[Abstract]

We newly identified tephra layers sandwiched by soil layers between AD 1663 (Kanbun age) and 1769 (Meiwa age) tephras at several sites on the northwestern flank of Usu volcano. The layers consist of pumice fall, ash fall and pyroclastic surge deposits. The presence of soil layers were confirmed by their carbon and nitrogen analysis. Petrographical and geochemical features of the pumice from newly recognized tephra largely differ from those of 1663 eruption, and are similar to those of 1769 one. However, on the basis of modal volume and chemical compositions of phenocrystic minerals, it can be possible to distinguish the pumice from those of the 1769 eruption. Considering the thickness of soil layers, the eruption would occurred at around end of 17th century. We name the eruption as the pre-Meiwa eruption, because petrological features of the pumice is similar to those of 1769 (Meiwa). The eruption was generated by distinct magma from other historic eruptive magmas. The sequence of the tephra suggests that the eruption started with phreatic eruptions followed by magmatic eruption to form plume. In the climactic stage, pyroclastic surge occurred with the formation of plume and its activity ended with phreatic eruptions. The finding of the pre-Meiwa eruption could provide important information to reveal not only relationship between eruption and dormancy but also evolution of magma systems.

Key words:

Usu volcano, pre-Meiwa eruption, historic eruptions, tephra, soil layer