Karst Topography

Karst (in German) topography is the name for the many landforms that developed on the surface and within soluble rocks as caves and other forms characteristic of a part of the Kras region of Slovenia, where the landforms developed on and in some 400 square kilometers of limestone (calcium carbonate) and were first studied many years ago. All rocks can be slightly soluble in water, with the result that various karstic landforms can also develop more slowly in the somewhat less soluble magnesium carbonate of dolomite or dolostone, as well as quite quickly in evaporate rocks such as rock salt or gypsum. In fact, some solubility can even occur slowly in silicate rocks such as sandstone, especially where it is cemented with carbonate minerals, or quartzite, basalt, and granite under favorable conditions and over long time periods. The processes of karstification of a region involve dissolution of the bedrock as various minerals are dissolved in the water. Limestone bedrock, the most common host rock of karst topography, is composed of the mineral calcite, which is subjected to the carbonation reaction in the presence of water with a small amount of dissolved carbon dioxide that makes a weak carbonic acid. As this slightly acidic water comes in contact with the limestone bedrock, it has the capability of dissolving and carrying away molecules in solution to make the karst topography. For karst topography to develop best, however, the carbonate must be thick, mechanically strong, and well jointed to allow concentrated percolation locations for the infiltrating water to carry away the solute loads. Chalk rock, for example, is quite a pure form of limestone, but it does not form caves or karst well because it is neither mechanically strong nor well jointed; instead it is soft and crumbly, therefore cave systems cannot become established.

Karst topography is terrain formed in soluble rocks that bears distinctive characteristic of drainage and relief unlike most other landscapes. Instead, it tends to have a rough surface with many enclosed depressions; the soil above the bedrock tends to be thin and patchy, with exposed bare rock that is etched into a great variety of irregular shapes, and much of its drainage is [Page 1650]subterranean where it flows through caves and grottos. True karst occurs where solutional processes dominate; in some parts of the world where karst processes create almost all the landforms of the region, it may be appropriate to refer to it as “holokarst.” On the other hand, fluviokarst occurs where the processes of rivers and solution are approximately equal in their activities. Large parts of the Midwestern United States, for example, are dissected by rivers running over and through carbonate rocks so that the intervening interfluves can have karst topography developed on and in them. Glaciokarst is formed in some areas of higher altitude and latitude where soluble carbonates occur beneath partial covers of ice so that as the glaciers wax and wane, their meltwaters can assist in the excavation of the subterranean topography. Psuedokarst landforms are developed by the very wide variety of nonsolutional processes that mimic some of those of true karst. Thus, granular disintegration of silicate rocks, hydraulic plucking and wave erosion of sea caves, lava flows that have internal lava tubes or caves where the molten rock flowed out, and mass movements that open out chambers underground and produce hummocky topography on the surface can all produce psuedokarstic landforms. Thermokarst is rather a misnomer in that it is not a karst at all, has no solutional component whatsoever, and instead is entirely due to the melting of permafrost and ground ice, although its surficial appearance of enclosed depressions may superficially resemble a true karst.

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