2.2 Glacial Erosion

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In what ways is this glacier creating distinctive landforms?

This glacier is modifying the landscape it’s flowing through. Glaciers erode and deposit telltale landforms. They show the direction a glacier flowed and how far it advanced. They create fantastic and unique features in mountain areas. Did glaciers leave clues where you live? If you live in the northern part of the United States, you might be able to find some. Would you know what to look for?

Erosion by Glaciers

Like flowing water, flowing ice erodes the land and deposits the material elsewhere. Glaciers cause erosion in two main ways: plucking and abrasion.

Plucking is the process by which rocks and other sediments are picked up by a glacier. They freeze to the bottom of the glacier and are carried away by the flowing ice.
Abrasion is the process in which a glacier scrapes underlying rock. The sediments and rocks frozen in the ice at the bottom and sides of a glacier act like sandpaper. They wear away rock. They may also leave scratches and grooves that show the direction the glacier moved. These grooves are called glacial striations.

Valley Glaciers

Valley glaciers create several unique features through erosion. You can see some of them in Figure below.

As a valley glacier flows through a V-shaped river valley, it scrapes away the sides of the valley. It carves a U-shaped valley with nearly vertical walls.

The glacier on the left is carving out the sides of a valley. The U-shaped valley on the right is what will be left when the glacier melts away. ^[2]

A hanging valley was cut off from the main valley by a larger glacier (SeeFigure below).

The waterfall on the right flows through a glacial valley. A larger glacier carved the valley the water falls into. There is a waterfall because this is a hanging valley. ^[3]

Acirque is a rounded hollow carved near the top of a mountain by a glacier (See Figure below). This is where the glacier begins. The highest cliff of a cirque is called the headwall.

On the left there are several cirques where glaciers are originating. A glacier melted and left behind cirques in the mountains on the right. ^[4]

An arête is a jagged ridge that remains when cirques form on opposite sides of a mountain. A low spot in an arête is called a col.
A horn, like the one seen in the Figure below, is a sharp peak that is left behind when glaciers erode all sides of a mountain.

The Matterhorn in Switzerland is the classic glacial horn. ^[5]

Vocabulary

Abrasion: The process in which a glacier scrapes underlying rock.
arête: A sharp ridge created by cirques on two sides.
cirque: A bowl near the top of a mountain where a valley glacier begins.
glacial striations: Long, parallel scratches carved into underlying bedrock by moving glaciers.
hanging valley: A cliff where a large glacier cut off the U-shaped valley of a tributary glacier.
horn: A pointy peak created by cirques on three or more sides.
plucking: Removal of blocks of underlying bedrock as meltwater seeps into cracks and freezes.

Summary

Glaciers are incredibly powerful agents of erosion.
Valley glaciers create very distinctive landforms like horns, cirques, and hanging valleys.
Glaciers pluck rocks from valley walls. This turns a V-shaped river valley into a U-shaped glacial valley.

Practice

Use this resource to answer the questions that follow.

Read Glacial Erosion athttp://nsidc.org/cryosphere/glaciers/questions/land.html

Describe glaciated valleys.
What are fjords?
What are cirques?
What are arêtes?
What are horns?

Review

Why do glacial striations show the direction a glacier moved?
Describe how these erosional features form: hanging valley, cirque, arete, and horn.
How does plucking create a U-shaped valley?

Image Attributions

^ License: CC BY-NC 3.0
^ Credit: (”left”) Image copyright totophotos, 2012; (”right”) Image copyright alet, 2012; License: CC BY-NC 3.0
^ Credit: (”left”) Image copyright Lizard, 2012; (”right”) Image copyright Volker Rauch, 2012; License: CC BY-NC 3.0