rivalta et al. - 2013 - explosive expansion of a slowly decompressed magma analogue. evidence for...

7/30/2019 Rivalta Et Al. - 2013 - Explosive Expansion of a Slowly Decompressed Magma Analogue. Evidence for Delayed Bubble Nucleation

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Explosive expansion of a slowly decompressedmagma analogue: Evidence for delayed bubblenucleation

Eleonora RivaltaGFZ GeoForschungsZentrum, Section 2.1, Physics of Earthquakes and Volcanoes, Telegrafenberg, DE-14473,

Potsdam, Germany ([email protected])

School of Earth and Environment, University of Leeds, Leeds, UK

Institute of Geophysics, University of Hamburg, Hamburg, Germany

Karen PascalSchool of Earth and Environment, University of Leeds, Leeds, UK

Jeremy PhillipsSchool of Earth Sciences, University of Bristol, Bristol, UK

Alessandro BonaccorsoINGV, Sezione di Catania, Piazza Roma 2, Catania, Italy

[1] While ascending in the plumbing system of volcanoes, magma undergoes decompression at ratesspanning several orders of magnitude and set by a number of factors internal and external to the volcano.Slow decompression generally results in an effusive or mildly explosive expansion of the magma, butcounterexamples of sudden switches from effusive to explosive eruptive behavior have been documentedat various volcanoes worldwide. The mechanisms involved in this behavior are currently debated, in

particular for basaltic magmas. Here, we explore the interplay between decompression rate andvesiculation vigor by decompressing a magma analogue obtained by dissolving pine resin into acetone invarying proportions. Analogue experiments allow direct observations of the processes of bubblenucleation and growth, flow dynamics, and fragmentation that is not currently possible with magmaticsystems. Our mixtures contain solid particles, and upon decompression, nucleation of acetone bubbles isobserved. We find that mixtures with a high acetone content, containing smaller and fewer solid particles,experience strong supersaturation and fragment under very slow decompressions, despite having lowviscosity, while mixtures with lower acetone content, with more and larger solid particles, degas

efficiently without fragmentation. We interpret our results in terms of delayed bubble nucleation due to alack of efficient nucleation sites. We discuss how a similar mechanism might induce violent, explosiveexpansion in volatile-rich and poorly crystalline low-silica magmas, by analogy with the behavior ofrhyolitic magmas.

Components: 11,253 words, 7 figures, 1 table.

Keywords: magma fragmentation; basaltic magma; analogue laboratory experiments; slow decompression; bubble nuclea-

tion; explosive volcanic eruptions.

Index Terms: 8428 Explosive volcanism: Volcanology; 8434 Magma migration and fragmentation: Volcanology; 8414

Eruption mechanisms and flow emplacement: Volcanology; 8439 Physics and chemistry of magma bodies: Volcanology;

8445 Experimental volcanism: Volcanology; 4302 Geological: Natural Hazards.

2013. American Geophysical Union. All Rights Reserved. 1

Article

Volume 00, Number 00

0 MONTH 2013

doi: 10.1002/ggge.20183

ISSN: 1525-2027


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Received 27 February 2013; Revised 20 May 2013; Accepted 22 May 2013; Published 00 Month 2013.

Rivalta, E., K. Pascal, J. Phillips, and A. Bonaccorso (2013), Explosive expansion of a slowly decompressed magma

analogue: Evidence for delayed bubble nucleation, Geochem. Geophys. Geosyst., 14, doi:10.1002/ggge.20183.

1. Introduction

[2] Sudden decompression of magma, induced forexample by the removal of mass from a volcanicedifice (flank collapse, landslides, glacier melting,lake drainage), has the potential to cause explosiveeruptions, depending on the amount of decompres-

sion and the volatile content of magma. A linkbetween explosive high-silica volcanism and slowdecompression of magma (e.g., induced by effu-sive activity) has also been suggested by decom-

pression experiments on rhyolite [Cashman et al.,2000; Castro and Gardner, 2008]. The established

physical explanation of slow decompression as atrigger for explosive eruptions is viscousrestraint: the induced expansion of gas bubblesmight be resisted by high viscous stresses in veryviscous magmas to such an extent that enough

pressure builds up within the bubbles to eventuallyrupture their walls, resulting in explosive expan-

sion. Additionally, laboratory experiments havesuggested that high-silica explosive eruptions dur-ing slow decompression might also exhibitdelayed bubble nucleation [Sparks, 1978; Man-gan and Sisson, 2000; Mourtada-Bonnefoi and

Laporte, 2002; Pinkerton et al., 2002; Mangan etal., 2004; Mangan and Sisson, 2005]: the nuclea-tion of gas bubbles during decompression may beretarded in poorly crystalline magmas by a lack ofefficient nucleation sites and slow volatile diffu-sivity, so that the magma becomes progressivelysupersaturated and eventually expands explosively

once a supersaturation threshold is reached andbulk vesiculation is triggered. Hurwitz and Navon[1994] studied the efficiency of different types ofcrystals in facilitating gas exsolution in rhyoliticmagma. They found that Fe-Ti oxides are veryefficient sites of nucleation, and their presencefavors equilibrium degassing during decompres-sion. On the contrary, magma with a low crystalcontent or containing crystals that are inefficientas nucleation sites, such as feldspar or quartz,requires large supersaturation to nucleate bubbles.

[3] Low-silica magmas can also erupt explo-sively. While a large majority of basaltic vol-

canic eruptions are effusive or mildly explosive,as in Strombolian or Hawaiian activity [Verg-noille and Mangan, 2000], basaltic volcanoesswitch occasionally to explosive activity ofgreater intensity, up to Plinian, sometimes withlittle warning [Williams and Self, 1983; Walkeret al., 1984; Coltelli et al., 1998; Doubik and

Hill, 1999; Gurenko et al., 2005; Hskuldssonet al., 2007]. Significant effort has been madein the last few years to understand violent ex-

plosive basaltic eruptions, investigating theeruption products [Polacci et al., 2001, 2003;Gurioli et al., 2008; Sable et al., 2009] andhow the physical properties of low-silica mag-mas change with volatile content [Polacci etal., 2006; Larsen, 2008; Metrich et al., 2009].The physics of the expansion and fragmentationof bubbly, low-viscosity fluids upon decompres-sion is still poorly understood; this has moti-

vated experimental analogue studies [Namikiand Manga, 2005, 2006, 2008] investigating thestyle of expansion of bubbly fluids as a functionof amount of decompression, decompressionrate, and conduit and magma parameters.Besides describing the phenomenology of thevarious expansion styles as function of vesicu-larity and decompression rate, those studiesoffer a quantitative physical model based onrates of bubbly liquid deformation for how sud-den decompression may lead to the fragmenta-tion of bubbly low-viscosity magma.

[4] The mechanisms by which slowly decom-pressed basaltic magmas can erupt explosivelyremain unclear. Decompression rates of the orderof 100400 Pa s1, typical of lava effusion, arenot commonly assumed to be potentially hazard-ous: lava effusion, particularly at basaltic volca-noes, is considered a low-risk eruptive style, andthe few laboratory experiments investigating thelink between slow decompression and explosivityfound that significantly higher rates were neededto observe fragmentation. Namiki and Manga[2006] decompressed at various rates bubbly flu-ids and observed fragmentation only for

RIVALTA ET AL. : SLOWLY DECOMPRESSED MAGMA ANALOGUE 10.1002/ggge.20183

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decompression rates larger than about 0.51 MPas1 ; Stix and Phillips [2012] obtained similarresults for a set of volatile-bearing gum rosinand acetone mixtures. However, counterexamplesof slowly decompressed basaltic systems thatunderwent violent explosive eruptions have beendocumented. Switches in the eruptive style in thesequence: Strombolian ! effusive ! high-energy explosive ! effusive have been inferred,for example, for the $2000 BP eruption at Xitlevolcano in the central Trans-Mexican VolcanicBelt [Cervantes and Wallace, 2003]. They havealso been observed at Stromboli volcano in 2003and 2007 [Calvari et al., 2011] in the followingsequence. Lava effusion started from fissures thatopened a few hundred meters below the summit,

while the usual low-energy explosive activityceased; lava effusion persisted for a few weeks,then suddenly an explosive paroxysmal event ofunusual energy (a 1 km sized eruption column)occurred, transporting to the surface magma withlow crystallinity and high volatile content from adeep reservoir, not tapped during normal Strom-

bolian activity. Such switches in erupting behav-ior are still unexplained. For the eruption atXitle, it has been suggested that a recharge eventinduced a sudden increase of magma overpres-sure in the conduit and an increased magmaascent rate [Cervantes and Wallace, 2003]. This

mechanism is not fully satisfactory, at least forStromboli, as lava flow certainly induced anincreased ascent rate, but the lava flow rate washighest in the initial phase of the effusion, and itsubsequently decreased systematically and signif-icantly, and was about an order of magnitudelower on the day of the paroxysm [Calvari etal., 2011]. Magma partitioning and simultaneouseruption of gas-rich magma from the vent and ofgas-poor magma from a fissure at the base of thecinder cone, proposed by Krauskopf [1948] forParicutin volcano, has also been suggested byCervantes and Wallace [2003] for Xitle.

Although at Stromboli the lava was flowing fromfissures located a few hundred meters below thesummit craters, a partition mechanism can beexcluded for the 2003 and 2007 paroxysms atStromboli, as the Strombolian activity at thesummit vents had stopped completely duringlava effusion.

[5] A few studies have offered physical mecha-nisms for mild to intermediate explosive expansionstyles at low-silica volcanoes. Namiki and Manga[2008] suggest that the stretching of the bubblycolumn of magma in the conduit during

decompression-induced expansion (or inertialfragmentation) might explain explosive basalticeruptions during slow decompression. Other exist-ing conceptual models [Vergniolle and Jaupart,1986; Parfitt and Wilson, 1995; Namiki andManga, 2006] explain the generation of Hawaiiansustained lava fountaining and mild to intermediateisolated Strombolian explosions [Aiuppa et al.,2011]. However, it is difficult to apply any of them,for example, to explain basaltic Plinian eruptions orto sudden switches from effusive to explosive erup-tive styles. Some authors suggest that the kineticsof bubble or crystal nucleation [Sable et al., 2006;Houghton and Gonnermann, 2008; Sable et al.,2009], or the dynamics of degassing [Schipperet al., 2010], may play a dominant role in explosive

eruptions of basaltic magma, and indeed in supersa-turated magmas, large quantities of energy arestored in a metastable equilibrium and can bereleased over short time scales. However, a concep-tual model of high-energy explosive eruptions at

basaltic volcanoes is still missing, and the fine-scalemechanisms able to cause the fragmentation oflow-viscosity magma without any sudden decom-

pression are poorly understood.

[6] We present here laboratory observations ofthe interplay between decompression and vesicu-lation rates from fast and slow decompressionexperiments using a magma analogue containingdissolved volatiles and solid particles. Asexpected, we observe that all mixtures fragmentduring sudden decompression, but some vesicu-late violently and fragment during decompres-sions as slow as 50400 Pa s1. Based on ourobservations, supported by elements of nuclea-tion theory and published petrological laboratoryexperiments, we propose that slow decompres-sion might induce strong supersaturation and

potential explosivity in basaltic magma if bubblenucleation is delayed by lack of crystals to actas nuclei or by general inefficiency of nuclea-

tion, as has been proposed for rhyolitic magmas.Our experimental observations support the ideathat delayed nucleation may turn slow decom-

pression of magma (e.g., induced in the conduitby lava effusion) into potential explosive behav-ior, provided the crystallinity of the magma is

poor or inefficient as an adjuvant for bubblenucleation. Our experiments suggest a possiblelarge-scale model for delayed bubble nucleationas a mechanism potentially leading to violent ex-

plosive eruptions at low-silica volcanoes, that wespeculatively apply to the 2003 and 2007 parox-ysms at Stromboli.


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2. Experimental Methods

2.1. Magma Analogue[7] Gum rosin-acetone (GRA) mixtures of differ-ent initial acetone concentration (here 1540 wt %acetone in gum rosin) were used as magma ana-logues, being prepared by solving brittle gum rosin

blocks [Fiebach and Grimm, 2000] into acetone ina continuously stirred and sealed glass flask forabout 24 h. Macroscopically, GRA mixturesappear purely liquid, although occasionally wevisually observed solid gum rosin particles in mix-tures of lower acetone concentration (


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The number density of crystals in GRA mixturesand the surface they offer as locus for nucleation

are important parameters in this study but imprac-tical to control, because they depend, along withacetone content, laboratory temperature, and pres-sure, also on the initial size distribution of thecrystals provided by the supplier, and on the his-tory of the stirring process. As a result, the solidfraction can vary by up to 1 order of magnitude forthe same acetone concentration, or even within thesame sample of mixture, as observed in the opticalmicroscope images (Table 1 and Figures 1a1c),also due to gravitational segregation, which is veryefficient for the largest particles. Consequently, weidentify the mixtures by their acetone mass con-

tent, over which we have a much closer control,remembering the quantity and dimension of the

particles is anticorrelated with acetone content inthe mixture. This anticorrelation means that wecannot explore a broad range of crystallinity for

both low and high acetone content.

[8] If decompressed below the vapor pressure pBof acetone at the relevant temperature (pB19.424 kPa at 1520C, see e.g., http://www.s-ohe.com/acetone.html), GRA mixtures experienceacetone bubble nucleation and bubble growth,the mixture expands, and the initial acetone con-

tent is reduced to a level depending mainly onthe final pressure reached and on the history ofdecompression [Mourtada-Bonnefoi and Mader,2004]. GRA mixtures with initial acetone con-centration $1530 wt % have often been used asa laboratory analogue for high-silica magmas indecompression experiments [Phillips et al., 1995;

Lane et al., 2001; Blower et al., 2001, 2002;Mourtada-Bonnefoi and Mader, 2004; Stix andPhillips, 2012] because of their large viscosityincreaseof several orders of magnitudeonreduction of acetone content (Figure 1f and Ta-

ble 1; see also Phillips et al. [1995]). Mourtada-

Bonnefoi and Mader [2004] measured reductionsof about one third and two thirds of the initial

acetone content in decompression experimentsresulting in nonexplosive expansion and frag-mentation, respectively, which for 1525 wt %GRA mixtures at 18C (the laboratory tempera-ture during our experiments) corresponds to aviscosity variation from about 0.11 Pa s toabout 102 106 Pa s (and up to 1013 Pa s for astronger volatile depletion). The strong viscosityvariation may be at least partially linked to thevariation in solid fraction, and in terms of rheol-ogy, GRA mixtures might behave as suspensions[Costa et al., 2009; Cimarelli et al., 2011]. Theend product is a dry, strong foam similar to

pumice. Blower [2001] and Blower et al. [2001,2002] compared scanning electron microscope(SEM) images of natural pumice and fragmented20, 25, and 30 wt % GRA samples from fastdecompression experiments, documenting

polyhedral-shaped bubbles (with vesicularity ofabout 90%) with a power-law bubble size distri-

bution, which they interpreted as originatingfrom continuous nucleation processes in a highlysupersaturated fluid, where slow diffusion limitsthe growth of nucleated bubbles so that newones nucleate in the regions of the melt leastdepleted in volatiles. In this study, we also

explored the behavior of 3540 wt % GRA mix-tures. Their initial viscosity (of the order of 102

Pa s, Figure 1f) increases after the experimentsto about 100 102 Pa s, and they retain enoughacetone during degassing to maintain high mobil-ity and scarce to absent polymerization. The end

product of fragmentation is a bubbly liquid massthat becomes more diluted and flows down theglass tube walls when pressure returns to atmos-

pheric and part of the acetone returns into solu-tion. The 30 wt % mixtures show anintermediate behavior (see sections 2.3 and 3.1

for more details). By way of comparison with

Table 1. Densities, Viscosities, and Crystallinities of GRA Mixtures and Acetonea

Sol (wt %) Density (kg m3) Viscosity (Pa s)Particle NumberDensity (mm2)

Mean InterparticleDistance (mm)

15 11506 50 12.956 0.123 10206 50 0.366 0.0230 10006 45 0.06956 0.0013 450 0.036 0.0135 9246 30 0.0266 0.005 120 0.046 0.0140 9006 30 0.0126 0.001 25 0.16 0.02Gum rosin $1100Pure acetone $790 $0.0003

aThe uncertainties are representative of the variability of the solutions characteristics for different stirring time and laboratory temperature. Theparticle number density and the mean interparticle distance were estimated by counting the particles in the region bordered in white in Figures 1a1c for 30, 35, and 40 wt % GRA, respectively. A detailed particle size distribution is reported in Figure 1 for a wider set of mixtures and particledimensions.


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http://www.s-ohe.com/acetone.htmlhttp://www.s-ohe.com/acetone.htmlhttp://www.s-ohe.com/acetone.htmlhttp://www.s-ohe.com/acetone.html


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initial and degassed viscosities of magmas, weestimate that to match the viscosity of degassedrhyolite and basalt (about 108 109 Pa s and 101

102 Pa s, corresponding to 35 wt % and 1013 wt % for our GRA 18C, respectively), andconsidering the previously observed reduction toone third of the initial content of GRA mixturesafter fragmentation, we require GRA with initialcontents of 1015 wt % and 3040 wt % ace-tone, respectively (Figure 1f).

[9] The diffusivity of acetone in 2030 wt % GRAmixtures at 20C varies approximately linearlywith acetone content from 0.28 to 2.8 1011 m2

s1 [Blower, 2001], which is comparable to thediffusivity of water in basaltic magmas at a tem-

perature of about 9001100C and of 700900Cin rhyolitic magmas, or to the diffusivity of CO2 ata temperature of 700900C in hydrated rhyoliticmagmas and of 12001400C in basaltic magmas[Baker et al., 2005]. The surface tension of GRAis in the range 0.0280.030 J m2 [Phillips et al.,1995], higher than the surface tension of pure ace-tone at our experimental temperatures, which isabout 0.0230.024 J m2. However, we observethat macroscopic (>$0.2 mm in radius) gum rosincrystals sinking in the mixtures are the source ofcontinuous bubble nucleation forp


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PCI board. We recorded the experiments using ahigh-speed video camera (up to 2000 frames/s, Red-lake Motionscope) and a conventional video camera(25 frames/s).

2.3. Decompression Rates

[16] The aim of this study was to explore thebehavior of magma analogues for decompressionrates slow enough that the time scale of decom-

pression is comparable to the time scales of bubblenucleation and bubble growth by diffusion,

because the effect of slow decompression on these

processes is poorly understood.

[17] Classical theory for homogeneous nucleationpredicts the following nucleation rates as a func-tion of supersaturation:

J 2n20VmD =kT

1=2

a0exp

163

3kTP2

1

where n0 is the number density of volatile mole-cules, Vm is the volume of a molecule, D is the vola-tile diffusivity in the mixture/melt, k is the

Boltzmann constant, T is the absolute temperature,

a0 is the mean distance between volatile molecules,is the surface energy, andP is the supersatura-tion pressure [Toramaru, 1995; Yamada et al.,2005; Mangan and Sisson, 2005]. Employingappropriate values for GRA mixtures (n0 3.11 1027 m3, Vm 1.22 10

28 m3, a0 6.85 1011

m, and P 20 kPa) yields unrealistically lowrates of the order of exp(2.5 108) m3 s1. Thismeans that GRA mixtures will require a very longtime to nucleation if this occurs homogeneously.

[18] A quantity often used to characterize bubblegrowth through diffusion in magma is the Pecletnumber for volatile diffusion [Toramaru, 1995;Gonnermann and Manga, 2007] :

Pedif

difdec 2

[19] It describes whether the time scale of diffusionof volatile into bubbles (SR)2/D, where S is thedistance between bubble centers, R is the bubble ra-dius, and D is the diffusivity, dominates over thetime scale of decompression decpm/(dp/dt) (melt

pressure divided by decompression rate). If Pedif>> 1, supersaturation occurs. Assuming that bub-

bles nucleate immediately on our solid particles(SR $ interparticle distance, see Table 1), dif$30 s (see Table 1). In our fast decompression experi-ments, dec $10

4 s, so that supersaturation isexpected. In the slow decompression experiments,dec pB/(dp/dt)$20400 s, which is of the sameorder as dif. Hence, for our slowest decompressionrates, slow decompression should be accompanied

by supersaturation only if at least some of our par-ticles are ineffective as nucleation sites. If the par-ticles are all ineffective as nucleation sites, strongsupersaturation is predicted. This is additional evi-dence for GRA mixtures that have a high acetonecontent and a low particle content behaving similarlyto crystal-poor basaltic magmas during vesiculation,and for GRA mixtures that have a low acetone con-

tent but a high particle content behaving similarly tocrystal-rich silicic magmas.

3. Experimental Results

3.1. Observations From Fast

Decompression Experiments: A Regime

Diagram for GRA Mixture Behavior

[20] Upon fast decompression, GRA mixtures

show a range of different styles of expansion

Figure 2. Shock-tube apparatus. A shock tube, containing

the sample, is connected to a steel vacuum chamber via a

pneumatically controlled sliding partition. The vacuum cham-

ber is evacuated by a vacuum pump and is fitted with a vac-

uum breaker that can be set to leak atmospheric air into the

chamber so that a prescribed linear decompression rate is

achieved. An approximately linear decompression rate can

also be achieved through a leak valve operated manually.

Pressure is measured at three locations: vacuum breaker,

between leak valve and vacuum chamber, and within the

shock tube.


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depending on acetone content and final pressurespf(Figure 3):

(1) Acetone exsolution/bubble nucleation (occur-ring at 20.525.5 kPa across all acetone con-centrations): a few bubbles form. Theyoccasionally ascend, and burst at the surface.The mixture does not expand significantly.

(2) Boiling (occurring at pressures in the range1120.5 kPa for all mixture compositions) :

bubbles form continuously, coalesce, ascend,and burst. The mixture expansion is verysmall (see movie at http://www.youtube.com/

watch?vVOJiZe_JlHA).(3) Foaming (occurring only for 15 and 23 wt %

GRA at pressures in the range 19 kPa): themixture rapidly forms bubbles at its surface tocreate a foam. The mixture/foam columnexpands at low energy, with velocities of theorder of a few cm/s or slower. No fragmenta-tion is observed (the foam does not separateinto discrete pieces). Foaming is seen only forlow acetone concentrations, because on reduc-tion of acetone content, those mixtures

become very viscous and inhibit the movement

of bubbles, which become trapped, coalesce,

and expand (see movie at http://www.youtube.com/watch?vhGTJHkIBLHo). For high ace-tone concentrations, we see vigorous boiling atthe same pressure, as more acetone is availablein the liquid state to maintain low viscosity andhigh bubble mobility. Both foaming and vigor-ous boiling allow different degrees of permea-

ble degassing of the mixtures. The progressiveexsolution observed during foaming and vigor-ous boiling also shows how acetone undergoes

phase transition gradually in GRA mixtures,over an extended time period, when below the

boiling point.(4) Fragmentation (occurring at p $


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pressures of about 5 kPa were reached. Some-

times, we observed a few bubbles at pressuresgreater than 25 kPa, and these were interpreted asair bubbles because the pressure was significantlygreater than acetone vapor pressure. When thisoccurred, we always observed acetone bubblenucleation at pressures 18.025.5 kPa followed by

boiling. However, during about half of the experi-ments, we did not observe any bubbles nucleatingat 25 kPa, nor boiling at 20 kPa or at lower pres-sures. The mixture remained stable and unchangeduntil pressures of about 710 kPa were reached,and then the mixture fragmented (see Figure 6aand movie at http://www.youtube.com/watch?

vhoOY9u68yHw). The fragmentation pressurewas approximately the samewithin experimentaluncertaintiesnot only for a specific GRA con-centration but also across the range of concentra-tions 3540 wt %. The fragmentation pressurecorresponds to that observed in fast decompressionexperiments (see Figure 3). The movies reveal the

primary mechanism of fragmentation in some ofour experiments: no nucleation of bubbles isobserved (Figure 5a) until one single bubbleappears at the surface (Figure 5b), it expands for afraction of a second until it is roughly 23 cm in

diameter (Figure 5c), then it explodes (Figure 5d)

and triggers fragmentation in the bulk of the mix-ture through a pressure wave (see horizontal whitearrows in Figures 5d5f, 5i, and 6). The fragmen-tation proceeds layer by layer [Cashman et al.,2000], with explosive expansion occurring on thesurface layer of the mixture (inclined white arrowin Figure 5e), and migrating downward as a frag-mentation layer (inclined white arrows in Figures5f5h) at approximately constant velocity (Figure5l) as the mixture is ejected upward. Bubble nucle-ation is a local and unstable process, which

becomes global only once energy is transferred tothe bulk and periphery of the fluid mass, for exam-

ple, mechanically through a pressure wave [Cash-man et al., 2000]. In our experiments, the surfaceof the mixture is a favored location for bubble

nucleation and expansion; in magma, nucleationoccurs internally in the melt with additional ex-penditure of energy.

[23] In Figure 7a, we display detailed observa-tions from a few significant experiments. The

blue curve (Exp J3, 40 wt %) is representative ofmost of our experiments leading to fragmenta-tion, with no bubble activity whatsoeverobserved, until the mixture fragments at about 8kPa. The red and green curves correspond toexperiments disrupted by the expansion of air

bubbles prior to acetone nucleation. The violetand orange curves correspond to similar decom-

pression rates leading to opposite results : the firstone degassed efficiently and did not fragment,while for the second one, we observed bubblesnucleating and later being reabsorbed, and no fur-ther nucleation was observed until fragmentation.We interpret this apparent lack of determinism asdue to the intrinsic stochasticity of the bubblenucleation process.

[24] We did not observe slow-decompression frag-mentation in 30 wt % GRA when decompressedmanually. However, if the decompression rate iskept constant electronically using the vacuum

breaker (third set of experiments), the same vari-ability (or lack of reproducibility) of results isobserved (Figure 7b), except that very low pres-sures can be reached without any nucleation.When we reached the lowest pressure possiblewith our equipment without observing any exsolu-tion, we applied a small vibration to the shocktube (the impact of a fingernail at the tube wall)and immediately observed very powerful fragmen-tation. In other words, it appears that highly local-ized accelerations (and possibly decelerations)during decompression can stimulate bubble

nucleation.

Figure 4. Summary of the results from slow-decompression

experiments. All experiments started at atmospheric pressure.

The mixture was decompressed at about 100400 Pa s1

. The

typical behavior of the mixtures was to show acetone exsolu-

tion in the pressure range 2025 kPa. Mixtures 30, 35, and 40

wt % sometimes did not display that behavior, and we

observed fragmentation at about 710 kPa. We always

observed nucleation at pB for


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[25] In summary, delayed nucleation, followed byfragmentation, is favored in our experiments if thedecompression proceeds at a regular rate, as weobserved for a higher percentage of the experi-ments, and for the 30 wt % mixture, if we used thevacuum breaker.

4. Discussion of the Experimental

Results

[26] In some of our slow-decompression experi-

ments, nucleation is retarded until significantly

below the boiling point, even if solid particles arepresent. Large supersaturation leads to explosiveexpansion in those cases. We interpret the permea-

ble outgassing of


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[27] The solid particles of gum rosin contained inour 30, 35, and 40 wt % mixtures seem, in gen-eral, to be inefficient nucleation adjuvants, giventhat we observe nucleation to be delayed. Thismight arise from a particularly unfavorable wet-ting angle of acetone bubbles onto gum-rosin solid

particles, due to the compositional similarity ofthe liquid part of GRA mixtures to the particles:

the liquid would strongly wet the particles, mak-ing them poor substrates for gas nuclei [Manganet al., 2004]. However, compositional similarity

between the liquid and solid phases should behigher for low acetone content, making supersatu-ration more likely for mixtures low in acetone;this is the contrary of what we observe, as nuclea-tion results to be more efficient for our 23 and 30wt % mixtures. Higher compositional similaritymight be counterbalanced by the availability ofmore, larger particles, which provide a highernumber of nucleation sites or a larger total solid

surface as support for nucleation. Therefore, it is

plausible that heterogeneous and efficient nuclea-tion takes place in $30 wt% GRA mixtures, due to scarce and inefficientnucleation sites. The layer-by-layer explosive

expansion we observe during fragmentationis consistent with large supersaturation of the mix-ture and a nucleation mechanism close to homoge-neous [Toramaru, 1995; Cashman et al., 2000],with a pressure wave propagating through thesupersaturated fluid triggering progressive massvesiculation.

[28] Explosive expansion during slow decompres-sion seems to occur at about the same pressure notonly for a specific acetone concentration, but forall 30, 35, and 40 wt % GRA, at least for a similardecompression history. The linearity of the

Figure 6. Frames from the high-speed camera movie for experiment J3, recorded at 500 frames/s. Theexpansion velocity is very high (see Figure 5) but lower than the one observed during fast decompression

experiments. In the first frames (352 and 354), a big bubble can be seen to grow and burst. In frame 358 frag-

mentation starts.


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Figure 7. (a) The decompression history during five runs of the experiments is plotted in different colors.

The acetone content for the individual runs is indicated in the inlet, along with the description of the symbolsused to indicate naked-eye observations at distinct times and corresponding pressures. The blue curve (Exp

J3, 40 wt %%) is representative of most of our experiments leading to fragmentation. The decompression rate

was about 200 Pa s1 in the region of interest (


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decompression rate (constant rather than acceler-ated or decelerated) seems to promote delayednucleation in our experiments over a relativelylarge range of decompression rates. Our data setdoes not allow us to resolve any possible depend-ence of the final fragmentation pressure on acetoneconcentration. It seems reasonable that this de-

pendence is weak or absent, given that the solubil-ity of acetone does not depend on concentration,and all the acetone becomes potentially availablefor exsolution immediately below the boiling pointof acetone, pB, so that the amount of supersatura-tion of the mixture has a dramatic increase as soonas p


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2002] and a lower surface energy and hence alower barrier to nucleation than high-silica melts,so that bubbles tend to form early during ascentand volatiles tend to exsolve efficiently [Manganet al., 2004]. On the other hand, the presence ofcrystals in basaltic magma has a smaller disruptive

power than for high-silica magmas: network-modifying cations and dissolved volatile mole-cules are very efficient in disrupting the stronglylinked framework of highly polymerized melts,

but less so in low-silica compositions [Manganand Sisson, 2005]. The wetting angle of bubblesonto the same type of crystals is larger for high-silica magmas than for low-silica magmas, whereit is small (but nonzero, as bubbles still tend nucle-ate on crystals; see e.g. Figure 2 in Mangan et al.

[2004]). Hence, the distinction between homoge-neous and heterogeneous nucleation becomesblurred for such less polymerized, low-silica mag-mas, as Mangan and Sisson [2005] demonstratedfor dacite as opposed to rhyolite. Therefore, itseems reasonable that the nucleation-facilitatingeffects of a low-energy barrier to nucleation inlow-silica magmas could be compensated by adiminished efficiency of crystals in supportingnucleation, making delayed bubble nucleation aviable mechanism for high-energy explosive erup-tions of volatile-rich, poorly crystalline basalts.Our experimental results, where delayed bubble

nucleation and mass vesiculation occurred morefrequently on low-viscosity magma analogues,with a molecular structure that does not tend to po-lymerize such as that of GRA with high acetonecontent, support this argument. Experimental datafor basalts are needed in order to confirm orexclude this hypothesis.

4.3. Comparison With Published

Experimental Studies on Slow

Decompression of Magma Analogues

[35] Analogue experiments on the effects of

decompression rate may be divided into two cate-gories: those that use volatile-bearing fluids as themagma analogue, where bubble nucleation takes

place during the experiments [e.g., Phillips et al.,1995; Lane et al., 2001; Stix and Phillips, 2012],and those that use bubbly fluids, where preexisting

bubbles are introduced into the fluid before theexperiment starts [e.g., Namiki and Manga, 2006].

None of the published experiments of either typehas evidenced any explosive behavior duringdecompressions as slow as in our experiments. Wenow compare our observations with those from

previous studies.

[36] If decompressed at slow decompression ratesin the laboratory, bubbly fluids expand with vari-ous nonexplosive styles [Namiki and Manga,2006], which we also observe when nucleation isefficient, and describe generically as foaming.Since we find that bubble nucleation (not studiedin those experiments, which involved preexisting

bubbles) controls fragmentation during slowdecompression, our experiments complement pre-vious findings, rather than conflicting with them.However, Namiki and Manga [2006] find that theheight of the bubbly column is an important pa-rameter for the outcome of slow decompression;they suggest from theoretical arguments that if the

bubbly column in a volcanic conduit reachesheight> 1 km, then decompression rates typical of

lava effusion (102

103

Pa s1

) may lead low vis-cosity magma to nonequilibrium expansion. It ischallenging to compare that theory with our obser-vations as we do not know how much of the initialacetone exsolves, and at what rate, during expan-sion. Nucleation has been in fact observed to take

place progressively in GRA mixtures, as happensfor magma undergoing sudden decompression[Blower et al., 2001, 2002].

[37] Stix and Phillips [2012] decompressed GRAmixtures at very slow rates (down to 2080 Pas1), in apparatus similar to ours but with acetone

concentration in the range 1530 wt %. Theyobserved different degassing styles at different

pressures but no fragmentation in any of theexperiments. However, they did not explore 35and 40 wt % mixtures, which are the ones in whichwe observe fragmentation if decompressed atthose rates. Also, they did not apply a constantdecompression rate either manually or with a vac-uum breaker, and their decompression rate was notconstant but decreased with time.

[38] Air bubbles present in our mixtures seem tosuppress supersaturation and favor diffuse nuclea-

tion, probably because these air bubbles representstable nuclei of gas accumulation and, by growing,they release free energy. This is consistent withresults from Mourtada-Bonnefoi and Mader [2004],who also used GRA mixtures and found that nucle-ation is very efficient if solid particles of variousmaterials are present in a magma analogue. In thelight of our results, those experiments did notobserve any large deviation from equilibrium, prob-ably because the particles added to those GRA mix-tures were efficient nucleation sites or weretrapping tiny air bubbles, or just because in thoseexperiments


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[39] Additional analogue experiments could bedesigned in order to improve the similarity tomagma, for example, it could be attempted to dis-solve two different volatile species into gum rosin,in order to check whether the resulting solubilitylaw is more progressive. In order to clarify thenucleation dynamics in GRA solutions, experi-ments could also be designed to explore in detailthe properties of GRA mixtures, for example, sur-face energy and wetting angles. Also, it would bedesirable to measure the wetting angle of acetone

bubbles on gum-rosin particles immersed in fluidGRA mixtures [see, e.g., Mangan et al., 2004].This requires microscope images of the mixture atthe exsolution pressure of acetone (2025 kPa), asacetone is liquid if pressure is atmospheric. An

experiment could also be designed to study theend products of fragmentation during slow-decompression versus foaming resulting from effi-cient nucleation and degassing.

5. Formulation of a Conceptual Model

of Delayed Bubble Nucleation in

Low-Silica Volcanic Systems

[40] In summary, our analogue experiments sug-gest that the idea that crystal-poor low-silicamagma, carrying insufficient and inefficient bub-

ble nucleation sites, may build up large supersatu-ration if slowly decompressed, should be furtherinvestigated.

[41] We propose the following conceptual model ofdelayed, nonequilibrium degassing of a high- andlow-silica volcano as a possible explanation for asudden change in the eruptive regime, from effusiveto explosive. During effusive activity, the magmaascending in the conduits is decompressed at a slowrate, and volatile-rich, crystal-poor magma willfeed the conduits from below. With slow ascentrates, the flow will have low Reynolds numbers

even for low viscosities, so that no turbulent mixingcan promote bubble nucleation. If crystallinity isvery low and if the crystals present are of thenucleation-inefficient types, the ascending magmamay undergo delayed nucleation, supersaturating

progressively and becoming increasingly metasta-ble. Magma could supersaturate even in presence ofexsolved bubbles, provided their number density issmall and the magma is not sufficiently depleted involatiles through diffusion. Mass vesiculation istriggered either when this magma batch reaches aspecific p (which could correspond to reaching a

specific level in the plumbing system) or when it

reaches a specific location where its peripherycomes into contact with stored magma with a highcrystal content, for example, in a shallow reservoir.This contact may induce bubble nucleation at the

periphery of the magma batch, be rapidly transmit-ted as a pressure wave throughout the whole vol-ume of supersaturated magma, and cause anexplosive expansion of the magma column involatile-coupled conditions. The explosive expan-sion may be accompanied or followed by masscrystallization, due to a sudden drop of the liquidustemperature [Hort, 1998]. The fragmentation sur-face propagates downward, layer by layer, until the

batch of supersaturated magma is exhausted. Thepower of the explosive expansion depends on thelevel of supersaturation p and on the volatile con-

tent. The duration of the explosive expansiondepends on the mass or height of column of super-saturated magma available and on the geometry ofthe plumbing system (the total energy will dependon the three factors). The reason why explosive ba-saltic eruptions are observed only episodically mayultimately result from the low likelihood of manysimultaneous conditions that need to be satisfied forthe mechanism to occur.

[42] We expect this mechanism to be generally rel-atively short lived and isolated (once the magma

batch is exhausted or the reservoir is empty, the ex-

plosive expansion ceases, and before a new explo-sive eruption occurs, the system needs first toregain stability and to accumulate volatile-richmagma, and then to undergo slow decompression)and to produce materials tapped directly fromdeeper reservoirs. This is consistent to whatobserved during the last 10 years of close observa-tion at Stromboli, where the usual mildly explosiveactivity is associated with high porphiritic magmafrom the upper reservoir, containing nucleation-facilitating crystals such as titanium and ironoxides, and where paroxysms, and to some extentmajor explosions, are associated with low porphir-

itic blond magma from a deep reservoir, wherenucleation-facilitating crystals are not found [e.g.,

Metrich et al., 2001; Pichavant et al., 2009].

[43] Earthquakes or any other form of pressurewave shaking supersaturated basaltic magmastored in conduits may also trigger delayed nuclea-tion (similar to the explosive expansion of oursupersaturated mixture resulting from the impactof a fingernail on the shock tube); if this occurs,the intensity of the response of the magmatic sys-tem should depend on the degree of supersatura-tion reached.


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[44] In the case of sudden decompression, massvesiculation and crystallization occur releasing atonce the energy provided by the decompression,while in case of slow decompression, the energy isstored slowly in the magma and released later in ashort time interval as a cascade effect.

6. Conclusions

[45] The conceptual model presented here is con-sistent with the physics of phase transition in mul-ticomponent mixtures and compatible withobservations from published results on decompres-sion of remelted magma samples. It offers a possi-

ble explanation for high-energy low-silica

explosive eruptions, which remain unexplained.Although petrological studies are required to dem-onstrate that delayed bubble nucleation followed

by explosive expansion can really apply to basalticsystems in general and specifically to a given erup-tions at a given volcano, this model suggests thatdecompression due to lava effusion, which is gen-erally considered a low-risk eruptive style, can

potentially trigger powerful explosive eruptions.The eruption process would actually be triggeredwhen decompression starts, but an explosive erup-tion would only occur when sufficient magma hasspilled from the conduit [Calvari et al., 2011], that

the pressure drop exceeds that capable of beingsustained by delayed nucleation, with the extrudedmagma volume being a proxy for the pressure dif-ferential p required for fragmentation.

Acknowledgments

[46] F. Maccaferri, S. Paillat, C. Cimarelli, and various stu-

dents from the Geological Fluid Dynamics Laboratory in the

School of Earth Sciences, University of Bristol, helped during

the experiments. Dan Morgan helped with the quantification

of crystallinity. Discussion with S. Lane and J. Neuberg and

insightful comments by M. Mangan and two anonymous

reviewers helped improving the manuscript. This project was

funded by the Italian Civil Defense Agency and the Istituto

Nazionale di Geofisica e Vulcanologia (project INGV-DPC

Paroxysm V2/03, 20072009) and by an ERC Starting Grant

(project CCMP-POMPEI). Logistic support by Cen/DTU

(Denmark) is gratefully acknowledged.

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