The 2007 explosive activity at Piton de la Fournaise volcano (La Reunion Island) : constraints on the eruptive processes by the volcanological study of the erupted deposits

The March-April 2007 Piton de la Fournaise (PdF) eruption was the most significant eruption on La Reunion island in historical times. The eruption was characterised by different styles of volcanic activity on different parts of the volcano. In particular, during the April eruptive phase, fluctuating high fountains occurred at a vent on the southeastern flank of the volcano at about 590 m a.s.l. (Piton Tremblet), and rootless littoral hydro-volcanic explosive activity occurred at the coast, whereas the volcano summit experienced caldera collapse. On the 5th - 6th April, the major caldera collapse occurred affecting the plateau and wall of the Dolomieu crater. Subsequent to caldera collapse events on the 6th and 12th April, an increase in the flank fountain activity was recorded. High discharge, 300-350 m fountain height, and textural change in the products were produced at Piton Tremblet. Lavas fed by the Piton Tremblet activity reached the coast and interacted with seawater generating hydro-volcanic explosive activity, which rapidly built a littoral cone. A link among these eruptive events and their chronology was suggested syn-eruptively due to change in the eruptive intensity and discharge, which showed significant increase, and textural change in the magma, passing from basalt to olivine-basalt to oceanite. This study provides the reconstruction of the complete chronology of the April 2007 eruptive phase, describes the eruptive dynamics involved in the different eruptive events and their peculiar characters, determines the link among different eruptive events, and investigates the PdF plumbing system. Available syn-eruptive records were analysed to reconstruct the chronology of the eruption. Available multidisciplinary monitoring data on the 2007 eruption were used. Deposits of the eruptive activity and their stratigraphy were analysed. Products from the lava fountain and the littoral cone were analysed in the lab for clast density, and grain size. Products from the entire range of eruptive activity were morphologically analysed for componentry. Microtextural observations were produced on the caldera collapse and flank fountain products. Quantitative microtextural analysis using image analysis method and stereological and volumetric data conversion of vesicularity was performed on 16 pyroclasts (16-32 mm diameter) from Piton Tremblet fountaining activity, selected among the modal densities within the diverse morphology range. Comparisons with other similar eruptive activity and/or eruptions occurred worldwide were investigated. The increase in discharge at 6th April and the reactivation on the 12th April of the fountaining at Piton Tremblet flank vent were linked with summit caldera collapse events. The collapses perturbed the pressure conditions within the volcano's plumbing system. At the summit, the caldera collapse activity showed the involvement of magma and hydrothermal system which produced phreatic and phreatomagmatic explosions and lavas, although gravity driven collapses and numerous landslides played the major role in the collapse dynamics. The magma source of the summit activity can be related to a pre-2007 intrusion emplaced at shallow depth below the eastern summit area and is not related with the Piton Tremblet magma source. Piton Tremblet fountaining dynamics were characterised by magma and volatiles coupling during ascent subsequent to volatile exsolution and bubble nucleation stage as inferred by quantitative vesicle data interpretation. Variations in degassing and eruptive activity were generated by deep magmatic inputs, caldera collapse events, and increasing crystal content in the magma. The rootless hydro-volcanic explosive activity occurred on the 6th April when channel-fed 'a'¿ívÖ lavas produced by Piton Tremblet interacted with seawater, and produced a nested littoral cone structure in four stages. The fragmentation was generated by steam explosions which produced three different particle components (vesicular, transitional and dense clasts) and three different particle transport types (tephra jet, fall and surge). Comparisons between the April 2007 PdF eruptive phase and similar events at other basaltic volcanoes worldwide showed that: 1) Piton Tremblet fountain activity is comparable with basaltic fountains at Kilauea (Hawaii), despite its lower maximum fountain height; however, the crystal content had an important role in modifying magma rheology and enhancing viscosity, and trapping bubbles leading to largely coupled ascent during fountaining activity; 2) the caldera collapse compared with other recent caldera collapses that occurred at different basaltic volcanoes (Fernandina, Miyakejima) is very similar in terms of caldera formation processes, eruptive features, and products, despite its smaller size; 3) the rootless hydro-volcanic activity was more powerful than the recent littoral activity occurred in Hawaii and comparable with the Hawaiian historical littoral cones. The evolution of the April 2007 eruption and the temporal links among the different volcanic stages demonstrate the connection of the magma plumbing system (which includes three main magma storage zones) at different levels and that perturbations of pressure within the system can drive dramatic increases of eruptive activity. The 2007 PdF eruption also showcased the range of possible volcanic hazards which could occur during a large eruption at a basaltic volcano, and showed how these hazards directly impacted the population of La Reunion island.