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[SCG54-07] Is the Iriomotejima NNE submarine volcano Irizuni?
Keywords:Iriomotejima, Active volcano, Submarine volcano
On October 31, 1924, exactly 100 years ago, an eruption occurred in the north-northeast offing of Iriomotejima Island in the Yaeyama Islands, Okinawa Prefecture, Japan, spewing a large amount of pumice into the sea surface. Captain Kano of the merchant ship Miyako Maru, who witnessed the eruption, left a detailed report, and based on this report, the location of the volcano is given as 24.8°N, 124°E in the 4th edition of the JMA's Compendium of Active Volcanoes in Japan. However, recent bathymetric surveys have revealed that the area is on the continental slope from the continental shelf to the Okinawa Trough, and that no volcanic bodies exist. On the other hand, Kato (1982) selected several locations of the Submarine Volcano NNE of Iriomotejima based on bathymetry and other data and conducted dredging, but no pumice or other volcanic ejecta were obtained. The Japan Coast Guard (JCG) reviewed the results of previous marine surveys on the occasion of the 100th anniversary of the eruption, and obtained a feeling that the rise called Irizuni might be the Submarine Volcano NNE of Iriomotejima, and conducted surveys in 2023 to obtain evidence to support this.
Irizuni is a submarine rise located 13.5 km north of Nojima Zaki on Iriomotejima Island. 2017 survey by the Japan Coast Guard survey vessel Takuyo showed that it was almost circular in shape, with a diameter of about 1 km at the bottom and a specific height of 40 m. It is flat and mound-like. To the east of the mound, there are two smaller, similarly shaped mounds and a landform that appears to have been washed down from Irizuni. The surrounding area is marked by fault displacement topography, but Irizuni has not been deformed at all. The summit is flat, but there are concentric patterns of high back scatter intensity, where the topography is slightly elevated. A patchy, highly back scatter area was observed in the center of the summit, and a plume rising from it was seen in the water column data of multi-beam echosounder. Dredging was also conducted at Irizuni in 1990 as part of the Basic Map of the Sea in Coastal Water (1:50,000) "Northern Part of Iriomote Sima" survey, and carbonate sediments consisting almost entirely of biological remains were obtained.
In 2023, based on the results of these surveys, reflection seismic survey using an air gun, topographic survey using a multi-beam echosounder, underwater camera photography and dredging were conducted to clarify whether Irizuni is the Submarine Volcano NNE of Iriomotejima. In the topographic survey, concentric and patchy high back scatter areas and upwelling plumes were found in almost the same locations as in 2017. Reflective surfaces with a shape reminiscent of a crater were observed at approximately 150 m below the seafloor. Dredging was conducted at the summit and slope of the mountain body, but only carbonate sands and rocks consisting mostly of biogenic remains were collected, and no evidence of volcanic activity such as pumice or lava was obtained.
Overall, the results of the above investigations indicate that it is highly unlikely that Irizuni is the Submarine Volcano NNE of Iriomotejima that erupted in 1924, but it is possible that it is an older volcano.
Irizuni is a submarine rise located 13.5 km north of Nojima Zaki on Iriomotejima Island. 2017 survey by the Japan Coast Guard survey vessel Takuyo showed that it was almost circular in shape, with a diameter of about 1 km at the bottom and a specific height of 40 m. It is flat and mound-like. To the east of the mound, there are two smaller, similarly shaped mounds and a landform that appears to have been washed down from Irizuni. The surrounding area is marked by fault displacement topography, but Irizuni has not been deformed at all. The summit is flat, but there are concentric patterns of high back scatter intensity, where the topography is slightly elevated. A patchy, highly back scatter area was observed in the center of the summit, and a plume rising from it was seen in the water column data of multi-beam echosounder. Dredging was also conducted at Irizuni in 1990 as part of the Basic Map of the Sea in Coastal Water (1:50,000) "Northern Part of Iriomote Sima" survey, and carbonate sediments consisting almost entirely of biological remains were obtained.
In 2023, based on the results of these surveys, reflection seismic survey using an air gun, topographic survey using a multi-beam echosounder, underwater camera photography and dredging were conducted to clarify whether Irizuni is the Submarine Volcano NNE of Iriomotejima. In the topographic survey, concentric and patchy high back scatter areas and upwelling plumes were found in almost the same locations as in 2017. Reflective surfaces with a shape reminiscent of a crater were observed at approximately 150 m below the seafloor. Dredging was conducted at the summit and slope of the mountain body, but only carbonate sands and rocks consisting mostly of biogenic remains were collected, and no evidence of volcanic activity such as pumice or lava was obtained.
Overall, the results of the above investigations indicate that it is highly unlikely that Irizuni is the Submarine Volcano NNE of Iriomotejima that erupted in 1924, but it is possible that it is an older volcano.