Page "Metamorphic rock" Paragraph 12

Metasomatism is the drastic change in the bulk chemical composition of a rock that often occurs during the processes of metamorphism.

It is due to the introduction of chemicals from other surrounding rocks.

Water may transport these chemicals rapidly over great distances.

Because of the role played by water, metamorphic rocks generally contain many elements absent from the original rock, and lack some that originally were present.

Still, the introduction of new chemicals is not necessary for recrystallization to occur.