The earth has music for those who listen. — George Santayana
3.1 Sources of Information
We cannot drill to the earth's centre. Knowledge comes from:
- Direct sources — rock samples from mining and deep drilling (deepest ~12 km)
- Indirect sources — earthquake seismic waves, meteors, gravitational field, magnetic field, and laboratory experiments on rocks
Seismic waves change speed and direction at layer boundaries, revealing internal structure.
3.2 Structure of the Earth
| Layer | Depth (approx.) | Composition |
|---|---|---|
| Crust | 5–70 km | Silica-rich rocks; continental (granite) and oceanic (basalt) |
| Mantle | ~2,900 km | Dense silicate rocks; upper and lower mantle |
| Outer core | Liquid iron-nickel | Generates earth's magnetic field |
| Inner core | Solid iron-nickel | ~6,370 km from surface at centre |
The Mohorovičić discontinuity (Moho) separates crust from mantle. The Gutenberg discontinuity separates mantle from core.
3.3 Earthquakes and Volcanoes
Earthquakes result from sudden release of energy along faults, usually at plate boundaries. The point underground is the focus; the point on the surface directly above is the epicentre.
Volcanoes occur where magma reaches the surface. Types: shield, composite (stratovolcano), and cinder cone.
India's Himalayas experience frequent earthquakes (e.g. 2015 Nepal quake felt across north India). The Deccan Traps are evidence of ancient volcanic activity.
3.4 Types of Rocks
- Igneous — formed from cooling magma (e.g. granite, basalt)
- Sedimentary — formed from compressed sediments (e.g. sandstone, limestone)
- Metamorphic — changed by heat and pressure (e.g. marble, slate)
The rock cycle links these types through geological processes.
Chapter Summary
- Interior studied via seismic waves, gravity, magnetism, and rock samples
- Layers: crust, mantle, outer core (liquid), inner core (solid)
- Earthquakes and volcanoes release energy from earth's interior
- Three rock types linked by the rock cycle
- Indian examples: Himalayan seismic zone, Deccan Traps
Exercises (NCERT)
- Exercise 1 — direct and indirect sources of earth's interior
- Exercise 2 — layers of the earth and discontinuities
- Exercise 3 — causes and effects of earthquakes; rock types