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Distribution of ocean and continents
Continental Drift Theory
- The Continental Drift Theory was proposed by Alfred Wegener in 1912. It suggests that all continents were once joined together in a supercontinent called Pangea and later drifted apart.
Evidence Supporting Continental Drift
1. Matching of Continents (Jigsaw Fit) – The eastern coastline of South America and the western coastline of Africa fit together.
2. Rocks of Same Age Across Oceans – Rock formations on continents like Brazil and Africa show a similar age and type.
3. Tillite (Glacial Deposits) – The same type of glacial deposits are found in India, South Africa, Australia, and Antarctica.
4. Placer Deposits – Similar gold deposits found in Ghana (Africa) and Brazil (South America).
5. Fossil Evidence – Identical fossils of Mesosaurus (a freshwater reptile) found in now-separated continents.
Forces Behind Continental Drift
1. Tidal Force – Wegener suggested that the gravitational pull of the Moon and Sun caused the continents to move.
2. Pole-Fleeing Force – Due to Earth’s rotation, landmasses moved away from the poles.
While Wegener’s explanation of movement was inadequate as believed by scientific community, his theory paved the way for the development of Plate Tectonics.
Post-Drift Theories
Sea-Floor Spreading Theory (Proposed by Harry Hess, 1961)
- Magma Rises at Mid-Ocean Ridges – Forms new crust as it cools.
- Crust Moves Away from the Ridge – Like a conveyor belt, carrying continents with it.
- Age of Rocks – Rocks near ridges are younger, and those farther away are older.
- Sediments found on ocean crust are thin as compared to continental one
- Infact ocean crust is younger (200 million year old)as compared to continental crust ( 3200 million year old)
Convection Current Theory
- Heat from Earth’s core causes the mantle to flow.
- It rises to the surface then cool down then again rises causing conventional current
- Convection currents move tectonic plates, leading to earthquakes, volcanic activity, and mountain formation.
Ocean Floor Configuration
1. Continental Margins
- Continental Shelf – A shallow, submerged edge of a continent.
- Continental Slope – A steep descent from the shelf to the deep ocean.
- Continental Rise – A gradual incline formed by accumulated sediments.
- Deep-Ocean Trenches – The deepest parts of the ocean, found at subduction zones.
2. Abyssal Plains
- Flat, extensive ocean floor areas covered by fine sediments.
3. Mid-Oceanic Ridges
- Underwater mountain chains formed by seafloor spreading.
- Central Rift System – A valley with volcanic eruptions and earthquakes.
Distribution of Earthquakes & Volcanoes
Earthquakes and volcanoes are concentrated along plate boundaries.
- Mid-Ocean Ridges – Plates move apart, causing shallow earthquakes.
- Alpine-Himalayan Belt & Pacific Rim – Deep earthquakes and intense volcanic activity.
- Ring of Fire – A highly active volcanic belt around the Pacific Ocean.
Sea-Floor Spreading & Plate Tectonics
- Volcanic Eruptions at Mid-Ocean Ridges – Create new oceanic crust.
- Magnetic Reversals – Identical magnetic stripes on both sides of ridges.
Age of Rocks –
- Oldest oceanic crust = 200 million years.
- Oldest continental rocks = over 3.2 billion years.
- Thin Oceanic Sediments – No sediments older than 200 million years.
Earthquake Patterns –
- Deep earthquakes occur in subduction zones.
- Shallow earthquakes occur at mid-ocean ridges.
Plate Tectonics
Types of Plates
Continental Plates – Carry landmasses.
Oceanic Plates – Mostly under oceans.
Major Plates
1. Antarctic Plate
2. North American Plate
3. South American Plate
4. Pacific Plate
5. Indo-Australian Plate
6. African Plate
7. Eurasian Plate
Minor Plates
- Cocos Plate (between Central America & Pacific Plate)
- Nazca Plate (between South America & Pacific Plate)
- Arabian Plate (Saudi Arabia region)
- Philippine Plate (east of the Philippines)
- Caroline Plate (between the Philippines & Indian Plate)
- Fiji Plate (northeast of Australia)
Types of Plate Boundaries
1. Divergent Boundaries (Spreading Sites)
- Plates move apart, forming new crust.
- Example: Mid-Atlantic Ridge.
2. Convergent Boundaries (Subduction Zones)
- One plate moves under another, forming volcanoes, trenches, and mountain ranges.
- Example: Java Trench.
3. Transform Boundaries
- Plates slide past each other horizontally.
- Mostly found in oceans, linking mid-ocean ridges.
Rate of Plate Movement
Fastest – Pacific Plate moves over 10 cm per year.
Slowest – The Arctic Ridge, moving less than 2.5 cm per year.
Forces Driving Plate Movement
- Convection Currents – Hot mantle rises, cools, and sinks, driving plate motion.
- Radioactive Decay & Residual Heat – Provide energy for mantle movement.
Movement of the Indian Plate
Boundaries
North – Collides with Eurasian Plate, forming the Himalayas.
East – Extends through Myanmar to the Java Trench.
West – Moves along Pakistan’s Kirthar Mountains to the Red Sea.
South – Bordered by an oceanic ridge.
Historical Movement
- 225 million years ago – India was an island near Australia.
- 200 million years ago – Began moving northward.
- 60 million years ago – Deccan Traps formed due to volcanic eruptions.
- 40–50 million years ago – Collided with Eurasia, forming the Himalayas.
- Present – The Himalayas are still rising.
