Lecture 6 - glacier erosion processes

Question 1

Alley et al (2003)

Answer 1

Gs erode, transport and deposit sediment rapidly

ice flow almost completely independent from bed slope, slope at ice bed interface > at ice air interface

but, perturbations of the surface slope can greatly affect sediment transport and erosion

headward growth of cirques, valley widening and downcutting portions of the valleys

proG sediment = sensitive recorders of climatic and glaciological changes

Question 2

Alley et al (2003) water association to erosion

Answer 2

rapid geomorphic activity (esp Gs with a lot of meltwater along beds, large and slide rapidly)

sediment discharge from highly erosive Gs is carried in subG streams

without drainage a thick subG till would be produced: protects G and reduces erosion

rapid subG drainage = overdeepenings = G bed rises in the direction of flow (cirques)

when bed decreases in elevation, excess heat not needed to maintain water below melting point can melt walls and enlarge channel

Question 3

Benn and Evans (2010)

Answer 3

different scales of erosional landforms (small to landscape)

importance of recognizing superposition of each of the scale features

different models of friction = Boulton, Hallet and Sandpaper

Question 4

Bennet and Glasser (2009 - chapter 5)

Answer 4

ice sheets are agents of erosions; pick up and transport material

decrease elevation beneath them and increase the margins due to deposition

3 types of glacial erosion = abrasion, plucking and meltwater erosion

Question 5

Bennet and Glasser (2009 - chapter 5): glacial abrasion

Answer 5

rock particles at base of G are moved across a bedrock surface, scratching it and wearing it down

basal contact pressure important control on abrasion (how hard is ice being pushed down onto bedrock)

• Boulton model: contact pressure related to the effective normal pressure (high when thick ice and low basal water pressure), so higher at cold based Gs with porous rocks. increase in effective normal pressure = increase in basal abrasion, friction might increase too and slightly slow the glacier
• Hallet model = contact pressure between clast in basal transport = rate at which ice flows towards the bed and forces the clast into the bed (is independent of the effective normal pressure), ice surrounds the particle. greater rate of basal sliding = increased basal abrasion = cold based gs do not slide over their bed and will not have the ability to erode them. concentration of debris within basal ice = controls the rate of abrasion, better when sparse (=concentrated on a smaller area)

Question 6

Bennet and Glasser (2009 - chapter 5): glacial plucking

Answer 6

G removes large chunks and fragment form the bed

propagation of fractures within the bedrock

unloading rock surfaces = may expand and fracture

entrainment = once broken. off the rock must be picked up and carried

Question 7

Bennet and Glasser (2009 - chapter 5): glacial meltwater erosion

Answer 7

either physical or chemical processes

depends on: rock type, discharge regime, amount of sediment carried

fluvial abrasion and cavitation

determining rates of erosion = direct observation beneath modern Gs, reconstructions in glaciated terrains or sediment volume calculations

G erosion is controlled by basal thermal regime