Dependence of the permafrost thickness and temperature regime on geological factors and processes

The effect of geological factors and processes on the permafrost thickness and temperature regime is essentially corrected by a heat balance of the upper layers of the lithosphere. This correction can take place as a result of the anisotropy of heat transfer conditions in rock which is dependent on the specific features of their composition, properties and structure, as well as on the redistribution of heat fluxes because of the occurrence of additional heat sources such as active zones of fractures, vapour-thermal springs, water-bearing horizons and zones and areas of vigorous chemical reactions with heat release (for example, oxidation of coal, of sulphides-bearing ores, etc.). Processes associated with heat absorption (for example, adiabatic expansion of gases rising to the surface, etc.) are also developed in the upper layers of the lithosphere. Fundamental changes in the freezing conditions for permafrost occur as a result of neotectonic movements which are responsible, for example, for sea transgressions and regressions, displacement of the water area of large lakes, changes in rate and character of sediment accumulation and spatial expansion and narrowing of denudation areas. These factors and processes affect the permafrost thickness and temperature regime in various ways.

Effect of special lithological features and moisture content of freezing ground

The composition and properties of the permafrost materials, through their moisture content, are responsible for expenditures of heat on moisture phase transition <2ph, and through the thermal properties {X,C). Increase of the moisture content causes increase of <2ph values and, consequently, decrease of the permafrost thickness. Analysis of the freezing through the years for a periodic, step change of temperature at the surface has shown that differences in permafrost thickness do not exceed 40-50% (Table 12.1). The depths of the permafrost can be changed by a factor of 1.5-2, on account of variations in geothermal gradient.

The depth of the permafrost depends essentially on the thermal conductivity of the freezing soil too. When considering seasonal freezing it was pointed out that its depth is directly proportional to thermal conductivity X. It is thought also that during permafrost formation the depth of freezing £Per cCa/A. At the same time, disturbance of this dependence under the effect of geothermal flux should be taken into consideration. According to Ku-dryavtsev's data the value of this disturbance does not exceed 10-15%.

In hard rocks where the values of X reach 10—12kJ (m hr°C)_1, permafrost thicknesses (all other factors being the same) are approximately 1.4-1.6 times greater than those of soils with A«4-5kJ (m hr°C)_1. Therefore similar thicknesses of the permafrost of hard rock and of loose deposits most likely indicate a different age for the formations, i.e. a greater age for the frozen units composed of loose deposits.

The geological structure of a region and particularly the specific features of the occurrence and composition of the freezing materials, their moisture content and heat conductivity, can have a very fundamental effect on the

Table 12.1. Variation of the permafrost thickness (m) depending on heat of water phase transition Qp h under temperature waves of various amplitudes at the surface A and the geothermal gradients g (after V.A. Kudryavtsev)

öph, kJm"3

¿per = 2°C

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