Edunes Online Education
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:
- Crust – Outer skin
- Mantle – Thick middle layer
- 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
🧠 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.
