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Report on Ioto (Japan) — 19 September-25 September 2001


Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
19 September-25 September 2001
Managing Editor: Gari Mayberry

Please cite this report as:

Global Volcanism Program, 2001. Report on Ioto (Japan). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 19 September-25 September 2001. Smithsonian Institution and US Geological Survey.

Weekly Report (19 September-25 September 2001)



24.751°N, 141.289°E; summit elev. 169 m

All times are local (unless otherwise noted)

At 1015 on 21 September a submarine eruption began off of the SE coast of Iwo-jima, an island inhabited by U.S. and Japanese military personnel approximately 1,300 km S of Tokyo. The eruption was preceded by isolated and continuous tremor beginning on the evening of 20 September. Visible evidence of the eruption consisted of seawater gushing several meters above sea level near the island's coast and an area of discolored seawater extending 300-400 m in length. During 1000-1100 approximately 30 earthquakes occurred in the active area; the typical rate is one or two earthquakes per hour. The climax of the eruption occurred during 1200-1500. At about 1300 water gushed several ten's of meters above sea level and steam rose to 100-300 m above the sea. JMA personnel observed seawater rising intermittently during 1515-1715 at two points 50 m apart and 150-200 m from the island's SE coast. The temperatures at these points were 33-34 ºC and 50 ºC, while the surrounding water was at 27 ºC. By 1500 ten earthquakes were recorded per hour. During 1600-1700 three eruption sites were visible; at one a pyroclastic cone was slightly above the sea surface. JMA reported that by the next day volcanic and seismic activity returned to usual levels, with zero to four earthquakes occurring per hour and no tremor events.

Geological Summary. Ioto in the central Volcano Islands portion of the Izu-Marianas arc lies within a 9-km-wide submarine caldera. Ioto, Iwojima, and Iojima are among many transliterations of the name. The volcano is also known as Ogasawara-Iojima to distinguish it from several other "Sulfur Island" volcanoes in Japan. The triangular, low-elevation, 8-km-long island narrows toward its SW tip and has produced trachyandesitic and trachytic rocks 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 accompanying 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 Motoyama plateau on the NE half of the island consists of submarine tuffs overlain by coral deposits and forms the island's high point. Many fumaroles are oriented along a NE-SW zone cutting through Motoyama. Numerous historical phreatic eruptions, many from vents on the west and NW sides of the island, have accompanied the remarkable uplift.

Sources: Volcano Research Center-Earthquake Research Institute (University of Tokyo), Japan Meteorological Agency (JMA), Reuters