Yukio HAYAKAWA, Fusao ARAI, and Tomohiro KITAZUME
The Edifice of Hiuchi Volcano was founded on the Mokake ignimbrite plateau which is directly underlain by the Nanairi pumice. They erupted from near the present summit successively about 350 ka. On the plateau, the Ozorizawa cone was first established about 100 ka. Next, the Shibayasugura cone having twin peaks, Shibayasugura (2356 m) and Manaitagura (2346 m), was built shifting slightly southeast. At 19 ka, the Jubeike lava flow and Kumazawa Tashiro lava dome were issued from the northeastern flanks. The latter was accompanied by a devastating blast. Southward-opening horseshoe depression is source of the Nushiri debris avalanche, which dammed up the Nushiri River and formed the Oze marsh about 8 ka. The Akanagure lava dome and flows immediately occupied the depression. The youngest volcanic feature at Hiuchi is the Miike lava dome, 300 m south of the Manaitagura peak. It emplaced during the 16th century, just before the Shirohikemizu flood descended the Hinoemata River on July 28, 1544. Hiuchi Volcano has erupted 17 x 1012 kg of magma since its birth, 350 thousand years ago. Average discharge rate is 4.9 x 1010 kg per thousand years, an order of magnitude smaller than that of the most active Japanese volcanoes.