effect of weight on glacial movement
- melt water underneath a glacier allows the glacier to slide over the ground. this is called basal sliding and its the main way warm glaciers move
- there's more melting around bits of rock protruding from the valley floor because there's more pressure on the ice. melt water can refreeze downstream of the obstruction where there's less pressure so the flow tends to be faster around the obstruction, and slower downstream
- glaciers move in an arc shape when they're in a hollow (by basal sliding) this is called rotational flow
- internal deformation is where the ice bends and wraps to flow downhill like a liquid its caused by ice crystals shifting past each other. its the main way cold glaciers move
- at the head of a glacier the valley is steep, so there's a strong gravitational force pulling the ice downwards. this makes the ice move more quickly. when ice moves quickly there's more tension which causes the ice to fracture into thick layers. the layers then slip downwards - this is called extensional flow.
- lower down the glacier the ice is moving more slowly because the valley is less steep. the faster ice from the head of the glacier pushed down on the slower ice and compresses it. the high pressure causes the ice to fracture into layers that slip forwards called compressional flow.
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- the main thing that determine the speed at which a glacier flows are the gradient of the valley floor, the thickness of the ice and the temperature at the base of the glacier
- the steeper the valley the faster the glacier will flow. the thicker the ice the faster it will flow, in a warm based glacier thicker ice exerts more pressure on the valley floor causing more melting which means faster flow. in a cold bade of glacier thicker ice means there's more internal deformation which makes it flow faster. the warmer the base the faster it will flow
- ice moves faster near the glaciers surface than at the base - friction at the base will slow the glacier down
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glaciers crack as they move down the valley
- stresses and strains cause cracks called crevasses to form in the glacier
- stress can be caused by extensional and compressional flow, calving or tension between the ice attached to the valley sides and back wall and the rest of the glacier
- the tension caused by the glacier pulling away from the ice attached to the back wall produces a big semicircular crevasse at the back of the glacier called the bergschrund
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- glaciers erode the valley floor and sides by plucking and abrasion
- plucking - ice in contact with rock surfaces can thaw slightly then refreeze around rocks protruding from the valley sides and floor. when the glacier moves forward it plucks the rocks away from the valley sides and floor.
- abrasion - debris carried along by the glacier can scrape material off the valley walls and floor
- melt water erosion shapes the valley floor
- glaciers can produce high quantities of melt water, making streams that are powerful enough to erode the valley floor and sides by normal fluvial processes
- the amount and rate of erosion is increased in areas of less resistant rock and if the glacier is thick or if its moving quickly. its also increased if there's lots of debris or if the debris is made of resident rock.
- weathering also contributes to the shaping of the glacial valley
- frost shattering breaks rocks off the back and side walls of the valley. melt water from snow gets into cracks in the valleywalls and then freezes - when it freezes it expands, so it exerts pressure on the rock and bits of rock get broken off. these bits of rock often fall onto or into the glacier (adding to debris that abrades the valley)
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- glaciers carry large loads of debris - this is material that the glacier has gathered by plucking, or bits of rock that have been broken off the back wall or Vally sides and fallen onto (or into) the glacier. debris ranges from fine sediment to huge boulders.
- there are three main ways debris is transported. sub-glacial material is carried on top of the glaciers surface. en-glacial material is carried within the body of the glacier. sub-glacial material is moved along at the base of the glacier.
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- the unsorted mixture of material deposited by the glacier is called till (sometimes called boulder clay) it includes everything from massiveboulder's down to pebbles and clay. glaciers drop any size till any ware
- lodgement till is spread onto the valley floor beneath the ice by moving glacier.
- ablation till is dropped by a glacier as it melts. the till is mainly deposited close to the glacier snout because this is where most ablation happens
- till points in the direction that the glacier is flowing
- till is often deposited at landforms called moraines.
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