Coral reefs are very large marine structures made of calcium carbonate (CaCO3) secreted by tiny animals known as stony corals and other marine animals inhabiting the reef. A coral head consists of thousands of genetically identical tiny animals known as polyps, which secrete calcium carbonate skeletons to support and protect their bodies. Coral reefs generally take from thousands to millions of years to develop and usually form in warm, shallow tropical waters with temperatures ranging from 23 to 29 ºC (73-84 ºF). There are three main types of coral reefs and several variants of these main types.
Coral reef formation
Coral reefs form when microscopic coral larvae adhere to a hard substrate or other hard surfaces under the marine environment, establishing large colonies of coral polyps which secrete calcium carbonate underneath and around them, and building the reef´s limestone structure. Each new generation of polyps adds a new layer of limestone, building reefs as massive as 40 meters (131 ft.) high and up to 100 km (62 miles) long. The coral reef´s structure grows vertically, tracking the Sun´s rays. After the coral structure reaches the upper surface waters, it starts to grow in a horizontal direction. Eventually, a great variety of marine organisms starts to inhabit the reef along with predators, providing home for 25 percent of all marine species.
Reef structures
Coral reefs have been on the face of the Earth for millions of years; however, the coral reefs that exist around the world today were formed during the last glacial period, when the melting of ice caused the sea levels to rise, flooding the continents and providing ecological conditions for stony coral to grow. Although coral reefs abound in the sunny, shallow, clear waters of the tropics, some reefs can be found in the open ocean, around volcanic islands and atolls, away from continental shelves. There are three principal types of reefs, which are classified based on their structure and their relation to nearby land features.
Fringing reefs
These types of reefs are the most common. They grow along the margins of the coastline, and may be separated from the shore by a lagoon (back reef zone). They may also lack a lagoon, in which case they develop a reef flat that extends from the reef crest to the shore. In other instances, a fringing reef may grow for hundreds of feet from the coast, containing various lagoons and a number of sea grass meadows and patch reefs. A fringing reef consists of a reef flat, which is the section facing the shore, and a reef slope, which is the region at the outer limits of the reef seaward.
Barrier reef
The formation of a barrier reef around a volcanic island is the half stage between the formation of a fringing reef and an atoll. While the landmass in an island subsides under its own weight, coral continues to grow around the island, forming a fringing reef, typically with a shallow lagoon dividing the island from the reef. Subsidence gradually continues over hundreds of thousands of years, forming a larger barrier reef, which is now farther from the island, with a bigger lagoon in between. In the end, the island totally subsides, creating an atoll.
Atoll
An atoll usually forms in the open ocean away from the shoreline, when a volcanic island gradually subsides, while a coral reef grows upward, creating a central lagoon, which separates the volcanic island from the barrier reef. Over time, subsequent subsidence of the central landmass brings the island underwater, while the barrier of corals remains, forming an atoll. The formation of a fringing reef may take 10,000 years, after the corals begin to attach around a volcanic island; over the following 100,000 years, the subsidence of the island will form a barrier reef, and within 30,000,000 years the entire submergence of the island will create an atoll.
Reef-building corals usually grow in the photic zone, comprising the region (100 meters or 328 ft.) into which sunlight can penetrate. Corals can thrive in temperatures between 24 and 30 ºC (75-86 ºF). Other coral species living in deeper ocean waters may withstand higher temperatures (40 ºC, 104 ºF). The growth of coral reefs is determined by changes in sea level. Coral reefs can grow vertically or expand seaward from shore. The construction of coral reefs is aided by some type of algae, which form a symbiotic association with the coral, known as zooxanthellae. Patch reefs are considered reef features of the three major types of reefs.
