Theory describing the mass balance in the Earth's crust, which treats all large portions of the crust as though they were floating on a denser underlying layer, about 70 mi (110 km) below the surface. In this theory, a mass above sea level is supported below sea level, so high mountains must be regions where the crust is very thick, with deep roots extending into the mantle. This is analogous to an iceberg floating on water, in which the greater part of the iceberg is under water.

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