It is because of subduction<span>: oceanic crust tends to get colder and denser with age as it spreads off the mid-ocean ridges. It gets so dense, that it sinks in the upper </span><span>mantle </span><span>(subduction). This is like a giant recycling system for oceanic lithosphere! Because continental crust is lighter than oceanic crust, continental crust cannot subduct. We therefore still have some very </span>old continental rocks<span> at the surface of the Earth.</span>
