Isostatic Subsidence
The crust floats buoyantly in the plastic asthenosphere, with a ratio of mass below the "surface" in proportion to its own density and the density of the asthenosphere. If mass is added to a local area of the crust (e.g. through deposition), the crust subsides to compensate and maintain isostatic balance. The opposite effect to Isostatic subsidence is known as isostatic rebound - the action of the crust returning (sometimes over periods of thousands of years) to a state of isostacy, such as after the melting of large Ice sheets or the drying-up of large lakes after the last ice age. Lake Bonneville is a famous example: Due to the weight of the water once held in the lake, the Earth's crust subsided nearly 200 feet (61 m) to maintain equilibrium, when the lake dried up, the crust rebounded. Today when you go to Lake Bonneville you will find that the center of the former lake is about 200 feet (61 m) higher than the distant edges.
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