Ol Doinyo Lengai is located near the southern end of the East African Rift in Tanzania. It is known for its unique, low temperature, carbonatitic lava. Eruptions have been recorded since the late 19th century, consisting of smaller tephra ejections and numerous carbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. Activity primarily occurs in the crater offset to the north about 100 m below the summit where hornitos (small cones) and pit craters produce lava flows and spattering. The current eruption period began in April 2017 and more recently has consisted of ongoing thermal activity in the summit crater (BGVN 47:08). This report updates similar activity during August 2022 through January 2023 using information primarily from satellite data.

Activity was relatively low during this report period. The MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system, based on analysis of MODIS data, showed a cluster of low-power thermal anomalies during August through early October (figure 220). There was a gap in activity during mid-October through mid-December 2022; one anomaly was detected in mid-December, and one occurred in late December. Only one anomaly was detected in early January 2023. A total of two thermal hotspots were recorded near the summit crater by the MODVOLC thermal algorithm using infrared satellite data acquired by NASA’s MODIS instrument on 10 August and 16 September 2022. Some of this thermal activity occurred in the center of the summit crater, based on in Sentinel-2 infrared satellite imagery, which may have represented ongoing lava effusions from the central hornito (figure 221).

Figure 220. Thermal activity during July 2022 through January 2023 at Ol Doinyo Lengai was detected more frequently during August through early October 2022, as shown in this MIROVA graph (Log Radiative Power). There was a gap in activity during mid-October through mid-December; two low-power anomalies were recorded during mid-December and late December, and one was recorded in early January 2023. Courtesy of MIROVA.

Figure 221. Sentinel-2 infrared satellite imagery of Ol Doinyo Lengai showed persistent thermal anomalies in the summit crater on clear weather days on 11 August 2022 (top left), 20 September 2022 (top right), 5 October 2022 (bottom left), and 28 January 2023 (bottom right). A single, large anomaly was visible in the center of the crater on 11 August and 20 September. On 5 October there were two smaller anomalies that filled the center and W part of the summit crater. On 28 January a single small anomaly was visible in the center of the summit crater. Images use “Atmospheric penetration” rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

A thermal anomaly was visible in the center of the summit crater on 6, 11, 16, 26, and 31 August. On 21 August there were two anomalies on the NW and NE sides of the crater. A small anomaly on the NE side of the crater was detected on 9 September. Hotspots were mostly obscured by weather clouds on 10 and 15 September but were still visible in the summit crater. The anomalies on the 20th (figure 221) and 30th of September were slightly larger in size compared to the one on the 9th and were also in the center of the crater. There were two anomalies detected in the center and on the W side of the crater on 5 October (figure 221). By 10 October there were three small anomalies located to the SW, NE, and central part of the crater, all roughly equal in size. There was only a total of two days during November and December where a very faint anomaly was visible, though clouds mostly obscured the view of the crater: on 19 November and 29 December. Two small anomalies were detected in the center of the crater on clear weather days on 8 and 28 January 2023.

Geological Summary. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: 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).