Explanation:

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.

Reason behind it:

1. Sea floor spreading:  Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.

2.  Subduction:  It is because 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 oceanic lithosphere! Because continental crust is lighter than oceanic crust, continental crust cannot subduct. We therefore still have some very old continental rocks at the surface of the Earth.