Geography Full A Level Notes on Glacial Erosion

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  • Created on: 27-11-12 06:43
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Glacial Erosion
As glaciers move over the landscape, they are able to change it through the processes
of weathering and erosion. The rock that is removed by these processes is then transported
by the glacier and deposited elsewhere. We do not understand precisely how the processes of
glacial erosion take place due to (i) difficulties in observing these erosion processes at work
beneath the glacier and (ii) difficulties in evaluating how much change has actually taken place
due to erosion through lack of knowledge of what the preglacial landscape was like.
However, glacial landscapes and their features have provided valuable evidence to support our
understanding of glacial erosion processes.
There are 3 main processes of glacial erosion.
1. ABRASION
This is where the bedrock underlying the glacier is eroded by
debris embedded in the base and sides of the glacier. As the
glacier moves over the bedrock, this material scrapes away
at the rock like sandpaper wearing it away. As it does so it
leaves behind scratches and grooves in the rock, known
as striations. Where these grooves are discontinuous but
regular in occurrence they are known as chatter marks. The
depth of the striations will be dependent on factors such as
resistance of thebedrock, as well as the fragments that are
undertaking the erosion. As the bedrock is eroded by
abrasion, further material may become entrained in the ice
increasing the amount of abrasion that is able to take place. Where fine material is embedded
in the base of the glacier it will act to 'polish' and smooth the bedrock below. Indeed as abrasion
takes place, rock material is ground down to produce a very fine 'rock flour'. The characteristic
bluegreen colour of glacial lakes and streams (opposite) is due to the high concentrations of
this rock flour in suspension.
Rates of abrasion are greatest where:
basal sliding can occur (coldbased glaciers do not abrade as they are frozen to the bed)
(however where there is excess meltwater under pressure, this may reduce the contact
between debris and bedrock reducing abrasion)
there is plenty of rock debris to act as 'cutting' agents in the abrasion process
the rock debris is more resistant than the bedrock it is abrading
there is a plentiful supply of debris at the base of the glacier
the glacier is travelling at a greater velocity across the bedrock
the ice is thick, enabling the embedded debris to exert greater pressure on the bedrock
below.

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PLUCKING
The process of plucking (also known as quarrying), results in the
removal of much larger fragments of bedrock than that undertaken by
abrasion. The process is most effective on well jointed rock and that
which has been preweakened by weathering processes such
as freezethaw and pressure release. Plucking occurs where ice is
at pressure melting point. As the meltwater produced refreezes (e.g. on the 'lee' side of a rock
obstacle) it entrains material in the base of the glacier.…read more

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Rates of glacial erosion
Stationary glaciers (coldbased glaciers) are much less erosive than warm basedglaciers. It is
the mobility of warmbased glaciers, due to the presence of meltwater, and the material that
they transport that facilitates greater amounts of erosion.…read more

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