Lehman Caves Geology Part III

This article was originally published in The Midden – Great Basin National Park: Vol. 19, No. 1, Summer 2019.

Internal layers exposed by condensation corrosion in a Lehman “turnip” stalactite.

NPS Photo by G. Baker

by Louise D. Hose, cave geologist

The following article is excerpted from a paper I prepared for the Great Basin National Park staff on the Geologic Story of Lehman Caves. In the last two issues, I wrote about Stage 1, the Sulfide-rich, Hypogenic Speleogenesis time and Stage 2, Secondary Deposits and Finding Stability in a New Environment. In this issue, I talk about Stages 3, Calcite Speleothems and the Pleistocene, and 4, Condensation-Corrosion Speleogenesis.

Stage 3 – Secondary Deposits – Calcite Speleothems and the Pleistocene
The most notable features in the cave to the average visitor are the abundant calcite speleothems, which appear to have formed almost entirely in the Pleistocene between 2.2 million and 7700 years ago. A couple dozen stalagmites have been dated and the majority grew between 125,000 and 250,000 years ago. The oldest date, however, is 2.2 million years old (Lachniet and Crotty 2017).

Eagle’s Wing in the Lodge Room provides a good example of a calcite speleothem corroded by acidic condensation.

NPS Photo by G. Baker

Stage 4 – Condensation-Corrosion Speleogenesis
Following the Pleistocene, the surface conditions dried and, probably after the natural entrance connected the cave to the surface allowing greater exchange of surface air with the cave air, condensation corrosion due to carbonic acid became the dominant cave process. It is likely still actively and slowly altering the cave’s appearance. Water vapor in the cave has picked up CO2 from the air then condensed on the walls, ceilings, and calcite speleothems as a film of carbonic acid. This moisture etches the marble bedrock or calcite speleothems. Condensation corrosion can preferentially etch some portions of a speleothem and reveal the internal crystal structure.

Dissolution along individual grain boundary in the marble bedrock produces disintegrated sandy or fluffy residue of mostly calcite on the walls and ceilings of the cave. This “punk” rock is soft, flaky, typically beige/tan or gray, and lines much of the walls and ceiling along the Lehman trails where dripstone and flowstone does not cover the bedrock.

Some early explorers recognized the soft nature of the rock and left inscriptions scratched into it, forsaking the need for carving tools or soot. The powdery material also sloughs off the walls and ceilings and accumulates onto the cave floor, similar to the process of the gypsum paste falling onto the floor when the cave first formed.

Condensate water also enters pores and fractures in the bedrock and speleothems or forms thin films of water and carries its dissolved content along the surface. As this water evaporates or temperatures change slightly, various speleothems may form. Much or most of the popcorn and aragonite frostwork in the cave likely formed through mechanisms related to condensation water. This manner of evaporation-and-condensation is most common in caves where circulation of air between entrances is limited and where there is little running water. It is typical of hypogenic caves in dry climates, both active and inactive. Lehman Caves certainly matches these descriptions.