Lecture 21

Namaskar. So, we were talking about rocks. There are three types of rocks. So, this is our last lecture on rocks. Metamorphic rocks is our last part. We will talk about it today. So, till now we have talked about minerals and rocks. So, we started with igneous rocks. After that, we talked about sedimentary rocks in the last lectures. Now, we will see metamorphic rocks, which are formed under high temperature pressure. And all these systems that we are seeing, igneous rocks, which we talked about, are intrusive and extrusive. And these rocks are formed by coming above magma. Some remain inside the ground and some come to the surface. And due to weathering, transportation, deposition and then solidification, cementation or lithification, sedimentary rocks are formed. And then this cycle, this plate, crust, which goes inside the earth, it is subjected to high temperature pressure. So, these metamorphic rocks are particularly formed due to high temperature pressure. Now, actually, what is metamorphism? The word metamorphism is used in many places. If we say that a person's nature is completely changed or so, I mean, this is an example. So, we say that you have metamorphism in you. So, there has been a very strong change in it. And the change is due to pressure and temperature. So, it is important that metamorphism is a change in the form of that happens in earth crust, uncrusted rocks in response to change in temperature and pressure. So, these two things are very important, which change the rocks, change the property of rocks, change the hardness, reduce the hardness. So, this is a process, which again we relate to plate tectonics. The rock cycle, which we were saying that the rock cycle is linked to it. So, the rock cycle is also linked to plate tectonics. So, because we talked about metamorphism, there is a plate tectonics, which is linked to it. So, because we said that there is a plate crust, when the crust goes inside the earth, it means this abduction is happening, at the time of this abduction, it reaches the high temperature pressure. So, this deformation is due to that only, this change happens. Now, unchanged rocks, here you will see below, this is your unchanged rock. Now, zonal of regional metamorphism, we will talk about this. Regional metamorphism, when we talk about it, it happens on a very large scale. So, as we say, region, means a very large area is covered in it. So, this is regional metamorphism. Contact metamorphism is, these two, means rocks are there, or contacts are there, in the middle of it, along with it. So, this is contact metamorphism. If this is a portion of a rock, or a portion of a plate, and this is one of yours. So, for example, if we see here, what is shown here also, so this portion will give you contact metamorphism. So, when there is contact between two portions. Now, generally it is seen that, due to the deformation, your rocks, like here it is told, limestone, limestone, we have seen that there is sedimentary rock in it. Sedimentary rock, when your limestone, if we put it in high temperature, or put it under pressure, then it will start deforming. So, this deformation will happen, and what will it become? It will become slag. So, in the same way, different types of rocks will develop. So, this whole thing that you have given here, this is called sedimentary rock. And this is about ancient crystalline, means crystallized rocks. But, if we see roughly, what is made of sedimentary rocks? If we deform our shale, then it will become a slate. If we further deform the slate, then it becomes a phyllite. We will tell you something about these rocks, what is their capacity. Then, it becomes a cyst, which we call it. And it is a sandstone, which is found in more quantity. If you deform it, and this is in high temperature pressure, which process overall is metamorphism. So, if it is metamorphosis, then it will become quartzite. And the old rocks, ancient crystalline rocks, if we deform it, then it will become nice. So, this is a process, which has different rocks, which change it. So, just a few days ago, we came back from Ladakh. So, in Ladakh, we saw that most of the rocks there, because this is a zone of Ladakh, which is your suture zone, we call it. It is found around it, where two plates collided initially, 50 million years back. So, after that, all these rocks, we find igneous rocks in it. But igneous rocks have not remained igneous, they have metamorphosed, which we also call ancient crystalline. And along with that, your sedimentary rocks, because there was Tethys sea there first, and its sediments, and the rocks formed from it, the sedimentary rocks formed from it, they have also deformed and they have also changed. So, the shale was moving towards the slate, it was limestone. So, we did not find much marble there, but some rocks, which are slate and then phyllite, all these are very much in excess, we find gneiss, granitic gneiss, all these are found there. So, the point is that if you see its area, then mainly what is it, that this kind of view is seen, when you go to the Himalayas. This is a very close-up photograph, but you can see, what kind of deformation has taken place in it, this is the whole folding. This is on a very small scale, we are still seeing it, but in a large scale, you can see here, in this area also, what kind of deformation has taken place. So, this kind of deformation, high temperature pressure. So, this kind of deformation, high temperature pressure. So, this kind of deformation. So, this kind of deformation. So, this kind of deformation. So, the So, the