The earth's crust (or lithosphere) is constantly changing, forming the world we know today. Volcanoes, earthquakes, and geysers have always revealed the dynamic nature of the lithosphere. But not until the twentieth century did geologists develop the theory of plate tectonics to explain the underlying forces that shape the face of the planet.

The lithosphere has an average thickness of 62 miles (100 km)—as thin as an eggshell compared to the 7,900 mile (12,740 km) diameter of the earth. The solid crustal plates that make up the lithosphere float on the asthenosphere, a layer of soft, molten rock. These plates do not form a rigid shell, but are on the move, driven by convection currents in the mantle.

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Plate Tectonics explains how these plates move and helps us understand why volcanoes and geysers appear. (Learn more about volcanoes and geysers.) It also explains other geologic phenomena.