METAMORPHISM

METAMORPHISM:
The term metamorphic is derived from the Latin term meaning, "change of form”. It can be defined as mineralogical, chemical & crystallographic changes in a solid-state rock, in response to new conditions of pressure & or temperature & or introduction of fluids, causing the existing rocks minerals to recrystalize. They may even become different minerals all together. It is important to realize that the rock remains in its solid state. Once rock minerals are melted, and then recrystallize, the new material will be igneous. Temperature change can range from 100 c to 900 c.
METAMORPHISM can be of four types:
· Regional metamorphism:
This type of metamorphism occurs in broad areas of earth’s crust. Regional metamorphism deforms the rocks strongly. This type of metamorphism is further classified in two categories Barrovian or Buchan type metamorphism depending upon temperature and pressure gradients.
· Impact metamorphism :
This type pf metamorphism occurs in ultrahigh pressures & low temperature which are the result of collision of a meteorite or extraterrestrial object or during an extremely violent volcanic eruption.
· Contact(thermal) metamorphism :
This occurs typically around igneous intrusive rocks. As a result of the temperature increase caused by igneous body itself. Magma fluids coming from the intrusive rock may also take part in contact metamorphism. Contact metamorphic rocks are known as hornfels and usually are fine grained.

· Hydrothermal metamorphism :
Hydrothermal metamorphism is the result of the interaction of a rock with high temperature fluid of distinct composition. This metamorphism results from reaction between protolith (original rock before transformation) & fluid. This kind of metamorphism is responsible for many metal deposits on earth.
We have studied metamorphism now we will be able to define metamorphism rocks more systematically.
Metamorphic Rocks results from mineralogical and structural adjustments of solid rocks to physical and chemical conditions differing from those under which the rocks originally formed. Temperature and pressure are important factor in the formation of metamorphic rocks.

Classification of Metamorphic Rocks:
Metamorphic rocks can be subdivided into foliated and non-foliated types. This is determined by the presence of minerals that are aligned parallel to each other. This results in a layered appearance. A non-foliated metamorphic rock generally consists of equal dimensional grains.
A rock can also be identified by its metamorphic grade. This is simply a description of the overall intensity of metamorphism the rock was subjected to. What this implies is that a low-grade metamorphic rock shows textural or mineralogic evidence of having been subjected to low pressures and/or temperatures.
Metamorphisms produced with increasing pressure and or temperature is known as PROGRADE METAMORPHISM. Conversely decreasing temperature and or pressure characterize RETROGRADE METAMORPHISM.
Slates, marbles & Quartzite are example of metamorphic rocks.
IGNEOUS ROCKS:
Igneous rocks are formed from solidification of molten magma erupted through the mantle or crust of earth. As this magma in earth is less dense then surrounding solid rocks, it rises up towards surface. This lava has two options depending upon it’s surrounding conditions either it may erupt at the surface from the volcano’s mouth or it may settle within the earth’s crust. This classifies the types of igneous rocks.
1. Intrusive or plutonic igneous rocks are the rocks, which results due to solidification of lava deep within earth.
2. Extrusive or volcanic rocksare the rocks, which results due to solidification or crystallization of lava on the outer surface of earth.
These two types of igneous rocks can be identified by visualizing them Since slow cooking of “intrusive rocks” promote the growth of mineral beds which can be identified without microscope. While in extrusive igneous rocks these minerals cannot be seen by naked eye. If molten magma cools at extremely high rates (as in extrusive igneous stones) disallowing crystallization, the result is a volcanic glass called obsidian.
Igneous rock textures .
Phaneritic
The size, shape and arrangement of the mineral grains of any rock is called its texture . Generally, if magma cools slowly, its mineral crystals will have more time to grow and the resulting rock will demonstrate a course or phaneritic texture. This texture will be more characteristic of intrusive rocks and you will be able to see the mineral grains .
Aphanitic
On the other hand, if the magma or lava cools quickly, the matrix of tiny crystals that are formed give the rock a fine or aphanitic texture. Fine textured rocks are usually volcanic or shallow intrusive. These rocks cool so quickly that you will usually not see the individual mineral grains with an unaided eye.
Porphyrhitic
A third type of texture occurs when a molten mass begins to cool slowly, and some crystals of one mineral begin to form. Sometimes this material is then moved (or erupted) and the remaining material cools quickly. The resulting rock will have large crystals that are in a finegrained groundmass. This is called a porphyrhitic texture, and the rock is porphyry.
Igneous rocks are also classified by the amount of silica they contain .
IGNEOUS ROCKS
ACIDIC
A ROCK THAT CONTAINS MORE THAN 66% SILICA
INTERMEDIATE
A ROCK THAT CONTAINS 52% TO 66% SILICA
BASIC
A ROCK CONTAINING 45% TO52% SILICA
ULTRABASIC
A ROCK CONTAINING LESS THAN 45% SILICA
As a rule, acidic and intermediate rocks form by the solidification of molten crystal material. Basic rocks more often form from molten material brought up from the mantle.
BY COMBINING THE TEXTURE AND THE CHEMICAL COMPOSITION OF A ROCK WE CAN DEFINE THE MOST COMMON IGNEOUS ROCKS.
COMMON IGNEOUS ROCKS

EXTRUSIVE
INTRUSIVE
ACIDIC
RHYOLITE
GRANITE
INTERMEDIATE
ANDESITE
DIORITE
BASIC
BASALT
GABBRO
ULTRABASIC

PERIDOTITE A melt goes through many stages as it solidifies in a process called fractionation. As the melt cools the minerals that generally crystallize first are those low in silica such as olivine and pyroxene. The remaining melt becomes more silica rich. This may then be erupted and form acidic rocks.