plate tectonics

Theory formulated in the 1960s to explain the phenomena of continental drift and sea-floor spreading, and the formation of the major physical features of the Earth's surface. The Earth's outermost layer, the lithosphere, is seen as a jigsaw puzzle of rigid major and minor plates that move relative to each other, probably under the influence of convection currents in the mantle beneath. At the margins of the plates, where they collide or move apart or slide past one another, major landforms such as mountains, rift valleys, volcanoes, ocean trenches, and mid-ocean ridges are created. The rate of plate movement is on average 2–3 cm/1 in per year and at most 15 cm/6 in per year.

The concept of plate tectonics brings together under one unifying theory many phenomena observed in the Earth's crust that were previously thought to be unrelated. The size of the crust plates is variable, as they are constantly changing, but six or seven large plates now cover much of the Earth's surface, the remainder being occupied by a number of smaller plates. Each large plate may include both continental and ocean lithosphere. As a result of seismic studies it is known that the lithosphere is a rigid layer extending to depths of about 50–100 km/30–60 mi, overlying the upper part of the mantle (the asthenosphere), which is composed of rocks very close to melting point. This zone of mechanical weakness allows the movement of the overlying plates. The margins of the plates are defined by major earthquake zones and belts of volcanic and tectonic activity. Almost all earthquake, volcanic, and tectonic activity is confined to the margins of plates, and shows that the plates are in constant motion (see plate margin).