CHAPTER 3: Interior of the Earth NCERT SOLUTION CLASS 11TH GEOGRAPHY| EDUGROWN NOTES

Short Answer Type Question:

Q1. What are body waves?

Answer

Body waves are earthquake waves which are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth.

Q2. Name the direct sources of information about the interior of the earth.

Answer

The direct sources of information about the interior of the earth are:

• Mining: It is the process of extracting valuable minerals from the earth. Surface rock or the rocks we get from mining areas provide most easily available solid earth material.

• Drilling: Scientists are working on two major projects such as “Deep Ocean Drilling Project” and “Integrated Ocean Drilling Project”. This and many deep drilling projects have provided large volume of information through the analysis of materials collected at different depths.
• Volcanic eruption: It forms another source of obtaining direct information. As and when the
molten material (magma) is thrown onto the surface of the earth, during volcanic eruption it becomes available for laboratory analysis.

Q3.Why do earthquake waves develop shadow zone?

Answer

Earthquake wave develop shadow zone because P and S waves follow a curved path inside the earth due to increasing capacity. The shadow zone of S-wave is much larger than that of the P-waves.

Q4. Briefly explain the indirect sources of information of the interior of the earth other than those of seismic activity.

Answer
The indirect sources of information of the interior of the earth are:

• Temperature, Pressure and Density: These increase with the increasing distance from the surface towards the interior in deeper depths.
The rate of change of Temprature, pressure and density are known. Knowing the total thickness of the earth, scientists have estimated the values of temperature, pressure and the density of materials at different depths.

• Meteors: These are not from the interior of the earth but the material and the structure observed in the meteors are similar to that of the earth. This make them another source of information about the interior of the earth.

• Gravitation force: It is greater near the poles and less at the equator. The gravity values also differ according to the mass of material. The uneven distribution of mass of material within the earth influences this value.

• Magnetic Field: Magnetic surveys also provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.

Q5.What are the effects of propagation of earthquake waves on the rock mass through which they travel?

Answer

The earthquake waves cause vibration in the body of the rocks through which they pass. P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation. As a result, it creates density differences in the material leading to stretching and squeezing of the material. Other three waves vibrate perpendicular to the direction of propagation. 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.

Q6.What do you understand by intrusive forms? Briefly describe various intrusive forms.

Answer

The lava that is released during volcanic eruptions on cooling develops into igneous rocks. These forms are called intrusive forms.

Various intrusive forms are:

• Batholiths: A large body of magmatic material that cools in the deeper depth of the crust develops in the form of large domes. These are the cooled portion of magma chambers.

• 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 these are located at deeper depths.

• 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. In case it develops into a saucer shape, concave to the sky body, it is called lapolith.

• Phacolith: A wavy mass of intrusive rocks located near the top of an anticline or the bottom of a syncline.

• 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.

• 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.

Long Answer Type Questions:

Q1.Explain different types of earthquakes.
Answer:
The various types of earthquakes are:-

• Tectonic earthquake: The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.
• Volcanic earthquake: A special class of tectonic earthquake is sometimes recognised as volcanic earthquake. However, these are confined to areas of active volcanoes.
• Collapse earthquake: In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
• Explosion earthquake: Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.
• Reservoir Induced earthquake: The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes. Sometimes earthquakes also occur in mines due to mining processes. Sometimes earthquakes also occur below the oceans on surface of the ocean causing tsunamis.

Q2.What are different sources of information | about the interior of the earth?
Answer:
Some of the direct sources are:-

• 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.
• Drilling: 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.
• Volcanic Eruptions: When molten material is thrown onto the surface of the earth during volcanic eruption it becomes available for analysis.
  Some of the indirect sources of information: Analysis of properties of matter indirectly provides information about the interior. Knowing the total thickness of the earth, scientists have estimated the values of temperature, pressure and the density of materials at different depths.
  • Meteors that at times reach the earth: However, it may be noted that the material that becomes available for analysis from meteors, is not from the interior of the earth. The material and the structure observed in the meteors are similar to that of the earth. They are solid bodies developed out of materials same as, or similar to, our planet. Hence, this becomes yet another source of information about the interior of the earth.
  • Gravitation: The gravitation force (g) is not the same at different latitudes on the surface. It is greater near the poles and less at the equator. This is because of the distance from the centre at the equator being greater than that at the poles. The gravity values also differ according to the mass of material.
  • Magnetic surveys: Magnetic surveys also provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.

Q3.Explain different types of earthquake waves.
Answer:

Earthquake waves are basically of two types body waves and surface waves.

• Body waves: These are generated due to the release of energy at the focus. They move in all directions in all directions travelling through the body of the earth. These are less destructive than the surface waves.
• Surface waves: The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface. These waves are more destruction. The surface waves are the last to report on seismograph. These waves are more destructive. They cause displacement of rocks, and hence, the collapse of structures occurs.
  There are two types of body waves. They are called P- and S-waves,
• P-waves: They move faster and are the first to arrive at the surface. These are also called ‘primary waves’. The P-waves are similar to sound waves. They travel through gaseous, liquid and solid materials.
• S-Waves: S-waves arrive at the surface with some time lag. These are called secondary waves. An important fact about S-waves is that they can travel only through solid materials. This characteristic of the S-waves is quite important. It has helped scientists to understand the structure of t he interior of the earth. Reflection causes was es to  rebound whereas refraction makes waves move in different directions. The variations in the direction of waves are inferred with the help of their record on seismograph.

Q4.Explain how does shadow zone emerge. Use a diagram.
Answer:
Earthquake waves get recorded in seismographs located at far off locations. However, there exist some specific areas where the waves are not reported. Such a zone is called the ‘shadow zone’. The study of different events reveals that for each earthquake, there exists an altogether different shadow zone.

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. Figures given below show the shadow zones of P- and S-waves.

Q5.Explain the interior structure of the earth.
Answer:
The interior structure f the earth can be classified into three parts:
1. Crust; (ii) Mantle; (iii) Core (i) Crust: It is the outermost solid part of the earth. It is brittle in nature. The thickness of the crust varies under the oceanic and continental areas. Oceanic crust is thinner as compared to the continental crust. The mean thickness of oceanic crust is 5 km w’hereas that of the continental is around 30 km. The continental crust is thicker in the areas of major mountain systems. It is as much as 70 km thick in the Himalayan region. It is made up of heavier rocks. It is having density of 3 g/cm3. This type of rock found in the oceanic crust is basalt. The mean density of material in oceanic crust is 2.7 g/cm3.

2. Mantle: The portion of the interior beyond the crust is called the mantle. The mantle extends from Moho’s discontinuity to a depth of 2,900 km. 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 has a density higher than the crust’s (3.4 g/ cm3). The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km. The lower mantle extends beyond the asthenosphere. It is in solid state.

3. Core: The earthquake wave velocities helped in understanding the existence of the core of the earth. The core- mantle boundary is located at the depth of 2,900 km. The outer core is in liquid state while the inner core is in solid state. The density of material at the mantle core boundary is around 5 g/cm3 and at the centre of the earth at 6,300 km, the density value is around 13g/cm3. The core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes also called the nife layer.

Q6.In how many groups are volcanoes classified on the basis of nature of eruption and the form developed at the surface? Explain.
Answer:
Volcanoes are classified on the basis of nature of eruption and the form developed at the surface into following types:

1. Shield Volcanoes: These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted. For this reason, these volcanoes are not steep. They become explosive if somehow water gets into the vent; otherwise, they are characterised by low-explosivity. Barring the basalt flows, the shield volcanoes are the largest of all the volcanoes on the earth. The Hawaiian volcanoes are the most famous examples. The upcoming lava moves in the form of a fountain and throws out the cone at the top of the vent and develops into cinder cone.

2. Composite Volcanoes: These volcanoes are characterised by eruptions of cooler and more viscous lavas than basalt. These volcanoes often result in explosive eruptions. Along with lava, large quantities of pyroclastic material and ashes find their way to the ground. This material accumulates in the vicinity of the vent openings leading to formation of layers, and this makes the mounts appear as composite volcanoes.

3. Calderas: These are the most explosive of the earth’s volcanoes. They are usually so explosive that when they erupt they tend to collapse on themselves rather than building any tall structure. The collapsed depressions are called calderas. Their explosiveness indicates that the magma chamber supplying the lava is not only huge but is also in close vicinity. The hills produced by them are similar to the one made by composite volcanoes.

4. Basalt Flood Provinces: These volcanoes outpour highly fluid lava that flows for long distances. Some parts of the world are cohered by thousands of sq. km of thick basalt lava flows. There can be a series of flows with some flows attaining thickness of more than 50 m. Individual flows may extend for hundreds of km. The Deccan Traps from India, presently covering most of the Maharashtra plateau, are a much larger flood basalt province. It is believed that initially the trap formations covered a much larger area than the present.

5. Mid-Ocean Ridge Volcanoes: These volcanoes occur in the oceanic areas. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins. The central portion of this ridge experiences frequent eruptions.