What are metamorphic rocks formed from?

Metamorphic rocks are formed through the process of metamorphism, which involves the transformation of existing rock types—either igneous, sedimentary, or even other metamorphic rocks—under extreme conditions of heat and pressure. This process alters the mineral composition and texture of the rock without the rock melting. The heat and pressure can cause physical and chemical changes, leading to the formation of new minerals that are stable under the new conditions.

For example, shale, a sedimentary rock, can be transformed into slate, a metamorphic rock, through the application of heat and pressure over geological time. This distinction is essential in understanding the rock cycle and the processes that lead to the formation of different rock types in geology.