Glacial erosion

Glacial erosion involves a variety of physical and chemical processes that are controlled by glaciological variables on a range of spatial and temporal scales. Abrasion is a commonly recognized process of glacial erosion, and results from clasts embedded in basal ice scratching and wearing bedrock, as a clast at the base of a glacier is forced toward and moved over the bed. Abrasion acts to reduce bed roughness and is involved in features that range from small-scale striations, to larger-scale features that include whalebacks, rock drumlins, and crag and tail. Plucking involves fracture of bedrock due to impact and differential stresses applied by the ice on bedrock, followed by entrainment of the material that is released by fracturing. Plucking typically occurs on the lee sides of obstacles and bedrock steps where tensile stresses fracture large fragments and basal ice entrains the fragments by freeze-on. Glacial plucking tends to maintain roughness of a bed by producing relief in contrast to the abrasional reduction of roughness. Plucking plays an important role in shaping many large-scale landforms; many classic features such as cirques and roches moutoneés are products of a combination of abrasion and plucking (Hambrey, 1994b). Meltwater is important in glacial erosion, both as a mechanism for removing sediment and clasts created by other physical processes, and in removing material by both physical subglacial fluvial processes and chemical erosion. Based on an understanding of how glacial erosion processes work, it is possible to summarize the major glaciological controls on patterns and rates of glacial erosion as: • Basal thermal dynamics Glacial erosion processes are only effective when the basal ice is sliding and there is meltwater, so warm-based ice is of prime importance to glacial erosion. Although there is some evidence that erosion beneath cold-based ice can occur, it is minimal compared to erosion under warm-based ice. Large-scale patterns of erosion are strongly influenced by areas of warm-based ice. • Basal ice–sliding velocity For abrasion, the ice-sliding velocity may play a role in the force that is applied to the bed (Hallet model) and in all models of glacial abrasion it is important in determining the rate at which a clast moves across the bed, and thus striation length per unit time. For plucking, high ice velocity increases the frequency of high stress contacts between the ice or clasts, increasing the rate of fracture of bedrock and producing more rock fragments. The sliding velocity is also an important term in the rate of removal of material produced by plucking or abrasion. Thus glacial erosion is strongly dependent on sliding velocity, and, in erosion equations, the velocity term may be raised to a power greater than 1 to reflect this strong dependence. Large-scale patterns of erosion are strongly influenced by areas of high sliding velocity. • Availability of meltwater and variability of meltwater pressure Meltwater plays an important role in the sliding velocity, but also can enhance fracture as a result of freeze-thaw effects in joints or fractures. In Boulton’s abrasion model meltwater pressure is important in reducing effective pressure and in plucking models variability of water pressures over short time periods can produce transient stresses and load effects that enhance rock failure. Meltwater also plays a role in the efficient removal of debris produced by other physical erosion mechanisms, and creates additional erosion through fluvial abrasion, cavitation, and chemical erosion. • Ice thickness Erosion process models that include effective pressure or normal pressure are dependent in part on ice thickness, and Boulton’s model of abrasion is particularly sensitive to ice thickness (abrasion increases initially with increasing ice thickness, but then decreases under large ice thicknesses because the large effective pressure causes deposition of basal sediment). Ice thickness also plays a part in controlling sliding velocity, which is important in almost all of the glacial erosion processes.