Geology

The Coordination Committee formed by GR No. Abhyas - 2116/(Pra.Kra.43/16) SD - 4
Dated 25.4.2016 has given approval to prescribe this textbook in its meeting held on
30.1.2020 and it has been decided to implement it from academic year 2020-21.

GEOLOGY

STANDARD TWELVE

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2020
Maharashtra State Bureau of Textbook Production and

Curriculum Research, Pune.

First Edition : © Maharashtra State Bureau of Textbook Production and Curriculum Research, Pune - 411 004.
2020 The Maharashtra State Bureau of Textbook Production and Curriculum
Research reserves all rights relating to the book. No part of this book
should be reproduced without the written permission of the Director,
Maharashtra State Bureau of Textbook Production and Curriculum
Research, ‘Balbharati’, Senapati Bapat Marg, Pune 411004.

Geology Subject Committee : Illustrations : Bhatu Bagale
Dr. N. J. Pawar (Chairperson), Pune Cover : Ganesh S. Dhavale
Dr. Satish Jagdeo Sangode, Pune Typesetting : DTP Section,
Mrs. Suneeti J. Malik, Pune Textbook Bureau, Pune
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Publisher
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Bureau, Prabhadevi,
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NATIONAL ANTHEM

Preface

Dear Students,

Heartfelt congratulations and welcome to standard XII !

Your study of the geology so far, has enriched you with knowledge about the
fascinating world of minerals, formed on and within our planet earth, the processes that
lead to their formation and to the creation and evolution of the magnificent landforms
from the lofty Himalayas to the deep ocean trenches. In your previous studies of the
subject, as a part of geography during your school years and as a separate subject of
geology in your XI standard, you have been introduced to the basic ideas about the
various geological processes.

This new edition of the book presents an extended view of the subject from an
earth and planetary sciences point of view. The various applied aspects of the subject
of geology have been presented to interesting and thought provoking writings on
hydrogeology, economic mineral deposits and geohazards. The science of data collection
and interpretation of satellite data has also been introduced. It gives me immense
satisfaction and pleasure to present this new edition of class XII text book.

Just like the XI standard text book, this book too is an accessible and comprehensive
guide to the important topics in geology, more so from an applied science point of view.
The topics are illustrated with examples and case studies from India in general and the
state of Maharashtra in particular. The book also includes numerous thought provoking
activities and questions.

Studying and understanding the dynamics of our planet, its resources, even the play
of internal forces and surface atmospheric phenomena which sometimes cause loss of
human lives and property, will surely lead to the formation of an aware and sensitive
citizen, who is equipped to work towards the betterment of the planet. You are encouraged
to use the QR code and links given within the text for achieving a better understanding
the various topics. This book is sure to cause many students to take up earth science as
a lifelong career.

Pune (Vivek Gosavi)
Date: 21 February 2020 Director
Bhartiya Saur : 2 Phalguna 1941
Maharashtra State Bureau of Textbook
Production and Curriculum Research, Pune

STD. XI Geology

Learning Outcomes

• The goal of this book is to introduce students with
fundamental knowledge of diverse applied fields in geology
like hydrogeology, palaeontology, stratigraphy, petrology,
geohazards and structural geology and remote sensing.

• In addition, the analytical exercises incorporated within the
text will imbibe the habit of scientific and analytical thinking
in the learner.

• Understand the origin and classification of the common rocks
found on the earth’s surface.

• Know the processes involved in the formation of various
surface landforms due to the interaction of the earths internal
forces with the crustal layers.

• Identify the common rocks and their underlying processes of
formation.

• Understand the science of palaeontology and fossil formation.

• Collect, illustrate and analyse basic geological information
from the field.

• Interpret geological maps and construct cross sections.

• Develop and aptitude for detailed understanding of rocks
and structures.

• Communicate observations and interpretation in a geological
report.

- For Teachers -

You have been teaching earth sciences in and therefore you are expected to utilize the
general and geology specifically, and as allotted number of periods fully. Do not
experienced mentors, we all know that the finish the chapter in a hurry.
subject of geology is interdisciplinary and
hence teaching geology is a challenging task  Major concepts of geology are complex and
that needs integration of both theoretical and need deep understanding. Hence, encourage
practical aspects. As the field of earth sciences group work putting in collective efforts.
has seen a continuous evolution in the accepted This will help the students to assimilate the
theories and due to the improved technologies, content without feeling the ‘burden of
leading to better understanding of natural learning’.
phenomena, the need to update and upgrade
the first edition of the book published in 2012  Facilitate peer learning as much as possible
was felt necessary. In this context, following by frequently reorganizing the class
guidelines should be strictly adhered to, to structure.
improve the overall teaching of the subject.
 Please do not teach the lessons in the book
 Please refer to Science and Geography by just reading them aloud.
textbooks from standard V to standard X
before using this textbook.  Follow the order of the chapters as given
in the content as the concepts have been
 It is expected that the teacher and student introduced in a sequence to facilitate
is well versed with the topics and subject knowledge building.
matter dealt with in the new edition of the
Std. XI textbook, published in 2019.  USE the highlighted boxed in texts titled
‘Do you know?’ for adding value and joy
 To begin with, get familiar with the to the process of understanding the subject.
textbook yourself and enhance the subject
by referencing and cross referencing the  The contents of ‘DO You Know?’ are NOT
concepts, adding case studies and updating to be used for evaluation.
new information.
 Use QR Code given in the textbook. Some
 Plan carefully and independently for the weblinks have been given at the end of the
teaching of the content and supervising book.
activities in each chapter.
 Teacher as well as students are expected to
 The present book has been prepared for use these references. These references will
constructive and activity-based teaching. surely help to explore knowledge beyond
the textbook.
 The teaching learning interactions, processes
and participation of all students is very  Please bear in mind that extra reading is
necessary and so is your active guidance. always helpful for in depth understanding
of the subject.
 Use geological aids for appropriate
understanding of the concepts.  Use thought-provoking, activity-oriented,
open-ended, multiple choice questions for
 Some chapters may be difficult to follow evaluation. Some examples are given at the
end of every chapter.

Contents

Sr. No. Name of the Chapter Page No.

1. The Dynamic Earth 1 - 12

2. Petrology 13 - 27

3. Palaeontology and Stratigraphy 28 - 43

4. Structural Geology 44 - 55

5. Economic Minerals and Rocks 56 - 72

6. Hydrogeology 73 - 82

7. Geohazards 83 - 97

8. Remote Sensing and GIS 98 - 115

* Practicals 116 - 127

* Glossary and Biblography 128

DISCLAIMER Note : All attempts have been made to contact copy right/s (©) but we have not heard from them. We will
be pleased to acknowledge the copy right holder (s) in our next edition if we learn from them.

Front Page : A mosaic depicting various chapters as well as the dynamic nature of our planet Earth.
Back Page : Calcite (nail head spar) on fine quartz crystals. Location Chandivli quarries, Mumbai, Maharashtra
Dimensions : 6.5cm × 2.5cm × 4.0cm,
Green Apophyllite - Location: Pashan quarries, Pune, Maharashtra, India.

Dimensions: 2.5cm × 2.0cm × 1.5cm

Acknowledgement : Back page photos by Mr. Arnav Samant

1   The Dynamic Earth

Introduction : layering (fig. 1.2). The heat loss occurs by
processes such as conduction, convection and
Away from the bustling noise of crowded advection. Whereas, the heat generated is mainly
cities or man-made activities; the Earth in the form of the primordial heat accrued during
appears like a stable, calm and peaceful the formation of the planet. Convection and
planet in tranquillity of nature. It is however advection being physical processes, the Earth
a highly dynamic planet similar to all other material in the form of magma is simultaneously
planets in the Solar system; and is orbiting undergoing differentiation and mixing to cause
around the Sun at ~ 30 km/s. It is also internally depletion of original magma that was available
dynamic, as evident from the phenomena of during the early part of planetary evolution.
plate tectonics operating through geological Convection plays a major role in driving the
ages. The Earth on its surface is undergoing plates, (fig. 1.2, 1.3) while advection gives rise
many dynamic processes as expressed by the to formation of ocean floor and other events like
activity of oceans, rivers, glaciers, winds and lava eruption on the surface.
atmospheric circulation. We can distinguish
the Earth’s dynamism into: a) the orbital or 0.4 MRE 0.5 0.6
planetary dynamics; b) the surface dynamics 0.3 MRE
or Earth surface processes; and c) the internal
dynamics as part of the evolution of the Earth. 0.2 MRE
This chapter describes the internal dynamics Moon
of the Earth. It is therefore, necessary to learn
about the internal structure/interior of the 0.1 MRE Radius
Earth in order to understand its expression
on the surface by the processes such as plate 4.56 Ga 0.7
tectonics. Formation
of core
The Interior of the Earth :
30 Ma journey of the Earth
The Earth evolved as a planet from Solar from a small planetesimal
nebula at about 4.56 Ga, and gradually shaped
into a spherical body (fig.1.1). With the ongoing 0.8
cooling, the outermost layer called the crust
was formed. The temperature of the universe is 1
of the order of -270 °C, and the Earth’s interior
is at >4000 °C. Presently the average surface 1 0.9
temperature of the Earth is 14°C - 15°C, and MRE
the temperature at the core of the Earth is 6000
°C. This results in temperature gradient with the Fig. 1.1 : Evolution of the Earth from a
Earth's interior. This temperature gradient of the planetismal state
Earth is however not linear and shows several
kinks that are influenced by the compositional Formation of the Earth from an asteroid
like body to a spherically layered Earth
system at ~4.53 Ga is one of the most dynamic
internal process the Earth has experienced in
its early stage of evolution. Note the step-wise
differentiation particularly at 0.3 MRE (mean
radius of the Earth), initiation of magma oceans
at ~0.5 MRE and segregation of the core at 0.7
MRE (fig 1.1). The overall size of the planet is
increased because of addition of mass as the
Earth cleared its own orbit.

1

Temp in degree C process facilitates layering of the Earth where
0 1000 2000 3000 4000 5000 6000 7000 8000 compositional and temperature stabilities are
achieved. These processes were more dynamic
1000 during the initial phase of evolution of the Earth.
Heavier elements like Fe and Ni sank towards
2000 Mantle the center of gravity of the Earth forming the
inner core.
Depth in Km3000 D prime layer
A) The Core:
Core Mantle boundary Core is divided into inner and outer core
as a result of P-T stability conditions of Fe-Ni
4000 Outer Core compounds wherein P increases the melting
5000 point of Fe-Ni compounds making the inner
core to remain solid. Lowering of the Pressure
Inner Core at about 5100 km results in melting of Fe-Ni
matter to produce the liquid outer core. The outer
6000 core being liquid, is therefore more dynamic and
the convection currents created are supposed to
  generate the Earth’s magnetic field.Information
about the core is obtained indirectly from
Fig. 1.2 : Internal layering of the Earth seismology.

Earth comprises mainly of silicate Do you know?
compounds (minerals) having definite crystal ThemagneticfieldgeneratedbytheEarth’s
structures and compositions that remain stable core, protects it's surface from the solar
at particular pressure (P) and temperature winds that are high energy electromagnetic
(T) conditions of the molten magma to radiations, of potentially harmfully charged
solid rocks. Molten magma is dynamic and particles. Without the magnetic field, the
experiences changing Pressure - Temperature Earth would be an inhospitable planet.
(P-T) conditions due to its upward/downward
movement (convection and advection). Change
in temperature with depth is influenced by
the convection current that governs physical
movement of the magma.

The Crust is divided into tectonic plates
interacting with each other

The mantle is zone of molten Solar Wind Magnetic Shield
silicates and other minerals.
The mantle heat drives Solar Auroral
CONVECTION CURRENT Flare Oval
which is the driving force
for the plates movement.
Mantle is 2900 km thick

Magnetic Field

The crust is of two Inner Core :
types i) Oceanic is The temperature at the centre of the Earth
denser, younger and reaches upto ~ 6,000°C and is largely considered
thinner, ii) Continental is to be the heat left over after the planet’s creation
older, thicker and less dense. (primordial heat), apart from other sources like
heat generated by radioactive decay. Inner core
Fig. 1.3 : Convection current governing the surface is ~1,200 km thick, and is about the same size
dynamics of the Earth

Convection also facilitates differentiation
of magma compounds, where heavier melt
(compounds) sink deeper into the Earth. This

2

as the Earth’s moon. Due to the tremendous silicon and oxygen with small amounts of iron,
pressure, material in the core does not melt and calcium, and aluminium to forms a typical
hence it remains solid. mineral known as perovskite (MgSiO3).
Upper mantle : The upper mantle consists
The Outer Core : of two different types of layers, namely
The outer core is ~2,200 km thick. lithosphere and the asthenosphere. The rigid
Density of the outer core is about 10 -12 gm/ lithosphere is composed of a rocky crust that is
cc. The temperature in the outer core ranges ~40 kms to 280 km thick and floats on top of
from 2,200°C to 4,900°C. The molten material the asthenosphere. Asthenosphere is ~180 km
comprises mostly of iron with little nickel. thick with temperature as high as ~1,450°C.
About 10% of it is made up of other elements, Asthenosphere is less rigid than the lithosphere
most likely oxygen and sulphur. and behaves plastically. Lithosphere mainly
Discontinuities are layered boundaries represents the rock composition known as
in the interior of the Earth discovered through peridotites, which contains the mineral olivine
seismic studies. and pyroxene. Peridotites are heavier than most
Boundary between inner core and outer of the crustal rocks and therefore they tend to
core is named as Lehmann discontinuity after sink into the upper mantle.
its discovery by the Danish seismologist Inge
Lehmann (fig. 1.4). The Crust :
The Crust is brittle and most fragile
Moho CRUST LITHOSPHERE outermost layer of the Earth and is the main
discontinuity ASTHENOSPHERE source of earthquakes. It represents mountains,
UPPER ocean floor and all the landforms and is part
MANTLE of lithosphere. Crust under the oceans is thin,
measuring 5 to 10 km and under land is between
Repetti MESOSPHERE 20 and 70 km. It is therefore, divided into two
Discontinuity types: oceanic crust and continental crust.
The oceanic crust found beneath the oceans
LOWER is composed of basalt, while the continental
MANTLE crust is largely made up of granites. However,
a large part of the continental crust is eroded
Guttenberg and transferred to the ocean basins in the form
Discontinuity of sediments which can later be recycled along
with the oceanic crust. Rocks as old as 3.8 Ga
OUTER BARYSPHERE can be found on the continental crusts; and are
CORE the source of information for the geodynamic
Lehmann history of the Earth. Continents are therefore a
Discontinuity INNER rich source of knowledge about the evolution of
CORE the Earth.
Earliest impressions of the dynamics of
Fig. 1.4 : Discontinuities in the interior of the Earth the Earth were philosophically routed and
several hypothesis were proposed. It was
B) The Mantle : during the early 17th century that the dynamics
Earth's Mantle has a thickness of about 2900 of the Earth were understood from geological
kms. It is the most important layer governing the
geodynamic processes of the Earth. Mantle is 3
divided into upper (~640 kms ) and lower mantle
(~2200 kms).

Lower mantle : It is more viscous than the
upper mantle and it convects slowly. It is known
to have temperatures as high as 2,200°C. Lower
mantle compositionally contains magnesium,

records. As discussed and hypothesized by ‘Gondwana’ (after the indigenous homeland
many philosophers and geologists during the of the Gond people of India). He defined it as
17th century, continents are floating over an ancient supercontinent ‘Gondwana Land’
the asthenosphere making the continental/ surrounded by the ancient ocean named
lithospheric plates. The most acceptable ‘Tethys’.
explanation of these observations has been
offered by the theory of ‘Continental Drift’. 1910 American physicist and glaciologist
Frank Bursley Taylor proposed the concept
The Continental Drift Theory: of ‘continental drift’ to explain the apparent
Internal dynamics of the Earth is expressed geological continuity of the American
by changes on its surface i.e., the crust. For over Appalachian mountain belt (extending
a long time, humans were unable to explain the from Alabama to Newfoundland) with
origin of various surface features of the Earth, the Caledonian Mountains of NW Europe
and it was during early 17th Century when the (Scotland and Scandinavia), occurring on two
geological idea of continental drift evolved opposite sides of the Atlantic Ocean.
as a hypothesis and a school of thought. It
faced many criticisms due to imagination and 1912 Alfred Wegener, the German
limitations to explain the driving forces. This meteorologist who spent his entire life to
was also due to the state of knowledge of science gather evidences re-proposed the theory of
and technology until the advent of geophysics continental drift. He compiled a considerable
(particularly seismology and magnetism) during amount of data, and suggested that during
the nineteenth century. However, the continental the late Permian, all the continents were
drift is the basic theory which evolved the concept once assembled into a supercontinent named
of plate tectonics, a revolution in geology. It is ‘Pangaea’, meaning all Earth. Pangaea
therefore necessary to learn both these aspects in began to break apart after the beginning
detail which are pivotal to understand many of of the Mesozoic Era, about 200 Ma ago,
different subjects within the scope of Geology. and continents then slowly drifted into their
current positions.
Do you know?
The theories of continental drift and plate 1937 South African geologist Alexander du
tectonics are evolved over four centuries as Toit supported to the theory by drawing maps
accounted in the time line below : illustrating a northern supercontinent called
Laurasia (i.e. the assembled land mass of
1620 Francis Bacon noted ‘conformable North America, Greenland, Europe and Asia)
instances’ along the opposite sides of the as explanation for distribution of coal-forming
mapped Atlantic coastlines. plants, and widely scattered coal deposits in
the Northern Hemisphere.
1858 Antonio Snider-Pellegrini suggested
that ‘the continents were linked during 1944 Wegener's theory was also consistently
Carboniferous Period’. He observed that plant promoted by an eminent British geologist and
fossils in coal-bearing strata of that age were geomorphologist Arthur Holmes in 1930s and
similar in both Europe and North America. 1940s, through his renowned book ‘Principles
of Physical Geology’.
1885 The famous Austrian geologist Edward
Seuss identified similarities between plant 1940–1960 The ocean floor topography
fossils from South America, India, Australia, was discovered through improvements in
Africa and Antarctica to suggest the name

4

geophysical surveying after World War II. Eurasia
Harry Hess (captain in the US Navy, later
professor at Princeton), proposed ‘Seafloor North America
Spreading’, a pivotal concept to continental
drift and plate tectonics South America Africa
India
1961 The American geologists Robert Dietz,
Bruce Heezen and Harry Hess proposed that Antarctica
the linear volcanic chains called mid-ocean Australia
ridges in the ocean basins are the sites of
ocean floor spreading. Fig. 1.5 : Reconstruction of the supercontinent Pangea

1963 Two British geologists, Fred Vine and He further gathered various evidences to
Drummond Matthews, finally proposed prove that the Continents were drifted to present
a hypothesis that convincingly explained positions. The evidences used in support of conti-
magnetic reversal stripes onto the ocean nental drift hypothesis are shown in (fig . 1.6): A) Fit
floor. They suggested that the new oceanic of the continents, B) Fossil similarities across
crust formed in the process of ocean floor continents, C) Matching of mountain ranges on
spreading acquired its magnetisation in different continents, D) Rock types, structural
the prevailing global magnetic field. By similarities and E) Paleoclimate evidences.
linking these observations to Hess's sea-
floor spreading model, they laid the most A
convincing foundation and proof for modern
plate tectonics. Africa

1965 Canadian Professor J. Tuzo Wilson South America
offered a fundamental reinterpretation
of Wegener's continental drift theory and B
became the first person to use the term ‘plates’
to describe the division and pattern of relative AFRICA INDIA Fossil evidence
movement between different regions of the of the Triassic
Earth's surface (i.e. plate tectonics). land reptile
Lystrosaurus
1960s - Present day. There was an increasingly
wide acceptance of the theory of plate SOUTH AMERICA AUSTRALIA
tectonics gaining a better understanding of the
boundaries and structure of the lithospheric ANTARCTICA
plates, with several modern tools of approach.
Fossil remains of Fossil remains of Fossils of the fern
Continental Drift Theory : Cynognathus, a the freshwater reptile Glossopteris. Found in
Alfred Wegener proposed this theory in Triassic land reptile Mesosaurus. all of the southem
1915 published in a book entitled ‘The Origin approximately 3m long. continents. Show that
of Continents and Oceans’. He proposed the they were once joined.
existence of Supercontinent called Pangaea that
began breaking apart about 200 million years 5
ago (fig. 1.5).

C Caledonian nents broke through the ocean crust, much like ice
breakers cut through ice was not convincing. The
Mountains renewed interest in continental drift was support-
ed by the ocean floor magnetic anomalies and the
North British Scandinavia apparent polar wandering. Records from ocean
America Isles Africa floor also explained the driving mechanism and
forces. This encompassed the foundation to an-
Appalachian other great theory called Plate tectonics.
Mountains
Plate Tectonics :
South During the 1950’s and 1960’s new
America technology permitted extensive mapping of
the ocean floor which mainly benefited the
D geoscience community to refine the continental
drift with scientific evidences. In 1963, Fred
North Europe Vine and D. Matthews linked the discovery of
America Africa magnetic stripes in the ocean crust near ridges
to Hess’s concept of seafloor spreading. They
South revealed that the ridges were spreading and
America creating new crusts (fig. 1.7); and if dated they
can find the rate of sea floor spreading. These
rates of seafloor spreading are accurate to the
drifting of adjoining continents.

FORMATION OF MAGNETIC ANOMALIES AT A MID-OCEAN RIDGE

4321 Present 1 2 3 4 Age before present
Normal Magnetic Polarity (millions of years)
Continental shelf

Matching ancient Calculated magnetic profile

rock assemblages assuming seafloor spreading

Reversed Magnetic Polarity

E

Mid Ocean Ridge Observed magnetic profile
form oceanographic survery

Africa

South
America

India

Lithosphere

Oceanlc crust Zone of magma injection, cooling, and http://usgs.gov
"locking in" of magnetic polarity

Antarctica Australia Enduring Resources for Earth Science Education - http://earthref.org/ERESE
http://earthref.org/cgi-bin/erda.cgi?n=212
Fig 1.6 : (A-E). Various evidences as foundation to the
theory of continental drift Fig. 1.7 : Symmetry of anomaly pattern with respect
to ridge depicting the centre of spreading in opposite
Major objection against the theory of conti-
nental drift was its inability to provide a mech- direction
anism capable of moving continents across the
globe. Wegener drift theory suggests that, conti- Plates are well defined by belts of seismicity
e.g. linear pattern of earthquake. Earthquakes
6 are generated as the plates interact. Thus all
the plates on the globe are interacting with each

other (fig. 1.8)

Fig. 1.8 : Major Lithospheric plates on the globe

Plate boundaries : of boundaries and new plate boundaries can be

All major interactions among individual created.

plates occur along their boundaries that are 1) Divergent plate boundaries :
classified into different types : (1) Divergent These boundaries are located along the
plate boundaries (also called constructive oceanic ridges where the ocean floor is spreading
margins); (2) Convergent plate boundaries and are called as constructive plate margins as
(destructive margins) and (3) Transform fault they form new plate material (fig. 1.9). Along
boundaries (conservative margins). Each plate well-developed divergent plate boundaries, the
is bounded by a combination of the three types seafloor is elevated forming the oceanic ridges.

Upwarping

A Continental crust Lithosphere

Rift valley

B

Linear Sea

C

Mid-oceanic ridge
Rift

D Continental crust Oceanic crust

Fig. 1.9 : Development of Midoceanic Ridge in the ocean

7

Continental rifts are a kind of divergent A Continental
plate boundaries where the landmass is split into
two or more smaller segments. The best example volcanic arc
for such a boundary can be seen in East African
rifts valleys (fig. 1.10) and the Rhine Valley in Trench
northern Europe.
Oceanic crust

SubductingOceanic Continental crust Continental
Ethosphere Melting Ethosphere
Asthenosphere
100 km

200 km

Volcanic island arc

B Trench

Oceanic crust

Continental crust
Oceanic Ethosphere Lithosphere

Asthenosphere Melting Oceanic 100 km
200 km
Subducting

C

Continental Suture Continental
lithosphere lithosphere

Oceanic plate 100 km
200 km
Asthenosphere

Fig. 1.11 : Types of convergent plate boundaries

Fig. 1.10 : East African Rift valley as an example of 3) Transform fault boundaries :
divergent continental margin Plates slide past one another without
creating/destroying any new lithosphere
2) Convergent plate boundaries : as transform faults (fig. 1.12). Most of the
Older portions of oceanic plates are transform faults join two segments of a mid-
returned to the mantle along the destructive ocean ridge as parts of prominent linear breaks
plate margins. The descending plate forms in the oceanic crust known as fracture zones.
an ocean trench called subduction zones (fig. Some interesting examples of Transform fault
1.11). The plate dips at an average angle of are the San Andreas fault (USA) (fig. 1.13) and
45° where it descends into the mantle. There the Alpine fault of New Zealand cutting through
are three types of convergent plate boundaries. continental crust.
A) Ocean – continent convergence
B) Ocean – ocean convergence Oceanic Ridge Crest
C) Continent – continent convergence
Transform Fault
8
Fracture
Zone

Fracture
Zone

Oceanic
Ridge Crest

Fig. 1.12 : Concept of transform fault

CANADA sedimentation. The younger mountains achieve
highest elevation as the rate of uplift exceeds
JUAN Souttle the rate of erosion e.g., Mount Everest (8848
DE FUCA WASHINGTON m). Different mechanisms of mountain building
process evolve into various types of mountains.
PLATE Portland There are four main types of mountains: i)
Mandoclno OREGON fold mountains, ii) fault-block mountains, iii)
transform volcanic mountains, and iv) relict mountain.
Fold Mountains are formed when two plates
Cape Mandocino CALIFORNIA NEVADA collide or a compressive force is developed within
a plate. Many of the world's great mountain
Point Arena U. S. A. ranges are fold mountains including the Andes,
Himalayas, and the Rockies.
Tomales Bay Fault-block Mountains are formed along
San Francisco fault lines and either side of the fault are called
Santa Cruz fault blocks. Some of the fault blocks are pushed
San Juan Bautista up, while others are pushed down resulting in the
difference in elevation. Satpura and Vindhyans
San Andress are considered as fault-block mountains.
Volcanic Mountains are caused by volcanic
Los Angeles activity when magma erupts all the way to the
surface of the Earth. The hot magma will cool
Fault ARIZONA and harden forming a mountain. Examples of the
volcanic mountains include Mount Fuji in Japan
PACIFIC OCEAN MEXICO and Barren island in the Bay of Bengal, the only
active volcano in India.
Gulf of California Relict type of mountains are formed by
differential erosion.
0 200 400 Km.

0 200 400 Mi.

Fig. 1.13 : San Andreas Transform Faults

Types of Mountains :
Mountains express the dynamics of the
interior of the Earth and hence are very significant
in understanding plate tectonics. They are positive
relief structures or landforms rising above the
surrounding land. Mountain building involves a
combination of movements and processes such
as upliftment, folding, deformation, faulting,
metamorphism, igneous activity, erosion and

Do you know?
Mountain ranges of India : India is a unique country representing almost all kinds of mountain
types of the world. Below is the summary of information on some of the important mountain
ranges of India. Students may search a map of India from reliable web resources to locate the
mountains listed in the table as an activity.

Mountains Salient Features
Karakoram Range A sub range of the Hindu Kush Himalaya, K2, the second highest peak in the world
is located here
Ladakh Range Famous Glaciers : Siachen Glacier, Biafo Glacier.
Zanskar Range Southeastern extension of the Karakoram Range. from the mouth of the Shyok
River in Ladakh to the border with Tibet
Boundary line between Ladakh region of Kashmir & remaining two regions of the
state i.e. Jammu and Kashmir
Highest peak : Kamet. Coldest place in India: Dras (The Gateway to Ladakh)
Famous Passes : Shipki, Lipu Lekh (Lipulieke), and Mana Pass

9

Pirpanjal Range Separates Jammu Hills to the south from the Kashmir (Kashmir Valley), beyond

which lie the Great Himalayas

Highest Point : Indrasan. India’s longest rail tunnel known as Pir Panjal Railway

Tunnel, Banihal road tunnel

Famous Passes : Pir Panjal Pass, Banihal Pass, Rohtang pass

Dhauladhar Range Spread in J & K and Himachal, with home to major hill stations like Kullu, Manali

(White Range) & Shimla

Highest peak : Hanuman ji Ka Tiba, or ‘White Mountain’

Shivalik Range Southernmost & Outer Himalayas also known as Manak Parbat in ancient times.

About 2,400 km long from Indus till Brahmaputra, with a gap of about

Aravali Range Means ‘line of peaks’, runs across Gujarat, Rajasthan, Haryana & Delhi, and

Mewar hills

Highest Peak : Gurushikhar, Mt Abu. Famous passes : Pipli Ghat, Haldi Ghat.

Kaimur Range Eastern portion of the Vindhya Range in MP, UP & Bihar, Parallel to river Son

Mahadeo Range Forms the central part of the Satpura Range, located in MP. Highest peak :

Dhoopgarh

Ajanta Range Maharashtra, south of river Tapi, sheltering caves of world famous paintings of

Gupta period

Rajmahal Hills In Jharkhand, made up of basaltic rocks, Point of Ganges bifurcation,

Garo Khasi Jaintia Continuous mountain range in Meghalaya a group of hills located to the south of

Hills, Mikir Hills the Kaziranga National Park (Assam), a part of the Karbi Anglong Plateau

Abor Hills Hills of Arunachal Pradesh , near the border with China, bordered by Mishmi and

Miri Hills

Mishmi Hills In Arunachal Pradesh with its northern & eastern parts touching China. Situated

at the junction of North Eastern Himalaya and Indo-Burma ranges

Patkai Range Also known as Purvanchal Range, consist of three major hills. The Patkai-Bum,

the Garo-Khasi-Jaintia and Lushai Hills

situated on India’s north-eastern border with Burma

Vindhya Range A complex, discontinuous chain of mountain ridges, hill ranges, highlands &

plateaus running through Madhya Pradesh, Gujarat, Uttar Pradesh and Bihar

Highest peak – Sadbhawna Shikhar.

Satpura Range A range of hills in central India. Passes through Madhya Pradesh, Gujrat,

Maharashtra, Chhattisgarh

Highest peak : Dhupgarh

Dalma Hills Located in Jamshedpur, famous for Dalma national park & minerals like iron ore

& manganese

Girnar Hills Gujarat

Harishchandra At Pune, acts as a water divide between Godavari & Krishna, hills made up of

Deccan basaltic lava

Balaghat range Between MP & Maharashtra, famous for manganese deposits.

Nilgiri Hills Referred as Blue mountains, a range of mountains in the westernmost part of

Tamil Nadu at the junction of Karnataka and Kerala

Hills are separated from the Karnataka plateau to the north by the Moyar River

and from the Anaimalai Hills & Palni Hills to the south by the Palghat Gap

10

Palani Hills Eastward extension of the Western Ghats ranges,adjoin the high Anamalai range
Anamalai Hills on the west, and extend east into the plains of Tamil Nadu
Cardmom Hills Also known as Elephant Hill. A range of mountains in the Western Ghats in Tamil
Pachamalai Hills Nadu and Kerala with highest peak Anamudi
Part of the southern, Western Ghats located in southeast Kerala and southwest
Tamil Nadu
Also known as the Pachais, Eastern Ghats in Tamil Nadu

Summary : activities such as plate tectonics.

• Surface of the Earth is an expression of the • Plate tectonics is one of the most important
dynamic processes that are occurring in the phenomenon which explains several important
interior of the Earth. processes occurring on the surface.

• The Earth's interior is divided into several • Mountain ranges are the locations where the
compositional and thermal layers interacting signatures of the Earth's internal dynamics
with each other by conduction, convection since geological past are evident and can be
or advection resulting into many dynamic studied readily by geologists.

EXERCISE

Q. 1. Choose the correct alternative : b) Volcanic mountains
c) Domal mountains
1) The difference between the temperature of d) Fold mountains
Universe and the interior of the Earth is of the
order of Q. 2. Very short answer :

a) 2000° C   b) 10000° C 1) How the convection currents maintain the
c) 600° C   d) 6000° C temperature of the mantle?

2) The lower boundary of mantle convection is set 2) What controls the temperature gradient of the
at about …… Km from the surface of the Earth. Earth?

a) 5000 km   b) 2000 km 3) What is the main driving force for plate motion?
c) 2900 km   d) 300 km
4) Why the radius of the Earth increased during its
3) Although the inner core and outer core are of very early stage of formation?
approximately the same composition (Fe-Ni),
the inner core remains solid due to Q. 3. Short answer :

a) Increase in melting point of the compound 1) Write down any two major points describing
due to increase in Pressure. the difference between Continental crust and
Oceanic crust.
b) Decrease in melting point of the compound
due to increase in Pressure 2) The Himalaya and Sahayadri are two different
types of mountains, justify this sentence.
c) Increase in melting point of the compound
due to decrease in Pressure 3) Mention any two most important properties of
asthenosphere.
d) The source of heat is in the outer core.
4) Give any two examples of the continental
4) When two plates collide, they are likely to landmasses that broke up as a result of break-
form up of the supercontinent PANGEA.

a) Fault-block mountains

11

Q. 4. Short answer : so that next we can ask what material will match the
density at any specified depth”.
1) Give any three evidences that indicate the
continents have drifted. Quoted from : The Inaccessible Earth: An integrated
view to its structure and composition By Brown and
2) Describe how magnetic strips on the ocean Musset p.2
floor explain the Plate tectonics.
1) What does the authors wants to indicate
3) Give an example of the divergent continental by saying “reasonable precision when
margin. determination of the density”.

4) Describe how relict type of mountains are 2) How is the density of such a large body like the
formed. Earth's is determined?

Q. 5. Long answer : 3) How the composition of the material at great
depths in the interior of the Earth is determined?
1) Describe what tectonic activity is undergoing at
the western margin of South American plate. 4) What is the interrelation of density, mass,
temperature and pressure?
2) Describe what are the limitations of Continental
drift theory and how the theory of Plate tectonics Q. 7. Unseen para :
resolved them.
“A theory of mantle convection is a dynamical
3) Describe different types of plate boundaries theory of geology, in that it describes the forces that
and exemplify then with the Indian-Australian give rise to the motions apparent in the deformation
plate. of the Earth's crust and in earthquakes and to the
magmatism and metamorphism that has repeatedly
4) Write a short note on the San Andreas Transform affected the crust. Such a dynamical theory is a
fault of North America. more fundamental one than plate tectonics, which
is a kinematic theory it describes the motions of
5) Give an account on the internal layering of the plates but not the forces that move them. Also plate
Earth. tectonics does not encompass mantle plumes, which
comprise a distinct mode of mantle convection”.
Q. 6. Assessing Unseen para :
Quoted from : Dynamic Earth: Plates, Plumes and
“To find out whether these changes are Mantle Convection - GEOFFREY F. DAVIES p.4
due to changes of composition, temperature or
other parameters, we turn to density, which is 1) In what way the author emphasizes that the
the most useful quantity that can be determined Earth's crust is linked to the mantle?
with reasonable precision. Density is deduced
by combining information from seismology with 2) Distinguish between some of the kinematic and
knowledge of the mass of the Earth, deduced from dynamic aspects of the Earth.
its gravitational attraction, and its moment of inertia,
determined from the movement, or precession, of its 3) How the author emphasize mantle plume to
axis of rotation. The resulting density variation with explain both dynamic and kinematic changes
depth inside the Earth is not uniquely determined, but of the Earth.
it is known within fairly close limits at most depths,


12

2  Petrology

Introduction : Minerals of the igneous rock are broadly
Petrology is the branch of geology, classified into two groups viz. i) Primary
which deals with the study of rocks. The and ii) Secondary. Primary minerals are the
term petrology is derived from the Greek minerals which have developed as a result
word ‘petros’ meaning rocks and ‘logos’ of crystallization from magma or lava (i.e.,
meaning study. Petrology, therefore, pyrogenetic minerals), while secondary
includes the mineralogical, textural and minerals are those which are formed after the
chemical description of rocks along with solidification of the rock either by alteration of
their geological origin, mode of occurrence the primary minerals or by precipitation from
and their relation to the physico-chemical solution, in voids or vesicles of rocks.
environment of the Earth. Petrological
studies are carried out under two major Primary minerals can be grouped into two,
heads as follows, viz, viz., 1) essential and 2) accessory.

a) Petrography : Description and systematic Essential minerals are necessary for the
classification of rocks classification, identification or nomenclature
of the rock. Accessory minerals are present in
b) Petrogenesis : Studies of the origin / small amounts and are therefore, not important
genesis of the rocks in the nomenclature e.g., in granite, quartz and
feldspar are essential minerals, while biotite and
Rock is an aggregate of minerals. Most / or hornblende are accessory minerals. Presence
rocks are composed of two or more minerals or absence of accessory minerals does not make
(e.g. granite). However, a few are composed any difference in nomenclature of rocks.
of a single mineral (e.g., dunite). Some rocks
may also contain fossil remains of plants Secondary minerals are those which are
and animals (fossiliferous limestone) or they formed from secondary solutions in cavities of
may be formed by recrystallization of pre- igneous rocks, most commonly found in volcanic
existing rocks (e.g. marble). igneous rocks, e.g., calcite, zeolites etc.

IGNEOUS PETROLOGY The minerals may be felsic or mafic. The
The first formed rocks on the Earth are felsic minerals are light in colour, with low
igneous/ primary rocks. The term igneous, is specific gravity, e.g., minerals from silica
derived from the Latin word ‘igneus’ meaning and feldspar groups. Mafic minerals are dark
fire. They are formed through cooling and coloured, with higher specific gravity, e.g.,
solidification of magma or lava. These rocks minerals belonging to pyroxene, amphibole,
may form with or without crystallization either mica, olivine groups and iron ores.
below the surface, as intrusive (plutonic) rocks
or on the surface as extrusive (volcanic) rocks. Rocks are described as a) phaneric or
Magma, may be derived from melting of pre- phanero-crystalline, if their minerals can be
existing rocks either in the mantle or in crust. distinguished separately with the naked eye, e.g.
One or more processes such as increase in a coarse grained rock like granite. b) rocks, in
temperature, decrease in pressure, or change in which the mineral grains cannot be distinguished
composition cause melting. by the naked eye, are described as aphanitic,
e.g. a fine grained rock like basalt or rhyolite.
(fig. 2.1)

13

Classification of Igneous rocks : coloured and are called melanocratic, such
1) Based on mode of occurrence : Igneous as, basic rocks which are rich in augite,
hornblende and olivine (e.g., basalt).
rocks formed at great depths inside the d) Ultrabasic rocks are very dark in colour and
Earth are called plutonic rocks. The name hence, are classified as hyper-melanocratic
is derived from Greek God of underworld (e.g. peridotite).
– Pluto. They have a slow rate of cooling
and are characterized by a coarse grained, a) Phaneric
equigranular texture. Granite is a typical
plutonic rock. b) Aphanitic
Rocks, which solidify on the Earth’s Fig 2.1 : Degree of crystallinity
surface from lava, are described as volcanic
rocks. The lavas may have erupted either Textures of Igneous rocks :
from a volcano or through fissures. A very Texture describes the mutual relationship
rapid rate of cooling of the lava makes of the minerals and /or glass, contained in a
these rocks fine grained or even glassy. rock. It takes into account relative amounts of
They show characteristic features such as crystalline and glassy matter, as well as the size,
vesicles, amygdales and flow structure. shape and the arrangement of minerals.
Volcanic rocks may show inequigranular a) Equigranular texture : If rock consists of
textures under the microscope, e.g., basalt.
The hypabyssal rocks consolidate at a mineral grains of almost equal size, it is
depth in between volcanic and plutonic said to possess an equigranular texture. As
depths. They show characters which are this texture is characteristically exhibited
intermediate between plutonic and the by granite it is also known as Granitic
volcanic rocks, e.g., dolerite. texture (fig. 2.2).
b) Inequigranular texture : If the rock
2) Based on SiO2 percentage : The percentage possesses mineral grains and other
of SiO2 in a rock decides the nomenclature constituents like glass having different
of the igneous rock as : sizes, it is said to exhibit inequigranular
texture e.g. porphy-ritic texture, where large
a) Acidic with SiO2 greater than 65% e.g. crystals called phenocrysts are embedded in a
granite; matrix of finer grains or glassy groundmass.

b) Intermediate with SiO2 - 55 - 65%; e.g.
syenite

c) Basic with SiO2 - 45 - 55% e.g. basalt and
d) Ultrabasic with less than 45% SiO2 e.g.

peridotite

3) Based on colour index :

a) Rocks rich in felsic minerals are often light
coloured and are termed as leucocratic
i.e., acidic rocks which contain essentially
quartz and alkali feldspar (e.g., granite).

b) Intermediate igneous rocks have a colour
index between acidic and basic rocks and
are called mesocratic (e.g., syenite).

c) Rocks rich in mafic minerals are dark

14

Porphyritic texture is typically shown by Structures of Igneous Rocks :
some volcanic rocks like basalt (fig. 2.3).
They are large and easily recognizable
Fig. 2.2 : Equigranular features to the naked eye. Structures provide
information about genesis of the rock.
Fig. 2.3 : Inequigranular (Porphyritic) Some of the common igneous structures are
as follows :
Tabular Classification of Igneous rocks :
i) Vesicular structure : Magma erupts on
Acidic Interme- Basic Ultra- the surface in the form of lava . When lava
diate basic is rich in gaseous content i.e., volatiles it
Silica> Silica erupts and gases escape into the atmosphere
65% <45% leaving behind the cavities of various
shapes and sizes. These cavities are called
vesicles and the resulting structure is called
vesicular structure e.g. vesicular basalt.
Vesicles can also be pipe shaped.

ii) Amygdaloidal structure : Vesicles can
subsequently get filled by secondary
minerals such as calcite, zeolites and
varieties of silica. Such filled vesicles are
called amygdales. The rock is then said
to exhibit amygdaloidal structure e.g.
Amygdaloidal basalt (fig. 2.4)

Depth of Silica Silica 45
occurrence 55 - -55 %
65 %

Plutonic Granite Syenite Gabbro Dunite

Hypa- Pegma- Dolerite Texture becomes finer

byssal tite

Volcanic Rhyolite Trachyte Basalt Fig. 2.4 : Amygdaloidal basalt

Mineral Ess- Ess- Ess- iii) Ropy structure : Lavas of basic
content Quartz, Feld- Feldspar, composition are mobile, due to low viscosity
Feld- spar. Augite and can flow greater distances. During this
Acc- Acc-Ol- Olivine process, their upper surface gets wrinkled
spar. Quartz resembling ropes. This structure is termed
Acc-Bi- ivine as ropy structure.
Quartz
otite, absent iv) Columnar structure : During rapid cooling
Horn- of basic lava, number of cooling centres are
blende developed. Lava tries to aggregate around
these centres. It is the result of contraction
Colour Leuco- Meso- Melano- Hyper of lava during cooling. Due to this, tensile
Index cratic cratic cratic melano- stress is developed. At right angles to the
cratic
15
[Ess - essential, Acc - Accessory]

     Quartz content reduces as Silica % decreases

stress direction, vertical joints or cracks are cover an area from a few meters to many square
formed. Such joints results in the formation kilometers. The Deccan plateau of western India
of hexagonal or polygonal columns, giving is the example of lava flows.
rise to columnar structure e.g. Columnar
basalt. B) Intrusive bodies :
The forms of Intrusive bodies are dependent
v) Pillow structure : This structure is most upon viscosity of magma and the structure of
commonly observed when hot lava erupts the rocks they intrude. Igneous intrusive bodies
under water. In this structure, the volcanic are classified as concordant and discordant
igneous body appears as pile of numerous depending on their structural relation with the
overlapping pillows or sacks stacked one host rock. Concordant bodies are more or less
above the other. parallel to the structure of the host rocks and
discordant bodies cut across the host rocks.
As the lava flows, its upper surface gets
solidified immediately due to contact with 1) Concordant igneous bodies :
water, hence crust of a pillow shows glassy a) Sill : Sill means shelf or slab of stone. It
texture.
is a tabular sheet of igneous rock intruded
Types of Igneous bodies : between and parallel to the existing strata.
The magma rising from mantle may or They are thin tabular sheets of magma
may not reach the surface of the Earth forming which have been intruded along the
extrusive or intrusive igneous bodies respectively bedding planes or foliations. They show
(fig. 2.5) nearly parallel upper and lower margins
and pinch out with distance. Their thickness
A) Extrusive bodies : may vary from a few meters to hundreds of
When lava travelling through fissures and meters and may extend laterally for few
volcanoes cools rapidly at the Earth's surface it kilometres. On the basis of their origin, sills
forms Extrusive igneous bodies. are differentiated into simple, multiple and
composite types. Simple sill is formed by
Lava flows : a single intrusion. Multiple sills are formed
The only extrusive form of igneous bodies by two or more intrusions. Composite sills
is lava flows. Their thicknesses may range consist of more than one rock type, formed
from few centimetres to hundreds of meters and

volcano

parasitic
cone

sill dyke laccolith
phacoliths formed in sill
saucer-shaped lopolith
anticlines and synclines

batholith

Fig. 2.5 : Concordant and discordant Igneous bodies

16

by more than one intrusive episode. several kilometres.
Dykes are frequently more resistant to
b) Laccolith : The term laccolith is derived
from the Greek word ‘lakkos’ meaning a erosion than the enclosing rocks and tend
cistern and ‘litho’ meaning stone. It is a to project as ‘walls’ above the surface.
bun shaped structure having a flat base and However, dykes can also be excavated
domed top and does not spread very far, out to form a ‘trench’ due to weathering
tends to heaps itself up around the orifice and erosion. Many a times dykes bake and
of the eruption. The strata above it are harden the adjacent country rock on either
generally lifted up in the form of an inverted side. They may occur as isolated bodies
bowl. Laccolith may be a few Kilometers or as ‘swarms’. Dykes may be vertical,
in diameter and about thousands of meters inclined, ring shaped, radiating or arcuate
thick. in nature. Outwardly dipping dykes from a
common centre are called as ‘cone sheet’.
c) Lopolith : Lenticular, centrally sunken
saucer or basin-like concordant igneous b) Batholith : The term Batholith is derived
bodies are called as lopoliths. In Greek from Greek word ‘bathos’ meaning
‘lopos’ mean basin. Generally, their depth. Batholiths are large scale igneous
thickness varies by 1/10th to 1/20th of their intrusions. They have extremely large
width, and diameters range from tens to dimensions, with steeply dipping walls.
hundreds of kilometers. The characteristic They extend on the surface up to thousands
shape is the result of sagging caused by the of kilometres with irregular outline. They
weight of intruding magma. are generally granitic in composition.
Stock and boss are offshoots of batholiths
d) Phacolith : The Greek word ‘phaco’ where stock is irregular in shape and boss is
meaning lens. These are crescent shaped more or less circular and both are less than
structures found in highly folded regions. 100 sq. kms in aerial extent. A roof pendant
They are formed at the crests and troughs is a pendant shaped body of country rock
of folds which are regions of weakness and hanging from the roof of the batholith.
tension. They exhibit doubly convex lens- Xenoliths are fragments of country rocks
like form. found within the batholiths.

2) Discordant igneous bodies : SEDIMENTARY PETROLOGY
Sedimentary rocks are one of the three main
Discordant igneous bodies show cross rock groups, along with igneous and metamorphic
cutting relationship with the structures of the rocks. They are formed as a result of :
host rocks. They are described as follows : 1) Deposition of the weathered remains of

a) Dyke : The word Dyke is derived from other rocks.
the Scottish term ‘dike’ meaning a wall of 2) Biogenic activity and.
stone. Dyke is a vertical or near vertical 3) Precipitation from solution.
wall-like body of igneous rock intruded
into the older rocks. They cut across the Formation of Sedimentary Rocks :
foliation or bedding of the country rocks. It Formation of sedimentary rocks is a slow
is a narrow, elongated, parallel sided wall process that may require millions of years.
of igneous rock. Its thickness varies from a Sedimentary rock formation begins when the
few centimetres to many meters. Similarly pre-existing rocks are exposed to weathering
its length can vary from a few meters to processes.

17

The formation of sedimentary rocks of pore spaces and water within them. The
involves five different processes as nshown in dissolved minerals get precipitated in these
fig. 2.6 : pore spaces to act as cement binding the
minerals.
1) Weathering : The first step is transforming
solid rock into smaller fragments or Classification of sedimentary rocks :
dissolved ions by physical and chemical By considering the manner in which the
weathering. detritus (particles of sediment) is distributed,
transported and deposited, a simple, tabular
2) Erosion : Erosion begins with the classification for sedimentary rocks, based on the
transportation of the weathered products products of weathering is depicted in table 2.1.
from their original location. This can take
place by gravity, running water, wind, or Products of Weathering
moving ice.
Residual Sedimentary deposits
3) Transportation : Sediment can be
transported by sliding down slopes, being (non-transported) (transported deposits)
picked up by the wind, or carried by running
water. Longer transportation of sediments e.g. Laterite,
results in to finer sediments.
Bauxite, Soils (based on type of transport)
4) Deposition : Sediments are deposited when
the energy of the transporting medium drops Transported in suspension Transported in solution
resulting in their settling. The final sediment
thus reflects the energy of the transporting (based on average grain size)
medium.
Rudaceous Arenaceous Argillaceous
5) Lithification and cementation :
Lithification is the process that converts (256 to 2 mm) (2 to 0.06 mm) (< 0.06 mm)
sediments into sedimentary rock. The
processes of compaction and cementation (boulders, pebbles (sand) (clay, dust and mud)
are involved in this change. Increased load
of overlying sediments results in reduction cobbles and gravels)

e.g. Conglomerate Sandstone Shale, Mudstone

Breccia (based on type of deposition)

Chemical deposits Organic deposits

(precipitation, (activity of animals
evaporation etc.) and plants)

e.g. Chemical limestone Coral / Shell limestone

Table 2.1: Products of Weathering

Erosion Deposition
Particles carried away Particles deposited
from their source by as loosely packed
water or wind sediment

Compaction
Particles squeezed
together under
great pressure

Key Compaction
Particles glued
  Increasing together as mineral
  pressure solutions harden

Fig. 2.6 Processes involved in formation of sedimentary rocks

18

1) Non-transported (Residual deposits) by rolling or creeping. e.g. Conglomerates-
2) Transported deposits in which constituent grains are rounded,
(fig. 2.8 a) and breccia in which constituent
1) Non-transported (Residual deposits) : grains are angular (fig. 2.8 b).
These are also known as sedentary
deposits formed due to accumulation a) Conglomerate
and consolidation of the materials which
were left behind as residue. These are the
insoluble products of rock weathering. e.g.
Laterite and bauxite (fig. 2.7. a and b).

a) Laterite

b) Bauxite b) Breccia
Fig. 2.7 : a and b : Examples of residual deposits Fig. 2.8 : a and b : Examples of Rudaceous rocks

2) Transported deposits : They are formed (ii) Arenaceous rocks (Arenites) : These
from the materials that have been rocks consist of sand-sized grains. They are
transported mechanically by saltation, transported by either saltation or suspension.
traction and suspension or chemically in Grain size ranges between 2 mm and
solution. Besides, some organic processes 1/16 mm. e.g. siliceous sandstone and (b)
also play an active role in the formation of ferruginous sandstone (fig. 2.9. a and b)
transported deposits. These are classified
into two groups : a) Siliceous sandstone

A) Clastic rocks   B) Non-clastic rocks b) Ferruginous sandstone
A) Clastic rocks : These detrital rock Fig. 2.9 : a and b: Examples of arenaceous rocks

fragments are carried and deposited by 19
mechanical means and later cemented. On
the basis of grain size, the clastic rocks are
further classified as :
(i) Rudaceous rocks (Rudites) : Very coarse
grained rocks with grain size more than 2
mm in diameter. Boulders, cobbles, pebbles
and gravel are transported by traction i.e.

Do you know? (b) Shale : It is made up of clay particles,
Wentworth’s scheme of classification usually transported in suspension e.g.
of grain size. Claystone, Mudstone, Shale etc. Their
grain size is finer than Silt rocks i.e., less
Size Name Group Consolidated than 1/16 mm. Shales normally exhibit
range of Rudaceous Rocks laminations (fig. 2.11).
(mm)
>256 particle Conglomerate, Fig. 2.11 : Shale
Boulder Breccia
>64 B) Non-clastic rocks : These are the rocks
Cobble formed due to chemical precipitation as
>4 well as due to the activity of biological
Pebble agents. They are of two types :

>2 Gravel i) Chemical deposits  ii) Organic deposits
i) Chemical deposits : They may be formed
>1 Very Sandstone,
coarse Arkose, Grit a) Due to evaporation of saturated solutions,
sand giving rise to deposits of Salt and Gypsum
(fig. 2.12)
>1/2 Coarse
sand

>1/4 Medium Siltstone
sand Claystone

>1/8 Fine
sand Arenaceous

>1/16 Very
fine
sand

>1/32 Coarse
silt

>1/64 Medium
silt

>1/128 Fine silt Argillaceous

>1/256 Very
fine silt

<1/256 Clay

(iii) Argillaceous rocks (Argillite) : a) Salt
(a) Siltstone : Constituent particles in siltstone

are finer than sand and coarser than clay.
These grains are transported by suspension.
(fig. 2.10).

Fig. 2.10 : Siltstone b) Gypsum
Fig. 2.12. a and b : Examples of chemical deposits
20

b) As a result of reaction between the e) Carbonaceous : Coal is a product of
components carried in solution e.g. siliceous deposition and burial of plant matter
(chert fig. 2.13 and flint), ferruginous (fig. 2.15).
and carbonate (limestone fig. 2.13 and
dolomite) deposits.

Fig. 2.14 : Fossiliferous Limestones

Fig. 2.13 : a) Chert

Fig. 2.13 : b) Limestone Fig. 2.15 : Coal

ii) Organic deposits : These are products of Texture and structures of sedimentary rocks :
accumulation of organic matter preserved 1) Clastic texture :
under suitable conditions. The deposition Loose detrital fragments derived from
may be bio- chemical or bio-mechanical. weathering of pre-existing rocks undergo erosion
Organic deposits are of five types : and transportation. They are finally deposited
and are bound together by fine grained particles
a) Siliceous : Radiolarian ooze; diatomites. known as the matrix. This arrangement of grains
and the matrix after compaction gives rise to
b) Calcareous : These deposits are formed clastic texture (fig. 2.16).
as a result of biomechanical as well as
biochemical processes eg. Fossiliferous Fig 2.16 : Conglomerate showing clastic texture
Limestone (fig. 2.14), Chalk, Marl etc.
2) Sedimentary Structures :
c) Phosphatic : Calcium Phosphate is utilized The process of deposition usually imparts
by certain organisms, especially fish and variations in layering, bed forms or other
brachiopod which are consumed by birds structures that give clues to the depositional
and bats. The bird and bats droppings environment.
accumulate in heaps and later get
transformed into ‘Guano’. 21

d) Ferruginous : These deposits are formed
by biochemical oxidation of Fe carried
in solution in bogs and deposited as bog-
iron ore.

I) Stratification and bedding : bedded strata are useful indicators of
a) Stratification : It is layering formed during current direction and tops and bottoms of
the beds (fig. 2.19).
deposition. Stratification results by changes
in depositional conditions with time. (fig.
2.17)

Fig. 2.17 : Bedding in sedimentary rocks Fig. 2.19 : Cross bedding

b) Bedding : The beds in sedimentary rocks iii) Graded Bedding : As current velocity
are evident because of differences in decreases, the larger or heavier particles are
mineralogy, clast size and degree of sorting deposited first, followed by finer particles
or colour of the different layers (fig. 2.17). (fig. 2.20 a and b). This results in bedding
showing a decrease in grain size from the
i) Planer bedding : It is the simplest bottom of the bed to the top of the bed. This
sedimentary structure formed generally helps in determining top and bottom of beds.
in all sedimentary environments and also
under a variety of depositional conditions Top Fine
(fig. 2.18).

Graded Medium
Ded

Coarse

Bottom
Fig. 2.20 : a) - Graded bedding

Fig. 2.18 : Planar bedding Fig. 2.20 : b) Graded bedding in sedimentary rocks

ii) Cross Bedding : It consists of sets of beds
that are inclined relative to each other.
The beds are inclined in the direction of
movement of the wind or water. Boundaries
between sets of cross beds usually represent
an erosional surface. It is very common
in beach deposits, sand dunes and river
deposits. Individual beds within cross-

22

IV) Ripple marks : It is the wavy pattern seen dehydration and melting of the earlier minerals.
on mud or sand deposits. Ripple marks are
produced by wave action when the sand Types of metamorphism :
is moved up and down in an oscillatory The predominant agent of metamorphism
motion by wind or water. Ripples can be defines the kind of metamorphism. Thus, the
symmetrical or asymmetrical (fig. 2.21). major kinds of metamorphism are :
(a) Cataclastic, (b) Thermal or contact and
Fig. 2.21 : Ripple marks in Sedimentary rocks (c) Dynamo-thermal metamorphism.

METAMORPHIC PETROLOGY : (a) Cataclastic metamorphism : It is a
Metamorphic, (Meta=other; morph=form) metamorphism in which directed pressure
rocks are formed when pre-existing rocks are (or stress) plays the major role. The directed
transformed into new rocks by heat, pressure pressure mechanically breaks down the pre-
and chemically active fluids below the Earth’s existing rocks giving rise to crushing and
surface or along the boundary of tectonic plates. fracturing, but without the formation of any
In general, metamorphism means a partial or new mineral in the affected rock. This causes
complete re-crystallisation of minerals in the the lamination of the argillaceous rocks to
pre-existing rocks and the production of new get deformed with the development of slaty
structures. Therefore, metamorphic rocks result cleavages (e.g. shale metamorphoses into
when the pre-existing rocks lose their stability slate).
due to the pronounced changes of temperature,
pressure and chemical environments, below (b) Thermal or contact metamorphism :
the shell of weathering and have re-established Rocks are thermally metamorphosed
stability by adjusting to the new environment. when they are intruded by magma and the
Thus, metamorphism is ‘response of solid high temperature of the magma heats the
rocks to the pronounced changes in temperature, country rocks. The pressure (either directed
pressure and chemically active fluids’. or uniform) effects are almost negligible
Agents of metamorphism : and recrystallization occurs due to thermal
Metamorphism results because of metamorphism. Hence, the calcareous rocks
the changes in the physical and chemical like limestone and arenaceous rocks like
environment of the original or pre-existing rock. sandstone develop a granulose structure when
The agents responsible for metamorphic changes thermally metamorphosed. Limestone forms
are : Marble and Sandstone forms Quartzite.
(a) Temperature, (b) Pressure and (c) Presence
of chemically active fluids. (c) Dynamothermal metamorphism :
Temperature plays a major role in the Dynamothermal metamorphism involves
chemical processes of metamorphism such as both high temperature and high pressure
effects at a high level. The pre-existing
rocks are therefore, subjected to more or less
complete recrystallisation and result in the
development of new structures. Foliation
is a characteristic feature and the typical
rocks are schists and gneisses.

Intensity of dynamothermal meta-
morphism increases with depth. Hence,
in the initial stages of metamorphism, the
argillaceous rocks like shales are altered

23

to Phyllites and with further increase They show interlocking grains of calcite
in temperature and pressure, schists are and quartz respectivelyof almost equal size
developed. At much higher temperature and with polygonal shapes.
pressure gneisses are formed.
b) Quartzite
Structures of metamorphic rocks : Fig 2.23 : Examples of Granulose structure
The structures of metamorphic rocks are
formed by the deformation and recrystallisation 3) Schistose structure : This structure is
of the minerals in the preexisting rocks. The marked by the linear or parallel arrangement
different metamorphic structures are 1) slaty of minerals in a direction perpendicular to
cleavage, 2) granulose, 3) schistose and the maximum stress (fig. 2.24)
4) gneissose.

1) Slaty cleavage : The slaty cleavage
results from the flattening and rotation
of mineral fragments under the action
of directed pressure (fig. 2.22). When
the argillaceous rocks are subjected to
cataclastic metamorphism it results in the
formation of slates, which are generally
rich in micaceous minerals.

Fig. 2.22 : Slate exhibiting slaty cleavage Fig. 2.24 : Muscovite exhibiting Schistose structure

2) Granulose structure : This structure is The schistose structure is developed
developed by thermal metamorphism in due to the unidirectional alignment of the
which original rock undergoes only re- minerals such as biotite, muscovite, chlorite
crystallisation. As there is no action of and talc, which are flaky and hornblende
directed pressure, the sedimentary rocks which is prismatic with a columnar or
like limestones and sandstones simply rod-like appearance. These structures are
recrystallize to give marble (fig. 2.23 a) and developed with high temperatures and with
quartzite (fig. 2.23 b) respectively. strong directed pressures or stress. Because
of directed pressure, the minerals form
a) Marble layers or folia arranged in parallel layers or
bands. This parallel alignment of the flaky
24 or the platy minerals characteristically
gives rise to foliation in metamorphic rocks,
which is also called as schistose structure.

4) Gneissose Structure : This structure is
developed when the original rock possesses
both flaky minerals and quartz. The flaky
minerals like mica give rise to a schistose

band while the hard and resistant minerals Building stones :
like quartz and feldspar recrystallize to For construction of large civil structures
form a granulose band. like dams, highways, bridges, tunnels etc, an
When both, granulose and schistose engineer must know the engineering properties
bands alternate with each other, they of rocks, such as strength, durability, colour,
give rise to gneissose structure which is a appearance, workability etc. These properties
composite structure (fig. 2.25). are very important because different rocks are
suitable for specific purposes and no rock is
Fig. 2.25 : Gneissose structure ideal or best suited for all kinds of constructions.
According to the need of construction a building
This structure develops due to the effect stone is cut and shaped. All tests are performed
of high temperature and directed pressure in the geotechnical lab in accordance with the
on the pre- existing rock. Bureau of Indian Standards Code (BIS).
For selection of good qualities of building
4) Importance of petrology in civil stones, following properties are important.
engineering : 1) Crushing strength, 2) Transverse strength,
In engineering geology, knowledge of rock 3) Porosity, 4) Density, 5) Abrasive resistance,
properties is important for the construction of 6) Frost and fire resistance, 7) Durability and
large structures. Rocks, which are competent, 8) Appearance.
durable and free from weak planes, are suitable
for foundation (e.g. granite, syenite, gabbro, 1) Crushing strength / Compressive strength
Quartzite sandstone). For flooring purpose, : It is the maximum load per unit area which
they should be resistant to abrasion. i.e. able a rock can withstand without undergoing
to withstand wear and tear. e.g. limestone failure/fracturing. Crushing strength should
and marble. Rocks durable to weathering are be greater than 100N/mm2.
appropriate for roofing e.g. slate, limestone.
Owing to its attractive colour and softness, 2) Transverse strength : It is the capacity of
marble is used for decorative purpose and for a stone to withstand bending loads. Shear
sculpture or face work of buildings. Laterites are strength is determined when the stone is
suitable for small-scale constructions. Neatly used as a column.
dressed sandstone can be suitable for walls.
For superstructure construction, rock should 3) Porosity : It is the ratio between the total
be easily workable and available in plenty. volume of pore spaces and total volume of
Thus, different rocks are suitable for different rock sample.
purposes, by virtue of their special physical
properties which are inherent and characteristic 4) Density : Density of a rock is the weight per
to them. unit volume. Bulk density refers to weight
per unit volume of a rock with natural
moisture content.

5) Abrasive resistance : It is the resistance
offered by a rock to mechanical wear
and tear i.e. abrasive resistance refers to
hardness of rocks.

6) Frost and Fire resistance : The freezing
of water in cracks may break rock into
angular fragments known as frost action.
Fire resistance of rocks becomes necessary

25

when they are used near furnaces. under the same conditions. It can be studied
under the branches of igneous, metamorphic,
7) Durability : It refers to the life of a and sedimentary petrology.
structure. Durability of a building stone • Igneous petrology focuses on the
is also susceptibility or resistance to the
weathering. composition, forms of igneous bodies and
texture of igneous rocks. Igneous rocks
8) Appearance : Rocks which are to be used include volcanic and plutonic rocks.
for face work should be decent, uniform in • Sedimentary petrology highlights the
colour, capable of retaining polish and free composition and texture of sedimentary
from clay, cavities, spots of other colour, rocks. They are classified as sandstone,
bands etc. Light colours are pleasing. siltstone and shale based on the grain size
and are bound together in a matrix of finer
Summary : material. These rocks are further classified
The study of rocks provide us with important based on the products of weathering into
information about the nature of the Earth's crust residual and transported deposits.
and mantle. In addition, it enables us to gain a • Metamorphic petrology deals with the
sense of the Earth's history, including tectonic composition and texture of metamorphic
processes that occurred over the long course of rocks which have undergone chemical,
geological time. Petrology relies heavily on the mineralogical or textural changes due to
principles and methods of mineralogy because change of pressure, temperature or both.
most rocks consist of minerals and are formed

EXERCISE

Q. 1. Select and write the correct answer : c) Quartzite d) Mica schist

1) An example of a discordant igneous intrusion is 6) Foliation in rocks is a characteristic feature
.................... . resulting from .................... .

a) Sill b) Lopolith a) cataclastic metamorphism
b) dynamothermal metamorphism
c) Laccolith d) Batholith c) thermal metamorphism
d) plutonic metamorphism
2) The hypabyssal equivalent of basalt is
.................... .

a) Gabbro b) Dolerite 7) a) Examples of concordant bodies are sill, dyke,
laccolith, lopolith.
c) Pegmatite d) Dunite
b) Examples of concordant bodies are sill,
3) A sedimentary rock in which the constituent laccolith, lopolith, batholith.
grains are rounded is .................... .
c) Examples of concordant bodies are sill,
a) Breccia b) Sandstone laccolith, lopolith.

c) Conglomerate d) Laterite d) Examples of concordant bodies are sill, dyke,
laccolith, lopolith, batholith.
4) One of the structures exhibited by sedimentary
rocks is .................... . 8) a) The wavy pattern seen on beach sands is
called ripple marks.
a) granitic b) schistose
b) The wavy pattern seen on beach sands is
c) clastic d) cross bedding called graded bedding.

5) Thermal metamorphism of sandstone results in c) The wavy pattern seen on beach sands is
the formation of .................... . called cross bedding.

a) Marble b) Slate

26

d) The wavy pattern seen on beach sands is exhibits a large variation in grain sizes and
called stratification. shapes.

9) a) The epizone is characterized by low 4) Give two examples of evaporites.
temperatures and strong uniform pressure.
5) What is stratification?
b) The epizone is characterized by low
temperatures and strong directed pressure. 6) What is graded bedding?

c) The epizone is characterized by high 7) What is meant by metamorphism of rocks?
temperatures and strong directed pressure.
8) Give the concept on the basis of which three
d) The epizone is characterized by high zones of metamorphism have been recognized.
temperatures and strong uniform pressure.
Q. 3. Answer in brief :
10) a) Syenite    i) Hypabyssal acidic rock. 1) How are extrusive igneous rocks formed? Give
b) Rhyolite   ii) Plutonic acidic rock.
c) Granite    iii) Plutonic intermediate rock. an example.
d) Pegmatite   iv) Volcanic acidic rock. 2) What are sills? Describe with neatly labelled

A) a) – iv); b) – i); c) – iii); d) – ii). diagram.
B) a) – ii); b) – iii); c) – i); d) – iv). 3) Write a short note on clastic texture with
C) a) – iii); b) – iv); c) – ii); d) – i).
D) a) – i); b) – ii); c) – iv); d) – iii). example and diagram.
4) What are organic deposits? Give examples.
11) a) Residual rock i) Shale 5) Describe slaty cleavage.
b) Chemical deposit ii) Phosphorite 6) What is meant by thermal/ contact
c) Argillaceous rock iii) Limestone
d) Organic deposit iv) Bauxite metamorphism?

A) a) – i); b) – iv); c) – ii); d) – iii). Q. 4. Write short notes :
B) a) – iii); b) – ii); c) – iv); d) – i). 1) How is equigranular texture formed in igneous
C) a) – ii); b) – i); c) – iii); d) – iv).
D) a) – iv); b) – iii); c) – i); d) – ii). rocks? Explain with diagram.
2) What is a laccolith? Describe its characteristics
12) a) Slaty cleavage   i) alternate light
  and dark coloured bands. with the help of a neatly labelled diagram.
3) What is meant by cross bedding? Explain in
b) Gneissose   ii) cataclastic
structure    metamorphism. brief with the help of a diagram.
4) What are residual deposits? Describe with
c) Schistose   iii) thermal
structure   metamorphism. examples.
5) Which are the major agents of metamorphism?
d) Granulose   iv) parallel arrangement
structure   of minerals. Describe their role in brief.
6) What is meant by cataclastic metamorphism?
A) a) – iv); b) – iii); c) – ii); d) – i).
B) a) – ii); b) – i); c) – iv); d) – iii). Q. 5. Describe the following :
C) a) – iii); b) – ii); c) – i); d) – iv). 1) Classify igneous rocks on the basis of mode of
D) a) – i); b) – iv); c) – iii); d) – ii).
occurrence/ depth of formation. Give examples.
Q. 2. Answer the following : 2) How are vesicular and amygdaloidal structures
1) What is a rock?
2) What is meant by the term ‘textures of rocks’? formed? Describe with diagrams.
3) Name the texture of igneous rocks which 3) Describe in brief, the processes involved in the

formation of sedimentary rocks.
4) Classify sedimentary rocks according to their

grain size, giving examples of each.
5) Explain schistose and gneissose structures with

the help of labelled diagrams.
6) Which are the three zones of metamorphism?

Describe in detail with diagram and examples.



27

3   Palaeontology and Stratigraphy

Introduction : Two major branches of palaeontology are
recognized,viz.PalaeozoologyandPalaeobotany.
Palaeontology is the study of ancient Palaeobotany deals with study of plant fossils
life that is preserved in the form of fossils. and palaeozoology with vertebrate and
‘Palaeo’ means ancient, ‘Ontos’ means life invertebrate animal fossils. Micropalaeontology
and ‘Logos’ is study. Initially, the term fossil is a specialized branch that deals with the study
was applied to any object dug up from the of microfossils. Paleoichnology comprises
ground. the study of Ichnofossils and/or Trace fossils.
Palynology or Palaeopalynology is yet another
Today the word fossil is used to refer only branch, which deals with the study of organic
to objects associated with prehistoric life walled microfossils like pollen, spores, seeds,
forms older tha 11,000 years. The term fossil seed coats etc.
was coined by the German mineralogist
Georgius Agricola (1494 -1555). Generally Do you know?
fossils are found in sedimentary rocks. 1) Aristotle, the Greek philosopher was one of
The term fossil is further extended to any
recognizable structure in sedimentary rocks the first to classify living things into either
that indicates some sort of life. Life on Earth plants or animals.
in its primitive form probably came into 2) Leonardo da vinci (1452-1519) stressed
existence about 3500 to 3600 million years that fossils were organic in nature and
ago. Currently, we are in geological time natural in origin.
interval known as the Holocene epoch, which 3) William Smith (1789-1839) a British
began about 11,000 years ago. Therefore, engineer was the first to note that there
any remains or evidence of ancient life from is an intimate relation between the fossils
before the Holocene epoch i.e. before 11,000 and sedimentary rocks containing them.
years ago definitely counts as a fossil. Fossils He realized the importance of fossils and
can range in size. Macrofossils can be held in further, described and defined sedimentary
the hand and examined with good hand lens. rock units on the basis of fossils contained
They can be several meters long and weigh in them. William Smith’s observation was
several tons like logs of petrified wood and the beginning of a new era as integrated
dinosaur bones. Microfossils are very small approach was significant in the study of
and require a microscope to study them. palaeontology and stratigraphy.
Their size can be in micrometres eg fossilised
pollen and spores. Prerequisites of fossilisation :
When an organism is buried quickly, there
Palaeontological studies deals with is less decay and a better chance for it to be
taxonomy, morphology of hard parts, their preserved as it does not come in contact with
nature, inter - relationship, environmental oxygen hard parts of organisms, such as bones,
implications, distribution, age and evolution shells and teeth have a better chance of being
of ancient life. fossilised. Number of prehistoric organisms
living in geologic past exceeds many billions.
28 It has been estimated that only one out of every

10,000 organisms leaves behind fossil record. Modes of preservation of organisms for
So far only about 91,000 species of fossils are fossilsation :
known. 1) Entire organism preserved : Rarely,

If an organism is to be preserved in the form the organisms are preserved in a virtually
of a fossil there are a set of certain prerequisites unaltered state. Ice, amber, and tar can
like : preserve entire organisms e.g. Woolly
1) It is necessary that the organism possesses mammoth preserved in ice from Siberia,
insects preserved in amber from Baltic
a skeleton of hard parts that can withstand countries (fig. 3.1), Dominican Republic,
decomposition. fossils from tar pits of Rancho La Brea,
California, USA.
2) Composition and structure of its hard parts
should be suitable for preservation. Fig. 3.1: Insect preserved in amber

3) Further, it should be covered or buried 2) Entire skeleton preserved : Most of the
quickly by some sediment (deposits), so vertebrate skeletons are entirely preserved
that dispersal and disintegration of hard due to differential solubility. Such skeletons
parts of skeleton is prevented. and shells once buried remain unaltered
e.g. entire skeletons of the giant reptiles
Possession of a skeleton or hard parts of the Mesozoic era such as Mesosaurus
and quick burial are the two most important braziliansis (fig. 3.2).
prerequisites for preservation that are most
commonly fulfilled in case of aquatic organisms. Fig. 3.2 : Entire skeleton of Mesosaurus braziliansis
After the death of an aquatic organism, its
body sinks to bottom of the water body. Later, 3) Petrification : The process of removal and
it is quickly buried by sediment, preventing replacement of organic matter by inorganic
destruction and dispersion. Chances of burial minerals thereby converting the material
and preservation of terrestrial or land animals 29
are higher when they die on the banks of lakes
or rivers. Rarely land animals may be covered
by volcanic ash and instantaneously preserved.
The probability of an animal being preserved as
a fossil is further governed by the composition
and structure of its hard parts. If the skeleton
consists of thin fragile shell, it can easily broken
and chances of preservation diminish; whereas
tough and strong skeletons are preserved
more likely to be preserved. It is not only the
nature of skeleton, but its composition which
determines the degree of fossilization. If the
skeleton is composed up of siliceous matter,
and calcium carbonate (invertebrates), chances
of fossilization are high whereas, phosphatic
vertebrate skeletons and chitinous skeletons of
insects have maximum chances of preservation,
as they dissolve with difficulties.

into rock is known as petrification. In Fig. 3.4 : impression of carbonised plant fossils
this process, the replacement takes place
molecule by molecule. Such fossils show 5) Imprints : Imprint as a mode of
internal and external details permitting the preservation is more common in case of
study of anatomy and morphology in detail. soft bodied organisms like jelly fish (fig.
A variety of minerals are known to cause 3.5). They leave behind their impression
petrification. The most common replacing on the surface of fine grained sediments.
material is silica, calcite, dolomite, pyrite These preserved impression are indicative
and accordingly, petrification may be of many morphological features.
a result of silicification, calcification,
pyritization. e.g. Petrified wood fossils
from Akal wood fossil park, Rajasthan,
National fossil wood park, Thiruvakkarai
T.N., National fossil wood park, Sathanur,
T.N, and petrified woods near Chandrapur,
Maharashtra (fig. 3.3).

Fig. 3.3 : Petrified wood Fig. 3.5 : Imprint of jelly fish

4) Carbonization : Process of carbonisation 6) Cast and Mould : Sometimes, open pores in
forms carbonized fossils. It is a process the rock let water and air reach the organism
of incomplete chemical decomposition or part of it, causing it to decay or dissolve,
of organic matter, reducing it to a carbon leaving behind a cavity in the sediment.
residue or a thin film. Parts of plants are This empty cavity is called a mould. A
usually preserved with this mode. Buried mould shows the original shape and the
broken parts of stems, leaves etc., undergo external morphology of the organism.
dehydration and decomposition loosing Later, sand or mud may fill the mould
nitrogen, oxygen and hydrogen. Carbon and harden, forming a cast of the original
contained within the remains of a soft- organism. A cast is a replica of the original
bodied animal or plant forms a stamp-like organism. This is the most common mode
impression on the sediment Organisms of preservation of invertebrate fossils e.g.
like graptolites and plant fossils from shells from Cretaceous of Tiruchirappalli
Gondwana are preserved as carbonized (TN), Bagh Beds (MP) and Jurassic of
fossils (fig. 3.4). Kutch (Gujrat) (fig. 3.6)

30

Fig 3.6: Cast and Mould of bivalve Do you know?

Types of fossils : Various kinds of trace fossils : Track : an
Broadly speaking, paleontologists divide impression made by a single foot.
fossils into two main groups :
1) Body fossils   2) Trace fossils Trackway : a number of tracks made
1) Body fossils : Body fossils are remains of during a single trip.

body parts of ancient animals, plants and Trail : an impression made by an animal
other life forms. They may be preserved without legs.
intact or fragmented (fig. 3.7a).
Burrows : a hole dug by a life form in loose
Fig. 3.7. a) Body fossil sediment (like mud).

2) Trace fossils : Trace fossils (sometimes also Borings : a hole dug by a life form into a
called ichnofossils) provide evidence about hard substrate (like wood or rock).
the movements and/or activities of ancient
organisms, but not necessarily about Coprolites : animal faecal pellets that have
their appearance. Tracks, trails, burrows, fossilized.
footprints, nests, worm burrows, gizzard
stones, and coprolites are considered to be Uses of fossils :
trace fossils. They range in size from worm
burrows to dinosaur footprints (fig. 3.7 b). Fossil records have facilitated palaeonto-
logists, stratigraphers and biologists to unravel
Fig. 3.7. b) Trace fossil the geological events. Many uses of fossils are
known. Some important uses are as follows :

1) Determination of the geological age of
strata : Geological age of the fossils is
the geological age of the rocks they are
contained in.

2) Information of Palaeo- environment
and history of deposition : Characters
of animals and plants from different
environments vary. Presence of vertebrate
fossils and plant fossils indicate terrestrial
environments like lakes, ponds, rivers,
flood plains, etc. Marine environments
are indicated by th exclusive presence of
fossils of corals, trilobites, brachiopods,
ammonoids, nautiloids etc.

3) Determination of palaeo-climate :
Terrestrial plants thrive in specific climatic
conditions. The occurrence of fossils of
such plants is evidence of the existence of
climatic conditions in the geologic past. For
example occurrence of palm fossils indicate
tropical climate.

31

4) Correlation of widely separated the general public as well as support
strata : Fossils are frequently used to sustainable economic development of the
determine relative or comparative ages of area, primarily through the development of
sedimentary rocks. Similarity of fossils in geological tourism.
widely separated rocks indicates equality in It is interesting to know that the first stamp
their age. So by associating similar fossils on fossils was issued by India. This stamp
among such rocks, they can be correlated. was issued to commemorate the Centenary
of Geological Survey of India in 1951. The
5) Reconstruction organic evolution : Fossil stamp features a fossil elephant: Stegodon
record is invaluable for reconstruction ganesa”, which was the probable direct
of organic evolution because change in ancestor of our modern day elephant. Such
characters of animals and plants is gradual. fossils were reported from Siwalik Hills of
This change is recorded in the fossils Himalayas.
found in successively younger strata. For
example, the earliest horse of Eocene age   
was the size of a dog with five toes. It was
succeeded by forms with lesser number of Terminologies used in palaeontology :
toes and increased height. Finally, modern 1) Index fossil : These are fossils which are
day horse has evolved. This is the evidence
of the evolution of modern day horse. easily recognisable, distinctive, widely
distributed and abundant. They existed for
6) Exploration of Petroleum and Coal a limited life span. (e.g Ammonites form
Reserves : Some microfossils like Mesozoic era, trilobites form Palaeozoic).
Foraminifera and Ostracoda are very Index fossils are used as stratigraphic
sensitive to the environment. Therefore, markers.
they are indicators of environments. Marine 2) Chemical fossils : When some organisms
reducing environment with rapid deposition, decompose they leave a characteristic
good compaction ratio, low current index chemical signature. Such chemical traces
and ideal temperature condition are essential provide indirect evidence for the existence
for the generation of hydrocarbons. All of past life.
these factors can be inferred with the help or 3) Pseudo fossils : These are visual patterns
microfossils. For exploration of petroleum, in rocks that are produced by naturally
deep drilling is required. Such drilling occurring geological processes rather than
must have stratigraphic control. For this biologic processes. They can easily be
purpose, accurate correlation of subsurface mistaken for real fossils, for e.g. dendritic
rocks is essential. Such correlation can be markings formed by filling of fissures in a
established by using microfossils, especially rock by manganese oxide, which are tree
Foraminifer and Ostracods. Plant fossils like.
are useful in the study of coal deposits. 4) Living fossils : A living fossil are present
day forms which also exists as fossils. (e.g.
Do you know?
Geo Parks aim to protect geodiversity
(rocks, minerals, landforms and fossils)
and promote geological heritage within

32

cyanobacteria, horseshoe crabs, ginkgo • Salkhan Fossils Park, Uttar Pradesh
biloba, cycads). • Akal Wood Fossil Park, Rajasthan
• Amkhoi fossil park, West Bengal
5) Reworked fossil : Reworking can happen • Raiyoli dinosaur fossil park, Gujrat
to any fossil. It simply means that the
fossil may have been removed from its Do you know?
original sedimentary layer and redeposited Narmada human
in a younger layer. A reworked fossil can On December 5, 1982, the geologist Arun
mislead a geologists about age of the rock Sonakia discovered the only known fossil
in which it was found of a human ancestor from South Asia on
the banks of the Narmada, at a place called
Do you know? Hathnora village in Sehore district, nearly
Lagerstatten Unique windows into the past 35 kilometres east of Hoshangabad in
These spectacular fossil deposits Madhya Pradesh.
represent an amazing ‘snapshot’ in time. Fossil skull of Narmada human, belongs
These extraordinary fossil deposits, where to the ancestor category of Homo erectus,
organisms are so well preserved that even who inhabited the Earth from 1.8 million
their soft parts remain as carbon films, are to 200,000 years ago and preceded Homo
referred to as Lagerstätten, a German word sapiens.
meaning ‘deposit places’.
Activity 1 :
Do you know? Recreation of Fossil Moulds and Fossil
Fossil parks in India Casts
Expected duration : Two, thirty minute
The Geological Survey of India (GSI) sessions on separate days.
currently maintains the following fossil Instructions :
parks: 1) Place some clay at the bottom of a small
• Siwalik Fossil Park, near Saketi, Himachal
container. This clay represents the bottom
Pradesh is notable for its life-size models of the ocean.
of the vertebrates that might have roamed 2) Press a shell or bone into the clay. This is
the Sivalik Hills 1.5-2.5 million years ago. a hard part of organism that was buried in
• Mandla Plant Fossils National Park, near the clay.
Dindori, Madhya Pradesh is a park that 3) Remove the shell or bone, and observe
attempts to preserve the fossil remains of the imprint in the clay. This imprint is a
a primordial forest that covered the region mould of the shell/ bone.
40—150 million years ago. 4) In a second container, prepare some
• National Fossil Wood Park, Tiruvakkarai plaster of paris by adding water and
in Tamil Nadu. mixing until it is creamy.
• National Fossil Wood Park, Sathanur, in 5) Pour the Plaster of Paris into the mould
Tamil Nadu. that has been created.

Other fossil parks in India include :
• Indroda Dinosaur and Fossil Park, Gujarat
• Ghughua Fossil Park, Madhya Pradesh

33

6) Next day, gently tap and remove the fossil The basic idea of stratigraphy is to arrange
cast. the rock units in chronological sequence, so that
continuous history of the Earth can emerge. This
7) Examine the fossil specimens from the is essential because rocks of different ages are
first investigation. Identify the fossils as found at different places.
moulds, casts, or other.
Principles of stratigraphy : upon three
Activity 2 :
Simulation of process of petrification : Stratigraphy is based
Expected duration: Thirty minutes session fundamental principles :
for initial set up, several days for observations
and thirty minutes session for conclusion and 1) Principle of Uniformitarianism : The
discussion. concept of uniformitarianism was given by
a Scottish geologist, James Hutton (1726-
We will model a process whereby the 1799). In its simplest form, it states that the
remains of a buried organism or part of an processes which have acted in the past, are
organism are replaced by minerals. essentially the same as those in operation
on the Earth today. It can be summarized
1) Cut two pieces of white sponge into a bone in the statement ‘present holds the key to
shape. One piece will be used to simulate the past’. By actual observation, we can
fossil formation and the other will be used see that layering and variation of grain size
for comparison. seen in the sediments, on the banks of the
modern stream, are the result of fluctuations
2) Fill a cup with hot water. Prepare in the velocity of the stream. Hence, we
a saturated solution of Epsom salts can summarize that the same phenomenon
(MgSO4) and add a few drops of food was responsible for the layering in ancient
colouring. fluvial sediments. Likewise, we know that
the ripple marks observed on beaches is
3) Pour the solution into a pan. a product of wave action. It is, therefore,
logical to conclude that ripple marks
4) Immerse one bone shaped sponge into the present in sedimentary rocks were formed
pan and observe the movement of water by the same process.
through the holes of the sponge.
Do you know?
5) Leave the pan untouched for several days
until the sponge is dry. James Hutton FRSE (Edinburgh 1797)

6) Examine the dry sponge. was a Scottish physician,

STRATIGRAPHY geologist, naturalist,

It is a branch of geology that deals with the chemical manufacturer and
study of stratified and sedimentary rocks with
reference to their description, identification, experimental agriculturalist.
content, correlation and extent, both horizontal
and vertical. ‘Stratum’ means a layer and His work helped to establish
‘graphy’ means description. Therefore,
stratigraphy is a descriptive study of layered or the basis of modern geology.
stratified (generally sedimentary) rocks.
His theories of geology and geologic time,
34
also called deep time, came to be included

in theories which were called plutonism and

uniformitarianism.

2) Order of Superposition : In 1669 Danish or stratigraphic position between two rock units
physician Steno on the basis of observation that are widely separated. Correlation may be
along the walls of the Arno Valley in local, regional, intra or inter basinal, short range
Italy suggested the concept of Order of or long range.
Superposition. He pointed out that in any
series of sedimentary layers, lying in a Methods of Stratigraphic correlation :
normal disposition; the rocks at the bottom
of a sequence are older than the rocks at the There are two methods of Stratigraphic
top of the sequence. However, where the correlation i) Lithological correlation and ii)
rocks are over folded and thrusted or faulted, Palaeontological correlation. It can be achieved
it is first necessary to determine whether the by a) Physical/ lateral continuity and b) presence
rocks are in their normal position or if are of marker horizon or key beds.
overturned. Most simple way of correlating strata
on local scale is by ascertaining, if there is a
3) Faunal Succession : The British surveyor, physical continuity of strata. If one can walk
William Smith (1769-1839), is regarded as physically from one locality to another on the
the father of modern stratigraphy and was same rock type, then correlation is established
the first to recognize the fact, that fossils by the principle of continuity of strata (fig.3.8).
could be used to correlate and date the
strata in which they were found. During the i) Lithological correlation :
course of his work, he noticed that the same
assemblage of fossils always occurred If succession of lithological units is
in the same rock layers. Fossil species in
layers above and below these layers were similar at different localities then there is every
distinctly different. The fossils occurred
in the same order in widely separated possibility that corresponding rock units are of
localities. This discovery of William Smith
led to the establishment of the ‘Law of the same age and can be correlated (fig. 3.9).
Faunal Succession’.
ABC
Stratigraphic Correlation : Correlation is a 1
method of finding equivalence in geological age
AB C2

3

4

5

6
Fig. 3.9 : Correlation based on lithological succession

Sandstone layer

A) Floor of sedimentary basin

B) Fig. 3.8 : Correlation of strata using the principle of continuity

35

In the lower part the lithological succession Do you know?
is similar and the beds of shale and sandstone
can be correlated in section A , B and C. It may Nicolas Steno (1638, 1686)
be noted that, there is a slight variation in the
thickness of these beds, at different localities. As Ascientist and a pioneer in both
we move upwards the limestone bed is missing anatomy and geology. Steno
in section c but correlated in Section A and B. If was trained in the classical
in a particular exposure only a small part of the texts on science. However, by
succession is seen, then there can be difficulty in 1659 he seriously questioned
correlation.When the horizons pinch or die out, accepted knowledge of the
the presence of a Marker Horizon or Key Horizon natural world. Importantly he questioned
is extremely useful. This bed is relatively thin explanations for tear production, the idea that
but persistent bed having distinctive lithological fossils grew in the ground and explanations
or structural characters, which permit its of rock formation. His investigations and his
identification without any difficulty. subsequent conclusions on fossils and rock
From the above discussion, it is evident formation have led scholars to consider him
that lithological characters are useful only to one of the founders of modern stratigraphy
limited extent. In areas where there is repetition and modern geology.
of beds of identical composition and where
marker horizons are missing, other methods like Standard Geological Time Scale :
Palaeontological correlation are more useful.
Age of the Earth is about 4600 million years.
ii) Palaeontological correlation : This enormous geological time is divided into
It is entirely based on fossils or fossil different durations. The successive sequential
assemblages. Organisms evolve with age and it is arrangement of geological time units for global
possible to differentiate between beds having the reference is called as Standard Geological
same lithology but with fossils of different age Time Scale (fig. 3.11). It is based on the
(fig. 3.10). Moreover a given species is likely to following criteria. The periods of non-deposition
be present at widely separated localities during a (and consequently of erosion) are called as
particular period. This enables palaeontological unconformities, that are related to widespread
correlation on a regional scale. tectonic activities. The regional unconformities
provide the natural boundaries for subdivisions
Lava flow 450 m.y. of geological time into different units. Fossils
are also of great importance in recognition of
Granite Section A Section B particular space of geological time. Radiometric
480 m.y. age analysis of older rocks formed before
biogenesis is also useful in this respect. The
Fig. 3.10 : Palaeontological Correlation Geological Time Scale is also based on principles
of stratigraphy that are discussed earlier.
36
The time units are Eon, Eras, Periods and
Epochs. The largest time unit is known as Eon.
It is successively divided into Eras, Periods and
Epochs.

Most commonly two Eons Pre Cambrian and
Phanerozoic are recognized. The former indicates
the time during which life was either absent,

obscure or very primitive and the latter suggests Ordovician, Silurian, Devonian, Carboniferous
the time when evolved life was in existence. and Permian. Carboniferous is again subdivided
The Pre Cambrian Era is further sub divided into Pennsylvanian and Missisippian.
into Haedean, Archaen and Proterozoic. Mesozoic Era is further subdivided into three
The second Eon Phanerozoic is subdivided periods, viz. Triassic, Jurassic and Cretaceous.
into three Eras viz. Palaeozoic Era containing Cenozoic Era is subdivided into three
ancient life, Mesozoic of middle life and Cenozoic periods, viz. Paleogene, Neogene and Quaternary.
contains modern/recent life. The life (animals/ Palaeogene Period consists of Palaeocene,
plants) of these eras (organic evolution) is shown Eocene, and Oligocene Epochs and Neogene
in (fig. 3.11). Period consists of Miocene and Pliocene Epochs.
The Palaeozoic Era is further subdivided into Quaternary Period is subdivided into Pleistocene
(from older to younger) six periods viz. Cambrian, and Holocene Epochs.

0 EraCenozoic Period Events
2.6 Quaternary Evolution of humans
50 Mesozoic
100 Neogene Mammals diversify
Millions of years ago 150 Paleogene
Paleozoic 200 Extinction of dinosaurs
250 Cretaceous First primates
Carbon- 300 First flowering plants
iferous350 Jurassic
400 First birds
450 Triassic Dinosaurs diversify
500
550 Permian First mammals
600 Pennsylvanian First dinosaurs
Mississippian Major extinctions
Reptiles diversify
Devonian First reptiles
Silurian Scale trees
Ordovician Seed ferns

Cambrian First amphibians
Jawed fishes diversify
Late Proterozoic
First vascular land plants
Sudden diversification
of metazoan families

First fishes
First chordates

First skeletal elements
First soft-bodied metazoans
First animal traces

650
Fig 3.11 : Geologic time scale, 650 million years ago to the present

37

Do you know? Major Events in Geological Past do you know

Era Period Geological Events Life (Animals-Plants)

Quaternary Early period known for ice age formation of Ape like man appeared in Pleistocene,
Palaeogene
and Neogene Laterites Aves, Fish, Important Age of man. Dicot-
Cretaceous
Cenozoic Monocot dominated.
Jurassic
Triassic Most of the Continents and oceans occupied their Age of mammals placental. marsupials,

Permian present positions, India collided with Asia, Alpine- elephants, horses, pigs, dear, cows,
Carboniferous
Himalayan orogeny, Himalaya-Alps formed, Red primates, flowering plants dominated.
Devonian
Silurian Sea opened.
Ordovician
Few phases of marine transgression General Great extinction of ammonites, dinosaurs.
Cambrian
elevation of land, fragments of Gondwanaland and Abundance of plants, extinction of cycades

Laurasia separated Andes and Rockys mountains plants Mammals evolved.

formed.

Mesozoic Fragmentation of Gondwanaland and Laurasia Age of dinosaurs Flying reptiles first bird

started, Atlantic ocean formed, Continental drift Archaeopterix. True mammals appeared

continued. Gymnosperms dominated.

Pangea broke into Gondwanaland and Laurasia, General recession in marine fauna Early

Period of marine regression and salinity crisis dinosaurs came into existence Egg laying

mammals, primitive flowering plants

Angiosperms appeared.

Siberia joined Laurassia-Gondwana, Formation of Reptiles diversified, brachiopod declined,

Pangaea, Ancient Atlantic closed. trilobites extinct. New land plants appeared

Gondwana was on South pole and covered by Early reptiles evolved from amphibians.

ice sheets, Gondwana collided with Laurassia, Amphibians were important. Luxuriant

Hercynian orogeny began, Appalachian mountains growth of bearing (gymnosperms)

formed. Spermatophytic plants Major Coal seams

formed.

Formation of Laurassia, Mountain building Golden age of fish. Arrival of amphibians.

activities on peak, Caledonian mountains formed, Vascular seedless pteridophytic plants

Palaeozoic Old Red Sandstaones of Europe formed. abundant Graptolites become extinct.

Caledonian orogeny. Baltica-Laurentia collided, Graptolites important, many trilobites

Formation of Caledonian mountains began, Siberia declined in number Jawless fish appeared.

drifted to North. First land Appeared.

Main continents converged-diverged, Gondwana Bush like graptolites are index fossils.

moved towards South pole, Baltica to South and Cambrian invertebrates continued

Siberia to North Probably by the end of Ordovician, first

land plants appeared

Six to Seven continents lying 60º N and 60º S, All major groups of invertebrate protozoans,

Present Antarctica was on Equator, there existed mollusca, trilobites, graptolites, sponges,

Panthalassic and echinoides, lapetus oceans. They corals, brachiopods, bryozoans present

were shallow Trilobites are index fossils. Calcareous

blue-green Algae present.

Ancient supercontinent Rodinia split and Ediacara fauna of Australia is important

Proterozoic fragmented, fragments reunited to form another Unicelluar life thrived. Stromatolites

supercontinent Pannotia, Panthalassic ocean (biosedimentary structures) important.

present. A chunk of Pannotia torned off into Burrows of soft bodied worms and

fragments. Era began with tectonic zonation with termites.

formation of geosynclinal and platformal types of

basins. Atmosphere-Hydrosphere purified.

Older shield areas formed. Extensive volcanism and Probably no life.

Archaean plutonism. Highest degree metamorphism. Earth

attained present shape. Ancient continents and

oceans formed. Differentiation of Earth into crust,

mantle and core. Accretion of Earth complete.

38

Do you know? Table 3.1 Major Lithostratigraphic units of
Peninsular India.
William Smith (1769 –1839) was an English
Era Period Litho. Unit
geologist, credited with
Quaternary Laterite-Bauxite
creating the first detailed, Cenozic
Tertiary
nationwide geological Cretaceous Tertiary of Assam, Gujarat,
Rajasthan, Maharashtra
map of U.K. It was only and East Coast.

late in his life that Smith Deccan Basalts.

received recognition for his Mesozoic Jurassic Upper Gondwana
Triassic Midde Gondwana
accomplishments, and became known as the

"Father of English Geology". Permian

Lower Gondwana

Brief outline of stratigraphy of Peninsular Carboniferous
India :
Devonian
Lithostratigraphy is one of the elements
of stratigraphy. It deals with organization of Paleozic Silurian
strata into sequential successive rock units
arranged in a chronological order, entirely Ordovician
based on lithological characters. It has its
own lithostratigraphic units, like; Supergroup, Cambrian
Group, Formation, Member and Bed. Many
comparable mutually related rock layers form Precambrian Vindhyan Supergroup
a Bed. Numerous Beds constitute a Member, Archaean Paleozic
several such Members give rise to Formation. Cuddapah Supergroup
Formation is the fundamental unit recognizable
and mapable in the field and can be distinguished Dharwar Supergroup
locally in a small area. Many such formations
make still larger units, covering larger area Peninsular Gneisses
called Group. An association of similar mutually
related Groups constitutes a Supergroup that Basement complex of
spreads over a very larger time span of the order South India-Karnataka-
of an Era. Sargur Schist complex.

Study of the Indian lithostratigraphic units Do you know?
with reference to their origin, distribution, content, Sir Thomas Henry Holland KCSI KCIE
age relationship and correlation is known as
Indian stratigraphy. Important lithostratigraphic FRS FRSE (22 November
units of peninsula India, according to their age 1868 – 15 May 1947) was a
and correlation are depicted in table 3.1. British geologist who worked
in India with the Geological
The important lithostratigraphic units of Survey of India, serving as
Peninsular India are described, in brief, with its director from 1903 to
reference to their distribution, age, lithological 1910. He later worked as an educational
characters, fossils and economic importance. administrator at Edinburgh University.

Dharwar Supergroup :

Dharwar Supergroup is named after
Dharwar district of Karnataka, where it is best
exposed. The term Dharwar system was coined
by R. Bruce Foot in1888.

39

Distribution : They occur in Dharwar-Mysore of quartzites, limestones, sandstones and slates
region. Older Dharwars are developed in without any fossils.
Bababudan Hills in Chikmagular region.
Younger Dharwars are observed at Chitradurga Fossils : These rocks are mostly non- fossiliferous
and Ranibenur regions.
Economic importance : Economic resources
Age : Archaean. of the Cuddapah system include limestone,
iron, manganese, copper, cobalt, nickel, barites,
Lithology : It is divided into three divisions asbestos, steatite, diamonds and other minerals.
as lower, middle and upper Dharwars. The
Lower Dharwar contains rhyolites, schists and Vindhyan Supergroup :
gneisses. The Middle Dharwar comprises of
the granite porphyries, basic and ultrabasic Vindhyan Supergroup of India is one of the
intrusive igneous rocks, volcanic products and largest and thickest sedimentary successions of
banded ironstones. The Upper Dharwar contains the world. Vindhyan Supergroup has been named
cherts, ferruginous silts, clay, conglomerates after great Vindhyan Mountain in Central India,
and quartzites. where these rocks are well exposed. It can be
separated into Lower and Upper divisions on
Fossils : No evidence of life. the basis of an unconformity marked at various
places.
Economic importance : They are known to
contain largest iron ore and manganese deposits. Distribution : These rocks cover an area of
The associated granitic rocks supply best quality about 1,00,000 sq. km. in Central India and are
of granites for decorative purpose as well as well exposed in the Son valley.
building material. Gold and chromite deposits
are also believed to belong to Dharwars. The Age : Upper Proterozoic.
other minerals include copper, lead, zinc, mica,
asbestos and kyanite. Lithology : This group consists of sedimentary
rocks such as sandstones, shales and limestones
Cuddapah Supergroup : with thickness often over 4000 m. to 6000 m.

Cuddapah Supergroup has been named Fossils : In the evolution of Peninsular India,
after the Cuddapah district of Andhra Pradesh, for the first time some structures indicative of
where it is best exposed in the form of crescent- primitive organisms have been reported.
shaped outcrop, covering about 42,000 Sq
km. area. These rocks are separated from the Economic importance : Shales from the
underlying Archaean rocks by the Eparchaean Vindhyan Supergroup contain workable pyrite
unconformity. mineralization. All types of limestone, sandstone
and shale are used as building materials. Huge
Distribution : The Cuddapah Basin of Andhra limestone deposits supply basic raw material to
Pradesh has an east-west width of about 140 km. cement industries. Most of the Indian diamonds
from Tadpatri to Nellore and is over 300 km. are obtained from Vindhyan Supergroup. They
north-south in length from Ongole to Tirupati are recovered either from Kimberlite pipes
Nagari Hills. The Cuddapah rocks are largely (volcanic necks) or from the diamond bearing
undisturbed except along the eastern margin, conglomerate horizons. Excellent quality of
where they are folded and faulted. The thickness glass sand is obtained from the weathered
of the Cuddapah formation is about 6100 m. sandstone found in Vindhyans.

Age : Lower Proterozoic. Gondwana Supergroup :

Lithology : The Cuddapah Supergroup consists Gondwana rocks are named after the Gond
tribe in Madhya Pradesh where these rocks are
40

best exposed. Subsequent to the deposition of the They thin out towards east.
Vindhyan, there was a break in sedimentation in
Peninsular India, till Carboniferous times. It was Age : Cretaceous- Eocene.
followed by the deposition, of thick sequence
of fluviatile sediments constituting Gondwana Lithology : The volcanic rock type generally
sediments. called as basalt is a fine grained, dark grey to
porphyritic rock with vesicular to amygdaloidal
Distribution : Gondwana rocks mostly occur structures. They contain a rich assemblage of
in the states of Madhya Pradesh, W. Bengal, secondary minerals, like; quartz and zeolites.
Chattisgarh, Odisha, Jharkhand, Telangana and Acidic and intermediate differentiates occur
Maharashtra. Isolated outcrops occur in Gujrat along the west coast of India. In the lower part, the
and Rajasthan. Gondwana Supergroup is today basalt flows are sometimes separated by fluvial
exposed in linear tracts largely corresponding sedimentary beds, called the intertrappeans. The
to the valleys of the river Narmada, Damodar, intertrappean beds show fresh water lacustrine
Mahanadi and Godavari. deposit. Best examples of such sandwiched
intertrappeans are in Malabar hills of Mumbai.
Age : Permo-carboniferous to Jurassic.
Fossils : Fossils of turtle, frogs, molluscs and
Lithology : They consist of alternation of some plant remains are reported.
sandstones, green sandstones, gritty sandstones,
shales, limestones and coal. Economic importance : Compact basalt is
extensively used for building constructions. Old
Fossils : Plant fossils such as glossopteris, temples, forts and buildings are built with basalt.
gangamopteris, conifers, cycades, and animal Deccan basalts host amazing zeolites and silica
fossils such as crustaceans, fish, insects, reptiles, bearing minerals (amethyst, quartz, chalcedony,
dinosaur (bones and eggs), coproliths and agates) are found as secondary minerals in the
amphibians are found. cavities within the deccan basalts. A thin capping
of bauxite is used as an ore of aluminium. Along
Economic importance : Gondwanas contain the coastal Konkan area of Maharashtra, the
thick coal seams which contribute practically all laterite blocks are used as building material.
of India's coal output. Gondwana Supergroup is
well known for its good steam and gas quality Cenozoic rocks :
of coal. Beds of oxides of iron ore products
have been worked out for blast furnace. Clay In Peninsular India, Cenozoic sedimentary
for terracotta, pottery. Sandstone is extensively rocks of marine origin are found in Rajasthan,
used as building material. along the onshore and offshore regions of
Gujarat and offshore regions of Maharashtra.
Deccan Volcanic Province (DVP) : These rocks are also developed along the east
coast in Krishna-Godavari and Cauvery basins.
Deccan Volcanic Province cover an area These are important because they form reservoir
of over 500,000 sq. km. in western India, with rocks for oil and natural gas.
flat-topped hills, hence also known as plateau
basalts. They also show step-like terrace form Distribution : These rocks occur in Rajasthan,
and consist of sub- horizontal flows. Maharashtra, Andhra Pradesh, Tamil Nadu and
Gujarat
Distribution : They occur almost in all parts
of Maharashtra and extend to cover parts of Age : Eocene to Pliocene.
adjacent states of Gujarat, Madhya Pradesh,
Telangana and Karnataka. Maximum thickness Lithology : Majority of the rocks are
of Deccan Basalts, (> 3500 m), is towards west. conglomerates, sandstones, limestones and
shales.

41