Tutorial:Ice Erosion
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[edit] Ice erosion
Ice erosion occurs through the movement of glaciers and ice sheets, whilst freeze-thaw processes also act to weaken and weather surrounding rock surfaces to produce shattered angular rocks and boulders.
Glaciers exert a massive force on the land. At the head of the glacier the accumulation of ice carves a bowl shaped cirque which has a step backwall and lower frontal lip. The glacier that spills out from this depression carves a broad U-shaped valley.
Ice scours uncolidated sediments and soils from rock surfaces, and can erode rock by a mixture of abrasion, crushing and fracturing and plucking.
[edit] Abrasion
Debris carried in the glacier can polish, scratch and groove the bedrock. Polished surfaces and parallel striations are common evidence of this type of erosion.
[edit] Crushing and fracturing
This results from pressure exerted not so much by the ice itself, but rather by the basal debris it carries. This can produce crescentic grooves called chattermarks in the bedrock surface.
[edit] Plucking
Plucking usually affects surfaces on the down-flow side of an obstruction. On the up-flow side immense pressures cause the basal ice to melt, which then refreezes in cracks in the bedrock on the down-flow side where the pressures are less. This widens cracks and can lead to the removal of large blocks or bedrock.
Other features associated with glacial erosion include:
- Hanging valleys: These are formed by tributary glaciers meeting the main glacier. The floors of these tributary valleys are not as deeply eroded and hence appear to ‘hang’ above the main valley.
- Truncated spurs: These are blunt ended ridges that are found along the flanks of the valley and slope down towards the valley middle. They are formed by the glacial erosion of spurs formed by more meandering streams and rivers.
- Arête: These are thin, ‘knife-like’ ridges of rock separating two glacial valleys or cirques.
- Roche moutonneé: These are valley floor rock landforms which have been abraded by ice flow on the upflow side giving a smooth polished elongated profile. However, on the downflow side they are blunt nosed and rough as here the rock has been eroded by plucking.
