The Earth's crust, which is Earth's outermost layer, is a thin layer. Comprised of two kinds of crusts, continental and oceanic crusts, it is merely 0.473 percent of Earth's mass. The ocean crust's thickness is around 7 km, while the continental crust's thickness is from 10 to 75 km.
The oceanic crust is made of dense rocks, like basalt, and carries water. The continental crust is made of lighter rocks, like granite, and carries land. The oceanic crust gradually sinks until it eventually enters the Earth's mantle; at the mantle, it melts. Later, as magma, it ascends again in the mid-ocean ridges. This is how Earth gets a new oceanic crust every 200 million years.