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Küppers, Ulrich (2005): Nature and efficiency of pyroclast generation from porous magma: Insights from field investigations and laboratory experiments. Dissertation, LMU München: Faculty of Geosciences



Enhanced knowledge of pre- and syn-eruptive processes is vital to deal with the increasing threat imposed to population and infrastructure by volcanoes that have been active historically and may potentially erupt in future. For many years, most of this knowledge was received from experiments on analogue materials and/or numerical models. In order to increase their significance and applicability for the “real” case, the natural complexity may not be oversimplified and the input parameters must be reliable and realistic. In the light of this, a close connection of field and laboratory work is essential. Volcanic eruptions may be phreatic, phreatomagmatic or magmatic, the latter scenario of which was addressed in this study. Rising magma is subject to decreasing lithostatic pressure. As a direct consequence, volatiles become increasingly oversaturated and bubbles will nucleate and grow depending on initial volatile content and melt viscosity. Both factors directly influence the diffusivity that limits the rate of bubble growth. Increasing amounts of bubbles increase the buoyancy difference to the surrounding rocks and lead to an acceleration of the rising melt-bubble mixture. Beside these limiting factors, the overpressure in the gas bubbles greatly depends on the magma’s ascent speed as it controls the residence time to conditions favourable to degassing (a combination of lithostatic pressure and magma temperature) and the time of overpressure reduction due to degassing. Volcanic eruptions occur when the bubbly melt can no longer withstand the exerted stress that derives from the overlying weight (lithostatic pressure), the expanding gas bubbles (internal gas overpressure) and different ascent velocities in the conduit (velocity profile). The melt will be fragmented and the gas-pyroclast mixture will be erupted. This study has combined close investigation of the deposits of the 1990-1995 eruption of Unzen volcano, Japan and detailed laboratory investigations on samples of this eruption and other volcanoes. The field work intended to reveal the density distribution of samples from within the eruptive products. Although all samples already underwent one eruption, their physical state (e.g. crystallinity, porosity) mostly remained close to sub-surface pre-eruption conditions due to their high viscosity and accordingly allowed their usage for the analysis of the fragmentation behaviour. In that purpose, rapid decompression experiments that simulate volcanic eruptions triggered by internal gas overpressure have been performed at 850 °C to evaluate fragmentation threshold and fragmentation efficiency. Laboratory investigations of that kind are one approach to bridge the gap between observational field volcanology and risk assessment as they reveal information on what can not be investigated closely but what is essential to know for realistic eruption models and the adjacent hazard mitigation. Changing the experimental conditions and close investigation of the artificial products reveals the influence of physical properties on the fragmentation behaviour. The density distribution inside a dome and the upper part of the conduit is crucial to the eruptive style of an explosive volcano. This information cannot be collected during an ongoing eruption but is important for future hazard assessment via modelling conduit flow and dome collapse/explosion behaviour. Therefore, the percentage of the mass fractions of all rock types in the primary and secondary volcanic deposits must be evaluated. For this purpose and at the lowest logistic effort, field-based density measurements have been performed on Unzen volcano, Japan. The resultant density distribution was found to be generally bimodal at constant peak values but changing peak ratios. The most abundant rock types at Unzen exhibited an open porosity of 8 and 20 vol.%, respectively. The porosity was found to be arranged in layers of cm- to dm-scale that were oriented subparallel to flow direction, i.e. subvertical within the conduit and flank-parallel within the dome lobes. The achieved results allowed for an internal picture of the dome during the last eruption of Unzen volcano. The evaluated picture of the density distribution within the uppermost parts of the conduit and the dome itself allowed for insights into and a better understanding of magma ascent and degassing conditions at Unzen volcano during its last eruption. Knowledge of the density distribution is additionally required to draw conclusions from the results of laboratory investigations on the fragmentation behaviour to the monitored behaviour of Unzen volcano during its last eruption. In the laboratory, the fragmentation behaviour upon rapid decompression has been investigated in a modified fragmentation bomb (Spieler et al., 2004). At 850 °C, initial overpressure conditions as high as 50 MPa have been applied to sample cylinders (25 mm diameter, 60 mm length) drilled from natural samples. In a first step, the minimum overpressure required to cause complete sample fragmentation (= fragmentation threshold, ΔPfr) has been evaluated. Results from samples of several volcanoes (Unzen, Montserrat, Stromboli, and Mt. St. Helens) showed that ΔPfr mainly depended on open porosity and permeability of the specific sample as these parameters were controlling pressure build-up and loss. The experiments further revealed that sample fragmentation was not the result of a single process but the result of a combination of simultaneously occurring processes as indicated by Alidibirov et al. (2000). The degree of influence of a single process to the fragmentation behaviour was found to be porosity-dependent. Further experiments at initial pressure conditions above ΔPfr and close investigation of the artificially generated pyroclasts allowed evaluating the fragmentation efficiency upon changing physical properties of the used samples. The efficiency of sample size reduction was investigated by grain-size analysis (dry sieving for particles bigger than 0.25 mm and wet laser refraction for particles smaller than 0.25 mm) and surface area measurements (by Argon adsorption). Results of experiments with samples of different porosities at different initial pressure values showed that the efficiency of fragmentation increased with increasing energy. The energy available for fragmentation was estimated from the open porosity and the applied pressure. A series of abrasion experiments was performed to shed light on the grain size reduction due to particle-particle interaction during mass movements. The degree of abrasion was found to be primarily depending on porosity and experimental duration. The results showed that abrasion may change the density distribution of block-and-ash flows (BAF) by preferentially abrading porous clasts. However, during the short drying interval prior to the analysis of the experimental pyroclasts, abrasion-induced grain-size reduction only played a minor role and was assumed to be negligible.