Tracking the growth and deformation of fissure phreatomagmatic eruptions: Insights from the ca. 3.9 ka Nisida eruption at Campi Flegrei caldera, southern Italy

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 past 15 k.y. Among these, only a few had a lateral propagation of the vent along an erup-tive fissure. In this work, we explore the ca. 3.9 ka phreatomagmatic eruption of Nisida that occurred in the southeastern caldera ring fault zone. Combining field and uncrewed aerial vehicle (UAV)-based structural analysis with the available seismic reflection data, we traced the different stages of growth of a tuff cone complex along an 1.4-km-long eruptive fissure. The eruption started within the ring fault zone and propagated first toward the northeast, and then toward the north-northeast, producing a dike-induced graben. We have evidence of multiple gravitational collapse events throughout the eruption, mainly accommodated by approximately west-northwest, south-dipping normal faults, and outward-dipping slip surfaces. An energetic maar-forming erup-tive phase produced inward-dipping circumferential normal to transtensive faults accommodating the vertical collapse. Post-lithification northeast-southwest and west-northwest normal to left-lateral faulting possibly suggests the reactivation of the buried caldera rim in historical times. Features of fissure explosive eruptions, such as those observed here, are rarely reported in the literature. This case study thus offers an exceptional opportunity to examine syneruptive volcano-tectonic processes. Furthermore, we reassessed the volume of the eruption, including the dismantled morphology, yielding an unprecedented value of 0.17 km³ dense rock equivalent (DRE), with the subaerial extent now mostly eroded. Our results suggest that most of the eruptions of the last eruptive epoch (5.5-3.7 ka) had similar sizes (Volcanic Explosivity Index of 4) considering the single eruptive episodes. This work could prove useful for improving volcanic hazard assessment at the active Campi Flegrei, which represents the world's highest-risk volcanic area.