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Geology of Mojave National Preserve: Hole in the Wall

Close-up look at tuff, the most common volcanic rock type you'll see at Hole in the Wall. Photo by M. Moreno, USGS
Close-up look at the Wild Horse Mesa tuff. Tuff is the most common volcanic rock you'll see at Hole in the Wall. Notice the variety of rock fragments welded together by a pinkish-orange ashy matrix.

Trapped beneath the surface...

By about 17.8 million years ago, the formation of rhyolite domes and flows that marked the first stage of volcanism had ended and the most dramatic and unusual phase of Hole in the Wall's eruptive history was just about to begin. While we usually think of eruptions issuing from a cone-shaped volcano, at Hole in the Wall, the story is quite different.

After a brief lull, the intensity of eruptions suddenly escalated. Three major eruptive episodes devastated the region during this second phase. Hidden not far beneath the surface, a mass of magma, crystals, and explosive gasses lay trapped. Over time, pressure within the gas-rich mass steadily increased. Finally the enclosing rock could not withstand the stress and the ground gave way in a horrendous eruptive explosion. Molten lava and huge fragments of rock were hurled upwards. Blocks of the enclosing rock up to 20 meters across were tossed into the air. Hot blocks, globs of lava and glowing ash littered the countryside, falling in thick layers that welded together as they reached the ground.

 

A slice through the Earth showing the caldera at Hole in the Wall
A slice through the Earth at Hole in the Wall. The black lines show where the caldera collapsed along faults. The gray rock represents older Mojave rocks that formed the landscape prior to the eruptions at Hole in the Wall. The brightly colored layers on top are volcanic rocks that flowed across the landscape or fell from the air as a thick blanket of volcanic debris (tephra). Click here to see entire image.

 

At the site of the eruption, the ground collapsed along numerous faults, forming a huge ring-shaped pit called a caldera. After the collapse lava and volcanic debris (tephra) partially filled the caldera.

It's hard to imagine the intensity of this event. No living thing could have survived within the blast zone. No eruption of comparable violence has occurred in recorded human history.

Map showing the original area coverd by the second stage of Hole in the Wall volcanic activity
This map shows the most violent stage of Hole in the Wall volcanic activity. The bright red area shows the wide swath of land smothered by lava flows and welded volcanic debris during the second phase of volcanism at Hole in the Wall. Even 17 million years later, much of the original rock produced by this event remains in the brown-colored areas shown on the map. Click here to see enlargement. Adapted from McCurry, 1995.

Then, for just a brief time, things quieted down on the surface. Below ground, however, pressure began to build again as more gas and magma collected beneath the caldera.

Twice more, when pressure built up to the point where the ground could no longer contain it, tremendous eruptions, only slightly smaller than the first, rocked the Mojave. Each time, thick layers of glowing volcanic debris decimated the landscape and the original caldera collapsed further.

Citation: McCurry, M., Lux, D. R., and Mickus, K. L., 1995. Neogene Structural Evolution of the Woods Mountains Volcanic Center, East Mojave National Scenic Area: San Bernadino County Museum Association Quarterly, v. 42, no. 3, p.75-80.

 

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