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: