An island or isle is a piece of land, distinct from a continent, completely surrounded by water. There are continental islands, which were formed by being split from a continent by plate tectonics, and oceanic islands, which have never been part of a continent. Oceanic islands can be formed from volcanic activity, grow into atolls from coral reefs, and form from sediment along shorelines, creating barrier islands. River islands can also form from sediment and debris in rivers. Artificial islands are those made by humans, including small rocky outcroppings built out of lagoons and large-scale land reclamation projects used for development.
Islands are host to diverse plant and animal life. Oceanic islands have the sea as a natural barrier to the introduction of new species, causing the species that do reach the island to evolve in isolation. Continental islands share animal and plant life with the continent they split from. Depending on how long ago the continental island formed, the life on that island may have diverged greatly from the mainland due to natural selection.
Humans have lived on and traveled between islands for thousands of years at a minimum. Some islands became host to humans due to a land bridge or a continental island splitting from the mainland. Today, up to 10% of the world's population lives on islands. Islands are popular targets for tourism due to their perceived natural beauty, isolation, and unique cultures.
Islands became the target of colonization by Europeans, resulting in the majority of islands in the Pacific being put under European control. Decolonization has resulted in some but not all island nations becoming self-governing, with lasting effects related to industrialization, nuclear weapons testing, invasive species, and tourism. Islands and island countries are threatened by climate change. Sea level rise threatens to submerge nations such as Tuvalu and the Marshall Islands completely. Increases in the frequency and intensity of tropical cyclones can cause widespread destruction of infrastructure and animal habitats. Species that live exclusively on islands are some of those most threatened by extinction.
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