Tephrostratigraphy and Eruption History during the Last 200,000 Years on the Yatsugatake Volcanic Chain, central Japan

OOISHI Masayuki

Key words

Yatsugatake Volcanic Chain, Yatsugatake Younger-period Tephra Beds, Eruption history, tephrochronology, Late Pleistocene 


   This study documents eruption history of the Yatsugatake Volcano from the tephrochronological viewpoint, to contribute the prevention of volcanic disaster. The Yatsugatake Younger-period Tephra Beds are identified as follows at Toyosato, Koumi area, central Japan; the Yt-Kw, Yt-Pm1, Yt-Pm2, Yt-Pm3 and Yt-Pm4 Tephra Beds, in ascending order. All tephra beds are composed of orange pumiceous grains by plinian eruption, involved by thin gray ash layer by vulcanian eruption except for the Yt-Pm4 Tephra Bed. The DRE (Dense Rock Equivalent) volume of Yt-Kw, Yt-Pm1, Yt-Pm3, and Yt-Pm4 are estimated to 1.1 km3, 0.4 km3, 0.7 km3 and 0.4 km3, respectively. The eruption of Yt-Kw, Yt-Pm1, Yt-Pm2, Yt-Pm3, and Yt-Pm4 occurred at about 153 ka, from 86 to 64 ka, from 64 to 46 ka, from 54 to 40 ka and at about 28 ka, respectively, based both on the stratigraphic position of widespread tephra beds and the thickness of intercalating loess beds.

   The Yt-Pm4 Tephra Bed was derived from the Kita-Yokodake Volcano that has nine lave flows. The Y5 or Y6 Lava Flow immediately followed the Yt-Pm4 Tephra Bed, indicating their eruption age at 28 ka. The Yt-KY Lapilli appear in andsol that overlies the Holocene Y9 Lava Flow. This stratigraphy strongly implies that the Yatsugatake Volcano erupted several times in Holocene at the latest. While, the Yt-Kw, Yt-Pm1, Yt-Pm2 and Yt-Pm3 Tephra Beds were derived from the Tengudake Volcano. The Yt-Kw Tephra Bed is probably correlative of the Ykl Lava Flow, and the Yt-Pm1, Yt-Pm2 and Yt-Pm3 Tephra Beds are probably correlative of the Tml or Tpl Lava Flows, in terms of ages of these tephras and lava flows.

   The tephra beds that described in this study are considered as useful chronological references to reconstruct Middle and Late Pleistocene history of the Yatsugatake Volcanic Chain and surrounding paleogeography. Especially, the Yt-Kw Tephra Bed can play an important role to determine the age of geomorphic surface around the Kanto Mountains during the Marine Isotope Stage 6. Future expected works is to precisely reveal distribution pattern, eruption age and source crater of each tephra bed.