CAVE

also called cavern, natural opening in the Earth large enough for human exploration. Such a cavity is formed in many types of rock and by many processes. The largest and most common caves are those formed by chemical reaction between circulating groundwater and bedrock composed of limestone or dolomite. These caves, called solution caves, typically constitute a component of what is known as karst terrain. Named after the Karst region of the western Balkan Peninsula extending from Slovenia to Montenegro, karst terrain in general is characterized by a rough and jumbled landscape of bare bedrock ledges, deranged surface drainage, and sinkholes, as well as caves. It should be noted, however, that there is considerable variation among karst areas. Some may have dramatic surface landforms but few caves. By contrast, others may have extensive cave development with little surface expression; for example, the Guadalupe Mountains of New Mexico, the site of Carlsbad Caverns and various other caves, have very few surface karst features. Karst landscapes are formed by the removal of bedrock (composed in most cases of limestone, dolomite, gypsum, or salt, but in some cases of such normally insoluble rocks as quartzite and granite) in solution through underground routes rather than through surface weathering and surface streams. As a result, much karst drainage is internal. Rainfall flows into closed depressions and down their drains. Further dissolution in the subsurface forms continuous conduits that serve as integrated drains for the rapid movement of underground water. The outlets for the water-carrying conduits often are springs of majestic size. Caves are fragments of such conduit systems, and some of them provide access to active streams. These caves may be completely water-filled; others are dry passages left behind by streams that cut to lower levels. Surface streams flowing from areas underlain by insoluble rock often sink when they reach the border of a karst region. These sinking streams form tributaries of the underground drainage system. any naturally formed subterranean cavity, including lava tubes and marine grottos. In many cases, a cave consists of a number of underground chambers, constituting a series of caverns. An assemblage of such caverns interconnected by smaller passageways makes up a cave system. Caves can be classified into two broad types according to origin: primary and secondary. Primary caves develop during the time when the host rock is solidifying. Those of the secondary variety originate after the host rock has been deposited or consolidated. Most caves are of the latter type. In certain instances, primary cavities are enlarged or further developed over geologic time by the mechanisms associated with secondary cave formation. Primary caves. These are best exemplified by lava tubes, cylindrically shaped tunnels that extend along the path of a lava field. Most caves of this type consist of a single, unbranched tunnel, but a few divide at intervals to form interweaving side passages that either rejoin the main channel or end in rockfall. Lava tubes are thought to form when the surface of a lava flow cools and solidifies. As the still molten interior of the lava field drains off, it leaves behind the distinctive cylindrical cavity. The pressure required to sustain the ceiling of the cavity is provided by hot gases. In some cases, the pressure of these fluids produces high, domed ceilings, whereas in others excessive pressure causes the gases to break through the roof to form crater-like vents. Lava tubes have various other characteristic properties, as for example lava pendants resembling stalactites that result from the drippage of the cooling ceiling or from remelting by hot gases. Another significant type of primary cave is the coral cave. Such cavities are formed when colonies of coral in shallow waters expand and unite, forming lacy or bulbous walls around a submarine void. When the shoreline rises or the sea level falls, a coral cave is exposed to wave action and weathering, which enlarges the void or obliterates it entirely. Secondary caves. Cavities of this kind are produced by mechanical and chemical processes. Although both types of processes are generally involved, one or the other tends to predominate. One common form of secondary cave of mechanical origin is the marine grotto. Such a cave develops where a sea cliff is exposed to wave action. Hydrodynamic pressure changes within the waves pull apart sections of the cliff face weakened by fracturing. Rock fragments and sand particles carried by the waves grind away soft rock material. Similar kinds of caves are quarried along the banks of streams and rivers by the mechanical action of water laden with silt and gravel. The most common type of secondary cave and of caves in general are those created by the chemical dissolution of a soluble host rock that has been weakened by fracturing and mechanical erosion. Mammoth Cave, Kentucky, and Carlsbad Caverns, New Mexico, are typical examples of such solution caves. Enormous cavities of this kind result when groundwater containing certain natural acids (e.g., those formed by the dissociation of water and carbonic acid) circulates along joints and faults in a calcareous rock such as limestone or dolomite and dissolves and corrodes it. This process, which occurs below the water table, gradually produces the honeycomb of passages and galleries characteristic of solution caves. As the water table drops and the cavities are drained, various kinds of speleothems, including stalactites, stalagmites, draperies, flowstones, and helictites are formed by the deposition of the carbonate minerals that had been dissolved from the surrounding limestone or dolomite. Additional reading Two introductory texts are Alfred Bgli, Karst Hydrology and Physical Speleology (1980; originally published in German, 1978), emphasizing caves; and J.N. Jennings, Karst Geomorphology, 2nd rev. ed. (1985), covering both caves and surface landformsa recommended introduction to karst. Discussion of cave geology, hydrology, mineralogy, and biology can be found in T.D. Ford and C.H.D. Cullingford (eds.), The Science of Speleology (1976), with a good chapter on volcanic caves. Carol A. Hill and Paolo Forti, Cave Minerals of the World (1986), is a systematic description of minerals and speleothems. Paul Courbon and Claude Chabert, Atlas des grandes cavits mondiales (1986), provides a definitive world list of long and deep caves, with maps and descriptive texts. Country-by-country review chapters on karst geology and surface landforms are contained in M. Herak and V.T. Stringfield (eds.), Karst: Important Karst Regions of the Northern Hemisphere (1972), concentrating mostly on Europe, the U.S.S.R., and the United States. William B. White