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تاريخ التسجيل : 26/08/2011
|Geology of Caves|| |
What is a cave?[/size]
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
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.
- 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
[b] 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
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
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.2]
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[/size][/b]