Week 6: Glacier erosion processes Flashcards
Glacier erosion =
detachment, entrainment and transport of rock/sediment from glacier/bed
What is glacier erosion linked to?
- Mechanisms/patterns of glacier flow and stability
- Landform/landscape evolution
e. g. MISI deep bed topography = deep bed due to glacier erosion
Glacier erosion reflects the balance between:
1) imposed shear stresses
2) strength of glacier bed
Controls on glacier erosion
BASAL SEDIMENT
ICE FLOW SPEED/WATER CONDITIONS/BASAL T
- all controlled by CLIMATE
ICE THICKNESS
BASAL SHEAR STRESS
ROCK STRENGTH/TYPE
EXISTING TOPOGRAPHY
SEDIMENT COVER
CRACKS IN ROCK
SEDIMENT CONCENTRATION IN BASAL ICE
LATITUDE
What determines material strength/resistance?
COHESION (chemical bonds/electrical forces)
FRICTIONAL STRENGTH (interlocking protuberance b/w surfaces)
What is shear stress?
Stress that ice exerts on a particle Highly variable in space and time = stress gradients - time of day - season - where in glacier
Shear stress > resistance =
Erosion = transport
Shear stress < resistance =
Deposition
Stress concentrations in cracks
Concentrate down into crack
Theoretical models of subglacial friction
- Coulomb ‘Boulton’ friction model
- Hallet friction model
- Sandpaper friction model
Coulomb ‘Boulton’ friction model
Assumes basal friction between bed and rock particles in basal ice due to effective normal pressure
Basal friction = effective normal P x internal friction angle
Effective normal P = ice overburden P - Pwater in cavities beneath particle
SO thick ice and low Water = high friction
Coulomb ‘Boulton’ friction model problem
Ice is viscous and deforms around particles diagram
Applies to some situations but not overall very realistic
Situations where Coulomb ‘Boulton’ friction model applies
- Rigid slabs of debris-rich basal ice
- Subglacial deforming layers without interstitial ice
- Particles in direct contact with bed
Hallet friction model dates
1979, 1981
Hållet friction model
Contact forces independent of ice thickness/subglacial Pw
Friction force = buoyant particle weight and drag force (because ice flows towards the bed)
Hållet friction model: why does ice flow towards the bed?
Melting, vertical strain or topographic bump
Hållet friction model: where are highest contact forces?
- large, heavy particles +/ basal ice melting
- rapid ice flow towards bed i.e. up-glacier side of bumps
Where does Hallet friction model apply?
To sparse basal debris (<50% vol) when particles spaced far apart
Sandpaper friction model reference
Schweizer and Ikea 1992
Sandpaper friction model
Ice does not flow around many debris particles, it acts like glue b/w them
- no contact b/w particles
- ice ‘envelops’ particles
- ice flowing around particles not influenced
Debris-rich ice deforms and moulds itself = contacts large area of bed
- Modified from Coulomb but lower friction than
Water-filled cavities still important
Does not assume buoyancy
When does sandpaper friction model work best?
High (>50%) debris concentrations with basal ice
Forms of erosion
ABRASION
QUARRYING (PLUCKING)
MELTWATER
Abrasion =
(1) Polishing (2) striation (Benn and Evans 2010)
Polishing =
removal of small protuberances
Striation =
small-scale bedrock scoring
Striae =
Cumulative effect of numerous brittle failures from rock particles dragging across bedrock/clasts
Striae development influenced by:
- Relative hardness of rock surface vs overriding clast
- Contact forces
- Velocity of clast relative to bed
- Debris concentration
- 10-30% i.e. low vol most effective - Erosion product removal
e. g. by meltwater - Basal debris availability
e. g. at stress concentrations
Quarrying/plucking =
removal of >1cm fragments
Governed by stress patterns but pre-existing weaknesses important = chattermarks
Lower Pw = higher stresses
- most likely at foot of lee-side cavities
Where do chatter marks occur/what are they?
1) Under large particles
2) Lee-side cavities
Quarrying/plucking scenario: rock bump
Water flows up, over and into cavity behind rock bump
Suddenly lower Pw
= freeze/stick on and carries material with it
+ve stresses =
Compression
-ve stresses =
Tension
Quarrying/plucking scenario: joint
Ice enters
Envelops clast
Prises rock apart and removes debris blocks
How is quarried/plucked material removed?
EMBEDDED AND TRANSPORTED BY ICE
HEAT PUMP EFFECT (Robin 1976)
- freeze on in cavity
ICE ENTERS JOINTS AND ENVELOPS CLASTS (Rea and Whalley 1994)
MELTWATER
Forms of meltwater erosion
Abrasion (corrasion)
Cavitation
Fluid Stressing
Solution
Bedrock structure
Abrasion (corrasion) =
erosion by friction
In sub/proglacial settings
High sediment load and turbulent flow
Cavitation =
turbulence = bubble growth/collapse = P and shock waves
Fluid stressing =
hydraulic stresses erode from repetitive stressing/unstressing
Solution; explanation
Particularly in carbonate-rich lithologies/high rainfall
Important CO2 sink during deglaciation (Sharp et al 1995)
Despite low T:
- high flushing rates
- increase CO2 solubility at low T = acidic meltwater
- rock flour availability
Solution and bumpy bed interactions
Stoss = high = dissolution
Lee = low = calcite precipitation
Bedrock structure effects
If not tilted = symmetrical e.g. cirque/U-shaped trough
Thickness
Orientation
Joint density/orientation
= nature/rates of meltwater erosion at different scales
Glacial erosion patterns; primary control
Basal thermal regime
i. e. WARM BASED =
- basal sliding =
- abrasion
- plucking
- meltwater erosion
N.B. warm-freezing = intense erosion
Glacial erosion patterns; secondary or local controls
Debris content
Bedrock geology
Ice velocity
Basal melt rate
Fluctuating water pressures
Previous erosion rates (i.e. pre-existing landscape)
Examples of cold-based erosion
Cuffey et al 2000: evidence of debris entrainment at -17’C under Meserve Glacier, Antarctica
Atkins et al 2002: geological evidence of erosion e.g. abrasion marks/deformation structures
Measuring glacial erosion rates
Direct observation
Landform evidence
Lab
In front of glacier
Thermochronometer
Measuring glacial erosion rates: direct observation
BOREHOLE
- camera = striations
- instrument = basal sliding
ICE TUNNEL
- between ice/rock base
- see surface and rate of change
- slow
Measuring glacial erosion rates: landform evidence
Nesje et al 1992: reconstruct preglacial landscape and compare
Measuring glacial erosion rates: Lab
Simulate glacier:
Circular chamber with rock at base, fill rest with ice and spin
Measuring glacial erosion rates: in front of glacier
Meltwater composition i.e. acidic/sediment composition
N.B. Obvious limitations and assumptions
Measuring glacial erosion rates: thermochronometer
Erode fast = deeper rocks exposed more quickly
Hallet et al 1996 erosion rate results
Polar glaciers 0.01 mm/a
Small temperate glaciers (Alps) 1
Large flowing glaciers (Alaska) 10-100
Frank Josef Glacier
Herman et al 2015
- catchment flows over distinctive metamorphic geology
- satellite date = ice flow speed above each rock type
- material in subglacial stream = identify relative eroded volume of each rock type
= rates increase with rainstorms
= non-linear relationship between erosion rates and ice velocity
FLAW: ice surface velocities measured not basal sliding velocity
Latitude control on erosion rate
Koppes et al 2015
Expect to increase with decreasing latitude (due to T)
15 tidewater glaciers studied over 10’ latitudinal spread
= climate/glacier thermal regime control rate more strongly than ice extent, ice flux or sliding speed
Numerical models of glacier erosion
Use one of 3 theoretical models as basis
Incorporate various other processes
Look at feedbacks/test impacts
Other processes incorporated into numerical models of glacier erosion
Hydrology
Sediment transport
Tectonics/isostasy
Hillslope diffusion
Problem with numerical models of glacier erosion
Rate poorly constrained
What is topographic steering?
Kessler et al 2008
+ve feedback initiated by ice being steered towards mountain passes
Enhanced erosion beneath thicker, faster ice = deepens
= enhances topographic steering
What is the effect of flushing sediment away?
Maintains bare rock vs sediment-laden ice contact
Allows erosion to continue more effectively
