Explain what is meant by the term ‘periglaciation’
The term ‘periglacial’ is applied to land areas, which lie around the margins of ice sheets and glaciers. These areas can be found in high latitudes and high altitudes. The landforms produced by periglacial processes occur in a range of cold, non-glacial environments, not always adjacent to present-day ice masses. Two main examples would be, the Polar lowlands, e. g. southern Greenland and Highlands, e. g. Rockies, Alps and Himalayas. Frost action and the resulting weathering and mass movement are found not only in periglacial regions. However, within these regions they are especially intense and dominant.
The most common environmental characteristic used to delimit present-day periglacial conditions is the presence of permafrost. Permafrost is perennially frozen ground, i. e. ground frozen continuously for two or more years. There are 3 main types of Permafrost: Continuous, is where the ground is completely covered. These areas are usually found in areas such as North Alaska. The second is discontinuous; this means there are patches of deeply frozen ground next to areas that are unfrozen and these are found in areas such as Iceland and South Canada.
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The last band is sporadic permafrost, where permafrost does exist but only in small patches. Overall, permafrost makes up 25% of the Earth’s surface and in Russia it is approximately 1500m deep. There are key variables, which influence the nature of the permafrost zone and the associated landforms. For example, climate, subsurface materials (soil, weathered regolith, rock type), amount, distribution and state of water (liquid or solid, i. e. frozen as ice), because the behaviour of water and ground ice can affect the volume and therefore pressure placed on the surrounding ground. This usually occurs in the ‘active layer’.
During the summer the active layer deepens and is often waterlogged. As the temperatures fall in the autumn, some of this water is transported by capillary suction towards two freezing fronts, i. e. from the ground surface downwards and from the permafrost upwards. Once there it freezes again, as lenses, often characterised as thin needle shapes, or layers within the pore spaces of the materials. This process is segregated ice. The freezing water in the ‘active layer’ is unevenly distributed; therefore as it freezes, the expansion in volume of the change from a liquid to a solid state will be uneven.
Spatial variations in sediment particle size and pressure variations in and around the trapped pockets of unfrozen soil and water also help to generate differential movement. These differential expansions result in movement of material (particles and water) by the process of horizontal frost thrusting and vertical frost heaving. The vertical movement is more pronounced, mainly because there is less resistance from the ground surface. The regular repetition of the freeze-thaw, expansion-contraction processes causes churning and movement of materials.
This movement causes two of the most distinctive groups of landforms found in periglacial regions, hummocks and patterned ground. A wide variety of hummocky landforms result from the uneven distribution of thrusting and heaving; patterned ground develops because particles of different sizes migrate through the ‘active layer’ at different speeds. Stone stripes are defined as ‘patterned ground with a striped pattern’, a sorted appearance due to parallel lines of stones and intervening strips of fine material orientated down the steepest available slope. They can vary greatly in size.
An example would be the small-scale stripe formations in the Venezuelan Andes. Different types of hummocks and mounds are ‘thufur’ and ‘palsas’. Thurfur are earth hummocks up to 0. 5m high and 1-2m in diameter. They occur in clusters, often with a regular spacing, to give a form of patterned ground, which may cover extensive areas. Their shape is influenced by the slope angle. On gentle slopes they are circular, but become elongated as gradients increase. Thurfur develop best in soils within uniform particle size. Palsas are larger mounds, most commonly 1-6m high and 10-30m wide.
They are widely and irregularly spaced and develop best on frozen peat bogs in areas of sporadic permafrost. The broad dome shape of palsas is caused by the formation of segregated ice and differential frost heave. An extreme form of a mound is the ‘pingo’. They are large mounds with permanent (perennial) ice cores. They are approximately circular in structure, 30-600m in diameter and 3-70m high. Two distinct types, closed-system pingos and open-system pingos, are related to the underlying permafrost.
Although pingos can grow as much as 1m per year, large examples may be thousands of years old, e. g. adiocarbon dating of two pingos in the Canadian Artic gave ages of 4500 and 7000 years. The largest concentration of pingos, about 1450, are found in Mackenzie Delta region of Canada. More features, which can develop in periglacial areas are frost-crack polygons. When ground temperatures continue to fall well below 0 degrees centigrade, the frozen ground material may contract. This causes sets of cracks to appear, often polygonal in arrangement. They are up to 10mm wide and 8m deep, with each polygon 5-30m in diameter. As the ‘active layer’ thaws during the summer, the cracks may fill with water.
In autumn, refreezing causes expansion and widening of the cracks and over time a crack may enlarge to become an ice wedge. Ice wedges are 2 -3m wide at the base and can extend below the ground up to 10m. The physical structure of this ice wedge is striped like onion-skin. This can also cause the process of frost heaving which I explained earlier, because over time the growing ice wedge will force the permafrost layer upwards towards the surface and therefore push the soil and rock upwards. This can create the landform ‘patterned ground’ which I have explained earlier.
However, the major feature of frost-crack polygons has occurred in the highland area of Maelifellssandur, southern Iceland. The landforms I have discussed above (e. g. hummocks, patterned ground) develop best on relatively flat terrain. On more strongly sloping surfaces in periglacial regions, a range of mass-movement processes occur, from gradual soil creep through flow surges and slope failures to abrupt rockfalls. A key mechanism is solifluction. First frost heave causes the gradual downslope movement of particles through the operation of the freeze-thaw cycle.
Second, gelifluction is the downslope creep caused as the ‘active layer’ thaws during the summer. Valley shapes are modified by solifluction, to produce valley cambering, stepped cross-profiles and slope-foot and valley-floor deposits. These deposits are widespread throughout the UK. In south-west England, the granite slopes of Dartmoor and Bodmin Moor have well-developed block streams and in southern England block streams are evident in the Marlbourough Downs of Wiltshire and the Portesham area of Dorset.