Journal of Volcanology and Geothermal Research, 17 (1983) 89--109 89 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

COMPUTER SI]~[LATION OF TRANSPORT AND DEPOSITION OF THE CAMPANIAN Y-5 ASH

WINTON CORNELL, STEVEN CAREY and HARALDUR SIGURDSSON

Graduate School of Oceanography, University of Rhode Island, Kingston, R.I.

02881 (U.S.A.)

(Received May 26, 1982; revised and accepted January 5, 1983)

ABSTRACT

Cornell, W., Carey, S., and Sigurdsson, H., 1983. Computer simulation of tran-

sport and deposition of the Campanian Y-5 Ash. In: M.F. Sheridan and F.

Barberi (Editors), Explosive Volcanism. J. Volcanol. Geotherm. Res., 17:

89-109.

Analyses of grain-size and modal composition of the Campanian tuff ash layer

(Y-5) from ll deep-sea cores have been carried out. This layer represents

ash fall that has been correlated with the 38,000 y.b.p. Campanian ignimbrite

(Thunell et al., 1979), a deposit formed by the largest eruption documented in

the Mediterranean region during the late Pleistocene (Barberi et al., 1978).

The bulk deposit is bimodal in grain-size and dominated by glass shards. The

calculated mean grain-size of the coarse mode of the individual size

distributions decreases with distance from the source and progressively

approaches a near-constant fine mode of approximately 13 microns. Distal

samples are unimodal in grain-size.

These data combined with a set of vertical profiles of wind (10 year average)

have been used as input to a computer model that simulates fallout of tephra.

Modelling indicates that the downwind variation of grain-size of the coarse mode

can be accurately reproduced with transport of ash between 5 and 35 km. The

observed fine mode of the deposit cannot, however, be generated by transport of

ash as individual particles at these elevations. Such transport would result in

deposition of virtually all of the fine ash beyond the studied area. Deposition

of fine ash within the studied distance of 1600 km from source can only occur by

fallout as particle aggregates from a high eruption plume or as individual

particles from co-ignimbrite ash clouds with a maximum elevation of

3 km. The large volume of ash in the fine mode (>70 wt.%) and the irregularity

in azimuth of low-level winds argue against major low-level transport of

co-ignimbrite ash. Rather, the ash may have been derived from both a plinian

eruption column and high-altitude clouds of co-ignimbrite ash, with settling of

fine ash as particle aggregates.

0377-0273/83/$03.00 © 1983 Elsevier Science Publishers B.V.

90

INTRODUCTION

One aim in the study of tephra deposits is the reconstruction of ancient

explosive eruptions. Such reconstructions strive to determine the properties

and dynamics of the eruption colwm and transport processes ca the basis of the

observed properties of the tephra deposit. The May 18, 1980 eruption of Mount

St. Helens provided an opportunity to develop and test a computer model of

tephra fallout constrained by observations of eruption column height, elevation

of major ash transport, lateral spreading of the eruption plume, atmospheric

wind profile, fallout area, volume, grain-size distribution, and modal

composition of the deposit (Carey and Signrdsson, 1982). The computer model

accurately simulates the downwind variation in grain-size, thickness and

composition of the Mount St. Helens deposit as far as 440 km from source. The

model also reproduces the thickness maximum of the deposit 327 km from source

and shows that fallout of particle aggregates has an important influence ca

grain-size characteristics, variation in model composition, and thickness of the

deposit. The deposition of particle aggregates produced a major fine mode in

the deposit.

In this paper we develop a quantitative model of the formation of the

Campanian ash layer (Y-J), which has a volume two orders of magnitude greater

than the Mount St. Helens 1980 ash-fall deposit. The - 38,000 y.b.p. Campanian

ash layer originated from a major eruption in the Phlegraean Fields caldera

(Rosi et al., this volume) and consists of both an ash-fall deposit mantling the

central and eastern basins of the Mediterranean Sea and a major ignimbrite on

land (Barberi et al., 1978). The Campanian tephra, identified in piston cores

from the Mediterranean, is correlated by composition of glass with the Campanian

ignimbrite on land (Thunnell et al., 1979). Sparks and Huang (1980) have shown

that the ash-fall layer is bimodal in grain size and that the median diameter of

the coarse mode decreases steadily with increasing distance from source. In

contrast, the median size of the fine mode changes little with distance from the

source. They proposed that a Plinian eruption colun~ supplied tephra composing

the coarse mode but ash clouds rising from pyroclastic flows produced the fine

mode. We evaluate various scenarios for transport and deposition of the

Campanian tephra, including fallout from a single-stage eruption column with and

without particle aggregation and transport as low-altitude co-ignimbrite ash.

DISTRIBUTION

The locations of cores containing the Campanian Y-5 ash are shown in Fig. I

and range from 430 to 1535 km from source. The isopach map of the deposit is

well constrained in the eastern Mediterranean, because of the good coverage of

core sites. Distribution and thickness of the tephra in the proximal area is

virtually unknown due to limited coring in the lonian, Tyrrhenian and Adriatic

Seas. The lack of core sites in the northern and central Aegean Sea precludes

determination of the north boundary of the layer in the distal area. There is,

9 1

I ~ ~:; ...: :: ' :~::%,':~ l ' " ~ ~ ' ' , . . . . l . . . . l ' ' '

o 200

42"N k,,~ I T A L Y ~ ADRIATIC t.. km ~ ~L. SEA ,

40°38 ° ~ ~ ~ , ,'~ ~' GREECF':! AEGEsAN(~'I, " ~,' :i: TURKEY

~'-~r~,, SEA / / - Y." ~ ~ "' 4 ; • i 0 ' :::::: ::: : :

, 6 • / I i . : :::::

• /' ~,~',~,,.,,...<",,...,.~-~ e / . . , ./ ~,,--" . .~,~,

IO°E 15 • 20 o 25 ° 30 °

Fig. I Isopach map of the Y-5 ash layer derived from distribution of eastern

Mediterranean piston cores. Core locations indicated by solid circles.

Triangles indicate samples used for grain-size analysis in this study.

Subrectangular areas refer to regions 1 to 7 in Table 1. Isopachs are in

centimeters.

evidence of thinning of the deposit to the north. Isopach contouring, based on

available piston core data thus cannot uniquely define a maximum thickness

axis. The 115 ° azimuth of the fallout axis shown in Fig. I is at best

approximate. We thus consider only the southern half of the deposit.

We divide the half-ellipse of the mapped deposit into 7 regions from the

proximal to distal areas in order to reconstruct the bulk grain-size

distribution in the study area and calculate tephra volume. The seven regions

(Table I) represent an area of 689,000 ~ and a tephra volume of 37 km S which

is roughly half the total ash-fall deposit. By comparison, the volume of the

ignimbrite on land is estimated to be 80 ~B (Rosiet al., this volume). The

composite volume thus exceeds 150 km S .

SamlDles and Techniques

Samples were obtained from the core collections of the University of Rhode

92

TABLE i

Volume and areal extent of tephra within regions of the half-ellipse isopach of

the Campanian layer (Fig. l)

Region area km 2 volume km 3

i 108.767 12.57

2 154,402 11.43

3 74,848 3.94

4 109,783 4.03

5 100,901 2.49

6 54,143 1.OO

7 85,921 1.10

Total half-ellipse 688,765 36.56

Total deposit 1.38x106 km 2 73.1 km B

Island and Lamont-Doherty Geological Observatory. Channel samples were taken of

the Y-5 layer in II cores (Table 2). Only the ash layer in core RC9-191

exhibited stratigraphic layering (Sparks and Huang, 1980). Ash coarser than 25

microns in diameter was wet-sieved at one-half phi intervals (phi units are

-log(base 2) of size in mm; Inman, 1952). Finer ash was analysed with a

Particle Data electro-resistance size analyzer (Muerdter et al., 1981). The

proportions of modal components (glass, lithics, felsic and mafic minerals) were

determined by counting particle types in each of the one-half phi size intervals

down to 25 microns. The size fraction less than 25 microns was assumed to have

the same modal proportions as the 25 micron size class.

GRAIN-SIZE DISTRIBUTION

In general, the Y-5 tephra samples are poorly sorted, bimodal, and exhibit a

coarse mode that progressively decreases in grain-size with distance from source

(Fig. 2) in accordance with the findings of Sparks and Huang (1980). For each

sample, the dividing point between a fine and coarse mode was chosen as the size

class with the minimum weight percent between the two respective modes. The

mean for the coarse mode (calculated by the method of moments) decreases

steadily from about 200 microns at 430 km to 30 microns at 1535 km from source

(Fig. 3). The mean of the fine mode shows much less variation, ranging from

about 15 microns at 430 km to lO microns at 1535 km (Fig. 3). The tephra layer

is unimodal in only two cores in the distal region and this may be related to

reworking, bioturbation, or merging of the two grain-size modes. The broad

geographic spread and limited number of samples do not allow us to stuay

across-lobe sorting by low-level winds. This effect has been shown to have

produced variations in grain-size characteristics across ash-fall lobes from

Mount St. Helens (Sarna-Wojcicki et al., 1981). Thunnell et al., (1979)

TABLE 2

Locations of cores sampled for Y-5 ash layer

ID on

Fig. 1 Core no.

Distance from

Latitude Longitude vent (km)

a RC9-191 38°i0 ' 18°00 '

b LYNCHII-3 35°02 ' 16°42 '

c TRI71-21 34°27 ' 20°08 '

d TR171-22 34°06 ' 21°22 '

e RC9-183 34°35 ' 23°25 '

f TR171-27 33°50 ' 26°00 '

g TR172-24 34°53 ' 28°28 '

h TR172-22 35019 ' 29°01 '

i TR172-11 34°08 ' 28°59 '

j TR172-12 33054 ' 29016 '

k TR172-19 34°43 ' 30°09 '

43O

660

86O

945

1025

1275

1405

1420

1450

1510

1535

93

Thickness Depth of

of layer layer in

(cm) core (cm)

4.0 306-310

6.0? 417-423

2.5 22-24.5

3.0 35-38

3.0 118.5-121.5

1.5 111.5-113

1.5 152-153.5

2.0 143-145

0.8 99.4-100.2

0.8 551.2-552

1.0 126.5-127.5

20 t 430 km

I-- Z I,iJ u 20 iv.

o. to

F- 'I-

F "d W 0 , , , , , , , ,

tO

0 i i i i i i i i

2 4 6 8 250 63 16 4

660

i i i i i I i I

I I I I I I I

i ! i ! I 1

4 6 8 250 63 16 4

GRAIN SIZE

i , , i i i i i

I 1275

i i 1 i ! i i i

i i r i i i l

4 6 8 Phi 250 63 16 4 micron~

Fig. 2 Grain-size distributions of the bulk Y-5 ash layer at various distances

downwind from the Campanian source area.

9 4

w N

Z

o 2 = E o.

8 1 - 7 -

1 6 - 6

3 2 - - 5

6 3 - - 4

1 2 5 - - 3

2 5 0 - zi

500 ~--

0

MEAN GRAIN S I Z E of COARSE ond F INE MODE ( m e t h o d o f m o m e n t s )

FINE MODE

• ; . - • oO~U N I MODAL

@ •

DE

i = = I I I I I I = = l I = L I

400 800 1200 1600

DISTANCE FROM CAMPANIAN SOURCE ( K m )

Fig. 3 Mean grain-size of the coarse and fine mode of the Y-5 ash layer as a

function of distance from the Campanian source area, in kilometers. Means

calculated by the method of moments (Folk, 1968). Open symbols indicate

unimodal samples.

recognize, however, a decrease in grain-size from south to north for the Y-5

ash.

The grain-size distribution of the Ca~panian tephra contrasts with that of

other studied layers of Italian origin. The Y-5 layer and three other tephra

layers in core RC9-191, 430 km from source, are compared in Fig. 4. The other

layers are unimodal, have modes near 19 microns, and lack a coarse mode, which

suggests that they fell from less voluminous plumes.

The Erain-size data for each of the seven regions (Fig. l) have been averaged

and weighted according to the mass in each region in order to obtain a bulk

grain-size distribution of the deposit within the study area (Fig. 5). This

distribution does not include the proximal deposit (0-400 km), which contains

the majority of the coarse material, and thus underestimates the coarse

component. The bulk grain-size distribution is bimodal, with modes at 3 to 3.5

phi (125-88 microns) and 6 to 6.5 phi (16-11 microns). More than half (57%) of

the ash is finer than 5 phi (32 microns). Similarly, 52% of the bulk grain-size

distribution of the May 18, 1980 Mount St. Helens deposit is finer than 5 phi

9 5

CORE RC 9-191 430 Km

l-- Z W (_~ 13: LLI O.

t-- " r L9

LLI

2 4

20

I0

2 3 4 5

Y-3 20

I i I ~ i 0 1

6 7 8 9 I0 Phi

20

I0

Y -5

2 3 4 5 6 7 8 9 5 0 0 250 125 65 52 16 8 4 2

I0 I

20

I 0

Y -Z

2 3 4 5 6 7 8 9 Phi

X - 2

0 i I I i I ] i I I

2 3 4 5 6 7 8 9 Phi 500 250 125 63 32 16 8 4 2 Microns

G R A I N S I Z E

Fig. 4 Grain-size distributions of the Y-3, Y-7, Y-5, and X-2 ash layers in core

RC9-191 (430 km from source).

(Carey and Sigurdsson, 1982).

MODAL CONSTITUENTS

Glass shards and pumice fragments make up 70 to 90% of the tephra (Fig. 6).

The glass shards are flat, gently curved, or Y-shaped (Fig. 7), and about 5 to

I0 microns in thickness. The habit indicates an origin as bubble walls,

resulting from extreme vesiculation of a relatively low-viscosity magma. The

tephra is of trachytic composition and calculated viscosities (Shaw, 1972) for

this magnm with 1% H20 are 6.2xi06 and 1.8xl05 poise at 850 ° and 1020°C

respectively. In comparison, the dacitlc Mount St. Helens magma was erupted as

microvesicular pumices. Calculated viscosities for the Mount St. Helens magma

with I% H20 are 4.4xi08 and 2.0xlO 7 at 830 ° and 950"C respectively.

In addition to platy glass shards, the Campanian tephra contains highly

96

14

I0

.,J <~ P" 8 0 I -

~6 o~ p-

~4 LIJ

2

I 2

500 250

m

b

0 4 5 7

I 000 125 63 :52 16 8

GRAIN S I Z E

8 9 P h i

4 2 M i c r o n s

Fig. 5 Total grain-size distribution of the Y-5 ash layer based on samples

between 430-1535 km from source.

elongate pipe-vesicular pumice fragments (Fig. 7), a variety of lithic

fragments, and crystals of sanidine, plagioclase, pyroxene, and phlogopite. The

relative proportions and downwind variation in these component abundances are

shown in Fig. 6.

DISPERSAL MDDEL OF CAMPANIAN TEPHRA

The atmospheric dispersal of tephra from the Campanian eruption has been

quantitatively modelled using a revised version of the computer model described

in Carey and Sigurdsson (1982). In the revised program, terminal settling

velocities of tephra components are calculated as a function of shape and

density at each one vertical kilometer increment of the fall trajectory,

beginning at the initial elevation of transport. The model incorporates

equations that take into account the vertical variations in atmospheric density,

viscosity, and temperature (R. S. J. Sparks and L. Wilson, written

communication ).

It has been shown that morphology significantly effects the terminal settling

velocity of a particle (Walker et al., 1971; Wilson and Huang, 1979). To

compensate for this effect we introduced a factor that adjusts the terminal

settling of a particle, calculated by equations of Sparks and Wilson (written

97

MODAL COMPONENTS BULK DEPOSIT

~oo -

8 0

I-- Z IJJ

(-) 6 0 ~C

L~ Q_

I.-- " r (D

W 4 0

20

~ " - ' ~ LITH ICS A D ' ~ ~

o , , 200 500 iooo 1500

DISTANCE FROM CAMPANIAN SOURCE (kin)

Fig. 6 Variation in modal composition of the bulk Y-5 ash layer as a function of

distance from a Campanian source area.

communication). This factor, based on the experimental work of Wilson and Huang

(1979), depends on particle diameter. It is only applied to particles with a

diameter greater than 44 microns, because of the diminishing effect of shape ca

fall velocities for smaller particles.

Particle density is an additional variable that complicates the calculation

of terminal settling velocities. Densities can be assumed to be essentially

constant for felsic and mafic crystals but for pumice and glass shards the

density varies with grain-size. Because manY of the glass shards with diameters

less than 125 microns in size are poorly vesicular and platy, we have assigned

98

Fig. 7 Scanning electron photomicrograph of a bubble-wall and pipe-vesicular

glass shard from the Y-5 ash layer in core RC9-191 (430 km from source). Scale

bar is i00 microns in length.

them a density of 2.3 g-cm -3 (the density of a trachytic liquid of Y-5

composition). Glass particles with a diameter between 125 and 180 microns were

assigned a density of 1.72 g-cm -3 (75% of magma density). Glass particles

larger than 180 microns were assigned a density of 1.15 g-cm -3 (50% of magma

density ).

Modelling of ash transport for the Campanian eruption requires some

assumptions about the vertical profile of wind. We assume that the atmospheric

conditions prevailing at the time of the Campanian eruption are analogous to

those occuring in the Italian region today. The 38,000 y.b.p, age of the Y-5

falls close to the isotope stage 3 (Thunnell et al., 1979; Cita and Ryan, 1978),

an interstadial. The coincidence of the age, however, with the cooling side of

stage 3 may suggest a better comparison to glacial conditions. The assumption

of equivalence of present and past atmospheric conditions, while clearly

imperfect, nonetheless provides a starting point in that present-day

meterological observations can put quantitative limits on various models of

tephra dispersal.

The lO-year data set of meteorlogical measurements collected over Brindisi,

Italy (N%O°30 ', E17°57 ' ) can be used to assess the present day atmospheric

structure and variability. The most frequent wind direction between 2 to 34 ]an

99

BRINDISI Mean-Annual Atmospheric Temperature

34 IO-Year Period

30

2o

Io

0 I I I I I I I % - - - J

- 6 0 @ - 4 0 ° 2 0 ° 0 " 2 0 =

TEMPERATURE, °C

Fig. 8 Mean annual atmospheric temperature profile (10-year period) recorded at

Brindisi, Italy.

elevation is rather constant for much of the year (Sept.-~y) with an azimuth of

265 ° (W-SW) (Figs. 8 and 9). In summer (June-Aug.) a significant change of wind

azimuth occurs above 14 kln, when the dominant direction is from the east (90°).

Below 2 km there is considerable seasonal variability in wind direction.

Low-level winds are from the WSW and SW for most of the year (Sept.-~y), but

from NW in summer (June-Aug.). The intensity o£ the most frequently occurring

wind direction varies both as a function of altitude and season (Fig. lO). Each

seasonal wind profile is typically S-shaped with maxima near 8 km (20-30 m/s)

and 34 km (25-35 m/s).

Linear regression equations were calculated for the major segments of the

seasonal-wind intensity profiles and the mean-temperature profiles (Table 3).

These equations were incorporated in computer models to simulate dispersal and

deposition of Campanian tephra during the four seasons of the year. Input to

the model consists of the bulk grain-size distribution of the eruption plume,

and the total modal proportions.

RESULTS

The computer model has been used to evaluate the most reasonable range of

100

E

: 7

0

I-- <~ > W /

i11

3 4

3 0

2 0

I 0 -

0 5 6 0 ('

Brindisi Seasonal Atmospheric Wind Direction tO-year period

• Winter _ • Spring

z~ S u m m e r

o A u t u m n

SCALE

520 ° 2 8 0 ° 2 6 0 ° 220 @ I 00 @ 6 0 @

MOST F R E Q U E N T WIND DIRECTION

Fig 9- Seasonal atmospheric wind direction based on the most frequently

occurring azimuth (10-year period) recorded at Brindisi, Italy.

eruption colmrm heights, wind conditions and processes of tephra transport that

can account for the observed bimodal grain-size distribution and variation in

modal composition in the Y-~ ash layer. The observed decrease of the mean

grain-size of the coarse mode with distance aw~y from source (Figs. 2 and 3)

suggests that the coarse tephra fell as individual particles during atmospheric

transport. As such, the coarse mode may provide constraints on the altitude of

tephra transport. To this end, we used the seasonal wind data (Fig. I0) to

constrain the upper level of major ash transport.

The observed gTain-size relations of the coarse mode are reproduced fairly

I01

TABLE 3

Seasonal wind velocity equations used in the FALLOUT model

Elevation Correlation

Season interval (km) Equation* coefficient

Winter

Spring

>18 V=1.3509 H-10.938 0.96

18-8 V=41.031-1.4088 H 0.98

< 8 V=2.9245 H+5.166 0.99

>18 V=i.1909 H-12.512 0.94

18-8 V=40.520-1.6712 H 0.97

< 8 V=2.5930 H+4.294 0.99

Summer >16 V=0.2458 H+2.546 0.79

16-10 V=66.1041-3.6455 H 0.99

<i0 V=2.5228 H+2.7144 0.99

Fall >18 V=1.2216 H-15.114 0.98

18-10 V=33.9024-1.4068 H 0.95

<lO V=1.6168 H+5.856 0.99

*-Derived from 10-year seasonal upper-wind averages collected for Brindisi,

Italy. Data Processing Division - USAFETAC - Air Weather Service (MAC).

Asheville, N.C. V is expressed as wind velocity in m/s and H in kilometers.

well by models which place upper limits of ash transport at 25 to 40 ~ for

autumn, 20 to 35 km for winter, and 25 to 35 km for spring (Figs. iI and 12). A

lower limit of ash transport of 5 km was used for all three of these solutions.

Transport of the coarse ash during summer can not have occurred because of the

reversal of wind direction above 14 km during that season (Fig. 9). A summer

eruption would therefore result in deposition of the tephra in the western

Mediterranean. The model for ash fall during spring gave the best fit to the

observed data for the coarse mode, with 67% of the actual data points (observed

means) falling between the curves predicted for an upper level of ash transport

at 25 to 35 ~m (Fig. 12). This modelling of settling of individual particles

indicates that ash finer than 22 microns should not fall within a distance of

1600 km from source; thus the ash comprising the fine mode (13 microns) should

not be deposited in this area.

The occurrence of a near~y constant fine mode in the deposit (Figs. 2 and 3)

suggests that an additional mechanism is needed to explain its deposition. This

relationship could be due to either: (1) particle aggregation of fine ash in the

eruption plume (e.g. Brazier et al., 1982; Carey and Sigurdsson, 1982) or (2)

co-ignimbrlte ash fall, as proposed by Sparks and Walker (1977). These two

hypotheses have been tested by the computer model.

102

54

50

E

i 0 - - 20

> I,I _J UJ

I 0

Seasonal Atmospheric Wind Velocity IO-year period

METERS / second

0 I0 20 30 4 0 I I I I

• Winter • Spring /k S u m m e r

o Autumn

0 ~ I I I I I 0 20 4 0 6 0 8 0

MEAN WIND VELOCITY, knots

Fig. i0 Seasonal vertical variation in mean wind velocity along the most

frequently occurring wind azimuth recorded at Brindisi, Italy.

Particle aggregation was modelled using the spring wind profile, which gave

the best fit in modelling of the coarse mode. Two models produced a mean for

the aggregated fine mode similar to the observed mean. In the first model of

aggregation 50% of the 63-44 micron ash, 75% of the 44-31 micron ash and 100% of

the less than 31 micron ash are treated as aggregate particles with a diameter

of 200 microns and a density of 0.2 g-cm -3 (model l; Fig. 12). This model

produced a fine mode with slight decrease of mean grain-size awsy from source

(Fig. 12). In the second model of aggregation, 25% of the 31-22 micron ash, 75%

103

. m

~" 3 Q.. v

ILl

N 2 (I)

5

Z

n -

4

AUTUMN

C O A R S E M O D E M E A N S

25 km ( r = 0 .949)

40 km ( r = 0 . 8 6 7 )

" OBSERVED/ • ( r = 0 . 8 5 4 )

t i i I i i i , I , i , J I J

WINTER ~.~.e~ 20 km •

(r =0.926)

35 km ( r = 0 . 9 0 4 )

32~

63p.

3 I- H ~ z 5 I ~ - ~ ~ I ~ ' '~ ~ ..... k, 0 8 S E R V E D

2 , , , I , , , , I , , , , I 250~

I00 500 1000 1500

D I S T A N C E F R O M S O U R C E ( k m )

Fig. ll Comparison of the FALLOUT-predicted and observed coarse mode means of

the Y-5 ash layer as a function of distance from source for the autumn (upper

part of figure) and winter (lower part of figure) seasonal wind data. Means

calculated by the method of moments. Shaded areas are linear regressions

through FALLOUT-predicted coarse mode means with maximum ash injection heights

at 25 and ~0 km for the autumn and 20 and 25 km for the winter. The lower limit

of injection is 5 km. Observed coarse mode means for the Y-5 layer are shown as

solid circles. The r values refer to correlation coefficients for linear

regressions.

of the 22-16 micron ash and 100% of the less than 16 micron ash (model 2; Fig.

12) are treated as aggregates of 200 microns diameter with a density of 0.2

g-cm -3. This model produced a fine mode with constant mean grain-size over the

1600 km of ash dispersal. In the second model, the fine-ash mode does not show

a decrease of mean grain-size with distance from source, but matches the

observed, nearly constant, fine mode. Agreement is also reasonable between the

104

v

uJ N

03

Z

< n- O

FINE MODE MEANS 3 km

CO-IGNIMBRITE MODEL Autumn-Winter-Spr ing ( r = O ~

OBSERVED • ........ ' ~ ' r 0 7 7 8 ' ~ ' . ~ , , . . ~ . . _ ? ~ m _ _

• ~ ( r = 0. 'r98")

, L , I , J J , I

8p.

16p.

32/.,.

AGGREGATION MODEL q 8 / . ~

/w-30 km Spring model 2 ( r = 0 . 0 2 3 ) ~......~.e @

'3'o de,, C,-- " . . . . . . . . . . . . = " " 16/.z

O ' -

COARSE MODE MEANS Spring 25 km •

( r=

- 6 3 / . ¢

, p r i s g 35 km

• ( r = 0 . 9 3 4 ) i

3 , BSERVED ~125N / j ( r = 0 . 8 5 4 )

2 ~ , , ~ I , , a , I , , , , I 1252~ I00 500 I000 1500

DISTANCE FROM SOURCE (km)

Fig. 12 Comparison of the FALLOUT-predicted and observed coarse and fine mode

means of the Y-5 ash layer as a function of distance from source. Shaded areas

are defined by linear regressions through the FALLOUT-predicted means for the

coarse and fine mode with maximum ash injection heights at 25 and 35 km for the

spring (lower part of the figure; coarse mode), 30 km for spring (fine mode

aggregation models 1 and 2; middle part of the figure) and 3 or 4 km for the

autumn, winter, and spring (fine mode co-ignimbrite model; upper part of the

figure). Observed coarse and fine mode means are shown as solid circles. The

r values refer to correlation coefficients for linear regressions.

105

I00

90

80

70

f- Z

6o r~ LU n

50 F- I (.9

W 40

30

20

0 200

PUMICE AND GLASS SHARDS o.

e-OBSERVED DEPOSIT

o-AGGREGATION MODEL (Spring 30- 5 kin)

&-Co-IGNIMBRITE MODEL (Spring 3-Okm)

FELSIC CRYSTALS

~o ~ -

o---

I i I

500 I000 1600

DISTANCE FROM SOURCE ( k m )

Fig. 13 Comparison of the FALLOUT-predicted and observed component proportions

of the bulk Y-5 ash layer as a function of distance from source. Two types of

models are presented: the co-ignimbrite and particle aggregation models for

spring wind conditions at the 3 and 30 km elevation maxima, respectively.

predicted and observed proportions of modal components (Fig. 13) for both

models.

To test if the fine mode could be produced by co-ignimbrite ash fall from low

elevations (Sparks and Walker, 1977; Sparks and Huang, 1980) we modelled the

dispersal of the bulk grain-size distribution at elevations of 5-0 kin, 4-0 km

and 3-0 ~m, with the spring, fall, and winter winds data, with the tephra

settling as individual particles. The summer wind profile was not considered,

as it has a polarity opposite to that of the deposit. The modelling shows that

mean grain-size of the fine ash of the modelled deposit accords well with that

of the observed deposit in the case of ash transport at 0 to 3 km height (Fig.

12). Agreement between the predicted and observed proportions of modal

components is also reasonable in this model (Fig. 13). The model is, however,

106

quite sensitive to height of dispersal and a change to 4 km or higher results in

a predicted fine mode which is much coarser than observed (Fig. 12).

DISCUSSION

Sparks and Huang (1980) present a volcanological model to account for the

occurrence and behavior of the coarse and fine gTain-size modes of the Y-5

deposit. They suggest that two sources of ash fall existed during the

eruption. The first was a high-altitude Plinian eruption column which provided

the majority of the coarse mode. Dispersal of the tephra by upper-level winds

resulted both in strong sorting and the transport of relatively large particles

great distances from source. A second source was attributed by them to ash

clouds generated from the top of pyroclastic flows, characterized by fine

grain-size and depletion of dense components such as crystals and lithics

(Sparks and Walker, 1977). These co-ignimbrite ash clouds are envisaged to rise

to great heights in order to account for the lack of lateral grain-size

variation in the fine mode of the deposit. However, because the base of these

clouds was near ground level, deposition of fine ash could also occur close to

source.

The results of our modelling of the Y-5 layer enables a rigorous evaluation

of the Sparks and Huang (1980) model. We show that the coarse mode of the

deposit can be accurately modelled with the present-day spring wind profile with

25-35 km as the upper limit of significant ash transport. These conditions

reproduce the downwind variation of the mean grain-size of the coarse mode. The

results agree with the Plinian eruption column phase envisaged by Sparks and

Hua~g (1980). However, the modelling also demonstrates that if all ash fell as

individual particles, no significant deposition of ash less than 22 microns in

size would occur within the 1600 km fallout area, and thus a fine mode would not

be generated under these conditions.

Our modelling of ash deposition indicates that a relatively constant fine-ash

mode (Figs. 2 and 3) can, in theory, be produced over a large area by either

deposition from low-level co-ignimbrite processes or the aggregation of

particles. If the fine-ash mode is generated solely by deposition of material

from co-ignimbrite ash clouds, then the modelling demonstrates that these clouds

could not have exceeded three kilometers in height. The co-ignimbrite modelling

indicates, furthermore, that no ash coarser than 32 microns (3 to 4 km upper

limit of transport) can be contributed by the co-ignimbrite ash fall to the

deposit at distances greater than 430 km (first sample locality). Sparks and

Huang (1980) had suggested that part of the coarse mode was derived from the

co-ignimbrite ash fall.

The process of particle aggregation is an alternative to low-level

co-ignimbrite ash transport for the generation of a pervasive fine-ash mode.

Carey and Sigurdsson (1982) suggest that particle aggregation caused the

"premature" fallout of fine ash and the production of a fine-ash mode in the May

18, 1980 Mt. St. Helens tephra. Modelling of aggregate transport for the Y-5

107

layer shows that a fine-ash mode resembling the observed mode could be produced

with 200 micron diameter particle aggregates transported at the same altitude as

the coarse mode.

The model of particle aggregation is, in our opinion, more likely to account

for the grain-size distribution in the Y-5 layer, rather than low-level

transport of co-ignimbrite ash. The three kilometer maximum elevation imposed

by the model for dispersal of eo-iguimbrite ash-fall seems low when the scale of

the Campanian eruption is considered. Our data for the bulk grain-size

distribution of the sampled deposit indicates that over 70% of the ash fallout

consists of the fine mode. If transport and deposition of much of the fine mode

was solely from co-ignimbrite ash clouds, then this implies that most of the

deposit was transported below three kilometers. Furthermore, the prevailing

direction of low-level winds over Italy and their highly variable direction over

the Mediterranean as a whole, makes it extremely unlikely that low-level

transport can produce such a large volume and polarized deposit. We do not

argue that co-ignimbrite ash clouds cannot be an important source of fine ash.

In the light of modelling presented in this paper, we believe, however, that the

two-source model presented by Sparks and Huang (1980) is not sufficient to

explain quantitatively the grain-size relations and transport of the Y-5 ash.

The eruption colunm of the Campanian event was more likely a complex

combination of a high-altitude Plinian column and high-altitude co-iguimbrite

ash clouds. Deposition of fine ash, both at proximal and distal sites, occurred

as a result of the low level base of the co-ignimbrite clouds and particle

aggregation of ash throughout the eruption colulmu and volcanic plume as a whole.

CONCLUSIONS

The 38,000 y.b.p. Campanian eruption produced the most extensive ash-fall

layer in the Mediterranean Sea. Deposition of tephra covered an area in excess

of 1.4xlO 6 ion 2 with a minimum volume of 73 ~3 (tephra). At distances up to

1535 km from source, the ash displays bimodal grain-size distribution, and a

coarse mode that migrates towards a relatively constant fine mode (13 microns)

with increasing distance from the source.

Computer modelling of the grain-size features of the deposit indicates that

downwind variation of the grain-size of the coarse mode can be accurately

reproduced with transport of ash between 5 and 35 km elevation. However, the

observed fine mode of the deposit can not be generated by transport of ash as

individual particles at these elevations but would result in transport of

virtually all of the fine ash beyond the studied area.

Deposition of fine ash within 1600 ~ can only occur by either "premature"

fallout of fine ash as particle aggregates from a high eruption plume or as

individual particles from co-ignimbrite ash clouds with a maximum elevation of

3 km. The low level (<3 km) constraint imposed by the modelling of

co-ignimbrite ash transport, the large volume of ash in the fine mode (>70% of

the deposit) and the irregularity of azimuth of low-level winds all argue

108

against low-level co-ignimbrite ash transport and indicate that particle

aggregation strongly influenced the origin of the observed fine-ash mode of the

Y-5 layer. The earlier depositional model of two volcanological phases for the

Y-5 ash layer (Sparks and Huang, 1980) appears to be inadequate as a

quantitative explanation of the grain-size features of the Y-5 deposit. Instead

the eruption colu~m was more likely a complex combination of material from both

Plinian and ignimbrite activity. Fine ash was deposited as a result of

aggregation of ash particles which were carried to great height. The ash may

have been derived from both the Plinian eruption column and high-altitude

co-ignimbrite clouds.

ACKNOWLEDGEMENTS

We are grateful to R.S.J. Sparks and L. Wilson for supplying us with

unpublished information c~ settling velocity equations. The paper benefited

greatly by a thorough review by R. B. Waitt and K. H. Wohletz. This work was

supported by National Science Foundation grant EAR-82-05955.

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