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Volcanoes belch 'slugs' from deep underground

Friday, 13 July 2007
Cosmos Online
Volcanoes belch 'slugs' from deep underground
Difficult to predict?: A 'Strombolian' eruption at Italy's Stromboli Volcano in October 2006.
Image: Science

SYDNEY: The explosive gases that make some volcanoes dangerous, come from deep underground, rather than near the surface as was previously thought, report European researchers.

The findings, detailed today in the U.S. journal Science, add to our understanding of why some volcanoes are particularly explosive and violent – and hint that eruptions of this sort may be difficult to predict from seismic activity alone.

Some powerful volcanic explosions, which have no clear warning signal, are thought to occur because large gas bubbles – known as ‘slugs’ – ascend faster than the surrounding magma; much like bubbles rushing out the top of a shaken-up bottle of cola or lemonade.

Origin of the slugs

These explosions are also associated with seismic activity around 250 m below the Earth’s surface, and previous research proposed that the two events were connected, with gas slugs forming at a similar depth.

To further investigate the origin of the slugs, Mike Burton from the National Institute of Geophysics and Volcanology in Catania, Italy led a team of experts who travelled to the Italian island of Stromboli – home to one of the most active volcanoes in the world.

The researchers carefully measured changes in the composition of gas escaping the volcano during both dormant and active periods. To do this from a safe distance, they used an innovative remote sensing technique using infrared light to take spectroscopic measurements. Spectroscopy measures the wavelengths of light absorbed or reflected by a gas to predict the chemicals it's made up of.

"Deep roots"

The team's analysis of the gas suggested that it came from deep within the Earth rather than at a depth of 250 m. They predict that slugs form about three kilometres beneath the volcano and their formation is not linked to earthquakes, thus making eruptions from these volcanoes more difficult to predict from seismic activity.

“Stromboli’s recurrent explosions have much deeper roots than previously inferred from geophysical data,” wrote the authors. They say this ‘decoupling’ of deep slug formation and explosion-related quakes may be common to many other volcanoes too, and determined in each case by the unique plumbing of the particular location.

“These results are important in helping to clarify the processes leading to the intermittent explosions at Stromboli and elsewhere,” commented Lionel Wilson, Emeritus Professor of Earth and Planetary Sciences at Lancaster University in England.

“The down side of all remotely-sensed measurements of this kind is that they [are less accurate], but with careful interpretation this is not too great a problem,” he said.