@synfully provided a thorough answer; I'll just add this helpful image from Wikipedia:

(Click to enlarge)
http://upload.wikimedia.org/wikipedia/commons/0/09/Pacific_Ring_of_Fire.png

That's what Johnny Cash fell into in his 1963 song

The "Ring of Fire" is an arc stretching from New Zealand, along the eastern edge of Asia, north across the Aleutian Islands of Alaska, and south along the coast of North and South America. The Ring of Fire is composed over 75% of the world's active and dormant volcanoes.

This is a listing of major volcanic areas in the Ring of Fire:

• In South America the Nazca plate is colliding with the South American plate. This has created the Andes and volcanoes such as Cotopaxi and Azul.
  
• In Central America, the tiny Cocos plate is crashing into the North American plate and is therefore responsible for the Mexican volcanoes of Popocatepetl and Paricutun (which rose up from a cornfield in 1943 and became a instant mountains).

• Between Northern California and British Columbia, the Pacific, Juan de Fuca, and Gorda plates have built the Cascades and the infamous Mount Saint Helens, which erupted in 1980.
  
• Alaska's Aleutian Islands are growing as the Pacific plate hits the North American plate. The deep Aleutian Trench has been created at the subduction zone with a maximum depth of 25,194 feet (7679 meters).
  
• From Russia's Kamchatka Peninsula to Japan, the subduction of the Pacific plate under the Eurasian plate is responsible for Japanese islands and volcanoes (such as Mt. Fuji).
  
• The final section of the Ring of Fire exists where the Indo-Australian plate subducts under the Pacific plate and has created volcanoes in the New Guinea and Micronesian areas. Near New Zealand, the Pacific Plate slides under the Indo-Australian plate.