A Model For Alternating Effusive and Explosive Rhyolite Volcanism: The -1350 A.D. Eruption of Tarawera, New Zealand

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9815042SelfThe c. 1350 AD activity of Mount Tarawera was the largest (5-10 km3 magma volume) eruption in New Zealand, and one of the largest rhyolitic eruptions in the world, during the last millennium. The eruption products are locally known as the Kaharoa formation. This event featured remarkable fluctuations in eruption style from highly explosive to effusive as a major pyroclastic fall deposit and several lava domes were emplaced. This collaborative study with New Zealand volcanologists will build on earlier mapping of Tarawera volcano to produce an integrated picture of the processes of a major rhyolite eruption. Detailed field mapping of the relationships between the Kaharoa lava domes, block and ash pyroclastic flow deposits, and careful logging of the layers in the fall deposit will determine the details of the dispersal, clast characteristics, and grain size distribution. Sampling of the eruption products for granulometric, grain morphologic, and crystal and vesicle size distribution studies will help determine the physical state of the magma upon eruption and eruption mechanisms. Textural analysis of the rhyolite pyroclastics and dome lava samples, when combined with geochemical data to be obtained in the collaborative part of this study, will reveal information about the physical state (degree of undercooling, degassing, crystallization) of the magma during various phases of the eruption and the control that this exerted on the changing eruption styles. Results of the study will be a general model for this type of volcanism worldwide, further understanding of the controls on variations in eruption style in lava dome-forming volcanism by magma degassing and effusion rate, and documentation of widely dispersed plinian deposits coincident with effusive rhyolitic activity. The results will also be an integral part of a volcanic hazard assessment for future recurrences of this type of volcanism, both in New Zealand and worldwide.