抄録
The 30 ka caldera-forming eruption of Ohachidaira in central Taisetsu volcano, Hokkaido, produced Plinian pumice-fall and pyroclastic-flow deposit that contain juvenile products of pumice, scoria, and banded pumice. We reconsidered the eruption history of the caldera-forming eruption on the basis of combined geological and petrological data. The pyroclastic-flow deposits are classified into two types based on petrological features: Hb-type and Px-type. We identified three representative outcrops of the main deposit types at the foot of Taisetsu volcano: the Oiwa outcrop of an Hb-type pyroclastic flow, the Obako outcrop of a Px-type pyroclastic flow and a Plinian pumice fall, and the Tenninkyo outcrop where a Px-type pyroclastic and an Hb-type pyroclastic flows are recognized in the upper and lower parts, respectively. We analyzed the glass compositions of juvenile pumices as means of discriminating the two types of pyroclastic-flow deposit and estimated the flow directions and distribution of the two types of pyroclastic flow. The Hb-type pyroclastic-flow deposit is distributed in north-westerly and south-westerly directions. On the other hand, the Px-type pyroclastic-flow deposit is distributed in north-easterly and south-westerly directions. There might be more than a few hundred years interval between the Hb-type and Px-type pyroclastic flows.
