According to the theory of plate tectonics, the lithosphere (earth's relatively hard and solid outer layer consisting of the crust and part of the underlying mantle) is divided into a few dozen plates that vary in size and shape; in general, these plates move in relation to one another. They move away from one another at a mid-ocean ridge, a long chain of sub-oceanic mountains that forms a boundary between plates. At a mid-ocean ridge, new lithospheric material in the form of hot magma pushes up from the earth's interior. The injection of this new lithospheric material from below causes the phenomenon known as sea-floor spreading. Given that the earth is not expanding in size to any appreciable degree, how can "new" lithosphere be created at a mid-ocean ridge? For new lithosphere to come into being in one region, an equal amount of lithospheric material must be destroyed somewhere else. This destruction takes place at a boundary between plates called a subduction zone. At a subduction zone, one plate is pushed down under another into the red-hot mantle, where over a span of millions of years it is absorbed into the mantle.