Caves invite you to use your imagination! The names of cave formations, collectively known as speleothems, may seem fanciful: moonmilk, soda straws, cave bacon, and cave popcorn. It makes you wonder whether cavers and speleologists are always hungry. You may also think that cave animals are quite hungry for they survive on a very limited food supply. On the other hand, there is plenty of water in most cave environments.
To discover what you might find in a cave, select a topic.
As you go farther into a cave, the ceiling may get lower. It may get so low that you'll have to crawl on your hands and knees. When this happens, you have entered a crawlway. As you continue crawling, you may come to the top of a chimney. This tall, rock structure is narrow enough that you can stretch your arms and legs across to opposite walls. Place your hands and feet on either side and carefully lower yourself using natural hand holds and foot holds. After exiting the chimney you may enter another crawlway, but you are bound to eventually come to a large room called a chamber. Looking around a chamber you're likely to see speleothems.
The features that arouse the greatest curiosity for most cave visitors are speleothems. These stone formations exhibit bizarre patterns and other-worldly forms, which give some caves a wonderland appearance. Caves vary widely in their displays of speleothems because of differences in temperature; overall wetness; and jointing, impurities, and structures in the rocks. In general, however, one thing caves do have in common is where speleothems form. Although the formation of caves typically takes place below the water table in the zone of saturation, the deposition of speleothems is not possible until caves are above the water table in the zone of aeration. As soon as the chamber is filled with air, the stage is set for the decoration phase of cave building to begin.
The term speleothem refers to the mode of occurrence of a mineral (i.e., its morphology or how it looks) in a cave, not its composition (Hill and Forti 1997). For example, calcite, the most common cave mineral, is not a speleothem, but a calcite stalactite is a speleothem. A stalactite may be made of other minerals, such as halite or gypsum.
Classifying speleothems is tricky because no two speleothems are exactly alike. Nevertheless, speleologists have taken three basic approaches: classification by morphology, classification by origin, and classification by crystallography. All three of these approaches have their problems (Hill and Forti 1997), so cavers often take a more practical approach that primarily uses morphology (e.g., cave pearls) but includes whatever is known about origin (e.g., geysermites) and crystallography (e.g., spar) when needed.
Cave Minerals of the World (Hill and Forti 1997) refers to 38 different types of speleothems and numerous subtypes and varieties. Let's not get bogged down in classifying speleothems but simply enjoy their exotic nature. Select a speleothem to learn more:
Cave balloons are round-shaped, thin-walled speleothems with gas inside of a mineralized, bag-like pouch. Cave balloons are rare and fragile. It is believed that balloons are short-lived; they quickly dry, crack, deflate, and change in luster, especially in low humidity environments. The extreme fragility of cave balloons probably accounts for the scarcity of this speleothem type.
Some caves in national parks are fortunate enough to house cave balloons. Jewel Cave has one of the best displays of cave balloons. Cavers have reported two dozen good specimens along with a few hundred other ones of all shapes, sizes, and lesser degrees of perfection (Cahill and Nichols 1991). The balloons in Jewel Cave are typically pearly to silvery-white, opaque, and have wall thicknesses of 0.0008 inches (0.02 mm) and diameters up to two inches (5 cm). They appear to have grown as little sacks - some of them distended, others puckered and deflated in appearance (Hill and Forti 1997).
There are not nearly as many cave balloons in the caves in the Guadalupe Mountains as in Jewel Cave. Six have been reported in Carlsbad Cavern. They have a satiny, pearly luster, are up to 0.6 inches (1.5 cm) in diameter, and have an estimated thickness of 0.0039 inches (0.1 mm) (Hill and Forti 1997). The balloons in the caves of the Grand Canyon are either poorly developed or in the state of decay (Hill and Forti 1997).
The origin of cave balloons is not well understood but is probably related to moonmilk with which cave balloons are usually associated. Moonmilk can exhibit high plasticity, depending on its water content. Cave-balloon growth may be initiated when solutions under pressure seep into cave passages along cracks or out of porous limestone walls. If these solutions encounter moonmilk blebs on cave walls or on the tips of coralloids, the ductile material expands in the same way as an inflatable party balloon.
Boxwork is so named because it resembles a maze of post office boxes. Intricate networks of fins or plates protrude in relief from bedrock walls, ceilings, speleothems, or floors. Boxwork can be composed of any mineral more resistant than its surrounding medium, but calcite is most common.
The best exposure of calcite boxwork are in the caves of the Black Hills, South Dakota, most remarkably in Wind Cave, a national park, where blades of crystalline material protrude outward from cave walls and ceilings for 24 inches (60 cm) or more (Hill and Forti 1997).
Perhaps one of the most nondescript of all speleothems is coatings, which line cave walls, ceilings, floors, and pools. Typically coatings unpretentiously serve as the backdrop for more showy speleothems, such as frostwork and helictites. Coatings are so unspectacular that very little attention is ever paid to them. However, many coatings are interesting because they are composed not only of the common minerals: calcite and aragonite, but also the rarer minerals that have magnesium and iron. Practically every cave mineral known can form as coatings (Hill and Forti 1997).
Columns are not stalactites nor are they stalagmites; they are both, together. When a stalagmite grows together with its counterpart feeder stalactite, a new speleothem is formed: a column or pillar.
Columns can reach gigantic proportions, sometimes over 65 feet (20 m) in height and diameter. Typically the largest columns are aligned along ceiling joints, where the greatest amount of water is dripping into the cave.
Coralloid (or corallite) is a catchall term describing knobby, nodular, botryoidal, or coral-like speleothems. After stalactites, stalagmites, and flowstone, coralloids are probably the most common speleothem type (Hill and Forti 1997). Coralloids range in size from tiny beads to globular masses a few feet (a meter) in diameter. Coralloids include cave popcorn, grapes, knobstone, coral, cauliflower, globularites, and grapefruit. Coralloids can form both in the open air and underwater.
When water drops flow down a sloped ceiling before dripping to the floor, calcite can build up in a line. These lines gradually form draperies or cave bacon. This type of speleothem is found in almost every cave in the world and is universally popular because of the close resemblance to its namesake (Nelson 2000). Iron oxide or organic solutions form the bacon-like stripes. As the formations grow, small undulations in the bedrock cause the draperies to become slightly curved. With time these curves become more and more accentuated so that the draperies become highly folded or furled along their lower edges. Dripstones may form at the bottom of draperies where the furls are steep enough for water droplets to fall to the floor.
Caves are greenhouses for flowers formed of cave minerals, typically gypsum. The crystal petals of these speleothems radiate out from a common center. The formation grows from a base rather than a tip like stalactites. Variations in crystal structure produce unique, curved, flower-like petals.
Cave flowers have been found in may caves throughout the world. Hovey must have been quite taken by the gypsum flowers when he saw them in Mammoth Cave in 1896 for he describes them in overtly flowery language (Hovey 1896):
"From a central stem gracefully curl countless crystals, fibrous and pellucid; each tiny crystal is itself a study; each fascicle of curved prisms is wonderful; and the whole blossom is a miracle of beauty... Floral clusters, bouquets, wreaths, garlands, embellish nearly every foot of the ceiling and walls... Clumps of lilies, pale pansies, blanched tulips, drooping fuchsias, sprays of asters, spikes of tube-roses, wax-leaved magnolias - but why exhaust the botanical catalogue? The fancy finds every gem of the green-house and parterre in this crystalline conservatory."
A fantastic gypsum flower display has also been found in Lechuguilla Cave, Carlsbad Caverns National Park, New Mexico.
Flowstone is one of the most common speleothems. It has been described as melted cake icing and frozen waterfalls. Flowstone is usually composed of calcite or other carbonate minerals, and deposition is in layers or bands. Where composed of calcite, individual flowstone layers may be very colorful: yellows, reds, and oranges.
Flowstone differs from coatings in that it deposits from flowing water and not from seeping water, but in reality, these two speleothem types are intergraded (Hill and Forti 1997).
Flowstone forms both in the open air and underwater and assumes a variety of forms. The most common of these is the petrified or frozen waterfall, also referred to as cascades, rivers, glaciers, or organ pipes. A well know example of waterfall flowstone is Frozen Niagara in Mammoth Cave. In places, flowstone can form on sedimentary rock, which is later washed away, leaving the flowstone as a canopy. Flowstone also forms in running streams where carbon dioxide degasses as water tumbles over rocks in a stream bed.
Frostwork is a spiny speleothem resembling cactus or thistle plants. It is the needle-like habit of aragonite that gives most frostwork its particular appearance. However, frostwork can be composed of calcite, opal, gypsum, other minerals, and ice. It is usually white but can also be other colors, including blue (Hill and Forti 1997).
Frostwork occurs worldwide in the caves of Australia, Brazil, Japan, Korea, Norway, Romania, South Africa, and the United States. The most common occurrence of frostwork is with coralloids, for example, clusters of needles frequently radiate from the tips of popcorn nodules. Frostwork can also be found on stalactites, walls, ceilings, ledges, and less occasionally on floors (Hill and Forti 1997).
Frostwork displays can be dazzling! They are among the most exquisite, fragile, and intricate of all speleothem types. Unfortunately, their beauty makes them prime targets for vandalism, and their delicate nature makes them easily destroyed by carelessness.
Helictites are contorted speleothems that twist in any direction, seemingly in defiance of gravity. The term helictite comes from the Greek root helix, meaning to spiral. Helictites have been compared to "the horrible, snaky tresses of Medusa" (Hill and Forti 1997). They have been described as threads, beads, worms, and antlers or twigs.
Helictites grow on cave ceilings, walls, and less often on cave floors. They typically grow on other speleothems, such as carbonate coatings, crusts, and sometimes on soda straws.
Regardless of size or shape, all helictites have one thing in common: they possess tiny central channels through which their extremities and diameters are fed and increased by seeping capillary water.
Moonmilk forms very fine crystals that vary in composition. Moonmilk's fine-grained particles become suspended in water, which gives the deposit the appearance of milk. Moonmilk is soft and pasty when wet, but crumbly and powdery when dry. Wet moonmilk looks and feels like cream cheese; dry moonmilk resembles talcum powder. For centuries, moonmilk has been used for medicinal purposes: a poultice to stop bleeding, for fevers, for diarrhea, and as an antacid.
Cave pearls are concentrically banded concretions that form in shallow cave pools. Cave pearls vary in shape; they can be spherical, cylindrical, irregular, cubical, or even hexagonal. They can range in size from smaller than a sand grain up to eight inches (20 cm) in diameter. Cave pearls have been compared to marbles, hailstones, cupcakes, cigars, oranges, pigeon's eggs, balls, and most of all, pearls from whence their name comes. The luster of cave pearls justified their naming back in 1874, but they can also be rather dull.
Cave pearls are known to occur in numerous caves, but exceptional displays exist in caves in the Guadalupe Mountains, New Mexico. In 1929, Hess described the fabulous cave pearls in the Rookery of Carlsbad Cavern (Hess 1929), which were once so numerous that specimens were handed out as souvenirs in the early days of commercialism (Hill and Forti 1997).
Cave pearls normally grow in shallow cave pools where water is dripping in from above or slowing flowing into the pool. Carbon dioxide is lost to the air, carbonate material precipitates around clastic particles in the pool, and excess precipitate material coats the floor and builds up into cups or nests around the pearls. Sand grains, bat bones, shell and wood fragments, or pieces of soda straws may act as nuclei for cave-pearl growth; all of these fragments become rounded as they grow into cave pearls of different shapes (Hill and Forti 1997).
Dripping water can agitate cave pearls, but it does not rotate them, round them, or polish them. Instead, cave pearls become round because the growing speed of the outer layer of the pearl is the same in all directions on account of the water in the pearl nest being supersaturated. A spherical shape is the structure that allows for the greatest amount of material for the smallest surface area; therefore, a round shape is naturally promoted even for pearls with highly irregular nuclei (Hill and Forti 1997).
Stalactites are usually composed of calcite, but they may consist of other minerals. All stalactites, whatever their composition, begin their growth as hollow soda straws. At first a water droplet collects on the cave ceiling by condensation or by water coming through a fracture in the rock. With the loss of carbon dioxide, a thin film of carbonate material precipitates and covers the surface of the drop. Similarly with evaporation in arid cave environments, a thin film of sulfate or other noncarbonate material precipitates over the surface of the drop. As the drop accumulates more water and becomes heavier, it begins to oscillate. This causes the film of material to move up toward the ceiling and to adhere there by surface tension. When the drop falls to the floor, this film is left on the ceiling as a round rim of material - the initial growth ring of a soda straw (Hill and Forti 1997). As drop after drop follows a similar path, an infinitesimal trace of material is left behind, and a hollow tube is created and eventually enlarged, as long as water continues to drip. It is not surprising, then, that the word stalactite is Greek for "oozing out in drops."
Stalagmites are convex floor deposits built up by water dripping from an overhead stalactite or from the cave ceiling. Because falling water droplets tend to splash, stalagmites spread out as they gradually build up from the floor. Hence, they do not have central, hollow tubes like stalactites. Stalagmites are usually larger in diameter than the stalactites above them and they generally have rounded tops instead of pointed tips. To help remember the difference between a stalactite and a stalagmite, think of how they are spelled: the one with a "c" (stalactite) hangs from the ceiling of a cave; the one with a "g" (stalagmite) builds up from the ground.
"That which drops" is the Greek meaning of stalagmites. When a drop of water falls from the ceiling or stalactite, it still has some material left in solution. When the drop hits the floor, carbon dioxide is given off and carbonate material is precipitated as a mound below the point of dripping; or, if a noncarbonate mineral, evaporation causes precipitation of mineral material.
Stalagmites can assume a fascinating variety of shapes and people have compared them to broomsticks, totem poles, toadstools, bathtubs, Christmas trees, beehives, coins and buttons, and even fried eggs!
Caves, especially their entrance zones, are often inhabited temporarily by animals that usually live above ground but occasionally move into caves for protection. Bears use caves for their long winter naps. Bats may remain in caves continuously throughout the winter when they are hibernating, but in the summer they rest in them only during the daylight hours. By contrast, other species live permanently in the dark zones of caves. Therefore, cave organisms can be categorized by how much time they actually spend in caves. They can also be segregated by where they actually live in caves: ceiling, walls, floor; entrance zone, twilight zone, dark zone.
To learn more about cave animals, select a topic.
Cave animals fit into three categories: trogloxenes, troglophiles, and troglobites. These categories are based on the amount of time cave animals spend in caves. Are they just visitors? Can they survive outside the cave environment? Do they spend their entire lives in caves?
Select a type of cave life to find out more:
Trogloxenes are cave visitors or temporary cave residents. They move freely in and out of caves. Trogloxenes is from the Greek words troglos (cave) and xenos (guest). Trogloxenes never complete their whole life cycle in caves. For trogloxenes, caves provide refuge from the elements, a cozy place to spend the winter, or an acceptable environment to bear their young.
Bats are probably the best known trogloxene. Skunks, raccoons, packrats, moths, frogs, beetles, some birds, and people are other examples of trogloxenes. Because these animals are not dependent on caves for their survival, they show no special adaptations to cave environments.
Troglophiles love caves. The name troglophile comes from the Greek words troglos (cave) and phileo (love). Troglophiles normally live in the dark zones of caves but they can and do survive outside caves, provided the environment is moist and dark. Earthworms are a good example; some types of salamanders, beetles, and crustaceans (such as crayfish) are also troglophiles. Some individual troglophiles may spend their entire life cycle in a cave, but other individuals of the same species live outside (Moore and Sullivan 1997).
There are life forms that live permanently in the dark zones of caves and are found exclusively in caves; they are called troglobites: from the Greek words troglos (cave) and bios (life). Troglobites cannot survive outside of the cave environment, and they have developed special adaptations from living their entire lives in caves. Because food sources in caves are meager, the sensory organs and physical adaptations of troglobites are devoted to sustaining energy and finding food: those that serve a benefit are enhanced; those not necessary are degenerated. In general, existing troglobites have evolved from troglophiles (Moore and Sullivan 1997).
Most troglobites are white to pinkish in color. They lack pigment (color) because they have no need for protection from the Sun's rays or for camouflage to hide from predators (Gee 1994). Many troglobites have no eyes or have eyes that are very poorly developed because eyes are not necessary in total darkness. Troglobites cannot afford to waste energy on unnecessary sensory organs, and it takes energy to maintain eyes.
Troglobites often have longer appendages and thinner shells than related surface forms. Adaptations such as these save energy, allowing troglobites to go for long periods without food, or assist in finding food. Blind cave fish have vibration receptors on their heads and sides to detect movements in the water and to guide them to prey. Troglobites include cave crayfish, cave shrimp, isopods, amphipods, millipedes, some cave salamanders, and many insects.
The organisms in caves are naturally segregated. For example, worms, beetles, millipedes, and springtails live in or near mud. Aquatic forms, such as flatworms, amphipods, isopods, and crayfish, live in water, or where humidity approaches saturation; the smaller aquatic animals may crawl over damp rocks (Moore and Sullivan 1997).Cave animals do not intermingle at random but are usually more or less concentrated in specific areas. The segregation within caves is of two types: stratification and zonation.
Stratification results from the gathering of individual species in localized areas of caves, such as floors, walls, or ceilings. For instance, cave crickets, spiders, and harvestmen (daddy longlegs) congregate on walls and ceilings, and bats ordinarily hang from the ceiling or from the crevices close to it.
Zonation applies to the preference shown by a given species for one of three zones in caves: the entrance zone, the twilight zone, or the dark zone. The entrance zone supports life that needs full sunlight in order to survive. The twilight zone extends from the entrance to the farthest point where light penetrates. The dark zone completely lacks light.
Table 1. Some typical inhabitants of stratification and zonation in caves. Table from Moore and Sullivan (1997) (Moore and Sullivan 1997).
|Entrance Zone||Twilight Zone||Dark Zone|
|Harvestmen (daddy longlegs)
|Harvestmen (daddy longlegs)
Microorganisms receive little attention in general biology text books, are largely ignored by most professional biologists, and are virtually unknown to the public except in contexts of disease or rot (Page 2000). Yet speleologists know that the workings of the biosphere on and below Earth's surface depend absolutely on the activities of the microbial world.
In cave environments, microorganisms are involved in the development of cave deposits such as moonmilk, in the production of food for cave animals, and in the breakdown of organic material.
Scientists divide microorganisms into heterotrophs and autotrophs with regard to their mode of life within caves. Both heterotrophs and autotrophs require carbon for their nutrition, but autotrophic bacteria can exist in areas devoid of organic materials; heterotrophic bacteria cannot. Autotrophs, in contrast to heterotrophs, are able to build the organic substances essential for life directly from inorganic raw materials. The particular kind of autotrophic bacteria found in cave environments are known as chemoautotrophs; they are able to obtain all the energy they need from the transformation of certain minerals into different ones.
Learn more about the important role that microorganisms play in a cave's energy-food chain.
Ideally a closed ecologic system with no organic input from the outside could exist in the dark zone of a cave. In this case, chemoautotrophs would derive the energy required for metabolism from cave minerals that are part of the walls, floors, and ceilings of caves.
However, most caves with established communities of cave organisms have heterotrophs (bacterial decomposers) that rely indirectly on sunlight. The heterotrophs decompose organic matter that is ultimately derived from sunlit areas. In caves, organic materials may come from waste material from cave-dwelling animals, dissolved matter of living organisms in drip water, or plant remains in cave streams. Organic material is also obtained from the fecal matter of animals that periodically spend time outside the cave, such as bats, cave rats, and crickets. Bacterial decomposers are eaten by protozoan that are eaten by aquatic cave-dwelling animals - such as flatworms, isopods, and amphipods - that are eaten by larger animals - such as crayfish, salamanders, and fish. Finally these aquatic animals release waste material that supports the bacterial decomposers that helped to initiate the chain. Now that's recycling!
Archaeologists have discovered cave drawings that are 30,000 years old in various parts of Europe and Asia. These drawings, typically of animals familiar to the artists, often display technical skill and artistic talent. The art emphasizes both hunted animals and dangerous animals, such as lions, bears, and rhinos. About 15,000 years ago, cave paintings began to display more refined skills and more colors. Archaeologists have also discovered clay statues and stone and ivory sculptures in these primitive art galleries.
Prehistoric paint was sometimes liquid and sometimes paste. Artists spread the paint with their fingers, with brushes made of reeds and hair, or with pads of moss. Their palettes were large flat bones. Sometimes they blew dry pigments onto the walls by means of hollow tubes. Their pigments were red or yellow iron oxide (ocher), burned bones, and black manganese minerals found in caves. The pigments were probably mixed with animal fat. In order to see in the dark while they painted, artists lit floor fires, pine torches, or stone lamps with marrow or fat for fuel. The lamps had wicks - perhaps made of moss - that produced a fairly bright light for several hours (Moore and Sullivan 1997).
The artists of these ancient cave paintings may have been camp tenders, people too old for hunting, or persons otherwise incapable of more active pursuits. Although we know how prehistoric artists made the colors for their pictures, we are not sure why they put their art in these cave chambers where no one lived. One likely reason is that these rooms were used for special ceremonies (Gee 1994). Maybe caves served as prehistoric museums? What do you think?
Going into caves for recreation is fun, but there are people who cave for science instead of just for sport. They are speleologists (from the Greek words spelaion, meaning caves, and logos, meaning study). Speleologists enjoy caves as much as anyone else, but probably have their noses pressed close to a speleothem, are keenly interested in a small fossil in the wall of the cave, or get very excited about some strange type of troglobite.
Most of the pioneers in the study of caves were European geologists and biologists. The chief efforts in the past were directed toward caves in Europe and North America. Today there are speleologists studying caves throughout the world.
Speleologists have provided us with most of our knowledge of caves: what they are and how they work. Some speleologists study cave animals; others study the geology of caves. There are speleologists who study underground rivers because of their importance to how caves form and to our drinking water. Other speleologists study tiny bacteria that live in caves, how speleothems form, ages of rocks in which caves form, or cave air and how it is exchanged with surface air.
If you want to become a speleologist, a degree program that focuses on mineralogy, geochemistry, and microbiology is a winning combination (Reed 2003).
People who explore caves are called cavers. They use to be referred to as spelunkers, but this is a word that is not used very much anymore by the people who really explore caves. Spelunker was a word made up using the Greek word "spelaion," meaning caves, and "lunker" ... well, no one is sure what a lunker is supposed to be. Anyway cavers just prefer to be called cavers!
Cavers are: truck drivers, doctors, teachers, scientists, roofers, construction workers, beekeepers, or lawyers. Just about any sort of person can be a caver. They can be tall or short, young or old, and live just about anywhere. Sometimes cavers travel many, many miles to explore caves in such remote places as Sarawak, Malaysia, or Irian Jaya, Indonesia, and live for months in tents deep in the jungle.
There are many reasons why people cave: the thrill of exploration, the enjoyment of taking photographs, or the satisfaction of mapping cave passages. There are even artists who take their paints and canvases into caves.
Most cavers enjoy the out of doors and seek out new caves to explore no matter where they may be located: in the mountains, deserts, or jungles. Most of them would rather be caving than doing anything else. After a good trip, cavers enjoy showing their photos and talking about the wonders they have seen. They are always thinking about their next caving trip.
Many caves throughout the world are filled with water. In the United States, Florida and Texas are known for their water-filled caves. There are also underwater caves in the Bahamas, Mexico, France, South Africa, and in the Canary Islands. Cave divers have explored and mapped miles of underwater passages worldwide; they have been down over 900 feet (275 m) deep in caves in Mexico.
When cavers want to explore these places they have to use scuba diving gear. As they swim through the caves, they unroll long reels of line so they can find their way back out of the cave. This kind of caving is very specialized and should never be attempted without a great deal of training. In underwater caves you cannot come straight up to the surface if you get into trouble. You must return the way you came in. Silt on the cave floor may be stirred up by the kicking of your fins and you will not be able to see at all, even with your lights on. If you lose your contact with your cave diving line you may loose your way and become lost and run out of air. Cave divers always stay in contact with their guide line and always pay attention to their air supply and the time shown on their underwater watches so that they will not become lost, stay too long in a cave, or run out of air. Experienced cave divers know that cave diving can be dangerous and always follow strict safety rules. The first rule you should know about is to leave cave diving to the experts.
Caves and Karst Index
Threats to Caves and Karst
Caves and Karst in National Parks
Challenge Your Understanding
Return to Views