PBS Online Savage Earth Logo Thirteen / WNET
Vertical Marker
Banner: The Restless Earth: Earthquakes
Back to Home
Hell's Crust
Article: Hell's Crust Sidebar One Sidebar Two Sidebar: Three Animation
The Ring of Fire

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 ring stretches from South America, where the Nazca plate dips beneath the South American plate, pushing up the Andes mountains, and then north up along the coasts of Central America and Mexico. In the Pacific Northwest the tiny Juan de Fuca plate, formed at a spreading center just to the west, is sinking (subducting) beneath the North American plate. This oceanic plate is blanketed with seafloor sediments, and its crust is water-logged. As it dips beneath the North American plate -- and before it melts completely -- the two plates can snag and then break free. The result is earthquakes, a characteristic of the Ring of Fire and other subduction zones.

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.

Click for full-size image

 The Ring of Fire

Design element

Article: The Earth at Work | Sidebar One: Probing the Depths | Sidebar Two: "Black Smokers" | Sidebar Three: Ring of Fire | ANIMATION
Hell's Crust: Our Everchanging Planet  | 
The Restless Planet: Earthquakes
Out of the Inferno: Volcanoes  |  Waves of Destruction: Tsunamis


PBS Online  |  Thirteen Online