CLASS 7 GEOGRAPHY CHAPTER 3 OUR CHANGING EARTH

KEYWORDS

1. Lithospheric plates - The  lithosphere is broken into a number of plates known as the lithoshepric plates.

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

3. Earthquake - When the lithospheric plates move, the surface of the earth vibrates. The vibrations can travel all around the earth. These vibrations are called earthquakes.

4. Focus - The place in the crust where the movement starts is called the focus. It is also called Hypocentre.

5. Epicentre - The place on the surface above the focus is called the epicentre. Vibrations of the earthquake travel outwards from the epicentre as waves.

6. Seismograph - An earthquake is measured with a machine called a seismograph.

7. Richter scale - The magnitude of the earthquake is measured on the Richter scale.

8. Weathering - is the breaking up of the rocks on the earth's surface.

9. Erosion - is the wearing away of the landscape by different agents like water, wind and ice.

10. Waterfall - When the river tumbles at steep angle over very hard rocks or down a steep valley side it forms a waterfall.

11. Meanders - As the river enters the plain it twists and turns forming large bends known as meanders.

12. Floodplains - When the river overflows its banks, leading to floods, it deposits layers of fine soil and other material called sediments along its banks. This leads to the formation of a flat fertile floodplain.

13. Delta - As the river approaches the sea, the speed of the flowing river decreases and it breaks up 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.

14. Sea Caves - Sea waves continuously strike at the rocks, cracks develop, which over time becomes larger and larger, leading to hollow cave like structure called the sea caves.

15. Sea Arches - As the sea waves continue to erode the sea caves, the cavities become bigger and bigger leaving only the roof, thus forming the sea arches.

16. Stacks - Further, erosion of sea arches leads to breaking of roof and only walls are left. These wall like features are called stacks.

17. Sea Cliffs - The steep rocky coast rising almost vertically above sea water is called sea cliffs.

18. Glaciers, are rivers of ice.

19. Glacial Moraines  - The material carried by the glaciers such as rocks big and small, sand and silt gets deposited. These deposits form glacial moraines.

20. Mushroom rocks - Wind erodes the lower section of rock more than the upper part. Therefore, in desert we can see rocks in the shape of a mushroom, commonly called as mushroom rocks.

21. Sand dunes - 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 called sand dunes.

22. Leoss - When the grains of sand are very fine and light, the wind can carry it over long distances. When such sand is deposited in large areas, it is called loess. 

 1. Why do the plates move? 

Ans. The movement of molten magma inside the earth results in the movement of plates. 

Q 2. What are exogenic and endogenic forces? 

Ans. The movement of the Earth is divided on the basis of the forces which cause them. So the forces that act in the interior of the earth are called as Endogenic forces and the forces that work on the surface of the earth are called as Exogenic forces. 

Q 3. What is erosion? 

Ans. Erosion is defined as the wearing away of the landscape by different agents like water, wind and ice. The process of erosion and deposition create different landforms on the surface of the earth. 

Q 4. How are flood plains formed? 

Ans. When a river overflows its banks, it results in the flooding of the area surrounding it. When it floods, it deposits a layer of fine soil and other material called sediments. Thus forming a fertile layer of soil called flood plains. 

Q 5. What are sand dunes?

Ans. When the wind blows, it lifts and transports sand from one place to another. When the wind stops blowing the sand falls and gets deposited in the low hill – like structures. These are called sand dunes. They are mostly found in desert areas.

Q 6. How are beaches formed? 

Ans. The sea waves deposit sediments along the seashores. This results in the formation of beaches. 

Q 7. What are the ox-bow lakes? 

Ans. When the river enters the plains it twists and turns forming large bends known as meanders. In due course of time, the meander loops start to cut off the river and form cut off lakes, known as the ox-bow lakes.

Give reasons

1. Some rocks have a shape of a mushroom because in desert, wind erodes the lower section of the rocks more than the upper section which makes base of the rock narrow and upper part wide, giving it the shape of a mushroom

2. Flood plains are very fertile because at times when river water overflows, it results in the flooding of the neighbouring areas. This deposits a layer of fine soil and other sediments at the banks of the river, which leads to fertile flood plains.

3. Sea caves are turned into stacks because when the cavities in the sea caves become bigger and bigger, only the roof of the caves are leftover, which forms sea arches. Further, due to erosion, the roof vanishes and only the walls stay intact turning the sea caves into stacks.

4. Buildings collapse due to earthquakes because when the lithospheric plates move, the surface of the earth vibrates and then these vibrations travel outwards from the epicentre in the form of waves, which leads to sudden movement and results in the collapse of buildings.