Taupō: an overview of New Zealand's youngest supervolcano
Simon J. BarkerColin WilsonFinnigan Illsley‐KempGraham S. LeonardEleanor MestelKate MauriohoohoB. L. A. Charlier
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Abstract:
Taupō volcano (New Zealand) is distinguished as the source of Earth's youngest supereruption (∼25.5 ka), with Lake Taupō occupying the resulting caldera. Taupō has also produced eruptions of a wide variety of sizes, styles and associated landscape responses over a ∼350 kyr period. Early Taupō (>54 ka) is poorly demarcated, merging with Maroa to the north, and is represented by widely scattered, geochemically distinct, effusive domes and explosive eruption products from vents all around the modern lake. Taupō had two independent magmatic systems from 54–25.5 ka, one that led to the Oruanui event focussed beneath the area of the modern lake and a second, northeast of the lake that has remained active to the present. Following the Oruanui supereruption, the rebuilt modern hyperactive Taupō magmatic system is primarily focussed beneath the lake and has generated 25 rhyolitic eruptions since ∼12 ka. The young rhyolite magmas come from an evolving silicic magma reservoir, but vary widely in their eruptive sizes and destructive potential. In the modern era Taupō experiences unrest every decade or so, but uncertainties remain over the nature of the magma reservoir and the processes that drive unrest or eruptive activity that require new geophysical data and interpretations.Keywords:
Caldera
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Peléan eruption
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The eruptive history and magma systems of large-scale explosive eruptions (VEI >5) in eastern Hokkaido, Japan, are reviewed on the basis of recently reported high-resolution tephrostratigraphy. More than 70 large-scale explosive eruptions have been recorded from the Akan, Kutcharo, Atosanupuri, and Mashu caldera volcanoes in the past 1.7 Ma. The total tephra volume of these eruptions is estimated to be approximately 1000 km3. The discharge rate increases remarkably from 0.2 km3/kyr to 2.0 km3/kyr at approximately 0.2 Ma. The discharge rate is still high owing to the recent frequent activity of the Mashu caldera. The silicic magma systems of the Akan, Kutcharo, and Mashu calderas formed independently. On the other hand, the magma of Atosanupuri is associated with that of Kutcharo caldera.
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Explosive volcanic eruption is one of the most hazardous natural phenomena. During explosive eruptions, a mixture of volcanic ash and gases is ejected from a volcanic vent into the atmosphere. For hazard risk assessment, it is important to comprehensively explain various observed data during eruptions and to understand the dynamics of explosive eruptions and the mechanism of volcanic ash dispersal. We have developed a pseudo-gas model of eruption cloud dynamics and ash dispersal. Our model has successfully reproduced the heights of eruption cloud and the distribution of fall deposits during large eruptions such as the Pinatubo 1991 eruption and those during small eruptions such as the Shinmoe-dake 2011 eruption. For more accurate estimates of volcanic hazard risks, two-way coupled models of multiphase flow are required.
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Transitions between explosive and effusive phases of silicic volcanic eruptions have been related either to stratification of volatiles in the source magma body or to the loss of volatiles through the permeable host rock of the conduit. One way to distinguish between these two models is to map and analyze the vesicular and glassy textures found in silicic lava flows. In this paper we present textural observations and isotopic evidence from active and Recent silicic lava flows which show that at least some vesiculation occurs during surface advance of extrusions, after magma has reached the earth's surface. This view is in contrast to the widely promoted “permeable foam” model, which states that all volatiles escape during ascent of the magma, and that all dense glassy material in lava flows forms from the collapse of pumiceous lava, i.e., that silicic lavas emerge as highly inflated foam flows. Such interpretations which claim that silicic lavas are completely degassed upon extrusion, and that all degassing must take place on the time scale of the eruption, neglect several important pieces of evidence, including the presence of obsidian in extremely small domes, and of vesicular zones in the interiors of silicic flows; the copious loss of volatiles through eruption plumes between eruptive phases; and direct observations of surface vesiculation during growth of the Mount St. Helens lava dome. The permeable foam model also implies the unlikely requirement that explosive‐to‐effusive transitions be associated with an increase in eruption rate. We present a more comprehensive model for the emplacement of silicic extrusions that allows for early gas loss during ascent, as well as late‐stage vesiculation. We then discuss how the redistribution of volatiles during surface flow can increase explosive hazards from silicic lavas days, weeks, or months after the lava emerges from the vent.
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Abstract Maar volcanoes are produced by subsurface phreatomagmatic explosions that can move vertically and laterally during an eruption. Constraining the distances that maar-forming explosions move laterally, and the number of relocations common to these eruptions, is vital for informing hazard scenarios and numerical simulations. This study uses 241 intact Quaternary maar crater shapes to establish global trends in size and spacing of explosion position relocations. Maar craters are sorted into shape classes based on the presence of uniquely identifiable combinations of overlapping circular components in their geometry. These components are used to recognize the minimum number of explosion locations responsible for observed crater shapes. Craters with unique solutions are then used to measure the size and spacing of the explosion footprints, the circular area of the largest crater produced by a single explosion of a given energy, that produce the crater shape. Thus, even in the absence of abundant observations of maar-type eruptions, the typical range, size and spacing of explosion positions are derived from maar crater shapes. This analysis indicates that most Quaternary maar eruptions involved at least three different explosion locations spanning distances of 200–600 m that did not always follow the trend of the dike feeding the eruption. Additional evaluation of larger maars, consistent with stratigraphic studies, indicates that centers of explosive activity, and thus the origin of ballistic and density current hazards, can move as many as twenty times during a maar-forming eruption. These results provide the first quantitative constraints on the scale and frequency of lateral migration in maar eruptions and these values can directly contribute to hazard models and eruption event trees in advance of future maar-type eruptions.
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One of the greatest remaining problems in modern volcanology is the process by which volcanic eruptions are triggered. It is generally accepted that eruptions are preceded by magma intrusion [ Sigurdsson and Sparks, 1978]. The degree of interaction between previously ponded magma in a chamber and newly intruded magma determines the nature and rate of eruption and also the chemistry of erupted lavas and shallow dykes. Here, we investigate the physics of this interaction. Volcano monitoring at its most effective is a synergy between basic science and risk assessment, while hazard mitigation depends on reliable interpretation of eruption precursors. The simple and much used Mogi model relates ground deformation (Δh) to changes in magma chamber volume. Gravity changes (Δg) combined with ground deformation provide information on magma chamber mass changes. Our new models predict how the Δg/Δh gradient will evolve as a volcano develops from a state of dormancy through unrest into a state of explosive activity. Thus by simultaneous measurement of deformation and gravity at a few key stations, magma chamber processes can be identified prior to the onset of conventional eruption precursors.
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