Types of Crust
The Earth has two kinds of crust. The ground that we stand on is called continental crust . It is raised above the waters and is very thick. Continental crust can be as thick as 30 km or so. There is also crust under the waters of the ocean, but it is thinner. This crust is called oceanic crust . This picture to the left shows both ocean crust and continental crust.
Continental crust descends deeper into the Earth than does oceanic crust. If you look again at the rotating globe at the start of this tour, you see that the continents are higher than the ocean floor. But under the surface of the Earth, the continents also go down deeper beneath the surface than does the oceanic crust. So for every mountain range reaching into the sky, there is an equally deep "root" with reaches down into the depths of the interior of the Earth. If you were to dig down through the Earth, you would start with oceanic crust, because it is thinner.