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Wednesday, 31 January, 2001, 22:26 GMT
Early warning for volcanic mudslides
Pinatubo spewing smoke USGS
Mount Pinatubo: Lahars could devastate the surrounding country
By environment correspondent Alex Kirby

A team of US scientists has found a way to predict lahars, the catastrophic mudslides sometimes triggered by volcanic eruptions.

Their early warning system depends on noting the difference between fresh rocks and older, weaker ones.

They developed it by analysing rocks on Mount Rainier in Washington State, associated with many lahars in the past. But it could be applied to other volcanoes, and could save many lives.

Lahar is an Indonesian word for a rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano, turning to mud as it flows downhill.

Deadly wave

Lahars form in several ways, chiefly by the rapid melting of snow and ice, intense rainfall on loose volcanic rock deposits, breakout of a lake dammed by volcanic deposits, and as a consequence of debris avalanches.

Armero mudslide USGS
Devastation hits Armero, November 1985
A small eruption in 1985 by the Nevado del Ruiz volcano in Colombia sent a torrent of muddy water through the town of Armero, 75 kilometres (45 miles) away. About 25,000 people lost their lives.

Mount Pinatubo, the volcano 90 km (55 miles) north-west of Manila in the Philippines, which erupted violently in 1991, is also a potential lahar threat.

The eruption spewed out large quantities of ash, which rain can transform into mud. A six-mile (10 km) permanent danger zone is now in force around the area.

Significant differences

US geologists have been concerned for some time about the persistent risk from lahars on Mount Rainier, which has been know to produce the mudflows in the past.

Some of the lahars from Mount Rainier travelled as far as the Pacific coast at Puget Sound, nearly 100 km (60 miles) away.

Now, a team of researchers, writing in the journal Nature, says there is enough unstable material on the flanks of the volcano to cause huge flows of debris "capable of reaching areas that are now densely populated".

It discovered significant differences between fresh volcanic rocks and older ones.

The fresh rocks are poor conductors of electricity, and as time passes they become hydrothermically altered by water and heat, which weakens them and makes them more liable to collapse and form a lahar.

Helicopter survey

Once the rocks have undergone this weakening process they develop pores which fill with water, and so they are better conductors. They are also slightly less magnetic than they were.

Mt St Helens eruption USGS
Lahars on Mount St Helens
The two types of rock have different effects on electro-magnetic signals, such as radio waves, beamed at and reflected from the mountainside.

The scientists used these characteristics to map regions of hydrothermal alteration on Mount Rainier, conducting aerial surveys with radio waves and sensitive magnetometers suspended from a helicopter.

Professor Bill McGuire is director of the Benfield Greig Hazard Research Centre, UK.

He told BBC News Online: "It seems a useful piece of work, using essentially relatively established geophysical methods to define areas of weak, altered rock that might contribute to lahar formation during a future eruption.

Reduced risk

"Torrents of water generated by the eruption melting the huge volumes of ice at the summit are most likely to form lahars where they pass over these zones of weak rock.

"This is because rock and debris would be more readily plucked from these zones than from less altered areas of the volcano's flanks.

"Identifying such weak zones should allow better lahar hazard zonation maps to be produced for Rainier and reduce the risk to the local population the next time Rainier reawakens."

Photographs courtesy of US Geological Survey

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See also:

11 Jan 01 | Asia-Pacific
Indonesian volcano 'could erupt soon'
20 Dec 00 | Americas
Mud slide fears over Mexico volcano
14 Oct 00 | Americas
Deadly mudslide at Ecuador volcano
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