USING IGNEOUS PETROLOGY TO INFORM FORECASTS OF VOLCANIC ACTIVITY

Rock samples provide some of the only direct evidence of magmatic processes that precede and accompany volcanic eruptions. Most data that volcanologists use to understand volcanic systems provide indirect evidence and require some level of inference to relate to the magmatic system itself. Volcanic rock samples can provide direct information about physical properties of magmas (volatile content, composition, crystallinity) and inform additional indirect estimates of pressures and temperatures of pre-eruptive magma storage conditions. They can also reveal rates of pre-eruptive processes, such as magma ascent, or the time between magma mixing and eruption. This information can allow better prediction of the nature of future volcanic eruptions—how large they will be, how long they will last, and the style/s of the eruption.

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.