Moving continents, plates, quakes and volcanoes

The Earth has layers: a thin crust, a thick mantle, a liquid outer core and a solid inner core. The crust and the solid top of the mantle make the lithosphere, which is broken into plates. Below it is the asthenosphere, hot, weak rock that flows very slowly: it is solid, partly melted in places, not a sea of liquid. Oceanic plates are thin and dense, continental plates are thick and less dense. Plates move a few centimetres a year, driven by convection currents in the mantle. Where plates meet we find three kinds of boundary. At a divergent boundary plates move apart and new crust forms: the Red Sea, where the African and Arabian plates separate, and the East African Rift. At a convergent boundary plates collide: two continental plates push up mountains such as the Himalayas; an oceanic plate going under a continental one makes mountains such as the Andes; two oceanic plates make island chains with volcanoes. At a transform boundary plates slide past each other, like the San Andreas fault. The Himalayas are higher; the Andes are longer.