NCERT Solutions For Class 7 Geography Social Science Chapter 3 Our Changing Earth
Question 1. Answer the following questions briefly:
(i) Why do the plated move?
(ii) What are exogenic and endogenic force?
(iii) What is erosion?
(iv) How are flood plains formed?
(v) What are sand dunes?
(vi) How are beaches formed?
(vii) What are ox-bow lakes?
Answer: (i) The plates move because of the movement of the molten magma inside the earth.
(ii) Exogenic forces. The forces that work on the surface of the earth are called as exogenic forces.
Endogenic forces. The forces that act in the interior of the earth are called as endogenic forces.
(iii) Erosion is the wearing away of the landscape by different agents like water, wind and ice.
(iv) During floods, layers of fine soil and other material called sediments are deposited on the river bank. This leads to the formation of a flat fertile flood plains.
(v) Sand dunes are low hill-like structures formed by the deposition of sand in the deserts.
(vi) Beaches are formed when the sea waves deposit sediments along the shores of the sea.
(vii) When the meander loop is cut off from the main river, it forms a cut-off lake. As its shape is like an ox bow, it is also known as ox-bow lake.
Question 2. Tick the correct answer:
(i) Which is not an erosional feature of sea waves?
(c) Sea cave.
(ii) The depositional feature of a glacier is
(a) Flood plain
(iii) Which is caused by the sudden movements of the Earth ?
(c) Flood plain.
(iv) Mushroom rocks are found In
(b) River valleys
(v) Ox bow lakes are found In
(b) River valleys
Answer: (i) – (a), (ii) – (c), (iii)-(a) , (iv) -(a), (v)-(b)
Question 4. Give reasons:
(i) Some rocks have a shape of a mushroom.
(ii) Flood plains are very fertile.
(iii) Sea caves are turned into stacks.
(iv) Buddings collapse due to earthquakes.
Answer: (i) In deserts, winds usually erode the lower section of the rock more than the upper part. Therefore, such rocks have narrower base and wider top, which take the shape of a mushroom.
(ii) Flood plains are formed by the deposition of fine soil and other material called sediments on the river banks. As the soil and sediments are brought by flood water, they are very fertile.
(iii) Sea waves strike at the rocks. As a result cracks develop which become bigger over time and hollow like caves are formed on the rocks. They are called sea caves. These cavities become bigger and bigger and a time comes when only the roof of the caves remain to form sea arches. Further erosion breaks the roof and only walls are left. These wall like features are called stacks. In this way, sea caves are turned into stacks.
(iv) Most of the buildings are not safe enough to resist the vibrations of the earthquakes. They are not made earthquake-proof. They collapse tearing apart due to shallow foundation and lack of adequate steel in the interior design.
Question 5. Activity
Observe the photographs given below. These are various features made by a river. Identify them and also tell whether they are erosional or depositional or landforms formed by both.
Solve the crossword puzzle with the help of given clues.
2. Loop like the bend of a river.
4. Solid form of water.
7. Moving mass of ice.
9. Sudden descent of water in the bed of a river.
11. Natural cavity on weak rocks formed by action of waves.
12. Embankment on a river that keeps it in its channel.
13. Large body of sea water.
14. Dry area where sand dunes are found.
15. Small hill of sand caused by the action of the wind.
16. Flat plain formed by river deposits during time of flood.
1. Rise and fall of water caused by friction of wind on water surface.
3. Flow of water in a channel.
5. Steep perpendicular face of a rock along a sea coast.
6. Debris of boulder and coarse material carried by glacier.
8. Crescent shaped lake formed by a meandering river.
10. Fine sand deposited by the action of the wind.
13. Isolated mass of rising steep rock near a coastline.
14. Alluvial tracts of land formed by the river deposits at the mouth of a river.
Earthquake-A case study
1. Read the ‘Earthquake – A case study’ given in the form of headlines that appeared in the newspapers after the quake. Arrange the events in the right sequence of their happening.
2. Imagine if a quake suddenly shook in the middle of the school day, where do you go for safety?
2. We would go for safety
- in the open
- under the gates
- would keep pillow or cushion on our heads.
INTEXT QUESTIONS WITH THEIR ANSWERS
Find out the names of a few rivers of the world that form a delta?
Name of some rivers (See the Map):
- Hawang Ho.
Very Short Answer Type Questions
Question 1 What do you know about the lithospheric plates?
Answer: The earth’s crust consists of several large and some small, rigid, irregularly— shaped plates, Le., slabs which cany continents and the ocean floor.
Question 2. How do the lithospheric plates move?
Answer: They move around very slowly, just a few millimeters each year.
Question 3. What is a volcano?
Answer: A volcano is a vent or opening in the earth’s crust through which molten material erupts suddenly.
Question 4. Define focus’ and ‘epicentre’. [Imp.]
Answer: The place in the crust where the movement starts is called the ‘focus’. The place on the surface above the focus is called the ‘epicentre’.
Question 5. Name the three types of earthquake waves.
- P waves or longitudinal waves
- S waves or transverse waves
- L wave or surface waves.
Question 6. What is a seismograph? [V. Imp.]
Answer: A seismograph is a machine which measures an earthquake.
Question 7. Name the scale on which the magnitude of the earthquake is measured.
Answer: Richter scale.
Question 8. Which earthquake is classified as a major earthquake?
Answer: An earthquake of 7.0 magnitude is classified as a major earthquake.
Question 9. Where is Victoria Falls located? [Imp.]
Answer: Victoria Falls is located on the borders of Zambia and Zimbabwe in Africa.
Question 10. What is a delta?
Answer: The collection of sediments from all the mouths forms a delta. It is triangular shaped landmass.
Question 11. Name some coastal landforms.
Answer: Sea caves, sea arches, stacks and sea cliff.
Question 12. How are glacial moraines formed? [V. Imp.]
Answer: The material carried by the glacier such as rocks big and small, sand and silt gets deposited. These deposits form glacial moraines.
Question 13. Name the two processes which wear away the landscape.
Answer: Weathering and erosion.
Question 14. What does the process of erosion and deposition create?
Answer: The process of erosion and deposition create different landforms on the surface of the earth.
Question 15. Name a few rivers of the world that form a delta?
Answer: Nile, Zaire, Ganga-Brahmaputra, Hwangotto, Murray-Darling, Amazon, etc.
Short Answer Type Questions
Question 1. Mention the work of ice.
Answer: Glaciers are rivers of ice which erode the landscape by destroying soil and stones to expose the solid rock below. Glaciers carve out deep hollows. As the ice melts they get filled up with water and become beautiful lakes in the mountains. The material carried by the glacier such as rocks big and small, sand and silt gets deposited. These deposits form glacial moraines.
Question 2. What is earthquake? What are some common earthquake prediction methods?[V. Imp.]
Answer: When the lithospheric plates move, the surface of the earth vibrates. The vibrations can travel all round the earth. These vibrations are called earthquakes. Some common earthquake prediction methods include studying animal behaviour, fish in the ponds get agitated, snakes come to the surface.
Question 3. Give an account of earthquake preparedness. [V. Imp.]
Answer: Earthquake is a natural calamity which we cannot stop. But we can minimise its impact if we are prepared before-hand.
During an earthquake, we should shift to some safe spot. We should hide under a kitchen counter, table or desk against an inside comer or wall. We should stay away from fire places, areas around chimneys, windows that shelter including mirrors and picture frames. Moreover, we should spread awareness amongst our friends and family members.
Long Answer Type Questions
Question 1. Explain the work of a river. [V. Imp.]
Answer: The running water in the river erodes the landscape. When the river tumbles at steep angle over very hard rocks or down a deep valley side it forms a waterfall. While entering the plain the river twists and turns and forms large bends which are known as meanders. Due to continuous erosion and deposition along the sides of the meander, the ends of the meander loop come closer and closer. In -due course of time the meander loop cuts off from the river and forms a cut-off lake, which is also called ox-bow lake. Sometimes, the river overflows its banks causing flood in the neighboring areas. As it floods, it deposits layers of fine soil and other material called sediments along its banks. As a result—fertile floodplain is formed. The raised banks are called levees.
As the river approaches the sea, the speed of the flowing water decreases and the river begins to break up into several streams known as distributaries. Then a time comes when the river becomes very slow and it begins to deposit its load. Each distributary forms its own mouth. The collection of sediments from all the mouths forms a delta, which is a triangular landmass.
Question 2. Give an account of the work of wind. [V. Imp.]
Answer: Wind is an active agent of erosion and deposition in the deserts. In deserts we often notice rocks in the shape of a mushroom, known as mushroom rocks. Winds erode the lower section of the rock more than the upper part. Therefore, such rocks have narrower base and wider top. 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 low hill-like structures. These are called sand dunes. When the grains are very fine and light, the wind can carry it over long distances. When such sand is deposited in large area, it is called loess.