Coral reefs are marine structures made of calcium carbonate (CaCO3) secreted by stony corals and other marine species living in the reef. Corals are colonial organisms composed of hundreds of thousands of individual animals known as polyps, which cluster in groups. Polyps secrete calcium carbonate skeletons which protect and give support to their bodies. Coral reefs may take from tens of thousands to millions of years to form and grow best in warm, shallow, clear waters with temperatures, ranging from 23-29 °C.
Coral reef formation
Coral reefs form when free-swimming larvae attach to submarine rocks or any other hard surfaces, forming colonies, along the edges of island or continents. The formed colonies grow into large groups of coral polyps which secrete calcium carbonate beneath and around them, contributing to the reef’s structure. A coral reef grows vertically following the sunlight. Once the coral reaches the surface sea level, it begins to grow horizontally. Over time, the abundance of marine life within the reef is broken down by the effect of ocean waves and eroding animals, forming complex coral reef ecosystems.
Reef structures
There are different types of reefs based on their morphology and their relation to the nearby land. Fringing reefs, the most common type of reef, project towards the sea, forming borders along the shoreline. Fringing reefs typically have a sand-filled channel between the reef and the shoreline. Barrier reefs form at at greater distances from the main shoreline and they usually have a deep lagoon which separates them from the adjacent land. Atolls form when a central volcanic island subsides below sea level, while the coral continues to grow upward, forming a central lagoon.
Time formation
When corals begin to attach and grow around an island, it may take approximatelly 10,000 years for a fringing coral reef to form. Over the next 100,000 years, the interior island will subside, forming a fringing reef with a central shallow lagoon in between the reef and main land. The submergence of the main land continues, and at approximately 30,000,000 years, the whole island will completely submerge under the sea, leaving a circular ring of growing coral surrounding a central lagoon.
Coral reef building species usually develop well in the photic zone, which is the region of the ocean where light can penetrate to a depth of approximately 100 meters (328 ft.). Reef-building corals can tolerate temperatures between 24 to 30 degrees C. Other coral species can tolerate temperatures of 40 degrees C, such as those found in deeper ocean waters. Most corals require salty water ranging from 32 to 40 parts per thousand and clear nutrient free water , so that the light from the sun can penetrate and permit the process of photosynthesis to occur.
Certain species of corals cease to exist at greater ocean depths. Corals are scarce in turbid or nutrient-rich waters. This is due to amount of suspended particles, preventing the penetration of sunlight. In certain colder regions or at depths beyong 70 meters, certain species of corals may develop around hard surfaces; however, their capacity to secrete calcium carbonate skeletons is significantly reduced. According to oceanworld.tamu.edu, Coral reefs grow about half of an inch every year, but if left undisturbed by man, a coral reef can grow large, such as The Great barrier Reef in Australia with 150 km (93 miles) across and 2,000 km (1,242 miles) long.
