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An island (or isle) is an isolated piece of habitat that is surrounded by a dramatically different habitat, such as water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island in a river or a lake island may be called an eyot or ait, and a small island off the coast may be called a holm. Sedimentary islands in the Ganges delta are called chars. A grouping of geographically or geologically related islands, such as the Philippines, is referred to as an archipelago.
There are two main types of islands in the sea: continental and oceanic. There are also artificial islands, which are man-made.
The word island derives from Middle English iland, from Old English igland (from ig or ieg, similarly meaning 'island' when used independently, and -land carrying its contemporary meaning; cf. Dutch eiland ("island"), German Eiland ("small island")). However, the spelling of the word was modified in the 15th century because of a false etymology caused by an incorrect association with the etymologically unrelated Old French loanword isle, which itself comes from the Latin word insula. Old English ieg is actually a cognate of Swedish ö and German Aue, and related to Latin aqua (water).
Relationships with continents
Differentiation from continents
There is a widely accepted difference between islands and continents in terms of geology. Continents are often considered to be the largest landmass of a particular continental plate; this holds true for Australia, which sits on its own continental lithosphere and tectonic plate (the Australian Plate).
By contrast, islands are usually seen as being extensions of the oceanic crust (e.g. volcanic islands), or as belonging to a continental plate containing a larger landmass (continental islands); the latter is the case of Greenland, which sits on the North American Plate.
Continental islands are bodies of land that lie on the continental shelf of a continent. Examples are Borneo, Java, Sumatra, Sakhalin, Taiwan and Hainan off Asia; New Guinea, Tasmania, and Kangaroo Island off Australia; Great Britain, Ireland, and Sicily off Europe; Greenland, Newfoundland, Long Island, and Sable Island off North America; and Barbados, the Falkland Islands, and Trinidad off South America.
A special type of continental island is the microcontinental island, which is created when a continent is horizontally displaced or rifted Examples are Madagascar and Socotra off Africa, New Caledonia, New Zealand, and some of the Seychelles.
A lake such as Wollaston Lake drains in two different directions, thus creating an island. If this island has a seashore as well as being encircled by two river systems, it becomes what might be called a subcontinental island. The one formed by Wollaston Lake is very large, about 2,000,000 km2 (770,000 sq mi).
Another subtype is an island or bar formed by deposition of tiny rocks where water current loses some of its carrying capacity. This includes:
- barrier islands, which are accumulations of sand deposited by sea currents on the continental shelves
- fluvial or alluvial islands formed in river deltas or midstream within large rivers. While some are transitory and may disappear if the volume or speed of the current changes, others are stable and long-lived.
Tectonic versus volcanic
Oceanic islands are typically considered to be islands that do not sit on continental shelves. Other definitions limit the term to only refer to islands with no past geological connections to a continental landmass. The vast majority are volcanic in origin, such as Saint Helena in the South Atlantic Ocean. The few oceanic islands that are not volcanic are tectonic in origin and arise where plate movements have lifted up the ocean floor above the surface. Examples are the Saint Peter and Saint Paul Archipelago in the North Atlantic Ocean and Macquarie Island in the South Pacific Ocean.
One type of volcanic oceanic island is found in a volcanic island arc. These islands arise from volcanoes where the subduction of one plate under another is occurring. Examples are the Aleutian Islands, the Mariana Islands, and most of Tonga in the Pacific Ocean. The only examples in the Atlantic Ocean are some of the Lesser Antilles and the South Sandwich Islands.
Another type of volcanic oceanic island occurs where an oceanic rift reaches the surface. There are two examples: Iceland, which is the world's second largest volcanic island, and Jan Mayen. Both are in the Atlantic.
A third type of volcanic oceanic island is formed over volcanic hotspots. A hotspot is more or less stationary relative to the moving tectonic plate above it, so a chain of islands results as the plate drifts. Over long periods of time, this type of island is eventually "drowned" by isostatic adjustment and eroded, becoming a seamount. Plate movement across a hot-spot produces a line of islands oriented in the direction of the plate movement. An example is the Hawaiian Islands, from Hawaii to Kure, which continue beneath the sea surface in a more northerly direction as the Emperor Seamounts. Another chain with similar orientation is the Tuamotu Archipelago; its older, northerly trend is the Line Islands. The southernmost chain is the Austral Islands, with its northerly trending part the atolls in the nation of Tuvalu. Tristan da Cunha is an example of a hotspot volcano in the Atlantic Ocean. Another hotspot in the Atlantic is the island of Surtsey, which was formed in 1963.
An atoll is an island formed from a coral reef that has grown on an eroded and submerged volcanic island. The reef rises to the surface of the water and forms a new island. Atolls are typically ring-shaped with a central lagoon. Examples are the Line Islands in the Pacific and the Maldives in the Indian Ocean.
Approximately 45,000 tropical islands with an area of at least 5 hectares (12 acres) exist. Examples formed from coral reefs include Maldives, Tonga, Samoa, Nauru, and Polynesia. Granite islands include Seychelles and Tioman.
The socio-economic diversity of tropical islands ranges from the Stone Age societies in the interior of North Sentinel, Madagascar, Borneo, and Papua New Guinea to the high-tech lifestyles of the city-islands of Singapore and Hong Kong. International tourism is a significant factor in the economy of many tropical islands including Seychelles, Sri Lanka, Mauritius, Réunion, Hawaii, Puerto Rico and the Maldives.
The process of de-islandisation is often concerning bridging, but there are other forms of linkages such as causeways: fixed transport links across narrow necks of water, some of which are only operative at low tides (e.g. that connecting Cornwall's St Michael's Mount to the peninsular mainland) while others (such as the Canso Causeway connecting Cape Breton to the Nova Scotia mainland), are usable all-year-round (aside from interruptions during storm surge periods).
Some places may retain "island" in their names for historical reasons after being connected to a larger landmass by a land bridge or landfill, such as Coney Island and Coronado Island, though these are, strictly speaking, tied islands. Conversely, when a piece of land is separated from the mainland by a man-made canal, for example the Peloponnese by the Corinth Canal, more or less the entirety of Fennoscandia by the White Sea Canal, or Marble Hill in northern Manhattan during the time between the building of the United States Ship Canal and the filling-in of the Harlem River which surrounded the area, it is generally not considered an island.
Another type of connection is fostered by harbour walls/breakwaters that incorporate offshore islets into their structures, such as those in Sai harbour in northern Honshu, Japan, and the connection to the mainland which transformed Ilhéu do Diego from an islet. De-islanded through its fixed link to the mainland, the former islet's name, Ilhéu do Diego, became functionally redundant (and thereby archaic) and the location took the fort as its namesake. Some former island sites have retained designations as islands after the draining/subsidence of surrounding waters and their fixed linkage to land (England's Isle of Ely and Vancouver's Granville Island being respective cases in point). Their names are thereby archaic in that they reflect the islands’ pasts rather than their present structures and/or transport logistics. Other examples include Singapore and its causeway, and the various Dutch delta islands, such as IJsselmonde.
Almost all of Earth's islands are natural and have been formed by tectonic forces or volcanic eruptions. However, artificial (man-made) islands also exist, such as the island in Osaka Bay off the Japanese island of Honshu, on which Kansai International Airport is located. Artificial islands can be built using natural materials (e.g., earth, rock, or sand) or artificial ones (e.g., concrete slabs or recycled waste).
Sometimes natural islands are artificially enlarged, such as Vasilyevsky Island in the Russian city of St. Petersburg, which had its western shore extended westward by some 0.5 km in the construction of the Passenger Port of St. Petersburg.
Artificial islands are sometimes built on pre-existing "low-tide elevation," a naturally formed area of land which is surrounded by and above water at low tide but submerged at high tide. Legally these are not islands and have no territorial sea of their own.
- Largest island: Greenland
- Largest island in a lake: Manitoulin Island, Ontario, Canada
- Largest island in a river: Bananal Island, Tocantins, Brazil
- Largest island in freshwater: Marajó, Pará, Brazil
- Largest sand island: Fraser Island, Queensland, Australia
- Largest artificial island: Flevopolder, the Netherlands (created 1969)
- Largest uninhabited island: Devon Island, Nunavut, Canada
- Most populous island: Java, Indonesia
- Lowest island: Franchetti Island, Lake Afrera, Ethiopia
- Island shared by largest number of countries: Borneo (Brunei, Indonesia, Malaysia)
- Island with the highest point: New Guinea (Puncak Jaya, 4,884 m, 16,024 ft), Indonesia
- Northernmost island: Kaffeklubben Island, Greenland
- Southernmost island (not fully surrounded by permanent ice): Ross Island, Antarctica
- Island with the most populated city: Honshu (Tokyo), Japan
- Most remote island (from nearest land): Bouvet Island
- Island with earliest known settlement: Sumatra (Lida Ajer cave), Indonesia
- Desert island
- Great wall of sand
- Island biogeography
- Island ecology
- Island country
- Island hopping
- Lake island
- List of ancient islands
- List of archipelagos
- List of artificial islands
- List of divided islands
- List of fictional islands
- List of island countries
- List of islands by area
- List of islands by body of water
- List of islands by continent
- List of islands by country
- List of islands by highest point
- List of islands by name
- List of islands by population
- List of islands by population density
- List of islands named after people
- Phantom island
- Private island
- River island
- Rock fever
- Small Island Developing States
- Tidal island
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