A volcano erupted without warning. Now, scientists know why.

Last year, one of the most dangerous volcanoes in Africa erupted without warning.

Somehow, Nyiragongo, a vertiginous volcano in the Congo, is always erupting: The mountain is crowned by a rare, persistent lava lake that is constantly fed by the magma churning below. But on May 22, 2021, his molten entrails found another route to the surface. They poured from fractures in the volcano’s flanks towards the metropolis of Goma, killing at least 31 people, injuring 750 others, displacing thousands more and leaving a trail of destruction in their wake.

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Now, in a new study published Wednesday in Nature, Delphine Smittarello, a geophysicist at the European Center for Geodynamics and Seismology in Walferdange, Luxembourg, and her colleagues have laid out how the eruption managed to ambush everyone.

Most volcanoes that are adequately monitored provide warning signals before they erupt. Magma breaking through rock creates distinctive types of earthquakes, deforms the earth as it rises, and releases noxious gases. Some volcanoes are so active that they always create remarkable chaos, but a distinct change in their usual, or “background,” behavior betrays their explosive ambitions.

Not the same for Nyiragongo in 2021. To the eyes of any pundit, it was business as usual.

“We couldn’t detect any dramatic changes that would tell us that an explosion is going to happen,” said Smittarello, the lead author of the new study.

Her team suspects that, before the outburst, magma intruded down Nyiragongo’s flank. But then, wait. Not only does the immobile magma remain silent, but the molten mass was already so close to the surface that if the flank had broken, it would have erupted immediately without the usual precursory scream.

And it was only a matter of time. On May 22, the flank — weakened over time by earthquakes and jagged and blackened by magma intrusions — gave way. For six hours the volcano wept from its freshly opened wounds.

This kind of unexpected eruption offers scientists a hard lesson: For every paradigm-changing secret they draw from their mountain subjects, “there are always things we don’t understand,” said Emily Montgomery-Brown, a geophysicist at the US Geological Survey. . Cascades Volcano Observatory which was not involved in the study. “It’s a good reminder not to get cocky.”

With its unusually fluid, fast-moving lava and ability to spew asphyxiating carbon dioxide gas into its surroundings, Nyiragongo is an extremely dangerous volcano that often endangers Goma, Congo, and Gisenyi, a contiguous city of Rwanda.

Nyiragongo’s flanking eruptions in 1977 and 2002 killed hundreds, but both were preceded by signs that magma was about to invade the surface: large earthquakes, strange convulsions in the lava lake and the eruption of nearby Nyamulagira volcano, underground magma pathways are in part winners with Nyiragongo’s.

Since 2015, a new seismic array has been set up in the area to listen to Nyiragongo’s magmatic music. Thanks in part to the endlessly bubbling lava lake, its soundtrack is as endless as it is powerful. Trying to distinguish unusual changes from the cacophony is like identifying a new voice in a huge crowd of people talking — not impossible, but extremely difficult.

Although the Goma Volcano Observatory has been beset by a myriad of political, technical and financial problems in recent years, its staff and global partners have been able to monitor the volcano during the eruption. And as far as they could tell, no precursors were detected before the 2021 eruption.

To make sure nothing had slipped under their radar, local and international scientists reviewed the scientific data they collected at the time, and their concerns were confirmed: Nyiragongo had not shown much seismic activity. His lava lake had not risen. His burp was par for the course and hadn’t changed shape significantly.

Even the most sophisticated observatory would not have seen this fire coming.

“This is a strange volcano,” said Benoît Smets, a geohazards expert at the Royal Museum for Central Africa in Tervuren, Belgium, and co-author of the study. Using traditional monitoring methods in Nyiragongo means “you won’t be able to detect these kinds of explosions.” And that makes this volcano even more dangerous than we thought.

Nyiragongo’s hidden potential is not unique. Other volcanoes can let their lava slide off landscapes relatively quietly, while others unleash unexpected bursts of steam. The hope is that by studying these eccentric explosions, one day – with the help of improved technological wizardry – some savior precursors will be identified.

But it is likely that we will never be perfect prophets of our volcanic future. “There may be things that we will never be able to predict,” Montgomery-Brown said.

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