Periglacial areas are those which, although not actually glaciated, are exposed to very cold conditions with intense frost action and the development of permantly frozen ground or permafrost.
As present, areas such as the tundra of northern Russia, Alska and Canada, together with a high mountainouos area such as the Alp, experience a periglacial climate. In the past however, as ice sheets and glaciers spread, many areas which are now temperate were subject to such conditions.
The climate of periglacial regions is marked persistently low temperatures. Summers are short but temperatures can sometimes reach about 15 degrees celcius. In winter, the temperatures remain well below zero and in some areas many fall below -50 at times.
Where subsoil temperatures remain below zero for atleast two consecutive years permafrost will occur. When summer temperatures rise above freezing the surface layer thaws from the surface downwards to form an active layer. The thickness of this layer depends on the conditions but may extends to 4m. As the ice in this layer begins to melt, large volumes of water are released. This water is unable to drain through the permafrost layer and, as low temperatures do not encourage much evaporation, the surface becomes very wet. On slopes as gentle as 2 degrees saturation of this upperlayer encourages soil movement downslope, a periglacial process known as solifluction. There are 3 kinds of permafrost:
- Continous permafrost- is found in the coldest regions, reaching deep into the surface layers. In Siberia today it is estimated that the permafrost can reach down over 1500m. In the very coldest areas there is hardly any melting of the upper layer.
- Discontinous permafrost- occurs in regions that are slightly warmerm where the ground isnt frozen as deep. Usually around 20-30m below the surface. There are also gaps in the permafrost under lakes, rivers and near the sea.
- Sporadic permafrost- is found where the annual temperatures are around or just below freezing. In these places permafrost only occues in the isolated spots where the climate is cold enough to prevent complete thawing of the soil during summer.
Freeze-thaw action -this process has already been descibed in detail on,previous card. In periglacial areas, screes develop at the foot of slopes as a result of frost shattering. On relatively flat areas, extensive spreads od angular boulders are left, which are known as blockfeild or felsenmeer (sea of rocks).
Nivation- takes places beneath a patch of snow in hollows, paritcularly on the north and east facing slopes. Freeze-thaw action and possibly chemical weathering, operating under the snow cause underlying rock to disintigrate. As some of the snow melts in spring, and weathered particles are moved downslope by the meltwater and by solifluction. Over some period of time this leads to the formation of nivation hollows, which when enlarged can be the beginnings of corries.
Solifluction- when the active layer thaws in the summer, excessive lubrication reduces the friction between soil particles. Even on slopes as shallow as 2 degrees, parts of the active layer begin to move downslope. This leads to solifluction sheets or lobes - rounded tounge like features often forming terraces on the sides of valleys.
Periglacial processes - 2
Frost heave - as the active layer starts to refreeze, ice cyrstals begin to develop. They increase the volume of the soil and cause an upward expansion of the soil surface. Frost heave is most significant in fine-grained material, and as it is uneven, it forms small domes on the surface.
Within the fine-grained material there are stones which because of thier lower specific heat capactiy, heat up and cool faster than the sourrounding fine material. Cold penertrating from the surface passes through the stones faster than through the sourrounding material. This means that the soik immediatly beneath a stone is likely to freeze and expand before the other material, pushing the stone upwards until it reaches the surface. On small domes, the larger stones move outwards, effectively sorting the material which, when viewed from above takes on a pattern. This patterned ground on gentler slopes takes the form of stone polygonsm, but where the ground is steeper, the stones moves downhill to form stone stipes.
Periglacial processes 3
Groundwater freezing- When the permafrost is thin or discontinuous, water is able to seep into the upper layer of the ground and then freeze. The expansion of this ice causes the overlying sediments to heave upwards into a dome shaped feature known as a pingo. This may rise 50m high. This type of pingo is referred to as an open-system or East Greenland type.
In low lying areas with continous permafrost on the site of small lakes, groundwater can be trapped by freezing from above and the permafrost beneath. As this water freezes it will expand, pushing up the overlying sediments into a closed system pingo or mackenzine type. Sometimes the surface of a pingo will collapse, leaving a hollow that is filled with meltwater.
Ground contraction- the refreezing of the active layer during winter causes the soil to contract. Cracks open on the surface in a similar way to cracks on the beds of dried up lakes. During melting the following summer, the cracks open again and fill with meltwater. As the meltwater contains fine sediment, this also begins to fill the crack. This process occurs repeatedly through the cycle of winter and summer, widening and deepening the crack to form and ice wedge, which eventually over a period of hundreds of years can become at least 1m wide and 2-3m deep. The cracking produces a pattern on the surface which when viewed from above is similar to polygons produced by frost heaving. These are therefore known as ice-wedge polygons.
Periglacial processes 4
Water and Wind Action- Water erosion is highly seasonal, occuring mainly in spring and summer when the active layer melts. This can cause short periods of very high discharge in rivers, bringing about far more fluvial erosion than would be expected given the relatively low mean percipitation. Drianage is typically braided because of the high amount of debris bring carried by meltwater streams.
Unobstructed winds blowing across periglacial landscapes can reach high velocities. They can cause erosion through abrasion and dislodge the fine unconsolidated materials that cover that area. The effects of erosion can be seen in grooved and polished rock surfaces and in stones shaped by the wind known as venifacts. The fine material of the outwash plain is picked up by the wind and carried long distances to be deposited elsehwere as extensive areas of loess.
In England loess deposits rarely more than 2m in depth cover parts of East Anglia and the London Basin where they are known as brick-earth deposits. In china, loess deposits are widespread and in places reaches depths of over 300m.