Internal Structure of Earth

Earth’s Interior With Special Reference to Seismology

1. Earth’s Interior With Special Reference to Seismology Mithun Ray Department of Geography Malda College (University of Gour Banga ) E-mail: [email protected]

2. What do you imagine about the nature of the earth? a solid ball !! or a hollow ball !!

3. Diameter of the Earth Equatorial= 12,756 km (7926 mile) Polar=12,714 km (7900 mile) Radius of the Earth Equatorial= 6378.1 km (3,963.2 mile) Polar= 6356.8 km (3949.9 mile) Average Radius 6371km (3958.8)

4. How do we know what’s inside the Earth? Deepest Mine In the world Mponeng Gold Mine located at Johannesburg, South Africa is the deepest mine in the world with an operating depth of 4 km

5. Kola Super deep Borehole (World’s deepest drill)

6. Sources of Information about the Interior Major Sources The studies of the paths and characteristics of earthquake waves travelling through the Earth, and The laboratory experiments conducted on the surface minerals and rocks at high pressure and temperature Other Sources The properties of surface rocks Earth's motions in the Solar System, Earth’s gravity and magnetic fields, and The flow of heat from inside the Earth.

7. Seismology Study of earthquakes and Seismic Waves that move through and around the earth

8. Seismic Waves /Earthquake waves

9. ‘P’ / Primary/ Pressure/ Compressional / Longitudinal Wave P waves travel faster than any other type of wave They can travel through fluid or solid materials. Material is compressed and stretched in the horizontal direction and the wave (disturbance) also travels in the horizontal direction. Ordinary sound waves in air are P waves  Least destructive

10. ‘S’ / Secondary / Shear / Transverse Wave ‘S’ waves travel more slowly than ‘P’ waves The material vibrates up and down i.e. perpendicular to the direction of wave propagation They can only travel through solid materials. However, they may propagate in liquids with high viscosity

11. A   Rayleigh wave   is a  seismic  surface  wave  causing the ground to shake in an elliptical motion, with no transverse, or perpendicular, motion. A Love wave  is a surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling.

12. BODY WAVE SURFACE WAVE

13. Reflection  involves a change in direction of waves when they bounce off a barrier.  Refraction  of waves involves a change in the direction of waves as they pass from one medium to another. 

14. Shadow Zone of ‘P’ wave The  shadow zone  of P-waves appears as a band around the earth between 103° and 142° away from the epicentre. This is because P-waves are refracted when they pass through the transition between the semisolid  mantle  and the liquid  outer core . However, the seismographs located beyond 142° from the epicentre, record the arrival of P-waves, but not that of S-waves. This gives clues about the solid  inner core .

15. S-waves do not travel through liquids. The entire zone beyond 103° does not receive S-waves, and hence this zone is identified as the  shadow zone  of S-waves. This observation led to the discovery of the  liquid  outer core . Shadow Zone of ‘S’ Wave

16. Shadow Zone of P-waves and S-waves a zone between 103° and 142° from epicentre was identified as the shadow zone for both the types of waves.

17. Internal structure of the Earth

18. CRUST The Crust is the outermost layer of the earth on which all living world exists This is a very thin layer. It is also a brittle layer. It is ranging from 5 km under the oceans to 100 km under the mountainous areas of continents. Usually, it’s about 40 km thick under the flat continents. The crust is about 0.5 % of the earth’s total mass. These rocks and minerals are made from just 8 elements. They are Oxygen (46.6%), Silicon (27.72%), Aluminum (8.13%), Iron (5.00%), Calcium(3.63%), Sodium (2.83%), Potassium (2.70%) and Magnesium (2.09%).

19. Continental Crust (SIAL): Composed of Silica (Si) and Aluminum (Al) Average density is 2.7 gm/ cm 3 This layer is mainly composed of Granite Oceanic crust (SIMA): Composed of Silica (Si) and Magnesium (Ma) Average density is 3.0 gm/ cm 3 This layer is mainly composed of Basalt

20. Isopach = lines of equal thickness of the crust

21. MANTLE The mantle is the middle layer. This makes up the largest volume of the Earth’s interior. It is almost 2900 kilometers in thickness. It is comprised of about 83% of the Earth’s total volume. It has an average density of 5.5 g/cm 3 It is also divided into two distinct layers as upper mantle and lower mantle.

22. Asthenosphere The upper portion of the mantle is called asthenosphere . The word astheno means weak. It is considered to be extending upto 400 km. It is the main source of magma that finds its way to the surface during volcanic eruptions. It is made up of peridotites .

23. CORE The core is found about 2,900 kilometers (1,802 miles) below Earth’s surface, and has a radius of about 3,485 kilometers (2,165 miles). The core is made of two layers: the outer core, which borders the mantle, and the inner core. The outer core is in liquid state while the inner core is in solid state. The core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes referred to as the NiFe layer.

24. বসু ও মাইতি,২০১৪

25. Discontinuity All layers of earth’s interior are separated from each other through a transition zone. These  transition zones are called discontinuities

26. Conrad Discontinuity :  Transition  zone between SIAL and SIMA. Mohorovicic Discontinuity:  Transition zone between the Crust  and Mantle. Repiti Discontinuity:  Transition zone between Outer mantle and Inner mantle. Gutenberg Discontinuity : Transition zone between Mantle and Core. Lehman Discontinuity : Transition zone between Outer core and Inner core.

27. Conrad Discontinuity:    The transition zone between thee upper and lower part of the lithosphere, is called as Conrad discontinuity. The name come from the Austrian geophysicist  Vector Conrad . Up to the middle 20 th century the upper crust in the continental region was seen to consist of felsic rock such as granite and the lower one consist of more magnesium rich mafic rocks such as basalt. Therefore, the seismologists of that time considered that Conrad discontinuity should correspond to a sharply defined contact between the chemically distinct  layers of  SIAL and SIMA.  In passing through the Conrad discontinuity the velocity of longitudinal seismic waves increases abruptly from approximately 6to6.5km/sec.  

28. Mohorovicic Discontinuity:   The transition zone  between the crust and mantle is called as mohorovicic discontinuity. The mohorovicic discontinuity was  discovered by Andrija Mohorovicic in the year of 1909. The Moho lies at the depth of 35km beneath the continents and 8km beneath the oceanic crust. The Moho separates both the continental crust and the oceanic crust from underlying mantle. The Moho lies almost  entirely within the lithosphere, only beneath the Mid Oceanic Ridge does it define lithosphere and asthenosphere boundary. Immediate above the Moho velocity of the P wave is 6km/sec and just below the Moho it becomes 8km/sec. Moho   is characterised  by up to 500km thick.

29. Gutenberg Discontinuity:    The mantle –core transition zone is called Gutenberg discontinuity. In the year of 1912  Weichert Gutenberg was discovered this discontinuity at the depth of 2900km beneath the earth surface. In this zone the velocity of seismic waves changes suddenly. The velocity of P wave decreases and S wave completely disappear at this depth. S wave shear material and cannot transmit through liquid. So, it is believed that the part of above the discontinuity is solid and part of beneath then discontinuity is liquid or molten form. This molten section is thought to be 700°c, hotter than the overlying mantle. It is also denser, probably due to a  greater percentage of iron.      

30. Repiti Discontinuity it is the transition zone between Outer mantle and Inner mantle. Lehmann discontinuity It is the transition zone between outer and inner core. The  Lehmann discontinuity  is an abrupt increase of  P -wave and  S -wave velocities at the depth of 220±30 km, discovered by seismologist Inge Lehmann.It appears beneath continents, but not usually beneath oceans, and does not readily appear in globally averaged studies. Several explanations have been proposed: a lower limit to the pliable asthenosphere, a phase transition, [ and most plausibly, depth variation in the  shear wave anisotropy.

31. Seismic Waves and the Interior of the Earth Reflection causes P and S waves to rebound whereas refraction makes waves move in different directions. The variations in the direction of these waves are inferred with the help of their record on seismograph. Change in densities greatly varies the wave velocity. By observing the changes in velocity, the density of the earth as a whole can be estimated. By the observing the changes in direction of the waves (emergence of shadow zones), different layers can be identified.

32. For both kinds of waves, the speed at which the wave travels also depends on the properties of the material through which it is traveling. Scientists are able to learn about Earth’s internal structure by measuring the arrival of seismic waves at stations around the world. For example, we know that Earth’s outer core is liquid because s-waves are not able to pass through it; when an earthquake occurs there is a “shadow zone” on the opposite side of the earth where no s-waves arrive. Similarly, we know that the earth has a solid inner core because some p-waves are reflected off the boundary between the inner core and the outer core.

33. By measuring the time it takes for seismic waves to travel along many different paths through the earth, we can figure out the velocity structure of the earth. Abrupt changes in velocity with depth correspond to boundaries between different layers of the Earth composed of different materials. The structure of Earth’s deep interior cannot be studied directly. But geologists use seismic (earthquake) waves to determine the depths of layers of molten and semi-molten material within Earth. Geologists are now using these records to establish the structure of Earth’s interior.

34. This Power Point Presentation (PPT) has been prepared especially for Undergraduate Students. The materials (Maps, Diagrams and Images) used in this presentation were collected and compiled mainly from variours academic blogs, research papers, books etc. The author has designed this PPT according to his convenience to deliver the lecture.

35. THANK YOU