CHAPTER - 03 OUR CHANGING EARTH
NOTES
The lithosphere is broken down into a number of plates known as the Lithospheric plates.
• The movement of these plates causes changes on the surface of the earth. The molten magma inside the earth moves in a circular manner. The forces that act in the interior of the earth are called endogenic forces, while the forces that work on the surface of the earth are called exogenic forces.
• Sudden movements like earthquake and volcanoes cause mass destruction over the surface of the earth.
• A volcano is a vent (opening) in the earth’s crust through which molten material erupts suddenly.
• The vibration in the plates of earth is called a earthquake.
• 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. Vibrations travel outwards from the epicentre as waves. Greatest damage is usually closest to the epicentre and the strength of the earthquake decreases away from the centre.
• Major Landforms:
(i) The landscapes are continuously worn away by two forces, weathering and erosion.
(ii) Weathering is the breaking down of the rocks on the earth’s surface.
(iii) Erosion is the wearing away of the landscape by different agents like water, wind and ice.
• Work of a River:
(i) When the river tumbles at a steep angle over hard rocks or down a steep valley side, it forms a waterfall.
(ii) As the river enters the plain, it twists and turns, forming large bends known as meanders.
(iii) At this point of time, the meander loop cuts off from the river and forms a cut-off called ox-bow lake.
(iv) During flooding, the river deposits layers of fine soil and other materials called sediments along its banks. This leads to the formation of a flat fertile plain called floodplain.
(v) The raised banks along the river are called levees.
• Work of Sea Waves:
(i) The erosion and deposition of the sea waves gives rise to coastal landforms.
(ii) Due to seawaves, hollow-like caves and formed on the rocks. They are called sea caves.
(iii) As cavities become bigger in size, only the roof of the caves remain, thus forming sea arches.
(iv) The erosion further breaks the roof and only walls are left. These wall-like features are called stacks.
(v) The steep rocky coast rising almost vertically above sea water is called sea cliff.
(vi) The sea waves deposit sediments along the shores forming beaches.
• Work of Ice:
(i) Glaciers are rivers of ice which erode the landscape by bulldozing soil and stones to expose the solid rocks below.
(ii) The material carried by the glaciers, such as big and small rocks, sand and silt gets deposited. These deposits form glacial moraines.
• Work of Winds:
(i) An active agent of erosion and deposition in the deserts is wind. It makes rocks in shape of a mushroom, called mushroom rocks. Winds erode the lower section of the rock more than the upper part. Therefore, such rocks have narrower base and wider top.
(ii) When the wind stops blowing, the sand falls and get deposited in low hill like structures. These are called sand dunes.
(iii) 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.
EXERCISE
Q1. Answer the following questions.
- Why do the plates move?
- What are exogenic and endogenic forces?
- What is erosion?
- How are floodplains formed?
- What are sand dunes?
- How are beaches formed?
- What are ox-bow lakes?
Answers:
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
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 |
Ans:
- 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.
- 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.
- 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.
- 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.
