Abstract
Rausu volcano in the central Shiretoko Peninsula, eastern Hokkaido, has produced three pyroclastic falls : Ra-1 (500-700 cal BP), Ra-2 (ca. 1,400 cal BP), and Ra-3 (ca. 2,200 cal BP), associated with eight pyroclastic flow deposits over the last 2,200 years. These tephras are rich in spongy glass shards with minor plagioclase, orthopyroxene, clinopyroxene, and opaque minerals. They are similar in mineral assemblage, but can be distinguished from each other by refractive indices of dehydrated glass. Tenchozan-a tephra (Ten-a : ca. 1,900 cal BP) from Tenchozan volcano, 4.5km southwest of Rausu volcano, contains abundant lithic fragments associated with flaky glass, plagioclase, orthopyroxene, and clinopyroxene. Its eruption is interpreted as phreatomagmatic. The volume of Ten-a is estimated to be about 0.02km3.
Cores were drilled in 2005 to obtain detailed records of Holocene tephras in peat bogs adjacent to Rausu and Tenchozan volcanoes using a peat sampler and geoslicer. Seven tephra layers were recognized and identified as Komagatake-c1 tephra (Ko-c1 : AD1856), Tarumae-a tephra (Ta-a : AD1739), Komagatake-c2 tephra (Ko-c2 : AD1694), Ra-1 or successive pyroclastic flow deposit, Mashu-b tephra (Ma-b : 774-976 cal BP), and Ichinonuma ash. Ko-c1, Ta-a, and Ko-c2 are the widespread marker tephras from Komagatake and Tarumae volcanoes, southwestern Hokkaido. Ma-b is from Kamuinupuri (a.k.a. Mashudake) volcano, 75km southwest of Rausu volcano. The source volcano and chronology of Ichinonuma ash are unknown. Mashu-l tephra (ca. 13,000 cal BP) is found in the southern part of Shiretoko Peninsula.
