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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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