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The Progression of Coral Reefs Stages

Coral first appeared in the fossil record more than 400 million years ago. Corals are anthozoans, the largest class of organisms in the phylum Cnidaria. Stony corals, called scleractinians, make up the largest order of anthozoans, and are the reef-building corals. While traveling in the South Pacific, Charles Darwin first proposed his theory of coral reef development. According to Darwin, coral reefs progress from fringing to barrier to atoll.

Geographic Distribution
- Most coral reefs are located between the Tropics of Cancer and Capricorn, in the Arabian Gulf, the Caribbean Sea, the Indian Ocean, the Pacific Ocean and the Red Sea, where environmental conditions are optimal. Worldwide, coral reefs cover approximately 110,000 square miles. Corals need both warm water and sunlight to thrive. The water temperature must be between 68 and 90 degrees Fahrenheit. Because the algae living in the coral polyps need sunlight for photosynthesis, you will find corals growing in the photic zone, where sunlight penetrates the ocean at depths up to 165 feet.
Formation
- New volcanic islands form when an underwater volcano erupts, forcing molten rock and ash up from the seabed. These cone-shaped islands are made entirely of rock and have no terrestrial or marine life. Once the island cools, flying, floating and swimming organisms arrive and begin to populate the island. Over time, thriving communities occupy every available habitat.
Fringing Reef
- Free-swimming coral larvae, called planulae, attach themselves to the submerged edge of the island. Many of the larvae die because the larvae need a solid surface on which to attach themselves. A new island provides the location that the larvae need to grow and expand. The larvae mature into individual coral called polyps. A coral reef, called a fringing reef, forms along the shoreline of this new island. This process can take up to 10,000 years.
Barrier Reef
- As the volcano that formed the island cools, it becomes denser. Over the next 100,000 years or so, the island begins to sink below the sea surface. The fringing reef separates from the shoreline. The coral reef continues to expand upward, forming a barrier reef. A lagoon forms between the top of the reef and the sinking island. As the polyps die, their skeletal remains composed of calcium carbonate form the bottom layers of the reef, while living coral continues to grow and reproduce in the upper part of the reef.
Atoll
- When the island disappears below the sea surface, it leaves a coral ring encircling the central lagoon. This is called an atoll, and this process may take as long as 30 million years. The coral reef continues to grow upward. In support Darwin's theory that atolls form as a fringing reef subsides, scientists have drilled through coral limestone before striking volcanic rock.

Nature