What is a cave?
 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,000 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,000 visited
Carlsbad Caverns in New Mexico, two of the most famous caves in the United
States.
Types of Caves
A simple classification
of caves includes four main types and several other relatively less important
types.
- Solution caves are formed in carbonate 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 joints 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.
How Caves Form
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.
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.
Minerals Found in Caves
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.
Uses of Caves
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.
Geological Survey Investigations
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.
Exploring Caves
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.
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. |
