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The processes are:
1. Endogenic
2. Exogenic
The direct sources of information about the interior of the earth are :
(i) Earth material or rock availed from mining areas and deep drilling projects.
(ii)Materials availed from the volcanic eruptions.
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The three types of earthquake waves are described below:
(iii) ‘L’ or Long Waves - These are surface waves. These do not go deep into the earth.
The indirect sources of information of the interior are :
(i)Density - The density of the sedimentary rocks is 2.7 and beneath the sedimentary rocks the density of igneous rocks is 3.5. This means that the density inside the earth is much higher. It is estimated that the core of the earth has the density of 11 to 12.
(ii)Temperature - There is a gradual increase in the temperature as depth increases. The rate of increase is 12°C per km at a depth of 100km. It is 2°C/km at a depth of 300 km and 1°C per km, below it.
(iii)Pressure- Like density and temperature, pressure also increases with the increase in depth inside the earth. It is due to overlying layers or depressions of the heavier material. It is estimated that the pressure at the centre of the earth is about 3500 kilobars.
(iv)The other indirect sources include gravitational magnetic field and the seismic activity. The gravitational force is greater near the poles and lesser at the equator.
(v)The gravity values also differ according to the mass of the material. Uneven distribution of the mass of material within the earth influences this value.
Gutenberg Discontinuity | Mohorovicic Discontinuity |
(i)The boundary between the mantle and core at the depth of 2900 km is called Gutenberg's discontinuity. | (i)The boundary between the crust and the mantle is called Mohorovicic discontinuity. |
(ii)It was observed by the seismologists, Beno Gutenberg in 1926. | (ii)It was observed by Yugoslav seismologist Mohorovicic in 1909. |
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Body Waves |
Surface Waves |
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(i) P and S waves are called body waves. |
(i)L waves are called surface waves. |
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(ii)These travel through the solid body of the earth. |
(ii)These move along the free upper crust of the earth. |
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(iii)These do not pass through liquid medium.
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(iii)These can pass through liquid medium and move at right angle to the surface of the earth. |
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Longitudinal Waves |
Transverse Waves |
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(i)In these waves, particles move to and fro in the direction of the waves. |
(i)In these waves, particles move at right angles to the direction of the wave motion. |
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(ii)These are known as Primary waves or P-waves or sound waves. |
(ii)These are known as Secondary waves or S-waves. |
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(iii)These waves can travel through liquid, solid and gaseous media. |
(iii)These waves travel only in solid media. |
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Crust |
Core |
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(i)This is the outermost layer of the earth. |
(i)This is the innermost layer of the earth. |
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(ii)It has an average density of 2.73. |
(ii)It has an average density of 17.2. |
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(iii)It covers about 0.5% by part of the earth. |
(iii)It covers about 83% by part of the earth. |
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(iv)It is made up of Silica and Aluminium. |
(iv)It is made up of Nickle and Ferrum. |
1. Direct observation is not possible :
The centre of the earth lies at a distance of about 6400 km from the surface of the earth. Man has been able to dig a hole only about 3 km deep for oil and the deepest mine of the world in South Africa is only 6 km deep.
2. High temperature : The temperature in the interior of the earth increases by 1°C for every 30 metres depth. Though this rate is not constant even then the temperature at the centre of the earth is about 2000°C. Because of this high temperature, the direct study of the interior of the earth is not possible.
So the interior of the earth is beyond the reach of direct observations and our knowledge about it is based upon indirect scientific evidences such as earthquake waves and meteorites, earth's gravitation and magnetic field.
There are three types of earthquake waves :
(i) ‘P’ waves or primary waves or longitudinal waves. These waves can travel through solid as well as liquid media.
(ii) The ‘S’ or secondary or transverse waves can travel in solid media only. These waves are slow in motion.
(iii) ‘L’ waves or surface waves move along the boundaries between solid, liquid and gaseous media. They do not go deep into the earth. They have a very low velocity.
The velocity of the waves increase if they travel through denser media. The velocity of first two types of waves increases with depth but only upto 2900 kilometres. Afterwards ‘S’ waves do not pass and ‘P’ waves pass at a reduced velocity. The velocity of ‘P’ waves passing through inner core again increases as compared to their passage through the outer core. Based on this finding the scientists have estimated that the iron and nickel of the core of the earth may be in liquid state.
Fig. 3.8. Earthquake Shadow Zones
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There are three types of earthquake waves :
(i) ‘P’ waves or primary waves or longitudinal waves. These waves can travel through solid as well as liquid media.
(ii) The ‘S’ or secondary or transverse waves can travel in solid media only. These waves are slow in motion.
(iii) ‘L’ waves or surface waves move along the boundaries between solid, liquid and gaseous media. They do not go deep into the earth. They have a very low velocity.
The velocity of the waves increase if they travel through denser media. The velocity of first two types of waves increases with depth but only upto 2900 kilometres. Afterwards ‘S’ waves do not pass and ‘P’ waves pass at a reduced velocity. The velocity of ‘P’ waves passing through inner core again increases as compared to their passage through the outer core. Based on this finding the scientists have estimated that the iron and nickel of the core of the earth may be in liquid state.
Fig. 3.8. Earthquake Shadow Zones
