Barrier reef in the context of "New Caledonia Barrier Reef"

An atoll (/ˈæt.ɒl, -ɔːl, -oʊl, əˈtɒl, -ˈtɔːl, -ˈtoʊl/) is a ring-shaped island, including a coral rim that encircles a lagoon. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical parts of the oceans and seas where corals can develop. Most of the approximately 440 atolls in the world are in the Pacific Ocean.

Two different, well-cited models, the subsidence model and the antecedent karst model, have been used to explain the development of atolls. According to Charles Darwin's subsidence model, the formation of an atoll is explained by the sinking of a volcanic island around which a coral fringing reef has formed. Over geologic time, the volcanic island becomes extinct and eroded as it subsides completely beneath the surface of the ocean. As the volcanic island subsides, the coral fringing reef becomes a barrier reef that is detached from the island. Eventually, the reef and the small coral islets on top of it are all that is left of the original island, and a lagoon has taken the place of the former volcano. The lagoon is not the former volcanic crater. For the atoll to persist, the coral reef must be maintained at the sea surface, with coral growth matching any relative change in sea level (sinking of the island or rising oceans).

Ono-i-Lau is a group of islands within a barrier reef system in the Fijian archipelago of Lau Islands (ono means "six" in the Fijian language). There are four central volcanic islands: Onolevu, Doi (or Ndoi) Lovoni and Ndavura, the uppermost parts of the volcanic edifice rising from the Lau Ridge slightly more than 1,000 metres (3,300 feet) below sea level and on which the reef and other islands are built. There are also three clusters of coral limestone islets, Yanuya (50 islets) and Mana (46 islets) on the barrier reef and Niuta (7 tiny islets). A sand cay, Udui, is not counted as one of the six “islands”.

The group forms one of the southernmost of the Lau Islands; it is located at 20.80° South and 178.75° West, and occupies an area of 7.9 square kilometres (3.1 square miles). It has a maximum elevation of 113 metres (371 feet). It is 90 kilometres (56 mi) south-southwest of Vatoa, the nearest largish island; the only land further south in the Lau Group is in the two small Tuvana islands of coral limestone and sand.

The Aleipata Islands are a group of four uninhabited islands off the eastern end of Upolu Island, Samoa, in central South Pacific Ocean. The islands are eroded volcanic tuff rings, and consist of a small northern pair on Upolu's barrier reef, and a larger southern pair outside it. Collectively the islands have an area of about 1.7 km (0.66 sq mi). Administratively, the islands are part of the Atua district. The islands are important for Samoa's biodiversity and are protected by the Aleipata Marine Protected Area.

A fringing reef is one of the three main types of coral reef. It is distinguished from the other main types, barrier reefs and atolls, in that it has either an entirely shallow backreef zone (lagoon) or none at all. If a fringing reef grows directly from the shoreline, then the reef flat extends to the beach and there is no backreef. In other cases (e.g., most of the Bahamas), fringing reefs may grow hundreds of yards from shore and contain extensive backreef areas within which it contains food and water. Some examples of this are Philippines, Indonesia, Timor-Leste, the western coast of Australia, the Caribbean, East Africa, and Red Sea. Charles Darwin believed that fringing reefs are the first kind of reefs to form around a landmass in a long-term reef growth process. The largest fringing coral reef in the world is the Ningaloo Reef, stretching to around 260 km (160 mi) along the coastline of Western Australia.