Anne Deysine, La Cour Suprême des Etats-Unis. Droit, politique et démocratie

Au moment où la France commence à se familiariser avec un nouveau type de contrôle de constitutionnalité depuis l’introduction de la « Question Prioritaire de Constitutionnalité » (QPC) en 2008, l’ouvrage d’Anne Deysine, juriste et professeur émérite de l’Université de Nanterre, tombe à point nommé pour expliciter les enjeux liés à ce contrôle ex post qui, aux États-Unis, est pratiqué depuis 1803. La décision historique de cette année-là créa le pouvoir moderne de « Judicial Review », donnant..

La recherche indépendante aux États-Unis et en France : le Council on Foreign Relations et l’Ifri

Cette brève monographie compare le développement d’un des plus anciens think tanks américain, le Council on Foreign Relations et l’Institut Français des Relations Internationales (Ifri). Le texte illustre ainsi les profon­des divergences entre les deux institutions, notamment au niveau de leur direction qui, dans le cas de l’Ifri, reste très personnalisée.This brief overview of two leading institutions in the field of policy-oriented research - the Council on For­eign Relations (CFR) in the US and the Institut Français des Relations Internationales (Ifri) – emphasizes the deep differences between their respective develop­ments. Unlike CFR, which successfully managed to evolve into a collective research entity, Ifri is still very much related to its founder, thus running the risk of being short-lived

Experiments on conduit flow and eruption behavior of basaltic volcanic eruptions

Multiphase flow in basaltic volcanic conduits is investigated using analog experiments and theoretical approaches. Depending on gas supply, large gas bubbles (gas slugs) may rise through basaltic magma in regimes of distinct fluid‐dynamical behavior: ascent of single slugs, supplied slugs fed from the gas source during ascent, and periodic slug flow. An annular flow regime commences at the highest gas supply rates. A first set of experiments demonstrates that the growth of gas slugs due to hydrostatic decompression does not affect their ascent velocity and that excess pressure in the slugs remain negligible. The applicability of theoretical formulae describing slug ascent velocity as a function of liquid and conduit properties is evaluated in a second set of experiments. A third set of experiments with continuous gas supply into a cylindrical conduit are scaled to basaltic conditions over Morton, Eotvös, Reynolds, and Froude numbers. Gas flow rate and liquid viscosity are varied over the whole range of flow regimes to observe flow dynamics and to measure gas and liquid eruption rates. Foam generation by slug bursting at the surface and partial slug disruption by wake turbulence can modify the bubble content and size distribution of the magma. At the transition from slug to annular flow, when the liquid bridges between the gas slugs disappear, pressure at the conduit entrance drops by ∼60% from the hydrostatic value to the dynamic‐flow resistance of the annular flow, which may trigger further degassing in a stored magma to maintain the annular flow regime until the gas supply is exhausted and the eruption ends abruptly. Magma discharge may also terminate when magma ascent is hindered by wall friction in long volcanic conduits and the annular gas flow erodes all magma from the conduit. Supplied slugs are found to reach much higher rise velocities than unsupplied slugs and to collapse to turbulent annular flow upon bursting at the surface. A fourth set of experiments uses a conduit partially blocked by built‐in obstacles providing traps for gas pockets. Once gas pockets are filled, rising gas slugs deform but remain intact as they move around obstacles without coalescence or significant velocity changes. Bursting of bubbles coalescing with trapped gas pockets causes pressure signals at least 3 orders of magnitude more powerful than gas pocket oscillation induced by passing liquid. Our experiments suggest a refined classification of Strombolian and Hawaiian eruptions according to time‐dependant behavior into sporadically pulsating lava fountains (driven by stochastic rise of single slugs), periodically pulsating lava fountains (resulting from slug flow), and quasi‐steady lava fountains (oscillating at the frequency of annular‐flow turbulence)

Dynamics of soap bubble bursting and its implications to volcano acoustics

In order to assess the physical mechanisms at stake when giant gas bubbles burst at the top of a magma conduit, laboratory experiments have been performed. An overpressurized gas cavity is initially closed by a thin liquid film, which suddenly bursts. The acoustic signal produced by the bursting is investigated. The key result is that the amplitude and energy of the acoustic signal strongly depend on the film rupture time. As the rupture time is uncontrolled in the experiments and in the field, the measurement of the acoustic excess pressure in the atmosphere, alone, cannot provide any information on the overpressure inside the bubble before explosion. This could explain the low energy partitioning between infrasound, seismic and explosive dynamics often observed on volcanoes

Toward continuous quantification of lava extrusion rate: Results from the multidisciplinary analysis of the 2 January 2010 eruption of Piton de la Fournaise volcano, La Reunion

International audienceThe dynamics of the 2–12 January 2010 effusive eruption at Piton de la Fournaise volcano were examined through seismic and infrasound records, time-lapse photography, SO2 flux measurements, deformation data, and direct observations. Digital elevation models were constructed for four periods of the eruption, thus providing an assessment of the temporal evolution of the morphology, the volume and the extrusion rate of the lava flow. These data were compared to the continuous recording of the seismic and infrasonic waves, and a linear relationship was found between the seismic energy of the tremor and the lava extrusion rate. This relationship is supported by data from three other summit eruptions of Piton de la Fournaise and gives total volume and average lava extrusion rate in good agreement with previous studies. We can therefore provide an estimate of the lava extrusion rate for the January 2010 eruption with a very high temporal resolution. We found an average lava extrusion rate of 2.4 m3s−1 with a peak of 106.6 m3s−1 during the initial lava fountaining phase. We use the inferred average lava extrusion rate during the lava fountaining phase (30.23 m3s−1) to estimate the value of the initial overpressure in the magma reservoir, which we found to range from 3.7×106 Pa to 5.9×106 Pa. Finally, based on the estimated initial overpressure, the volume of magma expelled during the lava fountaining phase and geodetic data, we inferred the volume of the magma reservoir using a simple Mogi model, between 0.25 km3 and 0.54 km3, which is in good agreement with previous studies

Populisms in the Americas

Over the last decade populism has emerged as a frequently key notion when analyzing political and social evolutions in a large number of European and American countries. Its usage, however, is on the whole negative in that more often than not, it designates a direct appeal to the people based on the resentment of elites and/or other social groups while at the same time advocating a ragbag of so-called “simple” and demagogical measures. It is a notion mainly used to denounce adversaries and th..

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

Flow-to-fracture transition in a volcanic mush plug may govern normal eruptions at Stromboli

Stromboli is a model volcano for studying eruptions driven by degassing. The current paradigm posits that Strombolian eruptions represent the bursting of gas slugs ascending through melt‐filled conduits, but petrological observations show that magma at shallow depth is crystalline enough to form a three‐phase plug consisting of crystals, bubbles, and melt. We combine a 1‐D model of gas flushing a crystalline mush with a 3‐D stress model. Our results suggest that localized gas segregation establishes hot conduits of mobile magma within a stagnant plug. The plug is prone to tensile failure controlled by gas overpressure and tectonic stress, with failure most likely beneath the observed vent locations. We hence argue that Strombolian eruptions are related to plug failure rather than flow. Our proposed three‐phase model of the shallow plumbing system may provide a promising framework for integrating geophysical, petrological, and morphological observations at Stromboli and in open‐system volcanism more generally

Populismes dans les Amériques

Au cours des dix dernières années, le populisme s’est affirmé comme une notion souvent clef dans l’analyse de l’évolution politique et sociale d’un grand nombre de pays européens ou américains. Son usage reste cependant très largement négatif car il désigne le plus souvent un appel direct au peuple basé sur le ressentiment contre les élites et/ou certains groupes sociaux, tout en prônant un arsenal de mesures « simples » et démagogiques. Il est d’ailleurs principalement utilisé pour dénoncer ..