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Abstract: Transitions between effusive and explosive styles of lava dome eruptions are likely accompanied by changes in lava rheology. The common presence of crystals in dome lavas produces a complex non-Newtonian rheology. Thus models of such complex rheology are essential for volcanic eruption models. Here, we have measured the rheology of natural Unzen lavas with a compressive uniaxial press operating at stresses between 1 and 70 MPa and temperatures between 940 and 1010 °C. Crystal-rich Unzen lavas are characterised by two essential rheological features which produce non-Newtonian effects. The first is an instantaneous response of the apparent viscosity to applied stress which requires that the magma be described as a visco-elastic fluid that exhibits shear-thinning. The second effect takes the form of a time-dependence of the viscosity at moderate to high stress (≥10 MPa). In this regime, the apparent viscosity slowly decreases as increasing fracturing of the phenocrysts and the groundmass occurs. Fragmentation of crystals and alignment of crystal fragments are observed to produce flow banding-effects which in turn lower the apparent viscosity of natural dome lavas. Ultimately, deformation may lead to complete rupture of the lava if the stress is sufficient. Cracking thus stands as an important process in natural dome lava rheology. The ubiquitous non-Newtonian rheology of dome lavas, observed experimentally here, needs to be adequately treated in order to generate appropriate eruption models. [Copyright &y& Elsevier]

Abstract: We perform a quantitative textural analysis of banding in obsidian from Big Glass Mountain (BGM), Medicine Lake Volcano, California and from Mayor Island (MI), New Zealand. The samples are compositionally homogeneous, with banding defined by variable microlite content (BGM) or vesicularity (MI). The spatial distribution of alternating light and dark bands has a 1/wavenumber power-spectral density, as well as multifractal characteristics. Through the use of simple mathematical models, we demonstrate that formation of the observed banding structure is consistent with a continuous deformational reworking of magma. Our results indicate that changes in crystallinity (vesicularity) were concurrent with magma deformation and are random multiples of the total amount of crystallinity (vesicularity) that was already present at any given time. Banded obsidian from BGM and MI can be found in close spatial proximity to brittle deformational textures. We therefore propose that a key element of banding formation in our samples may be magma brecciation or fragmentation. Fragmentation can result in variable rates of degassing of fragments, which in turn can lead to nonuniform crystal (vesicle) nucleation and growth rates. Repeated fragmentation, welding and subsequent viscous deformation can thus lead to banding formation. [Copyright &y& Elsevier]