The heat from radioactive processes within the planet's interior causes the plates to move, sometimes toward and sometimes away from each other. This movement is called plate motion, or tectonic shift.
Convection within the Earth's mantle causes the plates to move. ... The hot mantle rises up toward the surface (Figure below). As the mantle rises, it cools. At the surface, the material moves horizontally away from a mid-ocean ridge crest.
