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.
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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 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.
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.
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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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U.S. Department of the Interior |
U.S. Geological Survey