Chapter 2: INSIDE OUR EARTH

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🔵 INSIDE OUR EARTH

Edunes Online Education

🌍 Chapter 2: INSIDE OUR EARTH

Class 7 – CBSE Geography
Student: Subhangam

Earth is not just a planet we live on — it is a living, moving system. To understand earthquakes, volcanoes, mountains and continents — we must learn to think beneath the surface.

🌎 THINK LIKE A SCIENTIST: HOW DO WE KNOW ABOUT THE INTERIOR?

We cannot travel to the center of the Earth. The deepest mines go only about 4 km, while Earth’s radius is about 6371 km. So how do scientists know? They study seismic waves produced during earthquakes. These waves change speed and direction inside Earth — revealing its layered structure.

🧠 Memory Trigger: If waves change speed → material must change. Change in material → change in layer.

🧅 EARTH IS LIKE AN ONION (Layered Structure)

Imagine cutting an onion. You see layers arranged one inside another. Earth is similar — made of three concentric layers:

1. Crust – Outer skin
2. Mantle – Thick middle layer
3. Core – Deep center

🧠 Quick Order Trick: C → M → C Crust → Mantle → Core (Think: “Cool Mountains Collapse”)

🟤 1. CRUST – THE OUTERMOST LAYER

The crust is the thin outer skin where we live. All life exists only here.

• Thickness:
  • ~35 km under continents
  • ~5 km under oceans
• Thinnest layer of Earth

Even though we see mountains as huge, compared to Earth’s radius (6371 km), the crust is thinner than the skin of an apple.

Composition of Crust

• Continental Crust → Silica + Alumina = SIAL
• Oceanic Crust → Silica + Magnesium = SIMA

🧠 Memory Code: SI + AL = SIAL (Continents) SI + MA = SIMA (Oceans)

🟠 2. MANTLE – THE MIDDLE LAYER

The mantle lies just below the crust and extends up to 2900 km. It is the thickest layer.

• Made of hot, dense silicate rocks
• Partially molten in some regions
• Contains magma

Heat inside the mantle causes convection currents. These slow movements push tectonic plates. Plate movement causes:

• Earthquakes
• Volcanoes
• Mountain formation

🧠 Think Deep: No mantle movement → No plate movement → No mountains.

🔴 3. CORE – THE INNERMOST LAYER

The core begins at 2900 km depth and extends to Earth’s center. Radius ≈ 3500 km.

• Made mainly of Nickel (Ni) and Iron (Fe)
• Called NIFE

The core has:

• Extremely high temperature (like a furnace)
• Immense pressure due to overlying layers

The outer core is liquid. The inner core is solid due to very high pressure.

🧠 Memory Code: Ni + Fe = NIFE → Core

📊 SUMMARY TABLE – Layers of the Earth

Feature	Crust	Mantle	Core
Thickness	5–35 km	~2900 km	~3500 km radius
Main Elements	Silica, Alumina / Magnesium	Silicate rocks	Nickel, Iron
Nickname	SIAL / SIMA	–	NIFE
Special Features	Thinnest, life exists here	Hot, semi-molten, causes plate movement	Hottest layer, very high pressure

🌋 WHY SHOULD WE CARE?

Every earthquake you feel… Every volcano you see… Every mountain range on Earth… All are results of processes happening deep inside.

Interior structure explains:

• Continental drift
• Formation of oceans
• Earth’s magnetic field (from liquid outer core)

🧠 FINAL MEMORY MAP

Crust → Thin → Life exists Mantle → Thick → Movement Core → Hottest → Magnetic field Think from outside to inside: Surface → Heat → Energy

The Earth may look calm, but beneath your feet, it is full of energy, heat and motion.

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Prepared as per CBSE examination and NEP 2020 guidelines