Download - Petrology
Petrology
Kaustubh J. SaneHJD Institute of Technical Education, Kera,
• 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.
•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
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.
• 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
• 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
Holocrystalline
Holohyaline
Microcrystalline
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
Forms of Igneous Rocks
Extrusive
• Fissure eruption
• Central eruption
Intrusive
• Sills • Dykes • Lopoliths • Laccoliths• Phacoliths• Batholiths
Sedimentary rocks
• Sedimentary rocks are formed due to simple or complex mechanical and chemical processes.
• Origin of rocks– Provenance– Transportation – Deposition
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
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
Metamorphic rocks
• 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
• 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.
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
• 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.
• 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.
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
• Regional metamorphism-– Temperature and pressure affects a large regional
area.
– Shale---slate---phyllite---schist
• Dislocation metamorphism: