Ethiopia's Erta Ale basaltic shield volcano has had at least one active lava lake since the mid-1960s, and possibly much earlier. Two active craters (North Pit and South Pit) within the larger oval-shaped Summit Caldera have exhibited periodic lava fountaining and lava lake overflows over the years. A new eruptive event located about 3 km SE of the Summit Crater appeared on 21 January 2017. Activity at the eruption site increased during subsequent months, sending lava flows several kilometers NE and SW from a newly formed lava lake. This report discusses activity from February 2017 through March 2018 as the flows traveled as far as 16 km from the main vent. Information comes from satellite thermal and visual imagery, and photographs and reports from ground-based expeditions that periodically visit the site.

Summary of activity, February 2017-March 2018. The 21 January 2017 activity at Erta Ale was the first time a vent outside of the Summit Caldera has been observed (figure 50). The initial vent or vents created multiple lava flows that traveled generally NE and SW from their sources, creating at least one lava lake that persisted for about a year (figure 51). The flows began inside an older caldera at a location about 3 km SE of the South Pit Crater, but eventually overflowed the caldera rim in multiple directions. As the flow fields enlarged, thermal imagery captured hot-spots along the flows that were likely produced by breakouts, skylights into lava tunnels, and hornitos, as well as multiple surges of flows across the growing fields (figure 52). The imagery also showed the locations of the advancing flow fronts which had reached over 5 km SW of the source by August 2017 and over 16 km NE of the source by March 2018, eventually reaching the alluvial plain NE of Erta Ale. Thermal anomaly data indicated that the maximum thermal energy output happened in April 2017, gradually decreasing through March 2018. The far NE front of the northeast flow field was still active at end of March 2018.

When the new eruptive episode began, the lava lake at the South Pit Crater drained rapidly to around 80-100 m below the rim, according to visitors to the site a few weeks later. The crater was emitting a strong thermal signal by early March 2017 as the lake level rose again. Visitors in April witnessed a fluctuating lake level rising and falling by up to 20 m every 30 minutes over several days. The thermal signal remained strong at the South Pit Crater through March 2018. Due to significant political instability in the area, ground visits are intermittent, but high-quality photographs were taken in February 2017, December 2017, and January 2018 that show the new lava lake and parts of the new flow fields.

Activity during late January-March 2017. The new eruptive event at Erta Ale began in late January 2017 at the northern end of the Southeast Caldera located; the first lava flows observed were locatedabout 3 km SE from the main Summit Caldera (figure 45 (BGVN 42:07) and figure 50). Two separate vent areas appeared active initially. The northern vent sent lava flows to the NE for several kilometers and to the SW a much shorter distance. The southern vent sent a stream of lava to the S. By the end of January 2017 the North and South Pit Craters at the Summit Caldera were still thermally active, but the signals were much stronger from the new vent areas in the Southeast Caldera (figure 53). A faint thermal signal from about 5 km E of the northern vent suggested the extent of the new flows in that direction.

A small group of travelers led by Ethiopian geologist Enku Mulugeta visited Erta Ale during the first half of February 2017. They reported that within the main Summit Caldera, the hornito in the North Pit Crater had collapsed and the lava lake in the South Pit Crater was about 80-100 m below the caldera floor level. The eruption in the Southeast Caldera was still very active, and they photographed the sizable new lava field which contained numerous pahoehoe flows, actively spattering hornitos, and a large lava lake (figures 51, 52, and 54). During the following months activity remained high both at the new eruption site and at the Summit Caldera where the lava lake in the South Pit Crater gradually rose back up to about 50 m below the caldera floor. Culture Volcan annotated a series of Sentinel-2 satellite thermal images which show the progression of the lava flows through the following year.

By 10 March 2017 only the southern vent area was active inside the Southeast Caldera. It continued to feed the lava field; lava was actively flowing S from the vent towards the W rim of the Southeast Crater, and NE, breaking out from lava tubes which blocked the thermal signal until about 2.6 km NE of the vent (figure 55). Thermal signals from both the North and South Pit Craters were distinct and stronger than in late January.

A site visit to the South Pit Crater on 20 March 2017 demonstrated that the lake level had risen significantly since its drop in early February, and was once again actively convecting (figure 56). By the end of March 2017, satellite thermal imagery made clear the increasing thermal signal at the South Pit Crater, and in the Southeast Caldera, the major increase in effusion to the NE from the main vent. The width of the flow field had increased to about 1,400 m, and the farthest front was about 3,400 m NE from the vent (figure 57). The lava at the source measured about 180 x 75 m in size, suggesting a lava lake; a smaller overflow to the SW appeared to have reached the W rim of the Southeast Caldera by 30 March 2017 near the area where a new flow had first appeared in a 23 January 2017 satellite image (see figure 46, BGVN 42:07).

Activity during April-May 2017. In the next Sentinel-2 satellite image from 9 April (figure 58), the distance to the farthest front of the lava flow had increased to about 4,600 m from the lava lake, and a new flow had appeared a few hundred meters east of the lake that extended about 1,100 m ENE from its source. Lava also flowed SW from the source to the SW rim of the Southeast Crater, appearing to pond against and flow slightly beyond the rim.

A group visited Erta Ale during 11-15 April 2017 in collaboration with Addis Ababa University geologist Enku Mulugeta. They noted that fluctuating lava lake levels at the South Pit Crater were cycling every 30 minutes or so between 40 and 50 m below the caldera floor (figures 59 and 60). Lava tubes from the walls of the crater would feed the lake with fresh lava after it drained. Two coalesced hornitos, about 7 m high, were present in the NE part of the crater, emitting SO₂ gas and occasional lava. At the North Crater Pit, noisy degassing of SO₂ from several hornitos at the center of the solidified crust was apparent. Observers at the Southeast Caldera could see the lava lake with the top about 10 m below its crater rim, and minor fountaining during the night, but they were not able to get closer than about 700 m due to the active flows.

By the end of April 2017 satellite thermal imagery indicated that the northeast flow field at the Southeast Caldera extended more than 7 km NE from the lake and was curving towards the E (figure 61). The lava lake was still thermally active, as was the South Pit Crater to the NW.

Eleven days later, activity was quite different in the Southeast Caldera. Satellite imagery from 9 May 2017 (figure 62) showed a new, relatively narrow but bright lava flow moving NE for 2-3 km originating in a location slightly NE of the original lava lake; activity farther NE had diminished from the previous image. A subsequent image on 18 May looked similar, but by 19 May the narrow flow had been replaced by a much broader area of thermal anomaly in the region immediately E of the source. By 29 May 2017, the source of the lava appeared to have shifted several hundred meters SE of the earlier location, and a strong thermal signal once again extended NE across the northeast flow field from the new source for about two kilometers (figure 63).

Activity during June-August 2017. The rapidly changing flow field was significantly different again less than two weeks later in satellite imagery captured on 8 June 2017. Lava was flowing N, SE, and S across the northeast lava field, extending beyond the rim of the Southeast Caldera to the N and E. Another very strong thermal signal emerged from the SW corner of the Southeast Caldera where lava was flowing W and S outside the caldera rim forming a new southwest lava field (figure 64).

During June 2017, the most aggressive flow activity contributed to significant growth of the southwest lava field. By 28 June, infrared imaging detected flow fronts 4,500 m SW of the vent; they had extended to about 5,100 m, nearing the base of the SW flank of Erta Ale, by 5 July (figure 65). Flow activity also persisted in the northeast flow field with activity concentrated about 1.5 km NE of the vent on 28 June. Movement increased at the northeast flow field beginning in late June and it had extended to about 3.5 km NE of the lava lake by 5 July 2017.

Significant movement to the NE in the northeast flow field was apparent in satellite images beginning on 21 July 2017; the head of the flow had reached about 9.5 km from the lava lake by 28 July 2017, mostly focused in a narrow channel (figure 66). Activity decreased in the southwest flow field during July; the lava front had advanced only a few hundred meters by the end of July from its position on 5 July.

During August 2017, lava continued to flow from the Southeast Caldera lava lake in two directions. The northeast flow front extended to 12 km from the vent by 17 August and had reached over 14 km by 7 September. The southwest flow field, while it remained in roughly the same area, had a decreased but still significant thermal signature in early September, suggesting continued but diminished activity throughout the period (figures 67).

Activity during September-December 2017. In a Sentinel-2 satellite image from 26 September 2017, it was clear that the South Crater Pit was still thermally active, and that the southwest flow field had largely cooled with only a small area on its NW edge still producing a thermal anomaly (figure 68). In contrast, the northeast flow field had advanced about 1 km in the previous three weeks and was less than a kilometer from the edge of the valley alluvium. It finally reached the edge of the older lava field and began to advance across the alluvium NE of the volcano, more than 16 km from the lava lake, on 16 October 2017 (figure 69). Based on satellite imagery, Cultur Volcan interpreted that activity slowed significantly during November 2017, and while the thermal signal remained strong near the head of the flow, it did not advance significantly across the alluvium.

Visitors to the South Pit Crater in mid-December 2017 reported that its lava lake continued to be active and its level was about 60 m below the rim. They were also able to visit the Southeast Caldera lava lake, 2.7 km SE of the South Pit Crater, and take photographs from its rim; it was about 200 m long and 100 m wide and filled with slowly convecting lava (figures 70, 71). Satellite imagery from 25 December 2017 showed the active lake at the South Pit Crater, the active lake at the Southeast Caldera, and numerous skylights and overflows along the 16-km-long northeast flow field (figure 72).

Activity during January-March 2018. By mid-January 2018 thermal activity was concentrated a few kilometers back from the front of the northeast flow, about 12 km from the lava lake (figure 73). A Volcano Discovery tour group visited during 13-26 January 2018 and was able to access and photograph both the North and South Pit Craters and the new lake and flow fields around the Southeast Caldera with ground-based and aerial drone photography (figures 74-84).

By late March 2018 no thermal signal appeared in satellite imagery at the site of the Southeast Caldera lava lake, although the South Pit Crater was still visible. A large increase in the area of fresh flows and multiple thermal anomalies were present at the flow front of the northeast lava field 14-16 km from the former lava lake (figure 85). During the second half of March, the flow progressed several hundred meters out into the alluvial plain.

MIROVA thermal anomaly data. The MIROVA thermal anomaly data captures information about the distance of the anomalies from the summit as well as the radiative power released from Erta Ale. Both sets of information agree well with observations from the Sentinel-2 and Landsat satellite data. The plot of distance from the summit (figure 86) shows that during August 2016-mid-January 2017 the thermal anomalies were located very close to the summit point, representing heat flow from both the South and North Pit Craters within the Summit Caldera. Beginning on 21 January 2017, the jump in location of the anomalies corresponded with the beginning of the eruption in the Southeast Caldera. The MIROVA thermal anomalies progressed farther from the summit point during March and April 2017, when the northeast flow field was lengthening to the NE. The thermal signal jumps back closer to the summit point in early May corresponding to when new breakouts were spotted near the Southeast Caldera lava lake; the flows again traveled away from the lake during June and July 2017. Active lava flows from mid-August 2017 through March 2018 were visible in satellite imagery 12-16 km from the lava lake, which is reflected in the MIROVA data (figure 86).

The MIROVA data for the radiative power released from Erta Ale during August 2016-March 2018 also corresponds well with satellite and ground observations (figure 87). The levels of radiative power were moderate and constant during August 2016 to mid-January 2017 when only the lava lake and hornitos at the South and North Pit Craters were active (see also figure 47, BGVN 42:07). A moderate spike in the radiative power corresponds to the overflow of the South Pit Crater during 16-20 January 2017, followed by a large spike in radiative power on 21 January when the eruption started in the Southeast Caldera. This was followed by an extended period of increased radiative power as extensive flow fields formed in the Southeast Caldera. The graph is also able to distinguish the movement of the flows from near the Southeast Caldera lava lake to farther away and then near again during March-June 2017. The radiative power graph from 10 March 2017-9 March 2018 clearly shows a gradual decrease in the amount of radiative power over the period, suggesting a decline in flow activity, which corresponds well to satellite observations.

Geological Summary. The Erta Ale basaltic shield volcano in Ethiopia has a 50-km-wide edifice that rises more than 600 m from below sea level in the Danakil depression. The volcano includes a 0.7 x 1.6 km summit crater hosting steep-sided pit craters. Another larger 1.8 x 3.1 km wide depression elongated parallel to the trend of the Erta Ale range is located SE of the summit and is bounded by curvilinear fault scarps on the SE side. Basaltic lava flows from these fissures have poured into the caldera and locally overflowed its rim. The summit caldera usually also holds at least one long-term lava lake that has been active since at least 1967, and possibly since 1906. Recent fissure eruptions have occurred on the N flank.

Information Contacts: European Space Agency (ESA), Copernicus (URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus; Robert Simon, Sr., Data Visualization Engineer, Planet Labs Inc. (URL: http://www.planet.com/) [Images used under https://creativecommons.org/licenses/by-sa/4.0/]; Cultur Volcan, Journal d'un volcanophile (URL: https://laculturevolcan.blogspot.com); Toucan Photo (URL: http://www.toucan.photo/); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/); 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/); Stefan Tommasini (URL: http://vulkane-und-natur.de/).