Some interesting BBC clips:
•http://www.bbc.co.uk/learningzone/clips/the-formation-flow-and-retreat-of-glaciers/3248.html
•http://www.bbc.co.uk/learningzone/clips/the-franz-josef-glacier-formation-and-flow/3079.html
Ice formation
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When a climate starts to become colder, more precipitation in winter falls as snow. Summers also begin to shorten, so there is less time for the winter snow to melt. At first, this leads to permanent snow cover in upland areas, the lower edge of which is known as the snow line. As the climate continues to deterioraste, the snow lines moves down the slope.
At present, the snow line is at sea level in Greenland but at 6,000m on the equator. There is no permanent snow cover in the British Isles, so there is no snow line, but scientists estimate that if the Scottish mountains were 200-250m higher, there would be. In the northern hemisphere, the snow line is found at a higher altitude on south-facing slopes than those that face north, as the south-facing slopes receive more isolation.
Snow initially falls as flake, which have an open, feathery structure that traps air. As the snow accumulates, compression by the upper layers gradually turns the lower snow into a more compact from known as firn or neve. Meltwater seeps into the gaps and then freezes, further compacting the mass. As more snow falls, air is progressively squeezed out of the lower snow by the weight of the upper layers and after a period of some years (most experts put it between 20 and 40) a mass of solid ice develops. Where there is no summer metling, this process tkaes longer. During this period, the mass changes colour from white, indicating the presence of air, to a bluish colour, indicating that the air has been largely expelled. This is the ice that begins to flow downhill as a glacier.
Temperate (alpine) glaciers
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These glaciers melt in the summer, releasing huge amounts of meltwater. This acts as a lubricant, reducing friction. Temperate glaciers move by basal flow, extending/compressing flow, creep and surges. This type of glacier is more likely to erode, transport and deposit material.
Polar glaciers
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These occur in areas where the temperature is permanently below 0 degrees Celsuis, and therefore no melting occurs. Movement is slower than in temperate glaciers as they are frozen to their beds and thus move mainly by internal flow. Much less erosion, transportation and deposition occurs.
How do glaciers move?
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As ice moves downhill it does not always behave in the same way. It has great rigidity and strength, but under steady pressure it behaves as a plastic (mouldable) body. In contrast, when put under sudden compression or tension, it will break or shear apart. This give two zones within the glacier:
Upper zone: the ice s brittle, breaking apart to form crevasses
Lower zone: there is steady pressure. Here meltwater resulting from that pressure and from friction with the bedrock allows a more rapid, plastic flow. At depth in the glacier the melting point of the ice is raised slightly by the increased pressure. Basal ice is therefore more likely to melt at temperatures close to 0 degrees Celsuis (pressure melting point).
Types of ice movements
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Compressing flow – occurs where there is a reduction in the gradient of the valley floor leading to ice deceleration and a thickening of the ice mass. As such points ice erosion is at its maximum
Extending flow – occurs when the valley gradient becomes steeper. The ice acclerates and becomes thinner, leading to reduced erosion
Basal flow (sliding/slippage) – as the glacier moves over the bedrock, there is friction. The lower ice is also under a great deal of pressure and this, combined with the friction, results in some melting. The resulting metlwater acts as a lubricant, enabling the ice to flow more rapidly
Surges – these occur from time to time when an excessive build-up of meltwater under the glacier leads to the ice moving rapidly forward, perhaps by as much as 250-300m in one day. such surges represent a hazard to people living in the glacial valley below the snout
Internal flow – this occurs when ice crystals orientat themsleves in the directionof the glacier’s movement and slide past each other. As surface ice moves faster, crevasses develop. Internal flow is the main feature of the flow of polar glaciers as, without the presence of meltwater, they tned to be frozen to their beds
Creep – this occurs when dtrees builds up within a glacier, allowing the ice to behave with plasticity and flow.it occurs particularly when obstacles are met
Rotational flow – this occurs within the corrie (cirque), the birthplace of many glaciers. here ice moving downhill can pivot about a point, producing a rotatinal movement. This, combined with increased pressure within the rock hollow, leads to greater erosion and an over-deepening of the corrie floor
Ice movement
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Graph A) The sides of the glacier move slowly because they are in contact with the side of the valley and slow down because of friction. The
middle of the glacier is not restricted/slowed down by friction so flows much faster
Graph B) The bottom of the glacier is in contact with the valley floor so friction slows down the speed of the glacier. The surface of the
glacier is not in contact with any of the sides so therefore is not slowed down.
Basal melting pressures
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Warm based or temperate glaciers the melting point and the temperature of the ice remains fairly conistent as you go deeper.
In polar or cold based glaciers the temperature of the glacier increases the deeper you go due to the increased pressure and friction towards the base of the glacier.