Interior of the Earth

The earth,our homeland,is a dynamic planet.The earth’s surface has lofty mountains,high plateaus, large plains and deep valleys etc.The earth’s surface is constantly undergoing changes inside and outside.Have you ever wondered what lies in the interior of the earth?what is the earth made up of?Let us learn about this in detail.

The structure of the earth may be compared to that of an apple.On the bais of the study of earthquake waves the spherical earth is found to be three concentric layers.
They are:
1. The crust,
2. The mantle and
3.The core

1. The crust

The crust is the outermost layer of the earth.Its thickness varies from 5 to 30km.It is about 35km on the continental masses and only 5 km on the ocean floors.Despite greater thickness,the continental crust is less dense than the Oceanic crust because it is made of both light and dense rock types.The oceanic crust is composed mostly of dense rocks such as basalt.The crust comprises two of distinct parts.The upper part consists of granite rocks and forms the continents.It has the main mineral constituents of silica and alumina.So it is referred to as sial.It has an average density of 2.7g/cm³.The lower part is a continuous zone of denser basaltic rocks forming the ocean floors.Comprising mainly of silica and magnisium.It is therefore called Sima.It has an average density of 3.0g/cm³.The sial and the sima together from the earth’s crust.Since the sial is lighter than the sima,the continents can be said to be ‘floating’ on a sea of denser sima.

2. The Mantle

The next layer beneath the crust is called the mantle.It is separated from the crust by a boundary called Mohorovicic discountinuity.The mantle is about 2,900 km thick.It is divided into two parts.(i) The upper mantle with a density of 3.4 – 4.4g/cm³ extents down to 700 km.(ii) The lower mantle having a density of 4.4 – 5.5g/cm³ extends from 700 to 2,900 km.

3. The Core

The innermost layer of the earth is called the core.It is also known as barysphere.It is separated from the mantle by a boundary called Weichart-Gutenberg discontinuity.The core is also divided into two parts.

(i) The outer core , which is rich in iron,is in liquid state.It extends between 2,900 – 5,150 km.
(ii) The inner core, composed of Nickel and Ferrous (Nife),is solid in state.The central core has very high temperature and pressure.It extends from 5,150 km to 6,370 km.The average density of core is 13.0g/cm³

The Earth Movements

The lithosphere is broken into a number of plates known as the lithosphere plates.Each plate, oceanic or continental moves independently over the asthenosphere.The movement of the tectonic movements.The energy required to move these plates is produced by the internal heat of the earth.These plates move in different directions at different speed.

At places ,these plates move away from each other creating wide rifts on the earth’s surface.At some plates,these plates come closer and collide.When an Oceanic plate collides with a continental plate,the denser oceanic plate is forced below the continental plate.As a result of the pressure from above the rocks heats up and melts.The molten rocks rise again forming volcanic mountains along the continental edge.Alternatively,a trench may be formed between two plates
In some cases when two continental plates converage, neither plate can be forced under the other.Instead,folds may be created.Great mountain ranges like the Himalayas have been formed in this way.
The movement of these plates causes changes on the surface of the earth.The earth movements are divided on the basis of the forces which cause them.The forces which act in the interior of the earth are called as Endogenic forces and the forces that work on the surface of the earth are called as Exogenic forces.
Endogenic forces produce sudden movements and Exogenic forces produce sudden movements and Exogenic forces produce slow movements.Endogenic movements produce earthquakes and volcanoes that cause mass destruction over the surface of the earth.


A sudden movement of a portion of the earth’s crust which produces a shaking or trembling is known as an earthquake.The point where these vibrations originate is called the focus of the earthquake.

The point of the earth’s surface directly above the focus is called the epicentre of the earthquake.From the focus,the earthquake vibrations travel in different directions in the form of Seismic waves.
The earthquake waves are recorded by an instrument known as Seismograph.The magnitude of an earthquake is measured by the Richter scale.The numbers on this scale range from 0 to 9.

Causes of Earthquake

The chief cause of earthquake is the sudden slipping of the portion of the earth’s crust along fractures or faults.The movement of the molten rocks underneath the rocks apart.The sudden shifting of landmass causes upheavals in the crust of the earth sending vibrations or waves into the surrounding portions of the earth.Sometimes the surface of the earth itself cracks.

Effects of Earthquake

Earthquake may cause changes in the earth’s surface.Vibrations often set landslides in mountainous regions.A greater danger in an earthquake is the falling of buildings.Most of the houses which collapsed were made of mud and bricks and proved to be death traps.Underground water system is naturally disturbed by such movements.Fire is another great danger.

There are three types of earthquake waves:

P waves or longitudinal waves
S waves or transverse waves
L waves or surface waves

An earthquake which originates below or near the sea causes great disturbance in the water.The floods and waves cause great loss of life, sometimes more than the earthquake itself.Tsunami, a Japanese term,is the name given to the huge waves caused in the sea by an earthquake.Tsunamis are quite common along the coasts of Japan and other regions in the Pacific Ocean.

Distribution of Earthquake

The world’s distribution of earthquakes coincide very closely with that of volcanoes.Regoins of greatest sesimicity are circum-pacific areas,with the epicenters and the most frequent occurrences along the Pacific Ring of Fire.It is said that about 68% of earthquakes occur in this belt.Remaining 31%of earthquakes take place in the Mediterranean-Himalayan belt including Asia Minor,the Himalayas and parts of norh-West China.The remaining percent of earthquakes occur in Northern Africa and Rift valley areas of the Red sea and Dead sea.In India,the Himalayan region and the Ganga-Brahmaputra valley are prone to earthquakes.A number earthquakes have been experienced in this region.Some of them were very severe and caused extensive damage,e.g., the earthquake if Uttar Kashi in 1991 and Chamoli in 1999.The Deccan Plateau,which was supposed to be comparatively free from the dangers of the earthquakes,has experienced two severe earthquakes in the past,the Koyna(Maharashtra) earthquake in 1967 and the Latur earthquake in 1993.


A volcano is vent or an opening in the earth’s crust through which hot magma erupts from deep below the surface.The opening is usually circular in form.volcanic eruptions may also take place through a long crack or fissure through which steam and other materials flow out.
The molten rock material within the earth, together with gases, is called magma.after it rises to the surface,it is called as lava.

In course of time,lava and other materials flow out of a volcano accumulate around the opening and form a conial hill or a mountain vent is an opening or mouth of a volcano.The top of this cone is usually marked by a funnel-shaped depression,which is called a crater.If the crater of a volcano is of great size and is shaped like a basin,it is called a caldera.Calderas are caused by violent explosions which blow away entire tops of great cones.

Causes of volcanic Activity

The temperature increases as the depth increases at the rate of 1°C for every 32 metres.There is also great pressure.At a depth of about 15 km the pressure is about 5 tonnes per cm² of rock.Under these circumstances,the interior of the earth is in a semi-molten state called magma.The magma,under great pressure has the capacity to dissolve great pressure has the capacity to dissolve great volume of gas;some gases are also combustible.This makes volcanic Material burst forth through the weak spots in the earth’s crust.

Types of volcano

Volcanoes are classified according to their periodicity of eruptions and state of activity such as
1. Active volcano
2. Dormant vaolcano
3. Extinct volcano

Active volcano

Valcanoes that erupt frequently are called active volcanoes.Most of the active volcanoes lie in the Pacific Ring of Fire belt which lies along the Pacific coast.There are about 600 active volcanoes in the world,such as Mt.Stromboli in Mediterranean sea,st.Helens in USA,pinatubo in philipines.Mauna Loa in Hawaii is the world’s biggest active volcano.

Dormant volcano

These volcanoes have shown no sign of activity for many years but they may become active at any time.These are called sleeping volcanoes.Vesuvius mountain of Italy,Mt Fujiyama of Japan,Mt.Krakatoa of Indonesia are famous examples of this types.

Extinct volcano

A Volcano has not erupted in past 1000 years is often listed as Extinct Volcanoes.The top of extinct Volcanic mountains have been eroded.Mt Popa of Myanmar and Mt.Kilimajaro and Mt.Kenya of Africa are examples of extinct volcanoes.

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