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Over thousands of years, the hard, exterior skeletons of the corals build up to form a thick, ring-shaped coral reef.
Rain, wind, and waves weather and erode the volcanic island. Over time, it disappears into the sea.
The ocean breaks through the coral rim and fl oods the area that was once the volcanic crater, forming a shallow lagoon.
The volcanic island erodes completely, leaving an atoll, or ring of islands, around the lagoon.
APRIL 2016 19
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