Study of Glacier flow. Glacier Flow and Ice Profiles 10/8/09. Downslope transport of ice mass Accumula'on vs abla'on

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1 Glacier Flow and Ice Profiles What can we learn about glacier movement and subglacial processes? Study of Glacier flow Based on three lines of observamon: 1. Field evidence 2. Laboratory experiments 3. TheoreMcal arguments Defining the deformamon, viscous flow, and sliding of glacial ice under the force of gravity. UMass Geo 563, Fall 2009, 1&6 October Svalbard, 2005 Downslope transport of ice mass Accumula'on vs abla'on Glaciers transfer mass from the Accumulation zone to the ablation zone. Most rapid velocities reached at the ELA Why? Glaciers maintain their shape by flowing to carry away what has accumulated necessary to adjust their shape to maintain flow Ice has rheological propermes (smffness) Flow is determined by forces (stresses) applied Cartoons from ELA Accum Area dominated by extention, with deformation driving ice toward the bed of the glacier; Ablation area dominated by compressional flow. 1

2 Zone of extension driving particles downward ice Zone of compression driving particles upward ice Cartoons from 2

3 Field observations of motion: Complex crevasses shown on an active glacier. Note lower figure with strain ellipses Fractures perpendicular to strain. Can use insar methods to infer ice velocities and strain rates insar =interferometric Synthetic Aperature Radar Flow varies with depth over time Worthington Glacier Harper et al., 1998 Science Example from Ross Ice Shelf Joughin and Tulaczyk, 2002, Science. Crevasses can be tracked with repeated imagery. Scale is 0 to 800 meters / yr 3

4 What determines Glacier motion? Ice DeformaMon (creep) Ice flow law Temperature Fabric, grain size Chemistry, impurimes ions, bubbles, rocks parmcles MoMon over the Bed Basal sliding requires water Bed deformamon requires deformable substrate Movement at the surface (U s ) is the accumulamve effect of these processes, acmng singularly or in combinamon! Requires: Viscous flow, ElasMc response Brible behavior Avalanching Fracture / failure Stress Force per unit area ice in glacier subjected to weight of ice and the slope of of the ice surface Force = Mass x gravitational acceleration Strain change in shape of a material due to Stress. - elastic response recoverable deformation - permanent deformation brittle or ductile change with flow or creep Ed Waddingto nnotes Strain Rate = a b/a = x/yr Where a= original length b= remeasured length afer some interval of Mme 4

5 Ice Deforms as a non-linear viscous material Creep behavior of ice Creep movements within or between grains; May involve gliding by cleavage planes; recrystalization; and dislocations within crystalline lattice of the ice. See web.earthsci.unimelb.edu. au/wilson/ice1/index.html For movies of ice deformation Primary = strain rate decreases over time; grain hardening Secondary = steady state creep at minimum strain rate Tertiary = increasing creep with strain Final Steady state creep -- Glen s Flow Law Simple power relamonship, where strain ε s is described as : ε s = B σ n Where B = temperature dependent ice hardness parameter 0 deg C B= deg C B = deg C B= deg C b= σ = amt of Stress n= creep exponent (1=linear flow; = perfect plasmc but usually between 1.9 and 4.5 with a mean of 3) Cold ice more rigid and behaves slowly; temperate glacier ice deforms faster than polar glacier. 5

6 To model ice sheet flow think of ice as a slab spreading by creep under its own wt. Τ au = ρ g h (sin α) h α Tau for most glaciers = 0.5 to 1.0 bars = 50 to 100 kpa With 1 a best estimate close to yield strength of ice. BUT! For fast flow ice streams and outlet glaciers can be as low as 4-20 kpa Basal shear stress = ice density x gravitation acceleration x thickness x sin of the surface slope; Ice always flows in direction of surface slope. To compute ice thickness, Glacier bed condimons = h = Tau ρg(sin α) What does this mean? Normal Stress acmng in a vermcal plane and no water present: EffecMve Stress for situamons with water at the base: δ i = ρ i g h i δ e = ρ i g h i - p w 6