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Term Paper on Igneous Rocks
Term Paper # 1. Definition of Igneous Rocks:
Broadly speaking, all rocks that have formed from an originally hot molten material through the process of cooling and crystallisation may be defined as igneous rocks.
A very high temperature and a molten state are, therefore, two very important conditions for the original material from which the igneous rocks are believed to have been formed.
The hot molten material occurring naturally below the surface of the Earth is called magma. It is called lava when erupted through volcanoes. Igneous rocks are formed both from magma and lava. It may be mentioned here that magma is actually a hypothetical melt. It has not been possible to see it at its place of occurrence.
But it is assumed to get formed at great depths below the surface due to very high temperature related to a number of causes such as rise in temperature with depth and also occurrence of radioactive materials. Lava is, however, a thoroughly studied material that has poured out occasionally from volcanoes in many regions of the world again and again. It provides with ample proof of existence of the magma below the surface.
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. The solid and gaseous fractions, however, form only a small part of the magma or lava, which are predominantly made up of liquid material. In most cases magma is believed to be a sufficiently mobile melt.
In fact this mobility is one important quality that enables it to cool down to igneous rocks. This is because magma can exist as a melt as long as physical and chemical environment surrounding it remains unchanged. But as and when there is a change in one or more of these conditions (e.g. fall in temperature or pressure due to its upward movement), cooling and crystallisation of magma may start and end up with the formation of an igneous rock.
Igneous rocks are divided into following three sub-groups:
(a) 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.
The Deccan Traps of India spread over more than four lakh square kilometers in Peninsular India provide the best example of volcanic rocks. The Panjal Traps in Kashmir Himalayas provide another example. The basalts of Columbia Plateau and the Prana plateau of Brazil are other well-known examples.
(b) 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. They occur in good abundance in both the Peninsular and extra-Peninsular India. In the latter case they form the lower regions of deep Himalayan valleys.
(c) 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.
Term Paper # 2. Composition of Igneous Rocks:
(a) Chemical Composition:
Igneous rocks show a great variation in chemical composition and in fact no strict generalization is possible. Most reliable data is due to Clark and Washington who have shown that on an average the following elements (expressed in percentage terms of their oxides) are present in the igneous rocks.
From the above table, it is clear that silica is the dominant constituent of the igneous rocks. Subsequent chemical analyses as well as field observations have broadly confirmed this conclusion.
(b) Mineralogical Composition:
Igneous rocks like other rock groups are characterised by the abundance of only a few minerals. The following generalisation as given by Clark may be assumed as sufficiently accurate to indicate the relative abundance of different minerals in igneous rocks.
It is evident from the above table that most common minerals of igneous rocks are felspars, amphiboles (e.g. hornblende), pyroxenes (e. g. augite) and quartz. Others are present only in subordinate amounts.
Term Paper # 3. 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.
A number of factors control the formation of igneous rocks from magma or lava. Thus, the process of crystal formation (or crystallisation) may be slow or rapid; the magma may be rich in one constituent and poor in other constituents and further, it may be highly viscous or quite mobile and so on. All these factors lead invariably to various shapes, sizes and arrangements of the resulting minerals and hence produce a number of textures.
Term Paper # 4. Structures of Igneous Rocks:
Those features of igneous rocks that are developed on a large scale in the body of an extrusion or intrusion – giving rise to conspicuous shapes or forms are included under the term structures. They may be so well developed as to be recognized easily on visual inspection or they may become apparent only when thin sections of such rocks are examined under microscope. In the latter case they are termed microstructures.
Various types of structures developed in igneous rocks can be broadly grouped under three headings:
(a) Structures due to Mobility of Magma or Lava:
Magma or lava may be highly viscous, viscous, mobile or highly mobile. The mobility of the magma (or lava) is responsible for a variety of structures that the ultimate rock will acquire.
Some of these are:
i. The flow structures
ii. The pillow structures
iii. The ropy and blocky lava
iv. The spherulitic structure and
v. The orbicular structures.
i. The Flow Structures:
The Flow structures are defined by the development of parallel or nearly parallel layers or bands or streaks in the body of an igneous rock. The parallelism is caused by the flow of magma or lava during the process of crystallisation.
ii. The Pillow Structure:
This is characterized by the development of bulbous, overlapping, pillow like surfaces in the body of igneous mass. It is typical structure of rocks formed from mobile basaltic lava. In such a lava flow, its upper surface gets solidified while the lava beneath remains hot and capable of flowing.
But further flow is possible only when the crust formed at the top ruptures. The rupture occurs at the margins of previously congealed oval mass; from such ruptures lava flows out and cools down again forming fresh bulbous tops. The process is repeated producing overlapping bulbous masses.
iii. The Ropy and Blocky Lava:
These are structural variations developed in the volcanic rocks due to different mobility. Highly viscous “dry” lavas undergo very little movement after their eruption and before cooling. Their surfaces show broken and fragmented appearance. These are called the blocky lava. On the other hand, very mobile lava flows for considerable distance and cooling during flow process so that its upper surface is smoothly wrinkled rather than actually broken. The surface structure is then referred as ropy lava.
iv. The Spherulitic Structure:
It is distinguished by the presence of thin mineral fibers of various sizes arranged in perfect or semi perfect radial manner about a common centre. It is a common structure of acid volcanic and hypabyssal rocks. The structure results during crystallization of saturated lava or magma.
v. The Orbicular Structure:
It is a rare type of structure of igneous rocks. In this, a rock mass appears as if composed of ball like aggregations. Each ball is in turn composed of concentric shells of different minerals. This structure is shown by some granites.
b. Structures due to Cooling of Magma:
Many structures develop in igneous rocks on account of environment in which their cooling and crystallisation take place from the original melt.
The following deserve special mention:
i. The Jointing Structure:
Cooling of magma or lava is very often accompanied by development of cracks or joints in the rocks formed from these sources. These joints sometimes follow definite patterns. For instance, they may develop simply as horizontal sets of joints, quite closely spaced and dividing the rock into sheets. This type is called a sheet structure. Similarly, there may be developed three sets of joints mutually at right angles to one another.
The resulting rock mass gets divided into cubical blocks and the structure is termed as mural jointing. It is seen in granites. In basalts and many other volcanic rocks, there are developed polygonal cracks, similar to those developing in mud while drying and shrinking. These cracks continue depth wise for considerable thickness in the body of the rock dividing it virtually into polygonal blocks or columns. The structure so developed is often termed columnar structure.
ii. Rift and Grain:
These terms indicate two separate directions, often used by quarry men, in which the igneous rocks like granite can be broken from the main rock body with a comparative ease. The directions rift and grain, are at right angles to each other. The structure is thought to be due to presence and proper orientation of very minute cracks in some minerals of the cooled rock.
Much importance is attached to the crystal stresses that were in operation at the time of formation of the rock. It has been suggested that sheets of fluidal cavities are formed in quartz during crystallization as the first step. The rift and grain cracks of weakness are then caused along these fluidal cavities after the cooling provided the crystal stresses continue to operate.
iii. Vesicular Structure:
In many cases, lava is rich in gases at the time of eruption. The process of cooling and crystallisation is generally accompanied by the escape of these gases. This (escape of gases while cooling is going on) leads commonly to the formation of cavities of various sizes and shapes in the cooled mass. These cavities may remain empty while the entire mass of the rock is cooled down. The structure developed is then called, vesicular.
In other cases, liquids may seep into these cavities and cool down there forming secondary minerals of those specific shapes. These are called amygdaloids and structure of the rock as amygdaloidal. Scoria is a volcanic rock rich in such cavities, which are all empty making the rock very light. Many varieties of Panjal traps of Kashmir Himalayas show amygdaloidal structure in abundance.
iv. Miarolitic Structure:
Sometimes small and distinct cavities are formed during the crystallization of magma. These get filled with volatile components, which may enlarge them on the one hand and facilitate formation of unusual (rare) minerals in them on the other hand. These cavities often containing projecting crystals are called miarolitic cavities.
When they are present in good abundance in a rock, it is said to exhibit a miarolitic structure. Miarolitic cavities are distinguished from other cavities (e.g. geode) by the absence of any distinct wall between the minerals of the cavities and the rock in general.
c. Miscellaneous Structures:
The reaction and xenolithic structures may be mentioned under this category.
i. The Reaction Structure:
The Reaction structure is characterized by the presence in the rock of some incompletely altered minerals conspicuously surrounded on their borders by their alteration products. It often happens that some earlier formed minerals react with the magma during the subsequent stages of crystallization.
This reaction may be complete or incomplete. In the first case the original mineral disappears altogether. But when the reaction is incomplete, it (the original mineral) is surrounded on sides with the reaction product forming what is commonly called the reaction rim.
ii. The Xenolithic Structures:
The Xenolithic Structures are imposed on the igneous rocks because of incorporation of foreign material (e.g. from the host rock into which magma is intruded) into the magma during the process of crystallisation. The foreign fragments are termed xenoliths. These may arrange themselves in different patterns or they may get segregated in the crystallizing materials.
Term Paper # 5. Forms of Igneous Rocks:
Igneous rocks are formed from cooling and crystallisation of an originally hot molten natural material called magma (or lava when erupted from volcanoes). The cooled igneous masses occur in nature in a variety of shapes or forms.
As to what form 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, however, possible to divide the various forms of igneous intrusions into two broad classes:
i. Concordant and
This is based on the relationship of the igneous intrusion with the structure of the host rock.
i. Concordant Bodies:
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. The bedding planes in the sedimentary formations and cleavage planes in metamorphic rocks define such planes where the magma gets intruded and cools down without any attempt to cut across them.
Most important concordant forms are:
c. Lopoliths and
Those 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 commonly 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. Minor and local projections from big sills (and other concordant bodies) may rise above into the overlying strata. Such projections of magmatic composition are known as apophyses (singular: apophyx) and these should not be considered as an evidence against the concordant nature of the main intrusion.
Sills are commonly subdivided into following types:
(a) Simple Sills:
These are formed of a single intrusion of magma;
(b) Multiple Sills:
The sills which consist of two or more injections, which are essentially of the same kind of magma;
(c) Composite Sills:
The 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) Inter-Formational Sheets:
The sheets of magma injected along or in between the planes of unconformity in a sequence are specially termed as inter-formational sheets. These resemble the sills in all other general details.
The most common rocks composing the sills are intermediate and basic igneous rocks like syenites and gabbros. They may show aphinitic and porphyritic textures.
These are concordant, small sized intrusives 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.
Those igneous intrusions, which are associated with structural basins, that are sedimentary beds inclined towards a common centre, are termed as lopoliths. They may form huge bodies of consolidated magma, often many kilometers long and thousands of meters thick. 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.
Lopoliths, like sills, may be simple, complex or differentiated in character, the terms having same connotations.
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.
Laccoliths are further distinguished as asymmetrical when the roof rocks show different inclinations in different directions and inter-formational when these are injected along unconformities. They are commonly intermediate (silica content is between 45-66%) in composition and show great variation in texture.
ii. 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:
b. Volcanic necks and
a. Dykes (Dikes):
These may be defined as columnar bodies of igneous rocks that cut across the bedding plane or unconformities or cleavage planes and similar structures. Generally the 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 meters thick. Similarly, in length they may be anything between a few meters to many kilometers. In composition, dykes are generally made up of hypabyssal rocks like dolerites, porphyries and lamprophyres, showing all textures between glassy and phaneritic types.
Dykes generally tend to occur in groups or sets. Thus, the term dyke-set is used for a couple of parallel and closely spaced dykes. When the number of dykes occurring in a limited area is quite large, the term dyke swarm is used to express them collectively.
It is customary to classify dykes (like sills) as simple dykes, multiple dykes, composite dykes and differentiated dykes, the terms having the same significance as for sills.
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° – 40°. 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 upto 25 kms are also known.
Origin of Dykes:
Dykes are intrusions of magma into preexisting 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 case where magmatic pressure happens to be less than the lithostatic pressure (that due to overlying rocks), only the space made available by the original fractures is filled by the magma and determines the dimensions of the dyke.
b. 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.
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.
The Costa Rica Batholith of British Columbia is at least 2000 km long and 40-90-km wide on different sides. It is considered by many as the largest plutonic body in the world. The Idaho batholith and the Sierra Nevada Batholith of California are some other examples of batholiths.
When the surface area of batholith-like igneous mass is less than 100 km, it is commonly termed as stock. When such a stock has roughly circular outline (rather than irregular), it is further distinguished as a boss. Minor projections of igneous masses from the roofs of batholiths, stocks and bosses called apophyses are often observed passing into the overlying strata.
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.
Many views have been expressed regarding the emplacement of batholiths.
The most important theories are outlined below:
(i) Emplacement by Cauldron Subsidence:
According to this view, parts of country rock within a vertical ring dyke may fall into the underlying magma reservoir. The space thus created by the mega-subsidence, the cauldron subsidence, as it is called, gets subsequently filled with magma, which congeals to form batholiths, stocks and bosses.
(ii) Emplacement by Magmatic Stoping:
This view, forwarded by DALY, envisages engulfment of blocks at the roof of the magmatic reservoir. The roof blocks are first shattered due to expansion because of heating from below and then start sinking into the reservoir of magma below under the influence of thermal expansion and magmatic penetration.
It is held by many that huge granitic batholiths cannot be imagined to have been formed by simple process of cooling and crystallisation from such a large pre-existing magmatic reservoir. A new process called granitization has been suggested as the most plausible way to explain their formation.
Granitization may be broadly described as a set of processes by which already existing sedimentary and other rocks are changed into granite-like masses without actually passing through a magmatic stage.
The term envisages at least two fundamental conditions:
(a) The changes that convert the solid rocks into granitic bodies are essentially in-situ in character.
(b) The changes are brought about essentially in solid state.
Unlike the igneous intrusions, the igneous extrusions do not show much complexity in their form. They generally occur as widely spread, extensive flows covering enormous area and the existing topography. On cooling, these lava flows form solid sheets of rock. In many cases, the sheets may occur as layers laid one above another.
Often, there may be layers of other sedimentary materials deposited during the volcanic intermissions, which are called intertrappean layers. The total thickness of volcanic layers, piled one above another may reach many hundreds of meters. In some cases, the different flows can be distinguished from one another by the presence of intertrappean beds. In other cases, they may be completely welded together without any zone of separation being visible between them.
The volcanic flow sheets may be horizontal or slightly inclined depending upon the original topography and subsequent geological history of the area.
Term Paper # 6. Formation of Igneous Rocks:
Igneous rocks make up more than 90 per cent (by volume) of the crust of the Earth up to a depth of 10 km. It is, therefore, desirable to have a working knowledge about the processes thought to be involved in their formation.
At least three major questions have to be answered adequately to explain the formation of igneous rocks:
(a) What is the source material of igneous rocks?
(b) What is the process or (or processes) involved in the formation of these rocks from the source material?
(c) How great diversity in the nature of igneous rocks is to be explained?
A brief review of various possibilities (so far suggested) on the above questions leads to following general conclusions:
1. The Source-Magma:
All igneous rocks are formed from a hot molten material called magma when occurring below the surface or lava when flowing out from volcanoes. Nobody has actually observed the magma in its natural environment below the surface because of the great depths involved; even then its existence is fairly established from two factors- first, the temperature gradient and second, the volcanic eruptions. The temperature of the earth is established to increase with depth, on an average, to the extent of 30 degree centigrade per kilometer.
With such a rise, materials of the Earth may be assumed to be in a molten form at certain depths. Similarly, millions of cubic meters of lava have poured out from various volcanoes during the historic times. Many volcanoes are active even at present. This lava comes from within the earth and there has to be some source for huge outpouring. Hence, existence of magma at places below the surface of the Earth in magma chambers can be safely accepted.
Composition of Magma:
A study of chemical composition of igneous rocks reveals that 99.25 percent of an average igneous rock is made up of a few of nine elements only, namely, oxygen, silicon, aluminium, iron, calcium, sodium, potassium, magnesium and titanium. Naturally, these elements should represent the chemical composition of an average magma also.
It is, however, held that quite a few volatile components escape from the magma prior to or during the process of formation of rocks from the magma; and hence are not represented in the final rocks. Thus, no final conclusion can be drawn regarding the exact chemical composition of the magma.
As regards the physico-chemical constitution of the magma, it is believed that a few constituents are always dominating in the molten state.
Further, the magmatic constituents may be divided into two groups:
(i) Fixed Constituents:
The fixed constituents, which are characteristically refractory substances of higher melting points and low vapour pressures; examples, molecules of silica, alumina, magnesia etc.
(ii) Volatile Constituents:
The volatile constituents or the fugitives are characterised by high vapour pressure and include gases and vapours like those of water, hydrogen sulphide, hydrofluoric acid, carbon dioxide, sulphur dioxide, besides those of oxygen, nitrogen, and hydrogen.
2. The Process-Crystallization:
The principle involved in the formation of igneous rocks from magma or lava is that of crystallization, which may be defined as formation of solid crystals in a cooling melt with the change in its physico-chemical environment. Crystallisation always signifies a well-defined atomic arrangement in the solid resulting from a cooling melt.
The magma is assumed to pass through stages of nucleation followed by growth of crystals around these nuclei. The process is quite complicated and is controlled by a number of conditions. It results in the formation of minerals of different composition that ultimately make up the cooled solid mass – the igneous rock.
Crystallization is invariably linked with cooling of magma, and therefore, it starts from the outer margins of a magmatic body, which are to cool first. Crystallization centres are established in the cooling mass at different locations; different molecules are attracted from the melt to the nearby respective centres. Thus the crystals start growing and enlarging.
The grain size of the ultimate rock will be defined by following factors attending the process of crystallisation:
(a) Rate of Cooling:
Slow cooling results in the formation of coarse-grained crystals with well-defined shapes and faces provided enough molecules of that constituent are available. On the contrary, more rapid the rate of cooling, liner is the size of the crystals. This is because molecules of various constituents dispersed throughout a melt require sufficient time to move towards their respective centres of crystallisation.
When this time is not available, the resulting crystals contain only fewer molecules and hence remain small in size. In glass, for instance, cooling may take place at such a fast rate that virtually no time is available for the molecules to develop centres of crystallisation or to move towards them. Hence the resulting solid is practically without any crystallisation or atomic arrangement. It is simply a supercooled liquid.
(b) Molecular Concentration:
Different constituents in a magma may be present in varying molecular concentration: some making 50% and others 30% and still others as small as 10% and 5% and so on of the melt. Other conditions remaining the same, the components present in smaller concentrations may make smaller crystals.
(c) Viscosity of the Melt:
Higher viscosity retards the rate of diffusion of molecules towards the centres of crystallisation and thus growth of crystals in size is hindered. In highly viscous magmas, greater number of small sized crystals may be formed compared with mobile magmas that may contain only fewer crystals of bigger dimensions, other conditions remaining the same.
3. Phenomena-Associated with Crystallisation:
During the process of crystallisation from magma, certain other phenomenona are believed to play important role in defining the characters of the ultimate igneous rocks.
A brief mention of some of these will be quite useful:
Certain components of magma may crystallise out simultaneously at a certain stage during the process of crystallisation. It has been observed that this phenomenon takes place only when those constituents are present in certain constant proportions at a certain temperature.
The constant proportion in which two or more constituents crystallise out simultaneously from a melt is termed eutectic. A mineral formed as a part of eutectic system is no longer concerned with the equilibrium of the process of crystallisation of the rest of the melt. Its composition remains unchanged in the subsequent process.
(b) Mixed Crystals:
Quite a number of components of magma are miscible (i.e. can remain intimately mixed up) in all proportions. This leads to a peculiar process of reactions. Minerals formed at one stage from the magma, if not removed from the melt immediately after their formation by some natural process, react with the melt again and give rise to new minerals. Such minerals are called mixed crystals and abound in igneous rocks.
(c) Reaction Series:
In the mixed crystal system an earlier formed mineral reacts with the melt and forms a new mineral. The two minerals so related in the process form a reaction pair. A number of minerals may be related in this manner and when they are arranged in a proper order, they form a reaction series.
This series is actually based on experimental studies performed by N.L. Bowen on crystallisation of artificially prepared silicate melts broadly corresponding to basaltic magma. Although they represent crystallisation under laboratory conditions only, they are considered to represent the order of crystallisation of minerals in magmatic melts in a general way.
The Bowen’s Reaction Series is further distinguished into two types:
i. Continuous Series:
In this series, the atomic structure of the new minerals remains the same; there is only minor change in the chemical composition of the minerals so formed which takes place in a continuous manner. The best example of a continuous series is that of plagioclase felspars which gradually change from calcic to sodium felspars.
ii. Discontinuous Series:
In this series, new minerals with different chemical composition and different atomic structure are formed only at particular temperature. The Olivine Biotite series is the example established by Bowen.
The Bowen Reaction Series is shown graphically below:
The three minerals shown at the end of chart indicate the fact that from a melt these three minerals will form towards the last stages of crystallisation and they do not react with the remaining melt, if any.
4. Differentiation—Causes of Diversity:
Igneous rocks are known to occur in great variety. This may be explained by assuming that there exist as many types of magmas in nature as there are types of igneous rocks. This assumption is out-rightly rejected on theoretical as well as experimental observation. But the fact of diversity in nature of igneous rocks has to be explained. Various possibilities have been suggested in the past including about the number and nature of parent magmas.
It is, however, generally believed now that various types of igneous rocks have been formed from single parent magma, called the primary magma. The primary magma is believed to be basaltic in composition.
It is further believed that the diversity of igneous rocks formed from the primary magma can be explained by envisaging processes like:
(a) Differentiation and
It is the process by which an originally homogeneous and uniform magma splits up into different types of igneous rocks. It is argued that differentiation can take place before, during or even after the crystallisation of minerals in a magmatic melt. All possibilities have been suggested.
Those who believe in differentiation prior to crystallisation assume it to have taken place due to liquid immiscibility involving separation of liquid phases of contrasting composition in the parent magma. Subsequent crystallisation in these immiscible layers results in the formation of different rocks reflecting the composition of those layers. Though theoretically quite sound and possible, it is argued by many others that the phenomenon of liquid immiscibility is insignificant in causing differentiation on such a major scale as observed in igneous rocks.
Differentiation during crystallisation may be brought about by localization of the process of crystallisation and by localized accumulation of crystals. In the first case, crystallisation starts only at favourable locations, which can be cooling margins of a magmatic body.
This involves the diffusion of the particular molecules towards the crystallisation regions and corresponding deficiency of the remaining melt in those molecules. Thus, minerals of early crystallisation will be differentiated or separated from those of later crystallisation.
This process of fractional crystallisation involves a considerable amount of “molecular diffusion” or convection currents. This type of diffusion is both possible and an effective process but whether it could be held responsible for bringing out differentiation on a large scale is questioned by many.
Differentiation after crystallisation is possible in many ways.
The following processes are considered very important in this respect:
(i) Gravitational Differentiation:
This process involves the sinking or ‘setting’ of earlier formed crystals under the influence of gravity. The process is controlled by factors like specific gravity, shape and size of crystals on the one hand and viscosity of the melt on the other hand. Heavier and uniformly shaped crystals sink easily and quickly in lighter and less viscous melts.
(ii) Filtration Differentiation:
In this process, the solid phase (i.e. the crystals) is separated from the liquid phase (i.e. the melt) through the operation of lateral stresses. This process involves squeezing out of the liquid from the crystallizing melt and is known as filter processing. The squeezed out liquid may be injected into rocks farther away from the original source and crystallise there giving rise to new types of rocks.
The process of incorporation of the foreign materials, generally from the host rock, into the magmatic melts is termed as assimilation. This may lead to a change in the chemical composition of the magma, which on cooling may give rise to different types of rocks. Thus, assimilation is also thought as a process that can bring about the diversity in the character of igneous rocks.
It has been suggested, however, that the process of assimilation cannot be attached much importance in explaining diversity of igneous rocks because the capacity of most of the magmas to digest the foreign materials is generally limited, being, at the best, up to ten percent of their own mass.
Summarizing, it may be said that the origin of igneous rocks is a complicated question. These rocks are thought to have been formed from a single parent or primary magma, basaltic in composition and originally homogeneous and uniform in nature.
From this parent basaltic magma diverse types of igneous rocks are assumed to have evolved through the process of differentiation which might have been brought out by one or more of the processes like liquid immiscibility, fractional crystallisation, gravitational differentiation and filtration differentiation. Much importance is attached to the role of volatile constituents in the magma; their presence in reasonable concentration influences the crystallisation process and differentiation to a great extent.
Term Paper # 7. Engineering Importance of Igneous Rocks:
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 roadstones.
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.