EGU General Assembly 2020
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the Creative Commons Attribution 4.0 License.

A slip law for glaciers on deformable beds

Lucas Zoet1 and Neal Iverson2
Lucas Zoet and Neal Iverson
  • 1University of Wisconsin - Madison, Madison, United States of America (
  • 2Iowa State University, Ames, United States of America (

Slip of marine-terminating ice streams over beds of deformable till is responsible for most of the contribution of the West Antarctic Ice Sheet to sea-level rise. Flow models of the ice sheet and till-bedded glaciers elsewhere require a law that relates slip resistance, slip velocity, and water pressure at the bed. We present results of the first experiments in which pressurized ice at its melting temperature is slid of over a water-saturated till bed. Steady-state slip resistance increases with slip velocity owing to sliding of ice across the bed, but above a threshold velocity till shears at its rate-independent, Coulomb strength. These results motivate a generalized slip law for glacier-flow models that combines processes of hard-bedded sliding and bed deformation.

How to cite: Zoet, L. and Iverson, N.: A slip law for glaciers on deformable beds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12364,, 2020

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Presentation version 1 – uploaded on 21 Apr 2020
  • CC1: Comment on EGU2020-12364, Matteo Spagnolo, 08 May 2020

    Hi Lucas,

    I really enjoyed your presentation and experimental work. Is there a more in-depth analysis of the sediment deformation that is/will be published somewhere, besides what you already presented here?


    • AC1: Reply to CC1, Lucas Zoet, 08 May 2020

      Hi Matteo,


      Thanks for checking out the work.  Yes, using some results from this device and some from a new device at UW-Madison we've started analyzing the structures in the till. This is actually the subject of a virtual talk I'll be giving at North Central - GSA, which should be online around the May 18.  I can send you recording directly if you would like to see it.  But we’ve analyzed the till for easily observable structures that can observed in field and we also collected AMS and microstructure samples.  While we’ve collected those AMS and microstructure samples, we haven’t been able to analyze them yet because we’ve been kick out of the lab for the last 2 months.  Here is a link to the talk if you’re interested

      • CC2: Reply to AC1, Matteo Spagnolo, 08 May 2020

        Perfect, thank you Lucas and good luck with returning to the lab,


  • CC3: Comment on EGU2020-12364, Lev Tarasov, 08 May 2020

    Wondeful work folks in moving from our idealized approaches to data-grounded physics. Any insights or plans to investiigate one more complication that I've long worried about: the impact of pinning points (protruding through till) and dependence on the spatial scale thereof?  Is there any reason to suspect that a simple minded fractional area weighting of hard/soft bed drag contributions could be seriously off given resultant stress concentrations, flow of clasts around pinning points,...?


    • AC2: Reply to CC3, Lucas Zoet, 08 May 2020

      Hi Lev,

      These are good questions.  We could incorporate a mixed bed, like you talk about, in the experiment, but we’d have think carefully about how to scale such a thing.  Right now I don’t have plans to do exactly that but it could be worked into an experiment. 

      It could be as simple as a weighted average as you say but the drag response on the hard part if it were capable of supporting cavities could be nuanced with a different response than that of stuck clasts.  If the rigid part were small relative to deformable bed it not would likely matter much for the overall drag response but if it were large one could find themselves in a situation where a weighted average might be problematic because of the potential for different slip responses on hard sections would in part depend on the morphology of the hard section.