Oceanic crust is the layer of rock at the bottom of the ocean. It is formed when rock from the mantle melts and comes up at mid ocean ridges. As more crust gets formed, the older crust is pushed away from the ridges and eventually goes back down into the mantle at trenches near the edges of continents.

Since oceanic crust is constantly being recycled, it doesn't get much older than 200 million years. By comparison, continental crust can be billions of years old. Oceanic crust is made mostly of basalt and gabbro and is about 7 km (4 miles) thick. Continental crust, on the other hand, is often five times thicker.

On Earth there are 2 types of crust -- oceanic crust and continental crust. Continental crust is what's under our feet when we stand on land.

All our land masses are made of continental crust.

Oceanic crust is the rocks that are laying underneath our oceans. Oceanic crust is more dense (very heavy) than continental crust because it was made by lava/magma from the mantle (inner layer of the earth). It is black or very dark in color.

Oceanic crust is a rind of hardened magma, generally basalt on the surface with gabbro underneath, which separates the Earth's mantle below from the ocean above. It is created at the mid-oceanic ridges where the mantle wells up to form new crust, and is destroyed in subduction trenches, where the ocean crust slides underneath other crust and then sinks into the mantle where it is eventually absorbed.