brief study of igneous rocks

1. D.S.MODI, CIVIL DEPT.,
BVM ENGINEERING COLLEGE
VALLABH VIDYANAGAR
EXERCISE : 3
STUDY OF IGNEOUS ROCKS
Petrology: (Petra: Rocks and Logy: Logos: the science)
The branch of Geology dealing with various aspects of rocks such as
formation of rocks, chemical and mineralogical classification-
occurrence of rocks, their properties, textures, structures, etc.
Rocks:-
They are natural solid, massive and aggregate of minerals forming the crust
of the earth. They are regarded as building blocks of the Earth crust.
To The Geologists
Any mass of minerals matters whether consolidated or not, hard or even
soft irrespective of load bearing, which forms a part of the earth’s crust
is rock. Rock may consist of only one mineral species, they are called
monomineralic. But in nature rock are more usually consisting of
aggregates of minerals.
To The Civil Engineers
They regard rocks as something hard, consolidated and load bearing
material which where necessary has to be removed by blasting. This
concept also accords with the popular idea and the meaning of the
world.
There are three major /principal types of rocks on Earth’s crust. It is
based on way of formation:-
(1)Igneous Rocks (2)Sedimentary Rocks (3)Metamorphic Rocks
Igneous Rocks:-
All rocks that have formed from an originally hot molten material
through the process of cooling and crystallization may be defined as
igneous rocks. The source material for the formation of Igneous rocks is
Lava and Magma.
DSModi BVM (CIVIL) ENGINEERING GEOLOGY MANUAL -IGNEOUS ROCKS A.Y 2015-16

2. Magma Rocks:-
The hot molten material occurring naturally below the surface of the
earth is called magma.
The hot molten material when erupted through volcanoes as an eruptive
mass is lava. Igneous rocks are formed from both lava and magma.
Magma is actually a hypothetical melt. It has not been possible to see it
at its place and occurrences.
But it is assumed that it forms at a great depth [approximate 100 km
below the earth surface, at asthenosphere] due to very high temperature
and erupts at various region of the world again and again but remains
below the surface. The asthenosphere layer exists in Mantle zone, which
is plastic in nature having molten magmatic mass and also regarded as
hypothetical molten chamber. It is also assumed that the plates move on
this layer mainly responsible for plate tectonic activity.
Lava is thoroughly studied material and that has poured out
occasionally from volcanoes in many region of the world again and
again. It provides ample proof of existence of the magma below the
surface.
Magma or lava from which igneous rocks are formed may not be
entirely a pure melt; it may have a crystalline or solid fraction
thoroughly mixed with it. It consists of three components:
• A liquid portion, called melt that is composed of mobile ions
• Solids, if any, are silicate minerals that have already
crystallized from the melt
• Volatiles, which are gases dissolved in the melt, including
water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide
(SO2).
The magma can exist as a melt as long as physical and chemical
environment surrounding it remains unchanged. But as and when there
is a change in one or more of these conditions cooling and
DSModi BVM (CIVIL) ENGINEERING GEOLOGY MANUAL -IGNEOUS ROCKS A.Y 2015-16

3. crystallization of magma may start and end up with the formation of an
igneous rocks.
Igneous rocks are divides into three sub groups based on their
occurrence:-
• Volcanic Rock Hypabyssal Rock Plutonic Rock
Volcanic Rock:-
These are the igneous rocks formed on the surface of the earth by
cooling and crystallization of lava erupted from volcanoes.
Lava cools down very fast compare fine grained. The grain sizes formed
in these rocks are fine grained.
The cooling may take place on the surface or even under water of sea
and oceans. Ex: - Deccan traps of India – More than 4 lakh Sq. Km
spreaded in Peninsular India., the Columbia plateau and Prana plateau
of Brazil. Trap of Kashmir Himalayas.
Hypabyssal Rock:-
They are at intermediate depth at about 2 km. They exhibits mixed
characters of volcanic and plutonic rocks. Ex: - Porphyries.
Plutonic Rock:-
These are formed are considerable depth of 7 to 10 km below the
surface of the Earth. As these molten materials do not come
immediately with atmospheric contact, the rate of cooling is very slow.
Because of slow rate of cooling the rocks resulting from magma are
coarse grained.
These rocks are exposed on the surface of the earth as a consequence
and erosion of the overlying strata. Ex: - Granites, Gabbros are few
example
Composition: - Igneous rocks show great variation in chemical
composition.
Clark and Washington – Shown that on an average the following
elements (expressed in % of oxide) are present in the igneous rocks.
DSModi BVM (CIVIL) ENGINEERING GEOLOGY MANUAL -IGNEOUS ROCKS A.Y 2015-16

4. Chemical composition Mineralogical Composition:-
Texture of Igneous Rocks:-
The term texture is defined as the mutual relationship of different
mineralogical constituent in a rock.
It is determined by the size, shape and arrangement of these constituents
within the body of the rock.
The crystallization process from magma or lava may be slow or rapid.
The magma may be rich in one constituent and poor in other
constituents. The magma may be viscous quite mobile.
All these factors lead to various shapes, sizes and arrangement of the
resulting minerals and hence produce number of texture.
Factors explaining Texture:-
The three factors will primarily define the types of texture in a given
igneous rocks.
(A) Degree of Crystallization:- In an igneous rock all the constituent
minerals may be present in distinctly crystallized form and easily
recognized by unaided eye, or they may be poorly crystallized even
glassy or non crystallized. The resulting textures are:-
DSModi BVM (CIVIL) ENGINEERING GEOLOGY MANUAL -IGNEOUS ROCKS A.Y 2015-16
SiO2 59.14%
Al2O3 15.35%
FeO 3.80%
Fe2O3 3.08%
CaO 5.08%
MgO 3.45%
Na2O3 3.84%
K2O 3.13%
H2O 1.15%
Others 1.94%
Feldspar 59.5%
Pyroxene and
Amphibole
16.8%
Quartz 12.0%
Biotite 3.8%
Titanium 1.5%
Apatite 0.6%
Accessory
Minerals
5.8%

5. • Holocrystalline:-When all the constituents are distinctly
crystallized.
• Holohyalline: - When all the constituents are very fine in size and
glassy or non crystalline.
• Merocrystalline: - It is intermediate type. Some are crystallized
and some are glassy.
(B) Based on Granularity:- Average dimension of different
constituents in the given rock mass are Coarse, Medium, Fine.
Coarse size: > 5 mm Medium size: 1-5 mm Fine size: < 1 mm
(C) Based on Fabric:-This is a composite term expressing the relative
grain size of different mineral constituent in a rock as well as the degree
of perfection in the form of the crystals of the intermediate minerals.
• Equigranular: All the minerals are equal in size throughout the
rock mass
• In equigranular. All the minerals are not equal in size throughout
the rock mass
Texture of igneous rocks is primarily controlled by cooling rate of
magma
• Extrusive igneous rocks cool quickly at or near Earth’s surface
and are typically fine-grained (most crystals <1 mm)
• Intrusive igneous rocks cool slowly deep beneath Earth’s surface
and are typically coarse-grained (most crystals >1 mm)
DSModi BVM (CIVIL) ENGINEERING GEOLOGY MANUAL -IGNEOUS ROCKS A.Y 2015-16

6. Types of Textures in Igneous rocks:
Phaneritic: rocks with mineral grains that is large enough to be
identified by eye. Texture is typical of slowly cooled intrusive rocks.
Intrusive or plutonic rocks are typically Phaneritic. This means that
they are generally coarse grained and this texture is often quite uniform.
Grains can range in size but are often clearly visible to the naked eye
(>2-3 mm). Common example is granite.
Aphanitic: rocks with grain too small to be identified by eye. Texture
is most common in rapidly solidified extruded magma and marginal
facies of shallow intrusions. Extrusive or volcanic rocks are typically
aphanitic or glassy. This means that they are generally fine grained in
texture. Grains are typically 0.5 to 1 mm. Common example is Basalt.
In Glassy texture very rapid cooling of molten rock Resulting rock is
called Obsidian
Porphyritic texture:-
Many varieties are porphyritic. This means that the grain size is
bimodal, with a fine grained matrix surrounding larger grains that are
called phenocrysts. Minerals form at different temperatures as well as
differing rates large crystals, called phenocrysts, are embedded in a
matrix of smaller crystals, called the groundmass
Common example is Andesite.
Pegmatitic texture:-
DSModi BVM (CIVIL) ENGINEERING GEOLOGY MANUAL -IGNEOUS ROCKS A.Y 2015-16

7. A pegmatite is an extremely coarse-grained igneous rock (most crystals
>5 cm) formed when magma cools very slowly at depth form in late
stages of crystallization of granitic magmas
Pyroclastic texture:
Various fragments ejected during a violent volcanic eruption textures
often appear to more similar to sedimentary rocks
Mineralogical Classification:-
Rock forming minerals make the bulk of igneous rocks.
Two main classes of minerals in Igneous rocks:-
Felsic group of Minerals: - (Here Fel = Feldspars and Sic is for silica
bearing) they are Light in colour, lower in density and include Quartz,
Feldspar group and Feldspathoids group of minerals.
Mafic group of Minerals : (Here Ma= magnesium and fi= Fe bearing )
They are Dark in colour, heavier in density and have Ferro magnesium
minerals such as Mica, olivine, oxides and iron.
Colour Index: - For the Igneous rocks the colour index is
accepted and designated based on the dark colour minerals
present percentage wise in the given rock mass.
ROCK DIVISION COLOUR INDEX EXAMPLE
Leucocratic 1-30% Granite
Mesocratic 31-60% Gabbro
Melonocratic 61-100% Peridotite
‘Shand’ and others developed methods of classifying igneous rocks
saturated with silica:
DSModi BVM (CIVIL) ENGINEERING GEOLOGY MANUAL -IGNEOUS ROCKS A.Y 2015-16

8. • Oversaturated(ACIDIC): Silica % is > 66% + Free Silica +
Abundance of Quartz.
• Saturated (BASIC): Silica % is 45-66% and free silica is upto
10%.
• Intermediate: Rock term is used for silica % between 52-66%.
• Under Saturated (ULTRA BASIC): Silica % < 45% + No
Quartz.
• IGNEOUS ROCK CLASSIFICATION
Generalised Classification
Nature Acidic intermediate Basic Ultrabasic
Coarse
grained
Granite Syenite - Diorite Gabbro Peridotite
Dunite
Quartz
Porphyry
Syenite porphyry -
Diorite Porphyry
Dolerite -
Fine
grained
Rhyolite Trachyte -
Andesite
Basalt -
Amorphous Glass
Silica
(%)
> 65 55 - 65 45 - 55 < 45
BOWEN’S REACTION SERIES:-
DSModi BVM (CIVIL) ENGINEERING GEOLOGY MANUAL -IGNEOUS ROCKS A.Y 2015-16

9. ROCK CYCLE:
DSModi BVM (CIVIL) ENGINEERING GEOLOGY MANUAL -IGNEOUS ROCKS A.Y 2015-16

10. STRUCTURES IN IGNEOUS ROCKS:
Those features of igneous rocks that are developed on a large scale in
the body of an extrusion or intrusion.
Giving rise to conspicuous shapes or forms are included under the term
structures.
They may be so well developed as to be recognized easily on visual
inspection or they may become apparent only when thin sections of
such rocks are examined under microscope. They are microstructures.
Types of Structures
Structures developed in igneous rocks can be grouped under three
headings:
Structures due to mobility of magma or Lava:
The flow structures
The pillow structures the ropy and blocky lava
DSModi BVM (CIVIL) ENGINEERING GEOLOGY MANUAL -IGNEOUS ROCKS A.Y 2015-16

11. The spherulitic structures
Orbicular structures
Structures due to cooling of Magma:
Jointing Structure
Rift and Grain Structure
Vesicular Structure
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DSModi BVM (CIVIL) ENGINEERING GEOLOGY MANUAL -IGNEOUS ROCKS A.Y 2015-16