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Long Valley Caldera, Mono-Inyo Craters, and Mammoth Mountain
The Eastern Sierras and Mammoth Mountain are well known for their infinite supply
of recreational activity throughout the year. Excellent skiing, backpacking, boating, climbing,
off-road driving, hot spring swimming, hiking, fishing and camping
can all be accomplished in this fantastic natural playground. However, the forces
that created this beautiful place are still active, and are constantly changing the landscape.
Persistent seismic and
volcanic
activity over the last four million years has formed the spectacular Eastern Sierra landscape in the
Long Valley Caldera
and Mono Basin. The Glass Mountain volcanic eruptions
about 760,000 years ago
climaxed with a catacylsmic eruption of 600 cubic kilometers of rock and ash, causing a
two to three kilometer
depression of the magma chamber
roof beneath the surface to form the present 17 by 32
kilometer oval shaped
Long Valley Caldera. The Mono-Inyo craters
make up the youngest volcanic chain along the Eastern Sierra.
This system of craters was created by a fissure
system that extends from south of
Mammoth
Mountain through the western portion of the caldera to the north shore of Mono Lake. Repeated
eruptions of dacite and rhyolite
on the southwest rim of the caldera formed Mammoth Mountain, a cumulo-volcano.
The Mono Craters were formed by multiple
eruptions of high-silica rhyolite as
recently as 600 years ago. The Inyo Craters were formed by eruptions of low-silica rhyolite
as recently as 500 years ago. The most recent eruption occurred at Paoha Island in the middle of Mono
Lake 250 years ago. Eruptions tend to occur along this chain every 250 to 700 years.
Since 1980, a substantial amount of geologic unrest
within the
Long Valley Caldera and Mammoth Mountain. In May 1980, four earthquakes of magnitude six (three of which
struck in the same day) awakened the caldera. As a result, a
dome-like uplift occurred in the
caldera, raising the floor almost a foot. Ongoing swarms of earthquakes have induced the continued
swelling of the region, causing the area to rise nearly two feet over 100 square miles of land,
signifying that new magma is rising beneath the surface.
During the early 1990's, trees
began dying off at several places on Mammoth Mountain. Studies by USGS have discovered that the
trees are dying due to an excess of CO2 in the ground. In the late 1980's, geologists detected
many small earthquakes beneath Mammoth Mountain, causing a small body of magma to rise through a
fissure beneath the Mountain which prompted the release of volcanic gas. The excess amount of CO2 in
the soil interfers with other nutrients vital to plant survival. CO2 is heavier than air, and
seepage above the soil will loom over the ground, collecting in snow banks, depressions, and poorly
ventilated enclosures, posing a threat to people as well as the environment. The area of dead and dying trees has
covered more than 100 acres. Increased CO2 gas release and increased seismic activity are all
signs of an imminent eruption. Considering the volcanic history of the area, eruptions will likely
continue to occur, it is only a matter of when.
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