阿蘇4火山灰および阿蘇火砕流堆積物の古地磁気方位 Paleomagnetic Directions of the Aso-4 Ash and the Aso Pyroclastic Flows

この論文にアクセスする

この論文をさがす

著者

抄録

阿蘇火山は大型のカルデラをもつ複合火山で,約27万年前から約9万年前の間に,阿蘇カルデラを給源とする大規模な火砕流の噴出を含む噴火サイクルが4回(Aso-1,Aso-2,Aso-3,Aso-4)あったことが知られている.これらの火砕流の溶結凝灰岩を大分県と熊本県で採取し,古地磁気測定を行った.さらにAso-4については,中部・関東・東北地方の8地点で採取した降下火山灰,および宮崎県の5地点で採取した非溶結の火砕流堆積物についても測定を行った.<br>溶結凝灰岩の測定からは,それぞれの噴火サイクルで特徴的な古地磁気方位が得られた.Aso-1とAso-4はほぼ北向きの通常の正帯磁の方位を示し,Aso-2は異常に深い伏角(約70°),Aso-3は大きく東偏する偏角(約35°)を示すことがわかった.<br>Aso-4では,サブユニットの4Aと4Bで古地磁気方位に差がみられ,4Bの伏角は4Aより約5°浅くなった.広域型降下火山灰および宮崎の火砕流堆積物は,4Aの方位とほぼ同じであった.

Paleomagnetic measurements were made of the Aso pyroclastic flows of middle to late Pleistocene age in central Kyushu, and of the Aso-4 ashes in the Chubu-Kanto-Tohoku district. The Aso pyroclastic flows are divided into four major cycles: the Aso-1, -2, -3, and -4 flows in ascending order. The paleomagnetic directions obtained are characteristic of the individual cycles, and are all of normal polarity. The mean directions of the Aso-1 (Dm=3.4°E, Im=49.0°, α<sub>95</sub>=5.2°) and the Aso-4 (Dm=5.2°W, Im=42.0°, α<sub>95</sub>=5.2°) flows are similar to that of the present geomagnetic field (D=6°W, I=45°). On the other hand, the direction of the Aso-2 flow is characterized by a very steep inclination (Dm=2.5°W, Im=72.7°, α<sub>95</sub>=13.6°), and that of the Aso-3 flow by an extremely easterly declination (Dm=35.7°E, Im=51.2°, α<sub>95</sub>=3.7°).<br>The Aso-4 pyroclastic flow in Oita Prefecture is subdivided into two units: the Aso-4A and the Aso-4B flows in ascending order. The mean inclination of the Aso-4B flow is shallower than that of the Aso-4A flow by 5 degrees. Assuming that this directional difference was caused by geomagnetic secular variation (GSV), we can estimate the time interval between the Aso-4A and the Aso-4B flows to be about 50 years or more based on the standard GSV curve for southwestern Japan over the past 2, 000 years.<br>The directions of the Aso-4A welded tuffs in Oita Prefecture are consistent with those of the Aso-4 non-welded pyroclastic flow deposits in Miyazaki Prefecture, and with those of the Aso-4 co-ignimbrite ashes in the Chubu-Kanto-Tohoku district. This comparison was made on declination and inclination values for each flow reduced from their virtual geomagnetic poles (VGPs) to a reference point at 131°E, 33°N. The declination values have a range limited between 0.5°W and 5.4°W, and the inclination values are between 44.4° and 45.9°. Identical paleomagnetic directions for the widespread Aso-4 tephra indicate that the magnetizations were acquired in a field dominated by the same geomagnetic dipole field (DF) component in all three regions of the Japanese islands; the pole of the DF at the time of eruption is inferred to coincide with the mean VGP (A<sub>95</sub>=2.1°, N=21) located at 22.8°W, 83.3°N.

収録刊行物

  • 第四紀研究  

    第四紀研究 37(5), 371-383, 1998-12-01 

    Japan Association for Quaternary Research

参考文献:  31件

参考文献を見るにはログインが必要です。ユーザIDをお持ちでない方は新規登録してください。

被引用文献:  1件

被引用文献を見るにはログインが必要です。ユーザIDをお持ちでない方は新規登録してください。

各種コード

  • NII論文ID(NAID)
    10002510733
  • NII書誌ID(NCID)
    AN0034136X
  • 本文言語コード
    JPN
  • 資料種別
    ART
  • ISSN
    04182642
  • NDL 記事登録ID
    105863
  • NDL 雑誌分類
    ZM49(科学技術--地球科学--地質)
  • NDL 請求記号
    Z15-67
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
ページトップへ