Petrology

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Petrology

  1. 1. Petrology Kaustubh J. Sane HJD Institute of Technical Education, Kera,
  2. 2. • Rock is a natural solid massive aggregate of minerals forming the crust of earth. • The branch of geology dealing with various aspects of rocks such as their formation, classification and occurrence is called petrology. • A civil engineer has to deal with rocks during most of his life as materials for construction and as sites for engineering structures.
  3. 3. • Volcanic rocks • Plutonic rocks • Hypabyssal rocks Igneous rocks • Clastic rocks • Non-clastic chemically formed • Non-clastic organically formed Sedimentary rocks • Foliated • Non-foliated Metamorphic rocks
  4. 4. Igneous rocks • Broadly, all rocks which are formed from an original hot, molten material through the process of cooling and crystallization are defined as igneous rocks. • Hot molten material occurring naturally below the surface of earth is called magma, and which comes on surface and starts flowing along it is called as lava.
  5. 5. • Volcanic rocks- – Igneous rocks formed by the cooling and crystallization of lava erupted from volcanoes. – As lava cools down faster rate the grain size of crystals is fine and often microscopic. – The Deccan traps of India spread over more than 4 lakh km is best example of volcanic igneous rocks. – Rock types are; • Basalt, Rhyolite, Dacite, Trachytes. • Plutonic rocks- – Rocks which are formed at an considerable depths-generally between 7-10 km below surface are called as plutonic rocks. – Due to slow rate of cooling grains are often coarse grain. – Rock types are; • Gabrro, granites, charnockites. • Hypabyassal rocks- – These are formed at intermediate stage below the earth surface. – They show mixed characters of volcanic and plutonic rocks. Rock types
  6. 6. • Textures of Igneous rocks • Holocrystalline: crystallised • Holohyaline: very fine size or glass • Microcrystalline: intermediate size Degree of crystallization • Coarse: grains above 5mm • Medium: grains between 5 to 1 mm • Fine: less than 1mm Granularity • Panidiomorphic: euhedral crystal • Allotriomorphic: anhedral form • Hypidiomorphic: show crystal of euhedra, subhedra and anhedra form Fabric
  7. 7. Holocrystalline Holohyaline Microcrystalline
  8. 8. Structures of Igneous rocks Structure due to mobility of Magma • Flow structure: formation of parallel or nearly parallel bands of igneous bodies. • Pillow structure • Ropy lava • Spherulitic structure: arrangement of fibrous minerals in radial manner. Due to cooling of magma • Jointing structure • Vesicular structure • Miarolitic structures Miscellaneous structures • Reaction rings • Xenolithic structure
  9. 9. Forms of Igneous Rocks Extrusive •Fissure eruption •Central eruption Intrusive •Sills •Dykes •Lopoliths •Laccoliths •Phacoliths •Batholiths
  10. 10. Sedimentary rocks
  11. 11. • Sedimentary rocks are formed due to simple or complex mechanical and chemical processes. • Origin of rocks – Provenance – Transportation – Deposition
  12. 12. Texture of rocks • Size – Coarse- gravel – Medium- sand – Fine- clay • Boulder- minimum size 256mm • Cobble- between 64 to 256mm • Sand- less than 2mm • Silt- 1/16 to 1/256mm • Clay- less than 1/256
  13. 13. Rudaceous •Conglomarate: loosely cemented heterogeneous material consisting of cobbles and pebbles. •Breccia: coarser cemented angular fragments. Arenaceous • Sandstones: weathered sand sediments after natural compaction forms sandstones. • Ferruginous: red brown color sst. Presence of iron containing minerals in cementation. • Siliceous: sand grains are cemented with quartz. • Calcareous sst: cemented with calcareous material • Arkose: sst with 60% quartz and 40% feldspar. Argillaceous • Shale: laminated rock. Calcareous • Limestone: 93% CaCO3, 5% MgCO3; whitish color • Dolomite: reverse of limestone. Blackish color carbonaceous • Peat • Lignite • Bituminous • Anthracite
  14. 14. Metamorphic rocks
  15. 15. • Metamorphism denotes transformation of rocks into new type by recrystallisation of their constituents. • The changes in metamorphism is due to temperature and pressure conditions in crustal layers 0f earth. • Agents of metamorphism: – Temperature – Pressure – Chemically active fluids
  16. 16. • The temperature increases in deeper parts of crust. • Pressure developed due to gravity results in hydrostatic pressure. Which produces non- uniform pressure, which changes the shape. • Chemically active fluids are imp factors, they occupies void spaces and fissures.
  17. 17. Structures in metamorphic rocks • Cataclastic- – Develops due to breakdown of fragmental rocks by shearing. – More resistant minerals undergoes less crushing; other cases less resistant mineral undergoes greater crushing. – eg. mylonite
  18. 18. • Maculose- – These is shown mainly by argillaceous rocks under thermal and contact metamorphism. • Schistose- – Rocks show more or less parallel bands. – Flaky minerals like biotite and hornblende under temperature and pressure conditions form parallel layered arrangements resulting in schistose structure.
  19. 19. • Granulose- – These are formed due to presence of subhedral grain minerals. – It shows more or less uniform grain size. – E.g. marble and quartzite. • Gneissose- – Formed due to alteration of schistose bands and granulose structure. – E.g. granite gneiss.
  20. 20. Classification of rocks • Contact metamorphism – In this process rise of temperature is dominant factor. Thermal effects are influenced by the contact zones of country rocks of igneous or sedimentary types. – Eg. Limestone – marble sandstone – quartzite
  21. 21. • Regional metamorphism- – Temperature and pressure affects a large regional area. – Shale---slate---phyllite---schist • Dislocation metamorphism:

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