* Ocean crust is constantly being created and destroyed. At mid-ocean ridges, magma rises from the Earth's mantle, cools, and forms new oceanic crust. As the crust moves away from the ridge, it eventually gets subducted (pushed) back into the mantle at subduction zones.
* The process of subduction recycles ocean crust. This means that the oldest ocean crust is rarely found on the surface.
* The oldest ocean crust found is about 200 million years old. This is much younger than the oldest continental rocks, which are billions of years old.
In contrast to the constant renewal of ocean crust, continental crust is much more stable and can survive for billions of years. This is why continental rocks are much older than oceanic rocks.
