Topographic Map of Pacific Region. Tern Island is exactly in center of picture Coral reefs grow around the edges of these volcanoes in the shallow coastal areas. The coral animals produce rock-like homes for themselves, and these homes are the reef. When a volcano slips below the surface, an island might continue to exist if the coral, now growing on the top of the volcano, grows upward faster than the volcano slips downward. The surface of the atoll is sand produced by breakdown of the coral reef.
Atolls are all low islands, barely breaking the ocean's surface.
The great scientist Charles Darwin figured out how atolls come to be. Many of the Northwest Hawaiian Islands are atolls. Tern Island is one small part of the atoll called French Frigate Shoals . The entire atoll comes close to the ocean surface, but the water over the eastern half of the atoll is shallower than that over the western half. Only a few spots have accumulated enough sand on top of the coral reef to break the ocean surface.