Interior of the Earth
NCERT Solutions • Class 11 Geography • Chapter 31. Multiple Choice Questions
(i) Which one of the following earthquake waves is more destructive?
(c) Surface waves
Reason: Surface waves (L-waves) move along the earth’s surface and cause the displacement of rocks and collapse of structures, making them the most damaging.
Reason: Surface waves (L-waves) move along the earth’s surface and cause the displacement of rocks and collapse of structures, making them the most damaging.
(ii) Which one of the following is a direct source of information about the interior of the earth?
(b) Volcanoes
Reason: Volcanic eruptions bring molten magma from deep inside the earth to the surface, allowing direct sampling. Earthquake waves, gravity, and magnetism are indirect sources.
Reason: Volcanic eruptions bring molten magma from deep inside the earth to the surface, allowing direct sampling. Earthquake waves, gravity, and magnetism are indirect sources.
(iii) Which type of volcanic eruptions have caused Deccan Trap formations?
(b) Flood
Reason: Flood Basalt Provinces (like the Deccan Traps) are formed by highly fluid lava flowing over vast distances for long periods.
Reason: Flood Basalt Provinces (like the Deccan Traps) are formed by highly fluid lava flowing over vast distances for long periods.
(iv) Which one of the following describes the lithosphere:
(b) crust and upper mantle
Reason: The Lithosphere is the rigid outer layer of the earth, comprising the crust and the uppermost solid part of the mantle.
Reason: The Lithosphere is the rigid outer layer of the earth, comprising the crust and the uppermost solid part of the mantle.
2. Short Answer Questions
(i) What are body waves?
Body waves are earthquake waves that are generated due to the release of energy at the focus. They move in all directions travelling through the body of the earth. They are of two types: P-waves and S-waves.
(ii) Name the direct sources of information about the interior of the earth.
The direct sources are:
- Surface Rocks/Mining: Rocks obtained from mining areas (e.g., gold mines in South Africa).
- Drilling Projects: Deep Ocean Drilling Project and Integrated Ocean Drilling Project (deepest drill at Kola, Arctic is 12 km).
- Volcanic Eruptions: Magma thrown out during volcanic eruptions.
(iii) Why do earthquake waves develop shadow zone?
Shadow zones develop because of the variation in the properties of the earth’s interior layers.
S-waves cannot travel through liquids (outer core), creating a huge shadow zone.
P-waves are refracted (bent) when they pass from the mantle to the core due to density differences. These behaviors leave specific areas on the surface where seismographs record no waves. [Image of earthquake shadow zones]
S-waves cannot travel through liquids (outer core), creating a huge shadow zone.
P-waves are refracted (bent) when they pass from the mantle to the core due to density differences. These behaviors leave specific areas on the surface where seismographs record no waves. [Image of earthquake shadow zones]
(iv) Briefly explain the indirect sources of information of the interior of the earth other than those of seismic activity.
Indirect sources include:
- Density and Pressure Studies: Calculating the density of core based on earth’s total mass.
- Temperature: Increase in temperature with depth observed in mines.
- Meteors: They have similar material structure to earth (solid bodies).
- Gravitation: Gravity anomalies ($g$ varies at poles vs equator) indicate uneven mass distribution.
- Magnetic Field: Surveys reveal distribution of magnetic materials in the crust.
3. Long Answer Questions
(i) What are the effects of propagation of earthquake waves on the rock mass through which they travel?
Earthquake waves cause vibration in the rocks they travel through. The effects differ by wave type:
- P-Waves (Primary): They vibrate parallel to the direction of the wave. This creates density differences in the material leading to stretching and squeezing (compression and expansion) of the rock mass.
- S-Waves (Secondary): They vibrate perpendicular (at right angles) to the direction of wave propagation (vertical plane). This creates troughs and crests in the material, causing distortion.
- Surface Waves: These are the most damaging as they cause displacement of rocks and collapse of structures.
(ii) What do you understand by intrusive forms? Briefly describe various intrusive forms.
Intrusive Forms: When volcanic magma cools and solidifies within the crust (below the surface) before reaching the top, the resulting rock formations are called intrusive forms (Plutonic rocks).
Various Intrusive Forms:
Various Intrusive Forms:
- Batholiths: Large, dome-shaped bodies of magma cooled at great depths; forms the core of mountains.
- Laccoliths: Dome-shaped bodies with a level base, connected by a pipe-like conduit from below (mushroom shape).
- Lapolith: Saucer-shaped, concave body of lava.
- Phacolith: Wavy mass of intrusive rocks found at the base of synclines or top of anticlines.
- Sills/Sheets: Horizontal or near-horizontal layers of solidified magma. Thinner ones are sheets, thicker are sills.
- Dykes: Vertical wall-like structures formed when lava solidifies in perpendicular cracks/fissures (common in Deccan Traps).