DISSOLUTION OF LIMESTONE

   The solubility of calcite (and hence of limestone) in pure water is very low, but is vastly increased in the presence of carbon dioxide. This gas, dissolved in the water to produce carbonic acid, permits dissociation of calcium carbonate, and dissolution rates and loads are therefore directly related to carbon dioxide content. This accounts for the importance to limestone dissolution of plant growth; soil water contains greatly more carbon dioxide than stream waters. Further dissolution occurs due to mixing of saturated waters of different carbon dioxide content (see Mischungskorrosion), because of a nonlinear relationship between carbonate saturation and carbon dioxide content.

   This process is of major significance to continued dissolution within the phreas. Cold water can dissolve more carbon dioxide but, with respect to cave development, this climatic factor is overwhelmed by the higher organic activity producing more carbon dioxide in warmer environments. Loss of carbon dioxide, by diffusion into open air, causes water to precipitate calcite as speleothems. Limestone dissolution may also be achieved by organic acids or by strong acids, particularly sulphuric acid, though such effects are normally far less than that of carbon dioxide. Strong acid dissolution is probably involved in the inception of most underground drainage. Dissolution by sulphuric acid formed by oxidation of sulfide minerals or gases may be a major cave-forming process in some regions, and was largely responsible for the enlargement of Carlsbad Caverns and Lechuguilla Cave, New Mexico [9].