Myojinsho Reef

It is a reef located east of the Bayonnaise Rocks, about 60 km south of Aogashima Island in the Izu Islands of Tokyo, and is an active dacite volcano belonging to the Fuji volcanic belt. It is thought to be the central cone of a large double submarine volcano, with its base about 1,500 meters below sea level and the Bayonnaise Rocks as the western edge of its outer rim. It was named after the fishing boat Dai-Juichi Myojin-Maru, which discovered an undersea eruption and the birth of a new volcanic island on the morning of September 17, 1952 (Showa 27). New islands were formed from lava domes more than three times over the next year, but when they grew to an altitude of 200 to 300 meters, they sank below sea level in a major explosion. During the first explosion, the observation ship Dai-Go Kaiyo-Maru of the Hydrographic Department of the Japan Coast Guard was lost, and all 31 crew members died in the line of duty, making it the largest disaster in the history of volcanic observation in the world.

Since 1869 (Meiji 2), there have been over a dozen undersea eruptions recorded in the vicinity, and new islands appeared in 1896 and 1946. Several undersea explosions were also observed in 1970, forming wave-damaged reefs. Since then, discoloration of seawater, likely due to undersea volcanic activity, has been observed four times up until 1986.

[Akira Suwa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

東京都伊豆諸島青ヶ島の南約60キロメートルのベヨネース列岩の東方にある暗礁で、富士火山帯に属する石英安山岩の活火山。海面下約1500メートルに基底をもち、同列岩を外輪山の西縁とする二重式の一大海底火山の中央火口丘とみられる。1952年（昭和27）9月17日朝、海底噴火と新火山島誕生を発見した漁船第十一明神丸の名にちなんで命名。以後約1年間に3回以上も溶岩円頂丘の新島ができたが、標高200～300メートルに成長すると、大爆発で海面下に没した。その最初の爆砕期に海上保安庁水路部の観測船第五海洋丸が遭難、全乗員31人が殉職し、世界火山観測史上最大の惨事となった。

　付近では、1869年（明治2）以降、十数回も海底噴火が記録され、1896年と1946年にも新島が出没。1970年にも数回海底爆発が認められ、波浪(はろう)礁をなした。以後も1986年までに4回海底火山活動によるらしい海水変色が視認された。

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