Glacial isostasy is the process by which Earth's lithosphere (crust and part of the upper mantle) is physically pressed down under the weight of an ice sheet. When the ice mass is later reduced or removed and the weight is lifted, it allows the crust to rebound (return to its original position). Similar to a person sitting on a sofa, when weight is applied to the cushions, the cushions depress to accommodate the weight. As soon as the weight (stress) is removed, the cushions return to their original equilibrium. Earth's isostatic process is the same when weighed down under ice (or rock); it just takes longer to complete.
When large masses of ice cover Earth's surface, it causes the crust to undergo both elastic and plastic deformation. Elastic deformation is temporary movement; the crust will return to its original position once the stress is removed. Plastic deformation is permanent deformity; it does not resume its original shape. The crustal rock gets displaced as the mantle sinks beneath it. An ice sheet 3,281 feet (1,000 m) thick has the ability to depress the ground beneath it 902 feet (275 m). It takes several thousand years for the isostatic adjustment to take place because there is a time lag between when the glacier builds up and Earth's crust responds. When an ice sheet begins to shrink, lifting the weight back off the depressed landmass, the depressed area then begins the process of rebounding, rising back up again. This process can take several thousand years, starting out quickly, then slowing down.
Glacial isostasy comes into play concerning global warming when calculating sea-level rise from the melting of ice sheets and the thermal expansion of water. In some instances, isostasy may offset some of the effects of sea-level rise, if adjustment is occurring. The problem, however, is that if isostasy is progressing slower than sea level is rising, offsetting any negative effects would be minimal.
There is evidence of past isostatic rebound. In the Baltic Sea and the Hudson Bay area near Canada, which were both covered with an ice sheet 14,000 years ago, ancient beach ridges exist today around 1,000 feet (300 m) above sea level. Scientists at NASA and USGS believe this area is still actively rebounding, just at a much slower rate than it did shortly after the last ice age. The Northern Baltic Sea is rising almost 0.4 inch (1 cm) a year, which equals about 40 inches (1 m) per century. For Hudson Bay to reach its state of "equilibrium," geologists calculate it still needs to rebound about 492 feet (150 m).
Currently, in Antarctica, the weight of its ice sheet is so immense that it has pressed the continent 0.6 mile (1 km) into Earth's crust. In Greenland, its ice is so heavy that the land beneath it has also been pressed down into a dish shape. If these ice masses were to melt, the land beneath them would also undergo isostatic rebound.
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