Coral reefs are dynamic, evolving through time into different forms. During his voyages on the HMS Beagle, Charles Darwin first recognized the progressive development of coral reefs on volcanic islands. Volcanic islands subside; that is, they have a tendency to cool, condense, and sink through time. As they subside, tropical coral reefs grow upward along their margins.
The Hawaiian Islands-Emperor Seamounts chain is a classic example of this process. Active volcanic islands are found at the southeast end of the chain. Beyond Kaua‘i, however, islands are subsiding slowly and coral reefs have developed around the volcanic cores of islands. French Frigate Shoals, Midway, and Necker Islands are dominantly coral limestone accumulations, but deep sediment cores have revealed the volcanic origins of these islands.
The animation below shows the progressive development of fringing coral reefs into barrier reefs and ultimately into atolls.
This coral atoll in the South Pacific shows a well developed central lagoon and emergent reef crest. Active coral growth is on the margins of the island.
U.S. Department of the Interior | U.S. Geological Survey
