Short Question Answers : Interior of the Earth

Q. 1. Upto which depth, man's knowledge of structure of earth, is limited ?
Ans: Man's direct knowledge of the earth's interior is limited to only a few kilometres below the surface; the deepest boreholes (for example, the Kola Superdeep Borehole) reach about 12 km, which is negligible compared with the Earth's radius.

Q. 2. Name the three layers of the earth.
Ans: The three principal layers of the Earth are the crust, the mantle and the core.

Q. 3. What is the density and thickness of Crust ?
Ans: The thickness of the crust varies under oceanic and continental areas. Oceanic crust is thinner with mean thickness of 5 km. Continental crust is around 30 km thick and reaches 70 km in the Himalayan region.

Q. 4. What does the word NIFE stand for ?
Ans: NIFE refers to material rich in nickel (Ni) and iron (Fe), a composition typical of the Earth's core.

Q. 5. What is the density of NIFE ?
Ans: Density increases with depth. The core is made up of very heavy material mostly nickel and iron.

Q. 6. From which Greek word, the term volcano has been derived ?
Ans: A volcano is a place where gases, ashes and/or molten rock material (lava) escape to the ground.

Q. 7. What name is given to the molten matter within the earth ?
Ans: Molten rock beneath the Earth's surface is called magma.

Q. 8. What name is given to magma, when it appears on the surface of the earth ?
Ans: When magma reaches the surface, it is called lava.

Q. 9. Name the main parts of a volcano.
Ans: A volcano has a vent or opening through which lava, gases and ashes come out. It also has a crater.

Q. 10. Name two types of volcanic eruptions.
Ans: Volcanoes are classified on the basis of nature of eruption and form: shield volcanoes and composite volcanoes.

Q. 11. Name the three types of earthquake waves.
Ans: The three main types of earthquake waves are:
(i) Longitudinal waves or Primary waves (P-waves)
(ii) Transverse waves or Secondary waves (S-waves)
(iii) Surface waves, including Rayleigh waves (R-waves) and Love waves (L-waves).

Q. 12. What do you understand by the term 'Discontinuity' ?
Ans: A discontinuity is a boundary that separates layers of the Earth with different physical properties (for example, density or seismic velocity); such boundaries mark changes between the crust, mantle and core.

Q. 13. Define the term Nife.
Ans: Nife refers to the Earth's core material, composed largely of nickel and iron. The core extends from about 2,900 km depth to the centre of the Earth (about 6,371 km) and has a high average density of around 12-13 g/cm³.

Q. 14. Name the direct and indirect sources of information regarding the interior of the earth ?
Ans: Direct sources: (i) Mines (ii) Wells (iii) Boreholes
Indirect sources: (i) Temperature variations (ii) Pressure studies (iii) Density measurements (iv) Earthquake (seismic) waves (v) Meteorites (vi) Gravity measurements.

Q. 15. How are earthquakes measured ?
Ans: Earthquakes are described by two related scales:
(i) Magnitude (e.g., the Richter scale) measures the energy released by the earthquake; magnitude is given as a number (commonly quoted on a scale that ranges roughly from 0 upwards).
(ii) Intensity (e.g., the Mercalli intensity scale) describes the effects and damage at specific locations and is given on a scale from I to XII. Magnitude is an absolute measure of energy, while intensity depends on local factors and observed damage.

Q. 16. Describe the location and importance of 'Shadow Zone'.
Ans: Shadow zones are regions on the Earth's surface where seismic waves from an earthquake are not directly recorded. Their existence is explained by the refraction and reflection of seismic waves at boundaries inside the Earth:
(i) S-waves cannot travel through liquids, so they are absent beyond certain angular distances from the epicentre; this absence indicates a liquid outer core.
(ii) P-waves are refracted at the core-mantle boundary, creating P-wave shadow zones roughly between 104° and 140° from the epicentre on either side. The presence of these shadow zones provides strong evidence for the liquid outer core and for changes in internal structure.

Q. 17. What do you mean by Mohorovicic Discontinuity ?
Ans: The Mohorovičić discontinuity (or Moho) is the boundary that separates the Earth's crust from the underlying mantle. It is recognised by an abrupt increase in the velocity of seismic waves; this change was first detected by the Yugoslav seismologist Andrija Mohorovičić in 1909.

Q. 18. How is it that our knowledge is based on indirect observations about the structure of earth's interior ?
Ans: Direct observation of the Earth's interior is not possible beyond a few kilometres because temperature and pressure increase rapidly with depth. Most mines reach only about 3 km, and the deepest scientific boreholes reach about 12 km, which is tiny compared with the Earth's radius (~6,371 km). For this reason, scientists rely largely on indirect evidence such as seismic waves, volcanic eruptions, meteorite composition, gravity measurements and laboratory studies of rock behaviour under high pressure and temperature.

Q. 19. How do we know that the core of earth is liquid ?
Ans: The liquid nature of the outer core is inferred from seismic observations: S-waves (secondary waves) cannot travel through liquids and therefore are not recorded beyond the core; P-waves (primary waves) are strongly refracted at the core-mantle boundary. The presence of seismic shadow zones and the behaviour of seismic waves together indicate a liquid outer core.

Q. 20. What is a Seismogram ?
Ans: A seismograph is an instrument that records ground motions caused by earthquakes. The record produced by a seismograph is called a seismogram.