- The igneous and sedimentary rocks after they were first formed sometimes undergo
a change. When the change is considerable, the rock is said to have undergone
metamorphosis and the new rock is known as a metamorphic rock. The metamorphism is brought about by the action of water, heat or pressure or by the combined action of any one of these or all. The change brought about by water is hydro-metamorphism.
- The change brought about by heat is thermo-metamorphism. The change brought about by pressure is dynamo- metamorphism. The changes that are brought about are both physical and chemical in character. 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 or introduce new materials. By
the introduction of a cementing material like silica, lime or iron oxide, loose sand may be
turned into sandstone or sandstone into a quartzite. By the removal of certain constituents by percolating waters, basalt or granite may be converted into a laterite.
- The action of heat hardens the rock and develops new crystals in it. Crystalline
marble is produced this way from amorphous limestone by the action of heat and pressure.
- Due to pressure, the crystals of the original rock get pressed or flattened and the new rock is foliated. When foliation is slight, the layers are inseparable and it is called as gneiss. It foliation is complete with distinct and separable layers it is called as schist.