Forces of Nature: ThinkQuest 2000 (Team #C003603)
http://library.thinkquest.org/C003603/english/volcanoes/theringoffire.shtml
Because volcanoes usually occur on plate boundaries, where many zones of subduction (the area where the crustal layer descends into the mantle) occur, most of them are found at the edges of the continents. Roughly, they trace out the outline of continents on earth. Known as the Ring of Fire, these volcanoes form a discontinuous chain around the Pacific Basin. Most of the earth’s volcanic activity occurs beneath the sea, along the seams between crusts.