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Karst

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"Karst" redirects here. For the video game character in Nintendo's Golden Sun series, see Characters of Golden Sun.
A karst landscape
The karst hills of the Burren on the west coast of Ireland


Karst topography is a three-dimensional landscape shaped by the dissolution of a soluble layer or layers of bedrock, usually carbonate rock such as limestone or dolomite.[citation needed] These landscapes display distinctive surface features and underground drainages, and in some examples there may be little or no surface drainage. Some areas of karst topography, such as southern Missouri and northern Arkansas in the USA, are underlain by thousands of caves.

Different terms for karst topography exist in other languages - for example, yanrong in Chinese and tsingy in Malagasy - a notable exception being English (Jennings, Ch.1 p.1). 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 Paleoeuropean origin (karra - stone) and in antiquity it was called Carusardius in Latin. The Slovenian form grast is attested since 1177, and the Croatian kras since 1230.

Chemistry of karst landscapes

Karst lake (Doberdo' del Lago, Italy), from underground water springing into a depression. This lake has no surface inlet or outlet.

Karst landforms are generally the result of mildly acidic water acting on soluble bedrock such as limestone or dolostone. 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 may provide further CO2 to form a weak carbonic acid solution: H2O + CO2 → H2CO3. Recent studies of sulfates in karst waters suggests sulfuric and hydrosulfuric acids may also play an important role in karst formation.

This mildly acidic water begins to dissolve the surface and any 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 and accelerating the formation of underground karst features.

Somewhat less common than this limestone karst is gypsum karst, where the solubility of the mineral gypsum provides many similar structures to the dissolution and redeposition of calcium carbonate.

Karst formations

Surface karst.

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 dolines (closed basins), vertical shafts, 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.

The Witch's Finger stalagmite in Carlsbad Caverns

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.

A karst river 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).

Water drainage and problems

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.

Source of the river Loue, a karst spring.

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. There is an example of this in Lycoming County, Pennsylvania.

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 some difficuties 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

Pseudokarst refers to landscape features that 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.

List of notable karst areas

Africa

Asia

Phong Nha Cave in Phong Nha-Ke Bang, Vietnam.

Europe

North America

Canada

United States

Caribbean

Mexico

Oceania

See also

References

Jennings, J.N. Karst Geomorphology 2nd ed. Blackwell 1985 Sweeting, M.M. Karst Landforms Macmillan 1973

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