Interaction of pyroclastic density currents with human settlements: Evidence from ancient Pompeii Lucia Gurioli Istituto Nazionale di Geofisica e Vulcanologia, Via della Faggiola 32, 56126 Pisa, Italy M. Teresa Pareschi Elena Zanella Roberto Lanza Dipartimento di Scienze della Terra, Universita` di Torino, Via Valperga Caluso 35, 10125 Torino, Italy Enrico Deluca Marina Bisson Istituto Nazionale di Geofisica e Vulcanologia, Via della Faggiola 32, 56126 Pisa, Italy ABSTRACT Integrating field observations and rock-magnetic measurements, we report how a turbulent pyroclastic density current interacted with and moved through an urban area. The data are from the most energetic, turbulent pyroclastic density current of the A.D. 79 eruption of Vesuvius, Italy, which partially destroyed the Roman city of Pompeii. Our results show that the urban fabric was able to divide the lower portion of the current into several streams that followed the city walls and the intracity roads. Vortices, revealed by upstream particle orientations and decreases in deposit temperature, formed downflow of obstacles or inside cavities. Although these perturbations affected only the lower part of the current and were localized, they could represent, in certain cases, cooler zones within which chances of human survival are increased. Our integrated field data for pyroclastic density current temperature and flow direction, collected for the first time across an urban environment, enable verification of coupled thermodynamic numerical models and their hazard simulation abilities. Keywords: Vesuvius, pyroclastic density current, thermal remanent magnetization, deposits, magnetic fabric. INTRODUCTION Pyroclastic density currents, PDCs, are rapidly moving stratified currents of solid volcanic material and gases, at temperatures of hundreds of degrees centigrade (Branney and Kokelaar, 2002). Their velocity, density, tem-
perature, and loading with lethal projectiles make for deadly environments for humans and cause severe structural damage (e.g., Tanguy et al., 1998). Recent studies have investigated these effects (e.g., Lipman and Mullineaux, 1981; Blong, 1984; Sigurdsson et al., 1985;
Figure 1. Shaded relief map of Vesuvius region. White and red lines are isopachs (cm) related to dispersion of white and gray A.D. 79 Plinian fallout, respectively (from Sigurdsson et al., 1985); dashed gray line and arrows represent area invaded by A.D. 79 pyroclastic density currents (from Gurioli, 1999). Inset left—map of Pompeii ruins; yellow areas—portions of ruins still buried by undisturbed A.D. 79 deposits; dots—studied sections; inset upper right—location of Vesuvius area (circled) in Italy.
Newall and Punongbayan, 1996; Baxter et al., 1998; Valentine, 1998; Cioni et al., 2000; Gurioli et al., 2002; Druitt and Kokelaar, 2002; Luongo et al., 2003a, 2003b; Nunziante et al., 2003; Petrazzuoli and Zuccaro, 2004; Zuccaro and Ianniello, 2004; Spence et al., 2004). However, few quantitative data are available on the interaction of PDCs with urban environments. In this article we quantify and investigate the rapid variations in both flow directions and temperature across a PDC entering an urban area. We obtained these results by making descriptions and measurements in deposits cropping out within and around the archaeological ruins of Pompeii. The Roman town, 9 km southeast of Vesuvius (Fig. 1), Italy, was first covered by 2.5 m of air-fall pumice before being partially destroyed and buried by several PDCs (Fig. 1) during the A.D. 79 eruption (Lirer et al., 1973; Sheridan et al., 1981; Sigurdsson et al., 1985, 1990; Carey and Sigurdsson, 1987; Cioni et al., 1992, 1995; Luongo et al., 2003a). Here we present data related to the EU4pf deposits, which mark the abrupt switch from sustained Plinian activity to the caldera collapse phase of the A.D. 79 eruption (Cioni et al., 1992). These deposits were emplaced by a very energetic, turbulent, stratified PDC and display the highest degree of interaction with those structures that protruded through the Plinian fall deposits. METHODS Our initial field-based analysis involved detailed mapping (Pareschi et al., 2000), logging, and sampling of known and newly discovered outcrops within and around Pompeii (Fig. 1) to reveal variations in the geometry, texture, and sedimentary structure of the deposits. We then made rock-magnetic measurements to quantify the PDC flow directions and deposit temperatures. We obtained the flow direction by studying the anisotropy of magnetic susceptibility of the fine matrix of the deposits (Knight et al., 1986; MacDonald and Palmer, 1990; Hillhouse and Wells, 1991). More than 70 decimeter-sized samples were collected and 175 cylindrical specimens were obtained. The magnetic susceptibility and its anisotropy
q 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. Geology; June 2005; v. 33; no. 6; p. 441–444; doi: 10.1130/G21294.1; 4 figures.
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Figure 2. A.D. 79 deposits inside room (Castricio, site 10, Fig. 3). Right—nomenclature of A.D. 79 pyroclastic sequence (from Cioni et al., 1992, 2004). EU3—gray Plinian fallout deposit; EU3pf—pyroclastic density current (PDC) deposit from total collapse of Plinian column; EU4—lithic-rich, short-lived fallout layer; EU4pf—PDC deposit generated by total collapse of lithic-rich, short-lived column. EU4pf deposit is stratified and contains pieces of collapsed wall and large amounts of entrained debris (outlined in white contours).
were measured using a KLY-3 bridge and interpreted by equal-area stereographic projections. The mean flow direction at each site is given by the plunge of imbrication of the magnetic foliation, obtained through Jelinek (1977) statistics. Imbrication yields a better estimate of the flow direction than magnetic lineation, which may wander within the foliation plane and even be orthogonal to the flow direction (Tarling and Hrouda, 1993, and references therein).
Following Aramaki and Akimoto (1957), McClelland and Druitt (1989), and Cioni et al. (2004), the temperature of the deposits (Tdep; Cioni et al., 2004) was derived from the thermal remanent magnetization of the lithics and stripped roof tiles within EU4pf deposits. From these data we were able to estimate the temperature they reached after having been heated by the hot deposits. At least 5 lithic or tile fragments were collected from each unit at each site for a total of 269 and 85 samples,
Figure 3. Variations in EU4pf flow direction (anisotropy of magnetic susceptibility [AMS] data) and temperature (thermal remanent magnetization [TRM] data). Dots indicate sampled sites. Paleocontours (m) before A.D. 79 eruption are in black. AMS data: circles— equal-area projections of magnetic foliation; large arrow—site mean direction, given by sense of imbrication of magnetic foliation. TRM data: each temperature, in red, is defined by overlap of at least 10 individual specimens.
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respectively. The cores were demagnetized during progressive steps (10–14), with temperature intervals of 20–40 8C. The possibility of sampling building fragments makes the Pompeii deposits particularly suitable for such a study. As discussed in Evans and Mareschal (1986), Marton et al. (1993), Zanella et al. (2000), and Cioni et al. (2004), building fragments within the deposits are reliable archaeomagnetic thermometers because they were at ambient temperature when they were entrained by the current. Lithic fragments may be hotter than the current (e.g., if ripped from the volcanic conduit; Bardot, 2000) and thus provide an overestimated Tdep value. Finally, integration of data allowed us to construct a model for the behavior of PDCs entering and moving through a town. STRUCTURE INFLUENCES ON PYROCLASTIC DENSITY CURRENT DEPOSITION AND FLOW DIRECTION In the Vesuvius area, EU4pf deposits were emplaced by an aggradational, turbulent, stratified PDC that lasted for 8–10 min (Gurioli, 1999) and transported and deposited sediment in the style of a high-density turbidity current (Cioni et al., 2004). At Pompeii, as elsewhere, the deposits display an upward-fining sequence from massive to cross-stratified ash, capped by a pisolite-bearing ash bed (Fig. 2). These deposits draped Pompeii, thickening to a maximum of 3 m across roads or upflow of obstacles, and thinning to a minimum of 0.1 m over and around walls as well as downflow of obstacles. An increase in the incidence of internal structures, such as dunes or sigmoid lenses, as well as an increase in entrainment of eroded debris, such as pieces of walls, tiles, and human bodies, are clear evidence of local increases in turbulence that were caused by passage of the current over the rough urban surface. In addition, we defined the urban disturbance of EU4pf by quantifying its flow direction at the moment of its deposition, by examining the magnetic fabric of the fine matrix of the deposits. The data (Fig. 3) show how building-induced roughness was able to strongly influence flow directions. Analysis of outcrops located upflow of Pompeii show that the mean direction of the flow before it ran into the city was ;1708N. Only in open areas outside the city were the currents undisturbed, so that local flow direction was similar to that of the main flow (Fig. 3, black arrows). The irregular topography of the town, however, was able to divide the high-concentration lower part of the current into several streams that followed the external walls of the city and filled roads within the city (Fig. 3, green arrows). The most significant deviations were found in the deposits inside rooms or upflow and downflow of localized obstacles (such as
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walls), where in some cases local flow direction was opposite to the main flow direction (Fig. 3, orange arrows). Reverse motions have also been inferred from patterns in building damage such as door-hinge alignments (Luongo et al., 2003a). DEPOSIT TEMPERATURE VARIATION WITHIN POMPEII We also investigated the temperature of the deposits to record any thermal variation caused by the urban settlement in concomitance with the disturbance in the flow direction. Temperature data display large variability, ranging from 100 to 300 8C (Fig. 3). In nonurban areas around the volcano, all sites yield uniform temperatures of ;300 8C, suggesting thermal homogeneity of the transport system of the EU4pf deposits in the Vesuvius area (Cioni et al., 2004). However, a large decrease in temperature occurs within Pompeii, where the lowest values have been found in correspondence with the most deviated flow directions. Downflow of the city walls, where there are no morphological or urban disturbances, the deposit temperatures are high (Fig. 3, site 22). DISCUSSION: IMPACT OF THE PYROCLASTIC DENSITY CURRENT ON THE TOWN Invasion of Pompeii by EU4pf was almost instantaneous. If we assume flow velocities of 50–60 m/s (Esposti Ongaro et al., 2002), the EU4pf front took ,10 s to cross the town. The first building hit by EU4pf was Villa dei Misteri (Fig. 3, site 2a), a large, luxurious suburban villa located NE of Pompeii on steep slopes. Following Zajic et al. (2003), the upflow-oriented magnetic direction found on the downflow side of the villa (Fig. 3, site 2c) can be explained by generation of a horizontal vortex in the wake of a bluff body with a nonzero incidence angle of the main flow relative to the building. A few seconds later, the PDC entered the city. The current flowed around the external city walls (Fig. 3, sites 5, 6, and 7) and was channeled by street canyons and walls, traveling sometimes in a direction oblique to that of the prevailing flow (Fig. 3, site 10). It was also able to move up side streets oriented at right angles to the main flow direction (Fig. 3, site 24), a behavior similar to that found in wind-tunnel experiments (Theurer et al., 1996). At the time of the EU4pf impact, almost all of the houses had collapsed roofs, a result of the weight of pumice fallout (Macedonio et al., 1988). Site 12a (Fig. 3) is an example where three rooms with collapsed roofs are aligned parallel with the main flow direction, and thus represent rectangular cavities over which the flow moved (Fig. 4A). Inside each room (Fig. 4B), rock-magnetic measurements
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Figure 4. Rectangular cavities over which pyroclastic density current moved and interpretation of anisotropy of magnetic susceptibility (AMS) and thermal remanent magnetization data. A: Rooms 1, 2, and 3 in Castricio area (site 12a, Fig. 3). Black arrow—mean flow direction inferred for undisturbed upper part of current. B: Threedimensional sketch of rooms 1, 2, and 3, where deposits in room 2 are described in Figure 2. Light gray arrows— backflow directions from AMS, related to lower part of current; black arrow— mean flow direction of upper part of current; deposit temperature, Tdep, in bold. C: Computed wind field around group of identical buildings (modified from Xia and Leug, 2001). Height of reference building in wind tunnel, Ho, is in meters. Strong counterclockwise vortex is found between buildings, while upper current continues undisturbed.
indicate the development of a single vortex, similar to the skimming flow that develops between buildings when the ratio of the space between the buildings and their height is ,1 (Oke, 1987). The drastic temperature decrease within these rooms (from 300 8C to 100–160 8C, Fig. 4B) is evidence of a division between the main upper current and the vortices occurring inside the cavities at the base of the current (Fig. 4C). These vortices, which lasted minutes, could have induced mixing with the local ambient air, decreasing the temperature at the base of the current and, consequently, in the emplaced deposits. The lack of bodies at this site excludes a cooling effect of the deposits due to vaporization of human remains (Mastrolorenzo et al., 2001). The relatively
high amount of building materials also seems not to play an important role in cooling the deposits, as discussed in Cioni et al. (2004). About 10 s after the current encountered the NW city walls, the flow front reached the SW walls. It exited through the SW city gates and/ or passed over the 10-m-high walls. At Porta di Stabia (Fig. 3, site 21), the current passed through an arch constructed between two equal-height walls. Simulations that used a wind direction parallel to a central passage (Tutar and Oguz, 2002) predict that two large symmetrical vortices occur close to the central line of the passage at the arch exit, a scenario supported by the counterdirections obtained at Porta di Stabia in our rock-magnetic measurements. When the current passed over the city walls on the SW edge of the town (Fig. 3, site 25), a vortex with a vertical rotation plane, as suggested by the magnetic fabric of the deposit, developed downflow of the walls, at the base of the current. Such a situation is described by wind-tunnel experiment results (Murakami and Mochida, 1988; Tutar and Oguz, 2002), where vertically rotating vortices were observed to develop in the wake of bluff structures. In addition, the low Tdep of 100–120 8C downflow of the city wall (Fig. 3, site 25) indicates that large amounts of air were entrained during the 10 m jump over the city wall. Downflow of ancient Pompeii, the densest, lowermost portion (10–20 m high) of the EU4pf current regained its original physical characteristics, emplacing hot deposits once more (Fig. 3, site 22). However, the upper part of the current, above the level of the town’s structures, was completely unaffected. CONCLUSIONS The integration of field data and rockmagnetic measurements has allowed us to constrain the behavior of a turbulent PDC entering an urban area. Our data are in good qualitative agreement with the expected flow patterns predicted by engineering experiments and wind-tunnel simulations of single-phase flow. EU4pf had a significant impact on Pompeii, seriously damaging many structures. Given the modern, much denser urban fabric surrounding Vesuvius, the effects of any similar PDC invasion today would be more widespread. These results not only provide insights into the dynamics of PDCs entering urban areas, but also indicate the less exposed and relatively hazardous zones as well as scenarios for human populations affected by PDC, or during release of airborne pollutants or toxins. ACKNOWLEDGMENTS We are indebted to P.G. Guzzo, A. d’Ambrosio, G. Stefani, and A. Varone for archaeological assis-
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tance; A. Harris and J. Dehn for informal review; and R. Cioni, A. Neri, and M. Rosi for many discussions of these issues. We also thank K. Cashman and B. Houghton for their encouragement and L. Wilson, M. Ort, and V. Manville for their thorough and helpful reviews. M. Lanfranco and S. Ranieri contributed to field work. This work was supported by the European Commission, Project Exploris EVR1-CT-2002-40026, CNR-Progetto Finalizzato Beni Culturali, and the University of Torino. REFERENCES CITED Aramaki, S., and Akimoto, S., 1957, Temperature estimation of pyroclastic deposits by natural remanent magnetism: American Journal of Science, v. 255, p. 619–627. Bardot, L., 2000, Emplacement temperature determinations of proximal pyroclastic deposits on Santorini, Greece, and their implications: Bulletin of Volcanology, v. 61, p. 450–467. Baxter, P.T., Neri, A., and Todesco, M., 1998, Physical modeling and human survival in pyroclastic flows: Natural Hazard, v. 17, p. 163–176. Blong, R.J., 1984, Volcanic hazards: Sydney, Australia, Academic Press, 424 p. Branney, M.J., and Kokelaar, P., 2002, Pyroclastic density currents and the sedimentation of ignimbrites: Geological Society [London] Memoir 27, 143 p. Carey, S., and Sigurdsson, H., 1987, Temporal variations in column height and magma discharge rate during the A.D. 79 eruption of Vesuvius: Geological Society of America Bulletin, v. 99, p. 303–314. Cioni, R., Marianelli, P., and Sbrana, A., 1992, Dynamics of the A.D. 79 eruption: Stratigraphic sedimentological and geochemical data on the successions from the Somma-Vesuvius southern and eastern sectors: Acta Vulcanologica, v. 2, p. 109–124. Cioni, R., Civetta, L., Marianelli, P., Metrich, N., Santacroce, R., and Sbrana, A., 1995, Compositional layering and syn-eruptive mixing of periodically refilled shallow magma chamber: The 79 A.D. Plinian eruption of Vesuvius: Journal of Petrology, v. 36, p. 739–776. Cioni, R., Gurioli, L., Sbrana, A., and Vougioukalakis, G., 2000, Precursory phenomena and destructive events related to the Late Bronze Age Minoan (Thera, Greece) and A.D. 79 (Vesuvius, Italy) Plinian eruptions: Inferences from the stratigraphy in the archaeological areas, in McGuire, B., et al., eds., The archaeology of geological catastrophes: Geological Society [London] Special Publication 171, p. 123–141. Cioni, R., Gurioli, L., Lanza, R., and Zanella, E., 2004, Temperatures of the A.D. 79 pyroclastic density current deposits (Vesuvius, Italy): Journal of Geophysical Research, v. 109, p. 1–18. Druitt, T.H., and Kokelaar, B.P., eds., 2002, The eruption of Soufrie´re Hills Volcano, Montserrat, from 1995 to 1999: Geological Society [London] Memoir 21, 664 p. Esposti Ongaro, T., Neri, A., Todesco, M., and Macedonio, G., 2002, Pyroclastic flow hazard assessment at Vesuvius (Italy) by using numerical modeling. II. Analysis of flow variables: Bulletin of Volcanology, v. 64, p. 178–191. Evans, M.E., and Mareschal, M., 1986, An archaeomagnetic example of polyphase magnetization: Journal of Geomagnetism and Geoelectricity, v. 38, p. 923–929. Gurioli, L., 1999, Flussi piroclastici: Classificazioni e meccanismi di trasporto e di messa in posto [Ph.D. thesis]: Pisa, University of Pisa, 320 p. Gurioli, L., Cioni, R., Sbrana, A., and Zanella, E.,
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