Karst and Paleokarst

Karst is a term that is applied to terrain with distinctive landforms and underground drainage systems that form as a consequence of the solubility of certain rock types, particularly limestone, in water. Karst landscapes are sculpted largely by solution, whereas other rock types are sculpted largely by mechanical erosion. The word ‘karst’ has its origins in pre-Indo-European languages, from kar, meaning ‘rock.’ In Slovenia the word ‘kras’ (or ‘krs’), subsequently germanicized as ‘Karst,’ derives from the name of a barren stony limestone area near Trieste that is still considered the type area for limestone karst. Gypsum and rock salt may form karst, with extensive gypsum karsts known from Russia and the Ukraine, but their relatively high solubility renders such landforms more dynamic and, for rock salt, ephemeral in all but the most arid climates. In humid climates karst features may also develop, though rarely, on more weakly soluble rocks, such as granite or quartzite. Rock solubility and water are the primary factors in karst development. Arid climates, whether hot or cold, support little karst. Physical rock properties also are important in defining the nature of the karst features. Highly porous rocks seldom support welldefined karst features, which instead are favored by low porosity and good secondary permeability, in the form of fractures, which focuses the drainage into specific conduits through the karst rock. Removal of rock in solution allows the development of significant drainage through the rock, rather than across its surface as is largely the case for non-karst rocks removed by mechanical erosion. Consequently, karst landscapes generally lack well-developed surface drainage but have underground drainage conduits, or caves. As such a significant component of karst terrains typically lies beneath the surface, sometimes extending to depths of hundreds, or even thousands, of meters. Intimately associated with the dissolutional aspects of karst are depositional ones. The latter include clastic sediments that accumulate within caves and minerals precipitated from karst waters both above and below ground. Various subdivisions of karst are recognized. Relict karst is used to denote landforms inherited from earlier climatic or drainage regimes but still subject to modification by current conditions. Paleokarst refers to karst features buried by younger rocks and hence largely isolated from ongoing karst development. Where paleokarst is uncovered by later denudation it may be termed exhumed karst. Biokarst (Viles, 1988) encompasses small-scale sculpting of limestone by animals and plants, although the distinction between dissolutional sculpting (true biokarst) and mechanical excavation (bioerosion) is seldom made. Pseudokarst is, as its name implies, ‘false karst.’ Such features superficially resemble karst but form by quite different processes. It includes lava tubes, soil piping, and thermokarst, or cryokarst, formed by localized melting of permafrost. Karst geomorphology often is regarded as a specialist discipline of limited general relevance to geology or geomorphology. However,
12% of Earth’s terrestrial, ice-free surface is composed of limestone, with 7–10% supporting some form of karst landscape. Furthermore, as much as 25% of the world’s population may depend to some extent on karst water supplies. Consequently, the study of karst is crucial to understanding landscape and drainage development over a significant area of Earth’s surface (Ford and Williams, 2007; Gunn, 2003; Jennings, 1985; Waltham and Lowe, 2013).