Our Changing Earth - Solutions

 CBSE Class–VII Social Science

NCERT Solutions
Geography Chapter 3
Our Changing Earth

Q1. Answer the following questions.

  1. Why do the plates move?
  2. What are exogenic and endogenic forces?
  3. What is erosion?
  4. How are floodplains formed?
  5. What are sand dunes?
  6. How are beaches formed?
  7. What are ox-bow lakes?

Answers:

  1. The earth's crust consists of several large and some small, rigid irregularly shaped plates which carry the continents and the ocean floor. These are called lithospheric plates. These plates move around very slowly -just a few millimeters each year, because of the movement of the molten magma inside the earth. This magma moves in a circular manner. The movement of the plates causes changes on the surface of the earth.
  2. The earth's movements are divided based on the forces which cause them. The forces that work on the earth's surface are called exogenic forces while those that work in the earth's interior are called endogenic forces. Endogenic forces sometimes cause sudden movements and at other times produce slow movements. The erosional and depositional activities of wind, water, sea waves, and glaciers are examples of exogenic forces. Earthquakes, landslides, and volcanoes are examples of sudden movements of endogenic forces and they cause mass destruction on the surface of the earth. Mountains are formed due to slow movements of endogenic forces.
  3. The Earth's landscape is continuously being worn away by two processes. One is Erosion and the other is weathering. Erosion is the wearing away of the landscape by different agents like wind, water, and ice. The eroded material is carried away or transported by water, wind etc and eventually deposited. This process of erosion and deposition creates different landforms on the surface of the earth.
  4. As the river enters a plain it twists and turns to form large bends called meanders. Due to continuous erosion and deposition along the sides of the meander, a river sometimes overflows its banks. This leads to the flooding of neighbouring areas. As it floods, the river water deposits layers of fine soil and sediments on its banks leading to the formation of a flat, fertile floodplain.
  5. In sandy deserts, when the wind blows, it lifts and transports sand from one place to another. When it stops blowing, the sand particles fall and get deposited in low hill-like structures called sand dunes. They are temporary landforms.
  6. The erosional and depositional activities of sea waves give rise to different coastal landforms. A beach is one such coastal landform. It is formed when the sea waves deposit sediments along the seashore. People love spending time on the beach.
  7. An oxbow lake is a crescent-shaped river formed by a meandering river. During its journey through a plain, a river twists and turns to form meanders. An oxbow lake is a U-shaped lake that forms when a wide meander from the main stem of a river is cut off, creating a free-standing body of water. This landform is so named for its distinctive curved shape, which resembles the bow pin of an oxbow.

    Erosion and deposition occur constantly along the sides of a meander, causing its ends to come closer and closer. In due course of time, the meander loop cuts off from the river and forms a cut-off crescent-shaped ox-bow lake. It is called so because of its shape.

Q2. Tick the correct Answer.
(i) Which is not an erosional feature of sea waves?
(a) Cliff (b) Beach (c) Sea cave
Ans: (b) Beach

(ii) The depositional feature of a glacier is:
(a) Floodplain (b) Beach (c) Moraine
Ans: (c) Moraine

(iii) Which is caused by the sudden movements of the earth?
(a) Volcano (b) Folding (c) Floodplain
Ans: (a) Volcano

(iv) Mushroom rocks are found in:
(a) Deserts (b) River valleys (c) Glaciers
Ans: (a) Deserts

(v) Oxbow lakes are found in:
(a) Glaciers (b) River valleys (c) Deserts
Ans: (b) River valleys

Q3. Match the following.

(i) Glacier(a) Seashore
(ii) Meanders(b) River of ice
(iii) Beach(c) Rivers
(iv) Sand dunes(d) Vibrations of earth
(v) Waterfall(e) Hard bedrock
(vi) Earthquake(f) Deserts

Ans:

(i) Glacier(b) River of ice
(ii) Meanders(c) Rivers
(iii) Beach(a) Seashore
(iv) Sand dunes(f) Deserts
(v) Waterfall(e) Hard bed rock
(vi) Earthquake(d) Vibrations of earth

Q4.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) Buildings collapse due to earthquakes.

Ans:

  1. In deserts, one can see rocks in the shape of a mushroom—with a narrower base and a wider top. These are known as mushroom rocks. Such rocks are formed when the winds erode the lower section of a rock more than the upper part.Wind erosion is particularly common in the desert areas.
  2. Flood plains are formed as a result of the depositional activity of rivers. Rivers carry along with them eroded material like fine soil and sediments. When a river overflows its banks, it deposits the eroded material and creates flood plains. The deposited material is rich in silt or alluvium and makes the land fertile.
  3. Stacks are formed as a result of the erosional activity of the sea waves. When sea waves continuously strike rocks, cracks develop in them. As these cracks become larger and wider, hollow caves get formed on the rocks. These are called sea caves. As the waves keep striking the rocks, the cavities become bigger and bigger, with only the roof remaining at the end. Such structures are known as sea arches. Further erosion breaks the roof, and only walls remain. These wall-like features are known as stacks. Thus, sea caves are ultimately converted into stacks.
  4. Earthquakes are the sudden vibrations caused within the earth’s surface as a result of the movement of the lithospheric plates. Such vibrations can travel all round the earth. When they are of a high intensity, they cause damage to the things on the earth’s surface. Various human-made (e.g., buildings) and natural (e.g., trees) constructions can break down and collapse under the effect of the vibrations because they are situated on the Earth’s surface.The greatest damage usually happens closest to the epicentre and the strength of the earthquake decreases away from the centre. Although earthquakes cannot be predicted, the impact can certainly be minimised if we are prepared before hand.