Types and mechanisms of strombolian explosions: characterization of a gas-dominated explosion at Stromboli

RESEARCH ARTICLE

Types and mechanisms of strombolian explosions:characterization of a gas-dominated explosion at Stromboli

L. Leduc & L. Gurioli & A. Harris & L. Col &E. F. Rose-Koga

Received: 13 May 2014 /Accepted: 3 December 2014 /Published online: 24 January 2015# Springer-Verlag Berlin Heidelberg 2015

Abstract Textural and chemical analyses of bombs quencheddirectly from a normal explosion at Stromboli volcano (Italy)were integrated with coincident seismic, acoustic, and thermaldata. The data set defines a new gas-dominated type ofstrombolian eruption, named type 0. These events are char-acterized by high-velocity emission (150250 m s1) of a fewrelatively small, juvenile particles, with entrained non-juvenile clasts that previously fell back into the vent to bere-erupted. For the studied event, the explosion depth wasmore than 250 m deep, and the particles showed little resi-dence time in the shallow system. Slug ascent velocities overthe final 2035 m of magma-filled conduit, and the lowviscosity of the resident magma, are all consistent with simplebubble burst in a clean conduit. This conduit type anderuption style likely fit popular slug ascent and burst modelsused to explain strombolian eruptions. In contrast, theballistic-dominated type of explosions (type 1) are associatedwith larger proportions of stagnant material in the shallowsystem magma mix. We argue that the additional volume ofthis stagnant material pushes the free surface upward. Becauseof the larger volume of material available for entrainment intoa type 1 slug burst, which has to fragment through a thick capof degassed material, type 1 events tend to be rich in particles.In contrast, the less spectacular, gas-rich (type 0) events

have little material to entrain, thus being poor in lapilliand bombs.

Keywords Strombolian .Explosions .Gas .Bombs .Texture

Introduction

Two models have been proposed to explain strombolian ex-plosions: a collapsing foam model and a rise-rate-dependentmodel (Parfitt 2004). In the collapsing foam model, an accu-mulation of gas bubbles builds foam trapped by some con-striction in the conduit system. Upon reaching a critical thick-ness, the foam collapses to generate the gas slug. This entersand ascends the final section of the conduit to burst at themagma free surface (Jaupart and Vergniolle 1988, 1989;Vergniolle and Jaupart 1990). In the rise-rate-dependent mod-el, the rise rate of the magma is relatively slow compared withthat of the ascending bubbles, so that the bubbles have time togrow and coalesce to generate the slug (e.g., Wilson 1980;Wilson and Head 1981; Parfitt and Wilson 1995). The mainassumption of these models is that the magma rises in rheo-logically uniform and low-viscosity magma, so that slugs canburst cleanly at the magma free surface. This assumption hasbeen embraced by the geophysical community that hasinterpreted very long period (VLP) seismic events, associatedwith Strombolis normal explosive activity, as being associat-ed with slug generation (Ripepe et al. 2001; Chouet et al.2003). The sound and the high-frequency component of theseismic signal can then be related to the explosion itself(Ntepe and Dorel 1990), which results from the slug burstingat the surface of the magma column (Vergniolle and Brandeis1996; Vergniolle et al. 1996). The slug burst then propels amixture of hot gas and fragments up the empty section of theconduit, with the mixture registering a thermal signal upon

Editorial responsibility: M.R. Patrick

L. Leduc : L. Gurioli (*) :A. Harris : E. F. Rose-KogaLaboratoire Magmas et Volcans, Universit Blaise Pascal - CNRS -IRD, OPGC, 63038 Clermont Ferrand, Francee-mail: [email protected]

L. ColDipartimento di Scienze della Terra, Universit degli Studi diFirenze, 50121 Florence, Italy

Bull Volcanol (2015) 77: 8DOI 10.1007/s00445-014-0888-5

exiting the vent (Ripepe et al. 2001, 2002). During such low-intensity basaltic explosions, the type, style, and dynamics ofthe emission, together with the texture and chemistry of theejected particles, can reveal much about the explosion mech-anism (e.g., Rosi et al. 2006; Burton et al. 2007; Andronicoet al. 2008, 2013a, b, 2014; Col et al. 2010; Landi et al. 2011;Pistolesi et al. 2011; Miwa et al. 2009; Miwa and Toramaru2013; Gurioli et al. 2008, 2013, 2014; Lautze et al. 2013).

For the Stromboli volcano, in the Aeolian Islands (Italy)(Fig. 1), these studies have focused on what have becometermed, in decreasing order of magnitude and intensity, parox-ysms (Barberi et al. 1993), major and normal explosions(Barberi et al. 1993), and puffing (Harris and Ripepe 2007a).Within this classification, normal activity has been split into type1 explosions, dominated by coarse ballistic particles, and type 2events that consist of ash-rich plumes, with (type 2a) or without(type 2b) large numbers of ballistic particles (Patrick et al. 2007).Patrick et al. (2007) argued that the ash in type 2 eruptions mayoriginate from one of two sources. The first source is grinding,milling, and recycling of loose material that avalanches into thevent; the second source is due to rheological changes in theuppermost magma column, where increasing the viscosity ofthe magma may lead to fragmentation at a finer scale to createash. Type 2a and 2b explosions were shown to be a function ofthe overpressure of the bursting slug, type 2a involving higherbubble overpressures to send both juvenile and non-juvenileejecta to greater heights during higher energy eruptions(Patrick et al. 2007). For type 1 eruptions, recent studies haveshown that bombs and lapilli are passively entrained by the gasjet (Harris et al. 2013) and may result from failure and fragmen-tation of a high-viscosity, degassed cap (Gurioli et al. 2014).

In terms of textural features, the highly porphyritic (HP)scoriae associated with Strombolis normal explosions(Francalanci et al. 1999) have a population of spherical tosub-spherical (0.1 to 3 mm) vesicles and a sparser populationof large vesicles with diameters of up to 10 mm. The entirevesicle population has a number density that ranges from 102

to 104 mm3 (Lautze and Houghton 2005, 2007, 2008; Polacciet al. 2006, 2008, 2009; Cigolini et al. 2008; Col et al. 2010;Belien et al. 2010; Gurioli et al. 2014). Gurioli et al. (2014)studied two quenched bombs collected during a type 1 explo-sion to show that fresh magma was mingled with batches ofpartially-to-completely degassed, oxidized, and evolvedmagmawith high crystallinity and viscosity. This magma sat at the topof the conduit and played only a passive role in the explosiveprocess. The fresh, microlite-poor, vesiculated magma, howev-er, showed a response to the explosive event, by undergoingrapid decompression. Gurioli et al. (2014) thus suggested thatthe degassed, stagnant, oxidizedmagma forms a rheological capat the top of the conduit through which the fresh magma bursts.

We here define a new gas-dominated type of normalstrombolian eruption, named type 0, through sampling ofbombs and their textural and geochemical characterization,

together with the study of the associated geophysical signalsand thermal video. This integration of data allows us todiscuss the explosion mechanism and the associated shallowsystem conduit model. This new activity type is characterizedby high-velocity emission of a few, relatively small particles.This is likely associated with a slug bursting at the surface of afresh magma column, lacking the rheological cap of Gurioliet al. (2014) and with a relatively deep free surface.

Sampling and methods

Activity and sample collection

Sampling of lapilli and bombs was carried between May 23and 26, 2011. During this period, the frequency of explosionsat the southwest crater (SWC, Fig. 1) was relatively low, withone event every 2025 min, with some pauses being up to 4550min in duration. This compares with a typical SWC eruptionfrequency of seven events per hour (Harris and Ripepe 2007b).During May 23, explosions from the SWC were accompaniedby loud detonations. Emissions, however, were completelyinvisible to the naked eye and no particles were heard landing.That is, these were eruptions of pure gas with no ash, lapilli, orbombs exiting the vent. During the morning of May 25, a ventat the NE edge of the SWCwas generating rare, but impressive,bomb-loaded eruptions. Bombs and lapilli escaped the crater tothe SW to land on the crater outer flank 150200 m from thesource, where a fresh bomb and lapilli sheet was developing.Early in the afternoon of May 25, the eruption style from thesame vent changed to gas-dominated jets with weak particleloads. On May 26, explosions at the SWC remained gas-dominated, occasionally comprising gas-only jets.

OnMay 25 at 14:05 (GMT), an explosion occurred from theNE vent in the SWC (Fig. 1) after a particularly short inter-explosion period of 10 min. Visually, the emission was charac-terized by a cloud of gas with a diffuse load of relatively smallparticles. The jet was orientated to the SW, which was in thesame direction as thewind at that time. Thismeant that particlesescaped the SWC to land on its outer flanks, around 175 mfrom the launch point, where a sampling team of three peoplewere working (Fig. 1). Each person collected and quenched theparticle that landed closest to them. Quenching was carried outwithin a few seconds of collection using 1.5 L of bottled water.

Density and textural measurements

Bulk densities of the three samples were measured followingthe technique of Houghton and Wilson (1989) which requiresfirst weighing the samples in air. Each sample was thenwrapped in impermeable wax paper (of known weight) andweighed once immersed in water. The Archimedes principlewas then invoked to obtain sample bulk density. Sample

8 Page 2 of 15 Bull Volcanol (2015) 77: 8

vesicularity was then obtained using a dense rock equivalent(DRE) value of 2950 kg m3 as obtained for Strombolismagma by Gurioli et al. (2014).

FollowingGurioli et al. (2014), the largest vesicle populationsat the sample scale were imaged using a desktop scanner at 1200dpi, first by scanning the sectioned sample (at 2.5 magnifica-tion) and two thin sections taken from the same sample (at 5magnification). We imaged the two largest areas that seemed tohave contrasting textural characteristics in terms of content oflarge versus small vesicles and crystals. Larger magnifications(at 25 and 100) were captured through scanning electronmicroscopy (SEM) using chemical mapping (Fig. 2). Imageswere acquired using a JEOL JSM-5910LV equipped with amicroanalysis energy-dispersive X-ray spectrometer with asilicon drift detector. This type of detector has a count rateof several tens of kilo-counts per second, which allows com-plete chemical mapping in 10 min for a 114 dpi image. Theseelement maps were used because they greatly facilitate object/phase recognition for image preparation before textural char-acterization. For each thin section, six images were captured atthe 25 magnification, and two at the 100 magnification,so that 18 chemical maps (six for the 25 magnificationand 12 for the 100 magnification) were generated. Texturaldata were then obtained by processing Photoshop-enhancedimages using the Fast Object Acquisition and MeasurementSystem (FOAMS) software of Shea et al. (2010).

Geochemical analysis

Major element composition of the glass was measured usingthe electron microprobe SX-100 CAMECA of LaboratoireMagmas et Volcans in Clermont-Ferrand (France). The oper-ating conditions are detailed in Le Voyer et al. (2008) andsummarized here. An accelerating voltage of 15 kV and adefocused beam of 10 m with an 8-nA current were used.Peak counting times were 20 s per element. No effect of alkaliloss was observed on repeated analysis of the VG-A99 basal-tic glass standard under these conditions. The estimated ana-lytical error (3) for these measurements was

sampling period (Fig. 1). The camera was orientated N285 totarget SWC explosions. Images were acquired at 60 Hz, withthe camera being tilted downward at an angle of 2732.Given the camera-to-vent distance (290300 m), viewingangle relative to horizontal and pixel instantaneous field ofview (2.7 mrad), pixels were 1.840.06 m across. This cam-era has two gain settings, low which saturates at 150 C andmedium with saturation at 400 C (Harris 2013). We used thesecond setting. The velocity of particles, and the front of theexplosion plume, was extracted by manually tracking theposition of each feature through time. On the thermal imagery,the plume, a mixture of hot gas, aerosols, and fine (micron-scale) particles, was apparent as a broad, expanding, low-intensity thermal anomaly. In contrast, particles were apparentas small, discrete high-intensity thermal anomalies against theplume or sky background (Fig. 4). The image frequency of60 Hz meant that it was possible to follow each feature at time

steps of 17 ms. This, with the pixel spatial resolution, meantthat the measurement accuracyassuming an error of onepixel on the manually located feature locationwas 40 m s1.

Small, hot particles in the plume were apparent as single-pixel thermal anomalies. Sub-pixel particle sizes were esti-mated by applying the two-component mixture model ofHarris et al. (2012) whereby

p L ; T* L ; Tback = L ; Tparticle

L ; Tback 1

in which L(,T) is the Planck function for wavelength andtemperature T, T* is the pixel brightness temperature, Tparticleis the temperature of the sub-pixel hot particle (assumed at950 C), Tback is the temperature of the pixel background(taken from pixels surrounding the anomaly), and p is theportion of the pixel occupied by the hot particle. Multiplyingp by the pixel area (3.38 m2) gives the particle area. This will

Fig. 2 Block diagram showingthe strategy for the selection ofimages and extraction of vesiclesand crystals, from the scale of thecut sample to the thin section: (A)Bomb A and cut face from thewhole bomb (the cut face has thesame dimension as the bomb(~106 cm), (C) selection ofareas (~64 cm) for the thin-section preparation (STR11A_1);(E) selection of the six areas fromthe thin section for the 25imaging using the SEM, and the25 chemical maps; (G) two areason the 25 images used for the100 SEM images as used forchemical mapping; (B, D, F, andH) are tri-scale images createdduring each processing step usingAdobe Photoshop (black=vesicles, light gray=glass, darkgray=crystals)


be the dimension of the bomb in the air, before undergoingmodification due to deformation upon ground impact.

Results

The explosion

The explosion was laterally directed and oblique, the plumebeing directed toward the SSW at an angle of 51 relative to

the horizontal (Fig. 4). The thermal camera images show theemission to be dominated by an ash-free jet of hot gas, withonly three observed lapilli and bombs. This contrasts with theclassical model for normal explosions at Stromboli which aretypically viewed as being heavily loaded with hot bombs,lapilli, and/or ash (Chouet et al. 1974; Ripepe et al. 1993;Patrick et al. 2007; Taddeucci et al. 2012).

Two particles could be tracked (Fig. 4). Both had pixelbrightness temperatures of 455 C and were set against abackground at 18.5 C. This converts to a particle areaof 11030 cm2, equivalent to a circular object with a

Fig. 3 a Fifteen-minute-longrecord of the explosion of May25, 2011 at 14:05 (GMT) from theROC station giving seismicdisplacement (U/D), infrasonicpressure (Pr), and thermalamplitude (Th) for the explosionsampled here (blue box). b Zoomof a 1-min-long period centeredon the explosion: tsVLP, tsi, and tsthare the onset times of the VLP,infrasonic, and thermal signals,respectivelyas marked with reddashed lines. teth is the time atwhich the thermal signal ends

Bull Volcanol (2015) 77: 8 Page 5 of 15 8

diameter of 11.61.7 cm. This is a little larger than the dimen-sions of the samples collected (Fig. 6) and may be due todeformation (elongation and stretching) of the bomb duringflight, as opposed to squashing and flattening upon groundimpact. Particle velocities were as high as 25040 m s1 andshowed no change in velocity across the image field of view(Fig. 5a, b). This suggests that the particles were carried by thegas jet. Although, due to drag, the gas plume decelerates, theparticles launched with it maintain their high velocity, and donot decelerate, within the camera field of view, thereby outrun-ning the gas plume and continuing out of the frame at highvelocities (Harris et al. 2012). Such a plume dynamic is com-mon for mixtures of gas and small particles emitted duringnormal explosions at Stromboli (Harris et al. 2012). The upperlimit of our velocity assessment (290 m s1) approaches thespeed of sound and the high velocities of 230405 m s1 re-corded by Taddeucci et al. (2012). The plume itself was 30 mwide, tightly collimated, and jet like at the vent, developing intoa thermal after a distance of about 100m, at which point it had adiameter of 55 m. The plume front velocity decayed linearlyfrom around 200m s1 at the vent to 60m s1 at the image edge,this being around 250 m from the vent (Fig. 5c).

Textural observations

The three collected fragments have distinctive and differentcharacteristics (Fig. 6). The two bombs (bombs A and B) are

juvenile fragments. They are uniformly black to metallic grayand elongate with rough edges. The third fragment, coarselapilli, had a purplish color due to oxidation. We thereforeconsidered this fragment not to be fresh magma; we thus didnot use it for textural characterization.

Bombs A and B have a phenocryst assemblage ofclinopyroxene, olivine, and plagioclase. Two vesicle popula-tions were distinguished. The predominant populationconsisted of small spherical vesicles with diameters rangingfrom 0.5 to 2 mm. The second population comprised 4 to 12-mm-diameter vesicles that mainly inhabited central areas ofthe sample. The shape of these larger vesicles was irregularand sometimes clearly the result of coalescence of twovesicles.

Densities of the two juvenile samples were 1260 for sampleA and 1010 for B, while the recycled, non-juvenile scoria Chad a lower density of 890 kg m3 (Fig. 7). These values arewithin the range obtained by Lautze and Houghton (2007) forlapilli emitted at Stromboli during normal activity (Fig. 7).They belong to the medium density (MD) class of Lautzeand Houghton (2005). This indicates an absence of the dense,degassed magma described by Lautze and Houghton (2007)or the dense, microlite-rich magma described by Gurioli et al.(2014). Instead, these two bombs were characterized by ahighly porphrytic (HP) facies, a facies typically observed inscoria emitted during Strombolis normal activity (Francalanciet al. 1999). Francalanci et al. (2005) suggested that such dark-

Fig. 4 Thermal image sequenceof the explosion of May 25,2011 at 14:05 (GMT). The lastimage highlights the plume andthe two bombs identified


colored HP scoriae are erupted during normal explosive ac-tivity fed by a shallow reservoir at 3.5 km. This compares withlow porphyritic (LP), light-colored, volatile-rich pumices thatare only erupted from the deep reservoir, at 1011 km, duringthe most energetic eruptions (Francalanci et al. 2005), but cansometimes also be found in the products of normal activity(DOriano et al. 2011).

The vesicle number density values (Nv) are of the samemagnitude order (102 mm3) but vary within each same sample

Fig. 5 Evolution of velocity with time for the two bombs identified (a, b)and for the gas jet (c). The central dark gray area represents the meanvalue of the velocity field and the two dashed lines in c indicate the onsetof the jet

Fig. 6 a Photographs of the three samples. b Table giving the size of thesame samples. c Scanned images of the five thin sections, two each forjuvenile bombs A (STR11A_1 and STR11A_2) and B (STR11B_1 andSTR11B_2) and one for non-juvenile fragment C


(Table 1). Vesicle volumetric distributions for the four thinsections are all bimodal (Fig. 8a), which confirms the exis-tence of two distinct vesicle populations. The small vesiclepopulation covers a wide size range (from 0.01 to 1 mm) andis characterized by a main mode at 0.600.80 mm. The secondvesicle population is defined by a narrow size range (between1 and 710 mm) and has a less pronounced mode at 34 mmand sometimes an isolated, more or less pronounced, mode at10 mm. In Fig. 8b, we compare the cumulative vesicle volumefraction data of the four thin sections. Here, we see that thedistributions are similar for the small vesicle size populationbut different for the coarser vesicles. The crystal content,corrected for vesicularity, ranges between 37 and 49 %(Table 1), showing variations within the same sample andbetween the two juvenile bombs. The microlite content is verylow, less than 3 % (Table 1), and the oxidized olivines foundby Gurioli et al. (2014) are absent.

Chemistry and magma rheology

The glass of bombs A and B has three different colors (black,dark brown, and light brown, Fig. 9a). However, the threecolored glasses are chemically homogeneous (Fig. 9b,Table 2), with concentration variations less than the analyticalerror. Following the classification of LeBas et al. (1992), all ofthese glasses are basaltic trachyandesite with a shoshonitic

character (K2O >4 %). This is typical of the current, persistentactivity, at Stromboli (Fig. 9b; Mtrich et al. 2005; Landi et al.2006; 2009; Pioli et al. 2014).

This implies that the system is fed by viscous (~104 Pa s)partially degassed magma that contained, on average, 4055 vol% of plagioclase, clinopyroxene, and olivine in equi-librium with a shoshonitic (SiO2 ~52 wt%) residual melt(Mtrich et al. 2010). We can now calculate liquid viscosityusing our samples by applying the Vogel-Fulcher-Tammann(VFT) temperature-dependent viscosity relation given forStrombolis magma by Gurioli et al. (2014). This, for a liquidtemperature of 1200 C, gives 260 Pa s. Using this inthe Einstein-Roscoe relation, with our uncorrected crys-tal content of 1519 %, we obtain a liquid-crystalmixture viscosity of 530 to 670 Pa s. This increasesby a factor of 7 to 3700 to 4700 Pa s for a lower liquidtemperature of 1100 C.

Geophysical results

The explosive event was associated with a vertical displace-ment of 9.8106 m and a pressure of 50 Pa at the sensor. Thiscorresponds to 0.6 bar at the source, given attenuation effects(Lacanna and Ripepe 2013) and the distance to the explosionsource. The amplitude of the thermal signal was low (at7.6 C) and the thermal waveform was weak, consistent witha gas-dominated, low-emissivity, thermal event. The durationof the infrasonic signal was, however, quite long (Fig. 6b),which is typical of explosive events at SWC where fragmentsare coupled with a sustained degassing phase which generatesa long coda in the infrasonic signal (Ripepe et al. 1993;Marchetti and Ripepe 2005). The duration of both the thermaland infrasonic waveforms associated with the event was sim-ilar at around 6 to 8 s. The first signal to arrive was the VLP,whose arrival was recorded at 14:05:57. The thermal andinfrasonic signals, respectively, arrived 7 and 8 s after theVLP. Given the 1-s delay between the arrival of the thermaland infrasonic signals, and a typical exit velocity for the jet of200210 m s1 (Fig. 5c), we obtain a depth to the free surfaceof between 255 and 275 m. Given a VLP depth of 290 m

Table 1 Summary of textural features for each bomb

Samples Texture Dens (kgxm3) Ves (%) Ni N_ves N_ctx Ctx (%) Ctxcorr (%) Phenocorr (%) litescorr (%) NV (mm3) NVcorr (mm

3)

STR11A_1 HP 1260 54.3 20 351 275 19 41 39.8 1.2 285 1056

STR11A_2 HP 1260 54.3 19 324 310 17 37 36.6 0.4 203 700

STR11B_1 HP 1010 63.1 20 356 312 18 49 46.5 2.5 324 1705

STR11B_2 HP 1010 63.1 20 203 259 15 41 39.4 1.6 188 855

The table shows, for both samples: texture facies (Texture); bulk density (Dens); derived-density vesicularity (Ves); number of images processed for eachthin section (Ni); number of vesicle and crystal analyzed for each sample, respectively (N_ves, N_ctx); percentage of crystals (Ctx); vesicle-freepercentage of crystals (Ctxcorr); vesicle-free percentage of phenocrysts (Phenocorr); vesicle-free percentage of microlites (litescorr); total volumetricnumber density of vesicles (Nv); total volumetric number density of vesicles referenced to melt only (Nvcorr)

Fig. 7 Bulk densities of samples A, B, and C compared with the rangeobtained by (i) Gurioli et al. (2013) from 53 bombs from a majorexplosion and (ii) Gurioli et al. (2014) for two quenched bombscollected from a normal ballistic-dominated explosion at Stromboli.Density values of Lautze and Houghton (2005, 2007) are also reportedfor lapilli-sized samples. LD,MD, HD low, medium, and high density, asdefined by Lautze and Houghton (2005)


(Marchetti and Ripepe 2005), this means that there was be-tween 20 and 35m ofmagma between the VLP source and thefree surface. The 8-s delay between the VLP and infrasoundarrival, with the seismic path length distance (710 m), gives aslug ascent velocity (between the VLP source and the freesurface) of between 3 and 6 m s1.

Discussion

All of the data previously presented suggest that the slugascended the final 2035 m of magma-filled conduit at anaverage velocity of 3 and 6 m s1 to burst at a free surfacearound 265 m beneath the crater. This is quite deep by

Fig. 8 a Distribution of vesicle sizes in bombs A and B as a function ofvolume fraction. b Number of vesicles greater than a given size(cumulative number density referenced to the melt volume) versus sizeon a log-log plot for the four thin sections from bombs A and B. cDistribution of vesicle sizes in lapilli from normal type 1 explosions

sampled in 2002, as a function of volume fraction (modified fromLautze and Houghton 2007). d Distribution of vesicle sizes in a bombcollected in 2008 from a normal type 1 explosion, as a function of volumefraction (modified from Gurioli et al. 2014)


Strombolis standards. Ripepe et al. (2002) obtained burstdepths of 740 m and possibly as deep as 180 m. For theevent sampled by Gurioli et al. (2014), the depth of explosionwas around 100 m.

However, the slug ascent velocity obtained here (4.5 m s1)is substantially lower than those obtained in previous studies.Gurioli et al. (2014) obtained 25m s1, with Harris and Ripepe(2007a) obtaining 1070 m s1. Assuming that the slug fills arigid conduit with a radius (R) of 1 to 10 m, the slug ascent

velocity should be proportional to 0.48(Rg)1/2 (Batchelor1967). It should thus range between 1.5 and 4.75 m s1, witha limit of 3.5 m s1 if R is 5 m (Seyfried and Freundt 2000).Thus, ascent velocities obtained at Stromboli have been con-sidered generally too high to be explained by simple ascent ofa bubble through low-viscosity magma (Harris and Ripepe2007a). As a result, the model of Gurioli et al. (2014) explainshigh velocities (>10 m s1) as being due to propagation of afragmentation front across a highly viscous magma cap at the

Fig. 9 a Scan of the thin sectionof bomb A, with the threedifferent analyzed glass areashighlighted. b CaO/Al2O3 ratio ofglass versus K2O content of glass


Tab

le2

Major

elem

entcom

positio

nof

thethreeglasscolorsobserved

inthethin

sections

ofbombs

AandB

Sam

ple

STR11A_1

STR11A_2

Glass

Black

Darkbrow

nLight

brow

nBlack

Darkbrow

nLight

brow

n

wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3

SiO

252.88

1.04

52.51

1.04

52.34

1.03

51.59

1.02

51.83

1.03

51.90

1.03

52.55

1.04

52.29

1.04

51.79

1.03

51.42

1.02

52.41

1.04

52.70

1.04

TiO

21.67

0.11

1.62

0.11

1.68

0.11

1.64

0.11

1.58

0.11

1.64

0.11

1.64

0.11

1.69

0.11

1.65

0.11

1.61

0.11

1.73

0.11

1.58

0.11

Al 2O3

15.22

0.32

15.20

0.32

14.73

0.31

15.37

0.32

15.20

0.32

15.28

0.32

15.20

0.32

15.20

0.32

15.41

0.32

15.54

0.32

15.51

0.33

15.44

0.32

FeO

9.39

0.67

9.83

0.68

10.35

0.70

9.90

0.69

9.81

0.69

9.74

0.69

10.16

0.70

9.98

0.69

9.76

0.68

9.80

0.68

9.65

0.69

10.45

0.70

MnO

0.26

0.30

0.10

0.28

0.35

0.34

0.16

0.28

0.24

0.29

0.20

0.26

0.14

0.26

0.30

0.28

0.21

0.26

0.18

0.32

0.22

0.27

0.21

0.26

MgO

3.75

0.17

3.74

0.17

3.83

0.18

3.70

0.17

3.63

0.17

3.88

0.18

3.53

0.17

3.57

0.17

3.81

0.17

3.72

0.17

3.70

0.17

3.75

0.17

CaO

7.23

0.22

7.41

0.22

7.54

0.22

7.63

0.22

7.41

0.22

7.60

0.22

7.47

0.22

7.23

0.22

7.81

0.23

7.74

0.22

7.58

0.22

7.75

0.22

Na 2O

2.97

0.29

2.38

0.26

2.60

0.28

2.45

0.27

2.77

0.29

2.42

0.27

2.13

0.25

2.81

0.29

2.72

0.29

2.93

0.29

3.06

0.30

2.69

0.28

K2O

4.63

0.17

4.36

0.17

4.44

0.17

4.51

0.17

4.59

0.17

4.61

0.17

4.31

0.17

4.52

0.17

4.21

0.16

4.26

0.17

4.22

0.17

4.21

0.16

P2O5

1.09

0.21

1.09

0.21

1.15

0.22

1.19

0.22

1.11

0.22

1.04

0.21

1.25

0.23

1.22

0.22

1.07

0.21

1.32

0.23

1.18

0.22

1.02

0.21

Total

99.10

98.23

99.01

98.14

98.18

98.30

98.39

98.81

98.45

98.53

99.26

99.81

Sam

ple

STR11B_1

STR11B_2

Glass

Black

Darkbrow

nLight

brow

nBlack

Darkbrow

nLight

brow

n

wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3wt%

3

SiO

252.19

1.03

51.96

1.03

52.07

1.03

52.54

1.04

52.18

1.03

52.57

1.04

53.37

1.05

52.23

1.03

52.51

1.04

52.05

1.03

52.20

1.03

52.58

1.04

TiO

21.72

0.11

1.61

0.11

1.71

0.11

1.67

0.11

1.69

0.11

1.69

0.11

1.70

0.11

1.66

0.11

1.67

0.11

1.64

0.11

1.63

0.11

1.65

0.11

Al 2O3

15.53

0.32

15.29

0.32

15.23

0.32

15.10

0.32

15.39

0.32

15.14

0.32

15.32

0.32

15.32

0.32

15.06

0.32

15.24

0.32

15.04

0.32

15.37

0.32

FeO

9.53

0.68

10.17

0.70

9.72

0.68

9.69

0.68

9.54

0.68

9.91

0.69

8.81

0.66

9.29

0.67

10.25

0.70

10.43

0.71

9.74

0.68

9.38

0.67

MnO

0.21

0.32

0.13

0.33

0.27

0.33

0.02

0.27

0.24

0.28

0.14

0.32

0.29

0.26

0.07

0.28

0.27

0.30

0.30

0.31

0.25

0.29

0.20

0.29

MgO

3.57

0.17

3.56

0.17

3.47

0.17

3.43

0.17

3.57

0.17

3.58

0.17

3.48

0.17

3.60

0.17

3.68

0.17

3.36

0.17

3.42

0.17

3.54

0.17

CaO

7.63

0.22

7.46

0.22

7.57

0.22

7.37

0.22

7.34

0.22

7.41

0.22

7.13

0.22

7.87

0.23

7.43

0.22

7.65

0.22

7.60

0.22

7.46

0.22

Na 2O

3.07

0.30

2.51

0.27

2.90

0.29

2.59

0.28

3.29

0.31

2.97

0.29

3.25

0.31

2.69

0.28

3.19

0.31

2.82

0.29

2.96

0.30

2.34

0.26

K2O

4.31

0.17

4.23

0.17

4.22

0.17

4.27

0.17

4.34

0.17

4.28

0.17

4.53

0.17

4.16

0.16

4.37

0.17

4.26

0.17

4.31

0.17

4.21

0.16

P2O5

1.12

0.21

1.14

0.22

1.07

0.20

1.17

0.22

1.08

0.21

1.10

0.21

1.11

0.22

1.24

0.23

1.09

0.21

1.23

0.22

1.05

0.21

1.16

0.21

Total

98.90

98.06

98.23

97.85

98.64

98.78

99.01

98.14

99.51

98.99

98.20

97.88


top of the column. In contrast, the velocity obtained here fallswithin the range (1.5 and 4.75 m s1) of expected values forbubble ascent through low-viscosity magma in a conduit thatis 110 m wide.

The resulting emission is a gas-rich jet with rare, andrelatively small, lapilli and bombs launched at high (150250 m s1) velocities. Harris et al. (2012) showed that suchvelocities are due to a system where the ejecta are carried bythe gas phase, so that the ejecta velocities are a proxy for thegas phase. This seems consistent with our gas jet and particlevelocities being similar at the vent (Fig. 5).

Textural and chemical features of the sampled clasts allowus to observe and quantify the fresh, HP magma that com-prises the sampled bombs. We did not observe, in thesesamples, the microlite-rich magma described by Gurioliet al. (2014) or the dense facies described by Lautze andHoughton (2005, 2007, 2008). Our samples, however, showthe distinctive signature of expansion-coalescence, as shownby (i) the second population of coarse vesicles (Fig. 8a), (ii)the coarse-sized isolated peaks (Fig. 8a), and (iii) the cumula-tive histograms (Fig. 8b). This second population contains, onaverage, four times less vesicles than the first population. Thissuggests that the coalescence phenomenon was present, butlimited. This result is consistent with the findings of Lautzeand Houghton (2007, 2008) and Polacci et al. (2006, 2008,2009) who found that lapilli ejected during Strombolis nor-mal activity always have such a double-vesicle population(Fig. 8c). Therefore, the coalescence process is systematicfor ejecta emitted during normal activity, unless it is impededby the presence of stagnant material, as shown byGurioli et al.(2014). In such cases, the coalescence signature is absent in

the fresh, HP portions, of the bombs (Fig. 8d). Although wesee the presence of a coalescence signature and brown glass inour bombs, their textural features are consistent with freshmagma that has not undergone stagnation within the upperpart of the conduit, as well described by the matrix glasschemistry (Fig. 9b). It is also clear that the different glasscolors are not related to compositional changes associatedwith microlite formation. Because the darker glass is foundin the interior of the samples and the lighter glass in the rimregions (Figs. 6 and 9), the different color may simply be theresult of pyroclast cooling and conversion of Fe2+ to Fe3+(e.g., Tait et al. 1998). Oxidation may have caused theirglasses to become dark during transport and cooling.

A second element of interest is the presence of the non-juvenile scoria fragment. This clast has the same texturalmacro-scale features, in terms of scoriaceous appearance, asthe two fresh bombs, but shows pervasive oxidation (Fig. 6).We interpret this clast as old material that has undergoneoxidization due to contact with the atmosphere rather thanhigh temperatures. Recycling of material that has spent sometime in the conduit has already been documented forStromboli by Lautze and Houghton (2005, 2007, 2008) andGurioli et al. (2014). However, this is a record of a differenttype of entrainment in which cold material that has been lyingon the surface for some time, slumps into the vent to beentrained by the next eruption, as shown by the lithic ashparticles described by Lautze et al. (2012, 2013) andAndronico et al. (2013a).

All of these features point to this gas-rich eruption beingassociated with a bubble burst in relatively fresh magma. Thiscontrasts with the bomb-rich (type 1) eruption of Gurioli et al.

Fig. 10 a Comparison of thermalimages of the three explosiontypes (0, 1, and 2) defined fornormal activity at Stromboli. b Arevised conceptual sketch of thedifferent explosion types(modified from Patrick et al.2007), where convective overturnlikely causes the transition fromtype 0 to type 1, both of which aremodified by the presence ofrecycled, stagnant material lyingon the magma free surface


(2014) that burst through a cap of stagnant, degassed, andhighly crystalline magma. The much lower level of the freesurface for our gas-rich eruption, compared with that recordedduring bomb-rich eruptions, may result from convective over-turn. Such a process would remove the stagnant volume fromthe shallow system to leave a fresh column without thedegassed capping volume. Using the three-phase viscosityapproach applied by Gurioli et al. (2014), we find that themagma viscosity at the free surface during the gas-rich erup-tion was 0.81.8104 Pa s, as opposed to up to 18104 Pa sfor the ballistic-rich eruption sampled by Gurioli et al. (2014).The two column types thus have viscosities that are 103104 Pa s (gas rich) and 104105 Pa s (bomb rich). This mayrepresent a cycle between buildup of degassed mass andoverturn, during which the degassed mass is lost by density-driven convection. The removal of volume from the shallowsystem would result in a decrease in the level of the magmacolumn (Ripepe et al. 2002). This low level of the magmasurface in the gas-rich conduit could also explain the tightcollimation of the jet and possibly the lack of lapilli andbombs. However, the latter feature may also be due to athinner layer of fresh magma that fragments above the burst-ing bubble in the gas-rich case, as opposed to the capped casewhere there is a thicker layer of fresh and degassed magma.The paucity of erupted solid mass seems counterintuitive withthe fact that type 0 eruptions are among the most energeticsub-types of Strombolis normal activity but is consistent withthe conclusions of Taddeucci et al. (2013).

We have observational data that reveal that type 0 eventsare not uncommon at Stromboli. During the construction ofexplosion logs, we have, on occasion, noted loud jettingevents that are invisible by day, but apparent by night from atightly collimated blue gas flare. Unfortunately, though, ourdata set is not sufficiently large to judge how common type 0events are. But, from many hours of observations between1996 and 2014, our feeling is that they are much less commonthan type 1 and 2 eventswhich tend to dominate activity.Future studies now need help to determine the proportionalityof type 0, 1, and 2 events and how this changes through time.Given that type 0 activity is likely associated with the classicbubble burst model for explosive activity at Stromboli, defin-ing the commonality of such activity is, though, pressing: Ifthis activity type is rare, then the classical eruption modelconditions are also rare.

Conclusions

We define a new type of explosion at Stromboli: gas-dominatedtype 0 (Fig. 10). In such events, lapilli andbombs are rare or absent. Fragments are launched atseveral hundreds of meters per second in tightly colli-mated gas jets to which the particles are coupled. The

fragmentation depth is relatively deep, and those parti-cles emitted are juvenile and show little residence(degassing) time in the shallow system.

From studies of Strombolis erupted products, it is nowaccepted that the shallow conduit comprises a mixture ofvesicular and partly degassed magma (Corsaro et al. 2005;Burton et al. 2007; Lautze and Houghton 2005, 2007, 2008;Polacci et al. 2006, 2008, 2009; Andronico et al. 2008; Colet al. 2010; Mtrich et al. 2010; Belien et al. 2010; Schiaviet al. 2010; Bai et al. 2011; Pistolesi et al. 2011), plus totallydegassed, microlite-rich, oxidized magma (Gurioli et al.2014). This shallow system is located at the top of aconvecting conduit within which fresh vesicular magma risesthrough a sinking mass of dense degassed material (Allardet al. 1994; Harris and Stevenson 1997; Burton et al. 2007).The degree of overturn in this degassing system likely affectsthe exact mixture of magma types, style of activity, explosionmechanism, and depth of explosion.

We suggest that type 1 (ballistic dominated) eruptions areassociated with relatively large proportions of degassed, stag-nant, material in the shallow system mixture, the additionalvolume of which pushes the free surface (and explosionsource) upward. Given a larger volume of material to beentrained during the slug burst, which has to fragment throughthis cap, events tend to be spectacular and rich in particles. Atthe same time, the presence of this high-viscositymagma layerlikely forces the gas to fragment through the cap at a relativelyhigh propagation velocity (>10m s1). In contrast, type 0 (gas-dominated) eruptions are associated with a fresh conduit, theremoval of the degassed volume having caused the free sur-face (and explosion source) to migrate to a deeper level, just afew 10s of meters above the VLP source. In our case, the jetwas oblique, but we have also observed many vertical type 0eruptions by night, when they are apparent from tightly colli-mated blue-flare-lacking particles. Indeed, type 0 events tendto lack particles due to the small amount of juvenile materialavailable for accidental entrainment with the gas burst.Relatively slow ascent velocities for the slug are also consis-tent with simple bubble ascent and burst in a clean conduit.This low-viscosity, type 0, conduit (and eruption style) is thuslikely that which fits the popular slug ascent and burst modelstraditionally used to explain strombolian eruptions. Thepresence of the type 1 activity style (Gurioli et al. 2014)complicates such a simple model by introducing a degassedmagma cap with which the ascending slug interacts, and thereis likely a continuum of scenarios between the two end mem-bers (type 0 and type 1 explosions).

Acknowledgments We thank Christophe Constatin for the thin sec-tions prepared at Laboratoire Magma et Volcans, Clermont-Ferrand. Wethank J. Bernard for collecting one of the three precious samples. We alsothank the editor and D. Andronico and T. Shea for their corrections andsuggestions that really improved the paper, especially the figures, and L.Pioli for a few final precious discussions.


This research was financed by the French Government Laboratory ofExcellence initiative no ANR-10-LABX-0006, the Rgion Auvergne,and the European Regional Development Fund. This is Laboratory ofExcellence ClerVolc contribution number 110.

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http://dx.doi.org/10.1016/j.pce.2011.02http://dx.doi.org/10.1130/2013.2498(08)http://dx.doi.org/10.1029/2008GC002173http://dx.doi.org/10.1029/2004GL021406http://dx.doi.org/10.1029/2004GL02257http://dx.doi.org/10.1093/petrology/egp083http://dx.doi.org/10.1007/s00445-012-0685-yhttp://dx.doi.org/10.1130/G35844.1http://dx.doi.org/10.1029/2006GL026241http://dx.doi.org/10.1029/2008JB005672http://dx.doi.org/10.1016/j.jvolgeores.2009.12http://dx.doi.org/10.1016/j.jvolgeores.2009.12http://dx.doi.org/10.1029/2011GL050404http://dx.doi.org/10.1002/grl.50652

Types and mechanisms of strombolian explosions: characterization of a gas-dominated explosion at StromboliAbstractIntroductionSampling and methodsActivity and sample collectionDensity and textural measurementsGeochemical analysisGeophysical dataThermal camera data

ResultsThe explosionTextural observationsChemistry and magma rheologyGeophysical results

DiscussionConclusionsReferences

Documents

Types and mechanisms of strombolian explosions: characterization of a gas-dominated explosion at Stromboli