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 pre-glacial 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 the
bedrock,
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 blue-green
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 (cold-based 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. |
2. 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 pre-weakened by weathering processes such as freeze-thaw 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. As the glacier
continues to advance, the newly entrained material is prised out of the bedrock.
This material is then able to be used in the process of abrasion.
Rates of plucking are greatest where:
| the bedrock is well jointed; |
| meltwater is present enabling entrainment during refreezing |
| the ice is thick, creating greater frictional drag as the ice moves over the bed. |
3. SUB-GLACIAL MELTWATER EROSION
Meltwater under the glacier is able to erode both chemically and physically. Meltwater under the glacier is often travelling under pressure and may fluvially abrade the underlying bedrock using the sediment that it is carrying. This process is most effective where the suspended sediment is coarse. Meltwater may also be able to erode through the process of solution, this is particular effective in areas where the bedrock is chalk or limestone, and minerals in the rock becomes solutes, dissolved by the meltwater.
Weathering Processes
The weathering processes of freeze-thaw and pressure release (dilation) are important in the glacial erosion system as they help to 'prepare' the bedrock for erosion processes, by loosening and fracturing the rock providing weakness that can be exploited through processes such as plucking.
Freeze-thaw: this is where rocks are weakened as water entering joints, freezes and expands exerting pressures which prise the joints apart. Material which is loosened by this process may then be entrained to form basal debris used in the process of abrasion.
Pressure-release - As the glacier begins to melt, the pressure exerted by the glacier is reduced, for example on the 'down glacier' side of a roche moutonnée, and the underlying rocks fracture as dilation of the rock occurs.
Rates of glacial erosion
Stationary glaciers (cold-based glaciers) are much less erosive than warm based-glaciers. It is the mobility of warm-based glaciers, due to the presence of meltwater, and the material that they transport that facilitates greater amounts of erosion. There are a number of factors affecting rates of glacial erosion:
See Chapter 3 in Anderson, D. Glacial and Periglacial Environments for a more detailed outline of glacial erosion processes and factors affecting them (see references).