“Our Changing Earth,” explores the dynamic nature of Earth’s surface due to internal and external forces. It introduces lithospheric plates—rigid sections of Earth’s crust—and explains their movements caused by molten magma beneath. The chapter distinguishes between endogenic forces (internal) leading to sudden events like earthquakes and volcanic eruptions, and exogenic forces (external) responsible for gradual processes like weathering and erosion. Additionally, it examines how rivers, sea waves, glaciers, and wind shape various landforms, illustrating the continuous transformation of our planet’s landscape.
1. Lithospheric Plates
- Definition: The lithosphere (Earth’s outer layer) is broken into large pieces called lithospheric plates.
- Movement:
- Plates move very slowly, just a few millimeters each year.
- Caused by the movement of molten magma inside the Earth.
- Magma moves in a circular pattern (like boiling water in an activity).
- Effects: Plate movements change the Earth’s surface.
2. Forces Changing the Earth
- Endogenic Forces:
- Definition: Forces that work inside the Earth.
- Types:
- Sudden Movements: Earthquakes and volcanoes.
- Cause mass destruction on the surface.
- Slow Movements: Gradual changes over time.
- Examples:
- Earthquakes (vibrations from plate movement).
- Volcanoes (molten material erupting from vents).
- Exogenic Forces:
- Definition: Forces that work on the Earth’s surface.
- Examples: Weathering, erosion by water, wind, and ice.
3. Earthquakes
- Definition: Vibrations in the Earth’s crust caused by moving lithospheric plates.
- Key Terms:
- Focus: The point inside the crust where the movement starts.
- Epicentre: The point on the surface directly above the focus.
- Waves: Vibrations travel outward from the epicentre as waves.
- Damage:
- Greatest damage is near the epicentre.
- Damage decreases farther from the epicentre.
- Types of Waves:
- P Waves: Longitudinal waves (move like sound waves).
- S Waves: Transverse waves (move up and down).
- L Waves: Surface waves (travel along the surface).
- Measurement:
- Tool: Seismograph (machine that records earthquake strength).
- Scale: Richter Scale.
- 2.0 or less: Barely felt.
- Over 5.0: Can cause damage (things fall).
- 6.0 or higher: Very strong.
- 7.0 or more: Major earthquake.
- Example Activity:
- Boil water in a closed container with a lid.
- Place peas, a spoon, and beads on the lid.
- Observation: Lid shakes, beads roll, spoon vibrates—similar to Earth during an earthquake.
- Prediction:
- Cannot be predicted accurately.
- Local signs: Animals behave strangely (e.g., fish get agitated, snakes surface).
- Preparedness:
- Safe Spots: Under a table, desk, or against an inside wall.
- Avoid: Fireplaces, chimneys, windows, mirrors.
- Tips: Spread awareness to face disasters confidently.
4. Volcanoes
- Definition: A vent in the Earth’s crust where molten material (magma) erupts.
- Effects: Sudden eruptions cause destruction.
- Diagram: Fig. 3.2 shows a volcano.
5. Major Landforms
- Processes:
- Weathering: Breaking up of rocks on the Earth’s surface.
- Erosion: Wearing away of the landscape by water, wind, or ice.
- Deposition: Dropping of eroded material to form new landforms.
5.1 Work of a River
- Erosion:
- Waterfall: Formed when a river falls over hard rocks at a steep angle (Fig. 3.4).
- Deposition:
- Meanders: Large bends formed as the river twists on flat land.
- Ox-Bow Lake: A cut-off meander loop that becomes a lake.
- Floodplain: Flat, fertile land formed by sediment deposits during floods.
- Levees: Raised banks along the river from sediment.
- Delta: Sediments deposited at the river’s mouth as it splits into distributaries (Fig. 3.6).
- Diagram: Fig. 3.5 shows river features in a floodplain.
- Activity: Find names of rivers that form deltas (e.g., Nile, Ganges).
5.2 Work of Sea Waves
- Erosion:
- Sea Caves: Hollows in rocks formed by waves.
- Sea Arches: Caves grow, leaving only a roof.
- Stacks: Walls left after the roof of an arch breaks.
- Sea Cliff: Steep rocky coast rising above the sea.
- Deposition:
- Beaches: Sediments deposited along the shore.
- Diagram: Fig. 3.7 shows features made by sea waves.
5.3 Work of Ice
- Erosion:
- Glaciers: “Rivers of ice” that erode soil and rocks.
- Lakes: Deep hollows carved by glaciers, filled with water after melting.
- Deposition:
- Moraines: Deposits of rocks, sand, and silt left by glaciers.
- Diagram: Fig. 3.8 shows a glacier.
5.4 Work of Wind
- Erosion:
- Mushroom Rocks: Rocks with a narrow base and wide top due to wind erosion.
- Deposition:
- Sand Dunes: Low hill-like structures of sand (Fig. 3.9).
- Loess: Large areas of fine sand deposits (e.g., in China).
- Activity: Collect pictures of sand dunes from deserts.
6. Questions and Answers
6.1 Short Answer Questions
- Why do plates move?
- Plates move due to the circular motion of molten magma inside the Earth.
- What are exogenic and endogenic forces?
- Endogenic: Forces inside the Earth (e.g., earthquakes, volcanoes).
- Exogenic: Forces on the surface (e.g., weathering, erosion).
- What is erosion?
- Wearing away of the landscape by water, wind, or ice.
- How are floodplains formed?
- Rivers overflow, deposit fine soil (sediments) along banks, creating flat, fertile land.
- What are sand dunes?
- Low hills of sand formed by wind in deserts.
- How are beaches formed?
- Sea waves deposit sediments along the shore.
- What are ox-bow lakes?
- Lakes formed when a meander loop cuts off from a river.
6.2 Multiple Choice Questions
- Which is not an erosional feature of sea waves?
- (b) Beach (it’s depositional).
- The depositional feature of a glacier is:
- (c) Moraine.
- Which is caused by sudden movements of the Earth?
- (a) Volcano.
- Mushroom rocks are found in:
- (a) Deserts.
- Ox-bow lakes are found in:
- (b) River valleys.
6.3 Matching
- (i) Glacier → (c) River of ice
- (ii) Meanders → (d) Rivers
- (iii) Beach → (a) Sea shore
- (iv) Sand dunes → (h) Deserts
- (v) Waterfall → (g) Hard bedrock
- (vi) Earthquake → (e) Vibrations of Earth
6.4 Give Reasons
- Some rocks have a mushroom shape:
- Wind erodes the lower part more than the upper part in deserts.
- Floodplains are very fertile:
- Rivers deposit fine, nutrient-rich soil during floods.
- Sea caves turn into stacks:
- Waves erode caves until the roof collapses, leaving walls.
- Buildings collapse due to earthquakes:
- Vibrations shake the ground, weakening structures.
8. Crossword Puzzle
- Across:
- 7. Meander (loop-like river bend)
- 8. Ice (solid water)
- 9. Glacier (moving ice mass)
- Waterfall (sudden water drop)
- Sea cave (cavity by waves)
- Levee (river embankment)
- Sea (large seawater body)
- Desert (dry area with dunes)
- Sand dune (small sand hill)
- Floodplain (flat land by river deposits)
- Down:
- Wave (water rise/fall by wind)
- River (water flow in channel)
- Cliff (steep rock face by sea)
- Moraine (glacier debris)
- Ox-bow (crescent lake by river)
- Loess (fine sand by wind)
- Stack (steep rock by coast)
- Delta (land at river mouth)
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