Book Description

This thesis focuses on using spectroscopic gas emissions data to understand degassing processes occurring during low-intensity explosive activity at Santiaguito lava dome, Guatemala. It results in a refinement of the ultra violet camera spectroscopic technique; the proposal of a trigger mechanism for low-intensity explosions during lava dome growth; and the first successful modelling of an unsteady explosion plume. The ultra violet camera provides high spatial and temporal resolution spectroscopic measurements of volcanic 802, However, data collection is difficult under the non-ideal conditions typically encountered at silicic volcanoes. A new processing protocol is developed that corrects for plume-instrument distance and in-plume ash/aerosols, permitting data collection at 8antiaguito. The resulting dataset is used to constrain the trigger mechanism of 8antiaguito explosions. The lava dome is found to be continuously permeable, whilst explosions contain as little as 150 kg 802 - characteristics incompatible with gas pressurisation beneath an impermeable plug. Rather, the data support a model in which transient explosive degassing pathways are generated at the conduit margins through shear-driven fracturing. Rheological modelling shows that this process is likely to occur during even extremely slow extrusion of high-crystallinity intermediate magma, on account of the very high viscosity of such magma. This model is further supported by the discovery of tuffisite veins in Santiaguito dacite. Santiaguito explosions form short-duration plumes that may be used as observable proxies for larger Vulcanian plumes. Ultra violet camera data have been used to define time-varying boundary conditions for a cutting-edge unsteady plume model. This represents a first attempt to compare field observations of a volcanic plume with model predictions using field-derived boundary conditions. The resulting simulation closely recreates the observed plume ascent, in contrast to approximations made using steady and instantaneous plume models. This explicitly shows that unsteady effects are critical to understanding Vulcanian plume dynamics.