Colloquium: Geneviève Robert
Geneviève Robert
Ph.D. Candidate, Department of Geological Sciences, University of Missouri - Columbia
Abstract
Viscosity is one of the main controls on melt transport. Indeed, the viscosity of silicate melts can vary by several orders of magnitude over the temperature and dissolved water content ranges experienced in a given geologic setting. Additionally, the viscosity of most melts is not linearly dependent on temperature. Accurate determinations of viscosity over wide composition, temperature and water content ranges are needed in order to model magmatic and volcanic processes.
The Central American arc is active from Guatemala to Costa Rica. There are three active Guatemalan volcanoes, and they cover the compositional range from basalts and basaltic andesites (Fuego and Pacaya) to dacites and rhyodacites (Santiaguito). Fuego erupts daily producing frequent ash clouds and lava flows. Arc magmas receive a significant input of volatiles from subduction, and Fuego magmas typically contain 1-2 wt.% H2O. We measured the effect of H2O on the viscosity of basalt and basaltic andesite melts from Fuego volcano. The viscosity-reducing effect of water is most dramatic at low temperatures, and smaller than in silicic compositions. The viscosity of a calc-alkaline basaltic andesite magma with 2 wt.% H2O at 1200°C would increase by a factor of ~100 upon complete degassing during ascent. Heat capacity measurements on the same melts are in good agreement with the viscosity results. At eruption temperatures in calc-alkaline basaltic systems, the viscosity changes caused by degassing are not sufficient to allow for strain-induced fragmentation as is most typical of rhyolitic systems.
The ~8 My Grey’s Landing ignimbrite, Snake River Plain volcanic province, Idaho, is a pervasively rheomorphic, lava-like, metaluminous rhyolitic ignimbrite. Such extremely high-grade, lava-like welded ignimbrites are typically associated with large explosive eruptions with volumes up to 103 km3. The Grey’s Landing ignimbrite provides abundant field evidence supporting an aggradational deposition model and syn-deposition rheomorphism through the upward migration of a transient sub-horizontal ductile shear zone at the interface between the pyroclastic density current and the deposit. We used a combination of rheological experiments and thermo-mechanical modelling to test the syndepositional shear zone model. We show that syndepositional welding and ductile flow are achievable within a very restricted field of likely temperature-strain rate space, where rapid deformation is favoured by higher emplacement temperatures (>850°C). The field of ductile deformation is broadened significantly by accounting for strain heating, which permits a sustained temperature increase up to 250°C within the shear zone and helps to explain the pervasiveness of lava-like lithofacies in the Grey’s Landing ignimbrite.