OVPDLF reported that a sharp increase in seismicity at Piton de la Fournaise, increased gas emissions, and deformation recorded on 30 July prompted an Alert Level increase and an evacuation. A fissure eruption began the next day at 0920, preceded by 90 minutes of high seismicity and 80 minutes of major deformation; it was confirmed by a hiker and then by observation of a gas plume. A 1-km-long fissure opened in the NE part of the l'Enclos Fouqué caldera and produced dozens of lava fountains. Based on satellite images and gas data the flow rate was estimated to be 28 cubic meters per second initially and then 11 cubic meters per second later that day. A gas plume rose to altitudes of 3.2-3.5 km (10,500-11,500 ft) a.s.l. By the evening there were only five fountains, and a lava flow had traveled as far E as Plaine des Osmondes (NE part of the caldera). According to a news article, lava fountains were 40 m high, forming 20-m-high cones on 31 July. At 1115 on 2 August tremor stopped, after several hours of fluctuating intensity, and lava was no longer being effused.

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.

Sources: Observatoire Volcanologique du Piton de la Fournaise (OVPF), The Island Packet