Glaciers!

A Glacier is a mass of fallen snow, that over time has compressed into a large ice mass. Glaciers form when snow stays in one place long enough for it to turn into ice. The unique thing about glaciers, is their ability to move. Glaciers move under their own weight. They occupy about 10% of the world's land area. Scientist believe present day glaciers to be left over from the last ice age.

Glacier Vocabulary:

Word
Definition
Picture
Alpine Glacier A glacier confined to a mountain valley, which in most instances had previously been a stream valley.  
Arete A narrow knife like ride separating two adjacent glaciated valleys.  
Cirque An amphitheater-shaped basin at the head of a glaciated valley produced by frost wedging and plucking.  
Crevasse A deep crack in the brittle surface of a glacier.  
Drift The general term for any glacial deposit.  
Drumlin A streamlined asymmetrical hill composed of glacial till. The steep side of the hill faces the direction from which the ice advanced.  
End Moraine A ridge of till marking the former position of the front of the glacier.  
Esker Sinuous ride composed largely of sand and gravel deposited by a stream flowing in a tunnel beneath a glacier near its terminus.
Fiord A steep-sided inlet of the sea formed when a glacial trough was partially submerged.
Glacial Erratic An ice-transported boulder that was not derived from bedrock near its present site.
Glacial Striations Scratches and grooves on bedrock caused by glacial abrasion.
Glacier A thick mass of ice originating on land and recrystalzing of snow that shows evidence of past or present flow.
Ground Moraine An undulating layer of till deposited by as the ice front retreats.
Hanging Valley A tributary valley that enters a glacial through at a considerable height above its floor.
Horn A pyramid peak formed by glacial action in three or more cirques surrounding a mountain summit.
Ice Cap A mass of glacial ice covering a high upland or plateau and spreading out radially.
Ice Sheet A very large, thick mass of glacial ice flowing outward in all directions from one or more accumulation centers.
Kame A steep-sided hill composed of sand and garvel originating when sediment collected in openings in stagnant glacial ice.
Kettle Holes Depressions created when blocks of ice became lodged in glacial deposits and subsequently melted.
Lateral Moraines A ridge of till along the sides of of an alpine glacier composed primarily of debris that fell to the glacier from the valley walls.
Plucking The process by which pieces of bedrock are lifted out of place by a glacier.
Rock Flour Ground-up rock produced by the grinding effect of a glacier.
Stratified Drift Sediments deposited by glacial melt water.
Till Unsorted sediment deposited directly by a glacier.
Valley Glacier A glacier confined to a mountain valley, which in most instances had previously been a stream valley.(also know as a Alpine Glacier)
Valley Train A relatively narrow body of stratified drift deposited on a valley floor by melt water streams that issue from a valley glacier. No picture

Zone of accumulation

The upper portion of a glacier consisting of brittle ice. Also known as the zone of fracture.
Zone of wastage The part of a glacier beyond the zone of accumulation where all of the snow from previous winters melts, as does some of the glacial ice.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

How Glaciers Move

Glacier movement is referred to as flow. Glaciers flow in two main ways, one way being they move under their own weight which involves a plastic like movement of the glacier. "Ice behaves as a brittle solid until the pressure upon it is equivalent to the weight of about 50 meters (165 feet) of ice." (Tarbuck & Lutgens, 123) Once the ice becomes heavy enough, the glacier begins to flow as a plastic. The second way glaciers move is by the whole mass of ice slipping along the ground. Scientists believe the lower parts of glaciers move in this manner. One glacial feature that is caused by the movement of the glaciers are crevasses. Crevasses form because the upper most top of a glacier, known as the zone of fracture, is made up of brittle ice, and does not have enough ice on it to allow it to flow. Therefore, it rides on top of the heavier portions of the glacier. When the glacier travels over a rough surface the zone of fracture receives a lot of tension, causing the brittle ice to crack and form crevasses. Glaciers flow very, very slowly and different parts of the glaciers flow at different rates. "Flow is the greatest in the center of the glacier because of the drag created by the walls and floor of the valley." (Tarbuck & Lutgens, 123) The rate at which a glacier moves varies from glacier to glacier.

Glacial Erosion

Glaciers erode in two ways. One way glaciers erode is referred to as plucking. "Plucking occurs when melt water penetrates the cracks and joints alone the rock floor of the glacier and refreezes." (Tarbuck & Lutgens 127). When the water expands, it exerts a huge amount of leverage that pries rocks of all sizes loose. The rocks then become incorporated into the glacier. The second way glaciers erode is by abrasion. As the glacier moves along, the rocks it is carrying grind along the surface, as well as grinding themselves down. The crushed up rock produced by glaciers is known as rock flour. When mixed with water, the rock flour is termed glacial milk. When the glacier is carrying large fragments, long scratches or grooves can be made on the ground that the glacier passes over. The scratches or grooves are called glacial striations. When glaciers move through valleys once occupied by a stream, the ice modifies it in three ways: "The glacier widens, deepens, and straightens the valley, so that what was once a narrow V-shaped valley, is transformed into a U-shaped glacial through" (Trabuck & Lutgens 127). Features associated with glaciated valleys include, hanging valleys, cirques, aretes, horns, and fiords.

Glacial Deposits

As mentioned in the section about glacier erosion, glaciers pick up large loads of debris and transports them long distances. Drift is the term used to describe any and all sediments that have glacial origin. Drift is then divided into two more specific categories, till, and stratified drift. Till is described as sediment that is deposited directly from the glacier. Till is deposited as the ice in the glacier melts, and drops its load. Because till is deposited directly from the glacier, it is not sorted at all. The second type of drift, stratified drift, are sediments laid down by glacial melt water. Stratified drift is stored according to the size and weight of the sediment being deposited. The most widespread glacial features are known as moraines. There are different types of moraines such as medial moraines, lateral moraines, end (terminal) moraines, and ground moraines (for a description of each type of moraine see the vocab chart). Two more glacial deposits are out wash plains, and valley trains. They occur when a glacier begins to melt and loses its velocity. The glacier will deposit a big portion of its load in one section, making a ramp like surface. Very often within a out wash plain, or valley basin, glacial deposits such as, kettles, kames, drumlins and eskers can be found.

Causes of Glaciating

Scientists have yet to discover exactly what causes glacial ages. "Any theory that attempts to explain the causes of glacial ages must successfully answer two basic questions (1) What caused the onset of glacial conditions?...(2) What caused the alteration of glacial and interglacial stages that have been documented for the pleistocene epoch?" (Tarbuck & Lutgens, 137). Two possible theories for the causes of glaciating are, plate tectonics, and variations in the earths orbit.

Glacier Links!

Glacier quiz! www.oocities.org/kimiburke/quiz.htm
Really good glacier pictures http://tvl1.geo.uc.edu/ice/Glacier.html
Prentice-Hall glacier companion website http://wps.prenhall.com/
All About Glaicers http://nsidc.org/
Shockwave presentation about glaciers http://www.eoascientific.com
Glacial landforms and features http://www.glacier.rice.edu

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Sources used for this website:

Edward J. Tarbuck & Frederick K. Lutgens

EARTH SCIENCE eighth edition

1997 by Prentice Hall, Inc. New Jersey.