Carbon dioxide and water played key role in historic Mount Etna eruption

The plumbing systems of volcanos are vast and complex. But they aren't consistent, even in the same volcano.

A Cornell-led collaboration found very different mechanisms behind two historic eruptions of Mount Etna in Italy. Understanding these dynamics - combined with the techniques that revealed them - can help geologists assess the risk of future eruptions.

The findings published June 2 in Geochemistry, Geophysics, Geosystems. The first author is former postdoctoral researcher Maxim Gavrilenko.

The project was led by Esteban Gazel, the Charles N. Mellowes Professor in the Department of Earth and Atmospheric Sciences in the Cornell Duffield College of Engineering, whose research explores the way volcanos function - specifically what makes an eruption explosive and what are the mechanisms that control it.

Explosivity is determined by a range of factors, from the viscosity of magma to the volatiles - or the separating gasses - that are trapped inside it.

"Imagine a bottle of soda. If you open that bottle without agitating it, you can drink it, but if you shake it up, all the bubbles get separated really fast, and you have an explosion," Gazel said. "Volcanoes work in a similar way, and my lab is trying to quantify these processes."

The most crucial volatiles are water and carbon dioxide. For a long time, the geological community thought water was the primary volatile driver of volcanic eruptions, but in 2023 Gazel's group showed that carbon dioxide can trigger explosive eruptions. The researchers made that discovery by pioneering a new method that uses Raman spectroscopy to peer into crystals formed in magma and measure the tiny micron-sized bubbles that are roughly 1-10% the thickness of human hair.

"That technique gives us the density of CO2, and using a state equation we can transform that density into pressure, and pressure can be transformed into depth," Gavrilenko said. "Then we apply those techniques to these explosive eruptions, and we are able to reconstruct the plumbing system with an unprecedented precision."

Hoping to study a simplified system that involves mostly volatiles, the researchers selected Mount Etna, which, as volcanoes go, is a relatively gentle giant. However, it has had several aggressive explosions in the deep past. One of the largest on record came in 122 B.C. It was both "mafic" - with low-viscosity magma that's rich in magnesium and iron - and Plinian - the most explosive category of eruptions, named for Pliny the Elder, who first described them when Mount Vesuvius erupted in 79 A.D.

Collaborators and co-authors Terry Plank of Columbia University and Bruce Houghton of the University of Hawaii, Manoa, traveled to Mount Etna and conducted a systematic field sampling. After sequencing and measuring the magma crystals, the researchers determined that in the 122 B.C. eruption, magma from a depth of about 22 km slowly made its way toward the surface, but paused for several weeks at a shallow level of 2 to 5 km, where it gradually released gas before eventually erupting.

The team collected new data and compared their results with data from samples of an earlier eruption, known as the Fall Stratified event, nearly 4,000 years ago. In that case, the magma had risen quickly from a deeper level of the mantle, roughly 24 to 30 km, and erupted in a matter of hours, propelled by a much higher concentration of CO2.

"Some volcanoes are only high CO2, mostly in oceanic islands, and some volcanoes are mostly controlled by water, such as the ones in subduction zones. Etna is one of the few volcanoes in the world where you have the two volatile species competing," Gazel said. "This shows that at a certain threshold of CO2, the eruption will come from very deep and really fast, but when you have a higher threshold of water, then the process is controlled at shallow levels."

Gazel's team is now applying its method to volcanoes in Chile, Hawaii and many other locations.

"Ideally this should be done in every volcano on the planet," he said. "This is data we need for physical models of eruptions that are the base of risk assessment."

Mount Etna is not only an ideal test bed for parsing the deep intricacies of volcanic behavior; Gazel also appreciates its significance in Greek mythology, where it is known as the burial site of the giants Typhon and Enceladus, who were defeated by the Olympian gods.

"There may be the two giant mythological monsters under Etna," Gazel said. "And if you look at the plumbing system of the Plinian eruption, it's like Typhon, because it's elongated and serpentine, and the other one is Enceladus, because it's kind of smaller. If you work in Etna, it's hard not to be connected to history, classical work and great food."

Co-authors include postdoctoral researchers Kyle Dayton and Ellyn Huggins; and Anna Barth of University of California, Berkeley.

The research was supported by the National Science Foundation.