Nikko volcano is a large submarine volcano in the Volcano Islands chain of Japan. Discoloured water above the volcano has been observed almost every year between 1979 and 1990.
Nikko submarine volcano is a massive seamount that rises from nearly 3 km depth to within 391 m of the sea surface at the SE end of a submarine ridge segment extending from Minami-Iwo-jima island. Two large cones at the basaltic-to-andesitic volcano have been constructed on the NW and NE rims of a roughly 3-km-wide, flat-floored submarine caldera, whose rim is prominently displayed on the southern side, but largely buried on the north. A smaller cones lies on the SE caldera floor. The larger NW cone lies within a partially buried crater and displays hydrothermal activity. Discolored water was observed above Nikko seamount in July 1979, but no water discoloration has been observed during semi-regular seasonal reconnaissance flights since then. Hydrothermal venting was documented during a recent NOAA expedition.
