Petrology-formation of rocks

1. Subject code: 21306006
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Index
PETROLOGY & its definition
Geological classification of rocks
Rock cycle
Civil Engineering importance of Petrology
Igneous Rocks
Igneous Rock types
Forms of Igneous Rocks
Texture of Igneous Rocks
Structure of Igneous Rocks
Metamorphic Rocks
Structures in metamorphic rocks
Texture of Metamorphism Rocks
Scope of Metamorphism petrology
Sedimentary Rocks
Texture of sedimentary rocks
Structure of Sedimentary Rocks

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5. •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.
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6.  The rocks are classified in various ways based on different principles
such as Physical, Chemical ‘n’ Geological Classifications. Among the
different classifications. Geological classification of rocks is the most
proper because grouping of rocks is more logical , less ambiguous, orderly
and comprehensive.
 The Geological classification of rocks is based on their MODE OF
ORIGIN. Their classification is.....
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8. Petrology is very important from civil engineering
point of view, as it provides a proper concept and logical
basis for interpreting physical properties of rocks.
 The study of texture, structure, mineral composition,
chemical composition etc., gives all necessary details
regarding the strength, durability, colour, appearance,
workability, etc.,
These inherent characters of rocks are of chief concern
for a civil engineer to judiciously assess the suitability
occurring at project site for required purpose.
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9. These are primary rocks, Most
abundant rocks in the earth’s crust.
These are formed at a very high
temperature and pressure conditions
directly as a result of solidification
of magma or lava.
MAGMA: The term magma is
applied when the melt is
underground.
LAVA: The melt when it reaches
the earth’s surface and flows over it,
is called lava.
SOME IGNEOUS ROCKS
COOLING OF LAVA
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10. •EXTRUSIVE ROCKS
•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, Andesite.
•INTRUSIVE ROCKS
•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;
•Granites, Syenites, Diorites, Gabbros, Peridotites.
•Hypabyassal rocks-
–These are formed at intermediate stage below the earth surface.
–They show mixed characters of volcanic and plutonic rocks.
•Dolerites. 10
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11. Extrusive
Lava flows
Pyroclastics
Intrusive
Sills
Dykes
Lopoliths
Laccoliths
Phacoliths
Batholiths
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12. Textures based on the
1) Degree of Crystallinity
2)Granularity and
3)Shapes of Crystals.
The size, form and orientation of clasts or
minerals in a rock is called its texture. The
texture is a small-scale property of a rock,
but determined many of its large-scale
properties, such as the density, porosityy or
permeabililty
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13. In Igneous rocks the
common structures are :
Vesicular structure
Amygdaloidal structure
Columnar structure
Sheet structure
Flow structure and
Pillow structure 13
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14.  Metamorphism (from Greek:
Meta=change & morphic=form)
The change in pre-existing
rocks under the influence of
temperature, pressure and
chemically active solutions.
The metamorphic rocks
formed from igneous rocks are
called Orthometamorphic rocks .
Those formed from
sedimentary rocks are called
Parametamorphic rocks.
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15. 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
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16. •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. 16
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17. •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.
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18. 1) Crystalloblastic
2) Palimpsest Textures
3) Xenoblastic
4) Idioblastic Textures.
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19.  Metamorphic petrology is the study of the subsolidus
changes that a rock undergoes when exposed to
physicochemical conditions different from those prevailing
near the surface of the Earth.
 Metamorphism produces changes in mineral assemblages
and textures that reflect re-equilibration of the rock to the new
physicochemical conditions, i.e., minimization of the energy of
the system.
Metamorphism is commonly isochemical (except for volatile
components, e.g., H2O, CO2). Where the system is open, the
process is referred to as metasomatism. 19
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20. 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
influence on rocks.
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21. •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
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