CHAPTER – 4
PETROLOGY
(Refers Engineering and general geology by parbin singh)
INTRODUCTION:
Petrology is the branch of geology that studies rocks and the conditions under
which they form.
Petro = Rock Logos = Study
Petrology comprises of origin, association, occurrence, mineral composition,
chemical composition, texture, structure, physical properties, etc. of rocks.
Petrology has three subdivisions:
1. Igneous
2. Metamorphic
3. Sedimentary petrology.
Igneous and metamorphic petrology are commonly taught together because they
both contain heavy use of chemistry, chemical methods, and phase diagrams.
Sedimentary petrology is, on the other hand, commonly taught together
with stratigraphy because it deals with the processes that form sedimentary rock.
(http://www.kean.edu/~csmart/Structural/Lectures/01/Rock%20Review.htm)
Rock: Aggregated solids of minerals is called rock.
(Or)
A rock may be defined as “an unit of the earth’s crust
All rocks are made of two or more minerals, but minerals are not made of rocks.
There are three main types of rocks:
1. Igneous - formed when molten rock cools.
2. Metamorphic – rocks changed by the effect of heat and pressure.
3. Sedimentary – formed by the consolidation of sediments in the layered or
bedded rocks deposited in the ocean bottom or huge lakes, etc.
Characteristics of Igneous rocks:
A tough, frozen melt with little texture or layering; mostly black, white and/or
gray minerals; may look like granite or like lava
Characteristics of Metamorphic rocks:
Hardened sediment with layers (strata) of sandy or clayey stone; mostly
brown to gray; may have fossils and water or wind marks
Characteristics of Sedimentary rocks:
Tough rock with layers (foliation) of light and dark minerals, often curved;
various colors; often glittery from mica
ROCK CYCLE
The three different types of rocks are constantly being changed from one type
to another type through various geological processes.
IGNEOUS ROCK: Igneous rock formed through the cooling and solidification of magma
or lava. (http://www.geographynotes.com/rocks/igneous-rocks/igneous-rocks-texture-and-
classification-geology/5736)
Solidification into rock occurs either below the surface as INTRUSIVE rock or on the
surface as EXTRUSIVE rocks.
Classification on the basis of consolidation process:
INTRUSIVE IGNEOUS ROCK:
Intrusive igneous rocks crystallize below Earth's surface, and the slow cooling that
occurs there allows large crystals to form. Examples of intrusive igneous rocks
are diorite, gabbro, granite, pegmatite, and peridotite.
Intrusive igneous rocks that form at depth within the crust are termed plutonic
(or abyssal) rocks and are usually coarse-grained. Intrusive igneous rocks that form near
the surface are termed subvolcanic or hypabyssal rocks and they are usually medium-
grained. Hypabyssal rocks are less common than plutonic or volcanic rocks and often
form dikes, sills, laccoliths, lopoliths, or phacoliths.
EXTRUSIVE IGNEOUS ROCK:
Extrusive igneous rocks also known as volcanic rocks, are formed at the crust's
surface as a result of the partial melting of rocks within the mantle and crust. Extrusive
igneous rocks cool and solidify quicker than intrusive igneous rocks. Hence such rocks
are smooth, crystalline and fine grained. These rocks include andesite, basalt, dacite,
obsidian, pumice, rhyolite, scoria and tuff.
The Deccan traps, Panjal traps the best example of Volcanic rocks.
Elemen Oxide
t %
Chemical composition of Igneous rock: SiO2 = 40 – 75 %
SiO2 59.14
Al2O3 = 10 – 20 %
Al2O3 15.35
Oxides of Ca, Mg and Fe = More than 10 %
FeO 3.80
Classification On the basis of Silica Percentage:
Fe2O3 3.08
1. Ultrabasic Rocks: Silica < 45 %. Ex:- Peridotite.
CaO 5.08
2. Basic Rocks: Silica between 45 – 55 %. Ex:- Gabbro and Basalt.
MgO 3.49
3. Intermediate Rocks: Silica between 55 – 65 %. Ex:- Diorite.
Na2O 3.84
4. Acid Rocks: Silica > 65 %. Ex:- Granite.
K2O 3.13
Igneous Rock can also be classified as:
H2O 1.15
5. Oversaturated Rocks: These rocks crystalize from melts containing higher
Others 1.94
amount of Silica. They contain abundant quartz and alkali felspars.
6. Saturated Rocks: These rocks are formed when the amount of silica present in the melt is
just sufficient to form silicate minerals. Saturated igneous rocks do not contain quartz.
7. Undersaturated Rocks: These rocks crystalize from a melts which deficient in silica and
high in alkalies and aluminium oxide.
Mineral %
Mineral composition of Igneous rock:
Felspars 59.5
Pyroxenes & Amphiboles 16.8
Quartz 12.0
Biotite 3.8
Titanium 1.5
Apatite 0.6
Accessory Minerals 5.8
TEXTURES : Texture means the size, shape and arrangement of mineral grains in a rock.
The grain size depends on the rate of cooling of magma.
In general, The rate of cooling, slow – Coarse Grain and Vice Versa.
In the Study of Texture four points are considered –
1. Degree of crystallization
2. Size of grains
3. Shape of crystals
4. Mutual relation between mineral grains
STRUCTURES OF IGNEOUS ROCKS: Read Yourself
ENGINEERING IMPORTANCE:
Granites, syenites and dolerites are characterized by very high crushing strengths and
hence can be easily trusted in most of construction works.
Basalts and other colored igneous rocks, though equally strong, may not be used in
residentially building but find much use as foundation and roadstone.
The igneous rocks are typically impervious, hard and strong and form very strong
foundations for most of civil engineering projects such as dams and reservoirs. They
can be trusted as wall and roof rocks in tunnels of all types unless traversed by joints.
At the same time, because of their low porosity, they cannot be expected to hold oil or
groundwater reserves.
Peridotites and Pegmatites are valuable as they may contain many valuable minerals
of much economic worth.
SEDIMENATARY ROCKS: Theses rocks are formed by consolidation and cementation of
sediments deposited under water. Sedimentary rocks also include the rocks formed by
accumulation of chemically precipitated or originally derived material.
Sedimentary rocks are types of rocks that are formed by the deposition and subsequent
cementation of mineral or organic particles on the floor of oceans or other bodies of
water at Earth’s surfaces. Sedimentation is the collective name for processes that cause
these particles to settle in place.
Formation of Sedimentation Rocks:
The formation of sedimentary rocks takes place in the three stages:
1. Weathering and erosion of preexisting rocks
2. Sedimentation
3. Lithification and diagenesis : Lithification is a process by which soft and loose
sediments are converted into hard and firm rocks. In this process many physical and
chemical changes takes place within the sediment. This process is described as
diagenesis.
Sedimentary rocks occurs in layers and frequently contains fossils.
CLASSIFICATION:
1. Clastic Sediments
2. Nonclastics Sediments
CLASTIC ROCKS: Broken fragments of preexisting rocks ranging in size from small clay
particles to very large boulders. Decay and Disintegration, Transport and Sediments and
Gradual Deposition are the steps are the formation of clastic rocks. On the basis size of
grains , the clastic rocks are classified into three group:-
(a) Rudaceous Rocks: Formed by accumulation of gravels, pebbles and boulders. If the
grains are rounded, the rock is called “ Conglomerate” and if they are angular, the
rock is termed as “ Breccia”.
(b) Arenaceous Rocks: Composed of Sand grains. If the grains are rounded, the rock is
called “ Sandstone” and if they are angular, the rock is termed as “ Grif”.
(c) Argillaceous Rocks: Made up of very fine grain. Ex:- Shale and Mudstone.
NONCLASTIC SEDIMENTS: Formed by Chemical Precipitation of minerals from water or
by accumulation of remains of Animals and Plants. Classified into two groups:-
a) Chemically formed rocks
b) Organically formed rocks
CHEMICALLY FORMED ROCKS: Formed when mineral matter in solution is precipitated
from water, usually because of changes in water temperature or in the chemical content
of water.
a) Carbonated Rocks:- Formed by the chemical precipitation of CALCIUM
CARBONATES. “Lime stones” and “Dolomites” are the most abundant carbonate
rocks.
b) Salt Rocks:- Evaporation is the major process involved in the deposition of chemical
precipitates. The principal minerals of these deposits are chlorides and sulfates of Na,
K, mg and Ca. Ex: Rock-salt, Gypsum and anhydrite.
c) Ferruginous Rocks:- Formed by the chemical precipitation of IRON OXIDE. EX: Iron-
stone.
d) Siliceous Rocks:- Formed when SILICA is precipitated from water. Ex: Flint, Chert,
Jasper, Agate.
ORGANICALLY FORMED ROCKS: Composed mainly of remains of animals or plants.
e) Biochemical Rocks:- The biochemical sediment is produced when plants and
animals living under water, extract from it dissolved mineral matter, usually calcite,
to form shells or other hard parts. These shells accumulate on the ocean floor in great
quantities to form sedimentary rocks. Ex: Shell-limestone.
CHEMICAL COMPOSITION:
The range in chemical composition of sedimentary rocks is quite large and in this respect
they differ from igneous rocks.
A sandstone may contain as much as 99% silica.
Iron oxide content may be as high as 58%.
Pure limestones the CaO content may reach 55%.
ENGINEERING IMPORTANCE:
Sedimentary rocks cover a great part of the crust of the earth; they make up more
that 75% of the surface area of the land. It is with these types of rocks that an
engineers has to deal with in majority of cases.
Civil engineer has to see, for instance, if such rocks would withstand loads under
heavy construction and also, if they could be trusted in cuts and tunnels in highway
construction and also as reservoirs.
They are the most important rocks to act as natural reservoirs of oil and ground water
supplies.
METAMORPHIC ROCKS:
Formed from the older rocks when they are subjected to increased temperature, pressure and
shearing stresses at considerable depth in the earth’s crust. The older rocks may be either
sedimentary, igneous or other metamorphic rocks.
Agents of Metamorphism:
i) Heat: Due to temp increases inside or outside
ii) Uniform Pressure: Due to overlying rocks
iii) Directed pressure or stress: Operates during folding movements that accompany
mountain building.
iv) Chemically active fluids and gases: Due to passing of chemically active fluids and gases.
PROCESS OF METAMORPHISM:
1. Granulation: Crushing of rocks due to pressure and friction
2. Plastic deformation: After removal of stresses if it does not regain its original shape, its said
to be plastically deformed.
3. Recrystallization: Formation of new minerals or formation of new crystals. Recrystallization
causes mineralogical and textural changes in rocks.
4. Metasomatism: Process in which original composition of rocks are changed due addition and
removal of material under high temp. and pressures.
CLASSIFICATION OF METAMORPHISM:
Most metamorphism rocks form either at convergent plate boundaries or in the vicinity of
igneous intrusions. At convergent plate boundaries, crustal rocks are buried deep and
experience high pressure and temperatures. At the vicinity of igneous intrusions where
the surrounding rocks are heated by the ascending hot magma.
1. Cataclastic Metamorphism: Caused due to directed pressure(Shear force) and
results when rock bodies slide over each other along faults. Tectonic movements build
up tremendous pressure (but temp. remains low) causing the rocks to grind and
pulverize each other. Consequently, the rocks gets crushed into small pieces and is
deformed with formation of little new minerals. Ex: Braccia and Mylonite.
2. Dynamic Metamorphism: Also known as clastic metamorphism or impact
metamorphism or shock metamorphism or mechanical metamorphism or dislocation
metamorphism. Occurs when rocks are subjected to extreme pressure very rapidly
and increase in temp is very little. In this case a new rock is formed partly by the
mechanical effects of flow and partly by the growth of new minerals that develop in
the direction of flow. Ex: Slate, Phyllite and schist. Dynamic metamorphism is
produced on a comparatively small scale and is usually highly localized.
3. Contact Metamorphism: Also called as thermal metamorphism, it occurs around
igneous intrusions. In contact metamorphism heat plays dominant role and its
general effect is to promote recrystallization. The rate at which chemical reactions
take place during thermal metamorphism is exceedingly slow and depends on the
rock types and temperature involved. This kind of metamorphism is caused by
moderate pressure and extreme heat. Metamorphism is caused due to local heating
of the rock by intruded magma, in contact zones around igneous intrusions; as the
magma cools it heats up the surrounding rock. Due the temp. decline, a progressive
change of the mineral composition and texture of the rock can be found.
4. Plutonic Metamorphism: Uniform Static pressure and temp. dominate. It takes
place at great depths. Since at high static pressure there will be a reduction in
volume, there is a change in mineral composition and denser mineral are formed
during recrystallization.
5. Retrograde Metamorphism: Also known as regressive metamorphism, this occurs
when high temp metamorphic mineral assemblages change to low temp ones.
Strong differential movement or hydrothermal activity is needed to cause Retrograde
Metamorphism.
6. Regional Metamorphism: When directed pressure and heat act together in the
presence of hydrothermal fluid, the rocks are metamorphism over wider areas. It
takes place at great depths, such as root regions of fold mountains, where the stress
METAMORPHIC ZONES: Below the earth surfaces temp and pressure both increases
with increases depths.
1. Epizone: Occurs near the earth’s surfaces. Generally conditions of Cataclastic
metamorphism prevail.
2. Mesozone: Intermediate Zone. Generally conditions of regional metamorphism
prevail.
3. Katazone: Bottom Zone. Generally conditions of plutonic metamorphism prevail.
ENGINEERING IMPORTANCE:
Important source of building material.
Marble, Slate, Graphite contain schists etc. are the important building materials.
Metamorphism of impure lime stone produce talc, a very soft silicate mineral, that is
an important mineral filler in paints, rubber, paper and cosmetics.
Many economic deposits of gold, copper, tungsten and iron occur in metamorphic
rocks.
STRUCTURE AND TEXTURE OF SEDIMENTARY ROCK AND METAMORPHIC ROCKS:
Read yourself
