Thermal Relations Heat Flow Across Glaciers Boundaries

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Thermal RelationsHeat Flow Across Glaciers
Boundaries
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Thermal Relations Heat Flow Across Glaciers
Boundaries

• Footnotes for Diagram
• (1) varies considerably, depending upon
  geologic
• conditions
• (2) depends upon rate of flow
• (3) very important if surface is often above
  melting temp.
• (4) mostly reflected
• (5) from atmos., firn, snow
• (Note could be loss to atmos.)
• (6) incorporated in glacier
• (Note could be warmer or colder)

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Thermal RelationsThermal Classification

• ICE
• Cold ice below its
• pressure-controlled melting point
• Warm ice above its pressure-controlled melting
  point (above
• 0oC ??)

• GLACIERS
• Polar cold ice
• Temperate warm ice (except near-surface ice in
  winter)
• However, both types can occur in the same glacier.

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Thermal Relations Temperature Gradient Taku
Glacier
-17C
-10C
0C
Feb 8
Feb 28
June 4
July 15
65M
0C
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Thermal Relations Temperature Grad. NW Greenland
-50C
-25C
-5C
Winter
Mid-Aug.
-25C
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Thermal RelationsTemperature Gradient Antarctica
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Glacial MovementStress and Strain

• External force acting on unconfined soilid sets
  up 3 types of internal stresses compressive,
• tensile, and shearing.
• Because of stress, changes occur in shape,
  volume, or both.

Shearing Stress
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Glacial Movement Stress and Strain

• 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 MovementStress and Strain (Fig. 3.7B)
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Glacial MovementShearing 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.)

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Glacial Movement 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.

t 1.0 kg/cm2
Strain
t 0.1 kg/cm2 (elastic)
Time
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Glacial MovementGlen Flow Law

• 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
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Glacial MovementGlen Flow Law (Fig. 3.7C)
Vs
Vi
Vb
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Glacial MovementGlen Flow Law

• 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.

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Glacial MovementGlen Flow Law

• 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 MovementModels 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 Factors Influencing Ice Creep
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Glacial Movement Basal Slip

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

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Glacial MovementBasal Slip
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Glacier MovementBasal Slip - Processes
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Glacier MovementBasal Slip

• Regelation

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Glacial Movement

• 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
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Glacial Movement

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

Accumulation Zone
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Glacial Movement Direction of Flow and
Variations in Velocity

• 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 Compressive and Extending Flow

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

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Glacial MovementCompressive and Extending Flow
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Glacial Movement 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 Variations with Time

• 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 Behind
  stagnant ice. Thickness increases. Stress
  increases release occurs. Re-establishment of
  equilibrium profile.

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Glacial Movement Variations with Time

• Hydraulic lifting
• Water buildup along base. Pore pressure
  increases. Friction reduced until threshold
  crossed. Surge takes place.