Lava fountain driven by reservoir of gas.

In unprecedented measurements, volcanologists have pinpointed the source of volcanic gases driving a huge lava fountain from deep in Italy's Mount Etna.

Scientists have long debated the source of the gases that propel fountains of lava skyward during some eruptions. One theory suggests that the magma is boosted by gases fizzing from the material as it nears Earth's surface. Another view holds that the fountains are driven by reservoirs of gas that build up within a volcano's plumbing.

Mike Burton of the National Institute of Geophysics and Volcanology in Catania, Italy, and his colleagues used infrared sensors to analyze the gases within a lava fountain spewing from Mount Etna on June 14, 2000. During that 40-minutes episode of volcanic activity, a jet of lava arced at times to about 600 meters high.

When Mount Etna's lava is at normal atmospheric pressure, the gases that bubble out have a sulfur-to-chlorine ratio of about 3:1, says Burton. However, he and his colleagues found that the ratio in the June 14 lava fountain's gases averaged about 10:1, a proportion characteristic of gases emitted at a pressure about 350 times that of the atmosphere at sea level. The researchers present their findings in the Jan. 27 Nature.

The measurements suggest that the gas propelling this particular fountain had built up in a reservoir about 1.5 kilometers below the lava-filled crater, the researchers say. However, the finding doesn't rule out the view that other lava fountains are powered by fizzing lava closer to Earth's surface, says Burton.--S.P.