Volcanic unrest as seen from the magmatic source– Reyðarártindur pluton, Iceland

Abstract

Abstract How the Earth’s crust accommodates magma emplacement influences the signals that can be detected by monitoring volcano seismicity and surface deformation, which are routinely used to forecast volcanic eruptions. However, we lack direct observational links between deformation caused by magma emplacement and monitoring signals. Here we use field mapping and photogrammetry to quantify deformation caused by the emplacement of at least 2.5 km3 of silicic magma in the Reyðarártindur pluton, Southeast Iceland. Our results show that magma emplacement triggered minor and local roof uplift, and that magma reservoir growth was instead largely aseismic by piecemeal floor subsidence. The distribution and orientation of fractures and faults in the reservoir roof can be explained by magmatic overpressure, suggesting that magma influx was not fully accommodated by floor subsidence. The tensile and shear fracturing would have caused detectable seismicity. Overpressure eventually culminated in eruption, as evidenced by exposed conduits that are associated with pronounced local subsidence of the roof rocks, and modelling suggests the formation of an asymmetric graben at the volcano surface. Hence, the field observations highlight processes that may take place in the roots of silicic volcanoes, not accounted for in widely used models to interpret volcano unrest at active volcanoes.