Atoll in the context of Ontong Java Atoll

An island or isle is a piece of land, distinct from a continent, completely surrounded by water. There are continental islands, which were formed by being split from a continent by plate tectonics, and oceanic islands, which have never been part of a continent. Oceanic islands can be formed from volcanic activity, grow into atolls from coral reefs, and form from sediment along shorelines, creating barrier islands. River islands can also form from sediment and debris in rivers. Artificial islands are those made by humans, including small rocky outcroppings built out of lagoons and large-scale land reclamation projects used for development.

Islands are host to diverse plant and animal life. Oceanic islands have the sea as a natural barrier to the introduction of new species, causing the species that do reach the island to evolve in isolation. Continental islands share animal and plant life with the continent they split from. Depending on how long ago the continental island formed, the life on that island may have diverged greatly from the mainland due to natural selection.

Tuvalu (/tuːˈvɑːluː/ too-VAH-loo) is an island country in the Polynesian sub-region of Oceania in the Pacific Ocean, about midway between Hawaii and Australia. It lies east-northeast of the Santa Cruz Islands (which belong to the Solomon Islands), northeast of Vanuatu, southeast of Nauru, south of Kiribati, west of Tokelau, northwest of Samoa and Wallis and Futuna, and north of Fiji.

Tuvalu is composed of three reef islands and six atolls spread out between the latitude of 5° and 10° south and between the longitude of 176° and 180°. They lie west of the International Date Line. The 2022 census determined that Tuvalu had a population of 10,643, making it the 194th most populous country, exceeding only Niue and the Vatican City in population. Tuvalu's total land area is 25.14 square kilometres (9.71 sq mi).

A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as a reef, a barrier island or islands, a barrier peninsula, or an isthmus. Lagoons are commonly divided into coastal lagoons (or barrier lagoons) and atoll lagoons. They have also been identified as occurring on mixed-sand and gravel coastlines. There is an overlap between bodies of water classified as coastal lagoons and bodies of water classified as estuaries. Lagoons are common coastal features around many parts of the world.

The Hawaiian Islands (Hawaiian: Mokupuni Hawaiʻi) are an archipelago of eight major volcanic islands, several atolls, and numerous smaller islets in the North Pacific Ocean, extending some 1,500 miles (2,400 kilometers) from the island of Hawaiʻi in the south to northernmost Kure Atoll. Formerly called the Sandwich Islands by Europeans, the present name for the archipelago is derived from the name of its largest island, Hawaiʻi.

The archipelago sits on the Pacific Plate. The islands are exposed peaks of a great undersea mountain range known as the Hawaiian–Emperor seamount chain, formed by volcanic activity over the Hawaiian hotspot. The islands are about 1,860 miles (3,000 km) from the nearest continent and are part of the Polynesia subregion of Oceania.

The British Indian Ocean Territory (BIOT) is a British Overseas Territory situated in the Indian Ocean. The territory comprises the seven atolls of the Chagos Archipelago with over 1,000 individual islands, many very small, amounting to a total land area of 60 square kilometres (23 square miles). The largest and most southerly island is Diego Garcia, 27 square kilometres (10 square miles), the site of a Joint Military Facility of the United Kingdom and the United States. Official administration is remote from London, though the local capital is often regarded as being on Diego Garcia.

Mauritius claimed that the British government separated the Chagos Archipelago from Mauritius, creating a new colony, the British Indian Ocean Territory (BIOT). However, this was disputed by the United Kingdom, who said that the Chagos Islands had no historical or cultural ties to Mauritius, and that they were only governed during the colonial period from Mauritius (2191 km or 1361 miles away) as an administrative convenience. Mauritius further claimed that to avoid accountability to the United Nations for its continued colonial rule, the UK falsely claimed that the Chagos had no permanent population.

The Maldives consists of 20 natural atolls, along with a few islands and isolated reefs stretching from 7 degrees 10 minutes north to 0 degrees 45 minutes south. The largest of these atolls is Boduthiladhunmathi, while the atoll containing the most islands is Huvadhu.

Some atolls are in the form of a number of islands by time and in the form of isolated reefs, which could be classified as smaller atoll formations. All land above the surface in the Maldives is of coralline origin. The atolls of the Maldives form a quite regular chain and, especially in the northern and central atolls, an arrayed structure is apparent. There are broad and deep channels between some atolls. The origin of the word "atoll" itself is in the language of the Maldives. "Atoll" (from Dhivehi: atholhu) is now used in many languages worldwide.

French Polynesia (/ˌpɒlɪˈniːʒə/ POL-ih-NEE-zhə; French: Polynésie française [pɔlinezi fʁɑ̃sɛːz] ; Tahitian: Pōrīnetia farāni) is an overseas collectivity of France and its sole overseas country. It comprises 121 geographically dispersed islands and atolls stretching over more than 2,000 kilometres (1,200 mi) in the South Pacific Ocean. French Polynesia is associated with the European Union as an overseas country and territory (OCT). The total land area of French Polynesia is 3,521 square kilometres (1,359 sq mi), with a population of 282,596 as of September 2025 of which at least 205,000 live in the Society Islands and the remaining population lives in the rest of the archipelago.

French Polynesia is divided into five island groups: the Austral Islands; the Gambier Islands; the Marquesas Islands; the Society Islands (comprising the Leeward and Windward Islands); and the Tuamotus. Among its 121 islands and atolls, 75 were inhabited at the 2017 census. Tahiti, which is in the Society Islands group, is the most populous island, being home to nearly 69% of the population of French Polynesia as of 2017. Papeete, located on Tahiti, is the capital of French Polynesia. Although not an integral part of its territory, Clipperton Island was administered from French Polynesia until 2007.

Tokelau (/ˈtoʊkəlaʊ/ ; lit. 'north-northeast' or 'north wind'; known previously as the Union Islands, and, until 1976, known officially as the Tokelau Islands) is a dependent territory of New Zealand in the southern Pacific Ocean. It consists of three tropical coral atolls: Atafu, Nukunonu, and Fakaofo. They have a combined land area of 10 km (4 sq mi). In addition to these three, Swains Island (Olohega), which forms part of the same archipelago, is the subject of an ongoing territorial dispute, while being currently administered by the United States as part of American Samoa. Tokelau lies north of the Samoan Islands, east of Tuvalu, south of the Phoenix Islands, southwest of the more distant Line Islands, and northwest of the Cook Islands.

Tokelau has a population of approximately 2,500 people; it has the fourth-smallest population of any sovereign state or dependency in the world. As of the 2016 census, around 45% of its residents had been born overseas, mostly in Samoa or New Zealand. The populace has a life expectancy of 69, which is comparable to that of other Oceanian island nations. Approximately 94% of the population speak Tokelauan as their first language. Tokelau has the smallest economy of any nation. It is a leader in renewable energy, being the first 100% solar-powered nation in the world.

The Structure and Distribution of Coral Reefs, Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836, was published in 1842 as Charles Darwin's first monograph, and set out his theory of the formation of coral reefs and atolls. He conceived of the idea during the voyage of the Beagle while still in South America, before he had seen a coral island, and wrote it out as HMS Beagle crossed the Pacific Ocean, completing his draft by November 1835. At the time there was great scientific interest in the way that coral reefs formed, and Captain Robert FitzRoy's orders from the Admiralty included the investigation of an atoll as an important scientific aim of the voyage. FitzRoy chose to survey the Keeling Islands in the Indian Ocean. The results supported Darwin's theory that the various types of coral reefs and atolls could be explained by uplift and subsidence of vast areas of the Earth's crust under the oceans.

The book was the first volume of three Darwin wrote about the geology he had investigated during the voyage, and was widely recognised as a major scientific work that presented his deductions from all the available observations on this large subject. In 1853, Darwin was awarded the Royal Society's Royal Medal for the monograph and for his work on barnacles. Darwin's theory that coral reefs formed as the islands and surrounding areas of crust subsided has been supported by modern investigations, and is no longer disputed, while the cause of the subsidence and uplift of areas of crust has continued to be a subject of discussion.

A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms (usually microbes) which induce carbonate precipitation through their metabolism. Therefore, carbonate platforms can not grow up everywhere: they are not present in places where limiting factors to the life of reef-building organisms exist. Such limiting factors are, among others: light, water temperature, transparency and pH. For example, carbonate sedimentation along the Atlantic South American coasts takes place everywhere but at the mouth of the Amazon River, because of the intense turbidity of the water there. Spectacular examples of present-day carbonate platforms are the Bahama Banks under which the platform is roughly 8 km thick, the Yucatan Peninsula which is up to 2 km thick, the Florida platform, the platform on which the Great Barrier Reef is growing, and the Maldive atolls. All these carbonate platforms and their associated reefs are confined to tropical latitudes. Today's reefs are built mainly by scleractinian corals, but in the distant past other organisms, like archaeocyatha (during the Cambrian) or extinct cnidaria (tabulata and rugosa) were important reef builders.

American Samoa is an unincorporated and unorganized territory of the United States located in the Polynesia region of the south Pacific Ocean. Centered on 14°18′S 170°42′W / 14.3°S 170.7°W / -14.3; -170.7, it is 40 miles (64 km) southeast of the island country of Samoa, east of the International Date Line and the Wallis and Futuna Islands, west of the Cook Islands, north of Tonga, and some 310 miles (500 km) south of Tokelau. American Samoa is the southernmost territory of the United States, situated 2,200 miles (3,500 km) southwest of the U.S. state of Hawaii, and one of two U.S. territories south of the Equator, along with the uninhabited Jarvis Island.

American Samoa consists of the eastern part of the Samoan archipelago – the inhabited volcanic islands of Tutuila, Aunuʻu, Ofu, Olosega and Taʻū and the uninhabited Rose Atoll – as well as Swains Island, a remote coral atoll in the Tokelau volcanic island group. The total land area is 77 square miles (199 km), slightly larger than Washington, D.C.; including its territorial waters, the total area is 117,500 square miles (304,000 km), about the size of New Zealand. American Samoa has a tropical climate, with 90 percent of its land covered by rainforests. As of 2024, the population is approximately 47,400 and concentrated on Tutuila, which hosts the capital and largest settlement, Pago Pago. The vast majority of residents are indigenous ethnic Samoans, most of whom are fluent in the official languages, Samoan and English.

Palmyra Atoll (/pælˈmaɪrə/), also referred to as Palmyra Island, is one of the Northern Line Islands (southeast of Kingman Reef and north of Kiribati). It is located almost due south of the Hawaiian Islands, roughly one-third of the way between Hawaii and American Samoa. North America is about 3,300 miles (5,300 kilometers) northeast and New Zealand the same distance southwest, placing the atoll at the approximate center of the Pacific Ocean. The land area is 4.6 sq mi (12 km), with about 9 miles (14 km) of sea-facing coastline and reef. There is one boat anchorage, known as West Lagoon, accessible from the sea by a narrow artificial channel and an old airstrip; during WW2, it was turned into a Naval Air Station for several years and used for training and refueling. It was shelled by a submarine in December 1941, days after the Attack on Pearl Harbor, but was not the site of a major battle. Palmyra has, over time, had many of its islets merged together, so the actual amount of contiguous land depends on the tide and locations of sandbanks. For example, Strawn, Menge, and Cooper Islands are one contiguous island. Likewise, there are many shoals and coral heads on the atoll, which is ringed by a coral reef.

It is the second-northernmost of the Line Islands and one of three American islands in the archipelago, along with Jarvis Island and Kingman Reef. Palmyra Atoll is part of the Pacific Islands Heritage Marine National Monument, the world's largest marine protected area. The atoll comprises submerged sand flats along with dry land and reefs. It comprises three lagoons separated by coral reefs. The western reef terrace is one of the biggest shelf-reefs, with dimensions of 2 by 3 miles (3.2 by 4.8 km). Over 150 species of coral inhabit Palmyra Atoll, double the number recorded in Hawaii.

Ulithi (Yapese: Wulthiy, Yulthiy, or Wugöy; pronounced roughly as YOU-li-thee) is an atoll in the Caroline Islands of the western Pacific Ocean, about 191 km (103 nmi) east of Yap, within Yap State.

Kure Atoll (/ˈkʊəriː/; Hawaiian: Hōlanikū, lit. 'bringing forth heaven'; Mokupāpapa, 'flat island') or Ocean Island is an atoll in the Pacific Ocean 48 nautical miles (89 km; 55 mi) west-northwest of Midway Atoll in the Northwestern Hawaiian Islands at 28°25′N 178°20′W / 28.417°N 178.333°W / 28.417; -178.333. A coral ring 6 miles (9.7 km) across encloses a lagoon several meters deep. The atoll's largest island is called Green Island, and is a habitat for hundreds of thousands of seabirds in the present day and wildlife staff. A short, unused, and unmaintained runway and a portion of one building, both from a former United States Coast Guard LORAN station, are located on the island. Politically, it is part of Hawaii, though the nearest island is Midway, which is a separate unorganized territory. Kure Atoll, in addition to being the nesting grounds for tens of thousands of seabirds, has recorded several vagrant terrestrial birds, including snow bunting, eyebrowed thrush, brambling, olive-backed pipit, black kite, Steller's sea eagle and Chinese sparrowhawk. It is currently managed as a Wildlife Bird Sanctuary by the State of Hawaii's Department of Land and Natural Resource—Division of Forestry and Wildlife as one of the co-trustees of Papahānaumokuākea Marine National Monument with support from the Kure Atoll Conservancy. Kure is one of the westernmost islands of the Northwestern Hawaiian Islands.

Kure is seasonally inhabited by small crews of two to eight volunteers and biologists who work to restore and manage the native ecosystem.

The Hawaiian–Emperor seamount chain is a mostly undersea mountain range in the Pacific Ocean that reaches above sea level in Hawaii. It is composed of the Hawaiian ridge, consisting of the islands of the Hawaiian chain northwest to Kure Atoll, and the Emperor Seamounts: together they form a vast underwater mountain region of islands and intervening seamounts, atolls, shallows, banks and reefs along a line trending southeast to northwest beneath the northern Pacific Ocean. The seamount chain, containing over 80 identified undersea volcanoes, stretches about 6,200 km (3,900 mi) from near the Aleutian Trench off the coast of the Kamchatka peninsula in the far northwest Pacific to the Kamaʻehuakanaloa Seamount (formerly Lōʻihi), the youngest volcano in the chain, which lies about 35 kilometres (22 mi) southeast of the Island of Hawaiʻi.

The Hawaiʻi hotspot is a volcanic hotspot located near the namesake Hawaiian Islands, in the northern Pacific Ocean. One of the best known and intensively studied hotspots in the world, the Hawaii plume is responsible for the creation of the Hawaiian–Emperor seamount chain, a 6,200-kilometer (3,900 mi) mostly undersea volcanic mountain range. Four of these volcanoes are active, two are dormant; more than 123 are extinct, most now preserved as atolls or seamounts. The chain extends from south of the island of Hawaiʻi to the edge of the Aleutian Trench, near the eastern coast of Russia.

While some volcanoes are created by geologic processes near tectonic plate convergence and subduction zones, the Hawaiʻi hotspot is located far from plate boundaries. The classic hotspot theory, first proposed in 1963 by John Tuzo Wilson, proposes that a single, fixed mantle plume builds volcanoes that are then cut off from their source by the movement of the Pacific plate. This causes less lava to erupt from these volcanoes and they eventually erode below sea level over millions of years. According to this theory, the nearly 60° bend where the Emperor and Hawaiian segments within the seamounts was caused by shift in the movement of the Pacific Plate. Studies on tectonic movement have shown that several plates have changed their direction of plate movement because of differential subduction rates, breaking off of suducting slabs, and drag forces. In 2003, new investigations of this irregularity led to the proposal of a mobile hotspot hypothesis, suggesting that hotspots are prone to movement instead of the previous idea that hotspots are fixed in place, and that the 47-million-year-old bend was caused by a shift in the hotspot's motion rather than the plate's. According to this 2003 study, this could occur through plume drag taking parts of the plume in the direction of plate movement while the main plume could remain stationary. Many other hot spot tracks move in almost parallel so current thinking is a combination of these ideas.

The South China Sea Islands consist of over 250 islands, atolls, cays, shoals, reefs, and seamounts in the South China Sea. The islands are mostly low and small and have few inhabitants. The islands and surrounding seas are subject to overlapping territorial claims by the countries bordering the South China Sea.

The South China Sea Islands constitute a distinct tropical moist broadleaf forest terrestrial ecoregion and the South China Sea Oceanic Islands marine ecoregion.