Interior of The Earth - Solutions
CBSE Class 11 Geography
NCERT Solutions
Chapter 3
Interior of the Earth
1. Multiple choice questions.
(i) Which one of the following earthquake waves is more destructive?
(a) P-waves
(b) S-waves
(c) Surface waves
(d) None of the above.
Ans. (c) Surface waves. The two most important types of surface waves are Rayleigh waves and Love waves.
(ii) Which one of the following is a direct source of information about the interior of the earth?
(a) Earthquake waves
(b) Volcanoes
(c) Gravitational force
(d) Earth magnetism.
Ans. (b) Volcanoes. The magma which comes out from the interior of the Earth, gives us a lot of information about the interior of the Earth.
(iii) Which type of volcanic eruptions have caused Deccan Trap formations?
(a) Shield
(b) Flood
(c) Composite
(d) Caldera.
Ans. (b) Flood. The bulk of volcanic eruptions which occurred in the western ghats some 66 million years ago, caused the Deccan trap to form.
(iv) Which one of the following describes the lithosphere?
(a) Upper and lower mantle
(b) Crust and upper mantle
(c) Crust and core
(d) Mantle and core.
Ans. (b) Crust and upper mantle. It constitutes the hard and rigid outer layer of the Earth.
2. Answer the following questions in about 30 words.
(i) What are body waves?
Ans. A body wave is a seismic wave that moves through the interior of the earth, as opposed to surface waves that travel near the earth's surface. Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Therefore, it has been named as body waves. These are of two types: P-waves and S-waves. Each type of waves shake the ground in different ways.
(ii) Name the direct sources of information about the interior of the earth.
Ans. The knowledge about the interior of the Earth is obtained through direct sources and indirect sources. Some of the direct sources are:- (a) Mining: It is a process by which commercially variable valuable mineral resources are extracted from Earth's surface which includes precious stones, rocks and solid fuels.
(b) Drilling: The Ocean Drilling Program (ODP) is an international cooperative effort to explore and study the composition and structure of the Earth's ocean basins. Scientists world over, are working on two major projects such as "Deep Ocean Drilling Projects" and "Integrated Ocean Drilling Project". The deepest drill at Kola, in Arctic Ocean, has so far reached a depth of 12 km.
(c) Volcanic Eruptions: A volcanic eruption occurs when hot materials ( magma), from the interior of the Earth, are thrown out of a volcano. This becomes available for analysis.
(iii) Why do earthquake waves develop shadow zone?
Ans. A shadow zone is the area of the Earth from angular distances of 104 to 140 degrees from a given earthquake that does not receive any direct P-waves. Earthquake waves get recorded in seismographs located at far off locations. But there exists some specific areas where the waves are not reported. Such a zone is called a 'shadow zone'. The study of different events reveals that for each earthquake, there exists an altogether different shadow zone. It was observed that seismographs located at any distance within 105° from the epicentre, recorded the arrival of both P and S-waves. However, the seismographs located beyond 145° from epicentre, record the arrival of P-waves, but not of S-waves. Thus, a zone between 105° and 145° from epicentre was identified as the shadow zone for both the types of waves. The entire zone beyond 105° does not receive S-waves. The shadow zone of S-wave is much larger than that of the P-waves. The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicentre. The shadow zone of S-waves is not only larger in extent but it is also a little over 40 per cent of the earth surface.
(iv) Briefly explain the indirect sources of information of the interior of the earth other than those of seismic activity.
Ans. Analysis of properties of matter indirectly provides information about the interior
(a) Meteors: A new analysis of the chemical make up of meteorites has helped scientists work out when the Earth formed its layers. Although no one has ever been to the center of the Earth, we know from meteorites that the Earth has a center or core, made from nickle and iron metal.
(b) Gravitation: Issac Newton, calculated from his studies of planets and the force of gravity, that the average density of the Earth is twice than that of the surface rocks and therefore the Earth's interior must be composed of much denser material. Our knowledge of what is inside the Earth, has improved immensely since Newton's time.
(c) Magnetic field: The Earth's magnetic field is believed to be generated by electric currents in the conductive material of its core, created by convection currents due to heat escaping from the core. By this we can analyse the information about the interior of the Earth.
3. Answer the following questions in about 150 words.
(i) What are the effects of propagation of earthquake waves on the rock mass through which they travel?
Ans. Earthquake waves are basically of two types: Body waves and surface waves. Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Therefore these are called body waves. The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface. The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity. The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass. Surface waves are considered to be the most damaging waves. Seismographs located at any distance within 105° from the epicentre, recorded the arrival of both P and S-waves. However, the seismographs located beyond 145° from epicentre, record the arrival of P-waves, but not that of S-waves. Thus, a zone between 105° and 145° from epicentre was identified as the shadow zone for both the types of waves. The entire zone beyond 105° does not receive S-waves. The shadow zone of S-wave is much larger than that of the P-waves. The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicentre. The shadow zone of S-waves is not only larger in extent but it is also a little over 40 per cent of the earth surface.
(ii) What do you understand by intrusive forms? Briefly describe various intrusive forms.
Ans. The lava that cools within the crustal portions assumes different forms. These forms are called intrusive forms. Important intrusive forms are described below:
(a) Batholiths: A large body of magmatic material that cools in the deeper depth of the crust develops in the form of large domes. Batholiths are the cooled portion of magma chambers.
(b) Lacoliths: These are large dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below. It resembles the surface volcanic domes of composite volcano, only difference is that these are located at deeper depths.
(c) Lapolith: As and when the lava moves upwards, a portion of the same may tend to move in a horizontal direction wherever it finds a weak plane. It may get rested in different forms. If it develops into a saucer shape, concave to the sky body, it is called lapolith.
(d) Phacolith: A wavy mass of intrusive rocks, at times, is found at the base of synclines or at the top of anticline in folded igneous country. Such wavy materials have a definite conduit to source beneath in the form of magma chambers (subsequently developed as batholiths). These are called the phacoliths.
(e) Sills: The near horizontal bodies of the intrusive igneous rocks are called sill or sheet, depending on the thickness of the material. The thinner ones are called sheets while the thick horizontal deposits are called sills.
(f) Dykes: When the lava makes its way through cracks and the fissures developed in the land, it solidifies almost perpendicular to the ground. It gets cooled in the same position to develop a wall-like structure. Such structures are called dykes.
