How did Darwin explain atolls, and how did drilling prove him right?
Darwin worked out how atolls form before he had seen one; the proof, nearly a mile of coral over a volcano, came from drilling for nuclear tests.
▶ Start the storyCharles Darwin explained an as the last stage of a sinking island. A coral reef grows around a volcanic island. The island slowly , but the reef keeps growing upward to stay near the surface, until the volcano has vanished beneath the waves and only a ring of reef is left around a . Drilling at Enewetak Atoll in 1952 passed through about 1,400 metres of coral before reaching volcanic rock, which is just what the idea required.
Step 1: Volcanic island with a
Coral grows along its shore
Step 2: The island slowly sinks
The reef grows upward on old coral
Step 3: Barrier reef
A opens between reef and island
Step 4:
The volcano is gone; a ring of reef surrounds a
The mystery was real. Corals live in shallow water, yet atolls rise from the deep ocean. Some books Darwin carried on HMS Beagle said that atolls were corals growing on the rims of old underwater volcanic craters. Darwin disagreed. He thought of his idea in South America, before he had ever seen a coral island. After witnessing a volcano erupt and surviving the 1835 Concepción earthquake, he reasoned that while the land rose, large areas of the sea bed must be sinking, and it struck him that this could explain atolls.
He saw the idea in action at Tahiti. From its hills he looked across at the island of Eimeo, whose mountains rise from a calm ringed by breakers, and wrote that if you removed the central group of mountains, a lagoon island would remain. Published in 1842, his theory predicted a volcano's base under every lagoon.
It took more than a century to test. Royal Society expeditions in the 1890s drilled the Funafuti to 339 metres and found only coral, which settled nothing. In the 1950s the atomic tests at Enewetak brought deep drilling that finally Darwin. The full story turned out to be more complex than he imagined, and a rival model gives ice-age erosion a part in shaping the ring, but his core idea of sinking islands has survived.
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Recap
Corals grow upward as islands sink, and in the end only the ring of reef remains around a lagoon.
💡 A trick to remember it · A sinking volcano, a reef that will not drown: it climbs as the island falls until only the ring is left.
Surprising fact · Darwin devised the theory in South America, before he had ever seen a coral island.
Sources (3)
No source, no claim. Every fact in this lesson (19 claims) cites at least one of these.