Internal volcano-magmatic structure of Öræfajökull volcano, Iceland

Abstract Öræfajökull, one of Iceland’s most dangerous volcanoes which last erupted in 1727 and 1362 Common Era, began to show signs of significant unrest between 2017 and 2019. This activity led to a change in the aviation color code, the establishment of evacuation routes, and increased monitoring. The unrest was characterized by pronounced seismic activity, ground swelling, and heightened geothermal phenomena, including melting of ice and increased discharge of geothermal waters. This rare event at Öræfajökull has provided an invaluable and novel opportunity to better understand the internal structure of the volcano and processes responsible for the onset of the unrest. Here, we utilize both seismic tomography and geodetic modelling to locate important volcanic-magmatic features beneath Öræfajökull. Our conceptual model posits three distinct structural zones within the volcano: a geothermally active system at shallow depths, the start of a magma domain/melt zone between ~2 and 6 kilometers depth below-sea-level, and a zone of higher-velocity intrusive rocks below ~6 kilometers below-sea-level. This study establishes a critical baseline for future periods of unrest at Öræfajökull, highlighting the potential for intermittent increased activity over decades in complex, dormant volcanoes before a significant eruption ensues.