Hello everyone. Welcome to this NPTEL Structural
Geology course. This course is being offered
for the undergraduate students and designed
accordingly. The first two lectures, we will
cover the introduction and then slowly we
will jump into the other topics of the subject.
Before going into the actual course, we will
have some administrative parameters that we
will follow in this lecture and then we will
proceed. So before we start, I am your instructor.
My name is Santanu Misra. I am a faculty member
in the Department of Earth Sciences of IIT
Kanpur.
About my academic background, I studied Geology
in Jadavpur University. I had my Bachelor
degree, Master degree and also PhD degree
from Jadavpur University. Then I moved to
ETH Zurich in Switzerland for a post doc,
later I became a lecturer in the same institute.
I learnt mostly Experimental Rock Deformation
and worked on high pressure temperature deformation
behavior of rocks. Then I moved to another
beautiful country called New Zealand. The
institute was GNS Science. I worked there
on earthquakes and landslides phenomena.
In 2015 I decided to come back to India and
joined IIT Kanpur as a faculty member. My
research interest mostly includes primarily
experimental rock deformation, rock physics,
structural geology and tectonics. Throughout
this course or even later you can contact
me via this email. You also can call me during
the office time and to know more about my
research, my research group and other activities,
you can follow me via my webpage.
Saquib and Manab, these two are the TAs of
this course. They are teaching assistants.
Both of them are my PD students. They are
CSIR Senior Research Fellows in IIT Kanpur.
Saquib joined in 2015 and Manab as well. Saqib
works on petrology of structure of Nagaland
Ophiolite sequences, and Manab works in the
direction of rock physics and he works on
enhanced coal bed methane recovery via CO2
sequestration. About the study materials for
this course there are n number of books, there
are n number of online resources that you
can explore.
I particularly recommend these four books
and few online materials which will be helpful
for this course. And I also derive most of
the study materials from these four books
and these online materials. The first book
is the book of Structural Geology, Fundamentals
and Modern Developments written by Professor
S K Ghosh who was a Professor in Jadavpur
University, Kolkata. This book is little bit
of advanced level for an undergraduate student
but initial sections are written in a very
general way, in a very scientific way so that
one can understand.
What I like about this book, particularly
for the students of India, that the examples
of geological structures are cited from Indian
continent, or different Indian fields. So
therefore if you go to the field you can see
those structures and relate yourself. The
second and third book are the two classic
ones.
The second book is Structural Geology which
is the second edition of Twiss and Moores.
It covers entire span of structural geology.
And the third one of Professor John Ramsay,
Folding and Fracturing of Rocks is a classic
textbook of structural geology ever considered.
It is very important to have this book in
your library and it is worth reading book.
Fourth book is relatively new in the structural
geology field written by Professor Fossen.
What I like about this book is it is written
with examples of lot of applications; the
language is very easy to comprehend and understand.
And most importantly this book has fantastic
field photographs, colored field photographs
and in addition to this, a complimentary CD
of illustrations which are essentially helpful
to understand different structural features.
About online materials, I recommend these
three. So the first one is a textbook in pdf
by Professor Ray Patrice, the second one is
lecture note from Professor Jean-Pierre Burg.
And the third one is one YouTube lecture series
given by Professor Janos Urai of Aachen in
Germany.
In all these three materials you will get
excellent illustrations, very nice texts and
particularly for Professor Urai’s lecture
he has given lots of analog models and numerical
exercises which will be helpful for you. Needless
to mention, there are n number of, there are
series of online materials which are available.
You just have to type online that what you
are looking for, you just type the keywords
or the phrase or the sentences and you will
get series of suggestions from Google or whatever
search engine you use and you can figure out
what you are looking for. I am sure you will
get it. If not, you are always welcome to
contact me or the two teaching assistants
of this course.
This is the course template of this course
outline of this Structural Geology course.
The course is designed mostly following the
general undergraduate courses that is being
followed in India and globally. So it is a
twelve weeks course. Initially we learn at
least in this lecture and in the next lecture,
introduction and basic concepts of structural
geology. Then we will follow the certain different
aspects sequentially one after another to
cover different structural elements, their
measurements, stereographic projection. We
learn about strain and stress.
We learn about rheology and deformation mechanism
of rocks, then slowly we will go to the actual
real rock structures that we see in the field
like foliation and lineation, different types
of folds, their formation mechanisms, superposition
of folds, then boudinage and related structures,
fractures, joints everything. Then we move
to the ductile domain that is the ductile
shear zone which is very important in structural
geology and in general. And finally we will
end up with some notes on structural mapping,
summarize this course and do some discussions
for the future developments and studies. The
question that one should ask at the very beginning
that, why I should study structural geology?
Personally I like this subject very much.
We will see in course that this subject makes
you like a detective. Like you have something
in your hand, this is a puzzle. You have no
clue what happened in the past. So your challenge
or your task is to, whatever you have in your
hand, just looking at it, observing it, analyzing
it, you have to go and understand what has
happened in the past. So in a way, this is
a very challenging work and I like it very
much. Apart from this, you of course would
like to ask what are the job opportunities,
what are the different aspects that, what
is my use of studying structural geology in
the context of present day society? The answer
is it is significant.
So if you are structural geologist, your demand
is in many industries and also certainly in
academia. So you can be recruited or you can
be hired if you are good enough in exploration
and mining industries. You can be also hired
in litho-structural mapping and survey companies,
construction engineering and structural analysis
of different surface and sub-surface materials.
If you are interested to that, there are many
industries who are involved in this type of
work and they will certainly be interested
to hire you.
For natural hazard analysis, earthquakes,
landslides and so on, your job is secured
if you are good at it. Hydrogeology is also
one of the areas where structural geologists
are in high demand. And apart from this, I
can certainly join in academia and petroleum
industry and other places where you can work
on science and technology development.
As I said that structural geology is a subject
that unravels the past of the earth, the history
of the earth in a certain way and there is
no clear picture of that. We have to figure
out things what we have today in our hand.
So these three terms, predictions, uncertainties
and risks are somehow very much associated
with this subject. Whatever you do, you should
have some sort of prediction, some sort of
risk and some sort of uncertainty in your
discussions and in your results. So always
keep this in mind.
Ok, so what is structural geology? Now if
you have heard this term before then you must
have heard also these two terms that are being
always said or always used together with structural
geology. One is tectonics and another is geodynamics.
Now these three terms, structural geology,
tectonics and geodynamics, their origin comes
from Latin and Greek languages. So structure
is, comes from the Latin word struere that
means build, tektos is a Greek word from which
we have this word tectonics that means builder
and then dunamis is a Greek word which means
power or force. So you can see that if these
three terms, structural geology, tectonics
and geodynamics, well geology is ge that means
the earth. It is also a Greek word.
So these three terms as I was talking about,
structural geology, tectonics and geodynamics,
these three, from their origin of these three
words can suggest you that with the help of,
or with this action of power and force, how
you can build something and who is the builder
for that? So the structural geology is certainly
all about power, forces, building something.
And if you apply power and force, you have
to deform, you have to move something from
one point to another. So scientifically you
can finally conclude that the subject structural
geology together with tectonics and geodynamics
concerns in general with the shape, that is
the geometry, the displacements which is kinematics
and forces so mechanics in our earth and other
planetary bodies.
Now interestingly if you have these terms,
geometry, kinematics and mechanics, you can
certainly comprehend the fact that the subject
is highly interdisciplinary and it is indeed.
We take assistance, help and collaborate actively
with people from material science, mechanical
engineering, physicist, computer science and
remote sensing. Within the broader umbrella
of earth sciences we also collaborate with
geophysicists, petrologists, igneous, metamorphic
and sedimentary domains. We also take active
help from survey people, and of course nowadays
we are also taking people from Geodesy on
board. So this interdisciplinary nature of
the subject makes it highly broad and in overall
geology it makes it a complete science topic
together with physics, chemistry, maths and
biology.
Now these three subjects or which are commonly
used together, structural geology, tectonics
and geodynamics, these three form a very coherent
and interdependent sub-disciplines of geology.
And together with these three topics, we try
to understand that how these rocks, different
rock formations and earth systems in general,
crust, lithosphere, asthenosphere so on deform
and how do they deform via which processes?
You can understand when you see a rock which
is deformed. We will learn in this lecture
how to look at a deformed rock. It contains
a lot of information. A piece of rock gives
you a series of information. Your idea or
as a structural geologist or geologist in
general, your aim is to unravel this information
and use this information to study different
processes that happened at the past in the
earth, and also what could happen in the future.
So let us talk about these three topics, structural
geology, tectonics and geodynamics, what these
are? Whether these are different, whether
they are similar or if there is any difference
then where is this, where is the difference?
Well these individual topics, structural geology,
tectonics and geodynamics, from science point
of view; from approach point of view they
are very similar. Three of these subjects
essentially deal with displacement, forces
and kinematics, the geometry, shape etc. But
the fundamental difference between these three
topics are the scales of observation.
So structural geology generally we study in
field based, it is a field based discipline
and it operates from very microscale, about
100 microns or less to 100 meters or maximum
1 or 2 kilometers. So we can say that from
a grain to outcrop if you study rocks then
you are doing structural geology, of course
in the context of deformation. The tools that
are used to study structural geology include
field study, that is very important, rock
deformation experiments; you can do analogue
experiments and essentially numerical models.
In contrary tectonics is certainly a large
scale. As you can see the entire plate tectonics
discipline is pretty large. But it does not
involve what is happening at the bottom of
these plates. It just deals with the movement
of the plates, their mutual interactions and
so on. So tectonics in general deals from
about 100 meters to 1000 kilometers in scale.
In structural geology we learn that it is
below 100 meters. So tectonics is certainly
a large scale study of structural geology,
you can consider it this way. The tools we
use here are again field study, you can do
field work, you can do analogue experiments
and you can do numerical models.
Geodynamics is the subject that discusses
about the forces and processes that drive
the plate tectonics and the deformation of
the materials inside the earth. So you can
consider the mantle convection, plumes etc.
So as you can imagine the scale from just,
from the plate tectonics to where the plates
are to the core of the earth or at the core
mantle boundary the scale is huge. So it is,
it operates at the scale more than 100 kilometers.
And there is no way you can do field work
at core mantle boundary or even cross mantle
boundary. So there is no scope of doing field
work. So what tools we are left with are analogue
experiments and numerical models.
Now to study structural geology as I was talking
about, even for, within the subject of structural
geology apart from tectonics and geodynamics,
scale is something that is very important
that you always have to remember. Or always
have to take into account what is the scale
you are looking at, what is the scale of observation?
And structural geologists do it very, very
frequently. They jump from one scale to another.
Looking at a single grain, the deformation
of the single grain, one structural geologist
can immediately interpret an entire mountain
building process. So this is a fun, this is
a scale as well. Apart from the scale, there
are three pairs of terms. One is continuous
versus discontinuous. Second one is homogeneous
versus heterogeneous, and third one is isotropic
versus anisotropic. We are all familiar with
these terms but let us have a look at these
six terminologies in the context of studying
structural geology.
For the scales we generally cover three different
terms, microscopic, mesoscopic and megascopic.
As the name suggests, microscopic is something
that you observe under microscope, be it optical
microscope or electron microscope and we call
it microscopic scale. Mesoscopic scale is
something that you can cover just by a view.
So it is scale that to structure that can
be observed without the aid of the microscopes
on a hand specimen or a single outcrop and
so on.
So it is about 1000 meters or 1 kilometers
or something like that. And we call it outcrop
scale or outcrop study and then macroscopic
scale is something that you are doing a large
scale field study or regional scale field
observations, so this is greater than 1 kilometer
and so on. So it is to be completely exposed
in the outcrop that you may not get in the
field. You may get something here, something
there, in-between there is no rock exposure.
So it is your background, it is your intellectual
quality of the structural geology background
that how you can correlate from this outcrop
to that outcrop. And when you do that, you
are actually doing a macroscopic field observation
or macroscopic study, macroscopic scale study
of structural geology.
So here is an example of what do we understand
by scale. What I try to convey with this slide,
you have learnt probably already that if a
layer is horizontal or at any orientation
and if there is a layer parallel compression
this layer is ductile enough then it makes
a curved feature which is known as fold.
Now in these three images, in the first one
you can see that the width of the image from
the scale is given, is about 750 microns.
In these 750 microns from here to here, approximately
what you see this green material is an aggregate
of biotite mineral which is a kind of mica.
And you can see this biotite is not straight
here, it is folded. So there must be a layer
parallel compression here.
Now if I jump to the next image we see a very
similar structure which is fold but here the
scale is, or this distance in this entire
image is close to 50 meters. And if we look
at here, this distance is about 4 kilometer
and we almost see a very similar structure.
Now if I see fold in the first image and if
I see fold in the second or last image, then
they characteristically may be same, mechanically
they may be developed in a very similar way
but their scales are different.
So, therefore I was talking about, the concept
of scale is very important in structural geology.
And one has to jump from one scale to another
scale to solve the geometrical problems that
we see in the field and also in the experiments
and when you do observations under microscope.
Now about the continuity and discontinuity
of structures, this is something that is also
scale dependent. So for example this picture
here, you can understand that this is a layered
rock, we are not going into the fact that
how did it form and what it is, but we can
figure out certainly that it has alternate
dark and white colored bands. Now if I follow
any of these bands I see in this scale of
observation of this photograph I am looking
at, these layers are continuous. That is,
there is no discontinuity.
However when this layer got extended and it
formed a structure called boudinage we can
figure out that few of these layers are continuous
here, for example if I try to draw it here
but there are few layers. For example if I
take this little packet of layers, it comes
here then it vanishes and then it starts again
from somewhere here. So there is certainly
a discontinuity. This is something what we
call continuity and discontinuity, or continuous
and discontinuous.
We have few more examples and here we would
like to highlight the fact that why this continuity
and discontinuity are also scale-dependent?
For example here in the first image we see
this, this is a shear zone, ductile shear
zone and this layer, this black layer is continuous.
However if I consider this white layer in
the second image it is going like this and
then we have some other material inside and
then probably it continues somewhere here.
Now looking at it I have a discontinuity from
here to here, the layer is not continuous.
So this is a discontinuity. Here in this image,
you see again a layered rock and we have n
number of fractures which made these layers
discontinuous. Now at this scale of observation
I see them as a discontinuous layer, an individual
layer. But if I look it from far, I may not
see these fractures and I may consider this
as a continuous feature. So therefore continuity
and discontinuity in rocks are essentially
a function of the scale you are looking at.
Again you have another example where I try
to provide, for example here this is a little
complex structure. To understand it better
or to highlight its features I made a sketch
of this which is on the right side. And you
can see few layers which are marked by this
arrowhead are continuous, and few layers here,
these are getting discontinued. And also the
entire outcrop or entire image that we can
see, we can see a little discontinuous line
here that is separating by a sleeve. So there
are many ways you can produce discontinuity.
And that is also important to understand that
what is the reason for the discontinuity in
the structure you are looking at?
Coming back to two other terms that is homogeneous,
heterogeneous, isotropy and anisotropy, now
these are very classic terms which are being
used in almost all subjects. So in brief,
homogeneous materials are of uniform composition
throughout, or any properties that you are
looking which has uniform properties throughout
the material. And if that doesn't hold then
this is heterogeneous material.
Isotropic material is on the other hand, is
one which the physical properties are equal
in all directions. And if that does not happen
then it is anisotropic materials. You can
also consider it in a way that material properties
are independent of the direction in which
they are measured. We will learn more about
it with time but again I would like to remind
you the fact that this concept of homogeneity,
heterogeneity, isotropy and anisotropy are
again function of the scale.
So let us have some, have a look on some examples.
On the left side we have a photograph of a
sandstone, hand specimen of a sandstone. This
is the scale. So if it is 10 centimeters then
may be, this would be around 80 or 90 centimeters
altogether. And we see here that if we look
at the color, that is one of the physical
properties then color is mostly homogeneous.
It does not vary. The appearance is mostly
homogeneous, it does not vary.
And if we have some tools, if we can measure
some other properties like electrical conductivity
of rocks, hydraulic conductivity and so on,
then we might find that this material is very
much homogeneous. And if I make a thin section
of this little rock and then I observe it
in this scale then I figure out that it is
not at all a homogeneous material. So same
piece of rock I am looking at two different
scales, one is homogeneous, another is heterogeneous.
About this isotropy, anisotropy these two
are the photographs of two granite hand specimens.
The first one is sort of a massive granite.
You can see many different scales but statistically
if I consider this entire specimen then doesn't
matter if I am measuring a property from here
to here, that is in two different directions,
they would appear more or less same.
However in this sample if I try to measure
a property from here to here then I actually
encounter different layers. However if I measure
from here to here, the properties would remain
same because I am following the same material.
With time we will know this is known as transverse
isotropic material which is a layered material
and most of the cases our rocks are so. So
again the concept of isotropy and anisotropy
could be something that you are considering
with respect to the scale.
So we are almost at the conclusion of this
lecture. And what we learnt from this lecture
is very important when you see or when you
go to the field or when you see a photograph
of a deformed, of a rock sample, the first
question you should ask as a structural geologist,
am I looking at a deformed rock? And if yes,
then what is the scale of the structure I
am looking at? If this rock deformation or
different layers or different features that
I am looking at are homogeneous or heterogeneous,
and if this rock is isotropic and/or anisotropic?
So with this note I conclude this lecture.
And in the next lecture we will mostly learn
what are the different ways structural geologists
approach to look at deformed rocks. Thank
you very much and stay tuned.
