Distribution of Oceans and Continents

• Introduction

  • Surface area: 29% Land, 71% Water
  • Asymmetric distribution between hemispheres
  • Early belief: Fixed oceans and continents

• Continental Drift Theory

  • Proposed by: Alfred Wegener (1912)
  • Core Idea: All landmasses were one supercontinent
    • Name: Pangaea (Supercontinent)
    • Encompassing ocean: Panthalassa (Mega-ocean)
  • Timeline of Breakup:
    • Carboniferous period (~200 million years ago)
    • Split into Laurasia (North) and Gondwanaland (South)
    • Further fragmentation into modern continents
  • Evidence supporting the theory:
    • Jigsaw Fit (Matching coastlines like South America and Africa)
    • Rocks of same age across oceans (e.g., Brazil belt and Africa)
    • Tillite (Sedimentary rock from glaciers in tropical regions)
    • Placer Deposits (Gold deposits in Ghana without source rock in Africa, matching Brazil)
    • Distribution of Fossils (Identical plant/animal fossils like Mesosaurus on separated shores)
  • Force for Drift (Wegener's proposal):
    • Pole-fleeing force (due to earth's rotation/centrifugal force)
    • Tidal force (due to gravitational pull of moon and sun)
    • Critique: Oceans are too weak to break continental crust

• Convectional Current Theory

  • Proposed by: Arthur Holmes (1930)
  • Core Idea: Thermal convection currents in the mantle
  • Driver of movement: Radioactivity-generated heat
  • Laid groundwork for modern plate tectonics

• Mapping the Ocean Floor

  • Post-WWII echo-sounding technology revealed unexpected relief
  • Features discovered:
    • Mid-Ocean Ridges (active volcanic mountain chains)
    • Deep-sea Trenches (steep depressions)
    • Seamounts and Guyots
    • Magnetic anomalies parallel to ridges

• Concept of Sea Floor Spreading

  • Proposed by: Harry Hess (1961)
  • Key Mechanisms:
    • Volcanic eruptions at Mid-Ocean Ridges push crust apart
    • Oceanic crust continually created at ridges and consumed at trenches
  • Evidence:
    • Paleomagnetism (alternating normal/reverse magnetic stripes symmetrical to ridges)
    • Younger rock ages near ridges; older rocks near trenches/continents
    • Deep trenches show absence of thick sediment accumulation

• Plate Tectonics Theory

  • Unified theory introduced by McKenzie, Parker, and Morgan (1967)
  • Core Concept: Lithosphere broken into rigid slabs called Tectonic Plates
  • Types of Plates:
    • Major Plates (e.g., Eurasian, African, Pacific, Indo-Australian, North American, South American, Antarctic)
    • Minor Plates (e.g., Nazca, Cocos, Arabian, Philippine, Caribbean)
  • Types of Plate Boundaries:
    • Divergent Boundaries:
      • Plates move away from each other
      • Example: Mid-Atlantic Ridge (crust creation)
    • Convergent Boundaries:
      • Plates collide (Subduction zones, collision zones)
      • Types: Oceanic-Continental, Oceanic-Oceanic, Continental-Continental
      • Example: Himalayas (Continent-Continent collision)
    • Transform Boundaries:
      • Plates slide past each other horizontally
      • No creation or destruction of crust
      • Example: San Andreas Fault
  • Rates of Plate Movement:
    • Measured in cm/year (Slowest: Arctic Ridge ~2.5 cm/yr; Fastest: East Pacific Rise > 15 cm/yr)

• Movement of Indian Plate

  • Historical position: Part of Gondwanaland (with Australia, Africa, Antarctica, South America)
  • Journey:
    • Split and drifted northward (~140 million years ago)
    • Collided with the massive Eurasian Plate (~40-50 million years ago)
  • Consequence:
    • Subduction under Eurasia caused buckling of Tethys Sea sediments
    • Formation of the Himalayan mountain range and Tibetan plateau
    • Continued northward push creates ongoing tectonic activity and earthquakes in India