Journal of Geography (Chigaku Zasshi)
Online ISSN : 1884-0884
Print ISSN : 0022-135X
The 1970-activity of Myojin reef
Ryohei MORIMOTOJoyo OSSAKA
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JOURNALS FREE ACCESS

1970 Volume 79 Issue 6 Pages 301-320

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Abstract

On 29 th January 1970, Myojin reef, a submarine volcano, 420 km south of Tokyo (31°56.8'N, 139°59.5' E), known for its 1952-1953 great eruption especially for tragic accident of a missing vessel No. 5 Kaiyo-maru, began its present activity after 10 years quiescence since the last eruption of 21 st July 1960. According to oral communications from the eyewitnesses embarking on the fishing boats and to observations from aeroplanes including the writers' inspections, manifestations of the eruption on the sea surface were recognized at least five times e.g. 29 th January, 7 th February, 16 th and 17 th February, and 23 rd April.
Compiling all available informations obtained directly or indirectly from many eyewitnesses and of the writers' own observations, the writers could estimate the scale of each main submarine explosion and the time lag after each main explosion, primarily by observing the shape, size, and colour of the sea contaminated with the volcanic material, and secondly by identifying the floating volcanic ejecta on the sea.
As the results of the writers' estimation, eruption of 23 rd April was the largest of the five. Densely contaminated sea water and remarkable amount of floating pumice were still observable even five to ten hours after the largest eruption.
Immediately before the first explosion of the present activity, actual scene of the dangerous active volcano beneath the water was innocently caught by the fish-detecting instrument installed on the fishing boat No. 2 Shintoku-maru of Omaezaki port, Shizuoka Prefecture which passed over the active vent. Dome of the volcano and the volcanic spine protruding on it up to 50-70 m beneath the sea surface were clearly traced on the graph drown by the sounding machine, with ash cloud puffing from them.
Wave crusts due to underlying spine were once observed, but they disappeared probably due to destruction of the spine of the time of severe largest explosion on 23 rd April. The pumice blocks ejected by the last explosion were collected by several fishing boats at several points, which are exclusively two pyroxene dacite including dark more basic xenoliths. Result of chemical analysis of the white portion of one of these pumice blocks is shown as follows :
SiO2 : 68.31, Al2O3 : 14.69, Fe2O3 : 1.79, FeO : 2.54, MgO : 1.64, CaO : 3.44, Na2O : 3.80, K2O : 0.93, H2O+ : 1.05, H2O- : 0.29, TiO : 0.52, P2O5 : 0.09, MnO : 0.11, NaCl : 0.75, Total : 99.95 wt%.
It is similar to the white portion of the pumice ejected in previous eruptions.
Record of the fishing-detecting machine dated 18 th June by No. 3 Shintoku-maru of the same port makes clear that once formed spine was destructed owing to explosions which occurred near the foot of the spine.

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