Cryosphere Lecture 2

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Driving forces of ice mass change:
Mass balance and glacier Flow
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Accumulation zone
Accumulation > Ablation (+ve MB
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Ablation zone
Ablation > Accumulation (-ve MB)
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Equilibrium line divides
the two (Equlibrium line altitude - ELA
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Ice flows from
accumulation area to ablation area
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‘Mass balance’ For an ice mass to be ‘in balance’:
outputs (ablation) & inputs (accumulation) must be the same
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Mass balance is looked at in a sense of a
year ( aka positive/negative mass balance)
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Glaciers will grow with lower temperatures and
or greater snowfall (hence, lower ELA)
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Accumulation (gains/inputs)
Snowfall (Major) Wind-blown drift Avalanches Condensation (rime) Freezing rain or external water (Minor)
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Ablation (losses/outputs)
Calving (wet/dry) (Major) Melting Wind-blown drift Avalanches Evaporation/sublimation (Minor)
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Snow- Firn -
Ice
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Snow has a low density /high void content
50-300 km m-3
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Firn is compacted snow and has survived a melt season
400-830 kg m-3
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When Ice developes air passages are cut off
830-917 kg m-3
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Temperature is an important control on the transformation of
snow to ice (warmer = faster)
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Accumulation also important (more snow = more compression = faster) examples include
Upper Seward Glacier, Alaska= 3-5 years Site 2, Greenland dry snow zone = >100 years
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Ablation (Surface melt)
Function of energy balance (energy deficit or surplus at glacier surface) Takes into account solar radiation absorbed, vs emitted, heat transfer with air and latent heat
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Surface energy balance and ablation greatly influenced by supraglacial debris cover Typical albedo (radiation reflected):
clean ice (34-51%) dirty ice (15-25%) debris covered ice (10-15%)
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Thin debris =
= increased absorption and re-radiation = increased ablation
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Thick debris =
decreased conduction = increased protection = decreased ablation
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‘Dry’ calving
Ice breaking off glaciers in steep mountain environments
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‘Wet’ calving a function of:
Water depth Water temperature Water salinity Tidal variation Glacier crevassing Presence of surface meltwater
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Ice shelf basal melting
warm’ ocean water eroding base of floating ice Causes grounding line to retreat
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Grounded ice subglacial melting, function of:
Geothermal heat flux Ice velocity (frictional heating) Small amounts of melt, but important in ice flow
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Mass balance gradient:
Change in mass balance with altitude and the greater the mass balance difference, the faster the flow
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Maritime climate=
high acc. & abl. rates (e.g. Norway)
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Continental climate =
low acc. & abl. rates (e.g. Arctic Canada)
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A glacier is an accumulation of ice that flows under its own weight
...generally downhill (but not always)
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Ice is strong, but yields under stress such as
(>50 kPa)
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Ice flow has two components:
Internal deformation Basal sliding
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If left for a long period of time, with no input (snow) or output (melting / calving), it would act like
very thick treacle and spread out.
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However, the system has inputs and outputs...
Accumulation (snowfall) Ablation (melt, iceberg calving)
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Ice Flow
Provides dynamic link between accumulation and ablation zones Flow rate dependent on: magnitude of input and output
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Ice flow by internal deformation Ice can be considered a
fluid, though it is actually a solid => It flows Via deformation of ice crystals Easy to see when there is debris
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Driven by gravity: Steeper slope => faster flow =>
thinner ice
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Resisted by friction:
Valley bottom Valley sides
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Velocity/depth profile
Velocity decreases with depth Basal velocity in this case is zero, high basal drag
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Velocity/width profile
Velocity decreases at margins Velocity at margin is low, high lateral drag
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Controls on ice temperature
Surface energy balance Geothermal heat flux Frictional heat due to ice deformation and sliding Refreezing of meltwater at depth
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Ice flow velocity is a function of temperature
Warm ice deforms more readily than cold ice
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Ice temperature varies with depth
Surface air temp. controls ice temp. at the surface Geothermal heat flux controls temp. at the base Temp. in between is a function of ice flow and advection of cold ice from upstream
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Ice is not necessarily
0 °C at the base of the ice Ice melts at lower temperatures under higher pressures (regelation) Think of a ballerina spinning
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We can define a glacier by its temperature regime: Cold based
Only internal deformation – frozen to base
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Warm based
Ice at base is at pressure melting point => water is present
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Polythermal
A mixture of the above
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Factors controlling sliding:
bed roughness (size and frequency of obstacles quantity & distribution of water at bed amount of debris embedded in base of glacier
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Sediment deformation
Ice is often underlain by soft sediments rather than hard bedrock This can deform allowing ice to flow very fast Depends on water content of sediment Higher water content, the sediment is less strong and less able to resist deformation
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Velocity profile with basal sliding
The fastest glaciers slide with lubrication at the base (water or sediment)
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Other cards in this set

Card 2

Front

Accumulation zone

Back

Accumulation > Ablation (+ve MB

Card 3

Front

Ablation zone

Back

Preview of the front of card 3

Card 4

Front

Equilibrium line divides

Back

Preview of the front of card 4

Card 5

Front

Ice flows from

Back

Preview of the front of card 5
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