Nyiragongo (DRC) volcanic eruption of May 22, 2021: petrography of lava flows, seismicity and geothermal data relating to pre- and post-eruptive periods

Nyiragongo is one of the eight volcanoes of the Virunga volcanic chain located in the western branch of the East African Rift, more specifically in eastern Democratic Republic of Congo (DRC). The volcano's lava flows pose a threat to the city of Goma. Here we present seismographic, geothermal and petrographic data related to the May-June 2021 unrest and eruption of Nyiragongo. Seismicity was low prior to the eruption, with a daily average of 11 earthquakes from January 1 to May 22, 2021, the day of the eruption. Seismicity increased on May 22 2021, when the eruption began. During May 23 to September 28, 2021 the daily average number of earthquakes rose from 11 to 57 with the daily number of detected earthquakes peaking between May 22 and June 3, 2021. Prior to the eruption there was a moderate increase in ground temperature at the southern sites from February 27 to June 15 2021, but stability at the northern sites close to the volcano, and a constant radiant heat output from the crater. After the eruption, ground temperatures dropped from 23.5°C to 22.8°C; 23.7°C to 21.9°C from June 12 to June 20 2021 at the southern sites and from 26.0 °C to 24.5°C from May 26 to May 30 2021 at northern sites. The lava is glassy, with a low phenocryst content. The phases present in the basalt are: clinopyroxene, plagioclase, olivine and opaque minerals. In the foidites, the mineral phases are clinopyroxenes, plagioclases, nepheline and haüyne.