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Igneous rocks

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  • 1. Igneous Rocks Unit-III
  • 2. Igneous Rocks • Igneous Rocks - Forms of Igneous Rocks- Plutonic & Hypabasal, Common Igneous Rocks and relation of their Constituent Minerals, Classification of Igneous Rocks, Structure and Textures, Suitability of Igneous Rocks for Building and Foundation, Megascopic Description of Common Igneous Rock Types.
  • 3. Introduction • Among the different types of rocks, igneous rocks are most abundant. They are also known as primary rocks.
  • 4. Forms of Igneous Rocks • Igneous rocks are formed out of very hot lava or magma. The extruded (or thrown out) lava on solidification over the earth’s surface give rise to extrusive rocks. The magma on solidification below the earth’s surface give rise to intrusive rocks. The extrusive rocks occur as simple lava flow. The intrusive rocks, on the other hand, assume different forms (or shapes) which in turn are influenced by geological structures occurring in country rocks and fluidity (i.e.. viscosity) of magma.
  • 5. Forms of Igneous Rocks
  • 6. Forms of Igneous Rocks • Based on the relation of an igneous body with the attitude of associated country rocks, their forms are called concordant or discordant. If an igneous body is parallel to or occur along bedding planes of country rocks. Its forms is called concordant. On the other hand. If it cuts across the bedding planes of surrounding rocks, it is called discordant.
  • 7. Concordant or Discordant
  • 8. Forms of Igneous Rocks Influencing factor • Magma, after its formation under the influence of pressure exerted by a huge overburden, attempts to move upwards, forcefully. In its upward journey, when magma reaches sedimentary strata, their bedding planes offer convenient and suitable sites for magmatic intrusion or injection. Thus, the bedding planes play an important role in the formation of igneous bodies.
  • 9. Forms of Igneous Rocks
  • 10. Forms of Igneous Rocks • Another influencing factor in the types of form of an intrunsive igneous body is the nature of fluidity of magma. The fluidity or viscosity of magma is dependent on its silica content. Through magma has no fixed composition, it is usually either silica-rich or silica poor. Viscous magmas do not spread easily but have a tendency to pile up at a place. The basic magmas being less viscous easily spread over larger areas. The three typical forms of igneous intrusions, namely, sill, laccoliths and bysmalith, can be explained by this property of the viscous nature of magma.
  • 11. Forms of Igneous Rocks • The most common forms of intrusive igneous bodies as observed in the field are dyke, sill, laccoliths, lopolith, bysmalith, phacolith, chonolith, volcanic neck or plug, batholith, etc, lava flows and pyroclasts are forms of extrusive igneous bodies.
  • 12. Forms of Igneous Rocks
  • 13. Forms of Igneous Rocks Dykes • Dykes and sills are the most common forms of igneous rocks. Dykes are discordant, sheet-like, vertical or steeply inclined, intrusive igneous bodies, Since they are sheet-like, they are narrow in width and have nearly uniform thickness, i.e.. with parallel sides. They occur cutting across the bedding planes of the country rocks in which they are found.
  • 14. Dykes
  • 15. Forms of Igneous Rocks • During the forceful upward journey, magma intrudes through the discordant fractures, cracks, crevices, joints, shear zones or any other weak planes or zones. Subsequent solidification of this gives rise to dykes. Some dykes may serve as feeding channels or passages for other igneous bodies such as laccoliths, lopolith and sill. • Dykes may be horizontal or inclined or vertical. Steeply inclined or vertical dykes extending to greater depths are common in nature.
  • 16. Dykes
  • 17. Forms of Igneous Rocks • Dykes are important from civil engineering point of view for the following reasons. • They are undesirable at the sites of foundations of dams because they introduce heterogeneity in the region and also their sides turn out to be weak planes.
  • 18. Forms of Igneous Rocks • Dykes are like walls and act as barriers for the flow of underground water. Thus, like quartz veins, they interrupt the ground water movement which in good or bad potential of ground water in a region. • Dykes may give rise to new spring or seal old springs • Dykes may cause oil accumulation and thereby contribute to the occurrence of oil and gas deposits. • As dykes are hard, durable black and fine grained, they are used in making statues, sculptures, etc
  • 19. Dykes
  • 20. Forms of Igneous Rocks Sills • Sills are similar to dykes in being sheet-like, intrusive bodies but, unlike dykes, these are concordant. Sills are formed due to the penetration of magma into bedding planes of country rocks and their spreading capacity depends on the viscosity of magma, its temperature and the weight of the overlying rocks. • Basics magmas being more fluid and more hot, usually occur as sills. During sill formation, the ascending magma, under great pressure, pushes the overlying rocks upwards and accommodates itself by intruding and spreading along weak bedding planes.
  • 21. Sills
  • 22. Forms of Igneous Rocks • Sills being concordant, on consolidation, looks like beds themselves. The sills, which spread over larger areas, are generally thin and show uniform thickness. • Lava flows may resemble sill closely, because both are relatively thin, horizontal sheet-like igneous bodies spreading over larger areas. But they can be distinguished from one another easily as follows
  • 23. Sills
  • 24. Forms of Igneous Rocks • Sills produce baking effects on rocks on both sides, whereas in the case of lava flow, the baking effect is seen only on the lower side. • Lava flows show an irregular surface, whereas sills have more or less two flat and parallel sides. • Lava flows show vesicular character on the upper surface, whereas sills have more or less two flat and parallel sides.
  • 25. Forms of Igneous Rocks • Lava flows undergo quick cooling producing very fine grained igneous rocks, whereas sills cool down rather slowly and therefore produce medium and coarse grained igneous rocks. • Sills give out minor intrusions into the overlying rock masses, whereas lava flow do not.
  • 26. Forms of Igneous Rocks
  • 27. Forms of Igneous Rocks Laccolith • Like a sill, laccoliths is also a concordant igneous body. It has a nearly flat bottom but it is convex upwards, i.e. it is domed shaped. When viscous magma is injected along a bedding plane, as it cannot spread easily, it pushes up the overlying rocks and piles up more at the place where it reaches the bedding plane and thins out away from it. This gives the shape of an inverted bowl to the igneous body.
  • 28. Laccolith
  • 29. Forms of Igneous Rocks • Laccolith may also be formed when the supply of magma from beneath is more than that can be accommodated by lateral spreading. Laccolith is relatively a small igneous body. In the ground plan it is either circular or elliptical, depending upon weather the magma supply is from a cylindrical vent or an elongated fissure. Laccolith cause the overlying rocks folded upwards to take a dome-like shape.
  • 30. Laccolith
  • 31. Forms of Igneous Rocks Bysmalith • When the magma happens to be highly viscous, the lateral spread along the bedding plane will be very less and the intruding magma acquires a somewhat cylindrical shaped body. It is called “bysmalith”.
  • 32. Bysmalith
  • 33. Forms of Igneous Rocks • During the formation of bysmalith, since the overlying rocks are pushed up more or less vertically, they cannot bend unduly, but rupture along sides and number of actuate faults occur around bysmalith. The net result is that this igneous body appears as if it has punctured through the surrounding country rocks. Such a form appears partly concordant and partly discordant.
  • 34. Forms of Igneous Rocks Phacolith • When thick sedimentary are folded, along the crests and troughs, some empty spaces occur. These spaces are readily occupied if magmatic intrusion get assess to them. On solidification these appear as crescentic or lens-shaped across the axial plane. These igneous bodies are called “phacoliths” These are concordant igneous rocks.
  • 35. Phacolith
  • 36. Forms of Igneous Rocks Lopolith • This is a basin or saucer-shaped concordant intrusive body of enormous size. Its top is nearly flat and the bottom is convex downwards. In the way, it is just the reverse of laccolithh in shape. Lopolith is a very huge igneous body with a thickness approximately 1/10 to 1/ 20 of the diameter.
  • 37. Lopolith
  • 38. Forms of Igneous Rocks Volcanic Necks • These are the igneous bodies which seal the vents and conduits of ancient volcanoes. These are somewhat cylindrical in shape and vary in diameter from a few hundred metres to a kilometer or more. Volcanic necks are circular or elliptical or irregular in plan and vary in diameter. These are filled with crystalline rock or fragmentary material or both. Volcanic necks are also called volcanic plugs.
  • 39. Volcanic Necks
  • 40. Volcanic Necks
  • 41. Forms of Igneous Rocks Batholiths • These are the largest known intrusive igneous bodies characteristically occurring in mountain regions Batholiths have sides sloping away from each other which makes them larger and larger downwards and then extended to very great depths. • They occur as core bodies of folded mountains, and appear elongated along mountain ranges. By virtue of their position they appear as mountain roots. Since they are deep seated, only very prolonged erosion exposes them on the surface.
  • 42. Batholiths
  • 43. Forms of Igneous Rocks Chonolith • This is general term applicable to all other forms of igneous rocks. It includes any irregular form of igneous rocks and hence it does not have any specific shape.
  • 44. Forms of Extrusive Igneous Rocks Lava flow • On eruption of a volcano, lava simply flows on the surface and on consolidation gives rise to lava flows. These closely resemble sills in shape. Based on surface appearance, lava flow are described as block lava and ropy lava. Block lava is less mobile and has a rough and irregular surface. The ropy lava is more mobile and has wrinkled but smooth and shining surface.
  • 45. Forms of Extrusive Igneous Rocks
  • 46. Forms of Extrusive Igneous Rocks
  • 47. Forms of Extrusive Igneous Rocks
  • 48. Forms of Extrusive Igneous Rocks Pyroclasts • The rock fragments thrown out at the time of volcanic eruption are called pyroclasts. These are described variously, based on size and shape.
  • 49. Pyroclasts
  • 50. Forms of Extrusive Igneous Rocks • Bigger and angular fragments are called volcanic blocks. If they are somewhat rounded they are known as volcanic bombs. Smaller fragments are called lapilli or cinders. Fragments still smaller in size are called volcanic sand, volcanic dust and volcanic ash.
  • 51. Volcanic Bombs
  • 52. Volcanic Ash
  • 53. Volcanic Ash
  • 54. lapilli or cinders
  • 55. Volcanic blocks &Volcanic bombs
  • 56. Forms of Extrusive Igneous Rocks • A rock formed out of volcanic dust and ash is called volcanic sand, volcanic dust and volcanic ash. A rock formed out of volcanic dust and ash is called tuff. Volcanic agglomerate or volcanic breccias is the name given to the consolidation heterogeneous mass of pyroclastic material.
  • 57. Forms of Extrusive Igneous Rocks
  • 58. Forms of Extrusive Igneous Rocks
  • 59. Forms of Extrusive Igneous Rocks
  • 60. Miscellaneous Primary and secondary minerals • Primary minerals are those which have been formed directly from the solidification of magma or lava. These are also called pyrogenetic minerals. • Feldspar, pyroxenes, etc. are typical examples of this kind. Secondary minerals which have been formed due to weathering or metamorphism or from precipitation or evaporation or circulating of natural solutions. Bauxite, limonite, talc, chlorite, calcite, and opal are typical examples of secondary minerals.
  • 61. Primary and secondary minerals
  • 62. Miscellaneous Essential and Accessory Minerals • Both these are primary minerals. Essential minerals are defined as those which are necessary for naming or identification of rocks. These minerals will appear as major constituents and their characteristic occurrence is helpful in naming a rock. For example Quartz and feldspar. It means that both of these must occur predominantly in order to call a rock granite. If any one of these is either less or absent, the rock cannot be called granite.
  • 63. Miscellaneous
  • 64. Miscellaneous
  • 65. Miscellaneous • Accessory minerals are those which usually occur in a rock, but their presence or absence will not be considered in naming the rock. These minerals usually occur in small quantity in rock. Eg. hornblende and biotitic are the common accessory mineral of granite.
  • 66. Common Igneous rocks and relation of their constituents minerals • Generally, igneous rocks are named on the basis of the minerals present in them and their relative proportions. But minerals present in the rock do not bear any definite ratio depending on the composition of the parent magma, the same rock may show changes in proportions of essential minerals and types of accessory minerals. When essential minerals change considerably a rock is named differently. • The intercepts on any vertical gives the proportion of the constituents minerals for the rock concerned.
  • 67. Common Igneous rocks and relation of their constituents minerals
  • 68. Classification of Igneous Rocks • There are different types of classification of igneous rocks, Simple classification based on silica percent, silica saturation and depth of formation
  • 69. Classification of Igneous Rocks
  • 70. Classification of Igneous Rocks
  • 71. Classification of Igneous Rocks Classification Based on Silica Percent • The chemical composition of a rock is generally expressed in terms of different oxides like SiO2, Al2O3, Fe2O3, FeO, MgO, and CaO. Among different oxides silicon dioxide is always predominant in igneous rocks. • Since Silica percent is also responsible for the formation of different minerals and their association. It serves as a suitable basis for the classification of igneous rocks. When silica content exceeds 66 % the igneous rocks are called acidic, when it is 52- 66 %, the rock are called intermediate, The basic rock have 45 – 52 %.
  • 72. Classification Based on Silica Percent
  • 73. Classification of Igneous Rocks Acidic Igneous Rocks • These rocks are compositionally rich in silica, alumina and alkalis, but are poor in calcium, magnesium and iron. • They are composed of quartz, alkali feldspar and muscovite mica- representing the late stage of crystallization of magma. • They are leucocratic because these are rich in pale colored minerals and poor in dark magic minerals. Mafic minerals occur only as accessory minerals, in small quantities.
  • 74. Classification of Igneous Rocks • They have, characteristically, free primary quartz and are always devoid of unsaturated minerals. • They are relatively lighter rocks and have high melting points. • Granites and many pegmatites are typical examples of this group. Granites may have 70 % or more of silica content.
  • 75. Classification of Igneous Rocks
  • 76. Classification of Igneous Rocks Intermediate Igneous Rocks • These rocks may be lacking in free quartz completely or may be having very little of it. • These are mainly composed of alkali feldspar with a few accessory minerals. • They are leucocratic or mesocratic.
  • 77. Intermediate Igneous Rocks
  • 78. Classification of Igneous Rocks Basic Igneous Rocks • In these rocks, magic minerals occur as essential minerals i.e.. they occur as important constituents. • Quartz and olivine are generally absent, or any one of them may occur in small quantities. • Feldspar are of the plagioclase type. • Silica and alkalis are less and calcium and magnesium and ferrous iron are more in content when compared with acidic rocks. • The dominant occurrence of magic minerals makes these rocks to have a slightly higher specific gravity. • Gabbros, norite and basalt are examples of this group.
  • 79. Classification of Igneous Rocks
  • 80. Classification of Igneous Rocks Ultrabasic Rocks • If the acidic rocks represent one extreme, the ultrabasic rocks represents the other extreme. The important characters of this group are: • Free quartz is always absent • Unsaturated minerals and/ or magic minerals occur as essential minerals • These have the highest density among the different rock types. • Compositionally, these are the poorest in silica and the richest in magnesium.
  • 81. Classification of Igneous Rocks
  • 82. Classification Based on Silica Saturation • The Silica content of the Parent magma is responsible for the occurrence of saturated minerals or unsaturated minerals. In igneous rocks and quartz will never co-exist with unsaturated minerals. • When the parent magma is very rich in silica, only saturated minerals are formed and the surplus quantity of silica crystallizes as free quartz. This is the resulting rock will always have quartz and saturated minerals like feldspar. Unsaturated minerals like olivine will never occur in them
  • 83. Classification Based on Silica Saturation • When the parent magma has just enough silica for the formation of saturated minerals, the resulting rock will possess neither free quartz nor any unsaturated minerals.. Such rocks are characterized by the presence of saturated minerals like feldspar. These rocks are appropriately called saturated rocks. • When the parent magma has silica slightly less than what is required for the formation of all saturated minerals. That is, such rocks are composed of both saturated and unsaturated minerals. It is obvious that free quartz will always be absent in them.
  • 84. Classification Based on Silica Saturation • When the parent magma is highly deficient in silica, saturated minerals may not be formed at all and only unsaturated minerals will be occurring in the resulting rocks. This group also represents under saturated rocks. • The preceding grouping of rocks into oversaturated and under saturated categories depends on minerals associated which in tern is related to the silica content of magma.
  • 85. Classification based on depth of formation • According to this classification, igneous rocks are grouped into plutonic rocks, hyperbyssal rocks and volcanic rocks. Those igneous rocks which have formed under high pressure at great depths in the earth’s crust are called plutonic rocks. The igneous rocks formed on the surface are called volcanic rocks. The other igneous rocks which have formed at shallow depths are called hypabyssal rocks.
  • 86. Classification based on depth of formation • The plutonic rocks are formed (i) under great pressure, (ii) at high temperature and (iii)in the presence of huge quantity of volatiles. Great pressure ensure total crystallization of minerals formed and the hot surroundings slow down the process of solidification. This provides a lot of time for the mineral molecules to crystallize. The presence of volatiles considerably reduces the viscosity of magma. Which facilitates easy movement of mineral molecules in the body of magma. The net result of all these processes is the development of a characteristic coarse texture for plutonic rocks.
  • 87. Classification based on depth of formation
  • 88. Classification based on depth of formation • In the case of volcanic rocks, the physical conditions that prevail are • (i) Solidification under low pressure (only under atmospheric pressure) and in the absence of volatiles. • (ii) the underlying surface rocks are relatively cold overlying atmospheric gases circulate continuously. On the lava surface, cold atmospheric gases keep on circulating because the air above the lava flow is occupied by the dense cold air. Further, under these circumstances the associated volatiles also escape into the atmosphere which renders lava devoid of them; absence of volatiles results in high viscosity and low pressure resulting in the formation of fine grained or glassy matter. The net result is that the volcanic rock are fine grained.
  • 89. Classification based on depth of formation • The hyperbyssal rocks are formed at shallow depths under moderate temperature and pressure. Surrounding by neither cold nor very hot rocks, resulting in a medium rate of cooling. Naturally, such are medium grained, that is these are more coarse grained than volcanic rocks but more fine than plutonic rocks. Dolerite is the typical example.
  • 90. Classification based on depth of formation
  • 91. Structures and Textures • Structures and textures are physical features associated with the rocks. They are generally primary in nature. i.e.. they occur along with the formation of rocks and are important because. • (i) the contribute to the strength or weakness of rocks. • (ii) they serve as distinguished features of rock groups, and also, • (iii) They reveal the mode of origin of rock concerned. • The term structure used in this context is different from secondary geological structure like folds and faults which also occur in rocks.
  • 92. Structures and Textures • The term structure as commonly used refer to large-scale features or field characters of rocks such as bedding or columnar structure or pillow structure. • Textures on the other hand refers to smallscale features like porphyritic texture and interlocking texture which can be studied in hand specimen.
  • 93. Common Structures of Igneous Rocks • In igneous rocks the common structures are: • Vesicular Structure, amygdaloidal structure, columnar structure, sheet structure, flow structure and pillow structure.
  • 94. Common Structures of Igneous Rocks Vesicular Structure • This structure is due to the porous nature, commonly observed in volcanic rocks and is attributed to the following reasons. Magma is an intimate mixture of rock melt and volatiles (i.e.. gases). Eruption of any volcano is accompanied by flow of such melt and volatiles (i.e.. gases). Eruption of any volcano is accompanied by the flow of such melt on the surface. • Then the gases being lighter move upward and as they escape into the atmosphere create empty cavities of various sizes and shapes near the surface of lava flow. This cavities are called vesicles. Obviously, vesicles will be more nearer the top of lava flow and less at the base.
  • 95. Vesicular Structure
  • 96. Common Structures of Igneous Rocks • The degree of porosity in volcanic rocks due to the presence of vesicles varies widely. When the volcanic rock is highly porous and spongy in appearance, it is called scoria. The extreme cases of porosity produce a very light rock called pumice. It is usually pale grey in color and looks like a solidified form of foam or froth of lava. Pumice is so light that it floats on water.
  • 97. Common Structures of Igneous Rocks • The importance of this structure from the civil engineering point of view is as follows: • The vesicles, if plenty, make the rock hollow and less strong. Therefore, highly vesicular rocks are undesirable at foundation sites of huge civil construction like dams. • (ii) If the vesicles are interconnected with fractures, the rock become permeable too and behaves as an aquifer. Such a condition is good in terms of increasing ground water potential but the same is undesirable in tunneling as it may cause ground water problems.
  • 98. Common Structures of Igneous Rocks Amygdaloidal Structure • The vesicles which are empty to start with in the vesicular structure, are subsequent filled up by the deposits of surface waters or underground waters or hydrothermal solutions. Such infillings are called “amygdales”. When empty cavities are filled with amygdales, the vesicular structure is called an amygdaloidal structure
  • 99. Amygdaloidal Structure
  • 100. Common Structures of Igneous Rocks Columnar Structure • In this structure, the volcanic igneous rock appears to be made up of numerous parallel polygonal prismatic columns bundled together. This is the result of the contracting of lava during cooling. • Suppose , while flowing, lava occupies a depression on the earth’s surface. Then cooling of such lava commences from the surface downwards and the development of centres of contraction takes place on the cooling surface. The line joining these centres are the direction of great tensions. Therefore vertical cracks appear perpendicular to these lines. These cracks intersect each other and produce parallel polygonal prismatic columns. Ideal condition produces hexagonal columns.
  • 101. Columnar Structure
  • 102. Common Structures of Igneous Rocks Sheet Structures • In this structure, the rocks appear to be made up of number of sheets, because of the development of nearly horizontal cracks. This is the effect of erosion over rocks formed at a depth. • Plutonic rocks are formed under great depths, which means under great pressure (due to overburden). When erosion takes place, the overlying strata gradually disappears, ultimately exposing plutonic rocks on the surface.
  • 103. Sheet Structures
  • 104. Common Structures of Igneous Rocks • In this process, the earlier pressure no longer remain and this release or disappearance of pressure results in the development of joints and cracks, roughly parallel to the surface. These are sheet joints. As it may be inferred from the cause of the origin of this structure, these joints occur more prominently and closely spaced nearer the surface. With increasing depth, gradually, they become less significant and get more and more widely spaced. Ultimately, they ceases at considerable depths.
  • 105. Common Structures of Igneous Rocks
  • 106. Common Structures of Igneous Rocks Pillow Structure • In this structure, the volcanic igneous body appears as a pile of numerous overlapping pillow or sacks. It occurs particularly in soda-rich basaltic rocks known as spillities. • The pillows are generally interconnected and have glassy top. As the lava flows, its upper surface gets solidified, while the interior remains hot and fluid. Such a situation may result in the rupturing of the earlier formed thin crust and the draining out of unsolidified lava. This processes, when repeated, produces a pillow structure usually under submarine conditions.
  • 107. Pillow Structure
  • 108. Common Texture of Igneous Rocks • The term texture mainly refers to the mutual relationships of the constituents minerals in a rock in addition to crystalline, granualrity, and shapes of minerals in a rock. • Texture Based on the degree of Crystallinity • Igneous rocks are formed due to cooling and solidification of magma or lava. Crystallization of different minerals takes place when respective molecules in magma move to their centres of crystallization and arrange themselves in a definite pattern. If reasonable time is available, crystallization of different minerals take place from the melt giving rise to a rock.
  • 109. Common Texture of Igneous Rocks
  • 110. Common Texture of Igneous Rocks • But if sudden chilling of lava occurs, then there may not be any time for crystallization to take place. Under such conditions, lava solidifies as completely amorphous or glassy matter without any minerals. On the other hand, if cooling time is intermediate, then the resulting rock will be composed partly of glassy matter and partly of minerals.
  • 111. Common Texture of Igneous Rocks
  • 112. Common Texture of Igneous Rocks • Thus, depending on the nature of cooling the resulting igneous rocks are • (i) Completely crystalline • (ii) Completely glassy • (iii) partly crystalline and partly glassy Thus the preceding three different types of crystallization give rise to three textures of igneous rocks namely; • Holocrystalline,holohyaline, and merocrystalline.
  • 113. Common Texture of Igneous Rocks Texture Based on Granularity • Depending on the physical condition that had prevailed during the crystallization of magma, minerals grain occur in different sizes. The presence of volatiles, low viscosity, slow cooling and great pressure help to grow large minerals. • The following textures have been recognized based on the granularity of minerals. If minerals in the rock are big enough to be seen by the naked eye, the texture is described as phaneric texture. On the other hand, if minerals are too fine to be seen separately by the naked eye, the texture is described as aphanitic texture.
  • 114. Texture Based on Granularity
  • 115. Common Texture of Igneous Rocks Texture based on shape of crystal • These textures are of two different kinds; The first term of development of crystal faces or boundary outlines and the other with reference to the nature of the growth of the minerals. • When the mineral is completely bounded by crystal faces it is called “euhedral” ; when crystal faces are absent, it is called “anhedral” and when only a part of the mineral is bounded by crystal faces it is called “subhedral”. Euhedral minerals are rather rare in nature. The c/s of euhedral minerals as seen in their thin sections appear with polygonal boundary outline, whereas in the case of anhedral minerals, they appear rounded or surrounded in thin sections.
  • 116. Common Texture of Igneous Rocks
  • 117. Suitability of Igneous rocks for building and foundation • From the civil engineering point of view, the very purpose of studying petrology is to get a concept about what makes some rocks very competent and other less competent. Among the various types of rocks. Igneous rocks are inherently very competent and desirable for different civil engineering purposes.
  • 118. Suitability of Igneous rocks for building and foundation
  • 119. Suitability of Igneous rocks for building and foundation • The requirement of good building stones are; strength, durability, color, appearance, workability and availability. These properties of rocks except the last one are, in turn a function of their minerals, texture, structure, grain size, porosity and permeability.
  • 120. Suitability of Igneous rocks for building and foundation
  • 121. Suitability of Igneous rocks for building and foundation • The igneous rocks are composed mainly of silica minerals. Among various minerals, silicate minerals are the most durable. Further, rocks rich in silica content are pleasingly light colored as in the case of granite. • Since igneous rocks are formed out of solidification of a melt, they are necessarily dense, compact and massive. In other words, these rocks do not have any internal opening or hollow nature. This contribute to the strength and heaviness of these rocks. Further igneous rocks do not have an inhererent weakness due to the occurrence of bedding plane or mineral alignment.
  • 122. Suitability of Igneous rocks for building and foundation
  • 123. Suitability of Igneous rocks for building and foundation • Igneous rocks by virtue of their texture and minerals present in them have the ability to take very good polish and thus are become increasingly popular for face work .
  • 124. Megascopic Description of relatively common igneous rock types Granite • Granite is a plutonic igneous rock because it is formed due to solidification of magma at greater depth. It is holocrystalline and leucocratic rock because it is complete crystalline and light colored rock. Minerals present in granite • Granite is composed of only primary minerals. Among these, feldspar and quartz occur as essential minerals and common accessory minerals are such as hornblende, biotitic.
  • 125. Megascopic Description of relatively common igneous rock types
  • 126. Megascopic Description of relatively common igneous rock types Structure • Granite is compact, dense, massive and hard rock. But mural joints occur in some, dividing the rock into number of rectangular blocks, thereby facilitating the quarrying processes. Texture • Granites typically exhibit an interlocking, coarse grained texture. Granites are usually equigranular. But some shows inequigranular, textures and are called granite porphyries.
  • 127. Megascopic Description of relatively common igneous rock types Appearance of Granite • Granite is generally medium to coarse grained and grayish or pinkish in color. Mode of Occurrence and relative abundance • Granite rock occur in the form of very large igneous bodies such as batholiths, stocks, often they occur as cores of mountains ranges and are thus related to mountain-building activity.
  • 128. Granite
  • 129. Megascopic Description of relatively common igneous rock types Physical properties of Granite • Granite is massive, unstratified and dense, therefore it is very strong and competent • Granite has an interlocking texture, which keeps minerals firmly held and this cohesion contributes greater strength. • Granite is either equigranular or has porphyritic texture. • Since granite is massive and formed from melt it is neither porous nor permeable. So no saturation or percolation by water is possible.
  • 130. Physical properties of Granite
  • 131. Physical properties of Granite • Granite is very rich in silica; therefore it is very much resistant to decay • Presence of mural joint permit easy quarrying • Presence of rift and grain permit easy dressing. • Granites have the ability to take superb polish, and hence are becoming increasingly popular for face works of construction. • Granites offer reasonable fire and frost resistance, because minerals are not many and these rocks are free from fractures.
  • 132. Megascopic Description of relatively common igneous rock types Pegmatites • These are holocrystalline, phaneric-coares grained igneous rocks with an interlocking texture. • Many pegmatites are acidic and oversaturated. They resembles granites in mineralogy and hence are described as granite pegmatites.
  • 133. Pegmatites
  • 134. Pegmatites Minerals present in pegmatites • Granites pegmatites are mainly composed of alkali feldspar and quartz but may also be rich in muscovite and biotitic micas. Tourmaline, beryl, topaz, apatite and lepidolite minerals which are rich in rare volatiles also occur often. In addition to these, rare minerals of tin, arsenic etc. also occur in pegmatites. Thus pegmatites are storehouse of rare and valuable minerals.
  • 135. Pegmatites
  • 136. Megascopic Description of relatively common igneous rock types Mode of formation and occurrence of pegmatites • The peculiar grain size and mineral composition suggest that pegmatites are formed as product of solidification of final magmatic residues which are specially rich in volatile constituents. Structure and Texture • Pegmatites are rather less in occurrence and do not possess any specific structure diagnostic to them. But texturally, the minerals a large in size and interlocked Some of the constituent minerals develop very beautiful crystal outlines.
  • 137. Pegmatites Physical properties • Like granites, these rocks also have similar mineral content and interlocking texture . But from the civil engineering point of view, these rocks are not very useful because the extreme large minerals considerably influences the physical properties locally and hence the rock mass cannot behave uniformly throughout.
  • 138. Megascopic Description of relatively common igneous rock types Dolerite • The term dolerite was coined by Hauy to refer a dark, heavy, fine grained igneous rock. This was the most commonly found hyperbyssal rock. It is intermediate in composition and characteristically melonocratic. Miner logically and chemically it is similar to basalt. Mineral present in Dolerite • Dolerite is a rock, normally composed of feldspar and pyroxene as essential minerals. Iron oxides, hyperstene and biotitic occur as common accessory minerals. In general accessory minerals occur in very small quantities.
  • 139. Dolerite
  • 140. Megascopic Description of relatively common igneous rock types Mode of Occurrence • Very often, dolerite occur in nature as an intrusive rock i.e. as dykes (and less commonly as sills) in granites. These dark colored rocks are prominently noticed in the field by virtue of color contrast with surrounding granites which are light colored. Structure and Texture • Dolerite is very dense, massive and compact rock. It is neither porous nor permeable. It is relatively heavier than granite as it is richer in mafic minerals. The texture is generally equigranular, phaneric fine grained.
  • 141. Dolerite
  • 142. Megascopic Description of relatively common igneous rock types Physical Properties and Uses • Dolerite has all the merits and virtues possessed by granite, except its color. Since dolerites are more fine grained, they are stronger and more competent they are stronger and more competent than granites. They are suitable as railway ballast, concrete or bitumen aggregate, etc. As road metal, though they do not have good cementing values like limestone, they can be used if locally available.
  • 143. Megascopic Description of relatively common igneous rock types Basalt • The term basalt was first used by pliny to refer a black, ferruginous rock. Now the term is applied to very fine grained, black volcanic rock in which plagioclase feldspar and mafic minerals occur approximately in equal amounts. Minerals • Basalt is a simple mixture of Labradorite, augite(essential) and iron oxide. It is similar to dolerite in mineral content. Biotitic and hornblendes are the other rare accessory minerals. Structure and texture • Vesicular and amygdal structures are the most common in basalt. Columnar structures and flow structure are also observed in some cases.
  • 144. Basalts
  • 145. Megascopic Description of relatively common igneous rock types Availability and mode of Occurrence • Basalts are the most abundant among volcanic rocks. Their quantity is five times greater than all other volcanic rocks put together. They occur as flat or slightly inclined lava flow. Properties and Uses Massive basalts are highly durable and the strongest. This is because not only they are compact, hard and tough but also more fine grained than dolerite. For this reason basalts are extensively used as building stones. As a road metal, the basalts are excellent for macadam and bitumen roads. They are hard, tough and wear-resisting and have good binding properties.
  • 146. Thanks!
  • 147. Thanks!