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  1. 1. Petrology Unit-III
  2. 2. Petrology Petrology • Introduction, Shell Structure of Earth, Definition of Rocks, Classification of Rocks, Sequence of Formation of Different Groups of Rocks, Rock Cycle, Civil Engineering Importance of Petrology.
  3. 3. Introduction • Petrology means Study of Rocks ( Petro= rock; and logos = study) • The subject matter of petrology comprises of origin, association, occurrence, minerals composition, chemical composition, texture, structure, physical properties, etc. of rocks. • Petrolography deals with the descriptive part of the rock and petrogeny (geny= genesis= origin) deals with the mode of formation of rocks. This to gather make petrology.
  4. 4. Petrology
  5. 5. Shell Structure of the Earth • The crust (the outermost solid part) of our earth is made up of different kind of rocks. The thickness of the crust is approximately 35 km. Below this is separated from the mantle by the Moho discontunity. This rock crust is approximately called lithosphere (litho= rock; sphere= zone or shell). The surface of this crust is uneven with many depression and elevations. These depressions are filled with water and are variously called lakes, seas and oceans. This discontinuous body of water in association with lithosphere is called hydrosphere. On land masses of the lithosphere and in water bodies of the hydrosphere, all living creatures exist. This is described as the biosphere. All these, in turn are enveloped by a layer of air which is called the atmosphere.
  6. 6. Shell Structure of the Earth
  7. 7. Shell Structure of the Earth
  8. 8. The Earth's Composition
  9. 9. Shell Structure of the Earth • In the interior of the earth, i.e. below the lithosphere, the inaccessible part is called barysphere. Within this lies the asthenosphere ( or sphere of weakness) This Zone of somewhat plastic or semi-solid matter which makes isostacy possible by allowing necessary movement of matter from one place to another. The interior being possible by allowing necessary movement of matter from one place to another. The interior being very hot, it creates condition favorable for magma occurrence. This part where magma originates is called the pyrosphere. This part of mighty tectonic forces which are responsible for all major surface disturbances, i.e. either orogeny or epeirogeny. Hence that part of interior is called the tectonosphere.
  10. 10. Shell Structure of the Earth
  11. 11. Shell Structure of the Earth
  12. 12. Definition of a Rock. • In spite of the vast variety and complexities of rocks, a rock may be simply defined as “ an aggregate of minerals” Since the crust of the earth is composed of rocks, a rock may also be defined as “ a unit of the earth’s crust”. There are many mineral species occurring in nature. Since the occurrence of different minerals or different proportions of the same minerals can give rise to different rock types, there is scope to imagine innumerable varieties of rocks. There are different types of rocks but have the same minerals though in different proportions.
  13. 13. Definition of a Rock
  14. 14. Definition of a Rock. • In spite of the scope for the occurrence of a large number of rock types, close observation reveals that in nature only some rock types are common. Further, among the large variety of mineral species, only very few take part in the formation of different rocks. These minerals are approximately described as rock- forming minerals.
  15. 15. Definition of a Rock
  16. 16. Classification of Rocks • The rocks are classified in various ways based on different principles such as physical classification ( as Stratified and unstratified), chemical classification as (Calcareous, siliceous etc) geological classification (igneous, sedimentary , metamorphic) etc. Among the different classification, geological classification is the most proper because grouping of rock is more logical, less ambiguous, orderly and comprehensive. The geological classification of rock is based on mode of origin.
  17. 17. Classification of Rocks
  18. 18. Classification of Rocks Igneous Rocks (lignum= fire, meaning very hot) • As mentioned later, igneous rocks are the first formed rocks which had made up the primordial earth’s crust. For this reason these are called primary rocks, even though igneous rocks have been formed subsequent also. Igneous rocks are the most abundant rocks in the earth’s crust. In fact, their abundance is so much that their average composition closely tallies with the chemical composition of the earth’s crust itself. The igneous rocks are formed at a very high temperature directly as a result of solidification or assimilation.
  19. 19. Classification of Rocks
  20. 20. Igneous Rocks Magma and Lava • Both these refer to melt of rocks which are compositionally silicate rich minerals. The term magma is applied when the melt is underground. The same, when it reaches the earth’s surface and flows over it, is called lava. Thus based on the mode of occurrence, the melt of rocks is described either as magma or lava. Apart from this, a little compositional difference also occurs between the two. Magma is always associated with huge quantities of various volatiles. But when it flows on the surface, these volatiles escape into the atmosphere. Therefore the lava is devoid of such volatiles.
  21. 21. Igneous Rocks
  22. 22. Igneous Rocks Typical Characters • The igneous rocks are often characterized by the presence of crystalline minerals and an interlocking texture. These show indications of having been formed from a high temperature rock melt. They are usually massive, unstratified, unfossiliferous, have apophyses and often occur as intrusive cutting across other rocks which they might have heated, baked and altered. The volcanic igneous rocks are always extremely fine grained and they may be massive or vesicular or amygdaloidal.
  23. 23. Igneous Rocks
  24. 24. Igneous Rocks Formation, Occurrence and Crystallization of Magma • Magma is the parent material of igneous rocks. Anywhere on the earth, the temperature increases proportionally with the depth. The rate of increase of temperature of this kind, however, varies place wise and depth wise. So, it is natural to expect that at very great depth, the prevailing temperature must be capable of melting the rocks, thereby producing magma. This is one of the reasons for the formation of magma. The heat generated by processes such as radioactive minerals disintegration may also help this process. But below the earth, magma does not occur everywhere. This is because, just as temperature increases with depth, pressure also increase with depth due to increase of overburden. The effect of these two mutually associated phenomenon are different.
  25. 25. Igneous Rocks
  26. 26. Igneous Rocks • The rise in temperature tends to increase the volume of the material, whereas the rise in pressure tends to decrease the material. Hence depending upon the local conditions where the pressure effect is less than the effect of temperature effect, magma can form because the formation of hot magma from rocks has to be accompanied by necessary volume increase. If pressure is dominant, it will not permit increase in volume. That means magma is not allowed to form, even though the prevailing temperature may be capable of producing magma. In a view of these complications, and other factors, magma occurs below the earth only as isolated packets or chambers and not as continuous body. It is locally generated due to loss of pressure when deep seated fractures reach high temperature areas at a depth.
  27. 27. Igneous Rocks
  28. 28. Igneous Rocks • After its formation, the magma moves upward and gets surrounded by relatively colder rocks which results in gradual loss of heat from the body of the magma, because of this, solidification or crystallization of magma follows. The rate of cooling of magma depends on the size of magma chamber and other condition, Small bodies of magma will solidify quickly and directly producing homogeneous igneous rocks. But in case of huge magmatic bodies, the process of solidification is often accompanied by differentiation or assimilation or both. • Differentiation is a process whereby a magma, originally homogeneous, splits into parts of contrasted composition and then solidifies giving rise to rocks of different compositions. • Or sometimes two liquid magmas of different compositions may mix together forming new homogeneous magma. These phenomenon is called assimilation.
  29. 29. Igneous Rocks Sequence of Events in the Crystallization of Magma • Magma is composed of non-volatile and volatile constituents. With falling of temperature, when magma begins to crystallization, the non-volatile constituents come out to form the rock-forming minerals, i.e.. Mostly the silicates, which are characteristic of the usual igneous rocks this stage is called orthomagmatic stage. The separation of the non-volatile constituents leads to the concentration of the volatile components, with the result that the residual portion of the magma forms the pegmatite stage and the gaseous parts forms the pneumatolytic stage. The final consolidation of magma takes place from the last hot water-rich mineralized solutions. This is hydrothermal stage.
  30. 30. Igneous Rocks
  31. 31. Sedimentary Rocks • Sediments are the products of weathering. Since these are secondary materials (i.e. derived from pre-existing rocks), the rocks formed out of them are called sedimentary or secondary rocks. The origin of sedimentary rocks is totally related to the weathering influences on rocks. In-essence • (i) the intense decay of rocks, followed by leaching, leaves behind some insoluble, inert, porous, residual matter. • (ii) The disintegration produces loose rock debris or sediments of various sizes which on compaction or cementation produces the most common and abundant type of sedimentary rocks like shale and sandstone. • (iii) The leached out parts give rise to sedimentary rocks due to chemical processes such as evaporation and precipitation or deposited through the agency of organisms. Thus the origin of sedimentary rocks mainly takes place in four different ways in nature.
  32. 32. Sedimentary Rocks
  33. 33. Sedimentary Rocks • In lakes or seas, the sedimentary rocks occur in layers in a chronological sequence (i.e.. With the order of superposition which means older rocks occur at the bottom and successively younger rocks at the top. The layer differ from one another in minerals composition or chemical composition or in a set of such rocks, which are in sequence are called strata. • The contact planes of adjacent layers or beds are called bedding planes. Each bed indicates the sedimentation that had occurred at a stretch and each bedding plane indicates a brief period of non-deposition of sediments.
  34. 34. Sedimentary Rocks
  35. 35. Sedimentary Rocks Typical Characters • The general characteristics features of sedimentary rocks are • (i) lamination or bedding or stratification • (ii) cross-bedding or current bedding or torrential bedding • (iii) presence of cementing material • (iv) Occurrence of fossils • (v) Occurrence of tracks and trails • (vi) Occurrence of mud cracks, rain prints, etc. • (vii) porous, concretionary, nodular structures • (viii) Olitic, pusolitic and stalactite forms • (ix) ripple marks.
  36. 36. Sedimentary Rocks
  37. 37. Methamorphic Rocks • These are the third major group of rocks occurring in nature. Their mode of origin differs from that of igneous or sedimentary rocks. They are formed from any pre-existing rocks under the influence of temperature, pressure and chemically active solutions. The pre-existing rocks should naturally be either igneous or sedimentary rocks only. • Metamorphism literally means change. In the geological sense, the term is used to denote the occurrence of textural, mineralogical or chemical changes, etc. That have taken place in pre-existing rocks under the influence of temperature, pressure and chemically active solutions. Metamorphism occurs in rocks because of the two major groups of rocks, i.e.. Igneous and Sedimentary rocks, are formed under certain physical and chemical condition.
  38. 38. Metamorphic Rocks
  39. 39. Methamorphic Rocks Typical Characters • The metamorphic rocks generally have the following special features • (i) Some minerals like staurolite. Andalusite, sillimanite, which are products of metamorphism are found only in metamorphic rocks of course, these are not very common minerals. • Some structures and textures like lineation, foliation, schistosity and slaty cleavage are characteristic of most of the metamorphic rocks. • Usually, metamorphic rocks are composed of coursed grained minerals. But sometimes, the metamorphic rocks retain the texture and structure of parent igneous rocks or sedimentary rocks Such relict features are called palimpset structure and texture.
  40. 40. Methamorphic Rocks
  41. 41. Methamorphic Rocks Limits of Metamorphism • The normal concept of metamorphism is that “ changes in three factor of temperature, pressure and chemical environment”, disturb the physical and chemical equilibrium of a mineral assemblage, and metamorphism results from the efforts to establish a new equilibrium. The processes of weathering and cementation which take place at nearly normal surface temperature and pressure are generally not considered as true metamorphism. But according to some, these processes also represent metamorphism because they consider even alteration as equivalent to metamorphism were the metamorphic rocks.
  42. 42. Methamorphic Rocks
  43. 43. Methamorphic Rocks • Thus, from the preceding discussion it is clear that igneous, sedimentary and metamorphic rocks were formed in sequence, in the beginning. But it should be remembered that subsequently all these three groups of rocks have formed again and again as and when suitable conditions prevail.
  44. 44. Methamorphic Rocks Relative Abundance of Rock Types • If the metamorphic rocks are grouped with their parent rocks, the igneous group (i.e.. Igneous rocks and ortho-metamorphic rocks) comprises 95 % of the earth’s crust quantitatively; the remaining 5 % belonging to the sedimentary group. Among the different kind of igneous rocks, granites and basalts are the most abundant. Granites, which are typical plutonic rocks occur on the earth’s surface as continental bodies, mountain cores, etc. These are underlain by basaltic rocks, which are typical volcanic rocks, as ocean floors, Silicon and aluminium rich felsic minerals occur in granites and silicon and magnesium rich mafic minerals occur in basalts. Basically two types of magmas occur in the earth’s crust; granitic and basaltic.
  45. 45. Methamorphic Rocks
  46. 46. Metamorphic Rocks • Among the different groups of sedimentary rocks, nearly 80 % are argillaceous (shale's), 15 % are arenaceous (sandstones) and 5 % are calcareous (limestones) other rocks are less common. • As pointed out, the sedimentary group of rocks account quantitatively only for 5 % of rock of earth’s crust. But from the civil engineering point of view, sedimentary rocks are more abundant igneous rocks occupy a much lesser area, i.e. the remaining 30 % of the area of the earth’s surface.
  47. 47. Metamorphic Rocks • This is so because igneous rocks occur as huge voluminous bodies continuing to great depths. But sedimentary rocks occur as relatively thin veneers or beds, spreading over large areas. This explain how the quantitatively negligible sedimentary rocks occur over very wide area. For this reason, constructional works of civil engineering are more likely to rest upon the sedimentary rocks. Therefore, it is natural that details of sedimentary rocks are more important in civil engineering.
  48. 48. Metamorphic Rocks
  49. 49. Rock Cycle • The three group of rock, i.e.. Igneous, Sedimentary and metamorphic, which occurs in nature give place to one another as explained below • Igneous rocks, as a consequence of weathering, give rise to sedimentary rocks, further under the influence of metamorphism they also give rise to Ortho-metamorphic rocks. • Sedimentary rocks, when buried to great depths, form magma on melting and subsequently solidify to form igneous rocks and, under the influence of metamorphism, change over to para-metamorphic rocks. • The metamorphic rocks, like the igneous rocks, on weathering, give rise to sediments which form sedimentary rock later. The metamorphic rocks, like the sedimentary rocks, when buried to great depths due to tectonic activity will melt and give to magma, from which igneous rocks are formed. • Thus one group of rocks give rise to another in nature due to geological reasons and this phenomenon is called “Rock Cycle”
  50. 50. Rock Cycle
  51. 51. Civil Engineering Importance of Petrology • Petrology is very important from the civil engineering point of view, because this provides a proper concept and logical basis for interpreting physical properties of rocks. Thus, the study of texture, structure, mineral composition, chemical composition, etc. Gives all necessary details regarding the strength, durability, colour, chemical composition, etc. These inherent characters of rocks, occurring at concern for a civil engineer to judicious assess the suitability or otherwise of rock are of chief concern for a civil engineer to judiciously assess the suitability or otherwise of rocks, occurring at his project site for the required purpose. This is necessary because different kind of rocks are suitable for different specific purpose and no rock is ideal or best suited for all kind of purposes of construction.
  52. 52. Civil Engineering Importance of Petrology
  53. 53. Civil Engineering Importance of Petrology • For ex, granite-like rock being hard, competent, durable and free from weak planes are suitable for foundation purposes. Stones like Cuddapah, slab and Shahabad type limestone are suitable for flooring and roofing purpose. Marble by virtue of its attractiveness color and softness is the most sought after for face work, statue making, decorative and other sculptural work. Sandstone being easily workable can be neatly dressed and hence are suitable for construction of wall. • Thus different kind of rocks suit different purposes by virtue of their special physical properties which are inherent and characteristic to them. These details must be known so as to make the best use of available rocks at the construction site.
  54. 54. Civil Engineering Importance of Petrology
  55. 55. • Engineering and General Geology :By Parbin Singh • Textbook of Engineering Geology :N.Chenna Kesavullu References
  56. 56. Thanks !