Glacial Isostasy
Overview
Glacial Isostacy has played an important role in the formation of landscapes all around the world. The theory of glacial isostasy was fully accepted at the beginning at the 20th century (Kjemperud). Glacial isostasy not only dictates river drainages and lakes, it also can have a much broader impact. For instance, it has been speculated that glacial isostasy has played a role in the storage of hydrocarbons in the sedimentary layers (Kjemperud). This is because differential movements affect the crust especially the sediments and fluid content. Glacial isostasy has and will continue to affect the landscapes and economy of the world. Glacial isostacy affects places like Canada, Iceland, Swedan, Norwegia, and the United States
(Below) Here is a map that represents world wide glacial isostasy occurrences.Glacial isostacy is important because it effects vertical crustal motion, global sea levels and stress on the landscape. It also provides us with a means of seeing into the past as to what was happening way back when in geologic history. Knowing these facts can assist present day scientists with threats like earthquakes, natural resource availability and climate change.
map.png

Below is an image of the Donjek Glacier int the Saint Elias Range, YukonTerritory, Canada. 1985. This massive icesheet is very visibly changing the landscape as it creates this U shaped valley. However, its massive weight is also pressing down on the rigid lithosphere below, which is in turn deforming the ductile hot weak asthenosphere. Over the duration of this glaciers life, the mantle below will deform around the weight of this massive ice sheet.
Credit: Natural Resources Canada. Photograph by Douglas Hodgson. Copyright Terrain Sciences Division, Geological Survey of Canada.

glacier.pngThe Last Ice Age

The last glaciation maximum occurred 21,000 years ago at the end of the late Quaternary period. (Peltier)This was a 100kyr glaciation that covered all of Canada and much of the northern United States. It extended as far south as Chicago Illinois.(MSUE) During this time 32% of the land was covered in ice, and 30% of the oceans as well. (NSIDC)This glaciation period is characterized by at least four massive glacial retreats and advances that helped create the current landscape of the northern US. The ice sheet that covered North America is known as the Laurentide ice sheet, and it was nearly 4 Km thick.(MSUS) Eventually, when the glacier retreated it left the landscape completely transformed. The land that it had leveled out began to fill with the melt water from the glacier, forming the Great Lakes. However, because of isostasy, the land is not stagnant and continues to rise. Even today, the great lake region is rising at a rate of about 7.5 cm every 100 years.(MSUE)
ice_sheet.png(Right) is a picture that shows the extent of the Laurentide Ice Sheet. Because of the Laurentide Ice sheet and the formation of the great lakes an entire community based on these lakes has risen. Niagara falls brings in great tourists dollars, all while being considered one of the wonders of the world. The Lakes provide a way of life via fish production for many mid-western families, as well as providing huge amount of habitat for aquatic species.


Occurences

Glacial isostasy occurs as glacial rebound and subsidence. Subsidence occurs when the heavy ice sheets presses on the high viscosity mantle and the area around the glacier experience uplift. If the glacier is located on a region that has low viscosity mantle, it is argued that only subsidence will occur directly under the ice sheet. Glacial rebound occurs on both high and low viscosity mantle. (Pearson Prentice) Ice has an average density of .9 g/cm3 while mantle rock has an average density of 3.3 g/cm3. These densities leave potential for glacial depression to be as much as 275m under a 1000m ice sheet. However, this maximum depression is seldom hit because it can take upwards of 1000 years for complete isostatic adjustment to occur. Ice is much more susceptible to surface changes than rock, and thicknesses of ice sheets are never stagnant for such a large period of time.
rebound.png


(Above) This map is the combination of many GPS observations taken at the Canadian base network station. It demonstrates the uplift that is occurring in these areas. As anticipated, the further North you go, the more uplife that is occuring. This is due to the massive ice sheet, and this ice sheet was thicker the further north you went.

"Crustal Motion and Deformation Monitoring of the Canadian Landmass",
Geomatica[PDF,25.1 Mb,viewer]VOLUME 60 NUMBER 2, 2006 (CSRS Issue)



Evidence for post glacial rebound is primarily based on tilted and uplifted shorelines. Sedimentary strata are assumed to have been deposited in a horizontal layers, however when they are tilted, it is possible that crustal wrapping associated with high viscosity mantle rebound is present. This particular feature is seen prominently in the Great Lakes region. In this region Lake Superior experienced the deformation of the Laurentide glacier the most sever. This is apparent through its lower depths and its low topography relief.

great_lakes_profile.png
(Above) Is a profile of the Great Lakes region in the northern Unites States. This area was heavily impacted by the laurentide glaciation. The lakes slowly decrease in depth as you move further south. This is because as the glacier retreated less weight was impressed upon the southern sections compared to the more north areas.