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Glacial Erosion: Definition, Processes & Features

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  • 0:01 Glaciers & Glacial Erosion
  • 0:47 Glacial Erosion: Plucking
  • 1:30 Glacial Erosion: Abrasion
  • 2:15 Features of Glacial Erosion
  • 5:10 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Glaciers are huge blocks of ice that move along the landscape, carving distinct features along the way. Learn about the glacial erosion processes, plucking and abrasion, and the features they create, including cirque, horns, arête and roche moutonnee.

Glaciers & Glacial Erosion

If a block of ice the size of a mountain is moving toward you, what should you do? Well, if it were me, I would get out of the way. These moving mountains of ice do exist, and we call them glaciers. They tend to move very slowly, often only a few centimeters per day, so it would be a rare occasion for a human to get struck by a glacier. This is not true, however, for the landscape that lies in the path of a moving glacier. Land and rock cannot move out of the path of a glacier, so they are subjected to glacial erosion, which is simply the carving and shaping of the land beneath a moving glacier. Glacial erosion and the interesting landforms that result from this process are the subjects of this lesson.

Glacial Erosion

There are two main processes of glacial erosion. The first that we will talk about is plucking, which is defined as the erosion and transport of large chunks of rocks. As a glacier moves over the landscape, water melts below the glacier and seeps into cracks within the underlying bedrock. This water freezes and melts, weakening the bonds holding pieces of bedrock in place. These pieces of rock can now be picked up or plucked from their rocky base and carried along with the moving glacier. If you ever put an ice cube on your finger and felt the skin on your finger stick to the ice cube, then you have a fairly good idea of how plucking works.

Abrasion

Plucking removes rocks and by itself creates changes in the landscape, but plucking also contributes to the second process of glacial erosion, known as abrasion. Abrasion is defined as the erosion that occurs when particles scrape against each other. The enormous weight of the glacier, along with rocks and sediment plucked up and clinging to its belly scratch and carve the rock surface below. It's almost as if the moving glacier is sanding the rocks with abrasive sandpaper. As the glacier sands the rock, it leaves behind long scratches that form in the direction of the glacial movement called glacial striations. Seeing these scratch marks is a sure sign that a glacier once covered the land.

Features of Glacial Erosion

The scraping and plucking of glacial erosion creates a number of distinct landforms and features. If the glacier erodes into a mountain, it can create a cirque, which is a round hollow with steep sides. It's as if the glacier scoops out the side of the mountain like an ice cream scooper. If you look at a cirque, you might think it resembles an amphitheater; in fact, with a bit of imagination you could image circus animals and clowns putting on a circus at the base of a cirque.

When the glacier melts and moves on, it often leaves behind water that gets trapped in the cirque. A small mountain lake of trapped water is called a tarn. It might help you remember this term if you think of the two 'Ts,' 'Tarn' and 'Trapped.'

When we look at cirques and tarns, our eyes are drawn to the bowl-like inner structures that were carved out, but the peaks left behind from glacial erosion are just as impressive. If a mountain is eroded by several glaciers, we see the formation of a glacial horn, which is a good name because it looks like a horn jutting up from the earth. A glacial horn is defined as a sharp, angular peak formed when several glaciers erode a mountain in different directions. It's almost as if the glaciers sharpen the mountain into a point, just like ancient civilizations sharpened rocks into spears. The Matterhorn in Switzerland is a perfect example of a glacial horn.

In a somewhat similar fashion, if two glaciers slide down opposite sides of a mountain, we end up with a sharp, narrow mountain ridge called an arête. If you are an adventurous hiker, you would find walking along the ridge or an arête exhilarating, as it would provide a high-altitude scenic view. In fact, you would likely give an arête an 'A rating.'

We mentioned earlier that as ice moves over bedrock it can smooth the rock, cut striations into the rock and also pluck rocks from the foundation. The erosion can lead to the formation of a roche moutonnee, which is a mass of rock carved by a passing glacier. The rock formation is distinctive because it has a smooth, rounded side where the ice abrasively slid over it, followed by a steep, ragged side where rocks were plucked from the surface.

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