When ice-rich frozen soils thaw, and the ground ice melts and drains, surface subsidence is observed. The process of thermal subsidence is widely developed in the permafrost regions. It occurs both on seasonal and perennial

Fig. 5.7. Pseudomorphs along ice veins in the Ob river downstream: a - structure of enclosure of subaqueous type in the Kazantsev deposits (photo V.V. Baulin and L.M. Shmelev); b - structure of infilling in the Sartan deposits (photo L.M. Shmelev).

thawing of the frozen soils. Such subsidence normally accompanies the thawing of ice-rich soils (which have undergone heaving) in the seasonally freezing layer as well. Nonuniform development of the process gives rise to a variety of landforms and types of microrelief (collapse swallow holes, flat bottom depressions, alases, frost mounds and the like). Among manifestations of thermal subsidence in the thawing frozen soils, the process of perennial local thermal subsidence is of the greatest interest, namely ther-mokarst, which is accompanied by land subsidence and formation of negative landforms (Fig. 5.9a). The mechanism of this process consists of the consolidation of the thawing ice-rich soils or soils that contain monomineralic ice deposits, under the pressure of the thawing layer, and the expulsion of the contained moisture to drain to the surface or by an aquifer. Consolidation of soils takes place when the soils are soaked with water above saturation or as a result of the displacement of water and subsidence of blocks and layers of soil into fractures and cavities earlier occupied by the ice.

Fig. 5.8. Spot medallions: a in rick debris of alluvium of the Daldyno-Alakit interfluve (photo A.Y. Derevyagin); b - with polygonal heaving of stone material along the desiccation fissures (photo A.N. Kotov).

Fig. 5.9. Development of thermokarst: a - resulting from thawing of wedge ice after removal of sod cover on the I terrace above flood plain of the Tynda river (photo L.N. Maksimova); b - thermokarst lake in the foothills of the Cherskiy range - in the opening in the background an icing is seen (photo V.Ye. Afanasenko).

Fig. 5.9. Development of thermokarst: a - resulting from thawing of wedge ice after removal of sod cover on the I terrace above flood plain of the Tynda river (photo L.N. Maksimova); b - thermokarst lake in the foothills of the Cherskiy range - in the opening in the background an icing is seen (photo V.Ye. Afanasenko).

The prerequisite for the development of thermokarst is the availability of ground ice occurring in the form of monomineralic ice deposits or structure-generating ice in loose deposits. An adequate condition to give rise to thermokarst is such a change in heat exchange at the soil surface that either the depth of seasonal thawing begins to exceed the depth at which subsurface ice or ice-rich perennially frozen soils occur, or the mean annual temperature changes sign and perennial thawing of the frozen layers begins. General degradation of the frozen layers is associated not only with changes in heat exchange at the soil surface due to historical periods of climate warming, but also with changes in the constituents of the radiation-thermal balance of the surface, that is, with changes in the dynamics of vegetation development, modification of snow and water covers, with drying out of soils of the seasonally thawing layer and with other changes of elements of the geological and geographical setting. Man's activity is considered to be one of the main causes of the presently activated processes which are apparent in disturbance of the soil-vegetation cover leading to deeper penetration of the seasonal thawing (sometimes as much as 2-4 times).

The process of thermokarst development, as shown by V.A. Kudryavtsev, occurs in a different manner in the case of water withdrawal from a thermokarst depression and water inflow into a depression. In the case when water does not accumulate in the depression (draining type of thermokarst) the process slackens. Often, the thawed ice bodies are replaced by cavities that are not distinguishable in the terrain, i.e. they do not create thermokarst forms. When it gets colder such thermokarst usually ceases to develop or, if deposits of the seasonally thawed layer are eroded (are washed out by water), then thawing of the ground ice can resume and develop progressively. In this case thermokarst usually grades into the process of thermal erosion.

With an undrained thermokarst depression the process develops in another way. If water arrives in the depression, it accumulates solar heat thus leading to temperature increase of the water-body bottom materials; this, in its turn, usually promotes the deepening of the seasonally thawing layer. The latter is accompanied by further thawing of the monomineralic ice body, and the water body becomes deeper. In the long run, this can lead to complete thawing of the ground ice and the emergence beneath the water body of a closed sublacustrine talik (if the permafrost thickness is small the talik is open). Development of non-draining thermokarst is possible in any conditions, even with the most severe freezing.

Thermokarst landforms and types of microrelief are to a great extent dependent on the types of ice bodies and ice-rich soils subject to thawing, as

Fig. 5.10. Kettle lacustrine basin (khasyrei) and perennial frost mound. Southern Yamal (photo G.I. Dubikov).

well as on the peculiarities of ice development in the frozen soils, its forms and situation, etc. In West Siberia where thermokarst is mainly developed on sites of ice-rich marine, glacial-marine and glacial deposits, which comprise the strata with ground ice, the thermokarst basins are called khasyrev (Fig. 5.10). In Yakutia such basins which were formed after thawing of the 'ice complex' soils with syngenetic ice wedges are called alases. When syngenetic ice wedges are thawing (if there is no drainage from the subsidence depressions) thermokarst lakes are developed (see Fig. 5.9b) which are rather deep (up to 3-6 m) and vary in size (to several kilometres). With a flat bottom, the sites of active ice thawing are up to 10 m deep and more. When they dry out or are displaced (migrated) the alas basins form. Usually, alases and khasyrey develop on ancient depositional plains. Often, when thawing occurs in deposits with small polygonal-wedge ice, small lakes arise with depths of up to 1.5-2 m and having banks with rectangular outlines. When such lakes drain detrital polygonal mound-like landforms develop on their beds. If thawing out of ice wedges occurs under good drainage conditions and the ice wedge outlined blocks of soil are composed of rather hard material with low ice content, this gives rise to baydzherakhi - soil outliers (Fig. 5.11). If the ice wedges are replaced by soils then pseudomorphs of recurring ice wedges arise.

Fig. 5.11. Mounds (baydzherakhi) resulting from thawing of ice wedges in the Yuribey river valley, Gydan peninsula (photo G.I. Dubikov).

Thermokarst landforms are most widely developed in the subarctic belt of the northern coastal lowlands. Signs of the process become less distinct southwards and this is associated with less widespread ground ice. Beyond the limits of the permafrost regions only relict thermokarst landforms are encountered. On the depositional plain of Central Yakutia, characterized by a dry climate, the recent thermokarst landforms have a limited areal extent. Nevertheless, the great number of thermokarst lakes and alases in this region gives evidence of more active development of the process in the past.

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