The Earth which is our homeland is one of the members of solar system. It is one of the small inner or terrestrial planets with high density. It is the third planet outward from the sun. With its Satellites it lies between Venus and Mars.
When it is viewed from space it looks like a ball shining bright and blue. It is an oblate spheroid. It is pear shaped with an 18 metre rise at the North Pole and a 26 metre depression at the South Pole.
Among the nine planets, it is fifth in order of size. It has been proved through various experiments that earth is not a perfect sphere.
Hypotheses about the Origin of Planets
1. Nebula Hypothesis:
There are many hypotheses about the origin of earth and planets. But the hypothesis propounded by Laplace a french mathematician, is most popular. According to him, a giant slowly rotating cloud of gas and dust (or say nebula) with swirling motion contracted slowly under its own gravity.
In other words the original mass cooled and contracted. As a result of the law of conservation of angular momentum the rotational speed increased thereby spinning off gaseous material from the central mass by centrifugal force.
The central mass or the core of nebula evolved into a new star-the sun and the smaller units condensed into planets.
2. Collision Hypothesis:
There is another hypothesis about the origin of the Earth and planets called collision hypothesis, according to this hypothesis, the gravitational attraction of a passing star pulled away the gaseous material from the pre-existing sun.
This gaseous material after being pile away from the sun broke into small chunks and went flying as cold bodies into orbits in the plane of passing star around the sun.
The larger chunks, swept up small pieces of gaseous material by collision and gravitational attraction. Thus the planets came into existence.
In course of time, the earth has got its present shape through several phases and stages.
Interior of the Earth
Earthquake produce several kind of wave motions classified as body waves and surface waves.
1. Body Waves:
Body waves are called so because they travel through the solid body or the interior of the earth. They are of two types: Primary waves and Secondary waves.
(a) Primary (P) Waves also called longitudinal or compression waves, primary waves like sound waves pass through both liquid and solid material but they are fastest in solid. Since primary waves are fastest they are recorded first on the seismograph followed by secondary and surface waves.
(b) Secondary (S) Waves also called as transverse or distortional waves secondary waves like water ripples travel through only liquid material.
2. Surface (1) Waves:
Most destructive of all the waves, they are also known as long period waves. They move along the free surface of the earth and cover the longest distance. It is this wave which causes large scale destruction during earthquake.
Surface waves are of two types: Rayleigh waves and Love waves. They move more or less together but with different motions. Surface waves are also known as L waves.
Based on our analysis of differential speed of several types of seismic waves when they pass through the interior of the earth we can divide the earth into three layers viz therefore, the Mantle and the Crust.
1. The Crust:
Inside the upper surface of the earth, there is shallow crust the thickness of which ranges from 30 to 65 km. in continental areas and is about 10 km. beneath oceanic floor.
The uppermost part of crust is composed of sedimentary rocks; below the sedimentary cover there is layer of crystalline rocks consisting of granites and gneisses in the upper section and basaltic rocks in its lower section.
But under the oceanic floor the sedimentary layer is either thin or absent. Even there is no granite, only basaltic rocks are found. Thus the crust is divided into two layers:
SIAL AND SIMA
(a) SIAL—it is upper discontinuous and lighter layer of crust composed of sial that is Silicate and Aluminum.
(b) SIMA—it is the lower continuous and denser layer of the crust composed of SIMA that is silicate and Magnesium.
Mohorovic or Moho Discontinuity:
The contact zone of the crust and the mantle is called Mohorovicic discontinuity. In other words, the Primary waves near the surface travel at 6 km. per second but these waves increase in velocity gradually or suddenly to the base of the crust where they travel 7 km. per second.
This change or increase in velocity take place when Mohorocivic/ Moho discontinuity which separates the crust above from the mantle below. Generally Moho discontinuity exists at the depth of 45 to 60 km. but below the oceanic floor it exists at the depth of 5 to 7 km.
2. The Mantle:
As we have learnt that the Mantle is seprated from the Crust by the contact zone called Mohorocivic discontinuity the Mantle which is a thick intermediary lies above the core and below the crust. It is about 2900 kms thick.
The Mantle is solid as both P and S waves penetrate and travel through it. It is composed of dense and rigid rocks having magnesium and iron.
Based on our observation of earthquake waves the mantle is subdivided into two layer or parts, the Upper Mantle (Asthenosphere) and the lower Mantle.
(a) The Upper Mantle (Asthenosphere):
The Upper Mantle is also called asthenosphere. It extends from the crust to a depth of about 650 km. The lower part of Asthenosphere is solid like mesosphere (lower Mantle) but the upper part of asthenosphere is plastic and in molten condition.
Thus rocks in the upper Mantle behave as both elastic solid and plastic solid. It is plastic and elastic at the same time.
It means when stresses are applied there is change in shape but when they are removed it returns exactly to its former shape. The velocity of seismic waves slows down in the asthenosphero. Thus asthenosphere is referred to as the Low velocity zone.
(b) The Lower Mantle also called Mesosphere. This zone extends between 700 to 2900 km. Its density ranges from 4.0 to 3.5.
Gutenburg Discontinuity the zone which separates the Mantle from the core is called Gutenberg Discontinuity. It is at the mantle-core boundary when Primary waves decrease in velocity.
3. The Core:
From below 2900 km. to 6400 km exists the crore? It is 83% in volume. The S wave which can travel through only solid material suddenly disappears.
This shows that Swave cannot pass through the central part of the earth (i.e. the core) because it is made up of a medium which is not solid. As we have already learnt that S. waves cannot penetrate through a liquid medium.
Since S waves do not pass through earth’s core, it means that earth’s core is in liquid state whereas the mantle is sold. Seismic Primary waves which can pass through liquid, solid and gas slow down from 13.7 km. per second to 8 km. per second and bend as they enter the core.
Being bent they are not received in a zone of the core known as shadow zone. S waves don’t pass through the core as they can not penetrate through liquid. Only surface (L) waves are received in the shadow zone which lies between 103° and 143° distant from the focus of the earthquake.
Thus beyond 143° only P waves which travel through the core and surface waves which move along the surface are received. The core is primarily made up of nickel and iron in liquid condition.
Strangely P wave increase in velocity to some extent at the depth of 50-80 km. proving that interior (lower) core became solid (solidified iron) due to high pressure. It (interior core) is also known as bar sphere.
We can sum up the facts about the interior of the earth below:
Crust 1 %
Mantle 16% 32%
Core 83% 68%