Kaustubh J. Sane
HJD 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
• A civil engineer has to deal with rocks during
most of his life as materials for construction
and as sites for engineering structures.
• 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
– 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.
• Textures of Igneous rocks
• Holocrystalline: crystallised
• Holohyaline: very fine size or glass
• Microcrystalline: intermediate size
• Coarse: grains above 5mm
• Medium: grains between 5 to 1 mm
• Fine: less than 1mm
• Panidiomorphic: euhedral crystal
• Allotriomorphic: anhedral form
• Hypidiomorphic: show crystal of euhedra,
subhedra and anhedra form
Structures of Igneous rocks
Structure due to
mobility of Magma
• Flow structure:
parallel or nearly
parallel bands of
• Pillow structure
• Ropy lava
in radial manner.
Due to cooling of
• Jointing structure
• Reaction rings
Forms of Igneous Rocks
• Sedimentary rocks are formed due to simple
or complex mechanical and chemical
• Origin of rocks
Texture of rocks
– 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
•Conglomarate: loosely cemented heterogeneous material consisting of cobbles and pebbles.
•Breccia: coarser cemented angular fragments.
• 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.
• Shale: laminated rock.
• Limestone: 93% CaCO3, 5% MgCO3; whitish color
• Dolomite: reverse of limestone. Blackish color
• Metamorphism denotes transformation of rocks
into new type by recrystallisation of their
• The changes in metamorphism is due to
temperature and pressure conditions in crustal
layers 0f earth.
• Agents of metamorphism:
– Chemically active fluids
• The temperature increases in deeper parts of
• 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
– Develops due to breakdown of fragmental rocks
– More resistant minerals undergoes less crushing;
other cases less resistant mineral undergoes
– eg. mylonite
– These is shown mainly by argillaceous rocks under
thermal and contact metamorphism.
– 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.
– These are formed due to presence of subhedral grain
– It shows more or less uniform grain size.
– E.g. marble and quartzite.
– Formed due to alteration of schistose bands and
– 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
– Eg. Limestone – marble
sandstone – quartzite
• Regional metamorphism-
– Temperature and pressure affects a large regional
• Dislocation metamorphism: