Scientists unlock Mount Etna's volcanic plumbing, revealing why eruptions vary from weeks to hours

Mount Etna has erupted in radically different ways across its history. Some of its magma has crept upward slowly, stalling near the surface for weeks before finally breaking through. Other magma has raced from depths of nearly 30 kilometers straight to the surface in just hours. A team of researchers led by Esteban Gazel at Cornell University has now figured out why—and the answer lies in the invisible gases trapped inside the rock.

The team used a technique called Raman spectroscopy to examine microscopic gas bubbles preserved in crystals that formed inside ancient magma. These bubbles, only 1 to 10 percent the thickness of a human hair, hold a record of the pressure and depth at which they formed. By measuring the density of carbon dioxide and water vapor in these trapped pockets, the researchers could work backward to determine where the magma started its journey and how fast it moved. The method is straightforward in principle but requires extraordinary precision: transform gas density into pressure, pressure into depth, and depth into a map of the volcano's internal plumbing.

The researchers focused on two eruptions from Mount Etna's past. The first occurred in 122 B.C. and was among the volcano's most violent on record—a Plinian eruption, named after Pliny the Elder's account of Mount Vesuvius in 79 A.D. When the team reconstructed this event using their new technique, they found that magma began rising from about 22 kilometers below the surface. But instead of shooting straight up, it slowed dramatically. The magma stalled at a shallow depth of 2 to 5 kilometers and sat there for several weeks, gradually releasing gas, before the eruption finally occurred.

The second eruption they studied, known as the Fall Stratified event and dating to nearly 4,000 years ago, followed an entirely different script. Magma rose from 24 to 30 kilometers deep—even deeper than the 122 B.C. event—but it did not linger. It raced upward and erupted within hours. The key difference the researchers identified was the concentration of carbon dioxide. The rapid eruption was driven by much higher levels of CO2, while the slower, weeks-long ascent was dominated by water vapor.

This finding reframes how scientists think about volcanic explosiveness. For decades, researchers assumed water was the primary gas responsible for driving eruptions. Gazel's group showed in 2023 that carbon dioxide plays an equally important role, and this new work reveals how the balance between the two gases shapes the entire character of an eruption. When carbon dioxide reaches a certain threshold, it can push magma upward from great depths at remarkable speed. When water becomes the dominant volatile, the process unfolds more slowly, with magma stalling at shallower levels where it has time to degas gradually.

Mount Etna, Gazel noted, is one of the few volcanoes on Earth where these two gases compete for control. Some volcanoes are dominated almost entirely by CO2, typically those on oceanic islands. Others, like those in subduction zones where oceanic plates collide with continental crust, are controlled primarily by water. Etna sits in a rare middle ground where both gases matter, making it an ideal natural laboratory for understanding how their interplay shapes eruptions.

The implications extend far beyond Etna. Gazel's team is now applying the same Raman spectroscopy technique to volcanoes in Chile, Hawaii, and other regions around the world. The goal is ambitious: to eventually map the magma plumbing systems of every major volcano on the planet. If scientists can determine where magma begins its ascent, how quickly it moves, and which gases are driving that movement, they can build far more realistic models of how dangerous eruptions unfold. Those models form the foundation of volcanic risk assessment—the calculations that help governments and communities prepare for and respond to eruptions. The research was published in Geochemistry, Geophysics, Geosystems, with former postdoctoral researcher Maxim Gavrilenko as the lead author. The work was supported by the National Science Foundation.