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Figure 9. Cutaway view of the Earth showing the rocky mantle and iron core. The outermost layer consists of tectonic plates that are commonly about 100 km thick. Earthquakes occur within or at the boundaries of these plates. Although the mantle is solid, the rocks that comprise it act like a very viscous liquid and may move a few centimeters a year in great convection cells driven by temperature differences in the Earth. The plates move slowly with these currents. Spreading plate boundaries are thought to lie above areas of upwelling currents, and converging plate boundaries above areas where the currents move towards the center of the Earth. (See also Figure 10.)

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