This paper reports the results of a research on the relationship between Unzen volcano located on Kyushu Island in Japan and the DC-10 aircraft two-engine failure incident at FL370 (11,300 m) between Honolulu, Hawaii, USA and Fukuoka, Japan on 27 June 1991. The cause of the engine failure was presumed by the USGS to be an aircraft encounter with volcanic ash (hereafter referred to as “VA”) emitted from Unzen volcano. The authors doubted the presumption above, and examined the records of volcanic activities at Unzen volcano and related materials. As a result, it turned out that although the day of the aircraft encounter was one of the active days, pyroclastic flows on that day were not exceptional or extraordinary, and not likely to have a serious impact on aircraft. A PUFF model VA dispersion simulation using the volcanological observation data in the initial conditions showed that the maximum possible VA altitude was less than 13,000 ft. The second simulation, assuming that VA reached 37,000 ft by some mechanism, showed that VA concentration remained low and the calculated VA dose along the possible flight route was within the safe category of engine operations based on the DEvAC chart developed by Rolls-Royce. From all the data available, as well as comparisons with VA encounter cases at other volcanos in Japan, the possibility could not be found that the pyroclastic flows at Unzen volcano were related to the two-engine failure incident on 27 June 1991. The result of the research suggests that VA from Unzen volcano should not be considered the cause of the engine failure and that the true cause must be further investigated.

On August 13, 2021, Fukutoku-Oka-no-Ba Volcano, which belongs to the Volcanic Islands on the volcanic front of the Izu-Ogasawara Arc, erupted for the first time in 11 years since 2010. Fukutoku-Oka-no-Ba Volcano is one of the most active volcanoes in Japan, with major eruptions in 1904, 1914, and 1986-88 that formed new island, as well as small-scale eruptions in 1963(?), 1973-75, 1979(?), 1992, 1996-97, 2000, and 2004-2010. Observed eruptive activities include floating pumices, an eruption column, a vapor column, a water dome and a water column associated with phreatomagmatic explosion, strongly upwelling discolored water, and sulfur smell.

The 2021 eruption began with the sudden generation and growth of a white eruption column, followed by the generation of floating pumices, phreatomagmatic explosions, land formation associated with pumice ejection, and eject of giant pumices characteristic of underwater eruption. The eruption that was accompanied by floating pumices ejections ended on August 15, but volcanic activities with active discolored water and small floating particles continued until around October 2023.

Although any giant pumices have not been directly sampled, the white and massive objects at the base of the island show high temperatures in infrared images, suggesting that they are giant pumices. The water depth at the bottom of the crater for the 2021 eruption is estimated to be about 150 m, which is within the range of water depths of other underwater volcanoes where giant pumices have been found. On the other hand, the giant pumices deposited together with pumice fall deposits suggest that they were formed during explosive eruptions or that explosive and effusive eruptions occurred simultaneously.