An island or isle is any piece of land that is completely surrounded by water. Very small islands are called islets. It is also proper to call an emergent land feature on an atoll an islet, since an atoll is a type of island, although this convention is seldom adhered to. A key or cay is another name for a relatively small island or islet.
There are two ways in which islands can form. One way is when a slab of crust is being forced down into the Earth, it melts. Hot, molten material rises again into the crust, forming a series of volcanoes. These volcanoes can make a chain of islands called an `island arc`. The Aleutian Islands of Alaska were formed in this way.
The second way in which islands are formed is when there are hot spots in the lithosphere. Hawaii is an island of this type of island.
The Great Equatorial Ocean of Alpha Prime is littered with long chains of moderately-sized volcanic islands. These range in scale from areas the size of the Hawaiian chain to the continent of "Pangaea"-which rivals Australia. Yearly tsunami (giant waves) have scoured the tops of these seamounts leaving them virtually lifeless.
Continental 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 occur in river deltas and in large rivers. They are caused by deposition of sediment at points in the flow where the current loses some of its carrying capacity. In essence, they are river bars, isolated in the stream. While some are ephemeral, and may disappear if the river`s water volume or speed changes, others are stable and long-lived.
Volcanic islands are built by volcanoes. Mid-ocean examples are not 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.