第四紀研究
Online ISSN : 1881-8129
Print ISSN : 0418-2642
ISSN-L : 0418-2642
15 巻 , 2 号
選択された号の論文の3件中1~3を表示しています
  • 早津 賢二
    1976 年 15 巻 2 号 p. 55-65
    発行日: 1976/07/30
    公開日: 2009/08/21
    ジャーナル フリー
    Iizuna volcano (1871m) is formed at the southern end of the Myoko volcanoes which are situated in the northern part of the Fossa Magna. It is a Quaternary stratovolcano built on an anticlinal axis of basal Neogene strata, and has a horseshoe caldera with 2.5×2.0km in diameter at its summit. The volcanic cone is considerably dissected in contrast to the Myoko and Kurohime volcanoes. At the western to northern midslope of the cone, the five domes of Kenashiyama, Nakanomine, Takadekki, Tengudake, and 1340 m-ridge are arranged in NE-SW direction (Fig. 1).
    The growth-history of the Iizuna volcano may be divided broadly into the Ist and IInd stages, and the latter may be subdivided into the stratovolcano, caldera, and lava dome stages (Table 1). The eruptives from this volcano are full of variety such as lavas, pyroclastic (scoria, pumice, lapilli, and others) falls, pyroclastic flows, and volcanic mud flows. The rocks consist of basalt, pyroxene andesite, and hornblende andesite (Fig. 2). Augite-hypersthene andesite is predominant in volume.
    During the activity of the IInd stage, the mode of eruption and rock nature generally tend to change as follows, respectively: mode of eruption; eruption of scoria fall→alternated eruption of lava flows and pyroclastics→eruption of pyroclastic flows and falls, and viscous lave flows: rock nature; basalt→pyroxene andesite→horhblende andesite. The activity of this volcano began at the middle Pleistocene, and had already closed at least 32, 000 years ago.
  • 佐瀬 隆, 加藤 芳朗
    1976 年 15 巻 2 号 p. 66-74
    発行日: 1976/07/30
    公開日: 2009/08/21
    ジャーナル フリー
    The amount of plant opals and the constitutions of pollens in humic horizons of present and buried volcanic ash soils distributed in Tohoku, northern Kanto, eastern Tokai and Kyushu districts were investigated.
    The results obtained are summarized as follows;
    1) It is assumed that the constitutions of plant opals in the humic horizons of volcanic ash soils have an effect on a relationship between the amounts of plant opals (X) and organic carbon contents (Y). In a group having ratios, Panicoid (Pa.)+Festucoid (Fe.) vs. Sasaoid (S.), not less than 0.5, the relationship between X and Y is expressed in the following formula:
    Y=1.58X+3.92
    In another group having the ratios less than 0.5, the following formula is obtained:
    Y=0.63X+3.14
    This may be explained by the fact that Sasaoid grasses have higher amount of plant opal production per dry matter than other ones.
    2) The constitutions of pollens in the humic horizons of volcanic ash soils are characterized by high content of nonarboreal pollens (NAP), which allows an assumption that herbaceous plants were dominant in the vegetation concerned in formation of the humic horizons of volcanic ash soils.
    3) A maximum mean annual production of plant opal is shown by the humic horizons of Kirishima volcanic ash (Ks-d) soils which had been formed during a period from 4, 600y. B.P. to 3, 700y. B.P. (14C dates), possibly a part of the climatic optimum in the post glacial age. This period nearly agrees with that derived from buried humic horizons of volcanic ash soils in Hokkaido (SASE and KONDO, 1974).
  • 柏原 信, 広瀬 保志, 香河 正人, 菅 恵一, 春日井 昭, 山岸 一夫
    1976 年 15 巻 2 号 p. 75-86
    発行日: 1976/07/30
    公開日: 2009/08/21
    ジャーナル フリー
    1) 羊蹄第3軽石・スコリア層 (Yo. Ps-3), 羊蹄第2軽石・スコリア層 (Yo. Ps-2), 羊蹄第1軽石・スコリア層 (Yo. Ps-1) は, 支笏軽石流堆積物 (Spfl) と恵庭a降下軽石層 (En-a) の両層準の間にあり, とくにYo. Ps-1,-3は羊蹄山東麓から札幌-苫小牧低地帯におよぶ広い分布を示す.
    2) 札幌-苫小牧低地帯の周辺では, Yo. Ps-1とYo. Ps-3は, 従来恵庭火山起源のものと考えられてきたが, Yo. Ps-1,-2,-3はいずれも羊蹄火山起源であることをあきらかにすることができた.
    3) Yo. Ps-3の分布は, 羊蹄火山を西端とする長楕円形で分布軸の方向はほぼ真東であり, 75km以遠におよぶ. 20km以遠では, 下位より軽石層-スコリア層という一連の堆積を示すことが多い.
    4) Yo. Ps-2の分布は, 羊蹄火山を西端とする扇形で, 分布軸の方向はほぼ真東から10度南よりであり, 40km以遠に達する. 一般的には, 軽石とスコリアのまじった層であるが, 20km以遠では軽石層からなることが多い.
    5) Yo. Ps-1の分布は, 羊蹄火山を西端とする長楕円形で分布軸の方向はほぼ真東から10度北よりであり, 85km以遠に達する. 30km以遠では下位より粗粒火山灰層-軽石層-スコリア層という一連の堆積を示す.
    6) Yo. Ps-2,-3の軽石粒は, 普通輝石・しそ輝石安山岩質であるが, Yo. Ps-1の軽石粒は, 角閃石・普通輝石・しそ輝石安山岩質である.
    7) Yo. Ps-2, -3の降下年代は, およそ25,000~30.000y.B.P., Yo. Ps-1はおよそ15,000y.B.P. と推定される.
    8) Yo. Ps-3の下位には, 角閃石に富む降下軽石層がみられる. 今後の調査・研究により, これらがテフラ示準層となり羊蹄火山の活動がこれらの層準にまでさかのぼる可能性がある.
    9) Yo. Ps-1の上位には, 下位から第2黒色粗粒火山灰帯 (Ba-2), 第1黒色粗粒火山灰帯 (Ba-1) さらに北岡軽石・スコリア層 (K-Ps) がみられる. これらのテフラと羊蹄火山の溶岩流および段丘堆積物との層位関係についての調査は, 山体の形成史とこの地域の地形発達史をあきらかにするための重要な手がかりとなるであろう.
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