Notes Chapter 2 Class 9 Social Science CBSE Board

Shaping of the Earth’s Surface

1. Introduction

• Earth’s surface is constantly changing due to forces inside and outside the Earth.
• Theory of plate tectonics explains these changes and formation of landforms.

2. Plate Tectonics

• Theory given by W.J. Morgan.
• Earth’s crust is broken into several tectonic plates that move slowly over the semi-molten
  asthenosphere.
• Plates move a few centimetres per year.
• Responsible for mountains, earthquakes, volcanoes and major features of Earth.

3. Layers of the Earth

• Crust: Outermost layer (5–40 km thick).
• Mantle: Thick and hot layer below crust.
• Core: Innermost, extremely hot and heavy.
• Lithosphere: Crust + upper mantle (broken into plates).
• Asthenosphere: Semi-molten layer that allows plates to move.

4. Types of Tectonic Plates

• Continental plates (carry continents).
• Oceanic plates (carry ocean floors).
• Mixed plates (carry both).

5. Causes of Plate Movement

• Convection currents
  in the mantle: Heat from core makes material rise and sink, pushing the plates.

6. Types of Plate Boundaries

• Convergent Boundary: Plates move towards each other. – Forms fold mountains (e.g., Himalayas). – Causes volcanoes and earthquakes.
• Divergent Boundary: Plates move apart. New crust forms (e.g., Mid-Atlantic Ridge).
• Transform Boundary: Plates slide past each other. Causes earthquakes (e.g., San Andreas Fault).

7. Effects of Plate Tectonics

• Formation of mountains, valleys, ocean basins, volcanoes and earthquakes.
• Most earthquakes and volcanoes occur at plate boundaries.
• Ring of Fire: Area around Pacific Ocean with high seismic and volcanic activity.

8. Landforms

• Natural features on Earth’s surface.
• Formed by weathering, erosion, deposition and crust movement.
• Examples: Mountains, valleys, plateaus, plains, deserts, coastal features.

9. Weathering

• Breaking down of rocks into smaller pieces (no movement).
• Physical Weathering: By temperature changes, frost, wind.
• Chemical Weathering: Minerals react with water, air, acids.
• Biological Weathering: By plants, animals, micro-organisms.
• Helps in soil formation.

10. Erosion

• Wearing away and transport of materials by natural agents.
• Types: Water, wind, glacial, coastal erosion.
• Affects farming (loss of topsoil), houses, roads and livelihoods.

11. Agents of Gradation

Natural forces that level the Earth’s surface:

• Running water, glaciers, wind, waves, groundwater.

12. Landforms by Running Water

• Upper Course: V-shaped valleys, waterfalls, rapids.
• Middle Course: Meanders, oxbow lakes, floodplains.
• Lower Course: Deltas, levees, alluvial fans.
• Waterfall: River falls over steep cliff.
• Meander: Winding curve in river.
• Delta: Fertile land at river mouth (good for agriculture and fishing).

13. Landforms by Waves and Currents

• Form beaches, sand bars, sea cliffs, sea caves, arches, stacks.
• Beach: Sand/pebbles deposited by waves (tourism, fishing, natural protection).

14. Landforms by Glaciers

• U-shaped valleys, cirques, aretes, hanging valleys, fjords.
• Moraines: Deposits of rocks and soil (fertile soil, lakes for water and power).

15. Landforms by Wind

• Yardangs, ventifacts, deflation hollows, desert pavements.
• Dunes: Sand hills (barchan, longitudinal, star, parabolic) – act as barriers, used in tourism.

16. Landforms by Underground Water

• Karst topography in limestone areas.
• Caves, stalactites, stalagmites, sinkholes, underground rivers.
• Useful for water supply and tourism.

17. Landforms and Human Life

• Rivers and plains support agriculture and civilisations.
• Mountains act as barriers and allow cultural exchange.
• Coasts support trade, fishing and harbours.
• Landforms influence settlements, economy and culture.

18. Landforms and Disasters

• Landslides: Caused by heavy rain, earthquakes, deforestation, construction.
• Avalanches: Sudden snow slide on steep slopes due to heavy snow or temperature change.
• GLOFs (Glacial Lake Outburst Floods): Sudden release of water from glacial lakes.
• Dust Storms: Strong winds lift dry soil in desert areas.

19. Summary

• Internal forces (plate tectonics) create big features.
• External forces (weathering, erosion) shape smaller landforms.
• Humans depend on landforms but face disasters.
• Understanding helps in disaster management and sustainable living.