HR: 16:45h
AN: V34B-04 INVITED     [Abstracts]
TI: A Theoretical Model for Fragmentation of Highly Viscous Magmas in Shock-Tubes
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
AU: Mitani, N K
EM: mitani@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: Numerical results of conduit flow models for explosive volcanic eruptions strongly depend on criteria of magma fragmentation. The appropriateness of the fragmentation criteria has been experimentally tested using shock-tubes. However, because of difference in scales, phenomena observed in the experiments do not necessarily represent those of natural volcanic system. We theoretically investigated the fluid dynamics of decompression of viscous bubbly magmas in shock-tubes in order to compare these phenomena with different scales. We developed a coupled model for 1-dimensional time-dependent flow and bubble expansion. Gas-overpressure and hoop stress around each bubble are calculated by applying the cell model; a single bubble surrounded by a small shell of incompressible melt expands against viscous resistance of the melt. It is assumed that magma fragments and the flow changes from bubbly flow to gas-pyroclast dispersion when the hoop stress reaches a given threshold. The boundary between the two flow regions is defined as fragmentation surface. Initially a viscous bubbly magma at a high pressure is separated from air at the atmospheric pressure by a diaphram. As the diaphram is ruptured, a shock wave propagates into the air and a rarefaction wave propagates into the bubbly magma. As the front of the bubbly magma fragments, a narrow zone of steep pressure gradient develops just behind the fragmentation surface. As the zone of steep pressure gradient passes, the bubbly magma fragments due to rapidly decompression. As a result, the fragmentation surface together with the zone of steep pressure gradient propagates into the magma; in other words, self-sustained fragmentation occurs. Our theory suggests that the above fluid dynamical features can be explained by a combination of a traveling-wave type solution in the viscous bubbly flow region and a self-similar solution in the inviscid gas-pyroclast flow region. On the basis of these solutions, some simple formulae which predict fragmentation speed and fragmentation threshold as a function of magma properties and initial conditions are analytically derived. A scaling law which links the shock-tube experiments and Vulcanian explosions in nature has also been established.
DE: 3225 Numerical approximations and analysis (4260)
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005