Cave

I

Introduction

Cave, natural chamber or hollow beneath the surface of the earth or in the side of a hill, cliff, or mountain. Caves vary in size and shape. In some places, underground chambers and galleries may be linked to form cave systems that continue for hundreds of miles. Many caves have large openings to the surface or may have multiple outlets. Other caves may be totally shut off from the surface or may exist partly or entirely under water. Unusual mineral formations and life forms occur in caves, making these underground environments of special interest to both geologists and biologists.

II

Formation of Caves

Naturally formed caves evolve in various ways, in many cases as a result of the solvent action of water and compounds in it. Known technically as caves of solution, such chambers are most common in limestone formations, particularly in regions that have ample rainfall. The surface water in such regions contains carbon dioxide and acids derived from the organic constituents of the soil. Attacking the soluble limestone, this acidic water dissolves and carries the limestone away in solution. Over long periods of time, such action results in the formation of subterranean chambers. The depth of such chambers depends on the depth of the water table (see Water).

If after several unusually wet years the water table is rising, old cave chambers become flooded and new ones begin forming at higher levels. Likewise, during a long dry spell, chambers will begin forming at lower levels, closer to the declining water table. Over thousands of years, fluctuations such as these produce multi-level cave systems, as in Mammoth Cave National Park in Kentucky, where a subterranean stream flows through the lowest level. Underground rivers erode and transport sediments and rock fragments in a manner analogous to that of surface streams. If such action has been predominant, the cave is said to have been formed by mechanical abrasion.

Another way that limestone caves are thought to form is from the action of sulfuric acid produced by bacteria. Recent research indicates that some large caves such as Carlsbad Caverns formed in this way instead of by the slow dissolving effects of carbonic acid from carbon dioxide and water. Oil deposits deep beneath limestone formations released hydrogen sulfide gas. Underground microbes that derive energy from hydrogen sulfide helped create sulfuric acid in combination with oxygen and water. The strong acid solution dissolved the limestone, hollowing out the chambers and leaving gypsum formations, which are not found in caves formed mainly by water. Other caves where this sulfuric acid process is still taking place often have coatings of bacterial slime on the walls and ceilings.

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River action forms still another type of cave, commonly with a very large opening that gives it the appearance of a natural amphitheater. A river entrenched in a steep-walled canyon most actively erodes that portion of the canyon wall against which the current is strongest, as at a bend or in a meander. By erosion, solution, and quarrying, the river excavates a large quantity of rock, forming a large undercut area in the side of the canyon. With the passage of time the riverbed is lowered, and eventually the cave is left high in the side of the canyon. Such rock shelters were used extensively in what is now the southwestern United States by the prehistoric Native Americans known as Cliff Dwellers, who built their homes within them (see Cliff Dweller).

Wind action (eolian action) is partly responsible for the formation of small caves that are confined mostly to desert or semidesert regions. The action of windblown sand is one of several forces involved in the formation of these grottoes and caves in rock ledges and cliffs.

Lava caves can form during volcanic eruptions when molten rock runs beneath the surface of a lava flow that has hardened. When the liquid lava drains away, it leaves a hollow tube in the solid rock. Chambers or pockets of gas in lava can also form caves. Ice melting underneath lava can also hollow out caverns.

Ice caves sometimes occur in glaciers or in icebergs when meltwater drains through or under the ice, eroding it. Wave action against sea cliffs can erode the rock to form sea caves. Caves may also result from large boulders piled together by rockslides or when earthquakes create large cracks in rocks.

III

Cave Detection

The presence of caves in limestone regions may be detected by means of clues provided by the topography of the land. In such a region the roofs of large caverns may collapse and leave depressions and troughs at the surface of the ground. Natural bridges, another phenomenon of cave regions, may remain after the collapse of a tunnel bearing an underground stream. The Natural Bridge in Virginia is a classic example of this type of formation.

In the phenomenon known as disappearing streams, which is a common feature in areas underlain by caves, whole watercourses may vanish down sinks, or sinkholes, leading to the underground caverns. The sinks are indicative of caves below. Because of the capture of the surface waters by the subterranean drainage system, some cave regions have a rather dry, dusty, poorly vegetated appearance. Such regions are said to have a karst topography, a name derived from a famous cave region along the Adriatic Sea in Italy and Slovenia. Steep-walled sinks called cenotes, found in Yucatán, Mexico, constituted a chief source of water for the Maya peoples.

Caves are sometimes discovered underwater. In such cases, the caves were once dry but became completely flooded because of changes in water tables or in sea level. Mining operations sometimes encounter caves deep below ground.

IV

Features of Caves

Natural air conditioning occurs in large caverns if the temperature varies only a few degrees yearly, and the caves are more or less constantly ventilated with fresh air. These conditions are, in part, the result of complex meteorological phenomena, mainly variations in barometric pressure.

Caves formed by abrasion commonly consist of myriads of winding tunnels and former underground waterways that show many features analogous to those of surface streams, such as deposits of sand and gravel. Abrasion-formed caves normally lack the weird formations found in caves of solution.

In caves of solution, the dissolved lime carbonate is often precipitated in such a fashion as to form grotesque deposits. These secondary mineral formations that develop after most of the cave has been hollowed out are called speleothems, from Greek words meaning “cave deposit.” The best-known structures are the stalactites, which hang like icicles from the roofs of caves, and the stalagmites, which extend upward from the cavern floors (see Stalactite and Stalagmite). If the two growths meet and join, a pillar forms, helping to support the roof. Less well-known forms of carbonate deposition include flowstone and dripstone. Depending on dissolved mineral impurities brought into the cave by the groundwaters, the formations vary in color from alabaster white to hues of dusky red and brown. The dripstone formations may be exceedingly thin and translucent. Among rare formations is the helictite, a twisted, flowerlike variety of stalactite. Many cave formations are rather delicate and easily broken, and some of the best examples have been damaged or removed by unscrupulous cave explorers and visitors to public caves.

Limestone caves formed by sulfuric acid processes may have unusual crystals and mineral formations made of gypsum. The gypsum is hydrated calcium sulfate that forms when sulfuric acid dissolves calcium carbonate. Over time the gypsum may grow into delicate branching structures resembling flowers, hair, beards, or chandeliers. In other conditions the gypsum may form massive single crystals—selenite gypsum crystals discovered in a cave in Mexico in 2000 were up to 11 m (36 ft) long and one meter (3.3 ft) wide.