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Location of Plate Boundaries

Earthquakes and volcanoes, evidence of unrest in the Earth, help
locate the edges of plates. Earthquakes are distributed in narrow,
linear belts that circle the Earth.

Some of these belts have only shallow (0-20 miles; 0-35 km)
earthquakes, like the mid-Atlantic and east Pacific ridges. In
contrast, earthquakes in other belts, like western South America and
south-central Asia, are at shallow, intermediate (20-45 miles; 30-70
km), and deep (45-450 miles; 70-700 km) levels.

Volcanoes are also distributed in long belts that circle the Earth. A
dramatic example is the line of volcanoes that circles most of the
Pacific Ocean. This belt is known as the "Ring of Fire" because it is
the site of frequent volcanic eruptions.

The distribution of earthquakes and volcanoes coincides at most
locations. The Ring of Fire is an excellent example. Geologists
believe that areas of intense geologic activity, indicated by
earthquakes, volcanoes, and/or mountain building, mark the boundaries
between lithospheric plates. The distribution of earthquakes,
volcanoes, and mountain ranges define 7 large plates and 20 smaller
plates. The Nazca and Juan de Fuca Plates consist of only oceanic
lithosphere. The Pacific Plate is mostly oceanic lithosphere only a
small slice of continental lithosphere in southern California and Baja
Mexico. Most of the other plates consist of both oceanic and
continental lithosphere.
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References

1. file://localhost/vwdocs/vwlessons/plate_tectonics/part11.html
2. file://localhost/vwdocs/vwlessons/atg.html
3. file://localhost/vw.html
4. file://localhost/vwdocs/vwlessons/plate_tectonics/part13.html