HR: 1340h
AN: AE23A-0842    [Abstracts]
TI: Are Large Volcanic Eruptions Just Dirty Thunderstorms?
AU: * Williams, E R
EM: earlew@ll.mit.edu
AF: Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139
AU: McNutt, S R
EM: steve@giseis.alaska.edu
AF: Geophysical Institute, University of Alaska, Fairbanks, AK 99775
AB: A large number of volcanic eruptions are known to produce lightning, and eruption clouds in this category are comparable in size or larger than ordinary thunderclouds. Water substance is fundamental to explosive volcanism by exsolving from magma to the vapor phase, as the magma ascends beneath the volcano. A cubic meter of magma at depth contains about 100 liters of water (in condensed form). Simple calculations are performed that distribute all the available water over a spherical relaxation volume representing the explosion. The relaxation volume is given by E/Po where E is the available energy and Po is the ambient pressure against which the explosion expands. These results show that water concentrations in an eruption cloud can saturate the relaxation volume, guaranteeing the presence of condensate (liquid water and ice). In this environment, ice microphysics is applicable, volcanic ash and tephra may be ice-coated, and possibly the same viable mechanism known to be responsible for the electrification of thunderstorms is active. The gross electrical structures of large volcanic eruptions and the behavior of cloud-to-ground lightning are also consistent with the dirty thunderstorm hypothesis.
DE: 8409 Atmospheric effects (0370)
DE: 8414 Eruption mechanisms
DE: 3324 Lightning
DE: 3304 Atmospheric electricity
DE: 3314 Convective processes
SC: Atmospheric and Space Electricity [AE]
MN: 2004 AGU Fall Meeting