Frozen rocks as naturalhistorical geological formations

A few periods of active formation and existence of frozen ground alternating with periods of its disappearance or sharp decrease in extent can be discerned in the history of this planet's development. The distribution of frozen ground over the Globe in ancient epochs is associated with regions where ancient continental glaciations occurred and their moraine deposits are found. Therefore during the first half of the Earth's geological history (2.5 billion years ago) permafrost is unlikely to have been widespread because the platform massifs and continental areas were poorly developed while radioactive heating from the interior was still very great. However by the Early Proterozoic (2.1 - 2.5 billion years ago) the permafrost already existed on the North American continent and in Southern Africa. In the Late Proterozoic (600 - 1000 million years ago) it existed within Northern and Southern America, Greenland, Australia, Central and Southern Africa, the Russian Platform, Ural, Kazakhstan, Southern China and Korea. In the Palaeozoic (240 400 million years ago) the permafrost occupied (with interruptions) Cental and Southern Africa, Brazil, Southern America, Antarctica, mountain regions of India, Australia and the Arabian peninsula. In the Mesozoic and Early Cenozoic the frozen ground is unlikely to have been widespread. In the early Late Cenozoic (25 million years ago) cooling had occurred again and a set of glaciations took place while in the Pliocene and Pleistocene the perennial freezing of ground began which is still in progress. The regions of the widest permafrost development in this case were North America, Europe, Asia, Antarctica, Greenland. The most ancient traces of the permafrost in the deposits of the Early Pleistocene (more than 700 thousand years ago) and in those of the Late Pliocene (more than 1.8 million years ago) have been reported within the Kolyma lowland, Alaska and Canada.

Thus the data on known glacial events in the Earth's geological history point to a few large time intervals when the permafrost would have been widespread. The reasons for the irregular periodicity of the great glacial epochs and of permafrost development over the planet are still under discussion and poorly known. The development of permafrost was a possibility only when the ground was subjected to negative temperatures (below 0°C or 273.1 K). This caused the transition of ground water into ice and hence the transition of soils into the qualitatively new (frozen) state. The possibility or impossibility of negative temperatures in the surface layer of the lithosphere is determined by the relationships of the components of the Earth's energy (thermal) balance. It can be supposed that the periods of glacial epochs and permafrost originated as a result of the appropriate climate changes on the planet being predetermined first of all by astronomic factors and associated with particular tectonic developments as well. Thus using plate tectonics as a basis the largest periods of cooling on the Earth can be associated with the formation of gigantic continents (Laurasia, Gondwana, Pangaea, etc.) situated at one of the planet's poles.

Therefore frozen ground should be considered not as an exceptional phenomenon in the Earth's history, but as a natural-historical formation occurring repeatedly in the course of geological development in various parts of the planet. At the same time composition, structure, particular textural features and the other characteristics of frozen ground existing in ancient epochs are unlikely to have been identical to the present ones, being subjected to essential evolutional change in accordance with the irreversibility of evolution and types of lithogenesis. Such irreversibility in the Earth's history manifested itself in progressive replacement of the vulcanogenic sedimentary forms at first by humid, then by arid forms, and finally we can see a trend toward prevalence of the cryogenic type of lithogenesis over the others. At the same time the change from the predominantly chemogenic formation of basin sediments to chemogenic terrigenous and, beginning from the Cenozoic, to terrigenous biogenic ones is clearly followed. This is associated with a steady increase of the total area of platforms as well as with the migration of biota to dry land and with a sharp increase of total biomass.

The perennially frozen materials being rather specific formations, vary as far as their composition, cryogenic structure, type of cryogenesis, cryogenic age, temperature regime, thickness, ice content and other characteristics are concerned.

By frozen ground proper is usually meant geological formations characterized by negative temperature, by moisture contents exceeding that of unfrozen (pellicular bound) water Wunf under a given temperature and by ice cementing mineral particles and filling cavities, pores and fissures. Soils

(clastics, sand, clay and peat) as well as faulted or weathered magmatic, metamorphic and cemented-sedimentary rocks can constitute the frozen ground. Surface ice (river, lake, marine, glacial and other types) and underground ice (buried, wedge, segregated, sheet etc.) and snow accumulations are considered in this case as monomineral rocks while ice is considered as a specific mineral. Rocks with negative temperature, with moisture content less than Wun{ under a given temperature and without ice (monolithic magmatic, metamorphic, cemented-sedimentary rocks) are termed cryotic.

Among the variety of frozen and cryotic materials soils represent multi-component polyphase capillary, porous and often colloidal ground systems which are most complex subjects of investigation. Water, H20, usually occurs in three states of aggregation: in the form of ice, vapour and unfrozen water. The unfrozen water represents a portion of the bound water which has not frozen out, the content of which decreases as the negative temperature is lowered. Ice and unfrozen water are in steady dynamic equilibrium. Thus with increase in temperature the ice begins to melt and replenishes the unfrozen water content while with the lowering of temperature the ice content increases in a soil at the expense of the unfrozen water. Consequently the frozen ground is a highly dynamic system responding to any change in the external thermodynamic conditions. Frozen soils differ from unfrozen ones first of all by their solid nature, i.e. by ice cementation of mineral particles, and presence of particular (cryogenic) texture and structure. All this is conditioned by phase transition of ground water into ice at freezing and is accompanied by a large variety of complex physico-chemical processes. There is an associated movement (migration) of film water from the unfrozen to the freezing part of the ground, with coagulation and aggregation of the soil particles with shrinkage and loss of water content, below the freezing front and with dispersion, breaking, swelling and heaving of the ground in the frozen part by the wedging effect of the 9% volume increase on freezing of the moisture migrating to this part in thin films. Freezing of soils with moisture migration causes ground differentiation (segregation) with the massive frozen (mineral skeleton portion) and the visually observed migration-segregation ice layers (layered, reticulate, porphyritic-like, lenticular, etc.) forming a specific cryogenic structure. Cryogenic structures of hard rocks of various genesis and composition depend mainly on the nature of voids and their distribution (fissures, pores, cavities, etc.). Ice streaks and inclusions occupy those voids which were completely or partly filled with water before freezing. In the event that ice in the form of visual interlayers and separate inclusions is absent in frozen soils and occupies pore space in the form of ice cement only, the uniform cryogenic structure formed is termed massive. In essence, in this case, we are dealing with a cryogenic structure.

The essential differences between frozen and unfrozen soils are noted in their chemical mineral composition and degree of dispersion. This is associated with the specific character of geochemical and weathering processes proceeding within the permafrost zone. Thus the soils cemented with ice are characterized by a higher than usual carbon dioxide content, with reducing conditions and acidity clearly evident. This establishes favourable conditions for decomposition of silicates and migration of chemical elements, and for the formation of lower ion oxides and gley horizons. In these conditions hydrous mica and montmorillonite are formed, and such minerals as vivianite, pyrite, marcasite, siderite, etc. are accumulated. The specific geochemical situation of the permafrost zone (low temperatures, shortage of exchange and oxidizing processes, paludification, etc.) contributes to the preservation of vegetable and animal remains, formation of various hydrogenous compounds (methane, hydrogen sulphide) and completion of the process of humus formation at a less mature stage causing wide development not of humic acids but rather of mobile and aggressive fulvic acids and their organic mineral compounds (fulvates, chelates etc.). Sharp intensification of such processes as carbonation and sulphonation (precipitation of poorly soluble salts and formation of calcite, mirabilite and gypsum under decreasing temperature), occurs, with cryogenic 'desalination' of the permafrost pore solution and cryogenic concentration (especially of highly soluble salts) in subpermafrost waters, the formation of horizons containing salt water and brine having negative temperature (cryopegs) and with the reverse hydrochemical ground water redistribution, the formation of gas hydrates,

The manifest domination of physical weathering (cryoeluvium formation) over the chemical within the permafrost region causes wide distribution of poorly sorted clastic materials, cemented with ice and characterized by the clearly defined poorly sorted nature and heteroporosity of the mineral skeleton (cryogenic conglomerate, ice breccia, etc.). The fine-grained portion of the granulometric spectrum (particles smaller than 1 mm) within the permafrost zone has the distinctive feature of being high in dust content (up to 60% and higher). This is connected solely with the specific nature of the cryogenic weathering processes (with their repeated freezing and thawing) being accompanied by fracturing of sand particles and agglomeration (coagulation) of clay (colloidal) particles. It is precisely this process which provides an explanation for the wide development of loess-like deposits within the permafrost zone, characterized by predominance of particles of the dust size fraction (0.05 - 0.01 mm). As a consequence of the particular, unusual features of composition and cryogenic structure of frozen sediments they also have distinctly different properties as a whole, compared with unfrozen materials. In association with the strong manifestation of physical weathering (of the cryohydration type) and intensification of slope processes (of the freezing type), placer deposits of eluvial, solifluction, alluvial and other (mainly continental) genesis prevail among the sedimentary formations within the permafrost zone.

Ground ice occurs in frozen strata in the form of independent ice formations - the monomineral material (ice) is widely developed within the permafrost zone. Wedge ice is formed along frost (temperature) cracks, and ice bodies in various frost mounds, sheet ice deposits, etc. are widely distributed.

Numerous classifications of perennially frozen ground based on the subdivision of frozen materials with respect to any one or a few features, for example, with respect to ice content, cryogenic structure, permafrost genesis and age, temperature, permafrost thickness, etc., have been devised recently for the multicomponent composition of frozen ground and the inherent complex linkages responsible for its existence and development.

In a general approach permafrost is subdivided into epicryogenic and syncryogenic forms with respect to the type of freezing. Materials transformed into the perennially frozen state after completion of the accumulation of the sediments and their diagenetic modification (the process of transformation of sediment into rock), are epicryogenic. Epicryogenic rock units are formed mainly in the course of one-sided freezing from above in connection with global or regional cooling and build up their thickness with a deepening of the permafrost base. Syncryogenic rocks are formed as a rule from sedimentary (basin and continental) deposits on the existing frozen substratum when sediment accumulation and transition into the frozen state take place practically synchronously (simultaneously on a geological time scale). They are always underlain by epigenetically frozen materials and build up their thickness by the rising of the permafrost table as a result of sediments being progressively accumulated and freezing simultaneously into the perennially frozen state. Some researchers have described dia-cryogenic (parasyncryogenic) rock units formed recently in the course of freezing (from above and from the sides) of moisture-supersaturated unlithi-fied ground (newly deposited sediments and silts). Complex, diagenetic physical chemical processes in them preceding the freezing process are far from completion. Taliks in basin deposits freezing in the bottom conditions of shallowing water bodies are a good example. Various combinations of epicryogenetic, syncryogenetic and diagenetic strata in vertical section form polycryogenic strata, very widely distributed in the permafrost zone.

It should be noted that below the permafrost base in hard rocks as well as in sediments, materials having negative temperature but not containing ice often occur. This is usually associated with squeezing out of highly soluble salts (chlorides of calcium, magnesium, sodium, etc.) from pore solutions of the freezing rocks and their concentration in deeper horizons below the permafrost base. As a consequence highly mineralized sub-, intra- and suprapermafrost waters (up to 200 g/1 and higher) occur (in 'cryopegs') the freezing temperature of which depends on the concentration of the salts in the solution and is always well below 0°C. The thickness of such negative temperature strata with cryopegs varies in the range from the first ten to 500 or 700 m and greater and can be 1000- 1500 m and greater.

Ground freezing is shown to have occurred at various times in the territory of the former USSR. It is supposed that the permafrost of Pliocene-Eopleistocene age and formed 1-2 million years ago, is preserved in thick strata composing the lowlands of the Far North while within the remainder of the territory such strata are usually of Pleistocene age. During the thawing of permafrost in the Holocene climatic optimum (8 - 4.5 thousand years ago) the permafrost, termed relict (Fig. 4), was conserved at various depths from the surface. Later on, during the period of Late Holocene cooling new permafrost formed and a joining of the relict and the Late Holocene permafrost took place, mostly in the territory of Siberia and the Far North. There was no such joining of these permafrost horizons south of Western Siberia and in the European North, where therefore two layers of permafrost occur.

Frozen ground is usually subdivided with respect to lifetime into three varieties: 1) short-term frozen ground existing for hours, or days, and which extends from a few centimetres to the first ten centimetres in thickness; 2) seasonally frozen ground existing for a few months, which is from a few tens of centimetres to a metre or two in thickness; 3) perennially frozen ground existing for years, or hundreds and thousands of years, and which extends from the first meters to many hundred metres in depth.

The periodic change of ground surface temperature observed during the period of a year causes various thermal effects in near-surface ground, soils and rock strata. In places where there is no perennial freezing (the mean annual temperature of the ground is positive i.e. fmean >0°C) the seasonally freezing ground develops from the surface. The layer of seasonal freezing is underlain by thawed ground (or, in the far south) by ground which has not been frozen. And, vice versa, within the regions with permafrost, that is,

Fig. 4. Change of character of the permafrost from south to north: 1 - layer of seasonal ground freezing (a) and of seasonal thawing (b); 2 and 3 - contemporary continuous and discontinuous permafrost, respectively; 4 - relict continuous (a) and discontinuous permafrost (b); 5 and 6 - open and closed taliks, respectively; 7 - southern limits of present (a) and relict permafrost (b).

Fig. 4. Change of character of the permafrost from south to north: 1 - layer of seasonal ground freezing (a) and of seasonal thawing (b); 2 and 3 - contemporary continuous and discontinuous permafrost, respectively; 4 - relict continuous (a) and discontinuous permafrost (b); 5 and 6 - open and closed taliks, respectively; 7 - southern limits of present (a) and relict permafrost (b).

when fmean is below 0°C seasonal thawing occurs, i.e. the uppermost layer of frozen ground is thawed during the warm period of a year. The layer of seasonal thawing is always underlain by frozen ground. The depth of seasonal freezing mfr and seasonal thawing mtha varies usually from the first ten centimetres to a few metres. The maximal deviation of mean monthly ground surface temperature from the mean annual temperature fmean is termed the amplitude of temperature fluctuation A0. The value of A0 (physical value) is numerically equal to one half of the difference in mean temperature for the coldest and warmest months. Temperature fluctuations are diminished with depth z, i.e. damping of the amplitude of the annual temperature fluctuation takes place. The maximum depth at which the annual fluctuations are perceptible i.e. where Az = 0, is termed the depth of zero annual amplitude or the depth of penetration of annual temperature fluctuations, Han. The temperature of the ground over the period of a year is constant here (i.e. fz(i) = const) and is termed the mean annual temperature of the ground fmean. The depth of the layer of annual temperature fluctuations in the territory of the former USSR varies in the range from 5 to 20 m. Below this layer the ground temperature varies in accordance with the geothermal gradient. Within the area of development of permafrost the mean annual temperatures are in the range from 0°C to — 15°C and lower.

Perennially frozen ground is widely distributed now on the Globe and occupies about 25% of the Earth's continents and nearly 50% of the territory of the former USSR. Allowing for the seasonally frozen ground, the area occupied by frozen ground is as large as 50% of the Earth's continents and nearly 100% of the territory of the former USSR (Fig. 5). Great changes and discontinuity in time as well as space are typical of frozen ground. Thus

Fig. 5. Map of contemporary distribution of frozen ground on the Earth: 1-4 - permafrost (of types: 1 - glacier; 2 - ice; 3 - mountain; 4 - plain); 5-6 - seasonally frozen ground (5 - of humid; 6 - of arid type).

the short-term frozen and seasonally frozen layers are continuous, having the same upper boundary, the ground surface, while the lower boundary is situated at a not very great depth (from centimetres to the first metres). The structure of the perennially frozen ground is more complex because the upper boundary is situated at various depths from the ground surface on account of the processes of seasonal and perennial thawing (Fig. 4). In this connection the permafrost is subdivided into 'joining' permafrost, when the upper boundary coincides with the base of the layer of seasonal thawing, and 'non-joining' when there exists a layer of thawed ground between the table and the base of the seasonally freezing layer. When a number of permafrost layers one above the other are separated by thawed layers, 'layered' frozen strata are formed.

Continuous permafrost from the surface is usually developed in the northern regions only. However closed and open taliks often exist below large water bodies and, also in these regions, within the areas of intensive ground water circulation (see Fig. 4). The number and area of such taliks increases from the north southward where their formation depends in the majority of cases on the peculiar features of the ground surface radiation thermal balance. Regions or zones of practically continuous, discontinuous and island permafrost (Fig. 6) are recognized with respect to the nature of the

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