Perched on the northeastern edge of Sicily, Italy’s Mount Etna is a hyperactive volcano capable of producing incandescent lava flows as well as explosive, lightning-surrounded pyrotechnics. It’s also sliding into the Ionian Sea—and a new study provides fresh evidence as to why.

It’s been known for some time that the so-called Roof of the Mediterranean has been on the move. Etna is not slipping quickly; on average, its migration is happening at a rate several times slower than the growth rate of your fingernails. But geologists have been hunting for the exact cause of the volcano’s motion, since it’s linked to the risk that the fiery mountain may suffer a catastrophic collapse.

About a million people live on Etna’s slopes, and millions more reside on the coastlines across the Ionian sea. If part of the volcano near the shoreline becomes unstable and falls into the water, it could create mega-tsunamis that would devastate the shores of the eastern Mediterranean.

“A massive collapse would be a disaster for a vast and densely populated area,” says Boris Behncke, a volcanologist at the Etna Observatory at Italy’s National Institute of Geophysics and Volcanology who was not involved in the latest work. (Find out why people chose to live in the shadows of active volcanoes.)

Etna’s slippery slope

For their new study, published today in Science Advances, a team led by Morelia Urlaub at the Helmholtz Centre for Ocean Research in Kiel, Germany, deployed several underwater transponders around Etna’s southeastern flank, which they suspect is the most mobile section of the mountain.

These transponders contained pressure sensors that picked up on the slightest movements of the offshore flank. The devices also recorded their positions relative to each other, which meant that the team could detect movement of the flank compared to the more stable parts of the terrain.

According to the team, their results show that gravity is the primary force causing this flank of the volcano to move. Magma rising inside the volcano also plays a role, but the team thinks it has less of an overall effect on Etna’s seaward slide.

The new results “take us into the exciting realm of underwater monitoring for the first time at Etna,” says volcanologist John Murray of the U.K.’s Open University, who was not involved in the new work. Murray led a previous study that also looked at Etna’s slippage, and he says the new data are in line with his team’s observations, in that “magmatic forces are less important than gravitational spreading in the outward expansion of Etna.”

Until recently, many experts thought that shallow magma injections within the fiery mountain were the primary drivers of this volcano’s displacement. Indeed, during some of Etna’s eruptions, monitoring devices have recorded movements of tens of feet. This makes sense: Rising magma can inflate parts of the mountain, adding extra weight to sections of it and causing structural weaknesses to appear.

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But Etna’s southeastern flank tends to slip in fits and bursts, and not all of that motion is linked to internal, molten turmoil.

Keeping a close eye on things between April 2016 and July 2017, the latest monitoring effort detected one case of major movement around mid-May of 2017, when the volcano’s flank jutted forward into the sea by an inch or two. This activity coincided with the eight-day movement of a local fault.

The team agrees that rising magma does play a role, because other flank accelerations match up nicely with unambiguous intrusions of new molten material. But the fact that such huge deformations are also occurring far from the magma-dominated summit suggests that gravity is the star of the show—a notion shared by other research groups.

In April, Murray’s team reported on their work using hundreds of onshore GPS kits to assess Etna’s movement. Their data indicated that, from 2001 to 2012, Etna moved toward the Ionian Sea in a southeasterly direction at a rate of 0.6 inches (about 14 millimeters) every year. These researchers also suspect that gravity is the driving force, pushing Etna along on a layer of loosely packed sediments.

Gravity will bring you down

The April study suggested that the entire volcano was moving, but the new paper only looked at the southeastern flank. Still, with both studies in mind, “it seems that the consensus is shifting toward gravitationally driven sliding as the dominant mechanism” for Etna’s movement, says Urlaub.

The new study’s interpretations are quite reasonable, Behncke says, although he adds that the situation is complex, and it’s likely that contributions from gravitational pulls and magmatic movements vary with time. Both factors are also connected, with gravitationally driven flank movements allowing magmatic intrusions to take place.

“It's very difficult to make definitive statements unless the methods used by the authors are applied over a much longer period, encompassing a broader area,” he says.

There’s also the question of whether the southeastern flank movement could one day turn into a catastrophic collapse. Urlaub’s data indicates that it’s possible, although she notes that there’s not yet enough information to say for sure. Geologists need decades’ worth of monitoring data before they can tell the difference between normal and fast slippage.