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Igneous Rocks

Igneous Rocks
Rocks that have formed from an originally hot molten material through the process of cooling and crystallization may be defined as igneous rocks.

 

IGNEOUS ROCKS

 

                       Rocks that have formed from an originally hot molten material through the process of cooling and crystallization may be defined as igneous rocks.

 

Important Conditions For The Original Material

                       very high temperature and

 

                       a molten state

 

COMPOSITION

 

Magma

 

1 The hot molten material occurring naturally below the surface of the Earth is called magma.

2 It is called lava when erupted through volcanoes.

 

3 Igneous rocks are formed both from magma and lava.

4 It maybe mentioned here that magma is actually a hypothetical melt.

 

5 Lava is a thoroughly studied material that has poured out occasionally from volcanoes in many regions of the world again and again.

 

6 Magma or lava from which igneous rocks are formed may not be entirely a pure melt: it may have a crystalline or solid fraction and also a gaseous fraction thoroughly mixed with

it.

 

7 The solid and gaseous fractions, however, form only a small part of the magma or lava, which are predominantly made up of liquid material igneous rock.

 

Igneous rocks are divided into following three sub-groups

 

Volcanic rocks

 

        These are the igneous rocks formed on the surface of the Earth by cooling and crystallisation of lava erupted from volcanoes.

 

    Since the lava cools down at very fast rate (compared to magma), the grain size of the crystals formed in these rocks is very fine, often microscopic.

 

Further, cooling of lava may take place on the surface or even under waters of seas and oceans, the latter process being more common.

 

Plutonic Rocks

 

              These are igneous rocks formed at considerable depths-generally between 7-10 km below the surface of the earth.

 

              Because of a very slow rate of cooling at these depths, the rocks resulting from magma are coarse grained.

 

            These rocks get exposed on the surface of the earth as a consequence of erosion of the overlying strata.

            Granites, Syenites, and Gabbros are a few examples of Plutonic rocks.

 

Hypabyssal Rocks

 

               These igneous rocks are formed at intermediate depths, generally up to 2 kms below the surface of the earth and exhibit mixed characteristics of volcanic and plutonic rocks.

 

            Porphyries of various compositions are examples of hypabyssal rocks.

 

 

COMPOSITION

 

Mineralogical composition

 

Igneous rocks like other rock groups are characterised by the abundance of only a few, minerals.

S.No      Mineral                    (%)      S.No   Mineral        (%)     

                                                                          

(i)           Felspars         59.5  

(ii)          Pyroxenes & Amphiboles 16.8  

(iii           Quartz           12.0  

(iv)         Biotite            3.8                             

(v)          Titanium       1.5     

(vi)         Apatite          0.6     

(vii         Accessory Minerals            5.8     

 

 

1. TEXTURES OF IGNEOUS ROCKS

 

                       The term texture is defined as the mutual relationship of different mineralogical constituents in a rock. It is determined by the size, shape and arrangement of these constituents within the body of the rock.

 

Factors Explaining Texture

 

The following three factors will primarily define the type of texture in a given igneous

 

rock:

Degree of Crystallization

 

In an igneous rock, all the constituent minerals may be present in distinctly crystallized forms and easily recognized by unaided eye, or, they may be poorly crystallized or be even glassy or non- crystallized form.

 

                       The resulting rock textures are then described as:

 

(i)                                     Holocrystalline: When all the constituent minerals are distinctly crystallized;

 

(ii) Holohyaline: When all the constituents are very fine in size and glassy or non crystalline in nature.

 

       The term merocrystalline is commonly used to express the intermediate type, i.e. when some minerals are crystallized and others are of glassy character in the same rock.

   Rocks with holocrystalline texture are also termed as phaneric and the holohyaline rocks

 

are referred as aphinitic. The term microcrystalline is used for the textures in which the minerals are perceivably crystallized but in extremely fine grain.

 

Granularity

 

                       The grain size of the various components of a rock are the average dimensions of different constituent minerals which are taken into account to describe the grain size of the rock as a whole. Thus the rock texture is described as :

 

(i)           Coarse-grained. When the average grain size is above 5 mm; the constituent minerals

 

are then easily identified with naked eye.

 

(ii)                   Medium-grained. When the average grain size lies between 5 mm and 1 mm. Use of magnifying lens often becomes necessary for identifying ail the constituent mineral components.

 

(iii) Fine-grained. When the average grain size is less than 1 mm. In such rocks, identification of the constituent mineral grains is possible only with the help of microscope for which very thin rock sections have to be prepared for microscopic studies

 

Fabric

 

     This is a composite term expressing the relative grain size of different mineral constituents in a rock as well as the degree of perfection in the form of the crystals of the individual minerals.

 

     The texture is termed as equigranular when all the component minerals are of approximately equal dimensions and as inequigranular when some minerals in the rock are exceptionally larger or smaller than the other.

 

  Similarly, the shape or form of the crystals, which is best seen only in thin sections under microscope, may be described as perfect, semi perfect or totally irregular. The textural terms to describe these shapes are, respectively, euhedral, subhedral and anhedral.

 

                       An igneous rock may contain crystals of anyone type in a predominating proportion;

 

hence its fabric will be defined by one of the following three terms related to fabric:

 

(i)                      Panidiomrphi: when majority of the components are in fully developed shapes;

 

(ii)                   Hypidiomorphic: the rock contains crystals of all the categories: euhedral, subhedral or anhedral;

 

(iii)                 Allotriomorphic: when most of the crystals are of anhedral or irregular shapes

 

Types of Textures

 

These can be broadly divided into five categories:

 

. Equigranular textures

. Inequigranular textures

 

. Directive textures

. Intergrowth textures and

 

. Intergranular textures.

 

(1)                   Equigranular Textures

 

              All those textures in which majority of constituent crystals of a rock are broadly equal in size are described as equigranular textures.

 

             In igneous rocks, these textures are shown by granites and felsites and hence are also often named as granitic and felsitic textures

              In  the  granitic  texture,  the  constituents  are  either  all  coarse  grained  or  all

 

medium grained and the crystals show euhedral to subhedral outlines.

 

                    In the felsitic texture, the rock is micro granular, the grains being mostly microscopic crystals but these invariably show perfect outlines.

             Thus felsitic textures may be described as equigranular and panidiomrphic.

 

Orthophyric texture is another type of equigranular texture, which is in between

the granitic and felsitic textures. The individual grains are fine in size but not micregranular.

 

(2)                   Inequigranular Texture

 

                Igneous textures in which the majority of constituent minerals show marked difference in their relative grain size are grouped as inequigranular texture.

             Porphyritic and Poiklitic textures are important examples of such textures.

 

             Porphyritic Texture is characterised by the presence of a few conspicuously large sized crystals

 

(the phenocrysts) which are embedded in a fine-grained ground mass or matrix.

 

              The texture is sometimes further distinguished into mega-porphyritic and microporphyritic depending upon the size of the phenocrysts.

 

Difference in. molecular concentration

 

When the magma is rich in molecules of particular mineral, the latter has better chance to grow into big crystals which may get embedded in the fine-grained mass resulting from the deficient components.

 

 

Change in physico-chemical conditions.

 

                            Every magma is surrounded by a set of physico-chemical conditions like temperature, pressure and chemical composition, which influence the trend of crystallisation greatly.

 

                                 Abrupt and discontinuous changes in these textures may result in the formation of the crystals of unequal dimensions.

                           Thus, magma crystallizing at great depths may produce well-defined, large sized crystals.

 

                                  When the same magma (carrying with it these large crystals) moves upward, the pressure and temperature acting on it are greatly reduced.

 

                            Crystallisation in the upper levels of magma becomes very rapid resulting in a fine-grained matrix that contains the big sized crystals formed earlier.

 

 

 

 

 

Relative insolubility

 

      During the process of crystallisation, their crystal grains get enlarged whereas crystals of other soluble constituents get mixed up again with the magma; thus, the relatively insoluble constituents form the phenocrysts

 

And the soluble constituents make up the ground mass crystallizing towards the end.

 

(3)                   Directive Textures

 

  The textures that indicate the result of flow of magma during the formation of rocks

are known as directive textures.

 

     These exhibit perfect or semi perfect parallelism of crystals or crystallites in the direction of the flow of magma.

   Trachytic and Trachytoid textures are common examples.

 

     The former is characteristic of certain felspathic lavas and is recognised by a parallel arrangement of felspar crystals; the latter is found in some syenites.

(4)                   Intergrowth Textures

 

                 During the formation of the igneous rocks, sometimes two or more minerals may crystallize out simultaneously in a limited space so that the resulting crystals are mixed up or intergrown.

               This type of mutual arrangement is expressed by the term intergrowth texture.

               Graphic and granophyric textures are examples of the intergrowth textures.

 

                    In graphic texture, the intergrowth is most conspicuous and regular between quartz and felspar crystals. In granophyric textures the intergrowth is rather irregular.

 

(5)                   Intergranular Textures

 

     In certain igneous rocks crystals formed at earlier stages may get so arranged that polygonal or trigonal spaces are left in between them.

          These spaces get filled subsequently during the process of rock formation by crystalline or glassy

 

masses of other minerals.

 

The texture so produced is called an intergranular texture. Sometimes the texture is specifically termed intersertal if the material filling the spaces is glassy in nature.

 

2.FORMS OF IGNEOUS ROCKS

 

An igneous mass will acquire on cooling depends on a number of factors such as

 

(a)                   the structural disposition of the host rock (also called the country rock)

 

(b)                   the viscosity of the magma or lava

 

(c)                    the composition of the magma or lava

 

(d)                   the environment in which injection of magma or eruption of lava takes place.

 

It is possible to divide the various forms of igneous intrusions into two broad classes:

 

All those intrusions in which the magma has been injected and cooled along or parallel to the structural planes of the host rocks are grouped as concordant bodies.

 

Forms of concordant bodies Sills

 

                                  The igneous intrusions that have been injected along or between the bedding planes or sedimentary sequence are known as sills.

 

                               It is typical of sills that their thickness is much small than their width and length. Moreover, this body commonly thins out or tapers along its outer margins.

 

The upper and lower margins of sills commo11ly show a comparatively finer

grain size than their interior portions. This is explained by relatively faster cooling of magmatic injection at

 

these positions

 

In length, sills may vary from a few centimeters to hundreds of meters

 

Sills are commonly subdivided into following types:

 

(a)                   Simple Sills: formed of a single intrusion of magma;

 

(b)                   Multiple Sills: which consist of two or more injections, which are essentially of the same kind of magma;

 

(c)                    Composite Sills: which result from two or more injections of different types of magma;

 

(d)                   Differentiated Sills: these are exceptionally large, sheet-like injections of magma in which there has been segregation of minerals formed at various stages of crystallisation into separate layers or zones.

 

(e)Interformational Sheets: the sheet of magma injected along or in between the planes of unconformity in a sequence are specially termed as interformational sheets. These resemble the sills in all other general details.

 

                 These arecordant, small sized intrusive that occupy positions in the troughs and crests of bends called folds. In outline, these bodies are doubly convex and appear crescents or half-moon shaped in cross-section.

 

                  As regards their origin, it is thought that when magma is injected into a folded sequence of rocks, it passes to the crests and troughs almost passively i.e. without exerting much pressure.

 

Phacoliths


Lacocothis

 

                            These are concordant intrusions due to which the invaded strata have been arched up or deformed into a dome.

                            The igneous mass itself has a flat or concave base and a dome shaped top.

 

                                Laccoliths are formed when the magma being injected is considerably viscous so that it is unable to flow and spread for greater distances.

 

Instead, it gets collected in the form of a heap about the orifice of eruption. As the magma is injected with sufficient pressure, it makes room for itself by arching up the overlying strata.

 

                        Extreme types of laccoliths are called bysmaliths and in these the overlying strata get ultimately fractured at the top of the dome because of continuous injections from below.

 

Lopoliths

 

               Those igneous intrusions, which are associated with structural basins, that are sedimentary beds inclined towards a common centre, are termed as lopoliths.

.

 

              It is believed that in the origin of the lopoliths, the formation of structural basin and the injection of magma are "contemporaneous", that is, broadly simultaneous.

 

 

DISCORDANT BODIES

 

All those intrusive bodies that have been injected into the strata without being influenced by their structural disposition (dip and strike) and thus traverse across or oblique to the bedding planes etc. are grouped as discordant bodies.

 

Important types of discordant intrusions are dykes, volcanic necks and batholiths.

 

 

These may be defined as columnar bodies of igneous rocks that cut across the bedding plane or unconformities or cleavage planes and similar structures.

 

Dykes are formed by the intrusion of magma into pre-existing fractures.

 

It depends on the nature of magma and the character of the invaded rock whether the walls of the fracture are pushed apart, that is, it is widened or not.

 

            Dykes show great variations in their thickness, length, texture and composition.

 

            They may be only few centimeters or many hundreds of metes thick.

 

              In composition, dykes are generally made up of hypabyssal rocks like dolerites, porphyries and lamprophyres, showing all textures between glassy and phaneritic types.

 

            Cone sheets and Ring Dykes may be considered as the special types of dykes.

 

                The cone sheets are defined as assemblages of dyke-like injections, which are generally inclined towards common centres.

            Their outcrops are arcuate in outline and their inclination is generally between 30 o - 40 o .

 

            The outer sheets tend to dip more gently as compared to the inner ones

 

            Ring Dykes are characterised by typically arcuate, closed and ring shaped outcrops.

 

             These may be arranged in concentric series, each separated from the other by a screen of country rock.

 

They show a great variation in their diameter; their average diameter is around 7 kilometers. Few ring dykes with diameters ranging up to 25 kms are also known.

 

                    Origin of dykes It has been already mentioned that dykes are intrusions of magma into pre- existing fractures present in the rocks of the crust.

                These original fractures are generally caused due to tension.

 

                  Their original width might have been much less than the present thickness of the dykes.

 

                   This indicates widening of the cracks under the hydrostatic pressure of magmatic injection.

 

 

Volcanic Necks

                In some cases vents of quiet volcanoes have become sealed with the intrusions.

 

                Such congealed intrusions are termed volcanic necks or volcanic plugs.

 

                In outline these masses may be circular, semicircular, or irregular and show considerable variation in their diameter. The country rock generally shows an inwardly dipping contact.

 

Batholiths

 

  These are huge bodies of igneous masses that show both concordant and discordant relations with the country rock.

 

Their dimensions vary considerably but it is generally agreed that to qualify as a batholith the igneous mass should be greater than 100 square kilometers in area and its depth should not be

 

traceable. This is typical of batholiths: they show extensive downward enlargement

 

                 In composition, batholiths may be made of any type of igneous rock.

 

                     They also exhibit many types of textures and structures. But as, a matter of observation, majority of batholiths shows predominantly granitic composition, texture and structure.

 

 

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