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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.

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

 

Plate Tectonics

According to the new, generally accepted "plate-tectonics" theory, scientists believe that the Earth's surface is broken into a number of shifting slabs or plates, which average about 50 miles in thickness.

These plates move relative to one another above a hotter, deeper, more mobile zone at average rates as great as a few inches per year.

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.

From: Tilling, 1985, Volcanoes: USGS General Interest Publication