Introduction Shear stress movement and/or failure of material Shear strength resistance to movement and/or failure Complications 1. Stress-related Interactions between particles in transport and substrate surfaces Stress concentrations –protuberances –sharp corners (eg. steps) –control of failure location Stress gradients –failure where gradients are high

2. Strength-related Spatial and temporal variations Three alternative explanations/models of subglacial friction Coulomb Hallet Boulton Coulomb model  f =(p i -P w )tan  where  f = basal friction, p i = overburden pressure, P w = basal water pressure,  =internal angle of friction Friction proportional to normal pressure unrealistic because assumes ridgidity?

Hallet model ice will deform around particles –contact force  normal load –determined by: buoyancy (ratio of particle density and ice density) velocity toward the bed –Ice flow toward the bed melting due to geothermal heat and sliding melting dues to regelation sliding vertical strain –High friction: heavy particles, high melting rates

Sandpaper model Schwizer and Iken –Close contact between particles means that ice cannot deform around them –Ice is a matrix cement –Adjustment of Coulomb equation  f =(p i -sP w )tan  –where s = proportion of bed occupied by particles –lower friction than Coulomb –appropriate for debris concentrations >50%

Abrasion Rock scouring by material held in basal ice Benn and Evans distinguish Grooving Polishing –difference in scale Stress build up, failure,movement, stress build-up –a jerky motion

Controls on efficiency Relative hardness most effective when the tools are harder than the bed abrasion rates inversely proportional to bed hardness (Boulton's experiments) Normal stress tensile stress increases with normal stress note different controls of normal stress –Coulomb, Hallet, Schweizer and Iken Velocity high velocity = greater abrasion per unit time clast velocity < ice velocity –(ice creep and frictional drag  depends on normal stress)

Availability of bed material flow towards the bed in some locations –upstream of protuberances –conditions of high basal melt elevation of basal debris above bumps –decreasing particle - bed contact Debris concentration more debris, more abrasion? increase and decrease in abrasion with debris content –because high friction retards particles maximum efficiency at debris concentrations of 10-30% (modelling by Hallet) Debris evacuation decrease in abrasion if products are not removed –entrainment by ice –regelation –water flow

Quarrying Processes similar to abrasion but: larger scale (fracture of rock surfaces > 10mm) stress concentrations better known Stress concentrations and micro crack growth temporary stress increase in ice or by debris in ice Commonly: –lee side of obstacles –large particles at the bed The case of a low pressure cavity: Cavities and bed particles Normal stress –Maximum below a particle

Shear stress –Maximum shear below and ahead of a particle Compressive stress below the particle Tensile stresses ahead of the particl Outcomes: –chattermarks –fatigue (loading/unloading cycles) Other analyses some: stresses too low to explain rock failure –Boulton and Morland others: water pressure oscillations in cavities –Iverson

Entrainment Eroded particles removed from the site of detachment regelation ice flow into cracks and pores –the ice then deforms and the surrounded particle becomes mobile –friction reduction between particle and bed Mechanical incorporation of particles into basal ice

Erosion of soft beds Erosion and deforming beds The "effective" bed may not be the ice-bedrock or ice-sediment interface The behaviour of subglacial sediment remains poorly understood The idea: Shear stress exerted by the ice may exceed the shear strength of the sediment Deformation penetrates the substrate to the depth at which shear strength exceeds shear stress Pervasive deformation and forward movement sediment An efficient erosion mechanism Abrasion in a slurry An efficient transportation mechanism

Debris accretion Weertman's ice-debris accretion model for poly thermal glaciers junction between wet-based and cold based ice penetration of the 0^{\circ }C isotherm into the bed substrate frozen to the sole of the glacier decollement at the base of the frozen layer Accretion of proglacial sediments for glacier margins in cold environments not permafrost environment glacier flows onto/over partly frozen sediments frozen to the ice margin decollement and formation of thrust surfaces thrust block moraines

Basal Ice Formation regelaton diffusion Deformation ductile deformation –folding –boudinage brittle deformation –faulting –tensional failure –boudinage Outcomes mechanical mixing thickening of the basal zone