著者
小野 晃司 渡辺 一徳 星住 英夫 高田 英樹 池辺 伸一郎
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.40, no.3, pp.133-151, 1995
参考文献数
41
被引用文献数
21

We describe in this paper the character of ash eruption of Nakadake volcano presently going on and maintain that the products of the volcano during the recent geologic past are the main constituents of ashy soils distributed around, especially to the east of, the volcano. Nakadake volcano, the only active central cone of Aso caldera, mainly discharges black, sandy essential ash of basaltic andesite during its active period. The ash is the most voluminous constituent of the products of Nakadake through its activity of recent more than 20,000 years. We call ash eruption for the activity which discharges dominantly ash. The ash is divided into three groups : block-type ash, splash-type ash, and free crystals. The block-type ash, most common, is polyhedral surrounded by a few flat planes. It is formed by brittle fracturing of semi-solid top part of the magma column. While, the splash-type ash, discharged only during the very active phase in active periods, is derived from liquid magma which underlies the semi-solid top of the column. The ash is transported by gas stream from the magma in a quasi-steady state or intermittently, and is distributed around Nakadake volcano in a near-circular pattern by a low eruption column usually less than 1,000 m in height. Long-term thickness contours of the ashy soil from Nakadake volcano in three time intervals, separated by the present earth surface and three ash or pumice layers, of recent more than 20,000 years also show near-circular pattern but a little elongated to the east. 'Loam beds', mainly composed of decomposed and argillized volcanic ash, are said to be formed by accumulation of aeolian dusts during periods of no eruption in volcanic districts. But, loam beds (Akaboku) and black humic soils (Kuroboku) distributed around Aso volcano are composed mainly of primary fall-out deposits of ash or pumice along with aeolian dusts. Ash eruption of Nakadake mostly produces sandy ash rather continuously without long time break but in small rate of discharge. A correlation of detailed columnar sections eastwards of Nakadake crater shows most single strata of ashy soils, light brown- to black-colored, thin according to the increase of distance from the source crater. The ash, especially very fine ash distributed in the distal area, easily decompose and lose primary stratification to form a massive layer which is hardly discernible from aeolian loam beds by their close resemblance. Not only fine ash layers but Plinian pumice fall layers form 'loam' beds which are not distinguishable with adjacent aeolian beds by further decomposition. The thickness contours of the Kanto loam elongate and thin eastward from Fuji volcano over the Kanto plain. The thickness of loam beds changes regionally, thicker in volcanic areas and thinner in non-volcanic areas. These facts suggest contributions of primary pyroclastic falls for thickening of 'loam beds'
著者
高田 亮 大島 治 荒牧 重雄 小野 晃司 吉田 克史 梶間 和彦
出版者
The Volcanological Society of Japan
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.37, no.5, pp.233-250, 1992-11-15 (Released:2017-03-20)
参考文献数
34
被引用文献数
1

The subaerial history of Aogashima volcano (about 3 km3 in volume) was studied. The sequence of volcanic activity is summarized as follows: the growth of Kurosaki volcano (0.3 km3 in volume; basalt>andesite) in the northwestern area of Aogashima island; the construction of the main edifice of Main stratocone (basalt≫andesite) in the southeastern area; the fissure eruptions of Aphyric basalts (<0.1km3 in volume) on the northwestern flank; a surge activity (Ojiroike surge deposits) (basalt>ndesite) at about 3,000 y.B.P.; the eruptions of Kintagaura lavas (0.15 km3 in volume) filing the Southeasern basin, and airfalls (Yasundogo airfalls tephras) (0.4 km3 in volume; basalt>andesite) on the east and north flanks 3,000-2,400 y.B.P.; the occurrence of a debris avalanche (Nagashizaka debris avalanche deposits) associated with the formation of the Ikenosawa crater (1.7 km×1.5 km insize); the Tenmei (A.D. 1781-1785) eruption (0.08km3 in volume; andesite). Based on the historical records and the geological data obtained in this study, the sequence of the Tenmei eruption is restored as follows: according to historical records, a small ash eruption occurred in 1781; in 1783, the Tenmei eruption began with an explosive scoria effusion (Tenmei airfall tephras 1) associated with cone building (Maruyama pyroclastic cone); in 1785, ash fall continued intermittently for more than one month (Tenmei airfall tephras 2); finally, the Ikenosawa crater was filled with lava flows (Tenmei lavas 1 and 2). During the development of Aogashima volcano, magma paths were shifted over a distance of about 4 km. Some geological units derived from different magma paths have different, parallel trends of chemical composition in SiO2 vs. oxide diagrams. The chemical composition of magma changed with the shift in magma path. Each trend consists of a combination of plagioclase accumulation and crystallization. The magma-supply rate of Aogashima volcano was fluctuating in time and space with the growth of each geological unit, which may have led to the generation of andesite magma.
著者
松本 哲一 水垣 桂子 玉生 志郎 小野 晃司 北原 哲郎 品田 正一 笹田 政克
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.38, no.1, pp.1-13, 1993-04-01 (Released:2017-03-20)
参考文献数
21
被引用文献数
1

Taga Volcano, in northern part of the Izu Peninsula, central Japan, is a large Quaternary strato volcano made of andesite and basalt. The Hata Basalts underlying the Taga volcanic rocks is a pile of pyroclastics with some lava flows mainly of basalts which include the most mafic basalt in the late Tertiary to Quaternary volcanic rocks of north Izu and Hakone region. This Basalts has been treated as the late Tertiary product by previous workers. In order to clarify the volcano-stratigraphic relationship between the Hata Basalts and the Taga volcanic rocks, geologic investigations were carried out in and around the Tanna Basin, and the core and cuttings samples from three 500-600 m drill holes in the Tanna Basin were examined. The Hata Basalts is lithologically characterized by dominant pyroclastics, especially high-temperature oxidized, reddish-brown agglutinate, whereas the products of Taga Volcano are characterized by dominant lava flows. The Hata Basalts is overlain by the Taga volcanic rocks (TV4 and TV5) unconformably at the eastward of the Tanna Basin, and overlain by the Shimo-Tanna Shales and the Taga volcanic rocks unconformably at the westward. Petrography and major elements chemistry suggest that both the Hata Basalts and the Taga volcanic rocks are petrologically very similar to each other. K-Ar dating of two rock samples of the Hata Basalts and one sample of Taga Volcano revealed that they were products of almost the same age, around 600-700 ka. All the samples dated can be correlated to the Brunhes normal polarity epoch. We conclude that the Hata Basalts is the early stage products of Taga Volcano.
著者
渡辺 一徳 小野 晃司
出版者
日本地質学会
雑誌
地質學雜誌 (ISSN:00167630)
巻号頁・発行日
vol.75, no.7, pp.365-374, 1969-07-25
被引用文献数
6 9 5