Interior of the Earth Class 8 Geography Notes Maharashtra State Board
Boil half a liter of milk. When the milk comes to a boil, switch off the gas Keep a lid on the vessel. After ten minutes, remove the lid, take it to a side, and tilt it. Before boiling, the milk was in a liquid state. After it came to a boil, steam started coming out from it. After some time, a thick layer of cream develops over the milk. The temperature of the cream is lower than the milk below. Thus, it can be implied that the creamy layer cooled faster than the milk below it, which remained hot and in a liquid form. A similar process occurred when the earth cooled. The scientists unanimously believe that the Earth was formed out of the solar system itself. Initially, the earth was a gaseous hot balloon. It cooled down as it rotated around itself. The process of cooling took place from the outer surface towards the earth’s center. As a result, the outer layer (crust) of the earth becomes cooler and solid but as we move from the surface to the core, heat increases, and at certain depths, the interior of the earth is semi-liquid.
Man has always been inquisitive about what lies in the interior of the earth. A direct observation of the earth’s interior has not been possible as yet. Through various methods, scientists have tried to estimate the same. To infer about these, the study of the materials coming out of a volcanic eruption and the seismic waves is most important. Matter coming out of volcanic eruptions consists of hot magma, gases, steam, etc. When lava cools and solidifies, igneous rocks are formed. Through the estimates of the study of temperature, density, gravitational force, and pressure the interior of the earth was understood. For instance, when we go deep into a mine, we feel an increase in temperature. Similarly, the magma coming out of volcanoes from the earth’s interior is hot. Such estimations were made by the geologists.
Several earthquakes occur in various places on the earth every year. Seismic waves are generated. These waves travel through the interior of the earth. The study of their direction and velocity helps us to estimate the interior of the earth. Man has also tried to dig deep bore holes in the interior to know more about the same. Our earth was formed approximately 4.6 billion years ago. Initially, the earth was in a gaseous state. It started cooling through the process of radiation. The earth then liquefied. With time, the outermost part of the earth cooled first and became solid. This outermost layer of the earth is called the crust. Even today outer planets of the solar system are in a gaseous state.
The Composition of the Earth’s Interior:
While going from the earth’s surface to its core, major changes occur in temperature and density. Concerning these two elements, the earth’s interior can be divided into the following layers.
The uppermost layer of the earth’s surface is in a solid state and is called the crust. The thickness of the crust is not the same everywhere. On an average, it is 30 to 35 km thick. The thickness of the crust below the continents is from 16 to 45 km, 40 km under the mountain ranges, and less than 10 km under the oceans. As we move deeper under the surface, temperature increases. After that, the rate of increase in temperature reduces and again increases in the core. At the center of the earth, the temperature is around 55000 to 60000 C. The crust is the thinnest of all the layers when compared to the mantle and the core. It can be divided into two sub-layers.
Steps in the Process of Knowing the Interior of the Earth
The continents are mainly composed of Silica (Silica is the compound of Silicon Elements) and Aluminum. Because of their high proportion in this layer, the layer was earlier known as Sial. The density of continental crust is 2.65 to 2.90 gm/cm3. The thickness of this layer is approximately 30 kilometers. Granite rocks are mainly found in this layer.
This is the second layer of the crust. It is made up of silica and magnesium. It was earlier called Sima. This layer is approximately 7 to 10 km thick. The density of this layer is 2.9 gm/cm3 to 3.3 gm/cm3. In this layer, basalt and gabbro rocks are mainly found.
Elements found in various layers of the earth’s interior
There is a discontinuity between the continental crust and oceanic crust. It was first deduced by a scientist called Conrad and is named after him as Conrad discontinuity. There is a discontinuity between the crust and mantle too. This discontinuity was deduced by a scientist called Mohorovicic. It has been named after him as Moho discontinuity. There is a discontinuity between the mantle and core too. It has been named after Gutenberg, a scientist, who discovered it.
Below the crust lies the mantle. The mantle can be divided into two layers: upper and lower mantle. The upper layer is more in a liquid state. Here, one finds magma chambers. It is through these chambers that magma comes out on the earth’s surface during volcanic eruptions. This layer is also known as the asthenosphere. Epicenters of deep-seated earthquakes are usually found here. Mantle begins at a depth of around 42km from the earth’s surface. The internal energy released due to endogenetic movements occurring in this layer is responsible for mountain-building, rifts, volcanic eruptions earthquakes, etc.
In this layer, it is inferred that the temperature at a depth of 2400 to 2900 km depth would be around 2200°C to 2500°C. Here, there is a sudden change in the structure and density of the material. It is estimated that this layer extends upto a depth of 2870 km. The average density of this layer is 4.5 gm/cm3 and it increases with depth. Increasing pressure is the reason behind this. The density of the lower mantle is about 5.7 gm/cm3.
The core starts from a depth of about 2900 km from the earth’s surface. The part of the earth’s interior extending from the mantle upto the center of the earth is the core. The thickness of the core is about 3471 km. This layer can be divided into outer core and inner core.
The outer core extends from around 2900 km to 5100 km. The secondary waves cannot pass through the core. They get absorbed in this region. This has led scientists to believe that the core could be liquid or semi-liquid. The primary waves travel through this layer. But their speed reduces when passing through this layer. The density of the outer core is 9.8 gm/cm3. The temperature of the liquid outer core is around 5000°C. We have learned that the outer core of the earth’s interior is in a liquid state and the proportion of iron is more in this layer. Vertical currents originate in this liquid layer. This is another characteristic of this area.
The difference between the temperatures of the outer and inner core gives rise to vertical currents. The earth’s rotation gives them eddy (circular) motion. Electric currents develop in these spiral eddies of liquid iron and thus magnetic field is generated. It is sometimes called the geo-dynamo too. This magnetic field of the earth is functional even outside the earth’s surface for quite a distance. As a result, a cover develops around the earth because of the magnetic field. The earth’s atmosphere is protected from solar winds coming from the sun. The magnetic field thus developed around the earth, is called magnetosphere. This is the fifth and an important sphere of the earth.
The inner core extends from around 5150 km to a depth of around 6371 km (earth’s center). It is the core of the earth which is in a solid state. The density of this sphere is around 13.3 gm/cm3. Iron and nickel are the major elements found here. Therefore, this layer is also called Nife. As the materials in this layer are under extreme pressure, the inner core is in solid state. The temperature here is almost equivalent to the surface temperature of the sun.
Observe the velocity curves of the given seismic waves. The left curve in Figure ‘A’ shows the velocity of secondary (S) waves, while the curve on the right shows Primary (P) waves. Many changes are occurring in the curve of the secondary waves. The curve drawn with the help of dots shows the average velocity of the waves. The curve of the secondary waves seems to have ended around the depth of 2900 km. Changes are visible even in the curves of the primary waves. The velocity of the primary waves increases according to depth to 2900 km.
The velocity of secondary waves is around 6 to 8 km/sec around the boundary of the outer core and the curve has stopped there. These waves do not enter the core. At 2900 km, the velocity is around 12 km/sec. But when it enters the core, the velocity reduces to 8 km/sec. See this as shown in the dot curve. Based on these curves, the scientists have inferred the densities at various depths. The density curve of the Earth’s interior is shown in Figure ‘B’. The red curve in Figure ‘B’ shows gravitational force at various depths. The gravitational force increases at a certain depth from the surface and then reduces according to depth. At the centre, it is zero as is visible from the curve.
Good Maharashtra State Board Class 8 Geography Notes Interior of the Earth can simplify complex concepts and make studying more efficient.