plate

One of several relatively distinct sections of the lithosphere, approximately 100 km/60 mi thick, which together comprise the outermost layer of the Earth (like the pieces of the cracked shell of a hard-boiled egg).

The plates are made up of two types of crustal material: oceanic crust (sima) and continental crust (sial), both of which are underlain by a solid layer of mantle. Dense oceanic crust lies beneath Earth's oceans and consists largely of basalt. Continental crust, which underlies the continents and the continental shelves, is thicker, less dense, and consists of rocks that are rich in silica and aluminium.

Due to convection in the Earth's mantle (see plate tectonics) these pieces of lithosphere are in motion, riding on a more plastic layer of the mantle, called the asthenosphere. Mountains, volcanoes, earthquakes, and other geological features and events all come about as a result of interaction between these plates.