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tracts of land entirely surrounded by the waters of oceans, seas, lakes, or rivers. Islands differ from continents in that they are relatively small. They occur individually or in groups called archipelagoes.
The islands of oceans and seas are subdivided into continental islands, islands of the transitional zone, and oceanic islands. Continental islands, which rise above sea level from the underwater rim of the continents, include some of the world’s largest islands—Greenland, Kalimantan (Borneo), and Madagascar. They are usually composed of bedrock, although smaller islands may be formed through the accumulation of sediment by the action of waves and tides. Most of the islands of the transitional zones are volcanic; in some areas, however, there are mud volcano islands (for example, in the southern part of the Caspian Sea) and coral islands. Oceanic islands rise up from the ocean floor and mid-ocean ridges, and most are volcanic or coral islands.
The isolation of islands from continents determines the characteristics of their natural features, particularly their fauna and flora. Islands have fewer species than nearby continental areas lying in the same natural zone, and they have a high percentage of endemic species. This is especially true of oceanic islands.
The islands of rivers and lakes are divided into alluvial and erosion islands. The former are created through the accumulation of sediments in a channel or near the shore of a lake, and the latter are formed when river channels flow around an outlier composed of bedrock and alluvium or when a protruding segment
|Table 1. The world’s largest archipelagoes and islands|
|Area (sq km)|
|1Partly in the Pacific Ocean 2Partly in the Atlantic Ocean|
|Tierra del Fuego1||Indian Ocean||approx||72,000|
|Sri Lanka (Ceylon)||Arctic Ocean||65,600|
|Canadian Arctic Archipelago2||1,300,000|
of land near a lake is cut off by abrasion. (See Table 1 for the area of the world’s largest islands and archipelagoes.)