Glaciology L2: Glacier mass balance and glacier dynamics

What is glacier mass balance?
The year-by-year measure of the state of health of a glacier. "Difference between gains and losses over a given period of time- Benn and Evans, 1998)
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How is snow transformed into glacier ice?
Snow fall- accumulation- layers build over years- air is pushed out from bottom layers- compression- firn- compression-ice
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How do glaciers grow/shrink?
Grow- input of snow at surface +ve mass balance. Lose mass (-ve mass balance) by melting and iceberg calving.
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Where is the accumulation zone on a glacier?
Where precipitation occurs and snow persists- tends to be at higher elevations
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Where is the ablation zone on a glacier?
Tends to be at a lower elevation- warmer temperature-ice melt
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How do glaciers move?
Under the effect of gravity- deformation (occurs under own weight) and sliding (down rock bed)
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Why does redistribution of ice occur?
Mass is moved from a region of surplus to deficit in order to regain an equilibrium profile
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What is the balance velocity?
The rate at which ice is redistributed
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What are ice streams?
Sections/parts of an ice sheet/cap that flow more rapidly than the surrounding ice banded by areas of more slowly moving ice
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How can ice streams be identified?
Sheer margins may form at the edges- regions of significant crevasses- tearing prevalent
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What factors contribute to the "driving stress" (force which pushes ice flows downslope)?
Gravity; surface slope; ice thickness
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Define: ice creep
Internal deformation of individual ice crystals in response to an applied stress
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What can affect the rate of ice creep?
Crystal size, crystal orientation, bubbles. impurities, temperature
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To what extent does temperature affect ice creep?
Ice at 0 degrees celsius = 1000 x softer than ice at - 50 degrees C. Temperature= most important control- has much more of an impact than the other variables.
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Why does the measured velocity of a glacier often deviate from the norm?
Over short-term-changes can occur to the driving (stresses induced by weight of overlying ice and slope) and resistive (strength of glacial ice and friction with bed/valley sides) forces: balance between 2 changes- glaciers speed up/slow down
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What are the 3 mechanisms by which glaciers flow?
Ice creep, basal sliding, sediment deformation
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Why might glaciers not slide smoothly?
Beds= not smooth- rough @ different scales- obstacles
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How are the mechanisms that allow glaciers slide past obstacles?
Regelation, enhanced creep, presence of water
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What is regelation?
Means- "to freeze again". Most resistance occurs upslope of the obstacle- pressure melts ice to water. Water can flow over/around obstacle. Least resistance downslope of obstacle- less pressure-water refreezes
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When is regelation most efficient?
Around shall obstacles because latent heat is more easily advected back through obstacle to assist with moremelting.
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What is enhanced creep?
At upstream side of obstacle stress is increased so ice deforms more easily- flows over/around bedrock bump- no melting.
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When does enhanced creep occur?
Around larger obstacles because larger areas of ice experience enhanced shear stresses
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How does presence of water affect glacial flow?
Floods/submerges smaller obstacles- reduces friction-allows ice to flow more smoothly
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What is the controlling obstacle size?
A critical obstacle size where neither enhanced creep or regelation can occur efficiently. This is typically between 0.05-0.5m (5-50cm)
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How do you calculate total glacier velocity?
Creep+sliding+sediment deformation
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What us the velocity of a typical temperate valley glacier?
10 m/yr.
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What is the normal range of glacier velocities?
Up to 100 m /yr
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What is the world's fastest flowing glacier?
Jakobshavn Isbrae (Jakobshavn Glacier), Greenland- moves 700 m /yr
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What is the most important control on temperature variations in motion?
Distribution and pressure of water at the bed
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When might glacier velocity fluctuate?
Seasonally- faster flow in summer. Diurnally- faster flow during day on surface (supraglacial)- basal melting doesn't vary substantially over short timescales
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Other cards in this set

Card 2


How is snow transformed into glacier ice?


Snow fall- accumulation- layers build over years- air is pushed out from bottom layers- compression- firn- compression-ice

Card 3


How do glaciers grow/shrink?


Preview of the front of card 3

Card 4


Where is the accumulation zone on a glacier?


Preview of the front of card 4

Card 5


Where is the ablation zone on a glacier?


Preview of the front of card 5
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