HR: 0800h
AN: V41A-1364    [Abstracts]
TI: Flow Banding in Volcanic Rocks: A Record of Multiplicative Magma Deformation
AU: * Gonnermann, H M
EM: hmg@seismo.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley 307 McCone Hall, Berkeley, CA 94720-4767 United States
AU: Manga, M
EM: manga@seismo.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley 307 McCone Hall, Berkeley, CA 94720-4767 United States
AB: Banding in obsidian from Big Glass Mountain (BGM), Medicine Lake volcano, California and Mayor Island (MI), New Zealand provide a record with a 1/wavenumber power-spectral density and multifractal characteristics. The samples are compositionally homogeneous, with banding defined by variable microlite content (BGM) or vesicularity (MI). In both samples banding formation is well explained by continuous deformational reworking of magma and a concurrent change in crystallinity (vesicularity) that is a small random multiple of the total amount already present. Banding formation therefore represents a multiplicative process. We complement our spectral and multifractal analysis with several null-hypothesis test and propose repeated brittle deformation, concurrent development of textural heterogeneity (microlite content or vesicularity), reannealing, and viscous deformation as a viable process for the formation of flow banding in these samples. A brittle deformational component in a simple flow geometry, for example during magma ascent in the volcanic conduit, provides a suitable mechanism for (1) the spatial redistribution of textural heterogeneity over a broad range of length scales; (2) enhanced open-system magma degassing via a temporary network of highly permeable cracks and fractures; and (3) the development of spatially variable microlite content (vesicularity) through variable degassing of magma located at different proximities to cracks and fractures.
DE: 8414 Eruption mechanisms
DE: 8429 Lava rheology and morphology
DE: 5104 Fracture and flow
DE: 3250 Fractals and multifractals
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting