The processes of erosion - Glacial plucking (glacial quarrying)

Question 1

What is glacial plucking?

Answer 1

Glacial plucking is the mechanism by which glaciers detach or remove chunks of rock (of a range of sizes) from their beds.

Question 2

Describe the two steps that are involved in the process of plucking?

Answer 2

• Fracturing of bedrock, beneath the glacier as it moves
• The entrainment (picking up, or ‘capture’) of loose rock and its subsequent incorporation into the glacier

Question 3

Give some of the factors and processes which contribute to the fracturing of bedrock associated with plucking?

Answer 3

• Once glacial erosion has removed a significant depth of rock, the underlying rock will respond to stress relief by expanding and fracturing parallel to the erosion surface in a process known as pressure release or dilation jointing
• The erosion of overlying rock and its replacement by ice results in the application of pressure to the bedrock, causing it to fracture along lines of weakness
• The presence of meltwater under pressure beneath the glacier also widens joints and creates new fractures

Question 4

Give some factors which contribute to entrainment that is associated with plucking?

Answer 4

• Variations in pressure under the ice, associated with irregularities in the bedrock, create localised areas of pressure melting and regelation. On the stoss side of a bedrock irregularity, basal pressure is increased, leading to pressure melting. The water released flows around the protrusion and refreezes on the lee side where pressure is lower, causing rock to become frozen to the base of the ice.
• The presence of meltwater on the stoss side of the obstacle, where pressure is greater, reduces the frictional resistance that holds the rock in place, thereby pushing the loose rock away from the bed.

Question 5

When is plucking effective?

Answer 5

• Plucking is effective where the underlying bedrock is highly fractured and semi-permeable (leading to maximum meltwater at the glacier-bedrock interface)
• Where the ice is moving at higher velocities causing greater shear stress