What is an island?

Islands exist in oceans, seas, lakes and rivers. Some are as small as a rock with a palm tree. Others are huge: Greenland is the world's largest island, with over two million square kilometres. (Australia is much bigger, but it is counted as a continent.) A group of islands is an archipelago. Japan has thousands of islands, and Indonesia has more than seventeen thousand.

Islands form in different ways:

• Continental islands were once joined to a continent. When the sea rose after the last Ice Age, water filled the gap. Great Britain and Sri Lanka are examples.
• Volcanic islands grow from the sea floor when erupting volcanoes build up layers of lava until they poke above the water. Hawaii, Iceland and the Canary Islands are volcanic. See volcanoes.
• Coral islands form when tiny animals called corals build reefs in warm, shallow water. Over time, sand piles up on the reef. The Maldives are coral islands.
• River and lake islands are made when water splits around a bank of sand or rock.

Because islands are cut off, their wildlife can be unusual. Madagascar, off the east coast of Africa, is home to lemurs that live nowhere else. The Galápagos Islands have giant tortoises and finches that helped Charles Darwin develop his ideas about evolution. Island plants and animals can be at risk if new species arrive from outside.

Many countries are islands, including Japan, Cuba, Iceland, Ireland (shared with Northern Ireland), Madagascar and New Zealand. Some small coral islands are low and threatened by rising seas. An island is different from a peninsula, which is land surrounded by water on three sides. Find islands on a map in maps and learn about the waters around them in oceans.