The Pomici di Avellino eruption of Somma–Vesuvius (3.9 ka BP). Part II: sedimentology and physical volcanology of pyroclastic density current deposits
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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.
KeywordsPyroclastic density currents Pomici di Avellino Somma–Vesuvius Dynamic pressure Volcanic hazard
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 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.
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).
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).
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).
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).
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
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).
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.
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.
Parameters used for the calculation of velocity and concentration vs height of the sub-EU5b PDC
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.
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.
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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