Earth Structure

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In outward appearance, the earth is a nearly spherical ball with a radius of 6350 kilometers. Internally, the earth consists of three major layers. They are:

• Crust

• Mantle

• Core

The Earth's Crust

Continents and ocean basins (land beneath the oceans) are part of a rocky 'skin' that surrounds the main body of the earth. This skin is called the earth's crust. The crust of the earth is like the skin around an orange or an apple. The thickness of the crust varies from about 8 kilometers under the oceans to about 40 kilometers under the continents. Temperatures within the deepest parts of the crust may reach 870⁰ C - hot enough to melt rocks. The crust is made up of three kinds of rocks - igneous, sedimentary and metamorphic. All rocks on the earth's surface are made of minerals. Minerals are the most common solid materials found on earth. Minerals are themselves made up of basic chemical substances called elements. Rocks in the earth's crust consist mostly of two elements - silicon and oxygen. The next most common elements in the earth's crust are aluminium, iron, calcium, sodium and magnesium - in the decreasing order of abundance.

Earth's Crust

The three-quarters (3/4) of the earth's crust is covered with water. Over the earth's crust is a layer of atmosphere. Right from the day of formation of earth, its crust has been constantly broken and then reorganized to form new continents and oceans. It is thisthin and solidified crust of earth, which supports all life. The earth's crust is of lighter density as compared to the interior layers.

Mantle

The mantle is a thick layer of rock below the crust. It is the central region between the crust and the core of the earth. It is about 2,900 km deep. The rock in the mantle is made of silicon, oxygen, aluminium, iron and magnesium. The upper part of the mantle has a temperature of about 870⁰ C.

Mantle

This temperature gradually increases down through the mantle to about 4400⁰ C. The pressure inside the mantle increases with depth. Even under high pressure in the mantle, the rocks are usually in the solid state. Under extreme pressures and temperatures, however, some of the rocks near the bottom of the mantle tend to flow like coal tar. The density of mantle increases slowly as we go downwards. In the beginning of mantle, the density is about 4g/cm³. It goes on increasing until it reaches a value of about 6g/cm³ at the bottom of the mantle. Here, the mantle ends and a sudden increase in density occurs. It is here that the core of the earth begins.

The Core

The core consists of two parts: the inner core and outer core. The outer core begins about 2,900 kilometers below the earth's surface. Scientists believe that the outer core is about 2,250 kilometers thick and is made of melted iron and nickel. The temperature of the outer core ranges from about 4500⁰ C in the uppermost parts to about 6300⁰ C in the deepest parts.

Core (Region indicated by orange and red)

The ball-shaped inner core lies within the outer core and makes up the center of the earth. Scientists believe the inner core consists of solid iron and nickel. The temperature there may be as high as 7000⁰ C.

Thus, the core of earth consists of an inner solid sphere of highly compressed iron, surrounded by an outer shell of molten iron. The pressure at the center of the earth is extremely high, about 3.7 million atmospheres (3.7 x 10⁶ atm). It is because of this extremely high pressure that the iron in inner core can remain in the solid state despite the very high temperature.

Since the pressure is comparatively lower in the outer core, the iron remains in the molten form in the outer part of the core of the earth. Since the core of earth is very hot but the surface of earth is cooler, heat flows outwards from the hot interior of the earth to the surface of the earth. The flow of heat through the outer core of molten iron causes convection currents, which produce a magnetic field around the earth.

The inner core of the earth is very dense, its density being about 18 g/cm³. The density in outer core varies from about 10 g/cm³ just below the mantle to about 16 g/cm³ just before the inner core starts. The very high density of the core of earth is due to very high compression of the core material.

Scientists study the structure and composition of the inner parts of earth by indirect methods. The two important methods, which are used for this purpose, are:

• Study of waves produced during the earthquakes.

• Study of composition of meteorites, which fall on the earth.

Conditions for Life to Originate and Flourish on a Planet

All the planets do not have life on them. Earth is the only planet in the solar system having life on it. This is because certain conditions or requirements are necessary to enable life to originate and flourish on a planet. They are:

• The planet should have elements like carbon, hydrogen, nitrogen and oxygen, which are called the building blocks of life.

• The planet should have sufficient oxygen gas in its atmosphere required for the survival of living beings.

• The planet should have water.

• The temperature on planet should neither be too high nor too low.

• The planet should have a protective blanket like that of ozone layer around it.