The crust of the earth isn't one solid piece. It's made up of huge sections called plates. Continental plates contain major landmasses, and oceanic plates are under the major bodies of water. They fit together like a giant jigsaw puzzle. These plates are less dense than the magma in the upper mantle, and so they float on top.

Since the mantle is very hot, currents that flow through its molten magma cause it to flow in many directions. As a result, the plates move along with it. The movement of these plates is called plate tectonics. It is those movements that sometimes cause earthquakes.