Plate Tectonics - the idea that a planet's crust is broken into lithospheric "plates" that float on the semi-solid mantle and move due to convection currents in the mantle.

The idea originated with Alfred Wegener who wondered why the coasts of the eastern and western hemispheres look like they fit together like pieces of a puzzle.  His idea was known as Continental Drift.  This idea has been replaced by the modern concepts of plate tectonics.

4 types of interaction can occur at the margins of the plates:

1. Divergent margins - plates move away from each other.  This causes a rift zone where magma rises up from below the crust.  Example: the Mid-Atlantic Ridge.  This process creates new seafloor and accounts for seafloor spreading, which makes the Atlantic Ocean wider.

2. Convergent margins - plates move toward each other

   a. subduction - one plate is forced under the other.  Example: the Japan Trench, the northwestern US.  This process produces chains of volcanic islands such as the Aleutian Islands and produces volcanic mountains such as the Andes mountains of South America. 

   b. collision - two plates collide with little or no subduction, resulting in wrinkling of the crust.  Example: the Himalaya Mountains.

3. Transform fault margins - plates move parallel to each other.  The line of contact is called a fault.  The motion is very slow, and friction causes pressure to build.  When the pressure is released with a sudden movement, an earthquake occurs.  Example: the San Andreas Fault.