Metamorphic rocks form when existing rocks—whether sedimentary, igneous, or even other metamorphic rocks—are subjected to high heat, high pressure, and sometimes chemically reactive fluids, causing physical and chemical changes without melting the rock. This process, called metamorphism, typically occurs deep within the Earth or at tectonic plate boundaries where such conditions are common
. Key points about metamorphic rock formation include:
- Heat and Pressure: Elevated temperatures (above about 150–200°C) and pressures (often over 100 megapascals) cause the minerals in the original rock (protolith) to recrystallize and form new minerals and textures, transforming the rock's structure
- Sources of Heat: Heat can come from deep burial beneath the Earth's surface or from nearby magma intrusions that heat the surrounding rock (contact metamorphism)
- Pressure: Pressure arises from the weight of overlying rock layers (burial pressure) or from tectonic forces during plate collisions, which can also deform and align minerals, creating foliated textures in the rock
- Fluids: Hot, mineral-rich fluids can also facilitate chemical changes during metamorphism, although metamorphism can occur without fluid involvement (isochemical metamorphism)
- Types of Metamorphism:
- Regional metamorphism occurs over large areas during mountain-building processes where rocks are deeply buried and deformed.
- Contact metamorphism happens locally around igneous intrusions due to heat
- Examples of Metamorphic Rocks: Gneiss, schist, slate (foliated rocks), and marble, quartzite (non-foliated rocks)
In summary, metamorphic rocks form through the transformation of pre-existing rocks by heat, pressure, and chemical processes deep in the Earth's crust, without melting, resulting in new mineral assemblages and textures
