Inside Our Earth: CBSE Class 7 Geography Chapter 2 Notes

Under our feet lies a dynamic, constantly changing world. Just like an onion, the Earth is made up of several concentric layers nested inside one another. In this chapter of CBSE Class 7 Geography, we will venture deep below the surface to explore these fascinating layers—the Crust, the Mantle, and the Core. You will also learn about the variety of rocks and minerals that make up the Earth's crust, how they transform from one type to another through the majestic Rock Cycle, and why these resources are crucial for human life. Mastering the concepts of inside our earth class 7 ncert will not only help you ace your geography exams but will also change how you look at the mountains, soil, and stones around you. Let's begin our journey to the center of the Earth!

The Concentric Layers of the Earth

The Earth's structure is often compared to an onion or a hard-boiled egg because it consists of concentric, overlapping layers. The topmost layer where we live is the Crust. It is the thinnest of all layers, extending about 35 km on continental masses and only 5 km on ocean floors. Below the crust lies the semi-liquid Mantle, stretching to a massive depth of 2,900 km. Finally, at the very center, is the metallic Core with a radius of about 3,500 km. As you descend deeper into the Earth, both temperature and pressure rise drastically. The core experiences temperatures as high as 5,000°C, meaning everything at the center exists under extreme conditions.

Key Geographical Terms You Must Know

Sial
The main mineral constituents of the continental crust, standing for Silica (Si) and Alumina (Al).
Sima
The oceanic crust mainly consists of silica and magnesium; hence, it is called sima (Si-silica and Ma-magnesium).
Nife
The innermost core is mainly made of nickel (Ni) and iron (Fe, ferrous), hence called nife. It has very high temperature and pressure.
Fossils
The remains of the dead plants and animals trapped in the layers of rocks over millions of years.
Rock Cycle
The continuous process by which one type of rock changes into another under specific conditions in a cyclic manner.

The Rock Cycle: How Rocks Transform

  1. Cooling of Magma — Liquid magma cools down deep inside or on the surface of the Earth to solidify and form primary or Igneous Rocks.
  2. Weathering & Transport — These igneous rocks are broken down into small particles (sediments) by agents like wind, running water, and ice.
  3. Deposition & Compression — The loose sediments are transported and deposited in layers. These layers are compressed and hardened to form Sedimentary Rocks.
  4. Heat & Pressure Transformation — When igneous and sedimentary rocks are subjected to immense heat and pressure deep inside the Earth, they transform into Metamorphic Rocks.
  5. Melting Back to Magma — Under continuous extreme heat and pressure, these metamorphic rocks melt completely to form molten magma once again, ready to restart the cycle.

Exam Strategy & Word Origin Secrets

To score full marks on questions about rock types, remember their Latin and Greek word origins!

  1. Igneous comes from the Latin word Ignis, meaning fire (as they are formed from hot molten magma).
  2. Sedimentary comes from the Latin word Sedimentum, meaning settle down (as they settle from loose particles).
  3. Metamorphic comes from the Greek word Metamorphose, meaning change of form.

Also, do not confuse Sial (Continental) with Sima (Oceanic). Continental crust has Alumina (Sial), while Oceanic has Magnesium (Sima)!

Practice Questions with Solutions

  • Q: Why cannot we reach the center of the Earth? A: Step 1: Analyze the physical conditions of the Earth's interior. Step 2: Note the depth of the center, which is about 6,000 km below the ocean floor. Step 3: Point out that the temperature and pressure are exceptionally high, which would instantly melt any human or machine. Final answer: We cannot reach the center of the earth because it is 6,000 km deep, and has unbearable, extremely high temperatures and pressures that melt everything.
  • Q: What are the three types of rocks? Give one example of each. A: Step 1: Identify the main classifications based on their mode of formation. Step 2: List the three categories: Igneous, Sedimentary, and Metamorphic. Step 3: Provide a classic textbook example for each. Final answer: The three types of rocks are: 1. Igneous Rocks (e.g., Granite or Basalt), 2. Sedimentary Rocks (e.g., Sandstone), and 3. Metamorphic Rocks (e.g., Marble or Slate).
  • Q: Explain how sedimentary rocks are formed. A: Step 1: Explain the breakdown of existing rocks into smaller particles called sediments. Step 2: Explain how wind and water transport and deposit these sediments. Step 3: Detail how deposition builds layers and the pressure compresses them into hard rock. Final answer: Rocks roll down, crack, and hit each other to break into small fragments called sediments. These sediments are transported and deposited by wind and water. Over time, the loose sediments compress and harden to form layers of sedimentary rocks.
  • Q: What are the primary minerals found in the continental crust and oceanic crust? A: Step 1: Recall the acronyms used for the composition of both crusts. Step 2: Map 'Sial' to continental crust minerals (Silica and Alumina). Step 3: Map 'Sima' to oceanic crust minerals (Silica and Magnesium). Final answer: The continental crust is primarily made of Silica and Alumina (called Sial), while the oceanic crust consists mainly of Silica and Magnesium (called Sima).

Frequently Asked Questions

What is the thickness of the Earth's crust?

The Earth's crust is the thinnest layer. It is about 35 km thick on the continental landmasses and only about 5 km thick on the ocean floors.

What are fossils and in which rock type are they found?

Fossils are the preserved remains of dead plants and animals. They are found almost exclusively in sedimentary rocks because these rocks are formed by layers of accumulating sediment over long periods.

What is the difference between intrusive and extrusive igneous rocks?

Extrusive igneous rocks form when molten magma comes onto the Earth's surface and cools rapidly, creating fine-grained structures like basalt. Intrusive igneous rocks form when magma cools slowly deep inside the Earth's crust, forming large grains like granite.