Distribution of Oceans and Continents

In 1912, German meteorologist Alfred Wegener formulated the Continental Drift Theory regarding the distribution of oceans and continents:

• Pangaea & Panthalassa: Wegener postulated that around 250 million years ago (Carboniferous period), all modern continents were united into a single, colossal supercontinent called Pangaea (meaning "All Earth"), surrounded by a mega-ocean called Panthalassa ("All Water").
• The Splitting: Around 200 million years ago (Mesozoic era), Pangaea fractured into two massive landmasses: Laurasia (northern supercontinent: North America, Europe, Asia) and Gondwanaland (southern supercontinent: South America, Africa, Peninsular India, Madagascar, Australia, Antarctica), separated by the shallow Tethys Sea. Continued fracturing led to modern continents.
• Wegener's Postulated Driving Forces:
  • 1. Pole-Fleeing Force: Attributed to the centrifugal force caused by the Earth's rotation (responsible for the equatorial bulge).
  • 2. Tidal Force: Attributed to the gravitational attraction of the Moon and Sun pulling continents westward. (Physicists proved these forces were millions of times too weak to move continents, causing Wegener's theory to be initially rejected!).

1. 1. The "Jigsaw-Fit" of Opposing Coastlines: The shorelines of South America and Africa facing each other across the Atlantic show an astonishing geometrical fit. In 1965, Sir Edward Bullard used a computer to map the continental shelf at the 500-fathom line (approx. 900 meters depth), demonstrating a near-flawless fit!
2. 2. Rocks of Same Age Across Oceans: Radiometric dating proves that a belt of 2,000-million-year-old ancient crystalline rocks in eastern Brazil matches exactly with the ancient rock belt of West Africa.
3. 3. Tillite (Glacial Deposits): Sedimentary rock formed out of ancient glacial debris. Thick tillite deposits from the Carboniferous glaciation are found in six widely separated southern landmasses: India, Madagascar, Southern Africa, Australia, Antarctica, and South America. This proves that these tropical landmasses once clustered together near the South Pole!
4. 4. Placer Gold Deposits: Rich placer gold veins occur on the coast of Ghana (West Africa), but there are zero source gold-bearing quartz veins in Ghana! The original mother lode gold rocks are found across the Atlantic in Brazil, proving the two continents were conjoined when gold was deposited.
5. 5. Distribution of Fossils (Paleontological Proof):
   • Mesosaurus: A tiny, shallow-water freshwater reptile that could not swim in salty open oceans; fossils are found ONLY in eastern Brazil and southern Africa (South Africa's Karoo formation).
   • Glossopteris: A sub-polar fern tree fossil found widely across India, South Africa, Australia, and Antarctica.

A. Arthur Holmes' Convectional Current Theory (1930s):

British geologist Arthur Holmes resolved the fatal flaw in Wegener's theory (the driving force). He proposed that intense heat generated by radioactive decay in the mantle creates thermal convection currents in the asthenosphere. Rising convection currents push continents apart, while descending currents pull them together, providing the engine for continental motion!

B. Ocean Floor Topography:

Post-WWII sonar mapping shattered the myth that the ocean floor was a flat, muddy plain:

• Mid-Oceanic Ridges: Interconnected underwater volcanic mountain ranges stretching over 70,000 km across global oceans (e.g., Mid-Atlantic Ridge), featuring a central rift valley with continuous basaltic eruptions.
• Abyssal Plains: Deep, flat ocean floors blanketed by marine sediments between continental margins and ridges.
• Deep Ocean Trenches: Extremely deep, narrow chasms along ocean margins (e.g., Mariana Trench, 11,000 meters deep) where old oceanic crust plunges back down into the mantle.

C. Sea Floor Spreading (Harry Hess, 1961):

Geologist Harry Hess synthesized these discoveries into the Sea Floor Spreading hypothesis:

• Molten basaltic magma constantly wells up along the rift of Mid-Oceanic Ridges, solidifying to form brand-new oceanic crust.
• This newly created crust spreads symmetrically outward in both directions, pushing older sea floor away.
• Why Earth Doesn't Expand: The newly created crust at mid-ocean ridges is perfectly balanced by the destruction of ancient crust dipping down into deep ocean trenches (Subduction) and melting back into the mantle!
• Paleomagnetic Striping: Oceanic basalt records periodic reversals of Earth's magnetic field in alternating normal and reversed zebra-stripe bands symmetrical on both sides of mid-ocean ridges.

A. Theory of Plate Tectonics (1967):

Formulated independently by Dan McKenzie, Robert Parker, and W.J. Morgan:

• The Lithosphere (rigid outer crust and uppermost solid mantle, 100 km thick) is broken into seven major and dozens of minor rigid slabs called Tectonic Plates floating atop the plastic asthenosphere.
• The 7 Major Plates: (1) Antarctic, (2) North American, (3) South American, (4) Pacific (largest oceanic plate), (5) India-Australia-New Zealand, (6) African, and (7) Eurasian.
• Three Types of Plate Boundaries:
  1. 1. Divergent (Constructive) Boundaries: Plates pull apart from each other. Magma wells up, creating new crust (Mid-Atlantic Ridge, East African Rift Valley).
  2. 2. Convergent (Destructive) Boundaries: Plates collide. The denser plate plunges beneath the lighter plate into a Subduction Zone, forming deep trenches, volcanic island arcs (Japan), or massive fold mountains (Himalayas).
  3. 3. Transform (Conservative) Boundaries: Plates grind past each other horizontally along strike-slip transform faults (e.g., San Andreas Fault in California). Crust is neither created nor destroyed.

B. Movement of the Indian Tectonic Plate:

140 million years ago, India was an island landmass situated at $50^\circ\text{S}$ latitude in the southern hemisphere, separated from Asia by the vast Tethys Sea:

• During its northward flight across the Indian Ocean (drifting at 15 to 20 cm per year), India passed over the Reunion Volcanic Hotspot around 65 million years ago, pouring out the massive Deccan Traps flood basalts!
• Around 40 to 50 million years ago, the continental Indian Plate collided violently with the Eurasian Plate. Because both plates were continental, neither would subduct; the intervening Tethys marine sedimentary strata buckled, folded, and were thrust skyward to form the Himalayan Mountain Range, a process that continues today!