OVPF reported that a seismic crisis at Piton de la Fournaise began at 0736 on 2 July and was accompanied by rapid deformation. Volcanic tremor began at 0830, signifying the arrival of magma at the surface, and fissures opened on the E flank. OVPF recommended a change in the Alert Level to 2-1, the lowest of two sub-levels in “Alert 2: ongoing eruption” (inside the Enclos Fouqué caldera); Alert 2 is the third level on a four-color eruption scale. An overflight was conducted, and three fissures were located at an elevation of about 2,000 m in an area N of Piton Vouvoul. Two fissures were near each other, and one was located to the NE; all three trended NE-SW. Lava from the two at the higher elevation traveled ENE and lava from the third fissure traveled E. Tremor decreased sharply and during 1145-1230 no surface activity was visible. Tremor was variable and again increased; a fourth fissure opened at around 1750 at the top of the Grandes Pentes on the SE flank, around 1,500 m elevation. The fissure was about 500 m long, trended NNW-SSE, and produced lava flows that traveled E.

By 0430 on 3 July the SE flank fissure was the most active of the two fissure areas, with lava flows traveling longer distances to the E than from the higher E-flank fissures. In general, the lava emission rate fluctuated between 7 and 27 meters per second (m/s), averaging 12 m/s, based on satellite data. Field teams made visual observations during 0800-1000 on 3 July and noted that the E-flank fissures were no longer active, producing only gas emissions. The lava flows from those fissures had stopped at around 1,700 m elevation. Active lava fountaining was building several cones along the SE-flank fissure. The lava flows continued to advance, reaching 650 m elevation, in an area about 2.4 km from the nearest road. A sharp decline in volcanic tremor amplitude was noted at 1012 and remained at lower levels. During 3-4 July the lava emission rate fluctuated between 5 and 20 m/s based on satellite data, and the flow front advanced at a rate of about 40 meters per hour based on webcam images. By 1424 on 4 July the lava flow was about 3.5 km long based on satellite image analysis.

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.

Source: Observatoire Volcanologique du Piton de la Fournaise (OVPF)