Island

An island is a piece of land, distinct from a continent, that is completely surrounded by water. There is no fixed size threshold separating an island from a continent; continents are instead defined geologically as the largest landmass riding on a given tectonic plate. Islands form in several ways: continental islands break away from a continent through plate tectonics, oceanic islands never formed part of a continent and instead build up through volcanic activity or the growth of coral into atolls, and other islands accumulate from sediment along a shoreline as barrier islands or within a river channel. Because the surrounding water acts as a barrier to the arrival of new species, island life often evolves in isolation from mainland ecosystems, and roughly a tenth of the world's people now live on islands.