Central California - May/June 2009
Mammoth Lakes – Fire…
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A shaded relief map of the Long Valley Caldera area |
The Long Valley Caldera
was produced by a huge volcanic eruption that occurred about 760,000
years ago. Ash from the eruption traveled as far as Mexico to the south and
central Kansas and Nebraska to the east (map).
The ash fall covered the area presently occupied by Utah, Arizona, and
Colorado, along with parts of 10 other states! When the underground magma
chamber feeding the eruption emptied, the ground above collapsed forming a
depression (“caldera”) about
20x11 miles in size. This makes it one of the largest calderas on Earth.
Currently, this is one of three volcanic areas in the Continental US monitored
by the USGS (the other two being the Yellowstone Caldera
and the Cascade Mountain Range
in N. California, Oregon, and Washington).
Unfortunately,
the weather in this area was generally rainy (with some snow!) while I was
there. This made any backroad travel a little dicey. But the clouds broke a few
times, and I was able to visit/photograph some of the volcanic features of the
area.
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Two of the Mono
Craters. These are part of a chain of small volcanoes that erupted over
the last 3000 years or so. The latest eruption occurred in Mono Lake between
1715 and 1865, at the north end of the chain (just off the top of the above
map). |
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A shot of the side of Obsidian Dome (right). This Lava Dome reaches about
300ft above the surrounding terrain. It was formed by the slow extrusion of
thick lava over time. Depending on how much gas (little to none) the lava
contained, and how quickly it cooled, the resulting rock was either pumice or obsidian. Chemically, these
rock types are virtually identitical. |
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A banded bolder at the base of the dome. The consistency of the
rock varies from frothy pumice (white), to solid obsidian (black volcanic
glass)/ |
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More of the boulders at the base of Obsidian Dome. The large rock just left of center exhibits glare from it’s
smooth shiny surface. Pieces broken off this type of rock can be very sharp;
in effect they are pieces of broken glass. The Native Americans that
inhabited this region used these pieces of rock as sharp tools. And I was
very wary driving the graded roads around the dome. These sharp rocks could
very easily slice open a car tire. The rock at right below center has a very frothy surface. It
almost looks like the frozen foam of a carbonated beverage. |
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Hot Creek as it meanders through the caldera valley. The black
dots mid-ground are grazing cattle. In the distance are the snow covered
peaks of the Sierras. |
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Hot Creek, a little further downstream. The blue pools are
hydrothermal hot springs. Runoff from the Sierras percolated deep underground
and was heated by magma. It then rises and reaches these hot springs. Up
until a few years ago, swimming was allowed in this area. But increased
geothermal activity has made this a dangerous place to swim. Plumes of water
heated to nearly 200degF can emerge here. I know I would not want to be in
the water when that happens! The USGS has a good page describing the geology of the site here. |
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A closer shot of the geothermal pools. You can see the steam
rising from the water. |
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Hot Creek meandering just downstream of the hot springs. |
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This depression is known as the “Devil’s Punchbowl”. Here,
underground magma rose up and encountered the water table. The steam pressure
built up, and eventually the pressure became greater than the weight of the
overlying rocks. This resulted in a steam explosion (a “phreatic eruptiom”)
that excavated this crater. |
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Mammoth
Lakes – Fire… |
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All images copyright ©2009 by Al Hann All rights reserved |
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