NCERT
Exercise
Que 1
Choose the correct
answer from the four options given below :-
(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
ANS. (c) Surface waves
(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
ANS. (b) Volcanoes
(iii)
Which types of volcanic
eruptions have caused Deccan Trap formations?
(a)
Shield
(b)
Flood
(c)
Composite
(d)
Caldera
ANS. (b) Flood
(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
ANS. (b) Crust and upper mantle
(i) What are body waves?
ANS. The waves produced by an
earthquake are called Earthquake Waves. Geologic waves are generated
during the release of energy from the center of an earthquake’s origin and move in all directions in the interior of the earth. These
are also called body waves.
There
are three types of geological Or Body waves:-
1. P – Primary Waves,
2. S - Secondary Waves
3. L – Long Waves.
(ii) Name the direct sources of information about the interior
of the earth.
ANS.
1.
Mining
2.
Deep wells
3.
Excavating Pipelines
4.
Volcano Eruption
5.
Deep sea drilling
(iii) Why do earthquake waves develop shadow zone?
ANS. The area inside the Earth
where no seismic wave is recorded on a seismograph is called the shadow
region or shadow zone. The area between 105 degrees to 145 degrees
from the earthquake epicenter, where no seismic waves are recorded, forms a
shadow zone for P and S waves. The shadow zone of S waves is wider than the
shadow zone of P waves, which is more than 40% of the Earth's area. The
formation of the shadow region proves that the Earth's interior is made up of
heavy metals.
(iv) Briefly explain the indirect sources of information of the
interior of the earth other than those of seismic activity.
ANS.
In
addition to seismic activities, the following are the direct or indirect means
of giving information about the earth: -
1.
Density of different layers of the earth
2.
Rock pressure
3.
Temperature
4.
Composition of meteorites
5.
Gravity
6. Magnetic field.
Que 3 Answer the following questions in about 150 words:-
(i) What are
the effects of propagation of earthquake waves on the rock mass through which
they travel?
ANS. Different types of seismic
waves have different modes of transmission. For example, the direction of
vibration of P waves is parallel to the direction of the waves. This
transmission exerts pressure on the material in the same direction of motion.
As a result, there is a difference in the density of the substances. The
process of contraction and expansion takes place in rocks.
S
waves produce vibrations in the vertical plane at right angles to the direction
of the waves. Hence, they form bulges and troughs in the medium through which
they pass. Surface waves are considered the most destructive.
(ii) What do
you understand by intrusive forms? Briefly describe various intrusive forms.
ANS. When lava solidifies into rock layers below the crust before reaching the surface, various types of forms are formed, which are called intrusive forms.
Some of the important
interpolating figures are as
1. Batholith: - Whenever the hot magma generated
by the volcano cools down in the form of a large body and dome in the cracks
inside the earth crust before coming to the surface, then this shape or body
made of granite is called batholith. Huh. Sometimes these bodies appear on the
surface when the upper matter is removed by denudation (erosion) processes. For
example, "Devil's Tower" located in the Devil's Tower National
Monument located in the Utah region of America is a similar type of batholith
rock.
2. Lacolith:-
Whenever
hot magma in penetrating rocks gets deposited in a flat or pipe-like conveyance
tube, it is called lacolith. Their shape resembles the dome of a mixed volcano
found on the surface. Lacoliths are found at greater depths. Many such domed
hills made of granite rocks are found in the Karnataka plateau.
3. Lapolith:- If some part of the rising lava goes in the horizontal direction and freezes in the form of a saucer, then it is called Lapolyth.
4. Phacolith:-
When
magma freezes in the form of anticline and syncline in a wavy or flaky shape,
it is called a phacolith.
5. Sill or sheet:- The cooling of penetrating igneous rocks in the form of a sheet in a horizontal plane is called a sill or sheet. On the basis of the thickness of the deposit, it is divided into - low thickness deposits are called sheets and high thickness deposits are called sill.
6. Dyke:- When the lava flows into the cracks at approximately right angles to the surface and if it cools down to this stage, it forms a wall like structure, which is called a dike. Such figures are found in abundance in the Deccan trap of western Maharashtra.