Glacial Movement cont. Stress and Strain2

1
Glacial Movement (cont.)Stress and Strain-2

• Consider forces acting on a cubic cm of ice at
  base of glacier. See Fig. 3-7B.
• The force is the weight, acting vertically
  downward
• W ?gb where ? density of ice (0.9
  gm/cm3) g acceleration due to gravityFig.
  3.7C (cm/sec2) b thickness of glacier

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Glacial Movement (cont.)Stress and Strain (cont.)
3
Glacial Movement (cont.)Shearing Stress

• Now, the component of the weight acting parallel
  to the slope (Wt) is the shearing stress (?)
• ? ?gb sin?
• where ? surface slope
• ? ? increases with increase in b or ?.
• (? is usually 0.5 to 1.5 bars.)

4
Glacial Movement (cont.)Glen Flow Law of Ice
Creep

• Experiments in ice deformation have shown how ice
  behaves as it is deformed.
• It is not a Newtonian fluid.
• It is not a perfect elastic.
• It shows components of both of these, a style of
  deformation expressed as ice creep.

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Glacial Movement (cont.)Glen Flow Law (cont.)

• e A?nwhere
• e strain rate
• ? stress
• A a constant related to ice temperature
• n a constant with a mean value of approximately
  3

-0.02C
-6.7C
-12.8C
Strain
The strain rate is highly sensitive to shear
stress and less so to temperature.
Time
6
Glacial Movement (cont.)Glen Flow Law (cont.)
Vs
Vi
Vb
7
Glacial Movement (cont.)Glen Flow Law (cont.)

• Rate of strain governs that component of velocity
  due to internal motion.
• The rate of strain decreases upward. Why?
• However, each cube of ice moves forward by an
  amount equal to the sum of all strain rates
  below
• Can be considered as a piggyback effect.
• Therefore, the velocity must increase upward.
• 9.11/306 9.13/307

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Glacial Movement (cont.)Glen Flow Law (cont.)

• An equation for determining surface velocity -
  the glaciers maximum velocity Vs 1/32 (?g)3
  (sin?3) b4Rapid increase in V, with slight
  increase in b or ?.? must be steep near
  glaciers terminus because b is less.

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Glacial Movement (cont.)Models of Ice Creep

• Newtonian fluid has strain rate that is linear
  function of ?.
• Perfect plastic shows no deformation until
  critical (yield) stress then continuous
  deformation occurs.

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Glacial Movement (cont.)Factors Influencing Ice
Creep
11
Glacial Movement (cont.) Basal Slip-1

• Effectiveness depends primarily upon temperature
  of basal ice.
• What is the situation for Greenland? for
  Antarctica?Fig 2.5
• How can we determine the amount of slip? Fig. 3-7

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Glacial Movement (cont.)Basal Slip - 2
13
Glacier Movement (cont.)Basal Slip - 3Processes
14
Glacier Movement (cont.)Basal Slip - 4

• Regelation

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Glacial Movement (cont.)

• Direction of Flow and Variations in
  Velocity Flow occurs to distribute
  accumulation. Longitudinal
  Section Plan View

Zone of Accumulation
Equilibrium Line
Equilibrium Line
Zone of Ablation
16
Glacial Movement (cont.)

• Direction of Flow and Variations in Velocity - 2
• Maximum velocities occur at the equilibrium line.
• Due to increase in ice Q through accum. zone.

Accumulation Zone
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Glacial Movement (cont.) Direction of Flow
and Variations in Velocity - 3

• Transverse Sections Velocity decreases away
  Above EL from central axis.
• Below EL

0.12
0.08
0.04
0.02
V
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Glacial Movement (cont.) Compressive and
Extending Flow

• Compressive flow where velocity decreases loss
  of ice thickness - ablation zone where bed is
  concave 9.16/309 Thrust faulting 9.15/308
• Extending flow where velocity increases gain in
  ice thickness - zone of accumulation where bed
  is convex Normal faulting

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Glacial Movement (cont.)Compressive and
Extending Flow - 2
20
Glacial Movement (cont.)Variations with Time

• 1) Random Variations Due to local causes
  weather conditions, release of obstructed ice,
  thrust faulting
• 2) Seasonal Variations
• 3) Kinematic Waves Set up by increases in
  accumulation. Pulses move down-glacier at 2 to 5
  times the rate of ice flow. Greatest effects
  are in ablation zone. Amplitudes of up to 100 m.

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Glacial Movement (cont.) Variations with Time
- 2

• 4) Surges Sudden, spectacular movements. Velocit
  y increase of 10 to 100 times normal. No new ice
  added. Response to intrinsic threshold. Often
  periodic. Explanations Ice-damming Figs.
  3.16,3.17 Behind stagnant ice. Thickness
  increases. Stress increases release
  occurs. Re-establishment of equilibrium profile.