plate tectonics

plate tectonics Theory or model to explain the distribution, evolution, and causes of the Earth's crustal features. It proposes that the Earth's crust and part of the upper mantle (the lithosphere) consists of several separate, rigid slabs, termed plates, which move independently forming part of a cycle in the creation and destruction of crust. The plates collide or move apart at the margins, and these produce zones of earthquake and volcanic activity. Three types of plate boundary can be identified. At a constructive or divergent margin, new basaltic magma originating in the mantle injects into the plate. The crusts are forced to separate and an oceanic ridge is formed. At a destructive or convergent margin, plates collide and one plate moves under the other. This occurs along oceanic trenches. The recycling of crust by subduction results in the melting of some crustal material, and volcanic island arcs (such as the islands of Japan) are produced. Material that cannot be subducted is scraped up and fused onto the edge of plates. This can form a new continent or add to existing continents. Mountain chains are explained as the sites of former subduction or continental collision. At a conservative margin, plates move past each other along a transform fault. Plate movement is thought to be driven by convection currents in the mantle. See also seafloor spreading