The 9 x 5 km island of Nakanoshima is surrounded by coral reefs. A flat plateau separates the older dissected Sakiwaridake volcano on the south from the active andesitic cone of Ontake, which forms the northern half of the island. Ontake contains a summit crater that is filled with water during the rainy season. Sulfur deposits were mined at a SE-flank solfatara until 1944. Only very minor activity has been reported in historical time. A small mud eruption took place in 1914 at the summit crater, and "smoking" increased in 1949.