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, dolines (subsurface collapse of limestone causes shakeholes or when larger are called dolines), and caves. It has also been documented for weathering-resistant rocks, such as quartzite, given the right conditions. Subterranean drainage may limit surface water with few to no rivers or lakes.
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 (like limestone or dolostone) continues to break down, its cracks get bigger, 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 arrangements as more water will be able to flow through the region.
Dissolution mechanism
The carbonic acid that causes these 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, soil provides much more CO2 to form a weak carbonic acid solution, which dissolves calcium carbonate. The primary reaction sequence in limestone dissolution is the following:
In particular and very rare conditions such as the Lechuguilla Cave in New Mexico and the Frasassi Caves in Italy, the oxidation of sulfides leads to the formation of sulfuric acid that becomes the corrosion factor in karst formation.
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 karst valleys. Mature karst landscapes, where more bedrock has been removed than remains, may result in karst towers, or haystack/egg box 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 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 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 a half-mile down the canyon in a placid pool. A turlough is a unique type of seasonal lake found in Irish karst areas that 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 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. On February 13, 2014 a karst sinkhole collapsed and swallowed 8 rare Corvettes at the Corvette Museum in Bowling Green, Kentucky.
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.
Pseudokarst is 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. Arroyo Tapiado in Anza-Borrego Desert State Park has over two dozen mud caves east of San Diego, California.
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 (the Ozark Plateau Ozarks of southern Missouri and northern Arkansas). The Chocolate Hills are conical karst hills on the island of Bohol, Philippines and also a notable karst topography.
The Arbuckle Mountains of south central Oklahoma have some of the highest densities of karst features found in the United States. The intensely folded and faulted carbonate uplifted beds produce a sequence of limestone ridges and shale valleys. Waterfalls develop where creeks fall over a limestone ridge into a shale valley. Because the waters are rich in calcium carbonate dissolved from the karst system, large deposits of travertine have accumulated on the waterfalls where turbulence causes mineral precipitation. The most notable of these waterfalls is Turner Falls near the city of Davis.
List of terms for karst-related features.
• Abîme, a vertical shaft in karst that may be very deep and usually opens into a network of subterranean passages
• Cenote, a deep sinkhole, characteristic of Mexico, resulting from collapse of limestone bedrock that exposes groundwater underneath
• Foibe, an inverted funnel-shaped sinkhole
• Scowle
• Sinkhole
• Turlough (lake) (turlach), a type of disappearing lake characteristic of Ireland karst
• Uvala (landform), a collection of multiple smaller individual sinkholes that coalesce into a compound sinkhole
• Karren, bands of bare limestone forming a surface
• Limestone pavement, a landform consisting of a flat, incised surface of exposed limestone that resembles an artificial pavement
• Polje (karst polje, karst field), a large flat specifically karstic plain or field
• Karst fenster (karst window), a feature where a spring emerges with the water discharge abruptly disappearing into a sinkhole
Speleothems
Speleothems are formations in caves, common in karst landscapes. Speleothems made of pure calcium carbonate are a translucent white color, but often speleothems are colored by minerals such as iron, copper or manganese, or may be brown because of mud and silt particulate inclusions.
Dripstone is calcium carbonate in the form of stalactites or stalagmites
Stalactites are pointed pendants hanging from the cave ceiling, from which they grow. Soda straws are very thin but long stalactites having an elongated cylindrical shape rather than the usual more conical shape of stalactites. Helictites are stalactites that have a central canal with twig-like or spiral projections that appear to defy gravity and include forms known as ribbon helictites, saws, rods, butterflies, hands”, curly-fries, and “clumps of worms”. Chandeliers are complex clusters of ceiling decorations
Stalagmites are the “ground-up” counterparts of stalactites, often blunt mounds. Broomstick stalagmites are very tall and spindly. Totem pole stalagmites are also tall and shaped like their namesakes. Fried egg stalagmites are small, typically wider than they are tall
Columns result when stalactites and stalagmites meet or when stalactites reach the floor of the cave
Flowstone is sheet like and found on cave floors and walls. Draperies or curtains are thin, wavy sheets of calcite hanging downward. Bacon is a drapery with variously colored bands within the sheet. Rimstone dams, or gours, occur at stream ripples and form barriers that may contain water. Stone waterfall formations simulate frozen cascades
Cave crystals. Dogtooth spar are large calcite crystals often found near seasonal pools. Frostwork is needle-like growths of calcite or aragonite. Moonmilk is white and cheese-like. Anthodites are flower-like clusters of aragonite crystals.
Speleogens (technically distinct from speleothems) are formations within caves that are created by the removal of bedrock, rather than as secondary deposits. These include pillars, scallops, boneyard and boxwork.
Others. Cave popcorn, or cave coral, are small, knobby clusters of calcite. Cave pearls are the result of water dripping from high above, causing small “seed” crystals to turn over so often that they form into near-perfect spheres of calcium carbonate. Snottites are colonies of predominantly sulfur oxidizing bacteria and have the consistency of “snot”, or mucus. Calcite rafts are thin accumulations of calcite that appear on the surface of cave pools.