At the high pressures and temperatures found in the Earth's mantle, rock responds to stress by slow, creeping flow. The mantle, although solid, is a dynamic system. Convection is driven by density variations primarily caused by temperature variations and possibly also by compositional variations. Hot, relatively low-density material wells up to the surface, and cold, higher-density material plunges into the interior. This phenomenon influences the Earth's surface processes and drives the motion of the tectonic plates.