Plate tectonics- Unit 4

In the 19th century, scientists still believed that Earth’s crust was static.

Wegener proposed the idea of Pangaea and that the continents were once joined together.

Pangaea then broke up into two smaller supercontinents:

  • Laurasia- Asia, Europe and North America
  • Gondwana- Africa, South America, Antarctica, Australia and India.

EVIDENCE

  • Coastlines appear to fit together like a puzzle.
  • Fossil Correlation- fossils from some species can be found on opposite sides of major oceans (e.g Mesosaurus fossils in South America and Africa)
  • Rock and mountain correlation- identical rock composition and mountain structures found on opposite sides of oceans.
  • Paleoclimate data- Coal deposits (which suggest tropical climates) have been found in cold areas, glacial erosion has been found in warm areas.

PLATE TECTONIC THEORY

The theory that the Earth’s lithosphere (crust and outer mantle) is a number of tectonic plates that fit together like a jigsaw puzzle.

Continental crust

  • Thicker of the two crusts.
  • Less dense

Oceanic crust

  • Much thinner
  • More dense

Tectonic plates can move due to the forces caused by convection currents in the earth’s mantle.

  • Hot rock rises upwards and toward the crust.
  • The crust forces the hot rock to move outwards.
  • Rock cools and moves down towards the core.

EVIDENCE

Seafloor spreading

  • New oceanic crust is formed through volcanic activity at mid-ocean ridges and slowly moves from where it is formed.

Magnetic striping

  • When the earth’s magnetic field reverses, a new stripe, with different polarity, begins.
  • This led to the recognition of seafloor spreading.

PLATE BOUNDARIES

Convergent boundary- where two tectonic plates push together, can form volcanoes, ocean trenches, mountain ranges.

Divergent boundary- where two tectonic plates spread apart, can form mid ocean ridges, rift valleys, volcanoes, seafloor spreading.

Transform boundary- where two tectonic plates slide against one another, can cause earthquakes.

OTHER INFO

  • Oceanic crust goes under continental crust and this is called a subduction zone.