Physical Feature: Atoll formation

Atoll formation. Photo Credit: USFWS

The only coral atolls in the United States are found in the Northwestern Hawaiian Islands, and Kure Atoll is the northernmost atoll in the world. Atolls are ring shaped growths of coral surrounding a central lagoon, and are formed in tropical and sub-tropical seas. Corals grow in a ring around a high volcanic island, and as the volcano erodes and subsides back into the ocean the corals continue to grow, staying near the surface. After millions of years the volcanic rock sinks beneath the surface and only coral skeletons and live coral remain, creating atolls. At Midway Atoll this "coral cap" is between 500 and 1200 feet deep ("Drilling on Midway Atoll, Hawaii," Ladd, 1967).

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