USING IGNEOUS PETROLOGY TO INFORM FORECASTS OF VOLCANIC ACTIVITY
Two recent eruptions in Alaska provide contemporary examples of the contributions from petrology to volcanic monitoring. Deposits from phreatomagmatic explosions at Semisopochnoi volcano, which have been intermittent since 2018 and were sampled in May 2021, include a minor proportion of juvenile, strongly microlitic, basaltic andesite to andesite compositions (An45-85 plagioclase, opx, cpx, and ~66 wt.% SiO2 matrix glass). This crystal-rich juvenile material suggests slow ascent rates and that current products may have ascended into the upper crust during seismic swarms that began in 2015. Juvenile breadcrust bombs erupted during a May 2021 vulcanian blast at Great Sitkin are also crystal-rich, but with phases consistent with a middle andesite composition (An50-88 plagioclase, opx, and cpx phenocrysts, and ~77 wt.% SiO2 rhyolitic matrix glass). A possible interpretation of these bombs is that they represent shallowly stored “remnant” magma, that was pushed up and out by new magma ascending from depth. Continuing seismicity in July 2021 reveals ongoing unrest and possible continued magma ascent. This work demonstrates the utility of using the petrology and geochemistry of eruptive products to understand the pre-eruptive conditions of past, and possibly future, eruptions.