RESEARCH ARTICLE

Paroxysmal activity at Stromboli: lessons from the past

Antonella Bertagnini & Alessio Di Roberto &

Massimo Pompilio

Received: 22 June 2010 /Accepted: 27 February 2011# Springer-Verlag 2011

Abstract The persistent normal activity of Stromboli isoccasionally interrupted by sudden and highly energeticexplosive events called Strombolian paroxysms. Thesephenomena together with landslide-generated tsunamisrepresent the most hazardous manifestations of present-day volcanic activity at Stromboli. The most recentparoxysms, on 5 April 2003 and 15 March 2007, havedrawn attention to these energetic events because theysignificantly threatened inhabitants and tourists. Historicalaccounts and field evidence indicate, however, that evenlarger paroxysms, in terms of volume, dispersal of productsand intensity of explosive phenomena, occurred in therecent past. During these paroxysms incipiently weldedspatter deposits mantled the north and south rims of theSciara del Fuoco down to low elevations, extending muchfarther than the similar deposits from recent observedevents (5 April 2003 and 15 March 2007). In order toidentify, characterize and discriminate among products ofthese outstanding spatter-forming eruptions, more than 50stratigraphic sections were measured and sampled. Strati-graphic, sedimentological and radiometric (14C) dataindicate that only two paroxysms produced spatter thatreached very low elevations and inhabited areas: the firstoccurred in the 16th century and the last in AD 1930.Analysis of texture and deposit components reveals that theearly phases of the two eruptions were driven by distinctlydifferent eruptive dynamics. Both identified paroxysms areat least one order of magnitude greater than any similar

event observed by monitoring systems at Stromboli. Thesetwo large paroxysms were the most powerful volcanicevents at Stromboli in the last eighteen centuries.

Keywords Stromboli . Paroxysms . Spatter . Basalticexplosive volcanism . Volcanic hazard

Introduction

Stromboli is the northernmost island of the Aeolianarchipelago and represents only the uppermost portion(924 ma.s.l.) of a much larger edifice extending to a depthof 1,500–2,600 m below sea level (Di Roberto et al. 2008).

The volcano (Fig. 1) is famous for its persistent activitythat began in the present form between the 3rd and 7thcenturies AD (Rosi et al. 2000). Persistent “normal” activityconsists of rhythmic, short-lived, mildly energetic explo-sions that eject ash- to bomb-sized fragments to heights of afew hundred metres above the craters. Sporadic lava flowsoccur either from the summit craters or from eruptivefractures within the Sciara del Fuoco (SdF), a horseshoe-shaped structure on the NW side of the volcano thatdeveloped through a series of slope failures (Tibaldi 2001).

The persistent normal activity of Stromboli is occasion-ally interrupted by sudden and highly energetic explosiveevents called Strombolian paroxysms (Mercalli 1881).

Two paroxysms that occurred at Stromboli, on 5 April2003 and 15 March 2007, drew attention to this kind oferuption. Together with landslide-generated tsunamis, suchparoxysms represent the most hazardous manifestations ofpresent-day volcanic activity at Stromboli.

Paroxysms larger than those of 2003 and 2007 in termsof volume, dispersal of products and intensity of explosivephenomena have occurred in the 20th century, the most

Editorial responsibility: H. Delgado Granados

A. Bertagnini :A. Di Roberto (*) :M. PompilioIstituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa,Via della Faggiola,32-56126 Pisa, Italye-mail: diroberto@pi.ingv.it

Bull VolcanolDOI 10.1007/s00445-011-0470-3

famous being the eruption of 11 September 1930 describedin detail by Rittmann 1931.

Paroxysmal activity consists of sequences of explosionsfrom different craters lasting from a few minutes to days oreven weeks, as reported in November 1882 and April 1907(Bertagnini et al. 2008 and references therein). Cannon-shot-like detonations and window-breaking pressure waves areaccompanied by the formation of kilometre-high convectivecolumns of gas, and incandescent materials, generatingfallout of decimetre-sized bombs, lapilli and ash onto thevolcano slopes. During the largest paroxysms metre-sizedballistic lithic blocks and spatter are ejected onto the flanksof the volcano down to very low elevations, occasionallyreaching the villages of Stromboli and Ginostra (Barberi etal. 1993; Calvari et al. 2006; Rosi et al. 2006; Bertagnini etal. 2008).

In historic times, paroxysms have had a large impact oninhabited areas (the dramatic 11 September 1930 eruptionconvinced most of the 5,000 inhabitants to abandon theisland), and a good, almost continuous record of theseevents exists at least since the end of the 19th century.Information on the timing and type of eruptive events,associated phenomena (e.g. landslides, tsunamis) anddamage to inhabited areas were well described by eyewit-ness accounts and reported in scientific papers.

Despite this general knowledge, relevant volcanologicalparameters useful for understanding eruption dynamics and

assessing volcanic hazard (e.g. nature and dispersal oferuptive products, magnitude and intensity) cannot beeasily inferred from these reports. A notable exception isthe paper by Rittmann (1931) on the 1930 paroxysms,which contains a map of the deposits and reports the natureof eruptive products and morphological changes in thecrater and SdF areas based on fieldwork carried out shortlyafter the eruption. Petrographic data and analyses of the1930 samples collected by Rittmann are given in Hornig-Kjarsgaard et al. (1993).

In recent decades the volcanological community hasmade great efforts to study the products of large paroxysmswith the aim of defining the recurrence rate, magnitude andintensity of these catastrophic events.

Paleomagnetic investigations have shown that theseevents are not uniformly distributed on the curve ofsecular- and paleosecular variation of the magnetic field,but that they cluster into at least two groups: between 1400and 1600 AD and between 1900 and the present day(Speranza et al. 2004; Arrighi et al. 2004). These importantdata can be fully employed for stratigraphic correlations atthe volcano scale only when they are supported by moredetailed volcanostratigraphic investigations.

In this framework, we undertook a tephrostratigraphicstudy of the spatter deposits exposed on the middle-lowerflanks of the volcano in order to identify the mostsignificant eruptive events represented in deposit recording

STR78

STR06STR73

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La Petrazza

P.Lena

P.Lena

P.dell’Omo

Stromboli

Ginostra

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S.Bartolo

S.Vincenzo

Fig. 1 Aerial photograph ofStromboli volcano with the mainplace names. Solid circles showthe localities of stratigraphic sec-tions used for correlation

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the recent history of Stromboli. We identified key featuresthat allow both discrimination between deposit of distinctspatter-forming eruptions, and their stratigraphic correla-tions at the volcano scale.

By comparing these data with those from volcanologicalinvestigations carried out during recent paroxysms and ontheir deposits, it was possible to assess the size of pastcatastrophic paroxysmal eruptions and to make inferencesregarding their eruptive processes and dynamics.

Results clearly indicate that within the context of recentvolcanic activity at Stromboli, two major paroxysmal events,able to emplace spatter deposits at lower elevations and closeto inhabited areas, occurred in the last five centuries.

Present-day Stromboli magmas

The present-day activity of Stromboli is dominated by twobasaltic magmas differing chiefly in crystal and volatilecontents, and whose characteristics have remained con-stant since the onset of the present activity. NormalStrombolian activity and effusive events are fed by aHigh Porphyricity magma (HP) that resides in the upperportion of the feeding system (shallower that ~2 km;Francalanci et al. 1999, 2004 Métrich et al. 2001; Landi etal. 2004, 2006, 2009). The HP magma is a partially degassed(H2O <0.6 wt.%; CO2 <100 ppm; dl > S <1,300 ppm; Cl=1000–2006) potassium-rich (HK) shoshonitic basalt (SiO2=48.5–51.5 wt.%; K2O=1.9–2.5 wt.%; CaO/Al2O3=0.59–0.62) containing 45–55 vol.% euhedral crystals of plagio-clase (0.1–2.5 mm), clinopyroxene (0.5–5 mm) and olivine(0.1–4 mm) (Métrich et al. 2001, 2010; Bertagnini et al.2008).

Paroxysmal activity is typified by the emission of LowPorphyricity (LP), highly vesicular pumice associated withHP scoria. Many juvenile products are the result of intense,syneruptive mingling between LP and HP magma. The LPmagma is a volatile-rich (H2O=1.8–3.4 wt.%; CO2=890–1,890 ppm; S=1,660–2,250 ppm; Cl=1,660–2,030) HKbasalt (SiO2=48–49.5 wt.%; K2O=1.6–2.2 wt.%; CaO/Al2O3=0.68–0.81), containing <5 vol.% of <1 mm diopsideand homogeneous skeletal or euhedral olivine (Bertagnini etal. 2008; Pompilio et al. 2010; Métrich et al. 2010) andoriginating at a depth of 7–10 km.

According to Métrich et al. (2001), the HP magmaderives from the LP magma via crystallization mainlydriven by decompression and volatile loss at low pressure.

Rationale

The study of spatter deposits raises some general issuesderiving from their mechanism of emplacement. When

bombs are emplaced onto in a pre-existing spatter bombfield, as in the case of Stromboli, ejecta deriving fromdifferent eruptions could conceivably be found juxtaposedand not strictly and stratigraphically superposed, hamperingthe correlation of deposits.

At Stromboli, neither the mineral assemblage nor thechemical composition of the deposits can be used as adiagnostic tool to discriminate between products emittedduring different paroxysms since compositional variationsare, on the whole, rather small (Bertagnini et al. 2008).

A possible solution is the use of stratigraphic evidence toattribute each spatter outcrop to a specific eruptive episode,i.e. to find distinctive and recurring stratigraphic elementsin the spatter deposits that can help in attributing them todifferent eruptive events.

At Stromboli suitable stratigraphic evidence could be theassociation between fallout deposits erupted during the firststages of each paroxysm and the overlying spatter.

The characteristics of fall deposits can, in principle,provide information on possible triggers and the dynamicsof the eruption. They could possibly be used to identifydifferent paroxysms and then for stratigraphic correlationsat the volcano scale.

To this end, 80 stratigraphic sections were measured andsampled on natural exposures and in trenches dug on thevolcano flanks, yielding 42 fall deposit samples; specialattention was paid to those locations where historicalrecords report the deposition of spatter bombs and to thoseoutcrops where paleomagnetic sampling was carried out(Arrighi et al. 2004 and Speranza et al. 2004).

Further laboratory investigation of these samples includ-ed grain size and component analysis and scanning electronmicroscope (SEM) morphological and textural particlecharacterization.

Methods

Samples were wet-sieved at 1Φ (phi) intervals from −4 to 5Φ (16–0.032 mm). For each sample, the Median diameter(MdΦ), sorting coefficient (σΦ) and sedimentary coeffi-cients were calculated according to Inman (1952).

Component analyses were performed on −5 to 1 Φ (32–0.5 mm) grain size fractions.

About 2,000 fragments from the −5 to −1 Φ (32–2 mm)fraction of each sample weremacroscopically counted. The 0–1 Φ (2–0.5 mm) grain size fractions were mounted on thinsections and counted with a Swift point counting device; forgrain sizes between 0 and 1 Φ, a minimum of 500 fragments(up to >1,000) were counted on each thin section.

The sum of fractions analyzed for component analysisrepresents 42–96 wt.% of the whole sample, with a meanvalue of 76 wt.%.

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The nature, texture and morphological properties ofclasts forming samples were analyzed using a SEM-EDSPhilips XL30 scanning electron microscope (acceleratingvoltage 20 kV, beam current 1 nA, working distance10 mm) equipped with an energy dispersive X-ray analyzerat the Earth Science Department of Pisa University and thatof Siena University.

Results

Areal distribution and general stratigraphy

Spatter deposits occur on both sides of the SdF depressionwithin two spatter bomb fields, each a few hundred metreswide. Outcrops of these deposits are visible along the twofootpaths leading to the summit of the volcano along theSdF flanks (Figs. 1 and 2). The areal density of spatterdecreases with distance from the SdF rims, so that only afew isolated bombs occur off SdF at a distance of 300–400 m from the rims. The lowest-altitude deposits (from~150 m and ~190 ma.s.l. respectively on the northeast flankand the southwest flank of the SdF) comprise scatteredbombs (Fig. 2a). Moving upslope the spatter depositsbecome more continuous and above 400 ma.s.l., at thesummit of the volcano, they crop out as a decimetre-thick,nearly continuous accumulation (Fig. 2b, c).

In the summit area the spatter deposits occur as stacked,almost indistinguishable sequences representing the proxi-mal facies of several paroxysmal eruptions (Fig. 2d, e).Thick spatter deposits mantle the inner walls of the craterdepression, just below Pizzo Sopra la Fossa and from theBastimento saddle to La Fossetta depression (Fig. 2d, e).

Spatter-deposit fragments cover a wide spectrum ofmorphologies. Single, asymmetric cowpat bombs rangingfrom tens of centimetres up to ~1.5 m in diameter are themost common type, together with decimetre- to metre-sizedfolded and ribbon ejecta (Fig. 2).

Above 400 ma.s.l., single bombs are often stacked toform agglutinates several square metres wide (Fig. 2b) and>0.3 m thick; in rare cases bombs have coalesced, formingalmost continuous rheomorphic beds in which the originalparticle boundaries are almost obliterated (Fig. 2b).

Lithic blocks are often included in the spatter deposits;lithic clasts are subangular to subrounded in shape, andrange from holocrystalline lava fragments to spongy,hydrothermally altered volcanics.

Spatter deposits always lie conformably above the pre-existing ground slopes, showing very limited or no traces ofremobilization.

In dug trenches or in natural exposures, spatter layersusually cover a yellowish, massive, epiclastic ash depositranging from 20–30 cm to ~2 m in thickness. At least two

recognisable fallout layers, along with discontinuous align-ments of centimetric light-coloured pumice and blackscoriae, are embedded in this epiclastic deposit (Fig. 3).

The uppermost fallout bed (<20 cm below the base ofthe spatter sequences) is a few cm thick, dark grey toreddish finely laminated ash (Fig. 3).

The lowermost ash bed (about 1 m below the base of thespatter sequences) is thin and discontinuous fall deposit offine ash of rhyolitic composition. The best exposures of thishorizon occur along the old pathway to the volcano summitat about 350 ma.s.l. (WGS84 38°48.247′ N, 15°12.883′ E).Based on the major element composition, this horizon maybe ascribed to the last eruptions of Lipari (Mt. Pilato), andhas yielded a calibrated 14C age of AD 776 (+110/−90,Keller 2002). This finding confirms the age estimated byRosi et al. (2000) for the onset of Stromboli’s Present dayactivity but rules out the occurrence of a spatter-formingparoxysm at AD 550 as proposed by Arrighi et al (2004).

Epiclastic ash overlies a discrete scoria-fall deposit thatmay be ascribed to the Lower Sequence activity occurringbetween the 4th century BC and the 1th century AD (Rosi etal. 2000; Speranza et al. 2008) and the phreatomagmatic ash-fall deposits of Secche di Lazzaro (14C age of ~6000 yearsBP; Speranza et al. 2008) marking the final phase ofNeostromboli eruptive period (Bertagnini and Landi 1996).In some areas spatter piled up directly on Neostromboli lavas(Hornig-Kjarsgaard et al. 1993).

The above succession is summarised in the idealizedstratigraphic section of Fig. 3.

Spatter sequence stratigraphy: SSA and SSB

Two distinct spatter sequences can be identified bycombining field observations and the results of stratigraphicand component analyses (see below). The two sequences,respectively named Spatter Sequence A (SSA) and SpatterSequence B (SSB), have been recognized at several sites onthe flanks of the volcano between ~700 m and 150 ma.s.l.(Figs. 2 and 3). The sequences show a similar architecturecomprising a basal ash- to lapilli-sized fall deposit overlainby the spatter deposits.

Spatter Sequence A consists of up to 2×1.5 m-sized,single to agglutinated, nearly aphyric LP bombs (ribbon toconvoluted) with a pumiceous and microvesicular olive-green crust often showing a bread-crust texture. The glossy,dark interiors of bombs are characterized by the presence oflarge coalescent bubbles. Spatter bombs often pass laterallyinto alignments of pumice clasts having a centimetric todecimetric diameter. The basal fall deposit (Fall A) rangingfrom 2 cm to 20 cm in thickness, consist of fresh light-coloured fibrous to equant LP pumice, glassy black HPscoriae, and variable amounts of lithic fragments. Grain-size analyses reveal that samples range from coarse ash to

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a

b

dc

e f

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SdF

ab

d

e

f

c

Touristic footpath1 Km

N

Pizzo Soprala Fossa

Ginostra

Fig. 2 Spatter deposits croppingout on the southwest and north-east rims of SdF; a metre-sizedcowpat bombs that landed on adry-stone wall (SW ~150 ma.s.l.); b decimetre-thick, intenselyagglutinated spatter deposit (SW~380 ma.s.l.); c stratigraphicsection where the two SSA andSSB sequences crop out separat-ed by an epiclastic bed a fewcentimetres thick (SW ~405 ma.s.l.); d decimetre-thick spatterdeposits mantling the Bastimento(SW ~630 ma.s.l.) and e LaFossetta areas (NE ~700 ma.s.l.); fspatter accumulation (NE ~180 ma.s.l.); note the holes left by thepaleomagnetic drilling ofSperanza et al. (2004)

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medium lapilli (MdΦ=0.8/−2.5; White and Houghton2006) and that they are well to poorly sorted (σΦ=1.5–2.9; Table 1).

Spatter Sequence A is always found exposed along thesurface, and in at least two cases it accumulated on andpartially destroyed the dry-stone walls used to underpin anddelimit the small terraced plots of land typical of Aeolianagriculture (Fig. 2).

Spatter Sequence B is characterized by up to metre-sizeddark grey to black LP lava bombs with a 2–3 cm-thickglassy, microvesicular crust and a highly vesicular, sponge-like interior. In some outcrops the deposit is incipientlywelded (Sumner et al. 2005).

The basal fall deposit (Fall B) ranges from 2 to 10 cm inthickness and is mainly composed of medium ash to finelapilli-sized (MdΦ=1.8/−1.7; White and Houghton 2006),well to poorly sorted (σΦ=1.4–2.8; Table 1), black,lustrous HP scoria and subordinate lithic clasts and lightcoloured LP pumice. Black and flattened HP scoriaceousbombs ranging from a few centimetres to some decimetresin diameter are widespread within the basal layer. Charcoalsand burnt vegetation are commonly found at the base ofboth sequences.

In some outcrops SSA and SSB are exposed instratigraphic succession, with SSA always above SSB.They are usually separated by a few centimetres- todecimetres-thick bed of sometimes humified, epiclastic ash.

Component analysis and textural characterization

The following three principal components were identifiedin the samples: a) HP dark scoria (9–85 vol.%, Fig. 4a–c),b) LP pumice (3–73 vol.%; Fig. 4d–f and c) lithic clasts (4–39 vol.%; Fig. 5a–f).

HP lithotype fragments are mainly represented byvesicle-poor, pristine and fibrous achneliths. Crystals ofplagioclase and pyroxenes surrounded by fluidal glass arethe most common types, along with glass droplets andPele’s hair fragments (Fig. 4a, b). Vesicle-poor scoriafragments with blocky shapes are also abundant (Fig. 4c).

LP lithotype fragments are mainly represented by highlyvesicular pumices and display markedly different types ofvesicularity and shapes: a) blocky fragments with curvipla-nar external surfaces (large bubble septa) and smallspherical, non-coalescent bubbles; b) spongy fragmentswith amoeboid and coalescent vesicles, c) fibrous clastswith elongate vesicles (Fig. 4a, b).

Loose crystals of clinopyroxene and plagioclase arecommon. The presence of a thin film of glass showing thecomposition of HP glass indicates that they derive from theHP magma.

Fall A and B clearly differ in the ratio between LP andHP components (Fig. 6).

In Fall A, LP lithotype ranges from 36 vol.% to 85 vol.%, (LP/HP ratio: 0.6–6). Clasts resulting from the mingling

0

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Mt.Pilato C age ofAD 776 (+110/−90, Keller, 2002)14

SSA

SSB

Epiclastic deposit

Ash-lapilli fallout

Secche di Lazzaro deposit

Neostromboli lava

Lower sequence deposit

Spatter depositFig. 3 Idealized stratigraphicsection reconstructed fromstratigraphic trenches and natu-ral exposures

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of the HP and LP lithotypes often occur and were classifiedaccording to the abundance of the two lithotypes.

In Fall B, LP pumice ranges from 5 vol.% to 19 vol.%(LP/HP ratio: 0.05–0.2). No mingled clasts occur.

The lithic content ranges from 4 to 20 vol.% and 4 to39 vol.% in fallout A and B respectively (Table 1). In bothsamples, lithic fragments mainly consist of holocrystallinelavas and hydrothermally altered volcanics and altered HPscoriae (Fig. 5a–f).

Another distinguishing feature between fall deposits A andB is the ubiquitous presence of analcime-bearing lavas amongthe lithic clasts of fall deposit A (Fig. 5a–c). These clastscomprise porphyritic lavas (plagioclase + clinopyroxene +olivine) with a holocrystalline groundmass bearing 10–150 μm-sized, spherical crystals of analcime.

The presence of analcime microcrystals in Strombolivolcanic products was already noted (Hornig-Kjarsgaard et

al. 1993; Corazzato et al. 2008) and is exclusive to someNeostromboli lavas containing secondary analcime formedfrom a pre-existing igneous leucite.

Spatter sequences dispersal and volume estimation

Spatter Sequence A crops out on both sides of the SdF. On thenortheast flank SSA extends from an elevation of ~100 m toabout ~525 ma.s.l.; on the southwest flank SSA can bedistinctly traced from ~190 m up to ~680 m a.s.l (Fig. 7).

Spatter Sequence B mainly occurs on the northeast sideof the volcano, where it crops out from about 280 m. Onthe southwest flank of the SdF the sequence crops out atelevation >680 m (Fig. 7). The correlation of SSA and SSBat the volcano scale is shown in Fig. 7.

On the basis of the stratigraphic data and correlation, wetried to define the dispersal area and volume of deposits for

Table 1 GPS position, grainsize paramenters and component abundance of the fall deposit samples recovered from SSA and SSB stratigraphicsections

Sample Section Datum WGS 84 m a.s.l Inmann parameters Components vol.% SSA SSB

Lat. Long. MdΦ σΦ aΦ LP HP Lithic

PST12 3 38°48.051 N 15°12.961 E 520 −0,69 2,84 −0,46 7 81 12 ●PST166 6 38°48.318 N 15°12.853 E 295 −0,81 1,94 −0,40 5 85 10 ●PST167 6 38°48.318 N 15°12.853 E 295 −1,12 2,27 −0,04PST178 COR 38°48.206 N 15°12.877 E 380 −0,90 2,00 −0,45 17 77 6 ●PST179 COR 38°48.206 N 15°12.877 E 380 −0,65 1,39 −0,39PST180 COR 38°48.206 N 15°12.877 E 380 −0,30 2,09 0,05 33 53 14 ●PST181 73 38°48.299 N 15°12.850 E 313 1,35 1,76 0,52 10 50 39 ●PST182 73 38°48.299 N 15°12.850 E 313 −0,41 2,02 0,04

PST183 63 38°48.318 N 15°12.853 E 295 0,43 2,38 0,37

PST195 75 38°48.190 N 15°12.881 E 405 −0,35 1,40 −0,12 4 73 23 ●PST197 75 38°48.190 N 15°12.881 E 405 −1,10 2,23 −0,07 54 38 8 ●PST205 65 38°47.524 N 15°12.133 E 517 −2,02 2,60 −0,43 70 18 12 ●PST208 SPA04a 38°47.623 N 15°11.929 E 353 −0,12 2,75 0,10 51 37 12 ●PST210 73 38°48.299 N 15°12.850 E 313 −1,38 1,88 −0,37 64 32 4 ●PST212 73 38°48.299 N 15°12.850 E 313 −1,12 2,25 0,31 66 25 9 ●PST214 75 38°48.190 N 15°12.881 E 405 −0,50 1,83 −0,37 3 59 37 ●PST215 78 38°48.490 N 15°12.838 E 145 0,01 2,16 0,26 67 26 7 ●PST217 79 38°48.530 N 15°12.820 E 152 0,83 2,69 −0,19 73 13 14 ●PST222 SPA15a 38°47.701 N 15°11.702 E 180 −1,51 1,54 −0,75 33 59 8 ●ST514 62 38°48.005 N 15°12.950 E 546 −1,28 2,67 −0,68 16 77 7 ●ST516 73 38°48.299 N 15°12.850 E 315 −1,10 2,56 −0,31 40 52 7 ●ST529 77 38°48.170 N 15°12.924 E 400 −1,03 2,24 −0,02 33 58 9 ●ST534 6 38°48.318 N 15°12.853 E 295 −0,21 2,87 −0,10 38 50 13 ●ST538 SPA08a 38°48.170 N 15°12.886 E 419 −1,18 1,61 −0,45 12 84 4 ●ST540 SPA08a 38°48.170 N 15°12.886 E 419 −2,51 2,80 −0,35 49 31 20 ●ST542 65 38°47.524 N 15°12.133 E 517 −1,89 2,46 −0,38 40 46 14 ●ST546 66 38°47.466 N 15°12.339 E 681 −0,45 2,26 −0,24 9 70 21 ●

a Sections named as SPA correspond to sections where paleomagnetic sampling was carried out by Speranza et al. (2004)

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the SSA sequence, which shows a more complete set ofpreserved stratigraphic sections on both sides of the SdFand down to very low elevations on the volcano flanks. Theisopach map gives only a rough estimation, howevermainly because of the lack of thickness information insidethe SdF and since it does not take into account the lateraland distal parts of the deposit (thickness <1 cm), whichwere not preserved but likely represented a volumetricallysignificant part of the deposit.

The isopach distribution for lapilli and basal ash falldeposits indicates a dispersal axis oriented to thenorthwest and a volume (calculated according to themethod of Pyle 1989) possibly ranging between 106 and107 m3. This is minimum volume since the tephra possiblydeposited after spatter emplacement cannot be evaluated(Fig. 8).

The volume of the SSA spatter deposit was roughlyestimated assuming a continuous spatter agglutinate with anaverage thickness of 0.5 m for the area encompassing thetwo lobes on the sides of the SdF and extending down to400 ma.s.l. This calculation was based on the considerationthat the spatter above 400 ma.s.l. forms an almost

continuous agglutinate with a thickness >0.3 m, aspreviously mentioned. The resulting minimum spatter-deposit volume was on the order of 1×106 m3.

It was impossible to make a similar estimate of thedispersal area and volume of SSB due to the poorpreservation of sequences. Nonetheless, as its areal extentand average thickness are similar to those of SSA, weestimate a comparable volume of the order of ~106–107 m3.

Dating

To complete the characterization of spatter sequences, threesamples of charcoal included within the basal fall deposits,or just below the base of spatter sequences, were sampled attwo localities for radiocarbon dating. Analyses wereperformed by Beta Analytic Inc. (Miami, Florida, USA)with the standard radiometric method.

One sample was collected at the base of SSA and twosamples right below SSB.

The sample from SSA (marked with asterisk in sectionSPA04 in Fig. 7) was dated to 50±60 years BP,

500 mµ

500 mµ500 mµ

500 mµ

500 mµ

500 mµ

a b

dc

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Fig. 4 SEM backscatter imagesof juvenile fragments formingSSA and SSB fallout deposits. a,b vesicle-poor, pristine and fi-brous achneliths of HP lithotypefragments; c vesicle-poor, blockyHP clast; d highly vesicular LPspinose pumice with amoeboid,elongated and coalescentvesicles; e, f highly vesicular,blocky LP pumice characterizedby spongy vesicularity

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corresponding to ~1900 AD, whereas the two samples fromSSB (marked with asterisks in section STR3 in Fig. 7) wererespectively dated to 240±40 years BP and 250±30 yearsBP, corresponding to AD 1535–1545 (Cal. BP 415 to 405)and AD 1530–1550 (Cal. BP 420 to 400).

Discussion

Stratigraphic and sedimentologic investigations indicatethat two distinct spatter sequences (SSA and SSB) wereemplaced on the mid-lower flanks of the volcano.

0%

20%

40%

60%

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100%

PS

T21

4

PS

T19

5

PS

T16

6

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T12

ST

546

PS

T18

1

PS

T18

7

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538

ST

514

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T17

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534

ST

542

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PS

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5

PS

T20

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T20

8

wt%

HP

Lithic

LP

SSB SSAFig. 6 Fall deposit componentdistribution histogram. Note thesharp threshold between sam-ples containing ≤18% and thosecontaining >36% LP fragments(red dashed line)

500 mµ500 mµ

250 mµ

250 mµ250 mµ

100 mµ

a b

dc

e f

Fig. 5 SEM backscatteredimages of lithic fragmentsforming SSA and SSB falloutdeposits. a–c porphyritic lavafragments with a holocrystallinegroundmass bearing 10–150 μm-sized spherical crystalsof analcime (SSA); d hydro-thermally altered volcanic frag-ment with evident devitrificationat the rim of fragments andalong fractures; e, f holocrystal-line lava fragments

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These deposits may be ascribed to two large paroxysmsrepresenting the two most energetic eruptions in the recentactivity of Stromboli volcano. In the following section wediscuss how these two events fit into the framework of therecent eruptive activity of this volcano.

Dating and attribution

Recent paleomagnetic measurements on spatter deposits atStromboli volcano (Speranza et al. 2004) revealed thatpaleomagnetic directions are not uniformly distributed onthe curve of secular variation and that two distinct groupsof spatter can be identified. The paleomagnetic directions ofone group of spatter deposits are compatible with anemplacement age of AD 1400–1600, whereas those of theother agree with directions observed between 1900 and thepresent day (Speranza et al. 2004) which is quite consistentwith our 50 BP +/−60 yrs date.

Our stratigraphic analysis of spatter sequences, performedon outcrops sampled during paleomagnetic investigations,

reveals that the lowermost SSB includes only samples withpaleomagnetic directions compatible with a 16th century age.This result agrees with 14C dating and suggests that thelowermost eruption occurred about 400 years ago. Unfortu-nately, the lack of reliable historical accounts for this periodprevents the attribution of this eruption to a precise date. Ourfindings are not in agreement with the results of Arrighi et al.(2004) that argue for a large paroxysm in AD 550 on thebasis of archeomagnetic measurements. All spatter, but one(SPA15) deposits belonging to SSA yielded paleomagneticdirections compatible with the 20th century, in agreementwith radiometric dating.

Several large paroxysms have occurred on the island ofStromboli over the last century (for example 1906, 1916,1919, 1930, 1944; Barberi et al. 1993). These were reportedto have produced ash to lapilli fallout on the inhabited areasand, in some cases, spatter deposits beyond the summitarea.

Historical accounts and some scientific papers (Ponte1916, 1919, 1921, 1948; Riccò 1907, 1917; Rittmann

Fig. 7 Stratigraphic sections studied for the identification of SSA andSSB spatter sequences; the inset shows the relative position of eachsection. Asterisks mark the samples on which 14C age determinations

were performed by Beta Analytic Inc. (Miami, Florida, USA) with thestandard radiometric method

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1931) report that only the 22 May 1919 and the 11September 1930 paroxysms threw metre-sized spatterbombs down to the lowest elevations (~100 ma.s.l.) andinto inhabited areas.

During the 1919 eruption a few large bombs hit housesin the villages of Stromboli and Ginostra, whereas the finer-grained products were dispersed by strong winds to the SW,reaching the Sicilian coast (Ponte 1919). This dispersal isnot compatible with the northward dispersal observed inSSA.

Several lines of evidence suggest that SSAwas producedduring the 11 September 1930 paroxysm. The products ofthis event were mainly dispersed northward, as reported byRittmann (1931), who describes a 10 min-long, heavy rainof metre-sized spatter bombs, decimetre-sized scoria, lapilliand ash on the north-eastern slope of the volcano during thepeak phase of the eruption. Moreover, the nature, morphol-ogy and texture of the deposits forming SSA match those ofvolcanic products described by Rittmann (1931) for the 11September 1930 paroxysm.

Comparison with present-day paroxysms

As mentioned earlier, the study of spatter sequences atStromboli volcano raises some general issues regardingtheir mechanism of emplacement and the possible post-emplacement reworking or erosion.

A major issue concerning the two identified spattersequences is their general lack of lateral continuity, which

hinders the detailed and complete mapping of the depositsand the calculation of their volume. This is particularly truein the case of lapilli and ash fall deposits emplaced duringthe initial phases of the eruptions: these were preservedonly where associated with an overlying spatter bomb layer.

Bearing in mind these limitations, we compared ourresults with information on eruptions for which aneyewitness account exists and geological and geophysicaldata are available in order to make some inferences on therelative size and eruption mechanisms driving the studiedparoxysms.

Size of the eruptions

The first large paroxysm observed by a multidisciplinarymonitoring system, at Stromboli volcano, was the spectac-ular eruption of 5 April 2003. The results of this monitoringeffort and a complete characterization of the eruption werereported by Calvari et al. (2008).

The paroxysm of 5 April 2003, the most violent of the past50 years, consisted of several explosions and had a totalduration of about 373 s. During the main phase lasting ~38 s,multiple jets, comprising leading blocks with ashy contrailsfed an up to 4 km-high convective column. Metre-sizedballistic blocks were launched up to 1,400 m above the cratersand fell on the volcano flanks and on the village of Ginostra, atabout 2 km from the vent. The areal distribution was stronglyasymmetrical, in two narrow sectors oriented to NE andWSWrespectively, as was also the case in the large paroxysm of 11September 1930 (Rosi et al. 2006).

The fallout of coarse material (variably mingled LPpumice and lithic fragments) was mainly concentrated inthe summit portion of the edifice, whereas the ash- tolapilli-sized fraction drifted southwards with the windand was widely dispersed SSW in a narrow band (~20°)down to the sea (Rosi et al. 2006). Field surveys carriedout shortly after the eruption revealed that moderatelysorted and incipiently welded spatter spread across thesummit area, reaching a thickness of 1 m about 350 mfrom the vent (Rosi et al. 2006). Only a few scatteredspatter bombs were observed more than 400 m from thecraters (Pistolesi personal communication 2010) in thePizzo area.

From the isomass map a total erupted mass of 1.1–1.4×108 kg was calculated, corresponding to a volume of 2.2–2.4×105 m3 using a density of 500 kg/m3 (Pistolesi et al.2008). The average mass discharge rate was calculated in2.8–3.6×106 kg/s, with a possible peak at 1.0–1.2×107 kg/sin the 10 s-long initial gas thrust.

As pointed out by Rosi et al. (2006) and Pistolesi et al.(2008), emitted products, the impulsive nature of the event,and its eruption dynamics make the 5 April 2003 eruption akey case of Stromboli paroxysms.

1 Km

20 cm10 cm

4 cm2 cm

N

SdF

Vancori

Craters

P.Labronzo

P.Lena

Stromboli

Ginostra

10

20

3

22

4

23

4

Fig. 8 Maps showing pattern of isopachs for the SSA fall deposit.Numbers on spots represent measured thickness (cm)

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The comparison between deposits of 5 April 2003 andSSA (1930) and SSB (16th century) suggest that thesesequences were emplaced by eruptions driven by similardynamics. The dispersal patterns and thicknesses of bothSSA and SSB fall and spatter deposits are, however, verydifferent from those of the 5 April 2003 paroxysm.

Spatter deposits span a sector as large as 90°, extendingalmost continuously between the two rims of the SdF to adistance >1.5 km from the crater area and down to 150 m a.s.l, much farther than the 5 April spatter deposit, which wasrestricted to the crater area. In addition, the minimumvolume estimated for the SSA deposit (and possibly forSSB) is on the order of 106 m3, at least one order ofmagnitude larger than that of 5 April 2003. This suggeststhat SSA and SSB likely represent the most powerfulexamples known among this class of events.

Eruption mechanisms

A general model for the dynamics of paroxysmal explo-sions at Stromboli is not yet available and remains a matterof debate. However, it is now widely recognized that LPmagma plays an important role in triggering paroxysmalactivity.

The occurrence of paroxysms is generally considered tobe linked to the rapid ascent, decompression and fragmen-tation of a volatile-rich and almost aphyric LP magmaresiding at depth in the plumbing system (Bertagnini et al.2008 and references therein). Water and CO2 contents inolivine-hosted melt inclusions from paroxysms of differentscales (including the last April 5 2003 and March 15 2007paroxysms) indicate that LP magma emitted as pumiceduring paroxysms rises in equilibrium with its volatiles,under prevalently closed-system conditions from the stor-age zone lying at ~7–10 km depth (Métrich et al. 2010).During the last stages of their ascent, LP magmas interactand mingle with the almost degassed and crystal-rich HPmagma stored in the shallow part of the system.

Allard (2009) emphasizes the role of volatiles, proposinga gas-triggering mechanism. Paroxysms would be driven bythe fast ascent of CO2-rich gas pockets generated by bubblefoam growth and collapse occurring at deep levels in theplumbing system.

Whatever the general mechanism driving the paroxysmmay be, distinctly different eruptive dynamics took place inthe early phases of the two eruptions forming SSA andSSB. These distinct dynamics controlled the relativeabundance of HP vs. LP lithotypes and the differentlithic-clast populations within the ash and lapilli falldeposits of the studied paroxysms.

The relative abundance of HP vs. LP lithotypes cansimply be ascribed to initial differences in the volume of LPand HP magmas feeding the eruptions.

Alternatively, one could invoke different couplingmechanisms between volatiles and melts during pre-eruptive ascent. During the 1930 eruption (SSA), a LPmagma batch may have risen strictly coupled with its ownvolatiles, behaving as a “closed system” for most of itsascent (Cashman 2004), therefore following the dynamicsproposed for other paroxysms, including the 5 April 2003and 15 March 2007. Such behaviour is also supported bytextures of the juvenile fragments in the SSA falloutdeposit, which indicate that they formed by brittle frag-mentation of overpressured foam after a highly vesicular LPmagma batch rose through and mingled with the HPmagma, immediately before its explosive fragmentation(Fig. 9).

In contrast, in the 16th century paroxysm the rapidascent of a large volume of volatiles predated the arrivalof a slower-rising LP magma batch. At very shallowlevels the powerful gas outflow could have interactedwith HP magmas residing in the shallow part of thefeeding system, fragmenting it through the “inertialfragmentation” mechanism suggested by Houghton andGonnermann (2008 and references therein). We concludethat the onset of the 16th century paroxysm wasdominated by fire fountain activity that sprayed lava dropsonto the slopes of the volcano. Consequently, the SSB falldeposit representing this phase of the eruption mainlyconsists of fluidal fragments of HP magmas, with onlyminor LP fragments (Fig. 9).

The similar lithic-clast content in SSA and SSB falldeposits (SSA=4–20%; SSB=4–39%) revealed by compo-nent analysis implies that significant syneruptive craterexcavation occurred in the initial phases of the twoparoxysms. However, the different nature of lithic clastsin SSA and SSB fallout suggests the involvement ofdifferent portions of the upper conduit during the craterexcavation.

The presence of leucite-bearing lavas typical of Neo-stromboli volcanics among the lithic clasts of SSAfallout implies that, during that paroxysm, the craterwalls were disrupted and Neostromboli products wereintercepted; according to the geological map of Keller etal. (1993), these products are located just below the activecrater at a depth of less than 100 m. Significant crater-enlarging phenomena possibly involving Neostrombolileucite-bearing lavas have been reported for the 1930eruption (Rittmann 1931): an elongated explosion crater,160×90 m, with a maximum depth of 50 m (totalexcavation ~120 m) formed in the eastern portion of thecrater terrace, lowering the ground surface on average by70 m.

The absence of leucite-bearing lavas in the 16th centuryparoxysm (SSB) is probably linked to a shallower process,since the Neostromboli volcanics were not excavated and

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ejected. Alternatively, the 16th century paroxysms mayhave occurred after a period of quiescence or low levels ofsummit activity, which favoured intra-crater failure anddebris accumulation on the crater floor; in this case lithicclasts would have been derived from the remobilization andfragmentation of materials filling the crater before the onsetof the eruption and produced by non leucite-bearing RecentStromboli activity.

Concluding remarks

The integration of new detailed tephrostratigraphic findingswith available data, including historical accounts and present-day observations, led us to the following conclusions:

– only two distinct spatter sequences (SSA and SSB) canbe identified in the mid-lower Stromboli volcano flanksdown to ~150 ma.s.l.;

– paleomagnetic ages, 14C dating and stratigraphic con-siderations attribute these deposits to two paroxysms,

one occurring in the 16th century (SSB) and the otherin 1930 (SSA);

– although the two paroxysms are similar in terms ofdeposit thickness, dispersal and magnitude, we infer thatdistinctly different eruptive dynamics drove the earlyphases of the two eruptions, as revealed by the compo-nents of the deposits and the texture of fragments;

– judging from the dispersal of deposits and theirestimated volume, both studied paroxysms were atleast one order of magnitude larger than any paroxysmobserved by monitoring systems at Stromboli (e.g.April 5 2003).

– these large paroxysms represent the most powerfulvolcanic events in the context of Recent Stromboliactivity. Recurrence of such an event would pose aserious threat to people living on and visiting theisland, also considering that the area affected by spatterdeposition (SdF rims) comprises the old foot trailsleading to the volcano summit that are still popularamong hikers wanting to observe volcanic activity fromlower elevations.

HP

LP a

scen

ding

LP in

tera

ctin

gw

ith H

P (

min

glin

g)

LP+

HP

frag

men

tatio

n

<<deep>>craterization

LP a

scen

ding

HP

LP-derivedgas slug

HP

“in

ertia

lfr

agm

enta

tion”

HP+LP

HP

HP

HP

LP

Recent Stromboli

Lc-bearing Neostromboli

1930 16th century

LP fr

agm

enta

tion

LP

<<shallow>>craterization

Fig. 9 Schematic representation of processes driving the early phases of the two 1930 and 16th century eruptions

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Acknowledgements We thank J. Keller and an anonymous reviewerfor their positive and constructive comments. JDL White is alsoacknowledged for the final manuscript revision. G Giorgetti isacknowledged for helping in the SEM images acquisition. This workwas supported by V2—Monitoring and research activity at Stromboliand Panarea (2004–2006 INGV-DPC agreement); V2—Paroxysm(2007–2009 INGV-DPC agreement), and MIUR-FIRB—AdvancingInterdisciplinary Research Platform on Volcanoes and Earthquakes(AIRPLANE). Topolin topolin!

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