Island, or (usually poetic) Isle, a body of land surrounded by water. Islands differ from continents chiefly in size. A small island is called an islet. A group of islands or islets is called an archipelago. The Pacific Ocean has the greatest number of islands, mostly in archipelagoes.
|The world's 10 largest islands|
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Islands are formed in various ways. Continental islands, such as the British Isles, Madagascar, and Sri Lanka, were formed as land connecting them to the nearby continent sank or was eroded away, or was flooded by a rise in sea level. Oceanic islands are formed by geologic disturbances beneath the ocean, and are of two main types:
such as the Azores and the Hawaiian group, are the tops of volcanoes that rise from the sea floor.
such as the Solomon and Mariana groups, are accumulations of coral on top of rocks (usually of volcanic origin) that are slowly sinking into the sea.
Rivers form alluvial islands by depositing gravel and silt, as in the Mississippi and Nile deltas. The sea forms barrier islands by heaping sand along shallow coasts; the offshore islands of New Jersey and North Carolina are of this type.
Continental islands have, in general, the same kind of wildlife as the nearby continent. Wildlife on oceanic islands is limited to species that have been able to cross open water. Thus, birds and insects are common but mammals and reptiles are rare or unknown (unless introduced by humans). Plant life is also limited. Under these restrictive conditions, island creatures develop in unusual ways. For example, certain birds have become flightless in the absence of predators, and the coconut crab has developed powerful claws to open coconuts. Charles Darwin, who toured the Pacific in 1831-36, found that biological specialization on islands supported his evolution theory.