1. 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
2. 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
3. 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
4. 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
5. VARIETY OF FORMS OF IG EOUS 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
6. 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
7. 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.
8. 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
9. 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
10. • 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
11. 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.
12. 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
13. 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
14. 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
15. 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.
16. • 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
••PPhheennooccrryyssttss
••GGrroouunnddmmaassss
••MMeeggaapphheennooccrryyssttss
ooMMeeggaappoorrpphhyyrriittiicc tteexxttuurree
17. 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
18. 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.
19. 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)
20. 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)
21. 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
22. 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
23. 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
