Some islands are really parts of the continents, separated from the mainland by shallow seas. About 11,000 years ago, during the Ice Age, the sea-level was much lower than it is today, because so much water was frozen in ice sheets. At that time, Great Britain was joined ice mainland Europe. As the climate got warmer, the ice melted and the sea-level rose. In about 5500 BC, the rising waters encircled Great Britain, which has been an island ever since.
Other islands rise from the ocean deeps. Some, such as Surtsey, are volcanoes and some are made of coral. Atolls are circular or horseshoe-shaped groups of low coral islands that have formed on the tops of submerged volcanoes.