The "ring of fire" specifically refers to a zone of volcanos along the shores of the Pacific Ocean, approximately 10,000 kilometers in "diameter". I put diameter in quotes because the "ring" is not even close to a circle. The map on this web site will help: "http://www.geo.mtu.edu/ volcanoes/world.html All of these volcanos are formed above subduction zones, places where one plate dives beneath another and is consumed back into the mantle. As the plate descends, it is heated and water is driven off. Adding water to hot rocks under high pressure helps to break the chemical bonds that hold the rock together and the rock may melt, rise to the surface, and erupt as a volcano. In most places it is the Pacific Plate that is being consumed, but that is not true everywhere. For example, along the coasts of North and South America, small plates (such as the Nazca plate) lie between the Pacific plate and the subduction zone and it is the small plates that are being subducted.
Dave Smith
La Salle University
Philadelphia, PA