Unit 4: Glacial And Periglacial Systems

Essay 4

a) Differentiate between the terms active layer and permafrost.

The active layer is near to the surface and is subject to annual freezing during ‘winter’ and thawing in ‘summer’ due to fluctuations in temperature above and below 0ºC, so permitting movement under gravity. The depth of the active layer varies, but it is usually 0.5 to 5 metres thick. In comparison, permafrost is ground that is perennially frozen, ground that stays below 0ºC for two years or more; it does not rise above 0ºC in ‘summer’ therefore it cannot thaw, unlike the active layer. The depth of the permafrost varies like the active layer, but to a greater extent; the ground can be frozen down to approximately 400metres thick. The cover of permafrost can be continuous, discontinuous or sporadic and it can be found in areas such as Northern Canada and Siberia.

b)Examine the impact of ground conditions on the formation of distinctive periglacial landforms.

A periglacial area can be described as those ‘in which frost action and permafrost related processes dominate’. Ground conditions in a place such as this can be determined by the state of the active layer, its temperature, thickness, water content etc. and that of the permafrost, whether it is discontinuous, sporadic or continuous. All of these vary spatially and temporally. The formation of periglacial landforms can be seen as a system. The condition of the ground is one of the factors, or inputs, which affects the processes, or throughputs, and thus the landform, or output. Other inputs into this system which can affect the output could be the physical characteristics of the surrounding rocks and how they react to the environment around them, wind action and fluvial action.

When the ground in a periglacial area is wet, it is likely that a landform called a ‘pingo’ will occur. There are two different varieties: the closed system pingo, of which more than a thousand have been identified in the Mackenzie Delta of Northern Canada and the open system pingo, or the ‘East Greenland’ type. A lake on the surface of the ground in a periglacial environment serves as insulation to the water in the soil beneath it. A closed system pingo is so called because no water enters into it, hence ‘closed’, and it occurs under the ground of this lake once it has dried up. The water in the soil freezes, forms an ice core, and, as it has nowhere else to go, is pushed upwards by the rapidly advancing permafrost. This ice core domes the surface of the ground and forms a pingo. An open system pingo will usually form at valley bottoms, where water can drain into it. A body of water is trapped under-ground by the advancing freezing front and due to that and the permafrost beneath it, it freezes to form an ice core which is pushed up by the permafrost to form a pingo. Examples of open system pingos can be found in the Lowlands of the UK (The Wash Fenland) and in the West Midlands. When the ice core melts, the material held up by the core collapses into the centre, leaving a hollow, or a ‘pingo rampart’ and these sometimes fill with melt water, leaving a shallow lake. Pingos take a long time to form; a closed system pingo can take hundreds of years. They are extremely distinctive as they only occur in perigalcial landscapes; they vary in size: they can be tens of metres wide and high, although they rarely exceed fifty metres.

When the ground is extremely cold, ice wedge polygons may form. In extreme temperatures, usually in the winter, conditions are so cold that the frozen active layer and the permafrost will crack and split along weaknesses in the rock. These cracks will usually be in a roughly polygonal shape. During the ‘summer’ these cracks remain open and melt water will seep into them. These will then freeze and expand in the winter pushing the rock further apart. The process will then repeat in the summer. In some areas of the tundra, ice wedges are approximately ten metres across, although these probably took a few hundred years to form. An average diameter for ice wedge polygons is roughly thirty metres. If the climate warms enough for the ice wedge to melt completely, the wedge left behind is filled with sediment, forming a ‘fossillised ice wedge’, for example in Buchen, North East Scotland. Ice wedges and fossilised ice wedges are not particularly distinctive; they are under-ground which makes them more difficult to see and recognise. However, the lakes that sometimes fill up the dip in the centre are more significant, such as in Northern Canada. They are mostly polygonal in shape and so are fairly easily recognised. Ice wedge polygons most definitely change temporally; the cycle of freezing and re-thawing is not necessarily only seasonal, there may be a particularly cold few summers and so the ice will not melt , be added to, expand and so on. If the thawing of the surface is continuous and the ice wedges closely spaced, there may be large scale collapse of the surface which forms great depressions called alases. These often become the centre of drainage and form large lakes which may be several kilometres across, for example in Northern Canada. In Siberia, many formed next to each other and collapsed have formed troughs known as alas valleys which reach tens of kilometres in length.

Some landforms may be caused by the re-freezing of the active layer. Dependant on the type of material of the soil, frost lenses may form in a pore space. As they get larger its effects can be seen on the surface: it causes the surface to dome slightly, this is called frost heave. During the summer the lens may thaw and the ground will collapse. A repeated annual cycle of heave and collapse produces an irregular surface of marshy and badly drained hollows. It is described as thermokarst due to its similarity to the pitted surfaces of some limestone karst areas. It is therefore fairly significant or distinctive. A more significant landform caused by the re-freezing of the active layer and frost heave is the formation of ‘patterned ground’. Ice lenses tend to form under the larger rocks and pebbles in the ground. As the lens expands, the rock is pushed upwards, and when the lens melts in warmer temperatures the space beneath it is filled with finer sediment. Repetition of this process leads to larger boulders being pushed to the surface and, due to gravity, the rocks fall to the side of the dome created by the heave and form a roughly polygonal shape. Another explanation for the boulders rising to the surface is ‘frost pull’; the ice bonds to the top of the boulder and pulls it up towards the surface as it expands. Larger features often have a far greater depth and are related to much more severe freeze-thaw cycles and vice versa. This type of landform is fairly distinctive as it is on the surface and can therefore be easily seen; it is even more noticeable if the stone in the polygon is a different colour or type to the stone or earth around it. However, they become less distinctive the steeper the slope they rest on; the polygons elongate and eventually become lines which are more difficult to recognise.

When the topsoil, or the active layer, thaws in higher temperatures, a process called solifluxion may occur. The permafrost beneath the surface is extremely important in this process as it prevents the newly melted water in the active layer from draining down into it. The saturation of the active layer with melt water makes it extremely unstable as internal friction and cohesion is reduced. The soil is so unstable that it flows on even a small slope; flows of several kilometres have been seen on slopes of no more than 0.5 degrees. Vegetation cover disturbs and disrupts the flow making it easier to see: the terraces and lobes of solifluxion may be seen as either buckles in the ground or heaps of stones. Measurements of the flow are not easy to take due to unfriendly conditions, but a flow of less than ten centimetres per year in dramatic sporadic movements is considered common. Most movement will occur in spring when there is more melt water around, as in summer the water has often dried out. There are varying opinions, but some believe that solifluxion leads to asymmetrical valley shapes. The topsoil of one side of the valley, usually the side that faces the sun, slips down to the bottom creating one relatively gently slope and one relatively steep one. This landform is extremely noticeable, but maybe not extremely distinctive to periglacial areas; there are other ways asymmetrical valleys are formed.

Nivation occurs beneath snow patches, usually because of permafrost directly beneath it. The permafrost prevents melt water from entering the ground and causes it to sit on the surface. The water then is subjected to freeze-thaw action and creates depressions in the ground. The larger hollows may become nivation cirques as they then develop further with more snow and ice into corries. These hollows before they are enlarged by the corries are not particularly noticeable or distinctive; lots of processes form small depressions.

Other important processes in periglacial climates include weathering, wind action and fluvial action. Weathering is usually to do with freeze-thaw and vast blockfields and scree-slopes are created via this process. However, these are not particularly distinctive as freeze-thaw is not exclusive to periglacial climates. Wind action polishes surfaces and ‘carves’ boulders that exist in periglacial areas. The deposits are not easily discernable in current periglacial climates although they can be readily seen in previous periglacial landscapes. Fluvial action creates large channels, even though they may be frozen in winter. They also have to contend against high loads, so not much fluvial erosion takes place. That which does take place is not singular to periglacial areas either; anywhere there is a river will show these landforms.

The role of ground conditions in the development of distinctive periglacial landforms is the most important of all. However, there other factors that lead to different landforms, but these often do no lead to singularly periglacial landforms; many can be found elsewhere. Most of the ‘distinctive’ features are those that can only be formed when there are certain conditions in the active layer and in the permafrost.

Catherine Meeke