobium concentrations are found in carbonatite (a carbonate rock) intrusions in the East African Rift System. Both of these areas are failed rift arms.
The Oceanic Crust and Ophiolite Complexes
The sediments from the East Pacific Rise and the Red Sea contain high enrichments of iron and manganese. Similar metal- enriched sediments occur in the Troodos ophiolite complex in Cyprus. Copper ores occur in or above the basaltic pillow lavas in inland ophiolite successions in Cyprus, Turkey, and the Philippines. Chromite deposits are very common in the lower, serpentinized, ultramafic rocks of many ophiolites in Turkey, Greece, and Yugoslavia. Similar deposits are predicted to be present in mid-oceanic ridges and marginal basins.
Island Arcs and Cordilleran Mountain Belts
The new magmas that are generated by partial melting of a subducting lithospheric plate arise in the main arc of Cordilleran mountain belts and island arcs. These magmas give rise by fractional crystallization to volcanic and plutonic rocks and associated ore deposits. The lavas and granitic rocks change systematically in composition on passing inland from the trench, and the mineral deposits also have a zonal distribution. A general sequence (passing inland from the trench) of iron, gold, copper, molybdenum, gold, lead, zinc, tin, tungsten, antimony, and mercury occurs across the American Cordilleran margin and the arcs of the western Pacific.
Collisional Mountain Belts
Little is known in detail about the mineralization of collisional mountain belts. Having been through a stage involving subduction of an oceanic plate, these belts should contain many of the Cordilleran-type ore deposits, such as, in particular, all the types of ores found in ophiolite complexes like those in the Indus suture of the Himalayas and in the eastern Mediterranean. Tin is widely thought to occur in granites derived directly by the continental collision process; an example may be the Permian tin ores associated with the granites of southwestern England.
THE DRIVING MECHANISM
Although the relative distribution of rocks and structures with regard to plate boundaries is increasingly well understood, the underlying driving force responsible for the motion of plates is still unclear.
The most favored hypothesis is based on two considerations. First, more than 50 percent of the heat escaping from the Earth's interior does so via plate boundaries. Thermal energy derived at depth by the decay of radioactive materials is thus probably one fundamental driving force. Second, seafloor spreading and continental drift can best be explained by the action of convection currents operating in the uppermost 500 km (300 mi) of the mantle. The basic idea is that hot, partially molten materials upwell beneath the early rifts and the developing mid-oceanic ridges. The mantle materials move with the convective currents horizontally away from the ridges. The materials gradually cool and become denser as they pass toward the continents, where they sink back into the deeper mantle below subduction zones. Recent seismic studies of the mantle, however, somewhat complicate this picture by suggesting that such heat flow may be confined to the upper mantle.
Undoubtedly, many other variables affect or assist the process. Oceanic crust, for example, is denser than continental crust--a density difference that would facilitate the sinking of the downgoing plate. Computer and experimental models corroborate the convection-cell hypothesis.
HISTORY OF THE CONCEPT
The concept of plate tectonics has evolved over a long period. Early geological ideas were qualitative and speculative and were not widely accepted, but geophysical data obtained from the ocean floor more recently led to the construction of a quantitative model that has since emerged as the main conceptual framework for the Earth sciences.
The ideas of continental drift, seafloor spreading, and plate tectonics developed sequentially. The first major protagonist of continental drift was Alfred Wegener, who suggested as early as 1912 that one supercontinent, called Pangea, had broken up during the early Mesozoic and that the separate continents had then drifted to their present positions. He collated evidence to support his theory, such as the similarity of fossil fauna and flora in different continents and the continuity of geological structures and paleoclimatic belts.
Similar geological correlations between South America and Africa were advanced (1927 and 1937) by Alexander L. du Toit, who suggested the existence of two supercontinents--Laurasia in the north and Gondwanaland in the south--separated by the Tethys Sea. In 1929, Arthur Holmes envisaged that subcrustal convection currents were dragging two continents apart with consequent mountain building at the margin of a trench. Apparent confirmation of continental drift came in 1956 when Stanley Keith Runcorn and his colleagues established that the polar-wandering paths for North America and Europe diverge progressively from the present to the Triassic.
Beginning in the late 1950s, oceanographers began to discover magnetic anomaly stripes in the ocean floor; their significance, however, was not initially understood. The first major breakthrough came in 1960 when H. H. Hess suggested that new ocean floor was created at the mid-oceanic ridges and that the ocean evolved by seafloor spreading. The second came in 1963, when D. H. Matthews, F. J. Vine, and Lawrence Whitaker Morley proposed that the alternating magnetic anomalies in the ocean floor were caused by regular reversals in the Earth's magnetic field. Such geomagnetic reversals were previously known only in continental rocks. A further advance came in 1965 when J. Tuzo Wilson advocated the transform-fault mechanism to explain how oceanic plates slide laterally past each other.
The concept of plate tectonics came to fruition by 1970. The tectonic plates of the world were tentatively defined, along with their relative directions of movement and the rates of extension and compression at their boundaries. Major earthquake zones were correlated with the boundaries, and schematic models were developed for the evolution of island arcs, Cordilleran mountain belts, and Himalayan belts. The foundations of such mountain building were located at subducting and collisional plate boundaries.
Although plate tectonics remained somewhat controversial for a number of years, by the 1980s the actual movement of plates had been recorded by means of very long baseline interferometry (radio astronomy). Mechanisms of plate movement remain a subject of intense geophysical research, however, and the continental-drift aspect of plate movement continues to be controversial. Some researchers, for example, point out that studies indicating the depth of continental roots in the mantle rule out any simple linkage between plate activity and the actual form and movement of the continental bodies.
Brian F. Windley