During seafloor spreading, ocean rock is continuously created at mid-ocean ridges as magma rises to the surface and solidifies. This creates new oceanic crust. As the oceanic crust moves away from the ridge, it cools and becomes denser, eventually sinking back into the mantle at subduction zones where it is destroyed through the process of subduction. This cycle of creation and destruction of ocean rock plays a key role in the movement of tectonic plates and the shaping of Earth's surface.
