Petrology

Petrology
Petrology
• Introduction, Shell Structure of Earth,
Definition of Rocks, Classification of Rocks,
Sequence of Formation of Different Groups of
Rocks, Rock Cycle, Civil Engineering
Importance of Petrology.

Introduction
• Petrology means Study of Rocks ( Petro= rock;
and logos = study)
• The subject matter of petrology comprises of
origin, association, occurrence, minerals
composition, chemical composition, texture,
structure, physical properties, etc. of rocks.
• Petrolography deals with the descriptive part of
the rock and petrogeny (geny= genesis= origin)
deals with the mode of formation of rocks. This to
gather make petrology.

Shell Structure of the Earth
• The crust (the outermost solid part) of our earth is made up
of different kind of rocks. The thickness of the crust is
approximately 35 km. Below this is separated from the
mantle by the Moho discontunity. This rock crust is
approximately called lithosphere (litho= rock; sphere= zone or
shell). The surface of this crust is uneven with many
depression and elevations. These depressions are filled with
water and are variously called lakes, seas and oceans. This
discontinuous body of water in association with lithosphere is
called hydrosphere. On land masses of the lithosphere and in
water bodies of the hydrosphere, all living creatures exist. This
is described as the biosphere. All these, in turn are enveloped
by a layer of air which is called the atmosphere.

Shell Structure of the Earth
• In the interior of the earth, i.e. below the lithosphere, the
inaccessible part is called barysphere. Within this lies the
asthenosphere ( or sphere of weakness) This Zone of
somewhat plastic or semi-solid matter which makes
isostacy possible by allowing necessary movement of
matter from one place to another. The interior being
possible by allowing necessary movement of matter from
one place to another. The interior being very hot, it creates
condition favorable for magma occurrence. This part where
magma originates is called the pyrosphere. This part of
mighty tectonic forces which are responsible for all major
surface disturbances, i.e. either orogeny or epeirogeny.
Hence that part of interior is called the tectonosphere.

Definition of a Rock.
• In spite of the vast variety and complexities of
rocks, a rock may be simply defined as “ an
aggregate of minerals” Since the crust of the
earth is composed of rocks, a rock may also be
defined as “ a unit of the earth’s crust”. There
are many mineral species occurring in nature.
Since the occurrence of different minerals or
different proportions of the same minerals can
give rise to different rock types, there is scope to
imagine innumerable varieties of rocks. There
are different types of rocks but have the same
minerals though in different proportions.

Definition of a Rock.
• In spite of the scope for the occurrence of a
large number of rock types, close observation
reveals that in nature only some rock types are
common. Further, among the large variety of
mineral species, only very few take part in the
formation of different rocks. These minerals
are approximately described as rock-
forming minerals.

Classification of Rocks
• The rocks are classified in various ways based on
different principles such as physical classification
( as Stratified and unstratified), chemical
classification as (Calcareous, siliceous etc)
geological classification (igneous, sedimentary ,
metamorphic) etc. Among the different
classification, geological classification is the most
proper because grouping of rock is more logical,
less ambiguous, orderly and comprehensive. The
geological classification of rock is based on
mode of origin.

Classification of Rocks
Igneous Rocks (lignum= fire, meaning very hot)
• As mentioned later, igneous rocks are the first formed
rocks which had made up the primordial earth’s crust.
For this reason these are called primary rocks, even
though igneous rocks have been formed subsequent
also. Igneous rocks are the most abundant rocks in the
earth’s crust. In fact, their abundance is so much that
their average composition closely tallies with the
chemical composition of the earth’s crust itself. The
igneous rocks are formed at a very high temperature
directly as a result of solidification or assimilation.

Igneous Rocks
Magma and Lava
• Both these refer to melt of rocks which are
compositionally silicate rich minerals. The term
magma is applied when the melt is underground.
The same, when it reaches the earth’s surface and
flows over it, is called lava. Thus based on the mode of
occurrence, the melt of rocks is described either as
magma or lava. Apart from this, a little compositional
difference also occurs between the two. Magma is
always associated with huge quantities of various
volatiles. But when it flows on the surface, these
volatiles escape into the atmosphere. Therefore the
lava is devoid of such volatiles.

Igneous Rocks
Typical Characters
• The igneous rocks are often characterized by the
presence of crystalline minerals and an interlocking
texture. These show indications of having been formed
from a high temperature rock melt. They are usually
massive, unstratified, unfossiliferous, have
apophyses and often occur as intrusive cutting
across other rocks which they might have heated,
baked and altered. The volcanic igneous rocks are
always extremely fine grained and they may be massive
or vesicular or amygdaloidal.

Igneous Rocks
Formation, Occurrence and Crystallization of Magma
• Magma is the parent material of igneous rocks. Anywhere on the
earth, the temperature increases proportionally with the depth.
The rate of increase of temperature of this kind, however, varies
place wise and depth wise. So, it is natural to expect that at very
great depth, the prevailing temperature must be capable of melting
the rocks, thereby producing magma. This is one of the reasons
for the formation of magma. The heat generated by processes such
as radioactive minerals disintegration may also help this process.
But below the earth, magma does not occur everywhere. This is
because, just as temperature increases with depth, pressure also
increase with depth due to increase of overburden. The effect of
these two mutually associated phenomenon are different.

Igneous Rocks
• The rise in temperature tends to increase the volume of the
material, whereas the rise in pressure tends to decrease the
material. Hence depending upon the local conditions where
the pressure effect is less than the effect of temperature
effect, magma can form because the formation of hot magma
from rocks has to be accompanied by necessary volume
increase. If pressure is dominant, it will not permit increase
in volume. That means magma is not allowed to form, even
though the prevailing temperature may be capable of
producing magma. In a view of these complications, and
other factors, magma occurs below the earth only as isolated
packets or chambers and not as continuous body. It is locally
generated due to loss of pressure when deep seated fractures
reach high temperature areas at a depth.

Igneous Rocks
• After its formation, the magma moves upward and gets surrounded
by relatively colder rocks which results in gradual loss of heat from
the body of the magma, because of this, solidification or
crystallization of magma follows. The rate of cooling of magma
depends on the size of magma chamber and other condition, Small
bodies of magma will solidify quickly and directly producing
homogeneous igneous rocks. But in case of huge magmatic bodies,
the process of solidification is often accompanied by differentiation or
assimilation or both.
• Differentiation is a process whereby a magma, originally
homogeneous, splits into parts of contrasted composition and then
solidifies giving rise to rocks of different compositions.
• Or sometimes two liquid magmas of different compositions may mix
together forming new homogeneous magma. These phenomenon is
called assimilation.

Igneous Rocks
Sequence of Events in the Crystallization of Magma
• Magma is composed of non-volatile and volatile constituents.
With falling of temperature, when magma begins to
crystallization, the non-volatile constituents come out to form
the rock-forming minerals, i.e.. Mostly the silicates, which are
characteristic of the usual igneous rocks this stage is called
orthomagmatic stage. The separation of the non-volatile
constituents leads to the concentration of the volatile
components, with the result that the residual portion of the
magma forms the pegmatite stage and the gaseous parts
forms the pneumatolytic stage. The final consolidation of
magma takes place from the last hot water-rich mineralized
solutions. This is hydrothermal stage.

Sedimentary Rocks
• Sediments are the products of weathering. Since these are secondary
materials (i.e. derived from pre-existing rocks), the rocks formed out of
them are called sedimentary or secondary rocks. The origin of
sedimentary rocks is totally related to the weathering influences on rocks.
In-essence
• (i) the intense decay of rocks, followed by leaching, leaves behind some
insoluble, inert, porous, residual matter.
• (ii) The disintegration produces loose rock debris or sediments of
various sizes which on compaction or cementation produces the most
common and abundant type of sedimentary rocks like shale and
sandstone.
• (iii) The leached out parts give rise to sedimentary rocks due to chemical
processes such as evaporation and precipitation or deposited through the
agency of organisms. Thus the origin of sedimentary rocks mainly takes
place in four different ways in nature.

Sedimentary Rocks
• In lakes or seas, the sedimentary rocks occur in layers in a
chronological sequence (i.e.. With the order of superposition
which means older rocks occur at the bottom and successively
younger rocks at the top. The layer differ from one another in
minerals composition or chemical composition or in a set of
such rocks, which are in sequence are called strata.
• The contact planes of adjacent layers or beds are called bedding
planes. Each bed indicates the sedimentation that had occurred
at a stretch and each bedding plane indicates a brief period of
non-deposition of sediments.

Sedimentary Rocks
Typical Characters
• The general characteristics features of sedimentary rocks are
• (i) lamination or bedding or stratification
• (ii) cross-bedding or current bedding or torrential bedding
• (iii) presence of cementing material
• (iv) Occurrence of fossils
• (v) Occurrence of tracks and trails
• (vi) Occurrence of mud cracks, rain prints, etc.
• (vii) porous, concretionary, nodular structures
• (viii) Olitic, pusolitic and stalactite forms
• (ix) ripple marks.

Methamorphic Rocks
• These are the third major group of rocks occurring in nature.
Their mode of origin differs from that of igneous or
sedimentary rocks. They are formed from any pre-existing
rocks under the influence of temperature, pressure and
chemically active solutions. The pre-existing rocks should
naturally be either igneous or sedimentary rocks only.
• Metamorphism literally means change. In the geological
sense, the term is used to denote the occurrence of textural,
mineralogical or chemical changes, etc. That have taken
place in pre-existing rocks under the influence of
temperature, pressure and chemically active solutions.
Metamorphism occurs in rocks because of the two major
groups of rocks, i.e.. Igneous and Sedimentary rocks, are
formed under certain physical and chemical condition.

Methamorphic Rocks
Typical Characters
• The metamorphic rocks generally have the following special
features
• (i) Some minerals like staurolite. Andalusite, sillimanite, which are
products of metamorphism are found only in metamorphic rocks
of course, these are not very common minerals.
• Some structures and textures like lineation, foliation, schistosity
and slaty cleavage are characteristic of most of the metamorphic
rocks.
• Usually, metamorphic rocks are composed of coursed grained
minerals. But sometimes, the metamorphic rocks retain the
texture and structure of parent igneous rocks or sedimentary rocks
Such relict features are called palimpset structure and texture.

Methamorphic Rocks
Limits of Metamorphism
• The normal concept of metamorphism is that “ changes
in three factor of temperature, pressure and chemical
environment”, disturb the physical and chemical
equilibrium of a mineral assemblage, and
metamorphism results from the efforts to establish a new
equilibrium. The processes of weathering and
cementation which take place at nearly normal surface
temperature and pressure are generally not considered
as true metamorphism. But according to some, these
processes also represent metamorphism because they
consider even alteration as equivalent to metamorphism
were the metamorphic rocks.

Methamorphic Rocks
• Thus, from the preceding discussion it is clear
that igneous, sedimentary and metamorphic
rocks were formed in sequence, in the
beginning. But it should be remembered that
subsequently all these three groups of rocks
have formed again and again as and when
suitable conditions prevail.

Methamorphic Rocks
Relative Abundance of Rock Types
• If the metamorphic rocks are grouped with their parent rocks,
the igneous group (i.e.. Igneous rocks and ortho-metamorphic
rocks) comprises 95 % of the earth’s crust quantitatively; the
remaining 5 % belonging to the sedimentary group. Among the
different kind of igneous rocks, granites and basalts are the
most abundant. Granites, which are typical plutonic rocks occur
on the earth’s surface as continental bodies, mountain cores,
etc. These are underlain by basaltic rocks, which are typical
volcanic rocks, as ocean floors, Silicon and aluminium rich
felsic minerals occur in granites and silicon and magnesium rich
mafic minerals occur in basalts. Basically two types of magmas
occur in the earth’s crust; granitic and basaltic.

Metamorphic Rocks
• Among the different groups of sedimentary rocks,
nearly 80 % are argillaceous (shale's), 15 % are
arenaceous (sandstones) and 5 % are calcareous
(limestones) other rocks are less common.
• As pointed out, the sedimentary group of rocks
account quantitatively only for 5 % of rock of
earth’s crust. But from the civil engineering point
of view, sedimentary rocks are more abundant
igneous rocks occupy a much lesser area, i.e. the
remaining 30 % of the area of the earth’s surface.

Metamorphic Rocks
• This is so because igneous rocks occur as huge
voluminous bodies continuing to great depths. But
sedimentary rocks occur as relatively thin veneers
or beds, spreading over large areas. This explain
how the quantitatively negligible sedimentary rocks
occur over very wide area. For this reason,
constructional works of civil engineering are more
likely to rest upon the sedimentary rocks.
Therefore, it is natural that details of sedimentary
rocks are more important in civil engineering.

Rock Cycle
• The three group of rock, i.e.. Igneous, Sedimentary and
metamorphic, which occurs in nature give place to one another as
explained below
• Igneous rocks, as a consequence of weathering, give rise to
sedimentary rocks, further under the influence of metamorphism
they also give rise to Ortho-metamorphic rocks.
• Sedimentary rocks, when buried to great depths, form magma on
melting and subsequently solidify to form igneous rocks and, under
the influence of metamorphism, change over to para-metamorphic
rocks.
• The metamorphic rocks, like the igneous rocks, on weathering, give
rise to sediments which form sedimentary rock later. The
metamorphic rocks, like the sedimentary rocks, when buried to great
depths due to tectonic activity will melt and give to magma, from
which igneous rocks are formed.
• Thus one group of rocks give rise to another in nature due to
geological reasons and this phenomenon is called “Rock Cycle”

Civil Engineering Importance of Petrology
• Petrology is very important from the civil engineering point
of view, because this provides a proper concept and logical
basis for interpreting physical properties of rocks. Thus, the
study of texture, structure, mineral composition, chemical
composition, etc. Gives all necessary details regarding the
strength, durability, colour, chemical composition, etc. These
inherent characters of rocks, occurring at concern for a civil
engineer to judicious assess the suitability or otherwise of
rock are of chief concern for a civil engineer to judiciously
assess the suitability or otherwise of rocks, occurring at his
project site for the required purpose. This is necessary
because different kind of rocks are suitable for different
specific purpose and no rock is ideal or best suited for all
kind of purposes of construction.

Civil Engineering Importance of
Petrology

• For ex, granite-like rock being hard, competent, durable and
free from weak planes are suitable for foundation purposes.
Stones like Cuddapah, slab and Shahabad type limestone are
suitable for flooring and roofing purpose. Marble by virtue of
its attractiveness color and softness is the most sought after
for face work, statue making, decorative and other sculptural
work. Sandstone being easily workable can be neatly dressed
and hence are suitable for construction of wall.
• Thus different kind of rocks suit different purposes by virtue
of their special physical properties which are inherent and
characteristic to them. These details must be known so as to
make the best use of available rocks at the construction site.

• Engineering and General Geology :By Parbin Singh
• Textbook of Engineering Geology :N.Chenna Kesavullu
References

Petrology

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