That is the opbposite of synthesois reaction

The two basic reaction types are synthesis and decomposition. ... Decomposition reactions are the opposite of synthesis reactions because they take apart larger molecules and/or compounds. The generic equation for a synthesis reaction is A + B à AB, where A and B are the pieces that make up the compound/molecule AB.

igneous rocks which formed by the crystallization of magma at the surface of the earth called extrusive rocks. they are characterized by fine-grained textures because the rapid cooling at or near the surface did not provide enough time for large crystals to grow.