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
14
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
1
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.
33
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
2
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.
71
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
3
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].
4
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.
121
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
5
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).
6
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
7
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.
199
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].
8
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.
9
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
10
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
11
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).
288
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
12
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.
13
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).
14
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].
15
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
16
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.
417
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
17
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
18
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.
479
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.
488
489
References
490
Anderson T., and J.S. Flett, Report on the eruption of The Soufriere in St Vincent 1902 and on a
491
visit to Montagne Pelee in Martinique, Phil Trans R Soc London, 100, 353–553, 1903.
19
Influence of town on PDC temperature
492
493
494
495
Aramaki, S., and S. Akimoto, Temperature estimation of pyroclastic deposits by natural
remanent magnetism, Am. J. Sci., 255, 619-627,1957.
Bardot, L., Emplacement temperature determinations of proximal pyroclastic deposits on
Santorini, Greece, and their implications, Bull. Volcanol., 61, 450-467, 2000.
496
Bardot, L., and E. McClelland, The reliability of emplacement temperature estimates using
497
palaeomagnetic methods: a case study from Santorini, Greece, Geophys. J. Int., 143, 1, 39-
498
51, 2000.
499
500
Baxter, P.T., A. Neri, and M. Todesco, Physical modeling and human survival in pyroclastic
flows, Natural Hazard, 17, 163–176, 1998.
501
Baxter, P.J., R. Boyle, C. Paul, A. Neri, R. Spence, and G. Zuccaro, The impacts of pyroclastic
502
surges on buildings at the eruption of the Soufriere Hills volcano, Montserrat, Bull.
503
Volcanol., 67, 292–313, 2005.
504
Bernstein, R.S., P.J. Baxter, H. Falk, R. Ing, L. Foster, F. Frost, Immediate public health
505
concerns and actions in volcanic eruptions: lessons from the Mount St. Helens eruption,
506
May 18-October 18, 1980. Am. J. Public Health, 76, 25-37, 1986.
507
508
509
510
Carey, S., and H. Sigurdsson, Temporal variations in column height and magma discharge rate
during the 79 AD eruption of Vesuvius, Bull. Geol. Soc. Am., 99, 303-314,
1987.
Chadwick, R.A., Paleomagnetic criteria for volcanic breccia emplacement, Geol. Soc. Am. Bull.,
82, 2285-2294, 1971.
511
Cioni, R., L. Gurioli, A. Sbrana, and G. Vougioukalakis, Precursory phenomena and destructive
512
events related to the Late Bronze Age Minoan (Thera, Greece) and AD 79 (Vesuvius,
513
Italy) Plinian eruptions; inferences from the stratigraphy in the archaeological areas, in The
514
Archaeology of Geological Catastrophes, edited by W. G. McGuire et al., Geol. Soc. Spec.
515
Publ., 171, 123–141, 2000.
516
517
Cioni, R., L. Gurioli, R. Lanza, and E. Zanella, Temperatures of the AD 79 pyroclastic density
current deposits (Vesuvius, Italy), J. Geophys. Res., 109, 1–18, 2004.
20
Influence of town on PDC temperature
518
Cogné, J.P., Paleomac: a Macintosh application for treating paleomagnetic data and make plate
519
reconstructions, Geochem. Geophys. Geosyst, 4, 10007, doi:10.1029/2001GC000227,
520
2003.
521
Druitt, T.H., Pyroclastic density current, in The physics of explosive volcanic eruptions, edited
522
by J.S. Gilbert and R.S.J.Sparks, Geol. Soc. London, Spec. Publ., 145, 145-182, 1998.
523
Esposti Ongaro, T., A. Neri, M. Todesco, and G. Macedonio, Pyroclastic flow hazard assessment at
524
Vesuvius (Italy) by using numerical modeling. II. Analysis of flow variables, Bull. Volcanol.,
525
64, 178–191, 2002.
526
527
528
529
Evans, M.E., and G.S. Hoye, Archaeomagnetic results from southern Italy and their bearing on
geomagnetic secular variation. Phys. Earth Planet. Int., 151, 155-162, 2005
Evans, M.E., and M. Mareschal, M., An archaeomagnetic example of polyphase magnetization:
Journal of Geomagnetism and Geoelectricity, 38, 923–929, 1986.
530
Freundt, A., and M.I. Bursik, Pyroclastic flow transport mechanism, in From magma to tephra:
531
modelling physical processes of explosive volcanic eruptions, edited by A. Freundt and M.
532
Rosi, Elsevier, Amsterdam, 173-246, 1998.
533
534
Gurioli, L., Pyroclastic flow: classification, transport and emplacement mechanisms, Plinius, 23,
84-89, 2000.
535
Gurioli, L., M.T. Pareschi, E. Zanella, R. Lanza, E. Deluca, and M. Bisson, Interaction of
536
pyroclastic currents with human settlements: evidences from ancient Pompeii, Geology,
537
33, 6, 441–444, 2005.
538
Gurioli, L., E., Zanella, M.T., Pareschi, and R., Lanza, Influences of urban fabric on pyroclastic
539
density currents at Pompeii (Italy), part I: Flow direction and deposition, J. Geophys. Res.,
540
submitted.
541
Grubensky M.J., G.A., Gary, and J.W., Geissman, Field and paleomagnetic characterization of
542
lithic and scoriaceous breccias at Pleistocene Broken Top volcano, Oregon Cascades, J.
543
Volcanol. Geotherm. Res., 83, 93-114, 1998.
21
Influence of town on PDC temperature
544
Hoblitt, R.P., K.S., Kellogg, Emplacement temperatures of unsorted and unstratified deposits of
545
volcanic rock debris by paleomagnetic tecniques, Geol. Soc. Am. Bull., 90, 633-642, 1979.
546
Hort M., and J. Gardner, Constrains on cooling and degassing of pumice during Plinian volcanic
547
548
549
eruptions based on model calculations, J. Geophys. Res., 105, 25,981–26,001, 2000.
Horwell, C.J. and P.J. Baxter, The respiratory health hazards of volcanic ash: a review for
volcanic risk mitigation. Bull. Volcanol., 69, 1-24, 2006.
550
Hueda-Tanabe, Y., A.M., Soler-Arechalde, J., Urrutia-Fucugauchi, L., Barba, L., Manzanilla,
551
M., Rebolledo-Vieyra, and A., Goguitchaichvili, Archaeomagnetic studies in central
552
Mexico – dating of Mesoamerican lime-plasters, Phys. Earth Planet. Int., 147, 269-283,
553
2004.
554
555
Kirschvink, J.L., The least-squares line and plane and the analysis of palaeomagnetic data,
Geophys. J. Roy. Astron. Soc., 62, 699–718, 1980.
556
Lacroix, A. La Montagne Pelée et ses Eruption (Masson, Paris, 1904), 1904.
557
Luongo, G., A. Perrotta, and C. Scarpati, Impact of the AD 79 eruption on Pompeii, I. Relations
558
amongst the depositional mechanisms of the pyroclast products, the framework of the
559
buildings and the associated destructive events, J. Volcanol. Geotherm. Res., 126, 201-223,
560
2003a.
561
Luongo, G., A. Perrotta, C. Scarpati, E. De Carolis, G. Patricelli, and A. Ciarallo, Impact of the
562
AD 79 eruption on Pompeii, II. Causes of death of the inhabitants inferred by stratigraphic
563
analysis and areal distribution of the human causalities, J. Volcanol. Geotherm. Res., 126,
564
169–200, 2003b.
565
Marton, P., D.H. Tarling, G. Nardi, and Pierattini D.,An archaeomagnetic study of roof tiles
566
from temple E: Selinute, Sicily, Science and Technology for Cultural Heritage, 2, 131–
567
136, 1993.
22
Influence of town on PDC temperature
568
McClelland, E., Theory of CRM acquired by grain growth, and its implications for TRM
569
discrimination and paleointensity determination in igneous rocks, Geophys. J., Int., 126,
570
271-280, 1996.
571
McClelland E., C.J.N., Wilson, and L., Bardot, Palaeotemperature determinations for the 1.8-ka
572
Taupo ignimbrite, New Zealand, and implications for the emplacement history of a high-
573
velocity pyroclastic flow, Bull., Volcanol., 66, 492-513, 2004.
574
575
McClelland, E.A., and T.H. Druitt, Palaeomagnetic estimates of emplacement temperatures of
pyroclastic deposits on Santorini, Greece, Bull. Volcanol., 51, 16-27, 1989.
576
Nunziante, L., M. Fraldi, L. Lirer, P. Petrosino, S. Scotellaro, and C. Cicirelli, Risk assessment
577
of the impact of pyroclastic currents on the towns located around Vesuvius: A non-linear
578
structural inverse analysis, Bull. Volcanol., 65, 547–561, 2003.
579
Porreca, M., Applicazioni di metodi paleomagnetici per lo studio della messa in posto di flussi
580
piroclastici. Il caso delle unità vulcaniche recenti del cratere di Albano (Italia Centrale),
581
unpublished PhD Thesis , University of Roma Tre, 1-118, 2004.
582
583
584
585
Pullaiah G.E., E., Irving, L., Buchan, and D.J., Dunlop, Magnetization changes caused by burial
and uplift, Earth Planet. Sci. Lett., 28, 133-143, 1975.
Sigurdsson, H., S. Carey, W. Cornell, and T. Pescatore, The eruption of Vesuvius in 79 AD, Nat.
Geogr. Res., 1, 332-387, 1985.
586
Tait S., R. Thomas, J. Gardner, C. Jaupart, Constraints on cooling rates and permeabilities of
587
pumice in an explosive eruption jet from colour and magnetic property. J. Volcanol.
588
Geotherm. Res., 86, 79–91, 1998.
589
Tanaka, H., H., Hoshizumi, Y., Iwasaki, and H., Shibuya, Applications of paleomagnetism in the
590
volcanic field: a case study of the Unzen Volcano, Japan, Earth Planets Space, 56, 635-
591
647, 2004.
592
593
Tanguy, J.C., C. Ribiere, A. Scarth, and W.S. Tjetjep, Victims from volcanic eruptions: A
revised database, Bull. Volcanol., 60, 137–144, 1998.
23
Influence of town on PDC temperature
594
Tema E., I. Hedley, and P. Lanos, Archaeomagnetism in Italy: a compilation of data including
595
new results and a preliminary Italian secular variation curve, Geoph. J. Int. (in press) doi:
596
10.1111/j.1365-246X.2006.03150.x
597
598
Thomas, R.M.E. and R.S.J. Sparks, Cooling of tephra during fallout from eruption columns,
Bull. Volcanol., 54, 542-553, 1992.
599
Todesco, M., A. Neri, T. Esposti Ongaro, P. Papale, G. Macedonio, R. Santacroce, A. Longo,
600
Pyroclastic flow hazard assessment at Vesuvius (Italy) by using numerical modelling. 1.
601
Large-scale dynamics, Bull. Volcanol., 64, 155–177, 2002.
602
603
Watson, G.S. A test for randomness of directions, Mon. Not. Roy. Astornom. Soc. Geophys.
Supp., 7, 160-161, 1956.
604
Zanella, E., L. Gurioli, G. Chiari, A. Ciarallo, R. Cioni, E. De Carolis, and R. Lanza,
605
Archaeomagnetic results from mural paintings and pyroclastic rocks in Pompeii and
606
Herculaneum, Phys. Earth. Planet. Inter., 118, 227-240, 2000.
607
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.
612
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.
619
24
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.
623
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.
630
631
Figure 5. Pie diagram of the percentage occurrence of different types of fragments (see text for
632
further explanation).
633
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
638
639
Figure 7. Zijderveld diagrams of specimens of ballistic blocks from the fall-out deposit. For
640
symbols see figure 4.
641
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.
645
25
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.
653
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].
657
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].
661
662
Figure 12. Tdep variation of EU7pf I ash, EU7 II ash and EU8, within and around Pompeii.
663
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.
26