Mount Etna's dual eruption pathways reveal competing volcanic mechanisms

Mount Etna has been erupting for thousands of years, and scientists have watched it closely for decades. But a team from Cornell University has just upended how we understand what happens inside this volcano before it explodes. Two of Etna's most powerful ancient eruptions, they discovered, did not just differ in size or intensity—they were driven by entirely separate underground systems, operating at different depths and on completely different timescales. The research, published in Geochemistry, Geophysics, Geosystems, reshapes the way volcanologists think about how a single mountain can behave, with consequences for how we assess eruption risk not just at Etna but at volcanoes worldwide.

The key to understanding what makes an eruption violent lies in gases. Magma rising from deep in the Earth contains dissolved gases—mainly water vapor and carbon dioxide—and how quickly those gases escape determines whether an eruption unfolds gently or explodes. The analogy is straightforward: a sealed soda bottle shaken and opened suddenly releases its carbonation violently, while one opened calmly releases it slowly. For years, scientists assumed water was the primary culprit in most volcanic explosions. But in 2023, this same Cornell-led group showed that carbon dioxide could trigger equally explosive eruptions, opening an entirely new lens for understanding what happens in the magma chambers before the ground shakes.

The breakthrough came from a technique called Raman spectroscopy, which allowed researchers to study microscopic gas bubbles trapped inside crystals formed as magma moved through the Earth. These bubbles are impossibly small—only 1 to 10 percent the thickness of a human hair—yet they hold a record of the pressure and depth at which they formed. By measuring the density of carbon dioxide in these bubbles, scientists could calculate the exact pressure, which translates directly into depth. This gave them an almost step-by-step picture of how magma traveled underground during historical eruptions, reconstructing the volcano's internal plumbing with a precision that was not possible before.

The first eruption the team examined occurred in 122 BC and ranks among the largest ever recorded at Mount Etna. It was a Plinian eruption—the most explosive category, named after Pliny the Elder, who witnessed Vesuvius's famous 79 AD blast. The magma was mafic, meaning it was low in silica and relatively fluid, rich in iron and magnesium. Using their crystal analysis, the researchers found that magma began rising slowly from about 22 kilometers underground. But instead of continuing to the surface, it stalled at a shallow depth of 2 to 5 kilometers, where it remained for several weeks. During that pause, the magma gradually shed its dissolved gas. When it finally erupted, it did so violently, with water vapor as the dominant force driving the explosion—a relatively shallow, water-driven mechanism that scientists have long associated with volcanic blasts.

The second eruption told a starkly different story. Known as the Fall Stratified event and dating to roughly 4,000 years ago, it operated through an entirely different pathway. Magma did not stall or gradually lose its gas. Instead, it surged upward rapidly from a much greater depth—between 24 and 30 kilometers below the surface—and reached the surface within hours. The driving force was not water but carbon dioxide, present in much higher concentrations. Because carbon dioxide is less soluble in magma than water, it begins bubbling out at greater depths and builds pressure much faster as magma rises. When CO₂ levels are high enough, eruptions originate from far deeper underground, happen much faster, and provide far less warning than shallower, water-driven events.

What makes Mount Etna exceptional is its geological position. Most volcanoes around the world are dominated by one type of volatile system. Oceanic island volcanoes like those in Hawaii tend to have higher CO₂ levels. Volcanoes in subduction zones—where one tectonic plate slides beneath another, as in much of South America and Southeast Asia—tend to be water-dominated. Mount Etna sits in a rare middle ground where both volatile systems are active and competing, making it one of the best natural laboratories on Earth for studying how these two mechanisms interact. Professor Esteban Gazel, who led the project, emphasized that Etna's significance lies precisely in this intersection of two different volcanic worlds. The volcano is essentially running both systems simultaneously, and whichever one dominates at any given moment determines the character of the eruption that follows. This discovery has immediate implications: understanding which system is active at Etna at any particular time could improve forecasting not just for this volcano, but for how scientists assess eruption risk globally.