Thermokarst in Siberia and Its Influence on the Development of Lowland Relief

“Thermokarst” as a process is the melting of ground ice and the consequent formation of depressions. Thermokarst landforms depend on the tectonic regime of a region, the ground ice content, and the degree to which the permafrost equilibrium is disturbed. Thermokarst forms are especially prominent in the lowlands of the subnival region with permafrost. The authors distinguish two modes of thermokarst development—permafrost back-wearing and down-wearing—based on their investigations in Siberia. The first mode is characteristic of a more dissected relief. In this case permafrost back-wearing takes place and the process is characterized by development of gullies, thermocirques, and parallel retreat of steep walls with ice veins, resulting in a lower lowland level. The second mode of thermokarst development is due to permafrost melting from above and is typical of a flat undissected relief, mainly that of watershed regions. characteristic forms are depressions with steep slopes and flat floors (alases). Thermokarst valleys develop through coalescence of alases. Thermokarst processes destroy the lowland relief of large areas and create characteristic forms resulting in a lower lowland level. Thus thermokarst represents a special type of lowland development in permafrost conditions.