Plastic deformation (creep) - internal flow

• Intergranular displacement
  • slippage between ice crystals
  • recrystallization
• Intragranular displacement
  • movement within grains along crystal defects or cleavage planes

• Controlling variables
  • Glen's Flow Law obtained from laboratory experiments shows that the rate of deformation is related to shear stress and temperature.
    • E = ATⁿ where E = strain rate, A is a constant dependent on temperature, T=shear stress, and n is a constant varies between 1.5 to 4.2 with a mean of 3 (Patterson, 1994)
    • Temperature increases rates of intergranular slippage
      • Strain rate resulting from a given stress is 5 times greater at -10°C than at -25°C (Patterson, 1994).
  • Factors determining shear stress: T=phg sin Ø
    • ice density (p),
    • thickness of the ice (h)
    • slope of the ice surface (Ø)
    • and temperature (A)
  • Other variables influencing creep (Patterson, 1994)
    • crystal orientation: Because a large amount of movement is accomplished by basal gliding, an orientation of the basal plane parallel to the direction of flow favors faster rates of creep
    • water: facilitates grain boundary sliding and recrystallization
    • impurities
      • soluble impurities (HF, NH3, HCl, NaCl, volcanic acids)
        • Increase rates of creep by softening or disrupting the crystal lattice.

Crevassing - tensional fracturing (joints and normal faults)

• crevasses typically occur at surface (d<±30m, Benn and Evans, 1998) and are useful in determining internal variations in glacial flow (discussed later)
• types of crevasses
  • transverse crevasses formed by rapid extending flow, such as that induced by a rapid increase in gradient
  • marginal-cheveron: Produced by the stresses induced by rapid flow at the center of the glacier and the frictional drag of the valley walls.
  • longitudinal crevasses and splays: Formed under compressive flow or by strongly diverging flow such as occurs when ice flows out of the confines of a valley on to a piedmont floor or into a marine basin.

Thrust faulting

• Shear planes form under compressive flow conditions near glacier margins.

over a meltwater layer:

• occurs only in warm-based glaciers
• affected by: bed roughness and slope

enchanced basal creep

• accelerated obstacle related creep
• occurs in both cold and warm-based glaciers but
• because Temp and H₂O content increases rates of creep this process is faster beneath warm based glaciers.
• flow velocity increases with increasing obstacle size

Regelation sliding: pressure melting

• obstacle related melting: Ice melts on up-ice side where pressure is high. Regelelation occurs on the down-ice side were pressure is low. Theoretically latent heat released by freezing assists in further melting up ice.
• Occur beneath warm-based glaciers.
• Velocity decreases with obstacle size.
• Importance: Regelation freezes debris to the base of the ice.

Role of water in basal sliding

• inhbits adhesion of the ice to the bed
• necessary for regelation
• Importance of high subglacial water pressure
  • smooths the bed by submerging tiny obstacles that would normally create drag
  • causes a unequal distribution of stress which becomes concentrated on the stoss of of obstacles thereby increasing enhanced basal creep
  • lifts the glacier off the bed (hydraulic jacking)
  • exerts a force against the down-ice wall of subglacial cavities (hydraulic jack mechanism)
  • very high basal water pressure can cause the glacier to become unstable and surge

• Movement occurs within saturated basal till layer where high pore water pressure decreases the shear strength of the sediment.
• deformation takes place within top layers of sediment where effective stress is less
• occurs where meltwater cannot be efficiently removed from the bed

• vertical changes in velocity:
  • The greatest velocity occurs at the top of the glacier. However the greatest rate of deformation occurs at the base of the glacier. This is because the each layer is riding on the layer beneath it.
• vertical variations longitudinal direction
  • The accumulation zone experiences a downward component of vertical flow whereas in the ablation zone flow is upward.

Lateral (transverse) variations: Variations in flow that occur across the glacier and perpendicular to flow.

Diverging flow:

• Characteristics:
  • Flowlines diverge; lateral stresses are tensional; results in decrease in forward velocity
• Occurs where:
  • there is a transverse gradient in the slope of the ice (e.g. zone of ablation in a valley glacier)
  • streaming ice becomes unconfined

Converging flow:

• Characteristics:
  • Flowline converge; lateral stresses are compressive; results in an increase in velocity
• Occurs
  • in the zone of accumulation of an alpine glacier or
  • where unconfined flow becomes confined or channeled into a valley
• velocity: In alpine glaciers the greatest velocity occurs over the deepest portion of the channel (usually the center). Why?

variations in velocity:

• longitudinal variations in flow velocity reflect changes in discharge (flow increases down-ice to the equilibrium line and then decreases), slope, and cross-sectional area

Horizontal variations in flow:

Compressive Flow:

• Characterized by compressive longitudinal stresses
• Occurs in
  • the ablation zone,
  • where the bedrock floor is concave, and
  • where ice flows around an obstruction (e.g. enhanced basal creep) 

Extensional Flow:

• Characterized by tensional longitudinal stresses
  • occurs in
    • the zone of accumulation,
    • where there is an increase in gradient

sheet flow:

• Flow typical of unrestricted glaciers such as ice caps and ice sheets
• glacial flow is largely independent of underlying topography
• friction is only felt at the base of the glacier

streaming flow:

• constricted flow, such as in valleys
• glacial flow may or many not be controlled by underlying topography
• friction is felt at the base and margins of the glacier; flow is greatest at the center and decreases toward the margins (reflected in surface velocity profiles)

---

Effects of rock debris on flow:

• surface debris- acquire by avalanching-increases flow
• internal debris-decreases flow (debatable)
  
  

• The Crevasses Zone: Glacial velocity and surface elevation measurements in the Juneau Ice Fields, Scott Magee, Juneau Ice Fields Research Program

[Glacial and Quaternary Geology] [extended GeoIndex][QkRef][Geological Sciences] [Degree Programs] [Salem State College]
Lindley Hanson (email) Last Modified 3/20/03