Geo11 - Plate tectonics

• Alfred L. Wegener introduced the Continental Drift Theory in 1915
• The continental drift theory suggests that all continents were joined into a single large landmass called Pangaea around ~280 million years ago
• Continents further drifted apart until they reached their present positions today
• Wegener's evidences for continental drift were based on large scale geological and paleontological datasets
• Wegener's evidences for continental drift


  1. Fit of the continents

  2. Spread of certain index fossils

  3. Similar lithologies (rock types) across continents

  4. Paleoclimate evidences
• Wegener used the jigsaw puzzle-like fit of coastlines of some continents as evidence for continental drift
• Wegener noticed the spread of certain index fossils across continents separated by vast oceans as evidence for continental drift
• Similar lithologies (rock types) across continents were observed by Wegener as evidence for continental drift
• Wegener observed paleoclimate evidences such as glacial deposits in various continents as evidence for continental drift
• Paleoclimate evidences

  • Glacial deposits of the Carboniferous and Permian Karoo Glaciation found in South America, Southern Africa, India, Antarctica, and Australia
  • Tropical plant fossils and coal deposits observed in frozen regions such as the Canadian Arctic, Europe, and Asia
• Presence of glacial deposits in warm regions

  Assuming that the continents were once joined
• Regions with tropical plants and coal deposits

  • Possibly situated closer to the equator with a more tropical climate
  • Glacial deposits in several continents indicate they were closer to the southernmost hemisphere
• Wegener's continental drift theory was publicly criticized and rejected by scientists during his time
• Main reason for criticism of Wegener's theory
• Wegener proposed that continents were like icebergs floating on the heavier ocean crust and their movement was due to centripetal force of the Earth's rotation, lunar, and solar tidal forces
• Forces proposed by Wegener were considered too weak to move continents
• Wegener was considered an outsider in the field of geology due to his background in astronomy and meteorology
• Wegener's theory was dismissed by most geologists of the day until his death in an expedition in Greenland in 1930
• Seafloor spreading theory proposed by Harry Hammond Hess in 1960 and coined by Robert Dietz in 1961
• Seafloor spreading theory suggests that seafloor moves and carries the crust with it as it spreads from a central rift axis
• New crust is created at the rift axis by convection of the mantle, pushing older materials away
• Mapping of the seafloor's topography led to the discovery of oceanic ridges and trenches
• Young age of the seafloor (<200 My) was observed
• Rocks are older farther from the center of spreading
• Concept of magnetic reversals along mid-oceanic ridges supports seafloor spreading theory
• Paleomagnetism (Remnant Magnetism) is based on the earth's magnetic field produced by the geodynamo in the earth's outer core
• Magnetic references like north and south poles are different from the geographic north and geographic south
• Magnetic north is usually located around 11.5° away from the geographic north (true north) and is the direction where compasses point to
• Rocks usually contain magnetic minerals such as magnetite
• At high temperatures in the magma above the Curie temperature, magnetic minerals are randomly oriented
• Formation of rocks

  Magnetic minerals align themselves in the direction of the existing magnetic field at the time they were formed upon solidification below the Curie temperature
• Paleomagnetism is the record of Earth’s magnetic field through time
• Steeper dip angles indicate rocks formed closer to the magnetic poles
• Magnetic inclination is 0° in the equator and 90° in the poles
• Rocks with increasing age point to pole locations increasingly far from present magnetic pole positions
• Initially, it was thought that the position of the magnetic pole varied with time
• Further studies of pole position in different continents supported the hypothesis that the different continents moved relative to each other over time
• Magnetic inclination of different rock layers can be used to determine the movement of plates in the past
• A moving continent retains a record of changing paleomagnetic directions through time
• The resulting path of observed pole positions is called an “apparent polar wander path” (APWP)