Modelling and physico-chemical constraints to the 4.5 ka Agnano-Monte Spina Plinian eruption (Campi Flegrei, Italy)
Introduction
Plinian eruptions commonly show transitions from phases of sustained convective columns to phases characterized by partial or total collapse generating pyroclastic density currents (PDCs) (e.g., Orsi et al., 1992; Wohletz et al., 1995; Rosi et al., 2001; Cioni et al., 2015; Pensa et al., 2015). Furthermore, while many Plinian sequences show the “classic” transition from a lower (basal) fall deposit into Pyroclastic Density Current (PDC) deposits (Cas and Wright, 1987), many others have been described as related to more uncommon stop-start behavior (e.g. Christiansen and Peterson, 1981; Giordano, 1998; Adams et al., 2001; Yasui and Koyaguchi, 2004; Miyaji et al., 2011).
Factors that determine sustained column to PDC transitions include variations in mass discharge rate as a consequence of changing volatile contents, magma composition and/or crystallinity, which in turn affect the rheology of magma, changes in conduit geometry and magma-water interaction (e.g. Sparks, 1978; Wilson et al., 1980; Cashman and Mangan, 1994; Klug and Cashman, 1994; Lyakhovsky et al., 1996; Papale, 1999; Rust et al., 2004; Gurioli et al., 2005; Houghton et al., 2010; Shea et al., 2011, 2012; Vinkler et al., 2012; Pardo et al., 2014).
The link between eruption column and PDCs (i.e., if the PDC is related to a total or partial column collapse) is of great interest for modeling conduit dynamics aimed at volcanic hazard assessment and risk mitigation. Considerable advancements in such studies have been achieved in recent years (e.g. Esposti Ongaro et al., 2006; Dellino et al., 2007; de’ Michieli Vitturi et al., 2010d; Suzuki and Koyaguchi, 2012; Carazzo et al., 2015; Trolese et al., 2019). Despite that, the critical conditions that drive the collapse of a Plinian column still need to be thoroughly explored. Textural analysis of pyroclasts has been proved to provide reliable proxies for magma ascent dynamics (e.g. Toramaru, 2006; Gurioli et al., 2008; Piochi et al., 2008; Costantini et al., 2010; Shea et al., 2010), and, coupled with experiments or parameterization of magma rheology (e.g., Costa, 2005; Llewellin and Manga, 2005; Mader et al., 2013; Vona et al., 2016), may help in understanding and constraining the transitions in eruptive styles (Adams et al., 2006; Sable et al., 2006; Moitra et al., 2013; La Spina et al., 2016). The 4.5 ka trachytic Plinian eruption of Agnano-Monte Spina (A-MS; de Vita et al., 1999; Blockley et al., 2008; Smith et al., 2011) provides an excellent opportunity to study an uncommon explosive sequence related to at least three different eruptive pulses, each initiated by a buoyant phase later evolving into a collapsing phase. This eruption has been taken as reference eruption scenario for the next high scale eruption at Campi Flegrei (e.g. Orsi et al., 2004, 2009; Bevilacqua et al., 2015, 2017) by the Italian Civil Protection Agency and therefore it represents a very important event for the hazard studies of one of the most populated active volcanic areas on Earth.
In this study, we apply a multidisciplinary approach, using geological data (chemistry and texture of the emitted products) to set up the source term conditions for the modelling of the three major fallouts (A1, B1, D1) and of the related PDCs (A2, B2, D2) (de Vita et al., 1999; Dellino et al., 2001, 2004a). The aim of the study is to relate the textural and physical features of the deposit to intrinsic parameters of the magma during ascent along the conduit and to determine to what extent, if any, changes in magma properties may have affected the course of the A-MS eruption.
Section snippets
Campi Flegrei volcanic system and Agnano-Monte Spina eruption
The Campi Flegrei caldera is a nested structure which results from two main collapses that followed the Campanian Ignimbrite (39 ka; Fisher et al., 1993; Orsi et al., 1996; Rosi et al., 1996; Civetta et al., 1997; Fedele et al., 2008) and the Neapolitan Yellow Tuff eruptions (14 ka; Orsi et al., 1992, 1995; Wohletz et al., 1995; Deino et al., 2004). The activity during the last 15 ka was concentrated in three epochs (15-9.5, 8.6-8.2, 5.5-3.8 ka BP) separated by two quiescence periods (Di Vito
Sampling
We selected two key stratigraphic sections that well represent the volcanic stratigraphy of A-MS eruption: "Cavone degli Sbirri" (CS) and "Eremo dei Camaldoli" (EC) (Fig. 1A and Appendix A). These two sites are located at a distance of 2 and 5 km respectively from the eruptive center (Monte Spina). Both sections are located along the ENE-WSW axis of maximum dispersal of the main fallout deposits. The main fallout and PDC deposits have been sampled in the two sites. Bottom and top parts of the B1
Chemistry
The bulk rocks from the A-MS deposit ranges in composition from alkali-trachyte to trachyte (de Vita et al., 1999; Arienzo et al., 2010; Smith et al., 2011; Fig. 1c). Pumices are from near-aphyric to porphyritic (up to 13% of crystal in layer B1). Glass compositions of each A-MS layer and average analyses for all units are reported in Table 1 and Fig. 1c, respectively. The glass compositions, reported in Fig. 1c and Table 1, show a general decrease in silica content (from 62.3 to 59.8 wt% SiO2)
Numerical simulations of conduit dynamics during AM-S eruption
Based on the measured textural and chemical properties of the erupted products, we performed some 1-D numerical simulations using Confort 15 software (Campagnola et al., 2016a), in order to investigate the eruptive dynamics during magma rise and constrain the style and conditions of fragmentation, focusing in particular on the transitions from sustained to collapsing column. Confort 15 is an open source numerical model for steady-state magma flow through vertical and cylindrical conduits open
Conclusions
The 4.5 ka Agnano-Monte Spina eruption is the highest magnitude event of the past 5 ka at Campi Flegrei caldera. Despite the small volume of erupted magma (1 km3, Di Renzo et al., 2011), this event caused significant environmental impact over a very large area. According to different studies (e.g. Orsi et al., 2004; Bevilacqua et al., 2015), a future vent most likely will open in the area between the Agnano and San Vito plains and a future explosive eruption, in spite of the magnitude of the
Acknowledgements
This work was funded by the DPC-INGV Project V1-UR RM3 (resp. C. Romano). The Grant of Excellence Departments, MIUR-Italy (ARTICOLO 1, COMMI 314 - 337 LEGGE 232/2016) is also gratefully acknowledged. We thank Sergio Lo Mastro (LIME, Roma-tre, Italy) for the valuable technical support with SEM analyses. We also thank Stefania Sicola for valuable help with Raman data acquisition. Prof. Lucia Gurioli and an anonymous reviewer are strongly thanked for their thoughtful comments.
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