The Geology of Solution Caves
How Solution 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 the cracks and pores of the soil
and rocks and percolates deep into the ground. 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 saturation 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 the soil and decaying vegetation. The water, combining
chemically with the carbon dioxide, forms a weak carbonic acid solution.
This acid slowly dissolves the calcite, forming solution cavities, and
excavating passageways. The resulting calcium bicarbonate solution is carried
off in the underground drainage system.
It was once beleived that caves formed near the Earth's surface - above
the saturation 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
of ground water have shown that the first stage in cave development - the
dissolving of carbonate rocks and the formation of cavities and passageways
- take 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 valley deepens). 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 open bottle of soda pop). The acidity of the water is thereby reduced,
the calcium bicarbonate cannot remain in solution, and the calcite is deposited
as dripstone.
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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