Ice Sheet Flow
Ice Sheet Flow
= driven by its own weight
flows slowly (tens m/yr) throughout most of ice sheet
Driving
stress τd
= force that drives ice flow
= net balance of upstream & downstream forces
i.e. (pressure from upstream ice column)
-(pressure of downstream ice resisting motion)
Response to Driving Stress
Ice flows by vertical shearing
Conceptual model: stack of plates sliding wrt eachother
Slow flow at bottom
Faster flow @ top
Shear
Friction
= friction associated with shearing
balances driving stress
conceptual model: friction btwn individual plates due to shearing motion
Basal
sliding
Ice will slide if water = present at bed (underneath ice sheet)
Corresponding basal friction τb results from basal sliding --> counteracts driving stress
Ice Sheet
Flow Velocity
u = (2/5 A H Csliding) τd^3
A = Glen's Flow Constant
H = ice thickness
Csliding = sliding law constant
Glen's Flow Constant, A
= how malleable ice is
= f (ice temperature)
∴ slower flow when colder
Csliding
sliding law constant
= f (subglacial hydrology & geology)
Non-zero only when base @ melting point
i.e.sliding only occurs when water present @ bed
Determining Csliding: usually best fit value to fit model to observed velocities
Applicability ice flow velocity eqn
Applies to glaciers too
Tho parameters differ slightly
- Glaciers = warmer ∴ A=larger
- Driving stress = more uniform
Implications ice flow velocity eqn
- For a given A & Csliding
Thinner ice/gentler slope --> slower ice flow
- For given ice flow
Higher Csliding (weaker bed)
---> Thinner ice or flatter surface
Large scale
flow character
= how flow velocity eqn dictates ice sheet shape & flow character
Margin = faster ice flow
Interior = slower / little ice flow