地球科学：前触れなしに火山噴火が起こる仕組み

The 2021 eruption of Mount Nyiragongo in the Democratic Republic of the Congo, which occurred with no meaningful warning, may have been triggered by a rupture in the volcano’s edifice (the conical structure), a paper published in Nature suggests. The findings highlight the value of monitoring volcanoes, which may provide clues to how eruptions can be forecast in the absence of the usual precursory signals.

Mount Nyiragongo is an open-vent volcano with a large lava lake housed in the summit crater. Its eruption in May 2021 lasted approximately six hours, producing lava flows that led to about 220 people reported missing or dead and 750 injured, as well as infrastructure damage. The eruption appeared to have occurred without any precursors that could have provided a warning. Typically, volcanic eruptions are triggered by the ascent of magma to the surface and pressure build-up, which produces signals that can be detected. Two historical eruptions of Mount Nyiragongo, in 1977 and 2002, did have precursors, with seismic activity and eruptive activity reported before the main eruptions.

Delphine Smittarello and colleagues analysed the May 2021 eruption of Mount Nyiragongo and suggest that the eruption may have been triggered by an edifice rupture, which may have occurred because stress reached tensile strength (the maximum stress it could take before rupturing) or the structure weakened over time as a result of sustained stress and high temperatures. As the magma was already close to the surface and only had to travel a short distance before erupting, this left little time to detect the signals before the eruption 40 minutes later.

The authors note that these findings raise questions about the mechanisms behind Mount Nyiragongo’s eruptions and the possibility of more hazardous events due to its proximity to populated areas. More generally, they argue it highlights the issues raised by open-vent volcanoes for monitoring, early detection and risk management.