Cave

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Lechuguilla Cave, New Mexico, United States Lechuguilla Cave Pearlsian Gulf.jpg
Lechuguilla Cave, New Mexico, United States

A cave or cavern is a natural void in the ground, [1] [2] specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word cave can also refer to much smaller openings such as sea caves, rock shelters, and grottos, though strictly speaking a cave is exogene, meaning it is deeper than its opening is wide, [3] and a rock shelter is endogene. [4]

Weathering Breaking down of rocks, soil and minerals as well as artificial materials through contact with the Earths atmosphere, biota and waters

Weathering is the breaking down of rocks, soil, and minerals as well as wood and artificial materials through contact with the Earth's atmosphere, water, and biological organisms. Weathering occurs in situ, that is, in the same place, with little or no movement, and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity and then being transported and deposited in other locations.

Sea cave A cave formed by the wave action of the sea and located along present or former coastlines

A sea cave, also known as a littoral cave, is a type of cave formed primarily by the wave action of the sea. The primary process involved is erosion. Sea caves are found throughout the world, actively forming along present coastlines and as relict sea caves on former coastlines. Some of the largest wave-cut caves in the world are found on the coast of Norway, but are now 100 feet or more above present sea level. These would still be classified as littoral caves. By contrast, in places like Thailand's Phang Nga Bay, solutionally formed caves in limestone have been flooded by the rising sea and are now subject to littoral erosion, representing a new phase of their enlargement.

Rock shelter A shallow cave-like opening at the base of a bluff or cliff

A rock shelter — also rockhouse, crepuscular cave, bluff shelter, or abri — is a shallow cave-like opening at the base of a bluff or cliff. In contrast to solutional cave (karst) caves, which are often many miles long, rock shelters are almost always modest in size and extent.

Contents

Speleology is the science of exploration and study of all aspects of caves and the cave environment. Visiting or exploring caves for recreation may be called caving, potholing, or spelunking.

Speleology Science of cave and karst systems

Speleology is the scientific study of caves and other karst features, as well as their make-up, structure, physical properties, history, life forms, and the processes by which they form (speleogenesis) and change over time (speleomorphology). The term speleology is also sometimes applied to the recreational activity of exploring caves, but this is more properly known as caving or potholing, or by the uncommon American term spelunking. Speleology and caving are often connected, as the physical skills required for in situ study are the same.

Caving Recreational pastime of exploring cave systems

Caving – also known as spelunking in the United States and Canada and potholing in the United Kingdom and Ireland – is the recreational pastime of exploring wild cave systems. In contrast, speleology is the scientific study of caves and the cave environment.

Types and formation

The formation and development of caves is known as speleogenesis ; it can occur over the course of millions of years. [5] Caves can range widely in size, and are formed by various geological processes. These may involve a combination of chemical processes, erosion by water, tectonic forces, microorganisms, pressure, and atmospheric influences. Isotopic dating techniques can be applied to cave sediments, to determine the timescale of the geological events which formed and shaped present-day caves. [5]

Speleogenesis is the origin and development of caves, the primary process that determines essential features of the hydrogeology of karst and guides its evolution. It often deals with the development of caves through limestone, caused by the presence of water with carbon dioxide dissolved within it, producing carbonic acid which permits the dissociation of the calcium carbonate in the limestone.

It is estimated that a cave cannot exceed 3,000 metres (9,800 ft) in depth due to the pressure of overlying rocks. [6] For karst caves the maximum depth is determined on the basis of the lower limit of karst forming processes, coinciding with the base of the soluble carbonate rocks. [7] Most caves are formed in limestone by dissolution. [8]

Karst Topography formed from the dissolution of soluble rocks

Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant rocks, such as quartzite, given the right conditions. Subterranean drainage may limit surface water, with few to no rivers or lakes. However, in regions where the dissolved bedrock is covered or confined by one or more superimposed non-soluble rock strata, distinctive karst features may occur only at subsurface levels and can be totally missing above ground.

Limestone Sedimentary rocks made of calcium carbonate

Limestone is a carbonate sedimentary rock that is often composed of the skeletal fragments of marine organisms such as coral, foraminifera, and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). A closely related rock is dolomite, which contains a high percentage of the mineral dolomite, CaMg(CO3)2. In old USGS publications, dolomite was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolomites or magnesium-rich limestones.

Solvation attraction and association of molecules of a solvent with molecules or ions of a solute

Solvation describes the interaction of solvent with dissolved molecules. Ionized and uncharged molecules interact strongly with solvent, and the strength and nature of this interaction influence many properties of the solute, including solubility, reactivity, and color, as well as influencing the properties of the solvent such as the viscosity and density. In the process of solvation, ions are surrounded by a concentric shell of solvent. Solvation is the process of reorganizing solvent and solute molecules into solvation complexes. Solvation involves bond formation, hydrogen bonding, and van der Waals forces. Solvation of a solute by water is called hydration.

Caves can be classified in various other ways as well, including a contrast between active and relict: active caves have water flowing through them; relict caves do not, though water may be retained in them. Types of active caves include inflow caves ("into which a stream sinks"), outflow caves ("from which a stream emerges"), and through caves ("traversed by a stream"). [9]

Speleothems in Hall of the Mountain King of Ogof Craig a Ffynnon, a solutional cave in South Wales. HallOfTheMountainKings.jpg
Speleothems in Hall of the Mountain King of Ogof Craig a Ffynnon, a solutional cave in South Wales.

Solutional cave

Solutional caves or karst caves are the most frequently occurring caves. Such caves form in rock that is soluble; most occur in limestone, but they can also form in other rocks including chalk, dolomite, marble, salt, and gypsum. Rock is dissolved by natural acid in groundwater that seeps through bedding planes, faults, joints, and comparable features. Over time cracks enlarge to become caves and cave systems.

Chalk A soft, white, porous sedimentary rock made of calcium carbonate

Chalk is a soft, white, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite. Calcite is an ionic salt called calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells (coccoliths) shed from micro-organisms called coccolithophores. Flint (a type of chert) is very common as bands parallel to the bedding or as nodules embedded in chalk. It is probably derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction. Flint is often deposited around larger fossils such as Echinoidea which may be silicified (i.e. replaced molecule by molecule by flint).

Dolomite (rock) Sedimentary carbonate rock that contains a high percentage of the mineral dolomite

Dolomite (also known as dolostone, dolomite rock or dolomitic rock) is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite, CaMg(CO3)2. In old USGS publications, it was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolomites or magnesium-rich limestones. Dolomite has a stoichiometric ratio of nearly equal amounts of magnesium and calcium. Most dolomites formed as a magnesium replacement of limestone or lime mud before lithification. Dolomite is resistant to erosion and can either contain bedded layers or be unbedded. It is less soluble than limestone in weakly acidic groundwater, but it can still develop solution features (karst) over time. Dolomite can act as an oil and natural gas reservoir.

Gypsum mineral, calcium sulfate with bounded water

Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard/sidewalk chalk, and drywall. A massive fine-grained white or lightly tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire, and the Nottingham alabasters of Medieval England. Gypsum also crystallizes as translucent crystals of selenite. It also forms as an evaporite mineral and as a hydration product of anhydrite.

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 and underground drainage. Limestone caves are often adorned with calcium carbonate formations produced through slow precipitation. These include flowstones, stalactites, stalagmites, helictites, soda straws and columns. These secondary mineral deposits in caves are called speleothems .

The portions of a solutional cave that are below the water table or the local level of the groundwater will be flooded. [10]

Lechuguilla Cave in New Mexico and nearby Carlsbad Cavern 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 groundwater and forms H2SO4 (sulfuric acid). The acid then dissolves the limestone from below, rather than from above, by acidic water percolating from the surface.

Primary cave

Exploring a lava tube in Hawaii. Hawaiian lava tube.jpg
Exploring a lava tube in Hawaii.

Caves formed at the same time as the surrounding rock are called primary caves.

Lava tubes are formed through volcanic activity and are the most common primary caves. As lava flows downhill, its surface cools and solidifies. Hot liquid lava continues to flow under that crust, and if most of it flows out, a hollow tube remains. Such caves can be found in the Canary Islands, Jeju-do, the basaltic plains of Eastern Idaho, and in other places. Kazumura Cave near Hilo, Hawaii is a remarkably long and deep lava tube; it is 65.6 km long (40.8 mi).

Lava caves include but are not limited to lava tubes. Other caves formed through volcanic activity include rifts, lava molds, open vertical conduits, inflationary, blisters, among others. [11]

Sea cave or littoral cave

Painted Cave, a large sea cave, Santa Cruz Island, California Painted cave.jpg
Painted Cave, a large sea cave, Santa Cruz Island, California

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 to 50 metres (16 to 164 ft) in length, but may exceed 300 metres (980 ft).

Corrasional cave or erosional cave

Salt cave in Mount Sodom, Israel. Sodom Salt Cave 031712.JPG
Salt cave in Mount Sodom, Israel.

Corrasional or erosional caves are those that form entirely by erosion by flowing streams carrying rocks and other sediments. These can form in any type of rock, including hard rocks such as granite. Generally there must be some zone of weakness to guide the water, such as a fault or joint. A subtype of the erosional cave is the wind or aeolian cave, carved by wind-born sediments. [11] Many caves formed initially by solutional processes often undergo a subsequent phase of erosional or vadose enlargement where active streams or rivers pass through them.

Glacier cave

Glacier cave in Big Four Glacier, Big Four Mountain, Washington, ca. 1920 Big Four Glacier Ice cave.jpg
Glacier cave in Big Four Glacier, Big Four Mountain, Washington, ca. 1920

Glacier caves are formed by melting ice and flowing water within and under glaciers. The cavities are influenced by the very slow flow of the ice, which tends to collapse the caves again. Glacier caves are sometimes misidentified as "ice caves", though this latter term is properly reserved for bedrock caves that contain year-round ice formations.

Fracture cave

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 of stone. [12]

Talus cave

Talus caves are formed by the openings among large boulders that have fallen down into a random heap, often at the bases of cliffs. [13] These unstable deposits are called talus or scree, and may be subject to frequent rockfalls and landslides.

Anchialine cave

Anchialine 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 fauna.[ citation needed ]

Physical patterns

Geographic distribution

Domica Cave in Slovak Karst (Slovakia) Domica Cave 22.jpg
Domica Cave in Slovak Karst (Slovakia)

Caves are found throughout the world, but only a small portion of them have been explored and documented by cavers.[ citation needed ] 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 rough 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 square kilometres (390,000 sq mi)—has relatively few documented caves.

Records and superlatives

World's five longest surveyed caves

  1. Mammoth Cave, Kentucky, US [15]
  2. Sistema Sac Actun/Sistema Dos Ojos, Mexico [15]
  3. Jewel Cave, South Dakota, US [15]
  4. Sistema Ox Bel Ha, Mexico [15]
  5. Optymistychna Cave, Ukraine [15]

Ecology

Townsend's big-eared bats in a cave in California Townsends in music hall.jpg
Townsend's big-eared bats in a cave in California
Olms in a Slovenian cave Proteus anguinus Postojnska Jama Slovenija.jpg
Olms in a Slovenian cave

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 fully in caves) and accidentals (animals not in one of the previous categories). Some authors use separate terminology for aquatic forms (for example, stygobites, stygophiles, and stygoxenes).

Of these animals, the troglobites are perhaps the most unusual organisms. Troglobitic species often show a number of characteristics, termed troglomorphic, 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, and cave salamanders such as the olm and the Texas blind salamander.

Cave insects such as Oligaphorura (formerly Archaphorura) schoetti are troglophiles, reaching 1.7 millimetres (0.067 in) 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, 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.

Cave entrances often have typical florae. For instance, in the eastern temperate United States, cave entrances are most frequently (and often densely) populated by the bulblet fern, Cystopteris bulbifera .

Archaeological and cultural importance

Taino petroglyphs in a cave in Puerto Rico Taino petroglyph in cave.jpg
Taíno petroglyphs in a cave in Puerto Rico

Throughout history, primitive peoples have made use of caves. The earliest human fossils found in caves come from a series of caves near Krugersdorp and Mokopane in South Africa. The cave sites of Sterkfontein, Swartkrans, Kromdraai B, Drimolen, Malapa, Cooper's D, Gladysvale, Gondolin and Makapansgat have yielded a range of early human species dating back to between three and one million years ago, including Australopithecus africanus, Australopithecus sediba and Paranthropus robustus. However, it is not generally thought that these early humans were living in the caves, but that they were brought into the caves by carnivores that had killed them.

The first early hominid ever found in Africa, the Taung Child in 1924, was also thought for many years to come from a cave, where it had been deposited after being predated on by an eagle. However, this is now debated (Hopley et al., 2013; Am. J. Phys. Anthrop.). Caves do form in the dolomite of the Ghaap Plateau, including the Early, Middle and Later Stone Age site of Wonderwerk Cave; however, the caves that form along the escarpment's edge, like that hypothesised for the Taung Child, are formed within a secondary limestone deposit called tufa. There is numerous evidence for other early human species inhabiting caves from at least one million years ago in different parts of the world, including Homo erectus in China at Zhoukoudian, Homo rhodesiensis in South Africa at the Cave of Hearths (Makapansgat), Homo neandertalensis and Homo heidelbergensis in Europe at Archaeological Site of Atapuerca, Homo floresiensis in Indonesia, and the Denisovans in southern Siberia.

In southern Africa, early modern humans regularly used sea caves as shelter starting about 180,000 years ago when they learned to exploit the sea for the first time (Marean et al., 2007; Nature). The oldest known site is PP13B at Pinnacle Point. This may have allowed rapid expansion of humans out of Africa and colonization of areas of the world such as Australia by 60–50,000 years ago. Throughout southern Africa, Australia, and Europe, early modern humans used caves and rock shelters as sites for rock art, such as those at Giants Castle. Caves such as the yaodong in China were used for shelter; other caves were used for burials (such as rock-cut tombs), or as religious sites (such as Buddhist caves). Among the known sacred caves are China's Cave of a Thousand Buddhas [21] and the sacred caves of Crete.

See also

Related Research Articles

Stalactite elongated mineral formation which hangs down from a cave ceiling

A stalactite is a type of formation that hangs from the ceiling of caves, hot springs, or manmade structures such as bridges and mines. Any material that is soluble, can be deposited as a colloid, or is in suspension, or is capable of being melted, may form a stalactite. Stalactites may be composed of lava, minerals, mud, peat, pitch, sand, sinter, and amberat. A stalactite is not necessarily a speleothem, though speleothems are the most common form of stalactite because of the abundance of limestone caves.

Sinkhole Depression or hole in the ground caused by collapse of the surface into an existing void space

A sinkhole, also known as a cenote, sink, sink-hole, swallet, swallow hole, or doline, is a depression or hole in the ground caused by some form of collapse of the surface layer. Most are caused by karst processes – the chemical dissolution of carbonate rocks or suffosion processes. Sinkholes vary in size from 1 to 600 m both in diameter and depth, and vary in form from soil-lined bowls to bedrock-edged chasms. Sinkholes may form gradually or suddenly, and are found worldwide.

Stalagmite elongated mineral formation which forms on a cave floor

A stalagmite is a type of rock formation that rises from the floor of a cave due to the accumulation of material deposited on the floor from ceiling drippings. Stalagmites may be composed of lava, minerals, mud, peat, pitch, sand, sinter and amberat.

Mesa Elevated area of land with a flat top and sides that are usually steep cliffs

A mesa is an isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain. Mesas characteristically consist of flat-lying soft sedimentary rocks capped by a more resistant layer or layers of harder rock, e.g. shales overlain by sandstones. The resistant layer acts as a caprock that forms the flat summit of a mesa. The caprock can consist of either sedimentary rocks such as sandstone and limestone; dissected lava flows; or a deeply eroded duricrust. Unlike plateau, whose usage does not imply horizontal layers of bedrock, e.g. Tibetan Plateau, the term mesa applies exclusively to the landforms built of flat-lying strata. Instead, flat-topped plateaus are specifcally know as tablelands.

Krubera Cave cave

Krubera Cave is the second-deepest-known cave on Earth after the Veryovkina Cave. It is located in the Arabika Massif of the Gagra Range of the Western Caucasus, in the Gagra district of Abkhazia, a breakaway region of Georgia.

Carbonate rock

Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3) and dolomite rock, also known as dolostone, which is composed of mineral dolomite (CaMg(CO3)2).

Blue hole Marine cavern or sinkhole, open to the surface, in carbonate bedrock

A blue hole is a large marine cavern or sinkhole, which is open to the surface and has developed in a bank or island composed of a carbonate bedrock. Blue holes typically contain tidally influenced water of fresh, marine, or mixed chemistry. They extend below sea level for most of their depth and may provide access to submerged cave passages. Well-known examples can be found in South China Sea, Belize, the Bahamas, Guam, Australia, and Egypt.

Karst fenster An unroofed portion of a cavern which reveals part of a subterranean river

Karst fenster is a geomorphic feature formed from the dissolution of carbonate bedrock. In this feature, a spring emerges, then the discharge abruptly disappears into a sinkhole. The word fenster is German for 'window', as these features are windows into the karst landscape.

Ponor

A ponor is a natural opening where surface water enters into underground passages; they may be found in karst landscapes where the geology and the geomorphology is typically dominated by porous limestone rock.

Lava cave Cave formed in volcanic rock, especially one formed via volcanic processes

A lava cave is any cave formed in volcanic rock, though it typically means caves formed by volcanic processes, which are more properly termed volcanic caves. Sea caves, and other sorts of erosional and crevice caves, may be formed in volcanic rocks, but through non-volcanic processes and usually long after the volcanic rock was emplaced.

Solutional cave cave formed in soluble rock such as limestone, chalk, dolomite, marble, salt beds or gypsum

A solutional cave or karst cave is a cave usually formed in the soluble rock limestone. It is the most frequently occurring type of cave. It can also form in other rocks, including chalk, dolomite, marble, salt beds, and gypsum.

There are a number of terms that are used in connection with caves, caving and speleology. The following is an incomplete list.

The geology of Guam formed as a result of mafic, felsic and intermediate composition volcanic rocks erupting below the ocean, building up the base of the island in the Eocene, between 33.9 and 56 million years ago. The island emerged above the water in the Eocene, although the volcanic crater collapsed. A second volcanic crater formed on the south of the island in the Oligocene and Miocene. In the shallow water, numerous limestone formations took shape, with thick alternating layers of volcanic material. The second crater collapsed and Guam went through a period in which it was almost entirely submerged, resembling a swampy atoll, until structural deformation slowly uplifted different parts of the island to their present topography. The process of uplift led to widespread erosion and clay formation, as well as the deposition of different types of limestone, reflecting different water depths.

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