Tofua is a remote volcano in the central part of the Tonga Islands group that contains a 5-km-wide caldera; three post-caldera cones were constructed at the N end of a caldera lake. The easternmost cone has three craters that have produced young basaltic-andesite lava flows, some of which traveled into the caldera lake. The largest and northernmost of the cones, Lofia, has a steep-sided crater that is 70 m wide and 120 m deep and has been the source of historical eruptions dating back to the 18th century. Recent activity has been characterized by intermittent thermal anomalies in the Lofia crater (BGVN 45:08), which continues through this reporting period of August 2020 through July 2021. Information primarily comes from satellite data.

Summary of activity during August 2020-July 2021. No ash advisories were issued during the reporting period; however, intermittent hotspots were detected by Sentinel-2 thermal satellite imagery and Suomi NPP/VIIRS sensor data throughout the reporting period (figure 12). Anomalies from the MODVOLC system clustered during February 2021 through April and June. There was a notable increase in activity during mid-February through July, compared to the previous months, which included distinct sulfur dioxide plumes (figure 13) and consecutive days with thermal anomalies, according to NASA VIIRs satellite data.

Figure 12. Timeline summary of observed activity at Tofua from August 2020 through July 2021. For Sentinel-2, MODVOLC, and VIIRs data, the dates indicated represent detected thermal anomalies. For the SO2 data, the dates indicated represent detected sulfur dioxide plumes. White areas indicate no activity was observed, which may also be due to cloud cover. Each cell represents one calendar day. Data courtesy of Sentinel Hub Playground, HIGP, NASA Worldview using the “Fire and Thermal Anomalies” layer, and NASA Global Sulfur Dioxide Monitoring Page.

Figure 13. Weak but distinct sulfur dioxide emissions were detected from Tofua on 17 (top left), 18 (top right), 19 (bottom left), and 25 (bottom right) February 2021, each of which drifted W, based on data from the TROPOMI instrument on the Sentinel-5P satellite. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Sentinel-2 infrared satellite imagery and NASA Worldview detected intermittent thermal anomalies during August 2020 through July 2021 dominantly in the Lofia crater (figure 14); rare fires were noted along the NW coast of the island. Beginning in mid-February, the activity notably increased with a stronger thermal anomaly. This increase was accompanied by white gas-and-steam emissions, which included distinct sulfur dioxide plumes.

Figure 14. Sentinel-2 infrared satellite imagery showed a persistent thermal anomaly in the Lofia crater at Tofua during September 2020 through May 2021; the intensity of the anomaly notably increased during February through May and was occasionally accompanied by white gas-and-steam emissions. Images with "Atmospheric penetration" (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Geological Summary. The low, forested Tofua Island in the central part of the Tonga Islands group is the emergent summit of a large stratovolcano that was seen in eruption by Captain Cook in 1774. The summit contains a 5-km-wide caldera whose walls drop steeply about 500 m. Three post-caldera cones were constructed at the northern end of a cold fresh-water caldera lake, whose surface lies only 30 m above sea level. The easternmost cone has three craters and produced young basaltic andesite lava flows, some of which traveled into the caldera lake. The largest and northernmost of the cones, Lofia, has a steep-sided crater that is 70 m wide and 120 m deep and has been the source of historical eruptions, first reported in the 18th century. The fumarolically active crater of Lofia has a flat floor formed by a ponded lava flow.

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/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).