Isostasy is the vertical readjustment of the surface of the earth due to the addition or removal of weight.

Principles that apply to a floating object (taken from http://www-ocean.tamu.edu/~wormuth/labhtml/isostasy.html :
• gravity pulls down on the object, displacing fluid
• gravity also tries to force fluid back into its original place (the place occupied by the object)
• this creates an upward force, i.e. a buoyant force
• isostasy occurs when the buoyant force is equal to the gravitational force, i.e. isostatic equilibrium
Isostasy is commonly associated with the advance and retreat of glaciers. During these times, water is stored in massive ice sheets. In the theory of isostasy, a mass above sea level is supported by the same mass below sea level, so the huge weight from ice causes the land to subside. Once the ice melts, the land rebounds slowly overtime.

Think of a glass of ice and water. Push down one piece of ice, you'll see the other cubes rise with the water. When you remove the pressure the other cubes sink and the piece of ice you were pushing down slowly rises to the height of the other cubes.

Evidence supporting the idea of isostasy comes from Scandanavia. During the Pleistocene ice ages, the land there was once covered by a sheet of ice 3 kilometers thick, weighing down the crust. The ice melted some tens of thousands of years ago, and the crust is still rising slowly because isostatic balance has not yet been attained. In other areas formerly covered by ice sheets (around Hudson Bay and the coastline of Norway, for example), sea cliffs and beach ridges are now found nearly 300 m (1000 feet) above sea level.

I*sos"ta*sy (?), n. [See Iso-; Stasis.]

The state or quality of being isostatic. Specif. (Geol.),

general equilibrium in the earth's crust, supposed to be maintained by the yielding or flow of rock material beneath the surface under gravitative stress. By the theory of isostasy each unit column of the earth, from surface to center, has approximately the same weight, and the continents stand higher than the ocean beds chiefly because the material of the crust has there less density.

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