The crust is the outer shell of the solid earth. According to Arthur Holmes, the earth’s crust can only be defined with precision as consisting of the varied assemblage of rocks overlying the M-discontinuity (Moho) and thus forming an envelope surrounding the mantle.
Obviously the earth’s crust is made up of various types of rocks. On the lands the crust is covered with a blanket of soil or other loose deposits, such as desert sands. The earth’s crust is called the lithosphere (derived from the Greek lithos meaning stone).
It is of primary importance in understanding the earth’s various kinds of landforms. It is the crust on which we live and that forms the ocean floors and continents.
In addition, it is the rocks and other loose deposits of the crust which form the soils, the very basis of our existence. The mineral wealth, in a great variety, is also procured from the crust.
The thickness of the outer shell of the earth is not uniform. It varies from 8 or 9 km in the ocean basins to as much as 64 km. under some mountain systems. The average thickness of the crust on the continents is about 32 to 40 km.
As stated earlier, the crust is composed of various kinds of rocks and minerals that respond in different ways and at different rates in the land-shape forming processes.
It is because of this fact that the physical geographer must be familiar with different types of rocks and their primary characteristics.