Four types of Metamorphic agents

  1. Temperature
  2. Pressure
  3. Directive stress
  4. Chemically active fluid/metamorphic fluids


  • In regular continental crust, a typical geothermal gradient within the first 3 to 5 kilometers (2 or 3 miles) of Earth’s surface is about 25°C/km.
  • Promotes recrystallization/crystal growth.
  • Unstable mineral/s will decompose to form new stable mineral/s.
  • Devolatilization, dehydration, and decarbonization reactions may take place.
  • Temperature causes the rocks to easily overcome kinetic barriers toward attaining a new equilibrium.
  • Most metamorphism takes place between a temperature of around 150 to 850oC.
  • However, metamorphism can occur very locally at temperatures up to the solidus of dry basalt (>1000oC).


  • Here, pressure refers to confining pressure, equal in all directions.
  • There are pressure limits to the stability of minerals and mineral associations, just as there are temperature limits.
  • Most metamorphism takes place at pressures generally below 10 kbar corresponding to depths of 37 km depth, based on an average continental crustal density of 2.7 g/cm3.
  • However, in subduction zones near trenches and continental collision zone depths can reach 100 km or more (ultrahigh P >=30 kbar).

Directional Stress

  • This occurs most commonly in orogenic belts, extending rifts, or in shear zones (i.e., generally at or near plate boundaries).
  • affects the textures and structures in rocks but not the equilibrium mineral assemblage.
  • may also provide the impetus to overcome kinetic barriers to reactions.
  • Generally causes mineral grains to be aligned in a preferred orientation, resulting in foliation and lineation in metamorphic rocks.

Chemically active fluid/metamorphic fluids

  • Intergranular metamorphic fluids are usually dominated by H2O, but CO2 may also be present in some rocks.
  • Fluids affect the P-T stability condition for minerals; this is particularly important for low P-T conditions. Zeolite stability is strongly dependent on fluid composition. Zeolites may not appear if the fluid is too rich in CO2.
  • The motion of fluids may transport various chemical species over considerable distances.
  • The material exchange may occur between fluids and rocks; if substantial exchange occurs, this is “Metasomatism.”

Types of Metamorphism

  • Contact Metamorphism
  • Regional Metamorphism (High geothermal gradient)
  • Burial Metamorphism (Normal geothermal gradient)
  • Dynamic Metamorphism/High P-low T Metamorphism
  • Hydrothermal Metamorphism
  • Impact or Shock Metamorphism