Metamorphic rocks

Igneous and sedimentary rocks that have been altered from their previous condition through the combined action of heat and pressure below Earth's surface are called metamorphic rocks and the process is called metamorphosis.  The changes so brought about are both physical and chemical in character, which alter the structural features of rocks. Many times both the structural features and composition of original  rocks are affected simultaneously.  In some cases the metamorphism is so pronounced that the new rock looks quite different from the original.  The action of water tends to remove some material from, or to introduce new material like silica, lime or iron  oxides.