Our Changing Earth Class 7 Geography Chapter 3

summary of ncert class 7 chapter 3 geography
Meander

We know that the uppermost part of earth’s crust is known as lithosphere and this lithosphere is broken into many number of plates known as lithospheric plates. This plate moves around slowly like a few mm per year. This movement of lithospheric plates is because of the circular movement of molten magma inside the earth.

Due to these movement of plates earth’s surface got changed with time. these earth movements are divided on the basis of force by which it causes. These are two type –

Endogenic force

This force acts in the interior of the earth. these are further divided into Sudden forces and Diastrophic forces. Earthquake, volcano and landslide are some of the examples of sudden forces. On the other hand, building of mountain is a slow process, it takes millions of years. So it is an example of Diastrophic forces.

Exogenic forces

These are the forces that work on the surface of the earth. Erosional and depositional processes are considered in exogenic forces. For example, erosion and deposition can be done by river, wind, sea-waves, glaciers etc.

We can understand these force by their word origin i.e. Endogenic = Endo + Genic where endo means inside and genic means origin. Similarly, in exogenic, exo means outside and genic means origin. Let’s discuss some of the sudden movement under endogenic forces –

Volcano

Volcano is an opening vent in the earth’s crust through which molten matter comes out. The material which comes out is called as lava and when it is inside the earth’s crust or mantle it is called as magma. The top tip on earth’s crust formed due to volcano is called as crater.

Earthquake

earthquake barwal education

When the lithospheric plate moves the surface of the earth vibrate and this vibration is called as earthquake. Focus is the point earthquake started i.e. the origin of the seismic energy and the point Epicenter is situated just above the focus on the earth’s surface. Earthquake travel in form of waves there are three type of earthquake waves –

  1. P waves or longitudinal waves
  2. S waves or transverse waves
  3. L waves or surface waves

Measurement of earthquake is done by a machine called as seismograph and the magnitude is measured on the Richter scale. For example, earthquake that came in Bhuj, Gujrat was 6.9 on Richter scale.

MAJOR LAND FORMS

We have to understand two processes – weathering and erosion. Weathering is the breaking up of rocks on earth’s surface and erosion is wearing by some agents like water, wind and ice. The eroded material deposit somewhere and create some new landforms.

Work of River

The waterfall on a river erodes the landscape. When a river enters into plains it twists and turns forming large bends called as meanders. Because of continuous erosion at one side meander become larger and the meander loop cut off from the river and form a cut off lake which are also known as ox-bow lakes.

When river overflow it leads to flooding of the nearest area. Due to flood in that area it deposits layers of fine soil and sediments. Therefore, this area is known as fertile flood plains and the raised banks are called as levees. When a river reaches its drainage i.e. sea, its speed decreases and it break up into small streams called as distributaries. These distributaries deposit their load there and form delta.

The Angel falls of Venezuela is the highest waterfall in the world. Other famous falls are Niagara Falls and Victoria falls.

Work of Sea Waves

The erosion and deposition of sea waves leads to rise of coastal landforms. Sea waves continuously strike at rocks and crack develops. These cracks become larger and hollow like caves. When it erodes continuously these become larger and larger and only roof of rock remains at last. These are known as sea arches. When further erosion take place only wall remains and these are called as stacks. The steep rocky vertical coast of sea is known as sea cliff.

Work of Ice

Glacier are the agent of erosion and these are called as rivers of ice. Glacier erode landscape by bulldozing soil and stones. When the ice melt down in summers it forms beautiful lakes in mountains. The material eroded by glaciers in the form of rocks (small and big), sand and silt gets deposited and form glacial moraines.

Work of Wind

The only active agent of erosion in desert is wind. In desert the rocks are in mushroom shape and therefore called as mushroom rocks. With the blowing of wind, the sand gets lifted and transported to another place. When the wind stop blowing the sand get deposited and make low hills which are called as sand dunes. When the sand is very fine and it gets deposited in large area called as loess. Large deposits of loess found in china.

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