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Nyiragongo (1.52°S, 25.25°E) (Galle 2005) is a volcano situated on the western branch of the East African Rift System (EARS), 18 km north of Goma. It is one of 2 active volcanoes in the Virunga volcanic range, which is a highly alkaline area of the EARS. It is a steep (sides of 35°) stratovolcano and rises to an altitude of 3470m (Demant 1994). At the top of the volcano is a summit crater 1.3km wide with the world’s largest permanent lava lake (200m across) (Sawyer 2008). The lava lakes levels fluctuate considerably, producing the terraces visible on the inside of the crater. The volcano also featured in the BBC series Journey to the Centre of the Earth, presented by Richard Hammond.
The composition of the volcano is highly alkaline, and the lava lake is of a melilititic-nephilite composition. Alkaline volcanoes typically emit more SO2 and CO2 (Bailey 1990), making Nyiragongo a particularly good volcano to study in the VCO2 project given its extreme alkalinity. Chaigneau 1960 documented low, dense CO2 rich flows, and within the area Mazuku (low lying CO2 clouds) form. Mazuku (meaning evil wind in Swahili) are formed by CO2 degassing in lowland areas through fractures, due to the density of CO2 compared to the surrounding air, CO2 collects to lethal concentrations (Smets 2010). Another feature of Nyiragongo is the nearby lake Kivu. This lake is highly stratified and contains lethal quantities of CH4 and CO2 thought to be volcanic in origin (Gerlach 1980). At 400m the lake waters contain 22% CH4 and 77% CO2 (Burke 1963). Disturbances in the lake stratification could result in the release of the gasses resulting in mass mortality. Changes in lake stratification after eruptions have been noted (Allard 2003).
The lava lake appears to have been very long lived. From at least 1849 when the lava lake was first documented, the lava lake was present till the first documented eruption on 10th January 1977. In 1977 a set of N-S and E-W trending fissures opened up on the volcanoes flanks between 2700 and 2200 m altitude, where 22x 10^6 m^3 erupted, draining the lava lake (Demant 1994). Another, but smaller eruption occurred on the 21st June 1982, where magma erupted from 2 vents that were active in the 1977 eruption (Demant 1994). The latest eruption occurred in 2002 where a fracture opened on the volcanoes flanks between 2800 and 1700 m elevation resulting in the eruption of 20-30x 10^6 m^3 destroying large parts of the nearby city of Goma (Allard 2003). Nyiragongo is a particularly lethal volcano, not just due to the large amounts of CO2 released, but also due to the speed of the lava flows due to the lavas low viscosity. In 1977, lava flow speeds of around 60 km/hr were recorded (Sawyer 2008).
Nyiragongos magama appears to be strongly fractionated, olivines record Mg♯ 41-31 (Denaeyer 1965). There are thought to be two types of magmas, a Cr, Ni, MgO enriched magma and the main Nyiragongo suite (Platz 2004). The Nyirgongo suite endmember may originate from an olivine melilitite (Platz 2004) formed by partial melting at the base of the lithosphere within the dolomite-amphibolite-garnet stability field (Brey 1978). To best explain the MgO and CaO enrichment, but Al2O and SiO2 depletion seen in Nyiragongo samples, significant amounts of leucite and varying degrees of nepheline fractionation are thought to have occurred at shallow levels (Platz 2004). Shallow level magma storage is supported by the fact that there is a long-lived lava lake, for a lava lake to have stayed molten for such long periods of time; it must be connected directly to a magma chamber in order to provide sufficient thermal energy to prevent solidification.
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Gerlach T M, Chemical characteristics of the volcanic gases from Nyiragongo lava lake and generation of CH4-rich fluid inclusion in alkaline rocks. J. Volcanol. Geotherm. Res., 1980;8:177–189
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Le Guerne, Mechanism of Energy Transfer in the lava lake of Niragongo (Zaire), 1959- 1977. Journal volc and geotherm res, 1987;31:17-31
Platz T, Foley S F, Andre L. Low-pressure fractionation of the Nyiragongo volcanic rocks, Virunga Province, D.R. Congo. J. Volcanol. Geotherm. Res. 2004;136:269–295.
Sawyer G M, Carn S A, Tsanev V, Oppenheimer C, Burton M. Investigation into magma degassing at Nyiragongo volcano, Democratic Republic of the Congo. Geochemistry, Geophysics, and Geosystems (G3), 2008;9:2:Q02017.
Smets B, Tedesco D, Kervyn F, Kies A, Vaselli O, Yalire M M. Dry has vents (“ mazuku”) in Goma region (North-Kivu, Democratic Republic of Congo): Formation and risk assessment. Journal of African Earth Sci. 2010;58:5:787-798.