How do islands form?

Volcanic islands, such as Hawaii and Iceland, form when magma erupts on the ocean floor and builds up until it breaks the surface. The Hawaiian chain formed as the Pacific plate moved over a hotspot, creating a line of islands of different ages. Coral atolls, like those of the Maldives, grow as rings of reef around volcanoes that slowly sink. Continental islands, such as Great Britain and Madagascar, were once connected to larger landmasses and separated by rising sea levels or plate movements. Barrier islands form from sand deposited by waves and currents along coastlines.