The fizzy gases that cause some volcanoes to blow their tops appear to originate deep beneath the surface, a new study by a team of Italian geologists from the National Institute of Geophysics and Volcanology has revealed.

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.

Mike Burton and his team, who studied the Stromboli volcano on the small island north of Sicily, said the 'strombolian eruptions' - the fiery blobs the volcano shoots out 10 to 20 minutes in fountain-like geysers as high as 300 feet above the crater - actually begin much deeper underground; perhaps as deep as the base of the mountain.

The team studied the gases emitted from Stromboli's main crater and 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, they were able to determine precisely which gases were present.

Comparison of the gas emissions during and between the bursts revealed 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, said Burton.

"The gas phase driving the explosions preserves the memory of hotter-but also deeper-source conditions," he said.

The findings are scheduled for publication in the journal Science, reports National Geographic.