The Earth is so fragile. We sit on a thin layer of rock, under which is a seething mass of molten material. If the rock is very thin, the hot magma bursts out of the surface sometimes, cascading down a mountainside, like the pictures of Mount St Helen’s used in the movie Dante’s Peak. The Earth consists of rigid plates called tectonic plates, floating above a layer of molten magma. With a thickness of about 100 km, this ‘lithosphere’ is composed of an upper layer of crust (~7 km thick under the oceans, and ~35 km thick under the continents) and a lower, denser layer of the earth’s upper mantle. Underneath the lithosphere is a hot, mobile layer of partially molten rock lying within the earth’s upper mantle which is constantly being moved around by convection.
Hot mantle rises beneath mid-oceanic ridges, and cold, denser mantle descends at oceanic trenches. Sideways movement of the plates above these circular convection cells is rather like rigid blocks riding above a rotating conveyor belt. Here’s a little interactive demonstration
which goes with the picture.