What is the process called that forms metamorphic rocks?

The process that forms metamorphic rocks is known as the alteration of pre-existing rocks. This transformation occurs when existing rocks—either igneous, sedimentary, or other metamorphic rocks—are subjected to intense heat, pressure, and/or chemically active fluids. Under these conditions, the mineral composition and structure of the rocks can change significantly without the rock actually melting. This alteration may result in the formation of new minerals and a new texture distinct from the original rock.

For instance, shale, a sedimentary rock, can be transformed into slate, a metamorphic rock, by the application of heat and pressure over time. The presence of high temperatures can cause minerals within the shale to recrystallize, leading to a denser, more durable rock. This process is crucial in the rock cycle, showcasing how rocks can transition between different types as a result of geological processes.