The reason (or reasons) for Earth’s crustal movement is still somewhat of a mystery. The most accepted theory of plate movement is called continental drift, and the theory of its mechanism, plate tectonics. These theories suggest that the continental plates move laterally across the face of the planet, driven by the lower, more fluid mantle. At certain plate boundaries, molten rock from the mantle rises at a mid-ocean ridge (such as the Mid-Atlantic Ridge, a long chain of volcanic mountains that lie under the Atlantic Ocean); or its equivalent on land, the rift valley (such as the one in eastern Africa), the magma solidifying and moving away to either side of the ridge. At other plate boundaries, plates are pushed under an adjacent plate, forming a subduction zone, in which the crust sinks into the mantle again. And at other boundaries, plates just slip by each other, such as the San Andreas Fault in California, in which a part of the North American plate slides by the Pacific plate.

But not everyone agrees on these theories. One reason is because, although the idea of moving plates seems sound, the mechanisms for developing plate tectonics is not fully understood. Therefore, some scientists believe in continental drift, but not plate tectonics. Many of these scientists believe that the reason that the continents shift is that Earth is actually expanding, causing a false illusion of movement (although no one can explain why or how Earth is expanding). Another hypothesis is called “surge tectonics,” in which the features of Earth’s surface are explained by the sudden surge of plate movement, as opposed to a constant flow by the steady movement of the mantle. And still others suggest that the continents have always been in the same positions.

No one really can fully explain the reason for the continual movement of the plates. One thing is certain: the plates do move. Since the advent of Earth-orbiting observation satellites, scientists have been able to track the plates using sophisticated laser-ranging instruments that measure the minute movements.