Distinguish between abrasion and plucking

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6a)  Distinguish between the glacial erosion processes of abrasion and plucking.

Plucking occurs high on the glacier, well into the zone of accumulation, because for plucking to occur the temperatures have to have slight fluctuations around the melting point of the ice. Plucking occurs usually at the bottom of the glacier where, because of the pressure pushing down on the ice from above, the ice is at the pressure melting point. When there is a rock with a crack in it, the melted ice fills the crack and then refreezes. The refreezing exerts a pressure on the rock and weakens it but the water also freezes to the bottom of the glacier and is pulled away. When the frozen water in the crack is pulled away so is the rock.

Abrasion however can occur anywhere along the profile of the glacier. Abrasion is the sandpapering of material. Debris can be put into the glacier by many methods including plucking and the material is then dragged along the bed of the glacier. The coarse material scratches the surface and the finer material polishes the surface. The material caught in the glacier is eroded into a fine rock flour which gives glacial melt water its milky appearance.

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6b) Assess the relative importance of the factors influencing rates of glacial erosion.

There are several factors affecting the rates of glacial erosion including: presence of basal debris, debris size and shape, relative hardness of particles and bedrock, ice thickness and the velocity of ice.

Though a very important factor in erosion not all glaciers have a lot of basal debris. A temperate glacier, like the glacier d’Argentière, will have lots of basal debris from processes such as frost shattering and plucking high up in the profile of the glacier where as the ice caps in Greenland ...

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