7 Geography

Our Changing Earth

Learning Goals:

  • Changes in earth's surface
  • Volcano
  • Earthquake
  • Landforms

The lithosphere i.e. the solid crust is made up a large number of plates. These plates are called lithospheric plates. They move around very slowly – just a few millimeters each year. Their movement is because of the movement of the molten magma inside the earth. The molten magma inside the earth moves in a circular manner.


The lithospheric plates have the following features

  • Some of them are small in size whereas many of them are large – sized.
  • They are rigid
  • They are irregularly – shaped plates (slabs)
  • They carry continents and the ocean floor

The movement of the lithospheric plates is responsible for the changes on the surface of the earth. The earth movements are divided on the basis of the forces that cause them.


Earth Movements

Endogenic forces

The term ‘endo’ means inside and ‘genic’ means origin. The forces that are originated in the interior of the earth or, in other words, the forces that act in the interior of the earth are called endogenic forces. They are of two types:

  • Sudden Forces: As the name suggests, sudden forces cause instant / sudden movements. Examples of such movements are – earthquakes, volcanoes and landslides. These cause mass destruction over the surface of the earth.
  • Diastrophic Forces: These forces, as opposed to the sudden forces, cause slow movements. These cause a deformation in the earth’s crust especially in the form of folding, e.g., – mountain formation.

Exogenic forces

The term ‘exo’ means outside and ‘genic’ means origin. Therefore, exogenic forces are the forces that work on the exterior (surface) of the earth. These forces are erosional and depositional in nature.

  • The forces that cause erosion are called erosional forces. Erosion is defined as the removal of soil, sediments and rock fragments from the landscape. Wind, water and ice cause erosion.
  • Depositional forces cause deposition. The process of erosion stops when the transported materials fall out of the transporting medium (wind, water etc.) and settle on a surface. This process is called deposition.

The following are the erosional and depositional forces: river, wind, sea waves, glaciers.


Volcano

volcano
Fig: Volcano

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

Earthquake

earthquake
Fig: Earthquake

The vibrations caused on the surface of the earth when the Lithospheric plates move are called earthquakes. These vibrations can travel all around the earth.

The place in the earth where the movement starts is called the focus. It is the origin of the seismic energy. The place on the surface above the focus is called the epicentre. Vibrations travel outwards from the epicentre as waves.

The strength of the earthquake decreases away from the epicentre. In other words, the greatest damage is usually closest to the epicentre.

There are three types of earthquake waves:

  • P waves or longitudinal waves
  • S waves or transverse waves
  • L waves or surface waves

Earthquake preparedness

Although earthquakes cannot be predicted, the impact can definitely be minimized if we are prepared in advance. It is rightly said - ‘Prevention is better than cure’.

Some common earthquake prediction methods adopted locally by people is studying animal behavior which includes:

  • fish in the ponds getting agitated
  • snakes coming to the surface

It is recommended to spread awareness amongst our family and acquaintances so that they face the disaster confidently. Some of the do’s and dont’s of earthquake are:

  • Take shelter under a kitchen counter, table, desk or against an inside corner/wall.
  • Avoid areas around chimneys, fire places, windows that shatter including mirror and picture frames.

Next

Lesson Part 2

MCQ

Questions 1

Questions 2

NCERT Solution