A key or cay is another name for a relatively small island or  islet. An island in a river or lake is called an eyot. There are three main types of islands: continental islands, river islands, and
volcanic islands. There are also human-made or artificial islands. A grouping of related islands is called an archipelago.

Continental islands
Angel Island in the San Francisco BayContinental islands are bodies of land that lie  upon the continental shelf of a continent.Examples include Greenland and Sable  Island off North America, Barbados and Trinidad off South America Sicily off  Europe, Sumatra and Java off Asia, New Guinea and Tasmania off Australia.

River islands
River islands occur in river deltas and in large rivers. They are caused by deposition  of sediment at points in the flowwhere the current loses some of its carrying  capacity. In essence, they are river bars, isolated in the stream. While someare ephemeral, and may disappear if the river's water volume or speed changes, others  are stable and long-lived.

Volcanic islands
Hawai'i is a volcanic island. Wake Island is a volcanic island that has become an atoll.Volcanic islands are built  by volcanoes. Mid-ocean examples arenot part of any continent. One type of volcanic island is found in a volcanic island arc. These islands arise from volcanoes where the subduction of one plate under another is occurring. Examples include the  Mariana Islands, the Aleutian Islands Republic of Mauritius and most of Tonga in  the Pacific Ocean. Some of the Lesser Antilles and the South Sandwich Islands are  the only Atlantic Ocean examples.

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.

A third type of volcanic island are those formed over volcanic hotspots. A hotspot 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. Another hot spot in the
Atlantic is the island of Surtsey, which was  formed in 1963.

An atoll is an island formed from a coral reef that has grown on an eroded and submerged volcanic island. The reef rises to the surface of the water and forms a  new island. Atolls are typically ring-shaped with a central lagoon.

Examples
include  the Maldives in the Indian Ocean and Line Islands in the Pacific.