HR: 08:00h
AN: V21E-01 INVITED    [Abstracts]
TI: Total grain size distribution of particles fallen from the atmosphere following explosive ash eruptions
AU: * Rose, W I
EM: raman@mtu.edu
AF: Michigan Technological University, Geological Engineering & Sciences, Houghton, MI 49931, United States
AU: Durant, A J
EM: ajdurant@mtu.edu
AF: Michigan Technological University, Geological Engineering & Sciences, Houghton, MI 49931, United States
AB: We have collected grain size distribution data (-6 to 12 phi) for tephra-fall from five recent eruptions (Fuego 14 Oct 1974; Mount St Helens 18 May 1980; El Chichón 4 April 1982; Crater Peak/Spurr 18 Aug 1992 and Crater Peak/Spurr 16-17 Sept 1992) where ash was collected immediately after extensive subarial fallout up to several hundreds of kilometers from the source volcano. Total grain size distributions for each eruption were estimated using previously reported deposit distribution maps and the new GSD data, which has improved measurements of fine particles. Results indicate that all ash-fall samples have multimodal size distributions, with fine (<50 microns in diameter) particles dominating in distal regions. Most can be broadly described as having a coarse mode, which could reflect fragmentation by explosive vesiculation, and a fine mode that probably reflects milling in pyroclastic flows or in the vent. Fine particle fallout in these eruptions was greatly enhanced relative to single particle terminal velocities and occurred in <12 hours. The abundance of water in volcanic clouds, satellite remote sensing retrievals of ice and observations of mammatus support hydrometeor involvement in fine ash particle settling. In general, tephra generated in explosive eruptions cannot be characterized by a simple lognormal GSD. Fallout consists of abundant fine particles, which have previously been underestimated because they occur in distal areas and are not well preserved in the geological record. The total GSD data presented here provides a new basis for source parameters in eruption models and also for hazard evaluations since fine particles are important in assessing both aircraft and health hazards.
DE: 0370 Volcanic effects (8409)
DE: 8404 Volcanoclastic deposits
DE: 8408 Volcano/climate interactions (1605, 3309)
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting