we are going to deal with the origin and definition of the coal okay so coal is commonly known as a fossil fuel we all are aware of that and it is sometimes also referred to as black diamond in common language because it is very important source of energy and India has huge reserves of coal. So we rely a lot on the coal deposits, coal as a fuel and right from electricity generation to several other power plants for home consumption and then many other industries, coal. is a very important source of energy. But how is coal defined?
What is coal? Okay. What is the definition of coal in geological terms?
So I hope you all are aware that this the coal it is formed from the fossilized material of plants. Now remember when I say fossilize it means I am not talking about the imprints of leaves. I am saying the plants once they die and they get buried immediately under the sediment cover they get protected from the atmospheric oxygen and bacterial activity.
So this immediate burial of these plant material the dead plant material which gets protected from decomposition it leads to the formation of coal if you look at the uh the structure of the coal i am sure you all must have seen coal you will find that if suppose that this is a coal sample normally these coals they are all stratified they have these this whole mass is composed of these several layers individual layers which are basically the dead plant material getting deposited one over the other one over the other in time and then they get subjected to appropriate pressure and temperature and then they get converted to code So basically here we are getting two things. When it is we say plant material it means that it is organic material and secondly this layers the layered structure of this coal it gives impression that they have been deposited over each other or the sedimentation has taken place. So if we combine these two information that coal is composed of organic material and this organic material has been deposited in the form of sedimentation one over the other. So we can define the coal as an organosedimentary rock.
Organo sedimentary rock. So several people you can confuse is coal a mineral or is coal a rock. So I define coal as organo sedimentary rock.
I hope you understand this. All right. So moving ahead, the coal is defined as an organosedimentary rock, which contains which contains more than 50 percent. organic matter organic matter so if 50 percent is organic matter what is the other 50 percent these are basically the sediments or the other minerals which are present in the coal now if this organic material is less than 50 percent then what it can be called as they may be referred to as organic you shales. When the shaley material is more than 50% and organic material is less than 50% then they may be referred to as organic shales not coal.
So please keep it in mind that coal is defined as an organosedimentary rock which contains more than 50% organic matter. And how is coal formed? It is the result of what? So, the coal is a product of deposition. It is also a product of preservation.
Very, very important process. And coal is a product of conversion of dead vegetal material. Conversion of...
dead vegetal or dead plant material dead plant material and what is the most important factor in converting this plant material in coal it is the first one is the considerable or appropriate amount of time if the material which is getting preserved gets exhumed soon comes on the surface of the earth very soon it will not form coal after time then pressure and temperature these three are most important factors that lead to the formation of coal from dead plant material so once the plant dies it's uh you know it's this material gets preserved it first gets buried deposited preserved and then converted to coal but for that the duration of time should be sufficient the pressure and temperature should be appropriate the pressure and temperature they are direct impact of the overburden the sediments which are overlying the deposited organic material the deposition of the because as the sedimentary cover will go on getting thicker the plant material will be under huge load and that load will cause excess pressure and temperature okay so i hope you all understand this all right now For the formation of the coal there are several stages because we all are aware that coal it is of various types so we what are the different types of coals we have uh coals the most common common classification it is lignite bituminous and anthracite or this classification is based on the amount of carbon the percentage of carbon present in the code but even how it will reach all these how it will reach these stages it all depends on the first stage of you formation and that is peat. So peat formation it is the first stage of coalification. The process of formation of coal is known as coalification. So this coalification process it is a very large and broad terminology.
The Qualification Process Now, how are these peat formed? Because this is the first stage of formation of the coal. So, it is very important to understand how the peat is formed.
So, the peats are normally deposited in swamps or marshy areas. Daldali zameen, marshy areas. Formation of peat requires anoxic environments. Very very important factor.
Anoxic environments. Anoxic means very very less or no oxygen. Anoxic absence of oxygen.
So oxygen if present will cause decomposition of the plant material. Therefore anoxic environments are a must. for the formation of the peat and anoxic environments are very commonly found in swampy regions so let me tell you the reason why most of you must be knowing about the swamp marshes which we call in hindi as dal dal or daldali zameen so in the marshy areas what happens here the water gets logged means there is no entry and exit of water it is the whatever water goes inside gets locked over there this water logging leads to very poor ventilation when i say ventilation it means that uh the co2 will be you know it will be getting released oxygen will be coming in that is called as good ventilation but Once it is poor ventilation then oxygen gets used up and then no new or no fresh oxygen comes in.
This poor ventilation leads to the anoxic environment and this anoxic environment it protects plant material from decomposition. Alright, now there are three main factors which lead to the formation of a peat swamp. Okay, formation of peat swamp. There are three main factors. First one.
evolutionary development of flora flora means plants so we have to understand in which geological time what type of plants evolved because we all are aware that the coal is entirely made up of plant material so evolutionary development of flora is the first fact factor. Second factor is climate of course very very important because climate not only helps in the growth of plants but climate also is important for the preservation of the dead organic material and the third one is third one is geographical geographical and structural position structural position of the region means where we are studying the peat swamp that area what was its geographical position what was its tectonic evolution and how did it evolve okay So these are the three main factors that help in the formation of the peat swamp. So let us look at the first factor, the evolutionary development of flora. So if you look at this geological time scale, which deals about the higher plants, how did they evolve? So we'll talk from the lower Devonian, the early Devonian.
Here, the early Devonian, it was very, you know, it is very important for the growth of submerged plants in shallow lagoons. So submerged plants over here. So these are the different types of. plants the gymnosperms and angiosperms and how did they evolve but in devonian the silophytes p-s-i-l-o-p-h-y-t-e-s the early devonian saw the growth of silophytes what are silophytes they are submerged plants submerged in water somewhere they were submerged in shallow lagoons And these xylophytes, they gave rise to thin coal layers because they did not have any hard parts of the body as such because they used to live submerged in the water. Those plants used to live submerged in the water.
Water hyacinth type. So, such plants which did not have any hard stem, they gave rise to fine or thin coal layers. Then the late early Devonian, where the first land plants evolved. So let me add a slide here and we will keep going back to the previous picture. So early Devonian, it saw the growth of xylophytes which were submerged plants in shallow lagoons shallow lagoons and these xylophytes they gave rise to thin coal layers okay then late early devonian it saw the growth of first land plants then middle and late The middle and late Devonian they saw rapid spread of the land plants.
Rapid spread of the land plants. So these land plants were the one which gave rise to the true coal seam. Okay. They gave rise to the true coal.
further now after devonian the next important time period for the formation of the coal deposits was the early carboniferous as you all are aware that this name carboniferous has been given on the basis of extensive coal deposits so now In the early Carboniferous important and economically viable coal seams were formed. mining of the coal which is done from the economically viable coal seams, they started forming during the early carboniferous. This was a very very important time period.
Now the late carboniferous, the late carboniferous time period, the late carboniferous period. It is also known as bituminous coal period. Bituminous coal period. Because by the late Carboniferous, there were growth of rich forests. Like the Lepidodendrons and Sigillaria.
Okay. They are two important trees of the late Carboniferous time period. What are their names? rich forests with trees.
Very common example you should remember these names Lepidodendrons and Sigillaria. These are two important trees of the late Carboniferous and they were the ones which gave a rise to the bituminous coal because they are richer in the carbon percentage now the permian another important period for the coals here the coals have been predominantly formed from gymnosperms and here the coal seams coal seams they were formed predominantly from gymnosperms a very common example you should remember this name chordate chordates so the permian coal seams they were the result of gymnosperm now the jurassic and early cretaceous the jurassic and early cretaceous here Again gymnosperms and those important ones were ginkgophytes and psychedophytes and conifers. Jurassic and early Cretaceous periods, they had these gymnosperms which were ginkgophytes.
then cycadophytes and then conifers the conifer trees so these were the important gymnosperms which were the principal coal forming plant communities and between the early and late cretaceous most of the coal deposits were formed by the angiosperms because this was a time period rapid development of angiosperms to place. Okay. Now, compared to the Carboniferous flora, compared to the Carboniferous flora, the swampy plants of Mesozoic and Cenozoic, they are much more diverse and more specialized.
So, what I am trying to say is that the plant community during the Carboniferous, it was not that diversified. it was though it was very uh more you know very high in abundance but the number of new species were not evolved that is why they were less diverse while during the mesozoic and the cenozoic this swampy areas they have been inhabited by more diversified plant communities because you can see here that uh how the land plants they rapidly spread then the forests were formed with trees and then further gymnosperms the complex gymnosperms and then angiosperms they got evolved so it is very obvious that the carboniferous flora was less diverse than the cyanozoic flora okay so if you see over here you can yourself see in this diagram that during the from devonian to permian you the land plant communities were slowly evolving and then post-permean you can see that there was rapid spread and development of these plants of course they gave rise to the complex gymnosperms. So if you see here carefully in the Jurassic you can see the Ginkgo Psyda, Cycopsida all these were the rapidly growing plant communities. In Cretaceous you can see this Coniferopsida or Conifers, pine trees, Pinopsida.
So this tells us that they were the important plant forming communities during the Mesozoic. Why during the Cenozoic? Because you can see here from early to late Cretaceous, early to late Cretaceous, between this time period, there was a rapid evolution of angiosperms.
And during the Cenozoic, angiosperms became the most important plant communities. Okay. So this is how with the geological time, the coal seams started getting thicker. They became... true coal seams from the thin coaly layers okay so uh this is another uh important thing which you should know that the true coal seam formation took place only after the middle and late devonian when the plants they spread over the land the continents rapidly because it was that time period when the plant since the plants came on the land from getting submerged in the water they started getting thick stem thick trunks okay the woody tissues started coming in and that is why the true coal seam formation started taking place after Devonian but the Devonian coal seams they do not have any economic value you should keep this in mind so why did this happen during the Devonian The Devonian period was a time of great tectonic activity wherein the Gondwana and the Eumerica, Europe and America, they came together.
The continent Laurasia, it was created in the early Devonian by the collision of Laurentia and Baltica and then slowly the convergence of these two land masses were together so this was also the time period in the tropic of capricorn where now this hadley and ferrel cell where they are meeting today so this area was you know having lot of droughts and it was characterized by oxidized iron hematite deposits sea levels were high worldwide And much of the land lay under the shallow sea. So the land wasn't even available for the plants. That is why the land plants started coming only after the middle Devonian. So all these tectonic activity and the paleogeography, it played an important role in the development of the land plants and subsequently the coal seams. Now in the Carboniferous if you see how these land plants they started to grow.
So you can see very important plant communities in the Carboniferous. And a lot of them, a lot of them suddenly started to evolve. And that is why the coal formation of the Carboniferous led to the name Carboniferous period. As I told you that the upper Carboniferous is known as the bituminous coal period so if you see over here these trees lepidodendrons and sigillaria they were the major plant types the trees which could grow almost 30 meters in height and if you see this picture over here what the seed the coal seam which you are seeing over here you know this this coal seam this is the trunk of this tree you a whole tree trunk got preserved and this trunk is the the trunk which was found in that coal seam it was seven meter in height the entire trunk was seven meters so you can imagine that how this important trees which were of the late carboniferous period they led to the formation of the important coal seam that is why it is called as the bituminous coal period now This is the chordates. It is a gymnosperm.
and the permanent cold deposits have been formed from this gymnosperm cordage. The trunk of this tree of this gymnosperm was very important source for the formation of the polyphorus seams. It had lot of woody tissues in it. In the Mesozoic, the beads started forming from the angiosperm flora. angiosperm for us because post uh drastic and jurassic you can yourself see that how these angiosperms they simply radiated they suddenly started to evolve rapidly