Draft by Tom Rothhamel

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What is Isostasy?

The outer part of the Earth can be viewed as a water filled balloon - a thin, flexible cover over deformable material.   Pressure applied to the cover forces the fluid to move away until a new balance is reached.  Removal of the pressure allows the fluid to migrate back, restoring equilibrium.  In the context of glaciers, the pressure is the growth of a continental ice sheet - removal of the pressure is ice sheet decay.  The thin, flexible cover is the Earth's lithosphere.  The subsurface fluid is nearly molten rock at a depth of several tens to hundreds of kilometers (the asthenosphere).

Evidence of Isostatic Rebound

In areas formerly covered by ice sheets (around the Baltic Sea and Hudson Bay, for example), sea cliffs and beach ridges are now found nearly 300 m (1000 feet) above sea level!  ¹⁴C ages on marine shells and driftwood show that these features are postglacial (less than 14,000 years old).  They were formed at sea level and, even though eustatic sea level has risen, they have risen far more from isostasy.

Rates of Isostatic Rebound

Where rebound is well constrained by ¹⁴C ages, it usually occurs at an exponentially declining rate.  The half-recovery time is commonly several thousand years, thus recovery is still continuing around the Baltic Sea and Hudson Bay, albeit much more slowly than it did immediately following deglaciation.  [Watch the animation at top carefully!]  Seaports of hundreds of years ago may now be several kilometers inland and meters above sea level!

Partitioning Isostatic Rebound

from Andrews 1970

Total glacioisostatic recovery can be subdivided into three periods which operate in the order shown:

• Restrained Rebound (Rr)   takes place beneath a thinning ice sheet. As the ice thins the lithosphere will begin to gradually rise in response to decreasing pressure. [See animation.]  Evidence of this stage is not commonly preserved, as the land is still covered by ice.
• Postglacial Rebound (Pr) is the rebound recorded in an area after deglaciation. Postglacial uplift is the most obvious of the three types of recovery, following an exponential decay rate law (above).
• Residual Rebound (rr) is the rebound still to take place. The asthenosphere takes well in excess of 10,000 years to reach a new equilibrium following ice loading or removal.

Ongoing Glacioisostatic Recovery

A vivid example of residual rebound can be found in Central Scandinavia (left - from Flint, 1971). Repeated precise leveling shows recent deformation, at rates measured in millimeters per year. The northern Baltic Sea is rising nearly one centimeter a year.  This may not sound like much, but it accumulates at up to 10 cm (4 inches) per decade, one meter (about 40 inches) per century, and so on!  In order for isostatic equilibrium to be achieved, Hudson Bay still needs to rebound as much as 150 m. The amount of residual rebound, coupled with low and declining rates of recovery, imply that the lithosphere may not completely reach equilibrium within itself before the next glacial episode.

after Andrews (1975)

Depression beyond the Ice Margin

The weight applied to the crust is dispersed throughout the lithosphere. The lithosphere is so rigid that the weight is transferred across the crust resulting in a peripheral depression and a forebulge.  Around the periphery of the ice sheet margin up to a distance of 150-180 km, depression (>100m) occurs without ice loading.  This area can record relative sea level change without the complexity of glacial erosion or deposition.  The lateral displacement of mantle material from below the center of ice sheet loading results in the formation of an area of slight uplift (10 - 20 m) beyond the peripheral depression (the forebulge).