The sinking of cold ocean lithosphere directly drives the motions of mantle convection through slab- pull and ridge-push. Some scientists think mantle plums are involved in the upward flow of rock in the mantle. Slab-Pull is the force of gravity pulls old ocean lithosphere (relatively cold and dense) deep down into the mantle. Ridge- push the stiff ocean lithosphere is pulled down into the asthenosphere which is elevated near mid-ocean ridge. This downward slide is the result of gravity acting on the cool and dense ocean lithosphere.