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New Scientist
Aerial surveys
of volcanoes can help predict where deadly torrents of rock and
water might flow
Aerial surveys
of volcanoes can help predict where deadly torrents of rock and
water might flow, says a US team. This knowledge also improves
early warning systems for the events.
The torrents,
called lahars, are caused by the rapid melting of snow and ice
at a volcano's summit during an eruption. In 1985, a small eruption
of the Nevado del Ruiz volcano in Colombia caused a lahar that
killed 23,000 people in the town of Armero, 75 kilometres from
the summit. And other triggers, such as very heavy rainfall, can
create a sudden flow weeks or even months after an eruption.
A team at
the US Geological Survey has used magnetometers and radio waves
to probe the rocks that make up the flanks of Mount Rainer, a
volcano in Washington State.
They used
the data to produce a map showing the location of older and weaker
rock zones. These are the danger zones where lahars are likely
to form, says Carol Finn of the USGS.
"Lahars
come careering down the side of a volcano, so they are a big problem,"
says Hazel Rymer, a volcanologist at Open University, UK. "They
do tend to follow pre-existing channels, but it often isn't possible
to predict where they might start.
"These
techniques will be able to tell you not only where the old and
new rocks are, but how thick the deposits are - it sounds useful,"
she says.
Double probe
Studies of the valleys around Mount Rainier show that the volcano
has been the source of frequent lahars over the past 10,000 years,
says the USGS team. They used a helicopter to make their survey
of the volcano itself.
Fresh volcanic
rocks are poor conductors of electricity. But over time, their
pores fill with water, making older rocks better conductors. This
also makes them weaker and more liable to collapse to form a lahar.
The different conductivities of older and younger rocks can be
detected by bouncing radio waves off the surface of the slopes.
Older rocks
are also slightly less magnetic than more recently deposited rocks.
By combining
radio wave and a magnetic data, Finn's team were able to produce
a detailed map of the mountain's make-up. This allowed them to
predict where lahars are most likely to start.
The team has
set up a network of "listening" stations around the
volcano. Each station will measure the amplitude, frequency and
duration of ground vibrations. When the measurements exceed pre-programmed
thresholds, alerts will be sent to a base station computer.
Knowing where
the lahars are most likely to form helps them place the monitor
stations in the most vulnerable spots.
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