Influences of urban fabric on pyroclastic density currents at by dfgh4bnmu


									                                              Influence of town on PDC temperature

 1   Influences of urban fabric on pyroclastic density currents at Pompeii (Italy), part II:

 2   temperature of the deposits and hazard implications

 3   E. Zanella

 4   Dipartimento di Scienze della Terra, Università di Torino, Via Valperga Caluso 35, 10125, Torino, Italy

 5   ALP – Alpine Laboratory of Paleomagnetism, Peveragno, Italy

 6   L. Gurioli

 7   Istituto Nazionale di Geofisica e Vulcanologia, Via della Faggiola 32, 56126, Pisa, Italy

 8   Present address: Geology & Geophysics, University of Hawaii, 1680 East-west Rd, Honolulu HI 96822, USA

 9   M.T. Pareschi

10   Istituto Nazionale di Geofisica e Vulcanologia, Via della Faggiola 32, 56126, Pisa, Italy

11   R. Lanza

12   Dipartimento di Scienze della Terra, Università di Torino, Via Valperga Caluso 35, 10125, Torino, Italy

13   ALP – Alpine Laboratory of Paleomagnetism, Peveragno, Italy


15           During the AD 79 eruption of Vesuvius, Italy, the Roman town of Pompeii, was covered by

16   2.5 m fallout pumice and then partially destroyed by pyroclastic density currents (PDCs). Thermal

17   remanent magnetization (TRM) measurements performed on the lithic and roof tile fragments

18   embedded in the PDC deposits allow us to quantify the variations in the temperature (Tdep) of the

19   deposits within and around Pompeii. These results reveal that the presence of buildings strongly

20   influenced the deposition temperature of the erupted products. The first two currents which entered

21   Pompeii at a temperature around 300-360 ºC, show drastic decreases in the Tdep, with minima of

22   100-140 ºC found in the deposits within the town. We interpret these decreases in temperature as

23   being the result of localized interactions between the PDCs and the city structures, which were only

24   able to affect the lower part of the currents. Down flow of Pompeii, the lowermost portion of the

25   PDCs regained its original physical characteristics, emplacing hot deposits once more. The final,

26   dilute PDCs entered a town that was already partially destroyed by the previous currents. These

27   PDCs left thin ash deposits which mantled the previous ones. The lack of interaction with the urban

                                        Influence of town on PDC temperature

28   fabric is indicated by their uniform temperature everywhere. However, the relatively high

29   temperature of the deposits, between 140 and 300 ºC, indicates that even these distal, thin ash

30   layers, capped by their accretionary lapilli bed, were associated with PDCs that were still hot

31   enough to cause problems for unsheltered people.

32   KEYWORDS: Pompeii, temperature, magnetic fabric, pyroclastic density currents.


34   1. Introduction

35         Pyroclastic density currents (PDCs) are the primary cause of death during explosive eruptions

36   [Tanguy et al., 1998]. These currents are hot mixtures of gas, pumice and lithic fragments, ranging

37   in size from fine ash up to metric blocks and bombs [e.g. Freundt and Bursik, 1998; Druitt, 1998].

38   In the proximal locations their destructive effect is mainly due to their high momentum and

39   temperature [e.g. Baxter et al., 2005]. In distal locations, where the currents have already lost a

40   large portion of their solid load, their hazard is associated with their high concentration in fine ash

41   [e.g. Horwell and Baxter, 2006], coupled with their high velocity and temperature [e.g. Todesco et

42   al., 2002]. Thus, in order to assess the hazard posed by PDCs to human populations, it is extremely

43   important to understand PDC dynamics and their physical characteristics. In addition, educational

44   efforts, based on scientific analyses of PDC emplacement and their effects, will likely improve the

45   chances of survival of, or correct response to, future eruptions.

46         Observations made in villages and areas devastated by PDCs report evidence of sudden death

47   and survival among groups of people sheltering in the same house and even in the same room, as

48   during the eruption of Mt. Pelée, Martinique, in 1902 [Anderson and Flett, 1903,]. There is also

49   evidence for survival of certain individuals, when the majority of people in area impacted by PDC

50   were killed, as for example during the 1980 eruption of Mt St Helens (USA) [Bernstein et al., 1986]

51   or during the dome-collapse-fed pyroclastic flows of 1991 at Mount Unzen [Baxter et al., 1998]. In

52   addition, during the 1902 eruption of Mt. Pelée un-burnt material was found in close proximity to

53   incinerated objects [Lacroix, 1904]. The variable impact PDCs on human populations has been

                                         Influence of town on PDC temperature

54   explained at Montserrat in terms of survivors being located in areas marginal to the PDCs [Baxter et

55   al., 2005]. However, this explanation cannot be applied for unconfined, diluted PDCs, as shown by

56   Gurioli et al. [2005]. Further investigations are thus required if we are to understand these local

57   variations in PDC dynamics, and how a PDC interacts with a town and its human population.

58         We attempt to resolve this issue through analysis of the PDC deposits of the AD 79 eruption

59   of Vesuvius (Italy) which crop out within and around the Roman town of Pompeii. All PDCs that

60   entered the town, even the most dilute ones, were density stratified currents whose lower part

61   interacted with the urban fabric [Gurioli et al., submitted]. We use a combination of thermal

62   remanent magnetization (TRM) and anisotropy of the magnetic susceptibility (AMS) analyses to

63   obtain information regarding flow direction of these parent currents and deposit temperatures.

64   These data, when integrated with volcanological field investigations, reveal that the presence of

65   buildings strongly affected the distribution and accumulation of the erupted products; where the

66   results of our analyses from the single, most destructive unit in the sequence have been presented in

67   Gurioli et al. [2005]. Here, we build on these previous findings by now considered the TRM results

68   from the entire eruptive sequence cropping out within and around Pompeii to infer the temperature

69   of all of the deposits. This allows us to assess the cooling effect of the urban fabric on these currents

70   and the effect of these currents on the inhabitants.


72   2. Volcanological setting and sampling

73          Pompeii is located 9 km southeast of Vesuvius (Figure 1) and the following

74   reconstruction of events during the AD 79 eruption at that location is based on the analysis of

75   Sigurdsson et al. [1985], Carey and Sigurdsson, [1997] and Cioni et al. [2000]. The town first

76   experienced 7 hours of air fall comprising white pumice, lapilli and bombs (up to 3 cm in

77   diameter), with scarce lithic blocks of up to 3 cm in diameter. This emplaced unit EU2 (Figure

78   2). The town then underwent 18 hours of grey pumice, lapilli and bombs (up to 10 cm in

79   diameter) fallout to emplace EU3 (Figure 2); a deposit that is rich in lithic blocks of up to 7 cm

                                         Influence of town on PDC temperature

 80   in diameter. During this Plinian phase, some PDCs were generated by the discontinuous collapse

 81   of marginal portions of the convective column. Only the last of these events (EU3pf1, Figure 2)

 82   was able to reach the north-western edge of Pompeii, but did not enter the town itself. This PDC

 83   left only a 2-3 cm thick ash layer interbedded with the fallout deposit. The town was then

 84   completely covered by a 5-30 cm thick ash layer (EU3pf, Figure 2) emplaced by very dilute

 85   PDCs, derived from the total collapse of the Plinian column. Locally EU3pf was able to interact

 86   with obstacles of a few decimetres in height, suggesting that its denser, thin lower part was

 87   capable of only filling minor negative depressions [Gurioli et al., submitted].

 88          EU3pf was next mantled by a 3-6 cm thick, lithic rich (blocks up to 3 cm in diameter),

 89   grey pumice, lapilli and bomb (up to 3 cm in diameter) fall deposit (EU4, Figure 2), emplaced

 90   from a second, short-lived, lithic-rich column. The collapse of this second column generated the

 91   most powerful, turbulent, PDC of the eruption. This was able to partially destroy the town and

 92   left relatively coarse-grained, cross stratified, meter-thick deposits (EU4pf, Figure 2). EU4pf was

 93   able to interact with obstacles of a few meters in height, showing significant interaction with the

 94   town [Gurioli et al., 2005].

 95          Finally, the settlement was buried by a 1 m thickness of very dilute PDC and air-fall

 96   deposits (EU7 and EU8, Figure 2) consisting of ash and accretionary lapilli emplaced during the

 97   last, phreatomagmatic phase of the eruption. The EU7 sequence (Figure 2) comprises two

 98   centimetre-thick grain supported, lithic-rich lapilli beds, separated by a 1-to-3-cm thick cohesive

 99   ash layer, and capped by a coarse ash layer, which is in turn covered by a massive pisolite

100   bearing fine ash bed. EU8 then comprises an alternation of normally graded, ashy bedsets, each

101   up to 10 cm thick (Figure 2). Each bedset is characterised at its base by a massive-to-crudely

102   stratified facies followed by accretionary lapilli facies. In general, the EU7 and EU8 deposits

103   mantled a town that had already been severely damaged by the preceding currents (mainly

104   EU4pf) [Gurioli et al., submitted].

                                        Influence of town on PDC temperature

105          Within this sequence we sampled undisturbed sites in open country around the city, as

106   well as sites along the city walls and within the town, both along the roads and inside the rooms

107   (Figure 1). As described in Gurioli et al. [2005] we collected lithics and stripped roof tiles on

108   which we could perform TRM analyses. At least 5 lithic or tile fragments were collected from

109   each unit at each site giving a total of more than 200 samples. The tile fragments and the larger

110   lithic clasts (Figure 3) were first oriented using clinometer, magnetic compass and, whenever

111   possible, sun compass, before being removed from the outcrop. Usually two specimens were cut

112   from individual fragments to improve accuracy in the estimate of the deposition temperature

113   interval. The great majority of lithic clasts in the AD 79 deposits, and thus around 70% of our

114   samples, are fragments of a few mm in dimension (Figure 3). They were considered as un-

115   oriented samples, because their small size prevents accurate orientation. The presence of

116   fragments of plaster and roof tiles picked up and heated by the PDCs makes the Pompeii deposits

117   particularly suitable for such a study. As already discussed in Evans and Mareschal [1986],

118   Marton et al. [1993], Zanella et al. [2000] and Cioni et al. [2004], building fragments within the

119   deposits are reliable magnetic thermometers, because they were cold when they were picked up

120   by the PDC.


122   3. Measurement of deposition temperature using TRM

123       The thermal remanent magnetization (TRM) acquired during the cooling of a magmatic rock

124   records the polarity, direction and intensity of the Earth’s magnetic field at the time of cooling.

125   Such paleomagnetic information is widely used in geodynamics and stratigraphy. However,

126   thorough investigation of the TRM features may yield information on the physical processes

127   which led the remanence acquisition and help in understanding the emplacement mechanisms of

128   magmatic rocks. This applies to pyroclastic deposits, whose formation results from combination

129   of thermal and sedimentological processes. Paleotemperature investigation of pyroclastic rocks,

130   emplaced by PDCs, was pioneered by Aramaki and Akimoto [1957] and Chadwick [1971]. Their

                                        Influence of town on PDC temperature

131   assumption was straightforward. If a deposit was emplaced hot (where hot means at higher

132   temperature than the Curie point of magnetite), then the primary remanence of the embedded

133   lithic fragments would have been erased. A secondary remanence was then re-acquired during

134   the subsequent cooling. All fragments were magnetized at the same time and their TRM

135   directions are therefore well clustered. In contrast, if the emplacement temperature was cold each

136   fragment would have retained its own primary TRM acquired when the parent rocks formed. In

137   this case, the TRM directions are randomly distributed because of the chaotic movements during

138   emplacement.

139       Following Chadwick [1971], Hoblitt and Kellog [1979] applied a more quantitative

140   procedure, based on thermal demagnetization of the fragments. This technique allows derivation

141   of the TRM blocking temperature (Tb) spectrum and thus identification of the distinct TRM

142   components acquired in different temperature ranges, usually referred to as high-Tb and low-Tb

143   components. Such an analysis yields an estimate of the actual temperature reached by the

144   fragment upon re-heating within the pyroclastic material, this being the maximum unblocking

145   temperature (Tbmax) of the low-Tb component.

146       Further work has improved this methodology [e.g. McClelland and Druitt, 1989; Bardot,

147   2000] and have revealed some complications to the initial assumptions. According to the

148   simplest model, paleomagnetic estimates of the deposition temperature relies on two basic

149   assumptions:

150   1) During PDC transport and deposition heat is transferred from the hot gas and fine-grained

151   material to the cold clasts. For clasts larger than 2-5 cm, thermal equilibrium is mainly reached

152   during residence in the deposit rather than the emplacing current. In fact, the time required to

153   reach thermal equilibrium within the current is longer than the time of residence in the flow

154   [Cioni et al., 2004]. For this reason, rock magnetic investigations give an estimate of the deposit

155   (Tdep) rather than the emplacement (Temp) temperature.

156   2) The clasts’ remanence is a pure thermal remanent magnetization (TRM).

                                        Influence of town on PDC temperature

157   In cases where these assumptions are fulfilled, the natural remanent magnetization (NRM) of a

158   clast consists of two TRM components characterized by different blocking temperatures (Tb).

159   The high-Tb component pre-dates the PDC emplacement. This was acquired when the lithic clast

160   was originally formed. A part of this remanence is erased when the clast is re-heated within the

161   PDC and then acquired as low-Tb component when the PDC comes to rest and cools down.

162   Considering the full Tb spectrum of the clast, Tbmax is the temperature value which separates the

163   two components. It can be identified by stepwise thermal demagnetization because the directions

164   of the two components are different (Figure 4). The low-Tb direction is close to that of the

165   geomagnetic field at the time of the PDC emplacement, and it is the same in all clasts. The high-

166   Tb direction is fully random. According to the first assumption above, Tdep may thus be derived

167   from the mean Tbmax value derived from a number of clasts.

168       Estimation of Tdep, however, is not always as straightforward as in the above case

169   [Grubensky et al., 1998]. The first problem is that we do not know the thermal history of the

170   clast. It might have been either picked up cold along the slopes of the volcano, or ejected hot

171   from the conduct, possibly being even hotter than the PDC within which it became entrained. In

172   the second case, when the current stops the lithic fragment is still hotter than the surrounding

173   fine-grained matrix and the Tbmax value found from rock-magnetism will be higher than Tdep.

174       The second problem concerns NRM which, in addition to the thermal (TRM) components,

175   may also comprise chemical (CRM) and viscous (VRM) components. It has been shown by

176   McClelland [1996] that a chemical remanence (CRM) may develop due to mineralogical

177   changes during reheating. This will hinder the identification of the low-Tb and high-Tb

178   components (Figure 4b). VRM is typical of ferromagnetic grains with low relaxation time, and

179   thus low blocking temperature. According to theory [Pullaiah et al., 1975, Bardot and

180   McClelland, 2000] a VRM component acquired at 20 °C in the course of the ~1930 years

181   elapsed since the AD 79 eruption, is erased by heating at 125 °C in a time of 25 to 30 minutes,

182   typical of a thermal demagnetization step in the laboratory. It may thus be indistinguishable from

                                           Influence of town on PDC temperature

183   very low Tb secondary components. Moreover, a small VRM overprint often occurs in most

184   rocks. The presence of a VRM overprint could partially affect identification of the true, low-Tb

185   TRM component and hamper the determination of its direction by principal component analysis

186   [Kirschvink, 1980].

187       The problems outlined above do not always occur and when they do they can often be

188   overcome. In the case of the Vesuvius AD 79 eruption, archaeological remains are of great help.

189   Bricks and tiles, which have their own TRM typical of baked-clay artefacts, were picked up by

190   PDCs at the ambient temperature and could not be heated at values higher than Tdep. The

191   problems with CRM are also reduced sampling clasts of as different lithologies as possible,

192   because different lithology means different magnetic properties.

193       In conclusion, the various clasts collected at an individual site may have had different

194   thermal histories and have recorded more or less faithfully the Tdep. Following the approach of

195   Cioni et al. [2004], the thermal overprint of a group of clasts by a PDC is better represented by

196   what they have in common. The Tbmax values derived from each individual clast may differ from

197   each other because of the reasons summarized above, whereas the re-heating was a single event.

198   The traces it left, Tdep, must be consistent at the site scale.


200   4. Standard measurements and peculiar cases

201       A total of 379 specimens were measured at the ALP laboratory (Peveragno, Italy) using a

202   JR-5 spinner magnetometer, and Schonstedt and ASC TD-48 thermal demagnetizers. Small bits

203   were measured using the plastic box + Plasticine technique of Cioni et al. [2004]. Thermal

204   demagnetization was carried out in steps of 40 °C between a starting temperature of 100 °C and

205   a maximum of ~520 °C. Whenever sister specimens from individual clasts were available, a

206   second demagnetization was carried out using the same 40 °C steps but starting at 80 °C. The

207   data were then interpreted using the principal component analysis available as part of the

208   Paleomac program [Cogné, 2003].

                                         Influence of town on PDC temperature

209       On the basis of the demagnetization patterns, Cioni et al. [2004] distinguished four kinds of

210   thermal behaviour in clasts embedded within the AD 79 PDC deposits. Type A is characterized

211   by blocking temperatures higher than Tdep and its primary TRM is therefore not affected by the

212   re-heating. Type B has blocking temperatures lower than Tdep and its primary TRM is completely

213   erased during the re-heating. Type C is the archetype of lithic clasts with a TRM comprising two

214   components with distinct Tb spectra, which are well evident in the Zijderveld diagrams (Figure

215   4a). In Type D the two components are not very clear because the spectra more or less overlap,

216   show a zigzag pattern, or have points that are too close to each other to be well distinguished

217   (Figure 4b). Identification of Tbmax is thus straightforward in Type C, more complicated in Type

218   D. A similar classification was adopted by McClelland et al. [2004] whose Types 1a, 1b, 2 and 3

219   respectively correspond to the Types C, D, A, B types of Cioni et al. [2004]. In the present

220   paper, we add Type E, which comprises a few tile fragments which show no evidence of re-

221   heating. The normalized intensity decay curve (Figure 4c) shows that a fraction of their

222   ferromagnetic grains have low blocking temperatures. The clasts NRM directions, however, do

223   not change throughout demagnetization up to the highest values close to the Curie point and are

224   different from that of the ambient field in AD 79. In the example, the direction of the

225   characteristic remanent magnetization (ChRM), calculated using all steps and with maximum

226   angular deviation MAD = 1° (Figure 4c), is D = 356.5°, I = 2.3°, where D is declination and I

227   inclination. We have no explanation for the occurrence of these clasts and can only consider

228   them as outliers, but which bear witness to the fact that the temperature distribution within thin

229   pyroclastic deposits is far from uniform and can in fact be highly variable.

230       The pie diagram in Figure 5 summarizes the per cent occurrence of the five types in all

231   clasts investigated in the present paper. Type C accounts for about 50 % of the clasts, Type D for

232   about 45%, with the remainder are either being Type A or E. No Type B clasts were found in the

233   present investigation.

                                        Influence of town on PDC temperature

234       An uncommon case is given by the plaster fragment sampled at site 18 (Figure 1). Hueda-

235   Tanabe et al. [2004] have shown that plaster may be used as archaeomagnetic material. Small

236   ferromagnetic grains are free to move when plaster is applied to a wall or floor, orienting their

237   magnetic moment parallel to the Earth’s magnetic field and then being blocked when the plaster

238   dries. The vector sum of the NRM of a plaster specimen is thus given by the NRM of its

239   individual grains. The process is similar to the orientation of the grains of ferromagnetic

240   pigments in red coloured murals, which also have been shown to record the ambient field

241   direction at the time of painting [Zanella et al., 2000]. The plaster sampled at Pompeii is a kind

242   of pozzolana, one of the most outstanding results of Roman civil engineering. It was made from

243   lime and grains of volcanic rocks from the sandy deposits of neighbouring rivers. A large

244   fraction of grains are 1-2 mm in size (Figure 6a), too large to be effectively oriented by the

245   Earth’s field. Thus, it can reasonably be assumed that the remanence direction varies from grain

246   to grain. At a first glance the thermal demagnetization diagram looks quite odd (Figure 6b).

247   Because each grain has its own Tb spectrum, thermal demagnetization erases different fractions

248   of remanence in grains with different TRM directions, so that the direction of the resultant vector

249   varies randomly from one step to another, without any coherence. However, the low-temperature

250   demagnetization steps show a linear trend in the initial part of the Zijderveld diagrams (up to

251   180-200 °C in Figure 6b). This suggests that, in each individual grain, the fraction of remanence

252   with Tb < Tdep was erased when the plaster was re-heated to the Tdep of the pyroclastic material

253   filling the room and burying the walls. During cooling, all grains re-acquired a low-Tb

254   component showing the same direction, i.e. that of the Earth’s field, which is therefore coherent

255   throughout the specimen.

256       Another peculiar case is that of the lithics embedded in the fall deposits. The 16 lithics we

257   sampled were characterized by three TRM components: these being the expected high- and low-

258   Tb components, as well as an intermediate-Tb component with a distinct direction around

                                          Influence of town on PDC temperature

259   temperatures of 340 to 480 °C (Figure 7). No evidence for an intermediate-Tb component was

260   found in the tile fragments embedded in these deposits.

261       As discussed in the previous section, most of our samples were fragments too small to be

262   oriented in the outcrop. Out of a total of 379 specimens, 145 were oriented and 75 gave two

263   clearly isolated components whose directions could be transformed to the geographical reference

264   system. The directions of the high-Tb component were widely scattered, as expected (Figure 8);

265   those of the low-Tb were more clustered. The site mean ChRM direction (D = 350.4°, I = 61.8°,

266   Fisher’s semi-angle of confidence α95 = 7.4°) is close to the AD 79 Earth’s magnetic field

267   direction as given by archaeomagnetism [Tema et al., in press], even if its statistical definition is

268   lower than usual in paleomagnetic investigations. This is often the case in paleotemperature

269   investigations [McClelland et al., 2004; Tanaka et al., 2004]. In our case, this is mainly due to

270   the fact that the analysis of the low-Tb component relies on few clustered points due to the low

271   rate of decrease in the intensity of magnetization. Furthermore it may also be biased by a VRM.

272   Even if the low-Tb directions of individual clasts show some dispersion, their overall mean (D =

273   352.0°, I = 53.7°, number of specimens N = 75, length of the resultant vector R = 69; α95 = 4.9°)

274   passes the Watson [1956] randomness test and is consistent with the archaeomagnetic direction

275   (D = 354.3°; I = 58.0°; α95 = 1.7°) derived from various materials studied at Pompeii

276   (archaeological remains, fine-grained pyroclastics, lithic clasts) [Evans and Hoye, 2005; Tema et

277   al., in press and references therein].

278       Final estimation of paleotemperature was completed following the technique of Cioni et al.

279   [2004]. For each site and each eruptive unit, first the temperature interval which contains the

280   maximum unblocking temperature (Tbmax) of the low-Tb component of each individual clast was

281   derived from the demagnetization path. This was done by analysis of the normalized intensity

282   decay curve, the Zijderveld diagrams and the equal-area plot of directions. As a conservative

283   approach, directions separated by less than 15° were not considered as significantly different

284   [Porreca, 2004], because a small angular deviation might be due to post-depositional settling of

                                         Influence of town on PDC temperature

285   the fragment within the still unconsolidated rock. Tdep of each site and eruptive unit was then

286   estimated from the overlap range of the temperature intervals of the individual fragments

287   (Figures 9a, 9b, 9c1, 9c2, 9c3 and 9d).


289   5. The temperature of the AD 79 deposits around and within Pompeii

290          In the following discussion we present our data for the whole eruptive sequence present

291   in Pompeii, unit by unit (Figures 9a, 9b, 9c1, 9c2, 9c3 and 9d). Where ever possible, we compare

292   these results with the data obtained for the same units cropping out around Vesuvius by Cioni et

293   al. [2004].

294   5.1 The fallout sequence (EU2-EU3 and EU4)

295          We collected just a few samples from the three main fallout deposits (EU2-EU3 and

296   EU4), mostly to check whether they were emplaced hot and whether they were able to maintain a

297   temperature high enough to heat artefacts. As already shown by several authors [e.g. Thomas and

298   Sparks, 1992; Tait et al., 1998; Hort and Gardner, 2000] pumice clasts larger than 6 cm in

299   diameter suffer little heat loss during their fall and can be emplaced at temperatures within 10-20

300   % of their magmatic temperature [Thomas and Sparks, 1992]. Thus Plinian deposits, depending

301   on their grain size, thickness and distance from the vent, can remain sufficiently hot to pose

302   hazards to life and property [Thomas and Sparks, 1992]. For the fallout deposits in Pompeii, we

303   found that the white fallout deposit had a temperature high enough to warm the tiles up to 120-

304   140 °C (Figure 9a). Unfortunately, we could not collect any artefacts in the grey fallout, EU3,

305   but we can assume that this deposit was even hotter than the white one, because of its coarser

306   grained texture.

307          Within both the EU3 and EU4 fall deposits we also sampled lithic blocks. As previously

308   discussed, all these lithics are characterized by three components (Figure 7). We assume that,

309   while the high-Tb component was acquired during clast formation, the low-Tb component

310   represents the temperature of the lithic at the point at which it fell to the ground. If this

                                        Influence of town on PDC temperature

311   hypothesis is correct the lithic fragments reached Pompeii at a minimum temperature of 180-220

312   °C during the EU3 fallout and 220-260 °C during the EU4 fallout (Figure 9a). We do not fully

313   understand the meaning of the intermediate temperature component. It may represent heating

314   during passage as part of the gas-thrust section of the plume and/or cooling experienced in the

315   umbrella portion of the plume.

316   5.2 The first PDC entering Pompeii (EU3pf)

317          EU3pf deposits were sampled around and within Pompeii (Figure 10a). They show a

318   large variation in their Tdep, ranging from 140 to 300 °C (Figures 9b and 10a). Nowhere else

319   around Vesuvius do we find such high variability in EU3pf deposit temperatures [Cioni et al.,

320   2004] (Figure 10b). Even the coldest outcrops located north of Vesuvius are relatively warm in

321   comparison to those within Pompeii (Figure 10).

322          EU3pf in Pompeii shows the highest Tdep (240-300 °C) in the northern sector (Figures 9b

323   and 10a), outside the town. However, these values are lower than the 300 to 360 °C values

324   obtained for the EU3pf deposits emplaced upstream of Pompeii, in the proximal sector (Figure

325   10b). We explain these results as the consequence of uniform cooling experienced by the current

326   at this distance from the vent, due to the reduction of its total load which will decrease its

327   thermal energy.

328          Lowest temperatures were recorded within the town and in the western sector, where Tdep

329   drops to 140-220 °C. Slightly higher values have been found in rural areas south of Pompeii,

330   where the Tdep is 220-260 °C [Cioni et al., 2004] (Figure 9b and 10a). These temperatures show

331   that the EU3pf current, even if diluted and capable of only emplacing thin deposits, entered

332   Pompeii with a minimum temperature of 260-300 °C. The decrease in temperature within

333   Pompeii, indicates that the local interaction with the city structures had a cooling effect on the

334   lower part of the current. South of Pompeii, the EU3pf current was unable to restore the same

335   temperature it had before entering Pompeii, but it was still able to emplace hotter deposits than

336   found within Pompeii with temperatures of up to 220-260 °C.

                                        Influence of town on PDC temperature

337   5.3 The most powerful PDC (EU4pf)

338          Temperature data obtained from the thick deposits of EU4pf display the largest

339   variability in temperature at the scale of individual sampling sites, ranging from 100 to 320 °C

340   (Figures 9c1, 9c2, 9c3 and 11a). These values differ from those obtained from the same deposits

341   in non-urban areas around the volcano, (Figure 11b). In such non-urban locations, all sites yield

342   temperatures of around 300°C (260-340 °C), irrespective of their distance from the vent [Cioni et

343   al., 2004]. This relatively uniform temperature suggests a substantial homogeneity of the

344   transport system of the EU4pf deposits in the Vesuvius area [Cioni et al., 2004].

345          In contrast, a large decrease in temperature occurs within Pompeii. In sites examined

346   orientated along the axis of the flow direction (from the northwest edge of the city towards its

347   southern edge), temperatures generally decline. As found for EU3pf, EU4pf also has highest Tdep

348   values in the northern sector of Pompeii, where the Tdep range from 240 to 320 °C, with an

349   average value of around 280 °C. A low value of 200-240 °C was found up flow the Villa dei

350   Misteri, at site 2a (Figure 11a). We speculate that this anomalous value may be due to the

351   presence of some structure up flow of this area, or some morphological high, not now visible

352   because is covered by the deposits and the modern soil and vegetation.

353          Inside the town Tdep ranges from 100 to 220 °C, with an average value of 160 °C. Here

354   the lowest values are found in three rooms with collapsed roofs, aligned parallel with the main

355   flow direction. The lowest value that we found is located in the third of these three rooms, i.e.

356   the furthest down flow [Gurioli et al., 2005]. These low values are consistent with cooling due to

357   strong disturbances caused by the town and morphological features, such as the 3-meter-deep

358   cavities presented by rooms with collapsed roofs (Figure 11a, sites 10, 12a and 12b), the 10-m-

359   high cliff on the southern edge of the town (Figure 11a, sites 22c and 25), or collapsed walls

360   (Figure 11a, sites 11 and 16). Down flow of the city walls, where there are no morphological or

361   urban disturbances, the deposit temperatures are high once more (Figure 11a, sites 22a, 22b, 19a

362   and 19b).

                                         Influence of town on PDC temperature

363          These results show that the presence of the settlement resulted in substantial cooling of

364   the current over short distances. Roughness of the topographic/depositional surface increases the

365   ability of the basal portion of the flow to decouple from the main flow and to form local

366   vortexes, ingesting ambient air. Increases in turbulence, due to the surface irregularities caused

367   by the presence of the town, are evident from upstream particle orientations which develop

368   down-flow of obstacles or inside cavities [Gurioli et al., 2005] and from characteristic

369   sedimentary structures such as fines-poor, undulatory, lenticular bedded facies on the lee side of

370   the obstacles [Gurioli et al., submitted]. This would also cause air ingestion. Air ingestion into

371   the lower system of the EU4pf current during passage over the urban canopy is the most

372   reasonable cause of the observed strong temperature decreases. As shown in Cioni et al. [2004],

373   the very high thermal diffusivity of air with respect to magma, and the intimate mixing between

374   the air and gas-ash mixture, results in instantaneous thermal equilibrium during this process.

375   Furthermore, the EU4pf current lasted for 8-10 minutes [Gurioli, 1999]; because we witness

376   cooling this interval of time must be sufficient for the lower part of the current to entrain air and

377   undergo cooling.

378          The amount of building material entrained by the current seems not to play an important

379   role in cooling of the deposits, where we found no correlation between low temperatures and

380   amount of building material. Furthermore, tile-fragment-rich zones within EU4pf deposits are

381   present as small lenses (around 1-2 m long, 1 m high and less than 1 m wide) which probably did

382   not have sufficient volume or extent to cool the deposits.

383          The roof tiles have very high thermal conductivity, as a result they will heat up very

384   quickly. We estimate the characteristic time it takes to heat a cool object (roof tile) buried in a

385   hot medium (the deposit) from τ = D2/α, in which D is the object dimension and α is the thermal

386   diffusivity of the object. Thermal diffusivity is calculated using the density and specific heat

387   capacity for common brick [obtained from Holman, 1992], as well as the temperature dependent

388   thermal conductivity which we calculate for clay following Vosteen and Schellschmidt [2003].

                                           Influence of town on PDC temperature

389   This gives τ of ~1 minute for our smallest (1 cm) objects, increasing to ~1.5 hours for our largest

390   (10 cm) objects for heating from ambient to 200-400 °C. This means that all of our objects

391   should have equilibrated with the temperature of the deposit within 1.5 hours, with the smallest

392   objects reaching equilibrium in just a few minutes. Thus, although individual tile fragments

393   entrained within the deposit were heated quickly, they robbed insufficient thermal energy from

394   the surrounding hot body to cause significant cooling.

395          As shown in Gurioli et al [submitted], the urban canopy encouraged deposition and the

396   upper portion of the current was not able to fully restore the sediment supply to the lower

397   current. Our temperature results also show that the increase in surface roughness caused by the

398   presence of the town caused strong variations in temperature. However, these variations where

399   short-lived and confined to the lower part of the current. South and south east of Pompeii, at

400   distances of up to 8 km from the town, EU4pf was able to emplace hot deposits once more

401   [Cioni et al., 2004] (Figure 11b).

402   5.4 The final PDCs (EU7 and EU8)

403          Temperature data obtained from the thin deposits of EU7 and EU8 display the lowest

404   variability in temperature at the scale of individual sampling sites. In EU7 we sampled the ash

405   interlaid between the two lapilli beds (Figure 2d). This gives a temperature of between 210 and

406   260 °C, in agreement with the temperature range (180-240 °C) found by Cioni et al. [2004]

407   between the vent area and Pompeii for this deposit. The second ash layer shows a highest

408   temperature of 260-300 °C at Site 1 (Figure 12) but around and within Pompeii Tdep ranges from

409   180-260 °C, without showing strong variations within the city. This behaviour agrees with a

410   scenario within which there was interaction between this current and a town that was already

411   partially covered by the previous deposits [Gurioli et al., submitted].

412          EU8 was sampled at just one site because we had difficulties in finding lithic fragments

413   large enough to be suitable for measurement. It displays a Tdep of 180-220 °C. A fragment of tile

414   was also collected at site 16 (Figures 9d and 12) within the coarse grained ash layer of this

                                         Influence of town on PDC temperature

415   deposit. Even if the top of the deposit shows evidence of water condensation, the deposit at the

416   base had a temperature able to heat the tile up to 130-180 °C.


418   6. Implications for volcanic hazard

419           In the early afternoon of August 24 AD 79 a pumice fall began which lasted until early in

420   the morning of the following day. During this period Pompeii was covered by 3 meters of pumice.

421   Within 6 hours the roofs and parts of the walls of the buildings had collapsed under the pumice load

422   [Sigurdsson et al.,1985; Luongo et al., 2003a]. Luongo et al. [2003a, b] identify a significant

423   number of victims within this deposit (38 % of the total number of victims, estimated to be about

424   1150). They probably died as a consequence of building collapse. Our new data reinforce this

425   reconstruction, where we find that these deposits were emplaced with sufficient heat to be able to

426   heat cold materials to 140 °C. We can therefore speculate that in some sites they could have been

427   capable of causing damage by carbonisation of wood, such as roof beams, and skin burn upon direct

428   contact. This hazard scenario was made even more dangerous by the scattered rain of large, very

429   hot, lithic blocks.

430           In the early morning of August 25, the PDCs started to enter and devastate Pompeii

431   [Cioni et al., 2000]. In Pompeii, features of both EU3pf and EU4pf show that the two currents

432   were able to interact with the urban structures even in the first, very dilute case, suggesting that

433   the currents were stratified, and capable of interacting with objects of the same height as the

434   thickness of their depositional systems [Gurioli et al., submitted]. EU3pf interacted little with the

435   fabric of the town, due to its very thin depositional system. However, its content of fine ash and

436   relatively high temperatures would have made it hazardous to the human population, causing

437   asphyxia and lung damage. Recent studies [Luongo et al., 2003a, b] suggest that the first diluted

438   PDC (EU3pf) caused minimal damage in Pompeii. This is true for the building structures, but

439   our evidence indicates that even this current was extremely dangerous for the inhabitants [Cioni

440   et al., 2000]. Here we have been able to quantify this hazard, where the hazard results from the

                                         Influence of town on PDC temperature

441   temperature of this current and its composition of fine ash. Although the current left only a thin

442   deposit, the current itself had a thickness of more than 10 m (the height of the ridge on which

443   Pompeii was built). Furthermore, the current was hot, with a temperature of 140-300 °C. This

444   temperature would have been very close to the average temperature within the current at

445   Pompeii, an assumption made plausible by the abundance of fine-grained fragments that

446   comprised the PDC at this distance from the vent [Cioni et al., 2004]. Finally, the grain-size of

447   the EU3pf deposits indicates that 40-50 % of its mass was accounted for by particles of less than

448   0.1 mm in diameter [Gurioli et al., submitted]. Such a size represents dust that can be inhaled by

449   humans. Assuming a minimum fractional particle volume concentration of about 1-5 x10-4 for

450   this current and a minimum bulk density of 1000 kg/m3 for the particles [Freundt and Bursik;

451   1998], the concentration of fine materials is between 0.03 and 0.15 kg/m3. These values fall

452   within the concentration range for inhalable dust capable of causing asphyxia [0.1 kg/m3, Baxter

453   et al., 1998] at ordinary temperatures. A temperature of 200 °C and ash concentration of 0.1

454   kg/m3 are considered threshold values above which human survival is likely to be impossible

455   [Baxter et al., 1998]. Furthermore this atmosphere would have persisted for several minutes, as

456   suggested by the low terminal velocity of the fine particles and the aggradational model

457   proposed for this current [Gurioli, 2000; Gurioli et al., submitted]. Thus, this PDC was

458   extremely hazardous and, following Baxter et al. [1998], would have led to asphyxia and severe

459   lung damage.

460          Invasion of Pompeii by EU4pf was almost instantaneous. If we assume flow velocities of

461   50-60 m/s [Esposti Ongaro et al., 2002], in agreement with the average mean grain size of the

462   deposits [Gurioli et al., submitted], the EU4pf front took less than 10 s to cross the town. This

463   second, more concentrated current had a more profound influence on the town. Its dense

464   depositional system, coupled with its high velocity, was able to tear down some walls orientated

465   at right angles to the main flow direction (i.e. east-west trending structures). This is in agreement

466   with masonry vulnerability of the old Pompeii buildings that fall in the range of 1-5 kPa

                                         Influence of town on PDC temperature

467   [Nunziante et al., 2003] and simulated dynamic pressures of 1 kPa calculated for a PDC with a

468   mass effusion rate of 5 x 107 kg/s and at a distance of 7.5 km from the vent [Esposti Ongaro et

469   al., 2002]. The case of EU4pf is more clear-cut, in that it would have been completely lethal for

470   any inhabitants surviving the previous PDC, a consequence of its high velocity, density, mass

471   and temperature.

472          The final, dilute PDCs entered a town that was already partially destroyed by the previous

473   currents. At this stage of the eruption, all the remaining population were dead [Sigurdsson et al.,

474   1985; Cioni et al., 2000; Luongo et al., 2003a]. These currents just mantled the ruins left by

475   EU4pf without inflicting further damage upon the buildings. However, even though these

476   currents caused no further damage to Pompeii and its population, our data are significant in that

477   they show that even distal, thin ash layers can be emplaced by currents that are still hot enough

478   to cause problems for unsheltered people.


480   Acknowledgement.

481   We are indebted to P.G. Guzzo, G. Stefani , A. d’Ambrosio, A. Varone and C. Cicirelli for

482   archaeological assistance; G. Di Martino, B. Di Martino, A. Cataldo for their logistic help in the

483   archaeological sites; A.J.L. Harris for informal review. We thank C. Frola and E. Deluca for

484   some TRM measurements and their help in the field. M. Bisson for the GIS database of Pompeii.

485   A.J.L. Harris, E. Tema, S. Ranieri and M. Lanfranco contributed to field work. This work was

486   partially supported by the European Commission, Project Exploris EVR1-CT-2002-40026 and

487   the financial support of INGV and University of Torino.


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608   Figure captions

609   Figure 1. Ancient Roman town of Pompeii. Dots = sites of studied sections; light areas = portions

610   of ruins still buried by undisturbed AD 79 deposits. Inset upper right, shaded relief map of the of

611   Vesuvius region.


613   Figure 2. AD 79 deposits at Pompeii. On the left, the schematic stratigraphy according to the

614   nomenclature of Cioni et al. [1992, 2004]. The numbers in the brackets are the variation

615   thickness of the studied deposits. From a) to d) particulars of the deposits within and around

616   Pompeii (see figure 1 for site location). (1) White pumice lapilli and bombs. (2) Grey pumice

617   lapilli and bombs. (3) Massive to stratified coarse-grained ash and grey pumice lapilli. (4)

618   Accretionary lapilli in in coarse and fine ash matrix.


                                          Influence of town on PDC temperature

620   Figure 3. Hand sampling of lithic fragments and roman tiles from the PDC deposit matrix. The

621   orientation of the main face of the tile is traced. Inset: standard paleomagnetic specimens and

622   little bits embedded in the plasticine.


624   Figure 4. Stepwise thermal demagnetization of lithic clasts from the AD 79 deposits: a) type C

625   clast; b) type D clast; c) type E clast (see text for further explanation).

626   Left: Zijderveld diagrams . Symbols: full dot = declination; open dot = apparent inclination.

627   Right: equal-area projections of the directions of magnetization. Symbols: full/open dot =

628   positive/negative inclination. Directions in the Zijderveld diagrams are represented in the sample

629   reference system; in the equi-areal projections the geographic reference system is used.


631   Figure 5. Pie diagram of the percentage occurrence of different types of fragments (see text for

632   further explanation).


634   Figure 6. Stepwise thermal demagnetization of a plaster specimen from the AD 79 deposits.

635   a) plaster bit (front and transverse section).

636   b) normalised intensity deacy curve and Zijderveld diagram of specimen T19a. Symbols as in

637   Fig. 4


639   Figure 7. Zijderveld diagrams of specimens of ballistic blocks from the fall-out deposit. For

640   symbols see figure 4.


642   Figure 8. Equal-area projection of high-Tb (HT) and low-Tb (LT) component directions of the

643   oriented specimens from EU4 pf at site 12a. Symbols: full/open dot = positive/negative

644   inclination; star = site mean direction with α95 confidence ellipse.


                                         Influence of town on PDC temperature

646   Figure 9. Evaluation of the AD 79 deposits temperature (Tdep), by overlap of individual fragments

647   reheating temperature range (see text for further explanation). Types are shown left of the bar.

648   Color: black = lithic fragment (mainly lavas); grey = tile; black dot = plaster.

649      a) EU2, EU3 and EU4 fall deposits.

650      b) EU3pf deposits

651      c) EU4pf deposits

652      d) EU7pf I ash, EU7pf II ash and EU8 deposits.


654   Figure 10. Tdep variation of EU3pf:

655   a) within and around Pompeii.

656   b) around the Vesuvius area [modified from Cioni et al.,2004].


658   Figure 11. Tdep variation of EU4pf:

659   a) within and around Pompeii.

660   b) around the Vesuvius area [modified from Cioni et al.,2004].


662   Figure 12. Tdep variation of EU7pf I ash, EU7 II ash and EU8, within and around Pompeii.


664   Figure 13. Destructive effects of the AD 79 deposits in Pompeii: a) collapsed roof tiles in the

665   fall-out deposit (photo courtesy Soprintendenza di Pompeii); b) skeletons in the ash deposits

666   inside a room (photo courtesy Soprintendenza di Pompeii); c) collapsed wall by EU4pf; d)

667   bodies rolled in EU4pf deposits. In c) and d) the arrow indicates the main flow direction.


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