Karst

• 1. Introduction

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  Karst holds up to half of the world's hydrocarbon reserves, tucked away in the porous stone of ancient dissolved landscapes buried beneath younger rock. This porous, cave-riddled topography forms when soluble rocks like limestone and dolomite dissolve, leaving poljes on the surface and networks of sinkholes and caves below. Sometimes so much water vanishes underground that a karst landscape ends up with almost no rivers or lakes at all. There is even some evidence that harder rocks, such as quartzite, can develop the same dissolved features under the right conditions. A single word from the Balkans now names all of this, spread across continents. Where did that word come from, and what does it actually mean to live on ground this porous?

• 2. Naming Karst

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  The English word karst was borrowed from German Karst in the late 19th century, though the term had entered German usage far earlier. It described geological, geomorphological and hydrological features found across the Dinaric Alps, stretching from northeastern Italy above the city of Trieste, through southwestern Slovenia and the Balkan peninsula along the eastern Adriatic coast, to Kosovo and North Macedonia, where the Šar Mountains begin. Early topographical research described this zone, at its northwesternmost edge, as a plateau between Italy and Slovenia. In the local South Slavic languages, every variation of the word traces back to a Romanized Illyrian base, later reshaped in Slavic from the reconstructed korsъ into forms like kras and krš. Kras was first recorded in the 18th century, and the adjective kraški in the 16th century. As a proper noun, the Slovene form Grast appeared even earlier, first attested in 1177. Some modern languages kept the older, non-metathesized form, including Carso, Karst and karsti. The word's roots ultimately lie somewhere in the Mediterranean world. One theory connects it to the Proto-Indo-European root karra-, meaning rock; another ties it to the oronym Kar(u)sádios oros, cited by Ptolemy, and possibly to the Latin Carusardius.

• 3. Pioneers Of Karst Science

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  Johann Weikhard von Valvasor, a pioneer of karst study in Slovenia and a fellow of the Royal Society in London, introduced the word karst to European scholars in 1689. He used it to describe underground river flows in his account of Lake Cerknica. Jovan Cvijić advanced the field so far that he became known as the father of karst geomorphology. His 1893 publication Das Karstphänomen documented the dissolved landforms of the Balkans in detail. In 1918, Cvijić proposed a cyclical model for how karst landscapes develop over time. Karst hydrology did not emerge as its own discipline until the late 1950s and early 1960s, in France. Before then, the work of speleologists, the explorers of caves, had been dismissed as more sport than science, leaving the underground watercourses of karst caves scientifically understudied.

• 4. How Water Carves Stone

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  Karst develops most strongly in dense carbonate rock, such as limestone, when that rock is thinly bedded and heavily fractured. Chalk rarely develops karst well, since chalk is highly porous rather than dense, so groundwater flow spreads out instead of concentrating along fractures. Karst forms most readily where the water table sits relatively low, as in uplands cut by entrenched valleys, and where rainfall is moderate to heavy. Those conditions push groundwater downward quickly, which speeds the dissolving of bedrock, while standing groundwater instead grows saturated with carbonate minerals and stops dissolving anything further. The acid responsible for these features starts forming as rain falls through Earth's atmosphere, picking up carbon dioxide along the way. Once that rain reaches the ground, passing through soil adds still more carbon dioxide, produced by soil respiration. Some of this dissolved gas reacts with water to form a weak carbonic acid solution, which then dissolves calcium carbonate. Oxidation, though rare, can also play a role: it shaped the ancient Lechuguilla Cave in the US state of New Mexico, and remains active today in the Frasassi Caves of Italy. Sulfide oxidation offers another path to erosion. As oxygen-rich surface water seeps into deep, oxygen-starved karst systems, it reacts with sulfides such as pyrite or hydrogen sulfide already present, forming sulfuric acid. That acid then reacts with calcium carbonate, accelerating erosion within the limestone and ultimately forming gypsum.

• 5. Shapes Above And Below

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  On exposed surfaces, karst can wear away into small features such as solution flutes, runnels, limestone pavement split into clints and grikes, and kamenitzas, collectively known as karren or lapiez. At a medium scale, it can produce sinkholes or cenotes, vertical shafts, foibe shaped like inverted funnels, disappearing streams and reappearing springs. On a larger scale still, karst can shape limestone pavements, poljes and entire karst valleys. Where more bedrock has dissolved away than remains, the landscape can rise into karst towers, sometimes called haystack or eggbox landscapes. Underground, the same process builds complex drainage systems, extensive caves and cavern systems. Along limestone shores, especially in the tropics, erosion produces a sharp makatea surface above the sea's normal reach, undercut mostly by biological activity near sea level; some of the most dramatic examples appear at Thailand's Phangnga Bay and Vietnam's Halong Bay. Where water discharges its dissolved carbon dioxide, dissolved calcium carbonate can precipitate back out. Rivers emerging from springs can build tufa terraces from layers of calcite laid down over long periods, while inside caves, deposits of calcium carbonate and other minerals form the features collectively known as speleothems.

• 6. Karst In Disguise

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  Interstratal karst forms beneath a cover of insoluble rock, typically where a layer of sandstone sits over dissolving limestone. In the United Kingdom, extensive fields of dolines have developed at Cefn yr Ystrad, Mynydd Llangatwg and Mynydd Llangynidr in South Wales, beneath a cover of Twrch Sandstone overlying concealed Carboniferous Limestone; the last of these sites has been declared a site of special scientific interest. Kegelkarst, by contrast, is a tropical terrain of numerous cone-shaped hills, built from cockpits, mogotes and poljes without much fluvial erosion, and found in Cuba, Jamaica, Indonesia, Malaysia, the Philippines, Puerto Rico, southern China, Myanmar, Thailand, Laos and Vietnam. Salt karst, or halite karst, develops where underground salt dissolves, and can trigger surface depressions and collapses that pose a geo-hazard. Karst terrain is often difficult for people to cross, leaving its forests relatively undisturbed; the resulting high-pH soil favors unusual orchids, palms, mangroves and other plants. Paleokarst, a fossil version of karst preserved within the rock record, appears in surfaces exposed within the Clydach Valley Subgroup of the Carboniferous Limestone in South Wales, formed as older limestones weathered at the surface during pauses in deposition, before renewed sedimentation buried the irregular karst surface again, a cycle that repeated several times as sea levels fluctuated. Pseudokarsts merely resemble karst in form, formed instead by lava caves, granite tors such as Labertouche Cave in Victoria, Australia, paleocollapse features, and mud caves.

• 7. Underground Rivers And Aquifers

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  Rivers in karst country can disappear underground and reappear repeatedly, sometimes emerging under an entirely different name, as with the Ljubljanica, known as the river of seven names. This kind of resurfacing has a name of its own: a karst fenster, or karst window, is where an underground stream briefly surfaces between layers of rock, cascades a short distance, and vanishes again, often into a sinkhole. The Popo Agie River in Fremont County, Wyoming, offers another example: at a site called the Sinks, in Sinks Canyon State Park, the river vanishes into a cave within the Madison Limestone formation before rising again in a placid pool further down the canyon. In Ireland, seasonal lakes called turloughs form each year as water wells up from the underground system. Karst aquifers typically develop within limestone, as surface water carrying natural carbonic acid seeps into small fissures and gradually widens them, letting more water in and driving further enlargement. Countless small openings can store large volumes of water, while larger openings form conduits that drain the aquifer out to springs. Mapping these aquifers demands fieldwork to locate sinkholes, swallets, sinking streams and springs, since conventional methods like aquifer tests and potentiometric mapping fall short and must be paired with dye tracing, spring-discharge measurement and water chemistry analysis. The U.S. Geological Survey has used dye tracing to show that groundwater models assuming uniform porosity simply do not hold up in karst. Straight stream segments and sinkholes tend to align along fracture traces, and wells drilled along those traces are more likely to strike good water. Voids within karst aquifers can also grow large enough to cause collapse or subsidence, sometimes releasing contaminants catastrophically. Groundwater in karst areas moves far faster than in ordinary porous rock. In the Barton Springs Edwards aquifer, for instance, dye traces measured flow rates of 0.5 to 7 miles a day, or 0.8 to 11.3 kilometers a day, a speed that leaves these aquifers especially vulnerable to contamination.

• 8. Living With Karst

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  Karst hydrology supplies about 25% of the world's demand for drinking water, yet its rapid, unfiltered flow makes it just as easy to pollute as any surface stream, since cavernous, highly permeable karst formations offer little chance to filter out contaminants. Well water can turn unsafe after running straight from a sinkhole in a cattle pasture, bypassing the filtering an ordinary porous aquifer provides; sinkholes have often doubled as farmstead or community trash dumps, and failing septic tanks can send raw sewage directly into underground channels. Farming in karst country brings its own troubles: soils can be fertile and rainfall adequate, yet water slips through crevices so quickly that surface soil is often left parched between rains. Sinkholes can open gradually as surface cavities widen, or a cavern roof can give way without warning; such collapses have swallowed homes, cattle, cars and farm machinery, including part of the National Corvette Museum's collection in Bowling Green, Kentucky, in 2014. These landscapes appear across the globe. Australia's Nullarbor Plain is the world's largest limestone karst. Slovenia carries the world's highest risk of sinkholes, while the western Highland Rim in the eastern United States ranks second. Canada's Wood Buffalo National Park, in the Northwest Territories, holds its own karst sinkholes, and Mexico's Yucatán Peninsula and Chiapas host major karst regions of their own. Ireland's Burren is a karst limestone landscape in the west of the country, while the South China Karst, spread across Guizhou, Guangxi and Yunnan provinces, holds UNESCO World Heritage status. The field devoted to studying all of this, known as karst studies, karst science or karstology, spans biology, chemistry, ecology, geomorphology, hydrogeology, hydrology, politics and socio-economics, and counts William Morris Davis among its most prominent researchers.