Abstract
The widespread tephra Komagatake-g (Ko-g), which erupted from Komagatake volcano, is extensively distributed in southern to eastern Hokkaido. Ko-g is characterized by predominantly spongy glass and plagioclase with some orthopyroxene and clinopyroxene. The refractive index of dehydrated volcanic glass ranges from n=1.500-1.505. On the northeastern slopes of Mt. Komagatake, Ko-g is a coarse pumice-fall deposit (-20cm in diameter) with maximum thickness of 125cm. Even in eastern Hokkaido, 360km from the source, Ko-g has a thickness of 5cm and a grain size up to 1/2mm in diameter.
Based on several 14C dates obtained from different regions, the eruptive age of Ko-g can be determined to be 6, 500-6, 600 cal yrs BP.
Ko-g is an important key tephra for examining some issues in Hokkaido such as Jomon cultures, the mid-Holocene transgression, the chronology of tsunami sediments, and slope deposits in the Hidaka mountains.
