Karst topography is a geologic formation shaped by the dissolution of a layer or layers of soluble bedrock, usually carbonate rock such as limestone or dolomite, but has also been documented for weathering resistant rocks like quartzite given the right conditions.

Due to subterranean drainage, there may be very limited surface water, even to the absence of all rivers and lakes. Many karst regions display distinctive surface features, with sinkholes or dolines being the most common. However, distinctive karst surface features may be completely absent where the soluble rock is mantled, such as by glacial debris, or confined by a superimposed non-soluble rock strata. Some karst regions include thousands of caves, even though evidence of caves that are big enough for human exploration is not a required characteristic of karst.

Background

Karst topography is characterized by subterranean limestone caverns, carved by groundwater. The geographer Jovan Cvijić (1865–1927) was born in western Serbia and studied widely in the Dinaric Kras region. His publication of ''Das Karstphänomen'' (1893) established that rock dissolution was the key process and that it created most types of dolines, "the diagnostic karst landforms". The Dinaric Kras thus became the type area for dissolutional landforms and aquifers; the regional name ''kras'', Germanicised as "karst", is now applied to modern and paleo-dissolutional phenomena worldwide. Cvijić related the complex behaviour of karstic aquifers to development of solutional conduit networks and linked it to a cycle of landform evolution. Cvijić defined two main types of karstic area, holokarst, wholly developed, as in the Dinaric region along the eastern Adriatic and deeper inland in the Balkan Peninsula and merokarst, imperfectly developed with some karstic forms, as in eastern Serbia. Cvijić is recognized as "the father of karst geomorphology".

The international community has settled on ''karst'', the German name for Kras, a region in Slovenia partially extending into Italy, where it is called "Carso" and where the first scientific research of a karst topography was made. The name has an Indo-European origin (from ''karra'' meaning "stone"), and in antiquity it was called "Carusardius" in Latin. The Slovene form ''grast'' is attested since 1177, and the Croatian ''kras'' since 1230.. "Krš" – "Krsh" meaning in Croatian and in Serbian "barren land" which is typical feature in the Northern Dinaric limestone mountains could also be an origin to the word Karst.

Chemistry

Karst landforms are generally the result of mildly acidic water acting on weakly soluble bedrock such as limestone or dolostone. The mildly acidic water begins to dissolve the surface along fractures or bedding planes in the limestone bedrock. Over time, these fractures enlarge as the bedrock continues to dissolve. Openings in the rock increase in size, and an underground drainage system begins to develop, allowing more water to pass through the area, and accelerating the formation of underground karst features.

Main dissolution mechanism: carbonic acid

The carbonic acid that causes these features is formed as rain passes through the atmosphere picking up CO₂, which dissolves in the water. Once the rain reaches the ground, it may pass through soil that can provide much more CO₂ to form a weak carbonic acid solution, which dissolves calcium carbonate. The sequence of reactions involved in the limestone dissolution are the following:

:H₂O + CO₂ → H₂CO₃ :CaCO₃ → Ca²⁺ + CO₃^2– :CO₃^2– + H₂CO₃ → 2 HCO₃^– :CaCO₃ + H₂CO₃ → Ca²⁺ + 2 HCO₃^–

This is the main dissolution mechanism of calcium carbonate in limestone.

Secondary dissolution mechanism: sulfide oxidation

However, in particular and very rare conditions such as these encountered in the past in Lechuguilla Cave in New Mexico (and more recently in the Frasassi Caves in Italy), other mechanisms may also play a role. The oxidation of sulfides leading to the formation of sulfuric acid can also be one of the corrosion factors in karst formation. As O₂-rich surface waters seep into deep anoxic karst systems it brings oxygen which reacts with sulfide present in the system (pyrite or H₂S) to form sulfuric acid (H₂SO₄). Sulfuric acid then reacts with calcium carbonate causing an increased erosion within the limestone formation. This can be summarized by the cascade of the following reactions:

:H₂S + 2 O₂ → H₂SO₄ (sulfide oxidation) :H₂SO₄ + 2 H₂O → SO₄^2– + 2 H₃O⁺ (sulfuric acid dissociation) :CaCO₃ + 2 H₃O⁺ → Ca²⁺ + H₂CO₃ + 2 H₂O (calcium carbonate dissolution)

:CaCO₃ + H₂SO₄ → CaSO₄ + H₂CO₃ (global reaction leading to calcium sulfate) :CaSO₄ + 2 H₂O → CaSO₄ · 2 H₂O (hydration and gypsum formation)

As a result of this reaction the mineral gypsum forms.

Morphology

The karstification of a landscape may result in a variety of large or small scale features both on the surface and beneath. On exposed surfaces, small features may include flutes, runnels, clints and grikes, collectively called karren or lapiez. Medium-sized surface features may include sinkholes or cenotes (closed basins), vertical shafts, foibe (inverted funnel shaped sinkholes), disappearing streams, and reappearing springs. Large-scale features may include limestone pavements, poljes and blind valleys. Mature karst landscapes, where more bedrock has been removed than remains, may result in karst towers, or haystack/eggbox landscapes. Beneath the surface, complex underground drainage systems (such as karst aquifers) and extensive caves and cavern systems may form.

Erosion along limestone shores, notably in the tropics, produces karst topography that includes a sharp makatea surface above the normal reach of the sea and undercuts that are mostly the result of biological activity or bioerosion at or a little above mean sea level. Some of the most dramatic of these formations can be seen in Thailand's Phangnga Bay and Halong Bay in Vietnam.

Calcium carbonate dissolved into water may precipitate out where the water discharges some of its dissolved carbon dioxide. Rivers which emerge from springs may produce tufa terraces, consisting of layers of calcite deposited over extended periods of time. In caves, a variety of features collectively called speleothems are formed by deposition of calcium carbonate and other dissolved minerals.

Hydrology

Farming in karst areas must take into account the lack of surface water. The soils may be fertile enough, and rainfall may be adequate, but rainwater quickly moves through the crevices into the ground, sometimes leaving the surface soil parched between rains.

A karst fenster is where an underground stream emerges onto the surface between layers of rock, cascades some feet, and then disappears back down, often into a sinkhole. Rivers in karst areas may disappear underground a number of times and spring up again in different places, usually under a different name (like Ljubljanica, the river of seven names). An example of this is the Popo Agie River in Fremont County, Wyoming. At a site simply named "The Sinks" in Sinks Canyon State Park, the river flows into a cave in a formation known as the Madison Limestone, and then rises again a half-mile down the canyon in a placid pool. A Turlach is a unique type of seasonal lake found in Irish karst areas which are formed through the annual welling-up of water from the underground water system.

Water supplies from wells in karst topography may be unsafe, as the water may have run unimpeded from a sinkhole in a cattle pasture, through a cave and to the well, bypassing the normal filtering that occurs in a porous aquifer. Karst formations are cavernous and therefore have high rates of permeability, resulting in reduced opportunity for contaminants to be filtered out.

Groundwater in karst areas is just as easily polluted as surface streams. Sinkholes have often been used as farmstead or community trash dumps. Overloaded or malfunctioning septic tanks in karst landscapes may dump raw sewage directly into underground channels.

The karst topography itself also poses difficulties for human inhabitants. Sinkholes can develop gradually as surface openings enlarge, but quite often progressive erosion is unseen and the roof of an underground cavern suddenly collapses. Such events have swallowed homes, cattle, cars, and farm machinery.

The Driftless Area National Wildlife Refuge in Iowa protects ''Discus macclintocki'', a species of ice age snail surviving in air chilled by flowing over buried karst ice formations.

Pseudokarst

Pseudokarsts are similar in form or appearance to karst features, but are created by different mechanisms. Examples include lava caves and granite tors—for example, Labertouche Cave in Victoria, Australia and paleocollapse features.

Notable karst areas

Notable pseudokarst areas

North America

Belize

Great Blue Hole near the center of Lighthouse Reef, Belize

United States

Arroyo Tapiado in Anza-Borrego Desert State Park. Over two-dozen mud caves are found in this desert area east of San Diego, California.

See also

Abîme

Glaciokarst

Thermokarst

Speleology

Subterranean river

List of landforms

References

Further reading

Jennings, J.N., ''Karst Geomorphology'', 2nd ed., Blackwell, 1985, ISBN 0631140328

Sweeting, M.M., ''Karst Landforms'', Macmillan, 1973, ISBN 023103623X

External links

Speleogenesis Network, a communication platform for physical speleology and karst science research

Speleogenesis and Karst Aquifers – a large glossary of Karst related terms

Acta Carsologica – research papers and reviews in all the fields related to karst

SERA Karst Task Force – a non-profit resource organization dedicated to karst conservation and the clean up of cave and karst features

CDK Citizens of the Karst – Citizens of the Karst, a non profit NGO dedicated to the protection of the Puerto Rican Karst (English site available)

Category:Karst caves Category:Landforms Category:Cave geology Category:Geomorphology

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