On Sunday, June 3, 2018, at two in the afternoon, Marco Antonio Pamal, a native of Escuintla (Guatemala), sat down to eat with his family. The nearby Volcán de Fuego emitted a column of smoke from the early hours of the morning, but for communities used to living on the slopes of one of the most active volcanoes in the world, there was no reason to interrupt their routine. Then they heard a crash: when they went out, the avalanche of ash, sand and mud was already burying their town. Marco Antonio lost 17 family members that day; more than 300 Guatemalans died in an eruption that affected the entire country.
Although popular culture associates the destructiveness of volcanism with lava, pyroclastic flows – clouds of burning gas and rocky material that travel great distances very quickly – are much more dangerous. Versus the strombolian-type eruption that occurred on September 19, 2021 on La Palma, that of the Fire Volcano would reach the category of ‘Vesubian’, in memory of the phenomenon that buried Pompeii without giving its inhabitants time to escape.
“Most volcanoes erupt both effusively and explosively, the latter behavior being responsible for the majority of human pathway losses“, explains a new article published in the magazine Nature Geoscience, of the group Nature. Its authors, from the ETH Zurich research center in Switzerland, consider that the first great question of volcanology, “When will the next eruption of a volcano be?”, Can already be answered relatively precisely. The monitoring of seismic activity on La Palma, as a prelude to the eruption in Cumbre Vieja has shown it, they argue.
Therefore, they have considered answering the second: “Will it be a more effusive eruption, emitting viscous lava, or explosive, with kilometers of ash columns thrown into the atmosphere? “. It is a” headache “for researchers, confesses Oliver Bachmann, professor of Magmatic Petrology at the Swiss institute. Although explosive phenomena have not been the most characteristic recently in places like the Canary Islands or Hawaii, there are devastating examples beyond Volcán de Fuego: In the Philippines, the eruption of Mount Mayon killed 1,200 people in the 19th century, and that of Pinatubo in 1991 was powerful enough to alter the world’s climate.
The difficulty in predicting the behavior of a volcano is a constant, as volcanologists have reported on La Palma. But Bachmann and his colleagues started from one base: importance of gases dissolved in magma within the chamber. If the amount is high, the reduction in pressure during the ascent until it emerges as lava would facilitate the appearance of gas bubbles, “as would happen when shaking a bottle of champagne and uncorking it,” they explain. Unable to escape otherwise, these pockets would favor an explosive eruption. A magma in which less gas has dissolved, on the other hand, would lead to a more “calm” production of lava, as that of Cumbre Vieja was defined in the early days.
To test the hypothesis, the researchers analyzed data from 245 volcanic eruptions, reconstructing the temperature of the magma in the chamber, the amount of solid crystals present and what proportion of dissolved water, responsible for the formation of gas bubbles, could be found. They concluded that from 3.5% water dissolved in magma, the possibility of an explosive eruption became “very high”. The absence of crystals was also associated with this risk, as they form channels in the chimney that help the gas to escape.
A final surprise
Logic would lead to think that the higher the water content, the higher the explosiveness. But nothing about volcanoes is easy or predictable. Thus, they discovered that from a concentration of 5.5% of dissolved water, the risk of an explosive eruption decreased again. What would happen then is the gas bubbles would already be forming in the magmatic chamber before ascending through the chimney, acting as “buffers” of the pressure that accumulates underground.
Postdoctoral researcher Răzvan-Gabriel Popa, co-author of the work, explains it like this: these gas bubbles would be compressed by the emerging material from below, containing and slowing the pressure increase in the magmatic chamber. At the same time, this would allow the lava to absorb more heat. The lava would then have a higher temperature when it emerged and would be less viscous, which would allow the gas to escape more easily and would attenuate the explosiveness of the eruption.