Plate Tectonics

Look at a topo map of the earth

Note how the continents fit together.

The German meteorologist Alfred Wegener advocated "continental drift" in 1924. But the idea seemed absurd.

Thinking changed in the 1960's or so...
Some evidence:

• Fossil records are similar between South America and Africa.
• Some types of rocks match between South America and Africa.
• There is a ridge in the mid-Atlantic.

The favored theory now: plate tectonics

• the crust is divided into rigid plates
• the plates move (a few cm per year)
• the driving force is convection currents in the gooey mantle
• at plate boundaries there are several possibilities
  • manufacture of new oceanic plate
  • plates slide along each other
  • continental plates crash into each other
  • oceanic plate is subducted under a continental plate
    

    Look first at evidence from a great vacation spot.

    Then examine new plate formation.

    What happens when two continental plates push into each other?

    What about two plates moving parallel to each other?

    What happens when oceanic plate runs into continental plate?

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