Hachijojima is located on the volcanic front of the Izu-Ogasawara Arc and consists of two volcanoes, Higashiyama and Nishiyama which are built up by mainly basaltic lavas and pyroclastic materials. Topographic features and eruptive history of Higashiyama Volcano are complicated because the volcano has several old edifices and calderas. The activity of main stratovolcano of Higashiyama started probably several tens thousands years ago. The eruptive history of Higashiyama Volcano after completion of the main stratovolcano is divided into next five stages ; Noboryotoge (before 22,000 y. B. P.), Sueyoshi I (22,000 y. B. P.), Sueyoshi II, Nakanogo and Mitsune (10,000-4,000 y. B. P.) Stages. In Noboryotoge Stage thick pyroclastic deposits, mainly scoria fall, were deposited on the northeastern part of Higashiyama. In Sueyoshi I Stage a debris avalanche, pyroclastic flow and fall occured in succession. Nishihakuunzan Caldera might be formed as the result of this eruption. In Sueyoshi II Stage small eruptions and phreatic explosions lasted, and a new edifice, Mihara Volcano formed in Nishihakuunzan Caldera. In Nakanogo Stage scoria fall eruptions took place at the central crater and flank, and lavas and pyroclastic flows flowed down into southern part of Higashiyama. The latest activities of Higashiyama Volcano in Mitsune Stage were all flank eruptions. The total volume of volcanic products of Higashiyama Volcano is 14.3 km3, and the eruption rate during past 22,000 years is 0.22 km3/1,000 years.
