The rheology of crystallizing basaltic lavas from Nyiragongo and Nyamuragira volcanoes, D.R.C.

Nyiragongo (Democratic Republic of Congo) erupts fluid, fast-moving, foidite lavas. Nearby Nyamuragira fre- quently erupts less mobile tephrite lavas. Nyamuragira flows rarely threaten urbanized areas, but Nyiragongo flows threaten Goma (pop. ~900,000) and surrounding villages, resulting in fatalities in 1977 and 2002. We report new laboratory measurements of viscosity evolution during cooling and crystallization from both volcanoes by con- centric cylinder viscometry. Melt viscosity is ~33 Pa s at the liquidus (~1220°C) for Nyiragongo lavas, similar to Hawaiian basalts. Lavas remain fluid over ~75°C of undercooling (φc ≤ 0.05) before rapid crystallization forms a crystal network inhibiting flow. Melt viscosity is ~40 Pa s at the liquidus (~1260 ◦C) for Nyamuragira lavas, which remain fluid over ~110 ◦C of undercooling (φc ≤ 0.11) before the onset of rapid crystallization. Low viscosity al- lows thin Nyiragongo flows to reach >10 ms-1 even at subliquidus temperatures on low slopes, posing great risk to Goma.