Inside Our Earth

Source : NCERT & NIOS

·         The earth is made up of several concentric layers. Concentric means having a common centre.

·         The earth has one layer inside another.

Broadly, there are three layers of the earth:

1.   Crust

2.   Mantle

3.   Core

The Crust:

·         It is the outermost layer over the earth’s surface.

·         It is the thinnest of all the layers.

·         It forms 0.5-1.0 per cent of the earth’s volume.

·         About 35 km on the continental masses and only 5 km on the ocean floors.

·         Crust is the outer thin layer with a total thickness normally between 30-50 km.

·         The thickness of the crust varies under the oceanic and continental areas.

·         Oceanic crust is thinner (5-30 km thick) as compared to the continental crust (50- 70 km thick).

·         The continental crust is thicker in the areas of major mountain systems.

·         It is as much as 70 -100 km thick in the Himalayan region.

The main minerals constituting the continental mass are:

(granitic rocks , average density = 2.7g/cm^3)

1.   Silica

2.   Alumina

The continental mass is also called ‘sial’.

The name sial is derived from letters ‘si’ of silicon and ‘al’ of alumina.

The main mineral constituents of the oceanic crust are:

(Basalt, average density = 3 g/cm^3)

1.   Silicon

2.   Magnesium

The oceanic crust is called ‘sima’.

The root of the word sima is ‘si’ from silica and ‘ma’ from magnesium.

Mantle:

·         It is the layer just beneath the crust and extends up to a depth of 2900 km below the crust.

·         Average density = 3.4 g/cm^3.

·         It forms 83 per cent of the earth's volume.

·         The crust and the uppermost part of the mantle are called lithosphere.

·         Its thickness ranges from 10-200 km.

Asthenosphere:

·         The upper portion of the mantle is called asthenosphere.

·         The word astheno means weak.

·         It is considered to be extending up to 400 km.

·         It is the main source of magma that finds its way to the surface during volcanic eruptions.

·         It has a density higher than the crust’s.

Core :

·         It is the innermost layer i.e. it is below the mantle. Its radius is about 3500 km.

·         The core consists of outer core and inner core.

·         Accounts for 16 per cent of the earth's volume.

·         The outer core is liquid while the inner core is solid.

It is mainly made up of:

1.   Nickel

2.   Iron

The innermost layer is also called ‘nife’.

·         The name nife is derived from the word ‘ni’ from nickel and ‘fe’ from ferrous meaning iron.

·         The temperature and pressure of the central core is very high.

Sources of information about the Interior:

Direct Sources:

·         Deep earth mining and drilling reveals the nature of rocks deep down the surface.

·         Volcanic eruption forms another source of obtaining direct information.

Indirect Sources:

Depth:

·         With depth, pressure and density increases and hence temperature.

·         This is mainly due to gravitation.

Meteors:

·         Meteors and Earth are solar system objects that are born from the

same nebular cloud.

·         Thus they are likely to have a similar internal structure.

Gravitation:

·         The gravitation force (g) is not the same at different latitudes on the

surface.

·         It is greater near the poles and less at the equator.

·         This is because of the distance from the centre at the equator being greater than that at the poles.

·         The gravity values also differ according to the mass of material.

·         The uneven distribution of mass of material within the earth influences this value.

·         Such a difference is called gravity anomaly.

·         Gravity anomalies give us information about the distribution of mass of the material in the crust of the earth.

Magnetic field:

·         The geodynamo effect helps scientists understand what's happening inside the Earth's core.

·         Shifts in the magnetic field also provide clues to the inaccessible iron core.

Seismic activity:

Earth’s interior discontinuity:

1.   Conorod :  discontinuity between upper and lower crust.

2.   Mohorovicc: Discontinuity between lower crust and upper mantle.

3.   Repiti : discontinuity between upper and lower mantle.

4.   Guttenberg:  discontinuity between lower mantle and outer core.

5.   Lehmann:  discontinuity between outer core and inner core.

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