NCERT Summary: Our Changing Earth - Notes | Study Social Studies (SST) Class 7 - Class 7

Introduction

• The earth’s crust consists of several large and some small, rigid, irregularly-shaped plates which carry continents and the ocean floor, called Lithospheric plates.
  • These plates move around very slowly – just a few millimetres each year because of the movement of the molten magma inside the earth.
• The earth movements are divided on the basis of the forces which cause them:
  • Endogenic forces: The forces which act in the interior of the earth.
  • Exogenic forces: The forces that work on the surface of the earth.

1. Endogenic Forces

• Literal meaning: Endo means inside genic means origin.
• Endogenic forces sometimes produce sudden movements and at the other times produce slow movements.
• Sudden movements like earthquakes and volcanoes cause mass destruction over the surface of the earth.

Volcano

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

Earthquakes

• When the Lithospheric plates move, the surface of the earth vibrates whose vibrations can travel all round the earth. These vibrations are called earthquakes.
  • 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.
• Earthquakes cannot be predicted however, the impact can certainly be minimised if we are prepared before-hand.
• Some common earthquake prediction methods adopted locally by people include studying animal behaviour; fish in the ponds get agitated, snakes come to the surface.

Earthquake Preparedness

Where to take shelter during an earthquake —

• Safe Spot – Under a kitchen counter, table or desk, against an inside corner or wall.
• Stay Away from – Fire places, areas around chimneys, windows that shatter including mirrors and picture frames.
• Be Prepared – Spread awareness amongst your friends and family members and face any disaster confidently.

2. Exogenic forces

• Literal Meaning: Exo means outside genic means origin.

Major Land Forms

• The landscape is being continuously worn away by two processes
  • 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.

Work of a River

• The running water in the river erodes the landscape.
• 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.
• Due to continuous erosion and deposition along the sides of the meander, oxbow lake is created over time.
• When the river overflows its banks, it leads to the flooding of the neighbouring areas.
• The flooding results in deposition of layers of fine soil and other material called sediments along its banks which leads to the formation of a flat fertile floodplain.
• The raised banks are called levees.
• As the river approaches the sea, the speed of the flowing water decreases and the river begins to break up.
• The breaking of river into a number of streams called distributaries.
• The river becomes so slow that it begins to deposit its load.
  • 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. Cracks develop and with they become larger and wider. → Thus, hollow like caves are formed on the rocks which are called sea caves.
• Over time, these cavities become bigger and bigger only the roof of the caves remain which results in formation of sea arches.
• Further, erosion breaks the roof and only walls are left. These wall like features are called stacks.
• The steep rocky coast rising almost vertically above sea water is called sea cliff.
• The sea waves deposit sediments along the shores forming beaches.

Work of Ice

• A Glacier is a large mass of ice continuously moving over land surface which too erode the landscape by bulldozing soil and stones to expose the solid rock below.
• Glaciers carve out deep hollows. As the ice melts, they get filled up with water and become beautiful lakes in the mountains. 
• The material carried by the glacier such as rocks big and small, sand and silt gets deposited. These deposits form glacial moraines.

Work of wind

• An active agent of erosion and deposition in the deserts is wind.
• In deserts, rocks are presnt in the shape of a mushroom, commonly called mushroom rocks.
• Winds erode the lower section of the rock more than the upper part.
  • Therefore, such rocks have narrower base and wider top.
  • When the wind blows, it lifts and transports sand from one place to another.
  • When it stops blowing the sand falls and gets deposited in low hill – like structures which are called sand dunes.
• When the grains of sand are very fine and light, the wind can carry it over very long distances.
  • When such sand is deposited in large areas, it is called loess.
  • Large deposits of loess is found in China.