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 How Old Is Mammoth Mountain?

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عدد المساهمات : 1713
السٌّمعَة : 1
تاريخ التسجيل : 26/08/2011

07102011
مُساهمةHow Old Is Mammoth Mountain?

March 15, 2010 | San Francisco Chronicle

An article on the San Francisco Chronicle explores the age of Mammoth
Mountain, an area with a volcanic history in California, and considers
its future activity.


cover the ski slopes of Mammoth
Mountain this winter, the tumbled rocks and restless ground tell a story
of volcanic eruptions and earthquakes that shaped the region over
thousands of years.

Now, that story has been retold in fresh detail -
and with a new timetable - by a team of Stanford earth scientists. They
found that geological activity occurred much more recently than
previously thought to create the mountain and raise the chain of nearby
smaller volcanoes stretching south for 40 miles from Mono Lake to the
Mammoth area.

The mountain itself is an old volcano 11,000 feet
high at its summit that built its own bulk from eruptions about 68,000
years ago. In scientific terms, the mountain formed abruptly, "almost
certainly in less than 2,000 years," said geologist Gail A. Mahood, who
led the team of scientists.

The string of nearby volcanoes, whose eroded domes
are known as the Mono and Inyo craters, were formed much later - some
time after 9,000 years ago, Mahood said.

The scientists' findings were published last week in the Bulletin of the Seismological Society of America.
Helpful for the future

The work of Mahood's team
"provides a slew of new and accurate dates for eruptive events at Long
Valley that we can certainly apply for estimating the probability of
future events there," said Margaret Mangan of the U.S. Geological Survey
in Menlo Park. She is the scientist in charge of the Long Valley
Observatory, a network of instruments placed in the ground throughout
the Mammoth Mountain region to monitor geologic hazards.

To reach their conclusions, Mahood and her
colleagues conducted geologic surveys of the region over several years,
determining the age of the rocks by studying their radioactive isotopes.

To collect lava samples, Mahood herself hiked the
rough terrain of the mountainous survey area and climbed inside a broad
low-lying feature called the Long Valley Caldera.

The caldera is a huge crater 20 miles long and 11
miles wide, the sunken remains of a monstrous volcano that erupted
760,000 years ago in one of the most violent explosions on record. It
spread volcanic ash over what are now 14 Western states - as far east as
Kansas and Nebraska.

Mahood and her team of scientists identified four
relatively recent periods of activity they term "eruptive sequences"
that have shaped the scenic region over the past 190,000 years. The
oldest lasted until 160,000 years ago, she said, while the most recent
began about 9,000 years ago and continues today.

It was during one of those past eruption periods that Mammoth arose, she said.
"The main bulk of Mammoth Mountain, and the rocks
you stand on when you get out of the ski lift at the top, erupted over a
short period of time - almost certainly less than 2,000 years," Mahood
said.

Dating the mountain

That age makes the mountain much
younger than earlier surveys reported. Some past studies had suggested
it could have begun rising as long as 110,000 years ago in a series of
eruptions that lasted for more than 50,000 years.

To determine the ages of the ancient lava
throughout the region and the evidence of past earthquakes, Mahood and
her colleagues used a technology known as argon-argon dating, based on
the known time that one radioactive isotope of the gaseous element takes
to decay.

David P. Hill, a long-time research geologist at
the USGS in Menlo Park and recently retired scientist in charge of the
Long Valley Observatory, said Mahood's work provides the best dates yet
for the recurrence of volcanic events throughout the caldera region - a
timetable essential for assessing future dangers.

"I wouldn't be surprised if within 50 years or so
there were some kind of eruption in the area," Mahood said. But if any
eruptions do occur in the near future, she said, "they would be really
tiny."



Read more: http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2010/03/08/BAQ41C5JEI.DTL#ixzz0iEjxikWi
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