Glacier Motion chapter 4 Glacier flow n n

Glacier Motion chapter 4

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

Glacier movement n First studied in the Alps James Forbes, Mer de Glace above Chamonix, 1842 n Louis Agassiz & students – mapped the movements of Rhone Glacier, 1874 – 1882 n silver mine of middle ages near Chamonix is now buried by Argentierre Glacier n all were larger in 1500 s to 1800 s: Little Ice Age n n 1850 1900

Rhone Glacier?

Glacier movement n Motion glaciers flow, expand, contract n all motion is forward / downslope, outward n n n (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 n slower at base of ice compared to top of glacier n n velocity varies seasonally winter – upper moves faster (new snow) n summer – lower part moves faster due to more ablation & less resistance n

Balance velocity and discharge n n Discharge thru each n (wedge diagram) cross-section: n steeper mass balance gradient Q (x) = ( wx bx ) more mass transfer Balance (avg) velocity: higher Q and v v (x) = Q (x) / A (x) n not constant

Glacier movement: stress and strain n Motion n brittle fracture vs plastic flow n causes: gravity acting on ice mass on a slope n stress = forces pushing / pulling normal stress σ = i g d n shear stress = i g d sin n effective shear strength * = c’ + (pi – pw) σ tan φ n all proportional to depth (within glacier or at bed) n n strain = deformation of a body due to stresses

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

Glacier movement n Motion n zones of a glacier n zone of fracture: brittle ice n n crevasses: tension cracks, top ~ 30 – 60 m depth zone of flow – plastic behavior (internal deformation) n n n 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 movement n Motion n zones of a glacier n zone of fracture: brittle ice n n zone of flow: plastic behavior (internal deformation) n n n crevasses: tension cracks, to ~ 30 – 60 m in depth 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 n n n Long observed Results from rapidly-applied stress Form many distinctive patterns

Mechanics of crevassing n Observed patterns relate observed strain directly to the mechanics of stress couples

Crevasse examples n n Depth <30 – 40 m Tensional and marginal Terminal splays Complex systems

Crevasse examples

Icefalls

Icefalls

Glacier movement n Motion zones of a glacier: brittle fracture vs plastic flow n causes of flow: gravity acting on ice mass on a slope n n n temperate glacier will begin to flow when ~ 20 m deep on a 15° slope Movement types n most depend on the state & flow of heat among the glacier – ground – air – water

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

Kenneth G. Libbrecht, Caltech

Hermann Engelhardt Caltech

Hermann Engelhardt Caltech

Glacier movement n Movement types n internal deformation n plastic flow: internal creep n n melting & refreezing of ice crystals under stress sliding past one another faulting and folding n can vary up- / down-glacier with gross velocity (compressional vs extensional flow) n basal sliding n deformation of soft subglacial sediments n

Glacier flow n Creep quantified: Glen’s Flow Law (Nye) n strain rate is proportional to shear stress n έ=Aτn A = f (temp); 7 x 10 -18 to 7 x 10 -15 (at 0°C) n n = f (crystallinity ? ); 1. 5– 4. 2, use ~ 3 n shear stress proportional to height (depth) in glacier n n (V = k T 3 – ? )

Glacier movement n Movement types n internal deformation plastic flow: internal creep n faulting and folding n n basal sliding basal ice is near the pressure-melting point, water at the base of many glaciers lubrication n enhanced basal creep around bumps efficient flow n regelation creep: melting refreezing n temperate glaciers slide more than polar glaciers n n deformation of soft sediments below bed of glacier

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

Basal sliding (regelation) Univer Aber.

Glacier movement n Movement types internal deformation n basal sliding n deformation of soft sediments below bed of glacier n “Normal” glacier speeds ~ 0. 5 m – >300 m / yr n Surging glaciers: moving faster n

Planforms of observed flow n n Stakes across glacier Resurvey across time

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

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

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

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

Schematic mountain glacier n n Plan view Cross-section

Schematic mountain glacier n n Detailed section Terminus

Example – Malaspina Glacier n n Note accommodation of Malaspina and Agassiz glaciers into increasing space Longitudinal compression

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

Unsteady Flow II - Ogives

Unsteady Flow III – Kinematic Waves n n Thickening increases depth linearly Depth increases stress linearly Stress increases strain (flow) exponentially Therefore, a pulse propagates through the glacier

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

Glacier movement “Normal” glacier speeds ~ 0. 5 m – >300 m / yr n Surging glaciers: fast moving n n up to 110 m / day n (Kutiah Glacier, Pakistan – 11 km in 3 months) lasts 2 – 3 years n Hubbard Glacier, 1987 – Alaska n n n went from ~30– 100 m / yr 5 km / yr causes

Glacier movement “Normal” glacier speeds ~ 0. 5 m – >300 m / yr n Surging glaciers: fast moving – 100 s of m / day n n causes – not certain / more than one cause n n n 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

Surging Terminus

Summary of Flow Process I

Summary of Flow Process II

One more thing … Prediction of ice-sheet profiles (Nye, 1952) n Assume ice is a perfect plastic n yield strength ~ 100 k. Pa (± 50 k. Pa) n horizontal bed n altitude of ice surface at s inland from margin n n n h = (2 h 0 s) 0. 5 h 0 = / 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

Remember – flow is one-way!