Underground:
What and Who are in Caves?
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.
Openings - Crawlways,
Chimneys, and Chambers
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.
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
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.
There are
not nearly as many cave balloons in the caves in the
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
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).
Coatings
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
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.
Coralloids
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.
Draperies
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.
Cave Flowers
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
"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
Flowstone
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
Frostwork
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
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
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
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
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
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
Stalactites are the most common and most familiar of
all speleothems; they resemble icicles or carrots
hanging from cave ceilings. 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. Stalactites range in size
from small, slender, soda straws to thick, massive pendants tens of feet
(meters) long and wide.
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
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!
Cave Animals
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.
Types of Cave Animals
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
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
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).
Troglobites
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.
Segregation of Cave
Animals
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 |
|
Ceiling |
Mosquitoes |
Crane flies |
Bats |
|
Walls |
Centipedes |
Harvestmen (daddy longlegs) |
Harvestmen (daddy longlegs) |
|
Floor |
Cave rats |
Beetles |
Beetles |
Microorganisms
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.
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!
Cave Art
Archaeologists
have discovered cave drawings that are 30,000 years old in various parts of
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?
Speleologists
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
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).
Cavers
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
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.
Water and Cave Divers
Many caves
throughout the world are filled with water. In the
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.