Supervolcano brewing beneath Mount St Helens
Pastor Dale Morgan
*Supervolcano brewing beneath Mount St Helens*
10 June 2009 by David Shiga
New Scientist
IS A supervolcano brewing beneath Mount St Helens? Peering under the
volcano has revealed what may be an extraordinarily large zone of
semi-molten rock, which would be capable of feeding a giant eruption.
Magma can be detected with a technique called magnetotellurics, which
builds up a picture of what lies underground by measuring fluctuations
in electric and magnetic fields at the surface. The fields fluctuate in
response to electric currents travelling below the surface, induced by
lightning storms and other phenomena. The currents are stronger when
magma is present, since it is a better conductor than solid rock.
Graham Hill of GNS Science, an earth and nuclear science institute in
Wellington, New Zealand, led a team that set up magnetotelluric sensors
around Mount St Helens in Washington state, which erupted with force in
1980. The measurements revealed a column of conductive material that
extends downward from the volcano. About 15 kilometres below the
surface, the relatively narrow column appears to connect to a much
bigger zone of conductive material.
The column below Mount St Helens appears to connect to a huge zone of
conductive material
This larger zone was first identified in the 1980s by another
magnetotelluric survey, and was found to extend all the way to beneath
Mount Rainier 70 kilometres to the north-east, and Mount Adams 50
kilometres to the east. It was thought to be a zone of wet sediment,
water being a good electrical conductor.
However, since the new measurements show an apparent conduit connecting
this conductive zone to Mount St Helens - which was undergoing a minor
eruption of semi-molten material at the time the measurements were made
- Hill and his colleagues now think the conductive material is more
likely to be a semi-molten mixture. Its conductivity is not high enough
for it to be pure magma, Hill says, so it is more likely to be a mixture
of solid and molten rock.
Gary Egbert of Oregon State University in Corvallis, who is a
magnetotellurics specialist but not a member of Hill's team, is cautious
about the idea of a nascent supervolcano where Mount St Helens sits. "It
seems likely that there's some partial melt down there," given that it
is a volcanic area, he says. "But part of the conductivity is probably
just water."
If the structure beneath the three volcanoes is indeed a vast bubble of
partially molten rock, it would be comparable in size to the biggest
magma chambers ever discovered, such as the one below Yellowstone
National Park.
Every few hundred thousand years, such chambers can erupt as so-called
supervolcanoes - the Yellowstone one did so about 640,000 years ago.
These enormous eruptions can spew enough sunlight-blocking ash into the
atmosphere to cool the climate by several degrees Celsius.
Could Mount St Helens erupt like this? "A really big, big eruption is
possible if it is one of those big systems like Yellowstone," Hill says.
"I don't think it will be tomorrow, but I couldn't try to predict when
it would happen."
Further measurements probing the structure of the crust beneath the
other volcanoes in the area could help determine if the zone connects to
them all, Hill says. He presented his team's results on 27 May at the
Joint Assembly geophysics meeting in Toronto, Canada.