Karst


A karst landscape in Minerve, Hérault,France


The karst hills of The Burren on the west coast of Ireland
Karst topography is a geologic formation shaped by thedissolution of a layer or layers of soluble bedrock, usuallycarbonate rock such as limestone or dolomite,[1] but has also been documented for weathering resistant rocks likequartzite given the right conditions.[2]
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, withcenotes, 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.
Contents

• 1 Background
• 2 Chemistry
o 2.1 Main dissolution mechanism: carbonic acid
o 2.2 Secondary dissolution mechanism: sulfide oxidation
• 3 Morphology
• 4 Hydrology
• 5 Pseudokarst
• 6 Notable karst areas
• 7 Notable pseudokarst areas
o 7.1 North America
 7.1.1 Belize
 7.1.2 United States


Background


Doline in the causse de Sauveterre,Lozère, France.
Karst topography is characterized by subterranean limestone caverns, carved by groundwater. The geographerJovan Cvijić (1865–1927) was born in western Serbia and studied widely in the Dinaric Kras region. His publication ofDas 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 karst aquifers to development of solutional conduit networks and linked it to a cycle of landform evolution. Cvijić defined two main types of karst 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 karst 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 Sloveniapartially 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"),[3] and in antiquity it was called "Carusardius" in Latin. The Slovene form grast is attested since 1177, and theCroatian kras since 1230.[citation needed]. "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 lake (Doberdò del Lago, Italy) fed by an underground water source into a depression with no surface inlet or outlet
Karst landforms are generally the result of mildly acidicwater acting on weakly soluble bedrock such as limestoneor 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.[4]
[edit]Main dissolution mechanism: carbonic acid
The carbonic acid that causes these features is formed as rain passes through the atmospherepicking up CO2, which dissolves in the water. Once the rain reaches the ground, it may pass throughsoil 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+ + CO32–
CO32– + 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 ofsulfides 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 → SO42– + 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.[5]
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 flutes, runnels, clints and grikes, collectively called karren or lapiez. Medium-sized surface features may includesinkholes or cenotes (closed basins), vertical shafts, foibe(inverted funnel shaped sinkholes), disappearing streams, and reappearing springs. Large-scale features may includelimestone pavements, poljes and blind valleys. Mature karst landscapes, where more bedrock has been removed than remains, may result in karst towers, or haystack/eggboxlandscapes. 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, USA
Erosion along limestone shores, notably in the tropics, produces karst topography that includes a sharp makateasurface above the normal reach of the sea and undercuts that are mostly the result of biological activity or bioerosionat or a little above mean sea level. Some of the most dramatic of these formations can be seen in Thailand'sPhangnga 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 producetufa 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 nearOuhans 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 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 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 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 agesnail 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 inVictoria, Australia and paleocollapse features.
Notable karst areas
Main article: List of 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.