disasters

• The earth's surface is dynamic. Earthquakes, volcanic activity and other geological processes have been gradually changing it throughout millions of years
• Plate tectonics


  The key geologic theory that explains how the earth's surface had changed
• Alfred Wegener's theory of continental drift


  • Proposed that the earth's continents were once one - naming it Pangaea
  • Suggested that Pangaea began to break up about 200 millions years ago
  • Rejected due to lack of evidence
• Harry Hess's discovery


  • Used naval submarine detection technology to discover undersea volcanos at the Atlantic Ocean
  • Suggested that the seafloor was spreading outwards from the ridges
  • Confirmed in 1963 by Fred Vine and Drummond Matthews
• Types of tectonic plates


  • Continental plates (less dense, make up major land masses)
  • Oceanic plates (more dense, lie under the planet's oceans)
• Plate boundary


  The area where two plates meet
• Types of plate boundaries


  • Divergent (plates moving apart, new land formation)
  • Convergent (plates moving towards each other, land destruction)
  • Transform (plates sliding past each other, no land creation/destruction)
• Divergent plate boundary


  1. Plates move apart

  2. Magma rises from the mantle and reaches the earth's surface

  3. Magma solidifies and forms new crust

  4. Process repeats over time leading to volcano formation
• Seafloor spreading


  1. Base of continental crust heated and becomes less dense

  2. Crust begins to rise and forms a dome

  3. Dome fractures upwards and grows into an expanding rift

  4. Magma from the mantle emerges through the rift

  5. Magma cools and forms an oceanic crust, creating a mid ocean ridge
• New crust is continually being created due to seafloor spreading
• GPS satellites monitor the rate of seafloor spreading. The mid Atlantic ridge grows about 1"/year. Other seafloors grow almost 8"/year