E2 Ice Mechanics

Topic

Ice is a rheologically complex material: it behaves as an elastic solid over short timescales—the same timescales on which seismic waves propagate or ice undergoes brittle fracture—yet acts as a viscous fluid with variable internal flow resistance over long timescales, such as those governing the downslope flow of an entire glacier. The fundamental relationship describing ice deformation under shear states that the deformation rate is proportional to the applied stress raised to a power of approximately three; this strongly non-linear relationship causes ice to flow much faster as stress increases, concentrating flow into narrow zones where deformation is far more intense than elsewhere in the glacier.

Furthermore, the ease with which ice flows under a given stress depends heavily on its temperature, following a relationship where slight temperature rises yield disproportionately large increases in flowability: ice near its melting point can be one hundred to one thousand times softer than the cold ice found in polar regions. This thermal dependence links ice mechanics to thermodynamics: the heat dissipated internally by viscous deformation warms the ice, which softens it further; this softening increases deformation, leading to even greater heat dissipation—thereby closing a self-reinforcing positive feedback loop. In the warmer basal zones of a glacier, this feedback concentrates deformation within a thin, heat-activated layer. On short timescales, the elastic properties of ice determine the speed at which both compressional and shear seismic waves propagate through it; this propagation speed is precisely what seismic sounding exploits to infer a glacier's thickness and internal structure without the need for drilling.

When ice undergoes sustained deformation, the polycrystalline ice forming a glacier develops a preferred crystallographic orientation: each hexagonal ice grain within the polycrystalline mass rotates under shear stress until its principal crystallographic axis aligns perpendicular to the shear plane. This preferred orientation can reduce the ice's resistance to continued flow by up to a factor of ten, further reinforcing the flow concentration within the shear zones mentioned at the beginning of this description.

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