plate tectonic theory

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What is it?

• The Earth’s crust is divided into large tectonic plates that move on the semi-molten mantle.

• These plates' movement is driven by:

  • Convection currents in the mantle (heat from the core).

  • Slab pull.

  • Ridge push.

Key Evidence:

• Continental drift: Continents fit together (like South America and Africa), proposed by Alfred Wegener.

• Fossil evidence: Identical fossils on separate continents.

• Sea-floor spreading: New crust forms at mid-ocean ridges (e.g., Mid-Atlantic Ridge).

• Earthquakes and volcanoes: Occur along plate boundaries.

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Types of Plate Margins (Boundaries)

1. Constructive (Divergent) Margin

• Plates move apart (e.g., North American and Eurasian plates at the Mid-Atlantic Ridge).

• Magma rises, forming new crust.

• Creates volcanoes, mid-ocean ridges, and earthquakes (usually mild).

2. Destructive (Convergent) Margin

• Plates move towards each other. Types:

  • Oceanic + Continental: Oceanic plate subducts beneath the continental plate (e.g., Nazca and South American plates).

    • Creates composite volcanoes, ocean trenches, earthquakes, and violent eruptions.

  • Continental + Continental: Plates collide, neither subducts (e.g., Indian and Eurasian plates).

    • Forms fold mountains (e.g., Himalayas), causes earthquakes, but no volcanoes.

• 3. Conservative (Transform) Margin

• Plates slide past each other (e.g., San Andreas Fault in California).

• Crust is neither created nor destroyed.

• Causes earthquakes, often severe, but no volcanoes.