Standard: 7
Subject: Term 1 Social Science
Lesson: Interior of the Earth
புவியின் உள்ளமைப்பு
Interior of Earth
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The spherical earth is found to be three concentric layers.
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The earth’s surface has lofty mountains, high plateaus, large plains and deep valleys.
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The structure of the earth may be compared to that of an apple.
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The crust is the outermost layer of the earth greater thickness, the continental crust is less dense than the oceanic crust .
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The oceanic crust is composed mostly of dense rocks such as basalt.
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Earth is called as blue Planet.
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71% of the earth is covered by water.
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The crust comprises two of distinct parts.
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The upper part consists of granite rocks and forms the continents.
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Upper part has the main mineral constituents of silica and alumina. It is referred to as Sial.
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Sial has an average density of sial 2.7 g/cm^3.
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Lower part is a continuous zone of denser basaltic rocks forming the ocean floors, comprising mainly of silica and magnesium.
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Lower part is therefore called Sima.
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sima has an average density of 3.0g/cm^3.
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The sial and the sima together form the earth’s crust. Since the sial is lighter than the sima.
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The continents can be said to be ‘floating’ on a sea of denser sima.
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The next layer beneath the crust is called the mantle.
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Mantle is separated from the crust by a boundary called Mohorovicic discontinuity.
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The mantle is about 2,900 km thick.
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The innermost layer of the earth is called core.
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core is also known as barysphere.
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Core is separated from the mantle by a boundary called Weichart-Gutenberg discontinuity.
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The outer core, which is rich in iron, is in liquid state. It extends between 2,900 – 5,150 km.
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The inner core, composed of Nickel and Ferrous (Nife), is solid in state.
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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 13.0 g/cm^3
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The crust forms only 1% of the volume of the earth, 84% consists of the mantle and 15% makes the core.
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The radius of the earth is 6,371km.
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The lithosphere is broken into a number of plates known as the Lithospheric plates.
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Lithospheric plate, oceanic or continental moves independently over the asthenosphere.
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The movement of the Earth’s lithospheric plates is termed as tectonic movements.
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The forces which act in the interior of the earth are called as Endogenic forces .
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The forces that work on the surface of the earth are called as Exogenic forces.
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The asthenosphere is the part of the mantle that flows and moves the plates of the earth.
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A sudden movement of a portion of the earth’s crust which produces a shaking or trembling is known as an earthquake.
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The point where vibrations originate is called the focus of the earthquake
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The earth’s surface directly above the focus is called the epicentre of the earthquake.
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From the focus, the earthquake vibrations travel in different directions in the form of seismic waves.
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The earthquake waves are recorded by an instrument known as seismograph.
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The magnitude of an earthquake is measured by the Richter scale.
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The numbers on Richter scale range from 0 to 9.
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The chief cause of earthquake is the sudden slipping of the portion of the earth’s crust along fractures or faults.
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Earthquakes may cause changes in the earth’s surface.
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Vibrations often set landslides in mountainous regions.
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There are three types of earthquake waves:
1. P waves or longitudinal waves
2. S waves or transverse waves
3. L waves or surface waves
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Tsunami, a Japanese term, is the name given to the huge waves caused in the sea by an earthquake.
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On 26th December 2004, Tsunami in the Indian Ocean swept coastal area of Indonesia, India, Srilanka, Thailand.
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The world’s distribution of earthquakes coincide very closely with that of volcanoes.
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68 % of earthquakes occur in this belt.
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31 % of earthquakes take place in the Mediterranean-Himalayan belt including Asia Minor, the Himalayas and parts of north-west China.
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1% of earthquakes occur in Northen Africa and Rift valley areas of the Red sea and Dead sea.
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In India, the Himalayan region and the Ganga-Brahamaputra valley are prone to earthquakes.
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The earthquake of Uttar Kashi in 1991 .
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The earthquake of Chamoli in 1999.
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The Koyna (Maharashtra) earthquake in 1967.
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The Latur earthquake in 1993.
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A volcano is a vent or an opening in the earth’s crust through which hot magma erupts from deep below the surface.
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The opening is usually circular in form.
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Volcanic eruptions may also take place through a long crack or fissure
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The molten rock material within the earth, together with gases, is called magma.
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Magma rises to the surface, it is called as lava.
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vent is an opening or mouth of a volcano.
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The top of this cone is usually marked by a funnel shaped depression, is called a crater.
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The crater of a volcano is of great size and is shaped like a basin is called a caldera.
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The interior of the earth is in a semi-molten state called magma.
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The scientific study of valcanoes are called volcanology.
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People who study valcanoes are called volcanologists.
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magma rises slowly to the surface and spreads over a vast area is fissure eruption.
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Deccan Plateau in India.
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Colombian Plateau in North America.
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Besides lava, ash, steam, gases and pieces of rocks are also thrown out is explosive eruption.
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The terrible explosion on 27 th August 1883 in the island of Krakatoa, Indonesia is an example for explosive type of eruption.
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The viscosity of lava is determined by the amount of silica and water in magma.
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Highly viscosity lava is rich in silica and has little water.
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Low viscosity lava has little silica, but a lot of water.
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Barren island is situated in the Andaman Sea, and lies about 138 km northeast of the territory's capital.
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Barren island is only in active volcano along the chain from sumatra to myanmar. Last eruption occurred in 2017.
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Valcanoes that erupt frequently are called active volcanoes.
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Most of the active volcanoes lie in the Pacific Ring of Fire belt which lies along the Pacific coast.
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Mt. Stromboli in Mediterranean Sea, St.Helens in USA, Pinatubo in Philippines.
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Mauna Loa in Hawaii is the world’s biggest active volcano.
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Volcanoes have shown no sign of activity for many years but they may become active at any time are called Sleeping Volcanoes.
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Stromboli is known as the ‘light house of Mediterranean sea’
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Vesuvius mountain of Italy, Mt Fujiyama of Japan, Mt Krakatoa of Indonesia are Sleeping Volcanoes.
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A Volcano has not erupted in past 1000 years is often listed as Extinct volcanoes.
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Mt Popa of Myanmar , Mt.Kilimanjaro and Mt. Kenya of Africa are of extinct volcanoes.
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There are three major zones of volcanic activities in the world. They are:
1. The Circum – Pacific belt
2. The Mid continental belt
3. The Mid Atlantic belt
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Circum Pacific Belt convergent oceanic plate zone is termed as the Pacific Ring of Fire.
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Mid continental belt is the volcanic zone of convergent continental plate boundaries
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The volcanoes of Alpine mountain chains, the Mediterranean Sea and the fault zone of eastern Africa.
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The important volcanoes are Vesuvius, Stromboli, Etna, Kilimanjaro and Kenya.
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The Himalayas have no active volcanoes at all.
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Mid Atlantic Belt represents the divergent boundary of plates located along the mid Atlantic ridges.
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Volcanoes of this Mid Atlantic Belt area are mainly of fissure eruption type.
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Iceland is the most active volcanic area and is located on the mid-Atlantic ridge.
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St.Helena and Azores Island are fissure eruption volcanoes.
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