The Earth's crust recycles itself through subduction of crustal material into the mantle and upwelling of magma from the mantle. These processes are described by plate tectonics, a commonly accepted theory that explains the large-scale movements of the Earth's lithosphere.
The Earth's surface is divided into sections, called plates, that are in constant motion. When plates collide, the leading edge of one plate is subducted or forced beneath the other. Subduction lines are often marked by active volcanoes. Plates separate when magma rises from deep within the Earth and forces them apart. This creates mid-ocean ridges and rift valleys, which are also sites of volcanic activity.