Glaciers 2

How does a glacier stay the same size during flow(surface profile maintained)

o   Firn must be buried progressively deeper at the head of the glacier

o   Where there is net melting, material must reemerge in the ablation zone

o   Ice crystal flows downwards in the accumulation area, at the equilibrium line at bed-parallel and upwards in ablation zone

What are the processes of glacial flow?

• creep (internal deformation)

• fracture

• basal sliding and bed deformation

What is creep/internal deformation?

• occurs when firn is turned into ice under pressure

• ice crystals have parallel cleavage planes and tend to split in one particular direction

• Under pressure at depth, ice crystals align parallel to the shear stress in the ice and begin to slide past each other = down slope glacier flow

• as a non-newtonian fluid, ice’s viscosity deforms through strain and is dependent on the stress it is expeirencing

• controls - frictional/resisting forces and basal shear stress

• basal shear stress - controls how fast ice deforms based on ice thickness and surface slope

What is fracture?

• Ice can bend round corners/surfaces but is brittle at the surfaceàcrevasses can form as it moves

What is basal sliding and bed deformation?

• Require water at the ice-bed interface

• Ice slides over saturated sedimentàsediment deforms like plasticine

• Sediments slide over a layer of high pressure water

Warm- vs cold-based ice

• warm - thick and fast moving, high subglacial pressure, PMP is reached so basal ice melts, water present at bed, basal sliding is possible, erosion maximised

• cold - thin or slow-moving ice, low subglacial pressure, PMP not reached, basal ice remains frozen, no water at ice-bed interface, no basal sliding, very limited erosion

What is PMP

pressure melting point

What/where does erosion occur

o   Require refreezing(regelation) at ice bed interface

o   Pressure melting occurs on the up-glacier side of obstacles

o   Water refreezes on the lee-side of the obstacle

Erosional processes

• abrasion

• quarrying/plucking

Controls on rate of erosion

• limited by sediment supply and basal melting

• ice velocity near bed

• subglacial water pressure

• debris concentration in basal ice

• relative hardness of bedrock vs particles in ice

• duration of glaciation

Features of selective linear erosion

• Troughs separated by low-relief plateau areas

• Plateaus may have cold-based, thin ice

• Troughs are ice streams that reoccupy pre-glacial valleys

Abrasion vs Quarrying

• abrasion - occurs under thick ice, requires debris rich ice, sandpapering effect(smoothing), produces fine grained sediment, produces streamlined forms

• Quarrying/plucking - occurs under thinner ice, requires fluctuating water pressure at bed, ripping effect(roughening), produces blocks/boulders, produces stepped features(related to bedrock structure/weakness)

Types of erosional landforms

• grooves, gouges and chattermarks - intermediate scale landforms

• Roche moutonnees and Crag-and-tails