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Glaciers and Glaciation > Week 7: Landforms and landscapes from glacial erosion > Flashcards

Week 7: Landforms and landscapes from glacial erosion Flashcards

1
Q

Classification of erosional landscapes

A

SMALL = mm to a few m

INTERMEDIATE = m to a few km

LARGE = km to 10s km

LANDSCAPES = 100s km

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2
Q

Controls on erosion and form reference

A

Sugen and John 1976

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3
Q

Controls on erosion and form

A

Glaciological

Substratum

Topography

Time

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4
Q

Controls on erosion and form: glaciological

A

PRIMARY CONTROL

Stress (basal shear/normal)

Pw and routing

Ice flow direction/velocity

Thermal regime

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5
Q

Controls on erosion and form: substratum

A

Structure

Lithology

Degree of pre-glacial weathering

Permeability

e.g. before start of quaternary = warm and tropical = lots of cover by soft, friable, easily eroded material = thick, weathering mantle b/c deep weathering of Earth’s surface

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6
Q

Controls on erosion and form: topography

A

Morphology of glacier bed

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7
Q

Controls on erosion and form: time

A

Duration of glaciation

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8
Q

Small landforms (+reference)

A

Benn and Evans 1998

Striae

Rat tails

Gouges/fractures/chattermarks

P-forms

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9
Q

Small: striae =

A

Small scratches/thin grooves on bedrock/clast surface due to glacial abrasion

N.B. clast does more work than substrate “glacial polish”

  • not continuous
  • fine grained, hard rocks (quartzite)
  • cross-cutting
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10
Q

Striae: nailheads

A

No. of brittle failures that end abruptly (not continuous)

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11
Q

Striae: ice direction

A

Guided by topography

Thickens = more independent/diffluent flow

Thins during deglaciation = trapped in topography

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12
Q

Small: rat tails =

A

Small version of ‘crag and tails’

Down-ice from resistant nodules
Differential erosion around
Tail on lee side
UNAMBIGUOUS ice flow direction

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13
Q

Small: gouges/fractures/chattermarks =

A

Crescentric/lunate cracks due to fracture plane dip (=ice direction?)

Few cm wide

Often associated with grooves

Stick-slip behaviour (bed mosaic)

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14
Q

Crescentric =

A

Concave down ice

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15
Q

Lunate =

A

Concave up ice

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16
Q

Small: p-forms =

A

“Plastically moulded forms”
Smoothed depression eroded into bedrock

Vary in size

Transverse/longitudinal to ice flow or NO directional trend

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17
Q

P-forms possible mechanisms:

A
  1. Debris-rich basal ice
  2. Saturated till
  3. Subglacial meltwater
  4. Ice-water mixtures
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18
Q

P-forms, references:

A

SHAW 1988:
Similar to water-eroded scour marks

GOLDTHWAITE 1979:
Striae on surface = glacial

SHAW 1988:
Striae created after, tortuous appearance = not ice?

BOULTON 1974:
Observed with debris-rich basal ice in situ

REA AND WHALLEY 1984:
…and with ice turning corners

BOULTON 1974:
represent alternating erosion forms

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19
Q

P-forms, origin debate:

A

FLUVIAL EXPLANATION (Shaw)

  • scour mark appearance = analagous
  • striated = ice erosion after
  • tortuous = meltWATER (ice doesn’t flow like that)

ICE EXPLANATION (Boulton)

  • scour marks = fluvial erosion afterwards
  • striated = can not be formed by meltwater
  • tortuous appearance = evidence of ice flowing like that IN SITU
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20
Q

Intermediate landforms

A

Roche moutonees

Crags and tails

Channels

Tunnel valleys

Ice marginal channels

Pro glacial channels

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21
Q

Intermediate: roche moutonees =

A

Asymmetric bedrock bumps w/ abraded stoss faces and quarried lee faces

Vary <1m to 100s m

Shape = stress distribution across bedrock bump

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22
Q

Roche moutonees formation

A

Tend to form under thin ice with fluctuating Pw (Sugden et al 1992)

Bedrock structure important (Sugden et al 1992, Roberts et al 2005, Lane et al 2014)

  • no fractures/bedding planes = harder to erode into classic shape
  • oblique to ice flow = asymmetric
  1. Thick ice pushed down onto bed due to g
  2. Starts to thin = normal P drops off = meltwater
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23
Q

Are roche moutonees a production of deglaciation or peak glaciation?

A

DEGLACIATION?

Or immature pre-glacial landforms?

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24
Q

Whaleback =

A

Cousin of roche moutonees

No pluck phase, smooth
Coupled to bed during formation = unable to produce cavity to promote plucking

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25
Q

Intermediate: crags and tails

A

e.g. Edinburgh Castle/Royal Mile
= resistant bedrock crag with less resistant tapering tail

Tail length due to pressure shadow - due to velocity?

26
Q

Intermediate: nye channels

A

Cut directly into bedrocks

  • v high subglacial Pw
  • water in isolation and its erosive power

10-100s m long, 10s m wide

Follow hydraulic gradient (ice surface slope)
Local topography important

= linked cavity networks
- anastomosing patterns/shallow depressions

27
Q

What can nye channels be used for?

A

Reconstructing subglacial meltwater drainage and equipotential contours (Booth and Wallet 1993)

28
Q

Intermediate: tunnel valleys

A

Similar to nye channels but >100km long, 4km wide

Can be in isolation/dendritic/anastomosing

Commonly:

  • flat bottomed (i.e. dissipate completely into area of ~flat sea bottom)
  • steep sided
  • infilled with sedimentary sequence

N.B. huge meltwater drainage at bank-full conditions

29
Q

Tunnel valley debate

A
  1. S-S meltwater drainage over deformable bed towards end of glacial stage (Shoemaker 1986)
  2. Catastrophic meltwater drainage
    - e.g. N Sea lake build up at ice sheet sides = cut off drainage and changes direction
  3. Time-transgressive formation close to ice margin (Kristensen et al 2008)
    - end of glaciation meltwater forced out up reverse slope = supercooling
    - freeze in water and sediment = reverse clinoforms infill valley
    - need water going into body of certain elevation
30
Q

Clinoform =

A

Bedding form often associated with deltas

31
Q

Intermediate: ice marginal (lateral meltwater) channels

A

Drain and cut channels alongside ice margin and sub-marginally
Preferably alongside cold based ice

32
Q

What can lateral meltwater channels be used for?

A

Reconstructing marginal recession and ice sheet elevation (Greenwood et al 2007)

33
Q

Example of switching between lateral and subglacial channels

A

Lake District

Temperate/warm-based

34
Q

Intermediate: pro-glacial channels

A

Form by catastrophic draining of lake = channels/gorges
Can exploit previous subglacial (Nye) channels
Discrete spillways

35
Q

Discrete spillway =

A

Where water escapes from lakes over low points/cols

36
Q

Pro-glacial channel example

A

Channeled scablands Washington State, USA

(Largest are formed by jokulhlaups like this)

Baker and Bunker 1985

37
Q

Mega-grooves =

A

Straight grooves // to bedrock troughs

100s to 1000s m

Formed by glacial ice/meltwater = erosion

  1. Structurally controlled
  2. Structurally independent
    * *diagrams**

Relationship to fast flow e.g. ice streams

38
Q

Mega-ridges =

A

Straight ridges // to bedrock troughs

100s to 1000s m

Not necessarily related to fast flow; multiple glacial cycles = same repeated flow paths i.e. positive feedback

NOT diagnostic of “ice stream onset zones”

39
Q

Ice stream onset zones =

A

Flow converges = high velocity and bed erosion

= series of subglacial canyons/areas of subglacial scour

40
Q

Streamlined hard beds concept =

A

Influence of bedrock properties (hardness/fracture spacing/bedding) on occurence/character of mega-scale bedrooms

Erosion dominated by preferentially focussed abrasion/lateral plucking

Eyles et al 2012/2015
Krabbendam et al 2016

SHOWS THAT ICE STREAMING DOES NOT NEED A DEFORMABLE BED BUT CAN EQUALLY OCCUR ON A SMOOTH HARD BED

41
Q

Large scale landforms

A

Rock basins and overdeepenings

Troughs and fjords

Cirques

42
Q

Large: rock basins and overdeepenings =

A

Depressions commonly filled with post-glacial lakes

Form by quarrying/abrasion
+ enhanced by meltwater pressure variations (Hooke 1991, Kor et al 1991)

Can also occur in soft sediments (plucking/thrusting/advection of till)

43
Q

Controls on rock basins and overdeepenings

A

Glaciological variables

Substratum characteristics (structure/lithology)

44
Q

Formation of rock basins and overdeepenings

A

Downhill = extension
Hits bump = compression
Over bump = extension

Basal shear stress mostly over the over deepening
- associated with supercooling freezing sediment back onto glacier base = allows to rethicken

Grounding line instabilities = ice wants to lift
= increase flow velocity/surface fracturing/rate ice wants to flow back as it comes back over over-deepening

45
Q

Large: troughs and fjords

A

U-shaped, tend towards asymmetry
Some = pre-existing fluvial valleys/cut deep below SL

Jamieson et al 2008 + Kessler et al 2008 = +ve feedback linked to topographic steering and erosion proportional to ice discharge

46
Q

How long do fjords take to form?

A

1.2 Ma

47
Q

Large: cirques =

A

flat-floored/overdeepened basin connected to steep back wall by concave slope (Benn and Evans 1998)

48
Q

Types of cirque

A

Benn and Evans 1998:

  1. Simple
  2. Compound
  3. Cirque complexes
  4. Staircase
  5. Cirque troughs
49
Q

How do cirques form?

A

Develop from hollows - progressive back wall retreat and floor downcutting via:

  • quarrying
  • abrasion
  • weathering
  • mass movement

Thicken and merge across landscape = ice dome (flows irrespective of topography)

Develop from more than one episode of occupancy (Evans and Cox 1995)

50
Q

What are landscapes of glacial erosion based upon?

A

The regional association of landforms

51
Q

Landscapes of glacial erosion

A

Areal scouring

Selective linear erosion

Little/no glacial erosion

Alpine

Cirque

52
Q

Landscapes: areal scouring

A

Everywhere dominated by glacial erosion

e.g. Canadian Shield NW Scotland

  • streamlined bedrock knobs
  • roche moutonees
  • rock basins

Extensive warm-based erosion/basal sliding

Low relief areas

53
Q

Landscapes: selective linear erosion

A

In troughs with intervening plateau surfaces unmodified e.g. Scandinavia

  1. Fast, warm-based ice in troughs
  2. Slow, cold-based ice on plateau

Ice converges and moves through pre-existing corridors

54
Q

Landscapes: little/no glacial erosion

A

Unmodified pre-glacial landscapes survived glacial occupation
Protected under cold based ice

Characteristic dendritic pattern of periglacial features:

  • fluvial valleys
  • tors
  • blockfields
  • patterned ground
55
Q

Landscapes: little/no erosion debate

A

Subglacial Thermal Organisation (STO)
Kleman and Glasser 2007

Cold based ice vs no ice = ongoing debate
Exposure dating can help

56
Q

Landscapes: alpine

A

In areas subjected to repeated valley glaciation
= networks of glacial valleys/cirques separated by aretes

High relief, tectonically active mountain ranges e.g. Himalayas
Polythermal/warm-based

57
Q

Aretes =

A

Ice-free mountain peaks

58
Q

Landscapes: cirque

A

Independent cirques incised into upland terrain

Density/orientation/altitude = palaeoclimate indicators (Evans 1977)
Linked/associated with alpine landscapes

59
Q

Cirques are sensitive indicators of…

A

Climate regime

60
Q

CASE STUDY: Jakobshavn Isbrae, W Greenland FACTS

A

Sugen 1974 = landscapes of areal scour and selective linear erosion
Roberts et al 2005 = reconstruct ice sheet/stream evolution from various scales of erosional landforms

  1. Roche moutonees evolved over multiple cycles = changing striae direction (during deglaciation when ice became trapped into fjords)
  2. Whalebacks and RMs shape changed over time = different phases of glacial cycle
  3. Landscape has multiple glacial cycles locked within
61
Q

CASE STUDY: Jakobshavn Isbrae, W Greenland STAGES

A
  1. 1st advance
    - thick, fast ice
    - increased velocity lowers viscosity = suppresses bed separation
    - WB
  2. Deglaciation
    - thinning ice, velocity fluctuations
    - bed separation as cavities open = plucking
    - penetration of meltwater/air to base
    - Was modified to shorter RMs
  3. 2nd advance
    - thick, fast ice
    - RMs smoothed = multi-way smaller WBs over macro perturbations

Glaciers and Glaciation (29 decks)

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