Complex interaction between Strombolian and phreatomagmatic eruptions in the Quaternary monogenetic volcanism of the Catalan Volcanic Zone (NE of Spain)

Journal of Volcanology and Geothermal Research 201 (2011) 178–193

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Complex interaction between Strombolian and phreatomagmatic eruptions in theQuaternary monogenetic volcanism of the Catalan Volcanic Zone (NE of Spain)

Joan Martí a,⁎, Llorenç Planagumà b, Adelina Geyer c, Esther Canal b, Dario Pedrazzi a

a Institute of Earth Sciences “Jaume Almera”, CSIC, Lluis Solé Sabaris s/n, 08028 Barcelona, Spainb Tosca, Environment Services of Education. Casal dels Volcans, Av. Santa Coloma, 17800 Olot, Spainc CIMNE, International Center for Numerical Methods in Engineering, Edifice C1, Campus Nord UPC, Gran Capità, s/n, 08034 Barcelona, Spain

⁎ Corresponding author. Tel.: +34934095410; fax: +E-mail address: [email protected] (J. Martí).

0377-0273/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.jvolgeores.2010.12.009

a b s t r a c t

Article history:Received 1 September 2010Accepted 11 December 2010Available online 22 December 2010

Keywords:Catalan Volcanic Zonemonogenetic volcanismphreatomagmatismeruptive sequences

The Catalan Volcanic Zone (CVZ), at the NE of the Iberian peninsula, is one of the Quaternary alkaline volcanicprovinces of the European rifts system. The CVZ has been active during the last 12 Ma. Despite the fact thatthis volcanism is significant in extension and volume, and that eruptions have also occurred in Holocenetimes, it is mostly unknown compared to the contemporaneous alkaline volcanism in other parts of Westernand Central Europe. Volcanism younger than 0.5 Ma is mostly concentrated in an area of about 100 km2

located between the main cities of Olot and Girona. This basaltic volcanic field comprises more than 50monogenetic cones including scoria cones, lava flows, tuff rings, and maars. Magmatic eruptions range fromHawaiian to violent Strombolian. Phreatomagmatism is also common and has contributed to the constructionof more than a half of these volcanic edifices, frequently associated with the Strombolian activity but alsoindependently, giving rise to a large variety of eruptive sequences. We describe themain characteristics of thisvolcanism and analyse in particular the successions of deposits that form some of these volcanoes and discussthe potential causes of such a wide diversity of eruptive sequences. We find that the main cause of suchcomplex eruptive behaviour resides in the stratigraphic, structural and hydrogeological characteristics of thesubstrate above which the volcanoes were emplaced, rather than on the compositional characteristics of theerupting magma, as they do not show significant variations among the different volcanoes studied.

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1. Introduction

Monogenetic basaltic zones are common in many volcanicenvironments and may develop under very different geodynamicconditions (Francis, 1993; Connor and Conway, 2000; Walker, 2000).A particular characteristic of this type of volcanism is the largediversity of eruptive styles, morphologies and deposits that it maydisplay despite the usual monotony in magma composition(Houghton et al., 1999; Connor and Conway, 2000; Parfitt, 2004;Valentine and Gregg, 2008). Strombolian, violent Strombolian,subplinian and even plinian magmatic events are described ascommon in such volcanic environment. More complicated is thevariation in the eruptive styles when magma–water interactionoccurs, being this another common feature in many monogeneticvolcanic fields (Houghton et al., 1999; White and Houghton, 2000).

The physics of phreatomagmatismhas been studied in detail since the70 s and this has allowed to obtain several experimental and theoreticalmodels that constrain our understanding of theway inwhichmagma andexternal water interact and of the large diversity of deposits that thisexplosive interactionmay generate (Lorenz, 1973; Sheridan andWohletz,

1981, 1983; Wohletz, 1983; Wohletz and Sheridan, 1983; Wohletz andMcQueen, 1984; Wohletz, 1986; Lorenz, 1986, 1987; Zimanowski et al.,1991; Zimanowski, 1998; Morrisey et al., 2000). In monogenetic basalticvolcanism, phreatomagmatic activity may be related to the interaction ofmagma with surface and/or ground water, and the style and products ofthe resulting eruptions will depend on degassing patterns, magma ascentrates and degrees of interaction with external water (Wohletz, 1986;Houghton et al., 1999; Morrisey et al., 2000). Interpretation of deposits,including facies analysis,morphometric characterisationofpyroclasts, andgrain size distribution and component analysis, constitutes the essentialtool to identify and reproduce the sequence of events involved inphreatomagmatism and to evaluate its potential hazard in case of activeareas (Fisher and Waters, 1970; Heiken, 1971; Lorenz, 1973; Wohletz,1986; Lorenz, 1986, 1987; Brand and Clarke, 2009; Brand et al., 2009;Sottili et al., 2009).

Although there may exist clear similarities between the eruptiveactivity displayed by differentmonogenetic volcanic fields, it is also truethat important differences may arise when each succession of deposits(i.e.: eruptive sequence) is investigated in detail. These differences maybedue to changes inmagmacomposition (e.g.: volatile content),magmasupply rate, local tectonics, distribution and characteristics of aquifers,etc., from one volcanic field to another (Vespermann and Schmincke,2000;Walker, 2000). One example of this diversity amongmonogenetic

179J. Martí et al. / Journal of Volcanology and Geothermal Research 201 (2011) 178–193

volcanic fields is provided by theQuaternary volcanismdeveloped alongthe European rifts system (Fig. 1), which includes several volcanicprovinces all them related to the same geodynamic event but withdistinct local tectonicsand lithospheric and shallowgeological structures(Wilson and Downes, 1991, 1992; Downes, 2001).

One of the least known and understood regions of the Quaternaryalkaline volcanism in Europe is the Catalan Volcanic Zone (Martí et al.,1992) (Fig. 1). The age of this volcanism ranges from N12 Ma to earlyHolocene and it is mainly represented by alkali basalts and basanites(Cebriá et al., 2000). Small-sized scoria cones were produced duringmonogenetic short-lived eruptions associated with widely dispersedfractures of short lateral extent. Important phreatomagmatic events alsooccurred giving rise to awide diversity of eruptive sequences (Martí andMallarach, 1987). The total volumeof extrudedmagma in each eruptionis relatively small (0.01–0.2 km3), suggesting a relatively low magmasupply rate.

In this paper we present an outline of the Quaternary monogeneticvolcanism of the Catalan Volcanic Zone, in which we describe themain features of the eruptive activity of this volcanic field.We providea general characterisation of the erupted products in order to classify

Fig. 1. Simplified geological map of the Catalan Volcanic Zone and its surroundings (modifistudied area is indicated by a squared frame.

them in terms of eruption dynamics, rather than a thoroughdescription of the deposits and their interpretation in terms ofemplacement mechanisms, which by themselves would deserve aseparate study. We pay special attention to the diversity ofphreatomagmatic episodes that can be recognised in this volcanismand discuss the possible causes for the occurrence of such a largevariety of eruptive sequences in a rather small area and associatedwith a monotonous magma composition.

2. Geological setting of the CVZ and regional stratigraphy

Cenozoic alkaline volcanism is widely distributed along an extensiverifts system in central andwestern Europe, including the Rhenishmassifand Rhinegraben of Germany, the Massif Central of France, and thewestern PannonianBasin in Eastern Europe (Downes, 2001) (Fig. 1). Thecausalmechanism(s) of this rifts system is poorly understood. However,Hoernle et al. (1995) found that Sr, Nd and Pb isotope data for lavas fromcentral Europe to the eastern Atlantic Ocean and the westernMediterranean may be explained as the result of mixing between

ed from Guérin et al., 1985). In the inset, distribution of the European rifts system. The

180 J. Martí et al. / Journal of Volcanology and Geothermal Research 201 (2011) 178–193

several mantle components, one of which is a low-velocity component(LVC) common to the different regions.

Probably, the least known episode of Cenozoic alkaline volcanism inEurope is theone related to theValencia Trough(Martí et al., 1992;Muñozet al., 2005). The Valencia Trough is a NE–SW oriented Neogene basinlocated between the Iberian Peninsula and the Balearic promontoryoffshore of northeastern Spain (Fig. 1). TheValencia Troughhas a complexgeological history. Two main stages of magmatism have been identified(Martí et al., 1992). During Early to Middle Miocene time, the area wassubjected to compressional tectonics accompanied by calc-alkalinevolcanism. This was followed by a period of extensional tectonics andmafic alkaline volcanism from middle Miocene to Recent time. Thegreatest concentration of Middle Miocene to Recent volcanism in theregion is found in the Catalan Volcanic Zone (CVZ) at theNE of the Iberianpeninsula (Fig. 1). Despite the fact that this alkaline volcanism showsstrong similarities with the contemporaneous alkaline magmatism inother parts ofWestern and Central Europe, it has been poorly studied andits significance is still not well understood.

Available data indicate that themafic volcanic products of the CVZ, liketheparentalmagmasof the cumulatexenoliths, range fromstrongly silica-undersaturated tonearly silica-saturatedcompositions (Arañaet al., 1983;López-Ruiz and Rodriguez-Badiola, 1985; Martí et al., 1992; Cebriá et al.,2000). This region comprises a suite of intracontinental leucite basanites,nepheline basanites and alkali olivine basalts, which in most casesrepresent primary or nearly primary liquids (Cebriá et al., 2000). Thegeochemical characteristicsof these lavasarevery similar to theanalogouspetrologic types of other Cenozoic volcanics of Europe, which areintermediate betweenHIMUmantle, depletedmantle (DM) and enrichedmantle by mineralised deep fluids (EM1) (Cebriá et al., 2000; Downes,2001). Geochemical and isotopic signatures of magmas suggest theparticipation of at least a sublithospheric component and an enrichedlithospheric component with a K-bearing phase in the mantle source(Martí et al., 1992; Cebriá et al., 2000). Cebriá et al. (2000) proposed ageochemical model involving the generation of a hybrid mantlelithosphere source produced by the infiltration of the sublithosphericliquids intoenricheddomainsof themantle lithosphere, shortlybefore themelting event that generated the CVZ lavas.

The area has traditionally been divided into three different sub-zones: L'Empordà to the Northeast (N12–8 Ma), La Selva (7.9–1.7 Ma)to the south and La Garrotxa (0.5–0.01 Ma) to the west (Araña et al.,1983; Martí et al., 1992) (Fig. 1). The total volume of extrudedmagmaseems to increase progressively from the early episodes (L'Empordà)to the later ones (La Garrotxa). Thus, a progressive and concomitantincrease of the volume of magma generated, as well as an increase inthe degree of partial melting, can be observed in the geochemistry ofthe rocks from the CVZ (Araña et al., 1983; Martí et al., 1992). Somevolcanoes of the La Garrotxa sub-zone, contain ultramafic to maficxenoliths. The xenoliths comprise pyroxenites, melanogabbros,amphibolites and spinel lherzolites, the pyroxenites being the mostabundant. Pressure and temperature estimates for these xenolithssuggest that theymay have crystallised inmagma chambers located atthe crust–mantle boundary (Neumann et al., 1999), which accordingto geophysical estimates would be located at a depth of ~30 km(Gallart et al., 1984; Fernández et al., 1990; Gallart et al., 1991). Thesegeophysical studies also indicate that the CVZ is characterised by aregionally thinned lithosphere, about 60–70 km thick, by a highelevation and a high thermal gradient, suggesting that the area isaffected not only by the topographic load of the Pyrenees but also bythe opening of the Valencia trough. The local structure of the area iscomposed of a set of horsts and grabens bounded by a NW–SE systemof Neogene normal faults that determines the distribution ofvolcanism and the fluvial network (Saula et al., 1995).

The lithostratigraphic units that crop out in the studied area and thatform the substrate above which the volcanic edifices were emplacedcorrespond tomaterials of upper Palaeozoic, EoceneandQuaternary age.Due to the Alpine folding, the Neogene normal faulting system, and

further erosion, the substrate varies under each volcano. The oldest unitwe can recognise corresponds to the schist, gneiss, granodiorites andgranites of Permo-Carboniferous age. This unit is unconformablyoverlaid by the Eocene Formations that from base to top include: 1)the blue marls and gypsum of the Banyoles Formation; 2) themarls andbrown sandstones of the Bracons Formation; 3) the red sandstones,mudstones, and conglomerates of the Bellmunt Formation; 4) theglauconite sandstones and conglomerates of the Folgueroles Formation;andfinally 5) the grey sandstones andmarls of the Rocacorba Formation.Filling the bottom of the valleys and unconformably lying on theprevious units there are unconsolidated gravels, clay and sands andalluvial deposits, which together with lava flows and pyroclasticproducts form the Quaternary succession. The Palaeozoic terrains, theBellmunt Formation, and the Quaternary deposits constitute the mainaquifers of the area, but the base of the Folgueroles Formation and theBanyoles Formation may also act as aquifers in some sectors of thestudied area.

The age of the CVZ volcanism is not well constrained. Availabledata indicate that volcanic activity started about more than 12 Ma agoand continued till the beginning of the Holocene (Fig. 2). However,stratigraphic relationships suggest that younger eruptive events mayhave occurred.

3. Deposits successions and eruptive sequences

Thebest preserved outcrops of volcanic rocks from the CVZ are foundin La Selva and La Garrotxa sub-zones (Fig. 1). In this area more than 50well preserved volcanic cones can be recognised, and we have groupedthem into two different sectors, one at the north (N sector)corresponding to the Fluviá river basin, and one at the south (S sector)coinciding with the Ter river basin (Fig. 3). The main concentration ofvolcanic cones corresponds to the N sector, where there are more than30 cones, while the S sector contains no more than a dozen of them, butincludes the largest ones. The substrate on which these monogeneticvolcanoes stand differs from one sector to the other. At the north thevolcanic rocks lie on Tertiary sediments while towards the south theyrest in most cases directly on the granites and schists of the Palaeozoicbasement. In the studied area, volcanic activity occurred sporadicallyover a time period ranging from N500,000 to about 11,000 years ago,with aneruptive eventevery15,000 to20,000 years (Guérin et al., 1985).

Themost recent activity in theCVZwascharacterisedby thepresenceof small-sized scoria cones that were produced during short-livederuptions associated with widely dispersed fractures of short lateralextent. The total volume of extrudedmagma in each eruptionwas small(0.01–0.2 km3 DRE), suggesting that the magma available to feed eacheruption was also very limited. Strombolian and phreatomagmaticepisodes alternated in most of these eruptions giving rise to complexstratigraphic successions composed of a wide range of pyroclasticdeposits (Martí and Mallarach, 1987). The eruptive sequences that maybe deduced from these successions of deposits differ from one cone toanother and reveal that eruptions did not follow a common pattern,particularly for what concerns to phreatomagmatic episodes.

All the volcanoes studied were constructed during single eruptions(i.e.: they must be referred to as monogenetic) commonly includingseveral distinctive phases. We can consider two groups of volcanicedifices depending on whether or not phreatomagmatic activitycontributed to their construction. The volcanoes exclusively derivedfrom magmatic activity correspond to scoria cones, symmetrical orhorseshoe shaped, built by the accumulation of scoria, with occasionalemissions of lava flows. Examples of this type of activity are thevolcanoes of Puigalós, Puig de Martinyà, San Marc, Roca Negra, and PuigSubià (Fig. 4). Volcanic cones including phreatomagmatic activity aremuch more complex, although morphologically they are similar to thescoria ones. They may alternate phreatic phases produced by vapourexplosions that only erupted lithic clasts from the substrate, with typicalphreatomagmatic phases that generated a wide diversity of pyroclastic

Fig. 2. Age of the Catalan Volcanic Zone volcanism (data from Donville, 1976; Araña et al., 1983, and Guérin et al., 1985).

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density currents and fallout deposits, with typically Strombolian phasesincluding explosive and effusive episodes. The resulting eruptivesequences that can be deduced from the successions of deposits showsubstantial variations among the cones, indicating their differenteruptive behaviour. Examples of this type of activity are representedby the volcanoes of Santa Margarida, Croscat, Garrinada, Montsacopa,Can Tià and Cairat in the N sector (Fig. 4), and the volcanoes of Puigd'Adri, Puig de Banya del Boc, Clot de l'Omera, Granollers and SantDalmai in the S sector (Figs. 3 and 5). Table 1 summarises the differenteruptive sequences deduced for the studied volcanoes. We describe inthe later part some of the most representative examples of the recentCVZ volcanism.

3.1. Croscat and Santa Mararida volcanoes

The Croscat and Santa Margarida volcanoes are located at theinterior of an eroded anticline of Eocene rocks (Figs. 6 and 7). They arethe most representative edifices of the N sector of the studied area.Despite they have been traditionally considered as two separatevolcanoes with independent eruption dynamics (Mallarach, 1982;Martí and Mallarach, 1987), new stratigraphic data reveal that theybelong to the same eruptive episode. Part of the eruptive sequence ofthe Croscat volcano has been investigated in detail by Di Traglia et al.(2009) and Cimarelli et al. (2010), who pointed out the transitionbetween different eruptive styles in basaltic monogenetic volcanismand emphasised the role of phreatomagmatism in triggering violentStrombolian eruptions. However, new data from field work conductedin this study and water boreholes drilled in the vicinity of the Croscatvolcano reveal that the eruption history of this volcano is morecomplex than envisaged by Di Traglia et al. (2009) and Cimarelli et al.(2010), being intimately related to that of its neighbour SantaMargarida volcano (Figs. 7 and 8).

The Croscat and Santa Margarida volcanoes, together with LaPomareda cone, lie on a 3 km long eruption fissure oriented NW–SW(Figs. 6 and7). The eruption started at the southern endof thefissurewitha vent opening phreatomagmatic phase that excavated a relatively large(350 m across and 70 m deep) explosion crater in the Eocene substrate.This first explosive episode formed the Santa Margarida crater andgenerated amassive lithic rich pyroclasticflowdeposit, only visible on theeastern flank of the Santa Margarida volcano, and several widespreadmedium to coarse grained pyroclastic surges, and associated fine-ashdeposits, which covered most of the area forming the unit on which theCroscat succession built up (Figs. 8 and 9a). This phreatomagmatic phasewas followed by a short Strombolian phase that generated a thin, lithic-rich, coarse scoria lapilli fallout deposit that overlaid the previous depositsin the vicinity of the Santa Margarida crater. Immediately after this firstphreatomagmatic phase the eruption progressed along the central andnorthern sectors of the fissure with the extrusion of basaltic magma andgenerated massive spatter and occasionally rheomorphic, welded scoriaagglomerates, all them forming the first cone-building deposits of Croscatand La Pomareda. No more magma was erupted during this and furtherphases through the Santa Magarida crater.

Later on, the eruption concentrated in the central part of the fissure,changing from fissural (Hawaiian) to a central conduit (Strombolian)and giving rise to the construction of the rest of the Croscat scoria cone.The Croscat Strombolian activity generated twomain scoria fallout units(Fig. 8). The lower one conformably overlies the basal spatter and isformed by a several metres thick, poorly stratified coarse lapilli sizescoria depositwith several scoria bomb beds . The upper unit constitutesthemain volume of the Croscat cone and is formed by awell stratified tothinly laminated,mediumtofine lapilli size scoria deposit,more than tenmetres thick that contains sparse scoria bombs and blocks. The upperscoria lapilli unit also forms most of the intermediate to distal outcropstowards the east of the volcano, being recognised at distances farther

Fig. 3. Satellite image of the studied area with indication of N and S sectors described in the text.

182 J. Martí et al. / Journal of Volcanology and Geothermal Research 201 (2011) 178–193

than 5 km. It also covers thePomareda spatter and thephreatomagmaticdeposits and the explosion crater of SantaMargarida (Fig. 8). The contactbetween the lower and upper scoria lapilli units is clearly marked by achange in the internal stratification of the deposits and the grain-size ofthe scoria clasts. The vesicularity of the scoria lapilli clasts ranges from57% to 78% and their morphology is mostly characterised by irregularshapes with spherical vesicles but woody-shaped, highly vesicularstretched scoria fragments are occasionally present in the lower unit. Adetailed morphoscopic and textural analysis of the Croscat scoria hasbeen carried out by Di Traglia et al. (2009).

Towards the top the upper scoria lapilli unit is characterised by theappearance of centimetric-sized lithic clasts from old lavas, whichprogressively increase in abundance, thus defining a gradual changeto the uppermost unit of the Croscat pyroclastic succession. Thistransition is also defined by a decrease in the degree of vesicularity ofthe scoria lapilli and the appearance of blocky shaped ash fragmentswhich become predominant at the overlaying unit. These textural andcompositional characteristics suggest a transition from magmatic tophreatomagmatic activity during the last explosive episodes of theCroscat eruption, which is also supported by the sedimentologicalcharacteristics of the uppermost unit of the Croscat pyroclasticsuccession. This corresponds to a lithic-rich, thinly laminated unit,of a fewmetres thick, which extends for several kilometres to the eastchanging from planar to cross-bedded stratification from proximal todistal facies (Fig. 9b). The last eruptive phase of Croscat is representedby a lava flow the emplacement of which caused the breaching of thewestern flank of the cone. This lava covered an area of 5 km2 andflowed more than 10 km to the west, with an average thickness of

10 m. The total volume of magma (DRE) emitted during the Croscatand Santa Margarida eruption is of the order of 0.2 km3.

The gradual transition between the phreatomagmatic depositsfrom the Santa Margarida crater and the Strombolian scoria close tothe vent area indicates that some magma continued erupting for awhile without interacting with groundwater through the sameconduit. The absence of paleosoils and erosion surfaces betweenthese phreatomagmatic deposits and the fissure-vent derived scoriaand spatter that crops out at the base of the Croscat and La Pomaredasuccessions, indicates that both deposits were emplaced sequentially.This suggests that shortly after the first phreatomagmatic episodemagma migrated through the fissure towards the north–west andstarted to erupt along it forming the spatter deposits. After that,eruption concentrated at the middle sector of the fissure forming acentral conduit and gave rise to the construction of the Croscat cone.The products from the Croscat Strombolian phase also reached distalareas to the east mantling the whole Santa Margarida crater and itssurroundings.

This interpretation differs from that given by Di Traglia et al. (2009)and Cimarelli et al. (2010)who placed the first phreatomagmatic episodebetween the two scoria lapilli units of Croscat. According to this, theyproposed that the first phreatomagmatic phase occurred during thewaning of the scoria cone phase (lower lapilli unit) and suggested that itcaused a significant change in the eruption dynamics conditioning theoccurrenceof aviolent Strombolianphase (upper lapilli unit).However, asexplained previously, field andwater borehole stratigraphic relationships(Figs. 7 and 8) reveal that the first phreatomagmatic phase preceded theinitiation of the construction of the Croscat cone and corresponds to the

Fig. 4. Simplified geological and structural map of the N sector of the studied area with indication of the location and name of the main volcanic cones.

183J. Martí et al. / Journal of Volcanology and Geothermal Research 201 (2011) 178–193

vent opening phase that originated the Santa Margarida crater, lattermantled by the Croscat scoria (Fig. 8).

3.2. Can Tià

It forms part of the group of three cones (Fontpobra, Tuta de Colltort,and Can Tià) that are located close to the scarp formed by the westernmargin of the eroded Eocene anticline (Figs. 4 and 7). The Can Tiàvolcano corresponds to amaar-type construction,with a crater of 270 min diameter and 20 m deep that shows a flat bottom. It is asymmetricalshowing a higher rim sector towards the south. The stratigraphicsuccession of this volcano is only composed of pyroclastic deposits. Itincludes four different units from base to top (Fig. 8). The lower unitcorresponds to a 10 cm thick deposit composed of small (b1 cm)juvenile and lithic clasts in an ashy matrix, which rests unconformablyon a palaeosoil. The second unit is formed by a poorly stratified, non-welded, highly vesicular scoria lapilli deposit, up to 6 m thick, whichcontains a few lithic clasts, someof block size. The thirdunit conformablyoverlies the previous one and is a 1.5 m thick thinly laminated, wellsorted, fine-grained scoria lapilli deposit rich in lithic clasts of variable

size (≤2 to 30 cm) of Eocene red sandstones, with an interbedded ashlayer at theupper part. Thenumber of lithics increases gradually towardsthe top of the deposit, thus suggesting the initiation of a newphreatomagmatic phase. The uppermost unit of the succession showsa planar contact with the underlying scoria lapilli and corresponds to amassive pyroclastic flow deposit, up to 3 m thick, which containsabundant Eocene lithic clasts and highly vesicular scoria lapillifragments, all them surrounded by a lithic-rich, ash matrix nearlycompletely transformed into clay minerals, zeolites and iron oxides.

The succession of deposits observed in this volcano reveals how theexplosive activity initiated with a short explosive event, probably ofphreatomagmatic nature according to the composition of the resultingdeposit. Then, theeruption immediately changed intomagmatic (secondand third units), and again into phreatomagmatic (upper part of thirdunit and fourth unit). The succession of deposits and the distribution oflithics reveal that the first change in the eruptive behaviour (fromphreatomagmatic to magmatic) was abrupt but the second one (frommagmatic tophreatomagmatic)wasgradual. Also, thenatureof the lithicclasts found in these deposits clearly indicates a variation in the positionof the fragmentation level during the course of the eruption and provide

Fig. 5. Simplified geological and structural map of the northern side of the S sector of the studied area with indication of the location and name of the main volcanic cones.

184 J. Martí et al. / Journal of Volcanology and Geothermal Research 201 (2011) 178–193

the clues to understand the dynamics of this eruption (see Discussion).Although all the lithic clasts found correspond to Eocene rocks that formthe substrate below Can Tià volcano, they concentrate differentlydepending on each phase of the eruption. The first unit contains mostlylithics from the Bellmunt Formation, which constitutes themain aquiferin the area and is located at about 300 m below the surface. The lithicclasts found at the base of the second unit are grey sandstones from theRocacorba Formation, the uppermost stratigraphic unit in this sector ofthe studied zone. Towards the upper part of this unit lithics of brownsandstones from the Folgueroles Formation appear in a significant

Table 1Diversity of eruption sequences deduced from the deposit successions of the studiedvolcanoes.

Volcano type Eruptive sequence (from beginning to the end)

Puig d'Adri Phreatomagmatic–Strombolian–Phreatomagmatic–Strombolian–Hawaiian

Crosa de Sant Dalmai Phreatomagmatic–Strombolian–Phreatomagmatic–Strombolian

Santa Margarida–Croscat Phreatomagmatic–Strombolian–Phreatomagmatic–Hawaiian

Garrinada Hawaiian–Strombolian–Phreatomagmatic–HawaiianCan Tia Phreatomagmatic–Strombolian–PhreatomagmaticMontsacopa Hawaiian–Strombolian–PhreatomagmaticPuig de Banya de Boc Phreatomagmatic–Strombolian–HawaiianCairat Phreatomagmatic–StrombolianClot de l'Omera PhreatomagmaticRoca Negra Strombolian

proportion. The third unit is characterised by theprogressive appearanceagain of red sandstone lithic clasts belonging to the Bellmunt Formation,located deeper in the stratigraphic sequence of the area. These lithicclasts become clearly predominant towards the top of this unit andconstitute themain lithic fractionof the fourthunit,which represents theculmination of the secondphreatomagmatic phase andmarks the endofthe eruption.

3.3. Cairat

The Cairat volcano is located on the eastern flank of the erodedEocene anticline that limits the Olot depression at the north–east(Fig. 7). The Cairat is a maar-type volcano with a crater of 120 m ofdiameter excavated in the Eocene substrate. It is one of the fewexamples in the studied area nearly exclusively composed ofphreatomagmatic deposits. The pyroclastic deposits that form thisvolcanic edifice were preferentially emplaced to the north and southof the crater. The characteristics of the deposits and the nature of theabundant lithic clasts they content suggest that most of the eruptiveactivity of the Cairat volcano involved interaction of the eruptingmagma with groundwater from the main Eocene aquifer (Martí andMallarach, 1987). The succession of pyroclastic deposits of the Cairatvolcano is composed of a 20 m thick succession of lithic rich explosionbreccias and lapilli-sized fallout, and pyroclastic surge deposits(Fig. 9c and d)). The main characteristic of this succession of depositsis the presence of abundant lithic clasts from the Eocene basement,which in this area is formed (from top to base) by the Banyoles

Fig. 6. Panorama of the N sector with indication of the main volcanic cones and morphological features.

185J. Martí et al. / Journal of Volcanology and Geothermal Research 201 (2011) 178–193

Formation (blue marls), the Bracons Formation (grey sandstones andlutites), and the Bellmunt Formation (red sandstones and lutites).These lithic clasts range in size from a few centimetres up to 2 m.Although the distribution of the largest blocks is rather irregular theytend to concentrate towards the base of the sequence and in somelithic-rich units in the middle and upper parts. Some of the lithicsfrom the Bellmunt Formation are deeply hydrothermally altered.Juvenile fragments are less abundant than lithic clasts and correspondto poorly vesicular scoria lapilli, a few cauliflower bombs, and blockyshaped ash fragments.

The eruptive activity of the Cairat volcano mostly produced lithicbreccias,with amassive emplacement of ballistic blocks, and somemoreenergetic episodes that generated thinly bedded, pyroclastic densitycurrents. The locationof theventat ahill's crest,with steep slopes at bothsides, conditioned the accumulation of volcanic materials which wereaffected by continuous sliding until they redeposited on a more stableslope. This implied a continuous syn-depositional remobilisation of theoriginal pyroclastic products deposited on the highest parts. At thenorthern side the products of this continuous debris avalanching werechannelised inside a pre-existing gully where they eroded andincorporated part of the non-volcanic sediments that existed there.This also contributed to the large variety of lithic fragments found inthese pyroclastic deposits and the chaotic aspect of some of the units.However, it is still surprising the relative significant amount of lithicsfrom the Bracons and Banyoles formations, both older and locatedstratigraphically deeper than the Bellmunt Formation, the stratigraphicunit that constitutes the main aquifer in this area. The reason for theappearance in the deposits of this volcano of lithic clasts fromstratigraphic levels located below the aquifer that interacted with theerupting magma is purely tectonic, as in this particular site the action ofan Alpine thrust caused the inversion of the stratigraphic succession .

3.4. Garrinada and Montsacopa volcanoes

Garrinada and Montsacopa volcanoes are located in the city of Olotand, together withMontolivet (Fig. 4), form a looking/seeming orientedalignment of cones following a NE–SW direction. The tree cones do notbelong to the same eruption but correspond to three eruptive eventsseparated one from the other by several thousands of years. In all casesthe eruptive fissures that controlled the eruption of basaltic magmawere oriented NW–SE, so that the structural alignment that the threecones seems to define does not correspond to any tectonic feature.

While Montolivet volcano was entirely constructed by purelymagmatic activity, Garrinada and Montsacopa show a similarsequence involving a Strombolian phase at the beginning and aphreatomagmatic one at the end (Fig. 7). The Garrinada volcano hasbeen studied by Gisbert et al. (2009) who have carried out a detailedanalysis of its deposits and eruption sequence. Despite obvious

differences between the Montsacopa and Garrinada successions ofdeposits, which clearly indicate the existence of different eruptivepulses in each phase of these eruptions, both show a similar generalbehaviour for what concerns to magma/water interaction. In bothcases the eruption started and progressed for a while being purelymagmatic. However at about the middle of the eruption in the case ofthe Garrinada volcano, and towards the end of it in the case of theMonstacopa volcano, the eruptions changed into phreatomagmaticdue to the interaction of magma with a shallow aquifer located in theQuaternary unconsolidated sediments, as it is evidenced in both casesby the characteristics of the resulting deposits and the nature of thelithic clasts included (Gisbert et al., 2009). This phreatomagmaticactivity produced in both cases several lithic-rich explosion breccias,and pyroclastic density current deposits that represent differentmagma/water ratios (Gisbert et al., 2009). Montsacopa ended itseruption with this explosive activity. However, the Garrinada volcanoreturned to the magmatic activity with the emission of several lavaflows, not identified by Gisbert et al. (2009).

3.5. Clot de l'Omera and Puig de Banya del Boc

These two volcanoes are located at the S sector of the studied area(Figs. 5 and 10) and were originated during the same eruption. Puig deBanya del Boc is located on a normal fault that puts in contact Tertiarysedimentswith Palaeozoicmetamorphic rocks (Fig. 5). It corresponds to acone 120 m high with an elliptical crater and rests in part on Palaeozoicmetamorphic rocks. Magmatic and phreatomagmatic eruptive phasesoccurred during the construction of this volcano (Fig. 10). A vent openingphreatomagmatic phase characterised the onset of the eruption andgenerated a lithic-rich explosion breccia irregularly distributed and rich inPaleozoic fragments. Overlying this breccia there is a 3 m thick successionof thinly bedded, fine to coarse grained pyroclastic surge deposits thatshow characteristic high energy bedforms such as dunes and antidunes(Fig. 9eand f). Thesepyroclastic surgeswere radially emplacedaround thevent before being channelised into the main gullies close to the volcano.This phreatomagmatic phase ended with the emplacement of an up to20 m thick massive pyroclastic flow deposit, rich in Paleozoic lithicfragments, poorly vesicular juvenile scoria lapilli, and cauliflower bombs.The eruption continued with a magmatic phase that generated the cone-building scoria lapilli deposit that formed most of the Puig de Banya delBoc cone, covering most of the phreatomagmatic deposits which did notform any positive relief. This Strombolian scoria lapilli deposit is rich inperidotitic mantle nodules and granitic xenoliths. The eruption endedwith the emission of three lava flows emplaced towards de north andsouth of the volcano.

The Clot de l'Omera volcano originated on a conjugate fault of themain one on which the Puig the Banya del Boc formed and which onlyaffects Paleozoic terrains (Fig. 5). It is a maar-type volcano with an

Fig. 7. Synthetic stratigraphic sections of the volcanoes studied at the N sector. All the stratigraphic logs have been obtained using new field data. In the case of the Croscat–SantaMargarida stratigraphic section we have been able to use unpublished data from water boreholes provided by the Natural Park of La Garrotxa Volcanoes.

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elliptical crater of 550 by 450 m and 20 m deep that was entirelyexcavated in the Palaeozoic basement. The external slopes are rathergentle but the internal border is very steep. The succession of depositsthat form this volcanic edifice is composed of a 15 m thick alternationof lithic-rich pyroclastic surge deposits and explosion breccias, whichshow an asymmetric distribution having the maximum thicknesstowards the south. It represents a single phreatomagmatic eruptiveepisode that occurred at the same time than the phreatomagmaticphase of the Puig de Banya del Boc volcano.

3.6. Puig d'Adri

This volcano is located on theAdri normal fault,whichputs in contactPalaeocene and Eocene materials and is buried by Neogene sediments

towards the south (Fig. 5). The construction of the Puig d'Adri volcanoinvolved the superposition of three volcanic edifices, starting with theformation of a tuff-ring of 850 m in diameter, followed with thedevelopment at thewestern side of the tuff-ring of a scoria cone of smalldimensions, and ending with the construction of a new scoria cone thatformed the main relief of the volcano and covered most of the previousstructures.

The Puig d'Adri volcano shows one of the most complex eruptionsequences of the CVZ, which includes five different eruption phases. Theeruption started with a phreatomagmatic event that generated anirregularly distributed deposit of lithic-rich explosion breccia and asuccessionof lithic-richpyroclastic surgeswhichwere emplaced towardsthe south–east flowing for more than 5 km from the vent following themain gullies. This initial explosive phase was immediately followed by a

Fig. 8. Geological cross-section of the Croscat and Santa Margarida volcanoes. Note that the phreatomagmatic deposits (pyroclastic surges and phreatomagmatic ash) resulting fromthe vent opening phase of the Santa Margarida volcano appear below the Croscat pyroclastic succession, and how the Croscat scoria mantles the Santa Margarida crater and products.

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short Strombolian episode that generated a scoria lapilli deposit withlimited extend. The eruption activity returned to phreatomagmatic withhigher intensity than the previous phase, generating a new succession ofpyroclastic surge deposits similar to the former one, explosion breccias,and a pyroclastic flow deposit that flowed for more than 3 km towardsthe south inside the course of the Canet river (Fig. 9i and j). Most of thetuff-ring structure was constructed during this second phreatomagmaticepisode. The eruption continuedwith a sustained Strombolianphase thatgenerated a widespread scoria lapilli deposit around the main ventcoveringmost of the proximal phreatomagmatic products and giving riseto the construction of the main scoria cone. The eruption ended with aneffusive phase that generated two lavaflows that caused the breaching ofthe north-western flank of the scoria cone. One of the lavas flowed formore than 12 km towards the south (Fig. 5).

Most of the lithic clasts in the phreatomagmatic deposits of the Puigd'Adri volcano correspond to red sandstones and marls of the EoceneBellmunt Formation, indicating once again the significance of such unitas aquifer at a regional level. This stratigraphic unit is located severalhundredmetres belowPuig d'Adri. The size of the lithic fragments variesdepending on the deposit. In the pyroclastic surges such clasts are ofmillimetric to centimetric size while in the explosion breccias and thepyroclastic flow deposit they may reach several tens of centimetresacross. Juvenile fragments of basaltic composition are also present invariable proportions and sizes in these deposits. They include poorly-vesicular, fine to coarse grained scoria lapilli and blocky shaped ashfragments in all pyroclastic density current deposits, but also cauliflowerbombs in the pyroclastic flow and the explosion breccias. This size andcontent variation of the lithic and juvenile clasts suggest differentdegrees of fragmentation and a variablemagma/water ratio during thesephreatomagmatic episodes. The Puig D'adri pyroclastic surge depositsare well stratified, thinly laminated and show high energy sedimentarystructures (Fig. 9g and h), and are somewhat consolidated due to post-emplacement alteration of the juvenile fragments. The total thickness ofthe surge successionsmay reach2 m in somepoints but it is usually up to

1 m. The breccia deposits that appear at the base and top of the first andsecond phreatomagmatic phases, respectively, are characterised by thepresenceof large angular lithic clasts of the same composition than thosefrom thepyroclastic surges, andof a smaller proportion of juvenile scoriafragments of coarse lapilli size and sparse cauliflower bombs. Thesedeposits do not show internal stratification and their spatial distributionis rather irregular around the proximal areas. The pyroclastic flowdeposit is similar to those found in other volcanoes of the studied area(Puig de Banya de Boc,Garrinada, Arcs, etc.) (Martí andMallarach, 1987)and consists of large lithic clasts of Eocene rocks, up to 1 m in diameter,and decimetric highly to poorly vesicular scoria fragments, and fewcauliflower bombs in an abundant fine-grained matrix composed ofsmall lithics and juvenile clasts,which has beenmostly transformed intoclay aggregates, zeolites, and iron oxides. Lithic clasts tend to shownormal grading while the largest juvenile fragments show reversegrading. The deposit is composed of several flow units separated byplanar contacts, beingeachunit internallymassive. Thedeposit reaches amaximum thickness of 20 m. A particular characteristic of thispyroclastic flow deposit is the presence a crude, large-scale (up to 5 macross) columnar jointing, which allowed the vertical erosion of thedeposit by pervasive infiltration of meteoric water along the columnarjoints (Fig. 9j).

3.7. Crosa de Sant Dalmai

Thismaar volcano is locatedat theboundarybetweenLaSelva tectonicdepression, replenishedwith Pliocene andQuaternary sediments, and theTransversal chain formed by Palaeozoic granites and metamorphic rocks(Figs. 1 and 11). It is mostly composed of phreatomagmatic depositswhich form a circular tuff-ring around a shallow crater of 1250 m indiameter. The tuff-ring is asymmetrical, being higher at the west(maximum height of 50 m), where the internal and external slopes arealso steeper, than at the east (maximum high of 30 m). Also, the depositssurrounding the rim show awider distribution towards the east (Fig. 11).

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Fig. 10. Synthetic stratigraphic sections of the volcanoes studied at the S sector.

Fig. 9. Field photographs of pyroclastic deposits from the studied volcanoes: a) Lithic-rich pyroclastic flow deposit forming the base of the sequence at Santa Margarida volcano. Thisdeposit is conformably overlaid (contact is indicated by a discontinuous black line) by a lithic-rich Strombolian lapilli deposit that represents the last erupted product from the SantaMargarida crater. b) Distal facies of the pyroclastic surge deposits corresponding to the phreatomagmatic phase that marks the end of the explosive activity at the Croscat volcano. c)Example of a lithic-rich explosion breccia with some interbedded lithic-rich basaltic scoria horizons from the Cairat volcano. d) Well stratified alternance of explosion breccias andlithic-rich pyroclastic surge deposits from the Cairat volcano. e) Close view of a dry pyroclastic surge deposit showing cross-bedding from the Banya del Boc volcano. f) Alternation ofwell stratifiedwet and dry pyroclastic surge deposits. Note the presence of soft-sediment deformation and ballistic impacts on one of thewet deposits. g) Detail of a succession of fineand coarse grained pyroclastic surge deposits from the first phreatomagmatic phase at the Puig d'Adri volcano showing a well defined planar bedding. On top of these deposits and instratigraphic continuity there is a lithic-rich Strombolian lapilli deposit. h) Detail of a strongly indurated pyroclastic surge deposit from the second phreatomagmatic phase at thePuig d'Adri volcano showing a markedwave form at the base. i) Close view of the lithic-rich facies found at the base of a pyroclastic flow unit from the Puig d'Adri volcano. j) Detail ofthe pyroclastic flow deposits from the Puig d'Adri volcano emplaced into the course of the Canet river, showing large-scale (up to 5 m across) columnar jointing, which allowed thevertical erosion of the deposit by pervasive infiltration of meteoric water along the columnar joints.

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Fig. 11. Simplified geological map of the Crosa de Sant Dalmai volcano (modified from Martí et al., 1986).

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The age of this volcano is not well constrained. Although it is localised inthe la Selva sector (Figs. 1 and 2)wheremost of the outcropping volcanicrocks have ages older than 2Ma (Araña et al., 1983), it is evident from thestate of preservation of its morphology and juvenile components that theCrosa de Sant Dalmai volcano must be much younger.

The succession of deposits that form the Crosa de Sant Dalmai showsthe same stratigraphy all around the tuff-ring, thus suggesting thatmost of them were radially distributed from the vent. They reacheddistances of nearly 4 km towards the east and only of a few hundredmetres towards thewest. This asymmetrical distribution of the depositsseems to be related to different strength of the rocks that form thesubstrate at each side below the volcano. At the east the substratecorresponds to unconsolidated Pliocene and Quaternary gravels,whereas toward thewest the substrate is formed by Palaeozoic granitesand schists. This difference in rock strength may have played a majorrole during the eruption making it easier to excavate towards theeastern side in each explosion (Martí et al., 1986). The lowermost unitof the succession of deposits of the Crosa de Sant Dalmai volcano is notfully exposed and corresponds to N1 m thick coarse lithic-rich brecciawith blocks up to 70 cm in diameter of granites and schists withsubordinate scoria lapilli and cauliflower bombs. Above it there is auniform sequence composed of 22 alternating units of lithic-richexplosion breccia deposits and crudely stratified, coarse-grainedpyroclastic surge deposits (Figs. 10 and 12). The next unit in thestratigraphic succession corresponds to a 1 m thick Strombolian, non-welded scoria lapilli deposit, which is followed by six more alternatingunits of lithic-rich breccia and pyroclastic surges. The eruption endedwith a new Strombolian episode from a new vent opened at the interiorof themaar, which formed a small scoria cone and a lava flow emplacedinside the maar. The lithic clasts contained in the phreatomagmatic

deposits are always angular and may constitute up to 80% of thedeposit. The juvenile clasts are of basaltic composition and lapilli size,and are typically poorly vesicular except the scoria lapilli fragments thatform the Strombolian deposit in themiddle of the succession,which arehighly vesiculated as well as the ones that form the inner scoria cone.The presence of mantle derived nodules and inclusions is common inthe juvenile fragments, thus suggesting a rapid ascent of magma fromthe source/storage region.

4. Discussion

As it has been described in the previous section, the successions ofpyroclastic deposits at the youngest volcanoes of the CVZ suggest theexistence of diverse eruptive sequences characterising the eruptiveactivity of this monogenetic volcanic field (Table 1). This contrasts withthe relative compositional monotony of the magmas (alkali basalts andbasanites) involved. The physico-chemical characteristics, also includingthe degree of vesicularity and crystallisation, of the CVZmagmas are verysimilar in most of the studied volcanoes. They mainly correspond toleucite basanites, nepheline basanites and alkali olivine basalts, with aphenocrysts (olivine, pyroxene, and plagioclase) content up to 12%, withan aphiric tomicrocrystalline ormicrolitic groundmass, and a total watercontent up to 1.5% (Cebriá et al., 2000). The vesicularity of juvenile clastsin pure Strombolian deposits typically ranges between 65 and 85% inmost cases, and itmaydecrease till less than 40% in the phreatomagmaticones. Ascent velocities of the order of 0.2 m/s were calculated using thepresence of large mantle derived nodules and lower crust xenoliths insomeof these volcanoes (Martí et al., 1992). Densities in the range of 2.70to 2.87 g/cm3 and typical viscosities of theorder of 10–102 Pa shavebeencalculated using standard methods based on crystal content and rock

Fig. 12. Field photograph of the alternating lithic-rich explosion breccias and pyroclastic surge deposits that formmost of the stratigraphic succession of the Crosa de Sant Dalmai volcano.

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composition and assuming temperatures of the order of 1200–1300 °C.Despite some variations in the dynamics of the magmatic episodes maybe attributed to changes in magma flow conditions related to changes incrystallinity and vesicularity (gas content) of the erupting magma(Cimarelli et al., 2010), it is obvious that variations in magma physics isnot the main reason to account for the large diversity of eruptionsequences recorded in the CVZ when we also take into account thephreatomagmatic episodes.

In fact, as it can be deduced from the successions of depositsobserved in each volcano, differences in eruptive behaviour arerelated in many cases to the occasional interaction of the ascendingmagma with groundwater. Magma/water interaction is, thus, themain reason why a large number of these volcanoes significantlydeviate from the typical Hawaiian–Strombolian behaviour thatcharacterise some of them and monogenetic basaltic volcanoes ingeneral. The way and timing in which such magma/water interactionoccurred during the course of the CVZ eruptions may differconsiderably from one volcano to other. This contrasts with othermonogenetic volcanic fields where eruptions seem to follow a moregeneral pattern (Houghton et al., 1999; Valentine and Gregg, 2008;Brand and Clarke, 2009; Brand et al., 2009; Clarke et al., 2009; Sottiliet al., 2009). In the present case, the large diversity of eruptionsequences observed should be explained by variations in thestratigraphy and structure of the substrate beneath each volcanoand the hydraulic characteristics of each aquifer.

Two main sedimentary aquifers have interacted with CVZ magmascausing such a wide variety of phreatomagmatic episodes and eruptivesequences. One aquifer is located at an average depth of a few hundredmeters below the volcanic cones,while the other ismuch shallower, justa few tens of metres below the surface. The deep aquifer corresponds toEocene continental sediments, known as the Bellmunt Formation,composed of conglomerates, feldspar-rich sandstones and red mud-stones, and the shallow aquifer corresponds to volcanic and alluvialdeposits of Quaternary age, mostly formed by unconsolidated gravels,sands and clays and volcanic products (lavas and pyroclasts) fromformer eruptions. There is also a third aquifer that has played asignificant role in some of themost important eruptions (Puig de Banyadel Boc and Crosa de Sant Dalmai), which corresponds to the highlyfractured (granites and schists) Palaeozoic rocks. The depth of this lastaquifer varies depending on the local structure in each area but itmaybea few hundred metres deep or shallower.

For example, the eruptive sequence deduced for the Croscat–SantaMargarida volcanoes reveals how complex monogenetic volcanismmay be when phreatomagmatic episodes alternate with puremagmatic ones. In addition, in this particular case, eruption changedfrom a localised vent at the beginning, to a fissural vent and again to acentral vent different from the first one. The phreatomagmatic eventsthat we have identified at the beginning and at the end of theexplosive activity in this eruption correspond, respectively, to theinteraction of the erupting magma with two different aquifers. Thefirst phreatomagmatic episode, which corresponds to the SantaMargarida vent opening phase, was caused by the interaction of theascending magma with the Eocene aquifer located at about 250 m ofdepth below the paleosurface, as it is suggested by the abundance oflithic clasts of such lithology. The second phreatomagmatic event, atthe end of the explosive activity of Croscat, occurred by the interactionof the erupting magma with a shallow aquifer installed in Quaternaryunconsolidated deposits and lavas from previous eruptions.

The Can Tià eruptive sequence also illustrates a situation in whichmagma/water interaction occurred at different stages of the eruption,but in this case in the same aquifer. In the first phreatomagmaticphase magma/water interaction occurred by invasion of the Eoceneaquifer by the ascending magma. As magma pressure in the conduitwas still high enough to cross the aquifer and to avoid a massiveinteraction of water with magma, this was short and volumetricallysmall. Magma ascended to shallower levels, probably using otherparts of the same eruptive fissure, and continued its decompressionand consequent fragmentation without interacting with groundwa-ter. As eruption progressed during this Strombolian phase magmapressure decreased progressively in the conduit causing the progres-sive deepening of the fragmentation level. Groundwater from theEocene aquifer interacted again with magma when its pressure washigher than that of magma in the conduit, thus indicating that themagma column was already significantly vesiculated or fragmented.However, as it is indicated by the progressive appearance of lithics inthe lapilli deposit this was a gradual process, till the pressuredifference reached a threshold in which the entrance of water intothe magma conduit became massive. This caused an increase inexplosivity and the sudden enlargement (by erosion) of the conduit,and a subsequent increase in the eruption rate. The eruptedmixture ofrelatively cold lithics and fragmented magma was too dense and coldto be sustained by an eruption column and immediately collapsed on

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the vent forming a poorly expanded pyroclastic flow. Eruption couldend either because magma supply stopped or because the conduitcollapsed and blocked.

Another interesting example of the variation in the eruptiondynamics that can be offered by two close volcanoes corresponds tothe Clot the l'Omera and Puig de Banya de Boc volcanoes. Thephreatomagmatic activity that characterises the eruption of the Clotde l'Olmera volcano and the first eruptive episodes of the Puig deBanya del Boc volcano, and the sole presence of Palaeozoic lithicfragments in their deposits, indicate that there was an importantaquifer in the metamorphic rocks that form the substrate of thesevolcanoes. The synchronicity of both eruptions and the differenterupted products generated in both phreatomagmatic phases suggestthat the aquifer was intersected at the same time by the ascendingmagma in two different points of the same eruptive fissure and thatthe resulting magma/water interaction changed from one point toother. In the Puig de Banya del Boc the magma/water ratio increasedduring its phreatomagmatic episode passing from the generation ofdry pyroclastic surges to that of a wet pyroclastic flow. However, inthe Clot de l'Omera the alternation of dry pyroclastic surges andexplosion breccias suggest a pulsating sequence of phreatomagmaticexplosions in which magma/water interaction was intermittent butwith more or less the same ratio in each explosion.

In this sample of different eruptive behaviours it is also relevant tomention theeruption of Puig d'Adri volcano that, in a similarway than inthe Santa Margarida and Can Tià volcanoes, it also initiated with aphreatomagmatic event triggered when the ascending basaltic magmainteracted with a relatively deep aquifer. This first magma/waterinteraction episode suddenly stopped giving rise to a short Strombolianepisode that generated a restricted fallout scoria lapilli deposit, whichstarted as purely magmatic and gradually incorporated an increasingamount of lithics of Eocene rocks. This represents another example, asCan Tià and Croscat, of a progressive interaction of magma with anaquifer. In this case themagma/water interaction occurred in the Eoceneaquifer and increased progressively until giving rise to a newphreatomagmatic event. This was marked by the eruption of a newseries of pyroclastic surges and a final pyroclastic flow,which representsthe culmination of the magma/water interaction in Puig d'Adri volcano,and after which the eruption continued as purely magmatic.

Finally, the eruption of the Crosa de Sant Dalmai volcano is a classicalexample of phreatomagmatismcausedby the interactionof the ascendingmagma with a shallow aquifer. In this case the aquifer was probablyinstalled into the Quaternary unconsolidated sediments but it may havebeenalsoa significant contribution fromasecondaquifer installed into thefragmented Palaeozoic rocks, as it is suggested by the abundance ofangular lithic fragments of these lithologies. Magma supply wascontinuous during the whole eruption but the amount of water availablewas not constant, as it is indicated by the variations of the proportions oflithics and juvenile fragments in the resultingdeposits and thepresence ofpure Strombolian phases at the middle and end of the eruption.

All the investigated examples offer different case studies but confirmhow complex monogenetic basaltic volcanism may be even in arelatively small area when interaction of the erupting magma withgroundwater occurs. This is particularly relevant when there aredifferent aquifers, with different hydraulic characteristics, and whenthe substrate below thevolcanoesmay showa complex stratigraphy andstructure due to local tectonics, as it is the case studied in this paper. Infact, differences in substrate stratigraphy and rock strength may play asignificant role in the resulting eruptions and products, as has beenpointed out in other volcanic areas (Sohn and Park, 2005; Auer et al.,2007; Martín-Serrano et al., 2009). We do not want to go deeper in thisdiscussion on the exact mechanisms that have controlled magma/waterinteraction in each particular case, so that the reasons why eruptionsmay start violently with a phreatomagmatic episode or quietly with aHawaiian or Strombolian one will have to be considered in furtherstudies. However, we want to emphasise the importance of knowing, in

addition to magma physics and chemistry, the geology of the substratebelow the monogenetic volcanic fields in order to understand potentialeruption mechanisms when groundwater may be present. The largediversity of eruptive sequences deduced in the CVZ reveal that most ofthe variables that have controlled them depend on the local geologyrather than on magma composition, crystal content, vesiculation andfragmentation prior to explosive interaction with groundwater, whichdo not change significantly among the cases studied in this paper. This isparticularly relevantwhenwe try to conduct hazard assessment in theseareas. In addition to the initial problem that is usually imposed by thelack of a precise geochronology in long lived volcanic fields, we need toconsider thewide variety of potential eruptive scenarios thatmay occur,as it is the case in the CVZ. Phreatomagmatism is traditionally consideredunpredictable mainly because the hydrological characteristics of theterrain are not always well known. Magma/water interaction increasesexplosivity andmay significantly alter the course of amagmatic eruptiongiving rise to the appearance of more hazardous phenomena. Theexample of the CVZ illustrates quite well this relevant aspect ofphreatomagmatism, but also that having a good knowledge of thegeology and hydrogeology of the substrate it may not be sounpredictable.

5. Conclusions

We have presented the main characteristics of the youngest volcanicepisodes of the Catalan Volcanic Zone, a Quaternary alkaline volcanicprovince of the European rifts system. In this monogenetic volcanic fieldwe have recognised more than 50 well preserved volcanic edifices, inmost of which purely magmatic episodes (Hawaiian to violent Strombo-lian) alternatedwith phreatomagmatic ones, giving rise to a large varietyof eruptive sequences and corresponding successions of deposits. Weconclude that the diversity of eruptive sequences deduced reveal that themain cause of such complex eruptive behaviour resides in thestratigraphic, structural and hydrogeological characteristics of thesubstrate above which the volcanoes were emplaced, rather than onchanges of magma composition, crystal content, vesiculation andfragmentation prior to explosive interaction with groundwater.

Acknowledgements

We thank the Natural Park of the La Garrotxa Volcanic Zone and itsstaff to allow us to undertake this research and for all the support wehave always received from them. A. Geyer is grateful for her Beatriu dePinó́s post-doctoral fellowship 2008BPB00318. Constructive reviewsby Danillo Palladino, Karoly Nemeth and an anonymous reviewer aregreatly appreciated.

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