Temperature sections (temperature distribution with depth below ground surface) may be divided into two groups typical for stable and unstable permafrost (see examples in Fig. 5). The upper part of stable permafrost is cooled between -6 to -15 °C. At certain depths the temperature begins to increase due to the influence of terrestrial heat flow. It reaches zero at a depth of between 200-600 m.
In blocks of unstable (warm) permafrost the temperature in the entire frozen rock layer is not much below zero, on the order of - 0.2 to -0.5 °C. Unstable permafrost is usually formed in blocks of sedimentary rocks saturated with fresh water. In this case the frozen rocks contain ice, and the bottom permafrost boundary is a phase boundary. Such a situation is found in the Meso-Cenozoic depressions of Siberia. The speed of a moving phase boundary is about one order of magnitude slower than the speed of a moving thermal wave. Therefore the temperature field of the permafrost doesn't have enough time to follow the varying conditions at surface (in comparison with dry rocks or those saturated with mineralised waters). Therefore blocks of frozen rocks with a non-stationary temperature field and a thickness which is not appropriate to modern climatic conditions are preserved here for some time. The non-stationary temperature field is fixed by a break in the thermograms (temperature-depth profiles) at the depth of a phase boundary location.
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