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weathering,collective term for the processes by which rock at or near the earth's surface is disintegrated and decomposed by the action of atmospheric agents, water, and living things. Some of these processes are mechanical, e.g., the expansion and contraction caused by sudden, large changes in temperature, the expansive force of water freezing in cracks, the splitting caused by plant roots, and the impact of running water; others are chemical, e.g., oxidation, hydration, carbonization, and loss of chemical elements by solution in water. Weathering is important because it aids in the formation of soil and prepares materials for erosionerosion
, general term for the processes by which the surface of the earth is constantly being worn away. The principal agents are gravity, running water, near-shore waves, ice (mostly glaciers), and wind.
..... Click the link for more information. . Construction materials for buildings and roads are subject to weathering by water, carbon dioxide, aerosol gases, freeze-thaw cycles, and salt (see also formation of potholespothole,
in geology, cylindrical pit formed in the rocky channel of a turbulent stream. It is formed and enlarged by the abrading action of pebbles and cobbles that are carried by eddies, or circular water currents that move against the main current of a stream.
..... Click the link for more information. ). New techniques in road construction, allowing a minimum of weathering, more weather-resistant aggregates, and better building materials have lowered costs for maintenance and repair.
a process of the breakdown and alteration of rock under the conditions of the earth’s crust; caused by the effect of the mechanical and chemical action of atmospheric, ground, and surface waters and organisms.
In terms of the medium in which weathering occurs, a distinction is drawn between atmospheric and underwater weathering. A distinction is also made in terms of the type of weathering action on the rock. Physical weathering causes only the mechanical breakdown of the rock into fragments. Chemical weathering alters the chemical composition of the rock with the formation of minerals that are more resistant to the conditions of the earth’s surface. Organic or biological weathering leads to the mechanical decomposition or chemical alteration of the rock as a result of the vital activities of organisms. Soil formation is a unique type of weathering, and here biological factors play a particularly active role.
Rock weathering occurs under the effect of water (atmospheric precipitation or ground waters), carbon dioxide and oxygen, water vapor, atmospheric or subsurface air, seasonal and daily temperature fluctuations, and the vital activity of macro- and micro-organisms and the products of their decomposition. In addition to the listed agents, the rate and degree of weathering, the thickness of the weathering products, and their composition are also affected by the relief and geological structure of the terrain, as well as by the com-position and structure of the mother rock. The predominant mass of physical and chemical weathering processes (oxidation, sorption, hydration, and coagulation) occurs with the release of energy.
Usually the types of weathering occur simultaneously, but one or another of them predominates depending upon the climate. Physical weathering occurs chiefly under the conditions of an arid and hot climate and is related to sharp fluctuations in the rock temperature caused by the heating of sun rays (insolation) and the subsequent nighttime cooling. A rapid change in the volume of the surface parts of the rock here leads to their cracking. In areas with frequent temperature fluctuations around 0° C, the mechanical destruction of the rock occurs under the effect of frost weathering. When water that has penetrated into the cracks freezes, the volume of the water increases, and the rock splits. Chemical and organic weathering are confined chiefly to areas with a wet climate. The basic factors of chemical weathering are air and particularly water, which contains salts, acids, and alkalis. The water solutions circulating in the rock, aside from simple dilution, are also capable of causing complex chemical changes.
The physical and chemical processes of weathering occur in close relationship to the development and vital activities of animals and plants, as well as to the action of the products from their decay after death. A tropical or subtropical climate and an insignificant erosion of the terrain are the best conditions for the formation and conservation of weathering products (minerals) in situ. Here, the rock bed subjected to weathering shows a geochemical zonality (from the top to the bottom) expressed by a complex of minerals characteristic for each zone. The minerals are formed as a result of successive processes: decomposition of the rock under the effect of physical weathering, leaching out of the bases, hydration, hydrolysis, and oxidation. These processes often lead to the complete decomposition of the primary minerals and even to the formation of free oxides and hydroxides. Depending upon the degree of acidity and alkalinity of the medium and the participation of biogenic factors, minerals of varying chemical composition are formed-from those resistant in an alkali medium (in the lower horizons) to those resistant in an acid or neutral medium (in the upper horizons).
The diversity of weathering products represented by different minerals is determined by the composition of the minerals in the primary rock. For example, on ultrabasic rock (serpentinites), the upper zone is represented by rock where carbonates (magnesite and dolomite), cerolites and meerschaums are formed in the cracks. Then follow the horizons of carbonatization (calcite, dolomite, and aragonite), in the upper parts of which nickel cerolites or garnierite can form in the cracks; hydrolysis, to which the formation of nontronite and the accumulation of nickel (up to 2.5 percent NiO) is related; and silification (quartz, opal, and chalcedony). The zone of final hydrolysis and oxidation is com-posed of hydrogoethite (ocherous), goethite, magnetite and manganese oxides, and hydroxides (nickel- and cobalt-containing). The major deposits of nickel, cobalt, magnesite, and naturally alloyed iron ores are related to the weathering processes of this type of rock.
On the carbonatites that originally consisted of more than 90 percent calcium, ankerite or siderite, and a small quantity of mineral admixtures (pyroxenes, amphiboles, tantalonio-bates, and rare earth minerals), the end weathering products become friable. As a result of the oxidation of carbonates, iron hydroxides are formed, while the calcium and magnesium oxides undergo substantial leaching. This leads to an increase in the content of the mineral impurities that are stable under supergene conditions. Thus the fresh carbonates, even with an insignificant content of niobium, tantalum, rare earths, and phosphorus, can produce exploitable deposits of these elements with weathering. With the (physical) weathering of coal, the coal breaks down to blossom and loses its luster, and the thickness of the seams changes. With chemical weathering, the carbon and hydrogen content declines in the composition of the coals, and the oxygen in the organic mass increases. In addition, the moisture content of the coal rises, its caking capacity declines, and thermal conductivity falls.
In those instances when the weathering products do not remain in situ, but are carried from the surface of the weathering rock by water or wind, characteristic landforms often occur; they depend both upon the character of weathering and on the properties of the rock, in which the process manifests and emphasizes the particular features of the rock structure. For igneous rock (granites, diabases, and so forth), massive rounded forms of weathering are characteristic; for the foliated sedimentary and metamorphic rock there are steplike shapes (benches, recesses, and so forth). The heterogeneity of the rock and the varying resistance of the various parts of the rock to weathering lead to the formation of outliers in the form of solitary mountains, columns, towers, and so forth. In a wet climate, on the sloping surfaces of uniform and comparatively water-soluble rock like lime-stone, the running-off waters cut out irregular depressions separated by sharp prominences and crests. This forms an uneven surface, known as a cirque. In the process of the de-composition of residual weathering products, numerous soluble compounds are formed that are carried by the ground-waters into the water basins and either become part of the dissolved salts or are precipitated. The weathering processes lead to the formation of various sedimentary rocks, numerous minerals, including kaolins, ochers, refractory clays, sands, iron ores, aluminum, manganese, nickel, cobalt, gold and platinum placers, as well as oxidation zones of pyrite deposits with their minerals.
REFERENCESGinzburg, 1.1.“Obrazovanie drevnei kory vyvetrivaniia na territorii SSSR, ee mineraly i ikh svoistva.” Trudy iubileinoi sessii, posviashchennoi 100-letiiu so dnia rozhdeniia V. V. Dokuchaeva. Moscow-Leningrad, 1949.
Kazanskii, lu. P. Vyvetrivanie i ego roV v osadkonakoplenie. Moscow, 1969.
Vyvetrivanie i litogenez. Moscow, 1969.