One of the most famous Japanese tephra, Aira-Tanzawa ash, which erupted in Kyushu 1,000 km away at 25 ka and discovered by Machida and Arai in 1976, underlies the Murota pumice from Asama. The ash is composed mainly of silt-size glass, now partly altered into clay minerals. The thickness is about 10 cm, implying a minimum volume of 100 km3 (roughly equals to M7).
Between the ash and overlying BP0 pumice, a 5-cm thick heavily weathered massive ash layer is intercalated. This layer is interpreted as loess, which was emplaced as windblown dust during a dormant period of Asama. Another loess bed, 10-cm thick, is recognized between BP0 and overlying BP2 pumice. As accumulation rates of this kind of loesses have been measured 2-14 cm/ky at a number of volcanic regions in Japan, the dormant period represented by these two loess beds is estimated to be several hundred years each.
Below the Aira-Tanzawa ash, tephra from Asama are hardly found in the 3-m thick loess section upon the Neogene ignimbrite, suggesting the absence of explosive eruption of Asama before 25 ka.
Outcrop
Isopach map of the Aira Tanzawa ash (Machida and Arai, 1992)
Stop 1 Matori: View of Asama
