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The richness of the lagoon water is thought to be the result of deep ocean water percolating through the walls of the basement structure of the reef, bringing with it nutrients that previously had been locked away in geological storage.

The Formation of Reefs

Darwin's theory. British naturalist Charles Darwin first published his theory of coral reef formation in 1842, and it is still the dominant theory today. Darwin, in The Structure and Distribution of Coral Reefs, suggested that a fringing reef around the edges of an island would gradually grow outward, leaving a lagoon in its wake, and evolving into a barrier reef. If the island, over geological time, subsided, then what would be left would be an atoll:

"Now, as the island sinks down, either a few feet at a time or quite insensibly, we may safely infer from what we know of the conditions favourable to the growth of coral, that the living masses bathed by the surf on the margin of the reef, will soon regain the surface...

"Let the island continue sinking, and the coral-reef will continue growing up its own foundation, whilst the water gains inch by inch on the land, until the last and highest pinnacle is covered, and there remains a perfect atoll." (See diagram at right.)