Volcanoes occur along the edges of large tectonic plates. Wherever large tectonic plates collide, magma from the interior of the Earth’s crust gets pushed upward through a vent. The top of the vent is called a crater, and it is through this opening that volcanic eruptions take place.
The Ring of Fire contains the greatest percentage of the Earth's volcanic activity. It is a large area including the Pacific Ocean basin and the land at its edges from the Americas to east Asia and Australia. Most volcanic activity occurs in the mountain ridges under the ocean. However, eruptions occur in other parts of the world as well; for example, Iceland and Italy both have active volcanoes.