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Friday, January 14, 2011

What is karst?

Karst Morphology
Karst is the German form of the Indo-European word kar, which means rock. The Italian term is carso, and the Slovenian kras. In Slovenia, kras or krš means ‘bare stony ground’ and is also a rugged region in the west of the country. In geomorphology, karst is terrain in which soluble rocks are altered above and below ground by the dissolving action of water and that bears distinctive characteristics of relief and drainage (Jennings 1971, 1).
It usually refers to limestone terrain characteristically lacking surface drainage, possessing a patchy and thin soil cover, containing many enclosed depressions, and supporting a network of subterranean features, including caves and grottoes. However, all rocks are soluble to some extent in water, and karst is not confined to the most soluble rock types. Karst may form in evaporites such as gypsum and halite, in silicates such as sandstone and quartzite, and in some basalts and granites under favourable conditions (Table 8.1). Karst features may also form by other means – weathering, hydraulic action, tectonic movements, meltwater, and the evacuation of molten rock (lava). These features are called pseudokarst as solution is not the dominant process in their development (Gerrard, 2007)

Extensive areas of karst evolve in carbonate rocks (limestones and dolomites), and sometimes in evaporites, which include halite (rock salt), anhydrite, and gypsum. Figure 8.2 shows the global distribution of exposed carbonate rocks. Limestones and dolomites are a complex and diverse group of rocks (Figure 8.3). Limestone is a rock containing at least 50 per cent calcium carbonate (CaCO3), which occurs largely as the mineral calcite and rarely as aragonite. Pure limestones contain at least 90 per cent calcite. Dolomite is a rock containing at least 50 per cent calcium–magnesium carbonate (CaMg(CO3)2), a mineral called dolomite. Pure dolomites (also called dolostones) contain at least 90 per cent dolomite. Carbonate rocks of intermediate composition between pure limestones and pure dolomites are given various names, including magnesian limestone, dolomitic limestone, and calcareous dolomite.(Gerrard, 2007)

Karst features achieve their fullest evolution in beds of fairly pure limestone, with more than 80 per cent calcium carbonate, that are very thick, mechanically strong, and contain massive joints. These conditions are fulfilled in the classic karst area of countries bordering the eastern side of theAdriatic Sea. Chalk, although being a very pure limestone, is mechanically weak and does not favour the formation of underground drainage, which is a precondition for the evolution of medium-scale and large-scale surface-karst landforms.

Sources : Gerrard, John.2007.FUNDAMENTALS OF GEOMORPHOLOGY. New York : Routledge 270 Madison Avenue
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