plate tectonics

Continental Drift Theory

  • Alfred Wegener (1920s): Proposed the concept of continental drift.

    • Suggested all continents were once part of a single landmass called Pangaea about 200 million years ago.

    • Continents subsequently broke apart but did not explain why they drifted.

Evidence Supporting Continental Drift

  • Geographical Fit:

    • Continents like Africa and South America appear to fit together like puzzle pieces.

  • Fossil Evidence:

    • Found fossils of Mesosaurus, a freshwater reptile, on both Africa and South America.

      • Critics argued the animal could have swum across oceans—but it was a freshwater species, making this improbable.

    • Also discovered fossilized trees, leading to questions about their dispersed presence across continents.

  • Geological Evidence:

    • Appalachian Mountains in North America align with mountains in Greenland, Europe, and Africa, suggesting shared geological history.

  • Glacial Evidence:

    • Evidence of glaciers found in tropical regions like India, Africa, and South America, indicating these areas were once positioned near the South Pole.

Development of Plate Tectonics

  • Revival of Interests (1968): Research confirmed Wegener's theories and explained the mechanism behind continental drift, coining it plate tectonics.

Earth's Structure and Plates

  • Earth's Layers:

    • Crust: The thin outer layer of the Earth where we live, consisting of two types:

      • Continental Crust:

        • Composition: Varies by region, oldest rocks up to 4 billion years old.

        • Thickness: 22 miles on average, can reach 40 miles in mountainous regions.

        • Characteristics: Light and heterogeneous.

      • Oceanic Crust:

        • Composition: Primarily basalt; younger than continental crust, typically less than 200 million years old.

        • Thickness: Only 5 miles.

        • Characteristics: Heavier and homogeneous.

Major Tectonic Plates

  • Seven major tectonic plates:

    • Pacific Plate

    • North American Plate

    • South American Plate

    • African Plate

    • Eurasian Plate

    • Australian Plate

    • Antarctic Plate

Types of Plate Boundaries

  • Divergent Boundaries:

    • Plates move away from each other, creating a gap that allows hot mantle to fill in.

    • Forms new crust, creating features like mid-ocean ridges and rift valleys (e.g., the Red Sea).

    • Average seafloor spreading rate of 2 cm/year.

  • Convergent Boundaries:

    • Plates collide and one subducts beneath the other, leading to the formation of:

      • Subduction Zones: Denser oceanic crust sinks beneath lighter continental crust.

      • Mountain Ranges: When two continental plates collide, they create mountains (e.g., Himalayas).

    • Types:

      • Ocean-Continent: Oceanic crust subducts, forming trenches and volcanic arcs.

      • Ocean-Ocean: Denser ocean plate subducts, creating volcanic islands.

      • Continent-Continent: Both plates resist subduction, pushing up to form mountains without significant melting.

  • Transform Boundaries:

    • Plates slide past each other, characterized by friction and frequent earthquakes but no new crust or trenches formed.

    • Example: San Andreas Fault.

Driving Forces of Plate Movement

  • Convection in the Mantle:

    • Internal heat drives mantle convection; two theories explaining this:

      • Whole Mantle Convection: Heat from the core causes rising and sinking currents in the entire mantle.

      • Layer Cake Model: Separate convection currents in upper and lower mantle regions.

Conclusion

  • Moon's Structure: Lacks tectonic plates and convection, solid rock with no geologic activity.