* Plate Tectonics: The Earth's crust is constantly moving, driven by forces within the mantle. This movement is called plate tectonics.

* Seafloor Spreading: At mid-ocean ridges, new oceanic crust is formed as magma rises from the mantle and cools. This process pushes older crust away from the ridge, creating a continuous "conveyor belt" of seafloor.

* Subduction: As the seafloor moves away from the ridges, it eventually collides with continental plates. The denser oceanic crust is forced beneath the continental crust in a process called subduction. Subducted seafloor eventually melts back into the mantle.

This means:

* The oldest seafloor is continuously being recycled through subduction, making it significantly younger than continental rocks.

* Continental rocks, on the other hand, are much less likely to be subducted and therefore have had a longer time to accumulate.

Key Points:

* The oldest oceanic crust is around 200 million years old.

* The oldest continental rocks are over 4 billion years old!

This difference in age is a strong piece of evidence supporting the theory of plate tectonics.