What Is an Atoll? Darwin's Theory of the Ring of Coral
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From the air an atoll is a ring of white sand and green palms a few hundred metres wide around a lagoon of pale water, with the open ocean dark blue outside and nothing in the centre but water; from a boat it is a line of surf on the horizon. There are several hundred of them across the Pacific and Indian Oceans, and the puzzle they posed to the first naturalists was what held the ring up, since coral grows only in shallow water and the ring rises from an ocean floor thousands of metres down. Charles Darwin solved it in 1836, on the way home in the Beagle, with a theory that took a hundred and fifteen years to prove.
The puzzle
Reef-building corals are animals that live in symbiosis with algae, and the algae need light, so the corals grow only in the top fifty metres or so of warm, clear water. An atoll's reef is a living thing on that scale, yet it stands in mid-ocean where the seabed is four kilometres below, and drilling into the ring goes through hundreds of metres of dead coral before reaching anything else. Either the coral had grown up from a shelf just below the surface, which mid-ocean did not have, or the foundation had once been at the surface and was no longer, which required the ocean floor to move.
Darwin's answer
Darwin, who had been reading Lyell on the slow rise and fall of the Earth's crust, proposed that atolls were the last stage of a sequence that began with a volcano:
- •A volcanic island rises from the ocean floor, and corals colonise its shallow shores as a fringing reef
- •The island, its eruptions over, begins to sink slowly under its own weight and as the seafloor beneath it cools and subsides
- •The reef grows upward as fast as the island sinks, keeping its top in the sunlit water, and a lagoon opens between the reef and the shrinking island, which is now a barrier reef
- •The island disappears below the surface altogether, and the reef, still growing, remains as a ring around a lagoon, the atoll
The proof
Darwin's theory predicted that a drill through an atoll would pass through shallow-water coral all the way down to volcanic rock, and the alternative, that atolls grew on submerged platforms that had never been islands, predicted a thin cap of coral over something else. The test was expensive and took until 1952, when the American government, preparing to test hydrogen bombs at Enewetak in the Marshall Islands, drilled two holes through the atoll and hit basalt at 1,280 and 1,400 metres, after a kilometre and more of reef limestone that had all formed in shallow water, the oldest of it about 50 million years ago. The island beneath Enewetak had sunk more than a kilometre while the coral kept pace, and Darwin, who had never seen an atoll when he wrote the theory and saw one only at Cocos Keeling on the way home, was right in every particular.
Life on a ring
An atoll is land made by animals, and it is low, rarely more than three metres above the sea, built of coral sand and rubble thrown up by storms onto the reef's rim, held by palms and the pandanus, with a lens of fresh water floating on the salt beneath the sand that rain replenishes and that a drought or a storm surge can spoil. Polynesians and Micronesians settled the atolls of the Pacific from about two thousand years ago, sailing between them by star and swell, and the Maldives, Tuvalu, Kiribati and the Marshall Islands are nations made almost entirely of atolls, with the highest point in the Maldives at 2.4 metres. The lagoons are fisheries and harbours, the reefs are the breakwater that makes the land possible, and the whole system depends on the coral staying alive and the sea staying where it is.
What they face
The sea is rising at about four millimetres a year and the rate is increasing, and the corals that built and maintain the atolls are bleaching as the oceans warm, expelling their algae in heatwaves and dying if the heat lasts; the reefs of the Maldives lost most of their coral in 1998 and again in 2016. A reef that dies stops growing and stops repairing the rim, and a living reef can keep up with sea level only if the water is not too hot. Whether atolls will drown is argued, since many have grown in area over the last century as storms threw more sand onto them, but the fresh water beneath them is the first casualty of a higher sea, and the governments of Kiribati and Tuvalu have bought land abroad and negotiated the rights of citizens who may have to leave. The ring that Darwin explained is the landform most exposed to what the century is doing to the ocean.
The takeaway
An atoll is a ring of coral reef around a lagoon, formed, as Darwin proposed in 1836, when a volcanic island sinks slowly while the fringing reef around it grows upward to stay in the light, leaving a ring of living coral on a foundation of dead coral a kilometre thick over the drowned volcano; drilling at Enewetak in 1952 proved it. Atolls are low land made by animals, home to several nations, and threatened by the rising sea and the bleaching of the corals that maintain them.