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Glaciers

By Matthew Thorn

     A glacier is a large river of ice moving at a slow rate. Glaciers occur in areas where snow falls all year with little melt off. Glaciers move simply by gravity leaving a barren area behind them. Glaciers are the largest source of fresh water, second only to the oceans as the largest source of any water.

     Glacial ice is formed in a three step process. The first step is to form granular ice. Granular ice or nv is formed when snow falls, then is melted, and frozen many times. The second step is to create firn. Firn is formed when the weight of the snow above it compacts granular ice into firn. The final step is to form glacial ice. Glacial ice is simply even further compacted firn. To do this takes several years of compacting, under great pressure.

     Glaciers move because of gravity and a thin layer of water under the ice. The layer of water is formed by pressure. Most substance's melting temperature rises when under pressure, but for water it is exactly the opposite. The pressure on the ice is so great that it melts and slides on itself.

     Glaciers erode sediment through mainly three processes; abrasion, plucking, and bulldozing. Abrasion is when the sediment being pushed by the glacier rakes across the bedrock and is pulverized into what is called rock flour. The size of a particle must be from 0.002 to 0.00625 mm across. Plucking is when rocks are lifted or "plucked" from the soil. This occurs when water penetrates the space around the rock and freezes, lifting the rock into the glacier. Bulldozing is simply when the glacier pushes sediment in front of it.

     Glaciers deposit the sediment they carry in four ways. One way is till. Till is formed when a glacier stops moving and begins to melt. In the case of till, all the sediment in the glacier is simply left to rest where it falls. A second way is drumlins. Drumlins occur as hills of piled up sediment pushed out of the way as glaciers move. Another way is glacier erratics. They are rocks left behind by glaciers in erratic patterns (hence, the name), left behind by either being snagged, melting out, or for other reasons. The last way is stratified drift, which is the sediment carried out of the glacier by meltwater. It can be carried miles by braided streams (streams formed by meltwater).

     When glaciers reach the ocean, they float (look at an ice cube). They become icebergs and slowly deposit their remaining sediment over the ocean floor as they melt.

     There are many ways to find more information on glaciers and the geology of Antarctica, all available to anyone who has a library or the internet. One website I recommend is The British Antarctic Survey.