CBSE Class 7 Geography – 3. Our Changing Earth (Notes)

Lithospheric plate – The lithosphere is broken into a number of plates which are called the lithospheric plates (also called Tectonic plate).

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 millimetres 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:

There are two types of earth movements on the basis of the forces:-

Endogenic force

Exogenic force

 

             

Endogenic forces –  The forces that are originated in the interior of the earth are called endogenic forces.  

The term ‘endo’ means inside and ‘genic’ means origin.

They are of two type’s endogenic forces:

Diastrophic Forces –  Slow movements inside the earth is called diastrophic movements.

These forces, as opposed sudden forces 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. 

Sudden Forces – Cause instant / sudden movement inside the earth is called sudden movements. Examples of such movements are – earthquakes, volcanoes and landslides.

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

     

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.

Is rightly said – ‘Prevention is better than cure’.

Some common earthquake prediction methods adopted locally by people are studying animal behaviour which includes:

fish in the ponds getting agitated

snakes coming to the surface

Exogenic forces – The forces that work on the exterior (surface) of the earth are called exogenic force.

The term ‘exo’ means outside and ‘genic’ means origin.

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.

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.

Weathering: The breaking up of rocks on the earth’s surface due to air, water and heat is called weathering.

It creates rock fragments.

Erosion: The wearing away of landscape by different agents like wind, water and ice are called erosion. 

As mentioned earlier, it is the wearing away of landscape by different agents like wind, water and ice.

Role of water in erosion/ deposition:

River – A large amount of fresh water flowing continuously in a long line across land is called river.

The running water of the river erodes the landscape.

Waterfall – When the river tumbles at steep angle over very hard rocks or down a steep valley side is called waterfall.

Ox Bow Lake:

As the river enters the plain, it twists and turns forming large bends called meanders.

Due to continuous erosion and deposition along the sides of the meander, the ends of the meander loop come closer and closer.

When this happens, over a course of time, the meander loop cuts off from the river and forms a cut – off lake. Such a lake is also called an ox-bow lake.

Floodplain – An area of land built up from fine soil and other materials left by floods is called floodplains.

When the river overflows its banks, the neighbouring areas get flooded.

When the river floods, it deposits layers of fine soil and other material called sediments along its banks. This leads to the formation of a floodplain.

A floodplain is flat and fertile.

Levees: The raised banks of the river (due to overflowing) are called levees.

Delta – An area of low, flat land shaped like a triangle , where a river spilt and spread out into several branches before entering the sea are called delta.

When the river approaches the sea, it becomes very slow in its flow and begins to break up into a number of streams called distributaries.

The speed of the river water is so less that it starts depositing its load.

Each distributaries forms its own mouth.

A collection of sediments from all the mouths forms a delta.

Sea Waves – The up and down movements of water in the sea are known as sea waves.

The erosion and deposition by water in the form of sea waves leads to the formation of coastal landforms.

Some coastal landforms are as follows:

Sea waves continuously strike at the rocks. This leads to cracks.

The cracks become larger and wider overtime.

Thus hollow like caves are formed on the rocks. They are called sea caves.

As these cavities become bigger and bigger, only the roof of the caves remains, hence forming sea arches.

The erosion further breaks the roof and only the walls remain. These wall-like features are called stacks.

Sea cliff is the steep rocky coast rising almost vertically above sea water.

The sea waves deposit sediments along the shores forming beaches.

Role of ice in erosion/ deposition:

Glaciers – A large body of ice moving slowly down a slope or valley or spreading outward on a land surface is called glaciers.

Glaciers are ‘rivers’ of ice that too erode the landscape by bulldozing soil and stones to expose the solid rock below.

They create:

Deep hollows

Lakes in mountains: These are formed when the ice melts and the deep hollows get filled up with water

Glacial moraines: These are formed by the deposition of the material carried by the glacier such as rocks big and small, sand and silt.

Role of wind in erosion/ deposition:

An active agent of erosion and deposition in the deserts is the wind.

The landforms in the desert are dry and sandy.

Mushroom Rocks – Rocks that have narrow base and a broad top are called mushroom rocks.

 In deserts; we can see rocks in the shape of mushrooms called mushroom rocks.

The mushroom has a narrow base and a wider top.

Sand Dunes – The hill-like structures in the desert when the sand falls and gets deposited when the wind stops blowing are called sand dunes.

Loess – 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 deposit of loess is found in China.