How Lehman Caves Formed: Sulfuric Acid, Rising Water and the Gypsum Annex

Lehman Caves, in Great Basin National Park, is filled with beautiful decorations. Yet when park staff set out to create new exhibits, they found that the latest professional geological reports on the cave dated from the early 1960s — before most geologists had even heard of plate tectonics.

That early work, by U.S. Geological Survey geologists with caving ties (among them George Moore, soon to be president of the National Speleological Society), was good. But understanding of how caves form — speleogenesis — has moved on. In the 1960s and '70s, Lehman and similar eastern Nevada caves such as Old Mans and Crystal Ball were called "phreatic caves," and their carved features "boneyard." As cavers learned about hypogenic cave formation — caves carved by water rising from deep underground, studied in the Guadalupe Mountains, Europe, Mexico and elsewhere — many came to see Lehman as a product of such water. But almost nothing had been written about it and no focused study had been done.

In spring 2017 the park asked Louise D. Hose, a retired National Park Service geologist, to take a closer look. After a first trip that November and later visits, she wrote a paper on the cave's geologic story for park staff, excerpted in The Midden, a Great Basin National Park publication, in summer 2018.

Gypsum crust lines the Gypsum Annex, evidence of sulfur-driven cave formation in the cave's past (NPS photo).

An old cave, formed in an unusual way

Lehman's geology has been studied remarkably little compared with most major caves, so much remains uncertain. But one thing is clear: the cave is very old — millions of years — and has nothing to do with today's surface landscape. It also formed quite differently from the way most introductory geology textbooks describe caves forming.

Three points frame the story:

1. The whole cave may not have formed in the same way or at the same time.
2. There may have been several phases of cave-making, each by a different process. Four distinct phases of change are visible now, and the full story is probably more complicated.
3. The empty space had to exist before the decorations. The cave itself probably formed millions of years before most of its speleothems — stalactites, stalagmites, columns, shields — began to fill it.

The geologic time scale. Image from the National Park Service's page.

Stage 1: sulfuric acid from below

The best clues to the earliest stage lie off the tour route, in the cave's northwest corner, in a passage called the Gypsum Annex. It has few of the calcite decorations that crowd the rest of the cave, so the rock tells its story more plainly.

The evidence points to sulfide-rich, hypogenic cave formation. Warm, sulfur-rich water — probably also carrying plenty of carbon dioxide, like soda water — rose from deep in the earth through cracks in the Pole Canyon marble while the marble was still deeply buried (though after it had cooled from its last episode of metamorphism). Where the water and sulfur came from is unknown, but warm sulfur springs are still common in the Great Basin.

Sulfur-rich water does little to limestone until it nears the top of the water table and meets free oxygen. Then it forms sulfuric acid — the acid in car batteries — which is highly corrosive. So tubes filled with water began to form where sulfur-rich water from below mixed with oxygen-bearing water seeping down from the surface. The attack was fiercest at the top of those tubes, and the ceilings still show it:

• bubble trails — smooth grooves on overhanging walls and ceilings;
• cupolas — smooth domes in the ceiling;
• ceiling drains — round tubes in ceilings and high walls that look like drains but lead upward.

These features appear throughout the cave and mark it as a product of rising water. Carbon dioxide–rich water carves similar shapes as it nears the water table, but far less forcefully than sulfur-rich water.

How the gypsum formed

The walls and ceilings of the Gypsum Annex are mostly coated with gypsum crust. It formed after the water level dropped slightly and the passage filled with sulfur-laden air just above the water table — and the passage kept growing.

The rising water was warmer than the surrounding rock, and two things happened as it came up:

1. Gas bubbles formed. As pressure from the rock and water above decreased, bubbles of hydrogen sulfide or sulfur dioxide — and usually carbon dioxide — formed, ran along the ceiling of the flooded passage and escaped into the air-filled space above.
2. Acid attacked the walls. Near the air, oxygen converted the sulfur gases into sulfuric acid, which reacted with the marble to make gypsum (calcium sulfate) and bicarbonate.

The warm water also heated the air and its moisture above the cave walls' temperature, so sulfur-laden water condensed on the walls and ceilings, as in a steam room, turning their surfaces into gypsum.

That reaction releases a great deal of energy, and chemoautotrophic bacteria — microbes that live without sunlight or photosynthesis, powered by chemical reactions — thrive on it and speed it along. So instead of solid gypsum, the walls became coated with a paste of tiny gypsum crystals, sulfuric acid and abundant microbial life. When the paste dried and the bacteria died, the gypsum crust remained. It may once have lined the whole cave as the Snake Range rose and the water table fell.

Why the tour route has no gypsum

Gypsum dissolves easily in water. After the cave stopped enlarging, plenty of water from the surface found its way in, and as it flowed and dripped it built the calcite decorations visitors see today — and probably dissolved away any earlier gypsum. The northwest part of the cave has little calcite, which suggests a watertight layer above the Gypsum Annex kept surface water out and preserved its crust.

The later stages of the cave's history were to be described in future issues of The Midden.

Sources

• Louise D. Hose, "Lehman Caves: Little Understood but World-Class Cave," The Midden, Great Basin National Park, Vol. 18, No. 1 (Summer 2018), National Park Service. The article dates the cave to between 2.2 and 10 million years old, while its time-scale caption places the first stage between 17 and 2.2 million years ago, most probably 10 to 8 million, so no range is given here.