Report on Ioto (Japan) — 3 October-9 October 2001
Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
3 October-9 October 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, 3 October-9 October 2001. Smithsonian Institution and US Geological Survey.
24.751°N, 141.289°E; summit elev. 169 m
All times are local (unless otherwise noted)
No volcanic activity was observed at Iwo-jima after 22 September, but discolored seawater was occasionally visible along the SE coast until at least 10 October. The area of discolored water was smaller than when the submarine eruption began on 21 September. During 20 to about 28 September many earthquakes and tremor were detected. Floating pumice collected along Iwo-jima's coast after the eruption is being analyzed to determine if the eruption was magmatic or hydrothermal.
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.