A cave is a natural underground void large enough for a human to enter. Some people suggest that the term 'cave' should only apply to cavities that have some part that does not receive daylight; however, in popular usage, the term includes smaller spaces like sea caves, rock shelters, and grottos.
Speleology is the science of exploration and study of all aspects of caves. Exploring a cave for recreation or science may be called "caving", "potholing", or occasionally (only in Canada and the United States), "spelunking".
Types and formation
Caves are formed by geologic processes. These may involve a combination of chemical processes, erosion from water, tectonic forces, microorganisms, pressure, and atmospheric influences. Most caves are formed in limestone by solution.
- Solutional caves may form anywhere with rock that is soluble, and are most prevalent in limestone, but can also form in other material, including chalk, dolomite, marble, granite, salt, sandstone, fossilized coral and gypsum.
- The largest and most abundant solutional caves are located in limestone. Limestone dissolves under the action of rainwater and groundwater charged with H2CO3 (carbonic acid) and naturally occurring organic acids. The dissolution process produces a distinctive landform known as karst, characterized by sinkholes, sinking streams, and underground drainage. Limestone caves are often adorned with calcium carbonate speleothems (formations) produced through slow precipitation such as flowstone, stalactites, stalagmites, helictites, drapery, soda straws and columns. These secondary mineral deposits in caves are called speleothems.
- The world's most spectacularly decorated cave is generally regarded to be Lechuguilla Cave (New Mexico, USA). Lechuguilla and nearby Carlsbad Caverns are now believed to be examples of another type of solutional cave. They were formed by H2S (hydrogen sulfide) gas rising from below, where reservoirs of oil give off sulfurous fumes. This gas mixes with ground water and forms H2SO4 (sulfuric acid). The acid then dissolves the limestone from below, rather than from above, by acidic water percolating from the surface.
- Some caves are formed at the same time as the surrounding rock. These are sometimes called primary caves.
- Lava tubes are formed through volcanic activity and are the most common 'primary' caves. The lava flows downhill and the surface cools and solidifies. The hotter lava continues to flow under that crust, and if most of the liquid lava beneath the crust flows out, a hollow tube remains, thus forming a cavity. Examples of such caves can be found on Tenerife, Big Island, and many other places. Kazumura Cave near Hilo is a remarkably long and deep lava tube; it is 65.6km long.
- Blister caves are also formed through volcanic activity.
- Sea caves are found along coasts around the world. A special case is littoral caves, which are formed by wave action in zones of weakness in sea cliffs. Often these weaknesses are faults, but they may also be dykes or bedding-plane contacts. Some wave-cut caves are now above sea level because of later uplift. Elsewhere, in places such as Thailand's Phang Nga Bay, solutional caves have been flooded by the sea and are now subject to littoral erosion. Sea caves are generally around 5–50m in length but may exceed 300m.
- Glacier caves occur in ice and under glaciers, formed by melting. They are also influenced by the very slow flow of the ice, which tends to close the caves again. (These are sometimes called ice caves, though this term is properly reserved for caves that contain year-round ice formations).
- Fracture caves are formed when layers of more soluble minerals, such as gypsum, dissolve out from between layers of less soluble rock. These rocks fracture and collapse in blocks.
- Talus caves are the openings between rocks that have fallen down into a pile, often at the bases of cliffs.
- Anchihaline caves are caves, usually coastal, containing a mixture of freshwater and saline water (usually sea water). They occur in many parts of the world, and often contain highly specialized and endemic faunas.
- "Branchwork caves" resemble surface dentritic stream patterns; they are made up of passages that join downstream as tributaries. Branchwork caves are the most common of cave patterns and are formed near sinkholes where groundwater recharge occurs. Each passage or branch is fed by a separate recharge source and converges into other higher order branches downstream (Easterbrook, 1993).
- "Angular Network Caves" form from intersecting fissures of carbonate rock that have had fractures widened by chemical erosion. These fractures form high, narrow, straight passages that persist in widespread closed loops (Easterbrook, 1993).
- "Anastomotic Caves" largely resemble surface braided streams with their passages separating and then meeting further down drainage. They usually form along one bed or structure, and only rarely cross into upper or lower beds (Easterbrook, 1993).
- "Spongework caves" are formed as solution cavities are joined by mixing of chemically diverse water. The cavities form a pattern that is three-dimensional and random, resembling a sponge (Easterbrook, 1993).
- "Ramiform caves" form as irregular large rooms, galleries, and passages. These randomized three-dimensional rooms form from a rising water table that erodes the carbonate rock with hydrogen-sulfide enriched water (Easterbrook, 1993).
Geographic distribution of caves
Caves are found throughout the world, but only a portion of them have been explored and documented by cavers. The distribution of documented cave systems is widely skewed toward countries where caving has been popular for many years (such as France, Italy, Australia, the UK, the United States, etc.). As a result, explored caves are found widely in Europe, Asia, North America, and Oceania but are sparse in South America, Africa, and Antarctica. This is a great generalization, as large expanses of North America and Asia contain no documented caves, whereas areas such as the Madagascar dry deciduous forests and parts of Brazil contain many documented caves. As the world’s expanses of soluble bedrock are researched by cavers, the distribution of documented caves is likely to shift. For example, China, despite containing around half the world's exposed limestone - more than 1,000,000 km² - has relatively few documented caves.
Record lengths, depths, pitches and volumes
The cave system with the greatest total length of passage is Mammoth Cave (Kentucky, USA) at 591 km in length. This record is unlikely to be surpassed in the near future, as the next most extensive known cave is Jewel Cave near Custer, South Dakota, at 225 km.
The deepest known cave (measured from its highest entrance to its lowest point) is Voronya Cave (Abkhazia, Georgia), with a depth of 2,190 m. This was the first cave to be explored to a depth of more than 2 km. (The first cave to be descended below 1 km was the famous Gouffre Berger in France). The Gouffre Mirolda - Lucien Bouclier cave in France (1733 m) and the Lamprechtsofen Vogelschacht Weg Schacht in Austria (1632 m) are the current second- and third-deepest caves. This particular record has changed several times in recent years.
The largest individual cavern ever discovered is the Sarawak chamber, in the Gunung Mulu National Park (Miri, Sarawak, Borneo, Malaysia), a sloping, boulder strewn chamber with an area of approximately 600 m by 400 m and a height of 80 m.
Cave-inhabiting animals are often categorized as troglobites (cave-limited species), troglophiles (species that can live their entire lives in caves, but also occur in other environments), trogloxenes (species that use caves, but cannot complete their life cycle wholly in caves) and accidentals (animals not in one of the previous categories). Some authors use separate terminology for aquatic forms (e.g., stygobites, stygophiles, and stygoxenes).
Of these animals, the troglobites are perhaps the most unusual organisms. Troglobitic species often show a number of characteristics, termed troglomorphies, associated with their adaptation to subterranean life. These characteristics may include a loss of pigment (often resulting in a pale or white coloration), a loss of eyes (or at least of optical functionality), an elongation of appendages, and an enhancement of other senses (such as the ability to sense vibrations in water). Aquatic troglobites (or stygobites), such as the endangered Alabama cave shrimp, live in bodies of water found in caves and get nutrients from detritus washed into their caves and from the feces of bats and other cave inhabitants. Other aquatic troglobites include cave fish, the Olm, and the blind salamander.
Cave insects such as Oligaphorura (formerly Archaphorura) schoetti are troglophiles, reaching 1.7 mm in length. They have extensive distribution and have been studied fairly widely. Most specimens are female but a male specimen was collected from St Cuthberts Swallet in 1969.
Bats, such as the Gray bat and Mexican Free-tailed Bat, are trogloxenes and are often found in caves; they forage outside of the caves. Some species of cave crickets are classified as trogloxenes, because they roost in caves by day and forage above ground at night.
Because of the fragile nature of the cave ecosystem, and the fact that cave regions tend to be isolated from one another, caves harbor a number of endangered species, such as the Tooth cave spider, Liphistiidae Liphistius trapdoor spider, and the Gray bat.
Caves are visited by many surface-living animals, including humans. These are usually relatively short-lived incursions, due to the lack of light and sustenance.
Throughout history, primitive peoples have made use of caves for shelter, burial, or as religious sites. Since items placed in caves are protected from the climate and scavenging animals, this means caves are an archaeological treasure house for learning about these people. Cave paintings are of particular interest. One example is the Great Cave of Niah, in Malaysia, which contains evidence of human habitation dating back 40,000 years.
In Germany some experts found signs of cannibalism in the caves at the Hönne.
Caves are also important for geological research because they can reveal details of past climatic conditions in speleothems and sedimentary rock layers.
Caves are frequently used today as sites for recreation. Caving, for example, is the popular sport of cave exploration. For the less adventurous, a number of the world's prettier and more accessible caves have been converted into show caves, where artificial lighting, floors, and other aids allow the casual visitor to experience the cave with minimal inconvenience. Caves have also been used for BASE jumping and cave diving.
Caves are also used for the preservation or aging of wine and cheese. The constant, slightly chilly temperature and high humidity that most caves possess makes them ideal for such uses.
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