Petrology: Branch of Geology dealing with the study of rocks. It includes-

� Petrogenesis - origin and mode of occurrences as well as natural history.

� Petrography – dealing with classification and description of rocks.

PETROLOGY

•Is a branch of geology, which deals with study of rocks (Petro=rock, Logos=study)

ROCKS

IGNEOUS -most abundant -primary rocks -source is magma

or lava

SEDIMENTARY -thin veener above the Sial and Sima in Oceanic and Continental Crusts -secondary rocks

METAMORPHIC -proportion is similar to that of Igneous rocks -change of forms of Ig. and Sed. Due to Temprature, Pressure

and Chemical Fluids

James Hutton (1727–1797), the eminent 18th century gentleman farmer and

founder of modern geosciences, authored the concept of the rock cycle, which

depicts the inter-relationships between igneous, sedimentary, and metamorphic

rocks

IGNEOUS ROCKS: The rocks formed through volcanic action OR The rocks which

are derived from a molten mass “magma or lava”

SOURCE OF IGNEOUS ROCKS:

Magma: Molten mass comprising most abundant elements in earth –

Si, Al, Fe, Ca, Mg, K, H & O. Where the SiO2 is most abundant amongst all.

VOLCA�O: A gap in the Earth’s Crust where molten rocks and other material escape on

to the Earth’s surface

Temperature of Magma- 10400 to 12000 C

Influencing Factors for Magma Flow and Formation of Igneous Rocks

� Overlying sedimentary rocks as overburden plays important role in formation of igneous bodies eg: dykes, sills, laccoliths, bysmaliths, phacoliths, lopolith,

volcanic necks, batholiths and chonoliths.

� The beddings in the sedimentary rocks facilitates the magma to move through or intrude/inject through the weak planes

� Magma and lava

� Igneous rocks are the products of consolidation of magma or lava

Classification based on mode of formation:

� Intrusive Igneous rocks: Plutonic, Hypobasal

� Extrusive Igneous rocks: Volcanic rocks

Extrusive or volcanic rocks. 1 and 2 are the two lava-

flows.

� Fine grained igneous rocks: Volcanic

� Coarse grained igneous rocks: Plutonic

� Intermediate grain size: Hypobyssal

Important Feature of Igneous rocks

• Generally hard, massive, compact with interlocking

• Entire absence of fossils

• Absence of bedding plane

• Enclosing rocks baked

• Usually contain much feldspar

Mode of Occurrence or Forms of Igneous Rocks

The form, i.e. the size, shape of igneous bodies depends mostly on the following factors-

1. Mode of formation

2. Viscosity of magma, which in turn depends on the

� Temperature and

� Composition of magma

3. In relation with the surrounding rocks

� Physical characters of the invaded rocks

� Weight of the overlying rock mass in case intrusive bodies Structure.

Intrusive & Extrusive rock exhibit typical forms, which are characteristics to them

The form assumed by intrusive bodies depend upon major geological structures as faults,

fold and bedding plane etc and has cooled along the plane of weakness (like bedding

plane). Two major categories of intrusive bodies-

1.Concordant 2.Discordant rock bodies

• With respect to country rocks: Sedimentary, igneous and metamorphic

Discordant bodies- In this case intrusive mass to cut across the structure of preexisting

rock of the country. There are different types of discordant form in unfolded region as

well as in highly folded region.

Discordant body. D is he discordant body within the

country rocks

� VARIETY OF FORMS OF IG8EOUS ROCKS

Form depends on mode of emplacement within the country-rocks.

(a) In unfolded region

Dykes - Cross cutting relationship with country rocks. Dykes are commonly occur in

groups and such group may radiating, arcuate or any other pattern.

Different types of Dykes:

Arcuate and Ring Dykes- Dyke of arcuate outcrop occurring more or less in the form of

complete circle.

B-Arcuate B. Radiating Dykes

Cone sheets They are inwardly dipping (in the form of inverted co-axial plane) dyke like

masses with outcrop.

Ring dykes and Cone sheets appear circular in outcrop

Ring Dyke & Cone sheet

(b) In highly folded region

Batholiths- These are huge bodies of igneous intrusions that may show both concordant

and discordant relation with invaded rock. Batholiths of comparatively smaller dimension

are called stock and stocks of circular outcrop upon the surface are known as bossers.

Ethmolith: These are funnel shaped basic bodies with circular outcrop.

Harpolith: Sickle shaped basic bodies formed by stretching of strata after or during

injection.

Conolith: Any irregular intrusive body.

Volcanic neck –In some cases, vents of quite volcanoes have become sealed with

intrusions. Such intrusions are termed volconic neck

Concordont Bodies: These are intrusive bodies that run parallel to the structure of the

country rocks in which they occur.

Batholith. The large igneous body marked with B is a

batholith. On the surface, S is a stock and the small

outcrop B is a boss. R is the roof-pendant

Concordant body C is the concordant body within the country-rocks

In Unfolded Region:

• Sills- These are thin parallel sided tabular sheet of magma that has penetrated

along bedding planes, plane of schistocity, unconformities etc. These are also

doleritic in composition

Sill. The body marked with S is a sill. It runs parallel to

the rock bed

• Laccoliths –These are concordant intrusion due to which the invaded strata have

been arched or domed up. The mass itself has a flat or concave base and dome

shaped top.

• Lopoliths Those igneous intrusion associated with structural bain i.e. sedimentry

beds inclined towards a common center are termed lopoliths.

Laccoliths Lopoliths

• Bysmalith- Some time magma brake through the overlying rock bed and igneous

mass after consolidation is known as bysmalith.

In Highly folded Region:

• Phacoliths- Phacoliths are concordant plutonic bodies that lie parallel to the

bedding plane or foliation of folded country rock. They occur along the crests of

anticlines or the troughs of synclines in folded sedimentary strata

Phacoliths

Structures of igneous rocks

Structure of Igneous rock is large scale feature, which are dependent on several factors

like-

• Composition of Magma

• Viscosity of Magam

• Temperature and pressure at which cooling and consolidation take place

• Presence of gases and other volatiles

Igneous structures are mostly classified in to three major groups as follows-

• Mega structure

• Minor structure

• Micro structure

Mega Structure:

• Vesicular structure – developed by the escape of gases from magma after

cooling and consolidation

Vesicular and amygdaloidal structure in lava-flows. The lava-flow (white) contains

vesicles (marked with circles), some, of which have been filled-up with secondary

minerals. The dots represent the vesicles which have been fined-up subsequently.

o Amygdaloidal structure- However, vesicles thus formed and subsequently filled

in some low-temperature secondary minerals, such as calcite, zeolite , chalcedony

etc, these infillings are called amygdates. Lava containing the mygdates are said

to have aamygdaloidal structure.

• Sheet structure – cracks or joints are some times produced in the cooling of

magma or lava mass. These joints some time follow definite pattern. These joints

some times follow definite pattern. Thus may develop closely spaced sets diving

the rock in to thin sheets and producing termed sheet structure

• Platy structure Development of different sets of joints which give rise the only

plates of rock

Sheet Structure Platy structure

• Columnar structure- If the joints developed igneous rock in polygonal fashion,

dividing it in to columnar or prismatic block- Columnar structure. Or

As a consequence of contraction due to cooling a few sets of vertical joints

develop. Such joints bring about the formation of columns, which may be square,

rectangular, rhombic and hexagonal.

Rhombic column and hexagonal column

• Blocky lava- Lava of acidic composition due to their high viscosity do not flow at

greater distance.Common structural feature controlled by the mobility of lava.

Surface is rough.

• Ropy Lava: Lava of basic composition are quite mobile because of their low

viscosity they can flow to greater distance and after solidification it give smooth

surfacec

Blocky Lava Ropy Lava

• Pillow structures- Pillow like surface in the body of igneous rock. Pillow shaped

structure that are attributed to extension of lava under water (volcanic rock).

Pillow lava Flow lava

• Flow structure: These are plane result from flowage of magma with or without

crystal.

• Pumiceous - if vesicles are so abundant that they make up over 50% of the rock and the rock has a density less than 1 (i.e. it would float in water), then the rock is

pumiceous.

• Scoraceous- if vesicles are so abundant that they make up over 50% of the rock

and the rock has a density greater than 1, then the rock is said to be scoraceous.

Pumaceous

Minor Structure:

Primary Foliation: Some times many plutonic rocks are characterized by foliation

resulting from the parallel arrangement of platy of or ellipsoidal minerals.

Banding in rocks: These are also known as layered rock consisting of alternate bands of

different composition. It may result from lamellar flow, from settling of minerals from

crystallized magma.

Micro structure:

Coronas or reaction rims - often times reaction rims or coronas surround individual

crystals as a result of the crystal becoming unstable and reacting with its surrounding

crystals or melt. If such rims are present on crystals they should be noted

Myrmekitic structure: It is produced by an intergrowth of qtz and plagioclase feldspar,

where quratz occur as blebs or drop in plagioclase.

Orbicular - a texture usually restricted to coarser grained rocks that consists of

concentrically banded spheres wherein the bands consist of alternating light colored and

dark colored minerals

Spherulitic - a texture commonly found in glassy rhyolites wherein spherical

intergrowths of radiating quartz and feldspar replace glass as a result of devitrification.

Perilitic: These are curved, concentric lines of fracture, often seen in volcanic glass.

Theses are simply due to contraction of glassy mass on cooling.

Textures of igneous rocks

Reaction structure. The white grain with irregular

cracks is olivine. The rim around it is ade up of

pyroxene. This reaction rim is of primary origin and is,

therefore, a corona.

� Mutual relation between mineral grains or that between grains and glassy

materials

� Textures are generally microscopic

� Textural features depend upon size (granularity) and shape of mineral

grains

� Textural features are also dependent on proportion of crystals and glassy

materials.

Texture of igneous rock is a function of three important factor-

� Degree of crystallization

� Size of the grains or crystal

� Mutual relation of grains or of crystal and glassy matter

Crystallinity

� Holocrystalline- when made of minerals grains only

� Hemicrystalline – rock contains both crystalline and glassy matter

� Holohyaline – glass only

Degree of crystallization depends upon the following factors-

� Rate of cooling � Viscosity of magama � Depth of cooling � Volume of magma

Granularity

Phanerocrystalline- when individual crystals are visible to naked eye- coarse (>5mm),

medium (1-5mm) and fine(<1mm

Aphanitic- grains can not be distinguished

� Micro crystalline – individual crystal can be distinguished under the microscope

� Crypto crystalline- When individual crystal are too small to be seperately

distinguished even under microscope

� Mero crystalline- Intermediate in range

� Glassy – Where there is no crystallisation at all.

Glassy Aphanitic Phanerocrystalline

Conditions of development of perfect crystal forms

� Different minerals possess variable degrees of tendency of developing perfect crystal outline

� It is a function of the amount of available space within the crystallizing magma

Mineral grains depending on development of crystal outline

� Euhedral- well developed out line (idiomorphic/automorphic

� Subhedral- Crystal outline partially developed

� Anhedral- Crystal faces are absent

Mineral grains depending on exact form- With reference to the three dimension in

shape, crystals are classified as-

Equidimensional grains- Crystal found to have been developed equally along all

direction inside

Tabular grains- Better developed in two directions

Prismatic grains - Better developed in one direction

Eeqidimensional & Tabular Grain Prismatic

Textures in terms of relative size of grains:

� Equigranular texture- Panidiomorphic (all the crystals euhedral),

Hypidiomorphic (Crystals are subhedral) , Allotromorphic (anhedral

� Inequigranular texture- Grain size shows mark difference from grain to grain.

This is also known as seriate texture and are of the following type-

1. Porphyritic texture- In this larger crystals are enveloped in ground mass wich may

microgranular, micro crystaaline or even glass. This texture is characteristics of

volcanic and hypabasal rock. It origin may attributed to:

• Change in physio-chemical condition

• Molecular concentration

• Insolubility

Porphyritic texture.

The larger grains are the

phenocrysts and the smaller ones

together form the groundmass.

• Microphenocrysts

o Microporphyritic texture

o

o Glomeroporphyritic texture – phenocryst gather at one spot

o Vitrophyric: When the ground mass is glassy in porphyritic texture, it is called

vitrophyric texture.

o Felsophyric: Here ground mass is crypto crystalline n nature

� Conditions behind formation of Porphyritic texture

2. Poikilitic texture- In this case smaller crystal are enclosed in larger one without

common orientation. The enclosing crystal is known as olkocrysts and enclosed one

are called chadacrysts

Poikilitic texture. (ophitic texture)

The larger grain (oikocryst) has enclosed a few diversely oriented smaller grains

(chadacrysts) of another mineral.

� Conditions behind formation of Poikilitic texture

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•• GGrroouunnddmmaassss

•• MMeeggaapphheennooccrryyssttss

oo MMeeggaappoorrpphhyyrriittiicc tteexxttuurree

o Ophitic texture– Poikilitic texture in which euhedral plagiclase are partially or

completely enclosed by anhedral auguite. Ex. Dolerite

o Sub-ophitic texture: With equal size of plagioclase (feldspar grain and augite,

enclosure is partial.

o Granulo-ophitic texture- Here grains of plagiloclase are enclosed within a large

patch of pyroxene, and latter in turn is made up of a number of pyrogene grains.

3. Intergranular texture/ Intersertal texture – when the plagioclase laths are arranged

in triangular fashion and the polygonal interface left between crystals is having glassy

infillings, it is known as intersertal. However if the interspace is filled in with mineral

grains, it is called intergranular.

Intergranular texture Hyalo-ophitic texture

4. Directive texture: These are produced by flow in magma during their

crystallization.

o Hyalo-ophitic texture -Which is due to appreciable mixture of glass and feldspar

needles within the body of rock.

o Trachytic texture- Produced due to subparallel arrangement of plagioclase along

the direction of flow lava. Without evident flow it is known as felsitic magma.

Sub-ophitic

texture

Granulo-ophitic

texture

Trachytic texture Graphic texture

Graphic Structure : A structure consisting of intergrowths of quartz and alkali feldspar

(orthoclase) wherein the orientation of the quartz grains resembles cuneiform writing.

This texture is most commonly observed in pegmatites.

Patchy zoning - This sometimes occurs in plagioclase crystals where irregularly shaped

patches of the crystal show different compositions as evidenced by going extinct at

angles different from other zones in the crystal.

Oscillatory zoning - This sometimes occurs in plagioclase grains wherein concentric

zones around the grain show thin zones of different composition as evidenced by

extinction phenomena.

Moth eaten texture (also called sieve texture)- This sometimes occurs in plagioclase

wherein individual plagioclase grains show an abundance of glassy inclusions.

Perthitic texture - Exsolution lamellae of albite occurring in orthoclase or microcline

� Classification of igneous rocks

� Chemical composition

� Mineral composition

� Geological mode of occurrence

� Association and Genesis

On the basis of mineralogical composition

Since the relative amounts of the various minerals in a rock can be measured or estimated

with a fair degree of accuracy, this criteria is given more emphasis for the classification

of igneous rocks.

The minerals occurring in igneous rocks may be classed as

(a) Essential, (b) Accessory, and (c) Secondary.

The first two are the products of magmatic crystallization whereas the third one is formed

by the alteration of the primary minerals, i.e. (the 1st two), through the processes of

weathering, metamorphism etc.

(a) Essential minerals. These are the major constituents of the rock which help in the

diagnosis of the rocks types. The disappearance of these minerals would cause the

relegation of the rock to another type.

(b) Accessory minerals. These are the minor constituents of a rock and their presence bas

no bearing on the nomenclature of the rock.

Mineral commonly occurring in igneous rock may be broadly classified in to felsic and

mafic verities.

Felsic is a term derived from feldspar, felspathoid and silica.

Mafic; Ferromagnesian minerals- pyroxene, biotite and amphibolite

Chemical composition:

a) Acid igneous rocks. These rocks have more than 65% of the silica content, e.g.,

Granite, Grano-diorites •

(b) Intermediate rocks. These are having 55 to 65% or & sillica, eg/. syenite,& syenite.

(c) Basic rocks. Here the silica content is between 44 to 55%, e g., basic .

(d) Ultra basic rocks. In this case, silica content is less than 44%. Rocks like

anorthosite belongs to this category. Ultra basic rocks having higher magnesian contents

are known as ultramafics.

Shands and Holmes classified the igneous rocks as

(i) Supersaturated rocks. Also known as oversaturated rocks. Here, the excess of silica

crystallize as quartz.

(ii) Saturated rocks. They have just sufficient silica to form the stable silicate minerals

but no free quartz.

(iii) Under-saturated rocks. They contain insufficient silica and minerals like olivine,

nepheline, leucile etc. are therefore present.

Textural classification. (Mode of formation)

It is mostly based on the cooling history of the magma and the modes of occurrence of

these rocks.

Accordingly there are (a) Plutonic (b) Hypabyssal, and (c) Volcanic rocks.

The plutonic rocks are formed under deep seated conditions, where the temperature and

pressure are very high and the rate of cooling is very slow. Hence their texture is

holocrystalline and coarse.

The hypobasal group includes the rocks of dykes, sills and small laccoliths etc., which

occupies intermediate position in the crust between the plutonic and volcanic rocks. Their

textures is ,usually merocrystalline.

The volcanic rocks on the other hand are formed on the surface ,of the e3rthand due to

rapid rate of cooling their texture becomes holohyaline and fine grained.

Recent classification. ·It takes into account, factors like mineralogical composition,

chemical and textural characteristics etc. and is represented in tabular form. The

important features are as follows :

.

(i) Presence or absence of essential feldspar-classes Ist and 2nd.

(ii) Class-I:

Group.A- Acid Igneous Rocks.

Group-B- Intermediate'and Basic Rocks.

Group.C- Alkaline Igneous Rocks.

Group:D Ultrabasic and Ultramafics.

(iii) In Group-’B', there are two sub-groups, one is characterized by essential alkali

feldspar and the other by essential plagioclase.

In Group-’D', there are two sub-groups on the basis of presence or absence of felspathoid.

(iv) Class-II. There are two sub-groups on the basis of presence or absence of

felspathoids.

Igneous Rocks

Rocks can be grouped into families:

1. Granite family = felsic, light, contains quartz, feldspar, & muscovite.

(granite, rhyolite, obsidian, pumice)

1. Gabbro family = mafic, dark, contains feldspar, olivine, biotite. (gabbro, basalt, scoria)

1. Diorite family = intermediate composition and color. (diorite, andesite, obsidian)

TEXTURES: Expression of cooling history of respective magma.

Crystals and Glassy substances of like compositions –

o Difference in arrangement of atoms in space

o Difference in time of consolidation

o Crystalline substances − Denser packing of atoms and thus, higher Sp. Gr

Intrusive textures:

1. Fine-grained texture (Aphanitic) -- due to fast cooling (at or near surface)

2. Coarse-grained texture (Phaneritic) -- due to slow cooling at depth

3. Porphyritic texture -- coarse crystals (phenocrysts) surrounded by fine-grained

matrix (groundmass) forms due to initial slow cooling, then magma rising to (or

close to) surface and the remaining magma cooling quickly

Extrusive textures:

1. Glassy texture -- due to very rapid cooling -- magma cools so fast crystals don't

have time to form. Obsidian (volcanic glass) forms this way.

2. Vesicular texture -- full of rounded holes (vesicles) -- forms due to escape of gas

bubbles during cooling of lava. Pumice is a light-colored rock with this vesicular

texture.

3. Pyroclastic texture- chunks of molten material that fuse together

So, how does magma form?

• Magma forms in the lower crust and mantle in which high temperature melts the

rock and makes it into molten magma.

• Rock composition, pressure, and water content influence the melting temperature

of rocks also.

• When magma reaches the surface through a volcanic eruption, it is referred to as

lava.

Influences on rock melting temperature

Composition -- if rock contains a mixture of minerals it will melt at a lower temperature

compared to rocks that are mono-mineralic

Pressure -- high-pressure environments raise the melting temperature of rocks

Water content -- rocks that contain some water melt at lower temperatures than dry rocks

Magma composition

• The composition of the magma determines the composition of the rock that forms

• Igneous rock classification scheme shows that rocks vary in silica content (e.g.

quartz-rich rocks have higher silica content)

• Gabbros and basalts are low in silica, with bulk compositions less than about 60%

silica

• Granites and rhyolites are high in silica (bulk compositions with higher than 60%

silica)

Simple Classification of Igneous rocks

Acid or over

saturated rocks

with free Quartz

(SiO2 more than

66%)

Intermediate

rocks with no

free quartz (SiO2

between 66% to

55%)

Basic rocks

with

plagioclase and

pyroxenes

(mainly SiO2

between 55%

to 45%)

Ultra basic

rocks with

olivine,

pyroxene and

little or no

feldspar (SiO2

less than 44%)

Volconic Rhyolite

Dacite

Trachyte

Andesite

Phonolite

Basalt

Alkali basalt

Limburgite

Olivine basalt

Hypabasal Micro Granite

Pegmatite

Micro Synite

Micro Diorite

Dolerite

Lamprophyres

Plutonic Granite

Granodiorite

Synite Nephelene

synite Diorite

Gabbro

Anorthosite

Peridotite

Dunite

Picrite

Perknite