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Author for correspondence: Dougal D. Hansen, E-mail: [email protected]
1. 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). However, 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). For temperate ice, classic regelation and creep theory predicts the presence of a film due to ice pressure melting along grain boundaries (Nye, 1969, 1973; Kamb, 1970). However, considerations of premelting indicate a liquid film exists even at subfreezing temperatures (Gilpin, 1979; Dash and others, 1995; Rempel and Worster, 1999; Rempel and others, 2001; Rempel and others, 2004; Dash and others, 2006). Since ice is nowhere in contact with the clast, stresses in the ice must be transmitted to the rock through the fluid surrounding the clast. Fundamentally, discrepancies between interfacial models stem from their assumptions regarding the distribution of water pressure along the clast boundary and its controlling mechanisms.
To date, the competing models of Boulton (1974) and Hallet (1979b, 1981) have driven theoretical framing of subglacial rock friction (Fig. 1). Boulton (1974) first proposed that the bed-normal contact force beneath an abrading clast, Fc, is the product of the effective pressure in the film, N (i.e. cryostatic pressure, Pi, minus the water pressure, Pl) and the surface area of the particle, A, such that 1\[F_{\rm c} = F_{{\rm eff}} = AN{\rm,} \]where Feff is the contact force due to effective stress acting on the...