Seismic data suggest the presence of a 400 kilometer square-wide reservoir of magma located eight kilometers below the famous Mt. Vesuvius volcano in Italy, according to a report by Italian and French researchers in the 16 November issue of the international journal, Science.
Locating this reservoir won't help researchers predict exactly when the next Vesuvius eruption will take place, says Science author Paolo Gasparini of the Università di Napoli Federic II in Naples.
But, the discovery does point to a zone underneath the volcano to monitor for seismic clues, such as small earthquakes, that may signal an impending eruption, say the Science study authors.
"This also tells us that there is a huge amount of available magma under Vesuvius," says Gasparini. "It was really unexpected for the reservoir to be that size, so very wide and large. It underlies a very large area under the Neapolitan volcanoes."
Although Mt. Vesuvius' next eruption will probably be explosive, coming on the heels of a long quiet period, don't expect the magma reservoir to be drained. Most volcano experts believe that no more than 20 to 25 percent of available magma is released during any eruption, says Gasparini.
Mt. Vesuvius is relatively quiet now, trembling with mild earthquakes and venting low temperature volcanic gases in spots, but the volcano has a notable history of violent eruptions. Vesuvius is probably best known for its searing hot flows and rains of debris that buried the Roman cities of Pompeii and Herculaneum in 79 A.D.. Several significant eruptions have occurred in modern times, including the last major eruption in 1944 during World War II.
The Science researchers used a technique called seismic tomography to probe beneath Mt. Vesuvius, creating manmade explosions to generate seismic waves and tracing those waves as they travel through the crust. Data on the speed and direction of seismic waves collected by regional "listening" stations provide glimpses of the structure (and changes in structure) of the crust. Like the more familiar form of medical tomography, the CAT scan, seismic tomography combines data from these seismic station readings to build up a two- or three-dimensional picture of the crust.
Previous seismic work by the researchers, including a study published in Science (21 April 1996), hinted at a magma zone lurking beneath the volcano and the surrounding region. These experiments revealed an area of low seismic wave velocity within the crust beneath Mt. Vesuvius, where the waves were apparently slowed down and converted into different wave types.
Since seismic waves move more slowly through a liquid compared to a solid, and wave changes often occur at a boundary between two different geological layers, the scientists thought they had uncovered a possible melting zone within the local crust.
For the current study, the research team analyzed data from a more recent experiment--which included 1800 explosive shots fired from air guns onboard a ship in the Bay of Naples and tracking stations up to 90 kilometers away in the Apennine Mountains--to trace the extent of the magma reservoir.
Their analysis suggests that the reservoir may extend over 400 square kilometers and is buried eight kilometers deep in the crust, stretching beneath Vesuvius and other neighboring volcanoes such as Phlegraean Fields, site of the modern city of Naples.
Images of the reservoir reconstructed from the seismic data indicate that the magma lies in a nearly flat layer parallel to the crust's layers, following the conditions of neutral buoyancy, according to the researchers.
Magma is usually less dense, and therefore more buoyant, than the rocks it passes through on its rise towards the crust. Neutral buoyancy occurs when the magma reaches a spot where its density equals that of the surrounding rock and floats in a trapped layer.
"That's when it needs something else to reach the surface, like breaking up overlying rocks or building up gas pressure below," says Gasparini.
Isotope studies of Neapolitan volcanic magma show signs of significant mixing with the surrounding rock, suggesting that the reservoir isn't one continuous molten body. Instead, the reservoir might look more like a sponge, with the magma seeping through numerous fractures in the rock. The massive magma layer may feed into several smaller reservoirs that are closer to the surface and too small to identify with seismic techniques, say the Science authors.
The other members of the research team include Emmanuel Auger and Aldo Zollo at Università di Napoli Federico II and Jean Virieux at Géosciences Azur in Valbonne, France. This research was supported in part by the European Commission Division XII, Gruppo Nazionale di Vulcanologia, and MURST.