Flow banding in volcanic rocks: a record of multiplicative magma deformation

Banding in obsidian from Big Glass Mountain (BGM), Medicine Lake volcano, California, and Mayor Island (MI), New Zealand, provides 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 a continuous deformational reworking of magma and by 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 tests 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 the spatial redistribution of textural heterogeneity over a broad range of length scales, enhanced open-system magma degassing via a temporary network of highly permeable cracks and fractures, and the development of spatially variable microlite content (vesicularity) through variable degassing of magma located at different proximities to cracks and fractures.