HR: 1340h
AN: V53C-1584    [Abstracts]
TI: Rheology of Obsidian Flow: Emplacement Controlled by Final Water Degassing?
AU: * Hess, K
EM: hess@lmu.de
AF: LMU, Department of Earth and Environmental Sciences, Munich, 80333 Germany
AU: Lavallee, Y
EM: yanlavallee@hotmail.com
AF: LMU, Department of Earth and Environmental Sciences, Munich, 80333 Germany
AU: Castro, J M
EM: castroj@NMNH.SI.EDU
AF: Smithsonian Institution Smithsonian Institution Smithsonian Institution Smithsonian Institution Smithsonian Institution Smithsonian Institution, Department of Mineral Sciences P.O. Box 37012 NHB-119, Washington, DC 20013 AU: Noll, K
EM: kylenoll@uwm.edu
AF: University of Wisconsin-Milwaukee, Department of Geosciences P.O. Box 413, Milwaukee, WI 53201-0413 United States
AU: Cordonnier, B
EM: cordonnier@min.uni-muenchen.de
AF: LMU, Department of Earth and Environmental Sciences, Munich, 80333 Germany
AU: Mueller, S
EM: mueller@min.uni-muenchen.de
AF: LMU, Department of Earth and Environmental Sciences, Munich, 80333 Germany
AU: Dingwell, D B
EM: dingwell@lmu.de
AF: LMU, Department of Earth and Environmental Sciences, Munich, 80333 Germany
AU: Cameron, B I
EM: bcameron@uwm.edu
AF: University of Wisconsin-Milwaukee, Department of Geosciences P.O. Box 413, Milwaukee, WI 53201-0413 United States
AU: Spieler, O
EM: spieler@lmu.de
AF: LMU, Department of Earth and Environmental Sciences, Munich, 80333 Germany
AU: Fink, J H
EM: jonathan.fink@asu.edu
AF: Arizona State University, Department of Geological Sciences Box 871404, Tempe, AZ 85287-1404 United States
AB: We have systematically studied the rheology of seven calc-alkaline obsidian lava flows from: Lipari (Italy), Iceland, and Arkansas, Yellowstone, Newberry, Little Glass Mountain, Glass Mountain, and Big Glass Mountain (USA). These lavas were not re-melted but studied above their calorimetric glass transition temperature, thus they kept their original physical and chemical properties. The samples contain up to 10 % microlites and their magmatic water content is characteristically of the order of ~0.15 ±0.06 wt. % (determined by FTIR before and after each viscosity measurement).
The viscosity measured at effusive temperatures and one atmosphere varied between 10⁸ and 10¹² Pa s and no pronounced effects of crystallinity were detected. For a specific temperature, the viscosities of each lava varied by less than 1 log unit. The temperature dependence of viscosity was identical for all natural samples within the error range of the methods. Viscosities were in agreement with the predictions of the non-Arrhenian model of Hess and Dingwell¹.
The rheological resemblance of these obsidian lavas and their similar water content is most remarkable. Indeed, calc-alkaline obsidian lavas do not seem to reach water content less than ~0.09 wt. % in the process of emplacement. The main emplacement control on obsidian flow is commonly perceived to be the cooling rate. Indeed numerical models predict a long life to these thick flows which can retain heat for up to 10s of years². The Hess and Dingwell model estimates that the viscosity contrast of such a melt degassing from 0.5 to 0.15 wt. % water is approximately 1.6 log unit at effusive temperature. To achieve this increase in viscosity, the lava would need to experience a cooling of as much as 200°C. Here we argue that the final degassing acts as a ``chemical quench'', playing an important role in the emplacement of obsidian flow. ¹Hess KU, Dingwell, DB, Am. Min. 1996 ²Manley, CR, JVGR 1992
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8425 Effusive volcanism
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005