Convection currents
This hypothesis suggests that flow in the mantle is induced by convection currents which drag and move the lithospheric plates above the asthenosphere. Convection currents rise and spread below divergent plate boundaries and converge and descend along convergent. Three sources of heat produce the convection currents:
(1) cooling of the Earth's core
(2) radioactivity within the mantle and crust
(3) cooling of the mantle