Archipelagos form when underwater volcanoes continually produce molten magma that cools and builds up to eventually form a group of islands. Although archipelagos can form by erosion or by the movement or evaporation of water, volcanic activity is the most common cause of archipelago formation.
- Magma builds up
Where tectonic plates come together, underwater volcanoes can form. Hot spots are areas of intense heat in the middle of tectonic plates. Volcanoes can also form at hot spots as magma rises out of the sea floor.
- Tectonic plates shift over hot spots
When the tectonic plates move over existing hot spots, molten lava erupts from the Earth's crust. When molten magma hits the cooler water, it hardens and forms underwater volcanoes. If volcanoes exist where tectonic plates meet, the plates' movements can also cause these volcanoes to erupt.
- Islands form in an archipelago
Continual movement by the tectonic plates and the subsequent volcanic activity causes build up of cooled lava. Islands form after lava piles up over millions of years.
- Volcanic activity may or may not stop
For some islands, the tectonic plate may move away from the hot spot and the volcanic activity ceases. Other islands continue to have volcanic activity for millions of years.