Igneous rock, Engineering Geology, Semester IV GTU
Is a branch of geology, which deals with study of rocks
Importance of Petrology in civil
• It provides an opportunity to interpret the
physical properties of individual rocks, likewise:
texture, structure, mineral composition, chemical
• This helps in knowing the strength, durability,
colour, appearance, workability etc.
• These properties are very important for CE to
know because different rocks are suitable for
different purposes and no rock is ideal or best
suited for all purpose.
-source is magma
-thin veener above the
Sial and Sima in
-proportion is similar to
that of Igneous
-change of forms of Ig.
And Sed. Due to
All rocks that have formed from original hot, molten material
through the process of cooling and crystallization may be
defined as Igneous rock.
Magma and Lava
BROAD CLASSIFICATION OF IGNEOUS ROCKS
-Lava or Magma
Name comes from
Greek god of the
underworld - Pluto
Igneous rocks divided in three sub category
Cooling and crystallization of Lava..
Cools down very fast so the grain size of the crystal is very
e.g. Deccan trap
Ex : Rhyolite, Dacite, Trachytes and Basalts
This Igneous rocks formed at considerable depth (7-10 km
below earth surface)
Slow rate of cooling so Coarse grained
Exposed to surface due to removal overlying strata due to
Ex. Granites, Syenites, Gabbros
Formed at intermediate depth, 2 kms. Below the surface of
Mixed characteristics of Volcanic and Plutonic, Ex.
TEXTURE OF IGNIOUS ROCKS
Mutual relationship of diff. mineralogical constituents
(determined by size, shape and arrangement)
3 Factors which defines Textures..
Degree of Crystallisation
Holocrystalline , distinctly crystallized.( Also termed as
Merocrystalline , intermediate type
Holohyline , fine size, glassy (Also termed as aphinitic)
Granularity (average dimension of diff compo.)
Course ( > 5mm)
Medium (5mm to 1mm)
Fine (< 1mm)
Relative grain size of diff. minerals, and degree of perfection
in the form of the crystals
Equigranular or Inequigranular
Outline of individual crystals
Perfect : euhedral
Semi-Perfect : subhedral
Irregular : anhedral
If crystals of any one form or mixture of shape
Panidiomorphic : majority euhedral
Allotrimorphic : majority anhedral
Hypidiomorphic : all shapes
Types of Textures
Total five Textures..
Shown by Granite and Felcites so often name as
•Granitic texture (all course grained or all medium grained)
•Felsitic Structure , microgranular
• Orthophyric ( Between Granitic and Felsitic)
Porphyritic (presence of large size crystals (phenocrysts)
embeded in fine grained ground masses or matrix),
Caused due to
(i) Diff. in molecular concentration (ii) change in physio chemical
conditions and (iii) relative insolubility
Types of Textures
Poiklitic (presence of fine grained crystals within large xtals)
Indicates the result of flow of magma during formation.
Parallelism of crystals in direction of magma. Ex.Trachytic
and Trachytoid textures
During formation, two/more minerals crystallize
simultaneously. Ex. Graphic and Granophyric
Polygonal or trigonal spaces formed at early stage filled up
Structure of Igneous Rock
Features developed large scale in the body of an intrusion or
(A) Structure due to Mobility of Magma or Lava (highly
viscous, viscous and mobile, highly mobile)
(i) The flow structures :
Development of parallel layers or bands in the body of
Igneous mass due to flow of magma
(ii) The Pillow structure
Development of bulbous, overlapping, pillow like structures in
the body of Igneous rock.
Block Lava: is less mobile
i.e. more viscous and has a
Rough and irregular surface. When
cools down it result into angular,
chunky texture. This causes the lava to
thicken and to break apart as the flow
continues to move down slope.
Structure of Igneous Rock
(iii) The Ropy or Blocky lava
Developed due to different mobility
Highly viscous, broken surface : Blocky
Mobile lava, smoothly wrinkled surface : Ropy
(iv) The Spherulitic Structure
Thin minerals fibers of various size arranged in perfect or semi-perfect
(v) The Orbicular Structure.
Rare type. Rock mass appeared as if composed of ball-like segregation.
Each ball composed of concentric shell of different minerals
Ropy Lava: is more
mobile i.e. less
viscous, it has a
wrinkled but smooth
and shiny surface on
Structure of Igneous Rock
(B) Structure due to cooling of Magma
(i) The Joining Structures
Cooling of magmas often accompanied by development of
Sheet structure : horizontal sets, closely spaced
Mural jointing: three sets of joints , cubical blocks
Columnar structure : surface gets divided in polygonal cracks
(ii) Rift and Grain
Rock can be split with ease in two separate direction
(generally at right angle)
(iii) Vesicular Structure
Formation of cavities in the process of cooling and
crystallizations ( Due to lava rich in gases)
Structure of Igneous Rock
(iv) Miarolitic Structure
Small and distinct cavities formed during crystallization and
subsequently gets filled with volatile component then enlarge
and facilitate formation of minerals in them.
(C) Miscellaneous structure
(i)The Reaction Structure
Rocks surrounded on border by some incompletely altered
minerals forms this corona structure.
(ii) The Xenolithic structure.
Incorporation of foreign materials (from host rock) in to
magma during process of crystallization.
Forms of Igneous Rocks
Igneous rock formed from cooling and crystallization of magma, cooling
acquire variety of shapes which depends upon no. of factors
structural disposition of host rocks (Country rocks)
Viscosity of Magma or lava
Composition of Magma or lava
The environment in which injection of magma takes place.
Division in two broad class :
Concordant and Discordant
Concordant bodies : Intrusion in which magma, injected and cooled along
the structural plane of host rocks.
Most important concordant structure of the host rock:
Sills : injected along or between the bedding plane of sedimentary
sequences, thickness is much . Length may vary from few centimeters to
hundreds of meters.
Subdivided in to
(1) Simple sills : Single intrusion of magma
(2)Multiple sills : Two or more injections (of same kind)
(3) Composite sills : Two or more injections (of different kind)
(4) Differentiated sills: Exceptionally large, sheet like injection
of magma in which segregation of minerals into separate
layers or zones
(5) Interformational sills : The sheets of magma injected
along or in between the planes of unconformity in sequence.
Most common rocks composing sills are like Syenites and
Phacoliths : Occupy position in the trough and crests of crystal bends
called folds.(doubly convex, passes into crest or trough passively)
Lopoliths: associated with structural basins, that is sedimentary beds
inclined towards a common centre.
Many kms. Long and thousands of meter thick. (formation of basin and
injection of magma is simultaneous).
Like sills : simple, multiple, complex or differentiated in character.
Laccoliths : Invaded strata have been arched up or deformed in to dome.
The Igneous mass has flat or concave base.
Intrusive bodies that have been injected in to strata without
influence of structural deposition and thus traverse across or
oblique to the bedding planes
Dykes : Columnar bodies of igneous rocks that cut
across the bedding plane or unconformities.
Great variation in their thickness, length, texture and
Dykes occur in groups or sets (Dyke-set, Dyke-
Like sills classified as: Simple, Multiple, Composite
Cone sheets and Ring dykes.
Volcanic Necks : Vent of quiet volcanoes have
sealed with intrusions.
Batholiths : Huge bodies of igneous intrusion that
may show both concordant and discordant relations
with the invaded rock.(> 100 km2 area, depth not
CLASSIFICATION OF IGNEOUS ROCK
The Chemical composition of Rock
The mineralogical composition of the rock
The Textural variations
CHEMICAL CLASSIFICATIONS :
•Assumption that chemical composition of ultimate rocks
indicates magmetic relationship
•Most noteworthy by Petrologist : Cross, Iddings, Pirson and
Washington. : C.I.P.W.
•Standard set minerals called Norms selected. And Chem.
Compo. of given rock is defined in terms of this normative
Salient features of this classification
(A)Norm : Set of standard minerals of calculated chemical
composition. The norm minerals are divided in two groups :
Salic and Femic minerals
•The Salic minerals are : Quartz, Felspar, Orthoclase, Albite
and Anorthite, Lecuite, Nephelite, Corundum, Zircon and
•The Femic minerals are : Acmite, Diopside, Hypersthene,
Olivine, Magnatite, Chromite, Hematite, Pyrite, Rutile etc.
In the C.I.P.W. Classification, Igneous rocks are
divided into five classes, named according to SALIC
Prefixes “PER” and “DO” (High and moderate
No prefix when two groups are equally represented
Salic femic ratio Nomenclature for class
7.00 – 1.66 Dosalic
1.66 – 0.60 Salfemic
0.60 – 0.14 Dofemic
< 0.14 Perfemic
(C) Calculations :
Most important part of classification and involve frequent use
of tables for molecular weight, % weights of oxide and
molecular values etc.
Steps for result
•Rock chemical analyzed, data converted to oxides of
respective elements (with help of table)
•% weight of each oxide / molecular weight = Molecular
•The molecular numbers and various constituents oxides are
arranged in an orderly manner
•From molecular numbers, different normative minerals are
calculated according to sets of rules and with help of tables
•The % weights of different minerals are calculated from the
data obtain. From this the ratio between salic and femic
minerals can be easily computed and classified.
o Expression of chemical composition of parent
o Possible to recognize most common rock forming
minerals megaocopic or microscopic examination.
o Felsic Group: Light in colour, lower in density,
includes quartz, feldspar and felspathoids
o Mafic Group Dark in colour, heavier, Ferro-
magnesian group of elements. Represented in rock
by amphiboles, pyroxenes, micas, olivine, oxides of
oThen Rock division and color index.
Indicative of condition in which rock have
Phanerites: Course grained, can be
Aphanites: Fine grain constituents,
Glasses : Uncrystallized. Or glassy in
nature. Zero grain size
Also Plutonic, Volcanic and Hypabyssal
To Remember :
Composition and Classification
of Igneous Rocks
• Chemistry: e.g. % SiO2
• Mineralogy: e.g.
– Felsic (Feldspar and Silica)
– Mafic (Magnesium and Ferric)
Why last figure is so important
• The color and mineral distribution
indicate an increasing density and
melting temperature. Darker igneous
rocks generally weigh more and are
formed at higher temperatures and
pressures. This reflects the density-
stratification of the whole Earth!
Felsic Igneous Rocks:
-Igneous rocks rich in minerals high
in silica. They include:
Intermediate Igneous Rocks:
-Igneous rocks in between in
composition between felsic and mafic
igneous rocks. They include:
Mafic Igneous rocks
-very low silica content, and consist
primarily of mafic minerals. The most
common ultramafic rock is:
(You can see
Can see crystals. Usually
gray or pink. Can see
quartz - gray, glassy grains.
Can see feldspar - pink,
buff, or white.
Can see crysta ls with
somewhat more light colored
feldspar grains than dark
colored minerals. A mix of light
and dark but with no quartz. Salt
& pepper appearance.
Can see crystals — lots of
flat shiny cleavage surfaces.
Usually black to greenish
(You can NOT
see crystals, for
the most part)
Usually gray, pink,
pastel. Might see small
clear, rectangular crystals.
Light to dark gray. Normally has
small black crystals
Usually black or rust red.
May have some or lots of
gas bubble holes, some
holes may be filled. May
see small green grains.
OBSIDIAN: Black, red, green, GLASS
PERLITE: Usually pearly gray. May contain Apache Tears.
PUMICE: LOTS of gas bubble holes, very lightweight, will float on water. Abrasive.
(2 grain sizes)
ALL CRYSTALLINE IGNEOUS VARIETIES may exhibit porphyritic texture
TUFF: Compacted volcanic fragments generally less than 4mm diameter (ash)
VOLCANIC BRECCIA: Mixed tuff and angular large (>32mm diameter) fragments
AGGLOMERATE: Mixed ash and rounded/sub-rounded large (>32mm diameter) fragments
Composed of 90-
Composed of pre-
Composed of pre-
(such as hornblende)
Granite is a coarse-grained, light colored, intrusive igneous rock that
contains mainly quartz and feldspar minerals.
Diorite is a coarse-grained, intrusive igneous rock that contains a mixture
of feldspar, pyroxene, hornblende and sometimes quartz.
Intermediate Igneous rock of plutonic origin with 52-66
Composition : Rich in felspar plagioclase of sodic group,
also contain hornblende, biotite and some pyroxene
Texture: Resemblance to granite, course-medium grained.
But absence of Quartz
Occurrence : In form of small dykes, sills, stocks.
Generally it has a salt and
(about ½ black and ½ white).
Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with
other minerals such as hornblende, pyroxene and biotite.
Andesite - extrusive
ANDESITE is the fine-
grained equivalent of
DIORITE. It tends to
be a darker gray than
rhyolite and is often
Gabbro is a coarse-grained, dark colored, intrusive igneous rock that contains
feldspar, augite and sometimes olivine..
Gabbro - intrusive
GABBRO is a coarse-
grained rock that is high
is iron & magnesium-
olivene). The rocks will
be dark in color,
somewhat heavier than
granitic rocks and devoid
Black minerals are primarily amphibole (like
hornblende) and plagioclase feldspar.
Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly
of plagioclase and pyroxene.
Basalt - extrusive
BASALT occurs as thin to
massive lava. flows,
to thicknesses of
thousands of feet and
covering thousands of
square miles. The
volcanoes that produce
basaltic lavas are
relatively quiet, Basalt
is dark, fine-grained and
often vesicular (having
gas pockets). The
pockets may be filled
minerals, e.g. quartz,
zeolite minerals, calcite,
opal, etc. and then it is
called amygdaloidal (a-
Basalt - extrusive
Gases released near the surface
of a lava flow create bubbles or
vesicles that are “frozen” in
Obsidian is a dark-colored volcanic glass that forms from the very rapid
cooling of molten rock material. It cools so rapidly that crystals do not form.
Pegmatite is a light-colored, extremely coarse-grained intrusive igneous rock.
It forms near the margins of a magma chamber during the final phases of
magma chamber crystallization. It often contains rare minerals that
are not found in other parts of the magma chamber.
Peridotite is a coarse-grained intrusive igneous rock that is composed
almost entirely of olivine. It may contain small amounts of
amphibole, feldspar, quartz or pyroxene.
Pumice is a light-colored vesicular igneous rock. It forms through very rapid
solidification of a melt. The vesicular texture is a result of gas trapped in the
melt at the time of solidification..
Rhyolite is a light-colored, fine-grained, extrusive igneous rock that
typically contains quartz and feldspar minerals.
Volcanic Breccia - extrusive
VOLCANIC BRECCIA is
fragments) and forms in
explosive eruptions. It is
a mix of large angular
fragments and small ash.