island, relatively small body of land surrounded entirely by water. (As the oceans form a continuous mass of water on the earth's surface, all continents are islands in the strict sense of the word.) The largest islands on earth are, in descending order of size, Greenland, New Guinea, Borneo, Madagascar, Baffin Island, Sumatra, Honshu (largest of the islands of Japan), and Great Britain. Depending on their origin, islands are either continental or oceanic. Continental islands are created by rise in sea level where only the summits of coastal highlands remain above water; or by the sea breaking through an isthmus or peninsula and cutting the land from the mainland. Typical continental islands are Great Britain and Martha's Vineyard. Other islands emerge along coasts as barrier islands, such as the Outer Banks, off North Carolina. Oceanic islands can result from volcanic islands rising above the water, especially on or near a mid-ocean ridge, as when the island of Surtsey appeared along the Mid-Atlantic Ridge S of Iceland in 1963. Island arcs, such as the Aleutian Islands, result from magmatic activity associated with the convergence of lithospheric plates (see plate tectonics). Oceanic islands may also be the emergent tips of volcanoes (seamounts) formed by hotspots. Oceanic islands that result from coral growth on the summit of seamounts are called coral islands or atolls (see coral reefs). These low islands only occur in tropical ocean areas. Oceanic islands are generally characterized by low faunal diversity, consisting of a few sea birds and insects. Vegetation is usually more abundant, as seeds are carried from remote lands by wind, water currents, and birds.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.