The theory of plate tectonics also explains how Earth’s lithosphere is recycled over time. Tectonic plates are destroyed at subduction zones. Subduction zones are areas where a denser plate slides beneath a less dense plate. The denser plate sinks back into the Earth's hot mantle, then melts. New oceanic crust is formed at mid-ocean ridges. This happens through the process of seafloor spreading.
As new crust is forming at the spreading center, it pushes the older crust apart. Because of this, the oceanic crust contains symmetrical patterns of crustal rock ages. These rocks increase in age as their distance from the mid-ocean ridge increases.