Bulletin of the National Speleological Society - ISSN 0146-9517
Volume 24 Part 2: 95-106 - July 1962

A publication of the National Speleological Society

The Growth of Stalactites
George W. Moore

Abstract

Cave water is slightly acidic in the summer and slightly alkaline in the winter. Also, the carbon dioxide content of cave air is highest in the summer. These annual fluctuations result from seasonal variations in the production of carbon dioxide by microorganisms in the soil. Limestone is dissolved at the base of the soil zone in water rendered acidic by dissolved carbon dioxide and is deposited again when the soil water migrates downward into a cave and encounters the relatively low carbon dioxide content of the cave atmosphere. But the dissolving of the limestone is a slow process relative to the capacity of the water to dissolve. When the percolation of the water is also slow, stalactite growth is favored by the high concentrations of carbon dioxide produced in warm climates and during the warm season. When the percolation is fast under these conditions, however, the water will not be supersaturated when it reaches the stalactite, and growth cannot occur.

Water dripping from the tip of a stalactite deposits a central cylinder with vertical crystal orientation, whereas water flowing down the sides deposits radiating crystals. Stalactites on concrete structures have repeatedly given a false notion of the growth rate of cave stalactites, but radiocarbon dating, actual measurements, and study of annual growth increments show that the rate of elongation, though variable, averages about a quarter of a millimeter a year.

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