La Garrotxa Volcanism Guide 2012

A Field Guide to La Garrotxa Volcanic Zone

Volcanoes

Generalitat de CatalunyaDepartament d’Agricultura, Ramaderia,Pesca, Alimentació i Medi Natural

VolcanoesA Field Guide to La GarrotxaVolcanic Zone

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Biblioteca de Catalunya - Dades CIP

Volcanoes: A Field Guide to La Garrotxa Volcanic Zone BibliographyISBN 9788439388524I. Martí i Molist, Joan, 1957- II. Catalunya. Departamentd'Agricultura, Ramaderia, Pesca, Alimentació i Medi Natural III.Parc Natural de la Zona Volcànica de La Garrotxa (Catalunya)1. Vulcanisme – Catalunya – Garrotxa 2. Parc Natural de la ZonaVolcànica de La Garrotxa (Catalunya) – Guies551.21(467.1 Gt)(036)

Published byLa GarrotxaVolcanic Zone Natural Park

Legal deposit: B-11.374-2012ISBN 978-84-393-8852-4

Original title: El vulcanisme Guia decamp de la Zona Volcànica de laGarrotxa (2000,2001)

Title: Volcanoes A Field Guide to LaGarrotxa Volcanic Zone

© La Garrotxa Volcanic ZoneNatural Park and authors

© Traduccions i Tractament de laDocumentació, SL and Mike Lockwood

Digital VersionNatural Parc web page

Printed byAmpans, Manresa

1st editionOlot, April 2012

PhotographsPep CallísCover, figures 29, 34-37, 58-63, 66, 69, 73-76,85, 87, 95, 97, 98, 100, 102, 105, 106 and 114 (de-posited in La Garrotxa Volcanic Zone Natural ParkDocumentation Centre)

Albert PujadasFigures 28, 30, 33, 39, 40, 64, 66, 72, 78-80, 108,110 and 113

Joan MartíFigures 15, 27 and 31

Emili BassolsFigure 32

La Garrotxa Volcanic Zone NaturalPark Documentation CentreFigures 65, 67, 70 and 83

Maurice KrafftFigure 18

National Geographic Data CentreFigure 43

Llorenç PlanagumàFigures 71, 77 and 102

IllustrationsAlbert MartínezFigures 1, 2, 6-12, 15-17, 19-23, 25, 26, 38, 41, 42,44-50, 54, 56 and 57

Albert PujadasFigures 3-5, 13, 14, 24, 51-53, 55, 68, 107, 108, 109,110, 111, 112, 113, and 115-117

Llorenç PlanagumàFigures 81, 82, 84, 86, 94, 96, 99, 101, 103 and104 (Figures 82, 84, 86, 94, 96, 99, 101 and 104have been modified according to the Vulcà Projectgeological base)

Montse ViñasOriginal drawings for figures 88-93

Bibliographical references standardised and adapted by Montse Grabolosa

With the support of the environmentaleducation organisationsLa Cupp SCCL, Verd Volcànic andTosca


VolcanoesA Field Guide to La GarrotxaVolcanic Zone

Joan Martí i MolistJaume Almera Institute of Earth Sciences (CSIC), Barcelona

Albert PujadasGeodynamics Area Department of Environmental Sciences. University of Girona

Dolors Ferrés LopezLlorenç Planagumà GuàrdiaTosca. Collaborators with La Garrotxa Volcanic ZoneNatural Park

Josep Maria Mallarach CarreraOlot Foundation for Higher Education


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Just over 200 years ago, Francesc Xavier de Bolòs divul-ged the existence of the volcanoes in La Garrotxa to thescientific community for the first time. These volcanoes,whose eruptive activity had remodelled the landscape ofOlot and its valleys, have had a remarkable influence overthe centuries on local land-use and human activity.

The extensive quarrying undertaken in part of the volcanicarea from the 1960s to the 1980s provoked considerablesocial and scientific opposition, which eventually led to thepassing of a law in 1982 declaring the volcanoes a pro-tected area.

The conservation of this natural heritage is justified by thefact that this is the youngest volcanic area in the IberianPeninsula and one of the best preserved such areas incontinental Europe. The geomorphological features foundhere include volcanic cones, lava flows, barrage lakes andbasalt cliffs, and there are numerous sites where the geo-logical processes that have generated so many differentvolcanic morphologies can be easily observed in great de-tail.

Despite its legal protection, as part of the tasks implicit inthe organization and consolidation of the Natural Park itwas still necessary to halt the quarrying and to minimizeand restore the region’s damaged geological heritage. Amilestone was reached in 1995 with the restoration of themost emblematic volcano in the park, Croscat, not onlythe youngest volcano in the Iberian Peninsula, but also theone that has suffered most environmental impact.

Nevertheless, more in-depth knowledge was required inthe Park itself of the local volcanoes in order to build uponthe studies undertaken early in the twentieth century andthen reactivated in the 1960s. Initially, it was necessary toreview all previous work and develop a project for a com-prehensive study of the geology of the Catalan volcanicregion. The aim of this project, first contemplated in theearly 1990s, was to study various geological and geophy-sical aspects of the Park as a means to learning moreabout the region’s geological heritage in general.Eventually, in 1993 a project began that, despite its narro-wer scope, was still very ambitious. It was financed enti-rely by the Department of the Environment through LaGarrotxa Volcanic Zone Natural Park and executed by theSpanish National Research Council (CSIC) under the su-pervision of Dr Joan Martí, and would enable new geolo-

Foreword


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gists to be trained in the learning, management and rai-sing of awareness of the volcanoes of La Garrotxa.

The results of this project are included in this guide, whichin plain and simple terms provides new and valuable infor-mation for the study of the volcanoes of La Garrotxa. Thepublication of this guide is part of the Natural Park's ma-nagement strategy, approved in 2000, which will enableus to improve our knowledge of volcanic activity in the re-gion, plan research, preserve the Park’s geological andscenic values and increase awareness of the volcaniczone at local, national and international scales.

I hope that this guide, which has been painstakingly pre-pared following strict criteria, helps to increase awarenessof the value of this volcanic zone amongst teachers, uni-versity students and naturalists alike, thereby guarante-eing the knowledge, management and dissemination of aheritage that has been preserved for future generations.

Francesc Xavier Puig i OliverasDirector of La Garrotxa Volcanic Zone Natural Park


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Introduction 8

l1l Volcanoes 11

l1 l1 l What is a volcano? 12

l1 l2 l Magma genesis 14

l1l2l1l Where is magma generated? 15

l1 l3 l Magma ascent 17

l1l3 l1l How does magma ascend? 18

l1l3 l2l What happens to magma during its ascent? 19

l1 l4 l Eruptive activity 22

l1l4 l1l Why do eruptions occur? 23

l1l4 l2l Types of eruptive activity 24

l1 l4 l2 l1 l Effusive activity 24

l1 l4 l2 l2 l Explosive activity 24

l1l4 l3 l Volcanic materials 31

l1 l4 l3 l1 l Massive materials 31

l1 l4 l3 l2 l Fragmentary materials 34

l1 l4 l3 l3 l Types of pyroclastic deposit 35

l1l4 l4 l Volcanic morphology 39

l2 l Volcanism in Catalonia 41

l2 l1 l Distribution and evolution of volcanoes 42

l2 l2 l The Catalan volcanic field 45

L'Empordà Volcanic Zone 46

La Selva Volcanic Zone 46

La Garrotxa Volcanic Zone 46

l2 l3 l Rocks and magma 49

l2 l3 l1 l Minerals 50

l2 l3 l2 l Geochemical data 51Magma genesis and ascent

l2 l4 l Eruptions in La Garrotxa Volcanic Zone 53

l2 l4 l1 l Volcanoes and their phases of eruptive activity 54

l2 l4 l2 l Eruptive activity and volcanic edifices 57

l2 l5 l Volcanic materials 58

Contents


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l3 l La Garrotxa Volcanic Zone. Sites of volcanic interest 61

1 l Castellfollit de la Roca: lava flows 64

2 l El Cairat: pyroclastic breccia 66

3 l Sant Joan les Fonts: massive materials 68

4 l Montsacopa: cone morphology 72

5 l Croscat: cinder cone 74

6 l Turó de la Pomereda: an eruption sequence 76

7 l Santa Margarida: pyroclastic deposits 78

8 l Can Tià: eruption sequence 80

9 l Els Arcs Valley: pyroclastic flow 82

10 l Location and morphology of the volcanic cones as seen 84from Puig Rodó

11 l El Clot de l’Omera: maar 86

12 l Puig d'Adri: pyroclastic flow 88

13 l Puig d'Adri: pyroclastic surges 90

14 l The morphology of La Crosa de Sant Dalmai 92

15 l Pyroclastic surge and breccia of La Crosa de Sant Dalmai 94

Glossary 97

Bibliography 98

Map of La Garrotxa Volcanic Zone Natural Park Services 101

Environmental education organisations 102

Notes 104

Recommendations and indications for visitors 108


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This field guide presents a general but detailed view of themain features of La Garrotxa Volcanic Zone. It aims to bea useful tool for interpreting the landscape and geologicalprocesses in this volcanic zone and to provide the neces-sary tools for understanding from a geological perspectivesome of the most representative volcanic sites in the re-gion.

How significant is the presence of volcanoes in a regionsuch as this? In which geodynamic period should they beplaced? What is the origin and composition of volcanicrocks? What types of eruptions occurred? These are justsome of the questions this field guide answers.

Before offering an explanation of the volcanic history of LaGarrotxa, this guide takes a look at general concepts ofgeology and volcanology that relate to the subject matter.Therefore, we first examine magma, how it is generatedand reaches the surface, how its composition varies overtime, the mechanisms that give rise to volcanic eruptions,and the main features of eruptions and their resultingstructures.

This book consists of three parts:

1. Volcanoes. An explanation of the general aspects andbasic concepts of volcanism.

2. Volcanism in Catalonia. A brief description of thebasic features of the most recent volcanic activity in theregion.

3. Sites of volcanic interest. A description of 15 sites,the basis of a true field guide. The sites were selected ac-cording to the geological elements that can be observedand together exemplify the most remarkable features ofthe volcanoes found in Catalonia and, in particular, in LaGarrotxa Volcanic Zone Natural Park. Accessibility wasalso a taken into account so that visits would be fairly sim-ple. The selection of just 15 sites inevitably meant that ot-hers were omitted, many of which are also of great geolo-gical and educational interest, but far less accessible.

This guide can be used on many levels: the text is accom-panied by text boxes with a maroon background contai-ning explanations of concepts of interest such as magmaand the Earth's internal structure. The definition of termswritten in italics can be found in the glossary.

Introduction


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Although the information given is presented in a relativelysmall space, we hope that the reading of this guide duringa visit to the proposed sites will provide a general idea ofwhy and how volcanoes occur in this region, still one ofthe least known geological features of Catalonia.

The authors wish to thank the Catalan CartographicInstitute for the images and maps used in figures 54, 56,57 and 81, and the Natural Sciences Section of the LaGarrotxa Museum for the rock specimens appearing inthe photographs.


Volcanoes

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Everyone has some kind of idea, more or less exact, ofwhat a volcano is. Yet, when we try to explain this idea in‘scientific’ terms, the concept becomes less clear and inmany cases we have to resort to somewhat imaginativemorphological descriptions.

This definition makes it clear that a volcano is not merely itsfinal morphology, but rather is the culmination of a series ofgeological processes that involve the genesis, ascent anderuption of magma (Figs. 1 and 2).

Although on both geological and human time scales volca-noes represent relatively short periods of time, from just afew days to thousands of years, they are actually the resultof processes that last for hundreds of thousands or evenmillions of years.

Figure 2. Volcanic edifice

Figure 1. Volcanic system

What is a volcano?l1 l1 l

• • • A volcano is a vent in the Earth'ssurface through which molten rock (magma)generated within the Earth and, occasionally,non-magmatic material issue. Theaccumulation of these products around apoint source gives rise to shields or cones ofdiffering morphologies. • • •


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Magma

• • • Magma is a mixture of molten, mainly silicate, rock that containssolid particles in suspension (crystals and rock fragments) and dissolvedgases. • • •

The vast majority of the rocks we know ofare made of minerals belonging to the silicafamily, that is, minerals consisting of SiO4

anionic groups, isolated or bonded with ot-hers by metal cations (Fig. 3). For this rea-son, the magma resulting from the meltingof these rocks is also mainly silicate.Depending on the percentage of silica itcontains, magma is classed as either basic(less than 52%), acid (over 63%) or inter-mediate (between 52% and 63%).

Figure 3. SiO2 molecule

Physical propertiesDensity, viscosity and temperature are three of themost significant physical properties of magmathat determine the nature of the processes of as-cent and eruption. Density depends mostly on thechemical composition of the molten materials,whereas viscosity – the lava’s resistance to flow -also depends on the composition of the magmaand its temperature (Fig. 4).

Density varies according to the silica content(SiO2) of the magma. Basic magma with a lowersilica content has a higher density due to the gre-ater number of heavy metal cations it contains.

Acid magma is more viscous than basic magma,due to the larger number of bonds between its sili-ca molecules: the greater the temperature, thelower the viscosity, since heating favours molecularexcitation and makes it harder for bonds to form.

Basic magma reaches higher temperatures, of upto 1,100°C, while acid magma melts at700–800°C.

Figure 4. Variation in the composition and physicalproperties of magma


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Magma genesisl1 l2 l

Magma is formed inside the Earth, generally in the uppermantle, although occasionally it may be generated nearerthe surface in the Earth’s crust.

Molten material forms for a number of reasons that mayact individually or in combination: decompression, anincrease in temperature or greater presence ofwater (Fig. 5).

Magma is generated if asolid rock body is subject toa considerable increase in

temperature or if a rock, initially subject to very high tempe-rature and pressure, undergoes a great fall in pressure.However, in conditions of constant temperature and pressu-re, the assimilation of water by some of the minerals thatmake up the rock significantly lowers its melting point.

Figure 5. Causes of rock melting

• • • Magma genesisis the processwhereby the rocks inthe Earth's mantleand crust changefrom a solid to aliquid state. • • •

Melting affects only part of the rock. Rocks con-sist of various minerals, each with different meltingpoints at a given pressure. Magma genesis be-gins when the minerals with the lowest meltingpoints melt and then continues as the remaining

minerals in the rock also begin to melt. Thus, wealmost always speak of the partial melting of rockssince at any one time only some minerals meltand only in certain proportions (Fig. 6).

a. The melting process beginsin contact zones betweens largeminerals since it is here that thesmallest amounts of energy arerequired to melt the rocks.

b. The liquids generated are lessdense than the minerals that su-rround them. These liquids form anetwork of small interconnectedchannels and build up in certainareas until a minimum critical volu-me is reached; from this momentonwards, the liquids begin to as-cend due to their buoyancy.

c. Melting continues and the vo-lume of liquid increases andbuilds up near the roof of themelt zone. At the same time, theresidual solids compact down-wards, producing an increa-singly effective separation bet-ween solids and liquids.

Partial melting

Figure 6. Partial melting process


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Where is magma generated?The processes relating to magma formation can be explai-ned in the context of the theory of plate tectonics. Volcanicactivity and in general magmatic activity is not randomlydistributed over the Earth's surface, but is mostly concen-trated along the edges of tectonic plates. However, we findvolcanoes in places other than plate edges, both on landand at sea, which tells us that melting at a local scale alsotakes place (Figs. 7 and 9).

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In terms of the composition and density of its ma-terials, the Earth's interior is divided into three la-yers: core, mantle and crust (Fig. 8). As well, wecan define two external layers in terms of the rigi-dity of the materials:a. the lithosphere, made up of the crust and the

outermost part of the mantle, is fragile in beha-viour.

b. the asthenosphere, just below the lithosphe-re, represents the upper part of the mantle,which is plastic in behaviour and can flowwhen subject to great forces.

The theory of plate tectonics proposes a dynamicmodel of how the Earth works based on the factthat the lithosphere consists of a relatively smallnumber of plates floating independently of eachother on top of the asthenosphere.

The internal structure of the Earth

Figure 8. Internal structure of the Earth

Figure 7. Tectonic plates and the location of areas of volcanic activity


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Subduction zones

When two plates colli-de, one slides underthe other. When the co-oler lithosphere sinksinto the mantle, the lat-ter’s temperature is lo-wered. Melting still oc-curs, however, whenwater enters the mineralsystem of the mantle.This water, generatedby the dehydration ofsubducted minerals, lo-wers the melting pointof the minerals, therebyenabling part of therocks to melt eventhough the ambienttemperature has drop-ped considerably.

Oceanic ridges

Two lithospheric platesmove apart, which leadsto a decompression ofmaterial in the mantleand the melting of hugevolumes of solid rockthat then rise continuallytowards the dorsal axisof the ridge.

Hotspots

Volcanic regions far fromplate boundaries are ge-nerated by an anoma-lous increase in the tem-perature in the mantlecaused by a convectionrising in a single plumefrom the core-mantleboundary.

Rift zones

In inner plate areas, con-vection in the mantleleads to a thinning of thecrust and generates dis-tensive processes thatcan culminate in thecomplete rupture of thelithosphere and the for-mation of new oceanfloor. In some areas, thesplit in the lithosphere ispartial or does not occurat all; nevertheless, asystem of normal faultsdoes develop favouringthe ascent of magma.

Geodynamic environments of volcanism

Figure 9. Terrestrial lithosphere. Types of contact between tectonic plates

Plate boundaries Intraplate areas


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Magma will break off from the melt zone and rise whenthe volume of molten material is sufficient to overcomethe pressure exerted by the surrounding rocks.

In some cases magma rises to the Earth's surface di-rectly, almost without stopping, giving rise to individual,short-lived eruptions. Frequently, however, magma ac-cumulates in intermediate areas of the lithosphere inmagma chambers (Fig. 10), where it may solidify com-pletely or continue to rise to the surface.

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Magma ascent

• • • Magma ascent is the displacement of moltenmaterial from source areas to the surface anddepends on the volume of liquid initiallygenerated, its physical properties and the tectonicstructure of the surrounding area. • • •

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Magma chambersThese are reservoirs of molten rock that formwithin the lithosphere at depths of 1–60 kilo-metres, which are fed periodically by magmafrom melt zones. If they are connected to theEarth's surface, successive eruptions takeplace forming volcanoes or complex volcano-es with a long - but not necessarily continuous- periods of activity. This is the case of volca-noes such as Teide, Fuji, Etna and Vesuvius.

Magma ascent may halt within the Earth for re-asons related to crust structure and the distri-bution of tectonic forces at each point. Inareas of magma accumulation neutral densityexists, that is, the density of the magma isequal to that of the surrounding rocks.

Figure 10. A magma chamber


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How does magma ascend?Since liquids are less dense, differences in pressurebetween the magma and the surrounding rocks causemagma to rise. Magma ascent mechanisms are of twotypes: diapirs or dykes (tensile fractures) (Fig. 11).

Magmas generated in the upper mantle initially rise asdiapirs into shallow areas, where, due to the fragile be-haviour of the rocks, they move through fractures. Themobility of these relatively fluid basic magmas meansthat they can move through even narrow fractures.

Magmas generated in the crust are more acidic in com-position and consequently are more viscous. Given theirmobility, they can only rise as large diapirs. The move-ment of these magmas through narrow fractures is veryrare and only occurs under favourable structural condi-tions. Although they often reach the Earth's surface,masses of molten material build up in the crust formingbodies of rock known as plutons. Their subsequent soli-dification gives rise to plutonic igneous rocks.

Ascent through dykes occurs due to the pres-sure exerted by the magma as it rises towards thesurface. The molten material causes fractures towiden, which then close up again once themagma has passed through.

Diapirs are bodies of buoyant magma that pushthrough ductile rock in the lower crust or mantlethat deform on contact with the magma at hightemperature.

Figure 11. Ascent through dykes and diapiric ascent

Ascent through dykes diapiric ascent

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What happens to magma duringits ascent?Magma differentiates on its way to the surface, that is,its composition changes. Three principal mechanisms ofmagmatic differentiation occur during ascent: fractionalcrystallisation, magma mixing and assimilation of countryrock. These processes take place simultaneously or in-dividually and result in a broad range of chemical com-positions in the resulting magmas.

Fractional crystallisation

The pressure and temperature to which magma is sub-jected generally drop as it moves upwards. Under thesenew thermodynamic conditions, the various chemicalelements in the magma regroup and form increasinglystable structures that give rise to the first solid nuclei.These nuclei grow to form crystals separate from the li-quid, which has a different composition from the primarymagma.

This process may be repeated several times during theevolutionary history of the magma. Thus, from an initialmagma various different rocks (mineral aggregates) andresidual liquids, all of different composition, may form(Fig. 12a).

Magma mixing

As it rises to the surface, magma may mix with othermagmas of different composition and different physicalproperties. The end result will be magma with differentcharacteristics from the initial magmas (Fig. 12b).

Assimilation

In some cases, at higher temperatures, magma maypartially melt the surrounding rock and assimilate part ofits minerals, thereby again altering the original composi-tion of the magma (Fig. 12c).

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Figure 12a. Fractional crystallisation

Figure 12b. Magma mixing

Figure 12c. AssimilationInserted rock


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What rocks can tell us

Despite the relatively small number ofmelting mechanisms and places wheremelting takes place, the different types ofrock that melt in the source area, theexistence of degrees of partial melting

and the processes of magmatic differen-tiation all ensure that a wide range of dif-ferent types of magmas are formed.Consequently, the solidification of thesemagmas is the origin of the great diversityof volcanic and igneous rocks that arefound on the Earth's surface (Fig. 13).

Knowledge of the petrogenetic proces-ses that have occurred in the formation ofa certain type of rock is the basis of thedisciplines of petrology and geoche-mistry. Based on chemical, minerologicaland textural analyses, these two bran-ches of geology study where and howprimary magma was generated and itsevolution until it evolved into a certaintype of rock.

Figure 13. Classification of volcanic rocks

The content and proportion of the different chemi-cal elements in a rock provide information as tothe origin and compositional evolution of themagma from which it was formed.

The chemical composition of igneous rocks

Figure 14. Minerological and chemical analysis of a basalt, a trachyte anda rhyolite

The relationship between the majority ele-ments (those present in a proportion greaterthan 0.1%) and trace elements (content lessthan 0.1% and expressed in parts per million– ppm) reveals the changes in chemicalcomposition occurring in the magma and thedifferentiation processes that took place du-ring ascent.

Radiogenic isotopes and elements from therare earth group that appear in very smallquantities provide most information on the me-chanisms of magma genesis, and also com-plement studies of magmatic differentiation.


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Figure 15. Emplacement of different types of igneousbodies

I f magma reaches thesurface and causes aneruption, i t then beginsto cool very quickly.From this point onwards,the diffusion of elementsin the magma may becompletely inhibited and

give rise to rocks such as obsidian and pu-mice with a vitreous texture but no crystals.General ly, however, the typical texture ofthe resultant rocks is microcrystalline (con-sisting of very fine crystals). Some rocks areporphyritic in nature, a feature most charac-teristic of sub-volcanic rocks.

If the magma is locatedat more superficial levelsbut still within the Earth'scrust, i t forms intrusivebodies such as dykesand si l ls. The cool ingprocess is remarkablyrapid and prevents new

crystal l ine nuclei from growing. However,crystals developing deep down in more fa-vourable conditions will be more regular inshape and larger than the rest. The result isa texture known as porphyrit ic, wherebylarge, regular-shaped crystals (phenocrysts)are surrounded by a crystal l ine, generallymuch finer grained matrix.

When magma sol idif iesdeep down, the slowdrop in temperature fa-vours the dif fusion ofchemical elements andtherefore the addition ofnew material to the crys-tals that are being for-

med. This results in a crystalline rock with agranular texture containing large, similar-sized crystals.

Types of igneous rock and their texture

The texture of an igneous rock is definedby the characteristics of its mineralogicalcomponents (e.g. absolute and relativegrain size, shape and mutual geometricrelationships). Although some of theseaspects can be observed in the field, tex-ture analysis almost always requires theuse of a petrographic microscope.

The speed at which magma cools, deter-mined by the depth at which it solidifies,is reflected in the texture of the rock (Fig.15). Texture analysis thus reveals the sta-ges that the magma went through duringits solidification.


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Earth's internal dynamics is eruptive activity.Sometimes violent, sometimes more pacific, this isthe final stage of the volcanic process.

In the course of the formation of a volcanic region, up to fiveeruptive units can be differentiated according to the durationand/or style of the phenomena related to the exit of mate-rials onto the surface. The established hierarchy for theseunits from the least to the most important is as follows: erup-tive pulse, eruptive phase, eruption, eruptive epoch anderuptive period.

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Eruptive pulse

Eruptive phase

Eruption

Eruptive epoch

Eruptive period

A short event emitting volcanic materials lasting for just seconds orminutes. The deposition of the material expelled during this pulsegives rise to a layer or level.

A series of eruptive pulses lasting hours or days. The resulting depo-sit or series of deposits have similar granulometry, morphometry andcompaction.

The basic eruptive unit, lasting days, months or years that involves re-peated pulses or phases and forms a sequence of deposits. If twoeruptions from the same point source are to be regarded as discreteeruptions, enough time must elapse for soils to form or for non-volca-nic erosion processes to take place.

This unit covers several eruptions and may last hundreds or thou-sands of years, during which time one or various volcanic edifi-ces may form.

A succession of eruptive epochs, separated by periods of time longenough for tectonic phenomena such as folding and faulting to takeplace. This period may last thousands or millions of years and giverise to volcanic fields or regions.

• • • Eruptive activity involves a series of phenomena related to theemission of solid materials, liquids and/or gases onto the Earth's surfacefrom a point source. • • •

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Eruptive activity

Eruptive units


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Volatiles in magmaThe most common volatiles in magmas are water vapour (H2O), carbon dioxide (CO2) and sulphur dio-xide (SO2). The solubility of these gases depends on the pressure and temperature of the magma.

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Why do eruptions occur?An eruption starts when the pressure exerted by themagma within the volcanic conduit or magma chambersurpasses the lithostatic pressure. This increase in mag-matic pressure may be due to two factors, which mayoperate simultaneously or individually:

a. The injection of new magma from deeper areas in theEarth (the origin of most volcanic eruptions).

b. The supersaturation of gases (volatiles) in the magmaas it rises to the surface.

In volatile-poor basic magma, the increase in pressure isusually caused by the constant influx of new magma,whereas in acid magma it is due to a combination ofboth. Therefore, in superficial reservoirs of acid magmasupersaturated in gas, the arrival of new magma canprovoke an eruption.

Figure 16. Gas expansion in avolcanic conduit.

A process of magma coolingand crystallisation takes placein the magma chambers. Theresidual liquid is volatile-rich,as volatiles often cannot beeasily incorporated into crys-talline structures. Bubblesbegin to form that increasethe pressure in the magma.

As the magma rises to thesurface, lower lithostatic pres-sure means that the volatiles itcontains separate from the li-quid and form a separate gasphase. These volatiles formbubbles that increase in num-ber and size.

Figure 17. Gas expansion in amagma chamber.

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Types of eruptive activity The features of eruptive activity depend mainly on thevolatile content of the magma and therefore on its initialcomposition and its evolution during ascent. The activitytype can also be affected by the presence of water inthe place in which the magma is finally released. Thus,two types of eruptive activity, effusive and explosive, canbe identified.

l1 l4 l2 l1 l Effusive activityVolatile-poor magma leads to effusive eruptions (Fig. 18).The pressure exerted by gas bubbles inside the volcanicconduit is insufficient to fragment the magma and expelit into the air.

This type of activity is caused mostly by:

• The emission of basic and ultrabasic magmas, initiallyvery gas-poor.

• The degassing of acid magma due to the gradual es-cape of volatiles through fumaroles or steam erup-tions.

• Previous explosive eruptions in which most of thegases in the magma are lost in the conduit.

l1 l4 l2 l2 l Explosive activityExplosive eruptions are associated with volatile-rich mag-mas. During the explosion, gases concentrate in bub-bles and expand in the final part of the conduit. Thesebubbles interact with each other and isolate magmafragments. The sudden release of gas as the bubblesreach the surface causes a violent explosion that expelsfragments of lava. Sometimes, hydromagmatic explo-sions occur when magma enters into contact with water,causing the explosiveness to increase and the rocksaround the conduit to fragment.

Using as a basis a type of behaviour observed in activevolcanoes or in past eruptions, explosive eruptions areclassified into the following types: Strombolian,Vulcanian and Plinian, according to different degrees ofexplosiveness. Hydromagmatic eruptions also have dif-ferent degrees of intensity.

• • • Effusive activityis characterised bythe gentle andcontinued emissionof lava, the namegiven to magma onceit has emerged abovethe surface. • • •

• • • Explosiveactivity ischaracterised by thefragmentation andviolent expulsion ofmagma andoccasionally of thesurrounding rocks.The resultingfragments are calledpyroclasts. • • •

Figure 18. Emission of lava

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Strombolian activity

Stromboli, a volcano in the Aeolian Islands off the northcoast of Sicily, lends its name to a type of low-level eruptioncaused by gas mixed with escaping magma.

Strombolian activity consists of discrete explosions sepa-rated by periods that range from less than a second to se-veral hours. Each of these explosions or pulses comesabout as one or more bubbles of gas reach the surfacewhile the magma is at rest (Fig. 16). The result is the expul-sion of the magma fragments, which then build up aroundthe vent having described ballistic trajectories through theair (Fig. 19).

The pressure of the gas reaching the surface and its ascentthrough the magma depend on the physical properties ofthe magma. This activity is generally associated with basal-tic magmas with low viscosity in which bubbles rise to thesurface fairly easily.

Vulcanian activity

This type of activity is named after another volcano,Vulcano, also in the Aeolian archipelago; its name istaken from Vulcan, the Roman god of fire.

Vulcanian eruptions are highly explosive, but neverthe-less smaller and less violent than Plinian eruptions (Fig.20). The volume of the ejecta does not normally exceeda cubic kilometre and the eruption column is less than20-km high. However, the most distinguishing feature isthe occurrence of a series of short-lived explosions las-ting from minutes to a few hours. These explosions are

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Figure 19. Strombolian eruption


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caused when the conduit is blocked by rocks, by coo-led and consolidated magma or by debris from previouseruptions; if the pressure of the gases inside the conduitis strong enough the blockage is broken. This happenseither when there is an increase in the amount of mag-matic gas or, more frequently, when an aquifer is partiallyvaporised. Consequently, much of the ejecta result fromthe fragmentation of the blockage.

Andesitic magmas with their high viscosities often buildup and solidify in the neck of the volcano. If this occurs,domes form that block the conduit and trigger vulcanianactivity.

Plinian activity

This type of activity takes its name from Pliny theYounger, who wrote a detailed description of the erup-tion of Mount Vesuvius in AD 79.

Plinian eruptions are highly explosive and violent andeject huge volumes of fragments and volatiles (Fig. 21).Travelling hundreds of metres per second, pyroclastsand hot gases form a mushroom-shaped eruptive co-lumn that may reach heights of over 30 kilometres.

The column remains stable for as long as the ejectacontinue to be expelled with sufficient force from thevent. At the same time, part of the fragments fall in a py-roclastic shower around the vent. When the gas content

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Figure 20. Vulcanian eruption


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in the magma decreases, or if the radius of the vent in-creases due to erosion during the explosions, the speedat which the ejecta are released decreases and theeruptive column collapses, either partially or totally.

Collapse of this type provoke pyroclastic flows that movedown the sides of the volcanic cones at great speeds.

This type of activity is generally associated with acid magmas, dif-ferentiated in magma chambers in which they have evolved andbecome gas-enriched over a long period of time.

Hydromagmatic activity

During a magmatic eruption, the entry of water into the sys-tem can completely alter the style of eruptive activity andconsequently an initially gentle outflow of magma can sud-denly become extremely violent. This type of eruptive activitycan occur with both basic magmas and more evolvedtypes.

The more specific term phreatomagmatism is used to des-cribe the process of interaction between magma andgroundwater. In this case, the transfer of energy from themagma to the water may come about due to either conduc-tion (Fig. 25) o por contacto directo (Fig. 26).

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Figure 21. Plinian eruption

• • • Hydromagmaticactivity is theproduct of theinteraction betweenmagma or a sourceof magmatic heatand meteoric water,be it on the surface(seas, rivers or lakes)or groundwater(aquifers). • • •


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Magmatic eruptions

A good way of understanding how magma-tic eruptions occur is to contrast volcanicprocess with the opening of a bottle ofchampagne (Fig. 22):

a. Before the eruption, themagma is subject to pressure fargreater than atmospheric pres-sure and the volcanic gases aredissolved in the liquid.

b. When the conduit is unbloc-ked, there is an almost instanta-neous decompression of themagma, the gases expand andform bubbles.

c. The gases fragment themagma and force it out of theconduit in the form of splashesof lava that can reach greatspeeds.

a. The champagne in the bottleis subject to high pressure be-cause of the force exerted by thegas accumulating in the neck ofthe bottle. This high internal pres-sure means that, even thoughfermentation continues, no moregas can separate and so it is par-tially dissolved in the liquid.

b. On popping the cork, the gasthat has built up in the neck isreleased. The pressure in thebottle drops significantly andallows the gas dissolved in thechampagne to diffuse, separatefrom the liquid and form bubblesthat then grow rapidly.

c. The gases drag the liquid to-wards the neck of the bottle atgreat speed, fragmenting the li-quid and forcing it out in drops.

Once all the gas has escaped,the froth runs down the outsideof the neck of the bottle as itlacks the force to shoot out asbefore.

Figure 22. Representation of a magmatic eruption


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Hydromagmatic eruptions

Imagine a frying pan of hot oil on a kitchenstove in which a few drops of water acciden-tally land — the result is akin to a hydromag-matic explosion.

The relationship between the volume ofwater and magma that come into contactwill go a long way to determining the vio-

lence of the hydromagmatic eruption (Fig.24), as has been shown in laboratory ex-periments.

Just like the magma in an eruption, the hot oiltransfers its heat to the water, which vaporisesinstantly (Fig. 23). The resulting steam ex-pands, fragmenting the oil, which then spurtsout of the pan at speed in the form of splas-hes. The oil corresponds to the pyroclasts in avolcanic eruption.

However, if you throw a whole bucket of wateron the frying pan, the resulting reaction is verydifferent from the above. In this latter case, thelarger amount of water rapidly cools the oil andreduces the explosiveness of the interaction,which may become inexistent. This explainswhy underwater eruptions that occur in ridgeson the sea floor, for example, are not excessi-vely violent.

Figure 23. Simulation of a hydromagmatic eruption

Figure 24. Different types of volcanic deposits and edifices resulting fromhydromagmatic activity whose nature is determined by the relationship bet-ween the water interacting with the magma and the degree of explosivity orefficiency of the eruption. Wohletz and Sheridan (1983).


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Figure 25. Phreatic activity Figure 26. Phreatomagmatic activity

An intrusion of molten material can heat andvaporise an aquifer by thermal conduction wit-hout coming into direct contact. In this case,violent explosions may take place that expeljust the fragments of the rocks forming theaquifer without any magma being released tothe surface.

In the course of an eruption, groundwater mayenter into direct contact with the magma andbe instantly vaporised. This is only possible ifthe pressure of the gases in the magma insidethe conduit is lower than that exerted by thewater in the aquifer. Then, violent explosionsoccur that expel fragments of magma and ofthe rocks surrounding the conduit itself.

Figure 27. Eruption on Surtsey, Iceland

Surtseyan activityEruptive activity in Iceland is generally effusiveand Strombolian and involves the emission ofbasic magmas. However, in 1963 off the southcoast of Iceland Surtsey, a new volcanic is-land, was born. It was the result of a highly ex-plosive eruption caused when seawater ente-red the conduit and was vaporised instantly.This eruptive style, seen in the formation ofmany other volcanoes, is now known asSurtseyan activity.


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The study of volcanic rocks helps understand the trans-portation and deposition mechanisms they originatedfrom and therefore the type of eruptive activity involved.In this field of study, the geometric and textural relationsof the built-up material and its composition have to beanalysed.

l1 l4 l3 l1 l Massive materialsThese are compact bodies of homogeneous composi-tion resulting from the cooling of lava flows originatingfrom effusive eruptions. These rock bodies may be pre-sent in diverse forms depending on the initial viscosity ofthe magma. Variation in temperature during emplace-ment, the volume of material ejected and the features ofthe terrain where it is deposited (e.g. slope, irregularitiesand humidity) will also affect the final form they take.

The most fluid lavas are basic in composition and giverise to lava flows (Fig. 28). These represent continuousoutpourings of molten rocky material that slide alongthe flattest areas of land, potentially covering greatdistances.

Lava from acid magma is very viscous and normallybuilds up around the vent in the form of domes. In extre-me cases this type of lava is practically solid when itemerges and leads to the formation of pinnacles.

Lava flows

Lava flows can be distinguished by their lithology, morp-hology and the features of the site. These parametersvary according to the composition of the liquid magma,the speed of cooling of the flow and the features of theiremplacement. Lava flows can be classified by their ex-ternal appearance into two large groups: smooth andrough. The internal structure can be massive and com-pacted, or fractured by joints.

Internal structure of lava flows: retraction

Lava contracts considerably when it cools since it occu-pies less volume in a solid than in a liquid form. Thisleads to the development inside the massive body ofrock of various systems of fractures or cracks known asjointing. The main types of jointing are columnar and len-ticular (Fig. 29).

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Volcanic materials

• • • Volcanicmaterials consist ofall the solid, liquidand gaseousproducts expelledduring an eruption.We can distinguishbetween volatiles -gases that separatefrom the magma -and the materials thatform deposits,classified as eithermassive orfragmentary. • • •

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Figure 28. Solidified lava flow inthe Teide volcanic complex


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Lenticular or slab jointing occurs when the lava streamis moving, for instance when the flow is still being fedby the vent, and gas bubbles deposit in parallel planesin the direction of flow. As the lava cools, the planes fa-cilitate horizontal fracturing, which is most noticeable inthe centre of the lava flow.

Columnar jointing occurs when the lava flow is at rest.The difference in temperature between the very hotcentre and the top and bottom of the flow, which havealready cooled, causes convection cells to generate in-side the lava flow. These cells form perpendicular to thebase of the lava and develop vertical fractures, formingprismatic joints that split the rock into five- or six-sidedcolumns.

Spheroidal weathering, the internal structure that isoften present in the outermost parts of lava flows, can-not really be thought of as a type of jointing (Fig. 30).This flaking of concentric shells of lava is the result ofthe weathering of the volcanic rock caused as moistureslowly infiltrates through existing cracks. Another effectis white mottling, caused by the weathering of certainminerals in the rock.

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Figure 29. Columnar and lenticular jointing

Figure 30. Spheroidal weathering


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Very fluid lavas usually have a very smooth orundulating surface (figura 31). In some cases,due to slight turbulence inside the flow, thesurface may wrinkle or fold perpendicular tothe direction of flow, giving rise to rope lava.

Viscous lavas have a rough, irregular surfacemade up of broken lava blocks or clinker (Fig.32). The outermost layer of the lava flow coolsand forms a crust, which then breaks intoblocks as the lava underneath continues toflow. When the fragments are large, this is ca-lled block lava.

A single lava flow may exhibit diverse types ofmorphology. Thus, we frequently observe alava flow with an initial stretch with a smoothsurface, then an area of ropy lava that beco-mes increasingly irregular, followed by an areaof rough lava.

Submarine lava flows behave differently fromsub-aerial flows. Upon coming into contactwith the water, the lava cools suddenly and afairly plastic layer of glass is formed creatingblobs of lava. These blobs fall and roll downthe slope on top of each other and becomemisshapen, thereby forming what is known aspillow lava.

Lava flow morphology

Figure 31. Smooth lava (pahoehoe) Figure 32. Rough lava ('a'a)

Figure 33. Blister

BlistersWhen the lava flow flows over a lake or a we-tland, the water vaporises and a huge amountof gas is incorporated into the flow. Gas bub-bles rise inside the flow towards the surface,which is often semi-solid due to more rapidcooling. The build-up of bubbles in this areacauses pressure that can deform and evenbreak the surface of the lava flow. The resultare mounds, dozens of metres high known asblisters (tossols in Catalan) (Fig. 33).


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l1 l4 l3 l2 l Fragmentary materialsFragmentary materials consist of clasts generatedmainly by explosive eruptions. Gas bubbles form blobsof magma, which are expelled violently. In some cases,volcanic explosions can break part of the vent or chim-ney wall and the resulting fragments mix with the mag-matic clasts. Finally, all these materials are depositedforming pyroclasts (fragmentary deposits).

When an eruption is so violent that it cannot be obser-ved from close up, the study of pyroclastic ejecta is fun-damental to the understanding of the type of eruptiveactivity.

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PyroclastsThe word pyroclast comes from the Greekpyro, (fire) and klastos (broken). Each of thefragments, large or small, form part of the py-roclastic deposits and have their own particu-lar features.

Classification by size

Volcanic explosions give rise to fragments in avariety of sizes. Pyroclasts can be classifiedby size into three main groups: ash, lapilli andblocks (Fig. 34).

Ash has a diameter of less than 2 mm; lapilliare 2–64 mm, and blocks measure over 64mm.

Nature of fragments

Different types of clasts - juvenile or lithic - are dis-tinguishable according to their nature. Some py-roclastic deposits consist of only one type of frag-ment, while others consist of a mix of the two.

Juvenile fragments: also known as essentialfragments, derive directly from magma reachingthe surface.

Lithic fragments: these are fragments of therocks forming the vent that were ripped out by ex-plosions during the eruption. Lithic fragments canbe accessory, when they derive from rocks fromprevious eruptions, or accidental, when they arefragments of sedimentary, metamorphic or igne-ous rocks that form part of the volcanic substrate.

Figure 34. Classification of pyroclasts by size

LapilliBlocks Ash

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Other terminology used

Volcanic bombs: when fragments ofmagma the size of lapilli or blocks that arenot completely cooled when ejected, movethrough the air they are modified into roun-ded or spindle-shaped forms. If they havesuperficial cracks they are called bread-crustbombs. These are formed by the expansionof gas bubbles inside the still semi-moltenbomb when the surface has already cooledand is easily fractured (Fig. 35).

Scoria: juvenile pyroclasts, lapilli-sized or lar-ger, of irregular morphology that contain manyholes or vesicles. These fragments are basalticor basaltic-andesitic in composition and maybe semi-welded in deposits close to the ventbecause they were not completely solid whenthey were deposited (Fig. 36).

Pumice: juvenile fragments, generally lapilli-sized, acid in composition and pale-coloured.Pumice floats since it is highly porous and itsdensity does not exceed 1g/cm3 (Fig. 37).

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l1l4l3l3l Types of pyroclastic depositFragmentary materials build up different types of depo-sits according to the mechanisms of formation, transportand deposit in operation. We can distinguish three basictypes - pyroclastic fall, pyroclastic surge and pyroclasticflow deposits – that occur due to differences in the ge-nesis of the deposit.

Pyroclastic fall deposits

These are formed when ejecta from an eruption eitherfall freely and vertically having formed part of the erup-tion column or on a ballistic trajectory after being ejectedfrom the crater of the volcano (Fig. 38). Fall deposits mayshow gradation in size and laterally continuous parallelbanding. The further they land from the vent, the thinnerthe deposit and the smaller the fragments.

Figure 35. Volcanic bomb

Figure 36. Scoria

Figure 37. Pumice


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Types of fall deposit a. Strombolian fall deposits: the low energy ofthe eruption and high density of the fragmentsmean that the ejecta do not reach greatheights and fall to ground directly on a ballistictrajectory. This mechanism is characteristic ofStrombolian eruptions, in which fragmentsbuild up around the vent and form a volcaniccone.

b. Plinian pyroclastic deposits: whentheir density is low, fragments rise to conside-rable heights forming characteristic Plinianeruption columns. Finally, these materials fall ina shower of pyroclasts. Prevailing winds candisplace the cloud of materials that make upthe column and affect the emplacement of thepyroclasts. These deposits cover the landevenly and build up both in depressions andon higher ground (Fig. 39).

c. Hydrovolcanic deposits: in violent ex-plosions caused by the instantaneous eva-poration of water, part of the ejecta followsball istic trajectories. Unlike in Strombolianeruptions, the horizontal component in thesecases is much more important than the verti-cal component and the resulting build-up,which includes a considerable presence ofl ithic fragments, is known as pyroclasticbreccia (Fig. 40).

Figure 39. Plinian fall deposit

Figure 40. Pyroclastic breccia

Figure 38. Ballistic projection of pyroclasts and emplacement of a fall deposit


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Pyroclastic surge deposits

These deposits originate in turbulent gaseous flows thattransport horizontally small amounts of pyroclasts at su-personic speeds, close to the ground. The formation ofpyroclastic surges is associated mainly with:

a. the collapse of the outside of the column, which ismuch more diluted and colder than the centre;

b. annular explosions at ground level produced directlyin the vent that move radially.

These are high-energy flows and can move up slopes.Consequently, the deposits left by pyroclastic surgescover the underlying topography, although the most im-portant build-up of material occurs in valley bottoms (Fig.41). Such deposits are characterised by unidirectionalsedimentary structures and good granulometric classifi-cation. They often have an erosive base lying on the ma-terials of the substrate.

Pyroclastic flow deposits

These consist of fast-moving laminar flows of gas androck fragments that fill in depressions as they spread la-terally. Generally, they originate after the total or partialcollapse of a vertical eruption column and during empla-cement are accompanied by a huge ash cloud (Fig. 42).

The build-up of the materials transported by these flowsfills valleys and depressions. They normally have noclear stratification or any defined internal structure andare often compacted by secondary cementation. Theyare typical of explosive eruptions associated with diffe-rentiated magma, although they can also occur in basicvolcanism. Large pumice-rich pyroclastic flows areknown as ignimbrites.

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Figure 41. Emission and emplacement of a pyroclastic surge


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The deposits originating from pyroclastic flows and sur-ges are extreme manifestations of a wide range of diffe-rent types of emplacements and flows and many interme-diate forms can be found between these extremes.

Lahars

Lahar is an Indonesian word used to describe a water-satura-ted flow of volcanic debris or a mudflow. When large quantitiesof snow cover volcanoes or when their craters contain lakes, aneruption - however small - can cause huge slides of mud and

volcanic rock. These flowstravel at high speeds andcause rivers to break theirbanks and sweep awayeverything in their paths, fromvegetation and infrastructu-res, to vehicles and even en-tire villages. Lahar depositsare chaotic masses of volca-nic rock and other materialpicked up along the way.In the sequence of materialswe find volcanic deposits(lavas or pyroclastic rock) in-terspersed with sedimentarymaterials (Fig. 43).

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Figure 42. Pyroclastic flow deposit

Figure 43. Lahar emplacement


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The build-up of volcanic materials ejected close to thevent gives rise to the formation of one or several volca-nic edifices that are generally cone-shaped and variablein size. The morphology of volcanic constructions is clo-sely related to the type of eruptive activity and the episo-des that have taken place during the history of the vol-cano. Hence, we can classify volcanoes as either mo-nogenetic or polygenetic.

Monogenetic volcanoes

These volcanoes are formed in the course of a singleeruption, which can have several phases and pulses.The edifice constructed is simple and the main featuresinclude pyroclastic cones, tuff cones, tuff rings andmaars. A succession of different eruptive phases can re-sult in the superposition of several of these types of edi-fices in a single volcano.

Pyroclastic or cinder conesThey are the result of a Strombolian eruption and arebuilt mostly from cinder (scoria). The craters may be cir-cular or breached on one side. The horseshoe shapemay be due to the inclination of the vent, the presenceof prevailing winds that whip pyroclasts along in a givendirection, or to the expulsion of lavas that drag part ofthe pyroclastic deposits along with them. The flanks of acinder cone slope at an angle of 30–40°.

Tuff conesThese are formed from the interaction of magma andwater during a hydrovolcanic eruption. The materials for-med are mostly compact pyroclastic surges and flows.Craters are small and the flanks of the cone slope at20–25°.

Tuff ringsThese form as a result of a phreatomagmatic eruption.They consist of pyroclastic breccia, surges and flows.They have large craters and a low rim with flanks slopingat around 10°.

MaarsThese form as a result of a phreatomagmatic eruptionand are similar to tuff rings. In this case, the crater liesbelow the surrounding topography and the cone, for-med by pyroclastic surge and flow deposits, is very low.

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Volcanic morphology l1 l4 l4 l

Figure 44. Cinder cone

Figure 45. Tuff cone

Figure 46. Tuff ring

Figure 47. Maar


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Polygenetic volcanoes

These are formed from several eruptions over a long pe-riod, from thousands to millions of years. They are oftenassociated with intermediate or near-surface magmachambers where successive episodes of emptying andrefilling have taken place and where the primary mag-mas can evolve. The resulting edifices are known asstratovolcanoes and shield volcanoes.

StratovolcanoesAlso called composite volcanoes, they are associatedwith intermediate acid magma eruptions where explosi-ve and effusive activity alternates. Consequently, theyare formed by several layers of fragmentary depositsand lava flows. The edifice, which is large, may haveflanks with slopes of over 40°.

Shield volcanoesFormed from basaltic eruptions in which effusive activitypredominates. The edifice, formed by the accumulationof lava flows, is concave in shape and, as its name im-plies, resembles a shield. The cone are not very highand the flanks of the slope are at angles of less than10°, but in some cases the base may be over a hundredkilometres in diameter.

Both monogenetic and polygenetic volcanoes may havesmaller secondary edifices around them, clearly linkedto the activity of the main edifice, known as adventive orparasite cones.

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Collapse calderasIn volcanoes with magma chambers, in thecourse of an eruption large quantities ofmagma are ejected rapidly (phase a). Thepartial or total emptying of the magma cancause the chamber to collapse. This collapsereactivates the volcano and generates morehighly explosive phases (phase b). The end

result is a depression, kilometres wide,known as a collapse calderas (phase c). Theinternal walls that limit the depression are ver-tical and made mostly of ignimbrite depositsejected in phase b.

Figure 50. Formation of a collapse caldera

Figure 48. Stratovolcano

Figure 49. Shield volcano


Volcanism in Catalonia

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2. El vulcanisme a Catalunya-en baixa 4/6/12 07:09 Página 41

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The eruptive episodes that took place in La Garrotxaand in Catalunya in general during the Neogene andQuaternary were not simply a sporadic event. Theorigin of the series of volcanic morphologies androcks that constitute the Catalan volcanic field lieswithin a broader geodynamic context that affectsmuch of Western Europe.

Using as a basis the composit ion and dat ing ofvolcanic rocks, two erupt ive per iods have beenident i f ied in the western Mediterranean, and evi-dence of both ex is ts in the nor theast o f theIber ian Peninsula. The geological history of the re-gion is complex given the over lap of compressiveand distensive structures.

Distribution and evolution of volcanoes

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The first eruptive period occurred during theMiocene (24–18 Ma) and is characterised bycompressive tectonic conditions (Fig. 51). Theassociated magmatism was calc-alkaline, mostlyrepresented by sub-aerial volcanic manifestationsin Mallorca and, above all, by submarine featuressituated between the Balearic Islands and theIberian Peninsula. The origin of these eruptions isexplained by the presence of a subduction planesloping towards the Iberian Peninsula running NE-SW from the Balearic Islands to west of Corsicaand Sardinia.

From the Upper Miocene onwards, the situa-tion changed to one of distension that we findtoday (Fig. 52). This second cycle corres-ponds to the development of an intraplate riftaffecting Western Europe and associated withthe alkaline magmatic manifestations of theValencian, Els Columbrets and Catalan volca-nic fields. It is also worth mentioning that anumber of submarine volcanoes were formedduring isolated volcanic episodes occurring,for example, off the coast of Tarragona.

Figure 51. Western Mediterranean. Compression pe-riod. Calc-alkaline volcanism

Figure 52. Western Mediterranean. Distension period.Alkaline volcanism


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The European Cenozoic Rift System

During the Upper Miocene at the end ofthe Tertiary Period, an extensive processbegan in the western sector of the EurasianPlate that is still considered to be active. Asa result of the distensive forces operatingwithin the plate, a rift-type structure measu-ring over 2,000 km from the North Seacoast to the southern Iberian Peninsula hasdeveloped (figura 53). Within this rift there isa series of troughs and raised blocks thathave been created by the movement of a

number of large normal faults runningmostly NE-SW. The magma has taken advantage of theseweaknesses in the lithosphere to rise to thesurface and there are thus numerous vol-canic manifestations in both Eastern andWestern Europe associated with this riftsystem. The most important such volcanicfields ones are in Eiffel (Germany),Auvergne (France) and Catalonia.

Figure 53. The intracontinental rift system in Western Europe


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Figure 54. Geological cross-section of part of a tectonic trough with faults

The European rift system contains a seriesof discrete structures such as the ValenciaTrough and the fault trenches of the Gulf ofLion, the Têt and Tech rivers, and LaCerdanya. These two segments situated inthe northeast of the Iberian Peninsula havebeen displaced by a series of normal faultsthat lie perpendicular to those within the rift(Figs. 53 and 54). From west to east, thesefractures are known as the Amer, Llorà,Cartellà, Camós-Celrà, Juià, Riurà andVilopriu faults, which separate a series ofraised blocks (Les Gavarres, Les Guilleriesand the mountains of La SerraladaTransversal) and sunken blocks (L'Empordàand La Selva depression and the OlotTrough).

Most of the volcanoes in northeast Cataloniaare found on or close to one of these faults.


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The series of Neogene-Quaternary eruptive rocks in north-east Catalonia are situated in three volcanic areas:L'Empordà, La Selva and La Garrotxa. We can deduce fromthe geographical distribution of the eruptive features and thegeochronological data available that the magmatic activitybegan in the L’Empordà, moved south towards La Selvaand then finally reached La Garrotxa (Fig. 55).

The combination of the age of the volcanic phenomena inthe L’Empordà and La Selva and the effects of erosive pro-cesses explain why the volcanic edifices of these twozones have all but disappeared, and also why only the har-dest massive materials including fragments of lava flowsand collapsed chimneys are still recognizable.

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Figure 55. Map of NE Catalonia and geological table (modified from Saula et al.)


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L'Empordà Volcanic ZoneThis zone contains around fifty basalt outcrops, aswell as a number of trachyte outcrops, shared bet-ween the regions of L'Alt and El Baix Empordà, ofwhich the most important are located around LaBisbal d'Empordà, Rupià and Arenys d'Empordà.Most of these volcanic materials are covered byPliocene deposits and date from over 6 Ma, with theoldest being around 14 Ma.

Particularly noteworthy are the trachyte outcrops atVi lacolum and Arenys d'Empordà (L'Alt Empordà).These more evolved volcanic rocks have resultedfrom the cooling of magma that had undergone diffe-rentiation.

La Selva Volcanic ZoneThis region also comprises a series of around fiftybasalt outcrops, mostly around Maçanet de la Selvaand Riudarenes. The col lapsed chimneys of SantCorneli and Hostalric are its most interesting featuresand exhibit marked columnar jointing. In some parts,deposits of fragmentary materials originating from hy-dromagmatic eruptions can be identified.

Geochronological analyses have dated these rocksat 5–20 Ma.

La Crosa de Sant Dalmai on the northern rim of LaSelva depression is a well-preserved volcano thaterupted in more modern times.

La Garrotxa Volcanic ZoneThe youngest and best-preserved volcanoes inCatalonia are in La Garrotxa. Thirty-eight volcanoeshave been identif ied in La Garrotxa Volcanic ZoneNatural Park, with a further two in the Hostoles Valleyand five in the Llémena Valley (Fig. 56). A large num-ber of lava f lows and pyroclastic deposits (bothStrombolian and hydromagmatic in origin) are visita-ble (of particular interest in the Llémena Valley).

Despite some evidence of volcanic activity prior tothe Quaternary, available geochronological data si-tuates this volcanic zone at 350,000–10,000 yearsold and current estimates suggest that an eruptiveepisode has occurred approximately every 15,000years.


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Figure 56. Location of the volcanoes in La Garrotxa Volcanic Zone

1 La Canya 2 Aiguanegra3 Repas4 Repassot5 Cairat6 Claperols7 Puig de l'Ós 8 Puig de l'Estany 9 Puig de Bellaire

10 Gengí

11 Bac de les Tries 12 Les Bisaroques 13 the Garrinada 14 Montsacopa15 Montolivet16 Can Barraca 17 Puig Astrol 18 Pujalós19 Puig de la Garsa 20 Croscat

21 Cabrioler22 Puig Jordà 23 Puig de la Costa 24 Puig de Martinyà 25 Puig de Mar 26 Santa Margarida 27 Comadega28 Puig Subia 29 Roca negra 30 Simon

31 Pla sa Ribera 32 Sant Jordi 33 Racó34 Fontpobra35 Tuta de Colltort 36 Can Tià 37 Sant Marc 38 Puig Roig 39 Traiter40 Les Medes


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Figure 57. Location of the volcanoes in the Llémena Valley and La Selva depression

1 Crosa de Sant Dalmai 2 Puig d’Adri 3 El Rocàs4 Clot de l’Omera 5 Puig de la Banya del Boc 6 Granollers de Rocacorba 7 Puig Montner


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The composition of the rocks that make up the volcaniczone of La Garrotxa and the Catalan volcanic field in generalis relatively uniform. With the exception of the trachyte out-crops in L’Alt Empordà, all volcanic materials are composedof basalt and basanite, low in silica and high in sodium andpotassium. Therefore, as a whole the volcanic materials inCatalonia can be classified as alkaline. They are the result ofthe cooling of rapidly ascending basalt magmas and arecharacteristic of intraplate volcanic zones.

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Rocks and magmal2 l3 l

Figure 58. Sample from Olot

Basalt is a grey-black rock that, when not particularly vesicular, is very dense.

Figure 59. Sample from Vilacolum

The paler trachyte is porphyritic in texture (feldspar crystals).


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MineralsBasalt mineralogy is uniform and simple. In most cases,all that is visible – and only under a microscope - aresmall olivine, pyroxene and plagioclase feldspar phe-nocrysts in a microcrystalline or partially vitreous matrix,which is often rich in iron oxide (mainly magnetite). Otherminerals such as leucite and analcime are also presentin small quantities.

The very few mineralogical differences between basaltand basanite that exist are very hard to distinguish withthe naked eye. They are characterised by the presenceof small feldspathoid crystals such as leucite and gene-rally by a slight reduction in the percentage of silicondioxide (Fig. 13).

Unlike basalt rocks, trachytes have a high percentage ofsilicon oxide (over 60%) and are composed of large pla-gioclase crystals with some pyroxene and biotite. Underthe microscope, the trachyte matrix contains numeroussmall, elongated sanidine crystals, as well as titaniumand iron oxides.

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Observable minerals

Figure 60. Olivine

A pale green mineral with a glassylustre. It appears both in the form ofphenocrysts and as part of the matrix. Large crystals tend to beidiomorphic with regular sides corresponding to the facets of thecrystal.

Figure 61. Pyroxene

Dark with green tones. Pyroxenesare found both as phenocrysts andin the matrix. Most are titaniferousaugites and are often present inidiomorphic or sub-idiomorphicforms.

Figure 62. Plagioclase

A white mineral. This type of feldspar is generally subordinate inthe matrix and is only found exceptionally as a phenocryst..


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Geochemical dataMagma genesis and ascent

The geochemistry of the basalt rocks in the Catalanvolcanic field displays considerable homogeneity inmajor elements, including si l icon, aluminium, ironand calcium oxides. The only significant variationsare in the percentage of titanium oxide, attributableto variable temperatures in the magma when therocks were being formed.

Nevertheless, important variations exist between dif-ferent rocks in terms of the amount of trace elements(nickel, cobalt, chromium and strontium) and rareearth elements (lanthanum, cerium and neodymium)they contain. This variation in chemical compositioncoincides well with the three geographic regions -L’Empordà, La Selva and La Garrotxa - and revealsdifferences in the magma source area.

Variations observed in the geochemical analysis ofthe basalt rocks enable us to understand better thegenesis and ascent of the magmas that gave rise tovolcanism in Catalonia. The magma source areas aregenerally situated in the asthenospheric mantle, alt-hough the magmas that generated the volcanic fea-tures in L’Empordà come from an area that is morelithospheric.

The presence of these two source areas, the aste-nosphere and the lower lithosphere, can be linked tothe evolution of the European rift system. During theinitial extensive stages, the thinning of the lithosphe-re led to its decompression and partial melting. Thecrust was sti l l thick and pockets of magmas weretrapped in small chambers in which the Empordàtrachytes differentiated and formed. As the rift pro-gressed and the lithosphere grew thinner, the asthe-nosphere ascended and permitted less evolved mol-ten materials to rise.

In some cases, the almost total lack of contaminationof the basalts by rocks from the crust and their scantdifferentiation indicate that the ascent of the magmapockets from the source to the surface was rapid.

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EnclavesSome lava flows and pyroclastic deposits con-tain fragments of rock that were captured bythe magma during its ascent. These frag-ments, known as enclaves or xenoliths, con-sist mostly of plutonic rocks, although somemetamorphic or sedimentary rocks may alsooccur (Fig. 63). These enclaves usually con-sist of blocks (usually a few centimetres inlength) formed in the lithosphere (or in somecases in the mantle) that the magma tore outof the vent wall and then engulfed beforetransporting them to the surface.

In some cases, the lithic fragments found inpyroclastic deposits are also referred to ‘en-claves’, although, given their explosive origin,this term is somewhat of a misnomer.

Also of great interest is the presence of ul-trabasic xenoliths (Fig. 64) derived from themantle or from remains of magma differen-tiation in the basalts in the lower crust (e.g.Rocanegra, Puig de la Banya de Boc andPuig d'Adri volcanoes). These xenol i thswere denser than the basaltic l iquid, but,due to the speed of the magma’s ascent,were immersed and dragged to the surface.Calculations of the floatability of these frag-ments in magma show that an ascentspeed of around 0.2 m/s would have beennecessary to maintain the enclaves in sus-pension.

Figure 63. Enclave of plutonic rock: granitoid Figure 64. Ultrabasic enclave: dunite


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Each of the volcanoes in the La Garrotxa was formedduring a single eruption. Thus, they are monogenetic innature and were created by the ejection of a pocket ofmagma whose exhaustion marked the end of the volca-nic activity.

However, the various phases of activity present duringthe eruption are visible since they are marked by a chan-ge in the style of the magma's journey to the exterior, alt-hough the time-lapse between each of these phaseswas not sufficient for erosive stages or soil developmentto begin.

Eruptions in La Garrotxa Volcanic Zone

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Volcanoes and their phases oferuptive activityThe eruptive activity that gave rise to the volcanoesin La Garrotxa combined hydromagmatic and purelymagmatic phases that, despite the monotonouscomposition of the magma, has left a legacy of manydifferent volcanic features. The study of these volca-nic deposits has identified effusive, Strombolian andphreatomagmatic eruptive phases.

An oft-repeated evolut ionary process starts withStrombolian activity, which then evolves into effusiveactivity as the magma loses its gas (Fig. 65). The bestexamples of this evolut ion are the volcanoes ofCroscat, Montolivet and Sant Marc.

In other cases, the eruption starts with phreatomag-matic activity, which then develops into Strombolianand, finally, effusive activity; this is the case of thevolcanoes of El Traiter, La Garrinada and Puig d’Adri(Fig. 66).

More rarely, we find volcanoes that were formed froma single eruptive phase, either Strombolian (PuigAstrol; Fig. 67), or phreatomagmatic (El Clot del’Omera).

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Figure 65. Croscat with its horseshoe-shaped crater and lava flow, on which stands the famous beechwood of LaFageda d’en Jordà


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Eruptions in which the initial activity was Strombolianmay become phreatomagmatic if water enters the ventwhen the magma ejection loses intensity (e.g. Can Tià).Finally, there is also evidence of Strombolian phases in-serted into obvious phreatomagmatic sequences, whichcan occur if the water supply in the aquifer is momenta-rily exhausted.

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Figure 66. Puig d’Adri volcanoe

Figure 67. Puig Astrol volcanoe


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Despite the variety of possible combinationsof eruptive styles that occurs during aneruption, the most frequent case in the LaGarrotxa involving phreatomagmatic activityis as follows:

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Example of an eruption

Figure 68a.Phreatomagmatic eruptive phase

Figure 68b. Strombolian eruptive phase

Figure 68c. Effusive eruptive phase

The eruption starts with an explosive phrea-tomagmatic phase. Magma rich in juvenilegas is enriched by volatiles due to the eva-poration of the water in the subsoil. In thisearly stage, phreatomagmatic activity maybe interspersed with pure Strombolian pha-ses in which the water-magma interactionstops momentarily (Fig. 68a).

The ejection of new magma makes the ventwaterproof and so halts the phreatomagma-tic activity. Nevertheless, the magma in themagma pocket still contains enough gas togenerate Strombolian explosive activity(Fig. 68b).

Finally, when most of the juvenile gas hasbeen exhausted, effusive activity puts an endto the eruptive sequence. In this final stage,the eruption is placid and is characterised bythe emission of lava flows (Fig. 68c).


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Eruptive activity and volcanicedificesDuring an eruption, the alternation of types of activityoften leads to the formation of different, superimposedvolcanic edifices. For instance, in the Natural Park onthe volcano of Puig de Martinyà two subsequent cindercones cover much of a previous phreatomagmaticconstruction. Even so, the best examples of this type ofinterference between edifices originating from the sameeruption are the volcanoes of La Crosa de Sant Dalmai(Fig. 69) and Puig d'Adri. Both consist of edifices formedby Strombolian activity that have been superimposed onprevious phreatomagmatic structures.

On other occasions, volcanic edifices generated in thecourse of an eruption are partially or totally destroyed bysubsequent phases. The cinder cones of volcanoes suchas Croscat, Montolivet and Aiguanegra were partially des-troyed by lava flows during a final effusive phase (Fig. 70).The emission of magma through either the crater or thebase of the cone drags pyroclasts from one part of theedifice to another and, when seen from above, the finalshape of the crater resembles a horseshoe.

During Strombolian activity, the final part of the vent maybranch, allowing magma to be released through severalnew vents that form adventive or parasitic cones (1)such as those that surround Croscat (Fig. 71).

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Figure 69.La Crosa de Sant Dalmai volcanoe Figure 70. Rocanegra and Puig Subià volcanoe

Figure 71. Croscat volcanoe

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In La Garrotxa Volcanic Zone, rocks resulting fromeffusive activity vary very little due to the uniformityof their constituent magmas. Lava flows are grey-black and exhibit the fractures that are typical of co-lumnar and lenticular jointing and spheroidal weathe-r ing. The surface is usual ly smooth and the fewrough-surfaced lava flows ('a'a type) are hard to de-tect due to dense plant cover and human activity.

Explosive volcanic activity gives rise to a great diver-sity of pyroclastic deposits (Figs. 72 and 73). The vio-lence of these types of explosions and their origin,be they magmatic or hydromagmatic, determine thegranulometry of the pyroclastic rocks and their com-ponents.

In La Garrotxa several types of deposit are superimpo-sed as a result of a succession of different pulses andphases of eruptive activity (Fig. 74) and knowledge ofthe characteristics of each type of volcanic material isoften sufficient to identify them.

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Figure 72. Phreatomagmatic deposits from Puig d’Adri

Figure 73. Scoria deposits fromCroscat

Figure 74. Sequence of volcanicmaterials at La Pomereda


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Magmatic explosive activityStrombolian fall deposits

Juvenile fragments, highlyvesicular in general, mostlyblocks (bombs), with a varia-ble percentage of lapil l i. Iffound close to the vent, theyare welded into agglomerate.

Angular, highly vesicular,mostly lapi l l i-sized juveni lefragments. Often with layersof bombs, they are distribu-ted radial ly in a relat ivelysmall diameter from the ventand form the volcanic cone.

Angular, ash-sized vesicularjuvenile fragments. They de-posit radial ly around thevent, mostly far from thecone.

Figure 75. Strombolian fall deposits. Volcanic agglomerate

Figure 76. Strombolian fall deposits. Scoria

Figure 77. Strombolian fall deposits. Ash

Hydromagmatic explosive activity

Juvenile fragments and lithicsof varying size with notableblock content. Distributedaround the crater.

Juvenile fragments and lithics,ash-sized or fine lapilli.Fragments may show degreesof rounding and juvenile pyro-clasts are slightly vesicular.Highly dispersed with a highdegree of compaction.

Juvenile fragments and lithics,lapilli-sized and blocks enclo-sed in an ash matrix. They arecompacted and fill pre-exis-ting depressions.

Figure 78. Phreatomagmatic falldeposits. Breccia

Figure 79. Distributed around thecrater. Ash and lithics

Figure 80. Pyroclastic flow deposit. Volcanic tuff


La Garrotxa Volcanic ZoneSites of volcanicinterest

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3. Fitxes.en baixa 4/6/12 07:16 Página 61

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Interpreting the site descriptions

The descriptions are ordered by geographical cri-teria (Fig. 81) to facilitate their use in the field.

Two types of observation - landscape and volca-nic features – are described.Volcanic sites provide evidence of different typesof eruptive activity: effusive, Strombolian and hy-dromagmatic explosive.

Furthermore, each description contains details ofthe site’s location, the volcano whose eruption ori-ginated the deposits, the materials present andan interpretation of the observable sequence ormorphology.

Location of volcanic features

Sites can be divided into two groups: those withinLa Garrotxa Volcanic Zone Natural Park and thosein the Llémena Valley (as well as La Crosa de SantDalmai).

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Figure 81. Location of sites


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1 Castellfollit de la Roca:lava flows

2 El Cairat: pyroclasticbreccia

3 Sant Joan les Fonts:massive materials

4 Montsacopa: conemorphology

5 Croscat: cinder cone 6 Turó de la Pomereda:an eruption sequence

7 Santa Margarida: py-roclastic deposits

8 Can Tià: an eruptionsequence

9 Els Arcs Valley: pyro-clastic flow

10 Location and morp-hology of the volcaniccones as seen fromPuig Rodó

11 El Clot de l’Omera: amaar

12 Puig d'Adri: pyroclasticflow

13 Puig d'Adri: pyroclas-tic surge

14 La Crosa de SantDalmai: morphology

15 La Crosa de SantDalmai: pyroclasticsurge and breccia


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The town of Castellfollit de la Roca standsabout 7 km from Olot on a promontorybetween the rivers Turonell to the southand the Fluvià to the north.To reach Castellfollit from Girona, take theC-66 past Banyoles and then the A-26dual-carriageway past Besalú and turn offat the Castellfollit de la Roca exit. At the

entrance to the town there is an excellentview of the basalt cliff at the junction withthe road to Oix (km point 45). Park andtake Natural Park Itinerary 13 that leadsdown the river Fluvià (Fig. 82). Continuefor 500 m and then turn right over a woo-den footbridge to reach the foot of thebasalt cliff.

Castellfollit de la Roca:lava flows1

Point of interest l Observation of the basalt cliffActivity type l EffusiveAccess time on foot l 10 minutes

Location and access

The basalt columns are aresult of the superpositionof two lava flows and sub-sequent erosion by the ri-vers Fluvià and Turonell.This basaltic cliff stands50 m above the surroun-ding rivers at its highestpoint and extends for a fullkilometre; it provides anexcellent view of the inter-nal structure of a lava flow.The cliff has been rece-ding for thousands ofyears, mostly due to ero-sion by the river Fluvià andfrost weathering (freeze-thawing), which is all themore effective given theexisting jointing. Thesecracks are weak pointswhere weathering cantake place more effecti-vely, eventually leading tothe crumbling of the basal-tic columns. These arethen carried off when theriver is in spate and neverbuild up to stabilise thebase of the cliff.

Castellfollit de la Roca

Figure 82. Schematic geological map of Castellfollit de la Roca


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Interpretation

Alluvia from the rivers Fluvià and Turonelland two lava flows were deposited on topof the original Eocene substrate.Around 217,000 years ago lava from thevolcanoes on the Batet plateau flowedinto and along the valley of the formercourse of the river Fluvià to beyond thetown of Sant Jaume de Llierca. Then,some 192,000 years ago a second lavaflow flowed down the Turonell Valley fromthe Begudà volcanoes to Castellfollit de laRoca. In both cases differential cooling of

the lava gave rise to various differentiatedlayers within the lava flow. The time lapsebetween the two lava flows is marked bythe development of a soil and the deposi-tion of sedimentary materials that form alayer that clearly separates the two flows.To overcome this obstruction, the watersof the Fluvià and Turonell have eroded theboundary between the basalt materialsand the sedimentary rocks.

Description

The base of the cliff consists of layers ofEocene sandstone and marl overlain bygravels containing abundant limestone,sandstone and, exceptionally, basalt peb-bles.On top of these materials lies a 40-m-thick layer of black-grey basalt, althoughabout 9 m from the base of the volcanicmaterials there is a 0.2–1.5-m thick layerof clay and pyroclasts that are recognisa-ble by the abundance of plants growingthere (3). This layer divides the cliff intotwo parts:

a. The lower part has three clearly diffe-rentiated layers. The first has columnarjointing, but is partially covered by the ri-verside vegetation: it is 5.5-m thick and is

composed of prisms around 50 cm indiameter. The second layer has lenticularjointing and is 3.5-m thick. The final layeragain exhibits columnar jointing, but isless than a metre thick and the columnsare only 30 cm in diameter (1).

b. The upper part has four layers: the firstthree, each 5–9-m thick, exhibit markedcolumnar jointing, while near the top alayer about 9-m thick appears with well-developed spheroidal weathering (2).

Lava flows

Figure 83. Castellfollit de la Roca basalt cliff

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The eruption centre of the volcano of ElCairat lies on the ridge of the Sierra deMolera, which is connected to the vol-cano of Aiguanegra. During the 1980s,the pyroclastic materials emitted by thisvolcano were quarried at Can Barrancand today are visible in a number of dif-ferent sites.To reach the quarry, take the GI-522from Castellfol l it de la Roca to Sant

Joan les Fonts. A kilometre before SantJoan, a track turns south into the quarry.Park in the industrial estate on the otherside of the road and walk about 100metres up the track into the quarry andto the volcanic deposits visible in theexcavated banks (Fig. 84).

El Cairat: pyroclastic breccia 2

Point of interest l Can Barranc quarry Activity type l PhreatomagmaticAccess time on foot l 5 minutes

Location and access

The crater of El Cairat –maar-type in structure - isvisible from Begudà andfrom the Batet plateau. Itpossesses a crater ofaround 120 m in diameter,which is sunk into the su-rrounding Eocene sedi-mentary substrate. It isconsidered to be the onlyvolcano in the Natural Parkthat consists of a singleedifice of phreatomagma-tic origin. Its pyroclasticmaterials extend mostlynorthwards, althoughsome are also found southof the eruption centre. Theonly eruptive phase detec-ted was phreatomagmaticwith several intense stagesthat deposited a series ofpyroclastic materials thatare uncommon in the LaGarrotxa.

El Cairat

Figure 84. Schematic geological map of El Cairat


Figure 85. Barranc quarry

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Interpretation

The phreatomagmatic eruption of ElCairat ejected mostly pyroclastic brec-cias, although there is evidence of somemore violent pulses that generated pyro-clastic surges. The location of the erup-tion centre on a ridge-top with steep slo-pes on all sides permitted the build-up ofvolcanic materials: the ejected pyroclastsslid down the mountainside to a morestable area, where they were deposited.

Instantaneous and constant remobilisa-tion of the pyroclasts building up at thetop of the volcano occurred. During itsmovement on the northern flank, thisavalanche of fragmentary materials waschannelled into a gully, where heavy ero-sion swept away part of the sedimentslying on the stream-bed.

Description

The sequence of volcanic materials restson brown silt and clay, which appear nextto the track by a small spring. Abovethese layers lies a fragmentary volcanicdeposit noteworthy for its diverse granu-lometry, with clasts of a wide range ofsizes (from millimetres to metres across).This deposit is 10-m thick here and hasno clearly visible stratification, althoughalternating, various-sized fragments diffe-rentiate a series of layers of irregularthickness. These layers slope gentlynorthwards and are affected by normalfaults.

A detailed analysis of these volcanic de-posits reveals black juvenile fragmentswith little vesiculation, mixed with lithicsof diverse composition. The most plenti-ful lithics are the red clays and conglo-merates originating from the Bellmunt for-mation, the bluish marls from theBanyoles formation and the sandstones,silts and marls from the Bracons forma-tion, all of which are local Eocene sedi-mentary deposits.

Pyroclastic breccia


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These three sites in Sant Joan lesFonts are al l wel l worth visit ing. ElBoscarró lies on the right bank of theriver Bianya and was exposed by theworkings of a basalt quarry abandonedearly in the twentieth century. Along thesame river lies the abandoned quarry ofFontfreda. Finally, on the left bank ofthe Fluvià, at Molí Fondo, river erosionhas revealed a sequence of lava flows.

To reach Sant Joan les Fonts fromOlot, take the GI-522 towards LaCanya. From Girona, leave the A-26dual-carr iageway just after theCastellfollit viaduct and tunnels and fo-llow signs to Sant Joan les Fonts. Parkin the main square, from where thesites can be reached on foot alongNatural Park Itinerary 16 (Fig. 86).

Sant Joan les Fonts: massive materials 3

Point of interest l volcanic materials at El Boscarró, El Molí Fondo and Fontfreda Activity type l EffusiveAccess time on foot l 30 minutes

Location and access

The r iver Bianya f lowsinto the r iver Fluvià atSant Joan les Fonts.Erosion by these r ivershas uncovered the su-perposition of three lavaf lows that part ial ly oc-cupy the former r iver-beds. Quarrying for ba-salt in the early twentiethcentury has revealed theinteraction between thelava flows and their inter-nal structures and alsoallows us to reconstructthe geological history ofthe site.

Sant Joan les Fonts

Figure 86. Schematic geological map of Sant Joan les Fonts


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Description

El BoscarróThis site provides excellent views of dif-ferent types of jointing in the last of thethree lava flows that were channelledalong the Fluvià Valley. Five layers canbe distinguished: the lowest exhibits co-lumnar jointing with five- and six-sidedcolumns, 20–40 cm in diameter and2–3-m high; the second and fourth la-yers have slab jointing and are separa-ted by a third layer in which the massivematerial shows very few cooling cracks;finally, the fifth and uppermost layer, justbelow soil level, has been more alteredthan the others due to its proximity to thesurface and consequently exhibits clearspheroidal structures.On the other side of the quarry, we cansee where the river Bianya has beenchannelled along the point of contactbetween the volcanic materials andreddish Eocene sedimentary materials.

Molí FondoThe dam was built on top of the firstlava flow, which lies on the bed of theFluvià. To the right a certain degree ofcolumnar jointing can be seen in thebasalt, which is blue-grey in colour. Ifyou wander downstream along the riverbank, you walk on slabs that representthe base level of the second lava flow(1). In parts, the rough cinder base pro-trudes. On the nearby cliff, you can seethe rest of the lava flow exhibiting co-lumnar jointing. Just above lies a layerof sediment consist ing of sandstone

and basalt pebbles in a silt matrix (2).Final ly, at the top the third Boscarrólava flow is visible (3).

The cliffs at Fontfreda The visible layers here correspond tothe third lava flow that we visited at ElBoscarró. The lowest layer has obviouscolumnar jointing with columns over 3-m high and is crowned by an area oflenticular joint ing. Unl ike at ElBoscarró, a clear transit ion from onetype of jointing to the next is visible.

Superposition of lava flows

Figure 87. Feature Molí Fondo

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Interpretation

The first lava flow issuing fromthe volcanoes of the Batetplateau flowed down and intothe former bed of the Fluvià, fi-lling in part of the river basin.

The erosive activity of the rivergouged out a new riverbedfrom this lava flow and deposi-ted sediments on top.

Thousands of years later, theriverbed was occupied by asecond lava flow, whose originis still unclear.

Figure 88

Figure 89

Figure 90


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Over time, the river depositedmore sedimentary materials(silt, sand and pebbles) on topof the second lava flow andformed a river terrace.

About 133,000 years ago, athird lava flow covered theselatest alluvial sediments. Thisfinal lava flow originated fromLa Garrinada and stopped justpast the town of Sant Joan lesFonts.

Schematic diagram of Molí Fondotoday.

Figure 91

Figure 92

Figure 93


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Montsacopa, one of the four volcanoeswithin Olot itself, lies in the middle of thecity between La Garrinada to the north-east and Montolivet to the south-west.On top stands the chapel of SantFrancesc, built in the nineteenth century,and two watchtowers.

La Garrotxa Volcanic Zone Natural ParkItinerary 17 starts at the VolcanoMuseum, crosses the town and climbs tothe top of the volcano (Fig. 94). To shortenthe walk, you can park at the cemetery atthe base of the cone near an abandonedquarry and then walk up the steps to thecrater.

Montsacopa: cone morphology 4

Point of interest l Volcanic craterActivity type l StrombolianAccess time on foot l 10 minutes

Location and access

This volcano consists of a single, regular-shaped cinder cone. A walk around thecrater provides excellent views of the vol-canoes of Montolivet, Bisaroques and thethree craters of La Garrinada. The firstcrater, part of a tuff ring that originatedduring a phreatomagmatic phase, is visi-ble at its base. This tuff ring is almostcompletely covered by a cinder coneconstructed during the subsequentStrombolian phases, which also gave riseto the other two craters visible on top ofthe volcano, on its southern and northernslopes.Montolivet lies to the south-west andconsists of a cinder cone abutting ontothe ridge of La Pinya; its crater opens to-wards the north-east.To the south-east on the northern slopeof the Batet plateau stands Bisaroquesand its horseshoe-shaped crater. Judgingby the deposits found here, a number ofphreatomagmatic phases occurred du-ring its eruption. However, its cinder conewas formed during a subsequentStrombolian phase, but was then partiallydestroyed in the final stages of the erup-tion by a small lava flow that flowed north-wards towards the river Fluvià.

The volcanoes of Montsacopa, Montolivetand La Garrinada are positioned along asingle fracture through which the magmapenetrated on its ascent to the surface.

View from Montsacopa

Figure 94. Schematic geological map of the four Olotvolcanoes


Figure 95. Montsacopa

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Interpretation

There were at least two eruptive phases -one effusive, the other explosive - duringthe eruption of Montsacopa. During thefirst phase, a lava flow ran to the foot ofthe nearby ridge of Sant Valentí and alarge section with lenticular jointing is visi-ble next to Olot football club (although theproposed itinerary does not go there).The second phase was mostlyStrombolian, although the presence offragments that are not particularly vesicu-

lar in the upper layers of the sequence in-dicates the existence of phreatomagmaticpulses, which destroyed part of the lavaflow and created fragments that were de-posited as lithics in the pyroclastic surge.The Strombolian phase eventually builtthe cinder cone and the lack of a final ef-fusive phase with the emission of a lavaflow ensured that the crater's circularshape was preserved.

Description

The crater of Montsacopa is circular,about 120 m in diameter and 12-mdeep; its cone has sloping flanks andstands 94 m above the surroundingland. The bottom of the crater is f latand is currently cultivated.On the southern and south-easternflanks of the cone there are a numberof abandoned quarries, exploited in thesixteenth century for pyroclastic depo-sits that were mostly used in construc-

tion. In the quarry next to the cemeterythe different layers formed during theeruption have been exposed. Mostconsist of block- and lapilli-sized frag-ments, with the occasional encrustedbomb. These are highly vesicular juve-nile pyroclasts. However, at the top ofthe sequence of materials, despiteconsisting of solidif ied magma, someof the deposits exhibit incipient vesicu-lation and are mostly ash-sized.

Morphology of the cinder cone


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Croscat lies halfway between Olot andthe village of Santa Pau in a relatively flatarea bounded to the south by the Corb-Finestres ridge, to the north-east by themountain of Sant Julià del Mont and tothe north by the Batet basaltic plateau.The abandoned quarry on the northernflank of the volcano is a site of exceptio-nal interest that reveals the internal struc-ture of a cinder cone.

To reach Croscat, take the GI-524 to-wards Santa Pau and park in the Àreade Santa Margarida car park (on theright) 7 km after leaving Olot. Itinerary15 to Croscat and the Natural Park in-formation centre at Can Passaventstarts here (Fig. 96).

Croscat: cinder cone5

Point of interest l Quarry with volcanic depositsActivity type l StrombolianAccess time on foot l 20 minutes

Location and access

The top of Croscat stands160 m above the surroun-ding land (it is the tallestvolcano in the IberianPeninsula ) and its basemeasures 950 m in diame-ter. The symmetry of its co-nical-shaped cinder cone isdistorted by a horseshoe-shaped crater on its wes-tern flank.Croscat erupted in threephases, the first twoStrombolian and the last ef-fusive. The secondStrombolian phase built thecone and ejected pyro-clasts that covered the ne-arby volcanoes of SantaMargarida and Puig deMartinyà. The effusivephase generated a basanitelava flow that destroyed thesymmetry of the edifice andformed a horseshoe-sha-ped crater as it ran west for6 km. The beechwoodknown as La Fageda d’en

Jordà stands on this roughlava flow, which is dotted bynumerous blisters. Dating ofthe ejected materials at LaPomereda gives an age of

11,500 (±1,500 years) andCroscat is thus the most re-cent manifestation of volca-nic activity in the wholeCatalan volcanic field.

Croscat

Figure 96. Schematic geological map of Croscat


Figure 97. Croscat volcanic deposits

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Interpretation

The f irst phase of the eruption wasStrombolian and explosive and built thewelded scoria deposits recognisablenear the vent at the base of the se-quence. This activity then becamemore explosive and bui l t the cindercone. Init ial ly, the pyroclasts fel l inpractical ly horizontal layers, but withthe gradual growth of the cone the gra-dient of the deposits began to increa-se. Sporadically, when the release ofgases was less intense, bombs were

ejected. Finally, a lava flow was emittedfrom the eastern flank of the cone andran westwards towards Olot.The different colours of the pyroclastsare due mainly to thermal alterat ion.Hot gases released in the later stagesof the eruption caused oxidationaround the chimney, the hottest part ofthe volcano; the black-grey of the pyro-clasts thus changed to red-ochre.

Description

The quarry in Croscat was worked fromthe late 1950s to the early 1990s andtoday provides wonderful views of avast surface area of pyroclastic mate-rials, approximately 150 x 500 m. Theterracing on the right of the open facedates from when the quarry was activeand has helped stabilise the deposits.On the opposite side and in the middleof the deposits, however, landslips aremore frequent.It is easy to spot the different layers ofscoria, made up of irregular, highly vesi-

cular juvenile fragments that are mainly la-pilli-sized (Fig. 97). The gradient of theselayers increase as you move from thecentre to the outside of the cone. At thebase of the sequence, where bombs aremore abundant, different layers alternate.The materials are mostly dark grey orblack, although in the area closest to thecentre of the edifice they are reddishochre (1).In the lowest part of the former quarry,there is a layer of red welded scoria (2).

Structure of the cinder cone

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Next to Can Genís in the Massandell plainlies another former quarry, from which vol-canic materials emitted by the volcano ofTuró de la Pomereda were extracted. Thequarry walls reveal a sequence of pyro-clastic deposits covered by a lava flow.

To reach Turó de la Pomereda, take NaturalPark itinerary 1 from the Àrea de SantaMargarida car park towards La Fageda d’enJordà, which skirts the north side ofCroscat. Where the route forks at Can Pelat,take the right-hand track towards LaCanova. The abandoned quarry is visibleabout 20 m along on the left (Fig. 96).

El Turó de la Pomereda: an eruption sequence6

Point of interest l Volcanic materialsActivity type l Strombolian and effusiveAccess time on foot l 30 minutes

Location and access

El Turó de la Pomereda lies at the footof Croscat and this slightly raised areais one of the volcano’s five small ad-ventive or parasitic cones. Prior to thequarrying of the materials from its cen-tre, this small cone was tumulus-like inshape.A map shows that La Pomereda is alig-ned with Santa Margarida, Croscat and

Puig Astrol and lies on a fault that sup-posedly runs north-west to south-east.Its lava flow has been dated at 11,500years old and is thus the most recentmanifestation of volcanic activity in theCatalan volcanic field.

Turó de la Pomereda

Figure 98. La Pomereda volcanic deposits

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Interpretation

The eruptions of both La Pomereda andCroscat began with Strombolian phasesthat were not very explosive. In the cour-se of the eruption, scoria blocks in asemi-molten state were ejected and wel-ded together as they fell close to the vent.The initial phases of the eruptions at LaPomereda and Croscat both began in thisfashion and the deposits that formedconstitute a volcanic agglomerate.

The next phase was also typicallyStrombolian and gave rise to a deposit oflapilli-sized scoria and ash. The thinnessof these deposits, which here are foundvery near the vent, indicates that this se-cond phase was short lived. The finalphase was effusive and emitted a smalllava flow that partially covered the top ofthe underlying pyroclasts. The transfer ofheat from this layer to the lapilli belowcaused the pyroclasts to weld together.

Description

The south-western part of the quarryholds the best outcrop of volcanic ag-glomerate in the Catalan volcanic field(Fig. 98). This deposit consists of highlyvesicular juveni le fragments, mostlyblocks (bombs) with a variable percen-tage of lapilli. This scoria is welded andcontinuous towards the northwest; it islargely dark grey to black, although inparts reddish fragments appear.

On top of these fragments lies a 3-m-thick layer of dark grey scoria (1). Here,the clasts are mostly lapilli-sized (2), alt-hough there are some larger fragmentstowards the top. In the final 30 cm the la-pilli fragments are welded.Finally, a massive deposit (3) appears, so-mewhat channel-shaped and about 2-mthick in its middle. The base of this smallflow is scoria and its internal structure dis-plays columnar jointing with poorly defi-ned columns.

Massive and fragmentary materials

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Santa Margarida, one of the best-known volcanoes in La Garrotxa, lies atthe foot of a spur jutting south from theLleixeres r idge. Park in the Àrea deSanta Margarida car-park at the 8-kmpoint on the GI-524 (Olot to Santa Pau)and walk up the volcano.Itinerary 4 leading to the crater of SantaMargarida starts in the car park andheads towards Santa Pau; after just200 m, turn r ight along a track that

leads steeply up to the crater.However, the best way to see the vol-canic materials is to ignore this r ightturn and continue towards Mas el Crosand the eastern sector of the volcano.On the right-hand side of the road py-roclasts appear immediately, althoughthe best sequence of deposits is found400 metres further on (Fig. 99).

Santa Margarida: pyroclastic deposits 7

Point of interest l Volcanic deposits along the road to Mas el CrosActivity type l Phreatomagmatic and Strombolian Access time on foot l 15 minutes

Location and access

This phreatomagmaticvolcano, whose circularcrater is about 350 m indiameter and 70-m deep,stands on Eocene marls.I ts cone is not formedentirely of volcanic mate-rials, since the crater isimbedded in the pre-vol-canic stratum. In themiddle of the craterstands a Romanesquechapel, which has beenheavi ly restored in mo-dern times. The init ial Strombolianphase during the erup-t ion of Santa Margaridawas rather uneventfuland was quickly followedby phreatomagmatic acti-vity, which varied greatlyin intensity and at timeswas barely explosive atall. The vegetation cove-r ing the volcanic mate-rials somewhat hides the

small pyroclastic f low inthe south-eastern sectorof the volcano.

Santa Margarida

Figure 99. Schematic geological map of Santa Margarida


Figure 100. Outcrop on road to Mas el Cros

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Interpretation

The volcanic map of this sector of theVolcanic Zone reveals that not all thedeposits found here originated fromSanta Margarida.The base layers correspond to pyro-clastic surges expelled during the ph-reatomagmatic eruption. They weremainly dispersed eastwards by the in-teraction between the magma andwater in the aquifer in the Bellmunt for-mation (Eocene). The middle layer alsoconsists of deposits from SantaMargarida, but from a far less violentsubsequent eruption. This fall deposit

is practically Strombolian, although thepresence of lithics indicates there wassome degree of phreatomagmatic acti-vity. The dispersion of this material isradial, from the vent outwards. Finally, the scoria at the top of the de-posit corresponds to a Strombolian falldeposit that originated from Croscat, aki lometre away. From the absence ofany paleosol between these differentlayers we can deduce that the erup-tions of Croscat and Santa Margaridatook place simultaneously.

Description

Three types of volcanic materials appe-ar along the road to Mas el Cros, onesucceeding another from right to left asa result of the inclination of the layers(Fig. 100). On top of the silty pre-volca-nic soil, sits a layer of compacted ash.Then, black juvenile fragments appear,containing quite rounded, reddish-brown lithics (1). Finally, there is a layerof l ithic and juvenile lapil l i-sized frag-ments; the former consist mostly of red

sandstone, while the predominate frag-ments in the latter are black and slightlyrounded in shape with little vesiculation(2). The sequence is crowned by a de-posit that looks very like the previouslayer, only without any lithics, and con-sists of a f ine-grained scoria depositwith no stratification (3).

Pyroclastic fall and surge deposits

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The volcano of Can Tià takes its namefrom a farm close to the top of the Corb-Lleixeres ridge. Its vent stands is at thehead of the valley of Sant Iscle de Colltort,where many of its ejected materials are vi-sible. However, most of our observationswill be made in a small abandoned quarryright next to the house of Can Tià.To reach Can Tià, park opposite Can Xelat the 5-km point on the GI-524 (Olot to

Santa Pau road) and take Natural ParkItinerary 5, which leads straight to the vol-cano in about an hour (Fig. 101). On theway up, you pass over Eocene sedi-ments, initially reddish and then brownerin the uppermost strata, corresponding,respectively, to the Bellmunt andFolgueroles formations.

Can Tià: eruption sequence 8

Point of interest l Can Tià volcanic depositsActivity type l Phreatomagmatic and StrombolianAccess time on foot l 60 minutes

Location and access

This volcano lies next tothose of Fontpobra andLa Tuta and has a maar-type edifice with a circu-lar, 270-m-diameter cra-ter. Today, domestic ani-mals pasture this f lat-bottomed depression,around 20-m deep. Thelow cone is highest to thesouth. The Can Tià erup-tion had no effusive pha-ses and therefore all theejected materials are py-roclastic. Most are phre-atomagmatic, but someare the product of aStrombolian eruption.The largest pyroclasticdeposit has fil led part ofthe val ley of Sant Iscle,where there is a volcanictuff, possibly result ingfrom a pyroclastic flow.

The volcano

Figure 101. Schematic geological map of Can Tià and its volcanoes


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Interpretation

The eruption of Can Tià began with aStrombolian phase (Fig. 103a) that for-med a cinder cone built by scoria falldeposits.When the pressure in the vent drop-ped, the magma interacted with thewater in the Folgueroles aquifer, givingrise to a more violent phreatomagmaticeruption (Fig. 103b) with breccia andpyroclastic surges. These explosions inthe vent in this second phase destro-yed the cinder cone and the construc-tion of the maar began. The deepening of the area of water-magma interaction meant that waterfrom the Bellmunt aquifer could also in-tervene in the phreatomagmatic activity(Fig. 103c). In this phase of the eruption,a pyroclastic flow was formed from dif-ferent surge- and breccia-type flows.

Description

The sequence of deposits in the smallCan Tià quarry (Fig. 102) are around 10-m thick and contains two sets of frag-mentary materials.At the base (around 6–m thick) there isa black scoria deposit with no layering

made up of lapilli-sized fragments andblocks and is notably vesicular. A fewlithics measuring up to 10 cm in diame-ter are present (1). On top of the scoria lies a series of alterna-ting layers of breccia and ash. Here, the ju-venile fragments display incipient vesicula-tion and are slightly rounded. The mostabundant lithic fragments in the first layersare brown and correspond to the Eocenesandstones of the Folgueroles formation.The lithics in the breccia and ash in theupper part are mostly sandstone too, but arereddish in colour and originate from theBellmunt formation from the same geologicalepoch (2).Finally, there is a very compact tuff de-posit that can be followed downhill formore than a kilometre (3).

The eruption sequence

Figure 102. The quarry at Can Tià

Figure 103. Eruption sequence at Can Tià

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The small valley of Els Arcs lies on thenorthern slope of the Finestres ridge. Thebottom of the valley contains an interrup-ted series of outcrops of volcanic mate-rials that can be viewed in the streambednear Mas el Carrer.

To reach these deposits, park in SantaPau and follow Natural Park Itinerary 7 to-wards Els Arcs Valley. At Mas el Carrer,continue for about 10 m along a track tothe right that leads down to the stream-bed (Fig. 104).

Els Arcs Valley: pyroclastic flow9

Point of interest l Volcanic deposits at Mas el Carrer Activity type l Phreatomagmatic Access time on foot l 60 minutes

Location and access

Only the pyroclastic deposits emittedby this volcano are known, since itsvent has not yet been located. It maylie on the north-south fracture that hashelped mould the shape of this valley,although it is clear that the crater liesabove 475 m, the upper limit of the py-roclastic deposits. Numerous al luvialand piedmont sediments washed downfrom the northern slopes of theFinestres ridge have built up along theupper part of the valley and have pro-bably buried the volcano’s edifice.Sant Jordi had a number of different ac-tivity phases, of which the last genera-ted a deposit at least 1.7-km long with amaximum width of 350 m; it is thickestin its upper part (about 7.5 m). At theconfluence of the Arcs valley with theriver Ser, this deposit disappears underthe lava flows originating from other vol-canoes in the Santa Pau area.

Sant Jordi

Figure 104. Schematic geological map of Els ArcsValley


Water erosion in Els Arcs valley has re-vealed a complete sequence of mate-rials ejected by Sant Jordi. In the upperpart of the valley, the pyroclastic mate-rials rest on gravel with sandstone peb-bles and a sand and silt matrix.Three fragmentary deposits in 12 dis-cernible layers are visible (Fig. 105): atthe base lies a first deposit comprisingtwo very compacted layers, each mea-suring 5 cm, with ash-sized juvenilefragments and lithics (red sandstonefrom the Bellmunt formation). The upperlayer has coarser, lapilli-sized clasts (1)underlying a deposit of scoria includingsome ash with juvenile components andthe same red sandstone lithics (2). Thefinal deposit has four layers with a totalthickness of 7.5 m, with two layers atthe base, each 5-cm thick, consisting oflapilli-sized clasts and ash with juvenilecomponents and red sedimentary l it-hics. The most noticeable layers of thissequence are the topmost two, two-and four-metres thick, respectively. Bothare tuffs with juvenile fragments and lit-hics, some over 10 cm in diameter, andare embedded in a matrix of red ash (3).The base of the final layer is erosive andis flat-topped.

Figure 105. Outcrop El Carrer

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Interpretation

At least three phases occurred duringthe eruption of this volcano, of whichthe phreatomagmatic events were themost important.In the first, the water-magma interac-tion led to pyroclastic surges that for-med the base deposit. Then, the phre-atomagmatic activity was interruptedby a Strombolian phase, which emittedthe scoria. In this phase, however,small amounts of water entered thevent and caused small pyroclasticflows.

As the eruption was ending, the phrea-tomagmatic phase reactivated, genera-ting a pyroclastic flow that ran into theformer course of the valley. The two la-yers of tuff correspond to the two pul-ses that took place while the pyroclas-tic f low was being formed. The rapidemplacement of this flow meant that atthe front there was a significant inges-tion of cold air. This air heated up im-mediately due to the high temperatureof the flow and caused a series of ex-plosions, which created the pyroclasticsurges that gave rise to the layers for-ming the base of the third deposit.

Description

The pyroclastic flow

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Location and morphology of the volcaniccones as seen from Puig Rodó10

Point of interest l View from the viewpointActivity type l Strombolian and effusive Access time on foot l 20 minutes

Location and access

The road that leads to Xenacs offersexcellent views of the Bas valley (1), anagricultural plain that was once a lake.The lava flow emitted by Croscat randown towards Olot and dammed theriver Fluvià, giving rise to what is knownas a barrage lake. Over time, the lakegradually began to silt up and in eighte-enth century the marshy plain and la-goons were drained for cultivation.

From Puig Rodó, we can trace theroute of the Croscat lava flow acrossthe landscape by the woodland thatcovers it, mostly the beechwoodknown as La Fageda d'en Jordà (2).

The Bas valley

Figure 106. View from Puig Rodo at Xenacs

Puig Rodó (909 m) stands at the westernend of the Corb r idge where, from theXenacs Recreational Area, there are exce-l lent views of La Garrotxa Volcanic ZoneNatural Park, the main Pyrenees, the pre-Pyrenees, most of the Olot Trough and theBas Valley (Fig. 106).From Olot, take the C-152 through the townof Les Preses. After about 300 m, turn leftalong a road which climbs steeply in 5 km to

the car park in the recreational area. Fromhere, walk along the signposted path to thePuig Rodó viewpoint. The road is not viablefor coaches and is closed to all vehicles onweekdays, although an access permit canbe obtained from Les Preses Town Council.Natural Park Itineraries 10 and 11, starting inLes Preses, also climb up to Xenacs.

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Montolivetvolcano

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Racó volcano

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The main relief features

The depression bordered by L’AltaGarrotxa to the north, the Corb ridge tothe south, Sant Julià del Mont to the eastand La Collsacabra and Puigsacalm tothe west is known as the Olot Trough.Behind and to the sides of this depres-sion of tectonic origin stand most of thevolcanoes in La Garrotxa Volcanic Zone.The valleys we can see are all U-shapedbecause they were filled in by lava flowsemitted during eruptions or by sedimentsthat built up behind the barrage lakes for-med by lava flows. From Puig Redon we

can see almost the whole of the northernsector of the volcanic zone, including 14(8) of the park’s 40 volcanoes. A charac-teristic feature of the volcanic cones istheir shape and the form of their craters,either circular or horseshoe-shaped. Allare covered in woodland and almost al-ways stand out above the arable land thatreaches right up to their bases. Of note isthe Batet plateau (9) to the north-east, for-med by the build-up of successive lavaflows from the region’s oldest volcanoes,most of which have been eroded away.

The Olot Trough

On a clear day from Puig Rodó you cansee most of La Garrotxa, as well as partsof El Ripollès to the west and El Pla del’Estany and L’Alt Empordà to the east. Tothe north we can make out:

a. Axial Pyrenees (3): the backdrop tothe view north consists of the main ridgeof the Pyrenees, made up of ancientPalaeozoic rocks, whose highest peaksare snow-covered for much of the year.

b. Pre-Pyrenees and Sub-Pyrenees(L'Alta Garrotxa) (4): these mountains(1,000–1,500 m) lie in front of the AxialPyrenees and mostly consist of Eocenerocks that were intensely folded and af-

fected by faults during the Alpine oro-geny.

c. Mountains of La SerraladaTransversal: lying the closest to PuigRodó, these peaks include the Corb ridge(5) and are part of this transversal (N-Srunning) system, also made up entirely ofEocene rocks. These mountains consistof a series of raised and sunken blocks,the product of a system of normal faults,the highest of which are the peaks ofCollsacabra and Puigsacalm (6).The de-pression at the foot of Puig Rodó to thenorth corresponds to the Olot Trough (7).

Montsacopavolcano

Garrinadavolcano

Bisaroquesvolcano

Cabriolervolcanoes

Puig Astrolvolcano

Pujalósvolcano

Puig de laGarça

volcanoCroscatvolcano

Puig Jordàvolcano

Puig de laCosta

volcano

SantaMargaridavolcano

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A farm, Mas de la Pallonera, lies inside ElClot de l’Omera (clot = depression inCatalan), a circular depression on the leftbank of the river Llémena between the vi-llage of Llorà and El Pla de Sant Joan.To reach the area, from Girona take theGI-531 along the Llémena valley and atthe 15-km point, just before El Pla deSant Joan, a track runs to Mas de la

Pallonera (Fig. 107). Park in El Pla deSant Joan and walk along the track asfar as a clearing, from where there aregood views over El Clot de l’Omera. Ifyou continue along the track, a streamjust before the farm has interesting out-crops of volcanic materials.

El Clot de l’Omera maar11

Point of interest l View of the craterActivity type l Phreatomagmatic Access time on foot l 5 minutes

Location and access

This small volcanic edifice is partially co-vered by a lava flow originating from anot-her volcano, Puig de la Banya del Boc(Fig. 107). This latter volcano, encrustedinto the south-facing slope of La Serra deBoratuna, lies on the Llorà fault, whereTertiary sedimentary materials come intocontact with Palaeozoic metamorphicmaterials. During the formation of Puig dela Banya del Boc a series of differenteruption phases occurred. Initially, theeruption was phreatomagmatic, thenStrombolian and, finally, effusive. The ph-reatomagmatic phase emitted the pyro-clasts that can be seen along the banksof a stream, Torrent de Bosquerós, andthe river Llémena. As the same time asthese phreatomagmatic phases, El Clotde l’Omera erupted. Subsequently, theStrombolian phase of Puig de la Banyadel Boc began and built a cinder cone oflapilli and bombs with an elliptical crater.Finally, the effusive phase gave rise tothree lava flows, two of which flowed intothe former streambeds of Torrent deBosquerós (to the south-west) and CanPere Boé (eastwards). A third lava flowran south as far as the river Llémena and

created the agricultural plain today knownas El Pla de Sant Joan. Next to this plainstands El Clot de l’Omera, separatedfrom Puig de la Banya del Boc by ElsRasos de Llorà, a small hill of metamorp-hic rock.

El Clot de l’Omera

Figure 107. Geological diagram of El Puig de laBanya del Boc and El Clot de l’Omera


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Interpretation

The edifice of El Clot de l’Omera con-sists of a maar that was formed duringa single-phased phreatomagmaticeruption. The ejecta extend mainlysouthwards owing to the steep slopesof Els Rasos de Llorà to the north ofthe volcano, although this asymmetrycould have been caused by the inclina-t ion of the fracture from which themagma issued. The flat bottom of thecrater is due to the blocks of pyroclas-t ic materials that sl id down from thecrater rim. One of these blocks did notstabilise completely and evidence of itsmovement can be seen in the scar ofthe circular fracture in the streambehind Mas de la Pallonera.Alternating layers of ash deposits andbreccia are very visible due to the seriesof different pulses that occurred duringthe eruption. Some of the deposits con-tain a high percentage of lithics and canthus be attributed to phreatomagmaticpulses. The fact that most of the lithicsare metamorphic in origin suggests thata large aquifer existed in the substrateof the metamorphic rocks.

Description

The most notable feature of this volca-no is the crater in its single volcanicedifice, which abuts the southern slo-pes of Els Rasos de Llorà; on its innerwal ls metamorphic materials appearunderneath the pyroclastic deposits.Thus, the crater which lies below thepre-eruption land surface is flat-botto-med, and measures approximately 500m in diameter and is 20-m deep (Fig.108). Today, a drainage channel pre-vents the crater from flooding.The cone is partly covered by a lavaflow and is diff icult to see. However,there is a sequence of pyroclastic de-

posits around the crater that increasein thickness from north to south. In asmall streambed behind Mas de laPal lonera, a sequence of pyroclasticmaterials, 10-m thick in parts and con-sisting of a succession of breccia andash deposits, is visible. They are veryheterogeneous in composition due tothe size and type of l i thic fragments.These lithics are very angular in generaland originate from metamorphic rocks(e.g. schist and marble). A few basaltfragments are mixed in and mostlyshow little vesiculation.

Maar

Figure 108. El Clot de l’Omera and Puig de la Banyadel Boc

El Clot del’Omera

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Font de la Torre is a natural spring nearthe village of Canet d’Adri (El Gironès)that gushes out at the confluence oftwo streams, Riera de Rocacorba andTorrent de Rissec (Fig. 109). To reachthe spring from Girona, take the GI-531to Sant Gregori and about 3 km pastthis town, turn right on the GIV-5313 to

the village of Canet d’Adri. The streeton the left about 300 m after the centreof Canet d’Adri leads to Mas de laTorre. Park next to this farm and pickup the track to the spring in the bed ofRiera de Rocacorba.

Puig d’Adri: pyroclastic flow

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Point of interest l Volcanic materials deposited at Font de la TorreActivity type l Phreatomagmatic Access time on foot l 5 minutes

Location and access

This volcano stands at thefoot of the mountain ofRocacorba, between thevillage of Canet d’Adri andAdri. This is the eastern-most of the volcanoes inthe Llémena Valley and liesonly 7 km from the city ofGirona.Three superimposed vol-canic edifices (Fig. 109),which were built during dif-ferent eruption phases,can be identified. A cindercone reaching 408 m isthe most remarkable of thethree and is visible fromjust behind the church inCanet d’Adri.The products of this volca-no’s phreatomagmatic ac-tivity are numerous and va-ried. They are well disper-sed and there are depositsup to 5 km from the vent.An emission of lava in thelast stage of the eruptiongenerated a lava flow thatflowed 11 km to Domenyon the outskirts of Girona.

Puig d’Adri

Figure 109. Geological diagram of Puig d’Adri


The presence of abundant lithic frag-ments, along with a number of palaeo-magnetic studies that have determined anemplacement temperature for these ma-terials in excess of 550°C, are proof thatthis deposit is the product of the phreato-magmatic eruption of Puig d'Adri.Furthermore, the elongated shape andchannel-shaped cross-section suggestthat a pyroclastic flow swept down and fi-lled in the former river valley.Thus, during the first phreatomagmaticphase an important amount of water inte-

racted with the magma and the resultingexplosions ejected a dense pyroclasticflow that was channelled down the origi-nal valley of Riera de Canet. However,successive pulses in this phase genera-ted a series of sub-flows that gave rise tothe incipient layers visible in this deposit.Finally, a lava flow covered the depositedpyroclastic materials. Subsequently, thewater in the streams has eroded thesevolcanic products and exposed the se-quence of deposits (Fig. 111).

Compacted fragmentary materials (vol-canic tuff) are visible at Font de laTorre. This deposit contains juveni lepyroclasts and lithics (various millime-tres in diameter), surrounded by a finereddish-brown matrix. The black juveni-le fragments are of basaltic composi-t ion with l i t t le vesiculat ion. The mostplentiful lithics are red sandstone, alt-hough blue marls and pale grey calca-reous rock are also present.Although the deposit is fairly uniform incomposition, different layers are visibleand have been eroded - more eff i-ciently at the boundaries between la-yers - and give the outcrop a terracedeffect.This tuff appears along Riera de Canetfor about 3 km downstream from thespring and in places is 20-m thick. Ontop of this fragmentary deposit l ies alava flow, which can be seen clearly onthe left bank of Torrent de Rocacorbaand on the path that leads there.

Erosion by the streams (Riera deCanet, Torrent de Rocacorba andTorrent de Rissec) has created a seriesof deep pools that are unique in theCatalan volcanic field.

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Figure 110. Font de la Torre

Figure 111. Stages in the formation of the volcanic deposits at Font de la Torre.

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Interpretation

Description

The pyroclastic flow


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On the south-eastern flanks of Puigd’Adri stands a cork-oak wood knownas La Sureda d’en Toscà, which exhibitsfine examples of pyroclastic surges andbreccia deposits. Access from Girona isalong the GI-531 to Sant Gregori; 3 kmafter this town, turn right on the GIV-5313 to Canet d’Adri (Fig. 112).

In the village, take the road towards thehamlet of Collsacarrera. About 400 mbefore Col lsacarrera, park where atrack leads off to the r ight to CanToscà. Walk along the track for about25 m to where, just behind the bank onyour left, the outcrops begin.

Puig d’Adri: pyroclastic surges13

Point of interest l Volcanic materials in the Toscà cork-oak woodActivity type l PhreatomagmaticAccess time on foot l 15 minutes

Location and access

This eruption had f ivephases. The f i rst washighly explosive and ph-reatomagmatic and agreat deal of breccia andcinder were deposited;during this phase the tuffring was formed - edifice1 (Fig. 112). Two superim-posed cinder cones -edifices 2 and 3 - resul-t ing from subsequentStrombolian phases par-tially cover this first cons-truction. The tuff ring cra-ter is 850 m in diameterand the materials thatform the cone appearalong the road fromCanet d'Adri toCollsacarrera. The out-crop in La Sureda d’enToscà contains the bestexamples of these phrea-tomagmatic deposits.

The eruption

Figure 112. Schematic geological map of Puig d’Adri


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Interpretation

Although not visible at this site, mate-rials appear beneath the scoria that ori-ginated from the phreatomagmaticeruption. This scoria, so typical ofStrombolian activity, signif ies an inte-rruption in the phreatomagmatic phaseat the start of the eruption.Scoriaceous materials are normallyonly found as part of a cinder cone andso their location in this outcrop, farfrom the vent, can only be explained bythe remobilisation of the scoria by laterphreatomagmatic explosions.

The topmost layers of ash were depo-sited during the phreatomagmatic pul-ses and their lamination is due to thehigh energy present in the flow. Thesematerials are pyroclastic surge depo-sits and their compaction indicatesthat, when they were deposited, part ofthe water vapour in the flow condensedand caused them to be compacted.The breccia crowning the sequencecame from a series of less intense pul-ses in the phreatomagmatic phase.

Description

On close scrutiny and within just 20metres of each other materials deposi-ted in three groups in various layerscan be observed (Fig. 113). At the base of the sequence lies a sco-ria deposit formed almost entirely ofblack, highly vesicular lapilli-sized juve-nile fragments (1). There are no layerswithin this deposit, although at the topsome angular l i thics, mostly of redsandstone, appear (of various centime-tres in diameter).Covering the scoria are mill imetric la-yers of ash with considerable compac-tion (2), which gives these layers a cer-tain positive relief within the outcrop.The miniscule size of the fragmentsmeans that they cannot be identif iedwith the naked eye. However, with theaid of a magnifying glass you can seethat this ash contains a large propor-tion of lithic fragments of red sandsto-ne and some marl. The marked lamina-tion of the ash is obvious and the stra-tification is frequently crossed at a lowangle. A few coarser layers can be dis-tinguished between the ash layers.

Finally, at the top there is a series of la-yers of pyroclastic breccia; the largestof the pyroclasts indicates that thatsame lithic fragments are present as inthe ash (3). These layers are thickerand the fragments are looser. There isalso degree of lamination often markedby the presence of layers of ash.

The pyroclastic deposits of La Sureda d’en Toscà

Figure 113. La Sureda d’en Toscà pyroclastic surge

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3


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The quarry of Can Guilloteres used to ex-tract pyroclasts from La Crosa de SantDalmai, a volcano situated between thesettlements of Aiguaviva, Estanyol andSant Dalmai on the border between theregions of La Selva and El Gironès. FromGirona, take the road to Santa Coloma(GI-533) that goes through Aiguaviva.

After crossing the road to Estanyol, about1 km along on the right there is an openarea from where the volcanic materialswere once extracted. In the part farthestfrom the road you can climb a smallmound (about 5-m high) formed from py-roclasts, which offers a good view of thecrater of La Crosa de Sant Dalmai.

The morphology of La Crosa de Sant Dalmai

14

Point of interest l View from Can Guilloteres quarryActivity type l PhreatomagmaticAccess time on foot l 5 minutes

Location and access

This volcano lies is on theboundary between the de-pression of La Selva, filledwith Pliocene andQuaternary sediments,and the southern end ofthe Transversal mountainsystem, here representedby contacting granite andmetamorphic Palaeozoicrocks. The eruption waspredominantly phreato-magmatic, with a finalStrombolian phase. Theexact age of this volcano isunknown and, despitebeing in La Selva wherethe volcanic rocks are overtwo million years old, it isobvious from its excellentstate of conservation thatthis volcano was construc-ted no more than a fewhundred thousand yearsago. Due to its morphologyand size, La Crosa is re-garded as one of the mostspectacular volcanoes inCatalonia.

The low height of its edifi-ces and the fact that it liesin a relatively flat areamake it difficult to see theshape of this volcano.

La Crosa de Sant Dalmai

Figure 114. La Crosa de Sant Dalmai

1


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The morphologies descri-bed here belong to the vol-canic edifices that consti-tute La Crosa de SantDalmai. Analysis of the vol-canic deposits and sedi-ments reveals a series ofstages that gave rise to thecurrent relief features.The most important erup-tion of this volcano was theinitial phreatomagmaticphase, which built a maar-type edifice with a largecrater that initially wasmuch smaller (Fig. 115a).As the explosions causedby the interaction of waterand magma began to takeplace at greater depths(Fig. 115b), the diameter ofthe crater increased. Thesliding of pyroclastic mate-rials from the inside of thewalls into the centre of thecrater eventually made itfar bigger.When the phreatomagma-tic eruption ended aStrombolian phase beganthat built a cinder cone on

top of the northern edge ofthe maar (Fig. 115c). Thecrater of this edifice isopen to the south-east,possibly due to the emis-sion of a small lava flow inthe final stages of theeruption (Fig. 115d). When the volcanic activityhad ceased, the crater fi-lled with water, forming alake that slowly began tosilt up with lacustrine andcolluvial sediments (Fig.115d). Currently, an artifi-cial drainage system withtwo channels crossing thecone keep the crater dry.

Figure 115. Eruption sequence ofLa Crosa de Sant Dalmai

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Interpretation

Description

The view east from the top of the hil lreveals a circular depression, some1,250 m in diameter. The bottom of thisdepression (about 800 m wide) is flatand lies below the original ground level.It is currently used for crops and treeplantations. A l ine of hi l ls covered in

pine and holm oak woodland surroundsthe depression.On its northern side, a low hill penetra-tes sl ightly inside the depression (1)and abuts against the rim of the crater,which an aerial photo (Fig. 114) revealsas being horseshoe-shaped.

The morphology of the volcanic edifices


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La Crosa de Sant Dalmai lies betweenthe villages of Aiguaviva, Estanyol andSant Dalmai on the border between theregions of La Selva and El Gironès.From Girona, take the road to SantaColoma (GI-533) through Aiguaviva. Atthe 10-km point on this road, just befo-re the vi l lage of Sant Dalmai, park

where a track heads left (Fig. 110).From here walk about 200 m north-wards along a path through a field ofhazel trees to the abandoned CanCosta quarry. The volcanic depositsare not hard to find as they are about400-m long and 20-m high.

Pyroclastic surge and breccia of La Crosa de Sant Dalmai

15

Point of interest l Volcanic materials in the quarry at Can Costa Activity type l PhreatomagmaticAccess time on foot l 5 minutes

Location and access

La Crosa's cone is madeup of a sequence of py-roclastic deposits withlax dipping that spreadsout radial ly around thecrater. The cone is 203-m high to the south-west(Turó de Sant Llop) andthe layer of fragmentarymaterials is over 50-mthick. In the west,though, the cone is lessthan 200-m high and thedeposits are 30-m thick.The phreatomagmatic ex-plosions ejected a mixtu-re of magma fragmentsand country rock, whichwere distr ibuted asym-metr ical ly. Towards theeast, the dispersion rea-ched beyond what todayis the vi l lage ofVi lablareix, over 3.5 kmdistant, while to the westthe materials were ejec-ted only a few hundredmetres. This asymmetryin the emplacement off low materials is due to

the varying competenceof the subsoil (resistanceof the materials to the ex-plosions) and, for instan-ce, the Pl iocene sedi-ments found in the eas-tern sector are less com-petent than the meta-morphic rocks and grani-

te. However, it is possi-ble that the eastwardbias of the vent also ledto more volcanic pro-ducts being ejected inthat direction.

La Crosa phreatomagmatic deposits

Figure 116. Schematic geological map of Puig d'Adri


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Interpretation

During the phreatomagmatic phase of LaCrosa de Sant Dalmai eruption a seriesof pulses occurred, each of which for-med one or two layers of the sequence.The sequence of alternating breccia andash was determined by the availability ofwater in the area of interaction with themagma. Three stages in the phreato-magmatic pulses can be separated:

1. The aquifer was able to provideenough water for optimal water-magmainteraction. In this first stage, a conside-rable amount of water vaporised, gene-rating a pyroclastic surge that resultedin the ash deposit (Fig. 117; e.g. layer 2).

2. Less water was available in this stageand so the water-magma ratio waslower; as a result, the explosion wasless effective and generated a pyroclas-tic breccia deposit (Fig. 117; e.g. layer 3).At the end of this stage, there was al-most no water left in the country rock.

3. In the third stage, the aquifer was re-plenished and re-fed the area of interac-tion with the magma unti l there wasenough water to produce a new pyro-clastic surge.

These three stages were repeated suc-cessively over short intervals of time andled to the build-up of the sequence ofpyroclastic deposits exposed in thisoutcrop. The magma ascent was pro-bably continuous throughout the diffe-rent phases and therefore the rechar-ging of the aquifer that interacted withthe magma was rapid enough to main-tain the phreatomagmatic eruption. It islikely that the scoria deposit (Fig. 117,layer 23) was the product of aStrombolian phase occurring when thereplenishing of the aquifer in stage 3was insufficient to maintain the phreato-magmatic activity.

Description

Up to 30 alternating layers of breccia andash have been exposed in the quarry (Fig.117)) and vary in thickness from just a fewcentimetres to over a metre.At the base there is a layer over a metrethick, consisting mostly of large blocks oflithic fragments measuring 10 cm across(layer 1). On top lies a further series of la-yers with lithic and juvenile fragments me-asuring various centimetres across, andthen ash (layers 2 to 22). Next, comes ametre-thick layer of scoria with lapilli-sized fragments (layer 23). Finally, morealternating layers of breccia and ash ap-pear that are similar to the previous ones(levels 24 to 30), with at the base a brec-cia layer of 10-cm fragments.Thanks to the size of the fragments in thebreccia, the difference between the juve-nile clasts, which are black basalt, andthe lithics originating from different meta-

morphic and igneousrocks, are clearly visible.The most plentiful lithicsare granite, schist andporphyric deposits. Thejuvenile fragments showvery little vesiculation,except for those in thescoria (layer 23), whichare clearly more vesicu-lar. The lithic fragmentsare angular and in someparts account for 60% ofthe deposit.

Quarrying at Can Costa

Figure 117. Stratigraphiccolumn from the Can

Costa quarry


l1 l VolcanoesAAqquuiiffeerr

A water-bearing permeable geolo-gical formation in which groundwa-ter is stored and through whichwater can flow.

CCoouunnttrryy rroocckkThe rock surrounding the intrusionof another rock in the form of aseam, dyke, sill or pluton.

CCrryyssttaallA solid substance of defined che-mical composition, made up ofatoms or molecules arranged in aregular and periodic pattern in aspace that, in favourable condi-tions, may give flat surfaces knownas faces.

DDoommeeA mound-shaped protrusion withsteep flanks produced by an erup-tion of highly viscous, gas-poormagma, gradually expelled fromthe vent.

DDyykkeeA type of sheet intrusion of igneousrocks that cut discordantly acrossadjacent rock following existingfractures, which are generally verti-cal and measure tens to hundredsof metres in thickness.

GGeeoocchheemmiissttrryyThe science that studies the abun-dance and distribution of the solidmatter of the Earth or a celestialbody, and its composition

IIggnneeoouuss rroocckkRock formed by the solidification ofmagma in or outside the lithosphe-re.

IIssoottooppeeOne of two or more species ofatoms of a chemical element thathave the same atomic number (thesame number of protons), but adifferent number of neutrons.

JJooiinnttA fracture in a rock with no relativedisplacement of any part, the sur-face of which is usually flat and dif-fers greatly from the stratification.

LLiitthhoossttaattiicc pprreessssuurreeVertical stress imposed on a layerof soil or rock by the weight ofoverlying material.

MMeettaammoorrpphhiicc rroocckkssRock formed from pre-existing rockthat, with no intermediate liquidstage, has been transformed mine-ralogically and structurally in res-ponse to changes in physiochemi-cal conditions, temperature, pres-sure or shearing stresses.

PPeettrrooggeenniicc pprroocceessss Any of the processes that arise du-ring the formation of a rock.

PPeettrroollooggyyA branch of geology that deals withthe origin, history, occurrence,structure, chemical compositionand classification of rocks.

PPlluuttoonnA large body of intrusive igneousrock formed from magma coolingunder the Earth's surface.

SSeeddiimmeennttaarryy rroocckkA type of rock formed by the accu-mulation of material (e.g. mineralsor organic rock) on the Earth's sur-face within bodies of water.

SSiilliiccaattee mmiinneerraallA mineral formed of SiO4 tetrahe-dra.

SSiillllA body of igneous rock that intru-des between older rock layers.

TTeexxttuurree ooff rroocckkAlso known as microstructure, rocktexture refers to the relationshipbetween the constituent mineralsand vitreous material of an endo-genous or sedimentary rock.

l2 l Volcanism inCataloniaAAllkkaalliinnee rroocckk

Magmatic rock in which the so-dium oxide (Na2O) and potassiumoxide (K2O) combined are presentin a greater percentage than thealuminium oxide (Al2O3).

CCaallcc--aallkkaalliinnee mmaaggmmaaMagma with a SiO2 content betwe-en 55% and 61% and more so-dium and potassium oxide thancalcium oxide.

FFeellddssppaatthhooiiddFeldspathoids are a group of tec-tosilicate minerals made up ofSiO2, Na, K, Ca and Li that appearin place of feldspar when themagma is poor in SiO2.

NNeeooggeennee--QQuuaatteerrnnaarryyThe time period between 23 Maand the present.

RRaarree eeaarrtthh eelleemmeennttRare earth elements or metals area group of chemical elements thatinclude the lanthanides plus scan-dium and yttrium.

l3 l La Garrotxa Vo l c a n i cZone. Sites of volcanic interestEEoocceennee

The second epoch of the LowerTertiary, lasting from 56.5 to 35.4Ma.

FFoorrmmaattiioonnA unit of lithostratigraphy establis-hed in accordance with lithologicalcharacter.

BBaannyyoolleess FFoorrmmaattiioonnAn Eocene unit made up ofbluish marls.

BBeellllmmuunntt FFoorrmmaattiioonnAn Eocene unit mostly made upof clay, silt, marl, sandstone andred conglomerate.

BBrraaccoonnss FFoorrmmaattiioonnAn Eocene unit made up marl,sandstone and conglomeraterocks.

FFoollgguueerroolleess FFoorrmmaattiioonnAn Eocene unit made up ofsandstone.

97

Glossary

98

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FFeerrrrééss,, DD..;; PPllaannaagguummàà,, LLll..;;PPuujjaaddaass,, AA.. [[eett aall..]],, “Els nousvolcans del Parc Natural de la ZonaVolcànica de la Garrotxa”, Revista deGirona, Girona Provincial Council,vol. 188 (1998), pp. 32-41.

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MMaarrttíí,, JJ..;; AArraaññaa,, VV.., La volcanologíaactual. Madrid: Spanish NationalResearch Council (CSIC), 1993.578 pp. (Nuevas Tendencias ; 21).

MMaarrttíí,, JJ.., “El vulcanismeneogenoquaternari dels PaïsosCatalans”, in Història natural delsPaïsos Catalans: Geologia.Barcelona: Catalan EncyclopaediaFoundation, 1992,vol. II, pp. 360-371.

MMaarrttíí,, JJ.. [[eett aall..]], “Projecte degeologia de la zona volcànicacatalana: Informe final 1996”,Barcelona, Jaume Almera Instituteof Earth Sciences of the SpanishNational Research Council (CSIC),1996. Note: unpublished.

MMaarrttíí,, JJ..;; MMaallllaarraacchh,, JJ..MM..,“Erupciones hidromagmáticas en elvolcanismo cuaternario de Olot(Girona)”, Estudios Geológicos,[s.n.], vol. 43 (1987), pp. 31-40.

MMaarrttíí,, JJ.. [[eett aall..]], “Cenozoicmagmatism of the Valencia trough(western Mediterranean):relationship between structuralevolution and volcanism”,Tectonophysics, [Elsevier SciencePublishers], vol. 203 (1992), pp.145-165.

MMaarrttíí,, JJ.. [[eett aall..]], “Mecanismoseruptivos del volcán de la Closa deSant Dalmai (Girona)”, Anales defísica, Series B (special edition); pp.143-153.

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PPaallllíí,, LLll..;; RRooqquuéé,, CC.., El vulcanismede les comarques gironines (II-Gironès). [Map]. Girona: GironaProvincial Council; University ofGirona, 1995.

PPaallllíí,, LLll..;; RRooqquuéé,, CC.., El vulcanismede les comarques gironines (III-Alt iBaix Empordà). [Map]. Girona:Girona Provincial Council;Universidad de Girona, 1996.

PPaallllíí,, LLll..;; RRooqquuéé,, CC..,“Els afloramentsvolcànics a les comarquesgironines”, Revista de Girona,Girona Provincial Council, vol. 174(1996b), pp. 65-68.

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MMaarrttíí,, JJ.. [[eett aall..]],, “Complexinteraction between Strombolian anphreatomagmatic eruptions in theQuaternary monogenetic volcanismof the Catalan Volcanic Zone (NE ofSpain)”, Journal of Volcanology andGeothermal Research, ElsevierScientific Publishing Company, vol.201, issues 1-4, (Abril 2011), pp.178-193

Museu dels Volcans. [Guide]. Olot:Comarcal Museum of la Garrotxa;Caixa de Girona,1993. [24] pp.

NNeeoovvííddeeoo.. Els volcans de laGarrotxa [video recording]. Olot, TheVolcanic Region of La GarrotxaNatural Park (PNZVG), 1996. 1videotape (14 min.), colour (VHS),sound BS.

OOlliivveerr MMaarrttíínneezz--FFoorrnnééss,, XXaavviieerr.. ElParc Natural de la Zona Volcànicade la Garrotxa. Olot: Llibres deBatet, DL 2002. 72 pp. (Guies delsLlibres de Batet ; 11)

PPrraattss,, JJoosseepp MM.. El Parc natural dela zona volcànica de la Garrotxa[map-guide]. Barcelona: Generalitatof Catalonia., The Volcanic Regionof La Garrotxa Natural Park 1994.24 pp + 1 map-guide.

PPllaannaagguummàà GGuuààrrddiiaa,, LLlloorreennçç..Coneixem el que trepitgem?: elpatrimoni geològic de laGarrotxa.Olot: Museum ofVolcanoes: Culture Institute of Olotcity, DL. 2005. 36pp. + 1 opticaldisc (CD-ROM)

TTOOSSCCAA,, EEqquuiipp dd''EEdduuccaacciióóAAmmbbiieennttaall,, “Estratègia per a lagestió del vulcanisme al ParcNatural de la Zona Volcànica de laGarrotxa”, The Volcanic Region ofLa Garrotxa Natural Park (PNZVG),2000. 47pp.

Catalonia

AArrbbaatt,, SSííllvviiaa;; RRiiggaauu,, EEvvaa;; SSoolléé,,LLlluuííss,, Carícia de volcà [Girona]:Bescanó Council, Vilobí Council,1991. 94 pp.

“Dossier: El vulcanisme gironí”,Revista de Girona. Girona: GironaProvincial Council,1996 XLII Year,no. 174 (genuary-february 1996),PP. 58-93

MMaallllaarraacchh ii CCaarrrreerraa,, JJoosseepp MMaarriiaa..El Vulcanisme prehistòric deCatalunya. Olot: Alzamora, 1998.322pàg.

PPaallllíí ii BBuuxxóó,, LLlluuííss;; RRooqquuéé ii PPaauu,,CCaarrlleess.. El vulcanisme de lescomarques gironines [cartographicdocument]. [Girona]: ProvincialCouncil; Girona University. Area ofGeodynamics, DL 2007. 4 maps:col.; 57 x 70cm.

PPaallllíí ii BBuuxxóó,, LLlluuííss;; RRooqquuéé ii PPaauu,,CCaarrlleess.. El Patrimoni geològic de lesterres gironines: 300 elementssingulars. Girona: Universitat deGirona. Àrea de GeodinàmicaExterna, 2009. 425pàg.

PPuujjaaddaass,, AAllbbeerrtt [[eett aall..]].. Elvulcanisme de la Vall deLlémena.Girona: Universitat deGirona. Àrea de Geodinàmica,1997. 54pàg. (Dialogant amb lespedres ; 5)

PPuujjaaddaass,, AAllbbeerrtt [[eett aall..]].. Elvulcanisme de La Selva. Girona:Girona University. Area ofGeodynamics, 2000. 50pp(Dialogant amb les pedres ; 8)

Basic recommendedreading

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Books and unpublisheddocuments

TThhee VVoollccaanniicc RReeggiioonn ooff LLaaGGaarrrroottxxaa NNaattuurraall PPaarrkk,, Pla especialde La Zona Volcànica de LaGarrotxa: Aprovació definitiva, AcordGOV/161/2010, de 14 desetembre, pel qual s’aprovadefinitivament El Pla Especial de LaZona Volcànica de La Garrotxa,Barcelona: Generalitat of Catalonia,[2010]. 5 vol. And optical disc

PPrraattss SSaannttaafflloorreennttiinnaa,, JJoosseepp MM..;;PPllaannaagguummàà,, LLlloorreennçç;; OOlliivveerr,, XXaavviieerr..Entre volcans. [Olot]: Generalitat ofCatalonia. The Volcanic Region of LaGarrotxa Natural Park, 2007. 141pp.

RRCCRR AArraannddaa,, PPiiggeemm,, VViillaallttaaAA rr qq uu ii tt ee cc tt ee ss .. Les cases que nocriden = Las casas silenciosas =Tranquil houses: La casa de pagès alParc Natural de la Zona Volcànica dela Garrotxa. Olot: Generalitat ofCatalonia. The Volcanic Region of LaGarrotxa Natural Park, 2011. 118pp.

La recerca científica al Parc Naturalde la Zona Volcànica de la Garrotxa:1 9 8 2 - 1 9 9 2. Olot: Generalitat ofCatalonia. The Volcanic Region of LaGarrotxa Natural Park, 1993. 146pp.

Un Parc de contes: rondallesescrites pels estudiants de primàriade la Garrotxa per ser llegides iescoltades vora dels volcans. Olot:Generalitat de Catalunya. TheVolcanic Region of La GarrotxaNatural Park, 2009. 111pp.

Leaflets

El Centre de Conservació dePlantes Cultivades de Can Jordà .Barcelona: Generalitat of Catalonia.The Volcanic Region of La GarrotxaNatural Park, [2006].

Centre de Documentació = Centrode Documentación =Documentation Centre. Olot:Generalitat of Catalonia. TheVolcanic Region of La GarrotxaNatural Park, 2011.

12 indrets d'interès per visitar alparc: Parc Natural de la ZonaVolcànica de la Garrotxa. [Olot]:Generalitat of Catalonia. TheVolcanic Region of La GarrotxaNatural Park, [2007]. 1 leaflet, map;42x42 cm.

Itineraris pedestres: fageda d’enJordà; volcà de Santa Margarida;volcà del Croscat. Olot: Generalitat ofCatalonia. The Volcanic Region of LaGarrotxa Natural Park, 1996.No. 1.

Itineraris pedestres: Sender JoanMaragall (la Fageda d’en Jordà).Olot: Generalitat of Catalonia. TheVolcanic Region of La GarrotxaNatural Park, 1995. No. 2.

Itineraris pedestres: Olot; fagedad’en Jordà; Can Xel. Olot:Generalitat of Catonia. The VolcanicRegion of La Garrotxa Natural Park,1995. No. 3.

Itineraris pedestres: Santa Pau;volcà de Santa Margarida; Can Xel .Olot: Generalitat of Catalonia. TheVolcanic Region of La GarrotxaNatural Park, 1995. No. 4.

Itineraris pedestres: cingleres deCastellfollit. Olot: Generalitat ofCatalonia. The Volcanic Region ofLa Garrotxa Natural Park; CastellfollitCouncil, 1996. No. 13.

Itineraris pedestres: grederes delvolcà del Croscat. Olot: Generalitat ofCatalonia. The Volcanic Region of LaGarrotxa Natural Park, 1995. No. 15.

Itineraris pedestres: ruta de les TresColades. El Boscarró, el Molí Fondoi Fontfreda. Olot: Generalitat ofCatalonia; Sant. Joan les FontsCouncil, 1997. No. 16.

Itineraris pedestres: volcà delMontsacopa. Olot: Generalitat ofCatonia.; IMPC, 1997. No. 17.

Itineraris pedestres: Sant Feliu dePallerols, itinerari urbà. Sant Feliu dePallerols,1999. No.18.

Itineraris pedestres: valls de SantIscle i del Vallac: volcans i castells.Sant Feliu de Pallerols,1998. No.19.

Oferta pedagògica del Parc Naturalde la Zona Volcànica de laGarrotxa: curs 2011-2012. Olot,The Volcanic Region of La GarrotxaNatural Park (PNZVG), 2011. Maps

Maps

CCaarrttooggrraapphhiicc IInnssttiittuuttee ooff CCaattaalloonniiaa;;TThhee VVoollccaanniicc RReeggiioonn ooff LLaaGGaarrrroottxxaa NNaattuurraall PPaarrkk;; Cartavulcanològica de La zona volcànciade La Garrotxa. Barcelona.Cartographic Institute of Catalonia;Gological Institut of Catalonia, 2007

Municipi de Sant Feliu de Pallerols:map-guide [Olot] Generalitat ofCatalonia. The Volcanic Region of LaGarrotxa Natural Park, [1998-2001]

Booklets

L’Agricultura i la ramaderia al ParcNatural de la Zona Volcànica de laGarrotxa. Olot: generalitat ofCatalonia. The Volcanic Region of LaGarrotxa Natural Park, 2011. 37pp

Parc Natural de la Zona Volcànicade la Garrotxa = Parque Natural dela Zona Volcánica de la Garrotxa =Parc naturel de la Zone volcaniquede La Garrotxa = The VolcanicRegion of La Garrotxa NaturalPark.2nd ed. Barcelona: Generalitatof Catalonia. Natural Parks Service,2008. [10]pp.

Postcards and bookmarks

[Postcards]: Materials volcànics,Volcà del Croscat, Fageda d'enJordà, Volcà Montsacopa,Castellfollit de la Roca. [Olot]:Generalitat of Catalonia. TheVolcanic Region of La GarrotxaNatural Park, 2002.

[Bookmarks] Centre deDocumentació = Centro deDocumentación = DocumentationCentre. Olot: Generalitat of Catalonia.The Volcanic Region of La GarrotxaNatural Park, 2009 and 2011

Volcans de la Garrotxa [Gràfic].[Olot]: Generalitat of Catalonia. TheVolcanic Region of La GarrotxaNatural Park, [2011]. [36]bookmarks; maps; 6 x 21 cm

Posters

El vulcanisme estrombolià de laGarrotxa. Olot: The Volcanic Regionof La Garrotxa Natural Park(PNZVG), 1991.

L’arquitectura del volcànic. Olot:The Volcanic Region of La GarrotxaNatural Park (PNZVG), 1995.

El Parc Natural de la Zona Volcànicade la Garrotxa (panoramic). Olot:The Volcanic Region of La GarrotxaNatural Park (PNZVG), 1997.

Video/DVD

GGeeooVViirrttuuaall,, SSLL,, Volcans en 3D: volvirtual pel Parc natural de la ZonaVolcànica de la Garrotxa. [S.l.]:Generalitat of Catalonia. TheVolcanic Region of La GarrotxaNatural Park, [2008].

La Garrotxa Volcanic Zone Natural Park publications

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Map of the Services in La Garrotxa Volcanic Zone Natural Park

Natural Park Natural Reserve Built-up area

Documentation Centre

Natural Park Information Centre

Car- park

Signposted walking itinerary

Museum

Toilets

Picnic area

Viewpoint

Environmental Education Organisation

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Services accredited by La Garrotxa VolcanicZone Natural Park

The entities that collaborate with La Garrotxa Volcanic Zone Natural Park are largely smallbusinesses or groups of businesses that mainly work in the sector of environmental and cul-tural education in the Park. They are characterized by their commitment to providing qualityservices and their active collaboration with the Park in ensuring the successful protectionand improvement of its natural values and the sustainable exploitation of its resources

This quality service consists of:

• Discovery activities, knowledge of the environment and research in and around the Park’scentres of interest.

• Interdisciplinary work in various fields of study (essentially the environment and social rela-tionships).

• Activities with a maximum of 20 participants per guide/teacher• Guide/teachers with excellent knowledge of the local region and all accredited as guides

by La Garrotxa Volcanic Zone Natural Park. • Continual in-service training• Main area of operation in the Natural Park• All services covered by third-party insurance• Teaching material provided to be used by teachers before and after visits to Park• Active participation in scientific research and the protection of the natural and cultural va-

lues of the region

Secretary: BBeetthh CCoobboo

c/ Antoni Llopis, 6 1r 5a17800 Olot

Tel. (+34) 972 90 38 22(+34) 657 861 805

Fax (+34) 972 27 32 28e-mail: i n f o @ v e r d v o l c a n i c . c a tWeb page: w w w. v e r d v o l c a n i c . c a t

DDeessccrriippttiioonnCreated in 2003, Verd Volcànic aims to improve thequality of the services offered by the companies in theassociation: to stabilise the educational team, provideteam members with training in the various knowledgeareas and work to improve continually the serv i c e sprovided. It also undertakes to develop its activity in away which is consistent with the region's conserv a-tion and to ensure the protection of its natural andcultural values.

SSeerrvviicceess pprroovviiddeedd• Guided visits in Catalan, Spanish, English and

French.• Diagnostic surveys of local natural and cultural

heritage• Creation of tourist packages• Studies of flora and fauna• Design of footpath networks• Environmental and cultural technical assessment• International cooperation projects• Activities for school groups from half day to five-

day stays• Study programmes for schoolchildren from

countries such as Great Britain and Eire.

VERD VOLCÀNICLa Garrotxa Association for Environmental and Cultural Education

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Secretary: OOccttaavvii BBoonneett

Mas TarutAv. de Santa Coloma, s/n17800 Olot

Tel. (+34) 972 27 00 86Fax (+34) 972 27 04 55e-mail: [email protected] page: www.tosca.cat

Opening hours:9-14 h i 16-18 h

DDeessccrriippttiioonnTOSCA is a services company working to improvethe region through: education, communication, in-terpretation and socio-environmental information,the drawing up of technical studies and developingenvironmental action in La Garrotxa. TOSCA con-sists of a team of 10 professionals with back-grounds in Geology, Biology, Education and To u r i s mwith extensive experience in environmental matters.

SSeerrvviicceess• Management of the Educational and Information

Services of The Volcanic Region of La GarrotxaNatural Park.

• Development of educational activities and guidedwalks adapted to educational level. Preparationof teaching materials.

• Training programmes for teachers, environmentaleducators, students, and so on.

• Drawing up of studies on environmental educa-tion in protected natural areas.

• Design of geotourism trails.• Recovery of volcanic heritage.• Participation in projects on sustainability, agrobio-

diversity, sustainable tourism, studies of vulnera-ble areas, recovery of natural areas, and so on.

• Contributions to publications on environmentaleducation and sustainability.

T O S C AE n v i ronmental services in education and tourism

Secretary: EEsstteerr MMoorrcchhóónn

Avda. RepúblicaDominicana 3, bajos17800 Olot

Tel. (+34) 972 27 32 23Fax (+34) 972 26 22 33e-mail:[email protected]

DDeessccrriippttiioonnAn environmental education cooperative created in2006 formed by a team of graduates inEnvironmental Sciences, Psychology andGeography with extensive experience in the field ofenvironmental education.

SSeerrvviicceess• Environmental activities in La Garrotxa Vo l c a n i c

Zone Natural Park for schoolchildren of all ages • Environmental activities for adults including gui-

ded walks to some of the Natural Park’s bestknown sites (volcanoes of Croscat and SantaMargarida, Fageda d’en Jordà beechwood), butalso to some of its least known treasures (SantJoan les Fonts lava flows, Colltort Castle, chur-ches of La Serra del Corb)

• Languages: all the above activities can be carriedout in Catalan, Spanish, English, German orFrench

• Technical work including studies in the fields oftourism and education, coordination of trainingand education programmes.

LA CUPP, SCCL

Notes

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106

The Park has an extensive network ofsignposted itineraries that reach someof the most interesting sites in the re-gion.

The Natural Park consists mostly ofprivate property. Please ensure thatyou do not disturb residents.

Camping is forbidden within the Park.Nevertheless, there are numerouscamp sites, hotels and hostels in thePark where visitors can stay.

For reasons of safety and conserva-tion, the lighting of fires is strictly prohi-bited.

The maintenance service has to workhard to keep the most frequentedareas clean. Please use the bins ortake your rubbish home with you.

The capture and collection of animals,rock and mineral specimens andplants is forbidden in the Natural Park.

In a number of clearly signpostedareas access is limited to park servi-ces and residents. Vehicle accesshere is forbidden..

The Park information centres give spe-cial permits for those with reducedmobility in order to visit restricted areasby car.

Recommendations and indications for visitorsto the La Garrotxa Volcanic Zone Natural Park

Natural Park Services

Information Centres

Casal dels VolcansAv. de Santa Coloma, s/n17800 OlotTel. (+34) 972 26 81 12Fax (+34) 972 27 04 [email protected] SerraFageda d’en JordàCan PassaventCroscat Volcano

Documentation Centre

Opening hours: weekdays9.00 am to 2.00 pm.Visits by appointment only. Tel. (+34) 972 26 46 66Fax (+34) 972 26 55 [email protected]

Educational Services

Casal dels volcansAv. de Santa Coloma, s/n17800 OlotInformation and bookings:weekdays 9.00 am to 2:00 pm and 4.00 to 6.00 pmTel. (+34) 972 27 00 86 (+34) 972 26 81 [email protected]

Web PagesGeneral information: www.gencat.cat/parcs/garrotxa

Documentation Centre catalogue query: http://beg.gencat.net/

ISBN 978-84-393-8852-4

9 788439 388524

Generalitat de CatalunyaDepartament d’Agricultura, Ramaderia,Pesca, Alimentació i Medi Natural

La Garrotxa Volcanism Guide 2012

Documents

Transcript of La Garrotxa Volcanism Guide 2012