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lnternal Structure of the Earth

By : brainykey   August 31, 2013

INTERNAL STRUCTURE OF INDIAlnternal Structure of the Earth

The interior of the earth is not composed of homogeneous or uniform material. The structure of Earth can be visualized in two ways: chemically or by material properties. By comparing material strength, the layering of Earth is categorized as lithosphere, asthenosphere, upper mantle,lower mantle, outer core, and the inner core. Chemically, a tripartite arrangement for the earth has been suggested by Austrian geologist, Suess. The three concentric shells advocated by him on the basis of their density are outer Sial layer, inner Sima layer and the innermost nickel- iron core of Nife. The scientific study and analysis of various seismic waves of natural and man induced earthquakes revealed the mystery of earth’s interior and divided the earth in three different layers as explained below. The earth’s interior has three different layers; they are (i) the crust (ii) mantle and (iii) the core. These layers are distinguished on the basis of their (i) Physical and chemical properties, (ii) Thickness, (iii) Density (iv) Temperature , (v) metallic content and (vi) rocks. The geologic component layers of earths  are the following depths below the surface:

(A)   Earth’s crust:

all of the earth’s landforms ( mountains, plains, and plateaus) are contained within it, along with the oceans, seas, lakes and rivers. There are two different types of crust: thin oceanic crust that underlies the ocean basins and thicker continental crust that underlies the continents. These two types of crust are made up of different types of rock. The thin oceanic crust is composed of primarily of basalt and the thicker continental crust is composed primarily of granite. The low density of the thick continental crust allows it to “float” in high relief on the much higher density mantle below. The boundary between the crust and the mantle is MOHOROVICIC DISCONTINUITY.

 

(B)  Earth’s Mantle:

It is the thick, dense rocky matter that surrounds the core with a radius of about 2885 km. The mantle covers the majority of the Earth’s volume. This is basically composed of silicate rock rich in iron and magnesium. The mantle is less dense than the core but denser than the outer crust layer. This layer is separated from the core by Gutenberg-Wiechert Discontinuity. The outer and the inner mantle are separated by another discontinuity named Repetti discontinuity.  

(C) Earth’s Core:

Earth’s Core is thought to be composed mainly of an iron and nickel alloy. This composition is based on calculations of its density. The core is earth’s source of internal heat because it contains radioactive materials which release heat as they break down into more stable substances.   The core is divided into two different zones. The outer core is a liquid because the temperatures there are adequate to melt the iron-nickel alloy. However, the inner core is a solid even though its temperature is higher than the outer core. Here, tremendous pressure, produced by the weight of the overlying rocks is strong enough to crowd the atoms tightly together and prevents changing it to the liquid state.  

TYPES OF SEISMIC WAVES

There are two types of seismic waves, body wave and surface waves.

  • Body waves travel through the interior of the Earth. They follow ray paths refracted by the varying density and stiffness of the Earth’s interior which in turn, vary according to temperature, composition and phase.

Body waves are divided as

P waves (primary waves) are compression waves that are longitudinal in nature. These waves can travel through any type of materiat and can travel at nearly twice the speed of S waves. S-waves (secondary waves) are shear waves that are transverse in nature. These waves typically follow P waves during an earthquake and displace the ground perpendicular to the direction of propagation. S waves can travel only through solids, as fluids.(liquids and gases) do not support shear stresses. S waves are slower than P waves, and speeds are typically around 60% of that of P waves in any given material.

  • Surface waves are analogous to water waves and travel along the Earth’s surface. They travel slower than body waves. Because of their low frequency, long duration, and large amplitude, they can be the most destructive type of seismic wave.
 

There are two types of surface waves.

Rayleigh waves, also called ground roll, are surface waves that travel as ripples with motions that are similar to those of waves on the surface of water. The existence of these waves was predicted by ]ohn William Strutt, Lord Rayleigh, in 1885. They are slower than body waves. Love waves are surface waves that cause circular shearing of the ground.They are named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911. They usually travel slightly faster than Rayleigh waves, about 90% of the S wave velocity, and have the largest amplitude. The asthenosphere separates the strong, solid rock of the uppermost mantle and crust above from the remainder of the strong, solid mantle below. The combination of uppermost mantle and crust above the asthenosphere is called the lithosphere. The lithosphere is free to move (glide) over the weak asthenosphere. The tectonic plates are, in fact, lithospheric plates.

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