How Lehman Caves Settled After Its Water Drained: Gypsum, Mud and Fallen Rock

This is the second part of a geological history of Lehman Caves in Great Basin National Park, by retired National Park Service geologist Louise D. Hose, excerpted from a paper she prepared for park staff and published in the park's The Midden in winter 2018. The first part described Stage 1: how warm, sulfur-rich water rising from below dissolved the cave. This part covers Stage 2, the second of four stages she has documented — the cave finding stability in a new environment.

After the water left

The cave was completely drained of the rising water more than 2.2 million years ago — most likely about 8 million years ago. Before any of its spectacular decorations formed, three kinds of debris covered the floor:

1. Gypsum. As soon as the water table dropped below the passages, the gypsum paste coating the walls and ceilings fell, blanketing the floor with gypsum sand and dust. What stayed on the walls dried into a crystalline gypsum crust.
2. Mud, silt and sand. Some is the insoluble residue left from the dissolved marble, which settled out while the cave was enlarging; some washed in from the surface, especially near the natural entrance.
3. Breakdown — boulders and rock fallen from the ceilings and walls, as in the cave's Talus Room, where many of the boulders likely fell millions of years ago when the water drained.

Why rock fell

The collapse had several causes:

• Fractures. The walls and ceilings are crossed by a remarkable number of cracks, many of which appear to have widened since the cave formed — probably the biggest reason for all the breakdown. Joints and faults are weak zones prone to failure, and the faults are zones where stresses shift. They likely date to movements of the Snake Range Décollement and Basin and Range uplift.
• Earthquakes. Fault movement in and near the cave during Basin and Range tectonics shook the whole area, and breakdown likely came with those quakes.
• Losing the water's support. Without the partial buoyancy of water under the ceilings, or the pressure of a water-filled passage against the walls, the cave probably went through its most active collapse right after it drained, millions of years ago.
• Unloading. As the area rose and erosion stripped away most of the rock above, the release of that weight let joints and fractures open, making collapses easier.
• Crystal wedging. Gypsum is abundant in the cave. Turning limestone into gypsum — a much larger molecule — expands the rock. Along joints and cracks, that expansion pries the bedrock apart, dropping blocks to the floor and leaving blocky domes in the ceiling. The same process is common in Mammoth Cave.

Predicted frequency of damaging earthquake shaking (U.S. Geological Survey, undated).

Is it safe today?

Yes. The area now has a low risk of damaging earthquakes. It has been largely earthquake-free for about the last 8 million years, and the cave is more than 2.2 million years old, so its ceilings and walls have likely reached a stable state. They pose no more risk of spontaneous collapse than an average cave visit or a walk in the mountains anywhere. Most of the collapse happened millions of years ago, and more than 120 years of safe tours show the cave is as stable as most mountain recreation sites.

Sources

• Louise D. Hose, "Lehman Caves: Little Understood but World Class, Part 2," The Midden, Great Basin National Park, Vol. 18, No. 2 (Winter 2018), National Park Service. The source's photograph of the Talus Room is credited only to a named photographer and is not reproduced.