Island

Definition

If an island is a ground extent surrounded by water, there exists a not specified limit which separates an island from a Continent. Generally, this limit is fixed at the surface of the Australia; but the question of knowing if Australia is an island or a continent remains prone to debate.

Certain islands and almost islands are accessible to low tide and lose their insular character then. There still, it is possible to regard them or not as islands with whole share.

The toponyms can preserve the trace of an old island, like Lille in France, or proceed by analogy, the such French area of Île-de-France, extension of the old royal field of the Capétien S, which is not an island and whose inhabitants are called the “Franciliens”.

In the field of the ecology of the landscape

The true islands can shelter only little species when they are small, but starting from a threshold of size are big factors of Biodiversité, in particular by the Endémisme which it can allow and shelter. In an ecological network they can be used as ford for the species which can fly or which have good colonizing capacities on water or in the air. The species which live there are often smaller there (dwarf éléhants lived in Corsica and Sardinia).
As example, in the biogeographic vast domain that Mediterranean basin is the (3 million km2 approximately), the islands and small islands are only 4% (103.000 km ²) approximately of emerged surfaces, with 4000 small islands of less than 10 km2 and 162 islands of 10 km2 and more, distributed in a manner nonhomogeneous, where the biodiversity particularly regressed but still constitute a remarkable tank.

The concept of “island” is also used for métaphoriquement to describe all “isolate biogeographic” in which populations of living beings are insulated, forming islands with the biological direction of the term. (Sastre 1981). When process is in hand in an ecological context of Fragmentation, one speaks about “ecological Insularisation” ,

Cut

Insularity

The definition of the island like a ground extent surrounded by water results in gathering in the same category of the very dissimilar grounds, of the small island in the continent, from which the degree of insularity is very variable. This degree of insularity is difficult to define and is appreciated differently according to whether one is interested in the Géographie physical or human, with the economy and the Transport S, or the Biologie or the ecology.

Under the angle of the physical Geography, an author, François Doumenge, defined measurable criteria to appreciate the degree of insularity:

the “coastal index”, which is defined as the relationship between the length of the coasts and the surface of the island. This index is maximum, 2 or more, in the case of the Atoll S for which the presence of a central lagoon increases the length of the coasts appreciably. According to the relief (massive or very cut out) and the general shape of the island (more or less circular or lengthened) this index strongly varies. He considers that in lower part of 1/25 (1 km of coasts for 25 km ²), the island strongly has a character Continent Al
the “index of insulation”, definite like the relationship between the surface of the exclusive economic Zone of the 200 marine miles (ZEE) and that of the island. In the case of Clipperton (2 km ²), without any ground emerged in the ray of the 360 km, this index is very high. It decreases when islands are closer. With the lower part of 1/100, one cannot speak any more about insular insulation.

This author defines also a “index of Endémisme” which is the report/ratio of the full number of Taxon S (kinds, species and subspecies) of the insular settlement by the number of tax endemic. This index gives an idea of the importance of the Endémisme Végétal and Animal, i.e. of the biological insulation, which characterizes a given island.

The index of insulation of the Program of the United Nations for the environment (PNUE, 1998), is more complex. It consists in adding the square root of the distance from the island of equivalent or higher size nearest, the square root of the distance from the archipelago nearest and the square Racine of the distance from the continental country nearest. This index is highest (149) for the Easter Island. It is of 102 for Tahiti and 23 for the Corsica .

These indices do not take account of the human activities, of the importance of the population and the Accessibilité (for example presence of an international airport).

One can also wonder about the relevance of the island as a specific geographical object. A researcher, François Taglioni, led a study on small insular spaces in the world in order to revisit the concepts which touch in the islands and dynamic territorial that this category of spaces maintain between them.

Factor of vulnerability

The insular or insularized ecological systems (true islands or catches within the meaning of the ecology of the landscape) are often simplified and factor of endemism, and thus more vulnerable to the disturbances, in particular anthropic. In particular the introductions of species (voluntary or involuntary) there are often causes of biological Invasion seriously disturbing insular balances ecological, which has involved the disappearance of many species for a few centuries. HÖNER and GREUTER, 1988 showed that these invasions do not affect only the small islands, but also the large islands (Madagascar (GOODMAN, 1995), New Zealand (TOWNS and BALLANTINE, 1993) or Australia.).

Formation

Continental islands

The islands located on same the continental shelf are thus called as the Continent which is close for them. It is thus acted in fact of part of the same continent: it is the height of the sea level which makes that it is about an island (it is the case of the Great Britain which at the time of the last Glaciation was not any more one island). Certain islands are it besides only with high Marée (the Mont Saint-Michel or the Île of Noirmoutier for example). In these cases, the depth of the sea around them is (relatively) not very deep. (See on this subject the article Island accessible to low tide.)

The islands continents like the Australia or Madagascar which there are hundreds of million years were attached to the the Antarctic and of which the Tectonic plate drift on the surface of the sphere at a speed of a few centimetres per annum (1 cm per annum out of 100 million years = 1000 km). It was also the case of the Indian sub-continent which completes its course and slips under the eurasiatic plate and raises the latter at the point of collision which is the the Himalayas. For this reason the Himalayenne chain continues to rise.

Volcanic islands

There they are the accumulated lava of one or more Volcan S which emergent, forming the island. The depth of the sea in the neighborhoods can then be very large (several thousands of meters). The examples in the middle of the Océan geologically do not form part of any continent. A type of volcanic island is on an arc of volcanic islands. These islands emerge at the time of the Subduction of a plate by another. Examples are the the Northern Marianna Islands, the Aleutian Islands and the majority of the Tonga in the Pacific Ocean. Some of the Lesser Antilles and the Sandwich islands of the South are the only examples in the Atlantic Ocean.

Another type of volcanic island emerges when a oceanic Rift reaches surface. There are two examples: the Iceland, which is the largest volcanic island in the world and Jan Mayen, both in the Atlantic.

The last type of volcanic island is formed on the level of the hot spots volcanic. A hot spot is more or less stationary compared to the tectonic plate moving above him. Thus a chain of islands emerges when the plate moves. Over long periods, this type of island finally is eroded and submerged by the isostatic adjustment to become a underwater Mont. The movement of the plates on the level of a hot spot produces a line of islands directed in the direction of the movement of the plate. For example, the islands Hawaii, of Hawaii to the Atoll Kure, which extend then under surface from the ocean, take the direction of north on the level of the underwater Mont of the Emperor. Another chain with a similar orientation is the Archipel of Tuamotu; its part older and directed towards north is the islands of the Line. The chain more in the south is the islands Australes with the part directed towards north towards the territory of Tuvalu. The island Tristan da Cunha is an example of hot spot volcanic in the Atlantic Ocean.

A Atoll is an island formed starting from a coral Récif which was built on an eroded and submerged volcanic island. By the accumulation of corals and polyps on several hundred meters height, the reef emerges with water surface and form a new island. The atolls often have the shape of a ring with a Lagon central and not very deep. Examples are the Maldives in the Indian Ocean and Rangiroa in the Pacific.

River islands

The river islands appear in the deltas of river and in broad the River. They are formed by the deposit of Sédiment S at points where the current loses part of its intensity. By nature, they are parts of the rivers isolated from the current. Some are transitory and can disappear when the volume of water or the speed of the longevity and river changes while others are stable great.

History

Certain islands were deserted at the time of their discovery by Europeans. It is the case of Madeira, of the the Azores, the Mauritius, the Réunion, the Seychelles, Sainte-Hélène… Others were inhabited, but their first occupants were exterminated by the colonizers: it is the case of the Guanche S with the Canary islands, of the Indiens the Caribbean in the the Antilles or in Jamaica, of the Indiens Onas of the Ground of fire, of the indigenous in Tasmanie.

Insulation

Insulation, term whose etymology is attached to “island” via Italian insulated , and loneliness are often required in the islands, which it is voluntary or not:

voluntary insulation:

Paul Gauguin, with the Marquesas Islands,
Jacques Brel, in the Marquesas Islands,
Robert Louis Stevenson, in the Marquesas Islands, then with the Samoa

forced insulation:

Robinson Crusoé
the mutineers of the Bounty
Napoleon was assigned with residence with the isle of Elba, then with Sainte-Hélène,
the marshal Pétain was imprisoned with the island of Yeu
the captain Dreyfus and Papillon in the Devil's Island
Nelson Mandela with Robben Island close to the Cape

Penitentiaries establishments were installed in islands to limit the possibilities of escape:

Alcatraz in bay of San Francisco
the Bagne of Guyana in the islands of the Hello.
the Bagne of Noumea in the island of Nou (New Caledonia)
the Bagne of the Ile de Ré
the Bagne de Poulo Condor (Vietnam)

Literature

1844 : the count of Assembles-Cristo of Alexandre Dumas
1874: the mysterious Island of Jules Verne (see also the School of Robinsons and Two years of holidays )
1883: the Island with the treasure ( Treasure Island ) of Robert Louis Stevenson
1962: Island ( Island ) of Aldous Huxley
1962: the Island of Robert Blackbird
1979: the Island of another of Jacques Perry
1994: the Island of the day of before ( insulated It LED giorno PRIMA ) of Umberto Eco

See too

island|island

List of the islands by population
List of islands by surface
List of the lists of islands
Plate tectonics Peninsula
Island accessible to low tide

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