Lithosphere for TN exams | Tamil and English Free E-books

TNPSC has released its annual planner 2022. Notifications of various group exams can be expected as per schedule. Geography carries a significant weightage in most TN exams. Questions on Lithosphere are generally asked in TNPSC exams. We have come up with a study blog on the topic of Lithosphere for TNPSC to help you in your preparation.

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The shape of the Earth

The shape of the Earth is classified as an oblate spheroid or ellipsoid instead of a proper sphere because Earth’s circumference and diameter are not uniform.

Equator has a larger circumference and diameter compared to all other latitudes. The radius from the centre of the Earth to points in the same longitude is not same. This is because the poles are squished, resulting in a bulge at the Equator.

Coordination System

Imaginary lines are drawn on the globe to locate any place on Earth, set time zones, classify the climatic types etc.

Latitudes

  1. All parallel circles, from the Equator to the poles, are called parallels of latitudes.
  2. Latitudes are denoted in degrees, from 0° at the Equator to 90° at the poles.
  3. Difference between two neighbouring latitudes is always the same.

Important Latitudes Equator

It is an imaginary line that divides the globe horizontally into two equal parts. It divides the globe into Northern and Southern hemispheres.

Tropic of Cancer

  1. It is an imaginary line encircling the Earth at the latitude of 23 degrees 30 minutes North of the Equator.
  2. This is the northern boundary of the Tropics and the northernmost latitude at which the Sun can shine directly overhead.

Tropic of Capricorn

  1. It is an imaginary line encircling the Earth at the latitude of 23 degrees 30 minutes South of the Equator.
  2. This is the southern boundary of the Tropics and the southernmost latitude at which the Sun can shine directly overhead.
  3. The area situated between the Tropic of Cancer and the Tropic of Capricorn is referred to as Tropics or Torrid Zone.

Arctic & Antarctic Circle

  1. The Arctic and Antarctic Circles are imaginary lines located at 66.5 degrees latitude North and South of the Equator, respectively.
  2. While the Antarctic Circle experiences total darkness during Summer Solstice, the Arctic circle experiences total darkness during Winter Solstice.
  3. Arctic and Antarctic Circles are the northernmost and southernmost latitudes where the Sun is seen during Summer and winter solstice respectively.

Longitudes

It is a measurement system that defines the East-West stretch of the object on the Earth’s surface.

  • These are semicircles, and the distance between them decreases steadily from the equator to poleward until it becomes zero at the poles. All meridians meet at poles.
  • Prime Meridian (0 degrees) passes through Greenwich Observatory (near London).
  • One degree of Longitude equals 4 minutes, i.e., 360 degrees equals 24 hours.

International Date Line

  1. It is an imaginary non-linear line that serves as the ‘line of demarcation’ between two consecutive calendar dates.
  2. It passes through the mid-Pacific Ocean.
  3. A day is gained when the International Date Line is crossed from West to East and lost when it is crossed from East to West. For example, A person crossing the IDL from West to East at noon on Sunday will experience noon on Monday. So, the person gains a day.

Pole Star

A pole star is a star that is bright, closely aligned to the axis of rotation the Earth. It can be seen from the naked eye. Since it is located above the northern pole, it indicates the north direction.

The motion of the Earth

Earth has two types of motion- Rotation and Revolution.

Rotation

  1. When the Earth turns around an internal axis i.e., turns around its own axis, it is called a rotation.
  2. Earth rotates on a tilted axis, angle of 23.5 degrees with normal, around itself from west to the east direction in around 24 hours which is responsible for day and night.
  3. An imaginary line dividing the Earth with day and night is called a Circle of Illumination. It makes the right angle to the orbital plane.

Revolution

  1. When Earth circles an external axis, i.e., around the Sun, it is called a revolution.
  2. Earth revolves around the Sun in an elliptical orbit in around 365¼ days.
  3. The tilt of the Earth along with revolution causes a change in seasons.
  4. Revolution along with Rotation and Tilt of Earth’s axis results in variation in length of day and night.

Solistice

  1. Since the axis of the Earth is inclined to the plane ecliptic at an angle of 66 ½degrees, it gives rise to different seasons and varying length of the day and night and solstice.
  2. Solstice is the moment when the Sun’s apparent path is farthest north or south from Earth’s Equator. There are two solistice – Summer solistice and Winter solistice.
  3. Summer Solstice happens around 21st June. On this day, the Northern Hemisphere has the longest day and the shortest night and reverse conditions happen in the Southern hemisphere. On this day direct rays of the sunfall on the Tropic of Cancer.
  4. Winter Solstice happens on around 22nd December. On this day, the Southern Hemisphere has longer days and shorter nights. The reverse happens in the Northern Hemisphere. On this day direct rays of the Sun fall on the Tropic of Capricorn.
  5. From the Summer Solstice to Winter Solstice (21st June to 22nd December) the Arctic circle experiences the sunlight for almost six months and Antarctic circle experiences the darkness for six months and vice versa from Winter solstice to the Summer solstice.

Equinox

  1. It is the instant of time when the length of days and nights are equal.
  2. This occurs twice each year around 20 March and 23 September.
  3. Direct rays of the Sun fall on the Equator.

Interior of the Earth:

Based on the above observations and evidence the interior of the Earth is divided into three zones viz.

  1. Crust
  2. Mantle
  3. Core

1. Crust:

  • It’s the outermost solid part of the Earth. It’s hard in nature but can break easily.
  • The thickness of the oceanic crust is less compared to the continental crust. while that of the continental crust is around 30 km.

 Continental crust

  • The continental crust is thicker in the areas of major mountain systems.
  • The mean thickness is 5 km and is as much as 70 km thick in the Himalayan region.
  • It’s made up of heavier rocks having an average density of 3 g/cm3.

Oceanic Crust

  • The mean thickness of the oceanic crust is 5 km.
  • The mean density of material in the oceanic crust is 2.7 g/cm3.

2.  Mantle:

  • There is a sudden increase in velocity of seismic waves at the base of lower crust denoting a discontinuity between lower crust and upper mantle known as ‘Mohorovicic Discontinuity’.
  • The mantle extends from Mohorovicic discontinuity to a depth of 2,900 km.
  • It is formed largely of silica minerals rich in iron and magnesium.
  • It spreads over 2700 km, with an average density of 4.6 g/cm3,
  • It constitutes 83% of Earth’s volume and 68% of Earth’s mass.

Asthenosphere

  • The mantle contains a weaker zone known as Asthenosphere through which the molten rock materials find their way to the surface.
  • The material in the upper mantle portion is called magma. Once it starts moving towards the crust or it reaches the surface, it is referred to as lava.
  • The material that reaches the ground includes lava flows, pyroclastic debris, ash and dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorine, hydrogen and argon.
  • The concentration of radioactive material in this part is the reason for the formation of the asthenosphere.

Lithosphere: The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km. The lower mantle extends beyond the asthenosphere. It is in solid-state.

3.  Core:

  • The core-mantle boundary is located at the depth of 2,900 km; it is called as Weichert- Gutenberg Discontinuity.
  • It is further classified into Outer Core (liquid state) and Inner Core (solid-state).The core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes referred to as the NIFE LAYER (Nickel + Iron).
  • The density of the outer core is around 10 gm/cm3 and the density of inner core varies from 13.3 to 13.6 gm/cm3.
  • It accounts for 16% and 32% of the total volume and mass of the Earth respectively.
  • Connecting dot: The liquid outer core surrounding the solid inner core composed ofiron and nickel creates electrical currents when the Earth rotates on its axis which in turn creates the magnetic field of the Earth.

Theories associated with formation of continents

Continental Drift Theory

  • In 1912, a German meteorologist Alfred Wegener put forth the continental drift theory regarding the distribution of the oceans and the continents.
  • According to the theory, all the continents was a part single continental mass ‘PANGAEA’ and mega ocean surrounded this landmass called ‘PANTHALASSA.’
  • According to the theory, Pangaea landmass began to split around 200 million years ago. It first broke into two large continental masses as,
  • Laurasia– Northern components
  • Gondwanaland– Southern components. Indian subcontinent was part of it.
  • Subsequently, both of these components drifted away and in between a shallow sea known as Tethys sea emerged by filling up the water from Panthalassa.
  • Later on, Laurasia and Gondwana rifted and continued to break into various smaller continents that exist today.

Convectional Current theory

  • It was proposed by Arthur Holmes.
  • It says that movements due to radioactive elements operating in the mantle portion are the force behind the drifting of crust.

Sea Floor Spreading Theory

  • Mapping of ocean floor of the Earth and many paleomagnetic studies reveals,
  • Volcanic eruption along the mid oceanic ridges.
  • Rocks at equidistance on both sides of ridges.
  • Remarkable similarities such as: age, chemical composition, magnetic properties etc.
  • Thin oceanic floor

Based on the above facts, Henry Hess (1961) proposed Seafloor spreading which states,

  • Constant eruption causes rupture and pushes the floor apart.
  • New lava erupts and forms a new crust. Thus, spreads the oceanic floor.

Plate Tectonic Theory

  • The concept of seafloor spreading opened up a possibility to understand the configuration of the surface of the Earth.
  • Plate tectonic theory is a unifying theory of continental drift theory and seafloor spreading concept including thermal convection current theory.

Continental drift theory didn’t have acceptable explanations on how continents could move, the seafloor concept provided evidence of the movement of the seafloor while plate tectonic theory unified it with more acceptable evidences.

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