Plate Tectonics

Introduction to Plate Tectonics

• Plate tectonics is the unifying theory of geology.

• Explains phenomena such as volcanoes, earthquakes, and tsunamis as interrelated.

• Evolving science based on the concept of continental drift.

Historical Development

• Continental Drift:

  • Early ideas proposed by Edward Seuss and Alfred Wegener.

  • Idea of Pangaea, a supercontinent, introduced in 1915.

  • Alexander Du Toit introduced Laurasia and climatic paradox.

Evidence Supporting Continental Drift

• Continental Fit:

  • Modern continents fit together like a jigsaw puzzle, best along continental slopes.

• Fossil Evidence:

  • Shared fossils (e.g., Glossopteris, Mesosaurus) across distant continents.

• Similarity of Rock Sequences:

  • Similar rock sequences found in Carboniferous to Jurassic age across continents of Gondwana.

• Glacial Evidence:

  • Striations in bedrock indicate the historical movement of glaciers across continents.

Paleomagnetism

• Studies ancient magnetic fields to understand continental positions.

• Earth’s magnetic field captures the orientation of iron-bearing minerals during lava flow cooling.

• Magnetic reversals indicate that continents have shifted from their current positions.

Seafloor Spreading

• Proposed by Harry Hess in the 1950s, indicating that new oceanic crust forms along oceanic ridges.

• Evidence of symmetric magnetic anomalies and sediment age corroborates this theory.

Plate Tectonic Mechanism

• Lithosphere (rigid plates) floats on semi-plastic asthenosphere.

• Heat from the mantle drives lateral movement of lithospheric plates.

Types of Plate Boundaries

• Divergent: e.g., Mid-Atlantic Ridge

• Convergent:

  • Oceanic-Oceanic: e.g., Aleutian Islands

  • Oceanic-Continental: e.g., Andes

  • Continental-Continental: e.g., Himalayas

• Transform: e.g., San Andreas Fault