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

Etude expérimentale de la convection par les bulles et ses applications pour la cyclicité éruptive du volcan Erta Ale

Les produits solides de l'altération de la croûte continentale, et donc le matériel sédimentaire transporté par les fleuves, s'étalent sur un large spectre de taille de grains, de minéralogie et de chimie. Il en résulte un tri granulométrique des sédiments dans le fleuve. Les principaux affluents du bassin de l'Amazone, le plus grand fleuve du monde, ont été échantillonnés à plusieurs profondeurs d'eau et à différentes positions latérales. Concernant la phase dissoute, des hétérogénéités latérales de concentration en éléments majeurs et en composition isotopique du strontium, couplées à des arguments hydrodynamiques sur le mélange turbulent, suggèrent que la plupart des grands fleuves ne sont pas bien mélangés. La concentration en matières en suspension, ainsi que les compositions chimique et isotopique (strontium et néodyme) de la phase particulaire évoluent de façon importante avec la profondeur. Une nouvelle classification des éléments chimiques est établie au regard de leur comportement vis-à-vis du tri granulométrique et de l'altération. La comparaison de la chimie des sédiments de l'Amazone, sur toute la gamme de taille de grains, entre la sortie de la chaîne andine et l'embouchure, suggère des intensités d'altération des silicates relativement faibles durant le transport des sédiments à travers la plaine d'inondation. Enfin, les analyses de contenu en carbone organique particulaire et de son activité en carbone 14, montrent que l'oxydation du carbone fossile au cours du transport sédimentaire représente une source significative de CO2 atmosphérique, comparable avec l'altération des silicates à l'échelle du bassinRivers transport the residual products of continental weathering, that display a large range of grain size, mineralogical and chemical compositions. These grains are sorted within the river channel during transport to the ocean. The Amazon Basin main tributaries have been sampled along depth-profiles, at various lateral positions. In the dissolved phase, lateral heterogeneities of major species and strontium isotopic composition, along with constraints on turbulent mixing, suggest that most large rivers are not well-mixed with respect to their different tributaries. At all sites, suspended matter concentration significantly increases with depth. Chemical and isotopic compositions of suspended matter also vary greatly with depth. A new classification of chemical elements, following their behaviour with respect to sorting and weathering, is proposed. Comparison of the chemical compositon, along the whole grain size range, of Amazon sediments between the outlet of the Andes and the river mouths, suggests relatively low weathering intensities during transfer of sediments through the floodplain. Finally, using organic carbon content and 14C activity measurements, we demonstrate that fossil organic carbon oxidation during transfer through the floodplain results in a significant input of CO2 in the atmosphere, nearly countering silicate weathering at the basin scalePARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Gas-driven lava lake fluctuations at Erta 'Ale volcano (Ethiopia) revealed by MODIS measurements

International audienceThe long-lived lava lake of Erta 'Ale volcano (Ethiopia) is remotely monitored by moderate resolution imaging spectroradiometers (MODIS) installed on satellites. The Normalised Thermal Index (NTI) (Wright et al. Remote Sens Environ 82:135-155 2002) is shown to be proportional to the volume of the lava lake based on visual observations. The lava lake's variable level can be plausibly related to a stable foam, i.e. a mixture composed of densely packed non-coalescing bubbles in suspension within a liquid. This foam is trapped at the top of the magma reservoir, and its thickness changes in response to the gas flux feeding the foam being successively turned on and off. The temporal evolution of the foam thickness, and the resulting variation of the volume of the lava lake, is calculated numerically by assuming that the gas flux feeding the foam, initially constant and homogeneous since December 9, 2002, is suddenly stopped on December 13, 2002 and not restarted before May 2003. The best fit between the theoretical foam thickness and the level of the lava lake deduced from the NTI provides an estimate of both the reservoir radius, 155-170 m, and the gas flux feeding the foam, 5.5×10-3-7.2×10-3 m 3 s -1 when existing. This is in agreement with previous estimates from acoustic measurements (Bouche et al. Earth Planet Sci Lett 295:37-48 2010). The very good agreement between the theoretical foam thickness and that deduced from MODIS data shows for the first time the existence of a regime based on the behaviour of a stable foam, whose spreading towards the conduit ("wide" conduit condition), can explain the long-lived activity. Our predictive model, which links the gas flux at the vent to the foam spreading, could potentially be used on any volcano with a long-lived activity. The underlying gas flux and the horizontal surface area of the magma reservoir can then be deduced by combining modelling to continuous measurements of gas flux. The lava lake, when high, often shows regular rise and fall of its level. We have recognised a minimum of 26 very well marked cycles between January 2001 and December 13, 2002, corresponding to a typical return time of 10.8 ± 2.3 days and a gas volume of 8.3×105 ± 2.0×105 m 3. This corresponds to a gas volume fraction in the reservoir equal to 0.023-0.063 %. The yearly gas flux, estimated between December 13, 2002 and September 27, 2004, varies between 2.3×10-6 and 5.9×10-6 m 3 s -1 at the depth of the reservoir. The long-time series provided by infra-red sensors mounted on satellites could be used on any persistent volcano to detect potential periodic variations in the level of lava lakes or lava columns, providing that the vent has a funnel shape, as often, and is sufficiently large

Ecouter les volcans pour mieux les comprendre (volcans de la zone de subduction du Vanuatu)

Depuis une dizaine d'années, les mesures acoustiques s'imposent comme un outil permettant d'étudier les éruptions volcaniques. Dans le cas des éruptions basaltiques, la libération du gaz au niveau de la surface libre magmatique peut se faire de façon continue sous forme de dégazage passif, mais aussi discrète sous forme d'explosion liée à l'éclatement d'une bulle de gaz surpressurisée, formée dans le réservoir ou dans le conduit. Ce dégazage actif génère des signaux infrasonores énergétiques, dont la forme d'onde est remarquablement simple et très peu perturbée lors de sa propagation dans l'atmosphère. Une première partie de ce travail de thèse a consisté à la mise au point d'un programme de détection automatique des signaux acoustiques produits lors des éruptions basaltiques, ainsi que de calcul du volume de gaz actif libéré. Le principe est de pouvoir suivre à partir des données acoustiques l'évolution de la quantité de gaz libérée par les explosions, afin de contraindre les processus magmatiques du réservoir. Ces méthodes ont été utilisées pour étudier l'activité éruptive quasi-permanente de deux volcans du Vanuatu, Ambrym et Yasur. Le suivi acoustique du dégazaged'Ambrym, à partir des données d'un réseau de capteurs triangulaires situé à 3.5 km des cratères actifs entre 2008 rt 2010, a ainsi permis d'étudier le système d'alimentation superficiel et d'apporter une hypothèse sur la géométrie du réservoir magmatique sous-jacent. Les enregistrements acoustiques issus d'une station située à 500 m des évents actifs du Yasur ont enfin été analysés pour suivre de façon précise les fluctuations de volume de gaz libéré par l'activité strombolienne entre 2009 et 2011.Acoustic measurements can be a powerful tool to study volcanic eurptions. In the case of basaltic eruptions, the gas is continuously released at the magma free surface (passive degassing). It can also be released during explosive event, due to the bursting of an overpressure bubble of gas, which has been formed by coalescence of smaller bubbles in the magma chamber or in the conduit. This active degassing generates very energetic infrasonic signals whose waveform is remarkably simple and undistrubed during its atmospheric propagation. The first part of this thesis concerned the development of a program which automatically detects acoustic signal produced by basaltic eruptions, and calculates the associated gas volume. The principle is to follow from acoustic data the evolution of the amount of gas released bu the explosions, in order to constrain the dynamic of the magmatic reservoir. These methods were then used to study the almost permanent eruptive activity of two volcanoes in Vanuatu, Ambrym and Yasur. Acoustic monitoring of the active degassing of Ambryn, using date from a triangular network located at 3.5 km from the active craters between 2008 and 2010, has enabled to study the superficial plumbing system and to provide an assumption concerning the geometry of the magma chamber. Acoustics recordings from a station located at 500 m from the active vents of Yasur volcano were also analyzed to accurately follow the fluctuations of gas volume released by Strombolian activity between 2009 and 2011.PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Basaltic thermals and Subplinian plumes: Constraints from acoustic measurements at Shishaldin volcano, Alaska

International audienceAbstract The 1999 basaltic eruption of Shishaldin volcano (Alaska, USA) included both Strombolian and Subplinian activity, as well as a “pre-Subplinian” phase interpreted as the local coalescence within a long foam in the conduit. Although few visual observations were made of the eruption, a great deal of information regarding gas velocity, gas flux at the vent and plume height may be inferred by using acoustic recordings of the eruption. By relating acoustic power to gas velocity, a time series of gas velocity is calculated for the Subplinian and pre-Subplinian phases. These time series show trends in gas velocity that are interpreted as plumes or, for those signals lasting only a short time, thermals. The Subplinian phase is shown to be composed of a thermal followed by five plumes with a total expelled gas volume of $$\approx\!1.5 \times 10^{7}\;{\rm m}^{3}$$. The initiation of the Subplinian activity is probably related to the arrival of a large overpressurised bubble close to the top of the magma column. A gradual increase in low-frequency (0.01–0.5 Hz) signal prior to this “trigger bubble” may be due to the rise of the bubble in the conduit. This delay corresponds to a reservoir located at ≈3.9 km below the surface, in good agreement with studies on other volcanoes. The presence of two thermal phases is also identified in the middle of the pre-Subplinian phase with a total gas release of $$\approx \!4.3 \times 10^{6}\;{\rm m}^{3}$$ and $$\approx \!3.6 \times 10^{6}\;{\rm m}^{3}$$. Gas velocity at the vent is found to be $$\approx \!82\, {\rm m.s}^{-1}$$ and $$\approx \!90\, {\rm m.s}^{-1}$$ for the Subplinian plumes and the pre-Subplinian thermals respectively. The agreement is very good between estimates of the gas flux from modelling the plume height and those obtained from acoustic measurements, leading to a new method by which eruption physical parameters may be quantified. Furthermore, direct measurements of gas velocity can be used for better estimates of the $${\rm SO}_{2}$$ flux released during the eruption

Etude du son produit par la Soufrière de Guadeloupe et le Piton de la Fournaise (implications pour la dynamique éruptive et la surveillance volcanique)

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

Automatic detection and discrimination of volcanic tremors and tectonic earthquakes: An application to Ambrym volcano, Vanuatu

International audienceAn algorithm of automatic detection and discrimination of volcanic tremors, defined as events containing only P-type wave, and tectonic earthquakes, containing both P- and S-waves, is applied on data recorded on a 3-component STS-2 broad-band seismometer, installed from July to November 2000 at Ambrym volcano, Vanuatu. The algorithm consists in detecting P-waves and, if any, S-waves according to a modified version (Zhizhin, M., Rouland, D., Bonnin, J., Gvishiani, A., Burtsev, A., 2006. Rapid estimation of earthquake source parameters from pattern analysis of waveforms recorded at a single 3-component station, Bull. Seism. Soc. Am, 96, 6,2329–2347) of the Component Energy Comparison Method (CECM) proposed by Nagano et al. (Nagano, K., Niitsuma, H., Chubachi, N., 1989. Automatic algorithm for triaxial hodogram source location in downhole acoustic emission measurement. Geophysics, 54, 4, 508–513). The duration and maximum amplitude are automatically measured for each volcanic tremor and for each tectonic earthquake with corresponding duration-and amplitude-magnitudes. During the four-and-half months of observation, 1959 local and regional tectonic earthquakes with epicentral distances (estimated from S–P interval time) ranging from a few kilometres to about 800 km, and 14942 volcanic tremors with duration ranging from 10 to 2000 s, have been detected. The Gutenberg and Richter laws of tectonic earthquakes and volcanic tremors are examined according to their duration-and amplitude-magnitudes, and completeness of the corresponding catalogues is studied. The observations on the coherence of some long duration volcanic tremors, which display four different intermittencies (77 +/− 17 min, 165 +/− 10 min, 565 +/− 50 min, 1065 +/− 50 min), are interpreted as different types of degassing

Assessing and optimizing the performance of infrasound networks to monitor volcanic eruptions

International audienceWe propose a numerical modeling technique based on a frequency-dependent attenuation relation to assess, quantify and optimize the performance of any arbitrary infrasound network to monitor explosive sources such as volcanic eruptions. Simulations are further enhanced by including realistic sources and propagation effects. We apply our approach to both hemispheres by considering the Euro-Mediterranean and the Eastern Australian regions. In these regions, we use quasi-permanent infrasound signals from Mt. Etna recorded in Tunisia and from Mt. Yasur recorded in New Caledonia. These well-instrumented volcanoes offer a unique opportunity to validate our attenuation model. In particular, accurate comparisons between near-and far-field recordings demonstrate the potential of the proposed methodology to remotely monitor volcanoes. A good agreement is found between modeled and observed results, especially when incorporating representative 10 m s −1 wind perturbations in the atmospheric specifications according to previous campaign measurements. To optimize the network layout in order to ensure the best monitoring of the volcanoes, we proceed through a grid search to find optimum locations of an additional array. We show that adding one array at an appropriate location in both regions under study could significantly improve detections half of the year. The application of the proposed methodology can provide in near real-time a realistic confidence level of volcanic eruption detections, useful to mitigate the risk of aircrafts encountering volcanic ash