Evidence for continental drift & plate tectonics

• Structure of Earth; Core


  Centre of the Earth.

  Iron-nickel mass that gives Earth it’s magnetic field.

  Inner core is 1250 km thick.

  Outer core is liquid and is 2200 km thick.
• Structure of Earth; Mantle


  Makes up 80% of the volume of Earth.

  Consists of semi-solid rock containing silicon and oxygen.

  The asthenosphere extends from 100 km to 300 km, and has plastic properties that allow it to flow.

  Above this is the lithosphere which is rigid.
• Structure of Earth: Crust


  Outer shell made up of oceanic and continental crust.

  Oceanic crust is made up of dense basalt rock and is an average of 5 km deep.

  Continental crust is made up of mainly granite (less dense than basalt), and can be up to 100 km deep (under major mountain ranges).
• Convection currents are the continuous movement of a gas or liquid which when warmed rises and when cooled sinks.

  The current in the asthenosphere carries the solid lithosphere and crust (i.e. oceanic and continental plates).
• Evidence of sea-floor spreading

  The process of new oceanic crust forming at mid-ocean ridges due to the upwelling of magma from the asthenosphere.

  This pushes away older rock from the plate ridges.
• Biological evidence for continental drift

  Similar fossils reptiles found in both South America and South Africa.

  Similar fossil shellfish found in Australian and Indian limestones.
• Geological evidence for continental drift

  Mountain chains and some rock sequences on either side of oceans are very similar e.g. Northeast Canada and Northern Scotland.

  Continents such as South America and Africa fit together.

  India, Australia, Southern Africa, South America and Antartica all share effects of contemporaneous glaciation from 290 million years ago; this suggests they were all connected with the South Pole.
• Alfred Wegener invented the theory continental drift in 1912; in the Carboniferous period (250 million years ago), there was a single tectonic plate (Pangaea).
• Fossil records

  Similar fossils are found on separate continents (Australia & India).

  If they evolved independently they would be contradicting Darwin’s Theory of Evolution.
• Palaeomagnetism


  The magnetic orientation of iron particles within cooled solidified molten rock that has erupted.

  Direction of Earth’s magnetic field changes every 40000 to 500000 years.
• Age of sea-floor rocks

  In the 1960s an ocean drilling programme showed that the thickest and oldest sediments were near the continents and younger deposits were further out in the ocean, supporting sea-floor spreading.
• Evidence from ancient glaciations.

  Glacial deposits from the Permo-Carboniferous glacation are found in Africa, South America, Antarctica, India and Australia.
• Divergent Plate Boundaries; Rift valleys


  Formed when plates move apart in continental areas.

  Brittle crust fractures as sections move and areas of crust drop down between parallel faults to form a valley, e.g East African Rift Valley.
• Divergent Plate Boundaries; Ocean Ridges


  Formed when plates move apart in oceanic areas. The space between the plates is filled with basaltic lava from below to form a ridge. Volcanoes also exist along this ridge and may rise above sea level e.g Surtsey, South of Iceland.
• Oceanic-continental plate margins

  Different densities of plates

  Denser oceanic plate subducts under the continental plate.

  A deep ocean trench is formed at the plate boundary.

  Sediments and rocks fold and are uplifted along the leading edge of the continental plate.

  Continental crust buckles and mountain chains form e.g. the Andes.

  Faulting occurs in the Wadati–Benioff zone, releasing energy in the form of earthquakes.
• Oceanic-oceanic plate margins


  The slightly denser plate will subduct under the other, creating a trench.

  Descending plate melts, magma rises and chains of volcanoes (island arcs) are formed e.g. the Antilles.
• Continental-Continental plate margins


  Little if any subduction because of similar densities.

  Impact and pressure tends to form fold mountains e.g. the Himalayas.
• Conservative plate boundaries


  The two plates sliding past each other create violent earthquakes due to the additional build-up of pressure E.g. the San Andreas fault between the Pacific and North American Plates.