NCERT Solutions for Class 11 Geography Fundamentals of Physical Geography Chapter 3: Interior of the Earth

Chapter at a glance

Exercise Questions

Question 1 (i)

Which one of the following earthquake waves is more destructive?
(a) P-waves (b) S-waves (c) Surface waves (d) None of the above

Answer

3. Surface waves

Question 1 (ii)

Which one of the following is a direct source of information about the interior of the earth?
(a) Earthquake waves (b) Volcanoes (c) Gravitational force (d) Earth magnetism

Answer

Volcanic eruptions bring material from the interior to the surface; the others are indirect sources.

2. Volcanoes

Question 1 (iii)

Which type of volcanic eruptions have caused Deccan Trap formations?
(a) Shield (b) Flood (c) Composite (d) Caldera

Answer

2. Flood (basalt)

Question 1 (iv)

Which one of the following describes the lithosphere:
(a) upper and lower mantle (b) crust and upper mantle (c) crust and core (d) mantle and core

Answer

2. crust and upper mantle

Question 2 (i)

What are body waves?

Answer

Body waves are earthquake waves generated by the release of energy at the focus that travel through the body (interior) of the earth in all directions. They are of two types: P-waves (primary, longitudinal, fastest, travel through solids, liquids and gases) and S-waves (secondary, transverse, travel only through solids).

Earthquake waves that travel through the earth’s interior from the focus — P-waves and S-waves.

Question 2 (ii)

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

Answer

1. Surface rocks and rocks obtained from mining areas (e.g. gold mines of South Africa, 3–4 km deep).
2. Deep drilling projects (e.g. “Deep Ocean Drilling Project”, “Integrated Ocean Drilling Project”, Kola Superdeep borehole in the Arctic, 12 km).
3. Volcanic eruptions bringing magma (lava) to the surface.

Surface and mining rocks, deep drilling projects, and volcanic eruptions.

Question 2 (iii)

Why do earthquake waves develop shadow zone?

Answer

A shadow zone is an area where seismographs do not record certain waves. It develops because:

• S-waves cannot travel through liquids, so they do not pass through the liquid outer core; hence there is no S-wave record beyond about 105° from the epicentre.
• P-waves are refracted (bent) when they enter the liquid core, so they are not recorded in a belt between 105° and 145°.

Thus the shadow zones reveal the liquid nature of the outer core.

Because S-waves cannot pass through the liquid outer core and P-waves are refracted by it, leaving zones (beyond 105° for S; 105°–145° for P) with no record.

Question 2 (iv)

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

Answer

1. Temperature, pressure and density increase with depth; from known rates, scientists estimate conditions deep inside.
2. Meteors that fall to earth are made of materials similar to the earth’s interior (iron-rich cores).
3. Gravitation: gravity varies over the surface; gravity anomalies indicate differences in the distribution of mass in the crust.
4. Magnetic surveys give information about the distribution of magnetic materials in the crust.

Changes in temperature, pressure and density with depth; meteorites; gravity anomalies; and magnetic surveys.

Question 3 (i)

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

Answer

When earthquake waves travel through rocks, they cause the rock material to vibrate:

1. P-waves vibrate parallel to the direction of wave movement. They exert pressure on the material, causing compression and expansion (density differences) in the rock.
2. S-waves vibrate perpendicular to the direction of propagation; they create troughs and crests in the material.
3. Surface waves are the last to reach the surface; they move the ground up and down and side to side and are the most destructive, causing displacement of rocks and collapse of structures.

Wave velocity changes as they pass through materials of different density; they are reflected and refracted at boundaries. This behaviour has helped scientists infer the layers of the earth.

The effects on the surface include ground shaking, ground lurching, landslides, liquefaction of soil, avalanches, ground displacement, floods from dam failures, fires, tsunamis and structural collapse.

P-waves compress and stretch rock along their path, S-waves shake it at right angles (crests and troughs), and surface waves cause the most violent shaking; waves are reflected and refracted at layer boundaries, and at the surface cause shaking, landslides, liquefaction and collapse.

Question 3 (ii)

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

Answer

When magma cools and solidifies within the crust (below the surface), it forms intrusive (plutonic) landforms. They become visible when overlying rocks are eroded.

1. Batholith: a huge mass of magma cooled deep in the crust in the form of large domes; made of granite; forms the core of mountains.
2. Laccolith: a large dome-shaped intrusive body with a level base, connected by a pipe-like conduit from below (e.g. granite domes of Karnataka).
3. Lapolith: magma that cools in a saucer-shaped, concave basin.
4. Phacolith: wavy mass of intrusive rock found at the base of synclines or top of anticlines in folded rocks.
5. Sill: a near-horizontal sheet of intrusive rock parallel to the bedding planes; thin ones are called sheets.
6. Dyke: a wall-like, near-vertical intrusion cutting across the rock layers; common in the western Maharashtra (Deccan Traps), acting as feeders for eruptions.

Intrusive forms are magma bodies solidified inside the crust: batholiths (huge granite masses), laccoliths (domes), lapoliths (saucers), phacoliths (wavy, in folds), sills (horizontal sheets) and dykes (vertical walls).

Topic checklist

• Direct and indirect sources of information
• Earthquakes: waves, shadow zones, measurement, effects
• Structure of the earth: crust, mantle, core
• Volcanoes and volcanic landforms
• Intrusive forms

Total questions answered: 10 of 10