A reef is the limestone left by colonies of reef-building corals, which grow only where sunlight reaches them — within a few tens of meters of the surface. How a reef came by its shape is asked most sharply of atolls: rings of reef enclosing a lagoon, with no land at the center.
The account that asks least of the evidence is the planation account, which begins with the sea rather than the corals. When the sea stood lower, the flat tops of limestone banks lay in the open air. Rain, which dissolves limestone, ponded in their interiors and lowered them far faster than their freely drained edges, leaving a shallow saucer with a raised rim. When the sea rose again, corals settled on the rim, where the water was shallowest, and veneered it. On this account the ring is inherited rather than built: its outline was cut before the living reef arrived, the coral above is a skin no thicker than the range through which the sea has risen, a hundred meters at the outside, and the rock beneath is older material of other origin.
The subsidence account reverses the order, making the ring built rather than cut. A reef begins as a fringe around an island whose floor is slowly sinking. At the rim, where the light is best, the corals build upward at about the rate the floor goes down, so the living surface is held near the surface of the sea; the sheltered interior builds more slowly, falls behind, and is left as a lagoon. Given time enough the island passes below the water and the ring alone remains. The limestone beneath is then reef limestone throughout, every meter laid down in sunlit water and afterward carried below it, its thickness set by nothing but the distance the floor has sunk.
Both accounts still have their advocates. What is hard to reconcile with the planation account, however, is what the boreholes found: holes sunk through several Pacific atolls passed through hundreds of meters of limestone full of shallow-water corals — in one more than a kilometer — before reaching the volcanic rock beneath. No rise of the sea approaches a range of that order. And the flat tops of many submerged banks now lie far deeper than any coral can build.
