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Olivier Merle - One of the best experts on this subject based on the ideXlab platform.
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Anatomy of Piton de la Fournaise volcano (La Réunion, Indian Ocean)
Bulletin of Volcanology, 2012Co-Authors: Jean-françois Lénat, Patrick Bachèlery, Olivier MerleAbstract:The aim of this work is to propose a general model of Piton de la Fournaise volcano using information from geological and geophysical studies. Firstly, we make a graphical compilation of all available geophysical information along a W-E profile. Secondly, we construct a geological section that integrates both the geophysical information and the geological information. The lithosphere beneath Piton de la Fournaise is not significantly flexed, and the crust is underlain by an underplating body, which might represent the deep magma reservoir for La Réunion volcanism. Piton de la Fournaise is a relatively thin volcano lying on a huge volcanic construction attributed mostly to Les Alizés volcano. Indeed, if the differentiated rocks observed at the bottom of the Rivière des Remparts are the top of Les Alizés volcano, the interface with Piton de La Fournaise may be located at about sea level beneath the summit area. The endogenous constructions (intrusive complexes) related to Les Alizés and Piton de la Fournaise volcanoes represent a large volume. The huge intrusive complex of Les Alizés volcano probably rests on the top of the oceanic crust and appears to have a buttressing effect for the present eastern volcano-tectonic activity of Piton de la Fournaise. The early Piton de la Fournaise edifice was built around a focus located beneath the Plaine des Sables area. The center subsequently moved 5-6 km eastward to its current location. The dense, high-velocity body beneath the Plaines des Sables and the western part of the Enclos probably corresponds to the hypovolcanic intrusive complex that developed before the volcanic center shifted to its present-day position. Magma reservoirs may have existed, and may still exist, as illustrated by the March 1998 crisis, at the mechanical and density interface between the oceanic crust and the Les Alizés edifice. Strong evidence also exists for the presence of a shallower magma reservoir located near sea level beneath the summit. The March 1998 pre-eruptive seismic pattern (location and upward migration) seems to be evidence for a transfer of magma between the two reservoirs. The dominant structural feature of the central zone is a Collapse Structure beneath the summit craters, above the inferred magma reservoir near sea level. The Collapsed column constitutes a major mechanical heterogeneity and concentrates most of the seismic, intrusive, and hydrothermal activity because of its higher permeability and weaker mechanical strength.
Jean-françois Lénat - One of the best experts on this subject based on the ideXlab platform.
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Anatomy of Piton de la Fournaise volcano (La Réunion, Indian Ocean)
Bulletin of Volcanology, 2012Co-Authors: Jean-françois Lénat, Patrick Bachèlery, Olivier MerleAbstract:The aim of this work is to propose a general model of Piton de la Fournaise volcano using information from geological and geophysical studies. Firstly, we make a graphical compilation of all available geophysical information along a W-E profile. Secondly, we construct a geological section that integrates both the geophysical information and the geological information. The lithosphere beneath Piton de la Fournaise is not significantly flexed, and the crust is underlain by an underplating body, which might represent the deep magma reservoir for La Réunion volcanism. Piton de la Fournaise is a relatively thin volcano lying on a huge volcanic construction attributed mostly to Les Alizés volcano. Indeed, if the differentiated rocks observed at the bottom of the Rivière des Remparts are the top of Les Alizés volcano, the interface with Piton de La Fournaise may be located at about sea level beneath the summit area. The endogenous constructions (intrusive complexes) related to Les Alizés and Piton de la Fournaise volcanoes represent a large volume. The huge intrusive complex of Les Alizés volcano probably rests on the top of the oceanic crust and appears to have a buttressing effect for the present eastern volcano-tectonic activity of Piton de la Fournaise. The early Piton de la Fournaise edifice was built around a focus located beneath the Plaine des Sables area. The center subsequently moved 5-6 km eastward to its current location. The dense, high-velocity body beneath the Plaines des Sables and the western part of the Enclos probably corresponds to the hypovolcanic intrusive complex that developed before the volcanic center shifted to its present-day position. Magma reservoirs may have existed, and may still exist, as illustrated by the March 1998 crisis, at the mechanical and density interface between the oceanic crust and the Les Alizés edifice. Strong evidence also exists for the presence of a shallower magma reservoir located near sea level beneath the summit. The March 1998 pre-eruptive seismic pattern (location and upward migration) seems to be evidence for a transfer of magma between the two reservoirs. The dominant structural feature of the central zone is a Collapse Structure beneath the summit craters, above the inferred magma reservoir near sea level. The Collapsed column constitutes a major mechanical heterogeneity and concentrates most of the seismic, intrusive, and hydrothermal activity because of its higher permeability and weaker mechanical strength.
Simon Day - One of the best experts on this subject based on the ideXlab platform.
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Recent structural evolution of the Cumbre Vieja volcano, La Palma, Canary Islands: Volcanic rift zone reconfiguration as a precursor to volcano flank instability?
Journal of Volcanology and Geothermal Research, 1999Co-Authors: Simon Day, Herve Guillou, Juan Carlos Carracedo, P. GravestockAbstract:Abstract The Cumbre Vieja volcano is the youngest component of the island of La Palma. It is a very steep-sided oceanic island volcano, of a type which may undergo large-scale lateral Collapse with little precursory deformation. Reconfiguration of the volcanic rift zones and underlying dyke swarms of the volcano is used to determine the present degree of instability of the volcano. For most of its history, from before 125 ka ago to around 20 ka, the Cumbre Vieja volcano was characterised by a triple (“Mercedes Star”) volcanic rift zone geometry. The three rift zones were unequally developed, with a highly productive south rift zone and weaker NE and NW rift zones: the disparity in activity was probably due to topographic-gravitational stresses associated with the west facing Cumbre Nueva Collapse Structure underneath the western flank of the Cumbre Vieja. From 20 ka to about 7 ka, activity on the NW volcanic rift zone diminished and the intersection of the rift zones migrated slightly to the north. More recently, the triple rift geometry has been replaced at the surface by a N–S-trending rift zone which transects the volcano, and by E–W-trending en echelon fissure arrays on the western flank of the volcano. The NE rift zone has become completely inactive. This structural reconfiguration indicates weakening of the western flank of the volcano. The most recent eruption near the summit of the Cumbre Vieja, that of 1949, was accompanied by development of a west facing normal fault system along the crest of the volcano. The geometry of this fault system and the timing of its formation in relation to episodes of vent opening during the eruption indicate that it is not the surface expression of a dyke. Instead, it is interpreted as being the first surface rupture along a developing zone of deformation and seaward movement within the western flank of the Cumbre Vieja: the volcano is therefore considered to be at an incipient stage of flank instability. Climatic factors or strain weakening along the Cumbre Nueva Collapse Structure may account for the recent development of this instability.
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A past giant lateral Collapse and present-day flank instability of Fogo, Cape Verde Islands
Journal of Volcanology and Geothermal Research, 1999Co-Authors: Simon Day, S.i.n. Heleno Da Silva, Joao F. B. D. FonsecaAbstract:Fogo island is a large and extremely steepsided oceanic island volcano in the Cape Verde archipelago. It has a large (ca. 9 km across) east facing summit Collapse Structure, the Monte Amarelo Collapse, with a probable volume of at least 150–200 km3. For most of its history the Monte Amarelo volcano had a small but productive central vent complex and radial rift zones fed by laterally propagating dykes. Shortly before the Collapse the latter were replaced by north–south-trending arrays of en echelon, vertically propagating dykes. Since the Monte Amarelo Collapse the scar has partly filled with a new volcano, the Cha das Caldeiras volcano. The summit cone of this volcano, the Pico do Fogo, is a very young feature but has been abandoned in the most recent phase of activity, from the 18th century onwards. The same period has also seen the abandonment of earlier radial rift zones with laterally propagating dykes and their replacement with en echelon arrays of vents fed by vertically propagating dykes. These form an N–S-trending array within the older Collapse Structure and are associated with seaward displacement of the eastern flank of the volcano within the old Collapse Structure. The most recent eruptions, those of 1951 and 1995, appear to be associated with episodes of flank instability manifested in N–S surface fissuring and east facing normal faults. These recent structural changes in the volcano parallel those which took place in the Monte Amarelo volcano prior to its Collapse.
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Age and geometry of an aborted rift flank Collapse: the San Andres fault system, El Hierro, Canary Islands
Geological Magazine, 1997Co-Authors: Simon Day, Juan Carlos Carracedo, Herve GuillouAbstract:The catastrophic slope failures and landslides that occur in the final stages of lateral Collapses of volcanoes destroy much of the evidence for precursory deformation and the early stages of the Collapses concerned. Aborted or incomplete Collapse Structures, although rare, are rich sources of information on these stages of development of catastrophic Collapses. The San Andres fault system, on the volcanic island of El Hierro, is a relatively young (between about 545 and about 261–176 ka old) but inactive lateral Collapse Structure. It appears to represent an aborted giant landslide. It is developed along the flank of a steep-sided volcanic rift zone, and is bounded by a discrete strike-slip fault zone at the up-rift end, closest to the centre of the island. This geometry differs markedly from that of Collapse Structures on stratovolcanoes but bears some similarities to that of active fault systems on Hawaii. Although the fault system has undergone little erosion, cataclasites which formed close to the palaeosurface are well exposed. These cataclasites are amongst the first fault rocks to be described from volcano lateral Collapse Structures and include the only pseudotachylytes to have been identified in such Structures to date. Their development at unusually shallow depths is attributed to large movements on the fault in a single event, the inferred aborted landslide, and a lack of pressurized pore water. The absence of pressurized fluids in the slumping block may have caused the San Andres fault system to cease moving, rather than develop into a giant volcanic landslide. The recognition that the San Andres fault system is inactive greatly reduces the estimated volcanic hazard associated with El Hierro. However, the lack of evidence for precursory deformation prior to the aborted landslide event is disturbing as it implies that giant lateral Collapses can occur on steep-sided oceanic islands with little warning.
Patrick Bachèlery - One of the best experts on this subject based on the ideXlab platform.
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Anatomy of Piton de la Fournaise volcano (La Réunion, Indian Ocean)
Bulletin of Volcanology, 2012Co-Authors: Jean-françois Lénat, Patrick Bachèlery, Olivier MerleAbstract:The aim of this work is to propose a general model of Piton de la Fournaise volcano using information from geological and geophysical studies. Firstly, we make a graphical compilation of all available geophysical information along a W-E profile. Secondly, we construct a geological section that integrates both the geophysical information and the geological information. The lithosphere beneath Piton de la Fournaise is not significantly flexed, and the crust is underlain by an underplating body, which might represent the deep magma reservoir for La Réunion volcanism. Piton de la Fournaise is a relatively thin volcano lying on a huge volcanic construction attributed mostly to Les Alizés volcano. Indeed, if the differentiated rocks observed at the bottom of the Rivière des Remparts are the top of Les Alizés volcano, the interface with Piton de La Fournaise may be located at about sea level beneath the summit area. The endogenous constructions (intrusive complexes) related to Les Alizés and Piton de la Fournaise volcanoes represent a large volume. The huge intrusive complex of Les Alizés volcano probably rests on the top of the oceanic crust and appears to have a buttressing effect for the present eastern volcano-tectonic activity of Piton de la Fournaise. The early Piton de la Fournaise edifice was built around a focus located beneath the Plaine des Sables area. The center subsequently moved 5-6 km eastward to its current location. The dense, high-velocity body beneath the Plaines des Sables and the western part of the Enclos probably corresponds to the hypovolcanic intrusive complex that developed before the volcanic center shifted to its present-day position. Magma reservoirs may have existed, and may still exist, as illustrated by the March 1998 crisis, at the mechanical and density interface between the oceanic crust and the Les Alizés edifice. Strong evidence also exists for the presence of a shallower magma reservoir located near sea level beneath the summit. The March 1998 pre-eruptive seismic pattern (location and upward migration) seems to be evidence for a transfer of magma between the two reservoirs. The dominant structural feature of the central zone is a Collapse Structure beneath the summit craters, above the inferred magma reservoir near sea level. The Collapsed column constitutes a major mechanical heterogeneity and concentrates most of the seismic, intrusive, and hydrothermal activity because of its higher permeability and weaker mechanical strength.
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Pits, rifts and slumps: the summit Structure of Piton de la Fournaise
Bulletin of Volcanology Bull Volcanic Eruptions, 2007Co-Authors: Adam Carter, Benjamin Van Wyk De Vries, Karim Kelfoun, Patrick Bachèlery, Pierre BrioleAbstract:Abstract A clear model of Structures and associated stress fields of a volcano can provide a framework in which to study and monitor activity. We propose a volcano-tectonic model for the dynamics of the summit of Piton de la Fournaise (La Reunion Island, Indian Ocean). The summit contains two main pit crater Structures (Dolomieu and Bory), two active rift zones, and a slumping eastern sector, all of which contribute to the actual fracture system. Dolomieu has developed over 100 years by sudden large Collapse events and subsequent smaller drops that include terrace formation. Small intra-pit Collapse scars and eruptive fissures are located along the southern floor of Dolomieu. The western pit wall of Dolomieu has a superficial inward dipping normal fault boundary connected to a deeper ring fault system. Outside Dolomieu, an oval extension zone containing sub-parallel pit-related fractures extends to a maximum distance of 225 m from the pit. At the summit the main trend for eruptive fissures is N80°, normal to the north-south rift zone. The terraced Structure of Dolomieu has been reproduced by analogue models with a roof to width ratio of approximately 1, suggesting an original magma chamber depth of about 1 km. Such a chamber may continue to act as a storage location today. The east flank has a convex-concave profile and is bounded by strike-slip fractures that define a gravity slump. This zone is bound to the north by strike-slip fractures that may delineate a shear zone. The southern reciprocal shear zone is probably marked by an alignment of large scoria cones and is hidden by recent aa lavas. The slump head intersects Dolomieu pit and may slide on a hydrothermally altered layer known to be located at a depth of around 300 m. Our model has the summit activity controlled by the pit crater Collapse Structure, not the rifts. The rifts become important on the mid-flanks of the cone, away from pit-related fractures. On the east flank the superficial Structures are controlled by the slump. We suggest that during pit subsidence intra-pit eruptions may occur. During tumescence, however, the pit system may become blocked and a flank eruption is more likely. Intrusions along the rift may cause deformation that subsequently increases the slump's potential to deform. Conversely, slumping may influence the east flank stress distribution and locally control intrusion direction. These predictions can be tested with monitoring data to validate the model and, eventually, improve monitoring.
M Ugliano - One of the best experts on this subject based on the ideXlab platform.
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a two parameter criterion for classifying the explodability of massive stars by the neutrino driven mechanism
The Astrophysical Journal, 2016Co-Authors: T Ertl, Th H Janka, S E Woosley, Tuguldur Sukhbold, M UglianoAbstract:Thus far, judging the fate of a massive star (either a neutron star (NS) or a black hole) solely by its Structure prior to core Collapse has been ambiguous. Our work and previous attempts find a non-monotonic variation of successful and failed supernovae with zero-age main-sequence mass, for which no single structural parameter can serve as a good predictive measure. However, we identify two parameters computed from the pre-Collapse Structure of the progenitor, which in combination allow for a clear separation of exploding and non-exploding cases with only few exceptions ( 1‐2.5%) in our set of 621 investigated stellar models. One parameter is M4, defining the enclosed mass for a dimensionless entropy per nucleon of s = 4, and the other is 4 dm=drjs=4, being the mass-derivative at this location. The two parameters 4 and M4 4 can be directly linked to the mass-infall rate, ˙ M, of the collapsing star and the electron-type neutrino luminosity of the accreting proto-NS, L e / Mns ˙ M, which play a crucial role in the “critical luminosity” concept for the theoretical description of neutrino-driven explosions as runaway phenomenon of the stalled accretion shock. All models were evolved employing the approach of Ugliano et al. for simulating neutrino-driven explosions in spherical symmetry. The neutrino emission of the accretion layer is approximated by a gray transport solver, while the uncertain neutrino emission of the 1.1 M proto-NS core is parametrized by an analytic model. The free parameters connected to the core-boundary prescription are calibrated to reproduce the observables of Supernova 1987A for five di erent progenitor models. Subject headings: supernovae: general — stars: massive — hydrodynamics — neutrinos
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a two parameter criterion for classifying the explodability of massive stars by the neutrino driven mechanism
arXiv: Solar and Stellar Astrophysics, 2015Co-Authors: T Ertl, Th H Janka, S E Woosley, Tuguldur Sukhbold, M UglianoAbstract:Thus far, judging the fate of a massive star (either a neutron star (NS) or a black hole) solely by its Structure prior to core Collapse has been ambiguous. Our work and previous attempts find a non-monotonic variation of successful and failed supernovae with zero-age main-sequence mass, for which no single structural parameter can serve as a good predictive measure. However, we identify two parameters computed from the pre-Collapse Structure of the progenitor, which in combination allow for a clear separation of exploding and non-exploding cases with only few exceptions (~1-2.5%) in our set of 621 investigated stellar models. One parameter is M4, defining the normalized enclosed mass for a dimensionless entropy per nucleon of s=4, and the other is mu4 = d(m/M_sun)/d(r/1000 km) at s=4, being the normalized mass-derivative at this location. The two parameters mu4 and M4*mu4 can be directly linked to the mass-infall rate, Mdot, of the collapsing star and the electron-type neutrino luminosity of the accreting proto-NS, L_nue ~ M_ns*Mdot, which play a crucial role in the "critical luminosity" concept for the theoretical description of neutrino-driven explosions as runaway phenomenon of the stalled accretion shock. All models were evolved employing the approach of Ugliano et al. for simulating neutrino-driven explosions in spherical symmetry. The neutrino emission of the accretion layer is approximated by a gray transport solver, while the uncertain neutrino emission of the 1.1 M_sun proto-NS core is parametrized by an analytic model. The free parameters connected to the core-boundary prescription are calibrated to reproduce the observables of Supernova 1987A for five different progenitor models.