HR: 08:15h
AN: V51B-02    [PDF]
TI: Shear-Induced Fragmentation in Silicic Volcanism
AU: * Gonnermann, H M
EM: hmg@seismo.berkeley.edu
AF: University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720-4767 United States
AU: Manga, M
EM: manga@seismo.berkeley.edu
AF: University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720-4767 United States
AB: Fragmentation of magma, containing abundant gas bubbles, is considered as the defining characteristic of explosive eruptions. When viscous stresses associated with the growth of bubbles and the flow of the ascending magma exceed the strength of the melt, it breaks into disconnected fragments suspended within an expanding gas phase. While repeated effusive and explosive eruptions for individual volcanoes are common, the dynamics governing the transition between explosive and effusive eruptions remain unclear. Magmas for both types of eruptions originate from sources with similar volatile content, yet effusive lavas erupt considerably more degassed than their explosive counterparts. Recent observations suggest that magma fragmentation may not be restricted to explosive eruptions and we find corroborating evidence of magma fragmentation, reannealing and elongation of fragments into flow banding from obsidians from Big Glass Mountain rhyolite dome, California. One mechanism for degassing during magma ascent is the generation of intermittent permeable fracture networks through non-explosive fragmentation near the conduit walls. To gain insight into the mechanics governing fragmentation in silicic volcanoes, we have developed a numerical model for magma ascent in the volcanic conduit. The ascending magma (melt + bubbles) is modelled as steady, isothermal flow of a single-phase liquid at constant mass flux in a cylindrical conduit of constant radius. We specify a pressure, number density of bubbles, and relaxed Newtonian melt viscosity at the base of the conduit and solve the joint problem of bubble growth and magma ascent. Rather than include the transition to fragmentation and flow of fragmented magma, we determine the ascent distance above the conduit entry at which magma fragmentation by viscous shear should first occur. The model is quasi-one-dimensional and for a given depth computes the radially varying vertical component of magma velocity. We show that shear-induced fragmentation can occur in both effusive and explosive eruptions and is consistent with the observed conditions of volcanic systems, with the degassed nature of effusive silicic lavas, and with textural observations at the outcrop to microscale. We suggest that it may be important for magma degassing and inhibition of explosive behavior. This implies that, contrary to conventional views, explosive volcanism is not an inevitable consequence of fragmentation.
DE: 8414 Eruption mechanisms
DE: 8429 Lava rheology and morphology
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting