IITK NPTEL Structural Geology_Lecture 02: Introduction II [Prof. Santanu Misra]

Summary

This NPTEL Structural Geology course lecture delves into the methodologies of structural geologists, who interpret rock structures through field observation, laboratory experiments, and analytical modeling. It emphasizes the importance of scale in geological structures, the challenges of reconstructing non-reversible processes, and the significance of geometric, kinematic, and mechanical models in understanding rock deformations. The lecture also highlights the interdisciplinary nature of structural geology and the value of fieldwork, concluding with a look forward to studying structural elements in future sessions.

Takeaways

• 🔍 A structural geologist interprets rock structures by observing deformed rocks and explaining their present state through scientific observations and evidences.
• 🌋 The main job of a structural geologist includes dealing with complex interactions of natural elements, understanding large-scale geologic structures, and interpreting non-repeatable or non-reversible processes.
• 🧩 Geological reconstruction is a challenging task for structural geologists due to the limitations in time scales, missing data, and multiple overlapping deformations.
• 📏 There are three primary ways a structural geologist analyzes structures: geometric models, kinematic models, and mechanical models, which are considered direct methods of observation.
• 🔄 Kinematic models are an advancement from geometric models, focusing on the relative displacements and motions within a structure.
• ⚙️ Mechanical models, also known as dynamic analysis, are the most advanced and involve the interaction of material properties and forces, often studied through laboratory experiments.
• 🧪 Laboratory experiments, such as sandbox experiments and real rock deformation experiments, help structural geologists mimic and understand geological processes that occur over long time scales.
• 🌐 Geometric models are essential not only for understanding rock structures but also for practical applications like hydrocarbon reservoir exploration.
• 🏔️ The Himalayas are an example of a large-scale geological structure where kinematic models help understand the relative movements of tectonic plates and the formation of thrust faults.
• 📉 Dynamic models, including analogue and real rock deformation experiments, provide insights into the evolution of geological processes and the development of structures like folds and faults.

Q & A

What are the primary methods a structural geologist uses to interpret rock structures?

A structural geologist primarily uses field observation, laboratory experiments, and analytical and numerical modeling to interpret rock structures.

How do field observations, laboratory experiments, and analytical modeling complement each other in structural geology?

Field observations provide real-world data, laboratory experiments allow for controlled testing of hypotheses, and analytical modeling offers a way to simulate and understand complex geological processes that are not observable directly.

What is the significance of scale in structural geology?

Scale is crucial in structural geology as it influences the interpretation of structures. It helps in understanding the relative size and extent of geological features and processes, which can range from microscopic to continental.

How does a structural geologist approach the study of deformed rocks?

A structural geologist studies deformed rocks by observing their structures, understanding the processes that led to deformation, and using scientific evidence to explain their current state.

What is a geometric model in the context of structural geology?

A geometric model in structural geology is a qualitative or quantitative representation of the 2D or 3D orientation and arrangement of rock structures, which helps in understanding the deformation without involving material properties or forces.

Can you explain the concept of kinematic models in structural geology?

Kinematic models in structural geology focus on the relative movements and displacements within a geological structure. They help in understanding the direction and sense of movement without considering the forces or material properties involved.

What role do mechanical models play in structural geology?

Mechanical models, also known as dynamic models, involve the interaction of material properties and forces to understand the deformation processes. They are highly quantitative and often involve laboratory experiments or numerical simulations.

How are sandbox experiments used in structural geology?

Sandbox experiments are scaled-down models of natural processes used to simulate geological deformations like folding and faulting. They help in understanding the dynamics of mountain building and the evolution of geological structures.

What is the importance of real rock deformation experiments in structural geology?

Real rock deformation experiments, conducted under high pressure and temperature, mimic the conditions under which most geological deformations occur. They provide insights into the microstructural changes and the development of geological features.

How do structural geologists validate their interpretations and models?

Structural geologists validate their interpretations and models by comparing them with field observations and natural outcrops. This ensures that the models accurately represent the complexities and processes observed in nature.

What is the significance of understanding the non-repeatable or non-reversible nature of geological processes?

Understanding the non-repeatable or non-reversible nature of geological processes is significant because it emphasizes the importance of studying the end products of these processes. It also highlights the challenge of reconstructing past geological events and the need for careful interpretation of the available evidence.

Outlines

00:00

🔬 Introduction to Structural Geology

The speaker begins by welcoming the audience to the second lecture of the NPTEL Structural Geology course. They recap the previous lecture's discussion on how to examine rocks, focusing on whether they are deformed, homogeneous, heterogeneous, isotropic, anisotropic, continuous, or discontinuous, and the importance of scale. The lecture aims to explain the methodologies of a structural geologist, who observes deformed rocks and uses scientific evidence to explain their current state. The geologist's approaches are categorized into field observation, laboratory experiments (including rock deformation and analogue experiments), and analytical and numerical modeling. These tools, though limited, complement each other, allowing for a comprehensive understanding of rock structures. The lecture also touches on the challenges of geological reconstruction, the non-repeatable nature of geological processes, and the vast spatial and temporal scales involved.

05:01

🌋 The Role of Structural Geologists

This section delves into the role of structural geologists, who interpret and define structures based on scientific data. They deal with complex interactions of natural elements and the vast scales of geological structures, which can span millions to billions of years. The speaker emphasizes the challenge of interpreting these structures given their large scale in space and time, and the fact that most geological processes are non-repeatable or non-reversible. The lecture introduces three basic ways to observe structures: geometric models, kinematic models, and mechanical models. These are considered direct methods of observation. Geometric models are qualitative or sometimes quantitative, focusing on the 2D or 3D representation of structures. Kinematic models build on geometric models to understand the different motions within a structure. Mechanical models, the most advanced, involve the interaction of material properties and forces. The lecture also mentions indirect methods, where hypotheses about geological features are tested through models and experiments.

10:04

📏 Geometric Models in Structural Geology

The speaker explains geometric models, which are fundamental to structural geology. These models involve tracing layers in rocks to understand their deformation and continuity. Examples are given, including a fold with discontinuities and a syn-sedimentary deformation affected by faults. The speaker demonstrates how geometric models are constructed through photographs and field data, and how they can be applied at various scales, from small folds to large seismic reflection images of subduction zones. The importance of geometric models in understanding structures and aiding in fields like hydrocarbon exploration is highlighted.

15:07

🔄 Kinematic Models and Shear Sense Indicators

Kinematic models are discussed as a step beyond geometric models, focusing on the relative displacements and motions within structures. The speaker describes classic structures like delta structures and bookshelf structures, which are indicative of shear sense in structural geology. These models help in understanding the movement and interaction of different parts of a geological structure. The lecture also covers the application of kinematic models on a large scale, such as in the understanding of plate tectonics and the formation of the Himalayas.

20:07

🧪 Dynamic Models and Experiments in Structural Geology

This section introduces dynamic models, which involve the study of material properties and forces in geological processes. The speaker describes sandbox experiments that mimic the deformation of sedimentary layers, such as those in the Himalayas. The lecture shows a video of a sandbox experiment, demonstrating the formation of thrusts and the evolution of geological structures. The speaker also discusses analogue experiments with putty or plasticine layers to mimic folding processes and real rock deformation experiments under high pressure and temperature to study microstructures. The importance of validating interpretations from experiments with natural observations is emphasized.

25:09

📚 Conclusion and Future Topics

The speaker concludes the lecture by reviewing the key points discussed, emphasizing that structural geology is an interdisciplinary and quantitative science. The importance of field observation, understanding scale, and validating experimental models with natural data is highlighted. The speaker also previews the next lecture, which will focus on measuring structural elements such as lines and planes and their relationships.

Keywords

💡Structural Geology

Structural Geology is the study of the physical deformation of rocks and the processes that cause these deformations. It is a highly interdisciplinary subject that combines field observation, laboratory experiments, and analytical and numerical modeling to understand the forces that shape the Earth's crust. In the video, the lecturer emphasizes the importance of structural geology in interpreting the history and current state of rock formations, such as the Himalayas, and in geological reconstruction.

💡Deformed Rocks

Deformed rocks refer to rocks that have undergone changes in shape, size, or position due to geological processes such as folding, faulting, or shearing. The script discusses how structural geologists observe and interpret these deformations to understand the geological history and the forces that have acted upon the rocks. For instance, the lecturer describes how the structural geologist's job is to explain 'why and how they ended up at their present state'.

💡Homogeneous vs Heterogeneous

In the context of the video, homogeneous refers to rocks that have a uniform composition throughout, while heterogeneous rocks have varying compositions. These terms are important for understanding the physical properties and the way rocks respond to geological forces. The script mentions these terms as part of the initial questions a geologist should ask when examining a rock, indicating their relevance in structural geology.

💡Isotropic vs Anisotropic

Isotropic materials have uniform properties in all directions, whereas anisotropic materials exhibit direction-dependent properties. In structural geology, understanding whether a rock is isotropic or anisotropic is crucial for predicting its behavior under stress. The script introduces these concepts as part of the fundamental questions to ask when analyzing rock structures.

💡Geometric Models

Geometric models in structural geology are simplified representations of rock structures that help to visualize and interpret their geometry and spatial relationships. The script explains that these models are qualitative or sometimes quantitative and are used to understand the 2D or 3D representation of rock structures. An example from the script is the color-coding of layers in a fold to trace their continuity and deformation.

💡Kinematic Models

Kinematic models focus on the movements and relative displacements within a rock structure. They help to understand the direction and sense of deformation without considering the forces or material properties involved. The script describes kinematic models as the next stage beyond geometric models, where the structural geologist tries to understand the different motions at various parts of the structure.

💡Mechanical Models

Mechanical models, also known as dynamic models, involve the interaction of material properties and forces to explain the deformation of rocks. They are highly quantitative and often involve laboratory experiments or numerical simulations. The script mentions mechanical models as the most advanced among the three direct methods of structural geology study, providing insights into the forces and material behaviors that lead to rock deformation.

💡Field Observation

Field observation is the first step in structural geology, where geologists directly examine rocks and geological structures in their natural setting. The script highlights the importance of field observation as a basic tool for structural geologists, allowing them to gather primary data and observe rock structures in context. It is complementary to laboratory experiments and modeling.

💡Laboratory Experiments

Laboratory experiments in structural geology involve recreating geological processes under controlled conditions to study rock deformation and behavior. The script mentions real rock deformation experiments and analogue experiments as ways to understand processes that are not observable in the field, such as the slow rate of deformation in the Earth's crust.

💡Analytical and Numerical Modelling

Analytical and numerical modeling involves using mathematical and computational methods to simulate geological processes and structures. The script describes these models as tools that help structural geologists interpret complex geological phenomena that are not directly observable, such as mantle convection or plume dynamics.

💡Geologic Reconstruction

Geologic reconstruction is the process of piecing together the geological history of an area based on the evidence available. The script discusses the challenges of geologic reconstruction, such as the non-repeatable nature of geological processes and the incomplete record of past events, which require structural geologists to solve a 'series of jig-saw puzzles' to understand the past.

Highlights

Introduction to the methods of observing and interpreting rock structures in Structural Geology.

The importance of scale in understanding rock structures and the challenges it presents.

The role of a structural geologist in interpreting deformed rocks and their processes.

Categorization of approaches in structural geology: field observation, laboratory experiments, and analytical and numerical modeling.

The limitations and complementarity of field, laboratory, and analytical tools in structural geology.

Geological reconstruction as a challenge due to the non-repeatable nature of geological processes.

The significance of geometric models in structural geology for qualitative and quantitative analysis.

Kinematic models as an advancement over geometric models to understand relative displacements and motions.

Mechanical models, the most advanced, involving material properties and forces for dynamic analysis.

The application of geometric models in various fields such as seismology and hydrocarbon reservoir exploration.

The use of sandbox experiments to mimic natural processes and understand mountain building.

Real rock deformation experiments at high pressure and temperature to simulate natural conditions.

The evolution of structural geology from a descriptive to a quantitative science.

The necessity of field geology for structural geologists to observe and interpret rock features.

The validation of experimental and numerical model interpretations with natural observations.

Upcoming topics on different structural elements and their measurements in the next lecture.

Transcripts

Hello everyone. Welcome to this NPTEL  Structural Geology course. We are at our  

lecture number 2 and we will continue  today with this introduction.  

So where we stopped at our last lecture that  what are the different ways to look at the rocks,  

particularly, when you see a piece of rock or  a photograph the questions you should ask were,  

whether you are looking at a deformed rock,  whether it is homogeneous, heterogeneous,  

isotropic, anisotropic, whether the  structure is continuous or discontinuous  

and most importantly what is the scale? Now in this lecture we will mostly cover what  

are the different ways to observe and interpret  a rock structure? The question we would ask  

mostly today is what does a structural  geologist do or what are the different  

processes that a structural geologist follows  to interpret a rock structure? To answer this  

question what does the structural geologist do,  is essentially to observe deformed rocks in very,  

very intelligent way and explain with some  scientific observations and evidences why and  

how they ended up at their present state. And to do so, a structural geologist has many  

approaches. And one can categorize it in  three different sections. The first one  

is field observation, second one laboratory  experiments, it can be real rock deformation  

experiment or analogue experiment and of  course analog, and the third one is the  

analytical and numerical modelling. Now these three basic tools have their  

own limitations. We will learn it later but  at the same time these three tools are also  

complimentary to each other, that what you  cannot observe or understand from the field,  

you can understand if you perform an experiment in  the laboratory. So to continue this question again  

that the main job of the structural geologist is  to interpret and define on the basis of scientific  

data and at the same time a structural geologist  also does geologic reconstruction.  

So, in the first part. to interpret and  define structures, a structural geologist  

has to deal with very complex interaction  of natural elements. We will look at it,  

all these processes soon. Then most of the  geologic structures as you can imagine,  

these are extremely large in their extent.  For example, you can think of Himalaya or  

the complexity of South India and so on. And  these structures have formed with time scales  

that ranges from million to billion years.  And few processes are still happening.  

And with our human life span, we  cannot see it happening, right? So,  

we see the end product and almost fossilized. A  structural geologist has a challenge to interpret  

this large scale in space and time. And one more  important thing that structural geology has,  

which is somehow advantageous to us is that  most of the processes that happen in nature  

are non-repeatable or non-reversible in nature.  If something has happened then you cannot go  

back with time, in most of the cases. Geological reconstruction is another type  

of challenges because it has several limitations  and structural geologists have to solve a series  

of jig-saw puzzles. For example, as I was talking  about the, that it takes a lot of time to produce  

a structure. The rate of deformation in earth is  extremely slow in almost every case. There are  

of course few deviations of this. For example, if  you have an earthquake, or if you have an impact  

crater formation so that, these, there are few  things that happen very quickly but most of the  

other cases, the rate of deformation in earth  is extremely slow and it is almost impossible  

to track through a human life. And then you do  not have any initial picture. That how it was,  

how it looked before? You don't have the entire  picture as well, because some pieces are missing.  

You also cannot access all the areas, even if  you can access, maybe there is no exposure.  

And on the top of that, the deformation that  we look at on the surface, it is not a single  

deformation. That means one deformation has  happened sometime ago and then later some  

other deformation which is occurring from other  directions or in a completely different nature,  

it is either demolishing or overprinting the  previous deformation. So this is a challenge  

of a structural geologist to reconstruct and  interpret, separate out these deformations and  

what has happened at the first deformation,  what has happened in the second deformation  

and so on. So at one point, at one time this  is really challenging, sometimes frustrating  

for a structural geologist, but at the same  time it is extremely fun. And this is why  

structural geologist, most of the structural  geologists like their job very much.  

So there are three basic ways a structural  geologist looks at the structures in earth,  

or in nature. So these are geometric models,  kinematic models and mechanical models and you  

can classify them together as direct methods of  observations. To understand that geometric models,  

these are mostly qualitative and sometimes  quantitative. It does not involve any other  

detailed scientific, what it does, it simply  tries to interpret from what we are observing,  

its 2D or 3D representation, its orientation, how  it is, in a more comprehensive way. We will look  

at some examples later and these are  mostly done using some data obtained  

from field studies or some experiments. Kinematic model on the other hand is the next  

stage of the geometrical model where a structural  geologist after constructing the geometric models  

try to understand what are the different motions  at different parts of the structure? That which  

part has moved which way relative to the other  parts. That helps you in the next stage to  

understand the deformation, displacement,  motion of things. You can imagine plate  

tectonics which actually deals with the motion  of the plates, is a kind of kinematic model.  

The mechanical model which is the most developed  or the most advanced model among these three,  

or we call it dynamic analysis  which is highly quantitative;  

this model involves interaction of material  properties and forces and so on. So, these  

are mostly done by performing some experiments,  reconstructing the features in the laboratory and  

so on. These three geometric models, kinematic  models and mechanical models are very important  

direct methods of structural geology study. Then there is another method. We call it indirect  

method. In indirect method a structural geologist  may hypothesize a feature that Himalaya has formed  

by this, this and this and then he or she tries  to understand it by performing some models,  

experiments or numerical models. The structural  geologist collects data from the field and then  

either prove this hypothesis or disapprove this  hypothesis. So, this is some sort of models or  

ideas that most of the structural geologists  nowadays do and these are done mostly for  

the processes that we do not see. For example, we do not see the plumes,  

we do not see the mantle convections, so  there are many hypothesis and models by  

which a structural geologist or tectonics  expert or a geo-dynamic expert, they try  

to work on some indirect methods and these are  known as analytical models. In the following  

slides we will mostly focus on these geometric,  kinematic and mechanical models and I will try  

to give you a very basic idea that what are these,  how these are done and most importantly how these  

are being applied to the real field. Okay so  let us start with the geometric model.  

If you remember in the last lecture, I talked  about fold and this is a fold. So what we see  

here? These light yellow and dark brown alternate  layers, these are folded, and the thickness of  

these layers did vary during the deposition and  it remained intact while it got folded. Now if  

it was a simple fold then no problem, we can  simply trace one layer and figure out how did  

this fold happen, which layer is what. But this structure is somehow discontinued  

with some slips, for example this is one.  We can see from this relative displacements  

that it moved from one to another. But this  is part of the kinematic model which we are  

not looking at. But the challenge is to trace,  that if I look at this layer, this brown layer  

here then where it is in the other side. In  this case, it is simple, it comes here.  

Now similarly if I look at this brown layer  and again, I have a discontinuity here then  

my challenge is to find out whether it is this one  or the other one. Now whatever interpretation you  

make out of this, that whether this is this  layer or the other layer, you have to come  

up with some sort of logics and this logic  is the fundamentals of geometric model.  

For example, with this model, with this  photograph I prepared this geometric model.  

I did not cover all these layers but few of the  layers are color-coded and they are represented  

in this way. So this is a very simple example  how geometric models in structural geology are  

constructed. Can we apply it to a little  large scale? The answer is yes.  

Here we have one syn-sedimentary deformation  and you can see that this alternate black,  

white and few light brown layers got disturbed by  a series of faults. To understand it better what  

I try to do? I tried; I made a geometric model of  it which looks like this. And here you can better  

understand what are the different geometries of  the layers, which layer moved where, which layer  

is continued to where and so on. Now talking about the large scale,  

this is a seismic reflection image of a subduction  plate, a plate which is subducting under the sea  

floor. Now looking at it, apparently it looks  very homogeneous with some sort of topography  

and some other little features. Now creating  a geometric model of this reflection seismic  

data is a daily routine of a seismologist  and a structural geologist helps in this  

process significantly. So if someone who is  an expert of constructing geometric models  

apparently which is look extremely homogeneous  and most likely there is nothing but this person  

would interpret this structure in this way. So what you can see, it has a basal decollement  

and then series of thrusts are imagined from  this basal decollement with the reflection data  

and some velocity calculations, one can also  find if there are some different lithologies  

and so on. Again I repeat, this model does  not include any of the material parameters.  

I had some data and with this data I had to  interpret what has happened there. That is it.  

This is the geometric model. Can it be applied  in other fields? The answer is yes.  

What we see here, this is integration  of different sets of data. This is a  

computer-generated image. It involves millions and  billions of data. This is hydrocarbon reservoir  

somewhere in the North Sea. I took it from the  internet so dynamic graphics is the website,  

you can go and check. What we see here with this  geometric model, these red patches that we see  

at different areas, these are oil-rich areas. So once the geometric model comes up properly with  

the interpretation of different geophysical,  structural and other petrological data it is  

possible for an oil engineer to design the wells  and then it can actually intrude different parts  

of this, hydrocarbon reservoir and extractor.  So geometric model is essentially important  

not only for understanding the structures  but at the same time, after understanding  

the structures one can also explore simply by  constructing a perfect geometric model.  

Let's look at the kinematic models. As I said  kinematic models are one step advanced from the  

geometric models because after constructing  the geometry, the kinematic model includes  

the relative displacements, motions of the  observation we are making at. Now what we  

see here at the top photograph is a very  classic structure that we commonly observe  

in ductile shear zone. We will learn about it  later. This is known as delta structure.  

Now looking at it, if we try to construct this  geometric model without all these arrows and  

so on, this is how it should look like. Now it  is possible to look at this structure and with  

some other observations that this little ball  which is inside, maybe it is rotating this way,  

and therefore you have some sort of tail-like  features at the both ends. You can also construct  

this foliations or little layers outside  that concave embayment that also proves  

that while this little ball is rotating,  it was also dragging the other layers.  

Now again this model doesn't consider what is  this material of this ball, what is the material  

outside, and so on. It simply constructs first the  geometric model and then it considers that what  

was the relative motion and this is very important  as shear sense indicators in structural geology.  

There are many other shear sense indicators but  this is one of the most important ones.  

Now here is another one. This is microstructure  observed under microscope. You can see the scale  

here this, this length here is about 500  microns. What we see here, a mineral grain  

is broken along this lens and then there are  some sense of steps. Now these steps indicate  

that one grain has moved past another grain.  And just looking at this we can construct that  

this grain has moved down relative to this  and similarly others. Now this is a classic  

structure of structural geology in ductile shear  zone as well, in brittle ductile shear zone and  

this is known as bookshelf structure. So this is  how it happens. So if you have a series of books  

you tilt the books then they rotate past each  other. And this is how they slip individually.  

So this is a kinematic model again. This is a field photograph and looking at it,  

I color-coded the two layers with green and  orange and this displacement plane that has  

happened. So you can quickly, just looking at this  geometric model you can say that this entire block  

has moved this direction relative to the other  block. So once you interpret this, this you are  

actually working on a kinematic model. Now in large scale as I said, you know in the  

past lectures the tectonic model, the plate  tectonics is some kind of a kinematic model. Why?  

Because it doesn't involve the individual material  properties of the plates, what are their different  

properties and so on. It simply constructs  that how one plate moved relative to other,  

what was the mutual interaction between them  and so on. So this is how Himalaya is conceived,  

the development of Himalaya. So on the south side you can see,  

this is south, you have this Indian plate and then  here you have Tibetan plate and then Indian plate  

moved with time and then we have formed series  of thrust faults and then this is the present  

day configuration of Himalaya where we have  lot of faults, major faults are known as MCT,  

MBT, MFT, STD and so on and there is also  certainly a basal decollement which is MHT,  

Main Himalayan Thrust. Now this is a kinematic  model. It does not involve any sort of forces. It  

does not involve any sort of material properties,  the dynamics of these plates and so on. But if  

we have to involve that then actually we are  approaching to develop a dynamic model.  

Let us have a look. So this is again the  cross-section of Himalaya and what we see  

here that, in this side you have India, the  southern part, so here you have India and here  

you have Tibet. So in-between you had this  Tethyan sediments that got squeezed out from  

series of crusts, from series of thrusts and we  have this present configuration of Himalaya. Now  

to understand this entire process how did  it happen, one can perform a dynamic model  

by conducting sandbox experiment. So a sandbox experiment is a very simple  

experiment which is scaled down from the nature.  So this is a typical sandbox. What it does that,  

in this space you actually add series of layers  of sands. You add different colors to distinguish  

the different layers which represent or which  are analogous to the sedimentary layers and  

then you drag this belt so that these sand  bodies, entire sand bodies move from one  

side to another side and this is a block which  is analogous to the rigid Tibetan plate where  

the entire Tethyan sediments are hitting and we  would like to see what is happening here.  

Now what you see here, that we are involving  the materials. We would like to see what is  

happening with them. So this is a dynamic  model. We are involving some sort of  

forces as well. So let us have a look of  this movie and see how it works.  

So this alternate yellowish dark blue, then again  yellowish green, yellowish and the dark blue,  

these are different sand layers representing or  analogous to the sediments, sedimentary layers  

and you see deformation has already started.  The belt started moving from this side.  

What we see here with more displacements a  series of thrust folds are being formed. So  

these type of models not only discuss the  dynamics of the processes but it also tells  

you the evolution of the process, that how  from one setup, one geologic setup you actually  

achieved a different geologic setup just by  adding some sort of deformation.  

So this is a quick view of this entire experiment.  You can see series of thrusts being formed. They  

formed at different angles initially and while  they rotated they achieved a different angle.  

It also achieved some sort of elevation and  these are the very basic or key ideas of  

mountain building processes and one can observe  all these processes using a dynamic model.  

Now if we try to look at to another  geological process which is folding  

and we try to understand how one fold  interferes with another fold in nature,  

then one can perform the analogue experiments.  So here is an example that we have performed  

in the laboratory. What we see in Stage 1, we have  prepared one fold, one set of folds here where you  

can see the folds are here and these are the hinge  lines of the fold marked by the black lines.  

Now the folds are at an angle and then if  I compress it, this is what we have done,  

then we see some tiny folds are coming up. And  these experiments were performed to understand  

whether the new folds are generating are as  big as the previous one or they are small of  

the previous one or even larger than the previous  one and we figured out that no, these are small,  

at least these are very small in this case. But this is a dynamic model. We tried to,  

why? Because we tried to see, we tried to mimic  the layers of the geological features like the  

layers when we form folds. These are ductile so  we used some putty or plasticine layers which  

are ductile in nature. The scales and other things  are also maintained with respect to the geological  

scales. So this is again a dynamic model. Now, one can also do real rock deformation  

experiments at high pressure and temperature  because most of the deformation processes  

in nature, that happen at depth and there we  have pressure and temperature. So here is an  

example. A series of experiments performed  by one of my colleagues OK, in ETH Zurich  

with his famous Color marble experiments. So the first image what we see here is an intact,  

un-deformed color marble. You can  see the grain size is in equilibrium,  

more or less similar grain size. It has nice  triple junctions and so on. And then OK,  

sheared this at high pressure and temperature  and you can see, if this is the shear direction,  

he applied and with evolution of the strain, this  is shear strain 1, this is shear strain 2 and this  

is shear strain 11, you see first what you observe  is that the initial grain size of Color marble  

has reduced to a very fine grain size rock. And with time you can also see that here we are  

trying to develop some sort of directional layers,  okay in the rock and this is exactly what happens  

in nature and in experiments you can produce that  with real rocks. So this is a dynamic model.  

And based on that, with the stress-strain curve  one can actually understand that at which stage  

of this deformation what kind of features  you see in your microstructure.  

So we are at the end of this lecture so  some review comments from this introduction  

section that we had in this lecture and  the previous lecture. So structural geology  

is highly inter-disciplinary subject as  you have understood and it was initially  

little bit of descriptive subject but now  it is very much quantitative science.  

The key skill of a structural geologist is  essentially how you observe a particular  

feature in a piece of rock, or in an outcrop  and then based on your observation you try to  

answer these questions why, how and when.  One of the most important conclusions that  

we would like to make from this introductory  lecture that it is the rock that contains the  

information. You have to go and see the rock  in the field. So therefore field geology is  

very important for structural geologists.  The scale is one of the very important  

parameter to study structural geology. Whatever interpretation you do from a structure,  

it is important that you remember what is the  scale. Experiments, numerical models etc have  

their own limitations but they are excellent to  mimic the complexity of the nature and they also  

complement each other. Whatever interpretation you  do out of your experiments or numerical models,  

it is important that you validate them with  the nature. So I conclude this lecture.  

And in the next lecture we will start a new  topic that what are the different structural  

elements that we measured lines, planes and their  mutual relationships. Thank you and stay tuned.