Scientists in the field of tropical marine biology have found that coral reefs are often divided into 3 types: fringing reefs, barrier reefs, and atolls, which have special requirements for their development.
The coral reef is created when motile coral larvae attach themselves to a rock or other underwater object on the edges of islands. This is how the coral reef begins to form. When corals grow over long periods of time, they form a reef. The three reef types represent stages in the development of a coral reef over time.
Coral reefs grow around islands and continents near the coast. These are the reef types we see most often and are considered the youngest of the 3 reef types. They are separated from the coast by shallow lagoons.
The first stage of formation is when coral larvae attach to rocks or soil near shore. In some parts of the world, they are more likely to form where volcanoes have also formed, as flat walls are ideal for their formation. The larvae transform into polyps and secrete calcium carbonate, which forms their exoskeleton.
Calcium carbonate sediments secreted into the rocks provide a substrate for more polyps to attach. As more and more polyps accumulate and overlap over time, they form a coral reef. Other organisms with calcareous skeletons also add their remains to the reef when they die.
Because fringing reefs are the youngest and grow farther offshore, they tend to have less biodiversity. Some locations of these reefs include Kenya, Australia, and other parts of the coasts of Africa.
As the name suggests, this type of reef delimits the coast with a very wide and deep lagoon that separates the two structures. One of the largest and most famous barrier reefs in the Great Barrier Reef in Australia. This reef is approximately 1,200 miles long and has many complex reefs that make up it.
This type of reef is created when fringing reefs slowly coalesce and form a boundary along the shoreline. The calcium carbonate structures attract more polyps and the interstices fill up. It creates a line along the coast and a ring around an island. Large coral reefs are the rarest type of coral reef and are only found in a few select locations on Earth including Belize and parts of the South Pacific.
Smaller barrier reefs can be seen where islands are in the early stages of submersion. Most of the reefs around the Turks and Caicos Islands are barrier reefs formed by the precipitation of CaCO3 on the remnants of continental debris formed when North America separated from South Africa 200 million years ago (continental drift). Dissolved minerals in seawater form small particles that aggregate into the oolite rock that makes up most of the islands and coasts of the Turks and Caicos Islands.
An atoll reef is a ring-shaped coral reef or small reef island in a circle with a lagoon in the middle and is usually located in the middle of the sea. These types of reefs are formed when an island is completely (or almost) submerged in mid-ocean around a pre-existing structure (these islands are often the tops of underwater volcanoes) due to rising sea levels.
Most of the atoll reefs are in the Pacific Ocean where the water is warm and salty. They can grow from 12.6 km2 (Great Chagos Bank) to just 1.5 km2 (Ontong Java Reef). An atoll is a home to a greater diversity of marine life than previous reefs. More than a million species can live in one square kilometer of coral reef. The 3 types of reefs, fringing, the barrier, the atoll, arise in this order and represent the stages of coral reef development.
Patch reefs are small, isolated reefs that grow on the open floor of the island shelf or continental shelf. They are usually found between fringing reefs and barrier reefs. They are highly variable in size and rarely reach the surface.