Q. 1. Upto which depth, man’s knowledge of structure of earth, is limited ?
Ans. Upto a depth of 12 km.
Q. 2. Name the three layers of the earth.
Ans. Crust, mantle and core.
Q. 3. What is the density and thickness of Crust ?
Ans. 2.7 to 50 kms.
Q. 4. What does the word SIAL stand for ?
Ans. The crust consisting of silica and aluminium (SIAL).
Q. 5. What does the word SIMA stand for ?
Ans. SIMA is the layer below sial (Silica + Aluminium = SIMA).
Q. 6. What does the word NIFE stand for ?
Ans. The centre core of the earth (Nickle + Ferrous = NIFE).
Q. 7. What is the density of NIFE ?
Ans. 17 and 2500 km.
Q. 8. From which Greek word, the term volcano has been derived ?
Ans. From Greek word ‘Vulcan’ meaning God of the Underground.
Q. 9. What name is given to the molten matter within the earth ?
Q. 10. What name is given to magma, when it appears on the surface of the earth ?
Q. 11. Name the main parts of a volcano.
Ans. (i) Vent or opening
(ii) Volcanic pipe
Q. 12. Name two types of volcanic eruptions.
Ans. (i) Central eruption
(ii) Fissure eruption.
Q. 13. Name an active volcano in India.
Ans. Barren island near Andaman islands.
Q. 14. Name the largest active volcano in the world.
Ans. Mauna Loa (Hawaii islands).
Q. 15. Which volcano is known as the ‘lighthouse of the Mediterranean ?’
Q. 16. Which type of volcanoes are called ‘sleeping volcanoes ?’
Ans. Dormant volcanoes.
Q. 17. Name the three belts where volcanoes are found.
Ans. (i) Circum Pacific belt
(ii) Mid-world mountain belt
(iii) African rift valley.
Q. 18. Name the three types of earthquake waves.
Ans. There are three types of earthquake waves :
(i) Longitudinal waves or Primary or P-waves.
(ii) Transverse or Secondary or S-waves. (iii) Surface or Long R-waves.
Q. 19. What do you understand by the term ‘Discontinuity’ ?
Ans. The boundary separating the different layers of the earth is called Discontinuity. This demarcates the different layers of the earth having different densities.
Q. 20. Define the terms Sial and Sima.
Ans. Sial : It is the uppermost layer of the earth.
It consists of Silica and Aluminium. (Sial = Silica + Aluminium). Its average depth is 60 kms. It has an average density of 2.75.
Sima : The layer below the Sial is called Sima. It consists of Silica and Magnesium (Sima = Silica + Magnesium). Its average depth is 2800 kms. It has a density of 4.75.
Q. 21. Define the term Nife.
Ans. Nife : The core of the earth is called ‘Nife’. It consists of Nickel and Ferrous. It has an average depth of 3500 kms. It has a density of 13.
Q. 22. Name the direct and indirect sources of information regarding the interior of the earth ?
Ans. Direct Sources : (i) Mines (ii) Wells (iii) Bores Indirect sources : (i) Temp. (ii) Pressure (iii) Density (iv) Earthquake waves (v) Meteroites (vi) Gravity.
Q. 23. How are earthquakes measured ?
Ans. Measuring earthquakes. The earthquake events are scaled either according to the magnitude or intensity of the shock. The magnitude scale is known as Richter scale. The magnitude relates to the energy released during the quake while the intensity scale takes into account the visible damage caused by the event. The magnitude is expressed in absolute number 0 to 10. The intensity scale is named after Mercalli, an Italian seismologist. The range of intensity scale is from 1 to 12. The intensity relates to the actual damage caused by the quake. On the Richter scale, earthquakes with high intensity are rare.
Q. 24. Write a short note on ‘Barysphere’.
Ans. Barysphere. The central core of the earth is called Barysphere. It has a depth of 4980 km to 6400 kms. It has a density of 17. It consists of heavy minerals and is called core also. The core is metallic in nature.
Q. 25. Describe the location and importance of ‘Shadow Zone’.
Ans. Due to reflection and refraction suffered by earthquake waves, some areas do not experience any waves. Such areas are called ‘Shadow Zones’. From the epicentre, S-waves are deflected at an angle of 103° to 143° only. P-waves travel down to the boundary at the Core. These waves suffer reflection and refraction and emerge on the surface as P.K.P. waves. Thus two shadow zones on either side of the core are formed. These suggest the presence of the liquid core. These zones are free from earthquake waves. Only surface waves pass through it.
Q. 26. What do you mean by Mohorovicic Discontinuity ?
Ans. The abrupt changes in the velocity of seismic waves suggest that the crust is distinguished from the mantle. The surface of sudden increase in wave velocity which separates the crust above from the mantle below is called Mohorovicic discontinuity or M discontinuity. The P waves near the surface travel at about 6 km per second and increase to 7 km per second to the base of the crust. This is named after the Yugoslav seismologist Mohorovicic who first recognised the discontnuity in 1909.
Q. 27. How is it that our knowledge is based on indirect observations about the structure of earth’s interior ?
Ans. High temperatures are found in the interior on the earth. So direct observation is not possible as man cannot go deep down. Direct observation is limited only to a depth of 3 kms. Deepest mines go up to a depth of 3 kilometres. Due to increase in temperature with depth, we reach a very high temperature (2000°C) in the interior. Mines, wells and boreholes provide a limited knowledge. The depth of the deepest borehole (6 kilometres) is quite insignificant as compared to the radius of the earth (6,400 kms). So, we have to depend upon indirect sources of evidences like earthquake, waves, volcanic eruptions and meteorites.
Q. 28. How do we know that the core of earth is liquid ?
Ans. Three types of earthquake waves have different types of path and velocity. Both P and Swaves are reflected and refracted while passing through a different layer. S-waves cannot travel through liquid and have never been observed in the core. Hence, the core is considered liquid. The rigidity of the material of the core is zero, which is also a characteristic of a liquid core.
Q. 29. What is a Seismogram ?
Ans. Seismograph is an instrument which records earthquake waves. Study of Seismographic record is called Seismogram.
Q. 30. Name two types of surface waves.
Ans. Surface waves are those earthquake waves which move freely along the upper crust of the earth.
These produce disastrous effects of earthquakes. These are of two types : (a) Rayleigh Waves or R-waves. These are named after the English Physicist, Lord Rayleigh. These are similar to water waves travelling across the surface of a still pond when a pebble is tossed into water. Figure 2 shows Rayleigh waves.
(b) Love waves or L-waves. The second type of surface waves is the love wave. These are named after the Physicist A.E.H. Love. These move in a horizontal plane. These move at right angle to the direction of wave motion. Figure 3 shows Love Waves.