Geochemistry, Mineralogy, Volcanology News

Etna Under Pressure: Does Gas Buildup Foreshadow Eruption?

Pressure from both magma and gas can trigger eruptions. Monitoring degassing can help predict eruptions but only if the magma system is well understood first.

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With a yearslong monitoring effort, scientists have tracked the buildup and release of gas pressure beneath Italy’s Mount Etna leading up to its Christmas Eve eruption in 2018. According to new research, some types of gas built up continuously within the volcanic system for a year prior to the eruption, whereas pressure from other gases ebbed and flowed.

“We are now able to quantify the pressure accumulation in the magmatic reservoir while [the buildup is] developing,” said Antonio Paonita, a geochemist at Istituto Nazionale di Geofisica e Vulcanologia Sezione di Palermo in Italy and lead author of the study. “We are able to recognize a process occurring inside the volcano which causes eruptions and to state, ‘Ok, it is starting, it is growing, it is at critical levels!’ At Etna it lasts months to years.”

When combined with advanced theories for how magma recharges within Etna’s magmatic system, one of the most active in the world, this new insight into how pressure builds up will help scientists better understand and predict its eruptions.

Gases Track Magma Movement

What goes on inside a volcano’s magma reservoirs controls the volcano’s behavior at the surface. Magma and volatile gases flow into a chamber from deep within Earth and sometimes find nonexplosive ways out through conduits, sills, and dikes (as for magma) or degassing vents. Volcanoes tend to erupt when a magma chamber becomes overpressurized, which can happen when more magma and gas enter the chamber than leave it.

Pressure buildup in some volcanic systems can stretch for tens of years leading up to an eruption. Other systems, however, such as Kīlauea and the Alaskan volcanoes in the United States, the Aleutians that stretch between Alaska and Russia, Nyamuragira in the Democratic Republic of the Congo, and Mount Etna, “normally exhibit quicker dynamics and frequent eruptive activity” spanning months or years, Paonita explained. As theories of magma recharge of a reservoir become more sophisticated, surface and remote monitoring of volcanic degassing can help researchers estimate the overall state of pressurization within a volcanic system and anticipate its eruption potential.

Mount Etna is an excellent testing ground for such theories for a number of reasons: It erupts regularly, scientists have modeled its magma recharge system, and an extensive network of instruments continuously monitors the volcano’s degassing. With a combination of remote sensing and on-site sampling, the researchers analyzed the degassing patterns of carbon dioxide (CO2), sulfur dioxide (SO2), hydrochloric acid, and helium isotopes starting roughly a year prior to the 24 December 2018 eruption until about a year after.

“Different gases are released from magma at different depths along the [magma] ascent path,” Paonita explained. “Roughly speaking, each gas is then indicative of a range of depth where degassing is occurring and magma dynamics is acting.”

The team found that the helium isotope ratio emitted by Etna steadily rose from the first half of 2017 all the way up to the eruption, after which it dropped significantly. CO2 flux, which had been low and steady, increased in June 2018 and then fell and rose again cyclically until the eruption. SO2 flux, however, rose only in the weeks immediately preceding the eruption (from about 5,000 metric tons per day to about 12,000 metric tons per day) and fell back to typical levels after the eruption.

“Our study highlighted an imbalance between the amount of gas normally rising with the magma from the mantle beneath a volcano and that emitted in pre- and inter-eruptive phases,” Paonita said in a statement. “Recognizing and quantifying this ‘imbalance’ and its evolution almost in real time provides a new interpretative key for evaluating the ‘state of activity’ of the volcano.” The team published these results in Science Advances on 1 September.

Bespoke Volcanic Systems

Mount Etna looms over Sicily’s second-largest city, Catania; has erupted 50 times so far in 2021; and has grown 31 meters taller in the past 6 months. Can these results help volcanologists predict eruptions? Perhaps for Mount Etna, but probably not for other volcanoes.

Long-term surveillance of volcano degassing could anticipate an eruption, but its predictive power ultimately depends on how well we understand a particular magmatic system, Paonita said. A rise in the concentrations of CO2 and noble gases, for example, generally points to magma movement many kilometers below the surface, whereas a sharp rise in SO2 implies magma moving closer to the surface. “In theory, we can interpret degassing data with no basic knowledge of [a] system,” he said.

In reality, however, volcano behavior depends on the internal structure of a magmatic system, and although Mount Etna is an archetype of open-conduit volcanoes, each one is unique. “Our experience on that volcanic system—for example, long records of signals to be compared to its eruptive behavior—and our geological knowledge are precious and irreplaceable information for interpreting monitored signals in the right context.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

Citation: Cartier, K. M. S. (2021), Etna under pressure: Does gas buildup foreshadow eruption?, Eos, 102, https://doi.org/10.1029/2021EO163119. Published on 15 September 2021.
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