Claudio Scarpati

The Neapolitan Yellow Tuff — A large volume multiphase eruption from Campi Flegrei, Southern Italy

Abstractthe Neapolitan Yellow Tuff (NYT) (12 ka BP) is considered to be the product of a single eruption. Two different members (A and B) have been identified and can be correlated around the whole of Campi Flegrei. Member A is made up of at least 6 fall units including both ash and lapilli horizons. The basal stratified ash unit (A1) is interpreted to be a phreatoplinian fall deposit, since it shows a widespread dispersal (>1000 km2) and a constant thickness over considerable topography. The absence of many lapilli fall units in proximal and medial areas testifies to the erosive power of the intervening pyroclastic surges. The overlying member B was formed by many pyroclastic flows, radially distributed around Campi Flegrei, that varied widely in their eruptive and emplacement mechanisms. In some of the most proximal exposures coarse scoria and lithic-rich deposits, sometimes welded, have been identified at the base of member B. Isopach and isopleth maps of fall-units, combined with the distribution of the coarse proximal facies, indicate that the eruptive vent was located in the NE area of Campi Flegrei. It is considered that the NYT eruption produced collapse of a caldera approximately 10 km diameter within Campi Flegrei. The caldera rim, located by geological and borehole evidence, is now largely buried by the products of more recent eruptions. Initiation of caldera collapse may have been contemporaneous with the start of the second phase (member B). It is suggested that there was a single vent throughout the eruption rather than the development of multiple or ring vents. Chemical data indicate that different levels of a zoned trachyte-phonolite magma chamber were tapped during the eruption. The minimum volume of the NYT is calculated to be about 50 km3 (DRE), of which 35 km3 (∼70%) occurs within the caldera.

A facies interpretation of the eruption and emplacement mechanisms of the upper part of the Neapolitan Yellow Tuff, Campi Flegrei, southern Italy

This study focuses on the upper part, Member B, of the Neapolitan Yellow Tuff (NYT). Detailed measurements of stratigraphic sections within the unlithified ‘pozzolana’ facies show that Member B is composed of at least six distinct depositional units which each record a complex fluctuation between different styles of deposition from pyroclastic density flows. Six lithofacies have been identified: (1) massive valleyponded facies, the product of non-turbulent flows; (2) inverse-graded facies formed by flows that were turbulent for the majority of transport but were deposited through a non-tubulent basal regime; (3) regressive sand-wave facies, the product of high-concentration, turbulent flows; (4) stratified facies, the product of deposition from turbulent, low-particle-concentration, flows; (5) particle aggregate and (6) vesicular ash lithofacies, both of which are considered to have formed by deposition from turbulent, low-concentration flows. Although the whole eruption may have been phreatomagmatic, facies 1–4 are interpreted to be the product of dry eruptive activity, whereas facies 5 and 6 are considered to be of wet phreatomagmatic eruptive phases. Small-scale horizontal variations between facies include inverse-graded lithofacies that pass laterally into regressive sand-wave structures and stratified deposits. This indicates rapid transition from non-turbulent to turbulent deposition within the same flow. Thin vesicular ash and particle aggregate layers pass laterally into massive valley-ponded vesicular lithofacies, suggesting contemporaneous wet pyroclastic surges and cohesive mud flows. Three common vertical facies relations were recognised. (1) Massive valley-ponded and inverse-graded facies are overlain by stratified facies, suggesting decreasing particle concentration with time during passage of a flow. (2) Repeated vertical gradation from massive up into stratified facies and back into massive beds, is indicative of flow fluctuating between non-turbulent and turbulent depositional conditions. (3) Vertical alternation between particle aggregates and vesicular facies is interpreted as the product of many flow pulses, each of which involved deposition of a single particle aggregate and vesicular ash layer. It is possible that the different facies record stages in a continuum of flow processes. The deposits formed are dependent on the presence, thickness and behaviour of a high-concentration, non-turbulent boundary layer at the base of the flow. The end members of this process are (a) flows that transported and deposited material from a non-turbulent flow regime and (b) flows that transported and deposited material from a turbulent flow regime.

The eruption of The Breccia Museo (Campi Flegrei, Italy): Fractional crystallization processes in a shallow, zoned magma chamber and implications for the eruptive dynamics

Abstract The Breccia Museo Member (BMM) was formed by an explosive eruption that occurred in the SW sector of Campi Flegrei about 20 ka ago. The eruptive sequence consists of the Lower Pumice Flow Unit and the overlying Upper Pumice Flow Unit with its associated lithic Breccia Unit. Interlayered with the Breccia Unit is a welded deposit that mainly consists of spatter clasts (Spatter Unit). The products of this eruption range in composition from trachytic to trachyphonolitic with K 2 O decreasing from 9.5 to 7 wt.%; Na 2 O correspondingly increases from 2.6 to 7.2 wt.% with increasing differentiation (Nb from 23 to 122 ppm). The phenocrysts are mostly sanidine (Or 88-63 ) with subordinate plagioclase (An 33-27 ), clinopyroxene (Ca 47 Mg 44 Fe 9 to Ca 46 Mg 35 Fe 19 ), biotite, titanomagnetite, and apatite. The observed major- and trace-element variations are fully consistent with about 80% fractional crystallization of a sanidine-dominated assemblage starting from the least differentiated trachytes. The compositions of the erupted products are compatible with the progressive tapping of a shallow magma chamber that was thermally and chemically zoned. The incompatible trace elements indicate a slightly different magma composition with respect to trachytes of the Campi Flegrei mainland. The geochemical stratigraphy suggests that after an early eruptive phase during which the upper, most differentiated level of the magma chamber was tapped, the sudden collapse of the roof of the reservoir triggered drainage of the remaining magma, which ranged in composition from trachyte to trachyphonolite, and formed the Breccia Unit and the Upper Pumice Flow Unit. The strongly differentiated trachyphonolite composition of the spatter clasts also suggests that they likely originated from the uppermost part of the reservoir soon after the eruption of Lower Pumice Flow Unit and the collapse of the chamber roof. This is in agreement with the eruptive model proposed by Perrotta and Scarpati (1994).

The dynamics of the Breccia Museo eruption (Campi Flegrei, Italy) and the significance of spatter clasts associated with lithic breccias

Abstract The Breccia Museo Member is a pyroclastic deposit produced during an eruptive event that occurred in the southwestern sector of Campi Flegrei about 20,000 years ago. Two depositional units divided by a co-ignimbrite ash-fall deposit have been recognized. Facies variations in the deposits resulted from the interaction between the flow and paleomorphology, from the relative abundance of the lithic and juvenile components supplied by the source, and from the variations of the flow regime. The Lower Depositional Unit is a pyroclastic flow deposit characterized by a thick, coarse valley facies laterally grading into a thin, layered and fine-grained overbank facies. These different facies are due to the interaction between a density-stratified flow and topography. The more basal, high-concentration part of the flow was deposited along the axis of the paleovalleys (valley facies), whereas the upper, low-concentration part was deposited on the slopes (overbank facies). Vertical variations of the structures observed in the deposits of the Lower Depositional Unit resulted from flow unsteadiness during emplacement and, hence, on the variations of the suspended load fallout from the low-concentration upper part of the flow to the high-concentration boundary layer. The Upper Depositional Unit, made up of the Breccia, Spatter and Upper Pumice Flow Units, consists of horizons of lithic breccias and coarse welded spatter which thicken into the valleys. They are closely related to a gas-pipe-rich ash and pumice flow deposit. The strongly fines-poor character of the breccias and spatter beds is due to a very rapid segregation of the dense and coarse clasts and to the high rates of gas ascent through the hindered-settling zone in the basal part of the flow. After deposition of the majority of the dense and coarse material, the subsequent high-density depositional system came to rest immediately, thus yielding a pyroclastic flow deposit that is closely associated with the breccia. The presence of lithic breccia and spatter beds within the stratigraphic sequence is interpreted to reflect changes in the magma chamber structure during the eruption. Due to the drainage of part of the magma during the first phase of the eruption, the roof of the magma chamber collapsed and the lithostatic pressure fell below the magmatic pressure. We suggest that this new magmatic overpressure, not related to the expansion of the gas, generated fragmentation of poorly vesiculated magma inside the magma chamber. This triggered a new eruptive phase during which a mixture of spatter, pumice and collapse-produced lithic debris were erupted.

Impact of the ad 79 explosive eruption on pompeii, ii. causes of death of the inhabitants inferred by stratigraphic analysis and areal distribution of the human casualties

Detailed descriptions of the effects of explosive eruptions on urban settlements available to volcanologists are relatively rare. Apart from disease and starvation, the largest number of human deaths caused by explosive eruptions in the twentieth century are due to pyroclastic flows. The relationship between the number of victims related to a specific hazard and the presence of urban settlements in the area covered by the eruption has been shown. However, pyroclastic falls are also extremely dangerous under certain conditions. These conclusions are based on archaeological and volcanological studies carried out on the victims of the well-known AD 79 eruption of Vesuvius that destroyed and buried the Roman city of Pompeii. The stratigraphic level in the pyroclastic deposit and the location of all the casualties found are described and discussed. The total number of victims recovered during the archaeological excavations amounts to 1150. Of these, 1044 well recognisable bodies plus an additional group of 100 individuals were identified based on the analysis of several groups of scattered bones. Of the former, 394 were found in the lower pumice lapilli fall deposit and 650 in the upper stratified ash and pumice lapilli pyroclastic density currents (PDCs) deposits. In addition, a tentative evaluation suggests that 464 corpses may still be buried in the unexcavated part of the city. According to the reconstruction presented in this paper, during the first phase of the eruption (August 24, AD 79) a huge quantity of pumice lapilli fell on Pompeii burying the city under 3 m of pyroclastic material. During this eruptive phase, most of the inhabitants managed to leave the city. However, 38% of the known victims were killed during this phase mainly as a consequence of roofs and walls collapsing under the increasing weight of the pumice lapilli deposit. During the second phase of the eruption (August 25, AD 79) 49% of the total victims were on the roadways and 51% inside buildings. All of these inhabitants, regardless of their location, were killed by the unanticipated PDCs overrunning the city. New data concerning the stratigraphic level of the victims in the pyroclastic succession allow us to discriminate between the sequential events responsible for their deaths. In fact, casts of some recently excavated corpses lay well above the lower PDCs deposit, testifying that some of the inhabitants survived the first pyroclastic current. Finally, during the PDCs phase the victims died quite rapidly by ash asphyxiation. From the attitude of some casts, it seems that some people survived the initial impact of the second pyroclastic current and tried to support head and bust during the progressive aggradation of the deposit at the base of the current.

Impact of the AD 79 explosive eruption on Pompeii, I. Relations amongst the depositional mechanisms of the pyroclastic products, the framework of the buildings and the associated destructive events

Abstract A quantitative and qualitative evaluation of the damage caused by the products of explosive eruptions to buildings provides an excellent contribution to the understanding of the various eruptive processes during such dramatic events. To this end, the impact of the products of the two main phases (pumice fallout and pyroclastic density currents) of the Vesuvius AD 79 explosive eruption onto the Pompeii buildings has been evaluated. Based on different sources of data, such as photographs and documents referring to the archaeological excavations of Pompeii, the stratigraphy of the pyroclastic deposits, and in situ inspection of the damage suffered by the buildings, the present study has enabled the reconstruction of the events that occurred inside the city when the eruption was in progress. In particular, we present new data related to the C.J. Polibius’ house, a large building located inside Pompeii. From a comparison of all of the above data sets, it has been possible to reconstruct, in considerable detail, the stratigraphy of the pyroclastic deposits accumulated in the city, to understand the direction of collapse of the destroyed walls, and to evaluate the stratigraphic level at which the walls collapsed. Finally, the distribution and style of the damage allow us to discuss how the emplacement mechanisms of the pyroclastic currents are influenced by their interaction with the urban centre. All the data suggest that both structure and shape of the town buildings affected the transport and deposition of the erupted products. For instance, sloping roofs ‘drained’ a huge amount of fall pumice into the ‘impluvia’ (a rectangular basin in the centre of the hall with the function to collect the rain water coming from a hole in the centre of the roof), thus producing anomalous deposit thicknesses. On the other hand, flat and low-sloping roofs collapsed under the weight of the pyroclastic material produced during the first phase of the eruption (pumice fall). In addition, it is evident that the walls that happened to be parallel to the direction of the pyroclastic density currents produced during the second eruptive phase were minimally damaged in comparison to those walls oriented perpendicular to the flow direction. We suggest that the lower depositional parts of the pyroclastic currents were partially blocked (locally reflected) and slowed down because of recurring encounters with the closely spaced walls within buildings. Locally, the percentage of demolished walls decreases down-current, which has been interpreted as a loss in kinetic energy within the depositional system of the flow. However, it seems that the upper transport system by-passed these obstacles, then supplied new pyroclasts to the depositional system that restored its physical characteristics and restored enough kinetic energy to demolish the next walls and buildings further along its path.

Eruptive history of Neapolitan volcanoes: constraints from 40Ar–39Ar dating

The city of Naples can be considered part of the Campi Flegrei volcanic field, and deposits within the urban area record many autochthonous pre- to post-caldera eruptions. Age measurements were carried out using 40 Ar– 39 Ar dating techniques on samples from small monogenetic vents and more widely distributed tephra layers. The 40 Ar– 39 Ar ages on feldspar phenocrysts yielded ages of c . 16 ka and 22 ka for events older than the Neapolitan Yellow Tuff caldera-forming eruption (15 ka), and ages of c . 40 ka, 53 ka and 78 ka for events older than the Campanian Ignimbrite caldera-forming eruption (39 ka). The oldest age obtained is 18 ka older than previous dates for pyroclastic deposits cropping out along the northern rim of Campi Flegrei. The results of this study allow us to divide the Campi Flegrei volcanic history into four main, geochronologically distinct eruptive cycles. A new period, the Paleoflegrei, occurred before 74–78 ka and has been proposed to better discriminate the ancient volcanism in the volcanic field. The eruptive history of Campi Flegrei extends possibly further back than this, but the products of previous eruptions are difficult to date owing to the lack of fresh juvenile clasts. These new geochronological data, together with recently published ages related to young volcanic edifices located in the city of Naples (Nisida volcano, 3.9 ka) testify to persistent activity over a period of at least 80 ka, with an average eruption recurrence interval of ~555 years within and adjacent to this densely populated city.

The 1944 eruption of Vesuvius, Italy: combining contemporary accounts and field studies for a new volcanological reconstruction

We integrate the different contemporary sources together with new field data on the pyroclastic deposits to make a new volcanological reconstruction of the explosive phases of the 1944 Vesuvius eruption. We adopt the four successive phases of the eruption first defined by Imbo (1945), who made the most detailed contemporary description of the eruption: Phase 1 – effusive, Phase 2 – lava fountains, Phase 3 – mixed explosions and Phase 4 – seismic-explosive. Phase 1 consisted of four days of effusive activity. Phase 2 generated eight successive lava fountains which formed agglutinated spatter in a restricted area around the crater. At distances of > 1 km from the crater, reverse graded, well-sorted, scoria lapilli with up to 94 wt % juvenile material and calculations indicate a volume of 8.2 × 10 6 m 3 DRE (Dense Rock Equivalent) for Phase 2. A short pause in scoria fallout was observed that coincides with the transition between Phases 2 and 3 of the eruption. On the crater rim there is clear evidence for the different phases, owing to the stratification of the deposits; however, away from the crater, stratigraphic breaks suggesting any discontinuity in the eruptive activity are absent. The beginning of Phase 3 is marked by the appearance of abundant dense scoria fragments, coincident with the coarsest part of the lapilli. High-density scoria forms 10 wt % of juvenile material in Phase 2, increasing to 45% in the upper part of Phase 3. Isopach maps derived from field measurements indicate a mean volume of 40.2 × 10 6 m 3 DRE for Phase 3. Distal ash, mainly formed during Phase 3, was dispersed to the SE as far as Albania, and calculations yield a volume of 102 × 10 6 m 3 DRE. Intermittent activity associated with Phase 4 generated ash-rich plumes dispersed towards the SW and contemporary thickness descriptions yield a bulk volume of 4.2 × 10 6 m 3 (2.5 × 10 6 m 3 DRE). Small pyroclastic density currents (PDCs) were observed during Phases 3 and 4. The deposits (200 m from the crater rim) of these currents have been identified on the flanks of the cone. Thin, massive and poorly sorted ash layers, that occur up to 2.5 km from the crater rim, are interpreted to represent the distal facies of these PDCs. Mass discharge rate (MDR) estimates for the paroxysmal phase (end of Phase 2 and start of Phase 3) of this event are around 3.5 × 10 6 kg/s, however, this increases to > 10 7 kg/s if the mass of distal ash is taken into account. Column height estimates from fallout isopleths associated with the eruption9s paroxysmal phase are > 10 km. Based on the contemporaneous chronicles, we were able to define the type and extent of damage associated with the different styles (or temporal phases) of the eruption. Our calculations demonstrate that the present-day population at risk has doubled compared to 1944. The contemporaneous (and also subsequent) scientific literature underestimated the magnitude and intensity of this eruption and very little attention has been dedicated to the damage that occurred. We suggest that this is at least partly related to the extensive destruction of Neapolitan area and the deaths of tens of thousands of civilians related to the Second World War.

Erosional characteristics and behavior of large pyroclastic density currents

Factors influencing the erosive behavior of large pyroclastic density currents (PDCs), both mainly massive and thinly stratified, are not well understood. To investigate the parameters influencing the erosive behavior of PDCs produced during the flowing phase of large, caldera-forming Plinian (Campanian Ignimbrite) and phreatoplinian (Neapolitan Yellow Tuff) eruptions, we use scoured fall deposits at the base of, or interstratified with, PDC deposits from the Campanian region of Italy. At several localities, we calculated the depth of PDC erosion by comparing the measured thickness of eroded remnants to reconstructed thickness at each site (estimated by isopach mapping), and recorded the (1) distance from vent, (2) elevation of the locality, and (3) paleoslopes. Furthermore, we have considered how these factors can be influenced by outcrop exposure. Depth of erosion correlates with distance from the vent in low-relief landscape, while across very rugged topography the only related parameter is elevation. The different erosive patterns appear to show how pyroclastic currents interact with the topography in the surrounding terrain. When a PDC crosses relatively flat surfaces, it decelerates away from the vent, decreasing its erosive capacity; but when moving through steep terrain, a PDC accelerates down the valley, increasing its erosive capacity.

Chapter 5 The Campi Flegrei caldera boundary in the city of Naples

Abstract The Campanian Ignimbrite caldera occupies the Campi Flegrei region and part of the city of Naples. The previous caldera boundary throughout the northern periphery of Naples was merely inferred due to the lack of outcrops of proximal deposits associated with the Campanian Ignimbrite. The exact location of this important structural feature within the city of Naples is fundamental for the reconstruction of the volcanic evolution and hazard implications. New exposures and subsurface constraints reveal thick welded and lithic-rich successions overlying several monogenetic volcanoes. These proximal deposits are associated with the Campanian Ignimbrite and allow a better localization of the caldera boundary well inside the city of Naples, 2km south from the previous limit. The caldera rim in this sector partially coincides with a vent alignment that represents a structurally weak zone through which the caldera collapse occurred. The minor displacement (few tens of metres) of the top of the sedimentary succession, beneath the volcanic sequence near the caldera rim compared with 3km displacement of the top of the sedimentary succession in the central part of the caldera suggests the presence of a complex geometry of the caldera floor, which shows a piecemeal-like structure characterized by deeper blocks at the centre and shallower blocks to the sides.