Minerals From The Darkness Below

by

Kevin Faurot

Introduction

Basically, caves are a natural hole in the ground. Not just something like an ant hole but something that has rock formations underground. They take many years to form and form for many different reasons. The study of caves is known as speleology, which is greek for cave (Davies and Morgan, 2000). The following page was created as a project for a mineralogy course and will give you a view of some of these so called holes and inform you on how they are formed. Also, different minerals found in caves are listed and linked to sites so you can see what they might look like.

Table of Contents

Formation of caves Carlsbad Caverns Image Cave Minerals References and Links

Soda Straws as seen from below at Carlsbad Caverns.

Image taken from:
http://www.nps.gov/cave/tours/lower/straws.htm.

Formations of Caves

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, but solution caves have always been a source of wonder to humankind (Davies and Morgan, 2000). How do these extensive, complex, and beautifully decorated passageways develop?

Solution caves are formed in limestone and similar rocks by the action of water and can be thought of as part of a huge subterranean plumbing system (Davies and Morgan, 2000). 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 (Davies and Morgan, 2000).

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. However, rainwater 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 (Davies and Morgan, 2000).

This view left many cave features unexplained though. 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 (Davies and Morgan, 2000).

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 (Davies and Morgan, 2000).

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Carlsbad Caverns Image

Lake Chandalar. Just under seven feet deep.
Image taken from:
http://www.pbs.org/wgbh/nova/caves/
jewe_03.html#fea_top

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Cave Minerals

Many different minerals are found in caves. Some of these are very common, some rare. Images of some of the common minerals found in caves can be viewed by clicking on the underlined mineral names below. (These images are found at various external sites. Hit the back button on the menu at the top of the page to return here each time.)

Aragonite	Attapulgite
Azurite	Barite
Calcite	Celestite
Endellite	Goethite (Stalactite Formation)
Gypsum	Limonite (Stalactite Formation)
Magnesium	Malachite
Manganese	Nitrocalcite
Selenite	Halite (Sodium)

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Bibliography

Davies, W. E. and Morgan, I. M. September 5, 2000. Geology of caves. World Wide Web URL: http://wrgis.wr.usgs.gov/docs/usgsnps/cave/cave.html. US Geological Survey Western Earth Surface Processes Team and the National Park Service.

The National Park Service. July 12, 2002. Carlsbad caverns national park. World Wide Web URL: http://www.nps.gov/cave/galleries.htm. National Park Service.

Public Broadcasting Service. 2002. The Mysterious Life of Caves. http://www.pbs.org/wgbh/nova/caves/, from Nova and PBS.