Our Changing Earth

1. Answer the following questions

(i) Why do the plates move?
The plates move because of the movement of molten magma inside the Earth’s mantle. This molten magma moves in a circular manner (convection currents), which pushes and pulls the lithospheric plates very slowly — just a few millimetres each year.

(ii) What are exogenic and endogenic forces?

• Endogenic forces are forces that act in the interior of the Earth. They can be sudden (earthquakes, volcanoes, landslides) or slow (mountain building).

• Exogenic forces are forces that work on the surface of the Earth, caused by external agents like rivers, wind, sea waves, and glaciers. They include erosion and deposition.

(iii) What is erosion?
Erosion is the wearing away of the landscape by natural agents such as water, wind, and ice. The eroded material is transported and deposited elsewhere.

(iv) How are flood plains formed?
When a river overflows its banks during floods, it deposits layers of fine soil and sediments along both sides of its channel. Over time, this deposition creates a flat fertile area called a flood plain.

(v) What are sand dunes?
Sand dunes are low hill-like structures formed in deserts when the wind deposits sand in one place after carrying it over a distance.

(vi) How are beaches formed?
Beaches are formed when sea waves deposit sand, pebbles, and sediments along the shoreline.

(vii) What are ox-bow lakes?
Ox-bow lakes are curved lakes formed when a meander in a river is cut off from the main channel due to continuous erosion and deposition.

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2. Tick the correct answer

(i) Which is not an erosional feature of sea waves?
✅ (b) Beach

(ii) The depositional feature of a glacier is:
✅ (c) Moraine

(iii) Which is caused by the sudden movements of the earth?
✅ (a) Volcano

(iv) Mushroom rocks are found in:
✅ (a) Deserts

(v) Ox bow lakes are found in:
✅ (b) River valleys

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3. Match the following

Column A	Column B
(i) Glacier	(c) River of ice
(ii) Meanders	(d) Rivers
(iii) Beach	(a) Sea shore
(iv) Sand dunes	(h) Deserts
(v) Waterfall	(g) Hard bed rock
(vi) Earthquake	(e) Vibrations of earth

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4. Give reasons

(i) Some rocks have a shape of a mushroom.
In deserts, wind erodes the lower part of a rock more than the upper part because the sand particles carried by wind mostly hit the base. This creates a narrower base and a wider top, forming a mushroom shape.

(ii) Flood plains are very fertile.
During floods, rivers deposit fine soil, silt, and other minerals on the flood plain. This nutrient-rich sediment makes the soil highly fertile.

(iii) Sea caves are turned into stacks.
Sea waves first form caves in coastal rocks. Continuous erosion enlarges these caves into arches. Eventually, the roof of the arch collapses, leaving behind an isolated vertical wall called a stack.

(iv) Buildings collapse due to earthquakes.
Earthquakes produce strong vibrations that travel through the ground. If buildings are not designed to withstand these vibrations, the shaking causes structural damage, leading to collapse.

Major Landforms of the Earth

Major Landforms of the Earth

The surface of the Earth is constantly being shaped and reshaped by natural processes. Two key processes are responsible for changing the landscape:

1. Weathering – The breaking down of rocks into smaller pieces by natural forces such as temperature change, plants, animals, and chemicals.

2. Erosion – The wearing away of the landscape by moving agents such as water, wind, and ice.

After erosion, the eroded material is transported by these agents and eventually deposited elsewhere. The combined effect of erosion and deposition creates various major landforms.

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1. Work of a River

Rivers shape the land in three stages – upper course (erosion), middle course (erosion & deposition), and lower course (deposition).

A. Waterfalls

• Formed when a river flows over hard rock followed by softer rock, causing the softer rock to erode faster.

• Waterfalls are steep drops in the river’s course.

• Examples:

  • Angel Falls – Venezuela (Highest in the world)

  • Niagara Falls – On the Canada–USA border

  • Victoria Falls – On the Zambia–Zimbabwe border

B. Meanders and Ox-bow Lakes

• Meander: Large bends formed by a river in its middle and lower course due to lateral erosion.

• Over time, erosion on the outer banks and deposition on the inner banks make the loop sharper.

• Eventually, the loop is cut off, forming an ox-bow lake.

C. Floodplains and Levees

• During floods, rivers overflow their banks and deposit fertile alluvial soil, creating floodplains.

• Raised banks of deposited material along the river are called levees.

D. Delta Formation

• At the river’s mouth, speed decreases, and the river splits into distributaries.

• Deposition at the mouths forms a triangular or fan-shaped delta.

• Example: The Ganga–Brahmaputra Delta.

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2. Work of Sea Waves

Sea waves continuously erode and deposit material along coastlines, creating distinct coastal landforms.

• Sea Caves: Formed when waves erode cracks in coastal rocks.

• Sea Arches: When waves erode through a cave, leaving an arch-shaped opening.

• Stacks: When the roof of a sea arch collapses, leaving an isolated vertical rock column.

• Sea Cliffs: Steep rocky coasts rising sharply from the sea.

• Beaches: Formed when sea waves deposit sand and pebbles along the shore.

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3. Work of Ice (Glaciers)

Glaciers are “rivers of ice” that move slowly but have powerful erosive force.

• They erode by bulldozing soil and rocks, carving out deep hollows.

• When the ice melts, these hollows become lakes.

• Deposited material (rocks, gravel, sand) forms glacial moraines.

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4. Work of Wind

Wind is an active erosional and depositional agent in desert regions.

• Mushroom Rocks: Formed when wind erodes the lower parts of a rock faster than the upper parts, creating a narrow base and wider top.

• Sand Dunes: Low hills of sand formed by wind deposition.

• Loess: Fine, light sand particles carried over long distances and deposited in thick layers.

  • Example: Large loess deposits in China.

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5. Summary Table – Agents of Landform Formation

Agent	Erosional Landforms	Depositional Landforms
River	Waterfalls, Meanders	Floodplains, Levees, Delta
Sea Waves	Sea Caves, Arches, Stacks, Cliffs	Beaches
Glaciers	Glacial valleys, hollows	Moraines
Wind	Mushroom rocks	Sand dunes, Loess

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Conclusion

The Earth’s surface is in a state of continuous transformation. Rivers, sea waves, glaciers, and wind constantly modify the landscape through erosion, transportation, and deposition. These processes give rise to spectacular natural features, many of which have significant economic, environmental, and cultural value.

Chapter 3 – Earth Movements and Earthquakes

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Chapter 3 – Earth Movements and Earthquakes

1. The Lithosphere and Lithospheric Plates

The lithosphere is the solid outermost shell of the Earth, which includes the crust and the uppermost part of the mantle. Interestingly, the lithosphere is not a single unbroken sheet; instead, it is broken into a number of large and small pieces called lithospheric plates.

• These plates are like giant puzzle pieces covering the surface of the Earth.

• They are in slow motion—moving only a few millimetres per year.

Reason for movement:
Beneath the lithosphere lies molten magma in the mantle. This magma moves in a circular pattern (convection currents). The heat from Earth’s interior causes this molten rock to rise, move laterally, and then sink again, dragging the lithospheric plates along with it.

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2. Forces Causing Earth Movements

Movements on Earth’s surface are the result of two main types of forces:

A. Endogenic Forces (Internal forces)

These originate inside the Earth and cause changes on the surface.
They are of two types:

1. Sudden Forces – Act very quickly, causing immediate changes:

   • Earthquakes

   • Volcanoes

   • Landslides

2. Diastrophic Forces – Act very slowly over thousands or millions of years:

   • Mountain Building (e.g., formation of the Himalayas)

   • Plateau formation

B. Exogenic Forces (External forces)

These work on the surface of the Earth due to agents like:

• Rivers

• Wind

• Sea waves

• Glaciers

They cause erosion (wearing away of land) and deposition (laying down of material).

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3. Sudden Movements – Volcanoes and Earthquakes

A. Volcano

A volcano is a vent or opening in the Earth’s crust through which molten material (lava), ash, and gases escape from beneath the surface.

• Eruptions may be explosive or quiet.

• They can alter landscapes dramatically and cause widespread destruction.

B. Earthquake

An earthquake is the shaking or trembling of the Earth’s surface caused by the sudden movement of lithospheric plates.

Key Terms:

• Focus: The location inside the Earth where the earthquake originates.

• Epicentre: The point on Earth’s surface directly above the focus.

• Seismic waves: Vibrations that travel outward from the focus and epicentre.

Damage pattern:
The strongest shaking and most severe damage occur nearest to the epicentre, and the intensity decreases as you move away.

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4. Types of Earthquake Waves

1. P Waves (Primary or Longitudinal Waves)

   • Travel fastest.

   • Can move through solids, liquids, and gases.

   • Cause particles to move back and forth in the direction of the wave.

2. S Waves (Secondary or Transverse Waves)

   • Slower than P waves.

   • Can travel only through solids.

   • Move particles up and down or side to side, perpendicular to the wave’s direction.

3. L Waves (Surface Waves)

   • Travel along Earth’s surface.

   • Cause the most damage due to rolling and swaying motion.

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5. Measuring Earthquakes

• Instrument: Seismograph – records the intensity and duration of vibrations.

• Scale: Richter Scale – measures magnitude.

Richter Scale guide:

• 2.0 or less: Barely felt.

• Over 5.0: Damage possible (falling objects, cracks).

• 6.0 and above: Very strong, structural damage likely.

• 7.0 or more: Major earthquake, widespread destruction.

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6. Earthquake Prediction

Although exact prediction is not possible, certain traditional indicators have been observed:

• Sudden change in animal behaviour – fish become restless, snakes emerge from their holes.

• Groundwater level fluctuations.

• Unusual sky glows or sounds.

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7. Earthquake Preparedness

Preparedness can save lives and reduce damage.

Safe Spots during an earthquake:

• Under a sturdy table, kitchen counter, or desk.

• Against an inside wall or corner.

Places to avoid:

• Near fireplaces, chimneys, windows, mirrors, or picture frames.

Other precautions:

• Spread awareness among friends and family.

• Conduct drills to know how to react.

• Keep an emergency kit ready (water, torch, first-aid).

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8. Summary Table

Force Type	Nature	Examples
Endogenic	Internal	Earthquakes, Volcanoes, Mountain building
Exogenic	External	River erosion, Wind deposition, Glacial action

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Conclusion

The Earth’s surface is dynamic and constantly changing due to internal (endogenic) and external (exogenic) forces. Understanding how lithospheric plates move and how earthquakes occur helps us reduce damage through preparedness and awareness. While we cannot stop these natural processes, we can certainly learn to live more safely with them.