The Formation of Caves

The process of forming caves in soluble rock is very slow. It all begins with rain. As rain falls through the atmosphere, it absorbs a small amount of carbon dioxide. It gathers additional carbon dioxide as it moves through the soil. Water mixed with carbon dioxide is weak carbonic acid solution. As this solution of water and carbon dioxide seeps through the cracks and crevices, it dissolves the soluble rock and forms cavities and channels as it moves downward and laterally. After thousands of years of solution, underground rooms and chambers can be formed.

Not all cave passages in soluble rocks form in precisely the same way. The following is a discussion of three important modes of cave development:

(1) In many caves, underground streams have been very important. Water enters the cave system through sinkholes, losing streams, and other points on the surface. Most of the large springs of Kentucky are examples of such cave systems; of course, they are still filled with water.

(2) Quiet water solution - Sometimes water moves very slowly underground, rather than rushing along underground streams. Many of the caves formed this way are mazes of passages with many interconnections.

(3) Domes and pits can be formed by vertical solution. Water flowing vertically down cracks can develop passages almost like elevator shafts, except that the passages are circular or oval.

Cave Formations (Speleothems):

As rain water falls through the air, it picks up carbon dioxide. As the water soaks through the soil, it picks up more carbon dioxide which is being given off by decaying vegetation in the soil. Water and carbon dioxide form a mild acid, carbonic acid, which is the same as the fizz in a coke. As this acid seeps through limestone, it dissolves the calcium carbonate out of the limestone. This dissolved mineral is dripped into the cave, where the carbon dioxide is given off. The water will no longer carry the minerals, so they are deposited in the shape of tiny crystals. These crystals are known as calcite, or onyx.

Sometimes calcite is deposited as the drips hang from the ceiling of the cave, forming a stalactite. Other times the drops of water are still full of minerals as they drop to the cave floor and build up stalagmites. Beads of moisture sometimes deposit onyx on the cave walls in rough-looking clusters called cave coral. Water flowing slowly down the walls or across an incline in the cave form smooth looking onyx known as flowstone. Onyx formations can take many different shapes, depending on the way the water comes into the cave.

Karst Terrain:

A karst terrain is one in which solution, or dissolving of limestone, dolomite, gypsum or marble by water, plays a major role in land erosion. Karst areas are often characterized by a large number of caves, springs, sinkholes, and surface streams which disappear underground. Much like a piece of Swiss cheese coated with soil, central Kentucky contains these features and the unique environmental problems associated with them. All of these different types of water conduits contribute to the recharge of the groundwater system in the area.

There are two basic ways in which waters from the surface recharge groundwater supplies in karst areas. These are: (1) discrete recharge, and (2) diffuse recharge. Discrete recharge is very localized, and often involves large quantities of water which move rapidly into groundwater supplies. Storm water streams disappearing underground into sinkholes are an example of discrete recharge. Discrete recharge typically provides ineffective natural filtration and cleansing for water.

In contrast, diffuse recharge involves slower moving waters which seep through soil and rocks in route to groundwater supplies.