[Illustration: In the Pacific Northwest, the Juan de Fuca Plate plunges
beneath the North American Plate, locally melting at depth; the magma
rises to feed and form the Cascade volcanoes.]
According to the now 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.
The accompanying figure shows the boundaries of lithosphere plates that
are presently active. The double lines indicate zones of spreading from
which plates are moving apart. The lines with barbs show zones of
underthrusting (subduction), where one plate is sliding beneath another.
The barbs on the lines indicate the overriding plate. The single line
defines a strike-slip fault along which plates are sliding horizontally
past one another. The stippled areas indicate a part of a continent,
exclusive of that along a plate boundary, which is undergoing active
extensional, compressional, or strike-slip faulting.
[Illustration: Major tectonic plates of the Earth.]