The Ring of Fire

by Kathy Svitil

Alaska's frigid and remote Aleutian island chain, the towering Andes mountains of South America, and the tropical islands of Micronesia would seem to have little in common. In fact, these diverse areas are all part of the most volcanically and seismically active region on Earth, an area known as the "Ring of Fire." Three-fourths of Earth's active and dormant volcanoes -- including Mount St. Helens, which is featured in the program "Hell's Crust" -- lie along this arc, at the margins of the Pacific Ocean, where the large Pacific plate and other tectonic plates dive beneath yet other plates.
 

The subducting crust is wet, as crust goes, and that water helps to melt the mantle overlying the sinking plate. This forms blobs of magma -- slightly different than the magma that wells through at mid-ocean ridges -- which rise up toward the surface. Volcanoes form where the magma breaks through -- in this case, the volcanoes of the Cascades Range, including Mount St. Helens. Magma in subduction zones is particularly resistant to flow (viscous), and it is also quite gassy. So when it erupts, it does so in violent explosions -- as did Mount St. Helens, and as the other volcanoes in the Ring of Fire continue to do year after year.

Alaska's Aleutian islands, all volcanic in origin and formed from the subduction of the Pacific plate beneath the North American, represent the northern arc of the Ring of Fire. The ring then sweeps down along Russia's Kamchatka Peninsula and through Japan, where the Pacific plate dives beneath the Eurasian plate. That subduction is responsible for all of the Japanese islands and picturesque volcanoes like Mt. Fuji. The last section of the Ring of Fire is made up of Micronesia and New Guinea, where the Indo-Australian plate drops below the Pacific, and New Zealand, where the Pacific plate returns the favor, and dives below the Indo-Australian.