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Issue 475, 21 January 2002

The song of the volcano sent across the Atlantic from Montserrat to Leeds

Volcanoes don't only erupt – they sing, and in a new development by geophysicists at the University of Leeds, the sounds or seismic tremor made by a volcano thousands of miles across the Atlantic will be transmitted directly to Leeds, where they will be analysed to show the minute changes in pressure which foretell an imminent eruption.

Around 60 volcanoes are active each year around the world, with many others erupting unobserved on the ocean floor, and accurate predictions of volcanic activity is vital to prevent disaster.

Readings based on sound as well as movement should improve predictions of when a volcano might erupt, although scientists are already very close to achieving accuracy in terms of time. But more importantly, it may help in forecasting the size and type of eruption, especially in volcanoes which create a magma dome and release the most deadly weapon a volcano has: pyroclastic flows made up of lava full of hot rocks and gas which quickly expands, travelling much faster than normal lava flows.

Through monitoring the ground deformation and seismic signals on a volcano, geophysicist Dr Jurgen Neuberg (pictured below) of the school of earth sciences has found that it is possible to identify certain seismic events which indicate pressure within the volcano before it erupts. His approach is novel in that he uses broadbad seismic arrays rather than standard seismographs to measure volcanic activity. He can then analyse the sound made by the pressure changes within the volcanic system, and decompose it to show the harmonic content.

He explains: "Imagine someone clapping their hands close to a harp. The clapping will cause all the harp strings to vibrate to different degrees, and this whole range of frequencies is what we analyse in relation to the volcano."

"We did the pilot test on the Stromboli volcano near Sicily, which has a minor eruption every thirty minutes, so it's excellent for trying out new techniques. It was fantastic as well for undergraduate students to be part of ground-breaking research – the first successful broadband array. A broadband array shows the full spectra of sound which the volcano makes. It's like looking at a picture directly, whereas a traditional seismometer is like looking at it through blue glass."

Dr Neuberg is focusing much of his research on Montserrat (see pictures, above and below), where, after several hundred years of lying dormant, the Soufriere Hills volcano erupted in June 1997. Four to five million cubic metres of lava and rocks came down in a matter of minutes spreading across four square kilometres on the east side of the island, covering the whole town of Plymouth under ash and mud and claiming twenty casualties (see picture below).

He added: "Although most people talk of volcanoes erupting, the volcano on Montserrat actually 'collapsed'. The magma in the volcano is very viscous, and it wells up under the surface to form a bulge or dome on the side of the mountain. This dome is unstable and is weakened by hydrothermal alteration of the rock. Eventually the surface gives way under its own weight, releasing lava and producing pyroclastic flows."

Since 1997, the dome has continued to form and collapse, releasing pyroclastic flows on a regular basis. In 1998 the cycle appeared to stop, but the dome has since begun growing again. The process could go on for 10 years.

"It's very spooky to go to Plymouth now," Dr Neuberg recalls. "Everywhere is white and light grey with ash, and although most of the buildings are standing, the first two floors are filled with ash and mud. It's very sobering, and makes you realise how important it is to be able to predict these events as accurately as possible."

As part of a European project, MULTIMO, which he coordinates, Dr Neuberg plans to drill bore holes on Montserrat and equip them with a seismometer, a tiltmeter and a strainmeter. These will link to an observatory and then via a direct internet link to Leeds – allowing scientists at the University to see the output from the bore holes simultaneously online. The advantage of bore holes to surface measurements is that they are away from surface noise, and so can offer a clearer picture of the volcano without disturbances. Drilling for the bore holes will begin later this year.

With a team of researchers, he is also modelling the physical conditions inside the volcano, using computer simulation to reproduce the seismic wave propagation in gas charged magma. He hopes this will help to understand the signals and how they link to magma properties. "Scientists are already close to predicting when a volcano will erupt," says Dr Neuberg.

"The challenge now is to predict the size and nature of that eruption. This is the only way in which we will avoid the loss of life we saw on Montserrat, and have seen in similar, predicted eruptions such as that on St Helens in the USA, where several scientists died.

"We can't control volcanoes – our only hope is to understand them."

 


 
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