Dynamics of ash-dominated eruptions at Vesuvius: the post-512 AD AS1a event

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Bulletin of VolcanologyOfficial Journal of theInternational Association ofVolcanology and Chemistry ofthe Earth`s Interior (IAVCEI) ISSN 0258-8900Volume 73Number 6 Bull Volcanol (2011)73:699-715DOI 10.1007/s00445-010-0432-1

Dynamics of ash-dominated eruptions atVesuvius: the post-512 AD AS1a event

C. D’Oriano, R. Cioni, A. Bertagnini,D. Andronico & P. D. Cole

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RESEARCH ARTICLE

Dynamics of ash-dominated eruptions at Vesuvius:the post-512 AD AS1a event

C. D’Oriano & R. Cioni & A. Bertagnini & D. Andronico &

P. D. Cole

Received: 15 December 2009 /Accepted: 16 November 2010 /Published online: 13 January 2011# Springer-Verlag 2010

Abstract Recent stratigraphic studies at Vesuvius haverevealed that, during the past 4,000 years, long lasting,moderate to low-intensity eruptions, associated with con-tinuous or pulsating ash emission, have repeatedly oc-curred. The present work focuses on the AS1a eruption, thefirst of a series of ash-dominated explosive episodes whichcharacterized the period between the two Subplinianeruptions of 472 AD and 1631 AD. The deposits of thiseruption consist of an alternation of massive and thinly

laminated ash layers and minor well sorted lapilli beds,reflecting the pulsatory injection into the atmosphere ofvariably concentrated ash-plumes alternating with ViolentStrombolian stages. Despite its nearly constant chemicalcomposition, the juvenile material shows variable externalclast morphologies and groundmass textures, reflecting thefragmentation of a magma body with lateral and/or verticalgradients in both vesicularity and crystal content. Glasscompositions and mineralogical assemblages indicate that theeruption was fed by rather homogeneous phonotephriticmagma batches rising from a reservoir located at ~ 4 km(100 MPa) depth, with fluctuations between magma deliveryand magma discharge. Using crystal size distribution (CSD)analyses of plagioclase and leucite microlites, we estimate thatthe transit time of the magma in the conduit was on the orderof ~ 2 days, corresponding to an ascent rate of around 2×10−2 ms−1. Accordingly, assuming a typical conduit diameterfor this type of eruption, the minimum duration of the AS1aevent is between about 1.5 and 6 years. Magma fragmenta-tion occurred in an inertially driven regime that, in a magmawith low viscosity and surface tension, can act also underconditions of slow ascent.

Keywords Ash emission activity . Tephrite . Vesuvius .

Stratigraphy . Textural analyses

Introduction

Eruptions characterized by dominant ash emission areusually represented by long-lasting episodes of moderateto low-intensity activity associated with sustained orpulsating weak plumes, less than 5 km in height (Sparkset al. 1997; Cioni et al. 2008b). Ash in these plumes istransported by wind and can be traced up to hundreds of

Editorial responsibility: S. Nakada

Electronic supplementary material The online version of this article(doi:10.1007/s00445-010-0432-1) contains supplementary material,which is available to authorized users.

C. D’Oriano (*) : R. Cioni :A. BertagniniIstituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa,Via della Faggiola 32,56126, Pisa, Italye-mail: [email protected]

R. CioniDip.to di Scienze della Terra, Università di Cagliari,Via Trentino 51,09127, Cagliari, Italy

D. AndronicoIstituto Nazionale di Geofisica e Vulcanologia,Sezione di Catania,Piazza Roma 2,95123, Catania, Italy

P. D. ColeMontserrat Volcano Observatory,Flemmings,Montserrat, West Indies

P. D. ColeSeismic Research Centre,Trinidad and Tobago, West Indies

Bull Volcanol (2011) 73:699–715DOI 10.1007/s00445-010-0432-1

Author's personal copy

http://dx.doi.org/10.1007/s00445-010-0432-1

kilometers from their source. Recent examples of sucheruptive styles come from the 2001 (Scollo et al. 2007) and2002–03 (Andronico et al. 2008) long-lasting eruptions ofMt. Etna (Italy) and the April–May 2010 Eyjafjallajokull(Iceland) (Gudmundsson et al. 2010).

Recent stratigraphic studies at Vesuvius have highlightedthe presence of thick ash-dominated fall deposit sequencesrelated to such moderate to low intensity activity in theperiods: 1) between the 3,900 BP Avellino and 79 ADPompeii Pumice eruptions (AP1-AP6; Andronico and Cioni2002); 2) between the 79 AD Pompeii Pumice and 472 ADPollena eruptions (S. Maria Cycle; Andronico et al. 1995;Cioni et al. 2008a); 3) between the 512 AD and 1631 ADeruptions (Middle Age Activity: AS1-AS5 sequence;Andronico et al. 1995; Rolandi et al. 1998; Cioni et al.2008a, b); and 4) between the 1631 AD and 1944 eruptions(Arrighi et al. 2001).

The hazard posed by these eruptions is lower than thatassociated with pumice and ash deposition during largePlinian or Subplinian events (Santacroce 1987), but theareal distribution of their deposits, and their frequency inthe past demonstrate that it is important to include them inlong-term hazard assessment at Vesuvius aimed at land useplanning. Establishing an eruptive scenario for this type ofactivity is also fundamental for emergency planning,because they can have a large impact on infrastructureand the environment and both ash fallout and fallen ash canproduce negative health effects (Blong 1984; Rojas-Ramoset al. 2001; Horwell et al. 2003).

Deposits of this type of activity are characterized byrelatively monotonous successions of massive to laminatedash layers, ranging in thickness from a few millimeters todecimeters at a distance of 3–5 km from the vent. Theselayers are generally grouped in repetitive sequences,suggesting long-lasting, monotonous activity. Problemswith the interpretation of this type of activity arise becauseof the difficulty in separating tephra deposits from differenteruptive events. The general absence of evident disconti-nuities, such as erosional surfaces or humified beds in thesesequences, and the low rate of sedimentation that can resultfrom contemporaneous deposition and reworking of theash, make it difficult to interpret past deposits of this typeof activity. Also, the mechanism of ash production iscontroversial: morphological and textural features of fineash erupted during this kind of activity in the past havebeen interpreted as derived from both pure magmatic andphreatomagmatic fragmentation (Andronico and Cioni2002; Cioni et al. 2008b). All these aspects are addressedin this paper in order to reach a more complete understand-ing of the dynamics of these events at Vesuvius andworldwide.

To avoid any problems of deposit misinterpretation ormiscorrelation, we study the deposits of the AS1a eruption,

which are unambiguously identified stratigraphically bytheir position immediately above the Subplinian 512 ADeruption (Cioni et al. 2008b).

In this paper we provide a complete characterization(stratigraphic, textural, morphologic, petrographic andcompositional) of the AS1a ash fragments and deposits inorder to investigate the main processes controlling this typeof activity.

The detailed description of the analytical methods (grainsize analyses; sample preparation; morphological andtextural characterization of the ash; crystal size distributionanalyses; composition) is provided as Electronic Supple-mentary Material; ESM 1.

Stratigraphy

AS1a event: reference section and deposits

The deposits of AS1a eruption lie on top of the deposits ofan eruptive event, which has been recognized, based onhistorical chronicles and stratigraphic studies, as theSubplinian 512 AD eruption (also named AS1; Cioni etal. 2008a). A 10 cm-thick layer, composed of reworkedlight brown ash, characterized by an eroded upper surface,represents the boundary between the 512 AD deposit andthe overlying AS1a sequence.

The deposits of AS1a have been described in detail at sixlocalities, within a distance of 3–4 km from the vent(Fig. 1). Only one distal section, 5 cm-thick, has beenrecognized, at about 10 km from the vent.

The reference section is located in the outskirts ofTerzigno village (Crossodromo Quarry, section 1 in Fig. 1),where the deposit reaches its maximum thickness of 38 cm.At the investigated distance, there is little thicknessvariation over the 120° wide dispersal fan (Fig. 1).

We subdivide the AS1a event into 5 stages, correspondingto 5 layers (L1 to L5), each of which is formed by severalpulses (Fig. 1).

The base (L1) comprises a normally graded sequence offour ash fall beds, with a maximum total thickness of12 cm, from yellow, highly vesicular fine lapilli to very fineash.

Layer 2 (L2) is an 11 cm-thick, thinly laminated andnormally graded, medium to fine ash layer. Locally laminaeare interrupted and show wavy and convolute structures,suggesting wind-reworking.

Layer 3 (L3) is a marker bed in the sequence. It is alaterally continuous, indurated accretionary lapilli-bearinglayer with a thickness of 1 cm. The coarser fraction islargely represented by quasi-spherical, smooth-skinned,accretionary lapilli. Ash aggregates, crystals and irregularlyshaped, glassy shards constitute the ash fraction.

700 Bull Volcanol (2011) 73:699–715


Layer 4 (L4) is a 6 cm-thick, massive layer. The juvenilefraction is mainly composed of black, porphyritic tosubaphyric scoria, poorly to moderately vesicular, andpyroclasts show flattened shapes and pseudo-fluidalvesicles. This layer is characterized by the coarsest grainsize in the sequence, suggesting it was emplaced during amore intense stage of the eruption, possibly related toViolent Strombolian activity.

Layer 5 (L5) is 8 cm thick and formed of laminatedcoarse ash, interlayered with minor medium to fine ash,

with evidence of cross bedding at the top, inferred to berelated to wind reworking. Highly vesicular clasts dominateover low-vesicularity scoria.

Despite the small number of outcrops where the depositis unequivocally recognizable, an isopach map of the totalthickness was tentatively drawn in order to estimatedispersal and volume of the deposits. The isopach map(Fig. 1) indicates an eastward dispersal and suggests a ventposition in the area of the present cone. The minimumvolume (Pyle 1989; Fierstein and Nathenson 1992) of the

Fig. 1 Stratigraphic and sedimentological features of the ash depositsof AS1a eruption at Vesuvius, Italy (a). The inset in the bottom showsthe dispersal of the ash deposits and the locations studied in this study(b). The terminology proposed by Jenkins et al. (2007) is here used todescribe the AS1a volcanic event. Grain size analyses were performed

by combining results form sieving (for intervals from –2 to 3 Φ) andLaser-diffraction (for intervals from 3 to 12 Φ). Grain size parametersand description according to Inman (1952) and Cas and Wright(1987). F fine-grained; C coarse-grained; M medium-grained

Bull Volcanol (2011) 73:699–715 701


total deposit was estimated at about 1.3×108 m3, andconverted to a dense rock equivalent (DRE) of 7.7×107 m3,using a value of 1,480 kg m−3 for the bulk density of thedeposit (averaged over 4 samples (L2-L5) collected throughthe entire sequence) and 2,500 kg m−3 as the magmadensity. This volume (equivalent to a total mass of 1.92×1011 kg) corresponds to the sum of layers deposited duringsuccessive VEI 2–3 events, typical of Violent Strombolianand continuous ash emission events of Vesuvius (Cioni etal. 2008a).

Ash characterization

Textural and morphological features of the ash

Juvenile products show textural and morphological featuresvarying between two end-members: 1) light brown, highlyto moderately vesicular, subaphyric pumice, with a com-mon pseudofluidal structure of the bubbles and an irregularexternal shape, and 2) black to dark gray, very poorly tomoderately vesicular, crystal-rich porphyritic scoria, show-ing, in some cases, a glassy surface, and characterized byflat or rounded external shapes. Fragments with textural

features intermediate between these two end-members arevery common. In some cases, domains of black, felty-textured, holocrystalline scoria (10–50 μm), are included inthe highly vesicular, glassy fragments. The features of thefragments are strongly related to the grain size, with denseclasts being much more abundant in the finer-grainedportions. Rare lithic clasts are represented by altered, leucite-and pyroxene-bearing lavas and tuffs. Loose crystals ofleucite, phlogopite, clinopyroxene are rare in the ash fraction.

The combination of external morphology and clastoutlines, together with characterization of the internaltextures of the AS1a ash (vesicularity index and crystalcontent), were used by Cioni et al. (2008b) to distinguishthree main types of clast: spongy, fluidal (named “fused” inCioni et al. 2008b) and blocky. With respect to thatclassification, we identified two further types of clast(Fig. 2). Within the fluidal group we distinguished betweenhighly vesicular (fluidal-a) and poorly vesicular clasts(fluidal-b), while in the blocky group there are clasts withrounded outlines (blocky-a) and with angular forms(blocky-b). Abundance of spongy and fluidal clastsdecreases from L2 to L3, associated with an increase ofthe blocky, crystal-rich clasts (Table 1). The overlyingscoria lapilli layer (L4), related to the Violent Strombolian

Fig. 2 Morphological and textural features of the ash clasts in theAS1a eruption. For each type an SEM image of external shape of thewhole clast, processed as an outline of the clast and SEM image in

thin section are shown. For the fluidal and blocky clasts, examples ofthe a and b sub-type are shown

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activity, records a slight increase of the fluidal and spongyclasts, even though the blocky, crystal-rich clasts are stillabundant. At the top of the sequence, in L5, the deposit iscomposed of 73 vol.% of highly vesicular material (spongyand fluidal-a clasts) (Table 1).

Vesicles

Owing to the absence of lapilli in these deposits alldiscussion of particles vesicularity is limited to the vesiclepopulation finer than 1 mm (0 Φ).

Spongy and fluidal-a clasts have similar vesicularityindices, with a wide range among clasts, from 37 to 70 vol.% (Table 1). They have relatively abundant, generally large,lobate bubbles, with smooth outlines and incipient coales-cence (Fig. 2); the highest vesicularity is restricted to theclasts from L5. Bubbles are generally elongate (Width/Length, W/L is 0.5–0.7), becoming more spherical at thetop of the succession (W/L is 0.8). The diameter of thevesicles (weighted mean) becomes progressively larger withstratigraphic height, while the vesicle number density (NA)nearly halves passing from L2 to L3, and then progressivelyincreases toward the top of the succession. In all the analyzedsamples, the vesicle volume distributions are unimodal, withthe dominant peak (mode) at ~ 125 μm, shifting at the top(L5) toward ~ 200 μm (Fig. I; ESM 2; Table 1).

Fluidal-b and blocky-a clasts have similar groundmasstexture, showing vesicularity indices of 13–36 vol.%(Fig. 2; Table 1). The relatively few bubbles are nearlyspherical (W/L is 0.6–0.8) and show evidence of coales-cence (Fig. I; ESM 2). These clasts show uniform vesiclesfeatures through the entire succession. Clasts are charac-terized by the presence of at least two large bubbles,resulting from the coalescence of the smallest bubbles,which are variably distributed within the clasts. Thisresults in bimodal vesicle size distributions, with a peakat 80–120 μm, and a second peak at 200–500 μm (Fig. I;ESM 2).

Blocky-b clasts generally have a crystal content >90 vol.% and a low vesicularity index (16–36 vol.%). Theseclasts have many (high NA) irregularly shaped vesicleswith intermediate diameters, reflecting crystal growth onthe bubble walls, with evidence of coalescence (Fig. 2;Fig. I; ESM 2; Table 1). Fragments with a small amount ofgroundmass glass show round vesicles smaller than 5 μm,possibly reflecting the latest stages of vesiculation historyfollowing massive groundmass crystallization. Vesicle sizedistributions are highly asymmetrical toward the largestsizes, clearly evidencing an incipient process of coales-cence. The shape of the largest vesicles is suggestive ofthe coalescence of aligned bubbles along preferentialdirections (W/L 0.3–0.5).

Table 1 Abundance and related vesicle parameters for clast types from different ash layers produced by the AS1a eruption

Layer Clast-Type Clast-type abundance vol% Vesicularity vol% Vesicles diameter (μm) NA (mm−3) Vesicles aspect ratio (W/L)

L5 Spongy 55 39–70 90 590 0.5–0.8Fluidal-a 18

Fluidal-b 5 15–36 52 462 0.8Blocky-a 9

Blocky-b 13 17–26 65 1181 0.5–0.8



Blocky-b 23 19–35 66 669 0.65



Blocky-b 57 16–36 45 1030 0.5–0.7



Blocky-b 7 19–24 64 1289 0.8

Vesicle diameters calculated as weighted means, using the formula: x ¼P

wixiPwi, where w=frequency and x is the class size interval. NA=areal

density population number; W/L=Width/Length aspect ratio of vesicles

Bull Volcanol (2011) 73:699–715 703


Tab

le2

Major

elem

entcompo

sitio

nsof

bulk

rock,averagematrixglassesandcalculated

initial

melts(groun

dmasses+ph

enocrysts)

intheAS1a

erup

tion

MeasuredGlass

Com

positio

nCalculatedInitial

Melts

L4

L2(base)

L2(top)

L3

L4

L5

L2

(base)

L2

(top)

L3

L4

L5

Bulk

Rock

GlassyGM

Crystal

rich

GM

GlassyGM

Crystal

rich

GM

GlassyGM

Crystal

rich

GM

GlassyGM

Crystal

rich

GM

GlassyGM

Crystal

rich

GM

wt%

Mean

n=15

σn=1

Mean

n=15

σMean

n=8

σMean

n=18

σMean

n=2

σMean

n=14

σMean

n=11

σMean

n=15

σMean

n=3

σ

SiO

251.16

47.85

0.53

45.81

48.05

0.32

47.90

0.94

47.49

0.34

47.58

0.21

47.59

0.30

48.07

0.96

47.82

0.29

47.95

0.50

49.93

49.41

49.43

49.07

48.91

TiO

20.78

0.91

0.10

1.44

0.91

0.08

0.93

0.17

1.00

0.05

0.77

0.11

0.92

0.06

0.85

0.09

0.96

0.09

0.98

0.08

0.66

0.77

0.77

0.76

0.82

Al 2O3

19.43

20.21

0.33

19.65

20.29

0.25

20.52

0.58

20.03

0.31

20.16

0.13

20.15

0.19

20.43

0.83

20.14

0.19

19.95

0.56

21.45

20.86

20.90

20.83

20.57

FeO

7.40

8.29

0.33

11.21

8.42

0.29

9.05

1.41

8.54

0.35

9.95

0.26

8.21

0.31

8.50

1.50

8.30

0.20

9.09

0.25

5.95

6.46

6.28

6.58

6.74

MnO

0.15

0.32

0.10

0.31

0.27

0.08

0.29

0.09

0.26

0.08

0.36

0.06

0.24

0.08

0.32

0.10

0.24

0.07

0.40

0.11

0.20

0.16

0.16

0.17

0.16

MgO

2.24

1.23

0.11

1.00

1.22

0.09

1.05

0.25

1.33

0.16

0.84

0.05

1.39

0.08

1.04

0.34

1.35

0.11

0.67

0.03

1.06

1.42

1.35

1.34

1.62

CaO

7.04

7.74

0.40

5.44

7.68

0.27

6.33

1.40

8.13

0.31

6.44

0.16

7.95

0.47

6.27

1.62

7.78

0.45

4.54

0.60

6.26

7.08

6.76

6.85

7.57

Na 2O

3.47

6.77

0.77

9.23

6.64

0.45

7.83

1.10

6.74

0.46

7.64

0.09

6.57

0.30

7.69

1.47

6.84

0.60

9.93

0.80

4.74

4.48

4.52

4.97

4.88

K2O

7.98

5.42

0.24

4.66

5.25

0.34

4.87

0.23

5.12

0.47

4.93

0.03

5.70

0.57

5.49

1.03

5.30

0.42

5.03

1.29

8.84

8.46

8.89

8.42

7.80

P2O5

0.36

0.05

0.05

0.11

0.08

0.06

0.00

0.00

0.16

0.02

0.18

0.12

0.15

0.06

0.04

0.10

0.12

0.11

0.00

0.00

0.03

0.05

0.10

0.11

0.08

Cl

0.00

1.20

0.06

1.13

1.18

0.05

1.24

0.05

1.19

0.08

1.20

0.02

1.13

0.01

1.30

0.16

1.16

0.05

1.48

0.12

0.78

0.73

0.74

0.82

0.77

Bulkrock

byXRFandglasscompo

sitio

nby

energy

dispersive

X-ray

microanalysis.C

ompo

sitio

nsof

initialmagmas

werecalculated

byadding

thegrou

ndmassmineralcompo

sitio

nto

theaverage

residu

alglasscompo

sitio

nof

clastsforeach

layer,in

theprop

ortio

nsmeasuredusingim

ageanalysis:L2base=28

%Lct+4%

Pl+

3%Cpx

;L2top=26

%Lct+4%

Pl+

8%Cpx

;L3=29

%Lct+3%

Pl+

6%Cpx

;L4=22

%Lct+2%

Pl+

4%Cpx

;L5=21

%Lct+4%

Pl+

8%Cpx

.*G

M=grou

ndmass;*n

=nu

mberof

analyses;*σ

=standard

deviation

704 Bull Volcanol (2011) 73:699–715


Petrography and mineralogy

All the clasts show a similar porphyritic index, with 5–7%of phenocrysts of clinopyroxene (3 vol.% at the base and1 vol.% at the top of the sequence), leucite (1 vol.% at thebase and 3 vol.% at the top of the sequence) and phlogopite(2 wt.%). Microlites and microphenocrysts of leucite,pyroxene and plagioclase are the main phases in thegroundmass, with only minor Fe-Ti-oxide and apatite,occurring in the most crystal-rich products.

Leucite is present as isotropic microphenocrysts with aquasi-stoichiometric composition (K,Na)AlSi2O6. Micro-lites are also present in the groundmass as clusters smallerthan 20 μm or aligned along the rim of clinopyroxenemicrophenocrysts.

Plagioclase microlites generally occur as euhedral,elongate laths, with an average 2D aspect ratio (W/L) of1:6–9. Composition remains nearly constant from the baseto the top of the sequence, from An58–73Ab24–34Or1–10 inL2 to An51–74Ab23–40Or3–16 in L5, showing a weakcompositional variation from the core to the rim (Fig. IIa;ESM 2). Plagioclase microlites in sample L4 show thelargest core to rim variation up to An41Ab–41Or18.

Pyroxene microlites are characterized by euhedral,tabular habit, with average 2D aspect ratio of 1:5. Micro-lites are all zoned, from Wo49–54En22–35Fs14–26, withFe-rich rims, with a complex oscillatory zoning in thelarger microlites (Fig. IIb; ESM 2). Smallest crystals exhibitswallowtail morphologies as response to rapid growthduring the late stage of crystallization. Large, loose juvenilecrystals, characterized by the same compositional rangeshown by microlites, in many cases host melt inclusionsand leucite microcrystals.

Largely variable Ca-amphibole (from edenite toFe-edenite, to Fe-pargasite to pargasite), F-apatite, sodaliteand davyne have crystallized as accessory minerals,forming crystals <10 μm in size.

Bulk rock and glass composition

Bulk rock composition (Table 2) was obtained only on thecoarsest layer of the sequence (L4); the composition isphonotephritic and plots along the same line of descentdefined by the products of the preceding 512 AD eruption(Fig. 3). Mass balance calculations, assuming a simplefractionation process, indicate that the AS1a products can

Fig. 3 Major elements variationdiagrams for the bulk rock anddifferent clast types in the AS1aeruption and the bulk rocks ofthe 512 AD eruption. The AS1abulk rock composition is in linewith the trend of the 512 AD(see text for explanation). Thematrix glass compositions of theAS1a glassy clasts show littlevariations, while crystal richgroundmasses are highly scat-tered as a consequence ofmicrolites crystallization

Bull Volcanol (2011) 73:699–715 705


be derived from the crystallization of clinopyroxene (5%)and phlogopite (0.7%) starting from the most evolvedproducts of the 512 AD eruption.

Groundmass glass was analyzed from the different typesof clasts from L2 to L5. It has a foiditic composition, with alower SiO2 and higher alkali content than the bulk rock(Table 2; Fig. 3).

Groundmass glass of clasts, taken from the same layer andwith different textural features, shows a wide range in majorelement concentrations, which can be related to microlite

crystallization. In particular, CaO content varies from 7.21 to8.57 wt.% in glassy clasts to 3.80–8.31 wt.% in crystal-richclasts. Mass balance calculations indicate that this variabilitycan be explained by 25 wt.% of groundmass crystallization(10 wt.% leucite, 7 wt.% clinopyroxene, 7 wt.% plagioclase,1 wt.% oxides), without any significant compositionalvariation in the erupting magma. The up sequence composi-tional variation, measured by LA-ICP-MS on the glassy clasts,shows progressively more evolved melts (Table 3, Fig. 4).Variations in Nb, Ta, Th and U concentrations in glass can be

Table 3 Trace element compositions of the bulk rock and average matrix glasses. Bulk rock by XRF and groundmass glasses by laser ablation(LA-ICP-MS)

L2 (base) L2 (top) L3 L4 L5

ppm Bulk rock Mean(n=15)

σ Mean(n=15)

σ Mean(n=18)

σ Mean(n=14)

σ Mean(n=15)

σ

Li – 34 10.5 36 11.5 37 4.1 39 12.4 36 5.1

B – 74 47.9 65 17.2 50 13.6 61 51.4 86 35.2

Sc 5 10 9.5 3.47 1.5 4.88 1.7 5.35 5.3 2.86 1.1

Ti – 4637 503.3 5100 649.3 5533 476.3 4959 532.3 5421 814.5

V 192 189 30.5 189 24.3 213 16.3 199 23.1 197 34.5

Co 19 18 2.7 17 2.8 19 1.0 20 4.7 19 2.4

Zn – 94 15.4 99 23.9 105 8.7 98 31.3 183 135.3

Rb 311 109 63.0 125 48.6 111 34.2 116 35.4 164 73.8

Sr 1284 1556 222.0 1816 483.0 1653 109.9 1740 172.9 1857 276.0

Y 24 24 2.6 27 2.6 29 3.1 26 4.0 28 3.3

Zr 254 210 22.3 238 23.1 264 22.0 230 29.2 243 23.0

Nb – 72 10.7 73 12.9 71 4.7 73 11.1 85 17.5

Cs 19 3.55 2.3 4.52 2.3 3.43 1.6 3.44 1.4 5.77 3.8

Ba 1901 1844 249.8 1986 382.4 1937 80.9 2124 354.9 2452 547.6

La 72.2 84 8.8 91 13.2 86 1.5 88 13.7 100 10.4

Ce 139.8 153 19.5 158 17.9 150 1.3 156 24.1 163 18.1

Pr – 15 2.0 16 1.8 16 0.8 16 2.4 17 2.1

Nd – 50 5.6 58 6.5 51 8.5 56 13.2 58 6.2

Sm 9.26 7.78 1.9 10 1.0 9.34 2.4 9.49 2.0 10 2.2

Eu 2.36 2.17 0.3 2.46 0.3 2.26 0.8 2.35 0.6 2.44 0.5

Gd – 6.07 2.5 7.26 1.5 5.81 1.1 5.68 2.2 7.38 1.6

Tb 0.84 0.76 0.2 1.00 0.1 0.89 0.1 0.97 0.2 0.99 0.3

Dy – 4.26 0.8 5.16 0.9 6.02 0.3 4.56 1.5 5.31 1.1

Ho – 0.88 0.4 0.96 0.2 1.00 0.1 1.04 0.4 1.07 0.3

Er – 2.59 0.3 2.39 0.4 1.90 0.5 2.99 1.8 2.74 0.5

Tm – 0.41 0.2 0.36 0.1 0.40 0.2 0.49 0.2 0.41 0.2

Yb 2.37 3.00 1.2 2.58 0.4 4.39 0.7 3.08 1.5 3.37 1.2

Lu – 0.40 0.2 0.33 0.1 0.44 0.2 0.67 0.7 0.33 0.1

Hf 4.54 3.86 0.9 3.84 0.6 3.43 1.4 3.88 1.5 4.80 3.0

Ta 2.7 2.76 0.8 3.17 0.5 2.78 0.4 2.72 0.9 3.77 0.6

Pb – 59 15.8 60 15.1 67 14.7 69 18.1 76 23.0

Th 27.7 35 4.2 38 6.1 39 5.8 37 5.0 43 5.4

U 11.59 14 1.6 14 3.4 13 1.0 15 4.0 15 1.7

n number of analyses; σ standard deviation

706 Bull Volcanol (2011) 73:699–715


explained by a 15–20 wt.% of groundmass crystallization. Onaverage, the less evolved glass compositions are for clastsfrom the lapilli-bearing layer (L4), characterized by a lowergroundmass crystal content than those in other layers. As aconsequence, we infer that magma composition varied onlyslightly during the course of eruption.

The composition of volatiles dissolved in the ground-mass glass was estimated by EPMA analyses (water contentestimated by the difference method, Devine et al. 1995)(Table 4). Results indicate that H2O is 0.30±0.77 wt.%, Clis 1.53±0.16 wt.%, SO3 is 0.09±0.0.5 wt.% and F is 0.50±0.08 wt.%.

The melt inclusions (MI) trapped in pyroxene phenocrystsshow a large compositional range, within which it is possibleto identify two different groups. The first group of MI ischaracterized by CaO content ranging between 1.1 wt.% and3.0 wt.%. These MI are more evolved than the glasses stuck tothe crystals, suggesting that MI-hosting pyroxene were notcrystallized in the carrier melt, but they could representantecrysts derived from lateral portions of the reservoir(Davidson et al. 2007; Maclennan 2008). The second groupof MI is characterized by a compositional range close to that

of the groundmass glass of dense, crystal-rich fragments(CaO=4.84–7.89 wt.%) (Table 4). This slightly evolvedcomposition can be related to post-entrapment pyroxenecrystallization of less than 5 wt.% (Spilliaert et al. 2006).EPMA analyses indicate that some MI are not totallydegassed (H2O=2.40±1.05 wt.%, Cl=0.96±0.18 wt.%;SO3=0.38±0.13 wt.%; F=0.36±0.010 wt.%). By applyingthe empirical glass-cpx geothermometer of Cioni et al.(1999) for Vesuvius magmas, we obtain temperatures of1,044 °C, 1,001 °C and 989 °C for L2, L4 and L5 samples,respectively. These different values suggest that the magmainvolved in the eruption had been cooled progressively,consistent with the observed compositional variation.

Groundmass crystal size distribution

Five clasts were selected from each stratigraphic level forquantitative analysis of groundmass texture. Due to thelarge variability of clast types, only clasts with spongy andfluidal morphologies were analyzed, representing the mostabundant types in all the studied samples (Table 1). Theselected clasts are characterized by vesicularity indicesranging between 32 and 46 vol.% and crystal contentbetween 28 and 39 vol.%. Data reported in Table 5summarize the main 2D and 3D textural parametersobtained on each analyzed sample.

Leucite

Leucite is the most abundant microlite phase, reaching 25–30 vol.% (counting only microlites sized between 10 and150 μm). NA ranges between 216 and 304 mm−3; thelowest value is for L4. CSD curves are best-fit to a polyline(Fig. 5a), suggesting a multistage CSD history (Marsh1998). A two-segment line characterizes the CSD of theash-dominated stage of the eruption, L2, L3 and L5(Fig. 5a). The fine grained population (30.3–33.4 μm) hassimilar average size (3 Gτ) and nucleus density (n°) for L2and L3 (Table 5b), while it shows a substantially lower3 Gτ (12.9 μm) and higher n° (6.7×105 mm−4) in L5.Conversely, the larger population (62.7–133.5 μm) shows acontinuous decrease in leucite size and increase of n°toward the top of the sequence. This can be interpreted interms of a decrease in the growth rate or shorter growthtime.

The L4 lapilli-bearing layer is characterized by aconcave-up CSD, which fits well (R2=0.99) with a fractaldistribution (D) of 1.8 on a log-log plot (Higgins 2006a)(Fig. 5b). This CSD has been generally interpreted in theliterature to result from a process of continuous nucleationand growth, with D being a measure of the number ofnucleation events. For D=1.8, Blower (2001) suggests theoccurrence of at least 5 nucleation events.

Fig. 4 Trace element variation diagrams for the bulk rock and matrixglasses in the AS1a eruption

Bull Volcanol (2011) 73:699–715 707


Plagioclase

Plagioclase microlites represent 2–4 vol.% of groundmass,with the lowest value measured for the L4 sample.Conversion from 2D to 3D values (Higgins 1994) suggestsa tablet shape, with an aspect ratio between 1:6:7 and1:9:10 (Table 5a). CSDs (Fig. 5c) of the ash-bearing layers(L2, L3 and L5) are generally represented by a single linearsegment for the 10–90 μm size population. They arecharacterized by a nearly constant average crystal size,3 Gτ, and nucleus density, n° (25–29 μm and 2.6–4.7×105 mm−4, respectively; Table 5b). Sample L4 shows aconcave-up curved CSD, which fits well (R2=0.995) with afractal distribution having a dimension of 3.4 on a log-logplot (Higgins 2006a), possibly resulting from a highnumber of nucleation events (Blower 2001). The localizedoccurrence of large plagioclase crystals, absent in the othersamples, slightly affects the regularity of the CSD (Fig. 5c;right hand truncation effect).

Clinopyroxene

Clinopyroxene microlites represent 3–8 vol.% of thegroundmass. Microlite abundance is a function of thestratigraphic position of the analyzed sample and of thetexture of the ash. Microlites change from tablet-shapedwith 3D aspect ratio of 1:3:5 at the base of the succession(L2), to 1:6:7 in the lapilli-rich layer (L4) (Table 5a). As

shown in Fig. 5d, the CSD plots are strongly curved, andconcave-up throughout the whole range of microlite size(>10 μm). This fits well (R2=0.99) with fractal distribu-tions having dimensions varying between 2.6 and 3.2,related to the occurrence of repeated nucleation events(Blower 2001). Within the uppermost deposit of theeruption (L5) the NA halves (176.3 mm−4), due to theoccurrence of relatively few, large sized, microphenocrysts,suggesting that growth rather than nucleation of pyroxeneplayed the main role in this stage of the eruptions.

Discussion

Eruption dynamics

Stratigraphic, sedimentological, geochemical and texturaldata give a coherent picture of the evolution of the eruptedmaterial and of the processes which controlled the modes ofmagma discharge. The large variability in the morpholog-ical features of the juvenile products also suggests theprobable mechanism of magma fragmentation.

A characteristic feature of the AS1a deposits is theirfine-grained nature, including in proximal sites, and theirdeposition by several pulses of ash fallout. The alternationof massive and thinly laminated layers reflects thedevelopment in the atmosphere of variably concentratedplumes, which oscillated between pulses characterized by

Table 4 Chemistry of representative melt inclusions and the host minerals with the averaged matrix glasses

Melt inclusions Glasses L5- MI hosting pyroxene L4 - MI hosting pyroxene

MI-L5 MI-L4 MI-L2 L2 L4 L5 px1 px2 px3 px1 px2 px3 px4 px5 px6

SiO2 48.06 48.46 48.62 48.77 47.17 48.60 SiO2 42.47 48.86 41.94 45.56 50.48 46.76 45.23 46.75 46.70

TiO2 0.47 0.68 0.64 0.85 0.82 0.82 TiO2 2.21 1.14 2.42 1.48 0.86 1.46 1.75 1.31 1.18

Al2O3 20.57 19.79 19.93 19.61 19.32 19.27 Al2O3 12.14 5.91 11.67 8.47 4.51 8.30 8.96 7.98 7.51

FeO 5.50 7.34 5.31 7.65 7.63 7.57 FeO 12.00 6.63 11.53 9.08 5.54 9.16 8.87 8.86 8.19

MnO 0.22 0.28 0.15 0.22 0.25 0.25 MnO 0.16 0.15 0.17 0.09 0.15 0.17 0.16 0.17 0.10

MgO 0.83 1.15 1.34 1.19 1.66 1.37 MgO 7.76 12.77 8.04 10.56 13.94 10.66 10.57 11.11 11.42

CaO 4.79 5.14 5.23 6.62 7.95 7.05 CaO 23.46 23.97 23.76 23.87 24.25 23.66 24.39 24.35 23.92

Na2O 6.34 7.06 4.26 7.32 6.20 7.20 Na2O 0.36 0.35 0.37 0.27 0.20 0.32 0.29 0.32 0.26

K2O 10.04 6.59 9.59 5.26 5.68 5.62 Wo 53.63 50.96 53.90 52.20 50.43 51.68 52.87 51.97 51.68

P2O5 0.34 0.30 0.29 0.30 0.47 0.30 En 24.67 37.78 25.39 32.14 40.34 32.40 31.87 32.99 34.34

BaO 0.29 0.32 0.31 0.31 0.19 0.31 Fs 21.71 11.26 20.71 15.65 9.23 15.92 15.26 15.04 13.98

SrO 0.08 0.33 0.19 0.17 0.25 0.22

F 0.46 0.58 0.33 0.53 0.44 0.56

SO3 0.24 0.12 0.43 0.15 0.07 0.13

Cl 0.86 1.54 0.99 1.53 1.50 1.47

tot 99.07 99.65 97.60 99.93 99.60 99.72

H2O 0.93 0.35 2.40 0.07 0.40 0.28

708 Bull Volcanol (2011) 73:699–715


high-rate, continuous sedimentation (massive layers) versusdiscontinuous sedimentation (laminated layers) from short-lived plumes during prolonged periods of activity. Syn-depositional wind reworking was common, and particularlyaffected the laminated deposits.

The evidence shows that AS1a deposits were related tomoderate intensity activity mainly driven by magmaticexplosivity with a minor involvement of external fluids indepositional processes. In general deposits are poorlydispersed and do not show any features distinctive of wetdeposition (accretionary lapilli, vesiculated tuffs, softsediment deformation). Clear evidence for the involvementof external water is only shown by the thin, accretionarylapilli-bearing, L3 layer, characterized by juvenile frag-ments with an altered, yellowish external surface. Thedispersal of ash without a major involvement of lapilli-sizedmaterial can be interpreted as related to a low exit velocityof the eruptive plume from the vent (Houghton and Nairn1991). If gas flux from the vent was the transporting agentfor the fragmental material, the terminal velocity of the ashgives a lower limit to the velocity of the gas. Applying therelations given in Bonadonna et al. (1998), gas-thrustvelocities of only few meters per second are able to sustainand transport most of the ejected material.

The generation of large amounts of ash is generallyinterpreted as due to the disruption of highly vesicularmagmatic foams during high intensity eruptions, or fromenhanced fragmentation related to interaction of magmawith external water. The external morphology and texturalfeatures of the juvenile ash particles provide information onthe magma fragmentation processes and eruption dynamics.Spongy and fluidal clasts are the most abundant compo-nents, with subordinate dense, blocky, crystal-rich clasts. Asimilar assemblage has also been described for other ash-dominated eruptions, for example the 2001 eruption of MtEtna (Taddeucci et al. 2004), the 1994–1997 ash emissionactivity at Popocatepetl volcano, Mexico (Martin-DelPozzo et al. 2007), and the 2003–2005 ash emissions fromthe Nakadake crater lake, Aso Volcano, Japan (Miyabuchiet al. 2008). Ash-dominated eruptions characterized by thepresence of blocky, poorly vesicular glassy clasts have beenoften considered as the result of phreatomagmatic activity(Morrissey and Mastin 2000; Miyabuchi et al. 2008).Houghton and Wilson (1989) also suggested that magma-water interaction can trigger the explosive fragmentation ofa rising magma, suddenly interrupting vesiculation andpreventing crystallization, producing glassy clasts with ahighly variable vesicle content. The coexistence of variablycrystalline, vesicle-rich clasts and crystal-rich, dense blockyclasts does not, however, support an important involvementof external water. Morphological features of the clasts arealso not indicative of fragmentation by magma-waterinteraction, no evidence of typical phreatomagmatic fea-T

able

5Param

etersof

Crystal

size

distribu

tions

(CSDs)

formineralsin

selected

ashclast

samples

(spo

ngy

and

fluidalglasstypes).Crystal

fractio

n(φ),

vesicularity

index

(v.i.),

maxim

umnu

mberof

crystals(n

max),arealdensity

popu

latio

nnu

mber(N

A),aspect

ratio

(W/

L),

crystalaveragedo

minantsize

(3Gτ;

Cashm

an19

92),

nuclei

numberdensity

(mm-4).

Leucite

(Lct),plagicolase(Pl)andclinop

yrox

ene(Cpx

)”

ab

Layer

Φ(vol%)

v.i.

nmax

NA(m

m−3)

W/L

Lct

(smallestpo

pulatio

n)Lct

(largestpo

pulatio

n)Pl

lct

plcpx

lct

plcpx

lct

plcpx

lct

plcpx

3Gτ(μm)

n°(m

m−4)

3Gτ(μm)

n°(m

m−4)

3Gτ(μm)

n°(m

m−4)

L5

214

844

204

457

125

288

644

176

1:1:1

1:8:9

1:3:5

136.7*

105

631.7*

104

273.2*

105

L4

222

438

218

371

270

216

344

250

1:1:1

1:8:8

1:6:7

L3

292.5

646

258

406

280

304

478

330

1:1:1

1:9:10

1:5:5

301.0*

105

761.1*

104

292.6*

105

L2top

264

832

284

793

343

292

816

353

1:1:1

1:8:8

1:3:4

311.1*

105

946.6*

103

254.7*

105

L2base

284

345

306

636

233

284

590

216

1:1:1

1:6:7

1:3:4

339.1*

104

133

2.2*

103

283.6*

105

Lct

leucite;Plplagioclase;

Cpx

clinop

yrox

ene;

3Gτcrystalaveragedo

minantsize

(Cashm

an19

92);n°

nuclei

numberdensity

(mm

−4)

Bull Volcanol (2011) 73:699–715 709


tures such as pitting on the outer surfaces, deposition ofsecondary minerals, or presence of cracks from thermalcontraction (Heiken and Wohletz 1985; Cioni et al. 1992).

Hence, we suggest that the large heterogeneity observedin the texture of the juvenile material reflects fragmentationof a magma body with lateral and/or vertical gradients invesicularity and crystal content. Glass-bearing, vesicularclasts (spongy and fluidal) are derived from syn-eruptivelyvesiculating portions of the magma, while the microlite-rich, dense to poorly vesicular, blocky clasts are the productof the passive fragmentation of degassed magma, possiblyfrom stagnant zones along the margin of the conduit. Wesuggest that the smooth external surface of the fluidal clastsrepresents the inner walls of an interconnected network ofelongated bubbles through which the gas phase can moveeasily. This can lead to an open system degassing, favoredby the reaching of a permeability threshold in these parts ofthe magma (Fig. 7). Moreover, the deformed bubbles andthe smooth outer surfaces of fluidal clasts reflect the lowviscosity of the magma, suggesting ductile fragmentation.The general low abundance of lithic fragments is a furtherindication of the low energy of the explosive activity,which could not significantly erode the conduit–cratersystem.

Magma reservoir, ascent dynamics and eruption duration

The phenocryst and microlite paragenesis, and crystalliza-tion parameters such as growth and nucleation rates, ortimes of crystal growth, are used to constrain the possibledepth of the magma chamber, the crystallization history andthe transit time of magma in the conduit and the duration ofthe entire activity.

The composition of the erupting magma, as recalculatedby adding the groundmass mineral phases (in the propor-tions derived from image analysis) to residual glasses(Table 2), indicates that the most evolved products occurat the top of the sequence (Fig. 4). The observed mineralassemblage and the order of crystallization can be discussedusing some classical ternary phase diagrams. On theleucite-diopside-silica diagram (Fig. 6a), the recalculatedcomposition of erupting magmas is in agreement with afirst crystallization of diopside followed by leucite, at apressure ≥100 MPa. Accordingly, the coexistence ofphenocrysts of diopside and phlogopite, and only rareleucite, suggests crystallization around 100 MPa (Luth1967; Carmichael et al. 1974) in agreement with the resultsof experimental petrology for the products of the post-79 AD activity (Scaillet et al. 2008).

Fig. 5 Crystal size distribution(CSD) patterns of microlites influidal and spongy clasts in theAS1a eruption. a leucite CSDplot (natural logarithm of thepopulation density versus size)of clasts from L2 and L3; bleucite of clasts from L4 and L5;c plagioclase; d clinopyroxene.Error bars of the populationdensity are calculated by CSDcorrection software from thesquare root of the number ofintersections in each bin andpropagated to the other bins(Higgins 2006b)

710 Bull Volcanol (2011) 73:699–715


On the same diagram, matrix glass follows the diopside +leucite + liquid cotectic curve at 0.1 MPa, suggesting thatmicrolite crystallization occurred at lower pressure (Fig. 6a),probably following ascent-related decompression. Similarly,the assemblage of salic minerals in the groundmass (leucite +

sodalite + plagioclase) indicates crystallization around0.1 MPa (Fig. 6b; Carmichael et al. 1974). The crystalliza-tion of sodalite in place of nepheline in this magma may havebeen favored by the high Cl content (Baldridge et al. 1981).

The limited compositional variation of the eruptingmagma can be interpreted in terms of an increase ofgroundmass crystallization from the base to the top of thesequence. An exception to this general trend is representedby the L4 lapilli-rich layer, which is mainly constituted byhighly vesicular, glassy fragments with a low microlitecontent and poorly evolved glass composition, suggestingthe involvement of a crystal-poor and gas-rich magma,during the more intense stage of the eruption.

Size distributions of plagioclase and clinopyroxenemicrolites are characterized by a single population, whereasleucite has a CSD with two distinct populations, suggestingthat crystallization probably occurred in two different stepsof decompression accompanying magma ascent. The firststep caused an enlargement of the leucite stability field,forcing the melt along the cotectic leucite + diopside +liquid line during the final stage of magma ascent (Fig. 6a).

The CSD-derived parameters of plagioclase microlitesand of the finer-grained leucite population are constantthrough the sequence, suggesting similar growth conditionsduring the final step of ascent and decompression.

Based on this information, the time of crystal growth canbe inferred, and used as a proxy for the duration of themagma’s transit/residence in the conduit. For the leucitemicrolites we used a growth rate of 10−7 mms−1

corresponding to the maximum growth rate measured byShea et al. (2009) for the phonolite of the 79 AD eruption.The maximum value was chosen in consideration of thelow viscosity of the magma erupted during AS1a event.Results indicate a minimum growth time on the order of2 days. The same results are obtained for plagioclasemicrolites, when considering a similar growth rate (Fenn1977; Cashman and Marsh 1988; Shea et al. 2009).Assuming a 4 km long conduit (consistent with thecrystallization pressure derived from the mineralogicalassemblage) with a diameter between 5 and 10 m (Calvariet al. 1994; Coltelli et al. 1998; Parfitt 2004; Vergniolle etal. 2004; Bonaccorso 2006), an average magma ascent rateof about 2×10−2 ms−1 and a magma supply rate (MSR)between 1 and 4×103 kgs−1 can be estimated. Given a totalerupted mass of 1.92×1011 kg and considering the wholesequence of eruptive pulses responsible for the observeddeposits, the minimum duration of the eruption is estimatedas between 1.5 and 6 years.

Syn-eruptive degassing

The syn-eruptive budget of released volatiles can beestimated by balancing the content of volatiles in the

Fig. 6 Ternary phase diagrams showing the compositions of matrixglasses, bulk rock, and leucite microlites in the AS1a eruption. a Thesystem leucite-diopside-silica together with the corresponding norma-tive components of the glassy clast (full triangles) and crystal richclast (open triangles) matrix glass and bulk rock (open square)compositions; cotectic curve at P=0.1 MPa and 100 MPa arerepresented. b Phase equilibrium diagram of the system NaAlSiO4-KAlSiO4-SiO2-H2O for P=0.1 MPa, PH2O=100 MPa (modified fromDeer et al. 1965; Gittings 1979) together with the correspondingnormative components of the glassy clast and crystal-rich clast matrixglass, bulk rock and leucite compositions. In the diagram, each layer ischaracterized by a specific color: orange=L2 base; green=L2 top;magenta=L3; blue=L4; red=L5

Bull Volcanol (2011) 73:699–715 711


residual glass and the initial content of the magma, asderived from the EPMA analyses of melt inclusions inclinopyroxene. Data indicate that during the decompres-sion, Cl and F did not degas, as their increase in the residualglass is consitent with crystallization of 30–40% of micro-lites, similar to that measured in the ash fragments. Incontrast, water and sulphur were almost totally lost, and thisloss is estimated at 2.2 wt.% and 0.3 wt.%, respectively.The flux of volatiles emitted from the magma can becalculated by multiplying these quantities by the supplyrate. For the suggested range in MSR, the flux variesbetween 2–8×103 t/d and 0.3–1.1×103 t/d, respectively forH2O and SO3. These values are comparable with thosemeasured at Tungurahua volcano during the 1999–2006 ashemission activity (Arellano et al. 2008), but are lower than

those measured at Etna during the ash emission activity of2002–2003 (Andronico et al. 2005). For comparison, emis-sion rates of SO2 from Erebus volcano average 0.6×102 t/dduring periods of passive degassing (Sweeney et al. 2008).

A model for magma fragmentation

A possible model of magma ascent and fragmentation canbe proposed based on the different data discussed above(Fig. 7). Continuous magma degassing is needed to explainthe prolonged emission of ash. The presence ofinterconnected chains of vesicles recorded in the juvenilefragments suggests that the magma, at least in the upperpart of the conduit, behaved as a permeable liquid, allowingdegassing and groundmass crystallization. The abundance

Fig. 7 A possible model for magma ascent and fragmentation basedon external shape and bubble textures of the different types of ashfragments, CSD of microphenocrysts and microlites of plagioclase andleucite and bulk rock and groundmass composition. a The differenttypes of clasts, spongy, fluidal and blocky, represent different parts ofthe magma in the conduit (see text for explanation). b Theinterpretational illustration of CSDs is based on the results in Fig. 4.For plagioclase (25–29 μm) and leucite (30–33 μm) microlites, the

single population pattern shown with a red straight line is createdunder constant conditions throughout different eruption stages.Coarser microlites of leucite grown in the deeper level show anotherpopulation pattern represented by a green straight line. c Thecompositional areas of the glasses (residual melts) in equilibriumwith leucite and diopside in the deeper level (100 MPa) and theshallow level (0.1 MPa) are shown in green and red areas,respectively, onthe leucite-diopside-silica normative plots

712 Bull Volcanol (2011) 73:699–715


of clasts with fluidal external surfaces and highlyirregular outlines throughout the entire eruption suggeststhat the fragmentation of the low-viscosity melt waspossibly inertially driven by a sustained gas outflow,rather than being related to the brittle fragmentation of anoverpressured foam (Houghton and Gonnermann 2008;Namiki and Manga 2008). A vesicularity of 70 vol.% isneeded to reach a permeability large enough to allow asignificant free gas flux through low viscosity magmas,though this value significantly decreases in the presence ofvesicle deformation (Namiki and Manga 2008). Theabundant spongy and fluidal clasts are characterized byabout 40 and 70 vol.%, respectively, of mainly elongatevesicles. As a consequence, a possible mechanism for ashformation could be the tearing apart of low-viscosity, low-surface tension magma, by a continuous or semi-continuous flux of exsolving gas through a permeablezone developed at the top of the magma column. Thesmall amount of blocky, dense clasts could be related tothe passive, brittle fragmentation of the more rigidmarginal parts of the magma column. A similar processof turbulent gas outflow through an open network ofvesicles/fractures was first proposed by Yoshida andKoyaguchi (1999) as a possible cause of magma fragmen-tation in silicic magmas.

Namiki and Manga (2008) recently studied the con-ditions which promote permeable degassing or fragmenta-tion in low viscosity magmas. A main result of their studywas that the ability of magma to fragment is related to theReynolds number (Re) of the expanding magma in theconduit. For Re lower than 1 outgassing dominates. Forhigher Re, gas expansion drives fragmentation. In this case,for low gas velocities (10−2–10 ms−1), the formation of abuoyant, ash-charged plume is favoured instead of asustained fountaining. We suggest that either of theseconditions could also occur during moderate-intensityeruption of low-viscosity magmas, triggering and sustain-ing continuous ash emission activity.

Conclusions

Stratigraphic, petrographic, compositional and texturalinvestigations carried out on products of the ash-dominated, AS1a eruption allow us to define the probableprocesses which controlled the activity. The eruption wascharacterized by the emplacement of several thinly lami-nated deposits composed of abundant fine ash and contain-ing small amounts of coarse grained material. The almosttotal absence of lithics and accretionary lapilli together withthe micro-scale features of analyzed clasts argue againstsignificant magma-water interaction during the eruption.Data on the crystal size distribution suggest that the magma

was supplied to the surface under low-ascent rate con-ditions. During magma ascent, groundmass crystallizationwas controlled by degassing and cooling. These processeswere possibly simultaneously active during the AS1aeruption, with cooling becoming progressively more im-portant toward the final stages of the eruption, during whichjuvenile fragments, characterized by a larger volume ofmicrolite and a lower number density, formed. Vesicleshape and content, and ash morphology are suggestive ofmagma fragmentation driven by a continuous gas fluxthrough the top of the magma column. The gas velocitiesinferred are consistent with an ash-rich eruption plume andsubsequent ash fallout at low sedimentation rates over theslopes of the volcano.

An in-depth knowledge of the processes driving this type ofactivity represents a necessary step for the definition of theeruption scenario and for a correct hazard assessment of ash-dominated eruptions, which repeatedly occurred in the past4,000 years of activity at Vesuvius.We propose that this modelmay be extended to moderate-intensity eruptions dominatedby ash-emission activity at Vesuvius and elsewhere.

Acknowledgements This work was partially funded by the Italian“Dipartimento della Protezione Civile” in the frame of the 2004–2006agreement with Istituto Nazionale di Geofisica e Vulcanologia—INGV, project V3_4/11 “V3_4 - Vesuvio”, to R Cioni. The work of CD’Oriano was founded by MIUR project: Advancing InterdisciplinaryResearch Platform on Volcanoes and Earthquakes (AIRPLANE).Helpful comments were provided by S Nakada, JDL White, AVolentik and an anonymous reviewer and they are gratefullyacknowledged. We are grateful to P Pantani for graphical assistanceand A Cavallo for technical assistance in EMP analysis.

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