Class 7 Social Science Notes Chapter 3 (Our Changing Earth) – Our Environment Book

Alright class, let's delve into Chapter 3, 'Our Changing Earth'. This is a fundamental chapter for understanding the dynamic processes that shape our planet's surface, and concepts here often form the basis for questions in various government exams. Pay close attention to the forces at play and the landforms they create.

Chapter 3: Our Changing Earth - Detailed Notes

1. Introduction: The Dynamic Earth

• The Earth's surface is not static; it is constantly undergoing changes, both slow and sudden.
• These changes are driven by forces originating both inside the Earth (internal or endogenic forces) and on the surface of the Earth (external or exogenic forces).

2. Lithospheric Plates

• Definition: The Earth's crust is broken into several large and some small, rigid, irregularly shaped slabs of solid rock, which carry continents and ocean floors. These are called Lithospheric Plates.
• Movement: These plates move very slowly – just a few millimeters each year.
• Driving Force: The movement is caused by the circulation of molten magma inside the Earth in a circular manner.
• Consequences: The movement of these plates causes changes on the surface of the Earth, leading to earthquakes, volcanic eruptions, and mountain building.

3. Earth Movements: Forces Shaping the Land

Earth movements are divided based on the forces that cause them:

• A. Endogenic Forces (Internal Forces):

  • Definition: Forces that act in the interior of the Earth.
  • Effects: They sometimes produce sudden movements and at other times produce slow movements.
  • Types:
    • Sudden Forces: Cause mass destruction over the surface. Examples:
      • Earthquakes: Sudden shaking or trembling of the Earth's surface.
      • Volcanoes: Eruptions of molten material from the Earth's interior.
      • Landslides: Mass movement of rock, debris, or earth down a slope.
    • Diastrophic Forces (Slow Forces): Cause building of mountains and large landforms over long periods. (Less focus in this chapter, but good to know the term).

• B. Exogenic Forces (External Forces):

  • Definition: Forces that work on the surface of the Earth.
  • Effects: Cause wearing down and rebuilding of the land surface.
  • Processes:
    • Weathering: The breaking down of rocks on the Earth’s surface by various agents (temperature changes, frost action, plants, animals, human activities).
    • Erosion: The wearing away of the landscape by different natural agents like water, wind, and ice. The eroded material is carried away or transported and eventually deposited.
    • Deposition: The process by which eroded material (sediments) is laid down or dropped by agents like water, wind, or ice, creating new landforms.

4. Focus on Endogenic Forces: Volcanoes and Earthquakes

• Volcano:

  • Definition: A vent (opening) in the Earth’s crust through which molten material (magma) erupts suddenly.
  • Process: Molten rock material called magma inside the Earth rises towards the surface. When it reaches the surface, it is called lava. Along with lava, ash, gases, and rock fragments are also ejected.
  • Key Parts:
    • Magma Chamber: Large pool of molten rock beneath the surface.
    • Vent: The main opening through which magma erupts.
    • Crater: A bowl-shaped depression at the top of the volcano around the vent.
    • Lava: Molten rock that has erupted onto the Earth's surface.

• Earthquake:

  • Definition: When the Lithospheric plates move, the surface of the Earth vibrates. These vibrations are called earthquakes.
  • Key Terms:
    • Focus (or Hypocenter): The point inside the Earth's crust where the earthquake movement originates.
    • Epicentre: The point on the Earth's surface directly above the focus. The greatest damage usually occurs closest to the epicentre.
    • Seismic Waves: Vibrations that travel outwards from the focus in all directions. (P waves, S waves, L waves - L waves cause most surface damage).
  • Measurement:
    • Seismograph: An instrument used to detect and record earthquake vibrations (seismic waves).
    • Richter Scale: Measures the magnitude (energy released) of an earthquake. An earthquake of 2.0 or less is felt only a little. An earthquake over 5.0 can cause damage. 6.0 or higher is considered very strong, and 7.0+ is a major earthquake. (Note: Each whole number increase represents 10 times increase in amplitude and approx 32 times increase in energy release).
    • Modified Mercalli Scale: Measures the intensity (observed effects and damage) of an earthquake at a specific location.
  • Earthquake Preparedness:
    • Safe Spots: Under a kitchen counter, table or desk, against an inside corner or wall.
    • Stay Away From: Fireplaces, areas around chimneys, windows that shatter (including mirrors and picture frames), heavy objects.
    • Be Prepared: Spread awareness, have emergency kits ready.

5. Major Landforms created by Exogenic Forces (Weathering, Erosion & Deposition)

Exogenic forces continuously wear down (erode) and build up (deposit) the land surface. Key agents are:

• A. Work of a River:

  • Erosion:
    • Waterfall: Forms when a river tumbles at a steep angle over very hard rocks or down a steep valley side. Soft rocks underneath are eroded faster.
    • V-Shaped Valley: Rivers erode their bed and banks, carving deep valleys.
  • Transportation & Deposition (mainly in plains):
    • Meanders: As the river enters the plain, it twists and turns, forming large bends called meanders.
    • Ox-bow Lake: Due to continuous erosion and deposition along the sides of a meander, the ends of the loop come closer. Eventually, the river cuts through the neck of the loop, and the meander loop is cut off, forming a crescent-shaped lake called an ox-bow lake.
    • Floodplain: During floods, the river overflows its banks and deposits layers of fine soil and sediment (alluvium) on adjacent areas, creating a flat, fertile floodplain.
    • Levees: Raised banks formed by the deposition of coarser material along the river banks during floods.
    • Distributaries: As the river approaches the sea, its speed decreases, and it breaks up into numerous smaller channels called distributaries.
    • Delta: The collection of sediments deposited by the river at its mouth (where it meets the sea or a lake). Deltas are often triangular in shape.

• B. Work of Sea Waves:

  • Erosion: Sea waves continuously strike the rocks along the coast.
    • Sea Caves: Hollows formed by wave action on rocks.
    • Sea Arches: When cavities in coastal rocks become bigger, only the roof remains, forming sea arches.
    • Stacks: Further erosion breaks the roof of sea arches, and only walls are left. These wall-like features are called stacks.
    • Sea Cliff: Steep rocky coast rising almost vertically above seawater.
  • Deposition:
    • Beaches: Sea waves deposit sediments (sand, gravel) along the shores, forming beaches.
    • Bars/Spits: Linear deposits of sand/shingle extending from the land into the sea.

• C. Work of Ice (Glaciers):

  • Definition: Glaciers are slow-moving rivers of ice.
  • Erosion: Glaciers carve out deep hollows and U-shaped valleys (unlike V-shaped valleys of rivers) as they move. As the ice melts, these hollows may fill with water to form beautiful lakes.
  • Deposition: Glaciers carry rock material (sand, silt, boulders). When the ice melts, this material gets deposited.
    • Glacial Moraines: Deposits of rock debris (till) left behind by a melting glacier, often forming ridges or mounds.

• D. Work of Wind (Mainly in Deserts):

  • Erosion: Wind is an active agent of erosion in deserts.
    • Mushroom Rocks (Pedestal Rocks): Wind erodes the lower section of rocks more than the upper part, creating rocks with a narrower base and wider top, resembling a mushroom.
  • Deposition: Wind transports sand from one place to another. When the wind stops blowing, the sand falls and gets deposited.
    • Sand Dunes: Low hill-like structures formed by the deposition of sand. (Different shapes exist, e.g., Barchans - crescent-shaped).
    • Loess: When very fine and light sand is carried over long distances by wind and deposited in large areas, it forms loess. Large deposits are found in China.

6. Conclusion

The Earth's surface is a result of the continuous interplay between the internal (endogenic) forces that create relief features and the external (exogenic) forces that modify and sculpt them. This process of change is ongoing.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on the chapter for your practice:

1. The movement of the Earth's lithospheric plates is primarily caused by:
   a) Wind and water erosion
   b) Molten magma moving inside the Earth
   c) Gravitational pull of the moon
   d) Solar energy

2. The point on the Earth's surface directly above the origin of an earthquake is called the:
   a) Focus
   b) Crater
   c) Epicentre
   d) Vent

3. Which of the following is an example of an Endogenic force?
   a) River erosion
   b) Volcanic eruption
   c) Wind deposition
   d) Glacial melting

4. Mushroom rocks are typically found in:
   a) Glaciated areas
   b) River valleys
   c) Coastal regions
   d) Deserts

5. Meanders, ox-bow lakes, and floodplains are landforms created by the action of:
   a) Glaciers
   b) Sea waves
   c) Rivers
   d) Wind

6. The instrument used to measure the magnitude of an earthquake is called a:
   a) Thermometer
   b) Barometer
   c) Seismograph
   d) Anemometer

7. Which landform is created by the depositional work of glaciers?
   a) Sea Arch
   b) Delta
   c) Moraine
   d) Sand Dune

8. Sea caves, sea arches, and stacks are formed by the erosional work of:
   a) Wind
   b) Rivers
   c) Glaciers
   d) Sea waves

9. The breaking down of rocks on the Earth's surface is known as:
   a) Erosion
   b) Deposition
   c) Weathering
   d) Transportation

10. Molten rock material found inside the Earth is called:
    a) Lava
    b) Magma
    c) Sediment
    d) Loess

Answer Key for MCQs:

1. b
2. c
3. b
4. d
5. c
6. c
7. c
8. d
9. c
10. b

Study these notes thoroughly. Understanding the processes and the resulting landforms is crucial. Remember the difference between endogenic and exogenic forces, and the specific work done by each agent of erosion. Good luck with your preparation!