OVPF reported that the eruption at Piton de la Fournaise that began on 19 September was ongoing at a cone adjacent to the SW flank of Piton Kala Pélé during 28 September-4 October. The cone ejected lava to low heights above the rim. Lava flowed from the base of the cone in two main branches, to the SE and E, mainly through lava tubes, as far as 3 km. By 28 September the cone had grown to just over 8 m tall and around 27 m wide at its base. Average daily lava flow discharge rate estimates had a mean value of 8 meters per second at the beginning of the eruption but then stabilized at 2-4 meters per second; the flow rate increased during 28-29 September to more than 6 meters per second. Lava discharge rates were likely underestimated due to measurements hindered by weather conditions or flows obscured by tubes. Tremor levels and gas emissions also began increasing on 29 September and remained at high levels during the rest of the week. The vent at the top of the cone widened and a new, smaller cone formed on the S flank and produced lava flows. The volume of erupted lava was 2.6-5.4 million cubic meters by 30 September; peak discharge rates reached 20 meters per second at times.

Sulfur dioxide emission estimates derived from satellite data had increased from about 610 tons per day on 28 September to 1,525 tons per day on 1 October. A well-defined gas plume, denser than those seen during previous days, was identified in a 30 September satellite image drifting 300 km NW at an altitude of 3 km (10,000 ft) a.s.l. A more significant sulfur dioxide plume was identified in satellite data the next day, drifting as far as 400 km. Gas plumes drifted SW during 2-3 October. Sulfur dioxide emissions continued to increase and were about 2,500 tons per day on 3 October. The cone had grown to around 12 m tall and 43 m wide at the base.

During 3-4 October the ejection of lava above the cone became less intense, and the new smaller cone was only weakly active. The southernmost lava flow had reached 1,800 m elevation in an area 1.5 km NW of Nez coupé du Tremblet. During 4-5 October tremor levels fluctuated. Lava effusion increased, averaging 10 meters per second with peaks at 25 meters per second. Lava was ejected above the main vent, which was 23 m wide; the smaller vent was not active. The eruption stopped or paused at 0748 on 5 October based on visual observations and a sudden halt in tremor signals.

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)