Nyiragongo and Nyamuragira: a review of volcanic activity in the Kivu rift, western branch of the East African Rift System

摘要

Nyiragongo and Nyamuragira are two active volcanoes of the Western branch of the East African Rift in the Virunga area. They were built at the Kivu rift axis ca. 12,000 years ago and set above two tectonic steps separated by the Kameronze Fault. Both volcanoes have displayed a succession of intra-crater and flank eruptions that have been observed and documented since the end of the nineteenth century. Here, we have collated and reviewed these publications and reports. Nyiragongo is famous for its semi-permanently active lava lake, which at the time of writing (2020) was the largest in the world. During the construction of the main stratovolcano, which ended a few centuries ago with a caldera collapse, the lava composition changed from melilitite to leucite and then to melilite-bearing nephelinite. The historically active lava lake is believed to be directly fed from an upper intra-volcano reservoir, a shallow reservoir situated a few kilometres below the volcano in the granite basement, and a deeper intra-crustal magma chamber. Historic activity has been documented since 1894 and can be divided into eight stages, on the basis of sudden changes between lava filling and draining, with cycles of rising lava lake activity and overflows, followed by sinking and complete or partial drainage. Twice in recent history (in 1977 and 2002), major flank eruptions were accompanied by complete drainage of the lava lake and the upper plumbing system. The lava that filled the crater since 1948, and then again after the 1977 and 2002 drainage events have been calculated at a cumulative volume of around 324 x 10(6) m(3). In comparison, the 1977 and 2002 flank eruptions involved 47 x 10(6) m(3) of lava. The average annual output rate associated with crater filling is thus estimated at between 4 and 13 x 10(6) m(3). Lava lake behaviour changes from equilibrium, with alternation between gas pistoning and spattering regimes through disequilibrium with intermittent activity, to complete disappearance of the lava lake. These changes can be related to the conditions of the descent of dense degassed magma from the upper conduit into the shallow reservoir. However, since 1959, the chemical composition of the leucite and melilite-bearing nephelinite lavas has not significantly changed, which implies a magma supply from the same magma batch. Nyamuragira was characterised by a shield building phase of activity until a caldera collapse, ca. 300 to 500 years ago. The post-caldera phase has involved effusive activity in the caldera and at numerous flank fissures. The plumbing system consists of an upper reservoir roughly at the basement-volcano interface and averaging a volume of 400 x 10(6) m(3), a shallow upper-crust stratified reservoir, and a middle-crust mafic magma chamber. Volcanic activity has involved a succession of filling and emptying events at the upper reservoir. Lava volumes of historic eruptions reveal annual output rates averaging 14 x 10(6) m(3) between 1901 and 1976, and 40 x 10(6) m(3) between 1976 and 2012. A drastic increase in activity occurred in December 1976. This event coincided with the January 1977 flank eruption of Nyiragongo and resulted from a main tectonic event in the rift basement that improved the efficiency of magma ascent at both volcanoes. The historic lava composition can be related to six cycles of magma accumulation in, and withdrawal of, the upper reservoir from the shallow stratified reservoir.