Rheology of Crystallizing Basalts from Mt. Nyiragongo and Mt. Nyamuragira D.R.C.

Abstract:

Mt. Nyiragongo, located within the Virunga Volcanic Province on the western branch of the East African Rift, is known for its persistent lava lake activity as well as devastating eruptions in 1977 and 2002. The 2002 eruption caused a humanitarian crisis when channelized lava flows entered the nearby city of Goma killing 170 people and displacing ~350,000 others. These lavas have extremely low silica contents (39-42 wt.% SiO₂) and are very fluid, allowing flows to move rapidly away from the source. We have measured the rheology of lavas from Nyiragongo using a concentric cylinder viscometer at temperatures of ~1220, 1205, 1190, 1175, 1165, and 1145°C. Each experiment starts with a liquid viscosity measurement at 1500˚C, followed by cooling to the desired experimental temperature. The lava spends 10-12 hours at this temperature, with constant stirring, before measurements begin. After measuring at a range of strain rates, the lava is quenched by immersion of the Pt crucible in a water bath. The viscosity is ~32 Pas at the liquidus temperature of ~1220°C, increasing gradually to ~142 Pas at 1165˚C. These viscosity measurements are much lower than most other basaltic compositions including Hawaiian lavas which have a crystal fraction of ~42% and apparent viscosity of ~2000 Pas at 1169°C. Over this temperature range, crystal fraction varies little (1-5% spinel crystals). Interpolating between measurements of the melt viscosity by concentric cylinder and parallel-plate viscometry suggests that at 1165˚C, the viscosity of the starting melt would be ~63 Pas. Consequently, the change in viscosity is due primarily to cooling rather than either the physical or chemical effects of crystallization. The data were collected at strain rates between ~1 and 46 s^-1, and are well reproduced using a power-law model with exponents ~0.94 to 0.96. Below 1165˚C, crystal fraction and magma viscosity both increase rapidly. Further experiments at lower temperatures will quantify this rate of viscosity increase, and results will be compared to the more silicic Mt. Nyamuragira, only 15 km away.