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.
The ways that plates interact depend on their relative motion and whether oceanic or continental crust is at the edge of the lithospheric plate. Plates move away from, toward, or slide past each other. Geologists call these divergent, convergent, and transform plate boundaries.
At a divergent plate boundary lithospheric plates move away from each other. The mid-Atlantic Ridge, a topographically high area near the middle of the Atlantic Ocean, is an example of a divergent plate boundary.
At a divergent plate boundary lithospheric plates move away from each other. The mid-Atlantic Ridge, a topographically high area near the middle of the Atlantic Ocean, is an example of a divergent plate boundary.
At a transform plate boundary, plates slide past each other. The San Andreas fault in California is an example of a transform plate boundary, where the Pacific Plate slides past the North American Plate.
