Karst(redirected from karstic)
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Karst(kärst), Ital. Carso, Slovenian Kras, limestone plateau, W Slovenia, N of Istria and extending c.50 mi (80 km) SE from the lower Isonzo (Soča) valley between the Bay of Trieste and the Julian Alps. It is characterized by deep gullies, caves, sinkholes, and underground drainage—all the result of carbonation-solution. The best-known caves are at Postojna. The barren nature of the plateau deters human settlement. Rough pasture or forest covers much of the surface, and there is little arable land. The term karst is used to describe any area where similar geological formations are found.
(Kras), a limestone plateau in northwestern Yugoslavia (the western extremity is in Italy) north of the Istrian Peninsula. Maximum elevation, 643 m. There is a general development of karst forms of relief (sinks, funnel-like hollows, polje basins, sinkholes, and caves). Surface drainage is almost completely absent, and vegetation is sparse and grassy. Wine growing is carried on where the soils in the hollows are fertile. The world-famous Postojna Cave is located on the plateau. There is an institute for the study of caves in the city of Postojna.
(German Karst, from the name of the Karst plateau, or Kras, in Yugoslavia), phenomena in rock that is soluble in natural water and the process by which these phenomena are formed.
Karst is characterized by a complex of surface and underground forms and by the unique circulation and regime of underground water as well as a unique river network and lakes. It develops in calcareous and noncalcareous rock. Of the exposed and buried karstic rocks on the continents, up to 40 million sq km are calcareous rock, about 7 million sq km are gypsums and anhydrites, and up to 4 million sq km are rock salt. Calcareous rock dissolves in the presence of free carbon dioxide (CaCo3 + H2O + CO2 ⇄ Ca++ 2HCO3) or mineral or organic acids. Sulfate rock and rock salt can dissolve in water without the accompanying reactions. Karst develops under the combined influence of surface and underground water. The dissolving of the rock is frequently accompanied by mechanical water erosion. The conditions for erosion can be created by the solution of the contacts of the grains of the rock, freeing them from one another and facilitating water erosion.
The surface of karst terrain is characterized by small furrows, or karren, and closed depressions in the form of potholes, sinks, basins, poljes, natural wells and mines, and blind (closed off at the lower end) valleys and ravines. Particularly typical are the sinks—conical, basin- or dish-shaped, or irregular pits—with a diameter of 1–200 m and a depth of 0.5–50 m. On the bottom of the sinks and other depressions water-absorbing openings, or ponors, are found. The basins and sinks can be full of water at some times and dry up at others (periodically disappearing lakes). Basins with areas up to several hundreds of square kilometers, steep sides, flat bottoms, and disappearing streams and rivers are known as poljes.
In karst massifs various underground passageways, cavities, and caves form, frequently along fissures. The world’s longest caves are more than 100 km (for example, the Flint Ridge cave system in Kentucky in the United States or the Hôlloch cave in the Alps in Switzerland). Deep karst wells and natural shafts or chasms form passageways between surface and underground karst forms. The deepest caverns in the world are Pierre St. Martin, which is 1, 110 m deep (France-Spain), and Berger, which is 1, 122 m deep (Isère, France). In the USSR the Nazarov-skaia Cavern in the vicinity of Sochi in the Western Caucasus is about 500 m deep.
The complex of surface and underground karst forms is most fully expressed when the surface of the soluble rock is exposed; this is known as denuded karst. But the soluble rock can be covered over by a layer of soil and sod (then the exposed karren are absent); this is termed turf-covered karst. Mantled karst occurs when the surface is covered by insoluble loose sediments. (The surface is characterized by sinks and similar depressions that have developed owing to the washing of loose covering formations into the fissures and cavities of the karstic series.) Armored karst occurs when the surface is covered by insoluble rocky formations. (The only characteristic surface forms are sinkholes.) The soluble rock can be altogether buried beneath the nonkarstic series, in which case the karst forms are not expressed on the surface; this is termed buried karst. Unique karst land scapes are observed in tropical countries, frequently with characteristic outliers of limestone (cone karst and tower karst, for example). Karst processes in permafrost areas also occur in a unique manner.
Karst terrain has few surface watercourses. The rivers and streams frequently disappear into underground cavities and, having traveled a portion of their length in these caverns, again emerge on the surface in the form of large springs (vauclusian springs), mainly along the edges of the karst areas. The circulation of underground water is most intense in areas adjacent to valleys and in zones of tectonic disturbances, where karst develops more strongly. Under platform plain conditions, the interior parts (cores) of the divides are usually less karsted than the areas adjacent to the valleys.
Pseudokarst phenomena are externally similar to karst phenomena, and they occur in ice and permafrost (thermokarst) and in fragmental and porous soils (“clastokarst,” “clay karst,” “loess karst,” mechanical suffosion, and subsidences). A major role in the development of pseudokarst is played by physical processes such as the thawing of ice or the mechanical effect of running water. Processes related to the leaching of salts from loose soils or to the solution of the lime and gypsum cement of sandstones and conglomerates constitute the karst-suffosion (sapping) processes. In these rocks the water dissolves only the inclusions or the cement; the basic rock mass (the clay particles, sand, or shingle) is removed by the mechanical action of running water.
Karst complicates industrial, housing, and transport construction as well as the erection of hydroelectric power plants and reservoirs. (Instances of water filtering out of reservoirs and even the destruction of dams by karst phenomena are known.) The underground cavities and fissures reduce the strength of the ground, thereby making it necessary to carry out special investigations in constructing buildings, roads, and the like. In areas where karst is only slightly developed, housing of five stories can be built if the foundations are reinforced and reinforced-concrete belts are laid.
Karst frequently impedes the mining of minerals, but sometimes its draining properties are utilized. At the same time, certain minerals fill the karst cavities, forming deposits of lead, zinc and iron ores, bauxites, phosphorites, oil and fuel gases, and gold and diamond placers.
In the USSR well-developed karst areas exist in many regions on the East European Plain, in the Crimea, the Urals and the Cisurals, on the forward ranges of the Greater Caucasus, in Middle Asia (the Ustiurt plateau and the Karatau, Alai, Zerav-shan, and the Petr Pervyi ranges), areas in the Kazakh Melkosopochnik (low hills), the mountains of Southern Siberia, the Angara region, and the Far East. Abroad, karst has developed particularly extensively in Yugoslavia, which is considered to be the country of classic karst. It is also found in many other states of Western Europe, various regions of the United States, the West Indies (Cuba, Puerto Rico, and Jamaica), China (particularly in the Kwangsi Chuang Autonomous Region and Yunnan Province), on the Indochina peninsula, and elsewhere.
REFERENCESGvozdetskii, N. A. Karst, 2nd ed. Moscow, 1954.
Maksimovich, G. A. Osnovy karstovedeniia, vols. 1–2. Perm’, 1963–69.
Sokolov, D. S. Osnovnye usloviia razvitiia karsta. Moscow, 1962.
Karst i ego narodnokhoziaistvennoe znachenie: Sb. st. Moscow, 1964.
Tipy karsta v SSSR: [Sb. st.]. Moscow, 1965.
N. A. GVOZDETSKII