An area of green discolored water, 3-5 km long, was observed over the volcano during an overflight on 12 February. Subsequent overflights revealed additional water discolorations on 28 February, and 2, 3, and 4 March, although no discoloration was seen on 21 February. The 4 March discoloration appeared to have a source area 100 m across. Overflights have been conducted almost every month in the Izu and Volcano Islands by the JMSA. This was the first observed incidence of water discoloration since the mid-to-late 1970's, when bubbling, spouting, and discolored water were occasionally sighted.

Geological Summary. Periodic water discoloration and water-spouting have been reported over the Minami-Hiyoshi submarine volcano since 1975, when detonation sounds and an explosion were also reported. It lies near the SE end of a coalescing chain of youthful seamounts, and is the only vent with recorded activity. The reported depth of the summit of the trachyandesitic volcano has varied between 274 and 30 m. The morphologically youthful seamounts Kita-Hiyoshi and Naka-Hiyoshi lie to the NW, and Ko-Hiyoshi to the SE.

Information Contacts: JMSA.