火山.第2集
Online ISSN : 2433-0590
ISSN-L : 0453-4360
二一ラゴンゴ火山の1977年噴火とその噴出物の化学組成
中村 洋一青木 謙一郎
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ジャーナル フリー

1980 年 25 巻 1 号 p. 17-32

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Nyiragongo volcano (1°31’S, 29°15’E, 3,470 m in ht.) is located in the Birunga volcanic field, one of the most notable provinces of highly potassic rocks, in the Western Rift of the Eastern Africa Rift Valley. At 10 : 20 a.m. on January 10, 1977, Nyiragongo volcano erupted abruptly and ejected lava in the manner of fissure eruption from 4 flank rifts; Mudjoga, Shaheru, and Baruta rifts in an approximate N-S direction through the summit, and the other rift in the west flank of the summit. The lava flows spread over 34 km2 in total area. Only a few hours after the eruption, lava lake, which had been active in the summit caldera for the past 50 years, was lost completely. Before the present activity of Nyiragongo, an eruption had started on December 26, 1976, at Murara and Harakandi on the southern flank of Nyamuragira, an active volcano neighboring to the west of Nyiragongo. The activity lasted for 5 months and formed two spatter cones surrounded by new lava. The lava of the present eruption of Nyiragongo is aphyric nephelinite, which contains very small amounts of melilite, nepheline, Ieucite, and augite in phenocryst. The chemical composition of the lava is 39 wt.% SiO2 and 5.5 wt.% K2O and contains 25% lc, 24% ne, and 14% cs (CIPW normative minerals). This composition is the same as the lava of the lava lake, lost in the present eruption of Nyiragongo. It is clearly different from the new lava of Murara and Harakandi, which is porphyritic basanite, containing plagioclase, augite, and nepheline in phenocryst. The lava contains 45 wt.% SiO2 and 3.6 wt.% K2O, and contains 21% or, 8% an, and 11% ne. This composition is very similar to that of the lava in the 1957 eruption of Nyamuragira. The large difference in chemical composition of the above two lavas shows clearly that the present simultaneous eruptions in Nyiragongo and Murara-Harakandi, Nyamuragira, were caused separately by two different magmas.

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© 1980 特定非営利活動法人日本火山学会
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