HR: 0800h
AN: V31B-0485    [Abstracts]
TI: Deformation Timescales of Porous Volcanic Materials
AU: * Quane, S
EM: Steve.Quane@coloradocollege.edu
AF: Geology Department, Colorado College, 14 E. Cache La Poudre St., Colorado Springs, CO 80903, United States
AU: Friedlander, B
EM: bfriedlande@coloradocollege.edu
AF: Geology Department, Colorado College, 14 E. Cache La Poudre St., Colorado Springs, CO 80903, United States
AU: Robert, G
EM: grobert@eos.ubc.ca
AF: University of British Columbia, 6339 Stores Rd., Vancouver, B.C V6T 1Z4, Canada
AU: Lynn, H
EM: hlynn@coloradocollege.edu
AF: Geology Department, Colorado College, 14 E. Cache La Poudre St., Colorado Springs, CO 80903, United States
AB: We describe results from 20 high-temperature, constant strain rate and constant load deformation experiments on natural pyroclastic materials. Experiments were run unconfined and under variable H₂O confining pressures at temperatures between 650 and 900 C. Starting materials comprised 4.3 cm diameter, 6 cm length cores of sintered Rattlesnake Tuff rhyolite ash with starting porosities of 70 percent. Experimental displacement was controlled to achieve total strain values between 10 and 90 percent. In thin section, the deformed experimental end products exhibit striking similarities to all facies of natural welded pyroclastic rocks including variably flattened pumice fiamme and systematically deformed bubble wall shards. To quantify the amount of strain accumulation, we placed three manually rounded 1 cm diameter pumice lapilli at different heights in each experimental product. Axial ratios (x-axis dimension/y-axis dimension) of the deformed lapilli (fiamme) show a systematic increase with increased deformation. To further quantify strain, we measured flattening ratios of originally spherical bubble wall shards. These analyses are compared to similar measurements on natural samples to evaluate current methods of quantifying deformation in welded pyroclastic facies. Stress-strain and strain-time experimental results indicate that the glassy, porous aggregates have a strain- dependent rheology; the effective viscosity of the mixture increases non-linearly with decreasing porosity. Temperature, rather than stress is the dominant factor controlling the rheology of these materials. Results also indicate that the presence of moderate H₂O pressure allows for viscous deformation (e.g., welding) to occur at significantly lower temperatures than in anhydrous conditions. Results from these experiments are used to develop a constitutive relationship in which the effective viscosity of the experimental cores is predicted using melt viscosity, sample porosity and an empirically determined constant dependent on material properties. The real power of this new model is that now we can predict the timescale of formation of volcanic deposits that have undergone porosity loss by viscous deformation. Two examples we show are welding of ignimbrites and deformation in a volcanic conduit. Prediction of these poorly known timescales provides significant leverage for dynamic models detailing eruption and deposition of volcanic materials.
DE: 8404 Volcanoclastic deposits
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8434 Magma migration and fragmentation
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting