lecture 3 - glacier flow

Question 1

Alley et al (1986)

Answer 1

behavior and instability of an ice sheet depends on the dynamics of the large ice streams which drain it

ice stream B, west antarctic, glacial till deformation is primary mechanism for movement, doesn’t have a significant water beneath the ice and sed is permeable hence water can be drained

if till is not deforming, the velocity comes from the sliding of ice over a rigid substrate lubricated by water

Question 2

Bamber et al (2000)

Answer 2

90% of Antarctic ice sheet drained though a small number of fast moving ice streams and outlet glaciers

each drainage basin: has complex tributary systems, has fast flow feature which draws down ice from the inland sheet (which is cold based and flows by internal defo)

active ice streams have velocities 100-2000m/yr, require high strain rates in order to develop shear margins

flow rates in fast flow features are 5-10x greater than surrounding ice

Question 3

Engelhardt and Kamb (1998)

Answer 3

basal sliding causes rapid ice stream motion

WAIS moves 10-100x faster than surrounding ice

rapid movement = shear deformation of unfrozen, water saturated subG till

Question 4

Harrison and Post (2003)

Answer 4

climate (G thermal regime) and weather affect surge (initiation, termination and magnitude)
- could also be associated with the deformation of till in a soft bed situation (destruction of drainage system and rapid motion)

subG till found in all studied surge type glaciers, difficult to correlate

surges tend to initiate when there is little water available at the surface (winter) - surge termination when water is abundant

Question 5

Kamb et al (1985)

Answer 5

surge glacier can speed up to 100x its normal speed, then return to normal speed, may repeat within a range of 10-100 years

all at PMP during normal state so can’t be theory that basal ice warmed to melting point

= surge caused by extreme enhancement in basal sliding
- high basal water pressure (direct cause) therefore reduced friction

floods when the speed of the surging glacier slowed down

Question 6

Kamb et al (1985) Variegated glacier

Answer 6

Variegated glacier, Alaska; peak water pressure at Same time as velocity peak, thought to be enough to float the glacier, became very crevassed, lower end thickened as a bulge

• 95% motion from basal sliding, 5% from internal deformation
• more glacier flour in stream during surge phase

Question 7

Bennet (2003)

Answer 7

ice streams

• corridors of fast flowing ice, not in equilibrium with climate and catchment geometry, may exhibit surge behavior, bound by lateral ice (not rock)
• ice flow accelerates within topographically bound corridors

discovery of 5-6m layer of soft sediment beneath Whillians ice stream –> shift to concept of subG defo

Question 8

Stokes et al (2007)

Answer 8

sticky spots

Bed roughness may prevent the lateral migration of some ice streams

bedrock bumps unlikely to shut down ice streams, over long time scales erosion can reduce their amplitude