NCERT Class 9 Social Science Understanding Society India and Beyond Part 1 Chapter 02 Shaping of the Earths Surface PDF Download

Chapter 2: Shaping of the Earth's Surface

The Big Questions

1. What shapes the Earth's surface?
2. What is plate tectonics? What are the effects of plate movement?
3. How are landforms formed and how are they classified?
4. How are humans and other living beings connected to these landforms?
5. How do disasters associated with different landforms impact human lives?

The Earth's surface is not consistent; it is constantly being transformed by powerful forces acting from within and on the surface of the planet. One of the most important ideas that explains these changes is the theory of plate tectonics, which describes how large pieces of the Earth's crust move slowly over the molten mantle. The movement of these plates gives rise to various landforms, such as mountains, volcanoes, plains, and valleys. Understanding plate tectonics and landforms helps us explain natural phenomena like earthquakes, volcanic eruptions, and the formation of continents and oceans, and allows us to better appreciate the dynamic nature of the Earth.

Plate Tectonics

Plate tectonics is an important theory, given by W.J. Morgan, in earth science, that explains the movement of the Earth's crust. According to this theory, the outermost layer of the Earth is not one single piece but is broken into several large and small pieces called tectonic plates. These plates move slowly over the semi-molten layer beneath them and are responsible for major physical features and natural phenomena, such as mountains, earthquakes, and volcanoes.

The Earth is made up of three main layers: crust, mantle, and core. The crust is the outermost layer on which we live. Below the crust lies the mantle, which is very thick and hot. The core is the innermost layer and is extremely hot and heavy. The crust along with the upper part of the mantle forms the lithosphere. This lithosphere is broken into different tectonic plates. Beneath the lithosphere lies the asthenosphere, which is semi-molten and allows the plates to move.

[Figure 2.1: The Earth's interior (Given values are approximate), See in your textbook]

Tectonic plates are massive slabs of solid rock that move very slowly, usually a few centimetres per year. There are three main types of tectonic plates - continental plates, which carry continents; oceanic plates, which carry ocean floors; and mixed plates, which carry both continents and oceans. Some of the major tectonic plates of the world include the Pacific Plate, Eurasian Plate, African Plate, North American Plate, South American Plate, Indo-Australian Plate, and Antarctic Plate.

Teacher's Note

Remember that the lithosphere (crust plus upper mantle) is rigid and solid, while the asthenosphere below it is hot and partially molten. This difference is key: the rigid lithosphere is broken into plates that can move because they float on the softer, weaker asthenosphere beneath them. When you see diagrams, check which layer is which before answering questions about plate movement.

The movement of tectonic plates is caused by convection currents in the mantle. Heat from the Earth's core causes molten material in the mantle to rise, while cooler material sinks. This continuous movement creates convection currents that push and pull the tectonic plates, causing them to move in different directions.

[Figure 2.2: Movement of material below the crust, See in your textbook]

The edges where tectonic plates meet are called plate boundaries. There are three main types of plate boundaries. The first is the convergent boundary, where two plates move towards each other. When continental plates collide, they form fold mountains, such as the Himalaya. When an oceanic plate collides with a continental plate, the oceanic plate sinks beneath the continental plate, leading to volcanic activity and earthquakes. The second type is the divergent boundary, where plates move away from each other. In this case, magma rises from below and forms new crust, creating features, such as mid-ocean ridges. The Mid-Atlantic Ridge is a good example. The third type is the transform boundary, where plates slide past each other without creating or destroying crust. This type of movement mainly causes earthquakes, such as along the San Andreas Fault in the United States.

Teacher's Note

The three plate boundaries each cause different landforms and natural disasters. At convergent boundaries, you get mountains and volcanoes; at divergent boundaries, you get mid-ocean ridges and spreading zones; at transform boundaries, you get earthquakes. When a question asks what causes a particular feature or disaster, identify which boundary type is involved first, then the answer becomes clear.

Key Points

• The Earth has three main layers - crust, mantle, and core - and the lithosphere (made of crust and upper mantle) is broken into tectonic plates that move on the softer asthenosphere below.
• Tectonic plates move very slowly, driven by convection currents in the mantle where hot material rises and cool material sinks.
• There are three types of plate boundaries: convergent (plates colliding, forming mountains and volcanoes), divergent (plates separating, forming mid-ocean ridges), and transform (plates sliding sideways, causing earthquakes).

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