At the present time further accelerated development and differentiation of permafrost science is taking place. A number of scientific directions in geocryology have pretensions to being independent scientific disciplines (cryolithology, engineering geocryology, etc.). However, as is obvious from the foregoing, geocryology has to investigate practically all the aspects of formation and development of the ground in the qualitatively new (frozen) state. Therefore it is natural that the structure of this science is essentially the same as that of geology in miniature. Dynamic, lithogenetic, regional, historical and engineering geocryology are thus the constituents of geocryology (Fig. 7). Reasoning from this structure of geocryology it is clear how important and essential are its connections with all the geological disciplines as well as with the fundamental sciences (physics, chemistry, mathematics, mechanics) and with geographical and biological sciences (climatology, paleogeography, geobotany, soil science, etc.).
Dynamic geocryology deals with the processes of freezing and thawing, cooling and heating which control the formation and development of seasonally frozen as well as perennially frozen ground. Consideration of these and other processes proceeding in freezing, frozen and thawing ground amounts to a thermodynamic approach in terms of energy. Development of the thermodynamic and thermal-physical basis of formation of frozen ground is based on the study of heat exchange in the system 'atmosphere -lithosphere', of radiation-thermal and water-thermal balances, temperature regime and moisture phase transitions in the ground as well as of thermal processes in the upper layers of the Earth's crust. It is possible to study the dynamics (to reconstruct the history of formation and development or to forecast change) of perennially and seasonally frozen strata in time, in area of distribution and in depth, given the close correlation of thermal aspects of the ground freezing and thawing processes with the geologic-geographical conditions of the environment in which these processes proceed. Predictions are made in the context of natural historical evolution of the natural environment as well as in the context of technological disturbance of the natural environment at a global, regional or local scale. Such questions are
solved on the basis of physical and mathematical simulation and use of approximate and exact solutions to the problems of ground freezing and thawing (using computers), with regard to heat and mass exchange and internal and external sources of heat and ice formation in the range of negative temperatures. As a result the trend and character of the freezing process (degradational, stable, aggradational) are revealed, as well as the types of permafrost strata: short-, medium- and long-period degradation across the whole thickness; degrading in the upper part and aggrading in the lower part of the unit (and vice versa) with various periods of degradation and aggradation; aggrading across the whole thickness, etc. All this takes place against the background of development of the complex global crustal tectonic movements of various signs and amplitudes.
Another important part of dynamic geocryology is the study of, in essence, the thermal-physical, physico-chemical and mechanical principles of formation and development, and as well the forecasting of the permafrost geological processes resulting in geological phenomena (the specific permafrost topographic forms). All these processes and phenomena can be divided in a first approximation into three groups. The first group refers to permafrost geological processes and phenomena resulting from seasonal and perennial ground freezing and thawing, or cooling and heating (frost weathering, heaving, thaw settlement, frost fracturing, formation of ground and surface ice, thermokarst and polygonal topographic forms, cemetery mounds, frost mounds, etc.). The second group includes slope processes resulting from the effect of gravitational forces and causing various phenomena and topographic forms (sheet and differential solifluction, thermogenic and cryogenic detritus, subsidences, screes, rock streams, mountain terraces, stone fields and sorted stripes, mud- and solifluction flows, torrents, snow avalanches etc.). And finally, the freezing-geological processes and phenomena resulting from the effect of surface and ground waters, glaciers, snow patches, wind and other exogenous factors (wash-outs, transfer and accumulation of material in persistent water courses, glacial transport of material, nivation, thermal abrasion, thermal erosion, icing formation, sublimation and material transfer by wind, frozen ground erosion with help of thermal abrasion and collapse, bank retreat, etc.) belong in the third group. The study of the principles of permafrost geological processes, phenomena and topographic forms is not only of great general geological and lithological importance but also of practical interest as far as the engineering geology aspects are concerned.
Lithogenetic geocryology (cryolithology) reveals the general and specific principles of formation of frozen sedimentary materials and ice, their granu lar and chemical-mineral composition, texture and particular structural features, on the basis of the chemical, physico-chemical and mechanical processes in sediments in the permafrost regions, in the course of their stage by stage transformation. The main scientific thrusts in this part of geocryol-ogy are the following: research of material composition, textural-structural features and properties of frozen sedimentary material and ice (petrography and ice structures); investigation of the various genetic types, facies and composition, structure and properties with the aim of determining the genesis as well as mechanisms and the geomorphological and geological conditions of sedimentary deposition, the study of the formation and history of sedimentary materials within the permafrost regions. This is the study of the particular features of sedimentogenesis and transformation of a sediment into rock in the course of weathering, transfer, continental and basin accumulation and further diagenesis (sedimentary rock formation in the permafrost regions) on the basis of analysis of the stage of permafrost development. At the same time, investigating the principles of formation and development of frozen and cryotic magmatic, metamorphic and cemented sedimentary rocks, their structure and properties (petrogenetic geocryology, a subdivision for the future) should evidently be assigned to lithogenetic geocryology.
Since the formation and properties of frozen sedimentary materials are closely associated with the manner and conditions of their freezing as well as with the character and rate of sedimentation and tectonogenesis, lithogenetic geocryology attaches particular significance to the study of frozen bodies of various genesis. At the same time the synchronously and asynchronously epicryogenic and paleocryoeluvial continental masses, the syncryogenic masses of continental subaqueous and continental subaerial deposits, etc. can be recognized. The process of formation of sedimentary rocks within the permafrost zone and its results have proved to be so unique that researchers have recently begun to recognize the cryogenic type of lithogenesis (as a specific type). This type of cryolithogenesis is the youngest, appearing definitively only in the Proterozoic, becoming more important towards present time. It is characterized by a well-defined irreversibility of its evolution.
Regional and historical geocryology investigates zonal and regional features of the formation and development of seasonally and perennially frozen ground, their distribution, conditions of occurrence and temperature, structure and thickness of the frozen masses, and the permafrost geological processes and phenomena. Frozen ground and ice are classified with respect to their composition, cryogenic structure, genesis, age, and heat exchange conditions, and regionalization and mapping of the permafrost is conducted on this basis. Reconstruction of the history of frozen ground and its development within regions, continents and the Earth as a whole is an important aspect of this part of geocryology. The data on permafrost evolution and development now available are adequate mainly for the Quaternary period. The problems of distribution of frozen ground and its development (evolution) on the planet in the more ancient epochs still remain to be solved.
Engineering geocryology represents a particularly practical part of geocryology and deals with engineering and geological support for design, construction and operation of various engineering structures within the permafrost regions, for the carrying out and selection of the most reliable and economical ways for development of territory within the regions of seasonally and perennially frozen ground. All the specific types of engineering geological survey are thus conducted on the basis of regional engineering geocryology and should be carried out at the stage of predesign investigations. Compilation of engineering geocryological maps for various types of construction, prediction of engineering geocryological conditions in the course of economic development of a territory, as well as the development of general plans for harmonious exploitation of the environment within the permafrost zone recognizing changes of natural conditions, are the functional result of regional engineering geocryology.
Engineering geocryology surveys and studies (in the laboratory and field) of composition, cryogenic structure and physical- mechanical properties of frozen, freezing and thawing soils and ice as foundations, as materials and as the environment for engineering structures, are essential at the stages of technical project and shop drawings and in the effort to select the particular regions, sections and construction sites for economic development. Predicting the permafrost conditions and technological changes at the selected site as well as recommendations on the rational use of the geocryological environment and its protection are obligatory elements of an engineering geocryological survey.
At all the design stages and especially at the stage of working drawings it is necessary that primarily, attention should be given to the processes of interaction between frozen ground and constructions. The problems of thermal-physical and mechanical interactions between engineering constructions and frozen, freezing and thawing ground are solved on the basis of thermal physics and mechanics of frozen ground. Recently investigations of processes of physical and chemical interaction between subgrade and foundation have begun to receive attention and a number of questions have already been posed (of moisture redistribution, change in ice content, cryogenic structure and mechanical properties of soils adjacent to founda tions, of the chemical reactions and processes, corrosion of foundations, etc.). Specific engineering geocryological predictions for the behaviour of subgrade and foundation for the periods of engineering construction and operation of the structure are made on the basis of qualitative and quantitative investigations of interaction between the frozen ground and the construction. In case of unfavourable predictions, measures are developed which are intended to deliberately change (transform) the freezing-geological situation (temperature, composition, ice content, cryogenic structure and soil properties) and the frozen ground processes and phenomena, that is, measures to control the effects of permafrost are drawn up.
Specialized engineering geocryology is focused on the solution of problems of an engineering geocryological nature for reliable and economical construction and operation of particular engineered structures on frozen, freezing and thawing soils. One can recognize the following aspects of specialized engineering geocryology: construction (industrial, municipal, hydrotechnical, highway, pipeline etc.), mining (underground constructions, oil-gas fields, shafts, subways, quarries, tunnels, etc.) and agrobiological (forestry and agriculture, etc.). The experience being accumulated in the field of engineering geocryological surveying is recognized in the development of various kinds of rules, instructions, standards, regulations, methods and other normative documents (at Union, republic and departmental level) setting out the requirements for the conduction of investigations at various stages of design of engineering constructions on freezing soils.
And finally, taking into consideration the modern trends in the development of geocryology there is no escape from the conclusion that the more general science - planetary cryology - has begun to develop steadily using successively the knowledge and principles of investigation of the frozen Earth. The cryology of Mars is progressing rapidly, the foundations of the Moon's cryology are developed, the data on the cryology of the Jupiterian group of planets are being accumulated, etc. In other words we can speak about development of geocryology into planetary or cosmic cryology in this perspective.
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