Table of Contents
Magma, Volcanic rocks, IGNEOUS ROCKS, Plutonic Rocks, Hypabyssal Rocks
Igneous rocks are those that have developed from a hot, molten substance through the processes of cooling and crystallization.
Important Conditions For The Original Material
very high temperature and a molten state
1 Magma is the term for the naturally occurring, molten, hot material beneath the Earth’s surface.
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 (types of igneous rocks)
These igneous rocks were created on Earth’s surface when lava from volcanoes cooled and crystallized. The crystals formed in these rocks have very small, often microscopic grains because the lava cools down much more quickly than magma does. Lava may also cool on the surface or even below the surface of seas and oceans, with the latter process being more typical.
These rocks are igneous and were created at great depths, usually between 7-10 kilometres below the earth’s surface. The magma produces coarse-grained rocks because the rate of cooling is so slow at these depths. These rocks are exposed on the earth’s surface as a result of the underlying strata being eroded. Some examples of Plutonic rocks include granites, syenites, and gabbros.
These igneous rocks exhibit characteristics of both volcanic and plutonic rocks and are formed at intermediate depths, usually up to 2 km below the surface of the earth. Examples of hypabyssal rocks include porphyries of various compositions.
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 relationship between various mineralogical components in a rock is described as the term texture. It is based on how these components are arranged, sized, and shaped within the rock’s core.
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 insize 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.
The average dimensions of the various constituent minerals are taken into consideration to describe the grain size of the rock as a whole. This is known as the grain size of the various components of a rock. As a result, the rock’s texture is defined as:
(i) Coarse-grained. When the average grain size is above 5 mm; the constituentminerals
are then easily identified with naked eye.
(ii) Medium-grained. When the average grain size lies between 5 mm and1 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
This is a collective term that describes both the degree of crystalline perfection in the individual minerals’ crystals as well as the relative grain sizes of the various mineral constituents in a rock. When all of the component minerals in the rock have roughly equal dimensions, the texture is said to be equigranular. Conversely, it is said to be inequigranular when some minerals are noticeably larger or smaller than the others. Similar to how the shape or form of the crystals can be described as perfect, semi-perfect, or completely irregular under a microscope and is best seen in thin sections. Euhedral, subhedral, and anhedral are the respective textural terms used to describe these shapes. Any type of crystal may be present in an igneous rock 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
Equigranular textures are any textures in which the majority of a rock’s constituent crystals are roughly equal in size. These textures are frequently referred to as granitic and felsitic textures because they are found in igneous rocks where granites and felsites exhibit them. And The constituents of the granitic texture are either entirely coarse- or entirely medium-grained, and the crystals exhibit euhedral to subhedral outlines. The rock has a felsitic texture, which is microgranular with most of the grains being microscopic crystals that always have perfectly defined edges. Thus, equigranular and panidiomrphic textures can be used to describe felsitic materials. Another equigranular texture that falls between the granitic and felsitic textures is orthophyric.
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.
The relatively insoluble constituents form the phenocrysts because during the crystallization process, their crystal grains enlarge while crystals of other soluble constituents become re-mixed with the magma. And the ground mass crystallises toward the end with the soluble components.
(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
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 areessentially 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 inwhich 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 planesof 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.
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.
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.
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.
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.
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.
IMPORTANT IGNEOUS ROCKS
Definition Granites may be defined as plutonic light coloured igneous rocks.
These are among the most common igneous rocks.
Composition. Two most common and essential mineral constituents of granite are: Quartz and Felspar.
Quartz is always recognized by its glassy lustre, high hardness (H = 7), and cleavage less transparent white appearance.
Felspars making granites may be of two varieties: the potash felspars, commonly orthoclase and the soda-bearing felspars like albite and oligoclase.
Felspar microcline may also be present in some granites. Among the accessory minerals in granites, micas deserve first mention. Both varieties
(muscovite or white mica and biotite or black mica) are present in small proportions in most apatite, garnet and tourmaline.
Granites are generally coarse to medium grained, holocrystalline (phaneric) and equigranular rocks. Granitic, graphic, porphyritic and intergrowth textures are the most common types of textures met with in granites of different varieties.
As regards structures, granites occur in large massive bodies, often as batholiths, stocks and bosses beside in usual intrusive bodies like sills and dykes.
Many types of granites are distinguished on the basis of relative abundance in them of some particular accessory mineral.
For instance, when white mica, muscovite is present as a prominent accessory mineral, the granite may be distinguished as muscovite granite.
Similarly, when it is the black mica or biotite, which is a prominent accessory
mineral, the granite may be called a biotite-granite. When both the biotite and muscovite are present
Many types of granites are distinguished on the basis of relative abundance in them of some particular accessory mineral.
For instance, when white mica, muscovite is present as a prominent accessory mineral, the granite may be distinguished as muscovite granite.
Similarly, when it is the black mica or biotite, which is a prominent accessory mineral, the granite may be called a biotite-granite. When both the biotite and muscovite are present
Granites are the most widely distributed igneous rocks in the crust of the earth.
They occur chiefly as deep-seated intrusive bodies like sills, bosses, stocks and batholiths.
Their occurrence on the surface of the earth is attributed to prolonged weathering
and erosion of the overlying strata through historical times running over millions of years.
Megasacopic Identification. Granites may be identified in hand specimens by their:
(i) Light-coloured (leucocratic) appearance, such as grey, pink, brownish and yellowish.Some of the shades may take brilliant polish to make it eminently
suitable as a decorative building stone.
(ii) Coarse to medium-grained texture; fine-grained granites are rare specimens. (iii) Abundance of quartz and felspar orthoclase as essential minerals.
Granites find extensive use in architectural and massive construction where they are found in abundance.
These rocks have been used extensively in monuments and memorials, as columns and steps and as flooring in buildings.
Many minor granitic bodies occurring as sills and similar masses are clearly of igneous plutonic origin.
Their formation from parent magma through the normal process of cooling and crystallisation is easily accepted.
But exceptionally large bodies like batholiths and stocks and bosses running over hundreds of square kilometers close to or on the surface are not accepted by many as simple igneous
intrusions mainly because of their extensive dimensions.
These large granitic masses are believed by many to have been formed from pre-existing rocks through the process of granitization.
Following variations appear in the composition of these rocks:
the relative proportion of quartz (Si02) falls gradually so that in diorites it is reduced to a subordinate
felspar orthoclase, which is a dominant mineral in granites, is reduced in relative amount and replaced by felspar plagioclase in granodiorites.
In diorites, it is felspar plagioclase that makes the bulk of felspar constituent. A number of rock types get distinguished on the basis of this variation.
For example, adamellite is a variety of granodiorites that contains felspar orthoclase and plagioclase in equal proportion.
Definition. It is an intermediate type of igneous rock of plutonic origin with silica percentage generally lying between 52-66 per cent.
Diorites are typically rich in felspar plagioclase of sodic group (e.g. Albite).
Besides plagioclase and alkali felspars, diorites also contain accessory minerals like hornblende, biotite and some pyroxenes.
Quartz is not common but may be present in some varieties that are then specially named as quartz-diorites.
In texture, diorites show quite close resemblance to granites and other plutonic, rocks. They are coarse to medium grained and holocrystalline.
Diorites commonly occur as small intrusive bodies like dikes, sills, stocks and other irregular intrusive masses.
They also get formed at the margins of bigger igneous masses.
These are volcanic rocks in which plagioclase felspars (sodic and sub-calcic varieties like albite, andesine and labradorite) are the predominant constituents making the potash felspar only a subordinate member.
Besides plagioclase and potash felspars, andesites may contain small amount of quartz as well as biotite, hornblende, augite, olivine and hypersthene from the dark minerals giving them an overall grayish or darker appearance.
Andesites are known to be quite abundant volcanic rocks, next only to basalts and may occur as crystallized lava flows of extensive dimensions.
Petrologists are sharply divided over the origin of andesites. Some believe them to be the products of normal crystallisation from a mafic magma whereas others think that some andesites may be the products from mixed magmas or magmas enriched with fragments from the wall rocks.
The second view is supported by the presence of some foreign materials in andesites.
Syenites are defined as igneous, plutonic, even-grained rocks in which alkalifelspars (including orthoclase and albite) are the chief constituent minerals.
They may contain, besides these essential constituents, dark minerals- like biotite, hornblende, augite and some accessories
The most common felspars of syenites are orthoclase and albite; microcline, oligoclase and anorthite are also present in them in subordinate amounts.
In some syenites, the felspathoids (nepheline, leucite) also make appearance.
Common accessory minerals occurring in syenites are apatite, zircon, and sphene.
Quartz so common in granites is altogether absent or is only a minor accessory in syenites.
Syenites show textures broadly similar to those of granites, that is, they are coarse to medium- grained, holocrystalline in nature and exhibiting graphic, inter- grown or porphyritic relationship among its constituents.
These are igneous rocks of typically hypabyssal origin having formed as shallow sills and dykes
They may be regarded as equivalents of gabbros of plutonic origin and basalts of volcanic origin.
Dolerites are predominantly made up of calcic plagioclase (e.g. anorthite and labradorite).
Dark minerals like augite, olivine and iron oxide etc. are also present in good proportion in dolerites along with the plagioclase minerals.
Dolerites are mostly medium to fine grained rocks.
Ophitic and porphyritic textures are quite common in many dolerites.
Sills and dykes of doleritic composition have been recorded at many places associated with magmatic activity.
In the Singhbhum region of south Bihar, India, many doleritic dykes traverse the Singhbhum granites.
Basalts are volcanic igneous rocks formed by rapid cooling from lava flows from volcanoes either over the surface or under water on oceanic floors. They are basic in character. .
Basalts are commonly made up of calcic plagioclase felspars (anorthite and labradorite) and a number of ferro-magnesian minerals like augite, hornblende, hypersthene, olivine, biotite and iron oxides etc.
In fact many types of basalts are distinguished on the basis of the type and proportion of ferro- magnesian minerals in them.
Thus, for instance, Basanite is an olivine-rich basalt and Tepherite is an olivine-free type basalt.
The olivine free basalts, that are quite abundant in occurrence, are sometimes named collectively as Tholeiites.
Basaltic rocks form extensive lava flows on the continents and also on the oceanic floors in almost all the regions of the world.
In India, the Deccan Traps, which are of basaltic and related rocks, are spread over more than four hundred thousand square kilometers in Maharashtra, Gujarat, Madhya Pradesh and adjoining parts of Indian Peninsula.
These are exceptionally coarse-grained igneous rocks formed from hydrothermal solutions emanating from magmas that get cooled and crystallized in cavities and cracks around magmatic intrusions.
These rocks are searched for their containing big sized crystals of minerals. Some of these crystals may be gems and other precious minerals.
Pegmatites exhibit great variation in their mineral composition.
The granite pegmatites contain alkali felspars and quartz as the dominant minerals Crystals of some minerals in exceptionally big sizes have been found from pegmatites at many places.
Texture and Structure
Pegmatites do not show any special textures and structures except that they are invariably coarse grained and mostly inequigranular.
And In many pegmatites, the so-called complex pegmatites, a zonal structure is commonly observed.
In such cases, different minerals of pegmatite occur in different zones starting from the periphery and proceeding towards the centre.
In a five-zoned pegmatite, for instance, the outermost zone is made up of muscovite and felspar,. the second zone is of quartz and felspar, third zone of microcline and fourth of quartz. The central zone is ploymineralic containing albite and spodumene besides quartz and mica.
Petrologically, pegmatites of complex composition are known to occur.
First. Pegmatites have been formed from magmatic melts towards the endof the process of crystallisation, The hydrothermal factions left behind at this stage are capable of taking in solution all metallic and non-metallic components by virtue of their temperature, pressure and chemical reactivity. Most of the granite- and syenite -pegmatites are believed to have been formed through this mode.
Second. Pegmatites have formed due to replacement reactions between the hydrothermal solutions and the country rock through which these liquids happen to pass.
Hydrothermal liquids at elevated temperatures are considered quite effective in replacing original minerals by new minerals.
Pegmatites occur in a variety of forms as dykes, veins, lenses and patches of irregular masses.
Pegmatites are the source of many precious stones, gems, ores of rare-earths and heavy metals besides the industry grade muscovite mica.
These are igneous rocks of plutonic origin but characterized with a fine-grained, essentially equigranular, allotriomorphic texture.
Essential minerals of the aplites are the same as that of granites, that is, felspars and quartz.
They commonly occur as dykes and are formed from magmas that have different gaseous content compared to magmas from which granites are formed.
Panidiomrphic (in which most of crystals show perfect outline), fine grained and holocrystalline.
Lamprophyres show a great variation in their mineralogical composition.
Mostly they are rich in ferro-magnesian silicates. Important minerals forming lamprophyres are:
biotite, augite and other pyroxenes, hornblende and other amphiboles, felspars and olivine.
Many types of lamprophyres are distinguished on the basis of the type of felspar
and the dark minerals occurring in them.
Thus, Minette is, a lamprophyre containing felspar orthoclase and the black mica, biotite; Vogesite is another variety having felspar orthoclase and augite or hornblende.
The term peridotite is commonly used to express the ultra-mafic igneous rocks that are highly rich in a ferro-magnesian mineral OLIVINE, which has a composition of (Mg,Fe)Si04.
The chief characteristics of peridotites are:
(i) Low silica index; such rocks invariably contain less than 45% silica.
High colour index; rich as they are in dark minerals, the colour indexof peridotites is always above 70, generally in the range of 90-100.
Texture. Peridotites are generally massive and coarse grained in texture.
Varieties. A number of types of peridotites are distinguished on the basis of the accessoryminerals, e.g. hornblende-peridotite, pyroxene-peridotite etc. Kimberlite is a peridotite in which olivine is altered to serpentine.
Occurrence. Peridotites generally form sills and dykes of moderate size.
A number of modes of origin have been suggested for peridotites.
Hess believes them to be the products of primary peridotitic magma, a view that is very strongly objected by many others.
Another view holds them having been formed from a primary basic (basaltic) magma from which olivine and other mafic minerals were separated by some process.
A third possibility suggested regards the development of peridotite bodies simply as a result of hydrothermal (pneumatolytic) transport of their material and its subsequent reaction with rocks of appropriate composition.
Many of igneous rocks, where available in abundance, are extensively used as materials for construction.
Granites, syenites and dolerites are characterized by very high crushing strengths and hence can be easily trusted in most of construction works.
Basalts and other dark coloured igneous rocks, though equally strong, may not be used in residential building but find much use as foundation and road stones.
The igneous rocks are typically impervious, hard and strong and form very strong foundations for most of civil engineering projects such as dams and reservoirs.
They can be trusted as wall and roof rocks in tunnels of all types unless traversed by joints. At the same time, because of their low porosity, they cannot be expected to hold oil or groundwater reserves.
Some igneous rocks like peridotites and pegmatites are valuable as they may contain many valuable minerals of much economic worth.