Gary Swanson., age 15, of Loves Park, Illinois,, for his questions

What is isostasy?

Isostasy is a theory concerning the earth's crust. It is believed that there may be a system of weights and balances working to keep the weight of the earth's crust fairly even. There is a lot of evidence to support this idea, for the weight, though not necessarily the thickness,, does seem to be about the same. A high mountain area may, for instance, weigh no more than the dense layer of valley or sea bed at its feet.

The theory suggests that changes occur in the earth's crust to adjust this balance when it gets out of kilter. There is evidence to support the idea that mountains, volcanoes and other drastic earth movements occur to keep the weight of the crust even.