The Neapolitan Yellow Tuff eruption as the source of the Campi Flegrei caldera

11 The Neapolitan Yellow Tuff eruption as the source of the Campi Flegrei caldera Giuseppe Rolandi1, Massimo Di Lascio2, Roberto Rolandi3 1

Retired, University Napoli Federico II, Napoli, Italy; 2Consultant, Selfemployed Geologist, Battipaglia (Salerno), Naples, Italy; 3Dipartimento Scienze della Terra, Ambiente e Risorse, Universita` di Napoli-Federico II, Naples, Italy

Introduction Campi Flegrei is a large caldera that incorporates the western suburbs of Naples, in Southern Italy. It lies within a graben, at least 30 km wide, between the Apennines and the Tyrrhenian coast that has been subjected to a tectonic ESE-WNW extension during the Holocene (Milia et al., 2009). Formation of the caldera has been attributed to two episodes of major collapse during the eruptions of the Campanian Ignimbrite (CI; 250 km3 DRE [Dense Rock Equivalent]), 39 ka years BP, and of the Neapolitan Yellow Tuff (NYT; 50 km3 DRE), 15 ka BP (Rosi and Sbrana, 1987; Orsi et al., 1996, 1999; Deino et al., 2004; Acocella, 2008). More recent studies of the CI indicate that it was instead erupted from vents in the Campanian Plain, north of Campi Flegrei (De Vivo et al., 2009; Rolandi et al., 2003; Torrente et al., 2010; Milia and Torrente, 2011, Rolandi et al., 2019 this volume). The second interpretation is supported by stratigraphic interpretations of borehole data from the Campi Flegrei Deep Drilling Project, which shows that ground displacement in the Phlegraean area during the CI eruption was at least 10 times too small to account for caldera formation (De Natale et al., 2017). An immediate implication is that the Campi Flegrei caldera was formed during the NYT eruption alone. Here, we show that the available geological, structural, and geophysical data for the volcano support the conclusion that only the NYT eruption was responsible for the caldera collapse. We also argue that collapse Vesuvius, Campi Flegrei, and Campanian Volcanism. https://doi.org/10.1016/B978-0-12-816454-9.00011-0 Copyright © 2020 Elsevier Inc. All rights reserved.

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occurred through a combination of downsag and trap door mechanisms and that the pattern of postcaldera resurgence has controlled the distribution of intracaldera eruptions.

Separate sources for the Campanian Ignimbrite and Neapolitan Yellow Tuff Formation of the Campi Flegrei caldera was originally attributed to the CI eruption before the full extent of the NYT had been recognized (Rosi et al., 1983; Rosi and Sbrana, 1987). Later geological data have questioned this hypothesis (Rolandi et al., 2003). The simplest lines of evidence are the shallow depth and the small volume of the CI found within the caldera itself. First, interpretations associating Campi Flegrei with the CI anticipate a caldera collapse of about 1.5 km (Rosi and Sbrana, 1987; Orsi et al., 1996); second, by analogy with caldera-forming eruptions with volumes similar to the CI (250 km3 DRE), ignimbrite thicknesses of 1e2 km are expected within the caldera (Smith and Bailey, 1968; Steven and Lipman, 1976; Lipman, 1976, 2000). Neither condition has been satisfied. Stratigraphic data from the 501 m borehole of the Campi Flegrei Deep Drilling Project indicate subsidence related to the CI eruption of w0.1 km (De Natale et al., 2017), whereas additional deep boreholes on land and seismic surveys offshore suggest CI thicknesses w0.1 km (Pescatore et al., 1984; D’Argenio et al., 2004; Milia and Torrente, 2007; Sacchi et al., 2009, 2014; Aiello et al., 2012; De Natale et al., 2017). The results indicate that the Campi Flegrei caldera is too small to account for the size of the CI eruption. A more probable source is one or more fissures across the Campanian Plain, which extends for 30 km immediately north from Campi Flegrei (Rolandi et al., 2003, 2019, this volume). The formation and behavior of Campi Flegrei can thus be attributed to its response to the NYT eruption (Orsi et al., 1992; Scarpati et al., 1993).

The Neapolitan Yellow Tuff caldera The Campi Flegrei caldera is 12
16 km across, including both its subaerial and submerged sections (Fig. 11.1A). The subaerial portion of the caldera rim runs from north of the Quarto Plain along the base of the relicts of the Camaldoli and Posillipo hills, which reach 350 and 160 m a.s.l., respectively. At the base of Camaldoli Hill, an about 200 m thick section of volcanic rocks older than the NYT is exposed, including the famous 39 ka Piperno-Breccia Museo formation (De Lorenzo, 1904; Rittmann, 1950; Di Girolamo et al., 1984; Rolandi et al., 2003). The older rocks are not present at the base of Posillipo Hill, and their absence

Chapter 11 The Neapolitan Yellow Tuff eruption

accounts for the difference in height with Camaldoli. Both relicts are well preserved in sections facing toward the caldera along steep fault scarps, outlining a zone of arched ring fractures. Their outward-facing flanks have mean dips of 25 degrees and, following Dainelli (1930), we consider these to belong to the flanks of a volcanic edifice that was formed by the NYT eruption before caldera collapse occurred (Fig. 11.1B). The rim continues underwater, where it defines the limits of the Bay of Pozzuoli. Its southern boundary can be traced around the submerged volcanic edifices, from east to west, of the Nisida Banks, Pentapalummo Banks, and Miseno Banks (the first and last are distinct from the subaerial cones of Nisida and Capo Miseno) that are overlain by the 39 ka CI (Fig. 11.1A; Pescatore et al., 1984; D’Argenio et al., 2004; Milia and Torrente, 2007; Sacchi et al., 2009, 2014; Aiello et al., 2012). The trace of the western edge of the caldera from Cape Miseno through Bacoli town to Quarto in a northerly direction is recognized following the outcrops of NYT affected by collapse (Fig. 11.1A).

Distribution and alteration of the Neapolitan Yellow Tuff Pyroclastic deposits from the NYT eruption vary from loose pale gray material (the locally named Pozzolana) to the yellow, lithified tuff that gives the deposit its name (Fig. 11.2A,B). All the deposits were originally Pozzolana. Lithification is the result of secondary crystallization of zeolites (acicular crystals of phillipsite and rhombohedral chabazite; Fig. 11.2C), which form by the reaction of glass and susceptible mineral phases with saturated alkaline solutions at temperatures of 150e250 C and moderate to low pressures (Franco, 1974; De’ Gennaro et al., 1999; Gatta et al., 2010). The yellow color is produced by the oxidation of iron in the zeolites. Where water saturation has prevented oxidation (e.g., below the water table or sea level), the NYT has a pale-green color, but this changes to yellow on exposure to air. The rate of zeolitization is poorly constrained in volcanic environments, but observations of color changes in tephra deposits from the 1538 CE eruption of Monte Nuovo suggest that the yellow color can be acquired in about 100 years or less (Parascandola, 1946). Outside the caldera, the NYT is yellow and lithified to distances of about 6 km, but further away, it retains its original loose texture and gray color. At the larger distances, therefore, it appears that the Pozzolana was able to cool rapidly below the temperature threshold necessary for zeolitization (Fig. 11.2D; Scherillo, 1955; Scherillo and Franco, 1960). However, both gray and yellow facies are found among the few NYT outcrops within the caldera

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Figure 11.1 (A) Campi Flegrei caldera and structural sketch map of the area, (B) orthogonal profiles of the original Neapolitan Yellow Tuff (NYT) volcano, obtained by connecting the outer profiles of the volcanic relicts of Camaldoli and Posillipo hills, with the NYT thicknesses of cored NYT in the drill holes S10 and S16 (see Table A.1 and Fig. A.1 of Appendix), both shifted above the current sea level. The sections are referenced to an arbitrary but reasonable diameter of the crater.

(Di Giuseppe et al., 2017) (Fig. 11.2E). The yellow zeolite-rich NYT also occurs in coastal cliffs beneath Rione Terra (Fig. 11.2F), the historic center of Pozzuoli. The conditions favoring a mixed distribution of facies within the caldera are discussed below.

Caldera resurgence Gravity anomalies and stratigraphic relations show that the central portion of Campi Flegrei has been uplifted as a resurgent block (Giudicepitero, 1993; Orsi et al., 1996; Acocella, 2010; Capuano et al., 2013). By combining previous analyses with new geomorphological constraints, as well as data from gravity and

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Figure 11.2 (A) Ramping of Neapolitan Yellow Tuff (NYT) products on the Torregaveta volcano (Tg), 7 km from the source toward W, (B) ramping of NYT on the Vomero hill (Naples area), 4 km from the source toward E-NE; (C) crystals of zeolites (phillipsite and chabazite), present in the altered NYT; secondary electron image; (D) deposits of loose nonaltered NYT (Pozzolana ¼ P), outcropping 7 km from the source toward E (Ponti Rossie Naples). Almost continuously over the Pozzolana deposit are the products of c.13 ka Plinian eruption of Pomici Principali (PP), (E) red dashed line separates the unaltered Pozzolana present at the base of Posillipo fault scarp from the upper zeolite-rich NYT, (F) zeolite-rich NYT resurgent dome at Rione Terra cliff in Pozzuoli.

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offshore surveys, we have been able to define more precisely the extent of resurgence and the environmental conditions in which it took place.

Marine surveys in the Bay of Pozzuoli The Bay of Pozzuoli is a shallow depression that reaches depths of as much as 110 m across its southern half (Fig. 11.3). Seismic profiles show that, in the deepest part of the bay, the NYT is about 50 m thick. Its base rests on top of the CI (c.100e150 m thick), about 240 m below the sea floor, and its top is covered by about 100e120 m of loose pyroclastic deposits (Fig. 11.3A; Pescatore et al., 1984; D’Argenio et al., 2004; Milia and Torrente, 2007; Sacchi et al., 2009, 2014). Step changes in the position of the NYT along faults toward the SSE suggest that the southern part of the caldera collapsed by only 100e150 m (Fig. 11.3A; Sacchi et al., 2009, 2014). In contrast, the seismic profile near the coast to the NNE reveals evidence of significant uplift onland (Milia, 2010; Sacchi et al., 2009, 2014). Changes in the seismic profiles coincide with variations in Bouguer gravity anomalies across the caldera. Thus, the onland uplift overlaps a clear positive Bouguer gravity anomaly across a quasicircular subaerial area that incorporates the Agnano Plain and Pozzuoli, while the deeper part of the Bay of Pozzuoli coincides with a gravity low of 15 mgal (Fig. 11.2B; Capuano et al., 2013, and references therein). Following the method of Nielson and Hulen (1984), the gradients of the gravity changes outside the area of uplift (with a positive gravity anomaly) were extrapolated to infer the shape of Bouguer field after the NYT caldera collapse and before resurgence began. The reconstruction yields an estimated gravity minimum of about 30 mgal (Fig. 11.3C).

Borehole data in the subaerial part of the caldera Stratigraphies from 10 boreholes across the uplifted area (Fig. 11.4A and Appendix, Fig. A.1 and Table A.1) show that the NYT lies above or close to sea level in the area of positive gravity anomaly. The area is closed near the coast, coinciding with the 25 m isobaths (Fig. 11.2A). Geological profiles across the uplift, approximately EW and NEeSW, show thicknesses for the NYT between 20 and 990 m (Fig. 11.4B). In the profile AeB, taking into account the depth of 990 m as detected by data from drilling (S10, Table A.1), the scale of the total uplift is evident when the position of the base of the resurgent block is extrapolated according to an arbitrary extension of the base from 850 to 990 m (b.s.l.). In the

Figure 11.3 (A) Seismic profile obtained from the integration of two profiles presented by Sacchi et al. (2009, 2014). The trace of the profile of Fig. 11.2A has the same direction as the profile plotted in Fig. 11.2B, up to the uplifted Neapolitan Yellow Tuff (NYT) structure off Pozzuoli (see red arrow). The colored layers red, blue, and green are the beds of pyroclastic material that cover the NYT (light gray). Dark gray bed state the layers of Campanian Ignimbrite (CI) and pre-CI; (B) Faye anomaly map of the Campi Flegrei area, contoured at 1 mGal intervals; black dashed line shows the topographic margin of the caldera and red dashed line highlights the contour line of the resurgent area. (C) The present gravity signature along the cross section AeB is compared with the gravity signature (dashed line) that the caldera is postulated to have had immediately after the NYT eruption; vertical exaggeration of profile is about 100 times. Modified from Capuano, P., Russo, G., Civetta, L., Orsi, G., DAntonio, M., Moretti, M., 2013. The active portion of the Campi Flegrei caldera structure imaged by 3-D inversion of gravity data. Geochem. Geophisics Geosyst. 14, 4681e4697.

Figure 11.4 (A) The quasicircular area (5e6 km across) in the central part of the caldera is enclosed by the Main Ring Fault (MRF), within which drill holes indicate the Neapolitan Yellow Tuff (NYT) resurgent dome that extends above or near to the current sea level (see also Fig. A.1 in Appendix), (B) the morphology of the NYT dome rebuilt along the orthogonal profiles AeB and DeE traced in Fig. 11.4A. Note that the profile AeB, taking into account the depth of 990 m as detected by data obtained from drilling (S10, Table A.1), assumes an arbitrary but reasonable extension of the base of the resurgent structure placed at depth between 850 and 990 m. The profile DeE highlights the continuity of the roof of the NYT below the pyroclastic blanket deposited by the explosive postcaldera activity, (C) central resurgent area extending above sea level and lateral zones at different scales of collapse in the caldera area, constructed from seismic data profiles in the Pozzuoli Bay and from drill holes on the land (S0 to S29 in Table A.1).

Chapter 11 The Neapolitan Yellow Tuff eruption

profile DeE, the continuity of the roof of the resurgent block below the pyroclasts of the postcaldera explosive activity is evidenced. In Fig. 11.3C, through the data of the geophysical explorations in the bay of Pozzuoli and from the drilling data on the mainland is shown that, moving inwards from the caldera margin, the top of the NYT is found at depths increasing to 300 m b.s.l. before it abruptly emerges above sea level where the positive gravity anomaly occurs.

Onshore geomorphology of Campi Flegrei The area of central resurgence is flanked on its western and eastern sides by elongate depressions and marked along its southern margin by the raised marine terrace called La Starza. The smaller western depression covers the district of San Vito, between the area of uplift and the eastern side of Monte Gauro (Fig. 11.5). It is an endorheic basin (with drainage inland, not to the sea) formed by the aggradation of pyroclastic materials from the NYT and younger eruptions. To the southwest of Monte Gauro, another smaller endorheic basin has formed in the Toiano district and is today filled with 25e30 m of volcaniclastic material, as well as lake, marsh, and fossiliferous marine sediments. The eastern depression consists of the inland Fuorigrotta Plain, at about 50 m a.s.l., bordered by the low Coroglio coastal plain, lower than 25 m a.s.l (Russo et al., 1998). Its present form has evolved from an original marine environment, owing to the aggradation of pyroclastic deposits. The La Starza marine terrace is a cliff that rises to c.30 m a.s.l. and runs subparallel with the coast, between Monte Nuovo and Mt. Olibano lava dome (Cinque et al., 1985). It started to form as a basin at some time between 8 and 14 ka BP. The basin was a structural depression that became filled with alternating marine and terrigenous sediments (Cinque et al., 1985), which were deposited during alternations of marine incursions associated with the regional Flandrian transgression (Lambeck et al., 2011), and accumulations of pyroclastic deposits (Fig. 11.5, insets A and B). Incursions of the sea extended to at least the northern rim of Campi Flegrei, where, during excavation of the NapleseRome railway (Falini, 1950), fossil-rich marine sediments have been found between products erupted from the San Martino (c. 9.2 ka BP) and Montagna Spaccata (c. 5.8 ka BP) volcanic centers (Fig. 11.5). The La Starza sequence has since been exposed as the wall of a fault produced by net uplift until c.5.2 ka BP (Cinque et al., 1985; Rosi and Sbrana, 1987; Bellucci et al., 2006a,b).

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Figure 11.5 Geomorphologic map of the continental part of Campi Flegrei caldera showing depositional environments gradually formed around the resurgent dome and the extension of the La Starza marine deposits on the southern edge of the dome. Also shown is the position of marine levels underlying between the volcano of Montagna Spaccata (5.8 ka) and the volcano of San Martino (9.2 ka) (Falini, 1950). In the inset (A) is shown the stratigraphic structure of the La Starza succession drilled to depths of 116 m in the Neapolitan Yellow Tuff (NYT) substrate (modified from Russo, 2003). The drill penetrates the NYT block for about 16 m, so the thickness of the Starza succession is of about 100 m. The Starzas succession shows the alternation of sea levels with fossils (blue), sandy beach levels (gray), and of subaerial paleosols (brown). Both the paleosols sealing the fossiliferous levels of 11 and 6.2 ka have an age of 9 and 5.2 ka, respectively. The yellow and red colors of the drill hole points give the same indication as shown in Fig. 11.1A. In the inset (B) sea level rise in the last 15 ka is illustrated, according to data of Lambeck et al. (2011). The curve shows that the sea level rose to its highest uplift rate between 11 and 15 ka. After this period, the rate of ascent decreased and reached its minimum values around 5e6 ka, bringing it close to its current position.

Chapter 11 The Neapolitan Yellow Tuff eruption

Postcaldera volcanic activity At least 35 small eruptive centers that produced 66 eruptions have been identified within Campi Flegrei since the NYT eruption, the most recent of which produced Monte Nuovo in 1538 CE (Di Vito et al., 1999; Smith et al., 2012). Their volumes are w0.01e1 km3 DRE (Lirer et al., 1987) and their magmas are typically trachytes and alkali trachytes, with subsidiary amounts of latite and phonolite (Di Girolamo et al., 1984; Rosi and Sbrana, 1987; D’Antonio et al., 1999). The eruptions occurred between the NYT eruption and 8.2 ka BP and between 5.8 and 3.7 ka BP (Table A.2; Fedele et al., 2011; Di Renzo et al., 2011; Smith et al., 2012, Lirer, 2011). An interval without eruptions may also have occurred between 9.5 and 8.6 ka BP (Di Vito et al., 1999; Orsi et al., 2004; Isaia et al., 2009). Here, we prefer to view the eruptions as belonging to only two clusters, with an apparent hiatus from 8.2 to 5.8 ka BP that coincided with the uplift of the La Starza basin into a marine terrace. As noted also by Charlton et al. (2016), the distribution of eruptive centers changed after the hiatus to become concentrated around the central area, rather than the whole caldera (Fig. 11.6). Through a stratigraphic analysis, it is also clear that the volcano of S. Teresa, similar to the volcanoes of Capo Miseno and Nisida, is part of those volcanoes that in the second postcaldera phase were distributed on the whole caldera (Table A.2).

Discussion Formation of the Neapolitan Yellow Tuff caldera Although resurgence of the central zone of Campi Flegrei (Fig. 11.4C) has masked the original profile of the caldera, the style of uplift can provide insights into the mechanism of collapse (Elston, 1984; Lipman, 1984, 2000; Acocella and Funiciello, 1999; Cole et al., 2005). Primary constraints are provided by the shape of the Bouguer gravity anomaly (Fig. 11.3B), transects from borehole data (Fig. 11.4AeC), and evidence for marine transgression in cliff exposures. The zone of uplift is defined by the positive Bouguer gravity anomaly within the caldera and by the positions of the lateral basins and La Starza marine terrace (Figs. 11.2B and 11.5). Assuming the positive anomaly is a consequence of uplift, the prior profile of the caldera can be approximated by extrapolating together the gradients of the negative Bouguer anomalies outside the zone of uplift. The results suggest that, before uplift, the positive anomaly may originally have been the area of maximum negative anomaly (Fig. 11.3C) and, hence, was also the area of

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Figure 11.6 Distribution of eruptive centers of postcaldera volcanic activity: Red dots are the vents of 8e14 ka postcaldera periods; blue triangles are the vents of 3.7e5.8 ka period; black square indicates the vent position of the historic Monte Nuovo volcano (1538 CE). Inset (A) views in detail of the central area not affected by the 8e14 ka BP postcaldera volcanic activity; inset (B) views in detail of the central area affected by 5.8e3.7 ka BP postcaldera volcanic activity.

greatest initial collapse (Fig. 11.7A). The presence of marine fossils in the La Starza deposits confirm that sea entered the caldera after collapse. Although this does not indicate a precise depth, it is reasonable that subsidence reached about 100e150 m (Fig. 11.5A) because the shallow water fossils present in the La Starza’s cliff would have been adversely affected by greater depths (Ciampo, 2004).

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Figure 11.7 (A) The reconstruction of the boundary zones of Fig. 11.5A drawn considering the central area as collapsed, rather than as resurgent, (B) cross sections AeB and CeD of the Neapolitan Yellow Tuff (NYT) edifice shown in Fig. 11.1 (inset B) were used to reconstruct the geometry of the maximum collapse area along two orthogonal directions. Considering the profile AeB, we assume that the collapse of a cauldron 5e6 km across is about 900 m, putting it about 100e150 m below sea level. Dark and brown layers underlying the NYT are Campanian Ignimbrite (CI) and pre-CI deposits, respectively.

Tentative collapse profiles (Fig. 11.7B) can be obtained by combining the location of greatest collapse with the inferred surface profiles before collapse (Fig. 11.1) and the borehole cross sections (Fig. 11.3A,B). They suggest an approximately symmetrical E-W profile, with collapse occurring in steps down a sequence of normal faults. The orthogonal N-S profile is asymmetric by comparison, with fault-bounded collapse being focused in the northern half of the caldera. Thus, the E-W profile resembles a downsag morphology (Walker, 1984), whereas the N-S profile

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shows characteristics of trap door subsidence (Walker, 1984; Lipman, 2000). Caldera subsidence thus appears to have been controlled by a combination of mechanisms, rather than by a single process throughout. In contrast, movement of the inner zone of greatest collapse and subsequent resurgence may have been controlled by slip along a common system of ring faults, corresponding to the Main Ring Fault described for large calderas elsewhere (Lipman, 1976; Elston et al., 1975; Hildebrand, 1984). Invasion of the sea can account for the yellow tuff facies beneath Pozzuoli in the area of resurgence. As shown in Fig. 11.8A, entry of the sea may have cooled the upper levels of the NYT too quickly for zeolitization to occur. Secondary mineralization was thus developed only in the deeper levels of the NYT that

Figure 11.8 (A) Cartoon depicting the caldera divided into two areas, one where the Neapolitan Yellow Tuff zeolite alteration is absent (gray color), in contrast to both cauldron block and the caldera wall in the medium upper part, where both are completely affected by zeolite-forming process. This is shown in relation with their position with respect to the sea level of 15 ka ago. Also shown is a hypothetical failure surface in the caldera wall (red dashed curve) responsible for secondary caldera wall collapse; (B) cartoon depicting the present structure of the caldera in relation to the resurgent dome and with the presence of unaltered and altered areas by the zeolite-forming processes.

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cooled more slowly. Within the caldera, therefore, the NYT retained levels of gray Pozzolana on top of lithified yellow tuff (Fig. 11.8A). Subsequent uplift brought lithified tuff to the surface, where it is exposed beneath Pozzuoli (Fig. 11.8B). On land, the NYT was sufficiently thick around the caldera to permit slow cooling and lithification as, for example, the upper part of Posillipo Hill (Fig. 11.8A). Such conditions prevailed to distances of some 6 km from the caldera, beyond which the deposit had thinned enough to cool quickly and maintain its original Pozzolana characteristics (Scherillo, 1955; Scherillo and Franco, 1960). Boreholes in the Fuorigrotta-Coroglio plains adjacent to the caldera wall at Posillipo Hill show contradictory stratigraphic relations between the Pozzolana and yellow tuff (Fig. A.1). Only unaltered Pozzolana is found in the boreholes further from the caldera wall (Fig. 11.8B, borehole S0; Fig. A.1, boreholes S0, S3, S4, S6, S7), while yellow tuff is found on top of Pozzolana near the foot of the wall (Fig. 11.8B, borehole S1; Fig. A.1, boreholes S1, S2, S3). The difference can be explained by secondary collapse of the caldera wall after lithification had been completed (Fig. 11.8B). Collapse is favored by a near doubling in the density of overlying rock as secondary mineralization proceeds (from 800 kg m3 for Pozzolana to 1400e1500 kg m3 for yellow tuff). It has produced an outward flaring in the caldera wall to create a collar structure (Lipman, 2000). The instability exists today, so that the wall of Posillipo Hill remains vulnerable to collapse.

Caldera resurgence and intracaldera eruptions Uplift of the central block led to the formation around its margin of the San Vito-Toiano valleys to the west, the FuorigrottaCoroglio valleys to the east and the La Starza basin to the south. The two valleys are well structured and show features typical of high-energy depositional environments (Fig. 11.5). In contrast, sediments in the La Starza basin accumulated in a low-energy depositional environment. The difference could be explained by invoking a complex morphology of the resurgent block, comparable to a dome. The southern flank, less steep, had to be shaped with a wide ravine that allowed the deposition of the fossiliferous sequence and with the western and eastern sides steeper.

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The La Starza sediments show alternating sequences of fossiliferous marine sediments, pyroclastic deposits, and two main paleosols located respectively in the middle and at the top of the marine sequence (Fig. 11.5, inset A), whose age is 9 and 5.2 ka, respectively, which together indicate a large part of the events related to the dome history of repeated submergence and emergence from the sea (Cinque et al., 1985). At 15e18 m a.s.l. marine horizon, dated at 11e12 ka BP (Rosi and Sbrana, 1987), lies almost immediately below a paleosol dated at about 9 ka BP (Giudicepietro, 1993). The marine horizons are estimated to have formed 25e50 m b.s.l (Ciampo, 2004), when sea level was about 40 m lower than at 9 ka BP (Fig. 11.5, inset B). Hence, assuming that the 9 ka BP paleosol formed above sea level, the marine horizon must have been uplifted by some 65e90 m (25e50 m þ 40 m) between 11 and 12 and 9 ka BP, which is at a mean rate of 20e36 m per 1000 years. During the hiatus in eruptions between 8.2 and 5.8 ka BP, regional sea level rose by about 25 m (Fig. 11.5, inset B), sufficient for the sea to cover the caldera basin, depositing fossiliferous sediments as far north as the flanks of the San Martino volcanic cone (Fig. 11.5). The absence of volcanism may have additionally promoted marine incursion by favoring prolonged episodes of ground subsidence, possibly analogous to the subsidences of several centuries that have occurred at Campi Flegrei since Roman times (Bellucci et al., 2006a,b). Finally, formation of Montagna Spaccata 5.8 ka BP blocked the sea link with the Quarto depression, causing the San Vito Plain to become an endorheic basin (Fig. 11.5). The Montagna Spaccata eruption occurred shortly after the youngest marine level was formed at La Starza about 6.2 ka BP (Table A.2). It is associated with an interval of resurgence, again at an approximate mean rate of 30 m per 1000 years and faster than increases in sea level, until about 5.2 ka BP, the age of the paleosol now standing about 30 m a.s.l (Table A.2; Giudicepietro, 1993). The Montagna Spaccata eruption was an early forerunner of the most recent episode of intense volcanism in Campi Flegrei 3.7e5.2 ka BP (Table A.2). Of the 13 eruptive centers, 7 centers active during this interval occurred within 1 km of each other across the top of the uplifting block (Fig. 11.6, inset B). The key point is that the internal structure of the uplifted zone must have changed during the 5.8e8.2 ka BP hiatus. Before the hiatus, the structural coherence of the central part of the caldera was able to divert magma along the outer caldera rim and through any

Chapter 11 The Neapolitan Yellow Tuff eruption

surrounding regional faults (Smith and Bailey, 1968; Steven et al., 1984; Elston, 1984; Lipman, 1984, 2000) (Fig. 11.6, inset A). Magmatic pressure was built up, possibly by mechanisms related to stopping or raising a part of an underlying magma chamber (Elston, 1984; Lipman, 2000). Eventually, it became large enough to cause resurgence and, at the same time, to create new fractures that supplied the eruptions across its top during 3.7e4.8 ka BP.

Conclusions The structure of Campi Flegrei and the stratigraphy of its deposits are consistent with a single episode of collapse, triggered by the eruption of the NYT, 15 ka BP. The NYT may have produced a temporary volcanic edifice that was lost when collapse occurred by a combination of downsag and trap door subsidence. A hiatus in subsequent intracaldera eruptions occurred between 8.2 and 5.8 ka BP, coinciding with resurgence of a block, c.5 km across, within the main ring fault just ENE of the caldera’s center. Resurgence weakened the structural integrity of the block, which became a preferred location for younger intracaldera eruptions. Campi Flegrei’s history thus reflects the formation and evolution of a single resurgent caldera. As a result, further studies are required to better define the source region of the 39 ka BP CI, which has conventionally been viewed as the eruption that triggered formation of Campi Flegrei.

Acknowledgments M. Di Vito, G. Florio, A. Milia, and M. Sacchi are kindly thanked for providing useful discussions and for supplying some drill-hole stratigraphy, A.L. Doherty for correcting the English of the unrevised manuscript, and G. Finore for help in preparing the figures. We thank Aldo Di Bitonto (Soc. Geo-testing) for providing stratigraphic data on the NYT in the Pozzuoli area and A. Lima and B. De Vivo for critical reading of original manuscript. Reviews and language correction by Harvey E. Belkin and Christopher Kilburn considerably improved the final version of the chapter.

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Appendix A: supplementary data

Figure A.1 Stratigraphic sections for Neapolitan Yellow Tuff, obtained by drilling of Table A.1 and transect constructed by using the drilling stratigraphy.

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Table A.1 Source drilling data used in the Campi Flegrei, geographic coordinates, and references. The source data are also reported with progressive numbering.

Table A.2 Tabulated 40Ar/39Ar and 14C geochronological data from the Campi Flegrei postcaldera volcanic products. Reference sources for the geochronological data.