Secrets Still Smolder at One of the World’s Most Active Volcanoes

A crackling burst cut through the air on June 17, 1923, as Mount Etna—one of the world’s most active volcanoes—unleashed a mighty eruption. After burbling and shaking that fateful day, the peak convulsed, emitting an uproar like “the discharge of a thousand massed guns,” according to a dispatch from Rome to the Australian Mercury newspaper the next day.

Lava poured from a deep fissure on the volcano’s northeast flank, flooding nearby vineyards and pine forests with molten rock. As a hailstorm of searing hot stones set fields ablaze, the incandescent river reached a small hamlet on the outskirts of the Sicilian town of Linguaglossa.

“Lava started nibbling on the first houses,” says Boris Behncke, a volcanologist at the National Institute of Geophysics and Volcanology. The people of Linguaglossa hopped into action, rushing to the church to collect the statue of their protector saint, Sant’Egidio (known in English as Saint Giles). They carried the saint in a procession through the streets and to the edge of the crackling lava flow. And suddenly the lava seemed to halt, narrowly passing by the village’s edge.

While this eruption—and most others at Etna since—haven’t caused many deaths, the volcano is capable of intense destruction. During Etna’s most damaging eruption, in 1669, lava flowed from the peak for months, stretching more than ten miles away and entombing villages and fields that lay in its path.

Today, Etna still unleashes frequent fiery blasts, often with startling variety. Sometimes it spews torrents of lava, and other times it explodes with ferocious bursts. With some one million individuals living on Etna’s slopes, the volcano has become one of the best monitored in the world—and a proving ground for cutting-edge technology that can sensitively take the pulse of active geologic beasts.

“It’s really a volcanic playground for scientists,” says Behncke, who has been fascinated by Etna since his first visit as a student in 1989. At the time, Etna was erupting in spectacular fashion, shooting fountains of incandescent rock from fissures along its flanks and explosive bursts at its peak. “It was almost as though this volcano was showing me, ‘This is what you get,’” he says.

The more scientists study Etna, the more surprises it seems to reveal. But one of the longest-running mysteries of the volcano is why it formed at all.

Most volcanoes stand above a subduction zone, where a dense oceanic tectonic plate plunges beneath a more buoyant continental plate. As the oceanic plate descends, it releases small amounts of water that reduce the melting point of the rocks above, sparking the formation of magma that can erupt at the surface.

Etna sits near where the African plate meets the Eurasian plate, but from there the situation grows complex, Behncke explains. Some swaths of the African plate carry bits of thick, buoyant continental crust. So instead of plunging down into the mantle, these edges of the plate collide with Eurasia in a geologic battle that raises mountains. Off the shore of eastern Sicily, however, a section of the African plate plunges beneath the Eurasian plate, which drives the volcanic eruptions of the Aeolian Islands.

But Etna doesn’t sit above this plunging plate. Instead, it sits to the side. This unusual position has left scientists scratching their heads about where Etna’s magma comes from.

One possibility, Behncke says, is that this complicated mix of tectonic shifts is tugging parts of Earth’s crust apart, sparking formation of magma in the upper mantle. The nearby subduction of the African plate may also be a possible source of magma. As it plunges into the mantle, the slab seems to be tearing while it sinks at a steep angle, both of which could send the mantle welling up toward Etna.

Despite these and other theories, the precise source driving Etna’s fiery fury remains uncertain. As Behncke puts it: “It’s a big mess.”

Another important process shaping the volcano is its slow slide toward the Ionian Sea, creeping along at just a few centimeters each year. This shifting might help explain the frequency of Etna’s eruptions, says John Murray, a volcanologist at the Open University who has studied Etna for more than 50 years. His experiments reveal that such sliding creates a pattern of deep cracks similar to what’s seen on Etna today. These fissures provide conduits through which magma can rush to the surface, much like the northeast rift where the 1923 eruption burst through, Murray says.

At the same time, Behncke adds, the squish of magma to the surface can also contribute to the shifting peak. As the magma shoots upward, it shoves Etna’s flank further toward the sea.

No matter the magmatic source, Etna’s recent explosive vim and vigor—with more paroxysms in the last 25 years than the preceding 300—has made one thing clear: The volcano has much more firepower in store. “The activity at the moment is absolutely huge,” Murray says. “And it looks as if the eruption rate is still going to climb.”

The intensity of Etna’s modern activity is an acute reminder of the hazards of volcanoes around the world. Yet it’s also a boon for scientists’ ability to not only study the volcano’s inner workings but also develop better ways to communicate with communities at risk and test next-generation monitoring tools.

In recent years, Philippe Jousset, a geophysicist at German Research Center for Geosciences Potsdam, and his team used fiber optics to monitor Etna’s tiniest shivers. When light is pulsed down a fiber-optic strand, small imperfections along the cable bounce a fraction of that light back toward the source. Tiny geologic shakes jostle the cable, affecting the timing and properties of these reflected flashes, allowing scientists to locate vibrations in the ground.

Jousset and his colleagues installed a fiber-optic cable near Etna’s summit to track the volcanic rumbles. As Etna fell silent, the scientists were surprised to see a series of very small trembles that seem to originate from the movement of fluids and gas underground.

Scientists hadn’t previously spotted these signals at Etna in data collected by the volcano’s network of seismometers, which are traditionally used to monitor our planet’s tremors and quakes. But the discovery prompted a closer look at seismometer data, revealing one station near Etna’s peak that recorded a tiny bump corresponding to the geologic gurgle. Other stations were too far away to listen in, so the tiny tremble had been presumed to be noise. Fiber optics, however, provided a dense line of points from all along their length, helping scientists spot the small burbles.

Such signals could provide invaluable clues to the volcano’s subterranean happenings during periods of quiescence, when scientists have few ways to determine what will happen next. “There may be some signs telling us that we could get the next eruption at this location or that location,” Jousset says.

The sheer quantity of information about Etna and length of many of the records of study also hold promise for continued volcanic insights. For the Open University’s Murray, this will mark his 54th year surveying Etna through a technique called leveling. This method provides a detailed look at Etna’s changing shape through relative differences of surface height. The process requires that one person hold a graduated staff while another person stands at a distance to collect measurements through a telescope-like instrument sitting level atop a tripod.

While surface changes are now largely monitored using satellites, Murray’s work recently revealed important details that can still primarily be gleaned from leveling. Near the volcano’s summit, ash and volcanic rubble obscure the ground and cause constant shifts in the surface, making satellite analysis challenging. However, Murray’s decades of careful measurements show that Etna’s peak is slowly sinking—and more revelations likely await.

The data set “becomes more interesting the more you keep measuring,” Murray says. “I’m going to carry on for as long as I can.”