Volcanoes are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents,
along island chains, or beneath the sea forming long mountain ranges. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific "Ring of Fire." In the past 25 years, scientists have developed a theory -- called plate tectonics -- that explains the locations of volcanoes and their relationship to other large-scale geologic features.

Most of the world's active volcanoes are located along or near the boundaries between shifting plates and are called
"plate-boundary" volcanoes. However, some active volcanoes are not associated with plate boundaries, and
many of these so-called "intra-plate" volcanoes form roughly linear chains in the interior of some oceanic plates. The
Hawaiian Islands provide perhaps the best example of an intra-plate volcanic chain, developed by the
northwest-moving Pacific Plate passing over an inferred "hot spot" that initiates the magma-generation and
volcano-formation process. The peripheral areas of the Pacific Ocean Basin, containing the boundaries of several
plates are dotted by many active volcanoes that form the so-called "Ring of Fire".  The "Ring" provides excellent examples of "plate-boundary" volcanoes, including Mount St. Helens.

The boundary between the Pacific and Juan de Fuca Plates is marked by a broad mountain chain known as the Juan de
Fuca Ridge. Young volcanoes, lava flows, and hot springs were discovered in a broad valley less than 8 kilometers
wide along the crest of the ridge in the 1970's.

The east edge of the Juan de Fuca Plate is plunging beneath the North American Plate. As the denser plate of oceanic
crust is forced deep intothe Earth's interior beneath the continental plate, a process known as subduction, it
encounters high temperatures and pressures that partially melt solid rock. Some of this newly formed magma rises
toward the Earth's surface to erupt, forming an arc of volcanoes above the subduction zone.

From: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication; Tilling, 1985, Volcanoes: USGS General Interest Publication