Here is the notes for Class 7 NCERT Geography – Chapter 3: Our Changing Earth

Lithospheric plates

  • The earth’s crust consists of several large and some small, rigid, irregularly shaped plates (slabs) which carry continents and the ocean floor.
  • These plate move very slowly because of the movement of magma inside the earth.
  • 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.
    • Endogenic forces (endo- inside and genic- outside)
        • This forces which act in the interior of the earth.
        • Endogenic forces sometimes produce sudden movements and at the other times produce slow movements.
        • Sudden movements causes mass destruction over the surface of the earth.
        • Example, earthquake and volcano
    •  Exogenic forces (exo- outside and genic- origin)
        • This forces that work on the surface of the earth
        • Example, weathering, erosion, deposition

Volcano

  • It is a vent (opening) in the earth’s crust through which molten material erupts suddenly.

Earthquake

  • When the Lithospheric plates move, the surface of the earth vibrates.
  • The vibrations can travel all around the earth and causes earthquake.
  • The place in the crust where the movement starts is called the focus.
  • The place on the surface above the focus is called the epicentre.
  • Greatest damage is usually closest to the epicentre and the strength of the earthquake decreases away from the centre.

Three types of earthquake waves

  • P waves or longitudinal waves
  • S waves or transverse waves
  • L waves or surface waves
      • Seismograph- machine used to earthquake
      • Ritcher scale- measure magnitude of the earthquake

MAJOR LAND FORMS

  • The landscape is being continuously worn away by two processes – weathering and erosion.
  • 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 and ice.
  • The eroded material is carried away or transported by water, wind, etc. and eventually deposited.
  • This process of erosion and deposition create different landforms on the surface of the earth.
      • Angels Falls (Venezuela, South America): highest waterfall
      • Niagara falls on the border of Canada and USA in North America
      • Victoria Falls on the borders of Zambia and Zimbabwe in Africa.

Work of a River

  • When the river tumbles at steep angle over very hard rocks or down a steep valley side it forms a waterfall .
  • When the river enters the plain it twists and turns forming large bends known as Meanders.
  • Due to continuous erosion and deposition along the sides of the meander, the ends of the meander loop come closer and closer. In due course of time the meander loop cuts off from the river and forms a cut-off lake, also called an ox-bow lake.
How are levees formed?
  • When the river overflows its banks, leads to the flooding of the neighbouring areas. As it floods, it deposits layers of fine soil and other material called sediments along its banks. This leads to the formation of a flat fertile floodplain,thus raised banks are called levees.
How are distributries formed?
  • As the river approaches the sea, the speed of the flowing water decreases and the river begins to break up into a number of streams called distributaries.
How are deltas formed?
  • Each distributary forms its own mouth. The collection of sediments from all the mouths forms a delta.

Work of Sea Waves

  • The erosion and deposition of the sea waves gives rise to coastal landforms.
  • Seawaves continuously strike at the rocks.
  • Sea cliff: The steep rocky coast rising almost vertically above sea water is called sea cliff.
  • Sea caves: hollow like caves are formed on the rocks
How are sea arches are formed?
  • When the sea caves cavities become bigger and bigger only the roof of the caves remain, thus forming sea arches.
How are stacks are formed?
  • When erosion breaks the roof and only walls are left. These wall like features are called stacks. 

Work of Ice

  • Glaciers are “rivers of ice” which too erode the landscape by bulldozing soil and stones to expose the solid rock below.
How lakes are formed in between the mountain?
  •  Glaciers carve out deep hollows there. As the ice melts they get filled up with water and become beautiful lakes in the mountains.
How glacial moraines are formed?
  • The material carried by the glacier such as rocks big and small, sand and silt gets deposited. These deposits form glacial moraines.
Scroll to Top