Patterns Of Glacial Erosion

The primary or regional pattern of glacial erosion within a glacier or ice sheet is controlled by basal thermal regime (Figure 5.9). Only when an ice sheet or glacier is warm-based is meltwater produced in large quantities, and only when meltwater is abundant can widespread basal sliding and therefore glacial abrasion occur. As we have seen, glacial quarrying is also facilitated by the presence of meltwater. It is possible to predict where erosion is likely to occur within an ice sheet on the basis of its basal thermal regime (Figure 5.9) and the following predictions can be made.

1. Glacial erosion, in particular abrasion, will be most effective in areas where the ice is warm-based and basal melting is widespread.

2. Glacial erosion will be limited in areas where the ice is cold-based and there is no basal melting.

Ice flow

Cold-based Ice

"X

Warm-melting Warm-freezing

Glacial abrasion

Rock fracture Meltwater production

Ice flow

Warm-melting Warm-freezing

Glacial Limited amount

Glacial abrasion Rock fracture Meltwater production Glacial deposition

No erosion

Glacial Limited amount

Bedrock

Rock fracture abrasion and and some rock entrainment fracture of debris abrasion of glacial

Glacial abrasion Rock fracture Meltwater production Glacial deposition

NO EROSION

EROSION INTENSE LIMITED EROSION EROSION

EROSION

Figure 5.9 Schematic cross-section through an ice sheet showing the influence of the basal thermal regime on the processes of glacial erosion.

3. Glacial erosion, in particular quarrying, will be pronounced in areas where a zone of warm-based ice is replaced along a flow line by a cold-based zone. This is because meltwater flowing from the warm-based zone into the cold-based zone will refreeze, facilitating glacial quarrying and allowing basal debris to adhere to the glacier sole.

4. If ice sheet basal thermal regime evolves as a cold-based interior and a warm-based margin (Figure 3.13), then a belt of glacial erosion will advance across the landscape as the ice sheet grows. This belt of erosion will also retreat with the ice sheet as it decays.

While the primary or regional pattern of glacial erosion is controlled by basal thermal regime, the secondary or local pattern is influenced by a complex range of variables, which include the following.

1. Bedrock geology, because permeable lithologies may be associated with high values of effective normal pressure and therefore little glacial abrasion. Conversely, areas underlain by impermeable lithologies, or fractured and jointed bedrock will be easily quarried. Ice sheets flowing from areas with resistant lithologies into areas of softer lithologies will achieve more erosion.

2. Areas of fast-flowing ice will be zones of enhanced glacial abrasion. Consequently ice sheet margins that terminate in maritime climates will be more effective agents of erosion than those that terminate in more continental climates.

3. Warm-based zones of an ice sheet experience high levels of basal melting and therefore greater levels of glacial abrasion than areas with low rates of basal melting.

4. Areas subject to fluctuations in water pressure are likely to experience greater amounts of glacial quarrying than areas with little or no fluctuation.

5. It has been suggested that an ice sheet which grows in a previously glaciated area with an ice divide located in the same place as in the past may achieve less erosion than an ice sheet growing for the first time within an area. Once an efficient network of glacial troughs and discharge routes has been established there is less resistance to ice discharge and therefore less erosion. It follows therefore that in areas glaciated several times most of the glacial erosion may be completed during the first phase of glaciation.

It is these second-order variables that give rise to the complex and local variations we find within landscapes of glacial erosion. It is also important to note that there are a number of important feedbacks between glacier thermal regime, sediment supply, meltwater routing and the rates of sediment transport by meltwater that also determine the efficiency of glacial erosion (Box 5.7).

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