Convection within the mantle ie the thick layer below the crust, powers the movements of plate tectonics. The the conheat is generated mainly by the radioactive decay of elements within the mantle and the rock becomes molten, or magma. The magma rises to the surface at the mid-oceanic ridges and the circulation of the convection currents below the oceanic crust pushes the overlying crust towards the continents where the oceanic crust is subducted or pushed under the lighter continental crust.
