Following Etna's eruption in 2001, earthquakes continued, with severe shaking and a new river of lava appearing last fall. This is in contrast to other eruptions of the mountain over recent decades, each of which was followed by a quiet period lasting from several months up to two years, a team of Italian scientists reports the online edition of the journal Science.
"Mount Etna volcano has grown progressively more active during the past 30 years. This means that it is erupting more frequently and more intensely than during the last three centuries, and its magma output rate is increasing," said Domenico Patane of the National Institute on Geophysics and Volcanology in Catania.
With an increasing number of fractures in the rock inside the mountain, from the growing pressure of magma, the scientists warn that eruptions at Mount Etna "could become more frequent, voluminous and potentially hazardous in the near future."
Located on the northeastern part of the island of Sicily, off Italy's southern toe, Mount Etna's first recorded eruption was in 1500 B.C. and it has produced lava flows more than 150 times since then.
Small to moderate explosive eruptions have been known to take place at the summit, but Patane stressed that, in the 20th century, Etna eruptions tended to be lava flows rather than explosions.
Patane's team, from the Institute's centers in Catania and Rome, studied ground deformation and used tomography - which analyzes seismic waves moving through the ground - to determine various rock and magma formations beneath the mountain.
Ground swelling and earthquake activity indicate that since 1994 a huge volume of magma has intruded below the volcano, the team reported, leading to the current series of eruptions that began in 1998.
The magma rises in the area of two intersecting faults in the rock beneath the mountain, triggering earthquakes as the pressure increases.
The team found what they believe is a massive body of solid magma intruding from about one mile to about 11 miles deep beneath the mountain, causing a pressurization of the mountain's deep plumbing system which has fractured the rock. The rising liquid magma is moving through cracks and fissures and collecting in an area about 1.8 miles below the surface.
The report provides a new view of the processes involved in one of the world's most active volcanic systems, commented geophysicist Michael Hamburger of the University of Indiana.
Hamburger said the work is important because the team combined several types of research to provide a new and more detailed picture of the volcano's structure than previously available.
Michael F. Sheridan, a geologist at the State University of New York at Buffalo, said the Italians "assume a hard core that represents solidified magma beneath the volcano. Generally, people who interpret volcanoes look for a soft core. They're saying it's really a hard core filled with cracks and the magma is being squeezed up. So that could be a matter of controversy."
Sheridan said many scientists have studied Etna and some with other theories may disagree with Patane and his team. But, Sheridan added, their theory is coherent and "looks interesting to me."
Glen S. Mattioli, a geologist at the University of Arkansas, said the forecast for increasing future activity of the volcano seems "highly speculative."
However, he said "this is a solid piece of volcano seismology. The overall physical interpretations seem fairly sound ...."
Mattioli, Sheridan and Hamburger were not part of Patane's research team.
By Randolph E. Schmid