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INTRODUCTION TO PETROLOGY
Petrology
• Petrology is the study of rocks
• Petrology deals with the origin,
occurrence, mineral composition,
chemical and physical properties of
rocks
• Rocks are natural massive aggregates of
minerals, forming the crust of the Earth
• Petrology is further sub-divided into
Petrography and Petrogeny 2
Geological Classification of Rocks
• Igneous rocks
• Sedimentary rocks
• Metamorphic rocks
3
4
The Rock Cycle
georneys.blogspot.com
Texture & Structure
o Texture refers to the mutual relationship
of the different mineralogical constituents
in a rock
o Structure refers to the large scale features
or field characteristics of the rocks
Importance:
• contribute to the strength of the rock
• act as a distinguishing feature
• reveal the mode of origin of the rock 5
(a) Igneous rocks
• They are formed due to the cooling of
magma/lava
• They are called primary rocks, as they
are the first formed rocks that made up
the primordial Earth’s crust
• The basic classification of igneous rocks
is into extrusive and intrusive igneous
rocks6
Classification based on the depth of formation
o Volcanic rocks – formed on the surface of
the Earth
o Plutonic rocks – formed at considerable
depths
o Hypabyssal rocks – formed at
intermediate depths (<2km) 7
8
Granite, Intrusive
en.wikipedia.org
9en.wikipedia.org
Basalt, extrusive
Average mineralogical composition of Igneous rocks
Sl No
Mineral (%)
i Feldspars 59.5ii Pyroxenes and
Amphiboles16.8
iii Quartz 12.0iv Biotite 3.8v Titanium 1.5vi Apatite 0.6vii Accessory minerals 5.8
10
Factors defining textures of Igneous rocks
• Degree of Crystallization
1. Holocrystalline
2. Holohyaline
3. Merocrystalline
• Granularity
1. Coarse-grained
2. Medium-grained
3. Fine-grained11
Types of Textures
1. Equigranular
2. Inequigranular
1. Porphyritic
2. Poiklitic
3. Directive
4. Intergrowth
5. Intergranular
12
13
Equigranular texture -Granite
© Andrew Alden, 2007
© Andrew Alden, 2006
Porphyritic texture - Andesite
15© Andrew Alden, 2008
Poiklitic texture – feldspar
16
Intergrowth in Lunar Granite
Structures of Igneous rocks
• Structures due to mobility of
magma/lava
• Structures due to cooling of magma
• Miscellaneous structures
17
Structures due to mobility of magma
1. Flow structures 2. Pillow
structures
3. Ropy and blocky lava 4. Spherulitic
structures
5. Orbicular structures
18
Structures due to cooling of magma
1. Jointing structure
2. Rift and grain
3. Vesicular structure
4. Miarolitic structure
19
Miscellaneous structures
1. Reaction structure
2. Xenolithic structure
Forms of Igneous rocks
Refers to the shape in which cooled
igneous masses occur in nature:
Form is decided by:
o Structural disposition of host rock
o Viscosity and composition of magma or
lava
Types:
o Concordant
o Discordant 20
1. Concordant bodies
• Sills
• Phacoliths
• Lopoliths
• Laccoliths
21
22
Sill
en.wikipedia.org
23
Salisbury Crags, Edinburgh, Scotland, a sill exposed during the ice ages
en.wikipedia.org
24
Phacolith
en.wikipedia.org
25
Lopolith
en.wikipedia.org
26en.wikipedia.org
Laccolith
27en.wikipedia.org
Laccolith exposed by erosion of overlying strata in Montana
2. Discordant bodies
• Dykes/dikes
• Volcanic necks
• Batholiths
28
29en.wikipedia.org
Dike on the Baranof Cross-Island trail, Alaska
30
31bio-geo-terms.blogspot.com
32
Magmatic Intrusions
(b) Sedimentary rocks
33
• They are formed by the accumulation,
compaction and consolidation of
sediments
• They are secondary rocks, derived from
the sediments produced by the
weathering of pre-existing rocks
• The accumulation and compaction of
these sediments usually take place in
the presence of water
Environment of Formation
• Continental facies
• Transitional facies
• Marine facies
34
Varying Mineralogical Composition
Factors influencing mineralogical
composition:
o Nature of gathering ground
o Duration of transport
o Mixing up of sediments
o Allogenic and authigenic minerals
35
Textures of Sedimentary Rocks
Textures are determined by:
1. Origin of grains
o Clastic and non-clastic textures
2. Size of grains
o Coarse-grained - avg grain size
>5mm
o Medium-grained - avg grain size b/w 5
& 1mm
o Fine-grained - avg grain size <1mm
(contd.)
36
3. Shapes of grains
o Rounded, sub-rounded, angular & sub-
angular
4. Packing of grains
o Open-packed (porous) and densely
packed
5. Fabric of grains
o Described in terms of orientation of
longer axes of grains
6. Crystallization trend
o Crystalline granular & amorphous
textures
37
38
1. Mechanical structures
i. Stratification
ii. Lamination
iii. Cross bedding
iv. Graded bedding
v. Mud cracks
vi. Rain prints
vii.Ripple marks 39
40
Stratification in the Grand Canyon
en.wikipedia.org
41
Lamination in Travertine
en.wikipedia.org
42
Cross-bedding in Sandstone
43
http://www.earthsciences.hku.hk/shmuseum
Mud cracks
44
Rain prints
http://www.earthsciences.hku.hk/shmuseum
45
Ripple marks
2. Chemical structures
46
1. Concretionary structures
2. Nodular structure
3. Geode structure
3. Organic structures
1. Fossiliferous structure
2. Stromatolic structure
47
Concretionary structure
48en.wikipedia.org
Geode structure
Classification on the basis of mode of formation
49
o Clastic or Mechanically formed rocks
o Non-clastic rocks
o Chemically formed rocks
o Organically formed rocks
1. Clastic rocks
Type Sub-division Size
GravelsBoulders > 256 mm
Cobbles 16-256 mm
Pebbles 2-16 mm
SandsCoarse sands 0.5-2 mm
Medium sands 0.25-0.5 mm
Fine sands 0.0625-0.25 mm
Silts 1/256 – 1/16 mm
Clay < 1/256 mm
50
2. Non-clastic rocks
i. Chemically formed rocks
1. Siliceous deposits
2. Carbonate deposits
3. Ferruginous deposits
4. Phosphatic deposits
5. Evaporites
51
ii. Organic deposits
1. Carbonate rocks
2. Carbonaceous rocks
3. Phosphatic deposits
4. Ferruginous deposits
52
(c) Metamorphic rocks
53
• Metamorphism means change of form
• Metamorphism relates to the processes
responsible for the changes in a rock
under the influence of temperature,
pressure and chemically active fluids
• Metamorphic rocks are formed from pre-
existing rocks, under the influence of the
above factors
54
Quartzite, Prospect Mountain, Wheeler Peak, Nevada, The U.S.A
en.wikipedia.org
Terminology
• Ortho-metamorphic rocks - formed from
igneous rocks
• Para-metamorphic rocks – formed from
sedimentary rocks
55
Metamorphic Agents
• Temperature
• Pressure
• Chemically active fluids
56
1. Temperature
• Minerals are normally stable at
temperatures below 200° C
• Sources of heat for metamorphism:
– The internal heat
– The magmatic heat
• Metamorphic changes take place
between 300°C - 850°C
57
2. Pressure
• Pressure causing metamorphism is of
two types:
– Uniform pressure (due to over-
burden), acts vertically downwards
– Directed pressure (from orogenic
activity), can act in any direction
58
3. Chemically active fluids
• E.g.: water/steam, CO2, hydrofluoric acid
etc.
• These fluids act as carriers of chemical
components that drive the chemical
reactions with the minerals
• The pore fluids undergo expansion, with
rise in temperature
• Fluids present around rocks may react
with the minerals within them, at
elevated temperatures
59
Types of Metamorphism
1. Thermal metamorphism
2. Dynamic metamorphism
3. Dynamo-thermal/Regional
metamorphism
4. Metasomatism
60
(a) Thermal metamorphism
Refers to all metamorphic
processes in which heat plays a
predominant role.
o Contact metamorphism
o Pyro metamorphism
o Plutonic metamorphism
61
(b) Dynamic metamorphism
o Pressure/stress plays the predominant
role
o Also known as cataclastic, mechanical or
dislocation metamorphism
o Stress is more effective at higher levels
of the crust, where rocks are rigid and
brittle
o Pressure causes movement of and
interaction between rocks, resulting in
their mechanical breakdown – cataclasis
62
(c) Dynamo-thermal metamorphism
• Also known as Regional Metamorphism
• It refers to metamorphism under the
combined action of all the three agents
• Most prevalent of all metamorphic
processes
• Such conditions were available during
the mountain building activity, in the
history of the earth63
(d) Metasomatism
Refers to the formation of new
minerals by the chemical replacement of
the existing ones, under the influence of
chemically active fluids
The chemically active fluid may be
provided:
o from within the rock (mineral
metasomatism)
o from outside the rock (rock
metasomatism)
64
Types of Metasomatism
• Hydrothermal
• Pneumatolytic
• Additive
• Expulsive
65
Effects of Metamorphism
• Recrystallization
• Rock flowage
• Granulation
• Metasomatic replacement
66
Examples of Metamorphic changes
• Igneous rocks
Granites undergo dynamic
metamorphism, to form crush breccia
• Sedimentary rocks
Pure limestone, re-crystallizes
under conditions of contact
metamorphism, to marble67
Metamorphic Grades
• Represents the extent to which an
original rock has been changed by
metamorphism.
• The grades are indicated by the
presence of a set of index minerals
• Low grade
• Medium grade
• High grade
68
Metamorphic zones
Indicate the depth wise extension of
particular grades of metamorphism:
1. The Epizone (temperature < 300° C)
2. The Mesozone (temperature b/w 300° -
500° C)
3. The Ketazone
69
Mineralogical composition of Metamorphic rocks
Depends upon:
o The composition of the parent rock
o Type and degree of metamorphism
Types of minerals formed:
o Stress minerals
o Anti-stress minerals
70
Textures of Metamorphic rocks
• Crystalloblastic texture
• Palimpsest texture
71
Structures of Metamorphic rocks
1. Cataclastic structure
2. Schistose structure
3. Gneissose structure
4. Maculose structure
5. Granulose structure
72
73
A. Schistose structure
B. Granulose structure
C. Gneissose structure
A. Cataclastic
structure
B. Maculose
structure
C. Palimpsest
structure
Structures of Metamorphic rocks
© George Walter Tyrrell
Large scale structural features
• Rock cleavage
– Flow cleavage
– Fracture cleavage
• Schistosity
• Foliation
74
75
Gneiss, a foliated metamorphic rock
en.wikipedia.org
Classification of Metamorphic rocks
• Foliated rocks – rocks that show
parallelism in their mineralogical and
structural constitution e.g. slates,
phyllites
• Non-foliated rocks – characterized by the
absence of foliation
76
Reference
• Parbin Singh, Engineering and General
Geology, S K Kataria & Sons
• Chenna Kesavulu, N, Textbook of Engineering
Geology, MacMillan India
• Reddy, D V, Engineering Geology, Vikas
Publishing House
• Garg, S K, Physical and Engineering Geology,
Khanna Publishers
• Thompson, G R and J Turk, Introduction to
Physical Geology, Thomson Brooks/Cole 77