Petrology is the study of rocks, including their origin, composition, structure and distribution. There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks form from the cooling of magma either below or above the Earth's surface. Sedimentary rocks form through the compaction and cementation of sediments. Metamorphic rocks form from the alteration of existing rocks through heat, pressure, and chemically active fluids in the Earth's crust.

1. Petrology
Petrology is the branch of geology that deals with the study of origin,
composition, distribution and the structure of rocks.
Petrography: it is the branch of geology that deals with the description
and classification of rock, especially by microscopic examination of thin
section.
Petro genesis: deals with the origin and formation of rocks.

2. Rock
• It is naturally occurring solid aggregate of one or more minerals.
Rocks are of three types; igneous, metamorphic and sedimentary
rocks.

4. Igneous rocks
Igneous rocks are the primary rocks, which are formed due to cooling
and solidification of magma.
Magma is a hot viscous, siliceous melt, containing water vapour and
gases. Magma comes out from the greater depth below the earth
surface, such magma is called lava.

5. Classification of igneous rocks
A. On the basis of mode of occurrence
1. Intrusive rocks: slow cooling of magma inside the earth.
Slow cooling= big crystal
ex: granite , gabbro, pegmatite, diorite
Intrusive igneous rocks are classified as;
i. Plutonic rocks: formed at larger depths(7-10 km) below the surface of
the earth. Ex; gabbro, granite
ii. Hypabyssal rocks: formed by cooling of magma at shallow depth
below the earth´s surface (2-3 km). Ex; porphyries

6. 2. Extrusive(volcanic) rocks: cooling of magma at the surface of the
earth.
Fast cooling = tiny crystal
Ex: obsidian, Basalt, rhyolite, andesite

7. B. Chemical composition
• On the basis of silica content present in the rock;
Silica content Rock Example
66% Acidic Granite, rhyolite
52- 66% Intermediate Diorite, andesite
45- 52% Basic Gabbro, basalt
45% Ultrabasic peridotite

8. c. Basis of mineral composition
Felsic rocks Mafic rocks Intermediate rocks Ultramafic rocks
Rocks containing light
coloured minerals like
silica, feldspar and
muscovite.
Rock containing dark
colored minerals like
pyroxene, amphibole,
olivine, biotite, iron oxide.
They are rich in
magnesium and iron
Contains felsic as well as
mafic minerals.
Rocks containing mostly
mafic minerals. Low silica
content.
Example: Granite,
Rhyolite
* felsic- feldspar and silica
in abundance
Example: basalt, dolerite
and gabbro.
* mafic- magnesium and
ferric (iron) in abundance
Example: andesite, diorite Example: carbonatites

9. IUGS classification of igneous rocks
• Based on mineralogical composition (MODE)
• Quartz (Q), alkali feldspar (A) and plagioclase (P) is represented by the
corners of equilateral triangle, the length of whose sides are divided
into 100 equal parts. Any composition plotted at a corner, it
represents 100% of the corresponding minerals.
• Any point on the sides of triangle represents a mode of composed of
two adjacent corner components.

11. Structure of igneous rocks
1. Vesicular structure
2. Amygdaloidal structure
3. Columnar structure
4. Flow structure
5. Pillow structure
6. The spherulitic structure
7. The orbicular structure

15. Forms of igneous rocks
1. Concordant bodies
2. Discordant bodies

16. Types of concordant bodies
• Sill
• Phacolith
• Laccolith
• Lopolith

21. Types of discordant bodies
• Dyke
• Batholith
• Volcanic neck
• Stock and boss
• Xenolith

26. Texture of igneous rocks
Texture is the intimate mutual relationship of the constituents of rocks.
That is, deals with size, shape, nature and arrangements of igneous
rocks. Texture is best studied in thin section under microscope rather
than in the outcrop.
Required considerable factors for the description of texture;
1. Degree of Crystallization or Crystallinity
2. Crystal shape (Fabric)
3. Granularity (absolute size of the crystal, grain)

27. • Crystal : when the molecules in a rock have settled down into a stable
arrangement and with a recognizable pattern is called crystal.
• Glass : it is deformed state of crystallization, regarded as an
amorphous solid. In glass, the molecules have settled down into a
stable arrangement without any recognizable pattern

28. Major factors for the determination of
crystallinity
1. Rate of cooling of magma
2. Viscosity of magma
. Rapid cooling and high viscosity favour the formation of glass
. Slow cooling and low viscosity promote the formation of crystal

29. A. Crystallinity
Crystallinity is measured by the ratio subsisting between crystallized
and non crystallized matter.
Types of crystallinity
1. Holocrystalline
2. Holohyaline
3. Hemicrystalline/ hypocrystalline/ merocrystalline

30. 1. Holocrystalline
A rock composed entirely of crystal is said to be holocrystalline.
Holocrystalline (holo = complete) texture is characterized by deep-
seated or intrusive igneous rocks. Example: Granite, gabbro, syenite

31. 2. Holohyaline
• The rock which is entirely composed of glassy materials or glass is
known as holohyaline. Hyaline ( glassy or amorphous)
• They occur most often marginal facies of rock bodies but may occur
as lava (obsidian) or as dykes and sills (pitchstone).

32. 3. hemi/hypo/ merocrystalline
• When the rock is composed partly of crystals and partly of glass, the
term mero/Hypo/ Hemicrystalline is used. Example: Rhyolite,
Trachyte.

33. B. Crystal shape (Fabric)
• The fabric or pattern of the rocks depend on the shapes, relative size
and arrangement of the crystals.
i. Shape of the crystal
ii. Mutual relationship of crystal

34. i. Shape of crystal
• Crystal forms are described with references of the development of
their faces.
• i. euhedral: crystals are bounded by faces.
• Ii. Subhedral: crystals are partly bounded by faces.
• Iii. Anhedral: crystals are not bounded by faces.

35. ii. Mutual relationship
The fabric of a rock is influenced not only by the shape of the crystals,
but their relative size and their mutual arrangement.
Mutual relation classified as;
i. Equigranular
ii. Inequigranular

36. Equigranular texture
• Grains are more or less same size.
Granites show equigranular texture.

37. Inequigranular texture
• Grains are of different sizes.
i. Porphyritic texture:
When the larger crystals are embedded/ enveloped by smaller crystals
or groundmass.
ii. Poikilitic texture:
When the smaller crystals are enclosed within the larger ones.
iii. Ophitic texture:
It is special variety of poikilitic texture in which plates of augite enclose
numerous thin laths of plagioclage.

38. C. Granularity
• Deals with the absolute size of the crystals in igneous rocks ranges
from almost sub microscopic dimensions to crystal measurable in
yards.
• Types of granularity ;
a. Phaneric or phanerocrystalline
b. Aphanitic or microcrystalline
c. Cryptocrystalline

39. a. Phaneric or phanerocrystalline
• If the crystals are visible to the naked eye or by ordinary hand lens,
the rock is said to be phaneric or phanerocrystalline.
• Phaneric is further classified by;
i. Coarse Grain: ›5
ii. Medium Grain: 5 mm to 1 mm
iii. Fine grain: ‹1 mm

40. b. Aphanitic
• If the crystals are not visible to the naked eye or by ordinary hand
lens, the rock is known to be aphanitic.
• *crystals of rocks are visible only through petrographic microscope.

41. c. Cryptocrystalline
• Individual crystals are not distinguishable with the help of petrographic
microscope.
a. Intergranular texture:
When triangular or polygonal interspaces between the crystals are entirely
filled with granules of other minerals.
b. Intergrowth texture
When two minerals crystallize simultaneous, they may result intergrowth
texture. Example ; graphic ( intergrowth between orthoclase and quartz),
myrmekite (intergrowth between plagioclase and quartz).
c. Directive texture:
These textures indicate the result of flow of magma during the formation of
rocks. Example : trachytic texture.

42. Intergranular texture

43. Intergrowth texture

44. Sedimentary rocks
• The rock which are formed from the pre-existing rocks by
accumulation, compaction( presence of water) and consolidation of
sediments are called sedimentary rocks.

45. Classification of sedimentary rocks
• Based on the mode of formation;
1. Clastic rock
2. Non- clastic rock

46. 1. Clastic rock
• Clastic rocks are made up of pieces (clast) of pre-existing rocks.
• Pieces of rocks are loosened by weathering, and then sediments are
transported to some basin where they get piled up and lithified by
compaction and cementation to form clastic sedimentary rocks.
Name of sediment Size of sediment
Clay ‹1/256 mm
Silt 1/16 mm – 1/256 mm
Fine sand 1/4 mm- 1/16 mm
Medium sand 1/2 mm – 1/4 mm
Coarse sand 2 mm – 1/2 mm
Sand 2 mm – 1/16 mm
Gravel 2 mm - 16 mm
Pebble 64 mm- 16 mm
Cobble 256 mm – 64 mm
Boulder ›256 mm

47. On the basis of size of clasts, sedimentary
rocks are further classified as;
a. Rudaceous rocks : rocks consisting of coarse grained materials like
gravels, pebbles, cobbles and boulders.
b. Arenaceous rocks: rocks consisting of sand of variable grain size.
c. Argillaceous rocks: rocks consisting of sediment of very fine grain
size i.e. dust, mud, clay, silt etc.

48. 2. non-clastic rocks
• Formed from chemical and biological sedimentation process and
shows to evidence of derivation from pre- existing rocks or
mechanical deposition. Classified as;
• i. chemical sedimentary rocks
• Ii. Organic sedimentary rocks

49. Chemical sedimentary rocks Organic sedimentary rocks
Formed by the precipitation of soluble materials
followed by compaction and consolidation
Formed when large number of living things die, pile
up, cemented and compacted to form rocks.
Siliceous : containing silica. Eg, chert Carbonate: calcareous composition
Calcareous: containing carbonate. Eg, limestone Carbonaceous: carbon bearing rock
Ferruginous : containing iron Phosphatic : phosphate composition
Phosphatic: containing phosphate Ferruginous : iron composition
Evaporites: Gypsum

50. Structure features in sedimentary rocks
1. Stratification
2. Lamination
3. Graded bedding
4. Cross bedding
5. Ripple marks
6. Mud cracks
7. Rain print
8. Tracks and trails

51. Stratification
• Deposition of sediments into distinct layers or beds is called
stratification.

52. Lamination
• Thin bedding, less than 1 cm thickness are called lamination. The
individual layers are called laminae and are distinguished commonly
on the basis of difference in color.
• Commonly observed in fine grained sedimentary rocks like shale.

53. Graded bedding

54. Cross bedding
• Minor beds are inclined at an angle to major horizontal bedding
plane. Boundary between sets of cross bed represented an erosional
surface.
• Commonly found in shallow water and wind deposits.
• It indicates rapid changes in velocity and direction of flow of streams
or wind carrying sediments.
• Sandstone shows the cross bedding structures.

55. Cross bedding

56. Ripple marks
• Ripple marks are wavy undulation seen on the surface of bedding
plane. Marks indicate the orientation of water current and winds in
the past. Ripple marks may be symmetrical or asymmetrical.
• Symmetrical ripple marks are formed due to wave action and
assymetrical ripple marks are formed due to current actions. Eg best
observed in sandstone.

57. Ripple marks

58. Mud cracks
• Mud cracks are polygonal cracks formed due to alternate wetting and
dying of sediments. Commonly found in fine grained sedimentary
rocks. They form a network of fissures and they develop under
shallow water conditions only.

59. Rain print
• It is a shallow depression seen on the surface of some argillaceous
rocks due to the impact of rain drop.
• It is formed when a brief rain shower falls on a smooth surface of fine
grained sediment.

60. Tracks and trails
• Track and trails are the marking indicating the passage of some
animals or worms over a soft sediment, which is able to take and
retain impression.
• Footprints of amphibians, reptiles, birds, mammals may occur this
way.

61. Texture of sedimentary rocks
1. Origin of grains
2. Crystallization trend

62. Origin of grain
• Rocks formed by mechanical sedimentation process have clastic
texture. Rocks formed by chemical and organic processes have non-
clastic texture.
i. Clastic texture:
ii. Non- clastic texture
a. Size of grains
Coarse grained Average clast size › 5 mm
Medium grained 1- 5 mm
Fine grained ‹1 mm

63. b. Shape of grains;
i. Rounded
ii. Sub- rounded
iii. Angular
c. Packing of grains:
i. Openly packed
ii. Densely packed: clasts are packed tightly
d. Sorting of grains
i. Poorly sorted
ii. Well sorted

64. ii. Non-clastic texture
Macro crystalline Clast size ›20 micron
Microcrystalline Clast size ‹2o micron
Cryptocrystalline Clasts are to small to be
distinguished even under
microscope

65. 2. Crystallization trend
• Degree and nature of crystallized grains. Rocks may shows perfectly
interlocking grains giving rise to crystalline granular texture or they
may be made up of non- crystalline, colloidal particles when they are
termed as amorphous.

66. Metamorphic rocks
• Meta = change, morph = form
• It is a process that takes place when the heat and pressure deep in
the Earth´s crust cause rocks to change their original mineral
compositions and textures.
• Agents of metamorphism;
Heat
Pressure
Chemically active fluids and gases

67. Types of metamorphism
1. Thermal metamorphism (contact metamorphism)
2. Dynamic metamorphism
3. Dynamothermal metamorphism (regional metamorphism)
4. Geothermal metamorphism
5. Metasomatism

68. Thermal metamorphism( contact metamorphism)
• It is a process of formation of new mineral by reaction between the
contact rock and the escaping high temperature gaseous emanation
with other important materials from the magma chamber. The
magma must contain the ingredients of mineral deposit and must be
intruded at depth at the contact of reactive rocks.
• In this process, minerals grow haphazardly in all directions and the
metamorphic rocks acquire a granular fabric which is called hornfels
texture. Example, non- foliated rocks such as marble, quartzite and
hornfels.

70. Dynamic metamorphism( cataclastic metamorphism)
• Occur mainly due to direct pressure or stress is called cataclastic
metamorphism. With the little heat, when directed pressure acts,
rocks are forced to move past one another resulting in their crushing
and granulation. This kind of effect is called catalysis.

71. Dynathermal metamorphism ( regional
metamorphism)
• When directed pressure and heat act together in the presence of
migrating hydrothermal fluids, the rocks are metamorphosed over
wider area is called regional metamorphism. It can completely change
the mineralogy and texture of pre-existing rocks.
• It produces both foliated rocks such as slate, phyllite, schists, and
gneiss, and non – foliated rocks such as quartzite and marble.
• When shale are subjected to regional metamorphism, it changes to
slate in early stages, to schist in the middle stage and finally to gneiss
at the highest temperature of regional metamorphism.

72. Geothermal metamorphism
• Uniform pressure and heat are predominant is called geothermal
metamorphism.
• Usually, this will not bring out any important changes in the silicates
rocks. But oceanic salt deposits, known for variety of their minerals
have suffered considerable changes in this way.

73. Metasomatism
• It is the process in which original composition of rocks are changed
primarily by the addition or removal of material. This change is
caused by the movement of hydrothermal fluids through rocks usually
under high temperatures and pressures.

74. Grade of metamorphism

75. Metamorphic minerals
1. Stress minerals (direct pressure ):
Flat, tabular, elongated in nature and grow parallel to the direction of least
pressure. Example, mica, chlorite, talc, amphiboles, kyanite (regional
metamorphism)
2. Anti- stress minerals (uniform pressure):
Equidimensional in form , example: pyroxene, olivine, andalusite, silimanite.
3. Relict minerals;
Crystallization of minerals take place essentially in the solid state. Certain
minerals continue to survive in the metamorphic rocks.
These original minerals which have failed to react to the changed conditions
of temperature and pressure are called relict minerals.

76. Visible changes produced by metamorphism
in rocks

77. Classification of metamorphic rocks
Foliated rocks Non – foliated
Flat or elongated minerals are aligned in roughly
parallel planes.
Contain only one mineral
Platy or sheet like structure No structure
Contain different kinds of minerals Quartzite, marble, amphibolite
Slate, schist, gneiss

78. Structure of metamorphic rocks
1. Cataclastic structure
2. Schistose structure
3. Granulose structure
4. Gneissose structure
5. Maculose structure
6. Augen structure

79. Cataclastic structure
• It is produced under stress and in absence of high temperature,
whereby rocks are subjected to shearing and fragmentation. Only the
durable minerals partly survive the crushing force and the less
durable ones are powdered. When the resistant minerals and rock
fragments stand out in pseudoporphyritic manner in the finer
materials, its known as porphyroclastic structure.
• When the rocks are highly crushed into fine grained rocks, they are
called mylonites. All these structures are formed due to cataclasis,
they are as a whole known as cataclastic structure.

80. Schistose structure
• Formed under intense stress conditions.
• Characterised by the presence of parallel or sub parallel layers of
platy or flaky minerals. Example , schist

82. Granulose structure
• Formed by the recrystallization of minerals under uniform pressure
and high temperature.
• Presence of equidimensional minerals like quartz, feldspar and
pyroxene in layer proportion while the flaky minerals are either
absent or present in small proportion. Example, marble, quartzite

83. Gneissose structure
• Formed under the effect of directed pressure and heat. Characterised
by the presence of alternate bands of schistose ( dark colored) and
granulose ( light colored). Gneiss

85. Maculose structure
• Formed by incomplete recrystallization of minerals under high
temperature and pressure.
• Characterised by the presence of spotted appearance which may be
due to the development of large sized crystals( porphyroblast) within
the body of fine grained rocks. Example, hornfels

86. Augen structure
• Some resistant minerals gets deformed into thin lens like shape.

87. Rock cleavage
• Rock cleavage is seen in foliated rocks formed by the action of direct
pressure and generally accompanied by some recrystallization and
elongation with the constituent minerals in parallel arrangement.
• 2 types of cleavage;
1. Slaty cleavage ( parallel arrangement)
2. Fracture cleavage

88. Texture of metamorphic rocks
1. Crystalloblastic texture
Formed due to recrystallization of mineral grains in solid medium. All those textures
that have been newly imposed upon the rock during the process of metamorphism.
a. Idioblastic texture: crystal show perfect faces and forms.
b. Xenoblastic texture: crystals do not have any definite faces and forms.
2. Porphyroblastic texture:
When idioblasts occur as large crystals embedded in a fine grained groundmass.
3. Granoblastic texture:
In metamorphic rock, if the major constituents are granular or equidimensional.
4. Palimpest texture:
The remmant texture of the parent rock found preserved in the metamorphic rock.

89. Megascopic study of igneous/ sedimentary /
metamorphic rocks
• Granite
• Rhyollite
• Gabbro
• Basalt
• Pegmatite
• Syenite
• Shale
• Siltstone
• Limestone
• Sandstone
• Conglomerates
• Breccia
• Slate
• Phyllite
• Schist
• Gneiss
• Quartzite
• Marble