Strombolian and phreatomagmatic interaction

Complex interaction between strombolian and phreatomagmatic eruptions in the Quaternary monogenetic volcanism of the Catalan Volcanic Zone (NE of Spain) Joan Mart´ı, Llorenc¸ Planagum`a, Adelina Geyer, Esther Canal, Dario Pedrazzi PII: DOI: Reference:

S0377-0273(10)00392-6 doi: 10.1016/j.jvolgeores.2010.12.009 VOLGEO 4678

To appear in:

Journal of Volcanology and Geothermal Research

Received date: Accepted date:

1 September 2010 11 December 2010

Please cite this article as: Mart´ı, Joan, Planagum` a, Lloren¸c, Geyer, Adelina, Canal, Esther, Pedrazzi, Dario, 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 (2010), doi: 10.1016/j.jvolgeores.2010.12.009

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ACCEPTED MANUSCRIPT Complex interaction between strombolian and phreatomagmatic eruptions in the Quaternary monogenetic volcanism of the Catalan Volcanic Zone

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(NE of Spain)

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Joan Martí1, Llorenç Planagumà2, Adelina Geyer3, Esther Canal2, Dario Pedrazzi1

1. Institute of Earth Sciences “Jaume Almera”, CSIC, Lluis Solé Sabaris s/n, 08028 Barcelona, Spain.

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2. Tosca, Environment Services of Education. Casal dels Volcans, Av. Santa Coloma, 17800 Olot, Spain

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3. CIMNE, International Center for Numerical Methods in Engineering, Edifice C1, Campus Nord

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UPC, Gran Capità, s/n, 08034 Barcelona, Spain

Corresponding author: Joan Marti (joan.marti@ija.csic.es)

Paper submitted to the Journal of Volcanology and Geothermal Research, Special Issue on "Maars"

ACCEPTED MANUSCRIPT Abstract

The Catalan Volcanic Zone (CVZ), at the NE of the Iberian peninsula, is one of the Quaternary alkaline volcanic provinces of the European rifts system. The CVZ has been active during the last

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12 Ma. Despite the fact that this volcanism is significant in extension and volume, and that

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eruptions have also occurred in Holocene times, it is mostly unknown compared to the contemporaneous alkaline volcanism in other parts of Western and Central Europe. Volcanism younger than 0.5 Ma is mostly concentrated in an area of about 100 km2 located between the main

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cities of Olot and Girona. This basaltic volcanic field comprises more than 50 monogenetic cones including scoria cones, lava flows, tuff rings, and maars. Magmatic eruptions range from hawaiian

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to violent strombolian. Phreatomagmatism is also common and has contributed to the construction of more than a half of these volcanic edifices, frequently associated with the strombolian activity

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but also independently, giving rise to a large variety of eruption sequences. We describe the main characteristics of this volcanism and analyse in particular the successions of deposits that form some of these volcanoes and discuss the potential causes of such a wide diversity of eruption

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sequences. We find that the main cause of such complex eruptive behaviour resides in the stratigraphic, structural and hydrogeological characteristics of the substrate above which the

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volcanoes were emplaced, rather than on the compositional characteristics of the erupting magma,

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as they do not show significant variations among the different volcanoes studied.

ACCEPTED MANUSCRIPT Introduction

Monogenetic basaltic zones are common in many volcanic environments and may develop under very different geodynamic conditions (Francis, 1993; Walker, 2000; Connor and Conway, 2000). A

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particular characteristic of this type of volcanism is the large diversity of eruption styles,

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morphologies and deposits that it may display 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 as

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common in such volcanic environment. More complicated is the variation in the eruptive styles when magma-water interaction occurs, being this another common feature in many monogenetic

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volcanic fields (Houghton et al., 1999; White and Houghton, 2000).

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The physics of phreatomagmatism has been studied in detail since the 70s and this has allowed to obtain several experimental and theoretical models that constrain our understanding of the way in which magma and external water interact and of the large diversity of deposits that this explosive

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interaction may generate (Lorenz,1973, 1986, 1987; Sheridan and Wohletz, 1981, 1983; Wohletz, 1983, 1986; Wohletz and McQueen, 1984; Zimanowski et al., 1991; Zimanowski, 1998; Morrisey et

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al, 2000). In monogenetic basaltic volcanism, phreatomagmatic activity may be related to the interaction of magma with surface and/or ground water, and the style and products of the resulting

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eruptions will depend on degassing patterns, magma ascent rates and degrees of interaction with external water (Wohletz, 1986; Houghton et al., 1999; Morrisey et al., 2000). Interpretation of deposits, including facies analysis, morphometric characterisation of pyroclasts, and grain size distribution and component analysis, constitute the essential tool to identify and reproduce the sequence of events involved in phreatomagmatism and to evaluate its potential hazard in case of active areas (Fisher and Waters, 1970; Heiken, 1971; Lorenz, 1973, 1986, 1987; Wohletz, 1986; Brand and Clarke, 2009; Brand et al., 2009; Sottili et al 2009).

Although there may exist clear similarities between the eruption activity displayed by different monogenetic volcanic fields, it is also true that important differences may arise when each succession of deposits (i.e.: eruption sequence) is investigated in detail. These differences may be due to changes in magma composition (e.g.: volatile content), magma supply 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 among monogenetic volcanic fields is provided by the Quaternary volcanism developed along the European rifts system (Fig. 1), which includes several volcanic provinces all them related to the same geodynamic event but with distinct

ACCEPTED MANUSCRIPT local tectonics and lithospheric and shallow geological structures (Wilson and Downes, 1991; 1992; Downes, 2001).

One of the least known and understood regions of the Quaternary alkaline volcanism in Europe is

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the Catalan Volcanic Zone (Martí et al., 1992) (Fig. 1). The age of this volcanism ranges from > 12

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Ma to early Holocene and it is mainly represented by alkali basalts and basanites (Cebriá et al, 2000). Small-sized scoria cones were produced during monogenetic short-lived eruptions associated with widely dispersed fractures of short lateral extent. Important phreatomagmatic events also

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occurred giving rise to a wide diversity of eruption sequences (Martí and Mallarach, 1987). The total volume of extruded magma in each eruption is relatively small (0.01-0.2 km3), suggesting a

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relatively low magma supply rate.

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In this paper we present an outline of the Quaternary monogenetic volcanism of the Catalan Volcanic Zone, in which we describe the main features of the eruptive activity of this volcanic field.

We provide a general characterisation of the erupted products in order to classify them in terms of

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eruption dynamics, rather than a thorough description of the deposits and their interpretation in terms of emplacement mechanisms, which by themselves would deserve a separate study. We pay

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special attention to the diversity of phreatomagmatic episodes that can be recognised in this volcanism and discuss the possible causes for the occurrence of such a large variety of eruption

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sequences in a rather small area and associated with a monotonous magma composition.

Geological setting of the CVZ and regional stratigraphy

Cenozoic alkaline volcanism is widely distributed along an extensive rifts system in central and western Europe, including the Rhenish massif and Rhinegraben of Germany, the Massif Central of France, and the western Pannonian Basin in Eastern Europe (Downes, 2001) (Fig. 1). The causal mechanism(s) of this rifts system is poorly understood. However, Hoernle et al. (1995) found that Sr, Nd and Pb isotope data for lavas from central Europe to the eastern Atlantic Ocean and the western Mediterranean may be explained as the result of mixing between 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 in Europe is the one related to the Valencia Trough (Martí et al., 1992; Muñoz et al., 2005). The Valencia Trough is a NE-SW oriented Neogene basin located between the Iberian Peninsula and the Balearic promontory offshore of northeastern Spain (Fig. 1). The Valencia Trough has a complex geological history. Two main

ACCEPTED MANUSCRIPT stages of magmatism have been identified (Martí et al., 1992). During Early to Middle Miocene time, the area was subjected to compressional tectonics accompanied by calc-alkaline volcanism.

This was followed by a period of extensional tectonics and mafic alkaline volcanism from middle Miocene to Recent time. The greatest concentration of Middle Miocene to Recent volcanism in the

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region is found in the Catalan Volcanic Zone (CVZ) at the NE of the Iberian peninsula (Fig. 1).

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Despite the fact that this alkaline volcanism shows strong similarities with the contemporaneous alkaline magmatism in other parts of Western and Central Europe, it has been poorly studied and its

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significance is still not well understood.

Available data indicate that the mafic volcanic products of the CVZ, like the parental magmas of the

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cumulate xenoliths, range from strongly silica-undersaturated to nearly silica-saturated compositions (Araña et al., 1983; López-Ruiz and Rodríguez-Badiola, 1985; Cebriá et al., 2000;

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Martí et al., 1992). This region comprises a suite of intracontinental leucite basanites, nepheline basanites and alkali olivine basalts, which in most cases represent primary or nearly primary liquids (Cebriá et al., 2000). The geochemical characteristics of these lavas are very similar to the

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analogous petrologic types of other Cenozoic volcanics of Europe, which are intermediate between HIMU mantle, depleted mantle (DM) and enriched mantle by mineralised deep fluids (EM1)

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(Cebriá et al., 2000; Downes, 2001). Geochemical and isotopic signatures of magmas suggest the participation of at least a sublithospheric component and an enriched lithospheric component with a

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K-bearing phase in the mantle source (Martí et al., 1992; Cebriá et al., 2000). Cebriá et al (2000) proposed a geochemical model involving the generation of a hybrid mantle lithosphere source produced by the infiltration of the sublithospheric liquids into enriched domains of the mantle lithosphere, shortly before the melting event that generated the CVZ lavas.

The area has traditionally been divided into three different sub-zones: L'Empordà to the Northeast (>12-8 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 extruded magma seems to increase progressively from the early episodes (L'Empordà) to the later ones (La Garrotxa). Thus, a progressive and concomitant increase of the volume of magma generated, as well as an increase in the degree of partial melting, can be observed in the geochemistry of the rocks from the CVZ (Araña et al., 1983; Martí et al., 1992). Some volcanoes of the La Garrotxa sub-zone, contain ultramafic to mafic xenoliths. The xenoliths comprise pyroxenites, melanogabbros, amphibolites and spinel lherzolites, the pyroxenites being the most abundant. Pressure and temperature estimates for these xenoliths suggest that they may have crystallised in magma chambers located at the crustmantle boundary (Neumann et al., 1999), which according to geophysical estimates would be

ACCEPTED MANUSCRIPT located at a depth of ~30 km (Gallart et al., 1984, 1991; Fernรกndez et al., 1990). These geophysical studies also indicate that the CVZ is characterised by a regionally thinned lithosphere, about 60-70 km thick, by a high elevation and a high thermal gradient, suggesting that the area is affected not only by the topographic load of the Pyrenees but also by the opening of the Valencia trough. The

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local structure of the area is composed of a set of horsts and grabens bounded by a NW - SE system

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of Neogene normal faults that determines the distribution of volcanism and the fluvial network (Saula et al., 1995).

the

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The lithostratigraphic units that crop out in the studied area and that form the substrate above which volcanic edifices were emplaced correspond to materials of upper Palaeozoic, Eocene and

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Quaternary age. Due to the Alpine folding, the Neogene normal faulting system, and further erosion, the substrate varies under each volcano. The oldest unit we can recognise corresponds to

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the schist, gneiss, granodiorites and granites of Permo-Carboniferous age. This unit is unconformably overlaid by the Eocene Formations that from base to top include: 1) the blue marls and gypsum of the Banyoles Formation; 2) the marls and brown sandstones of the Bracons

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Formation; 3) the red sandstones, mudstones, and conglomerates of the Bellmunt Formation; 4) the glauconite sandstones and conglomerates of the Folgueroles Formation; and finally 5) the grey

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sandstones and marls of the Rocacorba Formation. Filling the bottom of the valleys and unconformably lying on the previous units there are unconsolidated gravels, clay and sands and

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alluvial deposits, which together with lava flows and pyroclastic products form the Quaternary succession. The Palaeozoic terrains, the Bellmunt Formation, and the Quaternary deposits constitute the main aquifers of the area, but the base of the Folgueroles Formation and the Banyoles Formation may also act as aquifers in some sectors of the studied area.

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

Deposits successions and eruption sequences

The best preserved outcrops of volcanic rocks from the CVZ are found in La Selva and La Garrotxa sub-zones (Fig. 1). In this area more than 50 well preserved volcanic cones can be recognised, and we have grouped them into two different sectors, one at the north (N sector) corresponding to the Fluviรก river basin, and one at the south (S sector) coinciding with the Ter river basin (Fig. 3). The main concentration of volcanic cones corresponds to the N sector, where there are more than 30

ACCEPTED MANUSCRIPT cones, while the S sector contains no more than a dozen of them, but includes the largest ones. The substrate on which these monogenetic volcanoes stand differs from one sector to the other. At the north the volcanic rocks lie on Tertiary sediments while towards the south they rest in most cases directly on the granites and schists of the Palaeozoic basement. In the studied area, volcanic activity

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eruptive event every 15,000 to 20,000 years (Guerin et al. 1985).

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occurred sporadically over a time period ranging from >500,000 to about 11,000 years ago, with an

The most recent activity in the CVZ was characterised by the presence of small-sized scoria cones

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that were produced during short-lived eruptions associated with widely dispersed fractures of short lateral extent. The total volume of extruded magma in each eruption was small (0.01-0.2 km3

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DRE), suggesting that the magma available to feed each eruption was also very limited.

Strombolian and phreatomagmatic episodes alternated in most of these eruptions giving rise to

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complex stratigraphic successions composed of a wide range of pyroclastic deposits (Martí and Mallarach, 1987). The eruption sequences that may be deduced from these successions of deposits differ from one cone to another and reveal that eruptions did not follow a common pattern,

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particularly for what concerns to phreatomagmatic episodes.

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All the volcanoes studied were constructed during single eruptions (i.e.: they must be referred to as monogenetic) commonly including several distinctive phases . We can consider two groups of phreatomagmatic activity contributed to their

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volcanic edifices depending on whether or not

construction. The volcanoes exclusively derived from magmatic activity correspond to scoria cones, symmetrical or horseshoe shaped, built by the accumulation of scoria , with occasional emissions of lava flows. Examples of this type of activity are the volcanoes of Puigalós, Puig de Martinyà, San Marc, Roca Negra, and Puig Subià (Fig. 4). Volcanic cones including phreatomagmatic activity are much more complex, although morphologically they are similar to the scoria ones. They may alternate phreatic phases produced by vapour explosions that only erupted lithic clasts from the substrate, with typical phreatomagmatic phases that generated a wide diversity of pyroclastic density currents and fallout deposits, with typically strombolian phases including explosive and effusive episodes. The resulting eruptive sequences that can be deduced from the successions of deposits show substantial variations among the cones, indicating their different eruptive behaviourExamples of this type of activity are represented by the volcanoes of Santa Margarida, Croscat, Garrinada, Montsacopa, Can Tià and Cairat in the N sector (Fig. 4), and the volcanoes of Puig d'Adri, Puig de Banya del Boc, Clot de l'Omera, Granollers and Sant Dalmai in the S sector (Figs. 3 and 5). Table 1 summarises the different eruptive sequences deduced for the studied volcanoes. We describe below some of the most representative examples of the recent CVZ

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Croscat and Santa Mararida volcanoes

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The Croscat and Santa Margarida volcanoes are located at the interior of an eroded anticline of

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Eocene rocks (Figs. 6 and 7). They are the most representative edifices of the N sector of the studied area. Despite they have been traditionally considered as two separate volcanoes with independent eruption dynamics (Mallarach, 1982; MartĂ­ and Mallarach, 1987), new stratigraphic

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data reveal that they belong to the same eruptive episode. Part of the eruption sequence of the Croscat volcano has been investigated in detail by Di Traglia et al (2009) and Cimarelli et al (2010),

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who pointed out the transition between different eruptive styles in basaltic monogenetic volcanism and emphasised the role of phreatomagmatism in triggering violent strombolian eruptions.

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However, new data from field work conducted in this study and water boreholes drilled in the vicinity of the Croscat volcano reveal that the eruption history of this volcano is more complex than envisaged by Di Traglia et al (2009) and Cimarelli et al (2010), being intimately related to that of its

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neighbour Santa Margarida volcano (Figs. 7 and 8).

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The Croscat and Santa Margarida volcanoes, together with La Pomareda cone, lie on a 3 km long eruption fissure oriented NW-SW (Figs. 6 and 7). The eruption started at the southern end of the

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fissure with a 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 and generated a massive lithic rich pyroclastic flow deposit, only visible on the eastern flank of the Santa Margarida volcano, and several widespread medium to coarse grained pyroclastic surges , and associated fine-ash deposits, which covered most of the area forming the unit on which the Croscat succession built up (Figs. 8 and 9a). This phreatomagmatic phase was followed by a short strombolian phase that generated a thin, lithic-rich, coarse scoria lapilli fallout deposit that overlaid the

previous deposits in the vicinity of the Santa Margarida crater.

Immediately after this first phreatomagmatic phase the eruption progressed along the central and northern sectors of the fissure with the extrusion of basaltic magma and generated massive spatter and occasionally rheomorphic, welded scoria agglomerates, all them forming the first cone-building deposits of Croscat and La Pomareda. No more magma was erupted during this and further phases 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

ACCEPTED MANUSCRIPT Croscat scoria cone. The Croscat strombolian activity generated two main scoria fallout units (Fig. 8). The lower one conformably overlies the basal spatter and is formed by a several metres thick, poorly stratified coarse lapilli size scoria deposit with several scoria bomb beds . The upper unit constitutes the main volume of the Croscat cone and is formed by a well stratified to thinly

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laminated, medium to fine lapilli size scoria deposit, more than ten metres thick that contains

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sparse scoria bombs and blocks. The upper scoria lapilli unit also forms most of the intermediate to distal outcrops towards the east of the volcano, being recognised at distances farther than 5 km. It also covers the Pomareda spatter and the phreatomagmatic deposits and the explosion crater of

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Santa Margarida (Fig. 8). The contact between the lower and upper scoria lapilli units is clearly marked by a change in the internal stratification of the deposits and the grain-size of the scoria

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clasts. The vesicularity of the scoria lapilli clasts ranges from 57% to 78% and their morphology is mostly characterised by irregular shapes with spherical vesicles but woody-shaped, highly vesicular

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stretched scoria fragments are occasionally present in the lower unit. A detailed morphoscopic and textural analysis of the Croscat scoria has been carried out by Di Traglia et al (2009).

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Towards the top the upper scoria lapilli unit is characterised by the appearance of centimetric-sized lithic clasts from old lavas, which progressively increase in abundance, thus defining a gradual

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change to the uppermost unit of the Croscat pyroclastic succession. This transition is also defined by a decrease in the degree of vesicularity of the scoria lapilli and the appearance of blocky shaped

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ash fragments which become predominant at the overlaying unit. These textural and compositional characteristics suggest a transition from magmatic to phreatomagmatic activity during the last explosive episodes of the Croscat eruption, which is also supported by the sedimentological characteristics of the uppermost unit of the Croscat pyroclastic succession. This corresponds to a lithic-rich, thinly laminated unit, of a few metres thick, which extends for several kilometres to the east changing from planar to cross-bedded stratification from proximal to distal facies (Fig. 9b). The last eruptive phase of Croscat is represented by a lava flow the emplacement of which caused the breaching of the western flank of the cone. This lava and covered an area of 5 km2 and flowed more than 10 km to the west , with an average thickness of 10 m. The total volume of magma (DRE) emitted during the Croscat and Santa Margarida eruption is of the order of 0.2 km3.

The gradual transition between the phreatomagmatic deposits from the Santa Margarida crater and the strombolian scoria close to the vent area indicates that some magma continued erupting for a while without interacting with groundwater through the same conduit. The absence of paleosoils and erosion surfaces between these phreatomagmatic deposits and the fissure-vent derived scoria and spatter that crops out at the base of the Croscat and La Pomareda successions, indicates that

ACCEPTED MANUSCRIPT both deposits were emplaced sequentially. This suggest that shortly after the first phreatomagmatic episode magma migrated through the fissure towards the north-west and started to erupt along it forming the spatter deposits. After that, eruption concentrated at the middle sector of the fissure forming a central conduit and gave rise to the construction of the Croscat cone. The products from

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the Croscat strombolian phase also reached distal areas to the east mantling the whole Santa

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Margarida crater and its surroundings.

This interpretation differs from that given by Di Traglia et al (2009) and Cimarelli et al (2010) who

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placed the first phreatomagmatic episode between the two scoria lapilli units of Croscat. According to this, they proposed that the first phreatomagmatic phase occurred during the waning of the scoria

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cone phase (lower lapilli unit) and suggested that it caused a significant change in the eruption dynamics conditioning the occurrence of a violent strombolian phase (upper lapilli unit). However,

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as explained above, field and water borehole stratigraphic relationships (Figs. 7 and 8) reveal that the first phreatomagmatic phase preceded the initiation of the construction of the Croscat cone and corresponds to the vent opening phase that originated the Santa Margarida crater, latter mantled by

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the Croscat scoria (Fig. 8).

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Can TiĂ

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It forms part of the group of three cones (Fontpobra, Tuta de Colltort, Can TiĂ ) that are located close to the scarp formed by the western margin of the eroded Eocene anticline (Figs. 4 and 7). The Can TiĂ volcano corresponds to a maar-type construction, with a crater of 270 m in diameter and 20 m deep that shows a flat bottom. It is asymmetrical showing a higher rim sector towards the south.

The stratigraphic succession of this volcano is only composed of pyroclastic deposits. It includes four different units from base to top (Fig. 8). The lower unit corresponds to a 10 cm thick deposit composed of small (< 1 cm) juvenile and lithic clasts in an ashy matrix, which rests unconformably on a palaeosoil. The second unit is formed by a poorly stratified, non-welded, highly vesicular scoria lapilli deposit, up to 6 m thick, which contains a few lithic clasts, some of block size. The third unit conformably overlies the previous one and is a 1.5 m thick thinly laminated, well sorted, fine-grained scoria lapilli deposit rich in lithic clasts of variable size (< 2 to 30 cm) of Eocene red sandstones, with an interbedded ash layer at the upper part. The number of lithics increases gradually towards the top of the deposit, thus suggesting the initiation of a new phreatomagmatic phase. The uppermost unit of the succession shows a planar contact with the underlying scoria lapilli and corresponds to a massive pyroclastic flow deposit, up to 3 m thick, which contains abundant Eocene lithic clasts and highly vesicular scoria lapilli fragments, all them surrounded by a

ACCEPTED MANUSCRIPT lithic-rich, ash matrix nearly completely transformed into clay minerals, zeolites and iron oxides.

The succession of deposits observed in this volcano reveals how the explosive activity initiated with a short explosive event, probably of phreatomagmatic nature according to the composition of the

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resulting deposit. Then, the eruption immediately changed into magmatic (second and third units),

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and again into phreatomagmatic (upper part of third unit and fourth unit). The succession of deposits and the distribution of lithics reveal that the first change in the eruptive behaviour (from phreatomagmatic to magmatic) was abrupt but the second one (from magmatic to phreatomagmatic)

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was gradual. Also, the nature of the lithic clasts found in these deposits clearly indicates a variation in the position of the fragmentation level during the course of the eruption and provide the clues to

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understand the dynamics of this eruption (see Discussion). Although all the lithic clasts found correspond to Eocene rocks that form the substrate below Can TiĂ volcano, they concentrate

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differently depending on each phase of the eruption. The first unit contains mostly lithics from the Bellmunt Formation, which constitutes the main aquifer in the area and is located at about 300 m below the surface. The lithic clasts found at the base of the second unit are grey sandstones from the

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Rocacorba Formation, the uppermost stratigraphic unit in this sector of the studied zone. Towards the upper part of this unit lithics of brown sandstones from the Folgueroles Formation appear in a

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significant proportion. The third unit is characterised by the progressive appearance again of red sandstone lithic clasts belonging to the Bellmunt Formation, located deeper in the stratigraphic

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sequence of the area. These lithic clasts become clearly predominant towards the top of this unit and constitute the main lithic fraction of the fourth unit, which represents the culmination of the second phreatomagmatic phase and marks the end of the eruption.

Cairat

The Cairat volcano is located on the eastern flank of the eroded Eocene 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 of diameter excavated in the Eocene substrate. It is one of the few examples in the studied area nearly exclusively composed of phreatomagmatic deposits. The pyroclastic deposits that form this volcanic edifice were preferentially emplaced to the north and south of the crater. The characteristics of the deposits and the nature of the abundant lithic clasts they content suggest that most of the eruptive activity of the Cairat volcano involved interaction of the erupting magma with groundwater from the main Eocene aquifer (MartĂ­ and Mallarach, 1987). The succession of pyroclastic deposits of the Cairat volcano is composed of a 20 m thick succession of lithic rich explosion breccias and lapillisized fallout, and pyroclastic surge deposits (Fig. 9c and 9d)). The main characteristic of this

ACCEPTED MANUSCRIPT succession of deposits is the presence of abundant lithic clasts from the Eocene basement, which in this area is formed (from top to base) by the Banyoles Formation (blue marls), the Bracons Formation (grey sandstones and lutites), and the Bellmunt Formation (red sandstones and lutites).

These lithic clasts range in size from a few centimetres up to 5 m. Although the distribution of the

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largest blocks is rather irregular they tend to concentrate towards the base of the sequence and in

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some lithic-rich units in the middle and upper parts. Some of the lithics from the Bellmunt Formation are deeply hydrothermally altered. Juvenile fragments are less abundant than lithic clasts and correspond to poorly vesicular scoria lapilli, a few cauliflower bombs, and blocky shaped ash

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fragments.

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The eruptive activity of the Cairat volcano mostly produced lithic breccias , with a massive emplacement of ballistic blocks, and some more energetic episodes that generated thinly bedded,

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pyroclastic density currents. The location of the vent at a hill's crest, with steep slopes at both sides, conditioned the accumulation of volcanic materials which where affected by continuous sliding until they redeposited on a more stable slope. This implied a continuous syn-depositional

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remobilisation of the original pyroclastic products deposited on the highest parts. At the northern side the products of this continuous debris avalanching were channelised inside a pre-existing gully

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where they eroded and incorporated part of the non-volcanic sediments that existed there. This also contributed to the large variety of lithic fragments found in these pyroclastic deposits of this

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volcano and the chaotic aspect of some of the units. However, it is still surprising the relative significant amount of lithics from the Bracons and Banyoles formations, both older and located stratigraphically deeper than the Bellmunt Formation, the stratigraphic unit that constitutes the main aquifer in this area. The reason for the appearance in the deposits of this volcano of lithic clasts from stratigraphic levels located below the aquifer that interacted with the erupting magma

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purely tectonic, as in this particular site the action of an Alpine thrust caused the inversion of the stratigraphic succession .

Garrinada and Montsacopa volcanoes

Garrinada and Montsacopa volcanoes are located in the city of Olot and, together with Montolivet (Fig. 4), form a looking/seeming oriented alignment of cones following a NE-SW direction . The tree cones do not belong to the same eruption but correspond to three eruptive events separated one from the other by several thousands of years. In all cases the eruptive fissures that controlled the eruption of basaltic magma were oriented NW-SE, so that the structural alignment that the three cones seems to define does not correspond to any tectonic feature.

ACCEPTED MANUSCRIPT While Montolivet volcano was entirely constructed by purely magmatic activity, Garrinada and Montsacopa show a similar sequence involving a strombolian phase at the beginning and a phreatomagmatic one at the end (Fig. 7). The Garrinada volcano has been studied by Gisbert et al

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(2009) who have carried out a detailed analysis of its deposits and eruption sequence. Despite

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obvious differences between the Montsacopa and Garrinada successions of deposits, which clearly indicate the existence of different eruptive pulses in each phase of these eruptions, both show a similar general behaviour for what concerns to magma/water interaction. In both cases the eruption

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started and progressed for a while being purely magmatic. However at about the middle of the eruption in the case of the Garrinada volcano, and towards the end of it in the case of the

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Monstacopa volcano, the eruptions changed into phreatomagmatic due to the interaction of magma with a shallow aquifer located in the Quaternary unconsolidated sediments, as it is evidenced in

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both cases by the characteristics of the resulting deposits and the nature of the lithic clasts included (Gisbert et al., 2009). This phreatomagmatic activity produced in both cases several lithic-rich explosion breccias, and pyroclastic density current deposits that represent different magma/water

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ratios (Gisbert et al., 2009). Montsacopa ended its eruption with this explosive activity. However, the Garrinada volcano returned to the magmatic activity with the emission of several lava flows, not

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identified by Gisbert et al (2009).

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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 de Banya del Boc is located on normal fault that puts in contact Tertiary sediments with Palaeozoic metamorphic rocks (Fig. 5). It corresponds to a cone 120 m high with an elliptical crater and rests in part on Palaeozoic metamorphic rocks. Magmatic and phreatomagmatic eruptive phases occurred during the construction of this volcano (Fig. 10). A vent opening phreatomagmatic phase characterised the onset of the eruption and generated a lithicrich explosion breccia irregularly distributed and rich in Paleozoic fragments. Overlying this breccia there is a 3 m thick succession of thinly bedded, fine to coarse grained pyroclastic surge deposits that show characteristic high energy bedfoms such as dunes and antidunes (Fig. 9e and 9f).

These pyroclastic surges were radially emplaced around the vent before being channelised into the main gullies close to the volcano. This phreatomagmatic phase ended with the emplacement of an up to 20 m thick massive pyroclastic flow deposit, rich in Paleozoic derived lithic fragments, 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

ACCEPTED MANUSCRIPT del Boc cone, covering most of the phreatomagmatic deposits which did not form any positive relief. This strombolian scoria lapilli deposit is rich in peridotitic mantle nodules and granitic xenoliths. The eruption ended with the emission of three lava flows emplaced towards de north and

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south of the volcano.

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The Clot de l'Omera volcano originated on a conjugate fault of the main one on which the Puig the Banya del Boc formed and which only affects Paleozoic terrains (Fig. 5). It is a maar-type volcano with an elliptical crater of 550 by 450 m and 20 m deep that was entirely excavated in the

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Palaeozoic basement. The external slopes are rather gentle but the internal border is very steep. The succession of deposits that form this volcanic edifice is composed of a 15 m thick alternation of

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lithic-rich pyroclastic surge deposits and explosion breccias, which show an asymmetric distribution having the maximum thickness towards the south. It represents a single phreatomagmatic eruptive

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episode that occurred at the same time than the phreatomagmatic phase of the Puig de Banya del Boc volcano.

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Puig d'Adri

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This volcano is located on the Adri normal fault, which puts in contact Palaeocene and Eocene materials and is buried by Neogene sediments towards the south (Fig. 5). The construction of the

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Puig d'Adri volcano involved the superposition of three volcanic edifices, starting with the formation of a tuff-ring of 850 m in diameter, followed with the development at the western side of the tuff-ring of a scoria cone of small dimensions, and ending with the construction of a new scoria cone that formed the main relief of the volcano and covered most of the previous structures.

The Puig d'Adri volcano shows one of the most complex eruption sequences of the CVZ, which includes five different eruption phases. The eruption started with a phreatomagmatic event that generated an irregularly distributed deposit of lithic-rich explosion breccia and a succession of lithic-rich pyroclastic surges which were emplaced towards the south-east flowing for more than 5 km from the vent following the main gullies. This initial explosive phase was immediately followed by a short strombolian episode that generated a scoria lapilli deposit with limited extend. The eruption activity returned to phreatomagmatic with higher intensity than the previous one, generating a new succession of pyroclastic surge deposits similar to the former one, explosion breccias, and a pyroclastic flow deposit that flowed for more than 3 km towards the south inside the course of the Canet river (Figs. 9i and 9j). Most of the tuff-ring structure was constructed during this second phreatomagmatic episode. The eruption continued with a sustained strombolian phase

ACCEPTED MANUSCRIPT that generated a widespread scoria lapilli deposit around the main vent covering most of the proximal phreatomagmatic products and giving rise to the construction of the main scoria cone. The eruption ended with an effusive phase that generated two lava flows that caused the breaching of the north-western flank of the scoria cone. One of the lavas flowed for more than 12 km towards the

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south (Fig. 5).

Most of the lithic clasts in the phreatomagmatic deposits of the Puig d'Adri volcano correspond to red sandstones and marls of the Eocene Bellmunt Formation, indicating once again the significance

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of such unit as aquifer at a regional level. This stratigraphic unit is located several hundred metres below Puig d'Adri. The size of the lithic fragments varies depending on the deposit . In the

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pyroclastic surges such clasts are of millimetric to centimetric size while in the explosion breccias

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and the pyroclastic flow deposit they may reach several tens of centimetres across. Juvenile

ACCEPTED MANUSCRIPT erosion of the deposit by pervasive infiltration of meteoric water along the columnar joints (Fig. 9j).

Crosa de Sant Dalmai

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This maar volcano is located at the boundary between La Selva tectonic depression, replenished

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with Pliocene and Quaternary sediments, and the Transversal chain formed by Palaeozoic granites and metamorphic rocks (Figs. 1 and 11). It is mostly composed of phreatomagmatic deposits which form a circular tuff-ring around a shallow crater of

1250 m in diameter. The tuff-ring is

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asymmetrical, being higher at the west (maximum height of 50 m), where the internal and external slopes are also steeper, than at the east (maximum high of 30 m). Also, the deposits surrounding the

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rim show a wider distribution towards the east (Fig. 11). The age of this volcano is not well constrained. Although it is localised in the la Selva sector (Figs. 1 and 2) where most of the

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outcropping volcanic rocks have ages older than 2 Ma (AraĂąa et al., 1983), it is evident from the state of preservation of its morphology and juvenile components that the Crosa de Sant Dalmai

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volcano must be much younger.

The succession of deposits that form the Crosa de Sant Dalmai shows the same stratigraphy all

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around the tuff-ring, thus suggesting that most of them were radially distributed from the vent. They reached distances of nearly 4 km towards the east and only of a few hundred metres towards the

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west. This asymmetrical distribution of the deposits seems to be related to different strength of the rocks that form the substrate at each side below the volcano. At the east the substrate corresponds to unconsolidated Pliocene and Quaternary gravels, whereas toward the west the substrate is formed

by Palaeozoic granites and schists. This difference in rock strength may have played a major role during the eruption making it easier to excavate towards the eastern side in each explosion (MartĂ­ et al., 1986). The lowermost unit of the succession of deposits of the Crosa de Sant Dalmai volcano is not fully exposed and corresponds to > 1 m thick coarse lithic-rich breccia with blocks up to 70 cm in diameter of granites and schists with subordinate scoria lapilli and cauliflower bombs. Above it there is a uniform sequence composed of 22 alternating units of lithic-rich explosion breccia deposits and crudely stratified, coarse-grained pyroclastic surge deposits (Figs. 10 and 12). The next unit in the stratigraphic succession corresponds to a 1 m thick strombolian, non-welded scoria lapilli deposit, which is followed by six more alternating units of lithic-rich breccia and pyroclastic surges. The eruption ended with a new strombolian episode from a new vent opened at the interior of the maar, which formed a small scoria cone and a lava flow emplaced inside the maar. The lithic clasts contained in the phreatomagmatic deposits are always angular and may constitute up to 80 % of the deposit. The juvenile clasts are of basaltic composition and lapilli size, and are typically

ACCEPTED MANUSCRIPT poorly vesicular except the scoria lapilli fragments that form the strombolian deposit in the middle of the succession, which are highly vesiculated as well as the ones that form the inner scoria cone.

The presence of mantle derived nodules and inclusions is common in the juvenile fragments, thus

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suggesting a rapid ascent of magma from the source/storage region.

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Discussion

As it has been described in the previous section, the successions of pyroclastic deposits at the

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youngest volcanoes of the CVZ suggest the existence of diverse eruption sequences characterising the eruptive activity of this monogenetic volcanic field (Table 1). This contrasts with the relative

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compositional monotony of the magmas (alkali basalts and basanites) involved. The physicochemical characteristics, also including the degree of vesicularity and crystallisation, of the CVZ

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magmas are very similar in most of the studied volcanoes. They mainly correspond to leucite basanites, nepheline basanites and alkali olivine basalts, with a phenocrysts (olivine, pyroxene, plagioclase) content up to 12 %, with an aphiric to microcrystalline or microlitic groundmass, and a

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total water content up to 1.5 % (CebriĂĄ et al 2000). The vesicularity of juvenile clasts in pure strombolian deposits typically ranges between 65 to 85 % in most cases, it may decrease till less

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than 40% in the phreatomagmatic ones. Ascent velocities of the order of 0.2 m/s were calculated using the presence of large mantle derived nodules and lower crust xenoliths in some of these

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volcanoes (MartĂ­ et al 1992). Densities in the range of 2.70 to 2.87 g/cm3 and typical viscosities of the order of 10-102 Pa s have been calculated using standard methods based on crystal content and rock composition and assuming temperatures of the order of 1200-1300ÂşC. Despite some variations in the dynamics of the magmatic episodes may be attributed to changes in magma flow conditions related to changes in crystallinity and vesicularity (gas content) of the erupting magma (Cimarelli et al., 2010), it is obvious that variations in magma physics is not the main reason to account for the large diversity of eruption sequences recorded in the CVZ when we also take into account the phreatomagmatic episodes.

In fact, as it can be deduced from the successions of deposits observed in each volcano, differences in eruptive behaviour are related in many cases to the occasional interaction of the ascending magma with groundwater. Magma/water interaction is, thus, the main reason why a large number of these volcanoes significantly deviate from the typical hawaiian-strombolian behaviour that characterise some of them and monogenetic basaltic volcanoes in general. The way and timing in which such magma/water interaction occurred during the course of the CVZ eruptions may differ considerably from one volcano to other. This contrasts with other monogenetic volcanic fields

ACCEPTED MANUSCRIPT where eruptions seems to follow a more general pattern (Hougthon et al., 1999; Valentine and Gregg, 2008; Brand and Clarke, 2009; Brand et al., 2009; Clarke et al., 2009; Sottili et al., 2009). In the present case, the large diversity of eruption sequences observed should be explained by variations in the stratigraphy and structure of the substrate beneath each volcano and the hydraulic

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characteristics of each aquifer.

Two main sedimentary aquifers have interacted with CVZ magmas causing such a wide variety of phreatomagmatic episodes and eruption sequences. One aquifer is located at an average depth of a

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few hundred meters below the volcanic cones, while the other is much shallower, just a few tens of metres below the surface. The deep aquifer corresponds to Eocene continental sediments, known as

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the Bellmunt Formation, composed of conglomerates, feldspar-rich sandstones and red mudstones, and the shallow aquifer corresponds to volcanic and alluvial deposits of Quaternary age, mostly

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formed by unconsolidated gravels, sands and clays and volcanic products (lavas and pyroclasts) from former eruptions. There is also a third aquifer that has played a significant role in some of the most important eruptions (Puig de Banya del Boc and Crosa de Sant Dalmai), which corresponds to

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the highly fractured (granites and schists) Palaeozoic rocks . The depth of this last aquifer varies

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depending on the local structure in each area but it may be a few hundred metres deep or shallower.

For example, the eruption sequence deduced for the Croscat-Santa Margarida volcanoes reveals

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how complex monogenetic volcanism may be when phreatomagmatic episodes alternate with pure magmatic ones. In addition, in this particular case, eruption changed from a localised vent at the beginning, to a fissural vent and again to a central vent different from the first one. The phreatomagmatic events that we have identified at the beginning and at the end of the explosive activity in this eruption correspond, respectively, to the interaction of the erupting magma with two different aquifers. The first phreatomagmatic episode, which corresponds to the Santa Margarida vent opening phase, was caused by the interaction of the ascending magma with the Eocene aquifer located at about 250 m of depth below the paleosurface, as it is suggested by the abundance of lithic clasts of such lithology. The second phreatomagmatic event, at the end of the explosive activity of Croscat, occurred by the interaction of the erupting magma with a shallow aquifer installed in Quaternary unconsolidated deposits and lavas from previous eruptions.

The Can TiĂ eruption sequence also illustrates a situation in which magma/water interaction occurred at different stages of the eruption, but in this case in the same aquifer. In the first phreatomagmatic phase magma/water interaction occurred by invasion of the Eocene aquifer by the ascending magma. As magma pressure in the conduit was still high enough to cross the aquifer and

ACCEPTED MANUSCRIPT to avoid a massive interaction of water with magma, this was short and volumetrically small.

Magma ascended to shallower levels, probably using other parts of the same eruptive fissure, and continued its decompression and consequent fragmentation without interacting with groundwater.

As eruption progressed during this strombolian phase magma pressure decreased progressively in

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the conduit causing the progressive deepening of the fragmentation level. Groundwater from the

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Eocene aquifer interacted again with magma when its pressure was higher than that of magma in the conduit, thus indicating that the magma column was already significantly vesiculated or fragmented. However, as it is indicated by the progressive appearance of lithics in the lapilli

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deposit this was a gradual process, till the pressure difference reached a threshold in which the entrance of water into the magma conduit became massive. This caused an increase in explosivity

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and the sudden enlargement (by erosion) of the conduit, and a subsequent increase in the eruption rate. The erupted mixture of relatively cold lithics and fragmented magma was too dense and cold

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to be sustained by an eruption column and immediately collapsed on the vent forming a poorly expanded pyroclastic flow. Eruption could end either because magma supply stopped or because the

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conduit collapsed and blocked.

Another interesting example of the variation in the eruption dynamics that can be offered by two

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close volcanoes corresponds to the Clot the l'Omera and Puig de Banya de Boc volcanoes. The phreatomagmatic activity that characterises the eruption of the Clot de l'Olmera volcano and the

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first eruptive episodes of the Puig de Banya del Boc volcano, and the sole presence of Palaeozoic lithic fragments in their deposits, indicate that there was an important aquifer in the metamorphic rocks that form the substrate of these volcanoes. The synchronicity of both eruptions and the different erupted products generated in both phreatomagmatic phases suggest that the aquifer was intersected at the same time by the ascending magma in two different points of the same eruptive fissure and that the resulting magma/water interaction changed from one point to other. In the Puig de Banya del Boc the magma/water ratio increased during its phreatomagmatic episode passing from the generation of dry pyroclastic surges to that of a wet pyroclastic flow. However, in the Clot de l'Omera the alternation of dry pyroclastic surges and explosion breccias suggest a pulsating sequence of phreatomagmatic explosions in which magma/water interaction was intermittent but with more or less the ratio in each explosion.

In this sample of different eruptive behaviours it is also relevant to mention the eruption of Puig d'Adri volcano that, in a similar way than in the Santa Margarida and Can TiĂ volcanoes, it also initiated with a phreatomagmatic event triggered when the ascending basaltic magma interacted with a relatively deep aquifer. This first magma/water interaction episode suddenly stopped giving

ACCEPTED MANUSCRIPT rise to a short strombolian episode that generated a restricted fallout scoria lapilli deposit, which started as purely magmatic and gradually incorporated an increasing amount of lithics of Eocene rocks. This represents another example, as Can TiĂ and Croscat, of a progressive interaction of magma with an aquifer. In this case the magma/water interaction occurred in the Eocene aquifer.

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and increased progressively until giving rise to a new phreatomagmatic event. This was marked by

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the eruption of a new series of pyroclastic surges and a final pyroclastic flow, which represents the culmination of the magma/water interaction in Puig d'Adri volcano, and after which the eruption

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continued as purely magmatic.

Finally, the eruption of the Crosa de Sant Dalmai volcano is a classical example of

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phreatomagmatism caused by the interaction of the ascending magma with a shallow aquifer. In this case the aquifer was probably installed into the Quaternary unconsolidated sediments but it may

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have been also a significant contribution from a second aquifer installed into the fragmented Palaeozoic rocks, as it is suggested by the abundance of angular lithic fragments of these lithologies. Magma supply was continuous during the whole eruption but the amount of water

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available was not constant, as it is indicated by the variations of the proportions of lithics and juvenile fragments in the resulting deposits and the presence of pure strombolian phases at the

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middle and end of the eruption.

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All the investigated examples offer different case studies but confirm how complex monogenetic basaltic volcanism may be even in a relatively small area when interaction of the erupting magma with groundwater occurs. This is particularly relevant when there are different aquifers, with different hydraulic characteristics, and when the substrate

below the volcanoes may show a

complex stratigraphy and structure due to local tectonics, as it is the case studied in this paper. In fact, differences in substrate stratigraphy and rock strength may play a significant role in the resulting eruptions and products, as has been pointed out in other volcanic areas (Sohn and Park, 2005; Auer et al., 2007; MartĂ­n-Serrano et al., 2009). We do not want to go deeper in this discussion on the exact mechanisms that have controlled magma/water interaction in each particular case, so that the reasons why eruptions may start violently with a phreatomagmatic episode or quietly with a hawaiian or strombolian one will have to be considered in further studies. However, we want to emphasise the importance of knowing, in addition to magma physics and chemistry, the s geology of the substrate below the monogenetic volcanic fields in order to understand potential eruption mechanisms when groundwater may be present. The large diversity of eruption sequences deduced in the CVZ reveal that most of the variables that have controlled them depend on the local geology rather than on

magma composition, crystal content, vesiculation and fragmentation prior to

ACCEPTED MANUSCRIPT explosive interaction with groundwater, which do not change significantly among the cases studied in this paper. This is particularly relevant when we try to conduct hazard assessment in these areas.

In addition to the initial problem that is usually imposed by the lack of a precise geochronology in long lived volcanic fields, we need to consider the wide variety of potential eruptive scenarios that

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may occur, as it is the case in the CVZ. Phreatomagmatism is traditionally considered unpredictable

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mainly because the hydrological characteristics of the terrain are not always well known.

Magma/water interaction increases explosively and may significantly alter the curs of a magmatic eruption giving rise to the appearance of more hazardous phenomena. The example of the CVZ

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illustrates quite well this relevant aspect of phreatomagmatism, but also that having a good

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knowledge of the geology and hydrogeology of the substrate it may not be so unpredictable.

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Conclusions

We have presented the main characteristics of the youngest volcanic episodes of the Catalan Volcanic Zone, a Quaternary alkaline volcanic province of the European rifts system. In this

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monogenetic volcanic field we have recognised more than 50 well preserved volcanic edifices in most of which purely magmatic episodes (hawaiian to violent strombolian) alternated with

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phreatomagmatic ones, giving rise to a large variety of eruption sequences and corresponding successions of deposits. We conclude that the diversity of eruption sequences deduced reveal that

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the main cause of such complex eruptive behaviour resides in the stratigraphic, structural and hydrogeological characteristics of the substrate above which the volcanoes were emplaced, rather than on changes of magma composition, crystal content, vesiculation and fragmentation prior to explosive interaction with groundwater.

Acknowledgements

We thank the Natural Park of the La Garrotxa Volcanic Zone and its staff to allow us to undertake this research and for all the support we have always received from them. A. Geyer is grateful for her Beatriu de Pinó#s post-doctoral fellowship 2008BPB00318. Constructive reviews by Danillo Palladino, Karoly Nemeth and an anonymous reviewer are greatly appreciated

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1) Simplified geological map of the Catalan Volcanic Zone and its surroundings (modified from Guerin et al., 1985). In the inset, distribution of the European rifts system. The studied area is

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indicated by a squared frame.

2) Age of the Catalan Volcanic Zone volcanism (data from Donville, 1976, Ara単a et al., 1983, and

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Guerin et al., 1985)

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

4) Simplified geological and structural map of the N sector of the studied area with indication of the

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location and name of the main volcanic cones

5) Simplified geological and structural map of the northern side of the S sector of the studied area

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with indication of the location and name of the main volcanic cones

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

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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-Santa Margarida stratigraphic section we have been able to use unpublished data from water boreholes provided by the Natural Park of La Garrotxa Volcanoes

8) Geological cross-section of the Croscat and Santa Margarida volcanoes. Note that the phreatomagmatic deposits (pyroclastic surges and phreatomagmatic ash) resulting from the 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.

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. This deposit 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 Santa Margarida 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

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presence of soft-sediment deformation and ballistic impacts on one of the wet deposits. g) Detail of

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a succession of fine and 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 in stratigraphic continuity there is a lithic-rich strombolian lapilli deposit. h) Detail of a

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strongly indurated pyroclastic surge deposit from the second phreatomagmatic phase at the Puig d'Adri volcano showing a marked wave form at the base. i) Close view of the lithic-rich facies

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found at the base of a pyroclastic flow unit from the Puig d'Adri volcano. j) Detail of the pyroclastic flow deposits from the Puig d'Adri volcano emplaced into the course of the Canet river, showing

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large -scale (up to 5 m across) columnar jointing, which allowed the vertical erosion of the deposit by pervasive infiltration of meteoric water along the columnar joints.

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

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1986)

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

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

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