Week 13: Glacial landsystems Flashcards

1
Q

Landsystem =

A

area of common terrain attributes, different from those of adjacent areas, in which recurring patterns of topography/soil/vegetation reflect underlying geology, past erosion and depositional processes and climate

(climate = glacial)

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2
Q

Process-form model =

A

conceptual, emphasises genetic interrelationships of specific landform-sediment associations at both local/regional scales in terms of known processes and form linkages

Another way of thinking about landsystems

Relates depositional/erosional forms to processes beneath ice sheet margins

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3
Q

Holistic approach to glacial landsystems

A

Geomorphology and sediments that characterise landscape are genetically related to the processes that generate them

Individual landforms/elements –> land facets –> landsystems

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4
Q

Different suites of landforms

A

Fringe

Marginal

Transitional

Inner

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5
Q

Concept development of landsystems

A

1970s-80s = styles of glaciation landsystems

  • subglacial
  • supraglacial
  • glaciated valley
  • Eyles 1983

1990s-present = continuum of styles and glaciation dynamics i.e.
- surging glacier
- ice stream
- plateau ice field
- Evans 2003
= relative importance of thermal regime and topography

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6
Q

Active temperate glaciers =

A

strong seasonal climatic control and predominantly warm bed

active i.e. flow maintained (moving forwards) during both advance and recession

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7
Q

Where are active temperate glaciers found?

A

Maritime locations e.g. Iceland/Alaska

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8
Q

Component domains of active temperate glaciers

A
  1. Marginal morainic
  2. Glacifluvial (+glacilacustrine)
  3. Subglacial
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9
Q

Characteristic formations in active temperate glaciers

A

Flutings/minor drumlines running into push moraines, representing retreat

LANDSYSTEM SIGNATURE INDICATIVE OF DEFORMING BED

Inset push moraines related to recession

Ponds between

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10
Q

Who came up with a landsystems model for actively receding temperate glaciers?

A

Evans and Twigg 2002

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11
Q

What does each ridge of an inset push moraine (active temperate glacier) represent?

A

One year’s worth of sediment being produced at margin

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12
Q

Surging glaciers =

A

short rapid surge/advance phase followed by longer quiescent phase

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13
Q

How are surging glaciers linked to the reorganisation of the subglacial drainage system?

A

Build up of subglacial meltwater until ice crosses threshold then decoupled from bed = surges

GIVES A CONSISTENT AND PREDICTABLE LANDSYSTEM

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14
Q

Examples of surging glaciers

A

Modern = Iceland
Tungnaarjokull

Pleistocene ice sheet = S margin of Laurentide Ice Sheet

  • Evans et al 1998/2008
  • surging global with thrust block moraines (glacitectonic compression) at outermost edge of lobe
  • megaflutings
  • crevasse-squeeze ridges
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15
Q

Surging glacier characteristics

A

Extensive crevassing
Catastrophic meltwater dischrage
Rapid snout advance
Fracturing in association around margin/within surge zone

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16
Q

The surging glacier landsystem (distal to proximal)

A
  1. Proglacial outwash fan
  2. Thrust block moraine
  3. Hummocky moraine
  4. Stagnating surge snout
  5. Flutings
  6. Crevasse-squeeze ridges
  7. Overridden thrust block moraine
  8. Concertina/zig-zag esker
  9. Crevasse-squeeze ridges emerging

Evans and Rea 1999, Evans et al 1999

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17
Q

How can you recognise crevasse-squeeze ridges in aerial photographs?

A

Sparse vegetation

  • starved of moisture on ridge summits
18
Q

Plateau Icefield examples

A

Iceland

Ellesmere Island, Canada

19
Q

What are plateau ice fields characterised by?

A

Cold-based plateau ice

Topographically constrained outlets and niche glaciers
- warm based?

Critical summit breadth concept

20
Q

Critical summit breadth concept

A

Manley 1955, 1959
Applied more recently to glaciation styles in the Lake District

y = height of summit over regional firn line (m)

x = breadth of summit (m)

21
Q

Concept of a plateau icefield

A

High altitude over large area
= colder climate
= thin, cold-based ice
= relatively inert

Moves to warmer based where spills over plateau
= topographically constrained outlets e.g. niche glacier

22
Q

Components of a plateau icefield

A
  1. Summit block fields
  2. Abraded bedrock
  3. Later (ice marginal) meltwater channels
23
Q

Plateau ice field; summit block field =

A

large area of intense frost shattering and thaw-freeze processes due to little action of inert ice in subsurface

24
Q

Plateau ice field; abraded bedrock =

A

where ice begins to pick up speed and flow around edge

25
Plateau ice field; lateral meltwater channels =
when glacier retreats back into centre of plateau = meltwater generated cuts
26
Why are plateau ice fields commonly mipmapped?
no evidence of glaciation on summit plateau other than potential meltwater channels BUT if you had ice in valley at a lower altitude it must have been present at higher altitude on plateau too
27
Trimline moraine =
line of moulders which shows where the ice margin was
28
Spatial/temporal continuum of plateau ice fields and valley glaciers
Depends on debris flux vs ice flux (Benn et al 2003)
29
English Lake District; glacial history/mapping
Sissons 1980 - no ice on summits - alpine style of glaciation - based on depositional evidence i.e. ablation zone Rea et al 1998, McDougal 2001/2014, Brown et al 2013 - plateau ice field concept Bickerdike et al 2016/2018 - "different styles of glaciation landsystems that occurred during Younger Dryas in upland areas of Britain" - Lake District = between plateau and alpine
30
Younger Dryas glaciation systems - decreasing glacier size/thickness - increasing topographic constraint AND EXAMPLES
Bickerdike et al 2016/2018 ICE CAP - Glen Lyon and surrounding valleys PLATEAU ICE FIELD - Monadhliath mountains, Scotland LOWLAND PIEDMONT LOBE - Lomond valley, Scotland ALPINE ICEFIELD - Isle of Mull, Scotland CIRQUE/NICHE GLACIERS - Snowdonia, Wales
31
When was the Younger Dryas?
12.8-11.5ka BP
32
Ice streams =
relatively fast moving streams of ice with sheets
33
Should ancient ice sheets have ice streams?
Yes, based on modern-day ice streams Potential location of major ice streams in N hemisphere ice sheets predicted by Denton and Hughes 1981
34
How were the two types of ice stream developed?
Dyke and Morris 1988 Used collected till samples for geochemical signatures = spatial distribution of bedrock 1. BOOTHIA TYPE 2. DUBAWNT TYPE
35
Boothia type ice stream
Two rock types (A/B) give rise to plume of erratic dispersal beneath faster flowing stream ice
36
Dubawnt type ice stream
Produces apparent plume because of restricted source area
37
Why are marine components of ice streams more common in modern settings?
Our ice sheets terminate in the ocean Clear break between streamline and non-streamline terrain at margin (clearer than terrestrial ice lobes/stream)
38
Example of marine based ice stream
Ice Stream B, WAIS
39
Example of terrestrial ice lobe/stream
Des Moines/Lake Michigan lobes
40
What happens to ice streams during ice sheet retreat?
Younger imprints on older