Petrologic predictions regarding future eruptive activity at Mount Hood, Oregon

Mount Hood, Oregon, represents a volcano that has a significant chance of erupting within the next few decades, but that has experienced no observed eruptions that provide direct geophysical or other constraints on eruption mechanisms and dynamics. In this case, petrological studies provide important insights into the potential nature of future eruptions, and these can be used to consider the geophysical and other signals that might accompany any renewed activity, and the timescales over which these might occur. In this contribution we present a summary of recent petrological work at Mount Hood and highlight data that provide insight into the likely nature of future eruptions. One of the most important features of Mount Hood lavas is the widespread evidence for magma mixing and mafic recharge. Andesites and low silica dacites from previous eruptive phases formed via mixing and hybridization between hotter ascending mafic magma and a long-lived crystal rich silicic magma or mush stored at shallow depths beneath the volcano. Mineral zoning studies show that mixing only occurs immediately prior to eruption, and we infer recharge of mafic magma into a shallow crustal magma storage zone is the predominant means by which eruptions of Mount Hood are initiated. Ascent of mafic magma and recharge would likely be accompanied by seismic, deformation and other detectable geophysical signals. Mineral barometry shows that amphiboles associated with shallow silicic magma formed at ~3-6 km, which we interpret to represent the depth of shallow silicic magma storage, and the depth at which recharge and mixing occurs. Amphiboles crystallized from mafic magma formed at ~10-16 km depth during magma ascent. Thus deeper earthquakes might accompany initial movement of mafic magma and more shallow seismic activity may occur during the convective overturn associated with recharge, mixing and final ascent of the hybridized magma. High-SiO2 melt inclusions in erupted lavas also contain low sulfur contents, thus mafic recharge and mixing should also be accompanied by the release of significant amounts of SO2 derived from mafic magma. Diffusion modeling based in mineral rim compositions suggests that the period between mafic recharge and eventual eruption and quenching is quite short - weeks to a few months at most, consistent with studies of other andesitic volcanoes. This provides an estimate of the potential time period that might elapse between detection of geophysical and other data indicative of recharge and mixing and magma reaching the surface.