Ocean

The oceanic plates beneath the ocean waters and above the earth's mantle are made up of igneous rock, formed from the magma, or molten rock, at mid-oceanic ridges. If you think of the plates as the skin of the earth, then the mid-oceanic ridge system is a 38,000-mile-long (64,000 km) cut in that skin. This cut bleeds lava, and never heals, because as soon as a "scab" of solidified lava develops over it, the two sides are pulled apart—allowing more molten rock to ooze forth into the ocean water. In this way, plates of oceanic crust are formed; one long thin section at a time bonds to the most recently made edges.