1G03 lecture 6 Plate Tectonics III W25 avenue2

Plate Tectonics Overview

• Definition: The theory that explains the movement of the Earth's lithosphere which is divided into tectonic plates.

Sea-Floor Spreading

• Key Concept: The ocean crust must be destroyed somewhere if the Earth is not expanding or contracting.

• Magnetic Reversals: Refers to switches in Earth's magnetic field, detected in bands parallel to Mid-Ocean Ridges (MORs).

• Evidence: Magnetic reversals can be dated using radioactive dating from land-based volcanoes.

• Age of ocean floor increases with distance from MORs, with the oldest being approximately 200 million years old.

• Rates of sea-floor spreading can be calculated based on distance from MORs.

  • Example Calculation: If 2000 km corresponds to 200 millionyears, the spreading rate is approximately 1 cm/year.

Plate Boundaries

• Types of Boundaries:

  1. Transform Boundaries: Two plates slide past one another; no crust is created or destroyed.

  2. Divergent Boundaries: Plates move apart (e.g., Mid-Ocean Ridges); new crust is created.

  3. Convergent Boundaries: Plates collide; crust is recycled.

Specific Examples

• Divergent Boundaries: Active volcanoes occur at divergence sites; formed in rift valleys (e.g., Red Sea, Gulf of California).

• Convergent Boundaries:

  • Ocean-Ocean: Example - Japanese Islands and Japan Trench.

  • Ocean-Continent: Example - Andes Mountains from Nazca Plate subducting under South American Plate.

  • Continent-Continent: Leads to high topography and mountain ranges like the Himalayas.

Geographical Context

• The North American Plate is bordered by:

  • Divergent boundary to the east.

  • Convergent and transform boundaries to the west (e.g., San Andreas Fault).

Implications of Plate Movements

• The movements of tectonic plates result in earthquakes, volcanic activity, and the formation of mountain ranges, affecting the Earth's landscape and environment.