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Glacier Motion. chapter 4. Glacier flow. “Without the flow of ice, life as we know it would be impossible.” Observed since 1700s Quantified: physical / mathematical relations. Glacier movement. First studied in the Alps James Forbes, Mer de Glace above Chamonix, 1842

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glacier flow
Glacier flow
  • “Without the flow of ice, life as we know it would be impossible.”
  • Observed since 1700s
  • Quantified: physical / mathematical relations
glacier movement
Glacier movement
  • First studied in the Alps
      • James Forbes, Mer de Glace above Chamonix, 1842
      • Louis Agassiz & students – mapped the movements of Rhone Glacier, 1874 – 1882
      • silver mine of middle ages near Chamonix is now buried by Argentierre Glacier
      • all were larger in 1500s to 1800s: Little Ice Age
          • 1850 1900
glacier movement5
Glacier movement
  • Motion
    • glaciers flow, expand, contract
    • all motion is forward / downslope, outward
      • (retreat is NOT “up-valley flow”)
    • motion usually not apparent: ~ 0.5 m to >300 m / yr
      • fastest where ice is thickest (~ ELA), w / water at base
      • slower at base of ice compared to top of glacier
    • velocity varies seasonally
      • winter – upper moves faster (new snow)
      • summer – lower part moves faster due to more ablation & less resistance
balance velocity and discharge
Balance velocity and discharge
  • Discharge thru each cross-section: Q (x) =  ( wx bx )
  • Balance (avg) velocity: v (x) = Q (x) / A (x)
    • not constant
  • (wedge diagram)
    • steeper mass balance gradient  more mass transfer  higher Q and v
glacier movement stress and strain
Glacier movement: stress and strain
  • Motion
    • brittle fracture vs plastic flow
    • causes: gravity acting on ice mass on a slope
    • stress = forces pushing / pulling
      • normal stress σ = i g d
      • shear stress  = i g d sin 
      • effective shear strength * = c’ + (pi – pw) σ tan φ
      • all proportional to depth (within glacier or at bed)
    • strain = deformation of a body due to stresses
what is flow
What is “flow”?
  • Manifestations of deformation (strain)
  • Mode
    • elastic
    • brittle
    • ductile
  • Character
    • homogeneous
    • inhomogeneous
  • Shear
    • pure
    • simple
glacier movement9
Glacier movement
  • Motion
    • zones of a glacier
      • zone of fracture: brittle ice
        • crevasses: tension cracks, top ~ 30 – 60 m depth
      • zone of flow – plastic behavior (internal deformation)
        • ice crystals slide past one another
        • especially if water present
        • in accum zone: flow down toward the bed
        • in abl’n zone: flow upward & outward
        • irregular movement, so cracks form in the ice above
glacier movement11
Glacier movement
  • Motion
    • zones of a glacier
      • zone of fracture: brittle ice
        • crevasses: tension cracks, to ~ 30 – 60 m in depth
      • zone of flow: plastic behavior (internal deformation)
        • ice crystals slide past one another
        • especially if water present
        • in accum zone: flow down toward the bed
        • in abl’n zone: flow upward & outward
        • irregular movement, so cracks in ice above it
    • causes of flow: gravity
brittle deformation crevasses
Brittle deformation – crevasses
  • Long observed
  • Results from rapidly-applied stress
  • Form many distinctive patterns
mechanics of crevassing
Mechanics of crevassing
  • Observed patterns relate observed strain directly to the mechanics of stress couples
crevasse examples
Crevasse examples
  • Depth <30 – 40 m
  • Tensional and marginal
  • Terminal splays
  • Complex systems
glacier movement19
Glacier movement
  • Motion
    • zones of a glacier: brittle fracture vs plastic flow
    • causes of flow: gravity acting on ice mass on a slope
      • temperate glacier will begin to flow when ~ 20 m deep on a 15° slope
  • Movement types
    • most depend on the state & flow of heat among the glacier – ground – air – water
what is flow really
What is “flow”, really?
  • Slip (planar)
    • external
    • internal – intragranular
  • Creep (intergranular)
  • Phase change (recrystallization)
glacier movement24
Glacier movement
  • Movement types
    • internal deformation
      • plastic flow: internal creep
        • melting & refreezing of ice crystals under stress
        • sliding past one another
      • faulting and folding
      • can vary up- / down-glacier with gross velocity (compressional vs extensional flow)
    • basal sliding
    • deformation of soft subglacial sediments
glacier flow25
Glacier flow
  • Creep quantified: Glen’s Flow Law (Nye)
    • strain rate is proportional to shear stress
      • έ = A τn
      • A = f (temp); 7x10-18 to 7x10-15 (at 0°C)
      • n = f (crystallinity ?); 1.5–4.2, use ~ 3
      • shear stress proportional to height (depth) in glacier
    • (V = k T3 – ?)
glacier movement26
Glacier movement
  • Movement types
    • internal deformation
      • plastic flow: internal creep
      • faulting and folding
    • basal sliding
      • basal ice is near the pressure-melting point,  water at the base of many glaciers  lubrication
      • enhanced basal creep around bumps  efficient flow
      • regelation creep: melting  refreezing
      • temperate glaciers slide more than polar glaciers
    • deformation of soft sediments below bed of glacier
slide27

Cold

Warm

Polythermal

Thermal Classification

J.S. Kite, WVU

glacier movement30
Glacier movement
  • Movement types
    • internal deformation
    • basal sliding
    • deformation of soft sediments below bed of glacier
  • “Normal” glacier speeds ~ 0.5 m – >300 m / yr
  • Surging glaciers: moving faster
planforms of observed flow
Planforms of observed flow
  • Stakes across glacier
  • Resurvey across time
observed flow plan and profile
Observed flow: Plan and profile
  • Plan View
    • parabolic
    • septum (ice streams)
  • Profile
    • exponential
    • non-zero at the bed
modes of profile flow
Modes of profile flow
  • Total velocity =
  • Internal velocity
    • laminar
    • sum of processes
  • + Basal slip
    • not if frozen to bed
  • + Bed deformation
    • if not rock
observed bed deformation

Shear Plane?

Observed bed deformation
  • Inferred from structures in till
  • Measured from markers emplaced in basal sediment and recovered
structures of glaciers
Structures of glaciers
  • What structures do you see here?[Grinnell Glacier]
  • Lenses, layers, fractures…
  • How do they form?
schematic mountain glacier
Schematic mountain glacier
  • Plan view
  • Cross-section
schematic mountain glacier37
Schematic mountain glacier
  • Detailed section
  • Terminus
example malaspina glacier
Example – Malaspina Glacier
  • Note accommodation of Malaspina and Agassiz glaciers into increasing space
  • Longitudinal compression
unsteady flow i
Unsteady Flow I
  • Flow is NOT constant
  • Varies with season (snow load increases the strain rate)
  • Varies with bed resistance = f(water)?
  • Varies unpredictably!
unsteady flow iii kinematic waves
Unsteady Flow III – Kinematic Waves
  • Thickening increases depth linearly
  • Depth increases stress linearly
  • Stress increases strain (flow) exponentially
  • Therefore, a pulse propagates through the glacier
unsteady flow iv surges
Unsteady Flow IV – Surges
  • Many glaciers (~10%) surge
    • Stagnant for years
    • Increase in thickness
  • Surge!
    • Decouple from the bed?
    • Surface fracturing
    • Thrusting?
glacier movement43
Glacier movement
  • “Normal” glacier speeds ~ 0.5 m – >300 m / yr
  • Surging glaciers: fast moving
    • up to 110 m / day
      • (Kutiah Glacier, Pakistan – 11 km in 3 months)
    • lasts 2 – 3 years
    • Hubbard Glacier, 1987 – Alaska
      • went from ~30–100 m / yr  5 km / yr
    • causes
glacier movement44
Glacier movement
  • “Normal” glacier speeds ~ 0.5 m – >300 m / yr
  • Surging glaciers: fast moving – 100s of m / day
    • causes – not certain / more than one cause
      • polar glacier becomes uncoupled from bed
      • stagnant ice dams up water in back, and floats the glacier; when water drains out, the surge stops
      • heavy precip = more accumulation
      • heavy avalanches = more accumulation
      • silting up of glacial tunnels and floating glacier – lots of lakes on surfaces before surge movement
one more thing
One more thing …
  • Prediction of ice-sheet profiles (Nye, 1952)
  • Assume ice is a perfect plastic
    • yield strength ~ 100 kPa (± 50 kPa)
    • horizontal bed
    • altitude of ice surface at s inland from margin
      • h = (2 h0 s) 0.5
      • h0 =  / i g  11  h = (22 s) 0.5
      • all in meters (can add sin  term for sloping bed?)
    • predicts parabolic profile
  • Good (not perfect) agreement with observed profiles