Important Questions: Interior of the Earth | Geography Class 11 - Humanities/Arts PDF Download

Very Short Answer Type Questions

Q1: Where does the asthenosphere lie?
Ans: The asthenosphere lies in the upper mantle, or below the earth’s crust.

Q2: What is the effect on ‘P’ waves at the mantle-core boundary?
Ans: The ‘P’ waves make an abrupt drop in velocity at the mantle- core boundary.

Q3: Which region in the earth’s interior is referred to as the low-velocity zone?
Ans: Asthenosphere in the upper part of the mantle is referred to as the low-velocity zone.

Q4: What is Love wave?
Ans: It is an earthquake wave that travels along the surface of the ‘ earth with a motion entirely horizontal.

Q5: What is a seismograph?
Ans: It is a sensitive instrument that records the intensity of vibrations of earthquake waves and helps in earthquake prediction.

Q6: What is the temperature of the core?
Ans: The temperature of the core is about 2000°C.

Q7: What is the depth of the earth’s crust?
Ans: 0-100 km. from the surface of the earth.

Short Answer Type Questions

Q8: What is asthenosphere?
Ans: The asthenosphere is the zone of hot rocks, believed to be in a plastic condition, underlying the solid lithosphere or the earth’s crust. It is sometimes termed as the soft layer of mantle or the low-velocity zone because the earthquake waves travel in it at reduced velocities.

Q9: Define surface waves along with their sub-types.
Ans: Hie earthquake waves that move along the free upper crust of the earth are called surface waves. Surface waves are of two types, viz, Rayleigh waves and Love waves. Rayleigh waves can be visualised as water waves travelling across the surface of a still pond after a pebble has been tossed into the water. But the motion in the Love waves is entirely horizontal, at right, angles to the direction of the wave motion. The Rayleigh and Love waves travel more or less the same length, but with different speeds.

Q10: How is the crust distinguished from the mantle?
Ans: The crust is distinguished from the mantle by the presence of an abrupt change in the velocity of seismic waves. This corresponds to the abrupt change in the rigidity of the rock from crust to mantle. This change in rigidity is due to the change in the mineral composition or in the physical state of rocks.

Q11: Describe the earth’s crust.
Ans: Earth’s crust is the topmost layer of the earth’s interior. It is also called the lithosphere. It has an average density of 3.0 g/cm. Its thickness varies from about 7 km. beneath the oceans to 70 km. under some parts of the continents. It is separated from the mantle below by Mohorovicic Discontinuity.

Q12: How do the rocks of the earth’s mantle behave when subjected to the earthquake waves?
Ans: The P’ waves move faster and make an abrupt drop in velocity of the mantle-core boundary, whereas ‘S’ waves move slowly and terminate at the mantle-core boundary. Through earth’s mantle (nearly ’ 2900 km) the speed of the earthquake waves is so high that only a very rigid and dense rock will satisfy the observed condition; the rocks behave as an elastic solid so that the mantle changes its shape when shear stresses are applied and returns exactly to its former shape when stresses are removed.

Q13: Describe the three types of earthquake waves.
Ans: These waves are :

• ‘P’waves or longitudinal waves-These are also known as primary waves. These travel in the direction of their movement. They can travel through solids as well as liquid and gaseous matter.
• ‘S’ waves or transverse waves – These are also known
• as secondary waves. These travel at a right angle to the direction of their oscillation. They can travel in solid ‘? medium only.
• ‘L’ waves – These are known as surface waves. These waves do hot go deep into the earth.

Long Answer Type Questions

Q14: Discuss how do seismic waves suggest layering of the earth’s interior.
Ans: Seismic waves, which are waves of energy caused by the sudden breaking of rock within the Earth or an explosion, are instrumental in revealing the layering of the Earth's interior. When seismic waves travel through the Earth, they behave differently as they encounter various materials, leading to their reflection, refraction, and absorption. These behaviors provide crucial information about the composition and structure of the Earth's layers.
1. P-Waves (Primary or Compressional Waves):

• Propagation: P-waves are the fastest seismic waves and can travel through both solid and liquid layers of the Earth.
• Reflection and Refraction: P-waves can be reflected and refracted when they encounter boundaries between different materials with varying densities. The angles at which they reflect and refract provide information about the Earth's composition.
• Absorption: P-waves are partially absorbed by liquid layers, indicating the presence of molten material beneath the Earth's crust.

2. S-Waves (Secondary or Shear Waves):

• Propagation: S-waves are slower than P-waves and can only travel through solid materials.
• Reflection and Absence in Liquid Layers: S-waves are completely absorbed by liquid layers. The absence of S-wave arrivals in certain areas during seismic events indicates the presence of molten material beneath the Earth's surface.
• Limited Reflection: S-waves experience limited reflection and refraction, which helps scientists identify the boundaries between solid layers.

3. Surface Waves:

• Propagation: Surface waves travel along the Earth's surface and are responsible for the damage caused during earthquakes.
• Revealing Layering: Surface waves are sensitive to the Earth's surface geology. Variations in surface wave amplitudes and velocities provide information about the properties of surface materials and subsurface layers.

4. Mohorovičić Discontinuity (Moho):

• Identification: The Moho is the boundary between the Earth's crust and the underlying mantle. Seismic waves experience a sudden increase in velocity at this boundary, indicating the transition from less dense crustal rocks to denser mantle rocks.

5. Gutenberg Discontinuity:

• Identification: The Gutenberg Discontinuity is the boundary between the Earth's mantle and the outer core. S-waves cannot travel through the liquid outer core, confirming the presence of a molten layer beneath the Earth's mantle.

6. Lehmann Discontinuity:

• Identification: The Lehmann Discontinuity is the boundary between the Earth's outer core and the inner core. P-waves experience a sudden increase in velocity at this boundary, indicating the transition from liquid to solid material in the inner core.

Q15: How do the waves of different types tell us about the changes in the nature of different layers of the earth’s interior?
Ans: By analyzing the behavior of seismic waves and studying the patterns of their arrival at seismometers during earthquakes, scientists can map the layering of the Earth's interior. This information helps geologists understand the Earth's composition, the movement of tectonic plates, and the dynamics of geological processes. Seismic waves are, therefore, crucial tools for investigating the deep structure of our planet.
Different types of seismic waves provide valuable information about the Earth's interior by exhibiting unique behaviors as they travel through various layers. These behaviors help scientists deduce the nature, composition, and boundaries of different Earth layers. Here's how different seismic waves convey information about the changes in the Earth's interior:
1. P-Waves (Primary or Compressional Waves):

Nature: P-waves are compressional waves that travel through both solids and liquids.
Indications:

• Velocity Changes: Changes in P-wave velocity indicate transitions between different materials. A sudden decrease in velocity might suggest a change from a solid to a less dense, more elastic material.
• Reflection and Refraction: P-waves reflect and refract at boundaries with different densities, revealing layer interfaces.
• Partial Absorption: P-waves are partially absorbed by liquids, indicating the presence of molten materials like magma or the Earth's outer core.

2. S-Waves (Secondary or Shear Waves):
Nature: S-waves are shear waves that only travel through solids, not through liquids or gases.
Indications:

• Absence in Liquids: S-waves are completely absorbed by liquids, indicating the presence of molten material beneath the Earth's surface.
• Limited Reflection: S-waves experience limited reflection and refraction, helping identify boundaries between solid layers.

3. Surface Waves:
Nature: Surface waves travel along the Earth's surface and cause most of the earthquake damage.
Indications:

• Sensitive to Surface Geology: Surface waves are sensitive to surface materials. Variations in surface wave amplitudes and velocities provide information about the properties of surface materials and subsurface layers.
• Love and Rayleigh Waves: Different types of surface waves, such as Love waves and Rayleigh waves, have different particle motion patterns and velocities, allowing scientists to infer the nature of subsurface materials.

4. Reflection and Refraction:

• Reflection: Seismic waves reflect at boundaries between layers with different densities and elastic properties. The angle of reflection provides information about the properties of the layers involved.
• Refraction: Seismic waves change direction (refract) when they pass from one layer to another with different velocities. The degree of refraction helps determine the velocity and density contrasts between layers.

5. Mohorovičić Discontinuity (Moho) and Other Boundaries:
Nature: Discontinuities are boundaries between Earth layers with distinct differences in composition and density.
Indications: Seismic waves experience abrupt changes in velocity and amplitude at these boundaries, indicating transitions between Earth's crust, mantle, outer core, and inner core.

Q16: Write short notes on:
(i) Shadow zone
Ans: It lies beneath the surface of the earth, i.e., in its interior. The seismic waves bend as they travel through the core and, therefore, ‘P’ waves are not directly received in a zone known as the shadow zone. Also, ‘S’ waves are not received there because they do not travel through the liquid outer core. Only surface waves are received in the shadow zone.

(ii) The earth’s crust
Ans: This is also known as the lithosphere. The crust is the outermost shell 1 of the earth. It consists of the surface granite SIAL and the intermediate basic SIMA layers. It is separated from the under layer MANTLE by the Mohorovicic Discontinuity. There are two kinds of crust – continental and oceanic. Continental crust has an average density of 3 g/cnt3, the average thickness of 35 to 40 km. (22 to 25 miles) with large areas older than 1500 million years. Continental crust is a complicated structure and has a variable composition. Oceanic crust is thinner than continental crust. Its average density is 2.7 g/cm3 and average thickness of only 6 km. (3.7 miles), with the simple layered structure of the uniform composition.

Q17: Distinguish between:
(i) Body Waves and Surface Waves
Ans: Body Waves:

• Nature: Body waves are seismic waves that travel through the Earth's interior, propagating in all directions.
• Types: There are two types of body waves - P-waves (Primary or Compressional waves) and S-waves (Secondary or Shear waves).
• Propagation: Body waves travel through the Earth, including its solid interior and sometimes through liquids and gases.
• Speed: Body waves are faster than surface waves.
• Effect: Body waves are responsible for the initial shaking felt during an earthquake.

Surface Waves:

• Nature: Surface waves are seismic waves that travel along the Earth's surface.
• Types: There are two main types of surface waves - Love waves and Rayleigh waves.
• Propagation: Surface waves do not penetrate deep into the Earth but move along its surface.
• Speed: Surface waves are slower than body waves.
• Effect: Surface waves cause most of the damage during an earthquake, as they produce strong shaking and ground displacement.

(ii) The crust of the earth and Core of the earth
Ans: Crust of the Earth:

• Composition: The Earth's crust is composed of rocks, minerals, and soil. It is relatively thin compared to the other layers.
• Depth: The crust is the outermost layer and is relatively shallow, making up only about 1% of the Earth's volume.
• Types: There are two types of crust - continental crust (found under continents, thicker but less dense) and oceanic crust (found under oceans, thinner but denser).
• State: The crust is solid and cool compared to the deeper layers of the Earth.

Core of the Earth:

• Composition: The Earth's core is primarily composed of iron and nickel. It is extremely dense and hot.
• Depth: The core is located beneath the Earth's mantle and constitutes a significant portion of the Earth's volume.
• Divisions: The core is divided into two layers - the outer core (liquid) and the inner core (solid).
• State: The outer core is in a liquid state due to high temperatures and pressure, while the inner core is solid due to even higher pressure, despite its extreme heat.

(iii) Gutenberg Discontinuity and Mohorovicic Discontinuity
Ans: Gutenberg Discontinuity:

• Location: The Gutenberg Discontinuity is the boundary between the Earth's mantle and the outer core.
• Effect on Seismic Waves: S-waves (shear waves) cannot pass through the liquid outer core, so their absence in certain areas during seismic events indicates the presence of the molten outer core.
• State: The outer core, below the Gutenberg Discontinuity, is in a liquid state due to extremely high temperatures.

Mohorovičić Discontinuity (Moho):

• Location: The Mohorovičić Discontinuity is the boundary between the Earth's crust and the underlying mantle.
• Effect on Seismic Waves: P-waves (primary waves) experience a sudden increase in velocity when they cross the Moho, indicating the transition from less dense crustal rocks to denser mantle rocks.
• State: Both the Earth's crust and the uppermost part of the mantle are solid above the Moho Discontinuity.

Q18: What is the main evidence in favour of the layered structure of the earth?
Ans: The scientists accept that the earth has a layered structure. The earth has three layers or shells :

• crust
• mantle
• core.

These layers are distinguished on the basis of their physical and chemical properties, i.e.,

• thickness,
• density,
• temperature,
• metallic contents and
• rocks.

Q19: Discuss the properties of the upper mantle.
Ans: The upper mantle is a crucial layer of the Earth's interior, situated beneath the Earth's crust and above the lower mantle. It possesses several distinct properties that play a significant role in the Earth's geological processes and tectonic activities:

1. Composition:

• Silicate Minerals: The upper mantle is primarily composed of silicate minerals, including olivine, pyroxenes, and garnet. Olivine is especially abundant and is considered a key mineral in the upper mantle.

2. Physical State:

• Solid State: The upper mantle is in a solid state, although it is capable of flowing over geological timescales. The solid but ductile nature of the upper mantle allows for convection currents, which drive plate tectonics and other geological processes.

3. Temperature and Pressure:

• High Temperature: The upper mantle is characterized by high temperatures, which increase with depth. The heat is primarily generated by the radioactive decay of elements within the Earth's interior.
• High Pressure: The upper mantle experiences significant pressure due to the weight of the Earth's overlying materials.

4. Rheology:

• Ductility: The upper mantle exhibits ductility, meaning it can flow slowly over time. This plasticity allows for the movement of tectonic plates, leading to phenomena such as earthquakes, volcanic eruptions, and mountain formation.
• Convective Flow: Convection currents within the upper mantle contribute to the movement of tectonic plates. Hot, less dense material rises, while cooler, denser material sinks, creating a cyclical flow pattern.

5. Seismic Properties:

• Seismic Velocity: Seismic waves, especially P-waves, travel faster through the upper mantle compared to the crust, indicating a higher velocity layer beneath the Earth's surface.
• Seismic Anisotropy: The upper mantle exhibits seismic anisotropy, meaning seismic waves have different velocities in different directions. This anisotropy provides information about the alignment of minerals and the mantle's deformation history.

6. Role in Plate Tectonics:

• Tectonic Plate Movements: The upper mantle is directly involved in the movement of tectonic plates. Convection currents within the upper mantle create forces that push, pull, and deform the Earth's crust, leading to the drifting of continents, earthquakes, and the formation of mountain ranges.
• Subduction Zones: Subduction zones, where one tectonic plate is forced beneath another, involve the sinking of oceanic crust into the upper mantle. This process leads to the formation of deep ocean trenches and volcanic arcs.

Q20: Write a short note on the earth’s core.
Ans: The Earth's core is a crucial and mysterious part of our planet, located beneath the Earth's mantle and above the inner core. Comprising primarily of iron and nickel, the core is about 2,900 kilometers (1,800 miles) beneath the Earth's surface. It is divided into two main parts: the outer core and the inner core.

• Outer Core: The outer core, about 2,300 kilometers (1,400 miles) thick, is in a liquid state. The extreme pressure and temperature cause the iron and nickel in this layer to remain in a molten state. The movement of molten iron in the outer core generates the Earth's magnetic field through a process called the geodynamo effect. This magnetic field is vital for life on Earth as it protects the planet from harmful solar radiation.
• Inner Core: Beneath the outer core lies the inner core, which has a radius of about 1,220 kilometers (760 miles). Unlike the outer core, the inner core is solid due to the incredibly high pressure, despite the temperature being even higher than the outer core. The pressure at the Earth's core is about 3.6 million times atmospheric pressure, and the temperature can reach up to 5,700 degrees Celsius (10,300 degrees Fahrenheit).

Understanding the Earth's core is vital for geologists and scientists because it holds key information about the Earth's composition and its geological history. Studying seismic waves (waves generated by earthquakes) has provided significant insights into the composition and properties of the Earth's core. These studies have revealed that the core is not uniform, and there are variations in composition and density, which contribute to the Earth's complex geological processes.