HR: 0800h
AN: V31D-0644    [Abstracts]
TI: An Analytical Study for 1-dimensional Steady Flow in Volcanic Conduits: Effects of Relative Motion between Gas and Liquid on Eruption Styles
AU: * Kozono, T
EM: kozono@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Koyaguchi, T
EM: tak@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: In gas-liquid multiphase flow, relative motion between gas and liquid largely affects the global features of bulk flow. We investigated the effects of relative motion between gas and liquid on eruption styles on the basis of a model for 1-dimensional steady flow in volcanic conduits. As magma ascends and decompresses, volatiles are exsolved and volume fraction of gas increases. As a result, magma fragmentation occurs and the flow changes from bubbly flow to gas-pyroclast flow. In our model, the vertical relative motion between gas and liquid is taken into account and a new transitional region ('permeable flow region') is introduced between bubbly flow region and gas-pyroclast flow region. In this region, both the gas and the liquid are continuous phases, allowing the efficient vertical escape of gas through the permeable flow structure. In the 1-dimensional steady conduit flow model, the length of the region before magma fragmentation (L_b) and the length of the region after magma fragmentation (L_g) are analytically given as a function of magma discharge rate (q). The steady solution for a given total length of the conduit (L_{total) is obtained by the relationship of L_b(q) + L_g(q) = L_total. We systematically investigated the effects of the relative velocity between gas and liquid on L_b(q) and L_g(q). The most important effect of the relative velocity is that the pressure at magma fragmentation decreases as q decreases or the relative velocity in the permeable flow region increases. This induces a significant decrease of L_g(q), whereas L_b(q) is not largely affected. Because of this effect, q decreases and the depth of fragmentation surface decreases for a given L_total. When the gas escapes sufficiently and the pressure at fragmentation is smaller than the atmospheric pressure, the flow reaches the vent without fragmentation, which corresponds to effusive eruptions. The condition for effusive eruptions to occur is analytically determined by the relationships of L_b(q) = L_total and L_g(q) = 0.
DE: 8400 VOLCANOLOGY
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005