The NSS Bulletin - ISSN 1090-6924
Volume 43 Number 2: 31-51 - April 1981

A publication of the National Speleological Society

Cavern Development by Thermal Waters
Stephen J. Egemeier


Several caves in Wyoming are forming by a process involving the replacement of limstone by gypsum and then solution of the gypsum. Artesian springs discharge thermal waters containing dissolved hydrogen sulfide into the caves. Atmospheric oxygen in the cave air dissolves into the spring waters and reacts with some of the hydrogen sulfide, producing sulfuric acid. The sulfuric acid reacts with limestone in the stream beds and dissolves it.

Much of the hydrogen sulfide in the spring waters escapes into the cave air. Some of it redissolves in water on the damp cave walls and ceiling where it is oxidized by dissolved oxygen in the water. Sulfur and sulfuric acid are produced. The acid attacks limestone and converts it to gypsum. Gradually a coating of gypsum up to half a meter thick forms on the cave walls and ceiling. Eventually the coating becomes so thick that it cannot support its own weight. As a result, some of the gypsum falls to the cave floor, where it is dissolved and removed by the cave stream. Passages are air-filled during cavern development. Caves that have formed by this process of "replacement-solution" are found in many states in the western United States.

The thermal springs in the cave are depositing metals in hydrogen sulfide-rich muds lining the springs. Small deposits of vanadium and uranium are are being formed perhaps in a manner similar to the process that formed the economic deposits of these metals in nearby caves in the Big Horn and Pryor mountains. Other metals deposited include silver, copper, rion, lead, and zinc.

The history and development of some caves will have to be re-studied, as all previous work was done without knowledge of the replacement-solution process. Gypsum crusts, dypsum deposits, and "phreatic" cavern features can be formed by this process in air-filled "vadose" caves.

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