Cochise College Student Papers in Geology

Geology Home Page physical geology historical geology planetary gems

Roger Weller, geology instructor

wellerr@cochise.edu

Glacial Erosion
by Kevin Demater
Physical Geology
Spring 2007

Glacial Erosion

Nothing in the world leaves such a noticeable impact on our Earth quite like glaciers. They tear away at the earth leaving amazing sites behind.

What is a glacier?

A glacier is a massive wall of ice and rock that is cemented together. They are either referred to as alpine glaciers (valley glaciers) or continental glaciers.

The difference between an alpine glacier and a continental glacier is quite simple: alpine glaciers are found in valleys and are tiny relatively to continental glaciers which cover vast amounts of land such as Greenland and Antarctica.

$C:\Documents and Settings\Kev\Desktop\usgsvglac.jpg$
Photograph by Bruce F. Molnia, USGS

How a Glacier Moves

A glacier needs to be between 150’ and 250’ feet thick to begin moving slowly. The speed at which it moves is determined by both the thickness of the ice and the angle of the slope. As a glacier moves away from a mountain a bergschrund is formed. A bergschrund is a large gap at the top of an alpine glacier between it and the mountain. As the glacier moves away the rocks cemented to it with the rocks are pulled away from the mountain. When a glacier pulls rocks away from mountain the process is known as glacial plucking. Glacial plucking may sound like it is pulling small rocks away but it is actually pulling both tiny rocks and huge boulders away from the mountain side. The process known as ice wedging occurs when ice forms in the cracks, splitting the rocks; when the glacier moves the rock breaks loose.

$C:\Documents and Settings\Kev\Desktop\6bergschrund.jpg$

(Diagram courtesy of R. Weller)

As a glacier moves it causes several things to happen to the surface of the earth.

Glacial Striations

Glacial striations are scratched caused by rocks picked up by the glacier being dragged across other boulders.

Glacial Groves

Similar to glacial striation glacial groves are caused by much larger rocks creating large gouges into the surface of the Earth.

$C:\Documents and Settings\Kev\Desktop\6trip-yosemite5.jpg$

(Photo courtesy of R. Weller)

Glacial Polish

Glacial Polish occurs through abrasion with the bedrock. Make a polished surface on part of it.

$C:\Documents and Settings\Kev\Desktop\6trip-yosemite2.jpg$

(Photo courtesy of R. Weller)

Not those are small examples of what glaciers do now on to the big stuff. Glaciers are what shape the mountains around us they create some of the most spectacular sites on our planet. Besides the above there are three things glaciers do to mountains that shape them. The create cirques, arêtes, and horns.

A cirque is formed by an alpine glacier on one side of a mountain. The valley resembles a scoop taken out of mash potatoes or an amphitheatre. Kettle lakes are often formed in cirques.

An arête is formed when a mountain has two glaciers on the sides of it as the glaciers move down the mountain they tear away at the sides of the mountain leaving a narrow ridge at the top of the mountain.

Horns are probably the most known of the three horns are made from glaciers being on 3 or more sides of a mountain as they move down they cut away at all sides of the mountain leaving a mountain in the shape of a horn. One of the most famous is the Matterhorn.

$C:\Documents and Settings\Kev\Desktop\Matterhorn_Riffelsee_2005-06-11.jpg$

Dirk Beyer, under GNU Free Documentation License

Hanging valleys are a unique system of valleys that flow into each other they are caused by alpine glaciers cutting into the surroundings. Valleys created by glaciers are U-shaped as opposed to V-shaped valleys which are made by rivers.

$C:\Documents and Settings\Kev\Desktop\6hanging.jpg$

(Diagram courtesy of R. Weller)

Erratics are another sign of erosion by glaciers. Have you ever seen a large boulder that doesn’t fit in the surroundings it is currently located in? Chances are it was probably left there after a glacier passed through the area and dropped it off.

Another sign of glacial erosion are drumlin often a glacier will carve a drumstick like hill out and deposit glacial till on it. Glacial till is the unsorted rocks that the glacier has picked up and dropped off. To go along with glacial till we have rock flower, rock flower are rocks that have been crushed to a powder like state by the glacier.

$C:\Documents and Settings\Kev\Desktop\6drumlin.jpg$

(Diagram courtesy of R. Weller)

Moraines are piles of glacial till that are left by glaciers depending where they are they have a slightly different name. Lateral moraines occur at the sides of glaciers, medial moraines are formed when two glaciers come together, and finally there is the terminal moraine which is at the bottom end of the glacier where it stops.

So conclude the different types of erosion a glaciers cause to our environment and how they shape the world around us.

Sources

http://library.thinkquest.org/TQ0311164/valglaerosionfeat.htm

http://www.usgs.gov

http://skywalker.cochise.edu/wellerr/aawellerweb.htm

http://www.nsidc.org/glaciers/questions/land.html

http://en.wikipedia.org/wiki/Glacier