plate tectonic theory

• Plate Tectonics Theory


  Continental Drift
• Alfred Wegener


  • German geologist and meteorologist
  • Proposed the first comprehensive theory of continental drift
  • Studied weather and the Earth
  • Based his theory on biologic and geologic evidence
• Pangaea


  A huge landmass that was surrounded by a single ocean Panthalassa
• Breakup of Pangaea


  1. Pangaea began to break up

  2. Continents slowly drifted to their present locations
• Wegener noticed that Africa and South America fit together almost like pieces in a giant jigsaw puzzle
• Wegener called his theory the Continental Drift Theory
• Evidence for continental drift

  • Climates of the distant past
  • Glacial debris and fossil remains in Argentina, Brazil, South Africa, Antarctica, India, and Australia
  • Skull of the Lystrosaurus reptile found in Antarctica
  • Magnetism in ancient rocks
• Rocks containing iron-bearing minerals have formed throughout geologic time
• When pole positions are plotted through time for both European and North American continents, the pole positions look alike but are separated by the ocean
• If the continents are put back together, the pole positions would fit
• Apparent polar wandering helps support continental drift
• Echo sounding


  1. Instrument on board ship sends out pulse of high-frequency sound waves

  2. Time for sound to reach seafloor and return is used to calculate water depth

  3. Structure of seafloor revealed by detonating underwater explosive and studying returning echoes
• Seafloor sampling


  1. Hollow tube dropped on long cable to seafloor, then pulled up with sediment samples

  2. Sediment samples studied for composition, age, fossil content, magnetization
• Ocean floors are geologically very young, dating back only about 135 million years, while continental rocks date back 4,000 million years
• There is a system of deep trenches in the Pacific Ocean, parallel to belts of earthquakes and volcanoes
• Narrow mid-ocean ridges can be found across all oceans
• Ocean floor spreading


  Bulging of ocean floor, opening cracks that allow basaltic material to move out, hardening to form new ocean floor and expand ocean basin
• Plate tectonics


  1. Earth's crust broken into large and small floating plates

  2. Plates carried by convection currents in mantle, may bump into each other

  3. Earthquakes common where plates slide past each other

  4. Undersea volcanoes found near spreading margins of moving plates

  5. At one plate boundary, molten rock rises to form new ocean floor

  6. At opposite boundary, plate slides underneath adjacent plate and melts in asthenosphere, forming oceanic trench
• Plate boundaries


  Convergent boundaries - plates push against each other

  Divergent boundaries - plates pull away from each other
• Convergent boundaries


  • Oceanic-oceanic
  • Oceanic-continental
  • Continental-continental
• Convergent boundaries


  1. Plate edges bent downward into a deep trench (subduction zone)

  2. Plate melts in asthenosphere

  3. Buoyant materials carried upward to form volcanoes and intrusive rock
• Subduction zone


  • Forms a trench along coasts of Peru and Chile
  • Forms the Andes mountain range at the end of the American Plate
• Some rocks in island arcs consist of lighter materials of the lithosphere plates carried downward to melt in the hot asthenosphere, then buoyantly carried upward again to form volcanoes and intrusive rock
• Continents are drifting across the globe and new land masses are being formed at the various land arcs
• If present trend continues

  • Atlantic Ocean will be wider, Pacific Ocean narrower
  • California will detach from North America
  • Arabian Peninsula will become part of Asia
  • East Africa will break away from Africa
  • West Indies island arc will become a land bridge between Americas and Western Pacific
• The face of Earth will probably continue to change just as it has been changing since it was formed