island
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Area of land surrounded entirely by water. Australia is classed as a continent rather than an island, because of its size.
Islands can be formed in many ways. Continental islands were once part of the mainland, but became isolated (by tectonic movement, erosion, or a rise in sea level, for example). Volcanic islands, such as Japan, were formed by the explosion of underwater volcanoes. Coral islands consist mainly of coral, built up over many years. An atoll is a circular coral reef surrounding a lagoon; atolls were formed when a coral reef grew up around a volcanic island that subsequently sank or was submerged by a rise in sea level. Barrier islands are found by the shore in shallow water, and are formed by the deposition of sediment eroded from the shoreline.
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