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Geography GE2011: Glacial and Periglacial Processes
Glacifluvial and glacilacustrine processes and landforms
Recommended reading
Benn, D.I. And Evans, D.J.A. (1998) Glaciers and Glaciation. Arnold, London, 98-139.
Marshak, S. (2001) Earth: Portrait of a Planet. Norton, New York, .
Murray, T. (2005) Glaciers and ice sheets. In Holden, J. (ed.) Physical Geography and the Environment. Pearson, Harlow, 440-446, .
1. Introduction
Structure of lecture
Sources of water in glaciers
Drainage routeways
Glacial runoff regime
Glacifluvial erosion
Glacifluvial deposition
Glacial lakes
NB Glacifluvial = fluvioglacialNigardsbreen, Norway, June 2004
2. Sources of water on glaciers
Surface sources: Surface melt of snow and ice
Rainfall runoff
Lake discharge
Basal and internal sources: Melt of basal ice by geothermal heat
Frictional melt at glacier bed
Internal friction
Advective heat flux from meltwater
Subglacial groundwater
Supraglacial streams Moulins and crevasses Englacial channels and conduits Marginal & sub-marginal channels Subglacial routeways:
Sheet flow (< 1mm thick) Channels Linked cavities Braided ‘canals’
Proglacial streams
3. Drainage routeways
Cold-ice glaciers do not have internal drainage routeways: supraglacial, marginal and proglacial routeways only.
Supraglacial stream
Subglacial stream
Drainage routeways (continued)….
Ice-marginal stream draining into a proglacial lake
Subglacial stream emerging at a portal at the glacier terminus
Drainage routeways (continued)….
Left: ice-marginal stream draining Mackensen Glacier, arctic Canada
Left: subglacial stream emerging from Nigardsbreen, Norway
Above: sub-marginal drainage at Schei Glacier, arctic Canada
4. Glacial runoff regime
Discharge of the Schei River, arctic Canada, June to August
No flow before
late June
Nival flood
Diurnal discharge
fluctuations Low air temperature in late July -
low discharge
Warmer air temperatures
Rainstorms
Only 2-3 months of runoff, but numerous flood events due to diurnal melt of snow and ice, and flashy response to rainstorms
Jokulhlaup:
Glacial outburst flood, usually caused by breaching of an ice dam or moraine dam.
Jokulhlaup on Sverdrup River, arctic Canada: discharge >2500 m3 s-1
Left: jokulhlaup caused by volcanic eruption melting ice cap, Iceland
Meltwater streams are important geomorphological agents:
• Hydrologic regime - numerous short-lived flood events.
• Abundant readily-entrained glacigenic sediment (till, etc) - high sediment load and effective bed abrasion.
• Subglacial streams flow rapidly under hydrostatic pressure.
5. Glacifluvial erosion
Glacifluvial erosion forms meltwater channels:
Marginal, submarginal and subglacial meltwater channels
Submarginal channel, Skye Subglacial channel, Lewis
Subglacial chutes
Form when meltwater drops down a moulin then flows laterally across the glacier bed:
Subglacial chutes at the foot of the Lomond Scarp
Moulin
Col channels
Form when a supraglacial or englacial stream is superimposed on to the underlying topography by ice-sheet downwastage:
Lake overflow channels
Channels cut by drainage of a glacier-dammed lake across a col:
Col channel, Strathrory, Easter Ross
Proglacial channels Formed by meltwater flowing from glacier termini.
Many present rivers flow in former proglacial channels.
Abandoned proglacial meltwater channel, Sør-Illabreen, Norway.
6. Glacifluvial deposition
Characteristics of glacifluvial deposits are similar to those of alluvial deposits:
1. Often stratified (bedded) due to changing flow conditions:
Glacifluvial deposits, Barrie, Southern Ontario
Glacifluvial deposits are a major source of sand and gravel for concrete, roads and other construction industries.
6.1 Depositional landforms
6.1.1 Ice-contact glacifluvial landforms
Origin of ice-contact glacifluvial landforms (from Strahler and Strahler, 2005)
Esker: course of a subglacial stream, infilled by glacifluvial sand and gravel.
Kame: glacifluvial sands and gravels dumped from an englacial or supraglacial position
Kame terrace formed at former glacier margin, Ellesmere Island, arctic Canada
Kame terrace
Proglacial outwash plain (sandur)
Kettle holes (kettles) are enclosed depressions formed by:
1. Burial of blocks of stagnant glacier ice under glacifluvial or glacial sediments during glacier retreat.
2. Slow melting of buried ice to form an enclosed hollow.
Developing kettle hole, Iceland
Ancient kettle hole, Glen Ling, Scotland
6.1.2 Proglacial glacifluvial landforms: sandar (outwash plains)
A sandur (plural sandar) is a glacifluvial floodplain.
Sandur plain (outwash plain) - topographically unconstrained (e.g. southern Iceland):
Valley sandur (outwash train) - forms a valley fill in mountain areas:
Characteristics of sandar
Braided channel network
Rapidly-changing flow - frequent avulsion
Downstream fining of gravel.
Sandar deposits laid down during rapid glacier retreat often form kettled sandar (pitted sandar).
Fluvial incision into sandar creates outwash terraces.
7. Glacial lakes (ice-dammed lakes)
Form where glacier ice forms a barrier to drainage:
Range in size from small ponds to inland seas: Glacial Lake Agassiz (North America) covered 2,000,000 km2.
Various locations: Marginal lakes in ice-free tributary
valleys
Trunk-valley lakes dammed by tributary valley glaciers
Glacier confluence lakes
Proglacial lakes.
Glacial lake dammed by Mackensen Glacier, Ellesmere Island: a trunk valley lake dammed by a glacier crossing a major valley.
Landforms and deposits associated with former
ice-dammed lakes
Shorelines formed at lake margins - usually indicate lake drainage via a outflow channel across a col.
Raised deltas, where streams entered the former lake.
Glacilacustrine deposits: fine-grained rhythmites containing dropstones.