Plate Tectonics

Plate tectonics is the theory that proposes that the earth's outer layer, the lithosphere, is divided into a dozen large, thick slabs, called plates, that are moving and changing in size; the interactions of these plates produce the major features on the earth's surface.

Traditionally the earth has been divided into three zones: the outer crust, the mantle, and the core at the center. Geologists now know that the earth's internal structure is somewhat more complicated than that simple three-part division indicates.. The crust and the upper part of the mantle are rigid, forming the lithosphere, the outer, brittle layer. Oceanic crust, basaltic in composition, is about 7 km thick, while continental crust can be 30 to 50 km thick and is composed for the most part of granitic rocks. The lithosphere is between 70

and 125 km thick. Below the lithosphere and extending downward to about 200 km below the surface is the asthenopshere, a hot, semi-molten layer that moves, driven by heat. Hot material moves toward the surface, cools, and drops downward forming a circulation pattern called a convection current. Thus the interior of the earth is a heat machine driving all of the internal movements.

The entire mantle is 2,900 km thick, and the earth has a radius of 6,370 km. Intense geologic activity takes place at the boundaries of the plates. Plate tectonics is the unifying theory that relates many geological features that appear to be independent of one another. The theory is as important to geology as the theory of relativity is to physics, the atomic theory is to chemistry, and the theory of evolution is to biology.

Although the theory of plate tectonics had a number of forerunners, the modern concept was proposed in the early 1960s; within ten years it had been accepted by most geologists.

According to the theory, the plates move on the underlying semisoft asthenosphere, driven by convection currents generated by the heat from within the earth. The plates are pulled away from each other, slide past each other, or move toward each other. Where the plates are pulled away from each other, at a "spreading boundary," submarine mountain ranges, called midoceanic ridges, are created. They result from the upflow of hot mantle material (magma), which pushes the lithosphere upward. More or less simultaneously, as the plates pull away, tension cracks develop along the ridge crest, into which magma squeezes, sometimes solidifying there or erupting out on the seafloor forming lava flows and volcanoes. As the plates move away from the ridge they cool, shrink, and sink, causing the ocean floor to deepen away from the ridge. Plates may be entirely oceanic (basaltic), partly oceanic and continental (granitic), or entirely continental in composition.

The boundary, where two plates slide past each other, is called a transform boundary. The San Andreas fault in California is an example of this type. Numerous earthquakes occurring along its boundary are a result of plate motion.

The third type of boundary is a convergent boundary, where plates move toward each other. Where one plate has a continent on it and the other ocean crust, the plate with the continents, which is less dense, will ride over the denser oceanic plate. Here convection currents and gravity aid in pulling the plate downward in a process known as subduction. As the oceanic plate descends into the hot mantle, the downward-arched crust forms an ocean trench, while the deeper parts of the plate heat up even more because of friction. As the plate moves deeper toward the asthenosphere, melting takes place and magma is created. Magma, less dense that the overlying rocks, works its way upward, sometimes cooling below the surface but also erupting onto it forming volcanoes and lava flows.

When these rocks are exposed to the atmosphere on the surface, the earth's external heat engine, driven by solar power, comes into play. The hydrological cycle, circulation of the oceans, the various types of erosion, and the weathering of rocks, all dependent directly or indirectly on the external heat engine, work to wear down the landforms created by the earth's internal movements.

—Sidney Horenstein

See also: Geological Time Scale; Geology, Geomor-phology, and Geography; Mountains; Oceans; Volcanoes


Gurnis, Michael. 2001. "Sculpting the Earth from Inside Out." Scientific American 284 (3): 40-47; Ham-

blin, W. Kenneth, and Eric H. Christiansen. 2000. The Earth's Dynamic Systems. Upper Saddle River, NJ: Prentice Hall; Keller, Edward A., and Nicholas Pinter. 1996. Active Tectonics: Earthquakes, Uplift, and Landscape. Upper Saddle River, NJ: Prentice Hall; Montgomery, Carla. 1996. Fundamentals of Geology, 3rd ed. New York: McGraw Hill Professional Publishing; Press, Frank, and Raymond Siever. 2000. Understanding Earth, 3rd ed. New York: W. H. Freeman and Company.

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