What causes the thickness of the earths crust to vary?

The thickness of the Earth's crust varies with location and ranges from 1 to 80 kilometers thick. The continental crust is 50 kilometers thick on average, while the oceanic crust typically reaches no more than 20 kilometers thick. The Earth's crust is the outermost of its three main layers.

Explanation:

Google

answer; ///i believe the correct answer to the above question is; ///(drainage basin;