On 22 May 2021, the Nyiragongo volcano (DR Congo) erupted. Lava flows destroyed part of the nearby city of Goma and thousands of people fled. In a study published in Nature, an international team of researchers shows that this eruption could not have been predicted.
An eruption that caused panic
Goma, 22 May 2021. Shortly before 7pm, the alarm was raised by the inhabitants of Goma, a border city in eastern DR Congo of about one million people, who witnessed the appearance of a lava flow from a fissure on the side of the Nyiragongo volcano.
The eruption came as a surprise to everyone. The confusion and lack of preparation made management difficult in the first hours of the crisis. As the lava flow devastated a densely populated area, intense seismic activity began, lasting several days and causing damage in Goma and the Rwandan border town of Gisenyi. Panic increased and resulted in the spontaneous evacuation of part of the population.
The eruption lasted only a few hours; lava flows caused damage, but on a much smaller scale than for the previous eruption in 2002, when nearly 10% of the city was destroyed.
An atypical eruption
'Volcanic eruptions are usually preceded by a pressure rise in the magma supply system, characterized by seismicity, an increase in gas emission and a swelling of the volcanic structure,' says Delphine Smittarello, a researcher at the European Centre for Geodynamics and Seismology in Luxembourg. 'To warn of an imminent eruption, volcanological observatories monitor seismicity, ground deformation and gas emissions that reflect the movement of magma in the Earth's crust.'
'Despite continuous monitoring by the network of ground-based instruments and satellite sensors, none of these usual signals were observed prior to the eruption of Nyiragongo volcano. Subsequent seismic records show that the first events indicating unusual activity began less than 40 minutes before the first lava flows. This very short time lapse between the first earthquakes and the eruption on the flank tells us that the magma took very little time to reach the surface because it was stored at a shallow depth,' says the researcher.
Adalbert Muhindo, director of the Goma Volcano Observatory (GVO), added: 'This is the first time such a seismic sequence has been recorded by a local network of sensors during an eruption of Nyiragongo. The detection of signals that occur only a very short time before the eruption provides important new knowledge that can be integrated into future monitoring methods for this volcano by our observatory in Goma, but possibly also for other volcanoes around the world that have large amounts of magma stored on the surface.'
'This behaviour of the volcano was all the more unexpected because the only two known historical eruptions, in 1977 and 2002, had been preceded by strong earthquakes (M>5) felt several weeks to a few days beforehand.'
The study also shows that a large amount of magma (243 million m³) has moved under Goma. 'The magma spread rapidly at shallow depths (less than 500m from the surface) from the volcano southwards, passing under Goma, and finally stopping under Lake Kivu', explains Benoît Smets, a researcher at the Royal Museum for Central Africa and specialist of Nyiragongo.
This magma behaviour poses new threats, says Benoît Smets: 'Our results suggest that once the magma is flowing at such a shallow depth, there is a risk of lava outflow in the middle of the town, a phreatomagmatic eruption (when the magma comes into contact with the cold water of Lake Kivu) or a limnic eruption of Lake Kivu, which is feared because of the huge amount of CO2 and methane dissolved in the deep water of that lake. These events are potentially much more dangerous than the three eruptions known so far.'
The Royal Belgian Institute for Space Aeronomy contributed to this study with satellite data, as part of a collaboration with:
- the Royal Museum for Central Africa (Belgium)
- the European Centre for Geodynamics and Seismology (Luxembourg)
- the National Museum of Natural History (Luxembourg)
The work includes the development of tools to improve volcanic acitivity monitoring, within the framework of the RESIST and VERSUS projects (Belspo's STEREO programme). On the one hand, the SO2 data from TROPOMI confirmed the absence of precursor signals. On the other hand, the analysis of aerosol data from the geostationary instrument SEVIRI has provided a better understanding of the magma progression beneath the city of Goma.
- Smittarello, D. et al. Precursor-free eruption triggered by edifice rupture at Nyiragongo volcano. Nature (2022).
- Precursor-free eruption triggered by edifice rupture at Nyiragongo volcano. NATURE.COM
- Nicolas Theys
- Hugues Brenot