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Week 6: Glacier erosion processes Flashcards

1
Q

Glacier erosion =

A

detachment, entrainment and transport of rock/sediment from glacier/bed

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2
Q

What is glacier erosion linked to?

A
  1. Mechanisms/patterns of glacier flow and stability
  2. Landform/landscape evolution
    e. g. MISI deep bed topography = deep bed due to glacier erosion
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3
Q

Glacier erosion reflects the balance between:

A

1) imposed shear stresses

2) strength of glacier bed

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4
Q

Controls on glacier erosion

A

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

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5
Q

What determines material strength/resistance?

A

COHESION (chemical bonds/electrical forces)

FRICTIONAL STRENGTH (interlocking protuberance b/w surfaces)

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6
Q

What is shear stress?

A 
Stress that ice exerts on a particle
Highly variable in space and time = stress gradients
- time of day
- season
- where in glacier
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7
Q

Shear stress > resistance =

A

Erosion = transport

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8
Q

Shear stress < resistance =

A

Deposition

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9
Q

Stress concentrations in cracks

A

Concentrate down into crack

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10
Q

Theoretical models of subglacial friction

A
  1. Coulomb ‘Boulton’ friction model
  2. Hallet friction model
  3. Sandpaper friction model
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11
Q

Coulomb ‘Boulton’ friction model

A

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

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12
Q

Coulomb ‘Boulton’ friction model problem

A

Ice is viscous and deforms around particles diagram

Applies to some situations but not overall very realistic

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13
Q

Situations where Coulomb ‘Boulton’ friction model applies

A
  1. Rigid slabs of debris-rich basal ice
  2. Subglacial deforming layers without interstitial ice
  3. Particles in direct contact with bed
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14
Q

Hallet friction model dates

A

1979, 1981

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15
Q

Hållet friction model

A

Contact forces independent of ice thickness/subglacial Pw

Friction force = buoyant particle weight and drag force (because ice flows towards the bed)

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16
Q

Hållet friction model: why does ice flow towards the bed?

A

Melting, vertical strain or topographic bump

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17
Q

Hållet friction model: where are highest contact forces?

A
  • large, heavy particles +/ basal ice melting

- rapid ice flow towards bed i.e. up-glacier side of bumps

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18
Q

Where does Hallet friction model apply?

A

To sparse basal debris (<50% vol) when particles spaced far apart

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19
Q

Sandpaper friction model reference

A

Schweizer and Ikea 1992

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20
Q

Sandpaper friction model

A

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

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21
Q

When does sandpaper friction model work best?

A

High (>50%) debris concentrations with basal ice

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22
Q

Forms of erosion

A

ABRASION

QUARRYING (PLUCKING)

MELTWATER

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23
Q

Abrasion =

A

(1) Polishing (2) striation (Benn and Evans 2010)

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24
Q

Polishing =

A

removal of small protuberances

25
Striation =
small-scale bedrock scoring
26
Striae =
Cumulative effect of numerous brittle failures from rock particles dragging across bedrock/clasts
27
Striae development influenced by:
1. Relative hardness of rock surface vs overriding clast 2. Contact forces 3. Velocity of clast relative to bed 4. Debris concentration - 10-30% i.e. low vol most effective 5. Erosion product removal e. g. by meltwater 6. Basal debris availability e. g. at stress concentrations
28
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
29
Where do chatter marks occur/what are they?
1) Under large particles | 2) Lee-side cavities
30
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
31
+ve stresses =
Compression
32
-ve stresses =
Tension
33
Quarrying/plucking scenario: joint
Ice enters Envelops clast Prises rock apart and removes debris blocks
34
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
35
Forms of meltwater erosion
Abrasion (corrasion) Cavitation Fluid Stressing Solution Bedrock structure
36
Abrasion (corrasion) =
erosion by friction In sub/proglacial settings High sediment load and turbulent flow
37
Cavitation =
turbulence = bubble growth/collapse = P and shock waves
38
Fluid stressing =
hydraulic stresses erode from repetitive stressing/unstressing
39
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
40
Solution and bumpy bed interactions
Stoss = high = dissolution Lee = low = calcite precipitation
41
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
42
Glacial erosion patterns; primary control
Basal thermal regime i. e. WARM BASED = - basal sliding = - abrasion - plucking - meltwater erosion N.B. warm-freezing = intense erosion
43
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)
44
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
45
Measuring glacial erosion rates
Direct observation Landform evidence Lab In front of glacier Thermochronometer
46
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
47
Measuring glacial erosion rates: landform evidence
Nesje et al 1992: reconstruct preglacial landscape and compare
48
Measuring glacial erosion rates: Lab
Simulate glacier: Circular chamber with rock at base, fill rest with ice and spin
49
Measuring glacial erosion rates: in front of glacier
Meltwater composition i.e. acidic/sediment composition N.B. Obvious limitations and assumptions
50
Measuring glacial erosion rates: thermochronometer
Erode fast = deeper rocks exposed more quickly
51
Hallet et al 1996 erosion rate results
Polar glaciers 0.01 mm/a Small temperate glaciers (Alps) 1 Large flowing glaciers (Alaska) 10-100
52
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
53
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
54
Numerical models of glacier erosion
Use one of 3 theoretical models as basis Incorporate various other processes Look at feedbacks/test impacts
55
Other processes incorporated into numerical models of glacier erosion
Hydrology Sediment transport Tectonics/isostasy Hillslope diffusion
56
Problem with numerical models of glacier erosion
Rate poorly constrained
57
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
58
What is the effect of flushing sediment away?
Maintains bare rock vs sediment-laden ice contact | Allows erosion to continue more effectively

Glaciers and Glaciation (29 decks)

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