Lehman Caves Geology, Part 3: Ice-Age Formations and Acid Air

Written from an article by Louise D. Hose, cave geologist, for The Midden (Great Basin National Park), Vol. 19, No. 1, Summer 2019 — the third part of her geologic story of Lehman Caves, after Stage 1 (sulfide-rich, hypogenic cave formation) and Stage 2 (secondary deposits and a new environment).

Internal layers of a Lehman "turnip" stalactite, exposed by condensation corrosion. NPS photo by G. Baker.

Stage 3: calcite speleothems of the Pleistocene

The features most visitors notice — the abundant calcite speleothems (cave formations) — formed almost entirely in the Pleistocene, between 2.2 million and 7700 years ago.

• A couple dozen stalagmites have been dated.
• Most grew between 125,000 and 250,000 years ago.
• The oldest is 2.2 million years old (Lachniet and Crotty, 2017).

The "Eagle's Wing" in the Lodge Room, a calcite speleothem corroded by acidic condensation. NPS photo by G. Baker.

Stage 4: condensation corrosion

After the Pleistocene the climate dried. Probably once the natural entrance opened and surface air began mixing with cave air, condensation corrosion by carbonic acid became the cave's main process — and it is likely still slowly at work.

How it works: water vapor picks up carbon dioxide from the air and condenses on walls, ceilings and formations as a thin film of carbonic acid, which etches the marble bedrock and calcite formations. It can eat some parts of a formation faster than others, revealing the crystal structure inside.

"Punk" rock: dissolving along grain boundaries turns the marble into a soft, flaky, sandy or fluffy residue, mostly calcite, beige, tan or gray, lining much of the walls and ceilings along the tour route where dripstone and flowstone don't cover the rock.

• Early explorers scratched inscriptions into it without needing tools or soot.
• It sloughs off onto the floor, much as gypsum paste fell when the cave first formed.

New formations from condensation: condensed water also seeps into pores and cracks or spreads in thin films, carrying dissolved minerals. As it evaporates or temperatures shift slightly, it deposits new speleothems. Much or most of the cave's popcorn and aragonite frostwork probably formed this way. The process is typical of caves with little airflow between entrances and little running water — especially hypogenic caves in dry climates, which Lehman Caves certainly is.

Sources

Written from "Lehman Caves Geology Part III," by Louise D. Hose, excerpted from her paper for Great Basin National Park staff, published by the National Park Service in The Midden and in the public domain; rewritten in hubnx's own words.