The Crust

The crust is the outermost solid shell of the earth — the thin, rocky skin on which all life and landforms exist. It is not a uniform layer; its thickness and character change dramatically depending on whether you are standing on a continent or sailing over an ocean.

The defining physical property of the crust is that it is brittle in nature. This means it does not bend or flow under stress; instead, it fractures, cracks, and breaks. This brittleness is why the crust is the layer where earthquakes originate and where rocks split along faults.

The most important point about the crust is that its thickness is not constant. It varies systematically between oceanic and continental areas, and this variation is one of the fundamental differences between the two types of crust.

• Oceanic crust is thinner. Its mean thickness is about 5 km.
• Continental crust is considerably thicker, with a mean thickness of around 30 km.

The continental crust is not uniformly thick either. It becomes thicker in regions of major mountain systems, where the crust has been compressed and uplifted over millions of years. The most striking example given is the Himalayan region, where the continental crust reaches a thickness of as much as 70 km — more than double the global average for continents. …