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L'identification des différentes formations quaternaires de la vallée de l'Orbieu, puis la reconstitution de leur géométrie, permettent de mettre en évidence les principales déformations tectoniques qu'elles ont subies. La néotectonique de cette région des Corbières a été active jusqu'à l'accumulation de la nappe alluviale de la basse terrasse weichsélienne. La datation des travertins de Graffan permettrait de caler la chronologie relative pour le système des terrasses de l'Orbieu. A geomorphological and sedimentological analysis makes it possible to identify the different Quaternary formations in the Orbieu valley. Then, the reconstruction of the longitudinal and transversal profiles shows the evolution of tectonic deformations since the formation settling. Neotectonics have been active up to the accumulation of the alluvial deposits of the Weichselian lower terrace. The dating of the Graffan travertines would allow adjust the Orbieu terraces system chronology.

La géologie connaît à partir de la fin du xviiie siècle un véritable essor dont les conséquences ne tarderont pas à apparaître dans la peinture de paysage de l’époque. Délaissant désormais les cabinets d’histoire naturelle, les naturalistes se mettent à chercher sur le terrain des indices pour comprendre le passé de notre terre et notamment son extrême ancienneté. Les artistes imitent leur démarche en se passionnant à leur tour aux phénomènes naturels comme les volcans, les glaciers ou tout simplement les falaises. Sous le prisme des découvertes effectuées en géosciences, il s’agira de déterminer l’influence de certaines théories scientifiques sur la peinture de paysage. Au nombre de quatre, ces doctrines contribueront dans une certaine mesure à moderniser ce genre pictural trop sclérosé par la tradition. Des personnalités comme Alexandre de Humboldt, Louis Agassiz, James Hutton ou encore George Cuvier ont avec leurs doctrines scientifiques profondément changé la vision de notre globe terrestre quant à son histoire, son évolution et ses mécanismes. Notre étude qui abordera de nombreuses facettes des sciences de la terre comme la glaciologie et la paléontologie tentera de mettre en évidence la portée de ces nouvelles disciplines sur le développement de la peinture de paysage de la fin du xviiie siècle jusqu’aux années 1860. From the final years of the eighteenth century, Geology enjoys a rapid development whose consequences won’t take long to react on the landscape painting of the time. Leaving from then on the study for the outer world, Naturalists set to look for clues on the ground to understand the past of our planet and particularly his old age. Artists imitate their reasoning and are fascinated as well by the naturals phenomenon like the volcanoes, the glaciers or simply the cliffs. Under the prism of the discoveries made in geosciences, it will be a matter to determine the influence of some scientific theories on the landscape painting. These doctrines will contribute to modernize this pictorial genre still too blocked by the tradition. Personalities such as Humboldt, Agassiz, Hutton and Cuvier have with their ideas profoundly changed the vision of our planet about his history, his evolution and his mechanisms. Our study which tackles some aspects of the geosciences like glaciology a paleontology will try to bring to the fore the range of these new scientific disciplines on the development of the landscape painting from the final years of the eighteenth century to the 1860 years.

[ES] El presente proyecto es una investigación teórica-práctica que plantea explorar la relación identitaria de los espacios territoriales aislados de carácter físico. Las islas, con su particular paisaje, es analizada a través de diferentes referentes sociológicos, antropológicos y plásticos para describir la evolución de su imaginario hasta la actualidad, desde las influencias obtenidas externamente. Mediante la exploración escultórica y fotográfica del paisaje se expone la relación de las formaciones volcánicas características de dichos espacios con el territorio, para ello analizamos simbólicamente como el asilamiento territorial, el horizonte y el volcán constituyen este paisaje tanto social, cultural y geológico, intervenido con las expresiones artísticas propias desarrolladas en esta investigación., [EN] The present project is a theoretical-practical investigation that proposes to explore the identity relationship of isolated territorial spaces of a physical nature. The islands, with their particular landscape, is analyzed through different sociological, anthropological and plastic references to describe the evolution of their imaginary to the present from externally obtained influences. Through the sculptural and photographic exploration of the landscape, the relationship of the characteristic volcanic formations of these spaces with the territory is exposed, for this we analyze symbolically how the territorial isolation, the horizon and the volcano constitute this social, cultural and geological landscape, intervened with own artistic expressions developed in this research.

International audience; An underwater volcanic eruption off the Vava'u island group in Tonga on 7 August 2019 resulted in the creation of floating pumice on the ocean's surface extending over an area of 150 km2. The pumice's far-reaching effects from its origin in the Tonga region to Fiji and the methods of automatic detection using satellite imagery are described, making it possible to track the westward drift of the pumice raft over 43 days. Level 2 Moderate Resolution Imaging Spectroradiometer (MODIS), Visible Infrared Imaging Radiometer Suite (VIIRS), Sentinel-3 Ocean and Land Color Instrument (OLCI), and Sentinel-3 Sea and Land Surface Temperature Radiometer (SLSTR) imagery of sea surface temperature, chlorophyll-a concentration, quasi-surface (i.e., Rayleigh-corrected) reflectance,and remote sensing reflectance were used to distinguish consolidated and fragmented rafts as well as discolored and mesotrophic waters. The rafts were detected by a 1 to 3.5°C enhancement in the MODIS-derived "sea surface temperature" due to the emissivity difference of the raft material. Large plumes of discolored waters, characterized by higher satellite reflectance/backscattering of particles in the blue than surrounding waters (and corresponding to either submersed pumice or associated white minerals), were associated with the rafts. The discolored waters had relatively lower chlorophyll-a concentration, but this was artificial, resulting from the higher blue/red reflectance ratio caused by the reflective pumice particles. Mesotrophic waters were scarce in the region of the pumice rafts, presumably due to the absence of phytoplanktonic response to a silicium-rich pumice environment in these tropical oligotrophic environments. As beach accumulations around Pacific islands surrounded by coral shoals are a recurrent phenomenon that finds its origin far east in the ocean along the Tongan trench, monitoring the events from space, as demonstrated for the 7 August 2019 eruption, might help mitigate their potential economic impacts.

Les nombreuses vallées qui découpent les flancs du Merapi sont caractérisées par des évolutions rapides, liées directement comme indirectement à l’activité du volcan. Dans une section de la rivière Gendol, située entre 6 et 11 km du sommet du volcan, les coulées et déferlantes pyroclastiques d’octobre et novembre 2010 ont tout d’abord comblé la vallée sur plusieurs dizaines de mètres d’épaisseur. Puis les lahars et l’érosion, induite par l’industrie d’extraction, ont contribué à l’incision et à l'élargissement du lit dans les dépôts pyroclastiques de 2010 ainsi que dans le substratum pré-éruptif. Cet espace, situé à l’amont de la Gendol, a fait l’objet d’un suivi topographique entre septembre 2010, soit quelques mois avant l’éruption, et avril 2014, 41 mois après celle-ci. Une analyse diachronique de la morphologie de la vallée à trois dates différentes a été permise par la conception de deux MNT, le premier en septembre 2010 construit par photogrammétrie Sfm, le second en août 2012 à partir de données LiDAR. Enfin, une étude de terrain effectuée en avril 2014 a permis de mettre à jour et compléter cette reconstitution. Cette étude met en évidence l’érosion verticale et latérale par les lahars, accentuée par la remontée progressive des zones d’extraction qui constituent le principal agent morphogène dans la vallée de la Gendol, mais également dans de nombreuses autres vallées du Merapi où les dépôts sont présents. The Merapi is an active stratovolcano in Central Java with a dense network of rivers located at the bottom of deep radial valleys, which rapidly evolves during and in the aftermath of the regular eruptions. In the wake of the 2010 eruption, the authors have monitored the evolution of a section of the Gendol Valley between 6 and 11 km to the summit of the volcano. In this reach, the pyroclastic density currents of October to November 2010 filled the valley with several tens of meters thick deposits, before being eroded by lahars and anthropic extraction in the quarries located at the bottom of the valley. The combination of these two processes has deepened and widened the valleys, cutting the 2010 pyroclastic-flow deposits and the pre-eruption material. The present contribution provides a quantitative analysis of the erosion in the Gendol valley. The results were acquired through a diachronic analysis of topographic data between 2010 – prior to the eruption – and April 2014. For this purpose, three topographical datasets were created: 2 DEMs were produced using Structure-from-Motion and LiDAR for respectively September 2010 and August 2012; and one field-survey was conducted in April 2014 with a handheld laser range-finder. The results have highlighted that lahars have heavily eroded vertically and laterally the valley, while the upstream progression of extraction sites locally increased erosion. Extractions are the main morphogen agent in the Gendolvalley but also in many other valleys of Merapi where deposits are present.

Most volcanic hazards depend on an injected dyke reaching the surface to form a feeder. Assessing the volcanic hazard in an area is thus related to understanding the condition for the formation of a feeder dyke in that area. For this latter, we need good field data on feeder dykes, their geometries, internal structures, and other characteristics that distinguish them from non-feeders. Unfortunately, feeder dykes are rarely observed, partly because they are commonly covered by their own products. For this reason, outcrops are scarce and usually restricted to cliffs, ravines, and man-made outcrops. Here we report the results of a study of feeder dykes in Tenerife (Canary Islands, Spain) and Iceland, focusing on their field characteristics and how their propagation is affected by existing structures. Although Holocene fissure eruptions have been common in both islands, only eleven basaltic feeder dykes have been identified: eight in Tenerife and three in Iceland. They are all well preserved and the relation with the eruptive fissure and/or the deposits is well exposed. While the eruptive fissures are generally longer in Iceland than in Tenerife, their feeders show many similarities, the main ones being that the feeder dykes (1) are generally sheet-shaped; (2) are segmented (as are the associated volcanic fissures); (3) normally contain elongated (prolate ellipsoidal) cavities in their central, topmost parts, that is, 2–3 m below the surface (with solidified magma drops on the cavity walls); (4) contain vesicles which increase in size and number close to the surface; (5) sometimes inject oblique dyke fingers into the planes of existing faults that cross the dyke paths; and (6) may reactivate, that is, trigger slip on existing faults. We analyse theoretically the feeder dyke of the 1991 Hekla eruption in Iceland. Our results indicate that during the initial peak in the effusion rate the opening (aperture) of the feeder dyke was as wide as 0.77 m, but quickly decreased to about 0.56 m. During the subsequent decline in the effusion rate to a minimum, the aperture decreased to about 0.19 m. At a later abrupt increase in the effusion rate, the feeder-dyke opening may have increased to about 0.34 m, and then decreased again as the effusion rate gradually declined during the end stages of the eruption. These thickness estimates fit well with those of many feeders in Iceland and Tenerife, and with the general dyke thickness within fossil central volcanoes in Iceland., Unidad de las Islas Canarias, Instituto Geológico y Minero de España, España, Department of Earth Sciences, University of London, Reino Unido

Both the genesis and rates of activity of shallow intraplate seismic activity in central Chile are poorly understood, mainly because of the lack of association of seismicity with recognizable fault features at the surface and a poor record of seismic activity. The goal of this work is to detail the characteristics of seismicity that takes place in the western flank of the Andes in central Chile. This region, located less than 100 km from Santiago, has been the site of earthquakes with magnitudes up to 6.9, including several 5+ magnitude shocks in recent years. Because most of the events lie outside the Central Chile Seismic Network, at distances up to 60 km to the east, it is essential to have adequate knowledge of the velocity structure in the Andean region to produce the highest possible quality of epicentral locations. For this, a N–S refraction line, using mining blasts of the Disputada de Las Condes open pit mine, has been acquired. These blasts were detected and recorded as far as 180 km south of the mine. Interpretation of the travel times indicates an upper crustal model consisting of three layers: 2.2-, 6.7-, and 6.1-km thick, overlying a half space; their associated P wave velocities are 4.75–5.0 (gradient), 5.8–6.0 (gradient), 6.2, and 6.6 km/s, respectively. Hypocentral relocation of earthquakes in 1986–2001, using the newly developed velocity model, reveals several regions of concentrated seismicity. One clearly delineates the fault zone and extensions of the strike-slip earthquake that took place in September 1987 at the source of the Cachapoal River. Other regions of activity are near the San Jose volcano, the source of the Maipo River, and two previously recognized lineaments that correspond to the southern extension of the Pocuro fault and Olivares River. A temporary array of seismographs, installed in the high Maipo River (1996) and San Jose volcano (1997) regions, established the hypocentral location of events with errors of less than 1 km. These events are clustered along no particular lineament approximately 25 km away from the San Jose and Maipo volcanoes. Recurrence intervals, based on a frequency magnitude relationship for lower magnitude events, indicate that earthquakes with magnitudes of 4.7 and 7 have a repeat time of 1 and 1200 years, respectively. Focal mechanisms of the two largest events indicate horizontal maximum and minimum compressive stresses with σ1 varying from a NW–SE orientation in the north to E–W at the southern extreme.

A seismic swarm which occurred in December 1994 near the Aoba volcano, Vanuatu, is investigated in detail by analyzing local and regional broad band seismic records. The seismic area is situated in a poorly instrumented region, although major seismic and volcanic events are possible. The data collected by four permanent short period stations (the nearest one being installed in the Espiritu Santo Island located 80 km from the volcano) allowed us to locate epicenters with a 2–3 km precision, and to document the present local seismicity. The quality of available regional broadband data allows us to determine reliable focal parameters, though the magnitude of the strongest event is 4.8. Data have been selected from the permanent Geoscope and Iris stations located in the south-western Pacific region, and from two temporary networks (one installed in New Caledonia and Vanuatu by I.R.D. and the E.O.S.T, and another one installed in Fiji and Tonga Islands by the Washington University, St Louis, USA). Results obtained either with local (epicentral distance d km ) body waves or regional (500 km ) surface waves lead us to conclude that the swarm is associated with the volcanic edifice, but is not of volcanic type according to the definitions given by Minakami [Phys. Volcanol. (1974) 1] and Schick [Bull. Volcanol. 44 (1981) 491]. The observed mechanisms evidence an important local influence of the volcano itself on the stress field and the observed near horizontal P axis argues against a strong increase of phreatic activity. In addition, seismic and volcanic observations are analyzed in the framework of the interpretation of earthquake swarms occurring in the vicinity of active volcanoes.

Tungurahua, one of Ecuador's most active volcanoes, is made up of three volcanic edifices. Tungurahua I was a 14-km-wide andesitic stratocone which experienced at least one sector collapse followed by the extrusion of a dacite lava series. Tungurahua II, mainly composed of acid andesite lava flows younger than 14,000 years BP, was partly destroyed by the last collapse event, 2955±90 years ago, which left a large amphitheater and produced a ∼8-km3 debris deposit. The avalanche collided with the high ridge immediately to the west of the cone and was diverted to the northwest and southwest for ∼15 km. A large lahar formed during this event, which was followed in turn by dacite extrusion. Southwestward, the damming of the Chambo valley by the avalanche deposit resulted in a ∼10-km-long lake, which was subsequently breached, generating another catastrophic debris flow. The eruptive activity of the present volcano (Tungurahua III) has rebuilt the cone to about 50% of its pre-collapse size by the emission of ∼3 km3 of volcanic products. Two periods of construction are recognized in Tungurahua's III history. From ∼2300 to ∼1400 years BP, high rates of lava extrusion and pyroclastic flows occurred. During this period, the magma composition did not evolve significantly, remaining essentially basic andesite. During the last ∼1300 years, eruptive episodes take place roughly once per century and generally begin with lapilli fall and pyroclastic flow activity of varied composition (andesite+dacite), and end with more basic andesite lava flows or crater plugs. This pattern is observed in the three historic eruptions of 1773, 1886 and 1916–1918. Given good age control and volumetric considerations, Tungurahua III growth's rate is estimated at ∼1.5×106 m3/year over the last 2300 years. Although an infrequent event, a sector collapse and associated lahars constitute a strong hazard of this volcano. Given the ∼3000 m relief and steep slopes of the present cone, a future collapse, even of small volume, could cover an area similar to that affected by the ∼3000-year-old avalanche. The more frequent eruptive episodes of each century, characterized by pyroclastic flows, lavas, lahars, as well as tephra falls, directly threaten 25,000 people and the Agoyan hydroelectric dam located at the foot of the volcano.

L'étude des laves du volcan Antisana (Equateur) révèle la présence de laves calco-alcalines, enrichies en Sr, Nb, Ti et P par rapport à des laves calco-alcalines normales. Certaines laves montrent également un appauvrissement net en terres rares lourdes, typique des adakites. Cependant, les conditions nécessaires à la genèse d'adakites n'étant pas réunies sous l'Antisana, nous proposons que ces roches à caractère adakitique soient issues de la fusion partielle d'un manteau hydraté, préalablement métasomatisé de façon hétérogène par des liquides adakitiques issus de la plaque plongeante. (Résumé d'auteur)

In order to investigate aquifers, several geophysical surveys have been carried out in the Baril area of the southern flank of Piton de la Fournaise volcano on Reunion in the Indian Ocean using audiomagnetotelluric (AMT), very-low-frequency (VLF) and self-potential (SP) methods. We present the results with emphasis on a comparison between SP data and the findings of geoelectric surveys. AMT soundings have indicated, from the surface downward, three layers : (i) resistive volcanic rocks, (ii) an intermediate resistivity layer, and (iii) a conductive basement attributed to a seawater-bearing aquifer. VLF measurements allow the mapping of the first layer apparent resistivity, and therefore its bottom, when the true resistivity is supposed to be isotropic and homogenous. When this assumption does not hold, only the SP method permits the mapping of this bottom. Because of the good agreement between the SP and electromagnetic results, we propose the SP method as the first tool that should be used in studying shallow hydrogeological structures in volcanic areas. (Résumé d'auteur)

A multidisciplinary research of a peat-bog localized 10 km away of Nevado Sabancaya volcano was done. Geomorphological study shows that the peat-bog formed in a closed depression dammed by an ancient terminal moraine of the Ampato glacier. 14C-ages demonstrate that the studied sequence corresponds essentially to the lower Holocene. Diatom determinations show that a lake flooded the peat-bog at about 8400 aBP. Sedimentological analyses allowed us to distinguish fluviatile layers and one or two tephra beds. The overall results show that the 1990 current eruption of Nevado sabancaya volcano occurred after a quiescence period of at least 8500 years., Une étude multidisciplinaire d'une tourbière située à 10 km du volcan Nevado Sabancaya a été réalisée. L'étude géomorphologique montre que la tourbière s'est développée dans une dépression fermée par une moraine terminale abandonnée par le glacier de l'Ampato. Des datations au 14C attestent que la séquence étudiée correspond essentiellement à l'Holocène inférieur. Les déterminations de diatomées indiquent qu'une nappe d'eau a inondé la tourbière vers 8400 aBP. Les analyses sédimentologiques ont permis de faire la distinction entre des couches alluvionnaires et une ou deux téphras. Il en résulte que l'éruption du Nevado Sabancaya, en cours depuis 1990, fait suite à une période de repos d'au moins 8500 ans., Juvigné Etienne, Thouret Jean CLaude, Gilot Etienne, Leclercq Louis, Gourgaud Alain. L'activité du volcan Nevado Sabancaya (Pérou) au cours de l'Holocène [Activity of Nevado Sabancaya volcano (Peru) throughout the Holocene]. In: Quaternaire, vol. 9, n°1, 1998. Hommes et volcans. pp. 45-51.

Cores were taken below 66 m of water from lacustrine deposits of the extinct basaltic maar named "Gour de Tazenat", Chaîne des Puys, Massif Central, France. Those cores allowed us to describe the environmental changes in the northern Chaîne des Puys (France) from the early Late Glacial to the present time. Geomorphological evolution of the catchment, along with the sedimentological investigations of lacustrine deposits are described. This shows that the only inlet of the lake has formed a delta; the volume of deltaic sediment is almost equal to the volume of the dell eroded by the post-eruptive backward erosion of the inlet. Since the delta is little developped, the lake (and the end of the eruption) should not be much older than Upper Weichselian in spite of TL-dates obtained on quartz grains from the adjacent tufiQ-ing and going back te about 120 ka. This paper deals also with sedimentological analyses of a 10 m long core that was taken in the muddy deposits of the flat bottom of the lake. That sequence corresponds to a period running from the early Late Glacial to the present time. The overall variations of clay-, water, organic matter contents and percentage of tree pollen throughout the core are correlated with each other. A thin tephra bed (volcanic ash-falls) was also found and linked with the Pavin volcano based upon trachytic composition of glass shards, along with the stratigraphical position within the Atlantic. That finding allowed us to enlarge the northern fan of the Pavin tephra over the entire Chaîne des Puys. New results of pollen analyses of the Atlantic to present time period shows the obvious development of Fagus (up to 50% of pollen sum). Later on, two phases of human deforestation are clearly recorded and attributed respectively to Neolithic then Celtic and subsequent populations; those two phases are separeted by a period of natural reforestation. Five sandy layers are interbedded in the Subboreal to Subatlantic mud; they should have anthropogene origine. The pollen analysis of the Late glacial to Boreal sequence was published elsewhere (Juvigné and Bastin, 199S). Nevertheless the main relevant results are summarized in this paper: Middle Dryas is particularly well recorded and the Late Dryas can be subdivided in three cold sub-oscillations. The same sequence contains two tephra beds: one is mugearitic within the Altered palynozone and dated at 10,280 yr BP and ; the other is trachytic within the Boreal and dated at about 8220 yr BP., Des carottes de sédiments lacustres ont été extraites du Gour de Tazenat. Un diagramme pollinique montre l'évolution naturelle de la végétation depuis l'Atlantique, ainsi que l'impact de deux phases de défrichements anthropiques. Dans l'ensemble, les variations des parts d'argile, d'eau et de matières organiques sont en corrélation positive depuis le Tardiglaciaire. Une mince couche de la téphra du volcan Pavin a été trouvée; elle permet d'étendre le lobe septentrional de cette retombée à l'ensemble de la Chaîne des Puys., Juvigné Etienne, Stach-Czerniak Alicja. Étude sédimentologique et palynologique des dépots lacustres tardiglaciaires et holocènes du Gour de Tazenat (Massif Central, France) [Sedimentological and palynological investigations of late glacial and holocene lacustrine deposits of the Gour de Tazenat (Massif Central, France)]. In: Quaternaire, vol. 9, n°1, 1998. Hommes et volcans. pp. 15-23.

Le Huaynaputina (Pérou méridional) comprend trois cratères au fond d'une caldeira d'avalanche éventrant un strato-volcan érodé, situé sur le rebord est d'un haut plateau. Il a été le site de la plus volumineuse éruption explosive (indice d'explosivité volcanique 6) de l'histoire dans les Andes centrales. L'épisode éruptif débuta le 19 février 1600, dura au moins 17 jours et mit en place, dans l'ordre stratigraphique : une retombée plinienne d'environ 8 km3 de lapilli ponceux et cendres dacitiques sur une zone supérieure ou égale à 85 000 km2; plusieurs retombées mineures de cendres; des déferlantes pyroclastiques précédant et intercalées dans des ignimbrites de 1 km3 environ, celles-ci étant canalisées dans toutes les vallées drainant le haut plateau; une retombée de cendres co-ignimbritiques,; une retombée très riche en cristaux, puis un dépôt cendreux. En outre, d'autres ignimbrites résultent du remaniement des téphras sur le plateau et dans les vallées; des coulées de débris ont dévasté la vallée du Tambo sur 120 km jusqu'à l'océan Pacifique. (Résumé d'auteur)

SPOT XS and Panchromatic images are used jointly in order to map the lava flows of the Nevado Sabancaya volcano (southern Peru). This mapping is achieved through two types of processing: the unsupervised multispectral clustering applied to the XS image, and some specific methods of image analysis (mathematical morphology, convolution filtering) applied to the Panchromatic image. The resulting images allowed us to identify flows and the two main morphological features of the flow areas: lava reliefs and flow lines. Therefore, our experience shows that image analysis can be a tool for thematic mapping which displays more capabilities than photointerpretation.

Surface geophysics has been used for a number of years to solve a variety of major groundwater exploration problems in coastal and island regions. The electrical properties of near‐surface rocks are highly dependent on porosity, degree of saturation, and pore fluid resistivity (Keller and Frischknecht, 1966). Because the resistivity of the formations decreases with increasing salinity of the water content, electrical methods, such as vertical electrical soundings, time‐domain electromagnetic soundings, and audiomagnetotellurics (AMT), are well‐suited for mapping changes in groundwater salinity and, in particular, for detecting a fresh water‐salt water interface.

[1] The identification and evaluation of trigger mechanisms for volcano flank instabilities and/or collapse represent a key issue for risk assessment in densely populated volcanic areas, as well as in long-distance settings, particularly in case of island or coastal volcanoes. Here we address quantitatively the effects of external (seismic) and inner (magmatic) forcing on the stress-strain state associated to flank instabilities at Mount Etna (Sicily, southern Italy) by means of a 2-D finite difference method numerical modeling. Modeled seismic actions include strong near-field, strong far-field, and low-magnitude near-field earthquakes. Magmatic actions consider the inner pressure changes induced by energetic lava fountains in the summit crater area and subvertical and oblique dike ascent below the summit area. Model results are validated in light of available monitoring data and recent eruptive activity. Numerical results show that the main strain effects are produced by high-magnitude near-field earthquakes (expected return time of ~103 yrs) and by vertical uprise of a magma dike below the volcano summit area. Maximum displacements in the order of tens of centimeters may involve the summit area, up to some 106 m3/m over some kilometers laterally. Stress releases up to 107 Pa may affect a limited portion of the magmatic conduit, thus favoring major effusive flank eruptions. Major catastrophic events, such as volcano flank collapse, should not be expected by applying, either individually or combined, the aforementioned actions.

International audience; Field evidence shows that pyroclastic flows can entrain blocks from underlying substrates formed by earlier geological events, yet, counterintuitively, they are less likely to erode unconsolidated layers of fine particles. Here we report laboratory experiments that reproduce these seemingly contradictory observations and also offer a means to infer pyroclastic flow velocity. Experiments demonstrate that the sliding head of a granular flow generates a dynamic upward pore-pressure gradient at the flow-substrate interface. Associated upward air flux is enough to fluidize a substrate of fines, so that particles are not entrained individually but the substrate instead is subject to small shear instabilities. In contrast, coarse particles forming a non-fluidized substrate are lifted at a critical upward force due to the pore-pressure gradient, according to their individual masses, which provides a basis for a model to calculate the flow velocity. Application to the 18 May 1980 pyroclastic flow deposits at Mount St. Helens (Washington State, USA) gives velocities of ∼9-13 m s-1 at ∼6-7 km from the vent on gentle slopes (

The ice-clad and fumarolic Nevado del Tourna volcano (4 ° 39′N, 75 ° 20′W) south of Nevado del Ruiz, is offset toward the southeast from the axis of the volcanic Ruiz-Tolima massif with respect to the major NE-trending strike-slip Palestina fault. It is composed of four units: (1) a pre-Tolima plateau-like basement of basaltic andesite lava flows of early Quaternary age; (2) a dissected, ancestral Tourna stratovolcano, cut by a presumed collapse caldera of middle Pleistocene age; (3) an older Tolima stratovolcano of late Pleistocene age, partly destroyed by a summit caldera; and (4) composite domes of the cone-shaped young and present Tolima. Young Tolima volcano is an andesitic and dacitic composite cone formed over the past 40,000 years within a 3-km-wide caldera that opened around 0.14 Ma. Deposits of welded and nonwelded pumice- and scoria-flows were emplaced toward the southeast (Rio Combeima) and northeast (Rio Totare). Repeated growth of lava domes over the past 16,000 years is witnessed by thick block-lava flows on the southern and eastern flanks and by block-and-ash or scoria-rich pyroclastic-flow deposits. This activity occurred during at least six eruptives stages as follows: El Placer, ca. 16,200-14,000 yr B.P.; Romerales, ca. 13,000–12,300 yr B.P.; Canalones, ca. 11,500–9750 yr B.P.; Mesetas, ca. 7200 – 4600 yr. B.P.; Encanto, ca. 3600 – 1700 yr B.P., and Nieves, historical. Interactions with the ice cap probably triggered debris flows that partly filled the Combeima and Totare valleys and formed the Holocene terraces on the upper Pleistocene volcaniclastic fans of Ibagueand Venadillo as much as 60 km from the source. The latest major activity was a plinian eruption, which deposited a pumice-fall layer ca. 3600 yr B.P. (0.5 km3 actual volume) mainly toward the west and northwest. Minor tephra-falls and debris flows occurred during the historical period before the reported 1918 and 1943 small (phreatic ?) events. A general hazard-zone map shows areas potentially affected by future eruptions both at Nevado del Tolima and at active Cerro Machin 12 km southward. The extent of areas likely to be affected by tephra-falls, debris flows, pyroclastic flows or surges, debris avalanches and lava flows is shown. Subplinian and plinian eruptions of Nevado del Tolima were used to represent the moderate and large events to be expected. 300,000 people live within a 35-km distance from those volcanoes, which have exhibited a behaviour more explosive than Nevado del Ruiz. Despite the small-sized ice cap, debris flows are the most probable hazard for even a minor eruption, because of the very steep slope gradient, and because of probable interactions of hot eruptive products with ice and snow. Additionally, scoria flows and debris avalanches can be directed toward the southeast and could be transformed into debris flows that would devastate the Combeima valley and suburbs of Ibaguecity, where about 50,000 people live

The Los Azufres volcanic complex has had a long eruptive history of ∼ 1.5 Myr or more (less than 3 Myr), subsequent to the building of a large Miocene and Pliocene andesitic complex. Since about 1.5 Ma, its development consists of two main periods of volcanic activity. The first period, from ∼1.5 to 0.8 Ma, has two magmatic cycles, about 200,000 years in duration, characterized by acidic, followed basaltic volcanism. Such a magmatic evolution suggests successive emptyings of a shallow reservoir in which differentiation previously occurred. About 0.6 Ma, major deep magma supplies were probably responsible for the resurgent doming which led to uplift of the southern part of the caldera. From ∼0.6 Ma to Present, the second period exposes volcanic products grading from basalts to rhyolites. Considering the recent age of the last ignimbrites (26,000 to 29,000 years), one can assume that the volcanic activity related to a voluminous differentiated magma body at shallow depth has not yet ended, especially in the southern area, in and around the resurgent zone occupied by the geothermal field.

SUMMARY Altimetric measurements of sea-surface heights have long been used to locate uncharted marine features. Newly available data sets (Geosat-GM, ERS-1 and Topex/Poseidon), now allow the topography of submarine volcanoes to be accurately restored and a method based upon inverse modelling is presented in this paper. This method allows a joint assimilation of altimetric data and shipborne profiles of bathymetry and/or gravity anomalies when available. Furthermore, uncertainties can be computed together with the topography, which constitutes a noticeable improvement since these parameters are still missing in the usual bathymetric data bases. Tests are performed using simulated data, in order to characterize the errors which may or must occur with the modelled topography of a previously unknown volcano, in particular the errors due to data uncertainties and the lack of precise knowledge of sensitive parameters that are used in the modelling. These errors can be maintained under the 100 m level (rms) for altimetric data with a measurement noise of 5 cm (rms) or less, which is achieved for Geosat, ERS-1 and Topex/Poseidon altimeters. An example of inversion of Seasat data is presented in the case of a seamount in French Polynesia.

In Vanuatu, Tongoa and Epi islands once formed part of a larger landmass, Kuwae, which was partly destroyed during a cataclysmic seismo-volcanic event that is recorded in local folklore. It led to the formation of a 12-kmlong and 6-km-wide oval-shaped submarine caldera with two distinct basins and a total area of - 60 km2 at the level of the rim. The age ofthis eniption, 1420-1430 A.D., and the structure of the related collapse are discussed and a composite log ( 143 m) of the pyroclastics surrounding the caldera is presented. They comprise thick hydromagmatic deposits belonging to a terminal hydromagmatic phase of the pre-caldera edifice, which grade upwards into two major sequences of pyroclastic flow deposits, clearly related to the caldera event. Collapse near the caldera edge was at least in the range 650 to 950 m, and may have been as much as 800 to 1100 m. The volume of rocks engulfed during the caldera formation is - 32-39 km3, suggesting the same volume of magma was erupted. Even if two coalescent collapse structures were formed, it is worth noting that the Kuwae caldera is not a reactivated structure, but the result of a single event of short duration which occurred in the first half of the Fifteenth century. This event is one of the seven biggest caldera-forming events during the last 10,000 years, and is comparable with the Santorini Minoan eruption and the Crater Lake eruption.

Because of intensive exploitation of the Goldberg volcano (West Eifel Volcanic Field, Germany), in the past three decades, additional information has been obtained on its volcanological history and its products. Two important explosive events are recorded in the stratigraphic pile. An attempt is made to trace tephra of the volcano, by sampling soils in plateau position all around the volcano. However, the sampled sites also contained minerals from the Laacher See tephra that blanketed the area as demonstrated by the occurrence of a layer in the nearby Bragphenn peat bog. We performed electronic microprobe analyses, to discriminate between the pyroxenes from Goldberg and those from the Laacher See. Since the quantities of minerals drop rapidly away from the volcano, the Goldberg tephra cannot be traced beyond some 6 km. Nevertheless the dispersion is more important in westwards so that further discovery of the tephra can be expected especially in Upper Belgium. Our results allow us to shift the minimal age of the eruption from 11.6 ka BP to 25 ka BP. En profitant de l’exploitation intensive du volcan Goldberg (Eifel occidental, Allemagne) au cours des trois dernières décennies, des informations nouvelles sont apportées concernant l’histoire volcanique de l’appareil et de ses produits. Deux importantes phases explosives sont attestées dans sa lithostratigraphie. En l’absence d’affleurements de téphra en place en dehors du volcan, la recherche des lobes est abordée par l’étude des concentrations de minéraux dans des sols actuels en position de plateau. Toutefois les sites étudiés contiennent également des produits du volcan du Laacher See qui sont retombés sur l’Eifel et l’Ardenne (Belgique), et dont nous avons découvert une lamine en place dans une tourbière locale (Bragphenn). En raison du mélange des téphras dans les échantillons de sols actuels, des analyses ont été réalisées au moyen de la microsonde électronique pour distinguer les pyroxènes du téphra du Goldberg de ceux du Laacher See. La méthode n’a pas permis de suivre le téphra à plus de 6 km du volcan, car la concentration de ses minéraux est trop faible au-delà. Néanmoins la dispersion étant plus importante vers l’ouest, c’est donc en haute Belgique que des découvertes ultérieures de ce téphra sont attendues. Nos résultats permettent de vieillir l’âge minimal de l’éruption de 11,6 ka BP à 25 ka BP.

SUMMARY To first order, the estimated thickness Te of the elastic part of the oceanic lithosphere increases linearly with the square root of the age of the lithosphere at the time of loading, Δt. In order to quantify this relationship in the particular case of volcanic loading, a synthesis of Te estimates reported both in this paper and in previous studies is conducted. Excluding the anomalously low estimates from the south-central Pacific, the values are very consistent for the three main oceans and follow the empirical relationship: Te (km) = (2.7 ± 0.15)½Δt (Ma). the relationship is used to predict the age of volcanoes when the age of the crust is known (for the Trindade chain in the south Atlantic Ocean) and to predict the age of the crust when the age of the volcanoes is known (for the Pacific plate east of the Tonga trench). the age estimates are in good agreement with the structural setting.

Dans ce travail, nous rapportons les compositions minéralogique et géochimique de couches de téphra présentes dans deux longues séquences sédimentaires prélevées dans le lac Icalma (Chili, Zone Volcanique Sud des Andes). Les deux séquences ont été datées par radiocarbone et couvrent les huits derniers millénaires. En raison de l’intense activité volcanique régionale et de l’effet des vents d’Ouest sur la dispersion de téphras, les sédiments du lac Icalma contiennent un nombre impressionnant de retombées téphriques. Afin de déterminer les compositions minéralogique et géochimique des magmas juvéniles respectifs, nous avons examiné les scories et/ou les ponces de taille supérieure à 420 μm, ce qui permet de s’affranchir de la contamination des téphras par le sédiment hôte. Nos résultats démontrent qu’un seul téphra se distingue nettement du reste de la série par ses compositions géochimique (rhyolithe) et minéralogique (essentiellement de l’hypersthène) particulières. Il s’agit du téphra Alpehue, émis par le volcan Sollipulli en 3000 a cal BP. La composition des autres téphras étant relativement homogène, une méthode de corrélation par zone a été utilisée et a permis de dégager deux unités minéralogiques et géochimiques distinctes, en plus du téphra Alpehue. Les téphras de l’unité supérieure consistent en andésite basaltique riche en olivine, tandis que ceux de l’unité inférieure sont des andésites à pyroxènes dominants. Cette démarche a permis de corréler les deux carottes sédimentaires entre elles, et de proposer des corrélations avec des enregistrements palustres et pédologiques plus distaux. In this paper, we document the mineralogical and geochemical composition of tephra layers identified in two long sediment cores (ICA-I and ICA-II) collected in Lake Icalma (Southern Volcanic Zone of the Andes, Chile). The sediment cores have been dated by radiocarbon and cover the last 8000 years. Due to the frequent regional volcanic activity and prevailing westerly winds, the sediments of Lake Icalma contain a large number of tephra deposits (134 in ICA-I and 98 in ICA-II). However, some of these tephras occur in reworked deposits and have therefore been excluded from the present tephrostratigraphic study. Based on the mineralogical composition of grains in the 105-420 μm fraction, it has been demonstrated that the tephra layers are highly contaminated by the host sediment. We therefore separated and analyzed the coarse-grained scoriae and (or) pumices (> 420 μm) to constrain the mineralogical and geochemical composition of the corresponding juvenile magmas. Our results demonstrate that only one tephra, the Alpehue pumice, clearly differs from the rest of the record by its original geochemical (rhyolitic glass) and mineralogical (essentially hyperstene) composition. This tephra was emitted by the Sollipulli volcano in 3000 cal yr BP. In addition, our results demonstrate that the mineralogy and geochemistry of successive tephra layers are generally very similar and don’t permit correlations of individual tephra layers. A method of correlation by group has therefore been used. This method clearly shows two distinct mineralogical and geochemical tephrostratigraphical units, in addition to the Alpehue tephra. Tephras occurring in the upper unit are composed of olivine-rich basaltic andesite, whereas those of the lower unit have a pyroxene-rich andesitic composition. Using this technique, we have been able to correlate the two sediment cores with each other, as well as with more distant peat and soil deposits. In each case the Alpehue tephra is the more reliable tephrostratigraphic marker. This paper demonstrates that the quality of lacustrine tephrostratigraphic records is affected by several parameters such as the direction of prevailing winds, the frequency of volcanic eruptions and the seismic activity. Using tephras deposited in such sedimentary environments for stratigraphic correlations requires the adjustment of classical methods of tephrostratigraphic investigation.

[1] Three independent ASAR interferograms spanning the August 2003 Piton de la Fournaise eruption, reveal a 3 by 3 km asymmetric pattern of range changes centred on the Dolomieu crater northern flank. It corresponds to 30 cm of displacement towards the satellite east of the eruptive fissures and 7 cm away from the satellite west of the fissures. Displacements are caused by dyke emplacement below fissures. We model the deformation using a 3D mixed boundary element method for elastic media. This consists of a dyke defined by six geometric parameters and an overpressure gradient. A neighbourhood algorithm was applied to explore this 7 dimensional parameter space. The best-fit model is a 57° eastward dipping dyke with a base lying around 1520 m a.s.l. The model provides new evidence of the dyke intrusion – related seaward displacements of the volcano eastern flank.

¿Cómo es el voleán Tungurahua? ¿Cuál ha sido su comportamiento histórico? ¿Qué tipo de fenómenos volcánicos se producen en él? ¿Cómo es vigilado por los técnicos? ¿Qué sucede en el actual proceso eruptivo? ¿Cuáles son los peligros ante los que la ciudadanía debe estar preparada? Este libro responde a estas inquietudes que actualmente son parte de la vida cotidiana de grandes sectores de población.

Wide-angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three-dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of La Réunion. The submarine part of the edifice has fairly low seismic velocities, without evidence for intrusives. An upper unit with a velocity-depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope. Between this unit and the top of the oceanic plate, imaged by normal incidence seismic reflection, a more homogeneous unit indicated by shadow zones on several wide-angle sections may correspond to lavas of a different nature, extruded underwater in the earlier phase of volcanism. Coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part, with limited evidence for some intracrustal intrusion. Deeper in the lithosphere, the presence of a layer of intermediate velocity between the crust and mantle is firmly established. It is interpreted as resulting from the advection of hotspot magmatic products, possibly partially molten, and of a composition for which the crust is a density barrier. The extensive wide-angle coverage constrains the extent of this body. It does not show the elongated shape expected from plate drift above a steady hotspot supply. Alternative propositions can hence be considered, for example, that La Réunion is caused by a solitary wave of hotspot material or by a young hotspot. The size of the underplate, 140 km wide and up to 3 km thick, corresponds to less than half the volume of the edifice on top of the plate. Copyright 1999 by the American Geophysical Union.

The largest explosive eruption (volcanic explosivity index of 6) in historical times in the Andes took place in a.d. 1600 at Huaynaputina volcano in southern Peru. According to chronicles, the eruption began on February 19 with a Plinian phase and lasted until March 6. Repeated tephra falls, pyroclastic flows, and surges devastated an area 70 × 40 km2 west of the vent and affected all of southern Peru, and earthquakes shook the city of Arequipa 75 km away. Eight deposits, totaling 10.2–13.1 km3 in bulk volume, are attributed to this eruption: (1) a widespread, ∼8.1 km3 pumice-fall deposit; (2) channeled ignimbrites (1.6–2 km3) with (3) ground-surge and ash-cloud-surge deposits; (4) widespread co-ignimbrite ash layers; (5) base-surge deposits; (6) unconfined ash-flow deposits; (7) crystal-rich deposits; and (8) late ash-fall and surge deposits. Disruption of a hydrothermal system and hydromagmatic interactions are thought to have fueled the large-volume explosive eruption. Although the event triggered no caldera collapse, ring fractures that cut the vent area point to the onset of a funnel-type caldera collapse.

The explosive activities of the volcanoes Stromboli (Aeolian Islands, Italy) and Yasur (Tanna Island, Vanuatu) produce low‐frequency seismic signals. Several types of such signals were identified both on Stromboli (Types Is and Us) and on Yasur (Types Iy, IIy, and IIIy). The dominant frequencies of these signals fall generally between 1 and 3 Hz although sometimes frequencies in the range 3–6 Hz also exist. The two volcanoes display similar seismic characteristics: the seismic signal associated with the strombolian explosion is accompanied by a forerunner signal that occurs several seconds before eruption and which corresponds to the time separating the formation of the gas pocket at different levels in the magmatic column and its reaching the surface. The similar spectral characteristics of the volcanic background seismic noise and the discrete signals suggest a common source.

International audience; An audiomagnetotelluric survey has been performed along an inactive flank of La Fournaise volcano massif in Reunion island, to study the subsurface resistivity structure. One-dimensional modelling of the AMT data at each site revealed an extensive low-resistivity (less than 10 ω m) zone at a few hundred meters below the surface. The significance of this unexpected conductive substratum is discussed in relation with the proposed impact of volcano-tectonic processes (caldera and landslides collapses), and a new interpretation of the geological structure is proposed. Moreover, it is likely that these conductors are poorly permeable argilaceous materials; then they coincide with a limit in permeability and determine groundwater behavior.

At Santa Maria Volcano (New Hebrides island arc), extensive ash and scoria flow deposits overlie the mainly effusive, pre-caldera cone. Hydromagmatic features characterize these deposits, the composition of juvenile clasts ranges from basalt to acid andesite/dacite (SiO2= 51–63.6%) with a dominant basaltic composition. The stratigraphic position of this pyroclastic series and its spatial distribution around a 8.5 km × 6 km wide caldera provide evidence of a relationship between this series and the caldera formation. In addition, these pyroclastic deposits are co-genetic to parasitic cones and lava flows developed along faults concentric to the caldera. Both series result from a compositionally layered magma reservoir, the subordinate differentiated magmas being the result of fractional crystallization from the basalts. A model of caldera formation which implies a large hydromagmatic eruption at the central vent and minor magma withdrawal by flank eruptions is proposed. This model emphasizes the importance of mafic hydroclastic eruptions in the caldera forming event and contradicts a model implying only quiet subsidence, a process often proposed for the formation of calderas in island are volcanoes of mainly mafic composition.

On 27 August 1883, the Krakatau volcanic complex (Indonesia) was the site of one of the most destructive historical eruptions. Most of the volcano was destroyed and a new caldera also formed during this catastrophic event. Since the date of the eruption, many geological studies of the superficial structures and eruption products have been carried out. A debate on the scenario of the eruption and the way the volcano collapsed has developed and still is unresolved. In order to assess the inner structure of the volcanic complex, we carried out a detailed land and marine geo-physical survey in the summer 1990. In this paper, bathymetry and gravity data collected during the survey as well as literature data are compiled and analysed. Bathymetric data show that the caldera is characterized by a flat seabottom at 240 m below sea level and by steep linear walls suggesting that the caldera collapse has been controlled by pre-existing features. Moreover, the build-up of the young active volcano, Anak Krakatau, on the very edge of the caldera could lead to mechanically unstable conditions which must be considered for hazard mitigation. The Bouguer anomaly of the volcanic complex is characteristic of volcanoes with an explosive behaviour. 3-D gravity modelling reveals the previously unknown geometry of the dense substratum of the proto-Krakatau and evidences the presence of a collapsed structure beneath the caldera filled up with low-density material. Finally, we point out a major weakness zone, oriented N150 ° on a line passing through the old and recent vents of the volcano. This zone could have guided both the development of the volcanic activity and the emplacement of the 1883 caldera. Furthermore, this weakness zone passing through the summit line of the pre-1883 Krakatau volcano has been introduced as a significant disruption surface of the volcanic edifice in the updated scenario of the 1883 eruption that we propose.

Cerro Machín es un volcán activo, caracterizado en el pasado por erupciones explosivas de tipo pliniano y ubicado en un sector de gran importancia económica. Desde la década de los noventa se han desarrollado varios modelos de simulación para determinar el área que podría resultar afectada en una nueva erupción, pero estos modelos se basan en programas con variables predeterminadas que no son fácilmente modificables. Por tanto, se diseñó una aplicación del modelo de simulación Model Builder de las herramientas del software ArcGIS© para facilitar el proceso de análisis de los flujos piroclásticos expulsados por ese volcán en tres eventos eruptivos identificados y modelados previamente, con el objetivo de crear una aplicación fácilmente adaptable a cambios en las variables de simulación y que pudiera ser empleada también para modelar otros volcanes. Los resultados obtenidos resultan compatibles y similares a los obtenidos en simulaciones previas, lo que indica que la aplicación desarrollada es funcional.

Datos geoquímicos y petrográficos acerca de las lavas anteriores de la caldera del Volcán de Fuego de Colima indican dos procesos magmáticos; cristalización fraccionada y mezcla de magma. Estos procesos pudieron suceder juntos, de tal manera que sus efectos se adicionan. Se distinguen tres tipos de mezcla: (i): mezcla entre nuevas inyecciones de magma profundo y minerales máficos acumulados en la parte inferior de la cámara magmática somera (ii): Mezcla, en la cámara, entre este magma juvenil (anteriormente diferenciado o no en una cámara más profunda, contaminando por olivino y clinopiroxeno o no) y un magma diferenciado de composición andesítica o dacítica. (iii) Mezcla por convección entre magmas ya diferenciados, por separación de minerales, o mezcla, o por ambos procesos, a diferentes niveles en la cámara somera.

Temperature measurements on shallow vertical profiles undertaken on Matthews and Hunter volcanoes of the New Hebrides arc (SW Pacific) demonstrate the absence of both unsteady and steady conductive abnormal flux at the location of the studied profiles. The reasons for this absence are explained in terms of limits in depth or magnitude for possible sources of heat inside the volcanoes. It implies that the magma chamber is of rather limited extent. This type of flux measurement has a low cost and it will be possible to implant a series of such temperature profiles on an edifice in order to obtain a map of the flux that could be widely used for the location of heat sources.

Two tephra layers were discovered in the "Godivelle-Nord" peat-bog in the Cézallier. Those tephra were dated at about 10 750 B.P., and it is shown that a time range of about two centuries should have passed between each eruption. The upper layer (tephra 4) is a potassic mugearite, and its mineral suite is characterized by labrador, augite, salite, and a little amount of kaersutite. The lower layer (tephra 5) is a potassic hawaiite, and its mineral suite is characterized by labrador, augite, and salite, they content about 1/3 of xenoliths including relativ high amount of kaersutite. It is supposed that the following volcanoes should be responsible for the eruption of tephra 4 and/or 5 : Lac Chauvet, Lac d'en Haut, Puy de Tartaret, Lac de la Fage. The discovery of tephra 4 and 5 allows us to distinguish two volcanic phases in the Cézallier and the southern Mont Dore : 1) the oldest one includes the eruptions of tephra 4 and 5, and occurred at about 10 750 B.P.; 2) the youngest one includes the eruptions of Montcineyre, Estivadoux, Montchal and Pavin-volcanoes, and occurred at about 6 000 B.P., Deux lamines de téphra ont été découvertes dans la tourbière de « Godivelle-Nord » dans le Cézallier. Leurs émissions ont eu lieu vers 10 750 B.P., à environ 2 siècles d'intervalle. La retombée supérieure (téphra 4) est une mugéarite potassique à labrador, augite, salite et une faible part de kaersutite. La retombée inférieure (téphra 5) est une hawaïte potassique à labrador, augite, salite; elle contient environ un tiers de xénolithes, et une part importante de kaersutite probablement énallogène. Les volcans suivants sont évoqués comme sources possibles des téphra : lac Chauvet, lac d'en Haut, Puy de Tartaret, lac de la Fage. La découverte des téphra 4 et 5 permet de distinguer deux phases éruptives dans le Cézallier et le Mont Dore méridional : 1) la plus ancienne, qui a eu heu vers 10 750 B.P., comprend l'émission des téphra 4 et 5; 2) la plus récente, qui date d'environ 6 000 B.P., groupe les éruptions des volcans du Montcineyre, d'Estivadoux, du Montchal et du Pavin., Juvigné Etienne. Deux retombées volcaniques tardiglaciaires dais le Cézallier (Massif Central, France) . In: Bulletin de l'Association française pour l'étude du quaternaire, vol. 24, n°4, 1987. pp. 241-249.

The recent synthesis about tephrochronology of the Late-Glacial and the Holocene of the Monts Dore, the Cantal, and the Cézalher are contradictory. The data from the literature are criticized and seven new 14C datings are presented. The results show that the explosive phase of the Montcineyre, Estivadoux, Montchal, and Pavin volcanoes lasted for approximately 150 to 200 years at about 6000 B.P Another three ash layers have also been dated : 1) the La Taphanel tephra (8780 B P) which were likely erupted by the maar Chopine; 2) the Godivelle-Nord tephra 4 (10300 B.P); 3) the Godivelle-Nord tephra 5 (10750 B.P)., Les synthèses téphrochronologiques récentes du Tardiglaciaire et de l'Holocène du Cantal, du Cézallier et des Monts Dore sont contradictoires. Ces données de la littérature sont critiquées, et sept nouvelles datations 14C de tourbe encadrant des téphra ont été réalisées. Les résultats montrent que la phase d'émission de téphra du groupe de volcans du Puy de Montcineyre, du maar d'Estivadoux, du Puy de Montchal et du lac Pavin a eu lieu vers 6000 B.P., et a duré entre 150 et 200 ans. Trois autres retombées ont aussi été datées : 1) les téphra de La Taphanel (8780 B.P.) probablement émises par le maar Chopine; 2) les téphra 4 de Godivelle-Nord (10300 B.P ); 3) les téphra 5 de Godivelle-Nord (10750 B.P.)., Gewelt Michel, Juvigné Etienne. Téphrochronologie du Tardiglaciaire et de l'HoIocène dans le Cantal, le Cézallier et les Monts Dore (Massif Central, France) : résultats nouveaux et synthèse. In: Bulletin de l'Association française pour l'étude du quaternaire, vol. 25, n°1, 1988. pp. 25-34.

The recent history of Popocatepetl shows that violent repetitive eruptions took place during four periods, each of them lasting 1000 to 2000 years. The first one occurred before 10,000 years B.P., the second between 10,000 and 8000 years B.P., the third from 5000 to 3800 years and the fourth period started 1200 years ago. Cataclysmic events of St. Vincent-type leading to ash and scoria pyroclastic flows, and numerous air-fall deposits, alternate with a few lava flows during these periods of intense activity. In spite of the apparent quiescence of the volcano, the last period is still continuing. Prehistoric and historic eruptions repeatedly formed large volumes of hot pyroclastic flows and air falls that extended 20 km southeastwards and 10 to 15 km northeastwards. Lava flows seem to be restricted to the summit area. Pyroclastic flows, ash and pumice falls, and lava flows, are likely to be produced on the century scale or even on the decade scale. The morphology of Popocatepetl’s slopes favours a wide distribution of the pyroclastic products, and many people live in sectors considered dangerous at the bottom of the volcano. In the case of pyroclastic flows similar to those of past eruptions, and directed towards the southeast, Atlixco (with 80,000 people) could be affected. If a Plinian eruption, like those produced just before the Hispanic Conquest, occurs with prevailing westerly winds, the whole region of Puebla could be affected. Risks presented by avalanches and lahars are also discussed.

On étudie, par des mesures en plongée sous-marine, les émergences en mer des écoulements souterrains dans le Piton de la Fournaise (île de la Réunion) dans le cadre de recherches sur les relations entre circulations souterraines, flux de chaleur et phénomènes magmatiques. On établit:- que l'ensemble des émergences se situe au niveau du rivage, et non en profondeur- que grâce à des mesures approchées des phénomènes de mélange des eaux, on peut estimer le débit moyen des résurgences en mer à 0,4 m3s-1 par kilomètre de côte sur la façade sud de l'île. Ce chiffre est cohérent avec des estimations indirectes obtenues à partir du bilan hydrique., The « Piton de la Fournaise » on the island of la Réunion receives as much as 6 to 10 m y-l of rainfall, which almost immediately infiltrates due to the highly pervious nature of the basalt (microfissures, scoria). Very few springs or rivers drain the systeml however, numerous marine outlets have been observed by infrared thermographic surveys. Some of these outlets can also be observed on SPOT images, although no infrared charmel is available on SPOT.In an attempt to study the role of underground flow in the thermal balance of the volcano and its relation with magmatic phenomena, we have tried to observe the nature of these outlets by underwater exploration and to evaluate their flow rate by salinity and temperature mesurements.Longitudinal and transverse salinity profiles were measured by divers using a salinity-conductivity meter with automatic correction of temperature, transported in a waterproof container, on those plumes that had been observed on airborne images or by helicopter surveys.All the observed plumes are indeed mixing zones of seawater and freshwater, with salinity ranging from 29.9 to 35.1 %o (ocean salinity is35.2%o at la Réunion). Their temperature is in general lower than that of the sea, which explains why they are visible on infrared irnages.It was found, however, that all outlets are systematically situated very close to the shore line, with an elevation between + 1 and - 2 m of sea level. In none of the observations were outlets found that could originate deeper down in the sea. Most observable oulets are located at the base of basalt flows, in natural « tunnels » under such lava flows. This is consistent with the existence of an abrupt interface between seawater and freshwater on the island, which has been observed in a fewboreholes, and which forces the freshwater flow upwards towards the shore line, even if the nature of the flow is very discontinuous in the basalt.Based on the salinity profiles, we have attempted to estimate the flow rate. We focus here on a particular outlet at Vincendo. We liken the development and mixing of the plume to what occurs in an estuary with low flow. It has been observed that three mechanisms control the mixing in an estuary: wind which creates currents and pushes freshwater toward the edges; tidal effects and waves creating currents and turbulent mixing due to rugosity of the sides and bottom; density difrerences between seawater and freshwater, the latter floating on top of the former. Three cases are generally considered:a - Seas without tide: the interface is stratified;b - Seas with small tides: stratification and mixing occur sirnultaneously;c - Seas with large tides: no stratification and regular mixing in the vertical dimension.The observed salinity profiles at Vincendo clearly indicate that we are in the second case at la Réunion, where the tide amplitude is small (0.7 m).Two dimensionless numbers are used in estuaries: the Richardson and Froude numbers (see expression in text) (FISHER et al., 1979). It has been observed that the transition from case a to c corresponds to Richardson numbers in the range 0.08 to 0.8. Assuming that salinity profiles are consistent with case b, i.e. a Richadson number in the range 0.25 - 0.80, we find that the freshwater flowrate should be in the range 0.020 - 0.260 m3. s-1.A second independent estimation can be obtained by observing that the average concentration gradient in the 7,000 m2 of the obseved mixing zone (30 x 40 m) is on the order of 0.3 kg. m-3. m-1. Selecting (from FISCHER et al., 1979) a turbulent dispersion coefficient for coastal watcrs over the scale of several thousand m2 of 2 to 5 x 10-3 m2. s-1, we can estimate the vertical dispersive flux over the mixing zone and, by mass balance, we obtain another estimate of the flux in the range 0.150 - 0.400 m2. s-1.We conclude that the flowrate is on the order of 0.150 m3.s-1, with a plausible range of 0.100 - 0.300 m3.s-1.These results were extrapolated to the entire South and East shores ofthe island by assuming that the flow rate of an outlet was proportional to its area as observed on infrared surveys. We obtain an average flux of 0,4 m3.s-1 km-l for the southern flank of the volcano. This ligure is consistent with a global estimate (0,6 m3.s-1km-l) obtained by a surface hydrologic balance over this part of the volcano. The difference can represent diffuse outlets into the sea.

Continuous monitoring of Cotopaxi volcano from January 1989 to September 1997 shows that it is subject to a steady shallow Long Period (LP) activity. LP events are concentrated in a column with a diameter of about 3 km and a height of about 12 km, between an elevation of 4 km and 8 km depth. They do not occur in swarms, nor have there been periods of complete calm. High frequency volcano-tectonic (VT) events are mostly located on the northeastern side of the LP column, with the maximum activity of both types of events coinciding in depth. LP activity cannot be related to unrest of the volcano. The most likely explanation for the continuous occurrence of LP events beneath Cotopaxi is that they are produced by the interaction of glacier thaw water and hot material at shallow depths. The relative spatial distribution of both type of events in two adjacent zones suggest that VT and LP activities are interconnected.