RHEOLOGY OF CRYSTALLIZING BASALTS FROM NYIRAGONGO AND NYAMURAGIRA VOLCANOES, D.R.C

Nyiragongo, a stratovolcano within the Virunga Volcanic Province on the western branch of the East African Rift, is known for 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 low silica contents (39-42 wt% SiO₂) and are very fluid, allowing flows to move rapidly down the volcano’s flanks. Rheology of lavas from Nyiragongo was measured using a concentric cylinder viscometer over a range of temperatures between 1221°C and 1145°C. The viscosity is ~41 Pa s at the liquidus temperature of ~1213°C (similar to Hawaiian lavas: 30 Pa s near the liquidus temperature of ~1230°C), increasing gradually to ~741 Pa s at 1145˚C.

Over this temperature range, crystal fraction remains low (φ_c ≤ 0.02) until ~50°C undercooled and only ~0.05 at 1145˚C, where the bulk melt viscosity would be ~138 Pa s. This change in viscosity is due mainly to cooling effects rather than physical or chemical effects of crystallization. The data were collected at strain rates between ~0.4 and 46.3 s^-1, and are well reproduced using a power-law model with exponents decreasing from 0.96 (1221°C) to ~0.78 (1145°C) with no detectable yield strength. Crystal fraction and lava viscosity both increase rapidly below 1145˚C, preventing further experiments.

Lavas from the neighboring shield volcano, Nyamuragira, show significant crystallization at temperatures closer to the liquidus and so pseudo-plastic behavior is also observed closer to the liquidus. Data are again well reproduced by a power-law fit with flow indices ranging between 0.89 (1255°C) and ~0.42 (1154°C). While still fairly fluid, having a liquid viscosity of ~46 Pa s, it is more viscous than Nyiragongo lavas and crystallizes more rapidly upon undercooling, reaching an effective viscosity of ~1110 Pa s at 1154˚C.

In recent years, the steep-sided stratovolcano Nyiragongo has erupted lava more fluid than erupted by the nearby shield volcano, Nyamuragira, producing fast moving flows that pose imminent danger to the inhabitants of Goma. Consequently, lava flow inundation hazard cannot be predicted solely by the morphology of the volcanoes from which the flows erupt.