Interior of the Earth
Chapter 2 — Question & Answer Solutions
Tick ✓ the correct options in the box.
(A) There are two layers in the crust.
(B) Which element is found in both mantle and crust?
(C) Which of these minerals are found in the core of the earth?
(D) The inner core is in which state?
(E) The outer core is made up of
(F) The layer of the earth on which we live.
(G) Which seismic waves can travel through a liquid medium?
Tell whether right or wrong. Correct the wrong statement.
Answer the following.
(A) What are the two parts of the crust? What is the basis of classification?
Ans:
- The continental crust and the oceanic crust are the two parts of the crust.
- The continental crust mainly consists of Silica (Si) and Aluminium (Al) in higher proportions. It is also called Sial.
- The oceanic crust is composed of Silica (Si) and Magnesium (Mg). It is also called Sima.
- The crust is classified mainly on the basis of its thickness, as well as the composition of its minerals.
(B) Why is the upper mantle called the asthenosphere?
Ans:
- The mantle lies below the crust.
- The upper portion of the mantle is called the ‘asthenosphere.’ The word ‘astheno’ means weak.
- It is in a semi-solid, plastic state, and it is from this weaker zone that molten rock material, or magma, finds its way to the surface during volcanic eruptions.
- The epicentres of deep-seated earthquakes are also found here.
- Therefore, the upper mantle is called the ‘asthenosphere.’
(C) The magnetosphere of the earth is a result of rotation. Explain.
Ans:
- The average temperature of the outer core of the earth is around 5000°C, while the average temperature of the inner core is around 6000°C. This difference in temperature results in the formation of vertical currents.
- The rotation of the earth gives an eddy (circular) motion to these currents.
- Electric currents develop in these spiral eddies of liquid iron, leading to the generation of the earth’s magnetic field. This magnetic field is called the magnetosphere.
- In this way, the magnetosphere is formed due to the rotation of the earth.
Draw neat diagrams, label them and explain.
(A) The interior of the earth
Ans:The earth’s interior has three main layers. The crust is the thin, solid outer layer (about 30 km thick on average) on which we live. Below it lies the thick mantle, which is solid but capable of very slow flow. At the centre lies the core, made of iron and nickel — its outer part is liquid, while the innermost part is solid due to extremely high pressure. The crust together with the rigid uppermost part of the mantle (about 100 km) is called the lithosphere.
(B) Magnetic pole and equator
Ans:The earth’s geographical axis (the line joining the North and South geographic poles, around which the earth rotates) does not exactly line up with its geo-magnetic axis (the line joining the north and south magnetic poles). Because of this, the geographical equator and the magnetic equator are also slightly tilted with respect to each other. This difference exists because the earth’s magnetic field is generated by the movement of molten iron in the outer core, which does not align perfectly with the earth’s axis of rotation.
Give geographical reasons:
(A) There is variety in the interior of the earth.
Ans:
- There are differences in the elements, temperature and pressure found in the three parts of the earth’s interior — the crust, mantle and core.
- Transitional zones are found between: (a) the continental crust and oceanic crust, (b) the crust and mantle, (c) the upper mantle and lower mantle, (d) the mantle and core, and (e) the outer core and inner core.
- Therefore, there are discontinuities in the interior of the earth.
(B) There is a correlation between the density of metals and their location in the interior of the earth.
Ans:
- The rocks found in the crust are mostly made up of the compounds silicon, aluminium and magnesium. These compounds are lighter in weight, so they are found in the upper layer of the earth’s interior.
- Elements like iron and nickel are found in the core of the earth. These elements are heavier in weight.
- Thus, metals with low density are found in the upper level of the earth’s interior, while metals with high density are found in the lower level — showing a clear correlation between density and location.
(C) The mantle is the centre of earthquakes and volcanic eruptions.
Ans:
- The rocks in the upper 100 to 200 km of the mantle melt due to intense heat, forming magma.
- Magma chambers are found in this layer of the earth.
- Due to heat and pressure, energy waves are created in the mantle. These energy waves move vertically, and magma rises to the surface during volcanic eruptions.
- Thus, the mantle is the centre of earthquakes and volcanic eruptions.
(D) The thickness of the crust below the continents is greater compared to the oceans.
Ans:
- The density of the crust below the continents is 2.65 to 2.90 g/cm³, while the density of the crust below the oceans is 2.90 to 3.3 g/cm³.
- Since the crust below the continents has comparatively low density, it keeps floating on the mantle and does not sink into it — as a result, its thickness remains high.
- Since the crust below the oceans has comparatively high density, it gradually sinks into the mantle, which keeps its thickness low.
- Thus, the thickness of the crust below the continents is greater compared to that below the oceans.
(E) The earth is protected because of the magnetosphere.
Ans:
- The ozone layer protects the earth from harmful ultraviolet radiation.
- The earth’s magnetosphere deflects most of the solar wind, whose charged particles would otherwise strip away the ozone layer.
- In this way, the earth is protected because of its magnetosphere.