Recent Mafic Eruptions at Newberry Volcano and in the Central Oregon Cascades: Physical Volcanology and Implications for Hazards
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Mafic eruptions have been the dominant form of volcanic activity in central Oregon throughout the Holocene. These eruptions have produced cinder cones, extensive lava flows, and tephra blankets. In most cases, the extent and volume of the tephra blankets has not been determined, despite the fact that future tephra production would pose considerable hazards to transportation, infrastructure, and public health. The economy of the region, which is largely based in tourism, would also be negatively impacted. For this reason, developing a better understanding of the extent and dynamics of tephra production at recent mafic vents is critical, both in terms of mitigating the hazards associated with future eruptions and in improving our scientific understanding of explosive mafic activity. Here I present detailed field and laboratory studies of tephra from recent mafic vents at Newberry Volcano and in the central Oregon High Cascades. Studies of Newberry vents show that eruption style is strongly correlated with eruptive volume, that extensive magma storage and assimilation occurred prior to the eruption of these vents, and that minimum pre-magmatic water content as recorded by plagioclase was 2.5 wt.%. Detailed mapping and physical studies of tephra deposits from High Cascades vents show that several recent eruptions produced extensive tephra deposits. These deposits are physically similar to well-documented historic eruptions that have been characterized as violent strombolian. At least one Cascade cinder cone (Sand Mountain) produced a tephra deposit that is unusually large in volume and characterized by uniformly fine-grained clasts, which is interpreted as evidence for syn-eruptive interaction with external water. Microtextural characteristics of tephra, along with an evaluation of possible water sources, support this interpretation. These investigations demonstrate that magma storage and eruption style at mafic vents is both variable and complex. Additionally, these studies show that cinder cones in central Oregon have the potential to erupt much more explosively than previously assumed. The results of this study will be an important tool for developing comprehensive regional hazard assessments. This dissertation includes previously published and unpublished co-authored material.