Rheology of crystallizing basalts from Nyiragongo and Nyamuragira volcanoes, D.R.C.
Abstract
Nyiragongo, a stratovolcano 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 [about]350,000 others. These lavas have unusually low silica contents (39-42 wt.% SiO2) and are very fluid, allowing flows to move rapidly down the volcano's flanks. The rheology of lavas from Nyiragongo was measured using a concentric cylinder viscometer over a range of temperatures between 1221[degrees]C and 1145[degrees]C. The viscosity is [about]33 Pa s at the liquidus temperature of [about]1213[degrees]C, similar to Hawaiian lavas (30 Pa s near their liquidus temperature of [about]1230[degrees]C), increasing gradually to [about]708 Pa s at 1145[degrees]C. Over this temperature range, the crystal fraction remains very low ([omega]c ? 0.02) until [about]50[degrees]C undercooled and is still only [about]0.05 at 1145[degrees]C, where the viscosity of the starting bulk melt would be [about]123 Pa s and that of the interstitial melt is [about]1323 Pa s. Consequently, the change in viscosity is due primarily to cooling effects rather than the physical or chemical effects of crystallization. The data were collected at strain rates between [about]0.4 and 46.3s-1 , and are well reproduced using a power-law model with exponents decreasing from 0.96 (1221[degrees]C) to ~0.78 (1145[degrees]C) and no detectable yield strength. Crystal fraction and lava viscosity both increase rapidly below 1145[degrees]C, preventing further experiments.
Degree
M.S.
Thesis Department
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OpenAccess.
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