Plate tectonics is a theory that explains the origin of most of the major features of the earth's surface. For example, the theory tells us why most volcanoes occur where they do, why there are high ridges and deep trenches in the oceans, and how mountains form. According to this theory, the earth has an outer shell made up of about 30 rigid pieces called tectonic plates. Some of these plates are gigantic. For instance, most of the Pacific Ocean covers a single plate. The plates move about on a layer of rock that is so hot it flows, even though it remains solid. The plates are moving very slowly relative to one another. They move at speeds up to about 4 inches (10 centimeters) per year.
The theory of plate tectonics developed from a theory of continental drift, presented in 1912 by German meteorologist Alfred Wegener. Wegener's theory proposed that the continents move about the surface of the earth. Wegener was not sure what caused continents to drift, however. His theory of continental drift became a subject of much debate among scientists.
However, evidence supporting the theory of continental drift gradually accumulated. In the late 1950's, scientists discovered that most earthquakes occur along lines parallel to ocean ridges and trenches. In 1960, Harry H. Hess, an American geologist, proposed a theory of what came to be called sea-floor spreading. In 1967, American geophysicist Jason Morgan and British geophysicist D. P. McKenzie independently proposed the idea that the earth's surface consists of a number of movable plates.
In 1969, the drillship Glomar Challenger completed its first scientific cruise. Material drilled from various locations on both sides of the Mid-Atlantic Ridge indicated that the age of the ocean crust was exactly as predicted by the analysis of paleomagnetism and sea-floor spreading. This discovery and the continuing accumulation of other evidence convinced most earth scientists that the theory of plate tectonics is valid.
Excerpt from the "Plate tectonics" article, The World Book Encyclopedia © 1999