Causes and Consequences of Holocene Mafic Explosive Volcanism in Central OR?

University Of Oregon Eugene, Eugene OR

Investigators

Abstract

Intellectual Merit: This project extends initial work by the PIs on small 'monogenetic' eruptions that are capable of explosive activity (with consequences for volcanic hazards), and for which eruption style and magma evolution depend in large part on the conditions of both gas and magma transport through the crust. This work has raised questions regarding mechanisms of gas migration, temporal evolution of magma delivery systems through the crust, and controls on fragmentation of crystalline mafic magma. It is proposed to examine links between physical and chemical processes active in small mafic systems by focusing on products of two Holocene eruptions: [1] Sand Mountain, OR (age = c. 3500 yr bp; volume = ~ 0.9 km3). This vent produced a large tephra sheet (>0.3 km3), and the uniformly fine (1-3 mm) grain size of the deposit raises interesting questions about magma fragmentation. [2] Cinder Cone, CA. This 1666 AD eruption is the youngest scoria cone in the Cascades, and serves as an example of syn-eruptive compositional evolution; moreover, episodes of tephra production can be related to specific lava flows, thus allowing evaluation of links between the physical and chemical evolution of the eruption. These two examples represent two end-member 'type' eruptions for hazard assessment (below). To complement the proposed field studies, analog experiments will be conducted to define three-phase flow regimes that control gas migration through crystal-rich magmas and to investigate the effect of crystals on magma expansion and fragmentation. Finally, experiments will be conducted to examine controls on temporary magma storage in sill complexes within the upper crust; the results of these experiments will have applicability not only to petrologic studies but also to volcano monitoring. Broader Impacts: An important broader impact of this work will be development of a comprehensive volcanic hazard assessment for central OR. This 'community-centric' hazard assessment will differ from traditional 'volcano-centric' assessments not only by incorporating multiple volcanic sources, but also in using data from the two 'type' eruptions to develop realistic tephra dispersion models for the range of wind conditions expected for the Bend OR region. Deposit data will be compiled in Google Earth (kml) format by students in the led PI's Geologic Hazards class. This project will also partially fund three graduate students to complete PhD dissertations that they started under the original grant: Dan Ruscitto is analyzing melt inclusion compositions as part of a larger study of the origin of subduction zone melts in central OR (with P. Wallace); Daniele McKay is studying the physical volcanology, petrology, and hazards of mafic eruptions from both Newberry Volcano and the central OR Cascades (with K. Cashman); Isolde Belien has conducted all analog experiments and will investigate links between gas migration, expansion, and fragmentation of crystal-rich magmas (with K. Cashman and A. Rempel). Additionally, both McKay and Cashman are helping to develop a new display at the Museum of Natural and Cultural History (UO) that will incorporate results from this work. Finally, this work involves collaborations with colleagues from the USGS (Michael Clynne; Cinder Cone), University of Geneva, Switzerland (Costanza Bonadonna; tephra modeling), and the University of Bristol, UK (Alison Rust & Jeremy Phillips; analog experiments).