In everyday consciousness, an island is a piece of land surrounded on all sides by water. This term refers to both huge tracts of land, for example, Iceland, and tiny patches of solid soil somewhere on the river that can completely disappear in the flood. By their origin, islands can be continental (created due to the movement of tectonic plates), volcanic (arising from the eruption of volcanoes), reclaimed (with a sandy base, which formed from a spit), artificial and, finally, distinguish coral islands. If it is clear with the first four groups that they occurred as a result of natural geotectonic processes or human activities, then what takes part in the formation of the coral island, because we know that coral is a living organism?
In the warm waters of the tropical seas these strange primitive creatures live, which feed on plankton caught by their tentacles. Each such animal builds a limestone cup for itself. The polyp dies over time, but the calyx remains. On its basis, a new polyp constructs a limestone dwelling for itself. And then the coral colonies grow upward, rushing behind the plankton up to the sun, until they reach sea level and begin to rise above the surface. This forms a coral island.
It will be natural to assume that these are low islands, since corals are marine animals, and they die on land. The optimal depth for them is about 30-10 meters, where no storms are felt and at the same time there is enough sunlight. Why, then, in real life can a coral island reach a height of several hundred meters? Yes, because the tectonic plates of the Earth are constantly moving. Fluctuations in the level of the seabed can raise the coral reef up or lower it into the realm of eternal darkness, by 200-300 meters. It is clear that over time the elevated island will be covered with soil and vegetation, but still its base will consist of countless limestone skeletons of long-dead polyps.
The coral island can be of two types: the island itself, which has the familiar form of elevation above the sea in the form of a rock, and the atoll. What are atolls and how are they formed? Pretty simple, albeit a long time. There are a lot of atolls in the Pacific Ocean, where, as you know, volcanic activity is very strong. Imagine an island representing a volcano’s vent that rises above the water. At its base, in the water, heated not only by the sun, but also by boiling magma in the bowels, polyps like to settle. They build reefs around the island, encircling it with a ring. Then, after centuries of such coral activity, a volcano erupts and ... the exploded mountain disappears into the abyss of water. And the corals remain! From an airplane, the atoll seems to be surrounded by a blue sea ring, inside which the lagoon turns green - all that remains of the once formidable volcano.
Of course, the volcano does not always plunge into the sea, and most often there are volcanic and coral islands. We see examples of such bio-magmatic symbiosis in the state of Tonga (Polynesia), which is a cluster of volcanic islands surrounded by coral reefs and atolls. And an example of a classic atoll that has an almost perfect ring shape is Bikini.
Coral island is a complex ecosystem. It can be said that reefs are the most densely populated of all marine habitats. The water temperature that never drops below + 20 degrees, the shelter that can be found in coral corridors, grottoes and thickets, the abundance of food attract many different creatures here. Polyps absorb plankton, and they, in turn, are gnawed by colorful parrot fish, butterfly fish, as well as the gluttonous starfish, the crown of thorns. Sea urchins and sea anemones settle in recesses of reefs. In the blooming coral garden, small amphiprions, known to us from the animated film "Nemo Fish", take refuge, and moray eels wait for their prey in caves from dead corals. Unfortunately, because of the turbulent human activity, corals become less and less, because polyps live only in very clean water. Even touching a coral can lead to its death. Therefore, while in tropical countries, never step on polyps, and especially not break fragile coral branches.