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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
Intermediate: crags and tails
e.g. Edinburgh Castle/Royal Mile = resistant bedrock crag with less resistant tapering tail Tail length due to pressure shadow - due to velocity?
26
Intermediate: nye channels
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
What can nye channels be used for?
Reconstructing subglacial meltwater drainage and equipotential contours (Booth and Wallet 1993)
28
Intermediate: tunnel valleys
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
Tunnel valley debate
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
Clinoform =
Bedding form often associated with deltas
31
Intermediate: ice marginal (lateral meltwater) channels
Drain and cut channels alongside ice margin and sub-marginally Preferably alongside cold based ice
32
What can lateral meltwater channels be used for?
Reconstructing marginal recession and ice sheet elevation (Greenwood et al 2007)
33
Example of switching between lateral and subglacial channels
Lake District | Temperate/warm-based
34
Intermediate: pro-glacial channels
Form by catastrophic draining of lake = channels/gorges Can exploit previous subglacial (Nye) channels Discrete spillways
35
Discrete spillway =
Where water escapes from lakes over low points/cols
36
Pro-glacial channel example
Channeled scablands Washington State, USA (Largest are formed by jokulhlaups like this) Baker and Bunker 1985
37
Mega-grooves =
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
Mega-ridges =
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
Ice stream onset zones =
Flow converges = high velocity and bed erosion = series of subglacial canyons/areas of subglacial scour
40
Streamlined hard beds concept =
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
Large scale landforms
Rock basins and overdeepenings Troughs and fjords Cirques
42
Large: rock basins and overdeepenings =
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
Controls on rock basins and overdeepenings
Glaciological variables Substratum characteristics (structure/lithology)
44
Formation of rock basins and overdeepenings
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
Large: troughs and fjords
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
How long do fjords take to form?
1.2 Ma
47
Large: cirques =
flat-floored/overdeepened basin connected to steep back wall by concave slope (Benn and Evans 1998)
48
Types of cirque
Benn and Evans 1998: 1. Simple 2. Compound 3. Cirque complexes 4. Staircase 5. Cirque troughs
49
How do cirques form?
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
What are landscapes of glacial erosion based upon?
The regional association of landforms
51
Landscapes of glacial erosion
Areal scouring Selective linear erosion Little/no glacial erosion Alpine Cirque
52
Landscapes: areal scouring
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
Landscapes: selective linear erosion
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
Landscapes: little/no glacial erosion
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
Landscapes: little/no erosion debate
Subglacial Thermal Organisation (STO) Kleman and Glasser 2007 Cold based ice vs no ice = ongoing debate Exposure dating can help
56
Landscapes: alpine
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
Aretes =
Ice-free mountain peaks
58
Landscapes: cirque
Independent cirques incised into upland terrain Density/orientation/altitude = palaeoclimate indicators (Evans 1977) Linked/associated with alpine landscapes
59
Cirques are sensitive indicators of...
Climate regime
60
CASE STUDY: Jakobshavn Isbrae, W Greenland FACTS
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
CASE STUDY: Jakobshavn Isbrae, W Greenland STAGES
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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