Suiyo, one of the Shichiyo Seamounts, named for the seven days of the week, lies south of Sofugan volcano. Suiyo ("Wednesday") is a basaltic-to-dacitic submarine volcano and lava dome that rises about 1,400 m from the sea floor to within about 1,000 m of the surface. The summit crater is 1.5 km wide and about 500 m deep. Major hydrothermal activity was observed in July 1991, with temperatures reaching more the 290°C, and the volcano was reclassified as active by the Japan Meteorological Agency.