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=(pi-Pw)tan

•where f = basal friction, pi = overburden pressure, Pw = 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=(pi-sPw)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

The role of basal temperature regime in influencing the

efficacy of various processes of erosion

Processes Basal Ice Temperature Regime

Warm Fluctuating Cold

Abrasion (due to basal melting)

Abrasion (obstacle related)

Fracture of fresh rock

Joint exploitation - freeze-thaw

Joint exploitation - dilatation

Debris entrainment - ice pressure

Debris entrainment - regelation

Meltwater - erosion

Meltwater - evacuation of debris

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