Volcanoes are found where the tectonic places that make up the Earth's surface are either pushing together or pulling apart. They are also found away from plate boundaries at "hot spots," where magma pushes through the Earth's crust.
Most of the Earth's volcanoes are undersea, along the Southeast Indian, Juan de Fuca, Pacific Antarctic, East Pacific, Mid-Atlantic, Southwest Indian and Central Indian ridges. Scientist estimate that 20 eruptions occur per year along these ridges, forming nearly a mile of new sea floor every year. The Mid-Atlantic Ridge is caused by divergent tectonic plates pulling apart. The ridges in the Pacific Basin form the Ring of Fire and are caused by convergent tectonic plates pushing together.
The Ring of Fire is a 25,000-mile, horseshoe-shaped series of oceanic trenches, volcanic belts and volcanic arcs. It starts east of Australia and runs westward along the islands of Indonesia. It then turns northward along the eastern coast of Asia until heading east near the eastern tip of Siberia. A southward run along the west coasts of North and South America completes the arc. The Ring of Fire is home to 452 volcanoes and 75 percent of Earth's active and dormant volcanoes. The Hawaiian Islands were formed by a "hot spot" where magma pushed through the Earth's crust and are home to three active volcanoes.
Mt. St. Helens, north of Seattle, Alaska's Cleveland Volcano and Mount Pinatubo in the Philippines are all active volcanoes located in the Ring of Fire. Yellowstone National Park also sits atop a supervolcano.