Tectonic Plates

• Alfred Wegener, a German scientist and meteorologist, noticed that the coastlines of South America and Africa fit very closely
• Alfred Wegener noticed the coastlines of South America and Africa fit very closely


  1915
• All continents were once joined together to form a supercontinent named Pangaea
• Pangaea began breaking up into smaller continents and drifted apart about 200 million years ago
• Alfred Wegener proposed his theory of continental drift


  1915
• Alfred Wegener presented a variety of evidence to support his hypothesis of continental drift: Similar plant and animal fossils, Matching rock types and mountain chains, Glacial evidence
• Fossil evidence: several of the same organism’s fossils were found on different landmasses. Wegener reasoned that these organisms couldn’t have crossed oceans presently separating the continents
• Matching rock types in several mountain belts that today are separated by oceans provided evidence for continental drift. E.g. The Appalachian mountains in coastal North America are similar in age
• No signs of land bridges connecting the continents were ever found
• Matching Fossils


  • cynognathus
  • lystrosaurus
  • mesosaurus
• Matching Rock Types


  • Matching rock types in several mountain belts that today are separated by oceans provided evidence for continental drift. E.g. The Appalachian mountains in coastal North America are similar in age and structure as those found in the British Isles and Scandinavia
• Glacial Evidence


  • Wegener found glacial deposits showing that between 220 million and 300 million years ago, ice sheets covered large areas of the Southern Hemisphere. Deposits of glacial till (sediment) occurred at latitudes that today have temperate and tropical climates: Southern Africa, South America, India, Australia. Glaciers cause grooves in the Earth’s bedrock as they move outward. Grooves and rock deposits left by ancient glaciers have been found on different continents very close to the equator. Wegener suggested that the glaciers were centered over the South Pole and the continents moved to their present positions later on
• Wegener’s hypothesis was rejected by most other scientists. He could explain and provide evidence that Pangaea once existed, however he could not describe a mechanism capable of moving the continents. He could not explain how or why continents move. Scientists who rejected Wegener and refused to believe that the continents drifted were referred to as Fixists
• Plate Tectonics


  Harry Hess was able to map the ocean floor using SONAR. Earthquake and magnetic field data were also now available. By 1967, these findings led to a new theory called Plate Tectonics which proved that Wegner was correct that the continents move. Sea-floor spreading was causing the plates to move. Sadly, Wegener was not alive long enough to see his contributions come to fruition
• Alfred Wegener contributed to plate tectonics
• Wegener noticed that the continents moved
• The supercontinent created was called Pangaea and it existed around 220-300 million years ago
• The movement of plates results in earthquakes, volcanic activity, mountain-building, and oceanic trench formation
• Sea-floor spreading is the process by which new oceanic crust is formed at mid-ocean ridges and then moves away from the ridge
• Convergent Boundaries: Oceanic + oceanic


  • When 2 oceanic crusts collide and one goes under the other
  • Creates a subduction zone
  • Associated with volcanic activity, island arcs and deep ocean trenches
• Convergent Boundaries: Oceanic + continental


  • Oceanic and continental crusts collide
  • Creates a subduction zone
  • Associated with deep ocean trenches and island arcs
• Convergent Boundaries: continental and continental

  • 2 continental plates collide
  • Creates mountains
  • Both plates are the same density and crunch together
• Divergent Boundaries 


  • Convection currents push molten material up
  • Causes 2 continental plates to separate
  • Creates a rift valley
• Transform Boundary


  • Is a result of other boundary movements
  • Causes plates to shift laterally or sideways
  • Associated with earthquakes at fault line
• Harry Hess discovered mid-ocean ridges
• Mid-ocean ridge

  Place in Earth's crust where magma rises through a volcanic vent
• Seafloor spreading

  1. New crust pushes older crust away from volcanic vent

  2. When oceanic crust spreads and new oceanic crust is formed
• Seafloor spreading is the last piece of evidence that supported plate tectonics
• Continents sit on plates
• Plate boundaries are the cracks between plates
• Types of Plate Boundaries

  • Convergent
  • Divergent
  • Transform
• Constructive plate boundaries


  1. Plates move apart

  2. Magma rises

  3. Fills the gap

  4. Forms volcanoes and new crust
• Destructive plate boundaries

  1. Oceanic plate subducts under continental plate

  2. Friction causes melting of oceanic plate

  3. Leads to earthquakes and volcanic eruptions
• Ways the movement of plate tectonics can be destructive


  • Transforming plate boundaries can cause earthquakes
  • Transforming boundaries can cause tsunamis
• Most earthquakes occur in Japan because it lies near the boundaries of 3 tectonic plates: The Pacific, Eurasian and Philippine Plates
• The movement of tectonic plates can be constructive when two plates move apart, allowing magma to rise up and fill the gaps
• Volcanoes


  • Found at plate boundaries
  • Where tectonic activity is high
• Locations of volcanoes


  • Convergent boundaries
  • Divergent boundaries
  • Hot spots
• Volcanoes forming at plate boundaries

  1. Tectonic plates collide and separate

  2. Magma forces its way to the surface
• Most volcanoes are found around the boundaries of tectonic plates, both on land and in the ocean