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Kelfoun K.,CNRS Magmas and Volcanoes Laboratory
Journal of Geophysical Research: Solid Earth

The rheology of volcanic rock avalanches and dense pyroclastic flows is complex, and it is difficult at present to constrain the physics of their processes. The problem lies in defining the most suitable parameters for simulating the behavior of these natural flows. Existing models are often based on the Coulomb rheology, sometimes with a velocity-dependent stress (e.g., Voellmy), but other laws have also been used. Here I explore the characteristics of flows, and their deposits, obtained on simplified topographies by varying source conditions and rheology. The Coulomb rheology, irrespective of whether there is a velocity-dependent stress, forms cone-shaped deposits that do not resemble those of natural long-runout events. A purely viscous or a purely turbulent flow can achieve realistic velocities and thicknesses but cannot form a deposit on slopes. The plastic rheology, with (e.g., Bingham) or without a velocity-dependent stress, is more suitable for the simulation of dense pyroclastic flows and long-runout volcanic avalanches. With this rheology, numerical flows form by pulses, which are often observed during natural flow emplacement. The flows exhibit realistic velocities and deposits of realistic thicknesses. The plastic rheology is also able to generate the frontal lobes and lateral levées which are commonly observed in the field. With the plastic rheology, levée formation occurs at the flow front due to a divergence of the driving stresses at the edges. Once formed, the levées then channel the remaining flow mass. The results should help future modelers of volcanic flows with their choice of which mechanical law corresponds best to the event they are studying. Copyright 2011 by the American Geophysical Union. Source

Merle O.,CNRS Magmas and Volcanoes Laboratory

A continental rift is conventionally described as a thinning process of the lithosphere ultimately leading to the rupture of the continent and the formation of a mid-oceanic ridge. Rifting is the initial and fundamental process by which the separation of two continents into two tectonic plates takes place. Previous classifications, particularly the one into "active" and "passive" rifting, are briefly presented, together with their limitations. The new classification presented here links continental rifts to the major plate tectonics structures which are at the origin of their formation. Thus, four types of rift can be defined: the subduction-related rift, the plume-related rift, the mountain-related rift and the transform-related rift. A number of examples representative of these four types of rift are then presented. This classification is shown to lie at the heart of our understanding of the major plate tectonic processes at work on Earth. © 2011 Elsevier B.V. Source

Roche O.,CNRS Magmas and Volcanoes Laboratory
Bulletin of Volcanology

The depositional processes and gas pore pressure in pyroclastic flows are investigated through scaled experiments on transient, initially fluidized granular flows. The flow structure consists of a sliding head whose basal velocity decreases backwards from the front velocity (U f) until onset of deposition occurs, which marks transition to the flow body where the basal deposit grows continuously. The flows propagate in a fluid-inertial regime despite formation of the deposit. Their head generates underpressure proportional to U f 2 whereas their body generates overpressure whose values suggest that pore pressure diffuses during emplacement. Complementary experiments on defluidizing static columns prove that the concept of pore pressure diffusion is relevant for gas-particle mixtures and allow characterization of the diffusion timescale (t d) as a function of the material properties. Initial material expansion increases the diffusion time compared with the nonexpanded state, suggesting that pore pressure is self-generated during compaction. Application to pyroclastic flows gives minimum diffusion timescales of seconds to tens of minutes, depending principally on the flow height and permeability. This study also helps to reconcile the concepts of en masse and progressive deposition of pyroclastic flow units or discrete pulses. Onset of deposition, whose causes deserve further investigation, is the most critical parameter for determining the structure of the deposits. Even if sedimentation is fundamentally continuous, it is proposed that late onset of deposition and rapid aggradation in relatively thin flows can generate deposits that are almost snapshots of the flow structure. In this context, deposition can be considered as occurring en masse, though not strictly instantaneously. © 2012 Springer-Verlag. Source

Ferot A.,CNRS Magmas and Volcanoes Laboratory | Bolfan-Casanova N.,CNRS Magmas and Volcanoes Laboratory
Earth and Planetary Science Letters

Experiments were performed under water-saturated conditions in the MFSH (MgO-FeO-SiO 2-H 2O) and MFASH (MgO-FeO-Al 2O 3-SiO 2-H 2O) systems at 2.5, 5, 7.5 and 9GPa, at temperatures from 1175 to 1400°C and H 2O initial abundance of 0.5-5wt%. One experiment was performed at 13.5GPa at a temperature of 1400°C in the MFSH system. Water contents were analyzed by Fourier transform infrared spectroscopy. Results show that Al contents in olivine and pyroxene in equilibrium with an aluminous phase decrease significantly with increasing pressure and decreasing temperature. The incorporation of Al enhances water incorporation in olivine and pyroxene, but only at pressures of 2.5 and 5GPa. At 7.5GPa (i.e. 225km depth) the pyroxene is monoclinic, indicating that in a hydrous mantle the orthoenstatite to clinoenstatite phase transition occurs at shallower depths than previously thought, which is more consistent with the Lehmann discontinuity than with the X discontinuity. The partitioning of water between pyroxene and olivine in the MFASH system decreases from a value of 2 at 2.5GPa (80km depth) to 0.9 at 9GPa (270km depth). At 13.5GPa and 1400°C, the water content of olivine is 1700±300ppmwt H 2O. The water partition coefficient between coexisting wadsleyite and olivine equals 4.7±0.7. We conclude that the water storage capacity of the upper mantle just above the 410km discontinuity is of 1500±300ppmwt H 2O. If we assume that the Low Velocity Layer observed near 350km is due to mantle melting, we can constrain the water content of the mantle at that depth to be ~850±150ppmwt H 2O. This new value is four times higher than previous estimates for the mantle source of Mid Oceanic Ridge Basalts.Finally, comparison of the depth ranges of the L and X seismic discontinuities and the water storage capacity of the upper mantle suggests that the L-discontinuity (180-240. km) is concomitant with a kink in the water storage due to the orthorhombic to monoclinic phase transition in enstatite, while the X-discontinuity (240-340. km) coincides with a kink in the water storage capacity due to dehydration of garnet. © 2012 Elsevier B.V. Source

The late-seventeenth century BC Minoan eruption of Santorini discharged 30-60 km3 of magma, and caldera collapse deepened and widened the existing 22 ka caldera. A study of juvenile, cognate, and accidental components in the eruption products provides new constraints on vent development during the five eruptive phases, and on the processes that initiated the eruption. The eruption began with subplinian (phase 0) and plinian (phase 1) phases from a vent on a NE-SW fault line that bisects the volcanic field. During phase 1, the magma fragmentation level dropped from the surface to the level of subvolcanic basement and magmatic intrusions. The fragmentation level shallowed again, and the vent migrated northwards (during phase 2) into the flooded 22 ka caldera. The eruption then became strongly phreatomagmatic and discharged low-temperature ignimbrite containing abundant fragments of post-22 ka, pre-Minoan intracaldera lavas (phase 3). Phase 4 discharged hot, fluidized pyroclastic flows from subaerial vents and constructed three main ignimbrite fans (northwestern, eastern, and southern) around the volcano. The first phase-4 flows were discharged from a vent, or vents, in the northern half of the volcanic field, and laid down lithic-block-rich ignimbrite and lag breccias across much of the NW fan. About a tenth of the lithic debris in these flows was subvolcanic basement. New subaerial vents then opened up, probably across much of the volcanic field, and finer-grained ignimbrite was discharged to form the E and S fans. If major caldera collapse took place during the eruption, it probably occurred during phase 4. Three juvenile components were discharged during the eruption-a volumetrically dominant rhyodacitic pumice and two andesitic components: microphenocryst-rich andesitic pumices and quenched andesitic enclaves. The microphenocryst-rich pumices form a textural, mineralogical, chemical, and thermal continuum with co-erupted hornblende diorite nodules, and together they are interpreted as the contents of a small, variably crystallized intrusion that was fragmented and discharged during the eruption, mostly during phases 0 and 1. The microphenocryst-rich pumices, hornblende diorite, andesitic enclaves, and fragments of pre-Minoan intracaldera andesitic lava together form a chemically distinct suite of Ba-rich, Zr-poor andesites that is unique in the products of Santorini since 530 ka. Once the Minoan magma reservoir was primed for eruption by recharge-generated pressurization, the rhyodacite moved upwards by exploiting the plane of weakness offered by the pre-existing andesite-diorite intrusion, dragging some of the crystal-rich contents of the intrusion with it. © 2014 Springer-Verlag Berlin Heidelberg. Source

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