M3 Slow Flow (Creep)

Topic

Slow flow is the mechanism by which ice deforms plastically under its own weight, without any sliding occurring at the base of the ice. This mode of slow flow dominates in cold glaciers—those with a base maintained below the melting point—and also in the interior regions of large polar ice sheets, far from the faster ice streams that cut through them. Deformation via slow flow occurs primarily because defects in the ice's crystal lattice shift under the applied stress; the cumulative displacement of these defects gradually produces macroscopic deformation throughout the glacier.

The same non-linear relationship between stress and strain described for ice in general—when integrated vertically across the thickness of a thin ice layer resting on a flat bed—reveals that the surface velocity resulting from this internal deformation increases with the fifth power of the ice thickness: if a glacier's thickness doubles, its flow velocity due to internal deformation increases by a factor of nearly thirty-two. Consequently, a thick glacier flows far more rapidly than a thin glacier resting on the same slope.

Ice temperature also strongly influences this flow velocity, following the same thermal dependency observed in ice generally: a glacier with an internal temperature of minus five degrees flows roughly three times more slowly than a glacier of identical geometry with an internal temperature of zero degrees.

Solifluction is the permafrost equivalent of this slow flow: water-saturated soil lying directly above the permafrost layer deforms slowly downslope, behaving much like a highly viscous liquid. Solifluction can occur on slopes of merely one or two degrees and is capable of displacing soil by several centimeters per year. Regelation, for its part, is a distinct mechanism of slow flow operating at the base of a temperate glacier: ice melts under the additional pressure encountered when meeting a bedrock obstacle, flows around that obstacle in liquid form, and refreezes once the excess pressure dissipates on the other side, thereby allowing the entire glacier to slide past irregularities in its bed. The rate of regelation flow increases with a steeper pressure gradient around the obstacle and higher thermal conductivity of the bedrock itself, thus—alongside internal deformation and solifluction—completing the three slow-flow mechanisms with which this description began.

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