BUBBLE SHAPES AND ORIENTATIONS IN OBSIDIAN LAVAS, PYROCLASTS, AND VENTS OF THE TWEED SHIELD VOLCANO, EASTERN AUSTRALIA

The Tweed volcano in eastern Australia consists of the eroded remnants of a former volcanic shield some 100 km in diameter constructed in the early Miocene. The volcano hosts an unusually high proportion of rhyolite lavas and pyroclastic units, which are heavily dissected and provide excellent stratigraphic exposure. To assess the flow conditions of both effusive and explosive eruptions of rhyolitic magma, we examine the shapes and orientations of deformed bubbles in obsidian samples from some of these well-exposed lavas, pyroclastic deposits, and their associated vents.

Bubble deformation in a highly viscous, low Reynolds number fluid, such as rhyolitic magma, is governed by the competing shear stress that deform the bubble and the surface-tension stresses that minimize interfacial area. The ratio of these stresses is the capillary number, Ca. Here we use the relationships between Ca and the shape and orientation of bubbles to evaluate the type and magnitude of shear stress associated magma flow. For each sample, the 3-D shapes of bubbles were measured using a petrographic microscope and two mutually perpendicular thin-sections cut perpendicular to the flow banding following the method outlined in Rust et al. (2003). Flow-induced bubble deformation is quantified as a dimensionless number, D, given by (l-b)/(l+b), where l and b are the semi-major and semi-minor axes of the deformed bubble. Bubble orientations in all samples are indicative of predominantly simple shear, with minor components of pure shear and bubble relaxation in some populations.Bubble deformation is modest in the obsidian lava, yielding shear stresses of a few kPa. In contrast, bubble geometries in obsidian clasts from pyroclastic flow and fall units record greater deformation and shear stresses between 50 and 90 kPa. Bubbles in samples from the vent, thought to be associated with a boulder tuff (agglomerate), yield intermediate shear stresses of 10 to 30 kPa, which somewhat surprisingly do not correlate with distance from the conduit walls. The moderate shear stresses for the vent samples are consistent with a fountain-fed origin for the boulder tuff.

Rust, A.C., Manga, M., and Cashman, K.V., 2003, Determining flow type, shear rate and shear stress in magmas from bubble shapes and orientations. J. Volcanol. Geotherm. Res. 122, 111-132.