How Sulfuric Acid May Have Carved Lehman Caves

Written from an article by cave geologists Harvey DuChene and Louise Hose.

Lehman Caves, with the acid pool basin in the Gypsum Annex. National Park Service.

Lehman Caves is one of the main attractions of Great Basin National Park. Absalom Lehman rediscovered it in 1885 and turned it into a show cave, and geologists have studied it since at least 1960. Since then, ideas about how caves form have changed a great deal — and part of Lehman Caves now looks like the work of sulfuric acid.

Two ways to make a cave

Epigenic caves are the most common. In the 1970s, researchers at Carlsbad Caverns National Park realized that most caves in the Guadalupe Mountains did not fit the Mammoth Cave model. Along with their deep origin, those caves held massive deposits of gypsum — a mineral left behind when sulfuric acid dissolves limestone. Pioneering work by geologists Stephen Egemeier, in the Kane Caves of Wyoming, and Carol Hill, in Carlsbad Cavern, produced the sulfuric acid theory of cave formation, and similar caves have since been recognized around the world.

The evidence in Lehman Caves

At least in the Gypsum Annex, the cave appears to have formed when sulfuric acid dissolved marble in the Pole Canyon Limestone along natural fractures, widening them into passages.

The gypsum itself is mostly gone from the known parts of the cave. But the acid left its mark in the bedrock: features called speleogens, formed while the acid was at work several million years ago. Together they point to an old pool of dilute sulfuric acid — what the authors call an acid pool basin.

The best example known in Lehman Caves is near the major junction leading into the Gypsum Annex, where a passage wall is distinctly undercut. From top to bottom:

Below the old waterline, the wall has no pseudo-scallops. The authors coined that name because true cave scallops are carved by flowing water; these hollows resemble them only on the surface.

How an acid cave works

Caves still forming this way, such as Cueva de Villa Luz in Mexico, show the process:

1. Warm springs rise to the water table — the level below which every void in the rock is full of water — carrying dissolved hydrogen sulfide.
2. The rising water has no free oxygen. Its hydrogen sulfide takes up whatever oxygen is available to form sulfuric acid, which reacts with the limestone or marble at once and is neutralized. What remains is weak sulfurous acid.
3. At the water table, gas escapes. Water vapor and some hydrogen sulfide rise into the cave air, which is warmer than the rock — heated by the springs and by the reaction itself.
4. Vapor condenses on the cool walls, picking up oxygen from the cave air. Hydrogen sulfide dissolves readily in water, so it is quickly absorbed back into this film — and becomes sulfuric acid.
5. On limestone, the acid reacts immediately, making a moist, acidic gypsum paste. Some acid-tolerant microbes can speed this up.

Think of a cold glass on a humid day. Moisture condenses on the outside, runs down in drops once there is enough of it, and leaves a puddle on the table. In an acid cave the glass is the wall and the puddle is the pool. The more wall there is above a pool, the more acid collects.

That is where each feature comes from:

• Acid running down the wall cuts the rillenkarren.
• Where the wall overhangs and moisture cannot flow, gypsum paste builds up until it falls, leaving a pseudo-scallop.
• In a closed hollow, the water pools. Acid in the pool — renewed by paste falling into it — dissolves the bedrock and slowly enlarges the basin. Most of the reaction happens at or near the surface, around the edges, which carves the notch and the overhung wall.

Questions that remain

The theory rests on two conditions, and neither holds at Lehman Caves today.

• The water table must have been higher. Most dissolution happens at or just above the water table, but the cave now sits well above it. When the pool was active, the floor of neighboring Snake Valley, if it existed, was higher too. The Snake Range is probably higher now than then, through regional basin-and-range uplift and/or isostatic rebound as a large mass of rock was removed from the top of the uplift. Detachment faulting exposed the range's core, and erosion of the layers above the cave's host rock continued. Uplift, rebound and erosion together lowered the water table relative to the cave.
• There must have been a source of hydrogen sulfide. No springs carrying it exist in or near the southern Snake Range today, though there are some elsewhere in the Great Basin of Nevada and Utah.

Many questions remain before anyone knows how Lehman Caves formed — and studying it is what brought them to light. Further research may explain how Lehman and the park's other caves formed, and shed light on the Great Basin's geologic history over the last 15 million years.

Sources

Based on "Evidence of a Sulfuric Acid Origin for Lehman Caves," by Harvey DuChene and Louise Hose, The Midden — Great Basin National Park, Vol. 20, No. 1, Summer 2020, published by the National Park Service and in the public domain; rewritten in hubnx's own words. The imported page held only an excerpt; the rest is from the full article on the park's website. The map is the Park Service's; Dave Bunnell's photograph of the acid pool basin is not reproduced.