Unoccupied aircraft systems (UAS) are developing into fundamental tools for tackling the grand challenges in volcanology; here, we review the systems used and their diverse applications. UAS can typically provide image and topographic data at two orders of magnitude better spatial resolution than space-based remote sensing, and close-range observations at temporal resolutions down to those of video frame rates. Responsive deployments facilitate dense time-series measurements, unique opportunities for geophysical surveys, sample collection from hostile environments such as volcanic plumes and crater lakes, and emergency deployment of ground-based sensors (and robots) into hazardous regions. UAS have already been used to support hazard management and decisionmakers during eruptive crises. As technologies advance, increased system capabilities, autonomy, and availabilitysupported by more diverse and lighter-weight sensors-will offer unparalleled potential for hazard monitoring. UAS are expected to provide opportunities for pivotal advances in our understanding of complex physical and chemical volcanic processes. Non-technical SummaryUnoccupied aircraft systems (UAS) are developing into essential tools for understanding and monitoring volcanoes. UAS can typically provide much more detailed imagery and 3-D maps of the Earth's surface, and more frequently, than satellites are able to. They can also make measurements and collect samples for geochemical analysis from hazardous regions such as volcanic plumes and near active vents. Through being quick to deploy, they offer key advantages during initial stages of volcano unrest as well as throughout eruptions. Data from UAS have already been used to support hazard management and decision-makers during crises. In the future, UAS will become increasingly capable of flying longer and more complex missions, more autonomously and with more sophisticated sensors, and are likely to become key components of broader sensor networks for monitoring and research.

The basalts of the 2021 Fagradalsfjall eruption were the first erupted on the Reykjanes Peninsula in 781 years and offer a unique opportunity to determine the composition of the mantle underlying Iceland, in particular its oxygen isotope composition (δ18O values). The basalts show compositional variations in Zr/Y, Nb/Zr and Nb/Y values that span roughly half of the previously described range for Icelandic basaltic magmas and signal involvement of Icelandic plume (OIB) and Enriched Mid-Ocean Ridge Basalt (EMORB) in magma genesis. Here we show that Fagradalsfjall δ18O values are invariable (mean δ18O = 5.4 ± 0.3‰ 2 SD, N = 47) and indistinguishable from “normal” upper mantle, in contrast to significantly lower δ18O values reported for erupted materials elsewhere in Iceland (e.g., the 2014–2015 eruption at Holuhraun, Central Iceland). Thus, despite differing trace element characteristics, the melts that supplied the Fagradalsfjall eruption show no evidence for 18O-depleted mantle or interaction with low-δ18O crust and may therefore represent a useful mantle reference value in this part of the Icelandic plume system.

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