Why is the seafloor so recent and the continental crust so old?
The oldest oceanic crust is about 260 million years old. This sounds old but is actually very young compared to the oldest continental rocks, which are 4 billion years old. Why is the seafloor so young? It is due to the process of subduction; 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 mantle (subduction). This is like a giant recycling system for the oceanic lithosphere. As the continental crust is lighter than the oceanic crust, the continental crust cannot subduct. We therefore still have some very old continental rocks at the surface of the Earth.