For billions of years the earth has been the leader in the recycling of earth materialsnamely the earth's own crust. How does this happen?
As rock is buried deeper into the earth, increasing pressure and temperature turn solid rock into an extremely hot, ooey-gooey liquid called magma. As magma moves toward the surface of the earth away from the core (because heat rises), it begins to cool and harden. Sometimes magma is poured and ejected from volcanoes and this magma is called lava. Lava and magma will continue to cool and harden, forming igneous rocks.
On the earth's surface, erosion will slowly break down and wash away all kinds of rocks, including igneous rocks formed from magma. The bits of rock produced by erosion are now called sediment (dirt) and is washed and blown away and deposited as sedimentary layers in our lakes, rivers, and oceans. Layer upon layer of sediments are deposited; these layers sink into the earth's surface under their own weight, becoming squished together and slowly forming sedimentary rocks.
As sedimentary rocks are pushed deeper and deeper into the crust, they are squeezed and reshaped by high temperatures and pressures. When rocks are squeezed and reshaped by heat and pressure, they are known as metamorphic rocks. As the now-metamorphic rocks continue to be pressed deeper into the earth's crust, still higher temperatures and pressures will melt and change them once again into...you guessed it...magma.
The recycling begins again as the magma moves toward the earth's surface, begins to cool and harden, is strewn from volcanoes and pours onto the surface as lava. This is known as the rock cycle.