Surface manifestations of hydrothermal fluids such as fumaroles and hot springs provide valuable information about the level of activity of a volcano during quiescent period. Geochemical study of gas and spring waters is useful to elaborate geochemical model for magmatic-hydrothermal system. Furthermore, temporal geochemical monitoring of these fluids with time provides a better understanding in processes occurring inside the volcano and can be useful to detect any changes in the activity of the magmatic-hydrothermal system. This thesis investigates two hydrothermal systems at Kelud and Papandayan volcanoes that are located at Java Island in Indonesia. Kelud is considered as one of the most dangerous volcanoes of Java because of its frequent eruptions. After the last eruption that occurred in 1990, a new lake rapidly filled the crater of Kelud volcano. Water samples collected since 1993 are near neutral Na-K chloride fluids and are typical of aged hydrothermal system where the acidity has been completely neutralized by fluid-rock interaction and where the emission of acid magmatic gases has stopped. Two sudden increases in lake temperature in 1996 and 2001 were accompanied by rapid changes in lake water compositions and suggest the existence of two hydrothermal systems feeding the lake: a shallow hydrothermal system dominated by Ca-Mg sulfate waters and a deepest aquifer with neutral alkali chloride waters. From 2001 to 2005, measurements of CO2 emitted by the surface of the lake were performed by using the accumulation chamber method modified in order to work at the surface of a crater lake. Two statistical methods were used to process data: the graphical statistical and stochastic simulation methods. The results of graphical statistical approach showed that two different degassing processes are acting at the lake surface: one corresponding to CO2 fluxes resulting from rising bubbles and the other corresponding to equilibrium diffusion of dissolved CO2 at the water-air surface. Total CO2 emission rate estimated by stochastic simulation ranges from 105 t/day for 2001 to 32 t/day for 2005. Thermal energy released by the lake was also estimated by using an energy balance model with a new constraint using the CO2 flux. The thermal flux decreased from 200 MW (2001) to 100 MW (2002) and then remained stable. Correlation between the chemical data of waters, the fluxes of CO2 and energy show that a constant decrease in the level of activity of the volcano since 1993 occurred although the lake temperature has been stable since 2003. Since the last magmatic eruption that occurred in 1772, phreatic eruptions occur on Papandayan volcano with the last one in 2002. The volcanic material ejected during this eruption is essentially made of altered rocks from within the hydrothermal system. The interaction of acid waters with the host rocks corresponds to an advanced argilic alteration. The chemical compositions of waters from Papandayan volcano and Kelud lake waters are contrasting. Indeed, the spring waters sampled since 1994 are acid sulfate-chloride waters and acid sulfate waters. The chemical and isotopic analyses of gases and waters suggest a significant magmatic contribution in SO2, HCl and HF to the hydrothermal system. The chemical composition of waters sampled after the 2002 eruption have provided information about origin of this eruption. Decrease in chloride concentration and in delta 34S of dissolved sulfates showed that the magmatic contribution in these fluids are less important and that the waters are likely to be formed by the condensation of steam (H2O, H2S) rising from a boiling aquifer.

<p>In the boreal forests of the Northern Hemisphere, time series of tree-ring width (TRW) and maximum density in the latewood (MXD) are highly correlated to local instrumental summer-temperature data and are thus widely used as proxies in high-resolution climate reconstructions. Hence, much of our present knowledge about climatic variability in the last millennium is based on tree-rings. However, many tree-ring records have a lack of data in the most recent decades, which severely hampers our ability to place the recent temperature increase in a longer-timescale perspective of natural variability.</p><p>The main objective of this thesis is to update and extend the Torneträsk TRW and MXD records in northern Sweden. Local instrumental climate-data is used to calibrate the new tree-ring records. The results show that TRW is mainly forced by temperature in the early growing season (June/July) while MXD has a wider response window (June – August) and has a higher correlation to temperature. Two reconstructions of summer temperature are made for (i) the last 7,400 years based on TRW, and (ii) the last 1,500 years based on a combination of TRW and MXD. The reconstructions show natural variability on timescales from years to several centuries. The 20th century does not stand out as a notably warm period in the long timescale perspective. A medieval period from AD 900 – 1100 is markedly warmer than the 20th century.</p><p>The environmental impact from a large explosive volcanic eruption in 1628/1627 BC is analysed in the tree rings of 14C-dated bog pines in south-central Sweden and in absolutely-dated subfossil pines from Torneträsk. The results show evidence of an impact in the southern site at approximately this time but no detectable impact in the North. </p><p>Subfossil trees of Fitzroya cupressoides in southern Chile were 14C-dated to approx. 50,000 years BP and amalgamated into a 1,229-year TRW chronology. This tree-ring record is the oldest in the world. The variability in this Last-glacial chronology is similar to the variability in present-day living trees of the same species. These results suggest that the growth–forcing mechanisms 50,000 years ago were similar to those at present.</p>

In the boreal forests of the Northern Hemisphere, time series of tree-ring width (TRW) and maximum density in the latewood (MXD) are highly correlated to local instrumental summer-temperature data and are thus widely used as proxies in high-resolution climate reconstructions. Hence, much of our present knowledge about climatic variability in the last millennium is based on tree-rings. However, many tree-ring records have a lack of data in the most recent decades, which severely hampers our ability to place the recent temperature increase in a longer-timescale perspective of natural variability. The main objective of this thesis is to update and extend the Torneträsk TRW and MXD records in northern Sweden. Local instrumental climate-data is used to calibrate the new tree-ring records. The results show that TRW is mainly forced by temperature in the early growing season (June/July) while MXD has a wider response window (June – August) and has a higher correlation to temperature. Two reconstructions of summer temperature are made for (i) the last 7,400 years based on TRW, and (ii) the last 1,500 years based on a combination of TRW and MXD. The reconstructions show natural variability on timescales from years to several centuries. The 20th century does not stand out as a notably warm period in the long timescale perspective. A medieval period from AD 900 – 1100 is markedly warmer than the 20th century. The environmental impact from a large explosive volcanic eruption in 1628/1627 BC is analysed in the tree rings of 14C-dated bog pines in south-central Sweden and in absolutely-dated subfossil pines from Torneträsk. The results show evidence of an impact in the southern site at approximately this time but no detectable impact in the North. Subfossil trees of Fitzroya cupressoides in southern Chile were 14C-dated to approx. 50,000 years BP and amalgamated into a 1,229-year TRW chronology. This tree-ring record is the oldest in the world. The variability in this Last-glacial chronology is similar to the variability in present-day living trees of the same species. These results suggest that the growth–forcing mechanisms 50,000 years ago were similar to those at present.

Volcanic tuffs and tuffaceous sediments are frequently associated with elevated As groundwater concentrations even though their bulk As contents (~ 5 mg kg-1; Savoie, 2013) are only marginally greater than the average crustal abundance of 4.8 mg g-1 (Rudnick & Gao, 2003). Thus, As mobilization must be facilitated by conditions particular to these rocks. Alkaline desorption, anionic competition, reactive glass dissolution, and reductive dissolution of iron oxides are proposed processes of As release from volcanic rocks. Geogenic As contamination of groundwater in the southern Willamette Valley in western Oregon has been well-documented since the early 1960s, and previous studies have identified the Little Butte Volcanics Series and Fisher and Eugene Formations as the source of As contamination. This study examines 19 samples from 10 units of ash flow tuffs and tuffaceous sediments within the Fisher Formation and Little Butte Volcanics Series, representing a range of weathering and devitrification, to determine conditions of mobilization and mineralogical constraints that control As release into solution. Leachate studies were conducted over a range of pH from 7 to 11, phosphate concentrations from 10 μM to 100 mM, and in time series from 4 to 196 hours. Results demonstrate that silicic volcanic tuffs are capable of mobilizing As in concentrations above regulatory limits at pH conditions produced naturally by the tuffs (pH 8-9) or with moderate concentrations of P (10-100 μM). Alteration products, e.g. zeolites and clays, appear to be the primary host phases for mobile As. Samples that do not contain these alteration products tend to produce concentrations of As well below regulatory limits and often below the instrument detection limits of this study. The type of alteration may influence As mobilization: tuffs containing more clays tend to mobilize As through surficial desorption, and tuffs containing more zeolites tend to mobilize As by dissolution or formation of colloids. Additionally, one volcaniclastic sample demonstrates that extremely elevated concentrations of As, up to 1000 μg/L are possible as a result of oxidative dissolution of As-bearing sulfide phases.

Dans l'objectif d'évaluer l'influence de la forêt sur l'hydrologie des bassins versant, une méthode de modélisation du bilan hydrique en milieu forestier a été développée et validée sur l'hydrosystème de Volvic. Cette méthode, facilement transposable, a permis de simuler l'impact hydrologique de scénarios de gestion forestière.La compréhension du fonctionnement de l'hydrosystème volcanique complexe de Volvic a été améliorée, depuis les processus de recharge jusqu'au fonctionnement des exutoires. La structure géologique complexe du site a été modélisée en utilisant l'ensemble des données disponibles et en mettant en œuvre une approche géologique génétique. Le fonctionnement hydrogéologique détaillé a ensuite pu être étudié dans plusieurs secteurs clés. L'hydrosystème de Volvic a été modélisé en utilisant le code MODCOU. Afin de prendre en compte les processus physiques spécifiques du site, des modules ont été développés et couplés à l'outil existant. Le modèle permet ainsi de simuler le débit à l'exutoire de l'aquifère. La modélisation et l'interprétation de données ont également permis de proposer un nouveau modèle conceptuel de fonctionnement hydrogéologique. La simulation du transport de traceurs tels que le chlorure et le 18O valide la fonction de transfert et les hypothèses retenues quant aux processus de recharge. Le modèle a ensuite été utilisé pour simuler des scénarios d'évolution climatique contrastés.Le modèle de fonctionnement l'hydrosystème de Volvic constitue ainsi un outil de gestion opérationnelle mais surtout un outil pilote permettant d'apporter des réponses à des questions scientifiques d'actualité relatives au rôle hydrologique de la forêt ou à l'impact du changement climatique à l'échelle de petits bassins versants de moyenne montagne. / In order to take account of hydrological forest role at the catchment scale, a method for evapotranspiration computation in forest context has been developed and validated on the Volvic site. This method, easily transposable, allows simulating forest management scenarios impact on hydrosystems recharge.Functioning understanding of the Volvic volcanic hydrosystem has been improved, from recharge processes to outlet functioning. The complex geological structure of the Volvic site has been modeled based on data interpretations and using a geological genetic approach. Detailed hydrogeological functioning was studied in several key sectors. The hydrogeological functioning of the Volvic hydrosystem has been modeled using the MODCOU code. In order to take account of the physical processes observed in Volvic, several modeling units has been developed and coupled to the existing tool. This model allows simulating the Volvic outlet discharge since 1985. This modeling allows suggesting a new conceptualization of hydrogeological functioning. The conservative transport modeling (chloride, 18O) validates the transfer function modeling and the evaluated altitudinal variation of climatic parameters. Then contrasted climate change scenarios were simulated.The Volvic hydrosystem functioning model constitutes an operational tool but also a pilot contributing to scientific progress in forest hydrology or in order to evaluate climate change impact on small mountainous watershed.

Dans le cadre de cette thèse, nous avons cherché à mieux comprendre le comportement des éléments traces – et notamment des éléments légers Li et B – lors des processus de dégazage magmatique par une approche géochimique, basée sur l'analyse de produits volcaniques naturels (laves, gaz, aérosols volcaniques) de deux volcans aux styles éruptifs très contrastés, le Piton de la Fournaise (Réunion) et le Lascar (Chili). Dans un premier temps, cette étude s'est intéressée au rôle des transferts de gaz dans le déclenchement des éruptions majeures et aux échelles de temps impliquées. L'étude des compositions en éléments traces des laves récentes (1998-2008) du Piton de la Fournaise nous a notamment permis d'identifier des anomalies transitoires en éléments volatils (p.e., Li, Cu, B, Tl, Bi, Cd) en début de l'éruption majeure d'avril 2007. La cinétique de fractionnement par diffusion des éléments explique les anomalies observées. Les courts laps de temps nécessaires pour fractionner par diffusion Li par rapport à Cd (minutes à quelques heures) et Bi par rapport à Cd (quelques heures à deux jours) soutiennent l'idée que les magmas ont subi des variations rapides de pression quelques jours avant l'effondrement du cratère du Dolomieu. Dans un second temps, ce travail de thèse a porté sur le dégazage passif du volcan Lascar. Le panache volcanique dilué a été échantillonné au cours de 3 missions d'échantillonnage, menées entre 2009 et 2012, et caractérisé en termes de gaz acides (SO2, HCl, HF) et d’éléments traces. Au cours de ces missions, des mesures de flux de SO2 par DOAS ont également été réalisées. Les données mettent en évidence deux sources principales qui contribuent à l'activité de dégazage observée en surface: un réservoir magmatique profond et un système hydrothermal superficiel. Les contributions des deux sources varient dans le temps en réponse aux changements de l'activité volcanique. Cette évolution temporelle a été démontrée non seulement avec des traceurs répandus comme le SO2 et HCl, mais aussi avec des éléments traces à la fois volatils et très mobiles tel que le B. Pour la détermination des teneurs en B de nos échantillons, nous avons développé une méthode de dosage très précise par dilution isotopique. Appliquées aux laves du Piton de la Fournaise, cette technique nous a permis d'estimer les quantités de B perdues lors des processus de dégazage magmatique (ϵB compris entre 10 et 30%) ainsi que leur dépendance aux conditions de dégazage (continu en système ouvert, processus pré-, syn- et post-éruptifs). Appliquées aux aérosols du Lascar, elle nous a permis de montrer que la volatilité du B est favorisée lors des processus hydrothermaux (interactions gaz-eau, gaz-roche). Enfin, appliquées à des enclaves de péridotites, cette technique nous a permis d'apporter des contraintes nouvelles sur le comportement du B dans le manteau terrestre et d'estimer la teneur en B du manteau primitif (0,26 ± 0,04 ppm). / This study is aimed at better understanding the behavior of trace elements – and notably those of light elements such as Li and B – during magma degassing processes. For this purpose, we used a geochemical approach based on the analysis of fresh lavas and volcanic aerosols from Piton de la Fournaise (Réunion) and Lascar (Chile) volcanoes, respectively. Firstly, this thesis work focused on the role of gas transfers in triggering major eruptions and the time scales involved. Trace element analyses of recent lavas (1998-2008) of Piton de la Fournaise reveal anomalous abundances of volatile elements (e.g., Li, Cu, B, Tl, Bi, Cd) a few days prior to the April 2007 summit collapse. The kinetic (diffusive) fractionation of elements accounts for the observed anomalies. The short time-scales required to fractionate Li from Cd diffusively (minutes to hours) and Cd from Bi (few hours to two days) support the idea that the magmas underwent rapid pressure variations a few days before the summit collapse.Secondly, this study concentrated on the quiescent degassing activity of Lascar volcano. Both major gaseous species and trace element enrichment in gas and aerosols collected in the sustained plume over the period 2009 to 2012 suggest the involvement of two main degassing sources with contrasted geochemical signatures: a deep magmatic reservoir and a shallow hydrothermal system. Contributions from these two dominant sources vary with time in response to changes in volcanic activity. This temporal evolution has been shown not only by well-known tracers such as SO2 and HCl, but also by a trace element both volatile and highly fluid-mobile such as B. To determine the bulk boron concentration of our samples, we have developed a robust low-blank method based on isotope dilution ICP-MS. Applied to lavas of Piton de la Fournaise, this technique allowed us to quantify the amount of B lost during magma degassing (10-30%) and its dependency on degassing conditions. Applied to aerosols of Lascar, it enabled us to show that B volatility is enhanced during hydrothermal processes (gas-water, gas-rock interactions). Finally, applied to fertile peridotite xenoliths, it led us to establish new constraints on the behavior of B during mantle processes and estimate a primitive mantle B content of 0.26 ± 0.04 ppm.

L'activité volcanique peut représenter une source naturelle de pollution atmosphérique. Cette pollution peut engendrer une dégradation de la qualité de l'air, affecter la santé humaine et perturber la sécurité aérienne. Le Piton de la Fournaise à La Réunion est l'un des volcans basaltique les plus actifs au monde. Ses éruptions sporadiques génèrent des panaches volcaniques essentiellement constitués de gaz et de nanoparticules qui se propagent dans l'atmosphère. En journée, la formation d'oxydants (photolyse) permet d'oxyder une partie du dioxyde de soufre en acide sulfurique. Les molécules d'H2SO4 peuvent réagir avec les molécules d'eau atmosphérique pour former des embryons via la nucléation binaire homogène. Puis, ces embryons grossissent grâce aux processus de condensation et/ou coagulation conduisant alors à la formation d'un aérosol volcanique submicronique. Cette thèse vise à observer, comprendre et modéliser les processus de Formation de Nouvelles Particules (FNP) au sein des panaches volcaniques. De ce fait, elle s'organise en deux parties. La première se base sur les données recueillies lors de la campagne multidisciplinaire STRAP menée à l’observatoire du Maïdo et au Piton de la Fournaise en 2015. Elle expose les résultats issus d’une double analyse de la fréquence et de l’intensité des événements de FNP à l’observatoire. Tandis que la première analyse s’intéresse aux processus en l’absence du panache volcanique, la seconde met en exergue les spécificités de la FNP liées à sa présence au Maïdo. La seconde partie s'axe autour de la modélisation d'abord 0D puis 3D des processus de FNP au sein des panaches volcaniques via le modèle atmosphérique Méso-NH. / Volcanic activity can be a natural source of air pollution. This pollution can lead to a deterioration in air quality, affect human health and disrupt aviation safety. The Piton de la Fournaise in Reunion Island is one of the most active basaltic volcanoes in the world. Its sporadic eruptions generate volcanic plumes consisting mainly of gases and nanoparticles that spread in the atmosphere. During the day, a part of the sulphur dioxide can be oxidized to sulphuric acid thanks to oxidants production (photolysis). H2SO4molecules tend to react with atmospheric water molecules and form clusters via homogeneous binary nucleation. Then, these clusters grow by condensation and/or coagulation processes leading to the formation of a submicronic volcanic aerosol. This thesis aims to observe, understand and model the New Particle Formation (NPF) processes within volcanic plumes. Consequently, it is organized in two parts. The first is based on the data gathered during the multidisciplinary STRAP campaign conducted at both the Maïdo Observatory and Piton de la Fournaise volcano in 2015. It presents the results from a dual analysis of the NPF events frequency and intensity at the observatory. While the first analysis focuses on processes in the absence of the volcanic plume, the second highlights the specificities of the NPF related to the presence of the plume at Maïdo. The second part focuses on 0D then 3D NPF processes modelling within volcanic plumes via the Meso-NH atmospheric model.

Small-scale volcanoes represent diverse group of landforms which vary in morphology, morphometry, and mechanisms of their formation. They are the most common volcanic form on Earth, and their existence and basic characteristics were also predicted for Mars. Availability of high-resolution image data now allows to search, identify and interpret such small volcanic features on the martian surface. This thesis extends our knowledge about the small-scale volcanoes with the following objectives: (a) to document the existence of martian analogues to some of the terrestrial volcanoes, in particular scoria cones, tuff cones, tuff rings and lava domes; (b) to establish their morphological and morphometrical parameters; and (c) to examine the effect of environmental factors, which differ on Earth and Mars, on the mechanisms of formation of the scoria cones. Interpretation of remote sensing images and digital elevation models reveals that scoria cones, tuff rings and cones, and lava domes exist on different parts of the martian surface and, in some cases, far away from previously well-known volcanic provinces. Scoria cones have been identified in the volcanic field Ulysses Colles situated within the Tharsis volcanic province; tuff cones and tuff rings have been found in the Nephenthes/Amenthes region at the...

L’archipel des Açores dans l’océan Atlantique est édifiées sur un épais plateau océanique, à proximité de la jonction triple entre les plaques Nord-américaine (Na), Nubienne (Nu) et Eurasienne (Eu). La formation du plateau et l’origine du volcanisme ont été le plus souvent attribués à la présence d’une instabilité mantellique. Cependant, la répartition et la morphologie des édifices volcaniques semblent avoir été grandement influencés par la déformation régionale liée à la migration de la frontière de plaque (Eu/Nu). En effet, la frontière serait passée d’une faille transformante aujourd’hui inactive, la zone de fracture est des Açores (EAFZ), à un rift ultra lent actif appelé le Rift de Terceira (TR).Lors de ce travail, nous utilisons le volcanisme comme marqueur de la déformation régionale. Nous nous intéressons particulièrement aux îles de S. Miguel et Graciosa, qui sont localisées à l’intérieur du TR, et à Santa Maria, une île volcanique éteinte qui se situe entre la EAFZ et le TR. De par leur position, ces trois îles constituent donc des cibles particulièrement appropriées afin d’étudier l’architecture et l’évolution de la frontière de plaque Eu/Nu durant les dernier Millions d’années. A partir de nouvelles données géomorphologiques, stratigraphiques, géochronologiques et tectoniques, couplées aux données bathymétriques et géophysiques disponibles, nous reconstruisons les étapes successives de construction et de démantèlement de ces îles puis discutons de leur signification géodynamique. Ces données sont ensuite complétées par des expériences de mécanique des fluides afin d’investiguer les liens possibles entre un panache mantellique, la migration de la frontière de plaque sur plusieurs échelles d’espace et de temps.Les résultats montrent que les édifices localisés dans le TR se construisent via des pulses volcaniques courts (<100 kyr) et relativement synchrones, séparés par des épisodes d’effondrements catastrophiques. Nous proposons qu’une telle évolution reflète des épisodes brefs et intenses de déformation régionale le long de la frontière de plaque active. La distribution des marqueurs tectoniques ainsi que leurs orientations N110 et N150 dans la partie Est de S. Miguel, nous conduit à proposer que l’extension oblique du TR est principalement accommodée par les failles bordières majeures du rift. Nous identifions une nouvelle tendance tectonique orientée N50° qui pourrait représenter des failles transformantes accommodant les variations d’obliquité du TR. L’activité de île de Santa Maria est ici datée entre 5.7 et 2.8 Ma. S. Maria a été façonnée par plusieurs effondrements sectoriels catastrophiques, le plus probablement déclenchés par les mouvements tectoniques régionaux. Nous identifions également une nouvelle structure de type graben reliant les îles de S. Maria et S. Jorge plus loin au NW. La forme de ce graben est semblable au TR et est située entre l’ancienne et la nouvelle frontière Eu/Nu. Nous interprétons ce graben comme un ancien rift transitionnel et donc comme une ancienne frontière de plaque Eu/Nu. A partir de nos données géochronologiques, nous proposons que la partie Est de ce rift transitionnel aurait migré vers la partie Est du TR entre 2.8 et 1.7 Ma.La migration de la frontière Eu/Nu a été interprétées par Vogt and Jung (2004) comme résultant de sauts successifs vers le NE de l’axe du Rift afin de maintenir sa position au dessus d’un point chaud fixe. Nos expériences de mécanique des fluides suggèrent que l’archipel des Açores, comme celui des Canaries, du Cap Vert, de Madère ainsi que les volcans sous marins de Great Meteor sont la signature en surface d’un groupe d’instabilités mantellique prenant naissance et remontant à partir du sommet d’un dôme thermochimique situé dans le manteau inférieur. De plus, Ces panaches secondaires pourraient être suffisamment faibles pour adapter leurs mouvements aux équilibres de forces pré-existants, notamment la structure et la morphologie de la lithosphère. / The Azores archipelago in the Atlantic comprises nine volcanic islands which developed on a thick oceanic plateau close to the Triple Junction between the North American (Na), the Nubian (Nu), and the Eurasian (Eu) lithospheric plates. The formation of the plateau and the origin of the volcanism remain controversial, but have been generally attributed to a plume-like mantle instability. However, the distribution of the volcanic edifices east of the Mid-Atlantic Ridge (MAR) appears greatly influenced by regional deformation associated with the northward migration of the Eu/Nu plate boundary from an extinct old transform fault, the East Azores Fracture Zone (EAFZ), up to the presently active ultra-slow Terceira Rift (TR). In this thesis, we use the volcanism as a marker for regional deformation. We especially focus on S. Miguel and Graciosa, which are located within the TR, and on S. Maria, an old volcanically extinct island located between the EAFZ and the TR. These three islands thus constitute particularly suitable targets to track the architecture and the evolution of the Eu/Nu plate boundary during the last few Myr. From new geomorphological, stratigraphic, geochronologic, structural/tectonic data, and existing bathymetric and geophysical data, we reconstruct the successive stages of growth and destruction of the islands, and discuss their geodynamic meaning. These data are then complemented by fluid dynamic modelling using laboratory experiments to examine the possible links between mantle instability, plate boundary migration and the development of the volcanism on various spatial and temporal scales.The new results on the islands show that the edifices located within the TR grew through short (<100 kyr) and partly synchronous volcanic pulses, separated by catastrophic sector collapses. We propose that such evolution reflects brief and intense episodes of regional deformation along the still active Eu/Nu plate boundary. The distribution of tectonic markers and the recognition of N110 and N150 tectonic structures in eastern S. Miguel leads us to propose that oblique extension in the TR is mainly accommodated by the master faults of the rift, and that the TR is presently not the locus of appreciable sea-floor spreading. Furthermore, we identify a new N050 trend, which may represent transform faults accommodating the variation in obliquity of the TR. The activity of S. Maria is here dated between 5.7 and 2.8 Ma. Like the recent islands, S. Maria experienced catastrophic flank collapses, most probably triggered by regional tectonics. We identify a new graben structure linking Santa Maria to the island of S. Jorge further NW. The shape of this graben is similar to the TR and it is located between the EAFZ and the current plate boundary. We interpret this graben as a former transient rift, and therefore an old Eu/Nu plate boundary. From the new data, we propose that the eastern part of the transient rift migrated to the eastern part of the TR between 2.8 Ma and 1.7 Ma.The overall migration of the Eu/Nu plate boundary to the north and the creation of the Azores plateau has been interpreted by Vogt and Jung (2004) as resulting from successive NE jumps of the rift axis to maintain its position over a fixed ‘hotspot’. Our fluid mechanics experiments suggest that the Azores, as Canary, Cape Verde, Madeira Islands and Great Meteor seamounts might be the surface signature of a cluster of mantle instabilities rising from the top of a large thermochemical dome located in the lower mantle. However, such secondary plumes present a strong time-dependence 5-40 Myr time scale. Moreover, they could be sufficiently weak to adapt their motions to the pre-existing force balances and morphology of the lithosphere. We therefore present a scenario of the Azores area evolution combining a triple junction and decompression melting buoyant material (i.e. such in volatiles and/or temperature) under a thickening lithosphere.

Les groupes ethniques minoritaires sont parmi les secteurs de la société qui sont menacés en permanence par des risques plus élevés de catastrophes. Le fondement d'un tel constat est les impacts négatifs disproportionnés de catastrophes passées. Il est de ce fait nécessaire d'intégrer tous les groupes ethniques en particuliers les minorités dans la réduction des risques de catastrophe (RRC). Pourtant, la communauté scientifique a peu étudié le rôle de l’ethnicité dans la vulnérabilité et la capacité des populations exposées à divers aléas. Ainsi les praticiens sur le terrain et les organisations non-gouvernementales (ONG), ne possèdent pas de méthodes et d’outils appropriés pour intégrer les minorités ethniques dans la RRC. En plus, des exemples de méthodologies et de politiques visant à rendre cet objectif opérationnel et institutionnalisés sont également limités. Cette étude vise à répondre à ces lacunes, en prenant l’exemple des communautés multiethniques autour de Mont Kanlaon situé sur l'île de Negros aux Philippines. La zone d'étude se caractérise par une grande diversité ethnique composé d'au moins trois grands groupes ethniques (Ilonggos, Cebuanos et Bukidnons). Les résultats de cette étude suggèrent que cette mosaïque constitue une dimension importante de la RRC puisque chaque groupe ethnique possède ses propres formes de vulnérabilité et de capacité face aux aléas volcaniques et d’autres origines. Ces minorités ethniques sont parmi les secteurs les plus vulnérables de la société philippine en raison de leur statut marginalisés. En outre, les résultats de cette étude suggèrent que chaque groupe ethnique possède des capacités issues en grande partie de ressources locales qui sont utiles aux fins de la RRC. / The ethnic minority groups are among the sectors of the society who are permanently threatened by higher risk of disasters. The basis of such claim is the disproportionate negative impacts of both major and minor disasters. There is thus a pressing need to integrate ethnic minority groups in disaster risk reduction (DRR). However, within the scientific community, there are limited studies in the current literature of disaster that investigated the role of ethnicity in shaping the vulnerability and capacity of the ethnic groups. The authorities, non-government organizations (NGOs), and researchers have limited defined methods and tools to make such as objective operational in the field. Finally, plans, actions and policies to make such objective institutionalized are also limited. This study aimed to fulfill those gaps. The study has adapted an integrated DRR framework and a methodology that follows the ideals and principles of Community-Based Disaster Risk Reduction (CBDRR). The present study focuses on the multiethnic communities surrounding Mt. Kanlaon located in the island of Negros, Philippines. The study area is characterized by a great ethnic diversity composed of at least three major ethnic groups (Ilonggos, Cebuanos, and Bukidnons). The results of the study suggest that this ethnic mosaic constituted a major dimension of DRR because each ethnic group portrays different vulnerabilities and capacities in the face of volcanic hazards and other natural hazards. These ethnic minorities are among the most vulnerable sectors of the Philippine society due to their marginalized and minority status. In addition, the results of this study also suggest that each ethnic group possesses capacities derived from local resources that are useful for the purpose of DRR.