Britannica
Sources used in this answer
- rock: Top Questions
- metamorphism
- foliation
- metamorphic rock
- metamorphic rock - Dynamic metamorphism
- metamorphic rock - Pressure
- metamorphic rock - Classification into four chemical systems
- igneous rock - Pegmatites and late-stage mineralization
- metamorphic rock - Metamorphic variables
- metamorphic rock - Hydrothermal metamorphism
How do metamorphic rocks form from existing rocks?
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geology
A rock formation called “faux amphibolite,” from the Nuvvuagittuq greenstone belt in northern Quebec, dated to 4.28 billion years ago.
Metamorphic rocks form through the alteration of existing igneous, sedimentary, or other metamorphic rocks when they are subjected to physical and chemical conditions that differ from those under which they originally formed. These changes occur in the solid state, meaning the rock does not melt during the process.123
The primary agents driving these transformations are:
- Temperature and Pressure: Increases in heat and pressure—often caused by burial, tectonic plate movements, or proximity to magma—force minerals to become unstable. They react with one another to form new, thermodynamically stable mineral assemblages.324
- Chemical Environment: The addition or subtraction of chemical components, often facilitated by fluids like water, can significantly alter a rock's mineralogy.325
- Mechanical Stress: Differential stress can cause mechanical deformation, leading to structural changes such as foliation, where mineral grains align along specific planes.367
Depending on the dominant conditions, this process is categorized into types such as contact metamorphism (heat-driven), regional metamorphism (large-scale pressure and temperature increases), and dynamic metamorphism (stress-driven).3