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

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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

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Granite, Intrusive

en.wikipedia.org

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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

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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

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Equigranular texture -Granite

© Andrew Alden, 2007

© Andrew Alden, 2006

Porphyritic texture - Andesite

15© Andrew Alden, 2008

Poiklitic texture – feldspar

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Intergrowth in Lunar Granite

Structures of Igneous rocks

• Structures due to mobility of

magma/lava

• Structures due to cooling of magma

• Miscellaneous structures

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Structures due to mobility of magma

1. Flow structures 2. Pillow

structures

3. Ropy and blocky lava 4. Spherulitic

structures

5. Orbicular structures

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Structures due to cooling of magma

1. Jointing structure

2. Rift and grain

3. Vesicular structure

4. Miarolitic structure

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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

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Sill

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Salisbury Crags, Edinburgh, Scotland, a sill exposed during the ice ages

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Phacolith

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Lopolith

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Laccolith

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Laccolith exposed by erosion of overlying strata in Montana

2. Discordant bodies

• Dykes/dikes

• Volcanic necks

• Batholiths

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29en.wikipedia.org

Dike on the Baranof Cross-Island trail, Alaska

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31bio-geo-terms.blogspot.com

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Magmatic Intrusions

(b) Sedimentary rocks

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• 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

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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

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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.)

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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

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1. Mechanical structures

i. Stratification

ii. Lamination

iii. Cross bedding

iv. Graded bedding

v. Mud cracks

vi. Rain prints

vii.Ripple marks 39

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Stratification in the Grand Canyon

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Lamination in Travertine

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Cross-bedding in Sandstone

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http://www.earthsciences.hku.hk/shmuseum

Mud cracks

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Rain prints

http://www.earthsciences.hku.hk/shmuseum

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Ripple marks

2. Chemical structures

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1. Concretionary structures

2. Nodular structure

3. Geode structure

3. Organic structures

1. Fossiliferous structure

2. Stromatolic structure

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Concretionary structure

48en.wikipedia.org

Geode structure

Classification on the basis of mode of formation

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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

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2. Non-clastic rocks

i. Chemically formed rocks

1. Siliceous deposits

2. Carbonate deposits

3. Ferruginous deposits

4. Phosphatic deposits

5. Evaporites

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ii. Organic deposits

1. Carbonate rocks

2. Carbonaceous rocks

3. Phosphatic deposits

4. Ferruginous deposits

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(c) Metamorphic rocks

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• 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

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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

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Metamorphic Agents

• Temperature

• Pressure

• Chemically active fluids

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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

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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

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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

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Types of Metamorphism

1. Thermal metamorphism

2. Dynamic metamorphism

3. Dynamo-thermal/Regional

metamorphism

4. Metasomatism

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(a) Thermal metamorphism

Refers to all metamorphic

processes in which heat plays a

predominant role.

o Contact metamorphism

o Pyro metamorphism

o Plutonic metamorphism

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(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

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(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)

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Types of Metasomatism

• Hydrothermal

• Pneumatolytic

• Additive

• Expulsive

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Effects of Metamorphism

• Recrystallization

• Rock flowage

• Granulation

• Metasomatic replacement

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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

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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

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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

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Textures of Metamorphic rocks

• Crystalloblastic texture

• Palimpsest texture

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Structures of Metamorphic rocks

1. Cataclastic structure

2. Schistose structure

3. Gneissose structure

4. Maculose structure

5. Granulose structure

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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

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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

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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