ROCKS PETROLOGY

Is a branch of geology, which deals with study of rocks

(Petro=rock, Logos=study)

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.

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

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

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

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

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.

COMPOSITION

Chemical Composition

SiO2 – 59.14 %, Al2O3 – 15.35 % ,

FeO - 3.8 %, Fe2O3 - -3.08%, CaO - 5.08%

MgO - 3.49%, Na2O3 – 3.84 % , K2O 3.13% ,

H2O 1.15 % and others

Mineralogical Composition

Felspar – 59.5 %, Pyroxine and Amphiboles –

16.8 % , Quartz – 12%, Biotite - 3.8 %,

Titanium 1.5%, Apatite 0.60 % and others..

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)

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

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

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

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.

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

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

Ropy Lava: is more

mobile i.e. less

viscous, it has a

wrinkled but smooth

and shiny surface on

cooling.

.

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)

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 Gabbros

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.

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

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.)

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 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.

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

(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.

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.

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

Composition Phaneritic Aphanitic Color index (% dark minerals)

Felsic Granite

Syenite

Monzonite

Rhyolite

Trachyte

Latite

10

15

20

Intermediate Granodiorite

Diorite

Dacite

Andesite 20

25

Mafic Gabbro Basalt 50

Ultramafic Peridotite 95

TABULAR CLASSIFICATION

Chief Mineral

Constituents

Acidic

Rocks

Intermediate

Rocks

Basic Rocks Ultra Basic

Rocks

Quartz &

Felspar

Felspar,

both

varieties

Plagioclase

felspar &

Iron &

magnesium

minerals

Iron &

Magnesium

minerals

Plutonic Rocks

(Massive & coarse

grained) GRANITE SYENITE GABBRO PERIDOTITE

Hypabyssal Rocks

(fine gr.) PORPHYRITIC ROCKS DOLERITE

Volcanic Rocks

(glassy,cellular) RHYOLITE ANDESITE BASALT

Silica % > 65 % 50 TO 65 % < 50 %

Colour Light coloured rocks Dark coloured rocks

Av. Sp. Gravity 2.6 2.8 3.1 3.6

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

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:

Granite

Rhyolite

Intermediate Igneous Rocks:

-Igneous rocks in between in

composition between felsic and mafic

igneous rocks. They include:

Granodiorite

Dacite

Diorite

Andesite

Mafic Igneous rocks -very low silica content, and consist

primarily of mafic minerals. The most

common ultramafic rock is:

Peridotite

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 crys ta ls wi t h 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

Granite is a coarse-grained, light colored, intrusive igneous rock that

contains mainly quartz and feldspar minerals.

Granite :

Composition :

•Quartz (glassy lusture, H=7, white transparent appearance

•Felspar - 1, Potash felspar (orthoclase), 2 Soda-bearing (abite, oligoclase)

•Mica (muscovite, biotite) and accessory minerals.

Texture : course-medium grained, holocrystalline, equigranular. Massive structure

Types : depend upon minerals & textures, eg. Muscovite-granite, biotite granite, hornblend, augite etc.

Use : ??

Granite - intrusive

quartz

feldspar

biotite

mica

Granite - intrusive

Pegmatite

Graphic Granite

Porphyritic

mica feldspar

Diorite is a coarse-grained, intrusive igneous rock that contains a mixture

of feldspar, pyroxene, hornblende and sometimes quartz.

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).

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

porphyritic, with

visible hornblende. hornblende

phenocrysts

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-

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.

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,

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.

Basalt - extrusive

Gases released near the surface

of a lava flow create bubbles or

vesicles that are “frozen” in

stone.

Vesicular (Scoria)

Amygdaloidal (*)

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 pyroclastic (fire-formed fragments) and forms in explosive eruptions. It is a mix of large angular fragments and small ash.

The end !

The following are

Spare pictures…

intrusive extrusive

Into the

Bedrock

On top (Exit)

(crystals)

(no crystals)