Origin & Structure of Earth
A Cloud of gas formed by a great initial explosion may have given rise to the universe, and the earth may have been formed from a small part of that cloud. An experiment can help us to understand how this is possible.
Fill a bottle with water and sand, and shake it vigorously. The sand is dispersed in the water because of the energy received from your hands , just as the original cloud of gas dispersed in space because of the energy received from the initial explosion. Now put the bottle on the table. The water gradually stops moving, and the attraction of the earth causes the sand to settle at the bottom of the bottle.
Once the energy from the explosion grew weaker, the forces of attraction made the cloud of gas thicken, forming countless gaseous spheres of every size. In the spheres, the attraction among the particles of the gas continued to thicken the matter. Pressures great enough to set off an atomic explosion were created inside the largest spheres. They exploded like bombs, and the stars were created.
In the smaller spheres, there was only enough pressure to form glowing balls of matter. This was the beginning of fiery heavenly bodies, planets like earth, which would one day cool. The structure of the earth is similar to that of an onion: a thin outer crust encloses a series of concentric rings that become hotter and hotter as they go in. Inside is the molten rock of the central nucleus, all that remains of the ancient ball of fire.
Thus, Five billion years ago the Earth was formed in a massive conglomeration and bombardment of meteorites and comets. The immense amount of heat energy released by the high-velocity bombardment melted the entire planet, and it is still cooling off today. Denser materials like iron (Fe) from the meteorites sank into the core of the Earth, while lighter silicates (Si), other oxygen (O) compounds, and water from comets rose near the surface. Click Here for an excellent animated history of the Earth
Inside the Earth
Today, the Earth is relatively stable. It can be divided into three parts, the core, mantle and crust.
The composition of the crust is quite different from the whole earth. It is much more siliceous, consisting of silicon, aluminum and lesser amounts of iron, magnesium, calcium, potassium and sodium, balanced by oxygen. The whole Earth contains much more iron.
The crust can further be divided into several parts: continental crust and oceanic crust. The later is denser and rides lower on the athenosphere. Hence, the placement of the oceans, the crust lies on the mantle which extends to a depth of 2885 kilometers and constitutes more than 80% of the earth’s volume, the mantle in turn surrounds the core.
The average density of each layer is as follows:
The Crust :: 2.80 grams/cm3.
The Mantle :: 4.93 grams/cm3.
The Core :: 10.93 grams/cm3
Why does the structure of the earth consist of concentric layers?
Take a piece of wood, an apple and a marble, and then put them in a pan filled with water. The marble immediately sinks to the bottom, and the apple and the piece of wood float. However one sticks out of the water more than the other. In the beginning our planet was a ball of glowing fluid matter and the mass (minerals) that composed it were all mixed up together. Gradually the densest mass (minerals) sank to the centre of the fiery ball, forming a nucleus; the less dense mass settled on top of the nucleus, and the lightest ones stayed on the surface, in the same way as the apple and the piece of wood. Thus the earth’s crust is composed of the lightest mass, and is like a raft floating on a hot inner nucleus.
Click here for more information on earth's interior
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