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Glaciers and Glaciation > Week 6: Glacier erosion processes > Flashcards

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
Q

Striation =

A

small-scale bedrock scoring

26
Q

Striae =

A

Cumulative effect of numerous brittle failures from rock particles dragging across bedrock/clasts

27
Q

Striae development influenced by:

A
  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
Q

Quarrying/plucking =

A

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
Q

Where do chatter marks occur/what are they?

A

1) Under large particles

2) Lee-side cavities

30
Q

Quarrying/plucking scenario: rock bump

A

Water flows up, over and into cavity behind rock bump
Suddenly lower Pw
= freeze/stick on and carries material with it

31
Q

+ve stresses =

A

Compression

32
Q

-ve stresses =

A

Tension

33
Q

Quarrying/plucking scenario: joint

A

Ice enters
Envelops clast
Prises rock apart and removes debris blocks

34
Q

How is quarried/plucked material removed?

A

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
Q

Forms of meltwater erosion

A

Abrasion (corrasion)

Cavitation

Fluid Stressing

Solution

Bedrock structure

36
Q

Abrasion (corrasion) =

A

erosion by friction

In sub/proglacial settings

High sediment load and turbulent flow

37
Q

Cavitation =

A

turbulence = bubble growth/collapse = P and shock waves

38
Q

Fluid stressing =

A

hydraulic stresses erode from repetitive stressing/unstressing

39
Q

Solution; explanation

A

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
Q

Solution and bumpy bed interactions

A

Stoss = high = dissolution

Lee = low = calcite precipitation

41
Q

Bedrock structure effects

A

If not tilted = symmetrical e.g. cirque/U-shaped trough

Thickness
Orientation
Joint density/orientation
= nature/rates of meltwater erosion at different scales

42
Q

Glacial erosion patterns; primary control

A

Basal thermal regime

i. e. WARM BASED =
- basal sliding =
- abrasion
- plucking
- meltwater erosion

N.B. warm-freezing = intense erosion

43
Q

Glacial erosion patterns; secondary or local controls

A

Debris content

Bedrock geology

Ice velocity

Basal melt rate

Fluctuating water pressures

Previous erosion rates (i.e. pre-existing landscape)

44
Q

Examples of cold-based erosion

A

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
Q

Measuring glacial erosion rates

A

Direct observation

Landform evidence

Lab

In front of glacier

Thermochronometer

46
Q

Measuring glacial erosion rates: direct observation

A

BOREHOLE

  • camera = striations
  • instrument = basal sliding

ICE TUNNEL

  • between ice/rock base
  • see surface and rate of change
  • slow
47
Q

Measuring glacial erosion rates: landform evidence

A

Nesje et al 1992: reconstruct preglacial landscape and compare

48
Q

Measuring glacial erosion rates: Lab

A

Simulate glacier:

Circular chamber with rock at base, fill rest with ice and spin

49
Q

Measuring glacial erosion rates: in front of glacier

A

Meltwater composition i.e. acidic/sediment composition

N.B. Obvious limitations and assumptions

50
Q

Measuring glacial erosion rates: thermochronometer

A

Erode fast = deeper rocks exposed more quickly

51
Q

Hallet et al 1996 erosion rate results

A

Polar glaciers 0.01 mm/a

Small temperate glaciers (Alps) 1

Large flowing glaciers (Alaska) 10-100

52
Q

Frank Josef Glacier

A

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
Q

Latitude control on erosion rate

A

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
Q

Numerical models of glacier erosion

A

Use one of 3 theoretical models as basis

Incorporate various other processes

Look at feedbacks/test impacts

55
Q

Other processes incorporated into numerical models of glacier erosion

A

Hydrology

Sediment transport

Tectonics/isostasy

Hillslope diffusion

56
Q

Problem with numerical models of glacier erosion

A

Rate poorly constrained

57
Q

What is topographic steering?

A

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
Q

What is the effect of flushing sediment away?

A

Maintains bare rock vs sediment-laden ice contact

Allows erosion to continue more effectively

Glaciers and Glaciation (29 decks)

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