Class 7 Social Science Notes Chapter 2 (Inside Our Earth) – Our Environment Book

Alright class, let's delve into Chapter 2, 'Inside Our Earth'. This is a fundamental chapter for understanding the planet we live on, and its concepts often appear in various government examinations. Pay close attention to the details.

Chapter 2: Inside Our Earth - Detailed Notes

1. The Earth's Interior: A Layered Structure

• Just like an onion, the Earth is made up of several concentric layers with one inside another.

• Understanding these layers is crucial.

  • a) Crust:

    • Definition: The uppermost, solid, and thinnest layer of the Earth.
    • Types & Composition:
      • Continental Crust: Forms the landmasses. It is thicker (about 35 km on average) and mainly composed of silica and alumina. Hence, it's called SIAL (Si-silica, Al-alumina). Granite is a common rock type.
      • Oceanic Crust: Forms the ocean floors. It is thinner (about 5 km on average) and mainly consists of silica and magnesium. Hence, it's called SIMA (Si-silica, Ma-magnesium). Basalt is a common rock type.
    • Significance: This is the layer we live on and interact with directly. It holds all landforms, oceans, and life.

  • b) Mantle:

    • Definition: Lies just beneath the crust.
    • Thickness: Extends up to a depth of 2900 km below the crust.
    • Composition & State: Primarily composed of silicate rocks rich in iron and magnesium. It's not fully solid or liquid but exists in a semi-molten state (viscous). This hot, dense material is the source of magma.
    • Significance: Convection currents within the mantle are believed to drive the movement of tectonic plates (though plate tectonics is discussed in higher classes, knowing the mantle's role is important).

  • c) Core:

    • Definition: The innermost layer of the Earth.
    • Radius: Approximately 3500 km.
    • Sub-layers & Composition:
      • Outer Core: Surrounds the inner core. It is in a liquid state due to extremely high temperatures overcoming the immense pressure. Composed mainly of iron and nickel.
      • Inner Core: The very center of the Earth. It is solid despite even higher temperatures because the immense pressure prevents the metals from melting. Composed mainly of iron (Fe) and nickel (Ni). Hence, the core is often referred to as NIFE (Ni-nickel, Fe-ferrous i.e., iron).
    • Characteristics: Characterized by extremely high temperature and pressure. The Earth's magnetic field is generated primarily in the liquid outer core.

2. Rocks and Minerals: Building Blocks of the Crust

• Rocks: Any natural mass of mineral matter that makes up the Earth’s crust. Rocks can vary in colour, size, and texture. They do not have a definite chemical composition.
• Minerals: Naturally occurring substances which have certain physical properties and a definite chemical composition. Rocks are made up of different minerals. Examples: Quartz, Feldspar, Mica, Gold, Iron Ore, Coal, Petroleum. Minerals are vital resources.

3. Types of Rocks

There are three major types of rocks, classified based on their formation:

• a) Igneous Rocks (Primary Rocks):

  • Formation: Formed from the cooling and solidification of molten magma (below the surface) or lava (on the surface). 'Ignis' is Latin for 'fire'.
  • Types:
    • Intrusive Igneous Rocks: Formed when magma cools down slowly deep inside the Earth’s crust. Slow cooling allows large crystals/grains to form. Example: Granite (used for grinding stones, kitchen counters).
    • Extrusive Igneous Rocks: Formed when molten lava erupts onto the Earth’s surface and cools down rapidly. Rapid cooling results in fine-grained texture. Example: Basalt (The Deccan plateau in India is made of basalt rocks).

• b) Sedimentary Rocks:

  • Formation: Formed from the accumulation, compaction, and cementation of sediments. Sediments are small particles derived from the breakdown (weathering and erosion) of pre-existing rocks (igneous, metamorphic, or other sedimentary rocks), remains of plants, and animals.
  • Process: Rocks are broken down -> transported by wind, water, ice -> deposited in layers -> layers get compressed and hardened (compaction & cementation) over time. 'Sedimentum' is Latin for 'settle down'.
  • Characteristics: Often occur in layers (strata). May contain fossils (the remains of dead plants and animals trapped in the layers of rocks).
  • Examples: Sandstone (formed from grains of sand), Shale (from mud/clay), Limestone (from calcium carbonate, often from shells/skeletons).

• c) Metamorphic Rocks:

  • Formation: Formed when existing igneous and sedimentary rocks are changed due to intense heat and pressure deep within the Earth. 'Metamorphic' means 'change of form'.
  • Process: The original rock's mineral composition and texture are altered without melting completely.
  • Examples:
    • Clay changes into Slate.
    • Limestone changes into Marble.
    • Granite changes into Gneiss.
    • Sandstone changes into Quartzite.

4. The Rock Cycle

• Definition: A continuous process through which old rocks are transformed into new ones. It illustrates the interrelationship between the three rock types.
• Process Explained:
  1. Molten magma cools and solidifies to form Igneous Rocks.
  2. Igneous rocks (and other rocks) are broken down by weathering and erosion into Sediments.
  3. Sediments are transported, deposited, compacted, and cemented to form Sedimentary Rocks.
  4. Igneous and Sedimentary rocks subjected to heat and pressure change into Metamorphic Rocks.
  5. Metamorphic rocks (and other rocks) under great heat and pressure melt down to form molten Magma.
  6. This cycle repeats continuously. Any rock type can change into any other rock type under specific conditions.

5. Uses of Rocks and Minerals

• Rocks: Used for building roads, houses, monuments (e.g., Red Fort - sandstone, Taj Mahal - marble); source of minerals; used in games (e.g., seven stones).
• Minerals: Used as fuels (coal, petroleum, natural gas - fossil fuels); in industries (iron, aluminium, gold, uranium); in medicine; in fertilizers; as gemstones.

Multiple Choice Questions (MCQs)

1. The innermost layer of the Earth is known as the:
   a) Crust
   b) Mantle
   c) Core
   d) SIAL

2. The rock which is made up of molten magma that cools down deep inside the Earth's crust is called:
   a) Extrusive Igneous Rock
   b) Sedimentary Rock
   c) Intrusive Igneous Rock
   d) Metamorphic Rock

3. The process of transformation of rocks from one type to another is known as:
   a) Rock Formation
   b) Rock Cycle
   c) Rock Weathering
   d) Rock Deposition

4. Which layer of the Earth is primarily composed of Silica and Magnesium (SIMA)?
   a) Continental Crust
   b) Mantle
   c) Inner Core
   d) Oceanic Crust

5. Fossils, the remains of dead plants and animals, are typically found in which type of rocks?
   a) Igneous Rocks
   b) Metamorphic Rocks
   c) Sedimentary Rocks
   d) Basalt Rocks

6. Granite is an example of:
   a) Sedimentary Rock
   b) Metamorphic Rock
   c) Intrusive Igneous Rock
   d) Extrusive Igneous Rock

7. Limestone under great heat and pressure changes into:
   a) Slate
   b) Marble
   c) Quartzite
   d) Gneiss

8. The Deccan Plateau of India is largely made up of which type of rock?
   a) Granite
   b) Sandstone
   c) Basalt
   d) Marble

9. The core of the Earth is mainly composed of:
   a) Silica and Alumina (SIAL)
   b) Silica and Magnesium (SIMA)
   c) Iron and Magnesium
   d) Nickel and Iron (NIFE)

10. What state is the Outer Core of the Earth believed to be in?
    a) Solid
    b) Liquid
    c) Gaseous
    d) Semi-molten

Answer Key for MCQs:

1. c) Core
2. c) Intrusive Igneous Rock
3. b) Rock Cycle
4. d) Oceanic Crust
5. c) Sedimentary Rocks
6. c) Intrusive Igneous Rock
7. b) Marble
8. c) Basalt
9. d) Nickel and Iron (NIFE)
10. b) Liquid

Make sure you revise these notes thoroughly. Understanding the Earth's structure and the rock cycle is essential not just for exams but also for appreciating the dynamic nature of our planet. Let me know if any part needs further clarification.