Karst Topography
Caves comprise a distinct landscape formation whose features attract both tourists and scientists alike. To begin with, four major components are needed in order for cave formation to occur: an underground limestone deposit, a fault which has fractured and broken up in portions of the limestone, high concentrations of carbon dioxide in the soil from living and decaying plants, and a steady flow of groundwater through the rock. Once all of these conditions are met, two different stages of cave formation take place.
The first stage is characterized by erosion and occurs as groundwater absorbs high concentrations of carbon dioxide as it moves through the rock, resulting in the creation of carbonic acid. As carbonic acid moves through the fractured limestone, it removes the calcium carbonate, or calcite, and enlarges the cracks until a cavern forms and eventually an opening at the surface is created. Once outside air enters the cavern, the carbon dioxide content of the water is reduced to normal levels and erosion ceases.
The cave then enters the second stage of formation where the deposition of calcite occurs, resulting in the numerous cave decorations or features that can be seen today. These decorations generally are categorized as either stalactites or stalagmites, but within each broad category numerous variations exist.
Stalactites
Stalactites are icicle-shaped deposits hanging from the roof of a cave which form from the deposition of calcium carbonate that is precipitated from mineralized water solutions.
Soda Straws
Long, narrow stalactites with a hollow middle are called "soda straws." When mineral-laden drops of water fall from the cave ceiling and make contact with the cave air, the minerals crystalize around the water droplet, deposting a thin ring of calcite while the water droplet falls to the cave floor. As water droplets continue to form and fall, additional calcite rings are deposited and form the hollow tube characteristic of the soda straw. Soda straws are the diameter of the water droplet and are very fragile. If the hollow center becomes plugged with debris, the water will begin flowing over the outside of the soda straw and will deposit the calcite in additional layers and create a more familiar cone-shaped stalactite. Because these deposits are formed by dripping water, the drip rate is particularly important. The formation of soda straws requires a slow drip rate so that the mineral crystals are able to stick together and grow.
Above, view of soda straw formations in Crystal Cave. Note the water droplets forming at the tips of these stalactites.
At left, a close up view of a mineralized water droplet at the base of a soda straw. A pen is included for scale. At right, more soda straw deposits with a field notebook shown for scale.
Stalagmites
Stalagmites are conical mineral deposits, usually composed of calcite, that are formed on the cave floor by the dripping of mineral-rich water solutions. These formations are the corresponding features found beneath stalactites. When water drips from the ceiling, not all minerals are deposited in the form of stalactities, but rather, some minerals remain in the water droplet. When the water droplet falls to the cave floor, any remaining minerals are deposited. Over time, the mineral deposits grow to form stalagmites which rise from the cave floor.
The image at left highlights an area of Crystal Cave where stalactites and stalagmites
are growing together. At right is a close up view of the column formation.
Flowstone
Flowstone is another feature commonly found in caves and is quite prevalent throughout Crystal Cave. Flowstone is a layered, sheet-like deposit of calcite that is formed where water flows down the walls or floors of a cave. Usually whitish in color and somewhat translucent, flowstone may contain beige or brownish layers which indicate the presence of iron impurities. In general, changes in the coloration indicate slight changes in the mineral composition of the water as depostition occurs. That is, different concentrations of certain minerals in the water will produce variations in the coloration of the flowstone.
In some cases, flowstone may also be referred to as "drapery" as the calcite deposits begin to overhang portions of the cave walls. The term "bacon" may also be used to describe these deposits especially when brown layers are present in the whitish flowstone. Flowstone forms very slowly, growing an average of only 1 inch every 100 years and is damaged easily by the oil from human fingers. Water will avoid areas that have been touched and calcite will no longer be deposited in these areas, thus hindering the growth of the formation.
Images of flowstone seen in Crystal Cave.
Influences on the presence and growth of cave features
The growth rate of formations and features within a cave is difficult to measure and size is not an indication of age. Many factors play a role in cave growth such as temperature, air movement, and the concentration of minerals in the earth above the cave. These factors in turn influence the presence of stalactites, stalagmites, and flowstone found within the cave. All of the deposits mentioned here are formed by the presence of dripping water, which therefore also makes the drip rate an important factor in the growth of these features. Finally, with the increasing interest and exploration of caves, humans also impact the cave formations. Artificial lights, dust, litter, and touching the formations all hinder the natural cave evolution.
