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Scheu, Bettina (2005): Understanding silicic volcanism: Constraints from elasticity and failure of vesicular magma. Dissertation, LMU München: Faculty of Geosciences
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Abstract

Volcanic eruptions are one of the most spectacular and dangerous natural phenomena. Volcanic activity can either be effusive, dominated by quiescent emission of lava or explosive, dominated by the eruption of pyroclastic material. A rapid transition between these two regimes is possible. possible. On a broad scale, a clear distinction of the eruption style can be drawn by magma composition. Large-scale basaltic eruptions are mostly effusive, whereas large-scale silicic eruptions are mostly explosive. Thus silicic volcanism possesses a severe hazard potential and can have devastating effects on human population. This work comprises experimental investigations of the fragmentation behavior of porous magma as well as of the propagation of elastic waves within this material. The analyses were conducted on samples from Unzen Volcano, Japan as well as samples from Soufrière Hills Volcano, Montserrat, West Indies. The elastic wave velocities of differently porous Unzen dacite samples were investigated with the use of a cubic multi-anvil press. The main results of this study show that porosity (density) and texture affect the elastic properties of samples at a given temperature. In particular, it can be stated: (1) Seismic velocities increase with pressure due to compaction and closing of microcracks. The Vp anisotropy decreases with pressure for the same reason. (2) Increasing the temperature also leads to higher elastic wave velocities and lower anisotropies. This must be highlighted as the inverse behavior is documented for the majority of rocks. The effect may be linked to reduction of pore volume and further closing of microcracks due to reduction of cooling tensions. At 600 °C mean Vp values of 4.31 - 5.64 km/s and mean Vs* values of 2.20 - 3. 32 km/s could be determined. (3) The velocity anisotropy can be linked to the texture of the samples: Those with a high anisotropy show a pronounced shape-preferred orientation of phenocrysts and microcrystals, sometimes in addition to layering within the groundmass of the sample. Since the crystals are typically aligned parallel to walls of volcanic conduits, the velocity normal to the walls is likely to be reduced. The data allows better estimates of the properties of silicic volcanic rocks at shallow depths within volcanoes, e.g. at conduit walls. These estimates are vital for computation of conduit models as well as the modelling of volcano seismic data, and may lead to an improved analysis of precursor phenomena in volcanic areas. The physical properties of magma within volcanic conduits and domes are crucial for modelling eruptions. This study comprises a detailed investigation of the fragmentation behavior (threshold and propagation speed) of differently porous sets of dacitic and andesitic samples derived from Unzen Volcano, Japan and Soufrière Hills Volcano, Montserrat, West Indies. The experiments were performed with a shock-tube based fragmentation apparatus and pertain to the brittle fragmentation process. The results show a strong influence of the open porosity and the initial pressure on the fragmentation behavior. The speed of fragmentation follows a logarithmic relationship with the pressure difference, the fragmentation threshold an inversely proportional power-law relationship with increasing porosity. In this study fragmentation speed values ranging from 15 - 150 m/s were observed for applied pressure differences of up to 40 MPa and open porosities from 2.5 - 67.1 %. The expansion of the pressurized gas in the vesicles largely provides the energy driving the fragmentation process. The fragmentation speed results of all analyzed samples show a close relationship to the energy density (fragmentation energy standardized to volume). A logarithmical increase of the propagation speed was observed with the energy density as soon as the energy threshold of 2.0 x 0.5 J/m³ was exceeded. The fragmentation speed is independent from the origin and composition of the samples, proving the governing role of the energy to the initiation a propagation of fragmentation process. Different fragmentation mechanisms were discussed and the layer-by-layer fragmentation due to vesicle bursting is concluded to be the main process responsible for the disintegration of vesicular rocks. The increased importance of fracturing due to the passing of the unloading wave after a rapid decompression could be proved for low porous samples. Further the influence of the sample’s permeability on the fragmentation behavior was evaluated. It could be shown that a high permeability hinders the initiation of a fragmentation and reduces the propagation speed of this process at a certain energy density. The fragmentation results were applied to the dome collapse events and Vulcanian events of the 1990-1995 Unzen eruption and the 1997 Vulcanian events at Montserrat. Large blocks with layers of various porosity were observed at the block-and-ash flow deposits of Unzen Volcano and support the model that a dome and dome lobes consist of areas of differing porosity. In addition, the samples gained from Montserrat, allow to postulate a porosity gradient within a volcanic conduit, with low porous magma close to the conduit walls. A layered composition of a dome and dome lobes, respectively, may lead to the fragmentation of single layers, followed by the collapse of the overlaying sections. These events could catalyze gravitationally induced dome collapse events leading to vigorous pyroclastic flows and / or trigger a sector collapse followed by an Vulcanian event. A porosity gradient at the magma in the conduit leads to a concave shape of the fragmentation surface and facilitates lateral fragmentation of dense magma close to the conduit walls. Conduit implosion may to occur during most explosive eruptions and is likely to influence the cessation or pulsation of the eruption. The slow magma ascent and extrusion rate at Unzen resulted in relatively dense extruded magma, as the magma could almost completely degas during the ascent. The low porosity of this magma causes a high fragmentation threshold of most material, which is too high for unassisted fragmentation. Therefore dome collapse events were the most abundant events of the 1990-1995 activity of Unzen Volcano, leading to numerous block-and-ash flows. Also the fragmentation-amplified collapse of dome lobes or parts of the dome are reasonable. This accounts especially for the long lasting collapse events with vigorous pyroclastic flows at the early stage of the eruption in June 1991, which were followed by minor Vulcanian events. Nevertheless a larger explosive event would have been possible, triggered by a landslide or a sector collapse of the dome. Similarly to Unzen, the first phase of activity of the recent eruption of Montserrat is characterized by numerous dome collapse events leading to violent pyroclastic flows. As the magma extrusion rate was quite high during this phase, the extruded magma was higher vesiculated compared to the Unzen magma and thus a more violent evolution of the eruption activity took place. Large dome collapse events frequently caused Vulcanian events, and even two cycles of Vulcanian activity from August to September 1997 occurred. The calculations of the fragmentation depth, reached by this explosions yielded about 1500 m, based on the laboratory gained fragmentation speeds, which is in good agreement numerical models and observations. In silicic volcanic systems the conduit seems not to be sharply defined. The conduit walls are more to be seen as a kind of transition zone between a hot, ductile and vesicular magma within the conduit and the host rock. The rocks forming this transition zone are assumed to be quite hot, but presumably below glass transition and react therefore solely brittle. Furthermore these rocks should be quite dense, compared to magma in the conduit, and heavily fractured due to the high shear strains this zone is presumably exposed to. The transition zone is less likely affected by a fragmentation event. Their rocks (magma as well as host rocks) are too dense to fragment due to pore pressure. The needed pressure difference is unrealistically high, for example an overpressure of 18 MPa would be needed to initiate the fragmentation of rocks with a porosity of 7.5 %. Nevertheless this material may be found in the deposits of explosive events, due to processes like conduit wall erosion or as remnant of dense lenses within more porous areas. Indeed, also fragmentation may take place, the most likely process fragmenting even this dense material is by lateral fragmentation, which may occur from a certain depth on behind a fast propagating fragmentation of highly vesicular magma at the center of the conduit. The style and progression of an eruption is depending on the properties of this vesicular magma. If its fragmentation threshold can be exceeded, an explosive event may take place. Otherwise the magma is extruded quiescent in a dome forming eruption. The transition zone bears important implications on the one hand for the explosive event as a lateral fragmentation of a certain area may cause cessation or pulsation of the event, on the other hand for the propagation of seismic signals related to the eruption. Within this transition zone as well as the nearby host rock a decisive change of temperature and porosity can be supposed. This leads to a significant shift of the elastic wave velocities within this zone, sometimes resulting in trapped waves within this zone as observed for Montserrat. Especially the abnormal velocity increase with increasing temperatures has to be mentioned. Thus implications for an overall view of a volcanic system are provided in this study, with the transition zone as the common link. The properties of the elastic wave velocities account for the for host rock as well as the transition zone and bear vital constraints for the interpretation and modelling of volcano seismic data. The results of the fragmentation experiments are applicable for dome rocks, the vesicular interior of a conduit as well as the transition zone and contain important implications for the modelling of conduit processes. Together the results of this study may contribute to a refined understanding of processes typical for silicic volcanism. This may allow an improved analysis of precursor phenomena in volcanic areas and consequently provide important constraints to the hazard and risk management.