island - Definition, Types

A piece of land totally surrounded by water, in an ocean, sea, or lake. It may be formed by: (1) remnants of former high land cut off from the mainland by a rise in sea level, or by subsidence, as with the islands of the Aegean, and the Western Is of Scotland; (2) volcanic eruptions on the ocean floor, as with the Hawaiian Is and Iceland; (3) deposition of sediment, as with the Frisian Is; and (4) coral islands.

Very small islands such as emergent land features on atolls are called islets. 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.

Definition

There is no standard of size which distinguishes islands from islets and continents. Any landmass surrounded by water could be considered an island.

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.

Types

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;

A special type of continental island is the microcontinental island, which results when a continent is rifted.

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.

Oceanic islands

Oceanic islands are ones that do not sit on continental shelves. One type of oceanic island is found in a volcanic island arc. 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. 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. 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. Examples include the Maldives in the Indian Ocean and Line Islands in the Pacific.