Computer simulations of the Tuscolano Artemisio's second pyroclastic flow unit (Alban Hills, Latium, Italy)

ELSEVIER Journal of Volcanology and Geothermal Research 61 (1994) 69-94

Journalof volcanology and geothermal research

Computer simulations of the Tuscolano Artemisio's second pyroclastic flow unit (Alban Hills, Latium, Italy)

G u i d o Giordano" , Flavio Dobran b,c

aUniversitil "'La Sapienza'" di Roma, Piazzale Aldo Moro 5, 00185 Roma, Baly bDepartment of Earth System Science, New York University, New York, NY 10003, USA

CIstituto Nazionale di Geofisica, Via di Vigna Murata 605, Roma, Italy

(Received April 16, 1993; revised version accepted January 12, 1994)

Abstract

Computer simulations of the second pyroclastic flow unit of Tuscolano Artemisio volcanic eruption were carried out using a complex thermofluid-dynamic two-phase flow model. The granular flow model accounts for the me- chanical and thermal nonequilibrium between the phases, particle collisions, and gas turbulence. The simulations involved actual topography of the volcano and a barrier placed at about twenty kilometers from the vent, several particle size classes, different vent diameters, and different gas and pyroclast velocities. The results from simulations show that smaller pyroclasts and larger mass eruption rates produce larger travel distances of pyroclastic flows. Computer simulations also predicted particle segregation from the flow at the break-in-slope of the volcano and at the base of the barrier, coignimbrite or phoenix columns above the flow, and flow reversals along the barrier slope. These results were found to be consistent with the general field characteristics of the second pyroclastic flow ofTuscolano Artemisio, including its large travel distance from the vent, and large deposit thicknesses at the break- in-slope of the volcano and at the base of the barrier. The simulations suggest the formation of fumarolic pipes and depositional surfaces along the barrier, and a mass eruption rate of about 109 kg/s.

1. Introduction

Recent advances in complex computer simulations of collapsing volcanic columns (Valen- tine and Wohletz, 1989; Valentine et al., 1991; Dobran et al., 1993) are permitting progres- sively detailed studies of the effects of eruption parameters on the generation and emplacement dynamics and thermodynamics of pyroclastic flows. A hot mixture of gas and pyroclasts exiting from a volcanic vent or crater penetrates into the atmosphere due to the high momentum of the jet. When this vertical momentum is exhausted at a certain height above the vent, the mixture

may ascend as a buoyant plume and generate a plinian eruption column or it may collapse, depending on whether the mixture density is lower or higher than the undisturbed air density at that height. A collapsing volcanic column produces a fountain above the vent and radially spreading pyroclastic flows. A pyroclastic flow may even- tually become unstable due to the buoyancy forces in the flow and generate one or more coignimbrite or phoenix columns (Dobran et al., 1993; Woods and Wohletz, 1991 ) on top of the flow. Depending on eruption parameters (vent diameter, pyroclasts sizes, gas and pyroclasts velocities, temperatures, and gas concentrations),

0377-0273/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0377-0273 ( 94 ) 00013-7

70 G. Giordano, F. Dobran / Journal of I/olcanology and Geothermal Research 61 (1994) 69-94

a collapsing volcanic column may exhibit fountain height oscillations, complex recycling of material from the collapsed column back into the jet thrust region, pulsation ofpyroclast mass flow- rate, and other processes (Valentine et al., 1991; Dobran et al., 1993; Neri and Dobran, 1993). The location of caldera rims around a vent can introduce further complicating features in the flow field depending on whether the column col- lapses on the inside or outside of the rims (Val- entine et al., 1992).

The results from complex numerical simulations of volcanic columns are consistent with the isothermal experiments performed in tanks with jets of fresh water and solid particles (Carey et al., 1988) and with downward injection of mixtures of methanol and ethylene glycol into a tank with fresh water (Woods and Cauldfield, 1992 ). This consistency and further verifications of models with laboratory data, which preserve the kinematic, dynamic, and thermal similitude conditions of actual volcanic columns, are very important because the models may then be employed to provide new physical constraints associated with transport and emplacement of pyroclastic flows (Fisher, 1983; Postma, 1986; Fisher et al., 1991; Branney and Kokelaar, 1992 ). As a further step forward in developing computer simulation capabilities for emplacement dynamics of pyroclastic flows produced by collapsing volcanic columns, it is necessary to ver- ify the models with some real volcanic eruptions. The volcanic column and pyroclastic flow models identified above assume, however, an axial sym- metry of flow emplacement and cannot be properly verified with asymmetrical volcanic eruptions. For this reason, it is necessary to test these models against eruptions which emplaced pyroclastic flows nearly symmetrically.

The second pyroclastic flow unit of Tuscolano Artemisio (IlptTA) located in the Alban Hills (Latium, Italy ) is ideally suited for verifying the results from complex pyroclastic flow models and for extracting new information from such models for better understanding of pyroclastic flow emplacement dynamics. This paper presents the results from computer simulations of several minutes of the emplacement of IlpfTA with an

important restriction that the modeling approach is limited to a single particle size. The results from simulations qualitatively reproduced several expected features of pyroclastic flows including: attainment of large travel distances of the flows; tendency of the flows to travel shorter distances when the particle size is large; concentration of pyroclastic material at the base of the flow, in the vicinity of the break-in-slope of the volcano, and at a morphological barrier at a large distance from the eruptive center; and the ability of flows to climb high obstacles and generate coignimbrite plumes above the flows. The re- suits from simulations were compared with some characteristics associated with different morphological features surrounding the volcano, and used to infer new flow characteristics associated with the IlptTA.

2. Characteristics of the second pyroclastic flow unit of Tnscolano Artemisio

During 0.5-0.36 Ma (million years ago), the Alban Hills volcanic complex produced at least four paroxysmal explosive events and related pyroclastic flow units (De Rita et al., 1988a). IlptTA marks the beginning of the second cycle at the central crater and is probably related to the collapse of the caldera. IlptTA, which was emplaced nearly symmetrically about the vent (Fig. 1 ), is the most widespread unit of the AI- ban Hills volcanic complex, with an outcrop area covering about 1600 km 2 around the present caldera (De Rita et al., 1988b). The maximum distance reached by the pyroclastic flow unit is about 33 km from the eruptive center and the deposit volume is at least 34 km 3. The unit has an age of 0.48 _+ 0.01 m.y. (Biddittu et al., 1979 ) and IlptTA was classified by Fornaseri et al. ( 1963 ) as a leucitite belonging to the High Potassic Se- ries of the Roman Comagmatic Province. It is stratigraphically composed of (De Rita et al., 1988b) (Fig. 2):

( 1 ) A basal fall deposit, found only in the east- ern sector of the volcano, which is composed of two layers of scoriae lapilli. The deposit is re- versely-graded with a maximum thickness of 70

G. Giordano, F. Dobran / Journal of Volcanology and Geothermal Research 61 (1994) 69-94 71

OUTCROP MAP OF II PYROCLASTIC FLOW OF TUSCOLANO - ARTEMISIO

ALBAN HILLS VOLCANIC COMPLEX

: ° o o

i " ' P " N ° o o

, o° , ~ . ~ ,

° N E M I k .

I =o ° o ° / \\ o° ° o

\ " ' , " ° 2 " ° ° ~ o

o °

TYRRHENIAN ~ \ / ° g°°°o" SEA ~ , ~ °

! " ~ \ \

O 10 km~" "" ~ L' ' \ - - - - -

SW m

COLLE DELLO STONIO

/ /

/ i

m

/ 1 /

/ /

m II PYROCLASTIC FLOW OF T.A.

", ,, OUTCROP LIMIT OF II PF. T.A.

APENNINES

WELLS A ~ A SECTIONS

'qf" "~',~ CALDERA RIM

1000 ], SECTION A-A 8 0 0 ~ . ,..

i!it so: NE m

NEMI . ~1000 SECTION B-B ~ ~ 8 0 0

~ ~ .............. ... ~oo

Fig. 1. Outcrop map of the second pyroclastic f low unit ofTuscolano A~emisio, Lat ium, Italy. Sections A - A and B-B show the geometry of deposits in two different radial directions from the eruptive center (De Rita et al., 1988b).

72 G. Giordano, F. Dobran / Journal oJ Volcanology and Geothermal Research 61 (1994) 69-94

PYROCLASTIC UNIT: SECOND PYROCLASTIC FLOW UNIT OF TUSCOLANO ARTEMISIO

LOCALITY: ALBAN HILLS VOLCANIC COMPLEX, LATIUM, ITALY

LITHOLOGY: MASSIVE, CHAOTIC, UNWELDED TUFF, WITH A COARSE-ASHY MATRIX, AND WITH LAVA, INTRUSIVE, AND THERMALLY METAMORPHOSED SEDIMENTARY XENOLITHS. FUMAROLIC PIPES ARE PRESENT.

AREA: 1600 km 2

MEAN THICKNESS: 21 m

MINIMUM VOLUME: 34 km 3

MEAN DEPOSIT DENSITY: 1250 kg/m 3

D.R.E. VOLUME: 17 km 3

MAXIMUM RUNOUT: 33 km

MAXIMUM HEIGHT CLIMBED: 250 m

ASPECT RATIO: 6 x 1 0 -4

PETROLOGY: HIGH K LEUCITITES

AGE: 0.48+0.01 m.y.a.

2001.tm PARTICLE DENSITY: 2000 kg/m 3

1000#m PARTICLE DENSITY: 1500 kg/m 3

16000pm PARTICLE DENSITY: 1150 kg/m 3

F--

z

O ..J LL

L) F- co < O O cc >- o_

E~ z 0 o LU CO

o " " ,

a: A S H - F A L L

i: I G N I M B R I T E

f~ F A L L

s: PALEOSOIL

Fig. 2. Stratigraphy of the second

cm, and can be considered as subplinian (Walker, 1973; Pyle, 1989).

(2) A main ignimbrite which is a scoriaceous deposit, showing a color ranging from brown-gray to violet, with a prevalent coarse-ash matrix, intrusive, and thermally metamorphosed sedimentary xenoliths. Most of the deposit is massive. The unit consists mainly of poorly vesiculated and roughly rounded scoriae. The deposit is a low-aspect-ratio ignimbrite (mean thickness/ mean length = 6 × 10- 4 ), with thicknesses ranging from 20 to 30 m; maximum thicknesses occur at the break-in- slope of the volcano, in front of the Apennine Mountains, and in paleovalleys

cycle of Tuscolano Artemisio volcano.

(Figs. 1 and 3 ). In these places the deposit dis- plays fumarolic pipes and columnar jointing. Where the flow was emplaced on morphological barriers it shows some flow unit boundaries. The ignimbrite is largely covered by a coignimbrite ash deposit that is massive to stratified, showing a thickness decreasing from two to one meters moving away from the vent, and development of a soil profile on top.

The IlpfrA is a low-grade unit (Walker, 1983 ), generally unconsolidated and in places indur- ated by zeolites. Near the Apennine Mountains, which are located about 20 km from the eruptive center, the unit surmounts heights of more than


LIMIT OF EXTENSION OF II PF T.A.

ISOPACH OF PYROCLASTIC FLOW (m)

APENNINES

CALDERA RIM

/ ' ) BREAK-IN-SLOPE

Fig. 3. Isopach map of the second pyroclastic flow unit (De Rita et al., 1988b). The figure also shows the break-in-slope of the volcano with a dot-dashed line. Notice the increase in deposit thickness after the break-in-slope and in front of the Apennines.

250 m and reaches altitudes of more than 440 m a.s.1. Here the deposits contain few lithics, and their maximum size is about 1 cm smaller than those measured at the same distances in areas not influenced by the topography (Fig. 4) (Gior- dano and Chiarabba, 1992). In general, lithic sizes decrease with an increasing distance from the eruptive center (Fig. 4), but show no size gradation vertically within individual stratigraphic sections. The granulometric analysis of the deposit shows a relatively low (less than 15 wt.%) abundance of fine ash (less than 62 am) , and in a Walker's diagram (Walker, 1971, 1983) the samples plot between the fall and pyroclastic flow fields (Fig. 5a). Moreover, on a plot of

weight fraction of particles less than 1 mm vs. weight fraction of particles less than 1/16 mm (Fig. 5b), the field data lie in the fines-depleted field, showing an increase in the fine ash content in distal facies. The mean size varies with distance between - 1 and + 1 ¢ (De Rita et al., 1988b). Even if this fines poverty could be related to the relative low vesicularity of the shards, as shown in scanning electron microprobe anal- yses carried out by De Rita et al. (1988b), the presence of a coignimbrite ash deposit, and the great mobility of the pyroclastic flow appear to indicate a depletion during flow. The granulometric curves approach log-normal profiles (Giordano and Rosa, 1992 ). Glass is always the

74 G. Giordano, F. Dobran /Journal of Volcanology and Geothermal Research 61 (1994) 69- 94

I

<

> <

10-

O O

• o • • !: ::.: :-.:.:!.:.;.., • • • II

0 i i i i I

10 14 18 22 26 30

DISTANCE FROM ERUPTIVE CENTER (km)

34

Fig. 4. Distribution of average maximum diameter of lithics with distance from the eruptive center. The circles refer to data taken in the absence of topographic barriers, triangles refer to data taken on the distal side of Colle dello Stonio (see text and Fig. 1 ), and squares refer to topographically high areas (modified from De Rita et al., 1988b).

principal component in the deposit (80-95 wt.%), whereas the crystal content ranges from 1 to 2 wt.%.

3. Summary of the physical model and input data for the simulations

3.1 Summary of the physical model

The physical model employed to produce simulations of the IlpfTA is described in Dobran et al. (1993), and for sake of brevity we omit the details of the method. The model involves the conservation of mass equations for the gas and pyroclasts phases, and for the water vapor and air which mix in the atmosphere after the vapor and pyroclasts are ejected from the vent. The momentum and energy balance equations model each phase separately allowing for the mechani- cal (velocity and pressure differences) and thermal (temperature differences) nonequilibrium effects between the phases. The gas turbulence is modeled by a turbulent subgrid scale model, whereas the particle-particle interaction in the flow is modeled by a granular flow model based on the kinetic theory. In application of kinetic theory to granular flows, the pressure and viscos- ity depend on particle collision frequency and is appropriate for dense mixtures occurring in pyroclastic flows. The particle collisions were not assumed to be perfectly elastic but the particles were assumed to be of a single size.

The solution of the mass, momentum, and en-

.!t / J t

. f " \ . FLOW

/ / 8 % ' , \ ' \ l" \. "\.

(a)

"X t "\ "...

l - / ' 4-" , o . . . . . . . . .-"

I FALL ~ ,

4 3 2 1 0 1 2 3 4

60"

50-

40-

30- , 0 /

20" FINES DEPLETED ~W/ ~ \ • / eo • o / . / "

FLOW / I ~"

10- < ~ ,~ - / T o.o_~.o.~_...~_..~.Z_ i / /~f °%- °- ? ' ~--eIT -- - o /°" "

0 l "-, ," ,

0 20 40 60 80

Mdo F 1

(b) // / /

FLOW . J ' / ' /

/ " - - " J ' / " / / . / " /

/

Fig. 5. Grainsize data of the second pyroclastic flow unit. (a) Median vs. standard deviation plot of Walker ( 1971 ) G iordano and Rosa, 1992). (b) F~ (weight fraction of particles less than 1 mm ) vs. F 2 (weight fraction of particles less than 1 / 16 m m ) plot (Walker, 1983 ). The triangles, full circles, and open circles refer to data taken between 12 and 18, 18 and 24, and 24 and 32 km from the vent, respectively.

I00

G. Giordano. F. Dobran / Journal of Volcanology and Geothermal Research 61 (1994) 69-94 75

ergy balance equations was carried out in a two- dimensional axisymmetric flow domain with variable grid sizes in order to maximize the computational resources and extents of domains. The cell size was 10 m in vertical direction close to the ground and in radial direction close to the vent. At large distances from the vent, the cell size was 100 m, in both radial and vertical directions. The simulations were carried up to 20 min over radial extents of 25-30 km and vertical extents from 3 to 5 kin. When the condensation of water vapor in the erupting plume occurred, the simulations were terminated because phase changes are not included in the governing equations. Complex simulations of volcanic columns also require adequate convergence studies before accepting the results from simulations. These studies were carried out by Dobran et al. ( 1993 ) with many different vent parameters and computational domain sizes involving the same physical model. This study established the ade- quacy of computational parameters employed in present simulations.

The initial conditions for the simulations were assumed to correspond to the standard atmosphere which has a vertical temperature and pressure stratification. The boundary conditions in the model corresponded to no mass, momentum, and energy fluxes across the axis of sym- metry passing through the center of the vent, and no slip condition along the slope of the volcano. No heat transfer from the flow to the ground was permitted, and at the vertical and radial outflow boundaries no gradients of the dependent vari- ables were allowed.

Computer simulations were made for various initial conditions as shown in Table 1. Graphical output was obtained by postprocessing the re- suits from computer simulations to show temporal and spatial variations of important physical parameters.

3.2 Input data for the simulations

Computer simulations of the I lpf rA included the topography of the volcano as shown in sections A-A and B-B of Fig. 1. This topography defines the volcanic vent location at an elevation

of 900 m a.s.1. (De Rita et al., 1988b). The slope of the volcano gradually levels off at a distance of 9 km from the vent. At about 19 km from the eruptive center, a topographic obstacle was simulated by a 250-m-high and 2-km-wide barrier with a triangular section. The input data for the simulations involved two-phase flow conditions at the vent and the vent geometry. These consist of vent diameter, volume fraction and average size of particles, pressure, and gas and particle velocities and temperatures (Table 1 ). We established these data from the field data, comparison with the historical eruptions of similar erupted magma volumes, as well as from pre- vious numerical simulations of large-scale volcanic eruptions (Sparks et al., 1978; Wohletz et al., 1984; Valentine and Wohletz, 1989; Dobran et al., 1993).

Mass eruption rate The mass eruption rate of the I lpfrA was es-

timated from the field data. I lpf rA deposited about 34 krn 3 of material (De Rita et al., 1988b). Using the mean deposit density of 1250 k g / m 3, we obtained a value of the DRE (Dense Rock Equivalent) volume of 17 km 3 which corresponds to a maximum discharge of 4.25X 1013 kg of leucititic magma. To establish the duration of the eruption it may be noted that the deposit lacks significant depositional discontinuities, ex- ept in isolated cases. This implies that I lpfrA emplaced during a single eruptive event. Erup- tions which produced deposits of l0 km 3 or larger are thought to have a duration of 3-30 hr (Wil- liams, 1941; Wilson et al., 1978; Fisher and Schmincke, 1984; Spera, 1984; Cas and Wright, 1987; Francis and Self, 1987). It was thus assumed that the MER (Mass Eruption Rate) ranged from 4× 108 to 4X l0 9 kg/s .

Column collapse height and velocity o f gas-pyroclast mixture at the vent

The determination of the column collapse height is commonly established in the literature by the method of energy line (Hsu, 1975; Sheri- dan, 1979; Sheridan and Malin, 1983 ). This line is constructed on a graph of the topography of the volcano before the eruption (H) vs. the max-

76 G. Giordano, F. Dobran /Journal of l~blcanology and Geothermal Research 61 (1994) 69-94

Table 1 Geometry and flow conditions of gas and pyroclasts at the volcanic vents of eruptions simulating the Tuscolano- Artemisio's II "d pyroclastic flow unit. The discharge rates and mean densities do not form the input data for the simulations since they can be computed from the information of other parameters in the table. The fountain heights of the collapsing columns were obtained from simulations.

No. Dv Tv Vv Pv %v Pp dp nt Zc (m) (K) ( m / s ) (MPa) (kg/m 3 ) (am) (kg/s) (m)

A 900 1373 150 0.1 0.015 2200 100 3 . I6× 109 1100 B 900 1373 150 0.1 0.015 2000 200 2.87 × 109 1100 C 900 1373 150 0.1 0.015 1700 500 2.45× 109 1100 D 600 1373 100 0.1 0.015 2000 200 8.52X 108 550 E 600 1373 100 0.1 0.015 2000 500 8.52X 108 550

imum distance traveled by the pyroclastic flow (L). A line constructed in this manner shows that the minimum collapse height of the eruption producing IlpfTA is Zc= 350 m above the vent. The maximum collapse height, about 3300 m above the vent, was estimated by taking into ac- count those topographic features surrounding the volcano which completely blocked the flow. The estimated range of the Heim coefficient, H/L, of 0.03-0.14 is in good agreement with other meth- ods of estimations. For example, Ui ( 1983 ) proposed a linear relation between log(H/L) and log (erupted volume ) which for erupted volumes exceeding 10 km 3 gives H/L<O.05. Sparks (1976) proposed an H/L diagram for ignimbrites, slides, and avalanches. The distances of 30 km reached by these flows correspond to vertical drops of 1000-2000 m, which by consider- ing the vent location in Fig. 1 of about 900 m a.s.1, results in 100-1100 m for the range of column collapse height above the vent.

The velocity of the erupting mixture of gas and pyroclasts can be estimated by using the following simple equation:

tar=- (2gZc) 1/2 ( 1 )

which for Zc=350-3300 m gives Vv=80-250 m / s .

Pyroclast size and density at the vent The present computer simulations require a

single particle size class which is probably the major drawback of the model, since actual pyroclastic flows contain wide ranges of particles. For

this reason, we employed various particle sizes, whose densities were determined experimen- tally, to test the influence of particle size on the pyroclastic flow dynamics. It should be realized, however, that such an approach of studying the flow dynamics is approximate, since the results from simulations involving constant particle sizes may not be additive due to the nonlinear effect produced by different size particles in a pyroclastic flow. Until polydispersed particle simulations are performed and properly verified with data, the results from simulations should be therefore interpreted with caution and not used to infer the local flow dynamics of pyroclastic flows. The choice of the particle size can be helped by the classification of Lowe ( 1982 ) who identified three main particle size populations in terms of their behavior in granular flows. The first population consists of clay, silt, and fine- to medium-grained sand-size particles (fine ash) which can be maintained, independent of their concentration, in suspension by the fluid turbulence alone. In the IIpfFA unit this particle class comprises 40-50 wt.%. The second population, made of coarse-grained sand (coarse ash) to small pebble-sized gravel (lapilli), is not fully suspended within dilute flows, whereas in concentrated suspensions having a wide range of particle sizes, these coarser grains can be supported by the combined effects of turbulence, hindered settling, and buoyant lift provided by the interstitial mixture of water and finer grained sediment. In the IIpfrA this particle population comprises 50-60 wt.% of the deposit. The third


grain size population of Lowe is comprised from pebble and cobble-sized clasts (lapilli to block sizes), which is practically absent in all observed outcrops of IIptTA. This classification appears to be useful for pyroclastic flows, since it is in good accord with the behavior of similar size classes ofpyroclasts (Sparks, 1976; Sparks et al., 1978; Brissette and Lajoie, 1990). Based on these considerations, the first particle size population was selected for the parametric study, because its transport mechanism is less conditioned by the presence of grains of other granulometric classes. Moreover, this particle size population tends to be in full suspension even in highly concentrated flows (Lowe, 1982). The pyroclast sizes chosen for computer simulations ranged from 100 to 500 /tm.

Pyroclast volume fraction and mixture density The pyroclast volume fraction at the vent can

be inferred indirectly by geophysical and petro- logical studies. Chiarabba (1993) and Chia- rabba et al. (1993) hypothesized from seismo- logical and geological data that the top of the magma chamber of IlptTA was located between 6 and 10 km. This information, combined with the known stratigraphy of the volcano (Funi- ciello and Parotto, 1978; Chiarabba, 1993 ), implies a magma chamber pressure between 150 and 200 MPa, which corresponds to the saturation solubility ofvolatiles of about 4 wt.% (Dolfi and Trigila, 1978, 1983 ). However, recent works show that the evolution of potassic Roman mag- mas develops nearly in absence of water (probably less than 1 wt.%) and in saturation of car- bon dioxide (Trigila et al., 1992). Preliminary data on another scoriaceous ignimbrite in Alban Hills (Fourth pyroclastic flow unit of Tuscolano Artemisio) also confirm this interpretation (R. Trigila, pers. commun., 1993). It would be in- consistent with these considerations to assume a large content of volatiles in the erupting magma of IlpfTA and we assumed instead a particle volumetric fraction ¢pv=0.015 which gives an exsolved gas mass fraction of X~v~0.005, depending on particle densities which change according to their size (Table 1 ). The mixture density was calculated according to:

Pm = ( 1 -- epv)Pgv +%vPp (2)

where the subscripts g and p represent the gas and pyroclast phases, respectively.

eyroclasl temperature at the vent The pyroclasts temperature at the vent was

chosen on the basis of the tephritic-leucititic magma as 1100°C (Dolfi and Trigila, 1978).

Vent diameter The vent diameter corresponds to the diame-

ter of the crater of the Tuscolano Artemisio volcano where the pressure of the exiting gas-particle jet is nearly balanced with the atmospheric pressure. This diameter was assumed to be in the range 600-900 m and is in good accord with morphometrical studies on alkaline stratovol- canoes (Pike and Clow, 1981; Cas and Wright, 1987), and with the mass eruption rate, mean density, and velocity at the vent as established above.

It is clear that some of the input parameters for simulations of the IlptTA are poorly con- strained. However, results from simulations and comparison with field observations can be used to further constrain these parameters as well as for providing new information of the pyroclastic flow dynamics. During each simulation, the vent parameters were held fixed at values listed in Ta- ble 1.

4. Results from computer simulations

The results from computer simulations involving the simulation A are shown in Figs. 6- 10, which illustrate the temporal and spatial distributions of pyroclast volumetric fraction, mixture density, gas and pyroclast velocities, and temperatures. The distribution of particle volumetric fraction of simulation A, which was simulated with a particle size of 100 am, is illustrated in Fig. 6 at 80, 180, 280, 380, 460, and 900 s after the onset of the eruption. At 80 s the column has formed a fountain, and a pyroclastic flow is moving down the slope of the volcano. After 180 s, the flow head reaches a distance of

78 G. Giordano, F. Dobran /Journal oJ Volcanology and Geothermal Research 61 (1994) 69-94

5-

o--1- 0

4 -

N 2 -

It = 80 s I

5 10 15 20 25

R (kin)

,---.3

0-

0 5 10 15 20 25

R (krn)

5

4-

f - . 3 -

N 2 -

1"

0

0

I t = 280 s

, , I , r , , ,

5 10 15 20 25

R (km)

S

4-

a 2 -

1-

0

0

t = .580 s

5 10 15 20 25

R (k~)

4 -

N 2 -

1"

0"

i ~ t = 460 s

5 10 15 20 25

R (kin)

5

4-

r - . -3-

N 2 -

1-

0

0

, i , , 1 i q , ,

5

= 900 s

10 15 20 25

R (km)

Fig. 6. Distribution of the particle volumetric fraction in the atmosphere at 80, 180, 280, 380, 460, and 900 s after the onset of eruption A. The contour levels shown are the exponents to the base 10 and, beginning from the outer region, correspond to - 8, - 6. - 5, - 4, - 3, - 2, - 1. The pyroclastic flow reaches the barrier at about 380 s and stops climbing it at about 460 s. At about

this time a phoenix cloud above the flow begins to develop.

:1+ 0

o

79

r , , , I i , , , I ' ' ' ' I ' ' ' ' I i T i ,

5 10 15 20 25

R (kin)

It = 8 0 s

. . . . l . . . . I . . . . I . . . . I ' ' '

5 10 15 20 25

R (kin)

5

4--

E

N 2 -

1-

0

0

t t = 1 8 0 s

G. Giordano, F. Dobran / Journal of Volcanology and Geothermal Research 61 (I 994) 69- 94

5 iI l,

i

1-

0 i , i , i , J l I I J J ~ ,

= 2 8 0 s

I I ' ' ' ' 1 ' ' ' ' I f ' ' '

5 10 15 20 25

R (km)

5

E

N 2 -

1"

0

0

t = 380 s

~ o

. . . . I ' ' ' ' I . . . . I " ' ' ' I ~ ' ~ ~

5 10 15 20 25

R (kin)

5 -

4-

N 2 -

1'

0.

o

I t = 4 6 0 s

5 10 15 20 25

R (kin)

$ -

E

N 2 -

1"

0

0

= 9 0 0 s

. . . . I . . . . I . . . . I ' ' ; ' I + ' '

5 10 15 20

R (km)

25

Fig. 7. Distribution of the radial mixture velocity of eruption A at 80, 180, 280, 380, 460, and 900 s after the onset of the eruption. The contour levels shown, beginning from the outer region, correspond to - 10, 10, 50, 75, and 100 m/s. The flow begins to change direction after climbing the barrier at about 460 s.

80 G. Giordano, F. Dobran I Journal of Volcanology and Geothermal Research 61 (1994) 69- 94

- 0.3'

N 0.2

0.0 17

0.5 ~ i . . . . .

0 . 4 - i ! ! i i i i i i i i ! i i ! , \ . . _ . . . . . . . . . . . _ .

~ y L - % : 1" - : L : : : ( , ' : : - L L ' . : : : '

0.1- ' - - - : 1:3 4 0 0 s

' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' '

18 19 20

R (kin)

21

0.5

0,4-

0.3-

N 0.2-

0.%

0.0 17

L

i , T ~ I ~ '--' ' I T T ~ , I 1 , ,

18 19 2O

R (kin) 21

: : , , ' , 0.4- - " '" '

_ . ' , ,

-~- 0.3- --~/'

N 0.2"

0.1-

0.0 17

. . . . , , _ , . ~ , ~ _ _ : ~

. . . . . . . . . . - -_: / . l -~-- - - - - - : ~ r ~

500 s

' ' ' ' I ' i , ' l ' ' ' ' l ' ' ' '

18 19 20

R (krn)

0 . 5 -

0 . 4 -

~- 0.3-

N 0.2 Z

0.I-

0.0- 17

l i t = 5 0 0 s

18 19 2O 21

R (km)

0.5 . . . . ',: ~ ~-~:- ; i~,'~ . . . . . .

I O.4

"E" 0.3 . . . . . . -" ~ - - " ' , ~ . . . . . . .

N 0.2 ; . . . . - . . . . . .

" - ' - "1 . . . . ,.,

o.I 6 0 0 s

0 .0 ' , , ' T I . . . . i . . . . , . . . . 17 18 19 20 21

R (kin)

0.5 ,' 1 / t = 6 0 0 s

0.4- I

, I

.#" 0.1-

0 . 0 " - - r , , , I . . . . T . . . . I . . . .

17 18 19 20

R (k~)

0 . 2 I -_ L ' _ - 5 ' > _ / : -_-2~- : : : _ d t" Z ; ; ;. :_;-_7- - a a ~--t-~ .-~_j_.H

o.1 9 0 0 s

0 .01 . . . . i . . . . I . . . . ~ . . . . 17 18 19 2O 21

R (kin)

0.5

0.4

~ -0.3

N 0.2

0.1-

0.0" 17 18 19 20

R (kin)

Fig. 8. Particle velocity vector plots and distribution of the mixture density of eruption A near the barrier at 400, 500, 600, and 900 s after the onset o f the eruption. The maximum flow velocities in the domains are reported in the boxes forming the inserts in the velocity vector plot figures. The contour levels of mixture density plots, beginning from the outer region, correspond to 0.8, 1.2, 10, 20, 50, 100, 200, 300, and 500 kg /m 3.

G. Giordano, F. Dobran I Journal of Volcanology and Geothermal Research 61 (1994) 69- 94 81

1000 -

(E 800-

v 600-

400- .

200-"

0-

0.0

~_t = 400 s

0.1 0.2 0.3 0.4 0.5

Z (km)

~" 150-

.~- 100-

> ~ 50-

N

-~ 0-

)~--50 - 0.0

I t = 4 0 0 s

q - -

I IX,\ \ \ / ,

0.1 0.2 0.3 0.4 0.5

Z (kin)

1000-

~ g00-

~ 600-

I 400-

G $ 2oo-

O - -

0 . 0

I t = 500 s

0.1 0.2 0.3 0.4 0.5

Z (kin)

~" 150

100"

>

50-

112

0-

-50 0.0

I t = 500 s

0.1 0.2 0.3 0.4 0.5

Z (kin)

1000

~E 800"

v 600-

I 400-

i 20o =

0 , , , , 0.0

I t = 600 s

i

i

0.1 0.2 0.3 0.4 0.5

Z (km)

~" 150-

100-

~ 50- ._x - 5

"~ O - g

-~ -50- 0.0

I t = 600 s

7x/ \ 7 " .

- ~ . . . . . .

0.1 0.2 0.3 0.4 0.5

Z (kin)

1000-

~E 800"

~'~ 600- v

I 400-

G 20o=

O" m

0.0

t = 900 s ~" 150-

~ 100- 8

~ 5o-

~ o -

8 N

4~ --50-

I t = go0 s

/ f ' \

0.1 0.2 0.3 0.4. 0.5 0.0 0.1 0.2 0.3 0.4 0.5

z (.m) z (kin)

Fig. 9. Vertical distr ibution o f mixture densi ty and radial mixture velocity at 400, 500, 600, and 900 s after the onset o f the eruption A. The full line corresponds to 7 km, short-dashed line to 9.5 km, medium-dashed line to 12 km, long-dashed line to 17 km, and dot-dashed line to 19.2 km from the vent. Note a significant mixture density increase with t ime at the base o f the barrier at 19.2 km from the vent. Also note that the lines in left-hand figures extending from the top reflect the effect o f topography.

R (km)

4-

f ~ 3 -

N 2 -

1-

0 0

G. Giordano, F. Dobran /Journal of Volcanology and Geothermal Research 61 (1994) 69-94

. . . . I ' ' ' ' I

5 10

t = 2 8 0 s

It = 8 0 s

I ' ' ' ' [ ' ' ' '

15 20 25

5

4-

N 2 -

1-

0

i t = 180 s

, , , , , , , , I I ' ' ' ' I ' ' ' ' I ' ' ' ,

5 10 15 20 25

R (km)

5

4-

N 2 .

1-

0

0

5 -

' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' '

5 10 15 20 25

R (kin)

O"

0

4 -

N 2 '

L i t_

t = 4 6 0 s

5 10 15 20 25

R (kin)

5 -

4-

E

N 2 -

1-

0-

82

0 5 10 15 20 25

R (kin)

5

4-

N 2 -

I-

0

0 5 10 15 20 25

R (krn)

Fig. 10. Panicle temperature distribution of eruption A at 80, 180, 280, 380, 460, and 900 s after the onset of the eruption. A contour level in the figure represents the difference between the particle temperature and the undisturbed atmospheric air temperature at zero elevation. The temperature contours, starting from the outer region, correspond to 0, 100, 500, 900, and 1000 = K. Note the high temperatures maintained by the pyroclastic flow near the ground and the cooling effect in the phoenix column produced by the back-flow shown in Fig. 8.

83

I ' ' ' i

10

R (kin)

N 2 -

1-

0 . . . . I . . . .

0 5 I . . . . I ' ' ' '

15 20 25

It = 6 0 s

' ' ' ' I ' ' ' ' I ' ' ' ' I

5 10 15

R (kin)

5

4-

I-

0

0

G. Giordano, F. Dobran / Journal of Volcanology and Geothermal Research 61 (1994) 69- 94

t = 120 s

20 25

5

4-

f ~ 3 -

N 2 -

1-

I t = 200 s

, , , , i , , ,

5

4"

N 2 -

1"

0" 0 I I ' ' ' ' I ' ' ' ' I ' ' ' '

0 5 10 15 20 25 0

R (kin)

V / [ t = 280 s

5 10 15 20 25

R (kin)

Fig. 11. Distribution of the particle volumetric fraction in the atmosphere at 60, 120, 200, and 280 s after the onset of eruption B. The contour levels shown are the exponents to the base 10 and, beginning from the outer region, correspond to - 8, - 6, - 5, - 4 , - 3 , - 2 , - 1. After about 200 s the pyroclastic flow practically stops ahead of the break-in-slope at 13 km from the vent, and several phoenix clouds are seen developing above the flow.

about 13 km from the vent. At 280 s, the flow has traveled about 17 km from the vent and its thickness has been reduced after the development of a coignimbrite cloud in connection with the break-in-slope of the volcano. The flow begins to climb the barrier at 380 s, and at 460 s it has reached a height of about 160 m on the obstacle. Beyond this time, the flow ceases to climb the obstacle and another coignimbrite column develops between 460 and 900 s. The distribution of the radial component of the two-phase flow mixture velocity, defined by:

U m = ( 1 - ep)pg u s + ~ppp Up ( 3 ) #m

where u s and up are the radial components of gas and particle velocities, at times corresponding to those of Fig. 6, is shown in Fig. 7. The flow main- tains a very high speed (more than 50 m / s ) until it hits the barrier at about 19 km from the vent. It subsequently slows down as it climbs the bar- tier, and at 460 s the flow descends from the barrier with a speed of about 10 m/s. The particle velocity vector plots and mixture density near the barrier at 400, 500, 600, and 900 s are illustrated

84 G. Giordano, F. Dobran / Journal of Volcanology and Geothermal Research 61 (I 994) 69-94

0 .6 ¸ . ~ •

0.5 - Zf f J ', ; ; ;

N 0.3-

0.2-

0.1 8

!

2 0 0 s

14

i

' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' '

9 I0 11 12 13

R (kin)

!

(a)

0 . 6 ~ - - .~ - .

0.5 ~_ ~ : c

E ~ " " ~ " ~ ~ ' - ,,~

N 0.3-

0.2"

f :

0 .1 . . . . i , I

8 9

2 8 0 s

J i ; , , , i , , , , i , , , , 1 , , , ' ,

10 11 12 13 14 R (kin)

0.6

0.5-

.--.0.4-

N 0.3"

0.2-

0.1 8

I t = 200 s

| @

, , , , l I, ~., i , , i, i ~ ,'i ,i , ,~ ,i t , , ,

9 10 11 12 13 14 R (km) (b)

0.6

0'5 I

,~, 0.4 Z

N 0.3-

0.2'

t -i ?8o s

i

0 .1 ' ' ' ' I . . . . I . . . . I ' ' ' ' I ' ' ' r I . . . .

B 9 I0 11 12 13

R (k~) 14

0.6

0'5 l

.-~ 0.4-

N 0.3-

0.2- n

I t = 200 s

0.1

II q

0.6

0"5 l

/-.0.4-

N 0.3'

0 .2 "

0.1 8 9 10 11 12 13 14 8

R (kin) ( C )

I t = 2 8 0 s

i " h i L~

1

, , , , i , , , , i , , , , i , , F , i , , , , i , , , ,

9 10 11 12 13 R (kin)

Fig. 12. The figure illustrates the effect of the break-in-slope of the volcano on the pyroclastic flow at 200 and 280 s after the onset of eruption B, in terms of the particle velocity (a), mixture density (b) , and dynamic pressure (c) plots. The maximum flow velocities in the domains are reported in the boxes forming the inserts in the velocity vector plot figures. The mixture density contour levels, starting from the outer region, correspond to 0.8, 1.2, 10, 20, and 50 kg/m 3. The contour levels of the dynamic pressure, reduced by the atmospheric pressure of undisturbed air at the same height, correspond to 1000, 5000, and 15,000 Pa. Very high values of dynamic pressure occur at a short distance away from the break-in-slope.

14


L 1"

I t = 4 0 s

0 - ~ 0 5 10 15 20 25

R (kin)

5

4--

~'2-

1-

0

0

I t = 8 0 s

T , , , I r T ]' i j r , , , | ] = i ' l ' [ [ '

5 10 15 20 25

R (k~)

5

I t = 120 s

I I i I I 1 i I ; I T f I i ] ] I [ { [ [ I I I

0 5 10 15 20 25

R (k~)

i I t = 180 s =

0 - ~ 0 5 10 15 20 25

R (~)

5

#-

N 2 -

I"

5-

4-"

N 2 -

I-

I t = 200 s

' ' ' ' I ' ' ' = I ' ' ' ' I ' ' ' " I ' , , ,

5 10 15 20 25

R ( ~ )

0 0-

0 0

t = 2 4 0 s

5 10 15 20 25

R (kin) Fig. 13. Particle volumetric fraction distribution in the atmosphere at 40, 80, 120, 180, 200, and 240 s after the onset of eruption C. The contour levels shown are the exponents to the base 10 and, beginning from the outer region, correspond to - 8, - 6, - 5, - 4 , - 3 , - 2 , - 1. It can be seen from the figure that the pyroclastic flow stops after about 120 s at 7 km from the vent and that it develops a large phoenix column.

86 G. Giordano, F. Dobran / Journal of Volcanology and Geothermal Research 61 (1994) 69- 94

400

(

E 300-

• ~ 2 0 0 -

~ 100- ._x

0

0.4

I t = 120 s

, , , , i , , , , i , , , , I , , , , i , , , , 1 , , , , I , , f , F , , , ,

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

z (kin)

150 .-~

E .._..

100- o 0

:>

5 0 -

"E 0- o N ,[,-

O "1-

- 5 0

0.4

/ \ /

f

/

/

t /

/

r t = 120 s

h . . . . . . . . .

\ \

\

\

\

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

z (km)

4 0 0

(

E 300-

~ 200-

lu

~ 100- ._x

t = 1 8 0 s

k

E v

o o

>

.x_

8 N 'E

150-

100-

50 -

O'

t 0 . . . . I . . . . I . . . . t ' - - ' Q ' ~ " ' I . . . . ~ . . . . I . . . . - 5 0

0.4- 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0.4

z (kin)

I t = 180 s

/ \ / \

\ /

\

\

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

Z (km)

4 0 0 1 5 0 -

( E 300-

~ 2 0 0 -

~ 100- ._x

t = 2 4 0 s E

v > ,

>

Q}

- 0

O .N

100-

5 0

0 ' ' ' ~ r l . . . . I . . . . . -- ' Q ' " ' ' I . . . . I . . . . I . . . . - 5 0 -

0 . 4 0 . 5 0 . 6 0 . 7 0 . 8 0 . 9 1.0 1.1 1.2 0 . 4

z (kin)

O-

t = 2 4 0 s

\ / \

/ \ / \

/ /

\

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

Z (km)

Fig. 14. Vertical distributions of mixture density and radial mixture velocity at 120, 180, and 240 s after the onset of eruption C. The full line corresponds to 1.5 km, medium-dashed line to 3 km, and long-dashed line to 6 km from the vent. The flow consists of a high concentration basal part moving slowly underneath a fast and less dense part.

80 s 3.0

t =

0

3.0

2.5-

2.0-

E ~,~ 1.5- N

1.0-

I

' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 . . . . I . . . . I ' ' ' ' 1 . . . . I

2 4 6 8 10 12 14

R (k~)

0.5-

0.0

0

3.0

t = 120 s

' ' ' ' 1 ' ' ' ' 1 ' ' ' ' I . . . . I ' ' ' ' 1 ' ' ' ' 1 . . . . I

2 4 6 8 10 12 14

R (k~)

, 3 . 0

2.5 =

. 2.0'

E ~ t5"

tO"

0.5-

O0

0

t = 200 s

, , , , i , , , , i , , , , i , , , , j , , , , l l , , , I , , , , i

2 4- 6 8 10 12 14

R (k~)

I t = 300 s 2.5"

2 .0

E t5"

N

1.0"

0.5-

0.0

G. Giordano, F. Dobran / Journal of Volcanology and Geothermal Research 61 (1994) 69- 94 87

' ' ' ' 1 ' ' ' ' 1 . . . . I . . . . I ' ' ' ' 1 . . . . I ' ' ' ' 1

0 2 4 6 8 10 12 14

R (kin)

Fig. 15. Particle volumetric fraction distribution in the atmosphere at 80, 120, 200, and 300 s after the onset of eruption D. The contour levels shown are the exponents to the base l0 and, beginning from the outer region, correspond to - 8 , - 6 , - 5 , - 4 , - 3 , - 2 , - 1.

in Fig. 8. At 400 s the velocities of particles at the barrier are reduced to low values (about 10 m / s ) but continue to climb almost to the top of the barrier located at about 300 m a.s.l. The flow along the barrier stops at about 500 s and changes the direction. During this flow reversal process, the particles at the base of the barrier accumulate and produce a density increase of the gas-particle mixture (Fig. 8 ). At 600 s, there is a consid- erable strengthening of the flow reversal process and a large accumulation of particles at the base of the flow above which develops a rapidly rising phoenix column. At 900 s, the flow begins to climb the barrier again. A more detailed vertical

distribution of the mixture density and radial mixture velocity at 400, 500, 600 and 900 s at different radial positions is shown in Fig. 9. Near the base of the barrier, at about 19.2 km from the vent at the lowermost 20 m of the flow, the mixture density rapidly increases with time and at 900 s it reaches a value of about 1000 kg/m 3. As described below, this density approaches a maximum particle packing density of 1400 kg/m 3. The vertical distribution of the radial velocity profiles at different radial positions from the vent is also shown in Fig. 9 and does not appear to be influenced by the above described flow reversal process, except near the base of the barrier. At

88

3.0

2.5-

G. Giordano, F. Dobran / Journal of Volcanology and Geothermal Research 61 (1994) 69- 94

2 . 0 ~

A E -~ 1.5- N

1.0-

0 . 5

0.0

0

I t = 4 0 s

2 4 6 8 10 12 14

R (l,,m)

3.0

l i t t = 80 s 2.5

2.0

1.0

0.5

0.0 . . . . I . . . . I 0 2 4 6 8 10 12 14

R (k~)

,3.0

2.5-

2.0-

~,~,E 1. 5 -

N

10--

0 . 5

0.0

0

i t = 120 s

, , , , l ~ , ~ , l , , , , l ~ , , ] , , ~ , l , , , , l , , , , I

2 4 6 8 10 12 14

R (kin)

3.0

2 .5

2 .0

E 1.5

N

1.0

0.5-

0.0

0

t = 180 s

' f ' ' l ' ' ' ' l ' ' ' ' l ' ' ' ' l ' ' ' ' l ' ' ' ' 1 ' ' ' ' 1

2 4 6 8 10 12 14

R (k~)

Fig. 16. Particle volumetric fraction distribution in the atmosphere at 40, 80, 120, and 180 s after the onset of erupt ion E. The contour levels shown are the exponents to the base 10 and, beginning from the outer region, correspond to - 8 , - 6 , - 5 , - 4 , - 3 , - 2 , - 1 .

this location the radial velocity profiles exhibit flow reversals which are associated with the pyroclastic flow climbing and descending from the barrier. Near the ground, the particle temperatures remain very high (about 1300 K) even at 19 km from the vent, whereas above the base of the barrier in the rising phoenix there is a strong cooling effect due to the entrainment of cold air (Fig. 10).

Simulation B (Figs. I 1-12) is similar to simulation A, except for its particle size and density which are 200 #m and 2000 kg/m 3, respectively. Plots of particle volumetric fraction distribution (Fig. I l ) show that by 120 s the column col-

lapses and the pyroclastic flow reaches the base of the volcano. After 200 s, the pyroclastic flow practically stops ahead of the break-in-slope located at 9 km from the vent, its head thickness reduces considerably, and several coignimbrite columns develop above the flow. Note that during the same time of 280 s the pyroclastic flow head of simulation A (Fig. 6) reaches a distance of about 17 km, indicating the importance of pyroclast size. A more detailed behavior of the pyroclastic flow near the break-in-slope is illustrated in Fig. 12. The particle velocity vector plots in this figure show the deceleration of the flow and strong upward motions of particles above the


flow at 280 s. In the region of flow deceleration the mixture density is larger than away from this region and it occurs after the break-in-slope (Fig. 12 ). The flow deceleration can be seen more effectively by a plot of the dynamic pressure, reduced by the pressure of the undisturbed atmosphere (Fig. 12c). This pressure is defined as:

2 2 ~ _ _ U m "~ V m

Pdy. = r * p m -~ Patm (Z) (4)

where Vm is the mean vertical component of velocity and is defined similarly as the radial mean velocity Um in Eq. (3).

The effect of the particle size of 500/zm and density of 1700 kg/m 3 was simulated by the simulation C with the results illustrated in Figs. 13 and 14. The particle volumetric fraction distributions shown in Fig. 13 demonstrate a pyro- elastic flow which stops on the slope of the volcano after few minutes at about 7 km from the vent. At about 120 s, a large phoenix column develops above the flow. Vertical distributions of mixture density and radial mixture velocity at 1.5, 3, and 6 km from the vent are shown in Fig. 14. The radial distance of 1.5 km corresponds to the impact distance of the collapsing column, 6 km corresponds to the approximate location of the phoenix, and 3 km represents an intermedi- ate distance from the vent. From the mixture density plots in Fig. 14, it is evident that this column is rapidly separating the particles from the gas with a maximum rate of particle concentration being located at about 3 km distance from the vent. This high particle concentration at the base of the flow produces very large vertical velocity gradients (Fig. 14). From this figure, it can also be seen that the pyroclastic flow consists of a high concentration (about 300 kg/m 3) basal part which is about 40 m thick and moving slowly, and of an overridding and low density (lower than l0 kg/m 3) flow which is moving at about 100 m/s.

Simulations D and E are similar to B and C in terms of granulometry but differ in terms of vent diameter and vent exit velocity. The 600-m diameter and exit velocity of 100 m/s produce a mass eruption rate for simulations D and E of 8.52 × l0 s kg/s. The column behaviors of simu-

lations D and E shown in Figs. 15 and 16 are dis- played in terms of the particle volumetric fraction distributions. The simulation D with a particle size of 200/zm shows a pyroclastic flow that stops after about 120 s and 7 km from the vent, whereas the simulation E with a particle size of 500/tm stops at about 80 s and 4 km from the vent. After stopping, both eruptions develop coignimbrite clouds which rapidly merge with the main clouds above the fountains.

5. Discussion

In this section, results from computer simulations of IlptTA will be compared with field observations and interpretations. These include the generation of pyroclastic flows, geometry of the deposit, ability of pyroclastic flows to surmount topographic barriers, lithics distribution, depositional structures, and the development of fumarolic pipes.

The geometry of the deposit shown in Figs. 1 and 3 shows two characteristic features of the IIpfTA: ( 1 ) the increased deposit thicknesses at the break-in-slope of the volcano and at the morphological barrier of the Apennines, and (2) the low aspect ratio. Regarding the first point, the results of simulations A (Figs. 6, 8, and 9) and B (Fig. 12) suggest that the break-in-slope of the volcano and the base of the barrier are areas where rapid sedimentation from the pyroclastic flow takes place. This effect is marked by the rapid density increase in a narrow basal zone of the pyroclastic flow (Figs. 8, 9, and 12 ) and the contemporaneous rise of coignimbrite columns. Even if a flow density increase does not necessar- ily equate with final deposit thickness, we hypothesize that such morphological features can induce important changes in the flow dynamics. In particular, the computer results appear to be consistent with field data where the simulated flow stops against the barriers. Here a progressive density increase of the flow with time at the base of the barrier is predicted. Such an effect occurs in the IIptTA as well as in the deposits of the Mt. St. Helens' 1980 directed blast and Taupo ignimbrite (Valentine, 1987; Fisher, 1990). The

90 G. Giordano, F. Dobran /Journal of Volcanology and Geothermal Research 61 (1994) 69-94

predicted mean density of about 1000 kg/m 3 at 900 s at the base of the barrier (Fig. 9 ) for simulation A reflects a mean value in a computational cell which is 10 m high. Closed packing of spherical particles with a density of 2200 kg/m 3 as used in the simulation, would yield a mean density of about 1400 kg/m 3, indicating that it is reasonable to hypothesize the sedimentation of particles from the flow or the development of a highly concentrated narrow basal zone of the pyroclastic flow during the eruption.

Regarding the aspect ratio of the ignimbrite, many good insights can be deduced from simulation A results (Figs. 7 and 8 ) where the radial velocity of the flow at 19 km from the vent is about 80 m/s even at 900 s after the eruption begins. If the barrier were absent, the pyroclastic flow would have traveled much larger distances before stopping [see Dobran et al. (1993) for simulations without barriers which show this effect ]. During the initial phase of research when we did not include the topography of the volcano, the distances reached by pyroclastic flows exceeded 20 km, which is consistent with the field deposits being found at about 30 km from the vent, where flow was unobstructed. The high kinetic energy of the pyroclastic flow enables it to climb the obstacle placed at 20 km from the vent. In Fig. 6 the pyroclastic flow climbs about 160 m which is in accord with the field observations which show that the I lpfrA surmounted the morphological barriers in this range of altitudes. This occurred, for example, in the Paleoaniene valley (De Rita et al., 1992 ), stretching from Ti- voli to Rome (Fig. 1 ), which had an axis perpen- dicular to the moving flow with topographic height difference from about 50 to 60 m. When the flow barriers were more than about 250 m high, as in the case of the Apennines in the east- ern sector (Fig. 1 ), the pyroclastic flow did not cross the barriers. The above considerations led us to speculate about the conditions which control the development of several depositional structures of IIpfTA, and particularly the effect of the substrate morphology.

The distribution of lithics with distance from the vent (Fig. 4) appears to be strongly influenced by the presence of the Apennines. In fact,

as the flow climbs the Apennines, the deposit is impoverished in coarse lithics. This field datum can be related to the decrease in radial momentum of the flow, which is reflected in the loss of particles (Figs. 8 and 9 ). A peculiar facies of the IlpfTA observed at the Apennines shows that when the flow was climbing the slopes it devel- oped several beds or flow units. Near the town of Poli, located in the Apennines at about 20 km from the vent (Fig. 1 ), the deposit has at least 10 beds from 20 to 100 cm thick. Elongate lithic clasts are imbricated, indicating a flow direction toward the source area (downslope). This may be related to the effect of barriers as shown in the velocity vector plots of simulation A (Fig. 8), where the flow first climbs the barrier and then reverses the direction as the coignimbrite cloud develops above the base of the barrier. The exis- tence of such beds or flow units in the field may be, therefore, related to the backflow during the emplacement of the deposit, as the dynamics of the pyroclastic flow changed during the eruption [pulses in pyroclastic mass flow rates as predicted by numerical simulations by Neri and Dobran (1993)], or by a combination of fac- tors, such as the shape of the barrier and change of eruption dynamics. The inward entrainment of air, caused by the growth of a coignimbrite plume above the barrier, may have also produced flow reversals.

It was noted by De Rita et al. (1988b) that fumarolic pipes in I lpfrA occur extensively only in paleovalleys and near Apennines where the deposit shows maximum thicknesses. The occur- rence of fumarolic pipes in ignimbrites has been associated with the mechanism of fluidization (Wilson, 1980), with the presence of water on the ground over which an ignimbrite is emplaced, and with the presence of entrained vege- tation which burns in the deposit (Cas and Wright, 1987). The results from computer simulations suggest, as an additional mechanism, that pipes may have been formed from rapid deposition of particles in front of obstacles (Fig. 9 ). Because of the predicted high rate of mixture density increase in this region (2.5 kg /m3-s ) , we hypothesize that the gas phase was trapped below the particles during deposition and that


subsequently this gas was drained through fin- ger-like instabilities (or pipes) created by the unstable density-stratified two-phase mixture.

The simulation A shown in Fig. 6, and to a lesser extent the simulation B (Fig. 11 ), appear to represent most closely the second pyroclastic flow unit of Tuscolano Artemisio. The only difference between these simulations is in the pyroclast sizes of 100 and 200 pm particles, respectively. Because of this particle size difference, the pyroclastic flow of the simulation B traveled only about 13 km as opposed to simulation A which would have traveled longer than 20 km if it were not for the presence of the barrier. The simulation C (Fig. 13 ) is different from simulations A and B only in the particle diameter of 500/tm, but its pyroclastic flow stops at about 7 km from the vent. As a consequence, the time required for the pyroclastic flow to reach its maximum distance decreases from about 400 s for simulation A (when the flow arrives at the barrier), to 220 s for simulation B, and to 180 s for simulation C. These results imply that larger size particles sep- arate more effectively from the transporting gas phase in the pyroclastic flow as a consequence of their earlier sedimentation closer to the vent than the smaller size particles. This is, of course, consistent with the basic physics of two-phase flows (Wallis, 1969).

The simulations D and E have lower mass eruption rates due to the lower exit velocities and vent diameters (Table 1 ). The effect of increasing the particle diameter from 200 #m in simulation D to 500/~m in simulation E is to produce lower travel distances of the pyroclastic flows (Figs. 15 and 16) which do not reach the break- in-slope of the volcano. By comparing the pyroclastic flow travel distances of all simulated eruptions it is evident that a larger mass eruption rate and a smaller particle size produce similar effects on the distances traveled by the pyroclastic flows. As a consequence, it is reasonable to hypothesize that the mass eruption rate of the IlpfrA should have been close to 109 kg/s.

The results from simulations involving constant particle sizes should, however, be used with caution in the interpretation of the emplacement dynamics of real pyroclastic flows until polydis-

persed particle simulations are carried out and verified with data. Different particle sizes should produce a nonlinear effect in local flow dynamics and consequently produce changes in the global flow, including the location and size of phoenix columns. The monodispersed particle size simulations of the second pyroclastic flow unit of Tuscolano Artemisio provide first order effects which are consistent with field data and thus provide an important verification of the employed physical model. As such, a refinement of the physical model which includes polydispersed particle sizes should not significantly af- fect the global pyroclastic flow behavior, and the monodispersed particle simulations should provide important limits associated with the emplacement dynamics of pyroclastic flows.

6. Summary and conclusions

The second pyroclastic flow unit of Tuscolano Artemisio was simulated numerically by a complex thermofluid-dynamic model which accounts for gas and pyroclast flow above the vent and along the slope of the volcano. Computer simulations involved actual topography of the volcano which included a barrier 2 km wide and 250 m high, located at a distance of about 20 km from the vent, and different vent diameters, particle sizes and densities, and gas and particle velocities. The input data for simulations were determined from the interpretation of field data.

The results from simulations were shown to be consistent with the characteristics of deposits and were employed to explain some flow emplacement characteristics of the pyroclastic flow unit. The column collapse heights in simulations ranged from 550 to 1100 m, with the pyroclastic flows traveling down the slope of the volcano at speeds exceeding 100 m/s. The distances reached by the pyroclastic flow fronts were shown to be influenced by the particle sizes and mass eruption rates. Smaller pyroclasts showed a tendency to produce longer travel distances of the flows because of their slower segregation from the flows than the larger particles. Larger mass eruption rates have a similar effect on the distances trav-

92 G. Giordano, F. Dobran / Journal of Volcanology and Geothermal Research 61 (1994) 69-94

eled by the pyroclastic flows, since they produce larger radial momenta of the flows. When these radial momenta are reduced by a break-in-slope of the volcano, a barrier, or by the distance from the vent, the pyroclastic flows produced phoenix columns at the top of flows.

The relationship between the gas-pyroclast thermofluid-dynamics and topography of volcano determined the following emplacement characteristics of the IlpfrA:

( 1 ) Geometry of the deposit. The widely dis- persed deposit of the IIpITA exhibits maximum thicknesses at the break-in-slope of the volcano, in paleovalleys, and in front of the Apennines. This is consistent with the predicted results from numerical simulations because of the reduction of pyroclastic flow radial momenta at such locations.

(2) Distance reached by the pyroclastic flow. The maximum distance reached by the IlptTA is about 30 km. The computer simulations showed that the pyroclastic flow would have traveled distances in excess of 20 km if it were not for the barrier at this location.

(3) Ability to climb hills. The IlpfTA easily crossed barriers in the range of about 100 m, and up to maximum heights of 250 m. In the simulations the pyroclastic flows climbed to the height of 160 m during the first 20 minutes of simulation. Moreover, it was possible to infer the following local influences of topography on the Ilpf-l'A deposit:

(1) Lithic distribution. The maximum lithic size of the Ilpfl'A behind the obstacle are smaller than in places where the deposit was not influenced by the topography. The results from simulations demonstrated that this may be ex- plained by the reduction of drag between the gas and pyroclasts caused by the reduction of the radial momentum of the flow.

(2) Depositional structures. Simulations showed that the presence of a barrier causes the ascent and descent of pyroclastic flow along the barrier slope due to the reduction of flow momentum caused by the barrier and inward air entrainment caused by the buoyantly rising phoenix column above the base of the barrier. Near

the Apennines, the IlptTA shows multiple flow units which may be related to flow reversals along the barrier which occurred several times during the simulation period.

(3) Fumarolic pipes. The abundant presence of fumarolic pipes in the deposits at the base of the Apennines and in paleovalleys may be ex- plained from simulations by the high rates of particle concentration at these topographical locations. This concentration may trap the gas below the depositing particles which subsequently escapes because of the unstable density-stratified two-phase mixture.

The main limitations of the reported computer simulations involve a single granulometric particle size, and absence of the fine scale flow resolution of pyroclastic flows caused by the large cell size (minimum 10 m) in numerical calcula- tions. Both of these limitations can be reduced at the expense of more powerful computational tools and are not associated with the limitations of the physical modeling approach.

The verification of results from complex computer simulations of pyroclastic flows requires modeling oriented stratigraphic data which implies the establishment of new quantitative methodologies in studying the pyroclastic flow deposits. Only in this way, and with intelligent use of computational resources, it will be possible in the future to develop real simulation capabilities of pyroclastic flows or volcanic columns.

Nomenclature

d D g H L

P R U

v

x

particle diameter vent diameter gravitational acceleration maximum drop distance maximum runout distance mass flow-rate pressure radial coordinate radial component of velocity vertical component of velocity mass fraction


Z vertical coordinate

Greek

volumetric fraction p density

- log2d (in m m )

Subscripts

atm undisturbed atmosphere c collapse dyn dynamic g gas m mixture p particle or pyroclasts v vent

Acknowledgements

This work was partially supported by the Gruppo Nazionale per la Vulcanologia of Italy. We wish thank A. Neri for his contributions, and one of us (GG) wishes to thank C. Rosa for useful discussions regarding the IIpfTA and the De- partment of Earth Sciences at the University of Rome for providing a support. We also wish to thank G.A. Valentine and K. Wohletz for providing useful comments during the review process of the paper.

References

Beget, J.E. and Limke, A.J., 1988. Two-dimensional kinematic and theological modeling of the 1912 pyroclastic flow, Katmai, Alaska. Bull. Volcanol., 50: 148-160.

Biddittu, I., Cassoli, P.F., Radicati di Brozolo, F., Segre, A.G., Segre Naldini, E. and Villa, I.M., 1979. Anagni, a K/Ar dated lower and middle Pleistocene site, Central Italy: preliminary report. Quaternaria, 21: 53-7 I.

Branney, M.J. and Kokelaar, P., t992. A rappraisal ofignim- brite emplacement: progressive aggradation and particu- late to non- paniculate flow transition during emplacement of high-grade ignimbrite. Bull. Volcanol., 54: 504- 520.

Brissette, F.P. and Lajoie, J., 1990. Depositional mechanism of turbulent nu6es ardentes (surges) from their grain sizes. Bull. Volcanol., 53: 60-66.

Carey, S.N., Sigurdsson, H. and Sparks, R.S.J., 1988. Exper- imental studies of particle-laden plumes. J. Geophys. Res., 93: 15,314-15,328.

Cas, R.A.F. and Wright, J.V., 1987. Volcanic Succession: Modern and Ancient. Allen and Unwin, London, U.K., 528 pp.

Chiarabba, C., 1993. Caratteristiche sismologiche e assetto strutturale profondo dell'area dei Colli Albani. Ph.D. Thesis, University of Rome (unpubl.).

Chiarabba, C. and Giordano, G., 1990. Genesi e modalith deposizionali delle piroclastiti della Media Valle dell'Aniene (Necks Auct. ). Rend. Soc. Geol. Ital., 13:129- 132.

Chiarabba, C., Cimini, G.B., Amato, A. and Iyer, H.M., 1994. Large teleseismic P-wave residuals variation in the Alban Hills Volcano, Central Italy. Ann. Geofis., in press.

De Rita, D., Funiciello, R. and Parotto, M., 1988a. Geologi- cal map of the Alban Hills volcanic complex (Vulcano Laziale). Gruppo Nazionale per la Vulcanologia-CNR, Roma.

De Rita, D., Funiciello, R. and Rosa, C., 1988b. Caratteris- tiche deposizionali della II colata piroclastica del Tusco- lano- Artemisio (Complesso Vulcanico dei Colli Albani, Roma). Boll. GNV, 4: 278-297.

De Rita, D., Funiciello, R. and Rosa, C., 1992. Volcanic ac- tivity and drainage network evolution of the Alban Hills area (Rome, Italy). Acta Vulcanol., 2:185-198.

Dobran, F., Neri, A. and Macedonio, G., 1993. Numerical simulation of collapsing volcanic columns. J. Geophys. Res., 98: 4231-4259.

Dolfi, D. and Trigila, R., 1978. The role of water in the 1944 Vesuvius eruption. Contrib. Mineral. Petrol., 67: 297-304.

Dolfi, D. and Trigila, R., 1983. Clinopyroxene solid solution and water in magma: result in the system phonolitic te- phrite-H20. Mineral. Mag., 47: 347-351.

Fisher, R.V., 1983. Flow trasformation in sediment gravity flows. Geology, l 1: 273-274.

Fisher, R.V., 1990. Transport and deposition ofa pyroclastic surge across an area of high relief: The 18 May 1980 eruption of Mount St. Helens, Washington. Geol. Soc. Am. Bull., 102: 1038-1054.

Fisher, R.V. and Schmincke, H.U., 1984. Pyroclastic Rocks. Springer, Berlin-Heidelberg, 456 pp.

Fisher, R.V., Orsi, G., Civetta, L., Heiken, G., Ort, M., Somma, R. and Pappalardo, L., 1991. Large ignimbrite eruption of the Phlegrean Fields Caldera: The Neapolitan Yellow Tuff and the Campanian Ignimbrite. G. Orsi and M. Rosi (Editors), IAVCEI Workshop on Explosive Vol- canism, Napoli, 218 pp.

Fornaseri, M., Scherillo, A. and Ventriglia, U., 1963. La re- gione vulcanica dei Colli Albani, Vulcano Laziale. CNR, Roma.

Francis, P. and Self, S., 1987. L'eruzione del Krakatoa. Le Scienze, 39: 66-77.

Funiciello, R. and Parotto, M., 1978. I1 substrato sedimen- tario nell'area dei Colli Albani. Geol. Rom., 17: 23-287.

Giordano, G. and Chiarabba, C., 1992. I depositi piroclastici

94 G. Giordano, F. Dobran /Journal of Volcanology and Geothermal Research 01 (1994) 69-94

della Media Valle dell'Aniene: correlazioni stratigrafiche con i prodotti del Vulcano Laziale. Studi Geologici Ca- merti, vol. spec. (1991/2), CROP 11: 351-355.

Giordano, G. and Rosa, C., 1992. L'influenza della topogra- fia sulle caratteristiche deposizionali della seconda colata piroclastica del Tuscolano Artemisio. Congresso GNV, Palermo.

Hayashi, J.N. and Self, S., 1992. A comparison of pyroclastic flows and debris avalanches mobility. J. Geopbys. Res., 97:9063-9071.

Hsu, K.J,, 1975. Catastrophic debris streams generated by rockfalls. Geol. Soc. Am. Bull., 86: 129-140.

Lowe, D.R., 1982. Sediment gravity flows lI: depositional model with special reference to the deposit of high-density turbidity currents. J. Sediment. Petrol., 52: 279-297.

Neff, A. and Dobran, F., 1993. Influence of eruption parameters on the thermofluid-dynamics of collapsing volcanic columns. J. Geophys. Res., in press.

Pierson, T.C., 1985. Initiation and flow behavior of the 1989 Pine Creek and Muddy River lahars, Mount St. Helens, Washington. Geol. Soc. Am. Bull., 96: 1056-1069.

Pike, R.J. and Clow, G.D., 1981. Revised classification of terrestrial volcanoes and a catalog of topographic dimension with new results on edifice volume. U.S. Geol. Surv., Open-file Rep. OF 81-1038.

Postma, G., 1986. Classification for sediment gravity flows based on flow condition during sedimentation. Geology, 14:291-294.

Pyle, D.M., 1989. The thickness, volume and grain size of tephra fall deposits. Bull. Volcanol., 51 : 1-15.

Sheridan, M.F., 1979. Emplacement of pyroclastic flows: a review. Geol. Soc. Am., Spec. Pap., 180: 125-136.

Sheridan, M.F. and Malin, M.C., 1983. Application of computer assisted mapping to volcanic hazard evaluation of surge eruption. Vulcano, Lipari and Vesuvius. J. Vol- canol. Geotherm. Res., 17: 187-202.

Sparks, R.S.J., 1976. Grain size variation in ignimbrites and implications for the transport of pyroclastic flows. Sedi- mentology, 23:147-188.

Sparks, R.S.J., Wilson, N. and Hulme, G., 1978. Theoretical modeling of generation movement and emplacement of pyroclastic flows by column collapse. J. Geophys. Res., 83: 1727-1739.

Spera, F.J., 1984. Some numerical experiments on the with- drawal of magma from crustal reservoir. J. Geopbys. Res., 89: 8222-8236.

Trigila, R., Gaeta, M., and Freda, C., 1992. Indicazioni ter- modinamiche e petrologiche sull'origine e evoluzione dei magmi potassici. Studi Geologici Camerti, vol. spec. (1991/2) ,CROPlI : 356-371.

Ui, T., 1983. Volcanic dry avalanche deposits: identification and comparison with nonvolcanic debris stream deposits. J. Volcanol. Geotherm. Res., 18:135-150.

Valentine, G.A., 1987. Stratified flow in pyroclastic surges. Bull. Volcanol., 49:616-630.

Valentine, G.A. and Wohletz, K., 1989. Numerical models of plinian eruption columns and pyroclastic flows. J. Geo- phys. Res., 94: 1867-1887.

Valentine, G.A., Wohletz, K. and Kieffer, S.W., 1991. Sources of unsteady column dynamics in pyroclastic flow erup- lions. J. Geophys. Res., 96: 2t,887-21,892.

Valentine, G.A., Wohletz, K. and Kieffer, S.W., 1992. Effects of topography on facies and compositional variation in caldera- related ignimbrite. Geol. Soc. Am. Bull., 104:154- 165.

Walker, G.P.L., 1971. Grain-size characteristics of pyroclastic deposits. J. Geol., 79: 696-714.

Walker, G.P.I,., 1973. Explosive volcanic e r u p t i o n s - a new classification scheme. Geol. Rundsch., 62:431-446.

Walker, G.P.L., 1983. Ignimbrite types and ignimbrite prob- lems. In: M. Sheridan and F. Barberi (Editors), Explo- sive Volcanism. J. Volcanol. Geotherm. Res., 17: 65-88.

Wallis, G.B., 1969. One-Dimensional Two-Phase Flow. McGraw Hill, New York, NY.

Williams, H., 1941. Calderas and their origin. Univ. Calif. Berkeley Publ. Geol. Sci., 25: 239-346.

Wilson, L., Sparks, R.S.J., Huang, T.C. and Watkins, N.D., 1978. The control of volcanic column heights by eruption energetics and dynamics. J. Geophys. Res., 83:1829-1837.

Wilson, C.J.N., 1980. The role offluidization in the emplacement of pyroclastic flows: an experimental approach. J. Volcanol. Geotherm. Res., 8: 231-249.

Wohletz, K.H., McGetchin, T.R., Sandford II, M.T. and Jones, E.M., 1984. Hydrodynamic aspects of caldera- forming eruption: numerical models. J. Geoph. Res., 89: 8269-8286.

Woods, A.W. and Cauldfield, C.P., 1992. A laboratory study of explosive volcanic eruptions. J. Geophys. Res., 97: 6699-6712.

Woods, A.W. and Wohletz, K.H., 1991. Dimension and dynamics of coignimbrite eruption columns. Nature, 350: 225-227.

Documents

Computer simulations of the Tuscolano Artemisio's second pyroclastic flow unit (Alban Hills, Latium, Italy)