what are periglacial processes?
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WHAT ARE PERIGLACIAL PROCESSES?
Learning destination: understand what permafrost, frost heave, groundwater
freezing, nivation and solifluction are and what landforms they create
Routes: Identify, describe and explain the formation of nivation hollows, ice wedges, patterned ground, pingos and solifluction
lobes
QUICK QUESTION… What glacial feature does this Wordsworth
poem describe?
“As a huge Stone is sometimes seen to lieCouched on the bald top of an eminence;Wonder to all who do the same espy,By what means it could thither come, and whence;So that it seems a thing endued with sense:Like a Sea-beast crawled forth, that on a shelfOf rock or sand reposeth, there to sun itself”
W Wordsworth 1807 The Leech Gatherer
WHAT ARE PERIGLACIAL AREAS? Areas that experience a cold climate, with intense
frost action and the development of permafrost Aka tundra areas 20-25% of the Earth’s surface is periglacial, e.g.
northern North America and Russia above the Arctic Circle
Experience conditions close to glacial (near an ice sheet e.g. Greenland). Peri = near. Consistently below 0 degrees celsius.
Temporally close to glacials (e.g. Highland Scotland)
Very cold climate with distinctive landforms May exhibit temperate climates today
Where are periglacial environments?• High altitudes – around ice masses in mountain ranges.
Also in high altitude plateau areas e.g. the Tibetan plateau and Bolivian plateau
• High latitudes e.g. northern Asia, North America and northern Europe
• Continental interiors – due to continentality, e.g. Siberia, central Asia
PERMAFROST Permanently frozen ground Impermeable Subsoil temperatures must remain below zero
for 2 years or more for permafrost to develop The extent, depth and continuity of the
permafrost layer varies through time according to fluctuations in climate
During summer, when air temperatures are above freezing, the surface layer thaws to form an active layer up to 4m deep.
The active layer gets easily waterlogged due to frozen ground beneath so will easily flow if there is a gradient - solifluction
3 TYPES OF PERMAFROST Continuous – found in coldest regions
(mean below -5) e.g. the Arctic where there is little thawing even in summer. Affects soil and rock to a depth of 700m in Canada and twice that in Siberia
Discontinuous – found in slightly warmer regions where freezing conditions do not penetrate to such great depths (20-30m). Discontinuous due to breaks around rivers, lakes and the sea. Patches are frozen.
Sporadic – mean annual temperatures are around or just below freezing, so permafrost appears only in isolated spots
EXAM QUESTIONS – JUNE 2010 Define the term ‘periglacial’ – 2 marks
(also in June 2011) Explain the annual changes in the active
layer above the permafrost – 4 marks
THE ANSWERS…
Describe the distribution of permafrost shown in Figure 3 – 4 marks
June 2012
MARK SCHEME JUNE 2012
PERIGLACIAL PROCESSES
FREEZE-THAW ACTION (FROST
SHATTERING)Blockfield
Felsenmeer
FREEZE-THAW ACTION (FROST SHATTERING) Provides a great deal of erosive material
in glaciers, so already studied In periglacial areas, screes develop at
the foot of slopes due to frost shattering On relatively flat areas, extensive
spreads of angular boulders are left, known as blockfield or felsenmeer (sea of rocks)
E.g. the Glyders, North Wales
FROST HEAVEPatterned ground
Stone polygons
Stone stripes
FROST HEAVE Results from the direct formation of ice
crystals in the soil as it starts to refreeze On freezing, fine-grained soils expand
unevenly upwards to form domes. As stones cool down faster than the
surrounding soil, small amounts of moisture in the soil beneath the stones freeze and turn to ice, expanding by 9% as they do so.
By repeatedly freezing and thawing over time, these ice crystals and lenses heave stones upwards in the soil.
FROST HEAVE CONT… In areas where temperatures fluctuate
between 0 degrees and -4 degrees C, the frost heaving and subsequent thawing is able to sort material to form patterned ground.
The larger stones move outwards down to the very low slopes of smaller domes because of their weight.
On gentler slopes stone polygons are created, but where the ground is steeper (greater than 6 ) L the stones are dragged downhill by gravity into more linear arrangements known as stone stripes.
Describe the patterned ground shown in Figure 2 and explain its formation – 6 marks
MARK SCHEME…
GROUNDWATER FREEZING
Pingos
GROUNDWATER FREEZING Freezing of water in upper layer of soil
where permafrost is thin or discontinuous leads to the expansion of ice within the soil
This causes the overlying sediments to heave upwards into a dome-shaped feature known as a pingo
Less than 50m in height, 0.5km across, found in sandier soils = open-system or East Greenland type
PINGOS Closed-system pingos or Mackenzie type pingos
are more typical of low-lying areas with continuous permafrost
On the site of small lakes, groundwater can be trapped by freezing from above and by the permafrost beneath as it moves in from the lakeside
Subsequent freezing and expansion of trapped water pushes the overlying sediments into a pingo form
If the centre collapses it may infill with water to form a small lake
Over a thousand of these pingos have been recorded in the Mackenzie delta (Canada)
TASK Draw 3 diagrams:1. Formation of an open-system (East
Greenland) type pingo2. Formation of a closed-system
(Mackenzie) type pingo3. A ruptured pingo
EXAM QUESTION – JAN 2012 Draw a labelled sketch to show
characteristics of a pingo and suggest an explanation for its formation – 7 marks
MARK SCHEME
GROUND CONTRACTION
Ice wedges
Ice wedge polygons
GROUND CONTRACTION Refreezing of the active layer during winter
causes the soil to contract and cracks open up on the surface
During melting the following summer, the cracks open again and fill with meltwater and its associated fine sediment, which helps to partially fill the crack
Repetition widens and deepens the crack to form an ice wedge up to 1m wide and 3m deep
A near polygonal pattern is produced on the surface, similar to frost heave polygons
Forms ice wedge polygons
ICE WEDGE POLYGONS
ICE WEDGE DEVELOPMENT
NIVATION Nivation hollows
NIVATION Occurs mainly between north and east
facing slopes beneath patches of snow in hollows
Frost action below snow which involves freeze-thaw and solifluction and meltwater
Freeze-thaw disintegrates underlying rock
During spring thaw, weathered particles moved downslope by meltwater and solifluction
Leads to nivation hollows which may be the start of corrie development
SOLIFLUCTIONSolifluction lobes
SOLIFLUCTION Summer thaw in the active layer
releases a lot of meltwater Water cannot percolate downwards due
to frozen ground so it saturates the soil It reduces internal friction between
particles making the soil mobile The soil flows even on slopes of only a
few degrees Leaves behind rounded tongue-like
features forming terraces on the side of valleys – solifluction lobes
SOLIFLUCTION LOBES Stepped features below vegetation,
pushed forward and rolled under like Caterpillar truck
Where vegetation is sparse, stones heaved to the surface are pushed to the front of the advancing lobe and form a small stone bank at the front of the lobe
Many parts of southern Britain experienced these conditions during the Quaternary ice age and these deposits, which filled in valleys are known locally as head (coombe in chalky areas)
WATER AND WIND ACTION
(AEOLIAN PROCESSES)
WATER… Periglacial areas are often open and
sparsely vegetated. This means that erosion by water and wind can be high.
Water erosion is seasonal, occurring in mainly spring and summer when the active layer melts.
This can bring short periods of high discharge in rivers bringing high levels of fluvial erosion.
Drainage is braided due to high amounts of debris being carried by meltwater streams.
WIND… Unobstructed winds reach high velocities Cause erosion through abrasion They dislodge fine, unconsolidated materials. Result in grooved and polished rock surfaces
and in stones shaped by the wind, called ventifacts
Fine material of outwash plain is picked up and carried long distances
It is deposited elsewhere as areas of loess Loess is found in many parts of North
America and Eurasia, just south of the Pleistocene ice sheet limit
AN EXAM QUESTION TO TEST YOU ON
ALL OF THE PROCESSES…
EXAM QUESTION – JUNE 2011 Outline periglacial processes likely to be
occurring around the glacier in Figure 2 – 5 marks
MARK SCHEME