Lecture 05

So, in the last lecture, we talked about geodynamics and we discussed how magnetic field is maintained. So, in the last lecture, we talked about geodynamics and we discussed how magnetic field is maintained and we discussed how magnetic field is maintained and we discussed how magnetic field is maintained and we discussed how magnetic field is maintained So, reversal in Earth's magnetic field are apparently random occurring on an average every few thousand years The change in polarity of Earth's magnetic field takes a few thousand years to occur in a very short time So, basically when water freezes, it expands to become a lower density. Thus, 90 percent of iceberg will be underwater. Similarly, a large mountain range, approximately 84 percent will be as part of the deep route. So, continents float high because they are made up of rocks of lower density. Then rocks of mantle or oceanic crust. So, if we take a crust and say it is a continental crust. So, horizontal compression forces cause a shortening and thickening of crust. So, horizontal compression forces cause a shortening and thickening of crust. So, horizontal compression forces So, this is a very good example and we can also talk about this in the Himalayas, because the height of our Himalayas is always increasing, it is not the same, because our plate movement is happening here, this is our Tibetan plate, for example, and this is our Indian plate. So, this pressure is being created and this is going up slowly, so when it goes up, then it will come down to the same depth, so these things are very important for us and it can also be the opposite, if we say that the height is increasing, then its depth will also increase here, so the root will get thicker, but if we assume that we are reducing its height, so this will also decrease, but it will take a lot of time to happen, it will take thousands and lakhs of years to happen, now what is the importance of this, so whenever it happens, it is decided that because our compression is happening, it is happening continuously, and as I said that the Indian plate and the Tibetan plate are colliding with each other, now we will talk about this later, so the height is increasing slowly, and that is why it is said that the height of the Everest does not remain the same, it increases day by day, it is increasing day by day, so we will see a little more about this, what will be its effect, so whenever this happens, it will try to come in equilibrium, so if we assume that we have eroded it, we have chopped it out from here, so it will go up, so let us look at it further, and we call it isostatic adjustment, because it will try to come in equilibrium again, and then it will be stable, so this is a very good example, this is shown in the first one, you have these mountains made here, and according to this your depth will be the same, and the depth will be in the same amount, as we were saying that it will be in the range of 20% to 80%, but assume that it has eroded, as I said that it has eroded from the top, erosion can happen due to many reasons, heavy rain, the season has become like this, the climate has come such that it is raining excessively, so it will start eroding it, so when it eroded, its height reduced, so what happened when the height reduced, it started to uplift, so this will create an isostatic balance, the higher you take it, the more depth it will have, the less you reduce, the less depth it will have, and if this erosion continues, then what will happen, the time will come that your whole area will become flat, and these lines are shown, as it was here earlier, so it was here, then the second time it eroded and came here, so the second one went here, the third one when it came here, so it came here, so basically what happened to you, it reduced here, its height reduced, it started to rise here, so this is called isostatic adjustment. So, as weight is removed from the top by erosion, the crust rebounds upward, it rebounds, because we were talking about the buoyancy effect, due to buoyancy effect, it will come up, where it was floating, but along with this, there is one more thing, which is there, that we did erosion here, so it came up and uplifted, but if we are increasing the weight by deposition, we are putting more material in it, then what will happen, it will sink, so we have to keep this thing in mind. So, here it is said that as weight is removed from the top by erosion, we are talking about deposition, so it rebounds upward. The process of erosion and isostatic uplifting will continue until mountain block reaches normal thickness. So, it will continue, until it comes to its equilibrium state. Eroded sediments cause adjacent area subside. Now, erosion happened, material went from here to the ocean, from here to the ocean, or it came to the land, so this area will subside, it will sink. So, it will happen in both ways. You will reduce the material, it will come up, you will increase the material, it will go down. The opposite is going to happen, it will come up, you will decrease the material, it will go down. So, you will have to keep it in mind that in this process, you are reducing the material, you are reducing the material. So, it will come to the ocean, you will reduce So, in this lecture, we have discussed about two things, one is isostasy and other is geodynamics. We have also discussed about interior of earth, so we have discussed about these two things. So, in this lecture, we have discussed about two things, one is isostasy and other is geodynamics.