| MadSci Network: Earth Sciences |
Dear Mikeyo, Stalactites are features of limestone caves and caverns. They hang from the cavern ceiling and resemble icicles. They are made of a mineral called calcium carbonate. Limestone itself is also made of calcium carbonate, and it is the properties of calcium carbonate that explain both the existence of the cavern and the presence of stalactites (and also stalagmites, which grow up from the cavern floor). Calcium carbonate dissolves easily in water, especially water that is slightly acidic. Rainwater is slightly acidic because rain dissolves the gas carbon dioxide from the atmosphere and becomes a very weak acid called carbonic acid. When rain falls, a proportion of the water soaks into the soil and down into the rocks to form groundwater. Because the rainwater is weak carbonic acid, the groundwater is too. If it soaks into limestone it will dissolve calcium carbonate from the rock, so that the groundwater percolating down carries dissolved calcium carbonate with it. On a large scale and over millions of years, groundwater may dissolve so much of the limestone that cavities, eventually linking up to form caves and caverns, will be formed underground. If a cavern exists, groundwater seeping down will eventually reach the roof and drip through into the cavern. Because the groundwater contains dissolved calcium carbonate, as each drop of water drips off the ceiling it will leave behind a tiny deposit of calcium carbonate. As you can imagine, given enough time and enough dripping water these tiny deposits will build up to form a stalactite, which slowly grows down from the ceiling. Large stalactites can take many thousands of years to form. Likewise, when the drop of water hits the floor of the cavern it will deposit any remaining calcium carbonate it contains, and slowly stalagmites will grow upward. This is the process that occurs in nature. But I have seen stalactites, probably also made of calcium carbonate, growing on the underside of a cement sidewalk on a riverbank. These were a few inches long and took just a few years to form, as they cannot be older than the sidewalk! Cement contains calcium carbonate, and in this case rainwater percolating over the sidewalk has dissolved some of the calcium carbonate and redeposited it as it dripped off the bottom to form these little stalactites. There are many websites promoting commercial caverns with beautiful pictures of stalactites and stalagmites. One of the most famous is Luray Caverns in Virginia; have a look at http://www.luraycaverns.com/ If you click on the Classroom/History link on the home page you will find a good description of how Luray Caverns formed, including the process that forms stalactites and stalagmites. I hope this answers your question. Best wishes, David Scarboro
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