Glacial Erosion

Processes of glacial erosion

Plucking

• Occurs when meltwater that accumulates at the base of the glacier due to friction seeps into joints in the underlying bedrock, originally created by freeze thaw action
• When the meltwater refreezes and expands, it fractures the bedrock
• This loose rock is then plucked from the ground and attached to the base of the glacier as it moves downslope

Abrasion

• When previously plucked material embeds itself in the glacier
• This then acts like sandpaper, scraping and eroding rock surfaces leaving scratches and grooves in it
• These grooves are known as striae or striations and show the direction of the ice flow

Factors affecting the rate of erosion:

Thickness of the ice

• Rate of erosion will be quicker when ice is thicker as it is heavier and has more power

Geology

• Different bedrock is eroded at different speeds – differential erosion
• Soft rock (Sedimentary) is eroded quicker than harder, more resistant rock

Gradient

• Rate of erosion is faster on steeper slopes

Landforms of Glacial Erosion

Cirque/Corrie/Coom

Example: Coumshingaun, Co. Waterford

• A cirque, also known as a corrie or coom, is a depression in a mountain, often considered the birthplace of a glacier.
• It typically has three steep sides, usually with a north-facing slope.
• A cirque may contain a glacial lake called a tarn.
• Cirques are formed by processes such as plucking, abrasion, rotational slip, and freeze-thaw action.
• The alternative freezing and thawing of water play a significant role in the formation of cirques.
• A bergschrund, which is a type of crevasse, can form due to freeze-thaw action within the cirque.
• An arête, which is a narrow ridge, may develop between cirques as the landscape erodes.
• Cirques are sometimes formed when two of them develop side-by-side or back-to-back.
• When three or more cirques erode back-to-back or side-by-side, they can create pyramidal peaks.
• This erosion process can result in an isolated peak at the center of the mountain

U-shaped valley and associated landforms

Example: Glendalough, Co.Wicklow

• A U-shaped valley, also known as a glacial trough, is formed by glaciers through processes like abrasion, plucking, and freeze-thaw action.
• Glaciers often take the easiest route as they move from upland areas down through valleys.
• The route is typically a pre-existing V-shaped river valley that is transformed by the glacier.
• As the glacier moves through the valley, it changes the shape from a V-shape to a U-shape by eroding the valley both vertically and laterally.
• U-shaped valleys are characterized by steep sides and flat floors.
• As glaciers cut through V-shaped valleys, they truncate interlocking spurs, leaving behind sharp ridges.
• Hanging valleys, which are tributary valleys, often form along the sides of the main U-shaped valley.
• Ribbon lakes, which are long, narrow lakes, can be found in glaciated U-shaped valleys.
• The advancing glacier abrades the landscape through differential erosion.
• Meltwater can accumulate within rock basins, where resistant rock on either side forms barriers called rock bars.
• If the ribbon lakes are connected by a river, they are referred to as paternoster lakes.
• Fjords are examples of drowned U-shaped valleys, typically formed when glaciers retreat and sea levels rise.
• U-shaped valleys are the result of glacier activity and melting.