karst

karst The term ‘karst’ is used to describe an area of limestone or other highly soluble rock in which the landforms are of dominantly solutional origin and in which the drainage is underground in solutionally enlarged fissures and conduits (caves). The word karst is derived from the Kras, a region is Slovenia, some 400 km² in extent, which was one of the first areas of well-developed limestone landforms to be investigated by scientists. The term has affinities with the Greek word chalis, the Latin calx, and the Italian carso. Karst has now become the generic term for all landscapes dominated by the distinctive suite of surface and subterranean features described above, even though the landscape of the Slovenian type-area is not particularly representative of other karst areas elsewhere.

The degree to which soluble rocks have been karstified varies greatly from place to place, ranging from areas such as parts of southern China or Indonesia, where almost all the landforms are understandable in terms of karstic processes (holokarst), to areas of limestone in which other geomorphological processes have been at least as important as dissolution in creating landforms. For example, fluviokarsts are the products of both rivers and karstic forces. Figure 1 shows the global distribution of carbonate rocks, most of which exhibit karstification at least to some extent.

Karst areas are relatively uncommon in Africa and Australasia but widespread in Europe (more than 12 per cent of the land surface) and in south-east Asia.

Karst landforms are developed on highly soluble rocks such as halite and gypsum, but their most widespread occurrence is where carbonate rocks, and especially pure limestones (calcium carbonate), crop out at the surface.

The geomorphological evolution of limestone terrains is uncomplicated in comparison with other rock types. A single process, chemical solution of the limestone by acidified rainwater, operates on a rock-type composed overwhelmingly of a single mineral (calcite). Thus virtually all of the rock is dissolved and then transported away as invisible solute load in a single operation. Little all no insoluble material accumulates to blanket the rock surface and to require mass-movement processes to move it downslope, as is the case with most other rock-types.

Humans have settled in karst regions since prehistoric times, apparently often in preference to other apparently more attractive areas. The barren terrain, patchy soils and vegetation, and the difficulties encountered in obtaining reliable supplies of surface water have not prevented the creation of high civilizations, such as those of meso-America and south China, in karstic terrains. Nowadays some 25 per cent of the world's population obtains its water supply from karst supplies, despite the high risk of contamination to which karst water is liable. Karst regions include some of the world's most spectacular scenic attractions, such as the Stone Forest of China, the travertine terraces of Pammukale in Turkey, and the great caves of Carlsbad and Mammoth in the USA. Increasingly these are becoming major, global tourist attractions and their integrity is accordingly under threat.

Most erosion of limestone is the result of the action of the dissolving action of dilute carbonic acid in rainwater on the limestone. Rainwater is slightly acidified by equilibrating with atmospheric carbon dioxide, but a much more significant source of carbon dioxide, and hence carbonic acid, is from soil-air dissolved by rainwater as it percolates through to the underlying limestone bedrock. Soil-air concentrations of carbon dioxide are commonly several orders of magnitude higher than atmospheric levels, and so therefore are carbonic acid concentrations in soil water.

In turn, the more acidic waters are able to dissolve larger amounts of limestone, and hence the most rapid and best development of karst landscapes is in areas which have abundant rain and thick soils rich in carbon dioxide: the humid tropics, for example. The character of the limestone is also important in the development of karst. Massive, very pure limestones karstify best and an essential prerequisite is that the main body of rock is impermeable. Thus, water can filter underground only through secondary openings in the rock such as joints and bedding planes. Such openings are spaced some distance apart in most limestones and these are the features that may become selectively enlarged into characteristic karstic landforms such as dolines, caves, swallow-holes, and gorges.

Water, ultimately derived from rainfall that is not evaporated (runoff), is essential to the formation of karsts, and almost all the major and minor landforms (Table 1) are readily understandable in terms of erosion or deposition of limestone by running or ponded water.

When an area of limestone is first exposed to external weathering agencies, perhaps after the removal of overlying non-carbonate rocks, the secondary openings in the rock will commonly be hairline fissures, able to engulf only seepages of rainwater. Initially a surface drainage system of streams is therefore likely to develop, with valleys being excavated as on non-limestones rocks, although in limestone regions the valleys are often gorge-like with steep sides. With time, progressively more of the runoff is able to sink underground as the openings are gradually enlarged by solution. Surface rivers will carry smaller and smaller flows and eventually may be active only during especially wet weather. Finally, the valleys become wholly dry and completely lose their original hydrological function. If the stream that formed the valley originated on adjacent impermeable rocks, a perennial stream sink (swallow-hole) may develop at the contact between the two rock-types. The surface stream will continue to deepen its valley upstream of the sink, and in time a reverse gradient or even a cliff will develop immediately down-valley of the point of engulfment forming a blind valley (Fig. 2b).

Table 1 A classification of karst landforms by size (micro to regional) and by location (surface or subterranean)

Scale of	Positive-surface	Negative-surface	Positive	Negative
landform	landforms	landforms	underground	underground
Small scale	Spitzkarren	Karren pits,	Calcite	Micro-
(millimetres to	(ridges and	runnels and	deposits	fissures
metres)	pinnacles)	cockles	(speleothems)
Medium scale	Cone and tower	Stream sinks	Calcite	Caves
(metres to	karst, tufa dams	enclosed hollows	deposits
hundreds of		(dolines, sinkholes	(speleothems)
metres)	uvalas)
Large scale	Karst plateaux,	Dry valleys,	None	Major caves
(100 m to	plains, and cliffs	gorges, poljes	and cave	and cave
kilometres)				chambers

In the floor of the dry valley and at other locations between valleys, points at which rainwater is funnelled into the ground from the immediate surroundings will experience greater solutional erosion and bedrock hollows will develop. Some of these may enlarge greatly by capturing the water feeding adjacent hollows and become fully formed enclosed depressions or basins termed dolines or sinkholes (USA), ranging in diameter from a few metres to several kilometres and with depths of up to several hundreds of metres. Figure 2 illustrates the decay of a river valley into enclosed depressions.

The extent to which a river (fluvial) system develops varies greatly between karst regions, but ultimately the process of karstification will destroy the surface drainage system and replace it with a landscape of enclosed basins of all sizes which serve to conduit runoff underground as efficiently and expeditiously as possible. These dolines are the equivalent of small valleys in a fluvial system, the waters from them uniting underground into the major stream draining the area. If river systems do survive it is usually because they have cut deep gorges and thus occupy the lowest parts of the karst region. Such rivers may be fed by the water of karst springs. The Causse plateaux of south-central France are an example of wholly karstic uplands separated by rivers such as the Tarn, flowing in deep gorges and draining the upland karsts.

The surface of a mature karst area seems chaotic, with hollows and hillocks of every size, little or no level ground, barren and often steep slopes. The ‘sense’ of the landscape is not obvious from above because the controls are underground. The rivers that erode and transport away the weathered limestone are in cave systems emerging from the large springs that characterize the margins of karst regions.

Caves are the equivalent of surface streams, but instead of cutting valleys cave streams dissolve an increasingly large tunnel through the rock, the stream eventually occupying only the lowest part of the passage. Cave streams may deposit insoluble sediments brought from the surface, but more typically underground deposits are of calcium carbonate reprecipitated from solution as stalactites, stalagmites, and other speleothem forms.

Karst landscapes are developed wherever soluble carbonate rocks outcrop and where surplus rainfall is available to dissolve the limestone. Karst landscapes are comparatively poorly developed in dry and cold areas which lack both water and carbon dioxide in soils. The best development is found in the wet, warm limestones of the tropics. Areas such as Indonesia and adjacent areas of south-east Asia, many of the Caribbean islands, much of central America, and southern China exhibit some of the world's finest karst landscapes, the product of extended karstic erosion under optimal conditions.

The landforms of karst range from tiny solutional etching and pits on exposed rock surfaces (karren) to flat-floored enclosed basins several hundreds of square kilometres in area enclosed by steep slopes some hundreds of metres high (poljes). However, a predominantly solutional origin for landforms on whatever scale is the essence of karsts.

D. Drew