HR: 0800h
AN: V41A-1362    [Abstracts]
TI: A High-load, High-temperature Deformation Apparatus For Volcanological Studies
AU: * Hess, K
EM: hess@lmu.de
AU: Dingwell, D B
EM: dingwell@min.uni-muenchen.de
AB: The need for an adequate understanding of the nature and extent of physico-chemical processes involved in explosive volcanism is considerable. In recent years much effort has been concentrated on rhyolitic melts under conditions relevant to explosive volcanism. Especially the description of single-phase rhyolitic melt properties such as the temperature, pressure and compositional dependence of viscosity has been greatly improved. Yet, modelling of the emplacement and eruption of silicic domes is still hampered by the lack of a sufficiently accurate rheological database for multi-phase lavas with crystals and vesicles and no simple expression can be used to describe their rheology. We have developed a unique high-load, high-temperature deformation apparatus for studying in situ the non-Newtonian flow behaviour of magmas. The apparatus accommodates samples that are up to 100 mm in diameter and 100 mm long, and can be used to run constant displacement rate and constant load experiments. The rig is ideal for volcanological studies because it uses experimental conditions that closely match those found in volcanic processes: temperature (25 to $1300 \deg$), stress (0 to $>$ 500 MPa), strain rates (10-6 to 10-2 s), and total strain (0 to 100%). The apparatus still has to be optimised, but we can already present some preliminary results. To study the flow behaviour of a reference melt, we performed a viscosity study at a constant temperature on a "NIST 710a" soda-lime composition. The sample was placed between the two pistons of the apparatus and heated up to the desired temperature ($609 \deg$) above the calorimetric glass transition temperature ($550 \deg$). After allowing the system to reach thermal equilibrium (6 hours) in a parallel plate type experiment, load was applied (10, 50, 100, 200, 250 kN), holding that load for several 10 seconds. For applied loads $<$ 100 kN (which correspond to stresses $<$ 80 MPa), the stress versus strain rate relationship always behaves linearly and the viscosity remains constant with time (Newtonian flow regime). If the applied load was $>$ 200 kN (which corresponds to stresses $>$ 160 MPa), the stress versus strain rate relationship curves and the apparent viscosity decrease with time (non-Newtonian flow regime). If the applied load is raised to 250 kN "hot" cracks are produced and the sample is partly fragmented. A major advance is, that we can use sample sizes in the range of several 10$^{5}$ mm$^{3}$, which means we are able to measure natural samples that contain large phenocrysts in cm size, like the Unzen conduit material. Finally, with this facility, it will be possible to measure reliable temperature distributions due to viscous heating in situ during the deformation process, using several thermocouples inside the sample. Experiments on obsidian are underway.
DE: 8429 Lava rheology and morphology
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting