Our last Bulletin report (BGVN 35:03) covered eruptive activity through the last eruptive episode, which ended 12 January 2010.

Beginning 14 August and through 10 September 2010, the Observatoire Volcanologique du Piton de la Fournaise (OVPDLF) recorded a slow but steady increase in the number and magnitude of earthquakes from Piton de la Fournaise. Inflation of the summit area began in late August. The following report is based on data received from OVPDLF. It discusses eruptions and related behavior as late as 10 December 2010.

On 13 September 2010, localized deformation W of the Dolomieu crater and a small number of landslides in the crater was observed. On 20 September instruments recorded a significant increase in the number of earthquakes located at the W and S of the Dolomieu crater, although their average magnitude was low.

On 24 September, OVPDLF reported the possibility of an impending eruption. During the night, a seismic crisis began with a series of several tens of earthquakes localized under the Dolomieu crater, which was associated with inflation (approximately 3 cm), especially close to the summit. The most significant deformations were measured on the rim and the N and S sides of the volcano, indicating a shallow magma body was distributed directly below the Dolomieu crater. After decreasing on 27 September, seismicity rose again by 29 September. Earthquakes were located at the base of the volcano, and inflation was noted particularly in the E. A significant number of landslides were detected in the crater. The Alert level remained at 1 ("probable or imminent eruption").

Beginning 7 October 2010, there was a steady increase in the number and magnitude of volcano-tectonic (VT) earthquakes. During 10-11 October the summit area inflated 3-7 cm, and an increase in the number of landslides in the crater was detected. The Alert level remained at 1.

Increased seismicity was again recorded on 14 October 2010, with a new seismic crisis of more than several hundred earthquakes. During this phase, significant ground deformation occurred near the summit, which generated numerous rockfalls inside the Dolomieu crater. At 1411, the seismicity moved toward the SE part of the volcano (Château Fort), and at 1910 an eruption began within the Enclos Fouqué, about 1.5 km SE of the Dolomieu crater rim. Lava fountaining occurred from four vents along a fissure. The Alert level was raised to 2 ("eruption in progress in the Fouqué caldera").

Eruptive activity continued on 15-16 October 2010, developing along a fissure. This eruption included low lava fountains and fed a lava flow moving to the ESE. Lava issued from an area close to the old Château Fort crater at the base of the SE flank of Dolomieu crater and remained within the Enclos Fouqué. Four small cones were active along the eruptive fissure; lava fountaining occured from three of them. A lava flow moved slowly about 1.6 km to the E and SE and approached the break in slope at the Grandes pentes. OVPDLF measured lava temperatures of ~1,100°C.

On 17 October 2010 explosions and degassing accompanied lava emissions. These explosions and degassing decreased on 18 October. The volcanic tremor also decreased to one-seventh compared to the beginning of the eruption. The number of VT events remained low (7/day); the strongest event occurred at 2323, a M 1.4 earthquake localized at about 1,600 m depth under the Bory summit crater. The base and the summit of the volcano remained in inflation. Preliminary estimation of the lava volume erupted was 600,000 m³.

During 19-21 October consistent eruptive activity continued, with weak emissions and small lava fountains at the main eruptive vents located along the eruptive fissure. Explosive activity and degassing decreased, and tremor remained stable. Lava flows extended ESE to ~2 km. Gas emissions decreased, but concentrated to the S and W of the fissure.

On 22 October 2010 eruptions continued, located close to the Château Fort area, in the southern portion of the Enclos Fouqué. During 22-26 October lava fountains and gas emissions originated from one vent, and lava traveled ESE. Gas emissions decreased significantly. At this point, only one cone was active and only a few lava fountains were observed. Volcanic tremor was stable. No earthquakes had been reported since the previous day. GPS ground deformation showed a weak deflation under the volcano.

A sudden increase in activity and tremor began on 27 October 2010 and continued on 28 October. On 29 October, observation made during a flight disclosed that a part of summit cone 3 (the only active cone) had collapsed. Some lava ejecta and gas emissions occurred from this cone, which also contained a small active lava pond. Lava from this cone fed a small, slow moving lava flow. This new lava field remained upstream of the cone named Gros Benard. On 31 October, OVPDLF reported that the eruption had ended.

On 9 December 2010, following a seismic crisis and inflation, a new eruption began from an eruptive fissure oriented N-S, just above the Mi-Côte peak, at ~2,500 m elevation, characterized by lava fountaining and two lava flows. Many small landslides occurred in the Dolomieu crater. Later that day lava flows from two fissures on the N flank of Piton de la Fournaise, ~1 km NW of the Dolomieu crater rim, traveled about 1.5 km N and NW. On 10 December 2010, seismicity and deformation measurements indicated that eruption of lava had stopped.

Geological Summary. Piton de la Fournaise is a massive basaltic shield volcano on the French island of Réunion in the western Indian Ocean. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three scarps formed at about 250,000, 65,000, and less than 5,000 years ago by progressive eastward slumping, leaving caldera-sized embayments open to the E and SE. Numerous pyroclastic cones are present on the floor of the scarps and their outer flanks. Most recorded eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest scarp, which is about 9 km wide and about 13 km from the western wall to the ocean on the E side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures outside the scarps.

Information Contacts: Laurent Michon and Patrick Bachélery, Laboratoire GéoSciences Réunion, Institut de Physique du Globe de Paris, Université de La Réunion, CNRS, UMR 7154-Géologie des Systèmes Volcaniques, La Réunion, France; Guillaume Levieux, and Thomas Staudacher, and Valérie Ferrazzini, Observatoire Volcanologique du Piton de la Fournaise (OVPDLF), Institut de Physique du Globe de Paris, 14 route nationale 3, 27ème km, 97418 La Plaine des Cafres, La Réunion, France (URL: http://www.ipgp.fr/fr/ovpf/actualites-ovpf/).