Week 13: Glacial landsystems Flashcards
Landsystem =
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)
Process-form model =
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
Holistic approach to glacial landsystems
Geomorphology and sediments that characterise landscape are genetically related to the processes that generate them
Individual landforms/elements –> land facets –> landsystems
Different suites of landforms
Fringe
Marginal
Transitional
Inner
Concept development of landsystems
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
Active temperate glaciers =
strong seasonal climatic control and predominantly warm bed
active i.e. flow maintained (moving forwards) during both advance and recession
Where are active temperate glaciers found?
Maritime locations e.g. Iceland/Alaska
Component domains of active temperate glaciers
- Marginal morainic
- Glacifluvial (+glacilacustrine)
- Subglacial
Characteristic formations in active temperate glaciers
Flutings/minor drumlines running into push moraines, representing retreat
LANDSYSTEM SIGNATURE INDICATIVE OF DEFORMING BED
Inset push moraines related to recession
Ponds between
Who came up with a landsystems model for actively receding temperate glaciers?
Evans and Twigg 2002
What does each ridge of an inset push moraine (active temperate glacier) represent?
One year’s worth of sediment being produced at margin
Surging glaciers =
short rapid surge/advance phase followed by longer quiescent phase
How are surging glaciers linked to the reorganisation of the subglacial drainage system?
Build up of subglacial meltwater until ice crosses threshold then decoupled from bed = surges
GIVES A CONSISTENT AND PREDICTABLE LANDSYSTEM
Examples of surging glaciers
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
Surging glacier characteristics
Extensive crevassing
Catastrophic meltwater dischrage
Rapid snout advance
Fracturing in association around margin/within surge zone
The surging glacier landsystem (distal to proximal)
- Proglacial outwash fan
- Thrust block moraine
- Hummocky moraine
- Stagnating surge snout
- Flutings
- Crevasse-squeeze ridges
- Overridden thrust block moraine
- Concertina/zig-zag esker
- Crevasse-squeeze ridges emerging
Evans and Rea 1999, Evans et al 1999
How can you recognise crevasse-squeeze ridges in aerial photographs?
Sparse vegetation
- starved of moisture on ridge summits
Plateau Icefield examples
Iceland
Ellesmere Island, Canada
What are plateau ice fields characterised by?
Cold-based plateau ice
Topographically constrained outlets and niche glaciers
- warm based?
Critical summit breadth concept
Critical summit breadth concept
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)
Concept of a plateau icefield
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
Components of a plateau icefield
- Summit block fields
- Abraded bedrock
- Later (ice marginal) meltwater channels
Plateau ice field; summit block field =
large area of intense frost shattering and thaw-freeze processes due to little action of inert ice in subsurface
Plateau ice field; abraded bedrock =
where ice begins to pick up speed and flow around edge
Plateau ice field; lateral meltwater channels =
when glacier retreats back into centre of plateau = meltwater generated cuts
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
Trimline moraine =
line of moulders which shows where the ice margin was
Spatial/temporal continuum of plateau ice fields and valley glaciers
Depends on debris flux vs ice flux (Benn et al 2003)
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
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
When was the Younger Dryas?
12.8-11.5ka BP
Ice streams =
relatively fast moving streams of ice with sheets
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
How were the two types of ice stream developed?
Dyke and Morris 1988
Used collected till samples for geochemical signatures = spatial distribution of bedrock
- BOOTHIA TYPE
- DUBAWNT TYPE
Boothia type ice stream
Two rock types (A/B) give rise to plume of erratic dispersal beneath faster flowing stream ice
Dubawnt type ice stream
Produces apparent plume because of restricted source area
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)
Example of marine based ice stream
Ice Stream B, WAIS
Example of terrestrial ice lobe/stream
Des Moines/Lake Michigan lobes
What happens to ice streams during ice sheet retreat?
Younger imprints on older
