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Glaciation. http://pubs.usgs.gov/factsheet/fs133-99/worldmap.gif. about 15 million square kilometres of the earth’s surface are covered with glaciers two types - alpine and continental Greenland and Antarctica both have continental glaciation
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about 15 million square kilometres of the earth’s surface are covered with glaciers • two types - alpine and continental • Greenland and Antarctica both have continental glaciation • continental glaciation is found at high latitudes; alpine is found at high altitudes
How glaciers form • two basic conditions are necessary for glaciation: (1) prolonged cold (2) ample snow – An ice age! Not all of the snow melts in the summer – so if it survives and when the next winter comes an accumulation of snow begins from year to year.
over time the overlying layers of snow cause the lower layers of snow to recrystallize (refreeze) and the pore space is reduced, ie., the snow becomes denser – the air is squeezed out • At the end of the winter, the snow that is left is called neve (50% of the air has been removed). If the snow survives the summer it is called firn.
it is called glacier ice when the pore space is reduced to about 5% and it is impervious to water • after about 60m of ice has been accumulated the pressure of the overlying layers causes the ice to flow outwards from the center of accumulation - this will happen sooner (thinner) on a slope • Remember the Pizza Doe example.
How ice erodes 1. scraps or scours - like sandpaper - rocks are embedded into the bottom of the ice sheet and erode the bedrock as it flows 2. conveyor belt - embedded rocks are carried in the ice and deposited in the front as the glacier flows
3. plucking - ice freezes onto the rock and pulls or plucks it away as it flows 4. bulldozer - material is pushed along in front
Mass balance • glaciers are in a state of mass balancemeaning that the rate of accumulation of ice is balanced by the rate of ablation (melting) • note that ice is transferred from the zone of accumulation to the zone of ablation by flowage • The Key is that no matter if the glacier is stationary, retreating or advancing snow is always being brought to the front or snout of the glacier.