Experimental constraints on subglacial rock friction

Dougal D. Hansen,Lucas K. Zoet

doi:10.1017/aog.2019.47

Abstract

AbstractSubglacial rock friction is an important control on the sliding dynamics and erosive potential of hard-bedded glaciers, yet it remains largely unconstrained. To explore the relative influence of basal melt rate, effective stress and ice temperature on frictional resistance, we conducted abrasion experiments in which limestone beds were slid beneath a fixed slab of ice laden with granitic rock fragments. Shear stress scales linearly with melt rate and cryostatic stress, confirming that both viscous drag and effective stress are first-order controls on the contact force in drained conditions. Furthermore, temperature gradients in the ice increase the contribution of viscous drag on basal shear stress. In all experiments, the relationship between melt rate and shear stress is best explained by a model that accounts for the effects of regelation and viscous creep on the bed-normal drag force. We interpret this to mean fluid flow around entrained clasts contributed to basal drag even at subfreezing temperatures. Incorporating premelting dynamics into the Watts/Hallet model for subglacial rock friction, we find that the predicted debris-bed drag decreases by approximately an order of magnitude, with a corresponding ~3.5 × increase in the transition radius. This is lower than we observe for ice slightly below the pressure melting point.

Highlights

Friction between entrained basal debris and the bed affects the erosion rates (Schweizer and Iken, 1992; Hallet and others, 1996; Koppes and Hallet, 2006; Herman and others, 2015; Koppes and others, 2015), sliding speed (Hallet, 1981; Shoemaker, 1988) and slip stability of glaciers (Zoet and others, 2013; McCarthy and others, 2017; Lipovsky and others, 2019)
We conducted drained laboratory abrasion experiments to investigate the relative influence of basal melt rate and effective stress on subglacial rock friction in two thermal regimes
Basal shear stress scales linearly with both melt rate at the ice–bed interface and applied normal stress, and slight depressions in ice temperature increase the contribution of bed-normal viscous drag

Summary

Introduction

Friction between entrained basal debris and the bed affects the erosion rates (Schweizer and Iken, 1992; Hallet and others, 1996; Koppes and Hallet, 2006; Herman and others, 2015; Koppes and others, 2015), sliding speed (Hallet, 1981; Shoemaker, 1988) and slip stability of glaciers (Zoet and others, 2013; McCarthy and others, 2017; Lipovsky and others, 2019). Basal ice is typically assumed to be debris-free in glacier flow models, meaning basal resistance arises solely from ice deformation around bedrock obstacles (Lliboutry, 1968; Nye, 1969; Iken, 1981; Zoet and Iverson, 2015, 2016). Observation shows that glaciers transport substantial bedload (Kirkbride, 2002), and theory and field measurements indicate subglacial rock friction may be significant, albeit poorly constrained (Iverson and others, 2003; Cohen and others, 2005). A central assumption in theories describing subglacial rock friction is that inclusions in ice are surrounded entirely by a thin water film (Boulton, 1974; Hallet, 1979b, 1981; Hindmarsh, 1996; Cohen and others, 2005; Emerson and Rempel, 2007). Discrepancies between interfacial models stem from their assumptions regarding the distribution of water pressure along the clast boundary and its controlling mechanisms

Methods

Results

Discussion

Conclusion