How and why do glaciers erode?

There are two main ways in which glaciers erode the bedrock that they are passing over, abrasion and plucking. Abrasion occurs when the debris being carried by the glacier moves to the base of the glacier and is carried across the bedrock. So long as the debris being carried is not sufficient to stop impede the movement of the glacier then there will be erosion of the bedrock by the debris. For there to be effective glacial erosion due to abrasion there must be a continuous flow of particles to the bed of the glacier, otherwise the debris becomes polished and ineffective.

The second main form of glacial erosion is 'plucking' this involves two processes, deformation and regelation. In deformation the glacier passes over the bedrock and surrounds a particle on the surface of the bed, the tractive force of the glacier then overcomes the frictional force holding particle into the bedrock and 'plucks' it away from the rock. Regelation on the other hand only removes small fragments of rock, when material carried by the glacier is incorporated into the bedrock, if there is differential movement within the lower layers of the ice then particles are forced to rotate and are ripped from the bedrock.

The amount of glacial erosion caused relies on a number of factors; the faster a glacier moves the greater the ice pressure will be and hence its erosive capacity; the speed of movement is due to the steepness of slope, the amount of available meltwater (not caught in irregularly shaped bedrock) to facilitate movement and also the net mass balance of the glacier. If the bedrock is permeable then meltwater will pass into the rock and will therefore not be available to assist in the glacial movement, so an impermeable or frozen bedrock is best for ensuring that the meltwater does not enter the bedrock. Yet if there is too much meltwater at the base of a glacier then it will be buoyed by the water and will move over the bedrock without eroding it, so there is a critical point of water volume, where there is optimum movement.

If the mass balance of the glacier is increased then the movement of the glacier will be increased proportionally. For there to be an increase in the mass balance then there must be more accumulation (inputs: e.g. direct precipitation or avalanches) than ablation (outputs: melting, evaporation, wind deflation or carving of ice from the glacier). In 1950 Shumski said that the mass balance of a glacier was 'the energy of glaciation', this can be seen (combined with Weertman's statement) to be the case by comparing the Franz Joseph and Meserve glaciers, where the higher mass balance of the Franz Joseph (due to the higher precipitation) leads to melting at the base because the glacier reaches the pressure melting point. This increased meltwater makes the glacier move faster, as does the increased size of the glacier, and so it erodes more than the cold based Antarctic glacier. There may also be seasonal differences within glaciers, where ablation or accumulation may increase or fall due to the seasonal change in weather conditions.

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