Island

From Wikipedia, the free encyclopedia

For other uses, see Island (disambiguation).

A small island in the Adriatic Sea

An island or isle is any piece of land that is completely surrounded by water. Very small islands such as emergent land features on atolls are called islets. A key or cay is another name for a relatively small island or islet. An island in a river or lake is called an eyot [ī´ǒt].

There are two main types of islands: continental islands and oceanic islands. There are also artificial islands. A grouping of related islands is called an archipelago.

The word island comes from Old English igland. However, the spelling of the word was modified in the 15th century by association with the Old French loanword isle.

1 Definition
2 Types
- 2.1 Continental islands
- 2.2 Oceanic islands
3 See also
4 External links

[edit] Definition

A small island near Fiji

There is no standard of size which distinguishes islands from islets and continents. Any landmass surrounded by water could be considered an island. As such, the largest island in the world is actually the super-continent of Africa-Eurasia.

Also, when defining islands as pieces of land that are completely surrounded by water, narrow bodies of water like rivers and canals are generally left out of consideration. 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 Sea. 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. However, generally cases such as these are not considered islands. Other examples of such coast-to-coast watersystems that are not considered to cut a land mass in two are the Caledonian and Forth and Clyde canals in Scotland and the Volga-Baltic Waterway in Russia.

This also helps explain why Africa-Eurasia can be seen as one continuous landmass (and thus technically the biggest island): generally the Suez Canal is not seen as something that divides the land mass in two.

[edit] Types

[edit] Continental islands

Angel Island in the San Francisco Bay

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.

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.

[edit] Oceanic islands

Hawai'i is a volcanic island.

Wake Island is a volcanic island that has become an atoll.

Oceanic islands are ones that do not sit on continental shelves. They are 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.

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.

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 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.

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.