Pre-eruptive storage conditions and eruption dynamics of a small rhyolite dome: Douglas Knob, Yellowstone volcanic field, USA

Microlite orientation in obsidian flow measured by synchrotron X-ray diffraction

“…The magnitude of strain is consistent with the previous investigations of microlite orientations and bubble deformation that suggested strains of the order of 2-3 for pure shear (Manga 1998;Castro et al 2002;Befus et al 2014Befus et al , 2015. These estimates all build on the solution of Jeffrey (1922) which is for motion of a single isolated particle.…”

“…A number of studies have used optical microscopy to determine the orientation distribution of microlites and then to quantify kinematics: the total strain and flow type that accompanied rhyolite flow emplacement (Castro et al 2002;Befus et al 2014Befus et al , 2015. Such measurements of microlite morphology are very time-consuming and rely on data from individual crystallites.…”

“…Microlites are oriented by flow, and their orientation distribution thus offers an opportunity to determine the type of flow and strain experienced by the melt prior to quenching (e.g., Castro et al 2002;Befus et al 2014Befus et al , 2015. Quantitative information, including the relationship between size and orientation, can be obtained with optical microscopy (e.g., Manga 1998;Castro et al 2004).…”

Rates of water exsolution and magma ascent inferred from microstructures and chemical analyses of the Tokachi–Ishizawa obsidian lava, Shirataki, northern Hokkaido, Japan

“…Rust et al (2003) determined shear rates and stress from bubble shapes and orientations, and spherulites in obsidian glass have been analyzed to estimate their thermal history during crystallization Watkins et al, 2009). However, detailed analyses of microlites are rare because effusive obsidian lava eruptions are hardly ever observed (e.g., Schipper et al, 2013) and few outcrops expose the interior structure of these lavas, although Befus et al (2014) analyzed samples from 10 locations across an obsidian dome at the Douglas Knob, Yellowstone volcanic field. Some drilling projects have described the structure of obsidian lava.…”

“…However, few studies contain quantitative textural data relating to the mechanical behavior and ascent rates for viscous obsidian lava eruptions, meaning that the question of whether the dominant outgassing process is brittle or ductile remains largely unresolved. A recent study by Befus et al (2014) estimated the decompression rate at Douglas Knob, in the Yellowstone volcanic field, at between 8 × 10 −6 and 3 × 10 −5 MPa/s, based on the analysis of microlites. Although the authors did not focus on the outgassing mechanisms, microlites in obsidian may preserve useful information about both magma ascent and outgassing mechanisms, and are worth analyzing.…”