An island is a land area completely surrounded by water. Familiar islands include Madagascar, the Hawaiian Islands, Greenland, Cuba, Ireland, Great Britain, and Manhattan. The Bahamas, Japan, Indonesia, Philippines, and New Zealand are nations made up of islands. A group of related islands, such as the Philippines, is called an archipelago.

There are two types of islands: continental and oceanic. Continental islands are areas of the continental shelf that have become isolated because of a rise in sea level. For example, Tasmania is part of the Australian continent that was cut off by the ocean waters forming the Bass Strait. Oceanic islands form as volcanoes erupt on the ocean floor. Iceland and the Hawaiian Islands are oceanic islands. Sometimes coral reefs form where ocean waters are shallow, eventually building up exposed island land called an atoll.

Climate change is also a growing threat to islands. Because they are so susceptible to rising sea level, islands are in danger of flooding should sea levels rise. Encroaching seawater may also contaminate fresh water sources on islands. The numerous islands of the Pacific, including those of Tuvalu, are threatened by rising sea levels. Warming seas also damage coral reefs and the diverse ecosystems they support.

  • Surtsey is a volcanic island south of Iceland that formed during a four-year eruption beginning in 1963. The island has been protected as a natural, uninhabited area since its emergence, and scientists have studied it closely. They have observed how the ocean carried seeds to establish plants there, as well as the arrival of birds and other animals that colonized the new land. What was barren rock in 1963 now has hundred of species of plants and animals.
  • The Galapagos Islands are famous for animals found nowhere else. Giant tortoises, marine iguanas, Galapagos penguins, and flightless cormorants call these islands home. The isolated animal populations of the Galapagos helped Charles Darwin refine his theory of natural selection when he visited the islands in 1835.
  • Coral reefs, which are the basis of island atolls, are habitats teeming with life. While they only cover 1% of the ocean floor, they are home to one-quarter of marine species. Pollution and warming seas threaten these precious ecosystems.
  • Rats introduced on 17 of Alaska's Aleutian Islands quickly spread and began preying on ground-nesting seabirds. Scientists studying the rats' effect found that islands with rats have far fewer birds than those where rats have not spread. Without the birds to keep their numbers in check, snails and other invertebrates multiply and clear the tidal zones of seaweed and algae. On islands where rats have not invaded, birds are plentiful and the ecosystem balanced.
  • Half of all animals known to have gone extinct in the past 400 years have been island dwellers; 90% of those were birds. Madagascar and Hawaii have large numbers of species native only to those islands; they also have very high numbers of endangered and threatened species.

a group of closely scattered islands in a large body of water.


a coral reef or string of coral islands that surrounds a lagoon.

continental island

land once connected to a continent but broken off by shifting tectonic plates.


part of a continent that extends underwater to the deep-ocean floor.


tiny ocean animal, some of which secrete calcium carbonate to form reefs.

coral reef

rocky ocean features made up of millions of coral skeletons.


body of land surrounded by water.


shallow body of water that may have an opening to a larger body of water, but is also protected from it by a sandbar or coral reef.

oceanic island

land formed from the eruption of a volcano on the ocean floor.


base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.


able to be influenced to behave a certain way.


an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.