2. Interior of the Earth
Complete solutions to the textbook exercise. Maharashtra State Board (Balbharati), Grade Eight Geography.
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Exercise
(A) There are two layers in the crust.
- (i) Inner and outer crust
- ✓ (ii) Continental and oceanic crust
- (iii) Surface and oceanic crust
- (iv) Mantle and Core
Answer: Continental and oceanic crust
(B) Which element is found in both mantle and crust?
- ✓ (i) Silica
- (ii) Magnesium
- (iii) Aluminium
- (iv) Iron
Answer: Silica (Note: Both Silica and Magnesium are present, but Silica is the dominant and most common element spanning the continental crust, oceanic crust, and mantle silicates).
(C) Which of these minerals are found in the core of the earth?
- (i) Iron-magnesium
- (ii) Magnesium-nickel
- (iii) Aluminium- Iron
- ✓ (iv) Iron-nickel
Answer: Iron-nickel
(D) The inner core is in which state?
- (i) Gaseous
- ✓ (ii) Solid state
- (iii) Semi-solid state
Answer: Solid state
(E) The outer core is made up of
- ✓ (i) Iron
- (ii) Gold
- (iii) Hydrogen
- (iv) Oxygen
Answer: Iron
(F) The layer of the earth on which we live.
- (i) Mantle
- (ii) Core
- (iii) Crust
- ✓ (iv) Continental crust
Answer: Continental crust
(G) Which seismic waves can travel through liquid medium?
- ✓ (i) Primary waves
- (ii) Secondary Waves
- (iii) Surface waves
- (iv) Oceanic waves
Answer: Primary waves
(A) The density of various materials is not the same in the interior of the earth.
Answer: Right
(B) The core of the earth's interior is made up of hard rock.
Answer: Wrong
Correct Statement: The outer core of the earth's interior is in a liquid state, while the inner core is in a solid state and is primarily made up of heavy metals like iron and nickel (Nife), not just hard rock.
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(C) Secondary waves cannot pass through outer core.
Answer: Right
(D) Continental crust is made up of silica and magnesium.
Answer: Wrong
Correct Statement: Continental crust is mainly made up of silica and aluminium (Sial). It is the oceanic crust that is made up of silica and magnesium (Sima).
(A) What are the two parts of the crust? What is the basis of classification?
The two parts of the crust are the Continental Crust and the Oceanic Crust. The basis of this classification is their thickness, physical composition, and density. The continental crust is thicker (16 to 45 km) but less dense, composed mainly of silica and aluminium (Sial). In contrast, the oceanic crust is thinner (around 7 to 10 km) but denser, composed predominantly of silica and magnesium (Sima).
(B) Why is the upper mantle called the asthenosphere?
The upper layer of the mantle is in a more liquid or semi-liquid state compared to the solid crust above it. It contains magma chambers where magma is formed and is a mechanically weak zone. Because this layer is partially molten, it is referred to as the asthenosphere.
(C) Magnetosphere of the earth is a result of rotation. Explain.
The outer core of the earth is in a liquid state and contains a high proportion of iron. The extreme difference in temperatures between the outer and inner core gives rise to vertical currents in this liquid layer. As the earth rotates, it gives these vertical currents an eddy (circular) motion. These spiral eddies of liquid iron generate electric currents, which in turn generate a massive magnetic field around the earth. This functional magnetic field that extends far outside the earth's surface forms a protective cover known as the magnetosphere.
(A) The interior of the earth
Note: Please draw Figure 2.2 from page 12 of your textbook. Draw a wedge or a circle showing the distinct layers.
Explanation: The interior of the earth is divided into three main layers based on temperature, density, and material composition:
1. Crust: The solid, outermost, and thinnest layer. It is divided into the Continental Crust (Sial: silica and aluminium) and Oceanic Crust (Sima: silica and magnesium).
2. Mantle: Lies below the crust and extends up to 2870 km. It is divided into the upper mantle (asthenosphere, containing magma) and the solid lower mantle.
3. Core: The innermost part starting from 2900 km. It consists of a liquid outer core and a solid inner core made of heavy metals (Iron and Nickel, also known as Nife).
(B) Magnetic pole and equator
Note: Please draw Figure 2.3 (B) from page 13 of your textbook showing the Earth as a magnet with its magnetic field lines.
Explanation: The earth acts like a giant magnet due to the geo-dynamo effect originating in the liquid outer core. The diagram illustrates the Geographical Axis (the true north-south axis of rotation) and the Geo-Magnetic Axis, which is slightly tilted away from the geographical axis. The Magnetic Equator runs roughly perpendicular to this magnetic axis. The magnetic field lines emerge from the magnetic poles and curve around the earth, creating the magnetosphere which shields the planet from solar winds.
(A) There is variety in the interior of the earth.
As we travel from the earth's surface toward its centre, there are major progressive changes in temperature, pressure, and density. Due to these extreme variations, the earth's materials exist in different physical states (solid crust, semi-liquid upper mantle, liquid outer core, and solid inner core) and consist of different chemical compositions (light silicates at the surface to heavy iron and nickel at the core). This results in a distinct, layered variety within the earth's interior.
(B) There is correlation between the density of metals and their location in the interior of the earth.
During the earth's formation and cooling, heavier materials sank toward the center while lighter materials floated toward the surface. As a result, the outermost layer (crust) contains lighter elements like silica and aluminium with a lower density (approx 2.65 to 3.3 gm/cm³). As depth increases, the density increases due to extreme pressure and the presence of heavier metals. The core contains the heaviest metals, iron and nickel, with the highest density (up to 13.3 gm/cm³ in the inner core).
(C) Mantle is the centre of earthquake and volcanic eruptions.
The upper layer of the mantle, called the asthenosphere, is in a semi-liquid state and houses massive magma chambers. Extreme heat and endogenetic movements within this layer generate immense internal energy and pressure. When this energy finds a weak point in the solid crust above, it forcefully pushes the magma out, causing volcanic eruptions. The sudden release of this built-up energy also causes the crustal plates to shift, generating deep-seated earthquakes.
(D) The thickness of the crust below the continents is less as compared to oceans.
Note: The premise of this statement in the question is factually incorrect based on the textbook data.
Correction & Reason: Actually, the thickness of the crust below the continents is greater than it is below the oceans. The continental crust has an average thickness of 30 to 35 km (up to 45 km or more under mountain ranges), while the oceanic crust is much thinner, generally less than 10 km thick. However, the oceanic crust is denser than the thicker continental crust.
(E) Earth is protected because of the magnetosphere.
The earth's magnetic field extends far beyond its physical surface, creating an invisible shield known as the magnetosphere. The sun constantly emits highly charged, dangerous particles known as solar winds. When these solar winds approach the earth, the magnetosphere deflects them and prevents them from stripping away our atmosphere. Without this protective cover, the earth's atmosphere would be destroyed, and life would not be possible.