Cave Geology
Introduction
A cave is a natural opening in the ground extending beyond the zone of
light and large enough to permit the entry of man. Occuring 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 archeaeology,
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 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 10,000 years ago during the chill of the last 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 670,000 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 significant types.
-
Solution
Caves are formed rock which is rich in carbonate and sulfate
minerals
such as limestone, dolomite, marble, and gypsum by the action of slowly
moving ground water that dissolves the rock to form features
such as 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.
-
Glacier Caves are formed by melt water which excavates drainage
tunnels through ice.
-
Lava Caves are tunnels or tubes in lava formed when the outer surface
of a lava flow cools and hardens while 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 attack
the weaker portion of rocks lining the shores of oceans and large lakes.
Such caves testify to the enormous pressures exerted by waves and the corrosive
power of wave-carried sand and gravel.
-
Tectonic Caves are formed by fault movement or hillside cleaving
due to erosion of the underlaying rock. Fractures occur in the rock layers
and slip apart leaving rooms and passages. These caves tend to be small.
-
Talus Caves are formed when rocks pile up at the base of a hill
or cliff. Natural tunnels are formed by when water washes out the dirt
and smaller debris that collected as the pile was formed. These caves tend
to be very small and unstable.
-
Eolian Caves are formed in desert areas by the sandblasting effect
of silt or fine sand being blown against a rock face. Some of these caves
are spectacular in size but 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.
-
Wind caves are not named for the mode of origin but for the
strong air currents that blow in or out of the cave as the atmospheric
pressure changes.
-
Ice caves are not so much a type of cave as a description
of the conditions inside. These caves are usually either solution caves
or lava tubes in which ice forms and persists thoughout all or most of
the year.
Also see: [Solution
Caves] [Cave
Features] [Cave
Minerals]
Source material quoted from:
U.S. Department of the Interior / U.S. Geological Survey publication
Geology of Caves ISBN 0-16-036026-9
Written by: W. E. Davies and I. M. Morgan
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