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Soft/Deformable Bed Sliding (V. Fast Flow (Since: (Only discrete regions…
Soft/Deformable Bed Sliding
V. Fast
Flow
Something else must oppose driving stress
Basal stress cannot balance driving stress
Since:
Only discrete regions
exhibit fast flow & low basal stress
Antarctica still exists
(all ice would've been lost if whole AIS had no basal stress)
Soft bed (deformable) 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
Role water
pressure
Low water p
Tight packing sediment particles
Do not respond strongly to shearing force
Lotsa effective stress btwn grains (pushing against eachother)
High water p
Waterlogged sediment with water in pores btwn grains
Loose packing of sediment grains
Water pressure supports some sediment load
∴ if shearing force applied
Particles move around eachother
Sediments deform
Yield stress
= maximum limit for (basal) shearing stress
Independent of shearing rate (sliding velocity)
Yield stress implications
With increasing sliding velocity
Basal stress remains constant once yield stress reached
∴ If bed is too weak & flow velocity = high
--> basal stress cannot balance driving stress
Why bed weak? High water pressure and/or soft sedimentology
Infinite
velocity ?
Both hard & soft bed sliding have limits
soft bed - relates to yield stress
hard bed - relates to cavitation
Where basal stress cannot balance driving stress @ a given location
∴Velocity equation would predict infinite velocities
Its assumptions must be wrong
Something else must oppose driving stresses
Something must oppose driving stress...
To prevent infinite velocities
Horizontal stresses
oppose driving stresses in fast flowing areas