hello everyone welcome to the jio ecologist I am Dr Krishnan and you have been watching my videos on geography and several other topics of environment so in this session we are going to learn about a very interesting Theory that's called thermal convection current theory now why are we going to learn about this because this theory is one of the important pivots in geographical history that changed the course of thinking of the geomorphologists and earth scientists because of its important things that were related to the internal structure of the earth and how it is associated to the landforms importantly mountain building so before this theory propounded in 1928-29 by sarath Holmes there were lots of Scholars right from 1850s onwards who talked about various Concepts that could actually tell us about the formation of various landforms on earth right from the theories of isostasy that we have learned in the previous lectures as well right from the concept of Airy rat and several others also important only we talked about lithium greens tetrahedral hypothesis then division model of cycle of erosion and also importantly Taylor and Wegener's hypothesis of continental drift up till 1915 but remember before this conventional current theory came to being other theories were almost criticized for not being aptly relevant for the explanation of the origin of mountains and volcanoes on Earth's surface so convectional current theory gave the best scientific understanding which was different from the previous theories for example before this the latest Theory which was acceptable in larger scientific Community was wagoner's Theory but remember Wegener's theory was highly criticized because he gave this Theory but could not explain it the reasons were supposed to be related to Tidal hypothesis or it was related to pole fleeing Force which was criticized so conventional current theory of Sir Arthur Holmes becomes one of the most important theory in understanding this mechanism of isostatic adjustment as well as at the same time also understanding about the formation of various Mountain chains across the globe and also later on when in 1960s Harry has propounded the theory of seafloor spreading and later on Morgan and pitcher duso Wilson all these Scholars came for plate tectonics remember that was the time when this Theory got its validation again right so that's why we must understand convection current theory so now before we go ahead don't forget to subscribe to our Channel and also share the videos with others as well so after this music we'll be right on the theme [Music] thank you so now let's learn about thermal convection current theory by Sir Arthur Holmes now why is it important because Arthur Holmes was the first Earth scientist to grasp the mechanical and thermal implications of mantle convection now remember the relationship between the crust and the mantle was very much important right from 1850s from the concept of Aryan Pratt has been have already learned in the previous lectures as well right so what happened just before this was that Alfred Wagner proposed finally the concept that we know by the name of continental drift theory but what happened he could not explain it and it was highly criticized because he said that these movement of continents were actually triggered by tidal forces it means gravitational pull of sun and moon but remember these large chunks or large building blocks of the earth that is Crystal lithospheric blocks could not be moved right by just tidal Force so it was not accepted as as well as this full fleeing force was also highly criticized so there was a void in the knowledge in terms of the exact knowledge that how these mountains are created how volcanism happens so that is the time when Arthur Holmes postulated his thermal convection current theory which was talking about majorly the origin of these relief features on Earth's surface right so that's why it acquired or it got the importance because there was a void in the knowledge of these geomorphic expression on Earth surface so now let's elaborate further more so Arthur Holmes talked about the driving force of mountain building now what is this driving force it's very important to understand that the Earth's interior gives this driving impetus or driving force so Arthur home said that thermal convection currents originating deep within the Earth are the driving forces for the surface mountain building that we observe right so he talked that main source of the origin of convective currents is excessive Heat in the substratum now the word here is substratum substratum is below the strata of the crust that is in the asthenosphere which is partially in molten as we have already learned in the internal structure so if you have not watched the videos on internal structure and isostasy I would suggest do watch that video to get more clarity on this particular topic so in fact the whole Theory depends exclusively on the mechanism of thermal convective currents that's very much important now here is important that he discussed three different layers in the Earth where different convective zones are present so according to homes the Earth consists of three zones or three layers and namely upper layer is made of granodiorite that is up to 10 to 12 kilometer intermediate layer is 20 to 25 kilometer made of amphibo light and lower layer of ecologide so this was the stratification presented by Arthur homes now if You observe this particular graph it's saying silica content here right and here is the percentage of mineral by volume so if you look at this geological graph for that matter it says that maximum silica content but minimum in terms of availability so amphibos are minimum in terms of availability but maximum are here in terms of silica content same is Olivine and pyroxine what happens here silica content is lesser and lesser as you go away right here but what happens their presence is more than what is amphibos so if you can pause the video here and look into this particular diagram you'll understand what that these intrusive and extrusive form of rocks and minerals have particular volume and also silica content depending upon in which layer they are formed by which particular process of thermal convection they originate that's important here so what is the difference between granite and granodurite now it's important here to discuss these three layers as homes said so Granite contains mostly of potassium feldspar and has low percentage of dark iron and magnesium minerals but at the same time if you look into this Gran onto your right it is darker remember it has more plagioclass feldspar that is calcium and sodium content and it also has darker minerals that's important so Granada do you write the first layer as you say and then amphibolide which was intermediate layer as he talked so amphibolite is basically what rock of convergent plate boundaries it's very important to understand where two plate boundaries converge there this is majorly formed where heat and pressure cause regional metamorphism so amphibolite is the resultant of regional metamorphism the intermediate layer where one layer goes subduction and there it forms when magma originates so that is where this important metamorphism takes place and mafic igneous rocks such as Basalt and gabro goes under this metamorphism to form amphibulide or metamorphism of clay Rich sedimentary rocks as well like Marlo gravic also form this particular amphibodite that is the intermediate layer now comes to the lower layer of ecologite now what is this Echo guide eclogite is a metamorphic rock formed when mafic igneous rocks are subjected to very high pressure obviously because it is under influence of so much of pressure from the above layers that's why it is important to understand that it's under high pressure so ecologite forms at pressure greater than those typical of the crust and they are unusually dense rocks acrogate can play an important role in driving convection within the solid earth that's why it is important so this is the three layer stratification which was stopped by Arthur Holmes that these are the layers after which he said that these layers and these particular kind of rocks are formed because of particular kind of thermal convection so let's elaborate and see then further he has grouped these particular rocks into three layers in or maybe you can set two particular zones one is the crystal Zone one is the mental zone so three layers are crust consisting of upper and middle or intermediate layers and crystalline upper part of lower layer substratum that is representing molten part of lower layer and crust and substratum are composed of Cl and SEMA remember this adverse OS hypothesis classification of CLC manife so if you look into this image What You observe this is the Crystal Part this particular part right and here is the mental part in which crust some ducks at particular places and also it also goes away from each other at certain places so where trust moves towards each other and crust goes away from each other as we have already learned in plate tectonics in details as well right so this is because some place convection current are Divergent some place convectional current are convergent and this is the main reason because of which you have trenches here then you have certain places volcanism here then you have made Oceanic ridges here so these are the surface Expressions which was given in terms of the explanation through this particular theory of convection current now the convection current depends on two factors us now factor is important here is the thickness of the crust near equator and the poles and uneven distribution of radioactive elements in the crust now remember radioactive elements are the prime source of energy as well so ascending that is going upward convective current originate under crust near the equator because of Greater thickness of crust now it's important that where thickness of crust is greater it means material is coming upward and it is solidifying this was the important part of the theory while at the same time where crust is going downwards it is descending it means descending convection currents are originating under polar crust because of shallower depth so equatorial region and polar region were also explained like this and if you can observe here so this is where you have accent that is going upward and diverging Force while at the same time there is something called convergent plate boundary right so magma generation is here and escaping of heat from the mantle is towards upward so this is what actually changes the entire scenario of or the geomorphic expression of the Earth's surface right that's important to remember now there are two situations of rising convective currents here one and two so the crustal Mass where two Rising convective currents diverge in opposite directions it is stretched and thinned now if it is diverging into Direction so it is stretching and it is thinner portion here right thus Divergent convective currents cause Continental drifts which was you know explained using this Theory so now continental drift explanation comes to convection current theory 1929 and when two lateral convective currents originate under continental and oceanic crust that is converge so compressive forces generated that causes subsidence now here is important point to remember that subsidence in the crystal zones give birth to geocent lines now remember geosyncline Theory we have also studied in details so geosync lines and closing of C's so if You observe carefully these two images one is where accent current is here here is thin crust right while it is here going towards each other it is moving downwards so convergence is here and what happens here descending current right and when descending current is there there can be a depression formed as well and GEOS and clients can form so that is important to remember now looking into this particular geosynclineal hypothesis and stages that we have already done so look here here is this particular expression where the convection current is going in descent it is going downwards so this is where depression is created like geosyncline between these two Crystal blocks right and in the second stage here the geosync lines actually gets folding because pressure from this side and pressure from this side goes here so it starts to fold so here you have fold mountains right so folding ranges this is how in different stages geosyncline's sedimentation orogenesis and Rise of mountains that we know by the name of glyptogenesis remember cobras work so glyptogenesis has been discussed in this particular Theory by validating it through convection current so convection current inside the earth results into this mountain building ridges and also volcanism at the same time so according to homes the cyclic pattern of convective currents and related Mountain buildings pass through three phases of or stages if you see which is given in this diagram so you can pause the video here and you can draw this diagram for yourself as well for the greater explanation now when this Theory gets more validation coming in 1929 this Theory he got more validation right in 1960s why because here this scholar called Harry Hess the great oceanographer he actually propounded the concept of sea flow spreading now look into this image magma comes upward and here it goes this side and this side and this is where new crust is found and here is the middle Oceanic Ridge right so this was the important aspect that actually proved that he was right Arthur Holmes was right in 1929 by saying that these ridges rise here trenches are formed somewhere mountains are formed on Earth surface because of convectional current so thermal conventional current theory got its validation after Harry has propounded the theory of seafloor spreading and later on plate tectonic theory clarified the entire scenario so now we know why Arthur Holmes thermal convection current theory of 1928-29 is important in terms of geoma pathological concept so now when we have discussed in details about the thermal convection current theory of Serato homes in this video in these sessions to come we'll be talking more on several such Grand theories in geomorphology which are very much important in understanding the mechanism of geomorphic Expressions landforms on Earth surface so if you have not subscribed to our channel do subscribe and also share the videos with others stay safe stay tuned best wishes