PLATE TECTONICS

(Geology) The concept that both continents and ocean basis are only the emergent parts of large pieces or plates of the earth's surface. It is generally agreed that the global surface can be divided into at least twenty discrete plates (seven major and many minor) with each plate moving in a different direction from that of its neighbor. It is this motion that creates the variety of features of the earth as well as leads to instability along the plate edges. The motion of the plates is believed to be caused by tremendous heat and pressure built up beneath the relatively thin veneer of the overlying plates. The motion of the plates is characterized by spreading centers whereby molten rock is forced to the surface to form new crustal rocks, and collision zones where plates meet and the older, heavier plate is forced beneath the newer, lighter plate to be turned into a molten state once again deep beneath the earth's surface. This subduction process builds up the mountains along the collision line and results in considerable seismic activity. The seven major plates are named for the continents or oceans and include Pacific, Eurasian, African, Australian, North American, South American, and Antarctic. Also see Plate Boundaries.