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What Is Glacial Plucking?

Andrew Kirmayer
Andrew Kirmayer

Climate change generally refers to atmospheric warming and changing weather patterns that may be brought on by man-made processes and emissions. The climate has gone in cycles throughout history and glaciation is typically one example. Large areas of ice have shaped the landscape in many parts of the world, reshaping rock formations by processes such as abrasion and glacial plucking. Pieces of bedrock can break off as a glacier passes, and become frozen within the ice. The rocks typically travel along the base of the ice, and new features in the bedrock are often formed as the ice sometimes repeatedly advances and retreats.

Glacial plucking often occurs as the advancing ice pushes against fractured bedrock. Additional breaks can form as a result of the stress, while warmer temperatures and friction from the movement of the glacier can cause melting. Water can then get into other cracks in the rock, weaken it from force or by re-freezing, and break off more pieces.

Glaciers pick up and move bedrock in an activity known as glacial plucking.
Glaciers pick up and move bedrock in an activity known as glacial plucking.

Abrasion is another process that affects the bedrock as a glacier passes over it. The ice, as well as debris in it, can erode various surfaces. Small particles can polish bedrock, while large pebbles and boulders typically leave scratch marks that can be examined to determine the motion of a glacier. The concentration and hardness of the entrained rocks as well as the speed of the ice typically influence the degree of erosion. Abrasion and glacial plucking sometimes happen at the same time, and is often identified by a bullet-shaped rock formation.

Glacial plucking can happen in zones, where large amounts of rock are broken away. Nearby is often a lake that is formed when the glacier melts and the water pools in that area. Abrasion is commonly seen on the other side of this lake, while a rock formation called a moraine typically marks the farthest point a glacier has advanced to.

Evidence of glacial plucking and other types of erosion can be found throughout the world. It is often seen in the United States as well as Canada as a result of ice sheets that formed during the last ice age, called the Wisconsin glaciation period. Rock formations produced by glaciers are commonly found in Connecticut and Iowa, while the land in what is now Finland and Sweden was shaped during a previous ice age. Many landscapes have been shaped by glacial plucking and other processes, while some modern day islands were formed by glaciation as well.

Frequently Asked Questions

What is glacial plucking?

Glacial plucking, also known as quarrying, is a process where a glacier erodes the underlying rock as it moves. The glacier freezes onto the bedrock and, as it advances, it pulls away chunks of rock, incorporating them into its ice mass. This process is facilitated by water that infiltrates cracks, freezes, and expands, loosening the rock for easier plucking.

How does glacial plucking contribute to landscape formation?

Glacial plucking plays a significant role in shaping landscapes by carving out valleys and creating jagged mountain terrain. As glaciers move and pluck rocks from the Earth's surface, they leave behind deep depressions and steep-sided valleys, such as U-shaped valleys, which are characteristic of glacially sculpted landscapes.

What conditions are necessary for glacial plucking to occur?

For glacial plucking to occur, there must be a glacier present with sufficient mass and movement. Additionally, the presence of water at the glacier's base is crucial, as it promotes freezing and thawing cycles that weaken the bedrock. Cold temperatures that allow the glacier to remain frozen to the rock are also essential for effective plucking.

Can glacial plucking affect the size of a glacier?

While glacial plucking itself does not directly affect the size of a glacier, it can influence the amount of debris carried within the ice. This debris can enhance the glacier's ability to erode the landscape further downstream, potentially affecting the glacier's morphology and the extent of its erosive power.

What evidence can indicate past glacial plucking in a landscape?

Evidence of past glacial plucking includes the presence of striations and grooves on bedrock surfaces, polished rock faces, and the deposition of erratics—large boulders that differ from the local geology and were transported by glacial action. U-shaped valleys and truncated spurs are also indicative of this erosive process.

How does glacial plucking differ from glacial abrasion?

Glacial plucking and abrasion are both erosive processes, but they operate differently. Abrasion occurs when the glacier's embedded debris scrapes and grinds against the bedrock, effectively sanding it down. In contrast, plucking involves the glacier pulling away blocks of rock. Both processes often work in tandem to sculpt the landscape beneath a glacier.

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    • Glaciers pick up and move bedrock in an activity known as glacial plucking.
      By: elnavegante
      Glaciers pick up and move bedrock in an activity known as glacial plucking.