DescriptionVesiculation of a rhyolitic melt
Although pumice is an end-member product of gas-rich explosive volcanism, the process of bubble growth which leads to the formation of pumiceous textures are not well constrained. Vesiculation in rhyolitic melts is a primary control on some of the largest explosive eruptions. This study presents the results of a series of experiments which have utilised hot-stage microscopy techniques to track vesicle growth in an initially vesicle-poor rhyolitic melt. Using rhyolitic obsidian erupted from Chaiten, Chile in 2008 (containing ~1.38 wt. % H2O), thin wafers were held at atmospheric pressure for periods of between 5 minutes and 2 days in the hot-stage, at temperatures between 575 oC and 875 oC. In-situ vesiculation was directly observed and the growth of individual bubbles measured using image tracking code in MATLAB. It was found that bubble growth rates increased with both temperature and bubble size. The average growth rate at the highest temperature of 875 oC is ~1.27 m s-1, compared with the lowest observed growth rate of ~0.02 m s-1 at 725 oC; below this temperature, no growth was observed. Average growth rate Vr follows an exponential relationship with temperature and melt viscosity where Vr ≈ exp (0.0169T) and Vr ≈ exp (-1.202). The extent of diffusive degassing from wafer surfaces was estimated with simple diffusion models. Diffusive loss was found to be negligible during brief high-temperature experiments but became increasingly important in slower, lower temperature experiments. Several stages of bubble growth were directly observed, including initial relaxation of deformed existing bubbles into spheres, extensive growth of spherical bubbles, and, at higher temperatures, close packing and foam formation. An advantage of the imaging techniques used here is that bubble-bubble interactions can be observed in-situ at a scale of 2 to 3 microns. Evolving bubble number densities (BND) with time were determined, allowing nucleation rates to be estimated. Maximum observed BNDs were 3.4 × 1012 m-3 with maximum increases of around 160 % observed in samples with lower initial vesicularity (< 5.7 × 1011 m-3). Experimentally determined rates of nucleation, growth and coalescence assist in the reconstruction and vesiculation history of quenched products and in models of magma vesiculation at shallow levels.
John Browning, Hugh Tuffen and Mike James Lancaster University, Lancaster Environment Centre VMSG Student poster prize winner
|7 Jan 2013 → 9 Jan 2013
|Bristol, United Kingdom