- Tokyo Geographical Society
- 地学雑誌 (ISSN:0022135X)
- vol.105, no.4, pp.431-447, 1996-08-25 (Released:2009-11-12)
We made a detailed tephrochronological study in the western and southern foots of Hakone Volcano and revealed the eruptive histories of Hakone and other volcanoes since 240 ka. We recognized four widespread tephras, Aira-Tn Ash (AT), Sanbe-IkedaAsh (SI), Kikai-Tozurahara Ash (K-Tz) and Ontake 1 Pumice (On-Pm1) as well as the tephras from the adjacent volcanoes, such as Hakone-Mishima Pumice (Hk-CC4), the pyroclastic flow associated with Hakone-Tokyo Pumice (Hk-TPfl), and Hakone Da-5 Pumice from Hakone Volcano, Fuji Susono-1 Scoria and Fuji Susono-2 Scoria (F-Sul and F-Su2) and Fuji-Yoshioka Pumice from Fuji Volcano, and the tephras from the Higashi-Izu monogenetic volcano field. Eruption ages of F-Su2, F-Sul, Hk-CC4 and F-YP are estimated to be 36 ka, 37 ka, 43 ka and 85 ka, respectively, by loess-chronometry. The discharge mass and eruptive magnitude M are estimated to be 3.7 × 1011 kg and 4.6 for Hk-CC4, and 4.2 × 1011 kg and 4.6 for Da-5 Pumice on the basis of their isopach maps. The eruption frequency in the study area is 0.11 times/ ky for the last 240 ky in average, showing an abrupt increase to 0.53 times/ky during 120 ka-135 ka. In this period, both Hakone Volcano and the Higashi-Izu monogenetic volcano field seem to have been active simultaneously.The loess thickness, which is interbedded between Hk-CC4 (43 ka) and Hk-TPfl (52 ka), tends to increase toward the eastern foot of Fuji Volcano. The age-depth diagram shows that the accumulation rate of loess is nearly constant in each locality except for the rapid increase at about 50 ka in the Kannami area. The linear accumulation of loess supports the validity of loess-chronometry. The 50 ka increase of loess accumulation rate seems to be associated with enlargement of barren plains, which were located in the eastern of Fuji Volcano and were probably the main source of loess materials.