The processes responsible for ice movements

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A. Describe the processes responsible for ice movements. (20)

There are three main processes which are responsible for ice movements, these are; Internal deformation, basal sliding and subglacial bed deformation.

Basal sliding this is the process by which a warm based glacier undergoes thawing at its base. This is due to the melting point of the ice being lowered by increased pressure and friction caused by the ice. This in turn causes the accumulation of a thin film of water along the subglacial base. A part of basal sliding is ice creep, Ice creep is where the ice flows around or over obstacles on the bed. The subglacial surface is often rough and full of obstacles such as areas of resistant rock. When a glacier meets an obstacle on the bed it does not often flow straight over or around the obstacle, there is a slight pause before the ice negotiates the obstacle. The reason for the pause is because the obstacle protruding into the base of the ice mass causes an increase in pressure within the ice on the upstream face of the obstacle. thus increasing the rate of ice deformation around the obstacle, allowing the ice mass to smoothly pass over or around the object. (which can form a roche moutonee)

Another process that controls the flow of a glacier is a process called sub glacial bed deformation. This is where lose sediment at the bed of the glacier moves enabling the glacier to move more freely over the bed. In order to do this the subglacial bed has to be a warm based glacier, i.e. the glacier has to have a film of melt water at the base. The deformation occurs when the water pressure in the pores or spaces between the sediment grains increases sufficiently to reduce the resistance between the other grains. This allows the sediment grains to flow relative to one another as a slurry like mass. The weight of the glacier then shears the sediments and this leads to a constantly deforming bed beneath the ice mass upon which the ice mass moves.

Internal deformation, is a process which individual grains/crystals of ice change shape due to shear gravitational stress of the glacier. The slope angle and the depth of the ice greatly effect this rate of deformation. The mechanism for this form of movement is very similar to that described is ice creep. It involves the elongation of ice crystals which causes the displacement of ice crystals relevant to each other. Although each grain of ice may only move a less than a millimetre the length of the glacier can change dramatically if more ice crystals deform across its length.

B. Explain what factors influence the rate of glacier ice movement. (25)

There are many factors which effect rate of glacier ice movement, these factors also effect the rate of erosion. The faster the rate of erosion the faster moving the ice is.

Firstly the slope angle, the steeper the slope angle the higher velocity of ice movement this is called extensional flow as often the ice extends as it expands. And likewise the lower the slope angle and the more gentle the slope the lower ice velocity is and often on gentle slopes compressional flow occurs where the ice is forced together compacting the ice further. Since slopes are generally more common in upland areas that is where the majority of glacial erosion occurs.

Secondly climatic factors. The climate of the area of glaciation determines whether the glacier is a warm based or cold based glacier. If it is a polar glacier on an ice cap the majority of the ice is frozen to the bedrock it lies on. Because it is physically attached to the rock these types of glaciers have a very low velocity. And because the glacier has a very low velocity (of about 1-2cm a day.) only one factor which causes ice movements other than slope angle and that is internal deformation this mainly occurs in polar regions such as the artic. However a warm based glacier is formed of land with warmer geothermal currents, such as in the U.K. Warm based glaciers on average tend to move faster than cold based glaciers at about 2-3m per day.

Warm based glaciers are formed when the geothermal heat and friction are greater than the pressure caused melting point of the ice, thus creating a thin lubricating film of water over the ice. This film then promotes the process of basal sliding when the glacier slides over bedrock . Climatic fluctuations, these fluctuations cause periods of rapid glacial advance and retreats called surges. The more often the diurnal temperature variations around 0’C occur the faster many erosional processes like freeze thaw occur. And likewise the colder the weather and the more precipitation the faster the rate of accumulation and so the faster the rate of glacial advance/growth.

Geology, an important factor which governs the rate at which a glacier moves is that of the underlying bedrock of the area. This can make a large difference to the rate of ice movement as a more resistant bedrock would provide a harder surface for glaciation to erode. Whether this is at a nivation stage where the corrie/cirques is about to be formed or when previous landscapes such as interlocking spurs block the path of glacial advance this rock may be such as granite. Again in opposite a softer less resistant rock such as sandstone may be easily eroded and so the glacier may move more quickly as the landscape provides less physical resistance.

Depth of glacier, the deeper the glacier the faster the rate of glacial advance. This is because more force upon ice crystals promotes internal deformation.

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