welcome we'll start with class 7 Geography chapter 3 that's changing our earth now this class totally focuses on the evolution of landforms when we talk about evolution of landforms there are two basic ways under which we classify movement of Earth they can be either endogenic forces or exogenic forces now genic means origin or Genesis means origin so when I talk about origin it can be either endogenetic or exogenic exogenetic means those which are affected by outer phenomenas or outside phenomenas are endogenetic which have which are affected by the inner phenomenas or inner changes so it origin which is based from within itself or inside the earth system would be known as endogenic forces however factors that affect the origin of land from from outside would be exogenic forces so when I say exogenic forces I can say wind water rain so all these are external forces which are affecting the formation of landform the structures of mountains the rivers uh the basic landscape of Earth however there are certain forces which are internal for example a volcano coming out or you have kind of earthquake movements that could be seen so all those are internal forces which are endogenic in nature so whenever we talk about Earth movements we classify forces as either endogenic or exogenic exogenic as I said could be either erosional or depositional depositional means whatever sediments have weathered away or have eroded away are finally getting settled down and that is known as deposition so it can be by action of River Wind sea waves or glaciers now when we talk about endogenic forces this could be all of the sudden or these could be slow and gradual over the course of time when I say it's slow and gradual it can be formation of mountain ranges formation of Himalayas for example is a classic could be a classic example of diastrophic forces which is a kind of very slow process which has taken place over centuries and centuries however you have certain forces which are sudden so eruption of earthquake is all of sudden you have landslide that is occurring in the mountain areas it is a kind of Sudden Change in the landscape volcanic activity in a region is all of the sudden so all these are kind of sudden phenomenas now what we will focus in today's class we have covered all these in great detail in individual classes what we will be covering today is a kind of summary of all these under under one single head so we'll start with the first one that is volcano will focus on the highlights of each for more details on each topic we have the respective links here so you can refer those now the first one that is volcano now volcano erupts when the inner material gets too hot and it's unable to keep its eye itself within the mountain region as a result it erupts when it erupts there is flow of magma and this magma flows out through an opening which is known as vent the material what comes out is known as lava the uppermost section where you have a kind of Lake formation once the material has erupted is known as crater and finally once you have the volcanic activity that freezes off or is vanished you have formation of lake that takes place and such formations are known as Crater Lakes when this crater gets bigger it's known as Caldera the inner section of the mountain where you have the origin of volcanic activity is known as the magma chamber and this is the basic reservoir of all the material that flows in into the volcano now the next is earthquakes now earthquakes when we classify earthquakes we can understand the various waves that come in in three basic classification the p waves S waves and L waves the p waves are longitudinal waves s waves are transverse waves and L waves are surface waves which could be filled over the surface of the Earth so what happens during the process of volcano earthquake is you have undress that happens on the surface and that unrest is governed by a kind of focus point which is below the surface of Earth now this is the focus point which is below the surface of the Earth and this is I can say the origin of earthquake now the perpendicular line where it meets the straight line onto the surface would be the epicenter or the main area of earthquake spread and then you would have the seismic waves that could be fed around the epicenter so epicenter would form the center of the seismic waves so seismic waves means the earthquake waves so seismicity is another term that we use for earthquake and these seismic waves as they move away from the epicenter they become weak so the waves which are closest to the epicenter would be the strongest and as you move away from the epicenter the effect of earthquake would gradually and slowly decline now register scale is the scale through which you measure the intensity of earthquake uh it's a kind of logarithmic scale and its intensity increases 10 times from say five if I say it's an if it's an intensity of 5 at Richter scale and then of 6 that means this intensity of 6 is nearly 10 times when I say the intensity at 5 so this is how we understand the register scale and now some important tips for preparedness uh there are certain safe spots that you must recognize so whenever you are standing against a corner of the wall the inside most region is the safest because when there would be a kind of wall that is falling it would be towards the outer side so if you are inside a room inside a closed space the safest area is standing closest to the corner or closest to the wall because that would be the safest region under the kitchen counter or under the table would be another certain safe spaces you should always stay away from fires away from ignitable um regions and spread awareness among the family members so these are some of the important highlights that we cover for earthquakes now next is the difference between weathering and erosion now when I say you have a kind of big rock let's say this is a rock now this rock breaks up into small rocks and this process is known as weathering so weathering in simple terms is breaking up of bigger rocks into smaller particles however now these particles would be transferred to far off places by the action of wind water sea waves or glaciers the four regions that we have talked about are once they are transferred or moved from one place to the next place this process is known as erosion so erosion is the process of movement of the fragmented particles of rock from one place to another place so I repeat again the difference between weathering and erosion is important weathering is simply a breaking up so for example let's say I have this a pen and if I break this if I cut this or divide this into small particles that would be a phenomena of weathering however if each small particle that I have broken this pen into is moved from one place to another by any agent say by wind or water or whatsoever that movement from place a to place B would result in the process that would be known as erosion so that's the basic difference between the concept of weathering and erosion now we have talked about the exogenic forces when we started the lecture now all these what we will be discussing now are part of exogenic forces or external forces that affect the formation of landform now the first external force is River action so underwater we will be talking about two one is River action and one is C waves action so River action what happens is water as we know the river behaves like a human being it's in the stage of Youth maturity and old age so when it's in the stage of Youth it is very active and as it moves towards its old age it becomes very slow and Mild so when it's in its youthful stage you would see features like waterfall so what would happen is there is alternate arrangement of hard rock soft rock and you would have differential erosion that means soft rock would be eroded more as compared to Hard Rock so there would be depressions that would be seen in soft rock and then you would have formations like this and that would lead to formation of waterfalls worldwide the highest waterfall is Angel Falls in Venezuela now there are some waterfalls which form the marketing lines between two Nations for example Niagara Falls is a demarcation between Canada and United States you have Victoria Falls which is a demarcation between Zambia and Zimbabwe in Africa then you have during the youthful stage since there is a lot of movement of water you would have a kind of wavy movement and this wavy movement is what is known as meanders it has lots of turns and twists along the side of meanders you would have flood plains that would be seen and there would be raised embarkments that would be seen on both the sides which are known as levees now sometimes what happened this Meandering river which can be seen here cuts off from its main course and this section establishes itself as a separate Lake and is it surface size shape of a kind of Oxbow and hence it is known as oxbow lake and finally in the Old Stage the river meets the ocean body and it's become it becomes very slow and sluggish so as a result it carries a lot of sediment which it has collected through the path from youthful stage and maturity stage and that sediment slowly and gradually deposits when it reaches the final stage and therefore towards the final stage you have formation of what is known as Delta that is take that takes place now India is unique to itself because in India you can see the major rivers that drain towards East have formation of Delta so you have Krishna godavari mahanadi kaveri which have Delta formations however the reverse which drain towards West that is into the Arabian Sea they drain directly in the form of estuaries and they do not form Deltas we will go into these details later when we go further with the ncrt classes for higher standards for now uh you have the formation which is Delta now the next important concept is C action now when we we talked about how River erodes the structures and there are various formations that takes place so whenever we talk about any structure be it River be it sea waves there would be two kinds of features that are seen first is erosional and second is depositional so erosional means because of the action of sea waves for example you have action of C waves and you have a kind of rock mass that is seen here there is erosional activity and there is formation of caves that means within the rock there is certain section that is eroded away and it's weird away and now you can have a kind of cave where you can pass by so it's a kind of erosional feature similarly you have centered certain depositional features for example what we talked about in case of Delta is a depositional feature now when we talk about C waves you have primarily the erosional features we are talking about so firstly you have sea caves that is formed if this C cave is extended and it forms a kind of tunnel through and through so you have a c cave and I can see from this end to this end it would be called as Arch and once you have the upper body that kind of disintegrates and breaks down from the remaining region you have formation which is known as stack so stack is a single piece of pillar that stands into the sea into the ocean that we could see now glaciers glaciers are also known as rivers of ice they are slow moving uh ice masses that move along now when River glaciers move along they create feature which is known as U-shaped valley so when you have action of river which is thinner and Sleek you have the mountain areas which are uh which are bifurcated and you have formation of valleys which are of the shape of V so with the river action you have formation of v-shaped valleys however due to the shape of the equation and the bulkiness of the glacier you have formation of valleys which are u-shaped so that's the first important difference between Glacier valleys and river valleys now when Glacier moves along it carries certain deposits along with it and the deposits that Glacier carries along is known as morains now these Moorings could be towards the end of the glacier so they are known as endurance they could be towards the side which are known as little Moorings in the center which are known as medial Moraine so on and so forth so these are the common depositional features that we talk about and finally you have the wind action when we talk about wind action it's primarily seen in the desert areas where wind is predominant so you have huge amount of sand that is blown from one region to another and deposited such depositions are known as Lewis's losses are commonly seen in China then you have another structures which are known as sand dunes we have covered the detailed classification of sand dunes in our video so you can refer those and then you have mushroom rocks which are structures again with differential erosion as you could see so you have kind of hard rock soft rock Arrangement and there are further more scientific explanations which we have covered here in this lecture so what happens is the the soft rock which is at the lower section erodes at a much higher Pace as compared to the Hard Rock and there is a kind of mushroom that is seen in the desert region and this formation of mushroom by rocks is known as what is we call as mushroom rocks so it's a very kind of very interesting phenomena that is seen in desserts you have big structures which seem like mushrooms but they are not really mushrooms but they are made up of frogs kind of Rocky structure so that is what is mushroom rocks and it's again a kind of erosional feature that is seen due to wind action so with this we cover the major uh parts of wind actions and kind of exogenic and endogenic forces and how landform has evolved over the course of time uh we'll be covering more chapters in further lessons you can subscribe to our channel for more updates have a good day