All kinds of natural ice that form independent bodies and accumulations can be regarded as a monomineral rock. They develop under condi tions of negative temperature of land and sea surface, atmosphere and lithosphere. Distribution of ice on the Earth's surface and in the crust is extremely nonuniform. Major masses of ice are concentrated on the land surface, mainly as glaciers and ice sheets. With an areal extent of only 3.1% (in relation to the Earth's surface) they constitute over 97% of all the mass of natural ice. The bulk of glacier ice is found in the Arctic and Antarctic zones, where ice thickness can reach 4 km and over. In the lithosphere ice occurs in the uppermost horizons (first hundreds of metres) making up in total about 2% of the ice mass on the globe. Ice on water surfaces is mainly represented by sea ice, icebergs and snow cover. Their share of the total ice mass does not exceed 0.2%. Atmospheric ice formed in air or on various surfaces from the air comprises, according to the data of Y.M. Kotlyakov, about 18% of the water vapour mass and 0.03% of the atmospheric mass. Their volume in the total mass of natural ice is minimal.
There are many classifications of natural ice. In the classification by P.A. Shumskiy, 28 kinds of ice are distinguished, and classified in three groups: ice resulting from freezing of water (congelation), sedimentary ice (snow cover) and metamorphic ice (glacier ice). Two large groups - land surface ice and ground ice - reflect the conditions of natural ice formation and occurrence of ice bodies in relation to solid earth materials.
Land ice can be subdivided, in the light of geocryological objectives, into newly deposited and re-deposited snow covers, metamorphosed snow covers and névé basins, ice of water bodies and streams, icings and glacier ice. Each group is distinguished by the mechanism of formation of the ice. Newly deposited and re-deposited snow covers consist of fine ice crystals (snow) and their fragments. Metamorphosed snow cover and snow patches comprise old snow transformed in the course of re-crystallization and having different size of grains. Ice of water bodies and streams forms under the influence of heat flow directed perpendicular to the freezing surface. Structural-textural features of this ice are dictated by the conditions of hampered growth of crystals, anisotropy in the rate of their growth, the presence of different admixtures in the water, as well as by the state of the water mass. Icings result from layer-by-layer freezing of water flowing out onto the surface of ground or ice, and they show a distinct lamination parallel to the surface of accumulation. Glacier ice formed of snow cover after its compaction and re-crystallization is characterized by a heterogeneity of structures and properties. As shown by observations in a glacier section, there are three stages of its formation: diagenesis of the snowpack, formation of firn and its transformation into ice.
Ground ice encountered in the form of large accumulations in the upper part of the lithosphere takes the form of ice bodies of thickness 0.3-0.5 m and more. Among these, different kinds of ice are distinguished: wedge, intrusive, migration, cave and buried ice. Wedge ice is formed in fractures arising in the frozen rock filled with moisture and has a heterogeneous composition with admixtures, and vertical and inclined banding. Intrusive ice forms in soils by crystallization of free ground water injected under pressure, and usually it comprises sheet-like, lens-like and stock forms, and often these have many air inclusions of various shapes and sizes. Migration ice is formed as a ground component if favourable conditions are created for freezing and migration of water towards the front of ice formation, this giving ice bodies several metres thick. Ice formations in caves have various origins. Among them are infiltration ice that forms in cavities, crust icing and ice stalactites and stalagmites; ablimation ice that forms ice strings and hoar frost in caves; snow ice forms snow patches, and in combination with infiltration often forms glaciers in caves; buried ice is the remnants of ice formed at the surface of the ground and having various origins (river, lacustrine, glacier, etc.), covered by a layer of sedimentary deposits that prevent its thawing.
Wedge ice comprises those types of ice which fill the fractures in weathered rocks (fracture-type) and form ice wedges in loose deposits. In the latter case they are the component of the Quaternary deposits called in the literature polygonal wedges or recurring ice wedges: they are called polygonal as they form a distinct polygonal grid; their section is shaped as a wedge or vein. They are also called recurring wedges owing to the multiple-repeated process of ice formation in vertical frost cracks that periodically develop in the same place (contraction hypothesis).
The types of ice under consideration (transparent in thin section) have various degrees of opacity, the colour is whitish, grey or brown, and they contain both mineral and organic admixtures, which according to the data of P.A. Shumskiy, form 3-5% of the total mass and 1-1.7% of the total volume of the ground. The ice has bubbles; the volume of gas-filled cavities makes up 4-6% of the total volume, and the bubbles have a globular or elongated shape (cylinder, pear-shaped, etc.). Among gaseous inclusions there are the authogenic (those separating from water during freezing) and xenogenic or foreign (air contained in interstices between ablimation crystals of ice). The latter type of gaseous inclusion is the most prevalent (2-4 % of total ice volume). Besides, xenoliths of surrounding materials are also encountered, being similar to fine mineral admixtures in composition. The arrangement of mineral admixtures and gaseous inclusions determines the vertical banding of the ice, but it is not always discernible. Vertical arrange ment of mineral admixture bands is assumed to be the result of the expulsion of soil particles by ice crystals growing from the walls of the frost fissure inwards towards the axial plane where the water is freezing. The majority of wedge ice varieties are slightly saline with salt content ranging from 0.01 to 0.1 g F1, i.e. salinity is similar to that of ultra-fresh surface water of the permafrost regions and atmospheric precipitation. Wedge ice density is dependent on the amount of gaseous inclusions and mineral admixtures, ranging mainly between 0.85 and 0.90g cm-3. Porosity is usually 2-A%, in rare cases 8%.
The prevailing vertically banded structure of the wedge ice is determined by the pattern of accumulation and freezing of the elementary (annual) ice vein. The vein ice structure is xenomorphic-granular, plate-like and hy-pidiomorphic-granular. The orientation of the main optical axes of the ice crystals is often chaotic, and if it has an ordered pattern it is parallel to heat flow directed subhorizontally.
Therefore, water that fills a vertical frost crack is being frozen from both sides, from the walls towards the centre. The developing elementary ice vein consists of two vertical rows of crystals. Consequently, the maximum possible size of crystals is equal to half the width of the frost crack. There is a steady reduction of ice crystals from the top downwards, which corresponds to diminished frost cracking in the same direction. The size of the ice crystals is predominantly lxl cm, the maximum being 1.5 x 2.0 cm. Their size is also dependent on the temperature of wall cooling and diminishes with depth. There is also a dependence between ice crystal size and the age of wedge ice. Crystals enlarge with time owing to the process of 'ice metamor-phism'. As shown by the observational data of V.V. Rogov, the size of crystals in recently formed wedges is 2-3 times less than that of the Pleistocene ones. Among the wedge ice types there are epi- and syngenetic.
Epigenetic ice wedges are formed in those sedimentary rocks which become frozen after having been accumulated and transformed (compacted) from above. The predominant size of the wedges does not exceed 3-5 m vertically, and 1.5-2 m wide at the top. The vertical dimension of epigenetic ice wedges is determined by the depth of the frost cracks' penetration into the frozen ground reaching 5-7 m, rarely 10 m; hypothetically, 12-15 m is possible. The ice in elementary frost fissures is formed of hoar frost crystals, snow pack in winter, and water that infiltrates in summer. An indispensable condition of ice-wedge formation is the penetration of the frost cracks below the maximum depth of the seasonally thawing layer. A cross-section of the typical epigenetic ice wedge looks like an inverted triangle with the base being smaller than the sides. In the lower part of the wedge, at the apex of the triangle, there are tongues, 'offshoots' of the wedge. Ice/soil contacts in the wedge are usually distinct, even sharp, often ferruginated and a fringe of pure transparent ice 1-2 cm thick is sometimes traced along them. The layers of the surrounding material in the vicinity of contact with the wedge, especially in the upper, widened parts, are often curved towards the top.
The following features are typical of the ice structure of epigenetic wedges. The colour is whitish, milk-like, sometimes brownish. The texture is distinct vertically banded; each layer begins from the more or less horizontal upper surface of the wedge. There is alternation of relatively pure ice 1-2 mm thick (up to 5 mm) and bands of ice enriched in mineral admixtures, vegetation residues and gaseous inclusions. The latter have an elongated shape being stretched, parallel to the axial suture of the elementary ice wedge. In the centre of these wedges the amount of gaseous inclusions is at a maximum reaching 4-5%, near the lateral contacts it is reduced to 1-3% of the volume. The diameter of the ice crystals does not exceed 1 cm. Often, subvertical bands are arranged fan-like; in the middle of the wedge the bands are vertical, while at the edges they are inclined, parallel to the lateral contacts with the surrounding soil.
Syngenetic ice wedges that grow synchronously with accumulation of sediment can have enormous size: 50-80 m (even more) vertically and 810 m horizontally. The shape of the wedges is usually composite, with expansions and narrowings and they often have a multi-stage form. At maximum growth, ice wedges become dominant constituents of the frozen material in general, soil is arranged in the form of vertical 'earth veins' or 'columns' narrowing towards the top between the ice network. In areal extent the ice occupies 60-70% of the ground at such sites.
Typically syngenetic ice wedges are in contact with ice-rich surrounding sediments. The layers of the latter, as a rule, are steeply curved upwards. This is assumed to be the result of the growing ice wedge squeezing the ice-rich soil near the contacts. In this case a direct correspondence should exist between the width of the ice wedge and the steepness of soil curvature near the contacts: the wider the wedge, the steeper is the curvature. However, this rule is not always valid, which is the reason why some investigators (Ye.M. Katasonov, A.I. Popov et al.) criticize the mechanism of deformation described.
The structure of ice of syngenetic wedges has a number of typical attributes making it possible to distinguish them from epigenetic ones (Fig. 9.3). There is always an admixture present in the ice of large quantities of soil particles and vegetation residues, especially in the lower and middle parts of the wedges. Bubble-structure is typical of the ice: gaseous inclusions are b c b c
shaped as spherical bubbles forming chains elongated vertically. At the same time vertical banding is poorly expressed and not always distinguishable visually, while in the epigenetic wedges it is revealed by accumulations of soil particles and vegetation residues. The ice texture of syngenetic wedges is similar to that of epigenetic wedges, but ice crystals in them are 2-3 times larger. The ice structure in the uppermost part of syngenetic ice wedges has a number of special features. Among these are vertical orientation of crystals, the small amount of soil and gaseous inclusions and the absence of vertical banding. This is explained by melting of the wedge ice near its upper surface forming migration ice layers which get frozen as if welded to its 'head'. N.N. Romanovskiy considers this process to provide 'frontal' growth of ice wedges upwards along with growth of vertical elementary wedges. A fringe of pure transparent ice about 10 cm thick is also traced near the lateral contacts. V.I. Solomatin and other investigators assume this to be associated with formation of segregated ice conditioned by the availability of horizontally oriented temperature gradients under the cooling influence of the open frost cracks.
The basic attributes of syngenetic ice wedges according to P.A. Shumskiy, B.I. Vtyurin, T.N. Kaplina, N.N. Romanovskiy et al., are as follows: 1) a great vertical length that exceeds substantially the maximum possible fracturing, even in the most favourable conditions; 2) undulating lateral contacts and outcrops of the upper ends of elementary ice veins and 'welding' to lateral contacts of big schlieren (belts) of segregated ice in ice-rich ground. The belts look as if they rest on benches ('shoulders') on the side of the wedges, resulting from variations in the rate of wedge growth and sediment accumulation of the surrounding deposits. In addition, there are a number of distinctive attributes to syngenetic ice wedges to which different authors assign different significance. Apart from the characteristic attributes of the wedges themselves there are certain features in the structure of the surrounding deposits which give evidence of the simultaneous upward growth of wedges and accumulation of sediment: 1) change of facies composition of deposits between wedges horizontally from the middle of the inter-wedge space with maximum peat content towards lateral contacts where the soil admixture increases; 2) periodic development of peat lenses in the vertical section of 'earth columns' between wedges, with overlying mineral deposits; 3) in general similar conditions of accumulation of facies.
The southern boundary of occurrence of recent syngenetic ice wedges in alluvial, marshland and slope deposits is found much further to the north than that of epigenetic wedges. In the present natural setting, ice wedges grow syngenetically on flood-plains of rivers, periodically inundated laida coasts, bottoms of water-logged kettle lacustrine basins where peat is being accumulated and in the talus-solifluction trains at the foot of slopes. However, recent syngenetic ice wedges have much smaller size than the Pleistocene ones; their vertical length usually does not exceed 10-15 m. The majority of authors (Ye.M. Katasonov, A.I. Popov, B.I. Vtyurin, N.N. Romanovskiy et al.) are of the opinion that the biggest syngenetic wedges developed in the low-temperature ground of continuous permafrost were formed in the course of accumulation of high flood-plain alluvial deposits. Other authors (N.A. Shilo, Yu.A. Lavrushin, S.V. Tomirdiaro et al.) support the involvement of eolian, slope and other processes.
Intrusive ice represents intrusive accumulations resulting from the intra-soil freezing and crystallization of free ground water injected into the frozen or freezing ground under pressure. They form deposits of lens-like, layers and column shapes similar to intrusive bodies of magmatic rocks (batholiths, laccoliths, sills, dikes and the like; Fig. 9.4). Intrusive ice is most typically expressed in cores of frost mounds (hydrolaccoliths), the biggest among them are 40-50 m high with diameter ranging from several tens to several hundreds of metres. In Yakutia such mounds are called bulgunniakh, in North America, pingos.
Injection of ground water and soil/water masses can occur many times thus leading to the composite structure of intrusive ice formations. They have been found to be confined in the majority of cases to the contact of clay and sandy-silty-clay deposits with underlying coarse-grained sandy and sand-pebble formations; more rarely they are encountered within strata of fine-grained soils. Typically, intrusive ice appears pure and transparent, but in the base of the intrusion there are tongues and streamlets of fine mineral particles and near contacts there are xenoliths and isolated particles of soil,
Fig. 9.4. Sheet ice in marine Quaternary deposits. The Yamal peninsula, Ney-to lake (photo by G.I. Dubikov).
vegetation residues and peat. Sometimes, there are pebbles and boulders in the transparent ice.
The frost mound structure as distinguished by Sh.Sh. Gasanov has an external glass-like shell that contains a small amount of admixtures, and an internal core with a great amount of mineral admixtures and air inclusions. Sometimes, the layers of glass-like and bubble-containing ice alternate. Air inclusions encountered in the intrusive ice are often arranged layer-by-layer, parallel to the roof of the intrusion; they also form vertical, slanting and non-oriented accumulations. In the large ice intrusions of the laccolith type there is sometimes a radial arrangement which leads to a radiating-fibrous texture of ice resulting from bending of layers on heaving.
The structure of intrusive ice in the seasonal mounds differs from that in the large hydrolaccoliths. In the seasonal mounds the ice is pure, transparent and has a distinct vertical pan-automorphic-granular structure. In the large hydrolaccoliths the structure is allotriomorphic-granular with large grains of ice (1-16 cm cross-section) and chaotic crystallographic orientation.
Reformed intrusive ice is distinguished as a specific variety of intrusive ice. According to Sh.Sh. Gasanov, this ice forms intrusive layers 6-8 m thick with 300 m cross-section in the upper horizons of the frozen series. Initially, the ice is formed under the influence of hydrostatic pressure of ground water, then, as a result of pressure there is migration of water and liquified soil out of the closed system.
The basic attribute of the reformed intrusive ice structure is an alternation of layers of pure bubble-rich ice and ice-rich soil owing to which a laminated texture arises, which is often deformed. In this case the ice consists of folded parallel layers and those having different orientations.
Migration ice can also form large accumulations in the ground. Often they are called segregation (i.e. separation) ice, that is, ice forming schlieren. However, when they are large monolithic ice bodies, the term segregation seems to be inappropriate, as the ice is not separated into layers, but forms a single body.
This type of ice formation is considered at present to be relevant in the formation of large sheet, lens-like deposits of ground ice and even frost mounds (2). However, this viewpoint is criticized by other investigators. A specific type of segregation was distinguished, characterized by the action of pressure migration. It can be stated for sure that migration ice is a predominant constituent of the frozen material within the limits of the so-called convex peat mounds.
The composition and structure of ice in the deposits associated with pressure-less migration of moisture are similar to those in the ordinary schlieren (segregational) ice. Its colour is, as a rule, white if it is transparent and pure. The ice contains bubbles of air, and soil and vegetation inclusions. The shape of the bubbles is irregular, pear-shaped and sometimes they form fibrous tubular accumulations. The soil in general shows layers of relatively pure ice alternating with ice containing admixtures.
The structure of the ice is hypidiomorphic-allotriomorphic and prismatic-granular. The prevailing orientation of the basic optical axes of the ice crystals is ordered and perpendicular to the freezing front with crystals mainly shaped as plates or columns. Their cross-section can be 3-10 cm. Ice density in the deposits of migration ice, like that of ordinary segregational-schlieren ice, ranges, according to P.A. Shumskiy, from 0.9140 to 0.9168 g cm-3.
It is thought that in especially favourable conditions, when there is seepage of surface water from rather deep lakes (that do not freeze completely in winter), ice of similar type can form sheet deposits 20-30 m thick and several hundred metres long. Usually, they have no expression in the recent topography. The ice of the deposits is transparent and laminated by virtue of the nonuniform distribution of air bubbles and organic-mineral admixtures, and it is coarse-grained. Its structure is allotriomorphic-granular.
Principles of the formation and development of the frozen strata and layers of seasonal freezing and thawing
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