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Glacier Flow in Vertical Profile (Actual ice velocity (Increases with…
Glacier Flow in Vertical Profile
Glacier flow in vertical profile
Strain rate ↑ w ice thickness
∴ Greatest strain rate @ bed
i.e. Most ice deformation
= concentrated in basal layers
where shear stress = highest
Greatest cumulative deformation
@ surface
= sum of all dislocations in ice column
∴ Highest velocities @ surface
Haut Glacier D'Arolla Deformation
Almost all flow/def occurred in
lowermost 40 m of ice nr bed
Msmt:
Drilled borehole
Go back year later
See how much cable moved
Velocity Gradient
High
near bed
Low
near surface
Actual ice velocity
Increases with distance from bed
∴ Max velocity @ surface
= Validated by deformation studies
of boreholes drilled through ice
FIND EXAMPLES
Msmt Ice Velocity
Flow velocity = measured @ surface
Represents
MAXIMUM
velocity
Caveat Predicted Deformation Profiles
Predicted def profiles within glaciers
approximate
those recorded in glaciers
Correspondence rarely exact
hard to get data
ice rheology = v complex
shearing/faulting/thrusting may occur & disturb def profile
Glens Flow Law
only approx, but best we can do atm
Faulting / shearing
occurs if creep cannot adjust sufficiently rapidly
to stresses set up within ice
i.e. to accomodate flow
over bed steepening
Fault type = f (compressive vs extensive)
Generates crevasses & thrusts
A Value
Multiplier, A,
varies enormously
∴ Ice flow modelling =
very hard
Since precise rheology not known
e.g. GrIS borehole
Shows need to use different A values
Softness multiplier varies with Z
Recent ice much
harder
vs Holocene (Ice Age) ice