Lava extrusion that began 3 February from the N side of the updomed summit region that surrounds Bory and Dolomieu craters continued until 25 February. After about 13 hours of seismicity, fissures opened on the SW side of the summit area and began to eject lava. The eruption was continuing as of 3 March.

Activity N of the summit, 3-25 February. During the first few days of the eruption, lava was extruded from a series of radial fissures in the N summit region. By 6 February, lava fountaining was confined to a spatter cone at 2350 m altitude at the lower end of a fissure that opened 4 February. Lava flows emerged from one or two vents 300 m down slope from the active spatter cone and moved 1.3 km to the E. Fountaining was most intense 10 February (30 m high) and 18 February (100 m high). About 19 February, a small lava lake formed inside the active cone. Lava fountains rose a few m above the lake surface. A 2-m-diameter vent high on the cone emitted blue and yellow flames 3-4 m high. The spatter cone partially collapsed 20 February. Lava overflowing the collapsed area formed a front 100 m wide.

Fountaining and extrusion of lava flows began a rapid decline on 23 February and stopped on the 25th. Several million cubic meters of lava were extruded 3-25 February.

Activity SW of the summit beginning 26 February. Seismographs at Réunion's volcano observatory began to record a series of small (about M 1) local earthquakes around midnight of the night of 25-26 February. Earthquakes became increasingly frequent that morning and by 1230 were occurring once every 15 seconds under the summit's Bory Crater. Harmonic tremor started at 1300 and the beginning of eruptive activity was observed at 1306. Two minutes later, a large, black cloud rose to 2 km height. Two en-echelon radial fissures, trending N74°E, opened on the SW side of the updomed summit region. The upper fissure, 200-300 m long, extended from 2,400 to 2,250 m altitude. The lower fissure, offset 100 m from the base of the upper fissure, extended 100 m farther downslope. Lava fountains rose to 15 m height from the entire length of the upper fissure, while fountains from the lower fissure were 50-60 m high. After half an hour, lava from the two fissures had merged into a single aa flow 2 km long that spread onto the caldera floor and moved toward the S caldera wall. Mid-afternoon outflow rates from the two fissures were ~300 m³/s (~1 x 10⁶ m³/hour), much higher than at any time during the N summit region activity earlier in the month. The lava was an aphyric basalt, as was the 3-25 February material. By about 1800, lava fountaining along the upper fissure was concentrated at its lower end, where a cone was growing. Seismicity ended within a few hours of the start of eruptive activity on 26 February, a pattern similar to that observed at the beginning of the eruption 3 February.

Lava fountaining along the entire lower fissure continued until 0200 on 27 February, then was limited to the middle of this fissure, where a cone formed. The rate of lava outflow declined to 60 m³/s by the morning of the 27th and 10 m³/s the following day. Fountaining from the upper fissure stopped 28 February but continued from the lower fissure, building a 15 m-high spatter cone. Two other spatter cones formed along the lower fissure 1 March, with activity concentrating at one of these, also 15 m high, on 2 March. The rate of lava production remained at 10 m³/s as of 2 March [but see 6:4], feeding a slow-moving lava flow that was incandescent for the upper 1.5 km of its length.

Further References. Bachelery, P., Blum, P.A., Cheminée, J.L., et al., 1982, Eruption at le Piton de la Fournaise Volcano on 3 February 1981; Nature, v. 297, p. 395-397.

Blum, P., Gaulon, R., Lalanne, F., and Ruegg, J., 1981, Sur l'Evidence de precurseurs de l'eruption du Volcan Piton de la Fournaise a la Réunion (Fevrier 1981): C. R. Acad. Sci. Paris, v. 292, serie II, p. 1449-1455.

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: M. Krafft, Cernay; L. Stieltjes, BRGM, Réunion; Volcano Observatory of Réunion.