index

Erosional Processes

Montana State University
Glacial Geology
April 17, 1999

Index of topics discussed in this page

Abrasion	Regelation	Quarrying and Plucking	Pro-Glacial erosion
Pressure and Erosion	Ice Movement and erosion	Erosional Landscape(intro)

Arasion: During abraison, particles drug across the bedrock are constantly being ground up into rock flour. For abraison to continue, a constatn renewal of fresh particles at the base of the glacier is needed. Like sand paper, if you use it too long the grains all wear down and you have no "tooth" left to sand with. It has been suggested by Sugden that if rock flour is not removed from the base, the debris will build up, decreaseing rates of erosion by allowing the ice to slide over the soft bed of ground up rock flour. Conditions beneath warm based ice provide enough basal melt to wash-out ground up particles, providing room for new debris at the base.

Plucking and Quarrying: The process of plucking is not dependent on melt water to erode. Pressure melting as the ice passes over and around protrusions can provide the needed water to fill crack and crevasses. The water is then frozen to the bed as soon as conditions area below the PMP. Plucked and quarryed debris are then entrained in the basal ice and can be used in abrasion and regelation(see next section).
A glacier or ice sheet that has a polythermal base, will greatly enhance the processes of quarrying and plucking. The alternating areas of warm and cold below the ice speed up fracturing of bedrock thereby providing more debris that can be plucked up and transported away. The central portion of the Canadian sheild is an excellent example of these processes. The kettle lakes that dot the landscape are interpretted by Sugden(1978) as holes where quarrying and plucking processes were enhanced by a poylthermal based Laurentide ice sheet.

Regelations: The entrainment and net movement of debris from the basal surface upward toward the surface of a glacier/ice sheet is known as regelation. The image above shows the entrainment and movement of debris at the basal surface. Regelation is extrememly productive in polythermal bases areas. this is due to the freeze-thaw action that helps the produce fresh debris and then cement them onto the basal ice. Regelation may remove debris that could be used in abrasion, but is continues to erode in other ways.
All the debris moving in close proximity to the rock bed alows for eroding of obstacles that have pushed up into the ice. The layer of regelation, which can be between 5 and 55% debris and 0.5-5m thick, grinds off the top of the obstacles as it passes around and over. Although the layer is destroyed at each of these obstacle, it reforms on the lee side with some fresh debris in tow. Regelation is depent on the net vector of debris movement. If there is no re-freezing to the ice then abrasion occurs and requires melt water to flush out the system.

Pro-glacial effects: Pro-glacial conditons play a significant role in ersional processes and rates. Areas where ice thrusting has pushed up chunks of ice rich ground, the advancing ice passivly accretes this onto the base, reducing erosion of bedrock. Drummlins are common in these areas. Hart suggestes these type of drummlins wer fomed by large eratics that stuck out of the basal ice. The eratics ploughed up the ground infornt of them and left a trail of boulder pavement behinde them. The ice itelf coforms readily to the depression behinde the eratic.

Pressure Effects: Above certain pressure limits erosion rates slow due to the cryostaticpressure of the verlying ice. In physics we learn than for an object to move friciton must be overcome. If too much force is applied normal to the sliding surface, movement is impared. The idea applies to ersional processed beneath a glacier as well. When thecryostatic pressure becomes too great, little movement occurs and erosion is minimized. Basal meltwater is a key factor in negating the cysostatic pressure. Enough water below a glacier will bouy it up and allow sliding to occur. ( Melt waterbv is further discussed by Melissa Boyson).
Conversely, if too little pressure is applied, erosion is significantly decreases because there is not enough force ginding the debris into the bedrock to erode it very well. An example of this is the Hudson Bay region and some of the now deglaciated islands in Northern Canada. Here tors and areas of low erosion can be found. The tors indicate passive ice, where little movement occured. These areas are also beleived to have been host to cold based ice that forze to the bed and casued little erosion. Another thought is that the area had a thin cover of ice that could not easily erode the bed.

Ice Movement: The movement of the ice is key in erosional processes. It allows transport of eroded debris, the emplacement of fresh debris for abrasion, movement of ice through a polythermal base area. It also can change the ersoinal process occuring in areas. When ice piles up in an area of thin, cold based ice, it increases the pressure and may reach the PMP. This allows for the processes of abrasion, plucking, quarrying and regelationto operate.
In alpine systems, this differentiation of erosional areas may help explain the formaiton of u-shaped valley's. The trough created by a glacier funnels meltwater to the center, below the thickest area o ice. The water can bouy up the ice and decrease the cryostatic pressure. This decreases erosion in the center of the trough. Along the side walls of the trough, the ice is thinner causing cooler basal temperatures. Any basal water is easily drained off, or frozen to the bed. All erosion process can operated readily along hte sides. The higher erosion rates carve out the base of the "U" shape.

Looking at patterns of erosion beneathe the Laurentide ice-sheet, Sugden(1978) noted that patterns seemed to follow area of certatain types of erosion. Following is a short list of his classification of landscapes.

Arial Scour	Selective Linear Erosion	Non-erosion
Area of high erosion due to polythermal conditions of melt/freeze. Areas usually show plucking and quarrying as main ersoive style. Lots of kettle lakes. example: Candaian shield surrounding Hudson Bay.	The large u-shaped troughs cut into realtively flat, low relief areas. Considered the result of concentrated areas of abrasion, quarrying and plucking and thicker ice. Topographic lows in the area. example: edges of greenland and Svalbard.	Areas where little erosion is present although the area was once covered by ice. Tors may be found. Considered the result of cold based ice with little movement. example: central candadian shield.

Authored by Kathryn Clapp