Records of, and controls on, temporal variations in activity at arc volcanoes
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Many attributes of volcanic activity, whether physical, geochemical/petrological, or geophysical, change over timescales from minutes to millions of years. Understanding the nature of these variations and their controls is essential both for hazard assessment and to understand volcanic and magmatic processes. Arc volcanism entails a relatively broad range of eruption styles and magma compositions, and thus wide-ranging hazards and a variety of eruption records. Despite their significance, records of activity for many arc volcanoes are inadequately characterised, so any temporal variability is poorly constrained. In this thesis, I investigate temporal variations in different styles of activity (dome-forming, Plinian-style, and effusive/glaciovolcanic) on a range of timescales (days–years, millennia, and hundreds of ky, respectively), from diverse records (seismicity, tephra deposits, and effusive eruption products), at four arc volcanoes. I analyse seismic time-series from Soufriere Hills Volcano (Montserrat) and Volcan de Colima (Mexico) using two statistical techniques, identifying temporal variation in the extent of long-range correlations and randomness in these data, which has potential to inform real-time monitoring and constrain eruptive processes. I also present new composition data for and review previous studies of tephra deposits from Volcan Hudson (Chile), revising its explosive eruption history and finding previously unrecognised shifts in the magma composition erupted through the Holocene. I also describe and analyse sequences of lithofacies from effusive eruptions of a range of magma compositions from Volcan Sollipulli (Chile), inferring varying extents of interaction with ice during their emplacement, and so changes in eruption style through time. These case studies highlight the importance of detailed characterisation of temporally varying records of volcanic activity, and some of the limitations of and uncertainties in these records and their interpretation.Keywords:
Volcanic hazards
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Empirical thesis.%%%%%%%%%%%%Chapter One. Introduction and motivation – Chapter Two. Insights into eruption dynamics from the 2014 pyroclastic deposits of Kelut volcano, Java, Indonesia, and implications for future hazards – Chapter Three. Eruption frequency, style and composition variations at Kelut volcano, Indonesia, based on 1500 years of tephra records – Chapter Four. The pre-eruptive magma storage system of the 1990 and 2014 Plinian, and 2007-2008 effusive dome-forming eruptions at Kelut volcano, Indonesia : insights into cyclic transitions in eruptive style – Chapter Five. Conclusions and future work – Appendices.%%%%Stratigraphic, petrological, mineralogical and geochemical records of volcanism are vital to understanding and forecasting the magmatic processes governing the cyclic effusive-explosive eruptive styles and patterns. Understanding the full range of eruptive behaviour at a volcano is fundamental in helping to reduce the disastrous impacts of eruptions to nearby vulnerable populations. Java (Indonesia) contains a large concentration of historically active and deadly volcanoes. However, few of these have well-constrained stratigraphic records that allow the variety of eruption styles and the eruption frequency to be estimated. Kelut volcano is one of the more deadly volcanoes in Indonesia, yet its geological record is poorly known. This thesis aims to improve our knowledge of Kelut, by examining detailed records of Kelut’s pyroclastic deposits and geochemistry, including both the most recent eruption (2014) and a series of units emplaced over the last 1500 years.%%%%The dynamics and chronology of individual eruptions of Kelut are complex. From AD ~560 to1800’s, glass shard compositions reveal that magmatic processes are the first-order control on eruptive behaviour. Mafic recharge events triggered the most explosive episodes. The system was maintained by variable rates and volumes of mafic magma influxes. In the early record, recharges were small or infrequent, and enough time between eruptions allowed homogenised high-Si magmas to erupt. More frequent eruptions took place from AD 1000 and variability of magmas was greater. From AD 1920 to 1990, eruptions tapped a continuously evolving magma system.The homogeneity of compositions from recent effusive-explosive rocks suggested that regular, small injections of gas-rich magma allowed for efficient mixing, buffering the system for some time. During this phase, other processes were of greater influence on eruptive styles. In some cases the presence of a conduit-capping lava dome inhibited gas release, eventually leading to overpressure and triggering a more explosive eruption, e.g., 2014. In the 2014 eruption, there is also potential evidence of mafic intrusion before eruption. This sudden intrusion left little trace,except in feldspar-rim compositions. Since 1920, explosive eruptions occured every ~21 years at Kelut. If the alternate explosive/effusive cycle holds, the next eruption of Kelut is likely to be a period of lava dome effusion.%%%%%%%%1…
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Chaitén Volcano erupted unexpectedly in May 2008 in one of the largest eruptions globally since the 1990s. It was the largest rhyolite eruption since the great eruption of Katmai Volcano in 1912, and the first rhyolite eruption to have at least some of its aspects monitored. The eruption consisted of an approximately 2-week-long explosive phase that generated as much as 1 km3 bulk volume tephra (~0.3 km3 dense rock equivalent) followed by an approximately 20-month-long effusive phase that erupted about 0.8 km3 of high-silica rhyolite lava that formed a new dome within the volcano’s caldera. Prior to its eruption, little was known about the eruptive history of the volcano or the hazards it posed to society. This edition of Andean Geology contains a selection of papers that discuss new insights on the eruptive history of Chaitén Volcano, and the broad impacts of and new insights obtained from analyses of the 2008-2009 eruption. Here, we summarize the geographic, tectonic, and climatic setting of Chaitén Volcano and the pre-2008 state of knowledge of its eruptive history to provide context for the papers in this edition, and we provide a revised chronology of the 2008-2009 eruption.
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