Huge plates of crustal and upper mantle material (lithosphere) “float” on more dense, plastically flowing rocks of the asthenosphere. The “depth” to which a plate, or block of crust, sinks is a function of its weight and varies as the weight changes. This equilibrium, or balance, between blocks of crust and the underlying mantle is called
isostasy. The taller a block of crust is (such as a mountainous region), the deeper it penetrates into the mantle because of its greater mass and weight. Isostasy occurs when each block settles into an equilibrium with the underlying mantle. Blocks of crust that are separated by faults will “settle” at different elevations according to their relative mass (Figure
The isostatic relationship is maintained as the crustal surface changes. For example, as a mountain range block erodes, the block will rise—it is not as heavy because the material is eroded away, and it does not need to “ride” as low in the mantle. The eroded material is deposited as sediment on the adjacent thinner continental blocks, which increases their weight, and they then sink farther into the plastic asthenosphere. Areas that are tectonically stable tend to be isostatically balanced. The viscosity of the mantle can be calculated based on the rates of the isostatic adjustment of the crustal blocks.
The development of thick ice sheets during the Pleistocene epoch warped the underlying crust downward into the mantle, an isostatic adjustment in response to the great weight of the ice. After the ice melted, the weight was removed from the crust and it began to slowly rise back to its preglacial position. This isostatic process, called crustal rebound, is still in progress in the Great Lakes area of the United States.
Some geologists believe that plate subduction generates large bodies of magma that adhere to the bottom of the continental mass and cool, locally thickening the crust. In order to maintain isostasy, the crust would then have to rise through the formation of a mountain range. This idea has not been widely accepted, however.