Bulletin of Volcanology

, Volume 72, Issue 5, pp 559–577

The Pomici di Avellino eruption of Somma–Vesuvius (3.9 ka BP). Part II: sedimentology and physical volcanology of pyroclastic density current deposits

Authors

    • CIRISIVU, c/o Dipartimento Geomineralogico
  • R. Bonasia
    • CIRISIVU, c/o Dipartimento Geomineralogico
    • Istituto Nazionale di Geofisica e Vulcanologia
  • P. Dellino
    • CIRISIVU, c/o Dipartimento Geomineralogico
  • D. Mele
    • CIRISIVU, c/o Dipartimento Geomineralogico
  • M. A. Di Vito
    • Istituto Nazionale di Geofisica e Vulcanologia
  • L. La Volpe
    • CIRISIVU, c/o Dipartimento Geomineralogico
Research Article

DOI: 10.1007/s00445-009-0340-4

Cite this article as:
Sulpizio, R., Bonasia, R., Dellino, P. et al. Bull Volcanol (2010) 72: 559. doi:10.1007/s00445-009-0340-4

Abstract

Pyroclastic density currents (PDCs) generated during the Plinian eruption of the Pomici di Avellino (PdA) of Somma–Vesuvius were investigated through field and laboratory studies, which allowed the detailed reconstruction of their eruptive and transportation dynamics and the calculation of key physical parameters of the currents. PDCs were generated during all the three phases that characterised the eruption, with eruptive dynamics driven by both magmatic and phreatomagmatic fragmentation. Flows generated during phases 1 and 2 (EU1 and EU3pf, magmatic fragmentation) have small dispersal areas and affected only part of the volcano slopes. Lithofacies analysis demonstrates that the flow-boundary zones were dominated by granular-flow regimes, which sometimes show transitions to traction regimes. PDCs generated during eruptive phase 3 (EU5, phreatomagmatic fragmentation) were the most voluminous and widespread in the whole of Somma–Vesuvius’ eruptive history, and affected a wide area around the volcano with deposit thicknesses of a few centimetres up to more than 25 km from source. Lithofacies analysis shows that the flow-boundary zones of EU5 PDCs were dominated by granular flows and traction regimes. Deposits of EU5 PDC show strong lithofacies variation northwards, from proximally thick, massive to stratified beds towards dominantly alternating beds of coarse and fine ash in distal reaches. The EU5 lithofacies also show strong lateral variability in proximal areas, passing from the western and northern to the eastern and southern volcano slopes, where the deposits are stacked beds of massive, accretionary lapilli-bearing fine ash. The sedimentological model developed for the PDCs of the PdA eruption explains these strong lithofacies variations in the light of the volcano’s morphology at the time of the eruption. In particular, the EU5 PDCs survived to pass over the break in slope between the volcano sides and the surrounding volcaniclastic apron–alluvial plain, with development of new flows from the previously suspended load. Pulses were developed within individual currents, leading to stepwise deposition on both the volcano slopes and the surrounding volcaniclastic apron and alluvial plain. Physical parameters including velocity, density and concentration profile with height were calculated for a flow of the phreatomagmatic phase of the eruption by applying a sedimentological method, and the values of the dynamic pressure were derived. Some hazard considerations are summarised on the assumption that, although not very probable, similar PDCs could develop during future eruptions of Somma–Vesuvius.

Keywords

Pyroclastic density currentsPomici di AvellinoSomma–VesuviusDynamic pressureVolcanic hazard

Introduction

Pyroclastic density currents (PDCs) represent the most hazardous events of explosive volcanism (e.g. Cas and Wright 1987; Carey 1991; Branney and Kokelaar 2002; Sulpizio and Dellino 2008). They form when a mixture of gas and solid particles, after eruption from the crater, collapses to the ground under the effect of gravity or during gravity-driven collapse of domes. Both triggering mechanisms generate a shear current that moves at high velocity across the ground (e.g. Burgissier and Bergantz 2002; Branney and Kokelaar 2002).

Pyroclastic density current deposits are very abundant in the geological record of explosive volcanoes. Their devastating potential has been demonstrated by historic eruptions and their hazardous effects demonstrated in densely populated areas surrounding active volcanoes (Todesco et al. 2002; Cioni et al. 2004; Baxter et al. 2005; Gurioli et al. 2005).

The PdA Plinian eruption is one of the four Plinian events that occurred at Somma–Vesuvius since 22 ka BP (Andronico et al. 1995; Santacroce et al. 2008). It devastated the area surrounding the volcano during the Early Bronze Age, as indicated by 14C age measurements (3,590 ± 25 14C yr BP, 3,900 ± 40 cal yr BP; Andronico et al. 1995) and archaeological finds (Albore Livadie 1980; Cioni et al. 2000).

The PdA eruption has been the focus of some volcanological, petrological and geochemical studies that are listed in the companion paper (Sulpizio et al. 2010), which serves also as reference for the stratigraphy and eruptive dynamics of the PdA eruption.

In particular, the PdA eruption was characterised by an extensive final phreatomagmatic phase that dispersed PDCs over a wide area of the Campanian Plain northwestward from Somma–Vesuvius (Arnò et al. 1987; Di Vito 1999; Cioni et al. 1999, 2000; Mastrolorenzo et al. 2006; Sulpizio et al. 2008a, 2010; Di Vito et al. 2009). These PDC deposits are among the most widely dispersed in the Somma–Vesuvius explosive record, and, together with the EU4 deposits of the following AD 79 eruption (Gurioli et al. 2005), represent the only PDCs that flowed well beyond the break in slope between the volcano and the surrounding plain. If emplaced during a present-day eruption, they would affect a very densely inhabited area that comprises the eastern outskirts and part of central Naples. Therefore, the detailed study of the eruptive dynamics, sedimentology and physical properties of these PDCs is needed for hazard mitigation in the area, and may serve as an example for mitigation strategies in other densely inhabited areas close to active volcanoes. In addition, the study of PdA PDCs contributes to the understanding of transport mechanisms and depositional behaviour of these complex volcanic phenomena.

This paper explains the detailed stratigraphic reconstruction, description of lithofacies association and architecture of the PDC deposits of the PdA eruption. This data is used to derive a sedimentological model for PDCs generated by different eruptive dynamics, driven by both magmatic and phreatomagmatic fragmentation. The key physical parameters (particle concentration, flow velocity and dynamic pressure) of the main PDCs were calculated from field and laboratory data by applying the model of Dellino et al. (2008).

General outline of the Pomici di Avellino eruption

The present paper uses as general stratigraphic framework the companion paper (Sulpizio et al. 2010), which establishes five eruptive units (EUs; Fig. 1) emplaced during an opening, Plinian and final phreatomagmatic phase of the eruption. The opening phase produced a double bedset of white pumice lapilli and brown fine ash (EU1a and EU1b) emplaced by fall from short-lived columns of low to medium height (13–21.5 km; Sulpizio et al. 2010) and dispersed to the northeast. Partial to total column collapses emplaced small PDC deposits along the northwestern slopes close to the eruptive vent. The Plinian phase emplaced pumice fall deposits (EU2, EU3 and EU4) dispersed towards the northeast (Fig. 1) from 23- to 31-km-high eruption columns (Sulpizio et al. 2010). EU2 is reversely graded and composed of white pumice lapilli and rare lithics, whereas EU3 is massive and composed of grey pumice lapilli and abundant lithics with a thin, massive ash layer at the top. Only one small PDC was emplaced during EU3, which crops out along the northern slopes of the volcano. EU4 is massive and includes grey pumice lapilli and abundant lithics and loose crystals. The minimum bulk volume of the fall deposits (EU1–4) is 1.4 km3, with the main part supplied by EU3 (1 km3; Sulpizio et al. 2010).
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Fig. 1

Composite stratigraphic succession of the Pomici di Avellino eruption with brief description of lithology and sedimentary characteristics of PDC deposits, isopach maps and eruptive dynamics. a Column showing the three eruptive phases and the five eruption units; b thickness distribution of EU1 PDC deposits; c thickness distribution of EU3pf deposits with the tentative isopach of 10 cm; d isopach map of EU5 deposits

The phreatomagmatic phase was dominated by pulsating phreatomagmatic explosions (EU5; Fig. 1), producing a succession of both PDC and minor fall deposits. PDC deposits of EU5 are mainly dispersed to the west and northwest up to about 25 km from the volcano (Fig. 1). Proximal PDC deposits are mainly massive, metres-thick, dune-bedded and coarse-grained, passing to horizontally stratified beds in the surrounding plain. Accretionary lapilli-bearing and vesicular ash beds are exposed on the eastern and southeastern slopes of the volcano, which were sheltered by the Mt Somma caldera wall at the time of the PdA eruption (Sulpizio et al. 2010).

Sedimentology of PDCs

PDCs were generated during EU1, EU3 and EU5 under different eruptive mechanisms, encompassing a broad spectrum of transport mechanisms and flow behaviour (Fig. 1). The PDC deposits are exposed mainly on the northern and western slopes of Mt Somma and in the plain north of the volcano (Fig. 1). Fine ash and accretionary lapilli-bearing fine ash deposits occur on the eastern and southern slopes of the volcano due to the sheltering effect of the Mt Somma caldera wall, which at the time of eruption extended to the southern part of the volcanic edifice (Sulpizio et al. 2010). The generation mechanisms of the PDCs varied from column collapse to pyroclastic fountaining, although radial expansion of over-pressurised mixture might have played a role during emplacement of EU5 PDCs (Fig. 1; Sulpizio et al. 2010). In the following sections, the sedimentological characteristics of the different PDCs, along with grain size and componentry of the deposits (Table A1.2), will be illustrated and discussed in detail.

EU1 PDCs

The EU1 PDC deposits are only seen in a limited area of the northwestern sector of the volcano slopes (Fig. 1b), where they crop out at the base of the coarse-grained, breccia fall deposits of the following EU2 and EU3 (Fig. 2). They are poorly sorted deposits and comprise a twofold succession (EU1a and b; Fig. 2) of massive to diffusely stratified beds overlain by coarse ash. EU1a deposits comprise a basal lapilli-bearing bed rich in lithic fragments that appears massive when viewed close up, but which shows an impersistent stratification when viewed from a certain distance (lithofacies isL; Fig. 2a; Table A1.3). The upper half of the EU1a deposit comprises diffusely stratified pumice and lithic lapilli and coarse ash (lithofacies dsLA; Fig. 2a; Table A1.3), which passes vertically to massive and diffusely stratified coarse and fine ash with sparse lapilli (lithofacies dsAL/mAL; Fig. 2a; Table A1.3). The EU1a deposit thins rapidly down-slope and laterally of site A12 (Fig. 2c, d), passing from a thickness of about 1 m (site A12) to about 50 cm (sites A9 and U; Fig. 2b–d), and showing transition to massive, lapilli-bearing beds overlain by massive coarse to fine ash deposits (lithofacies mL and mAL; Fig. 2c, d; Table A1.3). In the most proximal exposure (site A12; Fig. 2b), the EU1b deposits comprise a stacked succession of massive, poorly sorted, inversely graded lapilli-bearing beds (lithofacies mLA(il) and mL(il); Fig. 2a; Table A1.3) overlain by a diffusely stratified bed of coarse ash and lapilli (lithofacies dsLA; Fig. 2a; Table A1.3). Massive, coarse to fine ash with sparse lapilli (lithofacies mAL; Fig. 2a; Table A1.3) crops out at the top of the EU1b succession at site A12. Similarly to the EU1a deposits, the EU1b succession thins rapidly down-slope and laterally to site A12, passing from a thickness of about 1.5 m (site A12) to about 50 cm (sites A9 and U; Fig. 2b–d). Down-slope and laterally to site A12, the EU1b PDC deposits show transition to a massive lapilli-bearing bed (lithofacies mL; Fig. 2d; Table A1.3) overlain by diffusely stratified ash and lapilli (lithofacies dsLA; Fig. 2d; Table A1.3) and fine to coarse massive ash at the top (lithofacies mAL; Fig. 2d; Table A1.3).
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Fig. 2

Stratigraphy and sedimentological model for EU1 PDC deposits. a Exposure of EU1 deposits at section A12 with lithofacies codes of the different beds. The white triangles indicate reverse grading of coarse clasts; b map of exposure of EU1 deposits. The black dot indicates the inferred vent; c picture of the EU1–3 deposits at section U; d close-up view of the EU1 deposits at section U with lithofacies codes of the different beds; e sedimentological model for the EU1a deposits; f scheme of the lithofacies association inferred to be deposited by the flow of (e); g sedimentological model for the EU1b deposits; h scheme of the lithofacies association inferred to be deposited by the flow of (g)

Both EU1a and EU1b PDCs were originated from the collapse of a low and short-lived convective column, which also deposited fall beds over the northeastern volcano slopes (Sulpizio et al. 2010).

Interpretation

The vertical lithofacies association of EU1a deposits observed at site A12 (Fig. 2a) indicates temporal changes of the dominant physical flow process in the flow-boundary zone. The occurrence of lithofacies isL at the base of the deposit is suggestive of a flow-boundary zone dominated by grain interactions. The poor sorting and the massive appearance are suggestive of sedimentation from a rapidly depletive current in which the rate of supply (Rs) was higher than the rate of deposition (Rd). This would have favoured the development of a highly concentrated zone at the base of the current dominated by grain interactions, although shearing was still capable of inducing intermittent traction that produced impersistent stratification (lithofacies isL; Branney and Kokelaar 2002; Fig. 2e). The passage to lithofacies dsLA indicates a decrease in content of coarse-grained and heavy particles in the flow-boundary zone, with turbulence able to interact with depositing particles and to produce the diffuse stratification observed (Fig. 2e, f). Finally, lithofacies dsAL/mAL records the waning stage of the current, with the flow-boundary zone dominated by gentle settling of particles from suspension with reduced lateral transportation (Fig. 2e, f).

The vertical lithofacies association of EU1b (Fig. 2c, d) describes a slightly different story with respect to the EU1a. The repetitive occurrence of lithofacies mLA(il) and ML(il) at the base of the succession is indicative of a flow-boundary zone in which there developed discrete granular-dominated pulses that aggraded in steps (Fig. 2g, h). Each single pulse was dominated by particle–particle interactions that promoted the observed inverse grading of larger clasts (Fig. 2c, g and h) through kinetic sieving and kinematic squeezing processes (Le Roux 2003; Sulpizio et al. 2007; Sulpizio and Dellino 2008). The drag force exerted by the overriding flow on some large clasts that deposited on the front of the pulses (fall blocks; e.g. Nemec 1990) caused them to roll and become orientated with longer axis perpendicular to the current motion (Fig. 2a). The passage to lithofacies dsLA indicates a flow boundary dominated by particle–particle interaction, with turbulence able to interact with particles during deposition to produce the observed diffuse stratification (Fig. 2g, h). The lithofacies mAL at the top of the EU1b succession records the waning stage of the PDC, during which the flow-boundary zone was dominated by gentle settling of fine particles from suspension (Fig. 2g, h).

EU3pf

The EU3pf deposits crop out in the northern volcano slopes (Fig. 1c), where they are interbedded with the EU3 fall deposits (Fig. 3a, b). They show lateral variation of both thickness and sedimentary characteristics from the most proximal exposures (sites A12, V, U and A9; Figs. 3c and 1c) to those located on the northern volcano slopes (Figs. 1c and 3a, b). The most proximal deposits comprise poorly sorted massive to impersistently stratified lapilli and ash (lithofacies mLA and isAL; Fig. 3a; Table A1.3), with a coarse ash matrix and faint reverse grading of lithic blocks.
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Fig. 3

Stratigraphy and some physical parameters of the EU3pf deposits. a Picture of EU3pf deposits at section A12 with lithofacies codes of the different beds; b picture of the EU3pf deposits at section A6 with lithofacies code; c map of exposure of EU3pf deposits. The yellow-shaded area indicates the occurrence of the coarse-grained deposits (lithofacies mLB and isAL of (a)), while the brown-shaded area indicates the occurrence of fine-grained deposits (lithofacies mAL of (b)); d frequency distribution histograms of grain-size data for three samples from EU3pf deposits. For the sample AV 93 146, the component analysis is also reported; e selected grain-size parameters for the three analysed samples of EU3pf deposits

These coarse-grained deposits (ca. 1 m at site A12) rapidly thin laterally and show a transition to massive coarse ash beds (lithofacies mAL; Fig. 3b; Table A1.3) a few centimetres thick that are dispersed on the lower northern slopes of the volcano (Fig. 3c). The EU3pf originated from the partial collapse of the sustained column that dispersed the EU3 deposits (Sulpizio et al. 2010; Fig. 4).
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Fig. 4

Sketch of the eruptive dynamics responsible for the emplacement of EU3pf deposits and for the geographic distribution of the different lithofacies. The Plinian column produced EU3

Grain-size analysis and components

Three grain-size analyses were carried out on deposits of lithofacies mLB and mAL. The samples show a coarse-grained matrix (F2 < 5%; Fig. 3e) and uni- to tri-modal bulk grain-size distributions. Ignoring particles coarser than −4 ϕ, the grain size distributions appear almost Gaussian in shape (skewness close to zero; Fig. 3e), with main modes between −0.74 ϕ and −1.74 ϕ, and moderate to good sorting (Fig. 3e). The Gaussian-like shape improves with distance from the source, and the distribution of the most distal sample has very similar values of Mdϕ and Mz (Fig. 3e).

Despite the filtering of coarse grain sizes (mainly lithic blocks), lithic fragments are the most abundant component, with juvenile and loose crystals subordinate (Fig. 3e).

Lithic fragments are the most abundant component of the coarser grain sizes (Fig. 3e).

Interpretation

Vertical lithofacies changes EU3pf deposits observed at site A12 (Fig. 3a) indicate changes in the flow-boundary zone with time. Lithofacies mLB at the base of the deposit suggests a flow-boundary zone dominated by grain interactions. The poor sorting and the massive appearance are suggestive of sedimentation from a rapidly depletive current in which the rate of supply (Rs) was higher than the rate of deposition (Rd). This would have favoured the development of a highly concentrated zone at the base of the current, which promoted the onset of grain segregation through kinetic sieving and kinematic squeezing processes that gave origin to the faint reverse grading of large clasts (Fig. 3a). The passage to lithofacies isAL indicates a decrease in coarse-grained and heavy particles in the flow-boundary zone in which shearing remained greatly reduced by grain interactions but was able to produce intermittent traction responsible for the impersistent stratification and the orientation of some large clasts (Fig. 3a).

The occurrence of lithofacies mAL on the lower northern slopes of the volcano (Fig. 3b, c) indicates deposition from a dilute current-tail ash cloud, in which lateral transport was reduced and the particles mainly settled vertically (fallout regime; Branney and Kokelaar 2002). The decoupling of the fine-grained, turbulent ash cloud from the basal, coarse-grained underflow accounts for the observed different areas of exposure of lithofacies mLB/isAL and lithofacies mAL. In particular, distribution elongate to the northeast of the area, and characterised by lithofacies mAL (Fig. 3c), suggests that after decoupling the ash cloud was driven by lower atmosphere winds blowing from the SW (Fig. 4).

EU5 PDCs

The EU5 deposits were produced during the final phreatomagmatic phase of the eruption and comprise four sub-units (Fig. 1a–d), which mantle the volcano slopes (mean slope in excess of 25°), the volcaniclastic apron (slope between 10° and 5°) and the plain north of the Somma–Vesuvius (slope between 2° and 0°). They show isopachs elongate to the northwest and strong lateral variation of thickness passing from the northwest to the east and south volcano slopes (Fig. 5). Sedimentary characteristics show sharp changes from the proximal outcrops, dominated by massive to diffusely stratified deposits (lithofacies mAL and dsAL; Fig. 6b, c; Table A1.3), to the finer grained exposures of the plain (lithofacies altAL and altcfA; Figs. 7 and 8) and to the lapilli-bearing massive ash that crops out on the eastern and southern slopes of the volcano (lithofacies maccrA and mA; Fig. 9; Table A1.3). In particular, the outcrops on the northwestern volcano slopes are dominated by thick, coarse-grained deposits, which in very proximal exposures show dune bedding with wavelengths of tens of metres (lithofacies lensBL, section V; Fig. 6c; Table A1.3). Massive deposits also dominate the outcrops on the northern volcano slopes within 3 km from the vent (i.e. section A6–7; Fig.6b). A significant change in lithofacies is shown by the outcrops beyond the break in slope between the volcano and the surrounding plain to the north. The outcrops to the northwest (sections AVL2, 6, 5 and 11; Fig. 7) are dominated by alternating beds of lapilli and ash (lithofacies altLA; Fig. 7b–d; Table A1.3), which pass to massive ash or alternating coarse and fine ash at increasing distance from the vent (lithofacies mA and altcfA; Fig. 7e, f; Table A1.3). The most distal sites in this area are located on small topographic heights (sections AVL5 and 11; Fig. 7a) and show a prevalence of fine-grained material with fine ash beds that locally contain accretionary lapilli. A similar lateral lithofacies transition is shown by outcrops located in the plain north of Somma–Vesuvius (Fig. 8a), which are dominated by alternating coarse and fine ash with minor massive ash or alternating lapilli and ash (lithofacies altcfA, mA and altLA; Fig. 8b, c; Table A1.3) up to 14 km from the vent (section AVL13; Fig. 8a). The exposure of section AVL13 is located within an archaeological excavation of a Bronze Age village partially buried by EU5 PDCs (Di Vito et al. 2009). Here, the deposits of EU5a are a few decimetres thick and comprise massive, fine ash that thickens against the obstacles and entered the village huts (Fig. 8e, f). Analysis of magnetic anisotropy of EU5a deposits demonstrated that their emplacement was strongly influenced by the village structures (Di Vito et al. 2009). Human footprints and animal hoofprints were recognised at the top of EU5a, EU5b and EU5d deposits. EU5b, EU5c and EU5d are a few decimetres in thickness and thicken against obstacles, but did not penetrate the village huts due to their grain size in the range of coarse ash (Di Vito et al. 2009; Fig. 8e, f). Section AVL4 is located at a similar distance from the vent as section AVL1 (Fig. 8a), but shows overall finer grained beds (Fig. 8d). This finding is consistent with the position of AVL4 on a local topographic high. The most distal extremity of this transect (section AVL14; Fig. 8a) is located on the plain north of the Poggioreale hill and shows three main beds of accretionary lapilli-bearing ash with interbedding of two thin, coarse ash beds (lithofacies maccrA and mcA; Fig. 8g; Table A1.3). Exposures on the eastern and southern volcano slopes (red-shaded area between sections B and A10; Fig. 9a) comprise four main beds of accretionary lapilli-bearing fine ash (lithofacies maccrA; Fig. 9b; Table A1.3), sometimes interbedded by thin beds of massive, vesicular ash (lithofacies mvA; Fig. 9b; Table A1.3).
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Fig. 5

Isopach map of EU5 deposits. The red dots indicate the investigated stratigraphic sections, and numbers and letters the site codes. The black dot indicates the inferred vent area

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Fig. 6

Northeastward transect of proximal–distal exposures for EU5 deposits. a Map showing the location of the selected stratigraphic sections; b exposure of EU5b and c deposits at section A6–7 with lithofacies codes of the different beds; c exposure of EU5 deposits at section V with lithofacies codes of the different beds; d exposure of EU5 deposits at section AVL12 with lithofacies codes of the different beds

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Fig. 7

Northwestward transect of medial–distal exposures for EU5 deposits. a Map showing the location of the selected stratigraphic sections; b exposure of EU5deposits at section AVL2 with lithofacies codes of the different beds; c close-up view of lithofacies altLA from EU5b deposits; d exposure of EU5 deposits at section AVL6 with lithofacies codes of the different beds; e exposure of EU5 deposits at section AVL5 with lithofacies codes of the different beds; f panoramic view of EU5 deposits at section AVL11

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Fig. 8

Northward transect of medial–distal exposures for EU5 deposits. a Map showing the location of the selected stratigraphic sections; b exposure of EU5deposits at section AVL1 with lithofacies codes of the different beds; c exposure of EU5 deposits at section AVL7 with lithofacies codes of the different beds; d exposure of EU5 deposits at section AVL4 with lithofacies codes of the different beds; e exposure of EU5 deposits at the Bronze Age village of Afragola (Di Vito et al. 2009; section AVL13) in an area free of human artefacts. Lithofacies codes of the different beds are also reported; f exposure of EU5 deposits at the Bronze Age village of Afragola (Di Vito et al. 2009; section AVL13) close to a village hut. Note the abnormal thickening and the lateral discontinuity of the different beds. Lithofacies codes of the different beds are also reported; g exposure of EU5 deposits at section AVL14 with lithofacies codes of the different beds

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Fig. 9

Exposures of EU5 deposits on the eastern slopes of the volcano. a Map showing the area dominated by the exposure of lithofacies maccrA (pink-shaded area); b exposure of EU5 deposits at section B with lithofacies codes of the different beds; c exposure of EU5 deposits at section A10 with lithofacies code

All the EU5 PDCs originated from pyroclastic fountaining or from expansion of over-pressurised pyroclastic mixture at the vent, since long-lived sustained columns were not formed during the final phreatomagmatic phase (Sulpizio et al. 2010).

Grain-size analysis and components

Sixty-five grain-size analyses were carried out on deposits of various lithofacies from stratigraphic sections located between 2.8 and 11.5 km from the vent (Table A1.2). Most of the samples show prevalence of fine and coarse ash, with very few samples in which lapilli are predominant (Fig. 10a; Table A1.2). The abundance of coarse ash is also attested to by the F1 vs. F2 diagram (Fig. 10b) in which the values of F2 are always below 66% (Table A1.2). Grain-size distributions are almost unimodal and well sorted in the coarse ash beds, whereas most are polymodal and poorly sorted in the fine-grained beds (Fig. 11). It is noteworthy that the distribution and the main modes of the fine-grained part of the samples (finer than 2 ϕ) remain almost constant in both proximal and distal samples (Fig. 11; Table A1.2). Most of the samples with unimodal grain-size distribution are symmetrical or very slightly skewed, while the asymmetrical samples are all positively skewed (Table A1.2). Main modes cluster around a few values in both proximal and medial–distal samples from all the three sub-EUs sampled (Fig. 11; Table A1.2).
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Fig. 10

Grain-size characteristics of the sampled EU5 deposits. a Three-component grain-size variations in the deposits. Black labels indicate EU5a deposits, red labels indicate EU5b deposits and blue labels indicate EU5c deposits; b plot of the wt% finer than 1/16 mm (F2) versus the wt% finer than 1 mm (F1)

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Fig. 11

Grain-size and component variation from section AVL2 to section AVL7

Lithic fragments are the most abundant component in almost all analysed samples from both proximal and medial–distal outcrops (Fig. 11; Table A1.2). Dense juvenile fragments are always prevalent over vesicular ones, which are always below 5% by weight (Table A1.2). Loose crystals are abundant (5–25% by weight; Table A1.2), with salic crystals prevalent with respect to femic ones.

When analysed separately, the grain-size distributions of the different components show the predominant influence of lithic and dense juvenile distributions with respect to vesicular juvenile material and crystals, while the latter component increases its influence on grain-size distribution in medial–distal samples (Fig. 11). Median diameter values are similar in lithics and dense juvenile components, and these are generally coarser than those of salic and femic components (Table A1.2).

Interpretation

The lateral and vertical lithofacies association of EU5 deposits indicates eruption dynamics dominated by pyroclastic fountaining, without any long-sustained column (Fig. 12).
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Fig. 12

Sketch of the eruptive dynamics responsible for the emplacement of EU5 deposits and for the geographic distribution of the different lithofacies

Vertical lithofacies changes remain almost monotonous in EU5a–c, which indicates similar depositional mechanisms through time. On the other hand, the complex lateral changes in lithofacies can be interpreted in the light of changing physical conditions in the flow-boundary zone influenced by morphological complexities.

Thick, massive and lenticular deposits (lithofacies lensBL, lensL, mAL; Fig. 6b, c; Table A1.3) associated with diffuse to cross-stratified deposits (lithofacies sAL, xSA, dsAL; Fig. 6b, c; Table A1.3) dominate on the western volcano slopes (red-shaded area in Fig. 13). High Rd and Rs have favoured the development of a highly concentrated zone at the base of the current with suppression of turbulence and deposition of massive lithofacies. The occurrence of stratified lithofacies indicates episodes of increasing flow turbulence, with very large eddies that interacted with depositing and already deposited material to develop the giant dune bedding and the lenticular shape of massive deposits (Fig. 6c). At the same time, increasing flow turbulence reduced Rd and Rs in the flow-boundary zone, with deposition of stratified lithofacies (Fig. 6b, c).
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Fig. 13

Geographic distribution of the main lithofacies associations of EU5 deposits

The passage to the thinly stratified lithofacies altLA (orange-shaded area in Figs. 13 and 7b) is located beyond the break in slope between the volcano sides (mean slope in excess of 25°) and the volcaniclastic apron (slope between 10° and 5°). Changes imposed by deposition in proximity to a break in slope may induce major flow transformations in dry and fluid-bearing currents, documented in many papers for dry granular flows (e.g. Denlinger and Iverson 2001; Felix and Thomas 2004), volcaniclastic flows (e.g. Zanchetta et al. 2004b), PDCs (e.g. Macias et al. 1998; Cole et al. 2005; Gurioli et al. 2007; Sulpizio et al. 2007, 2008b; Sulpizio and Dellino 2008) and turbidity currents (e.g. Mulder and Alexander 2001; Gray et al. 2005). Beyond the break in slope, flow braking induces deposition of material in the flow-boundary zone and partial transfer of momentum to turbulence generation and elutriation of fines (Fig. 14a). At this point, the part of the flow above the flow-boundary zone can respond in two ways: (1) its bulk density is still greater than the surrounding ambient fluid and it propagates further as an independent gravity-driven current (Fig. 14a); or (2) its bulk density is less than the surrounding ambient fluid and it lofts convectively and stops (Sulpizio and Dellino 2008). The fate of a gravity-driven current is then a function of its mass and grain size, the local curvature at the break in slope and efficiency in energy transformation. All these parameters control both the amount of material remaining in the transport system as the flow crosses the break in slope and the subsequent physical characteristics of the surviving gravity-driven current. Case (1) applies to the EU5 deposits, since stratigraphy indicates that the current survived the passage across the break in slope and the massive deposition that occurred on the volcano sides. This implies that the grain sizes in the flow beyond the break in slope were those maintained in suspension by flow turbulence during transportation on the volcano slopes. The surviving flows had a different sediment load with respect to the parent proximal flows, which is reflected in the sudden decrease in content of coarse-grained and heavy particles in the flow-boundary zone and the finer grain size of the deposits. The greater selection of sediment load is also demonstrated by the grain-size analyses from section AVL2 downflow (Fig. 11) in which the distributions have main modes peaked around very similar values for both coarse-grained and fine-grained populations of particles (Fig. 11; Table A1.2).
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Fig. 14

Sedimentological model for the EU5 PDC deposits. a Flow behaviour when passing over the first break in slope between the volcano sides and the apron and scheme of the lithofacies association inferred to be deposited by the flow; b flow behaviour when passing over the second break in slope between the apron and the alluvial plain and scheme of the lithofacies association inferred to be deposited by the flow; c sketch explaining the lithofacies altLA and altAL in the case of a continuous aggradation of deposits from a sustained, unsteady current (from Branney and Kokelaar 2002, modified); d sketch explaining the lithofacies altLA and altAL in the case of a stepwise aggradation of different pulses developed within PDC

The lateral thickness continuity at the scale of the outcrop, the good sorting of the lapilli beds and the planar contact between the lapilli–ash pairs (Figs. 7 and 8) are peculiar and unusual characteristics of EU5 deposits in medial–distal areas, which require a sedimentological interpretation. Branney and Kokelaar (2002) interpreted similar bedded lapilli tuff deposits as the result of deposition during small-scale unsteadiness due to development of short-lived traction carpets periodically swept away by the impingement of turbulent eddies from higher in the current (Fig. 14c). In this model, the flow-boundary zone fluctuates between granular flow-dominated to fall-dominated, and this might explain the observed deposit characteristics. Another interpretation of the observed planar bedding might be proposed by connecting the development of pulses within the currents with the better selection of their sediment load. In this model, each pulse developed density stratification, with the flow-boundary zone dominated by granular flow (Fig. 14d). Since the lapilli-sized material is well selected and lacks gravel and boulders, the appearance of the deposits is massive, grain supported and well sorted, with very scarce intergranular matrix. The fine ash at the top of the lapilli beds marks the waning phase of the pulse with deposition of the finer grained material (Fig. 14d). The constant lateral thickness and the planar bedding can be explained by taking into account the flat morphology of the plain over which the currents spread, which prevented the development of any perturbation due to substrate roughness.

The passage from dominance of lithofacies altLA to lithofacies altcfA (yellow-shaded area in Fig. 13) approximately coincides with a subtle change in slope from the volcaniclastic apron (with mean slope between 10° and 5°) to the flat alluvial plain (slope between 2° and 0°; Fig. 14b). This slight change in the substratum slope further diminished the gravity component of the motion of the currents, the majority of which were no longer able to transport lapilli-size particles. From this point onward, the deposits of EU5a, EU5c and EU5d contain mainly coarse and fine ash, with the notable exception of only the part of sub-EU5b characterised by lithofacies altLA as far as section AVL13 (Fig. 8e, f).

The occurrence of lithofacies maccrA on the eastern and southern slopes of the volcano (brown-shaded area in Fig. 13) indicates deposition from a very dilute ash cloud, in which lateral transport was reduced and the particles mainly settled vertically (fallout regime; Branney and Kokelaar 2002). This dramatic change in lithofacies was caused by the presence of the morphological obstacle of the Mt Somma caldera wall, which blocked the basal, coarse-grained part of the currents and caused the stripping of the upper, finer grained, more diluted flows that gently settled beyond the obstacle (Fig. 12).

Fluid-dynamic characteristics and impact parameters of the EU5 PDCs

The flow behaviour of EU5 PDCs has been reconstructed by means of the method proposed by Dellino et al. (2008), which uses a system of equations implemented by the combination of the turbulent-boundary-layer-shear-flow theory and the laws of sediment mechanics. It allows calculation of the velocity, density, concentration and dynamic-pressure profiles of the stratified current. In particular, we applied the “two-component method”, which solves the system of equations starting from the features of two components sampled in the stratified layers of the deposits. In the EU5 case, we used data regarding vesicular juvenile particles and loose pyroxene crystals (femic crystals in Table A1.2 and Fig. 11). The method was applied to the coarsest bed of sub-EU5b, which exhibits traction features in the ash beds above the massive basal deposits (lithofacies lensL + sA at section A6–7, Fig. 6b, and lithofacies altLA at section AVL7, Fig. 8c). In principle, the “two-component method” applies to any bed that shows traction features, irrespective of its grain size. In the case of EU5 PDCs, the coarsest bed of sub-EU5b represents the most intense current in the whole stratigraphic succession, and therefore its physical parameters can be considered as an upper limit for the entire eruption.

Some basic concepts and equations of the model are illustrated in Appendix 2, while the full details can be gained from the original paper (Dellino et al. 2008).

For the PdA eruption, the model was applied to the reconstruction of the physical characteristics of EU5 PDCs, because they are the best exposed and most widespread. Two localities, positioned after the break in slope, were selected to characterise the down-current evolution of the dilute flows. One is about 3 km and the other about 11 km from the vent. Data were obtained from the analysis of particles sampled in the stratified layers, beds which include sets of alternating coarse and fine ash, the most abundant lithofacies after the break in slope. The data are presented in Table 1. Figure 15 shows velocity, particle volumetric concentration, density and dynamic-pressure profiles for the two localities. From the comparison of the profiles, the down-current decrease in velocity and dynamic pressure is evident, while concentration always remains much lower than 1%, as expected for diluted PDCs. It is noteworthy that in the lower 10 m, i.e. in the part of the current where much of the mechanical energy is concentrated, velocity is typically several tens of metres per second and dynamic pressure several kilopascals; these results fall within the highest range of values reported in the literature for PDCs (Valentine 1998).
Table 1

Parameters used for the calculation of velocity and concentration vs height of the sub-EU5b PDC

Locality

djuv (mm)

dpx (mm)

ρjuv (kg/m3)

ρpx (kg/m3)

ψjuv

ψpx

3 km

3.070

1.604

2,244

3,221

0.417

0.601

11 km

0.930

0.390

2,043

3,278

0.468

0.559

djuv equivalent diameter of the median size of juvenile fragments, dpx equivalent diameter of the median size of pyroxene crystals, ρjuv density of juvenile fragments, ρpx density of pyroxene crystals, ψjuv shape factor of juvenile fragments, ψpx shape factor of pyroxene crystals

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Fig. 15

Particle volumetric concentration, density, velocity and dynamic-pressure profiles of EU5 density currents at: a distance of 3 km from the vent; b distance of 11 km from the vent. The solid line represents the average solution, the dashed line the maximum solution and the dotted line the minimum solution

It is noteworthy that the dynamic pressure calculated for the PdA PDCs is one order of magnitude higher than that of the Pollena sub-Plinian eruption of Vesuvius (2–4 kPa on average at 4 and 7 km from the vent; Dellino et al. 2008), which is in the range of the reference type of eruptions on which the Somma–Vesuvius hazard assessment is based (Santacroce et al. 1998; Sulpizio et al. 2005). This demonstrates that the EU5 PDCs not only had a wider dispersal area compared to those of other Vesuvius eruptions but also that the intensity of these PDCs was much higher.

By integration of the profiles, the impact parameters values can be calculated at various heights in the current, which are useful for the estimation of their damage potential. For example, as impact parameters, we can consider particle volumetric concentration and dynamic pressure, as these are good proxies of the current’s destructive power (Dellino et al. 2008). They are calculated by integration over the respective gradients and represent the average value for the first 2 m of the current for particle volumetric concentration and for the first 10 m of dynamic pressure (Fig. 16).
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Fig. 16

Map showing the values of the impact parameters of EU5 density currents as calculated at two selected localities. Pdyn(10m) average dynamic pressure in the first 10 m of the current. C(2m) average particle volumetric concentration in the first 2 m of the current

For particle concentration, the values are 0.0043 and 0.0029 for the proximal and distal localities, respectively, which demonstrate the down-current decrease in concentration even at a level close to the base of the current. Nevertheless, the concentration values are high enough that, combined with temperature, they preclude the possibility of human survival. But, aside from the damage potential of particle concentration, the mechanical impact of the current is more important for assessing the currents’ destructive power. In fact, in the proximal area the dynamic pressure is so high (about 48 kPa) that if currents of this kind were to impact constructions with resistance such as that reported for the Vesuvian area (Spence et al. 2004), they could totally destroy buildings and infrastructures. The values of about 6 kPa at 11 km from the vent also indicate that the dynamic pressure would still be high enough at this distance to damage buildings considerably. At about 15 km from the vent (section AVL 13; Fig. 8), the EU5b deposition was disturbed by the village huts and fences, preventing the application of the sedimentological method for extracting the physical parameters of the currents (Di Vito et al. 2009). However, the archaeological data demonstrate the exertion of very low dynamic pressure by the moving flows, since village huts still stood after PDC emplacement (Di Vito et al. 2009).

This is evidence that, in some cases, dilute PDCs of phreatomagmatic origin can have a wide dispersal area and maintain high velocity, high dynamic pressure and therefore high destructive power, for over 10 km from the vent. This is at odds with the behaviour of other dilute PDCs such as those due to base surges, which typically form during phreatomagmatic eruptions of maars, tuff rings and tuff cones, but spread less widely, and are of minor intensity, than those of EU5. We think that the difference might be attributed to the higher magma-feeding rate and especially to the very wide conduit (or network of conduits) that characterised the phreatomagmatic phase of the PdA eruption, which is also typical of calderic phreatomagmatic eruptions (De Astis et al. 1997). In particular, the phreatomagmatic phase of the PdA eruption bore witness to the interaction of water from the regional aquifer stored into the Mesozoic carbonates that surrounded the magma chamber and the residual, unerupted magma (Sulpizio et al. 2010). This condition allowed a “massive” pre-eruptive mixing and interaction of water and magma (Zimanowski et al. 1991), which was orders of magnitude higher, in terms of magma and water volume, when compared to typical examples of the phreatomagmatic eruptions of maars, tuff rings and tuff cones.

Summary and some hazard consideration

Combined use of field investigation and laboratory analyses allow the detailed reconstruction of the eruptive and transportation dynamics of the PDCs of the PdA eruption of Somma–Vesuvius. PDCs were generated during all the three eruptive phases, with eruptive dynamics driven by both magmatic and phreatomagmatic fragmentation. Flows generated during phases 1 and 2 (magmatic fragmentation) have small dispersal areas and affected only parts of the volcano slopes. Lithofacies analysis demonstrates that the flow-boundary zones were dominated by granular-flow regimes, which sometimes show transition to the traction regime. In turn, PDCs generated during the eruptive phase 3 (phreatomagmatic fragmentation) were the most voluminous and widespread in the whole Somma–Vesuvius eruptive history, and affected a wide area around the volcano with deposit thicknesses of a few centimetres as far as 25 km from the source. Lithofacies analysis shows that the flow-boundary zones of EU5 PDCs were dominated by granular flows and the traction regime. When flowing on the volcano slopes, the EU5 PDCs were highly turbulent and able to maintain in suspension abundant particles in the size-range fine lapilli-coarse ash. This abundance of relatively coarse particles transported in suspension on the volcano slopes allowed the flow to survive through the passage over the break in slope between the volcano sides and the volcaniclastic apron, which induced braking of the basal concentrated parts of the flows and the deposition of the massive bed exposed at foothill in the northern and western volcano sectors. The suspension load of the PDCs propagated independently from this area onward to form new flows that carried a more selected particle population. Pulses were developed within the single currents and deposition occurred stepwise on both the volcano slopes and on the surrounding volcaniclastic apron and alluvial plain.

Physical parameters were calculated for the sub-EU5b PDCs, and show dynamic pressure in excess of 40 kPa on the volcano slopes, reduced to less than 10 kPa on the plain. These values are in agreement with archaeological finds showing that wooden huts were still standing after the passage of the PDCs at about 15 km from the source.

These data and considerations are helpful when dealing with hazard assessment, which is based on the expected range of probability that similar currents and impacts will occur in the future. If superimposed upon present-day urbanisation, the dispersion of EU5 deposits would affect a densely inhabited area that encompasses the eastern outskirts of the city of Naples, with several hundred thousand people exposed to lethal effects related to PDC passage and deposition.

Although an explosive eruption similar to the PdA is not probable when activity is renewed at Somma–Vesuvius (Santacroce et al. 1998; Marzocchi et al. 2004), the EU5 PDCs can be considered as the “maximum safety” event for addressing the maximum expected impact on the circumvesuvian area. As an example, the values of dynamic pressure yield information that can help in land-use planning, since they are the maximum calculated for the Somma–Vesuvius area for dilute PDCs. Another important point of concern for volcanic hazard mitigation is knowledge of the particle volumetric concentration profile of PDC, because it is this, in addition to dynamic pressure, which provides important information on the impact of such hazardous events on human health. It is trivial to state that a current with a dynamic pressure in excess of a few kilopascals is lethal for people both inside buildings and outdoors. However, in distal reaches, where the currents significantly slow down, they can maintain a toxic particle volumetric concentration near the ground surface. There is not a solid background on the effect that ash held in suspension by density currents has on human health, and only very recently are data emerging on the effect of ash fall (Horwell and Baxter 2006). In any case, exposure to the calculated concentrations can be extremely hazardous, with severe respiratory effects that are amplified by the prolonged exposure to fine ash due to their remobilization from fall deposits by lower atmosphere winds.

Acknowledgements

This research was partially funded by 2005–2007 INGV–DPC projects. We are grateful to Jim Bishop for revising the English text. Lucia Gurioli and Gert Lube are acknowledged for the careful revision of the manuscript. The Associate Editor James White is warmly thanked for the careful final editing of the manuscript.

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