Notes: Wed. 13 Nov

More on glaciers

Review: how glaciers move (last lecture). said that most glaciers are at the pressure melt point at their bases (see discussion of basal slip from last lecture).

Ice is a mineral with a well-defined atomic structure. Deformation occurs on planes of weakness in the ice - called crystal gliding.

In order for a glacier to survive, precipitation must accumulate on some part of the glacier. This precipitation is snow, and it accumulates best at high latitudes and high altitudes, where temperatures are cold. The snow accumulates at the head of the glacier, the part at higher elevation. Typically, snow will not fall below a certain elevation in the mountains. This elevation is a consistent line across the whole mountain range and is called a snow line. The only time there is extensive precipitation below this line is if glaciers extend below it.

Glaciers need snow to form, but not all places with snow will form glaciers.
Snow is deposited in layers and can be deformed by wind just like sand dunes.
Ice that does not move is not a glacier.
Glacial ice is dense and bluish in color. It is brown and dirty-looking near the base because it picks up debris as it moves.
There are layers within the glacier called foliation. Foliation is a result of deformation.

Classification of glaciers & their landforms

Glaciers can be classified by their extent. There are continental or mountain (also called valley or alpine) glaciers. Glacial depositional landforms are composed of three general types of material: till (unsorted, unstratified; deposited by glacier), outwash (sorted, stratified sands and gravels; deposited by water), or lake clays (see end of previous lecture for desciptions).

Continental glacial landforms:

See the figures in your book to see what these features look like.

Where the Ohio and Missouri Rivers join the Mississippi is the approximate southern boundary of the last ice sheet that covered the Great Lakes and northern USA.

Valley glacier landforms:

Valley shapes and erosion:

A glacier that fills a valley is similar to a river. It flows "downstream" and can join with other valley glaciers.
You can tell if a valley was formed by a river or glacier from the shape of the valley. A glacier is more effective than a river at eroding a valley. A river creates a V-shaped valley (in cross-section), because it only erodes along the center of the valley floor. A glacier creates a U-shaped valley, because it is contact with both the valley walls and valley floor and therefore erodes more. The size of a glacial valley is proportional to how big the glacier was.
Hanging valleys are U-shaped valleys which join the main valley but are at much higher elevation than the main valley floor. They are formed by a tributary glacier (which occupied the hanging valley) merging with the main glacier (which occupied the larger, main valley).

One last feature of glacial ice itself is common to valley and continental glaciers: crevasses, or deep cracks within the ice.

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