TYPE OF METAMORPHISM

 TYPES OF METAMORPHISM

Metamorphism can take place in several environments where special conditions exist like pressure,temperature, stress, conditions, or chemical environments.by this different conditions application

 several different types of metamorphism  are :-

1.  Contact metamorphism
2.  Burial metamorphism
3.  Dynamic metamorphism
4.  Regional metamorphism
5.  Hydrothermal metamorphism
6.  Subduction related metamorphism
7.  Shock metamorphism

•  Contact Metamorphism  ( alias thermal metamorphism) - 

    

                                                                       Figure 1.1  

Occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion. Since only a small area surrounding the intrusion is heated by the magma, metamorphism is restricted to a zone surrounding the intrusion, called a metamorphic aureole. Outside of the contact aureole, the rocks are un-metamorphosed.

 The grade of metamorphism increases in all directions toward the intrusion. Because of temperature differences in the surrounding rock and the intruded magma are larger at shallow levels in the crust, contact metamorphism is usually referred to as high temperature, low pressure metamorphism. The rock produced is often a fine-grained rock that shows no foliation, called a hornfels.

•  Burial Metamorphism - 

       

                                                                        Figure 1.2 ( Serpentinite)

        When sedimentary rocks are buried to depths of several hundred meters, temperatures greater than 3000°C may develop in the absence of differential stress. New minerals grow, but the rock does not appear to be metamorphosed .

                The main minerals produced are the  Zeolites. Burial metamorphism overlaps, to some extent, with diagenesis, and grades into regional metamorphism as temperature and pressure increase.

• Dynamic Metamorphism ;-

                  

                                                            Figure 1.3 ( Mushroom rocks)

                    This type of metamorphism is due to mechanical deformation, like when two bodies of rock slide past one another along a fault zone. Heat is generated by the friction of sliding along the zone, and the rocks tend to crushed and pulverized due to the sliding.  

                    Dynamic metamorphism is not very common and is restricted to a narrow zone along which the sliding occurred. The rock that is produced is called a mylonite.

• Regional Metamorphism -

                          

                                                             Figure 1.4 ( Garnet schist )

This type of metamorphism occurs over large areas that were subjected to high degrees of deformation under differential stress. Thus, it usually results in forming metamorphic rocks that are strongly foliated, such as slates, schists, and gneisses. 

                The differential stress usually result,  lts from tectonic forces that produce a compression of the rocks, such as when two continental masses collide with one another. 

                Thus, regionally metamorphosed rocks occur in the cores of mountain ranges or in eroded mountain ranges, where they are subjected to higher temperatures and pressure. 

               Most  regionally metamorphosed areas can be divided into zones where a particular mineral, called an index mineral, is characteristic of the zone.

              The zones are separated by lines (surfaces in three dimensions) that mark the first appearance of the index mineral. These lines are called  isograds (meaning equal grade) , represent lines (really surfaces) where the grade of  metamorphism is equal.

• Hydrothermal Metamorphism -

                  

                                                             Figure 1.5 ( Chlorite)

 Near oceanic ridges where the oceanic crust is broken up by extensional faults, sea      water can descend along the cracks. Since oceanic ridges are areas 

               where new oceanic crust is created by intrusion and eruption of basaltic magmas,          these water rich fluids are heated by the hot crust or magma and become hydrothermal              fluids. 

             The hydrothermal fluids alter the basaltic oceanic crust by producing hydrous minerals like chlorite and talc. Because chlorite is a green colored mineral the rocks hydrothermal metamorphic rocks are also green and often called greenstones.

•    Subduction Related Metamorphism -

                                      

                                                                          Figure 1.6

At a subduction zone, the oceanic crust is pushed downward resulting in the basaltic crust and ocean floor sediment being subjected to relatively high pressure. But, because the oceanic crust by the time it subducts is relatively cool, the temperatures in the crust are relatively low. Under the conditions of low temperature and high pressure, metamorphism produces an unusual blue mineral, glaucophane. Compressional stresses acting in the subduction zone create the differential stress necessary to form schists and thus the resulting metamorphic rocks are called blueschist

• Shock Metamorphism -

          

                                                               Figure 1.7 ( Fulgurite)

When a large meteorite collides with the Earth, the kinetic energy is converted to heat and a high pressure shock wave that propagates into the rock at the impact site. 

        The heat may be enough to raise the temperature to the melting temperature of the earth rock. 

     The shock wave produces high enough pressure to cause quartz to change its crystal structure to more dense polymorph like coesite or stishovite.

          Ancient meteorite impact sites have been discovered on the basis of finding this evidence of shock metamorphism.