Anatomy of a volcanic vent: new insights into the intra-crateric pyroclastic deposits of the Astroni volcano (Campi Flegrei)

Spatter and scoria cones, as well as tuff ring and tuff cones, are characteristic products of single or multiple small-volume eruptions (<1km3) [e.g. Nemeth et al. 2015] that typically exhibit a great variety of depositional features. These volcanic edifices form from the accumulation of erupted products proximal to their vent. In this scenario, the Astroni volcano (Campi Flegrei, Italy) represents the only documented example of well-preserved multistage vent in the most recent epoch of activity (4.8–3.8 ka) of Campi Flegrei caldera, reactivated by seven, relatively short-lived explosive eruptions [Isaia et al. 2004]. It represents a case study as a complex and highly dynamic depositional environment in which a wide spectrum of volcanic processes operated simultaneously or sequentially, not only during the active lifetime of the volcano but also during its periods of quiescence. The volcano and its deposits have been extensively studied and multiple interpretations were reported in the scientific literature [e.g. De Lorenzo and Riva, 1902; Rittmann, 1950; Scherillo, 1954; Rosi and Sbrana, 1987; Di Filippo et al. 1991; Di Vito et al. 1999; Isaia et al. 2004, 2022; Tonarini et al. 2009] with a general reconstruction of the volcanic and magmatic history [Isaia et al. 2004]. Most interpretative models are based on medial to distal outcrops, where the Astroni sequences are more readily observable. Here, we present a sedimentological analysis of ultra-proximal lithofacies combined with grain size, component and petrochemical data for the well-exposed intra-crateric Astroni pyroclastic deposits. Our aim is to reconstruct the architecture of the edifice and investigate the overall processes at play within the crater. We used a very detailed stratigraphic approach to identify several lithostratigraphic units separated by either thin paleosols or irregular to steep, erosive unconformities. The sequences are composed of matrix-supported, massive to diffuse bedded, lithic brecciae and coarse-grained pumice-rich deposits alternating with well-stratified, laterally persistent, fine to coarse ash layers and fine to coarse lapilli deposits. Undulating to cross-bedded, low-angle truncations, laminae and few centimeters thick lenses of pumice clasts are common in the stratified ash layers. Plane-parallel, stratified accretionary lapilli bearing deposits and lithified units are also observed. Laterally impersistent, clast-supported and well-sorted layers of dominant grey lapilli to block, pumice clasts are locally interbedded in the sequences at several stratigraphic heights. Outsized lava and pumice ballistic bombs, up to 40 cm in diameter, are also very common forming deep, mostly symmetric impact sags. The systematic survey, conducted through 10 measured sections only few tens of meters apart, allowed us to study the vertical and lateral facies variations of the different lithostratigraphic units. This investigation revealed a wide range of predominant pyroclastic density currents (PDCs) and minor fallout transport regimes and emplacement mechanisms in the vent setting. They include: ballistic impacts associated with short-lived to sustained, unstable to buoyant eruption column phases and contemporaneous fountaining activity. Unsteady, channelized to unconfined PDCs varying from dilute, turbulent MISCELLANEA INGV n. 83 Abstract Volume 6a Conferenza A. RittmannCatania, 18 | 20 Settembre 2024 83 suspensions with tractional bedload flow boundary zone to high-concentrated fluid-modified granular flows developed. The presence of several unconformities of variable time-duration and nature, suggests that weathering, erosion and/or crater wall collapse events also occurred during the periods of quiescence between the different Astroni eruptions, allowing a complex stratigraphy and intra-crateric history to be reconstructed. Our research provides a very close up view of the extremely complex, multiphase and multiscale phenomena and unpredictable dynamics occurring at a volcanic vent that might be considered in future hazard and risk assessments, especially in calderas with uncertain vent opening locations. The work in progress also emphasizes the importance of considering external, non-eruptive subaerial processes (such as erosion, collapse and remobilization events) that may occur at any time and are highly common at the vent, in terms of potential hazards.