Metamorphic rocks are formed from existing rocks that undergo changes due to heat, pressure, or chemically active fluids. There are two main types: foliated and non-foliated metamorphic rocks. Foliated rocks, like schist and gneiss, have a layered or banded appearance due to the alignment of minerals under pressure. Non-foliated rocks, such as marble and quartzite, do not exhibit this layering and are typically formed from the alteration of a single mineral or a mixture of minerals. The texture and composition of these rocks depend on the original rock type and the conditions of metamorphism.