Tracking the growth and deformation of fissure phreatomagmatic eruptions: insights from the ~3.9 ka Nisida eruption at Campi Flegrei caldera

Monogenetic eruptions characterize volcanic activity in caldera volcanic fields. One remarkable example is Campi Flegrei (southern Italy), which has experienced over 70 moderate-sized explosive eruptions in the last 14 kyr. Among these, only a few had a lateral propagation of the vent along an eruptive fissure. In this work, we have deepened into the ~3.9 ka phreatomagmatic eruption of Nisida that occurred in the southeastern caldera ring fault zone. Combining field and UAV-based structural analysis with the available seismic reflection data, we traced the different stages of growth of a tuff cone complex along a ~1.4 km long eruptive fissure. The eruption started within the ring fault zone and propagated first towards northeast, and then towards NNE, producing a dike-induced graben. We have evidence of multiple gravitational collapse events throughout the eruption, mainly accommodated by ~WNW, south-dipping normal faults, and outward-dipping slip surfaces. An energetic maar-forming eruptive phase produced inwarddipping circumferential normal to transtensive faults accommodating the vertical collapse. Post-lithification NE-SW and WNW normal to left-lateral faulting possibly suggests the reactivation of the buried caldera rim in historical times. All these features of fissure explosive eruptions are rarely reported the in literature, making this case study an exceptional site to disclose syn-eruptive volcano-tectonic processes. Furthermore, we have reassessed the volume of the eruption, including the dismantled morphology, yielding an unprecedented value of 0.17 km3 DRE, with the on-land extent nowadays mostly eroded. Our results suggest that most of the eruptions of the last eruptive epoch (5.5-3.7 ka) yield similar sizes (VEI 4), considering the single eruptive episodes. This work might be useful to improve the volcanic hazard assessment at the restless Campi Flegrei, which represents the world's highest-risk volcanic area.