Cryolithozone

(from “cryo”; the Greek lithos, “stone”; and “zone”), part of the cryosphere, the uppermost layer of the earth’s crust, which is characterized throughout the entire year or for at least a short time (but not less than 24 hours) by a freezing temperature in soils and rocks and the presence of, or possibility of, the existence of underground ice.

The principal characteristic of the cryolithozone is the occurrence of processes in the temperature interval that includes the freezing point of water (0°C). Depending on the frequency with which soil and rock temperatures pass through 0°C in the course of the year, the cryolithozone is divided into the layer of short-term and seasonal freezing and thawing, or the “active layer,” and the perennial cryolithozone. In the active layer the temperature 0°C, which separates the frozen and thawed states of moist soils, occurs twice a year—at the beginning and end of the cold period (not counting thaws and frosts). In the perennial cryolithozone the temperature of the rocks is below or at 0°C not just for the whole year but for many years (sometimes hundreds and even thousands). Along with perennially frozen soil and rock and underground bodies of ice (which together constitute the frozen zone of the lithosphere) the perennial cryolithozone contains rocks that are waterless and saturated with concentrated solutions with negative temperatures but have no ice inclusions.

The isothermal surface with a temperature of 0°C serves as the lower boundary of the cryolithozone. The continuity of the cryolithozone is disrupted by lenses of taliks of various origin that have a positive temperature all year round and do not freeze. Spatially the cryolithozone encompasses the mountain structures rising above the snow line on all continents, all the altitudinal climatic belts of the polar, subpolar, and moderate latitudes, and all the latitudinal climatic belts except the equatorial and, partially, the tropical and subtropical belts where the phenomena of freezing of moist soils or the cooling of dry sands and fissured rocks below 0°C are sporadic and related only to radiation cooling.

In the vast continental areas of the cryolithozone where the mean annual surface temperature is positive, only the seasonally frozen (active) layer is widespread. In areas where the mean annual temperatures of the earth’s surface are below the freezing point, the cryolithosphere includes both the active layer and all the formations of the perennial cryolithozone. In the area where permafrost is found, the active layer is called the seasonally thawing or seasonally thawed layer, while outside this area it is called the seasonally freezing or seasonally frozen layer. On the boundary of the area where perennially frozen strata are found, the mean annual temperatures of the earth’s surface may deviate from 0°C, which leads to periodic or episodic formation and degradation of frozen pereletoks (intergelisols), the rudimentary stages of the perennial cryolithozone. In areas where the mean annual temperature of the earth’s surface is close to 0°C on the negative side, the perennial cryolithozone is island-like.

The polar, subpolar, and high-mountain areas of the cryolithozone are characterized by cryogenic and postcryogenic processes and phenomena. These processes and phenomena include cryogenic weathering, cryolithogenesis, cracking and plastic deformation of frozen rocks, swelling of soils and loose rocks, freezing out of large-fragment material onto the surface, subsidences and thermokarst, solifluction and cryogenic caving of rocks from slopes, nivation and altiplanation, intensified lateral erosion, and abrasion of icy beds. There are certain relief forms associated with these processes, including exaration and nivation forms (troughs, cirques), gravitational and solifluction forms (slope terraces, landslides, cave-ins, mud avalanches), extrusive forms and swellings (ice sills, hydrolaccoliths, rock streams), thermoabrasive forms, polygonal forms, and periglacial forms.

The term “cryolithozone” was proposed by P. F. Shvetsov in 1955, although the need to differentiate the zone of the lithosphere with a subfreezing temperature had been demonstrated earlier, for example, in the works of the Russian and Soviet scientists L. Ia. Iachevskii (1889), M. I. Sumgin (1927), and N. I. Tolstikhin (1941).