Neapolitan Yellow Tuff Caldera Research Articles

Resurgent uplift at large calderas and relationship to caldera-forming faults and the magma reservoir: New insights from the Neapolitan Yellow Tuff caldera (Italy)

Resurgence uplift is the rising of the caldera floor, mainly due to pressure or volume changes in the magma reservoir. Identifying resurgence structures and understanding their relationship to the magmatic reservoir is challenging. We investigate the resurgence structures of the Neapolitan Yellow Tuff caldera (Italy) by integrating bathymetric data, high-resolution seismic profiles and Differential Synthetic-Aperture Radar Interferometry data. Our results show that the resurgent area is manifested as 1) a central dome constituted by two main blocks bounded by NNE-SSW trending faults, 2) an apical graben developed on top of the most uplifted block, 3) a peripheral zone including several uplifted and tilted blocks, bounded by inward-dipping faults. The onset of the uplift of the central dome occurred through re-activation, in reverse motion, of normal faults formed during the caldera collapse, and located in the peripheral zone. During periods of unrests, the blocks of the central dome move independently at different velocities, and the peripheral zone accommodates the deformation. The restless behaviour of the caldera is the result of a shallow magmatic reservoir located at 3.5 ± 0.7 km and characterised by a width that roughly corresponds to the extension of the overlaying resurgent area. Defining the caldera-forming fault system and identifying the area involved by the resurgence is crucial for estimating depth and width of the magma reservoir, and predicting the caldera behaviour during periods of unrest by localising possible vents and sectors that will deform. This knowledge contributes to the evaluation of the volcanic hazard.

Geological framework of the Bagnoli-Coroglio coastal zone and continental shelf, Pozzuoli (Napoli) Bay

ABSTRACT Bagnoli-Coroglio is a Site of National Interest (SNI) facing the Pozzuoli Bay that has been the place of an industrial activity for more than a century (1854-1985). The modern coastal geology of this area results from the recent evolution of an antecedent volcanic landscape that originated after a caldera collapse following the eruption of Neapolitan Yellow Tuff (NYT) of Campi Flegrei (15 ka). After the NYT eruption, a shallow marine depression formed within the caldera collapse area that underwent progressive filling-up, due to volcaniclastic sediment input from the caldera borders, within a semi-enclosed source-to-sink system. The integrated interpretation of the Digital Terrain Model (DTM) of the Bagnoli-Coroglio area and the high resolution reflection seismic profiles acquired off the Pozzuoli Bay suggests that the present-day coastal plain of Bagnoli-Fuorigrotta and conterminous continental shelf represent the latest infilling phase of the annular depression (caldera ‘collar’) between the structural border of the NYT caldera and the inner caldera resurgent dome. The stratigraphic architecture and morpho-bathymetry of the Bagnoli inner shelf provide a record of the complex interplay between eustatic sea level changes, volcano-tectonic (ground/seafloor) deformation, and supply of volcaniclastic sediments, with associated coastline shifts over the last millennia.

VS of the uppermost crust structure of the Campi Flegrei caldera (southern Italy) from ambient noise Rayleigh wave analysis

Shear wave velocities (VS) are defined in the uppermost 1–2km of the Campi Flegrei caldera through the non-linear inversion of the group velocity dispersion curves of fundamental-mode Rayleigh waves extracted from ambient noise cross-correlations between two receivers. Noise recordings, three months long, at 12 seismic stations are cross-correlated between all couples of stations. The experiment provided successful results along 54 paths (inter-stations distance), of which 27 sampled a depth > 1km.VS contour lines are drawn from 0.06km b.s.l. to 1km depth b.s.l. and show difference between the offshore (gulf of Pozzuoli and coastline) and the onshore areas. At 0.06km b.s.l., the gulf of Pozzuoli and the coastline are characterized by VS of 0.3–0.5km/s and of 0.5–0.7km/s, respectively. Such velocities are typical of Neapolitan pyroclastic soils and fractured or altered tuffs. The inland shows VS in the range 0.7–0.9km/s, typical of Neapolitan compact tuffs. Velocities increase with depth and, at 1km depth b.s.l., velocities lower than 1.5km/s are still present in the gulf and along the coastline while velocities higher than 1.9km/s characterize the eastern sector (grossly coincident with the Neapolitan Yellow Tuff caldera rim), the S. Vito plain and the area between Solfatara and SW of Astroni.Such features are much more evident along two cross-sections drawn in the offshore and onshore sectors by integrating our VS models with literature data. Our models join previous noise cross-correlation studies at greater scale at depths of 0.7–0.8km, hence the picture of the Campi Flegrei caldera is shown up to a depth of 15km. VS of about 1.7km/s, corresponding to compression velocities (VP) of about 3km/s (computed by using the VP/VS ratio resulted in the inversion), are found at depths of 1.1km, in the centre of the gulf of Pozzuoli, and at a depth of about 0.7km b.s.l. onshore. An increment of VS velocity (~1.9–2.0km/s) is locally observed onshore which might be attributed to a layer of tuffs and tuffites interbedded with thin lava beds, according to the correlation of VS with stratigraphies in the deep drillings of S. Vito.

The Campi Flegrei caldera (Italy): Formation and evolution in interplay with sea-level variations since the Campanian Ignimbrite eruption at 39 ka

To date, the origin of the Campi Flegrei caldera is still under debate and may be related to (1) a single caldera collapse associated with the Neapolitan Yellow Tuff (NYT) eruption, (2) two subsequent caldera collapses associated with the NYT and the preceding Campanian Ignimbrite (CI) eruptions forming a nested-caldera complex, or (3) not related to a caldera collapse after all. Here, we study the submerged portion of the caldera, which has favored a marine depositional setting and, thus, represents an ideal location for the reconstruction of its formation history, utilizing multichannel seismic data. Volcanic deposits and edifices were seismically distinguished from sedimentary successions, and the stratigraphy could be refined and extended back to the Campanian Ignimbrite eruption at ~39ka.High-resolution multichannel reflection seismic data revealed the existence of a nested-caldera complex formed during the CI eruption at ~39ka and the more recent NYT eruption at ~15ka. A ring-fault bounding an inner caldera collapse structure was clearly imaged. It appears that this inner ring-fault was initially activated during the CI caldera collapse and later reactivated during the NYT caldera collapse with different amounts of subsidence. The NYT caldera probably formed during an asymmetrical collapse with a maximum subsidence of ~75m in the offshore portion. The vertical displacement related to the CI caldera collapse may be significantly larger.The submerged caldera depression accommodates post-eruption sediments. Within this high-resolution archive, two major unconformities developed at ~8.6ka and 5ka, when resurgence-related uplift exceeded the rate of sea-level rise concurrent with the emersion of the La Starza terrace. A previously unknown post-collapse submarine volcanic mound located between Nisida Island and Nisida Bank probably formed between 4.8 and 3.7ka. Also, the Penta Palummo Bank appears to be constructed of at least two monogenetic volcanic edifices, the Penta Palummo volcano formed at ~100ka and a younger mushroom-shaped unit formed between 39 and 15ka. The main outcome of this study is a conceptual evolutionary model, providing novel insights on the formation of the Campi Flegrei nested caldera in the course of two large-scale eruptions (CI and NYT) with associated caldera collapses along mutual (i.e. reactivated) faults and subsequent caldera resurgence. As both the genesis and subsurface architecture of the Campi Flegrei caldera are still hotly debated topics in literature, our discoveries can be regarded as a substantial advancement in the understanding of the Campi Flegrei volcanic area.

The Neapolitan Yellow Tuff caldera offshore the Campi Flegrei: Stratal architecture and kinematic reconstruction during the last 15ky

Abstract In this study we integrate high-resolution swath bathymetry, single channel reflection seismic data and gravity core data, to provide new insights into the shallow structure and latest Quaternary to Holocene evolution of the submerged sector of the Neapolitan Yellow Tuff (NYT) caldera (Campi Flegrei) in the Pozzuoli Bay. The new data allow for a reconstruction of the offshore geometry of the NYT caldera collapse–ring fault system, along with the style and timing of deformation of the inner caldera resurgence. Our interpretation shows that the NYT eruption (~ 15 ka BP) was associated with a caldera collapse bounded by an inward-dipping ring fault system. The ring fault system consists in a 1–2 km wide fault zone that encircles an inner caldera region ~ 5 km in diameter and is often marked by the occurrence of pore fluids ascending through the fault zone, up to the seafloor, particularly in the western sector of the bay. A shallow magmatic intrusion along the ring fault zone was also detected offshore Bagnoli in the eastern part of the Pozzuoli Bay. Following the NYT eruption, the inner caldera region underwent significant deformation and resurgence with a maximum cumulative uplift of the offshore structure in the order of 180 m. The net uplift rate of the caldera resurgent dome was ~ 9–12 mm/year during the period 15.0–6.6 ka BP. The style of deformation of the resurgent structure can be described in terms of a broad doming, accompanied by subordinate brittle deformation, mostly concentrated in a small apical graben at the summit of the resurgent dome. Chronostratigraphic calibration of seismic profiles obtained by three tephra layers cored in the Pozzuoli Bay indicates 5 to 25 m of post-Roman differential subsidence and tilting towards ESE of the inner caldera resurgence, as recorded by the drowning of the infralittoral prograding wedge below the present-day storm wave base.

The active portion of the Campi Flegrei caldera structure imaged by 3‐D inversion of gravity data

We present an improved density model and a new structural map of the Neapolitan Yellow Tuff caldera, the active portion of the nested Campi Flegrei caldera. The model was built using a new 3‐D inversion of the available high‐precision gravity data, and a new digital terrain and marine model. The inversion procedure, based on a variable‐depth lumped assembling of the subsurface gravity distribution via cell aggregation, gives better defined insights into the internal caldera architecture, that well agree with the available geological, geophysical, and geochemical data. The adopted 3‐D gravity method is highly efficient for characterizing the shallow caldera structure (down to 3 km depth) and defining features related to regional or volcano tectonic lineaments and dynamics. In particular, the resulting density distribution highlights a pronounced density low in correspondence of the central portion of the caldera with a detail not available till now. The joint interpretation of the available data suggests a subsurface structural setting that supports a piecemeal collapse of the caldera, and allows the identification of its headwall. Positive gravity anomalies localize dense intrusions (presently covered by late volcanic deposits) along the caldera marginal faults, and the main structural lineaments both bordering the resurgent block and cutting the caldera floor. These results allow us to both refine the current geological‐structural framework and propose a new structural map that highlights the caldera boundary and its internal setting. This map is useful to interpret the phenomena occurring during unrest, and to improve both short‐term and long‐term volcanic hazards assessment.

Crystal fractionation, magma step ascent, and syn-eruptive mingling: the Averno 2 eruption (Phlegraean Fields, Italy)

The 3.7 ka year-old Averno 2 eruption is one of the rare eruptions to have occurred in the northwest sector of the Phlegraean Fields caldera (PFc) over the past 5 ka. We focus here on the fallout deposits of the pyroclastic succession emplaced during this eruption. We present major and trace element data on the bulk pumices, along with major and volatile element data on clinopyroxene-hosted melt inclusions, in order to assess the conditions of storage, ascent, and eruption of the feeding trachytic magma. Crystal fractionation accounts for the evolution from trachyte to alkali-trachyte magmas; these were intimately mingled (at the micrometer scale) during the climactic phase of the eruption. The Averno 2 alkali trachyte represents one of the most evolved magmas erupted within the Phlegraean Fields area and belongs to the series of differentiated trachytic magmas erupted at different locations 5 ka ago. Melt inclusions record significant variations in H2O (from 0.4 to 5 wt%), S (from 0.01 to 0.06 wt%), Cl (from 0.75 up to 1 wt%), and F (from 0.20 to >0.50 wt%) during both magma crystallization and degassing. Unlike the eruptions occurring in the central part of the PFc, deep-derived input(s) of gas and/or magma are not required to explain the composition of melt inclusions and the mineralogy of Averno 2 pumices. Compositional data on bulk pumices, glassy matrices, and melt inclusions suggest that the Averno 2 eruption mainly resulted from successive extrusions of independent magma batches probably emplaced at depths of 2–4 km along regional fractures bordering the Neapolitan Yellow Tuff caldera.

The magmatic feeding system of the Campi Flegrei caldera: Architecture and temporal evolution

To develop a model of both the structure and evolution of the Campi Flegrei caldera (CFc) magmatic feeding system, geochronological, geochemical and Sr, Nd, Pb and B isotopic data of representative volcanic products of the past 15 ka have been combined with geophysical and melt inclusion literature data, structural setting and dynamics of the resurgent caldera. According to previous petrological data, the CFc magmatic feeding system consists of a deep reservoir, in which mantle-derived K-basaltic parental magmas differentiate to shoshonite, latite and trachyte, through combined crustal contamination and fractional crystallization processes, and shallow reservoirs where the evolved magmas further differentiate and mingle/mix before eruptions. The Sr, Nd, Pb, and B isotope data allow recognition of three distinct magmatic components. One component is believed to be residual magma from the Neapolitan Yellow Tuff (NYT) caldera forming eruption. The NYT component ( 87Sr/ 86Sr of 0.70750–53, 143Nd/ 144Nd ratio of ca. 0.51246, 206Pb/ 204Pb of ca. 19.04 and δ 11B of ca. –7.9‰), has been the most prevalent component over the past 15 ka being mixed, in most cases, with the other two components. One of these other components is best recognized in the Minopoli 2 magma, first erupted 10 ka ago. Minopoli 2 magma is shoshonitic in composition and is the most enriched in radiogenic Sr ( 87Sr/ 86Sr of ca. 0.70860) and unradiogenic Nd and Pb ( 143Nd/ 144Nd ratio of ca. 0.51236, 206Pb/ 204Pb of ca. 18.90), and is characterised by δ 11B value of ca. –7.32‰. The third component is trachytic in composition and has higher 206Pb/ 204Pb (ca. 19.08), lower 87Sr/ 86Sr (ca. 0.70726) and δ 11B (−9.8‰) and higher 143Nd/ 144Nd (ca. 0.51250), with respect to the NYT component. This third component is best recognized in the Astroni 6 magma and did not appear until ca. 4 ka. The identified isotopically distinct magmatic components were erupted in different sectors of the CFc. During both I (< 14.9–9.5 ka) and II (8.6–8.2 ka) epochs of volcanic activity, magmas similar to the NYT component, and those resulting from mixing between Minopoli 2 and NYT components were erupted from vents located mostly on the marginal faults of the NYT caldera. During the III epoch (4.8–3.8 ka) magmas either similar to NYT, or resulting from mixing between Astroni 6 and NYT components were erupted from vents located along faults bordering the La Starza resurgent block and, subordinately, the NYT caldera. Moreover, magmas resulting from mixing between Minopoli 2 and NYT components were erupted from vents located along NE–SW regional faults activated during caldera resurgence. The inferred present structure of the feeding system is characterised by a deep reservoir, whose top is at about 8 km depth, that hosts shoshonitic–trachytic magmas. Remnants of the NYT magma reside at shallower depth in different sectors of the crust underlying CFc, and were sometimes intercepted by volatile-rich magmas of deep provenance during the three epochs of CFc volcanic activity.

The 2004–2006 uplift episode at Campi Flegrei caldera (Italy): Constraints from SBAS‐DInSAR ENVISAT data and Bayesian source inference

We investigate the 2004–2006 uplift phase of Campi Flegrei caldera (Italy) by exploiting the archive of ascending and descending ENVISAT SAR data acquired from November 2002 to November 2006. The SBAS‐DInSAR technique is applied to generate displacement mean velocity maps and time series. An appropriate post‐processing step is subsequently applied to map the areas whose temporal deformation behavior is correlated with that of the maximum uplift zone. Our results show that the deformation also extends outside the volcanological limits of the Neapolitan Yellow Tuff caldera, without significant discontinuities. The DInSAR data are inverted by considering a finite spheroid and an isotropic point‐source. The inversion results suggest that the new uplift is characterized by a source location similar to the previous small uplift event of 2000 and to the long term subsidence of the 1990's. In particular, the source is located at a depth of about 3.2 km and very close to the city of Pozzuoli (about 800 m offshore, to the SW); the associated volume variation is about 1.1 106 m3/year.

The present state of the magmatic system of the Campi Flegrei caldera based on a reconstruction of its behavior in the past 12 ka

New geochemical and Sr-isotope data have been acquired on samples representative of volcanic units erupted inside the resurgent Campi Flegrei caldera (CFc) over the past 12 ka. These data, integrated with previous published petrological, and with newly acquired geochronological, volcanological and geothermal data, shed light on the nature and timing of the processes that controlled the evolution of the Phlegraean magmatic system. In the past 12 ka, three isotopically and geochemically distinct magmatic components were erupted at the CFc as either homogeneous or mixed magma batches. One component, Campanian Ignimbrite component (CIc) ( 87 Sr/ 86 Sr =0.70735–0.70740), is similar to the trachytic magma extruded during the first phase of the Campanian Ignimbrite (CI) eruption (37 ka). A second component, Neapolitan Yellow Tuff component (NYTc) ( 87 Sr/ 86 Sr =0.70750–0.70757), is similar to the latitic–alkali–trachytic magma batches extruded during the course of the Neapolitan Yellow Tuff (NYT) eruption (12 ka). A third component, Minopoli component (MIc) ( 87 Sr/ 86 Sr ≈0.7086), is similar to the trachybasaltic magma of the Minopoli 2 (MI) eruption (9.7 ka). These components were erupted as either single batches of magma, or mixed CI–NYT or MI–NYT batches of magma, through vents located either along the structural boundary of the NYT caldera or inside the NYT caldera, mainly on portions of the resurgent block under extensional stress. The CI and NYT components represent residual portions of older, large-volume magma reservoirs which have fed eruptions since about 60 and 15 ka, respectively. The least-evolved MI component was erupted only during the 12–9.5 ka and 8.6–8.2 ka epochs of activity, through vents located on a NE–SW regional fault system. This component could represent a deeper reservoir tapped by the NE–SW regional fault system reactivated after the NYT caldera collapse. Deeper MI and shallower CI and NYT magmatic systems interacted by mixing among batches of magma during their rise to surface. Overall, the data suggest that the CFc magmatic system today is characterized by the presence of two larger, independent reservoirs, filled by residual portions of the CI and NYT magmas. These generated many smaller, shallower pockets of evolved magma, that fed most of the eruptions that occurred in the CFc over the past 12 ka. Moreover, a deeper reservoir (MI), tapped by the NE–SW regional fault system, provided batches of less-evolved magma that mixed with magma present in the shallower pockets.

Volcanism and deformation since 12,000 years at the Campi Flegrei caldera (Italy)

The Campi Flegrei caldera is a restless, nested structure resulting from two major collapses related to the Campanian Ignimbrite (37,000 years BP) and the Neapolitan Yellow Tuff (12,000 years BP) eruptions, respectively. Detailed stratigraphical, structural, volcanological and 14 C (AMS) geochronological studies, devoted to the reconstruction of the volcanic and deformational history of the Campi Flegrei caldera in the past 12,000 years have been carried out. The results of these studies show that in this time span, intense both volcanic and volcano-tectonic activity was confined inside the Neapolitan Yellow Tuff caldera. Volcanism was concentrated in epochs of intense activity, alternating to periods of quiescence. The I epoch lasted from 12,000 to 9500 years BP giving rise to 34 explosive eruptions, each every 70 years on average. During the II epoch, dated between 8600 and 8200 years BP, six explosive eruptions took place at an average interval of 65 years. The III epoch lasted from 4800 to 3800 years BP and produced 16 explosive and four effusive eruptions which followed each other at mean intervals of 50 years. Eruption vents of the I epoch were located mostly along the marginal faults of the Neapolitan Yellow Tuff caldera, while those of the II epoch aligned on the northeastern sector of this margin. During the III epoch volcanism was confined in the northeastern sector of the Neapolitan Yellow Tuff caldera floor. The caldera floor is disjointed in blocks with variable vertical movements by fault and fracture systems mainly trending NE–SW and NW–SE. The still active resurgence of the caldera floor began soon after its collapse. Onset of both II and III epoch of activity coincides with increase in resurgence rate of La Starza marine terrace, the most uplifted part of the resurgent block.

The restless, resurgent Campi Flegrei nested caldera (Italy): constraints on its evolution and configuration

We have collected new stratigraphic data in the Campi Flegrei caldera through surface geological mapping and study of cores drilled to various depths. We integrate these data with geological, geochronological and geophysical constraints available in the literature to reconstruct the volcanic and deformational history of the caldera. The caldera is a resurgent, nested structure resulting from two main collapses related to the Campanian Ignimbrite (37 ka) and the Neapolitan Yellow Tuff (12 ka) eruptions. While the structure is affected by a broad subsidence, ongoing local resurgence and unrest occur inside the younger, nested caldera structure. Although most of the caldera floor consists of multiple structural blocks due to long-term deformations, magmas were able to erupt only in one sector of the resurgent part. The caldera has shown signs of unrest during the past 25 years, with short-term deformations that have generated a net uplift of 3.5 m. The short-term deformations are related to the Neapolitan Yellow Tuff caldera resurgence and interpreted as the net result of brittle and ductile components. The long-term deformations likely represent the sum of the permanent, brittle component of each short-term deformational event.

The Neapolitan Yellow Tuff, a large-magnitude trachytic phreatoplinian eruption: eruptive dynamics, magma withdrawal and caldera collapse

The results of a detailed stratigraphical and volcanological study has allowed the definition of the Neapolitan Yellow Tuff (NYT) as the largest known trachytic phreatoplinian eruption with an estimated volume of not less than 30 km3 DRE. The NYT sequence can be subdivided in a lower and an upper member. Sedimentological and textural variations in time and space indicate that different eruptive and depositional mechanisms operated during the eruption. The lower member eruptive phase was characterized by alternating phreatoplinian and magmatic explosions while the upper member eruptive phase was characterized by a random sequence of phreatomagmatic and magmatic explosions. Particles produced by the same explosion were deposited contemporaneously by fallout and surge mechanisms either at different or at the same distance from the vent. Compositional variations show that the NYT sequence cannot be modeled as an inverted compositionally zoned magma body with magma becoming more basic in the course of the eruption. The chamber, composed of three geochemically distinct magma layers, was tapped at different depths according to the eruption dynamics. Each phreatoplinian explosion tapped concurrently all the magma layers while magmatic explosions tapped only one of them. The onset of the NYT caldera collapse is well constrained between lower and upper member eruptive phases. The collapse, which is only gravimetrically detectable, occurred inside the larger Phlegrean Fields caldera, which therefore can be defined as a nested caldera.