Interior Of The Earth

Surface waves is the last kind of wave that the energy released from an earthquake transforms into. Surface waves are generated when the body waves interact with the surface rocks. It causes the displacement of rocks, and hence, the collapse of structures occurs. Therefore, these are the most destructive among the earthquake waves.

Volcanoes and volcanic eruptions occur when the molten material (magma) is thrown onto the surface of the earth. These erupted materials provide a wealth of information regarding the composition of the interior of the earth, when available for laboratory analysis.

The Deccan Trap formation, in Maharashtra, were created by large flood basalt provinces. When erupted this highly fluid lava flows for long distances covering thousands of sq.kms.

Lithosphere comprises of the Crust and Upper most solid mantle, and it ranges from 10-200 kilometre.

(i) Earthquake triggers tremors through the interior of the Earth, these move in the form of waves radiating from the focus in all directions. Body waves are a type of earthquake wave, along with surface waves. 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, and hence the name body waves.

(ii) The direct sources of information about the interior of the earth are:

Rock samples

The most easily available solid earth material is surface rock or the rocks from mining areas. Apart from mining and surface rocks, scientists have taken up various projects to penetrate deeper depths to explore the conditions in the crustal portions. Many deep drilling projects have provided large volume of information through the analysis of materials collected at different depths.

Volcanoes

Volcanic eruption is a major source of direct information about the interior of the earth. As and when the molten material (magma) is thrown onto the surface of the earth, during volcanic eruption, it becomes available for laboratory analysis.

(iii) Earthquake waves develop shadow zones because of the properties of each (P and S) waves as well as the core. Outer crust is liquid and hence S-waves, which can travel only through solid material, is blocked and cannot move across the core. P-waves, which can move faster and travel through gaseous, liquid and solid material, gets refracted inside the core, and in the process P-waves fails to manifests itself in certain regions.

Thus, S-waves are not found beyond 105° from focus of an Earthquake, whereas P-waves are absent between 105° and 145° from the focus. Therefore, the regions between 105° and 145° forms a shadow zone for both kinds of earthquake waves.

(iv) Altogether there are 5 different indirect sources of information of the interior of the earth. Excluding the seismic activities these are:

1. Analysis of properties of matter indirectly.

Mining activity have proved that temperature and pressure increase with depth. It is also known that the density of the material increases with depth. Knowing the total thickness of the earth, scientists have estimated the values of temperature, pressure and the density of materials at different depths.

2. Meteors

The material and the structure observed in the meteors are similar to that of the earth. They are solid bodies developed out of materials same as, or similar to, our planet. However, the material available for analysis in such a way is not from the interior of the earth.

3. Gravitation

The gravitation force (g) is not the same at different latitudes on the surface, it differs from the poles to the equator. This is because the distance from the centre of the earth varies based on the latitude (less at poles, maximum at equator). The gravity values also differ based on the mass of material. Thus, the gravity measured at different places is influenced by these factors (latitude and mass). When these gravitation readings differ from the expected values, we get gravity anomaly. These anomalies give us information about the distribution of mass of the material in the crust of the Earth.

4. Magnetic Field

Magnetic survey provides information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.

(i) An earthquake occurs with the release of energy along a fault, and the rocks along a fault tend to move in opposite directions carrying the released energy through them in the form of waves. Earthquake waves are of two types – body waves and surface waves.

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. The two types of body waves – P and S waves – varies in their mode of propagation (longitudinal for P waves and transverse for S waves) and differs in the medium through which they can travel. The velocity of waves changes as the travel through materials with different densities – the denser the material, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities. This has helped scientists to understand the structure of the interior of the earth.

Surface waves are generated when the body waves interact with the surface rocks. These waves move along the surface and are the last to report on seismograph. They cause displacement of rocks, and hence, the collapse of structures occurs.

(ii) The lava that is released during volcanic eruptions on cooling develops into ingenious rocks. This cooling can take place either on reaching the surface or while the lava is still in the crustal portion. The lava that cools within the crustal portions assumes different forms, called intrusive forms.

Some major intrusive forms are:

Batholiths

These are the cooled portion of the magma chambers. They develop in the form of large domes, and appear on the surface only after the denudational processes remove the overlying materials. They cover large areas, and at times, assume depts that may be several kilometres.

Lacoliths

These are large dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below.

Lapolith, Phacolith and Sills

When lava moves upwards, a potion 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, it is called lapolith.

At times it is found at the base of synclines or at the top of anticline in folded igneous country, such wavy materials have a definite conduit to source beneath in the form of magma chamber. These are called the phacoliths.

The near horizontal bodies of the intrusive igneous rocks are called sill or sheet, depending on the thickness of the material.

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 structure called dykes.