Oceanic Crust

Oceanic crust covers about 60 percent of the Earth's surface. Oceanic crust is thin and young—no more than about 20 km thick and never older than about 180 million years. Everything older has been pulled underneath the continents by subduction. Oceanic crust is born at the midocean ridges, where pressure upon the underlying mantle is released and theperidotite there begins to melt in response. The part that melts becomes basaltic lava, which rises and erupts while the remaining peridotite becomes depleted.

Continental Crust

Continental crust is thick and old—on average about 50 km thick and about 2 billion years old—and it covers about 40 percent of the planet. Whereas almost all of the oceanic crust is underwater, most of the continental crust is exposed to the air.

The continents slowly grow over geologic time as oceanic crust and seafloor sediments are pulled beneath them by subduction.