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Basal Sliding Physics (Ice stream (narrow regions fast flow near margins…
Basal Sliding Physics
Ice stream
narrow regions fast flow near margins of ice sheet
= fast moving areas of ice sheet
' Rivers of ice' - discharge funnelled in channels
Export most mass (snow) from interior to ocean
(little surface melt in Antarctica)
Types
Basal Sliding
Hard bed sliding
Deformable bed sliding
Both require water @ bed
Reality: both occur in many places
Ice Motion
Ice sheet
high basal friction
mostly frozen to bed ∴ no basal sliding
motion via
internal def
shearing of upper ice layers wrt lower
∴ gradation in ice motion from bottom to top
Ice stream
∴ basal sliding - motion at bed possible
lower basal friction
Ice shelf
No basal fricition since ice = floating
V. fast flow
Ice Flow Around Obstacles
Mechanism depends on size of obstacle
Regelation - smaller obstacles (cm)
Enhanced creep - larger obstacles (m)
Hard vs
soft bed
Hard bed
sliding
Thin lubricating water layer @ bed
Frictionless @ microscale
Hard bed = uneven
--> obstacles provide friction
Ice flows around obstacles due to driving stress
Push on upstream side (stoss) more than lee
Obstacles push back
= basal stress
τb= force from bed exerted on ice
Soft (deformable) bed sliding
Poorly sorted sediments (till)
underlie many glaciers/ice sheets
Sediment deformation can allow v fast basal motion
Requires subglacial water @ bed
Sediments deform providing:
Shearing force applied
Water pressure = high enough
Ice shelves
floating extensions of ice sheets
very
fast flowing
Can assume
ice = sliding
If ice = moving fast (> 50-100 m/yr)
Since max velocity for
ice frozen @ bed ~ 13 m / yr
Even if ice
V thick H=1000 m
V steep a=0.01
Malleable A (T=-5°C)