Keywords:
Tephrochoronology in the southwest part of basaltic Izu-oshima
volcanic island during last 20,000 years, off Tokyo, Pacific Ocean
*Takehiko Suzuki1, Satoshi Terayama1, Jun Matsukaze1, Yuichi Kato1
1. Faculty of Urban Environmental Sciences, Tokyo Metropolitan University 
The Izu-oshima volcano (110 km SSW of Tokyo) in northern Izu-Bonin Arc, is one of the most active
Quaternary volcanoes in Japan. This volcano originating from basaltic magma has frequently erupted; that
is, 12 times of major eruptions occurred after 2 ka. Studies of its eruption history have been carried out by
Nakamura (1964), Kawanabe (2012) and so on. However, precise ages of eruption preceding the caldera
formation occurred at 1.8 ka have not been determined. Here, we re-examine the chronological
framework of eruption history of the Younger Edifice of pre-caldera. 
Fall-out tephras are typically exposed along so-called Great Cut of Beds in SW part of the island. Previous
studies by Tazawa (1990) and Uesugi et al. (1994) have described this outcrop, numbering each tephra
layer using serial numbers (O1–95 and Osb96–127). All tephras except O55 and O58 are proximal fall-out
tephras composed of scoriaceous lapilli and/or scoriaceous ash. Due to the large number of tephras and
lithological similarity, it is difficult to confirm each tephra defined by previous studies except several
characteristic ones such as O95, O58, and O55. O55 and O58 are rhyolitic tephras from Nijima island and
Omuro-dashi submarine depression, respectively (Saito et al., 2007). 
Uesugi et al. (1994) estimated the age of the lowest tephra to be 25 ka or older by using 15 radiocarbon
ages. However, this estimation is not reliable because it was determined using two oldest conventional
radiocarbon ages (non-calibration) of 14–13 ka for the same horizon (just above O64) and under the
assumption that the frequency of the tephra occurrence (154 years interval) is constant. 
In this study, radiocarbon dating was performed for organic tephric loess collected from 9 horizons above
O55 and 16 under O55. The oldest radiocarbon age is 18,598–18,287 cal BP, determined at the horizon
4 m below O95. The age of O95, the most prominent scoria fall deposit, is most likely to be 16–17 ka
judging from several radiocarbon ages. It is 2400–3700 years younger than the estimated age (19.4–19.7
ka) by Uesugi et al. (1994). We also recognized widespread K-Ah tephra using volcanic glass
geochemistry. Two radiocarbon ages determined at 45 cm above and 55 cm below K-Ah are 6,882–6,747
cal BP and 8167–8021 cal BP, respectively. These ages are concordant with revised age of K-Ah
(7,307–7,196 cal yrs BP; McLean et al., 2018), indicating the age of horizon detected K-Ah is older than
estimated age of 6,330±85 yBP by Uesugi et al. (1994). These results indicate that the interval between
O95 and K-Ah is shorter (67–75%) than previous estimation, suggesting higher frequent occurrence of the
eruptions in this period. 
Izu-oshima, eruption history, carbon dating 
SVC38-01Japan Geoscience Union Meeting 2019
©2019. Japan Geoscience Union. All Right Reserved.- SVC38-01 -