The eruptive activity on 26 November was followed by an increase in seismicity on 2 December, mainly centered below the summit crater (Dolomieu) at 700-1,500 m depth. The number of seismic events increased on 6 December at 0635, and half an hour later a N-S fracture opened in the W part of Dolomieu Crater, N of the 29 December 1985 cone. A second phase began 1 hour later (at 0803) inside the 29 March pit crater. Lava emanating from the fracture and a new cone in the SE part of Dolomieu filled the pit crater, already partially filled by 13 July and 12 November lava. Lava emission continued at a low rate but ceased from the initial fracture on 8 December. During December, almost 1.0 x 10⁶ m³ of aphyric basalt was extruded onto the crater floor. The lava covered 2/3 of the floor with an average thickness of 1-2 m. Significant degassing accompanied the activity.

On 6 January, activity decreased in the four cones within Dolomieu and hornitos formed in the SE part of Dolomieu at the site of the lava-filled pit crater. The same day, a new eruptive phase began at the summit craters. Weak seismic events, barely detectable through the tremor, were recorded at summit stations beginning at 0855/0900. Seismicity was associated with strong deformation of the NE part of the summit of the Fournaise structure. At 1211, after eight summit events, a 700-m-long fracture opened on the slopes of the Osmondes Valley, NE of the summit, W of Piton de Crac, and very close to the April-May 1981 eruptive fissure. Lava flows and degassing were still being observed as of 15 January and the lava front had descended to 500 m altitude in the Grande Pentes region.

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