caves A cave is a natural cavity in bedrock which acts as a conduit for water flow between input points, such as sinking streams or soil percolation water, and output points, such as springs or seepages. This three-dimensional complex of surface closed depressions, subterranean conduits, caves, and springs is known as karst terrain. Once the developing cave conduit has a diameter larger than 5–15 mm, the basic form and hydraulics do not change much, although the diameter can be as much as 30 m in the case of caves in New Guinea or Sarawak. This minimum diameter threshold allows turbulent flow, optimizing both the solution of rock and the effective transport of sediment through the conduit. These conduits are principally formed by the dissolution of the rock in acidified water, most commonly circulating in limestone and other carbonate rocks, such as dolomite, but dissolution also occurs in evaporites such as gypsum and halite, and in silicates such as sandstone and quartzite. Caves can also develop by other processes: by weathering, the hydraulic action of waves, tectonic movements, and glacial melt water, and the evacuation of molten rock in lava flows.

The longest caves known are found in flat-lying limestones or gypsum and have been the scene of systematic exploration and mapping over many decades. Most of the world's known deep caves are in Europe, especially in the younger mountain ranges of western and central Europe. But cave exploration in the tropics is starting to yield very significant caves, especially in China, Mexico, New Guinea, and Sarawak. The five longest and deepest caves are listed in Table 1.

All the caves listed in Table 1 are active, that is they are still enlarging as a result of dissolution by acidified water. In limestone caves, the main source of this acidity is carbon dioxide dissolved to form weak carbonic acid in meteoric waters, although sulphuric acid may be important in thermal waters. The rate of dissolution depends on limestone purity, water velocity, the concentration of dissolved gas, and whether the chemical system is open (a water-air interface is present in the conduit) or closed (no water-air interface is present in a flooded conduit). The karst drainage system may be divided into a number of zones, each of which has distinctive hydraulic, chemical, and hydrological characteristics (Fig. 1). These zones are not mutually exclusive, and under flood conditions one zone may temporarily gain the properties of the one below it in the sequence. The uppermost zone, or epikarst, contains the surface and soil as well as the subcutaneous zone of weathered rock and enlarged fissures. In this zone percolating water picks up carbon dioxide from root respiration and soil bacteria and becomes acidified. Below this is the endokarst, the zone in which most active conduits are found. This is usually divided into the vadose or unsaturated zone (free air surfaces) and the phreatic or saturated zone (water-filled with no free air surfaces). The upper portion of the vadose zone, where discrete threads of water in the subcutaneous zone join to form percolation streams, is often termed the percolation zone. In recognition of the transient boundaries between some zones, the lowest portion of the vadose zone within the range of fluctuations in flood-water level is the epiphreatic zone (or temporary phreatic). Below the vadose zone is the permanently water-filled phreatic zone. Here tubular conduits are still actively forming and may rise or fall tens of metres along the line of least resistance—the hydraulic gradient—from the input points or stream sinks to the outputs at the springs. There are three types of water flow in karst drainage systems: conduit, fissure, and diffuse. The water flowing in a cave stream is typical of very rapid conduit flow. Water dripping from straws and stalactites in the roof is obeying less rapid fissure flow carried by the fractures, including joints, in the rock mass. Water seeping through the intergranular pores of the rock and emerging as wet patches on the roof is governed by slow diffuse flow. The magnitude and relative importance of each type in a karst hydrological system will depend on the porosity and fissure density in the host rock.

Table 1. Longest and deepest known caves

Cave	Country	Length/
depth (m)
Longest caves
1. Mammoth, Cave System	USA	563 270
2. Optimisticheskaya	Ukraine	208 000
3. Jewel Cave	USA	193 200
4. Holloch	Switzerland	175 150
5. Lechuguilla Cave	USA	161 900
Deepest caves
1. Lamprechtosofen-Vogelschacht	Austria	1632
2. Gouffre Mirolda/Lucien Bouclier	France	1610
3. Réseau Jean Bernard	France	1602
4. Torca del Cerro	Spain	1589
5. Pantyukhinskaya	Georgia	1508

Many caves have been abandoned by the waters that formed them and are now inactive, except for the slow processes of ceiling and wall collapse and infilling by cave deposits (known as speleothems), forming features such as stalactites and stalagmites. Many of these inactive caves are now perched hundreds of metres above valley floors, owing to incision (cutting down by streams). These drained caves offer Earth scientists a library of environmental history which parallels and complements the information to be gained from ice and deep-sea cores and lake deposits. In particular, we can examine the relationships between geology, landscape denudation, and cave development, and can date cave levels using radiometric techniques such as uranium series disequilibrium.

Mammoth Cave in Kentucky, USA, is currently the longest cave in the world, and displays the role of geology and landscape denudation in determining the shape and orientation of cave passages. Its development and hydrology have been studied by Arthur Palmer and Jim Quinlan over many years. The cave system is formed in limestones of Mississippian (Early Carboniferous) age which dip gently to the north-west. The limestone forms a broad plain of low relief, the Pennyroyal Plateau, punctuated by closed depressions, or dolines, and dissected by several sinking streams draining to the Green River. The border of the limestone plain is an upland composed of limestone ridges capped by sandstones and other insoluble rocks. The Mammoth Cave system lies under these ridges, occupying a sequence of limestones 110 m thick. The Ste Geneviève Limestone in the middle of the sequence contains most of the cave passages.

Most canyons and tubular conduits in Mammoth Cave are highly concordant with the bedding of the limestones. Thus the main entrance passage follows the same beds of the Ste Geneviève Limestone for several kilometres. Passages tend to run parallel to the strata because there are very few joints and they rarely intersect more than one bed. In the south-eastern area of Mammoth Cave abrupt changes in passage level are associated with faults and major joints. The three most common passage types in Mammoth Cave are canyons, low-gradient tubular passages, and vertical shafts. The tubular passages are generally elliptical, reflecting the control of bedding. These are of phreatic origin, trending along the strike of the beds, and their sinuosity is controlled by local variations in the orientation of the controlling bedding plane. Former siphons in the passages are often visible. The vadose canyons are much modified by collapse but can be seen to drain down-dip. The numerous vertical shafts in the cave are controlled by major joint intersections, with inflowing water perched on bedding planes or relatively resistant beds.

A phreatic tube named ‘Cleaveland Avenue’ in Mammoth Cave illustrates geological control very well. It is about 1500 m long with almost no slope (4 m km⁻¹), and the limestone beds exposed in its walls show little variation along its length. Broad bends in the passage are caused by two gentle folds that push the tube to the west on the nose of an anticline and to the east into the trough of a syncline. The concentration of large passages at certain levels is a combination of stratigraphic control and the progressive downcutting of the Green River. Thus the passages cluster at three distinct elevations: 180, 168, and 152 m above sea level. These passages were backflooded by the river at the time when their levels were accordant, leaving the underground equivalents of flood plains and terraces. These deposits, and the stalagmites that cap them, have have been dated by a combination of palaeomagnetism and uranium series dating methods, and indicate that the cave has formed over the whole of the Quaternary period.

If the dissolution process has operated efficiently through time, then extensive caves will be found in a limestone massif. If that process has been severely interrupted by glaciation, aridity, or sea-level change, then these effects will be reflected in cave morphology. Thus the extensive caves of the arid Nullarbor Plain, Australia, probably formed as large phreatic tubes under conditions of greater rainfall in the Tertiary. Deepening of the caves has been enhanced by lowering of sea level in the Pleistocene, and today there are extensive flooded tunnels, explored by divers for just over 6000 m. In regions where climatic change has been minimal, such as the humid tropics, variation in cave development may reflect regional uplift patterns. In the karst towers of Guilin, China there are caves at various levels—right up to the summits—which have been abandoned by their streams as the valleys have incised into rapidly rising plateaux. Finally, the fluctuations in sea level during the Quaternary have also produced cave development below the present mean sea level, such as the Blue Holes of the Bahamas and the Great Barrier Reef.

Once the products of surface and underground processes enter the cave system, they are likely to be preserved with minimal alteration for tens of millennia, perhaps even millions of years. In the near-constant temperature and humidity of the cave, weathering processes are reduced in intensity compared with the surface environment. Caves can thus be regarded as natural museums in which evidence of past climate, geomorphic processes, vegetation, animals, and people can be found.

Caves overrun by glaciers can accumulate valuable records of the ice ages. Paul Williams of the University of Auckland, New Zealand has studied the record of glacial advances preserved within Aurora Cave, Fiordland. The overflow from the remote Lake Orbell sinks into this steep active cave system to emerge at a spring on the shore of Lake Te Anau, lying in a deep glacial trough. The cave began to form at least 230 000 years ago, and in it sequences of glaciofluvial gravels are interbedded with dateable flowstones and stalagmites. In the past 230 000 years seven glacial advances have filled the valleys with hundreds of metres of ice and brought gravels into the cave; the last glaciation is dated at about 19 000 years ago. This provides direct evidence lacking from surface deposits, which have been eroded by successive glaciations. Older deposits near by hold the promise of extending the glacial record of New Zealand well back into the Middle or Early Pleistocene.

David Gillieson

Bibliography

Ford, D. C. and and Williams, P. W. (1989) Karst geomorphology and hydrology. Unwin Hyman, London.
Gillieson, D. (1996) Caves: processes, development and management. Blackwell Scientific Publications, Oxford.
Jennings, J. N. (1985) Karst geomorphology. Blackwell Scientific Publications, Oxford.
White, W. B. (1988) Geomorphology and hydrology of karst terrains. Oxford University Press, New York.

71. Caves

caving

the term for speleology used by professionals.

speleology, spelaeology

the branch of geology that explores, studies, and describes caves. —speleologist, spelaeologist, n. —speleological, spelaeological, adj.

spelunker

a person who explores caves as a hobby. —spelunk, v.