Saturday, 11 June 2011

Glaciers Part 3 - Why are valley glaciers so erosive?

Ice movement is needed for erosion to occur. In flow ice behaves plastically as stresses within it increase with movement; this takes place by a mixture of basal slippage (this is when, during summer, limited melting occurs and lubricates the base of the glacier, thereby allowing it to move more freely) and internal flow. At its base the ice creeps plastically around obstacles such as rocks. It also melts on the upstream side of the obstacle which aids movement, only to refreeze again after passing over or around the obstacle. Internally the force of gravity means that individual crystals of ice flow within the glacier. Therefore, the pattern of crevasses tell us a great deal about the stresses and strains which are occuring within the glacier as it moves.
In the corrie

In the valley
The two main processes of glacial erosion are abrasion and plucking.


Rock fragments embedded in the nase of the glacier are foced, by the weight and pressure of the ice above, into the bedrock below as the ice moves. These pieces of rock are angular; their sharp edges are driven into the bedrock. Exposed bedrock surfaces are scarred with fine grooves or more deeply scratched by striations, which run in parallel lines and act as a good indicators of the direction of the ice movement. If smaller-sized debris is trapped in the bottom of the glacier, upstream rock surfaces are more likely to be polished. In all cases the height of the exposed rock is reduced and it is worn down.

The rate of abrasion is greatest when certain favourable conditions are present. One is a great thickness of ice, which will fill both the floor and the sides of the previous river valley and exert great pressure on a wide area of exposed bedrock. A second condition is'rapid' speed of flow. Not only is meltwater frequently presentat the base of these warm glaciers, but by beginning in high mountains the glaciers are flowing down steep gradients as well. A large ice budget also helps. Much new accumulation is matched by much additional ablation meaning rapid ice through-flow. Beign funnelled down an already existing valley is another contributory factor to rapid  and sides. This is confined to a zone close to the rock and is called Blockschollen Flow. The loose rocks in the glacier are dragged against the base and sides. The unevenness of th efloor over which the glacier is moving produces sequences of extending and compressing flow. As the glacier flows into a hollow the effect if increased gradient is to speed up the glacier movement and the ice becomes thinner. On leaving the hollow there is a reduction in the gradient, which leads to the glacier slowing down and to the ice thickening. The greater pressure on the stones trapped in the base of the glacier from the increased weight of ice above leads to greater abrasion, which in turn increases the depth of the hollow, thereby emphasizing further the uneveness of the valley floor.

Diagram showing Normal Flow and Blockschollen Flow

Diagram showing Extending and Compressing Flow - as with all the diagrams I have
included in the glaciers post, I apologise for the size of my writing as I am known for having
quite small handwriting as I am not great at writing big - I hope you can read the labels though!!!

Clearly the characteristics of the rock over which the ice is flowing were of great importance to the rate of abrasion. Softness and plentiful jointing were great aids to abrasion. Also many of the river valleys down which the glaciers passed had been affected by periglacial processess for a prolonged period before the glaciers arrived. Frost action preceded glacial action, leading to deep shattering of rock on the valley floors and sides, which made glacial abrasion easier and quicker and provided the tools with which to do more work.


 Formation of a Roche moutonnee

This is the pulling away of fragments of bedrock. From time to time glacier ice freezes and sticks to the bare rock below it. Later blocks of rock are pulled away by ice movement. Its effective operation relies upon alternate freezing and thawing taking place in the base, which is most likely to happen in two types of location where high rates of plucking are favoured. One location is where ice passes over a rock step on the valley floor. At this point the glacier is likely to be thinner and heavily crevassed. Both water and warmer tepmeratures may be able to penetrate to the rock face more regularly to create the ideal conditions for plucking. The second  is around an obstacle such as a hard rock outcrop. The increased weight of the ice upon the upstream side of such an obstacle often leads to pressure melting, even where the temperature is below freezing point. The water freezes on the downstream side of the obstacle where pressure is reduced. In both locations, the presence of weaknesses in the rock, especially joints, is of great importance. Water flowing into these is trapped there by refreezing, thereby helping to loosen pieces of rock over time. These are the chunks of rock plucked away by the forward thrust of glacier movement.

Other erosion factors:-
Frost action is a type of physical weathering and it operates whenever there are changes in temperature above and below freezing point occurs. The greater the frequency of these changes, the faster it operates. diurnal changes between night freezing and daytime thaw are most frequent in early and late summer, at the change in seasons and so freeze-thaw occurs rapidly and most effectively at this time. On rock outcrops, protuding above the glacier, it is the main process of denudation. It also effects rocks on the bed and sides of glaciers that are well jointed.

Frequent references have been made to freeze-thaw weathering, without which valley glacier erosion could not be so effective. Other factors are believed to help glacial erosion. One is pressure release. Ice has only one third of the density of rock. After ice erodes and removes rock, the weight of the ice is less than that of the rock removed. As pressure is released, rocks expand slightly and cracks develop running parallel to the surface. This is called sheeting. These are weaknesses into which water can penetrate, increasing the oppurtunities for freeze-thaw to occur and providing more shattered rock for the ice to remove. Another factor is erosion by sub-glacial meltwater streams. Meltwater works its way down through the oce into underwater streams, which flow under conditions of hydrostatic pressure in the glacier's base. The potent mixture of ice pressure and large amounts of water makes the streams perfectly capable of undertaking considerable fluvial erosion and cutting steep-sided V-shaped valleys under the ice. Some valleys are recognised by their uneven floors because, uniquely under hydrostatic pressure, streams can locally flow uphill.

There is lively debate about the relative effectiveness of abrasion and plucking. Some researchers have suggested that abrasion does no more than scrape, scratch and smooth off rock surfaces. They contend that plucking is more effective. Plucking undoubtedly operates most effectively where pre-existing rock fractures are present. It has been discovered that a joint giving blocks between one and seven metres in size seems to favour the maximum removal of bedrock by ice. Others argue that in the absence of jointing even soft rocks may be able to resist ice erosion by plucking. How could the many, very deep rock basins found on the glaciated valleys be formed if not by ice abrasion? The dark colour of many rivers fed by melting glaciers is due ot the ready availability of load materials at the glacier's snout. However, no one disputes the vital part played by freeze-thaw in the preparation of the rock for abrasion, plucking and transport by valley glaciers.

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