The Earth's Interior & Plate Tectonics
The layered structure of the Earth, how seismic waves reveal it, the major discontinuities, and the theory of plate tectonics with its three boundary types.
Key Takeaways
- The Earth has three broad layers — crust, mantle and core — separated by discontinuities.
- S-waves cannot pass through liquids, which is how we know the outer core is liquid.
- Plate tectonics explains earthquakes, volcanoes and mountain-building through three boundary types.
Core concept
We cannot dig to the Earth's centre, so our knowledge of its interior comes mostly from indirect sources — chiefly the behaviour of seismic (earthquake) waves, along with meteorites, gravity and the magnetic field. These reveal a layered structure of increasing density toward the centre.
Static foundation — the layers
Structure of the Earth's Interior
Seismic Waves — the X-ray of the Earth
| Wave | Type | Travels through | Speed |
|---|---|---|---|
| P-wave (Primary) | Longitudinal (push-pull) | Solids, liquids AND gases | Fastest — arrives first |
| S-wave (Secondary) | Transverse (side-to-side) | Solids ONLY | Slower |
| Surface wave (L-wave) | Along the surface | The crust | Slowest — most destructive |
The 'shadow zone' proof
S-waves are not recorded beyond about 105° from an earthquake's epicentre — the S-wave shadow zone. Since S-waves cannot travel through liquids, this is the key evidence that the outer core is liquid. The two main boundaries are the Mohorovicic (Moho) — crust to mantle — and the Gutenberg — mantle to core.
The Story of Plate Tectonics
Continental Drift (Wegener, 1912)
Alfred Wegener proposed that a single supercontinent, PANGAEA (surrounded by Panthalassa), broke apart and drifted — but he could not explain HOW.
Seafloor Spreading (Hess, 1960s)
Harry Hess showed new crust forms at mid-ocean ridges and spreads outward — providing the mechanism.
Plate Tectonics
The lithosphere is broken into rigid PLATES that float and move on the plastic asthenosphere.
The driving force
CONVECTION CURRENTS in the mantle (Arthur Holmes) carry the plates, causing earthquakes, volcanoes and mountains at their edges.
The Three Plate Boundaries
| Boundary | Movement | Result / Example |
|---|---|---|
| Divergent (constructive) | Plates move APART | New crust forms — Mid-Atlantic Ridge, East African Rift |
| Convergent (destructive) | Plates move TOGETHER (subduction) | Mountains, trenches, volcanoes — Himalayas, Andes |
| Transform (conservative) | Plates slide PAST each other | Earthquakes — San Andreas Fault |
Relevance & current linkage
Plate tectonics explains the Himalayas (Indian plate colliding with the Eurasian plate — still rising) and India's earthquake risk. (Add any recent major earthquake or a study on Himalayan seismicity if relevant.)
Prelims trap zones
- S-waves travel only through solids; P-waves through all media — the basis of the shadow zone.
- Moho = crust–mantle; Gutenberg = mantle–core — don't swap them.
- Divergent = constructive (new crust); Convergent = destructive — a common confusion.
Knowledge Check
2 questions · check your understanding
1. The liquid nature of the Earth's outer core is inferred from the fact that:
2. The Himalayas were formed at which type of plate boundary?
Prelims Pointers
- The Mohorovicic (Moho) discontinuity separates the crust from the mantle; the Gutenberg separates the mantle from the core.
- P-waves (primary) travel through solids, liquids and gases; S-waves (secondary) only through solids.
- Continental drift was proposed by Alfred Wegener; seafloor spreading by Harry Hess.
- Convection currents in the mantle (Arthur Holmes) drive plate movement.
Mains Angle
- 'Plate tectonics is the unifying theory of physical geography.' Discuss.
- Explain how the study of seismic waves has revealed the Earth's internal structure.
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