Islands are land masses completely surrounded by water.
They range in size from a few feet of sand to a huge continent, such as Australia. Three basic activities will create an island. A piece of land may break off from a continental plate and float into the ocean or other body of water.
Australia, for example: Some 200 million years ago, when the supercontinent Pangaea split up, Australia gradually moved to its present position, as an island. Or water may rise up and cut off a piece of land.
The Scilly Isles off the coast of England used to be attached to the mother country. As a result of flooding at the end of the last ice age, the sea level rose and cut off these granite islands.
Perhaps the most stupendous islands are those created by volcanic eruption. The molten layer of Earth, the mantle, pushes its way up through an oceanic plate. When it cools and solidifies, an island is left behind.