Aso-4 Ash : A Widespread Tephra and its Implications to the Events of Late Pleistocene in and around Japan

• MACHIDA Hiroshi

  Dept. Geography, Tokyo Metropolitan Univ.

• ARAI Fusao

  Dept. Geology, Gunma Univ.

• MOMOSE Mitsugu

  Dept. Geography, Komazawa Univ.

Bibliographic Information

Other Title

• 阿蘇 4 火山灰 : 分布の広域性と後期更新世示標層としての意義
• 阿蘇火山灰--分布の広域性と後期更新世示標層としての意義
• アソ カザンバイ ブンプ ノ コウイキセイ ト コウキ コウシンセイ シヒョウ

Abstract

The Aso-4 pyroclastic flow is one of the largest of late Pleistocene age in Japan covering extensive areas of Kyushu around the Aso caldera and western part of Honshu. We have recently discovered a distal ash which has very similar petrographic and chemical properties to the flow deposits at several localities of Japan, and is assumed to be an airfall part of the Aso-4 unit. The ash, named the Aso-4 ash, is a crystal-vitric fine-grained ash, consisting of rhyodacitic bubble-walled glass shards, and brown hornblende and orthopyroxene (bronzite) as mafic phenocrysts. Its identification is carried out by combined parameters ; refractive index of glass ranges 1.506-1.511, low refractive index of orthopyroxene (γ=1.699-1.701) and relatively high index of hornblende (n₂=1.687-1.688). In addition glass in the ash is characteristically rich in potassium. These properties are similar to those of the Aso-4 pyroclastic flows. It is suggested that the ash is a coignimbrite airfall deposit of fine-grained materials from the upper part of an eruption column formed at the same time as the Aso-4 flow. The ash mantles extensive areas from Kyushu to Hokkaido and is also recognized and identified in several piston cores from the Sea of Japan and the northwest Pacific Ocean. The fallout area of this ash is now known to attain approximately 4×10⁶ km². It is striking that in the eastern part of Hokkaido, ca. 1700 km distant from the source, the ash with the thickness of 15 cm is well preserved immediately below the Kutcharo pyroclasitc flow deposits II/III. The bulk volume of this ash is estimated to be more than 400 km³, possibly larger than that of the flow deposits. The stratigraphic position of this ash in standard sequence of South Kanto shows that it occurs after the Obaradai interstade (ca. 80 ka) and before the Misaki interstade (ca. 60 ka). Thus the Aso-4 ash provides a very important datum plane in late Pleistocene sequence in and around Japan.

Journal

• SECOND SERIES BULLETIN OF THE VOLCANOLOGICAL SOCIETY OF JAPAN

  SECOND SERIES BULLETIN OF THE VOLCANOLOGICAL SOCIETY OF JAPAN 30 (2), 49-70, 1985

  The Volcanological Society of Japan

Details

• CRID: 1390282681099376512
• NII Article ID: 110002989968
• NII Book ID: AN10512786
• ISSN: 04534360
• ISSN: 24330590
• DOI: 10.18940/kazanc.30.2_49
• NDL BIB ID: 3052851
• Web Site: https://ndlsearch.ndl.go.jp/books/R000000004-I3052851
• Text Lang: ja
• Data Source:
  • JaLC
  • NDL
  • CiNii Articles
  • Crossref
• Abstract License Flag: Disallowed