Karst

A karst landscape in Minerve, Hérault, France
Škocjan Caves, Slovenia
The Puerto Princesa Underground River, Philippines
Global distribution of major outcrops of carbonate rocks (mainly limestone, except evaporites)
The karst hills of The Burren on the west coast of Ireland
El Torcal de Antequera, Andalusia, Spain

Karst topography is a landscape formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves.[1] It has also been documented for more weathering-resistant rocks, such as quartzite, given the right conditions.[2] Subterranean drainage may limit surface water, with few to no rivers or lakes. However, in regions where the dissolved bedrock is covered (perhaps by debris) or confined by one or more superimposed non-soluble rock strata, distinctive karst surface developments might be totally missing, the features occurring at subsurface levels.

The study of karst is considered of prime importance in petroleum geology since as much as 50% of the world's hydrocarbon reserves are hosted in porous karst systems.[3]

Etymology

The English word karst was borrowed from German Karst in the late 19th century.[4] The German word came into use before the 19th century.[5] According to the prevalent interpretation, the term is derived from the German name for the Karst region (Italian: Carso), a limestone plateau above the city of Trieste in the northern Adriatic (now located on the border between Slovenia and Italy, in the 19th century it was part of the Austrian Littoral).[6] Scholars disagree, however, on whether the German word (which shows no metathesis) was borrowed from Slovene.[7][8]

The Slovene common noun kras was first attested in the 18th century, and the adjective form kraški in the 16th century.[9] As a proper noun, the Slovene form Grast was first attested in 1177,[10] referring to the Karst Plateau—a region in Slovenia partially extending into Italy, where the first research on karst topography was carried out. The Slovene words arose through metathesis from the reconstructed form *korsъ,[9] borrowed from Dalmatian Romance carsus.[10] Ultimately, the word is of Mediterranean origin,[10] believed to derive from some Romanized Illyrian base.[9] It has been suggested that the word may derive from the Proto-Indo-European root karra- "rock".[10][11] The name may also be connected to the oronym Kar(u)sádios oros cited by Ptolemy, and perhaps also to Latin Carusardius.[9][10]

Early studies

Karstic limestones as statuary in Shenyang Imperial Palace, Shenyang, China

Johann Weikhard von Valvasor, a pioneer of the study of karst in Slovenia and a fellow of the Royal Society for Improving Natural Knowledge, London, introduced the word karst to European scholars in 1689, describing the phenomenon of underground flows of rivers in his account of Lake Cerknica.[12]

Jovan Cvijić greatly advanced the knowledge of karst regions, so much that he became known as the "father of karst geomorphology". Primarily working with the karstic regions of the Balkans, Cvijić's 1893 publication Das Karstphänomen describes landforms such as karren, dolines and poljes.[3] In a 1918 publication Cvijić proposed a cyclical model for karstic landscape development.[3][13] Karst hydrology emerged as a discipline in the late 1950s and early 1960s in France. Previously, the activities of cave explorers, called speleologists, had been dismissed as more of a sport than a science, meaning that underground karstic caves and their associated watercourses were, from a scientific perspective, understudied.[14]

Chemistry

Karst lake (Doberdò del Lago, Italy) fed by an underground water source into a depression with no surface inlet or outlet
Doline in the causse de Sauveterre, Lozère, France

The development of karst occurs whenever acidic water starts to break down the surface of bedrock near its cracks, or bedding planes. As the bedrock (typically limestone or dolostone) continues to degrade, its cracks tend to get bigger. As time goes on, these fractures will become wider, and eventually a drainage system of some sort may start to form underneath. If this underground drainage system does form, it will speed up the development of karst formations there because more water will be able to flow through the region, giving it more erosive power.[15]

Dissolution mechanism

The carbonic acid that causes karstic features is formed as rain passes through the atmosphere picking up carbon dioxide (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 primary reaction sequence in limestone dissolution is the following:

H2O + CO2 → H2CO3
CaCO3 + H2CO3 → Ca2+ + 2 HCO−
3
Lijiang River, Guilin, China

In particular and very rare conditions such as 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 oxygen (O2)-rich surface waters seep into deep anoxic karst systems, they bring oxygen, which reacts with sulfide present in the system (pyrite or hydrogen sulfide) to form sulfuric acid (H2SO4). Sulfuric acid then reacts with calcium carbonate, causing increased erosion within the limestone formation. This chain of reactions is:

H2S + 2 O2 → H2SO4 (sulfide oxidation)
H2SO4 + 2 H2O → SO2−
4 		+ 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)

This reaction chain forms gypsum.[16]

Morphology

Limestone pavement in Dent de Crolles, France

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 solution flutes (or rillenkarren), runnels, limestone pavement (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 karst 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 at 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

A karst spring in the Jura mountains near Ouhans in eastern France at the source of the river Loue

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 occurs when an underground stream emerges onto the surface between layers of rock, cascades some distance, 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 800 m (1⁄2 mi) down the canyon in a placid pool. A turlough 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. 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 also poses difficulties for human inhabitants. Sinkholes can develop gradually as surface openings enlarge, but progressive erosion is frequently unseen until the roof of an underground cavern suddenly collapses. Such events have swallowed homes, cattle, cars, and farm machinery. In the United States, sudden collapse of such a cavern-sinkhole swallowed part of the collection of the National Corvette Museum in Bowling Green, Kentucky in 2014.[17]

Interstratal karst

Interstratal karst is a karstic landscape which is developed beneath a cover of insoluble rocks. Typically this will involve a cover of sandstone overlying limestone strata undergoing solution. In the United Kingdom extensive doline fields developed at Mynydd Llangynidr across a plateau of Twrch Sandstone overlying concealed Carboniferous Limestone.[18]

Kegelkarst

Kegelkarst is a type of tropical karst terrain with numerous cone-like hills, formed by cockpits, mogotes, and poljes and without strong fluvial erosion processes. This terrain is found in Cuba, Jamaica, Indonesia, Malaysia, the Philippines, Puerto Rico, southern China, and Vietnam.[19]

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. Mud Caves are an example of pseudokarst.

Notable karst areas

The world's largest limestone karst is Australia's Nullarbor Plain. Slovenia has the world's highest risk of sinkholes, while the western Highland Rim in the eastern United States is at the second-highest risk of karst sinkholes.[20][21]

Mexico hosts important karstic regions in the Yucatán peninsula and Chiapas.[22]

Many karst-related terms derive from South Slavic languages, entering scientific vocabulary through early research in the Western Balkan Dinaric Alpine karst.

See also

References

  1. ↑ What is Karst, University of Texas at Austin
  2. ↑ Geomorphological Landscapes of the World.
  3. 1 2 3 Ford, Derek (2007). "Jovan Cvijić and the founding of karst geomorphology". Environmental Geology. 51: 675–684. doi:10.1007/s00254-006-0379-x.
  4. ↑ Shorter Oxford English Dictionary. 2002. Vol. 1, A–M. Oxford: Oxford University Press, p. 1481.
  5. ↑ Seebold, Elmar. 1999. Kluge Etymologisches Wörterbuch der deutschen Sprache, 23rd edition. Berlin: Walter de Gruyter, p. 429.
  6. ↑ Online Etymology Dictionary
  7. ↑ Pfeiffer, Dieter. 1961. "Zur Definition von Begriffen der Karst-Hydrologie." Zeitschrift der Deutschen Geologischen Gesellschaft 113: 51–60, p. 52
  8. ↑ Pörtner, Rudolf. 1986. Bevor die Römer kamen: Städte und Stätten deutscher Urgeschichte. Rasatt: Pabel-Moewig Verlag, p. 88.
  9. 1 2 3 4 Snoj, Marko. 2003. Slovenski etimološki slovar. 2nd edition. Ljubljana: Modrijan, p. 318.
  10. 1 2 3 4 5 Bezlaj, France (ed.). 1982. Etimološki slovar slovenskega jezika, vol. 2, K–O. Ljubljana: SAZU, p. 82.
  11. ↑ Gams, I., Kras v Sloveniji — v prostoru in casu (Karst in Slovenia in space and time), 2003, ISBN 961-6500-46-5.
  12. ↑ Paul Larsen, Scientific accounts of a vanishing lake: Janez Valvasor, Lake Cerknica and the New Philosophy, 2003.
  13. ↑ Cvijić, Jovan (1918). "Hydrographie souterraine et évolution morphologique du Karst". Recueil des travaux de l'institut de géographie alpine (in French). 6 (4): 375–426. Retrieved June 5, 2017.
  14. ↑ Gilli, Éric; Mangan, Christian; Mudry, Jacques (2012). Hydrogeology: Objectives, Methods, Applications. Translated by Fandel, Choél. CRC Press. p. 7.
  15. ↑ "What is Karst (and why is it important)?". Karst Waters Institute.
  16. ↑ Galdenzi, S.; Cocchioni, M.; Morichetti, L.; Amici, V.; Scuri, S. (2008). "Sulfidic ground water chemistry in the Frasassi Cave, Italy" (PDF). Journal of Cave and Karst Studies. 70 (2): 94–107.
  17. ↑ Mood somber, repairs uncertain as Corvette museum opens. CNN.com
  18. ↑ "Archived copy" (PDF). Archived from the original (PDF) on 2015-11-20. Retrieved 2013-03-03.
  19. ↑ Whittow, John (1984). Dictionary of Physical Geography. London: Penguin, 1984, p 292. ISBN 0-14-051094-X.
  20. ↑ Austin Peay State University : Harned Bowl work not to blame for new sinkhole, say experts
  21. ↑ What is Karst topography and why should you care? - Clarksville, TN Online
  22. ↑ Mora, L., Bonifaz, R., López-Martínez, R., (2016). "Unidades geomorfológicas de la cuenca del Río Grande de Comitán, Lagos de Montebello, Chiapas-México" (PDF). Boletín de la Sociedad Geológica Mexicana (in Spanish). 68 (3): 377–394.

Further reading

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