A limestone cave is a natural wonder. Visitors who know nothing about geology marvel at the stalactites hanging from these caves, and at the stalagmites jutting up from the floor to meet them – a weird, spiky, phantom chamber of white rock buried deep under the earth. The wonder is only increased after learning the geology behind them.
It is amazing because carbonic acid, identified as the agent at work, is a very weak acid indeed. Not only is it weak but the amount of it found in rainwater, that which carries the corrosive agents that forms these caves, is very small too. To say that the rainwater seeping into limestone beds carves out magnificent caves systems is like saying Michelangelo used a feather to carve out his sculpted masterpiece David. It would have taken a very long time, and speleologists (those who study caves) estimate that limestone caves are formed over millions of years.
There is a neat school experiment demonstrating stalactites in formation even while watching, which affords a great illustration of what is happening in the formation these caves. Limestone is a far weaker base than the baking soda used in the experiment. Carbonic acid in rain water dissolves the limestone, which is the principal means by which limestone caves are formed.
The first requisite is a bed of limestone rock, which may be deeply buried beneath the earth’s surface. Water from rainfall may seep deep down into the layer through fault lines in the bed. These openings may eventually turn into cataracts, or even underground waterfalls due to the corrosive action of carbonic acid broadening the passage. The slow corrosive action may also work its way horizontally into the main bed of limestone giving rise to cave chambers. These chambers may extend all the way underground until a opening to the surface is located, where either a spring or a cave mouth is formed.
The process begins with the dissolving of carbon dioxide in rainwater. Carbon dioxide only forms 0.03 percent of the atmosphere, and neither is it readily soluble in water, which gives a measure of the minuscule proportion of carbonic acid in rainwater.
The corrosive action is only part of the story. Once corrosion has opened up a large enough opening for water to flow, mud and silt carried along with the water play a large role is the process of carving. Eventually the openings are wide enough so that the underground streams may be called rivers, at which stage the caves propagate much faster.
Stalactites and stalagmites only form in those chambers that have become deprived of the main flow. In other words, a deeper chamber has started to receive the main flow and is turning into a proper cave. The deprived chambers are now only receiving a dripping flow of water. Limestone starts to be deposited at the point of regular drip. After steady build-up and accumulation the stalactite formation materializes. Deposit also accumulates at the point of impact on the chamber floor where stalagmites accrue. Eventually both formations meet at a point and become columns. So it appears that these spectacular chambers, the highlight of a spelunking trip, are merely a sideshow to the actual formation of limestone caves.
