2. Importance of Petrology in civil
Engg.
• It provides an opportunity to interpret the
physical properties of individual rocks, likewise:
texture, structure, mineral composition, chemical
composition etc.
• 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.
3. 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
4.
5. IGNEOUS ROCKS
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
6. BROAD CLASSIFICATION OF IGNEOUS ROCKS
Volcanic rocks
(Extrusive rocks)
-Lava or Magma
flows
Intermediate rocks
(Hypabyssal rocks)
Plutonic rocks
(Intrusive
rocks)
Name comes from
Greek god of the
underworld - Pluto
7. Igneous rocks divided in three sub category
Volcanic rocks
Cooling and crystallization of Lava..
Cools down very fast so the grain size of the crystal is very
fine.
e.g. Deccan trap
Ex : Rhyolite, Dacite, Trachytes and Basalts
8. Plutonic rocks
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
erosion
Ex. Granites, Syenites, Gabbros
Hypabyssal Rocks
Formed at intermediate depth, 2 kms. Below the surface of
earth.
Mixed characteristics of Volcanic and Plutonic, Ex.
Porphyries.
11. 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
phaneric)
Merocrystalline , intermediate type
Holohyline , fine size, glassy (Also termed as aphinitic)
Microcrystalline
Granularity (average dimension of diff compo.)
Course ( > 5mm)
Medium (5mm to 1mm)
Fine (< 1mm)
12. Fabric
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
13. Types of Textures
Total five Textures..
Equigranular Texture
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)
Inequigranular Texture
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
14. Types of Textures
Poiklitic (presence of fine grained crystals within large xtals)
Directive Texture
Indicates the result of flow of magma during formation.
Parallelism of crystals in direction of magma. Ex.Trachytic
and Trachytoid textures
Intergrowth Textures
During formation, two/more minerals crystallize
simultaneously. Ex. Graphic and Granophyric
Intergranular Textures
Polygonal or trigonal spaces formed at early stage filled up
by crystallization
15. Structure of Igneous Rock
Features developed large scale in the body of an intrusion or
extrusion
Types
(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.
16. 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.
17. 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
radial manner.
(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
18. Ropy Lava: is more
mobile i.e. less
viscous, it has a
wrinkled but smooth
and shiny surface on
cooling.
.
19. Structure of Igneous Rock
(B) Structure due to cooling of Magma
(i) The Joining Structures
Cooling of magmas often accompanied by development of
cracks/ joints
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)
20. 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.
21. 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.
22.
23. 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
Gabbros
24. 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.
25. Laccoliths : Invaded strata have been arched up or deformed in to dome.
The Igneous mass has flat or concave base.
Discordant bodies.
Intrusive bodies that have been injected in to strata without
influence of structural deposition and thus traverse across or
oblique to the bedding planes
26. Important types
Dykes : Columnar bodies of igneous rocks that cut
across the bedding plane or unconformities.
Great variation in their thickness, length, texture and
composition.
Dykes occur in groups or sets (Dyke-set, Dyke-
swarm)
Like sills classified as: Simple, Multiple, Composite
and differentiated
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
traceable.)
27.
28.
29.
30. CLASSIFICATION OF IGNEOUS ROCK
The Chemical composition of Rock
The mineralogical composition of the rock
The Textural variations
31. 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
minerals.
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
Halite
•The Femic minerals are : Acmite, Diopside, Hypersthene,
Olivine, Magnatite, Chromite, Hematite, Pyrite, Rutile etc.
32. Nomenclature:
In the C.I.P.W. Classification, Igneous rocks are
divided into five classes, named according to SALIC
FEMIC ratio
Prefixes “PER” and “DO” (High and moderate
respectively.)
No prefix when two groups are equally represented
Salic femic ratio Nomenclature for class
>7.0 Persalic
7.00 – 1.66 Dosalic
1.66 – 0.60 Salfemic
0.60 – 0.14 Dofemic
< 0.14 Perfemic
33. (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
number
•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.
34. MINEROLOGICAL CLASSIFICATION
o Expression of chemical composition of parent
magma.
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
iron etc.
oThen Rock division and color index.
35. TEXTURAL CLASSIFICATION
Indicative of condition in which rock have
formed.
Phanerites: Course grained, can be
identified megascopically.
Aphanites: Fine grain constituents,
Microscopic examination.
Glasses : Uncrystallized. Or glassy in
nature. Zero grain size
Also Plutonic, Volcanic and Hypabyssal
39. To Remember :
Composition and Classification
of Igneous Rocks
• Chemistry: e.g. % SiO2
• Mineralogy: e.g.
– Felsic (Feldspar and Silica)
– Intermediate
– Mafic (Magnesium and Ferric)
– Ultramafic
40.
41. 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!
43. Intermediate Igneous Rocks:
-Igneous rocks in between in
composition between felsic and mafic
igneous rocks. They include:
Granodiorite
Dacite
Diorite
Andesite
44. Mafic Igneous rocks
-very low silica content, and consist
primarily of mafic minerals. The most
common ultramafic rock is:
Peridotite
45. LIGHT COLORED
Felsic
INTERMEDIATE
COLORED
DARK COLORED
Mafic
VERY DARK
COLORED
Ultramafic
COARSE-
GRAINED
(You can see
different
minerals)
GRANITE:
Can see crystals. Usually
gray or pink. Can see
quartz - gray, glassy grains.
Can see feldspar - pink,
buff, or white.
DIORITE:
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.
GABBRO:
Can see crystals — lots of
flat shiny cleavage surfaces.
Usually black to greenish
black.
FINE-
GRAINED
(You can NOT
see crystals, for
the most part)
RHYOLITE:
Usually gray, pink,
pastel. Might see small
clear, rectangular crystals.
Sometimes banded.
ANDESITE:
Light to dark gray. Normally has
small black crystals
BASALT:
Usually black or rust red.
May have some or lots of
gas bubble holes, some
holes may be filled. May
see small green grains.
GLASSY
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.
PORPHYRITIC
(2 grain sizes)
ALL CRYSTALLINE IGNEOUS VARIETIES may exhibit porphyritic texture
FRAGMENTAL
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
PERIDOTITE:
Composed of 90-
100% olivine
PYROXENITE:
Composed of pre-
dominately pyroxene
AMPHIBOLITE:
Composed of pre-
dominately amphiboles
(such as hornblende)
CLUES TO
IGNEOUS ROCKS
COLOR
TEXTURE
46. Granite is a coarse-grained, light colored, intrusive igneous rock that
contains mainly quartz and feldspar minerals.
51. Diorite is a coarse-grained, intrusive igneous rock that contains a mixture
of feldspar, pyroxene, hornblende and sometimes quartz.
52. DIORITE
Intermediate Igneous rock of plutonic origin with 52-66
silica%
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.
feldspar
biotite
Generally it has a salt and
pepper appearance
(about ½ black and ½ white).
53. Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with
other minerals such as hornblende, pyroxene and biotite.
54. Andesite - extrusive
ANDESITE is the fine-
grained equivalent of
DIORITE. It tends to
be a darker gray than
rhyolite and is often
porphyritic, with
visible hornblende.
hornblende
phenocrysts
55. Gabbro is a coarse-grained, dark colored, intrusive igneous rock that contains
feldspar, augite and sometimes olivine..
56. Gabbro - intrusive
GABBRO is a coarse-
grained rock that is high
is iron & magnesium-
bearing minerals
(pyroxenes, amphiboles,
plagioclase feldspar,
olivene). The rocks will
be dark in color,
somewhat heavier than
granitic rocks and devoid
of quartz.
Black minerals are primarily amphibole (like
hornblende) and plagioclase feldspar.
57. Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly
of plagioclase and pyroxene.
58. Basalt - extrusive
BASALT occurs as thin to
massive lava. flows,
sometimes accumulating
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
with secondary
minerals, e.g. quartz,
zeolite minerals, calcite,
opal, etc. and then it is
called amygdaloidal (a-
mig-duh-loy-dal) basalt.
59. Basalt - extrusive
Gases released near the surface
of a lava flow create bubbles or
vesicles that are “frozen” in
stone.
Vesicular (Scoria)
Amygdaloidal
(*)
60. 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.
61. 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.
62. 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.
63. 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..
64. Rhyolite is a light-colored, fine-grained, extrusive igneous rock that
typically contains quartz and feldspar minerals.
65. Volcanic Breccia - extrusive
VOLCANIC BRECCIA is
pyroclastic (fire-formed
fragments) and forms in
explosive eruptions. It is
a mix of large angular
fragments and small ash.
