Periglacial Processes
Periglacial
was
originally used to describe places near to or on the fringe of ice sheets or
glaciers.
Now it is defined more widely, usually referring to areas:
With permafrost;
With some seasonal temperature variation where the mean temperatures for at
least some period in the summer rise above 0ºC;
Where freeze-thaw cycles dominate the landform creating processes;
Which have a distinctive ecosystem adapted to the periglacial environment.
Such areas as these now make up approximately 25% of the world’s total
land area! |
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Key terms
Permafrost - Permanently frozen ground where soil temperatures have
remained below 0 °C for at least 2 years
Continuous Permafrost - Summers so cold that there is only a very
superficial surface melting of the ground. It has been estimated to reach up to
a depth of 1500 metres. Mean annual air temperatures of below -5 °C all year,
and as low as -50 °C.
Discontinuous Permafrost
- Found is slightly warmer areas so there are
islands of permanently frozen ground separated by small pockets of unfrozen less
cold areas. Slightly warmer zones due to proximity of surface water (rivers,
lakes, and the sea). Mean annual temps of between -1 °C and -5 °C
Sporadic Permafrost - Found when mean annual temperature is just below 0ºC and the summer
temperatures reach several degrees above but isolated pockets of permanently
frozen ground remain below the surface.
Active Layer - summer temperatures sufficient to melt the surface layer of
permafrost. This layer can be very mobile. It varies in thickness depending on
latitude and vegetation cover.
Talik
- Any unfrozen
material within the permafrost zone.
Generally, as you progress from the poles Southwards (in the northern
hemisphere) or Northwards (in the Southern Hemisphere) the permafrost will
change from continuous to discontinuous to sporadic to none.
The depth of permafrost will shrink and the depth of the active layer
will increase. This is of course
influence by local variations such as mountain ranges, lakes and ocean currents.
Processes;
Nivation -
The effects of snow on a
landscape. These include abrasion and freeze-thaw. Furthermore, melted snow
triggers mass movements such as solifluction and slope wash. These processes may
produce the shallow pits known as nivation hollows. In time, these
hollows may trap more snow and may deepen further with more nivation so that
cirques or thermocirques are formed.
Frost heave,
The upward
dislocation of soil and rocks by the freezing and expansion of soil water.
Frost push occurs when cold penetrates into the ground. Large stones become
chilled more rapidly than the soil. Water below such stones freezes and expands,
pushing up the stones. Frost pull can alter the orientation of a large
stone causing it to stand upright. This occurs when ice creeps downwards from
the surface. The growth of ice crystals on the upper part and the drying of the
soil around the lower part cause the stone to be pulled into a more vertical
inclination.
Solifluction -
This is the mass movement of soil and regolith affected by alternate freezing and thawing.
It is characteristic of saturated soils in high latitudes, both within and beyond the
permafrost zone.
Hydraulic pressure – the movement of water through a rock profile
due to pressure
Hydrostatic pressure – growth of an ice lens due to the
attraction of water molecules
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