The stress-dependent rheology of watery, temperate ice in tertiary creep
Date
2023-12
Authors
Schohn, Collin Michael
Major Professor
Advisor
Iverson, Neal R
Reber, Jacqueline E
Moore, Peter L
Committee Member
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Altmetrics
Abstract
Temperate ice constitutes the fastest-flowing parts of ice sheets and most of the ice of valley glaciers. Liquid water between ice grains may exceed 1% by volume. Although temperate, watery ice is thought to play a major role in fast glacier flow, no laboratory experiments have been conducted to tertiary (steady) creep with temperate ice that contains greater than 1% water. A single earlier study in which ice was sheared to tertiary creep at water contents no greater than 0.8% indicated ice softening by water, with an assumed stress exponent of n=3 in the power-law creep relation for ice.
In this study, a large ring-shear device is used to shear lab-made, polycrystalline ice at its pressure-melting temperature to tertiary creep (strains > 0.07), under confined compression and at either a constant stress or strain rate. The ice ring is 0.9 m in outside diameter, ~0.2 m thick, and 0.2 m wide, allowing realistic mean grain sizes to be studied (7.8 ± 5.6 mm). A tiltmeter frozen into the ice records strain rate to avoid measuring slip of warm ice across platen surfaces during shear. Watery ice (1.38±0.41%) is sustained with high confining pressure (~1.5 MPa). Water content is measured by inducing freezing fronts at the walls of the ice chamber, recording with thermistors arrival times of the cold wave as it moves radially inward, and then finding the water-content value that best-fits the solution of the relevant Stefan problem to arrival times.
Results indicate linear-viscous deformation (n=1.0) at stresses less than 0.16 MPa and nonlinear deformation with n=3.1 at higher stresses up to the limit of the apparatus, 0.24 MPa. At low stresses (≤0.16 MPa), pressure-melting and refreezing at grain boundaries (grain-scale regelation) (n=1)—likely rate-limited by heat flow across grains driven by stress and associated temperature heterogeneity at grain boundaries—dominates deformation. The shape-preferred orientation of grains plunges, on average, ~15º up-shear, consistent with refreezing and associated grain elongation parallel to the least compressive principal stress at 45º, accompanied by mechanical rotation during shear. Crystalline a-axes are subparallel to grain long axes, consistent with the expectation that refreezing of water should be most rapid along a-axes. Above shear stresses of 0.16 MPa, the influence of dislocation creep (n=4) increases, driving the stress exponent higher to the observed value of n=3.1.
Thus, two stress-dependent, micro-deformation mechanisms are inferred for watery ice: grain-scale regelation (n=1) and dislocation creep (n=4). Grain sizes, stresses, strain rates and water contents studied here require no extrapolation for application to temperate parts of glaciers. Ice in shear margins of ice streams and near the beds of glaciers is commonly under deviatoric stresses less than 0.16 MPa and is expected to be watery owing to high rates of shear heating. Use of n=1, rather than n=3 or n=4, in modeling temperate glacier ice is, therefore, recommended and could yield major differences in glacier flow velocity in response to environmentally-induced changes in stress.
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