Patrick Bachèlery

The off-shore continuation of an active basaltic volcano: Piton de la Fournaise (Réunion Island, Indian Ocean); structural and geomorphological interpretation from sea beam mapping

Abstract In 1984 a Sea Beam survey of the submarine east flank of Piton de la Fournaise was performed with R/V “Jean Charcot”. Three main types of volcanic or volcano-tectonic features have been identified: 1. (1) The subaerial NE and SE volcanic rift zones of Piton de la Fournaise do not extend more than about five kilometers offshore. Unlike typical Hawaiian rift zones, which form narrow (2–4 km) ridges extending tens of kilometers from the summit, the active rift zones of Piton de la Fournaise widen downslope, attaining more than 10 km at their front. 2. (2) The submarine extension of the Grand Brule slide is larger than the subaerial portion. The entire slide forms a 7 × 24 km scar bounded by two ramparts to the north and south. The slumped material may have moved as a debris flow, forming a large talus downslope of the slide. However, the submarine counterpart of the south area of the Grand Brule seems to be composed by a slumped block whose structure is apparently not disturbed. 3. (3) Another prominent feature is a conspicuous topographic high that occupies nearly all the center of the surveyed zone. This “cast flank submarine plateau” cannot be associated with any active structure of Piton de la Fournaise. Its surface generally dips gently (2–3°), and its northern and southern flanks are extensively cut by landslides. Cones of variable dimensions are observed on the plateau and farther to the east. Three hypotheses are examined to account for the origin of this morphology: (a) remnant flank of an ancestral Fournaise volcano associated with a large buried intrusion found by drilling beneath the Grand Brule; (b) distinct volcanic massif or (c) material of a huge and ancient landslide. The geophysical data show that the western part of this submarine plateau is reversely magnetized and associated with a moderate positive gravity anomaly. This confirms that, as suggested by the bathymetric analysis, this part of the plateau is relatively coherent. Conversely, the eastern portion of the plateau appears to be poorly magnetized and composed of low-density material, probably chaotic materials derived from landslides. The reversed magnetization of the western part shows that the whole structure is older than 0.7 Ma. These results show that the volcanic history of Piton de la Fournaise is more complex than previously thought.

April 2007 collapse of Piton de la Fournaise: A new example of caldera formation

Collapse calderas are frequent in the evolution of volcanic systems, but very few have formed during historical times. Piton de la Fournaise is one of the worlds most active basaltic shield volcanoes. The caldera collapse, which occurred during the April 2007 lateral eruption is one of the few large documented collapse events on this volcano. It helps to understand the mode and origin of caldera collapses in basaltic volcanoes. Field observations, GPS and seismic data show that the collapse occurred at an early stage of the eruption. The cyclic seismic signal suggests a step by step collapse that directly influenced the lateral eruption rate. Likely, the caldera results from the combined effect of (i) the progressive collapse of the plumbing system above the magma chamber since 2000, and (ii) the large amount of magma withdrawal during the early stage of the eruption by both a significant intrusion within the edifice and an important emission rate.

Constraints on magma transfers and structures involved in the 2003 activity at Piton de La Fournaise from displacement data

Piton de La Fournaise (Reunion Island) had shown 8 months of high activity during 2003, including four eruptions and one intrusion. The monitoring networks, provided by the Volcanological Observatory of Piton de La Fournaise, show two different kinds of behavior: strong and rapid deformations linked with the magma injections and a slight continuous intereruptive inflation of the summit area. Inversed modeling based on the deformation data, using a three-dimensional elastostatic boundary element code, combined with seismicity provides new insights into the volcanic feeding system involved in the 2003 activity. (1) The 2003 eruptions are fed from a pressurized magma chamber located beneath the summit at 300 m above sea level. (2) A vertical path was activated between the magma chamber and the surface during the December 2002 collapse, allowing the magma to rise into it and leading to the initial summit eruption in May 2003. (3) The August and September 2003 lateral eruptions had been fed by dikes intruding along the same vertical pathway, at an estimated velocity of 1.1 to 1.3 m/s before intruding the flank into the rift zones at a velocity of 0.2 to 0.8 m/s. This last point suggests a strong structural control on the dike pathway.

Dynamic dyke propagation deduced from tilt variations preceding the March 9, 1998, eruption of the Piton de la Fournaise volcano

During the hour preceding the March 9, 1998, eruption of the Piton de la Fournaise volcano, the deformation network of the Observatory recorded some large deformations in the summit area. A broadband seismic station of the GEOSCOPE global network, RER, is located about 8 km away from the summit of the volcano. Signals from that station may be interpreted as tilt changes. The combination of the above two kinds of signals allows, by using a tensile fault model, to constrain the geometry as well as some characteristics (volume, propagation velocity) of the dyke that intruded the summit area during the hour preceding the beginning of eruptive activity.

The beginning of the 1985–1987 eruptive cycle at Piton de la Fournaise (La Reunion); new insights in the magmatic and volcano-tectonic systems

Abstract The short eruption of June 14–15 1985 was the beginning of an important cycle of activity of Piton de la Fournaise that lasted more than two years and comprised twenty or so eruptive and intrusive phases. In this article we report and analyze the first events of this cycle. The June 1985 eruption was preceded by inflation and seismicity. A period of aseismic and slow inflation of the summit area was observed in early 1985. Seismic activity began in mid-May, and increased at the end of the month. Simultaneously, the rate of deformation increased. The seismic events clustered at depths ranging from 1 to 2.5 km, beneath the same area as the center of inflation, northwest of the main summit crater (Dolomieu). The shallowness and small lateral extension of the seismicity, and the short wavelength of the deformation showed that the focus of magmatic pressure was shallow and restricted to a small volume. The outbreak was preceded by a less than one hour intrusive crisis characterized by intense seismic activity and by rapid deformation. The intrusion reached the surface along a SW-NE system of fissures in line with the epicentral zone. The detailed analysis of the seismic and deformation patterns during the intrusive sequence, allow us to propose that the near surface path of the intrusion was guided by an existing tensional fissure system, a hypothesis that explains why the eruption did not occur closer to the epicentral zone. The eruption produced about one million cubic meters of transitional aphyric basalt of a composition similar to that of the lavas erupted since 1979. About one month later, an intrusion not followed by an eruption occurred on July 9; it was the first event of this type observed at Piton de la Fournaise since the installation of the Volcanological Observatory in 1980. The seismicity and deformation patterns during this intrusive sequence were similar to those observed before all the outbreaks since 1980. The main seismic zone was beneath the central to south part of Dolomieu crater, but the deformation showed that the intrusion eventually migrated toward the east and stopped at shallow depth, at about 1 km east of Dolomieu. As for the intrusion associated with the June 14 eruption, the path of the July 9 intrusion also seemed to be guided by a preexisting fissure system. As the July 9 summit crisis was ending, a new crisis began on the east flank of the volcano on July 10. It was the first seismic crisis observed in this area. It surpassed the previous summit seismic crises by two orders of magnitude in the number of earthquakes as well as the energy released. Initially, most hypocenters were concentrated at about 3 km ENE of the summit and at depths of 2–5 km beneath sea level. Later, after the climax of the crisis, the hypocenter zone spread over the entire Grandes Pentes area. We suggest that this flank crisis corresponded to the release of compressional stresses accumulated in the central area of the volcano during the repeated intrusions that occurred over the preceding years. The detailed analysis of the events reported in this article brings new insights in the shallow magma reservoir and in the tectonic behaviour of the volcano. The magma reservoir is inferred to be composed of several units rather than a single volume unit. The storage units are distributed over an area that corresponds about to that of Dolomieu crater and at depths between 1 and 2 km or less to at least 2.5–3 km. This reservoir complex has probably not been resupplied since 1977. The asymmetric tectonic behaviour of the volcano, already recognized from surface tectonic features, has been confirmed by the deformation and seismic observations. The contractional stresses accumulated in the central zone to accommodate the inflation of the reservoir and the emplacement of the intrusion, are released by preferential displacement of the free flank (sea-facing flank). A study of this mechanism is important for the recognition of future eruptive and tectonic activity.

Hierarchical organization as a diagnostic approach to volcano mechanics: Validation on Piton de la Fournaise

Self-organized systems are often used to describe natural phenomena where power laws and scale invariant geometry are observed. The Piton de la Fournaise volcano shows power-law behavior in many aspects. These include the temporal distribution of eruptions, the frequency-size distributions of induced earthquakes, dikes, fissures, lava flows and interflow periods, all evidence of self-similarity over a finite scale range. We show that the bounds to scale-invariance can be used to derive geomechanical constraints on both the volcano structure and the volcano mechanics. We ascertain that the present magma bodies are multi-lens reservoirs in a quasi-eruptive condition, i.e. a marginally critical state. The scaling organization of dynamic fluid-induced observables on the volcano, such as fluid induced earthquakes, dikes and surface fissures, appears to be controlled by underlying static hierarchical structure (geology) similar to that proposed for fluid circulations in human physiology. The emergence of saturation lengths for the scalable volcanic observable argues for the finite scalability of complex naturally self-organized critical systems, including volcano dynamics.

Real time monitoring of vertical ground deformations during eruptions at Piton de la Fournaise

Tilt changes related to a summit dike eraplacement at Piton de !a Fournaise volcano have been recorded by a 4 continuously recording electronic tiltmeters network during the April 18, 1990 eruption. The real-time processing of these data at the Observatory allows to calculate during the emplacement the relative position of the inflation center from the tilt vectors supplied by the nearby stations. Two main steps have been identified-an initial subvertical intrusion step, followed by a lateral dike migration from the summit cone. This monitoring allows to calculate a high average intrusion velocity of about 0.2 re. see-1 which can related to the highly fracturated pattern of the summit area.

Eruptive mechanism on Piton de la Fournaise volcano associated with the December 4, 1983, and January 18, 1984 eruptions from ground deformation monitoring and photogrammetric surveys

Abstract Two photogrammetric aerial surveys were carried out over the summit area of the basaltic shield volcano Piton de la Fournaise (Indian Ocean), one survey in 1981 and the second one in 1984. During this time, only two eruptions occurred; both the December 4, 1983, and the January 18, 1984, eruptions opened fissures on the southwestern part of the summit. Because of the slight and erratic ground deformation measured on the dry-tilt network, by continuous tilt stations, and within the geodetic network between 1981 and November 1983, and between February 1984 and June 1984, ground deformations revealed by the two photogrammetric surveys can be essentially associated with the two eruptions. Large displacement vectors were obtained, up to 40 cm. Horizontal displacement vectors indicate a northeastward ground deformation of the southwestern part of the cone where the effusive vents opened. A more diffuse uplift along the main fracture zones that cut the volcanic edifice was observed, while east of these fractures only slight—less than 10 cm—and opposite displacements were observed. This displacement field can be associated with the main geological structures of the dome of Piton de la Fournaise volcano. Some consequences of the observed displacement field may be outlined for the volcanic observational ground deformation networks. To estimate the displacement field revealed by the photogrammetric surveys, a simple model of dyke intrusion has been computed. This model is based on dislocations and takes into account the main fracture zones. Good agreement is observed between computed and observed data in the area of the effusive vents. Some disagreement remains in the northwestern part of the survey area, where horizontal deformations are small and erratic, and also in the northern part, where an uplift was observed that can be associated with the northern active fracture zones.

The December 4, 1983 to February 18, 1984 eruption of Piton de la Fournaise (La Reunion, Indian Ocean): Description and interpretation

Abstract On December 4, 1983 an eruption started at vents located 1.5 km southwest of the summit of Piton de la Fournaise at the base of the central cone. After 31 months of quiescence this was one of the longest repose period in the last fifty years. The eruption had two phases: December 4 to January 18 and January 18 to February 18. Phase 1 produced about 8 × 10 6 m 3 of lava and Phase II about 9 × 10 6 m 3 . The erupted lava is an aphyric basalt whose mineralogical and geochemical composition is close to that of other lavas emitted since 1977. The precursors of the December 4 outbreak were limited to two-week shallow (1.5–3 km) seismic crisis of fewer than 50 events. No long-term increase was noted in the local seismicity which is very quiet during repose periods and no long-term ground inflation preceded the eruption. Outbreaks of Phases I and II were preceded by short (2.5 hours and 1.5 hours) seismic swarms corresponding to the rise of magma toward the surface from a shallow reservoir. Large ground deformation explained by the emplacement of the shallow intrusions, was recorded during the seismic swarms. A summit inflation was observed in early January, before the phase II outbreak, while the phase I eruption was still continuing. Piton de la Fournaise volcanological observatory was installed in 1980. Seismic and ground deformation data now available for a period of 4 years including the 1981 and the 1983–1984 eruptions, allow us to describe the physical behavior of the volcano during this period. These observations lead us to propose that the magma transfer from deep levels to the shallow magma reservoir is not a continuous process but a periodic one and that the shallow magma reservoir was not resupplied before the 1981 and 1983–1984 eruptions. Considerations on the eruptive history and the composition of recent lavas indicate that the reservoir was refilled in 1977.

A new model for the evolution of La Réunion volcanic complex from complete marine geophysical surveys

Results from recent marine geophysical surveys offer a new perspective for characterizing the evolution processes of volcanic islands. In 2006, cruises FOREVER and ERODER 1 investigated the submarine flanks and the surrounding abyssal plain of La Reunion (Indian Ocean) to obtain for the first time a complete geophysical survey of the area. Combined analyses of these data reveal major differences in the evolution of the two emerged volcanoes, Piton des Neiges and Piton de la Fournaise. We show that debris avalanche deposits extend on the abyssal plain only offshore the active Piton de la Fournaise volcano attesting the occurrence of large flank-collapse events. The absence of such deposits offshore Piton des Neiges and the presence of compressive structures within the sedimentary unit below the edifice support a mechanism of slow deformation of this volcano, such as sliding or spreading. The slow deformation of Piton des Neiges has led to numerous secondary submarine slope instabilities and favored some unconfined turbidity flows which generated large sediment waves running downward all around the island. This study proposes a new model using the most complete marine data set available: slow deformation controls the evolution of Piton des Neiges whereas Piton de la Fournaise (formed on the flanks of a pre-existing edifice) experienced catastrophic, large flank-collapse events.