Glacier Motion

1. Glacier Motion chapter 4

2. Glacier flow • “Without the flow of ice, life as we know it would be impossible.” • Observed since 1700s • Quantified: physical / mathematical relations

3. 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

4. Rhone Glacier?

5. 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

6. 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

7. 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

8. What is “flow”? • Manifestations of deformation (strain) • Mode • elastic • brittle • ductile • Character • homogeneous • inhomogeneous • Shear • pure • simple

9. 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

11. 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

13. Brittle deformation – crevasses • Long observed • Results from rapidly-applied stress • Form many distinctive patterns

14. Mechanics of crevassing • Observed patterns relate observed strain directly to the mechanics of stress couples

15. Crevasse examples • Depth <30 – 40 m • Tensional and marginal • Terminal splays • Complex systems

16. Crevasse examples

17. Icefalls

18. Icefalls

19. 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

20. What is “flow”, really? • Slip (planar) • external • internal – intragranular • Creep (intergranular) • Phase change (recrystallization)

21. Kenneth G. Libbrecht, Caltech

22. Hermann Engelhardt Caltech

23. Hermann Engelhardt Caltech

24. 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

25. 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 – ?)

26. 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

27. Cold Warm Polythermal Thermal Classification J.S. Kite, WVU

29. Basal sliding (regelation) Univer Aber.

30. 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

31. Planforms of observed flow • Stakes across glacier • Resurvey across time

32. Observed flow: Plan and profile • Plan View • parabolic • septum (ice streams) • Profile • exponential • non-zero at the bed

33. Modes of profile flow • Total velocity = • Internal velocity • laminar • sum of processes • + Basal slip • not if frozen to bed • + Bed deformation • if not rock

34. Shear Plane? Observed bed deformation • Inferred from structures in till • Measured from markers emplaced in basal sediment and recovered

35. Structures of glaciers • What structures do you see here?[Grinnell Glacier] • Lenses, layers, fractures… • How do they form?

36. Schematic mountain glacier • Plan view • Cross-section

37. Schematic mountain glacier • Detailed section • Terminus

38. Example – Malaspina Glacier • Note accommodation of Malaspina and Agassiz glaciers into increasing space • Longitudinal compression

39. Unsteady Flow I • Flow is NOT constant • Varies with season (snow load increases the strain rate) • Varies with bed resistance = f(water)? • Varies unpredictably!

40. Unsteady Flow II - Ogives

41. 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

42. Unsteady Flow IV – Surges • Many glaciers (~10%) surge • Stagnant for years • Increase in thickness • Surge! • Decouple from the bed? • Surface fracturing • Thrusting?

43. 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

44. 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

45. Surging Terminus

46. Summary of Flow Process I

47. Summary of Flow Process II

48. 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

49. Remember – flow is one-way!