It seems logical that if Sedimentary and Igneous rocks are being converted into Metamorphic rocks, there should be more and more Metamorphic rocks and less of the other kinds,over long time periods. This does not happen because progresses such as weathering and erosion or melting also breaks down rocks. Melting rocks become new Igneous rocks, and rocks that have been broken down into small particles by weathering and erosion settle into layers of sediment and form new sedimentary rocks. Together, these processes are known as the rocks cycle. For example, Metamorphic rocks that are forced deep below Earth's surface may be melted when they come in contact with magma, where as part of the same Metamorphic rock that remains on the surface may be weathered down to sand.
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