What defines metamorphic rocks?

Metamorphic rocks are defined by their formation process, which involves the transformation of existing rock types through exposure to high temperatures and pressures within the Earth's crust. This process, known as metamorphism, alters the mineral composition and texture of the rock without the rock actually melting.

The key aspect of metamorphic rock formation is that they start as pre-existing rocks, which can be either igneous, sedimentary, or even other metamorphic rocks. As these rocks are subjected to extreme conditions, their original structures and mineralogical characteristics change, resulting in new rock types with unique properties. For instance, shale can transform into schist or gneiss under sufficient heat and pressure.

Other processes mentioned, such as solidification of magma, compaction of sediments, and crystallization of minerals, describe how igneous and sedimentary rocks are formed, but they do not pertain to the metamorphic process. Therefore, the formation of metamorphic rocks through the effects of heat and pressure on pre-existing rocks clearly defines them within the rock cycle, distinguishing them from other rock categories.