Evidence of a Sulfuric Acid Origin for Lehman Caves

This article was originally published in The Midden – Great Basin National Park: Vol. 20, No. 1, Summer 2020.

Map of Lehman Caves showing the acid pool basin in the Gypsum Annex.

by Harvey DuChene and Louise Hose, Cave Geologists

Lehman Caves is one of the key attractions in Great Basin National Park. In 1885 Absalom Lehman rediscovered the cave and made it into a show cave. Since at least 1960, the cave has been the subject of geologic investigation. Our knowledge of how caves are formed has come a long way since 1960, and we now know that there are multiple ways that caves can develop. The most common caves are epigenic, which means that they form near the surface of the earth as a result of limestone dissolution by carbonic acid. These caves are part of regional drainage systems. A great example is Mammoth Cave in Kentucky, and there are many thousands of similar caves in karst regions throughout the world. More than a dozen caves in Great Basin National Park are epigenic.

In the 1970s, researchers in Carlsbad Caverns National Park realized that caves of the Guadalupe Mountains did not fit the epigenic model that had been established for Mammoth Cave. They discovered that most caves in the Guadalupe Mountains were formed by processes that took place inside the earth and were not connected to surface drainage systems.

Figure 2. Acid pool basin in Gypsum Annex of Lehman Caves. Features characteristic of acid pool basins are rillenkarren, pseudo-scallops, an incised water line, the overhung wall, and the pool basin.

Dave Bunnell

These caves are hypogenic, which means that they were formed by aggressive (acidic) water rising from deep-seated sources. They also found evidence of a significant chemical reaction mostly in the form of massive deposits of gypsum, a mineral that forms as a byproduct of the dissolution of limestone by sulfuric acid. Pioneering work by geologists Stephen Egemeier in the Kane Caves of Wyoming and Carol Hill in Carlsbad Cavern led to the sulfuric acid theory of cave formation and the subsequent recognition of similar caves all around the world. One place that contains evidence of development by sulfuric acid is Lehman Caves. Lehman Caves, at least in the Gypsum Annex (Fig. 1), displays evidence that it was formed when sulfuric acid dissolved marble in the Pole Canyon Limestone along natural fractures, enlarging them and creating cave passages. Gypsum, the byproduct of the reaction between sulfuric acid and limestone (or marble), has mostly been removed from the known parts of Lehman Cave. However, there are telltale signs that gypsum was once present. These signs include an array of bedrock features (speleogens) that formed when the sulfuric acid event was happening several million years ago. These speleogens include rillenkarren, pseudo-scallops, incised water lines or notches, and overhung pool margins (Fig. 2). Taken together, these features are indicators that there was once a pool of water that contained dilute sulfuric acid. We call these ancient pool sites acid pool basins.

This is an excerpt from a longer article. To read more, please visit this Park webpage