Metamorphic rocks form with heat and pressure, and were originally igneous or sedimentary rocks, but with the added heat and/or pressure, have metamorphosed. The pressure between two plates pushing together would form metamorphic rocks, so when fold mountains are formed, they are metamorphic. When a denser plate meets a less dense plate, and one is forced under, the heat of the magma that is forced up along with the pressure forms metamorphic rocks.
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