Long mountain ranges on the bottom of the sea, where two continental plates are drifting apart.

At the oceanic ridges, hot magma from the mantle is dwelling up to the surface, forming new oceanic crust. This results in pushing apart the two adjacent continental plates. The formation of new oceanic crust has to be accompanied by the melting down of old crust at other places (the deep-sea trenches) These processes (the slow convection of parts of mantle and crust) is powered by the thermal difference between the interior and the surface of the earth.