Evolution and dynamics of the open‑vent eruption at Arenal volcano (Costa Rica, 1968–2010): what we learned and perspectives

On 29 July 1968, there was a violent reactivation of Arenal volcano. The resulting westward-directed lateral blast eruption left two villages destroyed and 78 people dead. The activity continued as a long-lasting, open-vent eruption that evolved into seven recognisable phases refecting changes in magma supply, explosive activity and cone evolution, and ended in October 2010. Here, we review this activity, the geophysical approaches applied to understanding it and the open questions resulting from these insights. The eruptive dynamics were characterised by almost constant lava efusion, degassing, strombolian and vulcanian explosions and infrequent pyroclastic density currents. In this study, the total rock dense equivalent volume of lava and tephra erupted is calculated at 757±77 Mm3, while the volume of the lava fow feld is 527±58 Mm3. Typical seismic activity included harmonic and spasmodic tremors, long-period events and explosion signals with frequent audible “booms”. The decline of the eruptive activity started in 2000, with a decrease in the number and size of explosive events, a shift from long to short lava flows along with the collapse of lava fow fronts and the subsequent formation of downward-rolling lava block aprons, the frequent growth of dome-like structures on the summit and a gradual decrease in seismic energy. Multiple geological and geophysical studies during this 42-year-long period of open-vent activity at Arenal resulted in many advances in understanding the dynamics of andesitic blocky lava fows, the origin and diversity of pyroclastic density currents and seismic sources, as well as the role of site efects and rough topography in modifying the seismic wavefeld. The acoustic measurements presented here include two types of events: typical explosions and small pressure transients. Features of the latter type are not usually observed at volcanoes with intermediate to evolved magma composition. Explosions have diferent waveforms and larger gas volumes than pressure transients, both types being associated with active and passive degassing, respectively. This body of data, results and knowledge can inform on the type of activity, and associated geophysical signals, of open-vent systems that are active for decades.El 29 de julio de 1968 se produjo una violenta reactivación del volcán Arenal. La explosión lateral dirigida hacia el oeste dejó dos pueblos destruidos y 78 personas muertas. La actividad continuó como una erupción de larga duración a conducto abierto que evolucionó en siete fases reconocibles, las cuales reflejaron cambios en el suministro de magma, la actividad explosiva y la evolución del cono, y terminó en octubre de 2010. Aquí revisamos esta actividad, los enfoques geofísicos aplicados para entenderla, y las preguntas abiertas que resultan de este conocimiento. La dinámica eruptiva se caracterizó por una efusión de lava casi constante, desgasificación, explosiones estrombolianas y vulcanianas, e infrecuentes corrientes de densidad piroc- lástica. En este estudio, el volumen total de lava y tefra erupcionada en equivalente de roca densa se calcula en 757 ± 77 Mm3 , mientras que el volumen del campo de lavas es de 527 ± 58 Mm3 . La actividad sísmica típica incluía tremores armónicos y espasmódicos, eventos de largo periodo y señales de explosión con frecuentes bums audibles. El declive de la actividad eruptiva comenzó en el año 2000, con una disminución del número y el tamaño de los eventos explosivos, un cambio de coladas de lava largas a cortas, junto con el colapso de los frentes de colada de lava y la subsiguiente formación de abanicos de bloques de lava que se desplazaban ladera abajo, el crecimiento frecuente de estructuras tipo domo en la cima, y una disminución gradual de la energía sísmica. Los múltiples estudios geológicos y geofísicos realizados durante este período de 42 años de actividad a conducto abierto en el Arenal, dieron lugar a muchos avances en la comprensión de la dinámica de las coladas de lava blocosas andesíticas, el origen y la diversidad de las corrientes de densidad piroclástica y las fuentes sísmicas, así como el papel de los efectos de sitio sísmicos y la topografía accidentada en la modificación del campo de ondas sísmicas. Las mediciones acústicas presentadas aquí incluyen dos tipos de eventos: explosiones típicas y pequeños transitorios de presión. Las características de este último tipo no suelen observarse en volcanes con una composición de magma intermedia o evolucionada. Las explosiones tienen formas de onda diferentes y volúmenes de gas mayores que los transitorios de presión, y ambos tipos están asociados con la desgasificación activa y pasiva, respectivamente. Este conjunto de datos, resultados y conocimientos puede enseñarnos sobre el tipo de actividad y las señales geofísicas asociadas, de los sistemas a conducto abierto que permanecen activos durante décadas.Institut de Physique du Globe de ParisUniversidad de Costa Rica///Costa RicaUCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela Centroamericana de Geologí

Stabilité et instabillité de la lithosphère continentale

PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Structure and activity of the geothermal field of Hvalfjörour (Iceland) from brittle tectonic, geothermal and paleostress analysis

International audienceThis paper presents the results of brittle tectonic, palaeostress inversion, and hydrothermal mineralisation studies of the Hvalfjörður low-temperature geothermal field in Southwest Iceland. This geothermal field (including two pronounced thermal anomalies) is located in the highly altered core area of an extinct and deeply eroded Tertiary central volcano. Most of the geothermal water appears to be conducted by vertical extension fractures. Palaeostress analysis indicates a rather complex stress history, with four major trends of extension involving normal and strike-slip faulting modes as well as dyke injection. Analysis of the data on the relative chronology indicates that these four regimes were closely intricate in time and space. The most important regime is a NW-SE, rift-perpendicular extension related to the oceanic rifting in Iceland. This trend partly controls the past (Pliocene-Early Pleistocene) hydrothermal activity in the Hvalfjörður area; however, an E-W rift-oblique extension also occurred with a N-S trending fracture system including normal faults, dykes and veins that show higher levels of hydrothermal mineralisation. Currently, there is strong hot-water convection, producing a thermal anomaly, in this N-S-trending fracture system. Our study highlights the paleostress evolution and the development of fracture systems in Hvalfjörður, including the past geothermal history; nevertheless the most efficient tool in geothermal prospection in such complex area remains the shallow geothermal survey. It has proved successful in many localities in Early Pleistocene to Miocene rocks where no surface indication of geothermal activity exists

Convection in an internally heated stratified heterogeneous reservoir

International audienceThe Earth's mantle is chemically heterogeneous and probably includes primordial material that has not been affected by melting and attendant depletion of heat-producing radioactive elements. One consequence is that mantle internal heat sources are not distributed uniformly. Convection induces mixing, such that the flow pattern, the heat source distribution and the thermal structure are continuously evolving. These phenomena are studied in the laboratory using a novel microwave-based experimental set-up for convection in internally heated systems. We follow the development of convection and mixing in an initially stratified fluid made of two layers with different physical properties and heat source concentrations lying above an adiabatic base. For relevance to the Earth's mantle, the upper layer is thicker and depleted in heat sources compared to the lower one. The thermal structure tends towards that of a homogeneous fluid with a well-defined time constant that scales with is the Rayleigh-Roberts number for the homogenized fluid. We identified two convection regimes. In the dome regime, large domes of lower fluid protrude into the upper layer and remain stable for long time intervals. In the stratified regime, cusp-like upwellings develop at the edges of large basins in the lower layer. Due to mixing, the volume of lower fluid decreases to zero over a finite time. Empirical scaling laws for the duration of mixing and for the peak temperature difference between the two fluids are derived and allow extrapolation to planetary mantles

Fundamentals of laminar free convection in internally heated fluids at values of the Rayleigh–Roberts number up to

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Evolution and dynamics of the open-vent eruption at Arenal volcano (Costa Rica, 1968-2010): what we learned and perspectives

International audienceOn 29 July 1968, there was a violent reactivation of Arenal volcano. The resulting westward-directed lateral blast eruption left two villages destroyed and 78 people dead. The activity continued as a long-lasting, open-vent eruption that evolved into seven recognisable phases reflecting changes in magma supply, explosive activity and cone evolution, and ended in October 2010. Here, we review this activity, the geophysical approaches applied to understanding it and the open questions resulting from these insights. The eruptive dynamics were characterised by almost constant lava effusion, degassing, strombolian and vulcanian explosions and infrequent pyroclastic density currents. In this study, the total rock dense equivalent volume of lava and tephra erupted is calculated at 757 ± 77 Mm3, while the volume of the lava flow field is 527 ± 58 Mm3. Typical seismic activity included harmonic and spasmodic tremors, long-period events and explosion signals with frequent audible "booms". The decline of the eruptive activity started in 2000, with a decrease in the number and size of explosive events, a shift from long to short lava flows along with the collapse of lava flow fronts and the subsequent formation of downward-rolling lava block aprons, the frequent growth of dome-like structures on the summit and a gradual decrease in seismic energy. Multiple geological and geophysical studies during this 42-year-long period of open-vent activity at Arenal resulted in many advances in understanding the dynamics of andesitic blocky lava flows, the origin and diversity of pyroclastic density currents and seismic sources, as well as the role of site effects and rough topography in modifying the seismic wavefield. The acoustic measurements presented here include two types of events: typical explosions and small pressure transients. Features of the latter type are not usually observed at volcanoes with intermediate to evolved magma composition. Explosions have different waveforms and larger gas volumes than pressure transients, both types being associated with active and passive degassing, respectively. This body of data, results and knowledge can inform on the type of activity, and associated geophysical signals, of open-vent systems that are active for decades

The Earth’s mantle in a microwave oven: thermal convection driven by a heterogeneous distribution of heat sources

International audienceConvective motions in silicate planets are largely driven by internal heat sources and secular cooling. The exact amount and distribution of heat sources in the Earth are poorly constrained and the latter is likely to change with time due to mixing and to the deformation of boundaries that separate different reservoirs. To improve our understanding of planetary-scale convection in these conditions, we have designed a new laboratory setup allowing a large range of heat source distributions. We illustrate the potential of our new technique with a study of an initially stratified fluid involving two layers with different physical properties and internal heat production rates. A modified microwave oven is used to generate a uniform radiation propagating through the fluids. Experimental fluids are solutions of hydroxyethyl cellulose and salt in water, such that salt increases both the density and the volumetric heating rate. We determine temperature and composition fields in 3D with non-invasive techniques. Two fluorescent dyes are used to determine temperature. A Nd:YAG planar laser beam excites fluorescence, and an optical system, involving a beam splitter and a set of colour filters, captures the fluorescence intensity distribution on two separate spectral bands. The ratio between the two intensities provides an instantaneous determination of temperature with an uncertainty of 5% (typically 1K). We quantify mixing processes by precisely tracking the interfaces separating the two fluids. These novel techniques allow new insights on the generation, morphology and evolution of large-scale heterogeneities in the Earth's lower mantle

There is no rush to upgrade the tennis racket in young intermediate competitive players: The effects of scaling racket on serve biomechanics and performance

International audienceINTRODUCTION: Scaling the equipment of young athletes is justified by the constraints-led approach introduced in motor learning. The aim of the present study is to analyze the effect of racket scaling on the serve biomechanics and performance parameters for young tennis players (between 8 and 11 years-old). METHODS: Nine young intermediate competitive tennis players (age: 9.9 ± 1.0 years) performed maximal effort flat serves with three different rackets (scaled 23 inches, scaled 25 inches and full-size 27 inches) in a randomized order. A radar measured ball speed while shoulder and elbow kinetics and upper and lower limb kinematics were calculated with a 20-camera optical motion capture system. Repeated measures ANOVAs were used to analyze the effect of the three rackets on ball speed, percentage of serve in, serve kinematics and kinetics. RESULTS: No significant differences in ball speed, maximal racket head velocity and percentage of serve in were observed between the three rackets. The lowest maximal upper limb kinetics and the highest upper limb maximal angular velocities were obtained with the scaled 23 inches racket. DISCUSSION: Using scaled rackets has the advantage to decrease shoulder and elbow loadings without reducing serve performance. Consequently, the present results incite tennis coaches and parents to not upgrade too soon the size of the racket in young intermediate tennis players to avoid overuse injury risks in the long term. Our results showed that the full-size 27 inches racket induced higher lower limb kinematics. As a consequence, occasionally serving with a fullsize racket can be a sparingly interesting intervention to help young tennis players to intuitively and immediately increase their leg drive action, allowing a more functional representation of the elite junior serve

Microwave-heating laboratory experiments for planetary mantle convection

International audienceThermal evolution of telluric planets is mainly controlled by secular cooling and internal heating due to the decay of radioactive isotopes, two processes that are equivalent from the standpoint of convection dynamics. In a fluid cooled from above and volumetrically heated, convection is dominated by instabilities of the top boundary layer and the interior thermal structure is non-isentropic. Here we present innovative laboratory experiments where microwave radiation is used to generate uniform internal heat in fluids at high Prandtl number (>300) and high Rayleigh–Roberts number (ranging from 104 to 107), appropriate for planetary mantle convection. Non-invasive techniques are employed to determine both temperature and velocity fields. We successfully validate the experimental results by conducting numerical simulations in three-dimensional Cartesian geometry that reproduce the experimental conditions. Scaling laws relating key characteristics of the thermal boundary layer, namely its thickness and temperature drop, to the Rayleigh–Roberts number have been established for both rigid and free-slip boundary conditions. A robust conclusion is that for rigid boundary conditions the internal temperature is significantly higher than for free-slip boundary conditions. Our scaling laws, coupled with plausible physical parameters entering the Rayleigh–Roberts number, enable us to calculate the mantle potential temperature for the Earth and Venus, two telluric planets with different mechanical boundary conditions at their surface

Waziers le Bas-Terroir (plaine de la Scarpe, nord de la France): synthèse des études sur les observations 2011-2015 et mise en évidence d’un enregistrement sédimentaire continu de la fin du Saalien (SIM 6) à l’optimum Eemien (SIM 5e).

International audienceThe Waziers Formation consists of a silty alluvial deposit topped by calcareous tuff and peat. It is preserved in a small palaeo- channel of the Scarpe-Scarbus river, recognised over several hundred metres long at the edge/entrance of the Scarpe river plain and dated to the end of the Saalian (SIM 6) and the Eemian (SIM 5e). The Waziers Formation was preserved by a later defluviation and the sealing by fluvio-aeolian deposits of the late Weichselian Pleniglacial (SIM 2). The synthesis of all the data and studies (lithology, palaeontology, geochronology, reconstruction of palaeotemperatures) provides a coherent picture of the evolution of the area since the end of the Saalian around 130,000 years ago to the end of the Weichselian Pleniglacial. At the base, sandy-gravelly (indicating dynamic flows) and then silty alluvium (Member 1) was deposited in the valley bottom at the end of the Saalian glacial period in a context of incipient forest reconquest. A water body then develops, allowing the sedimentation of a lacustrine chalk deposited in a Characeae marsh, followed by a peat bog (Member 2, ca. 1.5 m thick) in a forest context. A channelled stream still flows through the bog, at least intermittently, locally eroding and depositing silty alluvium. The lower part of Member 2 records the transition from the Saalian glacial (end of MIS 6) to the Eemian interglacial (beginning of MIS 5) in a context of continuous sedimentation with increasing organic content. The upper part of Member 2 records the Eemian climatic optimum in a typical peat facies, followed by the onset of climatic degradation. At the top of Member 2 (and that of the Formation) is the limit of a major erosion that did not allow the recording of the end of the Eemian interglacial. At the base of Member 3 (ca. 3 m thick) flows concentrated in deep gullies on frozen ground dismantle and rework the underlying deposits. Subsequently, flows (attributed to the Weichselian Pleniglacial) deposit a sheet of chalk gravels over the whole site in a wider active band (likely braiding). Most of Member 3 consists of fluvio-aeolian silty-sandy deposits and a thin terminal loessic deposit. The quality of the sedimentary record, with in particular continuous depo- sition at the time of the climatic transition from the Saalien to the Eemian, and the abundance of palaeontological elements (wood and plant macro-remains, macro- and meso-fauna, microfauna, avifauna, malacofauna...) in the Eemian deposits ensure the Waziers Formation is an exceptional testimony for the palaeoenvironmental and palaeoclimatic reconstructions. The presence of levels testying to recurrent human occupations in Members 1 and 2 (with isolated flakes, heaps of debitage, traces of anthropic actions on fauna) make this site and sequence a reference for the understanding of the human settlement dynamics from the MIS 6 (Saalian) to the MIS 5e (Eemian) in Northern France but also broadly in North-Western Europe.La Formation de Waziers est constituée d’un dépôt alluvial limoneux surmonté de tuf calcaire et de tourbe. Elle est conservée dans un petit paléo-chenal de la rivière Scarpe-Scarbus, reconnu sur plusieurs centaines de mètres de long à l’entrée de la plaine de la Scarpe et daté de la fin du Saalien (SIM 6) et de l’Eemien (SIM 5e). La Formation de Waziers a été préservée à la faveur d’une défluviation ultérieure à son dépôt puis de son scellement par des dépôts fluvio-éoliens de la fin du Pléniglaciaire weichselien (SIM 2). La synthèse de l’ensemble des données et études (lithologie, paléontologie, géochronologie, reconstitution des paléotem- pératures) fournit une image cohérente de l’évolution de la zone depuis la fin du Saalien vers 130 000 ans environ jusqu’à la fin du Pléniglaciaire weichselien. À la base, les alluvions sablo-graveleuses (témoignant d’écoulements dynamiques) puis limoneuses (Membre 1) se déposent dans le fond de vallée à la fin du glaciaire saalien dans un contexte d’amorce de reconquête forestière. Un plan d’eau se développe ensuite, permettant le dépôt d’une craie lacustre dans un marais à Characées puis d’une tourbière (Membre 2, env. 1,5 m d’épaisseur) dans un contexte forestier. Un cours d’eau chenalisé circule encore dans la tourbière, au moins par intermittence, érodant et déposant localement des alluvions limoneuses. La partie inférieure du Membre 2 enregistre la transition du glaciaire Saalien (fin SIM 6) à l’Interglaciaire eemien (début SIM 5e) dans un contexte de sédimentation continue de plus en plus organique. La partie supérieure du Membre 2 enregistre l’optimum climatique eemien dans un faciès de tourbe typique, puis l’amorce d’une dégradation climatique. Au toit du Membre 2 (et celui de la Formation) est observée la limite d’une érosion majeure qui n’a pas permis l’enregistrement de la fin de l’Interglaciaire eemien. À la base du Membre 3 (env. 3 m d’épaisseur), des écoule- ments concentrés dans des ravines profondes sur sol gelé démantèlent et remanient les dépôts sous-jacents. Ensuite, des écoulements (attribués au Pléniglaciaire weichselien) déposent une nappe de graviers de craie sur l’ensemble du site dans une bande active plus large (probable tressage). La majeure partie du Membre 3 est constituée de dépôts limono-sableux fluvio-éoliens et d’un mince dépôt loessique terminal. La qualité de l’enregistrement sédimentaire de la formation de Waziers, avec notamment un dépôt continu au moment de la transition climatique du Saalien à l’Eemien, et l’abondance des éléments paléontologiques (bois et macrorestes végétaux, macro et méso-faune, microfaune, avifaune, malacofaune…) dans les dépôts eemiens font de celle-ci un témoin exceptionnel pour les recons- titutions paléoenvironnementales et paléoclimatiques de cette période. La présence de niveaux témoignant d’occupations humaines récurrentes dans les Membres 1 et 2 (éclats isolés, amas de débitage, traces d’actions anthropiques sur faune) font de ce site et de cette séquence une référence pour la compréhension des dynamiques de peuplement humain entre le SIM 6 (Saalien) et le SIM 5e (Eemien) dans le Nord de la France et plus largement dans le Nord-Ouest de l’Europe