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Metamorphism & Physical Structure (Burial depth required for ice…
Metamorphism & Physical Structure
Dry
Snow Zone
Metamorphism proceeds via:
Gradual ∆ ice crystal structure & void size / shape
due to increasing pressure @ depth
Percolation + Wet Snow Zones
Meltwater
increases crystal metamorphism rate
Crystals melt at boundaries
--> Increases packing density & contact
∴ Rapidly speeds up ice formation
i.e. ice can form in 1 summer
Time for ice formation
Antarctica
~3500 years
160m depth
Alaska
5 years
~ 2m depth
Opposite extremes
Burial depth required for ice formation
Depends on thermal zone
Dry snow
= 50 - 160 m
No melt e.g Antarctic interior
∴ Ice accumulation takes
much longer
Percolation
= 30 - 50 m
Lotsa melt, percolation & refreezing
Wet snow
= 2 - 30 m
Whole snowpack saturated ∴ rapid ice formation
e.g. Alps
Glacier Phys Structure
Glacial ice =
far from hogeneous
Exhibits layers/planes
Stratification
Foliation
Debris layering
Fracturing
of various forms/sizes
= Critical for understanding glacial processes & info derived from glaciers
Stratification
= layering developed from deposition of snow
Like sedimentary layers
∴ Each layer = isochron
Radargram
Strata picked out in radargram
Due to variation in
ice density
ice purity e.g. debris content
or other
Foliation
Folding developed during flow
Debris layering
Layring between ice/firn/snow layers
Debris sourced from
above e.g. tephra/atmos dust
below e.g. basal debris layers
Bubble conc
= high in pure, blue ice
Khumbu Glacier, Nepal
= Glacier beneath
thick debris layer
Debris layers
control ablation
How?
Protects ice from temperature fluctuations
Fracturing
More fracture -->
greater stresses glacier experiences
Often stress from flow velocity
Greater velocity
-> more tensile stress
Lotsa fracture @ margins
as rapid ice velocity
∴shear band develops there
Crevasses
Generally form ⊥ principle extension axis