Eruption Mechanisms of the Puy De Lemptégy (Auvergne, France) Deduced from Its Volcano Stratigraphy

FACULTEIT WETENSCHAPPEN Vakgroep Geologie en Bodemkunde

Eruption mechanisms of the Puy de Lemptégy (Auvergne, France) deduced from its volcano stratigraphy.

Lieselotte Wallecan

Academiejaar 2010-2011

Scriptie voorgelegd tot het behalen van de graad Van Master in de Geologie

Promotor: Prof. Dr. Patric Jacobs Co-promotor: Prof. Dr. Matthieu Kervyn Leescommissie: Dr. Gerald Ernst, Dr. Karen Fontijn

This paper would not be ended successfully without the support and help of some people.

- Dr. Matthieu Kervyn. Dr. Kervyn gave me the opportunity to learn more about volcanoes and supported me on the fieldwork in the Auvergne. I got the chance to meet important scientific people who are working and studying worldwide volcanoes (e.g. Greg Valentine). Also thanks for the corrections of this paper and the discussions of the interpreted results. - Prof. Dr. Patric Jacobs. He gave me the possibility to use different laboratory equipment and the support during this research period. - Dr. Karen Fontijn. She explained and helped me with the different methods of the laboratory analyses. - Dr. Gerarld Ernst. He gave me the opportunity to discuss my observations and interpretations and gave me an introduction in the volcanology science. Also I want to thank him for the support during the difficult time I had when my best friend died. - Prof. Dr. Benjamin van Wyk de Vries. For the geological introduction of the Chaîne des Puys and the Puy de Lemptégy. - Audrey Delcamp. For the geological introduction of the Puy de Lemptégy and the research work she had carried out at Puy de Lemptégy. - Prof. Dr. Marlina Elburg. For the help with the ICP-OES measurements. - Prof. Dr. Peter Van den haute. For his contribution in the research and preparing of the thin sections. - Daniëlle Schram. For the practical help during the laboratory analyses. - My parents: Danny Wallecan and Dorinne Dezegher. For their financial support during my geology studies at the University of Ghent. - My partner: Maertens Pieter. For the support during my studies and the emotional support during our relationship. - Elke Vangampelaere †02/10/2010. I want to thank her for the support during my studies and the pleasant moments we shared during our student life. This paper is attributed to Elke Vangampelaere who died in a plane crash on 02/10/2010. She was a very special person and my best friend for ever and ever. “Pilots never die, they disappear behind the horizon.”

TABLE OF CONTENT

1 Introduction ...... 1 2 Objectives ...... 4 3 Regional geology and tectonics ...... 5 3.1 The Central Massif ...... 5 3.2 Volcanic constructs of the Chaîne des Puys ...... 7 3.3 The morphology of the Chaîne des Puys ...... 8 3.4 Volcanism in the Chaîne des Puys...... 9 3.5 Structural geology of the Chaîne des Puys...... 10 4 Scoria cones of the Chaîne des Puys...... 10 4.1 Morphology ...... 10 4.2 Eruption dynamics and internal structure ...... 11 4.3 Neighbouring cones of Puy de Lemptégy...... 15 4.3.1 Puy de Petit Sarcoui ...... 16 4.3.2 Puy des Goules ...... 16 4.3.3 Puy des Gouttes ...... 16 4.3.4 Puy de Côme ...... 17 4.3.5 Le Grand Sarcoui ...... 18 4.3.6 Puy de Chopine ...... 18 4.3.7 Le Clierzou ...... 19 4.3.8 Le Pariou ...... 19 4.3.9 Le Chaumont and Puy de Fraisse ...... 20 4.4 Puy de Lemptégy...... 20 5 Methodology of fieldwork and laboratory work...... 25 5.1 Fieldwork ...... 25 5.2 Grainsize distribution ...... 27 5.3 Density measurements ...... 28 5.4 Thin sections ...... 30 5.5 ICP-OES ...... 30 6 Stratigraphy of Lemptégy II: field and laboratory results for reference section ...... 32 6.1 Litholog 2 section 3: stratigraphy ...... 34 6.2 Litholog 1 section 3: stratigraphy ...... 36 6.3 Litholog 3 section 3: stratigraphy ...... 40 6.4 Correlation of the lithologs from key section 3 ...... 42 6.5 Key section 3: interpretation ...... 46 6.6 Density results and interpretation ...... 48 6.7 Grainsize distribution and sorting ...... 53 6.8 Geochemical analysis ...... 56 6.9 Thin sections ...... 59 7 Lemptégy II: stratigraphy of remaining sections ...... 63 7.1 Observations lithologs section 1 ...... 63 7.1.1 Section 1 litholog 2: stratigraphy ...... 63 7.1.2 Section 1 litholog 3: stratigraphy ...... 64 7.1.3 Section 1 litholog 5: stratigraphy ...... 68 7.1.4 Section 1: interpretation lithologs...... 70 7.1.5 Section 1: correlation lithologs ...... 73 7.2 General introduction of section 2 at Lemptégy II ...... 75 7.3 Observations of the lithologs section 2 from Lemptégy II ...... 76 7.3.1 Section 2 litholog 3: stratigraphy ...... 76 7.3.2 Section 2: interpretation lithologs...... 82 7.3.3 Section 2: correlation lithologs ...... 82 7.4 Observations of the lithologs section 4 of Lemptégy II...... 84 7.4.1 Section 4 litholog 2: stratigraphy ...... 86 7.4.2 Section 4: interpretation and correlation lithologs ...... 87 7.5 Observations of lithologs section 5 of Lemptégy II ...... 89 7.5.1 Section 5 litholog 1: stratigraphy ...... 90 7.5.2 Section 5: interpretation and correlation lithologs ...... 91 7.6 Observations of the lithologs section 6 from Lemptégy II...... 92 7.6.1 Section 6 litholog 1: stratigraphy ...... 95 7.6.2 Section 6: interpretation lithologs...... 96 7.6.3 Section 6: correlation lithologs ...... 96 7.7 Tectonic structures of Puy des Gouttes ...... 97 8 Statistical and spatial distribution of volcanic bomb at Lemptégy II...... 98 8.1 Introduction ...... 98 8.2 Statistical distribution of the bomb characteristics of Lemptégy II ...... 99 8.3 Dimensions of the bombs ...... 103 9 Discussion: eruption dynamics of Lemptégy II ...... 107 9.1 Introduction ...... 107 9.2 First cone growth: Lemptégy I and Puy des Gouttes ...... 107 9.3 Eruption dynamics of Lemptégy II ...... 108 9.3.1 Paleo relief caused by the Lemptégy I scoria cone ...... 108 9.3.2 Eruption chronology of the Lemptégy II eruption ...... 110 9.3.3 Other observed features at the Lemptégy II scoria cone ...... 116 9.4 Strombolian eruption and pyroclastic fall deposits...... 116 10 Conclusion ...... 122 11 Nederlandse samenvatting ...... 125 12 REFERENCES ...... 132 13 APPENDIX ...... 137

LIST OF FIGURES

Figure 1: Geological history of the Central Massif and the Chaîne des Puys (de Goër de Herve et al., 1999)...... 5 Figure 2: Morphology map of the Chaîne des Puys (Boivin et al., 2004)...... 6 Figure 3: Part of the geological map of the Chaîne des Puys (red cross: Puy de Lemptégy) (http://planet-terre.ens- lyon.fr/planetterre/objets/img_sem/XML/db/planetterre/metadata/LOM-Img200-2007-05- 28.xml), visited on 02/02/2011)...... 7 Figure 4: Neighbouring cones of Puy de Lemptégy (modified from de Goër de Herve et al., 1999) ...... 16 Figure 5: Puy des Goules © Bruno Monginoux (from www.photo-paysage.com (visited on 02/02/2011) ...... 16 Figure 6: Puy de Côme (Boivin et al., 2004) ...... 17 Figure 7: Puy de Chopine (right) and Puy des Gouttes (left) (Boivin et al., 2004)...... 18 Figure 8: Panorama view at the Chopine deposits at Lemptégy...... 19 Figure 9: Geological map of Puy de Lemptégy (Delcamp, 2005 modified geological map Boivin et al., 2004) ...... 21 Figure 10: View at Puy de Lemptégy (Delcamp, 2005) (photograph made by Nathalie) ...... 21 Figure 11: View at a spatter cone at Lemptégy (From Delcamp 2005)...... 23 Figure 12: Plan view of Lemptégy II (modified from Delcamp, 2005) ...... 24 Figure 13: DEM map with GPS points measured (red dots) (image from Stephane Petit) ..... 26 Figure 14: pictures of different steps of lab procedure for density measurements. Picture A: preparation, picture B: sample weight, picture C: sample weight + coating and picture D: principle of Archimedes; sample + coating in water (red circle: position of sample) ...... 30 Figure 15: Overview picture of the Lemptégy scoria cone (entire width of cone ~400m and arrow indication for nord) (Photograph made by Nathalie) ...... 32 Figure 16: Overview picture Lemptégy scoria cone (arrow indication of Nord) with location of sections and tracks (see legend) (Photograph made by Nathalie) ...... 33 Figure 17: section 3 litholog 2: stratigraphy and description of main clasts (grainsize = maximum (unless otherwise stated)) ...... 34 Figure 18: picture A: granite pieces incorporated in scoria clast (scale ~16cm), picture B: fragile clasts layer H (left), typical scoria clast (right) (scale ~16cm) and picture C: lava flow (scale: grey bar ~40cm (seen right side at picture) ...... 38 Figure 19: section 3 litholog 1: stratigraphy and description of main clasts (grainsize = maximum (unless otherwise stated)) ...... 39 Figure 20: section 3 litholog 3: stratigraphy and description of main clasts (grainsize = maximum (unless otherwise stated)) ...... 41 Figure 21: correlation of lithologs from key section 3: green lines = correlation lines, red question marks = uncertain correlation, red lines = no correlation ...... 44 Figure 22: Overview picture key section 3 with indication of lithologs, stratigraphic lines (phases indicated as full red lines with black arrow), uncertain stratigraphic line (red dotted lines), magmatic structures (full green line) and lens-shaped layers (green dotted lines) (length of section ~200m and height ~35m) ...... 45 Figure 23: intrusion observed at section 3: red lines indicating deformed layers, green line contouring the intrusion (scale (grey bar photograph) = 1.70m) ...... 47 Figure 24: Location of samples taken for density analyses (orange dots at litholog), chemical analyses (red dots at litholog), thin sections (green dots at litholog) at different stratigraphic levels in section 3 ...... 48 Figure 25: plot of bulk density results, samples section 3 ...... 49 Figure 26: location density samples from section 4 litholog 2 ...... 50 Figure 27: plot of bulk density results, samples section 4 ...... 51 Figure 28: locations (purple dots) of samples taken from section 4 (left) and section 3 (right) for grainsize analyses ...... 53 Figure 29: Plot of the Inman parameters Md (Φ) and σ (Φ) from the samples ...... 54 Figure 30: Md (Φ)/ σ (Φ) plot for some Strombolian pyroclastic fall deposits. Solid circles are samples collected from scoria cones, and crosses are from downwind fall deposits. The red points at the Md (Φ)/ σ (Φ) are my own samples (After G.P.L Walker and Croasdale 1972, with additions for cone deposits after Houghton & Hackett (1984), and J.V.Wright unpub. Data from Santorini. (Modified from Cas and Wright 1988) ...... 55 Figure 31: Location of geochemical samples CA S8.9 and CA R12 (orange squares) and thin sections samples (red squares) from section 3 ...... 57 Figure 32: geochemical analyses presented at TAS diagram for Lemptégy I and II (de Goër de Herve et al., 1999) (green square: plot of own geochemical data) ...... 58 Figure 33: photographs thin sections: picture A, B (polarized light) and C: sample TS S7; picture D, E and F (polarized light): sample TS S1; picture G and H (polarized light): sample TS S2 ...... 61 Figure 34: photographs of thin sections: picture A and B (polarized light): sample TS S8; picture C and D (polarized light): sample TS RS ...... 62 Figure 35: Overview photograph of section 1 ...... 65 Figure 36: Overview photograph of section 1: green lines: magmatic related structures (cryptodome), black lines: stratigraphic contacts, red lines: faults and dotted black lines: uncertain stratigraphic contacts (scale: length of section ~140m) ...... 65 Figure 37: Overview photograph of section 1 with location of the lithologs (purple lines) and detailed pictures of some structures: picture A: sharp contact between coarse pyroclasts and the fine material (scale: grey bar ~1.70m), picture B: thin (~10cm) fine grained layer that is cut off by fault F2 (scale: grey bar ~40cm) and picture C: detail of fault F3 (scale: grey bar ~40cm)...... 66 Figure 38: section 1 litholog 2: stratigraphy and description of main clasts (grainsize = maximum (unless otherwise stated)) ...... 67 Figure 39: section 1 litholog 3: stratigraphy and description of main clasts (grainsize = maximum (unless otherwise stated)) ...... 67 Figure 40: section 1 litholog 5: stratigraphy and description of main clasts (grainsize = maximum (unless otherwise stated)) ...... 68 Figure 41: Section 1 litholog 3: bomb with black coloured centre and vesicles (left), at the right: red coloured centre with elongated vesicles (pencil ~16cm)...... 68 Figure 42: correlation lithologs section 1: green dotted lines (tracks), full black lines (correlation lines), red question marks (no correlation) and black dotted lines (correlation between lithologs except for litholog 4) ...... 69 Figure 43: section 2 litholog 3: stratigraphy ...... 77 Figure 44: Overview photograph section 2 ...... 78 Figure 45: overview section 2 with stratigraphic contacts (full red lines; dotted red line (estimated correlation), black square (location of photograph) (scale: purple square (~1m)) . 78 Figure 46: photograph of picture A on Figure 45, western part of section 2 with stratigraphic contacts (full red lines, dotted red line (estimated contact)) and location of litholog 1 (green line) (grey bar (central on photograph) length ~1.70m) ...... 79 Figure 47: photograph of picture B on Figure 45, central part of section 2 with stratigraphic contacts (full red lines, dotted red line (estimated contact)), location of litholog 2 (purple line) and tectonic structures (full green lines) ...... 80 Figure 48: photograph of picture C on Figure 45, eastern part of section 2 with stratigraphic lines (full red lines, dotted red line (estimated contact)), location of litholog 3 (purple line) . 81 Figure 49: Correlation lithologs from section 2 (black lines: correlation lines) ...... 83 Figure 50: Overview photograph section 4 (scale: purple square height ~1m) ...... 84 Figure 51: left: detailed picture of soil layer between the deposits of Puy des Gouttes and Lemptégy II (scale: hammer ~28cm); right: detailed picture of irregular layer with the greyish rounded clasts ...... 84 Figure 52: Overview photograph of section 4 (full red lines: stratigraphic contacts, dotted red lines: uncertain stratigraphic contacts; black lines: location of lithologs) and correlation of lithologs from section 4 (black lines: correlation lines) ...... 85 Figure 53: Section 4 litholog 2: stratigraphy ...... 87 Figure 54: Overview photograph section 5 of Lemptégy II (scale: length ~100m) ...... 89 Figure 55: photograph section 5 with stratigraphic contacts (red line), location lithologs (white lines) and correlation ...... 90 Figure 56: section 5 litholog 1: stratigraphy ...... 90 Figure 57: Detail picture litholog 1 section 5: dirt in between the pyroclasts (scale: grey bar ~40cm)...... 91 Figure 58: overview photograph southern part of section 6 with stratigraphic contacts (red line) and location litholog 1 (green line) (scale: length ~50m) ...... 93 Figure 59: overview photograph central part of section 6 with stratigraphic contacts (red line) and location litholog 2 (green line) (scale: length ~30m) ...... 93 Figure 60: overview photograph northern part of section 6 with stratigraphic contacts (red lines) and location litholog 3 (black square) (scale: length ~15m) ...... 94 Figure 61: Correlation of lithologs section 6 (black lines: correlation lines) ...... 94 Figure 62: section 6 litholog 1: stratigraphy ...... 95 Figure 63: detailed photograph of lava flow (pencil ~16 cm for scale) ...... 97 Figure 64: photograph of faulting occurring at the Puy des Gouttes deposits (section 3) (red lines: stratigraphy; green line: fault) (scale: grey bar ~1.70m) ...... 97 Figure 65: colour features of bombs observed at all stratigraphic levels at section 1 ...... 99 Figure 66: relationship between vesicles shape, size and the colour pattern of the bombs present at section 1 ...... 100 Figure 67: colour features of bombs observed at all stratigraphic levels at section 3 ...... 101 Figure 68: relationship between vesicles shape, size and the colour pattern of the bombs present at section 3 ...... 101 Figure 69: relationship between vesicles shape, size and the colour pattern of the bombs present at section 4 and 5 ...... 102 Figure 70: Plot of longest axis and smallest axis, data of the measured bombs at section 1 plotted within the reference ellipticity isogons ...... 103 Figure 71: Plot of longest axis and smallest axis, data of the measured bombs at section 3 plotted within the reference ellipticity isogons ...... 104 Figure 72: Plot of longest axis and smallest axis, data of the measured bombs at section 4 and 5 plotted within the reference ellipticity isogons ...... 105 Figure 73: picture A: elongated vesicles; picture B: irregular vesicles and red interior of bomb; picture C: elongated and large round vesicles; picture D and E: ribbon crust pattern and picture F: protruding crust pattern (cauliflower) (scale for picture A-E: pencil ~16cm and picture F: grey bar ~40cm) ...... 106 Figure 74: sketch of grain avalanching, downwards accumulation of larger clasts, upwards slope accumulation of smaller clasts ...... 109 Figure 75: antiform orientation of bombs ...... 112 Figure 76: small intrusion observed at section 3 (black circle), full red lines show alignment of lava channel, dotted red lines show the estimated location of the branches from the lava channel resulting into intrusion like features (scale: purple square with height ~1 m) ...... 113 Figure 77: key lithologs chosen for sections showing which layers are related to the different stages in the Lemptégy II eruption, the Lemptégy II eruption phase 1 is not shown at this picture because of minor volume of pyroclastic material deposited and the red circle shows the Lemptégy II eruption phase 3 which is only found at section 3 (local change in eruption style) ...... 119 Figure 78: complete correlation of the Lemptégy II lithologs (see table 11 for eruption phases and deposits) (black lines: correlation lines, black dotted lines: estimated correlation, red question marks: no correlation) ...... 120 Figure 79: sketch of the growth evolution of the Lemptégy 2 scoria cone during the different eruption phases, during phase 1 only a small cone is developed with a minor volume of pyroclastic material. From phase 4, pyroclastic material will also be accumulated at the sections located further from the Lemptégy II vent...... 121

LIST OF TABLES

Table 1: elevation values for the DEM map…….……………………………………………25

Table 2: description of sorting for pyroclastic deposits (Cas and Wright, 1987)…………….49

Table 3: Average bulk density of 10 clasts taken at different stratigraphic level in section 3..50

Table 4: average bulk density of 10 clasts taken at different stratigraphic level in section 4...52

Table 5: Dense Rock Equivalent density from samples of section 3 and 4…………………..54

Table 6: Description of the samples from section 3………………………………………...... 54

Table 7: sorting results of grainsize samples from section 3………………………………....54

Table 8: sorting results of grainsize samples from section 4………………………………....54

Table 9: sorting coefficient for pyroclastic deposits (Cas and Wright, 1987)………………..54

Table 10: former geochemical analyses from Lemptégy: first and third column (de Goër de Herve et al., 1999), second and fourth column (Boivin et al., 2004)…………………………58

Table 11: geochemical analyses of samples from section 3 Lemptégy II (sample S7.1 located at highest height and sample S6.3 located at lowest height (Figure 24)) …………………….59

Table 12: Legend of abbreviations used in the graphics, large (≥ 4 mm) and small (< 4mm).99

Table 13: summary of the different eruption phases of Lemptégy II and related deposits….118 1 Introduction The Chaîne des Puys is a relatively well preserved Quaternary volcanic cone field, located in the Massif Central. It contains 100 eruptive centres which include lava domes, protrusions and maars. “Monogenetic” scoria cones are very common volcanoes and they tend to have a complex eruptive history. The Chaîne des Puys is the most northern and most recent volcanic unit of the Massif Central in France. The Chaîne des Puys does not only have a historical significance by means of recent eruptions, it is probable that there will be future eruptions associated with this volcanic field. One of these scoria cones belonging to the Chaîne des Puys is Puy de Lemptégy. This scoria cone is studied to have a better understanding of the eruptive history of Puy de Lemptégy.

The purpose of this study is to deduce the eruption dynamics of the Lemptégy II scoria cone from its stratigraphy. The scientific investigations carried out on the Lemptégy scoria cone began when the excavation of the pouzzolane d’Auvergne came to a halt. Due to the excavation processes, the Puy de Lemptégy became a great opportunity to study the intern mechanisms of a volcano. The investigations carried out on the Lemptégy scoria cone were done by the scientists of “Laboratoire Magma et Volcans” established at Clermont-Ferrand. Investigations done by Delcamp (2005) lead to the creation of a plan view map of the Lemptégy scoria cone focussing on intrusive rocks. The need for a complete documentation and mapping of the stratigraphy of the entire cone was high, therefore field research was carried out in July 2010 to provide new information about the complete stratigraphy of the Lemptégy II scoria cone.

So far no information was available to document the stratigraphy of the Lemptégy II deposits. Lithologs were needed to document the stratigraphy, therefore the quarry was divided in several sections where the outcrops of the Lemptégy II deposits were present. Accompanied by the fieldwork, laboratory analyses were carried out. These laboratory analyses are used to obtain a more detailed description of important pyroclasts (e.g. pyroclasts constituting the key layers) and related to the results of the field observations, document the Lemptégy II eruption type. The grainsize and density analyses were carried out to obtain more information about eruption dynamics and to characterize typical processes occurring during an eruption (e.g. grain avalanches). Geochemical investigations were done to document the geochemical composition of the Lemptégy II deposits and to reveal if certain processes (e.g. weathering)

1 could have an influence on the geochemical composition. With this new information a complete stratigraphy was set up and the eruption history could be interpreted for the Lemptégy II scoria cone.

Chapter 2 of this thesis gives an overview of the objectives that have to be answered in this study to obtain the main objective: deduce the eruption mechanisms of the Puy the Lemptégy form its volcano stratigraphy.

In chapter 3 of this thesis “Regional geology and tectonics” a brief description is given about the regional geology of the Massif Central focussing on a more detailed presentation of the Chaîne des Puys (e.g. morphology, volcanic constructs and structural geology), the most recently active volcanic zone of the Massif Central.

The following chapter gives an overview of the scoria cones of the Chaîne des Puys. The first part deals with a general description of scoria cones in terms of morphology, eruption dynamics and the observed internal structure. The second part of this chapter takes a closer look at the scoria cones surrounding the studied scoria cone: the Puy de Lemptégy. The main characteristics described for the surrounding scoria cones are: age, location, geochemical composition and the eruption type. The last part of this chapter gives a short introduction about the Puy de Lemptégy and summarize briefly former investigations.

The fifth chapter deals with the methodology of the fieldwork and the laboratory analyses.

The results of the fieldwork and the laboratory analyses can be found in chapters 6 and 7. Chapter 6 contains the description of a reference section at the Lemptégy II scoria cone. The observations made in the field are presented as lithologs coupled with a detailed description and interpretation of the observed features. The laboratory analyses are presented in the second part and contain the results of grainsize analyses, density measurements, geochemistry (ICP-OES) and petrographic analyses (thin sections). These analyses are mainly carried out on samples from the reference section. Chapter 7 presents the lithologs made from the other sections coupled with the observations and interpretations. For each section separately correlation between the different lithologs was carried out.

Chapter 8 presents the results of the observations carried out on the volcanic bombs observed at the Lemptégy II scoria cone.

The results obtained and presented in chapters 6, 7 and 8, are thereafter discussed and interpreted in chapter 9 to obtain an integrated stratigraphy for the Lemptégy II scoria cone and an interpretative model for the eruption dynamics in relationship with the obtained stratigraphy.

A general overview of all the analyses and obtained results can be found in the last chapter constituting the study (chapter 10). Chapter 10 gives a brief summary of all the performed analyses and the main results for the Lemptégy II scoria cone related to its stratigraphy and eruption dynamics. This chapter provide the answers to the research question.

2 Objectives The main objective of this study is to understand how the Lemptégy scoria cone built up and what kind of eruption style occurred. An intermediate objective is to obtain a complete stratigraphy of the Lemptégy II deposits which is exposed at the Lemptégy quarry in order to constrain the eruptive sequence and dynamics. The main method used to meet the stratigraphic issue is to drawn lithologs during the fieldwork. To find the answer on the main objective, there are some sub-objectives that has to be answered.

A first step in this study is to make observations at the Lemptégy scoria cone, which are gathered during the fieldwork. The main focus is to separate the different stratigraphic layers from each other with their own specific volcanic clasts. To interpret the eruption dynamics we have to characterize the different pyroclasts constituting the Lemptégy II deposits. The existence of typical pyroclasts is related to the eruption style occurring at the Lemptégy II scoria cone. The last step in gathering the observations is to obtain a statistical and spatial distribution pattern of volcanic bombs and reveal their main characteristics.

After collecting the observations the next part of the study can be accomplished. This part will incorporate the analysis of the field observations and to do a more detailed research of some pyroclasts. Correlation between the different stratigraphic layers is required, to set up a stratigraphic map. Important issue for this correlation part is trying to identify and document existing stratigraphic markers present in the stratigraphic sequence. Beside the analyse of the field observations also laboratory analyses are performed on samples taken from the Lemptégy II deposits. These analysis gave a more detailed understanding in terms of the grainsize distribution of the key layers and other stratigraphic layers. With the density analysis we are able to characterize the pyroclasts their density and interpret peculiarities about these density values. A last step in this research part is to confirm previous geochemical research done at Lemptégy II deposits with new samples taken from the Lemptégy II deposits.

All the data from the field observations and analysis have to be interpreted to obtain the main objective of this study. The main focus of this study is to interpret the eruption type and post- eruption events that create the typical morphology observed at the Lemptégy scoria cone. Also investigate whether several eruption phases occurred (multiple events) or not. The last step is to reconstruct the eruption history of the Lemptégy II scoria cone in a chronology order. All the data collected and analysed in the different parts of this work will contribute in a better understanding of the eruption dynamics of scoria cones.

3 Regional geology and tectonics

3.1 The Central Massif The Chaîne des Puys is the most northern and most recent volcanic unit of the Massif Central in France. The Massif Central forms together with the Armorican Massif (Brittany and Normandy) one of the two big basement massifs in France. The geological history of the Massif Central began around 400 million years ago with the start of the Hercynian orogeny which affected most of Europe with widespread thrusting, deformation and metamorphism of the basement rock to schist and gneiss lithologies (Jung, 1946). It is a rather complex geological history.

During the geological time a lot of tectonic processes affected the area of the Central Massif in France. During Devonian and Carboniferous time a sequence of mountains were built up during the Hercynian orogeny. The formation of the pre-volcanic basement which consist of granites and metamorphic rocks, is the root of the geological history of the Central Massif (Boivin et al., 2004).

Figure 1: Geological history of the Central Massif and the Chaîne des Puys (de Goër de Herve et al., 1999).

Figure 2: Morphology map of the Chaîne des Puys (Boivin et al., 2004). At the end of the Palaeozoic times there was nothing left of the Hercynic chain mainly caused by extensional movements and an erosion surface was developed (Figure 1). That pre-Triassic erosion surface remains in a steady state behaviour until the late Eocene. From that time onwards, the peneplane fragmented under influence of extensional movements, those are the precursor of the aperture of the western Mediterranean sea. The Hercynic faults, initially influenced by N-S compression, were reactivated which causes the subsidence of the Limagne – and Olby graben (Boivin et al., 2004).

The subduction of the African Plate beneath Europe, resulting in the Alpine orogeny, was the next major event to affect the area, dominating the Cenozoic time and leading to the uplift of the basement and mantle beneath Europe and in particular the Massif Central (Wilson and Downes, 2006). During the Oligocene crustal thinning together with extensional stresses in the lithosphere (Merle and Michon, 2001a; Boivin et al., 2004) led to the formation of the European Cenozoic Rift System and the grabens of Limagne, St. Flour, Aurillac and Le Puy (Nehlig et al., 2001), subsequently filled with erosion products (detrital sediments followed by sequences of marl and limestone) from adjacent uplifted areas over the following 20 Ma. The extensional movements did not create new reliefs but the basement of the Limagne graben was subducted at a height of 3000 meters against the surface of the Plateau des Dômes (Boivin et al., 2004).

As said before, during Cenozoic times extension of the European crust took place, followed by lithospheric mantle thinning (Merle and Michon, 2001a; 2001b) which caused probably mantle melting by decompression during the Miocene (Hoernle et al., 2002). This magma rose along the reactivated Hercynian faults, leading to the initiation of intra-plate, alkaline volcanism in the Auvergne (Wilson and Downes, 2006; Boivin et al., 2004). The main phase of volcanism in the Auvergne, resulting into the major volcanic edifices, occurred during the 6 late Miocene-Pliocene. In this time frame, the building of the central volcanoes of Cantal and Monts-Dore and the basaltic massifs of Aubrac and Devès-Velay started (Nehlig et al., 2001). The most recent volcanism is the Chaîne des Puys: the formation of fissure-related strombolian scoria cones, lava domes and maars during the Pleistocene and Holocene, between 90.000 and 6.000 years ago (Boivin et al., 2004).

3.2 Volcanic constructs of the Chaîne des Puys

Figure 3: Part of the geological map of the Chaîne des Puys (red cross: Puy de Lemptégy) (http://planet-terre.ens- lyon.fr/planetterre/objets/img_sem/XML/db/planetterre/metadata/LOM-Img200-2007-05-28.xml), visited on 02/02/2011). The Chaîne des Puys is a relatively well preserved Quaternary volcanic cone field consisting of almost 100 eruptive centres which include lava domes, protrusions and maars (Shields, 2009). They have been the subject of dedicated investigations by volcanologists since the 18th century yet little of the region‟s volcanology is understood, including the crustal structure beneath the Chaîne des Puys and the age of the majority of edifices (Shields, 2009). The youngest eruption is thought to be only 4400 years ago from tephra dating, yet the source of

7 the deposit remains unknown (Shields, 2009). The Chaîne des Puys does not only have a historical significance by means of recent eruptions, it is probable that there will be future eruptions associated with this volcanic field. Therefore the area is under constant seismic surveillance by the Observatoire de Physique du Globe de Clermont Ferrand (pers. comm., van Wyk de Vries, 2010).

3.3 The morphology of the Chaîne des Puys The name La Chaîne des Puys is caused by the fact that all volcanic devices are lined up along a pre-existing structure of the crystalline basement (Plateau des Dômes) (Boivin et al., 2004). The Plateau des Dômes is located between two tectonic grabens La Limagne in the east and L‟Olby – vallée de la Sioule – in the western part (Figure 2) (Boivin et al., 2004). The Chaîne des Puys is a north-south aligned feature of approximately 60 km long from Puy de Chalard in the north to Puy de Montcineyre in the south, but only 2 to 4 km wide (Figure 3) (Shields, 2009). The identified volcanic features include approximately 100 Quaternary eruptive centres: mostly scoria cones but also lava domes, maars or explosion craters resting on a granite-gneissic horst of the Plateau des Dômes (Shields, 2009). The formation and structure of scoria cones is discussed in detail in chapter 4.

The other constructs, will be briefly introduced in this paragraph. Volcanic domes are rounded, steep-sided mounds built by very viscous magma, usually either dacite or rhyolite in composition. Such magmas are typically too viscous (resistant to flow) to move far from the vent before cooling and crystallizing. Domes may consist of one or more individual lava flows. Lava domes are also referred to as volcanic domes (Fink and Anderson, 2000). A maar is a low-relief, broad volcanic crater cutting about 10 m to more than 500 m deep into the pre- eruption surface. Maars contain well-bedded ejecta (beds dipping < 25°) that decrease rapidly in thickness away from the rim. Maar deposits contain a large abundance of non-juvenile components (country rock) and the deposits are mainly emplaced by base surges and fallout (Vespermann and Schmincke, 2000). The explosions are usually caused by the heating and boiling of groundwater when magma invades the groundwater table (Vespermann and Schmincke, 2000). Preserved maars are often filled with water forming circular lakes. The Chaîne des Puys setting is a typical scoria cone field, which can often contain hundreds of volcanic centres, located in relatively flat areas. Scoria cones can be also present on the flanks or near the base of larger strato-volcanoes or shield-volcanoes (Kervyn et al., 2010). The average elevation of the Chaîne des Puys is approximately 1000 m but the volcanic edifices

8 are less than 300 m high except for the lava dome of Puy de Dôme, located near the middle of the chain with a height of 465 m (Shields, 2009).

3.4 Volcanism in the Chaîne des Puys. The oldest eruptions date back to 90 ky ago: the Chanat and St-Hippolyte cones. The recent eruptions are only 8.500 years (e.g. La Vache and Lassolas cones). But 10 à 15 km towards the south, at the border of Monts Dore and Cézalier, where the youngest volcanoes of France are located: the Lac Pavin group which have ages of ~7.000 years (de Goër de Herve et al., 1999). The Petite Chaîne des Puys, consisting of four scoria cones located west of Sioule river, belongs to an older period of volcanism in comparison with the main Chaîne des Puys (Shields, 2009). The Chaîne des Puys is composed of many edifices built up each by an individual eruption that typically did not last longer than a few weeks or months (de Goër de Herve, 1999). Volcanism far from the edges of tectonic plates, such as Chaîne des Puys, is rare. Most of the time volcanoes occurred in divergent plate tectonics (e.g. mid ocean ridges) or subducting plates. What is the geological explanation for the occurrence of volcanism at the Chaîne des Puys in an intraplate tectonic setting? In section 3.1 the crustal extension and rifting processes affecting this region of France was mentioned. Volcanism in the Auvergne is attributed to both extension and plume related thinning of the mantle-lithosphere (Merle and Michon, 2001a; Merle and Michon, 2001b). Evidence for the presence of an upwelling mantle plume beneath central Europe as well as passive extensional rifting comes from many sources. Seismic tomography studies show a broad cone-shaped low velocity structure about 200 km in diameter beneath the central part of the Central Massif, correlating with the volcanic areas of Cantal, Monts-Dore, Chaîne des Puys and Devès-Velay (Granet et al., 1995). Gravimetry shows a strong negative Bouguer anomaly of -70 mGal beneath the Massif Central (Shields, 2009), heat flux measurements show high heat flow of up to 105 mWm-2 (Shields, 2009). The composition of the volcanic rocks show a significant lower mantle source component (Hoernle et al., 2002). All of these phenomena can be explained by the ascent of a mantle diapir causing the Chaîne des Puys volcanism.

The lava flows of the Chaîne des Puys display a characteristic inverted relief. Lava is relatively resistant to erosion and protecting also the underlying rock against it. The lava flows were initially deposited on the topographic lower river valleys. During uplift periods, relief inversion occurred causing the lava flows to stands as the topographic highs (pers.comm

9 van Wyk de Vries, 2010). The Miocene and Pliocene lavas erupted prior to the construction of the Chaîne des Puys, for example, Les Côtes de Clermont, Montagne de la Serre and the Plateau de Châteaugay, are – as indicated previously – at higher altitudes than the early Quaternary flows (Shields, 2009).

3.5 Structural geology of the Chaîne des Puys. From a structural geological perspective, the Auvergne, which is generally heavily faulted, contains two major faults (e.g. the Limagne fault and The Sillon Houiller fault), both in the region of the Chaȋne des Puys (Shields, 2009). The current morphology and tectonic structure of the Plateau des Dômes is caused by a reactivation of the faults during the Miocene (Boivin et al., 2004). That reactivation caused an elevation of the horst Plateau des Dômes and intense incision of its margin as shown in the eastern part of the Plateau des Dômes (Figure 2) (Boivin et al., 2004). The Sillon Houiller fault and its many branches, which extend over 250 km and oriented NNE-SSW, separate the west of the region from Limousin (Shields, 2009). The kinematics are still debated as the fault displays normal motion and transform-like sinistral movements as well as thrust fault characteristics (Thiery et al., 2009). The slope gradient from the Sioule valley to the western margin of the Chaîne is gradual, with a vertical increase of approximately 100 m over 3 km to an altitude of more or less 850 m (Shields, 2009). The other topographic irregularity is the north-south trending normal Limagne fault, approximately 150 km in length, where the vertical offset creating a 600 m relief between the Limagne basin and the Plateau des Dômes (Shields, 2009). The Limagne fault shows a dislocated trace, sometimes quite invisible when it is covered by lavas form the Chaîne des Puys (Figure 2).

4 Scoria cones of the Chaîne des Puys.

4.1 Morphology Monogenetic basaltic volcanoes are the most common type of continental volcanoes. They range in morphology from small scoria cone volcanoes where much of the material was erupted by explosive mechanisms with variable proportions of lava flows, to small shields and chains of shields (Valentine et al., 2008). Scoria cones are small volcanic landforms which can be found in two types of volcanic provinces: upon flanks of major volcanoes (e.g. Mauna Kea (Hawaii) and Kilimanjaro (Tanzania)) or as independent edifices in relatively flat-lying volcanic fields (e.g. Puy de Lemptégy) (Settle, 1979).

If we consider the morphology of scoria cones, we can conclude that scoria cones have in general equal characteristics in the world. A scoria cone is a small cone shaped volcanic hill whose top contain a bowl-shaped crater (Wood, 1980). These hills are built up by the accumulation of typically lapilli-sized pyroclasts with lesser amount of volcanic bombs and blocks ejected around the vents during Strombolian eruptions (Lockwood and Hazlett, 2010). The size distribution of most fragments is depending on the strength of the explosion: small scoria and ash is produced by the violent expansion of the volatiles driving the eruption. The gas content decreases towards the end of the eruption and results in the ejection of larger fragments that can well on deposition (called spatter) (Wood, 1980).

There have been a lot of morphometric studies carried out on scoria cones. The standard morphometric parameters used in these studies are: H (cone height), Wco (cone width) and

Wcr (crater width) (Wood, 1980). Scoria cones range in size from a few tens to a few hundred of meters in height, with diameters 5 - 6 times their heights. Porter (1972) was one of the first scientist to conclude that the ratio H/Wco is constant (~0.18) for Hawaiian cones. Later this relationship has been confirmed and supported by other examples worldwide. Due to erosion, the ratio H/Wco decreases with time (related to the age of the scoria cone; older ones will have lower H/Wco than younger ones) (Favalli et al., 2009) and is influenced by rainfall, climate and cementation rates (Vespermann and Schmincke, 2000). Scoria cones range from 0.25 to

2.5 km in in cone width, with a median of 0.8 km. Cone heights equal 0.18 Wco, varying from a few meters and rising to as much as 424 m such as Paricutin volcano, Mexico. Crater diameters equal on average 0.4 Wco (data from Encyclopedia of volcanoes: Vespermann and Schmincke, 2000).

4.2 Eruption dynamics and internal structure The final morphology of a scoria cone depends on following factors: total volume of erupted material, range and median value of ejecta velocity, ejection angles, wind speed and direction, nature and size of particles, vent geometry and spacing (Wood, 1980; Vespermann and Schmincke, 2000). Most of these factors are due to the eruption dynamics of a scoria cone. These factors will be described in more detail for a general reference scoria cone. The total volume of erupted material is the controlling factor on cone size (Wood, 1980). Ejection velocities range from a few tens to a few hundreds of meters per second, increasing with cone size and magma volatile content (Wood, 1980). High velocities build large cones and the smaller ones are built up by less energetic eruptions (Wood, 1980). This implies a higher weight percentage of volatiles for large magma volumes than for small volumes. Wind

11 strength and direction strongly influence cone shape. In extreme cases most scoria is deposited downwind of a vent. Most of the time the wind is not strong enough to distort the trajectories of the larger particles that form the main cone but does concentrate ash in the downwind direction (Wood, 1980).

A key point is that the median particle size of the material normally is in the range of 1-4 cm instead of 10 cm or more as reported in the classic scoria cone growth modelling of McGetchin et al. (1974) (Riedel et al., 2003). Most of the modelling work on ejecta construct growth has focussed on scoria cones and on the role of ballistic ejection. This is often called Strombolian activity. The key assumption of these models is a typical particle size of 10 cm in diameter, smaller particles will however not follow ideal ballistic trajectories as they are influenced significantly by the gas drag (Riedel et al., 2003). Most ejected fragments follow however short near-ballistic trajectories, falling near the vent and rolling downslope until they come to a state of rest which is equal to the angle of repose. Slope angles of scoria cones are in the order of 25 to 38° (generally 30-33°) (Lockwood and Hazlett, 2010). Scoria cone grows vertically until slope angles are too steep to sustain equilibrium. Following processes can occur to retain again equilibrium: failure of the scoria cone through slumping and sliding of the collapsed mass, or particle avalanching resulting in reverse grading and undulating deposit layers (Sumner, 1998).

Scoria cones are typically the products of Strombolian eruptions (Valentine et al., 2008). First a short definition of the pyroclastic fragments that will be discussed here is needed. Spatter agglutinates are formed by the instantaneous flattening of hot, soft pyroclasts upon landing and sometimes they are welded together (Wolff and Sumner, 2000). The resultant deposit is an agglutinate or spatter pile; particle outline is in part retained. Ash are non-welded, loose pyroclasts smaller than 2 mm in diameter, lapilli is between 2 and 64 mm and bombs are those pyroclasts that are larger than 64 mm. Scoria is a pyroclast containing vesicles that are typically several millimetres in diameter (Wolff and Sumner, 2000). A juvenile clast is a volcanic particle derived directly from magma reaching the surface in contrast to the blocky fragments which are fragments of solid rocks (diameter larger than 64 mm) (Schmidt and Schmincke, 2000).

Strombolian eruptions, like Hawaiian, are common forms of basaltic volcanic activity (Valentine et al., 2008). The difference between Strombolian and Hawaiian explosions is related to the mechanical way that gas exsolution occurred (Valentine et al., 2008). In

Hawaiian eruptions, gas exsolution takes place in the form of numerous, small, escaping vapour bubbles that cause the magma to fountain continuously or flow out passively. During Strombolian eruptions, gas exsolution happens by means of the development of very large bubbles in the conduit, leading to rhythmic moderately explosive discharges as they escape (Valentine et al., 2008). The explanation for the contrasting styles of vesiculation generally relates to variable magma ascent rates. Strombolian explosions involve low-viscosity magmas. The slow rising magma develops larger bubbles that have the opportunity to coalescence and grow to large dimensions (several metres in diameter depending on conduit diameter) (Gonnermann and Manga, 2007; Francis and Oppenheimer, 2004). Significant dynamical interactions between bubbles result in bubble coalescence to form gas slugs (Houghton and Gonnermann, 2008). Large gas slugs (~large gas pockets (Vergniolle and Mangan, 2000)) rise to the surface at much faster rates than the melt (due to a greater buoyancy in comparison with the magma), resulting in a relatively volatile-depleted magma in the shallow conduit (Lautze and Houghton, 2005; 2006) and Strombolian style eruptions when the over pressurized bubbles bursts at the surface of the magma column (Vergniolle et al., 1996). Sparks (1978) considered the dynamics of bubble growth in detail. Bubbles grow fastest for low rates of magma ascent, high diffusion coefficients, high volatile contents and low values of the solubility constant (amount of volatiles that can be dissolved in the magma). On bursting, large bubbles produce high gas velocities, so the difference in explosivity between small and large scoria cones may be due to bubble size. This suggests that large cones, with their greater explosivity, are fed from deeper magma chambers than the smaller cones. The depth of the magma chamber correlates also with the separation distance between different scoria cones for example large scoria cones have large separation distances which is related to a deep magma chamber (Wood, 1980). That conclusion is observed by Settle (1979) at Paricutin, San Francisco volcanic field, and Nunivak Island (Alaska), where scoria cones have larger median basal diameters and greater average spacing between the cones than the relatively smaller scoria cones on Etna.

Strombolian activity generally takes place at open vents where fluid lava is rising at a high level in the conduit (a few tens of meters below the rim) (Lockwood and Hazlett, 2010). During the eruption some blobs of lava are thrown up and striking the walls of the conduit near the rim. Some of these blobs are still molten and form spatter agglutinate whereas others are cooled during their flight and reaching the ground in a solid condition (Lockwood and Hazlett, 2010). If magma level rises, more of the ejecta will exit the vent in a partly fluid

13 condition, piling up around the vent as spatter (Lockwood and Hazlett, 2010). Initially clasts are deposited onto the pre-existing surface but a raised rim rapidly develops at the distance of maximum clast accumulation (Valentine et al., 2008). Repeated explosions continue to build this rim, and grain avalanching maintains the outer cone slopes close to the angle of repose (Valentine et al., 2008). Recycling of previously pre-erupted bombs (broken into smaller lapilli and blocks) rolling down the inner cone slopes back into the vent, to be re-erupted again with some amount of juvenile material during subsequent bursts (Valentine et al., 2008). As a result, proximal facies tend to be a mixture of roughly equant, angular to variably abraded lapilli and blocks along with fluidal or flattened bombs (Valentine et al., 2008). Multiple episodes of avalanching and changes in explosion characteristics during a single eruption result in internal unconformities as the rim changes locations. Sufficient juvenile material ejection during low, short-lived fountains, results in locally high accumulation rate and welded to partially welded facies that are limited to tens of meters in lateral extent (Valentine et al., 2008).

The type-locality of Strombolian intermittent eruptions, is Stromboli located off the southwest coast of Italy (Sicily). The most typical activity consists of explosive ejections of incandescent pyroclasts and spheroidal of fusiform bombs thrown to heights of a few tens to several hundred meters above the source vents (Valentine et al., 2008). This activity may or may not be accompanied by the discharge of lava flows. If lava emerge, it is more viscous than that of Hawaiian eruptions and forms somewhat shorter and thicker flows. Each explosion generates a minor ash plume which is usually less than 200 m high. The volume of material produced in each explosion is small, ~10-3 to 100 m3 (Valentine et al., 2008).

The material of scoria cones consists of welded and/or non-welded scoria, lapilli, bombs and minor amounts of loose coarse ash (Vespermann and Schmincke, 2000). Scoria cones have a basaltic composition and are therefore dark coloured. Because off high temperature oxidation some parts of the scoria cone can turn red. Through personal observations of scoria cones deposits, I observed that scoria cones (e.g. Puy de Lemptégy) are composed of following pyroclastic fragments: black and red-brown scoria, bombs, lithic fragments and minor amounts of ash. The scoria and bombs can have a varying degree of welding and vesicularity. Welding of fragments is the development of mechanical strength between hot pyroclasts which are in close contact causing them to stick together (Parfitt and Wilson, 2008). Larger materials are in a molten condition and can be strongly flatten to form “cow-dung (cow-pie)” bombs (Sumner et al., 2005). The large sized fragments landing on the outer slopes, roll

14 downwards to form a coarse-grained ring around the foot of the cone (Valentine et al., 2008). Most products of Strombolian eruptions are deposited in the proximal cone facies, with only limited fallout beyond the cones themselves (Valentine et al., 2008).

There are 2 main facies that can be distinguished in a scoria cone: an inner crater facies and an outer wall facies. The inner crater facies deposits have following characteristics. Starting with the lower crater facies which is built up of erupted lava spatter essentially hot on landing that welds together to form agglutinate gradual moving towards the upper crater facies containing round bombs in a poorly sorted matrix of lapilli overlain by well sorted lapilli layers of several meters thick (Vespermann and Schmincke, 2000). The transitional area (going from the upper crater towards the outer wall facies) contains scoria fragments which are welded together but they can still be recognized as individual pyroclastic fragments (Vespermann and Schmincke, 2000). The deposits of the outer wall facies are characterized by the start of a basal fine ash layer overlain by coarse-grained, poorly sorted breccia layers extremely rich in accidental fragments (Vespermann and Schmincke, 2000).

There must be some attention made on classifying volcanoes as a Stromboli type eruption. Characteristic features of Strombolian eruptions are that they consist of transient explosions, closely spaced in time, involving low viscosity, generally basaltic magma and occurring within volcanic systems which are open to the surface (Parfitt and Wilson, 2008). Stromboli‟s mild activity is interrupted at intervals of a few months to a few years by episodes of more violent eruptions. Over the decades, volcanologists developed a habit of using the terms Hawaiian and Strombolian to describe landforms. For example, scoria cones (as discussed above) are commonly referred to as “Strombolian cones”, and fissure-fed basaltic eruptions as “Hawaiian fissures”. This gives an assumption that a given volcanic landform, a scoria cone, is produced by a single, presumably well-understood, eruption process (Valentine et al., 2008). However it is possible that different eruption processes contribute in the formation of a single scoria cone (Riedel et al., 2003). In the following sections, we intent to describe the deposits from the scoria cone “Puy de Lemptégy” as objectively as possible and deduce the eruption style or multiple eruptions styles or the occurrence of transient eruptions styles.

4.3 Neighbouring cones of Puy de Lemptégy. Before discussing the volcano “Puy de Lemptégy”, the pyroclastic constructs found in its direct surrounding will be described. Some of these have left their imprints at Puy de

Lemptégy, we find the deposits from Puy des Gouttes, Puy de Côme and Puy de Chopine. We will discuss the pyroclastic constructs from the oldest eruption towards the youngest.

Figure 4: Neighbouring cones of Puy de Lemptégy (modified from de Goër de Herve et al., 1999) 4.3.1 Puy de Petit Sarcoui The age of the Petit Sarcoui is estimated at 31.5 ± 5.0 ka. The dating was carried out on a massif lava flow by using thermoluminescence at feldspars (Guérin, 1983). It is classified as a scoria cone, situated near the Grand Sarcoui. This scoria cone is partially destruct by later explosions (Boivin et al., 2004).

4.3.2 Puy des Goules The age of Puy des Goules (Figure 5) is estimated at 31.5 ka, erupted simultaneously with the Lemptégy I eruption. The pyroclastic construct is classified as a simple, closed (no crater visible) scoria cone. The composition of

the lava is trachy-basalt (SiO2 range from 48 to 51 weight%) (Boivin et al., 2004).

Figure 5: Puy des Goules © Bruno Monginoux (from www.photo-paysage.com (visited on 02/02/2011)

4.3.3 Puy des Gouttes The age of Puy des Gouttes (Figure 7) is estimated at ~30 ka. At the quarry of Lemptégy, several places with deposits of Puy de Gouttes were observed. Based on the observations of the deposits of Puy des Gouttes, there have been 2 types of eruption phases. The deposits resulting from the first type of eruption consist of mainly lapilli rich black coloured scoria, in some parts turning into red when deposited on the facies of Lemptégy I (see explanation

16 further in this paper). The other part of the Puy des Gouttes deposits is related to the final eruption of Puy des Gouttes. It contains brown ash mixed with lapilli (black and red) and a lot of small fragments of granite. In these deposits we can observe some sedimentary; ripple like structures, channels and unconformities. Afterwards these deposits are affected by tectonic processes (e.g. extensive and compressional tectonic regime). The sedimentary structures were interpreted as the sign that the final eruption phase of Puy des Gouttes was a phreato- magmatic phase (de Goër de Herve et al., 1999; Boivin et al., 2004).

4.3.4 Puy de Côme Puy de Côme (Figure 6) is a large scoria cone located at a distance of 2 km SSW from Lemptégy. With a cone height of 240-260 m, it is one of the most volumetric scoria cone of the Chaîne des Puys. Puy de Come has two nested concentric craters. The lava flow is described as an aa-lava or blocky lava and is extremely viscous (de Goër de Herve et al., 1999). At the Lemptégy quarry we observe the Puy de Côme deposit as a fallout deposit containing lapilli size scoria and greyish ash deposit above the final deposits of Lemptégy 2 (de Goër de Hervé et al., 1999). The bottom and top of the Côme deposit shows at some locations wavy character. The thickness varies from several decimetres to a few meters. The composition of the scoria and ash is trachy-andesite-basalt. The deposits of Puy de Côme have also sedimentary characteristics (e.g. undulations, internal discordance) and periglacial structures (e.g. soliflux) (de Goër de Hervé et al., 1999). Especially when we take a look at the ash deposits located at Lemptégy we

Figure 6: Puy de Côme (Boivin et al., 2004) observe no soil layer between Lemptégy II and Puy de Côme. When we take a closer look to the age of the eruption, there may have been two different eruption periods, one occurred 15.9 ka B.P (e.g. dating on a massif lava piece) and the last eruption dated 7.6 ka B.P (e.g. dating on a bomb by thermoluminescence on feldspar) (Guérin, 1983). When we correlate the ages with the geological time schedule we see that both ages agree with a periglacial environment, the Dryas period.

4.3.5 Le Grand Sarcoui The age of “Le Grand Sarcoui” is estimated at 12.6 ± 1.0 ka on a massive lava piece by thermoluminescence and U/Th disequilibrium (Miallier et al., 2003). Le Grand Sarcoui is the oldest trachytic dome of the Chaîne des Puys. The cone width is about 800 m and the cone height is 250 m. With a 60° southern slope, Le Grand Sarcoui has steep flanks. The deposits of Le Grand Sarcoui are described as ochre-red stratified tuff layers with basement material incorporated (granite) (Boivin et al., 2004). Above these tuff layers there is a chaotic mass composed of volumetrically blocs and trachytic fragments in a trachytic ash matrix (Boivin et al., 2004). The chaotic mass is the result of small pyroclastic flows (nueés ardentes). At the top of Le Grand Sarcoui we can find the very viscous lava flow (Boivin et al., 2004). After the end of the eruption the nord-western flank became unstable and collapsed, causing debris avalanches (Boivin et al., 2004).

4.3.6 Puy de Chopine The age of Puy de Chopine (Figure 7) is estimated at 9.72 ± 0.27 ka based on tephra deposits dated by radio carbon and palynology (Brousse et al., 1970; Verner and Rayal, 2000). Puy de Chopine is classified as a trachytic protrusion or lava dome (Boivin et al., 2004). The cone height is 160 m and the cone width is 500 m. The flanks are steeply dipping because of the very viscous lava which was unable to spread laterally. The eruption dynamics of Puy de Chopine are quite remarkable. Before the Chopine protrusion was developed, the eruption started with an explosive phase which created a maar within an already existing strombolian cone: Puy des Gouttes (Boivin et al., 2004). The explosive phase was accompanied with hot pyroclastic flows carrying blocks, lapilli, ash and fragments related to the destruction of a part of the Puy des Gouttes (Boivin et al., 2004). It contains also older fragments of the basement such as granite and schist. The deposits of Puy de Chopine has an estimated range of 2 to 5 km in all directions and covers an area of 30 km². In the quarry of Puy de Lemptégy, 6 deposits units of Puy de Chopine (LU1 to LU6) are

Figure 7: Puy de Chopine (right) and Puy des identified as continuous layers towards the top Gouttes (left) (Boivin et al., 2004) of the quarry (de Goër de Herve et al., 1999). The thickness of the total sequence ranges from 50 cm to 3 m in the nord-west of the quarry, containing the most complete section (Figure 8). Remarkable is the whitish colour of the deposit related to the trachytic composition of Puy de Chopine. Layer LU1 contains charcoal

18 fragments (wood) and has a chaotic appearance with angular blocs in a matrix of ash and lapilli. LU2 is mostly made of lapilli, ash and containing fragments of the eruption of Puy

Figure 8: Panorama view at the Chopine deposits at Lemptégy. des Gouttes: the red scoria, basaltic lava fragments and old basement pieces. The next layer LU3 is made up of alternating pale rose coloured ash and lapilli. Layer LU4 and LU5 are built of the same fragments (ash, lapilli, basement fragments and scoria of Puy des Gouttes). The youngest layer LU6 is strongly weathered and developing a soil layer (de Goër de Herve et al., 1999). These sequence is not the whole stratigraphy of Puy de Chopine, there are other layers deposited elsewhere in the region (near Vulcania and Marsat) (de Goër de Herve et al., 1999).

4.3.7 Le Clierzou The age of Le Clierzou is estimated around 9.0 – 9.5 ka (de Goër de Herve et al., 1999). It is again a trachytic lava dome. Because of the extreme viscosity of the lava, the lava flow appears in the radial fissures caused by the expansion of the dome (Boivin et al., 2004).

4.3.8 Le Pariou The age of Le Pariou is estimated at 9.3 ± 0.2 ka based on dating of massive lava pieces and tephra deposits using radio carbon dating and thermoluminescence (Brousse et al., 1970; Guérin, 1983). Le Pariou consist of a complex edifice, the eruption began as a phreato- magmatic phase with a trachytic composition followed by a Strombolian phase (de Goër de Herve et al., 1999).

4.3.9 Le Chaumont and Puy de Fraisse The age of both pyroclastic constructs is not known. Le Chaumont is located eastward of Puy des Gouttes and Puy de Chopine. Puy de Fraisse is located southeast of Puy de Lemptégy.

Both are basaltic in composition (Boivin et al., 2004).

4.4 Puy de Lemptégy. This project aims at defining a complete stratigraphy of the volcanic layers deposited during the eruption of Lemptégy II. First of all, we will give a brief introduction about Puy de Lemptégy and a short summary of the early investigations done by Audray Delcamp, (Delcamp, 2005) who provide us with a plan view map of the quarry and the intrusive complex.

The Puy de Lemptégy consist of two edifices: Lemptégy I and Lemptégy II. Before the excavation started in 1946, Puy de Lemptégy was a small strombolian cone with a cone height of 50 m (altitude 1019m). After the second World War, there was a great need for gravel to build up houses and roads (de Goër de Herve et al., 1999). The volcanoes of the Auvergne were of great importance to deliver the quantity of gravel needed: the volcanic scoria (the “pouzzolane d‟Auvergne”). The Puy the Lemptégy was transformed into a quarry and till 1970 extensively exploited. After 1970, the volcanic scoria did not need the requirements of the industry anymore and the excavation came to a halt. In the following years, scientist of the university of Clermont-Ferrand had a great opportunity to do detailed research at Puy de Lemptégy (de Goër de Herve et al,. 1999). During the excavation of the edifices, dykes and other features became visible for the eye and Puy the Lemptégy became a great opportunity to study the intern structures of a scoria cone. This characteristic was used in the 90‟s to organise public guided tours in the volcano quarry and the concept of Volcan à ciel ouvert was born. Each year, more than 50.000 tourists visit Puy de Lemptégy to understand and to observe the internal structure of a volcano (Figure 9 and Figure 10) (de Goër de Herve et al., 1999).

Figure 9: Geological map of Puy de Lemptégy (Delcamp, 2005 modified geological map Boivin et al., 2004)

Figure 10: View at Puy de Lemptégy (Delcamp, 2005) (photograph made by Nathalie) In the 90‟s, scientists did research to document the geochemical composition of certain cones including Lemptégy I and II. At page 56, we see the results of the geochemical analyses carried out at a key section from Lemptégy II in order to confirm the earlier geochemical results. These results pointed out that Puy de Lemptégy is a double scoria cone erupted at ~ 32 ka B.P with emission of trachy-basalt that led to the formation of the first scoria cone: Lemptégy I. At ~ 30 ka B.P, a trachy-andesite lava flow caused the partial collapse of the Lemptégy I edifice through the west and a second edifice was built: Lemptégy II (Delcamp, 2005).

The study made by Delcamp (2005) is mainly based on the second edifice, Lemptégy II. The main purpose was to create a plan view of the quarry with the exact locations of the dykes and other magmatic related features plus to document the mechanism and orientation of dyke emplacement (pers. comm., van Wyk de Vries and Delcamp, 2010). The investigation was accompanied by means of structural and micro-structural study of the quarry through field work and thin sections observations (Delcamp, 2005). Intrusions emplaced in this scoria cone appears in the form of large bulges of magma (~cryptodome), thin or thick dykes and a central conduit system (Delcamp, 2005). A cryptodome is formed by the uplifting of older rock as magma is intruded from below but never reaches the surface (Lockwood and Hazlett, 2010), and its location is at the west side of Lemptégy II edifice. Eruptive fissures and 3 spatter cones formed early in the Lemptégy II eruption. Fissure eruptions result when magma-filled dykes intersect the surface (these dykes are observed in the quarry). Spatter fed flows are associated with these features (Delcamp, 2005).

In the following paragraphs, we will discuss the features indicated on the map produced by Delcamp (Figure 12). Many dykes were documented, which can be organised in several types: thin dykes which have a thickness from 40 cm to 1.40 m (letter D, H, I, K, L, M and the central part of dyke A) and thick dykes, which have a thickness of 2 to 5 m. These thicker intrusions were seen to cause deformation of the overlying material (letter B, C and the western branch of dyke A which is oriented east-west) (Delcamp, 2005).

The bulges (indicated with numbers on the map) are swollen intrusions. The cryptodome (noted on Cr at the map) is the termination of dyke D coming from the centre of Lemptégy II and was flown to the exterior of the volcano but ended in a cryptodome (which is an accumulation of molten material just beneath the surface (Fink and Anderson, 2000)). The layers above are deformed (Figure 36, detailed observation of the cryptodome and the deformed layers) (Delcamp, 2005).

The spatter cones (number 1 and 3 on the map) are at a maximum of 5 m height. Massive and dark levels with rare elongated vesicles, pseudo fiammes and scoria are deposited around the hole (Figure 11). These layers can cover non-welded products as well as welded scoria. During a strombolian eruption products can flatten and become welded due to the impact or due to compression caused by later deposits (Delcamp, 2005). If weight and heat become too high the scoria can be remobilized and create a low expansion lava flow, a spatter-fed flow

(Sumner, 1998). This phenomenon can explain the massive layer formation around the spatter cones.

Figure 11: View at a spatter cone at Lemptégy (From Delcamp 2005). Based on the observations made by Delcamp (2005), we can conclude how magma flow occurred at Lemptégy II, how the dykes propagated and have a better understanding of the intrusive system of a scoria cone.

At Lemptégy II, most of the magma flowed from the exterior to the centre of the edifice. Propagation of the dyke occurred by magma infiltration through scoria rather than by crack opening (Delcamp, 2005). Propagation by crack opening will cool the magma more rapidly. Some dykes still had enough magma supply and they bulge either at their flanks (extremely low magma flow) or at their tips. These dykes are intermediate between dykes and bulges (Delcamp, 2005).

Another interesting feature at Lemptégy is the repartition of the bulges and dykes (Delcamp, 2005). Except from the cryptodome in the west, the bulges are concentrated at the north east of Lemptégy II centre and the dykes are located in the west. The eruptive fissures separate these two types of intrusions. The first activity centre, Lemptégy I, is to the east of Lemptégy II. Bulges are located in the northeast of Lemptégy II suggesting that the propagation of intrusions to the east was prevented by the Lemptégy I construct (Delcamp, 2005). Lemptégy I was no more active, but the stress field caused by the strength of rock material prevented intrusions to propagate towards Lemptégy I.

At Lemptégy, studying the intrusive system enable the reconstruction of the first stages of cone growth. Activity began with the formation of three spatter cones. Then the eruption continued along fissures (Delcamp, 2005). The cone of Lemptégy I blocked the expansion of the dykes east of Lemptégy II which caused the formation of bulges. Magma flow in the west was channelized through the dykes towards the centre. Small lava flows (spatter fed flows) are emplaced from spatter cones (Delcamp, 2005). One more extended lava flow was fed from the summit (from the eruptive fissure) or from the base (from a dyke that extend the eruptive fissure). This lava flow is responsible for the edifice collapse (Boivin et al., 2004) towards the south (Delcamp, 2005).

Figure 12: Plan view of Lemptégy II (modified from Delcamp, 2005)

5 Methodology of fieldwork and laboratory work.

5.1 Fieldwork In order to document the stratigraphy of Lemptégy II, fieldwork started in July 2010 at Lemptégy. The first week, I got an introduction tour of the “Chaîne des Puys” and the Lemptégy quarry by Prof. Benjamin Van Wyk De Vries together with two American volcanologists: M. Petrons and G. Valentine. The purpose of this tour was to give a brief description of the early research done at Lemptégy II. Most of this research was carried out by Delcamp (see section 4.4).

The available LIDAR (Light Detection and Ranging) data were used to construct a DEM (Digital Elevation Model) model with absolute height values. To obtain the absolute height values, we choose objects at Lemptégy which were clearly recognizable on the LIDAR elevation data to obtain X, Y and Z coordinates with the GPS. 17 points were used to reconstruct an absolute elevation model and to correlate points with equal elevation values. All points at the DEM map have now absolute elevation values (Table 1, Figure 13). The GPS coordinates were measured for a long time at 1 location point to obtain the mean value. This method is used because the accuracy goes down and gives a more accurate result in the X and Y values. The accuracy of the Z-values will be significantly larger (~5m).

Table 1: elevation values for the DEM map

Describing position Waypoint X Y Z(m) Accuracy (m) Top of the Lemptégy II conduit (5*5 square) 96 495810 5073824 978 3.5 Second track (cross section dyke 1) 98 495816 5073788 966 2.0 Small house 97 495805 5073725 972 3.0 Dyke 2 99 495805 5073827 967 4.0 Levee channel 100 495748 5073889 971 2.0 Bomb 2m high on the track 101 496028 5073869 967 3.9 Small top in the length of the stadion left lower corner 102 496008 5073828 965 2.0 Bomb near track 1 (upper track) 104 496018 5073791 965 3.8 Small hill 105 495988 5073764 958 3.5 dyke 106 495957 5073925 958 2.2 Down in center Lemptégy 1 107 495892 5073866 944 2.2 Stadium lower right corner 108 495889 5073829 955 2.2 Stadium upper right corner 109 495889 5073817 958 2.0 Stadium upper left corner 110 495924 5073813 958 / Stadium lower left corner 111 495925 5073822 954 / Tip cross section of train roads 112 495928 5073722 965 1.8 Tip cross section of train roads opposite side 113 495900 5073743 963 1.2

Figure 13: DEM map with GPS points measured (red dots) (image from Stephane Petit) To have a good approximation of the stratigraphy there is need to set up a lithology litholog of each section in the quarry. Therefore the quarry was divided into several sections (Figure 16), which show a well visible stratigraphy of the deposited Lemptégy II layers. From each section where a litholog was made, the GPS coordinates as well as the direction of the litholog were measured. Using the different tracks (made for the train visit and the walkers guided tour) it was possible to made lithologs covering the whole height of the quarry walls.

The quarry was divided in different sections, 6 in total, based on the appearance of the Lemptégy deposits. For each section at least three lithologs were made (Appendix A: legend for lithologs). A detailed description of each layer was carried out to obtain more information on sorting, grading, maximum grainsize of the clast types, quantitative proportion of the clasts, grain shape and thickness of the layers. All these observations were made in the field and afterwards samples were brought to Ghent university for laboratory analyses (e.g. sieving, density measurements and preparing samples for geochemical and thin section analyses). From each layer there were several pictures taken. Overview pictures of the different sections

26 were also taken. These photographs were afterwards adapted in Photoshop to create panorama pictures. These panorama pictures were used to draw stratigraphic contacts.

The description of the lithologs is carried out following the methodology described by Cas & Wright (Cas & Wright, 1987). Making lithologs is a powerful tool to correlate the different layers with each other from each section and set up a stratigraphic map for the whole quarry. From key layers, samples were taken to carry out some lab analyses (e.g. density, geochemical analyses).

For all the lithologs that are described in this paper I start from the oldest deposited layer towards the youngest (from bottom to top). For section 3 I will describe each litholog made. In this section samples were taken in order to deduce more information about the vesicularity, sorting, mineralogy and chemical composition of the pyroclasts. The mineralogy and geochemical composition of the deposits is carried out at different heights in the reference section, because it is the most complete section of the Lemptégy II deposits.

Separately from the observations of the typical clasts, also the bombs were described and measured. From these bombs the long and short axis were measured by using a tape measure (accuracy ~1cm) and a detailed observation was made of the vesicles, colour, crust and other features. Afterwards the ellipticity ratio of the bombs was calculated.

5.2 Grainsize distribution Grain size distribution analyses are made with a set of sieves with mesh sizes spaced at one- phi (Φ) intervals (where Φ = -log2d , d being the grainsize in millimetres). The 64, 32, 16 and 8 mm size classes were sieved in the field, and weighed on a portable balance to 0.1 grams. Sieving the largest clasts in the field was done to minimalize the material that has to be transported to Ghent, only the fine fraction (< 8mm) was transported back to Ghent for further sieving. There are no set rules governing the size of the sample that should be collected for a routine sieve analysis of pyroclastic or volcaniclastic deposits (Cas and Wright, 1987). In many cases this is determined by the practicalities of the amount of material that can be transported back to a field camp or laboratory. The sample size needed to give a representative grain size analysis of a deposit becomes larger with increasing maximum grainsize and is also larger if the sorting in a deposit is apparently poor. From the grainsize distribution we can conclude something about the sorting of the layers which depend on the emplacement mechanism and eruption dynamics (Table 2) (Cas and Wright, 1987).

Table 2: description of sorting for pyroclastic deposits (Cas and Wright, 1987)

Sorting (σφ) Pyroclastic deposits 0-1 Very well sorted 1-2 Well sorted 2-4 Poorly sorted > 4 Very poorly sorted

For this study, the sample weight was around 7 kilo. From each section (except section 1 (too fragile) and section 6 (not able to sample)) there were several samples taken for grainsize analyses. From the key section several layers were sampled to deduce if sorting changed or if the grainsize changed with height in the section. Only the grainsize distribution analysis of section 3 and 4 are presented in this study. These sections contain layers were the pyroclasts have equal characteristics.

After field sieving the remaining material was brought to Ghent to sieve mechanically with the sieve tower. Mechanical sieving results sometimes in breaking of the juvenile pyroclasts especially when these clasts are pumice. The sieved material used here was scoria material which is less suspicious for breaking. The sieving procedure was carried out at a low amplitude and the sieving time was no longer than 10 minutes. The material retained at each sieve was weighted with the analytical balance (accuracy 0.0001 gram). The results are presented in a histogram in which the weight percentage of the fractions are plotted against the corresponding phi-values. σφ is a sorting coefficient (Cas and Wright, 1987). More explanation about the σφ parameter is given in the paragraph discussing the grainsize distribution and sorting of the layers (page 53).

5.3 Density measurements From some layers samples were taken for density measurements. Especially from the fine- grained layers alternating the coarse-grained layers at the reference section (section 3). The purpose of the density measurements is mainly to deduce if the fine layers contain more or less dense material (e.g.: lithics, low vesiculary pyroclasts) than the coarser layers. Samples for density measurement were taken from an outcrop at key section 3 and at section 4 where both sections containing clearly visible fine-grained layers.

To obtain the bulk density of the different scoria fragments we use the principle of Archimede. First step in this procedure is to set up the measurement construction. We need a measuring cup of 250 ml filled with destillized water (and the accessories) that we put on the

28 analytical balance (Figure 14, picture A). Secondly, I start the measurements with weighting the dry fragment (Figure 14, picture B). If we want to know what the amount of vesicles is in the fragments, we have to make sure that the fragment is sealed off. One way of doing that is to use a coating layer which protects the fragment from the water which can otherwise infiltrate into the vesicles and dispersing the present air in the vesicles. To avoid the infiltration of water we use laboratory hand gloves. We put the fragments in the fingers of the glove and make sure everything is vacuum and holding that vacuum state during the measurement by use of a nylon wire to tie up the glove. Third step is to measure the weight of the fragment plus the coating (Figure 14, picture C). Fourth step is to bring the fragment on the disk of the hook hanging in the water and measuring again the weight. When the fragment has a density that is less than the water density we have to put our fragment under the disk of the hook. The fragment will create an upward force based on the principle of Archimedes (Figure 14, picture D). Last step in the procedure is to put our weights into a formula which allows us to obtain the density of the fragment.

Density formula:

With ρsa = density fragment

ρ1 = density water

msa = dry mass fragment

mspl = mass fragment + coating under water

mpa = mass coating

ρpa = density coating (~0.9 g/cm³)

The Dense Rock Equivalent density (DRE density) (powdered sample) can be estimated from the geochemical composition and calculating the vesicularity in terms of:

1 – (ρBULK / ρDRE) = vesicularity

Figure 14: pictures of different steps of lab procedure for density measurements. Picture A: preparation, picture B: sample weight, picture C: sample weight + coating and picture D: principle of Archimedes; sample + coating in water (red circle: position of sample) 5.4 Thin sections Microscopic observations of thin sections gives the possibility to have a closer look at the minerals and vesicles composing the rock. The samples used for the thin sections are from an outcrop located at the key section (section 3). From this outcrop several layers were sampled (scoria pyroclasts). The purpose was to investigate whether the mineralogy changed during the deposition of the layers or not. From this eruption we might expect no drastically change in mineralogy composition but the proportion of them relative to the amount of glass and vesicles present, can evolve. Knowing precisely the minerals that constitute the scoria, we can deduce some more information about the composition and cooling history of the parent magma.

5.5 ICP-OES From the thin section samples, there were samples taken for chemical analyses with ICP-OES (Inductively Coupled Plasma – Optical Emission Spectroscopy). That allows us to document the chemical composition of the material (whole rock composition), studying if there is an evolution in the chemical composition of the material during the eruption and to find out which minerals are related to this evolution.

ICP-OES is a standard procedure in order to deduce the geochemical composition of a rock. Beside the major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K en P) also some of the trace elements can be analysed: Sr, Ba, Zr, Cr, Co, V, Sc, Ce, Y, Ni, Zn, Cu, La, Nd, Dy and Yb.

Only 0.2 gram of powdered rock is needed for the analyse. We have pulverized a larger quantity of the sample material to obtain a representative bulk composition for each sample.

30 to 40 grams from each scoria sample was enough to obtain a representative analyse for the whole rock analysis (pers. comm., M. Elburg, 2011).

Before the actual geochemical analysis could be done, there were several preparation steps in the procedure. First the samples were pulverized with the jaw crusher until the fragments were less than 6 millimetres size. Cleaning the jaw crusher with high pressurized air and alcohol was needed to avoid contamination between the samples. The actual pulverizing happened in an agate mill, respecting the cleaning procedure with alcohol to avoid contamination between the different samples.

Before determining the composition with ICP-OES, the weight loss of the samples by ignition (loss on ignition LOI) was determined. In a first step the adsorbed water in the sample was evaporated at 100°C and the sample weighed after cooling. The second step was heating the samples till 950°C (mafic samples) for several hours and after cooling the samples were weighed again. The weight loss on ignition (LOI) gives an idea on how much water was present in the structure of some water bearing minerals. The LOI value will be influenced by weathering processes.

After this heating procedure, the pulverized samples were molten in a lithium-borate flux

(ratio sample/flux = 1/5) at 1050°C and dissolved in 2% HNO3. Thereafter the analytic solvent was atomized in a plasma where atoms and ions are excited. The excited state is short-lived and the atom or ion returns to his ground state by emitting a photon. This photon is atom- or ion- characteristic and is detected by the detector. Primary standards are used to set up a calibration line against which the samples are analysed (pers. comm., M.Elburg, 2011).

6 Stratigraphy of Lemptégy II: field and laboratory results for reference section In the following sections, the objective is to complete the map with the vertical sections seen at Lemptégy II. When there is a complete map available with the different layers accumulated during the eruption phases, a reconstruction of the Lemptégy II eruption can be made. A detailed description of the different deposits and related tectonic structures will be given in the next major part of this paper.

The lithologs of section 3 are the longest and most complete lithologs made from the Lemptégy II deposits. On the overview pictures I will highlight the most important stratigraphic lines within this long section (almost the entire width of the Lemptégy II scoria cone). Otherwise it would not be easy to distinguish all the layers observed in the field. I will use the lithologs to make a proper correlation. In the following paragraphs I will incorporate the density measurements, the sieving results and the major element chemistry carried out on samples from this key section.

Figure 15: Overview picture of the Lemptégy scoria cone (entire width of cone ~400m and arrow indication for nord) (Photograph made by Nathalie)

Figure 16: Overview picture Lemptégy scoria cone (arrow indication of Nord) with location of sections and tracks (see legend) (Photograph made by Nathalie)

6.1 Litholog 2 section 3: stratigraphy The second litholog (Figure 17) from section 3 is made at the transition with section 1. That is useful to correlate afterwards layers observed in section 1 with those from section 3.

Figure 17: section 3 litholog 2: stratigraphy and description of main clasts (grainsize = maximum (unless otherwise stated))

Layers A, B and C are observed from track 1 (Figure 17). Layer A consists of black scoria clasts. On Figure 17, clast size is ~15cm but scoria with a minimum size ~2 to 4mm were also observed. The layer shows normal grading towards the lava flow. The transition towards layer B is marked by the emplacement of a greyish lava flow (Figure 18, picture C). The lava flow is massive (few vesicles observed) and can be lateral followed for ~4m. Layer B contains for more than 95% red scoria material. In between these coarse scoria fragments much smaller (~3cm) scoria clasts are observed. Probably formed by the friction between the edges of the coarser fragments that breaks into smaller clasts. No grading visible and layer B is poorly sorted because of the very large range in particles size. Layer C is a chaotic deposit which is very poorly sorted and contains red coloured scoria clasts, greyish bombs and some angular fragments with a red crust and black interior. Maximum grainsize of scoria and angular clasts is ~10cm.

Moving upwards to the second track, I identified the following layers: D, E, F, G, H and I. Layers D and E contain red scoria clasts with uniform grainsize (~6cm) in both layers. Moving up towards layer E there is a gradual increase in the amount of black scoria clasts. At the top of layer E, a sharp contrast in the typical pyroclasts was observed. Here most of the fragments are red angular particles. Layers D and E are better sorted than the previous layers. The transition towards layer F is sharp and characterized by a very different particle type than all the other layers from this litholog. Except from layer H which contains identical particles An equal thickness ~5cm is observed for both layers. Both layers (F and H) are characterized by a yellow greyish colour with very fragile and small clasts ~0.5 – 1cm. The clasts are highly vesicular observed by the occurrence of large and long vesicles and low clast density (page 48). These vesicles are connected with each other with very thin rope like structures. Because these fragments are so fragile, they can be easily crushed with the fingers. There is a lot of fine sand sized material in between these clasts. Layer F can be followed over a long distance along section 3 crosscutting the different tracks unlike layer H which seems to be a lens- shaped structure. In between these thin layers, layer G is deposited. Easily recognized as a new layer because of the typical black scoria clasts and black angular clasts present. No grading visible and the layer G seems to be well sorted. There are almost no bombs present except from 1 that I observed (~20cm in diameter). Layer I contains the same pyroclasts as in layer G and approximately the same thickness ~80cm. The only difference is an increase in black angular clasts towards the top of layer I.

The following layers J and K are observed from the third track in the quarry. Layer J, a very thick layer (~4m) can be divided into 3 parts where the bottom and upper parts contain more black, angular clasts (~60 volume%) than the central part. The central part consists of black scoria (~8cm) and a minor amount of black angular fragments (15% of the total clasts in volume). Also black, horizontally lying bombs are present but less than in the bottom and upper parts. For this layer, if you hold the scoria fragments in the sunlight, the scoria reflects blue and purple colour. The transition towards layer K is gradual and observed by the occurrence of an increase in the amount of black angular fragments (95 volume%). This layer shows a reverse grading in its lower part and a normal grading in its upper part.

Layers L, M and N are observed from the fourth track. Layer L consist for more than 90% out of red scoria, a few are black coloured. The maximum grainsize is ~3cm for the red scoria and ~10cm for the black scoria. The transition towards the next layer, layer M is very irregular and sharp. Layer M contains ~90 volume% of red, angular fragments. The layer shows invers grading. The maximum grainsize class is ~6cm, the finer are ~3cm. The transition towards layer N is sharp. Observed by the change in particle type: from angular to scoria clasts in layer N. Layer N contains ~95 volume% red scoria clasts, showing reverse grading. The bombs are greyish and flat lying. A general remark for the three layers, the thickness does not stay equal due to the irregular transitions to the next layer.

The highest track in the quarry is track 5 from which a single layer can be observed (layer O). This layer contains ~85 volume% of red scoria clasts and a few black scoria clasts. No grading is observed. The presence of the red coloured flat lying bombs is remarkable.

6.2 Litholog 1 section 3: stratigraphy At section 3 there are 5 tracks where I can stand on to observe the different deposits (Figure 19). The total thickness of the deposits shown at the litholog is ~25m. Layers A, B and C are observed from the first track. Deposit A contains 3 layers with red coloured lapilli sized clasts (~6mm). Within these layers grading is observed, from normal to reverse towards normal grading. The transition between deposit A and layer C is made by the presence of a brown, thin (~2cm) soil layer (layer B). Layer C contains the typical scoria particles which I found at the different sections in the quarry. The scoria clasts have a very irregular form with sharp edges (which are found at almost all the layers of section 3 except at the most elevated deposit). The layer (layer C) contains also a minor part of black angular clasts (20% of the total amount of pyroclastic clasts).

On track 2 layers indicated as E and F at the litholog are observed. Layer E contains black scoria clasts and bombs which some of them are horizontally emplaced and other show an orientation towards the Lemptégy II vent. The transition towards layer F is gradual and not easy to observe. The main characteristic of layer F is that it contains a lot more bombs (mostly at the top of the layer) in comparison with layer E. These bombs are black coloured and horizontally emplaced. Signs of some dip or orientation are not observed. The scoria fragments are the same as those described in layer E.

Following layers (G, H and I) were observed from the third track. Layer G is a thick deposit ~4m and consists out of black scoria clasts characterized by irregular form and sharp edges. There are also a lot of broken bombs present which are broken into several black, angular denser clasts (~angular clasts) in comparison with the scoria clasts. Bombs who are not broken have an orientation towards the north, so dipping away from the Lemptégy II edifice. The transition towards layer H is sharp and characterized by the occurrence of different clasts in comparison with other layers. Layer H (Figure 19) is a thin (~50cm) but key stratigraphic level in this section. It is composed out of particles which are looking different in comparison with the other particles. These clasts are very fragile, they are easily crushed between the fingers. The maximum grainsize of this particles is ~1cm and a lot of finer material (fine sand grainsize) is located in between these coarser fragments. When I break the fragments open, the interior is mostly black coloured. On the contrary the exterior shows a yellowish greyish appearance. The clasts have a very high vesicularity and a low bulk density (page 48). There is a gradual transition towards the next layer, layer I. Layer I has a gradual increase in the amount of black angular clasts in comparison with the amount of the black scoria clasts present which is only 40%. The angular fragments have a maximum grainsize ~3cm for the bottom part of the layer, gradually increasing to a size of 7cm (reverse grading).

The deposits observed on the fourth track is divided into three layers (J, K and L). Layer J is a mix of all the tree types of pyroclastic fragments found at the Lemptégy II deposits. The black bombs are deposited horizontally, some show an orientation but it is difficult to say whether it is an in situ bomb or already moved by the excavation activity of the quarry. The amount of scoria and angular fragments deposited at this layer is roughly equivalent (~40 volume% each). Layer K and L consist for at least 90 % out of black angular fragments. It seems to be that both layers can be regarded as one deposit. The grainsize of the fragments changes within those layers. The fragments at the bottom of layer K have a maximum grainsize ~5cm and decreasing towards the top of the layer (~3cm) and gradual increasing in size towards the top

37 of the following layer L. K is thus normally graded and L is inversely graded. The thickness for both layers is ~50cm.

From track 5 only one massive layer (thickness ~8m) is observed, deposit M, containing different lenticular layers. In this part of the section the layer consists out of 50% dark grey scoria and 50% angular clasts. The scoria particles have a more regular form (quite rounded and abraded edges) than in the other layers observed in this section. When I hold the scoria fragments in the sunlight, they appear to contain more colours like purple and blue spots at the edges. The size of the smallest particles is ~3cm, the coarser clasts ~6cm. The bottom and upper part of layer M shows inverse grading. Centrally located in M there is a part that contains very small scoria clasts (~5mm), which are rounded and contain small, rounded vesicles. The maximum grainsize of the central part is ~2cm. Layers observed at deposit M are difficult to follow horizontally along the section. Over a certain distance they change towards a layer which contains more angular or more scoria clasts. Also the transition towards the more central part is very irregular and the thickness various constantly. The deposit M therefore appears as a stack of discontinuous lens-shaped deposits with gradual or sharp transition in particle size or type.

Figure 18: picture A: granite pieces incorporated in scoria clast (scale ~16cm), picture B: fragile clasts layer H (left), typical scoria clast (right) (scale ~16cm) and picture C: lava flow (scale: grey bar ~40cm (seen right side at picture)

Figure 19: section 3 litholog 1: stratigraphy and description of main clasts (grainsize = maximum (unless otherwise stated))

6.3 Litholog 3 section 3: stratigraphy From track 1, I only observed the deposits of Lemptégy I, (deposit A) (Figure 20). As the focus of this work is not of Lemptégy I, only a brief description of layer A is given. The Lemptégy I deposits are different from the Lemptégy II deposits in many ways. No grading is visible in the Lemptégy I deposit anywhere in the quarry. Lemptégy I contains a huge amount of bombs (70%) with various shape: welded bombs, spindle bombs, breadcrust bombs and others. The scoria fragments are almost everywhere red coloured and show the same characteristics as at the Lemptégy II deposits. A some places welding of the scoria clasts is observed.

Above layer A, a lapilli-rich deposit is observed (layer B). The deposit is composed of 5 thin (~20cm) and 2 thick (~60cm) layers. Thick layers have clasts of ~1cm and the thin layers have clasts with size ~5mm. Layers are well sorted and some show grading, especially the thick ones. The coarser grained layers are sticking out between the more finer grained ones. These layers contain comparable clast types. The clasts are well rounded, light grey lapilli clasts with small round vesicles. The uppermost layer is a thin (~1cm), brown soil layer which marks the transition towards the deposit of interest: the Lemptégy II deposits.

From track 3 onwards the following layers (M, N, O and P) are observed (Figure 20). Layer M, with a thickness of ~4m, consist of black scoria clasts with an irregular form. There are no indications for grading present in the layer. The bombs are black coloured with an obviously red crust. The size of the bombs is in the order of ~1m for the horizontal axes and ~50cm for the vertical axes. The bombs, present at the different sections, are described in a separated part of this paper (section 8). The transition towards layer N is associated with an increase in the amount of angular clasts (~10cm). Remarkable at this layer is the presence of intact bombs and bombs who are clearly broken in angular clasts whose external shape had been preserved. The transition towards layer O is characterized by a very sharp contact. The contact is easily observed by the occurrence of fragile material. The material has a yellowish appearance and the maximum grainsize is ~1cm. The vesicles are connected with each other by means of very thin rope like structures. Layer O has a thickness of approximately 30cm. The transition towards layer P is more regular and the material of layer P consists for more than 80 volume% out of black, angular clasts with a maximum grainsize of ~5cm. Again no grading is observed within layer P.

Figure 20: section 3 litholog 3: stratigraphy and description of main clasts (grainsize = maximum (unless otherwise stated))

Layer R (track 4) is the only layer where granite pieces (Figure 18, picture A) are found back into the scoria deposits. So this layer contains basement material (section 3). Most of the pyroclastic clasts found in layer R, are black scoria with irregular form. The following 2 layers, S and T can only be distinguished from each other based on the maximum grainsize of the pyroclasts. Both layers consist for more than 90 volume% out of black angular clasts. The maximum grainsize of the clasts in layer S, is ~4cm and for layer T ~3cm.

The upmost deposit of this section, layer U (track 5) is composed of many transitional layers. Some containing more greyish, angular fragments others containing more greyish, rounded (regular form) scoria clasts. One aspect is that those layers are difficult to follow along the section. Layer U seems to consist out of an alternation of lenticular layers with one time more scoria material present and the other time much more angular fragments. The maximum grainsize of the scoria fragments is ~2cm and they are more rounded than in other parts of this section. The angular fragments have a maximum grainsize of ~5cm but the edges are not as sharp as seen in other layers at the section. Overall the deposit seems to be more well sorted but nevertheless they contain bombs as well. But the amount of bombs is less than 2% of the total amount of pyroclasts building up this deposit.

6.4 Correlation of the lithologs from key section 3 On Figure 21 the lithologs are presented in the order of appearance from the western towards the eastern part of the section. The lithologs of section 3 contain certain layers that correlate very well with each other. Depositions seen from the first track of section 3 are obviously different for the lithologs, litholog 2 containing Lemptégy II deposits, litholog 1 contains the last deposited layers of Puy des Gouttes and litholog 3 contains those of Lemptégy I. Those layers that build up the Puy des Gouttes sequence are dipping 27° towards the west and the Lemptégy II have a dip of about 15° in the same direction. From track 4 onwards the Lemptégy II layers are deposited more horizontally with a dip of approximately 5° to the west. The tracks are artificial and almost horizontal.

Deposit B (litholog 3) and deposits A and B (litholog 1) contain both pyroclasts of Puy des Gouttes. The following layers are those from the Lemptégy II deposits. From the observations made for each litholog it seems to be that layers B, C, D and E from litholog 2; layers C, E, F and G from litholog 1 and layers M and N from litholog 3 can be aggregated together in one deposit: phase 1 (Figure 22). Those layers contain the same type of pyroclastic fragments:

42 scoria, angular clasts, broken and intact bombs. The only difference is the colour of the layers from litholog 2 which is red coloured. But the colour is not a correlation characteristic.

The following layer which can be well seen on the panorama photograph (Figure 22) and is observed at each litholog is the very thin (~50cm) layer containing the specific fragile and highly vesicular clasts, seen at litholog 2 as layer F and H, at litholog 1 as layer H and layer O in litholog 3. That yellowish layer is covered in all the lithologs by a layer containing more angular clasts. Exceptionally at litholog 2 this layer is observed twice. The characteristics of the clasts constituting this layer are equal for each litholog, the same is valid for the more angular layers. The thickness of both layers decreases towards the east. As well as the highly vesicular layer as the angular layer are taken together in a new deposit: phase 2.

Gradually moving up the sequence, we can follow again some layers at all the three lithologs: layer J (litholog 1) and layer R (litholog 3) can be correlate with each other based on the type of clasts present. There are a few difficulties observed at litholog 2. It seems that only layer J will correspond with the former layers regarding at the constituting clasts. So layers J, R and J (litholog 2) are layers building up a new deposit: phase 3 (Figure 22).

Looking again at lithologs 1 and 3, I observe again 2 layers in each litholog that resemble each other very well: layers K and L from litholog 1 and layers T and S from litholog 3. Both layers are containing angular clasts. In general the grainsize of the clasts seems to decrease to the east. If we look at litholog 2, there are two layers catching the eye because of their abundance of angular fragments: layer K and M. In between these layers, there is another layer deposited: layer L which contains more scoria particles. Because that layer is not found back at the other lithologs, this layer can be a lens-shaped layer which disappear towards the east along the section. Layer K is characterized by a sharp contact recognized by the occurrence of a different clast type in comparison with the underlying layer J. That sort of transition is also recognized by layer K (litholog 1) and layer S (litholog 3). The maximum grainsize of the clasts is ~8cm (layer K litholog 2), related with the decreasing grainsize towards the east. The thickness is ~50cm which is in the range of the other layers. I group these layers together in one deposit: phase 4 (Figure 22).

For the highest visible layers at the three lithologs, there can be only a correlation made between deposit M of litholog 1 and deposit U of litholog 3. Both deposits are characterized by alternating layers where once the major proportion of the fragments are angular and coarser, and otherwise where the major proportion is constituted out of smaller scoria

fragments. Overall this last deposit is different from the others in terms of sorting. These last deposit is called phase 5 (Figure 22). Looking at litholog 2 we see a totally different upper deposit: layer O and N where there are no observations of alternating layers observed. Unless we consider layers L, M, N and O as 1 unit. Then there is an alternation remarkable of layers containing more scoria clasts (layer L, N and O) and a layer containing more angular clasts (layer M). The correlation is weak because the lens-shape structure is absent and the grainsize of the clast is not comparable and they lack the well-rounded clasts.

General conclusions based on this correlation at section 3 is that litholog 2 contains at some levels different layers in comparison with lithologs 1 and 3. The main question here is whether litholog 2 has more affinity with section 1 (see further in this paper) especially from layer K observed from track 3 onwards. And where the transition happens at section 3 towards the deposits seen at litholog 1 and 3. Those 2 important questions will be answered as much as possible in the correlation section of the whole quarry as well as in the section of the reconstruction of the eruption dynamics of Lemptégy II.

Figure 21: correlation of lithologs from key section 3: green lines = correlation lines, red question marks = uncertain correlation, red lines = no correlation

Figure 22: Overview picture key section 3 with indication of lithologs, stratigraphic lines (phases indicated as full red lines with black arrow), uncertain stratigraphic line (red dotted lines), magmatic structures (full green line) and lens-shaped layers (green dotted lines) (length of section ~200m and height ~35m)

6.5 Key section 3: interpretation Some more general remarks for this reference section (Figure 15), is the presence of those angular clasts which are blocky to elongated in shape with angular edges. The angular clasts seems to be representing pieces of previously erupted larger clasts that were broken on impact or during grain avalanching and recycling in the vent. The transition towards the red coloured pyroclasts is located at section 3. Some structures observed at these red coloured deposits resembles those seen at section 1 (see further in this paper). Especially seen from track 3, a small intrusion is emplaced and causes the deformation of the layers above (Figure 23). The pyroclasts observed, are different from those seen at the black coloured parts of section 3. They are partially welded and resembles the pyroclasts observed at section 1. This colour change and the different clast type can be due to the closer location to the intrusion complex for section 3 (and also section 1). Because of this closer location the temperature of the clasts is much higher than clasts that were deposited further (cool off during flight) and the average size of the clast is larger.

In layer R (litholog 3), I found also granite pieces (Figure 18, picture A) incorporated in the pyroclasts. Because the basement beneath the Lemptégy scoria cone consist mainly of granite, it is normal to find these granitic pieces. Normally very big granite pieces can be found back, but previous research carried out on these granites has shown that most dissolved in the ascending magma and adding gas content to the critical magma (pers. comm., van Wyk de Vries, 2010). I only found little pieces back in layer R. Those granite pieces are carried out from the crust by the ascending magma. The granite pieces are affected by the intense heat of the magma (~1000°) and the granite breaks apart. So the granite is partially remelted, the water-bearing minerals releasing water and adding up a large amount of gas. The granite has a mousse like appearance due to dissolution of water-bearing minerals (e.g. amphibole, micas).

At the upper part of section 3, the remarkable alternating layers are easily observed. What is causing these alternations of coarser and finer layers and why we cannot follow these layers along the whole section? The thickness of these layers varies constantly. There is maybe one fine layer that can be followed along the section and continues further along section 2 and section 4 where it becomes thinner in thickness (page 84). If we look at phase 5, the first layer we observe, is a coarse-grained layer (for almost each part along the section) followed by a rather sharp transition towards a fine-grained layer. Conclusion based on these observations, is that at this deposit reverse grading occurred, what can be explained by several episodes of more intense explosions of the Lemptégy II scoria cone. There is also another explanation

46 bounded to this appearance. Some beds have geometries and characteristic features indicating an emplacement as grain avalanche. Layer geometries that are lenticular over meters of distance (with local evidence for erosion into underlying deposits) and/ or have clear reverse grading present, is a typical geometry revealing grain avalanching (Valentine et al., 2005). Grain avalanche point to the fact that cone slope had reached the angle of repose by then.

Another phenomenon observed at the last deposit, is the clasts constituting the fine layers are well rounded and light grey coloured. The recycled brittle clasts in the vent are influenced by ball milling processes and the result of those processes causes the clasts to be rounded and having a smaller grainsize. Ball milling processes are used to grind materials into finer material (e.g.: the laboratory procedure: the agate mill to crush the scoria material for the chemical analyses) (Valentine et al., 2005). Avalanching can also cause some amount of rounding. Both processes are probably responsible for the rounding of the clasts. At the same phase 5 we observe irregular traces of more yellow-brownish coloured scoria clasts. The first thing I thought of was the presence of sulphur traces but this interpretation did not hold for long because of the lack of the typical sulphurous smell. A more plausible explanation is the occurrence of weathering processes. Infiltration of meteoric or rain water could corrode the pyroclastic clasts.

Figure 23: intrusion observed at section 3: red lines indicating deformed layers, green line contouring the intrusion (scale (grey bar photograph) = 1.70m)

6.6 Density results and interpretation To quantify the observed vesicularity and grainsize from typical pyroclasts, laboratory procedures are carried out. For a few layers deposited at section 3, I took samples to record information about the density of the particles. Particles chosen for those measurements were all scoria clasts. In appendix B, the tables with the results of the density measurements are presented. The method and formula used to calculate the mean density of the sampled layer is discussed in the methodology section (page 25). Figure 24 shows the location of the samples identified in the lithologs. There were also samples taken from section 4, to investigate whether some layers of section 3 match with those of section 4 and especially for samples taken from track 5 at section 3. From the other sections no samples were taken.

Figure 24: Location of samples taken for density analyses (orange dots at litholog), chemical analyses (red dots at litholog), thin sections (green dots at litholog) at different stratigraphic levels in section 3

Table 3: Average bulk density of 10 clasts taken at different stratigraphic level in section 3

sample (stratigraphic level (figure 24)) mean density (g/cm³) standard deviation L1S3T2 0,7290 0,0826 L1S3T3Sa1 0,8089 0,0666 L1S3T3Sa2 0,6206 0,1450 L1S3T3SaA 1,4513 0,2587 L1S3T4Sa9 0,7468 0,1358 L1S3T5Sa4 1,4953 0,2947

L1S3T5Sa5 1,3163 0,2596

Figure 25: plot of bulk density results, samples section 3 Density measurements highlight 2 groups of mean density. The first group (samples L1S3T3SaA, L1S3T5Sa4 and L1S3T5Sa5 (red circle at plot)) has a mean density around 1.4 g/cm³ and the second group has a much smaller mean density around 0.73 g/cm³. The samples with a density higher than 1, were mostly found in the upper layers from section 3. Samples L1S3T5Sa4 and L1S3T5Sa5 are from fine grained layers containing small, round scoria clasts. The interpretation for those round fragments is related to ball milling processes (key section 3). They are probably the result of denser material (bombs) broken into smaller pieces during their residence time in the conduit. The lesser amount and smaller size of the vesicles in comparison with the other scoria clasts can cause this higher density.

The other samples have a density smaller than 1. A remark has to be made for some of these samples. Sample L1S3T3Sa2 from the yellowish key layer (Figure 24) has the lowest density (~0.62 g/cm³) in comparison with the other layers. In the field I described the scoria clasts as very light and fragile pyroclasts, so the density measurements confirm this observation. Directly above this yellowish layer, there is a layer containing more angular pyroclasts with density results of around 1.45 g/cm³. The heavier density is not received at the same manner as for sample L1S3T5Sa5 (layer containing no small, round scoria). The pyroclasts must have been derived from material which was denser and these clasts are likely to be bomb fragments. The bulk density will depend on the amount of degassing of the magma

Figure 26: location density samples from section 4 litholog 2

Table 4: average bulk density of 10 clasts taken at different stratigraphic level in section 4

sample (stratigraphic level (figure 26)) mean density (g/cm³) Standard deviation L2S4T3Sa4 0,8687 0,1798 L2S4T3Sa7 1,0364 0,3992 L2S4T3Sa6 0,7594 0,1449 L2S4T3aSa5 0,7297 0,0562

Figure 27: plot of bulk density results, samples section 4 The first thing catching the eye is sample L2S4T3Sa7 (red circle at plot). It is the only one having a mean density larger than 1. The other samples have a much lower density. If we compare them with the density measurements of section 3, almost similar density results are obtained except that high density, low vesicularity clasts are less frequent observed here. Some fragments of sample L2S4T3Sa7 (Appendix B) are very dense, even denser than those from section 3 (upper layers). Sample L2S4T3Sa7 comes from the greyish irregular bordered layer (Figure 26). This layer contain particles having the same characteristics of the particles from samples L1S3T5Sa5 and L1S3T5Sa4 found at section 3 (Figure 24).

A remark has to be made for the density results of sample L2S4T3Sa7 (see appendix B and Figure 26). Not all the clasts are small and well-rounded, there are clasts which are irregular and containing larger vesicles. These clasts have a lower density (< 1) (see appendix B). For samples (L1S3T5Sa5 and L1S3T5Sa4) taken from the thin, fine-grained layers found at the top of section 3 (Figure 24) and sample L2S4T3Sa7 found at section 4 (Figure 26), the density results are comparable (higher density).

The range of density values for section 4 and 3 is comparable except for the yellowish layer at section 3 (Figure 24) having the lowest density value and certain layers at section 3 contain denser pyroclasts.

After calculating the bulk density of the samples, now we can calculate the Dense Rock Equivalent density (DRE). The value of the DRE density is estimated from the geochemical composition of the samples which lays on the border of trachy andesite-basaltic and trachy andesite. The DRE values for these geochemical compositions are: 2.700 – 2.750 kg/m³ (Spera, 2000). From the bulk and the DRE density we can calculate the vesicularity of the samples.

At Table 5, the DRE density results are presented for the samples taken at section 3 and 4. Looking at these results we can deduce some general conclusions. The samples (section 3) with the highest density (L1S3T3SaA, L1S3T5Sa4 and L1S3T5Sa5) have also the lowest vesicularity ~45 – 51%. Sample L2S4T3Sa7 (section 4) has a vesicularity of ~62% which is larger in comparison with the dense samples from section 3. The vesicularity of all the samples (except the dense ones) seems to be ranging between ~70 - 73%. One exception is made for sample L1S3T3Sa2 (section 3) which have the largest vesicularity (~77%) and also the lowest bulk density (~0.62 g/cm³) in comparison with the other samples.

The density results for sample L1S3T3Sa2 confirm the former described observations. The vesicularity results of the samples confirm the vesicularity values observed for scoria clasts which are ranging between 70-85% (Vergniolle and Mangan, 2000).

Table 5: Dense Rock Equivalent density from samples of section 3 and 4

Section 3 ERD (g/cm³) Vesicularity (%) Sample 2,7 2,75 L1S3T2 0,7300 0,7349 73 73 L1S3T3Sa1 0,7004 0,7059 70 71 L1S3T3Sa2 0,7701 0,7743 77 77 L1S3T3SaA 0,4625 0,4723 46 47 L1S3T4Sa9 0,7234 0,7284 72 73 L1S3T5Sa4 0,4462 0,4563 45 46 L1S3T5Sa5 0,5125 0,5213 51 52

Section 4 ERD (g/cm³) Vesicularity (%) Sample 2,7 2,75 L2S4T3Sa4 0,6783 0,6841 68 68 L2S4T3Sa7 0,6161 0,6231 62 62 L2S4T3Sa6 0,7187 0,7239 72 72 L2S4T3aSa5 0,7297 0,7347 73 73

6.7 Grainsize distribution and sorting In the following paragraph, samples of certain layers of section 3 are taken to document the grainsize distribution and the sorting of the layers. In appendix C the weight and cumulative weight percent for each size fraction retained at the sieves are shown in tables. For the description of the method I refer to page 27. The Dplot program was used to set up the cumulative graphics. From these graphics, the Inman parameters MdΦ and σΦ were derived.

MdΦ is the median diameter of the pyroclastic particles. σΦ is the sorting coefficient and represent a value for the sorting of the sampled layers. The values for calculating MdΦ were taken from the cumulative graphics. Scrolling the cursor on the graphic until the Y coordinate was equal to 16, 50 or 84 (percentile 16 “P16”), the X value (grainsize in mm) was read from the graphic. The definition related to the term percentile, is the amount of clasts that have grainsizes smaller than 16 mm in the case of P16. Calculating Φ50, Φ16 and Φ84 is based on the earlier described formula: Φ = -log2d. Using the Φ84 and Φ16 values in following formula in order to calculate the deviation standard:

From section 3, 5 layers were sampled (Table 6). For section 4, 3 layers were sampled (Figure 28). At the lithologs, the location of the samples from section 3 and 4 are indicated.

Figure 28: locations (purple dots) of samples taken from section 4 (left) and section 3 (right) for grainsize analyses

Table 6: Description of the samples from section 3

Sample Description SD1 Yellowish layer O SD2 Coarser angular layer N located under layer O SD4 Coarser angular layer P located above layer O SD7 Layer consisting of small round particles at layer U

SD9 Layer consisting of larger and angular particles at layer U

Table 7: sorting results of grainsize samples from section 3

Sample DPLOT: P16 (mm) P50 (mm) P84 (mm) f 16 f 50 f 84 Md (f ) s (f ) SD1 2,61 11,60 25,49 -1,38 -3,54 -4,67 -3,54 1,64 SD2 13,98 22,51 29,47 -3,81 -4,49 -4,88 -4,49 0,54 SD4 6,77 17,92 29,47 -2,76 -4,16 -4,88 -4,16 1,06 SD7 1,95 6,63 14,27 -0,96 -2,73 -3,83 -2,73 1,44 SD9 4,38 16,5 27,69 -2,13 -4,04 -4,79 -4,04 1,33

Table 8: sorting results of grainsize samples from section 4

Sample DPLOT: P16 (mm) P50 (mm) P84 (mm) f 16 f 50 f 84 Md (f ) s (f ) SD11 1,07 4,76 15,19 -0,10 -2,25 -3,93 -2,25 1,91 SD12 3,87 12,87 29,47 -1,95 -3,69 -4,88 -3,69 1,46 SD13 2,96 9,24 26,57 -1,57 -3,21 -4,73 -3,21 1,58

Table 9: sorting coefficient for pyroclastic deposits (Cas and Wright, 1987)

Sorting (σΦ) Pyroclastic deposits 0 - 1 Very well sorted 1 - 2 Well sorted 2 - 4 Poorly sorted > 4 Very poorly sorted

Figure 29: Plot of the Inman parameters Md (Φ) and σ (Φ) from the samples

Figure 30: Md (Φ)/ σ (Φ) plot for some Strombolian pyroclastic fall deposits. Solid circles are samples collected from scoria cones, and crosses are from downwind fall deposits. The red points at the Md (Φ)/ σ (Φ) are my own samples (After G.P.L Walker and Croasdale 1972, with additions for cone deposits after Houghton & Hackett (1984), and J.V.Wright unpub. Data from Santorini. (Modified from Cas and Wright 1988)

For section 3 (Table 7) all layers have σΦ values between 1 and 2, they are interpreted as well sorted layers except for sample SD2 which has a σΦ value smaller than 1 and is said to be very well sorted. The major part of particles have grainsizes between 8 and 32 mm so almost no fine material is present (Appendix C). Except for sample SD7, having the largest weight percent for the smaller grainsize fraction (2 – 16 mm). This observation is consistent with the observations made in the field: sample SD7 comes from a fine-grained layer. The grainsize distribution seems to be decreasing going higher up in the litholog.

For the results of section 4 (Table 8) all the layers have σΦ values between 1 and 2. Comparing the results of section 4 with those of section 3, section 4 contains more finer material (4 - 32 mm) (appendix C) and have larger σΦ values. Same remark for sample SD7 can be made here for sample SD11, the grainsize distribution (1-16 mm) match the field observations: smaller pyroclasts in comparison with surrounding layers. In general at section 4, the weight percent of the smaller grainsize fractions is larger than for the same grainsize fractions from section 3. This is related to the larger distance between the section 4 deposits and the Lemptégy II edifice. At larger distances, finer material is deposited. All these can be interpreted as fall deposits except from the large clasts (bombs with grainsize > 10 cm) which follow most of the time ballistic trajectories.

On Figure 29, the values calculated for Md(Φ) and σ(Φ) were plotted. Md(Φ) values have a range from -5 to -2 and the σ(Φ) values are between 0.5 – 2. If we compare our own results

55 with the results plotted from strombolian pyroclastic fall deposits (Figure 30), we may conclude that our own samples fall in the range of those from the strombolian pyroclastic fall deposits as defined by Walker (Cas and Wright, 1987). Looking at the histograms (Appendix C), the samples seems to belong also to fall deposits. Size distribution histograms of fall deposits display a Gaussian shape and having a narrow size range. Our samples mainly fall in the size range of 16-32 mm for section 3 and in the range 4-32 mm for section 4. As for other fall deposits, the histograms of the grainsize distribution contain a strong central peak (Appendix C).

There must be some attention made here on these measurements, because it was very difficult to have enough sample material and make sure that the largest size fraction was not more than 10% of the total amount of weight sample. Only the layers with the finest grainsize (~partial grainsize distribution) were sampled. The total grainsize for Lemptégy II is coarser and would fall in the centre of the cloud of Walker (Figure 30). Also the Gaussian shape of the histograms is often unclear because the layers contain fragments with grainsize larger than 64 mm (~15cm). Taking in account different transport histories of the pyroclasts (e.g. recycling into the vent and break-up of fragments) will lead to a reduction in grainsize distribution. There are a lot of layers and sections not sampled because of the fragility of the fragments which would have created a distorted image of the results. Also the accessibility to take samples was sometimes difficult.

6.8 Geochemical analysis The geochemical analyses are carried out on samples (scoria) from section 3, to deduce the chemical composition of the magma and eventually observing a geochemical transition in relation with the observed stratigraphy. There are 8 samples taken from section 3 (location of samples indicated at Figure 24) in order to observe a geochemical variation during the different eruption phases of Lemptégy II. Another 2 samples from section 3 have no chronological/ stratigraphic interest. They are sampled from the upper layer at section 3: sample CA S8.9 (yellow greyish scoria) and CA R12 (red scoria) (Figure 31). Those samples are chosen to investigate if weathering (CA S8.9) or oxidizing/baking (CA R12) of samples can influence the geochemical composition.

CA S8.9 CA R12

TS S8 TS RS

Figure 31: Location of geochemical samples CA S8.9 and CA R12 (orange squares) and thin sections samples (red squares) from section 3 Before looking at the geochemical results carried out at our samples, former analyses by de Goër de Herve (1999) and Boivin (2004) can be found on Table 10. These geochemical results de Goër de Herve (1999) point the following observations (Figure 32):

- Lemptégy I: chemical composition lays in the trachy-basalt field in the TAS diagram (first column Table 10) - Lemptégy II: chemical composition lays at the border of trachy andesite basalt and trachy andesite in the TAS diagram (third column Table 10)

If we plot the results (Boivin, 2004) from the second and last column (Table 10) in the TAS diagram, the same conclusion is obtained as above. Now I want to investigate if my Lemptégy II samples a) display the same geochemical composition and b) if there is a trend present in relation with the stratigraphy.

The answer for question a is positive. If we plot the mean values in the TAS diagram with a mean SiO2 of 54.65 wt% and a mean Na2O + K2O of 7.33 wt%, the resulting chemical composition lays in the trachy andesite – basaltic field (Figure 32). Also the other major components seems to have minor deviations in comparison with the Lemptégy II data (Table

10) with Al2O3 showing the largest deviation of 0.37 wt% and MnO the smallest deviation of 0.03 wt%. The answer for question b is seen at Table 11. We see at Table 11, small variations in the major elements for each sampled layer/deposit but there is no prove of an increase or decrease of the major elements regarding their location. Also there are no remarkable variations observed for the 2 other samples CA S8.9 and CA R12. Except a small increase in the Al2O3 * and Fe2O3 oxides compared with the mean value is observed, related to the occurrence of secondary processes for these samples. Sample S0.10 has the same characteristics for these 2

57 oxides (slight increase in comparison with the other samples). Same observations are made for the other major elements: a minor variation occurs or none is observed. In general the magma is rather silica rich to form the Lemptégy II scoria cone. Due to the more silica rich composition, the magma is more viscous and may contain more gas which will have an influence on the eruption dynamics of Lemptégy II. The average silica content differ by 1% with the results from Boivin (2004). This small difference ensures that our own geochemical data lays completely in the trachy-andesite basaltic field at the TAS diagram.

Table 10: former geochemical analyses from Lemptégy: first and third column (de Goër de Herve et al., 1999), second and fourth column (Boivin et al., 2004)

bomb at Lemtpégy dyke Lemptégy lava flow Lemptégy clast type bomb at Lemptégy I I II II

SiO2 48,11 48,41 55,21 55,57

Al2O3 16,08 16,18 17,38 17,48

Fe2O3 11,88 11,95 8,55 8,40 MgO 6,05 6,09 2,67 2,75 CaO 9,38 9,44 6,12 6,24

Na2O 3,53 3,55 4,67 4,64

K2O 1,82 1,83 2,88 2,93

TiO2 2,37 2,38 1,50 1,51

P2O5 0,60 0,60 0,84 0,79 MnO 0,19 0,19 0,19 0,21 LOI / 0,00 / 0,00

H2O / 0,04 / 0,19 Total 100,00 100,66 100,00 100,71

Figure 32: geochemical analyses presented at TAS diagram for Lemptégy I and II (de Goër de Herve et al., 1999) (green square: plot of own geochemical data) 58

Table 11: geochemical analyses of samples from section 3 Lemptégy II (sample S7.1 located at highest height and sample S6.3 located at lowest height (Figure 24))

in rock SiO2 TiO2 Al2O3 Fe2O3* MnO MgO CaO Na2O K2O P2O5 LOI total S7.1 54,52 1,56 17,70 8,59 0,21 2,57 6,02 4,49 2,82 0,73 0,11 99,21 S9.7 54,57 1,54 17,41 8,49 0,21 2,58 6,01 4,52 2,88 0,73 0,08 98,95 S0.10 54,67 1,61 18,50 8,92 0,22 2,55 5,78 4,31 2,57 0,71 0,40 99,86 S4.6 54,26 1,52 17,40 8,45 0,21 2,55 5,98 4,51 2,83 0,73 0,04 98,46 S1.4 54,65 1,53 17,51 8,45 0,21 2,56 6,05 4,57 2,89 0,74 0,01 99,16 S2.11 54,83 1,54 17,60 8,51 0,21 2,56 6,01 4,58 2,89 0,74 0,06 99,48 S3.8 54,68 1,54 17,52 8,46 0,21 2,55 5,99 4,57 2,88 0,74 0,09 99,14 S6.3 55,05 1,53 17,51 8,45 0,21 2,56 6,02 4,43 2,91 0,74 0,10 99,41 MEAN 54,65 1,55 17,64 8,54 0,21 2,56 5,98 4,50 2,83 0,73 0,11 99,21

R12 54,62 1,59 18,16 8,79 0,22 2,58 5,84 4,30 2,62 0,72 0,31 99,45 S8.9 54,23 1,58 18,20 8,76 0,22 2,55 5,79 4,29 2,70 0,72 0,33 99,04

6.9 Thin sections From section 3, the same stratigraphic layers were sampled (scoria pyroclasts) as for the samples used for geochemical analyses (Figure 24) in order to receive more information about the composing minerals and vesicles. The main idea was to investigate whether we could observe a variation in the type of minerals present in each layer. The geochemical variations between the different samples was minor and random, yielding no relationship with the stratigraphic order of the layers. After studying these thin sections with a microscope under plane polarised light and cross polarised light, I was able to conclude that also no major variation occurred at mineralogical composition. Following minerals and textures were observed at some samples:

- Vesicles o The major part of all the samples was taken by the presence of vesicles with various form and size. Inside these vesicles, traces of weathering products were find (unable to identify) and very small crystals were present. - Volcanic glass o A brown orange coloured material, which is isotropic (black) under crossed polarized light, abundant in the matrix and can be found in some vesicles.

- Opaque minerals: magnetite (Fe304) o Brown coloured in plane polarized light, black at crossed polarized light. The opaque minerals are present as minerals in the matrix and as small inclusions in other minerals (plagioclase, clinopyroxene).

- Plagioclase (Ca,Na)(Al,Si)408: o Colourless mineral displaying poly synthetic twins under crossed polarized light. The plagioclase minerals appear as phenocrysts and as small needle like minerals in the matrix. The phenocrysts show typical resorption features. - Feldspar: o present as small phenocrysts and in the matrix. Single twinning present. - Clinopyroxene:

o Probably augite (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 (Boivin et al., 2004), twins and cleavage observed, present as phenocrysts and under plane-polarized light minerals are pale brown – greenish coloured.

- Amphibole NaCa2(Mg,Fe)5(Si,Al)8O22(OH)2: o The presence of opacite rims, brown orange coloured (pleochroism), opaque minerals as inclusions, as phenocrysts (large) and the cleavage (120°) are an indication of amphibole.

Those minerals are typical minerals present in trachy andesite volcanic rocks (Boivin et al., 2004) and they confirm the previously obtained geochemical result. All the samples contain a major amount of vesicles and only a minor part consist of crystals. The matric is fine grained with some larger phenocrysts present. In the paragraph below a more detailed description of some samples.

 Figure 33, sample TS S7 (4*0.10): sample consist out of 2 parts clearly seen at the thin section (yellow and black coloured part), picture A is a photograph from the more yellow (inner part of sample (interior)) coloured part. This part contains smaller vesicles and a fine grained matrix (plagioclase) and a larger amount of plagioclase phenocrysts in comparison with the other thin sections (picture B). Picture C is a photograph of the other part which contains larger vesicles and in lesser amount plagioclase phenocrysts. This sample is taken from the layer containing finer grained, well-rounded material and described as denser material which is related with the observation of the minor amount and smaller vesicles.  Figure 33, sample TS S1 (4*0.10): at picture D I observed some vesicles forming agglomerates with polygonal borders (red circle picture D). At picture E, I observed a large phenocryst of hornblende (pleochroism (pale green) and cleavage (green circle at picture E)) and a minor amount of small plagioclase phenocrysts (picture F).

 Figure 33, sample TS S2 (10*0.25): at picture G, I observed large and smaller vesicles, ranging from round to irregular forms. For almost all the samples of litholog 1, except for TS S7 and TS S1, the amount of vesicles stays constant. There are a few plagioclase phenocrysts to observe (picture H).  Figure 34, sample TS S8 (4*0.10): at picture A, I observed a lot of large and smaller vesicles with round to irregular forms. Also some vesicles seems to have small inclusions inside them (brown fragments, red circle at picture A). These are from the yellow coloured parts at section 3 (Figure 31) which maybe are weathered scoria material.  Figure 34, sample TS RS (10*0.25): at picture C, the vesicles show more oxidized rims (reddish material, probably a larger amount of oxides) in comparison with the other samples, due to the more oxidized state of these samples. The sample contains irregular, large and smaller vesicles. Some vesicles contain inclusions. Again the small plagioclase phenocrysts are observed (picture D).

Figure 33: photographs thin sections: picture A, B (polarized light) and C: sample TS S7; picture D, E and F (polarized light): sample TS S1; picture G and H (polarized light): sample TS S2

Figure 34: photographs of thin sections: picture A and B (polarized light): sample TS S8; picture C and D (polarized light): sample TS RS

7 Lemptégy II: stratigraphy of remaining sections

7.1 Observations lithologs section 1 The following section is section 1 (Figure 35 and Figure 36). For section 1, 3 lithologs will be described.

7.1.1 Section 1 litholog 2: stratigraphy The pyroclasts accumulated at section 1 are different from the other deposits described in further sections. All the deposits exposed here are the result of the Lemptégy II eruption. Litholog 2 (Figure 38) is located left of the cryptodome (Figure 37). There is ~2m in the lower part of the section where the outcrop is not visible due to slice material. Moving upwards, the first layer (layer K), contains red scoria clasts with a maximum grainsize ~15cm. 40 volume% of these scoria clasts have a smaller grainsize, ranging between ~2 - 4cm. The scoria itself has an irregular form and contain small round (sometimes large ellipsoidal) vesicles causing the light weight. In between the scoria clasts, angular red greyish coloured and heavier pieces are found containing white spots at their edges. Contrasting with the scoria fragments, these pieces contain very little vesicles. No grading is visible except for the angular pieces are restricted near the cryptodome. The proportion of scoria clasts increases gradually with increasing distance (upwards) from the cryptodome. Layer L contains red scoria clasts and angular pieces. The scoria seems to stick together to form several agglomerates of many small scoria clasts. The other type of pyroclasts present at layer L are bombs. Those are red coloured and small scoria clasts seems to be incorporated in the edges. No visible grading present. Layer M is limited at the bottom and top by flat lying, red coloured bombs. Between these bombs, red scoria clasts with maximum grain size ~15cm and smaller clasts ~3cm are present. The scoria clasts show no grading, the grainsizes are mixed up. Only the bombs show crude bedding. Some pieces are darker coloured: the interior is purple coloured with a red crust containing very little vesicles and size ~4cm. These pieces are quite angular, are different from the angular pieces found at layer K and L. Layer N (Figure 37 picture A) is a very thin (~10cm) layer consisting of very small (~5mm) black and red scoria clasts. Layer N seems to have a discontinuous behaviour. Both contacts of layer N are very sharp bounded by coarser clasts. The dip is approximately 33° towards the east. The last layer (layer O) contains red and black scoria fragments with maximum grainsize of ~3cm which gradually increases in grainsize towards the top. Most of the clasts at the top of this layer seems to be partially broken into angular pieces of different sizes, termed here blocky fragments. 63

7.1.2 Section 1 litholog 3: stratigraphy The observations of litholog 3 (Figure 39) begins at a lower level in the quarry than litholog 2. Layers with letter A, B and C are located between the dykes coming from the Lemptégy II edifice. The dykes will not be described here, for a brief description see the introduction of Puy de Lemptégy (page 20). Layer A is a scoria-rich layer. Some of the scoria are welded. Welding of small scoria is also observed at the surface of the bombs. Looking in more detail at the bombs, there is a difference in the amount of vesicles. The bombs having a black coloured centre, contain more round and larger vesicles than those with a red coloured centre, here the vesicles are elongated (Figure 41 and chapter 8). The deposit is a chaotic mass of scoria and bombs without any grading. Layer B has an accumulation of bombs with the same characteristics as in layer A. Not only bombs are present, also scoria clasts are deposited. There is a difference in the colour pattern compared with layer A and layer C, here the scoria are red inside covered by a black surface. The surface of the pyroclasts is not as sharp as in the former layers and shows characteristics of abrasive processes causing a more rounded surface. The maximum grainsize of the scoria is ~10cm. Layer C consists of red scoria clasts. Both layers (A and C) have the same angular, dense red coloured pieces covered by white spots at their edges as observed in litholog 2 (layer K and L). Within layers B and C no grading is observed. The transition for these 3 layers is mainly marked by the difference in clast type and clast size. At the bottom of layer D there is a gradual decrease in the proportion of the angular pieces replaced by a more scoria-rich layer (maximum clast size: ~10cm). The proportion of bombs and scoria clasts decreases in layer E resulting in more angular, red coloured clasts which may be bombs that broke on impact. In the upper layer (layer F), an increase in proportion of the scoria clasts (black and red) and bombs is observed.

S N

Figure 35: Overview photograph of section 1

Figure 36: Overview photograph of section 1: green lines: magmatic related structures (cryptodome), black lines: stratigraphic contacts, red lines: faults and dotted black lines: uncertain stratigraphic contacts (scale: length of section ~140m)

Figure 37: Overview photograph of section 1 with location of the lithologs (purple lines) and detailed pictures of some structures: picture A: sharp contact between coarse pyroclasts and the fine material (scale: grey bar ~1.70m), picture B: thin (~10cm) fine grained layer that is cut off by fault F2 (scale: grey bar ~40cm) and picture C: detail of fault F3 (scale: grey bar ~40cm)

Figure 38: section 1 litholog 2: stratigraphy and description of main clasts (grainsize = maximum (unless otherwise stated))

Figure 39: section 1 litholog 3: stratigraphy and description of main clasts (grainsize = maximum (unless otherwise stated))

7.1.3 Section 1 litholog 5: stratigraphy Layer K contains red scoria clasts with a maximum grainsize ~10cm within between these large clasts scoria of ~2cm and black bombs. Transition towards layer L is marked by the appearance of another clast type: red angular clasts (80 volume%). Going upwards in the sequence another layer is observed (layer M). The transition is characterized by an decreasing grainsize of the scoria clasts in layer M (~3cm) and an decrease in the proportion of the angular clasts (~3cm). M1 belongs to the same deposit of layer M but an gradual increase in the proportion of black scoria clasts is observed. Layer N is a very thin layer (~10cm) display a lens-shaped character due to its disappearance towards the south as well as towards the north at this location (Figure 37 picture C). It contains very small scoria clasts (~4mm). Layer O contains approximately 50 volume% black and red scoria clasts.

Figure 40: section 1 litholog 5: stratigraphy and description of main clasts (grainsize = maximum (unless otherwise stated))

Figure 41: Section 1 litholog 3: bomb with black coloured centre and vesicles (left), at the right: red coloured centre with elongated vesicles (pencil ~16cm).

Figure 42: correlation lithologs section 1: green dotted lines (tracks), full black lines (correlation lines), red question marks (no correlation) and black dotted lines (correlation between lithologs except for litholog 4)

7.1.4 Section 1: interpretation lithologs The presence of faults, fault-like structures and magmatic structures have a great influence in the Lemptégy II deposits observed at section 1. I will this first describe briefly the tectonic and magmatic structures. Afterwards I will describe the overall characteristics of the pyroclasts for the whole section related to eruption dynamics.

The cryptodome, seen at Figure 36, is a dome-shaped structure created by the accumulation of viscous magma just beneath the surface. Related to this accumulation process, some layers above the cryptodome are deformed (Fink and Anderson, 2000). Cryptodomes may have significant implications for volcanic hazards. The deformation of the north flank of Mount St. Helens in 1980 was triggered by the formation of a cryptodome. The injection of lava beneath the surface of Mount St. Helens fractured and destabilized the pre-existing rocks, eventually leading to the cataclysmic debris avalanche and explosion of the cryptodome on May 18, 1980 (Kilburn, 2000). The occurrence of these processes would not have happen in this case here. The magma is much closer to the surface and the overlying rock less resistant. It would cause only a minor flank landslide. The layers covering the cryptodome follow the same dome-shaped structure made by the cryptodome. Chronological speaking all the layers of section 1 were first deposited and afterwards deformed by the formation of the cryptodome. Which are the layers that are deformed by the cryptodome? From the fifth track onwards the layers observed at litholog 2 (layers K, L, M and N) are deformed by the cryptodome. Layers K, L, M and N (litholog 2) are dipping ~33° towards the south. At the northern part of the cryptodome, layers are dipping ~33° towards the north. These dip directions confirm the dome-shaped structure of the layers covering the cryptodome. The upper layer O of litholog 2 seems to be deposited horizontally covering the deformed layers.

The deformation is clearly visible around the cryptodome, but going further towards the north similar deformed layers were observed at 2 other locations. The first place is located in the central part of section 1 and the second place is located further to the north (Figure 36). The only difference at these places, is that the intrusion causing the deformation process is not outcropping. So beneath the deformed layers there must be an underlying intrusion present causing the deformation. The dip of the layers is about 20°. The layers affected by the underlying intrusion are those from litholog 3 (layers D and E) and from litholog 6 (layers S, R, G and P). Also at these locations the highest layers of the lithologs were deposited horizontally and covering the deformed layers.

2 sets of deposit colours are present at section 1 (Figure 36). The red coloured parts are most abundant at the section, but there are 2 parts in which the pyroclasts (scoria) are black coloured. There seems to be a relationship between the deformed and non-deformed layers. At the cryptodome the deformed layers are red coloured except for a greater amount of black coloured clasts in layer O. The same is observed at the other deformed layers where the amount of black coloured clasts gradually increases towards the top of section 1. The localities with completely black clasts are located between the deformed layers. The explanation for this phenomenon, has to be related to the presence of the magmatic intrusions. The emplacement of these magmatic intrusions happens when they are still very hot which causes baking of the already deposited layers (becoming red coloured). Some parts of the deposits are not affected by these magmatic structures and they preserve their initial black coloured appearance.

Besides magmatic structures, tectonic features like faults are present. One fault (fault F1 on Figure 36) is located at the left side of the cryptodome, fault F2 is located at the right side of the cryptodome and fault F3 is occurring at the most northern part of the section (Figure 36). Fault 1 is a step wise fault (consisting of 3 steps; en echelon fault) with an offset of ~3cm. It is characterized by very fine crushed material bordered at both sides by coarser fragments. Fault F1 could be represented by layer N at litholog 2. Based on the fine material found and the sharp contacts. Fault F2 is a vertical fault cutting off a very thin layer (~10cm). This thin layer has quite the same characteristics of layer N at litholog 2. The fact that I was not able to follow layer N towards the other side of the cryptodome (due to steep and high flank) makes it difficult to interpret the appearance for layer N as a fault or as a thin deposit. The layer affected by fault F2 is not found back at the other side of fault F2. The fault ends also abruptly at this thin layer because the coarser layer K (litholog 2) above the cryptodome is not affected by the vertical fault F2. The last fault, F3 (Figure 37 picture C), is the most clearly visible fault. It is a thrust fault affecting layers L, M, N and M1 of litholog 5 with an offset distance of ~50cm. The fault is dipping 10° towards the south and contain very fine (~2mm) material. What causes the development of these faults? Probably it is related to the tectonic stresses resulting from the emplacement of the magmatic structures (e.g. cryptodome for fault F1 and F2) at this places. Part of the deformation occurs through layer folding but also through faulting.

Before trying to correlate the lithologs with each other, I will discuss some of the typical pyroclastic fragments present at this section which are rather different from the other sections

71 which are described later on. The constituting fragments of this section are scoria, bombs and at certain places dense angular fragments. The very small distance between section 1 deposits and the Lemptégy II central vent is the main cause for the typical appearance of the pyroclastic fragments. Thereby there are two main facies that can be distinguished from each other in a scoria cone deposit: an inner crater facies and an outer wall facies. The deposits of the inner crater facies are much more heterogeneous than those of the wall facies. It seems that this section is a part of the inner crater facies and a transitional zone between the inner crater and outer wall facies. What are the observations supporting this interpretation? The scoria fragments are red coloured (baking processes) and different with the other sections is that most of the individual scoria fragments are welded together. This observation is also seen in general for a transitional area, the scoria fragments may be welded together but they can still be recognized as individual clasts what is observed in this section: a slightly welded deposit. The different layers making up this deposit seems to be more compacted in comparison with the observations made at other sections where the deposits shows rather a loose accumulation of pyroclasts. The bombs are characterized by a strong shortening of the vertical axes and lengthening of the horizontal axes what is termed agglutination (Sumner, 2005). The energy used during deformation is derived from the kinetic energy of the falling clast, rather than the weight of the overlying deposit (which causes compactional welding) (Sumner, 2005). Such bombs are called „cowpat bombs‟, these bombs are clasts with viscous/ brittle rims with a fluid interior. The character of the deposit is also strongly influenced by its accumulation rate. For cold/brittle clasts, the accumulation rate is irrelevant and scoria cone deposits are formed. For warm clasts (in this case here) low accumulation rates yield bomb beds (Sumner, 2005). The scoria and bombs show varying degrees of vesicularity and welding.

The angular greyish clasts found above the cryptodome can be fragments that are ripped of the conduit wall or they are broken fragments of a lava flow or a dyke (pers. comm., van Wyk de Vries, 2010). The most reasonable explanation in this context is a lava flow or dyke deposited before the emplacement of the cryptodome. These clasts are found in the layers deposited before the cryptodome. The cryptodome may not have blocked the surface. It is not responsible for clast fragmentation and the deposits overlying it (emplaced afterwards). The lava flow or dyke fragments had been ejected from an erupting vent early in the eruption. These fragments are also quit dense and contains less vesicles in comparison with the scoria

72 clasts. Their seems to be an alignment present of the vesicles (which are elongated) in the largest fragments. This kind of alignment was seen at other lava flows deposited in the quarry.

7.1.5 Section 1: correlation lithologs On Figure 42, 5 lithologs were made at section 1 of which 2 of them (litholog 3 and 4) are located between the dykes coming from the Lemptégy II edifice. The same figure contains also the different tracks present in the quarry where the observed layers were located.

Because of the presence of magmatic and tectonic structures some layers are deformed and only observed at 1 locality. Starting at the deepest point from section 1, the deposits are a chaotic structureless deposit of scoria, bombs and dense angular fragments. The presence of those dense fragments are a reference to correlate some of the layers. Layer K and L at litholog 2 contain a lot of these dense grey clasts. Considering the 33° dip of those layer, it shows correlation with layer C and D (litholog 3) on Figure 42. The deposits contain the same pyroclastic fragments as observed in litholog 2. When we go further towards the north, those dense fragments are also found at layers G and in a minor amount in layer H of litholog 4. Based on the presence of these dense fragments there is a correlation possible between layer D (litholog 3) with layer H (litholog 4). The steeply dipping layers become more horizontally and thinning rapidly towards the north where they are not exposed anymore. For the deposits observed from track 3a and 3b, the identified layers contain a lot of bombs, which have an orientation. That orientation is for one part of the bombs towards the north (dip ~15°) and for the other part towards the south (dip ~20°). Looking at the bombs from a distance, we see that they form a bow-shaped structure (seen at layer A and B in litholog 3). From litholog 4 the bow shaped structure of the bombs is missing and the bombs are horizontally deposited, creating a crude horizontally bedding.

From the fifth track upwards, the major difference between the lithologs is litholog 4 which has only 1 layer deposited. This layer J contains no grading and does not seems to have any connection with the other layers at the other lithologs. It is not affected by the magmatic structures which causes other layers to be deformed. It has some affinity (same pyroclastic fragments) with the black deposit towards the south and the right side of the cryptodome. It is possible that during the deformation of some layers, depression structures developed and that these gaps were filled afterwards with pyroclastic fragments.

The uppermost layer of this section is present at all the lithologs (indicated as layer O, F, O and U except for litholog 4). That layer is covering the other layers as a horizontal deposit

73 containing a mix of small red and black scoria fragments. Looking at litholog 2, the thin layer N, is cut off by fault F2 (if as said before it is not a fault plane itself) and is no more observed at the other lithologs. Layer M of litholog 2 is probably also cut off by fault F2, some of the fragments seems to be the same of layer E (litholog 3), so they can be correlate. The steep dipping of the layers at litholog 2 is probably due to deformation and uplift of these layers caused by the cryptodome. Layer L (litholog 2) and D (litholog 3) seems to correlate very well based on the fragments building up this layer. They contain both bombs with small scoria fragments incorporated in the bombs edges. Also small scoria fragments are welded together, as well as the presence of the dense angular fragments in both layers. The first explanation is already said in the former paragraphs, that fault F2 abruptly end at layer N, the deposits beneath N are not cut off by this fault F2 so layers L and D are the same deposit. Another explanation is based on the emplacement of the cryptodome. During this emplacement tectonic forces are build up and causing the already deposited layers to deform and lift up (pushed up) due to the expansion of the cryptodome. During this upwards movement fault F2 is developed (normal fault). Result of this movement is that the layers of litholog 2 underwent a displacement.

The last 2 lithologs are easier to correlate. At litholog 5 there is a fault present (fault F3 Figure 36) which is interpreted as a thrust fault. On Figure 37 picture C it is clearly seen that the layer in the south of fault F3 had shifted over its corresponding part located north of fault F3 (displacement ~40cm). Quite rare to find thrust faults at a location were probably uplift occurred by the emplacement of a cryptodome or intrusion. Normally we expect normal faults at the flanks. Maybe the total thickness of the affected was not outcropping and is fault F3 wrong interpreted as thrust fault. At litholog 5 layers M, M1, N and O are affected by this fault. Layer M1 belongs to the downwards moved block and layer N is the upwards moved block, in other words layers N and M1 are the same deposit displaced by fault 3. That deposit contains a huge amount of angular fragments and the same deposit can be found back at litholog 6 which are indicated as layers S and T with T containing a major amount of angular fragments. So layers N and M1 (litholog 5) and layers S and T (litholog 6) can be correlate with each other. In both lithologs there is an alternation of layers containing more scoria fragments and bombs with layers containing more angular fragments. At litholog 6 layer P correlates with layer L (litholog 5), containing also much angular fragments. Layer M (litholog 5) correlates with layers Q and R from litholog 6 which are scoria deposits in between the more angular fragments containing layers (layer P and T litholog 6). The

74 alternation of more scoria-rich layers towards more angular-rich layers, shows a certain comparison between the phase 5 (Figure 22) deposit interpreted at section 3.

From litholog 5 (layers M1, N and O) and 6 (layers U, T and S) seems to show a connection with litholog 2 (layer M, N and O) and 3 (layer E and F) (Figure 42). Based on the identical pyroclasts (and equal grainsize of these) found in layers O (litholog 2), F (litholog 3), O (litholog 5) and U (litholog 6) they can be correlate and interpreted as the last deposit seen at section 1. For the other mentioned layers, the layers are build-up of the same mix of pyroclasts (e.g. scoria (red and black), angular clasts and in lesser amount bombs (at lithologs 5 and 6). Also the grainsize decreases towards the north related to the larger distance from the Lemptégy vent. These last observed layers at the lithologs, seems to have also an alternation between more scoria-rich layers and more angular-rich layers (e.g. at litholog 3: considering uppermost part of layer D (scoria-rich), layer E (angular-rich) and layer F (scoria-rich)).

7.2 General introduction of section 2 at Lemptégy II The following paragraph describes section 2 (Figure 16) of the Lemptégy II deposits. Figure 44 gives an overview of section 2, the eastern and western parts of section 2 is not visible, therefore I refer to Figure 46 and Figure 48. The section was divided into 3 parts (eastern, central and western part), from each part a litholog was made. Figure 45 shows the main stratigraphic contacts between the different deposits of 5 eruptions, of which 3 deposits belonging to 3 surrounding scoria cones (Puy des Gouttes, Puy de Côme and Puy des Chopine).

A sequence of different deposits from bottom to the top can be established as follow (pers. comm., van Wyk De Vries, 2010). Lemptégy I is a very chaotic deposit containing a lot of bombs ranging from 5 cm to 2 m in diameter, scoria (red coloured) which containing a lot of granite enclaves (de Goër de Herve et al., 1999). These granite enclaves are torn of the substrate rock by the ascending magma. The basement (page 5) exists of granites and so it is lithological to find granite enclaves in Lemptégy I deposits. The chemical composition of Lemptégy I lies on the border of basaltic towards trachy-basaltic (de Goër de Herve et al.,1999; Boivin et al., 2004). The eruption of Lemptégy I and Puy des Gouttes started contemporaneous. The age of Puy des Gouttes and Lemptégy I is ~30.000 years old, which means both were indeed active at the same time. But the last eruption phase of Puy des Gouttes started when the Lemptégy I eruption ceased due to lacking of mixed deposits of Lemptégy I and Puy des Gouttes. The deposits of this last eruption phase covering the

Lemptégy I deposits in the quarry. At many places, the contact between the deposits of Lemptégy I and Puy des Gouttes is very irregular. The lapilli of Puy des Gouttes seems to fill small depressions caused by the deposits of Lemptégy I. The lapilli that are in contact with the Lemptégy I deposits are red coloured. Colour refers to the fact that the deposits from Lemptégy I were still hot enough to bake and oxidize the lapilli of Puy des Gouttes and therefore causing the red colour. On Figure 47, we observe a system of fractures in the deposits of Puy des Gouttes. Also these fractures tend to affect the deposits of Lemptégy I (de Goër de Herve et al., 1999) but I did not observe fractures at the Lemptégy I deposits. Another volcano has left his imprints in the quarry of Puy de Lemptégy this is Puy de Côme. I was unfortunately not able to take a closer look at these deposits because of the high height in the sequence where Puy de Côme was located. The deposits of Puy de Côme are covering the Lemptégy II deposits and underlying the deposits of Puy de Chopine. At distance, it looks like very fine black material, interpreted as an ash layer with a wavy contact (bottom and top) between Lemptégy II and Puy de Chopine. The distance of Puy de Côme relative to the Lemptégy scoria cone is ~2 km, so ash fall out can be considered here. In the central part of section 2, I could not trace the ash layer further towards the west. There are a few explanations for this observation. The first explanation is that the ash layer became invisible because of sliding down of fragments. Second one is that it was not deposited in the western part of the section. And the last option is that the Chopine layer was emplaced as a pyroclastic flow, surge or avalanche. These are energetic processes able to erode/rework loose underlying deposits. The first explanation is the most reasonable one for this phenomenon due also to the fact that the outcrop was not reachable for detailed observations. The deposits of Puy de Chopine were already described in this paper (page 18).

7.3 Observations of the lithologs section 2 from Lemptégy II

7.3.1 Section 2 litholog 3: stratigraphy Litholog 3 (Figure 43) is the most complete sequence in section 2 which contains the thickest part of the deposits of Lemptégy II.

Layers A and B are deposits that are quite different from the already observed Lemptégy II deposits. Layer A contains well rounded clasts with small vesicles. The maximum grainsize ranging from ~3mm to ~6mm. A gradual transition in the colour is observed: from red towards the yellow-brownish layer B. Layer B contains identical clasts (~3mm) as in layer A. Layer C contains totally different clasts in comparison with the former layers and the

76 transition quite sharp contact. In this part of the section, the overall part the deposits is an alternation between fine grained scoria clasts and more angular, larger clasts. Observing normal grading or reverse grading is difficult. I tried to divide the deposit in several layers as well as possible. Layer C contains black scoria with an irregular surface. There seems to be a correlation between the maximum grainsize of the scoria and the vesicles. Some scoria have a maximum grainsize of ~7cm and containing long elongated vesicles. More rounded vesicles are observed in scoria clasts with a maximum grainsize of ~3cm. The larger scoria clasts are observed at the top of layer C and are consistent with reverse grading. A gradual transition in the proportion of angular, greyish clasts is observed in layer D. No grading is observed. A few intact bombs are observed. Layer E contains more scoria clasts (~5cm) and reverse grading is observed. A gradual increase in the proportion of angular, greyish clasts (~4cm) is observed in layer F. Again no grading is observed. Layer F shows the same characteristics as layer D and also the thickness of both layers is equal (~2m). Layer G is a thin (~60cm) layer containing more scoria clasts (~6cm) and less angular greyish clasts. Above layer G, a very thin (~15cm) layer, layer H, is observed. That thin layer can be followed from the central part of section 2 (Figure 47) towards the eastern part (Figure 48) and appearing in section 4 (litholog 1 layer B* and litholog 3 layer I* (Figure 52)). Layer H shows reverse grading, maximum grainsize of ~2mm to ~1cm. The clasts are well rounded, black and containing small rounded vesicles. The highest layer of the sequence, layer I, contains scoria clasts ranging in size from ~2 to ~6cm and more angular clasts (~4cm). Layer I seems to share again the same characteristics of layer D and E. Above layer I, there is another part of deposits belonging to this section (Figure 48), but for that part I was unable to have a detailed look at the fragments because of the height. Based on the pictures taken, that deposit contains black scoria clasts. The smaller (~2cm) scoria clasts seems to be organised in lens-shaped structures bordered by the occurrence of coarser clasts.

Figure 43: section 2 litholog 3: stratigraphy 77

Figure 44: Overview photograph section 2

Figure 45: overview section 2 with stratigraphic contacts (full red lines; dotted red line (estimated correlation), black square (location of photograph) (scale: purple square (~1m)) 78

Figure 46: photograph of picture A on Figure 45, western part of section 2 with stratigraphic contacts (full red lines, dotted red line (estimated contact)) and location of litholog 1 (green line) (grey bar (central on photograph) length ~1.70m)

Figure 47: photograph of picture B on Figure 45, central part of section 2 with stratigraphic contacts (full red lines, dotted red line (estimated contact)), location of litholog 2 (purple line) and tectonic structures (full green lines)

Figure 48: photograph of picture C on Figure 45, eastern part of section 2 with stratigraphic lines (full red lines, dotted red line (estimated contact)), location of litholog 3 (purple line)

7.3.2 Section 2: interpretation lithologs Layer A and B belong to the deposits of Puy des Gouttes due to the maximum grainsize and form of the particles, the clasts are indicated as lapilli. Apparently there seems no soil layer present between layer B and layer C in comparison with key section 3 where transition between Puy des Gouttes and Lemptégy II is marked by a soil layer. Probably the soil layer has a very thin thickness and was not observed.

Litholog 3 divides the whole deposit into 2 parts. For this division layer H is considered as a key layer. The layers beneath layer H, show an alternation of more scoria-rich layers and more angular-rich layers with some indications of reverse grading. The scoria-rich layers contain also broken, angular clasts which are probably bombs breaking apart during landing. Layer H, as said in the observations can be found back in section 4 (see further in this paper). It is a very thin layer consisting of very small fragments (lapilli), which can be related to the intensity of the eruption. Layer H shows internal grading fine-grained at the bottom towards coarse-grained at the top. Fine grained layers are the result of an increase in intensity in the eruption dynamic. The thickness reveals that the period of increased intensity was relatively short. Afterwards the intensity ceases, resulting in an increase in particle grainsize. Above layer H, another part is deposit with shows equal characteristics of the part beneath layer H.

7.3.3 Section 2: correlation lithologs Litholog 1 and 2 contain thicker deposits of Puy des Gouttes in comparison with the deposits at litholog 3. The decrease in thickness is because the layers dip ~13° towards the east and disappearing at the central part of section 4. The oxidizing and baking process caused by the Lemptégy I deposit is clearly seen in the bottom layers for all the lithologs. For all the lithologs, no soil layer is observed between the Puy des Gouttes and the Lemptégy II deposits. The correlation between several layers of Puy des Gouttes are mainly based on the maximum grainsize of the clasts. The deposits have the same characteristics: well-rounded lapilli clasts with small vesicles, deposited in alternating layers where the maximum grainsize varies from ~2mm to ~1cm. The Lemptégy II part at litholog 1 consist out of 1 layer containing scoria clasts with maximum grainsize ~4cm and gradually increasing to ~10cm at the top. The greyish angular clasts are less abundant in comparison with the other 2 lithologs. The Lemptégy II deposit (litholog 1) correlates well with layers B and C (litholog 2) and with layer C (litholog 3). Major proportion of the clasts in these layers (B and C (litholog 2); C (litholog 3)) are scoria clasts. Layer B (litholog 2) correlates with layer C (litholog 3), based on equal size clast and clast type. Layers C and D (litholog 2) correlate with layer D (litholog

3) again based on clast size and type. But the greyish, angular and scoria clasts are more concentrated in distinct layers observed at litholog 2. Unlike at litholog 3 (layers D, F and I) the angular clasts are much more mixed with the scoria clasts. Maybe there is a transition/alternation of lens-shaped structures containing coarse angular material (litholog 2) and layers containing an equal proportion of both clast types (angular and scoria) (litholog 3). The alternation observed in litholog 2 has an affinity with the last deposit observed at section 3 (phase 5). Layer E (litholog 2) correlate with layers E and F (litholog 3). The bottom part of layer E which is scoria-rich correlates with layer E (litholog 3) which is also scoria-rich. The upper part of layer E correlates with layer F (litholog 3) based on the proportion of the clast types. Probably layer G (scoria-rich) can be correlate with the uppermost part of layer E (litholog 2) due to the transition to again a more scoria-rich part in layer E (Figure 49).

Section 2 will be divided into 3 parts: part A (C, D, E, F and G litholog 3), part B which contains the key layer (layer H) and the third part, part C (all the layers deposited above layer H). The same structure can be found back at section 4.

D C B

Figure 49: Correlation lithologs from section 2 (black lines: correlation lines)

7.4 Observations of the lithologs section 4 of Lemptégy II.

Figure 50: Overview photograph section 4 (scale: purple square height ~1m)

Figure 51: left: detailed picture of soil layer between the deposits of Puy des Gouttes and Lemptégy II (scale: hammer ~28cm); right: detailed picture of irregular layer with the greyish rounded clasts

Figure 52: Overview photograph of section 4 (full red lines: stratigraphic contacts, dotted red lines: uncertain stratigraphic contacts; black lines: location of lithologs) and correlation of lithologs from section 4 (black lines: correlation lines) 85

7.4.1 Section 4 litholog 2: stratigraphy Starting with the lowest layer (layer C) of litholog 2 (Figure 53). Layer C consists of red and black well sorted rounded clasts, grainsize ranging from ~2mm to ~6mm. The clasts have small rounded vesicles. The transition from layer C towards layer D is quite sharp and marked by the presence of thin (~2cm) red-brownish soil layer (Figure 51). Layer D contains rounded greyish-red scoria clasts with grainsize ranging from a few millimetres to 1-2 centimetres. There are also small white fragments found (~3mm). The transition between layer D and E is sharp. The maximum grainsize of the clasts (layer E) is ~8cm. When breaking the clasts, I observed that the edge of the particles contains more vesicles than the inner part. Layer E shows no signs of grading and it contains for 40 volume% fine grained brown-yellowish coloured sandy material. Layer E consists of scoria clasts (~4cm) and bombs which are broken into several pieces.

The transition along section 4 between layer E and F is not seen because of the track construct for the guide tour. Layer F contains vesicular scoria clasts (~4cm). The scoria clasts are rounded, the surface is less angular than observed in the previous layers at other sections. The few bombs present have a size of ~30 to ~40cm in diameter. They are light-grey coloured and again there are lot of broken fragments (~20 volume%) with angular sharp edges probably from bombs that broke on impact. These broken clasts and bombs are clustered in the central part of layer F with more scoria clasts present under and above that central part.

Again the transition between layer F and G is not observed at this location because of the small landslides occurring at the section wall. The transition between layer F and G could be observed at the other lithologs (litholog 1 and 3 Figure 52). In these lithologs the transition is marked by the presence of a thin layer (thickness ~10cm). These thin layers are indicated as layer B* (litholog 1) and I* (litholog 3). They contain well rounded clasts with grainsize ranging from 1-2 to 5mm. This layer can be followed along section 4 (except where small landslides occurs e.g. at litholog 2) and could be marked as a key layer.

Layer G is a well sorted layer with a small proportion of bombs (<10 pieces) observed for the whole section. It appears that these bombs are concentrated at the highest part of layer G. The black scoria clasts have a regular surface, more or less rounded but angular shapes are still present. The scoria grainsize range from 2 to 4cm. In layer G, I observed that there is another layer (part) present with a the very irregular contact and a constant thickness variation (Figure 51). The clasts are very different in comparison with the typical pyroclasts found in the

86 quarry. The clasts are characterized by a light-greyish colour, well rounded (ball-like clasts) with small round vesicles and the grainsize range from ~3mm to 1cm.

Figure 53: Section 4 litholog 2: stratigraphy 7.4.2 Section 4: interpretation and correlation lithologs Layer C is attributed to a part of the deposits of the Puy des Gouttes eruption. This is based on the characteristics of the clasts as described in the previous sections (section 3 and 2).

I described in detail the most complete litholog of section 4, now it is the purpose to correlate the layers from the other lithologs with each other. On Figure 52, the correlation between the different lithologs is shown. A good relationship between the layers can be seen except for layers C, D and E from litholog 2 due to weathering material and small landslides at the locations for litholog 1 and 3. I made a few places free to see whether there were completely different layers deposited in comparison with layers C, D and E (litholog 2). Overall layers C, D and E were also found at these places with a minor variation in thickness of the layers.

The overall dip of the Lemptégy deposits is ~13° away from the Lemptégy II vent (dip to the east). The major difference between the lithologs is the thickness variation of the layers. The thickness variation can be easily explained by looking at Figure 52. Looking at the deposits of 87

Lemptégy II, it is clear that it fills part of a depression. Layers in the northern part of the section dip towards the south, for the central part the layers become almost horizontal and in the southern part of section 4 they dip towards the north. Related to this depression the thickness variation correlates with the deposited layers. This means that the bottom layer (layer A) of litholog 1 becomes thicker in the south and the upper layer of litholog 1 (layer B) becomes thinner towards the south. In general section 4 can be divided in 2 parts: the division based on using key layer B* (litholog 1) and I* (litholog 3). The deposits above this key layer are thinning towards the south and the deposits underlying the key layer are thickening towards the south. The expressed morphology can be due to the paleo-morphology created by the previous deposits of Puy des Gouttes and Lemptégy I. We know that the upper contact of the Lemptégy I deposits are quite irregular. The Puy des Gouttes deposits are draped over this irregular contact. It is probable that at this location a depression was formed and filled up with the Lemptégy II deposits.

There were a few observations made at litholog 2 which have not yet a good explanation. The soil layer between the Puy des Gouttes layer (layer C litholog 2) and the deposits of Lemptégy II tells us that there was a break of several years between the 2 eruptions. A soil layer is a characteristic feature of warmer periods in earth‟s geological history. Plotting the age of Puy des Gouttes and Lemptégy II at the geological time scale, the conclusion is that both ages fall in the Pleistocene period (Calabrian 1.8-0.01Ma (ICS, 2009)). The Calabrian age is characterized by the occurrence of peri-glacial environments. The development of a soil in a peri-glacial environment is at a very slow rate. A thin soil layer may actually cover a long period of time (~2000 years).

Layer D (litholog 2) is a fine grained layer which support that the eruption phase depositing this layer, started with a small volume of erupted material but intense explosion. The intensity decreases when layer E was deposited (containing coarser material). As mentioned above, layer E contained a lot of sand sized material. It seems that layer E was affected by weathering processes. After the deposition, it is probable that rain water could infiltrated in the deposit causing the fragments to degrade.

Layer F consist mainly of scoria but the central part appears to contain more broken clasts especially broken bombs. If we try to find those fragments at the same location in the other lithologs we cannot localize them. This means that it is probably a lens-shaped layer as a result of clasts rolling down the cone slope (grain avalanche) or a local depression in the

88 deposit where coarser fragments are gathering. Some of these broken clasts were found in the other lithologs. Those clasts are more angular and not found in lens-shaped layers. The thin layers (B*, G* (estimated occurrence at litholog 2 due to continuous dispersal along section 4) and I*) are the expression of an increased intensity in the eruption dynamic of Lemptégy II based on the small clast size occurring in these layers. This key layer can be probably linked with the thin layer (layer H) observed in litholog 3 at section 2 . It can be also linked at 1 of the fine-grained layers deposited at section 3 (phase 5). The overall grainsize of the pyroclasts is decreasing at section 4 due to the larger distance from the Lemptégy II vent.

The highest layer (layer G) contains the irregular part with greyish coloured, well rounded clasts. This irregular bordered part can be continuously followed along section 4. From the observations made at the clasts, we can interpret these clasts as the result of the conduit was being blocked. It is known that the conduit can be blocked during several eruption phases. During these blocking, different clast types are mixed and polished against each other what leads to the abrasion of the clasts. Looking at the density measurements carried out on section 4, sample L2S4T3Sa7 is a sample obtained from this greyish part. The density of the clasts are ~1.03 g/cm³ and are quite dense in comparison with other sampled scoria clasts (Figure 27). This denser clasts can be little bits from the abrasion process between denser fragments. In general section 4 shows a decrease in the overall grainsize of the different clasts (scoria) with a large proportion between 4 and 32 mm (Appendix C).

7.5 Observations of lithologs section 5 of Lemptégy II Section 5 (Figure 54) is a very small, heavily weathered outcrop where little observations could be carried out. At the overview photographs (Figure 55) the stratigraphic contacts for Lemptégy I and II are indicated. The Puy des Gouttes deposits are not present at this sections or not outcropping due to occurrence of landslides.

Figure 54: Overview photograph section 5 of Lemptégy II (scale: length ~100m)

Figure 55: photograph section 5 with stratigraphic contacts (red line), location lithologs (white lines) and correlation

Figure 56: section 5 litholog 1: stratigraphy 7.5.1 Section 5 litholog 1: stratigraphy It was difficult to observe the layers deposited at this section because of the chaotic appereance of the clasts. I was able to distinguish 4 layers based on the change in proportion of the different type clasts. At litholog 1, layers A1 and A3 contain greyish, angular clasts and black scoria. Layers A1 and A3 are different from layer A2. Layer A2 contains more scoria particles (70 volume%) in comparison with the angular clasts present in layers A1 and A3. A remark had to be made here: it was not easy to follow layer A2 along section 5. This layer can

90 also be lens-shaped layer consisting of less angular clasts. At litholog 2 (Figure 55), the same layer (layer B2) seems to appear containing less angular clasts. The highest layer (layer A4) consists of small scoria clasts (~2cm).

The pyroclasts found at section 5 have the same characteristics for both lithologs. The scoria is black coloured with a maximum grainsize ranging from ~2.5 and ~6 centimetres and have an irregular surface. Holding the scoria in the sunlight, different colours (purple and blue) are observed. The vesicles range from small round ones to big irregular vesicles. The angular clasts have a maximum grainsize of ~6 to ~10 centimetres. No grading observed in the different layers.

At section 5, a lot of weathered material (40 volume%) is present in between the pyroclastic clasts. It is a dark brown, coarse sand sized material (~dirt) which can be found for the whole section (Figure 57).

Figure 57: Detail picture litholog 1 section 5: dirt in between the pyroclasts (scale: grey bar ~40cm) 7.5.2 Section 5: interpretation and correlation lithologs Over the geological time section 5, was probably more influenced by weathering processes in comparison with the other sections. At the other locations there is not such a large proportion of dirt found between the pyroclasts. There must be another factor present for this difference. Looking at Figure 54, not much vegetation is observed above the section, only some grass and a few bushes. The top of the other sections is bordered by trees. The difference in vegetation

91 can be the cause for easily infiltration of rain water into section 5. Rain water can also flow down and causing weathering of the pyroclasts.

Some characteristics of the layers in both lithologs (section 5) had been observed at section 2 (Lemptégy II deposits litholog 3). These Lemptégy II deposits were made up of alternating layers which containing more or less angular clasts. In litholog 1, the different layers are indicated as part A. Litholog 2 is almost identical to litholog 1 except the layers their thickness is different. The deposits exposed at section 5 are linked with the deposit found in section 4 (litholog 2 layer E Figure 53) based on the appearance of the dirt (sand sized material) and the clast types.

7.6 Observations of the lithologs section 6 from Lemptégy II. Section 6 is a long section at the quarry. It is located between the Lemptégy II and Lemptégy I vent. At section 6, deposits of 3 different volcanoes are observed: Lemptégy I, Puy des Gouttes and Lemptégy II (Figure 58 and Figure 59). The top of section 6 is covert by a lava flow which can be followed along the section until it disappears in the northern part of the section (litholog 3). Section 6 is divided into 3 parts: southern part (Figure 58), central part (Figure 59) and northern part (Figure 60).

On Figure 58, the Puy des Gouttes deposits are exposed as an alternation of fine and coarse grained layers. The coarse-grained layers are sticking out due to a larger resistance to weathering. On Figure 59, the layers of Puy des Gouttes are dipping ~6° towards the southeast. The Puy des Gouttes layers deposited beneath the Lemptégy II deposits are red coloured. On Figure 60, the lower layers of Puy des Gouttes are deposited directly on the Lemptégy I deposits (as observed at the other sections) and these layers are also red coloured.

Figure 58: overview photograph southern part of section 6 with stratigraphic contacts (red line) and location litholog 1 (green line) (scale: length ~50m)

Figure 59: overview photograph central part of section 6 with stratigraphic contacts (red line) and location litholog 2 (green line) (scale: length ~30m)

SE NW

Figure 60: overview photograph northern part of section 6 with stratigraphic contacts (red lines) and location litholog 3 (black square) (scale: length ~15m)

Figure 61: Correlation of lithologs section 6 (black lines: correlation lines) 94

Figure 62: section 6 litholog 1: stratigraphy 7.6.1 Section 6 litholog 1: stratigraphy The numbered layers in litholog 1 (Figure 62) are the Puy des Gouttes deposits. At section 6, the layers are well exposed to make observations. Layer 1 has an irregular contact with the Lemptégy I deposits. Remark made here is that those layers in contact with the Lemptégy 1 deposits do not have the typically red colour in comparison with the other sections (e.g. section 3 and 2).

The deposits from Puy des Gouttes are well sorted and the pyroclasts have almost an uniform grainsize. Layer 1 contains light-greyish, rounded clasts with small round vesicles and a maximum grainsize ranging from ~2 to ~4 millimetres. At layer 2, the maximum grainsize changes abruptly towards more coarser clasts (~1cm). Grading is observed for layers 3 till 8. Layer 3 shows reverse grading (maximum grainsize ~1cm). Layer 4 goes from reverse to normal grading. An uniform grainsize distribution is observed and layer 5 (~1cm). Reverse grading is observed at layer 6, normal grading at layer 7 and the last observed deposit (layer 8) has an uniform grainsize with red coloured clasts (~5mm). Gradually going upwards in the sequence the Puy des Gouttes deposits are changing from black coloured clasts towards more reddish ones.

The contact between the deposits of Puy des Gouttes and Lemptégy II is sharp. At this location no soil layer is observed. Layer A contains black scoria clasts (~90 volume%) with a maximum grainsize ranging from ~2 centimetres towards larger clasts (~8cm) at the top of layer A. There are bombs present in layer A, with a red coloured crust and a dark greyish

95 centre. Layer B contains the same clasts as in layer A but has a larger proportion (80 volume%) of red coloured scoria. Above these layers, a lava flow was deposited which is characterized by a serrated edge and elongated vesicles in the central part of the lava flow.

7.6.2 Section 6: interpretation lithologs The most remarkable phenomenon is the absence of the red coloured Puy des Gouttes deposits which are directly in contact with the Lemptégy I deposits. At the other sections those layers have the typically red colour caused by the still hot Lemptégy I deposits to bake these layers. There are a few explanations possible. The first one is that the Lemptégy I deposits were already cold when the Puy des Gouttes deposited his lapilli fall out layers. The second explanation is probably related to the distance between the Puy des Gouttes scoria cone and the location of this deposits in the Lemptégy quarry. Looking at Figure 16, section 6 is the most distant section seen from Puy des Gouttes. The Lemptégy I deposits had more time to cool off before they were covered by the Puy des Gouttes deposits. The red colour is observed at the highest layers (layer 7 and especially 8) of Puy des Gouttes. The whole deposit consist of lapilli clasts which are well sorted and have a regular round surface.

For the Lemptégy II deposits (layer A and B) the distance of the Lemptégy II vent is small (~20m). The clasts deposited near the vent are larger and this is observed for layers A and B (~8cm). In comparison with the other sections (2, 4 and 5 Figure 16) are located further away from the Lemptégy II vent and the clasts have a smaller (~4-32mm) mean grainsize (Appendix C). The lava flow, covering layers A and B, had probably a high temperature to bake the clasts. The result is that more red coloured clasts are present in layer B than in layer A. The lava flow itself is classified as an AA – lava flow type (pers. comm., van Wyk de Vries, 2010) (Figure 63). The lava flow has a dip of ~6° towards the southeast and was flowing towards the southeast. The lava flow is affected by oxidation processes causing the red spots on the surface.

7.6.3 Section 6: correlation lithologs All the Puy des Gouttes deposits can be found back in each litholog, with minor differences in thickness but with the same characteristics for the pyroclasts. Especially for litholog 2 and 3 almost no differences were observed. Also the Lemptégy II layers correlate well except for the thickness issue. At litholog 3, the lava flow is absent.

Figure 63: detailed photograph of lava flow (pencil ~16 cm for scale) 7.7 Tectonic structures of Puy des Gouttes At Figure 47, the green lines represent the tectonic structures present at the Puy des Gouttes deposits. At this specific location (section 2) a contemporaneous occurrence of compressional and extensional movements is observed. These tectonic movements are no real tectonic movements but these are gravitational movements. The clasts are deposited on a steep volcano flank. To sustain the angle of repose, some of these clasts began to slide downwards from the slope. Probably these small slides can cause the occurrence of very small earthquakes. These small seismic activities are frequently observed at volcanic regions. The time period for these tectonic activity is situated after the Lemptégy I and before the Lemptégy II deposits. Based on the observations carried out in the field, the normal and inverse faults do not seems to affect the deposits of the Lemptégy I and II deposits (also the paleo-soil is not affected) (http://planet-terre.ens-lyon.fr/planetterre/objets/img_sem/XML/db /planetterre/metadata/LOM-Img200-2007-05-28.xml).

Figure 64: photograph of faulting occurring at the Puy des Gouttes deposits (section 3) (red lines: stratigraphy; green line: fault) (scale: grey bar ~1.70m) 97

8 Statistical and spatial distribution of volcanic bomb at Lemptégy II

8.1 Introduction For the different sections in the quarry, I measured the dimensions of the bombs present at those sections. Not all the bombs were measured only those bombs that are in situ, not completely broken and size range from minimum 10 cm to maximum ~2m. Also a detailed description was carried out of the following characteristics: colour, vesicles, form and other observed external features. In the following paragraphs you will find tables with the information for each section separately but I will first give a short introduction of the different bomb types that can be present in the sections.

The different types of bombs are named according to their shape, which is a result of the fluidity of the magma from which they result.

- Ribbon or cylindrical bombs: They have a ribbon like appearance and typically tabular vesicles. The bombs are formed from high to moderately fluid magma, ejected as irregular strings and blobs. Those strings break into small segments falling intact on the ground surface (Francis and Oppenheimer, 2004). - Spherical bombs: Formed from high to moderately fluid magma. To get a sphere shape, the magma has to be cooled off significantly or rolling down the slope. In this case, surface tension plays a major role in pulling the ejecta into spheres (Francis and Oppenheimer, 2004). - Spindle or fusiform bombs: Same formation process as the spherical bombs but these bombs spin during their flight with gives these bombs an elongated or almond shape (Francis and Oppenheimer, 2004). - Cow-pat bombs: Formed when highly fluid magma falls from moderate height. Their name is due to the fact they splodging at the surface and form irregular round like disks resembling cow-dung. These bombs do not solidify before impact so they are still liquid when they strike the ground (Francis and Oppenheimer, 2004). - Bread-crust bombs: rounded or angular lumps with a smooth, glassy crust broken by deep cracks and fissures. They are formed when lumps of viscous, gas-rich lava are ejected from the vent: the outer crust chills quickly to form the glassy crust but the interior remains hot, and continues to vesiculate (Francis and Oppenheimer, 2004).

- Cored bombs: consisting of different rinds of lava enclosing a core of former consolidated lava. The core consist of accessory fragments recycled in the vent from a previous eruption sometimes non-volcanic (Francis and Oppenheimer, 2004).

8.2 Statistical distribution of the bomb characteristics of Lemptégy II At some sections, several bombs were observed and described. Following characteristics were observed: size of the vesicles, form of the vesicles, colour of the crust and the interior colour (Appendix D for a detailed description). I present the results in graphics for each section. Two type of graphics are made: one is a relationship based on the colour issue (oxidized/baked) of the bombs and the other graphic is created to investigate whether there is a relationship between the vesicles and the bomb colour.

Table 12: Legend of abbreviations used in the graphics, large (≥ 4 mm) and small (< 4mm)

letter characteristic type first letter size of vesicles large (L) small (S) large and small (L+S) second letter form of vesicles round (R) irregular (I) elongated (E) third letter colour of crust red (R) black (B) fourth letter colour of interior red (R) black (B)

From section 1, 41 bombs were observed (observations made on in situ bombs and clearly reachable for observation).

Figure 65: colour features of bombs observed at all stratigraphic levels at section 1 The major part of the bombs (28  68%) have a red crust and a black coloured interior (Figure 65). Figure 66 illustrates the relationship between the vesicle shape and size. The

99 highest peaks, show that these bombs have 1) large (≥ 4 mm) and round vesicles for the bombs characterised by a red crust and black coloured interior, 2) large and irregular formed vesicles (Figure 73 picture B) with the bomb characterised by a red crust and black interior and 3) small round vesicles with the same colour pattern as the former ones. Those bombs at section 1 are mostly characterized by the presence of a red coloured crust and black interior, together with large round or irregular formed vesicles which is related to a lighter density. They are interpreted as bombs which are cooled during flight but still having a warm outer shell. The size of the vesicles depends on how much gas has been exsolved and how long bubbles have been expanding – coalescing.

Figure 66: relationship between vesicles shape, size and the colour pattern of the bombs present at section 1 Figure 67 and Figure 68 are graphics made from the observations of section 3. 55 bombs were observed (observations made on bombs found in situ and clearly reachable for observation).

For most bombs found at section 3, the interior is black coloured (39  71%) with the majority containing a red crust (25  64%) and the remaining part contains a black crust (14  36%). The graph (Figure 68) displaying the vesicles characteristics shows the following results:

- 10 (18%) bombs containing large and small vesicles, large ones are irregular formed and the smaller ones are round, the bomb has a red crust with black interior - 5 (9%) bombs containing large round vesicles, a red coloured crust and black interior

100

- 5 (9%) bombs containing small round vesicles and the bomb is completely black coloured - 5 (9%) bombs containing small round vesicles, a red coloured crust and black interior

The only conclusion based on these results is that most of the bombs present at section 3 have a black interior. There seems to be no correlation between the size and form of the vesicles related with the colour pattern. Almost all possible types of vesicles (round, irregular and elongated (Figure 73 picture A) are present if we look at the complete data displayed at Figure 68.

Figure 67: colour features of bombs observed at all stratigraphic levels at section 3

Figure 68: relationship between vesicles shape, size and the colour pattern of the bombs present at section 3 101

The following graphics are made from the observations from section 4 and 5. 7 bombs at section 4 were observed and 8 at section 5 (observations made on in situ bomb and clearly reachable for observation).

The bombs at section 4 are almost completely black (a few are completely red) with the majority (4 67%) containing small round vesicles. All of the observed bombs at section 5 are completely black. On Figure 69, the majority of the black bombs contain small round vesicles for both sections. A few contain large (≥ 4 mm) and irregular vesicles.

Figure 69: relationship between vesicles shape, size and the colour pattern of the bombs present at section 4 and 5 Looking at the location of section 4 and 5 at the quarry (Figure 16), these sections are further away from the Lemptégy II edifice in comparison with section 1 and 3. The amount of bombs present at those further located sections decreases rapidly. For the bombs present at section 4 and 5, following characteristics are observed: the majority of the bombs is completely black coloured containing small, round vesicles. They are already completely cooled off before reaching the surface (black coloured) because they spend more time travelling through the air until landing on the surface.

The characteristics of the bombs recorded at section 1 and 3 highlight some differences with section 4 and 5: 1) both sections are located near the Lemptégy II vent, 2) looking at the colour prospective, there is an increase in the amount of bombs having a red coloured crust (due to oxidation/heat) and 3) an increase in the size (large ≥ 4 mm) of the vesicles. The difference in size and shape of the vesicles are caused by differently degassing of the magma

102 before or during bomb ejection. The bombs are still hot reaching the surface (closer to the Lemptégy II vent, less time to cool off).

Those observations and interpretations are carried out for the whole sections which are compared with each other so no stratigraphic correlation is made here for this part of the research.

8.3 Dimensions of the bombs From each section, the dimension of the bombs was measured. Measuring the long axis (a) and the smallest axis (b) by using a metre (cm scaled). To deduce the ellipticity of those bombs, I use the following formula: ellipticity = a / b with a: longest axis and b: the smallest axis.

If a and b are equal, the cross-section being a circle, then the ellipticity ratio is equal to 1. With increasing ellipticity, the ratio increases (> 1). The graphics below show a relationship between the size of the bombs and their shape. A set of reference isogons with ellipticity values ranging from 1 (shape = circle) to 4. These ellipticity isogons gives more information about the shape of the bombs: circular or what is the quantity of the ellipse shape. At appendix E, you will find the results of these ellipticity ratio in the last column. In the paragraph below, I will present the ellipticity ratio of each measured bomb in a plot for each section. Afterwards I will give a brief interpretation of the results.

Figure 70: Plot of longest axis and smallest axis, data of the measured bombs at section 1 plotted within the reference ellipticity isogons

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Figure 71: Plot of longest axis and smallest axis, data of the measured bombs at section 3 plotted within the reference ellipticity isogons Most bombs of section 1 have an ellipticity ratio much larger than 1, so this means that those bombs are flattened and developing an ellipse form rather than a circle (Figure 70). Most of the bombs have an ellipticity ratio between 2 and 4. For an ellipticity ratio equal at 2, this means that the length of the bombs is twice as large than the thickness. For an ellipticity ratio equal at 4, the longest axis is 4 times larger than the smallest axis and they are quite flattened. The longest axis is horizontally oriented in the section (following the stratigraphy). On Figure 71, a transition towards more circular bombs is observed. The majority of the bombs has still an ellipticity ratio between 2 and 4. Again the longest axis is oriented horizontally in this section. The more circular bombs are probably located higher and further towards the east of section 3 due to longer time period for cooling.

For section 4 and 5, the results are plotted on the same graphic due to a minor amount of measurements. Most of the bombs plot between the ellipticity ratio ranging from 2 to 4 but the overall size of the bombs is much smaller than the bombs observed at section 1 and 3. For section 4 the data has a larger spreading and the bombs have a larger size but equal ellipticity ratio as the bombs measured in section 5. The bombs of section 5 are more clustered and have a smaller size in comparison with the section 4 bombs but an equal ellipticity ratio (Figure 72). In both sections the longest axis is horizontally oriented.

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Figure 72: Plot of longest axis and smallest axis, data of the measured bombs at section 4 and 5 plotted within the reference ellipticity isogons Following statements can be made:

- Ellipticity ratio is between 2 and 4 - The majority of the present bombs has a greater tendency towards an ellipse, so the majority is flattened with an increase for the longest axis - Section 3 has a small increase in the amount of lower ellipticity bombs - Section 1 is characterized by the abundance of flattened bombs. This suggests that the interior of the bombs was still hot enough during landing and to deform the surface. Due to shape and size of these bombs and the temperature on landing, these bombs are indicated as cow-pie bombs (also some present at section 3)

For the crust pattern there seems no relationship present at which section typical crust patterns are most abundant and if the crust pattern can be linked to size, shape and vesicles characteristics present for these bombs. There a few bombs where the crust could be observed in detail. A few general remarks:

- Some bombs have a crust which looks as a typical ribbon structure (Figure 73 picture D and E) related with a minor amount of elongated vesicles: ribbon bombs - Another type of bombs have a protruding surface and have a more circular appearance: cauliflower bombs (section 3, 4 and 5) (only a minor part of the total amount of bombs observed have this typical appearance) (Figure 73 picture F)

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- Most bombs at the quarry (especially at section 1) are identified as cow-pat bombs, because of their typical flattening.

Figure 73: picture A: elongated vesicles; picture B: irregular vesicles and red interior of bomb; picture C: elongated and large round vesicles; picture D and E: ribbon crust pattern and picture F: protruding crust pattern (cauliflower) (scale for picture A-E: pencil ~16cm and picture F: grey bar ~40cm)

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9 Discussion: eruption dynamics of Lemptégy II

9.1 Introduction Thanks to the observations made on the field, I was able to make several lithologs in order to document the stratigraphic sequence of the Lemptégy II deposits present in the quarry. With these stratigraphic sequence I could derive several phases in the eruption dynamic of Lemptégy II. The occurrence of magmatic structures reveals also important information about the chronology of the deposits and the eruption dynamics. A detailed description of the different pyroclastic clasts was needed in order to interpret the eruption dynamics associated with this typical pyroclastic deposit. In the following chapter, I will give a chronological reproduction of the eruption history from the Lemptégy II scoria cone.

9.2 First cone growth: Lemptégy I and Puy des Gouttes In the eastern part of the Lemptégy quarry (Figure 15), the pyroclastic products of the Lemptégy I eruption were observed. Plotting the geochemical analyses in the TAS diagram, those analysis plot on the border of basalt to trachy-basalt (Figure 32). The Lemptégy I pyroclastic products consist of lava flows, a large variety of sizes and types of bombs and red or black scoria clasts. The pyroclastic deposits of Lemptégy I show almost no internal grading nor layering. The Lemptégy I deposits are the result of a low intensity eruption from multiple vents. This interpretation is stated with the irregular deposition of the Lemptégy I deposits. The occurrence of welded scoria and spatter is related to the existence of different spatter cones building up these deposits. Some lithologs containing pyroclasts of Lemptégy I, marked with a general name the Lemptégy I deposits.

The eruption of Lemptégy I is dated at a 32.000 years ago. Within the same period a neighbouring volcano began to erupt: the Puy des Gouttes. At different localities in the quarry, the pyroclastic products of Puy des Gouttes were observed. The Puy des Gouttes deposits overlying the Lemptégy I deposits. The contact between the pyroclasts of Lemptégy I and the Puy des Gouttes is quite irregular, with Puy des Gouttes clasts filling in depressions created by the Lemptégy I pyroclasts. Both volcanoes began to erupt simultaneously. The scoria cone of Puy des Gouttes is much more voluminous than the rather small scoria cone of Lemptégy I. When the eruption of Lemptégy ceased, the eruption of Puy des Gouttes was still continuing. The Puy des Gouttes deposits, with discontinuous layers filling the irregular depressions caused by the Lemptégy I eruption, found at the quarry are related to the last eruption phase of the Puy des Gouttes. The main observations is the colour of the Puy des

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Gouttes deposits: the red coloured pyroclastic products at the base and the black coloured upper deposits. The red coloured base suggests that the deposits of the already erupted Lemptégy I pyroclasts were still hot enough to bake those Puy des Gouttes deposits. The upper deposits are not affected by the heat of the Lemptégy I deposits, they are cold and black coloured. The sections (2, 3, 4 and 6) containing Puy des Gouttes deposits are shown in Table 13. The deposits of Puy des Gouttes are well sorted and consist of well rounded, lapilli sized pyroclastic products. These deposits are termed lapilli sized fall deposits.

When the Puy des Gouttes eruption was ceased and before the development of the second edifice at the Lemptégy scoria cone and the resulting eruption phases, a rest stage occurred. This rest stage is reflected by the presence of a soil layer (which is observed at some lithologs e.g. section 3 litholog 1) between the Puy des Gouttes deposits and the first pyroclasts of Lemptégy II. Unlike the irregular border between Lemptégy I and Puy des Gouttes, the transition towards the Lemptégy II deposits is sharp.

9.3 Eruption dynamics of Lemptégy II

9.3.1 Paleo relief caused by the Lemptégy I scoria cone Taken into account the presence of the Lemptégy I scoria cone at the Lemptégy quarry, the material erupted by the Lemptégy II eruption phases, will be deposited around the Lemptégy II vent. But because of the earlier eruption of Lemptégy I, the Lemptégy I deposits will block the lateral dispersal of the Lemptégy II deposits in the eastern part of the quarry. For the sections 2, 4 and 5, Lemptégy II deposits will only be accumulated when the height of the eruption column exceeded the already accumulated Lemptégy I deposits. The lock of the earlier Lemptégy II deposits further away suggest a low energy eruption at the start of the Lemptégy II eruption. Dykes also will be better developed along the western side of the Lemptégy II vent (which is observed) and will be stopped at the eastern part. This phenomena is termed the presence of a paleo-relief. Between the Lemptégy I and II vent systems, no pyroclastic deposits are observed because of the excavation processes carried out in the quarry, except for a small amount of pyroclastic material present at section 6.

The dip of the layers deposited at section 3 (Figure 22) is influenced by the existence of the older Lemptégy I scoria cone. The deposits (section 3) show steeper dip directions with increasing height towards the east and eventually becoming horizontal when reaching the top of the Lemptégy I scoria cone. The dip caused by Lemptégy I will create grain avalanching towards the west which results in larger clasts gathering at the western part of section 3

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(Figure 21 and description of lithologs section 3). Looking at the clast size observed at section 3, there is a gradual decrease of clast size towards the east which state the occurrence of grain avalanching at section 3 caused by the former cone slope of the Lemptégy I scoria cone. The dip caused by the slope of the new scoria cone, Lemptégy II, will induce grain avalanching towards the north (away from the vent) (section 3) creating coarser deposits downwards and away from the Lemptégy II vent. Some clasts will also turn back into the vent by grain avalanching on the inner slope of the Lemptégy II vent. The vent can be partially blocked by those avalanching pyroclasts. Erupting pre-erupted material forms deposits containing more angular pieces (Valentine et al., 2005).

In the description of the lithologs from section 2 (page 76) there is an alternation visible of more scoria-rich and more angular-rich layers with a gradual decrease in grainsize towards the top. Grain avalanching will occur, with dip direction towards the east (section 2) due to the paleo-relief of the Lemptégy I scoria cone and towards the northeast (for section 2 litholog 3 (Figure 48)) away from the Lemptégy II vent.

Grain avalanching at section 4 will be towards the east (away from the Lemptégy II vent). For section 4, also the depression (Figure 50) will create local grain avalanching with the central part becoming a reservoir for larger clasts. In section 1 the deposits have a chaotic appearance, especially the layers located between the dykes. It is rather difficult to prove grain avalanching occurred but it was probably the case. Observations show that hot scoria clasts rolled down the slope as welded scoria clasts were incorporated at the bombs surfaces.

Grain avalanching creates downwards an accumulation of larger clasts, grain avalanches upslope show a gradual decrease in grainsize and inverse grading vertically within the layer (Figure 74).

Figure 74: sketch of grain avalanching, downwards accumulation of larger clasts, upwards slope accumulation of smaller clasts

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9.3.2 Eruption chronology of the Lemptégy II eruption The observed pyroclastic layers are the result of different phases in the Lemptégy II eruption. Before discussing the several eruption episodes and there related pyroclastic layers which can be found at Table 13, a general interpretation in the eruption style of Lemptégy II will be given. The eruption style is built with my own observations done in the field and the analysis of the pyroclastic material.

The Lemptégy II scoria cone is constructed by several eruption changes of a Strombolian type eruption. The changes show a gradual evolution of the eruption dynamics throughout an eruption. These changes leave a trace in the sequence of accumulated pyroclastic material during the Lemptégy II eruption. In the section about scoria cones (page 10), several characteristics of pyroclasts resulting from a Strombolian eruption style were discussed. The main question that has to be answered is: “Are these Lemptégy II deposits indeed the result of a Strombolian eruption?”

The first activity of Lemptégy II (phase 1) began with the formation of 3 spatter cones followed by the occurrence of eruptive fissures (Delcamp, 2005). Spatter cones develop when highly fluid magma is ejected from the vent. The fire fountains become extremely dense meaning that the clasts cannot radiate their heat into the atmosphere (Francis and Oppenheimer, 2004). So the molten clasts hit the ground and fuse together to form agglutinated spatter cones. Because spatter is mostly fluid when it lands on the surface, the deposits are very irregular and weld together as they cool off. From these spatter cones, spatter fed flows can develop when hot spatter fragments are welded together on the ground and flow away. Those spatter fed flows are usually small lava flows. Spatter fed flows are general linked with Hawaiian eruption style but in a minor part they appear also in Strombolian eruptions (Vergniolle and Mangan, 2000). The start of the Lemptégy II eruption was a low intensity eruption (stated with the presence of spatter cones and spatter fed flows). Only a minor volume of pyroclastic material is deposited during this stage.

The most extensive lava flow extends from the eruptive fissure. That eruptive fissure generated a linear system of spatter cones (Figure 11). During the construction of the spatter cones and spatter fed flows, some pyroclastic fragments located at the top of the fire fountains are carried away downwind (Vergniolle and Mangan, 2000). The individual clasts cool down quickly by radiating their heat into the atmosphere. So they are chilled and solid whey they hit the ground where they accumulate as scoria deposits (Vergniolle and Mangan, 2000). These

110 scoria deposits can be found in the lithologs of section 6 (section 6 litholog 1, 2 and 3) and section 3 (section 3 litholog 2 layer A + lava flow). The deposits at section 6 are limited by the occurrence of the lava flow (flow direction towards the south). The first eruption phase of Lemptégy II is indicated as scoria fall deposit 1 and the initiation of a lava flow (Table 13).

Looking at the Lemptégy II deposits we can divide these deposits into 3 types: inner crater -, transitional - and outer wall facies (Vespermann and Schmincke, 2000). The inner crater facies is built up of lava spatter which is described in the former eruption phase I and represent only a minor volume of erupted material. The transitional facies is characterized by the occurrence of welded scoria clasts but they can still be recognized as individual pyroclastic fragments. The outer wall facies are characterized by coarse grained layers (see Eruption dynamics and internal structure). The transition from the transitional – towards the outer wall facies is observed at section 3 (section 3 litholog 2). The transitional facies is located in the western part of the section (section 3 litholog 2 Figure 17) and the outer wall facies, starting from the centre towards the eastern part of the quarry (section 3 litholog 1 (Figure 19) and 3 (Figure 20)).

After the first eruption phase of Lemptégy II the eruption became concentrated along a single vent and resulting in the development of a scoria cone from a more explosive strombolian activity. During eruption phase 2 (Figure 77) which produced scoria fall deposit 2, pyroclasts were accumulated at section 1 and 3 (Table 13). The deposits observed at section 3 resulting from phase 2 are characterized by the deposition of mainly bombs and scoria (angular fragments are accumulated in a lesser amount). Some bombs are broken (brittle) by impact. Also the scoria were cold on impact. Major part of the pyroclasts is coarse grained ~8-32mm (Figure 30 and appendix C) which reveals the bursting of gas bubbles in the conduit causing little fragmentation (Valentine et al., 2008). Less clear in these outcrop layers is the occurrence of grain avalanching. As said in the previous paragraph the grains avalanching caused by the over steepening of the outer cone slope may not be observed at this section. We have only one plane to look at (no 3D view at the deposits). The occurrence of grain avalanching caused by the older scoria cone Lemptégy I is observed at section 3 and is due to the paleo-relief created by the Lemptégy I scoria cone.

The pyroclastic layers observed at section 1 were also deposited during phase 2 and are characterized by red scoria which are partly welded together, flattened (cow-pat) bombs (red crust, black interior with vesicles) but also angular, dense clasts are observed (see description

111 lithologs section 1 page 63). Layers deposited above these dykes contain the same dense fragments (section 1 litholog 2). The dense clasts are probably the result from a post eruption event of Lemptégy II which erupted lava or dyke fragments near the cryptodome. The orientation of the bombs present between the dykes, show an antiform structure (Figure 75). This orientation can be due to the development of a tension field (e.g. compression and uplift) caused by the emplacement of the dykes/cryptodome. This tension field causes the bombs to be reoriented. These deposits are marked by the eruption phase 2 which produced the fall deposit 2 and afterwards the start of the deformation of the layers located between the dykes.

Figure 75: antiform orientation of bombs On the overview photograph of Lemptégy II (Figure 15), we see several dykes crosscutting some of the Lemptégy II deposits. Taking in account the relative chronology between dykes and the surrounding rocks, the dykes must be younger than the rocks that are affected by the dykes. The dykes are located in the western part of the Lemptégy II edifice (section 1). After the emplacement of the dykes, one dyke will probably lead to the formation/growth of a cryptodome. Several large intrusions were also settled in between the already deposited layers at section 1 (Figure 36). What is the dynamic or process giving existence to these intrusions and the cryptodome? As said in previous sections a cryptodome is a sort of lava dome where the magma never reach the surface. It is an accumulation of magma just beneath the surface. During the emplacement, it causes the deformation of the previous erupted material (Figure 36 and interpretation lithologs section 1). The material is being uplifted and deformed (plastically and brittle). There are 3 more locations found (2 at section 1 (Figure 36)) and 1 at section 3 (Figure 23)) where the erupted material is plastically deformed (and pyroclasts contain the same characteristics). But the deformation is weaker and the magmatic structures are not outcropped except for the small intrusion seen at section 3 (Figure 23) which seems to be located in the alignment of the levee-constrained lava channel (pers. comm., van Wyk de Vries, 2010). Probably the lava flowing along this lava channel was locked at a certain moment (e.g. cooling off became more resistant to flow) and the lava was accumulated as an

112 intrusion like feature. The presence of an intrusion located at the most northern part of section 1 can also be related to the occurrence of the lava channel, the lava channel seems to also in the alignment of the deformation structures seen at this part of section 1. Maybe the levee- constrained lava channel was split up into 2 directions and resulting in the formation of 2 intrusions at different localities (Figure 76).

Figure 76: small intrusion observed at section 3 (black circle), full red lines show alignment of lava channel, dotted red lines show the estimated location of the branches from the lava channel resulting into intrusion like features (scale: purple square with height ~1 m) The chronology of the dykes and the emplacement of the intrusions/cryptodome is difficult to observe in the field. The dykes affect only the lower layers deposited at section 1. The emplacement of the cryptodome and intrusions at section 1 affect the upper deposited layers of section 1 (except for the uppermost layer) by means of deformation structures (e.g. faults and folding). I assume that the emplacement of the dykes started first after the deposition of the layers affected by these dykes. The growth of the cryptodome started probably simultaneously with the emplacement of the dykes (after eruption phase 2). By the next eruption phase new pyroclastic layers are deposited at section 1 (phase 4 see further). The cryptodome reached its maximum volume (as observed in the field) after this eruption phase 4 and causes the layers to be deformed by the cryptodome. When the last observed layer at section 1 was deposited, the cryptodome had stopped growing. Interpretation related to the fact that the last deposit show no deformation features. Probably the same occurred for the intrusions located further along section 1.

When this eruption phase ceases, another phase (phase 3) (Figure 77) began with the accumulation of different pyroclast material in comparison with the other deposits. This thin, fragile reticulite layer can be followed in section 3 where it disappears towards the west (almost horizontal at section 3 litholog 2) and towards the east of section 3. This layer is

113 marked as a key layer because the pyroclasts are different from the other pyroclasts observed at other sections. This reticulite layer is not found at other sections. What are the eruption dynamics causing the development of this typical and local deposit? Based on literature description, the pyroclasts are identified as probably reticulite (reticulite deposit). The interpretation for reticulite is stated with my own observations and analysis. The thin sections show the presence of a polygonal border between the vesicles (Figure 33). Reticulite pyroclasts consisting of a polygonal network of glass and vesicularities of 95-99% (Wolff and Sumner, 2000). The vesicularity results (only ~77%) do not confirm the normal observed vesicularity for reticulite. The eruption dynamics related to produce reticulite are different from those depositing scoria clasts. Reticulite is a product resulting from high Hawaiian fire fountains. The formation of reticulite is mainly due to a difference in ascent rates, expansion of the gasses and a different bubble nucleation compared to scoria pyroclasts (Wolff and Sumner, 2000). Is this layer prove of a transition from a Strombolian eruption phase towards a more Hawaiian phase? Or is the eruption style not an end member but a transitional eruption style between a Hawaiian and Strombolian eruption style? There are other observations to prove the occurrence of a Hawaiian eruption (e.g. spatter cones and spatter fed flows) but these observations are not solid because they tend to occur also in Strombolian eruption phases.

The deposited layer above this reticulite layer consist essentially out of coarse and angular clasts. Those clasts reflect pieces of previously larger clasts that were broken on 1) impact, 2) by grain avalanching or 3) by recycling into the vent. If indeed the reticulite layer is the result of a Hawaiian like eruption, the coarse angular layer above the reticulite layer shows that a transition towards a more strombolian eruption phase (phase 4) was occurring. Because the deposit consist out of angular fragments, I refer to call it a vent cleaning event re-ejecting clasts that went back into the vent by grain avalanching.

The next eruption phase (phase 4) (Figure 77) is essentially Strombolian with the erupted material consisting out of bombs, loose scoria and angular fragments for the deposits observed at section 3. Some layers deposited during this phase have a larger amount of angular clasts in comparison with the other deposited layers. It is less clear seen at the deposit what causing this increase in angular clasts, but as seen before grain avalanching or breaking of large clasts by impact might be reasonable explanations. Also it requires a higher fountain to accumulate pyroclasts on a higher level slope. This means that the energy of the eruption

114 was larger than the former phases. The bursting events during this eruption phase will deposit scoria and bomb size clasts which would have enough time to cool off during their flight.

During this eruption phase new layers of pyroclastic fragments are deposited at section 1. In general, an alternation of coarser (angular broken clasts) and finer grained material (juvenile clasts) is observed. Flattened (cow-pat) bombs, partly welded scoria and scoria agglomerates (due to the hot clasts rolling down the slope causing small scoria particles sticking together) are present. With the observations of these pyroclasts we can conclude that they built up the transitional facies (Table 13). During this phase, material will also be accumulated on the sections located further away from the Lemptégy II vent. The previous phases were not able to deposit pyroclasts at the sections located further away from the Lemptégy II vent which may be due their lower energy. These sections (2, 4 and 5) are characterized by an uniform decreasing grainsize away from the Lemptégy II vent in comparison with the other sections (Appendix C). The height of the Lemptégy I and Puy des Gouttes deposits is exceeded so accumulation of scoria clasts could occur. These deposits located further away from the Lemptégy II vent are well sorted (appendix C), have a smaller average grainsize (~4-16mm) (Appendix C) and containing less bombs which is consistent with the relationship of dispersal area and vent location. The deposits belonging to this eruption phase can be found at Table 13.

The last eruption phase (phase 5) (Figure 77), clearly seen at section 3 (litholog 1 and 3) and to a lesser extent visible at litholog 2 and section 4, is an accumulation of fine and coarse grained material (scoria). Phase 5 (section 3 Figure 55) shows nice bedding structures associated with grain avalanching (e.g. lenses of coarse inversely graded clasts). At least 4 fine grained layers are observed. The alternation of finer and coarser grained layers, indicate a change in the intensity of the eruption (pers. comm., van Wyk de Vries, 2010). The fine- grained layers indicating an increase in the intensity of the eruption. The last eruption phase was one of changing quite rapidly of intensity. A pulsating eruption is typical of the last eruption stage (pers. comm., Matthieu Kervyn, 2011). These fine grained layers consist of well-rounded, less vesicular scoria fragments (Table 5) due to the ball milling processes which occurs in the vent. So during the last eruption phase, the material erupted was probably consisting out of recycled brittle clasts that were accumulated in the vent by grain avalanching.

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At sections 2 and 4, the last eruption phase is visible by the occurrence of a thin, fine-grained layer containing round, less vesicular scoria clasts related to the production of more fine particles due to ball-milling processes. These higher density can be due to the fragmentation of the brittle clasts and the abrasive processes occurring in the vent. Some layers deposited at section 1 show an equal alternation of scoria-rich and angular-rich layers as observed at section 3. Due to the lack of the well-rounded clasts, those layers are probably not produced by the last eruption phase of Lemptégy II.

On Figure 79 a rough sketch is made of the evolution of the Lemptégy II scoria cone during its different eruption phases.

9.3.3 Other observed features at the Lemptégy II scoria cone A layer of loose material can fold or fault. This deformation structures are observed at the layers deposited at section 1 (Figure 36). The time between depositing the pyroclastic material and the emplacement of the cryptodome or the intrusions must be quite rapid. Welding, flattened bombs (~cow-pie bombs), small scoria particles incorporated at the bombs surfaces and agglomerates of scoria particles (welded) are evidences to confirm those deposits were still hot enough to be deformed ductily. Afterwards a cooling phase probably began, which can be inferred from the brittle failure structures seen at section 1: faults (Figure 36). Not only the temperature has an influence on the deformation type also the amplitude of movement and the strain forces acting on the deposits contribute as a major influence on which type of deformation that will occur.

Faults are also excellent gateways for escaping gasses (H20) which causes the pyroclasts to be oxidized (turning black pyroclasts into red ones). The cause of the pyroclasts having a red coloured surface/interior is not yet for a 100% sure: baking processes can occur but also oxidizing of the clasts can happen. I assume that the baking process is related with convection processes, transporting the heat of the hot deposit into the cold deposit. Both processes can be active.

9.4 Strombolian eruption and pyroclastic fall deposits. In the introduction, the characteristics of Strombolian eruptions were presented. More details are discussed in following paragraph related to the Lemptégy II eruption. Strombolian eruptions are characterized by coarse clasts, much of the material is too coarse to be carried up in the buoyant plume region, but instead is distributed in ballistic fashion around the vent (Vergniolle and Mangan, 2000). The pyroclasts are predominantly scoria clasts, angular clasts

116 and bombs. Most of the deposits are mostly well sorted (grainsize distribution and sorting coefficients page 53). Grading is a characteristic of fluctuating vigour of the eruption. The fragmentation of magma occurs into discrete highly vesicular ejecta. At gas contents of about 70% small bubbles may coalesce, either in the magma reservoir or in the transport system, to produce large gas pockets that fill the entire width of the volcanic conduit (Vergniolle and Mangan, 2000). There seems to be a relationship between the size of vesicles present at bombs and the width of the volcanic conduit. These gas pockets are separated by smaller regions of bubbly magma and this regime is called slug flow (Vergniolle and Mangan, 2000). Strombolian explosions and the gas piston events observed between the eruptive episodes are the surface expression of slug flow. Both phenomena are due to the bursting of a large slug of gas at the top of the magma column (Vergniolle and Mangan, 2000).

Fall deposits are an alternation of coarser and finer grained layers. When these alternating layers show a sharp bedding plane, they refer to a spasmodic, non-sustained eruption (Houghton, Wilson and Pyle, 2000). Some sections display sharp bedding planes for these alternating layers (section 4 and some places at section 3). Lacking of these well-defined bedding planes refers to a sustained eruption phase (Houghton, Wilson and Pyle, 2000). Fall units seems to follow and drape pre-existing relief surfaces with slope angles up to 25-30°. When the slopes become steeper, material will began to roll, slide or avalanche down slope (Houghton, Wilson and Pyle, 2000). All these characteristics of fall deposits are present at the Lemptégy II scoria cone (see previous paragraphs).

The grainsize distribution and sorting coefficient calculated from Lemptégy samples are an indication for a Strombolian eruption style and the pyroclasts are accumulated by fall deposits (Figure 30 and Appendix C for grainsize distribution).

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Table 13: summary of the different eruption phases of Lemptégy II and the related deposits

eruption phase deposits section litholog and layers

Lemptégy I Lemptégy I deposits 2 litholog 1 and 2 3 litholog 3 (layer A) 6 litholog 1

Puy des Gouttes lapilli sized fall deposits 2 litholog 1, 2 and 3 (layers A and B) 3 litholog 1 (layers A) and litholog 3 (layers B) 4 litholog 2 (layers C) 6 litholog 1 (numbered layers), 2 and 3

rest stage soil layer 3 litholog 1 (layer B) and litholog 3 (between layer B and M) 4 litholog 2 (between layer C and D)

Lemptégy II erupion phase 1 scoria fall deposit 1 spatter cones and spatter fed flow 3 litholog 2 (layer A) 6 litholog 1 (layers A and B), litholog 2 (layers A, B), litholog 3 ( emplacement dykes (section 1) + start of growing cryptodome/intrusion Lemptégy II erupion phase 2 scoria fall deposit 2 1 litholog 2 (layers K and L), litholog 3 (A, B, C and D), litholog 4 (layers G, H and I) 3 litholog 2 (layers B, C, D and E), litholog 1 (layers C, E, F and G), litholog 3 ( layers M and N)

Lemptégy II eruption phase 3 reticulite deposit (mark layer) 3 litholog 2 (layers F and H), litholog 1 (layer H), litholog 3 (layer O)

Lemptégy II eruption phase 4 scoria fall deposit 3 1 litholog 2 (layers M and N), litholog 3 (layer E), litholog 5 (layers K, L, M, M1 and N), litholog 6 (layers P, Q, R, S and T) 3 litholog 2 (layers I, J, K, L, M and N), litholog 1 (layers I, J, K and L), litholog 3 (layers P, R, S and T) 2 litholog 1, litholog 2 and litholog 3 (layers C, D, E, F and G) 4 litholog 1 (layer A), litholog 2 (layers D, E and F) and litholog 3 (layer H) 5 litholog 1 part A and litholog 2 part B end of grow cryptodome + deformation layers section 1 Lemptégy II eruption phase 5 scoria fall deposit 4 3 litholog 1 (layer M) and litholog 3 (layer U) 2 litholog 3 (layer I) 4 litholog 1 (layer B), litholog 2 (layer G), litholog 3 (layer I)

thin layer well rounded clasts (mark layer) 2 litholog 3 (layer H) 4 litholog 1 (layer B*), litholog 2 (layer G*) and litholog 3 (layer I*)

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Figure 77: key lithologs chosen for sections showing which layers are related to the different stages in the Lemptégy II eruption, the Lemptégy II eruption phase 1 is not shown at this picture because of minor volume of pyroclastic material deposited and the red circle shows the Lemptégy II eruption phase 3 which is only found at section 3 (local change in eruption style) 119

Figure 78: complete correlation of the Lemptégy II lithologs (see table 11 for eruption phases and deposits) (black lines: correlation lines, black dotted lines: estimated correlation, red question marks: no correlation)

120

W E

Figure 79: sketch of the growth evolution of the Lemptégy 2 scoria cone during the different eruption phases, during phase 1 only a small cone is developed with a minor volume of pyroclastic material. From phase 4, pyroclastic material will also be accumulated at the sections located further from the Lemptégy II vent.

121

10 Conclusion The goal of this paper was to document the stratigraphy of the Lemptégy II deposits and to interpret its eruption dynamics. The main methods used were: fieldwork, density analyses, ICP-OES analyses, grainsize distribution and thin sections. During the fieldwork the main focus, was to set up the lithologs and indicating the different stratigraphic contacts between different deposits. The pyroclasts were observed in detail focussing on: maximum grainsize, shape, vesicularity, grading and specific features belonging to typical pyroclasts. Beside the description of the pyroclasts, also magmatic and tectonic related structures were observed and integrated in the eruption dynamics of Lemptégy II.

The Lemptégy II main pyroclasts were scoria, angular clasts and bombs. All these pyroclasts had a different range in terms of size, vesicularity and shape. The scoria clasts have vesicularity values (~70 - 73%) confirming the normal observed vesicularity (~70 – 85%) for scoria clasts. 2 stratigraphic layers were indicated as marker layers due to their typical pyroclasts (reticulite and typical small round, denser pyroclasts). The characteristics of the pyroclasts constituting the marker layers were stated with grainsize, density and geochemical analyses. The correlation of the identified stratigraphic layers was well-founded for several layers but at some localities layers could not be correlated with each other and at other locations the correlation was estimated. Within the Lemptégy II deposits, magmatic structures (e.g. cryptodome and intrusions) were intruded and led to typical deformation structures (e.g. folds and faults). The bombs were observed in terms of size, shape and vesicularity. Size and shape information was collected in graphics in terms of the ellipticity ratio. We can conclude that most of the bombs have a tendency towards a high ellipticity ratio. This high ellipticity ratio is mainly the result of the eruption dynamic of these bombs. Most bombs were still hot on landing and were flattened during landing (~cow-pie bombs). Bomb sizes were different for each section separately but the ellipticity ratio was rather constant. Normally we expect a tendency towards a smaller ellipticity ratio for bombs deposited at the sections further away from the Lemptégy vent. The explanation for this issue is not completely answered yet and need to be investigated in more detail.

The Lemptégy II deposits were influenced by the presence of the Lemptégy I paleo-relief. The presence of the paleo-relief gives the occurrence of special features: dipping of layers (key section 3) towards the vent, grain avalanches and the retarded accumulation of pyroclasts for the sections having a larger distance to the Lemptégy II vent and a larger time to overcome

122 the height of the Lemptégy I scoria cone. The cryptodome and intrusions had a local influence on the Lemptégy II deposits causing local deformation structures. The characteristics of the pyroclasts are also influenced by the distance of the Lemptégy II vent (e.g. welded scoria).

With all the information gathered from the fieldwork and laboratory analyses. The eruption type of the Lemptégy II scoria cone is indicated as a Strombolian eruption. This eruption type is confirmed by grainsize distribution analyses with a grainsize range of 4 to 16 mm and the sorting coefficients ranging from 1 to 2 for several layers. Strombolian eruptions are characterized by the intermittent bursts of large gas bubbles in the conduit which lead to several deposited layers with minor fragmentation for the pyroclasts. The Strombolian eruption type is also stated with literature research on previous investigated scoria cones (ref). Based on field observations and changings in pyroclasts types, there were at least 5 main eruption phases for the Lemptégy II scoria cone eruption. Within the last major eruption phase several changes in eruption intensity were deduced due to the occurrence of reverse grading and fine-grained pyroclasts layers. The major eruption type is Strombolian but the occurrence of the reticulite layer suggests that there was a slight transition towards a Hawaiian like eruption. Several analyses contradict the reticulite layer interpretation. The Lemptégy scoria cone has ~ 54.65 wt% of SiO2 (trachy-andesite basalt). This is a rather high silica content to have a transition towards a Hawaiian-like eruption style especially when this specific reticulite layer show an equal amount of SiO2 in comparison with the other layers. The vesicularity is ~77% which has a large deviation compared with the normal observed vesicularity values of 95-99%. More detailed research should be carried out to understand which pyroclasts constitute this layer and deduce the eruption or accumulation dynamics of this key layer.

During the eruption process the geochemical composition of the Lemptégy II deposits stays constant within the stratigraphic sequence. The weathering and oxidizing/baking processes had no or little influence on the geochemical composition.

Thanks to the occurrence of features (e.g. dykes) that are crosscutting other deposits, a chronological reconstruction of the eruption history could be established. Secondary processes also occurred at the Lemptégy II deposits during or shortly after the eruption and they are mainly influenced by the slope of the scoria cone (e.g. grain avalanches). The local features (e.g. cryptodome) produced deformation structures: faults and folds.

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With all the observations and interpretations, I was able to give an eruption history and the related dynamics for the Lemptégy II scoria cone. At least 5 major changes in the eruption dynamics of the Lemptégy II eruption were interpreted and leave their appearance in the stratigraphic record. These changes followed each other very rapidly. The first 2 eruptions were low energy eruptions, only pyroclastic material was accumulated near the Lemptégy II vent. After the first eruption phase, dykes were emplaced together with the growth of the cryptodome and intrusions. The third change in the eruption dynamics is only observed at 1 section (section 3) which is observed as the deposit of a reticulite layer. From the fourth eruption phase, the eruption became more energy rich and pyroclasts were accumulated at the sections located further away from the Lemptégy II vent. This eruption phase produced also the most voluminous pyroclastic deposits of the Lemptégy II eruption. Related with this eruption phase the growth of the cryptodome ended and causing the deformation of the deposited layers at section 1. The last eruption phase is characterized by an alternation of fine- and coarse-grained layers. This last phase is interpreted as a pulsating eruption changing rapidly in intensity. The lithologs represent the identified stratigraphic layers deposited during the Lemptégy II eruption. The identified stratigraphic layers were as well as possible correlated with each other to give an overview of the Lemptégy II deposits present at the Lemptégy quarry.

The reticulite key layer needs some further investigation to state whether it is actual reticulite or another pyroclast type. This can be done by using X-Ray Computed Micro Tomography (CT). This technique gives the opportunity to visualise the pyroclasts in a 3D view (e.g. analyse the vesicles in 3D view) and to carry out textural analysis.

This study can be relevant for other volcano fields. Observations done at the Lemptégy II pyroclasts can be found at other scoria cones and will help to the interpretation of the eruption dynamics.

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11 Nederlandse samenvatting

11.1 Inleiding Deze studie heeft als doel om de eruptiedynamiek van de Lemptégy II scoria cone te ontrafelen, afgeleid van zijn stratigrafie. Wetenschappelijk onderzoek in de Lemptégy groeve is pas gestart nadat de ontginning van de “pouzzolane d’Auvergne” gestopt werd in 1974. Dankzij de ontginning werd de interne structuur van de vulkaan blootgelegd en konden wetenschappers het interne mechanisme bestuderen.

Om de stratigrafie van de Lemptégy II afzettingen te kunnen interpreteren in relatie tot de eruptie dynamiek van de vulkaan was het noodzakelijk om eerste lithologs op te stellen. Deze werden tijdens het veldwerk in juli opgesteld. Delcamp (“Le Laboratoire Magma et Volcans”) heeft een eerste onderzoeksfase gestart met de bedoeling om een bovenaanzicht te reconstrueren van de groeve. Op dit “plan view” werd de lokalisatie van de Lemptégy II vent bepaald en de geobserveerde dykes werden uitvoerig gedocumenteerd.

De nood naar een verticale aanvulling (stratigrafie) was groot en dit is hoofdzakelijk mijn bijdrage in dit onderzoek.

In de volgende hoofdstukken komen de belangrijkste objectieven aan bod behandeld tijdens deze studie. U vindt er een korte inleiding in de geologische setting van het gebied meer bepaald: het Centraal Massief. Evenals een korte opsomming over de verschillende methoden en labo analyses die gebruikt werden gedurende dit onderzoek. Daarna wordt er verder aandacht besteedt aan de verworven resultaten van het uitgevoerd veldwerk en bijhorende labo analyses. Vervolgens worden de bekomen resultaten en interpretaties aan een uitvoerige discussie onderworpen om daarna te eindigen met een algemeen besluit.

11.2 Doelstellingen Eén van de doelstellingen van deze thesis is het opstellen van de stratigrafie van de Lemptégy II afzettingen. Mede met behulp van deze stratigrafie kunnen we de eruptie dynamiek van deze scoria cone bepalen wat als hoofddoelstelling van dit werk werd beschouwd. Om de hoofddoelstelling te bereiken, maken we gebruik van enkele bijkomende doelstellingen. Bij het opstellen van de lithologs worden een aantal stratigrafische lagen aangeduid als key layers met een gedetailleerde observatie van de vulkanische clasten die deze lagen opbouwen. Bepaalde vulkanische clasten worden door bepaalde eruptie typen geproduceerd en aan de hand van deze clasten kunnen we verschillende eruptiefasen en het eruptie type van deze

125 vulkaan gaan karakteriseren. Doorgaans wordt ook gebruik gemaakt van de resultaten verkregen uit de laboratorium analyses en gekoppeld aan lithologs. Naast de eruptie dynamiek zijn er ook een aantal secondaire processen aanwezig (e.g. verwering, deformatie, …). Door een antwoord te vinden op deze bijkomende doelstellingen kunnen we een bijdrage leveren in de eruptiedynamiek van de Lemptégy scoria cone.

11.3 Geologische setting De Chaîne des Puys is de meest noordelijke en recentste vulkanische eenheid gelegen in het Centraal Massief (Frankrijk). De geologische geschiedenis van het Centraal Massief begon 400 miljoen jaar geleden samen met de Hercynische orogenese. In Europa zorgde deze orogenese voor grootschalige deformatie processen en voor de metamorfose van het basement gesteente in schist en gneiss lithologie (Jung, 1946). Tijdens het Cenozoïcum, greep een grootschalige extensie van de Europese korst plaats gevolgd door de verdunning van de lithosferische mantel (Merle en Michon, 2001a; 2001b) wat in het Mioceen aanleiding gaf tot het opsmelten van de mantel door decompressie. Via de re-activatie van de Hercynische breuken, kon de magma zich naar de oppervlakte begeven wat aanleiding gaf tot de initiatie van intra-alkaline vulkanisme in de Auvergne (Wilson en Downes, 2006; Boivin et al., 2004). De meest recente vulkanische activiteit is de Chaîne des Puys met de vorming van fissure- related strombolische scoria cones, lava domes en maars tijdens het Pleistoceen en het Holoceen (Boivin et al., 2004). De Chaîne des Puys is een noord-zuid gerichte constructie met een lengte van ~60km en een breedte van maar ~2km met ongeveer 100 quartaire eruptie centrums waaronder hoofdzakelijk scoria cones (Shields, 2010).

Scoria cones zijn kleine vulkanische landvormen die in 2 verschillende vulkanische provincies geobserveerd kunnen worden. Ze kunnen voorkomen op de flanken van grotere vulkanen (schildvulkanen) (vb. Mauna Kea (Hawaï)) en als een onafhankelijke vulkanische constructie in relatief vlakke vulkanische provincies (vb. Puy de Lemptégy) (Settle, 1979). Een scoria cone is een kleine, kegelvormige vulkanische heuvel waarvan de top een boogvormige krater bevat (Wood, 1980). Deze vulkanische vormen zijn opgebouwd door de accumulatie van verschillende pyroclasts: scoria (welded or non-welded), vulkanische bommen en blocks tijdens Strombolische erupties (Lockwood en Hazlett, 2010). De korrelgrootverdeling van de pyroclasts is gerelateerd aan de intensiteit van de eruptie. Het materiaal dat tijdens de eruptie ge-ejecteerd wordt, volgt hoofdzakelijk ballistische of bijna- ballistische trajecten (Riedel et al., 2003). Scoria cones zijn hoofdzakelijk het resultaat van Strombolische erupties. Deze type erupties zijn gekenmerkt door de ontwikkeling van grote

126 gasbellen in de conduit wat aanleiding geeft tot ritmische, gematigde explosies wanneer deze gasbellen barsten aan de oppervlakte van de magma kolom. Tijdens de accumulatie van de vulkanische clasten kunnen zich lokaal grain avalanches voordoen. Scoria cones worden gekenmerkt door het voorkomen van 2 soorten facies: inner wall en outer wall facies met hun eigen specifieke vulkanische clasten.

11.4 Methode: veld- en labowerk Vooraleer het veldwerk startte, kreeg ik een korte inleiding in de Lemptégy groeve (onder begeleiding van Benjamin van Wyk de Vries). Eerder onderzoek werd uitgevoerd op de dykes aanwezig in de groeve (door Delcamp) met aandacht voor het exact lokaliseren van deze dykes. Het veldwerk bestond vooral uit het observeren en beschrijven van de Lemptégy II afzettingen en het opstellen van de lithologs.

De Lemptégy groeve toont de aanwezigheid van 2 scoria cones: Lemptégy I en Lemptégy II. De groeve werd opgedeeld in 6 secties (voornamelijk gebaseerd op het voorkomen van de Lemptégy II afzettingen) waarbij één sectie als referentiesectie werd beschouwd (meest volledige sectie). Voor deze secties afzonderlijk werden minstens 3 lithologs opgesteld. Bij de observaties van de pyroclasts werd rekening gehouden met volgende kenmerken: sortering, normale of omgekeerde gradering, maximale korrelgrootte, kwantitatieve verdeling van de verschillende types van pyroclasts en dikte van de geïdentificeerde lagen (Cas en Wright, 1987). Ook werden er meerdere foto‟s genomen van elke sectie afzonderlijk die dan gecombineerd werden tot één panoramafoto waarop de stratigrafische contacten en afzettingen aangeduid werden.

Een aantal samples werden verder geanalyseerd in het laboratorium aan de Universiteit Gent. Deze labo analyses betroffen: korrelgrootteverdeling, densiteitbepalingen (bulk en dense rock equivalent density (DRE), geochemische analyses (inductively coupled plasma – optical Emission Spectroscopy (ICP-OES)) en slijpplaat analyses.

11.5 Stratigrafie van de Lemptégy II afzettingen: resultaten en interpretatie Voor de referentiesectie werden 3 lithologs opgesteld. Bij het opstellen van de lithologs werden 3 sterk verschillende stratigrafische contacten vastgelegd. Niet enkel Lemptégy II afzettingen werden geïdentificeerd maar ook de afzettingen van 2 naburige vulkanen waaronder Lemptégy I en Puy des Gouttes. De afzettingen van Lemptégy I worden gekenmerkt door een grote verscheidenheid in korrelgrootte van de aanwezige bommen en bestaan uit roodgekleurde scoria clasts. De Lemptégy I afzettingen vertonen een chaotisch

127 uitzicht, de afzettingen vertonen geen enkel bewijs van gradering. Deze afzettingen zijn het resultaat van meerdere vents. Het contact tussen de Lemptégy I afzettingen en de Puy des Gouttes afzettingen is sterk onregelmatig. De eruptie activiteit van beide vulkanen zou gelijktijdig gestart zijn (ouderdom beide vulkanen ~30000 jaar). De laatste eruptiefase van de Puy des Gouttes zou begonnen zijn wanneer de Lemptégy I eruptie was beëindigd. Deze afzettingen worden ook in de andere secties waargenomen. De Puy des Gouttes clasten worden gekenmerkt door afgeronde lapilli-sized scoria clasts (~1cm) en de afzettingen vertonen interne gradering (normaal en omgekeerde gradering).

De Lemptégy II afzettingen waargenomen in de referentiesectie, worden gekarakteriseerd door volgende eigenschappen: scoria (sterk variërende korrelgrootte), bommen, hoekige clasten (fragmenten van bommen gebroken bij impact) en bepaalde lagen die sterk verschillende clasten bevatten. Deze lagen werden dan ook geïdentificeerd als key layers door hun sterke verscheidenheid aan clasten en de lagen konden over een lange afstand getraceerd worden. 1 laag werd omschreven als een reticulite laag. Deze clasten worden gekarakteriseerd door maximale korrelgrootte van ~1cm, vesicles zijn abundant aanwezig, clasten zijn fragiel (gemakkelijk te breken met de vingers) en de clasten vertonen een geelachtige kleur. De andere key layer bevindt zich bovenaan de sequentie. Deze laag bestaat uit scoria die een sterk bolvormig karakter hebben (sterk afgerond), kleine vesicles en een maximale korrelgrootte van ~1cm. Deze laag wordt ook waargenomen in andere secties. De afzetting die deze referentie laag bevat, bestaat uit een afwisseling van scoria-rijke en scoria-arme lagen en deze lagen worden gekenmerkt door het voorkomen van omgekeerde gradatie. Het voorkomen van deze afzetting wordt gerelateerd in een verandering in intensiteit van de eruptiedynamiek. De afgeronde, zwaardere clasten worden in verband gebracht met ball- milling processen in de conduit (Valentine et al., 2005). Het voorkomen van omgekeerde gradering wordt gerelateerd aan een vermindering in de eruptie intensiteit.

Algemeen wordt de referentiesectie gekenmerkt door lagen die een helling naar het westen vertonen van ~26°. Deze helling wordt voornamelijk bepaald door het paleo reliëf aanwezig in de groeve. Voordat de eruptie van Lemptégy II startte, was er in de Lemptégy groeve reeds een scoria cone aanwezig. Deze scoria cone is gerelateerd aan de eruptie van Lemptégy I. De accumulatie van de pyroclasts van Lemptégy I scoria cone creëren de basis voor het paleo reliëf. De Lemptégy II afzettingen worden afgezet op de aanwezige hellingsflank van Lemptégy I en naarmate de hoogte toeneemt in de Lemptégy II sequentie, wordt de hellingshoek groter. Deze helling zorgt voor het voorkomen van grain avalanches. Wat op

128 zijn beurt leidt tot het voorkomen van grote clasten in het westen van de referentiesequentie en naar het oosten een vermindering in korrelgrootte. Dit paleo reliëf zorgt er ook voor dat de secties die op een grotere afstand van de Lemptégy II scoria cone gelegen zijn pas later in de eruptie geschiedenis van Lemptégy II vulkanische clasten gaan accumuleren.

De sectie die het dichts gelokaliseerd is bij de Lemptégy II vent, vertoont een andere pyroclast samenstelling. Deze sectie bestaat voornamelijk uit rood gekleurde welded en non-welded scoria en horizontaal gelegen bommen (afgeplat) (Sumner, 2005). In deze sectie zijn ook enkele magmatische structuren aanwezig als ook tektonische. De aanwezigheid van de cryptodome (dome vormige structuur waarbij de magma nooit het oppervlakte heeft bereikt) leidt tot de deformatie van de lagen afgezet boven de cryptodome (plooi structuren en breuken). Dezelfde deformatie patronen zijn terug te vinden op andere locaties in deze sectie maar zonder dat de magmatische structuren zichtbaar outcroppen. De opvallende aanwezigheid van roodgekleurde vulkanische clasten wordt gerelateerd aan bak en/of oxidatie processen die zich voordoen in deze sectie en voor deze typische rode kleur zorgen. Deze rood gekleurde clasten komen meestal voor op plaatsen waar zich één of andere warmtebron bevindt (bv: lavastroom, intrusie en cryptodome). Ook in de referentiesectie komen deze roodgekleurde fragmenten voor waar er zich een intrusie voordoet met dezelfde deformatiestructuren.

Voor de verder afgelegen secties stellen we vast dat de korrelgrootte van de clasten sterk afneemt alsook de hoeveelheid bommen. In deze secties vinden we ook een key layer terug die gecorreleerd wordt aan de key layer gelokaliseerd in de referentiesectie (hoogste afzetting in de sequentie). Deze key layer bevat dezelfde clasten als geobserveerd in de key layer in de referentiesectie (hoogste afzetting in sequentie) en is een dunne laag van ongeveer ~3cm dikte. In deze secties bemerken we ook terug in de invloed van het aanwezige paleo reliëf van de Lemptégy I scoria cone (depressie opgevuld met Lemptégy II afzettingen). Dit leidt ook in deze sequenties tot grain avalanches.

De clasten die geanalyseerd werden in het labo zijn afkomstig uit de referentiesectie en uit een verder afgelegen sectie. Samples werden genomen uit de key layers en de andere afgezette lagen. Bij deze analyses (densiteit en korrelgrootte verdeling) werd vooral aandacht besteed voor de algemene samenstelling van de clasten van de referentiesectie en de specifieke kenmerken van de clasten uit de key layers. Ook werd aandacht gegeven of

129 verweringsprocessen en oxidatie/bak processen invloed hadden op de geochemische samenstelling van de Lemptégy II clasten.

11.6 Discussie Aan de hand van de opgestelde lithologs per sectie is het de bedoeling om de geïdentificeerde lagen per sectie te correleren met elkaar. Bepaalde lagen correleren uitstekend met elkaar op basis van vulkanische fragmenten aanwezig, dikte van de lagen en korrelgrootte. Andere lagen zijn minder makkelijk te correleren. Voor deze lagen werden verschillende interpretaties naar voor geschoven. De clasten die de reticulite laag (key layer) samenstellen, vertonen voor bepaalde analyses totaal verschillende resultaten als voor reticulite zou moeten worden geobserveerd (vb.: de vesiculariteit berekend voor de clasten aanwezig in deze laag is ~77% maar reticulite zou ongeveer 95% vesiculariteit moeten vertonen (literatuur)). Bepaalde zaken verwachten dus nog meer gedetailleerder onderzoek. Het voorkomen van de reticulite laag zou de uitdrukking zijn van een transitie van de eruptiedynamiek van de Lemptégy II scoria cone meer bepaald van een strombolisch eruptie type naar een Hawaïaans eruptie type toe om dan vervolgens weer over te gaan naar een strombolische eruptie. Op het einde van de eruptie geschiedenis van Lemptégy II zouden er meerdere veranderingen waargenomen worden in de eruptie intensiteit (hoogste afzetting referentiesequentie).

11.7 Besluit De hoofddoelstelling van deze thesis was: de eruptiedynamiek afleiden uit de opgestelde stratigrafie. De uitgevoerde labo analyses bevestigen inderdaad dat het om scoria afzettingen ging. Scoria heeft een vesiculariteit tussen 70-85% en onze samples beantwoorden aan deze waarden. De korrelgrootte distributie duidt aan dat de Lemptégy II afzettingen, afgezet werden onder de vorm van fall deposits. De gegevens van de samples werden geplot in een referentiegrafiek voor fall deposits van scoria cones geproduceerd door een strombolisch eruptie type. De geochemische samenstelling van de Lemptégy II afzettingen werden weergeven in een TAS diagram en de Lemptégy II afzettingen plotten in het trachy-andesiet basalt veld. Er werd gecontroleerd of andere processen (oxidatie en verwering) een invloed hadden op deze samenstelling maar dit werd als nihil beschouwd.

De aanwezigheid van magmatische structuren zorgden ervoor dat de Lemptégy II afzettingen gedeformeerd werden (plooien en breuken). Ook andere processen die zich voordeden tijdens/na de eruptie van de Lemptégy II scoria cone werden geobserveerd zoals grain avalanches. De Lemptégy II afzettingen werden ook op bepaalde locaties in de groeve

130 beïnvloed door eerdere eruptie van de Lemptégy I scoria cone en het door deze scoria cone gecreëerde paleo-reliëf. Dankzij het voorkomen van cross-cutting structuren (dykes) kon een chronologische afzettingsgeschiedenis bepaald worden. De eruptie van de Lemptégy II scoria cone bestond op zijn minst uit 5 verschillende eruptiefasen met elk hun karakteristieke kenmerken wat betreft de pyroklastische fragmenten. De eerste 2 eruptiefases zijn getypeerd door een lage energetische eruptie, de pyroklasten konden zich enkel accumuleren nabij de Lemptégy II scoria cone. De daaropvolgende fase (fase 3) werd gekenmerkt door een lokale accumulatie van reticulite klasten. Vanaf de vierde fase wordt de eruptie energierijker en kunnen er zich ook pyroklasten accumuleren op de verder afgelegen secties. De laatste fase is een pulsating fase en wordt gekenmerkt door het afwisselend voorkomen van fijn- en grofkorrelige klasten. Ook werden er faseovergangen tussen verschillende eruptie types meer bepaald van Strombolisch naar Hawaiiaans en opnieuw naar een Strombolisch eruptie type.

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12 REFERENCES Boivin P. (2004) Volcanologie de la Chaîne des Puys, 4th edition, with map 1:25000. EAVUC. Université Blaise Pascal. Clermont-Ferrand. p.179.

Cas R.A.F. and Wright J.V. (1987) Volcanic successions: modern and ancient: a geological approach to processes and successions. Allen and Unwin, Londen, p. 529.

De Goër de Herve A., Camus G., Lavina P. Michelin Y. Montel J-L. Tort M. (1999) Lemptégy volcan à ciel ouvert pour comprendre la Chaîne des Puys. 56p.

Delcamp A. (2005) The structure of dyke intrusions in Lemptégy quarry, Chaîne des Puys. Masters thesis, Blaise Pascal University, Clermont-Ferrand. p. 50.

Favalli M., Karatson D., Mazzarini F., Pareschi M.T., Boschi E. (2009) Morphometry of scoria cones located on a volcano flank : A case study from Mt. Etna (Italy), based on high- resolution LiDAR data. Journal of Volcanology and Geothermal Research 186 : 320-330.

Fink J.H., and Anderson S.W., 2000, Lava domes and coulees, in Sigurdsson, H., ed., Encyclopedia of volcanoes: San Diego, California, Academic Press, p. 307-320.

Francis P. and Oppenheimer C. (2004) Volcanoes, second edition, Oxford University Press. p.521.

Gonnermann H.M., Manga M. (2007) The fluid mechanics inside a volcano. Annual Review of Fluid Mechanics Vol. 39, 321–356.

Granet M., Stoll G., Dorel G., Achauer U., Poupinet G., Fuchs K. (1995) Massif Central (France): New constraints on the geodynamical evolution from teleseismic tomography. Geophysical Journal International 121: 33–48.

Hoernle K., Zhang Y.S., Graham D. (2002) Seismic and geochemical evidence for large-scale mantle upwelling beneath the eastern Atlantic and western and central Europe. Nature. 374 : 34 – 39.

Houghton B.F., Gonnermann H.M. (2008) Basaltic explosive volcanism: Constraints form deposits and models. Chemie der Erde Geochemistry 68, 117-140.

Houghton B.F., Wilson C.J.N, and Pyle D.M., 2000. Pyroclastic fall deposits, in Sigurdsson, H., ed., Encyclopedia of volcanoes: San Diego, California, Academic Press, p. 555-570.

132

Kervyn M., Ernst G.G., Carracedo J.C., Jacobs P. (2010) Geomorphometry of ‘monogenetic’ volcanic cones. Published online in 2011.

Kilburn C.R.J., 2000. Lava flows and flow fields, in Sigurdsson, H., ed., Encyclopedia of volcanoes: San Diego, California, Academic Press, p. 291-306.

Lautze, N.C., Houghton, B.F. (2006) Linking variable explosion style and magma textures during 2002 at Stromboli volcano, Italy. Bulletin of Volcanology. 69, 445–460.

Lautze, N.C., Houghton, B.F. (2005) Physical mingling of magma and complex eruption dynamics in the shallow conduit at Stromboli volcano, Italy. Geology 33, 425–428.

Lockwood John P. and W. Hazlett Richard. (2010) Volcanoes global perspectives. Wiley- Blackwell, Oxford. p. 539.

McGetchin, T.R., Settle, M. and Chouet, B.A. (1974) Cinder cone growth modelled after Northeast Crater, Mount Etna, Sicily. Journal Geophysical Research, 79: 3257-3272.

Merle O., Michon L. (2001a) The formation of the West European Rift; a new model as exemplified by the Massif Central area. Bulletin de la Societe Geologique de France 172, 2: 213-221.

Merle O., Michon L. (2001b) The evolution of the Massif Central Rift; spatio-temporal distribution of the volcanism. Bulletin de la Societe Geologique de France 172, 2: 201-211.

Nehlig P., Boivin P., de Goer de Herve A., Mergoil J., Prouteau G., Thiéblemont D. (2001) Les Volcans du Massif Central. Geologique. 130-131 : 66-91.

Parfitt Elisabeth A. and Wilson Lionel. (2008) Fundamentals of physical volcanology. Blackwell publishing, Oxford. p.230.

Porter S. (1972) Distribution, morphology, and size frequency of cinder cones on Mauna Kea volcano, Hawaii. Geological Society of America Bulletin 83 : 3607-3612.

Riedel C., Ernst G.G.J., Riley M. (2003) Controls on the growth and geometry of pyroclastic constructs. Journal of Volcanology and Geothermal Research 127: 121–152.

Shields J. (2009) The morphometry of the Chaîne des Puys. Masters thesis, Blaise Pascal University p. 50.

133

Schmidt R., and Schmincke H.-U., 2000, Seamounts and island building, in Sigurdsson, H., ed., Encyclopedia of volcanoes: San Diego, California, Academic Press, p. 383-402.

Settle M. (1979) The structure and emplacement of cinder cone fields. American Journal of Science 279 : 1089-1107.

Sparks, R.S.J. (1978) The dynamics of bubble formation and growth in magmas: a review and analysis. Journal of Volcanology and Geothermal Research, 3: 1-37.

Spera F.J. (2000) Physical properties of magma, in Sigurdsson, H., ed., Encyclopedia of volcanoes: San Diego, California, Academic Press, p.171-190.

Sumner, J.M., Blake, S., Matela, R.J.,Wolff, J.A. (2005) Spatter. Journal of Volcanology and Geothermal Research 142, 40–65.

Sumner, J.M. (1998) Formation of clastogenic lava flows during fissure eruption and scoria cone collapse: the 1986 eruption of Izu-Oshima Volcano, eastern Japan. Bulletin of Volcanology 60, 195–212.

Thiery V., Rolin P., Marquer D., Cocherie A., Fanning M. C. and Rossi P. (2009) Visean sinistral wrench faulting along the Sillon Houiller in the French Massif Central: Late Variscan tectonic implications. Bulletin de la Societe Geologique de France v. 180; no. 6; p. 513-528; DOI: 10.2113/gssgfbull.180.6.513.

Valentine, G.A., and Fisher, R.V., 2000, Pyroclastic surges and blasts, in Sigurdsson, H., ed., Encyclopedia of volcanoes: San Diego, California, Academic Press, p. 571–580.

Valentine G.A., Gregg T.K.P. (2008) Continental basaltic volcanoes – Processes and problems. Journal of Volcanology and Geothermal Research 177, 857-873.

Valentine G.A., Krier D., Perry F.V., Heiken G. (2005) Scoria cone construction mechanisms, Lathrop Wells volcano, southern Nevada, USA. Geology. Volume 33, number 8 p. 629-632.

Vergniolle, S., Brandeis, G., Mareschal, J.C. (1996) Strombolian explosions. 2. Eruption dynamics determined from acoustic measurements. Journal of Geophysical Research 101, 20449–20466.

134

Vergniolle, S., and Mangan, M., 2000, Hawaiian and Strombolian eruptions, in Sigurdsson, H., ed., Encyclopedia of volcanoes: San Diego, California, Academic Press, p. 447–461.

Vespermann, D., and Schmincke, H.-U., 2000, Scoria cones and tuff rings, in Sigurdsson, H., ed., Encyclopedia of volcanoes: San Diego, California, Academic Press, p. 683–694.

Wilson M., Downes H. (2006) Tertiary-Quaternary intra-plate magmatism in Europe and its relationship to mantle dynamics. Geol. Soc. London. 32 : 147-166.

Wood C.A. (1980) Morphometric evolution of cinder cones. Journal of Volcanology and Geothermal Research, 7: 387-413.

Wolff J.A., and Sumner J.M., 2000, Lava fountains and their products, in Sigurdsson, H., ed., Encyclopedia of volcanoes: San Diego, California, Academic Press, p. 321-330.

Websites http://planet-terre.ens- lyon.fr/planetterre/objets/img_sem/XML/db/planetterre/metadata/LOM-Img200-2007-05- 28.xml, Florence Kalfoun, Olivier Dequincey, Last modified 11-02-2010, Site Planet-Terre Service UNIS / Diffusion des savoirs ENS de Lyon 46 allée d'Italie 69364 Lyon cedex 07 www.photo-paysage.com, Bruno Monginoux

Additional references

Brousse R., Delibrias G., Labeyrie J., Rudel A. (1970) Eléments de chronologie des eruptions de la Chaîne des Puys. Bulletin de la Societe Geologique de France volume 11, numéro 6, 770-793.

Guérin G. (1983) La thermoluminescence des plagioclases, méthode de datation du volcanisme. Applications au domaine volcanique français: Chaîne des Puys, Mont-Dore et Cézallier, Bas-Vivarais. Doctorat d‟état, Université Pierre et Marie Curie, Paris VI, Paris, 253p.

Jung J. (1946) Géologie de l’Auvergne et de ses confins Bourbonnais et Limousins, Mémoires pour servir à l ‘explication de la Carte géologique détailée de la France. 1 vol. 372p.

135

Miallier D., Condomines M., Sanzelle S., Pilleyre T., Guittet J. (2003) Concordant TL and U- Th ages for a trachytique volcano in the Chaîne des Puys (French Massif Central). Quaternary Research Sciences, soumis,

Vernet G., Raynal J.-P. (2000) Un cadre téphrostratigraphique réactualisé pour la préhistoire tardiglaciaire et holocène de Limagne (Massif-Central, France). Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary Science Volume 330, Issue 6, p. 399-405.

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13 APPENDIX APPENDIX A: LEGEND OF LITHOLOGS

Legend of the symbols used in the lithologs. The size of the symbols varies (related to grain size of clasts observed at the deposit and different variations of colour are possible for the scoria, angular clasts and bombs (e.g. red scoria with black surface or black scoria with red surface)

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APPENDIX B: DENSITY MEASUREMENTS

1. Results of density measurements litholog 2 section 3

Sample: litholog 1 (L1) section 3 (S3) track 2 (T2)  L1S3T2

sample dry weight (g) dry weight with coating (g) wet weight +coating (g) mass coating (g) density rock (g/cm³) sect3 track1 1 5,0375 5,6038 -3,0800 0,5663 0,628118698 2 2,0521 2,4411 -0,8015 0,3890 0,733558548 3 2,9782 3,4038 -0,9875 0,4256 0,763461731 4 2,0779 2,3961 -0,5774 0,3182 0,796710911 5 1,2670 1,6115 -0,7353 0,3445 0,648190452 6 2,6096 3,0374 -0,6709 0,4278 0,810889948 7 2,0790 2,4860 -0,4188 0,4070 0,851698424 8 1,0629 1,3244 -0,5002 0,2615 0,696173251 9 1,4195 1,7347 -0,8710 0,3152 0,632266808 mean 0,729007641 standard deviation 0,082686268

Sample: litholog 1 (L1) section 3 (S3) track 3 (T3) sample 1 (Sa1)  L1S3T3 Sa1

sample dry weight (g) dry weight with coating (g) wet weight +coating (g) mass coating (g) density rock (g/cm³) sect3track3sample1 1 1,7397 2,0696 -0,3954 0,3299 0,83294084 2 1,3081 1,5780 -0,2917 0,2699 0,83725789 3 2,7292 3,1515 -0,7596 0,4223 0,79654538 4 2,7711 3,1395 -0,8166 0,3684 0,78479066 5 3,2313 3,5224 -0,4696 0,2911 0,88464511 6 2,3272 2,6438 -0,2228 0,3166 0,92961516 7 2,6704 3,0583 -0,9801 0,3879 0,74357252 8 3,1253 3,5151 -0,7421 0,3898 0,82090753 9 1,9254 2,2071 -0,6990 0,2817 0,74583911 10 1,2345 1,4975 -0,5336 0,2630 0,71325986 mean 0,80893741 deviation standard 0,06663421

Sample: litholog 1 (L1) section 3 (S3) track 3 (T3) sample 2 (Sa2)  L1S3T3 Sa2

sample dry weight (g) dry weight with coating (g) wet weight +coating (g) mass coating (g) density rock (g/cm³) sect3track3sample2 1 2,7935 3,2097 -1,1923 0,4162 0,712260935 2 3,8596 4,3674 -1,4423 0,5078 0,739056866 3 1,9508 2,3083 -1,9243 0,3575 0,510879252 4 2,7025 3,1182 -3,1896 0,4157 0,464284821 5 1,8521 2,2588 -1,5030 0,4067 0,562107071 6 3,2264 3,7062 -3,3899 0,4798 0,493732126 7 1,5628 1,8713 -2,0166 0,3085 0,442765632 8 1,2451 1,5758 -0,6800 0,3307 0,662509843 9 1,5247 1,7805 -0,2743 0,2558 0,865131359 10 1,4550 1,8430 -0,5287 0,3880 0,753441018 mean 0,620616892 deviation standard 0,145008632

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Sample: litholog 1 (L1) section 3 (S3) track 3 (T3) sample A (SaA)  L1S3T3 SaA

sample dry weight (g) dry weight with coating (g) wet weight +coating (g) mass coating (g) density rock (g/cm³) sect3track3sampleA 1 8,6060 9,0404 2,0570 0,4344 1,32954091 2 9,2725 9,7970 2,5990 0,5245 1,40779424 3 7,0204 7,4607 2,3446 0,4403 1,52402209 4 5,6460 5,9992 2,1007 0,3532 1,61752233 5 5,1254 5,4500 2,0564 0,3246 1,69748225 6 4,6745 5,0395 2,0005 0,3650 1,78325039 7 6,2190 6,7295 0,9797 0,5105 1,20530959 8 2,2189 2,5376 -0,1301 0,3187 0,96349890 9 5,2385 5,6013 2,0893 0,3628 1,69262507 10 4,8053 5,2050 1,0250 0,3997 1,29201283 mean 1,45130586 deviation standard 0,25873722

Sample: litholog 1 (L1) section 3 (S3) track 4 (T4) sample 9 (Sa9)  L1S3T4 Sa9

sample dry weight (g) dry weight with coating (g) wet weight +coating (g) mass coating (g) density rock (g/cm³) sect3track4sample9 1 3,0518 3,4512 -1,3565 0,3994 0,70240804 2 2,5222 2,8692 -0,3220 0,3470 0,90306579 3 3,0734 3,5450 -1,8600 0,4716 0,63245631 4 3,9782 4,3731 -0,6533 0,3949 0,87096130 5 2,0903 2,4816 -1,2020 0,3913 0,64632198 6 3,3299 3,6701 -0,4080 0,3402 0,90391782 7 2,2243 2,5304 -0,3827 0,3061 0,86839808 8 1,9413 2,2902 -1,7030 0,3489 0,54080871 9 0,9594 1,1837 -0,6348 0,2243 0,61419967 10 2,2623 2,6068 -0,6678 0,3445 0,78585482 mean 0,74683925 deviation standard 0,13580567

Sample: litholog 1 (L1) section 3 (S3) track 5 (T5) sample 4 (Sa4)  L1S3T5 Sa4

sample dry weight (g) dry weight with coating (g) wet weight +coating (g) mass coating (g) density rock (g/cm³) sect3track5sample4 1 9,2473 9,8634 3,4550 0,6161 1,624262196 2 8,0148 8,6531 1,1919 0,6383 1,193329379 3 5,0483 5,4449 1,9679 0,3966 1,6717756 4 10,0810 10,5018 4,3266 0,4208 1,775403738 5 2,6853 3,1674 -2,0259 0,4821 0,579624796 6 8,6760 9,1709 1,9300 0,4949 1,303403169 7 5,6002 6,0752 0,4053 0,4750 1,094850402 8 5,9665 6,3244 1,8840 0,3579 1,483642546 9 4,5656 4,9513 -2,2200 0,3857 0,680570782 10 5,1772 5,6891 -1,1700 0,5119 0,827351788 mean 1,2949544 deviation standard 0,381351657

Sample: litholog 1 (L1) section 3 (S3) track 5 (T5) sample 5 (Sa5)  L1S3T5 Sa5

sample dry weight (g) dry weight with coating (g) wet weight +coating (g) mass coating (g) density rock (g/cm³) sect3track5sample5 1 2,9087 3,1472 0,9401 0,2385 1,504520669 2 1,9255 2,2367 0,1050 0,3112 1,083306041 3 2,8737 3,1653 0,6853 0,2916 1,339021415 4 2,5048 2,8173 0,9363 0,3125 1,641110307 5 3,3103 3,6368 0,1239 0,3265 1,055537744 6 2,3463 2,6765 -0,2057 0,3302 0,937085044 7 2,0386 2,2973 0,6589 0,2587 1,516332523 8 1,5602 1,7739 0,3107 0,2137 1,278929956 9 1,4879 1,7656 0,1416 0,2777 1,136691509 10 1,8634 2,0829 0,7183 0,2195 1,670799494 mean 1,31633347 deviation standard 0,259673034

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2. Results of density measurements litholog section 4.

Sample: litholog 2 (L2) section 4 (S4) track 3 (T3) sample 4 (Sa4)  L2S4T3 Sa4

sample dry weight (g) dry weight with coating (g) wet weight +coating (g) mass coating (g) density rock (g/cm³) sect4 log2track3sample4 1 5,7187 6,2631 0,2502 0,5444 1,06215476 2 4,1005 4,5518 -0,8872 0,4513 0,83413240 3 4,4894 5,0034 -3,3496 0,5140 0,57938133 4 3,2786 3,5956 -1,0860 0,3170 0,76056794 5 3,1824 3,6418 -1,3404 0,4594 0,71483987 6 4,9244 5,3650 -0,1973 0,4406 0,97501598 7 4,1148 4,5524 -0,1726 0,4376 0,97508193 8 2,5971 2,9442 -1,3319 0,3471 0,67049671 9 3,2677 3,6870 0,1200 0,4193 1,05857010 10 2,9934 3,3450 0,1089 0,3516 1,05678983 mean 0,86870309 deviation standard 0,17982202

Sample: litholog 2 (L2) section 4 (S4) track 3a (T3a) sample 5 (Sa5)  L2S4T3a Sa5

sample dry weight (g) dry weight with coating (g) wet weight +coating (g) mass coating (g) density rock (g/cm³) sect4log2track3asample5 1 5,7482 6,2567 -1,3414 0,5085 0,82083973 2 4,7594 5,2009 -1,7820 0,4415 0,73623362 3 3,7799 4,1694 -1,2844 0,3895 0,75608677 4 3,1060 3,5182 -0,9253 0,4122 0,78280218 5 2,6567 3,0308 -1,5025 0,3741 0,64804320 6 5,3478 5,7821 -1,8565 0,4343 0,75050398 7 4,2429 4,7475 -2,0780 0,5046 0,68020917 8 1,9712 2,2971 -0,7435 0,3259 0,73928087 9 1,3632 1,6153 -0,5231 0,2521 0,73695674 10 2,6060 3,0102 -1,4877 0,4042 0,64651072 mean 0,72974670 deviation standard 0,05625350

Sample: litholog 2 (L2) section 4 (S4) track 3 (T3) sample 6 (Sa6)  L2S4T3 Sa6

sample dry weight (g) dry weight with coating (g) wet weight +coating (g) mass coating (g) density rock (g/cm³) sect4log2track3sample6 1 1,6762 1,9422 -0,9550 0,2660 0,64715603 2 2,9974 3,2909 -0,5853 0,2935 0,84800575 3 4,8214 5,2372 -1,0756 0,4158 0,82761589 4 2,1650 2,5097 -0,3099 0,3447 0,89264858 5 1,8757 2,2300 -0,6995 0,3543 0,74309810 6 2,2588 2,6925 -2,2223 0,4337 0,51181305 7 3,0168 3,3773 -2,5502 0,3605 0,54817730 8 3,1197 3,4453 -0,2538 0,3256 0,93893767 9 2,7529 3,1421 -0,7630 0,3892 0,79630386 10 4,5730 4,9960 -0,9429 0,4230 0,83981621 mean 0,75935724 deviation standard 0,14495240

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Sample: litholog 2 (L2) section (S4) track 3 (T3) sample 7 (Sa7)  L2S4T3 Sa7

sample dry weight (g) dry weight with coating (g) wet weight +coating (g) mass coating (g) density rock (g/cm³) sect4log2track3sample7 1 2,4819 2,8820 -1,0724 0,4001 0,71031373 2 4,6320 5,0664 -0,1067 0,4344 0,99190047 3 9,1666 9,5875 3,3468 0,4209 1,59469884 4 4,3923 4,7092 1,6105 0,3169 1,60640413 5 2,4150 2,7747 -0,6950 0,3597 0,79019444 6 3,9051 4,3287 -1,6200 0,4236 0,71596284 7 5,5631 5,9788 -0,6162 0,4157 0,91096270 8 2,4707 2,8572 -0,9669 0,3865 0,73110177 9 9,0030 9,4020 3,2980 0,3990 1,59731090 10 3,9000 4,3010 -1,6160 0,4010 0,71587602 mean 1,03647258 deviation standard 0,39923678

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APPENDIX C: GRAINSIZE DISTRIBUTION

1. Graphics and histograms from samples from section 5.

The data used to set up the histograms.

The data used to set up the cumulative plots.

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Sample: SD1, left: histogram and right: cumulative plot

Sample: SD2, left: histogram and right: cumulative plot

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Sample: SD4, left: histogram and right: cumulative plot

Sample: SD7, left: histogram and right: cumulative plot

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Sample: SD9, left: histogram and right: cumulative plot

2. Graphics and cumulative plots from samples from section 4.

The data used to set up the histograms.

Phi Grainsize (mm) wt% SD11 SD12 SD13 -6 64 0,00 10,55 10,09 -5 32 14,36 31,12 20,58 -4 16 18,10 25,77 23,83 -3 8 23,70 15,63 24,40 -2 4 17,91 10,32 12,58 -1 2 10,95 1,89 2,53 0 1 6,21 1,22 1,93 1 0,5 4,13 0,92 1,61 2 0,25 2,62 0,90 1,22 3 0,125 1,35 0,81 0,70 4 0,063 0,54 0,56 0,35 <0,063 0,13 0,31 0,16

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The data used to set up the cumulative plots.

Phi Grainsize (mm) cumwt% SD11 SD12 SD13 -6 64 100,00 100,00 99,98 -5 32 100,00 89,45 89,89 -4 16 85,64 58,33 69,31 -3 8 67,54 32,56 45,48 -2 4 43,84 16,93 21,08 -1 2 25,93 6,61 8,5 0 1 14,98 4,72 5,97 1 0,5 8,77 3,50 4,04 2 0,25 4,64 2,58 2,43 3 0,125 2,02 1,68 1,21 4 0,063 0,67 0,87 0,51 <0,063 0,13 0,31 0,16

Sample: SD11, left: histogram and right cumulative plot

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Sample: SD12, left: histogram and right cumulative plot

Sample: SD13, left: histogram and right cumulative plot

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APPENDIX D: GEOCHEMICAL ANALYSES

1. Results geochemical analyses.

Major elements: representation as weight percentage oxides

in rock SiO2 TiO2 Al2O3 Fe2O3* MnO MgO CaO Na2O K2O P2O5 LOI total BCR2(1) 53,72 2,27 13,21 13,75 0,20 3,51 7,09 3,10 1,81 0,35 99,01 BCR2(2) 53,73 2,28 13,26 13,79 0,20 3,53 6,98 3,13 1,82 0,35 99,05 JB2(1) 51,55 1,12 14,21 13,81 0,21 4,40 9,41 1,96 0,39 0,09 97,15

R12 54,62 1,59 18,16 8,79 0,22 2,58 5,84 4,30 2,62 0,72 0,31 99,45 S0.10 54,67 1,61 18,50 8,92 0,22 2,55 5,78 4,31 2,57 0,71 0,40 99,86 S1.4 54,65 1,53 17,51 8,45 0,21 2,56 6,05 4,57 2,89 0,74 0,01 99,16 S2.11 54,83 1,54 17,60 8,51 0,21 2,56 6,01 4,58 2,89 0,74 0,06 99,48 S3.8 54,68 1,54 17,52 8,46 0,21 2,55 5,99 4,57 2,88 0,74 0,09 99,14 S4.6(2) 54,26 1,52 17,40 8,45 0,21 2,55 5,98 4,51 2,83 0,73 0,04 98,46 S6.3 55,05 1,53 17,51 8,45 0,21 2,56 6,02 4,43 2,91 0,74 0,10 99,41 S7.1 54,52 1,56 17,70 8,59 0,21 2,57 6,02 4,49 2,82 0,73 0,11 99,21 S8.9 54,23 1,58 18,20 8,76 0,22 2,55 5,79 4,29 2,70 0,72 0,33 99,04 S9.7 54,57 1,54 17,41 8,49 0,21 2,58 6,01 4,52 2,88 0,73 0,08 98,95 s15.2(2) 54,38 1,52 17,48 8,41 0,21 2,56 6,03 4,55 2,91 0,74 0,09 98,79

Minor elements: representation in ppm

in rock Ba Sr Zr Y V Cr Ni Sc Co Zn Cu La Ce Nd Dy Yb BCR2(1) 677 337 185 35,5 409 21 12 32,7 35 139 14,0 27 46 31 7,6 4,4 BCR2(2) 682 339 188 35,8 410 18 14 32,7 34 138 13,8 28 47 23 6,9 4,4 JB2(1) 213 176 47 22,4 555 28 14 53,4 28 113 234,2 2 9 -22 4,4 4,4

R12 908 791 327 38,6 93 10 7 8,5 18 122 7,6 80 150 106 8,9 3,8 S0.10 933 811 314 39,1 93 13 8 8,6 18 124 9,0 81 152 101 7,7 3,8 S1.4 884 775 325 37,6 91 7 6 8,4 17 117 8,8 80 149 103 7,5 3,1 S2.11 898 783 340 38,0 91 7 5 8,4 17 118 9,4 80 152 95 7,5 3,7 S3.8 884 778 325 37,7 90 9 5 8,4 17 119 10,0 78 150 98 8,2 3,1 S4.6(2) 871 773 344 38,0 91 8 5 8,5 17 117 9,5 81 144 101 7,6 3,2 S6.3 885 778 304 37,8 90 10 6 7,9 17 118 10,1 79 148 96 7,6 3,2 S7.1 901 785 328 37,8 94 10 5 8,5 17 118 9,4 79 149 94 7,6 3,8 S8.9 919 788 320 38,3 93 10 6 8,5 18 124 10,1 82 149 101 8,2 3,8 S9.7 881 770 323 37,5 93 10 7 8,3 17 117 10,4 78 147 93 7,4 3,1 s15.2(2) 888 778 312 37,6 90 10 5 8,5 16 117 8,8 79 149 95 7,5 3,1

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APPENDIX E: BOMB CHARACTERISTICS

1. Bomb characteristics of section 1.

section longest axis (cm) smallest axis (cm) size of vesicles form of vesicles colour (crust) colour (interior) ellipticity index section 1 1 40 20 large round red black 2,00 2 60 20 small round black red 3,00 3 80 20 large and small round / / 4,00 4 120 30 large round red black 4,00 5 30 10 small round black red 3,00 6 50 20 small round black red 2,50 7 100 30 large round red black 3,33 8 70 38 small round / / 1,84 9 60 20 large elongated black red 3,00 10 50 30 large round red black 1,67 11 60 38 small round red black 1,58 12 110 30 large round red black 3,67 13 90 35 large irregular red black 2,57 14 100 28 large (cm) round red black 3,57 15 100 50 small round red black 2,00 16 90 40 small round red black 2,25 17 150 60 large round red black 2,50 18 70 38 large irregular red black 1,84 19 120 15 large elongated red black 8,00 20 200 40 large (cm) round red black 5,00 21 100 35 large (cm) irregular red black 2,86 22 40 10 large (cm) irregular red black 4,00 23 100 40 large (cm) irregular red black 2,50 24 50 30 / / red black 1,67 25 60 20 / / red black 3,00 26 50 28 / / red black 1,79 27 60 30 / / red black 2,00 28 50 20 / / red black 2,50 29 60 20 large round and elongate red black 3,00 30 40 20 large irregular red red 2,00 31 40 10 small round black black 4,00 32 60 35 large (cm) elongated red black 1,71 33 80 35 small round red black 2,29 34 35 15 large (cm) irregular red black 2,33 35 40 10 small round red red 4,00 36 55 22 large (cm) and small irregular and round red red 2,50 37 100 38 large round red black 2,63 38 50 20 large (cm) and small irregular and round red black 2,50 39 80 21 large (cm) and small elongated and round black black 3,81 40 40 10 small round black black 4,00 41 80 38 large elongated black black 2,11

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2. Bomb characteristics of section 3. section longest axis (cm) smallest axis (cm) size of vesicles form of vesicles colour (crust) colour (interior) ellipticity index section 3 track5 1 46 40 large (cm) irregular red black 1,15 2 30 10 large and small irregular and round red red 3,00 3 33 10 large and small irregular and round red black 3,30 4 60 28 large irregular red red 2,14 5 90 39 small round red red 2,31 6 100 40 large (cm) round red black 2,50 7 60 35 large (cm) and small irregular and round red black 1,71 8 100 25 large rounded red black 4,00 9 40 30 large (cm) and small irregular and round red black 1,33 10 80 30 large (cm) and small irregular and round red black 2,67 11 58 40 large (cm) and small irregular and round red black 1,45 12 50 40 large (cm) and small irregular and round red black 1,25 13 46 40 large (cm) and small irregular and round red black 1,15 14 50 40 / / / / 1,25 15 35 25 small round red black 1,40 16 55 30 large and small irregular and round black black 1,83 17 100 40 large (cm) elongated and round black black 2,50 track 4 18 80 30 large and small irregular and round red black 2,67 19 110 20 large (cm) irregular black red 5,50 20 80 20 large round red black 4,00 21 60 20 large and small irregular and small black red 3,00 22 120 30 / / / / 4,00 23 80 21 small round red black 3,81 24 30 10 small round black black 3,00 25 100 40 large and small elongated and round red black 2,50 26 200 60 large (cm) round black black 3,33 27 40 20 large (cm) round black black 2,00 28 30 20 small round black black 1,50 29 70 38 large (cm) round red black 1,84 30 60 40 small round red black 1,50 31 120 40 large (cm) round red black 3,00 32 80 30 small round red black 2,67 track3 33 150 50 large (cm) round black black 3,00 34 140 38 small round red black 3,68 35 100 30 large and small round black black 3,33 36 80 38 large (cm) round black red 2,11 37 90 40 large and small round red black 2,25 38 100 40 large (cm) and small irregular and round black black 2,50 39 50 20 small round black black 2,50 40 40 40 / / / / 1,00 41 120 40 / / / / 3,00 42 30 20 large (cm) elongated black black 1,50 43 100 28 large and small elongated and round black black 3,57 44 200 46 small round black black 4,35 track 2 45 40 30 large and small irregular and round red black 1,33 46 60 10 large and small irregular and round red black 6,00 47 50 20 small round black black 2,50 48 40 32 large (cm) irregular red black 1,25 49 40 32 large (cm) irregular red black 1,25 50 120 40 small rounded black red 3,00 51 120 50 large (cm) elongated and round black red 2,40 52 60 40 small round black red 1,50 53 70 25 large and small irregular and round red red 2,80 54 40 20 small rounded red red 2,00 55 110 30 small rounded red red 3,67

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3. Bomb characteristics of section 4. section longest axis (cm) smallest axis (cm) size of vesicles form of vesicles colour (crust) colour (interior) ellipticity index section 4 1 40 15 large and small elongated and round black black 2,67 2 60 20 small round black black 3,00 3 40 35 small round black black 1,14 4 160 52 large elongated and round red red 3,08 5 90 50 small round black black 1,80 6 45 10 small round black black 4,50 7 80 30 large round black black 2,67

4. Bomb characteristics of section 5. section longest axis (cm) smallest axis (cm) size of vesicles form of vesicles colour (crust) colour (interior) ellipticity index section 5 1 40 18 large (cm) irregular black black 2,22 2 40 21 large (cm) irregular black black 1,90 3 50 35 small round black black 1,43 4 38 12 large and small irregular and round black black 3,17 5 32 10 small round black black 3,20 6 40 15 small round black black 2,67 7 50 15 small round black black 3,33 8 50 15 small round black black 3,33

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