Mount Nyiragongo, one of the world’s most dangerous volcanoes, erupted without warning in May 2021 in Congo. Lava erupted from fissures and flowed down the mountain toward cities below, killing or injuring hundreds and killing or injuring hundreds more.
Scientists have cobbled together to see how that unexpected outbreak happened using information from monitoring stations mounted near the volcano in 2015. The data also suggest that the incident could have been even deadlier — emphasising the critical need to better understand this volcano’s specific hazards prior to the next eruption, according to volcanologist Delphine Smittarello and colleagues in the September 1 issue of Nature.
“Nyiragongo is unique in that 1 million people are living just at the foot of the volcano” Smittarello of the European Center for Geodynamics and Seismology in Luxembourg concurs. The mountain looms over both the Congolese city of Goma, which has a population of about 700,000, and the Rwandan city of Gisenyi, which has a population of about 83,000. “There are so many people so close to a very dangerous place.”
The last two eruptions of Nyiragongo, in 1977 and 2002, were preceded by days of distinct seismic rumblings strong enough to be felt by people living nearby. However, prior to the May 22, 2021 eruption, even sensitive monitoring stations near the volcano appeared to detect no clear warning signs of magma on the move underground.
In the summit crater of the volcano, there was a large lake of molten lava: By 2021, the lake had risen to near the crater’s rim. However, Smittarello believes that lake level alone does not always indicate an impending eruption. Levels had risen and fallen intermittently since 2002 as magma moved around the volcano’s deep plumbing. And the lake was still 85 metres below sea level in 2021, 85 meters below in 2002.
So Smittarello and her colleagues revisited the seismic and acoustic data from the monitoring stations. This time, the analysis discovered a series of small quakes that began just 40 minutes before the eruption. Detections of acoustic signals — low-frequency “infrasound” waves — started to rise half an hour later, just 10 minutes before the lava burst through, indicating that the volcano was about to erupt.
According to the researchers, the real eruption was likely caused by a tiny rupture that formed in the volcanic cone due to the accumulation of stress over time from the heat and pressure of the magma within. That would have been sufficient for the magma to pass through.
According to the researchers, the short lag time between the signals and the eruption was most likely due to the magma being so close to the surface. “What we monitor is the movement of the magma, not the presence or absence of magma,” Smittarello explains. Because of magma had such a short distance to travel, there was little warning.
The eruption persisted about six hours, but there was plenty of seismic activity for the next ten days, indicating that the magma was now on the move. Those real-time data showed something troubling: magma was moving underground, away from the summit, curving under the city of Goma and nearby Lake Kivu.
Based on the magma’s anticipated course, Goma municipal officials issued evacuation orders for tens of thousands of people who might be in the magma’s path. Meanwhile, experts were on the lookout for signs of a possible “limnic eruption” at Lake Kivu, a rare sort of disaster in which a toxic cloud of dissolved gases such as carbon dioxide and methane bursts from deep waters, drowning living organisms nearby. Such an explosion might have been produced by gas-rich magma seeping into the lake’s bottom. In any situation, “if [the magma] finds a passage to the surface, it’s a disaster,” according to Smittarello.
Thankfully, neither disaster occurred, according to Smittarello. “It was a fortunate situation.” But we have no idea why.”
The researchers were able to determine the actual position of the underground dikes after reanalyzing post-event seismic data. One dike beneath Goma was discovered to be as shallow as 450 metres deep. This is especially remarkable given that such a small magma conduit would be anticipated to generate a distinct mix of volcanic gases through the earth fractures.
According to geophysicist Michael Poland of the US Geological Survey, who is headquartered in Vancouver, Wash., and directs the Yellowstone Volcano Observatory, it is not uncommon for volcanic dikes to have no gassy signs of their presence. The magma might have lost a lot of its gases as it flowed up into the top lava lake; by the time it pushed into subsurface conduits, it could have been degassed.
However, that situation is concerning since there is one fewer warning signal of potential risk to the populations above, according to Poland, who was not part in the current study. It also raises additional problems, such as how such gas-poor magma might interact with Lake Kivu if it were to flow into the lake.
What the 2021 eruption demonstrates is that scientists must examine such topics in order to have a better knowledge of Nyiragongo’s characteristics — and to modify monitoring and hazard alerts accordingly, Poland says.
Every volcano has its own personality, and scientists must learn to recognise any warning signals that may present. In this situation, he explains, that may include the lava lake level. “At Nyiragongo, the old technique is less dependable.”
image credits: ALEX MILES/AFP/GETTY IMAGES PLUS
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