Periglacial Environments
Originally defined as the zone peripheral to glaciers. Now defined as
near-glacial in the sense of either location or conditions:
-
perennially frozen ground (permafrost)
-
seasonally-thawed ground (active layer)
-
incomplete vegetation cover of herbaceous plants and
dwarf trees
-
ground is snow free for part of the year
-
frequent fluctuations of air temperature across 0º
C
Permafrost
- ground with a temperature perennially below 0o
C
- Water is usually present when ground freezes and ice
is usually associated with permafrost
- pore ice
- ice that forms in pore spaces and fractures and cements the soil
matrix
- forms as freezing plane descends into the ground without
displacing soil
- segregated ground ice (sporadic permafrost)
Origin and distribution of permafrost
- with a mean annual temperature less than 0oC,
the depth of frost penetration exceeds the depth of thaw. if this
climate persists, permafrost depth increases each year to thicknesses of
several hundred metres, with maximum depths of about 1500 metres in parts of
Siberia.
- permafrost underlies about 20% of the earth's land surface or about
50% of canada, in three zones:
- continuous permafrost is everywhere except under deep lakes
- discontinuous
permafrost absent under water bodies and warmer sites (e.g.
south-facing slopes), north of about 55o N in Canada
- sporadic permafrost is preserved at scattered sites, e.g.
northern-facing slopes or peat bogs, where the peat prevents melting
(insulates) in summer
Geomorphic significance of permafrost
- confines water and frost to the active layer between
the permafrost table and the ground surface
- descent of the freezing plane from the surface pressurizes the soil
water, reducing the freezing temperature and maintaining the thawed
(active) during autumn freeze up
- the growth and decay of segregated ground ice causes frost heave and
subsidence
Challenges |
Opportunities |
Low Temperatures |
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Diesel freezes at -50 degrees Celsius. It is a common
practice to light a bonfire beneath a vehicle fuel tank to keep it from
freezing. Axle grease also freezes and is warmed with a blowtorch.
-
Pen ink freezes. Batteries lose power faster. Metal sticks to
skin.
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Oymyakon is an industrial centre on the banks of the river Lena in north-eastern
Russia. Temperatures here have plummeted to a record low of -71.2 degrees
Before the 1920s and 30s, Oymyakon was a seasonal stop for reindeer herders.
But the Soviet government, in its efforts to settle nomadic populations—claiming
they were difficult to control and technologically and culturally backward—made
the site a permanent settlement.
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Frost Action
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- dominant set of periglacial processes given freeze thaw cycles
and commonly wet conditions of the active layer
a. shattering (wedging, splitting)
- mechanical weathering (disintegration) caused by force of ice
and dense water in fractures
b. heave
- displacement of soil and rock with the growth of segregated
ground ice as free water migrates to the freezing plane (lower
vapour pressure)
- produces hills (pingos) with a
core of segregated ice
- frost pull
- the entire soil mantle expands, or is pulled up, as the
freezing plane descends from the surface
- with thawing, the cohesive matrix of fine material retracts,
filling the space beneath clasts and leaving them in a slightly
elevated position relative to the preceding thaw season
- this process can pull clasts towards the surface, but not
through it
- frost push
- ice forms beneath rock fragments (clasts) because of their higher thermal
conductivity (more rapid heat loss)
- thus the clasts are pushed towards and eventually through
the surface, because the cohesive matrix retracts into the
spaces under the clasts when the ice melts in the spring.
- Dealing with dead people is a problem. Buried coffins tend
to rise to the surface after several years.
- needle ice
- slender ice crystals that form at night in moist loamy
periglacial soils
- typically 1-3 cm in length, but up to 40 cm
- as the ice needles growth, the soil is dessicated and
disturbed and thus becomes more susceptible to wind and water
erosion
c. cracking
- thermal contraction of sediments and ice at very low
soil temperatures that occur with low air temperatures and a lack of
snow and vegetation cover
- water seeps from the active layer into vertical cracks up to a
metre or more in depth. this water freezes and then cracks,
because ice has less tensile strength than frozen ground
- the repeated cracking and growth of ice creates ice wedges,
segregated ice that is wedge-shaped in cross section
-
- frost creep
- soil creep is enhanced by expansion and contraction of the
active layer with freezing and thawing
- solifluction (gelifluction)
- slow flow of the active layer over the permafrost table
- occurs on slope of 5-20o
- above 20o, periglacial slopes are subject to more
rapid mass wasting
- rockfall and rock avalanches on steep rock slopes
- earthflow and debris flow in unconsolidated materials
- massive landsliding in thawing permafrost and ground ice
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Human attempts to exploit periglacial
regions commercially have only been made relatively recently.
It was not until the closing decades of the
19th century that gold discoveries and overpopulation in European
Russia led to substantial migration and settlement into Alaska and
Siberia respectively; exploitation of coal reserves in the Arctic
areas of Scandinavia also began about this time. During the 20th
century more systematic surveying has revealed the Arctic to be a
rich source of a wide variety of minerals, including oil and
natural gas, iron ore, nickel, lead, zinc, uranium, tin, diamonds,
and cryolite, as well as gold and coal. Exploitation of these
minerals has encouraged a new influx of migrants, particularly
since the 1960s.
In North America concern over the Arctic
environment began in the 1970s when a project to build oil and gas
pipelines through the Mackenzie valley of northern Canada was
postponed indefinitely. Plans to build the pipelines underground had
been rejected because of the risk of rupture from earth movements
following the melting of the permafrost. An alternative plan to
raise the pipelines on stilts also eventually had to be shelved
because it would have interrupted the migration of large mammals
like the caribou.
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Nivation |
-
-
- geomorphic activity enhanced by
snow that
persists into the melt season
- periglacial environments commonly have less snowfall than
warmer climates, especially temperate mountains, but the duration
of snow cover is long and thus snow has much ecological and
geomorphic significance
- wet snow and slush
avalanches
tend to be dense and full depth (unlike mid-winter powder
avalanches) and thus can be effective geomorphic agents on arctic
and alpine slopes creating, under extreme conditions,
avalanche
plunge pools
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Fluvial processes |
- much of the year, water is stored as snow and ice, however,
water is released violently during a short melt season
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Eolian (Wind) processes |
- favoured by incomplete vegetation cover, braided stream
deposits, cryoturbation and dessication (freeze drying) of surface
sediments, and exposure to strong winds
- thus many present and former periglacial environments are
mantled with loess (e.g. northern China; upper Mississippi basin,
mid-western US)
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- Loess deposits are unusually fertile (wheat belt in USA)
- Used as a means for building dwellings
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Human Action |
Lowering the permafrost table
- Removal of vegetation for construction means that in summer
more heat penetrates the soil and so the depth of thaw increases,
as does the likelihood of flooding
- Construction of centrally heated buildings has warmed the
ground underneath them, causing the buildings to subside
- Siting of oil, sewerage and water pipes in the active zone has
increased the rate of thaw, sometimes causing breaking of the
pipes as the ground moves
- Drilling for oil and gas poses problems because the heat from
the drilling fluid melts the permafrost
- Road construction upsets the delicate equilibium of many
slopes produced by solifluction
Raising the permafrost table
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Improved communications links and new
engineering techniques—for example the use of stilts to raise
buildings above ground level to overcome problems caused by
permafrost melting in the summer, or of utilidors, specially
designed and insulated overground conduits, to protect sewers and
water mains—have allowed the rapid growth of a number of small
settlements into sizeable towns and, in Scandinavia and Siberia,
the faster development of cities such as Murmansk, Yakutsk and
Tromsø. |
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