Metamorphic Petrology
Metamorphic petrology studies the changes in mineralogy, texture, and chemistry that rocks undergo in the solid state in response to heat, pressure, and fluids.
Definition
The branch of petrology concerned with the mineralogical and textural transformation of pre-existing rocks under changing temperature, pressure, and fluid conditions without bulk melting.
Scope
This area covers the principles of metamorphism: the concept of metamorphic facies and grade, the chemical reactions that produce new mineral assemblages, the textures and fabrics generated by recrystallization and deformation, and the contrasting settings of regional and contact metamorphism. It uses mineral assemblages to reconstruct the pressure-temperature-time paths of rocks.
Sub-topics
Core questions
- How do metamorphic facies relate mineral assemblages to pressure and temperature?
- What chemical reactions mark increasing metamorphic grade?
- How do textures and fabrics record deformation and recrystallization?
- How do regional and contact metamorphism differ in cause and product?
Key theories
- Metamorphic facies concept
- Eskola's principle holds that rocks of a given bulk composition equilibrating under a particular range of pressure and temperature develop a characteristic mineral assemblage, defining metamorphic facies that map directly onto conditions.
- Index minerals and metamorphic zones
- The first appearance of specific index minerals, such as chlorite, biotite, garnet, staurolite, kyanite, and sillimanite in metapelites, marks isograds that divide a terrain into zones of increasing grade.
Clinical relevance
Metamorphic petrology lets geologists reconstruct the burial, heating, and exhumation histories of orogenic belts, constrain plate-tectonic processes such as subduction and collision, and understand the formation of metamorphic ore and industrial mineral deposits.
History
George Barrow's late-nineteenth-century mapping of zoned metamorphism in the Scottish Highlands introduced index minerals; Pentti Eskola formalized the facies concept in 1920, and twentieth-century experimental and thermodynamic work calibrated facies and reactions to quantitative pressures and temperatures.
Key figures
- Pentti Eskola
- George Barrow
- Bernard Yardley
Related topics
Seminal works
- eskola1920
- winter2013
- bucher2011
Frequently asked questions
- What is the difference between metamorphism and melting?
- Metamorphism transforms rock in the solid state; once temperatures are high enough to produce significant melt, the process passes into anatexis and igneous petrology.
- What is metamorphic grade?
- A qualitative measure of the intensity of metamorphism, chiefly the peak temperature reached, expressed through index minerals and facies from low grade (greenschist) to high grade (granulite).