Geography GE2011: Glacial and Periglacial Processes
Periglacial Processes and Landforms
Recommended reading
Murray, T. (2005) Permafrost and
periglaciation. In Holden, J. (ed.)
Physical Geography and the
Environment. Pearson, Harlow, 468-87.
Strahler, A.H. and Strahler, A.N. (2002)
Physical Geography, 2nd Edition, Wiley,
New York, 426-434.
Strahler, A.H. and Strahler, A.N. (2005)
Physical Geography, 3rd Edition, Wiley,
New York, 437-444.
Pingos (ice cored hills), 78° N
Spitsbergen, March 2005
1. Introduction
Periglacial: ‘the conditions, processes and landforms
associated with cold nonglacial environments’
(Harris et al., 1988).
Periglacial environments:
1.
Active periglacial environments
2.
Relict periglacial environments.
Periglaciation: ‘the collective and cumulative effects of
periglacial processes in modifying the landscape’
(Ballantyne and Harris, 1994).
2. Permafrost
‘Ground (soil or rock) that remains at or below 0°C’.
Permafrost is a thermal condition - ice may not be
present.
Permafrost is overlain
by a zone of seasonal
freezing and thawing:
the active layer:
Permafrost classification:



Continuous permafrost
Discontinuous permafrost
Sporadic permafrost
Permafrost underlies
~26% of the Earth’s land
surface, including 82%
of Alaska and 50% of
Canada, much of
northern Siberia.
In North America:
Southern limit of continuous permafrost coincides
approximately with -6°C to -9°C MAAT isotherms.
Southern limit of discontinuous permafrost extends
to -1°C isotherm.
Resolute, 74°N:
Permafrost depth = 396 m
Active layer depth = 0.5 m
Hay River, 61°N:
Permafrost depth = 12-14 m
Active layer depth = 1.5-3.0 m
Thermal regime of permafrost and the active layer
3. Ground ice
‘Ice formed in freezing and frozen ground’
3.1 Ice lenses and massive ice beds
Ice lenses are formed by ice segregation
Massive ice beds form by: 1. Ice segregation
2. Hydrostatic pressure
3. Burial of glacier ice
Ice lens in a
block of
frozen soil
Massive
ground ice
beds
Ground ice landforms: pingos and palsas
Pingos formed
by growth of
massive ice by
hydrostatic
pressure
Pingos, Reindalen, Spitsbergen
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Palsas (ice-cored peat
mounds), Vallée des Trois
Baies, Québec-Labrador.
3.2 Ice wedges
Formed by thermal cracking of permafrost during winter.
Meltwater trickles into the
crack and freezes against
the permafrost.
Over long periods a thick
ice wedge develops.
Ice-wedge polygons, Ellesmere Island, arctic Canada
Ice wedges in continuous permafrost, arctic Canada
Thermokarst landforms
Thermokarst landforms are those produced by the thaw
of ground ice, and are found in areas where continuous
permafrost once existed (e.g. Great Britain).
4. Frost action in soils
Freezing processes in soil:
 Frost-susceptible soils: ice segregation (lenses)
 Non-frost-susceptible soils: pore ice only formed
Frost-susceptibility is determined by the size of voids in the soil
and thus by grain-size distribution:
Well-sorted sand (left) and
well-sorted sand with 10%
silt (right).
Frost-susceptible soils
generally contain 3-10% by
weight finer than 20 µm.
5. Frost heave
Definition: ‘the upward and outward movement of the
ground surface caused by formation of ice in the soil’.
1.Active layer thawed
2.Active layer frozen - no ice segregation: heave < 3%
3.Active layer frozen with ice segregation: heave up to 50%.
Large freezing pressures (up to 1 MPa) can develop
as the ground freezes and heaves.
6. Frost weathering of rock
= Breakdown of rock through repeated freezing and thawing
• Water in rocks expands by 9% on freezing
• Two effects:
• Macrogelivation: breakdown of rock into angular clasts
by water freezing in joints
• Microgelivation: small-scale breakdown of rock into
particles (silt, sand) by freezing of water in pores and by
formation of ice lenses - often causes rounding of rock
surfaces by granular disaggregation
• Landforms: frost-shattered bedrock, blockfields and other
forms of frost-weathered detritus.
Blockfields: areas of frostweathered bouldery debris,
lacking surface fines.
Gneiss blockfield with pegmatite
vein, Jotunheimen, Norway
Quartzite blockfield, An Teallach,
NW Scotland
NB: Many exposed boulder
surfaces have been rounded
by microgelivation.
Other types of frostweathered detritus
Sandy diamictons: clasts
embedded in a sand-rich
matrix (e.g. sandstones, most
granites)
Silty diamictons: clasts
embedded in a silt-rich matrix
(e.g. mica-schists, shales)
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Photo - JPEG decompressor
are needed to see this picture.
7. Frost Sorting and Patterned Ground
Frost sorting:
sorting of debris by freezing and thawing of the ground
Patterned ground: Terrain that exhibits surface patterning
Classification of patterned ground:
Sorted patterns: defined by alternation of fines and clasts
Nonsorted patterns: defined by vegetation or microrelief
Sorted patterns:
Sorted stripes, Faroe Islands
Sorted circles, Fauldalen, arctic Norway
Main sorten patterns: nets, circles, polygons and
stripes. Secondary forms: steps, ovals and garlands
Many forms grade into others. For example, as
gradient increases, nets become elongated forming
garlands or ovals, then stripes.
Patterned ground varies in size
from several centimetres to a
few metres:
Formation of sorted patterns
General model:
Two main models of patterned ground formation
1. Freezing model: during freezing, frost penetration
is uneven. Clasts are heaved upwards to the
surface and outwards towards cell margins.
2. Thaw model: during thaw an unstable density
configuration sets up convection cells in the soil.
8. Periglacial mass-movement: solifluction
Solifluction: ‘ slow downslope movement of soil due
to freezing and thawing of the ground’
= frost creep + gelifluction
(see Lecture 10: Soil Creep)
Solifluction landforms
Relict solifluction sheet, Ben Wyvis
Active solifluction lobes, Fannich Mts
Solifluction lobes, Måtind, Andøya
Ploughing boulders, Fannich Mts.
9. Periglacial Mass movement: Rock Glaciers
Rock glacier:
a thick lobate or tongue-shaped mass of debris that
has moved slowly downslope through deformation of
internal ice.
Two types:
Talus rock glaciers (form in permafrost areas)
Glacigenic rock glaciers (form in glacial environments
with a high debris supply)
Movement is by ice creep (like glaciers, but much
slower). Surface displacement: few cm yr-1 to 1 m yr-1
Talus rock glacier:
Ferintosh Creek
rock glacier, Ohau
Range, NZ.
Talus-derived
debris is carried
downslope through
deformation of a
massive ice bed in
permafrost.
Glacigenic (glacially-derived) rock glacier, Himalayas;
formed by burial of glacier under supraglacial debris.
Relict talus rock glaciers, Lyngen Peninsula, arctic Norway.
Relict talus rock
glacier, Cairngorm
Mountains
Talus rock glaciers formed in Scotland 12,000 years
ago, implying permafrost conditions at that time.