At 0900 on 23 May Tonga Geological Services reported that elevated activity at Tofua had ceased during the previous week. The characteristics of a thermal anomaly identified in a satellite image indicated a return to baseline levels. The Aviation Color Code was lowered to Green (the lowest color on a four-color scale), the Maritime Alert Level was lowered to Green (the lowest level on a four-color scale), and the Alert level for residents of Vava’u and Ha’apai remained at Green (the lowest color on a four-color scale).

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.

Source: Tonga Geological Services, Government of Tonga