Atoll

Atoll, ring-shaped coral island or several smaller islands as part of a coral reef surrounding a central lagoon. There are some 400 atolls in the world, most of which are in the western and central parts of the Pacific Ocean. The largest is Kwajalein in the Marshall Islands (120 km long).

The 19th-century naturalist Charles Darwin first proposed the theory of atoll formation. After a volcano erupts on the ocean floor, the lava emissions form new layers on the volcano’s outer surface, creating a conical mountain that becomes visible above the water’s surface as an island. Next, coral begins to grow in the shallow waters surrounding the volcanic island, eventually forming a coral reef. Over time, the volcano ceases to erupt and it begins to erode and subside; sea water then floods the space between it and the surrounding reef. Eventually, the volcano may collapse under its own weight, erode below the surface of the ocean, or be covered by a rise in sea level. Sea water fills the area where the crater of the volcano once stood, eventually forming an atoll.