4. Glacial Erosion - Glacial Erosion and Weathering Processes

Question 1

Explain the two main processes of glacial erosion.

Answer 1

Glacial erosion tends to occur in upland regions and is carried out by two main processes:

• **Abrasion **occurs where the material the glacier is carrying rubs away at the valley floor and sides. *{It can be likened to the effect of sandpaper or a giant file.} *The coarser material may leave scratches on the rock known as striations; the finer debris smoothes and polishes rock surfaces. The debris involved in abrasion is often worn down by the process into a very fine material known as rock flour.
• **Plucking **involves the glacier freezing onto and into rock outcrops. As the ice moves forward, it pulls away masses of rock. Plucking is mainly found at the base of the glacier where pressure and friction often result in the melting of the ice. It is also marked in well-jointed rocks and in those where the surface has been weakened by freeze-thaw action (frost shattering). Plucking leaves a very jagged landscape.

Question 2

Explain the two weathering processes.

Answer 2

There are two other processes associated with glacial action that produce the debris glaciers use in their erosive action. Both of these are weathering processes:

• Freeze-thaw action/**frost shattering **occurs in areas where temperatures rise during the day but drop below freezing at night for a substantial part of the winter. Water which enters cracks in the rocks freezes overnight. Ice occupies more space than water (just under 10% more) and therefore exerts pressure on the crack. As the process continues, the crack widens, and eventually pieces of rock break off. On steep slopes this leads to the collection of material at the base, known as scree. In a glacial valley, much of this material falls from the valley side onto the edges of the glacier and some finds its way to the base of the ice via the numberous crevasses which cross the glacier’s surface.
• **Nivation **takes place beneath patches of snow in hollows, particularly on north- and east-facing slopes. Freeze-thaw action and possibly chemical weathering, operating under the snow, cause the underlying rock to disintegrate. As some of the snow melts in spring, the weathered particles are moved downslope by the meltwater and by solifluction. Over some period of time, this leads to the formation of **nivation hollows **which, when enlarged, can be the beginnings of a corrie (cirque).

Question 3

State the major landforms that arise as a result of glacial erosion.

Answer 3

There are a number of major landforms which are mainly produced by glacial erosion. These include corries (cirques), arêtes, pyramidal peaks, glacial troughs, hanging valleys and truncated spurs.