The ground you're walking on is recycled. Earth's rock cycle transforms igneous rocks to sedimentary rocks to metamorphic rocks and then back again. The cycle is not a perfect circle, but the basic works like this: Magma from deep in the Earth emerges and hardens into rock (that's the igneous part). Tectonic processes uplift that rock to the surface, where erosion shaves bits off. These tiny fragments gets deposited and buried, and the pressure from above compacts them into sedimentary rocks such as sandstone. If sedimentary rocks get buried even deeper, they "cook" into metamorphic rocks under lots of pressure and heat. Along the way, sedimentary rocks can be re-eroded or metamorphic rocks re-uplifted. But if metamorphic rocks get caught in a subduction zone where one piece of crust is pushing under another, they may find themselves transformed back into magma.
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