Follow the Magma

At dawn on March 11, 1669, a crack 2 meters wide and 9 kilometers long gaped open on the southern flank of Mount Etna on the island of Sicily. Within a week, lava surged toward the stone walls surrounding the coastal city of Catania. The eruption killed most of Catania's 20,000 residents and destroyed much of the city, even filling the moat of its signature Ursino Castle with molten rock.

The 1669 event was arguably the fastest and largest lava flow ever documented for Etna, yet a future eruption could easily surpass it. The volcano has erupted about once a year during the last decade, and it releases not only molten flows but also spectacular explosions of lava that can fountain hundreds of meters into the air. While most of Etna's eruptions occur far from the million-plus people who now live on its flanks, a new fissure could open anywhere on its 40-kilometer-wide, crater-scarred expanse, and lava could take a village—or Catania—again.

Today, scientists watch Etna more thoroughly than any other volcano in Europe. Their data—which include seismic readings, gas measurements, thermal images, and more—can now warn of eruption days, weeks, and months ahead, giving authorities ample time to detour air and ground traffic, alert local communities, and prevent a catastrophic loss of life.

"Until 15 or 20 years ago, volcanology was basically a science made of visual observations," explains Salvatore Giammanco, a geophysicist at the Catania branch of Italy's National Institute for Geophysics and Volcanology, called INGV. "Today, by using technology, we can get an enormous amount of information, and that helps us make computer models of the behavior of the volcano." Together, the varied data give researchers a detailed picture of how the entire system is evolving in time and space.

Giammanco and his colleagues comb the slopes of Mount Etna for indirect clues to future eruptions. Like most volcanoes, Etna is located near the collision boundary of two tectonic plates-giant slabs of Earth's crust and uppermost mantle, or lithosphere. As the African Plate sinks beneath the Eurasian Plate, which underlies Etna, it releases extremely hot water, causing partial melting of the Eurasian Plate's upper mantle. This melted rock can exude through fractures to the surface, creating a volcanic eruption. The plate movement also produces lots of earthquakes. More than 75 seismic stations planted at various altitudes on Etna and the eastern coast of Sicily help scientists sense changes in the motion of the plates and reconstruct the movement of magma below. But detecting these vibrations is just one part of the prediction technique.

One reason magma rises is that it contains dissolved gases. These gases can rapidly expand, sometimes explosively, to force magma to the surface. The process is similar to opening a shaken can of soda or champagne: dissolved carbon dioxide expands rapidly into bubbles when pressure is released and then spurts out the opening. Before an eruption, the movement of the gases in the magma produces seismic tremors. "A volcanic tremor is a continuous vibration," says INGV seismologist Susanna Falsaperla. "A kind of breathing of the volcano itself." A change in the size of the tremor signal on a seismometer signals that magma is moving—perhaps even rising to the surface.

But volcanic gases can also be measured directly, because some gas seeps out of the four craters at Etna's summit and the soil on its flanks. The more magma below, the more gas above. "The summit typically releases between 2,000 and 3,000 tons of sulfur dioxide gas per day," says Giammanco. "Before an eruption it can reach 20,000 tons per day." Etna's gas emissions are monitored constantly. Six stationary sensors ring the summit—they are among the first deployed at any volcano—and constantly document gas concentrations and radio the data to a control room in Catania. To monitor gas emissions on the flanks, Giammanco and his colleagues must travel into fault zones with portable sensors in cars, in briefcases, or even mounted on top of helmets.

The ratio of one gas to another in these emissions can indicate how soon the volcano may erupt. Various gases require different amounts of pressure to remain dissolved in magma. As magma rises toward the surface, the surrounding pressure drops, so gases with low solubility (such as carbon dioxide and helium) come out first. Sulfur dioxide, hydrogen sulfide, and water vapor follow. At very low pressures, gases such as chlorine and fluorine release.

Scientists can use gas indicators to track magma as it rises to the surface and predict when it might break through. For example, when rising magma reaches about 15 to 20 kilometers below ground, it releases mainly carbon dioxide and helium. When the magma comes within 3 or 4 kilometers of the surface, it starts to release sulfur dioxide, hydrogen sulfide, and water vapor. As the magma continues to ascend, it sheds chlorine, and then fluorine. Increases in carbon dioxide typically occur six months to one month before eruption, while increases in sulfur dioxide precede eruptions by several weeks to several days. "And when we observe increases in fluorine from the craters," says Giammanco, "that means the magma is there—it's almost arrived at the surface."

At 3,350 meters high, Etna dominates Sicily's temperate landscape with a peak that is white with snow for most of the year. But magma rising underground can warm the volcano's surface rock. Researchers make regular helicopter flights over Etna with infrared cameras that can see this heat, while stationary cameras keep a constant watch on the summit. By indicating places where the ground is heating up, thermal imaging can reveal the presence of hidden fractures through which magma may be moving toward the surface.

Etna's volcanologists also collect many other kinds of data using a variety of instruments. GPS devices allow them to notice ground deformationbulging, stretching, or twisting of the earth. They also measure the length of existing fractures to note any changes. Among the more sophisticated tools is Doppler radar, which is not yet widespread among volcano observatories. The radar measures the velocity and quantity of ash and other material ejected during an eruption. Etna's emissions can reach as far as Africa and Greece. "We can use these data to simulate the eruption using computer models, and forecast dispersal of volcanic material in the atmosphere," explains Mauro Coltelli, the head of INGV.

Much of the data collected on Etna are transmitted, in real time, to INGV's central control room in Catania. There, a wall of digital data screens showing seismic, gas, thermal, and other indicators keeps the pulse of this active system 24 hours a day, like the monitors of a patient in surgery.

Prediction of volcanic eruptions is not an exact science, yet a monitoring system as advanced as Etna's can give authorities months, weeks, or hours of warning that an eruption may come. "Only when we see chlorine and fluorine and when seismic activity begins can we tell the civil defense 'Okay, this is itan eruption will start in a matter of hours,'" says Giammanco. "It's more difficult to say early on where the next eruption will occur. Location can only be identified some hours before it begins."

The monitoring system allowed INGV scientists to successfully alert airports several hours preceding Etna's most recent major eruption, which occurred in 2002 and 2003. Ash fell for two straight months on Catania, forcing people to carry umbrellas and wear masks to protect their lungs. The scientists also simulated the possible path of the lava flow, concluding that it would not endanger any towns. Because of their predictions, unnecessary evacuations were avoided.

But volcanologists caution that it is impossible to predict every eruption. Sometimes, the system will flag moving magma that doesn't erupt at all—it invades the crust but never reaches the surface. And in 2004, there was a "silent eruption," a slow flow of lava from an old fissure without the typical warning signs. "There was no change," notes Falsaperla. "That means that we must be aware of the possibility that an eruption could start even in this silent mode." Loud or quiet, Etna's restlessness shows no signs of flagging, and it needs constant technological vigilance so scientists can stay a step ahead.