RHEOLOGY OF MONO CRATER RHYOLITES: EFFECTS OF TEMPERATURE, CRYSTALLINITY AND WATER CONTENT
The most hydrous South Coulée samples have water contents around 1.3 wt.% and cross the glass transition at 550˚C. Extrapolating our measurements to 850˚C, the likely eruption temperature, suggests a viscosity less than 107 Pa.s for these hydrous samples. The driest South Coulée sample contains 0.1 wt.% water and has a viscosity of 1.6 × 1010 Pa.s at 850˚C, and the remelted sample containing 0.04 wt.% H2O has a viscosity of 5.0 × 1010 Pa.s at the same temperature. Nine samples from North Coulée have water contents between 0.20 and 0.31 wt.%, and viscosities at 850˚C between 1.3 × 109 and 3.2 × 109 Pa.s. Six samples from Northwest Coulée have water contents between 0.53 and 0.80 wt.% and viscosities at 850˚C between 3.2 × 107 and 6.3 × 108 Pa.s. The small crystal fractions result in minor viscosity increases relative to crystal-free lavas of between 10 and 20%.
We conclude that Mono Crater lavas probably contained at least 1.3 wt.% H2O and had a viscosity of less than 107 Pa.s at the time of extrusion. Dissolved water contents less than about 1 wt.% would result in significantly higher viscosity, which would prohibit extrusion before cooling and quenching occurred at the vent.