A series of eruptions from Woods Mountains, the mountain
range visible across the valley to the east, began about 18.5
million years ago when thick viscous, gaseous rhyolitic lava
began erupting (see for a graphic of the various
compositions of magma). This episode covered the area near the
volcano with as much as 200 meters of flows and tuff. A period
of quiet followed. As impressive as these first eruptions sounds
the next episode of eruptions was even more violent.
Deep in the earth, gasses are kept dissolved in the lava. As the
lava moves toward the surface, the gasses escape building up
pressure beneath the volcano. The same principal keeps carbon
dioxide dissolved in a carbonated drink until it is opened,
allowing the carbon dioxide to form bubbles.
About 17.8 million years ago, enough pressure
built up to completely blow apart the entire mountain. Blocks
of rock as large as 20 meters across were thrown across the
landscape. Huge clouds of volcanic ash blew out with the rocks
encasing the rocks as they settled to the ground. The
temperature of the ash was still so hot that it fused together
into the tuff found in these canyons. It is thought that the
size of the eruption completely surpasses any eruption in
human history.
Since the volcano explosively blew away the mountain and the
magma that was beneath it, what material that was left at the site
of the eruption collapsed down filling the void creating a caldera.
This is kind of opposite the common perception that volcanoes build
up large mountain.
Two more eruptions of this style occurred. Each time more tuff
blanketed the area and the caldera grew deeper.
The last eruptions occurred between 17.7 and
17.6 million years ago. The magma swelled the caldera up and
thick viscous lava oozed out in sticky flows. These flows can
be seen on the other side of the valley.
Logging requirements:
Send me a note with :
- The text "GC13KN9 Tuff at Hole-in-the-Wall" on the first
line
- The number of people in your group.
- The largest pieces of rock you can find embedded within the
tuff. They say there a some 20 meters across, so can you find
them?
- The number of different kinds of rock that are embedded in the
tuff (resort to colors and descriptions if you don't know what each
one is called). You should be able to find pieces of obsidian.
- From the informational panel(N35 02.641 W115 23.893), how fast
were the ash clouds moving during the erruptions?
The above information was compiled from the
following sources:
- Hole-in-the-wall, Mojave National Preserve,
Geology, USGS and NPS,
http://www2.nature.nps.gov/geology/usgsnps/mojave/hole1.html
Placement approved by the
Mojave National Preserve