The ocean floor is as ancient and permanent as the continents.

Continents preserve rocks that are billions of years old. Before plate tectonics, it seemed reasonable to expect the deep ocean floor to contain an equally ancient geological record.

Ocean exploration revealed something very different.

Sonar mapping found enormous mountain chains running through the oceans. These mid-ocean ridges are places where hot material rises and new basaltic ocean crust forms.

Magnetic surveys supplied a striking clue that the new crust moves away from the ridge. As basalt cools, magnetic minerals record the direction of Earth’s magnetic field. Because the field has reversed many times, the seafloor developed alternating magnetic stripes. The same stripe pattern appears on both sides of a ridge, as if new crust were being added in the middle and carried outward.

Other evidence matched that picture. Sediment becomes thicker farther from the ridges because older seafloor has had more time to collect it, and radiometric dating shows the basalt itself getting progressively older with distance.

Yet even the oldest surviving oceanic crust is young compared with ancient continental rocks.

The reason is recycling. At subduction zones, oceanic plates bend downward and sink back into the mantle. New ocean floor is continually created while old ocean floor is destroyed.

Ancient sediments can sometimes sit on younger ocean crust, but the basaltic crust beneath them is geologically temporary.