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Glaciers 06.12.18 (Glacier mass balance (The system wants to be in a mass…
Glaciers 06.12.18
Glacier mass balance
The system wants to be in a mass balance, from where it is being gained to where it is lost.
So during heavy winter snowfalls, glacier wants to advance to transfer mass.
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Negative net balance- glacier down-wasting and snout retreating- sometimes the glacier thins rather than retreats.
Tipping point of a glacier- the highest mass. This is important for the geological record as if position changes we can tell if the Glacier is advancing or retreating.
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Glacial system
Glacial mass balance- a natural open system- Things can come in and effect it.
Inputs- throughputs- outputs- accumulation- ice flow- ablation.
Glacial system key features 1. Glacial mass
- Glacial mass balance
- Glacial flow.
- Glacial mass accumulation
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Superimposed ice- meltwater coming in and refreezing- slope further up catches the sunlight, ice melts and water refreezes on glacier.
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Calving- glaciers that terminate in water, smaller masses of ice break off.
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The higher and warmer the water the more unstable the ice water boundary and the more ice is lost via this mechanism.
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Net balance gradient
Continental type glacier
Low accumulation, low ablation, glacier is relativity static (not gaining, nor loosing mass). Low dry areas.
Low balance gradients, slow flow e.g. major ice sheets, High Arctic glaciers.
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Hence we are most concerned about what they do, as they have the possibility to contribute the largest sea level change, due to the volume of water.
Maritime type glacier
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high accumulation, high ablation.
High balance gradient, fast flow. e.g Southern Alps, New Zealand
More mass must be moved through system to keep an equilibrium, meaning higher velocities and flow rates.
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Glacier shit
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Quarrying
Bedrock is under tension, ice is wanting to extend down the valley so its under tension (transfers the tension to the rock).
Weakness is exploited and put under tension, water is forced into the cracks, freezes causing pressure. Material wants to move to low pressure so its not under stress. Some gets frozen and dragged along.
U shaped valleys
What the ice does to the valley in terms of erosion, determines how the glacier glows at a later stage.
ice is flowing the fastest at the middle and bottom middle of the glacier. - where it has the least friction and stress with the valley sides
Where the ice is flowing the fastest is where the most erosion takes place, so the bottom of the glacier becomes very eroded. The valley eventually broadens out, so there's more capacity for the ice to not have friction with the valley sides, so ice movement speeds up over time and becomes more efficent.
Glacial flow
Creep
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Ice crystals have parallel cleavage planes and tend to split in 1 particular direction. Under pressure at depth, they align parallel to the shear stress of the ice (i.e. down valley) and begin to slide past each other producing down slope glacial flow. It becomes more organised as it becomes more compressed.
Creep has 2 components
- Basal slip- lubricates and occurs at the ice/ bed interface, favouring easy sliding. (only where there's water- its the lube)
- Internal deformation- under pressure ice deforms easily- major component of flow- even when there is no basal sliding due to lack of water.
Brittle zone- top of glacier where ice isn't under that much confining pressure. So wants to break, crack and fracture.
Plastic zone- Further down into the ice, pressure from above means it flows and behaves plastically.
Fracture
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Fracture- Fracture occurs when the stress placed on the ice cannot be accommodated by deforming and the ice fractures to form crevasses.
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