Igneous rocks



 Igneous rock approximately 90% of the earth crust but their great abundance is hidden on the earth surface by a relatively thin layer of sedimentary and metamorphic rocks.  Magma is a hot viscous siliceous melt containing water, vapor, and gases. It comes from the great depth below the Earth surface, is composed mainly of oxygen,  silicon, aluminum,  iron, calcium, magnesium, sodium and, magnesium.
IGNEOUS ROCKS

 when Magma compound on the earth surface, it loses, it's gassed such a Magma is called Lava.


  The occurrence of igneous rocks


Magma is produced deep in the earth crust their temperatures or of the order of 900 to 1600 degree Celsius. it being lighter than the surrounding rocks work its way to earth surface towards the surface on consolidation is a product to a major type of igneous rocks.

  Extrusive rocks and Intrusive rocks


 Extrusive rocks --  when Magma reaches the earth's surface, it causes a volcanic eruption. this eruption generates extensive Lava flow. The Rocks formed due to the solidification of lava are called extrusive rocks. The extrusive rocks are also known as Volcanic rocks. As the lava tends to cool and crystallize rapidly, the texture of Volcanic rocks is generally fine-grained or glossy. During the cooling of Lava, the volatile present in it escape into the atmosphere Volcanic rocks often contain gas cavities called the bicycles vesicles. these rocks sometimes show flow structures, which is the result of a moment n viscous Lava.  it is seen as lines or streaks of different color in rocks

Intrusive rocks  --  these are formed when magma crystallizes beneath the earth surface depending on the depth of formation.
                on the basis of the depth of the formation intrusive rocks are two types

      Plutonic rocks and hypabyssal rocks


 Plutonic rocks --  rocks crystallized at great depth are called plutonic rocks, which is deeply magma which is deeply buried the earth's crust, cool slowly with the retention of the volatile.  as a result, the mineral constituents crystallizing from its have time to grow to considerable size giving the rocks a coarse grain texture.

Hypabyssal rocks --  rocks are formed when magma solidifies close to the Earth surface these rocks occur at injections with the country locks their structures are usually finer grained than those of plutonic rock but coarser than those of Volcanic rocks.  the hypabyssal rocks commonly show porphyritic texture


Important features of igneous rock---


1-- igneous rocks are generally hard, compact, massive with interlocking grains.
2--- the entire absence of fossils.
3-- the absence of bedding planes.
4--- enclosing rocks are backed is usually contain much feldspar.


Textures  --- 


 Texture means the size, shape, and arrangement of mineral grains in a rock. the grain size of an igneous rock depends on the rate of cooling of magma . in general slower is the rate of cooling, the coarser is the grain of rock. In the study of the texture of ur, points are considered. these  points are :


1--Degree of crystallization, 2-- the size of grains    3 -- the shape of crystal and 4--mutual relation between mineral grains.


Degree of crystallization  --  on the basis of the degree of crystallization, textures of igneous rocks can be divided into the following groups.

Holocrystalline texture --  when a rock is made up entirely of crystals, its texture is described as holocrystalline.

Holohyaline texture --  when a rock is composed entirely of glassy material, its texture is called holohyline.

Microcrystalline  texture  --  when a rock is composed partly of crystals and partly of glass, the texture is called  macrocrystalline.

Degree of crystallization   depends on the following factors :

1--Rate of cooling
2--The viscosity of magma ( exam--  composition of the magma and the presence of volatile components).
3--Depth of cooling.
4--The volume of the magma.


Size of grains (granularity) – it refers to the grain size of the crystals presents in the igneous rocks. these  are


(     Phaneric  -- when individual crystals are visible to the naked eyes and are 


(i)                   Coarse grain – when the grain size is 5 mm or above.
(ii)                Medium grain – grain size is 1mm to mm.
(iii)               Fine Grain --  grains are smaller than 1mm in diameter.

(    Aphanitic – when individual grains cannot be distinguished with unaided vision and are

(i)                  Microcrystalline – when individual crystals are distinguishable only under a microscope.
(ii)                 Mero crystalline – intermediate in range.
(iii)               Cryptocrystalline – when individual crystals are too small to be separately distinguished, even under the microscope.
(iv)              Glassy  --  when there is no crystallization at all.

In generals pegmatitic and plutonic rocks are coarse-grained, hypabyssal rock s are medium grained and volcanic rocks are  either fine-grained or glassy .
                       In most natural glasses there are found a number of minute bodies of various shapes, which represent the beginnings of crystals.
They are
(a)    Crystallites – these are embryo crystals.

(b)   Microlites  -- minute crystals, which may exhibit the crystal outline appropriate to their mineralogical nature


The shape of crystals ---  the grains of igneous rock are called euhedral. if they show well-developed crystals faces and if the Crystal faces are partly developed they are described as subhedral. the anhedral is used for those grains in which the crystal faces are absent.


Mutual relations of grains ---  depending on mutual relations of grains textures of igneous rocks may be described into four major groups.

equigranular texture
inequigranular texture
directive textures and
 intergrowth textures


Equigranular texture -- igneous rocks containing mineral grains of more or less equal size are said to have an equigranular texture. equigranular textures are of the following types --


      Panidiomorphic textures--- when most of the grains are euhedral, the texture of rocks is called panidiomorphic. this texture is usually found in lamprophyre.


Hypidiomorphic texture--- when most of the crystals are subhedral, the textures are called hypidiomorphic. this texture is characteristics of many plutonic rocks such as granites and syenites.


 Allotriomorphic texture--- when the most are the crystals are anhedral. the texture is called allotriomorphic. this texture is found in some aplites.


     Micro-granular texture -- microcrystalline igneous rocks may also have an equigranular texture.  the crystals of these find grained rocks are commonly anhedral or subhedral. such a texture is called micro-granular texture

      Orthophyric  texture -- some highly feldspathic rocks such as  orthophyres and plagiophyers the possess  fine-grained panidiomorphic texture is called orthophyric textures 

Felsitic textures -- an igneous rock containing uniform mass of Cryptocrystalline matter is said to have a felsitic texture.


Inequigranular texture ---  igneous rock showing variations in the size of mineral grains are said to have inequigranular textures. these textures have many types which describe in bellow.


 Porphyritic textures -- when the igneous rock contains large crystals of some minerals set in a matrix which is much finer grained or even glasses, the textures are called porphyritic.

                                               
                                                     Porphyritic texture


texture in andesine 


                 the large crystal is called phenocrysts and finer grained material is called groundmass.
texture in dacite

igneous rock showing porphyritic texture is known as porphyry, such as granite porphyry, diorite porphyry, and rhyolite porphyry. this texture develops when some of the crystals grow to a considerable size before the main mass of the magma consolidates into finer and uniform grade material. this texture is found largely in volcanic and hypabyssal rocks.

Poikilitic texture --   when in a rock smaller crystals are enclosed within larger crystals without common orientation, the textures is called poikilitic texture.. this texture is commonly found in syenites and monzonites where orthoclase forms the host mineral.

poikilitic texture in  augite

Ophitic texture -- is a special type of poikilitic texture in which bigger crystals of augite enclose smaller laths of plagioclase .if the plagioclase laths are only partly enclosed in the larger grains of augite, the texture is called sub ophitic. the ophitic texture is characteristic of dolerites.

 Intergranular and instersertal textures ---  in many basalts plasioclase laths occur in such a way that they form a network with triangular or polygonal interspaces. these interspaces are filled with minutes grains augite olivine and iron oxide such a texture is called intergranular textures. when glassy or fine grained chloritic or  serpentinous  materials occur in the interspace ,the texture is called intersertal.


Directive texture -- the textures produced as a result of lava during their consolidation are called directive textures.  the major directive texture is as follows  --




 Trachytic texture -- certain volcanic rocks, such as trachytic contain felspar laths arranged in lines parallel to the direction of flow of Lava. such a texture is called the trachytic texture.

Hyalopilitic textures
 ---    in a volcanic rock if felspar laths are found intermixed with glass, the texture is called hyalopilitic texture.



Intergrowth --


graphic intergrowth-- these intergrowths make between orthoclase and quartz.

perthitic intergrowth -- these intergrowths make between albite and orthoclase.

myrmekite intergrowth --   these intergrowths make between quartz and feldspar.

granophyric is the graphics texture found in hypabyssal and volcanic rocks of granitic composition.