An island or isle is a tract of land that is completely surrounded by water, above high tide, and isolated from other significant landmasses, yet is not large enough to be called a continent. Very small islands such as emergent land features on atolls are called islets. A key or cay is another name for a small island or islet. An island in a river or lake may be called an eyot.
There are two main types of islands: continental islands and oceanic islands. A grouping of geographically and/or geologically related islands is called an archipelago. Whatever their formative history, islands are popular getaway spots for those seeking respite from the harried lives of the civilized world. The island environment offers such sanctuary. As such, islands are the most common vacation destinations.
The word island comes from Old English ī(e)gland (literally, "watery land"). However, the spelling of the word was modified in the fifteenth century by association with the etymologically unrelated Old French loanword isle, which was a component of paenīnsula, which meant "almost-island" . 
An island is any area of land which is smaller than the smallest continent and is entirely surrounded by water. Islands may be found in oceans, seas, lakes, or rivers. Islands closely grouped together are called an archipelago.
Islands range in size from huge landmasses to tiny river islets. The world's ten largest islands, in descending order of size are Greenland, New Guinea, Borneo, Madagascar, Baffin Island, Sumatra, Honshu, Great Britain, Victoria Island and Ellesmere Island. 
Generally, when defining islands as bodies of land that are completely surrounded by water, narrow bodies of water such as rivers and canals are left out of consideration. As a basic rule, the body of water itself must be broader than the piece of land. For instance, in France the Canal du Midi connects the Garonne river to the Mediterranean Sea, thereby completing a continuous water connection from the Atlantic Ocean to the Mediterranean. So technically, the land mass that includes the Iberian Peninsula and the part of France that is south of the Garonne River and the Canal du Midi is completely surrounded by water. For a completely natural example, the Orinoco River splits into two branches near Tamatama, in Amazonas state, Venezuela. The southern branch flows south and joins the Rio Negro, and then the Amazon. Thus, all of the Guianas (Guyana, Suriname, and French Guiana) and substantial parts of Brazil and Venezuela are surrounded by (river or ocean) water. These instances are not generally considered islands.
On the other hand, an island may still be described as such despite the presence of a land bridge, such as Singapore and its causeway or the various Dutch delta Islands, such as IJsselmonde. The retaining of the island description may therefore be to some degree simply due to historical reasons—though the land bridges are often of a different [[geology|geological] nature (for example sand instead of stone), and thus the islands remain islands in a more scientific sense as well.
It is generally accepted that there are two types of islands: Oceanic and Continental. Continental Islands are thought to have been connected to the nearby continent at some point in time, and separated either recently (in a geologic frame of reference) or in ancient eras. Oceanic Islands are those which were never connected to another body of land but formed in mid-ocean.
With the exception of the Seychelles Islands in the Indian Ocean, Oceanic Islands are always either coralline or volcanic in composition. The geological make-up of Continental Islands resembles that of continents with a variety of formations, most often comprised of varying ages of stratifed rock.
Continental islands are bodies of land that lie on the continental shelf of a continent. Examples include Greenland and Sable Island off North America; Barbados and Trinidad off South America; Great Britain, Ireland and Sicily off Europe; Sumatra and Java off Asia; and New Guinea and Tasmania off Australia.
Many of the world's larger islands are of the continental type. Greenland (840,000 square miles [2,175,000 square km]), the largest, is composed of the same materials as the adjacent North American continent, from which it is separated by a shallow and narrow sea. Likewise the world's second largest island, New Guinea (309,000 square miles [800,000 square km]), is part of the Australian continental platform and is separated from it only by the very shallow and narrow Torres Strait. 
A special type of continental island is the microcontinental island, which results when a continent is rifted. Examples are Madagascar off Africa; the Kerguelen Islands; and some of the Seychelles. Another subtype is an island or bar formed by deposition of sediment where a water current loses some of its carrying capacity. An example is barrier islands, which are accumulations of sand deposited by sea currents on the continental shelf. Another example is islands in river deltas or in large rivers. While some are transitory and may disappear if the volume or speed of the current changes, others are stable and long-lived.
There are some schools of thought that break continental islands into further categorization, those of Recent Continental Islands and Ancient Continental Islands. The differing physical characteristics between the two is the depth of sea between the island and the nearby continent. Recent Continental Islands are surrounded by more shallow seas, usually less than 100 fathoms deep, while ancient continental islands are closer to 1,000 or more fathoms deep. 
Recent Continental Islands are generally situated nearer to continents, but at times a distance of several-hundred miles is the case, as with Borneo, which is about 600 miles from Cambodia, the intervening sea being everywhere less than 100 fathoms deep. 
Though Ancient Continental Islands are always separated from the adjacent continent by a deep sea, they are many times no further from the continent than the more recent islands. For example, Madagascar is separated from Africa by a strait which is about 2,000 fathoms deep, and there is more than 1,000 fathoms between Haiti and the coast of South America. 
While recent and ancient continental islands bear the physical differences mentioned above, their zoological features are clearly defined as well. Recent Continental Islands are home to the same types of animals—birds, mammals and reptiles—that are represented on the mainland. The species are either the same or closely allied. Ancient Continental Islands however, claim as their occupants animals which are on the whole different from those of the mainland. The families and orders vary greatly from those on the continent while oftentimes animals which are present on these islands are missing from the continent. 
Oceanic Islands are those that do not sit on continental shelves but rise to the surface from the floors of the ocean basins. They are volcanic in origin. Lava accumulates to such thickness that it finally protrudes above the ocean surface. One type of 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 include the Mariana Islands, the Aleutian Islands, Republic of Mauritius and most of Tonga in the Pacific Ocean. Some of the Lesser Antilles and the South Sandwich Islands are the only Atlantic Ocean examples.
Another type of oceanic island occurs where an oceanic rift reaches the surface. There are two examples: Iceland, which is the world's largest volcanic island, and Jan Mayen—both are in the Atlantic Ocean.
A third type of 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 eroded down and "drowned" by isostatic adjustment, 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 then extends 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 being the atolls in the nation of Tuvalu. Tristan da Cunha is an example of a hot spot volcano in the Atlantic Ocean. Another hot spot 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 include the Maldives in the Indian Ocean and Line Islands in the Pacific.
Normally, Oceanic Islands are situated in mid-ocean and surrounded by very deep seas, examples of which are the Azores, St. Helena, and the Sandwich Islands. But there are some exceptions, such as the Cape Verde Islands, the Canaries, and Madeira, all of which are within 400 miles of the coast of Africa, while some of the Canary Islands are only 50 miles distant from it. Though these are in such close proximity, they are separated from the mainland by a depth of more than 1,000 fathoms. 
The study of island biogeography is a field within biogeography that attempts to establish and explain the factors that affect the species richness of a particular community. The field was begun in the 1960s by the ecologists Robert MacArthur and E.O. Wilson, who coined the term "theory of island biogeography," as this theory attempted to predict the number of species that would exist on a newly created island.
MacArthur and Wilson showed that the species richness of an area could be predicted in terms of such factors as habitat area, immigration rate, and extinction rate. This gave rise to an interest in island biogeography. The application of island biogeography theory to habitat fragments spurred the development of the fields of conservation biology and landscape ecology.
The theory of island biogeography holds that the number of species found on an island (the equilibrium number) is determined by two main factors, the effect of distance from the mainland and the effect of island size. These would affect the rate of extinction on the islands and the level of immigration.
Islands closer to the mainland are more likely to receive immigrants from the mainland than those farther away from the mainland. This is the distance-effect. The size-effect reflects a long-known relationship between island size and species diversity. On smaller islands the chance of extinction is greater than on larger ones. Thus, larger islands can hold more species than smaller ones. The play between these two factors can be used to establish how many species an island can hold at equilibrium.
Phantom islands are islands that were believed to exist, and appeared on maps for a period of time (sometimes centuries) during recorded history, but were later removed after they proved nonexistent. In contrast, lost lands are islands or continents believed by some to have existed during prehistory, often associated with ancient myths and legends.
Phantom islands usually stem from the reports of early sailors exploring new realms. Some arose through the mislocation of actual islands, or other errors in geography. For instance, Pepys Island was actually a misidentification of the Falkland Islands. The Baja California peninsula appears on some early maps as an island but was later discovered to be attached to the mainland of North America. Thule was perhaps actually discovered in the fourth century B.C.E. but was lost, and then later re-identified by ancient explorers and geographers as Shetland, Iceland, Scandinavia, or even as nonexistent.
Other phantom islands are probably due to navigational errors, the misidentification of icebergs, fog banks, or to optical illusions.
While many phantom islands appear never to have existed, a few (such as, perhaps, Thompson Island) may have been actual islands subsequently destroyed by volcanic explosions, earthquakes or submarine landslides, or low-lying lands such as sand banks that are no longer above water.
A desert island is an uninhabited or sparsely inhabited island. Such islands are commonly invoked in metaphor, literature, and the popular imagination, as a place where individuals or small groups of people find themselves marooned or castaway, cut off from civilization.
Note that an arid desert climate is not necessarily implied. "Desert" in this term implies non-inhabited or deserted. Definitions such as "desolate and sparsely occupied or unoccupied," or "a small tropical island, where nobody lives."
A tidal island is a piece of land that is connected to the mainland by a natural or man-made causeway that is exposed at low tide and submerged at high tide. Because of the mystique surrounding tidal islands many of them have been sites of religious worship, such as Mont Saint Michel with its Benedictine Abbey. Tidal islands are also commonly the sites of fortresses, due to their natural fortifications.
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