ArticlePDF Available

Abstract and Figures

Pyroclastic density currents are ground hugging gas-particle flows that originate from the collapse of an eruption column or lava dome. They move away from the volcano at high speed, causing devastation. The impact is generally associated with flow dynamic pressure and temperature. Little emphasis has yet been given to flow duration, although it is emerging that the survival of people engulfed in a current strongly depends on the exposure time. The AD 79 event of Somma-Vesuvius is used here to demonstrate the impact of pyroclastic density currents on humans during an historical eruption. At Herculaneum, at the foot of the volcano, the temperature and strength of the flow were so high that survival was impossible. At Pompeii, in the distal area, we use a new model indicating that the current had low strength and low temperature, which is confirmed by the absence of signs of trauma on corpses. Under such conditions, survival should have been possible if the current lasted a few minutes or less. Instead, our calculations demonstrate a flow duration of 17 min, long enough to make lethal the breathing of ash suspended in the current. We conclude that in distal areas where the mechanical and thermal effects of a pyroclastic density currents are diminished, flow duration is the key for survival.
Content may be subject to copyright.

Scientic Reports | (2021) 11:4959 | 
www.nature.com/scientificreports
The impact of pyroclastic density
currents duration on humans:
the case of the AD 79 eruption
of Vesuvius
Pierfrancesco Dellino1*, Fabio Dioguardi2, Roberto Isaia3, Roberto Sulpizio1 & Daniela Mele1
Pyroclastic density currents are ground hugging gas-particle ows that originate from the collapse
of an eruption column or lava dome. They move away from the volcano at high speed, causing
devastation. The impact is generally associated with ow dynamic pressure and temperature. Little
emphasis has yet been given to ow duration, although it is emerging that the survival of people
engulfed in a current strongly depends on the exposure time. The AD 79 event of Somma-Vesuvius is
used here to demonstrate the impact of pyroclastic density currents on humans during an historical
eruption. At Herculaneum, at the foot of the volcano, the temperature and strength of the ow were
so high that survival was impossible. At Pompeii, in the distal area, we use a new model indicating
that the current had low strength and low temperature, which is conrmed by the absence of signs of
trauma on corpses. Under such conditions, survival should have been possible if the current lasted a
few minutes or less. Instead, our calculations demonstrate a ow duration of 17 min, long enough to
make lethal the breathing of ash suspended in the current. We conclude that in distal areas where the
mechanical and thermal eects of a pyroclastic density currents are diminished, ow duration is the
key for survival.
e impact of pyroclastic density currents (PDCs) is generally attributed to the combination of ow temperature
and dynamic pressure13. e latter is expressed by the dynamic pressure,
that represents the lateral force per unit area acting on buildings and living bodies, where
is the gas-particle mixture density, ρs and ρg are particle and gas density, C is particle volumetric concentration
and U is current velocity. A complete symbol list is found in Table1.
Engineering investigations1,4,5 show that dynamic pressures higher than 5kPa produce signicant damage,
while pressures under 1kPa have minimal to no consequence on structures or infrastructures. Particle volumetric
concentration represents an important parameter too because dynamic pressure is proportional to it. Currents
moving in the vicinity of a volcano can have a high concentration of hot magmatic particles that confer high
temperature and high dynamic pressure to the ow. is can cause burning of buildings, breaking of windows
and toppling of walls, which make survival impossible6.
Concerning eects on humans, it is emerging that even in areas far from a volcano, where particle concentra-
tion, temperature and dynamic pressure strongly decrease, people engulfed in the ow have “high probability
of receiving fatal skin burns and inhalation injury of the upper and lower respiratory tract, unless the duration
is very brief7. e presence of ne-ash particles suspended in air for a long time, even in very small amounts,
can be very harmful to human health, and represents one major cause of injury2. Exposure to pure hot air at
200–250°C can be survived for 2–5 minutes8, but the presence of inhalable hot ne ash drastically reduces sur-
vival times. e exposure time therefore plays a major role in determining the impact of PDCs on human beings,
(1)
P
dyn =
1
2
ρmixU
2
(2)
ρmix =ρsC+ρg(1C)
OPEN
Dipartimento Di Scienze Della Terra E Geoambientali, Università Di Bari, Bari, Italy. British Geological Survey, The
Lyell Centre, Edinburgh, UK.    
Napoli, Napoli, Italy. *email: pierfrancesco.dellino@uniba.it
Content courtesy of Springer Nature, terms of use apply. Rights reserved
Vol:.(1234567890)
Scientic Reports | (2021) 11:4959 | 
www.nature.com/scientificreports/
but, until now, it has not been quantied2,7. We study here the famous AD 79 eruption of Somma-Vesuvius9,10
and we reconstruct, for the rst time, also the eect of ow duration on humans.
The 79 AD eruption of Vesuvius and associated deposits. e eruption started on October 24th,
with the deposition of a thin bed of ne ash to the east11. is short opening event heralded the main explosive
phase, which started around noon of October 24th with the formation of a 25km high eruptive column that,
favored by stratospheric winds, caused the propagation of a south-eastwardly dispersed volcanic plume. e
Roman towns and villages around Somma-Vesuvius and along the plume dispersal axis were covered by pumice
Table 1. List of symbols, with description and physical dimension.
Symbol Description Dimension
ArAggradation Rate m s−1
C0Reference particle concentration (0.7)
CParticle volumetric concentration
Ctot Total particle volumetric concentration
CdParticle drag coecient
Csf Depth-averaged concentration in the basal shear ow
Cpa Air specic heat J kg−1°C−1
Cpg Gas specic heat J kg−1°C−1
Cps Solid specic heat J kg−1°C−1
dparticle size mm
gGravity acceleration (9.81) m s−2
gReduced gravity m s−2
HTotal ow thickness m
Hdep Deposit thickness m
Hsf Shear ow height m
kVon Karman constant (0.4)
kssubstrate roughness m
Pdyn Dynamic pressure k Pa
PnParticle Rouse number
Pn*Normalized Rouse number
Pnavg Average Rouse number of the solid material
Pni Rouse number of the ith particle-size class
Pnsusp Rouse number at maximum suspension capacity
Ri0Richardson number
SrSedimentation rate kg m−2 s−1
tAggradation time s
Taair temperature °C
TgGas temperature °C
Tmix Temperature of mixture °C
Tssolid temperature °C
u*Flow shear velocity m s−1
UCurrent velocity m s−1
wtParticle terminal velocity m s−1
wti Terminal velocity of the ith particle-size class m s−1
yFlow vertical coordinate m
y0Basal lamina thickness m
αSlope angle Deg
ϕiWeight fraction of the ith size class Weight%
ρaAtmospheric density kg m−3
ρdep Deposit density kg m−3
ρgGas density kg m−3
ρmix Density of the gas–particle mixture kg m−3
ρsParticle density kg m−3
ρsf Shear ow density kg m−3
ρsi Density of the ith particle-size class kg m−3
τShear-driving stress of shear ow Pa
Content courtesy of Springer Nature, terms of use apply. Rights reserved
Vol.:(0123456789)
Scientic Reports | (2021) 11:4959 | 
www.nature.com/scientificreports/
lapilli and ash with thickness up to 3m at Pompeii9, which caused roof collapse of several houses. Aer a few
hours, the plume became unstable and partially collapsed, generating small volume PDCs that hit the slopes of
the volcano and buried the town of Herculaneum9,10 (Fig.1a,b). e main explosive phase ended in the morn-
ing of October 25th, with the eruption resuming aer a few hours with a high column that suddenly collapsed,
generating the most destructive PDC of the whole eruption (the EU4 unit), causing injuries up to 20km south
from the volcano10,11. e EU4 unit invaded Pompeii (about 10km from the vent), causing the death of people
not yet escaped from the town. Pompeii is a particularly important site for evaluating the impact of an eruption
on human beings, because during the eighteenth century excavations archaeologists found a way of producing
plaster casts of the victims, giving clues on the eect that the PDCs had on people12.
Our survey at the archaeological excavation of Pompeii allowed the visit of the site of Casa di Stabianus
(Regio I, insula 22), where in the perimeter of a house some corpses lay embedded by the sediment that formed
aer the passage of the ow that deposited the EU4 unit. e EU4 deposit rests on top of the fallout pumice bed
of the main explosive phase, meaning that the PDC entered the house through the openings and the collapsed
roof, and engulfed people that were resting in the house in the time interval between the two main phases of the
eruption12,13. e deposit consists of a 0.23m thick bed with internal stratication (Fig.1c), showing tractional
structures such as sand waves. ese are the typical features of deposits formed from a dilute current, where ash
particles are sustained by turbulence until they settle out of suspension and into a bed load1416. e deposit was
formed by continuous aggradation, i.e. by the stacking up of one ash lamina over the other, during the time-
integrated passage of the current.
Some preliminary indication of the impact that the PDCs had on human beings comes plaster casts of the
bodies that lay embedded in the ash layer (Fig.1d). ey show intact bodies without evidence of any traumatic
sign12 and suggest that the current did not possess a high dynamic pressure (i.e. high dynamic pressure). Fur-
thermore, clothes are preserved and show that the original texture was not burnt by the passage of the PDC,
Figure1. e PDC deposits of the AD 79 Vesuvius eruption. (a)—Map showing Herculaneum and Pompeii
locations (courtesy of Osservatorio Vesuviano); (b)—Herculaneum: the white arrow shows the massive bed
formed by the concentrated current that caused charring of woods (yellow arrow) and toppling of walls
(red arrows); (c)—Pompeii: the stratied layer with tractional structures that was formed by the stacking up
of laminae during suspension sedimentation from the dilute PDC, is shown; (d)—Pompeii: some corpses,
embedded in the ash layer, which show intact bodies and preserved dressings (white arrow), are shown.
Content courtesy of Springer Nature, terms of use apply. Rights reserved
Vol:.(1234567890)
Scientic Reports | (2021) 11:4959 | 
www.nature.com/scientificreports/
indicating a temperature below the clothes decomposition, which ranges between 130 and 150°C for silk and
wool, respectively17.
The impact parameters of the PDC at Pompeii. e approach we used to reconstruct of the impact
parameters is described in the method section, which includes a description of the equations, from (3) to (14),
up on which the model is based. e main data used as input in the model are reported in Table2. Here the
results of ow dynamic pressure, temperature and duration, as representing the main impacts, are illustrated.
Flow dynamic pressure. In order to illustrate how ow strength varies as a function of current height at Pom-
peii, the proles of particle concentration, density, velocity and dynamic pressure are shown on Fig.2. Results are
presented by means of three curves representing the minimum (16th percentile), the average (50th percentile)
and the maximum (84th percentile) solution of the probability density functions that were calculated with the
method of Dioguardi and Dellino18 (see the method section). Velocity, U, while increasing upward in the ow
(Fig.2a), reaches values in the range of a few tens m/s. Concentration, C, strongly decreases with height (Fig.2b),
and already in the rst few meters is lower than 0.001. e density prole, ρmix (Fig.2c), mimics the trend of
the concentration prole, and already in the lower two meters decreases rapidly upward to a value lower than
atmosphere, making the upper part of the current buoyant. e dynamic pressure Pdyn, which represents the
combination of velocity and density, has a maximum in the rst few decimeters (Fig.2d). Higher in the current,
dynamic pressure is lower than 1kPa. With these values, no severe mechanical damages are expected to struc-
tures, infrastructure or human bodies.
Flow temperature. Flow temperature of the current was calculated by using as input in Eq.(11) (see the method
section) the values of density, concentration, temperature and specic heat of the three components of the gas-
particle mixture, namely: magmatic gas, air and volcanic particles. e temperature of magmatic gas and of vol-
canic particles was set to 850°C, which is compatible with the 79 AD eruption composition19. Air temperature
Table 2. Pompeii deposit data used as input in the model.
Hdep (m) ρdep (kg/m3)djuv (mm) ρjuv (kg/m3)Cd juv dxx (mm) ρxx (kg/m3)Cd xx
0.23 1900 0.40 2200 1.73 0.19 3280 1.39
Figure2. Proles of the impact parameters representing the ow dynamic pressure. e curves refer to
the minimum (16th percentile), the average (50th percentile) and the maximum (84th percentile) of the
probabilistic model solution. (a)—Velocity proles. (b)—Particle volumetric concentration proles. (c)—
Density proles. (d)—Dynamic pressure proles. British Geological Survey (UKRI) 2021.
Content courtesy of Springer Nature, terms of use apply. Rights reserved
Vol.:(0123456789)
Scientic Reports | (2021) 11:4959 | 
www.nature.com/scientificreports/
was set to 18°C, which is a reasonable value for the Somma-Vesuvius area at sea level in the autumn season20.
Average density was set to 1700kg/m3 for the volcanic particles, to 0.2kg/m3 for volcanic gas at 850°C and
to 1.2kg/m3 for air at 18°C, respectively. e specic heats were set to 2200J/kg°C for volcanic gas, 700J/
kg°C for the volcanic particles and 1005J/kg°C for air. As for particle concentration, an average value of
0.001 was set, obtained by integrating the concentration prole of the average solution over ow height (see
Fig.2b) by means of Eq.(4). e relative concentrations of magmatic gas and air were obtained by means of
ρg=ρmCm+ρa(1Cm)
and by using as gas density the value calculated by the system of Eqs.(8) and (9). e
concentration values of air and volcanic gas resulted 0.941 and 0.058, respectively. By setting all parameters in
(11), a temperature of 115°C was obtained. Zanella etal.21 and Cioni etal.19 made measurements on the PDC
deposit at Pompeii, which indicated temperatures, at the time of deposition, ranging between 140 and 300°C,
which is consistent with the values obtained in this paper when considering that the temperature in the com-
pacted deposit can be a little higher than that of the dilute gas-particle mixture.
e low temperature that we calculated at Pompeii is due to the much higher content of cold atmosphere air
in the current, with respect to the hot magmatic gas. is is attributed to the air entrainment process that char-
acterizes PDCs along runout. It is the sum of the air entrainment that occurs at the turbulent interface between
the ow head and atmosphere, which is regulated by the Richardson number of the current
Ri
0=g
Hcosa
U
2 where
g
=
ρmixρa
ρa
g is the reduced gravity22, and of the entrainment due to the ingestion of air occurring upon the
impact of the eruptive column with the ground. e latter eect is particularly ecient in diluting magmatic
gas with atmosphere air in the vicinity of the volcano, as it has been reported both by experiments22,23 and by
observation of recent eruptions24.
Flow duration. Flow duration was calculated by using as input in Eqs.(12) and (13) (Method section) data
obtained both directly on the PDC deposit at Pompeii, and by means of laboratory analyses carried out on
ash samples. Among input, particle concentration, Rouse number and settling velocity are all functions of the
shear ow density, which was calculated in terms of a probability density function with PYFLOW v2.025. As a
consequence, the results of ow duration are also expressed in terms of probabilities. e average value of ow
duration was about 17min. is duration is quite long when compared to the couple of minutes considered as a
survivable time for people engulfed in a PDC, even at low temperature2,7.
Our ow duration represents the time during which the layer thickness was formed by continuous settling of
particles out of suspension. It does not take into consideration the waning phase of the current, where sedimenta-
tion could have been minimal and not completely recorded in the deposit layer, or any periods of nondeposition
through bypassing, or pulses of erosion.
Indeed, the time here calculated is to be considered as a minimum estimation. is ow duration represents,
therefore, the phase when the current had a signicant load of life-threatening ash.
Discussion and conclusion
e PDCs of the AD 79 eruption of Somma–Vesuvius show a major dierence between proximal and distal
areas in terms of impact. In the vicinity of the volcano the main eect was related to dynamic pressure and
temperature19,26. is conclusion is corroborated by our observations at Herculaneum (Fig.1a), where the cur-
rent le a massive layer formed by a highly concentrated and hot ow that was capable of breaking and toppling
thick walls and of charring wood (see Fig.1b). ese characteristics are indicative of a highly destructive event
that did not permit survival, as discussed by previous authors27,28.
e situation in distal locations, such as in Pompeii, 10km from the volcano, is quite dierent (Fig.1a).
Here, the thermal and mechanical aects. e thermal and mechanical eects of the dilute PDC drastically
diminished there. If we integrate the prole of the average solution of the dynamic pressure over the rst 10m
(a typical building height in the Vesuvian area) a value lower than 1kPa results. According to engineering
investigations2,3, no damage to walls should be expected with such a ow strength, which is consistent with the
fact that at Pompeii the walls of Roman buildings do not show evidence of damage12,29 related to the passage of
the PDC. Furthermore, the bodies embedded in the ash bed do not show any evidence of bone dislocations or
fracture, and the bodies look intact, which is consistent with the low ow strength. Even the clothing, whose
textures remain visible throughout the plaster casts, look intact. is is in agreement with the low temperature
(115°C) of the gas-particle mixture calculated in this study.
e average value of ow duration that we calculated is about 17min, which combined with the concentra-
tion of ash particles (about 0.001), was a long enough time to cause death by asphyxia at Pompeii. e recent
literature on the subject suggests, in fact, that the exposure to ne ash, even at a low particle concentration, can
be survived only for a couple of minutes7. e ow duration of PDCs can be shorter or longer than this, depend-
ing on the scale of the eruption. ere are reports of recent eruptions showing that in the marginal reaches of
the current, where the ow duration was only a few minutes, people were able to survive7. In other cases, longer
ow durations did not permit survival and death was caused by ne-ash inhalation7,30. Flow duration is a key
factor for assessing the impact of PDCs on human beings, especially in distal areas, where the primary risk to life
is asphyxiation, as at Pompeii. We agree with Baxter etal.7 that the emergency planning for explosive eruptions
should concentrate on the distal parts of PDCs where survival could be likely, and where the primary risk to life
is asphyxiation from ash inhalation, rather than thermal or mechanical injury.
For Pompeii, we were able to reconstruct ow duration using a novel method that was applied for the rst
time in this paper. Our method should be used to infer the probable duration of pyroclastic density currents in
future events, with this contributing to hazard assessment of active volcanoes.
Content courtesy of Springer Nature, terms of use apply. Rights reserved
Vol:.(1234567890)
Scientic Reports | (2021) 11:4959 | 
www.nature.com/scientificreports/
Method
e reconstruction of the impact parameters of PDCs is based on a ow mechanical model that starts with the
assumption that the current is velocity and density stratied15,31,32. In the stratied multiphase gas-particle cur-
rent, the basal part is a shear ow that moves attached to the ground and has a density higher than atmosphere.
e upper part is buoyant, because particle concentration decreases with height down to a value that, combined
with the eect of gas temperature, makes the mixture density lower than the surrounding atmosphere.
e inputs needed, in our model, for the calculation of the impact parameters at Pompeii are reported in
Table2. Some of the input data are obtained directly in the eld, such as deposit and lamina thickness. Deposit
density is obtained by weighing a known volume of deposit. Other data come from laboratory analyses on
samples extracted from the deposit. In the laboratory, rst, the grain-size distribution is determined, then from
each size class a sample of particles per each component (crystal, glass, lithics) is extracted, and density data are
obtained on such particle samples by means of pycnometers33. Particle shape parameters, which are needed for
the calculation of settling velocity, are obtained by image analysis methods34.
In a PDC, particles are mainly transported by turbulent suspension and sedimentation is controlled by a bal-
ance between ow shear velocity u*, which is controlled by uid turbulence and favors suspension, and particle
settling velocity, wt = (4gd(ρs −ρmix)/3Cdρmix)0.5, which favors sedimentation, where g is gravity acceleration, d is
particle size and Cd is drag coecient. e median of the grain-size distribution was used for particle size. e
capacity of a current to transport particles in suspension is quantied by the Rouse number35
P
n=
wt
ku
, where
k is the Von Karman constant (0.4). During sedimentation, it is assumed that the particles of dierent com-
position that form a lamina settle at the same aerodynamic conditions, e.g., with the same terminal velocity15.
erefore, by equating the settling velocity of the glass and crystal components in the deposit, and assuming
that sedimentation starts when Pn = 2.5, hence when wt = u*, ow shear velocity and density ρsf of the shear ow
can be calculated aer d, ρs and Cd are measured in the laboratory36. ese results are then input in a numerical
code18,25 and the current parameters are reconstructed. e velocity prole follows the equation of a turbulent
boundary layer shear ow moving over a rough surface37
where ks is the substrate roughness (measured in the eld as 0.1m at Pompeii) and y is ow height.
where C0 is the particle volumetric concentration at the reference height y0 and H is the total current thickness.
In this work, y0 is taken as the basal lamina thickness, hence C0 is the particle concentration in the lamina (0.7
in this paper). Assuming steady sedimentation, H is obtained by the ratio Hdep/Csf where Hdep is deposit thickness
and Csf is the depth-averaged concentration in the basal shear ow, which can be calculated by ρsf = ρs Csf + ρg(1-
Csf), when ρsf and ρg are known.
e shear ow height and density are obtained by solving the system of (5) and (6), which is valid for a
turbulent current
where
τ
is the shear-driving stress of the ow moving down an inclined slope of angle
, in our case 3.2°, meas-
ured in the eld.
e density prole, which is a function of concentration, particle density and gas density, is:
Gas density and Rouse number are obtained by solving numerically the following system:
Equation(8) states that atmospheric density,
ρa
, is reached at the top of the shear ow, Hsf, and Eq.(9) states
that the average density of the shear ow,
ρsf
refers to the part of the ow that goes from the reference level, y0,
to the shear ow top height, Hsf.
By combining the velocity and density proles, the dynamic pressure prole is nally obtained
(3)
U
y
=u
1
k
ln
y
ks
+8.5
(4)
C
y
=C0
y0
Hy0
Hy
yPn
(5)
τ
=
ρ
sf
ρ
a
gsinαH
sf
(6)
τ
=ρ
sf
u
2
(7)
ρ
mix
y
=ρg+C0
y0
H
y0
Hy
yP
n
ρsρg
(8)
ρ
a
y
=ρg+C0
y0
Hy0
HHsf
H
sf P
n
ρsρg
(9)
ρ
sf =
1
Hsf y0
Hsf
y0
ρg+C0
y0
Hy0
Hy
y
P
n
ρsρg
dy
(10)
Pdyn
y
=0.5U
2
y
ρ
mix
y
.
Content courtesy of Springer Nature, terms of use apply. Rights reserved
Vol.:(0123456789)
Scientic Reports | (2021) 11:4959 | 
www.nature.com/scientificreports/
e proles of the ow parameters are expressed in terms of a probability density function that depends on
the variance of particle characteristics. e model has been validated by experiments3 and already applied to
other eruptions15,33.
e temperature of a PDC is quantied as the weighted average between the relative proportions of the three
components that make up the gas-particle mixture, namely the volcanic gas and solid particles that issue from
the crater, plus the atmospheric air that is entrained by the current during its spreading. e temperature of the
mixture can be approximated by
where T and Cp are the temperature and specic heat (at constant pressure), respectively. e subscripts g, s and
a stand for gas, solid particle and air, respectively.
Concerning ow duration, in a PDC, sedimentation occurs at a rate
Sr
that represents the mass of particles
settling over a unit area in the unit time. Deposit thickness grows by aggradation of ash laminae during the time-
integrated passage of the current. e aggradation rate
Ar
, which is the rate at which deposit thickness grows, is
equal to the sedimentation rate divided by deposit density,
ρdep
.
e total time of aggradation, t, which is a proxy of ow duration, is equal to deposit thickness divided by the
aggradation rate, Ar, which is represented by the ratio of deposit density and sedimentation rate:
Deposit density and thickness are measured in the eld, consequently the only missing quantity for the cal-
culation of ow duration is the sedimentation rate.
Dellino etal. 38, recently proposed a model for the calculation of the sedimentation rate
with the subscript i referring to the ith particle-size class and n being the number of size classes of the grain-size
distribution of the sediment, with
φi
,
ρsi
and
Pni
being the weight percent, the density and the Rouse number of
the ith grain-size fraction, respectively. Pn* = Pnavg/Pnsusp is the normalized Rouse number of the current, i.e. the
ratio between the average Rouse number of the solid material in the current and the Rouse number at maximum
suspension capacity. e model considers the contribution of each size class of particles to the sedimentation,
and not the average grain size, because the solid load constituting a suspension current, especially in the case
of PDCs, is made up of a mixture of dierent components (lithics, glassy fragments and crystals) with dierent
size, density and shape, thus dierent terminal velocity. e average Rouse number of the solid material in the
current is calculated as the average of the particulate mixture,
When Pn* is higher than 1, a current has a particle volumetric concentration in excess of its maximum
capacity, e.g. it is over-saturated of particles, which favours sedimentation. When it is lower than 1, a current
has a particle volumetric concentration lower than its maximum capacity, e.g. it is under-saturated, and could
potentially include additional sediment in suspension by erosion from the substrate. For a specic discussion
see Dellino etal.38.
Received: 11 January 2021; Accepted: 16 February 2021
References
1. Spence, R. J. S., Baxter, P. J. & Zuccaro, G. Building vulnerability and human casualty estimation for a pyroclastic ow: a model
and its application to Vesuvius. J. Volcanol. Geotherm. Res. 133, 321–343 (2004).
2. Horwell, C. J. & Baxter, P. e respiratory health hazards of volcanic ash: A review for volcanic risk mitigation. Bull. Volcanol. 69,
1–24 (2006).
3. Dellino, P. et al. Experimental evidence links volcanic particle characteristics to pyroclastic ow hazard. Earth Planet. Sci. Lett.
295, 314–320 (2010).
4. Zuccaro, G., Cacace, F., Spence, R. J. S. & Baxter, P. J. Impact of explosive eruption scenarios at Vesuvius. J. Volcanol. Geotherm.
Res. 178, 416–453 (2008).
5. Zuccaro, G. & Leone, M. Building technologies for the mitigation of volcanic risk: Vesuvius and Campi Flegrei. Nat. Hazards Rev.
13(3), 221–232 (2012).
6. Jenkins, S. et al. e Merapi 2010 eruption: An interdisciplinary impact assessment methodology for studying pyroclastic density
current dynamics. J. Volcanol. Geotherm. Res. 261, 316–329 (2013).
7. Baxter, P. J. et al. Human survival in volcanic eruptions: thermal injuries in pyroclastic surges, their causes, prognosis and emer-
gency management. Burns 43, 1051–1069 (2017).
8. Buettner, K. Eects of extreme heat in man. J. Am. Med. Assoc. 144, 732–738 (1950).
9. Sigurdsson, H., Carey, S., Cornell, W. & Pescatore, T. e eruption of Vesuvius in 79 AD. Natl. Geogr. Res. 1, 332–387 (1985).
(11)
T
mix =
ρ
g
T
g
C
g
Cp
g
+ρ
s
T
s
C
s
Cp
s
+ρ
a
T
a
C
a
Cp
a
ρ
gCgCpg
+ρ
sCsCps
+ρ
aCaCpa
(12)
t
=
H
dep
Ar
(13)
S
r=
n
i
ρsiwti
φ
i
si
n
i=1φisi
Ctot

10.065 Pni
P
n
+0.1579
0, 7 +
φ
i+1
si+1
n
i=1φi+1si+1
Ctot

10.065 Pni
P
n
+0.1579
0.3
0.01.
(14)
P
navg =
n
i=1
PniCi/
C
Content courtesy of Springer Nature, terms of use apply. Rights reserved
Vol:.(1234567890)
Scientic Reports | (2021) 11:4959 | 
www.nature.com/scientificreports/
10. Gurioli, L., Cioni, R., Sbrana, A. & Zanella, E. Transport and deposition of pyroclastic density currents over an inhabited area: the
deposits of the AD 79 eruption of Vesuvius at Herculaneum, Italy. Sedimentology 49, 929–953 (2002).
11. Cioni, R., Gurioli, L., Sbrana, A. & Vougioukalakis, G. Precursory phenomena and destructive events related to the Late Bronze
Age Minoan (era, Greece) and AD 79 (Vesuvius, Italy) Plinian eruptions: inferences from the stratigraphy in the archaeological
areas, in e Archaeology of Geological Catastrophes, edited by W. G. McGuire etal. Geol. Soc. Spec. Publ. 171, 123–141 (2000).
12. Luongo, G. et al. Impact of the AD 79 eruption on Pompeii, II. Causes of death of the inhabitants inferred by stratigraphic analysis
and areal distribution of the human causalities. J. Volcanol. Geotherm. Res. 126, 169–200 (2003).
13. Scarpati, C., Perrotta, A., Martellone, A. & Osanna, M. Pompeian hiatuses: new stratigraphic data highlight pauses in the course
of the AD 79 eruption at Pompeii. Geol. Mag. 157, 695–700 (2020).
14. Branney, M. J. & Kokelaar, P. Pyroclastic Density Currents and the Sedimentation of Ignimbrites 27 (Geological Society, London,
Memoirs, 2002).
15. Dellino, P., Mele, D., Sulpizio, R., La Volpe, L. & Braia, G. A method for the calculation of the impact parameters of dilute pyroclastic
density currents based on deposits particle characteristics. J. Geophys. Res. 113, B07206 (2008).
16. Lube, G., Breard, E. C. P., Cronin, S. J. & Jones, J. Synthesizing large-scale pyroclastic ows: experimental design, scaling, and rst
results from PELE. J. Geophys. Res. Solid Earth 120, 1487–1502 (2015).
17. Quaglierini, C. Chimica delle bre tessili. Zanichelli (2nd ed.) pp. 354 (2012).
18. Dioguardi, F. & Dellino, P. PYFLOW: a computer code for the calculation of the impact parameters of Dilute Pyroclastic Density
Currents (DPDC) based on eld data. Comput. Geosci. 66, 200–210 (2014).
19. Cioni, R., Gurioli, L., Lanza, R. & Zanella, E. Temperatures of the A. D. 79 pyroclastic density current deposits (Vesuvius, Italy).
J. Geophys. Res. 109, B02207 (2004).
20. Rolandi, G., Paone, A., Di Lascio, M. & Stefani, G. e 79 AD eruption of Somma: the relationship between the date of the eruption
and the southeast tephra dispersion. J. Volcanol. Geotherm. Res. 169, 87–89 (2007).
21. Zanella, E., Gurioli, L., Pareschi, M. T. & Lanza, R. Inuences of urban fabric on pyroclastic density currents at Pompeii (Italy): 2.
Temperature of the deposits and hazard implications. J. Geophys. Res. 112, B05214 (2007).
22. Dellino, F., Dioguardi, F., Doronzo, D. M. & Mele, D. e entrainment rate of non Boussinesq hazardous geophysical gas-particle
ows: an experimental model with application to pyroclstic density currents. Geophys. Res. Lett. 46(22), 12851–12861 (2019).
23. Lube, G. et al. Generation of air lubrication within pyroclastic density currents. Nat. Geosci. 12(5), 381–386 (2019).
24. Trolese, M. et al. Very rapid cooling of the energetic pyroclastic density currents associated with the 5 November 2010 Merapi
eruption (Indonesia). J. Volcanol. Geotherm. Res. 358, 1–12 (2018).
25. Dioguardi, F. & Mele, D. PYFLOW_2.0: a computer program for calculating ow properties and impact parameters of past dilute
pyroclastic density currents based on eld data. Bull. Volcanol. 80, 28 (2018).
26. Giordano, G. et al. ermal interaction of the AD79 Vesuvius pyroclastic density currents and their deposits at Villa dei Papiri
(Herculaneum archaeological site, Italy). Earth Planet. Sci. Lett. 490, 180–192 (2018).
27. Mastrolorenzo, G. et al. Archaeology: Herculaneum victims of Vesuvius in A.D. 79. Nature 410, 769–770 (2001).
28. Petrone, P. et al. A hypothesis of sudden body uid vaporization in the 79 AD victims of Vesuvius (2020). PLoSONE 13(9), e0203210
(2018).
29. Gurioli, L., Zanella, E., Pareschi, M. T. & Lanza, R. Inuences of urban fabric on pyroclastic density currents at Pompeii (Italy): 1.
Flow direction and deposition. J. Geophys. Res. 112, B05213 (2007).
30. Nakada, S. Hazards from Pyroclastic Flows and Surges. In Encyclopedia of Volcanoes (eds Sigurdsson, H. et al.) (Academic Press,
Cambridge, 2000).
31. Valentine, G. A. Stratied ow in pyroclastic surges. Bull. Volcanol. 49, 616–630 (1987).
32. Brown, R. J. & Branney, M. J. Internal ow variations and diachronous sedimentation within extensive, sustained, density stratied
pyroclastic density currents down gentle slopes, as revealed by the internal architectures of ignimbrites in Te wnerife. Bull. Volcanol.
75, 1–24 (2013).
33. Mele, D. et al. Hazard of pyroclastic density currents at the Campi Flegrei Caldera (Southern Italy) as deduced from the combined
use of facies architecture, physical modeling and statistics of the impact parameters. J. Volcanol. Geotherm. Res. 299, 35–53 (2015).
34. Mele, D., Dellino, P., Sulpizio, R. & Braia, G. A systematic investigation on the aerodynamics of ash particles. J. Volcanol. Geotherm.
Res. 203, 1–11 (2011).
35. Rouse, H. An analysis of sediment transportation in the light of uid turbulence. Soil Conservation Services Report No. SCS-TP-25
(USDA, Washington, D.C., 1939).
36. Dellino, P. et al. e analysis of the inuence of pumice shape on its terminal velocity. Geophys. Res. Lett. 32, L21306 (2005).
37. Furbish, D. J. Fluid Physics in Geology 476 (Oxford University Press, New York, 1997).
38. Dellino, P., Dioguardi, F., Doronzo, D. M. & Mele, D. e rate of sedimentation from turbulent suspension: an experimental model
with application to pyroclastic density currents and discussion on the grain-size dependence of ow mobility. Sedimentology 66(1),
129–145 (2019).
Acknowledgements
e associate editor (Marco Viccaro), Greg Valentine and an anonymous reviewer greatly helped in improving
the manuscript. Soprintendenza Speciale per i Beni Archeologici di Pompei, Ercolano e Stabia is acknowledged
for the hospitality at the excavations. Part of the instrumentation was obtained by the grant PON 3a SISTEMA
of MIUR. is work is published with permission of the Executive Director of British Geological Survey (UKRI).
Author contributions
P.D. wrote the manuscript adn coordinated the reseach F.D. perfromed calculation R.S. and R.I. contributed in
the eld work DM helped in the laboratory analyses.
Competing interests
e authors declare no competing interests.
Additional information
Correspondence and requests for materials should be addressed to P.D.
Reprints and permissions information is available at www.nature.com/reprints.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and
institutional aliations.
Content courtesy of Springer Nature, terms of use apply. Rights reserved
Vol.:(0123456789)
Scientic Reports | (2021) 11:4959 | 
www.nature.com/scientificreports/
Open Access is article is licensed under a Creative Commons Attribution 4.0 International
License, which permits use, sharing, adaptation, distribution and reproduction in any medium or
format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the
Creative Commons licence, and indicate if changes were made. e images or other third party material in this
article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the
material. If material is not included in the article’s Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from
the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.
© e Author(s) 2021
Content courtesy of Springer Nature, terms of use apply. Rights reserved
1.
2.
3.
4.
5.
6.
Terms and Conditions
Springer Nature journal content, brought to you courtesy of Springer Nature Customer Service Center GmbH (“Springer Nature”).
Springer Nature supports a reasonable amount of sharing of research papers by authors, subscribers and authorised users (“Users”), for small-
scale personal, non-commercial use provided that all copyright, trade and service marks and other proprietary notices are maintained. By
accessing, sharing, receiving or otherwise using the Springer Nature journal content you agree to these terms of use (“Terms”). For these
purposes, Springer Nature considers academic use (by researchers and students) to be non-commercial.
These Terms are supplementary and will apply in addition to any applicable website terms and conditions, a relevant site licence or a personal
subscription. These Terms will prevail over any conflict or ambiguity with regards to the relevant terms, a site licence or a personal subscription
(to the extent of the conflict or ambiguity only). For Creative Commons-licensed articles, the terms of the Creative Commons license used will
apply.
We collect and use personal data to provide access to the Springer Nature journal content. We may also use these personal data internally within
ResearchGate and Springer Nature and as agreed share it, in an anonymised way, for purposes of tracking, analysis and reporting. We will not
otherwise disclose your personal data outside the ResearchGate or the Springer Nature group of companies unless we have your permission as
detailed in the Privacy Policy.
While Users may use the Springer Nature journal content for small scale, personal non-commercial use, it is important to note that Users may
not:
use such content for the purpose of providing other users with access on a regular or large scale basis or as a means to circumvent access
control;
use such content where to do so would be considered a criminal or statutory offence in any jurisdiction, or gives rise to civil liability, or is
otherwise unlawful;
falsely or misleadingly imply or suggest endorsement, approval , sponsorship, or association unless explicitly agreed to by Springer Nature in
writing;
use bots or other automated methods to access the content or redirect messages
override any security feature or exclusionary protocol; or
share the content in order to create substitute for Springer Nature products or services or a systematic database of Springer Nature journal
content.
In line with the restriction against commercial use, Springer Nature does not permit the creation of a product or service that creates revenue,
royalties, rent or income from our content or its inclusion as part of a paid for service or for other commercial gain. Springer Nature journal
content cannot be used for inter-library loans and librarians may not upload Springer Nature journal content on a large scale into their, or any
other, institutional repository.
These terms of use are reviewed regularly and may be amended at any time. Springer Nature is not obligated to publish any information or
content on this website and may remove it or features or functionality at our sole discretion, at any time with or without notice. Springer Nature
may revoke this licence to you at any time and remove access to any copies of the Springer Nature journal content which have been saved.
To the fullest extent permitted by law, Springer Nature makes no warranties, representations or guarantees to Users, either express or implied
with respect to the Springer nature journal content and all parties disclaim and waive any implied warranties or warranties imposed by law,
including merchantability or fitness for any particular purpose.
Please note that these rights do not automatically extend to content, data or other material published by Springer Nature that may be licensed
from third parties.
If you would like to use or distribute our Springer Nature journal content to a wider audience or on a regular basis or in any other manner not
expressly permitted by these Terms, please contact Springer Nature at
onlineservice@springernature.com
... The current starts breaking openings between 1 and 5 kPa, and at values higher than 5 kPa breaking of walls starts to occur. Particle volumetric concentration is a factor of hazard because ash particles in suspension in air are very harmful to breath 21,22 , even at temperatures lower than 200 °C and at concentration as low as 0.001 (that is typical of dilute PDCs), if exposure time is longer than a couple of minutes 21,23 . The sedimentation rate helps in defining hazard because it can be used for approximating flow www.nature.com/scientificreports/ ...
... The sedimentation rate helps in defining hazard because it can be used for approximating flow www.nature.com/scientificreports/ duration (t) of dilute pyroclastic density currents 23,24 (see the method section), which lasts until sedimentation is fed from turbulent suspension. In this paper we use data from bedforms of dilute pyroclastic density currents for reconstructing the impact parameters and contribute to hazard assessment. ...
... The model has been validated by experiments 13 and already applied to other eruptions 42 . Here it is summarized as adapted from Dellino et al. 23 . ...
Article
Full-text available
Pyroclastic density currents are ground hugging gas-particle flows associated to explosive volcanic eruptions and moving down a volcano's slope, causing devastation and deaths. Because of the hostile nature they cannot be analyzed directly and most of their fluid dynamic behavior is reconstructed by the deposits left in the geological record, which frequently show peculiar structures such as ripples and dune bedforms. Here, a set of equations is simplified to link flow behavior to particle motion and deposition. This allows to construct a phase diagram by which impact parameters of dilute pyroclastic density currents, representing important factors of hazard, can be calculated by inverting bedforms wavelength and grain size, without the need of more complex models that require extensive work in the laboratory.
... Strictly from the archaeological excavation point of view (beyond the main goal here), a comprehensive work on recent studies at Pompeii is the volume XXXII -2021 of Rivista di Studi Pompeiani, which we referred to. On the other hand, numerous volcanological studies conducted in Pompeii have investigated several aspects of the eruption, including: (i) stratigraphy (Sigurdsson et al., 1985;Cioni et al., 1990Cioni et al., , 1992Scarpati et al., 2020); (ii) influence of the urban structure on the pyroclastic current direction and impact (Luongo et al., 2003a;Gurioli et al., 2005aGurioli et al., , 2007Ruggieri et al., 2020); (iii) causes of death of the inhabitants (Giacomelli et al., 2003;Luongo et al., 2003b;Scandone et al., 2019); (iv) temperature of the pyroclastic currents (Cioni et al., 2004;Gurioli et al., 2005a;Zanella et al., 2007Zanella et al., , 2014, and sedimentation dynamics of such currents (Dellino et al., 2021); and (v) heat effects on frescoes (Marcaida et al., 2019). In this section, the interplay among local stratigraphy, sequence of volcanic events, damages and victims at Pompeii archaeological site caused by the eruption is reported (Fig. 7), by exploring the available literature (see also section 3). ...
Article
Full-text available
A full review of the 79 CE Plinian eruption of Vesuvius is presented through a multidisciplinary approach, exploiting the integration of historical, stratigraphic, sedimentological, petrological, geophysical, paleoclimatic, and modelling studies dedicated to this famous and devastating natural event. All studies have critically been reviewed and integrated with original data, spanning from proximal to ultradistal findings of the 79 CE eruption products throughout the Mediterranean. The work not only combines different investigation approaches (stratigraphic, petrological, geophysical, modelling), but also follows temporally the 79 CE eruptive and depositional events, from the magma chamber to the most distal tephras. This has allowed us first to compile a full database of all findings of those deposits, then to relate the products (the deposits) to the genetic thermomechanical processes (the eruption), and lastly to better assess both the local and regional impacts of the 79 CE eruption in the environment. This information leads to a number of open issues (e.g., regional environmental impact vs. local pyroclastic current impact) that are worthy of further investigations, although the 79 CE eruption of Vesuvius is one of the best studied eruptions in volcanology. The structure of the work follows three macro-categories, the historical aspects, the products, and the processes of the 79 CE eruption. For each investigation approach (from stratigraphy to modelling), all dedicated studies and original data are discussed. The open issues are then synthesized in the discussion under a global view of Plinian eruptions, from the magma setting to its dispersion as pyroclasts flowing on the surface vs. falling from the volcanic plume. In this way, a lesson from the past, in particular from the well-studied 79 CE eruption of Vesuvius, will be of help for a better synchronization of processes and products in future developments. Lastly, various aspects for volcanic hazard assessment of Plinian eruptions are highlighted from the tephra distribution and modelling points of view, as these large natural phenomena can have a larger impact than previously thought, also at other active volcanoes.
... Individuals analysed from Casa del Fabbro. The analysed human remains came from Room 9 of the Casa del Fabbro (Regio I, Insula 10, civic 7), and their position and orientation are compatible with instantaneous death due to the approach of the high-temperature volcanic ash cloud 17 . More than half of individuals found in Pompeii died inside their houses, indicating a collective unawareness of the possibility of a volcanic eruption or that the risk was downplayed due to the relatively common land tremors in the region 2 . ...
Article
Full-text available
The archaeological site of Pompeii is one of the 54 UNESCO World Heritage sites in Italy, thanks to its uniqueness: the town was completely destroyed and buried by a Vesuvius’ eruption in 79 AD. In this work, we present a multidisciplinary approach with bioarchaeological and palaeogenomic analyses of two Pompeian human remains from the Casa del Fabbro. We have been able to characterize the genetic profile of the first Pompeian’ genome, which has strong affinities with the surrounding central Italian population from the Roman Imperial Age. Our findings suggest that, despite the extensive connection between Rome and other Mediterranean populations, a noticeable degree of genetic homogeneity exists in the Italian peninsula at that time. Moreover, palaeopathological analyses identified the presence of spinal tuberculosis and we further investigated the presence of ancient DNA from Mycobacterium tuberculosis. In conclusion, our study demonstrates the power of a combined approach to investigate ancient humans and confirms the possibility to retrieve ancient DNA from Pompeii human remains. Our initial findings provide a foundation to promote an intensive and extensive paleogenetic analysis in order to reconstruct the genetic history of population from Pompeii, a unique archaeological site.
... We also identify a secondary hazard associated with the overspill: the production of increased suspended ash. These plumes although cooler than the main overspill and channelized flow still represent a major hazard by increasing the suspended ash in the air which over minutes of exposure can cause asphyxia (Dellino et al., 2021). ...
Article
Full-text available
Due to their mobility, high velocities, and common occurrence, small‐volume pyroclastic density currents (PDCs) represent a major hazard around volcanoes. Small‐volume events are particularly sensitive to topography and channelization into drainage basins. Understanding the flow transition initiated by avulsion or overspill from valley confined PDC to unconfined PDC is necessary to mitigate their damage. We present three‐dimensional multiphase models of channelized PDCs and link the generation of overspill currents to channel geometry (width, depth, and curvature). Our simulation geometries span the range of natural channels commonly found in stratovolcanic landforms. Channels help inhibit ambient air entrainment into the underflow and can maintain and deliver material with minimal cooling during transport. We show that the main avulsion mechanism can include significant portions of the insulated underflow layer from the channel leading to a dangerously hot overspilled current. In all sinuous channel simulations, the underflow of the current becomes superelevated when it encounters bends and is able to overwhelm channel walls. The amount of mass overspilled is a function of channel curvature. Superelevation of the current increases with channel curvature and decreasing channel width but is underestimated using traditional estimates of superelevation due to the lack of the free surface in these flows. We also identify the occurrence of buoyant plume and secondary overspill events due to cross stream flow within the channel which would produce complex deposits associated with overspill events.
... Individuals analysed from Casa del Fabbro. The analysed human remains came from Room 9 of the Casa del Fabbro (Regio I, Insula 10, civic 7), and their position and orientation are compatible with instantaneous death due to the approach of the high-temperature volcanic ash cloud 17 . More than half of individuals found in Pompeii died inside their houses, indicating a collective unawareness of the possibility of a volcanic eruption or that the risk was downplayed due to the relatively common land tremors in the region 2 . ...
Article
Full-text available
The numerical modeling of compressible multiphase flows is of high interest for several engineering applications. In this work, we focus on the study of pyroclastic flows arising from volcanic eruptive events. An accurate evaluation of the effects of this multiphase flow is of crucial importance for the Civil Protection for the preservation of urban settlements in volcanic areas. In this work, we propose a Finite Element formulation for the simulation of pyroclastic flows at conditions of thermal and kinetic equilibrium. This analysis belongs to the class of advection dominated problems, which are known to suffer from numerical instabilities. The required stabilization is provided by using a Variational Multiscale Method, typically used for monophase compressible flows and extended here for the first time to multiphase flows. The stabilized formulation is validated with benchmark problems for compressible flows, which are solved for both monophase and multiphase cases. A throughout comparison of the numerical results of the two different flows is also presented. Moreover, the numerical formulation is applied to the simulation of representative cases of pyroclastic flows considering large-scale computational domains and realistic material properties and initial thermal-kinematic conditions. The numerical analyses presented show the accuracy of the proposed method for the simulation of compressible multiphase flows and its suitability for risk assessment studies of urban settlements prone to be affected by pyroclastic gravity currents.
Preprint
Full-text available
Pyroclastic density currents are ground hugging gas-particle flows moving at high speed down the volcano slope. They are among the most hazardous events of explosive volcanism, causing devastation and deaths 1,2 . Because of the hostile nature they cannot be analyzed directly and most of their fluid dynamic behavior is reconstructed by the deposits left in the geological record, which frequently show peculiar structures such as bedforms of the types of ripples and dunes 3,4 . In this paper, we simplify a set of equations that link flow behavior to particle motion and deposition. This allows, for the first time, the build up of a phase diagram by which the hazard of dilute pyroclastic density currents can be explored easily and quickly by inverting bedforms wavelength and grain size.
Article
Full-text available
After 43 years of repose, Taal Volcano erupted on 12 January 2020 forming hazardous base surges. Using field, remote sensing (i.e. UAV and LiDAR), and numerical methods, we gathered primary data to generate well-constrained observed information on dune bedform characteristics, impact dynamic pressures and velocities of base surges. This is to advance our knowledge on this type of hazard to understand and evaluate its consequences and risks. The dilute and wet surges traveled at 50-60 ms −1 near the crater rim and decelerated before making impact on coastal communities with dynamic pressures of at least 1.7 kPa. The base surges killed more than a thousand livestock in the southeast of Taal Volcano Island, and then traveled another ~ 600 m offshore. This work is a rare document of a complete, fresh, and practically undisturbed base surge deposit, important in the study of dune deposits formed by volcanic and other processes on Earth and other planets.
Article
Full-text available
The entrainment rate of pyroclastic density currents is investigated by large‐scale experiments. The ground flows are initiated by the impact on the terrain of a dense gas‐particle fountain issuing from a cylindrical conduit, similarly to natural volcanic events. On impact, the excess density with respect to the surrounding atmosphere was up to 11.6 kg/m³, making the currents non‐Boussinesq. A power law model of the entrainment rate is developed, which is similar to that proposed for snow avalanches by Ancey (2004, https://doi.org/10.1029/2003JF000052) and is verified for the Richardson's number range between 0.25 and 5.95. Rapid changes of the entrainment are caused by (i) strong accelerations at the fountain impact on the ground; (ii) break in slope; and (iii) topographic obstacles. Such changes, together with the sedimentation rate, influence flow mobility. The use of the power law is suggested for modeling the motion of unsteady hazardous geophysical mass flows such as pyroclastic density currents and snow avalanches.
Article
Full-text available
Pyroclastic density currents are highly dangerous ground-hugging currents from volcanoes that cause >50% of volcanic fatalities globally. These hot mixtures of volcanic particles and gas exhibit remarkable fluidity, which allows them to transport thousands to millions of tonnes of volcanic material across the Earth’s surface over tens to hundreds of kilometres, bypassing tortuous flow paths and ignoring rough substrates and flat and upsloping terrain. Their fluidity is attributed to an internal process that counters granular friction. However, it is difficult to measure inside pyroclastic density currents to quantify such a friction-defying mechanism. Here we show, through large-scale experiments and numerical multiphase modelling, that pyroclastic density currents generate their own air lubrication. This forms a near-frictionless basal region. Air lubrication develops under high basal shear when air is locally forced downwards by reversed pressure gradients and displaces particles upward. We show that air lubrication is enhanced through a positive feedback mechanism, explaining how pyroclastic density currents are able to propagate over slopes much shallower than the angle of repose of any natural granular material. This discovery necessitates a re-evaluation of hazard models that aim to predict the velocity, runout and spreading of pyroclastic density currents.
Article
Full-text available
In AD 79 the town of Herculaneum was suddenly hit and overwhelmed by volcanic ash-avalanches that killed all its remaining residents, as also occurred in Pompeii and other settlements as far as 20 kilometers from Vesuvius. New investigations on the victims' skeletons unearthed from the ash deposit filling 12 waterfront chambers have now revealed widespread preservation of atypical red and black mineral residues encrusting the bones, which also impregnate the ash filling the intracranial cavity and the ash-bed encasing the skeletons. Here we show the unique detection of large amounts of iron and iron oxides from such residues, as revealed by inductively coupled plasma mass spectrometry and Raman microspectroscopy, thought to be the final products of heme iron upon thermal decomposition. The extraordinarily rare preservation of significant putative evidence of hemoprotein thermal degradation from the eruption victims strongly suggests the rapid vaporization of body fluids and soft tissues of people at death due to exposure to extreme heat.
Article
Full-text available
This paper presents PYFLOW_2.0, a hazard tool for the calculation of the impact parameters of dilute pyroclastic density currents (DPDCs). DPDCs represent the dilute turbulent type of gravity flows that occur during explosive volcanic eruptions; their hazard is the result of their mobility and the capability to laterally impact buildings and infrastructures and to transport variable amounts of volcanic ash along the path. Starting from data coming from the analysis of deposits formed by DPDCs, PYFLOW_2.0 calculates the flow properties (e.g., velocity, bulk density, thickness) and impact parameters (dynamic pressure, deposition time) at the location of the sampled outcrop. Given the inherent uncertainties related to sampling, laboratory analyses, and modeling assumptions, the program provides ranges of variations and probability density functions of the impact parameters rather than single specific values; from these functions, the user can interrogate the program to obtain the value of the computed impact parameter at any specified exceedance probability. In this paper, the sedimentological models implemented in PYFLOW_2.0 are presented, program functionalities are briefly introduced, and two application examples are discussed so as to show the capabilities of the software in quantifying the impact of the analyzed DPDCs in terms of dynamic pressure, volcanic ash concentration, and residence time in the atmosphere. The software and user’s manual are made available as a downloadable electronic supplement.
Article
Full-text available
This study of burns patients from two eruptions of Merapi volcano, Java, in 1994 and 2010, is the first detailed analysis to be reported of thermal injuries in a large series of hospitalised victims of pyroclastic surges, one of the most devastating phenomena in explosive eruptions. Emergency planners in volcanic crises in populated areas have to integrate the health sector into disaster management and be aware of the nature of the surge impacts and the types of burns victims to be expected in a worst scenario, potentially in numbers and in severity that would overwhelm normal treatment facilities. In our series, 106 patients from the two eruptions were treated in the same major hospital in Yogyakarta and a third of these survived. Seventy-eight per cent were admitted with over 40% TBSA (total body surface area) burns and around 80% of patients were suspected of having at least some degree of inhalation injury as well. Thirty five patients suffered over 80% TBSA burns and only one of these survived. Crucially, 45% of patients were in the 40-79% TBSA range, with most suspected of suffering from inhalation injury, for whom survival was most dependent on the hospital treatment they received. After reviewing the evidence from recent major eruptions and outlining the thermal hazards of surges, we relate the type and severity of the injuries of these patients to the temperatures and dynamics of the pyroclastic surges, as derived from the environmental impacts and associated eruption processes evaluated in our field surveys and interviews conducted by our multi-disciplinary team. Effective warnings, adequate evacuation measures, and political will are all essential in volcanic crises in populated areas to prevent future catastrophes on this scale.
Book
Fluid Physics in Geology is aimed at geology students who are interested in understanding fluid behavior and motion in the context of a wide variety of geological problems, and who wish to pursue related work in fluid physics. The book provides an introductory treatment of the physical and dynamical behaviors of fluids by focusing first on how fluids behave in a general way, then looking more specifically at how they are involved in certain geological processes. The text is written so students may concentrate on the sections that are most relevant to their own needs. Helpful problems following each chapter illustrate applications of the material to realistic problems involving groundwater flows, magma dynamics, open-channel flows, and thermal convection. Fluid Physics in Geology is ideal for graduate courses in all areas of geology, including hydrology, geomorphology, sedimentology, and petrology.
Article
A new stratigraphic survey of the pyroclastic deposits blanketing Pompeii ruins shows departures from prior reconstruction of the events that occurred inside the town during the two main phases (pumice fallout and pyroclastic density currents) of the ad 79 Vesuvius eruption. We document the depth and distribution of subaerial erosion surfaces in the upper part of the pyroclastic sequence, formed during two short-lived breaks occurring in the course of the second phase of the eruption. These pauses could explain why 50% of the victims were found in the streets during the pyroclastic density currents phase.
Article
Understanding the thermal behavior of pyroclastic density currents (PDCs) is crucial for forecasting impact scenarios for exposed populations as it affects their lethality and destructiveness. Here we report the emplacement temperatures of PDC deposits produced during the paroxysmal explosive eruption of Merapi (Central Java) on 5 November 2010 based on the reflectance of entombed charcoal fragments. This event was anomalously explosive for Merapi, and destroyed the summit dome that had been rapidly growing, with partial collapses and associated PDCs, since October 26. Results show mean reflectance values mainly between 0.17 and 0.41. These new data provide a minimum temperature of the flow of 240–320 °C, consistent with previous estimations determined from independent field, engineering, and medical observations published in the literature for this eruption. A few charcoal fragments recorded higher values, suggestive of temperatures up to 450 °C, and we suggest that this is due to the thermal disequilibrium of the deposits, with larger block-size clasts being much hotter than the surrounding ash matrix. Charring temperatures show no major differences between proximal and distal PDC deposits and are significantly lower than those that may be associated with a fast growing dome dominated by dense and vitric non-vesicular rocks. We therefore infer that the decrease in temperature from that at fragmentation (>900 °C) occurred in the very initial part of the current, <2 km from source. We discuss possible processes that allow the very fast cooling of these energetic PDCs, as well as the conservative thermal behavior shown during the depositional phase, across the entire depositional area.
Article
Large‐scale experiments generating ground‐hugging multiphase flows were carried out with the aim of modelling the rate of sedimentation, Sr, of pyroclastic density currents. The current was initiated by the impact on the ground of a dense gas‐particle fountain issuing from a vertical conduit. On impact, a thick massive deposit was formed. The grain size of the massive deposit was almost identical to that of the mixture feeding the fountain, suggesting that similar layers formed at the impact of a natural volcanic fountain should be representative of the parent grain‐size distribution of the eruption. The flow evolved laterally into a turbulent suspension current that sedimented a thin, tractive layer. A good correlation was found between the ratio transported/sedimented load and the normalized Rouse number, Pn*, of the turbulent current. A model of the sedimentation rate was developed, which shows a relationship between grain size and flow runout. A current fed with coarser particles has a higher sedimentation rate, a larger grain‐size selectivity and runs shorter than a current fed with finer particles. Application of the model to pyroclastic deposits of Vesuvius and Campi Flegrei of Southern Italy resulted in sedimentation rates falling inside the range of experiments and allowed definition of the duration of pyroclastic density currents, τdep, which add important information on the hazard of such dangerous flows. The model could possibly be extended, in the future, to other geological density currents as, for example, turbidity currents. This article is protected by copyright. All rights reserved.
Article
Pyroclastic density currents (PDCs) can have devastating impacts on urban settlements, due to their dynamic pressure and high temperatures. Our degree of understanding of the interplay between these hot currents and the affected infrastructures is thus fundamental not only to implement our strategies for risk reduction, but also to better understand PDC dynamics. We studied the temperature of emplacement of PDC deposits that destroyed and buried the Villa dei Papiri, an aristocratic Roman edifice located just outside the Herculaneum city, during the AD79 plinian eruption of Mt Vesuvius (Italy) by using the thermal remanent magnetization of embedded lithic clasts. The PDC deposits around and inside the Villa show substantial internal thermal disequilibrium. In areas affected by convective mixing with surface water or with collapsed walls, temperatures average at around 270 °C (min 190 °C, max 300 °C). Where the deposits show no evidence of mixing with external material, the temperature is much higher, averaging at 350 °C (min 300 °C; max 440 °C). Numerical simulations and comparison with temperatures retrieved at the very same sites from the reflectance of charcoal fragments indicate that such thermal disequilibrium can be maintained inside the PDC deposit for time-scales well over 24 hours, i.e. the acquisition time of deposit temperatures for common proxies. We reconstructed in detail the history of the progressive destruction and burial of Villa dei Papiri and infer that the rather homogeneous highest deposit temperatures (average 350 °C) were carried by the ash-sized fraction in thermal equilibrium with the fluid phase of the incoming PDCs. These temperatures can be lowered on short time- (less than hours) and length-scales (meters to tens of meters) only where convective mixing with external materials or fluids occurs. By contrast, where the Villa walls remained standing the thermal exchange was only conductive and very slow, i.e. negligible at 50 cm distance from contact after 24 hours. We then argue that the state of conservation of materials buried by PDC deposits largely depends on the style of the thermal interactions. Here we also suggest that PDC deposit temperatures are excellent proxies for the temperatures of basal parts of PDCs close to their depositional boundary layer. This general conclusion stresses the importance of mapping of deposit temperatures for the understanding of thermal processes associated with PDC flow dynamics and during their interaction with the affected environment.