The continents and oceans are not fixed; they are pieces of a puzzle in motion. — Alfred Wegener
4.1 Continental Drift
Alfred Wegener (1912) proposed continental drift: continents were once joined in a supercontinent Pangaea and drifted apart. Evidence:
- Matching coastlines (South America and Africa)
- Identical fossils (Mesosaurus) on separated continents
- Similar rock formations and glacial deposits
- Palaeoclimatic evidence
Wegener could not explain the mechanism of drift; the theory was initially rejected.
4.2 Sea-Floor Spreading
Harry Hess proposed that new oceanic crust forms at mid-oceanic ridges and spreads outward. Palaeomagnetic stripes on the ocean floor (alternating normal and reversed polarity) confirmed spreading.
Old crust is destroyed at ocean trenches through subduction. This explained how continents could move.
4.3 Plate Tectonics
The lithosphere is broken into rigid plates that float on the semi-molten asthenosphere. Plate boundaries:
| Boundary type | Movement | Features |
|---|---|---|
| Divergent | Plates move apart | Mid-ocean ridges, rift valleys |
| Convergent | Plates collide | Fold mountains, trenches, volcanoes |
| Transform | Plates slide past | Faults, earthquakes |
The Indian Plate collided with the Eurasian Plate ~50 million years ago, uplifting the Himalayas and forming the Indo-Gangetic plain.
4.4 Distribution of Oceans and Continents
Oceans cover ~71% of the earth's surface. Continents are concentrated in the Northern Hemisphere. The Pacific Ring of Fire is the most active zone of earthquakes and volcanoes.
India sits at the junction of three plates: Indian, Eurasian, and Indo-Australian—making it seismically vulnerable.
Chapter Summary
- Wegener's continental drift: Pangaea, fossil and coastline evidence
- Sea-floor spreading at ridges; subduction at trenches
- Plate tectonics explains earthquakes, volcanoes, and mountain building
- Indian Plate collision created the Himalayas
- Oceans cover most of the earth; plates are in constant motion
Exercises (NCERT)
- Exercise 1 — evidence for continental drift
- Exercise 2 — sea-floor spreading and palaeomagnetism
- Exercise 3 — types of plate boundaries with examples