Ash and small quantities of lava were emitted from summit area vents in November and early December. Eruptive episodes were preceded by shallow fracturing from 18 September to about mid-November in the NE part of Dolomieu cone. Fracturing was indicated by weak seismicity detected only by summit stations. Tiltmeters recorded a slow, minor deflation. In early November, fracturing increased and was centered 700 m under the middle of Dolomieu. Geodetic measurements indicated that the E part of Dolomieu was sliding down to the ENE, accompanied by opening of the crater's N part. On 12 November, a very short, intense, seismic crisis (from 1150 to 1248) was followed by continuous tremor and a day-long ash eruption in the 29 March pit-crater. The crater floor rose nearly 40 m as 30,000-40,000 m³ of ash were emitted. Seismic instruments indicated that shallow fracturing continued after the eruption. Weak seismicity continued until 17 November.

Tremor in the S part of the (Enclos) caldera began on 15 November and lasted for almost two days. Beginning on 17 November, depth, frequency, and magnitudes of seismic events increased. Magnitudes were greater than 1.0 and hypocenters were at 1,500 m depth beneath the E part of Dolomieu Crater.

From 19 to 20 November a shallow seismic crisis shook the summit area and for the next six days 10-12 events/day were recorded. During the night of 25-26 November, M 2.0-2.5 events occurred ~3,500 m below Dolomieu Crater. The next evening (1530-1604), after 12 hours without seismicity or significant deformation, a 30-minute fracturing episode near the pit-crater (accompanied by tilt) marked the emplacement of magma. At 1604 an E-W fracture opened 200 m below the rim on the E flank, producing only a small amount of lava. After 30 hours of tremor, seismic activity remained weak, similar to the September-October shallow fracturing.

From 2 to 6 December a shallow seismic crisis occurred in the summit area, At 0700 on 6 December, a fissure eruption accompanied by tremor began in the bottom of Dolomieu Crater, where two vents and a fissure were observed. As of 15 December the eruption continued, as one vent emitted small quantities of lava and violent degassing events occurred from spatter cones. Intense seismicity was still being recorded at summit stations.

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: H. Delorme, B. Gillet, and J. Delmond, OVPDLF; P. Bachelery, Univ de la Réunion; J-L. Le Mouel, J-L. Cheminee, A. Hirn, P. Blum, and J. Zlotnicki, IPGP.