When is an island not a continent?
GREENLAND is the largest island in the world, over two million square kilometres, most of it buried under an ice sheet up to three kilometres thick. Australia is bigger, but it is counted as a continent rather than an island, a line geographers drew by tradition rather than by any strict rule. After Greenland come New Guinea, BORNEO and MADAGASCAR. JAVA, in Indonesia, is the most crowded island on Earth, home to around 150 million people, more than live in Russia.
Islands form in very different ways. Some, like Madagascar and SICILY, are pieces of continent cut off by the sea. Others, like HAWAII and ICELAND, are the tops of volcanoes that grew up from the ocean floor. The Hawaiian islands sit in a line because the Pacific plate is slowly sliding over a fixed hot spot deep underground, which builds a new volcano, then carries it away and builds another. The Big Island is the youngest, and still growing; the oldest islands in the chain have worn away to nothing.
Rings of coral
An ATOLL is a ring of coral islands around a LAGOON, with nothing in the middle. In the 1830s, on the voyage of the Beagle, Charles Darwin worked out how they form. Coral grows as a reef around the shores of a volcanic island. Over hundreds of thousands of years the volcano slowly sinks and wears away, while the coral keeps growing upwards towards the light, until only a ring of reef is left around an empty lagoon. Drilling on Pacific atolls in the 1950s found volcanic rock deep beneath the coral, just as he predicted.
The MALDIVES, a chain of about 1,200 small coral islands in the Indian Ocean, is the lowest country in the world, with most of its land less than a metre and a half above sea level, which makes rising seas a real worry for its people. Our coral reef page covers the builders themselves.
Island animals, settlers and the rest of the grid
Animals and plants that reach an island can evolve in their own direction, cut off from the mainland. Madagascar split from Africa and then from India tens of millions of years ago, and around nine out of ten of its plant and animal species live nowhere else, including all the world's wild lemurs. On some islands small animals grew large and large ones shrank: Crete, Cyprus and Sicily were once home to dwarf elephants no bigger than a pony.
People reached some of the most remote islands on Earth by canoe, as our Polynesian mythology page explains. The first settlers reached TAHITI and the islands around it well over a thousand years ago.
The Caribbean islands of CUBA, JAMAICA and the BAHAMAS, the Mediterranean isles of CRETE, CYPRUS, SARDINIA and CAPRI, and the Pacific and Indian Ocean islands of FIJI, BALI and SUMATRA fill the rest of the grid. Capri is famous for its Blue Grotto, a sea cave where sunlight passing through the water beneath the entrance lights the whole cave a glowing blue. Many are TROPICAL, but some of the largest islands, like Greenland, lie inside the Arctic Circle.