Ch – 3 Our Changing Earth

Important Points:

  • The lithosphere is broken into a number of plates known as Lithospheric plates.
  • These plates move just a few millimeters every year.
  • The molten magma inside the earth causes these movements.
  • The molten magma moves in a circular manner inside the earth.
  • The movement of plates bring changes on the surface of earth.
  • These changes are classified on the basis of the forces which cause them.
  • The forces which act in the interior of earth are known as endogenic forces.
  • The forces which act on the exterior of earth are known as exogenic forces.
  • Endogenic forces are further classified as sudden forces ( eg: earthquake, volcano ) and diastrophic forces ( eg: building mountains )
  • Exogenic forces cause erosional and depositional changes ( eg: river, wind, sea-waves, glaciers )
  • A volcano is a vent in the earth’s crust through which molten material erupts suddenly.
  • The vibrations felt on the earth’s surface, because of the movement of lithospheric plates, cause earthquakes.
  • The place in the curst where the movement starts is known as focus.
  • The place on the surface above the focus is known as epicentre.
  • Focus is the origin of seismic energy.
  • The three types of earthquake waves are:
    • P waves or longitudinal waves
    • S waves or transverse waves
    • L waves or surface waves
  • An earthquake is measured with the help of seismograph.
  • The magnitude of an earthquake is measured by Richter scale.
  • Weathering is the breaking up of the rocks on the earth’s surface.
  • Erosion is the wearing away of the landscape by different agents like water, wind, etc.
  • The process of erosion and deposition creates different landforms on the earth.
  • When the river tumbles at steep angle over very hard rocks or down a steep valley side it forms a waterfall.
  • As the river enters the plain, it twists and turns forming large bends known as meanders.
  • In due course of time, the meander loop cuts off from the river and forms a cut-off lake, also called an oxbow lake.
  • As flood occurs in lake, it deposits layers of fine soil and other material called sediments along its banks, which leads to the formation of flood plains.
  • The raised banks are called leeves.
  • As the speed of water decreases on approaching sea, the river begins to break up into a number of streams called distributaries.
  • The collection of sediments from the mouths of distributaries forms a delta.
  • When the sea waves strike continuously at the rocks, cracks develop and over time they become larger and hollow like caves, they are called as sea caves.
  • When the cavities of caves become bigger and only their roof remains, then it is known as sea arches.
  • When further erosion occurs, the roof breaks off and only arches remain, then it is known as stacks.
  • The steep rocky coast rising almost vertically above sea water is known as cliff.
  • Glaciers are rivers of ice which erode the landscape by bulldozing soil and stones to expose the solid rock below.
  • The material carried by the glacier such as rocks, sand and silt gets deposited and forms glacier moraines.
  • Wind is an active agent of erosion and deposition in deserts.
  • When wind stops blowing sand falls down and forms low hill-like structures known as sand dunes.
  • When such sand is deposited over a large area it is known as loess.
  • Sometimes wind erodes the lower section of the rocks more than the upper section, which leads to the formation of mushroom rocks.

Published by Priya Prakash

In the midst of winter, i found there was within me an invincible summer ~ Albert Camus

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