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.