HR: 14:55h
AN: V23E-06    [Abstracts]
TI: The in-situ production of ash in pyroclastic flows
AU: * Manga, M
EM: manga@seismo.berkeley.edu
AF: University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720, United States
AU: Dufek, J
EM: dufek@berkeley.edu
AF: University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720, United States
AU: Standish, D
EM: dstandish@berkeley.edu
AF: University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720, United States
AB: Abrasion and fragmentation of pumice clasts during the propagation of pyroclastic flows has long been recognized as a potential source for the enhanced production of volcanic ash, however its relative importance has eluded quantification (Walker, 1981). The amount of ash produced in-situ can potentially affect runout distance, deposit sorting, the volume of ash introduced in the upper atmosphere, and internal pore pressure. We conduct a series of laboratory experiments on the collisional production of ash that may occur during different regimes of pyroclastic flow transport. We further parameterize the experiments of Cagnoli and Manga (2004) to determine the rate of production of frictional ash. We find that the energy of these interactions is insufficient to create a fractal particle size distribution; rather a bimodal suite of large particles and 10-100 micron ash particles are typically produced Using these laboratory experiments we can develop a subgrid model for ash production that can be included in analytical and multiphase numerical procedures to estimate the total volume of ash produced during transport. We examine numerically a range of initial flow energies and bed slopes over which the flows propagate. To simplify the problem we consider flows starting with 1 cm pumice clasts that can be broken up into 100 micron ash. We find that for most flow conditions10-20% of the initial 1 cm clasts comminutes into ash with the percentage increasing as a function of initial flow energy. Most of the ash is produced in the high-energy regions near the flow inlet, although flow acceleration on steep slopes can produce ash far from the vent. Ash produced at the frictional base of the flow and in the collisional upper regions of the flow can be redistributed through the entirety of the flow, although frictionally produced ash accumulates preferentially near its source in the bed-load. As slope increases, the relative proportion of ash generated by friction increases relative to collisional ash production in the upper regions of the flow. For most flow conditions the ash produced causes the flows to travel further and also increases the pore pressure in the flows.
DE: 8404 Volcanoclastic deposits
DE: 8408 Volcano/climate interactions (1605, 3309)
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8440 Calderas
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting