La Garrotxa Volcanism Guide 2012

Volcanoes

A Field Guide to La Garrotxa Volcanic Zone

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

0. Primeres pàgines-en

1/6/12

13:49

Página 1

Volcanoes A Field Guide to La Garrotxa Volcanic Zone

0. Primeres pàgines-en

1/6/12

13:49

Página 2

Biblioteca de Catalunya - Dades CIP Volcanoes: A Field Guide to La Garrotxa Volcanic Zone Bibliography ISBN 9788439388524 I. Martí i Molist, Joan, 1957- II. Catalunya. Departament d'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 Zona Volcànica de La Garrotxa (Catalunya) – Guies 551.21(467.1 Gt)(036)

Published by La Garrotxa Volcanic Zone Natural Park

Illustrations Albert Martínez

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

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

Original title: El vulcanisme Guia de camp de la Zona Volcànica de la Garrotxa (2000,2001) Title: Volcanoes A Field Guide to La Garrotxa Volcanic Zone © La Garrotxa Volcanic Zone Natural Park and authors © Traduccions i Tractament de la Documentació, SL and Mike Lockwood Digital Version Natural Parc web page Printed by Ampans, Manresa 1st edition Olot, April 2012 Photographs Pep Callís

Cover, figures 29, 34-37, 58-63, 66, 69, 73-76, 85, 87, 95, 97, 98, 100, 102, 105, 106 and 114 (deposited in La Garrotxa Volcanic Zone Natural Park Documentation Centre)

Albert Pujadas

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

Joan Martí

Figures 15, 27 and 31

Emili Bassols Figure 32

La Garrotxa Volcanic Zone Natural Park Documentation Centre Figures 65, 67, 70 and 83

Maurice Krafft Figure 18

National Geographic Data Centre Figure 43

Llorenç Planagumà Figures 71, 77 and 102

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

Albert Pujadas

Llorenç Planagumà

Figures 81, 82, 84, 86, 94, 96, 99, 101, 103 and 104 (Figures 82, 84, 86, 94, 96, 99, 101 and 104 have been modified according to the Vulcà Project geological base)

Montse Viñas

Original drawings for figures 88-93

Bibliographical references standardised and adapted by Montse Grabolosa With the support of the environmental education organisations La Cupp SCCL, Verd Volcànic and Tosca

0. Primeres pàgines-en

1/6/12

13:49

Página 3

Volcanoes A Field Guide to La Garrotxa Volcanic Zone

Joan Martí i Molist

Jaume Almera Institute of Earth Sciences (CSIC), Barcelona

Albert Pujadas

Geodynamics Area Department of Environmental Sciences.

University of Girona

Dolors Ferrés Lopez Llorenç Planagumà Guàrdia

Tosca. Collaborators with La Garrotxa Volcanic Zone Natural Park

Josep Maria Mallarach Carrera Olot Foundation for Higher Education

0. Primeres pàgines-en

1/6/12

13:49

Página 4

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

The extensive quarrying undertaken in part of the volcanic area from the 1960s to the 1980s provoked considerable social and scientific opposition, which eventually led to the passing of a law in 1982 declaring the volcanoes a protected area.

The conservation of this natural heritage is justified by the fact that this is the youngest volcanic area in the Iberian Peninsula and one of the best preserved such areas in continental Europe. The geomorphological features found here include volcanic cones, lava flows, barrage lakes and basalt cliffs, and there are numerous sites where the geological processes that have generated so many different volcanic morphologies can be easily observed in great detail.

Despite its legal protection, as part of the tasks implicit in the organization and consolidation of the Natural Park it was still necessary to halt the quarrying and to minimize and restore the region’s damaged geological heritage. A milestone was reached in 1995 with the restoration of the most emblematic volcano in the park, Croscat, not only the youngest volcano in the Iberian Peninsula, but also the one that has suffered most environmental impact.

Nevertheless, more in-depth knowledge was required in the Park itself of the local volcanoes in order to build upon the studies undertaken early in the twentieth century and then reactivated in the 1960s. Initially, it was necessary to review all previous work and develop a project for a comprehensive study of the geology of the Catalan volcanic region. The aim of this project, first contemplated in the early 1990s, was to study various geological and geophysical aspects of the Park as a means to learning more about the region’s geological heritage in general.

Eventually, in 1993 a project began that, despite its narrower scope, was still very ambitious. It was financed entirely by the Department of the Environment through La Garrotxa Volcanic Zone Natural Park and executed by the Spanish National Research Council (CSIC) under the supervision of Dr Joan Martí, and would enable new geolo-

4

0. Primeres pĂ gines-en

1/6/12

13:49

PĂĄgina 5

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

The results of this project are included in this guide, which in plain and simple terms provides new and valuable information for the study of the volcanoes of La Garrotxa. The publication of this guide is part of the Natural Park's management strategy, approved in 2000, which will enable us to improve our knowledge of volcanic activity in the region, plan research, preserve the Park’s geological and scenic values and increase awareness of the volcanic zone at local, national and international scales.

I hope that this guide, which has been painstakingly prepared following strict criteria, helps to increase awareness of the value of this volcanic zone amongst teachers, university students and naturalists alike, thereby guaranteeing the knowledge, management and dissemination of a heritage that has been preserved for future generations.

Francesc Xavier Puig i Oliveras Director of La Garrotxa Volcanic Zone Natural Park

5

0. Primeres pàgines-en

1/6/12

13:49

Página 6

Contents

Introduction

8

l1l Volcanoes

11

l1l1l What is a volcano?

12

l1l2 l Magma genesis

14

l1l2l1l Where is magma generated? l1l3 l Magma ascent

15 17

l1l3 l1l How does magma ascend?

18

l1l3 l2l What happens to magma during its ascent?

19

l1l4 l Eruptive activity

22

l1l 4 l1l Why do eruptions occur?

23

l1l 4 l2l Types of eruptive activity

24

l1 l4 l2 l1 l Effusive activity

24

l1 l4 l2 l2 l Explosive activity

24

l1l 4 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

l1l 4 l 4 l Volcanic morphology l2 l Volcanism in Catalonia

39

41

l2 l1l 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 Magma genesis and ascent

51

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

6

58

0. Primeres pàgines-en

1/6/12

13:49

Página 7

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 from Puig Rodó

84

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 Glossary Bibliography

94 97 98

Map of La Garrotxa Volcanic Zone Natural Park Services

101

Environmental education organisations

102

Notes

104

Recommendations and indications for visitors

108

7

0. Primeres pĂ gines-en

1/6/12

13:49

PĂĄgina 8

Introduction This field guide presents a general but detailed view of the main features of La Garrotxa Volcanic Zone. It aims to be a useful tool for interpreting the landscape and geological processes in this volcanic zone and to provide the necessary tools for understanding from a geological perspective some of the most representative volcanic sites in the region.

How significant is the presence of volcanoes in a region such as this? In which geodynamic period should they be placed? What is the origin and composition of volcanic rocks? What types of eruptions occurred? These are just some of the questions this field guide answers.

Before offering an explanation of the volcanic history of La Garrotxa, this guide takes a look at general concepts of geology and volcanology that relate to the subject matter.

Therefore, we first examine magma, how it is generated and reaches the surface, how its composition varies over time, the mechanisms that give rise to volcanic eruptions, and the main features of eruptions and their resulting structures.

This book consists of three parts: 1. Volcanoes. An explanation of the general aspects and basic concepts of volcanism.

2. Volcanism in Catalonia. A brief description of the basic features of the most recent volcanic activity in the region.

3. Sites of volcanic interest. A description of 15 sites, the basis of a true field guide. The sites were selected according to the geological elements that can be observed and together exemplify the most remarkable features of the volcanoes found in Catalonia and, in particular, in La Garrotxa Volcanic Zone Natural Park. Accessibility was also a taken into account so that visits would be fairly simple. The selection of just 15 sites inevitably meant that others were omitted, many of which are also of great geological and educational interest, but far less accessible.

This guide can be used on many levels: the text is accompanied by text boxes with a maroon background containing explanations of concepts of interest such as magma and the Earth's internal structure. The definition of terms written in italics can be found in the glossary.

8

0. Primeres pĂ gines-en

1/6/12

13:49

PĂĄgina 9

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

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

9

0. Primeres pĂ gines-en

1/6/12

13:49

PĂĄgina 10

1. Els volcans.en BAIXA

1/6/12

13:55

Pรกgina 11

1 Volcanoes

1. Els volcans.en BAIXA

1/6/12

13:55

Página 12

11l lEls volcans Volcanoes

l1 l 1l What is a volcano?

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

• • • A volcano is a vent in the Earth's surface through which molten rock (magma) generated within the Earth and, occasionally, non-magmatic material issue. The accumulation of these products around a point source gives rise to shields or cones of differing morphologies. • • • This definition makes it clear that a volcano is not merely its final morphology, but rather is the culmination of a series of geological processes that involve the genesis, ascent and eruption of magma (Figs. 1 and 2).

Figure 1. Volcanic system

Although on both geological and human time scales volcanoes represent relatively short periods of time, from just a few days to thousands of years, they are actually the result of processes that last for hundreds of thousands or even millions of years.

Figure 2. Volcanic edifice

12

1. Els volcans.en BAIXA

1/6/12

13:55

Página 13

Magma

The vast majority of the rocks we know of are made of minerals belonging to the silica family, that is, minerals consisting of SiO4 anionic groups, isolated or bonded with others by metal cations (Fig. 3). For this reason, the magma resulting from the melting of these rocks is also mainly silicate.

Depending on the percentage of silica it contains, magma is classed as either basic (less than 52%), acid (over 63%) or intermediate (between 52% and 63%).

Figure 3. SiO2 molecule

Physical properties Density, viscosity and temperature are three of the most significant physical properties of magma that determine the nature of the processes of ascent and eruption. Density depends mostly on the chemical composition of the molten materials, whereas viscosity – the lava’s resistance to flow also depends on the composition of the magma and its temperature (Fig. 4).

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

Figure 4. Variation in the composition and physical properties of magma

Acid magma is more viscous than basic magma, due to the larger number of bonds between its silica molecules: the greater the temperature, the lower the viscosity, since heating favours molecular excitation and makes it harder for bonds to form.

Basic magma reaches higher temperatures, of up to 1,100°C, while acid magma melts at 700–800°C.

13

11l lEls volcans Volcanoes

• • • Magma is a mixture of molten, mainly silicate, rock that contains solid particles in suspension (crystals and rock fragments) and dissolved gases. • • •

1. Els volcans.en BAIXA

1/6/12

13:55

Página 14

11l lEls volcans Volcanoes

l1 l 2 l Magma genesis • • • Magma genesis is the process whereby the rocks in the Earth's mantle and crust change from a solid to a liquid state. • • •

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

Molten material forms for a number of reasons that may act individually or in combination: decompression, an increase in temperature or greater presence of water (Fig. 5).

Magma is generated if a solid rock body is subject to a considerable increase in temperature or if a rock, initially subject to very high temperature and pressure, undergoes a great fall in pressure.

However, in conditions of constant temperature and pressure, the assimilation of water by some of the minerals that make up the rock significantly lowers its melting point.

Figure 5. Causes of rock melting

Partial melting Melting affects only part of the rock. Rocks consist of various minerals, each with different melting points at a given pressure. Magma genesis begins when the minerals with the lowest melting points melt and then continues as the remaining

minerals in the rock also begin to melt. Thus, we almost always speak of the partial melting of rocks since at any one time only some minerals melt and only in certain proportions (Fig. 6).

Figure 6. Partial melting process

a. The melting process begins in contact zones betweens large minerals since it is here that the smallest amounts of energy are required to melt the rocks.

b. The liquids generated are less dense than the minerals that surround them. These liquids form a network of small interconnected channels and build up in certain areas until a minimum critical volume is reached; from this moment onwards, the liquids begin to ascend due to their buoyancy.

14

c. Melting continues and the volume of liquid increases and builds up near the roof of the melt zone. At the same time, the residual solids compact downwards, producing an increasingly effective separation between solids and liquids.

1. Els volcans.en BAIXA

1/6/12

13:55

Pรกgina 15

l1l 2 l1l Where is magma generated?

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

The internal structure of the Earth In terms of the composition and density of its materials, the Earth's interior is divided into three layers: core, mantle and crust (Fig. 8). As well, we can define two external layers in terms of the rigidity of the materials: a. the lithosphere, made up of the crust and the outermost part of the mantle, is fragile in behaviour. b. the asthenosphere, just below the lithosphere, represents the upper part of the mantle, which is plastic in behaviour and can flow when subject to great forces.

The theory of plate tectonics proposes a dynamic model of how the Earth works based on the fact that the lithosphere consists of a relatively small number of plates floating independently of each other on top of the asthenosphere.

Figure 8. Internal structure of the Earth

15

1 l Volcanoes

The processes relating to magma formation can be explained in the context of the theory of plate tectonics. Volcanic activity and in general magmatic activity is not randomly distributed over the Earth's surface, but is mostly concentrated along the edges of tectonic plates. However, we find volcanoes in places other than plate edges, both on land and at sea, which tells us that melting at a local scale also takes place (Figs. 7 and 9).

1. Els volcans.en BAIXA

1/6/12

13:55

PĂĄgina 16

1 l Volcanoes

Geodynamic environments of volcanism

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

Plate boundaries

Intraplate areas

Subduction zones

Oceanic ridges

Hotspots

Rift zones

When two plates collide, one slides under the other. When the cooler lithosphere sinks into the mantle, the latter’s temperature is lowered. Melting still occurs, however, when water enters the mineral system of the mantle.

This water, generated by the dehydration of subducted minerals, lowers the melting point of the minerals, thereby enabling part of the rocks to melt even though the ambient temperature has dropped considerably.

Two lithospheric plates move apart, which leads to a decompression of material in the mantle and the melting of huge volumes of solid rock that then rise continually towards the dorsal axis of the ridge.

Volcanic regions far from plate boundaries are generated by an anomalous increase in the temperature in the mantle caused by a convection rising in a single plume from the core-mantle boundary.

In inner plate areas, convection in the mantle leads to a thinning of the crust and generates distensive processes that can culminate in the complete rupture of the lithosphere and the formation of new ocean floor. In some areas, the split in the lithosphere is partial or does not occur at all; nevertheless, a system of normal faults does develop favouring the ascent of magma.

16

1. Els volcans.en BAIXA

1/6/12

13:55

Página 17

l1 l 3 l Magma ascent

• • • Magma ascent is the displacement of molten material from source areas to the surface and depends on the volume of liquid initially generated, its physical properties and the tectonic structure of the surrounding area. • • • In some cases magma rises to the Earth's surface directly, almost without stopping, giving rise to individual, short-lived eruptions. Frequently, however, magma accumulates in intermediate areas of the lithosphere in magma chambers (Fig. 10), where it may solidify completely or continue to rise to the surface.

Magma chambers These are reservoirs of molten rock that form within the lithosphere at depths of 1–60 kilometres, which are fed periodically by magma from melt zones. If they are connected to the Earth's surface, successive eruptions take place forming volcanoes or complex volcanoes with a long - but not necessarily continuous - periods of activity. This is the case of volcanoes such as Teide, Fuji, Etna and Vesuvius.

Magma ascent may halt within the Earth for reasons related to crust structure and the distribution of tectonic forces at each point. In areas of magma accumulation neutral density exists, that is, the density of the magma is equal to that of the surrounding rocks.

Figure 10. A magma chamber

17

1 l Volcanoes

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

1. Els volcans.en BAIXA

1/6/12

13:55

Pรกgina 18

l1l 3l1l

1 l Volcanoes

How does magma ascend?

Since liquids are less dense, differences in pressure between the magma and the surrounding rocks cause magma to rise. Magma ascent mechanisms are of two types: diapirs or dykes (tensile fractures) (Fig. 11).

Ascent through dykes

diapiric ascent

Figure 11. Ascent through dykes and diapiric ascent

Ascent through dykes occurs due to the pressure exerted by the magma as it rises towards the surface. The molten material causes fractures to widen, which then close up again once the magma has passed through.

Diapirs are bodies of buoyant magma that push through ductile rock in the lower crust or mantle that deform on contact with the magma at high temperature.

Magmas generated in the upper mantle initially rise as diapirs into shallow areas, where, due to the fragile behaviour of the rocks, they move through fractures. The mobility of these relatively fluid basic magmas means that they can move through even narrow fractures.

Magmas generated in the crust are more acidic in composition and consequently are more viscous. Given their mobility, they can only rise as large diapirs. The movement of these magmas through narrow fractures is very rare and only occurs under favourable structural conditions. Although they often reach the Earth's surface, masses of molten material build up in the crust forming bodies of rock known as plutons. Their subsequent solidification gives rise to plutonic igneous rocks.

18

1. Els volcans.en BAIXA

1/6/12

13:55

Pรกgina 19

l1l 3l 2l What happens to magma during its ascent?

1 l Volcanoes

Magma differentiates on its way to the surface, that is, its composition changes. Three principal mechanisms of magmatic differentiation occur during ascent: fractional crystallisation, magma mixing and assimilation of country rock. These processes take place simultaneously or individually and result in a broad range of chemical compositions in the resulting magmas.

Fractional crystallisation The pressure and temperature to which magma is subjected generally drop as it moves upwards. Under these new thermodynamic conditions, the various chemical elements in the magma regroup and form increasingly stable structures that give rise to the first solid nuclei.

These nuclei grow to form crystals separate from the liquid, which has a different composition from the primary magma.

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

Figure 12a. Fractional crystallisation

Magma mixing As it rises to the surface, magma may mix with other magmas of different composition and different physical properties. The end result will be magma with different characteristics from the initial magmas (Fig. 12b).

Assimilation

Figure 12b. Magma mixing

In some cases, at higher temperatures, magma may partially melt the surrounding rock and assimilate part of its minerals, thereby again altering the original composition of the magma (Fig. 12c).

Figure 12c. Assimilation Inserted rock

19

1. Els volcans.en BAIXA

1/6/12

13:55

PĂĄgina 20

1 l Volcanoes

What rocks can tell us Despite the relatively small number of melting mechanisms and places where melting takes place, the different types of rock that melt in the source area, the existence of degrees of partial melting

and the processes of magmatic differentiation all ensure that a wide range of different types of magmas are formed.

Consequently, the solidification of these magmas is the origin of the great diversity of volcanic and igneous rocks that are found on the Earth's surface (Fig. 13).

Knowledge of the petrogenetic processes that have occurred in the formation of a certain type of rock is the basis of the disciplines of petrology and geochemistry. Based on chemical, minerological and textural analyses, these two branches of geology study where and how primary magma was generated and its evolution until it evolved into a certain type of rock.

Figure 13. Classification of volcanic rocks

The chemical composition of igneous rocks The content and proportion of the different chemical elements in a rock provide information as to the origin and compositional evolution of the magma from which it was formed.

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

The relationship between the majority elements (those present in a proportion greater than 0.1%) and trace elements (content less than 0.1% and expressed in parts per million – ppm) reveals the changes in chemical composition occurring in the magma and the differentiation processes that took place during ascent.

Radiogenic isotopes and elements from the rare earth group that appear in very small quantities provide most information on the mechanisms of magma genesis, and also complement studies of magmatic differentiation.

20

1. Els volcans.en BAIXA

1/6/12

13:55

Pรกgina 21

Types of igneous rock and their texture

If magma reaches the surface and causes an eruption, it then begins to cool very quickly.

From this point onwards, the diffusion of elements in the magma may be completely inhibited and give rise to rocks such as obsidian and pumice with a vitreous texture but no crystals.

Generally, however, the typical texture of the resultant rocks is microcrystalline (consisting of very fine crystals). Some rocks are porphyritic in nature, a feature most characteristic of sub-volcanic rocks.

If the magma is located at more superficial levels but still within the Earth's crust, it forms intrusive bodies such as dykes and sills. The cooling process is remarkably rapid and prevents new crystalline nuclei from growing. However, crystals developing deep down in more favourable conditions will be more regular in shape and larger than the rest. The result is a texture known as porphyritic, whereby large, regular-shaped crystals (phenocrysts) are surrounded by a crystalline, generally much finer grained matrix.

When magma solidifies deep down, the slow drop in temperature favours the diffusion of chemical elements and therefore the addition of new material to the crystals that are being formed. This results in a crystalline rock with a granular texture containing large, similarsized crystals.

Figure 15. Emplacement of different types of igneous bodies

21

1 l Volcanoes

The speed at which magma cools, determined by the depth at which it solidifies, is reflected in the texture of the rock (Fig. 15) . Texture analysis thus reveals the stages that the magma went through during its solidification.

The texture of an igneous rock is defined by the characteristics of its mineralogical components (e.g. absolute and relative grain size, shape and mutual geometric relationships). Although some of these aspects can be observed in the field, texture analysis almost always requires the use of a petrographic microscope.

1. Els volcans.en BAIXA

1/6/12

13:55

Página 22

l1 l 4 l

1 l Volcanoes

Eruptive activity One of the most obvious manifestations of the Earth's internal dynamics is eruptive activity.

Sometimes violent, sometimes more pacific, this is the final stage of the volcanic process.

• • • Eruptive activity involves a series of phenomena related to the emission of solid materials, liquids and/or gases onto the Earth's surface from a point source. • • • In the course of the formation of a volcanic region, up to five eruptive units can be differentiated according to the duration and/or style of the phenomena related to the exit of materials onto the surface. The established hierarchy for these units from the least to the most important is as follows: eruptive pulse, eruptive phase, eruption, eruptive epoch and eruptive period.

Eruptive units Eruptive pulse

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

Eruptive phase

A series of eruptive pulses lasting hours or days. The resulting deposit or series of deposits have similar granulometry, morphometry and compaction.

Eruption

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

Eruptive epoch

This unit covers several eruptions and may last hundreds or thousands of years, during which time one or various volcanic edifices may form.

Eruptive period

A succession of eruptive epochs, separated by periods of time long enough for tectonic phenomena such as folding and faulting to take place. This period may last thousands or millions of years and give rise to volcanic fields or regions.

22

1. Els volcans.en BAIXA

1/6/12

13:55

Pรกgina 23

l1l 4 l 1l Why do eruptions occur?

1 l Volcanoes

An eruption starts when the pressure exerted by the magma within the volcanic conduit or magma chamber surpasses the lithostatic pressure. This increase in magmatic pressure may be due to two factors, which may operate simultaneously or individually: a. The injection of new magma from deeper areas in the Earth (the origin of most volcanic eruptions). b. The supersaturation of gases (volatiles) in the magma as it rises to the surface.

In volatile-poor basic magma, the increase in pressure is usually caused by the constant influx of new magma, whereas in acid magma it is due to a combination of both. Therefore, in superficial reservoirs of acid magma supersaturated in gas, the arrival of new magma can provoke an eruption.

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

As the magma rises to the surface, lower lithostatic pressure means that the volatiles it contains separate from the liquid and form a separate gas phase. These volatiles form bubbles that increase in number and size.

A process of magma cooling and crystallisation takes place in the magma chambers. The residual liquid is volatile-rich, as volatiles often cannot be easily incorporated into crystalline structures. Bubbles begin to form that increase the pressure in the magma.

Figure 16. Gas expansion in a volcanic conduit.

Figure 17. Gas expansion in a magma chamber.

23

1. Els volcans.en BAIXA

1/6/12

13:55

Página 24

11l lEls volcans Volcanoes

l1 l 4l 2 l Types of eruptive activity The features of eruptive activity depend mainly on the volatile content of the magma and therefore on its initial composition and its evolution during ascent. The activity type can also be affected by the presence of water in the place in which the magma is finally released. Thus, two types of eruptive activity, effusive and explosive, can be identified.

Figure 18. Emission of lava

• • • Effusive activity is characterised by the gentle and continued emission of lava, the name given to magma once it has emerged above the surface. • • •

l 1l4 l 2l1l Effusive activity Volatile-poor magma leads to effusive eruptions (Fig. 18).

The pressure exerted by gas bubbles inside the volcanic conduit is insufficient to fragment the magma and expel it into the air.

This type of activity is caused mostly by: • The emission of basic and ultrabasic magmas, initially very gas-poor.

• The degassing of acid magma due to the gradual escape of volatiles through fumaroles or steam eruptions.

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

l 1l 4l2l2l Explosive activity • • • Explosive activity is characterised by the fragmentation and violent expulsion of magma and occasionally of the surrounding rocks.

The resulting fragments are called pyroclasts. • • •

Explosive eruptions are associated with volatile-rich magmas. During the explosion, gases concentrate in bubbles and expand in the final part of the conduit. These bubbles interact with each other and isolate magma fragments. The sudden release of gas as the bubbles reach the surface causes a violent explosion that expels fragments of lava. Sometimes, hydromagmatic explosions occur when magma enters into contact with water, causing the explosiveness to increase and the rocks around the conduit to fragment.

Using as a basis a type of behaviour observed in active volcanoes or in past eruptions, explosive eruptions are classified into the following types: Strombolian, Vulcanian and Plinian, according to different degrees of explosiveness. Hydromagmatic eruptions also have different degrees of intensity.

24

1. Els volcans.en BAIXA

1/6/12

13:55

Pรกgina 25

Strombolian activity Stromboli, a volcano in the Aeolian Islands off the north coast of Sicily, lends its name to a type of low-level eruption caused by gas mixed with escaping magma.

Figure 19. Strombolian eruption

The pressure of the gas reaching the surface and its ascent through the magma depend on the physical properties of the magma. This activity is generally associated with basaltic magmas with low viscosity in which bubbles rise to the surface fairly easily.

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

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

25

1 l Volcanoes

Strombolian activity consists of discrete explosions separated by periods that range from less than a second to several hours. Each of these explosions or pulses comes about as one or more bubbles of gas reach the surface while the magma is at rest (Fig. 16). The result is the expulsion of the magma fragments, which then build up around the vent having described ballistic trajectories through the air (Fig. 19).

1. Els volcans.en BAIXA

1/6/12

13:55

Pรกgina 26

1 l Volcanoes

caused when the conduit is blocked by rocks, by cooled and consolidated magma or by debris from previous eruptions; if the pressure of the gases inside the conduit is strong enough the blockage is broken. This happens either when there is an increase in the amount of magmatic gas or, more frequently, when an aquifer is partially vaporised. Consequently, much of the ejecta result from the fragmentation of the blockage.

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

Figure 20. Vulcanian eruption

Plinian activity This type of activity takes its name from Pliny the Younger, who wrote a detailed description of the eruption of Mount Vesuvius in AD 79.

Plinian eruptions are highly explosive and violent and eject huge volumes of fragments and volatiles (Fig. 21).

Travelling hundreds of metres per second, pyroclasts and hot gases form a mushroom-shaped eruptive column that may reach heights of over 30 kilometres.

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

26

1. Els volcans.en BAIXA

1/6/12

13:55

Página 27

in the magma decreases, or if the radius of the vent increases due to erosion during the explosions, the speed at which the ejecta are released decreases and the eruptive column collapses, either partially or totally.

1 l Volcanoes

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

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

Figure 21. Plinian eruption

Hydromagmatic activity During a magmatic eruption, the entry of water into the system can completely alter the style of eruptive activity and consequently an initially gentle outflow of magma can suddenly become extremely violent. This type of eruptive activity can occur with both basic magmas and more evolved types.

The more specific term phreatomagmatism is used to describe the process of interaction between magma and groundwater. In this case, the transfer of energy from the magma to the water may come about due to either conduction (Fig. 25) o por contacto directo (Fig. 26).

27

• • • Hydromagmatic activity is the product of the interaction between magma or a source of magmatic heat and meteoric water, be it on the surface (seas, rivers or lakes) or groundwater (aquifers). • • •

1. Els volcans.en BAIXA

1/6/12

13:55

Pรกgina 28

1 l Volcanoes

Magmatic eruptions A good way of understanding how magmatic eruptions occur is to contrast volcanic process with the opening of a bottle of champagne (Fig. 22):

a. Before the eruption, the magma is subject to pressure far greater than atmospheric pressure and the volcanic gases are dissolved in the liquid.

b. When the conduit is unblocked, there is an almost instantaneous decompression of the magma, the gases expand and form bubbles.

c. The gases fragment the magma and force it out of the conduit in the form of splashes of lava that can reach great speeds.

Figure 22. Representation of a magmatic eruption

a. The champagne in the bottle is subject to high pressure because of the force exerted by the gas accumulating in the neck of the bottle. This high internal pressure means that, even though fermentation continues, no more gas can separate and so it is partially dissolved in the liquid.

b. On popping the cork, the gas that has built up in the neck is released. The pressure in the bottle drops significantly and allows the gas dissolved in the champagne to diffuse, separate from the liquid and form bubbles that then grow rapidly.

28

c. The gases drag the liquid towards the neck of the bottle at great speed, fragmenting the liquid and forcing it out in drops.

Once all the gas has escaped, the froth runs down the outside of the neck of the bottle as it lacks the force to shoot out as before.

1. Els volcans.en BAIXA

1/6/12

13:55

PĂĄgina 29

Hydromagmatic eruptions

1 l Volcanoes

Imagine a frying pan of hot oil on a kitchen stove in which a few drops of water accidentally land — the result is akin to a hydromagmatic explosion.

Figure 23. Simulation of a hydromagmatic eruption

Just like the magma in an eruption, the hot oil transfers its heat to the water, which vaporises instantly (Fig. 23) . The resulting steam expands, fragmenting the oil, which then spurts out of the pan at speed in the form of splashes. The oil corresponds to the pyroclasts in a volcanic eruption.

However, if you throw a whole bucket of water on the frying pan, the resulting reaction is very different from the above. In this latter case, the larger amount of water rapidly cools the oil and reduces the explosiveness of the interaction, which may become inexistent. This explains why underwater eruptions that occur in ridges on the sea floor, for example, are not excessively violent.

The relationship between the volume of water and magma that come into contact will go a long way to determining the vio-

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

Figure 24. Different types of volcanic deposits and edifices resulting from hydromagmatic activity whose nature is determined by the relationship between the water interacting with the magma and the degree of explosivity or efficiency of the eruption. Wohletz and Sheridan (1983).

29

1/6/12

13:55

Pรกgina 30

1 l Volcanoes

1. Els volcans.en BAIXA

Figure 25. Phreatic activity

Figure 26. Phreatomagmatic activity

An intrusion of molten material can heat and vaporise an aquifer by thermal conduction without coming into direct contact. In this case, violent explosions may take place that expel just the fragments of the rocks forming the aquifer without any magma being released to the surface.

In the course of an eruption, groundwater may enter into direct contact with the magma and be instantly vaporised. This is only possible if the pressure of the gases in the magma inside the conduit is lower than that exerted by the water in the aquifer. Then, violent explosions occur that expel fragments of magma and of the rocks surrounding the conduit itself.

Surtseyan activity Eruptive activity in Iceland is generally effusive and Strombolian and involves the emission of basic magmas. However, in 1963 off the south coast of Iceland Surtsey, a new volcanic island, was born. It was the result of a highly explosive eruption caused when seawater entered the conduit and was vaporised instantly.

This eruptive style, seen in the formation of many other volcanoes, is now known as Surtseyan activity.

Figure 27. Eruption on Surtsey, Iceland

30

1. Els volcans.en BAIXA

1/6/12

13:55

Página 31

l1 l 4l 3 l Volcanic materials

l 1l4 l 3l1l Massive materials These are compact bodies of homogeneous composition resulting from the cooling of lava flows originating from effusive eruptions. These rock bodies may be present in diverse forms depending on the initial viscosity of the magma. Variation in temperature during emplacement, the volume of material ejected and the features of the terrain where it is deposited (e.g. slope, irregularities and humidity) will also affect the final form they take.

The most fluid lavas are basic in composition and give rise to lava flows (Fig. 28) . These represent continuous outpourings of molten rocky material that slide along the flattest areas of land, potentially covering great distances.

• • • Volcanic materials consist of all the solid, liquid and gaseous products expelled during an eruption.

We can distinguish between volatiles gases that separate from the magma and the materials that form deposits, classified as either massive or fragmentary. • • •

Lava from acid magma is very viscous and normally builds up around the vent in the form of domes. In extreme cases this type of lava is practically solid when it emerges and leads to the formation of pinnacles.

Lava flows Lava flows can be distinguished by their lithology, morphology and the features of the site. These parameters vary according to the composition of the liquid magma, the speed of cooling of the flow and the features of their emplacement. Lava flows can be classified by their external appearance into two large groups: smooth and rough. The internal structure can be massive and compacted, or fractured by joints.

Internal structure of lava flows: retraction Lava contracts considerably when it cools since it occupies less volume in a solid than in a liquid form. This leads to the development inside the massive body of rock of various systems of fractures or cracks known as jointing. The main types of jointing are columnar and lenticular (Fig. 29).

31

Figure 28. Solidified lava flow in the Teide volcanic complex

1 l Volcanoes

The study of volcanic rocks helps understand the transportation and deposition mechanisms they originated from and therefore the type of eruptive activity involved.

In this field of study, the geometric and textural relations of the built-up material and its composition have to be analysed.

1. Els volcans.en BAIXA

1/6/12

13:55

Pรกgina 32

1 l Volcanoes

Lenticular or slab jointing occurs when the lava stream is moving, for instance when the flow is still being fed by the vent, and gas bubbles deposit in parallel planes in the direction of flow. As the lava cools, the planes facilitate horizontal fracturing, which is most noticeable in the centre of the lava flow.

Figure 29. Columnar and lenticular jointing

Columnar jointing occurs when the lava flow is at rest.

The difference in temperature between the very hot centre and the top and bottom of the flow, which have already cooled, causes convection cells to generate inside the lava flow. These cells form perpendicular to the base of the lava and develop vertical fractures, forming prismatic joints that split the rock into five- or six-sided columns.

Spheroidal weathering, the internal structure that is often present in the outermost parts of lava flows, cannot really be thought of as a type of jointing (Fig. 30).

This flaking of concentric shells of lava is the result of the weathering of the volcanic rock caused as moisture slowly infiltrates through existing cracks. Another effect is white mottling, caused by the weathering of certain minerals in the rock.

Figure 30. Spheroidal weathering

32

1. Els volcans.en BAIXA

1/6/12

13:55

Pรกgina 33

Lava flow morphology

1 l Volcanoes

Figure 31. Smooth lava (pahoehoe)

Figure 32. Rough lava ('a'a)

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

Viscous lavas have a rough, irregular surface made up of broken lava blocks or clinker (Fig. 32) . The outermost layer of the lava flow cools and forms a crust, which then breaks into blocks as the lava underneath continues to flow. When the fragments are large, this is called block lava.

A single lava flow may exhibit diverse types of morphology. Thus, we frequently observe a lava flow with an initial stretch with a smooth surface, then an area of ropy lava that becomes increasingly irregular, followed by an area of rough lava.

Submarine lava flows behave differently from sub-aerial flows. Upon coming into contact with the water, the lava cools suddenly and a fairly plastic layer of glass is formed creating blobs of lava. These blobs fall and roll down the slope on top of each other and become misshapen, thereby forming what is known as pillow lava.

Blisters When the lava flow flows over a lake or a wetland, the water vaporises and a huge amount of gas is incorporated into the flow. Gas bubbles rise inside the flow towards the surface, which is often semi-solid due to more rapid cooling. The build-up of bubbles in this area causes pressure that can deform and even break the surface of the lava flow. The result are mounds, dozens of metres high known as blisters (tossols in Catalan) (Fig. 33) .

Figure 33. Blister

33

1. Els volcans.en BAIXA

1/6/12

13:55

PĂĄgina 34

l 1l4 l 3l2l Fragmentary materials

1 l Volcanoes

Fragmentary materials consist of clasts generated mainly by explosive eruptions. Gas bubbles form blobs of magma, which are expelled violently. In some cases, volcanic explosions can break part of the vent or chimney wall and the resulting fragments mix with the magmatic clasts. Finally, all these materials are deposited forming pyroclasts (fragmentary deposits).

When an eruption is so violent that it cannot be observed from close up, the study of pyroclastic ejecta is fundamental to the understanding of the type of eruptive activity.

Pyroclasts The word pyroclast comes from the Greek pyro, (fire) and klastos (broken). Each of the fragments, large or small, form part of the pyroclastic deposits and have their own particular features.

Classification by size Volcanic explosions give rise to fragments in a variety of sizes. Pyroclasts can be classified by size into three main groups: ash, lapilli and blocks (Fig. 34) .

Blocks

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

Lapilli

Ash

64 mm

2 mm

Figure 34. Classification of pyroclasts by size

Nature of fragments Lithic fragments: these are fragments of the rocks forming the vent that were ripped out by explosions during the eruption. Lithic fragments can be accessory, when they derive from rocks from previous eruptions, or accidental, when they are fragments of sedimentary, metamorphic or igneous rocks that form part of the volcanic substrate.

Different types of clasts - juvenile or lithic - are distinguishable according to their nature. Some pyroclastic deposits consist of only one type of fragment, while others consist of a mix of the two.

Juvenile fragments: also known as essential fragments, derive directly from magma reaching the surface.

34

1. Els volcans.en BAIXA

1/6/12

13:55

PĂĄgina 35

Other terminology used

Figure 35. Volcanic bomb

Scoria: juvenile pyroclasts, lapilli-sized or larger, of irregular morphology that contain many holes or vesicles. These fragments are basaltic or basaltic-andesitic in composition and may be semi-welded in deposits close to the vent because they were not completely solid when they were deposited (Fig. 36) .

Figure 36. Scoria

Pumice: juvenile fragments, generally lapillisized, acid in composition and pale-coloured.

Pumice floats since it is highly porous and its density does not exceed 1g/cm3 (Fig. 37) .

Figure 37. Pumice

l1l4l3l3l Types of pyroclastic deposit Fragmentary materials build up different types of deposits according to the mechanisms of formation, transport and deposit in operation. We can distinguish three basic types - pyroclastic fall, pyroclastic surge and pyroclastic flow deposits – that occur due to differences in the genesis of the deposit.

Pyroclastic fall deposits These are formed when ejecta from an eruption either fall freely and vertically having formed part of the eruption column or on a ballistic trajectory after being ejected from the crater of the volcano (Fig. 38). Fall deposits may show gradation in size and laterally continuous parallel banding. The further they land from the vent, the thinner the deposit and the smaller the fragments.

35

1 l Volcanoes

Volcanic bombs: when fragments of magma the size of lapilli or blocks that are not completely cooled when ejected, move through the air they are modified into rounded or spindle-shaped forms. If they have superficial cracks they are called bread-crust bombs. These are formed by the expansion of gas bubbles inside the still semi-molten bomb when the surface has already cooled and is easily fractured (Fig. 35) .

1/6/12

13:55

Pรกgina 36

1 l Volcanoes

1. Els volcans.en BAIXA

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

Types of fall deposit a. Strombolian fall deposits: the low energy of the eruption and high density of the fragments mean that the ejecta do not reach great heights and fall to ground directly on a ballistic trajectory. This mechanism is characteristic of Strombolian eruptions, in which fragments build up around the vent and form a volcanic cone. b. Plinian pyroclastic deposits: when their density is low, fragments rise to considerable heights forming characteristic Plinian eruption columns. Finally, these materials fall in a shower of pyroclasts. Prevailing winds can displace the cloud of materials that make up the column and affect the emplacement of the pyroclasts. These deposits cover the land evenly and build up both in depressions and on higher ground (Fig. 39) .

Figure 39. Plinian fall deposit

c. Hydrovolcanic deposits: in violent explosions caused by the instantaneous evaporation of water, part of the ejecta follows ballistic trajectories. Unlike in Strombolian eruptions, the horizontal component in these cases is much more important than the vertical component and the resulting build-up, which includes a considerable presence of lithic fragments, is known as pyroclastic breccia (Fig. 40) .

Figure 40. Pyroclastic breccia

36

1. Els volcans.en BAIXA

1/6/12

13:55

Pรกgina 37

Pyroclastic surge deposits

a. the collapse of the outside of the column, which is much more diluted and colder than the centre; b. annular explosions at ground level produced directly in the vent that move radially.

These are high-energy flows and can move up slopes.

Consequently, the deposits left by pyroclastic surges cover the underlying topography, although the most important build-up of material occurs in valley bottoms (Fig. 41) . Such deposits are characterised by unidirectional sedimentary structures and good granulometric classification. They often have an erosive base lying on the materials of the substrate.

Figure 41. Emission and emplacement of a pyroclastic surge

Pyroclastic flow deposits These consist of fast-moving laminar flows of gas and rock fragments that fill in depressions as they spread laterally. Generally, they originate after the total or partial collapse of a vertical eruption column and during emplacement are accompanied by a huge ash cloud (Fig. 42).

The build-up of the materials transported by these flows fills valleys and depressions. They normally have no clear stratification or any defined internal structure and are often compacted by secondary cementation. They are typical of explosive eruptions associated with differentiated magma, although they can also occur in basic volcanism. Large pumice-rich pyroclastic flows are known as ignimbrites.

37

1 l Volcanoes

These deposits originate in turbulent gaseous flows that transport horizontally small amounts of pyroclasts at supersonic speeds, close to the ground. The formation of pyroclastic surges is associated mainly with:

1. Els volcans.en BAIXA

1/6/12

13:55

Pรกgina 38

1 l Volcanoes

The deposits originating from pyroclastic flows and surges are extreme manifestations of a wide range of different types of emplacements and flows and many intermediate forms can be found between these extremes.

Figure 42. Pyroclastic flow deposit

Lahars Lahar is an Indonesian word used to describe a water-saturated flow of volcanic debris or a mudflow. When large quantities of snow cover volcanoes or when their craters contain lakes, an eruption - however small - can cause huge slides of mud and volcanic rock. These flows travel at high speeds and cause rivers to break their banks and sweep away everything in their paths, from vegetation and infrastructures, to vehicles and even entire villages. Lahar deposits are chaotic masses of volcanic rock and other material picked up along the way.

In the sequence of materials we find volcanic deposits (lavas or pyroclastic rock) interspersed with sedimentary materials (Fig. 43).

Figure 43. Lahar emplacement

38

1. Els volcans.en BAIXA

1/6/12

13:55

Página 39

l1 l 4l 4l Volcanic morphology

1 l Volcanoes

The build-up of volcanic materials ejected close to the vent gives rise to the formation of one or several volcanic edifices that are generally cone-shaped and variable in size. The morphology of volcanic constructions is closely related to the type of eruptive activity and the episodes that have taken place during the history of the volcano. Hence, we can classify volcanoes as either monogenetic or polygenetic.

Monogenetic volcanoes These volcanoes are formed in the course of a single eruption, which can have several phases and pulses.

The edifice constructed is simple and the main features include pyroclastic cones, tuff cones, tuff rings and maars. A succession of different eruptive phases can result in the superposition of several of these types of edifices in a single volcano.

Pyroclastic or cinder cones They are the result of a Strombolian eruption and are built mostly from cinder (scoria). The craters may be circular or breached on one side. The horseshoe shape may be due to the inclination of the vent, the presence of prevailing winds that whip pyroclasts along in a given direction, or to the expulsion of lavas that drag part of the pyroclastic deposits along with them. The flanks of a cinder cone slope at an angle of 30–40°.

Tuff cones These are formed from the interaction of magma and water during a hydrovolcanic eruption. The materials formed are mostly compact pyroclastic surges and flows.

Craters are small and the flanks of the cone slope at 20–25°.

Tuff rings These 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 sloping at around 10°.

Maars These form as a result of a phreatomagmatic eruption and are similar to tuff rings. In this case, the crater lies below the surrounding topography and the cone, formed by pyroclastic surge and flow deposits, is very low.

Figure 44. Cinder cone

Figure 45. Tuff cone

Figure 46. Tuff ring

Figure 47. Maar

39

1. Els volcans.en BAIXA

1/6/12

13:55

PĂĄgina 40

Polygenetic volcanoes

1 l Volcanoes

These are formed from several eruptions over a long period, from thousands to millions of years. They are often associated with intermediate or near-surface magma chambers where successive episodes of emptying and refilling have taken place and where the primary magmas can evolve. The resulting edifices are known as stratovolcanoes and shield volcanoes.

Stratovolcanoes Also called composite volcanoes, they are associated with intermediate acid magma eruptions where explosive and effusive activity alternates. Consequently, they are formed by several layers of fragmentary deposits and lava flows. The edifice, which is large, may have flanks with slopes of over 40°.

Figure 48. Stratovolcano

Figure 49. Shield volcano

Shield volcanoes Formed from basaltic eruptions in which effusive activity predominates. The edifice, formed by the accumulation of lava flows, is concave in shape and, as its name implies, resembles a shield. The cone are not very high and the flanks of the slope are at angles of less than 10°, but in some cases the base may be over a hundred kilometres in diameter.

Both monogenetic and polygenetic volcanoes may have smaller secondary edifices around them, clearly linked to the activity of the main edifice, known as adventive or parasite cones.

Collapse calderas In volcanoes with magma chambers, in the course of an eruption large quantities of magma are ejected rapidly (phase a). The partial or total emptying of the magma can cause the chamber to collapse. This collapse reactivates the volcano and generates more highly explosive phases (phase b). The end

result is a depression, kilometres wide, known as a collapse calderas (phase c). The internal walls that limit the depression are vertical and made mostly of ignimbrite deposits ejected in phase b.

Figure 50. Formation of a collapse caldera

40

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Pรกgina 41

2 Volcanism in Catalonia

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

PĂĄgina 42

l2l 1l Distribution and evolution of volcanoes The eruptive episodes that took place in La Garrotxa and in Catalunya in general during the Neogene and Quaternary were not simply a sporadic event. The origin of the series of volcanic morphologies and rocks that constitute the Catalan volcanic field lies within a broader geodynamic context that affects much of Western Europe.

2 l Volcanism in Catalonia

Using as a basis the composition and dating of volcanic rocks, two eruptive periods have been identified in the western Mediterranean, and evidence of both exists in the northeast of the Iberian Peninsula. The geological histor y of the region is complex given the overlap of compressive and distensive structures.

Figure 51. Western Mediterranean. Compression period. Calc-alkaline volcanism

Figure 52. Western Mediterranean. Distension period.

Alkaline volcanism

The first eruptive period occurred during the Miocene (24–18 Ma) and is characterised by compressive tectonic conditions (Fig. 51). The associated magmatism was calc-alkaline, mostly represented by sub-aerial volcanic manifestations in Mallorca and, above all, by submarine features situated between the Balearic Islands and the Iberian Peninsula. The origin of these eruptions is explained by the presence of a subduction plane sloping towards the Iberian Peninsula running NESW from the Balearic Islands to west of Corsica and Sardinia.

From the Upper Miocene onwards, the situation changed to one of distension that we find today (Fig. 52) . This second cycle corresponds to the development of an intraplate rift affecting Western Europe and associated with the alkaline magmatic manifestations of the Valencian, Els Columbrets and Catalan volcanic fields. It is also worth mentioning that a number of submarine volcanoes were formed during isolated volcanic episodes occurring, for example, off the coast of Tarragona.

42

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Pรกgina 43

The European Cenozoic Rift System number of large normal faults running mostly NE-SW. The magma has taken advantage of these weaknesses in the lithosphere to rise to the surface and there are thus numerous volcanic manifestations in both Eastern and Western Europe associated with this rift system. The most important such volcanic fields ones are in Eiffel (Germany), Auvergne (France) and Catalonia.

During the Upper Miocene at the end of the Tertiary Period, an extensive process began in the western sector of the Eurasian Plate that is still considered to be active. As a result of the distensive forces operating within the plate, a rift-type structure measuring over 2,000 km from the North Sea coast to the southern Iberian Peninsula has developed (figura 53). Within this rift there is a series of troughs and raised blocks that have been created by the movement of a

2 l Volcanism in Catalonia

Figure 53. The intracontinental rift system in Western Europe

43

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

The European rift system contains a series of discrete structures such as the Valencia Trough and the fault trenches of the Gulf of Lion, the Têt and Tech rivers, and La Cerdanya. These two segments situated in the northeast of the Iberian Peninsula have been displaced by a series of normal faults that lie perpendicular to those within the rift (Figs. 53 and 54). From west to east, these fractures are known as the Amer, Llorà, Cartellà, Camós-Celrà, Juià, Riurà and Vilopriu faults, which separate a series of raised blocks (Les Gavarres, Les Guilleries and the mountains of La Serralada Transversal) and sunken blocks (L'Empordà and La Selva depression and the Olot Trough).

2 l Volcanism in Catalonia

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

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

44

Página 44

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Página 45

l2l2l The Catalan volcanic field The series of Neogene-Quaternary eruptive rocks in northeast Catalonia are situated in three volcanic areas: L'Empordà, La Selva and La Garrotxa. We can deduce from the geographical distribution of the eruptive features and the geochronological data available that the magmatic activity began in the L’Empordà, moved south towards La Selva and then finally reached La Garrotxa (Fig. 55).

The combination of the age of the volcanic phenomena in the L’Empordà and La Selva and the effects of erosive processes explain why the volcanic edifices of these two zones have all but disappeared, and also why only the hardest massive materials including fragments of lava flows and collapsed chimneys are still recognizable.

2 l Volcanism in Catalonia

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

45

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Página 46

L'Empordà Volcanic Zone This zone contains around fifty basalt outcrops, as well as a number of trachyte outcrops, shared between the regions of L'Alt and El Baix Empordà, of which the most important are located around La Bisbal d'Empordà, Rupià and Arenys d'Empordà.

Most of these volcanic materials are covered by Pliocene deposits and date from over 6 Ma, with the oldest being around 14 Ma. Particularly noteworthy are the trachyte outcrops at Vilacolum and Arenys d'Empordà (L'Alt Empordà).

These more evolved volcanic rocks have resulted from the cooling of magma that had undergone differentiation.

2 l Volcanism in Catalonia

La Selva Volcanic Zone This region also comprises a series of around fifty basalt outcrops, mostly around Maçanet de la Selva and Riudarenes. The collapsed chimneys of Sant Corneli and Hostalric are its most interesting features and exhibit marked columnar jointing. In some parts, deposits of fragmentary materials originating from hydromagmatic eruptions can be identified.

Geochronological analyses have dated these rocks at 5–20 Ma. La Crosa de Sant Dalmai on the northern rim of La Selva depression is a well-preserved volcano that erupted in more modern times.

La Garrotxa Volcanic Zone The youngest and best-preserved volcanoes in Catalonia are in La Garrotxa. Thirty-eight volcanoes have been identified in La Garrotxa Volcanic Zone Natural Park, with a further two in the Hostoles Valley and five in the Llémena Valley (Fig. 56) . A large number of lava flows and pyroclastic deposits (both Strombolian and hydromagmatic in origin) are visitable (of particular interest in the Llémena Valley).

Despite some evidence of volcanic activity prior to the Quaternary, available geochronological data situates this volcanic zone at 350,000–10,000 years old and current estimates suggest that an eruptive episode has occurred approximately every 15,000 years.

46

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Página 47

2 l Volcanism in Catalonia

1 2 3 4 5 6 7 8 9 10

La Canya Aiguanegra Repas Repassot Cairat Claperols Puig de l'Ós Puig de l'Estany Puig de Bellaire Gengí

11 12 13 14 15 16 17 18 19 20

Bac de les Tries Les Bisaroques the Garrinada Montsacopa Montolivet Can Barraca Puig Astrol Pujalós Puig de la Garsa Croscat

21 22 23 24 25 26 27 28 29 30

Cabrioler Puig Jordà Puig de la Costa Puig de Martinyà Puig de Mar Santa Margarida Comadega Puig Subia Roca negra Simon

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

47

31 32 33 34 35 36 37 38 39 40

Pla sa Ribera Sant Jordi Racó Fontpobra Tuta de Colltort Can Tià Sant Marc Puig Roig Traiter Les Medes

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Página 48

2 l Volcanism in Catalonia

2. El

1 2 3 4 5 6 7

Crosa de Sant Dalmai Puig d’Adri El Rocàs Clot de l’Omera Puig de la Banya del Boc Granollers de Rocacorba Puig Montner

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

48

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

PĂĄgina 49

l2l3l Rocks and magma The composition of the rocks that make up the volcanic zone of La Garrotxa and the Catalan volcanic field in general is relatively uniform. With the exception of the trachyte outcrops in L’Alt Empordà , all volcanic materials are composed of basalt and basanite, low in silica and high in sodium and potassium. Therefore, as a whole the volcanic materials in Catalonia can be classified as alkaline. They are the result of the cooling of rapidly ascending basalt magmas and are characteristic of intraplate volcanic zones.

2 l Volcanism in Catalonia

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).

49

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

PĂĄgina 50

l2l 3 l1 l Minerals Basalt mineralogy is uniform and simple. In most cases, all that is visible – and only under a microscope - are small olivine, pyroxene and plagioclase feldspar phenocrysts in a microcrystalline or partially vitreous matrix, which is often rich in iron oxide (mainly magnetite). Other minerals such as leucite and analcime are also present in small quantities.

2 l Volcanism in Catalonia

The very few mineralogical differences between basalt and basanite that exist are very hard to distinguish with the naked eye. They are characterised by the presence of small feldspathoid crystals such as leucite and generally by a slight reduction in the percentage of silicon dioxide (Fig. 13).

Unlike basalt rocks, trachytes have a high percentage of silicon oxide (over 60%) and are composed of large plagioclase crystals with some pyroxene and biotite. Under the microscope, the trachyte matrix contains numerous small, elongated sanidine crystals, as well as titanium and iron oxides.

Observable minerals

Figure 60. Olivine A pale green mineral with a glassy lustre. It appears both in the form of phenocrysts and as part of the matrix. Large crystals tend to be idiomorphic with regular sides corresponding to the facets of the crystal.

Figure 61. Pyroxene Dark with green tones. Pyroxenes are found both as phenocrysts and in the matrix. Most are titaniferous augites and are often present in idiomorphic or sub-idiomorphic forms.

50

Figure 62. Plagioclase A white mineral. This type of feldspar is generally subordinate in the matrix and is only found exceptionally as a phenocryst..

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Página 51

l2l 3 l2 l Geochemical data Magma genesis and ascent The geochemistry of the basalt rocks in the Catalan volcanic field displays considerable homogeneity in major elements, including silicon, aluminium, iron and calcium oxides. The only significant variations are in the percentage of titanium oxide, attributable to variable temperatures in the magma when the rocks were being formed.

Nevertheless, important variations exist between different rocks in terms of the amount of trace elements (nickel, cobalt, chromium and strontium) and rare earth elements (lanthanum, cerium and neodymium) they contain. This variation in chemical composition coincides well with the three geographic regions L’Empordà, La Selva and La Garrotxa - and reveals differences in the magma source area.

The presence of these two source areas, the astenosphere and the lower lithosphere, can be linked to the evolution of the European rift system. During the initial extensive stages, the thinning of the lithosphere led to its decompression and partial melting. The crust was still thick and pockets of magmas were trapped in small chambers in which the Empordà trachytes differentiated and formed. As the rift progressed and the lithosphere grew thinner, the asthenosphere ascended and permitted less evolved molten materials to rise.

In some cases, the almost total lack of contamination of the basalts by rocks from the crust and their scant differentiation indicate that the ascent of the magma pockets from the source to the surface was rapid.

51

2 l Volcanism in Catalonia

Variations observed in the geochemical analysis of the basalt rocks enable us to understand better the genesis and ascent of the magmas that gave rise to volcanism in Catalonia. The magma source areas are generally situated in the asthenospheric mantle, although the magmas that generated the volcanic features in L’Empordà come from an area that is more lithospheric.

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Página 52

Enclaves Some lava flows and pyroclastic deposits contain fragments of rock that were captured by the magma during its ascent. These fragments, known as enclaves or xenoliths, consist mostly of plutonic rocks, although some metamorphic or sedimentary rocks may also occur (Fig. 63) . These enclaves usually consist of blocks (usually a few centimetres in length) formed in the lithosphere (or in some cases in the mantle) that the magma tore out of the vent wall and then engulfed before transporting them to the surface.

Also of great interest is the presence of ultrabasic xenoliths (Fig. 64) derived from the mantle or from remains of magma differentiation in the basalts in the lower crust (e.g.

Rocanegra, Puig de la Banya de Boc and Puig d'Adri volcanoes). These xenoliths were denser than the basaltic liquid, but, due to the speed of the magma’s ascent, were immersed and dragged to the surface.

Calculations of the floatability of these fragments in magma show that an ascent speed of around 0.2 m/s would have been necessary to maintain the enclaves in suspension.

2 l Volcanism in Catalonia

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

Figure 63. Enclave of plutonic rock: granitoid

Figure 64. Ultrabasic enclave: dunite

52

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Pรกgina 53

l2l4l Eruptions in La Garrotxa Volcanic Zone Each of the volcanoes in the La Garrotxa was formed during a single eruption. Thus, they are monogenetic in nature and were created by the ejection of a pocket of magma whose exhaustion marked the end of the volcanic activity.

However, the various phases of activity present during the eruption are visible since they are marked by a change in the style of the magma's journey to the exterior, although the time-lapse between each of these phases was not sufficient for erosive stages or soil development to begin.

2 l Volcanism in Catalonia

53

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Página 54

l2l 4 l 1l Volcanoes and their phases of eruptive activity The eruptive activity that gave rise to the volcanoes in La Garrotxa combined hydromagmatic and purely magmatic phases that, despite the monotonous composition of the magma, has left a legacy of many different volcanic features. The study of these volcanic deposits has identified effusive, Strombolian and phreatomagmatic eruptive phases.

2 l Volcanism in Catalonia

An oft-repeated evolutionary process starts with Strombolian activity, which then evolves into effusive activity as the magma loses its gas (Fig. 65) . The best examples of this evolution are the volcanoes of Croscat, Montolivet and Sant Marc.

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

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

More rarely, we find volcanoes that were formed from a single eruptive phase, either Strombolian (Puig Astrol; Fig. 67) , or phreatomagmatic (El Clot de l’Omera).

54

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Página 55

Figure 66. Puig d’Adri volcanoe

2 l Volcanism in Catalonia

Figure 67. Puig Astrol volcanoe

Eruptions in which the initial activity was Strombolian may become phreatomagmatic if water enters the vent when the magma ejection loses intensity (e.g. Can Tià).

Finally, there is also evidence of Strombolian phases inserted into obvious phreatomagmatic sequences, which can occur if the water supply in the aquifer is momentarily exhausted.

55

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Pรกgina 56

Example of an eruption Despite the variety of possible combinations of eruptive styles that occurs during an eruption, the most frequent case in the La Garrotxa involving phreatomagmatic activity is as follows:

2 l Volcanism in Catalonia

The eruption starts with an explosive phreatomagmatic phase. Magma rich in juvenile gas is enriched by volatiles due to the evaporation of the water in the subsoil. In this early stage, phreatomagmatic activity may be interspersed with pure Strombolian phases in which the water-magma interaction stops momentarily (Fig. 68a) .

Figure 68a.Phreatomagmatic eruptive phase

The ejection of new magma makes the vent waterproof and so halts the phreatomagmatic activity. Nevertheless, the magma in the magma pocket still contains enough gas to generate Strombolian explosive activity (Fig. 68b) .

Figure 68b. Strombolian eruptive phase

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

Figure 68c. Effusive eruptive phase

56

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Página 57

l2l 4 l2 l Eruptive activity and volcanic edifices During an eruption, the alternation of types of activity often leads to the formation of different, superimposed volcanic edifices. For instance, in the Natural Park on the volcano of Puig de Martinyà two subsequent cinder cones cover much of a previous phreatomagmatic construction. Even so, the best examples of this type of interference between edifices originating from the same eruption are the volcanoes of La Crosa de Sant Dalmai (Fig. 69) and Puig d'Adri. Both consist of edifices formed

by Strombolian activity that have been superimposed on previous phreatomagmatic structures.

On other occasions, volcanic edifices generated in the course of an eruption are partially or totally destroyed by subsequent phases. The cinder cones of volcanoes such as Croscat, Montolivet and Aiguanegra were partially destroyed by lava flows during a final effusive phase (Fig. 70).

The emission of magma through either the crater or the base of the cone drags pyroclasts from one part of the edifice to another and, when seen from above, the final shape of the crater resembles a horseshoe.

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

1

1

Figure 71. Croscat volcanoe

57

2 l Volcanism in Catalonia

Figure 70. Rocanegra and Puig Subià volcanoe

Figure 69.La Crosa de Sant Dalmai volcanoe

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

PĂĄgina 58

l2l5l Volcanic materials In La Garrotxa Volcanic Zone, rocks resulting from effusive activity vary very little due to the uniformity of their constituent magmas. Lava flows are greyblack and exhibit the fractures that are typical of columnar and lenticular jointing and spheroidal weathering. The surface is usually smooth and the few rough-surfaced lava flows ('a'a type) are hard to detect due to dense plant cover and human activity.

2 l Volcanism in Catalonia

Explosive volcanic activity gives rise to a great diversity of pyroclastic deposits (Figs. 72 and 73) . The violence of these types of explosions and their origin, be they magmatic or hydromagmatic, determine the granulometry of the pyroclastic rocks and their components.

Figure 72. Phreatomagmatic deposits from Puig d’Adri

Figure 73. Scoria deposits from Croscat

Figure 74. Sequence of volcanic materials at La Pomereda

In La Garrotxa several types of deposit are superimposed as a result of a succession of different pulses and phases of eruptive activity (Fig. 74) and knowledge of the characteristics of each type of volcanic material is often sufficient to identify them.

58

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Pรกgina 59

Magmatic explosive activity Strombolian fall deposits

Figure 76. Strombolian fall deposits. Scoria

Figure 77. Strombolian fall deposits. Ash

Juvenile fragments, highly vesicular in general, mostly blocks (bombs), with a variable percentage of lapilli. If found close to the vent, they are welded into agglomerate.

Angular, highly vesicular, mostly lapilli-sized juvenile fragments. Often with layers of bombs, they are distributed radially in a relatively small diameter from the vent and form the volcanic cone.

Angular, ash-sized vesicular juvenile fragments. They deposit radially around the vent, mostly far from the cone.

Hydromagmatic explosive activity

Figure 78. Phreatomagmatic fall deposits. Breccia

Figure 79. Distributed around the crater. Ash and lithics

Figure 80. Pyroclastic flow deposit. Volcanic tuff

Juvenile fragments and lithics of varying size with notable block content. Distributed around the crater.

Juvenile fragments and lithics, ash-sized or fine lapilli.

Fragments may show degrees of rounding and juvenile pyroclasts are slightly vesicular.

Highly dispersed with a high degree of compaction.

Juvenile fragments and lithics, lapilli-sized and blocks enclosed in an ash matrix. They are compacted and fill pre-existing depressions.

59

2 l Volcanism in Catalonia

Figure 75. Strombolian fall deposits. Volcanic agglomerate

2. El

vulcanisme a Catalunya-en baixa

4/6/12

07:09

Pรกgina 60

3. Fitxes.en baixa

4/6/12

07:16

Pรกgina 61

3 La Garrotxa Volcanic Zone Sites of volcanic interest

3. Fitxes.en baixa

4/6/12

07:16

Página 62

Interpreting the site descriptions

Location of volcanic features

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

Sites can be divided into two groups: those within La Garrotxa Volcanic Zone Natural Park and those in the Llémena Valley (as well as La Crosa de Sant Dalmai).

Two types of observation - landscape and volcanic features – are described.

Volcanic sites provide evidence of different types of eruptive activity: effusive, Strombolian and hydromagmatic explosive.

3 l La Garrotxa Volcanic Zone

Furthermore, each description contains details of the site’s location, the volcano whose eruption originated the deposits, the materials present and an interpretation of the observable sequence or morphology.

Figure 81. Location of sites

62

3. Fitxes.en baixa

4/6/12

07:16

Página 63

Volcanic sites

1 Castellfollit de la Roca:

2 El Cairat: pyroclastic

3 Sant Joan les Fonts:

4 Montsacopa: cone

5 Croscat: cinder cone

6 Turó de la Pomereda:

7 Santa Margarida: py-

8 Can Tià: an eruption

lava flows

9 Els Arcs Valley: pyro-

clastic flow

tic surge

massive materials

an eruption sequence

roclastic deposits

10 Location and morp-

11 El Clot de l’Omera: a

14 La Crosa de Sant

15 La Crosa de Sant

hology of the volcanic cones as seen from Puig Rodó

maar

Dalmai: morphology

Dalmai: pyroclastic surge and breccia

63

morphology

sequence

12 Puig d'Adri: pyroclastic

flow

3 l La Garrotxa Volcanic Zone

13 Puig d'Adri: pyroclas-

breccia

3. Fitxes.en baixa

1

4/6/12

07:16

Página 64

Castellfollit de la Roca: lava flows Point of interest l Observation of Activity type l Effusive Access time on foot l 10 minutes

the basalt cliff

Location and access entrance to the town there is an excellent view of the basalt cliff at the junction with the road to Oix (km point 45). Park and take Natural Park Itinerary 13 that leads down the river Fluvià (Fig. 82) . Continue for 500 m and then turn right over a wooden footbridge to reach the foot of the basalt cliff.

The town of Castellfollit de la Roca stands about 7 km from Olot on a promontory between the rivers Turonell to the south and the Fluvià to the north.

To reach Castellfollit from Girona, take the C-66 past Banyoles and then the A-26 dual-carriageway past Besalú and turn off at the Castellfollit de la Roca exit. At the

3 l La Garrotxa Volcanic Zone

Castellfollit de la Roca The basalt columns are a result of the superposition of two lava flows and subsequent erosion by the rivers Fluvià and Turonell.

This basaltic cliff stands 50 m above the surrounding rivers at its highest point and extends for a full kilometre; it provides an excellent view of the internal structure of a lava flow.

The cliff has been receding for thousands of years, mostly due to erosion by the river Fluvià and frost weathering (freezethawing), which is all the more effective given the existing jointing. These cracks are weak points where weathering can take place more effectively, eventually leading to the crumbling of the basaltic columns. These are then carried off when the river is in spate and never build up to stabilise the base of the cliff.

Figure 82. Schematic geological map of Castellfollit de la Roca

64

3. Fitxes.en baixa

4/6/12

07:16

Página 65

Lava flows Description The base of the cliff consists of layers of Eocene sandstone and marl overlain by gravels containing abundant limestone, sandstone and, exceptionally, basalt pebbles.

On top of these materials lies a 40-mthick layer of black-grey basalt, although about 9 m from the base of the volcanic materials there is a 0.2–1.5-m thick layer of clay and pyroclasts that are recognisable by the abundance of plants growing there (3) . This layer divides the cliff into two parts:

composed of prisms around 50 cm in diameter. The second layer has lenticular jointing and is 3.5-m thick. The final layer again exhibits columnar jointing, but is less than a metre thick and the columns are only 30 cm in diameter (1). b. The upper part has four layers: the first three, each 5–9-m thick, exhibit marked columnar jointing, while near the top a layer about 9-m thick appears with welldeveloped spheroidal weathering (2).

a. The lower part has three clearly differentiated layers. The first has columnar jointing, but is partially covered by the riverside vegetation: it is 5.5-m thick and is

2

3 1

Figure 83. Castellfollit de la Roca basalt cliff

the lava gave rise to various differentiated layers within the lava flow. The time lapse between the two lava flows is marked by the development of a soil and the deposition of sedimentary materials that form a layer that clearly separates the two flows.

To overcome this obstruction, the waters of the Fluvià and Turonell have eroded the boundary between the basalt materials and the sedimentary rocks.

Alluvia from the rivers Fluvià and Turonell and two lava flows were deposited on top of the original Eocene substrate.

Around 217,000 years ago lava from the volcanoes on the Batet plateau flowed into and along the valley of the former course of the river Fluvià to beyond the town of Sant Jaume de Llierca. Then, some 192,000 years ago a second lava flow flowed down the Turonell Valley from the Begudà volcanoes to Castellfollit de la Roca. In both cases differential cooling of

65

3 l La Garrotxa Volcanic Zone

Interpretation

3. Fitxes.en baixa

2

4/6/12

07:16

Página 66

El Cairat: pyroclastic breccia Point of interest l Can Barranc quarry Activity type l Phreatomagmatic Access time on foot l 5 minutes

Location and access Joan les Fonts. A kilometre before Sant Joan, a track turns south into the quarry.

Park in the industrial estate on the other side of the road and walk about 100 metres up the track into the quarry and to the volcanic deposits visible in the excavated banks (Fig. 84) .

The eruption centre of the volcano of El Cairat lies on the ridge of the Sierra de Molera, which is connected to the volcano of Aiguanegra. During the 1980s, the pyroclastic materials emitted by this volcano were quarried at Can Barranc and today are visible in a number of different sites.

To reach the quarry, take the GI-522 from Castellfollit de la Roca to Sant

3 l La Garrotxa Volcanic Zone

El Cairat The crater of El Cairat – maar-type in structure - is visible from Begudà and from the Batet plateau. It possesses a crater of around 120 m in diameter, which is sunk into the surrounding Eocene sedimentary substrate. It is considered to be the only volcano in the Natural Park that consists of a single edifice of phreatomagmatic origin. Its pyroclastic materials extend mostly northwards, although some are also found south of the eruption centre. The only eruptive phase detected was phreatomagmatic with several intense stages that deposited a series of pyroclastic materials that are uncommon in the La Garrotxa.

Figure 84. Schematic geological map of El Cairat

66

3. Fitxes.en baixa

4/6/12

07:16

Pรกgina 67

Pyroclastic breccia Description A detailed analysis of these volcanic deposits reveals black juvenile fragments with little vesiculation, mixed with lithics of diverse composition. The most plentiful lithics are the red clays and conglomerates originating from the Bellmunt formation, the bluish marls from the Banyoles formation and the sandstones, silts and marls from the Bracons formation, all of which are local Eocene sedimentary deposits.

The sequence of volcanic materials rests on brown silt and clay, which appear next to the track by a small spring. Above these layers lies a fragmentary volcanic deposit noteworthy for its diverse granulometry, with clasts of a wide range of sizes (from millimetres to metres across).

This deposit is 10-m thick here and has no clearly visible stratification, although alternating, various-sized fragments differentiate a series of layers of irregular thickness. These layers slope gently northwards and are affected by normal faults.

Interpretation Instantaneous and constant remobilisation of the pyroclasts building up at the top of the volcano occurred. During its movement on the northern flank, this avalanche of fragmentary materials was channelled into a gully, where heavy erosion swept away part of the sediments lying on the stream-bed.

The phreatomagmatic eruption of El Cairat ejected mostly pyroclastic breccias, although there is evidence of some more violent pulses that generated pyroclastic surges. The location of the eruption centre on a ridge-top with steep slopes on all sides permitted the build-up of volcanic materials: the ejected pyroclasts slid down the mountainside to a more stable area, where they were deposited.

67

3 l La Garrotxa Volcanic Zone

Figure 85. Barranc quarry

3. Fitxes.en baixa

3

4/6/12

07:16

Página 68

Sant Joan les Fonts: massive materials Point of interest l volcanic materials at El Boscarró, El Molí Fondo and Fontfreda Activity type l Effusive Access time on foot l 30 minutes

Location and access To reach Sant Joan les Fonts from Olot, take the GI-522 towards La Canya. From Girona, leave the A-26 dual-carriageway just after the Castellfollit viaduct and tunnels and follow signs to Sant Joan les Fonts. Park in the main square, from where the sites can be reached on foot along Natural Park Itinerary 16 (Fig. 86) .

These three sites in Sant Joan les Fonts are all well worth visiting. El Boscarró lies on the right bank of the river Bianya and was exposed by the workings of a basalt quarry abandoned early in the twentieth century. Along the same river lies the abandoned quarry of Fontfreda. Finally, on the left bank of the Fluvià, at Molí Fondo, river erosion has revealed a sequence of lava flows.

3 l La Garrotxa Volcanic Zone

Sant Joan les Fonts The river Bianya flows into the river Fluvià at Sant Joan les Fonts.

Erosion by these rivers has uncovered the superposition of three lava flows that partially occupy the former riverbeds. Quarrying for basalt in the early twentieth century has revealed the interaction between the lava flows and their internal structures and also allows us to reconstruct the geological history of the site.

Figure 86. Schematic geological map of Sant Joan les Fonts

68

3. Fitxes.en baixa

4/6/12

07:16

Página 69

Superposition of lava flows Description El Boscarró This site provides excellent views of different types of jointing in the last of the three lava flows that were channelled along the Fluvià Valley. Five layers can be distinguished: the lowest exhibits columnar jointing with five- and six-sided columns, 20–40 cm in diameter and 2–3-m high; the second and fourth layers have slab jointing and are separated by a third layer in which the massive material shows very few cooling cracks; finally, the fifth and uppermost layer, just below soil level, has been more altered than the others due to its proximity to the surface and consequently exhibits clear spheroidal structures.

On the other side of the quarry, we can see where the river Bianya has been channelled along the point of contact between the volcanic materials and reddish Eocene sedimentary materials.

3

2

1

Figure 87. Feature Molí Fondo

and basalt pebbles in a silt matrix (2) .

Finally, at the top the third Boscarró lava flow is visible (3) .

The cliffs at Fontfreda The visible layers here correspond to the third lava flow that we visited at El Boscarró. The lowest layer has obvious columnar jointing with columns over 3m high and is crowned by an area of lenticular jointing. Unlike at El Boscarró, a clear transition from one type of jointing to the next is visible.

3 l La Garrotxa Volcanic Zone

Molí Fondo The dam was built on top of the first lava flow, which lies on the bed of the Fluvià. To the right a certain degree of columnar jointing can be seen in the basalt, which is blue-grey in colour. If you wander downstream along the river bank, you walk on slabs that represent the base level of the second lava flow (1) . In parts, the rough cinder base protrudes. On the nearby cliff, you can see the rest of the lava flow exhibiting columnar jointing. Just above lies a layer of sediment consisting of sandstone

69

3. Fitxes.en baixa

4/6/12

07:16

PĂĄgina 70

Interpretation The first lava flow issuing from the volcanoes of the Batet plateau flowed down and into the former bed of the FluviĂ , filling in part of the river basin.

Figure 88

The erosive activity of the river gouged out a new riverbed from this lava flow and deposited sediments on top.

3 l La Garrotxa Volcanic Zone

Figure 89

Thousands of years later, the riverbed was occupied by a second lava flow, whose origin is still unclear.

Figure 90

70

3. Fitxes.en baixa

4/6/12

07:16

PĂĄgina 71

Over time, the river deposited more sedimentary materials (silt, sand and pebbles) on top of the second lava flow and formed a river terrace.

Figure 91

About 133,000 years ago, a third lava flow covered these latest alluvial sediments. This final lava flow originated from La Garrinada and stopped just past the town of Sant Joan les Fonts.

Figure 92

Schematic diagram of MolĂ­ Fondo today.

3 l La Garrotxa Volcanic Zone

Figure 93

71

3. Fitxes.en baixa

4

4/6/12

07:16

PĂĄgina 72

Montsacopa: cone morphology Point of interest l Volcanic crater Activity type l Strombolian Access time on foot l 10 minutes

Location and access La Garrotxa Volcanic Zone Natural Park Itinerary 17 starts at the Volcano Museum, crosses the town and climbs to the top of the volcano (Fig. 94). To shorten the walk, you can park at the cemetery at the base of the cone near an abandoned quarry and then walk up the steps to the crater.

Montsacopa, one of the four volcanoes within Olot itself, lies in the middle of the city between La Garrinada to the northeast and Montolivet to the south-west.

On top stands the chapel of Sant Francesc, built in the nineteenth century, and two watchtowers.

3 l La Garrotxa Volcanic Zone

View from Montsacopa This volcano consists of a single, regularshaped cinder cone. A walk around the crater provides excellent views of the volcanoes of Montolivet, Bisaroques and the three craters of La Garrinada. The first crater, part of a tuff ring that originated during a phreatomagmatic phase, is visible at its base. This tuff ring is almost completely covered by a cinder cone constructed during the subsequent Strombolian phases, which also gave rise to the other two craters visible on top of the volcano, on its southern and northern slopes.

Montolivet lies to the south-west and consists of a cinder cone abutting onto the ridge of La Pinya; its crater opens towards the north-east.

To the south-east on the northern slope of the Batet plateau stands Bisaroques and its horseshoe-shaped crater. Judging by the deposits found here, a number of phreatomagmatic phases occurred during its eruption. However, its cinder cone was formed during a subsequent Strombolian phase, but was then partially destroyed in the final stages of the eruption by a small lava flow that flowed northwards towards the river FluviĂ .

Figure 94. Schematic geological map of the four Olot volcanoes

The volcanoes of Montsacopa, Montolivet and La Garrinada are positioned along a single fracture through which the magma penetrated on its ascent to the surface.

72

3. Fitxes.en baixa

4/6/12

07:16

PĂĄgina 73

Morphology of the cinder cone Description tion. In the quarry next to the cemetery the different layers formed during the eruption have been exposed. Most consist of block- and lapilli-sized fragments, with the occasional encrusted bomb. These are highly vesicular juvenile pyroclasts. However, at the top of the sequence of materials, despite consisting of solidified magma, some of the deposits exhibit incipient vesiculation and are mostly ash-sized.

The crater of Montsacopa is circular, about 120 m in diameter and 12-m deep; its cone has sloping flanks and stands 94 m above the surrounding land. The bottom of the crater is flat and is currently cultivated.

On the southern and south-eastern flanks of the cone there are a number of abandoned quarries, exploited in the sixteenth century for pyroclastic deposits that were mostly used in construc-

Figure 95. Montsacopa

lar in the upper layers of the sequence indicates the existence of phreatomagmatic pulses, which destroyed part of the lava flow and created fragments that were deposited as lithics in the pyroclastic surge.

The Strombolian phase eventually built the cinder cone and the lack of a final effusive phase with the emission of a lava flow ensured that the crater's circular shape was preserved.

There were at least two eruptive phases one effusive, the other explosive - during the eruption of Montsacopa. During the first phase, a lava flow ran to the foot of the nearby ridge of Sant ValentĂ­ and a large section with lenticular jointing is visible next to Olot football club (although the proposed itinerary does not go there).

The second phase was mostly Strombolian, although the presence of fragments that are not particularly vesicu-

73

3 l La Garrotxa Volcanic Zone

Interpretation

3. Fitxes.en baixa

5

4/6/12

07:16

Página 74

Croscat: cinder cone Point of interest l Quarry with volcanic Activity type l Strombolian Access time on foot l 20 minutes

deposits

Location and access To reach Croscat, take the GI-524 towards Santa Pau and park in the Àrea de Santa Margarida car park (on the right) 7 km after leaving Olot. Itinerary 15 to Croscat and the Natural Park information centre at Can Passavent starts here (Fig. 96) .

Croscat lies halfway between Olot and the village of Santa Pau in a relatively flat area bounded to the south by the CorbFinestres ridge, to the north-east by the mountain of Sant Julià del Mont and to the north by the Batet basaltic plateau.

The abandoned quarry on the northern flank of the volcano is a site of exceptional interest that reveals the internal structure of a cinder cone.

3 l La Garrotxa Volcanic Zone

Croscat The top of Croscat stands 160 m above the surrounding land (it is the tallest volcano in the Iberian Peninsula ) and its base measures 950 m in diameter. The symmetry of its conical-shaped cinder cone is distorted by a horseshoeshaped crater on its western flank.

Croscat erupted in three phases, the first two Strombolian and the last effusive.

The second Strombolian phase built the cone and ejected pyroclasts that covered the nearby volcanoes of Santa Margarida and Puig de Martinyà. The effusive phase generated a basanite lava flow that destroyed the symmetry of the edifice and formed a horseshoe-shaped crater as it ran west for 6 km. The beechwood known as La Fageda d’en

Figure 96. Schematic geological map of Croscat

Jordà stands on this rough lava flow, which is dotted by numerous blisters. Dating of the ejected materials at La Pomereda gives an age of

74

11,500 (±1,500 years) and Croscat is thus the most recent manifestation of volcanic activity in the whole Catalan volcanic field.

3. Fitxes.en baixa

4/6/12

07:16

Pรกgina 75

Structure of the cinder cone Description cular juvenile fragments that are mainly lapilli-sized (Fig. 97). The gradient of these layers increase as you move from the centre to the outside of the cone. At the base of the sequence, where bombs are more abundant, different layers alternate.

The materials are mostly dark grey or black, although in the area closest to the centre of the edifice they are reddish ochre (1).

In the lowest part of the former quarry, there is a layer of red welded scoria (2).

The quarry in Croscat was worked from the late 1950s to the early 1990s and today provides wonderful views of a vast surface area of pyroclastic materials, approximately 150 x 500 m. The terracing on the right of the open face dates from when the quarry was active and has helped stabilise the deposits.

On the opposite side and in the middle of the deposits, however, landslips are more frequent.

It is easy to spot the different layers of scoria, made up of irregular, highly vesi-

1

2

Figure 97. Croscat volcanic deposits

ejected. Finally, a lava flow was emitted from the eastern flank of the cone and ran westwards towards Olot.

The different colours of the pyroclasts are due mainly to thermal alteration.

Hot gases released in the later stages of the eruption caused oxidation around the chimney, the hottest part of the volcano; the black-grey of the pyroclasts thus changed to red-ochre.

The first phase of the eruption was Strombolian and explosive and built the welded scoria deposits recognisable near the vent at the base of the sequence. This activity then became more explosive and built the cinder cone. Initially, the pyroclasts fell in practically horizontal layers, but with the gradual growth of the cone the gradient of the deposits began to increase. Sporadically, when the release of gases was less intense, bombs were

75

3 l La Garrotxa Volcanic Zone

Interpretation

3. Fitxes.en baixa

6

4/6/12

07:16

Página 76

El Turó de la Pomereda: an eruption sequence Point of interest l Volcanic materials Activity type l Strombolian and effusive Access time on foot l 30 minutes

Location and access To reach Turó de la Pomereda, take Natural Park itinerary 1 from the Àrea de Santa Margarida car park towards La Fageda d’en Jordà, which skirts the north side of Croscat. Where the route forks at Can Pelat, take the right-hand track towards La Canova. The abandoned quarry is visible about 20 m along on the left (Fig. 96).

Next to Can Genís in the Massandell plain lies another former quarry, from which volcanic materials emitted by the volcano of Turó de la Pomereda were extracted. The quarry walls reveal a sequence of pyroclastic deposits covered by a lava flow.

1

3 l La Garrotxa Volcanic Zone

Figure 98. La Pomereda volcanic deposits

Turó de la Pomereda Puig Astrol and lies on a fault that supposedly runs north-west to south-east.

Its lava flow has been dated at 11,500 years old and is thus the most recent manifestation of volcanic activity in the Catalan volcanic field.

El Turó de la Pomereda lies at the foot of Croscat and this slightly raised area is one of the volcano’s five small adventive or parasitic cones. Prior to the quarrying of the materials from its centre, this small cone was tumulus-like in shape.

A map shows that La Pomereda is aligned with Santa Margarida, Croscat and

76

3. Fitxes.en baixa

4/6/12

07:16

Pรกgina 77

Massive and fragmentary materials Description On top of these fragments lies a 3-mthick layer of dark grey scoria (1). Here, the clasts are mostly lapilli-sized (2), although there are some larger fragments towards the top. In the final 30 cm the lapilli fragments are welded.

Finally, a massive deposit (3) appears, somewhat channel-shaped and about 2-m thick in its middle. The base of this small flow is scoria and its internal structure displays columnar jointing with poorly defined columns.

The south-western part of the quarry holds the best outcrop of volcanic agglomerate in the Catalan volcanic field (Fig. 98) . This deposit consists of highly vesicular juvenile fragments, mostly blocks (bombs) with a variable percentage of lapilli. This scoria is welded and continuous towards the northwest; it is largely dark grey to black, although in parts reddish fragments appear.

2

3

The next phase was also typically Strombolian and gave rise to a deposit of lapilli-sized scoria and ash. The thinness of these deposits, which here are found very near the vent, indicates that this second phase was short lived. The final phase was effusive and emitted a small lava flow that partially covered the top of the underlying pyroclasts. The transfer of heat from this layer to the lapilli below caused the pyroclasts to weld together.

The eruptions of both La Pomereda and Croscat began with Strombolian phases that were not very explosive. In the course of the eruption, scoria blocks in a semi-molten state were ejected and welded together as they fell close to the vent.

The initial phases of the eruptions at La Pomereda and Croscat both began in this fashion and the deposits that formed constitute a volcanic agglomerate.

77

3 l La Garrotxa Volcanic Zone

Interpretation

3. Fitxes.en baixa

7

4/6/12

07:16

PĂĄgina 78

Santa Margarida: pyroclastic deposits Point of interest l Volcanic deposits along the road Activity type l Phreatomagmatic and Strombolian Access time on foot l 15 minutes

to Mas el Cros

Location and access leads steeply up to the crater.

However, the best way to see the volcanic materials is to ignore this right turn and continue towards Mas el Cros and the eastern sector of the volcano.

On the right-hand side of the road pyroclasts appear immediately, although the best sequence of deposits is found 400 metres further on (Fig. 99) .

Santa Margarida, one of the bestknown volcanoes in La Garrotxa, lies at the foot of a spur jutting south from the Lleixeres ridge. Park in the Àrea de Santa Margarida car-park at the 8-km point on the GI-524 (Olot to Santa Pau) and walk up the volcano.

Itinerary 4 leading to the crater of Santa Margarida starts in the car park and heads towards Santa Pau; after just 200 m, turn right along a track that

3 l La Garrotxa Volcanic Zone

Santa Margarida This phreatomagmatic volcano, whose circular crater is about 350 m in diameter and 70-m deep, stands on Eocene marls.

Its cone is not formed entirely of volcanic materials, since the crater is imbedded in the pre-volcanic stratum. In the middle of the crater stands a Romanesque chapel, which has been heavily restored in modern times.

The initial Strombolian phase during the eruption of Santa Margarida was rather uneventful and was quickly followed by phreatomagmatic activity, which varied greatly in intensity and at times was barely explosive at all. The vegetation covering the volcanic materials somewhat hides the

Figure 99. Schematic geological map of Santa Margarida

small pyroclastic flow in the south-eastern sector of the volcano.

78

3. Fitxes.en baixa

4/6/12

07:16

Pรกgina 79

Pyroclastic fall and surge deposits Description sandstone, while the predominate fragments in the latter are black and slightly rounded in shape with little vesiculation (2) . The sequence is crowned by a deposit that looks very like the previous layer, only without any lithics, and consists of a fine-grained scoria deposit with no stratification (3) .

Three types of volcanic materials appear along the road to Mas el Cros, one succeeding another from right to left as a result of the inclination of the layers (Fig. 100) . On top of the silty pre-volcanic soil, sits a layer of compacted ash.

Then, black juvenile fragments appear, containing quite rounded, reddishbrown lithics (1) . Finally, there is a layer of lithic and juvenile lapilli-sized fragments; the former consist mostly of red

3 2

1

Figure 100. Outcrop on road to Mas el Cros

Interpretation

79

3 l La Garrotxa Volcanic Zone

is practically Strombolian, although the presence of lithics indicates there was some degree of phreatomagmatic activity. The dispersion of this material is radial, from the vent outwards.

Finally, the scoria at the top of the deposit corresponds to a Strombolian fall deposit that originated from Croscat, a kilometre away. From the absence of any paleosol between these different layers we can deduce that the eruptions of Croscat and Santa Margarida took place simultaneously.

The volcanic map of this sector of the Volcanic Zone reveals that not all the deposits found here originated from Santa Margarida.

The base layers correspond to pyroclastic surges expelled during the phreatomagmatic eruption. They were mainly dispersed eastwards by the interaction between the magma and water in the aquifer in the Bellmunt formation (Eocene). The middle layer also consists of deposits from Santa Margarida, but from a far less violent subsequent eruption. This fall deposit

3. Fitxes.en baixa

8

4/6/12

07:16

Página 80

Can Tià: eruption sequence Point of interest l Can Tià volcanic deposits Activity type l Phreatomagmatic and Strombolian Access time on foot l 60 minutes

Location and access Santa Pau road) and take Natural Park Itinerary 5, which leads straight to the volcano in about an hour (Fig. 101). On the way up, you pass over Eocene sediments, initially reddish and then browner in the uppermost strata, corresponding, respectively, to the Bellmunt and Folgueroles formations.

The volcano of Can Tià takes its name from a farm close to the top of the CorbLleixeres ridge. Its vent stands is at the head of the valley of Sant Iscle de Colltort, where many of its ejected materials are visible. However, most of our observations will be made in a small abandoned quarry right next to the house of Can Tià.

To reach Can Tià, park opposite Can Xel at the 5-km point on the GI-524 (Olot to

3 l La Garrotxa Volcanic Zone

The volcano This volcano lies next to those of Fontpobra and La Tuta and has a maartype edifice with a circular, 270-m-diameter crater. Today, domestic animals pasture this flatbottomed depression, around 20-m deep. The low cone is highest to the south. The Can Tià eruption had no effusive phases and therefore all the ejected materials are pyroclastic. Most are phreatomagmatic, but some are the product of a Strombolian eruption.

The largest pyroclastic deposit has filled part of the valley of Sant Iscle, where there is a volcanic tuff, possibly resulting from a pyroclastic flow.

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

80

3. Fitxes.en baixa

4/6/12

07:16

Página 81

The eruption sequence Description made up of lapilli-sized fragments and blocks and is notably vesicular. A few lithics measuring up to 10 cm in diameter are present (1) .

On top of the scoria lies a series of alternating layers of breccia and ash. Here, the juvenile fragments display incipient vesiculation and are slightly rounded. The most abundant lithic fragments in the first layers are brown and correspond to the Eocene sandstones of the Folgueroles formation.

The lithics in the breccia and ash in the upper part are mostly sandstone too, but are reddish in colour and originate from the Bellmunt formation from the same geological epoch (2).

Finally, there is a very compact tuff deposit that can be followed downhill for more than a kilometre (3) .

The sequence of deposits in the small Can Tià quarry (Fig. 102) are around 10m thick and contains two sets of fragmentary materials.

At the base (around 6–m thick) there is a black scoria deposit with no layering

3 2 1

Figure 102. The quarry at Can Tià

Interpretation

Figure 103. Eruption sequence at Can Tià

81

3 l La Garrotxa Volcanic Zone

The eruption of Can Tià began with a Strombolian phase (Fig. 103a) that formed a cinder cone built by scoria fall deposits.

When the pressure in the vent dropped, the magma interacted with the water in the Folgueroles aquifer, giving rise to a more violent phreatomagmatic eruption (Fig. 103b) with breccia and pyroclastic surges. These explosions in the vent in this second phase destroyed the cinder cone and the construction of the maar began.

The deepening of the area of watermagma interaction meant that water from the Bellmunt aquifer could also intervene in the phreatomagmatic activity (Fig. 103c) . In this phase of the eruption, a pyroclastic flow was formed from different surge- and breccia-type flows.

3. Fitxes.en baixa

9

4/6/12

07:16

PĂĄgina 82

Els Arcs Valley: pyroclastic flow Point of interest l Volcanic deposits Activity type l Phreatomagmatic Access time on foot l 60 minutes

at Mas el Carrer

Location and access To reach these deposits, park in Santa Pau and follow Natural Park Itinerary 7 towards Els Arcs Valley. At Mas el Carrer, continue for about 10 m along a track to the right that leads down to the streambed (Fig. 104).

The small valley of Els Arcs lies on the northern slope of the Finestres ridge. The bottom of the valley contains an interrupted series of outcrops of volcanic materials that can be viewed in the streambed near Mas el Carrer.

3 l La Garrotxa Volcanic Zone

Sant Jordi Only the pyroclastic deposits emitted by this volcano are known, since its vent has not yet been located. It may lie on the north-south fracture that has helped mould the shape of this valley, although it is clear that the crater lies above 475 m, the upper limit of the pyroclastic deposits. Numerous alluvial and piedmont sediments washed down from the northern slopes of the Finestres ridge have built up along the upper part of the valley and have probably buried the volcano’s edifice.

Sant Jordi had a number of different activity phases, of which the last generated a deposit at least 1.7-km long with a maximum width of 350 m; it is thickest in its upper part (about 7.5 m). At the confluence of the Arcs valley with the river Ser, this deposit disappears under the lava flows originating from other volcanoes in the Santa Pau area.

Figure 104. Schematic geological map of Els Arcs Valley

82

3. Fitxes.en baixa

4/6/12

07:16

Pรกgina 83

The pyroclastic flow Description Water erosion in Els Arcs valley has revealed a complete sequence of materials ejected by Sant Jordi. In the upper part of the valley, the pyroclastic materials rest on gravel with sandstone pebbles and a sand and silt matrix.

Three fragmentary deposits in 12 discernible layers are visible (Fig. 105) : at the base lies a first deposit comprising two very compacted layers, each measuring 5 cm, with ash-sized juvenile fragments and lithics (red sandstone from the Bellmunt formation). The upper layer has coarser, lapilli-sized clasts (1) underlying a deposit of scoria including some ash with juvenile components and the same red sandstone lithics (2) . The final deposit has four layers with a total thickness of 7.5 m, with two layers at the base, each 5-cm thick, consisting of lapilli-sized clasts and ash with juvenile components and red sedimentary lithics. The most noticeable layers of this sequence are the topmost two, twoand four-metres thick, respectively. Both are tuffs with juvenile fragments and lithics, some over 10 cm in diameter, and are embedded in a matrix of red ash (3) .

The base of the final layer is erosive and is flat-topped.

3

1

2

Figure 105. Outcrop El Carrer

Interpretation

83

3 l La Garrotxa Volcanic Zone

As the eruption was ending, the phreatomagmatic phase reactivated, generating a pyroclastic flow that ran into the former course of the valley. The two layers of tuff correspond to the two pulses that took place while the pyroclastic flow was being formed. The rapid emplacement of this flow meant that at the front there was a significant ingestion of cold air. This air heated up immediately due to the high temperature of the flow and caused a series of explosions, which created the pyroclastic surges that gave rise to the layers forming the base of the third deposit.

At least three phases occurred during the eruption of this volcano, of which the phreatomagmatic events were the most important.

In the first, the water-magma interaction led to pyroclastic surges that formed the base deposit. Then, the phreatomagmatic activity was interrupted by a Strombolian phase, which emitted the scoria. In this phase, however, small amounts of water entered the vent and caused small pyroclastic flows.

3. Fitxes.en baixa

10

4/6/12

07:16

Página 84

Location and morphology of the volcanic cones as seen from Puig Rodó Point of interest l View from the viewpoint Activity type l Strombolian and effusive Access time on foot l 20 minutes

Location and access Puig Rodó (909 m) stands at the western end of the Corb ridge where, from the Xenacs Recreational Area, there are excellent views of La Garrotxa Volcanic Zone Natural Park, the main Pyrenees, the prePyrenees, most of the Olot Trough and the Bas Valley (Fig. 106) .

From Olot, take the C-152 through the town of Les Preses. After about 300 m, turn left along a road which climbs steeply in 5 km to

the car park in the recreational area. From here, walk along the signposted path to the Puig Rodó viewpoint. The road is not viable for coaches and is closed to all vehicles on weekdays, although an access permit can be obtained from Les Preses Town Council.

Natural Park Itineraries 10 and 11, starting in Les Preses, also climb up to Xenacs.

3

Racó volcano

Montolivet volcano

6 8 1

8

3 l La Garrotxa Volcanic Zone

Figure 106. View from Puig Rodo at Xenacs

The Bas valley From Puig Rodó, we can trace the route of the Croscat lava flow across the landscape by the woodland that covers it, mostly the beechwood known as La Fageda d'en Jordà (2) .

The road that leads to Xenacs offers excellent views of the Bas valley (1) , an agricultural plain that was once a lake.

The lava flow emitted by Croscat ran down towards Olot and dammed the river Fluvià, giving rise to what is known as a barrage lake. Over time, the lake gradually began to silt up and in eighteenth century the marshy plain and lagoons were drained for cultivation.

84

3. Fitxes.en baixa

4/6/12

07:17

Página 85

The main relief features fected by faults during the Alpine orogeny.

On a clear day from Puig Rodó you can see most of La Garrotxa, as well as parts of El Ripollès to the west and El Pla de l’Estany and L’Alt Empordà to the east. To the north we can make out:

c. Mountains of La Serralada Transversal: lying the closest to Puig Rodó, these peaks include the Corb ridge (5) and are part of this transversal (N-S running) system, also made up entirely of Eocene rocks. These mountains consist of a series of raised and sunken blocks, the product of a system of normal faults, the highest of which are the peaks of Collsacabra and Puigsacalm (6).The depression at the foot of Puig Rodó to the north corresponds to the Olot Trough (7).

a. Axial Pyrenees (3): the backdrop to the view north consists of the main ridge of the Pyrenees, made up of ancient Palaeozoic rocks, whose highest peaks are 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 Axial Pyrenees and mostly consist of Eocene rocks that were intensely folded and af-

Montsacopa volcano

Garrinada volcano

Bisaroques volcano

Cabrioler volcanoes

Puig Astrol Pujalós volcano volcano

4

8

9 8

8

Puig de la Garça volcano

8

Croscat Puig Jordà volcano volcano

8

8

8

8

7 8

2

8

Puig de la Santa Costa Margarida volcano volcano

8

8

5

can see almost the whole of the northern sector of the volcanic zone, including 14 (8) of the park’s 40 volcanoes. A characteristic feature of the volcanic cones is their shape and the form of their craters, either circular or horseshoe-shaped. All are covered in woodland and almost always stand out above the arable land that reaches right up to their bases. Of note is the Batet plateau (9) to the north-east, formed by the build-up of successive lava flows from the region’s oldest volcanoes, most of which have been eroded away.

The depression bordered by L’Alta Garrotxa to the north, the Corb ridge to the south, Sant Julià del Mont to the east and La Collsacabra and Puigsacalm to the west is known as the Olot Trough.

Behind and to the sides of this depression of tectonic origin stand most of the volcanoes in La Garrotxa Volcanic Zone.

The valleys we can see are all U-shaped because they were filled in by lava flows emitted during eruptions or by sediments that built up behind the barrage lakes formed by lava flows. From Puig Redon we

85

3 l La Garrotxa Volcanic Zone

The Olot Trough

3. Fitxes.en baixa

11

4/6/12

07:17

Página 86

El Clot de l’Omera maar Point of interest l View of the crater Activity type l Phreatomagmatic Access time on foot l 5 minutes

Location and access Pallonera (Fig. 107) . Park in El Pla de Sant Joan and walk along the track as far as a clearing, from where there are good views over El Clot de l’Omera. If you continue along the track, a stream just before the farm has interesting outcrops of volcanic materials.

A farm, Mas de la Pallonera, lies inside El Clot de l’Omera (clot = depression in Catalan), a circular depression on the left bank of the river Llémena between the village of Llorà and El Pla de Sant Joan.

To reach the area, from Girona take the GI-531 along the Llémena valley and at the 15-km point, just before El Pla de Sant Joan, a track runs to Mas de la

3 l La Garrotxa Volcanic Zone

El Clot de l’Omera This small volcanic edifice is partially covered by a lava flow originating from another volcano, Puig de la Banya del Boc (Fig. 107) . This latter volcano, encrusted into the south-facing slope of La Serra de Boratuna, lies on the Llorà fault, where Tertiary sedimentary materials come into contact with Palaeozoic metamorphic materials. During the formation of Puig de la Banya del Boc a series of different eruption phases occurred. Initially, the eruption was phreatomagmatic, then Strombolian and, finally, effusive. The phreatomagmatic phase emitted the pyroclasts that can be seen along the banks of a stream, Torrent de Bosquerós, and the river Llémena. As the same time as these phreatomagmatic phases, El Clot de l’Omera erupted. Subsequently, the Strombolian phase of Puig de la Banya del Boc began and built a cinder cone of lapilli and bombs with an elliptical crater.

Finally, the effusive phase gave rise to three lava flows, two of which flowed into the former streambeds of Torrent de Bosquerós (to the south-west) and Can Pere Boé (eastwards). A third lava flow ran south as far as the river Llémena and

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

created the agricultural plain today known as El Pla de Sant Joan. Next to this plain stands El Clot de l’Omera, separated from Puig de la Banya del Boc by Els Rasos de Llorà, a small hill of metamorphic rock.

86

3. Fitxes.en baixa

4/6/12

07:17

Página 87

Maar Description The most notable feature of this volcano is the crater in its single volcanic edifice, which abuts the southern slopes of Els Rasos de Llorà; on its inner walls metamorphic materials appear underneath the pyroclastic deposits.

Thus, the crater which lies below the pre-eruption land surface is flat-bottomed, and measures approximately 500 m in diameter and is 20-m deep (Fig. 108) . Today, a drainage channel prevents the crater from flooding.

The cone is partly covered by a lava flow and is difficult to see. However, there is a sequence of pyroclastic de-

posits around the crater that increase in thickness from north to south. In a small streambed behind Mas de la Pallonera, a sequence of pyroclastic materials, 10-m thick in parts and consisting of a succession of breccia and ash deposits, is visible. They are very heterogeneous in composition due to the size and type of lithic fragments.

These lithics are very angular in general and originate from metamorphic rocks (e.g. schist and marble). A few basalt fragments are mixed in and mostly show little vesiculation.

Interpretation The edifice of El Clot de l’Omera consists of a maar that was formed during a single-phased phreatomagmatic eruption. The ejecta extend mainly southwards owing to the steep slopes of Els Rasos de Llorà to the north of the volcano, although this asymmetry could have been caused by the inclination of the fracture from which the magma issued. The flat bottom of the crater is due to the blocks of pyroclastic materials that slid down from the crater rim. One of these blocks did not stabilise completely and evidence of its movement can be seen in the scar of the circular fracture in the stream behind Mas de la Pallonera.

Alternating layers of ash deposits and breccia are very visible due to the series of different pulses that occurred during the eruption. Some of the deposits contain a high percentage of lithics and can thus be attributed to phreatomagmatic pulses. The fact that most of the lithics are metamorphic in origin suggests that a large aquifer existed in the substrate of the metamorphic rocks.

Puig de la Banya del Boc

El Clot de l’Omera

87

3 l La Garrotxa Volcanic Zone

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

3. Fitxes.en baixa

12

4/6/12

07:17

Página 88

Puig d’Adri: pyroclastic flow Point of interest l Volcanic materials Activity type l Phreatomagmatic Access time on foot l 5 minutes

deposited at Font de la Torre

Location and access the village of Canet d’Adri. The street on the left about 300 m after the centre of Canet d’Adri leads to Mas de la Torre. Park next to this farm and pick up the track to the spring in the bed of Riera de Rocacorba.

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

3 l La Garrotxa Volcanic Zone

Puig d’Adri This volcano stands at the foot of the mountain of Rocacorba, between the village of Canet d’Adri and Adri. This is the easternmost of the volcanoes in the Llémena Valley and lies only 7 km from the city of Girona.

Three superimposed volcanic edifices (Fig. 109) , which were built during different eruption phases, can be identified. A cinder cone reaching 408 m is the most remarkable of the three and is visible from just behind the church in Canet d’Adri.

The products of this volcano’s phreatomagmatic activity are numerous and varied. They are well dispersed and there are deposits up to 5 km from the vent.

An emission of lava in the last stage of the eruption generated a lava flow that flowed 11 km to Domeny on the outskirts of Girona.

Figure 109. Geological diagram of Puig d’Adri

88

3. Fitxes.en baixa

4/6/12

07:17

PĂĄgina 89

The pyroclastic flow Description Compacted fragmentary materials (volcanic tuff) are visible at Font de la Torre. This deposit contains juvenile pyroclasts and lithics (various millimetres in diameter), surrounded by a fine reddish-brown matrix. The black juvenile fragments are of basaltic composition with little vesiculation. The most plentiful lithics are red sandstone, although blue marls and pale grey calcareous rock are also present.

Although the deposit is fairly uniform in composition, different layers are visible and have been eroded - more efficiently at the boundaries between layers - and give the outcrop a terraced effect.

This tuff appears along Riera de Canet for about 3 km downstream from the spring and in places is 20-m thick. On top of this fragmentary deposit lies a lava flow, which can be seen clearly on the left bank of Torrent de Rocacorba and on the path that leads there.

Figure 110. Font de la Torre

Erosion by the streams (Riera de Canet, Torrent de Rocacorba and Torrent de Rissec) has created a series of deep pools that are unique in the Catalan volcanic field.

Interpretation

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

89

3 l La Garrotxa Volcanic Zone

racted with the magma and the resulting explosions ejected a dense pyroclastic flow that was channelled down the original valley of Riera de Canet. However, successive pulses in this phase generated a series of sub-flows that gave rise to the incipient layers visible in this deposit.

Finally, a lava flow covered the deposited pyroclastic materials. Subsequently, the water in the streams has eroded these volcanic products and exposed the sequence of deposits (Fig. 111).

The presence of abundant lithic fragments, along with a number of palaeomagnetic studies that have determined an emplacement temperature for these materials in excess of 550°C, are proof that this deposit is the product of the phreatomagmatic eruption of Puig d'Adri.

Furthermore, the elongated shape and channel-shaped cross-section suggest that a pyroclastic flow swept down and filled in the former river valley.

Thus, during the first phreatomagmatic phase an important amount of water inte-

3. Fitxes.en baixa

13

4/6/12

07:17

Página 90

Puig d’Adri: pyroclastic surges Point of interest l Volcanic materials Activity type l Phreatomagmatic Access time on foot l 15 minutes

in the Toscà cork-oak wood

Location and access In the village, take the road towards the hamlet of Collsacarrera. About 400 m before Collsacarrera, park where a track leads off to the right to Can Toscà. Walk along the track for about 25 m to where, just behind the bank on your left, the outcrops begin.

On the south-eastern flanks of Puig d’Adri stands a cork-oak wood known as La Sureda d’en Toscà, which exhibits fine examples of pyroclastic surges and breccia deposits. Access from Girona is along the GI-531 to Sant Gregori; 3 km after this town, turn right on the GIV5313 to Canet d’Adri (Fig. 112) .

3 l La Garrotxa Volcanic Zone

The eruption This eruption had five phases. The first was highly explosive and phreatomagmatic and a great deal of breccia and cinder were deposited; during this phase the tuff ring was formed - edifice 1 (Fig. 112) . Two superimposed cinder cones edifices 2 and 3 - resulting from subsequent Strombolian phases partially cover this first construction. The tuff ring crater is 850 m in diameter and the materials that form the cone appear along the road from Canet d'Adri to Collsacarrera. The outcrop in La Sureda d’en Toscà contains the best examples of these phreatomagmatic deposits.

Figure 112. Schematic geological map of Puig d’Adri

90

3. Fitxes.en baixa

4/6/12

07:17

Página 91

The pyroclastic deposits of La Sureda d’en Toscà Description On close scrutiny and within just 20 metres of each other materials deposited in three groups in various layers can be observed (Fig. 113) .

At the base of the sequence lies a scoria deposit formed almost entirely of black, highly vesicular lapilli-sized juvenile fragments (1) . There are no layers within this deposit, although at the top some angular lithics, mostly of red sandstone, appear (of various centimetres in diameter).

Covering the scoria are millimetric layers of ash with considerable compaction (2) , which gives these layers a certain positive relief within the outcrop.

The miniscule size of the fragments means that they cannot be identified with the naked eye. However, with the aid of a magnifying glass you can see that this ash contains a large proportion of lithic fragments of red sandstone and some marl. The marked lamination of the ash is obvious and the stratification is frequently crossed at a low angle. A few coarser layers can be distinguished between the ash layers.

3

2

1

Figure 113. La Sureda d’en Toscà pyroclastic surge

Finally, at the top there is a series of layers of pyroclastic breccia; the largest of the pyroclasts indicates that that same lithic fragments are present as in the ash (3) . These layers are thicker and the fragments are looser. There is also degree of lamination often marked by the presence of layers of ash.

Interpretation

91

3 l La Garrotxa Volcanic Zone

The topmost layers of ash were deposited during the phreatomagmatic pulses and their lamination is due to the high energy present in the flow. These materials are pyroclastic surge deposits and their compaction indicates that, when they were deposited, part of the water vapour in the flow condensed and caused them to be compacted.

The breccia crowning the sequence came from a series of less intense pulses in the phreatomagmatic phase.

Although not visible at this site, materials appear beneath the scoria that originated from the phreatomagmatic eruption. This scoria, so typical of Strombolian activity, signifies an interruption in the phreatomagmatic phase at the start of the eruption.

Scoriaceous materials are normally only found as part of a cinder cone and so their location in this outcrop, far from the vent, can only be explained by the remobilisation of the scoria by later phreatomagmatic explosions.

3. Fitxes.en baixa

14

4/6/12

07:17

PĂĄgina 92

The morphology of La Crosa de Sant Dalmai Point of interest l View from Can Guilloteres Activity type l Phreatomagmatic Access time on foot l 5 minutes

quarry

Location and access After crossing the road to Estanyol, about 1 km along on the right there is an open area from where the volcanic materials were once extracted. In the part farthest from the road you can climb a small mound (about 5-m high) formed from pyroclasts, which offers a good view of the crater of La Crosa de Sant Dalmai.

The quarry of Can Guilloteres used to extract pyroclasts from La Crosa de Sant Dalmai, a volcano situated between the settlements of Aiguaviva, Estanyol and Sant Dalmai on the border between the regions of La Selva and El Gironès. From Girona, take the road to Santa Coloma (GI-533) that goes through Aiguaviva.

3 l La Garrotxa Volcanic Zone

La Crosa de Sant Dalmai This volcano lies is on the boundary between the depression of La Selva, filled with Pliocene and Quaternary sediments, and the southern end of the Transversal mountain system, here represented by contacting granite and metamorphic Palaeozoic rocks. The eruption was predominantly phreatomagmatic, with a final Strombolian phase. The exact age of this volcano is unknown and, despite being in La Selva where the volcanic rocks are over two million years old, it is obvious from its excellent state of conservation that this volcano was constructed no more than a few hundred thousand years ago. Due to its morphology and size, La Crosa is regarded as one of the most spectacular volcanoes in Catalonia.

1

Figure 114. La Crosa de Sant Dalmai

The low height of its edifices and the fact that it lies in a relatively flat area make it difficult to see the shape of this volcano.

92

3. Fitxes.en baixa

4/6/12

07:17

Pรกgina 93

The morphology of the volcanic edifices Description pine and holm oak woodland surrounds the depression.

On its northern side, a low hill penetrates slightly inside the depression (1) and abuts against the rim of the crater, which an aerial photo (Fig. 114) reveals as being horseshoe-shaped.

The view east from the top of the hill reveals a circular depression, some 1,250 m in diameter. The bottom of this depression (about 800 m wide) is flat and lies below the original ground level.

It is currently used for crops and tree plantations. A line of hills covered in

Interpretation top of the northern edge of the maar (Fig. 115c) . The crater of this edifice is open to the south-east, possibly due to the emission of a small lava flow in the final stages of the eruption (Fig. 115d).

When the volcanic activity had ceased, the crater filled with water, forming a lake that slowly began to silt up with lacustrine and colluvial sediments (Fig. 115d) . Currently, an artificial drainage system with two channels crossing the cone keep the crater dry.

Figure 115. Eruption sequence of La Crosa de Sant Dalmai

93

3 l La Garrotxa Volcanic Zone

The morphologies described here belong to the volcanic edifices that constitute La Crosa de Sant Dalmai. Analysis of the volcanic deposits and sediments reveals a series of stages that gave rise to the current relief features.

The most important eruption of this volcano was the initial phreatomagmatic phase, which built a maartype edifice with a large crater that initially was much smaller (Fig. 115a) .

As the explosions caused by the interaction of water and magma began to take place at greater depths (Fig. 115b) , the diameter of the crater increased. The sliding of pyroclastic materials from the inside of the walls into the centre of the crater eventually made it far bigger.

When the phreatomagmatic eruption ended a Strombolian phase began that built a cinder cone on

3. Fitxes.en baixa

15

4/6/12

07:17

Página 94

Pyroclastic surge and breccia of La Crosa de Sant Dalmai Point of interest l Volcanic materials Activity type l Phreatomagmatic Access time on foot l 5 minutes

in the quarry at Can Costa

Location and access where a track heads left (Fig. 110) .

From here walk about 200 m northwards along a path through a field of hazel trees to the abandoned Can Costa quarry. The volcanic deposits are not hard to find as they are about 400-m long and 20-m high.

La Crosa de Sant Dalmai lies between the villages of Aiguaviva, Estanyol and Sant Dalmai on the border between the regions of La Selva and El Gironès.

From Girona, take the road to Santa Coloma (GI-533) through Aiguaviva. At the 10-km point on this road, just before the village of Sant Dalmai, park

3 l La Garrotxa Volcanic Zone

La Crosa phreatomagmatic deposits La Crosa's cone is made up of a sequence of pyroclastic deposits with lax dipping that spreads out radially around the crater. The cone is 203m high to the south-west (Turó de Sant Llop) and the layer of fragmentary materials is over 50-m thick. In the west, though, the cone is less than 200-m high and the deposits are 30-m thick.

The phreatomagmatic explosions ejected a mixture of magma fragments and country rock, which were distributed asymmetrically. Towards the east, the dispersion reached beyond what today is the village of Vilablareix, over 3.5 km distant, while to the west the materials were ejected only a few hundred metres. This asymmetry in the emplacement of flow materials is due to

Figure 116. Schematic geological map of Puig d'Adri

the varying competence of the subsoil (resistance of the materials to the explosions) and, for instance, the Pliocene sediments found in the eastern sector are less competent than the metamorphic rocks and grani94

te. However, it is possible that the eastward bias of the vent also led to more volcanic products being ejected in that direction.

3. Fitxes.en baixa

4/6/12

07:17

Pรกgina 95

Quarrying at Can Costa Description morphic and igneous rocks, are clearly visible.

The most plentiful lithics are granite, schist and porphyric deposits. The juvenile fragments show very little vesiculation, except for those in the scoria (layer 23), which are clearly more vesicular. The lithic fragments are angular and in some parts account for 60% of the deposit.

Up to 30 alternating layers of breccia and ash have been exposed in the quarry (Fig. 117) ) and vary in thickness from just a few centimetres to over a metre.

At the base there is a layer over a metre thick, consisting mostly of large blocks of lithic fragments measuring 10 cm across (layer 1). On top lies a further series of layers with lithic and juvenile fragments measuring various centimetres across, and then ash (layers 2 to 22). Next, comes a metre-thick layer of scoria with lapillisized fragments (layer 23). Finally, more alternating layers of breccia and ash appear that are similar to the previous ones (levels 24 to 30), with at the base a breccia layer of 10-cm fragments.

Thanks to the size of the fragments in the breccia, the difference between the juvenile clasts, which are black basalt, and the lithics originating from different meta-

Figure 117. Stratigraphic column from the Can Costa quarry

Interpretation 3. In the third stage, the aquifer was replenished and re-fed the area of interaction with the magma until there was enough water to produce a new pyroclastic surge.

During the phreatomagmatic phase of La Crosa de Sant Dalmai eruption a series of pulses occurred, each of which formed one or two layers of the sequence.

The sequence of alternating breccia and ash was determined by the availability of water in the area of interaction with the magma. Three stages in the phreatomagmatic pulses can be separated: 1. The aquifer was able to provide enough water for optimal water-magma interaction. In this first stage, a considerable amount of water vaporised, generating a pyroclastic surge that resulted in the ash deposit (Fig. 117; e.g. layer 2) .

2. Less water was available in this stage and so the water-magma ratio was lower; as a result, the explosion was less effective and generated a pyroclastic breccia deposit (Fig. 117; e.g. layer 3) .

At the end of this stage, there was almost no water left in the country rock.

95

3 l La Garrotxa Volcanic Zone

These three stages were repeated successively over short intervals of time and led to the build-up of the sequence of pyroclastic deposits exposed in this outcrop. The magma ascent was probably continuous throughout the different phases and therefore the recharging of the aquifer that interacted with the magma was rapid enough to maintain the phreatomagmatic eruption. It is likely that the scoria deposit (Fig. 117, layer 23) was the product of a Strombolian phase occurring when the replenishing of the aquifer in stage 3 was insufficient to maintain the phreatomagmatic activity.

3. Fitxes.en baixa

4/6/12

07:17

Pรกgina 96

Glossary l 1 l Volcanoes Aquifer A water-bearing permeable geological formation in which groundwater is stored and through which water can flow.

Country rock The rock surrounding the intrusion of another rock in the form of a seam, dyke, sill or pluton.

Crystal A solid substance of defined chemical composition, made up of atoms or molecules arranged in a regular and periodic pattern in a space that, in favourable conditions, may give flat surfaces known as faces.

Dome A mound-shaped protrusion with steep flanks produced by an eruption of highly viscous, gas-poor magma, gradually expelled from the vent.

Dyke A type of sheet intrusion of igneous rocks that cut discordantly across adjacent rock following existing fractures, which are generally vertical and measure tens to hundreds of metres in thickness.

Geochemistry The science that studies the abundance and distribution of the solid matter of the Earth or a celestial body, and its composition Igneous rock Rock formed by the solidification of magma in or outside the lithosphere.

Isotope One of two or more species of atoms of a chemical element that have the same atomic number (the same number of protons), but a different number of neutrons.

Joint A fracture in a rock with no relative displacement of any part, the surface of which is usually flat and differs greatly from the stratification.

Lithostatic pressure Vertical stress imposed on a layer of soil or rock by the weight of overlying material.

Metamorphic rocks Rock formed from pre-existing rock that, with no intermediate liquid stage, has been transformed mineralogically and structurally in response to changes in physiochemical conditions, temperature, pressure or shearing stresses.

Petrogenic process Any of the processes that arise during the formation of a rock.

Petrology A branch of geology that deals with the origin, history, occurrence, structure, chemical composition and classification of rocks.

Pluton A large body of intrusive igneous rock formed from magma cooling under the Earth's surface.

Sedimentary rock A type of rock formed by the accumulation of material (e.g. minerals or organic rock) on the Earth's surface within bodies of water.

Silicate mineral A mineral formed of SiO4 tetrahedra.

Sill A body of igneous rock that intrudes between older rock layers.

Texture of rock Also known as microstructure, rock texture refers to the relationship between the constituent minerals and vitreous material of an endogenous or sedimentary rock.

l 2 l Volcanism in Catalonia Alkaline rock Magmatic rock in which the sodium oxide (Na2O) and potassium oxide (K2O) combined are present in a greater percentage than the aluminium oxide (Al2O3).

Calc-alkaline magma Magma with a SiO2 content between 55% and 61% and more sodium and potassium oxide than calcium oxide.

Feldspathoid Feldspathoids are a group of tectosilicate minerals made up of SiO2, Na, K, Ca and Li that appear in place of feldspar when the magma is poor in SiO2.

Neogene-Quaternary The time period between 23 Ma and the present.

Rare earth element Rare earth elements or metals are a group of chemical elements that include the lanthanides plus scandium and yttrium.

97

l 3 l La Garrotxa Volcanic Zone. Sites of volcanic interest Eocene The second epoch of the Lower Tertiary, lasting from 56.5 to 35.4 Ma. Formation A unit of lithostratigraphy established in accordance with lithological character.

Banyoles Formation An Eocene unit made up of bluish marls.

Bellmunt Formation An Eocene unit mostly made up of clay, silt, marl, sandstone and red conglomerate.

Bracons Formation An Eocene unit made up marl, sandstone and conglomerate rocks.

Folgueroles Formation An Eocene unit made up of sandstone.

Bibliography Specialised bibliography Araña, V. [et al.], “El volcanismo neógenocuaternario de Cataluña: caracteres estructurales, petrológicos y geodinámicos”, Acta Geológica Hispánica, [University of Barcelona; Jaume Almera Institute of Earth Sciences], vol. 18 (1983), no.

1, pp. 1-17.

Cas, R.A.F.; Wright, J.V. Volcanic Successions: Modern and Ancient.

London: Chapman & Hall, 1987.

521 pp.

Donville, B., Géologie Néogène et âges des éruptions volcaniques de la Catalogne orientale, Toulouse [Paul Sabatier University], 1973. 3 volumes. Note: unpublished doctoral thesis.

Ferrés i López, D., “Caracterització de l’activitat estromboliana a la ZVG: Caracterització del volcà Croscat”,The Volcanic Region of La Garrotxa Natural Park (PNZVG), 1995.

Ferrés, D.; Planagumà, Ll.; Pujadas, A. [et al.], “Els nous volcans del Parc Natural de la Zona Volcànica de la Garrotxa”, Revista de Girona, Girona Provincial Council, vol. 188 (1998), pp. 32-41.

Francis, P., Volcanoes: A planetary perspective. USA: Clarendon Press, 1995. 443 pp.

Guerin, G.; Benhamou, G.; Mallarach, J.M., “Un exemple de fusió parcial en medi continental: El vulcanisme quaternari de Catalunya”, Vitrina: publicació del Museu Comarcal de la Garrotxa, La Garrotxa Regional Museum, vol. 1 (1985), pp. 19-26.

López-Ruiz, J. ; RodríguezBadiola, E., “La región volcánica Mio-pleistocena del NE de España”, Estudios Geológicos, [s.n.], vol. 41 (1985), pp. 105-126.

Lewis, C. J.; Baldrige, W. S.; Asmeron, Y., “Neogene asthenosphere-derived volcanism and NE-directed extension in NE Spain: Constraints on the geodynamic evolution of the western Mediterranean”, Eos Trans.

AGU, 79 (17), Spring Meeting Supplement, S 336-S 337, 1998.

Mallarach, J.M.; Martí, J.; Claudin, F ., “Primeres aportacions sobre el vulcanisme explosiu d'Olot”, Revista de Girona, Girona Provincial Council, vol. 121 (1987), pp. 69-74.

Mallarach, J.M., Carta geològica de la regió volcànica d'Olot: Litologia i geomorfologia=Geological map...

[Map]. E.1:20.000. Olot: City Council, 1982.

Mallarach, J.M., El vulcanisme prehistòric de Catalunya. Girona: Girona Provincial Council, 1998.

322 pp.

Martí, J.; Araña, V., La volcanología actual. Madrid: Spanish National Research Council (CSIC), 1993.

578 pp. (Nuevas Tendencias ; 21).

Martí, J., “El vulcanisme neogenoquaternari dels Països Catalans”, in Història natural dels Països Catalans: Geologia.

Barcelona: Catalan Encyclopaedia Foundation, 1992,vol. II, pp. 360371.

Martí, J. [et al.], “Projecte de geologia de la zona volcànica catalana: Informe final 1996”, Barcelona, Jaume Almera Institute of Earth Sciences of the Spanish National Research Council (CSIC), 1996. Note: unpublished.

Martí, J.; Mallarach, J.M., “Erupciones hidromagmáticas en el volcanismo cuaternario de Olot (Girona)”, Estudios Geológicos, [s.n.], vol. 43 (1987), pp. 31-40.

Martí, J. [et al.], “Cenozoic magmatism of the Valencia trough (western Mediterranean): relationship between structural evolution and volcanism”, Tectonophysics, [Elsevier Science Publishers], vol. 203 (1992), pp.

145-165.

Martí, J. [et al.], “Mecanismos eruptivos del volcán de la Closa de Sant Dalmai (Girona)”, Anales de física, Series B (special edition); pp.

143-153.

Neumann, E. R. [et al.], “Origin and implications of mafic xenolits associated with Cenozoic extensionrelated volcanism in the València Trough, NE Spain”, Mineralogy and Petrology, [Springer-Verlag], vol. 65 (1999), pp. 113-139.

Pallí, Ll.; Roqué, C., El vulcanisme de les comarques gironines (IIGironès). [Map]. Girona: Girona Provincial Council; University of Girona, 1995.

98

Pallí, Ll.; Roqué, C., El vulcanisme de les comarques gironines (III-Alt i Baix Empordà). [Map]. Girona: Girona Provincial Council; Universidad de Girona, 1996.

Pallí, Ll.; Roqué, C.,“Els afloraments volcànics a les comarques gironines”, Revista de Girona, Girona Provincial Council, vol. 174 (1996b), pp. 65-68.

Planagumà i Guàrdia, Ll.; “El vulcanisme freatomagmàtic de la serra del Corb”, The Volcanic Region of La Garrotxa Natural Park (PNZVG), 1995.

Pujadas, A.; Mallarach, J.M., “El vulcanisme de la Vall de Llémena”, Revista de Girona, Girona Provincial Council, vol. 174 (1996), pp. 77-81.

Pujadas, A., El vulcanisme de la Vall de Llémena. Girona: University of Girona, 1997. vol. 5, 67 pp.

(Dialogant amb les Pedres ; 5).

Pujadas, A.; Pallí, L., “Fosa de Olot”. In Pallí, L. and Roqué, C. (ed.). Avances en el estudio del Cuaternario español, Girona, (1999), pp. 346-356.

Ros, X.; Palomar, J.; Gaete, R. , “Estudi geotècnic del cingle de Castellfollit de la Roca”,The Volcanic Region of La Garrotxa Natural Park (PNZVG), 1996.

Saula, E.; Picart, J.; Mató, E. [et al.], “Evolución geodinámica de la fosa del Empordà y las sierras transversales”, Acta Geológica Hispánica, University of Barcelona; Jaume Almera Institute of Earth Sciences, vol. 29 (1996), pp. 55-75.

Sheridan, M. F.; Wohletz, K. H., “Hydro-volcanism: Basic considerations and review”, Journal of Volcanology and Geothermal Research, [Elsevier Science Publishers B.V.], vol. 17 (1983), pp.

1-29.

Tournon, J., “Les roches basaltiques de la province de Gerona (Espagne); basanites à leucite et basanites à analcime”, Bull. Soc. Fr. Minéral. Cristallogr., [s.n], vol. 92 (1969), pp. 376-382.

Ziegler P.A., “European Cenozoic rift system”, Tectonophysics, [Elsevier Science Publishers], vol.

208 (1992), pp. 91-111.

Basic recommended reading Garrotxa Aragonès Valls, Enric. Descobrint el vulcanisme quaternari a la Garrotxa: de les observacions precientífiques als primers estudis geològics (S. XVI-XIX). [Barcelona]: Council.

Culture Institute, 2001. pp [77]-125.

Bassols Isamat, Emili, “Els volcans salvats”, Revista de Girona no. 251, 2008, pp 60-65.

GeoVirtual, SL, Volcans en 3D: vol virtual pel Parc natural de la Zona Volcànica de la Garrotxa. [S.l.]: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, [2008].

C artographic Institute of Catalonia; Geological Institute of Catalonia; The Volcanic Region of La Garrotxa Natural Park. Carta vulcanològica de la zona volcànica de la Garrotxa [cartographic document], Barcelona: Cartographic Institute of Catalonia, 2007 Mallarach i Carrera, J. M.; Riera i Tussell, M., Els volcans olotins i el seu paisatge: iniciació a la seva coneixença segons nou itineraris pedagògics. Barcelona: Serpa,1981. 250 pp.

Mallarach i Carrera, J. M., Els Volcans. Girona Provincial Council.

Caixa de Girona,1989 (Revista de Girona logbook ; 21).

Catalonia Oliver Martínez-Fornés, Xavier. El Parc Natural de la Zona Volcànica de la Garrotxa. Olot: Llibres de Batet, DL 2002. 72 pp. (Guies dels Llibres de Batet ; 11) Prats, Josep M. El Parc natural de la zona volcànica de la Garrotxa [map-guide]. Barcelona: Generalitat of Catalonia., The Volcanic Region of La Garrotxa Natural Park 1994.

24 pp + 1 map-guide.

Planagumà Guàrdia, Llorenç.

Coneixem el que trepitgem?: el patrimoni geològic de la Garrotxa.Olot: Museum of Volcanoes: Culture Institute of Olot city, DL. 2005. 36pp. + 1 optical disc (CD-ROM) TOSCA, Equip d'Educació Ambiental, “Estratègia per a la gestió del vulcanisme al Parc Natural de la Zona Volcànica de la Garrotxa”, The Volcanic Region of La Garrotxa Natural Park (PNZVG), 2000. 47pp.

A rbat, Sílvia; Rigau, Eva; Solé, Lluís, Carícia de volcà [Girona]: Bescanó Council, Vilobí Council, 1991. 94 pp.

“Dossier: El vulcanisme gironí”, Revista de Girona. Girona: Girona Provincial Council,1996 XLII Year, no. 174 (genuary-february 1996), PP. 58-93 Mallarach i Carrera, Josep Maria.

El Vulcanisme prehistòric de Catalunya. Olot: Alzamora, 1998.

322pàg.

Pallí i Buxó, Lluís; Roqué i Pau, Carles. El vulcanisme de les comarques gironines [cartographic document]. [Girona]: Provincial Council; Girona University. Area of Geodynamics, DL 2007. 4 maps: col.; 57 x 70cm.

P allí i Buxó, Lluís; Roqué i Pau, Carles. El Patrimoni geològic de les terres gironines: 300 elements singulars. Girona: Universitat de Girona. Àrea de Geodinàmica Externa, 2009. 425pàg.

Pujadas, Albert [et al.]. El vulcanisme de la Vall de Llémena.Girona: Universitat de Girona. Àrea de Geodinàmica, 1997. 54pàg. (Dialogant amb les pedres ; 5) Pujadas, Albert [et al.]. El vulcanisme de La Selva. Girona: Girona University. Area of Geodynamics, 2000. 50pp (Dialogant amb les pedres ; 8)

Martí, J. [et al.], El vulcanisme: guia de camp de la zona volcànica de la Garrotxa, 2nd edition. Olot: The Volcanic Region of La Garrotxa Natural Park, 2001. 322 pp Martí, J. [et al.], “Complex interaction between Strombolian an phreatomagmatic eruptions in the Quaternary monogenetic volcanism of the Catalan Volcanic Zone (NE of Spain)”, Journal of Volcanology and Geothermal Research, Elsevier Scientific 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.

Neovídeo. Els volcans de la Garrotxa [video recording]. Olot, The Volcanic Region of La Garrotxa Natural Park (PNZVG), 1996. 1 videotape (14 min.), colour (VHS), sound BS.

99

La Garrotxa Volcanic Zone Natural Park publications Books and unpublished documents T he Volcanic Region of La Garrotxa Natural Park, Pla especial de La Zona Volcànica de La Garrotxa: Aprovació definitiva, Acord GOV/161/2010, de 14 de setembre, pel qual s’aprova definitivament El Pla Especial de La Zona Volcànica de La Garrotxa, Barcelona: Generalitat of Catalonia, [2010]. 5 vol. And optical disc Prats Santaflorentina, Josep M.; Planagumà, Llorenç; Oliver, Xavier.

Entre volcans. [Olot]: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, 2007. 141pp.

RCR Aranda, Pigem, Vilalta Arquitectes. Les cases que no criden = Las casas silenciosas = Tranquil houses: La casa de pagès al Parc Natural de la Zona Volcànica de la Garrotxa. Olot: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, 2011. 118pp.

La recerca científica al Parc Natural de la Zona Volcànica de la Garrotxa: 1982-1992. Olot: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, 1993. 146pp.

Un Parc de contes: rondalles escrites pels estudiants de primària de la Garrotxa per ser llegides i escoltades vora dels volcans. Olot: Generalitat de Catalunya. The Volcanic Region of La Garrotxa Natural Park, 2009. 111pp.

Leaflets

El Centre de Conservació de Plantes Cultivades de Can Jordà .

Barcelona: Generalitat of Catalonia.

The Volcanic Region of La Garrotxa Natural Park, [2006].

Centre de Documentació = Centro de Documentación = Documentation Centre. Olot: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, 2011.

12 indrets d'interès per visitar al parc: Parc Natural de la Zona Volcànica de la Garrotxa. [Olot]: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, [2007]. 1 leaflet, map; 42x42 cm.

Itineraris pedestres: fageda d’en Jordà; volcà de Santa Margarida; volcà del Croscat. Olot: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, 1996.No. 1.

Itineraris pedestres: Sender Joan Maragall (la Fageda d’en Jordà).

Olot: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, 1995. No. 2.

Itineraris pedestres: Olot; fageda d’en Jordà; Can Xel. Olot: Generalitat of Catonia. The Volcanic Region of La Garrotxa Natural Park, 1995. No. 3.

Itineraris pedestres: Santa Pau; volcà de Santa Margarida; Can Xel .

Olot: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, 1995. No. 4.

Itineraris pedestres: cingleres de Castellfollit. Olot: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park; Castellfollit Council, 1996. No. 13.

Itineraris pedestres: grederes del volcà del Croscat. Olot: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, 1995. No. 15.

Itineraris pedestres: ruta de les Tres Colades. El Boscarró, el Molí Fondo i Fontfreda. Olot: Generalitat of Catalonia; Sant. Joan les Fonts Council, 1997. No. 16.

Itineraris pedestres: volcà del Montsacopa. Olot: Generalitat of Catonia.; IMPC, 1997. No. 17.

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

Itineraris pedestres: valls de Sant Iscle i del Vallac: volcans i castells.

Sant Feliu de Pallerols,1998. No.19.

Oferta pedagògica del Parc Natural de la Zona Volcànica de la Garrotxa: curs 2011-2012. Olot, The Volcanic Region of La Garrotxa Natural Park (PNZVG), 2011. Maps Maps Cartographic Institute of Catalonia; The Volcanic Region of La Garrotxa Natural Park; Carta vulcanològica de La zona volcància de La Garrotxa. Barcelona.

Cartographic Institute of Catalonia; Gological Institut of Catalonia, 2007 Municipi de Sant Feliu de Pallerols: map-guide [Olot] Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, [1998-2001]

100

Booklets L’Agricultura i la ramaderia al Parc Natural de la Zona Volcànica de la Garrotxa. Olot: generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, 2011. 37pp Parc Natural de la Zona Volcànica de la Garrotxa = Parque Natural de la Zona Volcánica de la Garrotxa = Parc naturel de la Zone volcanique de La Garrotxa = The Volcanic Region of La Garrotxa Natural Park.2nd ed. Barcelona: Generalitat of Catalonia. Natural Parks Service, 2008. [10]pp.

Postcards and bookmarks [Postcards]: Materials volcànics, Volcà del Croscat, Fageda d'en Jordà, Volcà Montsacopa, Castellfollit de la Roca. [Olot]: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, 2002.

[Bookmarks] Centre de Documentació = Centro de Documentación = Documentation Centre. Olot: Generalitat of Catalonia.

The Volcanic Region of La Garrotxa Natural Park, 2009 and 2011

Volcans de la Garrotxa [Gràfic].

[Olot]: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, [2011]. [36] bookmarks; maps; 6 x 21 cm Posters El vulcanisme estrombolià de la Garrotxa. Olot: The Volcanic Region of La Garrotxa Natural Park (PNZVG), 1991.

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

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

Video/DVD GeoVirtual, SL, Volcans en 3D: vol virtual pel Parc natural de la Zona Volcànica de la Garrotxa. [S.l.]: Generalitat of Catalonia. The Volcanic Region of La Garrotxa Natural Park, [2008].

Map of the Services in La Garrotxa Volcanic Zone Natural Park

Natural Park Natural Reserve Built-up area

Natural Park Information Centre

Toilets

Car- park

Picnic area

Signposted walking itinerary

Viewpoint

Documentation Centre

Museum

Environmental Education Organisation

101 101

Services accredited by La Garrotxa Volcanic Zone Natural Park The entities that collaborate with La Garrotxa Volcanic Zone Natural Park are largely small businesses or groups of businesses that mainly work in the sector of environmental and cultural education in the Park. They are characterized by their commitment to providing quality services and their active collaboration with the Park in ensuring the successful protection and 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’s centres of interest.

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

• 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 values of the region

VERD VOLCÀNIC La Garrotxa Association for Environmental and Cultural Education Secretary: Beth Cobo c/ Antoni Llopis, 6 1r 5a 17800 Olot Tel. (+34) 972 90 38 22 (+34) 657 861 805 Fax (+34) 972 27 32 28 e-mail: info@verdvolcanic.cat Web page: www.verdvolcanic.cat

Description Created in 2003, Verd Volcànic aims to improve the quality of the services offered by the companies in the association: to stabilise the educational team, provide team members with training in the various knowledge areas and work to improve continually the services provided. It also undertakes to develop its activity in a way which is consistent with the region's conservation and to ensure the protection of its natural and cultural values.

Services provided • 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 fiveday stays • Study programmes for schoolchildren from countries such as Great Britain and Eire.

102 102

LA CUPP, SCCL Secretary: Ester Morchón Avda. República Dominicana 3, bajos 17800 Olot Tel. (+34) 972 27 32 23 Fax (+34) 972 26 22 33 e-mail: lacupp@gmail.com

Description An environmental education cooperative created in 2006 formed by a team of graduates in Environmental Sciences, Psychology and Geography with extensive experience in the field of environmental education.

Services • Environmental activities in La Garrotxa Volcanic Zone Natural Park for schoolchildren of all ages • Environmental activities for adults including guided walks to some of the Natural Park’s best known sites (volcanoes of Croscat and Santa Margarida, Fageda d’en Jordà beechwood), but also to some of its least known treasures (Sant Joan les Fonts lava flows, Colltort Castle, churches of La Serra del Corb) • Languages: all the above activities can be carried out in Catalan, Spanish, English, German or French • Technical work including studies in the fields of tourism and education, coordination of training and education programmes.

TOSCA Environmental services in education and tourism Secretary: Octavi Bonet Mas Tarut Av. de Santa Coloma, s/n 17800 Olot Tel. (+34) 972 27 00 86 Fax (+34) 972 27 04 55 e-mail: info@tosca.cat Web page: www.tosca.cat Opening hours: 9-14 h i 16-18 h

Description TOSCA is a services company working to improve the region through: education, communication, interpretation and socio-environmental information, the drawing up of technical studies and developing environmental action in La Garrotxa. TOSCA consists of a team of 10 professionals with backgrounds in Geology, Biology, Education and Tourism with extensive experience in environmental matters.

S ervices • Management of the Educational and Information Services of The Volcanic Region of La Garrotxa Natural Park.

• Development of educational activities and guided walks adapted to educational level. Preparation of teaching materials.

• Training programmes for teachers, environmental educators, students, and so on.

• Drawing up of studies on environmental education in protected natural areas.

• Design of geotourism trails.

• Recovery of volcanic heritage.

• Participation in projects on sustainability, agrobiodiversity, sustainable tourism, studies of vulnerable areas, recovery of natural areas, and so on.

• Contributions to publications on environmental education and sustainability.

103 103

Notes

104 104

105

Recommendations and indications for visitors to the La Garrotxa Volcanic Zone Natural Park The Park has an extensive network of signposted itineraries that reach some of the most interesting sites in the region.

The maintenance service has to work hard to keep the most frequented areas clean. Please use the bins or take your rubbish home with you.

The Natural Park consists mostly of private property. Please ensure that you do not disturb residents.

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

Camping is forbidden within the Park.

Nevertheless, there are numerous camp sites, hotels and hostels in the Park where visitors can stay.

In a number of clearly signposted areas access is limited to park services and residents. Vehicle access here is forbidden. .

For reasons of safety and conservation, the lighting of fires is strictly prohibited.

The Park information centres give special permits for those with reduced mobility in order to visit restricted areas by car.

106

Natural Park Services Information Centres

Educational Services

Casal dels Volcans Av. de Santa Coloma, s/n 17800 Olot Tel. (+34) 972 26 81 12 Fax (+34) 972 27 04 55 pnzvg@gencat.cat Can Serra Fageda d’en Jordà Can Passavent Croscat Volcano

Casal dels volcans Av. de Santa Coloma, s/n 17800 Olot Information and bookings: weekdays 9.00 am to 2:00 pm and 4.00 to 6.00 pm Tel. (+34) 972 27 00 86 (+34) 972 26 81 12 contractacio@tosca.cat

Documentation Centre Opening hours: weekdays 9.00 am to 2.00 pm.

Visits by appointment only.

Tel. (+34) 972 26 46 66 Fax (+34) 972 26 55 67 wgrabolo@gencat.cat

Web Pages General 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 Catalunya Departament d’Agricultura, Ramaderia, Pesca, Alimentació i Medi Natural