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 A cave
is a natural opening in the ground extending beyond the zone of light and
large enough to permit the entry of man. Occurring in a wide variety of rock
types and caused by widely differing geological processes, caves range in
size from single small rooms to interconnecting passages many miles long. The
scientific study of caves is called speleology (from the Greek words spelaion
for cave and logos for study). It is a composite science based on geology,
hydrology, biology, and archaeology, and thus holds special interest for
earth scientists of the U.S. Geological Survey.
Caves have been natural
attractions since prehistoric times. Prolific evidence of early man's
interest has been discovered in caves scattered throughout the world.
Fragments of skeletons of some of the earliest manlike creatures
(Australopithecines) have been discovered in cave deposits in South Africa,
and the first evidence of primitive Neanderthal Man was found in a cave in
the Neander Valley of Germany. Cro-Magnon Man created his remarkable murals
on the walls of caves in southern France and northern Spain where he took
refuge more than 1O,OOO years ago during the chill of the ice age.
Interest in caves has
not dwindled. Although firm figures for cave visitors are not available, in
1974 about 1.5 million people toured Mammoth Cave in Kentucky, and more than
67O,OOO visited Carlsbad Caverns in New Mexico, two of the most famous caves
in the United States.
A simple classification
of caves includes four main types and several other relatively less important
types.
- Solution caves are formed in carbonat and
sulfate rocks such as limestone, dolomite, marble, and gypsum by the
action of slowly moving ground water that dissolves the rock to form
tunnels, irregular passages, and even large caverns along jointss and
bedding planes. Most of the caves in the world-as well as the
largest-are of this type.
- lava caves are tunnels or tubes in lava
formed when the outer surface of a lava flow cools and hardens while the
molten lava within continues to flow and eventually drains out through
the newly formed tube.
- Sea caves are formed by the constant
action of waves which attacks the weaker portions of rocks lining the
shores of oceans and large lakes. Such caves testify to the enormous
pressures exerted by waves and to the corrosive power of wave-carried
sand and gravel.
- glacier caves are formed by melt water
which excavates drainage tunnels through the ice. Of entirely different
origin and not to be included in the category of glacier caves are
so-called "ice caves," which usually are either solution caves
or lava caves within which ice forms and persists through all or most of
the year.
In desert areas, some
shallow caves may be formed by the sandblasting effect of silt or fine sand
being blown against a rock face. These eolian caves, some of which are
spectacular in size, are surpassed in number by caves of other origins in
most deserts. More common even in the driest deserts are sandstone caves
eroded in part by water, particularly if the sandstone is limy. Caves
commonly known as "wind caves," such as the one in Wind Cave
National Park in South Dakota, are named not for the mode of origin of the
cave but for the strong air currents that alternately blow in or out of the
cave as the atmospheric pressure changes. Most wind caves are, in fact,
solution caves.
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The melt-water streams
draining out along the floor of a glacier cave or the surging, pounding waves
at the mouth of a sea cave offer immediate evidence of the origin of these
caves. Solution caves, however, have always been a source of wonder to man.
How do these extensive, complex, and in some places beautifully decorated
passageways develop?
Solution caves are
formed in limestone and similar rocks by the action of water; they can be
thought of as part of a huge subterranean plumbing system. After a rain,
water seeps into cracks and pores of soil and rock and percolates beneath the
land surface. Eventually some of the water reaches a zone where all the
cracks and pores in the rock are already filled with water. The term water
table refers to the upper surface of this saturated zone. calcite (calcium carbonate),
the main mineral of limestone, is barely soluble in pure water. Rainwater,
however, absorbs some carbon dioxide as it passes through the atmosphere and
even more as it drains through soil and decaying vegetation. The water,
combining chemically with the carbon dioxide, forms a weak carbonic acid solution.
This acid slowly dissolves calcite, forms solution cavities, and excavates
passageways. The resulting calcium bicarbonate solution is carried off in the
underground drainage system.
It was once believed
that caves formed near the Earth's surface-above the saturated zone-where the
water moved downward through the cracks and pore spaces. This view, however,
left many cave features unexplained.
Why, for instance, are
cave passages nearly horizontal, in places crossing folded or tilted rock
structures? How would horizontal passages form at several different but
persistent levels? Recent studies of the movement and chemistry of ground
water have shown that the first stage in cave development-the dissolving of carbonate
rocks and the formation of cavities and passage-ways-takes place principally
just below the water table in the zone of saturation where continuous mass
movement of water occurs.
A second stage in cave
development occurs after a lowering of the water table (the water table
normally sinks as the river valleys deepen). During this stage, the solution
cavities are stranded in the unsaturated zone where air can enter. This leads
to the deposition of calcite, which forms a wide variety of dripstone
features.
The chemical process
causing deposition of calcite is the reverse of the process of solution.
Water in the unsaturated zone, which dissolved some calcite as it trickled
down through the limestone above the cave, is still enriched with carbon
dioxide when it reaches the ventilated cave. The carbon dioxide gas escapes
from the water (just as it escapes from an opened bottle of soda pop). The
acidity of the water is thereby reduced, the calcium bicarbonate cannot
remain in solution, and calcite is deposited as dripstone.
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Cave
Features
The decorative dripstone
features are called speleothems (from the Greek spelaion for cave and theme
for deposit). When these structures are highlighted by lanterns or electric
lights, they transform a cave into a natural wonderland.
The most familiar speleothems
are stalactites and stalagmites. stalactites hang downward from the ceiling
and are formed as drop after drop of water slowly trickles through cracks in
the cave roof. As each drop of water hangs from the ceiling, it loses carbon
dioxide and deposits a film of calcite. Successive drops add ring below ring,
the water dripping through the hollow center of the rings, until a pendant
cylinder forms. Tubular or "soda straw" stalactites grow in this
way; most are fragile and have the diameter of a drop of water, but some
reach a length of perhaps a yard or more. The large cone-shaped stalactites
begin as these fragile tubes and then enlarge to cones when enough water
accumulates to flow along the outside of the soda straws. Deposition of calcite
on the outside of the tubes, most of which are near the ceiling and taper
downward, results in the familiar cone shapes.
stalagmites grow upward
from the floor of the cave generally as a result of water dripping from
overhanging stalactites. A column forms when a stalactite and a stalagmite
grow until they join. A curtain or drapery begins to form on an inclined
ceiling when the drops of water trickle along a slope. Gradually a thin sheet
of calcite grows downward from the ceiling and hangs in decorative folds like
a drape. Sheets of calcite that are deposited on the walls or floor by
flowing water are called flowstone. Rimstone dams are raised fence-like
deposits of calcite on the cave floor that form around pools of water.
Helictites are curious
twisted or spiraling cylinders or needles. They apparently develop when water
seeps through the ceiling so slowly that slight chemical or physical changes
can cause reorientation of the crystal structure of the calcite or gypsum.
Cave corals, also formed by slowly seeping water, are small clusters of
individual knobs.
Most cave passages
contain deposits of material that have been washed into the cave. This
material, known as cave fill, varies from sand and clay to stratified gravel.
The pebbles in these deposits often are highly polished or frosted and
sometimes are as large as 6 inches in diameter. Cave fills are particularly
noteworthy because they contain materials that reflect a geologic history and
a record of past climates of the surrounding area.
Rock material produced
by the collapse of the ceiling or walls of a cave is called breakdown and may
range in size from plates and chips to massive blocks. Most breakdown present
in caves today appears to have occurred thousands of years ago. It is
generally associated with the early history of cave development.
The size and depth of
many caves in the United States are impressive. Seven caves have more than 15
passage miles. The longest is the Flint-Mammoth Cave system in
Kentucky with more than
169 miles. The other six are Jewel Cave in South Dakota (54.4 miles), Organ
Cave in West Virginia (32 miles), Wind Cave in South Dakota (28.7 miles),
Cumberland Caverns in Tennessee (23.2 miles), Sloan Valley Cave system in
Kentucky (22.4 miles), and Crevice Cave in Missouri (20.8 miles).
The deepest cave in the
United States is Neff Canyon in Utah. There, a depth of 1,189 feet below the
entrance is reached along a steeply sloping 1,700-foot passage. The second
deepest cave is Carlsbad Caverns in New Mexico; its lowest point is 1,022
feet below the entrance. Ellisons Cave system in Georgia, a close rival of
Carlsbad, has a depth of nearly 1,000 feet.
The largest cave room is
in Carlsbad Caverns, where the Big Room covers 14 acres. This room is 1,800
feet long and ranges up to 1,100 feet wide. The maximum height of the ceiling
is 225 feet. The size of the Big Room, the length of the caverns (14.9 miles,
the 11th longest in the United States), and the depth probably make Carlsbad
the biggest cave in the United States.
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Many interesting
minerals are found in caves in addition to the calcite which forms the major
features. Aragonite, a calcium carbonate mineral similar to calcite but not
as common, often occurs in intricate needles known as anthodites. Gypsum
(calcium sulfate) and related calcium sulfate minerals are next to calcite in
abundance. Some caves, although they are developed in limestone, have
extensive passages lined with fine, curling growths of gypsum flowers. In
other caves, selenite (a less common variety of gypsum) forms long
transparent rods or nests of fibrous crystals. Sulfates of sodium and
magnesium are also found in caves, although they are less conspicuous than
gypsum. Iron minerals in the form of oxides (limonite) and hydroxide
(goethite) occur in caves and in some places form stalactites. Manganese
minerals in caves are commonly present as thin, sooty coatings on walls and
ceilings and in earth fills. Nitrocalcite (calcium nitrate) is abundant in
earth fills in many caves, but individual fragments are generally
microscopic. Barite (barium sulfate) and celestite (strontium sulfate) also
occur in earth fills. In some solution caves, clay minerals exist in
relatively pure forms; these include the less common varieties attapulgite
and endellite.
In deep caves
encountered during mining operations, a number of ore minerals have been
found in the decorative wall draperies. Most common are azurite and malachite
(forms of copper carbonate). About 50 other minerals also have been reported
in cave deposits.
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Studies are underway in
Europe to extend the use of caves for domestic cold storage, air
conditioning, and water supply purposes. A large cave in southwestern
Virginia is used as a natural tunnel by the Southern Railway.
From the early 19th
century through the Civil War, caves in Kentucky, Tennessee, Virginia, West
Virginia, Alabama, Georgia, Arkansas, and Missouri were important sources of
nitrates, an essential ingredient of gun powder. Surface or near-surface
accumulations of nitrate salts form coatings on rock walls, fill cracks and
crevices, and mingle with cave earth. The origin of the nitrate salts is not
clearly understood, but the salts are believed to result from the action of
nitrifying bacteria on organic matter or humus. Although no accurate records
of production were kept, it has been estimated that over 15,000 tons of niter
earth (producing 200 tons of potassium nitrate) were removed from Mammoth
Cave in Kentucky between 1811 and 1814.
Caves have also been a
source of bat guano, a material mined as a phosphate fertilizer in the
Southern United States and Mexico. In general, the largest deposits have
occurred in limestone caves within the flight range of the Mexican
free-tailed bat.
Scientists value caves
as natural underground laboratories. Of paramount importance is the fact that
caves and other solution cavities in limestone have a direct bearing on the
underground water system. Cavernous limestone strata are among the most
productive aquifers (water-bearing beds) in the United States and are
therefore important sources of water. Because of this, U.S. Geological Survey
research programs concerned with limestone regions commonly include studies
of the path, rate of flow, amount, and quality of water circulating through
caves and hidden passageways.
Geological engineers and
others concerned with ground stability are aware that regions underlain by
cavernous limestone present special construction problems. Studies of the
subsurface conditions are especially important in areas of limestone and
gypsum because of the danger of ground failure and subsidence.
There are about 17,000
known caves in the United States. They occur in every State except Rhode
Island and Louisiana. About 125 caves have been opened to the public for
study and enjoyment. Of these, 15 are in national parks or monuments, and 30
are in State parks. The remainder are privately owned and operated. Most of
these caves are in the Appalachian Mountains, the Ozark Mountains, the Black
Hills, and the limestone regions of Kentucky, Tennessee, and Indiana.
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Caution!
Exploring newly
discovered or unattended caves can be extremely dangerous! Through
experience, a set of safety rules has evolved that-if observed-may prevent
accidents. If you plan to go cave exploring:
- Always tell
someone where you are going and when you can be expected to return;
obtain permission from the owner of the cave for the visit.
- Respect gates,
whether they are in the field or at the cave entrance.
- Never enter a
cave alone.
- Always carry
several sources of light; do not depend solely on flashlights.
- Make sure you
have proper equipment in good working condition.
- Never go beyond
your physical and technical capabilities.
- For the sake of
conservation, keep visits to a minimum.
- Better yet, meet
with knowledgeable and experienced cavers. Association with a group of
experienced spelunkers is the best safety insurance that you can have.
Caves are natural
features and should be protected, but many have been vandalized by careless
visitors or damaged by poorly planned commercial development. Some caves have
been stripped of speleothems which took thousands of years to form and in
many places will not form again. All should try to prevent this random destruction
of these natural wonderlands. Follow the footprints of others; look but don't
touch; bring away only photographs; leave no evidence of your visit.
- Folsom, Franklin,
1962, Exploring American Caves-Their History, Geology, Lore, and
Location; A Spelunker's Guide (rev. ed.): Collier Books, New York, 319
p.
- Harrison, D. L.,
1970, The World of American Caves: Reilly and Lee Books (Division of
Henry Regnery Co.), Chicago, Illinois, 152 p.
- Mohr, C. E., and
Sloane, H. N., eds., 1955, Celebrated American Caves: Rutgers University
Press, New Brunswick, New Jersey, 339 p. Descriptions of about 20 caves
or cave areas.
- Moore, G. W., and
Sullivan, G.N., 1978, Speleology-The Study of Caves: Zephyrus Press,
Teaneck, New Jersey, 2nd edition, 150 p.
- Sloane, H. N.,
and Gurnees, R. H., 1966, Visiting American Caves: Crown Publishers,
Inc., New York, 246 p. Listing and data on commercial caves.
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