Tofua is located in the central part of the Tonga Islands group and 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 recorded eruptions dating back to the 18th century. The current eruption period began in October 2015 and recent activity has consisted of mostly intermittent thermal anomalies in the Lofia crater and sulfur dioxide emissions (BGVN 46:08). This reporting period covers persistent thermal anomalies in the Lofia crater during August 2021 through July 2022. Information primarily comes from satellite data.

Summary of activity during August 2021-July 2022. Intermittent hotspots were detected by Sentinel-2 infrared satellite imagery, the MODVOLC infrared satellite data using NASA’s MODIS instrument, and Suomi NPP/VIIRS sensor data (figure 15). Sentinel-2 imagery showed a bright thermal anomaly in the Lofia crater 2-5 times each month during the reporting period (figure 16). A total of 14 thermal alerts were detected by the MODVOLC system 1-3 times every month of the reporting period. Many of these hotspots were also reflected in the Sentinel-2 and Suomi NPP/VIIRS data. There was a total of 114 days of hotspots during the reporting period that ranged from 3-14 each month in Suomi NPP/VIIRS data.

Figure 15. Timeline summary of thermal activity at Tofua detected using satellite data from August 2021 through July 2022. The dates indicated represent detected thermal anomalies for Sentinel-2, MODVOLC, and VIIRS data. 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 - MODVOLC Thermal Alerts System, and NASA Worldview using the “Fire and Thermal Anomalies” layer.

Figure 16. Sentinel-2 infrared satellite imagery showed a persistent bright thermal anomaly (bright yellow-orange) in the Lofia crater at Tofua during August 2021 through July 2022. Images were taken on 15 August 2021 (top left), 24 September 2021 (top middle), 17 January 2022 (top right), 12 April 2022 (bottom left), 12 May 2022 (bottom middle), and 16 July 2022 (bottom right). 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 Worldview (URL: https://worldview.earthdata.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).