Detailed gravity study of the offshore structure of Piton de la Fournaise volcano,Reunion Island

We present an interpretation of gravity data acquired in 1984 by the French R/V Jean Charcot on the submarine part of the eastern flank of Piton de la Fournaise volcano. We comment on the Bouguer anomaly map and give a quantitative interpretation of three gravity profiles. The main results are that a gravity high over Grand Brûlé, the lower subaerial part of the eastern flank, does not extend far offshore and that an anomalous topographic feature, discovered in 1982 on the submarine eastern flank, is characterized by a large negative anomaly. We propose three hypotheses to explain the origin of this anomaly, i. e., it marks the site of a new volcano, or it is a consequence of lateral volcanism from a volcano older than Piton de la Fournaise, or more probably, it represents a great landslide deposit.     

Eruptive history of the Piton de la Fournaise volcano, Reunion Island, Indian Ocean

In order to establish a general chronology of the volcanic evolution and to determine the temporal succession of the structural units, potassium-argon measurements were made on 15 samples selected as a function of their stratigraphical position on Piton de la Fournaise volcano.The rocks of Réunion Island are essentially oceanic and basaltic lavas of two shield volcanoes: the central, now extinct Piton des Neiges and the more recent, still active, Piton de la Fournaise. Piton de la Fournaise volcano is generally thought to have been developed unconformably on the southeastern flank of the Piton des Neiges volcano. Previous studies have shown four successive phases and three calderas in the construction of Piton de la Fournaise.The subaerial basaltic shield-building lavas of Piton de la Fournaise appear to be older than previously thought: at least 530,000 y. old instead of 360,000 years. In terms of their duration and erupted volumes, the four successive phases are not equivalent. The duration of the first two phases is 240,000 years (from 530,000 to 290,000 y. B.P.) and 155,000 years (from about 220,000 to 65,000 y. B.P.). The duration of the third phase is less than 60,000 years and the fourth phase may actually be an episode of the third. The two volcanoes, Piton des Neiges and Piton de la Fournaise, were active simultaneously for at least 500,000 years. The evolution of Réunion Island appears to be consistent with activity along a developing rift. The evolution of Piton de la Fournaise is mainly linked with the structural development of the shield and to large-scale slumpings due to instability of the slope.     

Anatomy of Piton de la Fournaise volcano (La Réunion, Indian Ocean)

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.     

Internal structure of Piton de la Fournaise volcano from seismic wave propagation and earthquake distribution

Arrival times of seismic waves from local earthquakes are inverted for both locating the source and defining the 3-D velocity heterogeneity of Piton de la Fournaise.The lateral heterogeneity of the 2632 m high edifice is resolved as a high-velocity plug, 1.5 km in diameter, surrounded by a low-velocity ring, which may be interpreted as due to the construction of Fournaise on the flank of the older volcano Piton des Neiges. Wave mode conversion detected on three-component seismograms provides evidence for boundaries of contrasted velocities.Pre-eruptive swarm earthquakes cluster in the high-velocity zone, under the Dolomieu summit crater. Low strength and cohesion of the surrounding material account for the lack of seismicity for the final 1–3 km radial flow of magma to the vents in Enclos Fouqué.Beneath the high-velocity plug the existence of a body with low velocity for P, and even for S, waves is well constrained. However, the walls and base are poorly defined because of the lack of deep earthquakes for sampling. The few earthquakes that are located in this depth region usually occur at a depth of around 1.5 km below sea level in the region of the cone. This can be considered providing the upper constraint on the lower limit of the aseismic part of the low-velocity body. The coincidence in time of their occurrence with the swarms above sea level and the eruptions suggests magmatic activation of the low-velocity aseismic volume 1.5 km below sea level under the high-velocity plug of the cone. Further down, the concentration of seismicity in two swarms, between 2 and 4 km, under the eastern flank does not allow the structure to be sampled effectively.     

Geoelectrical structure of the central zone of Piton de la Fournaise volcano (Réunion)

 A study of the geoelectrical structure of the central part of Piton de la Fournaise volcano (Réunion, Indian Ocean) was made using direct current electrical (DC) and transient electromagnetic soundings (TEM). Piton de la Fournaise is a highly active oceanic basaltic shield and has been active for more than half a million years. Joint interpretation of the DC and TEM data allows us to obtain reliable 1D models of the resistivity distribution. The depth of investigation is of the order of 1.5 km but varies with the resistivity pattern encountered at each sounding. Two-dimensional resistivity cross sections were constructed by interpolation between the soundings of the 1D interpreted models. Conductors with resistivities less than 100 ohm-m are present at depth beneath all of the soundings and are located high in the volcanic edifice at elevations between 2000 and 1200 m. The deepest conductor has a resistivity less than 20 ohm-m for soundings located inside the Enclos and less than 60–100 ohm-m for soundings outside the Enclos. From the resistivity distributions, two zones are distinguished: (a) the central zone of the Enclos; and (b) the outer zone beyond the Enclos. Beneath the highly active summit area, the conductor rises to within a few hundred meters of the surface. This bulge coincides with a 2000-mV self-potential anomaly. Low-resistivity zones are inferred to show the presence of a hydrothermal system where alteration by steam and hot water has lowered the resistivity of the rocks. Farther from the summit, but inside the Enclos, the depth to the conductive layers increases to approximately 1 km and is inferred to be a deepening of the hydrothermally altered zone. Outside of the Enclos, the nature of the deep, conductive layers is not established. The observed resistivities suggest the presence of hydrated minerals, which could be found in landslide breccias, in hydrothermally altered zones, or in thick pyroclastic layers. Such formations often create perched water tables. The known occurrence of large eastward-moving landslides in the evolution of Piton de la Fournaise strongly suggests that large volumes of breccias should exist in the interior of the volcano; however, extensive breccia deposits are not observed at the bottom of the deep valleys that incise the volcano to elevations lower than those determined for the top of the conductors. The presence of the center of Piton de la Fournaise beneath the Plaine des Sables area during earlier volcanic stages (ca. 0.5 to 0.150 Ma) may have resulted in broad hydrothermal alteration of this zone. However, this interpretation cannot account for the low resistivities in peripheral zones. It is not presently possible to discriminate between these general interpretations. In addition, the nature of the deep conductors may be different in each zone. Whatever the geologic nature of these conductive layers, their presence indicates a major change of lithology at depth, unexpected for a shield volcano such as Piton de la Fournaise. Received: 3 November 1999 / Accepted: 15 September 1999     

The basalts of Réunion Island,Indian Ocean

Réunion consists of two shield volcanoes, Piton des Neiges (3069 m) and Piton de la Fournaise (2631 m). The former is extinct and deeply eroded, so that its internal structure is clearly displayed. The deepest accessible part of the pile is a strongly zeolitised agglomerate (Cirque Agglomerate) made up mainly of olivine-basalt fragments. This is covered by a thick sequence of oceanite and olivine-basalt flows (Oceanite Series), which in turn is overlain by feldsparphyric basalts and lavas of intermediate composition (Differentiated Series). An intricate plexus of intrusions, ranging in composition from picrite to quartz-syenite, is exposed in the core of the volcano. Piton de la Fournaise is still active, and is producing oceanites and olivine-basalts generally similar in character to the Oceanite Series lavas of Piton des Neiges. New chemical data on the « primitive » basalts of both volcanoes are presented, and a brief comparison is made with the Hawaiian tholeiites. It is concluded that the Réunion « primitive » basalts are best described as transitional between tholeiitic and alkaline.     

Pb,Hf and Nd isotope compositions of the two Réunion volcanoes (Indian Ocean): A tale of two small-scale mantle “blobs”?

Pb, Hf, Nd and Sr isotopes of basaltic lavas from the two Réunion Island volcanoes are reported in order to examine the origin of the sources feeding these volcanoes and to detect possible changes through time. Samples, chosen to cover the whole lifetime of the two volcanoes (from 2 Ma to present), yield a chemically restricted (compared to OIB lavas) but complex distribution. Réunion plume isotopic characteristics have been defined on the basis of the composition of uncontaminated shield-building lavas from the Piton de la Fournaise volcano. The average ?_Nd, ?_Hf, ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb isotope ratios calculated for this component are + 4.4, + 9.1, 0.70411, 18.97, 15.59 and 39.03, respectively. In Pb–Pb isotope space, each volcano defines a distinct linear trend but slight variations are also detected within the various volcanic sequences. The Piton des Neiges volcano yields a distinct and significantly more scattered isotopic distribution than Piton de la Fournaise for both Pb, Hf and Nd isotope tracers. A principal component analysis of the Pb isotope data from Piton de la Fournaise reveals a major contribution of the C and EM-1 components (with a clear Dupal flavor) as main components for the modern Réunion plume. The same components have been identified for Piton des Neiges but with a stronger participation of a depleted mantle component and a weaker EM-1 contribution. The compositional change of the lavas erupted by the Piton des Neiges and Piton de la Fournaise volcanoes is attributed to the impingement of two small-scale blobs of plume material at the base of the Réunion lithosphere. Compared to other hot-spots worldwide, in particular Hawaii and Kerguelen, magmas beneath Réunion are generated from a considerably more homogeneous, compositionally more primitive plume higher in ²⁰⁶Pb. Although shallow-level contamination processes have been locally detected they did not alter significantly the composition of the plume magmas. This is tentatively attributed to mantle dynamics producing small, high-velocity blobs that ascend rapidly through the lithosphere, and to the lack of a well-developed magma chamber at depth in the lithosphere.     

Evolution structurale du volcan actif du Piton de la Fournaise,Ile de la Réunion — Océan indien occidental

This structural study shows that the Piton de la Fournaise volcano was built over four periods separated by 3 calderas. Each stage, dated by K/Ar and CI4 data, and characterized by its own stratigraphy, intrusive system and collapses, is analysed in detail. The stratigraphical study shows lithological and petrological units within some of these stages. The lavas of Piton de la Fournaise are alkaline basalts ranging in composition from picrite to hawaiite. The feeder dikes systems are radial and converging to the volcanic paleocenters of each period. However, the majority of intrusions and surface cones are concentrated along rifts named « Reunion type » because of there wideness. The uplift of magma in these rift zones causes displacement and sumpling of the unsupported seaward flank of the volcano. Collapse structures with variable diameter, formed at different phases of the volcano history. Some are compared to calderas in relation to an intermediate magma chamber, others seem to be due to the bulge and strecht of the massif. The 3 calderas of great size (8–15 km) separating each stage are related to a lower and larger magmatic chamber. This geological study of Fournaise leads us to purpose an evolutive pattern of the volcano based on paleogeographical and paleostructural reconstitutions. The first Fournaise was built over a rift trending N 120 of the old neighbouring volcano of Piton des Neiges. The activity of this rift progressively decreased all through time with the development of a curved intrusive system where most eruptions took place. As in the Hawaiian rifts, the influence of gravitational stresses is invoked to explain the migration of the intrusive zones.     

Geothermal model of the Salazie Cirque (Reunion Island): Volcanic and structural implications

A programme of geothermal exploration on Réunion Island resulted in the siting of an exploration geothermal borehole in Salazie Cirque. The borehole attained a depth of 2108 m and encountered temperatures of 192 ± 8°C; no aquifer was found.The geological information obtained made it possible to propose a stratigraphy for the deep underlying rocks beneath the Salazie Cirque on the Piton des Neiges and thus to interpret the events that followed the emergence of the island. A large gabbroic mass was intersected and found to comprise several successive intrusions, the principal one of which is continuously zoned from melagabbro to monzonite. Modeling of geophysical data on the drillhole results enabled the distribution of the different geological units to be interpreted.A geothermal model is proposed taking into account the dynamic evolution of the hydrothermal processes that preceded and accompanied emplacement of the gabbro responsible for the geothermal anomaly underlying the three cirques that sculpt the flanks of the Piton des Neiges.     

Huge landslide blocks in the growth of piton de la fournaise, La réunion, and Kilauea volcano, Hawaii

Piton de la Fournaise, on the island of La Réunion, and Kilauea volcano, on the island of Hawaii, are active, basaltic shield volcanoes growing on the flanks of much larger shield volcanoes in intraplate tectonic environments. Past studies have shown that the average rate of magma production and the chemistry of lavas are quite similar for both volcanoes. We propose a structural similarity — specifically, that periodic displacement of parts of the shields as huge landslide blocks is a common mode of growth. In each instance, the unstable blocks are within a rift-zone-bounded, unbuttressed flank of the shield. At Kilauea, well-documented landslide blocks form relatively surficial parts of a much larger rift-zone-bounded block; scarps of the Hilina fault system mark the headwalls of the active blocks. At Fournaise, Hilina-like slump blocks are also present along the unbuttressed east coast of the volcano. In addition, however, the existence of a set of faults nested around the present caldera and northeast and southeast rift zones suggests that past chapters in the history of Fournaise included the slumping of entire rift-zone-bounded blocks themselves. These nested faults become younger to the east southeast and apparently record one of the effects of a migration of the focus of volcanism in that direction. Repeated dilation along the present set of northeast and southeast rift zones, most recently exemplified by an eruption in 1977, suggests that the past history of rift-zone-bounded slumping will eventually be repeated. The record provided by the succession of slump blocks on Fournaise is apparently at a relatively detailed part of a migration of magmatic focus that has advanced at least 30 km to the east-southeast from neighboring Piton des Neiges, an extinct Pliocene to Pleistocene volcano.     

Growth and collapse of the Reunion Island volcanoes

This work presents the first exhaustive study of the entire surface of the Reunion Island volcanic system. The focus is on the submarine part, for which a compilation of all multibeam data collected during the last 20 years has been made. Different types of submarine features have been identified: a coastal shelf, debris avalanches and sedimentary deposits, erosion canyons, volcanic constructions near the coast, and seamounts offshore. Criteria have been defined to differentiate the types of surfaces and to establish their relative chronology where possible. Debris avalanche deposits are by far the most extensive and voluminous formations in the submarine domain. They have built four huge Submarine Bulges to the east, north, west, and south of the island. They form fans 20–30 km wide at the coastline and 100–150 km wide at their ends, 70–80 km offshore. They were built gradually by the superimposition and/or juxtaposition of products moved during landslide episodes, involving up to several hundred cubic kilometers of material. About 50 individual events deposits can be recognized at the surface. The landslides have recurrently dismantled Piton des Neiges, Les Alizés, and Piton de La Fournaise volcanoes since 2 Ma. About one third are interpreted as secondary landslides, affecting previously emplaced debris avalanche deposits. On land, landslide deposits are observed in the extensively eroded central area of Piton des Neiges and in its coastal areas. Analysis of the present-day topography and of geology allows us to identify presumed faults and scars of previous large landslides. The Submarine Bulges are dissected and bound by canyons up to 200 m deep and 40 km long, filled with coarse-grained sediments, and generally connected to streams onshore. A large zone of sedimentary accumulation exists to the north–east of the island. It covers a zone 20 km in width, extending up to 15 km offshore. Volcanic constructions are observed near the coast on both Piton des Neiges and Piton de la Fournaise volcanoes and are continuations of subaerial structures. Individual seamounts are present on the submarine flanks and the surrounding ocean floor. A few seem to be young volcanoes, but the majority are probably old, eroded seamounts. This study suggests a larger scale and frequency of mass-wasting events on Reunion Island compared to similar islands. The virtual absence of downward flexure of the lithosphere beneath the island probably contributes to this feature. The increased number of known flank–failure events has to be taken into consideration when assessing hazards from future landslides, in particular, the probability of landslide-generated tsunamis. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Magmatic evolution of the basaltic shield volcanoes of Reunion Island

Chemical data are presented for the basic lavas of the two volcanic shields, Piton des Neiges and Piton de la Fournaise, which comprise Reunion Island. In addition, data for cumulate xenoliths have been used to predict mineral/melt distribution coefficient values for the Reunion magmas.The younger volcanic shield, Piton de la Fournaise, comprises two lava sequences, the >0.5−0.2-m.y. B.P. Primary Shield lavas, and the <0.2-m.y. B.P. Caldera Series lavas. Fractional crystallization models for these lavas indicate that olivine is the major fractionating phase during the evolution from the parental basalt composition to the average basaltic liquid. Only during the evolution of the older, Primary Shield lavas has the common fractionation of an ol + cpx + plag + mt assemblage resulted in the eruption of hawaiitic, ankaramitic and feldspar-phyric lavas. The restriction of the Caldera Series liquids predominantly to olivine fractionation and the extensive cotectic fractionation during the evolution of the Primary Shield sequences is interpreted in terms of the maturity of the volcanic center. The younger stages of evolution involve high magma input into a well-developed feeder and reservoir system, thus maintaining the liquids above a cotectic surface. Whereas, during the evolution of the Primary Shield lavas, lower magma input rates into a less well-developed feeder system increased the probability of the fractionating liquid attaining a cotectic surface. Fractional crystallization accounts for all the chemical variation observed for the Piton de la Fournaise basaltic magmas. The analytical data are closely comparable to the rare earth element (REE) and trace element fractionation curves predicted by least-squares calculations, this supports the use of such models in quantitative evaluation of fractional crystallization.A preliminary survey of Sr isotope values indicates that the oldest (>2 m.y. B.P.) lava sequences of Piton des Neiges may be derived from a source which was isotopically distinct from that of the <2 m.y. B.P. lavas of both volcanic shields. These latter sequences are remarkably consistent in both isotopic and trace element abundance implying a homogeneous source material and an invariable partial melting process. Partial melting calculations indicate that the basaltic lavas have been derived by 5–10% melting of a garnet-poor peridotite (cpx/gt 9). Systematic differences in the light- and heavy-REE patterns between similar basaltic provinces are interpreted to be a result of variation in the nature of the phases buffering the entry of light- and/or heavy-REE into the melt during partial fusion.     

Recurrence of major flank landslides during the last 2-Ma-history of Reunion Island

New detailed swath bathymetry and backscatter data corroborate the existence of four large bulges on the submarine flanks of Reunion Island. These fan-shaped promontories are 20–25 km wide at the coastline and 70–150 km across the seafloor 40–50 km offshore. Their surfaces are characterized by a speckle sonar pattern, indicating the presence of large blocks up to several hundred meters across. Each bulge results from the superposition of multiple landslide deposits whose older ones are dissected and delimited by erosive channels as much as 200 m deep and 20 km long. The submarine flanks of Reunion Island are thus mostly built by accumulation of debris avalanche fans. Morphologic and geologic evidence define large subaerial source areas for these mass-wasting events. In particular, inferred headwalls of most landslides having affected the Piton des Neiges massif generally coincide with the boundaries of its cirques (Mafate, Salazie, and Cilaos), whereas recurrent landslides have resulted in the formation of large concentric amphitheatre structures through the Piton de la Fournaise massif. Thus, about 15 slide events accompanied growth of the Reunion Island shield since 2 Ma.Editorial responsibility: J. Stix     

Calderas,landslides and paleo-canyons on Piton de la Fournaise volcano (La Réunion Island,Indian Ocean)

Based upon a re-interpretation of previous data and a new field campaign, a structural evolution is proposed for the early history of Piton de la Fournaise volcano from 500,000 to 50,000 years. Conceptually, it is shown that the formation of a caldera in which lava flows are contained inside the caldera depression, gives time for erosion to excavate deep canyons on the external slopes of the volcano, for example, the Rivière des Remparts, the Rivière Langevin and the Rivière de l'Est canyons on Piton de la Fournaise volcano. These canyons are infilled when lavas, filling the caldera and overflowing its rim, are able again to flow on the external slopes of the volcano. In the past, this excavating/infilling process has occurred twice following the formation of the Rivière des Remparts and Morne Langevin calderas. The formation of the third caldera, the Plaine des Sables caldera, was followed by the excavation of the current canyons. In addition to this process, two large landslides have been documented in the field. The first, which happened about 300,000 years ago, is apparently the first episode of the break up of Piton de la Fournaise volcano, predating the formation of the four large calderas. The second landslide, which occurred 150,000 years ago and is considered to be less extensive, has carried away the entire southern flank of the Rivière des Remparts caldera.     

Stress, failure and fluid flow deduced from earthquakes accompanying eruptions at Piton de la Fournaise volcano

In the present episode of eruptive activity, evidence from seismicity for sustained magma inflow from depth into the edifice of Piton de la Fournaise is lacking. Pre-eruptive main deformation and shallow seismicity help to identify very small volumes of magma that are in motion beneath the rim of the Dolomieu summit crater, and oriented along the azimuth of the future vents. Small magma pockets may reside in the cone above sea level, or may be expelled repeatedly, due to crystallisation in a small, low-velocity, aseismic region below sea level under the high-velocity central plug of the cone in which pre-eruptive earthquake swarms are located. In cross-section the hypocentres define two steep sheets diverging from the aseismic zone at sea level towards 1.5 km above sea level (or 1 km beneath the 2632 m high cone). However, failure induced by increased pressure in the suggested chamber does not account for the observed focal mechanisms.The occurrence and timing of magma transport are attested by eruption, and seismic activity may be related to magma transport. Focal mechanisms document strike-slip, not normal faulting or tensile failure. Vertical propagation of the edge of a feeder dike may enhance strike-slip motion above the edge, in a region where effective normal stress is decreased by thermally induced groundwater flow. The strike-slip mechanisms could also be caused by a tensile-shear widening of the horizontal section of vertical conduits.Fournaise strike-slip earthquakes occur in two orientations, with P axes orthogonal between them, within a single pre-eruptive event. Earthquakes are distributed in the same volume but mechanisms switch from one to another type systematically with time, indicating a reversal of stress conditions. The orientations of P axes with respect to the epicentral trend suggest that in the later parts of events leading to eruptions, a compression of the medium occurs after a dilation in the first part. The activated zone might respond successively to the arrival and the departure of the magma on its way from the reservoir at depth to the vent, radial to the cone.     

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.     

March 1986 eruptive episodes at Piton de la Fournaise volcano (Reunion Island)

From May 1985 to April 1986 five discrete eruptions have occurred at Piton de la Fournaise volcano. On March the 17th, a sixth episode began with four distinct stages. They took place along the southeast rift zone of the volcano, from the summit to the sea coast. It was the first rift zone eruption in the south since 1800 A.D. and the first ever monitored at Piton de la Fournaise volcano.Three fissural vents opened at decreasing altitude emitting about 12 to 15 × 10⁶ m³ of olivine basalts between 19th March and 1st April. Strong seismic activity was accompanied by deformation of the summit area, and large-scale variations of the magnetic field. A summital event characterized the end of the flank activity with collapse of a new pit-crater and outflow of small amounts of degassed aphyric basalt.     

The interplay between collapse structures,hydrothermal systems,and magma intrusions: the case of the central area of Piton de la Fournaise volcano

We explore the possible relationships between a structural heterogeneity, the hydrothermal system, and the intrusive activity at Piton de la Fournaise volcano. Geological and geophysical data show that as the result of repeated collapses (the last one in 2007), a cylinder of faulted, fractured, and crumbled rocks must exist between the surface and the top of a magma reservoir at about sea level. This structure constitutes a major geological heterogeneity. An obvious spatial correlation exists between this column of fractured and brecciated rock and the location of (1) most of the seismic activity, (2) a low-resistivity dome, (3) a huge self-potential anomaly, (4) thermal evidence of hydrothermal activity, and (5) the root of magma intrusions. The dominant factors that make this structural heterogeneity a trap for the activity are probably its higher permeability and its weaker mechanical strength. Evidence exists for the presence of an active hydrothermal system confined in this permeable zone. The long-term stability of the activated zone above sea level and the similarity of the pre-eruptive crises, in spite of the inferred large perturbation of the magmatic system in 1998, suggest a common triggering mechanism for all the eruptions since at least the first data recorded by the observatory in 1980. This mechanism can be purely magmatic, resulting from the pressurization of a reservoir, but we also propose that the hydrothermal system may play a role in the development of volcanic instabilities. A qualitative model is proposed to explain the triggering of magma intrusions by hydrothermal processes, and its speculative aspects are discussed. This work represents a first attempt to integrate the structural and dynamic information in a unified framework at Piton de la Fournaise.     

Caldera rim collapse: A hidden volcanic hazard

Following the emblematic flank collapse of Mount St Helens in 1981, numerous models of flank sliding have been proposed. These models have allowed to largely improve the understanding of mechanisms involved in such landslides, which represent a tremendous risk for populations living around volcanoes. In this article, a new mode of landslide formation, related to buried calderas, is described. The model emphasizes the paramount importance of the hidden ring fault that, even when the caldera is buried, still remains a plane of weakness in the core of the edifice. Under certain conditions, this plane of weakness becomes activated as the upper part of a pre-existing critical slip surface and is used in the emplacement of huge landslides which travel downslope at a very high velocity. A natural example is taken from Piton de la Fournaise Volcano (La Réunion Island, Indian Ocean). It reveals that the primary cause triggering caldera rim collapse is partial unbuttressing of the flank of the volcano. In the natural example, this occurs through regressive erosion that excavates deep canyon in the direction of the buried caldera but other mechanisms may exist. On account of the large volumes of material involved in caldera rim collapse as well as their long runout distances, such a volcanic hazard should be taken into account on every volcano where buried calderas are suspected.     

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

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.