As lava extrusion N of the summit was ending, seismic activity resumed on 25 February. By 0400 on the 26th, 50 events had been recorded, 20 with magnitudes greater than 1. Seismographs registered an additional 521 local earthquakes, 111 of M 2 or greater, between 0400 and 1304. Harmonic tremor began as two en-echelon fissures, trending 215°, opened ~800 m SW of Cratère Bory at 1304. Aphyric basalt flows had reached 2 km in length by the next morning when lava was pouring from the fissures at a rate of 600 m³/s (2 x 10⁶ m³/hour). By 1 March, the outflow rate had declined to about 20 x 10⁶ m³/s and a 400-m-long segment of the flow was moving through a lava tube. Lava fountains reached an average height of 20-40 m. Blue and green flames were nearly always visible over the fissures. Three vents remained active 2 March, but by 4 March fountaining continued from only one vent, reaching an average height of ~20 m. By the 4th, lava had reached the break in slope above the caldera wall and most of the flow traveled through tubes. The flow reached the S caldera wall, at 1,900-2,000 m altitude, on 7 March.

Seismicity, which had remained quiet since 26 February, resumed on the morning of 7 March when 500 events were recorded, all with magnitudes <1.0. On 9 March, 700 more events, again with magnitudes <1.0, were recorded between 0819 and 1651 from a seismic station on the N side of the caldera. Between the 13th at 1000 and the 15th at 2344, the same station recorded 50 more events, 22 of which had magnitudes of 1 or more.

The lava flows spread laterally as the rate of outflow slackened 16-21 March. Lava reached 1800 m altitude and the flow front was ~5 km from the vents. The activity declined progressively, ending completely 25-26 March. A levelling network W of the summit was reoccupied 11 March, indicating that a net inflation of 11 microradians had occurred since the last survey on 13 February.

After about 2 hours of local seismicity, a 1-km-long fissure trending 060° opened on 1 April at 2141, between 1,900 and 1,600 m altitude about 2 km NE of the updomed summit crater area (figure 6). Olivine basalt lava flowed rapidly downslope 1 and 2 April, stopping at 480 m altitude ~2.3 km W of the coast highway. On 3 April, effusive activity was limited to two vents along the fissure at 1,650 and 1,600 m altitude. Intense explosive activity ejected lava fragments to an average height of 50 m, building two cones, while numerous flows (as many as a dozen at once, 200-300 m in length) moved as much as 1 km toward the N caldera wall. Activity at the upper vent stopped 6 April. Many phreatomagmatic explosions occurred 7 April, of which two were observed between 1200 and 1700. A large quantity of cauliflower bombs and ash were ejected.

Poor weather prevented observations for the next 13 days. By 22-24 April, when weather had cleared, tephra ejections were reaching an average altitude of only 30 m. Lava flowed from the ends of tubes, forming numerous small tongues of aa and pahoehoe tens of m in length. A 2 km by 300 m field of lava had formed by the 24th.

On 3 and 4 May the explosive activity ceased as effusive activity weakened. Only three aa flows persisted in the lava field, one 300 m long, the other two only 10 m in length. By the night of 4-5 May, all eruptive activity appeared to have ended.

Heavy rains stopped some of the seismometers from working, but the 1-2 instruments that remained functional recorded no seismicity after the premonitory swarm on 1 April. No significant changes in tilt were measured.

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: J. LeMouel and J-L. Cheminée, IPG, Paris; M. Krafft, Cernay.