Nikko has shown no signs of activity since July 1979.

Geological Summary. Nikko submarine volcano is a massive seamount that rises from nearly 3 km depth to within 392 m of the ocean surface near the N end of the Mariana Volcanic Arc. 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 cone is present on the SE caldera floor. The larger NW cone lies within a partially buried crater and displays hydrothermal activity. Discolored water was observed in July 1979, but none has been observed during semi-regular seasonal reconnaissance flights since then. Hydrothermal venting was documented during a NOAA expedition.

Information Contacts: JMSA, Tokyo; JMA, Tokyo.