There is no absolutely reliable early warning system for volcanic eruptions. But before a mountain explodes, it emits warning signals days before: earthquakes occur, volcanic gases rise, the underground magma chambers fill up and literally inflate the mountain. However, when the Nyiragongo erupted in the Democratic Republic of the Congo on May 22, 2021, the volcanic mountain did not even produce these signs. Now a working group led by Delphine Smittarello from the European Center for Geodynamics and Seismology in Walferdange has found out how the eruption occurred near Lake Kivu. In the journal "Nature", the researchers write that a huge magma layer had formed at a comparatively shallow depth. Therefore, during the volcanic eruption, the lava could then come to the surface relatively quickly.
The Nyiragongo is located in the Democratic Republic of Congo, not far from the border with Rwanda. The volcano rises at 3470 meters above sea level, and its cone is filled with a 200 to 250 meter wide, always bubbling lava lake. Near the volcano is the city of Goma on the Congolese side and the city of Gisenyi in Rwanda. Before the strong outbreak in 2021, the Nyiragongo spat in 1977 and 2002. Both times, the outbursts announced a few weeks to days before by strong earthquakes. Unlike in May 2021: Only 40 minutes before the eruption did the monitoring system transmit a strong volcanic activity. As a result, a lava flow poured out of the mountain flank that devastated a settlement area. Thousands of people left their houses panicked, hundreds of people were injured, around 220 died.
The research group led by geophysicist Smitarello now analyzed the measurement data recorded in 2021. Why did the Nyiragongo show the typical warning signals so late? "This very short period of time between the first earthquakes and the outbreak on the volcanic flank shows us that the magma needed very little time to reach the surface, as it was already stored at a low depth," says Smittarello, according to a press release from the European Center for Geodynamics and Seismology. Apparently, a huge magmblase of 243 million cubic meters had accumulated under the city of Goma. At a depth of just about 500 meters, the magma moved in a so -called dyke - in a flat layer.
"The formation of such a dyke without previous signs does not agree with conventional interpretations of volcanic dykes, which usually occur with the huge penetration of pressurized magma from a central reservoir into the flanks of the volcano, " explains Emily K. Montgomery-Brown of the US Geological Survey in Vancouver, USA, in an accompanying commentary in "Nature". So experts have not yet documented an outbreak like the one at Nyiragongo.
According to the researchers, the dyke with magma only came to hold on the nearby Kivu lake. For Benoît Smets from the Royal Museum of Central Africa in Tervuren, Belgium, who also worked on the »Nature« study, this represents a new, previously unknown risk of the volcano: »Our results indicate that as soon as Magma Deeps flows, the risk of a lava discharge in the middle of the city, a phreatomagmatic eruption-if the magma comes into contact with the cold water of the Kivu lake-or a limnic eruption of Lake Kivu. «Such events are much more dangerous because Explosive than the eruptions in 1977, 2002 and 2021.
