How does metamorphic rock generally form?

Metamorphic rock generally forms from existing rock that undergoes a transformation due to increased heat and pressure, which can occur deep within the Earth's crust. This process alters the mineral composition and structure of the original rock without the rock melting, distinguishing metamorphic rock from igneous and sedimentary rocks. The transformation can lead to the formation of new minerals and textures, primarily due to the recrystallization of the original minerals under these extreme conditions.

For example, shale, a sedimentary rock, can be transformed into schist or gneiss when subjected to significant heat and pressure, showing the distinctive changes that define metamorphic processes. This ongoing geological process is essential for understanding the rock cycle and how different types of rocks interrelate through various environmental conditions.