The Solfatara eruption of Campi Flegrei caldera (Italy): Eruptive products and dynamics of a gas-driven to magmatic explosive event

Small- to moderate-volume eruptions at caldera systems often involve complex and rapidly evolving dynamics. The Campi Flegrei caldera in southern Italy has experienced >60 small- to moderate-volume eruptions since the last major caldera-forming event at ca. 15 ka. Among these, the ca. 4.3 ka Solfatara eruption provides a particularly valuable case study for investigating the dynamics of explosive activity in calderas hosting hydrothermal systems. In this study, we reconstruct the eruption sequence and dynamics of the Solfatara eruption using field data, componentry, grain size, ash morphology, vesicle texture quantification, and glass geochemistry. We identify three main eruption phases that indicate a clear transition in eruption styles: a gas-driven phase (Phase 0), a transitional phase from gas-driven to magmatic activity marked by phreatomagmatic explosions (Phase 1), and a magmatic-dominated phase (Phase 2), each associated with specific depositional units. Unit 0, deposited during Phase 0, consists of a fine, indurated ash layer with a high content of free crystals and altered lithic clasts interpreted as originating from the sudden decompression of hydrothermal fluids driving a phreatic explosion. Unit 1, deposited during Phase 1, shows alternating fine and coarse ash beds with abundant altered lithic clasts and clear presence of juvenile clasts (∼35 wt%), mainly deposited by dilute pyroclastic density currents (PDCs). These were generated through repeated explosions over short periods of time driven by magmatic activity, yet with an important contribution of hydrothermal fluids (magma-water/fluids interactions). In Unit 2, emplaced during Phase 2, the deposits become richer in juvenile clasts (up to 64 wt%) and include juvenile-rich lapilli fall beds and proximal lithic-rich tuff breccias alternating with PDC-related ash and lapilli layers, indicating a shift to a magmatic-dominated, volatile-driven explosive activity. We estimate a total erupted volume of 0.053 km3 (whole deposit), with ∼0.013 km3 dense rock equivalent of juvenile magma erupted. The Solfatara eruption shows the complex behavior and hazard of small- to moderate-volume eruptions, in which transitions from gas-driven to magmatic activity can occur rapidly and produce diverse, hazardous phenomena. These findings have important implications for the reconstruction of eruption scenarios, emergency planning, and hazard assessment at Campi Flegrei and at other active calderas worldwide.