Describe and explain the variety of landforms found in periglacial areas
The term periglacial literally means ‘around the ice’. Periglacial regions are characterised by persistently low temperatures, but are not covered by glacial ice. The processes associated with periglacial regions, usually take place in areas where the mean annual air temperature is below 3i??C. Such areas usually experience short cool summers and long cold winters. At present, areas such as the Tundra of northern Russia, the USA and Canada, together with high mountainous regions such as the Alps, experience a periglacial climate.
There are a number of different processes, which produce these periglacial landforms. These processes are frost action including shattering (wedging and splitting), heave and cracking, mass movement including solifluction, nivation and fluvial processes. Frost action is one of the most important processes in periglacial regions, which results in the physical breakdown of rocks into finer particles as water turns to ice and expands. Frost action can also cause the ground surface to expand upwards by up to 5cm per year, a process known as frost heave.
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In some periglacial areas, frost heave produces irregular, hummocky surfaces covered with small frost mounds. Frost action is responsible for the development of patterned ground, an array of small-scale, geometric features found at the surface of a regolith that has been disturbed by frost action. The group includes circles, polygons, and nets, which normally occur on level or gently sloping surfaces, and steps and stripes, which are found on steeper gradients. Both sorted and non-sorted varieties are recognized.
The sorted varieties are typically outlined by coarse, stony material. There are two types of patterned ground. There are small dome shaped polygons and stripes due to frost sorting and there are large polygons with raised edges due to frost wedging/ice contraction. The first type, existing due to the process of frost sorting, is a result of the heave/thrust of stones to the surface, causing surface sediments to be up domed, followed by surface sorting. This occurs on slopes of less than 6i??, and the polygons can be 1-5m in diameter.
Elongated stripes are formed on slopes with a gradient over 6i??. In autumn and winter, ice layers form in the fine debris and raise the dome even further. There is fossil patterned ground in Breckland in Norfolk, which consists of polygons with a diameter over 10metres. The second type, produced by frost wedging/ice contraction, occurs in areas where there is continuous permafrost (below -6i??C). During the winter, ground contraction and cracking produces an irregular polygonal pattern. Firstly, freezing occurs in random areas around the freezing centre.
Then the surrounding rock is drawn towards these freezing centres causing the rock to crack in an irregular polygonal pattern. In the early stages these are just hairline cracks but can reach up to 40mm across. In the spring or summer following, the active layer thaws and water seeps into the crack, which is surrounded by permafrost leaving a vain of ice in the permafrost. After successive years, more water is added and the vain enlarges to a wedge. An ice wedge is a tapering mass of pure ice, which is 1-2m across and is commonly 10m deep and occasionally 30m deep.
Sediments either side of the wedge get distorted upwards to create parallel ridges each side of the wedge. Preserved ice wedges are found in Wolverhampton embedded in the boulder clay. If thawing of the surface is continuous, ice wedges may melt and create small depressions. In areas where the wedges are particularly numerous and closely spaced, large-scale collapse of the surface may ensue to create major depressions, called alases. These hollows are likely to become centres of drainage, allowing lakes to form in them, which will prevent the permafrost from re-establishing itself at the surface.
In Siberia, co-alescence of many adjacent alases has created linear troughs, known as alas valleys, which reach tens of kilometres in length. A pingo is a small, dome-shaped isolated hill, measuring up to 60m in height and 300m across. In the centre of a pingo is an accumulation of ice, which often melts leaving a meltwater lake. These pingo’s are formed in two ways, either closed system/Mackenzie type or open system/east Greenland type. The first type is formed when a shallow lake uses stored heat from the summer sun to insulate the ground below, so there is no permafrost able to form surrounding the lake.
Instead of permafrost, saturated unfrozen sands surround the lake. Sediments and plants succession then infill the lake. The winter cold then penetrates and begins to freeze the talik (the unfrozen areas). Hydrostatic pressure and ice pressure cause the water to migrate upwards and freeze forming a lens of ice in the lake sediments. The lens grows and the surface is domed up forming a pingo. An example of a pingo formed in this way is the Mackenzie Delta in Northern Canada. The second type is formed in valley bottoms especially along spring lines. It is common in areas of discontinuous permafrost.
Areas of talik begin to freeze away from the edges and the water in the pores freezes causes ice pressure and hydrostatic pressure to be exerted on the frozen part. This pressure causes the water to be forced upwards through cracks and as it moves it freezes as a lens of solid ice. Over time more water is squeezed out turning to ice resulting in the ice lens growing. This forms the pingo. Pingos formed in this way are found in the Yukon valley in Alaska. Once fully formed, by either method, the surface sediments of the pingo may crack due to continual updoming.
During the summer thaw, the sediments of the active layer slip down the sides of the dome, which may expose the ice core or create a crater in the top of the pingo which may fill up with water in the summer. When the ice is exposed in this way, it begins to melt, when melting dominates the pingo decays. Or if an old pingo, and no water is being added, decays an ognip is left. This is a circular rampart of sludged material with a hollow/basin where the pingo once was. This may also fill with water. Another periglacial process, which creates landforms, is mass movement.
Mass movement occurs when gravity and/or water saturation causes downslope movement of weathers material. The most dominant of mass movement processes is solifluction. This is the slow downhill movement of surface sediments in saturated conditions. Solifluction occurs on slopes with min angle of 2i??. It is prevalent in permafrost areas because of the seasonal thawing of the ground to produce an active layer. It can cause complete mixture of rocks and soils at a particular site. Clays may form in a series of lobes. Other materials form sheets.
Solifluction terraces are step-like features with `risers’ up to 15 m high and `treads’ up to 500 m long. The solifluction sometimes occurs under a vegetation mat, which is pushed forwards and rolled under. This produces a turf-banked lobe. Stone banked terraces form where the vegetation cover is less continuous and stones are more frequent in the active layer. The terraces tend to be smaller, up to 5 m high and 50 m long. The stones accumulate along the lobate front of the terrace because winter freezing pushes them to the surface and moves them down the slope when the terrace in general, is frozen solid.
Although both these features are common in permafrost zones, they form in other areas where winter temperatures are low. They can be found at about 800 m in Scotland in the Cairngorms. Solifluction lobes are tongue-shaped masses of active layer with a gentle terrace and steep frontal scarp (1-6 m) where resistance from adjacent materials causes the mass to bulge, which occurs in fields where lobes overlaps one another to from a staircase slope profile composed of alluvium, angular debris from fines to boulders.