Lithosphere & Mantle To find how much depression has occurred from ice loading the following equation can be used:

                                h(pI/pR) = Bed depression

Where:    h is the ice thickness
               pI is the density of ice
               pR is the density of rock (lithosphere and asthenosphere)

An ice load on the lithosphere is analogous to a weight being placed on the center of a sheet of wet particle board. The particle board will sag directly under the weight. As distance increases out from the center of the applied weight the particle board will be less and less deformed.

The rigidity of the lithosphere and its thickness under the ice as well as ice thickness (mass) controls the extent of depressional radius. Where the particle board is supported on all four corners and is open underneath, the lithosphere is floating on the mantle. In order for the ice to depress the lithosphere enough pressure, downward force, must be exerted by the ice to displace the mantle.

The lithosphere, being rigid, is deformed beyond the ice boundaries. The force of the ice on the lithosphere is greatest under the ice maximum, however, like the particle board illustration the depression extends past the immediate location of the applied mass. This principle is responsible for large depressional basins. When the ice thins or is removed completely the mantle flows back into place causing the basin to uplift.

Loading and unloading ~ “compensation between the topography and mass.” Andrews

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Types of Isostatic Uplift
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Glacial Isostasy Indicators

The Marine Limit is a narrow zone marking the maximum level to which the sea transgressed across an area. As sealevel has increased, it is important to add ~30m to marine limit estimates when calculating isostatic uplift for the last 10,000 years.

If an ice body retreats quickly marine limits increase in elevation towards the former ice center. If retreat is slow, marine limits will decline proximal to the former ice center.

In locations that experienced the encroachment of late glacial seas into glacio-isostaticaly depressed basins and then were uplifted forming an inland water body or a bog, raised beaches and deltas have been found. Radio-carbon dating can be used to find the approximate age of the uplifted feature if wood peat, whale bone or marine crustaceans are present. Raised shingle ridges are also dateable land forms found at the marine limit.

When estimating regional or global isostatic rebound it is important to remember that uplift rates for one particular region, such as the Canadian Arctic do not correspond with uplift rates anywhere else on the globe.

It is hard to determine precise uplift rates because uplift events take place over long periods of time. There may be little or no isostatic uplift over 1000's of years, then one catastrophic event raises the lithosphere 10's to 100's of meters. However it is more probable that a steady rate of uplift remains dynamically constant throughout.

Ice flow equilibrium is reached much faster than isostatic equilibrium, this relates to periodicity and the Milankovitch cycles. Therefore isostatic rebound may still be occurring as the next ice age begins.

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References:

Andrews, J.T., Glacial Systems, an approach to glaciers and their environments, Wadsworth Publishing co. Belmont, Ca. 1975.

Hughes, Terence, J., Ice Sheets, Oxford University Press, New York, 1998.