Darwin's elegant theory of atolls may have the story partly backwards
Charles Darwin pictured each atoll as a coral necklace left behind when a volcano sank beneath the waves. It is a beautiful idea, and still widely taught, yet newer drilling suggests many rings were shaped instead when falling ice-age seas exposed flat reefs to rain that ate out their middles.
An atoll is a ring of coral, often topped by low islets, enclosing a lagoon. There are about 440 of them, mostly in the Pacific and Indian oceans, and they exist only where water is warm enough for reef-building corals. Their land is tiny next to their total area and rarely rises more than 5 metres. The word itself comes from atholhu in Dhivehi, the language of the Maldives, which reached French as atollon in 1625 and was then taken up by Darwin.
Darwin drew on his voyage aboard HMS Beagle from 1831 to 1836 and published his explanation in 1842. A fringing reef hugs a volcanic island; as the island slowly subsides, the reef keeps growing upward and becomes a barrier reef, as around Bora Bora; finally the peak vanishes and only the ring and its lagoon remain. Where water is too cool for coral to keep pace, a sinking island becomes a seamount instead. The boundary latitude is called the Darwin Point.
To test such ideas the Royal Society drilled into Funafuti in Tuvalu in 1896, 1897 and 1898, with William Johnson Sollas leading the first expedition and Edgeworth David the second. Later researchers proposed the antecedent karst model: during glacial lowstands, flat-topped reefs stood exposed, and rain dissolved their interiors faster than their rims, leaving saucers that corals recolonised when the sea returned. Work by A. W. Droxler and colleagues found that reefs buried beneath modern atolls were flat-topped, not rings, supporting that view.
The extremes are telling. Kure Atoll, at about 28 degrees north, is the northernmost, while Aranuka in Kiribati sits just 13 km from the Equator. Bermuda, often claimed as the northernmost, is called a pseudo-atoll, kept warm enough by the Gulf Stream. Huvadhu in the Maldives holds 255 islands, more than any other atoll.
Source: Atoll
