tectonic plates

The Earth’s crust is composed of massive plates of irregularly shaped solid rock, both continental and oceanic lithosphere. Plate size can vary greatly, from a few hundred to thousands of kilometers across. These plates float on and travel independently over the mantle. Much of the Earth's seismic activity occurs at the boundaries of these plates.

Adapted from http://pubs.usgs.gov/gip/dynamic/tectonic.html

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