The karst, Karst or karst relief, is a form of topography whose origin is due to weathering processes due to dissolution of soluble rocks such as limestone, dolomite and gypsum. These reliefs are characterized by presenting an underground drainage system with caves and drains.
The word karst comes from the German Karst, A word used to refer to the Italian-Slovenian area Carso, where karst landforms abound. The Royal Spanish Academy approved the use of both words "karst" and "karst", with equivalent meaning.
Limestone rocks are sedimentary rocks mainly composed of:
Dolomite is a sedimentary rock made up of the mineral dolomite, which is double carbonate of calcium and magnesium CaMg (CO3)two.
Gypsum is a rock composed of hydrated calcium sulfate (CaSO4.2HtwoO), which may contain small amounts of carbonates, clay, oxides, chlorides, silica and anhydrite (CaSO4).
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The chemical processes of karst formation basically include the following reactions:
COtwo + HtwoO → HtwoCO3
HtwoCO3 + HtwoO → HCO3- + H3OR+
Thief3 + H3OR+ → Catwo+ + HCO3- + HtwoOR
COtwo + HtwoO + CaCO3 → 2HCO3- + ACtwo+
CaMg (CO3)two + 2HtwoO + COtwo → CaCO3 + MgCO3 + 2HtwoO + COtwo
Karst relief can form within or outside of a host rock. In the first case it is called internal karst, endocarstic or hypogenic relief, and in the second case external karst, exocarstic or epigenic relief.
The underground water currents that circulate within the beds of carbonate rocks, dig internal courses within the large rocks, through the dissolution processes that we have mentioned..
Depending on the characteristics of the scour, different forms of internal karst relief originate.
Dry caves are formed when internal water currents leave these channels that have dug through the rocks..
The simplest way of being dug by water inside a cave is the gallery. The galleries can be widened to form "vaults" or they can be narrowed and form "corridors" and "tunnels". They can also form "branched tunnels" and rises of water called "siphons".
During the period when the water has just left its course inside a rock, the remaining galleries are left with a high degree of humidity, exuding drops of water with dissolved calcium carbonate..
When the water evaporates, the carbonate precipitates into a solid state and formations appear that grow from the ground called "stalagmites", and other formations grow hanging from the ceiling of the cave, called "stalactites".
When a stalactite and a stalagmite coincide in the same space, joining, a "column" is formed within the caves..
When the roof of the caves collapses and collapses, "canyons" are formed. Thus, very deep cuts and vertical walls appear where surface rivers can circulate..
The dissolution of limestone by water can pierce the rock at its surface and form voids or cavities of different sizes. These cavities can be a few millimeters in diameter, large cavities several meters in diameter or tubular channels called "lapiaces".
As a lapiaz develops sufficiently and generates a depression, other karst landforms appear called "sinkholes", "uvalas" and "poljes".
The sinkhole is a depression with a circular or elliptical base, whose size can reach several hundred meters.
Frequently, water accumulates in the sinkholes which, by dissolving the carbonates, digs a funnel-shaped sink.
When several sinkholes grow and join in a great depression, a "grape" is formed.
When a large depression with a flat bottom and dimensions in kilometers is formed, it is called "poljé".
A poljé is in theory an immense grape, and within the poljé there are the smallest karst forms: uvalas and sinkholes..
In the poljés a network of water channels is formed with a sink that empties into groundwater.
In karst formations there are intergranular spaces, pores, joints, fractures, fissures and ducts, whose surfaces can be colonized by microorganisms.
In these surfaces of the karst reliefs, three photic zones are generated depending on the penetration and intensity of the light. These zones are:
The different forms of life and their adaptation mechanisms are directly correlated with the conditions of these photic zones.
The entry and twilight zones have tolerable conditions for a variety of organisms, from insects to vertebrates..
The dark zone presents more stable conditions than the superficial zones. For example, it is not affected by the turbulence of the winds and maintains a practically constant temperature throughout the year, but these conditions are more extreme due to the absence of light and the impossibility of photosynthesis..
For these reasons, deep karst areas are considered nutrient poor (oligotrophic), as they lack photosynthetic primary producers..
In addition to the absence of light in endocarstic environments, in karst formations there are other limiting conditions for the development of life forms.
Some environments with hydrological connections to the surface can suffer floods; desert caves can experience long periods of drought and volcanic tubular systems can experience renewed volcanic activity.
In internal caverns or endogenic formations, a variety of life-threatening conditions can also occur, such as toxic concentrations of inorganic compounds; sulfur, heavy metals, extreme acidity or alkalinity, lethal gases or radioactivity.
Among the microorganisms that inhabit endocarstic formations we can mention bacteria, archaea, fungi and there are also viruses. These groups of microorganisms do not present the diversity that they show in surface habitats.
Many geological processes such as iron and sulfur oxidation, ammonification, nitrification, denitrification, anaerobic sulfur oxidation, reduction of sulfate (SO4two-), methane cyclization (formation of cyclic hydrocarbon compounds from methane CH4), among others, are mediated by microorganisms.
As examples of these microorganisms we can cite:
Some exokarst formations contain deltaproteobacteria spp., acidobacteria spp., Nitrospira spp. Y proteobacteria spp.
In the hypogenic or endokarstic formations species of the genera can be found: Epsilonproteobacteriae, Ganmaproteobacteriae, Betaproteobacteriae, Actinobacteriae, Acidimicrobium, Thermoplasmae, Bacillus, Clostridium Y Firmicutes, among others.
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