Introduction to Petrology

INTRODUCTION TO PETROLOGY

Petrology •

Petrology is the study of rocks

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Petrology deals with the origin, occurrence, mineral composition, chemical and physical propertiess of rocks propertie

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Rocks are natural massive aggregates of minerals, forming the crust of the Earth

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Petrology is further sub-divided into Petrography and Petrogeny  2

Geological Classification of Rocks •

Igneous rocks

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Sedimentary rocks

•

Metamorphic rocks

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The Rock Cycle

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Texture & Structure o

Texture refers to the mutual relationship of the

different mineralogical constituents 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

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act as a distinguishing feature feature

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reveal the mode of origin of the rock 5

(a) Igneous rocks •

They are formed due to the cooling of magma/lava

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They are called primary rocks, as they are the first formed rocks that made up the primordial Earth’s crust

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The basic classification of igneous rocks is into extrusive and intrusive igneous rocks 6

Classification based on the depth of formation o

Volcanic rocks

–

formed on the surface of the

–

formed at considerable depths

Earth o

Plutonic rocks

o

Hypabyssal rocks

–

formed at intermediate

depths (5mm

o

Medium-grained Medium-graine d - avg grain size b/w 5 & 1mm

o

Fine-grained

- avg grain size 256 mm 16-256 mm 2-16 mm 0.5-2 mm 0.25-0.5 mm 0.0625-0.25 mm 1/256 – 1/16 mm < 1/256 mm

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2. Non-clastic rocks i. Chemically formed rocks 1. Siliceous de deposits 2. Carbonate de deposits 3. Fe Ferrru rugi ginnou ouss de deppos osit itss 4. Ph Phos osph pha ati ticc de deppos osit itss 5. Evaporites

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ii. Organic deposits 1. Carbonate rocks 2. Carbonaceous ro rocks 3. Ph Phos osph pha ati ticc de deppos osit itss 4. Fe Ferrru rugi ginnou ouss de deppos osit itss

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(c) Metamorphic rocks •

Metamorphism  means change of form

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Metamorphism relates to the processes responsible responsibl e for the changes in a rock under the influence of temperature, pressure and chemically active fluids

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Metamorphic rocks are formed from preexisting rocks, under the influence of the above factors 53

Quartzite, Prospect Mountain, Wheeler Peak, Nevada, The U.S.A

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Terminology •

Ortho-metamorphic Ortho-metamor phic rocks - formed from igneous rocks

•

Para-metamorphicc rocks Para-metamorphi

–

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 temperature temperaturess 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 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.

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These fluids act as carriers of chemical components that drive the chemical reactions with the minerals

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The pore fluids undergo expansion, with rise in temperature

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Fluids present around rocks may react with the minerals within them, at elevated temperatures temperatures 59

Types of Metamorphism 1. Th Ther erma mall meta metamo morp rphi hism sm 2. Dy Dyna nami micc meta metamo morp rphi hism sm 3. Dyn Dynamo amo-th -ther ermal mal/Re /Regio gional nal metam metamorp orphis hism m 4. Metasomatism

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(a) Thermal metamorphism Refers to all metamorphic processes processes in which heat plays a predominant role. o

Contact metamorphism

o

Pyro metamorphis metamorphism m

o

Plutonic metamorphism

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(b) Dynamic metamor metamorphism phism 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 Metamorphism

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It refers to metamorphism under the combined action of all the three agents

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Most prevalent of all metamorphic processes processes

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Such conditions were available during the mountain building activity, in the history of the earth 63

(d) Metasomat Metasomatism ism 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

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Pneumatolytic

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Additive

•

Expulsive

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Effects of Metamorphism •

Recrystallization

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Rock flowage

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Granulation

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Metasomatic replaceme replacement nt

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Examples of Metamorphic changes •

Igneous rocks Granites undergo dynamic metamorphism, to form crush breccia

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Sedimentary rocks Pure limestone, re-crystal re-crystallizes lizes under conditions of contact metamorphism, to marble 67

Metamorphic Grades •

Represents the extent to which an original rock has been changed by metamorphism.

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The grades are indicated by the presence of a set of index minerals •

Low grade

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Medium grade

•

High grade

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Metamorphic zones Indicate the depth wise extension of particular grades of metamorphism: 1. Th The e Epiz Epizon one e (tem (tempe pera ratu ture re < 300 300 C) °

2. Th The e Mes Mesozo ozone ne (te (tempe mperat rature ure 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 70

Textures of Metamorphic rocks •

Crystalloblasticc texture Crystalloblasti

•

Palimpsest texture

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Structures of Metamorphic rocks 1. Ca Cata tacl clas asti ticc stru struct ctur ure e 2. Sc Schi hist stos ose e str struc uctu ture re 3. Gn Gnei eiss ssos ose e str struc uctu ture re 4. Ma Macu culo lose se st stru ruct ctur ure e 5. Gr Gran anul ulos ose e str struc uctu ture re

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Structures of Metamorphic rocks

A.

Schistose structure

A. Cataclastic structure

B.

Granulose structure

B. Maculose structure

C.

Gneissose structure

C. Palimpsest structure © George Walter Tyrrell

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Large scale structural features •

Rock cleavage –

Flow cleavage

–

Fracture cleavage

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Schistosity

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Foliation

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Gneiss, a foliated metamorphic rock

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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

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Chenna Kesavulu, N, Textbook of Engineering Geology , MacMillan India

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Reddy, D V, Engineering Geology , Vikas Publishing House

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Garg, S K, Physical and Engineering Geology , Khanna Publishers

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Thompson, G R and J Turk, Introduction to Physical  Geology , Thomson Brooks/Cole

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