Mantle convection

Mantle convection is the slow creeping motion of Earth's rocky mantle in response to perpetual gravitationally unstable variations in its density. Material near the surface of Earth, particularly oceanic lithosphere, cools down by conduction of heat into the oceans and atmosphere, then thermally contracts to become dense, and then sinks under its own weight at convergent plate boundaries. This subducted material sinks to some depth in the Earth's interior where it is prohibited, by inherent density stratification, from sinking further. This stoppage creates a thermal boundary layer where sunken material soaks up heat via thermal conduction from below, and may become buoyant again to form upwelling mantle plumes.