Host: The Association of Japanese Geographers
Five Holocene marker tephras from Hokkaido Island, i.e. Tarumae-a tephra (Ta-a, AD1739), Komagatake-c2 tephra (Ko-c2, AD1694), Mashu-b tephra (Ma-b, 774-976 cal BP), Tarumae-c tephra (Ta-c, 2500-2800 cal BP), and Mashu-d1 tephra (Ma-d1, 3267-3368 cal BP) are identified in the sedimentary sequences of coastal lowlands on Kunashiri and Shikotan Islands, southern Kuril Islands, on the basis of the refractive index and major element analyses of tephra glass shards. Ko-c2 and Ta-c are the most distinguishable ash fall deposits, providing valuable time markers to examine Holocene issues such as paleotsunamis.
In addition, more than 12 tephra layers are described and presumed to be the products of volcanoes in Kunashiri Island. Many of them contain glass shards showing relatively low refractive index (n=1.480-1.490) and high SiO2 content (77-79%) in comparison with the widespread tephras from Hokkaido Island. These tephras are classified into seven types according to TiO2-K2O contents. They are presumably supplied from three or more volcanoes in Kunashiri Island.
