Many archipelagos are formed through volcanic activity. A hot spot in the Earth's crust spews out magma, which cools and solidifies into rock. As the volcano grows, it eventually protrudes above the ocean surface to form an island. When tectonic plate movement carries it away from the hot spot, the volcano shuts down and another one begins to form. Archipelagos can also be created by erosion, or when sea levels fall to expose underlying rock formations.
An archipelago can consist of hundreds of islands and islets, or thousands. The Malay Archipelago, for example, contains over 20,000 land masses. While some island chains are hot and covered in lush vegetation, others are barren, cold and uninhabitable. Some islands still have active volcanoes.
Volcanic archipelagos are mainly formed along tectonic plate boundaries and around intra-plate hot spots, such as the Hawaiian Islands and the Philippines. However, large island groups exist all around the world.
