Our Changing Earth (NCERT Class 7 Geography Chapter 3 Notes)

Earth Movements

Lithospheric Plates
The lithosphere is broken into several small, rigid, irregularly shaped plates known as lithospheric plates. These plates carry continental and oceanic floors. These move slowly a few millimetres each year. Thus changing surface of the earth. The plate movement can be attributed to the movement of molten magma beneath these plates.

Earth Movement
The earth movements can be divided on the basis of cause forces. These forces are endogenic forces and exogenic forces.

Endogenic forces

1. Endogenic forces are forces which act in the interior of the earth.
2. Endogenic forces at times produce sudden movements such as earthquakes and volcano eruption and at other times, slow movements such as mountain formation.
3. Sudden movements like earthquakes and volcano eruption are the cause of mass destruction.

Volcano

1. A volcano is an opening (vent) in the earth surface through which molten lava suddenly erupts on the surface.

Also read: Volcanoes in India

Origin of Earthquake

Earthquake

1. When lithospheric plates move, the earth surface vibrates. These vibrations are called earthquakes.
2. The spot in the crust where these vibrations start, known as focus.
3. The place on the surface above the focus is known as the epicentre.
4. The vibrations move outward from the epicentre in the form of waves.
5. There are three types of earthquake waves: 1. P waves or longitudinal waves, 2. S waves or transverse waves and 3. L waves or surface waves.
6. The maximum damage happens usually closest to the epicentre. The intensity of the earthquake decreases from the centre.
7. The magnitude of the earthquake is scaled by the Richter scale and measured with help of seismograph.
8. An earthquake of 2.0 or less can be classified as normal tremor. An earthquake of 5.0 and above can cause damage. An earthquake of 6.0 and above is classified as a strong earthquake and an earthquake of 7.0 and above is classified as the major earthquake.

Earthquake Preparedness

1. Pick a Safe spot: Under kitchen counter, table or desk against an inside corner or wall.
2. Stay away from fire, chimneys and glass windows.
3. Be prepared and spread awareness among your friends and relatives.

Exogenic forces

1. The exogenic forces are forces which act upon the exterior of the earth. They continuously shape the earth surface by weathering, erosion and deposition.
2. The process of breaking up of rocks into smaller pieces on earth surface is known as weathering whereas erosion is the process of wearing of landmass with the agency of water, wind and ice. The eroded or weathered material is carried by the wind or water and deposited somewhere else. This process creates and shapes different landforms on earth.

Features made by a River

Work of a River

1. As the river falls from very steep valley sides, it forms the waterfall.
2. As the river enters the plain, it forms large bends known as meanders due to the complexity of path.
3. In due time, meander loop cuts off the river and forms a cut-off lake known as an ox-bow lake.
4. Sometimes, the river floods and deposit silt and sediments on the plain area. The area is known as floodplains.
5. The raised banks due to over deposition of sediment and silt are known as leeves.
6. As the river approaches the sea, it slows down and breaks up into several streams known as distributaries. At this time, the river becomes so slow that it deposits a large chunk of sediment and silt at its mouth. This led to the formation of a delta.

Works of Sea Waves

1. Sea caves are the result of continuous striking of sea waves on rocks.
2. When these cavities or sea caves becomes bigger, in such a way that the roof of the sea cave remains, it leads to the formation of sea arches.
3. With further erosion, when roof breaks down and only walls remain, the wall structures are known as stacks.
4. The steep rocky coast rising vertically known as sea cliffs while they deposit sediment on shores to form beaches.

Works of the Ice

1. Glaciers are known as the rivers of ice. They erode landmass by bulldozing soil and stones to expose the solid rocks.
2. They carve deep hollows, which result in lakes as ice melts.
3. The material carried by glaciers gets deposited somewhere. These deposits are known as the glacier moraines.

Works of the Wind

1. Wind is an active agent of erosion and deposition in deserts.
2. Mushroom rocks are formed due to the action of winds. Here, winds erode the lower section of the rock more than the upper section.
3. In deserts, winds carry the sand from one spot and deposit it on another spot. When it stops blowing, it leads to the formation of sand dunes.
4. When sand is deposited on a large section of the area, it is called loess.