UNIT-III.

PETROLOGY
 Unit-III. Petrology
Petrology is the branch of geology that studies rocks, and the conditions
in which rocks form.
• A rock is defined as a consolidated mixture of minerals.
 By consolidated we mean hard and solid.
 A mixture of minerals implies the presence of more than one
mineral grain, but not necessarily more than one type of mineral.

• A rock can be composed of only one type of mineral (e.g., limestone

is commonly made up of just calcite), but most rocks are composed
of several different types of minerals.

• It is very important to understand the difference between rocks and

minerals.
• A rock can also include non-minerals, such as the organic matter
within a coal bed, or within some shales.
 Unit-III. Petrology
Classification of Rocks
• According to their origin, rocks are divided into three groups, namely,
the igneous, metamorphic and sedimentary rocks.
formed from the cooling of a magma (i.e.,
IGNEOUS ROCKS
from molten rock)
formed when weathered fragments of other
SEDIMENTARY
rocks are compressed and cemented together
ROCKS

formed by alteration (due to heat, pressure

METAMORPHIC
and/or chemical action) of a pre-existing
ROCKS
igneous or sedimentary rock
• Some key factors that affect the engineering attribute and consequently
the engineering behavior of a rock are the rock texture and the
dominant weathering process.
 Unit-III. Petrology
The Rock cycle
• The inter-relationships between rock types can be summarized on what
is known as the rock cycle diagram.
 Unit-III. Petrology
IGNEOUS ROCKS
• Igneous rocks are formed when hot molten rock material called
magma solidifies.
=> Magmas are developed when melting occurs either within or beneath
the Earth’s crust, that is, in the upper mantle.
• Igneous rocks are composed principally of silicate minerals.
• Of the silicate minerals, six families are quantitatively by far the most
important constituents:
o Olivine [(Mg,Fe)2SiO4],
o Pyroxene [e.g. augite, (Ca, Mg, Fe, Al)2(Al,Si)2O6],
o Amphiboles [e.g. hornblende, (Ca, Na, Mg, Fe,Al)7-8(Al,Si)8O22(OH)2],
o Mica[e.g. muscovite, KAl2(AlSi2)10(O,F)2; and biotite,
K(Mg,Fe)2(AlSi3)O10(OH,F)2],
o Feldspars (e.g.orthoclase,KAlSi3O8; albite,NaAlSi3O8; and anorthite,
CaAl2Si2O8), and silica minerals (e.g. quartz, SiO2) -
 Unit-III. Petrology
Igneous rock identification and classification
• according to their mode of occurrence, Igneous rocks may be
divided into Intrusive and Extrusive types.
• In intrusive type, the magma crystallizes within the Earth’s crust,
whereas
• In the extrusive type, magma solidifies at the surface, having
erupted as lavas and/or pyroclasts from a volcano.
• The intrusions have been exposed at the surface by erosion.
• They have been further subdivided on the basis of their size, that is,
into major (plutonic) and minor (hypabyssal) categories.
 Unit-III. Petrology
Criteria to identify igneous rocks
i. Conditions of magma cooling:

Crystallization at great depth slow solidification

Intrusive rocks

Crystallization at the surface fast solidification

Extrusive rocks
 Unit-III. Petrology
Cont’

− Ability of magma to move toward the surface depends on the presence of preferential
pathways or fractures
− Earth crust is not very deformable majority of magma never reaches the surface
 Unit-III. Petrology
ii. Magma composition:
Magma composition depends on the composition of the pre-
existing rock. For example:
 If pre-existing rock comes from continental crust: Si, O, Al
 If pre-existing rock comes from oceanic crust: Si, O, Mg, Fe
Magma Solidifie Chemical Temperature Viscosity Gas
Type d Rock Composition Content
1 Basaltic Basalt 45-55 SiO2 %, 1000-1200 oC Low Low
high in Fe, Mg,
Ca, low in K, Na
2 Andesitic Andesite 55-65 SiO2 %, 800-1000 oC Intermediate Intermediate
intermediate in Fe,
Mg, Ca, Na, K
3 Rhyolitic Rhyolite 65-75 SiO2 %, low 650-800 oC High High
in Fe, Mg, Ca,
high in K, Na.
Unit-III. Petrology
Cont’
Unit-III. Petrology
Cont’
 Unit-III. Petrology
Intrusive igneous rocks

Minor Intrusions :
1. Dikes
2. Sills

Major Intrusions :
1. batholiths,
2. Stocks
 Unit-III. Petrology
Volcanic Activity and
Extrusive Rocks

=> Volcanic activity is a surface

manifestation of a disordered
state within the Earth’s interior
that has led to the melting of
material and the consequent
formation of magma.

• Volcanic zones are associated

with the boundaries of the
crustal plates.
 Unit-III. Petrology
Texture of Igneous Rocks
• Igneous rocks are also classified according to their texture and their composition.
• In describing texture we are generally referring to the average size of the mineral
grains present, but other important characteristics include the relative sizes
=>(i.e., whether a mineral is present in large grains relative to other minerals) and
the presence or absence of cavities.
In terms of grain size and texture, igneous rocks are described as:
GLASSY no mineral grains or crystals are actually present
The crystals are present, but they are too small to distinguish by the naked
APHANITIC
eye
Individual mineral grains can be distinguished by the naked eye (average
PHANERITIC
grain sizes range from 2 mm to 5 mm or more

PEGMATITIC most of the mineral grains are greater than 1 cm across

PORPHYRITIC There are large crystals of one of more minerals set within a groundmass of
OR finer-grained material. Eg. Porphyritic feldspars in some granites, for
MEGACRYSTS instance, may be 5 to 10 cm long.

PYROCLASTIC There are angular fragments of volcanic rock within a finer-grained matrix
 Unit-III. Petrology
Texture of Igneous Rocks
• Intrusive igneous rocks are generally crystalline (i.e., phaneritic and
more rarely pegmatitic) because they have had a long time to cool. The
crystals, which are large enough to see with the naked eye, are mostly
angular or irregular in shape. Intrusive porphyritic textures are
formed in cases where some minerals have crystallized from magma
over a long period.
• Extrusive (i.e., volcanic) rocks can be glassy, aphanitic, porphyritic or
pyroclastic. Extrusive porphyritic textures result when some minerals
have crystallized from magma over a long period, and then a volcanic
eruption takes place, so that the rest of the magma suddenly cools and
crystallizes. Pyroclastic textures result when fragments of rock and
glass are ejected explosively during an eruption and then accumulate on
the ground around the vent.
 Unit-III. Petrology
Texture of Igneous Rocks

Porphyritic textured igneous rock Glassy textured igneous rock Glassy rock like obsidian examples of
phaneritic rocks.

This is an example of a pyroclastic

Aphanitic texture Pegmatitic texture texture
 Unit-III. Petrology
Texture of Igneous Rocks
• In many cases glassy volcanic rocks are also vesicular; this means that
they are full of air cavities created by the gases in the magma..
 Unit-III. Petrology
SEDIMENTARY ROCKS
• Sedimentary rock is a rock resulting from the disintegration and
decomposition of any rock type when it is transported, redeposited, and
partly or fully consolidated or cemented into a new rock type.
• The sedimentary rocks generally are formed in quite definitely arranged
beds, or strata, which can be seen to have been horizontal at one time
although sometimes displaced through angles up to 90 degrees.
Origins of Sedimentary Rocks:
Sedimentary rocks are formed from sediments through the following
processes : Weathering, Erosion, Transportation, Deposition
(sedimentation), Burial and Diagenesis (lithification).
 Sedimentary rocks can be composed of sediments created from any of three
great families of rock (igneous, sedimentary, and metamorphic).
 Sedimentary rocks are also created by the precipitation of minerals from
solution by either organic or inorganic chemical processes or commonly a
mixture of both.
 Unit-III. Petrology
Cont’
• The surface processes of the rock cycle that are important in the
formation of sedimentary rocks are summarized here:
i. Weathering : is the general process by which rocks are broken down
at Earth’s surface to produce sediment particles.=> Chemical,
physical and biological weathering, are the general processes by
which rocks are broken down on the Earth’s surface.

ii. Erosion: refers to processes that dislodge particles of rock produced

by weathering and move them away from the source area. Erosion
occurs most commonly when rainwater runs downhill.

iii. Transportation: refers to processes by which sediment particles are

moved to sink areas. Transportation occurs when water, wind, or the
moving ice of glaciers transport particles to new locations downhill
or downstream.
 Unit-III. Petrology
Cont’
iv. Deposition (also called sedimentation) refers to processes by which
sediment particles settle out as water currents slow, winds die down,
or glacier edges melt to form layers of sediment in sink areas. In
aquatic environments, particles settle out, chemical precipitates form
and are deposited, and the bodies and shells of dead organisms are
broken up and deposited.

v. Burial: occurs as layers of sediment accumulate in sink areas on top

of older, previously deposited sediments, which are compacted and
progressively buried deep within a sedimentary basin. These
sediments will remain at depth, as part of Earth’s crust, until they are
either uplifted again or subducted by plate tectonic processes.

vi. Diagenesis refers to the physical and chemical changes caused by

pressure, heat, and chemical reactions by which sediments buried
within sedimentary basins are lithified, or converted into sedimentary
rock.
 Unit-III. Petrology
Sedimentary Rocks Cont’

Fig: several surface processes of

the rock cycle contribute to the
formation of sedimentary rocks.
 Unit-III. Petrology
Diagenesis/Lithification
• Lithification is he process which convert sediments into sedimentary
rocks.
• Diagenetic processes include not only cementation and compaction but
also solution and redeposition of material, to produce extremely strong
or very weak rock.
• There are two major methods of lithification:
1. Compaction
2. Cementation

1. Compaction: The weight of the overlying sediments compresses the deeper

sediments, reducing pore space and squeezing out water. Clays and muds contain
up to 60 % water, but only 10% after compaction. Eventually, the sediments are
compacted into rock

2. Cementation: chemical precipitation of mineral material between grains

(SiO , CaCO , Fe O ) binds sediment into hard rock.
Unit-III. Petrology
 Unit-III. Petrology
Sedimentary Rocks Cont’
How to Classify & identify sedimentary rocks?
• Sedimentary rocks can be divided into two principal groups, namely,
the clastic (detrital) or exogenetic, and the non-clastic or endogenetic
types.
 1st criterion: Origin of sediments
• Clastic/Detrital sedimentary rocks: Made up of solid particles that come from
the disintegration of existing rocks(=erosion/weathering)

Ex. Conglomerate
Gravel accumulation,
compacted and cemented
 Unit-III. Petrology
Sedimentary Rocks Cont’
How to Classify & identify sedimentary rocks?
 1st criterion: Origin of sediments
• Chemical or Biochemical sedimentary rocks: Made up of materials that come
from biological or chemical processes (accumulation of plants and animal
organisms, precipitation of minerals from an oversaturated medium, etc.)

Ex. Limestone
Precipitation of CaCO3 (calcite)
by marine organisms
 Unit-III. Petrology
Sedimentary Rocks Cont’
 2nd criterion: Grain size (for clastic sedimentary rocks)

Particle size Sediment Rock

>2 mm Gravel Conglomerate (breccia or pudding stone)

2 – 0.062 mm Sand Sandstone

0.062 – 0.0039 mm Silt Siltstone

< 0.0039 mm Clay Claystone, shale

• The strength of conglomerate Conglomerate

depends on the cement material.

Conglomerate: pudding stone

Conglomerate: breccia (angular
(rounded gravels)
gravels)
 Unit-III. Petrology
Sedimentary Rocks Cont’
 2nd criterion: Grain size (for clastic sedimentary rocks)
Sandstone
• High content in quartz rapid abrasion of drilling tools
• Nature of the cement: calcareous, siliceous or argillaceous
• Generally high permeability (10-6 – 10-9m/s)
• Used as building stone

Sandstone house – Condroz, Belgium

 Unit-III. Petrology
Sedimentary Rocks Cont’

 2nd criterion: Grain size (for clastic sedimentary rocks)

Claystone / Shale
• 50-60% of sedimentary rocks
• Plastic behaviour
• Low permeability (10-6 – 10-
9m/s)

• Possible layered appearance/

lamination due to parallel
organization of clay minerals
• Subject to swelling
• Raw material for bricks industry
and for the manufacture of
cement
 Unit-III. Petrology
Sedimentary Rocks Cont’

 3rd criterion: Chemical elements involved in the processes

• Ca -Carbonate rocks (ex. Limestone)

• Na, Ca -Evaporite rocks (ex. Gypsum, Anhydrite, Halite)

• Si –Silica rocks (ex. Chert/Flint)

• Organic sedimentary rocks (ex. Coal)

 Unit-III. Petrology
Sedimentary Rocks Cont’

CARBONATE ROCKS
The dominant biological sedimentary rock lithified from carbonate sediments is limestone, which
is composed mainly of calcite. Various types of limestone are as follows:
1. Bio-chemical limestone = Fossiliferous limestone
 Coral limestone
 Chalk
2. Chemical limestone (precipitation of calcite)
 Oolitic limestone
 Micritic limestone
 Calcareous tufa or travertine
 Stalactite/Stalagmite
3. Detrital limestone: same process as breccia
 Unit-III. Petrology
Sedimentary Rocks Cont’
CARBONATE ROCKS CONT’
 Unit-III. Petrology
Sedimentary Rocks Cont’
CARBONATE ROCKS CONT’
 Unit-III. Petrology
Sedimentary Rocks Cont’
CARBONATE ROCKS CONT’
 Unit-III. Petrology
Sedimentary Rocks Cont’
CARBONATE ROCKS CONT’
Engineering implications of limestone

• Low hardness (low hardness of calcite)

• Soft, but often massive rock good for foundations/construction/excavation in many
cases
• High permeability (10-6 – 10-9m/s)
• Low solubility in water

• Alkalinity: limestone neutralize acid rain, leading to retention of heavy metals

Good to prevent them from flowing into spring waters (>< granite, sandstone)
 Unit-III. Petrology
Sedimentary Rocks Cont’
CARBONATE ROCKS CONT’

Industrial uses of limestone

• Lime industry
• Construction materials
• Crushed for ballast, road base,
etc.
• In glass, paper industries
• Etc.
 Unit-III. Petrology
Sedimentary Rocks Cont’

OTHER CARBONATE ROCKS

Dolostone
• CaCO3 is converted into
Ca,Mg(CO3)2
• Dissolved by acidic water, but
less affected than limestone
Marl
• Detrital clay + CaCO3
• Harder than claystone, but
remains plastic
• Less soluble in water than
limestone
 Unit-III. Petrology
Sedimentary Rocks Cont’
EVAPORITE ROCKS
• Evaporates form through inorganic precipitation (chemical process) of
minerals from evaporating seawater and/or water in arid-region lakes
that have no or few freshwater provision.
 Unit-III. Petrology
Sedimentary Rocks Cont’
EVAPORITE ROCKS CONT’
 Unit-III. Petrology
Sedimentary Rocks Cont’
EVAPORITE ROCKS CONT’
 Unit-III. Petrology
Sedimentary Rocks Cont’
SILICA ROCKS
 Unit-III. Petrology
Sedimentary Rocks Cont’
ORGANIC SEDIMENTARY ROCKS
 Unit-III. Petrology
Sedimentary Rocks Cont’
 Unit-III. Petrology
METAMORPHIC ROCKS
• Metamorphic rocks are formed when existing parent rocks are
transformed (metamorphosed) by heat and pressure deep below the
surface of the earth or along the boundary of tectonic plates.
=> Metamorphic rocks were once sedimentary, igneous, or another
metamorphic rock. These rocks are physically deformed and
chemically changed due to different temperatures and pressures.

• During metamorphism, rocks may fold, fracture, or even partially

melt to a viscous state and flow before reforming into a new rock.
Metamorphic rocks change in appearance, mineralogy, and
sometimes even chemical composition from their parent rock
source.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
• Metamorphic rocks record how temperature and pressure affected an
area when it was forming. Metamorphic rocks are best identified
when looking at the rock as you see them in nature. You can clearly
see the deformation and features that are characteristic of an entire
area.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Causes of Metamorphism:
• The three primary causes of metamorphism include one or more of
the following conditions: heat, pressure, and/or chemically active
fluids.
1. Heat
• Heat provides energy for chemical reactions to proceed resulting in
new minerals to form from original minerals in the source rock.
• Heat provides the energy that enables individual ions in the rock to
mobilize and migrate between other ions recrystallizing and forming
into new minerals.

=> Metamorphic changes can occur with increasing or decreasing

temperature:
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Causes of Metamorphism Cont’:
 Prograde refers to mineral changes that take place during an increase in
temperature,
 Retrograde refers to mineral changes that take place during a decrease
in temperature.
=> Heat involved in metamorphism comes from two main sources:
 Heat transferred during contact metamorphism from magma or igneous
intrusions.
 Progressive temperature increase associated with geothermal gradient
(The rate at which the temperature increases as you go deeper into the
Earth’s crust) as rocks are transported to greater depths below the
Earth’s surface.
• The geothermal gradient averages 30oC per kilometer increase with
depth (but it can vary from 20oC to 50oC per kilometer of depth)
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Causes of Metamorphism Cont’ :
2. Pressure
• Pressure equals force per unit area: (Pressure = F/A) and increases
with depth as the weight and thickness of the overlying rocks
increases.
• Pressure during metamorphism is manifested by two different forces:
body force (confining pressure) and surface force (differential
stress).
 Body force —forces are applied equally in all directions (gravity and
weight), as a result individual grains are compressed closer and
closer together. Extreme confining pressures that occur at great
depths may even cause ions in the minerals to recrystallize and form
new minerals.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Causes of Metamorphism Cont’ :
 Surface force — Operates across a surface and occurs when rocks
are compressed or extended along a single plane (push-pull forces).
As a result, the rocks are shortened or extended in the direction the
pressure is applied.
3. Chemically Active Fluids
• Chemically active fluids that are present between mineral grains
during metamorphism act to facilitate ion movement and the re-
crystallization of existing and new minerals.
• Higher temperatures increase the reactive capability of ion-rich
fluids. When these fluids come in contact with mineral grains, the
grains readily dissolve because of differential chemical potentials,
and ions migrate to areas of lower potential eventually
recrystallizing.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
The Role of Parent Rocks in Metamorphism
• Parent rocks provide the minerals and ion sources that are
transformed into new minerals and rocks.
• In most cases the new metamorphic rock has the same chemical
composition as the parent rock that they formed from.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Types of Metamorphism
There are three ways that metamorphic rocks can form. The three types
of metamorphism are Contact, Regional, and Dynamic metamorphism.
1. Thermal or Contact Metamorphism:
• Occurs when parent rock is intruded by magma (usually an igneous
intrusion). Metamorphic changes under these conditions are
primarily the result of temperature changes associated with the
intruding magma.
• Additionally, when hot ion-rich water circulates through fractures in
a rock, it can also cause chemical changes to the parent rock.
• These heat-driven, chemical reactions occur with igneous activity
and the presence of water.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Types of Metamorphism Cont’
• Contact metamorphism produces non-foliated (rocks without any
cleavage) rocks such as marble, quartzite, and hornfels when
magma comes in contact with an already existing body of rock.

• The area affected by the contact of magma is usually small, from 1 to

10 kilometers.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Types of Metamorphism Cont’

2. Regional Metamorphism:
• It occurs when rocks are subjected to both heat and pressure on a
regional scale.
• It is caused by burial deep in the crust and is associated with large
scale deformation and mountain building.
• It is the most widespread form of metamorphism.
• Regional metamorphism involves both the processes of changing
temperature and stress.
• Regional metamorphism usually produces foliated rocks such as
gneiss and schist.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Types of Metamorphism

3. Dynamic Metamorphism:
• It occurs when rocks are subjected to extreme pressure very
rapidly. It also occurs because of mountain-building.

• Metamorphism does not involve any transformation to magma

• Metamorphism affects:
 The mineralogy (type of minerals)
 The texture (crystal arrangement)
 The structure (in-situ geometry)
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Classification of Metamorphic rocks

a) Metamorphic rocks can be classified by how much

metamorphic changes have undergone:

 Low-grade metamorphism involves lower temperature and

compressional forces that result in less overall change to the
parent rock. In many cases, after low-grade metamorphism,
changes of the parent rock may still be easily distinguishable.
 High-grade metamorphism results in a total transformation of
the parent rock into a new rock whereby its original parent-rock
source is difficult to identify.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Classification of Metamorphic rocks
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Classification of Metamorphic rocks

b) Metamorphic rocks are classified by their texture and

composition, which is also related to their grade.

• The visible character most useful for the classification of metamorphic

rock is the presence or absence of foliation.
• Metamorphism creates new textures on the rock it alters, in general
the grain size of crystals increase as the grade of metamorphism
increases.
• There are three major types of metamorphic rock textures: Foliation
(Foliated metamorphic rocks), Granoblastic (Non-foliated
metamorphic rocks), and Large-crystal.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
 Foliation is a set of flat or wavy parallel planes produced by directed
stress deformation. Metamorphic minerals can be compressed,
elongated and/or rotated by being forced into preferred orientations.

 Existing minerals keep their random orientation if force is uniformly

applied by confining pressure.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Examples of foliated rocks: Slate, Phyllite, schist,
gneiss and Migmatite.

Figure : Foliated rock

 Unit-III. Petrology
Metamorphic Rocks Ctd’
What are politic rocks?
When shale is metamorphosed, it produces pelites or politic rocks.

• Shale originated as a rock composed of clay and quartz minerals. If the

metamorphism is not intense, recrystallized minerals such as muscovite
line up perpendicular to the stress direction and form slate. If the
intensity of the metamorphism increases, phyllite forms; at even higher
intensity, schist forms; finally, with even higher temperatures and
pressures, gneiss is the eventual result.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Types of metamorphic rocks
Amphibolite is a type of metamorphic rock that forms through re-
crystallization under conditions of high viscosity and directional
pressure. It is a non-foliated (non-layered) rock composed of
“amphiboles” and plagioclase, usually with little quartz.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Types of metamorphic rocks
Hornfels is a fine-grained non-foliated metamorphic rock with no
specific composition. It is produced by contact metamorphism under
intense heat that usually “bakes” magma in a chamber such as sill or
dike.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Types of metamorphic rocks
Phyllite is a foliated (layered) metamorphic rock that is made up
mainly of very fine-grained mica. The surface of phyllite is typically
lustrous and sometimes wrinkled. It is intermediate in grade between
slate and schist.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Types of metamorphic rocks
Gneiss is foliated metamorphic rock that has a banded appearance and
is made up of granular mineral grains. It typically contains abundant
quartz or feldspar minerals.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Types of metamorphic rocks
Marble is a non-foliated metamorphic rock that is produced from the
metamorphism of limestone. It is composed primarily of calcium
carbonate (Calcite mineral: CaCO3).
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Types of metamorphic rocks
Quartzite is a non-foliated metamorphic rock that is produced from the
metamorphism of sand stone. It is composed primarily of quartz
minerals.
 Unit-III. Petrology
Metamorphic Rocks Ctd’
Types of metamorphic rocks
Schist is a foliated metamorphic rock. It often contains significant
amount of mica which allow the rock to split into thin pieces. It is
intermediary rock between phyllite and gneiss. If there is evidence of
chlorite it is called “chlorite schist”. It sometimes contains small crystals
called red garnets.

Garnet schist Chlorite schist

UNIT-IV. PHYSICAL GEOLOGY