Island, any area of land smaller than a continent and entirely surrounded by water. Islands may occur in oceans, seas, lakes, or rivers. A group of islands is called an archipelago.
Islands may be classified as either continental or oceanic. Oceanic islands are those that rise to the surface from the floors of the ocean basins. Continental islands are simply unsubmerged parts of the continental shelf that are entirely surrounded by water. Many of the larger islands of the world are of the continental type. Greenland (840,000 square miles [2,175,000 square km]), the largest island, is composed of the same materials as the adjacent North American continent, from which it is separated by a shallow and narrow sea. Likewise the world’s second largest island, New Guinea (309,000 square miles [800,000 square km]), is part of the Australian continental platform and is separated from it only by the very shallow and narrow Torres Strait. A slight warping of the sea bottom in the vicinity of Torres Strait would be sufficient to join New Guinea to Australia; conversely, a slight rise in sea level may submerge a hilly coast and leave the hilltops remaining as small islands just offshore (such as those off the coast near Boston and the islands off the Maine coast).
The islands that rise from the floors of the ocean basins are volcanic. Lava accumulates to enormous thickness until it finally protrudes above the ocean surface. The piles of lava that form Hawaii rise as high as 32,000 feet (9,700 metres) above the ocean floor.
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