The theory of isostasy allows geologists and cryologists to understand the growth or retreat of ice sheets, based upon the movement of the land in response to its increasing or decreasing glacial load.

Earth's continental crust floats on a geological layer called the mantle. A continental ice sheet weighs a great deal, so an advancing ice sheet will cause the crust to subside beneath it. When the ice melts, the land it had depressed will rebound upward.

These vertical motions of hundreds of meters control the growth and retreat of the glaciers, as well as the directions in which meltwater drainage flows. Models of mantle deformation based on data from the last glaciation can be applied to areas beneath contemporary ice sheets to better understand their dynamics.

During the survey of Mount Everest, investigators found that their plumb bob was not attracted by the mass of the Himalayan mountain range as much as they had expected. They concluded that the mountain range was supported by some sort of mass deficiency beneath it. This led to the theory of isostasy, which holds that Earth's mantle is denser than its crust, which floats upon it; it is thicker or less dense beneath mountain ranges. This theory has been found to be generally true, except for some unusual circumstances such as the trenches near subduction zones. These exceptions have been explained by treating the surface of the Earth as an elastic plate that flexes and spreads out gravitational loads (Ruddiman, 2008).

The continental ice sheets that developed in Canada and Northern Europe placed enormous loads on the crust of the Earth beneath them. Just as a boat rides lower in the water when fully loaded than when empty, the crust of the Earth sank further into the mantle to accommodate this additional load. The flexure produced by this motion on the elastic plate caused parts of it just beyond the loads to rise up as a forebulge. When the ice melted, the load disappeared, areas that had been depressed began to rise up, and the forebulge started to subside. These changes in elevation are often referred to as glacial isostatic adjustments. Although the ice disappeared thousands of years ago, the crust continues to move vertically today, because the mantle is incredibly viscous. Glacial rebound analysis suggests it has a viscosity between 1020.5 and 1022 Pascal seconds. (For comparison, the viscosity of water is 10-3 Pascal seconds, and the vicosity of molasses is 104 Pascal seconds.)

Waves coming ashore often cut terraces, erode cliffs, and produce beaches at elevations close to sea or lake level. If the land rises relative to these levels, a series of terraces may result. Organic remains can be used to date these terraces, and geologists have discovered that terraces of the same age at different locations are not at the same elevation. Those closest to where the glacial load was greatest are higher, as they have experienced greater isostatic rebound. Furthermore, terraces formed long ago will show a greater slope, presently, than those formed more recently. By studying such patterns of strandlines, a history of isostatic rebound can be reconstructed.

For example, areas around Hudson Bay, in Canada, have already rebounded over 300 meters, continue to rise at rates on the order of 1 centimeter per year, and probably have another 100 meters of uplift to go before isostatic balance is obtained (Fowler, 2005).

The history of isostatic rebound in a given area can be used to make estimates of mantle viscosity and of how this viscosity changes with depth. Computer models can help fill in the gaps where data are absent. Where there is a significant lack of data, suites of scenarios may be obtained, all of which fit the model equally well, and all of which are internally consistent. As additional data become available, some scenarios will be eliminated, and the preferred scenario can shift rather dramatically.

For example, preferred scenarios in the mid-1990's envisioned 4 kilometers of ice in Greenland but only 2.5 kilometers of ice in central Canada during the last glacial maximum. A few new data points led to revisions in 2004 that reversed this distribution and placed parts of Greenland in the area of the glacial forebulge.

These scenarios are used by climate scientists to try to determine how rapidly the ice sheets on Greenland and Antarctica might be shrinking. The elevation of the top of the ice sheets and the gravitational attraction at points above the ice sheets can be determined from satellites (Peltier, 2001). However, to calculate how much the volume of ice is changing, the elevation change of the bottom of the ice sheet must also be considered, and isostatic adjustment models are used to estimate this change.

Isostatic adjustments may also have influenced the growth and retreat of the continental glaciers. The accumulation of ice occurs much more rapidly than can be accommodated by isostatic subsidence. Just as the crust of the Earth is still rebounding thousands of years after the load was removed, it subsided over a period of thousands of years during and after the addition of the load. These changes in elevation, frequently called "bedrock lag," affect local climatic conditions and thus the growth or retreat of the continental ice sheets.

Another important consequence of isostatic adjustment concerns drainage of the large lakes at the southern margin of the continental glacier in North America. The route and ultimate destination of the drainage were likely important factors influencing global temperatures near the end of the last glaciation. These lakes consisted of freshwater, most at 4œ Celsius, which would have floated on the more saline water in the North Atlantic. Such a freshwater cap would be unlikely to sink initially, and it would take a long time for the winds to mix it sufficiently to permit sinking. The cap may have slowed or turned off the thermohaline circulation, also called the global conveyor belt or the meridional overturning circulation (MOC). The North American lakes drained into the Atlantic Ocean through the Mississippi River Valley, the Hudson River Valley, and the St. Lawrence River Valley, as well as draining northwest to the Arctic Ocean. Isostasy provides a theory to explain the vertical movements of Earth's crust as the load of continental ice sheets is added or removed from large areas.

It permits us to estimate volume changes of contemporary ice sheets by providing estimates of the crustal movements taking place beneath them. By constraining drainage histories of the glacial lakes that formed near the end of the last glaciation, it makes it possible to determine where and when cold, freshwater pulses may have flooded out over the oceans, with their potential radically to modify Earth's climate.

References

1. Fowler, C. M. R. The Solid Earth: An Introduction to Global Geophysics. 2d ed. New York: Cambridge University Press, 2005.

2. Peltier, W. R. "Chapter 4: Global Glacial Isostatic Adjustment and Modern Instrumental Records of Relative Sea Level History." In Sea Level Rise: History and Consequences, edited by B. C. Douglas, M. S. Kearney, and S. P. Leatherman. San Diego, Calif.: Academic Press, 2001.

3. Ruddiman, William F. Earth's Climate Past and Future. 2d ed. New York: W. H. Freeman and Company, 2008.

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