Question.2. Answer the following questions in about 30 words.

(i) What are body waves?
Answer. Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. When the body waves interact with the surface rocks and generate new set of waves these are called surface waves.

(ii) Name the direct sources of information about the interior of the earth.
Answer. Some of the direct sources are:

(a) Mining
(b) Deep Surface Drilling
(c) Volcanic Eruptions

(iii) Why do earthquake waves develop shadow zone?
Answer. The earthquake waves develop shadow zone because:

1. The ‘P’ and ‘S’ waves followed a curved path in the interior of the earth which proves that the density increases inside the earth.
2. ‘S’ waves get disappeared in core of the earth and ‘P’ waves get refracted which result there is no wave, which is called shadow zone.

(iv) Briefly explain the indirect sources of information of the interior of the earth other than those of seismic activity.
Answer.

1. Meteors that fall to earth represent same of the original, diverse material that formed planets billion of year ago. By studying meteors we can learn about early conditions and process in the solar system’s history.
2. Gravity: Gravity of the earth varies at different latitudes. Gravity anomalies give us information about the distribution of mass of the material in the crust of the earth.
3. Magnetic surveys also provide information about the distribution of magnetic materials in the crustal portion.

Question.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?
Answer. The earthquake waves affect the rock mass through which they travel. Propagation of earthquake wave is not similar.

1. P-waves exert pressure on the material in the direction of the propagation. Consequently, it creates density difference in the material. Other waves vibrate perpendicular to the direction of propagation.
2. The direction of vibration of S-waves is perpendicular to the wave direction in the vertical plane. They create trough and creat in the material through which they pass.
3. The surface wave or L-waves are the most dangerous waves and vibrate in perpendicular direction to the surface in horizontal plane.

(ii) What do you understand by intrusive forms? Briefly describe various intrusive forms.
Answer. The lava when erupts does not come out of the surface every time. When it cools within the crustal portions is called intrusive forms. Some of the intrusive forms are as under:

1. Batholiths: A large body of magmatic material that cools in the deeper depth of the crust develops in the form of large domes. Batholiths are the cooled portion of magma chambers.
2. Lacoliths: These are large dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below. It resembles the surface volcanic domes of composite volcano, only these are located at deeper depths.
3. Lapolith: When the lava moves upwards, a portion of the same may tend to move in a horizontal direction wherever it finds a weak plane. It may get rested in different forms. In case it develops into a saucer shape, concave to the sky body, it is called Lapolith.
4. Sills: The near horizontal bodies of the intrusive igneous rocks are called sill or sheet, depending on the thickness of the material. The thinner ones are called sheets while the thick horizontal deposits are called sills.
5. Dykes: When the lava makes its way through cracks and the fissures develop in the land, it solidifies almost perpendicular to the ground. It gets cooled in the same position to develop a wall like structures.