North-Central Section - 42nd Annual Meeting (24–25 April 2008)

Paper No. 2
Presentation Time: 1:00 PM-5:00 PM


ROMINE, William1, WHITTINGTON, Alan1 and HOFMEISTER, Anne2, (1)Department of Geological Sciences, University of Missouri, Columbia, MO 65211, (2)Department of Earth and Planetary Sciences, Washington University, Campus Box 1169, St. Louis, MO 63130,

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.