Because of its interaction with the outside atmosphere partial melting result in a magma having a composition different from the rock from which it was derived.
As existing crustal rocks melt in the presence of heat from magmas, partial melting also takes place. Existing rocks melt during this process, allowing the resulting magma to be more felsic and less mafic than the underlying rock. If a portion of the rock that the magma travels through is absorbed, we say that the magma has become polluted by the crust. Unless the substance being added has the same chemical composition as the magma, each of these processes would result in a change in the magma's chemical makeup. As a result, magma forms that vary from the original rock in composition.
Because minerals with a higher silica content have lower melting temperatures, partial melting results in a melt that contains more silica than the original rock.
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