Volcanic "Fizz" That Triggers Explosive Eruptions Starts Deep

Richard A. Lovett
for National Geographic News
July 12, 2007
The fizzy gases that cause some volcanoes to blow their tops—like champagne bubbles popping a cork—appear to originate deep beneath the surface, a new study suggests.

Scientists have long known that during an eruption, gases fizz out of magma as the molten rock rises to the surface.

But in some types of magma, small bubbles coalesce into larger, gaseous "slugs" that rise upward, causing fiery bursts when they reach the surface.

Such is the case at Italy's Stromboli volcano, a 3,050-foot (925-meter) cone that sits on a small island north of Sicily (Italy map).

The volcano has been continuously active for at least 2,000 years, earning it the nickname "Lighthouse of the Mediterranean."

Every 10 to 20 minutes Stromboli shoots fiery blobs in fountainlike geysers as high as 300 feet (100 meters) above the crater.

These bursts, dubbed strombolian eruptions, are associated with seismic activity not far beneath the crater, so volcanologists had assumed that the slugs formed in that region.

But the new study, to be published in tomorrow's issue of the journal Science, found that the belches actually begin much deeper underground—perhaps as deep as the base of the mountain.

Learning more about Stromboli is important, the scientists said, because the mountain produces much larger explosions several times a year (see a photo of a recent Strombli eruption). These violent blasts pose a major hazard for tourists and scientists observing the peak from a nearby overlook.

"Therefore, improved understanding of the processes controlling the different types of explosions at Stromboli is … a high priority," the team wrote.

Gases From the Deep

Mike Burton of Italy's National Institute of Geophysics and Volcanology led the team that studied the gases emitted from Stromboli's main crater.

For the latest research, Burton's team examined the spectrum of infrared light emitted by red-hot blobs of magma.

By finding out which wavelengths were absorbed by the air above the crater, the scientists were able to determine precisely which gases were present.

And by comparing gas emissions during and between bursts, they found differences in composition indicating that the gas slugs originated at a depth of about 10,000 feet (3,000 meters) beneath the crater.

That is approximately the base of the mountain, where it rests on the seabed.

"The gas phase driving the explosions preserves the memory of hotter—but also deeper—source conditions," the scientists wrote.

Volcano Plumbing

Other scientists studying Stromboli are pleased with the findings.

"It's a nice complement to what seismology can do," said Bernard Chouet, a geophysicist with the U.S. Geological Survey in Menlo Park, California.

Prior research used a network of seismic stations to determine that the conduit feeding the volcano flares wider about 800 feet (250 meters) beneath the surface, Chouet said.

Rather than originating in this region as previously believed, gas slugs rise quietly from below until they reach the area and have more room to expand, the new study suggests.

"That produces a piston-like action of the liquid 250 meters [820 feet] down," Chouet said.

Together, the study and older seismic models are producing a better overall picture of the volcano's plumbing, he noted.

The work has global significance, because other volcanoes are known to produce strombolian eruptions.

Learning more about the source of the gas can also help researchers better understand why some volcanoes produce relatively benign eruptions while others are more explosive.

"It's providing an added component," Chouet said, "to the model of slug formation."

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