6 comments on “Nyiragongo and Nyamuragira Volcanoes, Congo

  1. Thanks for an amazing post about two dangerously underestimated volcanoes But here’s my query why is Nyiragongo the shape it is? A steep sided stratovolcano, when , given its highly fluid magma, it should be a classic shield like its northern neighbour I’ve puzzled about this for years, and have never seen an explanation -not even from Haroun Tazieff, who wrote a whole dam’ book about Nyiragongo. Any ideas?

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    • Howdy Michael –

      It’s gotta be the composition of the magma. Ol Doinyo Lengai has very cool, very runny carbonatite lavas yet has a fairly steep stratovolcano cone. Also would like to take a look at the remaining cones to the east of the pair. Appears activity has migrated west over time. I think the other volcanoes can tell us something. Cheers –

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      • Then again, Ol Doinyo’s carbonatite lavas are a pretty recent development: the bulk of the cone is made up of conventional silicate rocks (or it would have disappeared by now; natrocarbonatites are water soluble) Nephelinite, phonolite and LOTS of tuff Alkalic, yes, but frequently explosive. So maybe there’s a similar explanation here, with an early growth phase of explosive activity. If so, there would be widely distributed tephra layers in the vicinity (there are for OL) You’re right, examining the older Virunga volcanoes might give clues AFAIK they seem to be stratocones like Nyiragongo rather than shields

        Easier said than done, though, since the area has been in a war zone for most of the last 50-odd years 😦

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        • It’s a tough neighborhood.

          Almost wondering if there is viscosity point where the cone building switches from strato (very lo viscosity), to shield (lo viscosity), to strato again (higher viscosity). Mode of eruption also is a factor – fissures? Dikes? Sills? Spatter cones? Have no idea how all this hangs together. As usual, there end up being more new questions than answers, which is pretty cool.

          Best to you and yours. Cheers –

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    • My idea was that Nyiragongo has perhaps seen other modes of eruptions in its earlier days. If the part between the two rift arms had been a microplate and attached itself later, N. might have been near the sea, and probably situated much more south (in moderate climates) anyway. That could mean more explosive volcanism.

      This paper (DOI 10.1007/BF00279728) describes the crater walls: “…a 15 m thick coarse chaotic pyroclastic deposit containing large rounded blocks of lava (up to a few meters in diameter) is found. … uniform, finely stratified and non-welded cinder. Such a deposit is indicative of highly explosive eruptions, the color and the complex stratification being typical of hydromagmatic style eruptions.”

      So, if there had been phreato-magmatic explosions and tephra was deposited alternating with lava flows, this would have been the recipe for the build-up of a stratovolcano. Perhaps even an ancient subduction scenario could have played a role. This paper (DOI: 10.1016/j.chemgeo.2008.11.010) mentions components in the modern lavas that could stem from a long subducted plate.

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