What are the major types of Metamorphism? What are the Contact, fault zone metamorphism, plutonic metamorphism?

 Types of metamorphism: The main types of metamorphism are-

1)      Contact metamorphism: Contact metamorphism is also called “thermal metamorphism”. This metamorphism is caused due to local heating of rocks by the intrusion of hot igneous bodies nearby.  The zone of metamorphic rocks which occurs surrounding the intrusion is called “aureole”.  In contact, metamorphism heat plays an important role and its general effect is to promote recrystallization. In this process, minerals grow haphazardly in all directions and the metamorphic rock acquires a granular fabric which is called the “hornfels texture”. Contact metamorphic rocks do not show schistosity.

    During contact metamorphism transfer of magmatic vapors and gases from an igneous body into the country rocks often takes place. These emanations react with the country rocks and form new minerals such a process is called the “Pneumatolytic metamorphism”. A localized burning or baking effect may be produced at the contact of an igneous body and the country rocks. This effect is described as the “pyrometamorphism”.

 

2)      Fault zone metamorphism: Rocks along geologic faults are deformed due to pressures associated with shearing, compression, or extensional stresses, with minor changes due to heat. Fault zone metamorphism is a type of dynamic metamorphism (A metamorphism which is associated with high pressure with little increase in temperature, is called dynamic metamorphism.). Shallow faults (close to the earth's surface) may grind nearby rocks into smaller, angular fragments called fault breccia, or may develop a fine clay powder called fault gouge. Fault gouge forms by the chemical alteration of fault breccia. The rocks that form adjacent to the fault in deeper sections are called mylonite. The minerals in mylonitic rocks are deformed due to shear stresses associated with movement along the fault.

 

3)      Cataclastic Metamorphism: The metamorphism, in which only the directed pressure or lateral stress plays the dominant role, is called Cataclastic metamorphism. These stresses are caused by earth movements such as folding and faulting. They operate mainly in the upper Earth’s crust at which the temperature is moderately low. Due to these stresses, the rocks are crushed, ground, and deformed. New rocks thus formed, which are known as Cataclastic rocks. Examples of Cataclastic rocks are “mylonite”, “fault breccia”.

 

4)      Plutonic metamorphism: At great depth below the earth’s surface static pressure and high temperature operate together. The metamorphism caused by these factors is called the “Plutonic metamorphism”. High static pressure favors a reduction in volume. Hence during recrystallization mainly denser minerals are formed. The metamorphic rocks produced in this way commonly have an even-grained texture. Such rocks are called “granulites”