TO MANY, Earth feels like a static place. The rock beneath our feet seems solid and unchanging. Yet, over billions of years, the surface of the Earth has actually changed dramatically and its interior composition altered, all thanks to plate tectonics. Shifting plates of crust move on thick layers of flowing - but still solid - rock called the mantle. This constant shifting not only moves continents, but also recycles material from the surface back towards the planet's deep interior.
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