Atoll - Kansho (English spelling) atoll

Japanese: 環礁 - かんしょう（英語表記）atoll

A donut-shaped coral reef with no island in the center. Although there are some slightly elevated areas, most of the outline is only a few meters above sea level. The reef flat is formed by the washing up of rock fragments broken by the waves, forming a ring shape. The width of the reef flat is about 100 meters, and the reef flat can be washed away by large waves. The central lagoon is very shallow, but the reef flat is connected to the open sea by a channel where it ends. The slope of the atoll on the open sea side can be more than 45 degrees to a considerable depth. Atolls are mainly found in the southern Pacific Ocean and the Indian Ocean, and some of the larger ones are over 60 km in diameter, and are particularly common in the Marshall Islands. There is a theory about the origin of atolls that was published by C. Darwin in 1842. Darwin thought that the fringing reefs around volcanic islands sank and grew as barrier reefs separated from the volcanic islands, and the central volcanic islands also sank below sea level to form atolls. Recent coral reef drilling surveys and artificial earthquake surveys in various locations have provided evidence to support this idea. Drilling has revealed that Funafuti Atoll is entirely made up of limestone, including coral reefs, down to a depth of about 350m, and Bikini Atoll is entirely made up of limestone, including coral reefs, down to a depth of about 800m. In 1960, drilling into Eniwetok Atoll revealed basalt at a depth of about 1,300 to 1,400m, confirming the existence of a volcanic island. Meanwhile, an artificial earthquake survey has suggested that Bikini Atoll also has a volcanic island at a depth of about 1,300m.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

ドーナツ状に，中央部に島のないサンゴ礁。やや小高い部分もあるが，輪郭の大部分は海抜数m程度で，波で砕かれた岩片が打寄せられて現れた礁原が，環状を呈する。礁原の幅は 100mぐらいで，大波のときには礁原が洗われることもある。中央の礁湖はごく浅いが，礁原が切れた水道で外海と通じる。外海側の環礁の斜面は，かなりの深さまで 45°以上の傾斜のことがある。環礁はおもに太平洋南部とインド洋に分布し，大きなものは径が 60kmをこえるものもあり，マーシャル諸島に特に多い。環礁の成因については，C.ダーウィンが 1842年に発表した説がある。ダーウィンは火山島のまわりの裾礁 (きょしょう) が沈降し，火山島から離れた堡礁となって成長し，中央部の火山島も海面下に没して環礁ができたと考えた。近年の各地のサンゴ礁試錐調査や人工地震による調査で，この考えを裏づける資料が得られている。試錐ではフナフチ環礁が約 350mまで，ビキニ環礁が約 800mまで，全部がサンゴ礁などの石灰岩であることがわかった。 1960年のエニウェトク環礁試錐では約 1300～1400m下に玄武岩が出て，火山島が確認された。一方人工地震による調査では，ビキニ環礁も約 1300mの深さに火山島があるとみられている。

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