Periglacial
From Wikipedia, the free encyclopedia
 |
This article does not cite any references or sources. (March 2008) Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed. |
Periglacial is an adjective referring to places in the edges of glacial areas, normally those related to past ice ages rather than those in the modern era. That is to say, at the time in question, the area was not buried by flowing ice but was subject to severe freezing.
A periglacial lake is one formed where the natural drainage of the topography is obstructed by an ice sheet, ice cap or glacier.
Contents |
[edit] Periglaciation
Periglaciation is the corresponding noun. It means 'periglacial conditions', that is principally, an area of permafrost - intense freezing, perhaps with freeze/thaw of the surface. That is to say, the surface layer melts briefly in summer. Periglaciation occurs near mountain glaciers. At lower levels it forms a zone of cold around continental glaciers in areas of high latitudes, covering perhaps 20% of the earth’s land surface.
[edit] Effects
Periglacial conditions in the Pleistocene created landscapes and geological conditions moulded by frost action; the repeated freezing and thawing of material over many years. Around a third of the earth's land surface can be considered as having been subject to periglacial conditions at some time.
Periglaciation results in a variety of ground conditions but especially those involving irregular, mixed deposits created by ice wedges, solifluction, gelifluction, frost creep and rockfalls.
The resulting ecological community is known as tundra.
[edit] Factors affecting location
- Latitude – temperatures tend to be higher towards the equator. Periglacial environments tend to be found in higher latitudes. Since there is more land at these latitudes in the north, most of this effect is seen in the northern hemisphere. However, in lower latitudes, the direct effect of the sun's radiation is greater so the freeze-thaw effect is seen but permafrost is much less widespread.
- Altitude – Air temperature drops by approximately 1 °C for every 100 m rise above sea level. This means that on mountain ranges, modern periglacial conditions are found nearer the Equator than they are lower down.
- Ocean Currents – Cold surface currents from polar regions, reduce mean average temperatures in places where they exert their effect so that ice caps and periglacial conditions will show nearer to the Equator as in Labrador for example. Conversely, warm surface currents from tropical seas increases mean temperatures. The cold conditions are then found only in more northerly places. This is apparent in western North America which is affected by the North Pacific current. In the same way but more markedly, the Gulf Stream affects Western Europe.
- Continentality – Away from the moderating influence of the ocean, seasonal temperature variation is more extreme and freeze-thaw goes deeper. In the centres of Canada and Siberia, the permafrost typical of periglaciation goes deeper and extends further towards the Equator. Similarly, solifluction associated with freeze-thaw extends into somewhat lower latitudes than on western coasts.
[edit] Landforms associated with periglacial environments
Coombe and head deposits Coombe deposits are chalk deposits found below chalk escarpments in Southern England. Head deposits are more common below outcrops of granite on Dartmoor.
Patterned Ground is stones which form circles, polygons and stripes. Local topography affects which of these are expressed. A process called frost heaving is responsible for these features.
Solifluction lobes are formed when waterlogged soil slips down a slope due to gravity forming U shaped lobes.
Blockfields or Felsenmeer are large angular blocks created by freeze-thaw action. A good example of a blockfield can be found in the Snowdonia National Park, Wales. Blockfields are common in the unglaciated parts of the Appalachian Mountains in the northeastern Untited States, such as at the River of Rocks or Hickory Run Boulder Field, Lehigh County, Pennsylvania.
Other landforms include:
