Difference between revisions of "Island" - New World Encyclopedia

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[[Image:Mokuaeae island.jpg|right|thumb|300px|Mokuaeae Island, off Kilauea Point, Kauai, Hawaii.]]  
 
[[Image:Mokuaeae island.jpg|right|thumb|300px|Mokuaeae Island, off Kilauea Point, Kauai, Hawaii.]]  
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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 [[atoll]]s 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 [[ait|eyot]].
  
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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.
  
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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 [[Etymology|etymologically]] unrelated Old French loanword ''isle'', which was a component of ''paenīnsula'', which meant "almost-island" . <ref> ''Dictionary.com''. [http://dictionary.reference.com/browse/Island Island]. Retrieved April 21, 2008. </ref>
  
An '''island''' or '''isle''' is any piece of land that is completely surrounded by water, above high tide, and isolated from other significant landmasses. Very small islands such as emergent land features on [[atoll]]s are called '''[[islet]]s'''. A '''key''' or [[cay]] is another name for a small island or islet. An island in a river or lake may be called an [[ait|eyot]], [[International Phonetic Alphabet|IPA]] {{IPA|[aɪət]}}.
 
There are two main types of islands: '''continental islands''' and '''oceanic islands'''. There are also [[artificial island]]s. A grouping of geographically and/or geologically related islands is called an [[archipelago]].
 
 
The word ''island'' comes from [[Old English]] ''ī(e)gland'' (literally, "watery land"). However, the spelling of the word was modified in the [[15th century]] by association with the [[Etymology|etymologically]] unrelated Old French loanword ''isle''.<ref>[http://dictionary.reference.com/browse/Island Island]. Dictionary.com. Retrieved October 6, 2007.</ref>
 
  
 
== Definition ==
 
== Definition ==
[[Image:Island near Fiji.jpg|right|thumb|200px|A small [[Fiji]]an island.]]
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[[Image:Island near Fiji.jpg|thumb|300px|A small [[Fiji]]an island.]]
[[Image:Island.jpg|right|thumb|200px|A small island in the [[Adriatic Sea]]]]
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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 [[ocean]]s, [[sea]]s, [[lake]]s, or [[river]]s. Islands closely grouped together are called an [[archipelago]].  
[[Image:Manhattan.jpg|thumb|300px|Manhattan Island, New York. One of the world's most densely populated islands at 67,000 residents per square mile.]]
 
 
 
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 [[ocean]]s, [[sea]]s, [[lake]]s, or [[river]]s. 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 (Canada)|Victoria Island]] and [[Ellesmere Island]]. <ref>''Lexicon Universal Encyclopedia''. 1989. Lexicon Publications, Inc. New York, New York. ISBN 0717220257</ref>
 
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 (Canada)|Victoria Island]] and [[Ellesmere Island]]. <ref>''Lexicon Universal Encyclopedia''. 1989. Lexicon Publications, Inc. New York, New York. ISBN 0717220257</ref>
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[[Image:Island.jpg|left|thumb|275px|A small island in the [[Adriatic Sea]]]]
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Generally, when defining islands as bodies of land that are completely surrounded by water, narrow bodies of water such as [[river]]s and [[canal]]s 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 River|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.
  
Generally, when defining islands as bodies of land that are completely surrounded by water, narrow bodies of water such as [[river]]s and [[canal]]s 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 River|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.
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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 (island)|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.
 
 
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 (island)|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 geological nature (for example sand instead of stone), and thus the islands remain islands in a more scientific sense as well.
 
  
 
==Types of islands==
 
==Types of islands==
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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]].
 
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 America]]n 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 [[Australia]]n continental platform and is separated from it only by the very shallow and narrow [[Torres Strait]]. <ref>Encyclopædia Britannica. 2008. [http://www.britannica.com/eb/article-9042921 island] ''Encyclopædia Britannica Online''. Retrieved April 21, 2008.</ref>
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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 America]]n 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 [[Australia]]n continental platform and is separated from it only by the very shallow and narrow [[Torres Strait]]. <ref>Encyclopædia Britannica. 2008. [http://www.britannica.com/eb/article-9042921 island] ''Encyclopædia Britannica Online''. Retrieved April 21, 2008.</ref>
  
 
A special type of continental island is the '''microcontinental island''', which results when a continent is [[rift (geology)|rift]]ed. Examples are [[Madagascar]] off [[Africa]]; the [[Kerguelen Islands]]; and some of the [[Seychelles]]. Another subtype is an island or [[shoal|bar]] formed by deposition of sediment where a water current loses some of its carrying capacity. An example is [[barrier island]]s, which are accumulations of [[sand]] deposited by sea currents on the continental shelf. Another example is islands in [[river delta]]s 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.
 
A special type of continental island is the '''microcontinental island''', which results when a continent is [[rift (geology)|rift]]ed. Examples are [[Madagascar]] off [[Africa]]; the [[Kerguelen Islands]]; and some of the [[Seychelles]]. Another subtype is an island or [[shoal|bar]] formed by deposition of sediment where a water current loses some of its carrying capacity. An example is [[barrier island]]s, which are accumulations of [[sand]] deposited by sea currents on the continental shelf. Another example is islands in [[river delta]]s 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''.
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[[Image:Manhattan2.jpg|thumb|500px|[[Manhattan|Manhattan Island]], [[New York]], one of the world's most densely populated islands at 67,000 residents per square mile.]]
 
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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. <ref name=Wallace> Alfred R. Wallace, October 15, 1879. [http://www.wku.edu/~charles.smith/wallace/S316.htm Islands, as Illustrating the Laws of the Geographical Distribution of Animals (S316: 1879/1880)] ''Western Kentucky University''. Editor Charles H. Smith. Retrieved April 19, 2008. </ref>
 
 
 
 
----
 
 
 
 
    Recent Continental Islands are always surrounded by a shallow sea (almost always less than 100 fathoms deep), so that they are connected with the continent by a submarine bank, and an elevation of 600 feet would again join them to it. They are generally situated near to continents, but sometimes at a distance of several [[p. 5]] hundred miles, as in the case of Borneo, about 600 miles from Cambodia, the intervening sea being everywhere less than 100 fathoms deep.  
 
 
 
    Ancient Continental Islands are always separated from the adjacent continent by a deep sea, though they are usually no further off than the more recent islands. Thus, Madagascar is separated from Africa by a strait which is about 2,000 fathoms deep, and there is more than 1,000 fathoms between Hayti and the coast of South America.  
 
 
 
    Oceanic Islands are usually 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. Thus, the Cape de Verde Islands, the Canaries, and Madeira, are all within 400 miles of the coast of Africa, while some of the Canary Islands are only 50 miles distant from it; but even these are separated from the mainland by a depth of more than 1,000 fathoms.  
 
  
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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. <ref name=Wallace/>
  
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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]]. <ref name=Wallace/>
  
[[Image:Angelskyview.jpg|thumb|left|[[Angel Island, California|Angel Island]] in the San Francisco Bay]]
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While recent and ancient continental islands bear the physical differences mentioned above, their [[zoology|zoological]] features are clearly defined as well. Recent Continental Islands are home to the same types of [[animal]]s—[[bird]]s, [[mammal]]s and [[reptile]]s—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. <ref name=Wallace/>
  
 
=== Oceanic islands ===
 
=== Oceanic islands ===
[[Image:Hawai'i.jpg|right|thumb|The islands of [[Hawaii (island)|Hawai'i]] are volcanic islands.]]
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[[Image:Hawai'i.jpg|right|thumb|230px|The islands of [[Hawaii]] are volcanic islands.]]
[[Image:Wake Island.png|right|thumb|[[Wake Island]] is a volcanic island that has become an [[atoll]].]]
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[[Image:Wake Island.png|right|thumb|230px|[[Wake Island]] is a volcanic island that has become an [[atoll]].]]
Oceanic islands are ones that do not sit on continental shelves. They are [[volcano|volcanic]] in origin. 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.
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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 [[volcano|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.
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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 [[Hotspot (geology)|hotspot]]s. 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 [[isostasy|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 Atoll|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 the atolls in the nation of [[Tuvalu]]. [[Tristan da Cunha]] is an example of a hotspot volcano in the Atlantic Ocean. Another hot spot in the Atlantic is the island of [[Surtsey]], which was formed in 1963.
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A third type of oceanic island is formed over volcanic [[Hotspot (geology)|hotspot]]s. 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 [[isostasy|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 Atoll|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.
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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 Ocean|Pacific]].
  
==Phantom islands==
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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|Cape Verde Islands]], the [[Canary Islands|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. <ref name=Wallace/>
[[Image:Map by nicolo zeno 1558.jpg|thumb|The [[Zeno map]] of [[1558]] showing [[Frisland]] – a phantom island in the [[North Atlantic]]]]
 
  
'''Phantom islands''' are [[island]]s 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.
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==Island biogeography ==
 
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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 [[ecology|ecologists]] [[Robert MacArthur]] and [[Edward Osborne Wilson|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.  
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 4th century B.C.E. but was lost, and then later reidentified 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.  
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MacArthur and Wilson showed that the [[biodiversity#Species richness|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]].  
  
While many phantom islands appear never to have existed, a few (such as, perhaps, [[Thompson Island (South Atlantic)|Thompson Island]]) may have been actual islands subsequently destroyed by volcanic explosions, earthquakes or submarine landslides, or low-lying lands such as [[shoal|sand bank]]s that are no longer above water.
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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]].
  
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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.
  
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==Descriptive island terms==
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===Phantom islands===
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[[Image:Map by nicolo zeno 1558.jpg|thumb|The [[Zeno map]] of 1558 showing [[Frisland]]—a phantom island in the [[North Atlantic]]]]
  
==Island biogeography ==
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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 [[myth]]s and legends.
The theory of island biogeography holds that the number of species found on an island (the '''equilibrium number''') is determined by two 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. The equilibrium number of an island close to [[Africa]] is going to be larger than that of one found in the mid-[[Atlantic]]. 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.
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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.  
  
The theory of island biogeography was tested by Wilson and his student [[Daniel Simberloff]] in the [[mangrove]]s off [[Florida]]. Small islands of mangroves were surveyed then fumigated with [[methyl bromide]] to clear their [[insect]] and [[arthropod]] communities. The islands were then monitored to study the immigration of species to the islands (the experimental equivalent of the creation of new islands). Within a year the islands had been recolonized, and had reached equilibrium, with islands closer to the mainland having more species, as predicted.
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Other phantom islands are probably due to navigational errors, the misidentification of [[iceberg]]s, fog banks, or to optical illusions.  
  
Research conducted at the rainforest research station on [[Barro Colorado Island]] has yielded a large number of publications concerning the ecological changes following the formation of islands, such as the local extinction of large predators and the subsequent changes in prey populations.
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While many phantom islands appear never to have existed, a few (such as, perhaps, [[Thompson Island (South Atlantic)|Thompson Island]]) may have been actual islands subsequently destroyed by [[volcano|volcanic]] explosions, [[earthquake]]s or submarine landslides, or low-lying lands such as [[shoal|sand bank]]s that are no longer above water.
  
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===Desert island===
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[[Image:Desertisland.jpg|thumb|250px|right|A desert island in [[Palau]].]]
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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.
  
'''Factors that Influence Island Communities'''
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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."  
*  Degree of isolation (distance to nearest neighbor, and mainland)
 
*  Length of isolation (time)
 
*  Size of island (larger area usually facilitates greater diversity)
 
*  Climate (tropical versus arctic, humid versus arid, etc.)
 
*  Location relative to ocean currents (influences nutrient, fish, bird, and seed flow patterns)
 
*  Initial plant and animal composition if previously attached to a larger land mass (e.g., [[marsupials]], [[primates]], etc.)
 
*  The species composition of earliest arrivals (if always isolated)
 
*  Serendipity (the impacts of chance arrivals)
 
* Human activity
 
  
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===Tidal island===
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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 [[place of worship|religious worship]], such as [[Mont Saint Michel]] with its [[Order of Saint Benedict|Benedictine]] [[Abbey]]. Tidal islands are also commonly the sites of [[fortresses]], due to their natural fortifications.
  
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The former Bennelong Island in [[Sydney]], [[Australia]] was developed into [[Bennelong Point]] and is now the location of the [[Sydney Opera House]].
  
 
==Notes==
 
==Notes==
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* Quammen, David. 1997. ''The Song of the Dodo: Island Biogeography in an Age of Extinctions''. Scribner. ISBN 0684827123
 
* Quammen, David. 1997. ''The Song of the Dodo: Island Biogeography in an Age of Extinctions''. Scribner. ISBN 0684827123
 
* Stommel, Henry. 1984. ''Lost Islands: The Story of Islands That Have Vanished from Nautical Charts'', University of British Columbia Press. ISBN 0774802103
 
* Stommel, Henry. 1984. ''Lost Islands: The Story of Islands That Have Vanished from Nautical Charts'', University of British Columbia Press. ISBN 0774802103
* Allan A. Schoenherr, C. Robert Feldmeth, Michael J. Emerson. 2003. ''Natural History of the Islands of California''. University of California Press.
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* Schoenherr, Allan A., C. Robert Feldmeth, Michael J. Emerson. 2003. ''Natural History of the Islands of California''. University of California Press.
 
;Online sources
 
;Online sources
 +
* ''Australian Broadcasting Corporation''. [http://www.abc.net.au/nature/island/islands1.htm Island Life] Retrieved April 19, 2008.
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* ''Global Oneness Commitment''. [http://www.experiencefestival.com/a/Island_-_Continental_islands/id/5594775 Continental Islands] Retrieved April 19, 2008.
 
* ''Graphic Maps''. [http://www.worldatlas.com/webimage/countrys/io.htm Islands of the World] Retrieved April 19, 2008.
 
* ''Graphic Maps''. [http://www.worldatlas.com/webimage/countrys/io.htm Islands of the World] Retrieved April 19, 2008.
* ''WWF''. [http://www.panda.org/news_facts/education/middle_school/habitats/oceanic_islands/index.cfm Oceanic Islands] Retrieved April 19, 2008.
 
 
* Roberson, Don. May 25, 2007. [http://montereybay.com/creagrus/islands.html Oceanic Islands] ''Creagrus''. Retrieved April 19, 2008.
 
* Roberson, Don. May 25, 2007. [http://montereybay.com/creagrus/islands.html Oceanic Islands] ''Creagrus''. Retrieved April 19, 2008.
* ''Global Oneness Commitment''. [http://www.experiencefestival.com/a/Island_-_Continental_islands/id/5594775 Continental Islands] Retrieved April 19, 2008.
 
 
* Wallace, Alfred R. October 15, 1879. [http://www.wku.edu/~charles.smith/wallace/S316.htm Islands, as Illustrating the Laws of the Geographical Distribution of Animals (S316: 1879/1880)] ''Western Kentucky University''. Editor Charles H. Smith. Retrieved April 19, 2008.  
 
* Wallace, Alfred R. October 15, 1879. [http://www.wku.edu/~charles.smith/wallace/S316.htm Islands, as Illustrating the Laws of the Geographical Distribution of Animals (S316: 1879/1880)] ''Western Kentucky University''. Editor Charles H. Smith. Retrieved April 19, 2008.  
* ''Australian Broadcasting Corporation''. [http://www.abc.net.au/nature/island/islands1.htm Island Life] Retrieved April 19, 2008.
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* ''WWF''. [http://www.panda.org/news_facts/education/middle_school/habitats/oceanic_islands/index.cfm Oceanic Islands] Retrieved April 19, 2008.
  
 
==External links==
 
==External links==
{{Wikisource1911Enc|Island}}
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*[http://www.un.org/Depts/los/convention_agreements/texts/unclos/part8.htm Definition of island] from United Nations Convention on the Law of the Sea. Retrieved October 6, 2007.
*[http://www.un.org/Depts/los/convention_agreements/texts/unclos/part8.htm Definition of island] from United Nations Convention on the Law of the Sea. Retrieved October 6, 2007.
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*[http://islands.unep.ch/isldir.htm Listing of islands] from [[United Nations]] Island Directory. Retrieved October 6, 2007.
*[http://islands.unep.ch/isldir.htm Listing of islands] from [[United Nations]] Island Directory. Retrieved October 6, 2007.
 
  
 
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[[Category:Physical sciences]]
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Revision as of 08:00, 8 March 2009


Mokuaeae Island, off Kilauea Point, Kauai, Hawaii.

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" . [1]


Definition

A small Fijian 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. [2]

File:Island.jpg
A small island in the Adriatic Sea

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.

Types of islands

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

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. [3]

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.

Manhattan Island, New York, one of the world's most densely populated islands at 67,000 residents per square mile.

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. [4]

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. [4]

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. [4]

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 animalsbirds, 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. [4]

Oceanic islands

The islands of Hawaii are volcanic islands.
Wake Island is a volcanic island that has become an atoll.

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. [4]

Island biogeography

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.

Descriptive island terms

Phantom islands

The Zeno map of 1558 showing Frisland—a phantom island in the North Atlantic

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.

Desert island

A desert island in Palau.

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."

Tidal island

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.

The former Bennelong Island in Sydney, Australia was developed into Bennelong Point and is now the location of the Sydney Opera House.

Notes

  1. Dictionary.com. Island. Retrieved April 21, 2008.
  2. Lexicon Universal Encyclopedia. 1989. Lexicon Publications, Inc. New York, New York. ISBN 0717220257
  3. Encyclopædia Britannica. 2008. island Encyclopædia Britannica Online. Retrieved April 21, 2008.
  4. 4.0 4.1 4.2 4.3 4.4 Alfred R. Wallace, October 15, 1879. Islands, as Illustrating the Laws of the Geographical Distribution of Animals (S316: 1879/1880) Western Kentucky University. Editor Charles H. Smith. Retrieved April 19, 2008.

References
ISBN links support NWE through referral fees

Print sources
  • Firestone, Clark Barnaby. 1924. The Coasts of Illusion: A Study of Travel Tales, Harper Books.
  • Gaddis, Vincent. 1965. Invisible Horizons, Chilton Books. New York.
  • Larson, Edward J. 2001. Evolution's workshop: God and science on the Galápagos Islands. New York: Basic Books. ISBN 0465038107
  • MacArthur, R. H. and Wilson, E. O. 1967. The Theory of Island Biogeography. Princeton, N.J.: Princeton University Press.
  • Menard, Henry W. 1986. Islands. Scientific American Library series, no. 17. New York: Scientific American Library. ISBN 0716750171
  • Newmark, W. D., A land-bridge island perspective on mammalian extinctions in western North American parks, Nature, 325, 430 - 432 (29 January 1987)
  • Nunn, Patrick D. 1994. Oceanic islands. The Natural environment. Oxford, UK: Blackwell. ISBN 0631178112
  • Pilkey, Orrin H., and Mary Edna Fraser. 2003. A celebration of the world's Barrier islands. New York: Columbia University Press. ISBN 9780231119702
  • Quammen, David. 1997. The Song of the Dodo: Island Biogeography in an Age of Extinctions. Scribner. ISBN 0684827123
  • Stommel, Henry. 1984. Lost Islands: The Story of Islands That Have Vanished from Nautical Charts, University of British Columbia Press. ISBN 0774802103
  • Schoenherr, Allan A., C. Robert Feldmeth, Michael J. Emerson. 2003. Natural History of the Islands of California. University of California Press.
Online sources

External links

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