Ocean floor rocks are. Continental rocks

crust has been forming on Earth for over 4 billion years, all of the sea floor older than about 200 million years has been recycled by plate tectonics. Continental crust is not subducted and destroyed, so very old continental rocks have survived.

Although oceanic crust has been forming on Earth for over 4 billion years, all of the sea floor older than about 200 million years has been recycled by plate tectonics. Continental crust is not subducted and destroyed, so very old continental rocks have survived.