RHEOLOGY OF MONO CRATER RHYOLITES: EFFECTS OF TEMPERATURE, CRYSTALLINITY AND WATER CONTENT

The nature of volcanic processes, including rate of magma ascent, exsolution of volatiles, explosivity, and flow distance, is highly dependent on the viscosity of the associated magma. We present measurements of the viscosity of Quaternary rhyolitic lava flows from Mono Craters, California. We quantify the effects of temperature, dissolved water content, crystallinity and vesicularity on viscosity. We use the parallel plate and concentric cylinder methods to obtain data in the viscosity range of 5 × 10³ to 8 × 10¹² Pa.s, from superliquidus conditions to the glass transition. The investigated obsidian samples, collected from three different flow lobes, contain between 0.1 and 1.3 wt.% H₂O, and less than 5 vol.% crystals. We also remelted one sample from each lobe in a muffle furnace to produce nearly anhydrous glass.

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.