Distinguish between the terms active layer and permafrost - Geography Essay Example
Permafrost is the name given to ground (soil or rock) that remains at or below 0ï¿½C for at least 2 years. It is usually perennially frozen and it underlies about 20% of Earth’s land surface. Continuous permafrost implies MAAT c. <-6oC to -8oC whereas Discontinuous permafrost implies MAAT c. -1oC to -8oC. Permafrost is characterised by several different things; frost shattering of material, growth off ground ice (upheaval displacement), accelerated wind erosion (high wind, low vegetation cover), thermal erosion by fluvial activity, gelifluction (down-slope flow of soil saturated by meltwater from thawing ice) and solifluction (mass wasting by combination of gelifluction & freeze-thaw).
Permafrost may extend 1m to less than 400m below the surface. On the surface of permafrost is a layer which is frozon in winter but thaws in summer called the active layer. There are some key features of an active layer. When melting occurs, because the frozen ground beneath is impermeable, the active layer becomes waterlogged. Flows become major processes in this mobile layer. With the onset of winter, freezing progresses from the surface downwards. Unfrozen mobile materials are therefore trapped under increasing pressure and become contorted. Melting ice and snow lead to high stream discharges in the short summer.
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Explain the role of sub-surface conditions in the formation of distinctive periglacial landforms
Ground ice – ice crystals and lenses (frost-heave) – sorted stone polygons (stone
circles and strips: patterned ground
– Ground contraction – ice wedges with unsorted polygons:Patterned ground
– freezing of groundwater – Pingos
Frost-heave includes several processes which cause either fine-grained soils such as silts and clays to expand to form small domes, or individual stones within the soil to be moved to the surface (Figure 5.5). It results from the direct formation of ice, either as crystals or as lenses. The thermal conductivity of stones is greater than that of soil. As a result, the area under a stone becomes colder than the surrounding soil, and ice crystals form. Further expansion by the ice widens the capillaries in the soil, allowing more moisture to rise and to freeze. The crystals, or the larger ice lenses which form at a greater depth, force the stones above them to rise until eventually they reach the surface.
During periods of thaw, meltwater material under the uplifted stones, pr them from falling back into their original positions. In areas of repeated freezing (idealy where temperatures fall to between -4 -6ï¿½C) and thawing, frost-heave both lifts and sorts material to form pattemed ground on the surface (Figure 5.6). The larger stones, extra weight, move outwards to form, flat areas, stone circles or, more accur polygons. Where this process occurs with a gradient in excess of 2ï¿½, the sto slowly move downhill under gravity! elongated stone stripes.
Ground contraction The refreezing of the active layer during the severe winter cold causes the soil to contract. Cracks open up which are similar in appearance to the irregularly shaped polygons found on the bed of a dried-up lake. During the following summer, these cracks open close or fill with melt- water and, sometimes, also with water and wind- blown deposits. When the water refreezes, during the following winter the cracks widen and deepen to form ice wedges (Figure 5.7).
This process is repeated annually until.the wedges, which underlie the perimeters of the polygons, I grow to as much as 1 m in width and 3 m in depth. Fossil ice wedges, i.e. cracks filled with sands and silt left by meltwater, are a sign of earlier periglacial conditions (Figure 5.9). ~t Patterned ground (Figure 5.8) can, therefore, be produced by two processes: frost-heaving (Figure 5.6) and ground contraction (Figure 5.7). Frost-heaving results in small dome-shaped polygons with larger stones found to the outside of the circles, whereas ice contraction produces larger polygons with the centre of the circles depressed in height and containing the bigger stones. The diameter of an individual polygon can reach over 30 m.
shallow discontinuous permafrost? MAAT<-4oC to -5oC
Blockfields & scree
* frost wedging, shattering etc.
* esp. on high ground, mountain tops
e.g. Cairngorms, Lake District mountain tops
Frost weathering – frost shattering/freeze thaw – Blockfields, talus (scree), tors
Frost weathering includes the process frost shattering creating landforms such as blockfields, tors and talus (scree). In Periglacial areas mechanical weathering is far more significant then chemical weathering, with freeze- thaw being the dominant process. On relatively flat upland surfaces the extensive spread of large, angular boulders, formed in situ by frost action, are known as blockfields. Scree (or talus) develops at the foot of a slope, especially those composed of well-jointed rocks prone to frost action. These slopes composed of large quantities of angular fragments of rock and typically have an angle of rest of about 35 degrees.
Snow – nivation – nivation hollows
Meltwater – soliflucation, soluflucation sheets, rock streams
– Streams, braiding, dry valleys in chalk
* intense weathering & seasonal meltwater saturation
may form low ‘terraces’ of head deposits
Wind – windblown – loess (limon)
Defintion: any area with a tundra climate, such as mountainous areas in mid-latitudes, or where frost processes are active or permafrost occurs in some form.
The high Cairngorms is an alpine-arctic environment in which frost-related processes operate effectively. Two generations of periglacial features may be recognised, features active today or earlier in the Holocene and relict features from the Pleistocene
Active features are found both on the plateau and in the valleys. Due to the effects of erosion by the last ice sheet, relict features are confined to the Cairngorm plateau.
* ?origin, climatic conditions, vegetation enhancement
* not nec. permafrost
* occur in seasonally frozen, non-continuous & continuous permafrost MAAT -12 – -20oC
* Depends on matrix : Sands & gravels MAAT < -6oC, Soil MAAT < +1oC – < -1oC
Patterned ground: MAAT <0OC – -2OC
* stone sorting -frost heave & ice formation, wind ?permafrost
* large-scale sorted patterns, e.g. polygonal ground ?permafrost MAAT <0oC to -2oC
* Map extent of features
* Date feature – directly? or indirectly (age of adjacent material)
* Reconstruct spatial & temporal distribution
1. Earlier evidence liable to be destroyed or v. modified by later glaciations
2. Glacier reoccupation
3. Subsequent modification/reworking by, e.g. periglacial processes
4. Outer margins of ice sheets beyond present-day land limit, i.e. submerged
5. Ice marginal features may not have been formed or have been eroded after glaciation
6. Dating erosion surfaces & clastic sediments
Problems in understanding processes
1. Interpretation – (insufficient knowledge of analogues – processes, products & climatic implications
2. Equifinality – some glacial landforms resemble ice-marginal features, e.g. Muir of Dinnet, Dee valley, Scotland kame terraces, eskers + moraine.
The first periglacial landform identified is the frost shattered slope with scree. The slope exists because of geomorphological process. It may be the valley sides, or a particularly large outcrop of base rock. Assuming it is a steep slope, there will be very little substantial vegetation cover. This will remove any insulation and moisture removing abilities. The water passing the slope may be stored in cracks, caused by chemical or exfoliation weathering. This water freezes during the diurnal temperature variation, and expands by 10%. This widens the crack.
The daily repetition of this cycle, with fresh input each day results in a frost shattered rock face. When the crack widens, and the rock can no longer support its own weight, it falls under gravity to the base of the slope, where it forms scree, or talus. Since the entire rock face is being acted upon in this way, then a sizeable amount of jagged, angular material of assorted size forms a scree deposit, resulting the landform shown.
The block field is located near to the frost shattered slope. It forms when frost shattered material from a nunatak or peak under periglacial influence deposits material in an unsorted fashion, by glacial process. The material is usually of large size, ranging from 0.5 to 10m in diameter. It is unsorted, and usually randomly arranged. The size and angularity of the material, combined with the unnatural position make fluvial deposition very unlikely. Thus, a block field is formed.
The stone polygons, garlands and stripes form when ice wedges create patterns in the ground. Ice wedges work on a very similar principle to frost shattering. The main difference is that the ground does not crack, so the sediment around the wedge is pushed up, forming a small cone. The stone polygons form as stones roll away from the ice wedge centre, to the periphery of the wedge cone, and to the periphery of each wedge cone. Stripes and garlands occur when the wedges form close to an incline, in which case the stones roll away down the incline, and stretch out in to a garland. Stripes occur when the stone polygons are subjected to consequent movement through solifluction, which elongates them in one axis, usually parallel to the downslope direction.
Solifluction lobes are usually found downslope, with the centre in an axis such that it is parallel to the downslope direction. When the upper layer, the active layer, of permafrost thaws in the summer, the resulting mass is a viscous semi-liquid semi-solid unsorted ooze. This can move downhill under the influence of gravity, since it has a low friction coefficient and a potential energy gradient, enabling it to move on slopes as shallow as 2ï¿½.
Thus, it moves at a relatively slow rate in a process known as creep, downhill. Any soil or vegetation which has developed above the active layer will be taken along on the surface, and it will be transported downslope. When the ooze ceases to move, either due to it running out of potential energy, or the refreezing of the active layer, then a solifluction lobe is formed. This is very similar in appearance to a small cliff, up to 5m high. It extends up valley, and may present a “tongue” layout in plan view.
(c) Study Figure 9. Suggest reasons for both the increased depth of the active layer and the ground level subsidence after the clearance of the vegetation. 
As the vegetation is cleared, the shade it provided from the insolation is removed. Thus the sunlight can act on the active layer for a greater period of time. This melts more of the active layer, and the resulting warmth reaches lower in to the soil. Since the active layer was previously super saturated, on draining it will lose 10% of its volume through reduction to water, and a further 25% as the excess drains away. This explains the 5 m ground level subsidence apparent from the diagram. The increase in active layer is due to the increased solar penetration which thaws more upper permafrost, creating a larger active layer, by approximately 25 m.
Permafrost produces distinctive periglacial landforms.
Tundra is the ‘classic’ periglacial environment.
The study of these features is called geocryology.
May be continuous (permanent), or discontinuous (seasonal), and extend 1m to >400m below the surface.
The upper active layer near the surface thaws during summer, and produces distinctive thermokarst features.
Frost weathering, frost wedging, gelifluction, and solifluction are common processes.
Patterned ground, frost polygons are common landforms.
Permafrost is a very significant fact of life in the Northern Hemisphere:
– 50% of Canada
– 50% of Russia
– 80% of Alaska
are underlain by permafrost.
This causes considerable engineering difficulties.