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For more things of this name, see Island (disambiguation).

A small island

An island is any piece of land smaller than a continent and larger than a rock, that is completely surrounded by water. Very small islands are called islets. Although seldom adhered to, it is also proper to call an emergent land feature on an atoll an islet, since an atoll is a type of island.

There are three main types of islands: continental islands, river islands, and volcanic islands.

Groups of related islands are known as archipelagos.

Continental islands

Continental islands are bodies of land that are connected by the continental shelf to a continent. That is, these islands are part of an adjacent continent and are located on the continental shelf of that continent. Examples include Greenland and Sable Island off North America, Barbados and Trinidad off South America, Sicily off Europe, Sumatra and Java off Asia, Papua and Tasmania off Australia.

A special type of continental island is the microcontinental island, which results when a continent is rifteded. The best example is Madagascar off Africa. The Kerguelen Islands and some of the Seychelles are also examples.

Another subtype is the barrier island: accumulations of sand on the continental shelf.

River islands

River islands occur in river deltas and in large rivers. They are caused by deposition of sediment at points in the flow where the current loses some of its carrying capacity. In essence, they are river bars, isolated in the stream. While some are ephemeral, and may disappear if the river's water volume or speed changes, others are stable and long-lived.

Volcanic islands

Volcanic islands are built by volcanoes. Mid-ocean examples are not geologically part of any continent. One type of volcanic 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 Marianas Islands, the Aleutian Islands, 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 volcanic 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.

The last type of volcanic island are those formed over volcanic hot spots. A hot spot 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.

See also