Tofua Island in the central part of the Tonga Islands group is the emergent summit of a large stratovolcano that contains a 5-km-wide caldera. Three post-caldera cones were constructed at the northern end of a caldera lake. The easternmost cone has three craters that produced lava flows, some of which have traveled into the caldera lake. The largest and northernmost of the cones, Lofia, has been the source of previous eruptions dating back to the 18th century. The current eruption period began in October 2015 and has more recently consisted of thermal activity in the Lofia crater (BGVN 48:02). This report updates activity from February through July 2023 using information primarily from satellite data.

Occasional 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 19). Sentinel-2 infrared imagery showed a bright thermal anomaly in the Lofia crater once during February, four times during April, three times during May, twice during June, and twice during July (figure 20). A single thermal alert was detected by the MODVOLC system on 11 June 2023. There was a total of 29 days during February-July when hotspots were detected in Suomi NPP/VIIRS data, with detections ranging from 1-7 each month.

Figure 19. Timeline summary of thermal activity at Tofua detected using satellite data from February through July 2023. 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 Copernicus Browser, HIGP - MODVOLC Thermal Alerts System, and NASA Worldview using the “Fire and Thermal Anomalies” layer.

Figure 20. Infrared (bands B12, B11, B4) satellite images showing a persistent bright thermal anomaly (bright yellow-orange) in the Lofia crater at Tofua during 2 April 2023 (top left), 17 May 2023 (top right), 26 June 2023 (bottom left), and 21 July 2023 (bottom right). The black color at the bottom center of the images is the crater lake. Courtesy of Copernicus Browser.

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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).