KARST: WEATHERING PROCESSES AND LANDSCAPES - ENVIRONMENT - 2025

The karst, karst or karst relief, is a form of topography whose origin is due to weathering processes by dissolving the soluble rocks limestones, dolomites 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.

Figure 1. Mountains of Anaga, Tenerife, Canary Islands, Spain. Source: Jan Kraus via flickr.com/photos/johny

Limestone rocks are sedimentary rocks mainly composed of:

• Calcite (calcium carbonate, CaCO ₃).
• Magnesite (magnesium carbonate, MgCO ₃).
• Minerals in small quantities that modify the color and degree of compaction of the rock, such as clays (aggregates of hydrated aluminum silicates), hematite (ferric oxide mineral Fe ₂ O ₃), quartz (silicon oxide mineral SiO ₂) and siderite (iron carbonate mineral FeCO ₃).

Dolomite is a sedimentary rock made up of the mineral dolomite, which is double carbonate of calcium and magnesium CaMg (CO ₃) ₂.

Gypsum is a rock composed of hydrated calcium sulfate (CaSO ₄.2H ₂ O), which may contain small amounts of carbonates, clay, oxides, chlorides, silica and anhydrite (CaSO ₄).

Karst weathering processes

The chemical processes of karst formation basically include the following reactions:

• Dissolving carbon dioxide (CO ₂) in water:

CO ₂ + H ₂ O → H ₂ CO ₃

• The dissociation of carbonic acid (H ₂ CO ₃) in water:

H ₂ CO ₃ + H ₂ O → HCO ₃ ^- + H ₃ O ⁺

• The dissolution of calcium carbonate (CaCO ₃) by acid attack:

CaCO ₃ + H ₃ O ⁺ → Ca ²⁺ + HCO ₃ ^- + H ₂ O

• With a resulting total reaction:

CO ₂ + H ₂ O + CaCO ₃ → 2HCO ₃ ^- + Ca ²⁺

• The action of slightly acidic carbonated waters, producing the dissociation of the dolomite and subsequent contribution of carbonates:

CaMg (CO ₃) ₂ + 2H ₂ O + CO ₂ → CaCO ₃ + MgCO ₃ + 2H ₂ O + CO ₂

Factors necessary for the appearance of karst relief:

• The existence of a limestone rock matrix.
• The abundant presence of water.
• The appreciable CO ₂ concentration in the water; this concentration increases with high pressures and low temperatures.
• Biogenic sources of CO ₂. Presence of microorganisms, which produce CO ₂ through the respiration process.
• Enough time for the action of the water on the rock.

Mechanisms for dissolution of host rock:

• The action of aqueous solutions of sulfuric acid (H ₂ SO ₄).
• Volcanism, where lava flows form tubular caves or tunnels.
• Physical erosive action of sea water that produces marine or coastal caves, due to the impact of waves and undermining of cliffs.
• Coastal caves formed by the chemical action of seawater, with constant solubilization of host rocks.

Geomorphology of karst reliefs

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.

Figure 2. Karst relief in Covadonga, Asturias, Spain. Source: Mª Cristina Lima Bazán via

-Internal karst or endocarstic relief

The underground water currents that circulate within beds of carbonaceous rocks, are digging 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

Dry caves are formed when internal streams of water leave these channels that have carved through the rocks.

Galleries

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”. “Branched tunnels” and rises of water called “siphons” can also be formed.

Stalactites, stalagmites and columns

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 water droplets 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 meet in the same space, uniting, a "column" is formed within the caves.

Cannons

When the roof of the caves collapses and collapses, "canyons" are formed. Thus, very deep cuts and vertical walls appear where surface rivers can flow.

-External karst, exocarstic or epigenic relief

The dissolution of limestone by water can pierce the rock on 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".

Dolinas

The sinkhole is a depression with a circular or elliptical base , the size of which can reach several hundred meters.

Frequently, in the sinkholes water accumulates that by dissolving the carbonates digs a sink in the shape of a funnel.

Grapes

When several sinkholes grow and join in a great depression, a "grape" is formed.

Poljés

When a large depression with a flat bottom and dimensions in kilometers is formed, it is called a “poljé”.

A poljé is in theory a huge 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.

Figure 3. Cueva del Fantasma, Aprada-tepui, Venezuela. (Observe the people on the left side of the image for size reference). Source: MatWr, from Wikimedia Commons

Karst formations as life zones

In the karst formations there are intergranular spaces, pores, joints, fractures, fissures and ducts, whose surfaces can be colonized by microorganisms.

Photic zones in karst formations

In these surfaces of the karst reliefs, three photic zones are generated depending on the penetration and intensity of the light. These zones are:

• Entrance area: this area is exposed to solar irradiation with a daily day-night lighting cycle.
• Twilight zone: intermediate photic zone.
• Dark area: area where light does not penetrate.

Fauna and adaptations in the photic zone

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 wind turbulence 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 poor in nutrients (oligotrophic), since they lack photosynthetic primary producers.

Other limiting conditions in karst formations

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 flooding; 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.

Microorganisms of the endocarstic areas

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 (SO ₄ ^2-), methane cyclization (formation of cyclic hydrocarbon compounds from methane CH ₄), among others are mediated by microorganisms.

As examples of these microorganisms we can cite:

• Leptothrix sp., Which effects iron precipitation in the Borra caves (India).
• Bacillus pumilis isolated from Sahastradhara caves (India), mediating calcium carbonate precipitation and calcite crystal formation.
• The filamentous sulfur oxidizing bacteria Thiothrix sp., Found in Lower Kane Cave, Wyomming (USA).

Microorganisms of the exocarstic zones

Some exokarst formations contain deltaproteobacteria spp., Acidobacteria spp., Nitrospira spp. and proteobacteria spp.

Species of the genera: Epsilonproteobacteriae, Ganmaproteobacteriae, Betaproteobacteriae, Actinobacteriae, Acidimicrobium, Thermoplasmae, Bacillus, Clostridium, and Firmicutes, among others, can be found in hypogenic or endokarst formations.

Landscapes of karst formations in Spain

• Las Loras Park, designated a World Geopark by UNESCO, located in the northern part of Castilla y León.
• Papellona Cave, Barcelona.
• Ardales Cave, Malaga.
• Santimamiñe Cave, Empty Country.
• Cave of Covalanas, Cantabria.
• Caves of La Haza, Cantabria.
• Miera Valley, Cantabria.
• Sierra de Grazalema, Cádiz.
• Tito Bustillo Cave, Ribadesella, Asturias.
• Torcal de Antequera, Malaga.
• Cerro del Hierro, Seville.
• Massif de Cabra, Subbética Cordobesa.
• Sierra de Cazorla Natural Park, Jaén.
• Anaga Mountains, Tenerife.
• Massif of Larra, Navarra.
• Rudrón Valley, Burgos.
• Ordesa National Park, Huesca.
• Sierra de Tramontana, Mallorca.
• Monastery of Piedra, Zaragoza.
• Enchanted City, Cuenca.

Landscapes of karst formations in Latin America

• Lakes of Montebello, Chiapas, Mexico.
• El Zacatón, Mexico.
• Dolinas de Chiapas, Mexico.
• Cenotes of Quintana Roo, Mexico.
• Cacahuamilpa Grottoes, Mexico.
• Tempisque, Costa Rica.
• Roraima Sur Cave, Venezuela.
• Charles Brewer Cave, Chimantá, Venezuela.
• La Danta System, Colombia.
• Gruta da Caridade, Brazil.
• Cueva de los Tayos, Ecuador.
• Cura Knife System, Argentina.
• Madre de Dios Island, Chile.
• Formation of El Loa, Chile.
• Coastal area of ​​the Cordillera de Tarapacá, Chile.
• Cutervo Formation, Peru.
• Pucará Formation, Peru.
• Umajalanta Cave, Bolivia.
• Polanco Formation, Uruguay.
• Vallemí, Paraguay.

References

1. Barton, HA and Northup, DE (2007). Geomicrobiology in cave environments: past, current and future perspectives. Journal of Cave and Karst Studies. 67: 27-38.
2. Culver, DC and Pipan, T. (2009). The biology of caves and other subterranean habitats. Oxford, UK: Oxford University Press.
3. Engel, AS (2007). On the biodiversity of sulfidic karst habitats. Journal of Cave and Karst Studies. 69: 187-206.
4. Krajic, K. (2004). Cave biologists unearth buried treasure. Science. 293: 2,378-2,381.
5. Li, D., Liu, J., Chen, H., Zheng, L. and Wang, k. (2018). Soil microbial community responses to forage grass cultivation in degraded karst soils. Land Degradation and Development. 29: 4,262-4,270.
6. doi: 10.1002 / ldr.3188
7. Northup, DE and Lavoie, K. (2001). Geomicrobiology of caves: A review. Geomicrobiology Journal. 18: 199-222.