Ch-4: Distribution of Oceans & Continents
1. Continental Drift Theory
Proposed by Alfred Wegener (1912).
- Pangaea: Super continent (All Earth).
- Panthalassa: Mega Ocean (All Water).
- Around 200 million years ago, Pangaea split into Laurasia (North) and Gondwanaland (South).
Evidence for Drift:
- Jig-Saw Fit: Africa and South America coastlines match perfectly.
- Rocks of Same Age: Ancient rocks (2000 mya) in Brazil match those in West Africa.
- Tillite: Glacial deposits in India match Africa, Antarctica, Australia (Gondwana System).
- Placer Deposits: Gold in Ghana coast (no source rock) comes from Brazil plateau.
- Fossils: Mesosaurus (reptile) found only in S. Africa and Brazil (separated by ocean).
Force for Drift (Wegener):
1. Polar Fleeing Force (Rotation of Earth).
2. Tidal Force (Attraction of Moon/Sun).
*Note: Scientists found these forces inadequate.*
2. Post-Drift Studies
Convectional Current Theory (Arthur Holmes, 1930):
Currents in mantle generated by radioactive heat cause movement.
Sea Floor Spreading (Hess, 1961):
Volcanic eruptions at Mid-Ocean Ridges push crust apart. Ocean floor spreads but doesn't shrink (consumed at trenches).
3. Plate Tectonics (1967)
Lithosphere is divided into 7 Major Plates (e.g., Eurasia, Pacific, India-Australia) and Minor Plates (e.g., Nazca, Arabia).
Types of Plate Boundaries:
- Divergent: Move apart (New crust formed). Ex: Mid-Atlantic Ridge.
- Convergent: Move together (Crust destroyed). Ex: Himalayas.
- Transform: Slide past (No creation/destruction). Ex: San Andreas Fault.
Movement of Indian Plate
India was a large island off Australia coast. Collided with Asia ~40-50 million years ago -> Himalayas formed. Height is still rising!
NCERT Solutions
Q1. Multiple Choice Questions
(i) Who proposed Drift Theory?
Ans: (a) Alfred Wegener
(ii) Polar fleeing force relates to?
Ans: (c) Rotation of the earth
(iii) Which is NOT a minor plate?
Ans: (d) Antarctica (It's a major plate).
(iv) Not considered for Sea Floor Spreading?
Ans: (c) Distribution of fossils (This was Wegener's evidence).
Q2. Short Answers (30 words)
(i) Forces suggested by Wegener?
Wegener suggested two forces: 1. Polar-fleeing force (due to earth's rotation) and 2. Tidal force (due to attraction of the moon and the sun).
(ii) Convection currents initiation?
These currents are generated due to radioactive elements causing thermal differences in the mantle portion.
(iii) Diff: Transform vs Convergent/Divergent?
In Transform boundary, crust is neither produced nor destroyed (sliding). In Divergent, crust is created. In Convergent, crust is destroyed.
Long Answers & Project
Q3. Long Answers (150 words)
(i) Evidences of Continental Drift
Wegener gave several proofs:
1. Jig-Saw Fit: Africa and S. America fit together.
2. Rocks of Same Age: 2000 million yr old rocks match across the ocean.
3. Tillite: Gondwana system glacial deposits found in India, Africa, Falkland, etc. prove similar history.
4. Placer Deposits: Gold in Ghana (Africa) derived from Brazil (S. America).
5. Fossils: Identical species (Mesosaurus) found on separated continents.
(ii) Drift Theory vs Plate Tectonics
Drift Theory (Wegener): Said Continents move through the ocean floor. Forces (Tidal/Rotation) were weak. Main idea: Pangaea.
Plate Tectonics: Says Lithospheric Plates move (continents are just part of plates). Driving force is Convection Currents in Mantle. Explains Sea Floor Spreading, Volcanoes, and Earthquakes better.
Project Work: Earthquake Collage
Idea for Collage:
Collect pictures of destruction from newspapers/internet:
1. Collapsed Buildings (Nepal Earthquake).
2. Cracked Roads (Gujarat).
3. Tsunami Waves (2004 Indian Ocean).
Paste them on a chart and label "Nature's Fury".