Learn about the theory of continental drift, sea floor spreading, and plate tectonics. Understand the Earth's dynamic processes that shape continents, oceans, and seismic activity.

Continental Drift Theory : Alfred Wegener

• Continental Drift

  The theory of continental drift, proposed by Alfred Wegener in 1912, explains the distribution of oceans and continents. According to this theory:

  

  

  • All continents were once part of a single supercontinent called Pangaea, meaning "all earth."
  • Pangaea was surrounded by a mega-ocean named Panthalassa, meaning "all water."
  • Approximately 200 million years ago, Pangaea began to break apart:
    • Laurasia: The northern continental mass.
    • Gondwanaland: The southern continental mass.
  • Laurasia and Gondwanaland further fragmented to form the continents as we know them today.

  

• Evidence Supporting Continental Drift

  

  • The Matching of Continents (Jig-Saw Fit):
    • The coastlines of Africa and South America fit remarkably well.
    • In 1964, Bullard used a computer program to align the Atlantic margins at the 1,000-fathom line, showing a near-perfect match.
  • Rocks of Same Age Across Oceans:
    • Radiometric dating reveals similarities in rock formations across continents.
    • A 2,000-million-year-old rock belt from Brazil matches with rocks in western Africa.
    • Marine deposits along the coastlines of South America and Africa date back to the Jurassic age, indicating the absence of an ocean before that period.
  • Tillite (Glacial Deposits):
    • Tillite is a sedimentary rock formed from glacial deposits.
    • Gondwana sedimentary sequences in India have counterparts in Africa, Madagascar, Antarctica, Australia, and the Falkland Islands.
    • These similarities indicate extensive glaciation and a shared geological history among these landmasses.
  • Placer Deposits:
    • Rich placer gold deposits are found along Ghana’s coast, but no source rocks exist in the region.
    • The gold-bearing veins are found in Brazil, suggesting that Ghana’s deposits originated from Brazil when the continents were adjacent.
  • Distribution of Fossils:

    

    • Identical species of land and freshwater plants and animals are found on continents separated by oceans today.
    • Lemurs: Found in India, Madagascar, and Africa, suggesting a once-contiguous landmass called Lemuria.
    • Mesosaurus: A small reptile adapted to shallow brackish water, found only in South Africa and Brazil, now 4,800 km apart with an ocean between them.

Wegener's Forces of Drifting

• Force for Drifting

  Alfred Wegener suggested two forces responsible for the drifting of continents:

  • Polar-Fleeing Force:
    • Related to the Earth's rotation.
    • Caused by the equatorial bulge due to centrifugal force from Earth's rotation.
  • Tidal Force:
    • Attributed to the gravitational pull of the Moon and the Sun, creating tidal movements.
    • Wegener proposed these forces acted over millions of years to cause drifting.
  • These forces were later deemed inadequate by most scholars.
• Post-Drift Studies

  Advancements in geological studies, especially post-World War II, provided new insights:

  • Convectional Current Theory:
    • Proposed by Arthur Holmes in the 1930s.
    • Suggested convection currents in the mantle, driven by radioactive heat, could cause continental movement.
    • These currents provided a plausible mechanism for Wegener's theory.
  • Mapping of the Ocean Floor:
    • Post-WWII expeditions revealed that the ocean floor is not a flat plain but features:
      • Submerged mountain ranges (mid-oceanic ridges).
      • Deep trenches near continental margins.
    • Rock dating showed that oceanic crust is much younger than continental crust.
    • Rocks on either side of mid-oceanic ridges displayed symmetry in composition and age.
• Ocean Floor Configuration

  The ocean floor is segmented into three major divisions based on depth and relief:

  • Continental Margins:
    • Transition zones between continental landmasses and deep-sea basins.
    • Include features such as:
      • Continental shelf.
      • Continental slope.
      • Continental rise.
      • Deep-oceanic trenches.
  • Abyssal Plains:
    • Extensive flat plains located between continental margins and mid-oceanic ridges.
    • Composed of sediments deposited from continental margins.
  • Mid-Oceanic Ridges:
    • Interconnected mountain chains beneath the oceans, forming the longest system on Earth.
    • Features:
      • A central rift system at the crest, marked by volcanic activity.
      • A fractionated plateau and flank zones along its length.
• Distribution of Earthquakes and Volcanoes

  

  • Seismic activity maps reveal:
    • Lines of seismic activity along the mid-oceanic ridges, extending across the Atlantic and Indian Oceans.
    • Branches diverging south of India:
      • One branch towards East Africa.
      • Another merging with seismic lines from Myanmar to New Guinea.
  • Shallow-focus earthquakes occur along mid-oceanic ridges, while deep-focus earthquakes are found along:
    • The Alpine-Himalayan belt.
    • The Pacific Rim (Ring of Fire).
  • Volcanic activity follows a similar pattern, with the Pacific Rim hosting numerous active volcanoes, hence its name "Ring of Fire."

Sea Floor Spreading Theory

• Concept of Sea Floor Spreading

  

  Post-drift studies, especially ocean floor mapping and palaeomagnetic studies, provided key insights that led to the development of this concept:

  • Key Observations:
    • Volcanic Activity at Mid-Oceanic Ridges:
      • Volcanic eruptions are common along mid-oceanic ridges.
      • Large amounts of lava are brought to the surface, forming new oceanic crust.
    • Symmetry of Rocks:
      • Rocks on either side of the ridge crest show similarities in:
        • Period of formation.
        • Chemical composition.
        • Magnetic properties.
      • Rocks near the ridge crest are the youngest, with normal magnetic polarity.
      • Rock age increases with distance from the ridge crest.
    • Younger Oceanic Crust:
      • The oceanic crust is significantly younger than continental crust.
      • Oceanic crust rocks are no older than 200 million years, while some continental rocks are as old as 3,200 million years.
    • Thin Ocean Floor Sediments:
      • Ocean floor sediments are unexpectedly thin.
      • If ocean floors were as old as continents, thicker sediment columns would be expected.
      • No sediment column found is older than 200 million years.
    • Earthquake Depths:
      • Mid-oceanic ridges: Earthquake foci are shallow.
      • Deep trenches: Earthquake foci are deep-seated.
  • Hess’s Hypothesis (1961): Sea Floor Spreading

    

    • Mechanism:
      • Volcanic eruptions at the ridge crest cause the oceanic crust to rupture.
      • New lava emerges, pushing the existing crust outward on both sides.
      • The ocean floor spreads as a result.
    • Younger Age of Oceanic Crust:
      • Confirms that new crust forms continuously at the mid-oceanic ridges.
    • Consumption of Oceanic Crust:
      • As one ocean floor spreads, it does not shrink other oceans.
      • Older oceanic crust sinks into deep trenches, where it is consumed.

      

Theory of Plate Tectonics

• Plate Tectonics

  The concept of plate tectonics was introduced in 1967 by McKenzie, Parker, and Morgan. It describes the movement of lithospheric plates over the asthenosphere.

  • Key Concepts
    • Definition of Tectonic Plate:

      

      • Massive, irregularly shaped slab of solid rock.
      • Composed of both continental and oceanic lithosphere.
      • Plates move as rigid units horizontally over the asthenosphere.
    • Structure of the Lithosphere:
      • Includes the crust and upper mantle.
      • Thickness varies:
        • 5–100 km in oceanic regions.
        • About 200 km in continental regions.
    • Classification of Plates:

      

      • Major Plates:
        • Antarctica and the surrounding oceanic plate.
        • North American plate (including part of the western Atlantic floor).
        • South American plate (including part of the western Atlantic floor).
        • Pacific plate (largely oceanic).
        • India-Australia-New Zealand plate.
        • African plate (including the eastern Atlantic floor).
        • Eurasian plate (including adjacent oceanic areas).
      • Minor Plates:
        • Cocos plate (between Central America and the Pacific plate).
        • Nazca plate (between South America and the Pacific plate).
        • Arabian plate (Saudi Arabian landmass).
        • Philippine plate (between the Asiatic and Pacific plates).
        • Caroline plate (north of New Guinea).
        • Fiji plate (northeast of Australia).
  • Types of Plate Boundaries

    

    • Divergent Boundaries:
      • New crust is generated as plates pull away from each other.
      • Example: Mid-Atlantic Ridge, where American plates separate from Eurasian and African plates.
    • Convergent Boundaries:
      • Crust is destroyed as one plate dives beneath another.
      • Occurs in three ways:
        • Between an oceanic and a continental plate (subduction zone).
        • Between two oceanic plates.
        • Between two continental plates.
    • Transform Boundaries:
      • Plates slide horizontally past each other.
      • Crust is neither created nor destroyed.
      • Transform faults are perpendicular to mid-oceanic ridges.
  • Rate of Plate Movement
    • Determined using magnetic strips parallel to mid-oceanic ridges.
    • Rates vary:
      • Slowest: Arctic Ridge (<2.5 cm/year).
      • Fastest: East Pacific Rise (>15 cm/year).
  • Force Behind Plate Movement
    • Convection Cells:
      • Hot mantle material rises, spreads, cools, and sinks back into the mantle.
      • Forms a continuous cycle called convective flow.
    • Heat Sources:
      • Radioactive decay.
      • Residual heat from Earth's formation.
    • Driving Force:
      • Slow movement of hot, softened mantle drives plate motion.

Drifting of Indian Plate

• Movement of the Indian Plate

  The Indian plate consists of Peninsular India and parts of the Australian continental landmass. It has been involved in significant tectonic activity, including its northward drift, collision with the Eurasian plate, and the formation of the Himalayas.

  • Plate Boundaries
    • Northern Boundary:
      • Marked by the Himalayas.
      • Forms a subduction zone due to continent-continent convergence with the Eurasian plate.
    • Eastern Boundary:
      • Extends through the Rakinyoma Mountains (Myanmar).
      • Continues towards the island arc along the Java Trench.
      • Includes a spreading site east of Australia in the SW Pacific Oceanic Ridge.
    • Western Boundary:
      • Follows the Kirthar Mountains of Pakistan.
      • Extends along the Makrana coast to the Red Sea rift southeastward via the Chagos Archipelago.
    • Southern Boundary:
      • Boundary with the Antarctic plate.
      • Marked by a W-E oceanic ridge near New Zealand.

  

  • Geological History of the Indian Plate
    • Initial Position:
      • India was a large island located off the Australian coast.
      • Separated from Asia by the Tethys Sea about 225 million years ago.
    • Northward Journey:
      • Began around 200 million years ago after Pangaea broke apart.
      • Position of India:
        • 140 million years ago: Located at approximately 50°S latitude.
        • Continued moving northward, approaching the Eurasian plate.
    • Collision with Eurasia:
      • Occurred 40-50 million years ago.
      • Resulted in the rapid uplift of the Himalayas.
    • Deccan Traps Formation:
      • Outpouring of lava began around 60 million years ago.
      • Coincided with the Indian plate being near the equator.
    • Himalayan Uplift:
      • Began after the collision with the Eurasian plate (~40 million years ago).
      • The process is ongoing, with the Himalayas still rising in height.