plate tectonics

PLATE TECTONICS, the theory that the lithosphere (the outer part of solid Earth) is divided into a small number of plates that float on and travel independently over the Earth's mantle. Much of the Earth's seismic activity and volcanism, along with mountain-building processes, occurs at the boundaries of these plates.

The surface of the Earth is composed of about a dozen large plates and several small ones. Within each plate the rocks of the terrestrial crust move as a rigid body, with only minor flexuring and few manifestations of seismicity and volcanism. The margins of the plates are defined by narrow bands in which 80 percent of the world's earthquakes and volcanoes occur. There are three types of boundaries. The first of these is a very narrow band of shallow earthquakes caused by tensile stresses that follow exactly the crest of the 80,000-kilometer- (48,000-mile-) long, active midocean ridges. The second boundary type occurs in areas where these ridges are offset. Earthquakes are much more violent along faults at such sites and result from the plates on either side of the faults grinding laterally past one another in opposite directions. Earthquakes forming the third boundary are distributed more diffusely but include all of the world's deep earthquakes (i.e., those originating at depths greater than 145 km) and are associated with extremely narrow zones in which the ocean floor descends below its normal depth to as much as 10.5 km below sea level--the oceanic trenches. Across this margin, the maximum earthquake depths systematically increase along a dipping plane, with shallower earthquakes associated principally with the volcanic activity that borders each trench.

The ridge-crest earthquakes originate because of the tension created when the plates on either side move in opposite directions. This movement also releases the pressure on the underlying hot rocks, causing them to begin melting. The resulting magmas rise to form volcanoes (such as those in Iceland), which then solidify and later fracture as the tensional forces reassert themselves. Such new volcanic rocks thus become added to the edge of each plate, which grows at these "constructive" margins. The evidence for plate motion is not only the nature of the earthquakes but also the age of the volcanic oceanic rocks. Dating can be achieved by using both the fossil content of the sediments overlying the volcanic rocks and the time record represented by the anomalies in the magnetism of the rocks, which can be detected by ships sailing on the ocean surface. These show that the youngest volcanic rocks are at the crests of the midocean ridges and the oldest are in the deepest areas, i.e., the oceanic trenches. Nowhere, however, are such rocks older than 190 million years, indicating that all older oceanic rocks must have been destroyed.

The trench margin is termed "destructive" because this is the region where the oceanic rocks are subducted (carried down) into the mantle along the dipping plane. Where subduction occurs along a continental edge, volcanism distorts the continental rocks, forming such mountain chains as the Andes. Elsewhere, volcanism creates island arcs, as in the southwestern Pacific. The composition of the volcanoes and their mineralization changes systematically with depth to the dipping plane, but their overall composition is that of continental crustal rocks. The destructive margins are thus regions where continental crustal rocks are created but oceanic rocks are recycled back to the mantle. The density of continental rocks is too low for them to be subducted, so if they are carried to a trench they will eventually collide, giving rise to mountain chains such as the Alps and Himalayas, which formed when Africa and India, respectively, collided with Europe and Asia.

Although the lateral extent of the plates is well defined, their thickness is less certain. At the crest of the oceanic ridge they are very thin, but heat-flow and seismic evidence suggest that their base increases rapidly with depth, reaching 48-57 km (30-36 miles) within about 9-19 km of the crest. By about 960 km distance from the crest the base has increased to 115 km. A plate may be subducted at any thickness but rarely exceeds 145 km. Each plate is composed of rigid mantle rocks with oceanic crustal rocks, but not necessarily those of the continental variety (e.g., the Pacific plate is devoid of continental rocks). The zone of rigid crustal and mantle rocks is termed the lithosphere to distinguish it from the deeper asthenosphere, where mantle rocks are at a higher temperature and so deform plastically when subjected to tectonic stresses. The continental lithosphere is not consistently underlain by an asthenosphere. Moreover, the presence of volcanic rocks such as diamond-bearing kimberlites indicates that here the Earth's lithosphere is at least 190 km thick, so that mantle flow, which causes plate motions, must occur at even greater depths.

The movements of the mantle result from the need to transfer to the Earth's surface the heat generated within it by radioactive decay, and hence convective patterns vary with time. This is shown by changes in the location of past plate margins. The subduction that formed the Western Cordillera of North America largely ceased 10 million years ago, although some activity continues to produce volcanoes (e.g., the continuing eruptions of Mount Saint Helens in Washington) and earthquakes in Alaska.

Over time scales of hundreds of millions of years, changes in mantle convection initiated the formation of the Atlantic and Indian Oceans by splitting preexisting continents that were grouped as two major blocks, Laurasia and Gondwanaland, some 160 to 180 million years ago. Similarly, past continental collisions are recorded by largely eroded mountain chains, such as the Appalachian Mountains of eastern North America and the Caledonian-Hercynian Mountains of Europe and Africa, which were formed when these continents collided on successive occasions. The rate of mantle convection depends essentially on the square root of heat production within the mantle. This means that convection rates must have been at least twice as fast about 3 billion years ago, when the radiogenic heat being produced was about five times greater than today. The surface expressions of such motions, however, may have been different. There are no continental rocks more than 4 billion years old, possibly because the lithosphere was thin and was recycled without generating continental rocks. The nature of plate tectonic activity during most of the Earth's history is still uncertain, and models of the way in which it would be reflected in the continental rocks are highly speculative.