Eruption dynamics of cyclic vulcanian explosions from Galeras volcano, Colombia

Extended periods of volcanic unrest characterised by repeated, or cyclical, vulcanian explosions are common at arc volcanoes worldwide. Such explosions are thought to occur following the emplacement of a dense, degassed, lowper-meability plug of highly crystalline and highly viscous magma in the shallow conduit. Despite advances in the understanding of the eruptive dynamics of these systems, the ascent, degassing and crystallisation processes that modulate the timing and magnitude of these explosions remain incompletely understood. In this thesis, geochemical, textural and experimental methods are used to study a rare suite of time-constrained andesitic ballistic bombs produced by the explosions of Galeras volcano, Colombia, from 2004-2010. Time-constrained samples such as these are rare as it is often not possible to sample between vulcanian explosions, yet sampling was undertaken by the Colombian Geological Survey (Servicio Geológico Colombiano) during this time, providing an unusual opportunity to link samples with time-constrained monitoring observations. The thesis presents a description and classification of bomb samples (Chapter 2), coupled with geochemical analyses of groundmass glass and plagioclase feldspar microlite compositions in order to establish a model of the typical magma plug stratigraphy prior to vulcanian explosions (Chapter 3). These plugs are found to be stratified with respect to groundmass crystallinity, vesicularity, melt composition and melt volatile content. Magma plugs at Galeras volcano during this period were excavated to a maximum depth of approximately 500 m in any individual explosion, and the stratification in magma properties gave rise to three distinct morphological types of ballistic bombs: dense, scoriaceous and inflated. Groundmass plagioclase textures varied systematically throughout the course of the eruptive period due to increasing magma ascent rates (Chapter 3). Magma plugs emplaced prior to small-volume explosions that were typically preceded by long repose times feature low number densities of large, prismatic plagioclase microlites due to modest magma ascent rates (average decompression rate of 1 MPa/h), and these explosions were associated with the presence of an extruded lava dome at the surface. In contrast, high number densities of smaller, tabular crystals occur in magma plugs emplaced prior to larger-volume explosions that were typically preceded by short repose times, due to higher magma ascent rates (an average decompression rate of 10 MPa/h). Dense and scoriaceous bomb types representing the most degassed region of the magma plugs were further investigated by measuring connected porosity and gas permeability, and using X-Ray micro-tomography to characterise the topology of the porous network in densifying, high-crystallinity andesite (Chapter 4). The relationships between key topological properties of the porous network and the relationship between porosity and permeability are described, in order to facilitate future modelling of the densification process. The rheology of dense and scoriaceous bomb types was also investigated using high-temperature (940 °C and ambient pressure) uniaxial compression tests (Chapter 5). Samples featuring low number densities of prismatic microlites showed a higher bulk viscosity and were more shear-thinning than samples featuring higher number densities of tabular microlites. This work suggests that variations in magma ascent rate produce systematic changes in crystal micro-textures that control magma bulk viscosity, densification rate and plug permeability, and exert an important control on the magnitude and timing of cyclic vulcanian explosions. This model explains variations in average SO2 flux recorded prior to explosions at Galeras volcano, due to plug permeability differences caused by rheological nuances, and explains when and why transitions to effusive dome building occurred, as dome-building phases will be promoted by higher magma permeability and associated efficient outgassing. This new conceptual model explicitely links magma ascent rates, crystal micro-textures, porous micro-textures, macroscopic permeability, magma rheology, explosion repose times and volumes, as well as recorded seismicity and surface measurements of SO2 flux. This model could potentially be applied to understanding extended periods of cyclic vulcanian explosions and dome-building at other arc volcanoes and has important implications for hazards during periods of low and high magma ascent rates. For example, periods of high magma ascent rate should favour the rapid development of low-permeability plugs (the "plug-forming regime") and lead to larger explosions (~105-106 m2) with shorter repose periods (~ tens of days). Periods of lower magma ascent rate should favour the slower development of comparatively high-permeability plugs or lava domes (the "dome-forming regime"), and may lead to lava dome construction accompanied by small-scale vulcanian explosions (~104-105 m2) with longer repose times (~ hundreds of days). Whereas frequent, larger-scale vulcanian explosions during the plug-forming regime are expected to produce more severe hazards related to explosive activity (shock waves, ballistics, ash plumes and pyroclastic density currents) than periods of infrequent small-scale explosions during the dome-forming regime, hazards from lava dome collapse (block-and-ash flows and decompression-induced explosions) should be anticipated in this latter case, demonstrating the diverse and evolving nature of hazards at plug-and dome-building volcanoes, and the challenge of monitoring them.