Structure of the Earth
The study of the passage of seismic waves through the earth has helped in knowing the structure of the earth’s interior and in defining the physical properties of various layers. The seismic waves travel at different velocities depending on the nature of the layer in which they are traveling. Thus they not only indicate the position of each layer but also give clues as to its composition. On the basic of seismic investigation the earth can be divided into four major layers.
The outer superficial layer of the earth is called the crust. It extends down to 30 or 40 kilometerbeneath continents and to about 10 km beneath ocean basins. At the bottom of the crust, the velocity of earthquake waves increases abruptly as they enter into a denser layer called mantle. The crust-mantle boundary is known as Mohorovicic discontinuity.
In the continental regions, the crust can be divided into two layers. The upper layer which is less dense and granitic in character is known as sial, while the lower layer which is basaltic in character is known as sima. The term sial represents rocks containing silica and magnesia, under oceans only sima layer is found and the dial layer is absent.
The mantle is located beneath the earth’s crust and has a thickness of about 2900km. it has been divided into two layers (1) Upper Mantle (2) Lower Mantle. The boundary between these is at about 700km depth. The average mineral composition of the upper mantle is similar to ultra basic rocks like peridotite. This zone provides lava for oceanic eruption. In the lower mantle the density of the material increases rapidly to 4.3 x 103 kgm-3 and in the deeper parts it rises to about 5.5 x 103 kgm-3. It has been suggested that the lower mantle consists of a mixture of peridotite and minerals of higher density.
The upper mantle contains a most important zone called asthenosphere. It is located at depth between 50 to 100 km. in the asthenosphere the velocity of S-waves decreases thereby indicating that this zone consists partly of melted rocks. Most basalt originates in this zone. It is believed that the plastic material of the asthenosphere moves and carries along the lithosphereplates. The outer solid portion of the earth existing above the asthenosphere is calledlithosphere. The lithosphere includes part of the upper mantle and crust.
The boundary between the mantle and the core is at a depth of about KM. this boundary is marked by an abrupt reduction in the velocity. P-waves as well as by the disappearance of the S-waves.
Outer Core of Structure of the Earth
The outer core was discovered when it was found that P-waves were bent inward thereby producing a shadow zone at the surface. Since the S-waves do not pass through the outer core, it is concluded that it may be in the liquid state.
Inner Core of Structure of the Earth
The inner core extends form about 5150 km to the earth’s centre at 6371 km. it transmits P-waves at a higher velocity which indicates that it is in the solid state. Both the solid and liquid portions of the core are believed to consist of iron and nickel similar to iron meteorites and have a density about 1.2×103 kgm-3