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NCERT Class 7 Social Science Chapter 23 Our Changing Earth
Also, you can read the NCERT book online in these sections Solutions by Expert Teachers as per Central Board of Secondary Education (CBSE) Book guidelines. CBSE Class 7 Social Science Solutions are part of All Subject Solutions. Here we have given NCERT Class 7 Social Science Chapter 23 Our Changing Earth and After, NCERT Class 7 Social Science Textbook of Our Pasts – II: History, Social and Political Life – II: Civics, Our Environment: Geography. for All Chapters, You can practice these here.
Our Changing Earth
Our Environment [Geography]
QUESTIONS WITH ANSWERS
I. Answer the following questions:
Q.1. Why do plates move?
Ans: The plates move as the tremendous heat generated inside the core of the earth cause the rocks in the mantle to melt. These molten rocks expand, rise and push the plate away from each other. When they cool down, they sink and bring plate towards each others.
Q.2. What are Exogenic and Endogenic forces?
Ans: Exogenic forces: The forces that work on the surface of the earth are called exogenic forces.
Endogenic forces: The forces that acts in the interior of the earth are called endogenetic forces.
For example: Earthquake, volcanic eruption, etc.
Q.3. What is erosion?
Ans: Erosion is the wearing away of the landscape by different agents like water, wind and ice.
Q.4. How are flood plains formed?
Ans: A flood plain is a flat land adjacent to a stream or a river that experience occasional or periodic flooding. These plains are formed during floods, when the river deposit sediments brought with them on the river bank.
Q.5. What are sand dunes?
Ans: Sand dunes are the hills of sand deposited by the action of winds in deserts.
Q.6. How are beaches formed?
Ans: Beaches are formed by the eroded material deposited by the waves along the sea coast.
Q.7. What is ox-bow lake?
Ans: 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, called an ox-bow lake.
II. Tick the correct answer:
Q.1. Which is not an erosional feature of sea waves?
(c) Sea cave.
Ans: (a) Beach.
Q.2. The depositional feature of a glacier is _______.
(a) Flood plain.
Ans: (c) Moraine.
Q.3. Which is caused by the sudden movement of the earth?
(c) Flood plain.
Ans: (c) Volcano.
Q.4. Mushroom rocks are formed in _______.
(b) River valley.
Ans: (a) Desert.
Q.5. Ox-bow lakes are found in _______.
(b) River valley.
Ans: (b) River valley.
III. Match the following:
|(i) Glacier||(a) Sea shore|
|(ii) Meanders||(b) River of ice|
|(iii) Beach||(c) Rivers|
|(iv) Sand dunes||(d) Vibrations of earth|
|(v) Waterfall||(e) Hard bed rock|
|(vi) Earthquake||(f) Deserts|
|(i) Glacier||(c) Rivers|
|(ii) Meanders||(b) River of ice|
|(iii) Beach||(a) Sea shore|
|(iv) Sand dunes||(f) Deserts|
|(v) Waterfall||(e) Hard bed rock|
|(vi) Earthquake||(d) Vibrations of earth|
IV. Give reasons:
1. Some rocks have shape of a mushroom
Ans: Wind action is most prominent in desert. When strong winds laden with coarse sand particles strike the surfaces of the rock mass. It get errored. The base eroded more than the rock. As wind direction keeps on changing regularly. The rock get eroded from all sides. Eventually it resembles like a mushroom with narrow base and wide top.
Q.2. Flood plains are very fertile.
Ans: Flood plains are very fertile as they are formed by silt brought by river.
Q.3. Sea caves are turned into stacks.
Ans: As the cavities of sea caves become bigger and bigger only the roof of the caves are left, thus forming arches. Erosion of sea arch breaks the roof and only walls are left these wall like features are called stack.
Q.4. Building collapse due to earthquake.
Ans: Most of the buildings are not safe enough to resist the vibration of the earth. They are not made earthquake proof. Thus they collapse when the movement in tectonic plates under the surface of the earth which send out vibration in all the directions causes an effect to anything built on the crust.
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.
VI. For Fun:
Solve the crossword puzzle with the help of given clues.
Across: 2. Meander.
15. Sand- dune.
16. Flood plain,
Down: 1. Wave.
7. Ox-bow lake.
SOME OTHER IMPORTANT QUESTIONS FOR EXAMINATION
Very Short Answer Type Questions
Q.1. What are plate tectonics?
Ans: The theory of plate tectonics states that the Lithosphere is broken into six large and several small blocks called plates. The whole process of plate movements / motions is referred to as plate tectonics.
Q.2. How are the plate movement classified?
Ans: The movement is classified into three categories. These movements are convergence, divergence and conservative movements.
Q.3. What are endogenic processes?
Ans: The processes that act in the interior of the earth are called endogenic processes.
Q.4. What is magma?
Ans: The hot molten material in the interior of the earth is called magma.
Q.5. What is a volcano?
Ans: Volcano is a rocky vent in the earth’s crust, nearly circular in shape through which its products like, rock materials, gases and water comes forth violently.
Q.6. On the basis of periodicity of eruptions, how are volcanoes are classified?
Ans: On the basis of periodicity of eruptions, volcanoes can be classified into:
(a) Active Volcanoes.
(b) Dormant Volcanoes.
(c) Extinct Volcanoes.
Q.7. Define earthquake.
Ans: The sudden vibration on the surface of the earth is referred to as earthquake.
Q.8. What do you know about the following:
Ans: (i) The place in the crust where the movement of the earth starts is called the focus.
(ii) The place on the surface above the focus is called the epicentre.
Q.9. How is earthquake measured?
Ans: An earthquake is measured with a machine called a seismograph. The magnitude of the earthquake is measured on the Richter scale.
Q.10. Define the concept weathering.
Ans: Weathering refers to the breaking up of the rocks on the earth’s surface.
Q.11. What is the meaning of erosion?
Ans: Erosion refers to wearing away of the landscape by different agents like water, wind and ice.
Q.12. How is a waterfall formed?
Ans: The running water in the river erodes the landscape. When the river tumbles at steep angle over very hard rocks or down a steep valley side it forms a waterfall.
Q.13. Name the highest waterfall.
Ans: Angel Falls of Venezuela in South America.
Q.14. Which movements results in volcanic eruptions and earthquakes?
Ans: Sudden movements results in volcanic eruptions and earthquakes.
Q.15. Name the scale used to measure the magnitude of an earthquake.
Ans: The magnitude of the earthquake is measured on the Richter scale.
Q.16. Why is wind more prominent in deserts?
Ans: Wind action is more prominent in deserts because there is hardly any vegetation to check the velocity of wind.
Short Answer Type Questions
Q.1. What do you know about endogenic processes?
Ans: The processes that act in the interior of the earth are called endogenic processes. These processes are responsible for the formation of fold mountains, like the Himalayas in Asia and the Alps in Europe. Endogenic processes are also responsible for seafloor spreading, along the Indo-Australian Plate in the Indian Ocean between the continents of Africa and India.
Q.2. How earthquakes and volcanoes modify the surface of the earth?
Ans: The sudden vibration of the earth surfaces cause large scale deformations at the crust of the earth. These deformations lead to the formation of mountains, ocean basin, etc. Sometimes molten rocks, gases and steam reach the earth’s surface through hole or vent in the earth’s crust. Successive eruptions from a single vent build up a mountain known as volcano. The collection of erupted rock material around opening results in the building up of a volcanic cone. The direct effects of earthquakes are the changes in the earth’s surface vibrations in the crust may lead to development of cracks or fissures in the rocks. Land may subside along such fissure or cracks and it may lead. to sudden changes in the level of sea or land.
Q.3. What is a seismograph? How earthquake is measured?
Ans: An instrument used to measure or record the seismic/earthquake waves is called Seismograph. An earthquake is measured on the Richter Scale. An earthquake over 5.0 magnitude can cause damage from things falling. A 6.0 or higher magnitude is considered very strong and 7.0 is classified as major earthquake.
Q.4. What is volcanic eruption? On the basis of periodicity how, volcanic eruption is classified?
Ans: A sudden eruption of the volcano through a rocky vent in the earth’s crust, is called a volcanic eruption. On the basis of periodicity of eruptions, volcanoes can be classified into:
(a) Active Volcanoes.
(b) Dormant Volcanoes.
(c) Extinct Volcanoes.
Q.5. List the factors which affect the rate of weathering and erosion.
Ans: Following factors affect the rate of weathering and erosion:
(i) Temperature of a place.
(ii) Vegetation cover.
(iii) Change in land use pattern.
(iv) Types and slope of soil.
(v) Rainfall of a place.
Q.6. What are different types of earthquake waves?
Ans: Earthquake waves are of three types. These are given below:
(i) P-waves or longitudinal waves or Primary waves.
(ii) Secondary waves, also called Transverse or S-waves.
(iii) L-waves or Long waves, also called Rayleigh waves.
Q.7. Name some of the common earthquake prediction methods adopted by people.
Ans: Some common earthquake prediction methods adopted by people include:
(a) Studying animal behaviour.
(b) Fish in the ponds gets agitated.
(c) Snakes come to the surface.
Q.8. What are meanders?
Ans: As the river enters the plains, it twists and turns, forming large bends known as meanders.
Q.9. What do you understand by erosion? What are the main agents of erosion?
Ans: Erosion is the wearing away of the landscape by different agents. The eroded material is carried away or transported by water, wind etc. and eventually deposited. Main agents of erosion are: wind, water and ice.
Long Answer Type Questions
Q.1. Describe the preparedness to minimise the affect of an earthquake.
Ans: Following steps should be taken to minimise the affect of an earthquake:
Safety Measures: General safety measures in case of an earthquake are briefly stated as follows:
(a) Take urgent note of warnings issued by government departments over radio, television and in newspapers.
(b) Move out if you feel it is safer, but do not panic when an earthquake occurs.
(c) When outside, don’t stay under a tree or near the walls of a building.
(d) Don’t rush outside or go to a balcony, unless you feel your house is going to collapse. Stay near the door or crawl under a strong table but stay away from walls.
Q.2. What do you know about exogenic processes?
Ans: Exogenic processes work on the surface of the earth and are responsible for weathering. Weathering is refer to the process that breaks rocks into smaller fragments and cause erosion, through which rock fragments, after being worn away are transported by agents of denudation like running water, wind and glaciers. The Ural Mountains lying between Europe and Asia have decreased in height due to these processes.
Exogenic processes are also known as external forces, because they wear away, transport and deposit the products of weathering on the earth’s surface.
Q.3. What is the importance of drainage pattern of a river?
Ans: The importance of drainage pattern of river is:
(a) It throws light on the evolution of river systems.
(b) It shows the flow of water in the river for a part or whole of the year.
(c) If the drainage pattern is well developed river may have enough water to reach the sea or make an area of lake inland lakes or salt lakes.
Q.4. Differentiate between the following:
(a) Endogenic and Exogenic forces.
Ans: Endogenic and Exogenic forces
(b) Weathering and Erosion.
Ans: Weathering and Erosion
Q.5. Name the erosional and depositional features found in coastal areas. Explain how they are formed.
Ans: The erosion and deposition of the sea waves give rise to coastal land forms such as sea caves, sea arches, stacks and sea cliff. These can be explained as follows:
(a) Sea caves: Sea waves continuously strike at the rocks as a result of which cracks are formed. Over time these cracks become larger and wider. Thus hollow like caves are formed. They are called sea caves.
(b) Sea arches: As the cracks or cavities become bigger and bigger only the roof of the caves remain, thus forming sea arches.
(c) Stacks: Erosion breaks the roof and only walls are left. These walls like features are called stacks.
(d) Sea cliff: The steep rocky coast rising almost vertically above the sea water is called sea cliff.
(e) Beaches: The sea waves deposit sediments along the shores forming beaches.
Higher Order Thinking Skill
Q.1. How the action of wind in desert helps in the formation of Mushroom rocks and Sand dunes?
Ans: Mushroom rocks: In a sandstorm, the wind is able to make the sand grains bounce up to about 1 metre above the ground. When the sand collides with the rock, it wears away the surface. The rock is eroded only at its base and the rock takes an appearance of a mushroom.
Sand dunes: Deposition by wind produces extensive dunes. A dune is a hill or a mound of sand deposited by the wind. Dunes are common in all sandy regions such as sandy deserts and sandy beaches.
Q.2. Describe the features caused by deposition work done by the wind.
Ans: Several features are caused by the depositional work done by the wind.
(a) During sandstorm, the wind carries large amounts of sand grains. As the wind slow down its energy, it starts dropping its load of sand. Such hills of sand deposited by the wind are called Sand dune.
(b) When dunes have a long, gentle slope on the windward side and a steep slope on the leeward side, forming a crescent shape, they are called Barchans.
(c) Sand dunes can move or migrate over time. When fine sand particles are deposited over large areas they form Loess plains.
Q.3. Give a detailed account of the features formed by the work of Ice.
Ans: Features formed by the work of ice can be explained as follows:
(a) Glaciers are rivers of ice which erode the landscape by bulldozing soil and stones to expose the solid rocks below.
(b) Glaciers carve out deep hollows.
(c) As the ice melts they get filled up with water and become beautiful lakes in the mountains.
(d) The material carried by the glaciers such as rocks big and small, sand and silt gets deposited. These deposits form glacial moraines.
Value Based Questions
Q.1. How volcanoes are useful to us?
Ans: Volcanoes are useful to us as,
(i) The material thrown out of volcanoes form the fertile soil.
(ii) They brings out useful minerals from beneath of the earth, which are used in different industries.
(iii) They are useful for the scientists to understand the nature of the interior of the earth.
Q.2. The alluvial delta regions are very fertile. Give reason.
Ans: The alluvial delta regions are very fertile as they are formed by the silt deposited by the river. The soil deposited in these regions is rich in minerals which adds to the productivity of the soil. For example, Ganga-Brahmaputra delta is the largest fertile and density delta region of the world.
OBJECTIVE TYPE QUESTIONS
1. Fill in the blanks:
1. Sudden movements like _______ and _______ cause mass destruction over the surface of the earth.
Ans: Earthquakes, Volcano.
2. The forces that work on the surface of the earth are called as _______.
Ans: Exogenic forces.
3. There are _______ types of earthquake waves.
4. Although _______ cannot be predicted, the impact can certainly be minimised if we are prepared before-hand.
5. ______ are rivers of ice which too erode the landscape.
6. An active agent of _______ and ______ in the deserts is wind.
Ans: Erosion, Deposition.
II. Write True or False for each of the following statements:
1. The lithosphere is broken into a number of plates known as the lithospheric plates.
2. Endogenic forces sometimes produce sudden movements and at the other times produce slow movements.
3. The highest waterfall is Angel Falls of Venezuela in North America.
4. The steep rocky coast rising almost vertically above sea water is called stacks.
5. The place in the crust where the movement starts is called the epicentre.
III. Look at the given below diagram and explain the following features in brief;
(a) Sea Cave
(c) Sea Arch
(b) Sea Cliff
Ans: (a) Sea Cave: Sea waves continuously strike at the rocks. Cracks develop. Over time they become larger and wider. Thus, hollow like caves are formed on the rocks. They are called Sea Caves.
(b) Sea Arches: As the cavities of the sea cave, become bigger and bigger, only the roof of the cave remain, thus, forming Sea Arches.
(c) Sea Cliff: The steepy rocky coast rising almost vertically above sea water is called Sea Cliff.
(d) Stacks: Erosion of sea arch breaks the roof and only walls are left. These wall like features are called Stacks.
IV. Multiple Choice Questions (MCQs): Choose the correct option:
1. The opening at the top of a volcanic mountain through which lava erupts is called:
(b) Mountain building.
(c) Volcanic eruption.
Ans: (c) Volcanic eruption.
2. The opening at the top of a top a volcanic mountain through which lava erupt is called _______.
Ans: (c) Magma.
3. Folding is caused by a force of _______.
Ans: (b) Upliftment.
4. Sand dunes are _______.
(a) Hill-like structure.
(b) Cave-like structure.
(c) Wall-like structure.
(d) Roof-like structure.
Ans: (a) Hill-like structure.
5. An earthquake is measured with the help of _______.
Ans: (a) Seismograph.
6. Loess is found in _______.
Ans: (c) Deserts.
7. Which places should we stay away from during earthquake?
(a) Fire places.
(b) Area around chimneys.
(c) Area around window.
(d) All of these.
Ans: (d) All of these.
8. A good example of volcano is _______.
(a) Mt. Everest.
(b) Mt. Fujiyama.
(c) Mt. Nanda Devi.
(d) Mt. K₂.
Ans: (b) Mt. Fujiyama.
9. Meanders are usually formed in ______.
(b) River valley.
Ans: (b) River valley.
10. Mushroom rocks are usually found in _______.
Ans: (d) Deserts.
V. Fill in the blanks:
1. Forces which act in the interior of the earth are called _______.
Ans: Endogenic forces.
2. ______ is a landform formed at the mouth of the river.
3. Hills of sand formed due to action of _______ in desert are called sand dunes.
4. _______ is a vent in the earth’s crust through which molten magma erupt suddenly.
5. Fine sand deposited by action of wind in a desert is called _______.