Caves and Caverns

One of the most memorable geology field trip I've made was a visit that several classmates and I made to Carlsbad Caverns National Park. We hiked into the caves, descending deeper into the earth as millions of Mexican free-tail bats clung to the ceiling. As we reached the 'bottom', the lowest point in the cave accessible to visitors, a ranger had us stop and find places to stand. Once the path was closed ahead of us, and behind, and we were all ready, the ranger cut the electricity. Immediately we were in darkness so black you could not see your hand inches from your face. After a minute or so, the lights were turned on, and we continued our trip through the caverns; but the sense of absolute darkness that I experienced has never left me.

Caves and caverns have long been a source of wonder, amazement, lodging, and even have served as art galleries to humans for hundreds of thousands of years. No one can say when the first modern human ventured into a cave, but we know from archeological evidence that caves were often central to early human societies. They were places to bury the dead and to paint pictures whose meanings have been lost to time. Today, most of us visit caves to explore a part of our world and be fascinated by the natural beauty they contain. But caves continue to be used for other purposes, usually as storage for all manner of material and wastes.

Natural caves and caverns usually form in areas with large deposits of limestone. The limestone is easily eroded by the acidic ground water, which widens the naturally occurring joints, fractures, and bedding planes. (See my article: Weathering Part 1: Physical Weathering for more on how water works to erode the rocks around us.) As the water flows through the limestone it dissolves some of the calcite, the main mineral in limestone, widening the fractures. Over time the open spaces formed by the widening fractures connect creating large cave systems.

Caves are not only places for erosion. Deposition often occurs at the same rate that the limestone is eroded. Deposits are of calcite (and a few other minerals) and build up deposits called dripstone. These deposits are formed in the reverse of the chemical process that originally created the cave. As water flows through a cave, or as it drips off the ceiling, some of the dissolved carbon dioxide in the water is released into the air of the cave. The loss of the carbon dioxide makes the water more saturated in calcite, and as the water drips, some of the calcite is left behind. When the water drops hit the cave floor, more carbon dioxide is lost, causing more calcite to be deposited. This process of deposition forms the features most of us associate with caves, stalactites and stalagmites. Stalactites hang from the ceilings, usually resembling icicles in form. Stalagmites are cone-shaped deposits that form on the floor, usually directly below a stalactite. Over time, with a continued source of water, the stalactite will continue to grow downward, and the stalagmite will grow upward, eventually joining together to form a column. There have been hundreds of names applied to many of the dripstone deposits found in individual cave systems. Names like soda straw and drapery are commonly used to describe different dripstone deposits. In some cave systems water will flow across the cave floor, depositing calcite in the same manner. These calcite deposits are often ribbon-like or sheetlike, and are commonly referred to as flowstone deposits.

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