A volcano erupted without warning. Now scientists know why.

Last year, one of Africa’s most dangerous volcanoes erupted without warning.

In a way, Nyiragongo, a dizzying Congo volcano, keeps erupting: the mountain is crowned by a rare, persistent lava lake, constantly fed by churning magma below. But on May 22, 2021, its molten innards found another way to the surface. They oozed out of fissures in the volcano’s flanks towards the metropolis of Goma, killing at least 31 people, injuring 750 others, displacing thousands and leaving a trail of destruction in their wake.

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

Most properly monitored volcanoes provide warning signals before they erupt. Magma working its way through rock creates different types of earthquakes, deforms the land as it rises, and releases noxious gases. Some volcanoes are so active that they always cause noticeable havoc, but a marked change in their usual or “background” behavior betrays their eruptive ambitions.

Not so for Nyiragongo in 2021. It was business as usual for any expert.

“We couldn’t see any dramatic changes that would tell us there’s going to be an outbreak,” said Smittarello, the lead author of the new study.

Her team suspects that magma seeped under Nyiragongo’s flank before the attack. But then it waited. Not only does immobile magma remain silent, but the molten mass was already so close to the surface that if the flank broke off without the usual precursor, it would have erupted immediately.

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

This type of unannounced eruption teaches scientists a harsh lesson: For every paradigm-shifting mystery they extract from their mountainous subjects, “there are always things we don’t understand,” said Emily Montgomery-Brown, a geophysicist at the US Geological Survey’s Cascades Volcano Observatory who 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 suffocating carbon dioxide gas into its surroundings, Nyiragongo is an extraordinarily dangerous volcano, frequently endangering Goma in Congo and Gisenyi, a neighboring town in Rwanda.

Nyiragongo’s flank eruptions in 1977 and 2002 killed hundreds, but both were preceded by signs that magma was about to rush to the surface: large earthquakes, strange tremors in the lava lake, and the eruption of nearby Nyamulagira volcano, partially engulfing its underground magmatic lanes are with Nyiragongos.

Since 2015, a new seismic array has been constructed in the region to listen to the magmatic music of Nyiragongo. Thanks in part to its endlessly bubbling lava lake, its soundtrack is as endless as it is loud. Trying to hear unusual changes in the cacophony is akin to identifying a new voice in a gigantic crowd of people talking—not impossible, but extremely difficult.

Although the Goma Volcano Observatory has been plagued by myriad political, technical, and financial problems in recent years, its staff and global partners managed to monitor the volcano at the time of the eruption. And as far as they could tell, no precursor signals were spotted before the 2021 eruption.

To make sure they hadn’t missed anything, local and international scientists reviewed the scientific data collected at the time, and their concerns were confirmed: Nyiragongo had not shown any particular seismic activity. His lava lake hadn’t worked. His belch had been normal, and it hadn’t changed shape much.

Even the most modern observatory could not have seen this conflagration 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. The use of traditional monitoring methods on Nyiragongo means that “you cannot detect these types of eruptions”. And that makes this volcano even more dangerous than previously thought.

Nyiragongo’s stealth abilities are not unique. Other volcanoes can discharge their lava relatively quietly from rugged landscapes, while others unleash unexpected bursts of steam. The hope is that by studying these eccentric eruptions – aided by enhanced technological wizardry – some life-saving precursors will one day be discovered.

But it is possible that we will never become perfect prophets of our Vulcan future. “There can be things that we can never predict,” Montgomery-Brown said.

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