Plate Tectonics Overview

Layers of Earth

• Crust: Outermost layer, part of the lithosphere
  • Two types:
    • Oceanic: Dense, thin, mainly basalt
    • Continental: Less dense, thick, mainly granite
• Lithosphere: Rigid outer layer, includes crust and upper mantle
• Asthenosphere: Below the lithosphere, plastic-like, part of the mantle
• Mantle: Composed of semi-fluid rock, convection currents occur
• Core:
  • Outer Core: Molten rock
  • Inner Core: Solid rock

Tectonic Plates

• Earth is covered by tectonic plates (7 large, several small)
• Plates move due to convection currents in the mantle

Plate Movements

• Divergent Boundaries: Plates move apart, new crust forms
• Convergent Boundaries: Plates move towards each other, crust is destroyed
  • Oceanic-Oceanic: One plate subducts, forms trench and volcanoes
  • Oceanic-Continental: Oceanic plate subducts, forms mountains or volcanoes
  • Continental-Continental: Plates collide, form mountain ranges
• Transform Boundaries: Plates slide past each other, cause earthquakes

Sea Floor Spreading

• Occurs at divergent boundaries, creates ocean ridges
• Example locations:
  • Mid-Atlantic Ridge: Spreads 2.5 cm/year
  • East Pacific Rise: Spreads 8-12 cm/year
• Older crust found away from ridges, new crust near ridges

Convection in the Mantle

• Convection currents in the asthenosphere created by heat from the core
• Hot mantle rises, spreads, cools, and sinks, driving plate movement

Plate Boundary Landforms

• Mountain Ranges: Form at convergent boundaries (e.g., Andes, Himalayas)
• Ocean Ridges: Form at divergent boundaries (e.g., Mid-Atlantic Ridge)
• Ocean Trenches: Form at subduction zones (e.g., Mariana Trench)
• Volcanoes: Often form at convergent boundaries (e.g., Mount Etna, Kilauea)

Continental Drift Theory

• Proposed by Alfred Wegener, 1915
• Pangaea: Supercontinent existing 250 million years ago
• Split into Laurasia (North) and Gondwana (South), further into current continents
• Continents continue moving; North/South America westward, Australia northward

Questions for Review

1. What is a tectonic plate?
2. What causes the movement of tectonic plates?
3. Differences between oceanic and continental crust?
4. Compare movements at plate boundaries.
5. How might the Atlantic and Pacific Oceans change over time?
6. What is sea floor spreading and associated landforms?
7. How does crust age change relative to ocean ridges?
8. Compare sea floor spreading rates at different locations.
9. What is convection and its role in plate movement?
10. Details of convergent, divergent, and transform boundaries.
11. What is the Continental Drift Theory, and how have continents moved?