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 which 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 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 (see below).
Primary caves
Exploring a lava tube in Hawaii
Some are formed at the same time as the surrounding rock. These are called primary caves.
- Lava tubes are formed through volcanic activity. They are
the most common primary caves. Lava flows downhill and the surface cools and solidifies. The lava
now flows under this crust, until the eruption ends. If the liquid lava inside the crust flows out, a hollow tube remains.
Numerous lava tubes are found on Hawaii (Big Island).
Kazumura Cave near Hilo is the longest and deepest
lava tube of the world and also the eighth longest cave of the United States. 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 like 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 which
contain year-round ice formations).
- Solutional caves may form anywhere with rock which 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 most common process of cave formation is karstification, which is the solution of rocks by rain water.
- 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.
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 formations produced through slow precipitation, including the most common and
well-known stalactites and stalagmites. 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 acid rising from below, where reservoirs of oil give off
sulfurous fumes, rather than by acidic water percolating from the surface.
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 caves
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.[1]
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.[2] 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 deepest individual pitch (vertical drop) within a cave is 603 m in the
Vrtoglavica cave in Slovenia, followed by Patkov Gušt (553 m) in the Velebit mountain, Croatia.
The largest individual cavern ever discovered is the Sarawak chamber, in the Gunung
Mulu National Park (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 ecology
Townsend's Big-eared bats in a cave
Cave-inhabiting animals are often categorized as troglobites (cave-limited species),
troglophiles (species which can live their entire lives in caves, but also occur in other
environments), trogloxenes (species which 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 faeces of bats and other cave
inhabitants. Other aquatic troglobites include cave fish and the Olm.
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.
Archaeological and social importance
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, which contains evidence of human habitation dating back 40,000
years.[3]
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.
See also
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References
External links
zh-yue:山窿
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