Experimental constraints on subglacial rock friction

Hansen, Dougal; Zoet, Lucas K.

doi:10.1017/aog.2019.47

Cited by 11 publications

(29 citation statements)

References 69 publications

(151 reference statements)

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“…This result highlights the importance of the state of drainage at the bed and may explain why mild sensitivity to normal stress on the ice, although not detected here, has been recorded in some past experiments (Hansen & Zoet, 2019;Zoet et al, 2013). These experiments had smaller beds, and in one case, holes drilled through the rock bed (Hansen & Zoet, 2019). Maximum lengths of flow paths in the film along the bed to atmospheric pressure were a factor of ∼4.0 shorter than in the current experiments, such that an increase in normal stress on the ice would have produced a smaller increase in cavity water pressure and hence a greater increase in contact force and friction.…”

Section: 1029/2020jf005718supporting

confidence: 78%

“…Larger clasts would be too large a fraction of the ice chamber's width to inhibit wall effects, and smaller particles would make observations of bounding ice difficult. The till clasts are primarily subrounded (Powers, 1953) and generally less angular than gravel‐sized clasts not sourced from till used in past debris‐bed friction experiments (Hansen & Zoet, 2019; Iverson, 1990). The number of clasts in each size range is chosen to follow that observed by Hooke and Iverson (1995) in basal tills, using a fractal dimension of 2.9.…”

Section: Methodsmentioning

confidence: 99%

“…However, in these experiments with clasts in ice, as well as in other experiments with glacier ice containing finer debris particles (< 1.25 mm) (Zoet et al, 2013), a weak dependence on total normal stress on the ice was measured; bulk friction coefficients for debris‐ice mixtures were 0.03–0.17. Moreover, in previous experiments with cobble‐ and gravel‐sized spheres and clasts, water‐filled cavities formed to various extents between these inclusions and the bed (Hansen & Zoet, 2019; Iverson, 1990). Evidence of regelation ice was universally absent in these experiments.…”

Section: Introductionmentioning

confidence: 93%

“…Journal of Geophysical Research: Earth Surface past debris-bed friction experiments (Hansen & Zoet, 2019;Iverson, 1990).…”

Section: 1029/2020jf005718mentioning

confidence: 99%

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Controls on Subglacial Rock Friction: Experiments With Debris in Temperate Ice

2020

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Glacier sliding has major environmental consequences, but friction caused by debris in the basal ice of glaciers is seldom considered in sliding models. To include such friction, divergent hypotheses for clast-bed contact forces require testing. In experiments we rotate an ice ring (outside diameter = 0.9 m), with and without isolated till clasts, over a smooth rock bed. Ice is kept at its pressure-melting temperature, and meltwater drains along a film at the bed to atmospheric pressure at its edges. The ice pressure or bed-normal component of ice velocity is controlled, while bed shear stress is measured. Results with debrisfree ice indicate friction coefficients < 0.01. Shear stresses caused by clasts in ice are independent of ice pressure. This independence indicates that with increases in ice pressure the water pressure in cavities observed beneath clasts increases commensurately to allow drainage of cavities into the melt film, leaving clast-bed contact forces unaffected. Shear stresses, instead, are proportional to bed-normal ice velocity. Cavities and the absence of regelation ice indicate that, unlike model formulations, regelation past clasts does not control contact forces. Alternatively, heat from the bed melts ice above clasts, creating pressure gradients in adjacent meltwater films that cause contact forces to depend on bed-normal ice velocity. This model can account for observations if rock friction predicated on Hertzian clast-bed contacts is assumed. Including debris-bed friction in glacier sliding models will require coupling the ice velocity field near the bed to contact forces rather than imposing a pressure-based friction rule.

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Section: 1029/2020jf005718supporting

confidence: 78%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 93%

“…Journal of Geophysical Research: Earth Surface past debris-bed friction experiments (Hansen & Zoet, 2019;Iverson, 1990).…”

Section: 1029/2020jf005718mentioning

confidence: 99%

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Controls on Subglacial Rock Friction: Experiments With Debris in Temperate Ice

2020

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“…These processes result in a logarithmic healing curve for tectonic faults. Creep of ice is a well‐established mechanism for ice deformation around larger asperities that characterize the glacier bed (Zoet & Iverson, 2016), but for debris‐laden ice, the clast‐bedrock contacts dominate sudden frictional slip (Hansen & Zoet, 2019; Thompson et al, 2020; Zoet, Carpenter, et al, 2013). In order to assess the relationship between healing at the ice‐bed interface during times of no slip (stick) and the stress drop associated with the sudden slip, the logarithmic healing rate measured in Zoet, Carpenter, et al (2013) for the same experimental conditions was applied (Figure 3b).…”

Section: Discussionmentioning

confidence: 99%

Application of Constitutive Friction Laws to Glacier Seismicity

Zoet

Ikari

Alley

et al. 2020

Geophysical Research Letters

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While analysis of glacial seismicity continues to be a widely used method for interpreting glacial processes, the underlying mechanics controlling glacial stick-slip seismicity remain speculative. Here, we report on laboratory shear experiments of debris-laden ice slid over a bedrock asperity under carefully controlled conditions. By modifying the elastic loading stiffness, we generated the first laboratory icequakes. Our work represents the first comprehensive lab observations of unstable ice-slip events and replicates several seismological field observations of glacier slip, such as slip velocity, stress drop, and the relationship between stress drop and recurrence interval. We also observe that stick-slips initiate above a critical driving velocity and that stress drop magnitude decreases with further increases in velocity, consistent with friction theory and rock-on-rock friction laboratory experiments. Our results demonstrate that glacier slip behavior can be accurately predicted by the constitutive rate-and-state friction laws that were developed for rock friction. Plain Language Summary Glacier beds and tectonic faults may at first appear to be quite different, but they share important characteristics. In both cases, motion may be smooth (aseismic creep) or earthquake-producing "stick-slip." A powerful physical constitutive relationship called rate-and-state friction has been developed to understand earthquakes and smooth slip on tectonic faults. Laboratory experiments reported here simulate glacier-bed motion by sliding debris-bearing ice over a rock plate under conditions that are typical for glacier beds. They produce the first laboratory icequakes. Transitions between steady and stick-slip motions are generated by controlling shearing velocity and other conditions, as predicted by rate-and-state friction theory. Future studies can thus apply this physical framework to glacier slip, helping to understand ice motion and its potential to accelerate sea level rise in a warming world. Furthermore, because motion at the glacier bed is often much easier to study than tectonic faults, additional observations of glaciers may provide useful insights into earthquake behavior.

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