A rigid segment of the earth's crust which can ‘float’ across the heavier, semi-molten rock below. The plates making up the continents—continental plates—are less dense but, at up to 35 km deep, are thicker than those making up the oceans—the oceanic plates—which are up to 5 km deep. Thus a plate is a part of the lithosphere which moves over the plastic asthenosphere. The boundary of a plate may be a constructive, destructive, conservative, or, more rarely, a collision margin.
The theory of plate tectonics submits that the earth's crust is made up of six large plates: the African, American, Antarctic, Eurasian, Indian, and Pacific plates, and a number of small plates, the chief of which are the Arabian, Caribbean, Cocos, Nasca, Philippine and Scotia plates. The movement of plates causes global changes, such as continental drift and a remodelling of ocean basins and the creation of major landforms: oceanic ridges, fold mountains, island arcs, and rift valleys, together with earthquakes and volcanoes, which occur at a destructive plate boundary where one plate plunges below another. The causes of plate movement are still the subject of controversy, but it is known that while plates may move away from constructive margins at speeds of up to 6 cm per year, they may be consumed at destructive margins at up to 15 cm per year.
Plates
