Of paramount importance in geological activity, is the role of flowing water, glaciers and other agents of the environment. Among these, the processes brought about by the action of intermittent and permanent watercourses prevail, along with those resulting from sheet wash by rainfall and meltwater, leading to conveyance of enormous quantities of sedimentary material.
Naturally, the impact of the flow of water is not merely the transport of material. Creeks and rivers contribute greatly to erosion and accumulation. Downcutting and lateral erosion play a key role in the formation of river valleys. Within the permafrost regions lateral erosion dominates over down-cutting, both in rivers on plains and in mountains, thus leading to river meandering and development of meander lakes. The proportion of down-cutting relative to lateral erosion is determined by a number of factors. The frozen state of the banks and the high content of ice of soils (up to 80%) that compose them contributes to the intensification of the lateral erosion process because of the thermal and mechanical impact of water flow and solar radiation on the frozen soils. Among manifestations of this impact is displacement of materials thawed from bank slopes into rivers as well as the formation of various niches resulting from thermal abrasion and thermal erosion due to undercutting of ice-rich soils by streams or waves. At the same time, downcutting (as compared with lateral erosion) is hampered by intense inflow in river catchments of talus deposits and solifluction material from slopes which leads to the overloading of river beds, thus causing stream migration sideways and bank undercutting. Small streamlets may thus become totally filled with the material.
Sheetwash of fine material in the permafrost regions has a number of specific features and occurs by transport of rain and meltwater from firn. The wash of fine particles along slopes is facilitated by the raindrops of summer precipitation and by slow thawing of snow cover. Downslope wash of silty-sandy-clay materials is intensified when the presence of higher ice content and schlieren cryogenic structures drastically reduce the structural strength of the thawing soils.
On slopes with rudaceous material a sheet wash of fine earth usually becomes dispersed (suffosion) into the slope (mainly on steep slopes) or grades into illuvial deposits (on gently sloping hillsides and in hollows). The processes of downslope wash and leaching may bring about formation of rather thick strata of deposits at the foot of slopes and on their gently sloping parts, deposits known as detritus aprons, perluvium, and the like.
A substantial role in regions of frozen ground is played by the action of intermittent watercourses, i.e. the action of stream flow from rainfall and melt water. This process of linear erosion named thermal erosion, along with sheet wash, greatly intensifies the formation of diluvial sediments and plays a significant role in conveyance of weathering products down slopes. The specific nature of thermal erosion by intermittent streams implies a combination of mechanical (cutting) action of water and thermal influence. The mechanism of denudation is associated with accelerated thawing of the frozen soils and their subsequent erosion.
Thus, in analysis of the process attention should be given to the resistance of soils to erosion, the interrelationship between erosion rate and rate of thawing of the underlying frozen soils as well as to the erosive energy of flowing water. It is evident that when there is a thawed layer of some thickness beneath a stream flowing in frozen soils, then with greater mechanical energy of flow the erosion front will catch up on that of thawing. In the longer term there will be the moment when the erosion front reaches the front of thawing. Further increase of the flow energy will not contribute to the intensification of erosion, since the process of erosion is completely controlled by the rate of thawing. Therefore, the capacity of frozen soils to erosion is determined, on the one hand, by the structural strength of thawing soils and on the other hand, by thermal-physical parameters of the frozen soils which determine the rate of their thawing at a constant water temperature. Accordingly, higher temperature of water flow leads to the intensification of erosion of the frozen soils.
The activity of intermittent streams in the permafrost regions is accompanied by the formation of dissected relief which is most typical of marginal parts of terraces lying above floodplains. Observations have shown that man-induced disturbance of soil and vegetative cover on sites having different elevations leads to erosional channels which sometimes grade into gullies. The latter land-forms are usually typical where vehicle tracks run down the sloping surface. In the beginning they are of canyon shape or V-shape when the rate of their growth lengthwise reaches 10-20 m yr ~ and sometimes (in the bottom of ravines and hollows) it is as high as 100-150m yr-1. Rapid growth of gullies diminishes with time, which indicates their stabilization when the ever-increasing cryogenic washout from the slopes into bottoms of gullies prevents their further deepening. The bottoms of such gullies flatten while still retaining a steep slope, which is why their length seldom exceeds 1 km. Based on the available data, it is possible to determine the length of time for gulley stabilization. This takes 20-30 years after erosional landforms develop. Regions in which there is rapid growth of gullies include the northern part of West Siberia, the north of Yakutia, the region of the Baikal-Amur railway, etc.
Of specific interest is the process of destruction (erosion) in the permafrost regions of frozen coastal lands under the influence of the mechanical energy of waves and water temperature which cause cliffs to retreat due to washout and leaching. This process, which is widely developed in the permafrost regions along the banks of rivers, lakes and reservoirs as well as on the sea coast, is known as thermal abrasion. The process becomes active if ice-rich soils and ground ice are exposed. Owing to heat exchange with the water the frozen soils rapidly thaw, and the layer that has thawed gradually slides exposing the frozen ground. Thus direct contact is promoted between the frozen ground and water and there is rapid destruction. Such banks stabilize with time provided the ice content of soils that compose the banks is low (lower than a critical value). Stabilization of the banks enclosing water bodies and disturbed by thermal abrasion, is possible when lowering of water level results from the enlargement of the basin of the water body caused by the same thermal abrasion, or by erosional incision of outflowing streams.
When soils are uniformly saturated with ice, thermal abrasion activity is dependent on water temperature and, to a greater extent, wave action. The sea coast is subject to more intense wave action. As regards inland water bodies the wave action is dictated both by the force of winds and the size of these water bodies. In conditions of unidirectional prevailing summer winds the lakes are aligned in the same direction. These are usually called oriented lakes. The greatest intensity is observed on sea coasts and banks of large lakes in the extreme north-east of Russia and in the north of Yakutia where the widely developed ice-rich banks are subject to the most active wave cutting. Thus, lowland banks on the northern margin of the Yana river delta retreat at a rate of 16-20m yr~1 and the coast of the Laptev Sea retreats at a rate of A-6myr~1. The thawing rate of ice-rich banks of lakes in Central Yakutia amounts to 7-10myr-1, while thermokarst lakes in the Anadyr tundra migrate at a rate of lOmyr-1. Intense erosion of the banks of the northern water bodies is associated with washing-out of the submarine bank slope composed of the frozen ice-rich soils, under the influence of both mechanical and thermal energy of the moving water and is characterized by three specific features. The first one is that the intensity of the washing-out of the frozen soils is a function of temperature; the second one is that the volume of sediments settling on the submarine slope due to washing-out of ice-rich soils is smaller than that of the frozen soils eroded. The third feature lies in the fact that settlement of the frozen soils at thawing leads to deepening of the water body thus contributing to the further development of thermal abrasion. The role of the thermal factor in thermal abrasion is conditioned by the ice content of the frozen soils. Thus, if banks are composed of soils containing no ice the thermal factor does not operate at all. The bank will retreat until a limiting equilibrium profile of the submarine bank slope is formed. If the banks are predominantly composed of ice, the thermal factor is practically 100% effective in the process of thermal abrasion. In this case the bank will retreat continuously.
A typical phenomenon of the permafrost regions zone are ice bodies which arise and grow only during the frost season of a year. They form from different sources of water: ground water, subsurface water, river water, lacustrine water (often icings have a mixed recharge). The flat-convex ice bodies - icings - result from multiple inputs of these waters onto the surface and their freezing in lamina. Icings have an impact on the re-distribution of surface runoff and terrain giving rise to specific deposits ('icing alluvium'), they are capable of exerting a detrimental impact on engineering structures. Often icings arise as a result of the altered freezing conditions associated with construction and operation of different structures and other features of land development. Severe and extreme continental climates with cold win ters with low snowfall are most favourable for the development of icings.
Water flows onto the surface because of the increased hydrodynamic pressure as a result of seasonal freezing along watercourses; there is higher hydrostatic pressure with the freezing of lakes and sublacustrine taliks. That is, seasonal freezing reduces the cross-section for flow of the surface and subsurface waters of rivers and streams. There arises a hydrodynamic pressure. Under the influence of this pressure the frozen roof (river ice or frozen soil) is broken and water flows out onto the surface. Water flows in different directions and freezes, and flow is blocked until a further break occurs. With such cyclicity, laminated ice bodies of various size and thickness form on the soil surface. This depends on the number of cycles, volumes of flowing water, i.e. on the reserves of ground water and the soil-freezing situation. Similarly, icings can form because of hydrostatic pressure arising in the freezing of lakes and closed sublacustrine taliks. The rate of icing formation varies within an annual cycle. In autumn and early winter the rate typically is slow but steadily increasing (early stage of formation). During the winter period when the area and volume of the icing increase uniformly, the stage of maturity of the icing begins. The stage of maturity continues from mid-winter till early spring. After a persistent increase of mean diurnal temperatures above 0°C the stage of erosion is initiated.
Icings are subdivided according to the sources of recharge, location in the terrain, time of existence, size, etc. There are three types of icings: hydrogenous (surface water), hydrogeogenic (subsurface water) and heterogeneous (mixed surface and subsurface water) These types are, in their turn, subdivided into kinds. Thus, for example, icings of hydrogenous type comprise the following kinds: water of rivers, creeks, lakes, snowmelt, glaciers and seas. In the majority of cases icings are confined to valleys of rivers and creeks (valley, floodplain or terrace), or to slopes, proluvial debris cones, glaciers and glacier-adjacent sites. There are annual icings which completely thaw by the end of the summer season, and perennial ones. When small thin, disconnected lenses of ice remain at the end of the summer season they are called summer-surviving. These icings are well distinguished in aerial and satellite images made after the complete disappearance of snow cover.
The size of such icings varies between very small (area under 103 m2) and enormous (area over 107 m2 and volume over 107 m3). Enormous icings can be 10 m thick with a length reaching several tens of kilometres. If their thickness exceeds 5-6 m such icings are, as a rule, perennial (Fig. 5.15). The size of icings is dependent on the source of their recharge. Very small ice bodies form on account of the meltwater from the seasonally thawing layer and perched water, while big and enormous ice bodies ('tarynn') form on
account of the underground subpermafrost flow of water of mixed deep and subpermafrost flow origins.
These are masses of ice formed of precipitation as a result of snow accumulation perennially exceeding thaw at a negative basal ground surface temperature. There are areas of recharge and of ablation (thawing) in glaciers and they are divided by the recharge boundary. Glaciers form above the snow line, which is the level of earth surface above which snow accumulation prevails over thawing. The snow line is lower in cold and humid regions and goes up to high elevations in warm and dry regions. Thus, on the Arctic islands and Antarctic continent glaciers may extend below sea level, in the mountain systems of the equatorial zone they exist at altitudes of 4500 m and over.
The size and thickness of glaciers increases from the equator towards poles and from sea level towards high altitudes of mountains: size ranging between 0.1km2 and 1 million km2 and thickness from several metres to several kilometres. A variety of processes and phenomena are associated with glaciers, which contribute to the formation of specific landforms and types of sediments through erosion and accumulation. The role of glaciers in formation of deposits is unique, because the vast volume of ice which is highly viscous moves along valleys or over the orographic elements thus eroding and conveying particles of practically any size without sorting. Glacial erosion occurs in the following way: 1) by means of removal of loose material resulting from weathering; 2) by means of corrasion - destruction and removal of bedrock as rock waste, either frozen into the bottom of glacier or dragged by the glacier along its bed; 3) by means of pulling out, when the glacier plucks the blocks of bedrock bounded by fractures and conveys them within the ice. Under the influence of glaciers the V-shaped young valleys are transformed into U-shaped ones having flat bottoms. Deposits conveyed by the glacier create different shapes, for example, ramparts stretching for many kilometres - eskers - and cone-like hills - kames.
These are stabilized accumulations of snow and ice (sometimes sliding slowly) that have remained after the thaw during the warm season of a year on sites of negative soil surface temperature. On tundra plains, snow patches remain in shaded hollows and beneath steep escarpments of valleys, whereas in mountains they survive on cornices and escarpments of slopes within the mountain-tundra belt. Snow patches reach 2-4 m in thickness, rarely more, while the thickness in perennial névé basins ranges between 5 and 10 m and over.
The impact of névé basins manifests itself in the formation of deposits and is associated with various processes known under the common name of nivation. One group of the processes is responsible for the preparation (making available) of soil material for conveying it downwards from the snow patch, i.e. the formation of weathering products. Such preparation is facilitated by negative temperatures existing beneath the snow patches, by total absence of vegetation and highly wetted soils resulting from thawing of snow patches. The second group of processes is associated with the conveyance and accumulation of the weathering products.
They are composed of loose snow formed mainly in the upper parts of erosional cuts. The initial stage of snow accumulation starts if equilibrium is maintained between the angle of slope of the snow surface and that of internal friction in snow. If a critical value of snow thickness is exceeded and the shear strength of the snow is small there arise zones of the snow cover in an unsteady state thus causing development and fall of snow avalanches down the slope. On hillsides of the Cherskiy and Alatau ranges, for example, avalanches on the average carry the same amount of rudaceous material per year as rock falls usually do.
These are short-term rapid flows carrying water, ice, snow and muddy materials. Conditions that promote snow-ice mudflows are similar to those of ordinary mudflows: accumulation of large volumes of loose material, inflow of water and the slope of the hillside. The carrying capacity of mudflows is rather great. Rudaceous materials can be in a suspended state in the snow mudflow; moreover their density exceeds greatly that of the mudflow itself.
In conditions of polar and extreme continental climate, wind-born (aeolian) transportation of fine earth occurs widely. The process takes place both in winter and in summer, being most intensive in winter when fine-grain material is blown out by strong winds on sites with disturbed or no vegetation cover and where frozen soils are less cohesive (ice-cement cohesion) due to ice sublimation in them. Therefore, the amount of material transported in winter is dependent on wind velocity, intensity and on depth of sublimation. The conveying capacity of wind is one three-hundredth that of water, that is why wind can transport only sandy or more fine particles. In conditions of severe climate aeolian processes play an important role in the formation of a thick ice-rich sequence of deposits on coastal lowlands in the north-east of Russia which is called the 'yedoma' complex.
As a conclusion it should be noted that a general dependence of exogenous geological processes in the permafrost regions on heat exchange gives rise to features that are specific in their development and appearance. Of importance is the latitudinal and altitudinal zonation of the occurrence of the freezing-geological processes: heaving, icing formation, thermokarst, solifluction and the like which are modulated by the zonation of mean annual temperatures of the ground, depths of seasonal thawing and freezing as well as by the zonal distribution of thicknesses of the frozen ground, development of ice-rich frozen soils etc. Important, too, is the role of regional factors, especially with respect to the forms of the processes.
Composition, cryogenic structure and properties of frozen rocks
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