Stalactite

Formation and type

Stalactites are formed by the deposition of calcium carbonate and other minerals, which is precipitated from mineralized water solutions. Limestone is the chief form of calcium carbonate rock which is dissolved by water that contains carbon dioxide, forming a calcium bicarbonate solution in underground caverns. The chemical formula for this reaction is:

Every stalactite begins with a single mineral-laden drop of water. When the drop falls, it leaves behind the thinnest ring of calcite. Each subsequent drop that forms and falls deposits another calcite ring. Eventually, these rings form a very narrow (0.5 mm), hollow tube commonly known as a "soda straw" stalactite. Soda straws can grow quite long, but are very fragile. If they become plugged by debris, water begins flowing over the outside, depositing more calcite and creating the more familiar cone-shaped stalactite. The same water drops that fall from the tip of a stalactite deposit more calcite on the floor below, eventually resulting in a rounded or cone-shaped stalagmite. Unlike stalactites, stalagmites never start out as hollow "soda straws." Given enough time, these formations can meet and fuse to create columns.

Stalactites can also form in lava tubes, although the mechanism of formation is very different.

Concrete

Stalactites can also form on concrete, and on plumbing where there is a slow leak and limestone (or other minerals) is in the water supply, although they form much more rapidly there than in the natural cave environment (description and experiments see literature).

The way stalactites form on concrete is due to different chemistry than those that form naturally in limestone caves and is the result of the presence of calcium oxide in concrete. This calcium oxide reacts with any rainwater that penetrates the concrete and forms a solution of calcium hydroxide. The chemical formula for this is:

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Over time this calcium hydroxide solution reaches the edge of the concrete and, if the concrete is suspended in the air, for example, in a ceiling or a beam, then this will drip down from the edge. When this happens the solution comes into contact with air and another chemical reaction takes place. The solution reacts with carbon dioxide in the air and precipitates calcium carbonate.

Origin of the term

Photo gallery

See also

References

External links

Category:Speleothems