Structure and Present State of the Astroni Volcano in the Campi Flegrei Caldera in Italy Based on Multidisciplinary Investigations.

Despite its known reconstructed volcanic history, the structural setting and present state of the Astroni Volcano of the Campi Flegrei Caldera in Italy are still poorly defined. Through structural, geophysical, and geochemical investigations, we elucidate the structure and present volcanic activity of the Astroni Volcano, hosting which hosts tuff cones, scoriae cones, lava domes, and lakes on the crater floor. A volcano-tectonic analysis focused on the entire volcano edifice, coupled with electrical resistivity tomography of the shallower part of the Astroni crater, revealed the main rock formations, faults, and possible fluid patterns within the first 100 m depth. Two main NE–SW and NW–SE strike-fault sets were imaged using electrical-resistivity modeling and measurements along the wall of the volcanic edifice; they likely delimit a maar-like structure resulting from the highest energetic subplinian Astroni 6 eruption event and acted as magma pathways during the late eruptive activity stage. A 3D view of the reconstructed resistivity model revealed both deep root-conduit-like structures and shallower dome-like shapes for volcanic edifices on the crater floor. Gas and carbon compositions in the NNE sectors of the Astroni Lago Grande are similar to those of the Solfatara fumarole fluids, suggesting common hydrothermal origin and a possible link with a deep hydrothermal reservoir. This fluid-emission area along the border of the younger volcanic structure exhibits a +40 °C maximum soil-temperature anomaly. The proposed volcano-tectonic architecture should improve the unrest scenarios in case of reactivation in this Campi Flegrei Caldera sector and the monitoring strategies for the Astroni Volcano.