An island is any piece of land smaller than a continent and larger than a rock, that is completely surrounded by water. Very small islands are called islets. Although seldom adhered to, it is also proper to call an emergent land feature on an atoll an islet, since an atoll is a type of island.
There are three types of islands: continental islands, river islands, and volcanic islands.
A special type of continental island is the microcontinental island, which results when a continent is rifteded. The best example is Madagascar off Africa. The Kerguelen Islands and some of the Seychelles are also examples.
Another subtype is the barrier island: accumulations of sand on the continental shelf.
Another type of volcanic island occurs where an oceanic rift reaches the surface. There are two examples: Iceland, which is the world's largest volcanic island, and Jan Mayen—both are in the Atlantic.
The last type of volcanic island are those formed over volcanic hot spots. A hot spot is more or less stationary relative to the moving tectonic plate above it, so a chain of islands results as the plate drifts. Over long periods of time, this type of island is eventually eroded down and "drowned" by isostatic adjustment, becoming a seamount. Plate movement across a hot-spot produces a line of islands oriented in the direction of the plate movement. An example is the Hawaiian Islands, from Hawaii to Kure, which then extends beneath the sea surface in a more northerly direction as the Emperor Seamounts. Another chain with similar orientation is the Tuamotu Archipelago; its older, northerly trend is the Line Islands. The southernmost chain is the Austral Islands, with its northerly trending part the atolls in the nation of Tuvalu. Tristan da Cunha is an example of a hotspot volcano in the Atlantic Ocean.
