Ioto (硫黄島, also known as Iwo-jima) volcano is a triangular-shaped, flat, 8 km long and up to 4.5 wide volcanic island stretching NE-SW. It is surrounded by steep cliffs under the sea, which belong to a 9-km-wide submarine caldera. The volcano is located 1250 km south of Tokyo in the central Volcano Islands of the Izu-Marianas arc.
The summit of Iwo-jima is Suribachi-yama at 160 m.
Parts of the caldera have been experiencing remarkably strong uplift (up to more than 1 meter per year and averaging 25 cm / year over several centuries). There is strong hydrothermal activity at present, at fumaroles along a NE-SW zone cutting through Moto-yama. Many phreatic eruptions occurred in historic times, mostly from vents on the west and NW sides of the island.
Iwo-jima was the site of an infamous WWII battle, and the location of a famous image of the raised US flag.
Ioto, Iwo-jima, and Io-jima are among many spellings of the same name, which means "Sulfur Island", as many other volcanic islands in Japan. Another name, to distinguish it better is Ogasawara Io-jima.
Ioto volcano has produced trachyandesitic and trachytic lavas that are more alkalic than those of other Izu-Marianas arc volcanoes.
The island has undergone dramatic uplift for at least the past 700 years caused by resurgent doming of the caldera. A shoreline landed upon by Captain Cook's surveying crew in 1779 is now 40 m above sea level. The Moto-yama plateau on the NE half of the island consists of submarine tuffs overlain by coral deposits and forms the island's high point.
During 1911 to 1952, uplift was 11 cm per year and reached 33 cm per year between 1952 and 1968. More recent studies show that the center of the Iwo-jima caldera is experiencing subsidence while the surrounding areas are uplifted.
- Smithsonian / GVP volcano information
- Ukawa et al (2005) "Long-term geodetic measurements of large scale deformation at Iwo-jima caldera, Japan", Journal of Volcanology and Geothermal Research, Volume 150, Issues 1-3, Pages 98-118
- NEWHALL, C. G. & DZURISIN, D. (1988) "Historical Unrest at Large Calderas of the World", Volumes 1 & 2, USGS Bulletin 1855. x + 598 pp., v + pp. 599-1108.