New
research results suggest that magma sitting 4-5 kilometers beneath the surface
of Oregon’s Mount Hood has been stored in near-solid conditions for thousands
of years. The
time it takes to liquefy and potentially erupt, however, is surprisingly
short–perhaps as little as a couple of months.
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The
key to an eruption, geoscientists say, is to elevate the temperature of the
rock to more than 750 degrees Celsius, which can happen when hot magma from
deep within the Earth’s crust rises to the surface.
It
was the mixing of hot liquid lava with cooler solid magma that triggered Mount
Hood’s last two eruptions about 220 and 1,500 years ago, said Adam Kent, an
Oregon State University (OSU) geologist and co-author of a paper reporting the
new findings.
“These
scientists have used a clever new approach to timing the inner workings of
Mount Hood, an important step in assessing volcanic hazards in the
Cascades,” said Sonia Esperanca, a program director in NSF’s Division of
Earth Sciences.
“If
the temperature of the rock is too cold, the magma is like peanut butter in a
refrigerator,” Kent said. “It isn’t very mobile.
“For
Mount Hood, the threshold seems to be about 750 degrees (C)–if it warms up
just 50 to 75 degrees above that, it greatly increases the viscosity of the
magma and makes it easier to mobilize.”
The
scientists are interested in the temperature at which magma resides in the
crust, since it’s likely to have important influence over the timing and types
of eruptions that could occur.
The
hotter magma from deeper down warms the cooler magma stored at a 4-5 kilometer
depth, making it possible for both magmas to mix and be transported to the
surface to produce an eruption.
The
good news, Kent said, is that Mount Hood’s eruptions are not particularly
violent. Instead of exploding, the magma tends to ooze out the top of the peak.
A
previous study by Kent and OSU researcher Alison Koleszar found that the mixing
of the two magma sources, which have different compositions, is both a trigger
to an eruption and a constraining factor on how violent it can be.
“What
happens when they mix is what happens when you squeeze a tube of toothpaste in
the middle,” said Kent. “Some comes out the top, but in the case of
Mount Hood it doesn’t blow the mountain to pieces.”
Read
more at the NSF
Research.gov.
Peanut
butter and Mount
Hood images via Shutterstock; combined by Robin Blackstone.