An atoll is a type of low coral island of the tropical oceans. The atolls are composed of a barrier reef, one or more islands called motu formed by accumulation of sand in the back of the reef and surrounding a central depression. Depression can be part of the emerging island or part of the sea (lagoon) or, more rarely, a closed chamber filled with fresh, brackish or very saline. For an atoll to form, the climatic conditions of environment must meet three important conditions. The temperature should be high enough, there must be a lot of light (shallow water) and the environment must be rich in oxygen. These three conditions are frequently found in the tropics, on the fringe of volcanic islands. In addition to climatic conditions, certain geological conditions must also exist for the atoll to form. Initially, there must be volcanic activity on an oceanic plate, like that of the Pacific. If the volcano reaches a height sufficient to emerge and form an island, a coral reef can begin to form around it. This reef is built at shallow depths, where there is plenty of light. This is the initial stage of the formation of an atoll called "fringing reef". According to the theory of plate tectonics, the more an oceanic plate moves away from the ridge that forms it, the more its density rises, it cools and it sinks, taking with it the hardware it supports. The material that sinks to the plate is a volcano. If the rate of production by the coral reef is sufficient to counter the sink rate of the volcano and thus remain on the surface, it will form a barrier island. This is the second stage in the formation of an atoll. As the plate moves laterally and the volcano sinks, the reef grows vertically. Over time, often several million years, the volcano disappears completely under water and what remains on the surface is the coral reef ring-shaped, this is the stage of the atoll itself. arrier islands are low ridges of sand parallel the coast at distances from 3 to 30 kilometers offshore. They formed in many ways. Some of them originated as spits that were subsequently severed from the mainland by wave erosion or by the general rise of sea level in the last episode of glaciation, while others were created when turbulent waters in the line of breakers heaped up sand that had been scoured from the bottom. Finally, some of them could have been sand dune ridges that originated along the shore during last glacial period, when sea level was lower.