Piton de la Fournaise, located in the SE part of La Réunion Island in the Indian Ocean, has been producing frequent effusive basaltic eruptions on average twice a year since 1998. The activity is characterized by lava fountains and lava flows, and occasional explosive eruptions that shower blocks over the summit area and produce ash plumes. Almost all of the recent activity has occurred within the Enclos Fouqué caldera, with recent eruptions in 1977, 1986, and 1998 at vents outside of the caldera. The most recent eruptive episode lasted 18 hours on 13 July 2018. This report summarizes activity during September-November 2018 and is based on reports by Observatoire Volcanologique du Piton de la Fournaise (OVPF) and satellite data.

After deformation had ceased in early August, inflation resumed in the beginning of September (figure 145) accompanied by low-level seismicity. From 1 to 12 September CO₂ concentrations at the summit had decreased, followed by an increase during 12-20 September. A seismic crisis was reported on 0145 on 15 September that included 995 shallow (less than 2 km depth) volcano-tectonic earthquakes recorded in less than four hours. This was accompanied by rapid deformation of up to 24 cm.

Figure 145. Horizontal displacement at Piton de la Fournaise recorded in October 2018 at the OVPF permanent GPS stations located inside the caldera. The source for the deformation was located at a depth of 1-1.5 km below the Dolomieu crater. Courtesy of and copyright by OVPF/IPGP.

The eruption began at 0435 on 15 September with a fissure opening and erupting lava on the SW flank near Rivals crater. This new fissure was about 300 m downstream, and was a continuation of, the 27 April-1 June 2018 fissure. Volcanic tremor rapidly and steadily declined once the eruption began, which is commonly observed during eruptions of Piton de la Fournaise. An observation flight that day showed five fissures with lava fountains reaching 30 m high in the center of the fissure system (figure 146). By 1100 two main lava flows had merged further downflow and traveled 2 km from the fissures. During the first hours of the eruption the estimated time-averaged discharge rate was 22.7 and 44.7 m³/s.

Figure 146. An overflight at Piton de la Fournaise at 1100 on 15 September 2018 showed that five fissures had opened and two main lava flows had merged and extended to 2 km. The lava fountain in the center of the fissures reached 30 m high. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du samedi 15 Septembre 2018 à 16h45).

A survey on the 15th recorded multiple lobes at the end of the lava flow and flow rates of 1-5 m³/s (figures 147 and 148). Three vents remained active on 16 September and a spatter cone was being constructed around them. The lava effusion rate was measured at 2.5-7 m³/s. SO₂ levels were elevated and the resulting gas plume was dispersed towards the W. On the 17th the lava flow was still high on the flank and moving E.

Figure 147. The lava flow of the 15 September 2018 eruption of Piton de la Fournaise as seen on 17 September. The top images are photographs of the active fissure and the location of the lava flow as it progresses towards the SE, and the bottom images are thermal infrared images of the lava flow. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du samedi 17 Septembre 2018 à 17h30).

Figure 148. The active vent at Piton de la Fournaise producing a lava flow with flow rates of 1-5 m3/s on 15 September 2018. The opening of the vent is towards the south and a degassing plume is visible. Courtesy of and copyright by OVPF/IPGP.

By 18 September a cone had developed and was open to the south, producing lava fountaining and feeding the lava flow (figure 149). The lava flow had extended to 2.8 km from the vent, with the active flow front about 500 m from the southern wall of the caldera. The flows advanced several hundred meters by the 21st and the height of the cone was 30 m on the eastern side where a near-vertical wall had formed (figure 150). The cone contained three active lava fountains.

Figure 149. A spatter cone being built around the new vent on Piton de la Fournaise on 18 September 2018 at 1230 local time. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du samedi 18 Septembre 2018 à 17h00).

Figure 150. The active vent on Piton de la Fournaise with spattering activity on 21 September 2018 at 1615. The wall of the cone on the left of the photograph is nearly vertical and was 30 m high. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du samedi 21 Septembre 2018 à 20h00).

Fallout of Pele's hair was reported in the Grand Coude area on 22 September. The cone remained open to the south and a deep channel had formed with lava tubes observed close to the cone (figure 151). Three lava fountains continued to feed the lava flow towards the S, then the SE, with a flow rate of 1-3 m³/s.

Figure 151. The eruption fissure at Piton de la Fournaise on 22 September at 1100 local time. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du samedi 22 Septembre 2018 à 17h15).

By 26 September the fissure system had evolved into a single cone and the opening towards the south had closed, leaving a circular vent and a lava lake (figure 152). Observations on the 26th showed that lava tubes were developing and feeding outbreak flows 150-300 m away from the cone. During 24-30 September the surface lava flow rate varied from 0.5 to 5.3 m/s, but this was expected to be higher in the lava tubes. By the 27th the majority of the lava was feeding from within the vent area into lava tubes that continued to feed breakout flows several hundred meters from the cone. On the 30th a small lava flow was also visible at the foot of the cone and spattering was seen low above the cone (figure 153).

Figure 152. A view of the active cone and lava flow on Piton de la Fournaise on 25 September 2018. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du samedi 26 Septembre 2018 à 17h00).

Figure 153. An explosion producing spatter that is added to the new cone on Piton de la Fournaise. Photographs taken around 1100 on 29 September 2018. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du samedi 30 Septmeber 2018 à 15h00).

The surface lava flow rate ranged from less than 1 and up to 4 m³/s on 1-2 October, with the majority of the activity still taking place in lava tubes with some small breakout flows (figure 154). There was a reduction in surface activity on 2-3 October along with a change from continuous degassing to the emission of discrete gas plumes ("gas pistons") that were accompanied by a sharp increase in tremor (figure 155). Observations on the 4th noted that spattering at the vent was minor and rare. No breakouts were observed.

Figure 154. The surface activity of Piton de la Fournaise at 1030 on 2 October 2018. The activity was focused at a single vent and a cone had developed on top of the initial fissure. A white degassing plume and incandescent lava are seen at the vent, but the majority of activity is below the surface in lava tubes. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du samedi 3 Octobre 2018 à 14h00).

Figure 155. Thermal infrared imaging of the Piton de la Fournaise eruptive site and active lava flow field taken from Piton Bert at 1050 on 8 October 2018. Courtesy of OVPF/IPGP (Bulletin d'activité du samedi 8 Octobre 2018 à 16h00).

Limited activity continued from the 5 to 7 October surface activity remained low, with minor spattering and few breakouts. Lava continued to flow within the lava tubes and degassing was visible at the surface above them. From 30 September to 8 October the lava had traveled 1.8 km E within lava tubes and emerged as a breakout along the northern flow (figure 156). The south and central flow-fronts had not advanced during this time.

Figure 156. The progression of the Piton de la Fournaise lava flow from 30 September (red) to 8 October 2018 (blue) as determined by InSAR satellite data. There are three main lobes, with the activity focused at the northern lobe during this time. Courtesy of OVPF/IPGP (Bulletin d'activité du samedi 8 Octobre 2018 à 16h00).

On 14 October no lava channels were visible on the surface and only small breakouts were observed (figure 157). Activity continued in lava tubes and strong degassing persisted from both the vent and main lava tubes (figure 158). On the 18th OVPF/IPGP reported continued strong degassing and a small lava channel that had formed out to a few tens of meters from the cone (figures 159 and 160).

Figure 157. OVPF sampling a lava breakout on Piton de la Fournaise 600 m from the lava flow front at 1015 on 14 October 2018. Courtesy of OVPF/IPGP (Bulletin d'activité du samedi 14 Octobre 2018 à 13h00).

Figure 158. The Piton de la Fournaise eruption site at 0945 on 14 October 2018. At this point most of the activity is confined to lava tubes, with the main lava tube marked by degassing moving away from the degassing vent to the left of the photograph. Courtesy of OVPF/IPGP (Bulletin d'activité du samedi 14 Octobre 2018 à 13h00).

Figure 159. A white gas plume at the active vent of Piton de la Fournaise on 18 October 2018. Courtesy of OVPF/IPGP (Bulletin d'activité du samedi 18 Octobre 2018 à 17h00).

Figure 160. The eruptive vent and active lava flow on Piton de la Fournaise at 1130 on 18 October 2018. Courtesy of OVPF/IPGP (Bulletin d'activité du samedi 18 Octobre 2018 à 17h00).

By 25 October the lava flow rate was still low with no further extension of the flow boundary, SO₂ emission from the vent were low (close to or below the detection limit), CO₂ levels were decreasing, and the intensity of the tremor had stabilized at a very low level for about 24 hours (figure 161). At this point the lava field was essentially composed of lava tubes with a maximum recorded surface temperature (maximum integrated pixel temperature) of 71°C (figure 162). This low level of activity continued during the 26-28th with a small amount of surface lava activity about 1 km from the vent. Over 29-31 October the surface activity was extremely low with no fresh lava observed and only degassing at the vent. The eruption was declared over at 0400 on 1 November after 47 days of activity.

Figure 161. Plot of Real-time Seismic-Amplitude Measurement (RSAM), an indicator of the volcanic tremor and intensity of the Piton de la Fournaise eruption, from 15 September to 25 October 2018. The increase in RSAM beginning on 3 October was due to a change in degassing regime due to the gradual closure of the eruptive vent as the cone grew. The RSAM values stabilized after 24 October; the eruption ended on 1 November. Courtesy of OVPF/IPGP (Bulletin d'activité du samedi 4 Octobre 2018 à 16h30).

Figure 162. An ASTER infrared satellite image of Piton de la Fournaise showing the lava flow in the SE caldera area on 25 October 2018. At this time, the lava field is essentially composed of lava tubes and it has a maximum surface temperature of 71°C. Cooler temperatures are darker and hotter temperatures are shown as white. Courtesy of OVPF/IPGP.

Thermal observations during the September-November eruption showed the evolution of the lava flow and the reduction in surface temperatures when the activity was dominated by lava tubes (figure 163). The sharp increase in thermal anomalies detected by the MIROVA algorithm showed the onset of lava effusion, and the anomalies tapered off as the flow field cooled down (figure 164). The estimated volume of lava produced from 15 September to 17 October was 9-19 million m³, but this is lower than the actual erupted volume due to the lava tube activity. There were 459 MODVOLC thermal alerts from 15 September to 25 October.

Figure 163. Infrared Sentinel-2 images showing the progression of the active areas of the Piton de la Fournaise lava flow (bright yellow-orange) during September and October 2018. Images courtesy of Sentinel Hub Playground.

Figure 164. The MIROVA plot of thermal energy from Piton de la Fournaise shows three eruptive episodes in 2018: 27 April-1 June, a one day event on 13 July, and 15 September-1 November. Thermal signatures continue beyond the eruption dates as the lava flows cool. Courtesy of MIROVA.

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: Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, 14 route nationale 3, 27 ème km, 97418 La Plaine des Cafres, La Réunion, France (URL: http://www.ipgp.fr/fr; Twitter: https://twitter.com/ObsFournaise); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).