Karst (in German) topography is the name for the many landforms that developed on the surface and within soluble rocks as caves and other forms characteristic of a part of the Kras region of Slovenia, where the landforms developed on and in some 400 square kilometers of limestone (calcium carbonate) and were first studied many years ago. All rocks can be slightly soluble in water, with the result that various karstic landforms can also develop more slowly in the somewhat less soluble magnesium carbonate of dolomite or dolostone, as well as quite quickly in evaporate rocks such as rock salt or gypsum. In fact, some solubility can even occur slowly in silicate rocks such as sandstone, especially where it is cemented with carbonate minerals, or quartzite, basalt, and granite under favorable conditions and over long time periods. The processes of karstification of a region involve dissolution of the bedrock as various minerals are dissolved in the water. Limestone bedrock, the most common host rock of karst topography, is composed of the mineral calcite, which is subjected to the carbonation reaction in the presence of water with a small amount of dissolved carbon dioxide that makes a weak carbonic acid. As this slightly acidic water comes in contact with the limestone bedrock, it has the capability of dissolving and carrying away molecules in solution to make the karst topography. For karst topography to develop best, however, the carbonate must be thick, mechanically strong, and well jointed to allow concentrated percolation locations for the infiltrating water to carry away the solute loads. Chalk rock, for example, is quite a pure form of limestone, but it does not form caves or karst well because it is neither mechanically strong nor well jointed; instead it is soft and crumbly, therefore cave systems cannot become established.
Karst topography is terrain formed in soluble rocks that bears distinctive characteristic of drainage and relief unlike most other landscapes. Instead, it tends to have a rough surface with many enclosed depressions; the soil above the bedrock tends to be thin and patchy, with exposed bare rock that is etched into a great variety of irregular shapes, and much of its drainage is subterranean where it flows through caves and grottos. True karst occurs where solutional processes dominate; in some parts of the world where karst processes create almost all the landforms of the region, it may be appropriate to refer to it as “holokarst.” On the other hand, fluviokarst occurs where the processes of rivers and solution are approximately equal in their activities. Large parts of the Midwestern United States, for example, are dissected by rivers running over and through carbonate rocks so that the intervening interfluves can have karst topography developed on and in them. Glaciokarst is formed in some areas of higher altitude and latitude where soluble carbonates occur beneath partial covers of ice so that as the glaciers wax and wane, their meltwaters can assist in the excavation of the subterranean topography. Psuedokarst landforms are developed by the very wide variety of nonsolutional processes that mimic some of those of true karst. Thus, granular disintegration of silicate rocks, hydraulic plucking and wave erosion of sea caves, lava flows that have internal lava tubes or caves where the molten rock flowed out, and mass movements that open out chambers underground and produce hummocky topography on the surface can all produce psuedokarstic landforms. Thermokarst is rather a misnomer in that it is not a karst at all, has no solutional component whatsoever, and instead is entirely due to the melting of permafrost and ground ice, although its surficial appearance of enclosed depressions may superficially resemble a true karst.
Bare karst occurs where the soluble bedrock is exposed directly to the atmosphere, and covered karst has the bedrock beneath mantles of regolith sediments and soil. Free karst has drainages that go unimpeded directly to the ocean, whereas impounded karst is surrounded by other less permeable lithologies so that the drainages have to exit in the karst area with a different sort of hydrological system.
From the standpoint of hydrology and depth in the Earth, karst is divided into the surface and near-surface zones of the epikarst and the deeper subsurface zones of the endokarst. Epikarst is the karstic region in the vadose zone above the water table where there are air-filled openings as well as some water in the openings. Epikarst occurs from the surface and soil of the cutaneous zone, as well as the regolith and enlarged fissures of the subcutaneous zone. In these zones, close to the surface, precipitation is the chief input and evapotranspiration an output. At the base of the epikarst, there is a continuum of percolation involving seepage, trickles, and flows, up to full cave streams. Endokarst, below the epikarst, has the vadose zone of unsaturated percolation and water flow at its very top down into the phreatic zone of saturated water flow below the water table wherein water-filled fissures and pores occur, as well as fully water-filled caves. Outflow springs from the endokarst and epikarst areas occur at many different levels on the surface around higher topography with caves inside it.
Linear caverns in karst areas are vadose caves, which are above the water table; shallow phreatic, epiphreatic, or water table caves, which occur right at the water table; or phreatic caves, which are entirely below the water table. These basic cave types also have variants in which irregular cave passage loops occur, either entirely up and down beneath the water table as deep phreatic with loops or phreatic with loops that project a little way above the water table so that they become air filled for short distances. In addition, there are mixed loop and epiphreatic caves that have substantial parts both above and below the water table.
Because karst landforms are unusual in appearance and are distributed all over the world, so that a huge and confusing lexicon of descriptive and attempted genetic terms has arisen in many languages to describe them. Multilingual synonyms abound. The following is a brief exposition of some of the better-understood forms, although it should be realized that fine divisions between some landforms may reflect continua of form and process such that absolute differentiation between types may be somewhat ambiguous at times.
Karst landforms are many and highly varied. Perhaps the best way to consider these landforms is by their size scale, as well as whether or not they are on the surface or underground as subterranean landforms. In addition, karst landforms can be classified based on whether they are positive landforms that resulted from rock having been removed from around them or from material that was deposited to make them or negative landforms wherein material has been removed to leave an opening of some kind.
On the surface where the larger landforms occur, which are most commonly observed as being of karst origin, perhaps the most common are the sinkholes, or dolines. Five main kinds of dolines occur, the solution, the suffusion, the collapse, the subsidence and the stream-sink types. Solution dolines occur where the dissolving of the bedrock is concentrated at some intersection of joints or some other favorable circumstance where solution was concentrated. Insoluble residues can eventually clog the drainage so that a pond doline results. Suffosion dolines are those formed by the processes of a through wash where sediments are carried away both in solution as well as by suspension so that a depression results that is generally fairly well choked with debris. Collapse dolines, such as the famous vertically sided cenotes in the Yucutan in Mexico, are those where the roof of a cave collapses. Over time, with no further collapse, the vertical walls will degrade and become less steep so that bowl- or cone-shaped dolines can result. Subsidence dolines result where a quite soluble rock such as rock salt dissolves and allows the settling or subsidence of overlying sediment or rock without any obvious fracturing or breaking of it. Stream-sink dolines or ponors form where streams flow down into swallow holes in carbonate rocks and then pass underground.
Uvalas are compound sinkholes that have developed a larger depression complex. As the size increases, a polje (plural polja)—a large, flat-floored, and closed depression—is formed. Both uvalas and polja commonly have permanent or ephemeral streams in them that disappear into dolines. Polja can be subdivided into a number of different types depending on their relationships to the surrounding geology and hydrological systems. For example, border polja have rivers coming into them from outside nonkarst areas that are controlled by the external water tables so that lateral planation and alluvial deposition occur in the polje. Structural polja have downfaulted zones of permeable soluble rocks juxtaposed against insoluble rocks. Baselevel polja have regional water tables that intersect the ground surface inside them so that they flood periodically.
An eggbox topography forms where the limestone is thick and the water table deep so that deep solutional sinkholes form close together. A cone karst develops where the sinkholes coalesce together, and the region is dominated by projecting residual relief rather than by closed depressions. The result is a polygonal pattern of ridges around separate dolines. Cockpit karst has lower ridges and residual pepino hills, whereas tower karst has huge residual hill towers or mogotes (haystack hills) that can be steep enough or with vertical and overhanging sides to form a pinnacle karst.
Fluvial aspects of karst topography beyond those mentioned above include blind and halfblind valleys, steepheads or pocket valleys, dry valleys, meander caves, natural bridges, and tufa and travertine deposits. Half-blind valleys are those where a stream develops a swallow hole and downcuts a small amount upstream from the sink to leave the downstream portion only occasionally still hosting flood waters. A blind valley, on the other hand, is one that is closed off at its lower end by a cliff or steep slope facing upstream, below which the stream disappears into the sinkhole of a cave opening. Dry valleys have no surface stream in them, although they once did until the original stream that formed them found and exploited some subterranean exit point to abandon them. Steepheads and pocket valleys are those where a river emerges from a cave at the head of a valley. Meander caves are those where the outer bend of a meander in a karst area undercuts a limestone valley side so that a large overhanging cliff and cave system develops beneath. Karst natural bridges form where cave roofs collapse upstream and downstream from the bridge, where subterranean stream piracy has diverted water beneath them, or where rivers dissolve their way through narrow bands of limestone that cross their paths. Tufa and travertine deposits can be produced by rivers in karst areas because water bodies there are commonly supersaturated with dissolved carbonate. Any disturbance to the water in rapids, waterfalls, or agitated flow through aqueous vegetation or the like on the surface may allow a small amount of carbon dioxide to escape from the water or a bit of water to evaporate, which then can cause some calcium carbonate to precipitate from the water. This carbonate buildup occurs in two main forms: (1) the porous and low-density tufa, which commonly has many root and stem casts inside it where the carbonate came out of the solution and precipitated around water plants, and (2) travertine, which is compact, of higher density, crystalline, and commonly banded and layered where variations in the water have changed the crystallization character or composition somewhat over time. Together, tufa and travertine can accumulate in masses large enough to constitute dams, mounds, sheets masses, and many other forms.
An additional discussion of karst includes a discussion moving down in scale to the smaller forms. A general term for many types of these smaller forms includes karren, which are a highly diverse group of small-scale features and sculpted forms of the bedrock that are exposed on the surface of the ground or that appear inside caves. Bare karst forms can be produced by wetting caused by rain or snowmelt hitting and flowing over the surface or dripping onto and seeping into it to carry away dissolved load. The small landforms that these processes produce are quite common in soluble carbonates but may also be produced on silicate rocks where certain mafic minerals (e.g., amphibole, pyroxene, and some micas) can weather more quickly to produce some solution and granular disintegration from differential decomposition that allows development of similar appearing landforms. The smaller karst landforms produced by solution can be subdivided into (a) bare karst forms produced by surface wetting, (b) bare karst forms produced by concentrated surface runoff, (c) partly covered karst forms, (d) covered karst forms, and (e) polygenetic forms or assemblages of karren. In many cases, it may be quite difficult to determine whether or not a form was produced while it was covered with moist regolith or saprolite above it and later exhumed or whether it was generated entirely while partially or fully exposed to the atmosphere, or some odd combinations of both these possibilities.
The bare karst forms produced by wetting are scale dependent and include the following, in more or less increasing size: (a) micropits and etched surfaces are produced by precipitation falling on gently sloping or flat bare rocks to form small pits and rills less than 1 millimeter deep and not larger than a few centimeters; (b) solution ripples and fluted scallops are shallow, ripple-like flutes that form on steep to vertical surfaces perpendicular to the direction of the water flow over them; (c) solution flutes or rillenkarren are longitudinal hollows that start at the top of a rock crest and run down the steep rock surface with either rounded (silicate) or sharp (carbonate) ribs between them; (d) solution bevels are smooth and flatish tiny risers and treads in stair-step forms; (e) solution runnels or rinnenkarren are the larger solution hollows that result from larger volumes of overland flow; (f) decantation runnels are related to solution runnels that result from the dripping of acidified water from point sources upslope that are either in direct contact with the surface or not, or still exist as a present-day source or not, so that the resulting runnels in the rock can meander and/or reduce in size downslope; and (g) decantation flutings are similar to decantation runnels but have water sources at their heads that are diffuse sheet flows from upslope.
On the other hand, the bare karst forms that are instead produced by concentrated surface runoff also include, in order of more or less increasing size, the following: (a) microfissures follow thin small joints up to several centimeters in length but less than 1 centimeter in depth; (b) splitkarren or solution fissures occur along joints, veins, or stylolite solution features formed between bedding plains that are several centimeters to several meters long and some centimeters deep; (c) solution slots or grikes and kluftkarren are fairly large solutional features developed along structural discontinuities such as large and connected joints or faults and can ultimately result in quite accessible and spacious bogas openings in the karst, as well as karst corridors and karst streets; (d) clints are the tabular blocks left between grikes (solution slots); and (e) solution spikes or spitzkarren are the steep projections between grikes.
The partly covered karst forms that occur include (a) solution pits, which are round-bottomed or tapered forms, and (b) solution pans, which are dish- or basinal-shaped depressions formed on flat rock surfaces resulting from bedding planes or joints, which may have sides that overhang or carry solution flutes above bottoms covered with organic remains, clay, silt, sand, and pebbles. These two forms of solution pits and pans together are the most common of all karren forms. In addition, other partly covered karst forms are (a) undercut solution runnels (hohlkarren), which are similar to runnels but become larger with depth as a result of the damper conditions at their bases due to accumulations of soil and/or humus, and (b) solution notches (korrosionkehlen), which are inwardly curved lateral depressions or recesses produced by chemical etching from damp soil directly abutting the bedrock.
The fully covered karst forms have acidic sediment and soil above them in which the moisture is retained, and this helps them develop as (a) rounded solution runnels or rundkarren, which are smoothed out by their contact with the cover materials; (b) cutters, which are soil-covered solution slots (grikes) that are wider at the top and taper at depth; and (c) solution pipes, shafts, or wells, which are conical or cylindrical holes that can develop along joints or in an isolated fashion in unjointed carbonates such as chalk.
Polygenetic karst forms are commonly assemblages of different karren types. They include the following: (a) karren fields, exposed tracts of karren that may be as large as several square kilometers; (b) limestone pavements, where numerous soil-covered or bare solution slots (grikes) separate exposed rock surfaces (clints) that can be covered with karren forms, which either developed in the subterranean environment or subaerially, or both but in sequence; (c) pinnacle karsts and stone forests, a spectacular karst landscape with thin and pointed rock remnants that can be 45 m (meters) tall and 20 m wide at the base, although some tropical forms are known to stand as high as 120 m, with near-vertical sides and sharp, sawtooth tops; (d) ruiniform karsts, a type of karst wherein the upstanding tabular clints in between the well-developed, wide-solution slots are so well developed that after extensive soil erosion only a few upstanding blocks protrude upward like ancient ruined buildings scattered out in a former cityscape; (e) corridor karsts, which develop where large solution slots grow bigger and form aligned or crisscrossing steep-sided valleys that may develop into labyrinth karsts or giant grikelands; and (f) coastal karrens, which develop in the common carbonates along shorelines so that solution notches or nips develop in the rock exposed above shoe platforms, as well as a dense and pervasive variety of pits, pans, and spikes that may have been partly aided by boring and grazing organisms that can assist in producing biokarsts and partly by wave action, wetting and drying, salt weathering, or some plant activity to produce phytokarst.
Carbonation, Caverns, Geomorphology, Groundwater
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